Soichiro Mitsui

Application of two-wavelength optical heterodyne alignment system in XS-1

This article presents the alignment performance of the two- wavelength optical heterodyne alignment system in the x-ray stepper XS-1. The alignment accuracy obtained by the double- exposure method with a single mask and a Si trench wafer was better than 20 nm. The dependence of the alignment accuracy on Si trench depth indicated that the two wavelengths compliment each other and ensure a 3(sigma) of less than 20 nm. The alignment capabilities for other processed test wafers were also investigated by mix-and-match exposure. For etched SiO2 and poly-Si film on a Si trench, an accuracy below 20 nm was obtained. For AlSiCu film sputtered on etched SiO2, there appeared systematic alignment offsets depended on die position, which are thought to be due to a wafer-induced shift. The systematic offset errors were eliminated by the use of send-ahead wafer and corrections for individual offsets on each die, and thus the alignment accuracy was improved to 20-40 nm for each alignment axis. The two-wavelength heterodyne alignment system of the XS-1 has sufficient potential for 130-nm lithography and below.

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.

Development of a mask-scan electron beam mask writer

Mask-scan strategy for writing large complex patterns such as oblique lines and contact holes with assist bar and serif is more effective for decreasing the number of shots than is variable shaped beam (VSB) strategy and character projection strategy. In a simple case, the number of shots for writing the oblique lines can be as small as 1/600 of that of VSB strategy. Oblique parallel lines of 250 nm width and assist bar of 120 nm width were projected using the mask-scan strategy.

Evaluation of new x-ray stepper, the XRA

The exposure performance has been evaluated for the new x-ray stepper, the XRA, which is equipped with global alignment and magnification correction systems. Dose uniformity in the exposure field of 3.9%, stage accuracy of less than 20 nm, and good linearity and stability between the magnification change and applied force were obtained. For the 100 nm node, both critical dimension (CD) control of 10 nm and overlay accuracy of less than 30 nm were obtained using two 4 Gbit masks of the gate and contact hole by the double exposure method. Possible improvements of the dose uniformity and mask pattern CD will result in more accurate CD control. By compensating for the alignment offset, overlay accuracy of 23 nm for the 70 nm node is also expected.

X-Ray Mask Distortion Induced in Back-Etching Preceding Subtractive Fabrication: Resist and Absorber Stress Effect

The influence of resist and absorber stress distributions on X-ray mask distortion induced during back-etching preceding subtractive fabrication is analyzed experimentally and simulated. The stress distribution (gradient) in a resist and/or that in an absorber film causes larger pattern displacement rather than the film average stress. Some resists have considerably high stress after coating on a wafer, and this stress also changes during exposure, causing local pattern displacements. However, use of chemically amplified resist systems, in which the reaction after exposure is limited to a small amount acid generation, might solve this problem. Low-stress positive-tone resists should thus be developed.

Evaluation of Replicated Dynamic Random Access Memory Cell Patterns using X-Ray Lithography

We have evaluated the exposure latitude and the mask linearity of the lines and spaces (L/S), the isolated lines and the cell patterns used for DRAMs for various sizes down to 0.12 µ m. The exposure latitude was larger than 10% for all types of patterns with sizes larger than 0.12 µ m, using a mask of contrast of 3.7 in a mask-to-wafer gap of 20 µ m. We found that the mask linearity is insufficient and that mask pattern bias is required. The image shortening effect in the cell pattern was also evaluated. The results showed that the value of the shortening is higher when the design rule is smaller, the pattern density is sparser and the mask-to-wafer gap is larger. The value of the shortening in a 0.12-µ m- design- rule cell pattern was 13 nm, which is small enough to be corrected by simply giving a bias in the mask pattern.

Evaluation of Mask Soaking Performance in a Thermally Stabilized Vacuum Chamber in an Electron Beam Mask Writer

A temperature difference between a mask and a writing environment, which occurs during mask loading, is one of the most significant sources of the mask image placement error. The temperature of the mask has to be adjusted in a thermal chamber. To realize a rapid and highly accurate thermal soaking process, we constructed a new thermally stabilized vacuum chamber with an improved thermal soaking efficiency and evaluated its performance. The chamber is equipped with the top and bottom thermal stabilization plates (TSPs). It was found that the soaking time is largely dependent on the emissivities of the TSPs. The soaking relaxation time of less than 0.4 h is achieved. The thermal stability of the mask after soaking is 0.004°C (3σ) for 12 h. It was also found that a high TSP emissivity is effective in reducing the influence of the temperature variation of the environment on the mask, and the temperature variation of the mask is reduced to approximately 28% of that of the environment. A quick soaking and stable temperature holding of the mask are realized.

Environment-proof writing chamber for next generation electron beam mask writing system

We have developed an environment-proof writing chamber for an electron beam mask writing system. The chamber is made of low thermal expansion metal. The measured size dependence on the chamber temperature is more than one order less than in the case of iron or stainless steel. The results of a numerical study indicate that this chamber can suppress the influence of environmental temperature change on the positioning error to nanometer level or below. This chamber is suitable for a 70-nm generation mask writing system and beyond.

Stage tracking of a mask-scan EB mask writer test stand

A stage tracking function has been developed for a mask-scan EB mask writer. Position error of EB mask on an EB-mask-stage induces position error of projection beam on the EB-mask and the position of a writing pattern. The position of the EB-mask is measured by a laser interferometer. The shift from the aimed position is fed back to a mask selection deflection and a main deflection. The velocity of EB-mask stage and specimen-stage is also fed back to the deflection. The deflection control unit for the stage tracking has been made and the tracking function confirmed from the test memory of the unit. Using the unit, scanning writing patterns have been obtained with step and repeat stage mode.

Evaluation of a new X-ray stepper XRA

A new X-ray stepper XRA, which is equivalent to β-machine for proximity X-ray lithography (PXL), was installed at ASET Amagasaki-branch, and we have started the evaluation of its performance. The present alignment accuracy using global alignment method is 5.3±18.2 nm (x) and 3.8±30.5 nm (y). We evaluate the magnification correction function installed on XRA, and obtain a good linearity between the magnification and the applied force. By considering the dependence of the beam position and its size to the stored current, the critical dimension (CD) repeatability is improved from 15.5 to 5.5 nm (3σ).