单晶硅表面钝化层在低能电子辐照前后的SIMS和XPS分析

为了研究低能电子辐照对单晶硅器件表面钝化材料中产生的化学微结构的变化,在轻掺杂P型单晶硅基底上制作了三种表面钝化膜,分别是单一SiO₂钝化膜、SiO₂/Si₃N₄复合钝化膜、硼硅玻璃/Si₃N₄复合钝化膜,开展了表面钝化单晶硅在最大能量70 keV的加速器电子束下的辐照实验。样品在空气气氛下辐照6 h,用二次离子质谱(SIMS)测试了辐照前后三种表面钝化膜中Si、N、B的纵深变化,同时用Ar离子刻蚀X射线表面光电子能谱(XPS)对Si元素的化学结合状态进行测试分析。结果表明:对单一SiO₂钝化的轻掺杂P型材料,辐照在SiO₂/Si界面产生明显的材料结构变化,界面附近的SiO₂不再是完整化学计量比,而是SiO_x(x<2);对SiO₂/Si₃N₄复合钝化的轻掺杂P型材料,辐照对SiO₂     

Experimental Observations of the Chemistry of the SiO2/Si Interface

Changes in silicon surface preparation prior to thermal oxidation are shown to leave a signature by altering the final SiO2/Si interface structure. Surface analytical techniques, including XPS, static SIMS, ion milling, and newly developed wet-chemical profiling procedures are used to obtain detailed information on the chemical structure of the interface. The oxides are shown to be essentially SiO2 down to a narrow transitional interface layer (3-7 ?). A number of discrete chemical species are observed in this interface layer, including different silicon bonds (e.g., C-, OH-, H-) and a range of oxidation states of silicon (0 ? +4). The effect of surface preparation and the observed chemical species are correlated with oxide growth rate, surface-state density, and flatband shifts after irradiation.     

Mechanisms of Charge Buildup in MOS Insulators

The radiation sensitivity of MOS devices has been recognized to be the result of a charge buildup caused by the sweepout of electrons and the trapping of holes following hole-electron pair production by ionizing radiation. Holes have been shown to have a finite, although small, mobility in thermally grown SiO2. Trapping of holes takes place near the Si-SiO2 interface, possibly by oxygen vacancies in the oxide. Normal thermally grown SiO2 possesses only small concentrations of electron traps. Electron traps have been shown to be generated, however, in the oxide by ion implantation, by irradiation with nonpenetrating electrons, and by exposure of the surface of the oxide to negative ions from a corona discharge. Although Na+ and Li+ ions have been shown to be mobile at room temperature in SiO2, contamination can be kept to levels where ionic charge buildup is negligible. The role of contaminants in the formation of hole traps, however, remains to be determined.     

The Influence of Silicon Surface Defects on MOS Radiation-Sensitivity

The relationships of the electrical properties of irradiated MOS structures to processed-induced surface defects have been investigated. It has been found that the radiation-induced perturbations in oxide space-charge and interface states relate directly to the density of oxidation-induced stacking faults and edge dislocations. The density of such surface defects depends on both the structural properties of the starting silicon and the fabrication procedures. Oxidation-induced stacking faults are strongly related to the oxidation conditions, such as temperature, time, and ambiance. High-frequency CV, quasi-static CV, and ac conductance techniques were used to determine the interface state densities and flat-band voltage distributions. It has been found that, in addition to interface states, exposure to ionizing radiation causes gross non-uniformities in trapped positive space-charge. Using secondary ion mass spectrometry (SIMS) on an MOS sample possessing a high density of stacking faults, it has been shown that positive charge-species drift to the SiO2/Si interface and form clusters in a random fashion similar to that of the defects delineated by differential etching. Based on the known relationship of surface generation lifetime on surface defects pre-irradiation lifetime has been demonstrated as an effective electrical parameter for the prediction of the total-dose radiation sensitivity of MOS devices.     

AN ABNORMAL BEHAVIOR OF NANO—CRYSTALLINE IN SiO2 SUBSTRATE

Temperature dependence of the hyperfine field of Fe nonocrystalline in SiO2 matrix prepared by using an ion implantation and subsequent heat treatment can not be described by a formula for the isolated Fe nanocrystalline.The large stresses or chemical bond force to which interface atoms may be subjected lead to increase effective anisotropic constant.In addition a little concentration of oxygen atoms contained in the crystalline might enhance the byperfine field.     

X-ray irradiation of ion-implanted MOS capacitors

He⁺ ion-implanted metal-oxide-semiconductor (MOS) capacitors with two different oxide thickness have been irradiated by X-rays and the depth distribution of the implant damage in the Si–SiO₂ structures have been examined. The efficiency of X-ray annealing of electronic traps caused by implantation and changes in charge populations are reported. The experiment shows that (in the case when defects introduced by implantation are located at the Si–SiO₂ interface) only defects corresponding to the deep levels in the Si can be affected by X-ray irradiation. When defects introduced by ion implantation are located deeper within the Si substrate complete annealing of these defects is observed.     

Radiation-Induced Hole Trapping and Interface State Characteristics of Al-Gate and Poly-Si Gate MOS Capacitors

Using the techniques of thermally stimulated current and high-frequency C-V measurements, the device processing dependence of radiation-induced hole traps and interface states in MOS capacitors has been investigated. It was found that the fabrication of poly-Si gates caused all investigated gate oxides to resemble those grown by pyrogenic techniques. The role of postoxidation annealing (POA) in hole trap formation and interface state generation was investigated. The measurements imply that POA causes a "weakening" of bonds in SiO2 which can become hole traps, and, in the presence of OH, lead to the annihilation of certain types of hole traps while generating new types of hole traps and interface states. Possible models and interface state distributions are discussed. Midgap voltage shifts were found not to be generally valid measures of hole trapping in these MOS devices.     

Interface-State Generation in Thick SiO2 Layers

Evidence is presented that interface states are produced by radiation in some MOS structures with thick SiO2 layers (e.g., field oxides) by a mechanism different from the slow two-stage process previously identified in thinner radiation-hardened gate oxides. Production of interface states by the new process occurs promptly after irradiation, is independent of oxide field and polarity, and takes place with nearequal efficiency at room temperature and 77 K.     

Permanent Radiation Effects in Hardened Al2O3 MOS Integrated Circuits

Complementary-symmetry metal-oxide semiconductor (COS/MOS) inverter circuits have been fabricated with aluminum oxide as channel insulator. This oxide was formed either by plasma-anodization of Al or by vapor deposition at 450°C using Al-isopropoxide. The RCA CD-4007 circuit configuration was used for these studies. The inverters were subjected to ionizing radiation with the applied gate biases encountered under normal operation (0 and +10 volts). For all units tested, the shifts in the inverter characteristics were due to shifts in the threshold voltages of the individual transistors. No measurable deterioration in gain due to interface state generation was observed. These results confirm the expected radiation hardness measured previously with MOS capacitors. Based on the net voltage shifts in the transfer characteristics of the complementary inverters, plasma-grown A12O3 shows an increase of more than an order of magnitude in hardness over units with SiO2 channel oxide. The hardness of units with deposited A12O3 was found to be intermediate to these. The increased hardness of A12O3 over SiO2 is attributed, at least in part, to the presence of trapping centers for negative charge. This serves to balance the positive trapped oxide charge so that a considerable reduction in the net oxide charge is attained.     

Hole Transport and Recovery Characteristics of SiO2 Gate Insulators

The transient response, or flat-band voltage recovery, in a number of pedigreed SiO2 gate insulator MOS capacitors following exposure to a pulsed 13-MeV electron beam was studied as a function of time, temperature, and applied bias. A quantitative comparison of the response characteristics of the different oxides is made. It is found generally that the response consists of two stages. The first (early time) stage in most cases encompasses most of the recovery and is dominated by hole transport through the insulator film to the interface. This hole transport recovery stage in all the oxides studied is well described by the stochastic model of hopping transport based on a continuous time random walk used previously in the analysis of hole transport in SiO2. It is further shown that all the main features of the hole transport are consistent with a model of the holes moving via polaron hopping between localized sites in the SiO2. The second or long-term stage of recovery commences after cessation of the hole transport in the oxide and is found to vary significantly in its importance and time of onset among the various oxides. This stage involves a radiation-induced buildup of interface states and possibly, in some cases, an annealing of a trapped-hole distribution near the SiO2/Si interface.