Shear strength of metal-sapphire contacts

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Abstract
The shear strength of polycrystalline Ag, Cu, Ni, and Fe contacts on clean (0001) sapphire has been studied in ultrahigh vacuum. Both clean metal surfaces and surfaces exposed to O2, Cl2, and C2H4 were used. Auger electron spectroscopy was used to assess both the cleanliness of the surfaces and the approximate coverage of O, Cl, and C after exposure to the gases. The results indicate that there are two sources of strength of Al2O3‐metal contacts: an intrinsic one that depends on the particular clean metal in contact with Al2O3 and an additional one due to intermediate films. The shear strength of the clean metal contacts correlated directly with the free energy of oxide formation for the lowest metal oxide, in accord with the hypothesis that a chemical bond is formed between metal cations and oxygen anions in the sapphire surface. Contacts formed by metals exposed to chlorine exhibited uniformly low shear strength indicative of van der Waals bonding between chlorinated metal surfaces and sapphire. Contacts formed by metals exposed to oxygen exhibited enhanced shear strength, in accord with the hypothesis that an intermediate oxide layer increases interfacial strength. It is suggested that the strong oxide‐oxide bond is due to the establishment of interfacial bonds associated with complex oxide (spinel structure) formation. It was also found that C2H4 exposures increased the strength of the Fe contact, but had no effect on the other contacts.

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