Kazuya Kamon

Fast and accurate optical proximity correction based on aerial image simulation

Because optical lithography requires precise CD control, we developed a fast, accurate proximity correction method based on aerial image simulation. Simple formulas using a linear combination of simulated aerial image intensities both at and around mask edge were found effective for fast, precise CD prediction. Using the developed CD prediction and the fine biasing correction methods, we verified that various two-dimensional patterns printed by an i- line stepper using modified illumination and binary intensity mask are satisfactorily corrected; i.e., CD deviations from designed values, line shortening and feature deformations are effectively reduced.

IMPROVEMENT OF FOCUS AND EXPOSURE LATITUDE BY THE USE OF PHASE-SHIFTING MASKS FOR DUV APPLICATIONS

Three types of phase-shifting mask designs are studied with respect to their suitability to print periodical L/S structures. The evaluation criteria are DOF, exposure latitude, linearity, and image contrast and slope of the intensity profile. Mask-making issues are also taken into account. The investigation is based on both simulations and experimental results. A fully transparent shifter causing an optical shift of 180 degrees is considered. A negative tone photoresist is used for the exposures with a KrF excimer laser stepper (248 nm).

Shape effects of edge-line phase shifter on light intensity contrast

By using two-dimensional simulation, dependence of light intensity contrast on numerical aperture (NA), coherence factor (a) of i-line stepper, shifter-width, phase error and slope angle of phase shifter edges have been investigated. For the slope angle of 90° and the shifter phase of 180°, the highest contrast is obtained for NA=0.65 and a=0.3. As the slope angle becomes to be small, contrast degrades remarkably for high NA(=0.65). On these simulation, 0.18pm resolution limit of isolated space pattern is successfully realized using an image reversal resist process.

Effect of shifter edge angle and lens aberration on the pattern profile in the edge-line phase-shift method

Origin of asymmetrical resist patterns, which had been observed in the edge-line phase shift lithography, has been investigated by simulations and experiments concerning the affects of shifter edge angle and stepper lens aberration. It has been found that the asymmetry of resist patterns has been caused by coma aberration of stepper projection lens and enhanced as the shifter width becomes narrower. Furthermore, the effect of shifter edge angle has been proved equivalent to the effect of narrowing the shifter width.

New phase-shifting mask structure for positive resist process

One of the problems in applying the phase-shifting method in the positive resist process is the resist bridge generated at the phase-shifter edge. This problem has occurred in the past because the light intensity decreased to zero due to the interference at the phase-shifter edge. In order to solve this problem, we propose a new phase-shifting mask structure containing an intermediate phase-shifter. This intermediate phase-shifter will change the phase of the light by 90 degrees and will be placed at a peripheral edge of the conventional phase-shifter on the transparent substrate. The effect of this mask structure is demonstrated. A 0.3 micrometers lines and spaces pattern is successfully resolved without resist bridge, and the DOF at a 0.35 micrometers lines and spaces pattern is 1.2 micrometers wide. It is also demonstrated that this mask structure is effective on patterns such as LOCOS.

Estimation of optical proximity effect caused by mask fabrication error

To get wide lithography latitudes in ULSI fabrication, an optical proximity correction system is being widely used. We previously demonstrated that the optical proximity effect is highly dependent on beam interference conditions. By using an aperture with a spindle shaped opaque region and a controlling interference beam number optimized for imaging, we can obtain a high correction accuracy of less than +/- 0.01 micrometers for all kinds of pattern. To put the optical proximity correction into practical use, we must fabricate the corrected mask either by an EB or a laser writing system. But during mask writing, there is another problematic proximity effect. The optical proximity effect caused by mask fabrication error is becoming a serious problem. In this paper, we estimate the optical proximity effect caused by mask fabrication error. For EB writing, the mask feature size of 0.35 micrometers line changes dramatically in a space less than 0.8 micrometers in size; this is not tolerable. For a large pitch pattern, modified illumination reduces the DOF to 0 micrometers . Otherwise, laser writing stably fabricates a mask feature size for a 0.35 micrometers line, and the modified illumination reduces the optical proximity effect. This resist feature fluctuation is binary, so, correcting the mask pattern is easy. Although, it was wrongly thought that for larger pitch pattern, the DOF was reduced by the modified illumination, the DOF reduction actually came from the combination of the two proximity effects. Using an accurate mask produced by a laser writer, we do not observe any DOF reduction in modified illumination. Moreover, this has led to development of an optical proximity correction system with EB proximity correction.

Optical proximity effect of a next-generation superresolution technique

A new super resolution technique with wide applicability and high resolution enhancement has been developed. Various optical properties of the new optics have been analyzed by some optical simulations. Using a spontaneously optimized source shape for the mask pattern itself leads to better applicability with the new optics. From experimental verification, the DOF of the new optics is 1.5 times wider than the modified illumination. An alignment accuracy of 0.1 micrometers (on mask) is acceptable between the first and second mask. According to simulation results, the optical proximity effect is reduced by the new optics. These features offer significant merits to photolithography technology. Therefore, we estimate the optical proximity effect by optical image measurements and simulations. From the simulation result, we can use the third source aperture to control the optical proximity effects. For conventional illumination, a 0.35 micrometers L/S image is formed by two beam interference. For the new optics, the same pattern is imaged by four beam interference because the UV light is diffracted twice by the mask pattern. While the optical proximity correction system was being developed, it was confirmed that the pattern size fluctuation is sufficiently smaller for four or more beam interference. The experimental results of an x-z image profile for 0.35 micrometers , 0.4 micrometers or 0.5 micrometers line and any space are consistent with simulation results.

Time-dependent simulation of acid and product distributions in chemically amplified resist

In KrF or ArF resist processing, a chemically amplified resist is widely used for ULSI device fabrication. Due to the catalytic reaction of generated acid, decomposition of a positive resist or cross linking of a negative resist is amplified during post-exposure baking. In order to take into account these characteristics during resist simulation, a resist simulator based upon the percolation theory is developed, and the acid and product distributions during post exposure baking are iteratively calculated. Thus, we can conclude that the acid and product distribution in resist film are time dependent. Moreover it is necessary to develop using percolation theory a resist simulator that can take into account macroscopic feature changes from microscopic molecular structural change. The dissolution rate curve and distribution of acid diffusion length are derived with percolation theory. Then the distribution of the product that corresponds to decomposition in a positive resist is calculated. When we increase the acid thermal diffusion enough to reduce the standing wave effect while keeping it small enough not to reach neighboring patterns, the contour lines of product distribution from the thermal catalyst reaction move vertically rather than horizontally. (This is not a molecule movement.) By using these features, the resist rectangularity is improved and the DOF is chemically enlarged.

Application of phase-shift mask to GaAs IC fabrication process

To obtain stable light contrast, a pattern accuracy, alignment accuracy and irradiation resistance of SOG have been investigated, and pattern layout has been optimized for the subresolution PSM and edge-line PSM. Satisfactory pattern accuracy and no deterioration in transmissivity index during exposure of 500,000 J/cm2 have been confirmed on fabricated PSMs. Applying these PSMs to GaAs IC process, a fine gate finger pattern of 0.3 micrometers has successfully been formed without generation of undesirable pattern in the joint area of gate finer and large area pad.

Optical proximity correction for super-resolution technique

In order to support next generation ULSI devices, some super resolution techniques are developed. The super resolution technique is effective for smaller pattern but not for larger pattern. This is because the optimum dose is changed, due to the pattern characteristic. However, the z-image profile has sufficient focus latitude. To overcome this problem, the optical proximity correction (OPC) is effective. This phenomenon is observed in the conventional illumination as well as the other super resolution technique. Thus, we developed the OPC system. Using the parallel processing system, we can correct the memory device data in about 2 days. The active region reduction due to the optical diffraction was preferably compensated by the OPC system. Therefore, the OPC system can be applied to the practical use. The OPC system is applicable to the super resolution. Consequently, the applicability of the super resolution technique is significantly enhanced.