Note: Descriptions are shown in the official language in which they were submitted.
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A PRESSURE TRANSD~CER HAYING ELECTRICALLY
SHIELDED PIEZORESISTIVE SENSORS
TECHNICAL FIELD
The present invention relates to pressure trans-
ducers generally and particularly to semiconductor pres-
sure sensors having piezoresistive sensors diffused
therein.
BACK~ROUND ART
Transducers which produce a resistance change in
response to an applied pressure change have heretofore
included thin film strain transducers as well as semi-
conductor transducers.
Semiconductor pressure transducers are known
wherein the strain gauge resistors are formed on one
side of a monocrystalline semiconductor diaphragm such
as Silicon having diffused therein a dopant such as
boron. The sensing resistors are usually arranged on
the diaphragm to form a bridge with the sensing resis-
tors being`simultaneously in tension and compressionwhenever the diaphragm is deflected by a force or
pressure applied to the surface of the diaphragm.
Many configurations of resistance elements on such
Silicon diaphragms are known and examples of such
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various configurations may be found in U.S. Patent No.
3,537,319 as well as U.S. Patent No. 3,697,918.
Regardless of the resistance configuration, such
pressure transducers utilizing diffused resistance strain
gauges suffer from both long and short term drift problems
which are caused by an accumulation of charges on the
transducer surface hav;ng the straing gauges diffused
therein. Such surface charges may arise from polariza-
tion of fluid in contact with the transducer~ from exter-
nal sources of voltage such as common mode voltage; as
well as other such phenomena. The effec~ of a surface
charge on the transducer may be descri~ed as follows.
Charges on the surface of an oxide layer covering a sem~-
conductor ~ill induce image charges in the semiconductormateri al . For exampl e, i f the semiconductor ~s N-type, a
negative surface charge induced on the ~ransducer surface
will repel negative charges in the semiconductor caus~ng
a reduction in the number of electrons in that area.
This is called "depletion" because the majority carriers
in the semiconductor are depleted in that area. If a
positive surface charge is induced on the surface of the
oxide over the N-type transducer, an increase in the
number of electrons in that area will occur. This action
is termed "accumulation". The same effects occur with
P-type material~ however the polarity of the voltages is
reversed. In the case of the diffused piezoreslstive
strain gauge, both-N- and P-type material are present.
The background material is N-type and the diffused resis-
tors are P-type. When the strain gauge structure is sub-
jected to the effect of surface chargPs on the oxide
transducer surface the diffused resistors may change
resistance and reverse leakage currents may increase
resulting in eventual changes in the effective output of
the strain gauge transducer.
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This forementioned phenomenon was observed by the
inventors in the use of diffused piezoresistive pressure
transducers as the pressure sensing element in a fluid-
filled absolute pressure transmi-tter. When a voltage
5 source was connected between the housing of the pressure
transmitter and either the -~ or - terminal of the source
a slow change in the output of the pressure transmitter
was detected having a time constant of between one-half
and one hour. When the polarity of the voltage was
reversed the output of the transmitter shifted in the
opposite direction. The inventors suspected that a slow
polarization of the fluid inside the housing of the
pressure transmitter was occurring. Substitution of an
AC electric source for the DC source reduced but did not
eliminate the mentioned effect.
SUMMARY OF THE INVENTION
The present invention solves the forementioned
problems associated with the known semiconductor diffused
piezoresistive pressure transducer devices as well as
others by providing an electricaLly shielded ~iffused
piezoresistive pressure transducer which is insensitive
to any surface charges developed on the surface of the
transducer.
In accordance with the present invention there is
provided a pressure transducer having shielded piezore-
sistive strain gages comprising a base material of N-Type
material, a piezoresistive P-Type material formed into
said N-Type base material, an oxide layer extending over
said P-Type piezoresistive material to act as an elec-
trical insulator for said piezoresistive material, alayer of electrically conductive material extending over
said oxide layer covering said piezoresistive material
but being electrically isolated there-from, a direct
current voltage source of a substantially constant
potential value having a positive polarity side, and said
direct current voltage source having its positive polarity
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side elec-trically connected to hoth said electrically
conductive ma-terial layer and said piezoresistive material
to maintain said electrically conductive layer extending
over said piezoresistive material at a known constant
potential with respect to said piezoresistive material
preventing the accumulation of any varying surface charge
on said electrically conductive layer by draining of the
surface charges into said voltage source.
In specific embodiments of the invention, the layer
of electrically conductive material was made from Chrome-
Gold
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metallization layers and ~he layered coating shield was
limited to areas covering the pressure sensitiYe resis- -
tors of the pressure transducer.
In view of the foregoing it will be seen that one
aspect of the presPnt invention is to provide an elec-
trically shielded pressure transducer.
Yet another aspect of the present invention is to
provide a pressure transduc~r insens;tive to surface
charges which may develop on the transducer.
Yet another aspect of the present invention is to
provide a pressure transducer wherein any surface charges
are bled away through a suitable electrical connection of
the transdueer shield.
These and other aspects of the present invention
will be more clearly understood after a review of the
following description of the preferred embodiment when
considered w~th the accompanying drawings.
BRIEF D~SCRIPTION OF THE DRA~IINGS
2~ Fig. 1 ;s a top v;ew of the pressure transducer of
the present invention.
Fig. 2 is a seotional side view of the pressure
transducér taken along section 2-2 of Fig. 1.
Fig. 3 is an electrical schemat;c diagram showing
the electrical connection of the surface charge shield
of the pressure transducer of Figs. 1 and 2.
DESCRIPTION OF THE PREFERRED EMBCDIMENT
Referring now to the drawings wherein the showings
are for purposes of d;sclosing a preferred embodiment of
the invention and are not intended to l;mit the
invention thereto, Figs. 1 and 2 show a semiconductor
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~iezoresistive pressure transducer assembly 10 intended
for use in a known fluid-filled pressure transmitter (not
shown) which monitors the res;stance change of the
transducer assembly 10 in response to pressure variations
to thereby provide an indication of the pressure applied
to the transducer assembly 10.
The pressure transducer assembly 10 has a base 12
consisting of bulk N type Silicon into which a P type
~ ron material is diffused to form a series of pressure
~ant'irsensitive resistors R. In the present assembly 10 five
individual resistive patterns are diffused with the two
~, centrally located resistors R~ and R'c being the
~' resistors under compression in the use of the transducer
assembly 10 in the pressure transmitter. The end
rPsistors Rt and R't located along the periphery of the
transduce~ assembly 10 in line with the compressive
resistors Rc and R'c are the resistors which will be
under tension in the use of tl)e transducer assembly 10
in the pressure transmitter. A fifth resistor Rk is
located at right angles to the pressure sensitive resis-
tors R to make the fifth resistor Kk pressure insensi-
tive. The resistor Rk is diffused into the base 12 and
may be used as a temperature compensation resistor.
In the diffusion process whereby a dopant is di F-
fused into the N-type Silicon base to form the P-type
diffused resistors a coating of Silicon-dioxide is
formed along the entire face of the base into whi~ch the
resistors are diffused. This Silicon-dioxide coat;ng
acts as an electrical insulator and a protective coat-
ing to the semiconductor device formed by the diffusion
of impurities into the N-type base to form diffused
resistors Rc; R'c; Rt, R'~. This Silicon-dioxide coat-
ing is selectively removed in a manner that will be
described later to leaYe a coating 14 over the surface
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o~ the base except in th~ electrical cont~ct areas 16
for resistors Rc; R'Ci Rti R't and to facilitate elec-
trical connection of Rc to Rt.
Consi~ering the structure so far, it will be seen
that a functioning semiconductor pressure transducer
assembly can be had by providing electrical contact to
~he contact points 16 in the areas where the Silicon-
dioxide has been removed allowing the measure of the
resistance changes across Rc; R'c; Rt and R'~. However,
as was mentioned in the prior art description surface
charge build-up may cause either depletion or accumula-
tion whlch would tnterfere with the steady state conti-
nuous operation of the transducer assembly 10 mounted
into a pressure transmitter. To allevlate this problem;
a coating of Chrome material 18 having a thickness be-
tween 750 to 1000 Angstroms is deposited over the resis-
tors Rc; ~'~; Rt; R't followed by a coat of Gold material e.
20 having a thickness of approximat-ely 5000 Angstroms
over thé Chrome material. Thle Chrome 18 material is de-
posited first since the Gold 20 does nat adhere as easily
to the N-type Silicon 12 and Silicon-oxide 14 whereas
Chrome 18 does. On the other hand, Gold 20 adheres very
well to a Chrome 18 coating and since it is the Gold 20
coating that is desirable for corrosion resistance and
high electrical conductivity the Chrome 18 coating is
used as an intermediate layer. The temperature compen-
sation resistor Rk is not coated with the Chrome 18
Gold 20 shielding layer since it does not come into
play in the ac~ual pressure measurement as do the resis-
tors Rc and Rt. ~or convenience in fabrication, contact
to the diffused resistors Rc; R'c; Rtg R't is made with
the same Chrome-Gold coating as is used for the shie`ld,
leads being extended across the surface of the Silicon-
dioxide to large pads for attachment of wires from an
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external source.
In mounting the transducer assembly 10 in a
pressure transmitter, the transducer assembly 10 is
usually bonded to a hollow support structure 17 which
allows pressure to be applied to the resistors Rc; R'c,
Rt; R't- To facilitate the alignment of the transducer
assembly 10 to this support structure 17 a ser;es of
alignment rlngs ?2 are formed on the face of the
transducer assembly 10 defining the inner circum-
ference 19 of the support structure 17 and requiring
the alignmen$ of this circumference defined by the
rings 22 with the actual circumference 19 of the
suppor~ structure 17.
Referring now to Fig. 3, it will be seen that
when the transducer assembly 10 is mounted in a pres-
sure transmitter the following electrical connections
are made to only the sensing resistors Rc and Rt as
well as to the Chrome 18 Gold 20 shielding layer
coverlng the sensing resistors Rc and Rt. In the
present embodiment only the resistor in compression Rc
is used in c~njunction with its adjoin;ng resistor in
tension Rt. A DC voltage source of approximately 10
volts is applied across the series connected resistors
Rc and Rt and the output of the res;stor ;n tension Rt
is monitored. As the pressure applied to the trans-
ducer assembly 10 is varied, the resistance of the
resistors Rc and Rt also varies in a known ~anner there
by allowing the voltage and/or current variation across
the resistor Rt to provide the indication of pressure.
As is seen, the Chrome 18 Gold 20 shields covering the
resistors Rc and Rt are electrically connected by a
jumper wire 24 with the two shields then being con-
nected by a connection 26 to the posit;ve side of the
10 volt DC power source. The connection 26 in the
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F~g. 1 embodiment is actual'ly one of the runners 20
going to the external c~rcumference pad for external
wire ronnection. This çonnection of the Gold 20
5 Chrome 18 shield to the same power source as is power-
ing the resistors Rc and Rt seems to hold the shield
at a fixed potential w;th respect to the base and pre-
vents the accumulation of any surface charge over the
res;stors Pc and Rt which would change the~r effective
values and the output from the sensor in the manner
previously described. As a consequence, no ch~nging
image charges from any surface charge is possible since
any excess charge tending to induce a shield voltage
different than the 10 volt measuring circuit voltage
lS would be drained off into the voltage source. As a re-
sult the previously discussed drift problems normally
assoc~ated with such sensing resistors subject to sur-
face charge accumulation ;s effectively eliminated.
Next referring 'back to Figs. 1 and 2, a clear
understanding of the p~essure transducer assembly 10
will be had after the following d~scussion of the
method 'whereby the assembly 10 is manufactured.
As mentioned earlier, P-type resistors are dif-
fused into the N-type base to form resistors R~; R'c;
Rt~ ~'t- As a consequence of this diffusion process,
a coating of Silicon-dioxide is formed on the entire
top surface 2~ of the N-type Silicon-material 12. To
provide electrical contacts f'or the resistors Rc; R'c,
Rt; R'ti and Rk, a series o~ points 16 is etched away
using known etching techniqures extending down to the
P-type Silicon material forming the piezoresistors Rc;
R'c; R~; R't as well as the temperature compensating
resistor Rk to provide electrical contacts therefore.
The Silicon-dioxide material is also etched away in
the area extending from the circumference 30 all the
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way to the end of the sides 32 of the'pressure trans-
ducer assembly 10. This etched away area provides a
contact area used to easily connect the Chrome-Go1d
5 shield 18, 20 extending over the resistor Rt with the
Chrome-Gold shield 18, 20,extending over the resistor
Rc by connecting the jumper ~J;re ?4 between the area
" outside of the circumference 30 and the shield 18, ?0
over the resistor Rc. It will be noted that the shield
10 18~ 70 over the resistor,~ will be connected to the
etched area extending beyond the circumference 30
through runners 34 formed during the latter described
, process. To form the surface shield over the resistors
, Rc, R'c, Rt; R't a coating 18 of Chrome is vacuum~
15 evaporated on the entire surface of the base 12 includ-
', ing the etched and unetched portions. This well-known
vacuum-evaporation process is cont~nued unt~l the Chrome
coatlng 18 ls deposited to a thickness of between 750 to
1000 Angstroms. Chrome ls used since lt has excellent
adhering qualities to the N-type Sil~con material as
well as the Silicon-dioxide m,aterial as well as provid-
~ng the' desired amount of electrical conductivity. A
5000 Angstrom layer of Gold film 20 is next vacuum-
evaporated on to the entire Chrome film 18. Gold pro-
vides excellent adherence to IChrome as well as eorro-
sion resistance and excellent electrical conductiYity.
The reason that Gold 20 is not deposited directly on to
the N-type Silicon and the Silicon-dioxide surface is
that Gold although desirable for purposes of corrosion
resistance and electrical conductivity provides rela-
tively poor adherence to the forenamed surfaces while
providing excellent adherence to the Chrome surface 18.
Next, photoresist material is applied to the entire
Gold surface 20 and the photoresist covered surface is
then exposed through a mask which allows the areas
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shown as 209 and 22 in Fig. 1 to be exposed while
masking the areas shown as 14 in Fig. 1. The mask is
then removed and the photoresist material is developed
in a known manner which removes the unexposed photo-
resist material ~rom the a,reas designated as 14 in
Fig. 1. Next, known etching materials are used to
etch away the Gold in areas which are not protected by
the photoresist material followed by etching away the
Chrome from these same areas not protected by photo-
resist materialO Separate etchan~ materials are used
to individually etch away the Gold and the Chrome
sînce a universal etchant effective in simultaneously
; removing both of these materials has not been found
to be effective. The forego~ng steps leave a finished
transducer assembly 10 as shown in Figs, 1 and 2.
Although a topcoating could be now placed over the
entire'assembly, the need for such has not been found
to be necessary.
Certa~n modifica~ions and improvements will
occur to those skilled in the art upon reading this
speciFication. It will be understood that all such
improvements and modifications have been del eted
~ herein for the sake oF conciseness and readability but
- 25 are properly covered within the scope of the following
claims.
