Teruaki Okino

Nikon EB stepper: its system concept and countermeasures for critical issues

The imaging concept of electron projection lithography (EPL) with silicon stencil reticle is explained. A silicon membrane thickness of 1 - 4 micrometer is suitable for the reticle. A scattering contrast of greater than 99% is expected. Nikon EB steppers dynamic writing strategy of discrete exposure on a sub-field by sub-field basis with deflection control of the electron beam is explained. The basic system configuration of EB stepper is introduced. Examples of error budget for CD variation and Overlay/Stitching are shown. Nikons policy for countermeasures for critical issues such as proximity effect correction, sub-field/complementary stitching and wafer heating influence are explained. For extensibility down to 70 nm and below, both exposure tool and reticle should be improved.

Direct measurement of Coulomb effects in electron beam projection lithography

We describe a direct measurement of the image blur and defocus induced by Coulomb effects (stochastic and global space charge) in electron beam projection lithography. The Nikon 100 kV electron beam projection experimental column was used for the experiments. This column has similar values of electron optical parameters to those of electron beam projection lithography (EPL) systems. The Coulomb effect image blur and defocus were directly measured by a knife-edge method. The experimental data of Coulomb effect image blur and defocus as functions of beam current are shown. The experimental results show excellent agreement with our Monte Carlo simulation results.

Nikon EB Stepper: the latest development status

The latest development status of EB Stepper is reported. The experimental data include the latest resist image data exposed by 100keV electron beam, mask error factors and dosage margins at several backscattered electron levels, transmission data of continuous membrane reticles, and recommended structures for alignment marks, etc. The basic studies related to system design are also explained, those are the strategy for the management of reticle deformation and the stitching accuracy in overlaid layers, etc. Through these data, the resolution capability down to 50nm technology node is clearly shown and alignment/stitching capability is also described. The requirement to a continuous membrane reticle is indicated from experimental data.

130 kV High-Resolution Electron Beam Lithography System for Sub-10-nm Nanofabrication

An electron beam lithography (EBL) system, CABL-UH, with a 130 kV high acceleration voltage has been developed that succeeded in minimizing beam size by minimizing Coulomb blur. This system has a short single-stage electron beam (EB) gun with an alignment function of two extractor centers to minimize Coulomb blur. This gun has also succeeded in thoroughly avoiding microdischarges. By adopting this EB gun and many other techniques, high resolution and long-term high stability have been achieved and an extremely fine pattern (4 nm line) has been delineated.

High accuracy aerial image measurement for electron beam projection lithography

A direct means of measuring an image blur of electron beam projection lithography (EPL) tools is described. An aerial image sensor used for the image blur measurement was fabricated and evaluated. The signal to noise ratio (SNR) was very high and the signal contrast was 97%. The measured image blur, defined as the distance between 12% and 88% of the beam edge profile, under the optimum condition was 13 nm and the measurement repeatability was 3 nm (e sigma). The measurement error due to the sensor was extremely small, and a quantitative measurement of the image blur can be realized using this technique. The application of this technique to a system calibration is demonstrated. Focus and astigmatism were measured and the optimum settings of focus coils and stigmators were determined with an excellent repeatability. The potential for this technique to provide an automated self-calibration system on the EPL tools is clearly shown.

Full-field exposure performance of electron projection lithography tool

Electron projection lithography (EPL) is a realistic technology for the 65nm node and below, as a complementary technology of optical lithography especially for contacts and gate layers because of its high resolution and large process margin. Nikon has developed an EPL exposure tool as an electron-beam (EB) stepper and the first generation EB stepper; NSR-EB1A is now almost completed as an R&D tool for the 65nm technology node. Using a ϕ200mm reticle, a 20mm×25mm exposure field is realized. Full-field exposure performance of NSR-EB1A is shown. A 70nm isolated line and 1:1 nested lines are simultaneously resolved, as are 50nm 1:2 nested lines. 60nm contact holes are resolved with a depth of focus over a 10μm range and dosage window over ±6%. Stitching accuracy is about 20nm (3σ) and the single machine overlay is about 30nm (mean + 3σ). These data mean sufficient performance for device manufacturing of the 65nm technology node. The concept of a large subfield is one candidate for resolution and throughput e...

First dynamic exposure results from an electron projection lithography tool

Electron projection lithography (EPL) is one of the promising technologies below the 65 nm node, especially for contact hole and gate layers. Nikon is developing an EPL exposure tool as an electron beam (EB) stepper and the first generation EB stepper is now being manufactured. The voltage of 100 kV is adopted for electron beam acceleration. The subfield size is 0.25 mm×0.25 mm on the wafer and the deflection width of the electron beam is 5 mm on the wafer. The magnification of the projection optics is 1/4. A 5 mm×25 mm area from the φ200 mm reticle can be exposed by the combination of beam deflection and stage scanning motion (dynamic exposure). This area is called “a mechanical stripe.” After one mechanical stripe exposure, the reticle and wafer stages turn around and the next exposure of the adjacent mechanical stripe starts as a scan and stitch stage motion. Finally, a 20 mm×25 mm exposure field from the φ200 mm reticle is exposed. We report the first dynamic exposure in the history of EPL although on...

Data of scattered electron characteristics in 100-kV EB stepper

Nikon is developing an Electron Beam (EB) stepper as one of the next-generation lithography systems for feature sizes of less than 100 nm. As a reticle for the EB stepper using a high power EB (acceleration voltage: 100 kV, current on reticle: 100 (mu) A), a scattering stencil reticle with a grid-grillage structure has been investigated, EB projection experimental column which operates a high power EB was constructed. Some experimental data of scattered electron characteristics using the EB projection experimental column are given as follows: (1) Scattering contrast of 99.9% can be obtained using 100 kV electron beam (membrane thickness; 2 micrometer, aperture half angle onto reticle; 2 mrad). (2) Changes of resist pattern width of 1:1 and 1:2 lines and spaces are around 40% and around 20% respectively due to the proximity effects by backscattered electrons form the silicon substrate. (3) Contrast of EB mark detection for the system calibration, the reticle alignment, and the wafer registration is obtained. Comparing with the values that be obtained by theoretical calculation, some of experimental data gave good agreement.

Investigation of proximity effect correction in electron projection lithography (EPL)

An electron projection lithography (EPL) system which projects reticle patterns onto a wafer will be applied to sub 100 nm lithography. Requirements for line width accuracy are very strict as feature sizes are less than 100 nm. For electron beam lithography, proximity effect corrections have always been an important issue for accurate feature width control. In this paper characteristics of several correction methods are examined, and appropriate correction methods for 100 kV EPL are introduced. Employing the shape correction method burdens the reticle pattern preparation system much more than other methods. Therefore a calculation method suitable for 100 kV EPL where the backscatter radius is very wide ((beta) b approximately equals 30 micrometer) and the forward scatter radius is narrow ((beta) f approximately equals 7 nm) has been developed. The calculation of deposition energy by the backscattered electron beam is carried out with a coarse grid but wide range. The calculation of the combined effect of the electron scattering blurs from the features is carried out only within a narrow range. The correction calculation is carried out using both of these results. Using this method, accurate and fast calculations can be achieved. Employing the GHOST correction method increases total exposure cost. The practical GHOST correction methods may also be improved. An additional correction method named shape correction with GHOST is also shown.

Symmetric Magnetic Doublet Optics with Dynamically Compensated Field Aberration for Reducing Image Projection System.

Aberrations calculated from the electron trajectories are compared with those estimated from the third-order aberration coefficients. Field curvature and radial and azimuthal distortions are consistent with each other for moderate radial value R. In order to optimize lens positions, a method is adopted which makes the lens condition under which the principal ray trajectory from the largest objective radius R o crosses the optical axis at its normal position equal to that under which the trajectories diverging from the same Ro focus on the image. For the 50 cm object-to-image distance optics, residual aberrations are calculated through the trajectory calculation after the corrections of lens current, crossover and beam position. When the main and subfield sizes are 20 mm and 0.25 mm square, the residual field curvature, astigmatism, and radial and azimuthal distortions are 13.1 nm, 6.4 nm, 19 nm, and 8.35 nm, respectively, when the beam semiangle is 0.1 mrad, and the beam energy is 100 keV.