Oxynitride gate dielectric and method of forming

#The present invention generally relates to a gate dielectric for a semiconductor device and method of forming and more particularly, relates to an oxynitride gate dielectric structure that contains nitrided regions with controlled concentrations of nitrogen for use in a semiconductor device and method of forming the structure.

#Conventional gate dielectrics used in semiconductor memory devices consist of a thin SiO2 layer. A recent trend in semiconductor processing has been the inclusion of a small concentration of nitrogen in the gate dielectric layer. It has been found that the nitrogen provides the beneficial effects of reducing channel hot electron damage and reducing boron out diffusion from the polysilicon gate into the channel. The inclusion of nitrogen further raises the dielectric constant so that a nitrided film has a lower leakage current than a pure oxide film that has an equivalent capacitance. Even though nitrogen has beneficial effects on gate insulators, too large a nitrogen concentration may be undesirable. Large nitrogen concentrations can cause unacceptable shifts in Vfb, as well as degradation in other properties of the dielectric.

#There is little flexibility in the control of the concentration and depth distribution of nitrogen in most methods for introducing nitrogen into SiO2. Chemical methods of introducing nitrogen rely on the reaction between a nitriding agent such as NO or NH3 with the silicon substrate or a previously grown oxide. The resulting films have a large concentration of nitrogen at the Si/SiO2 interface where the chemical reaction takes place. The reaction is self-limiting by the nitrogen since the nitrided layer acts as a diffusion barrier to oxygen and thus prevents further gas species from reaching the Si/SiO2 interface. This provides an additional benefit of using nitrogen in a nitriding process, i.e., the nitrided films are more uniform in thickness than conventional oxides. The uniformity in thickness is demonstrated by the smaller distribution of electrical characteristics measured at different sites across a wafer surface treated with, for example, NO oxidation.

#These and other objects, features and advantages of the present invention will become apparent from the following detailed description and the appended drawings in which:

#FIG. 1 is an enlarged, cross-sectional view of a semiconductor gate stack incorporating the present invention oxynitride gate dielectric.

#FIG. 2A is an enlarged, cross-sectional view of the present invention gate dielectric after an oxynitride layer is first formed on a silicon substrate.

#The present invention discloses a method for forming an oxynitride gate dielectric in a semiconductor device that has controlled concentration profile of nitrogen in the oxynitride layer. The method further enables a substantially pure silicon dioxide layer to be formed in a reoxidation process in the presence of oxygen and a halogenated species between an oxynitride layer and a silicon substrate. The oxynitride layer may be formed by either passing at least one gas that contains nitrogen and/or oxygen over the surface of a silicon substrate or by depositing in a chemical vapor deposition technique. The at least one gas that contains nitrogen and/or oxygen can be flown into a reaction chamber at a sufficiently high flow rate and chamber temperature such that a nitrogen-rich oxynitride layer which has a nitrogen concentration in the range between about 0.1 and about 50 atomic percent can be formed. The at least one gas may be NO, N2O, NH3 or O2. The at least one gas may be flown into a reaction chamber to react with a silicon surface at a pressure between about 1 mTorr and about 20 Atm. The temperature of the silicon surface may be kept between about 500° C. and about 1200° C. The at least one gas may also contain one gas selected from NO, N2O and NH3, and one gas selected from O2 and N2O.

#The method may be carried out for forming a gate dielectric in a CMOS device by first forming an oxynitride film and then re-oxidizing the film in an oxidizing ambient such as O2 containing a halogenated species such as HCl, CH2Cl2, C2H3Cl3, C2H2Cl2. CH3Cl and CHCl3. The resulting structure formed consists of an oxide layer on top of an oxynitride layer which has a controllable nitrogen content, on top of another oxide layer that is formed on a silicon substrate. The thicknesses of these layers may vary between about 1 Å and about 40 Å.

#The oxynitride film can be formed by a standard thermal treatment, such as a reaction between NO, N2O and NH3 with a silicon substrate, possibly in combination with an oxidizing gas such as O2 or N2O. Alternatively, the oxynitride film can be formed by a CVD process, including but not limited to plasma CVD, remote plasma CVD, rapid thermal CVD and low pressure CVD.