Friday, June 13, 2014

Common Ways to Electrically Heat a Semiconductor Chuck - The Basics

A semiconductor wafer platform, know as a chuck, is used to support and hold in place (usually by means of applied vacuum), a silicon wafer. A chuck heater is used to uniformly and accurately distribute heat (and cooling) to a semiconductor wafer chuck.  This is done during the manufacturing, characterization, testing, and failure analysis of semiconductor wafers.

Semiconductor wafers contain many electronic devices or electronic circuits, known as dies. Each die has to be carefully tested through a range of temperatures. Precise and uniform temperature control is a requirement. The simplest, and most common way to apply heat (and cooling) is through a chuck incorporating an electric resistance heating element.

There are several designs of resistance heated chucks. One approach is to use a tubular heating element which is cast inside of an aluminum casting, and then precision machined to the specifications of the chuck.

A tubular heater has a single nichrome wire, centered inside a metallic tube which terminates at either end of the tube. The tube is then back-filled with magnesium oxide. The assembly is then rolled down for compaction and optimum heat transfer from the nichrome wire to the metallic sheath. The finished tubular element then is shaped to optimize the size and thermal profile of the aluminum casting. The tubular element is then placed in a mold where molten aluminum is then poured into the mold, cooled then released and the rough chuck heater is then sent to precision machining.

A similar process is to use another form of resistance heater called a cable heater. A cable heater is similar to the tubular heater, with the exception that the nichrome element makes the trip down to the end of the tube, then “U” turns and makes it’s way back, allowing for termination at one end of the element. It is then drawn (mechanically stretched) down to much smaller diameters, thus providing much tighter coil patterns and better thermal profiles. Another characteristic of cable heaters is that they can be of other cross sectional shapes than round, such as square.

One other method to apply resistance heat to a semiconductor chuck is via an etched circuit heating element sandwiched between to thin sheets of a mica, elastomer or polymer. These heating elements can provide an excellent thermal profile, but are limited to the temperatures and watt densities of the surrounding encapsulating material. Another concern for using these types of “thin foil” heaters is possible outgassing from the encapsulating material itself.

Belilove Company is a pioneer in the use of applied resistance heat in the semiconductor industry. If you have any question regarding any kind of semiconductor heating application, please feel free to contact us at (510) 274-1990 or email customerservice@belilove.com.

photo credit: oskay via photopin cc