Shuichi Tamamushi

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.

Reticle flexure influence on pattern positioning accuracy for reticle writing

This paper presents a method for estimating the influence of reticle flexure on pattern positioning accuracy, and evaluates the method by measuring patterned reticles. Reticle flexure causes the pattern shift which occurs by stretching or compression of the reticle surface. A height-mapping function of an electron beam (EB) writing system and a measuring machine are used to calculate the pattern shift due to reticle flexure. The bent shape of a reticle on the EB-writing system differs from that on the measuring machine, so that the patten shifts on the two machines are different. The pattern shifts caused by the bent shape difference were excluded from the measurement result of pattern positioning errors. The values of pattern positioning accuracy evaluation parameters, x, y-scaling and orthogonality, are calculated among several reticles (5 inches, 0.09 inches thick). The deviations of these three values are reduced to less than 50% of their uncompensated values.

Performance improvement in electron-beam reticle writing system

Several experiments in order to improve throughput and accuracy have been carried out on electron beam reticle writing system which adopts variably shaped beam, vector scanning, and continuously moving stage. Stage speed optimization process by the stripe is introduced to reduce the writing time loss which arises from constant stage speed through writing a reticle. As a result, writing time decreases to 2/3 on average and the throughput of 2 reticles of 64 Mbit DRAM class per hour can be realized. Substrate clamping configuration for writing and measuring machine affects the substrate flexure and deteriorates the global positioning accuracy. The change of clamping point number form 4 to 3 for each machine improves the reproducibility of global distortion to 47% or more. The multipass writing method is effective to reduce stripe stitching error and fluctuation of the main-field position. In the case of multiplicity of 4, stripe stitching error and fluctuation of the main-field are 20 nm and 13 nm, respectively. The writing time ratio compared with single-pass writing is 1.6 even in 4-pass writing. Therefore, throughput should also be emphasized in view of accuracy improvements.

Analysis of pattern shift error for mask clamping measured by Nikon XY-31

Pattern measurement repeatability of metrology tools must be evaluated precisely to warrant higher pattern placement accuracy, according to a budget of pattern shift errors effected by the initial deformation of a substrate, clamping conditions, etc. As first steps, we focused on our metrology tool, Nikon XY-3i. Pattern measurement repeatability was usually evaluated to measure a referential pattern of a single mask repeatedly. For taking tilting variations on each of mask setting into account, we divided the coordinates of measured data into some error factors. Besides, we proposed sag correction method to eliminate tilting variation for precisely evaluation. This method was effective to unify each of the referential planes on measuring. Sag correction was effective to diminish in variations of orthogonality error factor and trapezoid error factors and deviations (3(sigma) ) of measurement repeatability. Therefore, we succeeded to get the quantitative budget of measurement repeatability for our metrology tool.

Writing accuracy of EBM-3500 electron beam mask writing system

A high accuracy electron beam writing system EBM-3500 has been developed for 130 nm node lithography technology. The EBM-3500 is based on its predecessor EBM-3000 system and incorporates new features to improve writing accuracies. Based on the extensive error analyses of the EBM-3000, several important improvements in such areas as ground noise and stray magnetic field reductions, among others, have been made. Thanks to these improvements, EBM-3500 achieves high accuracies to satisfy the present and future technology requirements.

The Effect of Down-Flow Cleaning Process on Materials Used in an Electron Beam System.

In order to search for an appropriate material for an electron beam (EB) system to which in situ cleaning is applied, we investigated the particle generation due to the cleaning process and beam drift due to charging of the processed electrodes. In order to study the particle generation, we observed the change of surface morphologies of materials caused by the cleaning process. Particles are generated on the surfaces of Ag and BeCu. Particle generation is not observed on the surfaces of Ti, Cu, BeCu (Au-plated), Au, Al, Al (Au-plated), Al (Ni plated) and Pt. In order to study the charging effect, we measured the beam drift after cleaning the electrodes. The beam drift is small when Au and Ti electrodes are used and large when Ni plated electrodes are used. Therefore, Au and Ti are suitable materials for the EB system to which the in situ cleaning process is applied.

Development of an in-situ cleaning system for an e-beam reticle writer

We have newly designed and constructed a unique electron optical column installed with an in-situ cleaning system, applying the down-flow ashing process with a mixture O2 and CF4. We carried out in-situ cleaning using designed system, and confirmed that beam drift which is caused by charging up of a contamination layer was reduced.

Practical approach to evaluating mask CD uniformity patterned by a variable-shaped beam

CD uniformity to be patterned by electron-beam (EB) writing system with a variable-shaped beam was evaluated. The experimental EB writing system, EX-8D, was used under conditions of current density of 20 A/cm2 and acceleration voltage of 50 keV. Quartz reticles coated with positive tone resist ZEP7000TM (Nippon Zeon Co., Ltd.) were applied. Test patterns of 1-micrometer-width design were written by shaped beam shots of 1 micrometer square with different exposure doses. Since higher measurement repeatability was confirmed, line width of test patterns without shot stitching points was measured by Nikon XY-3I with a circle-spot probe of 1 micrometer. Line width of clear patterns on resist film was measured after development, and line width of clear patterns on chrome (Cr) film of one mask was measured at same points after wet-etching. The other mask was measured at the same points after dry-etching process by conventional reactive ion etching (RIE). Certain comparisons in this study indicate the importance of evaluating CD uniformity on Cr film after dry- etching process. Expect for resist heating contribution by four-pass writing method, the uncertainty of CD error was quantified as follows: 4 nm (3(sigma) ) on resist film at the applied dose of 19 (mu) C/cm2, and 4 nm (3(sigma) ) on Cr film at the applied dose of 27 (mu) C/cm2.