Lithography S
P
E
C
I
A
L
F
O
C
U
S
Photomask Blanks Quality and Functionality Improvement Challenges for the 130 nm Node and Below by Masao Ushida, Hideo Kobayashi, Kunihiko Ueno, HOYA Corporation
Sub-wavelength lithography will use various types of resolution enhancement techniques (RET) on reticles, such as embedded attenuated phase-shift mask (EAPSM) and OPC to extend refractive reduction optics to the 130 nm node and below. There are significant difficulties that confront mask makers as well as photomask blanks manufacturers. This article explores the development status of photomask blanks and reviews issues to be solved.
Quartz substrate
It is required that the photomask substrate should have a suitable transmittance at each exposure wavelength used. For the 157 nm, as is generally known, new synthetic quartz seems to be the best candidate and must be used for a suitable transmittance and sufficient exposure durability. In addition, it has been suggested another new synthetic quartz might be necessary, even at 193 nm. The blanks suppliers’ mission is to prepare superior quality substrate for film deposition and resist coating and eventually for reticle manufacturing, although characteristics of the substrate, with the exception of flatness, are dependent on quartz manufacturers. Several types of glass defects on substrate, such as scratches, pits, sleeks and micro-cracks, are typically residue not removed completely by polishing, damages by particle contamination during polishing process or damages during cleaning, particularly ultrasonic. Figure 1 shows a typical glass defect, a socalled “micro-crack,” that is 0.4 µm long, 0.06 µm wide and 0.01 µm deep. The micro-crack also can be caused by mega-sonic cleaning that is now almost standard in the mask-cleaning process. In some cases, such a small and shallow defect cannot be found even by a reticle inspection system because of poor contrast, especially due to its depth.
Careful visual inspection is still used to inspect blanks substrate for glass defects, because there is no automated scanning inspection system with a suitable sensitive inspection “speed” for blanks manufacturing. There is a giant gap in inspection speed permitted between reticle and blanks manufacturing: a couple of “hours” for reticle versus a couple of “minutes” for blanks. With respect to flatness, 0.5 µm flat material with a 0.25 µm flat requirement might be available in the near future. Figure 2 shows a 6025 substrate with a 0.18 µm contour map in a 146 mm square area.
Figure 1. A typical glass defect.
Autumn 2000
Yield Management Solutions
47