Kag Hyeon Lee

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.

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.

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.

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.

Novel antireflective structure for metal layer patterning

IN lithographic processing to define patterns on the high reflective substrate, ARLs (anti-reflective layers) not only enable better line width control but also realize designs that were previously impossible to print. So far, several anti-reflective films like TiN, SiOxNy:H, and organic films for the high reflective substrate have been studied. In this paper, we suggest the novel anti-reflective structure for metal layer patterning, which is Al(aluminum)/SiO2 stack structure. the reflectivity and the resist absorption rate are simulated for the I-line, and ArF lithography. The simulated thickness of ARL(Al) and ARL(SiO2) for zero reflectivity on the wavelength of 365 nm was 12.6 nm and 95.2 nm respectively, and on the 193 nm was 20.4 nm and 98.8 nm. The process latitude according to the thickness variation of the deposited ARL(Al) and ARL(SiO2) films, and the results of the lithography experiment were discussed.

Alignment system for ArF excimer-laser-based step-and-scan system

ArF excimer laser exposure tool is expected as a workhorse in gigabit DRAM mass production era. It can resolve 0.18 micrometer or finer patterns due to its short wavelength of illumination light. Also, the step-and-repeat photolithography system is changing to more complicated step-and-scan system. On the other hand, this in turn requires alignment system to work to tighter budgets. In the 0.18 micrometer optical lithography performance level, overlay error should be maximum 40 nm. In this paper, we report the theory and design parameters of the alignment system for home made ArF excimer laser based step & scan system. We have examined the advantages of our alignment system, and have implemented a trade-off strategy. Our discussion includes an overview of the alignment system which composed of reticle alignment system, wafer alignment system (off-axis and TTL) and auto focus/leveling system.

Performance analysis of ArF excimer laser lithography optics

In ArF excimer laser lithography, catadioptric systems are widely used as projection optics. These systems consist of a mirror, a polarizing beam splitter, quarter wave plates and corrector lenses. Combination of a polarizing beam splitter and a quarter wave plate is used to separate the forward and backward beam and to remove the central obscuration. The life time of the ArF excimer laser optics is limited by the well-known defect, the compaction of fused silica. The compaction is proportional to the square of energy density, and the life time is given by the inverse of compaction. If we make the image space working distance longer, the energy density becomes smaller and the life time becomes longer. We have designed two examples of catadioptric systems and analyzed their optical performances and life times. Also we have reviewed the relationships between the working distance and the size of optical component, however, there is a strong relationship between both of them. The longer working distance makes the optical component larger. It has the significant meaning in optics design, because the big sized optical material, which can be used at 193 nm, is very rare. Especially, the material for prism of polarizing beam splitter is hard to attain. Hence there should be some compromises between the working distance and the size of optical components.

Base-line error-free non-TTL alignment system using oblique illumination for wafer steppers

In this paper, an alignment method with oblique illumination and oblique detection named ZBAS (Zero Base-line Alignment System) is proposed and its characteristics and applications are analyzed. It detects the position error from alignment mark at the exposure position even though it is a non-TTL method. The alignment mark, a diffraction pattern with a 4 micrometers width (8 micrometers pitch), is illuminated with a He-Ne laser beam at an incident angle of 75 degree(s) and the first order diffracted beam at this is detected as an alignment signal at a diffraction angle of 60 degree(s). The peak value of the alignment signal with appropriate electrical system is about 20 mV and the signal sensitivity to the wafer position is 0.5 mV/0.1 micrometers . The theoretical analysis of ZBAS was carried out with respect to the effects of pattern depth and resist thickness. The result shows the stable detection is realized by a 2- wavelength (670,780 nm) illumination, in which the variation of the calculated signal is 27% in the range of 0.6 approximately equals 1.2 micrometers of pattern depth, 37% in the range of 0.6 approximately equals 1.2 micrometers of resist thickness. In conclusion, the ZBAS was verified as a potential alignment system in excimer laser stepper, which has a smaller base-line error than the conventional non-TTL method.