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071012: Managing mature fabs
Ed’s Threads 071012
Musings by Ed Korczynski on October 12, 2007

Managing mature fabs
Associated with SEMICON Europe 2007, the Fab Manager’s Forum gathered representatives of Europe’s semiconductor fabs to discuss operations of primarily mature fabs. Michael Lehnert, of Renesas Semiconductor, presented examples of the benefits derived from fault detection besides yield improvement in mature fabs. Renesas Semiconductor Europe Landshut (RSEL) has a 200mm line with 13-15k wspm running 0.5 to 0.15 µm for MCUs (the line was originally a DRAM line).

Fault Detection and Classification (FDC) is a challenge for a fab running several hundred products, with 10 to 100 parameters/tool resulting in up to 5 GB/day of data. With 300,000 SPC charts and 10 entries per chart, and with the data normally distributed and applied 3-sigma limits with a 0.3% false alarm rate, a fab must handle ~1000 false alarms every day. Manufacturing engineers need to change how they work, spending more time with abstract analyses looking at computer screens, and less time crawling through the fab poking at tools. Monitoring facilities parameters such as gas flows and pressures may provide additional relevant data streams.

FDC improvement in wafer-scrap yield was expected, but an additional benefit has been in engineering productivity, with gathering time reduced and more accurate data. Spare-parts and consumables evaluation is now easier, so there is greater confidence in being able to change to less expensive sources when possible. Greater confidence allows for reduction in sampling frequency and reduces the need for dummy wafers. Better preventative maintenance (PM) planning—for example monitoring the filament current in an implanter—results in reduced consumables costs, equipment uptime, and even turn-around time (TAT) due to greater tool availability.

Dr. Detlef Nagel, Sr. Director Product Engineering, Qimonda Dresden discussed how to manage APC in worldwide DRAM fabs. Future business requires an evolution from APC to predictive process control (PPC), which will in turn require a revolution in data-mining, multi-variate control, and yield prediction. Technology complexity can be kept under control by generic run-to-run (R2R) controllers and virtual metrology.

Qimonda uses SMIC and Winbond as foundries to balance production, along with their own fabs in Europe, the US, and Malaysia. Fast distribution of knowledge is a problem due to regional cultural differences, and the inherent difference between development and volume fabs. One innovative solution is the use of a network of senior equipment engineering specialists, with individuals responsible for an assigned toolset within some areas of expertise. This worldwide captive network improves equipment throughput and reliability at Qimonda fabs; there is traditional information exchange with the foundry partners but not the expert knowledge.

Peter Schaffler, global yield enhancement manager for TI, talked about yield enhancement in the Freising fab. It was originally a 3” Bosch fab, and has been continually upgraded to the current level of 0.2µm processing on 200mm wafers; the line runs CMOS/BiCMOS with 20k active reticles used on 400k wafers/year. TI now does tool qualification with product wafers, challenging costs and tool availability. Sampling strategy directly affects your costs: too much wastes expenses, while too little guarantees lost yield. Typical these days is 10-20% of lot starts, but sampling frequency should be determined by the number of lots at risk and the complexity of the mask level, which results in tool-specific dynamic sampling. Or course, an efficient data analysis system is needed to provide macros for data drill-downs using tool, parametric metrology, final electrical test, and other data sets. Proper charting and visualization in an interactive GUI allows new analyses to be done.

Single-wafer tracking allows for the extraction of yield-loss signatures like the wafer number in the lot, first or last wafer effects, and different lots with single wafer excursions. For example, electrical-test data that may originally show no signature can be sorted to obtain a clear clustering of parameters into groups of five wafers, which in turn could point directly at a TEL furnace which was the only toolset running batches of five.

A breakout session on the dynamics of the used equipment market provided a fantastic perspective on the status of the current market. In addition to third-party brokers, OEMs now provide refurbished tools with full one-year warrantees for typically 40-80% of the original selling price. As always, the price is set by markets: the price to acquire the tool, cost to properly refurbish, and the customer demand for the tool. At the high-end of pricing, the used tool is sold with all new tool specs and it may then be considered as almost just another new tool for capacity. If the market forces align in certain ways, even a used 150mm tool may be sold for US$2M.

If you buy through a broker, it is somewhat common to then have to purchase a use license (often for the software) from the OEM. These licenses can range from $10k to $700k for complex tools; and are the single greatest hurdle for customers of 3rd-party brokers. The consensus was that licenses are not unreasonable in principle, but customers really expect to receive some value in terms of software upgrades and service support for their payment. Service-contracts from OEMs certainly minimize the risk of working with used or refurbished tools, regardless of the seller.

Hallway discussions with equipment brokers revealed that they’re tracking a tremendous number of 200mm tools which are planned to be decommissioned over the next 1-2 years. How the industry will absorb these tools remains to be seen, but with SECS/GEM interfaces and modular sub-system designs, it’s likely that most of these tools will remain productive somewhere in the world.


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071012: Managing mature fabs

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Anonymous Ray Bunkofske said...

FDC is a proven contributor to the efficiency of the semiconductor manufacturing process. Modern FDC applications available from multiple suppliers provide a solid means to address the concerns mentioned in your Blog. Instead of making separate charts for every parameter, product and recipe combination it is much preferred to build multivariate models where the parameters are normalized in the background so as to greatly reduce the number of charts. There is no reason for more than two or three charts per tool-chamber combination to provide fault detection. Once a fault is detected then additional text or charts appropriate to the fault can be displayed to guide the resolution of the problem. Data analyzed in this way not only provides superior fault detection with virtually 0% false alarms, it enables prediction of several down stream metrics such as metrology results (film thickness, etch rate) and end of line electrical test results such as threshold voltage or overlap capacitance. Combine the signals from the tool with data from auxiliary sensors such as chamber impedance or full spectrum OES and you have a very powerful diagnostic and predictive tool. None of these capabilities are difficult to implement but they do require some care and preparation, something that could take six months to a year and it is difficult to get over this activation barrier. These efforts do not require huge investments in software, infrastructure or people, just a management with enough vision to stay the course and take advantage of the benefits as they materialize. Once through this phase of the program the system should pay back in 6-12 months through improved process capability and reduced time to detect.

Tue Oct 16, 09:29:00 AM PDT  

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Ed's Threads is the weekly web-log of SST Sr. Technical Editor Ed Korczynski's musings on the topics of semiconductor manufacturing technology and business. Ed received a degree in materials science and engineering from MIT in 1984, and after process development and integration work in fabs, he held applications, marketing, and business development roles at OEMs. Ed won editorial awards from ASBPE, including interviews with Gordon Moore and Jim Morgan, and is not lacking for opinions.