Tensile Strength and Fracture Toughness of Surface Micromachined Polycrystalline Silicon Thin Films Prepared Under Vario
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Mat. Res. Soc. Symp. Proc. Vol. 505 01998 Materials Research Society
EXPERIMENT Thin Film Tensile Tester The thin film tensile tester has a new grip system. The free end of a cantilever beam shaped specimen is fixed to a probe by electrostatic force generated by the voltage between the specimen and the probe. The procedure of the tensile testing using electrostatic force is shown in Figure 1. While the tensile load is applied, mainly the friction force acted between the two surfaces in contact holds the specimen free end. In this test, the specimens can be handled as fabricated on the silicon wafer and can be set to and removed from the tester without touching
the thin film. The tester is constructed in the sample chamber of a scanning electron microscope (SEM) for alignment of the probe to the specimen and testing observation. Figure 2 shows the tester. All the tensile testings are performed in vacuum. Insulating film| Specimen
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Substrate b)
Tensile force
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Power iply
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Figure 1 Procedure of tensile testing using electrostatic force, a) Probe is closed to specimen. b) Specimen is fixed to probe by electrostatic force. c) Tensile force is loaded to the specimen. d) Fractured free end is released by applying the opposite pole voltage.
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SEM Chamber Figure 2 Thin film tensile tester
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Sample eparation The poly-Si thin film that we have tested is a silicon film crystallized from an amorphous film. The "crystallized" poly-Si films have the following advantages. First, the internal stress of the film is tensile, which is suitable for micromachined devices. Second, the grain size of the film can be controlled by the annealing temperature. The crystallization occurs above 600*C, and the grain size becomes small with increasing the annealing temperature [5]. The specimens were fabricated with the surface micro-machining process. LPCVD amorphous silicon film is deposited by Si2H6 at 520°C on the substrate, where LPCVD Si3N 4 film (thickness 0.2 pIm) and plasma CVD nondoped silicate glass (NSG) film (thickness 2 pm) have been deposited as an insulating layer and a sacrificial layer, respectively. The amorphous film is crystallized by annealing in nitrogen at 600, 700, or 1000°C. Additional annealing at 1000°C is performed on the 600°C and 700'C annealed films. The film is then patterned to the specimen with photolithography and reactive ion etch (RIE), and the aluminum pad on the fixed end is fabricated. The specimen free end has many small etching holes to shorten sacrificial etching time. The sacrificial layer is then removed by wet etching with hydrofluoric acid (HF) and dried using p-dichlorobenzene without sticking of the specimen to the substrate. The microstructure of the films is observed by SEM. To determine the grain size of the film, the film is etched by the Wright etching solution before the SEM observation. Tensile Testing of Polysilicon Thin Films Figure 3 shows the thin film specimen. The tested part of the specimen is 30, 100, 300 pm long, 2 pm wide and 2pm thick. These dime
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