Energy deposition by soft x-rays - an application to lithography for VLSI

The energy deposited in x-ray irradiated low atomic number materials bounded by high atomic number elements can be much greater than the bulk dose. This effect has been studied for x-rays above 10 keV but the region below this energy has not been explored. The development of x-ray lithography for the mass production of integrated circuits with submicron linewidths, and the associated problems of energy deposition in sensitive regions of processed circuits, photoresists, and x-ray masks, has motivated the study reported here of dose enhancement effects in the low energy region. Computations were based upon a semiempirical model of energy deposition and charge transfer that had previously been successfully applied to soft x-ray emission (below 10 keV) and to energy deposition at high energies. It was found that x-ray absorption edges dominate the qualitative and quantitative features of enhancement effects at energies below 10 keV. The result is abrupt changes in the magnitude of the enhancement with small changes in photon energy. The combined effect of absorption edges and enhancement, lead to a factor of ten difference in the peak dose to sensitive device regions, depending upon the characteristic x-ray line selected. The enhancement has a particularly strong effect upon the contrast achievable with x-ray masks. This is due to the fact that dose enhancement, in effect, sensitizes the photoresist to bremsstrahlung. These studies indicate that for the configurations investigated, operating at x-ray line energies below 1.5 keV offers a distinct advantage from the point of view of minimizing device damage and achieving a desirable degree of contrast.