Mechanical properties of boron-doped single-crystal silicon microstructures

High aspect ratio single crystal silicon resonant beams oriented with angles of 0, 11.25, 22.5, 33.75, and 45 degrees to the Si 100 direction were fabricated by using high density plasma etching and dissolved wafer techniques. The silicon was doped with boron with a concentration of >= 5 X 1019 cm-3. The moduli of elasticity of the boron doped silicon were calculated from the measured resonant frequencies of the fabricated cantilever beams. They were approximately 1.56, 1.60, 1.66, 1.81 and 1.92 X 1012 dyne/cm2 with respect to the five different orientations mentioned above. These values are smaller than those calculated theoretically for intrinsic single crystal silicon. The built-in stresses in the clamped-clamped bridges were also found to be orientations. The stress variation is attributed to the variation of the modulus of elasticity and the difference in thermal expansion coefficients between silicon and the glass substrate to which the beam anchors are bonded.