Tensile testing of polysilicon

Tensile specimens of polysilicon are deposited on a silicon wafer; one end remains affixed to the wafer and the other end has a relatively large paddl e that can be gripped by an electrostatic probe. The overall length of the specimen is less than 2 mm, but the smooth tensile portion can be as small as 1.5 x 2 mu m in cross section and 50 mu m long. The specimen is pulled b y a computer-controlled translation stage. Force is recorded with a 100-g l oad cell, whereas displacement is recorded with a capacitance-based transdu cer. Strain can be measured directly on wider specimens with laser-based in terferometry from two small gold markers deposited on the smooth portion of the specimen. The strength of this linear and brittle material is measured with relative ease. Young's modulus measurement is more difficult; it can be determined from either the stress-strain curve, the record of force vers us displacement or the comparison of the records of two specimens of differ ent sizes. Specimens of different sizes-thicknesses of 1.5 or 3.5 mu m, wid ths from 2 to 50 mu m and lengths from 50 to 500 mu m-were tested. The aver age tensile strength of this polysilicon is 1.45 +/- 0.19 GPa (210 +/- 28 k si) for the 27 specimens that could be broken with electrostatic gripping. The average Young's modulus from force displacement records of 43 specimens is 162 +/- 14 GPa (23.5 +/- 2.0 x 10(3) ksi). This single value is mislead ing because the modulus values tend to increase with decreasing specimen wi dth; that is not the case for the strength. The three methods for determini ng the modulus agree in general, although the scatter can be large.