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Flare has become a significant problem for low K 1 lithography. Several authors have reported measurement of flare in projection lenses. 1, 2, 3 Most of the work is based on the Flagello-Kirk method using resist clearing dose. To measure the flare reliably and accurately using this method the contribution of the process needs to be understood. In this paper we present data looking at the influence of such effects on the measured flare.Flare or scattered light is caused by many sources in the imaging system and does not take a predictable path to the image plane. 1 There are many errors that can creep in to Flagello-Kirk measurement that increase the flare and aberrations attributed to the projection lens. 2, 3, 4 We review some of the process conditions that the lithographer can control to minimize the impact of such errors. These approaches also point to ways of reducing flare seen by product wafers. Experiment and resultsTraditionally flare calculation has been done as the ratio of the intensity in the dark region to that in the bright region, I F / I O , figure1. The dose ratio conversion to a flare estimate is based on the exposure time needed to clear the PAG/PAC in both regions using the following relationship. Energy = Intensity * exposure time % F l a r e = { I F / I O }*100 = { E 0 / E C }*100 = { t O / t C }*100In general this has been assumed to be a fixed quantity. Although substrate and the lens coating have been mentioned as possible sources of flare there is very little published data on their magnitude. The simplest way to test the constant flare assumption is to perform the Flagello-Kirk test on a resist swing curve. The test was performed on a 0.75NA ArF Scanner. As seen from figure 2 we have a flare swing curve. The energy trapped in the resist varies as a function of thickness and hence dose to clear the resist has a swing curve form. The remaining light is partly absorbed in the substrate and partly reflected back in to the lens. The reflected light returns to the wafer as flare but its amplitude does not change in tandem with the dose to clear and so we get a flare swing curve.An obvious way to desensitize the process from such interference phenomenon is to eliminate the reflected light. This can be done by means of a Bottom Anti-Reflective Coat, BARC. Figure 3 shows the result of using a BARC. The swing curve effect was suppressed but the measured flare increased. A review of the film stack shows that the for a resist refractive index of 1.7 the air-resist interface reflectivity is 0.067 and is independent of the resist thickness. By lowering the interface index mismatch we can reduce the reflectivity and reduce the flare light. We tested this hypothesis by using a Top Anti-Reflective Coat, TARC, with an index of 1.45. The TARC thickness was chosen by simulation, figure 4, to minimize the reflectivity into air due to thin film effects. The results shown in figure 5 indicate we were successful in reducing the measured flare by almost half.Since thin film effects of the wafer has suc...
Unpolarized light has traditionally been used for photolithography. However, polarized light can improve contrast and exposure latitudes at high numerical aperture (NA), especially for immersion lithography with an NA > 1.0. As polarized light passes through a reticle, any birefringence (BR) in the reticle material can cause a change in the orientation or degree of polarization, reducing the contrast in the final resist image. This paper shows the effects of reticle BR on dry and immersion imaging for 193nm lithography. The BR magnitude and orientation of the fast axis were mapped across several unpatterned mask blanks, covering a range of BR from 0 to 10 nm/cm. These reticles were printed with a series of open areas surrounded by test structures. The BR was measured again on the patterned reticles, and several locations were selected to cover a range of magnitudes at different orientations of the fast axis. Dry and immersion imaging were evaluated, looking at BR effects on dense lines and contact structures. Mask error enhancement factor (MEEF), line edge roughness (LER), and dose and focus latitudes were studied on line/space patterns. Dose and focus latitudes and 2-D effects were studied on contact patterns. Based upon these results, the effect of reticle BR on CD is minimal, even for BR values up to 10 nm/cm.
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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