High Temperature ASICs

Tekmos has two approaches to high temperature operation.  Our basic 0.35u ASIC process performs well up to 200ºC operation.  Above there, the leakage currents become noticeable and may affect circuit operation.  The leakages become excessive above 220ºC, and may damage the circuit.

Tekmos also has a 1.0u, fully depleted SOI process. In this process, the junction area is reduced down to just a small vertical stripe along the source and drains of transistors. This almost 100:1 reduction in junction area results in a corresponding reduction of junction leakage, and allows us to operate easily at 250ºC, and in certain cases about 3100ºC. Our highest operating temperature achieved in our labs has been 320ºC.

Reliability

IC reliability studies are run at elevated temperatures in order to accelerate failure mechanisms, which provides a prediction for device reliability at lower temperatures.  Running ASICs at high temperature means that the parts are likely to fail sooner, and we cannot run them at even higher temperatures to predict their reliability.

Based on actual measurements, we can easily provide 1000 hours of operation at temperature, and with 10,000 hour operation being a reasonable goal.

Packaging

Tekmos recommends using ceramic packages for high temperature operation.  We have measured plastic packages at 210ºC, and noticed visible package degradation after 2500 hours.  Since most high temperature applications require high reliability, it makes sense to use ceramic packages.

Bonding

Bonds are one of the main failure mechanisms for high temperature reliability.  Gold wires on aluminum pads can create intermetallic compounds (the purple plague) that cause opens in the bonding.  Better choices are aluminum bonding for aluminum pads, or replacing the aluminum pads with a gold – nickel- palladium mixture.  We can discuss the options with the customer on a case by case basis to pick the best choice for their application.

Testing

In addition to normal testing, Tekmos tests all high temperature parts at the rated temperature.  Our preferred approach is to use gang testing in a high temperature oven.  Temperature forcing probes can damage the testers with excess heat.  This is another case of evaluating each design to determine the appropriate high temperature test strategy.

EEPROM Data Retention

EEPROMs, as well as Flash memories, lose their charge faster at high temperature.  There are two ways to address this:

During the manufacturing process, there is a high temperature bake that serves as a data retention test.  A pattern is written into the EEPROM, and then the wafer is stored for 24 hours at an elevated temperature.  Normally, this temperature is 250ºC.  Tekmos uses 310ºC for our high temperature products.

The other method for improving data retention is in the application.  EEPROMs must be treated as dynamic memories.  They must be refreshed periodically.  The refresh rate depends on the operating temperature.  In general, a weekly refresh will work, but there is no disadvantage to performing it more often.  We have done experiments where we refreshed parts over 1,000,000 times without damaging the part.