Wafer-Level Strength and Fracture Toughness Testing of Surface-Micromachined MEMS Devices
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at. Res. Soc. Symp. Proc. Vol. 605 @2000 Materials Research Society
samples with mm-scale dimensions revealed bend strengths of 512 MPa for CVD-produced SiC [13] and 400 MPa for hot-pressed 13-SiC [14]. Also, single crystal P3-SiC whiskers had a tensile strength of 8.4 GPa and a fracture toughness, Kic, of 3.23 MPa[m, when tested along the direction [15]. We have previously demonstrated a surface-micromachined electrostatic comb-drive polysilicon actuator which is capable of generating over 0.50 mN of force, and we have used this actuator to determine the maximum bend stress and the fatigue behavior of notched polysilicon fracture mechanics specimens containing surface-micromachined notches [16]. We have also made preliminary measurements of the fracture toughness of polysilicon, using the same actuator [17]. In this report, we extend our technique of wafer-level testing using microfabricated electrostatic actuators to include bend strength measurements (using surface-micromachined notched specimens) of amorphous silicon and poly-SiC. We also report an improved technique of determining the fracture toughness of polysilicon and present results for wafer-level tensile testing of poly-SiC. EXPERIMENT All of the devices in this investigation were fabricated using standard surfacemicromachining procedures with 100 mm diameter silicon (100) substrates. A completed device, including the electrostatic actuator with 1450 pairs of comb fingers and the integrated fracture mechanics specimen, is shown in the scanning electron micrograph (SEM) in Figure 1. All of the fracture specimens shown in the following figures were attached to similar actuators. A polysilicon single-edge notched specimen is shown in Figure 2a, with a higher magnification image of a specimen after testing in Figure 2b. A sharp-cracked specimen made by a two-mask process is shown in Figure 3a, and a sharp-cracked specimen made by an improved one-mask process is shown in Figure 3b. The tensile specimen is shown in Figure 4a, with higher magnification images (tilted upwards by 45 degrees) before testing in Figure 4b and after testing in Figure 4c.
a)
Figure 1. Surface-micromachined device for b) fracture mechanics and strength investigations, comprised of a specimen and an integrated Figure 2. Single-edge notched polysilicon bend electrostatic comb-drive actuator. test specimen a) before and b) after testing. 26
a) D) Figure 3. Micromachined fracture mechanics specimens with sharp cracks formed by indentation: a) fabricated using a two-mask process, b)fabricated using a one-mask process (this specimen is cropped in the image; the released end is 400 Rm long).
b) a) Figure 4. a) Micromachined tensile test specimen; b) higher magnification image (tilted upwards by 45 degrees) showing the free-standing poly-SiC tensile test beam and the polysilicon beneath the rest of the structure; c) the poly-SiC tensile test beam after testing.
c)
The process steps for the polysilicon devices included: thermal growth of the -2.5 jim thick release oxide, LPCVD of the st
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