Hai Bin Chung

Design of illumination system for ArF excimer laser step-and-scanner

In a lithography tool, illumination uniformity on the wafer surface is important, because the variance of intensity on the wafer surface makes it hard to control the line width of the pattern. An illuminating system for ArF excimer laser step- and-scanner has been designed and assessed. The system showed good illumination homogeneity in both of the reticle and pupil plane with a reasonable light transmission efficiency. The goal of design is the uniformity within plus or minus 1% on the reticle surface. In order to achieve the goal, the output beam of the excimer laser with nonuniform intensity distribution was re-shaped by using a beam expander which is composed of 4 cylindrical lenses, and the zoom lens varying the beam size according to the aperture of flys eye lens integrator. The flys eye lens integrator consists of 208 lenses and generates the good homogeneity in the reticle plane. The effective light sources, i.e. the images made by flys eye lenses, were projected onto the entrance pupil of the combined system of projection and relay lenses. The exposure field at reticle plane is 104 X 20 mm2, and is defined as the image of reticle blinder by the 1x relay optics. The designed illumination system showed good performance by simulation and it will be adequate to the ArF excimer laser step-and-scanner under development.

Effect of Degree of Coherence in Optical Lithography Using Dummy Diffraction Mask

The effects of degree of coherence in optical lithography using the dummy diffraction mask are investigated by simulation and experiment. Influences on resolution and depth of focus of the repeated and isolated patterns are simulated for the several coherence factors of the illumination. The process margins are also simulated through the exposure vs. defocus diagram. The lithographic performances of the complex periodic patterns are experimented. As degree of coherence (DOC) decreases, the contrast benefit is increased for line and space(L/S) patterns size less than 0.5 λ/NA where NA is the numerical aperture, and the depth of focus(DOF) benefit is increased for L/S patterns size less than 0.65 λ/NA. The smaller L/S patterns size is, the greater the influence of DOC is. In the isolated patterns of sub-resolution type, better lithographic performances could be obtained for the smaller coherence factor.

Probe-beam scan-type autofocus system using position-sensing detector for sub-half-micron lithography tools

The depth of focus in sub-half micron lithography is about plus or minus 5 micrometers , several factors, such as field image curvature, circuit topography, wafer flatness error and auto-focus errors, reduce the usable focus margin. To minimize these focus margin decreasing factors, multi-point focus detection and/or field chip leveling control should be required before every exposure. Even the cases, global deviations of wafer and chuck flatness errors are being corrected with the good field leveling system, the focus sensing accuracy must stay in the plus or minus 0.075 micrometers region for all the exposure field. In the conventional optical auto- focus systems, small spot sizes probe beam was used, and the focus signal can be affected strongly by local variations in reflectance, scattering on the wafer surface and wafer flatness. Whereas probe beam scan type auto-focus system, newly developed in this work, has a large focus measurement area, and it averages out the errors which decrease the usable depth of focus. In this work, a new optical auto-focus system for sub-half micron lithography tools will be presented and its characteristics and applications will be discussed. It is composed of a probe beam scan mirror and a position sensing detector, and is insensitive to the structures on the wafer surface. Also, the theoretical analysis of focus measurement error, per the probe beam width and wafer pattern topology will be discussed.

Optimum combination of source, mask, and filter for better lithographic performance

We have tried to find optimum combinations of source, mask and pupil filter suitable for all kinds of mask pattern with critical dimension (CD) of 0.18 micrometers by aerial image simulation. The annular aperture radii of deep UV source (wavelength 193 nm) are controlled to give optimum annular illumination for equal line/space pattern. The optimized illumination aperture has inner radius of (sigma) in equals 0.525 and outer radius of (sigma) out equals 0.600. Under this illumination, various transmittances of halftone mask are investigated as mask variation and phase distribution of pupil filter is modulated annularly. The aerial image characteristics are compared among eight combinations of illumination, mask, and filter in order to find the most effective combination for each typical mask pattern such as equal line/space, isolated line, isolated space and contact hole. The effective method is annular illumination for line/space, pupil filter for the other patterns. The combination of annular illumination + binary intensity mask + pupil filter is recommended for simultaneous printing of all kinds of pattern.

Electron Beam Damage in the SiN Membrane of an X-Ray Lithography Mask

The damage caused as a result of exposure to an electron beam of 20 to 50 kV acceleration voltage on the SiN membrane of an X-ray mask has been investigated. It determined that the optical and the mechanical properties of this material are modified and may potentially limit its use as a membrane in an X-ray mask structure for high density memory devices of giga bit dynamic random access memory (DRAM) level. In particular, after exposure to the electron beam of a 50 kV acceleration voltage and a dosage of 900 µC/cm2, the optical transmission of the SiN membrane fell by about 17% in the wavelength of 633 nm and the change of the out of plane distortion(OPD) on the 16 ×16 mm2 membrane was observed. The difference in the mechanical deflection before and after exposure to the electron beam of 20 kV to 50 kV acceleration voltage on the membrane area of 800 ×800 µm2 was about 500 A to 200 A which was measured by the α-step (Tencor 200) with the stylus force of 19.6 dyn.

Performance of small-field 193-nm exposure system

A small field ArF excimer laser based exposure tool has been designed and fabricated for the 193 nm lithography process research and exposure tool development. The projection optics based upon Schwartzchild concept has a specification of 3 mm field diameter, X5 reduction ratio, and 0.5 NA. The exposure tool uses an unnarrowed ArF excimer laser as a light source, and uses a flys eye homogenizer to produce a reticle illumination uniformity of < +/- 5% RMS/pulse. The results of preliminary exposure with PMMA resist coincided with simulation and expectation, and advanced imaging tests carrying out for various resists. In this paper, we report the detailed system parameters and characterization data of the small field ArF excimer laser exposure tool and some of advances in 193 nm lithography that have been achieved with the system.

Optimization of electron-beam lithography for super-low-noise HEMTs

This paper reports on electron beam lithography for super low noise HEMT (high electron mobility transistor)s. For the optimization of the electron beam lithography for the HEMTs, the SPACING (spacing between the footprint and the head patterns) and the dosage for the head and the footprint to define 0.1 micrometers T-shaped gates with wide head at 30 kV acceleration voltage were investigated by experiment and Monte Carlo simulation. We also compared the single exposure method with dose split method. The footprint of 1 pixel line is to be 0.1 micrometers and the head size to be larger that 1 micrometers when the dosage of the footprint is 700 (mu) C/cm2, the SPACING is 3 pixels and the head dosage is 60 (mu) C/cm2. Using this optimized technique, a T-shaped gate of 0.1 micrometers level with a high ratio of gate head size to footprint, larger than 10, was obtained, and a AlGaAs/InGaAs/GaAs pseudomorphic HEMT with 0.13 micrometers T-shaped gate was successfully fabricated. The HEMT device exhibited very low noise figures of 0.34 dB and 0.49 dB at 12 GHz and 18 GHz, respectively.

Design of illumination aperture for ArF exposure system with wide exposing latitude

We report on the optimum design of illumination aperture with high throughput on ArF exposure system. The quadrupole illumination in the modified illumination methods has a problem to decrease throughput due to the shortage of transmission light passing the pole of the aperture comparing with the conventional illumination. In this paper, we suggest the new quadrupole structure to define 0.15micrometers line and space patterns with high throughput on the ArF exposure system with 0.55 NA.

New technology for enhancing depth of focus using birefringent material

We propose a new technology for enhancing the depth of focus in sub-quarter-micron optical lithography. For this work, we introduce a new projection optical system that includes a birefringent optical component. The quartz crystal as a birefringent materials makes the incident light be resolved into the predetermined polarized direction of the component. By this concept, the focus latitude is increased by the birefringent optical component, which can create two or more focal planes at slightly different positions along the light axis. In addition, the exposure can be done at each focal plane simultaneously by the polarization condition of exposure light. In this paper, we report the basic ideas, and the simulation results that the focus latitude for patterns with a feature size of a sub-quarter-micron can be increased more than tow times using new technology comparing with the conventional method.

Correlation of the vector diffraction effect on the optical lithographic image of a binary mask with magnification of the projection system.

We show quantitatively through a computer simulation the effect ofvector diffraction on the image in optical lithography. Thesimulation was made with a new program, and the variation ofdiffraction as the magnification was varied was calculated for variousnumerical apertures and degrees of defocus. The diffraction at themask improves the images of lines across the polarizing direction, andthe diffraction at the lens improves the images of the lines along thepolarizing direction. We investigated the difference between imagesof lines along and across the polarizing direction as the magnificationincreased. Our result shows that the effect of the lens is sodominant that the images of lines along the polarizing direction arealways better.