Recent rapid progress in the technologies of extreme ultraviolet lithography (EUVL) is ensuring that EUVL will be a primary candidate for the next generation lithography beyond 32-nm node. However, realization of defect-free reflective mask blank is still counted as one of the most critical issues for high volume production in EUVL. Asahi Glass Company (AGC) has developed comprehensive technologies for manufacturing EUVL mask blanks from figuring and polishing glass substrate to cleaning, multilayer coating, and evaluating its performances by making use of our long and wide experience in providing high quality processed glass substrates and coatings for electronic devices. In this paper, we will present the current status of each aspect of EUVL mask blank development in AGC toward the specifications required for high volume production. In the effort to meet the specifications, we have introduced a number of key technologies that can be divided into three regions, which are materials, glass processings, and evaluations. We have developed state-of-the-art processes and tools for manufacturing EUV mask blanks, such as a new polishing technique for extremely flat substrate, a new cleaning recipe and tool for low-defect substrate, and a newly developed deposition tool for ultra-low defect and higher EUV reflective coating with our new optical thin film materials for multilayer coating. Furthermore, in order to clarify their performances, we also introduced a wide variety of evaluation techniques such as flatness and roughness measurement of substrate, a defect inspection, and EUV reflectometry as well as defect analysis techniques which help us eliminate printable defects in EUVL mask blanks.
In extreme ultraviolet (EUV) lithography technology, ultra low thermal expansion material is required as photomask substrate. We have previously developed Ti-doped silica glass which exhibits both ultra low coefficient of thermal expansion (CTE) and high homogeneity for EUV substrate. On the other hand, we have been investigating other candidate materials which have low CTE, from the viewpoint of structural chemistry. Silica glass is well-known as a low thermal expansion material and the reason is explained that in the open structure of silica glass two factors, expansion and shrinkage, compete with each other with increase in temperature. The network of silica glass consists of tetrahedra like quartz crystal. In this structure, Si is stably present with a valence of 4 and a coordination number of 4. We have carried out an atomistic simulation and estimated the volume change of oxide materials which may have the same structural transformation mechanism as SiO2. As a result, the volume of SnO2 with quartz structure (quartz-SnO2), in which Sn was present with a valance of 4 and a coordination number of 4, decreased with increase in temperature, that is, the density of quartz-SnO2 increased. Thus, it was indicated that the glass with lower CTE than that of silica glass could be obtained with substituting Sn for Si. Based on this hypothesis, we have prepared Sn-doped silica glass by Asahi silica glass producing method. The synthesized Sn-doped silica glass exhibited lower CTE than that of an ordinary silica glass.
For 157 nm lithography the pellicle material will be most probably a 800 μm thick inorganic (fluorine doped fused silica) plate instead of a standard thin (~ 1 μm) organic (polymer) film. The thickness of the pellicle makes it an additional optical element in the 157 nm exposure tool. This puts tight requirements on the optical properties of the pellicle. One of the largest challenges is to control the pellicle induced overlay errors that result from small variations in pellicle flatness. A local tilt of 12 μrad already introduces an image displacement of 1 nm. This paper deals with the theoretical understanding of the pellicle indued overlay errors. It shows the relation between offline pellicle flatness measurements and exposure tool overlay performance. Two potential solutions are presented to obtain the pellicle within the desired overlay specification. System overlay corrections in combination with a new mounting strategy based on 'correctable pellicle shapes' seem to make the desired overlay specification (≤ 1 nm) feasible. The proposed 'one-dimensional' pellicle shape seems to be very promising. Distortion data, as obtained from exposures on a 193 nm system with and without pellicle, indicate that the proposed solution for automatically and fully correcting for a non-flat pellicle is feasible.
Projection photolithography at 157 nm is now under research as a possible extension of current 248 nm and planned 193 nm technologies .We have found that a thin-film fused silica glass pellicle would be available to 157nm lithography because of its high durability to F2 laser irradiation . In this paper, we present the performance of the hard pellicle made of AQF. Transmission is 97.6 percent when AR films are coated on both surfaces, and its uniformity at 157.6 nm is better than +/- 0.2 percent, and birefringence is within 1 nm. We developed a new evaluation system of a hard pellicle bending in horizontal position and in vertical position. We achieved less than 1um sagging with 800um thickness membrane and glass frame made of modified fused silica in horizontal position.
Identifying a functional pellicle solution for 157-nm lithography remains the most critical issue for mask technology. Developing a hard pellicle system has been a recent focus of study. Fabrication and potential pellicle-induced image placement errors present the highest challenges to the technology for meeting the stringent error budget for manufacturing devices in the 65-nm regime. This paper reports the results of a comprehensive proof-of-concept study on the state-of-art hard pellicle systems, which feature 800-mm thick modified fused silica pellicles and quartz frames. Pellicles were fabricated to ensure optical uniformity and flatness. Typical intrinsic warpage of these pellicles was close to the theoretical limit of 4.0 mm under a gravitational load. Quartz frames had bows less than 1.0 mm. The advantage of quartz frames with matched thermal expansion was demonstrated. An interferometric facility was developed to measure the flatness of the mask and pellicle system before and after pellicle mounting. Depending on the mounting process as well as mounting tool characteristics and techniques, variations were observed from pellicle to pellicle, mount to mount, and mask to mask. A redesign of the mounter and mounting process has significantly improved pellicle flatness. Finite element models were also generated to characterize the relative importance of the principal sources of pellicle-induced photomask distortions. Simulation results provide insightful guidance for improving image quality when employing a hard pellicle.
Changes in optical absorption of lithographic-grade SiO2 glass containing hydrogen were examined when irradiated intermittently by ArF excimer laser (6.4 eV) under a simulated operating mode of lithography laser. Absorption intensity at 6.4 eV was increased during the irradiation and was decreased gradually after the termination of the irradiation. However, when re-irradiated, the absorption intensity was recovered instantly to the extent just before the termination of the irradiation. These observed phenomena could be explained by the formation and restoration of E' centers. E' centers were generated through two kinds of processes, dissociation of strained Si-O-Si bonds and dissociation of photo-induced ODCs, while E' centers were converted into SiHs by chemical reaction with hydrogen in SiO2 glass. The phenomenon of fast re-darkening was due to photolysis of SiHs into E' centers.
Projection photolithography at 157 nm is now under research as a possible extension of current 248 and planned 193 nm technologies. We have succeeded in the development of the modified fused silica glass 'AQF' for 157 nm lithography. In this paper, we present the performance of the newest material; AQF/Ver.2.1. Transmission and its uniformity at 157 nm is better than 78 +/- 1.5 percent, and birefringence is within 2 nm. We also have developed hard pellicle with 300 micrometers thickness. Its transmission is over 92 percent when AR films are coated on both surfaces. This hard pellicle also has a very good durability to F2 laser.
Projection photolithography at 157 nm is now under research as a possible extension of current 248 nm and planned 193 nm technologies. We have succeeded in the development of the modified fused silica glass 'AQF' for 157 nm lithography. In this paper, we present the performance of the newest material; AQF/Ver. 2.1. Transmission and its uniformity at 157 nm is better than 78 +/- 1.5 percent, and birefringence is within 2nm. The surface flatness is less than 0.5 micrometers , and surface defects over 0.4 micrometers in size are free.
Projection photolithography at 157 nm is now under research as a possible extension of current 248 nm and planned 193 nm technologies. Although the conventional silica glass used for 248 nm and 193 nm lithography cannot be applied for 157 nm lithography because of its low transmittance, we have already developed the modified fused silica glass 'AQF' for 157 nm lithography. In this paper, we report on the fabrication of 'AQF' 6 inch photomask substrate. 157 nm transmission and its uniformity is better than 75 plus or minus 3%, and flatness is less than 0.5 micrometer. Defects over 0.4 micrometer in size are free. The physical and mechanical properties are also shown to be similar with our conventional silica glass 'AQ.'
Projection photolithography at 157 nm is now under research as a possible extension of current 248 nm and planned 193 nm technologies. However, the conventional silica glass used for 248 nm and 193 nm lithography cannot be applied for 157 nm lithography because of its low transmittance. In order to develop the modified silica glass for 157 nm lithography, the transmittance in the vacuum-UV region and the optical properties induced by vacuum-UV irradiation were investigated. The OH group and the ODC in the silica glass markedly affect the initial transmittance at 157 nm and the former also affects the resistance to vacuum -UV irradiation. The hydrogen bonded OH group was observed after vacuum-UV irradiation. From these results, the new silica glass 'AQF' for 157 nm lithography has been successfully developed with a high internal transmittance at 157 nm and a excellent resistance to F2 laser.