A STUDY OF PULSED-LASER BUMP FORMATION ON SMOOTH GLASS SUBSTRATES

CO2-laser produced micro-bumps on glass substrates are not only of int erest in tribological applications, e.g. zone texturing of substrates as discussed in this paper, but for other applications such as micro-o ptics fabrication. These bumps are positive protruding domes with net volume gain. Mass conservation implies a local density decrease inside the bump. We have studied this density variation using nanoindentatio n to probe local hardness. Our findings indicate a clear and significa nt decrease in bump hardness (up to 30%) relative to the background gl ass - consistent with this local change in density. SIMS and electron microprobe analysis indicate no chemical changes between the bump surf ace and backrground regions. Regardless, the glass texture zone displa ys excellent bump durability and contact start/stop performance. Addit ionally, the role played by the chemical surface strengthening and its effect on bump height has been investigated. For chemically engineere d surface stresses ranging from 100 to 500 MPa, the bump height increa ses with increasing surface stress at a rate of 0.064 nm/MPa Extrapola tion of the plot to zero stress confirms a non-zero thermal expansion contribution to the bump height - consistent with our theory for bump; generation. Reflectivity measurements have shown that the optical cons tants vary little with changing surface stress, implying that the lase r coupling to the surface is essentially unaltered.