Wafer Inspection System: Calibration and Measurement
Wafer surface scanners are widely used in semiconductor industries to detect particle contamination on wafers. The performance characteristics of a PMS SAS-3600 wafer surface scanner have been evaluated using ideal polystyrene latex (PSL). It was also used to measure irregularly shaped silicon particles. Three sizes of PSL spheres (0.199μm, 0.3μm, and 0.426μm) were used to study the sizing accuracy and counting efficiency of this wafer scanner. The results show that this scanner sizes 0.199μm and 0.3μm PSL reasonably accurate. But it cannot size 0.426μm PSL very well. The background noise lever was so high that this scanner could not measure particles near its nominal lower detection limit: 0.1μm. It over-counted Si particles by 28%. An electrostatic enhanced wafer deposition chamber was used to prepare the test wafers. Both the theoretical calculations and experimental results show that electrostatic force increases the deposition velocity by 2 orders of magnitudes for particles smaller than 1μm. The sedimentation is the dominant mechanism for particles larger than 1μm.
Sizing Accuracy, Counting Efficiency, Lower Detection Limit and Repeatability of a Wafer Surface Scanner for Ideal and Real‐World Particles
The performance characteristics of a Tencor Surfscan 4000 wafer surface scanner have been evaluated using ideal polystyrene latex (PSL) spheres and irregularly shaped real-world particles of Si and SiO 2 . The particles were uniform in size and were deposited on bare silicon wafers and used as standard calibration wafers to study the scanner response. Particles in the 0.1 to 1.0 μm diam range were used. The sizing accuracy, counting efficiency, lower detection limit, and count repeatability of the scanner were studied systematically. The full Maxwell's electromagnetic equations also have been solved numerically on a supercomputer to obtain the light-scattering cross section of the particles on bare silicon wafers
The effects of electrostatic and inertial forces on the diffusive deposition of small particles onto large disks: Viscous axisymmetric stagnation point flow approximations
Fine particles can deposit on microelectronic parts during their manufacture and cause damage. Prediction of the rates of deposition is important in planning to reduce such losses. Analytic expressions were developed and Monte Carlo simulations (Brownian dynamics) were carried out to predict deposition rates of sub-micron particles in viscous axisymmetric stagnation point flow. Diffusive, inertial and electrostatic deposition mechanisms were incorporated in the analysis. The electrostatic forces considered included the Coulombic force on a charged particle in an electric field and the image force on a charged particle near a conducting plane. Analytical solutions to the convective-diffusion equation, in the absence of inertial effects, showed that for a Boltzmann charge equilibrium, the electrostatic image force did not appreciably enhance deposition for a free-stream velocity of 50 cm s−1. However, the Coulomb electrostatic attraction of a charged particle towards a charged surface greatly enhanced deposition, assuming a field from the disk approximately equivalent to having a disk 20 cm in diameter (80 pF capacitance) at 2000 V. Some areas for future study are suggested by these results. Brownian dynamics simulations carried out for particles of the order of 1 μm in diameter at a much higher velocity (30 m s−1) demonstrated an interaction between inertial and diffusive deposition that produced inertially enhanced deposition at Stokes values below the critical value, St < 0.15. Where inertia and electrostatic effects were negligible, the simulations matched the known solution for stagnation flow deposition due to diffusion. In the Appendix is presented a derivation of the Langevin equation governing single-particle motion in a fluid that is not uniform in temperature as well as in velocity.