Takeshi Irita

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.

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.

Simulation of readout signals with near-field SIL

We investigate the readout signal with near field SIL by using vector diffraction theory. Readout signal from phase-change medium and optical ROM is analyzed. Air gap dependence and polarization dependence is discussed.

Pattern distortion of the stencil reticle caused by stress of silicon membrane and resist on the reticle

Silicon stencil reticle has been developing for the EB stepper, which is the electron beam projection lithography system for 70nm node generation and beyond. The reticle distortion is affected by stress such as silicon membrane stress and resist stress on a reticle in their fabrication. To analyze pattern distortion using finite element method (FEM), the image placement (IP) and the critical dimension (CD) errors of the stencil reticle were measured at every step of reticle fabrication processes. It was found that the resist stress is the key factor of IP error in the membrane process. In the wafer process, the IP errors are mainly related to silicon membrane stress. IP and CD errors of 200mm stencil reticle in both processes are discussed using FEM. The calculation results show CD errors are caused by the stress of silicon membrane. Moreover, it is discussed that CD error depends on pattern shape and density even on the stress-controlled reticle blanks.

Direct measurement of chromatic aberrations induced by SiNx continuous membrane mask

In this article, we present experimental results that quantify the magnitude of chromatic aberrations induced by a SiNx continuous membrane mask. We used a continuous membrane mask, which consists of a TaSi scatterer layer on a SiNx membrane layer of 100 nm thickness, for the experiments. The beam edge profiles were measured directly by the knife-edge method, which can measure the image blur with the repeatability of 3 nm (3 sigma). The effect of electron energy loss induced by inelastic scattering is clearly shown in the measured beam edge profiles. The magnitude of chromatic aberrations is quantified from the measured beam edge profiles considering lens aberrations and measurement errors. The results show that lower atomic number Z and lower density materials and thinner membrane are required on the mask membrane for a high resolution lithography.

High-accuracy aerial image measurement for electron beam projection lithography

A direct means of measuring the image blur of electron beam projection lithography (EPL) tools is described. We developed an aerial image sensor using a Si membrane knife-edge and a transmitted electron detection technique. The aerial image sensor is designed to increase signal amplitude and signal contrast in order to yield a large signal to noise ratio even under a low beam current density condition. The image blur can be quantified accurate to a few nanometers because the measurement error due to the sensor is extremely small. The aerial image sensor was installed in Nikons electron beam projection experimental column and was evaluated. The measured image blur, defined as the distance between the 12% and 88% points of the beam edge profile, under the optimum condition was 13 nm, and the measurement repeatability was 3 nm (3 sigma). 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 excellent repeatability. The potential for this technique to provide an automated self-calibration system on EPL tools is clearly shown.

Highly accurate CD control at stitching region for electron-beam projection lithography

For electron beam projection lithography system, it is one of the most important issues to stitch desired patterns accurately. We have found a way to stitch the patterns with high accurate critical dimension by a pattern edge deformation that moderates a stitching error by as much as 2.5 times compared with no-deformed edge.

A Drying Method Using Naphthalene for Flexible Microgripper Fabrication

A new technique for the microfabrication of a flexible microgripper is presented. In order to avoid sticking between the microgripper fingers due to capillary force, a drying method using naphthalene as a drying solvent (the naphthalene drying method) is proposed and demonstrated. Using this method, a new type of flexible microgripper was successfully fabricated without any sticking between the fingers. The advantages of the naphthalene drying method include simplicity, compatibility with atmospheric pressure, and suitability to batch fabrication.