As device features shrink, the maximum particle size that is tolerable in its manufacturing and operation phases will also shrink. It is estimated that over 50% of the yield losses in semiconductor industry are due to microcontamination.

According to SEMATECH’s International Technology Roadmap for Semiconductors (Lithography Roadmap, 2000 Update), the minimum feature sizes in the near future are projected to be 130 nanometers (nm) in the year 2002, 100 nm in 2005, and 70 nm in 2008.¹ It is often expressed that if the diameter of a particle exceeds about one-quarter of the minimum feature size, the particle could cause a fatal device failure.

Removal of nanoparticles is especially challenging since, as the particle size decreases, the adhesion stress between the particle and substrate increases by a fractional power law. The stress at the interface can become so intense that the required removal force per unit area becomes extremely high in nm range and plastic deformation may occur at the interface. The removal of nanoparticles is a nontrivial problem in micro- and nano-manufacturing.

Dry Laser Cleaning

The dry laser cleaning (DLC) method utilizes the thermo-elastic coupling effect produced by the direct irradiation of a substrate with a laser pulse. When a laser pulse irradiates a substrate, the portion of the absorbed pulse energy, which is dependent on material optical properties, transforms into thermal energy. This creates a rapid thermal expansion of the irradiated surface, which results in elastic waves that propagate through the surface of a substrate. As the elastic waves travel, the surface of substrate experiences an acceleration field, which causes the particles that are attached to the surface to accelerate accordingly. Particle detachment occurs when the resultant removal force acting on the particle due to the surface acceleration is greater than the adhesion forces holding the particle on the surface.

Our simulations of the surface accelerations due to pulsed laser irradiation have been performed using the finite element analysis package, ABAQUS^®. The radial distribution of maximum acceleration generated on a Silicon substrate is summarized in Figure 1.² The Q-switched Nd:YAG laser parameters used in the simulations were specified as follows: 5 ns pulse width and 370 mJ pulse energy. The calculated minimum out-of-plane acceleration needed to detach a 500 nm silica particle from a silicon substrate is approximately 5x10⁹ m/s². Data presented in Figure 1 indicate that with the above specified laser parameters, this magnitude of acceleration can be achieved.