Characterizing best focus for lithographic patterns is a very common task. It has been observed that the estimated best focus changes considerably with substrate type and substrates change quite frequently in process development. Such effects are seen even when the resist thickness is not altered. In this paper we will present data to identify the cause of the change and throw some light on the interaction between substrate and scanner leveling system. keywords: Focus, substrate, interaction, tilt. IntroductionBest focus shift from the focusing of the resist is given by resist thickness divided by twice the refractive index, figure 1. 1 For a given feature and resist thickness, this offset from the aerial image is fixed. Any simulator capable of doing vector simulations in resist will account for this.Simulations of best focus are insensitive to the underlying substrate composition. In reality the best focus is dependent on the substrates. Since the DOF margin of future nodes are expected to be in the sub-300nm region, understanding the sources of such variations is critical to operating within the available margin.All photolithographic exposure systems, scanner or stepper, use some mechanism to detect the wafer surface. The most common method currently employed is to use an optical system with low angle of incidence and detection, figure 2. Any movement of the wafer in the vertical direction causes the leveling light to be incident at a point in the horizontal plane that is displaced from the nominal position. This has the same effect on the reflected light and its displacement is recorded by a detector. The light used can be either monochromatic or broadband. 2, 3, 4 Interference from the underlying stack and pattern affect the interpretation of the displacement and hence the resist surface and the nominal focus, figure 3. Sensor and wafer stack variations along the exposure field also give rise to focal plane tilts. Monochromatic leveling systems are more likely to be affected by this. 3 The following experiment was set up to determine the magnitude of such effects. Experimental setup and ResultsBossung curves of a semi-dense pattern were used since these are more sensitive than dense features to focus variations. The feature tested was binary 95nm 1:9 L/S oriented in the scan direction. The exposure was done on KrF scanners of NA = 0.80 with annular illumination of 0.85/0.55 outer/inner sigma setting. The resist thickness used was 3300A and the BARC thickness was 620A. Best focus is determined at the left, center and right of each field. For each field position the focus curve is measured on 5 fields. The data for each focus at each field position is averaged. Best focus is determined by a 2nd order polynomial fit to the mean curve, figure 4. For this test focal plane curvature was accounted for in the tilt calculation. Data was collected from 4 scanners, two of each with monochromatic and broadband light leveling systems.The test was repeated on the following substrates. a ) 3 , 0 0 0 A B P S G b ) 1...
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