Designing systems to test large numbers of semiconductor or electronic devices in a factory environment where each individual device under test generates significant thermal load is not a trivial task. Examples of such devices include but are not limited to high-speed microprocessors, laser diodes, and linear high-power amplifiers.
From a semiconductor manufacturers standpoint, factory floor space often comes at a premium. A testing system that is capable of testing the largest number of semiconductor devices in the smallest factory floor footprint is therefore highly desirable. Removing large amounts of heat from high-dissipation devices in a very small footprint is problematic, however. Air-cooling devices undergoing testing require large heat sinks with a corresponding large surface area and size. Water-cooling devices undergoing testing require a lower corresponding heat exchanger area than air, but is still not the smallest solution possible and also has the intrinsic risk of damaging sensitive electronics in the test system should a leak develop.
One solution for cooling heat generating devices utilizes a dual-phase refrigerant where the physical process of evaporation (latent heat of vaporization) absorbs significantly more heat than air or fluid can alone for the same corresponding heat exchanger size. Cooling schemes that depend on either a refrigerant cycle or Carnot heat cycle have long been in existence. Various semiconductor test system manufacturers have used refrigerant cycles wrapped around gas compressors for their central plants.
A key difference between a true refrigerant cycle and a Carnot heat cycle is nothing more than the thermodynamic direction that heat moves: low to high versus high to low. In a refrigerant cycle a compressor takes low pressure gas, compresses it generating high-pressure gas and heat, and then removes the heat in the condenser on the high-pressure side of the system. A Carnot heat cycle on the other hand, uses a fluid pump to pressurize liquid refrigerant into the evaporators. Heat then flashes the liquid refrigerant into gas and then a water sub-cooled condenser downstream condenses refrigerant gas on the low-pressure side of the system to complete the cycle. A classic example of a Carnot heat cycle is a fossil or nuclear-fueled steam power plant generating electricity.
Some of the common problems experienced by current thermal systems used in semiconductor testing environments include refrigerant leaks, poor chiller mean-time-before-failure (MTBF) and inconsistent thermal performance as the device-under-test (DUT)/system thermal load parameters changes. Other problems associated with some thermal systems include the inability to charge the thermal system while the system is operational and the inability to control suction pressure to increase cooling temperature differential.