Study on Process Flow In Mems

Many high-performance devices have been built using RF-MEMS including shunt and series switches (1, 2, 3), variable capacitors (4, 5), inductors (6, 7), and low-loss and variable transmission lines (8). However, these devices were developed in processes customized to build individual devices. RF-MEMS holds great promise for improving performance and increasing the integration of the RF front-end of wireless systems. The resulting narrow range of applications will not generate the volumes needed to justify the required foundry capital investment nor that needed to stabilize the process itself. While some standard processes exist (9, 10), they are unsuited for RF applications due to their resistive materials.

High performance passives are usually needed in groups within a RF sub-system, such as in tunable filters or in phase shifters. A process suitable for integration must be able to build all of these in one chip where each of the devices maintains its high performance and where the various devices can be interconnected with low loss to maintain the high-performance of the overall circuit. Another obstacle has been high-temperature processing that limited monolithic integration with active circuitry. This paper presents a flexible manufacturing process flow that addresses these shortcomings and details high performance devices created in the process.

RF-MEMS require high quality metals and insulators for state-of-the-art well-controlled RF characteristics. All of our materials needed to have the following properties: Low deposition temperature (300 C) to allow post-processing on CMOS High etching selectivity Low stress and stress gradients Precise patterning techniques Low cost deposition of thick (> 3 µm) layers Available inter-layer adhesion layers The sacrificial material has been chosen to provide uniform smooth layers, good step coverage, fast complete removal from thin gaps, formation of stacked patterned layers for vertical topography and additional functionality when intentionally protected from release etch. There are three basic applications of metal in RFMEMS: buried conductors, exposed electrodes, and electrical contacts. Our choice of copper for the buried conductors was driven by: Highest bulk conductivity (> 4 * 107 S/m) Corrosion resistance to processing steps Available CMP process

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Since electrodes and electrical contacts are both exposed, we have chosen to use the same metal for both. Our choice of Au-based metal was driven by: Low contact resistance (< 0.5 ohms) Low contact force (< 200 uN) High melting temperature (> 800C) Low self-adhesion Low catalytic activity The choice of insulators was constrained by: Low loss tangent (< .003) and DC conductivity Low dielectric constant (< 5) High breakdown field High mechanical yield strength Insulators considered included alumina, high-resistivity silicon, silica, various polymers and GaAs. Alumina and silica both fulfill all of the necessary requirements. Silica was chosen due to immediate process availability within the candidate foundries. Another consideration is the choice of substrate. The process enables device designs that are relatively substrate independent. Thus we can fabricate high performance passives, switches and other MEMS directly on top of functional electronic circuits, both passive and active, even on a resistive substrate. The complete range of compatible substrates is limited by thermal expansion, although these are not severe due to the low temperatures. All initial work has used low-resistivity (< 10 ohm-cm) silicon substrates.

The nominal process stack is shown in Figure 4.1. The overall flow is divided into three main sections: substrate connect, thick metal and thin metal. The substrate connect layer is used to make electrical connections to underlying circuitry and provide a planarized surface for further processing. The thick metal section, used primarily for passives and interconnects, is composed of copper embedded in silica. This is built up in 7 µm thick layers composed of a nominal 3.5 µm sheet conductor and a 3.5 µm stud interconnect. Each layer is planarized using CMP. The nominal process uses two of these layers, yielding 14 µm of copper and/or silica, although more or fewer layers can be used to provide optimal complexity/cost tradeoffs.

The thin metal section is composed of three layers of gold 0.5 µm thick that are used for electrodes and contacts, two layers of sacrificial copper that are 1.5 µm and 0.5 µm thick respectively and a 2 µm silica mechanical layer.

The process and the material properties determined from test structure extractions have been captured in CoventorWare™ for visualization and modeling of the 3D structure leading to straightforward electromechanical, damping and thermal simulations using CoventorWare™ and RF analyses using Ansoft’s HFSSTM.

1. G.-L. Tan and G. M. Rebeiz, “A DC-Contact MEMS shunt switch,” IEEE microwave and wireless components letters, Vol. 12, No.6, June 2002, pp. 212-214. 2. J. J. Yao and M. F. Chang, “A surface micromachined miniature switch for telecommunications applications with signal frequencies from dc up to 4GHz,” Transducers 95 Eurosensors IX, Th 8th IntI. Conf. on Solid-State Sensors and Actuators, Stockholm, Sweden, June 25-29, 1995, pp. 384-387. 3. C. Goldsmith, et al., “Characteristics of micromachined switches at microwave frequencies,” 1996 IEEE MTT-S Digest, pp. 11411144. 4. A. Dec, et al., Micromachined Electro-Mechanically Tunable Capacitors and Their Applications to RF ICs, IEEE Transactions on Microwave Theory and Techniques, Vol. 46, No. 12, December 1998.

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5. DJ. Young, et al., A Low Noise Rf Voltage-Controlled Oscillator Using On-Chip High-Q Three-Dimensional Coil Inductor and Micromachined Variable Capacitor, Proceedings of the Solid-State Sensor, Actuator, and Microsystems Workshop, Hilton Head Island, SC, June 8-11, 1998. 6. F. Lihui, et al., “High Q multiplayer spiral inductor on silicon chip for 5-6 GHz,” IEEE electron device letters, Vol. 23, No.8, Aug. 2002, pp.470-472. 7. J.-B. Yoon, et al., “CMOS-compatible surface-micromachined suspended-spiral inductors for multi-GHz silicon RF Ics,” IEEE electron device letters, Vol. 23, No.10, Oct. 2002, pp. 591-593. 8. L. P. B. Katehi and G. M. Rebeiz, “Novel micromachined approaches to mmics using low-parasitic, high-performance transmission media and environments,” 1996 IEEE MTT-S Digest, pp. 1145-1148. 9. MUMPS process through Cronos, http://www.memsrus.com 10. Richard Nass, MEMS are finding homes in unexpected places, Portable Design, August 2003 11. RF MEMS family switches offered for mobile systems integrations, Portable Design. June 2001 12. SuMMiT process through Sandia 13. Lilian G Yengi (2005). Systems biology in drug safety and metabolism: integration of microarray, real-time PCR and enzyme approaches. Pharmacogenomics 6: 2, 185-192. 14. Luca Mologni, Carlo Gambacorti-Passerini (2005). PNAs as novel cancer therapeutics. International Journal of Peptide Research and Therapeutics 10: 3, 297. 15. Stich N., A. Gandhum, V. Matyushin, J. Raata, C. Mayer, Y. Alguel, T. Schalkhammer (2002). Phage display antibody-based proteomic device using resonance-enhanced detection. In: Journal of Nanoscience and Nanotechnology 2 (3-4): p. 375-381. 16. Stich N., A. Gandhum, V. Matyushin. C. Mayer, G. Bauer, T. Schalkhammer (2001). Nanofilms and nanoclusters: Energy sources driving fluorophores of biochip bound labels. In: Journal of Nanoscience and Nanotechnology 1 (4): p. 397-405. 17. Technical paper, “Development of integrated microfluidic system for genetic analysis”, Journal of Microlithography, Microfabrication and Microsystems, October, 2003. 18. L.N. Invin, “Gene expression in the hippocampus of behaviourally stimulated rats: analysis by DNA microarray”, Molecular Brain Research, 96 (2001), pp. 163-169. 19. M. KOh and J. G. Liao, “Gene expression profiling by DNA microarrays and metabolic fluxes in Escherichia coli”, Biotechnology Progress, 16 (2000). 20. J. van Brunt (2003), “Pharmacogenomics gets clinical”, Signals Magazine (online), 25 April 2003, available at http://www.signalsmag. com/signalsmag.nsf. 21. B. Windle and A Guiseppi-Elie (1003). “Microarrays and Gene Expression Profiling Applied to Drug Research”, Burger’s Medicinal Chemistry, 6th edition (Ed. Donald J. Abraham, PhD), John Wiley & Sons, Inc. 22. E. Williams (2001). “The Policy and Ethics of DNA Chip Technologies”, Bioethics and the Impact of Genomics in the 21st Century: Pharmacogenomics, DNA Polymorphism and Medical Genetic Services, (Eds N Fujiki, S. Masakatu and D. Macer), Tsukuba, Japan: Eubios Ethics Institute, pp. 104-110. R. Compano, Nanotechnology 12 -85 (2001)