Focus monitor technology for attenuated phase shift masks under annular illumination has been developed for a in-line quality control. The focus monitor pattern on a reticle employs a pair of grouped lozenge-shaped opening patterns in the attenuated phase shifting region. Since the phase shifting angles of light passing through the first and second opening patterns are 90° and 0°, respectively, the best focus position for the first pattern shifts to that of the second pattern. Subtraction of the length of the patterns is a linear function of the actual focal position printed on the wafer. The linear function is insensitive to further mask phase error. Therefore, the effective focal position can be extracted by measuring that subtracted from the measured length. High resolution of 10 nm defocus was achieved using this technique.
Desirable wafer edge flatness was investigated to obtain optimum free-standing wafer edge shape for photolithography. In order to obtain the criteria of free-standing edge shape, we clarified the desirable post-chuck flatness at edge sites in advance. We investigated a desirable free-standing wafer edge, taking into consideration both the wafer and wafer holder shape. Firstly, to obtain a desirable post-chuck wafer edge shape, the vicinity of wafer edge after chucking was modeled, and SFQR was simulated. Secondly, a shape in the vicinity of free-standing edge shape was modeled, and the edge flatness after chucking was simulated. And finally, the simulated flatness was compared with the desirable post-chucked wafer edge shape, and we could obtain desirable free-standing wafer edge shape. Individual measurement of the free-standing back-side and front-side surfaces as well as the thickness of the edge position was found to be necessary for accurate estimation of the post-chuck edge shape.
A focus monitor technology for attenuated PSM under annular illumination has been developed as an in-line quality control. The focus monitor pattern on a reticle employs a pair of grouped lozenge-shaped opening patterns in attenuated phase shifting region. Since the phase shifting angles of the light passing through the first and second opening patterns are 90 degrees and 180 degrees, respectively, the best focus position for the first pattern shifts to that for the second pattern. The subtraction of the length of the patterns is a linear function of the actual focal position printed on the wafer. Therefore, the effective focal position can be extracted by measuring the subtraction of the measured length. A high resolution of 10-nm defocus could be achieved by this technique.
We have developed in-line dose and focus monitoring techniques for the detailed analysis of critical dimension error and accurate process control. From exposed wafers, effective does and focus are measured with specificed monitor marks built on a reticle. The contributions of effective dose and focus to critical dimension error on device chips were clarified in a fabrication proces of 110 nm isolated pattern with a KrF scanner. The critical dimensions error was described as a function of effective dose and focus, which include various process fluctuations. We could determine whether current exposure settings such as dose input and focus input were adequate or not. Based on the experimental data, we estimated the benefit of simultaneous Run-to-Run control of dose and focus. The estimation clarifies that it realizes total critical dimension control including Run-to-Run and intra-Run.
We have established effective dose metrology using a dose monitor mark named the effective dose meter, which has no focus response. By placing the effective dose meter onto the scribe line in a device reticle, in-line monitoring of the effective dose on a product has been realized. The effective dose meter is designed to monitor the effective dose as a resist line length whose dimension is detectable with an optical measurement tool. The design is considered to have no impact on both reticle fabrication and wafer processing. By monitoring with the effective dose meter, the contribution of effective dose error to critical dimension variation is obtained independently of focus error. Dose budget analysis from the in-line effective-dose monitor clarifies the current process ability on reticle linewidth variation and resist processing uniformity. This paper describes the mark design and the analysis result of the in-line effective dose monitor in device fabrication with KrF lithography.
Wafer-induced focus error is investigated for analysis of our focus budget in photolithography. Using a newly developed wafer monitor, NIWF-300 (Nikon Corp.), we directly measure surface flatness of the wafer placed on wafer holder with vacuum chuck. Single site polished Si wafers were evaluated with NIWF-300 and a conventional flatness monitor. We also investigated the effect of wafer holder using a ring-shape wafer support and a pin-shape wafer support. As a result, we found wafer shape measured in a freestanding condition does not represent surface flatness of the wafer on a holder. The holder has an impact on the wafer surface. The increase of adsorption ratio between wafer and holder improves the surface flatness.
We have established the effective dose metrology using a dose monitor mark named the effective dose-meter that has no focus response. By arranging the effective dose-meter onto scribe line in a device reticle, the in-line monitor of effective dose on product has been realized. The effective dose-meter was designed to monitor effective dose as a resist line length whose dimension is detectable with an optical measurement tool. The design is considered not to impact on both reticle fabrication and wafer processing. By monitoring the effective dose-meter, the contribution of effective dose error to critical dimension variation could be obtained independently with focus error. Dose budget analysis from in-line effective dose monitor made clear the current process ability with respect to reticle linewidth variation and resist processing uniformity. This paper describes the mark design, and the analysis result of in- line effective dose monitor in device fabrication with KrF lithography.
An accurate measurement technique for effective exposure dose in optical microlithography has been developed. The effective exposure dose can be obtained by a dose monitor mark in a photomask named effective dose-meter, consisting of plural segments including grating patterns with a pitch below the resolution limit and different duty ratios gradually. Since the effective dose-meter does not resolve on a wafer but it makes flood exposure with the dose as a function of the duty ratio, residual thickness of the photoresist after development changes according to the duty ratio. Therefore, the effective exposure dose can be obtained by grasping the duty ratio of the grating patterns in the effective dose-meter corresponding to the position that the photoresist had cleared completely. A calibration technique utilizing an aerial image measurement system also has been proposed to avoid the influence of intra- wafer process variation. The advantages of this method are (1) completely focus-free, (2) the effective dose-meter is small enough to ignore the influence of the intra-wafer process variation on the accuracy, and (3) highly dose resolution of less than 0.5%. It was found that this technique function effectively. The variation of the effective exposure dose in a wafer in the current krypton-fluoride-excimer-laser lithography process was measured as a demonstration of this technology.
A methodology for measuring the effective illumination source shift in exposure tools has been established. A grating-pinhole mask is placed upside-down on mask stage, and exposed. This mask consists of square pinholes with 80 micrometers square and 2D square lattices in these pinholes. The pitch of the grating pattern is suitably designed so that the 1st-order diffraction beams can illuminate the edge of the pupil of the projection optics. Both the shape of illumination source and the silhouette of the pupil of the projection optics are projected on the wafer located by normal photoresist. A conventional optical microscope is available for easily observing the photoresist patterns. The grating-pinhole consisting of attenuated phase-shifting structure has found to be also effective to measure both effective coherence factor and intensity non-uniformity of effective illumination source.
A simulated annealing, one of the optimization algorithms, has been applied to the optical proximity correction. Using attenuating phase shifting mask with annular illumination, the stepper condition as well as mask design has been optimized to correct line shortening for typical cell array pattern of dynamic random access memory. A cost function has been designed to reflect the desired dose-window and depth of focus obtained by ED-tree analysis. The performances of escape from a local minimum and the convergency have been demonstrated. Then all parameters that dominated the optical proximity effect have been optimized to minimize the cost function under practical constraints.
Phase contrast lithography using an annular-shaped phase only filter on the pupil plane of the projection optics was found to enlarge depth of focus for lines and spaces, isolated lines, spaces and hole patterns. The lines and spaces and isolated lines prefer an annular illumination. On the contrary, higher coherent illumination was effective for isolated spaces and hole patterns using the same phase modulation. It is not necessary to change the phase filter for each lithographic level. Phase contrast lithography gives us larger depth of focus in combination with halftone phase shifting mask. The phase filter does not have the problem of heating, and has high efficiency of exposure light. Several simulation results are presented, and the possibility of the phase contrast technology becoming a candidate for quarter micron lithography is discussed.
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