CA1125049A - Silicon transducer - Google Patents

Abstract

J. C. Greenwood 37 (Rev) TRANSDUCER
AND METHOD OF MAKING THE SAME

ABSTRACT OF THE INVENTION
A silicon transducer including a silicon frame with one or more lands extending from a diaphragm or the like. The lands are interconnected by two thin strips formed integrally with the lands.
The strips are essentially the transducer. The transducer is constructed by etching a boron doped wafer with a mixture of catechol, ethylene diamine and water.

Description

-1 - J. C. Greenwood 37 (Rev.) TRANSDUCER
AND METHOD OF MAKING T~IE SAME

BACKGROUND OF THE INVENTION
-This invention relates to transducers, and more particularly to electrical semiconductor transducers of the acoustic type.
SUMMARY OF THE INVENTION
According to one aspect of the invention, there is provided a pressure transducer comprising a flexible diaphragm supported in a frame of the same material as the diaphragm;
first and second lands extending from one surface of said diaphragm; and a plurality of filaments stretched and fixed between said lands, changes in the configuration of said diaphragm causing corresponding changes in the resonant frequency of vibration of said filaments.
According to another aspect of the invention, there is provided the method of making a silicon pressure transducer, said method comprising the steps of: selectively boron doping both faces of a silicon wafer; etching one face of said wafer to define a groove; and selectively etching the other face of said wafer opposite said groove so as to define a silicon diaphragm on which two lands are mountedl said lands supporting a plurality of filaments s-tretched therebetween.
BRIEF DESGRIPTION_OF THE DRAWINGS
In the accompanying drawings which illustrate exemplary embodiments of the present invention:
Fig. 1 is a perspective view of a semiconductor resonant filament transducer;
FigO 2 is a top plan view of the transducer of Fig. l;
Fig. 3 is a cross-sectional view of the transducer taken through a resonant filament; and Fig. 4 is a schematic diagram of an oscillator drive circuit for use with the transducer.

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-la- J. C. Greenwood 37 (Rev.) DESCRIPTION OF THE PREFERRED EMBODIMENT
The principle on which the transducer functions is analogous to the variation with tension of the resonant frequency of a stretched string. The resonant fre~uency of such a transducer is a direct function of an applied force with temperature variation a second order effect. The electrical output of the transducer is in a form particularly suitable for signal processing by logic circuitry or by a microprocessor.
Referring to Figs. 1 and 2, a transducer 11 is shown which is formed by selective etching of a silicon wafer doped with boron.
In Fig. 2 parts that remain after the etching process are shown.

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-2~ JO CO Greenwood 37 (Rev) The fabrication of selectively etched silicon devices de-pends upon the phenomenon of inhibition of certain etching steps by a concentration of dopant boron higher than 4 x 1019 atoms/ccO
There is an abrupt change in the etch rate from that normal for undoped silicon to substantially zero at this boron concentration level so that the thickness of an unet~hed r~gion is defined pre-cisely by the boron difusion depth~ The process is more fully d2s¢ribed in U~Ko Specification No~ 1~211~496 ~J. C. ~reenwood 61 ~he wafer is doped through a mask with boron in those areas in lG which etching is not required~ and the wafer is then etched with a mixture of catechol~ ethylene diamine and water to form the transducer ~tructure shown in Figs~ 1 and 20 The transducer 11 includes a pair of lands 1~ (as opposed to grooves) protruding from a silicon diaphragm 13 supported in a rectangular silicon frame 10 and linked by a pair of filaments 14 when pressure is applied. In practice of course a plurality of such devices is disposed on a silicon wafer.
Asl contrary to con~entional semiconductor fabrication techniques~ both major surfaces of the semiconductor wafer are ~tched~ means must be provided for handling a process wafer by its rim portions only~ Thus the transducers formed on a wafer must be disposed towards the central region leaving the rim portion free~
In the apparatus used for the ~tching process, the silicon wafer on which the devices are to be formed is mounted by its rim 2~ portion on a glass carrier and is then sealed via an O-ring again~t a shoulder at one end of a tubeu A clamp ring ~r gland nut metal with a reverse thread on the tube secures the wafer in position.
Etch solution can then be pvured into the cup thus formed and act on one 3ide only o~ the wafer. In some applications the glass

3~ carrier may be replaced by a vacuum chuck arrangement.
In a typical transducer fabrication process~ the silicon wafer is cleaned in hydrofluoric acid, caro's acid and water and is then treated to a boron diffusion from both sides~ The front of the wafer is masked with an evaporated aluminum layer wi~h the reverse ace masked and etched in a phosphoric acid etch to define the face of the diaphragmO The aluminum coating has ~ photoresis~
on it~ and is etched with a phosphoric and~or nitric acid mixture~
Th~ silicon i~ et~hed by plasma etching to a depth greater than has be2~ r~ndered insoluble in a sele¢tive etch by the boron diffusionO

-3 J~ C. Greenwood 37(Rev) Thi~ is done on both sides of the slice~ which is then etche~ in a selective etch to define the first configuration of the device.
The various etching techniques will be apparent to those skilled in the art~ but the following rules should be observed:
lo The etch rate of catechol-diamine-wat~r is substantial-ly slower in ~he clll~ crystallographic direction than in any otherO
To a first approximation the rate in the <111> direction can be reg~rded as zero~
2, A concave face tends to be opened up to give a hollow bounded by the slowest etching <111> 50ctahedral) faces; thus a pinhole in a protective oxide coating on a ~111> orientation slice gi~es rise to a square pyramidal etch pit~
3. A convex face tend~ to give a solid bounded by the fastest e~ching faces which are the 24 ~331~ faces.
40 An irregularity~ such as might be caused by ~aulty masking, in a <111> face tends to be straightened out whereas the same sort of irregularity in a fast etching face does not~
5. The cleanest c331> fast etching faces are obtained whe~ one edge only is adjacent to another fast etching face~ tha other edge5 being adjacent to unsoluble material or ~111> planesO
Irregular shapes result from other alignments although not every combination of adjacent ~aces has been t:ried.
6. On ~100> slices a variety of corner shapes can be obtained by putting compensating spikes on the mask. The angle of the spikes does not appear to be cri~ical although a 1 in 3 slope gives good results~ The length of a spike is related to the etch time~ which is determined by the thickness of the slice. Normally the etch time should be greater than is needed just to reach the other side, so that any irregularities are cleaned up. If the etch time is 20% greate~ than is needed to reach the other side~
a nearly square corner is obtained by making the length of the spike 20~ longer than the thickness of the sliceu If no spike i~
,, used~ the corner is chamfered~ Intermediate sized spikes give intermediate results~ -7. If a part of the surface is to be undercutJ care has to be taken that this process is not stopped by ~ facesO
~or example if a bxidge is to be undercut on a ~100> orientation slice~ the bridge must be at an angle to the ~ ace~ and ~3t be ~ufiei2ntly n2rr~w~

~4~ J~ CD GreenwoOd 37 (~evl In Fig. 3, which i5 a cross section of part of the transducerj we see one of the resonant strips at 20, which i8 spaced from the lower surface 21 of the deviceO On the lower surface 21 there are ~he devices driving electrode 22, its pick-up electrode 23~ and a guard electrode 24 therebetween.
In use the transducer~ which functions as ~ pressure gauge, is mounted by its rim or frame against a source o~ pressure to be measured~ The filaments are excited at their resonant vibrational frequency~ e.g~ by a circuit o~ the type shown in 10 Fib. 4, this frequency being determined by the pressure differen~e across the diaphragm~ Changes in pressure cause corresponding chang2s in the tension of the filament 14 and hence ~hanges in their resonant fr2quency~
Conveniently the transducer may be excited electro-~tatically~ the circuit of FigO 4 being intended for this purpose~as thi~ provides substantially no damping of the filamentO
To reduce the capacitive coupling between input and output to a minimum, two resonant strips~ as shown in Figs~ l and 2~ are used~ those being driven in antiphaseO The preampli-2G ier ~ET~s are mounted on the transducer itself~ The circuit ofFig~ 4 shows amplifiex stages which give an overall gain of about loa~ The second of these stages is an AGC ~tage and the third is a phase splitter with unity gainO Tpe fourth stag~ i~
an optional ~tage to get an increased amplitude outputO
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Claims (4)

-5- J. C. Greenwood 37 (Rev) WHAT IS CLAIMED IS:

1. A pressure transducer comprising a flexible diaphragm supported in a frame of the same material as the diaphragm; first and second lands extending from one surface of said diaphragm; and a plurality of filaments stretched and fixed between said lands, changes in the configuration of said diaphragm causing corresponding changes in the resonant frequency of vibration of said filaments.

2. A transducer as claimed in claim 1, in which said material is silicon.

3. The method of making a silicon pressure transducer, said method comprising the steps of: selectively boron doping both faces of a silicon wafer; etching one face of said wafer to define a groove; and selectively etching the other face of said wafer opposite said groove so as to define a silicon diaphragm on which two lands are mounted, said lands supporting a plurality of filaments stretched therebetween.

4. A process as claimed in claim 3, in which said selective etching is effected with a mixture of catechol, ethylene diamine and water.