Thin Film Adhesion Characterization by Microwedge Scratching of Precracked Fine Lines
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microwedge, and the lateral stress was measured. While values of G, are obtained, there is a strong dependence of the strain energy release rate G on the amount of friction between the thin film and the substrate at the microwedge tip (as opposed to the crack tip). Also, because spallation due to beam bending occurs before the crack length becomes stable, the value found can at best be regarded as an upper bound for GQat the spalled length of the thin film beam. In the present study, carbon precracks of known lengths at the end of thin film fine lines are formed. The friction between the film and the substrate at the indenter tip is shown to be negligible. Therefore, G is well known. In addition, mode mixity depends on precrack length. Differences in crack path behavior have been observed and are attributable to mode mixity differences. SAMPLE PREPARATION AND MECHANICAL TEST PROCEDURE The geometry of the situation to be discussed is shown in Fig. 1. A thin film fine line is mechanically probed by an asymmetric microwedge, while the normal and lateral stresses are monitored. In order to realize the configuration, test structures were fabricated in a clean room utilizing standard microelectronics fabrication equipment. A two level mask set was designed to construct thin film fine lines of various line widths and lengths overlapping very thin C precracks by lengths from I to 50 gim. The processing sequence to build the structures was as follows:
821 Mat. Res. Soc. Symp. Proc. Vol. 356 0 1995 Materials Research Society
Fig. 1 Schematic of the situation discussed in this paper
Fig. 2 SEM of the asymmetric gwedge
1) Obtain silicon wafers, 10-20 ohm-cm, p-type. 2) Grow 1 g.m SiO 2 layer by wet oxidation at 1000TC. 3) RF-sputter deposit diamond-like-carbon, 5 W/cm2 , 20 mtorr, 150A.
4) 5) 6) 7) 8) 9) 10)
Expose and develop photoresist to define C precracks. Dry 02 etch at 60 mtorr, 175 W in plasma etcher. Strip photoresist in heated H2SO 4. RF-sputter deposit tungsten at 13 mtorr, 10 W/cm 2 to thicknesses of 0.5 and 1.5 pgm. Expose and develop photoresist so that W is aligned to4 C precracks. Wet etch in KFe 3(CN) 6, KH 2PO4, and KOH solution . Strip photoresist in acetone.
Isotropic etching of the C layer in step 5 is done to obtain a sharp precrack. The system of W on Si0 2 was chosen as a model system because interfacial cracking is brittle in nature, so that modeling by linear elastic fracture mechanics is valid. Also, this system has been used for interconnect in the microelectronics industry. Because W is elastically isotropic, Young's modulus is independent of grain texture. Young's modulus was found to be about 400 GPa by the method of Oliver and Pharr5. Average residual stress was measured by wafer curvature to be 100 MPa (tensile), negligible compared to stresses induced by the microwedge. Because of the high melting temperature of W, further low temperature treatments (< 300 0C)will not affect the stress. 6 An asymmetric diamond wedge of 20 pgm width was purchased from Imetra . The included angle on the
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