We evaluated the chromium-shielding attenuated phase shift mask (Cr-shield att-PSM) for the fabrication of fine hole patterns in 157-nm lithography. The transmittance of the phase shifter was set at 5% to achieve the best performance for 70- to 90-nm-diameter holes. Simulation and experimental results indicated that the optimum distance a between the pattern edge and the Cr-shield edge changed depending on the size and pitch of the holes. The optimum distance a for sub-70-nm-diameter holes was zero, which meant the binary mask gives the best depth of focus. In the case of 80-nm-diameter holes, the conventional att-PSM proved to be the best option for 1:1 hole patterns. For 1:2 hole patterns, the optimized distance a was 60 to 70 nm. For isolated hole patterns, the optimum distance a was 45 nm. After optimizing distance a, we confirmed the side-lobe control capability of the Cr-shield att-PSM through exposure experiments. The elimination of side-lobes greatly improved the resolution. Furthermore, we found that the mask linearity was improved through use of a Cr-shield att-PSM.
157-nm lithography processes together with optimization of mask feature size and illumination conditions and chromeless mask (CLM) of mesa-type were used to fabricate a 45-nm gate by combining a high numerical aperture (NA) lens with off-axis illumination (OAI) and using Si-containing resist. It was observed that the minimum pitch for forming a 45-nm line was 140-nm. It was also shown that quadrupole illumination was the optimum OAI condition and the optimum mask feature size for forming a 45-nm line of 200-nm pitch was between 50 nm to 55 nm. In these conditions the normalized image log-slope value was about 3.0. It was demonstrated that a 45-nm SRAM gate with a depth of focus of 150 nm could be fabricated by combining these resolution enhancement techniques with high NA lithography and Si-containing resist. Furthermore the 45-nm SRAM-gate pattern was successfully transferred with a bi-layer process. From these results it was proven that fabrication of 45-nm node device could be achieved by using CLM with high NA lithography.
Aberration metrology is critical to the manufacture of quality lithography lenses in order to meet strict optical requirements. Additionally, it is becoming increasingly important to be able to measure and monitor lens performance in an IC production environment on a regular basis. The lithographer needs to understand the influence of aberrations on imaging and any changes that may occur in the aberration performance of the lens between assembly and application, and over the course of using an exposure tool. This paper will present a new method for the detection of lens aberrations that may be employed during standard lithography operation. The approach allows for the detection of specific aberration types and trends, as well as levels of aberration, though visual inspection of high resolution images of resist patterns and fitting of the aberrated wavefront. The approach consists of a test target made up of a 180-degree phase pattern array in a “phase wheel” configuration. The circular phase regions in the phase wheel are arranged so that their response to lens aberration is interrelated and the regions respond uniquely to specific aberrations, depending on their location within the target. This test method offers an advantage because of the sensitivity to particular aberration types, the unique response of multiple zones of the test target to aberrations, and the ease with which aberrations can be distinguished. The method of lens aberration detection is based on the identification of the deviations that occur between the images printed with the phase wheel target and images that would be produced in the absence of aberration. This is carried out through the use of lithography simulation, where simulated images can be produced without aberration and with various levels of lens aberration. Comparisons of printed resist images to simulated resist images are made while the values of the coefficients for the primary Zernike aberrations are varied.
The potential for extending the numerical aperture (NA) in order to develop devices beyond the 45-nm node has been investigated using a 157-nm microstepper exposure tool at 0.90NA (third generation) and verifying the resolution limit of several different resolution enhancement techniques. It was observed that with 157-nm lithography at 0.90NA a 60-nm line and space (L/S) and a 50-nm isolated line could be formed by using an attenuated phase shifting mask (Att-PSM), and that a 50-nm L/S and a 35-nm isolated line could be formed by using an alternating phase shifting mask (Alt-PSM). The influence of the flare for the same pattern sizes was more severe for the L/S pattern rather than isolated line. However, it was the most difficult to image an isolated line with an Att-PSM, which was limited with a tolerance to the flare of less than 1%. Furthermore, the requirement of more than 0.93 for lens NA was confirmed in order to fabricate half pitch 65-nm node device with Att-PSM and half pitch 45-nm node device with Alt-PSM. Results obtained in the pattern formation of 45-nm node with an Alt-PSM confirmed that a 35-nm line could be formed down to 140-nm pitch, a 40-nm line could be formed down to 135-nm pitch, and a 45-nm line could be formed down to 100-nm pitch. It has been demonstrated that 157-nm lithography could find application to half-pitch 65-nm and 45-nm node devices.
Chromeless Phase Lithography is known as an effective resolution enhancement technique for isolated line patterns. We fabricated a chromeless phase lithography mask for 157-nm lithography, and evaluated the lithographic performance using a 0.90 numerical aperture 157-nm microstepper. To obtain the best resolution, illumination condition was optimized to conventional illumination with 0.7 partial coherence (σ) using lithography simulation. In the exposure experiment, 30-nm-wide isolated line, 30-nm-wide 140-nm-pitch line-and-space, and 30-nm-wide static random access memory (SRAM) gate patterns were resolved. Further lithography simulation results indicated that the resolution limit of 24-nm would be obtained by eliminating the image degradation factors such as the aberration, flare, and central obscuration.
The bilayer process we developed for 157-nm lithography uses a fluorine-containing silsesquioxane-type resist (F-SSQ). Gate fabrication is done by using a F-SSQ(90 nm)/organic film(200 nm)/poly-Si(150 nm)/SiO2(10 nm)/Si structure. The organic film works well as an anti-reflecting layer. Using a microstepper with a numerical aperture of 0.90 and optimizing the resist thickness, we made a 50-nm 1:1 line-and-space (L/S) pattern by using an alternative phase-shifting mask and made a 45-nm SRAM by using a chromeless phase lithography mask. Neither resist pattern footing nor undercutting was observed on the organic film. The reactive ion etching (RIE) selectivity between the F-SSQ and the organic film was sufficient (about 7), the resist pattern was transferred to the underlayer, and both 50-nm 1:1 L/S and 45-nm SRAM gate patterns were made using the organic film as an etching mask. Contact hole (C/H) fabrication is done by using a F-SSQ(105 nm)/organic film(400 nm)/tetraethyl orthosilicate (TEOS)-SiO2(1200 nm)/Si structure, and we made a 75-nm 1:1 C/H pattern by using the microstepper with a binary mask. The RIE selectivity was sufficient (about 15) for making high-aspect-ratio contact holes, and we made a 75-nm 1:1 C/H pattern in 1200-nm-thick TEOS. This bilayer process is thus promising for making 65-nm-node semiconductor devices.
Fluorinated polymers are key materials for single-layer resists used in 157-nm lithography. We have evaluated the potential of fluorinated polymer-based resists from the viewpoint of critical dimension (CD) control, using a 0.90 numerical aperture (NA) 157-nm micro-stepper with an alternating phase shift mask (alt-PSM). A resolution limit of 55-nm line-and-space patterns was obtained and the bake temperature dependence of the CD was found to be less than 2 nm/°C. We further evaluated these resists using a 0.80-NA FPA-5800FS1 157-nm scanner for full-field imaging with an alt-PSM. With these resists, 60-nm line-and-space patterns were resolved, and a depth of focus (DOF) of more than 400 nm for 100- and 80-nm line-and-space patterns was confirmed. The CD variation across the wafer for a 100-nm 1:1 dense line pattern was 3.3 nm (3σ). Although there is still a need to improve line edge roughness and dry etching resistance, in terms of CD control the fluorinated polymer-based resists have demonstrated sufficient potential for mass-production of 65-nm-node semiconductor devices and beyond.
The TaSiOx attenuated phase-shifting mask (Att-PSM) has strong potential for durability against laser irradiation and good lithographic performance in 157 nm lithography. However, the resist resolution limit and depth of focus (DOF) are deteriorated by side-lobe patterns generated near the contact hole. This is because the side-lobe intensity generated near the light-transmitting region becomes larger in sub 100 nm contact holes. To minimize the effect of side-lobes and improve lithographic performance, we evaluated an Att-PSM with a chrome light-shielding layer and optimized the transmittance of its attenuated phase-shifting film. In an optical simulation, we investigated the effect of the side-lobe intensity on the resist region (i.e., a reduction in resist thickness). The light-shielding film was placed on the attenuated phase-shifting film to prevent the side-lobe pattern, and its effect on the imaginary resist pattern was simulated. We found that the distance between the patterning edge of the hole and that of the light-shielding region must be greater than 90 nm to fabricate a 100 nm isolated hole without side-lobe patterns. The side-lobe intensity could be controlled using the chrome-shielding-type Att-PSM, and the lithographic performance (such as resolution limit and DOF) was enhanced.
Alternating Phase Shifting Mask (Alt-PSM) technology is one of the most effective Resolution Enhancement Technology (RET). It has been used for current optical lithography and will be used for 157nm lithography also. Considering about topographic structure of Alt-PSM, current etched quartz with undercut structure will be very difficult to be applied for 157nm Alt-PSM because undercut structure limits mechanical durability at narrower chrome width. To solve this problem, Side-wall Chrome Alternating Aperture Mask (SCAAM) is proposed. This structure has the characteristics of “There is no undercut”, “Ideal topographic structure for lithography (All quartz steps are covered by chrome film which means very few refracted light at quartz side-wall will go through chrome film and affect printing results compared with conventional etched quartz type Alt-PSM)”. We fabricated SCAAM type Alt-PSM for 157nm lithography and printed by using 157nm microstepper with a 0.85-NA lens. In this report, we will show preliminary printing results of using SCAAM and which will be compared with the results of using conventional etched quartz type Alt-PSM.
We evaluated the requirements for 65-nm SRAM gate fabrication using attenuated phase shifting masks (att-PSM). Off-axis illumination (OAI) and att-PSM, together with optical proximity correction (OPC) were used as resolution enhancement techniques (RETs) for ultimate resolution. It was shown that the photolithographic parameters of the transmittance of the att-PSM and the illumination conditions for optimum conditions were a transmittance of between 15 and 20% and 3/4 annular illumination. The exposure latitude was simulated to be more than 10.9% at 300-nm defocus for a critical dimension (CD) specification of 10%. It has been demonstrated that a 65-nm SRAM-gate, with a line and space (L/S) ratio limited to 1:2 at the minimum pitch, could be fabricated with sufficient depth of focus (DOF). The pattern transfer was accomplished with a bi-layer process, in which the reactive ion etching (RIE) selectivity between a silicon-containing resist and an organic film is very high. This bi-layer process enabled the application of a very thin resist layer. The conditions described in this paper proved successful for the fabrication of a 65-nm SRAM gate with a good pattern profile despite the resist thickness of less than 120nm.
A phase-shifting mask (PSM) is one of the most effective resolution enhancement technologies to improve the resolution limit and process margins such as exposure latitude (EL) and depth of focus (DOF). The attenuated phase-shifting mask (Att-PSM) is the most practical PSM, because it has a simple structure and can be easily fabricated. However, it is very difficult to evaluate the impact of using Att-PSMs on the resolution limit and process margin, under the condition of both a shorter wavelength and higher numerical aperture (NA). The reason is that the resolution improvement of the Att-PSM is very small under the above condition. In this study, we investigate the impact of using the Att-PSM instead of a binary mask under the conditions of shorter wavelength (157-nm) and higher-NA (0.85-NA). We evaluated the resolution limit by both aerial image simulation and exposure experiment. The aerial image simulation confirmed that the resolution improvement in the line and space pattern that can be expected from an Att-PSM of 5% transmittance diminished by decreasing wavelength and increasing NA. In particular, when a wavelength of 157-nm and an NA of 0.85 are used, we obtained a 6% resolution improvement compared to the binary mask. In the exposure experiment, we obtained an 11% resolution improvement when using a TaSiOx-type Att-PSM of 5.7% transmittance. From these results, we found that the Att-PSM can be used to fabricate smaller size features even shorter wavelength of 157-nm and the higher NA of 0.85.
157-nm lithography is being investigated for the sub-65nm technology node of semiconductor devices. Many efforts have been reported on the exposure tool, the F2 laser, the resist materials, the resist processing and the mask materials. A critical component for the success of this 157-nm lithography is the availability of high numerical aperture (NA) lenses that lead to higher resolution capability and a larger process margin. It was reported in a previous article that a 0.85 high NA 157-nm microstepper has demonstrated a resolution capability of 55 nm dense line and space features in combination with an alternating phase shirting mask and using a 120nm thick fluoropolymer resist. The influence of the intrinsic birefringence of the CaF2 lens material on the wavefront aberrations of the projection optic was also experimentally confirmed. In this paper, the effect of the wavefront errors on the imaging performance will be discussed from an evaluation of the short-range flare and the local area flare present in the high numerical aperture (NA) lens.
157 nm lithography is being investigated for the sub-70 nm technology node of semiconductor devices. Many efforts have been reported on the exposure tool, the F2 laser, the resist materials, the resist processing and the mask materials. A critical component for the success of this 157 nm lithography is the availability of high numerical aperture (NA) lenses that lead to higher resolution capability and higher process margin. In this paper, we describe our recent evaluation results of a high precision 157 nm Microstepper with 0.85 NA lens combined with simulation analysis of the lithographic performance. The details of the evaluation results discussed here include the resolution limit of the high NA lens and the possible effects of intrinsic birefringence upon the lithographic performance.
TaSiOx is expected to be the most effective film material for use in attenuated phase shifting masks (Att-PSMs), in terms of both its durability under irradiation and its lithographic performance in 157-nm lithography. In this study, we optimized the transmittance of 5.5 percent and evaluated the effectiveness of TaSiOx by both aerial image simulation and exposure experiment in order to evaluate the material's potential for 157 nm lithography. Through the aerial image simulation, it was confirmed that aerial image intensity of side lobes was less than half of that needed for resolving patterns by transmittance of 5.5 percent. In an exposure experiment, the resolution, depth of focus (DOF), and mask error enhancement factor (MEEF) were evaluated for hole patterns. The result of this evaluation was that we were able to fabricate a pattern of 100-nm diameter isolated holes without side lobes and obtain a better than 200-nm DOF and MEEF greater than three with a 5.5 percent TaSiOx type Att-PSM. This study has confirmed that TaSiOx type Att-PSMs have strong potential for application in the fabrication of 100-nm hole patterns by 157-nm lithography.
In Selete, we have developed various resolution-enhancement technologies (RETs) such as the alternating phase shifting mask (alt-PSM), attenuated-PSM (att-PSM), and off-axis illumination (OAI). The alt-PSM, for example, reduces the k1 factor and extends the lithographic performance. A problem concerning the alt-PSM is the difference in the transmitted light intensities of the non-phase-shifting region and the phase-shifting region which can cause critical-dimension (CD) placement error. The transmitted light intensities of the two regions can be made equal by side-etching, in which the quartz (Qz) is undercut by wet-etching at the side of the transmitting region. We sought to optimize the mask structure in terms of a high numerical aperture (NA) through a simulation using two kinds of structures with a 157 nm exposure wavelength. The structures were a single-trench structure and a dual-trench structure, with each trench dug in the transmitting region. To attain a high NA (NA equals 0.85), we tried to optimize the parameters of the Cr film thickness, the amount of the undercut (side-etching), and the phase shift. The evaluated line pattern sizes were 70 nm (line/space size equals 70/70 nm, 70/140 nm, 70/210 nm, and 70/350 nm) and 50 nm (line/space size equals 50/50 nm, 50/100 nm, 50/150 nm, and 50/250 nm) at the wafer. Further, using the optimized mask, we calculated the lithographic margin of a sub 70 nm pattern through a simulation. For the 70 nm line patterns, we found that it will be difficult to fabricate precisely a 70 nm line patten using a mask with a single- trench structure. And we also found that the most suitable conditions for the dual-trench structure mask were a 90 nm undercut, a 100 nm Cr film thickness, and a 180 degree(s) phase shift. The exposure latitude at a depth of focus (DOF) of 0.3 micrometers , simulated using the optimized mask, was 5.3% for the 70/70 nm pattern, 3.6% for 70/140 nm 16.0% for 70/210 nm, and 29.3% for 70/350 nm. As the pitch widened, the exposure latitude increased for the 70 nm line patterns. Using the optimized dual-trench mask for 157 nm lithography, it will be able to keep the EL more than 3% at DOF of 0.3 micrometers for a 70 nm line pattern.
l57nm lithography is being investigated for the sub-7Onm technology node of semiconductor devices. Many efforts have been reported on the exposure tool, the F2 laser, the resist materials, the resist processing and the mask materials1. A critical component for the success of this 157nm lithography is the availability of high numerical aperture (NA) lenses that lead to higher resolution capability and higher process margin. In this article, we describe our recent evaluation results of a high precision 157nm Microstepper with 0.85 NA lens combined with simulation analysis of the lithographic performance. The details of the evaluation results discussed here include the resolution limit of the high NA lens and the possible effects of intrinsic birefringence upon the lithographic performance.