Renesas Advanced Technology: Quality Improvement

Technology: 1 of 2

Renesas Drives Continuous Quality Improvement: Using Knowhow to Find Grains of Sand Growth and diversification in the use of semiconductors continues apace, as these components find their way into an ever-widening range of device types. Meanwhile, the need for high quality and reliability remains as firm as ever. Renesas is incessantly working to improve quality and reliability, starting from the design stage of products. In conjunction with this effort, we are also devoted to achieving continuous improvements in production processes.

Dedicated to True Quality Improvement

As a solutions provider, we are always aware that solutions cannot be optimal unless they also include the high quality and reliability that each customer requires. We use reliability metrics, including the defect rate (in ppm: parts per million), FIT (failures in time: number of fails during operation) and so on. Our targets for these metrics are set separately for each product, in accordance with the product’s intended use. In pursuing reliability, we draw on deep experience in screening our product and removing potential failures prior to shipment.

To deliver true reliability, it is not sufficient to check for defects only after production is finished; we must also focus on identifying and correcting potential problems during the manufacturing process. By improving processing steps that might otherwise drive up the defect rate, we can reduce the number of defects, raise the production yield, and drive the quality up. At Renesas, we continuously inspect our processes, using the well-known Japanese concept of "kaizen" (i.e. continuous improvement in PDCA cycles) to identify and remove issues that might otherwise impede the output quality (See Figure 1).

Figure 1: Renesas Approach to Quality Improvement: Continuous Cycle of Process Inspection

Figure 1. Renesas Approach to Quality Improvement: Continuous Cycle of Process Inspection

Renesas Knowhow to Quality Improvement #1 ―Wafer Testing―

Let’s briefly review how semiconductor production processes relate to defect generation. Remember that production is carried out by forming a great number of circuits on a large wafer (up to 300-mm across) of ultra-high-purity silicon, and then cutting the wafer into chips (dies) and mounting each chip into a package. Circuit formation can be disrupted at any step along the way by introduction foreign matter on the wafer.

To catch problems early, we conduct numerous tests on wafers during the trial production stage—an essential step toward working out difficulties before moving into the mass production stage. Accordingly, we begin thinking about testing during the design stages—where we decide how to include test circuits into the design so that we can subsequently carry out testing on completed circuits.

Digital circuitry consists of both sequential circuits (mainly flip-flops) and combinational circuits. In general, chain testing is used for sequential circuits, and scan testing is used for combination circuits. In chain test mode, the flip-flops are connected in a circuit that skips over the intervening combinational circuitry, enabling rapid testing to ensure that all flip-flips are behaving as expected. Scan testing, in contrast, makes use of specially designed input patterns. Full testing of all combinations would require a huge number of patterns, at very high cost. Renesas, however—backed by years of experience—has developed great expertise in designing tests that are highly effective at reasonable cost.

Renesas Knowhow to Quality Improvement #2 ―Effective TEG Testing―

As part of the semiconductor development phase—before mass production starts—test wafers are put through the production process, and the resulting product subjected to scan and chain testing as described above. By analyzing the results, the designers can identify the types of defects that occur; they adjust the production steps accordingly, and then repeat the testing. Since production of each set of wafers takes several weeks, considerable time is required to confirm that adjustments are effective.

Another method for reliability is the use of TEG (test element group) circuits, which are essentially subsets of the full circuitry, placed on wafers specifically for the purpose of testing. TEG testing enables effective analysis of potential production problems. TEG circuits are an extremely effective means of analyzing device characteristics (See Figure 2).

Figure 2: Benefits of TEG Approach, when Applied by Experts

Figure 2. Benefits of TEG approach when applied by experts

Use of TEGs and test wafers makes it possible to identify and develop countermeasures to production issues. A modern LSI chip may hold over a 100 million transistors—roughly equivalent to the number of blades of grass covering a major league baseball field. And it's quite possible that a defect equivalent to the extent of just a single grain of sand may cause failure of the chip itself. This fact, however, also implies that it is sometimes possible to greatly reduce the failure rate simply by correcting these very tiny defects. Renesas brings very strong expertise and capabilities to TEG testing, just as we do to scan testing.

Renesas Expertise Also Backs Quality of Contractor Output

In line with Business Community Planning efforts that took root following the Great East Japan Earthquake, Renesas has started consigning some production to outside manufacturers. But we continue to take the same approach to quality management for all of our products—whether manufactured in-house or outside.

When we contract with an outside producer, therefore, we don’t just hand over the drawings and wait for the finished product. While the manufacturer takes responsibility for production, Renesas maintains responsibility for quality. There are many issues, therefore, that we work out together.

Note that different outside manufacturers may use different equipment and different production methods. Accordingly, some of the problems that occur during production will be specific to a particular manufacturer; but others will be common to many or all manufacturers. In all cases, Renesas uses its deep experience and knowhow to analyze these problems and work out appropriate countermeasures—backed by our deep familiarity not only with production processes but also with instrumentation, design, and other upstream issues. While the term fabless has gained considerable currency, in practice it would be difficult to produce high quality output without sufficient knowhow in production and design processes.

In contrast to Renesas, companies that simply hand the drawings off to the contractor—and then wait for the product to return—are unable to identify problems in the production process itself. They may try to screen out defects from the finished output, but this is not sufficient to secure real quality improvements. At Renesas, therefore, we keep in touch with the process, and work together to solve problems. In this way we get the same high quality from both in-house and outside production facilities.

Delivering the Quality Our Customers Demand

Our very low and continuously improving MCU defect rate shows just how effective our efforts at quality improvement have been. As Figure 3 makes clear, we are pushing the rate down steadily, year after year. We are also working, of course, to reduce our FIT—the number of fails that occur over time while our products are in use by our customers.

Renesas semiconductors are used in a wide variety of industries and device types: in cars, industrial machinery, household appliances, and much more. Each usage has its own quality requirements, which our company—backed by long and deep experience—is uniquely qualified to meet. In more demanding domains, we apply leading-edge technology to deliver products with customer’s required quality. Our powerful quality-improvement technologies enable us to fully respond to the varying needs of our many customers.

Figure 3: Renesas Automotive MCUs; Low and Consistently Improving Defect Rate

Figure 3: Renesas Automotive MCUs; Low and Consistently Improving Defect Rate