Yong-Ho Oh

Resolution enhancement through optical proximity correction and stepper parameter optimization for 0.12-μm mask pattern

As the design rule of semiconductor microchips gets smaller, the distortion of a patterned image due to the optical proximity effect (OPE) becomes the limiting factor in the mass production. We developed an optical proximity correction (OPC) program that can be applied to a strong or attenuated phase shift mask as well as to a binary mask. The OPC program named OPERA is based on a stochastic approach as other rule-free OPC programs, but it has tow remarkable points. Firstly, proper cost function and optimization strategy enable us to achieve very closely clustered mask pattern that could be manufactured at a reasonable cost. Secondly, OPERA can carry out the optimization of illumination parameters for any modified illumination methods, such as, annular or quadrupole using the critical dimensions information of mask patterns.

Manufacturability evaluation of model-based OPC masks

A systematic method for the model-based optical proximity correction in presented. This is called optical proximity effect reducing algorithm (OPERA) and has been implemented to TOPO, an in-house program for optical lithography simulations. Comparing simulational results as well as experimental results, we found that OPERA is not only suitable for shape restoration but also for resolution enhancement. However, the resulting optimized patterns have a high degree of complexity and this brought up a number of issues for mask manufacturing. First, data volume and exposure time were dramatically increased for conventional e-beam file formats. This was solved by using the MODE6 format that preserves data hierarchy. Second, due to excessive shot divisions, a variable-shaped beam machine could not finish the exposure process. A raster-scan beam machine successfully finished the exposure. Finally, a die-to-die inspection was performed but many false defects that do not affect wafer printing were defected. This will be solved by a new type of tool that inspects a mask by evaluating its aerial image.

Filtering characteristics and grating images of multiphase grating optical low-pass filters

Although the grating optical low-pass filter (GOLF) has many advantages over the traditional birefringent low-pass filter, it is not adopted in commercial digital cameras because the shadow image of the grating is visible, especially when the F -number of a camera is large. We design and manufacture grating optical low-pass filters with multiple phases to compare their filtering characteristics and shadow images. From the simulation of the Fresnel diffraction pattern of a 1-D multiphase grating, we obtain the optimum width and phase of each grating step that minimizes the visibility of the Fresnel diffraction pattern. We design 2-D multiphase structures by extrapolating the 1-D structure. Four types of GOLF are manufactured with a semiconductor process and installed into a digital camera to compare grating images. We find that the visibility of grating images can be reduced by increasing number of phase steps.

Comparison of a grating optical low-pass filter with a birefringent filter

The characteristic of 2D grating optical low-pass filter (GOLF) was compared to that of birefringent low-pass filter (BLF). The modulation transfer function (MTF) of the optical system that consists of lens and GOLF is theoretically derived by taking all orders of diffracted beams into consideration. MTF of a 2 phase chess-board type GOLF and a 3-phase GOLF was compared to that of BLF. The 3 phase GOLF with 9 center beams of equal strength has the best filtering characteristic, and thus removes most moire fringe, but resolution degradation is severe compared to BLF. The two- phase GOLF with phase difference of 180 degrees that is similar to BLF in terms of beam distribution has medium characteristic of 3-phase GOLF and BLF. And, 3 phase GOLF of phase difference of 90 degrees has similar characteristics, too. These three GOLFs are made and experimented by attaching them to a digital camera. The experimental result coincides with the theoretical development.

Design of Multi-phase Holographic Optical Low-pass Filter for the Improvement of the MTF Characteristics

We studied the spatial filtering characteristics of a multi-phase hologram optical low-pass filter(HOLF). Using the Monte-Carlo based hologram generation program, we designed holograms whose diffraction patterns are circular shaped 21 beams and calculated the diffraction efficiencies and MTFs. 4-phase HOLF have the same diffraction efficiency as that of 2-phase HOLF. The MTF graphs of the two are also nearly alike. But 8-phase HOLF shows higher efficiency than those previously discussed and has larger MTF values in the low frequency region. 16-phase HOLF has just a little better characteristics than 8-phase. Considering the errors which can arise in the process of making holograms, 8-phase HOLF fits the goal of improving the resolution of spatial filter. We also fabricated 8-phase HOLF and .measured MTF The experimental results agree well with the theoretical expectations.

Grating Optical Low-Pass Filter with Near Perfect Stop Band Characteristics

The characteristics of a two-phase grating optical low-pass filter(GOLF) that has phase difference π were investigated. Through the modulation transfer function (MTF) calculation of GOLF, we found that the grating filter the phase steps of which have the width ratio of 1:3 cuts off high spatial frequencies above the Nyquist frequency almost completely. To verify the theoretical development, we manufactured GOLF on a glass plate through semiconductor process and measured its MTF with an optical transfer function (OTF) measurement system. The measurement agrees with the calculation within measured spatial frequency range.

Optical proximity correction of critical layers in DRAM process of 0.12-μm minimum feature size

We studied whether the critical layers of 0.12 micrometer DRAM could be processed with optical lithography techniques assuming ArF excimer laser as a light source. To enhance the aerial image fidelity and process margin, phase shift mask (PSM) patterns as well as binary mask patterns are corrected with in-house developed optical proximity correction (OPC) software. As the result, we found that the aerial image of the critical layers of a DRAM cell with 0.12 micrometer design rule could not be reproduced with binary masks. But, if we use PSM or optical proximity corrected PSM, the fidelity of aerial image, resolution and process margin are so much enhanced that they could be processed with optical lithography.

Correction of image distortion of CGH with a large diffraction angle

Most CGH programs use a model equation based on the diffraction angle. Therefore, if the diffraction angle is large enough, the image on a flat screen is distorted. To correct the distortion, we created the model equation from diffraction theory and verified it through experiment. We also suggest a design method that compensates for the distortion without changing the CGH program.

Effects of the Phase Error on the MTF Characteristics of Binary-phase Hologram Optical Low-pass Filter

When we design a binary phase holographic optical low-pass filter (HOLF), the phase difference is generally set to be to optimize the diffraction efficiency. However, the phase difference of real HOLF mostly deviate from by the error in the fabrication process. The deviation causes the (0,0)-th order diffracted beam to increase, which results In raising the diffraction efficiency. To study the effects of the phase error on the performance of HOLF, we calculated the MTF of HOLF for various phase differences. The results show that the phase error of 10 makes little change in the filtering characteristics of HOLF. Considering the filtering by lens and CCD, the effects of the phase error becomes much smaller. To confirm it experimentally, we fabricated HOLFs for various phase differences. After installing it in a digital camera, we take picture of test targets and observe the Moire fringes and the resolution. The results agree with our prediction.

Modified Illumination with a Concentric Circular Grating at the Backside of a Photomask

Modified illumination techniques have been used to enhance the resolution of the sub-wavelength lithography. But, since they shield the central part of incident light, the light efficiency is seriously degraded, which in turn reduces the throughput of a lithography process. In this research, we introduced an annular illumination structure that enhances the light efficiency with a concentric circular grating at the backside of a photomask. The efficiency of the structure was theoretically analyzed.