EFFECTS OF AMBIENT GAS ON PHOTOACOUSTIC DISPLACEMENT MEASUREMENT BY LASER INTERFEROMETRIC PROBE

The effect of gas adjacent to the sample surface on the photo-acoustic displacement (PAD) measurement was studied by using an extremely sens itive laser interferometric probe with a sensitivity of 0.1 picometers . For silicon, the PAD signal measured at atmospheric pressure increas ed about 18% as compared to the signal obtained in vacuum, and varied by less than 0.7% for a change in pressure of 5% around 1 atm. It is s hown, by a simple theoretical model, that the variation of the PAD is caused by a change in refractive index of the gas and the real PAD can be accurately obtained by correcting this effect.