Yung-Joon Jung, Mehmet R. Dokmeci, Jin-Goo Park
Date of Award
Doctor of Philosophy
Department or Academic Unit
College of Engineering. Department of Mechanical and Industrial Engineering.
Cleaning, Contamination Control, Laser induced shockwave, Particle
Engineering | Mechanical Engineering
As surface contamination by submicron particulates becomes a critical issue in semiconductor industry, efforts have been made to develop noncontact effective laser-based cleaning techniques. The laser shockwave cleaning (LSC) process is one of the promising techniques that has been demonstrated to be effective for inorganic particle dry removal. In the cleaning process, the surface is not directly exposed to irradiation of laser beam, which eliminate the possibility of potential surface damage. In addition, a relatively large area can be cleaned by a single laser pulse. However, even though this technology has those distinct advantages, LSC technique has a difficulty in removing organic particles, which removal mechanism hasn't been found yet. Recently, the laser shockwave cleaning technique has been combined with ultraviolet (UV) laser for organic particle removal. But, when UV laser cleaning is used in this hybrid approach with LSC, optimization is required to ensure damage free cleaning. This is required whenever a new surface is used.
The reason for ineffectiveness in organic particle removal using LSC has been investigated. The shock wave pressure is sufficiently strong to deform soft particles such as organic particles increasing the contact radius between the particle and the substrate, which leads to higher adhesion force. The deformation of organic particle occurs during LSC process has been verified with high angle SEM images of 300nm PSL particles exposed to laser shockwaves. For theoretical calculation of the contact radius between 300nm PSL particles and Si substrate, the shockwave speed has been measured using newly designed two-probe beam deflection method and The Maugis-Pollock theory is applied. The predicted contact radius agrees with the experimental measurements. Removal moment ratio of PSL particle and silica particle has been also analyzed when applying LSC. It shows that silica particles can be easily removed by LSC, while soft (PSL) particles smaller than 1ìm in diameter will be harder to remove. It has been also verified experimentally with three different sized PSL particles such as 300nm, 600nm and 2μm. The removal efficiency of 2μm PSL particles is more than 90%, while in the case of 300nm and 600nm PSL particles, they are 30% and 60% respectively.
In order to remove organic particles, a wet laser shockwave cleaning (WLSC) technique has been refined successfully for wafer scale cleaning as well as for more efficient particle removal. This technique utilizes the advantage of using water to reduce the adhesion force by an order of magnitude, utilize the double layer repulsive force, eliminate the capillary force encountered in dry LSC and increase the drag force by increasing the medium density (by three orders of magnitude). In order to evaluate cleaning performance of the wet laser shockwave cleaning technique, removal of either organic particles or inorganic particles using different size particles have been investigated and compared to original LSC. Complete WLSC removal of 300nm PSL particles as well as 280nm silica particles was achieved. In addition, 28nm PSL particles were successfully removed by WLSC. The removal mechanism for wet laser shockwave cleaning has been investigated using computational fluid dynamics as well as shadow-graphic photography. Removal moment ratio for PSL and silica particle removal using WLSC has been also analyzed and compared to experimental results, but it shows the disagreement between them in the case of PSL particles. Based on the assumption that the water could be compressed due to the shockwave pressure perpendicular to the substrate leading to a decrease in the water film thickness, the removal moment ratio has been reanalyzed. As a result, it has been shown that when the thickness of the water film decreases to less than 1μm, 28nm PSL particles can be successfully removed.
Kim, Tae-Hoon, "Nanoscale particle removal using conventional and hybrid laser shockwave cleaning" (2010). Mechanical Engineering Dissertations. Paper 13. http://hdl.handle.net/2047/d20000983
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