Publications

A non-contaminating, non-contact method to open glass-cap type MEMS (Micro-electromechanical systems) packages by separating the silicon substrate from the glass cover using a CO2 laser is presented. Current methods for opening these packages are cumbersome, can lead to sample contamination and are not easily done under vacuum. The package is placed in an evacuated chamber connected to gas-sampling equipment and processed through a ZnSe (transparent to 10.6 μm laser radiation) window. Laser-induced heating promotes initiation and propagation of cracks in the cover glass or at the glass/Si interface resulting in separation of the cover from the substrate. Two techniques are discussed. First, local perimeter heating of the package creates a compressive stress zone, surrounded by a tensile stress zone. Tensile zone motion relative to natural or artificially induced flaws promotes selective crack growth and propagation leading to complete separation. Second, overall heating of the package creates a coefficient of thermal expansion (CTE) difference. In both techniques the sudden release of stored residual stresses may be sufficient to "flip" the lid off the substrate. Careful tuning of the process (temperature rise and energy density) is necessary to minimize or eliminate chip debris and avoid package degassing which confuses gas analysis.

Recently, the evaporative recoil pressure effect induced by high intensity laser irradiation on molten zone motion in welds has become increasingly appreciated. Theory indicates that so-called conduction mode welds are in fact rarely encountered. Given that shapes and sizes of fusion zones are so dependent upon recoil force, the ability to model fusion zone behavior requires correct implementation of the physics involved, particularly as size scales decrease and surface energy effects increase in relative magnitude. Our presentation discusses validation experiments supporting such model development. Two techniques are discussed, a calibration method using sensitive piezoelectric force gauges, and a more general tool using a microphonic method. Each technique has advantages and disadvantages, which will be discussed. For example, while the piezo force gauge technique is readily understandable, it requires a very lightweight sample in order to avoid smearing of the force signal. However, when the sample size becomes very small, other phenomena begin to affect the gauge, giving apparently negative force measurements! The microphonic technique can be applied to actual welds, but needs careful consideration as well to eliminate comb-filtering, echoes and sample ringing. Measurements on 304L will be presented and discussed relative to contemporary theories.

Careful characterization of laser beams used in materials processing such as welding and drilling is necessary to obtain robust, reproducible processes and products. Recently, equipment and techniques have become available which make it possible to rapidly and conveniently characterize the size, shape, mode structure, beam quality (Mz), and intensity of a laser beam (incident power/unit area) as a function of distance along the beam path. This facilitates obtaining a desired focused spot size and also locating its position. However, for a given position along the beam axis, these devices typically measure where the beam intensity level has been reduced to I/ez of maximum intensity at that position to determine the beam size. While giving an intuitive indication of the beam shape since the maximum intensity of the beam varies greatly, the contour so determined is not an iso-contour of any parameter related to the beam intensity or power. In this work we shall discuss an alternative beam shape formulation where the same measured information is plotted as contour intervals of intensity.

Computational materials simulations have traditionally focused on individual phenomena: grain growth, crack propagation, plastic flow, etc. However, real materials behavior results from a complex interplay between phenomena. In this project, the authors explored methods for coupling mesoscale simulations of microstructural evolution and micromechanical response. In one case, massively parallel (MP) simulations for grain evolution and microcracking in alumina stronglink materials were dynamically coupled. In the other, codes for domain coarsening and plastic deformation in CuSi braze alloys were iteratively linked. this program provided the first comparison of two promising ways to integrate mesoscale computer codes. Coupled microstructural/micromechanical codes were applied to experimentally observed microstructures for the first time. In addition to the coupled codes, this project developed a suite of new computational capabilities (PARGRAIN, GLAD, OOF, MPM, polycrystal plasticity, front tracking). The problem of plasticity length scale in continuum calculations was recognized and a solution strategy was developed. The simulations were experimentally validated on stockpile materials.