By Douglas G. Sutherland and David W. Price
Author’s Note: This is the sixth in a series of 10 installments that explore fundamental truths about process control—defect inspection and metrology—for the semiconductor industry. Each article in this series introduces one of the 10 fundamental truths and highlights their implications. Within this article we will use the term inspection to imply either defect inspection or a parametric measurement such as film thickness or critical dimension (CD).
In previous installments we discussed capability, sampling, missed excursions, risk management and variability. Although all of these topics involve an element of time, in this paper we will discuss the importance of timeliness in more detail.
The sixth fundamental truth of process control for the semiconductor IC industry is:
Time is the Enemy of Profitability
There are three main phases to semiconductor manufacturing: research and development (R&D), ramp, and high volume manufacturing (HVM). All of them are expensive and time is a critical element in all three phases.
From a cash-flow perspective, R&D is the most difficult phase: the fab is spending hundreds of thousands of dollars every day on man power and capital equipment with no revenue from the newly developed products to offset that expense. In the ramp phase the fab starts to generate some revenue early on, but the yield and volume are still too low to offset the production costs. Furthermore, this revenue doesn’t even begin to offset the cost of R&D. It is usually not until the early stages of HVM that the fab has sufficient wafer starts and sufficient yield to start recovering the costs of the first two phases and begin making a profit. Figure 1 below shows the cumulative cash flow for the entire process.
What makes all of this even more challenging is that all the while, the prices paid for these new devices are falling. The time required from initial design to when the first chips reach the market is a critical parameter in the fab’s profitability. Figure 2 shows the actual decay curve for the average selling price (ASP) of memory chips from inception to maturity.
Consequently, while the fab is bleeding money on R&D, their ability to recoup those expenses is dwindling as the ASP steadily declines. Anything that can shorten the R&D and ramp phases shortens the time-to-market and allows fabs to realize the higher ASP shown on the left hand side of Figure 2.
From Figures 1 and 2 it is clear that even small delays in completing the R&D or ramp phases can make the difference between a fab that is wildly profitable and one that struggles just to break even. Those organizations that are the first to bring the latest technology to market reap the majority of the reward. This gives them a huge head start—in terms of both time and money—in the development of the next technology node and the whole cycle then repeats itself.
Process control is like a window that allows you to see what is happening at various stages of the manufacturing cycle. Without this, the entire exercise from R&D to HVM would be like trying to build a watch while wearing a blindfold. This analogy is not as far-fetched as it may seem. The features of integrated circuits are far too small to be seen and even when inspections are made, they are usually only done on a small percentage of the total wafers produced. For parametric measurements (films, CD and overlay) measurements are performed only on an infinitesimal percentage of the total transistors on each of the selected wafers. For the vast majority of time, the fab manager truly is blind. Parametric measurements and defect inspection are brief moments when ‘the watch maker’ can take off the blindfold, see the fruits of their labor and make whatever corrections may be required.
As manufacturing processes become more complex with multiple patterning, pitch splitting and other advanced patterning techniques, the risk of not yielding in a timely fashion is higher than ever. Having more process control steps early in the R&D and ramp phases increases the number of windows through which you can see how the process is performing. Investing in the highest quality process control tools improves the quality of these windows. A window that distorts the view—an inspection tool with poor capture rate or a parametric tool with poor accuracy—may be worse than no window at all because it wastes time and may provide misleading data. An effective process control strategy, consisting of the right tools, the right recipes and the right sampling all at the right steps, can significantly reduce the R&D and ramp times.
On a per wafer basis, the amount of process control should be highest in the R&D phase when the yield is near zero and there are more problems to catch and correct. Resolving a single rate-limiting issue in this phase with two fewer cycles of learning—approximately one month—can pay for a significant portion of the total budget spent on process control.
After R&D, the ramp phase is the next most important stage requiring focused attention with very high sampling rates. It’s imperative that the yield be increased to profitable levels as quickly as possible and you can’t do this while blindfolded.
Finally, in the HVM phase an effective process control strategy minimizes risk by discovering yield limiting problems (excursions) in a timely manner.
It’s all about time, as time is money.
1) Process Watch: You Can’t Fix What You Can’t Find, Solid State Technology, July 2014
2) Process Watch: Sampling Matters, Semiconductor Manufacturing and Design, September 2014
3) Process Watch: The Most Expensive Defect, Solid State Technology, December 2014
4) Process Watch: Fab Managers Don’t Like Surprises, Solid State Technology, December 2014
5) Process Watch: Know Your Enemy, Solid State Technology, March 2015
About the authors:
Dr. David W. Price is a Senior Director at KLA-Tencor Corp. Dr. Douglas Sutherland is a Principal Scientist at KLA-Tencor Corp. Over the last 10 years, Dr. Price and Dr. Sutherland have worked directly with over 50 semiconductor IC manufacturers to help them optimize their overall inspection strategy to achieve the lowest total cost. This series of articles attempts to summarize some of the universal lessons they have observed through these engagements.