Fabrication of a Nanoimprint Lithography Mask for Improved Infrared Detectors

FABRICATION OF A NANOIMPRINT LITHOGRAPHY MASK FOR IMPROVED INFRARED DETECTORS

Isha Datye Faculty Mentor: Dr. Sanjay Krishna Graduate Student Mentor: John Montoya The Center for High Technology Materials The University of New Mexico, Albuquerque, NM 87131

Undergraduate Student of The Department of Electrical and Computer Engineering The University of Illinois at Urbana-Champaign Urbana, IL 61801

ABSTRACT

Infrared photodetectors will require new technologies on the pixel level to provide spectral information for the development of polarimetric and color images. Current infrared photodetectors have nearly identical pixels over a broad spectral range, resulting in black-and-white images. Scientists have been researching the idea of an infrared retina, which is similar in function to cones in the human eye, to produce multi-color images. A multi-color infrared camera system can be accomplished by tuning individual pixels to a specific infrared “color” with the aid of resonant structures patterned onto a photodetector’s surface. In addition, a resonant structure can also improve a detector’s detectivity (D*, a measure of the signal to noise ratio) or increase the operating temperature. One of the key limitations of present day technology is the difficulty in making deep subwavelength structures on a large scale. This research paper will focus on the fabrication of a nanoimprint lithography mask to pattern resonant structures on a scale that would make this multi-color technology ready for mass fabrication. Research has been conducted on nanoimprint lithography and it has proven to be a very efficient method to pattern structures on substrates with nanoscale precision. The goal for this project was to pattern complicated resonant structures on a substrate for the development of a multi-color infrared camera.

1. INTRODUCTION visible light, which has wavelengths from about 400 nm to 750 nm. Infrared detectors, The infrared region of the electromagnetic photodetectors that respond to infrared spectrum has wavelengths from 0.75 radiation, have made significant microns to 1000 microns, longer than that of improvements since they were first developed. Although there are several photodetectors. As one can see, these images different divisions within the infrared are based on the intensity of light to provide region, the mid-wavelength (MWIR) and a false color image. Next generation long-wavelength (LWIR) infrared regions photodetectors will be able to provide are most important for infrared detector spectral information based on the technologies, since they include the wavelength of light. wavelengths at which most objects emit radiation. For example, humans emit radiation at a wavelength of 10 microns, which is in the LWIR range [1]. There is an increased emphasis on obtaining hyperspectral and hyperpolarimetric sensitive detectors for night vision, missile tracking, medical diagnostics, and environmental monitoring applications [1- 3]. The development of frequency-selective surface technology on the pixel level has the a) b) potential to provide enhanced infrared detection for a desired wavelength of radiation. Since it was first developed in the Figure 1. (a) [18] Infrared images of the human 1960s, infrared imaging technology, body showing areas of muscle pain in the back especially in the area of focal plane arrays, and post-operative inflammation in the knee. has made significant improvements in (b) [19] Infrared detector images showing a jet producing an image. A focal plane array, a helicopter and jet engine. device that converts an optical image into an electrical signal that can then be processed 1.1 Background or stored, is the core of a long wavelength imaging sensor [14]. The first generation In the past decade, new infrared detector consisted of a single pixel or a one- technologies, such as quantum dot infrared dimensional array of pixels that required a photodetectors (QDIPs), quantum well mechanical sweep to produce a two- infrared photodetectors (QWIPs), quantum dimensional image [1]. The second dots-in-a-well infrared photodectors generation now consists of a two (DWELL), and superlattice structures (SLS), dimensional array of pixels to produce an have been developed with ever increasing image, eliminating any need for moving operating temperatures. Infrared parts [1]. Third generation infrared cameras photodetectors that can operate at room will consist of a two dimensional array of temperature can significantly reduce their pixels that can pass spectral information at cost of operation and therefore expand their room temperature, which is similar in use for everyday applications [2]. QWIPs, function to cones in the human eye [1]. generally made with GaAs materials [17], Although significant improvements have are already well-known and are available been made by the infrared detector commercially [3]. However, they have many community, infrared images that are truly shortcomings and are generally thought to multi-color are not readily available. A few be inferior to QDIPs [2]. For example, examples are given in figure 1 to QDIPs do not require diffraction gratings to demonstrate images taken with conventional couple normally incident light [2]. Because of this, there is one step less in the fabrication of QDIPs than in the fabrication are still interested in exploring new of QWIPs. QDIPs are similar to QWIPs in technologies that can take even better structure; the quantum well is substituted images and can incorporate more elements with a quantum dot [17]. QDIPs are such as color, polarization, and dynamic generally constructed with InAs dots on range. GaAs substrates [17]. An image of a QDIP structure is shown in figure 2. QDIPs can operate at higher temperatures as a result of having a lower dark current [2].

Figure 3. Image of a quantum dots-in-a-well structure, showing the InAs quantum dot in an InGaAs quantum well.

Figure 2. [20] The image on the left shows a 10- layer InGaAs/GaAs QDIP structure, and the image on the right shows a diagram of a QDIP in Currently, all of the pixels in an infrared an electric field. camera are nearly identical, creating black- and-white images instead of images of different colors [15]. Scientists would like to The DWELL structure, a cross between change this by integrating multispectral QDIPs and QWIPs with InAs quantum dots capability on the pixel level. An example of in an InGaAs quantum well [3], has also multispectral imaging is shown in figure 4. been proven to have low dark currents, and They have been researching the concept of higher operating temperatures [3]. Similar to an infrared retina, which would act similarly QDIPs, these detectors allow normal to the cones in a human eye in the incidence, which ultimately provides better information conveyed in the images [1]. In control over the operating wavelength [16]. order to create this infrared retina, either A DWELL structure is shown in figure 3. plasmonics or matamaterials can be used. InAs/GaSb type-II strain layer superlattices Current infrared detector technology, can in also operate at higher temperatures, have a general, be improved with the aid of higher detectivity, high efficiency, and some resonant structures of a given design to multi-color capability [2], although not on a provide a higher operating temperature, large scale. This is the most promising spectral information on the pixel level, and a technology, but it also the most expensive. higher detectivity [3]. This project will focus on the development of a nanoimprint 2. MOTIVATION FOR PROJECT lithography mask for the fabrication of these resonant structures. Initially, a simple Although the current infrared detector nanoimprint lithography mask will be technologies have made many created to imprint an array of posts. These improvements in their images, researchers simple structures can aid in the fabrication of surface plasmon (SP) diffraction gratings, the beam and the surface normal to the which have been shown to enhance the sample. In electron beam lithography (EBL), signal received by an infrared photodetector a beam of electrons is scanned in a pattern [13]. A simple nanoimprint lithography across a surface with photoresist to create mask can be used to establish a fabrication small structures in the resist that can then be process for the creation of nanoimprint transferred to the substrate. masks with complicated features, such as metamaterials. Metamaterials are artificial materials that have properties usually not found in nature. They offer a huge advantage over SP structures because they can be created for pixels with a small size [5]. They can be engineered to have a negative index of refraction, meaning that the light is bent around the object rather than transmitted through or reflected away from (a) it [5]. Narrow-band perfect absorbers can be created because of the electric permittivity and magnetic permeability of metamaterials [5]. They can control and interact with infrared radiation only if their structures have wavelengths similar to those of the infrared light waves with which they interact [5]. (b)

3. RESEARCH OBJECTIVE Figure 4. (a) [21] In the top right picture, an object is detected that wouldn’t be seen with the human eye. (b) [22] In the bottom three pictures, different parts of This project focused on the fabrication of a a flame can be seen with an infrared detector. mask through different forms of lithography, such as interferometric and electron beam lithography, to pattern metamaterial EBL is generally used to make integrated structures. Lithography is a technique used circuits and masks [10, 12]. Although useful to transfer patterns onto a substrate. for creating interesting, repetitive patterns, Interferometric lithography is a process in electron beam lithography cannot be which a laser beam is split into two beams, performed on a large scale because it is one going directly to the sample and one extremely slow and expensive [10]. going to mirror and then reflected to the However, it can be used on a single sample sample, thereby creating an interference and then nanoimprint lithography can be pattern [8]. The pitch of grating d is used utilized on a mass scale. Nanoimprint determined by the formula lithography, as opposed to electron beam lithography, is relatively simple, fast, and inexpensive [9]. In addition, it has a high d = throughput and resolution [4]. Nanoimprint lithography (NIL) can be performed in three where is the wavelength of the laser beam, different ways, the first called thermal NIL, n is the index of refraction of the medium the second called ultraviolet NIL, and the (in air, n is 1), and is the angle between third called substrate conformal imprint lithography (SCIL) [4]. The general process the images taken by an infrared detector. As for NIL is shown in figure 5. In thermal mentioned before, metamaterial structures NIL, a mold is pressed into a resist on a have a higher absorption efficiency, which substrate at a high temperature, the substrate will enhance the optical signal to an infrared and mold are cooled while pressed together, detector and produce more electrons, which and the mold is released from the substrate, will allow the infrared detector to operate at leaving a pattern on the substrate [9]. In higher temperatures [5]. ultraviolet NIL, a mold is pressed into a UV resist on a substrate, the resist is cured by exposure to UV light, and the mold is released from the substrate, leaving the pattern on the substrate [4]. SCIL, developed by Philips Research and Suss MicroTec, is an improved version of UV NIL by providing ways to get even better resolution and patterning over larger areas [4]. In addition, SCIL provides a low force and a low temperature processing condition for nanoimprint lithography, which is an advantage for the fabrication of focal plane arrays [4]. Photolithography, a simpler method used to transfer a pattern to a Figure 5. [23] Schematic of NIL fabrication process. photoresist, would not be beneficial to use in this case; it is diffraction limited, so it doesn’t allow feature sizes as small as those Metamaterial structures also have the needed for the fabrication of metamaterial potential to create a more narrow spectral structures that can be obtained using IL and response, which will improve multispectral EBL [14, 15]. We wanted our feature sizes imaging [11]. This technology will be to be around 250 nm. Our plan was to begin different from other infrared detector with interferometric lithography to establish technologies because the mask patterned the fabrication process for a mask and to with these structures can be used on any pattern simple structures, and then continue infrared detector, rather than only on with electron beam lithography to pattern specific infrared detectors. more complicated metamaterial structures. Finally, the mask from EBL would be used 4. METHOD as a mold for mask fabrication through nanoimprint lithography. This will We began with a 530 nm thick quartz wafer, ultimately help to replace the optical filter since quartz is stronger than many other and put it on the chip level, which is desired materials. In addition, it is clear, making it since the optical filter can be very expensive easier for us to see through it for mask and bulky. Also, the optical filter transmits alignment in later steps. Using a dicing saw, light of certain wavelengths and blocks the we inscribed 230 nm thick cuts into the rest of it, producing only one color at a time wafer to make it easier to later cleave into [13]. By patterning metamaterial structures square samples. We deposited about 35 nm on the pixels, the different geometry of the of chrome on the side of the wafer without structures will produce different colors in the dicing lines using metal evaporation. We used chrome because it is a strong metal for the dry etching process, and the IL process our samples under an SEM to see if we were allows metallic nanostructures to be successful. Our final recipe for the RIE was transferred easily [6]. Next, we used a 10 mTorr chamber pressure, 10 sccm spinner to spin-coat ICON-16 anti-reflection oxygen gas, a radiofrequency (RF) power of coating, which prevents reflections from the 15% of 200 W, and an etch time of 1.75 quartz wafer, at 2700 RPM, and did a soft minutes. bake at 200°C for 60 seconds. We then spin- coated SPR-505A photoresist, a material coated on a surface to create patterns, at 3000 RPM and did a soft bake at 95°C for a) 60 seconds. At this point, we cleaved the wafer into separate square samples along the b) c) lines we inscribed with the dicing saw. This way, we had multiple samples to test for different exposure times using interferometric lithography. The laser that d) e) we used had a beam with a wavelength of 355 nm, frequency of 60 Hz, and energy of 75 mJ. The pitch of grating for our samples was 500 nm. Initially, we exposed a few f) g) samples without rotating the sample in the sample holder to create one dimensional lines. We tested exposure times varying from 5 to 13 seconds, and with each sample we incremented the time by 2 seconds. After h) i) each exposure, the sample was placed on a hot plate heated to 110°C for 60 seconds, and then developed using MF-702 for 60 seconds. The developer removes the photoresist from the parts of the sample not j) k) exposed by the laser beam and leaves Figure 6. Visual representation of the interferometric patterns across the surface of the sample. At lithography process. a) quartz substrate, b) chrome this point we looked at our samples under a deposition, c) ARC coating, d) photoresist coating, e) scanning electron microscope (SEM) to see IL, f) developing sample, g) dry etching ARC, h) wet how the lines looked. Once we found a etching chrome, i) removing ARC and photoresist, j) sample with solid, unconnected lines, we dry etching quartz, and k) removing chrome. used that exposure time to pattern two dimensional posts on new samples. We once The next step in our procedure was to wet again took SEM images to examine the etch the chrome. We used a wet etchant, posts. The next step in the fabrication of the called CEP-200, and tested different etch mask was dry etching the ARC, which was times—10, 15, and 20 seconds—on a few done with a reactive ion etching (RIE) samples. After looking at these samples machine. In this process, gases and plasma under an SEM, we determined that the are introduced in a chamber. We used chrome needed to be etched for at least 20 oxygen gas for the dry etching. We tested a seconds, because we could still see the few different etch times and then looked at chrome on the wafer with the etch times less than 20 seconds. At this point in our CINT, using the procedure outlined above. experiment, we didn’t have enough samples A few SEM images confirmed that the to test the dry etching of the quartz, so we patterns were transferred to the quartz. repeated the entire process—dicing saw cut Although the next step of our project was to lines, metal deposition, interferometric fabricate many masks using the sample from lithography, dry etching the ARC, and wet electron beam lithography as a mold through etching the chrome. Then, we removed the nanoimprint lithography, we unfortunately photoresist and ARC by using another were not able to access the machines and plasma dry etching machine with oxygen materials needed to do so. gas. Since our facility, the Center for High Technology Materials (CHTM), does not 5. RESULTS have the capability to dry etch quartz, we had to go to the Center for Integrated We were able to successfully make one Nanotechnologies (CINT) at Sandia dimensional lines and two dimensional posts National Laboratories to use their machine using interferometric lithography, as seen in for dry etching. We used a Trion Tech the SEM images in figure 7. The lines and fluorine dry etching machine with a chamber posts have a diameter of 250 nm. pressure of 10 mTorr, ICP radiofrequency of 350 W, RIE radiofrequency of 35 W, 45 sccm CF4, 5 sccm Ar, 5 sccm O2, and an etch time of 120 seconds. We went back to CHTM to take some SEM images to see if the etching worked. After removing the chrome with the chrome wet etchant and taking a few more SEMs, we had completed the process for the fabrication of a mask. A visual process for the fabrication of a mask Figure 7. The sample on the left was exposed for using IL is shown in figure 6. We then 7 seconds, and the sample on the right was applied this process to the fabrication of a exposed for 3.5 seconds, and then rotated and new mask using electron beam lithography exposed for another 3.5 seconds. to pattern more complicated shapes than posts. We began with a new quartz wafer on After creating the patterns in the photoresist, which we spin-coated a photoresist, called we dry etched the ARC to transfer the poly methylmethacrylate (PMMA), pattern to the ARC. We tested a few specifically for electron beam lithography, at different etch times, and even though 3000 RPM. Next, we went to CINT to use different etch times may have successfully their electron beam lithography machine. dry etched the ARC, some of the times This process was quite complicated and took reduced the diameter of the posts a few hours to complete, unlike significantly. We wanted the posts to still be interferometric lithography, which only similar in diameter to how they were before takes a few minutes. We did a direct write to the dry etching. We determined that 1.75 create the pattern on our substrate. We minutes was the etch time that correctly developed the sample using MIBK diluted transferred the pattern to the ARC and kept 1:3 for 1 minute. After this, we deposited the diameter of the posts, as shown in figure around 35 nm chrome using a metal 8. We knew we had dry etched the ARC evaporator and dry etched the quartz at because we could see a slight horizontal line showing the separation between the determine the etch rate, we took a quartz photoresist and ARC. substrate that had half of the quartz exposed and the other half with 35 nm of chrome. We dry etched the sample for 5 minutes and measured an etch depth of 0.5 microns using an alpha-step machine. For our samples, we dry etched the quartz for 2 minutes and achieved an etch depth of 200 nm. An example of this is shown in figure 10. We wanted close to a 1:1 ratio between the diameter of the post (250 nm) and the depth Figure 8. Both of these images are SEMs of a of the post (200 nm). sample after the anti-reflection coating was dry etched for 1.75 minutes using oxygen gas. The image on the left is the top view and the image on the right is the side view of the same sample.

We then wet etched the chrome using different etch times and took SEM images of the different samples. After comparing the images, we believe that 20-30 seconds was enough time to wet etch the chrome, as seen below in figure 9.

Figure 10. Titled view of the substrate after we dry etched the quartz. There is still chrome on the posts, but we believe we were able to successfully etch the quartz.

After using the SEM at CHTM, we went to a different facility, The Center for Micro-

Engineered Materials, to use their SEM to Figure 9. SEM image of a sample that was wet obtain different types of images. We took etched for 20 seconds to remove the chrome. The some secondary electron (SE) images and quartz can be seen in between the posts. The some back-scattered electron (BSE) images photoresist can be seen on top of the posts (the to get new information from the images, as darker spots). shown in figure 11. SE images show the topographical contrast in the images, and After wet etching the chrome, we removed BSE images show the contrast in the the photoresist and ARC using a plasma RIE material. We performed electron beam machine with oxygen gas. Then, we went to lithography on a sample to pattern a CINT to dry etch the quartz. We dry etched metamaterial structure. Although we were approximately 100 nm per minute. To able to transfer the pattern to the quartz successfully, the patterns did not look adhesion of the photoresist. This can be seen exactly as we intended them to look. in figure 14, where parts of the cross do not appear. One way to prevent this is to clean the substrate thoroughly before applying photoresist, by baking the substrate for a long period of time and then cleaning it with acetone and isopropanol. Although our results were not perfect, we were still able to show that it is possible to pattern these metamaterial structures on a quartz substrate.

Figure 11. Both pictures are top views of the substrate after the quartz was dry etched. The top image is an SE image, and the bottom image is a Figure 12. Image of the metamaterial structure BSE image. that we patterned on the quartz substrate.

The metamaterial structure that we 6. CONCLUSION attempted to pattern on the substrate is shown in figure 12. The patterns were In this paper, we have outlined the current distorted and non-uniform, as seen in figures infrared detector technologies and the 13 and 14, but we were not completely sure reasons for continued research in this field. why this happened. This may have happened We discussed the need to increase the due to surface charging from the electron functionality of pixels on an infrared camera beam lithography machine. We believe by patterning metamaterial structures reducing the current and coating the sample through lithography to enhance the function in gold prior to performing EBL would of the focal plane arrays and improve the reduce the charging effects. If we had more optical signal to an infrared camera. We time, we would vary the parameters to try to have shown that it is possible to pattern perfect the EBL process. This process would these structures and that, if produced on a take another two weeks to perfect, since mass scale using nanoimprint lithography, there are still many unknowns, such as level they show much promise in ultimately of current and amount of gold. Another improving infrared detectors. This reason why the patterns appear to be technology is different than the other disconnected and non-uniform is poor technologies mentioned earlier in this paper, since it can be used on any infrared detector; color, and multispectral capabilities in the it is not specific to a certain type of infrared images. detector. 7. FUTURE WORK

If we had more time to continue this research project, we would attempt to improve the process for electron beam lithography. We would like to be able to successfully pattern metamaterial structures on the quartz substrate. After obtaining the required materials for nanoimprint lithography, we will be able to use this quartz substrate as a mold for NIL to fabricate masks on a large scale. Then, we will be able to use the masks from NIL to print patterns on the focal plane arrays in infrared detectors. NIL can be used to create Figure 13. Top view of the pattern after dry subwavelength grating patterns on the mask etching the quartz. The pattern is non-uniform [7]. Currently, EBL can only be and some parts of the crosses are disconnected. implemented for substrates of small sizes. If we could perform EBL on a large sample, then we could fabricate a mask that could be used for any infrared detector. The final step in this project would be to actually construct devices with multi-color and multispectral capabilities using the metamaterial structures we fabricated.

8. ACKNOWLEDGEMENTS

I would like to thank the National Science Foundation and their Research Experience for Undergraduates program for giving me the opportunity to perform undergraduate research. I would like to thank my faculty mentor, Dr. Sanjay Krishna, and my

Figure 14. 30 degree tilted view of the graduate student mentor, John Montoya, for metamaterial structure patterned on the quartz all their help and guidance in my project substrate. throughout these ten weeks. In addition, I would like to thank some of the other researchers at CHTM, including Dr. Alex As mentioned before, these metamaterial Raub, for helping me become familiar with structures have the potential to improve the some of the steps in the IL process, Xiang images taken by an infrared camera by He, for helping me learn how to use the laser incorporating polarization, dynamic range, and for helping with the dry etching process, and Ajit Barve, for helping me take many of Journal of Colloid and Interface Science, the SEM images in this paper. I would also vol. 360, issue 1, pp. 320-323, 2011. like to thank CHTM at UNM and Sandia [9] Tao H, Landy N I, Bingham C M, Zhang National Laboratories for allowing me to use X, Averitt R D and Padilla W J. A their facilities throughout the summer. Metamaterial Absorber for the Terahertz Regime: Design, Fabrication and 9. References Characterization. Opt. Express 75 7181, 2008. [1] S. Krishna. The Infrared Retina. J. Phys. [10] Harriott, Lloyd R. Electron Beam D: Appl. Phys, vol. 42, pp. 1-6, 2009. Lithography. American Physical Society, [2] A. V. Barve, S. J. Lee, S. K. Noh, and S. APS/AAPT Joint Meeting, April 18-21, Krishna. Review of current progress in 1997, abstract #K7.02. quantum dot infrared photodetectors. Laser [11] Lammel G, Schweizer S, Schiesser S, Photonics Rev. (in press). and Renaud P. Tunable optical filter of [3] Krishna, S., Gunapala, S. D., Bandara, S. porous silicon as key component for a V., Hill, C. & Ting, D. Quantum Dot Based MEMS spectrometer. J. Microelectromech. Infrared Focal Plane Arrays. Proc. IEEE 95, Syst. 11 815-28, 2002. 1838–1852, 2007. [12] Digital Focal-Plane Arrays. Rep. [4] Ji, R.; Hornung, M.; Verschuuren, M. A.; Lincoln Laboratory (MIT). Web. van de Laar, R.; van Eekelen, J.; Plachetka, [13] S. J. Lee, Z. Ku, A. Barve, J. Montoya, U.; Moeller, M.; Moormann, C. UV W. Jang, S. R. J. Brueck, M. Sundaram, A. Enhanced Substrate Conformal Imprint Reisinger, S. Krishna, and S. K. Noh. A Lithography (UV-SCIL) Technique for monolithically integrated plasmonic infrared Photonic Crystals Patterning in LED quantum dot camera. Nat. Commun. 2:286 Manufacturing Microelectron. Eng. 2010, doi: 10.1038/ncomms1283, 2011. 87, 963-967. [14] S. R. J. Brueck, Optical and [5] N. I. Landy, S. Sajuyigbe, J. J. Mock, D. interferometric lithography— R. Smith, and W. J. Padilla. A Perfect nanotechnology enablers, Proc. IEEE 93, p. Metamaterial Absorber. Physical Review 1704, 2005. Letters, volume 100, issue 20, pages [15] A. Boltasseva and V. M. Shalaev. 207402/1-207402/6, 2008. Fabrication of optical negative-index [6] K. Du, I. Wathuthanthri, W. Mao, W. metamaterials: Recent advances and Xu, and C. Choi. Large-area pattern transfer outlook. Metamaterials2, 1–17, 2008. of metallic nanostructures on glass [16] S. Krishna. Quantum dots-in-a-well substrates via interference lithography. infrared photodetectors. Journal of Physics Nanotech 22:285306, 2011. D: Applied Physics, 38, p.p 2147., 2005. [7] Ahn S-W, Lee K-D, Kim J-S, Kim S H, [17] H. C. Liu. Quantum dot infrared Lee S H, Park J D and Yoon P W. photodetector. Opto-Electron. Rev. 11(1), Fabrication of subwavelength aluminium 2003. wire grating using nanoimprint lithography [18] "Infrared Camera Image Gallery." and reactive ion etching. Micro. Infrared Camera and Night Vision Eng. 78/79 314-8, 2005. Superstore. Web. [8] L. Gao, L, Lin, J. Hao, Weifeng Wang, [19] "FLIR Thermography - Infrared R. Ma, H, Xu, J, Yu, N. Lu, Wenchong Cameras & Thermal Imagers." FLIR. Web. Wang, L. Chi. Fabrication of split-ring [20] Fu, Lan, P. Kuffner, H. Hoe Tan, and resonators by tilted nanoimprint lithography. Chennupati Jagadish. "Using Controlled Interdiffusion to Make a Two-color Quantum Dot IR Photodetector | SPIE Newsroom: SPIE." SPIE - the International Society for Optics and Photonics. SPIE, 2006. [21] Dereniak, E. Snapshot Polarimetry. Photograph. [22] R. Rehm, M. Walther, J. Schmitz, J. Fleibner, J. Ziegler, W. Cabanski and R. Breiter. Dual colour thermal imaging with InAs/GaSb superlattice in mid-wavelength infrared spectral range. Elec. Lett., 42(10), 2006. [23] "Nano Imprint Lithography: A Giant Leap for Miniature Manufacturing - News - NRC-CNRC." National Research Council Canada: From Discovery to Innovation / Conseil National De Recherches Canada : De La Découverte à L'innovation. National Research Council Canada, 05 Aug. 2005.