Hitoshi Sunaoshi

The development of full field high resolution imprint templates

Critical to the success of imprint lithography and Step and Flash Imprint Lithography (S-FIL®) in particular is the manufacturing 1X templates. Several commercial mask shops now accept orders for 1X templates. Recently, there have been several publications addressing the fabrication of templates with 32nm and sub 32nm half pitch dimensions using high resolution Gaussian beam pattern generators. Currently, these systems are very useful for unit process development and device prototyping. In this paper, we address the progress made towards full field templates suitable for the fabrication of CMOS circuits. The starting photoplate consisted of a Cr hard mask (≤ 15nm) followed by a thin imaging layer of ZEP 520A. The EBM-5000 and the EBM-6000 variable shape beam pattern generators from NuFlare Technology were used to pattern the images on the substrates. Several key specifications of the EBM-6000, resulting in improved performance over the EBM-5000 include higher current density (70 A/cm2), astigmatism correction in the subfields, optimized variable stage speed control, and improved data handling to increase the maximum shot count limitation. To fabricate the template, the patterned resist serves as an etch mask for the thin Cr film. The Cr, in turn, is used as an etch block for the fused silica. A mesa is formed by etching the non-active areas using a wet buffered oxide etch (BOE) solution. The final step in the template process is a dice and polish step used to separate the plate into four distinct templates. Key steps in the fabrication process include the imaging and pattern processes. ZEP520A was chosen as the e-beam resist for its ability to resolve high resolution images. This paper documents the resolution and image placement capability with the processes described above. Although ZEP520A is slow relative to chemically amplified e-beam resists, it is only necessary to pattern 1/16th the area relative to a 4X reduction mask. Write time calculations for 1X templates have also been performed, and are compared to 4X photomasks.

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.

Correction technique of EBM-6000 prepared for EUV mask writing

Image placement (IP) errors caused by electro-static chuck (ESC) and non-flatness of mask are additional factors in writing extreme ultra-violet (EUV) mask, and minimizing their influences is being fervently addressed. New correction technique of EBM-6000 has been developed for EUV mask writing based on the conventional grid matching correction (GMC) without ESC to obtain good reproducibility to satisfy users requirement to develop EUV mask at an early stage.

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.

Contamination Charging up Effect in a Variably Shaped Electron Beam Writer

We investigated the beam drift on the second shaping aperture caused by the charging up of contamination layers on shaping deflector surfaces in an EX-8D variably shaped beam (VSB) writer. A surface charging model which is based on the movement of the secondary electron (SE) charge-up area in a deflector in response to the direction and strength of the field was adapted to explain the beam drift phenomenon. It was found that SE charge-up causes the beam drift and its amount depends on the amount of SEs moving between the electrodes. It was also found that by removing the contamination layers using a downflow cleaning process the beam drift is significantly reduced even if SEs move between the electrodes.

Key improvement Schemes of Accuracies in EB Mask Writing for Double Patterning Lithography

Double pattering or exposure methodologies are being adopted to extend 193nm optical lithography. These methodologies require much tighter image-placement accuracy and Critical Dimension (CD) controls on mask than the conventional single exposure technique. Our experiments indicate that the global image placement drift induced by the time elapsed in mask writing is the dominant factor that degrades image-placement accuracy. In-situ grid measurement method is being proposed to suppress this time dependent drift. Resist charging effect is also an important error factor. While it can be reduced by charge dissipation layer (CDL), further feasibility study is required for using CDL to overcome certain side-effects pertaining to CDL. High dose resist improves local CD uniformity and pattern fidelity. However, mask writing time becomes longer with lower sensitivity. To satisfy conflicting issues, throughput and CD uniformity, high sensitivity CAR which has short acid diffusion length is desirable. Shortening acid diffusion length is essential for achieving good pattern resolution as well as good CD uniformity. This paper will address the results of error source analyses and key schemes of accuracy improvements in photo-mask manufacturing using NuFlare Technologys EB mask writers.

Controlling linewidth roughness in step and flash imprint lithography

Despite the remarkable progress made in extending optical lithography to deep sub-wavelength imaging, the limit for the technology seems imminent. At 22nm half pitch design rules, neither very high NA tools (NA 1.6), nor techniques such as double patterning are likely to be sufficient. One of the key challenges in patterning features with these dimensions is the ability to minimize feature roughness while maintaining reasonable process throughput. This limitation is particularly challenging for electron and photon based NGL technologies, where fast chemically amplified resists are used to define the patterned images. Control of linewidth roughness (LWR) is critical, since it adversely affects device speed and timing in CMOS circuits. Imprint lithography has been included on the ITRS Lithography Roadmap at the 32 and 22 nm nodes. This technology has been shown to be an effective method for replication of nanometer-scale structures from a template (imprint mask). As a high fidelity replication process, the resolution of imprint lithography is determined by the ability to create a master template having the required dimensions. Although the imprint process itself adds no additional linewidth roughness to the patterning process, the burden of minimizing LWR falls to the template fabrication process. Non chemically amplified resists, such as ZEP520A, are not nearly as sensitive but have excellent resolution and can produce features with very low LWR. The purpose of this paper is to characterize LWR for the entire imprint lithography process, from template fabrication to the final patterned substrate. Three experiments were performed documenting LWR in the template, imprint, and after pattern transfer. On average, LWR was extremely low (less than 3nm, 3σ), and independent of the processing step and feature size.

Beam-monitoring system using microapertures for electron-beam lithography

A new beam-monitoring system for electron-beam lithography is proposed, which can be used for the variably shaped beam (VSB) method, the character projection (CP) method and the electron- beam mask (EB mask) projection method. The system is composed of micro-apertures and a detector placed below the micro- aperture, which is installed at a focal plane of a mask writer. The micro-apertures are formed on a 1-micrometer-thick Si film on which two 200-nm-thick W layers are deposited. A shaped beam is scanning over the micro-aperture, and the electrons that pass through the micro-aperture are directly detected with the detector, so that the two-dimensional shape and size of the beam are measured. The contrast and the signal-to-noise ratio obtained by this system are greatly superior to those obtained by the conventional mark-scanning method.

Fabrication of micro-marks for electron-beam lithography

We have developed micro-marks to be used for precise adjustment of the electron optics in electron-beam writers. The micro-marks fabricated are the dot-mark of W on a Si substrate, the hole-mark on a W layer, and the hole-mark on a Au layer. With these marks, it is possible to precisely measure the edge resolution as well as the two-dimensional shape and size in the variably shaped beam (VSB) and the character projection (CP) lithography. Measuring the edge resolution of shaped beam in 50 keV electron-beam writer with these marks, we found that the hole-mark on a W layer gave the best resolution and contrast. These results show that these marks are applicable to lithography of 0.15 µm or below.