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Introduction
Sacrificial polysilicon surface micromachining is emerging as a technology that enables the mass production of complex microelectromechanical systems by themselves, or integrated with microelectronic systems. Early versions of these micromachined systems-on-a-chip found application in the commercial world as acceleration sensors for airbag deployment - for example, ADI's ADXL50. There were two advances in manufacturing techniques for micromachined systems-on-a-chip that made possible great leaps ahead in the complexity of the systems. The first was a three-layer polysilicon micromachining process which included a fourth polysilicon electrical interconnect layer. The other was a single-layer (+ second electrical interconnect level) polysilicon surface micromachining process integrated with 1.25 micron CMOS. Samples of systems-on-a-chipbuilt in these processes are pop-up mirrors and multi-axis accelerometers.

Integrated Microelectronic/Micromechanical Systems-on-a-Chip
A great deal of interest developed in manufacturing processes that make possible the monolithic integration of MicroElectroMechanical Systems (MEMS) with driving, controlling, and signal processing electronics. This integration promises to improve the performance of micromechanical devices, as well as reduce the cost of manufacturing, packaging, and instrumentation for these devices, by combining the micromechanical devices with an electronic sub-system in the same manufacturing and packaging process. For example, Analog Devices developed and marketed an accelerometer¹ which demonstrates the viability and commercial potential of integration. They accomplished this by interleaving, combining, and customizing their manufacturing processes that produce the micromechanical devices with those that produce the electronics. In another approach, researchers at Berkeley² developed a modular integrated approach in which the aluminum metallization of CMOS is replaced with tungsten to enable the CMOS to withstand subsequent micromechanical.

As summarized in a review paper by Howe4, micromechanical structures require long, high-temperature anneals to ensure that the stress in the structural materials of the micromechanical structures is completely removed. 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.

Figure 1

Figure 1. Micromachined resonators (left) next to their CMOS driving electronics (right) fabricated using the embedded micromechanics integration process.

A unique micromechanics-first approach4, which overcomes the planarity issues of building the MEMS before the CMOS, was developed at Sandia. In this approach, micromechanical devices were fabricated in a trench etched on the surface of the wafer. After these devices were complete, the trench was refilled with oxide, planarized using chemical-mechanical polishing, and sealed with a nitride membrane. The wafer with the embedded micromechanical devices was then processed using conventional CMOS processing. Additional steps were added at the end of the CMOS process in order to expose and release the embedded micromechanical devices. Completed devices are shown in Figure 1. A cross-section of this technology is shown in Figure 2. This technology was named as one of the recipients of the 1996 R&D 100 Award.

Important developments resulted from a collaboration with designers from the Berkeley Sensor and Actuator Center (BSAC). BSAC designs for inertial measurement units (three-axis acceleration5 and three-axis rotation rate6,7) were built using Sandia's Integrated MicroElectroMechanical Systems (IMEMS) Technology. Wafer lots of devices were fabricated, and further information on the accelerometers fabricated with this technology is reported in the next section.

Figure 2

Figure 2. A schematic cross-section of the embedded micromechanics approach to CMOS/MEMS integration.

Inertial Sensors
One of the principal commercial products fabricated using surface-micromachining, is inertial sensors, examples of which are Analog Devices' ADXL1508 and Motorola's XMMAS40GWB9. The primary application of these accelerometers is as airbag-deployment sensors in automobiles, but they are also being used as tilt or shock sensors. (In the following discussion, please note the use of g for gravitational acceleration and the word "gram" for mass). The Motorola device is a ±40g (full scale) single-axis accelerometer with a noise floor of 400 mg (400 Hz bandwidth, peak) and has an analog output.9 The Analog Devices accelerometer is available as either a single (ADXL150) or dual-axis device (ADXL250), has a ±50g full scale output, an analog output, and a noise floor of 10 mg (100 Hz bandwidth, rms).8 Please note that the use of peak vs. rms noise specifications together with the difference in bandwidth specifications for the devices makes a direct comparison difficult.

Important developments resulted from a collaboration with designers from the Berkeley Sensor and Actuator Center (BSAC). BSAC designs for inertial measurement units (three-axis acceleration5 and three-axis rotation rate6,7) were built using Sandia's Integrated MicroElectroMechanical Systems (IMEMS) Technology. Wafer lots of devices were fabricated, and further information on the accelerometers fabricated with this technology is reported in the next section.

The application of these types of accelerometers as inertial measurement units is limited by the need to manually align and assemble them into three-axis systems, the resulting alignment tolerances, their lack of on-chip A/D conversion circuitry, and their lower limit of sensitivity. In order to overcome some of these limitations, a three-axis, force-balanced accelerometer was designed at U.C. Berkeley5 to be fabricated using the integrated MEMS/CMOS technology described in the previous section. This three-axis accelerometer system-on-a-chip is shown below in Figure 3.

Figure 3

Figure 3. Three-axis accelerometer micrograph with labeling of functional units as reported by Lemkin et al5

The performance of the device is summarized in Table 1 below. Approximately an order of magnitude increase in sensitivity is seen over the commercial devices described previously. The accelerometer chip also includes clock generation circuitry, a digital output, and photolithographic alignment of the sense axes. Thus, this system-on-a-chip is a realization of a full three-axis inertial measurement unit that does not require manual assembly and alignment of sense axes.

Although the bias stability of this accelerometer system has yet to be assessed, the noise numbers indicate that there are potential commercial applications for this system such as:

  • automotive control
  • automotive diagnostics
  • automotive navigation
  • virtual reality environmental sensing.10

A combined X/Y-axis rate gyro and a Z-axis rate gyro were designed by researchers at U.C. Berkeley and fabricated in this manufacturing process to yield a full six-axis inertial measurement unit on a single chip. The estimated size for this system was approximately 4 mm by 10 mm.

 

Noise Floor
g(Hz))

Dynamic Range
(dB, 100 Hz Bandwidth)

Proof Mass (µgram)

Resonant Frequency (kHz)

Power Dissipation (mW)

Sampling Rate (kHz)

X-axis

110

84

0.38

3.2

45

500

Y-axis

160

81

0.26

4.2

45

500

Z-axis

990

70

0.39

8.3

45

500

Table 1. Performance of the three-axis accelerometer as reported by Lemkin, et al.5

Multi-Level, Planarized Polysilicon Systems-on-a-Chip

Microengine
Micromechanical actuators have not seen the wide-spread industrial use that micromechanical sensors have achieved. Two principal stumbling blocks to their widespread application have been low torque and difficulty in coupling tools to engines. Sandia National Laboratories developed devices that overcome these difficulties. Our three-layer polysilicon micromachining process11,12 made possible the fabrication of devices with increased complexity that greatly enhanced the ability to couple tools to engines.

The three-layer process includes three movable levels of polysilicon in addition to a stationary layer for a total of four layers of polysilicon. The polysilicon layers are separated from one another by sacrificial oxide layers. A total of eight mask layers are involved in this process. An additional friction-reducing layer of silicon nitride is placed between the layers that form bearing surfaces. Inset (lower right) in Figure 4, is a cross-section view of a bearing formed between two layers of mechanical polysilicon. The balance of the Figure 4 illustration is a picture of two sets of comb-drive actuators that drive a pair of linkages that then drive a pair of rotary gears. The whole system is driven by transmitting drive signals that are 90° out of phase with each other to the comb drive actuators. The small gear has been operated at speeds in excess of 300,000 revolutions per minute. The operational lifetimes of these small devices can exceed 8x108 revolutions. The small gear is shown driving a larger (1.6 mm diameter) gear13 in Figure 4. This large gear has been driven at speeds as fast as 4800 rpm.

Figure 4

Figure 4. Two sets of linear comb-drive actuators drive the small gear (shown in cross-section in the inset). The smaller gear drives a 1.6 mm diameter shutter gear in the lower left of the photo. The inset (lower right) is a focused ion-beam (FIB) cross-section image of the small gear.

Microtransmission
To increase the torque available from a rotary drive, a multi-layer microtransmission was developed.14 This transmission, shown in Figure 5, employs sets of small and large gears mounted on the same shaft that mesh with other sets of gears to transfer power while providing torque multiplication and speed reduction. The structure in Figure 5 shows the output gear of a microengine, similar to the one in Figure 4, meshed with a linear rack to provide linear motion with a high degree of force.

Figure 5

Figure 5. Small and large gears mounted on the same shafts mesh with other sets of gears to transfer power while providing torque multiplication and speed reduction.

Micromirror
The microengine in combination with the microtransmission can be used to drive a pop-up mirror up, out of plane. With the torque increase resulting from this combination, the mirror can be elevated by the mechanism, alone – without manipulation from the outside using probes.(Figure 6).

micromirror

Figure 6. The mirror can be elevated by the mechanism, alone – without manipulation from the outside using probes

Summary
A technology involving micromachined devices embedded below the surface of a wafer, prior to fabrication of microelectronic devices, was developed and applied to build complex sensor systems on a single chip. A three-layer polysilicon process made possible intricate coupling mechanisms that link linear comb-drive actuators to multiple rotating gears. This technology has been used to build devices such as microengines, microtransmissions, and micromirrors. These devices were also combined to yield intricate mechanical systems-on-a-chip.

References

  1. W. Kuehnel and S. Sherman, "A surface micromachined silicon accelerometer with on-chip detection circuitry," Sensors and Actuators A, vol. 45, no. 1, pp. 7-16, 1994.
  2. W. Yun, R. Howe, and P. Gray, "Surface micromachined, digitally force-balanced accelerometer with integrated CMOS detection circuitry,"Proc. of the IEEE Solid-State Sensor and Actuator Workshop `92, p. 126, 1992.
  3. R. Howe, "Polysilicon integrated microsystems: technologies and applications,"Proc. Transducers ';95, pp. 43-46 (1995).
  4. J. Smith, S. Montague, J. Sniegowski, J. Murray, and P. McWhorter, "Embedded micromechanical devices for the monolithic integration of MEMS with CMOS", Proc. IEDM '95, pp. 609-612, 1995.
  5. M. Lemkin, M. Ortiz, N. Wongkomet, B. Boser, and J. Smith, "A 3-axis surface micromachined sigma-delta accelerometer," Proc. ISSCC '97, pp. 202-203, 1997.
  6. T. Juneau, et. al, "Dual axis operation of a micromachined rate gyroscope," to be presented at Transducers '97, Jun. 1997.
  7. W. Clark, R. Howe, and R. Horowitz, "Z-axis vibratory rate gyroscope," Micromachining Workshop III: Technology and Applications, Sept. 1996. Juneau
  8. Analog Devices, Datasheet: "ADXL150/ADXL250 rev.0" Norwood, MA, 1996.
  9. Motorola, Datasheet: "XMMAS40GWB/D," Phoeniz, AZ, 1996.
  10. B. Barbour, J. Elwell, and R. Setterlund, "Inertial instruments: Where to now?" Proc. AIAA Guidance and Control Conf., pp. 566-574, 1992.
  11. R. Nasby, J. Sniegowski, J. Smith, S. Montague, C. Barron, W. Eaton, P. McWhorter, D. Hetherington, C. Apblett, and J. Fleming, "Application of chemical-mechanical polishing to planarization of surface-micromachined devices," Proc. Solid-State Sensor and Actuator Workshop, pp. 48-53 (1996).
  12. J. Sniegowski, "Multi-level polysilicon surface micromachining technology: Applications and Issues," to be presented at ASME International Mechanical Engineering Congress and Exposition (1996).
  13. J. Sniegowski and E. Garcia, "Surface-micromachined geartrains driven by an on-chip electrostatic microengine," IEEE Electron Device Letters, vol. 17, no. 7, p. 366 (1996).
  14. J. Sniegowski, S. Miller, G. LaVigne, M. Rodgers, and P. McWhorter, "Monolithic geared-mechanisms driven by a polysilicon surface-micromachined on-chip electrostatic microengine," Proc. Solid-State Sensor and Actuator Workshop, pp. 178-182 (1996).
  15. J. Sniegowski, "Micromachining Technology for Advanced Weapon Systems,"Proc. Government Microcircuit Applications conference (GOMAC '97), Las Vegas, NV, March 10-13, 1997, pp. 36-39.

 

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