Localized Laser Transmission Bonding for Microsystem Fabrication and Packaging
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Journal ofJournal Manufacturing of Manufacturing Processes Processes Vol. 6/No.Vol. 1 6/No. 1 2004 2004 Localized Laser Transmission Bonding for Microsystem Fabrication and Packaging S. Theppakuttai, D.B. Shao, and S.C. Chen, Dept. of Mechanical Engineering, The University of Texas at Austin, Austin, Texas, USA. E-mail: [email protected] Abstract Introduction This paper reports a method using localized laser heating Bonding is a widely used method in microelec- to bond silicon and glass wafers directly. A pulsed Nd:YAG tromechanical systems (MEMS) fabrication and pack- laser (1064 nm, 12 ns) is transmitted through the glass wafer and absorbed by the silicon wafer. Bonding is realized when aging. The excellent mechanical strength of silicon the glass wafer is in immediate contact with the silicon wafer. combined with the electrical insulation, chemical A continuous wave He-Ne laser (633 nm, 20 mW) is used for durability, and optical transparency of glass makes probing the transient melting and resolidification of the sili- the silicon-glass couple the most employed material con surface upon pulsed laser heating. This transmission bonding process is conducted locally while the entire wafer combination in MEMS. Another reason for this com- is maintained at a low temperature, which is especially use- bination is the very small difference in the coeffi- ful in fabricating or packaging temperature-sensitive materi- cient of thermal expansion between glass and silicon als or devices. The bonded areas are studied in detail using (Rogers and Kowal 1995). Anodic bonding and fu- a scanning electron microscope (SEM) and a chemical anal- ysis is done to understand the bonding mechanism. Numer- sion bonding are the most common techniques used ical simulation is also carried out using the finite element for silicon-glass and silicon-silicon wafer joining, method to predict the local temperature change of both the respectively. The anodic bonding method utilizes a glass wafer and the silicon wafer during laser heating. high electric field (about 1000 V), and fusion bond- ing is done in a furnace at temperatures as high as Keywords: Laser Bonding, Pulsed Laser Heating, Silicon Wafers, Glass Wafers 1100°C. Therefore, both techniques tend to damage the prefabricated microelectronics or pre-seeded Nomenclature biomolecules and thus exclude the use of thermally sensitive materials for micro and meso devices. c = specific heat (J/kgK) Moreover, both anodic and fusion bonding are con- h = specific enthalpy (J/m3) 2 ducted along the entire wafer and so they do not I = intensity (W/m ) provide an effective solution to integrating discrete J = pulse energy (J) micro or nano structures onto a chip to realize lab- k0 = thermal conductivity (W/mK) 3 on-a-chip or other integrated microsystems. Fur- Qab = heat generation (J/m ) thermore, in cases where hermetic sealing is R = reflectivity desired, the residual gases trapped in the hermetic r = cylindrical coordinate cavity can induce plastic deformation in thin sili- T = temperature (K) con membranes as the gases expand due to the t = time (s) high temperature associated with the bonding pro- z = coordinate along propagation (m) cess (Henmi et al. 1994). ␣ = absorption coefficient (1/m) 3 To overcome the abovementioned drawbacks, = density (kg/m ) researchers have been trying to find reliable bond- ing processes that can be conducted under a low Subscript temperature. Unfortunately, these new processes are g = glass highly dependent on the bonding materials, surface p = pulse treatment, and surface flatness (Bower and Chin Si = silicon 1997; Bower, Ismail, and Roberts 1993). An alter- 24 Journal of Manufacturing Processes Vol. 6/No. 1 2004 native approach to bonding based on the concept of this reason, they are first cleaned in ethanol solution localized heating has been introduced in recent followed by rinsing in deionized water. The samples years. Lasers, with their selectivity, precision, and are then dried with nitrogen gas. The clean samples low heat distortion, are an attractive choice for pro- are then loaded in between two aluminum plates with viding localized heating. Laser transmission joining an opening in the center, and pressure is applied is a fast and single-step process with “direct-write” continuously until the end of the bonding process capability. This approach makes it possible to bond by bolting the plates together. The tight contact of microstructures and pre-immobilized biomolecules both the joining partners caused by the pressing de- for sensing, actuation, and even computing in vice ensures elimination of small air gaps as well as BioMEMS. A pulsed 355 nm laser was used for glass- good heat conduction to the glass. When locally to-silicon bonding in the presence of indium as an heated by the laser radiation, the glass is softened intermediate layer. To achieve selective bonding, a due to heat conduction from the silicon surface and, masking material was also used (Luo and Lin 2002). after cooling, the adhesion of the joint compounds Both pulsed and continuous wave (CW) 1064 nm results in a tight joining. lasers have been employed for the bonding process The schematic of the experimental setup is shown in the presence of aluminum or gold as an interme- in Figure 1. A 1064 nm pulsed Nd:YAG laser of 12 diate bonding layer (Mescheder et al. 2002; Wild, ns pulsewidth is used for the bonding process. The Gillner, and Poprawe 2001). However, in most cases, output of the laser beam passes through an aperture the presence of an intermediate bonding layer is and a beam splitter, which is used to split the inci- undesirable as it increases the size of the device and dent laser beam into two—one part to be used for complicates the fabrication of many devices. Also, bonding and the other for measuring the energy of little work has been done to investigate the physical pulses used for bonding. The sample is mounted on mechanism of the laser-assisted bonding process for a 3-D stage and the laser beam is focused to a spot micro and meso systems. size of 800 µm diameter by using a plano-convex In this paper, a 1064 nm pulsed nanosecond laser lens (diameter = 2.54 cm and focal length = 5.08 is employed for the bonding process in the absence cm). By adjusting the working distance between the of an intermediate layer. The bonding area is probed lens and the sample, the bonding area can be varied. for obtaining more information about the melting The bonding process depends on the melting area process. A low-powered CW He-Ne laser is used for as well as melting duration of silicon and the subse- in situ monitoring to provide information about the quent heat conduction to glass. Therefore, to ensure melting time based on the change in the reflectivity proper bonding, an in situ monitoring of the melting of silicon (Xu, Grigoropoulos, and Russo 1996; Hatano et al. 2000). To understand the localized Beam Aperture bonding process, it is essential to have a theoretical splitter or numerical model that could calculate the tempera- Laser Mirror ture distribution. In this work, the temperature pro- files during laser bonding are calculated using the Energy Oscilloscope finite element method. meter Experimental Setup Photodetector The samples used for bonding were silicon (Nova Lens Electronic Materials, Texas) and Pyrex™ glass He-Ne (Bullen Ultrasonics, Ohio) wafers of thickness 500 Filter µm. The surface roughness of the silicon and glass Lens samples to be joined is in the nanometer scale (< 2 nm), as the wafers were well polished. Before load- 3-D stage ing the samples, care is taken to ensure that the Figure 1 samples are clean and free of any contaminants. For Schematic of Experimental Setup 25 Journal of Manufacturing Processes Vol. 6/No. 1 2004 process is very important. A low-powered CW He- fect in this work, the temperature profile along the Ne laser (633 nm, 20 mW) is used as a probing beam direction perpendicular to the incident direction has for online monitoring. To make sure the measure- to be considered, and thus a two-dimensional, tran- ment of melting time is accurate, the detection beam sient heat conduction model is established. A volu- is focused onto the heating area. This is achieved by metric heat-generation source with an exponentially expanding the He-Ne beam using a beam expander decaying distribution in the propagation direction and focusing it using a plano-convex lens with a and with Gaussian distribution in the lateral direc- shorter focal length (diameter = 2.54 cm and focal tion is used to more accurately study the local tem- length = 2.54 cm). The incident probing beam re- perature distribution. The absorption coefficient of flected by the silicon sample is collected by a fast silicon at 1064 nm is small, and this justifies the as- silicon PIN photodiode with 1 ns rise/fall time. The sumption of volumetric heat generation. photodiode is connected to a digitizing oscilloscope The schematic of the heat transfer through glass- with 500 MHz bandwidth and 1 GSa/s sampling rate silicon is shown in Figure 2. Pyrex™ glass is virtu- operated at single-shot acquisition mode (1 ns time ally transparent to the laser beam at the wavelength resolution). To eliminate the emission signal and considered. Thus, the incident laser beam is prima- other reflected wavelengths from the heated silicon rily absorbed by silicon. Heat is conducted to the surface, optical filters were used, which only trans- glass from the molten silicon, thereby softening the mit the probing laser wavelength. The heating pulse glass at the silicon-glass interface. Upon solidifica- is used as a trigger to capture the reflected detecting tion, bonding is realized between the two layers.