Takashi Kamikubo

High-Accuracy Proximity Effect Correction for Mask Writing

A high-accuracy proximity effect correction method for high-precision masks has been developed to satisfy current and future requirements. In this paper, we explain the primary features of this method and the theories on which it is based. The developed formula for obtaining the optimum correction dose is expressed in the form of either iterations or an infinite series of functions. The advantage of this formula is that it quickly converges to the sought value, bringing about high-accuracy proximity effect correction with a high calculation speed. A coarse graining method (covering pattern density and representative figure methods) for reducing calculation time is explained. This method has been adopted for an EX-11 series and has been used for mask writing from the 180 nm design rule onward.

Advanced electron-beam writing system EX-11 for next-generation mask fabrication

Toshiba and Toshiba Machine have developed an advanced electron beam writing system EX-11 for next-generation mask fabrication. EX-11 is a 50 kV variable-shaped beam lithography system for manufacturing 4x masks for 0.15 - 0.18 micrometer technology generation. Many breakthroughs were studied and applied to EX-11 to meet future mask-fabrication requirements, such as critical dimension and positioning accuracy. We have verified the accuracy required for 0.15 - 0.18 micrometer generation.

Mask process correction (MPC) modeling and its application to EUV mask for electron beam mask writer EBM-7000

In electron beam writing on EUV mask, it has been reported that CD linearity does not show simple signatures as observed with conventional COG (Cr on Glass) masks because they are caused by scattered electrons form EUV mask itself which comprises stacked heavy metals and thick multi-layers. To resolve this issue, Mask Process Correction (MPC) will be ideally applicable. Every pattern is reshaped in MPC. Therefore, the number of shots would not increase and writing time will be kept within reasonable range. In this paper, MPC is extended to modeling for correction of CD linearity errors on EUV mask. And its effectiveness is verified with simulations and experiments through actual writing test.

EBM-9000: EB mask writer for product mask fabrication of 16nm half-pitch generation and beyond

EBM-9000 equipped with new features such as new electron optics, high current density (800A/cm2) and high speed deflection control has been developed for the 11nm technology node(tn) (half pitch (hp) 16nm). Also in parallel of aggressive introduction of new technologies, EBM-9000 inherits the 50kV variable shaped electron beam / vector scan architecture, continuous stage motion and VSB-12 data format handling from the preceding EBM series to maintain high reliability accepted by many customers. This paper will report our technical challenges and results obtained through the development.

Modeling of charging effect and its correction by EB mask writer EBM-6000

The impending need of double patterning/double exposure techniques is accelerating the demand for higher pattern placement accuracy to be achieved in the upcoming lithography generations. One of the biggest error sources of pattern placement accuracy on an EB mask writer is the resist charging effect. In this paper, we provide a model to describe the resist charging behavior on a photomask written on our EBM-6000 system. We found this model was very effective in correcting and reducing the beam position error induced by the charging effect.

Electron Beam Mask Writer EBM-7000 for Hp 32nm Generation

Optical lithography is facing resolution limit. To overcome this issue, highly complicated patterns with high data volume are being adopted for optical mask fabrications. With this background, new electron beam mask writing system, EBM- 7000 is developed to satisfy requirements of hp 32nm generation. Electron optical system with low aberrations is developed to resolve finer patterns like 30nm L/S. In addition, high current density of 200 A/cm2 is realized to avoid writing time increase. In data path, distributed processing system is newly built to handle large amounts of data efficiently. The data processing speed of 500MB/s, fast enough to process all the necessary data within exposure time in parallel for hp32nm generation, is achieved. And this also makes it possible to handle such large volume dense data as 2G shots/mm2 local pattern density. In this paper, system configuration of EBM-7000 with accuracy data obtained are presented.

Proximity Effect Correction For Electron Beam Lithography: Highly Accurate Correction Method

A new formula for proximity effect correction is discussed. The formula is represented by a series expansion. When infinite terms are used, the formula gives accurate optimum correction doses. The correction accuracy of the new formula is evaluated for the worst case scenario and compared with the conventional formula. It is shown that (1) the new formula suppresses correction errors to less than 0.5% for the deposited energy and (2) dimensional errors are less than 4 nm, even if only the first 3 terms are calculated for critical patterns. By using the new formula, the proximity effect correction can be carried out with sufficient accuracy, even for making reticles of 1 Gbit or higher-capacity DRAMs.

Correction of resist heating effect on variable shaped beam mask writer

Abstract. The specifications for critical dimension (CD) accuracy and line edge roughness are getting tighter to promote every photomask manufacturer to choose electron beam resists of lower sensitivity. When the resist is exposed by too many electrons, it is excessively heated up to have higher sensitivity at a higher temperature, which results in degraded CD uniformity. This effect is called “resist heating effect” and is now the most critical error source in CD control on a variable shaped beam (VSB) mask writer. We have developed an on-tool, real-time correction system for the resist heating effect. The system is composed of correction software based on a simple thermal diffusion model and computational hardware equipped with more than 100 graphical processing unit chips. We have demonstrated that the designed correction accuracy was obtained and the runtime of correction was sufficiently shorter than the writing time. The system is ready to be deployed for our VSB mask writers to retain the writing time as short as possible for lower sensitivity resists by removing the need for increased pass count.

New electron optics for mask writer EBM-7000 to challenge hp 32nm generation

Semiconductor scaling is expected to continue to hp32nm and beyond, accompanied by explosive data volume expansion. Required minimum feature size at hp 32nm will be less than 50nm on the mask, according to ITRS2007(1). EBM 7000 is a newly designed mask writer for the hp32 nm node with an improved electron optical column providing the beam resolution (10 nm measured in situ) and beam current density (200 A/cm2) necessary for cost effective mask production at hp32nm node. In this paper we report on column improvements, the in situ beam blur measurement method and writing results from EBM 7000. Written patterns show dose margin (CD change [nm] / 1 % dose change) of .94 nm /1 % dose for line/space arrays using chemically amplified resist PRL009 and our standard processing. Using a simple model to relate the measured beam intensity distribution to the measured dose margin, we infer an effective total blur of 30 nm, dominated by a contribution of 28 nm from the resist exposure and development process. Further evidence of the dominance of the process contribution is the measured improvement in dose margin to .64 nm/% dose obtained by modifying our standard process. Even larger process improvements will be needed for successful fabrication of hp22nm masks.

Study of heating effect on CAR in electron beam mask writing

Heating effect was evaluated for EBM-6000 which is operated at high current density of 70A/cm2 and acceleration voltage of 50kV. FEP171 as widely used for current productions and lower sensitivity resists are tested. Lower sensitivity resist is one of key items to achieve highly accurate Local critical dimension uniformity (LCDU) because of shot noise reduction. CD variations in experiment are compared with simulated temperature changes induced by heating effect. Then, the ratio, ΔCD/ΔT, is found mostly constant for every resist, 0.1 nm/C°. Writing conditions are estimated to meet CDU spec of hp45 generation for a worst case pattern, i.e. 100% density pattern. For FEP171, the maximum shot size of 0.85 μm shot size at 2pass writing mode is sufficient. It should be reduced to 0.5 μm at 2pass writing mode for every lower sensitivity resist. When 4pass writing mode is used, the maximum shot size of 0.85 μm is available. Writing conditions and writing time for realistic patterns are also discussed.