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A novel pressure sensor structure for integrated sensors
Sensors and Actuators, A21-A23 (1990) 62-64
62
A Novel Pressure Sensor Structure for Integrated Sensors YAOLING
WANG,
LITIAN
LIU, XINYU ZHENG
and ZHIJIAN
LI
Instatuteof Mcroelectronrcs, Tmghua Unwersaty,Bevmng(Chum)
Abstract
A novel pressure sensor ‘slhcon box’ structure IS proposed, fabtrcated by one-sided processmg technology Tests and analyses show that, while the excellent performance of smgle crystal slhcon diaphragm sensors 1s maintained, the new techmque has advantages for both discrete and mtegrated pressure sensors of high yield, good umforrmty, low cost and feaslhhty of mass production This structure also provides some reduction of thermal stress caused by the rmsmatch between the chp and the substrate occurrmg m a &con cup structure 1. lotr&clion The angle crystal slhcon diaphragm structure made by back-etchmg a &con wafer IS the most important pressure sensor structure widely used for different purposes Though improvements of this techmque have been made m different respects and the performances have been improved, a senes of problems mtrmslc to the slhcon-cup-hke structure and back-etchmg technology remam unsolved [ 1,2] Among these problems are the large ahgnment error of the pressure cavity with the plezoreslstlve elements on the diaphragm, relatively low unifornuty of pressure senatwlty, ddiiculty m protecting the sensing elements from unexpected corrosion dunng the long time backetching, comphcated equipment needed for double-sided lithography, back-etching and electrostatic bonding, large die area, low yreld and low effiaency In addition to these, the technololpcal compatlbdlty of the conventional &con dlaphragm pressure sensors wth MOS or bipolar electronics necessary for an integrated pressure sensor 1s not good One approach to overcome these drawbacks IS to measure the pressure by a polyslhcon dlaphragm sensor, where the single crystal dlaphragm and the pressure cavity are replaced by a layer of polyslhcon diaphragm and a cavity formed by etching the sacnfiaal dlelectnc layer, respectively However, because of the poor piezo092~4247/90/%3 50
resistive charactenstlcs of polyslhcon, this effort does not gve good results Reported m this paper IS our recent work on constructing novel single crystal sihcon pressure sensors with a new diaphragm structure ma& by one-sided processing technology As wdl be mentioned below, both device performances and fabncatmg feaslblhty are greatly Improved 2. stNctnrealldl+Wes&gProcedures Our new sensor 1s different from the conventional s&con-cup-like pressure sensors because of Its full slhcon box structure and processmg technology taken only on one side of the wafers Indeed, both absolute and guage pressure sensors are avadable by ths technique For the absolute pressure sensor, the pressure cavity 1s completely sealed by the silicon matenal thus forming a vacuum ‘box’ Whde for the guage pressure sensor, a pressure inlet hole 1s opened at the backside of the chtp The structures and the corresponding processing procedures are shown m Fig 1 In Fig l(a), a (100) onented wafer, the ‘substrate wafer’, IS etched by KOH or EDP on the polished surface to form a shallow cavity In the next step, a (100) orrented 2-4 Q cm n-type wafer, the ‘diaphragm wafer’, 1s directly bonded together with the substrate wafer by a S-S1 direct bonding (SDB) techNque [ 31 In ths step, the cavities on the substrate wafer are sealed by the closely Joined surfaces of the two wafers The ambient pure oxygen existing m the cavltles IS fully exhausted m the thermal process to form vacuum references The bon&d &aphragm wafer 1s then thinned and polished m a conventional way to get the desired thickness m the range 20-60 pm according to the sensmvlty reqmred (see Fig l(c)) Three points are now evident first, m the fabncatlon of the pressure cavities, there 1s no need for the troublesome double-sided hthography and back-etchmg, secondly, the depth of the cavltles 1s not cntlcal, it has nothmg to do vvlth the final sensitlvtty, and, finally, to a device manufacturer the bonded wafer shown m Fig l(c) is almost the same as the general sdlcon wafer In other words, after this 0 Elsevler Sequola/Prmted m The Netherlands
(4 Rg 1 Structures and processmg procedures of novel pressure sensors (a) the etched substrate wafer ( ) and the diaphragm wafer ( ), (b) the two wafers bonded together by SDB techmque, (c) the diaphragm wafer thmned and pohshed to the desired thickness, (d) cross section of an absolute pressure sensor, (e) cross section of a guage pressure sensor, adhered to a glass plate
stage the yield of the ducrete or mtegrated sensors fabncated on the bonded wafer should be the same as that of conventional ICs of a slmllar scale Then, plezoreslstors or other pressure sensmg elements are made on the &aphragm of the ‘s&on box’ The surrounding &con area can be used for the penphery clrcults All these are fulfilled by standard planar technology except for the know-how technique for cavity ahgnment Shown m Fig l(d) is the cross sectlon of such a discrete sensor with four plezoreslstors on Its diaphragm If the etching depth m Fig l(a) 1ssufficiently large so that the cavities can penetrate to the backside of the substrate wafer, guage pressure sensors shown m Fig l(e) will be made Figure 2 shows an SEM photograph of a cloven absolute pressure Sensor It 1s observed that the alignment of the cavity with the p~ezoresistors 1s fairly good For this reason, the sensmg elements can be put very close to the edge of the diaphragm, say, less than 10 ym away from it In a conventlonal sicon-cup-like structure, the tolerance of the space between the sensing elements and the diaphragm edge must be much larger because double-sided hthography 1s less accurate and the positron of the diaphragm edge 1s somewhat ambiguous Dunng the process of back-etching, the size of the diaphragm becomes smaller and smaller urlth increasing depth The final size of the diaphragm is determined not only by the etchmg time and rate but also by the thickness of the wafer Therefore, htgh precision m sensing element localization is an important
Wg 2 SEM photograph of a test chtp The 800 x 800 x 15 pm vacuum cavity IS cloven Three kmds of sensmg element are fabncated on the &aphragm and that of other cavmes
advantage of the new sensor It means that, with the same diaphragm sue, the new sensor w111have hgher sensltlaty and better umfornuty Another impact of the new structure on pressure sensor manufacturmg 1s that the slgmficance of the electrostatic bonding techmque 1s partially ehmmated As the cavity of the absolute pressure sensor 1s fully sealed by slhcon, the thermal stress of the chip itself IS mtrmslcally neghgble Therefore, if the chip 1sadhered to its package by some soft matcnal, the problem of thermal stress can be reduced to a rmmmum 3. Expenmental Resalts The diaphragm dimensions of the ‘&con box’ pressure sensor under test were 800 x 800 pm square and about 6Opm m thickness Four plezoreslstors which composed a Wheastone bndge were made by double lmplantatlon of B+ The average concentration of the p-type regon was 3 x 10’scmm3 For slmphaty, the sensors were adhered to DIP packages by epoxy The power supply was 5 V d c It was estimated that the total tolerance of the measurement system was 01% toO2% As expected, the sensors exhibited good hnearlty as do conventional sillcon-cup-like sensors The data gven m Table 1 show a nonhneanty error not larger than 0 13% of the full-scale output (FSO) Indeed, slmrlar lmemty was also observed m a pressure range up to 1 MPa T’he senslt.tlvltyat 26 “C was 2 157 mV/V/lO’Pa By measurement before and after applying a pressure of 0 6 MPa for 5 h, the repeatablhty was determined as 0 109% FSO These parameters are comparable with those of commercial pressure sensors On the other hand, the temperature
64
TABLE 1 Pressure response of a s111conbox sensor at different temperatures (mV) Temp (“C) 26 50 80 100
Pressure ( IO5Pa, relative to atm) 0
04
08
12
16
20
10 68 10 34 9 79 9 43
1492 14 51 1374 1326
19 26 18 12 17 78 17 12
23 61 22 86 21 70 20 96
2195 26 99 25 60 2411
32 19 31 18 29 52 2851
OUTPUT
(MV) $5: mc 1M c
30 -
0
L Rdatlra
a PmLIBare
12
I6
20
Sensrtlvlty (mV/V/lOs Pa)
RMS error (% FSO)
2 157 2083 1 974 1908
0 10 006 0 13 011
the conventional &con-cup-hke sensors by then full slhcon box structure and front-processing technology As demonstrated by our work, the sensors possess the excellent pressure sensing charactenstlcs of smgle crystal slhcon and, m the meantime, have the potential of high sensltmty, good umfornuty, small die area, high yield, low temperature coefficient and low cost Because of the good technologuzal compatiblhty of the new structure with standard planar technology, this new technique 1s expected to contnbute much to batch-fabncatlon or even mass production of mtegrated pressure sensors
(10%)
Fig 3 Pressure responses of a new pressure sensor at different temperatures
coefficient of the sensltlvlty was shghtly larger In the range between 26 “C and 100 “C, the pressure sensltlvlty had a temperature coefficient of - 1 2 x 10-3/“C, which probably comes from mappropnate adhermg matenal and would be greatly improved d the epoxy were replaced by some soft adhesive matenal Shown m Fig 3 are the pressure responses at different temperature
4. Discussion and Conclusions Novel single crystal silicon pressure sensors have been realized They are datmgmshed from
Acknowledgements
The authors would hke to thank Associate Professor Fe1 Gmpu and Zhang Zhongmm for their help m device fabncatlon and techmcal suggestions This project 1ssupported by the National Saence Foundation of Chma References I H Guclcel and D W Bums, Fabncabon techmques for Integrated sensor nucrostructurcs, Tech Digest, IEEE IEDM, (1986) 176- 179 2 S Sugyama, T Suzulu, K Kawahata, K Shnnaoka, M Talogawa and I Igarashl, Micro-diaphragm pressure sensor, Tech Dtgest, IEEE IEDM, (1986) 184- 187 3 M Shnnbo, K Furukawa, K Fukuda and K Tanzawa, Sdlcon-to-s&con duect bondmg method, J App/ Whys , 60 (1986) 2987-2989
62
A Novel Pressure Sensor Structure for Integrated Sensors YAOLING
WANG,
LITIAN
LIU, XINYU ZHENG
and ZHIJIAN
LI
Instatuteof Mcroelectronrcs, Tmghua Unwersaty,Bevmng(Chum)
Abstract
A novel pressure sensor ‘slhcon box’ structure IS proposed, fabtrcated by one-sided processmg technology Tests and analyses show that, while the excellent performance of smgle crystal slhcon diaphragm sensors 1s maintained, the new techmque has advantages for both discrete and mtegrated pressure sensors of high yield, good umforrmty, low cost and feaslhhty of mass production This structure also provides some reduction of thermal stress caused by the rmsmatch between the chp and the substrate occurrmg m a &con cup structure 1. lotr&clion The angle crystal slhcon diaphragm structure made by back-etchmg a &con wafer IS the most important pressure sensor structure widely used for different purposes Though improvements of this techmque have been made m different respects and the performances have been improved, a senes of problems mtrmslc to the slhcon-cup-hke structure and back-etchmg technology remam unsolved [ 1,2] Among these problems are the large ahgnment error of the pressure cavity with the plezoreslstlve elements on the diaphragm, relatively low unifornuty of pressure senatwlty, ddiiculty m protecting the sensing elements from unexpected corrosion dunng the long time backetching, comphcated equipment needed for double-sided lithography, back-etching and electrostatic bonding, large die area, low yreld and low effiaency In addition to these, the technololpcal compatlbdlty of the conventional &con dlaphragm pressure sensors wth MOS or bipolar electronics necessary for an integrated pressure sensor 1s not good One approach to overcome these drawbacks IS to measure the pressure by a polyslhcon dlaphragm sensor, where the single crystal dlaphragm and the pressure cavity are replaced by a layer of polyslhcon diaphragm and a cavity formed by etching the sacnfiaal dlelectnc layer, respectively However, because of the poor piezo092~4247/90/%3 50
resistive charactenstlcs of polyslhcon, this effort does not gve good results Reported m this paper IS our recent work on constructing novel single crystal sihcon pressure sensors with a new diaphragm structure ma& by one-sided processing technology As wdl be mentioned below, both device performances and fabncatmg feaslblhty are greatly Improved 2. stNctnrealldl+Wes&gProcedures Our new sensor 1s different from the conventional s&con-cup-like pressure sensors because of Its full slhcon box structure and processmg technology taken only on one side of the wafers Indeed, both absolute and guage pressure sensors are avadable by ths technique For the absolute pressure sensor, the pressure cavity 1s completely sealed by the silicon matenal thus forming a vacuum ‘box’ Whde for the guage pressure sensor, a pressure inlet hole 1s opened at the backside of the chtp The structures and the corresponding processing procedures are shown m Fig 1 In Fig l(a), a (100) onented wafer, the ‘substrate wafer’, IS etched by KOH or EDP on the polished surface to form a shallow cavity In the next step, a (100) orrented 2-4 Q cm n-type wafer, the ‘diaphragm wafer’, 1s directly bonded together with the substrate wafer by a S-S1 direct bonding (SDB) techNque [ 31 In ths step, the cavities on the substrate wafer are sealed by the closely Joined surfaces of the two wafers The ambient pure oxygen existing m the cavltles IS fully exhausted m the thermal process to form vacuum references The bon&d &aphragm wafer 1s then thinned and polished m a conventional way to get the desired thickness m the range 20-60 pm according to the sensmvlty reqmred (see Fig l(c)) Three points are now evident first, m the fabncatlon of the pressure cavities, there 1s no need for the troublesome double-sided hthography and back-etchmg, secondly, the depth of the cavltles 1s not cntlcal, it has nothmg to do vvlth the final sensitlvtty, and, finally, to a device manufacturer the bonded wafer shown m Fig l(c) is almost the same as the general sdlcon wafer In other words, after this 0 Elsevler Sequola/Prmted m The Netherlands
(4 Rg 1 Structures and processmg procedures of novel pressure sensors (a) the etched substrate wafer ( ) and the diaphragm wafer ( ), (b) the two wafers bonded together by SDB techmque, (c) the diaphragm wafer thmned and pohshed to the desired thickness, (d) cross section of an absolute pressure sensor, (e) cross section of a guage pressure sensor, adhered to a glass plate
stage the yield of the ducrete or mtegrated sensors fabncated on the bonded wafer should be the same as that of conventional ICs of a slmllar scale Then, plezoreslstors or other pressure sensmg elements are made on the &aphragm of the ‘s&on box’ The surrounding &con area can be used for the penphery clrcults All these are fulfilled by standard planar technology except for the know-how technique for cavity ahgnment Shown m Fig l(d) is the cross sectlon of such a discrete sensor with four plezoreslstors on Its diaphragm If the etching depth m Fig l(a) 1ssufficiently large so that the cavities can penetrate to the backside of the substrate wafer, guage pressure sensors shown m Fig l(e) will be made Figure 2 shows an SEM photograph of a cloven absolute pressure Sensor It 1s observed that the alignment of the cavity with the p~ezoresistors 1s fairly good For this reason, the sensmg elements can be put very close to the edge of the diaphragm, say, less than 10 ym away from it In a conventlonal sicon-cup-like structure, the tolerance of the space between the sensing elements and the diaphragm edge must be much larger because double-sided hthography 1s less accurate and the positron of the diaphragm edge 1s somewhat ambiguous Dunng the process of back-etching, the size of the diaphragm becomes smaller and smaller urlth increasing depth The final size of the diaphragm is determined not only by the etchmg time and rate but also by the thickness of the wafer Therefore, htgh precision m sensing element localization is an important
Wg 2 SEM photograph of a test chtp The 800 x 800 x 15 pm vacuum cavity IS cloven Three kmds of sensmg element are fabncated on the &aphragm and that of other cavmes
advantage of the new sensor It means that, with the same diaphragm sue, the new sensor w111have hgher sensltlaty and better umfornuty Another impact of the new structure on pressure sensor manufacturmg 1s that the slgmficance of the electrostatic bonding techmque 1s partially ehmmated As the cavity of the absolute pressure sensor 1s fully sealed by slhcon, the thermal stress of the chip itself IS mtrmslcally neghgble Therefore, if the chip 1sadhered to its package by some soft matcnal, the problem of thermal stress can be reduced to a rmmmum 3. Expenmental Resalts The diaphragm dimensions of the ‘&con box’ pressure sensor under test were 800 x 800 pm square and about 6Opm m thickness Four plezoreslstors which composed a Wheastone bndge were made by double lmplantatlon of B+ The average concentration of the p-type regon was 3 x 10’scmm3 For slmphaty, the sensors were adhered to DIP packages by epoxy The power supply was 5 V d c It was estimated that the total tolerance of the measurement system was 01% toO2% As expected, the sensors exhibited good hnearlty as do conventional sillcon-cup-like sensors The data gven m Table 1 show a nonhneanty error not larger than 0 13% of the full-scale output (FSO) Indeed, slmrlar lmemty was also observed m a pressure range up to 1 MPa T’he senslt.tlvltyat 26 “C was 2 157 mV/V/lO’Pa By measurement before and after applying a pressure of 0 6 MPa for 5 h, the repeatablhty was determined as 0 109% FSO These parameters are comparable with those of commercial pressure sensors On the other hand, the temperature
64
TABLE 1 Pressure response of a s111conbox sensor at different temperatures (mV) Temp (“C) 26 50 80 100
Pressure ( IO5Pa, relative to atm) 0
04
08
12
16
20
10 68 10 34 9 79 9 43
1492 14 51 1374 1326
19 26 18 12 17 78 17 12
23 61 22 86 21 70 20 96
2195 26 99 25 60 2411
32 19 31 18 29 52 2851
OUTPUT
(MV) $5: mc 1M c
30 -
0
L Rdatlra
a PmLIBare
12
I6
20
Sensrtlvlty (mV/V/lOs Pa)
RMS error (% FSO)
2 157 2083 1 974 1908
0 10 006 0 13 011
the conventional &con-cup-hke sensors by then full slhcon box structure and front-processing technology As demonstrated by our work, the sensors possess the excellent pressure sensing charactenstlcs of smgle crystal slhcon and, m the meantime, have the potential of high sensltmty, good umfornuty, small die area, high yield, low temperature coefficient and low cost Because of the good technologuzal compatiblhty of the new structure with standard planar technology, this new technique 1s expected to contnbute much to batch-fabncatlon or even mass production of mtegrated pressure sensors
(10%)
Fig 3 Pressure responses of a new pressure sensor at different temperatures
coefficient of the sensltlvlty was shghtly larger In the range between 26 “C and 100 “C, the pressure sensltlvlty had a temperature coefficient of - 1 2 x 10-3/“C, which probably comes from mappropnate adhermg matenal and would be greatly improved d the epoxy were replaced by some soft adhesive matenal Shown m Fig 3 are the pressure responses at different temperature
4. Discussion and Conclusions Novel single crystal silicon pressure sensors have been realized They are datmgmshed from
Acknowledgements
The authors would hke to thank Associate Professor Fe1 Gmpu and Zhang Zhongmm for their help m device fabncatlon and techmcal suggestions This project 1ssupported by the National Saence Foundation of Chma References I H Guclcel and D W Bums, Fabncabon techmques for Integrated sensor nucrostructurcs, Tech Digest, IEEE IEDM, (1986) 176- 179 2 S Sugyama, T Suzulu, K Kawahata, K Shnnaoka, M Talogawa and I Igarashl, Micro-diaphragm pressure sensor, Tech Dtgest, IEEE IEDM, (1986) 184- 187 3 M Shnnbo, K Furukawa, K Fukuda and K Tanzawa, Sdlcon-to-s&con duect bondmg method, J App/ Whys , 60 (1986) 2987-2989