Jai-Cheol Lee

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.

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.

EDLICS: a new relaxation-based electrical circuit simulation technique

Event-Driven Local-Interactive Circuit Simulation (EDLICS) is a new approach to the transient simulation of electrical circuits. Unlike the simulation methods which are based on differential equations, EDLICS directly models the physical behavior of electrical components. The EDLICS approach propagates incremental changes in circuit variables (voltage, current, and charge) as events similar to the behavior of event-driven logic simulation. An electrical component receives an event (either voltage, current, or charge) from a node, converts it to another event and delivers it to a neighboring node. The transient behavior of a circuit is analyzed by evaluating capacitors with current flow for charge buildup. EDLICS is the generalization of relaxation-based simulation, although it originated in a different perspective. The Kirchhoff Current Law, Kirchhoff Voltage Law, and capacitor current integration formula are used interactively in the Gauss-Seidel fashion. Since EDLICS is based on the true behavior modeling of electrical components, it is capable of handling circuit components and circuit configurations which other relaxation-based simulators cannot. The EDLICS technique is applied to a MOS circuit simulator, EDsim. EDsim is one to two orders of magnitude faster than the latest version of SPICE3 and compares favorably with iSPLICE3.<<ETX>>

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.

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.

Monte-Carlo-Based Automatic Design of Chromeless Phase Shift Mask

The optical proximity effect correction algorithm (OPERA) program based on the Monte-Carlo method has been used for optical proximity correction (OPC) and phase shift mask (PSM) generation for low-NA projection lithography systems. To apply OPERA to high-NA systems, we adopted an imaging model based on a vector diffraction theory. Also, we developed a new convergence algorithm that does not reduce the degree of freedom and prevents the convergence process from falling into a local minimum. The new program, OPERA-V, is at least 10 times faster than the previous version. OPERA-V not only improves the computation time of traditional OPC, but can also be used as an automatic design tool for creating chromeless PSMs.