FACILITY REPORT

History, teamwork come together at Siemen's Dresden Fab

DRESDEN, GERMANY—Siemens Microelectronics Center (SIMEC) is one fabrication facility that can actually claim a place in history. "Before the [Berlin] Wall came down, this was used as a military site," recounted Johann "Hans" Harter, director of fab operations. "The place where the modules are standing was a tank exercise area [for the Soviet army], so we had quite a bit of pollution. There was actually a gun turret laying in the woods a little bit down the hill . . . . This part of Dresden has a long military history."

Before building the 8-inch advanced-process fab, old army barracks were demolished, and the grounds were meticulously cleared of waste and debris, with about 8 hectares turned back into heath and woods. A green consciousness pervades the manufacturing activities as well, with an emphasis on reuse, recycling, and clean disposal of water and chemicals. For example, the plant reuses 70% of its DI water, according to Harter.

SIMEC also stands as a symbol of reunified Germany, the first fab built in what was once the German Democratic Republic, or GDR. More than 85% of the 2600 employees come from the former Communist country, according to Harter, with nearly 80% from the surrounding state of Saxony. A poignant irony is that during the Soviet occupation, the locals were forbidden to enter this one-time military zone. When Siemens dedicated the site, it was the first time many Dresdeners had ever visited it.

Even SIMEC's facility layout symbolizes, in its own way, the coming together of a divided people. It features two integrated 5400-square-meter Class 1 ballrooms—Modules 1 and 2—which are largely equipped with the same tool sets. "Each area has a base structure with tools: litho, etch, wet, furnace, and so on—with two exceptions," explained Harter. "We have consolidated implantation in Module 1(except for 300 mm) and deep-UV litho in Module 2. Both modules have i-line litho, though, which is used for all layers in the 0.35-µm processes and noncritical layers in 0.25 µm." Memory, flash, embedded, and logic chips are manufactured at SIMEC. A small assembly-and-packaging back-end is attached to the plant, with a capacity for handling about 30% of the modules' output, according to Harter.

Between the two modules lies a central support building. The fourth floor houses parts cleaning and production control. All wafer lots start in production control where they are laser scribed and stored in stockers before being sent out into the line. Most of the third floor of the support building contains the chemical mechanical polishing (CMP) bays. "This was done mainly for logistics reasons, both for wafer transport and the supply and waste treatment of slurries," the fab director explained. Single- and multihead polishers buff the wafers with tungsten, oxide, and nitride mixes, and then the wafers are taken off-line for post-CMP brush cleaning. Carriers from CMP are sent upstairs to be cleaned before reentering other parts of the fab, Harter noted. As for post-CMP wafer characterization, metrology manager Uwe Belz said that the procedures are performed in the respective inspection areas of the modules themselves, adjacent to the other process bays.

The subfab and basement areas are filled with a tangle of piping for the gas, chemical, and water systems as well as the usual air-handling tools, chillers, and abatement apparatuses. Electrical engineer Gottfried Zomack cited the impressive airflow statistics associated with large, ballroom-type fabs: in SIMEC's case, 5.6 million cubic meters of air per module is handled using 32 fan towers, which translates into 260 hourly air changes. As for the air exhaust vented through the fume scrubber, Zomack joked: "The air we are pushing to the outside is cleaner than that being taken in." The power supply has been improved since the fab started operations in 1995; a modular setup features six transformers per module running off a main grid, ensuring continuous, high-quality electricity. The gas cabinets are bunkered on one side, with an automatic safety system and easy access at the rear of the area, while large chemical cabinets occupy another section of the lower floors.

Unusual Cleanroom Protocol

Another curious fact about SIMEC is that it is one of the few Class 1 fabs with a gloveless cleanroom protocol. For the most part, workers go about their tasks without the discomfort of skintight gloves, except in cases where they must come into contact with the wafer cassettes. Detlef Mitrach, cleanroom technology manager at SIMEC, explained how wafer cassettes are moved manually with slot-in handle-grips (when they are not being conveyed by some part of the fab automation system), with a glove receptacle near most tool bays for those clearing the cassettes from the equipment itself.

The third floor of SIMEC's central support building contains the CMP bays, which were placed there mainly for logistical reasons.

Harter related why Siemens decided on its gloveless policy after it conducted several studies into the use or nonuse of gloves and determined that the pluses of improved wafer handling outweighed the minuses of possible contamination. "We did extensive experiments and we found that by using gloves, people might get a wrong sense of security that if they are wearing gloves, they can just grab the wafers. So by not using gloves they know they have to keep their distance from them. And it's much better for the skin." Near the entrance to Module 1, Mitrach also pointed out a screen for a new computerized information system that features cleanroom protocol and other behavior tips, with more information being added regularly. Posted nearby were defect status and weekly defect-trend charts as well as meeting summaries with actions taken for each product.

300-mm Pioneers

The Semiconductor300 joint venture between Siemens and Motorola will undoubtedly earn the facility a place in the semiconductor history books. The heavily publicized 300-mm pilot line began operations in January, and tools are being brought in on a regular basis. The project partners expect to start fully integrated wafers by the end of the year. About 1800 square meters of Module 2 is occupied by the new unit.

When I walked through the SC300 portion of the facility in late March, an intermittent strip of blue cleanroom tape on the raised floor demarcated the area—a hop-step to the right and you were in 200 mm, back to the left and you had reentered the 300-mm zone. (The section will be segregated by cleanroom walls eventually.) Metrology tools made up the majority of the larger-wafer tools visible, with newly arrived deposition and phototrack tools in various states of assembly, massive furnaces fitted with automation interfaces, and a huge wet bench looming in the background also in view.

One obvious question about Semiconductor300 is how Siemens handles access by Motorola people into the rest of SIMEC. "We're still working on that," confided Harter. "The plan right now is to ask for a different color for their cleanroom garments and have a badge that indicates who they are. The computer systems of the two companies are separated completely by firewall techniques, providing them access to data that are used jointly but clearly separating the other projects. They are supposed to stay in the 300-mm areas and use the direct access to the garment area; they are not supposed to be found in any other place in the cleanroom. If they ask for analytical services, for example SEM cross sections, clearly then we have to use joint tools. [The Siemens personnel] are told to clean up their desks and not show cross sections of other products before the Moto guys are allowed to sit with them at the SEM."

In addition to the Motorola personnel now in the facility, there are close to 1000 vendor company employees servicing SIMEC at various times. The long list of equipment and materials suppliers doing business with SIMEC includes many big fish (Applied, Novellus, Canon, TEL, KLA-Tencor) as well as many smaller fry. Bare wafer lots come from most of the main silicon houses, rather than just one or two.

Ion implantation equipment has been consolidated in SIMEC's Module 1.

As in most leading-edge fabs, Siemens personnel have a close relationship with their vendors. "We have companies who work very closely with us, who actually have process engineers on-site working together with our process engineers in improving tools," said Harter. "Others provide service around the clock, others are coming in on call." One area where the supplier company owns its service is gas supply. "The whole gas farm is outsourced to Air Liquide; they run it on our premises." In each vendor relationship, the bottom line is a major factor in deciding the level of on-site participation. "It depends on whether it is economically feasible for them to be here or not."

Problem Solving, Not Finger-Pointing

SIMEC's organizational structure fosters teamwork and an atmosphere of problem solving, rather than finger-pointing. "We have very good communication between the departments," Harter explained. One key element is the daily 9 a.m. meeting. "Every sector and department goes through and reports what they're doing for the day, what happened overnight, if any wafers were lost, the reasons why, and actions taken or if actions need to be taken. This is often the start of any focus teams, which are then ad hoc set up."

The nine-member defect team, led by Ralf Schütten, convenes a weekly meeting attended by as many as 20 engineers. "We work together with the process integration department and also with product engineers and production. From each group, one representative will attend these meetings. We make the in-line measurements and collect all the data on the product wafers, and the production department gets their results from their equipment control data. We come together and look for defect density trends on the product wafers. We talk about the actual problems, look for possible root causes, and define the next activities needed to reduce defect density in the line.

"For troubleshooting, we have more meetings during the week. When a problem in the line comes up, we come together with the responsible process engineers, we have a short meeting, maybe one hour, and we define the next activities. When we make a true correlation and we know OK, we have a problem with one special tool, the process engineer who is responsible for the tool comes to us."

Schütten's defect detectives work closely with Uwe Belz's metrology group. "Metrology covers the whole characterization tool set. We take care of all metrology issues as well as the manufacturing part of the inspections. On the other side, where we have physical parameter measurement, we are somewhat differently organized. There, we have process engineers taking care of metrology tools, not the operators, because those tools are spread around the fab. They [the metrology people] should be where they are needed, not at a central place, so the operators of the other departments are doing the measurements to control their processes themselves. So metrology is taking care of defect inspection, maintenance of the tools, and the manufacturing and engineering of physical parameter measurements."

SIMEC's defect team uses a short-loop inspection methodology.

Belz's comments about the spread of the metrology tools throughout the fab becomes clear as we proceed through the modules. "As you can see, in each process finger there are small bubbles with the metrology tools in them and the departments themselves are responsible for those bubbles. In addition, we have tools that are used less frequently or are used by different departments, like AFM, FTIR. . . . It is also somewhat of an evaluation zone here. An in-line FIB was in evaluation for our physical failure analysis, so a decision is made to buy one and the final installation will be in Module 2."

The Almighty Defect Catalog

One important tool in any well-run, high-yield fab is the defect catalog. Process engineer Dieter Gscheidlen, one of the defect investigators at SIMEC, showed me how their vast database on a Lotus Notes—driven system works. "For each workstream number you have the defects that are typical to that number; if you click here, then this page opens. The operator can see what the defects look like and can compare them with what he sees under the microscope. If, for example, you have SEM images, he can see where it might come from, the defect codes, and what he has to do if he has this problem on the wafer. This is available for each defect type."

"We usually divide it into general defect types that occur in every layer, and we have some specific defect types which might only occur on a certain workstream operation of the wafer. . . . We have a relatively low number of defect classes. We try to cover most defects by the general failure classes and only if we have a very specific or very critical defect that we cannot cover by the general classes, then we give it its own class. We always have to take care that the work for the operators is as simple as possible; if you have a very large number of defect classes it will be very hard to classify correctly." One extra notation on the on-screen system mentioned by Gscheidlen is the use of an exclamation mark to highlight a critical defect, alerting the team to find the root cause as soon as possible.

SIMEC's team uses a short-loop defect inspection methodology to help solve problems. The information gathered from inspections and reviews of both bare and product wafers is fed back into the process steps before the wafers exit the line for final test. The enormous databased catalogs contain particle generation data garnered from tools during the startup of the fab. The Yield Manager software (from the Knights Technology subsidiary of Electroglas) handles the analysis of the wafer inspection and review data.

Schütten explained their sampling strategy. "Typically, we start with 100% sampling during ramp-up of a technology, and we measure 5 wafers per 25-wafer lot. Now with 64-Mb DRAM, we start with a review of wafers that have a defect count higher than the limit, and also we will review good wafers to get more information about excursions of the defect density and the baselines. . . . We have much data about tool fingerprints measured with bare wafers; here we find the signature of the wafer handler"—pointing to an on-screen chart—"we have this information for many tools. We use this to compare it with the defect on the product wafer, so we know where it came from and the possible root cause."

When asked how often they run across an anomaly they haven't seen before, Gscheidlen responded: "It becomes more and more seldom—we don't have so often the situation where we see a very new defect." Harter added that, in the case of a "new phenomenon showing up, they put it on the wall so everyone can see it."

Visiting the Value

Engineers as well as higher-level managers and executives realize the importance of staying in touch with what's going on in the fab, Harter stressed. "When his time allows it, the head of our business division always likes to go into the fab to understand what's going on. He's very eager on that. . . .We encourage the people to go into the line and work with the people who have done the measurements and have a look themselves at what's going on. That's one reason why I'm carrying my trainer [building sweatsuit] in my office, so I can switch easily . . . so I can go into the fab if I have a half-hour to spare in the evening before I leave or when I'm in on the weekend. We try to convey the message that the line is the area where the value is generated, not the office building." — Tom Cheyney

Photos by Ed Shvartzman

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