Sangpyo Kim

The CD measuring repeatability enhancement by intensity gradient

As required CD (critical dimension) measuring accuracy is tighter, it is necessary to enhance the repeatability of CD-SEM on photo-mask, by optimizing charge up, scan speed, beam size, acceleration, current and temperature control. CD-SEM shows sparkle noise which degrades the image of CD-SEM. And defocus is also getting the source of worse gauge R&R. We evaluated the effect of defocus and noise on CD repeatability by extracting CD from gradient value of image after anisotropic nonlinear diffusion filtering on SEM image. As SEM image is measured after averaging the intensity of image on range of interest (ROI) to remove scan noise, anisotropic nonlinear diffusion (AND) which has different diffusivity according to direction, is efficient tool to get smooth pattern without averaging. This smoothing technique is effective in measuring isolated pattern on mask which is difficult to measure around corner. Some simple CD measuring algorithms are available to get better CD repeatability. Using the maximum intensity and gradient of image, we were able to measure CD on various shaped patterns with enhanced repeatability.

Comparison of analysis techniques for aerial image metrology on advanced photomask

The standard method for defect disposition and verification of repair success in the mask shop is through the utilization of the aerial imaging platform, AIMSTM. The CD (Critical Dimension) deviation of the defective or repaired region as well as the pattern shift can be calculated by comparing the measured aerial images of this region to that of a reference. Through this analysis it can be determined if the defect or repaired region will be printed on the wafer under the illumination conditions of the scanner. The analysis of the measured aerial images from the AIMSTM are commonly performed manually using the analysis software available on the system or with the help of an analysis software called RV (Repair Verification). Because the process is manual, it is not standardized and is subject to operator variations. This method of manual aerial image analysis is time consuming, dependent on the skill level of the operator and significantly contributes to the overall mask manufacturing process flow. AutoAnalysis (AA), the first application available for the FAVOR® platform, provides fully automated analysis of AIMSTM aerial images [1] and runs in parallel to the measurement of the aerial images. In this paper, we investigate the initial AutoAnalysis performance compared to the conventional method using RV and its application to a production environment. The evaluation is based on the defect CD of three pattern types: contact holes, dense line and spaces and peripheral structure. The defect analysis results for different patterns and illumination conditions will be correlated and challenges in transitioning to the new approach will be discussed.

Analysis of resist pattern collapse and optimization of DUV process for patterning sub-0.20-μm gate line

We analyzed the local pattern collapse by KLA-2132 in patterning gate line of 0.18 micrometers on the Poly-Si/WSix/Si3N4, where the thickness variation of Si3N4 (Nitride) film affected on the substrate reflectivity. By thickness split experiments of organic bottom anti- reflective layers (ARLs), we showed the effect of thickness variation of Nitride on the resist pattern collapse. We investigated the contribution of various factors to the pattern collapse. First of all, we focused on the CD variation due to substrate reflectivity variation to remove patterns of tolerable aspect ratio. In order to obtain better CD uniformity by tight reflectivity control considering the thickness variation of Nitride film, we optimized anti-reflective layer process using inorganic ARLs. As an inorganic ARL, we used PECVD SiOxNy:H(SiON) of which optical constants were changed by deposition conditions. We compared typical positive-tone DUV resists, of acetal based with environmentally stable chemically amplified photoresist type, to clarify the effect of resist and organic bottom ARL materials.

Advanced repair solution of clear defects on HTPSM by using nanomachining tool

As the mask specifications become tighter for low k1 lithography, more aggressive repair accuracy is required below sub 20nm tech. node. To meet tight defect specifications, many maskshops select effective repair tools according to defect types. Normally, pattern defects are repaired by the e-beam repair tool and soft defects such as particles are repaired by the nanomachining tool. It is difficult for an e-beam repair tool to remove particle defects because it uses chemical reaction between gas and electron, and a nanomachining tool, which uses physical reaction between a nano-tip and defects, cannot be applied for repairing clear defects. Generally, film deposition process is widely used for repairing clear defects. However, the deposited film has weak cleaning durability, so it is easily removed by accumulated cleaning process. Although the deposited film is strongly attached on MoSiN(or Qz) film, the adhesive strength between deposited Cr film and MoSiN(or Qz) film becomes weaker and weaker by the accumulated energy when masks are exposed in a scanner tool due to the different coefficient of thermal expansion of each materials. Therefore, whenever a re-pellicle process is needed to a mask, all deposited repair points have to be confirmed whether those deposition film are damaged or not. And if a deposition point is damaged, repair process is needed again. This process causes longer and more complex process. In this paper, the basic theory and the principle are introduced to recover clear defects by using nanomachining tool, and the evaluated results are reviewed at dense line (L/S) patterns and contact hole (C/H) patterns. Also, the results using a nanomachining were compared with those using an e-beam repair tool, including the cleaning durability evaluated by the accumulated cleaning process. Besides, we discuss the phase shift issue and the solution about the image placement error caused by phase error.

Novel CD control of HTPSM by advanced process for sub-20nm tech

As the design rule of the semiconductor shrinks, the CD MTT (Critical Dimension Mean-to-Target) specification for photomask becomes tighter. So, more precise control of CD MTT is required. We have investigated the CD MTT control and applied it to the attenuated PSM (Phase Shift Mask) successfully for several years. We can control the CD MTT of MoSi pattern by measuring Cr/MoSi pattern to estimate MoSi pattern CD and additional etch to shrink MoSi pattern as reported in previous study. At first, the MoSi pattern CD can be estimated with the Cr/MoSi pattern CD because the CD gap between MoSi pattern and Cr/MoSi pattern is relatively constant. Additional MoSi etch is performed to shrink the MoSi pattern CD after then. The CD gap alwasys exists and the variation of the CD gap is enough small to be not considered in conventional photomask production until now. However, the variation of the CD gap is not ignorable in case of sub-20 nm tech. In this study, we investigated new method to measure MoSi pattern CD before Cr strip process to eliminate the CD gap between MoSi pattern and Cr/MoSi pattern. To eliminate the CD gap, we attempt three solutions – 1) Optimize etch process to perform perfect Cr/MoSi pattern profile without the CD gap, 2) Improve CD measurement accuracy by developing new SEM measuring mechanism, 3) Develop of new process to modify Cr/MoSi pattern profile to be measured without the CD gap. It was found that the CD gap can be eliminated and MoSi pattern CD can be measured perfectly. Finally, MoSi pattern CD control was improved because of CD gap elimination.

The study of develop optimization to control various resist defect in Photomask fabrication

To reduce the pattern size in photomask is an inevitable trend because of the minimization of chip size. So it makes a big challenge to control defects in photomask industry. Defects below a certain size that had not been any problem in previous technology node are becoming an issue as the patterns are smaller. Therefore, the acceptable tolerance levels for current defect size and quantity are dramatically reduced. Because these defects on photomask can be the sources of the repeating defects on wafer, small size defects smaller than 200nm should not be ignored any more. Generally, almost defects are generated during develop process and etch process. Especially it is difficult to find the root cause of defects formed during the develop process because of their various types and very small size. In this paper, we studied how these small defects can be eliminated by analyzing the defects and tuning the develop process. There are 3 types of resist defects which are named as follows. The first type is ‘Popcorn’ defect which is mainly occurred in negative resist and exists on the dark features. The second type is ‘Frog eggs’ defect which is occurred in 2nd process of HTPSM and exists on the wide space area. The last type is ‘Spot’ defect which also exists on the wide space area. These defects are generally appeared on the entire area of a plate and the number of these defects is about several hundred. It is thought that the original source is the surface’s hydrophilic state before develop process or the incongruity between resist and developer. This study shows that the optimizing the develop process can be a good solution for some resist defects.

The method of quartz damage recovery in the photomask repair process

As the pattern size became gradually smaller, the defect detectability of the photomask inspection tool was more improved. For these reasons, we have to repair various defects more precisely. By improving the mask yield through the repair process, we can reduce the cost of mask fabrication. In this study, we studied the defect called quartz damage which distorts the AIMSTM (Arial Image Measurement System) intensity of the repaired pattern and causes the scrap of the photomask. The quartz damage is generally observed when the abnormal defects like particles were repaired in the poor repairing condition. The quartz damage occasionally results in repair errors and affects the AIMS intensity. Currently there is no clear solution for recovering the quartz damage. As a result, it is very difficult to get the high quality photomask if the quartz damage is generated on the photomask. Therefore, it is important to find a method of recovering the quartz damage for producing the high quality photomask. In this paper, we demonstrated that the quartz damage can be recovered through the TEOS (Tetraethoxysilane) gas deposition. Also we investigated the effect on the recovery of the quartz damage of various parameters such as the type and the depth of the quartz damage as well as the repair conditions of the TEOS gas deposition.

Optimized Multigrid Strategy for Accurate Flare Modeling with Three-Dimensional Mask Effect in Extreme-Ultraviolet Lithography

Extreme-ultraviolet (EUV) lithography has many critical challenges regarding its implementation in the semiconductor industry. One of the main challenges is flare, the unwanted total integrated light scattering at the wafer level, which reduces the critical dimension and imaging performance. Therefore, EUV flare has been intensively studied and has been compensated by a rule-based method for many years. However, there are few results with regard to developing more accurate and feasible flare-modeling techniques to enable us to satisfy the criteria of the sub-22 nm half pitch (HP) technology node and beyond. In this work, we studied an improved flare-modeling technique considering the interaction of scattered EUV light with a three-dimensional EUV mask topography in order to obtain an accurate flare distribution and an optimized multigrid strategy for generating a flare map over the full-field scale. Also, we proposed a flare-modeling technique based on the pedestal model, which we developed using novel effective reflection coefficients in order to achieve sufficient accuracy. Such an approach is thought to be needed instead of the conventional pattern density approach in preparation for upcoming advanced HP technology nodes or for different absorber materials and illumination angles. Lastly, the need for a flare map shift to compensate for the mask defocus error is introduced and some flare evaluations of mask patterns used in the exposure-dose-monitoring technique were performed.

A study on irregular growing defect mechanism and removal process

Main Topics of a photomask have been CD(Critical Dimension), Overlay and Defects. In side of defects, technique suppressing growing defects which are occurring on a mask surface becomes as important as defect control method during mask manufacturing process. Conventional growing defects arise out of combination of sulfuric ion on a mask surface and environmental facts such as pellicle ingredient, humidity and etc. So Mask cleaning process was driven to reduce sulfuric acid on a mask surface which source of growing defects. And actually various cleaning process has been developed through the elimination of sulfuric acid such as DI, O3 cleaning. Normally Conventional growing defects are removed using DI, SC1 or SPM cleaning according to incidence. But recently irregular growing defects are occurred which are completely distinct from conventional growing defects. Interestingly, irregular growing defects are distributed differently from conventional on a mask. They spread in isolated space patterns and reduce the transmittance so that space pattern size continuously decreased. It causes Wafer Yield loss. Furthermore, irregular growing defects are not fully removed by cleaning which is traditional removal process. In this study, we provide difference between conventional and irregular growing defects based on its characteristic and distributed formation. In addition, we present and discuss removal and control technique about irregular growing defects. For elemental analysis and study, diverse analysis tool was applied such as TEM for checking Cross-Section, AFM for checking the roughness of surface, EDAX, AES, IC for analyzing remained ions and particles on the mask and AIMS.

Simulation analysis of backside defects printability in 193nm photolithography

Backside defects a few micrometers in size are serious concern in lithography because they can degrade the image quality on a wafer. It was known that defects attached on the backside affected the printing images on a wafer by locally altering the partial coherence (σ) and the transmitted intensity of the illumination. The ability to detect and to simulate their impact of defects on the backside is one of the key components in ensuring quality of photomask. The purpose of this study is to determine the minimum size of defects on the backside which would be affected printability in 193nm photolithography. It was investigated to the influence of wafer critical dimension (CD) variation according to illumination and NA, that of refraction according to defect size. For this study, a reticle was designed to include line and space patterns, contact patterns and isolated patterns on the front side. And the type of defects attached on the backside was made of chrome to investigate the relation between transmittance of backside defects and its printability. The correlation of measurements made with UV and DUV-based inspection system; simulation performed with a 193nm aerial image measurement system. Besides the allowable size of backside defects was determined using the criterion of a maximum intensity variation of 10%.