Chi-n Lu

Mask defect management in extreme-ultraviolet lithography

Abstract. We present a series of baseline techniques for inspection, cleaning, repair, and native defect mitigation of extreme ultraviolet (EUV) masks. Deep-ultraviolet inspectors are capable of inspecting patterns down to about 45 nm in pitch on wafer. Cleaning methods involving both chemical and physical forces have achieved good particle removal efficiency while minimizing absorber shrinkage and have realized 90% PRE in removing particles from the backside of an EUV mask. In addition, our compensation method for native defect repair has achieved partial success.

Fundamental Study of Droplet Spray Characteristics in Photomask Cleaning for Advanced Lithography

The fundamentals of droplet-based cleaning of photomasks are investigated and performance regimes that enable the use of binary spray technologies in advanced mask cleaning are identified. Using phase Doppler anemometry techniques, the effect of key performance parameters such as liquid and gas flow rates and temperature, nozzle design, and surface distance on droplet size, velocity, and distributions were studied. The data are correlated to particle removal efficiency (PRE) and feature damage results obtained on advanced photomasks for 193-nm immersion lithography.

Sub-20-nm node photomask cleaning enhancement by controlling zeta potential

As semiconductor manufacturing advances to sub-20-nm nodes, specification (size < 50 nm) for extremely fine particles on photomasks is getting more and more stringent. Photomask cleanliness, which seriously impacts manufacturing cycle time and productivity, is a serious challenge in the development of sub-20-nm node mask cleaning process. Several cleaning approaches, including the use of chemical and physical forces, are widely used in mask cleaning. In this study, we focus on the chemical force through zeta potential (ZP). ZP indicates the degree of repulsion between the particles and the mask surface (mostly quartz). In the nano-scale, stronger repulsion means easier removal of particles from mask surfaces. By controlling ZP of different chemicals from -10 mV to -150 mV in the cleaning process, the particle removal efficiency (PRE) is further improved by about 10%, especially for extremely fine particles. The ZP measurement methodology for different cleaning chemicals on quartz surface is also carried out. ZP is a helpful index in evaluating the performance of new chemicals for mask cleaning. To enhance photomask cleaning for sub-20-nm nodes, the chemical force needs to be increased because the physical force has been constrained to avoid pattern damages, especially when much smaller assistant features are commonly used to gain a greater lithography process window. How to choose a suitable cleaning approach for the next generation mask cleaning is very critical.

Improvement of etching selectivity for 32-nm node mask making

As the geometry of semiconductor devices continue to scale down, high-NA imaging will be used to enhance the resolution. Sub-resolution assistant features are used to gain depth of focus at the wafer. One of the challenges in patterning small assistant features during mask fabricating is resist collapse. Reducing resist thickness is one of the solutions. This necessitates an increase in the selectivity of chromium (Cr) to photo-resist (PR). The selectivity determines the PR remaining on the mask after Cr etching. Insufficient remaining PR will induce pinhole-type clear defect and poor line edge roughness (LER). In this paper, the Cr-to-PR selectivity was studied under induced couple plasma (ICP) and quasi-remote plasma environment. PR remaining, etching bias, and critical dimension uniformity (CDU) are the main subjects for evaluation. To understand the etching behavior for higher selectivity, design of experiment (DOE) L4 by Taguchi method is used to find the dominating factors. By adopting the optimized etching recipe, the resist can be thinned down to effectively improve its collapse margin, especially for smaller assistant features. The results show that 72-nm assistant features on mask can be patterned for early 32-nm node development. This paper also suggests several approaches that can be used to reduce the required resist thickness, such as hard-mask, film thickness reduction, and etcher hardware modification.

Key indexes of the effectiveness of mask surface treatments

A proper surface treatment, such as O2 plasma, helps to improve particle removal efficiency (PRE) because of the formation of hydrogen bonding between particles, water and the mask surface after treatment. The effectiveness of surface treatments cannot be determined only by the static wettability after processes. More key indexes should be considered. In this paper, we report our findings on the relationship between surface treatments on photomasks and the resulting wettability. In addition, added defects after the treatment and the cleaning process were inspected with a 193- nm KLA inspector on 193-nm immersion and EUV photomasks, which consist of SiO2, MoSi, Cr, Ta-based absorber and Ru. Based on our work, three indexes can be built for determining the effectiveness of surface treatments. The first is to check whether the surface becomes super-hydrophilic after treatment. The second is to determine the efficiency of surface treatments on enhancing wettability. The last is to quantify the added watermark count after the surface treatment and the cleaning process. With a proper surface treatment, watermarks can be greatly eased. These three indexes can quickly determine possible effective methods for treating the surfaces of different materials.

Physical force optimization for advanced photomask cleaning

We investigated methods to extend the damage-free process window for fragile Sub-Resolution Assist Features (SRAF) in mask cleaning using MegaSonic and binary spray techniques. Particle removal efficiency (PRE) was found to increase by 8% and damage reduced from 7 ppm to 0 ppm with the optimization of the spray droplet characteristics through liquid media control. MegaSonic damage was eliminated completely from 10 ppm to 0 ppm by varying physical and chemical properties of the cleaning media. Since particles in the deep trenches are very difficult to remove using droplet spray alone, a combination of MegaSonic and Binary Spray processes was tested. The acoustic effects generated through the MegaSonic combined with optimized droplet impact showed an improvement of 4% in PRE of hard-to-remove trench particles. Overall, the improved process points to a promising solution for overcoming the roadblock in mask cleaning for the advanced mask cleaning.

Sub-20nm node photomask cleaning enhanced by controlling zeta potential

As semiconductor manufacturing advances to sub-20nm node (i.e., minimum feature pitch) technology, specifications with respect to ‘dust’ on photomasks are becoming ever more stringent. Photomask cleanliness is essential to high-quality lithography, and flaws seriously affect manufacturing cycle time and productivity. For example, detecting a single newly introduced particle following pellicle (protective cover) mounting results in a loss of one to two days for demounting, repair, and cleaning. A major challenge in developing sub-20nm node mask cleaning processes is removing extremely fine particles (<50nm) from the mask surface. In the past, we have attempted to adjust the nozzle flow and spray force (physical force), but improvement is limited and the method damages the mask pattern. We have also tried increasing the concentration of the cleaning chemical (chemical force), which degrades the photomask film. Here, we describe an alternative mask-cleaning method based on chemical force induced by so-called zeta potential (ZP). Extremely fine contaminating particles derive mostly from the deionized water used in chip fabrication and from cleaning-tool piping. Whether a particle adsorbs onto a mask surface in chemical solution depends on the balance between van der Waals and electrostatic interactions. Van der Waals interactions increase as two surfaces get closer to each other. In contrast, electrostatic interactions result from the electrical potential, or ZP, between the mask and particle surfaces. The behavior of ZP is determined by chemical properties such as pH, electrolyte concentration, and surface energy. As shown in Figure 1, the chemical force between particles and the mask surface changes in different ZP environments.1, 2 Figure 1. Zeta potential (ZP) induces chemical force on both the particle and mask quartz (Qz) surfaces. (a) Positive ZP: attraction between particles and the quartz surface. (b) Negative ZP: repulsion between particles and the quartz surface.

Study of loading effect on dry etching process

Nowadays, the CD (Critical Dimension) control on masks manufacturing plays an important role in photolithography process for 90-nm node technology and below. The process performance of photolithography will degrade severely even when the mask CD error is small. One of the most important process-induced mask CD errors comes from the dry etching process. With the loading effect due to environment pattern variations, isolated and dense patterns have different etching biases. Furthermore, the loading effect can induce an overall CD variation called global loading effect contributed from the pattern density change in large areas and a CD variation on individual monitor pattern called micro-loading effect contributed from various feature dimensions in the near region. The micro-loading effect can also be classified as the “nearest spacing” effect which is dependent upon the space between the nearest neighbor pattern and the monitor pattern, and the “nearest neighbor” effect which is dependent upon the size of the nearest neighbor feature around the monitor pattern. All of these effects enlarge the total range of mask CD linearity and proximity errors. In this paper we report the result of the global loading effect and micro-loading effect by varying pattern densities and feature dimensions nearby. With the design of test pattern, the global loading effect and the micro-loading effect can be separated. The CD variation dominated by the micro-loading effect in the dry etching process is observed. This large etching bias change resulted from the micro-loading effect is consistent with the depletion of radical species in the narrow space during the etching process.

Mask-making study for the 65-nm node

The specifications of mask critical dimension (CD) have become much tighter for sub-100nm nodes to satisfy wafer CD uniformity requirements. The small patterns produced by aggressive optical proximity correction compound the difficulty, thereby necessitating the use of e-beam writers in combination with chemical amplified resists (CARs). Challenges of resists include post coating delay (PCD), post exposure delay (PED) in vacuum, and strong post exposure bake (PEB) sensitivity. CD errors are classified into localized area and global ones; machines causing each type of errors are then identified. Focus variation and fogging effect have to be emphasized for the 65-nm requirements. Although the DOF of e-beam systems is larger than that of the optical systems, high current density and/or plate-to-plate deviation may cause focus variation to result in poor CD uniformity. Therefore, dosage optimization is necessary for getting the best focus. The fogging effective level is around 3~10 nm at various pattern loadings. The paper presents, quantitative results and the methodology leading to them.

Global CD uniformity improvement in mask manufacturing for advanced lithography

The control of global critical dimension uniformity (GCDU) across the entire mask becomes an important factor for the high-end masks quality. Three major proceses induce GCDU error before after-developing inspection (ADI) including the E-Beam writing, baking, and developing processes. Due to the charging effect, the fogging effect, the vacuum effect and other not-well-known effects, the E-Beam writing process suffers from some consistent GCDU errors. Specifically, the chemical amplified resist (CAR) induces the GCDU error from improper baking. This phenomenon becomes worse with negative CARs. The developing process is also a source of the GCDU error usually appears radially. This paper reports the results of the study of the impact of the global CD uniformity on mask to wafer images. It also proposes solutions to achieve better masks.