Room temperature reactions involving silicon dangling bond centers and molecular hydrogen in amorphous SiO2 thin films on silicon

When metal/oxide/silicon (MOS) devices are subjected to ionizing radiation, hot carrier damage, or any process in which charge carriers are present in the oxide, interface states are created at the Si/SiO, interfaceelm The presence of holes in the oxide is most effective in triggering the interface state generation process.879 Studies of the transient response of irradiated devices shows that most of the damage occurs in seconds to minutes after irradiation at room temperature.b6 Early studies of MOS radiation damage showed that the radiation tolerance of the devices was strongly affected by hydrogen in high-temperature processing steps.lM3 Studies of the interface state creation process4-6 have been interpreted by McLean.” In McLean’s model,” holes interact with the oxide to liberate a hydrogen species which then drifts to the Si/SiO, interface. At the interface, the hydrogen reacts to form silicon dangling bond interface state defects. The McLean model is clearly consistent with many experimental results.“6’11’12 Recent evidence suggests that molecular hydrogen may play a significant role in the radiation damage process. For example, the density of radiation induced interface states can be significantly enhanced by placing irradiated MOS devices in an ambient rich in molecular hydrogen.t3-I5 Although experimental evidence regarding hydrogen’s role in the radiation damage process is fairly compelling, it consists almost entirely of electronic measurements. There is little direct experimental evidence regarding the atomic scale structures involved in reactions of hydrogen and radiation damage centers in the silicon dioxide films on silicon. Reactions of particular interest would obviously involve SiO, hole trap sites and hydrogen and would proceed in a period of seconds to minutes at room temperature. In this letter we report evidence for several reactions involving E’ centers and hydrogen which take place when SiOZ films are exposed to hydrogen at room temperature. (The E’ center is an unpaired electron residing in an sp hybridized orbital of a silicon bonded to three oxygens;16 in thermal oxides in E’ is a hole trapped in an oxygen vacancy.“) These reactions take place within minutes after our very thick ( ~4800 A) oxide films are exposed to hydrogen. We think that these reactions may play a part in the MOS interface state radiation damage process for several reasons. ( 1) The E’ center is the dominant deep hole trap in MOS oxides;” (2) the reactions take place in a time scale which is of the correct order of magnitude expected in the damage process; (3) it is known that, or at least strongly suspected that, molecular hydrogen is present in irradiated MOS oxides; is (4) recent studies suggest that the radiation damage at the Si/SiO, interface may be triggered by a reaction involving hydrogen at the hole capture site.” Our study involves room temperature electron spin resonance (ESR) measurements of SIMOX (separation by implantation of oxygen) buried oxides. The 4800 A thick SIMOX buried oxides utilized in this study were prepared by implanting 1.8 x lOi8 oxygens/cm2 at an energy of 200 keV. The ion current density during deposition was 34 mA/cm2. The temperature of the substrate during implantation was 640 “C. The implant step was followed by a 5 h anneal at 1315 “C in an ambient of 99.5% Ar and 0.5% 0,. The silicon surface orientation was (100); the silicon is n-type with a resistivity of about 30 0 cm. A residual oxide layer formed by the high-temperature anneal, as well as the top silicon layer, were removed by etches in HF (oxide) and KOH (silicon) prior to the study. The etches were carried out at room temperature. After etching, the samples were cut into 3.5 mmX20 mm rectangles and, with the oxides protected, were subjected to a buffered HF etch at room temperature. This last etch removes mechanical edge damage. Earlier, we demonstrated that SIMOX buried oxides exhibit an extremely high ( 1018/cm3) density of E’ precursors.2o721 The density of E’ precursors is higher than that observed in thermal thin films. This increased E’ density makes it possible to observe several otherwise difficult to observe hydrogen E’ complexes. Although these oxides are not identical to the thermal SiO, films of MOS gate oxides, the reactions observed here are also likely to take place at some sites in thermal oxides. We subjected the Si/SiO, samples to -40 h of vacuum ultraviolet (hc/d<10.2 eV> irradiation from a deuterium lamp at room temperature. (A second set of samples were exposed to 210 Mrad of gamma irradiation from a Co6’ source. These gamma irradiated samples were not subjected to the KOH etch; they retained the 1 ,um silicon overlayer. The gamma irradiated samples and the VUV irradiated samples exhibit nearly identical ESR spectra.) A post-VUV irradiation trace is illustrated in Fig. 1 (a). A