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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.
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Inspection KLA-Tencor KT-365UV EPS algorithm (alpha version, prelimary results)
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Figure 4. Reticle defect inspection.
Figure 2. A 6025 substrate showing a 0.18 µm contour map in a 146 nm square area.
EAPSM film
EAPSM is one of the RETs used as optical lithography pushes smaller dimensions and shorter wavelengths. Figure 3 shows typical spectral transmittance and reflectivity of “six percent trans” of a new MoSi-based EAPSM film for the 193 nm. Engineering sample supply for preliminary evaluation was started late in 1999; and the film was qualified by several users for optical characteristics, uniformity and durability. A feature of the HOYA 193 nm EASPM film is that percentage trans becomes very high at longer waveOptical Properties
lengths, which possibly induces sensitivity degradation for reticle inspection. Figure 4 shows the sensitivity achieved by KLA-Tencor 365UV with ESP for each defect type. Reticle defect inspection in this instance was done on the film at the same sensitivity or detectability level as the binary film by modifying the algorithm. Inspection algorithms and light optics will improve, along with EAPSM defect inspection capability. In addition, repair results on the EASPM film are shown in Figure 5. Since the film is MoSi-based, existing repair techniques for deep UV EAPSM film are also available. HOYA MoSi-based 193 nm EAPSM film can provide required optical characteristics and process compatibility for etching, defect inspection and repair to the users of HOYA deep UV EAPSM film.
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system with high sensitivity, high contrast, superior resolution, superior post-coating delay (PCD) and post-exposure delay (PED) stability. It still is generally thought that CARs must be spun on just prior to exposure due to their very short life after coating, while mask makers have been procuring resist-coated blanks. The industry plans to stay the course, even with CARs, for the next generation; thus is also the mission of blanks suppliers. Exposure Delay time (days)
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For precise PCD stability (CAR blanks life), it is important to understand the main CD drivers in all mask-making processes. There are three major intervals (delays) in the mask-making process; i.e., PCD, PED, and post-PEB (post-exposure baking). Figure 7 shows CD movement on Hitachi Chemical RE-5153P in PCD, PED and post-PEB delay. PED for only a couple of hours induced a significant CD movement of over 100 nm, while two weeks’ storage (PCD) induced very small CD movements (less than 5 nm). Post-PEB delay had almost no impact on CD stability. The impacts of these intervals (delays) upon CD movements were similar on all positive-tone CARs examined. Storage in a dry-N2 purged box, on the other hand, improved CD stability in all process Develop. delays, as shown in the figure. In PCD, the CD movement was Delay time (days) significantly reduced to less than 10 nm, even for four weeks’ storage, by utilizing the controlled environment. In PED, the CD movement was also reduced but, unfortunately, not as significantly.
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Figure 6. CD movement of CARs.
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In order to study the feasibility for CARs-coated blanks supply, storage tests were done in a common storage condition to determine CARs film life after coating. Figure 6 shows CD movement of five CARs selected for four weeks’ storage. Sample blanks were put into HOYA regular shipping boxes (acrylic resin base). The boxes had been sitting in a clean room without seals for two to four weeks where NH3 concentration was 0.33 µg/m3 (at 40 percent RH, 22. RT). CD movement observed after two weeks’ storage was less than 5 nm on all CARs. However, four weeks’ storage induced a significant CD movement of roughly 50 to 75 nm.
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To find the main CD driver in the CARs mask making process, the dependence on each interval in process (PCD, PED and Delay after PEB) has been described by CD shift from the 1.0 µm target CD. Regular and dry-N2 purged box conditions where NH3 ~ 0.33 µg/m3 in 40 ~ 45RH percent and NH3<0.001 µg/m3 in less 10RH percent, respectively. Figure 7. CD movement due to inter val in process.
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The following are findings in the CAR blanks feasibility study: CARs possess sufficient PCD stability for advanced reticle fabrication if they are kept in a dry-N2 purged environment. Dry-N2 purged environment or equivalent (chemically clean environment) is necessary for blanks storage. On the other hand, CD movement due to PED can’t be neglected on CARs practical use. Especially for mean-to-target yield improvement, it is recommended PEB be done immediately after exposure, as it is a chemically
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clean environment for plate handling, storage or between exposure and PEB. A solution was discovered for CARs coated blanks life and supply as mentioned above, which had been considered the most serious issue for CAR blanks practical use. Summary
The defect guarantee level of blanks, especially for hard defects such as glass and chrome pinholes, has been behind the requirement until now. This is due to a giant gap in Permitted inspection â&#x20AC;&#x153;speedâ&#x20AC;?; i.e., inspection cost
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for blanks, as mentioned above. However, the advent of new inspection techniques other than the ordinary scattering, transmitting or reflecting light detection, look very promising for blanks manufacturers trying to catch up with requirements for the next few years. There is a movement for the mask manufacturers to furnish inspection tools for sorting blanks to improve reticlemanufacturing yield for defects. The ultimate goal is to supply blanks defect data on each single plate for highend reticle manufacturing with a high yield. By improving quality and functionality, photomask blanks for each lithography and mask-making generation can be supplied to the industry.
KLA-Tencor Trade Show Calendar September 19-21
Diskcon, San Jose, California
September 25-27
AEC/APC, Lake Tahoe, California
October 3-5
ITC, Atlantic City, New Jersey
November 28-30
Fall MRS, Boston, Massachusetts
December 6-8
SEMICON/Japan, Makuhari, Japan
February 27-28, 2001
SPIE-Microlithography, Santa Clara, California