Current flash memory technology is facing more and more challenges for 45nm and 32nm node technology. To get good
CD and yield control, optimized RET, OPC modeling and DFM techniques have to be applied . To enhance process
window (PW) and better CD control for main features, assist features (SB) have to be used. Simulation and wafer
evaluation show that the SB CD performance is very critical. Based on OPC simulation, we can get a very good
prediction about the CD size and placement of assist features. However, we can not always get what we want from mask
suppliers. For 45nm node technology and beyond, The SB CD size (~ 20nm at 1X) has almost pushed to the current
mask process limit. Wafer fabs have a very big concern about the stability of linearity signatures from different
suppliers and different products in order to keep high accuracy of OPC models. Actually the CD linearity signature
varies from one mask supplier to another and also varies from product to product. To improve the SB CD control, the
ideal goal is to make "flat" linearity for all mask suppliers. By working closely with TPI mask supplier, we come up
solutions to improve SB CD control to get "flat" linearity. Also technology development is causing more severe SB
printability, we proposed a methodology to use AIMS for predicting SB printability. Wafer results proved the feasibility
for these methodologies.
Early insertion of ArF nm lithography will occur at the 130 nm node in 2001. Process development for the 100nm node will also occur this year. Both aggressive gate length reductions and minimum pitch design rules below 250nm present immediate challenges for the new ArF technology. Gate line widths will approach one half of the wavelength of the exposure system.
The effect of mask critical dimension (CD) error for binary mask and attenuated phase shift mask (PSM) are investigated by simulation and experimental based data. For the large features, mask error factor (MEF) is approximately unit. But as the CD is closed to the resolution limit, the MEF value is rapidly increased. The MEF was dependent on the contact density. For example, dense contact has larger MEF value than isolated contact. Attenuated PSM has smaller MEF value comparing with binary mask because it is applied the positive mask bias in order to reduce the sidelobe printing. The sensitivity of mask CD error for NA and sigma variation was different from the contact density. For the isolated contact, MEF value was almost independent on the sigma value. However, the MEF was improved by high NA lines at the resolution limits both for the isolated and for the dense contact. According to these data, the mask CD control budget for the sub-quarter micron contact was considered.
In the experiment, various ion species are implanted to the developed images to improve etching and thermal flow resistance of Deep UV chemically amplified resists. Among various ion species, it is found that Argon ion did not affect the photoresist thickness and critical dimension after ion implantation. Much improved results could be obtained. Untreated contact hole patterns start flowing on 120 degrees C and finally are filled up on 130 degrees C with noticeable film shrinkage. On the other hand, Argon implanted contact hole patterns are standing still up to 170 degrees C without any thickness shrinkage and CD variation. Application of higher temperature results in the protrusions at the bottom of the resist profiles. Pattern deformation after dry etching process can be prevented. Cross-sectional SEM micrographs of the ion implanted contact hole patterns show clear interface between the hardened and the unhardened. SIMS analysis of the ion implanted photoresists reveal the presence of the ions at the surface of the substrate, not in the resist. Detailed mechanistic study will be discussed. Application of this process to bilayer resist process and a new antireflective layer has been tried and evaluated. And also the effects of accelerating energy and ion dose on reflective index of carbonized layer have been investigated. RI analysis shows the change of n and k value according to energy and ion dose. It can be speculated that the transparent matrix resin changed to highly absorbing amorphous carbon based materials. It is quite sure for that the n and k value can be controlled for the application of bottom antireflective layer. This new ARC material is very compatible to resist and est to strip, compared to conventional Organic Bottom ARC material. Thickness optimization for the ARC, application to real device and etching characteristics are under development in our lab.
An attenuated phase-shifting mask is favorable lithography technique for enhancing the depth-of-focus for isolated hole. However, it is restricted by sidelobe printing at dense array holes. To reduce the sidelobe printing, various methods such as surface insoluble layer, add an auxiliary hole, and optimization of NA and sigma were investigated. The method of surface insoluble layer was not effective for the dense array holes and CD uniformity was not improved. The method that adds an auxiliary hole at sidelobe position of highly dense array pattern can reduce the sidelobe printing completely, but mask CD and mask defect inspection as well as automatic layout of auxiliary holes for nonrepeating patterns in periphery area will be issued. In order to optimize the NA and sigma value, DOF and sidelobe printing were considered. Also CD control is studied by considering the CD linearity and optical proximity correction (OPC) as mask print bias is applied. Design rule for attPSM was suggested at optimized and fixed conditions.
An alternating phase shift mask is very effective to memory devices which have highly repeated patterns. In order to apply the alternating phase shift mask to real device, we have investigated the design problems such as proximity effect, phase contradiction, phase transition, and linewidth variation. We also design various hard defects in order to check defect printability on wafer. Using i-line lithography with an alternating phase shift mask, we obtain useful focus latitude of 1.2micrometers for bit line of 256M DRAM. Deep UV alternating phase shift mask is used for isolation patterns with design rule of 0.16micrometers . The experimental and simulation results for phase-induced problems and defect printability on wafer are described in detail.
Alternating phase shift mask (PSM) is very effective to memory devices which have highly repeated patterns. In order to apply the alternating PSM to a real device, we have investigated the design problems such as proximity effect, phase contradiction, phase transition, and linewidth variation. We also designed various hard defects to check defect printability on a wafer. Using i-line lithography (0.50 NA, 0.46 sigma) with alternating PSM, we obtained a useful DOF of 1.2 micrometer for a bit line of 256 M DRAM. The experimental and simulation results for phase-induced problems and defect printability on wafer are described in detail.
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.
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.
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.
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.
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.
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.