Test & Measurement


A short overview of the two most common cooling technologies for thermal chuck systems


In a world of ever-increasing demand for thermal wafer test, the big question is: how can we increase reliability and gain efficiency, while simultaneously lowering the cost of ownership and operation? Klemens Reitinger, managing director, ERS explains


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n the early 1970s temperature testing started to be performed on wafer level. The wafer sizes were roughly two inches in diameter and if temperature control was performed, it happened by way of a Peltier element inside the chuck, a thermo-electric device on which one side heats up and the other cools down depending on which direction the current flows. The applications were simple wafer sort during which the functionality of the chip is tested and confirmed prior to dicing and packaging in its individual component. This gives the manufacturer the opportunity to discard the bad dies early on in the production process thereby saving the packaging and final test costs for those devices, as well as to match the high quality of other devices in the market. Fast forward to today and the wafer size has increased to 300mm (12 inches) diameter, while the need for thermal test remains and if anything, is gaining momentum.


The simple heating concept behind most hot testing of semiconductor wafers today is the same as we see in everyday cooking applications. A resistance heater is built into a plate (i.e., chuck) that is slightly larger in diameter than the wafer that will be put on top of it during test. A good “hot chuck” is recognised not only by its upper temperature limit, but also by how fast it reaches the selected


temperature, how uniform the temperature is across its diameter, how rigid and flat the chuck plate is, and how electrically quiet it can remain during test. When the need for cold test began to increase with the integration of semiconductor devices in automobiles, as well as in space and outdoor applications like renewable energy, a common and very successful solution was introduced to the market that combined the resistance heater of the simple hot chuck together with the flow of a coolant through the chuck.


Cooling is critical To control the temperature of the chuck – and thus the temperature of the wafer under test – tool designers employ a number of technologies. The two most common chiller cooling technologies today are: synthetic cooling liquids and air.


Liquid


The use of synthetic cooling liquids such as perfluoropolyether (PFPE), sold under various brand names, is widespread and accounts for the majority of older cold chillers in operation. These inert liquids are chemically stable as long as the operating limits are not exceeded, and they can be used in a broad temperature range typically from about -70°C to +120°C. Because they are very poor conductors of electric current, they do not pose a danger


Figure 2: ERS’ AC3 air-cooling technology


of short-circuits in the case of leakage. In contrast to water, even to purified water, synthetic cooling liquids are rather costly, and the thermal performance is also not as good. Since these liquids evaporate over time even in closed circuits, the systems need regular refills. In addition, they require a complex infrastructure to handle their logistic and environmental implications. Though they are not highly toxic, these liquids are subject to hazardous waste legislation that adds to their already high operating costs. In case their operating temperature range is exceeded, there is a theoretical risk that they could disaggregate into their chemical constituents. These can include highly toxic and otherwise hazardous gases such as fluorine or chlorine, and therefore restricts the use of PFPE-based liquids to relatively low temperatures; at the same time, their utilisation is associated with high costs and technical complexity.


Air-cooling Figure 1: Comparison between costs of liquid cooling and air-cooling www.cieonline.co.uk


A newer invention uses air as the cooling medium since virtually every semiconductor manufacturing line has a ready supply of pressurised clean dry air (CDA) easily accessed at the wall utilities behind the equipment in the factory. When air is used as the coolant, it does not impose any limitations with respect to the temperature ranges required for semiconductor test, which differentiates it from water, other liquids and synthetic cooling agents. In addition to using it as a coolant, large amounts of CDA (or other very dry gases) will be necessary as an environmental purge to prevent frost in the wafer test area inside of the equipment. Failure to reliably prevent frost in the test area can result in


the destruction of the very expensive electrical probes used during cold wafer test. The requirement for environmental dry air (or gas) purge exists independent of whatever coolant is used to achieve the temperature in the wafer chuck. The drawback of air as a coolant is its


relatively low thermal capacity. However, given its availability and flexibility, this drawback does not have a high material impact. The heat exchangers used in ERS’ AC3 thermal system effectively cancels out the drawback of the lower thermal capacity, while benefitting from the smaller footprint, lower utility costs and application flexibility associated with using air as the coolant (Figure 1). The patented heat exchanging system on the AC3 extracts maximum cooling from the chilled air and then utilises that same air to purge the test area. This effectively reduces the consumption of air and electrical power (Figure 2). With test floor space at a premium, the


more compact air chillers have been gaining popularity over the last 10 years, as lower maintenance requirements and operating costs translate directly into productivity gains and uptime. As the need for cold test continues to increase in sync with automotive semiconductor applications, environmental responsibility and lowering power consumption continue to drive the demand for maximising efficiency. Reducing chemical consumption remains on top of the list for environmental health organisations and we can therefore expect that clean and green chiller technology will continue its march forward.


www.ers-gmbh.de Components in Electronics April 2018 41


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