Shintaro Kawata

Spatial coherence of KrF excimer lasers

The spatial coherence and the beam divergence at 248 nm of a KrF excimer laser were obtained experimentally. These results are in good agreement with the theoretical calculations based on a simple pulse-laser model and the van Cittert-Zernike theorem. The relation between the spatial coherence and the beam divergence was obtained theoretically and supported by experimental results. This expression is given as a function of the wavelength of the laser but includes no parameters related to the laser structure. It is shown that these theoretical results are applicable to various kinds of pulse laser.

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.

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.

Pattern displacement measurements for Si stencil reticles

We have fabricated Si stencil reticles that are employed by a new type of e-beam projection lithography system (EB stepper). We applied a stress reduction technique to the Si membrane to improve the pattern placement accuracy. The residual stress of Si membranes which were fabricated by anisotropic etching of B-doped Si wafers in KOH aqueous solution was reduced by annealing at 1150 °C. We carried out pattern-displacement measurements for a Si stencil reticle made of a Si membrane where the residual stress was reduced to 10 MPa, and we observed that the pattern displacement error was reduced to less than 20 nm. Furthermore, the pattern displacement in the stencil reticle had a high correlation with the displacement determined from a simulation based on a finite element model. However in the same reticle, we discovered additional, comparatively small displacements in random directions, which was not expected in a membrane that had a homogeneous tensile stress. As a cause of the pattern displacement in rand...

Stencil reticle repair for electron beam projection lithography

Repair of stencil reticles for electron beam projection lithography system is one of the critical issues on reticle manufacturing.Focused ion beamdeposition is studied as the method for repairing the clear defects of stencil reticle. The film deposition of diamondlike carbon (DLC) across the stencil pattern, on the sidewall of the stencil, and on the pre-etched slot pattern is demonstrated. The deposited DLC films have good properties as the repair material.Deposited patterns across the stencil pattern are imaged on the resist with the Nikon 100 kV experimental projection column. When the thickness of the deposited DLC film is more than 0.5 μm and the contrast aperture size of the projection column is 1.5 mrad, the thickness of the deposited pattern does not affect the critical dimension of the resist pattern imaged the repaired patterns. The profiles, the pattern size, and the electron scatteringproperties of DLC films are stable for 100 kV electron beam continuous irradiation (2 C/cm2 dosage; corresponding to half-year dosage on electron beam stepper). Moreover, the repaired pattern is not damaged by the wet megasonic cleaning.

Phenomenological analysis of carbon deposition rate on the multilayer mirror

It is very important to mitigate oxidation of multilayer mirrors (MLMs) and carbon deposition onto MLMs to extend the lifetime of EUV exposure tool. We focused on carbon deposition on Si-capped multilayer mirror. We made experiments of EUV irradiation to the multilayer mirrors using an EUV irradiation apparatus connected to a beam line (SBL -2) of synchrotron radiation facility Super-ALIS in the NTT Atsugi research and development center. Thickness of deposited carbon was obtained by using XPS. We investigated carbon deposition rates at various partial pressures of various organic species. Phenomenological analysis was applied to the obtained carbon deposition rate. Carbon deposition rate was proportional to the pressure at the proportional EUV intensity. Applying this normalization of the deposition rate and the EUV intensity, carbon deposition rate seems to behave according to each universal function for each hydrocarbon species.

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.

Illumination System Of An Excimer Laser Stepper

An illumination system suitable for an excimer laser stepper has been investigated. Unnecessary interference pattern(speckel) is reduced effectively by scanning the laser beam. We report spatial coherence of the lasers with different spectral line width, illumination system of the stepper, appearance of the interference pattern,its spacing and contrust and their relation to the illumination system and to the coherence of the laser. Then we report reduction of this pattern together with a simple method to measure its contrast.

Repair method on silicon stencil reticles for EB projection lithography

The stencil reticle is one of masks for EPL (Electron beam Projection Lithography). The ability to repair pattern defects in such masks is an essential requirement for their use. For clear defects, repair issues include deposition on the stencil pattern, electron scattering properties of the deposition material, stability of the repair against 100 kV electron beam (EB) bombardment and etching and pattern profile accuracy. For opaque defects, ion milling of the stencil material to high accuracy is required. In this paper, we discuss the repair deposition of clear defects up to 0.15 micrometer using a carbon-compound precursor and the critical dimension (CD) controllability of the exposed resist pattern with various thickness of repair depositions using Nikons EB experimental projection column (100 kV). In addition, using a Monte Carlo simulation of the electron scattering in the silicon membrane and the repair deposition, the beam contrast is estimated for EPL at contrast aperture (CAP) size. The resist CD can be controlled using more than 2 micrometer-thick Diamond like Carbon (DLC) deposited on the stencil reticle and shaped by focused ion beam (FIB). Profiles, pattern sizes and the electron scattering properties of DLC repairs are stable against 100 kV EB irradiation (about 2 C/cm2 dosage; corresponding to half-year dosage). These results show the possibility of DLC as the repair material for clear defects. The possibility of repairing opaque defect, which involves FIB milling of sizes from 0.2 to 0.4 micrometer, is also demonstrated.

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.