A study of pulsed-laser bump formation on smooth glass substrates

CO2-laser produced micro-bumps on glass substrates are not only of interest in tribological applications, e.g. zone texturing of substrates as discussed in this paper, but for other applications such as micro-optics 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 nanoindentation to probe local hardness. Our findings indicate a clear and significant decrease in bump hardness (up to 30%) relative to the background glass - consistent with this local change in density. SIMS and electron microprobe analysis indicate no chemical changes between the bump surface and backrground regions. Regardless, the glass texture zone displays excellent bump durability and contact start/stop performance. Additionally, the role played by the chemical surface strengthening and its effect on bump height has been investigated. For chemically engineered surface stresses ranging from 100 to 500 MPa, the bump height increases with increasing surface stress at a rate of 0.064 nm/MPa Extrapolation 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 constants vary little with changing surface stress, implying that the laser coupling to the surface is essentially unaltered.