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MPW eases analog/mixed-signal design

Posted: 01 May 2007 ?? ?Print Version ?Bookmark and Share

Keywords:MPW services design? IC design time? analog mixed-signal design?

By Wes Hansford
Metal Oxide Semiconductor Implementation System

Design strategy has a great impact on the time it takes to get analog and mixed-signal ICs to market. Even though many designs in this category are implemented in relatively mature technologies, 0.18?m and higher, the trend is toward rising mask and fabrication costs, which increases the likelihood that design respins will be needed.

While no two designs share the same goals or constraints, most have a couple of things in common: They need to be affordable at the outset, and they need to get to market quickly, either to generate revenue or to secure the next round of funding.

You can find competent tools to get you started for around $20,000. But how do you overcome the high costs of masks and wafers? The answer is to buy only part of a mask set and wafer run, a limited area that's enough to get your evaluation samples. Providers of multiproject wafer (MPW) services, including fabs and specialist service organizations, enable you to do this.

By understanding the nature of MPW services and adapting your design methodology to take advantage of them, you can save weeks or months of development time in getting a chip to market. Assuming you have the EDA tools in place, here's how to use MPW to accelerate your design.

  1. Realize that using an MPW service changes the cost structure and dynamics of your design work. On the first run, you don't need to fully process all wafers and get them packaged. It will often be possible to fix a design fault, or improve yield, by simply changing one or more metal layers and reusing the saved layers.

  2. This flexibility means that, for the first iteration, you should focus on getting your design working, not on maximizing yield, minimizing die size or other aspects of design-for-manufacture. Even the largest and most experienced chip companies now work on the basis that most of today's designs will need at least one respin.

  3. On the first run, build in extra circuit elements that allow you to fine-tune performance. In an analog circuit, for example, you could replace single fixed resistors or capacitors with a combination of smaller devices, allowing you to switch them in or out with a metal mask change. These additional elements are easily removed on the next run.

  4. Build in additional test structures on the first iteration. That will make the part easier and faster to test, again shortening time-to-market.

  5. Not all MPW services work the same way. Ensure that your MPW service does not restrict you to a "standard" 5-by-5mm tile. Also, where your device is below the minimum charged for die size, look for additional samples in lieu of this. That lowers costs or gives you more samples per dollar. MPW services are based on your only paying for the proportion of the wafer that your design occupies.

  6. Don't restrict yourself to the 40 or so devices produced in a typical MPW sample run. Because most of the cost, by far, is in the mask set (additional wafers are only about 1 percent of the initial run cost at 0.13?m), the incremental cost of producing perhaps 200 samples is very small. That means you can get samples to your customers for evaluation, as well as into your lab. This will accelerate customer acceptance and shorten time-to-market. In fact, you can economically produce up to 1,000 samples per MPW run.

  7. Make full use of open-cavity packages for the first run. Yesterday's clumsy and expensive ceramic open-cavity packages have been replaced by plastic versions that are available in the same outlines as their fully molded production counterparts. They will make your first devices much easier to probe, and they only affect device performance in the same way that the final packaging does, so you are able to test in real-world conditions.

Using MPW services is not just about cutting the direct costs of development. The major cost saving is in design time. By adapting design priorities to take advantage of the speed and reduced cost at which respins can be undertaken, the whole development cycle shrinks.

About the author
Wes Hansford
is deputy director of Metal Oxide Semiconductor Implementation System (Mosis). He can be reached at hansford@mosis.com.cts Division. David has a bachelor's of science in astrophysics from the University of Edinburgh in the U.K. and a master's of science in Astronautics and Space Engineering from Cranfield Institute of Technology in the U.K.




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