A Novel Mems Pressure Sensor With Mosfet On Chip

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 based on the MOSFET stress sensitive phenomenon, in which the source-drain current changes with the stress in channel region. Two MOSFET’s and two piezoresistors were employed to form a Wheatstone bridge served as sensitive unit in the novel sensor. Compared with the traditional piezoresistive pressure sensor, this MOSFET sensor’s sensitivity is improved significantly, meanwhile the power consumption can be decreased. The fabrication of the novel pressure sensor is lowcost and compatible with standard IC process. It shows the 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.

Figure 1. Two PMOSFET’s and two piezoresistors are connected to form a Wheatstone bridge. To obtain the maximum sensitivity, these components are placed near the four sides of the silicon diaphragm, which are the high stress regions. The MOSFET’s has the same structure parameter W/L, same threshold voltage VT and gate-source voltage VGS (equal to VG-Vdd). They are designed to work in the saturation region. The piezoresistors also have the same resistance R0.

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. MOSFET also has a stress sensitive phenomenon, in which the source current changes with the stress in channel region [4-6]. Some experimental applications e.g. MOS ring oscillator accelerometer have been reported [7]. In this paper, a novel MOSFET pressure sensor was reported, which used two PMOSFET’s and two piezoresistors to form a bridge circuit. The structure design and operating principle were demonstrated. The fabrication process was described. Measurement results of the sensor’s sensitivity and power show significant improvement compared with traditional piezoresistive pressure sensor. II.

DESIGN

Based on the stress sensitive effect of MOSFET, a new MOSFET-bridge-circuit structure is designed, as shown in

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Figure 1. (a) Schematic of novel MOSFET pressure sensor including two MOSFET and two resistors on the membrane to form a Wheatstone bridge, (b) The MOSFET-based bridge circuit, the output voltage is Vout=Vout1Vout2, the voltage source is Vdd.

When there is no forced pressure, the bridge is in balance. The balanced output V0 of each arm is:

V0 =

1 W 2 R0 ⋅ μ p 0 COX (VGS − VT ) . 2 L

(1)

As a result, the sensor output signal Vout is zero. When a pressure is forced on the membrane, the current and piezo-resistance in each bridge arm are changed. The variation of the PMOSFET current is proportional to the change of channel mobility Δμp, computed as:

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ΔI DS / I DS = Δμ μ0 = π l ⋅ σ l + π t ⋅ σ t ,

(2)

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 piezoresistance is also proportional to the resistor mobility change ΔμR, which can be expressed using a similar formula, as: ΔR / R0 = Δμ R μ R 0 = π l ⋅ σ l + π t ⋅ σ t ,

(3)

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: Vout 1,2 =

1 W 2 ( R0 ± ΔR ) ⋅ ( μ p 0 ± Δμ p ) COX (VGS − VT ) , (4) 2 L

therefore the sensor output is obtained as:

⎛ Δμ p Δμ R Vout = 2 ⎜ + ⎜μ ⎝ p 0 μR 0

⎞ ⎟⎟ ⋅ V0 . ⎠

(5)

Formula (5) shows that, Vout is proportional to the stress as well as the forced pressure. This is the operating principle of the novel MOSFET-bridge-circuit pressure microsensor. III.

Figure 2. Process flow of the MOSFET pressure sensor based on Al-gate post-IC process and bulk silicon MEMS process.

IV.

RESULTS AND DISCUSSION

The sensitivity and power of the MOSFET pressure sensor are deduced and measured, which are the most important performance parameter for the pressure microsensor. These parameters (using the subscript “MOS”) are compared with those of the traditional piezoresistive pressure sensor (using the subscript “res”). A. Performance parameters The sensor sensitivity is:

FABRICATION

The whole fabrication was based on Al-gate post-IC process, as shown in Figure 2. First, a (100)-oriented n-type silicon wafer (6~8Ωcm) was selected as start substrate, with SiO2 and Si3N4 layers on both sides, deposited by thermal oxidation and LPVCD methods. Second, the backside was etched in 33% KOH solution to form the silicon membrane. Third, a new field SiO2 layer is thermal oxidized. Fourth, the source and drain windows formed, and then high-dope boron was implanted to form the source and drain. Fifth, the gate area and piezoresistor windows formed. A low-dope boron implantation was performed to adjust the VT and form the piezoresistors. Then a thin gate SiO2 layer was thermal oxidized. Sixth, Al layer was sputtered and wet etched to form the interconnection. At last, Si-Au/Ti-Si bonding was performed in vacuum to form the pressure referential cavity. In all, five lithography steps were used. The whole process is low-cost and compatible with standard IC process.

SMOS =

⎛ Δμ p Δμ R Vout = 2⎜ + ⎜μ Vdd ⎝ p0 μR0

⎞ V0 . ⎟⎟ ⋅ ⎠ Vdd

(6)

The balanced power is computed as: PMOS = 2Vdd ⋅

⎛ V ⎞V 2 Vdd − VSD0 = 2 ⎜ 1 − SD0 ⎟ dd , R0 Vdd ⎠ R0 ⎝

(7)

where VSD0 is the balanced source drain voltage. To ensure the PMOSFET working in the saturation region, the VSD must meet the requirement of:

VSD > VSG − VT > 0 .

(8)

The output expression of traditional silicon piezoresistive pressure microsensor uses Wheatstone bridge circuit with four piezoresistors is: Vout =

Δμ R ΔR ⋅ Vdd = ⋅V . R0 μ R 0 dd

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(9)

The sensor sensitivity and balanced power are: Sres =

Vout Δμ R = , Vdd μR0

Pres =

(10)

Vdd 2 . R0

significantly. In region III (0.5 < α < 1), the sensitivity and power get both raised. However, the increase of sensitivity is more remarkable than that of power. At the upper limit of α (equals to 1), the MOSFET sensor’s sensitivity and power are raised by 300% and 100%, respectively.

(11)

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 = 2α (1 + β ) , S res

(12) Figure 3. The sensitivity and power dependence on α and β between MOSFET and piezoresistive sensors

PMOS = 2α , Pres

(13)

where V0 V W (VGS − VT ) 1 , = 1 − SD = R0 ⋅ μ p 0 COX Vdd Vdd 2 L Vdd 2

α=

β=

(π lσ l + π tσ t )MOS . (π lσ l + π tσ t )res

(14)

(15)

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 a material factor, symbolizing the ratio of stress sensitive degree between MOSFET’s and piezoresistors. It can be changed with different fabrication conditions. Ref [5] has reported that the β for PMOSFET is more than 0.5. So the typical regions α and β are: 0 < α < 1, β > 0.5 .

(16)

B. Measurement results The measured sensitivity and linearity error of MOSFET pressure sensor and the reference piezoresistive sensor are shown in Figures 4-5. The sensitivity of the fabricated MOSFET pressure sensor sample was 0.3mV/KPa, and the linearity error was 0.6% FS. These parameters were better 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, with the same power. When α is set as 0.4, the sensitivity of MOSFET sensor is improved by 89%, and meantime the power is decrease by 20%. The results show significant improvement of the new MOSFET pressure sensor. V.

The design, fabrication and measurement results of novel MOSFET pressure sensor are reported. The MOSFET sensor’s sensitivity is improved significantly; meanwhile the power consumption can be decreased. It shows the great promising application of MOSFET-bridge-circuit structure for the high performance pressure sensor. VI.

The factor (α, β) dependences of sensitivity and power between MOSFET and piezoresistive sensor is shown in 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 is equal to 0.28, the two sensors’ sensitivity are the same, but the MOSFET sensor’s power is decreased by 44.4%. In region II (0.28< α < 0.5), the power is still decreased, but the sensitivity become increased. When α is equal to 0.5, the sensors’ power are the same, but the MOSFET sensor’s sensitivity is raised by 100%. So its sensitivity is improved

CONCLUSION

ACKNOWLEDGMENT

The authors thank for Chinese National High Technology Project (863project 2006AA04Z372) support. REFERENCES [1] [2] [3] [4]

W. J. Fleming, IEEE Sensors J., 1, p.296 (2001). R. Schlierf, M. Gortz, T. S. Rode and K. Trieu, Transducers, p.1656 (2005). L. Lin and W. Yun, IEEE Proc. Aerospace Conf., p.429 (1998). D. Colman, R. T. Bate and J. P. Mize, J. Appl. Phys, 39, p.1923 (1968)

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[5] [6] [7] [8]

J. Neumeister, G. Schuster and W. V. Munch, Sens. Actuators, 7, p.167 (1985). A. T. Bradley, R. C. Jaeger and J. C. Suhling, IEEE Trans. Electron Devices, 48, p.2009 (2001). Z. Zhang, R. Yue and L. Liu, IEEE International Conference on Solid-State and Integrated Circuits Technology, p.1796 (2004). F. Fruett and G. C. Meijer, Electron. Lett., 36, p.173, (2000).

Figure 5. The sensitivity and the linearity error of the traditional pressure sensor sample. (a) the sensitivity is 0.2mV/KPa, (b) the linearity error is 0.62% FS.

Figure 4. The sensitivity and the linearity error of fabricated MOSFET pressure sensor sample. (a) the sensitivity is 0.3mV/KPa, (b) the linearity error is 0.6% FS.

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