Controlling the concentration and position of nitrogen in ultrathin oxynitride films formed by using oxygen and nitrogen
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ECR plasma. The UHV system is composed with main chamber, heater room and preparation chamber. The main chamber is separated from the heater room by quartz plate to prevent carbon contamination. The base pressure in the main chamber was about lxl0-9 Torr. Furthermore, the inside of the main chamber was covered by quartz to prevent metal contamination. The wafers were first annealed under UHV at a substrate temperature, Tu, of 900°C for 5 min in order to remove the native oxide layer and smooth the Si surface. The clean surface was then exposed at a TSub of 750'C to oxygen radicals or nitrogen radicals produced by the ECR plasma. A microwave discharge (2.45 GHz, 150 W) was applied to the gas stream at 5x 104 to 5x 10' Torr. Oxynitride films were formed in three ways: (a) by radical nitridation after radical oxidation, (b) by radical oxidation after radical nitridation, and (c) by supplying oxygen radicals and nitrogen radicals simultaneously. We then selected one of these radical oxidized nitride films produced by radical oxidation at 750'C for 30 min after radical nitridation at 750TC for 5 min and radical nitrided it again at 750'C for 30 min to make an oxynitride film with two
nitrogen-enriched layers: one at the polysilicon-SiO 2 interface, and the other at the SiO 2-Si interface. To investigate the structure of radical oxynitride films, we measured the oxynitride thickness by spectroscopic ellipsometry. To confirm the thickness determined ellipsometry, we also measured the thickness of some oxynitride films by making cross-sectional transmission electron microscope (TEM) images. The N concentration was estimated by using x-ray photoelectron spectroscopy (XPS). We measured the N Is, and 0 Is spectra of radical oxynitride films and then estimated the N concentration by using the following formula: IN [NSN xl00 [%1 [N] I i
(1)
+
SN
SO
Here IN,and Io are integration of N, and 0 spectra respectively, and SN, and So are the relative sensitivities of N, and 0 [11]. We also obtained a high-resolution depth profile of the sample by secondary ion mass spectroscopy (SIMS). A Cs' cluster was used as the primary bombarding species, the primary ion energy was set at 500 eV or at 1 keV, and the sputtering rate was less than 0.02 nm/sec. We also measured the interface roughness by atomic force microscopy (AFM). The AFM measurement was carried out in air after the oxynitride was removed by dipping the wafers in HF. RESULTS AND DISCUSSION The relation between the oxynitride thickness and the time the wafer was exposed to radical oxygen or radical nitrogen is shown in Fig. 1. In the radical nitridation after radical oxidation, we formed radical oxide at 5x10-3 Torr for 30 min and then nitrided it with radical nitrogen at 5x10 4 Torr for 5, 15, and 30 min. In the radical oxidation after radical nitridation, on the other hand, we formed radical nitride at 6x10 4 Torr for 30 min and then oxidized it with radical oxygen at 5x10 3 Torr for 5, 15, and 30 min. And when we supplied oxygen radicals and nitrogen radicals simultaneous
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