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Volume 34 (Jan 24, 2005)
The path traveled by the creators of the NP Series of ultra-low on-resistance power MOSFETs for automobiles (2/2)
Part 2: The difficulty of the conditions for automotive power MOSFETs
1.8m ohm achieved after 14-month trial
NEC Kansai Factory worked on a trial production of wafers based on the 0.25µm process, the impurities diffusion method for the silicon board and the four-wire connection. For the first two to three months, a total of 400 types of prototypes with 20 wafers and 20 various chips on each were produced. Then team members reviewed those prototypes one by one to select the best. Their first priority was to achieve the target value and then minimize quality unevenness at the mass production line. So prototyping and careful review were repeated to check the quality. There were 10 trial productions during 14 months and the development team members visited Kansai Factory each time. Sometimes they stayed two to three days to review. They checked a total of 4,000 prototypes and made improvements. Through all their efforts, including the adoption of a one-rank higher 0.25µm process, the impurities diffusion method for silicon board and the four-wire connection packaging, the process was finalized so that the actual product could be designed. At this point, a power MOSFET with on-resistance value of less than 2m ohm was designed. And the prototype was produced immediately in NEC Kansai.
According to paper calculations, the prototype was supposed to break the 2m ohm barrier, but none of the development team members believed that. Incredibly though, the prototype recorded anon-resistance value 1.8m ohm (Figure 2). Upon confirming the record, team members looked back on the past 14 months of agony and cherished the memories. "That accomplishment really filled me with emotion," said one of the team members remembering the moment.
Staff member,
Power Management Device Division,
MOSFET Development
Hiroyoshi Kobayashi, colleague of Manabu Yamada and one of the development team members, brought the finished sample to the lab to test it for automotive use. He connected a battery directly to it and then switched it on to pass electrical currents at 200A and 12V. The mold would be cracked in a very short time - and he would keep record of how long it was exactly. This was an important checkup to confirm if the device was good for a fraction of a second, until the safety circuit of electrical system of the automobile worked. The major concern was still the achievement of the target value, but another issue was safety because the crack could not be avoided throughout the test. So Kobayashi prepared a heavy protection around the test site, put a protection mask on himself, and then repeated the test. The prototype cleared this endurance test too.
Huge numbers of sample requests!
After much trial and error, the release of the first in industry* power MOSFET for automobiles with on-resistance of less than 2m ohm was announced at CEATEC in October 2003. The product had a response beyond all expectations. NEC Electronics received requests for samples from domestic and foreign automobile electrical equipment makers and provided a dozen makers with the samples. Usually sample distribution of a new product is 20-30 units per customer. At that time though, each maker ordered samples of a few hundred at one time, quickly depleting the stock and forcing the production of more samples. To keep delivery dates, scheduling management at each process line was important. Lead times were short. Besides, the packaging assembly line was overseas. So the team had to estimate delivery times, including overseas transport. Team members were too busy to enjoy the pleasure of having so many requests; if they failed to keep quality and delivery dates as requested, there would not be any further orders. From the end of 2003 through 2004, NEC Kansai Factory produced the samples at full capacity. The assembly factory overseas also worked hard to keep the line running, even during the Lunar New Year holidays in February. Initial sample distribution was done without major problems. The number of total distributed samples of the NP Series reached several ten thousands.
Mass production of the NP Series was partially started already. Domestic and foreign power-steering makers highly valued the products, saying, "With these power MOSFETs, at last we are able to offer power steering that we had only dreamed of for so long." Automobiles mounting ultra-low on-resistance power MOSFETs "NP Series" will appear in the market soon, and computerization of automobiles will be more accelerated.
* As of May 21, 2004
The end
