Transient laser-induced surface deformation of Si-based multilayer structures

ABSTRACT Transient thermal expansion, heat generation and conduction, nonlinear laser heating and subsequent meltingbehaviors of polysificon/oxide/silicon wafer 'sandwich' structures have been investigated via a laser pump-probe technique.Laser pmcessing, damage, and characterization of bulk semiconductor materials has been extensively investigated. However,less is known about the mechanisms of laser-induced damage for multilayer structures such as CCD imaging arrays. In orderto develop damage resistant sensors and better isolation for sensors used in laser environments, it is necessary to understandthe processes contributing to the laser hardness and damage of these sensors. We designed experiments to measure thetransient thermomechanical and laser-damage characteristics of multilayer systems that resemble CCD devices. The finalobjective was to gain some insight into the dominant mechanisms responsible for laser-induced degradation in CCD imagingarrays and devices of similar structures. Positive identification of a precursor event signaling the onset of catastrophic damageand identification of the weakest link elements on the sensor array will be essential for making design or processingmodifications to improve hardness. It is our goal in this paper to contribute to the above objectives by describinginvestigations of laser-induced damage to silicon-based multilayer structures. We performed single and multiple shotexperiments to obtain morphological damage thresholds for a sample set of various isolation oxide and polysilicon layerthickness. The transient photothermal deflection (TPD) technique that we employed consisted of a 10 ns pulsed Nd: YAGpump laser source at 1064 nm wavelength with a 10 Hz repetition rate. A cw HeNe laser was to probe the localized, time-dependent slope change of the illuminated surface. The deflection of the reflected probe beam was detected by a fast bicellphotodiode with temporal resolution of 20 ns. The deformation signals were recorded by a digital camera system and high-speed oscilloscope. The waveforms were later analyzed to extract the peak angular deflection and the vertical displacementbased on the geometry of the Gaussian irradiation profile. Vertical displacement down to a few nm could be detected. Measureddisplacement and surface temperature were then compared to computer simulations at different fluence levels for bulk silicon.They were found to be in excellent agreement to each other. In addition, single shot experiments were performed to obtaintheir respective damage onsets for various Si-based multilayer samples. Results indicated that the top polysilicon layerreceived the major portion of the photon energy deposited before substantial heat was conducted into the Si substrate. And ithad a lower melt onset than bulk silicon due to strong nonlinear absorption. Laser-induced damage to polysilicon layer wasalso related to film microstructures and degree of thermal isolation which depended on the oxide thickness. Measurement ofpeak surface deformation at subthreshold fluences gives insight into the thermomechanical processes which may play animportant role in multi-pulse condition.Keywords: transient photothermal deformation, silicon and polysilicon damage, isolation oxide, damage onset, transienttemperature gradient.