Give your gizmo the gift of sight
This is the third part of a three-part posting on chip design and how to reconcile it with the open source and DIY movements. (Part 1 is here and part 2 is here.) In this part I will discuss economics- How much does it actually cost to fabricate a chip? Can batching be used to make the cost accessible to a group of (hypothetical) casual chip designers?
Once a chip design is “taped out”, the chip foundry who manufactures them first creates a set of masks- anywhere from around 15 to upwards depending on the design and the manufacturing process. These are for a photolithography process that is vaguely similar to those used in PCB manufacture, however for chips the masks are both more precise and more expensive.
I won’t give exact costs or list specific foundries, but I’ve seen masks cost anywhere from under $10k to over $50k for a complete set. This is for a 0.5um or 0.6um process. Obviously the masks for the latest 32nm process would be much more.
Typically in chip design the foundry creates a set of masks for a “reticle”, a box-like region that gets replicated over and over across a whole wafer using a stepping process. A wafer itself is a disc of silicon less than a millimeter thick but generally tens of centimeters in diameter.
The typical reticle size I use is about 21mm x 21mm. (The masks themselves are much larger than this but the image is optically reduced during the manufacturing process.) You can fill up that reticle pretty much any way you want- you can put in a single 21mm x 21mm chip. If your chip size is just 2mm x 2mm you can put in a 100 of these into the reticle, so every reticle gets you 100 chips. You could also put in 100 different designs. This is where batching would come in.
There are companies that provide batching services. One of the oldest is the MOSIS service, run by ISI of the University of Southern California. MOSIS was set up originally with DARPA and NSF grants as a way to bring chip design to universities, and give students the ability to design and fabricate a chip, either as a classroom exercise or for a research grant. MOSIS also offered their services to industry. To this day they still offer these services and actually serve as a “store front” for several major chip foundries (ON-Semiconductor, TSMC, IBM, and others) for customers who want to prototype chips without paying for a whole set of masks.
The economics are essentially that everyone shares the cost of the tooling. Let’s say a mask set for a reticle has 10 different designs and cost $20k to make (a somewhat made up number)- that comes down to $2k per design- a much more reasonable number!
There are other services similar to MOSIS, and there are also individual companies that offer “multi-project runs” specifically for smaller customers that want to batch-prototype chips. So the batching concept is clearly established. In fact, whenever I do a run of silicon at Centeye I also place multiple designs on one reticle to get the most for my money.
Hypothetical Cost Breakdown
So let’s suppose a company wanted to get into the chip batching business. Let’s say the company decides to accept 2mm x 2mm size chips, and place 100 different designs onto a reticle. (The remaining 1mm slivers could be used for test circuits and quality control…) Looking at pure costs alone (e.g. neglecting stuff like “overhead”, “labor”, and “profit”), the numbers might look like this:
Mask set for a 21mm x 21mm reticle: $20,000
6”/150mm diameter wafers, set of 10 (approx 30+ reticles per wafer): $10,000
Dicing the wafer up into chips: $500 per wafer
First consider prototype quantities- I don’t know of any foundry that will make a single wafer- typically a set of wafers are manufactured in case one or a couple of them fail quality assurance inspections. For initial prototyping you would end up dicing just one wafer. Next comes packaging- most customers are not equipped to work with bare die, so they would probably want the chips in a DIP or similar package that they can then solder to a board or press into a breadboard- this would probably cost at most $20 per chip, at cost. Total cost per customer: ($20k + $10k + $500)/100 = $305 for about 30 chips, plus $20 per chip packaged.
Next let’s consider a slightly higher quantity price break, by dicing up all 10 wafers. The total cost per customer rises to ($20k + $10k + 10 x $500) = $350 for about 300 chips, not including packaging.
These numbers are encouraging. Of course, we have to assume 100 such customers can be found, and we have to consider the other costs to stay in business, but the above numbers should give you a starting point.
So where does that put us?
Once we factor in the cost of doing business, we get upwards to a thousand dollars for a batch-run prototype chip. This is a stiff amount compared to a batch-fabbed PCB. But it is not impossible- This amount is easily within the budget of a Kickstarter project, and there are hobbyists that would be willing to spend this amount on a chip fab. Certainly small companies could spend this amount. Also note that due to the nature of the chip manufacturing process, there could be several hundred individual chips available for use (or sale) if the design works. (There are a number of caveats, of course, which I didn’t mention here, but can discuss below if there is interest.)
I think one of the challenges, though, is overcoming the fear of spending money on a fabrication that doesn’t work. Getting back a PCB that doesn’t work is never fun; the stakes are higher for chips because of both the higher cost and the long lead times (generally six weeks or more). This is where the combination of good design tools and good design practices can help out- I think the “abstract layout” workaround mentioned in my last post, properly executed, could make “probability of success” sufficiently high for the DIY crowd.