Growth Rate and Characterization of Silicon Oxide Films Grown in N20 Atmosphere in a Rapid Thermal Processor

A rapid thermal oxidation process of silicon in N20 ambient was investigated using a commercially available reactor with a fixed wafer position and a gas flow parallel to the wafer surface. For such a configuration, thickness uniformities in the 2% range were obtained for the first time. The oxidation rate as a function of process temperature and time was investigated. A retardation in the N20 oxidation rate as compared to the oxidation in O2 ambient is explained by the formation of a nitrided interracial layer. A comparison of experimental results with an oxidation model calculation shows that this interface can affect either the oxidant diffusivity through the oxide or the reaction rate at the silicon surface. Infrared spectra of nitrided oxide films reveal a regular arrangement in the Si-O network, similar to that of high quality thermally grown oxides. A vibrational contribution to that spectrum from a Si-O-N subnetwork is displayed. The accumulated charge to breakdown on metal-oxide-semiconductor capacitors as a function of the injected current density revealed different slopes for oxides either thermally grown or grown in N20. Hence, the projected lifetime for devices with N20 grown oxide under low operating fields is extended by one order of magnitude in comparison with th e thermal oxide. Since complementary metal-oxide-semiconductor (CMOS) integrated circuits are scaled down into the submicron region, thermally grown SiO2 gate dielectrics show increasingly technological and reliability problems in the very thin thickness range. In this regard there are two major issues affecting the performance of MOS field-effect transistors (FETs). First, boron penetration from the p*gate through the SiO2-gate dielectric results in instabilities of the threshold voltage of p-channel MOSFETs. Second, if n-channel MOSFETs are subjected to high-field stress, a significant interface state generation will occur. The nitridation of SiO2 in NH3 atmosphere is the most reported method up to now to overcome these problems. 1-~ Unfortunately, hydrogen incorporation during the ni tr idation process introduces a large number of electron traps in the film. 4'~ Subsequent reoxidation reduces both the electron trapping and interface state generation, but some problems related to this multistep process still remain. 6'7 Nitridation of SiO2 in N20 atmosphere was also investigatedY The problem of incorporated hydrogen or OH groups is avoided, since only nitrogen can be built into the dielectric layer. The results are quite promising for the fabrication of future quarter-micron devices, but extended experimental results have to be obtained. A simple one-step process is the oxidation of silicon in N20 atmosphere either in a conventional furnace 8'~~ or in a rapid thermal processing (RTP) system. ~1-~ All reported data on these so-called N~O oxides show that electrical reliability and blocking properties are improved. ~8 However, an N20 rapid thermal oxidation (RTO) process was reported in Ref. 11 to produce high nonuniformities in the range of 10 to 19% (percentage standard deviation, 1 ~) in film thickness and composition. An improved thickness uniformity value of 7 % was reported recently, ~ but the nitrogen concentration was low (0 to 1%) in this case. These published data were observed in "conventional" RTP reactors with horizontal gas flow and fixed wafer positions. Recently, good thickness uniformities (< 5%) were reported for a modified RTP system, z7 where the wafer in the reactor chamber can be rotated in a vertical gas flow system. In this work an RTO process in N20 ambient with excellent thickness uniformities is presented. These values were achieved in a commercially available conventional RTP reactor. For the characterization of the growth process a temperature study was performed. The oxide quality and nitrogen incorporation was investigated using Fourier transform infrared (FTIR) and secondary ion mass spectroscopy (SIMS). To study the hot carrier immunity timedependent dielectric breakdown (TDDB) measurements were carried out on MOS capacitors. J. Electrochem. Soc., Vol. 141, No. 1, January 1994 9 The Electrochemical Society, Inc. Experimental The RTP system (AST-100) used for this study is designed to process single wafers with a diameter of 100 mm. With tungsten halogen lamps the wafer can be heated rapidly (1 to 400~ to a steady-state temperature of up to 1300~ The cooling rate is 70~ for a starting temperature of 1100~ In RTP systems the problems are mainly the temperature uniformity, the compensation of wafer edge effects, and the measurement and calibration of the temperature. ~ From the manufacturer a system was chosen with double-sided i l lumination of the wafer in a highly reflective water-cooled chamber in which an air-cooled coldwall quartz reactor is located. A schematic of the lampheated quartz reactor is shown in Fig. 1. The 21 lamps are arranged in two parallel banks with the rectangular quartz tube between them. It is possible to control the lamp power separately for six units consisting of three or four lamps. The temperature is measured with a pyrometer fixed under the reactor. The calibration of the pyrometer is carried out using a thermocouple-instrumented wafer. There are eight openings for the gas inlet at the front side of the reactor chamber, which shall insure a homogenous flow of the process gas across a wafer. The gas outlet is located at the back side of the chamber. The sample is placed on a quartz holder with three quartz pins. To compensate for temperature effects at the edge of a wafer, a silicon reflection ring can be used. The best oxide thickness uniformity was obtained after a fine tuning of the heating lamps and adjusting the silicon reflection ring on a level 1 mm below the silicon wafer surface as shown in Fig. 1. The oxidations were carried out on n-type wafers with a resistivity of 5-10 ~ cm, <100> orientation and 4 in. in size. The wafers were freshly etched in concentrated HF and dried with nitrogen or directly transferred from the storage box into the reactor chamber. Thin nitrided oxide layers were then grown in undiluted ultrapure N20, pure oxide layers in 02, with a flow rate of 5 liter/min. To understand the growth dynamics in an RTP system, a temperature range of 900 to 1150~ was covered. We determined the film thickness by ellipsometry, assuming an SiO2 bulk layer with a refractive index of 1.475. The deviation in film thickness which appears if the nitrided interface layer is