Monolithic Composite “Pressure + Acceleration + Temperature + Infrared” Sensor Using a Versatile Single-Sided “SiN/Poly-Si/Al” Process-Module

We report a newly developed design/fabrication module with low-cost single-sided “low-stress-silicon-nitride (LS-SiN)/polysilicon (poly-Si)/Al” process for monolithic integration of composite sensors for sensing-network-node applications. A front-side surface-/bulk-micromachining process on a conventional Si-substrate is developed, featuring a multifunctional SiN/poly-Si/Al layer design for diverse sensing functions. The first “pressure + acceleration + temperature + infrared” (PATIR) composite sensor with the chip size of 2.5 mm × 2.5 mm is demonstrated. Systematic theoretical design and analysis methods are developed. The diverse sensing components include a piezoresistive absolute-pressure sensor (up to 700 kPa, with a sensitivity of 49 mV/MPa under 3.3 V supplied voltage), a piezoresistive accelerometer (±10 g, with a sensitivity of 66 μV/g under 3.3 V and a −3 dB bandwidth of 780 Hz), a thermoelectric infrared detector (with a responsivity of 45 V/W and detectivity of 3.6 × 107 cm·Hz1/2/W) and a thermistor (−25–120 °C). This design/fabrication module concept enables a low-cost monolithically-integrated “multifunctional-library” technique. It can be utilized as a customizable tool for versatile application-specific requirements, which is very useful for small-size, low-cost, large-scale sensing-network node developments.

(a) illustrates the pressure-sensing SiN diaphragm in the absolute-pressure sensor. The diaphragm length, width and thickness are symbolized as 2b, 2a and h, respectively. Assuming 2b ≫ 2a ≫ h, the deflection and stress along the x-direction are [

S(1.4)
The stress distribution is shown in Figure S1(b). Figure S1. (a) Schematic of the rectangular pressure-sensing diaphragm and piezoresistors of the absolute-pressure sensor. (b) Stress distribution in the diaphragm and the piezoresistors' longitudinal-parts placement design.

OPEN ACCESS
To realize a Wheatstone-bridge piezoresistive sensing, four meander-shape poly-Si piezoresistors R 1 ~ R 4 are designed, with their longitudinal parts placed along the x-direction at the high-stress area for large sensitivity, as shown Figure S1(b). In detail, the length of tensile-stress area is 0.42a. Thus the longitudinal parts of R 1 and R 4 are laid at the tensile-stress areas of (−a, −0.58a) and (0.58a, a). Note that the length of longitudinal part l is bigger than 0.42a, therefore a minor part is laid outside the (−a, a) region. In contrast, both longitudinal parts of R 2 and R 3 are placed at the compressive-stress area of (−l/2, l/2). Each piezoresistor has n longitudinal segments, and (1−n) turning parts in which the current flows transversally. The total resistance, longitudinal resistance and effective transverse resistance of one piezoresistor are symbolized as R, R L and R T , respectively. Thus, R = R L + R T .
Since the strain of poly-Si strips are nearly the same as the strain of SiN, i.e., For R 2 and R 3 , Thus the Wheatstone-bridge output voltage turns to be Then the sensitivity is calculated as Based on the design parameters in this paper, we obtain The calculated sensitivity is 112 mV/MPa (V in = 3.3 V). Equation (1.14) gives a design guideline on the rectangular-diaphragm based pressure sensor, in which the effects of various structure and material parameters on the sensor performance are revealed.
Due to the residual in-plane tensile stress of the deposited LS-SiN diaphragm (about 100 MPa), the stiffness of the diaphragm will be increased and therefore the real sensitivity will be reduced. Then the ANSYS numerical analysis is implemented with considering the effect of residual tensile stress. Figure S2 illustrates the simulated stress distribution of σ SiN x (x) (with deflected shape), with 1 MPa pressure perpendicular to the diaphragm and an in-plane 100 MPa tensile stress. The σ SiN x (x) curve along x-direction is shown in Figure S3(a). In contrast, the simulated σ SiN x (x) distribution without considering the residual tensile stress is also shown in Figure S3   From the simulated stress data in Figure S3, the resistance changes (ΔR 1 and ΔR 2 ) are calculated according to (1.9) and (1.10). Note that the stress data in Figure S3(a) is firstly subtracted by 100 MPa before the calculation. Then we obtain the simulated sensitivity of the pressure sensor. With the supplied voltage (V in ) of 3.3 V, the sensitivity of the device without residual tensile stress is 71 mV/MPa. By considering the residual stress, the simulated sensitivity decreases to 55 mV/MPa. 2

Structure Design
where x varies in (0, L), a is the applied acceleration, ν SiN is the Poission's ratio, E sin is the Young's module of SiN, γ 1 (=0.295) is a coefficient expressing the additional contribution of the axial force to the beam stiffness, σ 0 is the residual tensile stress in the LS-SiN beam. As the beam is free along the y-direction, σ SiN y (x) is ignored. The stress distribution is shown in Figure S4(b). To realize a Wheatstone-bridge piezoresistive sensing, four poly-Si piezoresistors R 1 ~ R 4 are designed, with their longitudinal parts placed along the x-direction at the high-stress area for large sensitivity, as shown Figure S4(b). In detail, the length of tensile-stress area is 0.5L. Thus the longitudinal parts of R 1 and R 4 are laid at the tensile-stress areas of (0, l). In contrast, both longitudinal parts of R 2 and R 3 are placed at the compressive-stress area of (L−l, L). Each piezoresistor has two longitudinal segments, as well as one square turning part (size = W RT ) in which the current flows transversally. The total resistance, longitudinal resistance and transverse resistance of one piezoresistor are symbolized as R, R L and R T , respectively. Thus, R = R L + R T .
With the similar analysis in Part 1, we obtain the strain in poly-Si strips as

Performance Analysis
From the above discussion, we can calculate the pressure-induced resistance change of the piezoresistors. For R 1 and R 4 , For R 2 and R 3 , Thus the sensitivity is calculated as Based on the design parameters in this paper, we obtain For the residual stress, ANSYS numerical analysis is conducted, as show in Figure S5. It shows that the rotating structure design releases the deposition stress significantly, and the residual stress σ 0 at piezoresistor location is lowered to only about 5 MPa. Then the calculated sensitivity of accelerometer is 69 μV/g. Figure S5. Simulated residual stress distribution in the beam of accelerometer.

Finite Element Analysis on Infrared Detector
Numerical simulation by using ANSYS is conducted to simulate the temperature distribution over the structure of infrared detector under radiation. With the applied radiation-density of 6.5 mW/cm 2 , the temperature distribution at one-quarter of the structure are shown in Figures S6 and S7. Then we obtain that the temperature difference ΔT HC between the hot and cold ends is about 0.12 °C. Figure S6. Simulated temperature distribution over one-quarter of the infrared detector. Figure S7. Simulated temperature distribution from the absorbing membrane center to the cold-end.