MEMS and the mask muddle: Small isn’t cheap

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July 11, 2003 — As silicon chip features get smaller, the cost of making the masks that print them is exploding. At 90 nanometers, the next step down for chip makers, a set of masks costs $1.5 million. At the step beyond that, 65 nanometers, those masks will probably cost about $4 million. And a single chip can use 20 or more masks.

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“There’s a major, major problem for (mask makers) and an innovative solution is absolutely required in the near term,” said Pat Gardner, director of operations for the western region of SEMI, the semiconductor equipment manufacturers’ association.

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One innovation is to use MEMS-based micromirror arrays to create the masks. The arrays can cut the cost of mask sets and in the next few years they may help do away with masks entirely for small runs of chips.

Swedish company Micronic Laser Systems AB uses an array of about one million mirrors in its Spatial Light Modular (SLM) module. In current systems, the mirrors write the pattern on the mask. The result is a machine with about the speed of older methods, but the feature size of the newer technology.

One big advantage to micromirror technology is the array can write an entire section of the mask at once, unlike the latest devices. This means the micromirror pattern generator is much faster than competing e-beam technology, which creates the mask a feature at a time. Another advantage is that the technology scales in both speed and feature size.

“If you need a finer feature size you can shrink the size of the mirrors,” says Jorge Freyer, senior vice president of marketing and business development at Micronic. “For higher throughput you can increase the size of the mirror array.” According to Freyer, there is almost no limit on the size of the micromirror array because Micronic can either use larger arrays or additional smaller arrays. “The real limit today is how quickly you can get the data into the system.”

Micronic gets its micromirror arrays from the Fraunhofer Institute in Dresden, Germany. The current SLM has a million mirrors, each 16 by 16 microns. The next generation will have 8 million mirrors, each 8 by 8 microns, Freyer said. By using optical tricks and exploiting the 64-level gray scale of each mirror, the Micronic device can place an edge within 1 nanometer of its desired location.

Micromirror arrays aren’t a panacea. The cost of making a mask with a micromirror array is rising, but not nearly as quickly as costs using conventional means. Freyer said it is still more expensive to produce smaller feature sizes, but it costs about 30 percent less than with e-beam technology. At around $16 million each, Freyer said, the Micronic 6000 is ‘comparable’ to other mask-writing tools in these feature sizes.

The next logical step is to do away with masks entirely, and that is where the industry is going later in this decade. The result will be more expensive than making masks for high-volume chips, but Freyer said chip makers are telling him that at a rate of one wafer per hour the break-even point at current feature sizes of direct writing versus masks would be as few as 100 wafers. At 65 nanometers, he said, the break-even point would be around 50 wafers.

Micronic hopes to have a direct-write tool available by 2005 to work at the intermediate size of 90 nanometers. The company is currently looking for a stepper manufacturer to produce a direct write tool because it is important that the micromirror pattern generator appear to the data file just like a stepper. This would hold down costs and prevent a steep learning curve for mask makers.


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