Footprint Reduction of Automated Wafer Test Equipment

Introduction:

Clean room floor space in semiconductor wafer fabrication and test facilities is extremely costly to maintain. As a result, equipment manufacturers are continually seeking new methods for reducing the amount of space required for their equipment.

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Customer Challenge:

The customer provides automated measurement tools for assessing the dielectric coating properties of silicon wafers. Some optional measurement techniques require the wafer under test to be rapidly heated to a high temperature and then cooled back to ambient prior to measurement. The customer designed a separate self-contained enclosure to house a heat/cool station and related controls for this feature as an add-on option for the main test system. This separate enclosure required over two square feet of floor space and was over four feet tall. The rapid heat option was found to be much more popular than originally anticipated with almost all test system customers requesting it; however, these customers were disappointed about the extra floor space the option required. The customer needed a way to incorporate the heat/cool station functionality directly into the main tool and approached Single Iteration with this need. In addition, the customer also needed to increase system throughput while raising the operating temperature set point while maintaining temperature uniformity performance.

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Single Iteration Solution:

The customer's need involved many significant design challenges. To minimize changes in the configuration of the main test system, the customer needed to fit the current external capability into unused space inside the main chassis. This meant reducing the current sixteen cubic feet design envelope into a space less than two cubic feet in size. The increase in set-point temperature meant delivering 20 percent more power in about 12 percent of the current space. This essentially ruled out the use of separate heat and cool plates as there was not sufficient space for isolated chambers and the additional time for the machine robot to move the wafer between plates would violate the throughput requirement.

The primary driver for an effective solution was determined to be quickly delivering a large amount of heat to only the wafer while minimizing the heating of adjacent assemblies and the enclosure. Single Iteration recommended the use of a radiant heater located above the wafer as a good solution for the heat source. An infrared source could be quickly switched on and off; thereby, minimizing the compartmental heating load. However, the high operating temperature of the infrared element coupled with the small space meant that heater termination cooling; as well as, wafer cooling itself would be more difficult. In addition, the added cost of an active refrigeration system for cooling was considered prohibitive. A more innovative solution was required.

The ambient cooling set point was immediately identified as a major complexity (and cost) driver. Cooling to ambient in any reasonably short amount of time requires active refrigeration. Further investigation determined that the driver for this cooling specification was simply to return the wafer to a safe handling and processing temperature for other system components. Through thermal system modeling and analysis, Single Iteration determined that this set point could be safely increased without impact to the process or equipment. This eliminated the need for active refrigeration and allowed the consideration of a passive cooling plate. Further analysis determined the emissivity properties of the wafer would allow a small air gap to be used as an effective thermal barrier. This enabled development of an integrated single chamber heat/cool module that would fit in the available space. Placing the wafer on the passive cooling plate and using lift pins to raise the wafer during the heating cycle allowed rapid heating of the wafer without also heating the passive cooling plate underneath. Then retracting the lift pins to bring the wafer in contact with the passive cooling plate returned the wafer to the lower set point. By maintaining a positive pressure in the system enclosure and providing an external vent on the back of the heat/cool station, waste air flow from the main enclosure convection cooling system could be exhausted through the heat/cool module during wafer transfer. This served to cool the infrared heater termination and passive cooling plate between wafer thermal cycles.