Soo-Han Choi

Study of optical proximity effects using off-axis illumination with attenuated phase shift mask

The global proximity effects of densed line, semi-isolated line are studied for conventional illumination, off-axis illumination, and finally off-axis illumination in combination with attenuated phase shift masks which have transmittance of 4% and 8%, respectively, by experiments and simulations. To analyze the behavior of proximity effects, the lithographic performances of the super resolution technique are investigated comparing the cross-sectional view of resist pattern profile, useful depth of focus, and the curves of linewidth vs. defocus for 0.30 micrometers , 0.35 micrometers , and 0.40 micrometers pattern size, respectively. The global proximity effect is quantitatively analyzed by fitting the curve for densed line and isolated line to 2nd order polynomials. Off- axis illumination with attenuated phase shift mask is very effective to minimize the proximity effects for the pattern size less than 0.40 micrometers , and have useful depth of focus of 1.0 micrometers for 0.30 micrometers patterns.

Effect of pattern density for contact windows in an attenuated phase shift mask

An attenuated phase shift mask (PSM) is the most promising candidate for the high volume production lithography process among the various PSM types. It has been shown that attenuated PSM improves the lithographic performance such as depth of focus, especially in contact window by its edge enhancement. In this paper, the side lobe effect that restricts the lithographic performance of attenuated PSM and the light intensity distribution have been examined on changing the pattern density and the transmittance by experimental and simulation. The side lobe effect caused by proximity effect is very severe when pitch sizes are in the range of 0.7-0.9 micrometers for 0.35-0.45 micrometers contact hole on mask and it is enlarged by defocus exposure condition. The side lobe effect in this range of pitch size may forms the additional pattern in wafer, which restricts the application of attenuated PSM. The side lobe effect can be removed by additional pattern positioning at the center of four contact hole patterns, but simulation result of Exposure-Defocus tree (E-D tree) shows that lithographic performance of attenuated PSM is decreased by an auxiliary pattern. In the application of attenuated PSM in dense pattern, the relation between performance and side lobe effect is mutually contradictory.

Implementation of i-line lithography to 0.30 um design rules

The optical lithography is extending its life by combining high numerical aperture (NA) optics and shorter wavelength. The shorter wavelength lithography has required the new developments of related technologies. In particular, DUV resists require an entirely different resist chemistry. Much progress has been demonstrated in the field of transparent chemically amplified resists with high sensitivity. However, this DUV lithography ((lambda) equals 248 nm) has been delayed for mass production due to their limitations, such as (i) delay time effects, (ii) high cost ownership due to expensive resist materials and laser maintenance, and (iii) critical dimension (CD) variation over topography caused by multireflection of topographic features. On the other hand, i- line lithography ((lambda) equals 365 nm) has apparently been applied to 64M DRAM of 0.35 micrometers design rule, and attempted to 0.30 micrometers technology which corresponds to 2nd generation 64M DRAM or 1st generation 256 M DRAM. It might be achieved by combination of off-axis illumination (OAI), phase shift mask (PMS) and advanced resist process technique of i-line lithography. Therefore, i-line lithography can be more practical method rather than DUV lithography for the mass production. In this paper, we have optimized the i-line lithographic techniques for the various pattern shape and density for 0.30 micrometers design rule. Optimum duty ratio was tried to find for line and space, contact hole patterns. The basic rule is to keep the minimum Cr width over 0.30 micrometers mask. OAI have been applied to get higher contrast of line and space, and even contact hole patterns, and achieve good pattern fidelities of island patterns. By the implementation of OAI, process latitudes were greatly improved compared to that of conventional techniques. In order to optimize the process over the actual topography, optimum numerical aperture (NA) and aperture of the OAI were selected. In conclusion, 0.30 micrometers design rule device was successfully fabricated by optimizing the advanced i-line lithographic techniques.

Overlay and lens distortion in a modified illumination stepper

Optical lithography, when extended by phase shift mask technology and modified illumination techniques, is a promising technology for sub-half-micron devices. Modified illumination can improve the resolution limit and depth of focus, but the imaging profile is changed, with pattern type, direction, and density having an effect on the result. The uniformity of the illumination system also differs according to aperture type. Because lens distortion may be affected by the aerial image and structure of illumination optics, we can expect that a modified illumination system may affect lens distortion and overlay accuracy in a real process. A comparison of changes in overlay and lens distortion was done for different illumination conditions. Focus was varied for each combination. As a result, we can observe the variation of overlay error in a modified illumination system relative to the conventional system. To use modified illumination in sub-half-micron processes distortion error must be reduced.

Quantitive analysis of the proximity effect in optical lithographic process

As the density of VLSI circuits increases, the proximity effect has been one of the critical issues in optical lithography. In general, the linewidth difference between dense and isolated patterns corresponds to 0.08 micrometers when a conventional i-line single resist process using a 0.54 NA is applied to the half-micron geometry on a flat wafer. Therefore, this linewidth difference has significantly affected the process stability in the real process applications. This paper describes the dependency of the proximity effects on the pattern size, line and space duty ratio, kinds of substrate film, defocus effect during exposure, and resist process conditions related to the variation of the resist thickness and develop time. Critical dimension (CD) deviation caused by the different latent image contrast is also experimentally monitored using two different photoresists. A simulation is performed for the purpose of obtaining the optimum resist thickness to reduce CD difference caused by the variations of resist thickness in the real topography.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Fundamental analysis on fabrication of 256-MB DRAM using phase-shift mask technology

This paper describes the phase shift mask (PSM) effects in view of production using i-line lithography. For the PSM technology, it was hard to control process because the process condition was limited by the exposure tool. To fabricate the 256MB DRAM with 0.25 micrometers minimum feature size (MFS), we evaluated the PSM including attenuated type for conventional patterns and a 0.25 micrometers cell array using positive and negative tone phase shift mask for actual process. Furthermore, we applied various approaches to get a sufficient depth of focus (DOF) and high resolution using an i-line system with 0.57 NA, an off-axis illumination system, low partial coherence factor, and process in cases of alternating, subresolution, and attenuated type of phase shift mask. As a result, even if pattern delineation was possible, we should optimize design, topology structure, and process to get enough DOF margin, good uniformity, and high repeatability for device fabrication.

Accurate overlay control for 0.30-um i-line lithography

Errors induced by mask, stepper, process and measurement tool disturb the perfect registration. A new method to check the precision of the mask itself would be introduced. It could give more practical references for wafer processing. The amount of all other errors could be easily notified using the above method. It is shown that the thickness of mask is a quite important factor to decide the accuracy of pattern placement on mask. Thicker substrate has smaller distortion inherently, and is less deformed by the subsequent processes for the mask fabrication. Although the field size of the thicker one was almost twice than that of the thinner one, 250 mil quartz substrate had more accurately placed patterns than the 90 mil one. OAI is a kind of sources degrading registration accuracy, which can induce another lens aberration. This error should be compensated by magnification and rotation control of the chip. The misregistration was able to be suppressed less than 60 nm. The registration control of the contact hole is extremely important and difficult, especially when it has to be formed through multi-layers. A new method had been tried, which independently controls the overlays with respect to x and y direction, respectively. It was found to be a potential solution in this case.

Effective alignment techniques and their implementation to enhance total overlay accuracy on highly reflective films

Comparative analysis of the total overlay accuracy on aluminum (Al) and chemical vapor deposition tungsten (CVD-W) was carried out as a function of the available alignment techniques over the various process conditions. In the case of Al films, they were prepared at the various sputtering temperatures, thicknesses, and step heights. The total accuracy was also examined associated with standard and off-axis illumination (OAl). The effect of the sputtering temperature of single and dual Al deposition has been especially investigated in a view of surface roughness, which significantly affects total overlay accuracy. It was observed that CVD-W film can be an alternative of Al film. As the Al film deposition temperature decreases, it was found out that random requisition error with a large distribution was improved by implementing an off-axis alignment (OAA) technique with a visual broadband light source. Meanwhile, Al thickness was not so much critical to overlay accuracy comparing with Al sputtering temperature. And also scaling error determined by asymmetric alignment mark shape was minimized according to the optimization of exposure data file. Minimum detectable step height of alignment mark was down to 300 angstrom using a sensitive alignment techniques over the Al film.

Quarter-micrometer i-line lithography using an alternating phase-shift mask

Phase-shifting mask allows remarkable improvement of the resolution and depth of focus than is possible with conventional mask. In this paper, we examine the optimum coherence factor ((sigma) ) and numerical aperture (NA) by considering the process margins of conventional and alternating shifter L&S patterns on high NA i-line stepper and next we investigate the possibility to apply this optimum parameter in real devices of 0.25 micrometers - 0.35 micrometers design rules. We evaluate the process window, line and space duty ratio, CD difference by proximity effect, illumination uniformity, and neighboring linewidth variation with experimental and simulations including resist profile as well as aerial image. In this experiment, we obtained the DOF of 2.0 micrometers for 0.25 micrometers alternating shifter L&S with an optimum coherence factor on high NA i-line stepper and we can conclude that 256 Mb DRAM with 0.25 micrometers design rule could be printed with large DOF.

Magnetically enhanced reactive ion etching of silylated resist in O2/Ar mixtures

This study describes the etching behavior of the silylated resist in a magnetically enhanced reactive ion etcher under the pressure ranges from 3 to 10 mTorr. In a pure oxygen plasma, the resist undercut beneath the silylated mask layer and isotropic resist profile were generated independent of etch conditions. Compared to the tri-level-resist process, the resist undercut tended to be reduced in the top imaging process by silylation. It was found that the addition of Ar to an O2-plasma was effective to reduce the resist undercut due to the consumption of the silylated mask layer. However, in an Ar-rich plasma, the resist profiles appeared to be positively sloped by the excessive consumption of the silylated resist.