Bookdon - I scanned an article on masks from the SAL newsletter, "ONE STEP AHEAD", Vol. 2, No. 5 1997:
X-RAY LITHOGRAPHY RISK REDUCTION: MASKS
X-ray lithography (XRL) was first introduced more than 13 years ago. Like any other new technology, it was considered "high risk." Even one year ago, XRL was considered riskier than driving optical technology to its limits. Now, however, semiconductor manufacturers are taking another look at x-ray lithography's potential as the least venturous method of manufacturing state-of-the-art devices. There are a number of reasons for the change, but the most compelling is the overall cost of ownership.
Masks for x-ray lithography were initially difficult to manufacture and very expensive. It is now clear that x-ray masks can be manufactured: (i) they are being regularly delivered to customers, (ii) the cost is very reasonable when compared to optical masks; (iii) they are production-worthy; and (iv) the manufacturing infrastructure is rapidly solidifying.
IBM delivered its first x-ray mask in 1988 (see AMF History and Milestones). Since then, hundreds of masks have been manufactured. The first 0.18 micron mask was shipped in 1995 and the first 0.13
micron mask was shipped in 1996. IBM's Advanced Mask facility (AMF) can currently deliver defect-free 0.15 micron production masks in 8 weeks after receipt of order. Years of experience in building
x-ray masks have resulted in a proven mask manufacturing capability.
The cost of x-ray masks is approaching the cost of optical masks. For 0.25 micron design-rules, x-ray masks are more expensive than masks for 248 nm steppers. At 0.18 micron design-rules, the cost is comparable. At 0.13 micron, however, it is likely that delivering defect-free production x-ray masks is significantly less expensive than delivering an optical mask. The uncertainty is due to the fact that 0.13 micron production-type (full-field, defect-free) optical masks are not readily available for cost comparison.
One advantage of x-ray masks is that tolerances are about the same as optical masks with OPC, but the writing area on x-ray masks is much smaller. An x-ray mask of 50 x 50 mm prints the same field as a
4x optical mask which is 200 X 200 mm (of course, this comparison is not valid, because optical lenses are not capable of printing a 50 X 50 mm field with sub-quarter-micron resolution). The smaller area means less writing and inspection time.
An additional cost advantage of x-ray masks is that they can be readily inspected and repaired using commercially available equipment (see AMF History and Milestones). On the other hand, optical
masks for DUV exposures are difficult to inspect and repair. Moreover, not all visible mask defects are printable and not all printed defects are visible on a DUV mask.
X-ray masks are good for at least 40,000 exposures (the average optical mask is used for << 10,000 exposures). A recent change in absorber material eliminates gold from the process. The current
absorber is tantalum-based, an element which is acceptable in semiconductor manufacturing.
X-ray masks are SMIF-compatible and readily transported.
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I hope this answers some of the x-ray mask questions.
Betty |
