Laser Shock Cleaning for Particle Removal

The semiconductor industry has been searching for a completely dry cleaning process since the mid-1980s. Until now, no dry process has been able to remove inorganic particles effectively without inducing damage. Wet process, although effective, does use large amounts of water,1

Laser cleaning has been considered by many as a possible technique for the removal of particulate contaminants.2,3 However, the direct dry laser irradiation of wafers has not been shown to remove inorganic particles and could easily cause surface damage. Wet or (explosive evaporation) laser cleaning has been shown to be effective in the removal of organic particles, but it has been shown to damage patterned wafers.

In this article, we introduce a dry laser cleaning method using a shock wave generated just above the wafer surface. The particle removal is caused by the gas high velocity induced by the propagating shock wave.

Laser-induced shockwaves

The laser-induced shockwave cleaning (LSC) technique removes particles without any direct interactions between the laser beam and the surface.4-9 When the intense laser beam is focused just above the wafer surface, the gaseous constituents ionize and create an airborne plasma. A shockwave is subsequently generated by the rapid expansion of the plasma, and propagates spherically in all directions, including toward the substrate. Figure 1 shows a schematic diagram of how the plasma shockwaves are generated by a focused laser beam above the surface with a photo of laser-induced shockwaves in action. The plasma can be easily observed in the picture. If the force of the shockwave is larger than the adhesion force of the particle on the substrate, the particle will be easily removed from the surface. Figure 2 shows a schematic diagram of UV-assisted LSC setup used in the experiments. It is also important to mention the LSC throughput is high, since only five or six pulses are needed to clean a 200 mm wafer because of the large area cleaned per pulse.

Shockwaves generated by laser-induced plasma can be easily measured and visualized by the laser shadow photography method (Schlieren photography) shown in Figure 3. The figure shows the airborne plasma generation and the resulting shockwave propagation. The first frame shows the intense laser beam focused above the wafer, which is at the bottom of the photo. The rapid expansion of the plasma produces the intense shockwave (shown as a dark ring in the center image) outside the plasma. The shockwave then propagates spherically as shown in the right image, and impacts the surface. Particles impacted by shockwave can be removed by the shear velocity behind the shockwave. The velocity of the shockwave in Figure 3 is faster than 2000 m/sec. The velocity is strongly dependent on laser power density, height above the wafers and gas atmosphere, and can be as high as 10,000 m/sec.

Particle adhesion, removal forces

Figure 4 shows the particle removal mechanisms.10,11 The removal mechanism is called rolling because the removal force rolls the particle before separation. The removal moment ratio (MR) can be defined as the ratio of the removal moment (MR ) to the adhesion resisting moment (MA ):

If the removal moment ratio is larger than 1, more than 80% of the particles could be removed from the surface.

The moment ratio for the removal of silica particles on oxide wafers is calculated at different shockwave velocities — 2000 and 10,000 m/sec. Figure 5 shows the moment ratio as a function of particle size at different gap distance between laser focus point and the wafer. At 2000 m/sec, 100 nm particles can be removed. When the velocity is increased to 10,000 m/sec, particles as small as 30 nm can be removed. It indicates that nanometer-sized particles can be removed by LSC at these velocities. All the experiments and calculations are conducted in air at atmospheric pressure. Other gases could be used such nitrogen or argon, especially if oxidation is an issue.

Particle removal

A Q-switched Nd:YAG laser with a wavelength of 1064 nm is used to generate the shockwaves and remove the particles from wafers in a Class 1 cleanroom environment. The laser energy density at the focus point is ~1011 W/cm2 . A Tencor Surfscan 5500 and scanning electron microscopy (SEM) are used to evaluate cleaning performance. Particles in an IPA suspension are deposited by generating an aerosol using a standard on silicon and oxide wafers. Silica, alumina, polystyrene latex (PSL), copper, tungsten and gold particles are used for the cleaning experiments.

Figure 6 shows optical micrographs of a silicon wafer before and after LSC, and the removal efficiency of microscale metallic particles. The optical micrographs clearly show a large cleaned area (resulting from one laser pulse). Outside the cleaned area, particles could still be seen. Copper, tungsten and gold particles larger than 1 µm are removed effectively by LSC as shown in Figure 6b.

The removal of PSL particles using LSC only was not as effective. The removal of 0.6 µm PSL particles from bare silicon wafers is shown in Figure 7. Less than 68% of particles were removed by LSC. Much of this is due to the adhesion-induced deformation of PSL particles on the wafer.12 However, UV laser irradiation prior to LSC greatly improved the particle removal, effectively breaking any organic bonding between PSL particles and the silicon wafer. Subsequent LSC successfully removed 98.5% of the particles on the wafer.

Unlike PSL particles, silica and alumina particles can be effectively removed using LSC only as shown in Figure 8. Removal efficiencies higher than 95% were obtained for both silica and alumina particles using LSC only. UV laser irradiation prior to LSC improves the removal efficiencies to 99%, indicating that organic removal prior to particle removal can be very useful. Figure 9 shows a particle scan map of silica particles on silicon wafers before and after LSC.

The effect of the gap distance (the distance between the laser beam focus point and the wafer) on the removal efficiency was also investigated (Fig. 10). The figure shows the number of silica particles remaining on the wafer after cleaning. It shows that there is a significant decrease of the removal efficiency as the gap distance increases. The shockwave force decreases and consequently the particle removal efficiency as the gap distance increases.

Damage-free LSC

LSC was also applied to patterned wafers. Wafers with 0.12 µm patterns were contaminated with 40 nm silica CMP slurry and dried before being cleaned by LSC. Figure 11 shows SEM micrographs of wafer surfaces before and after LSC. Slurry particles were completely removed from surfaces without any pattern damage. After LSC treatment using a narrow gap distance of 5 mm, no damage was observed (Fig. 12). Unlike other wafer cleaning techniques, LSC is capable of selective removal of defects (Fig. 13).

Conclusions

LSC is an effective new dry cleaning technique for the removal of micro and nanoscale particulate contaminants from wafers. Unlike conventional laser cleaning, no surface damage is observed with LSC because of the use of shockwaves. Successful removal of submicron inorganic particles was carried out by the LSC process; this has been known to be impossible to achieve using conventional laser cleaning techniques (without substrate damage), in which the laser directly irradiates the surface.

Micron and submicron metallic, silica and alumina particles have been successfully removed from the wafer surface. However, PSL particles were not completely removed without prior UV irradiation. This is due to the particle deformation leading to a large increase in the adhesion force.

The velocity of the shockwave depends on the input power of laser beam, and the gap distance. A 200 mm wafer can be cleaned in 30 sec because of the shockwave's large cleaning area per pulse (~4 cm in diameter). This offers a large process throughput, which was a problem for conventional laser cleaning.

Patterned wafers with 0.12 µm linewidths were also cleaned with LSC, and no pattern damage has been observed. Selective cleaning could be easily performed using the LSC technique. LSC will deliver a new way to remove particulate contaminants in both front- and back-end processes, in cluster tools or in combination with other etch, thin-film deposition or other semiconductor processes.

Ahmed A. Busnaina is the William Lincoln Smith Chair Professor and Director of the Nanomanufacturing Research Institute and the NSF Center for Microcontamination Control at Northeastern University. He specializes in nanoscale defects removal, mitigation and characterization, wafer cleaning technology, chemical and particulate contamination in LPCVD and sputtering processes, particle adhesion and removal, submicron particle transport, deposition and removal in clean environments. He is also involved in the fabrication of micro and nanoscale structures and interconnects. He has provided consulting services for several companies such as IBM, Motorola, Eaton, GE, GM, DuPont, Corning, Seagate, Xerox, IPEC, FSI, AMTX, Lawrence Livermore National Laboratory, etc. He established the Microcontamination Research Laboratory in 1988, including a fully equipped Class 10 cleanroom, to provide innovative basic and applied research to the semiconductor industry and equipment manufacturers. The laboratory became the nucleus for the current NSF Center for Contamination Control.

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