The Role of Scattered Radiation in the Dosimetry of Small Device Structures

In very large scale integrated circuits (VLSI), multiple layers of different atomic number materials are separated by a few microns or less. When this type of structure is irradiated with high energy gamma rays, the transport of energy by energetic electrons must be taken into account. By comparing Monte Carlo calculations with experimental data, we show that it is essential to specify the low energy photon component of the gamma ray spectrum at the point of interest if accurate dose calculations are desired. In the case of silicon next to gold irradiated with a Cobalt-60 source, the silicon dose within a few microns of the interface can be twice as great as that derived from a calculation in which scattered radiation is neglected. The principal source of scattered radiation is the source material and holder, together with any intervening components such as a collimator. Backscattered gamma radiation, however, is also significant and subject to wide fluctuations under typical test conditions; it is most likely to produce large dose fluctuations within a few microns of an interface.