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heater resistors embedded between two

the heaters was first shown to be linear and with a custom drive circuit to provide

vacuum sensors was demonstrated.

The devices presented here are sim based, bilayer thermal transducers (similar to by Ataka, et al. [ 11) consisting of 60nm Ti/W

of Hitachi PIQ-L200 and a top 3.5 pm layer The process (see Appendix) uses a sacrificial 2 layer which is wet etched for release and a

these devices can be fabricated above fully comp wafers for future integration with control circui

between ambient and 195°C. The resis temperature measurements for several heater

I

40 80 120 160 Temperature ("C)

Fig. 1 : Plot of resistance versus temperature for several heater designs, showing interpolated TCR value (361 ppm).

A primary goal was to study the dynamic temperature responses of the transducer structures. Resistance can be computed during operation using many methods, but it was decided to use a constant-voltage drive (to avoid the thermal runaway possible with constant current drive with a positive TCR) with current sensing and direct analog resistance computation. A block diagram of the circuit used is shown in Fig. 2. An input offset voltage is summed with the drive signal to provide a sense current with which to measure the resistance when the drive voltage is zero. With the basic circuit, it is readily possible to explicitly measure resistance (and hence temperature, by calibrating to a known reference temperature) with a bandwidth several orders of magnitude greater than that of the thermal devices. For the multi-heater structures described below, multiple circuits of this type can be used to make real-time temperature measurements at a number of heater sites. By closing a feedback loop around the circuit , it is possible to make the input signal correspond to a desired temperature (and if the thermal model of a structure is well defined, this can in tum correspond to its physical position).

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Fig. 2: Simplified block diagram of the dynamic resistance measurement circuit. The analog computation unit used WQS an Analog Devices AD538 and all op-amps were Linear Technology type LTI056. Gain was adjusted to 1V = lkL2 using calibration resistors.

Multi-segmented actuator designs were fabricated that employ multiple, electrically controllable segments to provide additional degrees of freedom compared to previously reported thermal actuators (Fig. 3). The individual heated segments are isolated by regions with increased thermal resistance due to "cut-out'' regions in the structure. The dimensions of the structure are 2.1 mm from base to tip, 184 pm in width, and a total thickness of approximately 8.5 pm. The cut-outs are 58 pm X 46 pn sections between the four 425 pn long segments. Typical heater resistances were 2.4 kn.

aT

as shown in Fig. 4. The devices were also shown to actuate (over smaller ranges of motion) in water and isopropyl alcohol.

Fig. 3: Scanning electron micrograph of four multi- segmented thermal actuators.

Each segment could easily be actuated individually at power levels of =20 mW (each) for large deflections in room temperature air. By applying a bias power to all four segments and selectively de-energizing individual segments, the separate actuation Of each One be demonstrated

Fig. 4: Collage showing (top) all segments 08, (center} all segments biased, and (bottom) the distal (tip) segment ofl

334 TRANSDUCERS '95 . EUROSENSORS IX

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316 - 88

The thermal response of the four segmen s at 25 mTorr showed the expxted decreasing time constan nearer the base (due to decreased thermal resistance of cond ction). The heating and cooling time constants exhibited i arked asymmetry at the distal segment, becomin more symmetrical nearer the base. This asymmetry may be due to time constants associated with convection, an is the subject of further study.

Fig. 5 show$ the measured thermal distal heater at variqus pressures from 10

1 segment temperature versus pressure showed a response, as shown in Fig. 6, indicating that th can readily be used as pressure sensors. These re consistent with those, obtained using inorganic,

is the major heat loss mechanism. I

I I 300 T

2 5 0

g 200

b 2 c" 100

e ' 5 0

5 0

0 I . l" I . _ . . I )

0 2 4 6 10

Ttma (seconds)

Fig 5: Distal segmenf temperature versus time and ressure (2.2V squarewave drive signal). Both positive and egative overshoot is electronic artifact. F

DISCUSSION

In summary, we have demonstrated thermal actuators with multiple electrically-controllable segments. Through the use of a direct resistance measurement technique and a known heater temperature coefficient of resistance, dynamic measurements of heater temperature versus pressure was demonstrated as a means for both studying and controlling such devices.

ACKNOWLEDGEMENTS

We would like to acknowledge the assistance of N. Maluf and R. Reay. This work was funded by ARPA (grant no. N0014-92-J-1940-P00001).

REFERENCES

[ 13 M. Ataka, A. Omodaka and H. Fujita, "Fabrication and Operation of Polyimide Bimorph Actuators for a Ciliary Motion System," J. Microelectromech. Syst., vol. 2, no. 4, DE. 1993, pp. 146 - 150.

121 C. Mring, T. Grauer, J. Marek, M. S . Mettner, H.-P. Trah and M. Willmann, "Micromachined Thermoelectrically Driven Cantilever Structures for Fluid Jet Deflection," Roc. MEMS '92, Travemunde, Germany, Feb. 4 - 7, 1992, pp. 12- 18.

[2] H. Baltes and D. Moser, "CMOS Vacuum Sensors and Other Applications of CMOS Thermopiles," Roc. 7th Int. Conf. on Solid-state Sens. and Act., Yokohama, Japan, June 7-10,1993, pp. 736 - 741.

0 01 0 1 1 10 100 I 1000

0 Pressure (Torr)

Fig. 6: Plot of peak distal segment temperature versus pressure between 10 pTorr and one atmosphere (76 Torr).

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APPENDIX: PROCESS FLOW

L

S i 0

Aluminum TiW Resistor. Silicon

Silicon PIQ-3200 Nitride \ Polyimide\

A

B

C

D

E A) Initial stack of patterned sacrificial aluminum on LTO and SO,, selectively protected using PECVD silicon nitride and overcoated with PIQ-L200 polyimide and a top PECVD silicon nitride layer. B) Vias through the PIQ-L200 have been dry etched for metal-to-metal contacts and the top silicon nitride has been patterned to serve as a selective stiffening layer and to encapsulate the Ti,” resistors. C) Aluminum conductors, Ti,” resistors and a top PECVD silicon nitride have been deposited and patterned. D) The top PIQ-3200 polyimide and a final PECVD silicon nitride layer have been deposited and patterned (this silicon nitride layer protects the bond-pad areas during the sacrificial aluminum wet etch). The exposed polyimide layers have been dry etched to open the protected bond pads and the sacrificial aluminum layer. E) The sacrificial aluminum has been wet etched to release the thermal actuators, followed by a brief dry etch to remove the protective silicon nitride above the bond pads.

336 TRANSDUCERS ‘95 * EUROSENSORS IX

The 8th International Conference on Solid-state Sensors and Actuators, and Eurosensors IX. Stockholm, Sweden, June 25-29, 1995