Target heating during ion implantation

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Abstract
The temperature of ion-implanted silicon wafers has been measured as a function of time using an infrared pyrometer. Ion beam power density was varied from 0.10 to 200 W cm−2, and the target temperature was measured in the range from 100 °C to the silicon melting point. The resulting heating and cooling curves have been analyzed using a model which includes the effects of ion beam heating, radiative cooling, conductive cooling, and reradiation from the surroundings. For the system studied, conductive cooling in vacuum is less reliable and generally less effective than radiative cooling, unless conductive grease can be used. The model has also been generalized to the case of a group of wafers being scanned cyclically by the ion beam, and the calculated temperatures have been compared with experimentally measured values. Implanting N wafers cyclically is not equivalent to dividing the input power density by N, because the time required for the wafers to cool is typically less than the time between scans. However, it is possible to reduce the wafer temperature by many hundreds of degrees by choosing the scan rates properly. The model is intended as a guide to the design of new apparatus as well as to the use of existing ion implantation systems. Although silicon is used in the examples, the procedure can easily be generalized to other target materials.

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