Pulsed laser ablation in a cooled liquid environment

Pulsed laser ablation of a Si substrate in cooled KOH solution and a water-confined region is investigated. The liquid temperature is varied from room temperature down to 4°C. For laser ablation in water, the material removal rate reduces slightly as liquid temperature increases. However, the material removal rate for laser ablation in KOH solution is twice as high as that in the water at a cooled liquid environment (temperature ∼10°C). This enhancement results from a strong chemical interaction between the KOH solution and Si materials around the ablated crater during fast cooling of the substrate surface from the vaporization temperature after the laser ablation. It is also found that the material removal rate is reduced greatly in the KOH solution as the liquid temperature increases. It is because at a liquid temperature above 20°C, the KOH solution starts to interact strongly with Si when the two materials are in contact. The formation of Si(OH)x chemical debris blocks the incident laser beam to reach the Si surface and reduces the laser ablation effect. This result of the investigation is applied to set up a cooled KOH flow system for wafer thinning to thin substrate thickness down to 50 μm.