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The acronym MTBF stands literally for Mean Time Between Failures. However, time to failure is rarely measured on semiconductors today. Instead, most IC manufacturers measure a failure rate, in failures per hour, and then invert the number to calculate hours to failure. Since modern semiconductors rarely fail during life tests at moderate temperatures, the MTBF measuring process has evolved into a challenge of accumulating hours without failure (device hours). In most situations, the preferred units for presenting reliability are failure rates instead of MTBF. Since MTBFs are growing so large, and failure rates are shrinking so small, the unit of measure for failure rates has become a FIT. The acronym FIT comes from the phrase Failure unIT. Although it is an accurate description, it is not from the phrase "Failure In Time." The FIT is expressed as a rate of failures per billion device hours. One FIT is equivalent to 0.0001% per thousand hours. 10,000 FIT is equivalent to 1% per 1000 hours.

Semiconductor Failures

The acronym MTBF itself implies that there is a mean period of time at which failures regularly occur, and therefore failure rates are constant throughout the lifetime of each device. Contrary to popularized beliefs, semiconductors do NOT fail at constant failure rates. One number cannot be used to describe the expected failure rate for integrated circuits because they fail at different rates throughout their lives. Generally, semiconductors have a very low failure rate early in life, then have increasing failure rates as they wear out. At a point when about half of the devices fail in a group of circuits, the failure rate begins decreasing again. A very small part of an IC's population may fail early in life. These early failures have been associated with manufacturing or assembly defects. The early failures are sometimes called "infant" failures. As semiconductor reliability improves and more samples are stressed, the early failures become easier to detect and eliminate. TriQuint also utilizes feedback from high volume customers to identify causes of early failures so that defects can be continuously reduced. With long wear out lifetimes and very low rates of defects, TriQuint ensures high reliability.

Life Test Results

Figure 1 shows some example results of seven gallium arsenide integrated circuits life tested at TriQuint. Microwave, digital, and linear circuits of various levels of integration are depicted. Figure 1 clearly shows that each integrated circuit design has its own unique failure rate which remains low over its useful lifetime, increases at wear out, and then decreases afterward. The Figure 1 results are shown for a hotspot (usually the hottest FET on the die surface) of 150°C. This is the maximum rated temperature of operation for devices manufactured at TriQuint. Lower temperatures of operation result in considerably lower failure rates, and longer median lives. The failure rate goal at TriQuint is to have failure rates less than 100 FIT for the first 20 years of life at 150°C. For reference, twenty years is 175,320 hours. This goal is indicated by the star on Figure 1.

Figure 1. FIT Rate Summary.

Use Temp Activation Energy 0.3eV 0.7eV 1.0eV 1.6eV 150°C 121 FIT 19 FIT 4.4 FIT 0.19 FIT 125°C 72 FIT 5.8 FIT 0.78 FIT 0.012 FIT 100°C 40 FIT 1.5 FIT 0.11 FIT 0.00052 FIT

Table 1. Circuit FITs. "Chi-Square," 239 Devices, 9300 hours without failure.
Activation Energy @ 60% Confidence Level
1 FIT = 1 Failure/Billion Device Hours