Toshifumi Suganaga

In-situ aberration monitoring using phase wheel targets

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.

Effect of high numerical aperture lens on lithographic performance in 157 nm lithography

157 nm lithography is the most promising candidate for the post-193 nm lithography at the sub-70 nm technology node of semiconductor devices. In order to realize sub-70 nm resolution using 157 nm lithography, the critical components such as F2 laser, optics, photomasks, and resists have been studied extensively [Bloomstein et al., J. Vac. Sci. Techol. B 15, 2112 (1997); Rothschild et al., J. Photopolymer Sci. Technol. 13, 369 (2000); Rothschild et al. (unpublished); Itani and Wakamiya, Microelectron. Eng. 61–62, 49 (2002)]. Moreover, for improving the numerical aperture (NA) of the projection lens and thus the resolution capability at 157 nm, liquid immersion interference lithography at 157 nm has been studied and has obtained a minimum resolution of less than 70 nm lines and spaces [Switkes, Bloomstein, and Rothschild, Appl. Phys. Lett. 77, 3149 (2000); Switkes and Rothschild, J. Vac. Sci. Technol. B 19, 2353 (2001)]. In this article we will report on the feasibility study of a high NA (0.85 NA) projecti...

Wavefront aberration measurement in 157-nm high numerical aperture lens

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 with high numerical aperture lens for 70-nm technology node

Abstract not available.

157-nm lithography with extremely high numerical aperture lens for 45-nm technology node

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.

Sub-70-nm pattern fabrication using an alternating phase-shifting mask in 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.

157-nm chromeless phase lithography for 45-nm SRAM gate

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.

157-nm resist assessment by a full-field scanner

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.

Control of side-lobe intensity for attenuated phase-shifting mask in 157-nm lithography

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.

Optimization of the chromium-shielding attenuated phase shift mask for 157-nm lithography

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.