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Microfabrication services at INO Christine Alain*, Hubert Jerominek, Patrice Topart, Timothy D. Pope, Francis Picard, Félix Cayer, Carl Larouche, Sébastien Leclair, Bruno Tremblay INO ABSTRACT MEMS (Micro Electro Mechanical Systems) technology has expanded widely over the last decade in terms of its use in devices and instrumentation for diverse applications. However, access to versatile foundry services for MEMS fabrication is still limited. At INO, the presence of a multidisciplinary team and a complete tool set allow us to offer unique MEMS foundry-type services. These services include: design, prototyping, fabrication, packaging and testing of various MEMS and MOEMS devices. The design of a device starts with the evaluation of different structures adapted to a given application. Computer simulation tools, like IntelliSuite, ANSYS or custom software are used to evaluate the mechanical, optical, thermal and electromechanical performances. Standard IC manufacturing techniques such as metal, dielectric and semiconductor film deposition and etching as well as photolithographic pattern transfer are available. In addition, some unique techniques such as on-wafer lithography by laser writing, gray-scale mask lithography, thick photoresist lithography, selective electroplating, injection moulding and UV-assisted moulding are available to customers. The hermetic packaging and a novel patented wafer-level micropackaging are also applied. This multifaceted expertise has been utilized to manufacturing of several types of MEMS devices as well as complex instruments including micromirror-type devices, microfilters, IR microbolometric detector arrays, complete cameras and multipurpose sensors. Keywords: MEMS foundry, microelectroforming, microphotonic, thick resist, micromirrors, IR bolometers, bulk and surface micromachining, wafer level packaging. 1. INTRODUCTION Microfabrication or micromachining technology is the art of fashioning microscopic optical, fluidic, sensing or mechanical parts such as lenses, capillary tubes, beams, cantilevers, membranes and other suspended or non-suspended microstructures. Merging micromachining with various IC (Integrated Circuit) fabrication technologies permits to fabricate a wide variety of MEMS (Micro Electro Mechanical Systems). MEMS technology has expanded into a multi billion-dollar business over the last decade1. MEMS technology has already produced numerous microdevices and instruments that have allowed cost reduction, miniaturization and increased performances. However, the access to versatile foundry services for MEMS fabrication is still limited. Even if the fabrication of MEMS mainly relies on the techniques previously developed for the fabrication of ICs, foundries have rarely shown an interest in MEMS fabrication. A few North American and European foundries offer access to their MEMS fabrication capacity, mainly based on polysilicon as the mechanical layer and silicon oxide as the sacrificial layer. Some universities also offer the possibility to deposit and pattern a variety of thin films on a limited scale. INO, which is a Canadian, private, non-profit organization, has been working in the MEMS field for more than a decade. This corporation was created in 1985 and is located in the Québec Metro High Tech Park since 1988. At present, INO has 215 employees and its annual budget in 2001-2002 was over $25 M CAN. INO is ISO-9001 certified. It practices an entrepreneurial management style with its activities aimed at the private sector. INO pursues scientific excellence while focusing on technological activities that meet the needs of business. Its core expertise is schematically * email@example.com; phone 1 (418) 657-7006; fax 1 (418) 657-7009; http://www.ino.ca INO, 2740 Einstein Street, Québec, QC, Canada, G1P 4S4 illustrated in Figure 1. Availability of all these technologies under one roof creates an exceptional multidisciplinary environment suitable for development of specialty components, devices and instruments. A unique capability was developed at INO in terms of MEMS fabrication, where both bulk and surface micromachining technologies are available. Moreover, the use of polymers as the sacrificial layer, instead of silicon oxide, permits various shapes and materials to be used, while remaining fully CMOS compatible. The presence of a multidisciplinary team and a complete tool set allow to offer exceptional MEMS foundry-type services. These services include: design, prototyping, fabrication, packaging and testing of various MEMS and MOEMS devices. The present paper reviews the available services, from design to product. In addition to standard IC manufacturing methods, unique enabling technologies such as laser writing, gray-scale mask lithography, thick photoresist lithography, selective electroplating and polymer sacrificial layers are described. A new micropackaging technology that permits hermetic packaging of devices on a die or wafer level is also presented. Finally, examples of INO’s realizations are shown. Figure 1: INO’s core expertise. 2. SIMULATION TOOLS, DESIGN AND PROCESS FLOWS The design of a MEMS device starts with the evaluation of different structures adapted to a given application. The behavior of the structures is verified using performance computer simulation tools. INO mainly uses IntelliSuite, a software specifically created for MEMS design, simulation and optimization. IntelliSuite permits the modeling of various characteristics including mechanical, electrostatic and electromagnetic performance, either in static or in dynamic mode. ANSYS is utilized as a complement to IntelliSuite for thermal and mechanical analysis. In-house software is also used for more specific applications. An example is INOVOBOL, which can simulate the performance of IR bolometric detectors. Having the ultimate performances in mind during the simulations, variables such as the choice of dielectric and metallic materials, thin film thickness and length to width ratios have to be evaluated. The effect of elements like the combination of different optical characteristics or the matching between thermal expansion coefficients must not be neglected. Once the design variables are fixed, a complete photolithographic mask set is drawn with the software DW2000. This cad tool produces GDSII files, therefore providing full compatibility with photomask suppliers. In order to draw the mask set, the fabrication process must be carefully chosen and defined during the design phase. In particular, compatibility issues between materials and integration issues between MEMS and ICs have to be properly evaluated and understood. Bulk micromachining can be well suited for some applications where microbeams and microcantilevers are to be shaped. However, as holes must be etched in the substrate to release the microstructures, this technique is not fully compatible with CMOS. MEMS components must be built adjacent to the electronic circuit. This forces a horizontal integration between MEMS and ICs, which requires more chip space. Nevertheless, this method is suitable for fast prototyping of MEMS on bare substrates at lower cost. If needed, MEMS can be hybridized with CMOS later on. Bulk micromachining is also the easiest method to obtain flat surfaces and high aspect ratio cavities. On the other hand, surface micromachining eases the integration between ICs and MEMS. Chip space is saved, vertical integration is possible, allowing further miniaturization and cost reduction. The reliability of electrical contacts between ICs and MEMS is also better if directly integrated, not hybridized. Having the choice between bulk and surface micromachining therefore leads to maximum process versatility, limited only by the lithography accuracy and the material etching selectivity. INO’s microfabrication tool set permits to create a custom fabrication process for each application. A typical example of a complex process developed at INO is the fabrication of double stage microbolometers2. This process, fully compatible with CMOS and biCMOS, has 11 masking levels and 15 deposited thin films. The main design rules are described in Table 1. These rules were specifically developed for suspended VO2-containing structures having sacrificial layers with a total thickness of up to 3.0 µm. The minimum size and width are mainly defined by the type of photolithographic equipment used, the thickness of the masking photoresist, the anisotropy of the etch recipes and the etch selectivity between materials. In this specific case, the smallest feature size is 1.5 µm. The overlap limits are set in accordance with the achievable alignment accuracy and capping requirements of specific materials. The distance between suspended and non-suspended structures is set to a large number in order to avoid any edge effect during photolithography and etch steps. Figure 2 further explains the sample microbolometer fabrication process. In step 1, the first sacrificial layer is spun and defined on top of the microbolometer electrodes. These electrodes can be readily defined in the CMOS top metal, or can be deposited and patterned before the polyimide application. Step 2 illustrates the deposition of the first structural dielectric layer, shaping of the via holes and deposition of the bottom electrodes. In step 3, the 2nd dielectric layer is deposited and patterned. The dielectric-metal-dielectric sandwich forms an encapsulated arm that will be the first stage of the microbolometer. The second stage manufacturing starts with the deposition and patterning of the 2nd polyimide layer, as shown in step 4. Step 5 includes the deposition and patterning of a VO2 layer, encapsulated in 2 layers of dielectric. The second via holes, VO2 contacts and electrodes are made in step 6. The microbolometer platform is then encapsulated and patterned in step 7. Finally, the polyimide sacrificial layer is removed in step 8. The result is a double stage suspended structure, monolithically integrated on a read out circuit, as illustrated in Figure 3. The microbolometers are being built on 1.5 µm CMOS read out circuits from Mitel (now Dalsa Semiconductor). This type of MOEMS therefore needs a total of 25 masking levels to be realized. Feature description Minimum size Via and contact size 1.5 x 1.5 µm2 Post width in polyimide 4.0 x 4.0 µm2 Metal line width 1.5 µm Overlap of metal over via 1.0 µm Dielectric-metal-dielectric suspended structure width 3.0 µm Overlap of dielectric over metal 0.75 µm Overlap of dielectric over VOx 1.5 µm Gap between 2 suspended structures 2.0 µm Gap between suspended and non suspended structures 40 µm Table 1: INO’s main design rules for double stage microbolometer fabrication. Figure 2: Sample fabrication process flow for double-stage microbolometer fabricated at INO. Figure 3: Double stage VO2-based microbolometer fabricated at INO on a 1.5 µm CMOS read out circuit. 3. EQUIPMENT, MATERIALS AND FABRICATION Prototyping and fabrication of MEMS devices require a diversity of equipment, materials, physical structures and fabrication methods. At INO, microfabrication equipment is located in 4 600 square feet (~ 430 m2) of class 100, 1 000, and 10 000 clean rooms. The surface micromachining processes are fully compatible with CMOS technology, and permit the monolithic integration of mechanical, electrical or optical elements with CMOS circuits without inducing damage. MOEMS requiring up to 2 sacrificial layers and 15 masking layers with minimum feature size of 1.5 µm have been fabricated over circuits realized in Mitel’s 1.5 µm and 0.8 µm CMOS technologies (now Dalsa Semiconductor, Bromont, QC, Canada) and in AMI’s 0.6 µm CMOS technology (Pocatello, ID, USA). In both cases, the resistance of 1.5 x 1.5 µm2 via contacts between the CMOS circuit and the MOEMS parts was lower than 5 ohms. Processing of round, square and irregularly shaped substrates made of silicon, glass, quartz, alumina, lithium niobate, stainless steel and other materials with lateral dimensions of up to 8 or 10 inch is possible. Dielectric, metallic and semiconductor thin films are readily deposited and selectively etched using diverse techniques. The equipment used for thin film deposition of metals includes 3 sputtering systems and 2 e-beam evaporators. Metals such as Au, Ag, Al, Al alloys, Ti, Cr, Cu, Mo, MoCr, Ni, Pt and V can be deposited with thickness varying from 5 nm to 2 µm and uniformity in the 2 % range. The stress of some metal thin films, like aluminum, is either kept constant or dynamically varied during the deposition step. By inducing a gradient of stress to certain layers, it is possible to obtain flexible contacts as the ones illustrated in Figure 4. These contacts are well suited to multi-chip applications with a high electrical contacts density. Mixed vanadium oxides are deposited using a reactive sputtering technique to fabricate microheaters or IR bolometric detectors. These oxides3 have a semiconductor behavior, with a linear TCR (Temperature Coefficient of Resistivity) varying from 1 to 3.5 %, and a resistivity of 0.1 to 10 ohm•cm at 20 °C. Amongst them, VO2 is a specific oxide that shows a non-linear TCR with a phase transition between 50 and 70 °C. Thin or thick films of Ni, SnPb and In (up to 500 µm) are also deposited by electroplating. These electroformed metals show almost zero stress even if deposited in thick layers, which makes them an ideal material for microparts. Thick nickel shims are fabricated for replication of optical devices by injection moulding in plastics. In addition, nickel and tin-based solders are selectively plated in films of several tens of microns. In that case, SU-8 photoresist serves as a polymer mould during the electrodeposition. Figure 5: 10 x 10 µm2 polyimide CMYG color filters Figure 4: Flexible contacts permitting non-permanent fabricated by dry etching. retrievable electrical contacts for flip chip assemblies. Dielectric thin films are very important in microfabrication for their mechanical, electrical and optical properties. INO uses 2 techniques for dielectric thin film deposition. PECVD (Plasma Enhanced Chemical Vapor Deposition) of dielectrics is mainly used to fabricate mechanical parts or insulators on up to φ 8 in substrates. PECVD silicon nitride can be deposited with controlled stress varying from –150 to +300 MPa and refractive index from 1.92 to 2.20 at 633 nm wavelength. PECVD silicon oxide has a refractive index of 1.45 to 1.48 and a controlled stress of –300 to –50 MPa. For optical applications such as filters or anti-reflection coatings, RLVIP (Reactive Low Voltage Ion Plating) of dielectrics is employed. Various oxides can be deposited with a higher refractive index than the one that may be achieved with other techniques and mechanical characteristics approaching those of the bulk material. Typical optical coatings are SiO2, ZrO2, Y2O3 and HfO2. Stacking of these thin films permits to obtain any refractive index from 1.47 to 2.20 at 633 nm wavelength. Photoresists with thickness between 0.5 and 7 µm are mainly spun as standard masking materials. Negative or SU-8 photoresists are used to obtain photo imageable films with thickness of 1 to 100 µm and a patterning aspect ration of 5:1. Photo-imageable hybrid organoceramic materials are spun to fabricate microlenses of 80 µm thickness or less. Polymer sacrificial layers are mainly composed of polyimides of 1 to 20 µm thickness, but the photoresists are also used in some critical applications. Color filters are fabricated using either color polyimides or photoresists from Brewer Sciences. Figure 5 illustrates 10 x 10 µm2 pixels of a polyimide CMYG color filter fabricated by dry etching. Available photolithography tools include contact aligners for binary or gray-scale masks, a laser writer permitting on chip lithography and stations for holographic recording. The critical dimensions resolved are of 1.5 µm for the contact aligners, down to 0.3 µm for the holographic recording stations. An example of a 2D sinusoidal grating with 300 nm period fabricated at INO is illustrated in Figure 6. The gray-scale mask technology is particularly interesting because it permits to generate a continuous surface relief in photoresist using a single step exposure. This eliminates possible alignment errors typical for binary techniques and lengthy write times typical for laser writer or e-beam techniques. Gray-scale masks are helpful in the fabrication of refractive microlenses and other optical elements. However, laser writing technology remains a cost effective alternative for the fabrication of single custom elements. INO uses a custom-made laser writer having a 442 nm wavelength illumination source with a spot size of 1 to 3 µm. The beam intensity is modulated at 1 MHz to obtain up to 1 024 levels of exposure depth. The positioning accuracy is +/- 40 nm over a course of 250 x 250 mm2. The maximum writing speed is 100 mm/s, but typical write times are few hours for 1 cm2. The equipment used for thin film etching is composed of wet benches, RIE (Reactive Ion Etching) systems and a barrel plasma asher. Anisotropic or isotropic wet etching can be performed on most materials with high selectivity. Wet etching often permits to obtain an infinity of shapes and surface finish, depending on the etched substrate. Silicon by itself is an excellent mechanical material to work with using wet etching, essentially for bulk micromachining. Depending on the crystal orientation of the sample, by varying the concentration, nature and temperature of the etching solution, one can obtain several etch pit shapes4. The most popular are the v-grooves, used for optical fiber alignment. Wells and capillary tubes can be obtained by etching (100) silicon in KOH. Anisotropic wet etching of silicon is often used to release suspended microstructures. However, isotropic etching of silicon can also be used. Figure 7 to Figure 9 show examples of a miniature well, a capillary tube and micro hot plates fabricated by bulk micromachining of silicon at INO. Dry etching is primarily used in the case of surface micromachining. RIE systems are used for isotropic or anisotropic etching of quartz, silica, silicon oxide, silicon nitride, vanadium oxide, titanium and polyimide. The etching uniformity is 2 to 10 % on 6 inch substrates, depending on the etched material. Releasing of the suspended structure is made in a barrel plasma asher. Dry etching techniques can equally be applied to etching of structures in bulk substrates such as in the case of gratings and other micro-optical components. The RIE etching selectivity between photoresists and other bulk materials such as fused silica can be precisely adjusted to transfer a pattern obtained from a gray-scale mask. Figure 7: Miniature well. Figure 6: A 2D sinusoidal grating with 300 nm period. Figure 8: A 3 µm capillary tube. Figure 9: Micro hot plate. 4. PACKAGING, TESTING AND OTHER MANUFACTURABILITY ISSUES After fabrication of the MEMS on a wafer, dicing, packaging and testing of the devices are necessary. INO has a Disco DAD320 saw for precision dicing of silicon wafers, glass substrates, optical fibers and other materials. In a standard micropackaging scheme, diced MEMS are attached inside metal or ceramic packages using suitable epoxies. The bonding pads of the MEMS are then attached to the package leads using a Hybond 572-31 thermosonic wire bonder. Gold or aluminum wires of φ 5 to 35 µm are currently used. The devices are then ready to be tested and the packages are ready for sealing. Over the years, packaging has proven to be one of the most critical steps in the production of functional and reliable MEMS. One of the challenges is to obtain hermeticity of the device while keeping a window open to the outside world. INO has developed a strong expertise in that domain. One example is the packaging of IR focal plane arrays, where detectors are sealed under vacuum in a metal package equipped with an IR transmitting window made of Ge or ZnSe. Hermeticity for more than 4 years without getter refiring has been demonstrated in these packages, as shown in Figure 10. Other concepts have been developed to decrease the size and cost of the package while increasing its reliability and flexibility. One of them uses a novel enabling technology that permits hermetic micropackaging of devices on a die or a wafer level. Micro package parts, adaptable to many existing MEMS designs, are fabricated using the technology illustrated in Figure 11. In this technology currently developed by INO, the SU-8 photoresist serves as a polymer mould where nickel and tin-based solder layers are selectively electroplated in films of several tens of microns. The package consists of a miniature metal spacer, having a pump-out hole, mounted on a bolometric detector array. The metal spacer serves as a receptacle to the IR window. After pumping the assembly, the pump-out hole is plugged, keeping the detectors inside the package under vacuum. The process is very versatile and can easily be scaled up from die to wafer level packaging by electroplating trays of metal spacers that will perfectly fit on any specified chip design. Figure 12 is a picture of metal spacers specifically designed and fabricated on a wafer for INO’s 160 x 120 pixel bolometric detector array. Figure 10: Signal and noise level versus time showing long term hermeticity of a bolometric detector package. Figure 12: Tray of electroplated metal spacers designed for Figure 11: Hybrid micropackaging scheme by selective the packaging of 160 x 120 pixel bolometric detector arrays electroplating. at INO. The top picture is a SEM view of one micropackage corner. Once the MEMS devices are packaged, comprehensive testing must be done to confirm that the design goals were achieved. INO’s metrology equipment pool includes a scanning electron microscope, atomic force microscope, ellipsometer, FTIR, spectrophotometer, thin film stress measurement system, spectrophotometer, probing stations and IR test benches. Test chambers with capacities of 1.2 ft3 (0.034 m3) to 15 ft3 (0.425 m3) are available for environmental testing. Thermal cycles from –75 °C to +200 °C are programmable at a rate of up to 10 °C / minute. The humidity can be controlled from 20 % to 95 % for temperatures of 7 °C to 85 °C. An electrodynamic shaker can be used in combination with thermal cycles. Sine and random vibrations or shocks of up to 9 800 N can be induced at a frequency of 5 to 3 000 Hz. Another system can provide shocks to up to 1 500 g. Combining novel enabling technologies can lead to low cost applications and scalable manufacturing techniques. As explained earlier, binary lithography, gray-scale mask lithography or laser writing can be used to generate binary profiles or continuous surface relief in thick or thin photoresist layers. Microelectroplating, either in selective or non- selective fashion, permits to produce metallic shims and microparts. Typically, shims made of nickel have a roughness of 5 nm or less. Aspect ratios as high as 5 may be obtained in selective Ni electroplating using the SU-8 photoresist. Various three-dimensional microparts can be generated by stacking photoresist and microelectroplated layers. After the shims or microparts are fabricated, they are used as masters to perform injection moulding or embossing in plastics like acrylic and polycarbonate or hybrid organoceramic materials. These replication techniques are highly conformal. The achievable minimal feature size in injection moulding and embossing is < 25 nm. The achievable aspect ratio is 1 for injection moulding, 5 for hot embossing and larger than 20 for UV embossing. In particular, injection moulded optical elements are replicated with high fidelity and uniformity. The process is very stable, with a piece-to- piece variability of less than 2 %. When an elastomeric transparent master is fabricated from a nickel shim, UV embossing of optical devices becomes possible in hybrid organoceramic materials. INO’s hybrid organoceramic materials are thick crack-free and adherent films, stable up to 250 °C. An 80 µm layer of an organoceramic spun on a fused silica substrate offers a high transparency while keeping high power stability. Figure 13 and Figure 14 respectively show the injection-moulded DOEs and φ 72 µm organoceramic refractive microlenses fabricated at INO. These replication techniques were developed for manufacturing cost reduction and mass production of various miniature parts such as microlenses, DOEs, optical guided-wave structures and biomedical devices. Figure 13: Injection-moulded DOEs in plastic. Figure 14: φ 72 µm refractive microlenses in a hybrid organoceramic material. 5. REALISATIONS AND CONCLUSIONS Examples of MEMS devices fabricated in INO microfabrication facilities over the last decade are multiple and varied. Microphotonic devices such as diffractive or refractive optical elements and color sensitive imaging arrays, along with mechanical parts such as flexible contacts and micropackage elements have already been presented. Other examples of MEMS fabricated at INO include deformable micromirrors, light valves, IR bolometer arrays and bioprobes. INO has designed and fabricated micromirrors for different applications. Amongst them, flexure hinge micromirrors with surfaces of 100 x 100 µm2 to 300 x 300 µm2 have show a deflection angle of 2 to 4 degrees with 0.1 to 1 ms response times and an activation voltage of 20 V. An example of flexure hinge micromirrors fabricated at INO is shown in Figure 15. Figure 16 shows another type of micromirror with a reflecting surface of 200 x 300 µm2. This micromirror can be positioned to up to 45 degrees with an activation voltage of 50 V. Figure 15: Flexure hinge micromirrors. Figure 17: Standard 50 x 50 µm2 VO2-based IR bolometer. Figure 16: Large deflection angle micromirror. INO also has an extended expertise in the fabrication of custom VOx or VO2-based uncooled IR bolometric detector arrays. In addition to the 25 x 25 µm2 pixel previously illustrated in Figure 3, arrays of 39 x 39 µm2 and 50 x 50 µm2 pixels, shown in Figure 17, have been fabricated. The performance of these bolometers can be found elsewhere3. Unattended ground sensors, portable IR cameras, bolometric detectors for space applications and laser-beam profilers are some applications of these detectors. Other examples of the developed bolometer-based instruments are the fingerprint sensors. These bioMEMS have a slightly modified platform structure for mechanical strength and static discharge prevention. When a finger is brought in contact with these devices, only the fingerprint ridges touch the surface, creating a differential in temperature reading5. All these realizations are only examples showing that the combination of customer’s idea and INO’s expertise in design, computer simulation, prototyping, fabrication of mini series, packaging, performance and environmental testing will lead to a product. Building these devices using cost effective materials and fabrication methods like polymers, plastics, microelectroforming or hermetic wafer level packaging finally makes the economical application of MEMS a viable reality. REFERENCES 1 Nexus MST Market Analysis 1998 and 2002. 2 H. Jerominek et al, “Miniature VO2-based bolometric detectors for high-resolution uncooled FPAs”, Proceedings of SPIE, 4028, pp. 47-56, 2000. 3 T. D. Pope et al, “Commercial and Custom 160 x 120, 256 x 1, and 512 x 3 Pixel Bolometric FPAs”, Proceedings of SPIE, 4721, pp. 64-74, 2002. 4 K. E. Petersen, “Silicon as a Mechanical Material”, Proceedings of the IEEE, 70, No. 5, pp. 420-457, 1982. 5 F. Picard et al, “Nonimaging applications for microbolometer arrays”, Proceedings of SPIE, 4369, pp. 274-286, 2001.
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