UNIVERSITY OF CINCINNATI
DATE: November 25 2003
I, SREERAM APPASAMY , hereby submit this as part of the requirements for the degree of: MASTER OF SCIENCE in: ELECTRICAL ENGINEERING It is entitled: DEVELOPMENT OF HIGH THROUGHPUT PLASTIC MICROLENSES USING A REPLACEABLE INJECTION MOLD DISK
Approved by:
Dr. Chong H. Ahn Dr. Joseph T. Boyd Dr. Joseph H. Nevin
DEVELOPMENT OF HIGH THROUGHPUT PLASTIC MICROLENSES USING A REPLACEABLE INJECTION MOLD DISK
A thesis submitted to the
Division of Research and Advanced Studies of the University of Cincinnati
in partial fulfillment of the requirements for the degree of
MASTER OF SCIENCE
in the department of Electrical and Computer engineering and Computer Science of the College of Engineering
2003
by
Sreeram Appasamy
B.E, Madras University, India 2000
Committee Chair: Dr.Chong H. Ahn
ABSTRACT
The objective of this work is to develop plastic microlenses using high throughput injection molding techniques. In this work, plastic microlenses have been designed, fabricated and characterized for future applications in optical communications and biochemical chip detection systems.
Microlenses, that have wide ranging applications in image processing, displays, communications and biochemical detection have been researched for the past two decades. Various fabrication techniques have been developed using a wide range of materials.
Plastic as a material for fabricating microlenses has been investigated in recent years.
Soft lithography and hot embossing are examples of two of the techniques that have been used to fabricate plastic microlenses. The primary reason for using plastics as the material is their potential for high volume replication at a very low cost. However, a suitable technique for high volume fabrication of plastic microlenses has not been optimized yet.
This work has successfully implemented a fabrication technique that enables high volume replication of plastic microlenses using injection molding techniques. A replaceable injection mold disk was fabricated using metal electroforming. The initial microlens mold was realized using the photoresist reflow technique. The fabricated mold disk was used as the master mold for the injection molding process.
The microlenses were fabricated using two new plastics, COC and Poly IR 2 that have excellent optical transparency in the visible and infrared wavelength regions respectively.
The plastic microlenses were characterized in terms of their focal lengths and surface roughness.
i The plastic microlenses developed in this work hold lot of promise for numerous applications in optical communications and biochemical detection.
ii ACKNOWLEDGEMENTS
At the outset, I would like to thank the Almighty for giving me the strength and perseverance to successfully complete my graduate degree.
Dr.Chong Ahn, my advisor, is the primary reason why I have been able to successfully finish my master‘s degree. His vision and energy are qualities that have kept me going and I have been extremely lucky to have his immense guidance during my study. He has not only guided me through my master‘s thesis, but also helped mould me into a better person than what I was. He has a great sense of humor to go with his other
qualities and there have been several instances where he has broken a tense situation in
meetings with his humor. I would consider it an achievement if I imbibe even a small
percent of his wonderful attributes. It‘s been a huge learning experience working with
him and I am sure it will help me throughout my life.
I would also like to convey my deep gratitude to Jeff Simkins who has helped me
throughout my work and I don‘t know if I can meet a kinder and more helpful person in
the future. Robert Cole has been a great friend and guide and his friendship is something
I will always remember. Ram, my former roommate, has been a good friend and has
always come out with excellent suggestions to aid my work.
The people whom I have to thank most are my labmates who had made working at the
BioMEMS lab such a pleasure and I would treasure the memories forever. My sincere
thanks to Chuan, Sehwaan, Xiaoshan, Hyoung Jin, Anirudhha, Sukirti, Kai, Red,
JungYoup, Yasser, Jaephil, Rong, Chunyan, Alok, and Phalgun. They all have their own
iii unique sense of humour, particularly Chuan, Hyoung Jin, Anirudhha and Yasser, and it‘s been a total privilege to work among such friends. I will never forget the picnics; the soccer and great food particularly. I really really wish everyone attains great success and
prosperity in their careers and personal lives. I would also like to acknowledge Dr.Choi
and Dr.Kim for their guidance and advice.
I would like to especially acknowledge W eizhuo Li for working with me in setting up
the optical system . I would also like to thank Dr.Joseph Boyd for allowing me use his
laboratory and also being kind enough to help me with his suggestions and ideas. I would
also like to extend my thanks to Ron Flenikken and Don Dotson for helping me with
various things during the course of study.
On the personal front, I would start of by thanking my parents whose contributions I
really don‘t need to emphasize and also my brother who is also currently at the
University of Cincinnati.
I would also express my gratitude to my close friends Ismail Raja, Arun and Kumar
for being such great friends. My special thanks to Priya and Mala, who have been my
closest friends and will always be for their love and friendship. There have been so many
others who have put their own memorable imprints on my life and I thank all those for
their love, friendship and support.
iv TABLE OF CONTENTS
Abstract… … … … … … … … … … … … … … … … … … … … … … … … … .. i
Acknowledgements… … … … … … … … … … … … … … … … … … … … … iii
List of Figures… … … … … … … … … … … … … … … … … … … … … … .....3
List of Tables… … … … … … … … … … … … … … … … … … … … .… … .… 6
Chapter1: Introduction
1.1: MEMS Technology… … … … … … … … … … … … … … … … … … … ..… … ..7
1.2: Motivation for this work… … … … … … … … … … … … … … … … … … … … 8
Chapter 2: Background
2.1: Introduction… … … … … … … … … … … … … … … … … … … … … … … … … 10
2.2: Evolving fabrication techniques… … … … … … … … … … … … … ..… … … .. .11
2.3: Applications of microlenses… … … … … … … … … … … … … ..… … … … … 16
2.4: Microlenses for µTAS… … … … … … … … … … … … … … … … … … … … … .18
2.5: Conclusions… … … … … … … … … … … … … … … … … … … ...… … … .… … 20
Chapter 3: Fabrication of the Replaceable Injection M old Disk
3.1: Introduction… … … … … … … … … … … … … … … … … … … … … .… … … … 21
3.2 Photolithography and reflow to form microlenses… … … … … … … … … ..… 21
3.2.1 Melting on a hot plate… … … … … … … … … … … … … … … … … … … … 21 3.2.2 Melting using an oven… … … … … … … … … … … … … … … … … … .… ..25
3.3: Electroforming to fabricate the replaceable injection mold disk… … … ..… ..27 3.3.1 Electroforming … … … … … … … … .… … … … … … … … … .… … ..… ..27 3.3.2 Nickel electroforming… … … … … … … … … ..… … … ..… … … … … … 29 3.4 Conclusions… … … … … … … … … … … … … … … … … … .… … … … … … … .34
1 Chapter 4: M icrofabrication of Plastic M icrolenses Using Injection M olding
4.1: Introduction… … … … … … … … … … … … … … ..… … … … … … … .… … … … 35
4.2 Injection molding… … … … … … … … … … … … … … … … .… … … … .… … … 36
4.3 Plastic microlenses fabricated on COC (Cyclo Olefin Copolymer)… … ..… … .38 . 4.4 Plastic microlenses fabricated on Poly IR 2… … … … … … … … … … .… .. … ...43
4.5 Conclusions… … … … … … … … … … … … … … … … … … … … … … ...… .… … 44
Chapter 5: Characterization of Plastic M icrolenses
5.1: Introduction… … … … … … … … … … … … … … … … … … … … … … … … … … 46
5.2 Plano convex lens… … … … … … … … … … … … … … … … … … … … … … … … 46
5.3 Focal length determined by surface profile of microlens… … … … … … .… … 47
5.4 Characterization of microlenses on COC… … … … … … … … … … … ..… .… … .50
5.4.1 Focal length and spot size measurement by observation of focal plane… … … … … … … … … … … … … … … … … … … … … … … .… .50 5.4.2 Focal length measurement by fiber-microlens-fiber power coupling… … … … … … … … … … … … .… … … … … … … … … … … … … … ..52
5.5 Characterization of microlenses on Poly IR2… … … … … … … … … ...… … … ...58
5.6 Conclusions… … … … … … … … … ..… … … … … … … … .… … … … … … … … ...61
Chapter 6: Conclusions
6.1: Summary… … … … … … … … … … … … … … … … … … … … … … … … … … … .61
6.2 Future W ork… … … … … … … … … … … … … … … … … … … … … … … … … … .62
References… … … … … … … … … … … … … … … … … … … … … … … ..… ...64
2 LIST OF FIGURES
Figure 2.1: Formation of graded index lens by ion diffusion through a metal mask.
Figure 2.2: Microlens made by the photo thermal technique.
Figure 2.3: Microlens fabricated on silicon.
Figure 2.4: Fabrication of silicon microlens by bulk micromachining.
Figure 2.5: Microlenses made on PMMA using proton irradiation.
Figure 2.6: Schematic of use of microlens array to improve the efficiency of CCD array.
Figure 2.7: Schematic of microlens application in beam steering using an afocal pair of microlenses.
Figure 2.8: (a) Schematic of a µTAS chip using micro lens arrays for illumination and detection in parallel micro capillaries of a chemical chip. (b) A large NA microlens fabricated on a microfluidic chip to improve efficiency in a fluorescence detection system.
Figure 3.1: SEM pictures of photoresist microlenses: (a) 500 µm diameter microlens array and (b) close up of a 600 µm diameter microlens.
Figure 3.2: SEM pictures of photoresist microlenses: (a) 600 µm diameter microlens array and (b) 300 µm diameter microlens array.
Figure 3.3: SEM pictures of photoresist microlens: (a) 200 µm diameter microlens array (b) a 500 µm microlens array.
Figure 3.4: Schematic of the process flow to fabricate the replaceable injection mold disk.
Figure 3.5: Digital camera images of (a) electroformed disk before machining and (b) after machining and fit as injection mold disk.
3 Figure 3.6: SEM images of the replaceable injection mold disk: (a) 300 µm diameter mold cavity and (b) 500 µm diameter mold cavity.
Figure 4.1: Schematic of the injection-molding machine with the replaceable mold insert inside the mold cavity.
Figure 4.2: Schematic of the injection molding process.
Figure 4.3:(a) Photograph of the BOY 22A injection molding machine and (b) the electroformed mold fit as a replaceable injection mold disk.
Figure 4.4: Optical transparency curve comparing COC, PMMA and PC.
Figure 4.5: SEM pictures of COC microlenses: (a) a 200 µm diameter microlens array and (b) a close view of 200 µm diameter lens array.
Figure 4.6: SEM pictures of COC microlenses: (a) a 300 µm diameter microlens array (b) a close up of 300 µm diameter microlenses.
Figure 4.7: SEM pictures of COC microlenses: (a) a 400 µm diameter microlens array and (b) closer view of the same array.
Figure 4.8: SEM pictures of COC microlenses: (a) a 600 µm diameter microlens array and (b) a close up of a 600 µm diameter microlens.
Figure 4.9: AFM images: (a) Image of an area of a 300 µm lens having an rms roughness of 9 nm and (b) image of a rough area of a 600 µm lens having an rms roughness of 39 nm.
Figure 4.10: Transmittance curve for the Poly IR 2 plastic material.
Figure 4.11: SEM pictures of poly IR microlenses: (a) a 300 µm diameter microlens and (b) a 700 µm diameter microlens array.
Figure 5.1: Schematic of a Plano convex lens as a converging and collimating system.
4 Figure 5.2: Parameters for a Plano-convex lens.
Figure 5.3: Surface profile of a 700 µm diameter microlens.
Figure 5.4: Schematic of the experiment to measure focal length of the microlenses by imaging the focal plane.
Figure 5.5: CCD images of the focused spot using a white light source through a (a) 500 µm diameter and (b) 300 µm diameter COC microlens.
Figure 5.6: Schematic of the fiber-microlens-fiber power coupling experiment to measure the focal length of the microlens.
Figure 5.7: Digital image of the fiber-microlens-fiber power coupling experimental set up: (a) Image showing the input fiber aligned with the microlens and (b) image showing a mis-alignment between the input fiber and microlens.
Figure 5.8: Power coupled into the receiving fiber plotted versus the fiber-fiber separation in the fiber-microlens-fiber power coupling experiment.
Figure 5.9: Plot comparing the values of the focal lengths measured using different methods for various microlens diameters.
Figure 5.10: CCD images of the focused spot of an infrared 1.3µm wavelength laser source through a (a) 500 µm diameter and (b) 300 µm diameter Poly IR 2 microlens.
5 LIST OF TABLES
Table 5.1: Tabulated values of the focal lengths and spot diameters determined from the surface profile of the COC microlenses for various diameters.
Table 5.2: Tabulated values of the measured focal length and spot diameters for various microlens diameters by imaging the focal plane.
Table 5.3: Focal length obtained by fiber-microlens-fiber power coupling experiment for various microlens diameters.
Table 5.4: Tabulated values of the focal lengths determined from the surface profile and observation of the focal plane of poly IR2 microlenses for various diameters.
6 CHAPTER 1
INTRODUCTION
1.1 M EM S technology
Microelectromechanical systems (MEMS) technology is an enabling tool for
fabrication of components, devices and systems in the micro scale. MEMS has wide
ranging applications in a variety of industries [1] such as automotive sectors, life sciences
[2] and telecommunications, which have embraced MEMS as the cutting edge technology
that will ultimately result in miniaturized, cost effective and fast systems.
Among the various applications for MEMS, telecommunications has large demands
for the development of RF-MEMS and optical MEMS components. MEMS has enabled
fabrication of miniaturized optical switches, microrelays, optical wave-guides,
micromirrors and microlenses that are envisaged to play a pivotal role in further
miniaturization of telecommunication systems. These constitute what is known as optical
MEMS [3].
It is very important to elucidate on the challenges that MEMS faces, with packaging
considerations and standardization being of primary concern. MEMS has pretty much
took off from IC fabrication technology but it still has some way to go before maturing
into a standardized technology with set rules and techniques.
Since it took off from IC fabrication technology, MEMS initially adopted silicon as
the primary material for fabrication. However, recently glass [4] and polymers are also
being considered as desirable MEMS materials for microfluidics and optical
communications.
7 Plastics are increasingly being used as the material for MEMS devices [5]. Plastics in
addition to the obvious advantages of being cheap have the potential for mass production
using replication methods borrowed from macro-molding processes like injection
molding and hot embossing. Most plastics have excellent biocompatibility, optical
transparency and ease of surface modification that makes them an excellent choice for
microfludic applications as well as optical MEMS.
1.2 M otivation for this work
Microlenses have been actively researched since the advantages of photonics over
electronics were realized. Microlens research has opened up new application areas in
image processing, displays, communications and biochemical analysis. Various
fabrication techniques have been developed with a wide range of materials being
investigated.
Several research groups are investigating plastic as a material for fabricating
microlenses. Soft lithography, screen-printing and hot embossing are some of the
techniques that have been used to fabricate plastic microlenses.
The primary reason for plastics being a material of choice is their potential for both
fabrication flexibility and high volume replication. However, a fabrication technique that
enables high volume production of plastic microlenses has not been optimized.
This work proceeds to develop a new fabrication technique that enables high volume
fabrication of plastic microlenses. In this work, plastic microlenses were fabricated using
high throughput injection molding techniques using a fully replaceable injection mold
disk. Microlenses have been fabricated using two new plastic materials, Cyclo Olefin
8 Copolymer (COC) and Poly IR 2, for applications in the visible and infrared wavelengths respectively.
Chapter 2 gives a detailed background of the various techniques that have been
adopted to fabricate microlenses. Chapter 3 describes the fabrication of the replaceable
injection disk using metal electroforming. Chapter 4 describes the injection molding
process to make plastic microlenses while chapter 5 describes the characterization of the
fabricated plastic microlenses.
Chapter 6 derives a conclusion and also suggests scope for future improvements to
this work. A list of references is given at the end.
9 CHAPTER 2
BACKGROUND
2.1 Introduction
This chapter describes the background of microlenses, including the commonly used fabrication techniques and applications of microlenses.
Since the late 1980s there has been an upsurge of interest in the development of microlenses.
This mainly came about with the realization of the benefits of photonics over electronics. The technologies used to fabricate microlens arrays have varied significantly both in the techniques tried and in the quality of microlenses being produced. MEMS technology has been pivotal in realizing novel fabrication methods on different substrates.
The applications of microlenses are diversified in various fields like imaging, beam shaping, displays, fiber optics, and lately in BioMEMS. Beam shaping applications include beam homogenizing and fan out elements. Coupling applications include matching light into cores of optical fibers, concentrating light into active areas of CCD arrays, and collimation of light from laser diodes.
As the demand for lens arrays containing thousands of lenses with diameters of less than 1 mm has grown over the past twenty years, it has become clear that conventional methods for making lenses such as turning and polishing are not practical methods. MEMS technology has facilitated a large number of different methods to make individual microlenses or microlens arrays.
10 2.2 Evolving fabrication techniques
One of the earliest applications of microlenses was to couple light efficiently into optical fibers. Some of the earliest microlenses were fabricated on the tip of commercial optical fibers.
In 1974, L.G.Cohen and M.V Schneider [6] fabricated a single microlens on the surface of a
single mode optical fiber to increase the coupling efficiency for light coupling from a junction laser. The microlens was fabricated by coating photoresist on end of the fiber and then UV light
is guided through the core to expose the resist. The exposed photo resist takes a hemispherical
shape to form the microlens. They also fabricated a single microlens on the active surface of the
junction laser. Both these techniques increased the coupling efficiency.
Nippon Sheet Glass Company, in 1984 [7], pioneered the fabrication of microlens arrays
using what is called the ion exchange technique using glass substrates. It is based on the
principle that, by selectively diffusing a dopant with a refractive index higher or lower than that
of the substrate, a distributed planar microlens array can be obtained as described in figure 2.1.
In this way Nippon produced either large two-dimensional arrays of microlenses or linear arrays
for use in photocopiers and fax machines.
Ions diffuse into the substrate creating a high index layer
M etal layer
Low index substrate
Figure 2.1. Formation of graded index lens by ion diffusion through a metal mask.
11 The major disadvantages of this technique are high fabrication costs and long processing times. It is not possible to replicate the microlens shape onto any other material due to the fact
that there is not an actual physical microlens shape but only a lens effect produced due to
chemical modification.
Nicholas F. Borelli et al. [8] fabricated 2-D microlens arrays on photosensitive glass using
the photothermal technique. The glass when exposed to light and heated through a prescribed
thermal cycle, produces noble metal colloids which, when they attain a critical size, act as nuclei
for the growth of a crystalline micro-phase from the initially homogenous glass. Due to the
densification of the exposed regions relative to the unexposed regions a surface relief pattern is
produced. During the thermal cycle, greater stress in the denser exposed regions squeezes the
unexposed cylindrical region pushing it upwards as described in figure 2.2. Surface tension
makes the profile spherical. One of the unique advantages of this method is bi-convex
microlenses can be made if the glass is exposed from both sides.
Exposed region
Force due to densification
Spherical lens
Figure 2.2. Microlens made by the photothermal technique.
12 The major disadvantage is that at least a 15 µm gap must be present between individual microlenses to enable sufficient force to be developed to form the lenses. It also has the problem
of relatively long fabrication times.
Microlens arrays were also fabricated on silicon to improve the concentration of optical
energy onto CdHgTe photodiodes. C.L. Jones et al. did this work in 1991 [9]. Initially micro
lenses were fabricated on silicon using the well-known reflow method. (This method is also
utilized in this work and will be explained in detail in the following chapters).
Argon ions Photoresist pedestals
(a) Reflow (b)
(c) (d)
Figure 2.3. Microlens fabricated on silicon: (a) Photoresist pillars (b) Reflow to form microlens (c) Exposed to a beam of argon ions causing both the resist and the silicon to erode (d) Microlens shape transferred to silicon.
The sample is then exposed to a beam of argon ions. Both the resist and the lens material are
eroded, thus transferring the curved shape of the resist to the silicon as described above in figure
2.3. Fused silica is a good material to fabricate microlenses using this technique in the blue and
UV regions while silicon and GaAs are good materials if it is to be used in the IR region.
13 Recently, researchers at the Korean institute of Science and technology [10] utilized bulk micro-machining technology to fabricate microlenses on silicon. The fabrication involves boron diffusion after SiO2 patterning. The diffusion profile is a hemispherical shape. The silicon
substrate is then bulk etched with EDP.
SiO2 Silicon microlens on cantilever
Silicon substrate
(a) (c) Boron rich layer
(b)
Figure 2.4. Fabrication of silicon microlens by bulk micro machining: (a) SiO patterned and boron diffusion (b) SiO patterned for cantilever, and 2 2 boron diffusion is performed (c) Silicon substrate etched in EDP to form microlens on cantilever.
EDP doesn‘t etch the boron rich hemispherical layer and a microlens is formed on a
cantilever shape as shown above in figure 2.4. The focal length can be easily controlled with diffusion parameters and arbitrary shaped microlens can be fabricated by SiO2 patterning.
Plastics as a material in MEMS has been increasingly researched and applied to a variety of
MEMS devices. Plastics have the obvious advantages of cost effectiveness and most importantly
the potential for mass production using various replication techniques. Different techniques have
been investigated to develop plastic microlenses.
14 Pantalis, in 1992 [11] worked on lenses in the 1 mm œ 2 mm dimensions with a technique
somewhat similar to hot embossing. It involved drying a polycarbonate sheet and then heating to
its softening temperature. The softened polycarbonate is pressed against a stainless sheet full of apertures. Controlling the pressure applied, temperature and time of pressing can control the
radius of curvature of the lenses.
Maria Kufner et al. [12] developed a method in which deep proton irradiation of PMMA
(Polymethyl methacrylate) enables a monomer vapor to diffuse into the substrate and create a
surface relief profile. The PMMA substrate is irradiated with proton energies between 3 to 13
MeV through a metal mask containing an array of circular apertures so that the molecular chains
are cracked in the exposed areas. The PMMA is then exposed to an atmosphere of monomer
vapor, at temperatures between 90 0C and 95 0C so that the monomer diffuses into and swells the irradiated regions forming microlens profiles as described in figure 2.5.
Proton beam M onomer diffusion
PM M A
(a) (b) Domains with reduced molecular weight Expansion of irradiated domains
(c) (d) Figure 2.5. Microlenses made on PMMA using proton irradiation. (a) Proton beam irradiation (b) Reduced molecular weight regions created on PMMA (c) Diffusion of monomer (d) Expansion of irradiated regions to form microlenses.
15 X.J. Shen et al. [13] used a micro-plastic embossing process using silicon molds to fabricate
microlenses on polycarbonate. A polycarbonate substrate is embossed by using a silicon mold
insert with openings of 100, 120 and 200 µm in diameter. These holes were fabricated by deep
reactive ion etching process. Hot embossing involves applying pressure at a temperature greater
than the glass transition temperature of the plastic. Microlenses of different heights and
curvatures were fabricated in this manner.
Madanagopal V. Kunnavakam et al. have fabricated low cost microlenses using soft
lithography [14, 15]. UV curable epoxy is used as the material for fabrication.
All these methods discussed above have their own advantages and disadvantages. However,
all of the methods developed so far to fabricate microlenses involve long fabrication times and
are unsuitable for high volume production. Hot embossing is suitable for making many replicas
of a single master without damaging the master, but the cycle time is of the order of 30 minutes
making it incompatible with high volume production.
The rest of this chapter goes through the various possible applications of microlens arrays.
2.3 Applications of microlenses
Microlens arrays are used for collimating or focusing purposes (laser arrays, detector arrays,
sensors, optical interconnects, optical computing, etc), for illumination (flat panel displays, TV
projection systems, retro-reflectors, diffusers, etc), and for imaging (photocopiers, 3D-
photoraphy, signal and image processing, fiber couplers, microlens lithography, astronomy, etc).
16 CCD arrays consist of linear array of pixels each of which contains a photoreceptive cell and electronic components as shown in figure 2.6. Metallised photo shields usually shield the electronic components from the illuminating radiation. Placing a microlens array on top of the
CCD sensor concentrates the light into the photoactive detector area [16].
M icrolens
Photo detector Electronics
Figure 2.6. Schematic of use of microlens array to
improve the efficiency of CCD array.
There has also been tremendous interest in the application of microlens arrays in optical array interconnection [17]. Much of this interest has been generated by the interest in optical information processing, either for optical computing or telecommunications. The main criterion is the ability of the microlens array to direct an array of beams from a source array to a spatially variant detector array. This is also known as beam steering. Beam steering can be achieved using decentered microlens arrays. The principle of operation is to arrange the lenses as an afocal pair separated by the sum of their focal lengths. If the second lens is then displaced sideways, the output beam will be deviated by an angle dependant on the amount of displacement as described in figure 2.7.
17
Figure 2.7. Schematic of microlens application in beam steering using an afocal pair of microlenses.
Microlens photolithography [18, 19] is a new lithography method that uses microlens arrays
to transfer the patterns on a photomask to the photoresist coated on the substrate. Microlens
lithography provides a moderate resolution for almost an unlimited area. The imaging system
consists of stacked microlens arrays acting as micro objectives. Each micro objective images a
small part of the mask pattern onto the substrate. Potential applications for microlens lithography
include the fabrication of large area flat panel displays, color filters and micromechanics.
Microlens lithography is a non-contact method (> 500 µm distance from mask to substrate)
and permits photolithography for a very large area at a resolution of 2 to 5 µm. The large depth of focus provides reduced requirements to the flatness and alignment precision of the substrate.
2.4 M icrolenses for µTAS
Miniaturized total analysis systems, called µTAS cover a wide range of disciplines such as organic and analytical chemistry, biochemistry, genomics, proteomics etc. Most of these systems rely on some kind of optical detection systems that incorporate a complex arrangement of lasers,
18 detectors, filters, etc. Microlens arrays offer the potential to reduce the size and simplify the architecture of analytical systems.
M icrolens chip
M icrolens M icrofluidic chip
M icrofluidic channel
Detector array (a) (b)
Figure 2.8. (a) Schematic of a µTAS chip using microlens arrays for illumination and detection in parallel micro capillaries of a chemical chip. (b) A large NA microlens fabricated on a microfluidic chip to improve efficiency in a fluorescence detection system.
Figure 2.8 (a) above [20] illustrates a possible system that integrates a microfluidic chip with microlens arrays for illumination and detection. Researchers at Berkeley [21] have employed a similar concept to fabricate a high numerical aperture microlens on top of a microfluidic channel to gather more light emitted due to fluorescence as shown in figure 2.8 (b). They have claimed an increase in detection sensitivity.
19 2.5 Conclusions
This chapter described the development of various fabrication techniques for microlens
arrays. The techniques adopted have been varied and different materials have been investigated.
Microlens fabricated at the tip of an optical fiber is proven to be successful in increasing the
coupling efficiency from source to fiber. Microlenses have been fabricated on glass and silicon.
Microlenses on silicon are transparent to the infrared wavelength.
The major disadvantage in all these methods is the absence of potential for low cost high
volume production. A reliable technique for high volume production of plastic microlenses is yet
to be optimized. Hot embossing offers the scope for medium volume production but the time
involved in obtaining a single replicated part is significantly high.
Microlenses have wide ranging applications in CCD arrays, displays, and imaging. The
scope of microlenses integrated with µTAS is being actively explored and offers a whole new
area of research.
20 CHAPTER 3
FABRICATION OF THE REPLACEABLE INJECTION M OLD DISK
3.1 Introduction
This chapter describes the fabrication of the replaceable injection mold disk for the injection molding process. The reflow method, used to create photoresist microlens patterns is discussed followed by the electroforming process that forms the injection mold disk. This chapter details the various fabrication issues involved with appropriate
illustrations and scanning electron microscope (SEM) photographs. This chapter is
divided into two sections, (1) reflow method to form photoresist microlenses and (2)
electroforming to form the replaceable injection mold disk.
3.2 Photolithography and reflow to form microlenses
3.2.1 M elting on a hot plate
One of the simplest and easiest techniques to form microlenses is the photoresist reflow method. Popovic et al. [22] demonstrated this simple technique in 1988.
Cylindrical pedestals of photoresist are fabricated by photolithography. The pedestals are
then melted so that surface tension pulls these pedestals into a hemispherical shape. A
number of researchers [23, 24, 25, 26] have used this technique to produce microlens
arrays of various shapes including hemispherical, cylindrical and hexagonal.
21 The photoresist reflow method is an ideal method to produce microlenses as it offers
an excellent compromise between ease of manufacture, high potential for large-scale
replication, low lens aberration and high fill factor.
A 3-inch silicon substrate is used for optimizing the reflow process to form micro
lenses. The silicon substrate is cleaned using acetone and methanol for 5 minutes each,
followed by running DI water for 5 minutes. This is followed by a base clean where the
wafer is dipped in a 1:1:5 ammonium hydroxide, hydrogen peroxide, and DI water
solution at 75 0C. AZ 4620 (Clariant corporation), a thick positive photoresist is used for the process. The photo mask had 5 by 5 arrays of circular patterns of various diameters ranging from 200 µm to 700 µm. AZ 4620 is spun onto to the silicon substrate using a spread of 500 rpm for 10 seconds followed by spin of 400 rpm for 15 seconds. Soft bake is done in 65 0C oven for 4 minutes, followed by 100 0C on a contact hot plate for 4
minutes and again in a 65 0C oven for 4 minutes. This recipe gives a thickness of about
37-40 µm.
One of the problems encountered with a thick photoresist is the formation of edge
beads during spinning. Edge bead is the formation of a thicker photoresist layer around
the edges of the wafer compared to the center. The presence of edge beads will give rise to a gap between the contact mask and the substrate. This can cause UV light to diffract at the gap leading to overexposure and consequent change is the final pattern shape. The edge bead was avoided by using a pointed straw to remove the excess photoresist collecting at the edges during the spin. This method, though crude, seems to work well for AZ 4620. The exposure was carried out using a Logitech aligning system. The photoresist is exposed for around 81 seconds under UV light (365nm).
22 The photoresist was then developed using the AZ400K developing solution, diluted
with DI water in a 1:3.5 ratio to obtain the desired cylindrical pedestals. The pedestals
were then melted on a hot plate at 160 0C for 3 minutes. The pedestals pulled themselves into a hemispherical shape due to surface tension.
One significant problem that is observed is the damage to microlenses, apparently due
to trapping of bubbles in the photoresist structure. Some lenses clearly show a trapped
pocket of air while some others are totally destroyed.
This occurrence can be explained by the following arguments.
(1) The evolution of bubbles during the melting process can destroy the lens shapes.
Dan Daly offers an explanation in the textbook —Microlens arrays. — [27]. Gases
evolve during the exposure process and diffuse from the exposed part of the resist
into the unexposed parts forming gas pockets. There is also significant amount of
nitrogen released during the melting process itself. During melting these pockets
of gases expand and explode causing the resist structures to be damaged.
(2) Melting the photoresist pedestals on a hot plate may significantly aggravate the
problem. Hot plate heats from the bottom upwards causing formation of gas
pockets in the lower layer of the pedestals while the upper layer is still solid. This
prevents an escape route for the bubbles, which consequently get trapped or
expand and explode. Also, the melting time used is around 3 minutes that gives
very little time for the bubbles to diffuse out of the resist.
(3) It is also observed that lenses with larger dimensions, specifically 400 µm
diameter and above have more damaged lenses than the lenses with diameters of
200 µm and 300 µm. This can be explained by the fact that bigger lenses have
23 higher volume of resist and consequently aid formation of more gas pockets
during exposure and baking. Therefore upon melting on the hot plate, the bigger
lenses are more likely to be damaged or destroyed.
Figure 3.1 shows the SEM pictures of a 500 µm and 600 µm lens array. The 500 µm microlenses clearly have a distorted shape due to trapped gas bubbles while one of microlens of the 600 µm array is totally damaged.
500 µm 300 µm
(a) (b)
Figure 3.1. SEM pictures of photoresist microlenses: (a) 500 µm diameter microlens array and (b) close up of a 600 µm diameter microlens.
24 Figure 3.2 shows the difference in resist damage between the smaller (300 µm
diameter) and the larger array (600 µm diameter). It is clear that the bigger array is
mostly damaged while the smaller array shows no presence of gas pockets or damage.
1 mm 500 µm
(a) (b) Figure 3.2. SEM pictures of photoresist microlenses: (a) 600 µm diameter microlens array and (b) 300 µm diameter microlens array.
3.2.2 M elting using an Oven
From the above explained reasons, it can be concluded that small melt times and the bottom upwards heating using the hot plate are two of the major reasons for the damage of the microlens structures. This can be overcome by using a slow ramped heating profile inside an oven. This helps in two ways. One, a ramped profile maintains the resist in a
molten state for a long time giving enough time for the gas pockets to escape. Two, the
oven heats the resist uniformly avoiding the problems associated with melting on a hot
plate. The photoresist pedestals were formed as described in section 3.1.1. The patterned
pedestals were then placed in an oven. The pedestals were then slowly heated using a
ramped profile. The substrate was heated to 100 0C at a rate of 0.5 deg/minute, allowed to
25 stay at 100 oC for 15 minutes, then heated to 125 0C at the same rate and allowed for another 25 minutes and then heated to the final melting temperature of 165 oC. The
substrate was then allowed to cool down in the oven naturally.
Melting the photoresist pedestals in an oven using a slow ramped temperature profile overcomes the damage to the microlens structures due to formation of bubbles. The ramped temperature profile gives enough time for the gas bubbles to escape, as the photoresist stays molten for a longer period of time. Figure 3.3 shows the SEM images of
microlens arrays formed by melting in an oven.
The lenses melted on an oven have smaller sag than those melted on a hot plate. This
is due to the fact that melting in an oven over a long time causes a lot of solvent to
evaporate leading to a reduction in the volume of the resist. However, when melted on a
hot plate, the loss of solvent is minimal and thus the lenses are able to obtain a higher
aspect ratio than is possible with an oven for the same spin speeds.
1 mm 500 µm
(a) (b) Figure 3.3. SEM pictures of photoresist microlenses: (a) 200 µm diameter microlens array (b) a 500 µm microlens array.
26 3.3 Electroforming to fabricate the replaceable injection mold disk 3.3.1 Electroforming
Plastic microreplication essentially requires a master mold that has the negative
patterns of the final desired shape on the plastic. Master molds can be of any material.
The substrates that are commonly used as master molds for plastic microreplication
include silicon, stainless steel, plastics themselves and metallic molds. However, for
injection molding, metallic molds are preferred due to their ability to withstand high
temperatures and force that occurs during the process.
Micro patterns can be machined on metallic substrates using conventional machining
and drilling or using techniques like EDM (Electro discharge milling) [28]. These
methods offer several advantages such as the ease of fabrication of highly accurate
microstructures.
MEMS provides what is now commonly known as the LIGA technology for the
fabrication of master molds. LIGA [29, 30] is a German acronym that in essence means
X-ray lithography, electroplating and molding. LIGA enables fabrication of high aspect
ratio master molds that can be used for plastic micro-replication. Researchers using the
standard UV lithography followed by electroplating and molding have modified the name
of the process as UV LIGA [31, 32, 33]. There are however several issues associated with
master mold fabrication using UV LIGA. They are outlined below.
(1). One major problem associated with electroplating is non-uniform electroplating
thickness in channels having different dimensions. This occurs due to concentration of
flux lines in narrower channels leading to a higher rate of electroplating and consequently
27 a thicker electroplated layer as compared to wider channels. This can be a problem in applications where channel dimensions are extremely critical.
(2). Another issue that cannot be addressed by electroplating is fabrication of structures that do not have a regular shape like rectangular channels. One example is the hemispherical shape that also pertains to this work.
The solution to this is electroforming, instead of electroplating. [34, 35, 36].
Electroforming, as applied to microfabrication, means electroplating metal on a patterned substrate to a very large thickness to form an electroform that is then removed from the substrate. The electroform, that has the negative patterns of the final required shape, is then used as the master mold.
Electroforming is an ideal solution pertaining to this work for the fabrication of the injection mold disk.
The photoresist reflow method to fabricate microlenses is well researched and documented. It is proven to be the simplest and a highly repeatable method for obtaining high quality microlenses. It is also proven to work extremely well for producing microlens arrays with high fill factors and dimensions ranging from a 5 µm diameter to 1 mm diameters. Many microlenses applications require very closely packed microlens arrays, i.e. arrays with a high fill factor. Mechanical machining to fabricate a mold consisting of closely packed hemispherical cavities is highly restrictive. The reflow method is proven to be ideal for fabricating high fill factor arrays. The electroforming method is the best solution to fabricate a master mold that can consequently enable high volume production of plastic microlens arrays with high fill factor. The electroforming
28 method detailed in the next few pages enables the fabrication of a master mold without
forgoing the advantages of the photoresist reflow technique.
3.3.2 Nickel Electroforming
Electroforming involves deposition of a large thickness of metal on the substrate. The standard 500 µm thick silicon wafer will crack and break due to the stress exerted by the
growing nickel layer. Therefore a 3 mm thick silicon wafer was custom ordered (W afer
W orld) and used as the substrate.
Nickel or its alloys [37, 38] is usually used as the material for electroforming. Nickel
sulphamate is usually the solution used for electroforming applications due to the low
internal stress in the plated nickel layer.
Photolithography and reflow were used to fabricate the microlenses as described in
section 3.2.2. The electroplating process requires a conductive layer to be deposited if the
substrate itself is non conductive. Therefore a seed layer of titanium (50 nm) and copper
(250 nm) was deposited using an E-beam evaporator. The titanium is used to increase the
adhesion between the copper layer and the silicon substrate, as silicon has a poor
adhesion to copper. Nickel sulphamate bath was used as the solution for nickel
electroforming. The electroforming process flow is described in figure 3.4.
29
Photolithography to form photoresist pedestals Metal electroforming
Reflow to form microlens Backside planarization patterns.
Release substrate and Seed layer deposition machining electroform.
Figure 3.4. Schematic of the process flow to fabricate the replaceable injection mold disk.
Before going into the details of the process, it needs to be stressed that one of the most crucial issues in electroforming is stress control. W hen a metal is electroplated to a large thickness, the issue of tensile and compressive stress dominates the final electroform shape [39]. Some of the important parameters that determine the residual stresses in the electroplated layer are the pH of the electroplating bath, the temperature of the bath, and the components of the bath [40, 41, 42]. Suitable use of organic additives can also reduce the residual stresses in the deposited layer.
The electroplating was carried out in an 8 liter electroplating bath. The metallised silicon substrate was connected to the cathode and nickel pellets acted as the anode. The electroplating was carried out at a current density of 10 mA/cm2 and at a pH of 3.5-4.0.
30 As the electroplating goes on, the pH of the bath increases. The pH has to be continuously monitored as higher pH can increase the residual stress in the deposited layer. The pH was brought down to the desired range by addition of dilute sulphuric acid.
The electroplating was carried out for a period of 160 hours. The average electroplating rate was around 10 µm/hour. The final thickness of the electroplated layer was around 1.6 mm.
The backside of the electroform was then planarized. The electroform was then mechanically removed from the silicon substrate. The seed layers were removed by selective etchants. Titanium was etched out using a 2 % HF solution and copper was selectively etched out using cupric sulphate solution.
On visual inspection, the electroform was flat and very much suitable for use as an injection mold disk for plastic replication. The tensile and compressive stresses were reduced and caused no distortion of the electroform. The residual stresses in the electrodeposit were controlled by continuously monitoring the pH and temperature of the bath.
31 Figure 3.5 (a) shows the electroformed nickel layer.
(a) (b) F igure 3.5. Digital camera images of (a) electroformed disk before machining and (b) after machining and fit as injection mold disk.
Figure 3.5 (b) shows the electroform after it was separated from the substrate and
machined so as to fit as a mold disk in the injection molding machine. The different
arrays of hemispherical cavities are the negative patterns of the final desired microlens
arrays.
Embedded fragments of what appeared to be metal were observed in some of the
mold cavities. It is assumed that due to the mechanical process used to remove the
substrate from the electroform, the thin films of seed layers may have broken down into
smaller fragments and got embedded in the mold cavities. Some of the fragments were
brownish in color and they were etched out when the electroform was treated with a
copper etchant solution and agitated using an ultrasonic bath. This conclusively proves it
was copper and also corroborates the reasoning that the fragments embedded in the
cavities are the seed layers. However, other fragments that most probably were titanium
could not be etched away. Most of these fragments were observed in the bigger lenses
32 and it is expected that the bigger lenses will have a rougher surface compared to the smaller lenses.
Figure 3.6 shows the SEM images of the mold cavities. It shows the 300 µm and 500
µm diameter mold cavities.
(a) (b)
Figure 3.6. SEM pictures of the replaceable injection mold disk: (a) 300 µm diameter mold cavity and (b) 500 µm diameter mold cavity.
The fabricated electroform is then used as a replaceable injection mold disk as described in the following chapters.
33 3.4 Conclusions
A replaceable injection mold disk was fabricated using electroforming. The mold disk
is used for plastic replication using injection molding. The mold disk has the reverse
patterns of the final desired microlenses. The initial photoresist mold was fabricated
using the reflow technique.
This process combines the advantages of the reflow method for fabricating microlenses with the potential for high volume production. The reflow method used to produce microlenses has been extensively researched and documented, and is the simplest and most reliable method for producing microlens with diameters ranging from
5 µm to 1 mm. The electroforming method described in the chapter enables the fabrication of a mold disk and also incorporates the advantages of the reflow method.
The electroforming process was optimized for low stress deposits so that the shape of
the electroform is not distorted. This was achieved by monitoring the pH and temperature
of the electroplating bath.
The fabricated mold was machined to size to fit as an injection mold disk. Some of
the mold cavities have embedded fragments of metal which are the seed layers that broke
down due to the stress associated with the mechanical separation of the electroform from
the substrate.
34 CHAPTER 4
M ICROFABRICATION OF PLASTIC M ICROLENSES USING INJECTION M OLDING
4.1 Introduction
Plastic molding techniques have been available for a long time since the scope and usefulness of plastics as a material was realized. Plastics can be widely divided into three
major groups - thermoplastics, thermosets and elastomers. Of these, thermoplastics are
the most commonly used for molding purposes. Thermoplastics consist of long chain
molecules that have relatively strong intramolecular covalent bonds but relatively weak
intermolecular bonds. Thus, thermoplastics can be easily plasticised under quite mild
temperature conditions, rapidly shaped into complex products, and then resolidified by
cooling. The commonly used plastic processing techniques include extrusion process,
compression and transfer molding and injection molding.
Micro molding to produce plastic structures of the order of micrometers has been
borrowed from the macro plastic processing technology. Recent research in MEMS has
focused on plastic as an alternative material due to its low cost and potential for high
volume production [43, 44, 45]. W ith many plastics exhibiting good biocompatibility and
high optical transparency, they are envisaged to be the material of choice particularly in
microfluidics and telecommunications.
Hot embossing [46, 47] has been widely used for the fabrication of plastic based
MEMS devices. Hot embossing is relatively simple, cheap and gives good quality
35 replication. But the major disadvantage of hot embossing is its unsuitability for high
volume production due to its large cycle times.
Injection molding, or microinjection molding [48, 49] as applied to this work is a
much more efficient and versatile process, capable of rapid production of complex high
quality components with tightly controlled dimensional tolerances. This chapter describes
the microinjection molding process to fabricate plastic microlenses.
4.2 Injection molding
Injection molding involves the rapid injection of a metered portion of a polymer melt into a closed mold, where it is held under pressure until solidification occurs. The mold
cavity is then cooled down and the replicated plastic part is ejected out. The schematic of
the injection molding machine is figure 4.1.
The solid plastic, which is in the form of pellets, enters the injection unit via a feed
hopper. The reciprocating screw is not only capable of rotation in order to plasticise the
plastic feed but also capable of axial movement in order to inject the melt into the mold
cavity. The feed is plasticised by the combined actions of the reciprocating screw and the
heaters surrounding the injection unit. The barrel ends in a nozzle that opens into the
mold cavity. External heating of the barrel and the nozzle is provided by the zones
heating systems that enable the barrel temperature to be varied along its length.
A novel replaceable mold disk technique, developed by Ram et al. at the University
of Cincinnati [50] is applied in this work. In this method, the molding block is custom
designed to hold the replaceable mold disk. The molding block is heated with the help of
a heating fluid. A separate oil heating unit is added to the injection molding machine to
heat the mold block. The molding block should be kept at the demolding temperature of
36 the plastic. As the rapid cooling of the replicated plastic part causes cracking due to thermally induced stress.
Figure 4.2 illustrates the schematic of the injection molding process.
Hopper feed M olding block Plastic pellets
Reciprocating screws
Figure 4.1. Schematic of the injection molding machine with the replaceable mold insert inside the mold cavity.
Heater M old cavity Plastic melt
(a) (b) (c) (d)
Figure 4.2. Schematic of the injection molding process: (a) Replaceable mold disk placed in the mold cavity (b) Plastic melt injected and the mold closes (c) demolding the part from the mold cavity (d) replicated plastic part ejected.
37 Figure 4.3 shows the injection molding machine and the electroformed nickel mold
used as a replaceable injection mold disk.
Figure 4.3. (a) Photograph of the BOY 22A injection molding machine and (b) the electroformed mold fit as a replaceable injection mold disk.
4.3 Plastic microlenses fabricated on COC
The most important requirement for a plastic intended for optical applications is its
optical transparency in the intended wavelength. The most commonly used plastics have been PMMA (polymethyl methacrylate) and polycarbonate. This work introduces a new
material, COC for applications in visible and UV regions. Figure 4.4 shows the optical
transparency curve that compares COC, polycarbonate and PMMA. It can be seen that
COC is transparent to wavelengths from 300 nm upwards while polycarbonate and
PMMA are not. This makes COC an excellent choice for optical applications both in the
UV and visible region. Microlenses on COC are an ideal choice for future applications in
µTAS systems that integrate microoptics with total analysis systems as discussed in
chapter 2.
38
COC 100 80
) %
( 60
PC PM M A y
t i
s 40 n e t
n 20 I 0 0 200 400 600 800 1000 Wave length [nm] W avelength (nm)
Figure 4.4. Optical transparency curve comparing COC, PMMA and PC.
Also, COC has excellent flow behaviour and molding properties that makes it an excellent material for injection molding purposes. COC was melted and injected into the mold cavity. The process parameters for COC injection was optimized by Ram et al. [51] at the University of Cincinnati. The melt flow index for COC is 55g / 10minutes.The temperatures of the rear, center and front section of the barrel were 279 oC, 285 oC and
291 oC respectively. The temperature of the nozzle was 296 oC. The demolding temperature of the molding block was kept at 118 oC.
Plastic microlenses were fabricated using the electroformed replaceable mold disk.
The microlenses were observed under the microscope. The smaller lenses (300 µm and
200 µm diameters) had excellent surface quality. Some of the bigger lenses (600 µm and
700 µm diameters) seemed to have a rougher surface. This was expected as explained in chapter 3. The roughness on the mold cavities is replicated onto the plastic microlenses.
39 Figures 4.5 and 4.6 show SEM images of the smaller microlenses. The high surface quality of the lenses can be clearly seen.
Figures 4.7 and 4.8 show the SEM images show the 400 µm and 600 µm diameter microlenses .The surfaces roughness on the lenses is clearly seen.
500 µm 200 µm
500µm
(a) (b) Figure 4.5. SEM pictures of COC microlenses: (a) a 200 µm diameter array and (b) a close view of the 200 µm diameter lens array.
300 µm 100 µm
Figure 4.6. SEM pictures of COC microlenses: (a) a 300 µm diameter microlens array and (b) a close up of 300 µm diameter microlenses.
40 500 µm 500 µm
(a) (b)
Figure 4.7. SEM pictures of COC microlenses: (a) a 400 µm diameter microlens array and (b) closer view of the same array.
500µm 100µm
(a) (b)
Figure 4.8. SEM pictures of COC microlenses: (a) a 600 µm diameter microlens array and (b) a close up of a 600 µm diameter microlens.
41 The surface roughness on the microlenses is characterized using the AFM (Atomic
force microscope). A small 20 A 20 µm2 area of the 300 µm and 600 µm diameter
microlenses was scanned by the AFM probe. Larger areas could not be scanned due to
the size of the lenses. It is impractical to scan the whole surface of the microlens with the
AFM probe tip.
Figure 4.9 shows the AFM images for the 300 µm and 600 µm diameter microlenses.
The rms surface roughness of the scanned area for the smaller microlens was 9 nm and
for the bigger microlens was 39 nm.
(a) (b)
Fig ure 4.9. AFM images: (a) Image of an area of a 300 µm lens having an rms roughness of 9nm and (b) image of a rough area of a 600 µm lens having an rms roughness of 39 nm.
42 4.4 Plastic microlenses fabricated on Poly IR 2
Poly IR 2 is a plastic manufactured by Fresnel Technologies that is highly transparent in the IR region. Silicon is generally the material of choice for fabricating microlenses for applications in the IR region, specifically in CCD photodetectors. This work introduces
Poly IR 2 as new plastic material for fabricating microlenses for applications in the IR
region. Poly IR 2 is a flexible whitish plastic and was bought in the form of pellets for
injection molding.
Figure 4.10 shows the optical transmission curve for Poly IR 2 plastic material.
) % (
e c n a t
t i m s
n a r T
W avelength (µm)
Figure 4.10. Transmittance curve for the Poly IR 2 plastic material.
The same replaceable mold disk is used to fabricate the microlenses on the Poly IR 2
plastic. The injection molding is carried out as described in the previous sections. The
temperatures of the rear, center and front portions of the barrel were kept at 210 oC,
43 212oC and 216 oC respectively and the temperature of the nozzle was maintained at 221 oC. The mold cavity was heated to a temperature of 71 oC.
Figure 4.11 shows the SEM images of the Poly IR 2 microlenses. The first image
shows the close up of a 300 µm microlens. The second image shows a 700 µm diameter
microlens array. Due to the fact that the same mold has been used, the quality of surface
is very similar to that of the microlenses on COC. As can be observed from the images,
the smaller microlens have a superior surface quality compared to the bigger microlenses.
100 µm 1 mm
Figure 4.11. SEM pictures of poly IR 2 microlenses: (a) a 300 µm diameter microlens and (b) a 700 µm diameter microlens array.
4.5 Conclusions
Plastic microlenses were fabricated using injection molding techniques. This
replaceable mold technique is suitable for high volume production of plastic microlenses.
The turn around time for a single chip was the order of a minute.
Two new plastics, COC and Poly IR 2 have been used to fabricate the microlenses.
COC and Poly IR 2 have excellent optical properties and compatibility with the injection
44 molding process that would make them excellent choices for applications in optical
MEMS. COC is an ideal material of choice for applications in the visible and UV regions. COC is proven to be biocompatible and hence would be extremely suitable for applications integrating microlenses with DTAS systems. Poly IR 2 will be a good choice for applications in telecommunications. It‘s suitability with high volume fabrication techniques has been explored in this chapter though not thoroughly researched.
The fabricated COC microlenses have been characterized for their surface roughness using an AFM probe tip. The high surface quality of the 200 µm and 300 µm diameter microlenses compared to the bigger microlenses was expected, as discussed in chapter 3.
45 CHAPTER 5
CHARACTERIZATION OF PLASTIC MICROLENSES
5.1 Introduction
This chapter describes the characterization of the fabricated microlenses. The most important parameters for a lens are its focal length and the wavefront aberration. This chapter details the focal length measurement of the microlenses on COC and Poly IR 2 plastics. The wavefront aberrations have not been characterized for the fabricated microlenses.
This chapter describes the different experiments to measure the focal length of the
microlenses fabricated on COC and Poly IR 2 plastics. 5.2 Plano Convex lenses
Plano convex lenses are essentially defined from their shape. They are curved on the
outside with their other side flat. Plano convex lenses are primarily used for focusing or
collimating applications.
Figure 5.1. Schematic of a Plano convex lens as a converging and collimating system.
46 5.3 Focal length determined by surface profile of the microlens
h
2r R
f Figure 5.2. Parameters for a Plano-convex lens.
The expected focal length of the microlenses can be calculated by determining their radius of curvature.
The focal length is related to the radius of curvature of the lens by
f = R/ (n-1) (1)
where f: focal length of the lens
R: Radius of curvature
n: Refractive index of the material of the lens.
If we assume that the lens is perfectly spherical, then the height of the lens h is
h = R – (R2 – r2) ½ (2)
where h: height of the lens.
R: Radius of curvature.
r: Radius of the base of the lens.
47 From equation 2, it is clear that the radius of curvature can be calculated, and hence
the focal length, if the values of h and r are known. The values of h and r for the
fabricated microlenses are determined by surface profilometry.
The diffraction effects and spherical aberration determine the focussed spot diameter of a lens system. When imaging, the diffraction of light as it passes through the lens aperture limits the performance of the lens. The minimum spot diameter possible after diffraction is given by
SD = 2.44 ? f / # (3)
where SD is the spot diameter
? is the wavelength of the light
f / # is the f-number of the lens which is given by focal length
divided by the diameter of the lens.
Spherical aberration is caused due to different focus points for the paraxial and
marginal rays. Paraxial rays, i.e. the rays closer to the optical axis have a focal length
longer than those of the marginal rays, i.e. rays farther away from the optical axis.
The spot diameter limited by spherical aberration when a plano convex lens is used to
image a point source can be given as
SD (due to spherical aberration) = 0.067f / f/#3 (4)
Figure 5.3 shows a typical surface profile of a 700 µm diameter microlens. From the
surface profile, the height and diameter of the microlens can be obtained and used to
calculate the theoretical focal length.
48
80
70 60
t (µm) 50
40 30 20
Microlens heigh 10 0 0 100 200 300 400 500 600 700 800 900 1000
Microlens width (µm)
Figure 5.3. Surface profile of a 700 µm diameter microlens.
The calculated values of the focal lengths and focal spot sizes for the various microlens (COC) diameters are tabulated below in table 5.1. COC has a refractive index of 1.53.
Microlens diameter Calculated focal length Spot size diameter limited by
Diffraction Spherical aberration
200 µm 220 µm 1.7 µm 11.07 µm
300 µm 405.5 µm 2.08 µm 11.00 µm
400 µm 658 µm 2.54 µm 9.9 µm
500 µm 955.5 µm 2.95 µm 9.17 µm
600 µm 1.33 mm 3.42 µm 8.18 µm
700 µm 1.76 mm 3.88 µm 7.41 µm
Table 5.1. Tabulated values of the focal lengths and spot diameters determined from the surface profile of the COC microlenses for various diameters.
49 5.4 Characterization of microlenses on COC
The focal length of a lens is the most important parameter. The focal length of the
COC microlenses was measured using two different techniques. The two techniques will be detailed in the following pages.
5.4.1 Focal length and spot size measurement by observation of focal plane
The microlenses focus an incident beam of light at the focal point. The plane containing the focal point is called the focal plane. The schematic of this experiment is shown below in figure 5.4.
To TV monitor CCD camera
Microscopic objective
Focused spot Microlens array
White light source Mirror
Figure 5.4. Schematic of the experiment to measure focal length of the microlenses by imaging the focal plane.
50 The principle of the experiment can be explained thus. A laser or lamp is used as the source of visible light. A 45-degree mirror reflects the incident beam of light perpendicularly upwards towards the optical microscope where the microlens chip is placed. The chip and mirror are aligned so that the light beam hits one of the microlenses.
As the diameter of the smallest microlens is 200 µm, which is not very small, it is relatively simple to obtain a fairly accurate alignment.
The distance between the microlens chip and the mirror below is around 8 cm. Thus, with reference to the microlenses, that have expected focal lengths ranging from hundreds of microns order to a couple of millimeters, the object can be considered to be at infinity.
The microscope is connected to a CCD camera that in turn is connected to a TV monitor. Thus, whatever the microscope images is seen on the TV monitor. The microlens focuses the incident beam onto the focal plane. The beam focuses at the focal point and then diverges. The objective is moved vertically to visually observe the smallest spot of light. When the smallest spot of light is viewed on the TV monitor, it indicates that the microscope is imaging the focal plane. The light source is then switched off, and the illumination to the microscope is turned on. The microscope is then focussed on the surface of the microlens. As the object is assumed to be at infinity, the difference between the vertical distances moved is equal to the focal length of the microlens. The size of the spot is also measured using the ruler on the eyepiece of the microscope.
Figure 5.5 shows the imaged focussed spots in the focal plane of the 500 µm and 300
µm diameter microlenses.
51
Figure 5.5. CCD images of the focussed spot using a white light source through a (a) 500 µm diameter and (b) 300 µm diameter COC microlens.
The measured focal lengths and spot diameters of the microlenses for different
diameters are tabulated below.
Diameter Focal Length Spot diameters 200 µm 240 µm 8 µm 300 µm 450 µm 10 µm 400 µm 540 µm 14 µm 500 µm 810 µm 20 µm 600 µm 1.23 mm 24 µm 700 µm 1.85 mm 40 µm
Table 5.2. Tabulated values of the measured focal length and spot diameters for various microlens diameters by imaging the focal plane.
5.4.2 Focal length measurement using fiber-microlens-fiber power
coupling.
The focal length was also measured using fiber-microlens-fiber power coupling
method. The basic principle is this. A multimode optical fiber is used to couple light into
the microlenses. The light coming out the microlens is picked up by another similar
52 optical fiber. When both the fibers are at a distance 2f from the microlens, the maximum
power is coupled into the receiving fiber.
From the lens maker’s formula, the relation between the focal length of the lens and
the object and image distances is given by
1 / f = 1/ S2 - 1/ S1 (5)
where S1 is the distance of the object from the lens
S2 is the distance of the image from the lens.
If the object is placed at 2f then the image is also formed at 2f. The magnification
given by the ratio of S1 / S2 is equal to 1. Therefore the microlens forms an identical
image of the light source, i.e. fiber in this case at a distance 2f. Therefore an identical
fiber placed at 2f collects the maximum power. The focal length can then be obtained
from measuring the distance between the two fiber ends.
Figure 5.6 below depicts the schematic of the experiment.
Fiber To detector Laser Microscopic objective Fiber 2f 2f
Figure 5.6. Schematic of the fiber-microlens-fiber power coupling experiment to measure the focal length of the microlens.
The fibers used are multimode commercially available optical fibers with 50 µm core
and 250 µm cladding. The fibers are mounted on three axis precision aligners. Light is
coupled into the first fiber from a 633 nm Helium Neon laser using a 20x microscopic
53 objective. The coupled light emerges from the fiber at the other end and is coupled onto
the microlens. The microlens images the fiber end that acts as the light source. The
focussed light is coupled into the second receiving fiber whose other end faces a photo
detector. The power is measured for various positions of the two fibers. The power measured is then plotted as a function of the fiber-fiber distance.
The thickness of the substrate needs to be taken into account. The thickness of the substrate is 1.3 mm. Due to this, the measurement of focal lengths for the lenses with diameters 200 µm and 300 µm was not possible. This is because for the fibers to be placed at 2f, 2f should be greater than 1.3 mm.
Figure 5.7 shows the digital camera images of the fiber-microlens-fiber power coupling experimental set up. The first image shows the input fiber aligned to the microlens so that a focussed image is formed on the other side while the second image shows a mis-alignment leading to scattering of the input light.
Figure 5.7. Digital image of the fiber-microlens-fiber power coupling experimental set up: (a) Image showing the input fiber aligned with the microlens and (b) image showing a mis-alignment between the input fiber and microlens.
54
Figure 5.8 shows the plot of the calculated power coupled into the receiving fiber versus the fiber-fiber distances.
600
500
400
300
200
Coupled power (uw) 100
0 0 2 4 6 8 10 Fiber-fiber separation (mm)
Microlens diameter 400 µm Microlens diameter 500 µm
Microlens diameter 600 µm Microlens diameter 700 µm
Figure 5.8. Power coupled into the receiving fiber plotted versus the fiber-fiber
separation in the fiber-microlens-fiber power coupling experiment.
55 The fiber-fiber distance corresponding to the maximum power being coupled is equal to
4f. From the above graph, the focal length values determined for the microlenses of
different diameters are tabulated in table 5.3.
Microlens diameter Determined focal length
400 µm 550 µm
500 µm 750 µm
600 µm 1.25 mm
700 µm 1.9 mm
Table 5.3. Focal length obtained by fiber-microlens-fiber power coupling experiment for various microlens diameters.
The comparisons between the measured and calculated values show some discrepancies. The measured values of the focal lengths reasonably match that of the calculated values for the smaller microlenses. However, the discrepancies are higher for the lenses having diameters of 400 µm and above.
Figure 5.9 compares the focal length values determined by analyzing the surface
profile, obtained by imaging of the focal plane and by the fiber-microlens-fiber power coupling method.
56 2000
1800
1600
1400
1200
1000
800 Focal length (um) 600
400
200
0 0 100 200 300 400 500 600 700 800 Microlens diameter(um)
Focal length analytically calculated from the microlens surface profile
Focal length measured by imaging the focal plane.
Focal length measured by the fiber-microlens-fiber power coupling experiment.
Figure 5.9. Plot comparing the values of the focal lengths measured using different methods for various microlens diameters.
The comparison plots shows discrepancies between the focal length values obtained by different methods. The focal lengths measured from the experiments tally closely though they differ from the focal lengths estimated from the surface profile. The 200 µm and 300 µm diameter microlenses have an experimentally determined focal length that is very close to the estimated value from the surface profile.
57 The discrepancies can result from the following reasons
(1) Spherical aberration: In the first technique where the focal plane is imaged, due
to spherical aberration it is possible the smallest point of focus is not the actual
point of focus but just the point where the paraxial rays are focussed. Therefore
the experimentally measured focal point may be higher than the calculated value.
This seems to be true for the 200 µm and 300 µm microlenses whose shape is
closer to a sphere and thus may have more spherical aberration. From equation 4,
the spherical aberration is inversely proportional to the cube of the f-number.
Therefore, the smaller microlenses that have a smaller f-number have more
spherical aberration than the bigger microlenses.
(2) Surface roughness: The performance of the bigger lenses is probably affected by
the surface roughness though there is no data to corroborate this reasoning.
The spot diameters experimentally measured is also compared to the expected
spot size diameter calculated from equations (3) and (4). It can be concluded that the
200 µm and 300 µm diameter microlenses have a performance that is limited by
spherical aberration. The measured spot size diameters are actually smaller that what
is expected due to spherical aberration that dominates the smaller microlenses. This
probably can be accounted for by instrument inaccuracy and user error.
5.5 Characterization of microlenses on Poly IR 2
The focal lengths of the Poly IR 2 microlenses were measured by observing the focal plane as described previously. An infrared CCD camera was used to image the focussed spot of the IR light. Table 5.4 tabulates and compares the values of the experimentally
58 and calculated focal lengths. The measurement of the spot diameters was not possible, as the focussed IR light couldn’t be seen through the microscope eyepiece.
(a) (b)
Figure 5.10. CCD images of the focused spot of an infrared 1.3µm wavelength laser source through a (a) 500 µm diameter and (b) 300 µm diameter Poly IR 2 microlens.
Poly IR 2 has a refractive index of 1.52, very close to that of COC. Figure 5.10 shows the imaged focal spot of an infrared laser source through the Poly IR 2 microlenses.
Microlens diameter Calculated focal length Measured focal length 200 µm 215 µm 250 µm 300 µm 410 µm 420 µm 400 µm 665 µm 500 µm 500 µm 1 mm 805 um 600 µm 1.35 mm 1.22 mm 700 µm 1.78 mm 1.9 mm
Table 5.4. Tabulated values of the focal lengths determined from the surface profile and observation of the focal plane of Poly IR 2 microlenses for various diameters.
59 As the Poly IR 2 material has a refractive index very close to that of COC, and as the same mold is used for replication, the focusing performance is similar to that of COC.
5.6 Conclusions
The fabricated plastic microlenses were characterized for their focal lengths. Two different experiments were used for the focal lengths of the microlenses on COC. The focal lengths were determined by observing the focussed spot on the focal plane and also by the fiber-microlens-fiber power coupling method.
The experimentally determined focal lengths were compared to the calculated focal length from analyzing the surface profilometry data. The experimentally determined focal lengths for the 200 µm and 300 µm diameter microlenses were found to be higher than the calculated values. This possibly could be explained by spherical aberration that may dominate as these microlenses have a very low f-number. The bigger microlenses have an experimentally determined focal length value that is smaller than the calculated values.
The bigger microlenses will have lower spherical aberration due to their high f-numbers but the presence of surface roughness could probably be an explanation.
The focal lengths of the Poly IR 2 microlenses were characterized by imaging the focal plane. The comparison of the focal lengths between the values determined from the surface profile and from observing the focal plane shows discrepancies very similar to that of the microlenses on COC. Therefore, the explanation in the last paragraph can be applied to explain the discrepancies for the microlenses on Poly IR 2 plastics as well.
60 CHAPTER 6
CONCLUSIONS
6.1 Summary
A fabrication technique that enables high volume fabrication of plastic microlenses has been successfully developed and optimized. Plastic microlenses were successfully fabricated using high throughput injection molding techniques.
Microlenses were initially fabricated using the photoresist reflow method. The process
parameters to obtain reliable and repeatable microlens shapes using the reflow process were optimized for the AZ 4620 thick photoresist obtained from Clariant Corporation.
A negative pattern of the microlens patterned by the reflow method was obtained using electroforming a nickel layer to a thickness of 1.6 mm. The electroform was planarized and machined to fit as a replaceable injection mold disk in the injection
molding machine. The electroforming process was optimized for low stress deposits by
controlling the pH, temperature and current density of the electroplating bath.
The fabricated mold disk was used as the master mold for plastic replication using the
injection molding process. Two new plastics, COC and Poly IR 2, with high transparency
in the visible and infrared wavelength regions respectively, were used to fabricate the
microlenses.
The fabricated microlenses were characterized in terms of their surface roughness
using AFM (Atomic probe microscope). The surface quality was found to differ among
different microlenses with the surface roughness ranging from 9 nm to around 40 nm. It
61 was concluded that the source of the roughness was the fabricated injection mold disk
and that the roughness in the mold occurs due to the mechanical separation of the
electroform from the substrate.
The plastic microlenses were characterized for their focal lengths. The focal lengths
were experimentally determined by observing the focused spot on the focal plane and by
the fiber-microlens-fiber power coupling method. The experimentally determined focal
lengths were compared with the focal lengths estimated from the surface profile of the
microlenses.
The comparison showed a reasonably good matching. The small discrepancies were
possibly caused by spherical aberration of the microlenses, which is an important
parameter in determining the focal lengths.
6.2 Future W ork
This work has optimized a fabrication scheme to mass produce plastic microlenses.
The optical performance of the microlenses is pretty much limited by the photoresist reflow method. As the electroforming process and subsequent injection molding
faithfully reproduces the shape of the microlenses initially fabricated by the reflow
method, the optimization of the reflow technique is extremely crucial to control the
performance of the microlenses.
Any future work in this direction would need to incorporate rigorous analysis of the
reflow method and its optimization to fabricate microlenses for various focal lengths and
f-numbers. This can enable accurate modeling of the reflow process to obtain highly
optimized microlens shapes for various applications.
62 The microlens surface should be of the highest quality for it to perform within desired optical tolerances .The source of surface roughness on the microlens was due to the metal seed layers that break down into very small fragments due to the mechanical separation of the electroform from the substrate. A better way to separate the electroform from the substrate would go a long way in improving the surface quality of the microlenses.
The COC microlenses developed in this work hold lot of promise for applications in biochemical detection. Further work on integrating the microlenses with microfluidic chips can pave the way for applications in biochemical detection. Any future work in this direction would need to address the issues of alignment between the microlens and microfluidic chips. Alignment accuracy would be one of the most important issues for microlenses integrated with microfluidic chips.
Poly IR 2 is a promising material for applications in telecommunications as it is transparent to the communication wavelength of 1.3 and 1.65 µm. However, the injection molding parameters for Poly IR 2 need to be more thoroughly characterized as also the optical performance of the microlenses on Poly IR 2.
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