Optical Detection of Photoacoustic Pulses in Thin Silicon Wafers.
Abstract
A probe beam deflection method is used to study the propagation of photoacoustic longitudinal waves and plate modes in silicon wafers. Nanosecond acoustic pulses are produced in thin silicon wafers by absorption of weak nitrogen laser pulses causing no sample ablation. A HeNe probe laser is tightly focused on the same or on the opposite side of the wafer. Deflections of this probe beam are monitored by a photodiode located in the Gaussian wing of the reflected, diverging beam. The transient deflection signal can be observed on a Tektronix Mod. 7104 oscilloscope in a single shot mode at nanosecond time resolution. The absorption of the pump laser pulse causes a permanent surface distortion, which is the source of longitudinal and shear wave pulses propagating into the sample. In conclusion, the probe beam deflection technique is shown to be a fast and simple method for measuring acoustic pulse propagation in solids. It does not require elaborate servo mechanisms as the interferometric detection scheme while offering similar sensitivity. In addition, it does not require high surface quality of the medium under investigation. Due to its non-contact nature it is applicable to very hostile environments. Additional work is needed for quantitative understanding of the signal shape.
Document Details
- Document Type
- Technical Report
- Publication Date
- Jun 25, 1985
- Accession Number
- ADA156705
Entities
People
- A. C. Tam
- H. Sontag
Organizations
- International Business Machines Corporation (Armonk, NY)