Dick J. Chang

Stress measurement in MEMS using Raman spectroscopy

Raman spectroscopy is used as a non-contact method in measuring stresses at the surface of a crystalline structure or the crystalline-coated surface of an amorphous structure. The stress measurement capability is based on the relative frequency shift of Raman spectra when the crystal lattice is strained. The Raman spectroscopy has a resolution on the order of a few micrometer (micrometers ) which may be used to probe the local non-uniform stress distribution and thus address the material nonhomogeneity. This paper presents the Raman secular equation for general and cubic crystal systems and discusses the stress field effects to Raman frequency shifts and polarization. Experimental testing will include the calibration of the Raman signal versus mechanically applied stresses using single crystal strips, poly-silicon coatings deposited on different specimen configurations, and the stress measurements on a frequently used MEMS structure, cantilever beam, subject to electrostatic forces. Correlation of the experimental results with the analytical prediction will be addressed.

Stress Measurements in Silicon Microstructures

Raman spectroscopy is used as a non-contact probe of stress with high spatial resolution in micro-machined silicon structures. The motivation for this work is that reliability or cycle life can be substantially increased by understanding the origin of stress including residual stress. Excessive stresses induced by workmanship shortcomings or design constraints may be addressed by Raman measurements. In microelectronics, stress is known to play a significant role in interconnects which limits reliability, life, and ultimately cost of many circuits. We wish to demonstrate the utility of Raman spectroscopy as a tool for the development and design of silicon microstructures. The equations for a general 2D stress field are discussed. Calibration studies using macro-mechanical fixtures for single crystal silicon specimens under 2D stress field are presented. Our measurements show good agreement with the theoretical values and thus validate the approach taken. Stress maps of conventionally fabricated test structures, laser machined, and polysilicon structures are presented.

Damage modes and mechanisms of Si-Ge films under prompt soft X-ray radiation

Abstract Thermal and residual stresses in films exposed to sudden temperature changes are analyzed based on an elastoplastic-brittle idealization of film response. The results thus obtained are used to explain qualitatively the damage mechanisms of various failure modes observed in SiGe film deposited onto single-crystal silicon substrates exposed to soft X-ray radiation for a short duration of time.

On thermal and thermal deflection analyses in substrates during coating deposition

Abstract Thermal deflection of a cantilever substrate and a circular wafer during and after film deposition is analyzed. Simple and accurate expressions for the substrate temperature are obtained under the assumption of a constant heat flux at the front surface, heat dissipation according to Newtons law at the rear surface during deposition and heat dissipation at both surfaces after deposition. The solution is then applied to extract, from published measured total deflection history data, the deflection history of the substrate due to intrinsic stress only.