SCALPEL Masks Progress
Ruth DeJule, Associate Editor -- Semiconductor International, 12/1/1999
With the focus on extreme ultra-violet lithography (EUVL) and Scattering with Angular Limitation in Projection Electron beam Lithography (SCALPEL), Motorola has been developing fabrication technologies for next-generation lithography masks targeting the 70 nm technology node. Earlier this year, the company demonstrated an e-beam patterned, microprocessor-level EUV mask. Now, Motorola has completed a fabrication process yielding the first 200 mm SCALPEL mask fabricated for a microprocessor-level device with 0.18 µm minimum geometries (see Figure).
The complex mask is a silicon substrate with free-standing membranes that contain the device patterns. To form the mask blank, a low atomic number membrane material such as silicon nitride is deposited on both sides of the substrate. This film, approximately 100 to 150 nm thick and deposited by LPCVD, is fairly transmissive to electrons, thus minimizing scattering. On the front side, an etch stop layer is created by sputtering 10 nm of chrome. Next a high atomic number scattering material -- amorphous alloys tantalum silicide or tantalum silicon nitride, 20-30 nm thick -- is deposited with a magnetron sputtering.
Processing continues on the back side of the wafer. Here, a photolithography process is used to form struts that define the membranes. Standard reactive ion etch removes the nitride, exposing the silicon substrate. A patented wet etch process removes the silicon down to the nitride on the front side, forming free-standing membranes, 1.1 mm by 12.1 mm, supported by silicon struts. The completed mask blank is now ready for device patterning. Requiring a lithography step and using a Leica VB6 E-beam tool, device patterns are written on the front side. The chip data are fractured so sub-sections of the pattern are placed in individual membranes. The e-beam written pattern is then transferred into the scattering and etch stop layers, thus completing the mask. These patterns are subsequently stitched together in a step-and-scan type electron projection lithography (EPL) stepper. The finished mask is capable of printing dice as large as 24 mm x 17 mm.
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Fig. 1 A 200 mm SCALPEL mask with 0.18 mm microprocessor design rules contains
free-standing membranes, 150 nm thick. (Source:
Motorola) |
Handling the reticles has been a concern. However, a mask-handling technique that contacts only the edges has been effective in improving mask blank yields to 90%, according to Dr. Pawitter Mangat, advanced reticle technology development manager at Motorola. Over the past year, the lab has stockpiled approximately 70 mask blanks, which are continuously being used for process development.
Thus far, work has concentrated on setting up an
infrastructure for manufacturing the mask blanks at Motorola Labs (Physical
Sciences Research Laboratories) in Tempe, Ariz., in joint development with
Motorola's Semiconductor Products Sector (Advanced Products Research and
Development Laboratory in Austin, Texas). In the coming year, process
development will continue, but with greater focus on manufacturing issues;
statistical process control such as mask blank yields, defects and stress; and
uniformity of the films. Exposing the masks using an EPL stepper developed by
Lucent and IBM will enable them to be evaluated in terms of scattering
effectiveness. It also will provide learning to optimize the process and meet
specifications such as image placement, CD uniformities and defect requirements.
Pellicles, inspection, repair and metrology are yet to come. 'Despite successful
demonstrations and no foreseeable show-stoppers, as an industry, we still have a
lot of challenges ahead to make this a production-worthy technology,' noted
Mangat. 
