Anto Yasaka

Development of new FIB technology for EUVL mask repair

The next generation EUVL masks beyond hp15nm are difficult to repair for the current repair technologies including focused ion beam (FIB) and electron beam (EB) in view of the minimum repairable size. We developed a new FIB technology to repair EUVL masks. Conventional FIB use gallium ions (Ga+) generated by a liquid metal ion source (LMIS), but the new FIB uses hydrogen ions (H2+) generated by a gas field ion source (GFIS). The minimum reaction area of H2+ FIB is theoretically much smaller than that of EB. We investigated the repair performance of H2+ FIB. In the concrete, we evaluated image resolution, scan damage, etching rate, material selectivity of etching and actinic image of repaired area. The most important result is that there was no difference between the repaired area and the non-repaired one on actinic images. That result suggests that the H2+ GFIS technology is a promising candidate for the solution to repair the next generation EUVL masks beyond hp15nm.

Study of EUV mask defect repair using FIB method

We evaluated a new FIB-GAE (Focused Ion Beam-Gas Assisted Etching) repairing process for the absorber defects on EUVL mask. XeF2 gas and H2O gas were used as etching assist agent and etching stop agent respectively. The H2O gas was used to oxidize Ta-nitride side-wall and to inactivate the remaining XeF2 gas after the completion of defect repair. At the Photomask Japan 2008 we had reported that side-etching of Ta-nitride caused CD degradation in EUVL. In the present paper we report on the performance of defect repair by FIB, and of printability using SFET (Small Field Exposure Tool). The samples evaluated, were in form of bridge defects in hp225nm L/S pattern. The cross sectional SEM images certified that the newly developed H2O gas process prevented side-etching damage to TaBN layer and made the side-wall close to vertical. The printability also showed excellent results. There were no significant CD changes in the defocus characterization of the defect repaired region. In its defect repair process, the FIB method showed no signs of scan damage on Cr buffered EUV mask. The repair accuracy and the application to narrow pitched pattern are also discussed.

Photomask repair technology by using gas field ion source

Recently, most of defects on high-end masks are repaired with electron beam (EB). The minimum repairable dimension of the current state-of-the-art repair systems is about 20-30 nm, but that dimension is not small enough to repair the next generation masks. Meanwhile, new molybdenum silicide (MoSi) films with high cleaning durability are going to be provided for an alternative technology, but the etching selectivity between new MoSi and quartz under EB repair process is not high enough to control etching depth. We developed the focused ion beam (FIB) technology that uses light ions emitted from a gas field ion source (GFIS). In this study, the performance of our developed GFIS mask repair system was investigated by using new MoSi (HOYA-A6L2). Specifically, the minimum repairable dimension, image resolution, imaging damage, etching material selectivity and through-focus behavior on AIMS were evaluated. The minimum repairable dimension was only 11 nm that is nearly half of that with EB. That result suggests that GFIS technology is a promising candidate for repairing the next generation masks. Meanwhile, the etching selectivity between A6L2 and quartz was 6:1. Additionally, the other evaluations on AIMS showed good results. Those results demonstrate that GFIS technology is a reliable solution of repairing new MoSi masks with high cleaning durability.

Application of vector scanning in focused ion beam photomask repair system

With continuous reduction in linewidth of the VLSI devices, the pattern integrity of photomasks becomes considerably more important than ever. Consequently, requirement for the defect repair technology on photomasks is more severe and strict. Focused ion beam (FIB) technology has been widely used for defect repairing in photomask industry. Therefore, the performance of the FIB mask repair tool has to be improved especially in repair accuracy and precision. The FIB repair processes are classified into two kinds; one is additive repair using FIB induced deposition for missing patterns, the other is subtractive repair using gas assisted FIB etching for extra patterns. In both processes, precursor gas is applied onto the processing area through a small nozzle. Thus, the repair processes are controlled by the FIB irradiation and the precursor gas supply. Important characteristics of the repairs, such as size, shape, and placement of the repair area, are defined by the FIB scanning control. As conventional FIB ...

Damage analysis of EUV mask under Ga focused ion beam irradiation

EUV mask damage caused by Ga focused ion beam irradiation during the mask defect repair was studied. The concentration of Ga atom implanted in the multilayer through the buffer layer was calculated by SRIM. The reflectivity of the multilayer was calculated from the Ga distribution below the capping layer surface. To validate the calculation, a multilayer sample was irradiated with Ga FIB, and then EUV reflectivity was measured. The measured reflectivity change was in good agreement with the calculated value. An aerial image of patterns with Ga implanted region was simulated. The impact of the estimated Ga absorption on the linewidth of 32 nm hp line pattern was found to be less than 1 nm.

FIB mask repair technology for electron projection lithography

We have studied stencil mask repair technology with focused ion beam and developed an advanced mask repair tool for electron projection lithography. There were some challenges in the stencil mask repair, which were mainly due to its 3-dimensional structure with aspect ratio more than 10. In order to solve them, we developed some key technologies with focused ion beam (FIB). The transmitted FIB detection technique is a reliable imaging method for a 3-dimensional stencil mask. This technique makes it easy to observe deep patterns of the stencil mask and to detect the process endpoint. High-aspect processing can be achieved using gas-assisted etching (GAE) for a stencil mask. GAE enables us to repair mask patterns with aspect ratio more than 50 and very steep sidewall angle within 90±1°precisely. Edge placement accuracy of the developed tool is about 14nm by manual operation. This tool is capable to achieve less than 10nm by advanced software. It was found that FIB technology had capability to satisfy required specifications for EPL mask repair.

Performance of GFIS mask repair system for various mask materials

We have developed a new focused ion beam (FIB) technology using a gas field ion source (GFIS) for mask repair. Meanwhile, since current high-end photomasks do not have high durability in exposure nor cleaning, some new photomask materials are proposed. In 2012, we reported that our GFIS system had repaired a representative new material “A6L2”. It is currently expected to extend the application range of GFIS technology for various new materials and various defect shapes. In this study, we repaired a single bridge, a triple bridge and a missing hole on a phase shift mask (PSM) of “A6L2”, and also repaired single bridges on a binary mask of molybdenum silicide (MoSi) material “W4G” and a PSM of high transmittance material “SDC1”. The etching selectivity between those new materials and quartz were over 4:1. There were no significant differences of pattern shapes on scanning electron microscopy (SEM) images between repair and non-repair regions. All the critical dimensions (CD) at repair regions were less than +/-3% of those at normal ones on an aerial image metrology system (AIMS). Those results demonstrated that GFIS technology is a reliable solution of repairing new material photomasks that are candidates for 1X nm generation.

FIB mask repair technology for EUV mask

We evaluated a FIB-CVD (Focused Ion Beam-Chemical Vapor Deposition) process for repairing clear defects on EUV masks. For the CVD film, we selected Carbon material. Our simulation result showed that the properties of wafer-prints depended on the density of the carbon films deposited for repairing the clear defects. Especially, when the density of carbon film was higher than that of graphite the properties of the wafer-prints came out to be almost same as obtained from Ta-based absorbers. For CVD, in this work we employed typical carbon based precursor that has been routinely used for repairing photomask patterns. The defects created for our evaluation were line-cut defects in a hp225nm L/S pattern. The performance of defect repair was evaluated by SFET (Small Field Exposure Tool) printability test. The study showed that the focus characteristic of repaired region deteriorated as the thickness of the deposition film decreased, especially when the thickness went below the thickness of the absorber. However, when the deposition film thickness was same as that of the absorber film, focus characteristic was found to be excellent. The study also revealed that wafer-print CDs could be controlled by controlling the CDs of the deposition films. The durability of deposition films against the buffer layer etching process and hydrogen radical cleaning process is also discussed.

Evaluation of defect repair of EUVL mask pattern using FIB-GAE method

We utilized a newly developed low acceleration voltage FIB (Focused Ion Beam) system and evaluated the process for repairing the absorber layer on EUVL mask. During the etching of the absorber layer, which is a step in conventional repair technique, a phenomenon of side-etching of Ta-nitride layer with XeF2 gas was observed. This phenomenon was considered to be caused by the isotropic etching of the Ta-nitride layer with XeF2 gas. We then added another gas for etching and evaluated the new process to prevent the side-etching of Ta-nitride layer. In this paper, we will report four evaluation results of EUVL mask pattern defect repair using FIB-GAE (Gas Assisted Etching). The first one is the problem of pattern topography after conventional repairing process and the reaction mechanism of gas assisted etching of Ta based absorber. The second evaluation result is addressed in two parts. One is the evaluation of a new gas assisted etching process that employs an additional gas that has an ability to control the etching rate of absorber layer. The second part addresses the repairing accuracy of EUVL mask pattern. The third is the basic etching performance e.g. etching rate of Ta based absorber, Cr based buffer, and Si based capping layer. The fourth and the last evaluation is the application of the newly developed gas assisted etching process on programmed bridge defect in narrow pitched L/S patterns.

Ga implantation and interlayer mixing during FIB repair of EUV mask defects

EUV mask damage caused by Ga focused ion beam irradiation during the mask defect repair was studied. The concentration of Ga atom implanted in the multilayer through the buffer layer and distributions of recoil atoms were calculated by SRIM. The reflectivity of the multilayer was calculated from the Ga distribution below the capping layer surface. To validate the calculation, Ga focused ion beam was irradiated on the buffer layer. The EUV reflectivity was measured after the buffer layer etching process. The measured reflectivity change was considerably larger than the one predicted from the absorption of light by the implanted Ga. The large reflectivity loss was primarily due to the absorption of light by chromium silicide residue which was generated by the intermixing of the buffer and the capping layer. Both lowering of the acceleration voltage and using thicker buffer layer were found to be effective in reducing this intermixing. The reduction of the reflectivity loss by using thicker buffer layer was confirmed by our experiments. An aerial image of patterns with etching residue formed by the intermixing was simulated. When the thickness of the intermixed layer happened to be 8 nm and the size of the resulting residue was larger than 100 nm, then the impact of the estimated absorption by the residue on the linewidth of 32 nm hp line pattern became more than 5 %.