405 Hilgard Ave., Los Angeles, CA 90095. ABSTRACT. Using a single maskless postprocessing step we have developed an accelerometer in a standard ...
Monolithic multiple axis accelerometer design in standard CMOS Brett Warneke, Eric Hoffman, Kristofer S.J. Pister University of California, Los Angeles, Department of Electrical Engineering 405 Hilgard Ave., Los Angeles, CA 90095
ABSTRACT Using a single maskless postprocessing step we have developed an accelerometer in a standard commercial CMOS process capable of a sensitive axis parallel or perpendicular to the die surface. Our postprocess is realized using xenon difluoride (XeF2) as a bulk etchant. The combination of this etchant and the standard CMOS process allows realization of cantilevers with piezoresistive sensors in all spacial coordinates from a widely-accessible source and at a minimal cost. Fabrication of accelerometers for all three axes and associated electronics on a single piece of silicon reduces the cost of three-dimensional acceleration detection while increasing sensor reliability. Keywords: accelerometers, micromachined sensors, CMOS sensors, XeF2 etchant, piezoresistive sensors, aluminum hinges, 3D microstructures 1. INTRODUCTION Many designs for silicon micromachined accelerometers have been developed since the first silicon accelerometer was reported by Roylance and Angell in 19792 and various fabrication approaches have been utilized, including bulk micromachining3, surface micromachining4, wafer bonding5, and dissolved wafer micromachining. Recently, a couple of monolithic b/2 triaxial accelerometers3,6 and several proLb cesses that integrate CMOS electronics with sensors7,8,14 have been developed. However, many of these methods require a custom process that increases the cost and development W Pit time of the sensors, while decreasing the availability of the fabrication technology. Lp
Commercial foundry CMOS, a well-developed, low-cost process with high availability, has been demonstrated to have micromachining possibilities using maskless postprocessing9. By stacking an active area, all metal contact cuts, and the overglass cut in the Layer Thicknesses Overglass = 1 µm Metal2 = 1.15 µm Ox2 = 0.65 µm Metal1 = 0.6 µm Ox1 = 0.85 µm Poly2 = 0.4 µm Poly ox = 0.08 µm Poly = 0.4 µm Field ox = 0.6 µm
Piezoresistor
Proof Mass Aluminum Hinges
Plate Oxide Beam piezoresistor
Bulk Si
Figure 1: Piezoresistive sensor diagram with dimensional variables and cross-section of the plate and oxide beam.
layout, bare silicon is exposed when the chips return from the foundry10. When the chips are subsequently etched in EDP, TMAH, or XeF2 suspended structures are released. XeF2 etching1 allows the implementation of aluminum hinges and thus the use of 3D microstructures in the CMOS process, permitting the same basic accelerometer design to have a sensitive axis either parallel or perpendicular to the die surface. A multi-axis sensitive sensor and microelectronics can then be integrated on a single piece of silicon. 2. SENSOR DESIGN Our sensor is implemented in an Orbit 2µm CMOS process and is based on the piezoresistive properties of polysilicon. A proof mass is created by stacking all oxide, polysilicon, and metal layers together to form a rectangular plate (figure 1) 5.73 mm thick with an average density of 2.53 gm/cm3 and surrounding it with ‘pit’ - the stack of active area, contact cuts, and overglass cuts that provides bare silicon for post-processing. This plate is suspended by oxide beams containing polysilicon piezoresistors connected to the bulk of the wafer. These piezo resistors sense movement of the plate by changing resistance proportionally with the strain induced by the bending of the beams. This results in a configuration which is primarily sensitive to acceleration perpendicular to the plate. To facilitate measurements parallel to the wafer plane, the beam can be attached to a hinged oxide support rather than the bulk. This hinged oxide support allows rotation of the proof mass and beam assembly out of the wafer creating a sensitive axis parallel to the wafer plane. Figure 2 shows a scanning electron micrograph (SEM) of two accelerometers set up to measure acceleration along two axes.
Figure 2: SEM of released accelerometers. The assembled one is sensitive to the axis parallel to the surface, while the other is sensitive to the perpendicular axis. polysilicon aluminum
Figure 3: Cross-sectional diagram of an aluminum trace running between oxide plates and contacting a polysilicon piezoresistor. The oxide and first metal overetch into the silicon substrate causes the trace to be recessed.
The aluminum hinges are created by laying out the second metal layer over the pit region, leaving lines of bare aluminum that are released as the XeF2 etches the underlying silicon. As shown in figure 3, the oxide and first metal overetch into the silicon substrate causes a dip in the metal hinge. Using micromanipulators, the hinges can be plastically deformed at one of the corners of the dip, causing the hinged structure to remain in the bent position1. During fatigue testing the hinges have been shown to bend from 0o to 90o and back again as many as 5 times without breaking. To further increase the rigidity of the hinge, braces may be placed on the structure to lock it into place after assembly, as shown in figure 4. Traditionally, piezoresistive accelerometers have been implemented using bulk micromachining, due to the need for a large proof mass to achieve reasonable sensitivities; however, our maskless CMOS post-processing technology
Figure 4: 300 µm tall deflection structures. The one on the right demonstrates the use of braces to help support 3D microstructures.
Resistance vs. Deflection for thinned CMOS cantilevers 2.00
Figure 5: SEM of an accelerometer with bond wires providing 9.6 µgm of additional proof mass. The plate has been bent up slightly with the hinges.
% Change in Resistance
1.00 Beam 7 Fit: slope= -0.039 Beam 5 Fit: slope= 0.049
0.00
Beam 1 Fit: slope= 0.0326 Beam 4 Fit: slope= 0.0411
-1.00
Lb -2.00
t
-40.00
z piezoresistor
Figure 8: Cross-sectional diagram of an oxide beam with embedded polysilicon piezoresistor showing dimensional variables for analysis.
-20.00
0.00
20.00
40.00
60.00
Deflection (µm)
Figure 7: Test results from bending beams of several different thicknesses and differing relative positions of the piezoresistor within the thin film stack. Fixed base
Thinned cantilever with piezoresistor
restricts us to releasing thin film surface structures piezoresistor metal 2 aluminum that are relatively light. One method to increase the mass while maintaining automation capability in a production process involves opening up long strips of passivation along opposite sides of the proof mass, Figure 6: Cross-section of thinned beam #7 illustrating the use of gate oxide to change the beam thickness and relative position of essentially leaving long bond pads. Before the XeF2 the piezoresistor. etch bond wires are run across the plate from one pad to another shown in figure 5. Using a manual bonder, we have been able to put 14 bond wires on a 500 µm long plate, adding 9.6 µgm to the proof mass for an increase of 5.5 times. A mask-level improvement that we are exploring involves decreasing the thickness of the beams, since the sensitivity is proportional to the inverse cube of the beam thickness. By adding various oxide cuts and substituting gate oxide for field oxide, we have been able to decrease the beam thickness and change the relative position of the piezoresistor within the beam (figures 6 and 7). Table 1 illustrates the performance improvements achieved using this technique. The ideal thicknesses are calculated from the film thicknesses for a typical Orbit CMOS cross-section. The actual thicknesses of the oxide were measured with a Nanospec instrument and are substantially less than the ideal thicknesses due to the overetching of the overglass and via cuts, as demonstrated by the two cantilevers that were completely etched away. The beams with metal layers had metal lines down their length, versus a complete metal layer on the beam, to evaluate the possibility of running wires onto the proof mass. Without the protection of the overglass, several of the wires broke loose from the underlying oxide during etching or under pressure from the micromanipulator probe tip used in the tests. This appears to have affected the measurements since there is quite a scattering of points. Hysteresis in the metal further contributed to the scattering, since measurements were taken over repeated bending of the beams. The thickness of the metal was not included in the ideal values below to allow comparison with the measured oxide thicknesses..
Table 1: Thinned CMOS Cantilevers
Beam #
Layers
( ∆R ) ⁄R -------------------∆z
Ideal Thickness (µm)
Actual Thickness (µm)
(%/µm) 0.033
Comments
1
field ox, poly1, ox1, ox2
2.5
1.80
only an overglass cut
2
field ox, poly1, ox1
1.85
0.19
The piezoresistor did not work. Probably damaged by overetch.
3
field ox, poly1, ox1, ox2, overglass
3.5
3.13
standard oxide beam
4
field ox, poly1, ox1, ox2, metal2
2.5
1.81
0.041
5
field ox, poly1, ox1, metal1, ox2, metal2
2.5
1.80
0.049
6
gate ox, poly1, ox1
1.3
0.014
7
gate ox, poly1, ox1, metal1, ox2, metal2
1.95
1.26
8
gate ox, poly1, ox1, metal1
1.3
0.019
did not survive Orbit processing -0.038 did not survive Orbit processing
3. FABRICATION Our devices were fabricated through the MOSIS foundry service using Orbit Semiconductor’s 2 micron double poly, double metal CMOS N-well and P-well processes. When the packaged chips return from the foundry, we add the bond wires to the proof mass, then perform the maskless XeF2 etch. With an etch rate between 1 and 10 µm/minute, the XeF2 etch takes between 5 and 25 minutes depending on the design and etch conditions. Next, any 3D structures on the released chip are assembled by hand and the device is ready to be tested. Similar CMOS microstructures have been observed to electrostatically assemble during scanning electron microscopy. Research is currently being done to try and duplicate self assembly conditions in order to design an electrostatic assembly process. As discussed by Chang et al. in these proceedings, XeF2 has many features that make it a desirable etchant11. Those features that facilitate this accelerometer design in particular are the high etch rate, aluminum/oxide selectivity, and gentle gas phase etch. Additional benefits include inexpensive apparatus and lesser toxicity, both of which contribute to decreasing the production cost of these devices. 4. TRANSDUCER ANALYSIS 4.1 Responsivity We assume that the proof mass can be treated as a rigid plate and the two beams are treated as a single cantilever. Referring to the dimensions given in figures 1 and 8, the strain at any point x along the beam is given by ( x )ε( x ) = zM -------------EI
(1)
where E is Young’s Modulus (7.6 x 1010 Pa for silicon dioxide), I is the area moment of inertia of the beam with respect to the neutral axis, and M(x), the bending moment that is being sensed, is given by L M ( x ) = ma -----p + ( L b – x ) ≈1 --- maL p 2 2
(2)
assuming it is fairly constant in the beam and is determined by the mass m of the plate concentrated at the center of the plate acting on the moment arm Lp/2. A Wheatstone bridge is used to sense the stress induced resistance change by placing the piezoresistors on the two beams on opposite legs of the bridge. When driven by a voltage VE, the signal output will then be ∆R 1 ∆V ≈VE ------- = V E --- Gε 2R 2
(3)
where R is the nominal value of the bridge resistors and G ≈− 20 is the n-type polysilicon gauge factor12, 13. The responsivity of the sensor is then given by
3GzL p m R V = ∆V ------- = -------------------- VE 3 a Et b
(4)
4.2 Sensitivity If we design the system such that the dominant electronic noise is due to the Johnson noise of the Wheatstone resistors by using a low noise amplifier, the sensitivity will be given by
8k B TR S i = -------------------RV
(5)
4.3 Dynamic Range We assume that the strain limit for the beam is 1%. The maximum acceleration is then 0.01 × EI × 2 a max = -------------------------------zL p M
(6)
and the dynamic range in dB, including a safety margin SM from the fracture strain, for a bandwidth BW is a max ⁄( SM ) DR = 20 log --------------------------- Si BW
(7)
4.4 Resonant Frequency The mechanical damping in the beam can be modelled as a spring-mass system with a spring constant of 3
Et b k = -------------3 4L eff
(8)
where Leff is the effective length of the beams. Since the plate is not completely rigid, it will contribute to the actual length of the cantilever. The natural frequency is then given by ω =
k ---m
(9)
Table 2 lists the theoretical performance values for several accelerometer designs. All designs in the table have a dynamic range of 128dB in a 100 Hz bandwidth and use the following parameters: VE =1.5V, b=10µm, Lb=20µm, W=500µm, Lp=500µm, T=300K, and R=1KΩ . Table 2: Calculated Accelerometer Performance Si g ⁄( Hz )
amax (g)
f0 (Hz)
3.50x10-5
1.65x10-4
4.29x103
734.5
1.51x10-4
3.82x10-5
996
354.0
t (µm)
z (µm)
m (µg)
RV (V/g)
3.5
0.95
2.72
3.5
0.95
11.71
Comments
wire bonded mass; tested device
Accelerometer Frequency Response Normalized to ADXL50 Reference
Roll-Over Test 10000.00
-640.00 Roll-over data Cos (x)
1000.00
Responsivity (mv/g)
Amplifier Output (mV)
-660.00
-680.00
100.00
-700.00
10.00
-720.00 0.00
100.00
200.00
300.00
10.00
400.00
100.00
1000.00
Frequency (Hz)
Angle between sensitive axis and g (degrees)
Figure 9: Accelerometer roll-over test with ideal response.
Figure 10: Accelerometer frequency response normalized to the ADXL50 reference sensor mounted on the test fixture.
Table 2: Calculated Accelerometer Performance t (µm)
z (µm)
m (µg)
RV (V/g)
2.5
0.45
2.72
2.5
0.45
11.71
Si g ⁄( Hz )
amax (g)
f0 (Hz)
4.55x10-5
1.27x10-4
3.30x103
443.4
thinned beams
1.96x10-4
2.94x10-5
766
213.7
thinned beams with wire bonded mass
Comments
5. RESULTS For the tests below we connected the output of the Wheatstone bridge to an external instrumentation amplifier with a gain of 800. To demonstrate DC functionality, we performed a simple roll-over test, letting gravity provide an acceleration of +1g to -1g as the sensitive axis was rotated with respect to g (figure 9). From this we see that the sensor’s responsivity is 4.1x10-5 V/ g and is fairly linear. The mismatch between the initial and final values is likely due to temperature drift. Figure 11 illustrates the frequency response test setup where we utilized an 8” wide range audio speaker as a shaker table. An ADXL50 configured with a 1kHz roll-off was used as a reference accelerometer. Figure 10 shows the frequency response normalized to the ADXL50 output. From the first resonance at 395 Hz, we can back-calculate to find that Leff = 483 µm, which is less than Lp+Lb, but shows that the length of the plate is a significant factor. Figure 12 shows the output of a spectrum analyzer that was used to examine the output of the ADXL50 and our device when the speaker was driven at 480 Hz. This plot and the frequency response shows that we have some higher order harmonics in either the test setup or the accelerometer. 6. CONCLUSION We have fabricated an accelerometer in a foundry CMOS process with a single maskless post-process etch in XeF2. Our design facilitates a monolithic triaxial accelerometer with integrated electronics. We have demonstrated the functionality of
the sensor, but are still in the process of fully characterizing it. Theoretically it was shown that the tested designs are practical, but have not yet reached the maximum achievable performance. This technology allows us to build accelerometers very cheaply with a highly available process and performance suitable for the high volume sensor markets. 7. ACKNOWLEDGMENTS
Figure 11: Accelerometer frequency response test setup
This work was supported in part by the California MICRO program. We would like to thank Jeff Surdilla for building the frequency response test bed, Ezekiel Kruglick for his work on 3D microstructures in CMOS, including the devices in figure 4, and Makoto Miura for bonding the accelerometers. 8. REFERENCES 1. E. Hoffman, B. Warneke, E. Kruglick, J. Weigold, K.S.J. Pister, “3D structures with piezoresistive sensors in standard CMOS,” in Proceedings IEEE Micro Electro Mechanical Systems Workshop, pp. 288-293, Amsterdaam, January 1995. 2. L. Roylance and J. Angell, “A batchfabricated silicon accelerometer,” IEEE Transactions on Electron Devices, vol. ED-26, no. 12, pp. 19111917, December 1979.
Figure 12: Spectrum analyzer plot for ADXL50 (top) and our accelerometer (bottom) when driven at 488 Hz. Harmonic peaks due to test board vibrations.
3. G. Andersson, “A novel 3-axis monolithic silicon accelerometer,” in Proceeding 8th International Conference on SolidState Sensors and Actuators (Transducers ‘95), pp. 558-561, Stockholm, Sweden, June 25-29, 1995. 4. P. Barth, F. Pourahmadi, R. Mayer, J. Poydock, and K. Petersen, “A monolithic silicon accelerometer with integral air damping and overrange protection,” in Technical Digest, IEEE Solid-State Sensor and Actuator Workshop, Hilton Head Island, South Carolina, pp. 35-38, June 1988. 5. V. McNeil, M. Novack, and M. Schmidt, “Design and fabrication of thin-film microaccelerometers using wafer bonding,” in Proceedings 7th International Conference on Solid-State Sensors and Actuators (Transducers ‘93), pp. 822-825, Yokohama, June 7-10, 1993. 6. T. Mineta, S. Kobayashi, Y. Watanabe, S. Kanauchi, I. Nakagawa, E. Suganuma, and M. Esashi, “Three-axis capacitive accelerometer with uniform axial sensitivities,” in Proceeding 8th International Conference on Solid-State Sensors and Actuators (Transducers ‘95), pp. 554-557, Stockholm, Sweden, June 25-29, 1995.
7. H. Seidel, U. Fritsch, R. Gottinger, and J. Schalk, “A piezoresistive silicon accelerometer with monolithically integrated CMOS-circuitry,” in Proceeding 8th International Conference on Solid-State Sensors and Actuators (Transducers ‘95), pp. 597-600, Stockholm, Sweden, June 25-29, 1995. 8. M. Offenberg, F. Lärmer, B. Elsner, H. Münzel, and W. Riethmüller, “Novel Process for a monolithic integrated accelerometer,” in Proceeding 8th International Conference on Solid-State Sensors and Actuators (Transducers ‘95), pp. 589-592, Stockholm, Sweden, June 25-29, 1995. 9. M. Parameswaran, H.P. Baltes, Lj. Ristic, A.C. Dhaded, and A.M. Robinson, “A new approach for the fabrication of micromachined structures,” Sensors and Actuators, 19:289-307, 1989. 10. J. Marshal et al, “Realizing suspended structures on chips fabricated by CMOS foundry processes through the MOSIS service,” NISTIR 5402, U.S. National Institute of Standards and Technology, Gaithersburg, MD 20899, June 1994. 11. F. Chang, R. Yeh, G. Lin, P. Chu, E. Hoffman, E. Kruglick, and K.S.J. Pister, “Gas-phase silicon micromachining with xenon difluoride,” SPIE ‘s 1995 Symposium on Micromachining and Microfabrication, In Press, Austin, USA, Oct. 23-24, 1995. 12. P.J. French and A.G.R. Evans. “Polycrystalline silicon strain sensors,” Sensors and Actuators, 8:219-225, 1995. 13. J.Y.W. Seto. “Piezoresistive properties of polycrystalline silicon.” J. Applied Physics, 47(11):4780-4783, 1976 14. T.A. Core, W.K. Tsang and Steven J. Sherman. “Fabrication Technology for an Integrated Surface-Micromachined Sensor,” Solid State Technology, 39-47, October 1993.
