Thin Film Fine Line Work of Adhesion by Microwedge Indentation
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"**Sandia
National Laboratory, Livermore, CA 94551
ABSTRACT The mechanics for plane strain geometry are presented for thin film fine lines by considering the case of microwedge indentation of a fine line subject to no residual stress. The analysis is separated into three parts depending on the absence or presence of buckling. During indentation, unbuckled or double-buckled configurations may exist, while single-buckling may occur when the indenter tip is removed. A microwedge tip was employed in the indentation testing of unstressed if-sputtered tungsten fine lines to demonstrate some of the analysis. INTRODUCTION The thin film fine line is an extremely important and common structure in the integrated circuit, optical electronics, magnetic recording industries. Although its main purpose is to serve an electrical, optical, or magnetic function, it can only do so if it adheres to the substrate. However, it has not yet been evaluated as a fracture mechanics specimen in its as-fabricated form, although it is more likely to detach from the substrate than the planar film from which it was formed. For planar thin films, indentation has been used to evaluate thin film adhesioni- 3 by an axisymmetric analysis. While a Vickers indenter tip is convenient for planar thin film adhesion evaluation, a microwedge is the natural fit for the case of a thin film fine line because of the plane strain geometry. Although it is advantageous to evaluate adhesion before etching, a unique advantage of the planar geometry is that crack lengths will be much greater due to the higher driving force relative to the axisymmetric geometry. This allows for good accuracy in measuring crack length using optical Nomarski microscopy, which is rapidly accomplished for nontransparent films. Further, with a Nanoindenter 4 , as was used in this work, or a continuous microindenter 5, good alignment of the
indenter tip to fine lines can be done on a routine basis, and loads in the micronewton and penetrations in the nanometer regime are can be measured. Therefore, there is no need to translate a hardness estimate into a depth based on a load reading when evaluating experimental results. Because the microwedge is typically wider than the fine line, alignment is not critical. Also a nonzero tilt of the microwedge is allowable in achieving a reasonable analysis, as the analysis depends on the depth penetration measurement rather than a load value. That is, the microwedge may strike the substrate as well as the fine line without necessarily requiring retesting. However, it is important to evaluate precisely the tilt so that the indentation volume V0 may be known accurately, as adhesion depends on V0,2. Tilts of less than 10 were routinely observed in this work. Another advantage of this technique is that the interfacial crack of may be observed without precision cross section techniques by tilting in the SEM. INDENTATION OF AN UNSTRESSED THIN FILM FINE LINE The geometry of the situation to be discussed is shown in Fig. 1. During indentation, driving forces fo
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