The degradation of porous methyl silsesquioxane (MSQ) films (k=2.2) by plasma irradiation has been investigated using several kinds of gas chemistries, which are used for photoresist strip. The porous MSQ is easily degraded by O2 plasma ash process and the dielectric constant increases largely, even by using the condition that minimizes the damage to nonporous SiOCH films. As the results of several kinds of plasma irradiation, it is clarified that N2 plasma minimizes the degradation of porous MSQ. O2 and H2 plasmas extract methyl (CH3) groups from the porous MSQ films and degrade even the deep part from the film surface. On the other hand, N2 plasma changes the quality of only the film surface and suppresses the degradation of the inside of the film. Furthermore, the small amount of H2 addition to N2 plasma is effective in the increase of the ash rate and the removal of the deposition on the sidewall without the remarkable increase of the dielectric constant. Finally, the optimized N2/H2 ash process minimizes the damaged layer thickness of the sidewall with single damascene structures.

The fabrication of a dark-field phase shifter edge mask that improved the lithography process latitude for fine transistor gate patterns was examined. The applicability of this mask was confirmed by analyzing of phase shift mask fabrication yield. The pattern formation method used both a dark-field phase shifter edge mask and a clear-field binary mask. It was clarified that the fabrication yield of the phase shifter edge mask was dependent on the transparent area of the phase shifter region. Clear-field phase shifter edge masks were fabricated with difficulty because of shifter defects. On the other hand, the fabrication yield of the dark-field phase shifter edge mask was about two times that of the clear-field phase shifter edge mask. This mask was applied to an actual logic device. Resist patterns were fabricated with a line width accuracy of 0.15 ± 0.01 µm and a focus budget of ±0.4 µm. The dark-field phase shift mask exposure method also improved the lithography process latitude significantly.

The effects of the “electron shading” charge build-up at the bottom of holes are investigated using fluorocarbon gas plasma. The etch rates of the electrically conductive films such as phosphorus-doped polysilicon at the bottom of the holes change depending on whether the films are patterned or not. This is caused by the decrease of the low-energy ions which reach the bottom of the holes due to positive charging of the underlying layers. Furthermore, the potential at the bottom of the contact holes is investigated using metal-nitride-oxide-silicon (MNOS) capacitors. The positive charging due to the electron shading effect is measured. In order to reduce the electron shading charge build-up, the pulse-modulated plasma is investigated. The selectivity to the underlying layer increases upon using pulse-modulated plasma.

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We investigated the mechanism for differences in etched depth between isolated and dense via holes of a damascene structure using a SiOCH film (k=2.8). In Ar∕CHF3∕N2 and Ar∕C4F6∕N2 plasmas, the depth of an isolated (4.8μm pitch) hole was more than 40% varied from a dense (200nm pitch) hole. The difference between dense and isolated holes was found to become larger with increasing air exposure time of the wafer. The amount of H2O in the SiOCH film also increased with increasing air exposure time based on thermal desorption spectroscopy (TDS) analysis. Therefore, we investigated the effects of H2O addition to the plasmas. The investigation revealed that the etched depths of dense holes became similar to those of isolated holes with H2O addition. Based on these results, we concluded that the mechanism for the difference between isolated and dense holes was as follows. When the capping layer on SiOCH is etched, the H2O contained in SiOCH desorbs in etched holes and affects etching reactions. The etching characteristics of isolated holes are significantly affected, since the amount of desorbed H2O in isolated holes is larger than that in dense holes.