CA1170077A - Semiconductor pressure sensor with slanted resistors - Google Patents
Semiconductor pressure sensor with slanted resistorsInfo
- Publication number
- CA1170077A CA1170077A CA000411049A CA411049A CA1170077A CA 1170077 A CA1170077 A CA 1170077A CA 000411049 A CA000411049 A CA 000411049A CA 411049 A CA411049 A CA 411049A CA 1170077 A CA1170077 A CA 1170077A
- Authority
- CA
- Canada
- Prior art keywords
- strain
- diaphragm
- resistor
- ridge
- resistors
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
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- Measuring Fluid Pressure (AREA)
- Pressure Sensors (AREA)
Abstract
D-4,935 C-3301 SEMICONDUCTOR PRESSURE SENSOR
WITH SLANTED RESISTORS
Abstract of the Disclosure A diaphragm is formed in a silicon chip by etching a rectangular cavity in one side thereof and piezoresistive resistors are formed in the other surface of the chip to sense stress changes on the diaphragm due to pressure changes. At least one resistor is placed along the edge of the diaphragm where a sharp stress peak occurs. To avoid the problem of inaccurate placement of the resistor relative to the peak, the resistor is slanted with respect to the stress ridge at a small angle of 10°
to 20°. This makes the resistor placement and cavity alignment much less critical thereby assuring greater uniformity of response from one sensor to another at the expense of signal size for a given pressure change on the device.
WITH SLANTED RESISTORS
Abstract of the Disclosure A diaphragm is formed in a silicon chip by etching a rectangular cavity in one side thereof and piezoresistive resistors are formed in the other surface of the chip to sense stress changes on the diaphragm due to pressure changes. At least one resistor is placed along the edge of the diaphragm where a sharp stress peak occurs. To avoid the problem of inaccurate placement of the resistor relative to the peak, the resistor is slanted with respect to the stress ridge at a small angle of 10°
to 20°. This makes the resistor placement and cavity alignment much less critical thereby assuring greater uniformity of response from one sensor to another at the expense of signal size for a given pressure change on the device.
Description
7(~
D-4,935 C-3301 SEMICONDUCTOR PRESSURE SENSOR
WITH SLANTED RESISTORS
This invention relates to semiconductor pressure sensors and more particularly to such sensors having a pressure responsive diaphragm with piezo-resistive resistors thereon for sensing pressureinduced strain in the diaphragm. It is well known to fabricate a pressure sensor by etchiny a cavity in one side of a silicon ~-- chip to form a diaphragm and then form on the dia-phragm surface several strain sensing resistors for connection into a strain measuring bridge. Ideally at least one re~sistor lies along a high strain peak or ridge which runs parallel to the edge of the dia-phragm and very close to the edge. A resistor thus placed takes maximum advantage of the strain pattern on the diaphraym to obtain maximum signal change for a given pressure change. The peak of the strain pattern is so sharp, however, that alignment of the resistor placement with respect to the cavity edge is ~'O critical. That is, if the resistor is placed a relatively short distance from the strain peak, it can lie in a relatively low strain region. Thus, small variances from one pressure sensor to another result in unacceptably large variances in resistor response to pressure. Thus, low yields of pressure sensors is the consequence of poor uniformity of response. It is desired, however~ to achieve a high yield of pressure sensors so that they can be manu-factured economically in high volumes.
It is, therefore, an object of this i~vention to provide a semiconductor pressure sensor having a resistor configuration which takes advantage of the high strain ridge adjacent the diaphragm edye and yet does not have a critical placement.
~ 3~
It is another object of the invention to provide pressure sensor st.ructure which is conducive to more uniform response and thus higher yields during production than is obtained with prior art devices.
This invention is carried out by providing a semiconductor chip containing a cavity defining a diaphragm such th~t a high strain ridge occurs along an edge oE the diaphragm, a plurality of strain sensing resistors on the diaphragm, at least one of the resistors being elongated and slanted across the high stress ridge at a small angle to the ridge so that if there are small variations in resistor placement relative to the ri~dge, the resisto.r still will cross the:peak and only minor variations in resistor response to diaphragm strain will xesult.
;~ The above and other advantages will be : made more apparent from the:following specification taken in conjunction with the accompanying:drawings wherein like reference numeraIs refer to like parts and wherein:
:~ : : Figure 1: i6 a top view of a pressure sensing semiconductor chip with resistor placement according to the~prior art. : ~
Figure 2 lS a cross sectional view oE the pressure sensor of Figure 1. ~ ~
: Figure 3 is a curve ilIustrating the pattern:
of pressure induced strain along the surface of the ~emiconductor pre5sure sensor.
Figure 4 is a plan view of a portion of a pressure sensor With resistor configuration according to the invention, and Figure 5 is a diagram illustrating the - geometrical relationships of the semiconductor pressure sen50r according to the invention.
~:17(~
Figures 1 and 2 illustrate a semiconductor pressure sensor accordiny to the prior art which comprises a silicon chip 10 having a cavity 12 etched therein on one side to form a diaphragm 14 of about l mil tnickness so that it readily responds to changes in differential pressure applied across the diaphragm.
The edge 16 of the diaphragm is generally rectangular in shape and encompasses an area of typically 30 or 40 mils on each side. According to the usual prac-tice, a pair of resistors 18 extend in a directiongenerally toward the center of the diaphragm to measure the diaphragm strain in the radial direction and a second pair of resistors 20 extend parallel to the diaphragm edge ~o measure the strain in the so-called tangential direction. The resistors com~
prise thin elongated~strips about l mil wide and sufficient length, say 10 to 15 mils, to provide a~desired resistance and are connected to contact areas 22 at either end which have at least a portion 29 lying beyond the diaphragm edge 16. The resistors 18 are each formed in two parallel segments connected by contact areas 24 on the diaphragm. The resistors and contact areas together comprise island-like regions of one conductivity type in a silicon surface of the opposite conductivity type formed by ion - implantation or diffusion.
Figure 3 diagrammatically illustrates the strain measured at various points along the sensor chip beginning at a point outside-the diaphragm about 9 mils from the edge and then proceeding toward khe diaphragm center. That is, the diaphragm edge 16 occurs at 9 mils on the diaphragm. Thus, it is seen that a sharp strain peak occurs just inside the diaphragm edge about 1 r.lil from the edge. The peak 26 actually forms a ridge since it extends along the 7~
diaphragm edge and parallel thereto as shown by a broken line 26 in Figure l. Ideally the resistor 20 extends along the high strain ridye 26 in order to obtain a maximum signal response for any change in pressure. The accuracy of placement of the resistor 20 right on the ridge 26 depends not only on the accuracy of registration of various masks used in resistor formation and in the cavity etching step but also in the control of the etching. The : lO actual size of the cavi.ty 12 and thus the position of the edyes 16 can vary somewhat accordiny to how well the etching..is controlled. Thus, the distance o~ the resistor 20 from the edge 16 may vary. As : can be seen from the curve-of Figure 3, if the :~ 15 resistor placement misses-the strain xidge 26 by a:
: small amount, say 1 mil, then the:resistor sensitivity to dlaphragm pressure ~ill be dramatically reduced : . and may very well be unacceptable, particularly where t is desired to fabricate a large number of sensors having somewhat~.uniform response characteristics.
As illustrated in Figure 4,~ the criticality o~ resistox placement is very much lessened by slanting the:resistor 30 at a small angle with ~ respect to the diaphragm edge 16 and there~ore to : 25 the~strain ridge 26. :This allows for some variation in resistor placement with respect to the edge 16 by keeping at least.a~portion:of the resistor 30 in :~ the high strain zone. Since some of the resistor -will be in relatively low strain regions of the curve of Figure 3, the overall signal change for a glven pressure change will be less than the ideal case : of Figure 1. However, the uniformity of signal .
change from one sensor to another wi.ll be much greater so that the overall yield of the sensor fabrication process will be greater. The angle between the resistor 30 and the strain ridge 26 should be as small as is practical in order to maximize the signal change and the particular angle depends upon the accuracy with which the etching and other geometry controlling steps can be con-trolled~ The yeometrical relationships are exem-plified in Figure 5 where the nominal cavity edge 16 determines the position of the nominal strain ridge 26 relative to the ixed resistor 30 which is --- 10 centered on the nominal ridge 26 and is positioned at an angle 4 thereto. Due to manufacturing tolerances, the actual cavity edge and strain ridge position will vary from the nominal position by a distance of plus or minus "a". The lines 26' and 26'~' spaced a distance "a" on either side of the nominal strain ridge 26 indicate-the acceptable limits of ridge position from sensor to sensor to define a tolerance~band. The ends "b" and "c" of the slanted resistor 30 are positioned just outside the strain ridge limits 26' and 26 " so that wherever the strain ridge occurs within those limits, the resistor 30 will traverse the tolerance band including the strain ridge 26~. The actual size of the angle depends upon the length of the resistor 30 and the production tolerance "a" which determines the posi-tion o~ the resistor 30 relative to the strain ridge 26. Thus, for a given tolerance "a", the angle ~ will be smaller for longer resistors 30 and it will be larger for shorter resistors 30 By keeping the tolerance "a" as small as possible the angle Q is al~o kept small to optimize the resistor response to strain. Where the tolerance "a" is 1.5 mils and the resistor 30 length is 10.4 mils or 14.5 mils, for example, the angle ~ is about 17 or 12 respectively. Depending on the tolerance afforded by the fabrication process and the design length of the resistor, the angle ~ may, as a practical matter fall ~ithin the range of 10 to 20. Even a small angle 0 may effect a significant increase in yield, say a 10% improvement. Since the actual s~rain ridge 26 varies in its position relative to the resistor 30 and the strain curve is unsymmetrical, the resistor response to strain will also vary but by an amount which is small compared to the corr sponding variations occurring in the prior art configuration of Figure 1. That is, the slanted resistor, since it always covers the strain peak 26 as well~as lower s-train values, provides a relatively uniform response to strain within acceptable manufacturing tclerances even though the signal size i5 less than that of the resistor 20 of the Figure 1 configuration.
It will thus be seen that according to the present invention a semiconductor pressure sensor is~provided which has a greatér ease of manufacturing to produce sensors sufficiently uniform in response to pressure to provide a high yield of sensors during manufacture thereof.
D-4,935 C-3301 SEMICONDUCTOR PRESSURE SENSOR
WITH SLANTED RESISTORS
This invention relates to semiconductor pressure sensors and more particularly to such sensors having a pressure responsive diaphragm with piezo-resistive resistors thereon for sensing pressureinduced strain in the diaphragm. It is well known to fabricate a pressure sensor by etchiny a cavity in one side of a silicon ~-- chip to form a diaphragm and then form on the dia-phragm surface several strain sensing resistors for connection into a strain measuring bridge. Ideally at least one re~sistor lies along a high strain peak or ridge which runs parallel to the edge of the dia-phragm and very close to the edge. A resistor thus placed takes maximum advantage of the strain pattern on the diaphraym to obtain maximum signal change for a given pressure change. The peak of the strain pattern is so sharp, however, that alignment of the resistor placement with respect to the cavity edge is ~'O critical. That is, if the resistor is placed a relatively short distance from the strain peak, it can lie in a relatively low strain region. Thus, small variances from one pressure sensor to another result in unacceptably large variances in resistor response to pressure. Thus, low yields of pressure sensors is the consequence of poor uniformity of response. It is desired, however~ to achieve a high yield of pressure sensors so that they can be manu-factured economically in high volumes.
It is, therefore, an object of this i~vention to provide a semiconductor pressure sensor having a resistor configuration which takes advantage of the high strain ridge adjacent the diaphragm edye and yet does not have a critical placement.
~ 3~
It is another object of the invention to provide pressure sensor st.ructure which is conducive to more uniform response and thus higher yields during production than is obtained with prior art devices.
This invention is carried out by providing a semiconductor chip containing a cavity defining a diaphragm such th~t a high strain ridge occurs along an edge oE the diaphragm, a plurality of strain sensing resistors on the diaphragm, at least one of the resistors being elongated and slanted across the high stress ridge at a small angle to the ridge so that if there are small variations in resistor placement relative to the ri~dge, the resisto.r still will cross the:peak and only minor variations in resistor response to diaphragm strain will xesult.
;~ The above and other advantages will be : made more apparent from the:following specification taken in conjunction with the accompanying:drawings wherein like reference numeraIs refer to like parts and wherein:
:~ : : Figure 1: i6 a top view of a pressure sensing semiconductor chip with resistor placement according to the~prior art. : ~
Figure 2 lS a cross sectional view oE the pressure sensor of Figure 1. ~ ~
: Figure 3 is a curve ilIustrating the pattern:
of pressure induced strain along the surface of the ~emiconductor pre5sure sensor.
Figure 4 is a plan view of a portion of a pressure sensor With resistor configuration according to the invention, and Figure 5 is a diagram illustrating the - geometrical relationships of the semiconductor pressure sen50r according to the invention.
~:17(~
Figures 1 and 2 illustrate a semiconductor pressure sensor accordiny to the prior art which comprises a silicon chip 10 having a cavity 12 etched therein on one side to form a diaphragm 14 of about l mil tnickness so that it readily responds to changes in differential pressure applied across the diaphragm.
The edge 16 of the diaphragm is generally rectangular in shape and encompasses an area of typically 30 or 40 mils on each side. According to the usual prac-tice, a pair of resistors 18 extend in a directiongenerally toward the center of the diaphragm to measure the diaphragm strain in the radial direction and a second pair of resistors 20 extend parallel to the diaphragm edge ~o measure the strain in the so-called tangential direction. The resistors com~
prise thin elongated~strips about l mil wide and sufficient length, say 10 to 15 mils, to provide a~desired resistance and are connected to contact areas 22 at either end which have at least a portion 29 lying beyond the diaphragm edge 16. The resistors 18 are each formed in two parallel segments connected by contact areas 24 on the diaphragm. The resistors and contact areas together comprise island-like regions of one conductivity type in a silicon surface of the opposite conductivity type formed by ion - implantation or diffusion.
Figure 3 diagrammatically illustrates the strain measured at various points along the sensor chip beginning at a point outside-the diaphragm about 9 mils from the edge and then proceeding toward khe diaphragm center. That is, the diaphragm edge 16 occurs at 9 mils on the diaphragm. Thus, it is seen that a sharp strain peak occurs just inside the diaphragm edge about 1 r.lil from the edge. The peak 26 actually forms a ridge since it extends along the 7~
diaphragm edge and parallel thereto as shown by a broken line 26 in Figure l. Ideally the resistor 20 extends along the high strain ridye 26 in order to obtain a maximum signal response for any change in pressure. The accuracy of placement of the resistor 20 right on the ridge 26 depends not only on the accuracy of registration of various masks used in resistor formation and in the cavity etching step but also in the control of the etching. The : lO actual size of the cavi.ty 12 and thus the position of the edyes 16 can vary somewhat accordiny to how well the etching..is controlled. Thus, the distance o~ the resistor 20 from the edge 16 may vary. As : can be seen from the curve-of Figure 3, if the :~ 15 resistor placement misses-the strain xidge 26 by a:
: small amount, say 1 mil, then the:resistor sensitivity to dlaphragm pressure ~ill be dramatically reduced : . and may very well be unacceptable, particularly where t is desired to fabricate a large number of sensors having somewhat~.uniform response characteristics.
As illustrated in Figure 4,~ the criticality o~ resistox placement is very much lessened by slanting the:resistor 30 at a small angle with ~ respect to the diaphragm edge 16 and there~ore to : 25 the~strain ridge 26. :This allows for some variation in resistor placement with respect to the edge 16 by keeping at least.a~portion:of the resistor 30 in :~ the high strain zone. Since some of the resistor -will be in relatively low strain regions of the curve of Figure 3, the overall signal change for a glven pressure change will be less than the ideal case : of Figure 1. However, the uniformity of signal .
change from one sensor to another wi.ll be much greater so that the overall yield of the sensor fabrication process will be greater. The angle between the resistor 30 and the strain ridge 26 should be as small as is practical in order to maximize the signal change and the particular angle depends upon the accuracy with which the etching and other geometry controlling steps can be con-trolled~ The yeometrical relationships are exem-plified in Figure 5 where the nominal cavity edge 16 determines the position of the nominal strain ridge 26 relative to the ixed resistor 30 which is --- 10 centered on the nominal ridge 26 and is positioned at an angle 4 thereto. Due to manufacturing tolerances, the actual cavity edge and strain ridge position will vary from the nominal position by a distance of plus or minus "a". The lines 26' and 26'~' spaced a distance "a" on either side of the nominal strain ridge 26 indicate-the acceptable limits of ridge position from sensor to sensor to define a tolerance~band. The ends "b" and "c" of the slanted resistor 30 are positioned just outside the strain ridge limits 26' and 26 " so that wherever the strain ridge occurs within those limits, the resistor 30 will traverse the tolerance band including the strain ridge 26~. The actual size of the angle depends upon the length of the resistor 30 and the production tolerance "a" which determines the posi-tion o~ the resistor 30 relative to the strain ridge 26. Thus, for a given tolerance "a", the angle ~ will be smaller for longer resistors 30 and it will be larger for shorter resistors 30 By keeping the tolerance "a" as small as possible the angle Q is al~o kept small to optimize the resistor response to strain. Where the tolerance "a" is 1.5 mils and the resistor 30 length is 10.4 mils or 14.5 mils, for example, the angle ~ is about 17 or 12 respectively. Depending on the tolerance afforded by the fabrication process and the design length of the resistor, the angle ~ may, as a practical matter fall ~ithin the range of 10 to 20. Even a small angle 0 may effect a significant increase in yield, say a 10% improvement. Since the actual s~rain ridge 26 varies in its position relative to the resistor 30 and the strain curve is unsymmetrical, the resistor response to strain will also vary but by an amount which is small compared to the corr sponding variations occurring in the prior art configuration of Figure 1. That is, the slanted resistor, since it always covers the strain peak 26 as well~as lower s-train values, provides a relatively uniform response to strain within acceptable manufacturing tclerances even though the signal size i5 less than that of the resistor 20 of the Figure 1 configuration.
It will thus be seen that according to the present invention a semiconductor pressure sensor is~provided which has a greatér ease of manufacturing to produce sensors sufficiently uniform in response to pressure to provide a high yield of sensors during manufacture thereof.
Claims (4)
1. A pressure sensing element comprising a semiconductor chip containing a cavity defining a diaphragm subject to deflection in response to pressure, the diaphragm having an elongated high strain zone adjacent and parallel to the diaphragm edge wherein upon-diaphragm deflection a localized high strain peak is produced in the high strain zone, and a plurality of piezoresistive resistors in the chip for sensing strain in the diaphragm, at least one of said resistors for measuring the strain in the strain zone parallel to the diaphragm edge, said one resistor being elongated and slanted across the high strain zone with the longitudinal resistor axis-disposed at an angle in the approximate range of 10° to 20° with respect to the said zone, so that small variations in resistor placement relative to the strain zone result in only minor variations in resistor response to diaphragm strain.
2. A pressure sensing element comprising a semiconductor chip containing a cavity defining a diaphragm subject to deflection in response to pressure, the diaphragm, upon deflection, having an elongated high strain ridge adjacent and parallel to the diaphragm edge, and a plurality of piezoresistive resistors in the chip for sensing strain in the diaphragm, at least one of said resistors for measuring the strain in the strain ridge parallel to the diaphragm edge, said one resistor being elongated and slanted across the high strain ridge with the longitudinal resistor axis disposed at an angle in the approximate range of 10° to 20° with respect to the said ridge, so that small variations in resistor placement relative to the strain ridge result in only minor variations in resistor response to diaphragm strain.
3. A pressure sensing element comprising a semiconductor chip containing a cavity defining a diaphragm subject to deflection in response to pressure, the diaphragm, upon deflection, having an elongated high strain ridge adjacent and parallel to the diaphragm edge wherein due to manu-facturing tolerances the strain ridge occurs anywhere within a predetermined tolerance band defined by limits parallel to the diaphragm edge, and a plurality of piezoresistive resistors in the chip for sensing strain in the diaphragm, at least one of said resistors for measuring the strain in the strain ridge parallel to the diaphragm edge, said one resistor being elongated and slanted across the high strain ridge and traversing the tolerance hand with both ends of the resistor dis-posed just outside the limits so that small variations in resistor placement relative to the strain ridge result in only minor variations in resistor response to diaphragm strain.
4. A pressure sensing element comprising a semiconductor chip containing a cavity defining a diaphragm subject to deflection in response to pressure, the diaphragm, upon deflection, having an elongated high strain ridge adjacent and parallel to the diaphragm edge, and a plurality of piezoresistive resistors in the chip for sensing strain in the diaphragm, at least one of said resistors for measuring the strain in the strain ridge parallel to the diaphragm edge, said one resistor being elongated and slanted across the high strain ridge with the longitudinal resistor axis disposed at an angle large enough to significantly increase yields during manufacture thereof compared to resistors parallel to the strain ridge and small enough to have good resistor response to diaphragm strain, so that small variations in resistor placement relative to the strain ridge result in only minor variations in resistor response to diaphragm strain.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000411049A CA1170077A (en) | 1982-09-09 | 1982-09-09 | Semiconductor pressure sensor with slanted resistors |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000411049A CA1170077A (en) | 1982-09-09 | 1982-09-09 | Semiconductor pressure sensor with slanted resistors |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1170077A true CA1170077A (en) | 1984-07-03 |
Family
ID=4123549
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000411049A Expired CA1170077A (en) | 1982-09-09 | 1982-09-09 | Semiconductor pressure sensor with slanted resistors |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1170077A (en) |
-
1982
- 1982-09-09 CA CA000411049A patent/CA1170077A/en not_active Expired
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| MKEX | Expiry |