Fracture Mechanical Evaluation of GaAs Wafers
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Fracture Mechanical Evaluation of GaAs Wafers M. Schaper1, M. Jurisch2, F. Bergner1, R. Hammer2, Dresden University of Technology, Institute of Materials Science, D-01062 Dresden, Germany 2 Freiberger Compound Materials GmbH, D-09599 Freiberg, Germany 1
ABSTRACT Strength and fracture toughness of (100) oriented GaAs wafers are analyzed by fracture and indentation testing. Using finite element method (FEM) critical fracture stresses are calculated from the fracture loads of wafers tested under biaxial bending, whereas atomic force (AFM), scanning electron microscopy (SEM) and acoustic C-scan microscopy of deformation and cracking patterns around micro- and nano-indentations provided information on yielding, crack initiation and crack growth resistance. Through fracture mechanical evaluation of these results critical defect sizes are derived, which are tolerable without strength degradation. It is shown, that a fracture strength of at least 800MPa can be achieved by careful fabrication even of 6" wafers. This figure is much higher then the prescribed minimum strength level estimated to avoid premature failures during wafer handling and processing routes. INTRODUCTION GaAs is an attractive semi-conducting material, especially in applications where a high carrier mobility is desired. One of the main drawbacks of GaAs is its extreme brittleness[1, 2, 3] with a fracture toughness being only half that of Si. Consequently, wafer breakage is one of the main factors determining the yield in device manufacturing. Because wafer breakage may initiate from scratches well below the µm range, careful thermal treatments as well as etching and polishing procedures are applied to remove residual surface damage. Similarly as surface scratches, sub-micron sized structural defects or inhomogenities as localized dislocation clusters or precipitates may initiate premature fracture of wafers during handling and processing. Therefore, wafer strength represents a sensitive measure of wafer quality as well as an essential pre-requisite for R&D activities related technological processes of wafer preparation as sawing, grinding, damage etching. Supplementary to critical fracture stress, instrumented indentation testing in the micro- and nano-ranges provides a powerful means for measuring near-surface properties of brittle materials. Combined with scanning electron microscopy (SEM) and atomic force microscopy (AFM) of deformation and cracking patterns around indentations data for initial yielding, crack initiation and crack growth resistance may be derived. A fracture mechanics based compilation of measured strength and crack growth resistance data enables an assessment of the defect tolerance of the material through calculation of a critical defect size, which is tolerable without strength degradation. The aim of our work has been to evaluate the critical defect size of GaAs based on fracture mechanics principles. To reach this goal the indentation fracture toughness and the fracture strength of GaAs wafers were measured for various gro
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