Kinetic and X-ray Photoelectron Spectroscopic Studies of the Thermal Nitridation of Si(100)
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KINETIC AND X-RAY PHOTOELECTRON SPECTROSCOPIC STUDIES OF THE THERMAL NITRIDATION OF Si(100)* C.H.F. PEDEN, J.W. ROGERS, JR., D.S. BLAIR, AND G.C. NELSON Sandia National Laboratories, Albuquerque, NM 87185-5800 ABSTRACT The thermal nitridation of Si(100) by NH3 and N2H 4 has been studied by X-ray photoelectron (XPS) and Auger electron (AES) spectroscopies. The pressure dependence of the rates as a function of reaction time has been measured. It has been found that the growth of the first monolayer (ML) of nitride is mediated by a surface reaction step. For subsequent growth, diffusion of one or more of the reacting species becomes an important process in determining the rate of reaction. Such species may be substrate Si diffusing to the vacuum/Si3N4 interface, or possibly network nitrogen diffusing into the Si substrate. The independence of the reaction rate on NH 3 or N2 H4 pressure at long reaction times rules out a mechanism involving molecular diffusion of the nitriding gas to the Si3N4/Si interface in a manner similar to the oxidation of Si by 02 or H2 0. Careful analysis of the Si(2p) XPS spectra reveals the presence of a unique Si species with a Si(2p) binding energy intermediate between elemental Si and Si in Si 3 N4 . Further, the relative intensity of the Si(2p) features due to this species, and the angular dependence of the XPS peaks indicate that they result from a ML of Si at the outermost surface layer, on top of the growing Si 3 N4 film. INTRODUCTION AND EXPERIMENTAL Future scaled VLSI devices will require very thin (5100 A) gate insulators of high quality. The particular material requirements for these thin insulators have been discussed recently by Moslehi and Saraswat [1]. In this paper, these authors also review the preparation and properties of thin Si 3 N4 films which are a promising alternative to SiO 2 as device dimensions are reduced. Generally it has been found that the growth of Si 3 N4 thin films by the thermal nitridation of Si with dinitrogen (N2 ) or ammonia (NH3 ) is initially rapid, followed by a marked decrease in rates until growth nearly stops at a maximum thickness of about 40 A [1-5]. This behavior can, in fact, be useful in controlling the thickness of insulator films based on Si3N4 [1]. To understand the growth and chemical state of such films in more detail, we have performed kinetic and spectroscopic studies of the thermal nitridation of Si(100) using hydrazine (N2 H4 ) or NH 3 as the gas source for nitrogen. (We have previously found [6] that initial growth with N2 H4 is significantly more rapid than with NH In this short 3 .) report, we present the results of XPS studies and briefly describe some of the kinetic data [7]. In addition, we compare and contrast the kinetic results for thermal nitridation with those previously measured for the thermal oxidation of Si under "dry" (02) and "wet" (H20) conditions in order to compare the mechanism of reaction for oxide and nitride film growth. Both n- and p-type, doped Si(100) single crystals (20o-cm resistivity) from commercially a
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