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Unformatted text preview: Surface nitridation of silicon dioxide with a high density nitrogen plasma R. Kraft, a) T. P. Schneider, W. W. Dostalik, and S. Hattangady Semiconductor Process and Device Center, Texas Instruments Inc., Dallas, Texas 75243 ~ Received 20 February 1997; accepted 16 May 1997 ! A high density nitrogen plasma generated with a helicon plasma source has been used to incorporate approximately 15 at. % nitrogen into the top 0.5 nm of a silicon dioxide layer. The surface nitridation was accomplished in 10 s with a high flux of low energy ions which were extracted from the high density nitrogen plasma and accelerated in the plasma sheath towards the surface an electrically floating silicon dioxide surface. A rf compensated Langmuir probe was used to measure the nitrogen ion energy and ion current density as a function of the nitrogen pressure and source power. The nitrogen ion energy, ion current density, and exposure time determine the nitrogen range and dose into the silicon dioxide surface. This process may be advantageous for nitriding the gate oxide in advanced complementary metal–oxide semiconductor process flows. © 1997 American Vacuum Society. @ S0734-211X ~ 97 ! 14204-4 # I. INTRODUCTION Plasma nitridation is a useful technique for modifying the physical characteristics of materials. 1,2 For example, in the complementary metal–oxide semiconductor ~ CMOS ! pro- cess flow it may be desirable to nitride the surface of a ma- terial to change its electrical properties, oxidation resistance, etch resistance, or diffusion barrier properties. When the ni- tridation process described in this article was applied to a 4.0 nm gate oxide in a 0.18 m m CMOS process flow, 3,4 boron penetration from the doped polysilicon gate was suppressed. This was accomplished with ~ 1 ! no degradation in n-channel or p-channel mobility, ~ 2 ! drive currents which were equivalent to, or exceed that of control oxide, ~ 3 ! little flat band voltage shift, and ~ 4 ! no significant variation in midgap interface trap density from that of control oxide. The attractive feature of plasma nitridation for CMOS ap- plications is that high nitrogen concentrations can be con- fined very close to the top surface of the nitrided material. Compared with typical high thermal budget nitridation pro- cesses, such as with N 2 O or NO, 5,6 the plasma nitridation process incorporates nitrogen much closer to the surface and does not put any nitrogen at the oxide/silicon interface. Com- pared with ion implantation nitridation, plasma nitridation can be performed at much lower ion energies and much higher ion current densities. In particular, a low pressure, high density nitrogen plasma generated in an inductively coupled, electron cyclotron, or Helicon source can deliver ion current densities of 5 mA/cm 2 and ion energies of 10 eV over large areas such as a 300 mm wafer. High nitrogen concentration can be attained in a short period of time, thus making the plasma nitridation process suitable for high vol-...
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