New Particle Monitoring Strategies in UPW for 300 mm Fabs

Process failures caused by particles in Ultra Pure Water (UPW) are relatively infrequent events, but with the move to 300 mm wafers and 90 nm line widths, they can have significant financial impact. Most particle monitoring strategies in the past focused on the use of the mobile and inline high sensitivity 0.05 micron particle counters. This paper examines the opportunities to improve performance, track data trends, and reduce cost by using a combination of both high sensitivity particle counters (to 0.03 microns) and less expensive lower sensitivity (0.1 microns) counters with very high sample volumes. The scope of this paper details the impact of size sensitivity, sample volume, and zero count level on particle counter performance. Practical monitoring considerations and particle distribution differences between poorly filtered and highly filtered systems are discussed. Introduction UPW is a vital component in semiconductor fabs. It is used in chemical make-up, CMP processing, and wafer cleaning and rinsing. In 300 mm fabs, as much as 1500 gallons of water are used to fabricate one wafer. As critical dimensions have decreased to 90 nm and smaller, the requirement for higher purity water and the need to monitor smaller particle sizes has increased. In order to fully understand the benefits and drawbacks of any particle monitoring strategy, it is critical to understand the behavior of particles in UPW and some basics of particle counter design. Two of the most significant points are that particle concentrations in UPW are very low and that the volume of sample measured per unit of time (sample volume) by the particle counters can be very small. These two factors can result in poor counting statistics and poor particle counter performance. The push from the semiconductor industry to identify particles of smaller and smaller size resulted in the development of very high sensitivity instruments but at the cost of measurement precision due to small sample volume. Only recently has instrumentation been developed that is capable of meeting the strict sensitivity requirements while also improving the size of the sample volume. Since both particle counter size sensitivity and sample volume affect data variability, different counter designs may optimize one of these variables over the other. With an understanding of particle size distributions in UPW, less sensitive and less expensive particle counters can be quite useful in particle monitoring programs. Particle Concentrations and Size Distribution in UPW Systems Two criteria are needed to specify a particle concentration: 1) the number of particles/volume of fluid and 2) the minimum size of those particles. Particle concentrations in UPW system are very low, often less than 0.5 particles/mL for particles ≥ 0.05 microns. New 300 mm facilities are striving to achieve concentrations < 0.2 particles/mL for particles ≥ 0.05 microns. Particle concentrations are much greater for small particles when compared to particle concentrations for larger particles. In fact, particles distributions in filtered UPW systems follow a very predictable power law distribution, usually ~ 1/diameter as established by Knollenberg and Veal (1). This distribution results in a ratio of cumulative particle counts to particle size as shown in Table 1. Due to the exponential relationship between the number of particles and the particle size, graphs of particle size distribution are best expressed logarithmically (Fig.1). Typical Particle Size Distribution in UPW Particle Size (μ) Ratio of Particle Counts