Silicon optical pressure sensor

Sensorsand Actuators A, 32(1992) 628-631

628

Silicon optical pressure sensor J. A. Dziuban,

A. Gorecka-Drzazga

and U. Lipowicz

Institute of Electron Technology, Technical University of Wroclaw, Wroclaw (Poland)

Abstract A novel optical-fiber silicon pressure sensor with a bossed membrane has been fabricated. The pressure deflects the micromachined thin membrane and moves the end of a fiber illuminating the ppn junction fabricated near the edge of the membrane in a standard IC process. A photovoltage proportional to the degree of deflection of the membrane is generated in the junction. A nontypical output voltage versus inlet pressure characteristic has been obtained.

Introduction

Although many types of silicon pressure sensors have been described in the literature [ 1,2], some of which are produced in large quantities [3], new constructions are currently being developed [4-81. Among them, silicon fiber-optic pressure sensors [9, lo] play an important role. A thin silicon membrane has good mechanical properties and may be easily and repeatably deflected in proportion to the pressure of gases and liquids. The displacement of the membrane may be simply defined by a light intensity measurement in the system shown in Fig. 1. The silicon fiber-optic pressure sensor presented here works like the device in ref. 10 but has an electrical output signal. It regains no electrical supply and is destined to work as an on-off pressure detector. The principle of the sensor operation is schematically shown in Fig. 2.

The sensor consists of the following parts (Fig. 3)*: (1) A silicon micromachined die with a bossed thin membrane, V-groove and the light-sensitive p-n junction

0924-4247/92/35.00

1. Light modulation

15pm

in passive fiber-optic

I

silicon sensors.

lp

Fig. 2. Principle of a fiber-optic silicon sensor with electrical output.

(2) A silicon support.

Construction

*The construction was partially designed during Gordon’s tics Session in Rabka in 1989 [II, 121.

Fig.

Synec-

(3) A standard 150 pm diameter optical fiber with a movable end ‘looking’ at the p-n junction. (4) A metallic case with pneumatic and electrical connections. In the device, the movable end of the fiber illuminates the p-n junction proportionally to the membrane deflection caused by the pressure of liquids or gases. Light is transmitted to the p-n junction area by the fiber from an external light source, which may be a halogen lamp, the sun or @I 1992 -

Elsevier Sequoia. All rights reserved

629

-

glue

3 -

fiber

1 support

light

Photomasks

Fig. 3. Technical

realization

used in the sensor fabrication

of the sensor.

a laser. Any movement of the fiber end involves changes in the diode illumination and eventually changes the d.c. photovoltage generated in the p-n junction.

Fabrication process There are three main steps of the sensor fabrication: (1) Structure

micromachining

We use 3” 5 R cm n-type ( lOO)-oriented doublesided polished silicon wafers. A thick silicon oxide layer ( 1.8 pm) is produced in a high-temperature Hydrox process. After the first photolithography, prolonged etching in KOHH,O-ISA mixture (80 “C) forms a three-dimensional die (2) with a V-groove and bossed membrane (20 pm thick, 6 mm x 6 mm). The extension of the etching time allows the membrane to be overetched and the silicon support ( 1) to be formed. (2) Light-sensitive diode preparation A standard IC process, including wet oxidation, boron diffusion and Al layer sputtering, is used. Three photomasks are used. The layout of all the masks used in the fabrication process is presented in Fig. 4. (3) Sensor packaging

The sensor die (2) is bonded with support ( 1). The 150 pm fiber is glued to the V-groove with a 100 pm gap between the end of the fiber and the

Fig. 5. Sensor in operation

with 0.6 bar under the membrane.

edge of the p-n junction. After that, all parts are glued to a case and the electrical connections are bonded. The structure is then encapsulated and sealed. The front view of the sensor with the membrane strongly deflected down is shown in Fig. 5. Epoxy glue drops are seen in the middle of the membrane and near the left edge of the frame. The fiber, p-n junction with metallic contact and Al connecting wire are shown. The bossed area lies

630

on the surface of the metallic case; the total movement of the membrane is 0.5 mm.

Results

We have obtained some wafers with many dies. After carefully checking the parameters of the p-n junctions (I- V characteristics, photosensitivity, etc.), the main technical parameters of the sensor have been measured. The sensor has a nontypical characteristic with a monotonic increase (0 to 0.38 bar) and decrease (0.68 to 1.3 bar) of the output voltage versus increasing inlet pressure (Fig. 6), and with two slopes corresponding to two sensitivities: +240 mV/bar and - 80 mV/bar, respectively. The zero-pressure offset voltage is high; its minimum value is 90 mV and strongly depends on the fiber positioning in the device (the gap between the end of the fiber and the p-n junction) and the illumination level. The sensor shows good sensitivity. It seems that it should have good temperature-dependent parameters, but additional measurements must be done. The sensor works properly in the pressure ranges - 1 to 0 bar and 0 to + 1.6 bar (overpressure 3 bar), but the sensor membrane must be moved up for both pressure regions. Typical technical parameters of the sensor with the external end of the fiber directly illuminated by a 100 W halogen lamp (with no coupling system) are given in Table 1.

TABLE

I. Selected parameters

of silicon optical pressure sensor

Parameter

Value

Dimensions of die (mm) Output signal” (V) Sensitivity= (mV/bar) Overpressure (bar)

10 x 8.5 x 0.5 up to 1 d.c. photovoltage +240 and -80 3 light +90 0.38/0.6

SUPPlY

rJoffse: (mV) Pressure on/off (bar)

“Output signal sensitivity will change with light intensity variation; additional LToffsetvalue depends on the fiber-optic adjustment in the structure.

Conclusions

The sensor works properly in a wide range of pressure values. It has good electrical and pneumatic parameters, a high output voltage signal and sufficient overpressure protection. The high offset voltage depends on the illumination level and the accuracy of production. The sensor needs no electrical supply, which may be very useful in many applications. It has a nontypical output voltage versus inlet pressure characteristic with two slopes and a plateau.

Acknowledgement

The authors wish to thank Dr Anna Sankowska for her helpful cooperation.

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0.6

1.2

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PIbar

Fig. 6. The output sensor.

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vs. inlet pressure

characteristic

of the

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