Ž .Thin Solid Films 383 2001 321�324

Manufacturing of surface micromachined structures for chemicalsensors

Carmen Moldovana,� , Byong-Hak Kimb, Stefan Raibleb, Victor Moagara

aNational Institute for Research and De�elopment in Microtechnologies, Center of Ser�ices for Micromachining Technologies,P.O. Box 38-160, 72225 Bucharest, Romania

bInstitute of Applied Physics, Auf der Morgenstelle 10, D-72076 Tubingen, Germany¨

Abstract

This paper presents the technological process, the simulation and the microfabrication of polysilicon structures used as masssensitive sensors for gas detection. The microstructures are double-ended and released, polysilicon microbridges, realised bysacrificial layer technique. The surface micromachining compatibility with the integrated circuits microfabrication is discussedand the bridge oscillation observed. � 2001 Elsevier Science B.V. All rights reserved.

Keywords: Surface micromachining; Chemical sensors

1. Introduction

The device presented in this paper consists of a� �resonant polysilicon microbridge 1 and the measure-

ment circuitry to pick up the resonance frequency ofthe bridge. By coating the bridge with sensoractivematerial it is possible to use these devices as mass

� �sensitive gas sensors 2 .The microbridge oscillation is activated electrostati-

cally and detected capacitively. The resonant structureis achieved by a sacrificial layer technique. The senseand drive electrodes were patterned by phosphorusimplantation and together with the polysilicon mi-crobridge they form variable capacitors; capacitychanges due to gap variations can be used to detect the

� �bridge oscillations 1 . The capacity detection is per-formed by a highly accurate CMOS detection circuit onthese devices.

The electronic circuits for excitation-detection are� �integrated on the same chip with the microbridge 3 ,

Ž .having the main aim to amplify the small current nA

� Corresponding author. Tel.: �40-14908412; fax: �40-14908238.Ž .E-mail address: cmoldovan@imt.ro C. Moldovan .

offered by the variable capacitors from detection way.Fig. 1 shows an optical microphotograph of the reso-nant sensor chip with electronics integrated on thesame chip, having an area of 1.5�1.4 mm2.

2. Determination of the resonant frequency of thepolysilicon microbridge using the ANSYS program

The ANSYS dynamic analysis shows the resonancefrequency and the proper modes of vibration ofdouble-ended microbridges. Fig. 2 indicates the firstand the second mode of vibration of a double-endedpolysilicon microbridge having the following dimen-sions: 500 �m length, 30 �m width, 1 �m thickness. Auniform distributed load on the bridge is considered.Analysing bridges with different lengths we obtain theresonance frequency in the range of tens KHz�fewMHz depending of the microbridge length and the

Ž .proper mode of vibration Table 1 . Table 1 indicatesthe values of resonant frequency for four modes ofvibration of microbridges having the length 100�500�m. The simulation data are used to calculate the

Ž .quality factor Q and for the design of the mi-crobridge.

0040-6090�01�$ - see front matter � 2001 Elsevier Science B.V. All rights reserved.Ž .PII: S 0 0 4 0 - 6 0 9 0 0 0 0 1 6 0 6 - 0

( )C. Moldo�an et al. � Thin Solid Films 383 2001 321�324322

Ž . Ž .Fig. 1. Sensor chip top view: a bridge; b electronics; 1,2 pads for driving the structure.

Table 1 indicates the decrease of the resonant fre-quency with the increase of the microbridge length andthe increase of the resonant frequency for superiormodes of vibration.

3. Microdevices manufacturing technique

The microdevice was realised in surface micro-machining technique, including the deposition of thinfilms and the use of a sacrificial layer. For the combina-tions construction material�sacrificial layer we choose

Ž . � �polysilicon�silicon oxide PSG pair 1 . The technologi-cal steps for processing a double-ended clampedpolysilicon bridge are based on CMOS aluminium gate

� �technology 3 .

3.1. Technological steps

Ž .N-Type wafers 5�7 � cm are used. Drive and sense

Table 1The resonant frequency for four modes of vibration function of thebridge length

L F1 F2 F3 F4Ž . Ž . Ž . Ž . Ž .�m KHz KHz KHz KHz

100 883.9 2453 4864 5215.7150 390.7 1080 2191 3577200 219.2 611.2 1241 2008.6250 140 357.7 786.1 1278.3300 97 269 533 888350 71.3 197.4 3959 652400 54.5 151 299 507450 43 119 237 392500 34.9 96.4 192 319

electrodes are defined by phosphorus implant in thep-well region. In the same step, the n transistors are

Ž . Ž .Fig. 2. The first a and the second b mode of vibration of adouble-ended polysilicon microbridge: 500 �m length, 30 �m widthŽ .ANSYS analysis .

( )C. Moldo�an et al. � Thin Solid Films 383 2001 321�324 323

defined, followed by p transistor configuration. Theboron diffusion defines the source and drain of p MOStransistors. The configuration of the silicon oxide grownduring the boron diffusion realise the physical support

Žof the resonant bridge. A phosphosilicate glass 10.wt.% P O to be used as a sacrificial layer is deposited2 5

and patterned. On top of PSG oxide the resonantmicrobridge will be constructed. After growth of a

ŽMOS silicon oxide, the contact windows metal-poly-.silicon, metal-diffusion are opened, followed by a

metallic layer deposition. The metallic layer has therole to interconnect the drive-sense electrodes with thesignal processing circuits. A phosphosilicate glass depo-sition step follows, which is patterned to uncover theresonant bridge and retain both ends of the bridgeunderneath. Sacrificial layer etching frees the bridge,and thus the microresonator becomes functional. Thisarrangement is the best choice to minimise the totalnumber of masking steps that must be added to thestandard CMOS process. Sixty-two fabrication stepsand 12 masks were necessary to fabricate the sensorchip in total. The greatest advantage of this process is

Ž .Fig. 3. SEM picture of the poly-Si microbridge: a after the sacrifi-Ž .cial oxide etching; b the clamped end aspect. Bridge dimensions: 1

�m thickness, 30 �m width and 500 �m length.

Ž . Ž .Fig. 4. a Released poly-Si microbridge in oscillation. b SEMpicture of the encapsulated sensor.

its perfect compatibility with CMOS technology andthat it does not requires additional equipment.

3.2. Specific processes for microstructure fabrication

The polysilicon microbridge is 1 �m thick. Thepolysilicon deposition for the bridge was realised byLPCVD, followed by lithographic configuration and

Ž .planar plasma CF �O etching. The sacrificial oxide4 2was then removed in BHF 10:1, 70 min to release a

Ž .30-�m-wide bridge Fig. 3 .For better stability the hard bake process for the

resist mask was repeated after every 20 min of etchingof the sacrificial layer. The released microbridges canbe destroyed by a usual procedure of rinser�dryer. Therinse is recommended in a batch DI water, and the dryis realised on a hot-plate or in an oven. No sticking ofthe microbridge with the substrate was observed. The

Ž . Žmicrobridge is released Fig. 3a and free of stress no.deflection was observed .

Considering Fig. 3b, the re-design of the device isnecessary, to improve the lay-out dimensions for sacri-ficial layer and polysilicon bridge. The lithographic

( )C. Moldo�an et al. � Thin Solid Films 383 2001 321�324324

processes must be reconsidered to avoid the sub�superexposure, the alignment errors and the overetch-ing. Fortunately no influences in polysilicon bridgeoscillations were observed due to the clamped end

Ž .aspect Fig. 3b .

4. Testing the bridge oscillation

The bridge oscillation was observed under the elec-Ž .tronic microscope SEM Fig. 4a or optical microscope

Žby applying an AC voltage to the drive electrode pads.1,2 in Fig. 1 . Therefore, we can conclude that the

microbridge was released. This method can be used onŽ .the wafer or on the encapsulated structures Fig. 4b to

discriminate between released and unreleased mi-crobridges.

The sensor was put in a metal package without a capŽ .Fig. 4b . The package remains open for easy tests andobservation of the bridge oscillation, for coating a

sensitive layer and characterising the sensor in a harshenvironment. The electronic circuit area is protectedwith phosphosilicate glass.

5. Conclusion

A microbridge structure together with electronic cir-cuits on the same chip was realised. The surface micro-machining processes were optimised. Small and repro-ducible microstructures can be obtained due to theplanar technology. A silicon-based sensor was obtainedwhich can be tested in various applications.

References

� �1 R.T. Howe, Resonant Microsensors, Transducers’87, 1987, pp.380�385.

� � Ž .2 R.T. Howe, R.S. Muller, IEEE Trans El. Dev. April 1986 174.¨� �3 C. Moldovan, V. Moagar, F. Craciunoiu, Vapour concentration

or pressure resonant sensor in CMOS technology, MME’98,Ulvik, Norway, 1998, pp. 268�272.