• Document: Wafer Inspection System: Calibration and Measurement
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Wafer Inspection System: Calibration and Measurement Author: Xiaoliang Wang Professor: David Pui TA: Keung Shan Woo Lab Members: Xiaoliang Wang, Meghan Kearney, Bruce Mehdizadeh, Bob Chenny Lab Date: April 24, 2002 Lab Location: EE Cleanroom Report Date: April 25, 2002 Wafer Inspection System: Calibration and Measurement Abstract 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. Introduction: Particle contamination on the semiconductor is a very important problem in integrated circuit fabrications. As the feature sizes shrink to smaller than sub-micron dimensions, particle contamination on the wafer is the leading cause of products yield loss [1]. Figure 1 shows a relation of failure rate affected by particle with DRAM memory capacity or minimum feature size. It shows that 95% of failure is caused by particle contamination at 16 Mbit DRAM [2]. 1 Figure 1. DRAM failure rate caused by particles contamination [2] The wafer surface scanner is the state-of-the-art instrument to monitor defects on wafers. There are various methods to measure the number and type of defects on wafers, including chemical (in-situ residual gas analysis), optical (film thickness sensors and laser-scattering detectors) and beyond (x-ray and electron beam scattering) [3]. By far, the most common and sensitive instruments for measuring particles are those based on light scattering, which are called wafer surface scanners. Such scanners are essential for many applications, such as inspection of incoming bare silicon wafers, measuring particle contamination added by processing equipment, and evaluation of the efficiency of wafer cleaning systems [4]. The specifications of several commercial available wafer scanners are listed in Table 1. The mechanism of a wafer scanner is similar to that of the airborne or liquidborne particle laser particle counters. The wafer is swept by a laser beam. The light is scattered by particles and the surface. The photodetector collects the scattered light and converts it to a voltage signal, which is a strong function of the particle size and refractive index. Therefore, light scattering is a high-resolution method for both sizing and counting, especially for sub-micrometer sized particles. Today’s instrumentation can count single particles with effective light-scattering diameter as small as 0.05µm [5]. The major performance parameters of the wafer scanner are: sizing accuracy, counting efficiency, lower detection limit, and counting repeatability [6]. 2 Table 1 Wafer Surface Scanners1 Wafer Surface Scanners Incident Collection Lower Manufacturers Model Laser Measurement Range Angle Angle Detection Surfscan 4000 2mW He-Ne -45º ~ -5º 0.006-1024µm 2 Tencor Instruments 0º 0.2µm (PSL) (0.6328µm) 5º ~ 45º (11 Channels) 2400 Charleston Road Surfscan 5500 Mountain View, CA 94043 Surfscan 6200 Tel.) 415/9696767 Surfscan 7000 Fax.) 415/9696371 Estek Products Division WIS-8500

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