X-RAY PHOTOEMISSION ANALYSIS AND ELECTRICAL CONTACT PROPERTIES OF NF3 PLASMA CLEANED SI SURFACES

The removal of native silicon oxide on [100] silicon with an electron cyclotron resonance (ECR) excited NF3 plasma is demonstrated. In situ x-ray photoemission spectroscopy verifies removal of the oxide and shows that a residue remains on the surface after exposure to the plasma. The residue is about 1.2 nm thick with the approximate formula Si6F8ON2 When analyzed with a uniform overlayer model. X-ray photoemission spectra of the residue show fluorine and oxygen in at least two different bonding states and a unique nitrogen having a diamagnetic bond. Chemical bonding in the residue is ascribed to F(x)-Si, F(x)-Si-O, Si-O-Si, and N2-O-Si species, where x = 1, 2, and 3. A distinct high-energy peak is identified in the quasicore level F 2s transition that is attributed to a small amount of interstitial fluorine having diffused into the silicon lattice. The residue is stable at room temperature in both vacuum and under hydrogen, but when exposed to room ambient, it and the substrate appear to oxidize accounting for a loss of both fluorine and nitrogen. Heating the residue to 640-degrees-C in vacuum causes a significant loss of fluorine and nitrogen also. A possible mechanism accounting for the formation of the residue is proposed assuming that there is a difference in the decay time of the atomic species in the plasma. The ECR cleaning process is integrated into a sputtered TiN/Ti metallization sequence to show the effect of in situ chemical and physical plasma cleaning methods on the electrical contact properties of devices with submicron dimensions and high-aspect ratios. The specific contact resistance of Ti to n+-polycrystalline Si and to TiSi2/n+-, p+-[100] Si is found comparable to that achieved with an ECR excited Ar plasma cleaning where a surface residue is not produced.