Effect of specimen size on Young's modulus and fracture strength of polysilicon

The microstructure of polysilicon specimens of varying size was examined and tensile tests were conducted to determine if the measured modulus and strength depend on the size of the specimen. All specimens were from the same MUMP's 25 run at MCNC, and the thicknesses were 1.5, 2.0, and 3.5 mum. Microstructure was examined in specimens as narrow as 2 mum and ranging up to 20 mum in width. The tensile specimens tested were 6, 20, or 600 mum wide and 250, 1000, or 4000 mum long. Nothing in the transmission electron microscopy (TEM) observations indicates any effect of specimen size on the microstructure; the columnar grains are fine (0.2-0.5 mum) and uniformly distributed. The widths of all specimens were found to differ from the specified mask values, and a more pronounced variation was measured for the smaller specimens. Three different approaches are used to measure Young's modulus, and they all give a value of 158 +/- 10 GPa with no evidence of substantial effects of specimen size. However, the strength does increase somewhat as the total surface area of the test section decreases-from 1.21 GPa +/-0.08 GPa to 1.65 +/- 0.28 GPa-reflecting the fact that the larger specimens have more surface flaws. Test techniques and procedures are briefly presented along with detailed analyzes of the results.