A Novel MEMS Pressure Sensor with MOSFET on Chip Zhao-Hua Zhang *, Yan-Hong Zhang, Li-Tian Liu, Tian-Ling Ren Tsinghua National Laboratory for Information Science and Technology Institute of Microelectronics, Tsinghua University Beijing 100084, China [email protected] Abstract—A novel MOSFET pressure sensor was proposed Figure 1. Two PMOSFET’s and two piezoresistors are based on the MOSFET stress sensitive phenomenon, in which connected to form a Wheatstone bridge. To obtain the the source-drain current changes with the stress in channel maximum sensitivity, these components are placed near the region. Two MOSFET’s and two piezoresistors were employed four sides of the silicon diaphragm, which are the high stress to form a Wheatstone bridge served as sensitive unit in the regions. The MOSFET’s has the same structure parameter novel sensor. Compared with the traditional piezoresistive W/L, same threshold voltage VT and gate-source voltage VGS pressure sensor, this MOSFET sensor’s sensitivity is improved (equal to VG-Vdd). They are designed to work in the significantly, meanwhile the power consumption can be saturation region. The piezoresistors also have the same decreased. The fabrication of the novel pressure sensor is low- resistance R . cost and compatible with standard IC process. It shows the 0 great promising application of MOSFET-bridge-circuit structure for the high performance pressure sensor. This kind of MEMS pressure sensor with signal process circuit on the same chip can be used in positive or negative Tire Pressure Monitoring System (TPMS) which is very hot in automotive electron research field. I. INTRODUCTION Piezoresistive pressure microsensor is one kind of the most widely used pressure sensors for automotive, aerospace, biomedicine, and many other applications [1-3]. It is usually composed of a silicon membrane and a Wheatstone bridge circuit with four piezoresistors. The piezo-resistances change with the stress and therefore output the pressure information. Figure 1. (a) Schematic of novel MOSFET pressure sensor including two MOSFET and two resistors on the membrane to form a Wheatstone bridge, MOSFET also has a stress sensitive phenomenon, in (b) The MOSFET-based bridge circuit, the output voltage is Vout=Vout1- which the source current changes with the stress in channel Vout2, the voltage source is Vdd. region [4-6]. Some experimental applications e.g. MOS ring oscillator accelerometer have been reported [7]. In this paper, When there is no forced pressure, the bridge is in a novel MOSFET pressure sensor was reported, which used balance. The balanced output V0 of each arm is: two PMOSFET’s and two piezoresistors to form a bridge circuit. The structure design and operating principle were 1 W 2 VRC=⋅μ () VV −. (1) demonstrated. The fabrication process was described. 000OXGST2 p L Measurement results of the sensor’s sensitivity and power show significant improvement compared with traditional As a result, the sensor output signal V is zero. piezoresistive pressure sensor. out When a pressure is forced on the membrane, the current and piezo-resistance in each bridge arm are changed. The II. DESIGN variation of the PMOSFET current is proportional to the Based on the stress sensitive effect of MOSFET, a new change of channel mobility Δμp, computed as: MOSFET-bridge-circuit structure is designed, as shown in 1-4244-2581-5/08/$20.00 ©2008 IEEE 1564 IEEE SENSORS 2008 Conference Δ = Δμμ =πσ ⋅ + πσ ⋅ , (2) IIDS/ DS 0 l l t t where σl and σt are the parallel and vertical stress in the channel; πl and πt are the parallel and vertical channel piezoresistive coefficient, respectively. The change of piezo- resistance is also proportional to the resistor mobility change ΔμR, which can be expressed using a similar formula, as: Δ = Δμμ =πσ ⋅ + πσ ⋅ , (3) RR/ 00llttRR where σl and σt are the parallel and vertical stress in the resistor bar; πl and πt are the parallel and vertical piezoresistive coefficient, respectively. According to the different current direction placing, the bridge becomes unbalance. The μp of M1 and the μR of R2 get increased with the stress, in opposition, the μp of M2 and the μR of R1 are decreased. Then the two arms’ outputs become as: Figure 2. Process flow of the MOSFET pressure sensor based on Al-gate post-IC process and bulk silicon MEMS process. 1 W 2 VRRCVV=±Δ⋅±Δ()()μμ () −, (4) out 1,22 0pp 0 OXL GS T IV. RESULTS AND DISCUSSION therefore the sensor output is obtained as: The sensitivity and power of the MOSFET pressure sensor are deduced and measured, which are the most ⎛⎞Δμ Δμ important performance parameter for the pressure VV=+⋅2⎜⎟p R . (5) out⎜⎟μμ 0 microsensor. These parameters (using the subscript “MOS”) ⎝⎠pR00 are compared with those of the traditional piezoresistive pressure sensor (using the subscript “res”). Formula (5) shows that, Vout is proportional to the stress as well as the forced pressure. This is the operating principle A. Performance parameters of the novel MOSFET-bridge-circuit pressure microsensor. The sensor sensitivity is: III. FABRICATION Δμ VV⎛⎞p Δμ The whole fabrication was based on Al-gate post-IC S ==out2⎜⎟ +R ⋅ 0 . (6) MOS VV⎜⎟μμ process, as shown in Figure 2. First, a (100)-oriented n-type dd⎝⎠pR 0 0 dd silicon wafer (6~8Ωcm) was selected as start substrate, with SiO2 and Si3N4 layers on both sides, deposited by thermal The balanced power is computed as: oxidation and LPVCD methods. Second, the backside was etched in 33% KOH solution to form the silicon membrane. VV− ⎛⎞ V V2 =⋅dd SD0 =− SD0 dd , (7) Third, a new field SiO2 layer is thermal oxidized. Fourth, the PVMOS221 dd ⎜⎟ source and drain windows formed, and then high-dope boron R0dd0⎝⎠VR was implanted to form the source and drain. Fifth, the gate area and piezoresistor windows formed. A low-dope boron where VSD0 is the balanced source drain voltage. To ensure implantation was performed to adjust the VT and form the the PMOSFET working in the saturation region, the VSD piezoresistors. Then a thin gate SiO2 layer was thermal must meet the requirement of: oxidized. Sixth, Al layer was sputtered and wet etched to form the interconnection. At last, Si-Au/Ti-Si bonding was VVV> − >0 . (8) performed in vacuum to form the pressure referential cavity. SD SG T In all, five lithography steps were used. The whole process is low-cost and compatible with standard IC process. The output expression of traditional silicon piezoresistive pressure microsensor uses Wheatstone bridge circuit with four piezoresistors is: ΔR Δμ VV= ⋅ =R ⋅ V . (9) out ddμ dd R00R 1565 The sensor sensitivity and balanced power are: significantly. In region III (0.5 < α < 1), the sensitivity and power get both raised. However, the increase of sensitivity is V Δμ more remarkable than that of power. At the upper limit of α S =out =R , (10) (equals to 1), the MOSFET sensor’s sensitivity and power res V μ ddR 0 are raised by 300% and 100%, respectively. V 2 =dd . (11) Pres R0 To compare the performance between the MOSFET sensor and the traditional piezoresistive sensor, the same resistors and membrane size, i.e., the same stress distribution, are used. According to above computations, it is obtained that S MOS =+21α ()β , (12) S res Figure 3. The sensitivity and power dependence on α and β between MOSFET and piezoresistive sensors P MOS = 2α , (13) B. Measurement results P res The measured sensitivity and linearity error of MOSFET where pressure sensor and the reference piezoresistive sensor are shown in Figures 4-5. The sensitivity of the fabricated 2 MOSFET pressure sensor sample was 0.3mV/KPa, and the VV ()VV− αμ==−=0 SD 1 ⋅ W GS T , (14) linearity error was 0.6% FS. These parameters were better 1 RC00pOX VVdd dd2 LV dd than those of the traditional piezoresistive pressure sensor. When α is set as 0.5, the sensitivity of MOSFET sensor is improved by 145% compared with the piezoresistive sensor, ()πσ+ πσ with the same power. When α is set as 0.4, the sensitivity of β = ll ttMOS . (15) ()πσ+ πσ MOSFET sensor is improved by 89%, and meantime the ll ttres power is decrease by 20%. The results show significant improvement of the new MOSFET pressure sensor. α is a circuit factor, expressing the working point of the bridge, which can be adjusted by changing the size design and process parameters. The upper limit of α is near to 1. β is V. CONCLUSION a material factor, symbolizing the ratio of stress sensitive The design, fabrication and measurement results of novel degree between MOSFET’s and piezoresistors. It can be MOSFET pressure sensor are reported. The MOSFET changed with different fabrication conditions. Ref [5] has sensor’s sensitivity is improved significantly; meanwhile the reported that the β for PMOSFET is more than 0.5. So the power consumption can be decreased. It shows the great typical regions α and β are: promising application of MOSFET-bridge-circuit structure for the high performance pressure sensor. 01,0.5<<α β > . (16) VI. ACKNOWLEDGMENT The factor (α, β) dependences of sensitivity and power The authors thank for Chinese National High Technology between MOSFET and piezoresistive sensor is shown in Project (863project 2006AA04Z372) support. Figure 3. Take β equal to 0.8 as an example, in region I (α < 0.28), the sensitivity and power of MOSFET sensor are both decreased compared with the piezoresistive sensor. When α REFERENCES is equal to 0.28, the two sensors’ sensitivity are the same, but [1] W. J. Fleming, IEEE Sensors J., 1, p.296 (2001). the MOSFET sensor’s power is decreased by 44.4%.