Here is the article from the SAL newsletter, Vol. 3, No. 5 1998.
THE X-RAY ADVANTAGE IS COST OF
OWNERSHIP
X-Ray lithography is attractive because of its simplicity. The theory, the equipment, and the lithography process are elegantly simple. The result is a real cost-of-ownership and profitability advantage.
The theory is one of carefully crafting a high-quality mold, then easily and inexpensively replicating it. A mask containing an image (the mold) and a wafer to be patterned are held in close proximity, but not in contact, as a beam of x-ray energy is scanned across the image (replicating the image on the mask). The wafer is then moved (stepped) with respect to the mask, and the exposure scan is repeated. Cost and complexity are in the x-ray mask, not in the process.
The capital equipment for XRL is also very simple, much simpler than optical lithography equipment. An XRL system consists of an x-ray source and a stepper without a lens. (in optical lithography, the lens is an expensive and critical component of the stepper.) The source may be a point source, much like current DUV laser systems, or a highly reliable compact storage ring (synchrotron).
The robust process of XRL is perhaps its most important contribution to low cost-of-ownership. Even at 100 nm design rules, no resolution enhancement techniques are needed: no optical proximity correction, no phase shifting, no anti-reflection coatings, and no top surface imaging techniques. This means fewer process steps, less equipment, and lower personnel costs. The cost-ofownership advantage and the potential profitability of XRL are further enhanced by its
process latitude and image fidelity with excellent edge roughness.
ASET Cost of Ownership Study
In 1995, ASET members compiled a cost-of-ownership study using DRAM devices as the evaluation vehicle. This study was recently updated and published in the July 1998 issue of Semiconductor International. The study concluded that, mostly due to lower process costs, XRL
has a small cost-of-ownership advantage over 248 nm DUV for I Gbit DRAM devices. For 4 Gbit DRAM devices, the cost-of-ownership for XRL is about 25% lower than for 193 nm DUV. See table below for details. The study assumed that mask costs for XRL and optical lithography are equal for the same ground rules.
(Table won't reproduce here. The following is the text explaining the results of the table:)
Assuming 40,000 wafers/mask, 200 mm wafers for i-line and 256 Mb 248 nm (KrF) steppers, 300 mm wafers for all others, results show a cost savings for XRL over 193 nm (ArF).
Process Comparison
Dr. Hisatsune Watanabe, VP, R&D Group, NEC Corporation compared several lithography technologies at the March 1998 ASET meeting in Kyoto, Japan. Relevant manufacturing and process parameters for 248 DUV, 193 DUV and XRL are shown on the radar graphs at the top of
page 7. (These tables can't be copied here.)
The key conclusion of this study is that, compared to DUV, XRL simplifies the process and improves the process latitude, and dramatically enlarges the exposure field. XRL throughput is
based on current DUV resists; optimized x-ray resists will shorten exposure times and increase throughput.
Source Maintenance
Synchrotron sources are very inexpensive when the cost is distributed over 10 to 20 steppers. To maintain and run a synchrotron ring costs about $500,000 to $1,000,000 annually, including on-site skilled technicians and spare parts.
X-ray point sources such as the NanoPulsarTM from SAL have two consumables (anode and cathode) that need to be replaced after approximately 1,000 wafers. The actual time to replace and recalibrate is less than one hour. Replacement costs are estimated to be similar to the cost of maintaining a 248 nm DUV laser source and replacing stepper optics as necessary. The maintenance costs of a 193 nm DUV laser are estimated to be 2-3 times higher than a 248
DUV laser.
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