![]() ![]() Light reflected from the second surface serves as the reference while a portion of the light passes through the flat to interrogate the wafer (test flat). The wedge angle ensures that reflections from the first surface of the flat do not contribute to the interferometric signal. This interferometric technique compares the wafer to a reference wedge (or reference flat) of very high quality and flatness. The basic design used to measure topography on bare wafers is similar to the Fizeau interferometer shown in Figure 3. Extremely precise interferometric tools have been developed to measure such variations in wafer shape prior to processing. Such deformations can affect pattern imaging at nm scales. For example, a wafer can bend or the chuck (either electrostatic or pneumatic) that supports a wafer can produce indentations in the wafer at points of contact. To help counter such effects, inspection tools for sub-100 nm defect detection employ highly sophisticated optical spatial filtering, analysis of polarization of the scattered signal, and specialized signal processing algorithms to detect defects in the presence of surface haze.īare wafer topography is measured for many reasons. Surface chemical contamination from sources such as ambient humidity can also contribute to reductions in SNR. Sub-100 nm inspection for non-patterned wafers is complicated by issues of scale with SNR being a critical parameter in determining an inspection system's detection limits for particles and other defects on wafer surfaces. As might be expected, a high degree of precision and accuracy is required in the motion control of the wafer stage and the optical components in systems used in these applications.Īs inspection tools are required to sense and quantify ever smaller particles, the impact of factors such as surface microroughness (haze) begins to influence the detectability of small particles owing to reductions in the SNR of the scattered light signal. Sophisticated image analysis algorithms are claimed by some manufacturers to achieve sub-20 nm sensitivity. These tools employ the same basic operating principles as tools designed for larger scale defect detection but use DUV illumination-enhanced optical systems. ![]() Sub-100 nm inspection tools are currently used in manufacturing environments to provide quality assurance on incoming wafers and for process tool monitoring and qualification for high volume manufacturing. Note that the complexity of scattering from patterned surfaces reduces the overall photon flux to the detector, resulting in longer integration periods for wafer inspection. It may use bright-field and/or dark-field imaging, depending on the application. Patterned wafer inspection is a much slower process. ![]() In general, dark-field inspection is preferred for non-patterned wafer inspection since high rastering speeds are possible and this enables high wafer throughput. This method requires highly accurate and repeatable rotary and linear motion control of the wafer stage and optical components. This map provides information on defect size and location and on the condition of the wafer surface due to issues such as particle contamination. In wafer inspection tools, the light intensity is electronically recorded using a PMT or CCD and a map of the scattered or reflected intensity over the wafer surface is generated, as shown in Figure 2. The rotational position of the wafer and radial position of the beam define the position of the defect on the wafer surface. Depending on the illumination arrangement, the scattered light can be detected directly (dark-field illumination) or as a loss in intensity in the reflected light beam (bright-field illumination). When the laser beam encounters a particle or other defect on the surface of the wafer, the defect scatters a portion of the laser light. This type of reflection is referred to as specular reflection. The laser light is reflected from the surface as it would be from a mirror, as is shown in Figure 1. A laser beam is radially scanned over the surface of a rotating wafer to ensure that the beam is projected onto all parts of the wafer surface. ![]() The basic principle used for defect detection on non-patterned wafers is relatively simple. 219.Device manufacturers use optical detection systems to inspect wafers and masks for particles and other types of defects and to determine the position of these defects in an X-Y grid on the wafer. Properties of Crystalline Silicon, edited by R. Howlader, Proceedings of the 54th Electronic Components and Technology Conference ( IEEE, New York, 2004), p. ![]()
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