Tadahito Fujisawa

Optimization of partially coherent optical system for optical lithography

A new approach, based on the optimization algorithm of ‘‘simulated annealing,’’ is applied to maximize depth of focus for a partially coherent optical system. Optimization is carried out with practical constraints for optical lithography. A phase contrast lithography, consisting of an annular effective source and an annular phase filter on the pupil, is proposed as a definite result of the optimization procedure.

Measurement of effective source shift using a grating-pinhole mask

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.

Novel in-situ focus monitor technology in attenuated PSM under actual illumination condition

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.

Run-to-Run CD Error Analysis and Control with Monitoring of Effective Dose and Focus

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.

Effective exposure-dose measurement in optical microlithography

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.

Effective exposure-dose monitor technique for critical dimension control in optical lithography

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.

Highly accurate and precise measurement technique for effective exposure dose

The recent advantage of a measurement technique for effective exposure dose (EED), which is performed by monitoring the residual thickness of the photoresist, has enabled us to elucidate errors consuming the dose margins. The EED describes total thickness variation caused by non-uniformity of illumination, post exposure baking, and non-uniformity of development, and so on. In this paper, the principles for EED measurement are discussed. The accuracy and precision of this technique is shown. Furthermore, the dynamic range of this technology is considered.

CD control with effective exposure dose monitor technique in photolithography

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.

Analysis of wafer flatness for CD control in photolithography

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.

Mask defect specification in the spacer patterning process by using a fail-bit-map analysis

We obtained the acceptable mask defect size for both opaque and clear defects in the spacer patterning process using the fail-bit-map analysis and a mask with programmed defects. The spacer patterning process consists of the development of photoresist film, the etching of the core film using the photoresist pattern as the etching mask, the deposition of a spacer film on both sides of the core film pattern, and the removal of the core film. The pattern pitch of the spacer film becomes half that of the photoresist. Both the opaque defect and the clear defect of the mask resulted in a short defect in the spacer pattern. From the fail-bit-map analysis, the acceptable mask defect size for opaque and clear defects was found to be 80nm and 120nm, respectively, which could be relaxed from that in ITRS2008. The difference of the acceptable mask defect size for opaque and clear defects comes from the difference of the defect printability at the resist development.