Therma-wave dose sensitivity considerations, including energy dependence

Therma-wave (TW) dose measurement is one of the standard technologies used in the semiconductor field to monitor the process of ion implantation. One of the main advantages of this technique, in addition to its nondestructiveness and the lack of additional processing, is the wide range of implant conditions (dose and energy) over which the technique is effective. However, it has long been recognized that there can be substantial variations in the dose sensitivity of this technique depending on the TW value. The plot of dose sensitivity (defined as % change TW / % change dose) vs. therma wave value results in a “V” shaped function, usually demonstrating a minimum value near 600-700 therma wave units. This dose sensitivity minimum can significantly effect the ability to properly monitor implant dose variations near this minimum. In order to better characterize this sensitivity variation, a matrix of wafers was implanted with variations in both dose and energy with sufficient dose resolution to provide useful sensitivity data. The variation in dose for boron implants was from 5×10¹¹ to 1×10¹⁴ ions/cm² and in energy from 3 to 500 keV. Wafers were also implanted with phosphorus and arsenic over a similar dose range with somewhat different variations in the energy range. The basic sensitivity characteristic reported in the literature was demonstrated with a “V” shaped pattern being evident. What was unexpectedly observed was that there was a change in the value of the minimum of the sensitivity as the implant energy was varied. For the conditions studied there was a significant change in the minimum sensitivity value as the implant energy was changed, ranging from 0.083 at 3 keV to 0.223 at 2 MeV. In addition it appeared that the location of the minimum, with respect to the TW value may have also shifted, varying from near 500 TW units to almost 700 TW units. The significance of these variations in the therma wave sensitivity response at different energies is discussed along with possible causes