Fabrication and Electrical Characterization of Low-Temperature Polysilicon Films for Sensor Applications

Abstract

The development of low-temperature piezoresistive materials provides compatibility with standard silicon-based MEMS fabrication processes. Additionally, it enables the use of such material in flexible substrates, thereby expanding the potential for various device applications. This work demonstrates, for the first time, the fabrication of a 200 nm polycrystalline silicon thin film through a metal-induced crystallization process mediated by an AlSiCu alloy at temperatures as low as 450 °C on top of silicon and polyimide (PI) substrates. The resulting polycrystalline film structure exhibits crystallites with a size of approximately 58 nm, forming polysilicon (poly-Si) grains with diameters between 1-3 µm for Si substrates and 3-7 µm for flexible PI substrates. The mechanical and electrical properties of the poly-Si were experimentally conducted using microfabricated test structures containing piezoresistors formed by poly-Si with different dimensions. The poly-Si material reveals a longitudinal gauge factor (GF) of 12.31 and a transversal GF of -4.90, evaluated using a four-point bending setup. Additionally, the material has a linear temperature coefficient of resistance (TCR) of -2471 ppm/°C. These results illustrate the potential of using this low-temperature film for pressure, force, or temperature sensors. The developed film also demonstrated sensitivity to light, indicating that the developed material can also be explored in photo-sensitive applications.

Keywords: gauge factor; low-temperature; metal-induced crystallization; polyimide; polysilicon; temperature coefficient of resistance.

Figure 1

Illustration of the MIC process for non-silicide forming systems, which starts with (a) the diffusion of the Si atoms into the metal layer, followed by (b) the nucleation of the atoms into a crystalline structure, and finishing with (c) the achievement of a continuous poly-Si layer.

Figure 2

SEM pictures of the poly-Si film obtained by annealing the AlSiCu(200 nm)/a-Si(250 nm) at 450 °C for 8 h (a) on top of a Si wafer and (b) on top of PI 4110-HD layer.

Figure 3

SEM picture of the poly-Si obtained by annealing the AlSiCu (200 nm)/a-Si (250 nm) at 450 °C for 12 h on top of a PI layer.

Figure 4

XRD measurements of the poly-Si film obtained showing a crystalline peak in the direction <111> at 2θ = 28.43° (a) directly on top of a Si wafer and (b) on top of PI HD-4410.

Figure 5

BSE images of the poly-Si film obtained by annealing the AlSiCu (200 nm)/a-Si (250 nm) at 450 °C for 8 h (a) on top of a Si-wafer and (b) on top of PI 4110-HD layer.

Figure 6

(a) Test structure containing three piezoresistors and the conductive paths for external measurements. (b) Illustration of the disposition of the three piezoresistors in each device: the top piezoresistor has a rotation of 0°, middle piezoresistor has a rotation of 90° and the bottom piezoresistor has a rotation of 135°.

Figure 7

Flow-chart illustration of the fabrication process of the device containing poly-Si piezoresistors for electrical and mechanical characterization of the developed material: (i) developed poly-Si; (ii) patterning of the poly-Si film; (iii) AlSiCu sputtering; (iv) AlSiCu patterning and (v) deposition and patterning of SiO₂ passivation layer.

Figure 8

I–V characteristic of the poly-Si piezoresistors with dimensions (a) L/W = 5, (b) L/W = 10 and (c) L/W = 20 extracted under dark and light environments.

Figure 9

Electrical resistance stability measured for 6 h in a climate chamber at a temperature of 25 °C and humidity of 50% with input voltage signal of 5 V and 10 V for piezoresistors with L/W = 5, 10 and 20 normalized with the nominal resistance at 5 V for each piezoresistor.

Figure 10

Resistance variation with temperature for piezoresistors with L/W = 5, 10 and 20.

Figure 11

Illustration of a four-point bending set-up for the piezoresistive behavior characterization.

Figure 12

Machined pieces for the (a) load beams and (b) support beams. (c) Positioning of the device in the set up.

Figure 13

(a) Resistance change with strain of the piezoresistors oriented at 0° to extract the longitudinal GF. (b) Resistance change with strain of the piezoresistors oriented at 90° to extract the transversal GF.