An overview of the major sensor and actuator projects using the micromachining capabilities of the Microelectronics Development Laboratory at Sandia National Laboratories is presented. Development efforts are underway for a variety of surface micromachined sensors and actuators. A technology that embeds micromechanical devices below the surface of the wafer prior to microelectronics fabrication has also been developed for integrating microelectronics with surface micromachined micromechanical devices

Smith, J.H.

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Interkama, Nov. 1995. Microsystem Technology Development at Sandia National Laboratories James H. Smith developed by researchers at U.C. Berkeley' for use as catalytic gas sensors, flow sensors, and thermal- conductivity pressure gauges have been fabricated using a single-level doped polysilicon process. A sacrificial oxide is patterned to form both the anchor layer and a stiction- reducing dimple level. A scanning electron micrograph (SEM) of a differential pair of filaments is shown in Figure 2. One of these Naments is passivated with silicon nitride while the other is coated with a platinum catalyst. These filaments have been used to detect combustible gas mixtures and can clearly detect levels as low as 100 ppm of H2 in air. The filament pairs consume milliwatts of power when operated in a continuous mode and can be operated in pulsed mode to reduce the average power consumption to An overview of the major sensor and actuator microwatts. projects using the micromachining capabilities of the A pressure sensor technology similar to one Microelectronics Development Laboratory at Sandia previously developed at u. of Wisconsin' has been National Laboratories is presented. Development efforts developed at the MDL3 based upon a silicon nitride layer as are undernay for a variety of Surface ~ c ~ o m a c h i n e d  the diaphragm material. A sacrificial oxide underneath this Sensors and actuators. A technology that embeds diaphragm layer is etched away using HF-based chemistries Integrated Micromechanics, Microsensors and CMOS Technology Department Sandia National Laboratories Albuquerque, NM 87185-1080 USA ABSTRACT micromechanical devices below the surface of the wafer prior to microelectronics fabrication has also been developed for integrating microe~ectronics with SUrfXe leaving a cavity beneath the diaphragm. ~n additional silicon nitride layer is used to seal the cavity in near- vacuum conditions (approx. 200 mTorr). Polysilicon micromachined micromechanical devices. piezoresistors are deposited on the diaphragm to sense the diaphragm strain that results from changes in ambient 1. FACILITIES pressure. A completed, 100 micron diameter pressure sensor is shown in Figure 3. A planar version of this sensor is presently under development. The Microelectronics Development Laboratory (MDL), shown in Figure 1, at Sandia National Laboratories is a 30,000 square foot, class 1 semiconductor fabrication facility located in Albuquerque, NM. The MDL is a modern, well-equipped CMOS fabrication facility with both 2 micron and 0.5 micron CMOS technologies. The facility has been adapted to enable the advancement of other technologies, such as micromechanics, in addition to the continued development of sub-micron CMOS. These other technologies benefit from the wide variety of equipment and Processes in existence to Support the basehe  CMOS, but they must also maintain a degree of compatibility with CMOS manufacturing Processes SO that they do not contaminate those processes. In the area of micromechanics, the MDL, has development projects in both surface and bulk couple tools to engines. micromachining, although the surface micromachining constitutes the majority of the MDL'S efforts and is emphasized here. 3. ACTUATORS Micromechanical actuators have not seen the wide- spread industrial use that micromechanical Sensors have achieved. T~~ principal stumbling block to their widespread application have been low torque and difficulty in coupling tools to engines. The MDL has development projects that are overcoming these issues. A steam-based actuation mechanism4 generates orders of magnitude higher force per unit chip area than conventional electrostatic actuators. Also, a three-level polysilicon micromachining process' enables the fabrication of devices with increased degrees of complexity that greatly enhance the ability to This three-level process includes three movable levels of polysilicon in addition to a stationary level for a total of four levels of polysilicon. These levels are each separated by sacrificial oxide layers. A total of 8 mask levels are used in this process. An additional friction- reduction layer of silicon nitride is placed between the layers that form bearing surfaces. The inset (lower right) to based on ~CrOmechanical technologies. Surface Figure 4 illustrates a bearing formed between two layers of micromachined POlySiliCOn filaments Similar to those mechanical poly. The overall photo in Figure 4 shows two ?his work, performed at Sandia National Laboratories, was supported by the U.S. Department of Energy under contract DE-AC04-94AL8.5000. 2. SENSORS The m L  has fabricated a number of Sensors J DISCLAIMER .Ah1 This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes-any legal liability or responsi- bility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Refer- ence herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recom- mendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. I comb-drive actuators6 driving a set of linkages to a set of rotary gears. This engine can be rotated by applying sinusoidal driving forces 90’ out of phase with each other to each of the comb-drive actuators. Operation of the small gears (shown in the inset) at rotational speeds in excess of 200,000 revolutions per minute has been demonstrated. The operational lifetime of these small devices exceed 8x108 revolutions. This smaller gear is shown driving a larger (1.6 mm diameter) gear7 in Figure 4. 4. CMOS/MICROMECHANICS INTEGRATION Finally, the task of integrating micromechanics with the controlling CMOS is being undertaken. As recently summarized in a review paper by Howe*, micromechanical structures require long, high-temperature anneals to assure that the stress in the structural materials of the micromechanical structures has completely relaxed. On the other hand, CMOS technology requires planarity of the substrate to achieve high-resolution in the photolithographic process. If the micromechanical processing is performed first, the substrate planarity is sacrificed. If the CMOS is built first, it (and its metallization) must withstand the high- temperature anneals of the micromechanical processing. This second alternative was chosen by researchers at U. C. Berkeley’ and has been examined at the MDL. In this approach, the standard aluminum metal used in CMOS is replaced with tungsten. Since tungsten is a refractory metal, it withstands the high-temperature processing. However, a number of issues remain unsolved concerning the adhesion of the tungsten layer and the unwanted formation of tungsten silicides. Despite these issues, the MDL has fabricated integrated devices with functioning control electronics, although both device yield and performance were less than optimal. A unique micromechanics-first approach” has also been developed at Sandia. In this approach, micromechanical devices are fabricated in a trench etched on the surface of the wafer. After these devices are complete, the trench is refilled with oxide, planarized using chemical-mechanical polishing, and sealed with a nitride membrane. The wafer with the embedded micromechanical devices is then processed using conventional CMOS processing. Additional steps are added at the end of the CMOS process in order to expose and release the embedded micromechanical devices. A cross-section of this technology is shown in Figure 5. Completed devices are shown in Figure 6.  5. SUMMARY sensors and actuators using micromechanical processing techniques. Combustible gas detectors based on hot polysilicon filaments and pressure sensors based on sealed nitride diaphragms have been produced. A three-level polysilicon process enables intricate coupling mechanisms that link linear comb-drive actuators to multiple rotating gears. A new technology where micromachined devices are embedded below the surface of a wafer prior to fabrication of microelectronic devices has also been developed for integration of microelectronic and micromechanical devices. 6. ACKNOWLEDGEMENTS This paper outlines the work of a number of people at the Microelectronics Development Laboratory including C. Barron, E. Garcia, P. McWhorter, S .  Montague, J. Murray, A. Ricco, J. Sniegowski, and H. Weaver. 7. REFERENCES IC. Mastrangelo and R. Muller, “A constant-temperature gas flowmeter with a silicon micromachined package,” Solid-state Sensor and Actuator Workshop, pp. 43-46 (1988) *H. Guckel and D. Burns, ‘Tlanar processed polysilicon sealed cavities for pressure transducer amys,” ZEDM, pp. 223-225 (1984). 3W. Eaton and J. Smith, “A CMOS compatible surface micromachined pressure sensor for aqueous applications,” Proc. of Smart Structures and Materials ‘95, voL 2448, pp. 258-265, SPIE (1995). 4J. Sniegowski, “A microactuation mechanism based on a liquid- vapor surface tension,” Abstracts of Late News Papers of the 7rh International Conference on Solid-state Sensors and Actuators, p. 12 (1993). ’J. Sniegowski and E. Garcia, “Microfabricated actuators and their applications to optics,” Proceedings of SPZE Photonics West ‘95, SPIE (1995). 6W. Tang, T-C. Nguyen, and R. Howe, “Laterally driven polysilicon resonant microstructures,” Sensors and Actuators, vol. ’J. Sniegowski and E. Garcia, “Surface-micromachined geartrains driven by an on-chip electrostatic microengine,” submitted to ZEEE Electron Device Letters. *R. Howe, “Polysilicon integrated microsystems: Technologies and applications,” Proc. Transducers ’95, pp. 43-46 (1995). ’W. Yun, R. Howe, and P. Gray, “Surface micromachined, digitally force-balanced accelerometer with integrated CMOS detection circuitry,” The ZEEE Solid-state Sensor and Actuator Workshop ‘92, p. 126 (1992). ‘OJ. Smith, S. Montague, J. Sniegowski, and P. McWhorter, “Embedded micromechanical devices for the monolithic integration of MEMS with CMOS,” to be presented at I995 ZEDM. 20, pp. 25-32 (1989). Sandia’s Microelectronics Development Laboratory has developed and is advancing a broad range of Skd ia  National Laboratories in Albuquerque, NM. Figure 2. Two polysilicon filaments for use as a combustible gas detector. The upper filament is passivated with silicon nitride. The lower filament has been selectively coated with a platinum catalyst. Figure 3. An SEM of a surface micromachined pressure sensor. The pressure sensor uses polysilicon piezoresistors on a nitride diaphragm over a vacuum cavity to sense changes in ambient air pressure. ring Fig the gear shown in the inset. This smaller gear drives a 1.6 mm diameter shutter in the lower left of the photo. Inset (lower right) shows a focused ion-beam cross-sectional image of the small gear. CMOS DeYlcBArea Mlcmmechanlcal Devlce Area 4 c - c Figure 5. micromechanics approach to CMOSMEMS integration. A schematic cross-section of the embedded Figure 6. Micromachined resonators next to their CMOS driving electronics fabricated using the embedded micromechanics integration process. 

Microsystem technology development at Sandia National Laboratories

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Microsystem technology development at Sandia National Laboratories

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