SAN MATEO, Calif. The rapid increase in the cost of photomask sets for advanced IC processes has made a lot of news in recent months. When word got around that a mask set for leading-edge 130-nanometer processes could cost more than $1 million and for 90-nm processes, $2 million, the semiconductor industry went into a collective state of shock. But behind the scenes forces are converging on the problem with a variety of solutions.
Perhaps the most obvious, given the huge number of transistors that can be fit into the real estate covered by a reticle, is to share the masks. The technique, commonly referred to as multiproject wafers, has been used as a means of prototyping and to provide limited runs for university projects for years. The acknowledged leader in this area has been a relatively low-profile not-for-profit organization, the Mosis Service.
Originally funded by the Defense Advanced Research Projects Agency, Mosis was conceived in the early 1980s to give researchers subsidized access to a foundry. Cut loose from Darpa funding in 1994, the organization still serves universities and research centers. But it is also emerging as an important access point into the foundry industry for commercial ventures needing test chips, prototype runs or increasingly moderate production runs at sharply reduced mask costs.
Working closely with both clients and foundries including Taiwan Semiconductor Manufacturing Co. and IBM Microelectronics, Mosis packs a number of client designs into a single mask set and then has the wafers processed at the appropriate fabrication facility. The company now can return anything from wafers to dice to fully packaged ICs from the run. The service is available for anything from a run of 40 test chips to a few thousand production ICs. Available processes include TSMC's 180-, 250- and 350-nm CMOS logic and mixed-signal processes, and IBM's 250- and 500-nm silicon germanium BiCMOS processes, among others.
The addition of SiGe means a new relationship between Mosis and its clients, said deputy director Wes Hansford. "A lot of our growth now is coming from commercial design teams exploring their technology options," he said. "They may want to compare what can be done in CMOS and SiGe, for example, and we give them an affordable way to actually build chips in each process and compare them."
Hansford said that the service is also being used to manage the perceived risks involved in new circuit development, particularly with mixed-signal designs. A design team may create a set of test patterns intended only for cell characterization. Or it may tape out an entire functional block of its intended system-on-chip (SoC) to examine not only the parametric characteristics of the devices, but the overall functional behavior. Either way, the die area involved is quite small, and the mask cost for a conventional run would be prohibitive.
Even using the foundries' existing prototype services, such as TSMC's CyberShuttle, can present a problem for such small objects. For example, CyberShuttle divides the maximum reticle area into fixed regions, and the user has to pay for some integer number of regions. Mosis, in contrast, hand-packs the designs into the reticle, so it does not have any fixed size or aspect ratio requirements, said Hansford.
Although characterization and prototyping are the conventional uses of the service, Hansford said that there is also a growing use of Mosis as a production shop. With a minimum run of even 200-mm wafers in 180 nm yielding a huge number of dice, many designs may never reach what TSMC would regard as production. So the ability to slip a job into the multiproject wafer flow and pull out a few thousand dice can be a design team's only access to advanced processes.
While Mosis slashes mask costs by sharing mask sets, the mask vendors themselves are addressing the issue of spiraling cost in a different way: education. "Traditionally, design teams just did their design, ran the design rule checks and if everything passed, they pitched the thing over the wall to the mask shop. They didn't know or care what happened next," said Dan Del Rosario, chief executive officer of Photronics Inc. (Jupiter, Fla.). "But in the region starting at about 130 nm, that needs to change. There are significant decisions that the design team and the foundry make that can substantially influence mask costs. And those should not be transparent to the SoC designers."
The main issue, Del Rosario said, is that different mask layers have different resolution requirements. Usually the poly, contact and active-area masks will have the finest features. Masks for the upper metal layers can be comparatively crude.
In larger geometries, when even the most detailed masks still had feature sizes near the wavelength of the stepper light source, all the masks could be produced on the same equipment using the same technology. But now, the critical-layer masks require the most expensive and slow equipment and demand the most difficult techniques.
"There are now three categories of masks," Del Rosario said. "Out of a set of 32, about two-thirds will be noncritical and about a third will be critical. You can produce those masks on scanning-laser equipment with pretty good throughput. But two to three of the masks will be extremely critical. They will require directed e-beam equipment. Now you can be talking about 24 hours just to expose the mask on the e-beam system.
"Much of the cost of the mask set depends on just how much you demand of those critical masks. And that in turn depends on the process often on details of the process that are not readily visible to the design team."
For instance, Del Rosario said, a foundry may decide to save on costs by using a 248-nm stepper instead of a state-of-the-art 193-nm one to do the poly. But that forces the mask shop to use the most difficult available mask technology: hard phase shifting. If the fab employs the 193-nm steppers, the mask shop may be able to use aggressive optical proximity correction (OPC) instead of hard phase shifts, at a considerable savings to the design team.
"A typical binary mask using aggressive OPC may cost $20k," Del Rosario said. Moderate phase-shift techniques using a halftone mask cost $50,000. "If you require hard phase shifting, that's going to cost you $130k. It makes a big difference."
The problem forces designers to consider issues about which they have been blissfully indifferent. Choices of stepper, mask type and writer type all interact with one another and with the design rules, and those differences can show up in the range and performance of library cells available to the designers. As dimensions continue to shrink these choices will also have consequences on yield and on long-term reliability. "Part of the answer has to be an integration of the decision-making processes, all the way from design through fabrication," Del Rosario said.