Nobuaki Ogushi

Critical Dimension Control in Synchrotron Radiation Lithography Using a Negative-Tone Chemical Amplification Resist

Total critical dimension (CD) controllability for 0.14 µ m line-and-space in SR lithography was evaluated for all wafer levels. The evaluation was carried out for the CD accuracy between the wafers, in the wafer and in the exposure field. The influence of the exposure process instability to the CD accuracy was also evaluated. The instability of the post exposure baking (PEB) temperature and the post exposure delay (PED) time affects the CD accuracy, and they were estimated to be less than 5 nm, respectively. The CD accuracy at the same point on the X-ray mask was 5.4 nm in the wafer and 10.5 nm between the wafers. It was found that the CD accuracy between the wafers was degraded by the inaccurate exposure dosage caused by the daily change of the SR beam distribution in the vertical direction. In the exposure field, the CD instability due to SR beam nonuniformity was 4.1 nm and that due to the X-ray mask was 15 nm. Consequently, the total CD controllability is presently estimated to be 19.5 nm for all wafer levels and the improvement of the dose repeatability and X-ray mask CD control is required to achieve the CD accuracy of less than 11 nm.

LPP-based reflectometer for characterization of EUV lithography systems

An EUV reflectometer, based on a laser-produced plasma (LPP) light source, has been developed for characterization of EUV lithography systems. The reflectometer consists of the LPP light source, a prefocusing toroidal mirror, a grating monochromator, a polarizer, a beam intensity monitor, a refocusing toroidal mirror and a sample stage. The LPP light source is driven by a Nd:YAG laser; the laser beam is focused onto a copper tape target. A debris mitigation system that uses a rotating shutter was developed. Higher-orders formthe grating monochromator were suppressed to less than 0.2% of incident beam intensity by total reflection of three grazing incidence mirros. In order to compensate for beam intensity instability, a beam intensity monitor using a grating beamsplitter was installed between the refocusing mirror and the sample. Beam intensity instability can be corrected to less than 0.1% by using the beam intensity monitor.

Evaluation of exposure dose repeatability in synchrotron radiation lithography

The stabilization of exposure dose repeatability is one of the key factors in reducing the critical dimension variation in synchrotron radiation lithography. We have evaluated and improved the exposure dose repeatability of the full field exposure system, which employs a cylindrically convex mirror. The beam size spreads with the stored current in the storage ring, which causes the major exposure dose errors. The maximum exposure dose error is evaluated to be ±4.0% according to the ray trace simulation, and it is expected to reduce the error to ±0.6% using exposure time compensation. We have evaluated the improvement of the exposure dose repeatability experimentally by using the stabilized resist process. Critical dimension variations of less than ±2 nm between six wafers exposed over several days were obtained. This result indicates that the practical exposure dose error is almost consistent with the calculated value, having a variation of less than ±1.0%.