Takeyuki Yamada

Improvement of total quality on EUV mask blanks toward volume production

Total quality on EUV mask blanks have to be improved toward future volume production. In this paper, progress in EUV blank development and improvement in flatness, bow and ML blank defects as critical issues on EUV blanks were reported. Steadily progress in flatness improvement was made in the past five years by improving polishing processes. A LTE substrate with a high flatness of 78 nm PV in 142 mm square area was achieved in average. Annealing process was developed to make small bow of less than 600 nm after ML coating. It was confirmed that annealed ML blank has stable performance in bow and centroid wavelength values through mask making process. Small bow of less than 300 nm was successfully demonstrated using annealing process and a CrN back side film with high compressive stress. Low defects of 0.05 defects/cm2 at 70 nm SiO2 sensitivity inspected by a Lasertec M1350 was demonstrated on a multilayer (ML) blank with a LTE substrate as best. Small defects over 50 nm in a M7360 were effectively reduced by improvement of polishing process consisting of local polish, touch polish and cleaning.

Process development for EUV mask production

Absorber layer patterning process for low reflectivity tantalum boron nitride (LR-TaBN) absorber layer and chromium nitride (CrN) buffer layer were improved to satisfy high resolution pattern and high level critical dimension (CD) control. To make 100nm and smaller pattern size, under 300nm resist thickness was needed because of resist pattern collapse issue. We developed absorber layer dry etching process for 300nm thickness resist. Absorber layer patterning was done by a consequence of carbon fluoride gas process and chlorine gas process. We evaluated both gas processes and made clear each dry etching character. Sufficient resist selectivity, vertical side wall, good CD control and low buffer layer damage were obtained. Then, we evaluated how buffer layer dry etching affects EUV reflectivity. Finally, we evaluated EUV mask pattern defect inspection and defect repair. Sufficient contrast of mask pattern image and good repair result were obtained using DUV inspection tool and AFM nano-machining tool, respectively.

Recent performance of EUV mask blanks with low-thermal expansion glass substrates

A high flatness of 50 nm, zero defects at more than a size of 30 nm and a high reflectivity of more than 66% for extreme ultraviolet (EUV) light are critical issues related to EUV mask blanks. In this paper, progress on these issues and the recent performance of EUV blanks is reported. Steady progress in defect reduction was achieved in the past six years by improving fabrication processes. When inspected by a Lasertec M1350, defect quality as low as 0.02 defects/cm2 at 70-nm sensitivity was demonstrated on a multilayer (ML) blank with a quartz (QZ) substrate. A QZ substrate with a high flatness of around 90 nm peak-to-valley (P-V) on both sides and a high defect quality of 0.006 defects/cm2 at 60-nm sensitivity was obtained using a newly developed polishing process consisting of local polishing, touch polishing and cleaning. The cleaning process was developed for low thermal expansion (LTE) glass to reduce the defects associated with it. Using the cleaning process, the ULETM substrates showed defectivity similar to the QZ substrates. An average flatness of 117 nm P-V, and best flatness of 84 nm P-V on the front side and 56 nm P-V on the back side were obtained on ULE substrates using the new polishing process. Multilayer (ML) blanks with a high defect quality of 0.08 defects/cm2 at 80-nm sensitivity were produced on a ULE substrate. The ML blanks, consisting of 50 bilayers, have high peak reflectivity of more than 66% and excellent uniformity of less than 0.04 nm in centroid wavelength, which meets the desired specifications.

Development of new chrome blanks for 65-nm node and beyond

For advanced reticle fabrication, a resist thinning technique continues a promising trend of the resolution enhancement. To bring out thin resist performances, a new chrome absorber has been developed for the second layer of 193nm att-PSM. The new chrome absorber is thinner and has a higher dry-etch rate than our current products, such as NTAR5. This new chrome absorber can utilize a super thin resist application because of a reduction in dry-etching time. Additionally, a technique of film stress reduction was also developed to reduce placement shift by film stress relaxation. The new chrome absorber with super thin resist (TF blanks) exceeds current products in the mask-making metrics of resolution and CD performance. This performance will meet the requirements of 65nm-node and beyond.