MOS characteristics of ultrathin SiO2 prepared byoxidizing Si in N2O

MOS characteristics of ultrathin gate oxides prepared by furnace oxidizing Si in N₂O have been studied. Compared to control oxides grown in O₂, N₂O oxides exhibit significantly improved resistance to charge trapping and interface state generation under hot-carrier stressing. In addition, both charge to breakdown and time to breakdown are improved considerably. MOSFETs with N₂O gate dielectrics exhibit enhanced current drivability and improved resistance to g_m degradation during channel hot-electron stressing     

Effects of residual surface nitrogen on the dielectric breakdowncharacteristics of regrown oxides

Effects of residual surface nitrogen, remaining on the Si surface after stripping off tunneling oxynitrides (N₂O-grown or NH ₃-nitrided oxides), on the quality of the regrown gate oxides are studied. Residual surface nitrogen is observed to reduce the breakdown field and degrade the time-dependent dielectric breakdown (TDDB) characteristics of the subsequently grown gate oxides. Results show that oxide regrowth in N₂O, rather than O₂, can significantly suppress these undesirable effects     

High-field-induced leakage in ultrathin N2O oxides

Stress-induced leakage current (SILC) is studied in ultrathin (~50 Å) gate oxides grown in N₂O or O₂ ambient, using rapid thermal processing (N₂O oxide or control oxide, respectively). MOS capacitors with N₂O oxides exhibit much suppressed SILC compared to the control oxide for successive ramp-up, constant voltage DC, and AC (bipolar and unipolar) stresses. The mechanism for SILC is discussed, and the suppressed SILC in N₂O oxide is attributed to suppressed interface state generation due to nitrogen incorporation at the Si/SUO₂ interface during N₂O oxidation     

Effects of growth temperature on TDDB characteristics of N2O-grown oxides

Effects of oxide growth temperature on time-dependent dielectric breakdown (TDDB) characteristics of thin (115 Å) N₂O-grown oxides are investigated and compared with those for conventional O₂-grown SiO₂ films with identical thickness. Results show that TDDB characteristics of N₂O oxides are strongly dependent on the growth temperature and, unlike conventional SiO₂, TDDB properties are much degraded for N ₂O oxides with an increase in growth temperature. Large undulations at the Si/SiO₂ interface, caused by locally retarded oxide growth due to interfacial nitrogen, are suggested as a likely cause of degradation of TDDB characteristics in N₂O oxides grown at higher temperatures     

Rapid thermal anneal of gate oxides for low thermal budget TFT's

The performance and reliability of deposited gate oxides for thin film transistors (TFT's) has been studied as a function of rapid thermal annealing (RTA) conditions. The effect of temperature ranging from 700 to 950°C and the annealing ambients including oxygen (O₂), argon (Ar), and nitrous oxide (N₂O) is investigated. Improvement in charge to breakdown (Q_bd) is seen starting from 700°C, with marked increase at 900°C temperature and above. The N₂O and Ar ambients result in higher Q_bd compared to O₂ ambient and we attribute this to reduced interfacial stress. Fourier Transform Infrared spectroscopy (FTIR) is used to qualitatively measure the stress. The bias temperature instability is decreased by RTA. The TFT characteristics are significantly improved with RTA gate oxide. The RTA-Ar anneal at 950°C results in the lowest trap density in TFT's as measured from charge pumping technique     

High quality gate dielectrics grown by rapid thermal processingusing split-N2O technique onstrained-Si0.91Ge0.09 films

Thermal stability and strain relaxation temperature of strained Si _0.91Ge_0.09 layers has been investigated using double crystal x-ray diffraction (DCXRD). High quality gate oxynitride layers rapid thermally grown on strained Si_0.91Ge_0.09 using N₂O and the split N₂O cycle technique below the strained relaxed temperature is reported. A positive fixed oxide charge density was observed for N₂O and split-N₂ O grown films. The O₂ grown films exhibit a negative fixed oxide charge. The excellent improvements in the leakage current, breakdown field and charge-to-breakdown value of the N₂O or split-N₂O grown films were achieved compared to pure O₂ grown films     

P-channel MOSFET's with ultrathin N2O gate oxides

The performance and reliability of p-channel MOSFETs utilizing ultrathin (~62 Å) gate dielectrics grown in pure N₂O ambient are reported. Unlike (reoxidized) NH₃-nitrided oxide devices, p-MOSFETs with N₂O-grown oxides show improved performance in both linear and saturation regions compared to control devices with gate oxides grown in O₂. Because both electron and hole trapping are suppressed in N₂O-grown oxides, the resulting p-MOSFETs show considerably enhanced immunity to channel hot-electron and -hole-induced degradation (e.g., hot-electron-induced punchthrough)     

High-field breakdown in thin oxides grown in N2O ambient

A detailed study of time-dependent dielectric breakdown (TDDB) in N₂O-grown thin (47-120 Å) silicon oxides is reported. A significant degradation in breakdown properties was observed with increasing oxide growth temperatures. A physical model based on undulations at the Si/SiO₂ interface is proposed to account for the degradation. Accelerated breakdown for higher operating temperatures and higher oxide fields as well as thickness dependence of TDDB are studied under both polarities of injection. Breakdown under unipolar and bipolar stress in N₂O oxides is compared with DC breakdown. An asymmetric improvement in time-to-breakdown under positive versus negative gate unipolar stress is observed and attributed to charge detrapping behavior in N₂O oxides. A large reduction in time-to-breakdown is observed under bipolar stress when the thickness is scaled below 60 Å. A physical model is suggested to explain this behavior. Overall, N₂O oxides show improved breakdown properties compared with pure SiO₂     

Effect of postoxidation annealing on the reliability of rapidthermal thin gate oxides

The reliability of thin gate oxides grown by rapid thermal oxidation in O₂ followed by one and two step postoxidation annealing (POA) in N₂ was studied. The one step POA was carried out by switching O₂ into N₂ immediately after oxidation without changing temperature, while the two step POA was cooled down first and subsequently heated to the same temperature as oxidation in N₂. It was experimentally observed that the oxide thickness increases significantly with the POA time in one step POA, while the oxide thickness shows very little change during two step POA. The interfacial properties and the oxide breakdown endurance can be improved by the two step POA. Also, the radiation hardness of oxide is less degraded by the two step POA than by one step POA. The effect of oxide thickness variation due to POA is chiefly responsible for the observation and is important to thin gate oxides     

Furnace nitridation of thermal SiO2 in pureN2O ambient for ULSI MOS applications

Furnace nitridation of thermal SiO₂ in pure N₂ O ambient for MOS gate dielectric application is presented. N₂O-nitrided thermal SiO₂ shows much tighter distribution in time-dependent dielectric breakdown (TDDB) characteristics than thermal oxide. MOSFETs with gate dielectric prepared by this method show improved initial performance and enhanced device reliability compared to those with thermal gate oxide. These improvements are attributed to the incorporation of a small amount of nitrogen (~1.5 at.%) at the Si-SiO₂ interface without introducing H-related species during N₂O nitridation     

Post-polysilicon gate-proces-induced degradation on thin gate oxide

The post-polysilicon gate-process-induced degradation on the underlying gate oxide is studied. The degradation includes an increase in the electron trapping rate and a decrease in the charge-to-breakdown, Q_bd, of the gate oxide. It is found that N₂O nitrided gate oxide is more robust than O₂ gate oxide in resisting the degradation. Also, to grow a thin polyoxide on the polysilicon-gate in N₂O rather than in O₂ lessens the degradation on the underlying gate oxide. It is nitrogen, which diffuses through the polysilicon gate and piles up at both polysilicon/oxide and oxide/silicon-substrate interfaces, that improves the oxide quality for the N₂O process     

Thin fluorinated gate dielectrics grown by rapid thermal processingin O2 with diluted NF3

The authors report the application of rapid thermal processing (RTP) to the fabrication of ultrathin (~10 nm) high-quality fluorinated oxides in O₂+NF₃ (100 ppm diluted in N₂). NF₃ was used as the F source gas and was introduced either prior to rapid thermal oxidation (RTO) or with O₂ during the initial stage of RTO. The oxidation rate was enhanced because of the presence of NF₃. In addition, F depth profiles in fluorinated oxides were dependent upon the process conditions. The electrical characteristics of MOS capacitors have been studied and correlated with the chemical properties. The initial interface state density (D_t) was found to decrease with F incorporation. The results suggest that the interfacial F incorporation plays a major role in determining the interface hardness for both hot-electron and radiation damages     

MOS characteristics of fluorinated gate dielectrics grown by rapidthermal processing in O2 with diluted NF3

Rapid thermal processing (RTP) was applied to the fabrication of the ultrathin (~10 nm) high-quality fluorinated oxides in O₂+NF₃. NF₃ (diluted in N₂) was used as the F source gas and was introduced either prior to rapid thermal oxidation (RTO) or with O₂ during the initial stage of RTO. The electrical characteristics of MOS capacitors have been studied and correlated with the chemical properties. It was found that SiO₂ with a small amount of F incorporated shows reduced interface state generation under F-N injection, whereas excessive F incorporation is detrimental     

Thickness dependence of boron penetration through O2 and N2O-grown gate oxides and its impact on threshold voltagevariation

We report on a quantitative study of boron penetration from p⁺ polysilicon through 5- to 8-nm gate dielectrics prepared by rapid thermal oxidation in O₂ or N₂O. Using MOS capacitor measurements, we show that boron penetration exponentially increases with decreasing oxide thickness. We successfully describe this behavior with a simple physical model, and then use the model to predict the magnitude of boron penetration, N_B, for thicknesses other than those measured. We find that the minimum t_ox required to inhibit boron penetration is always 2-4 nm less when N₂O-grown gate oxides are used in place of O₂- grown oxides. We also employ the boron penetration model to explore the conditions under which boron-induced threshold voltage variation can become significant in ULSI technologies. Because of the strong dependence of boron penetration on t_ox, incremental variations in oxide thickness result in a large variation in N_B , leading to increased threshold voltage spreading and degraded process control. While the sensitivity of threshold voltage to oxide thickness variation is normally determined by channel doping and the resultant depletion charge, we find that for a nominal thickness of 6 nm, threshold voltage control is further degraded by penetrated boron densities as low as 10¹¹ cm^-2     

Electrical properties of composite gate oxides formed by rapidthermal processing

In this study, oxide stacks formed by combinations of rapid thermal chemical vapor deposition and rapid thermal oxidation have been investigated as gate dielectrics. This was achieved by performing various types of in situ rapid thermal oxidations both prior to and after oxide deposition to form composite stacked structures. The oxidation ambient and temperature was varied to study the effect on electrical properties such as mobility, leakage current, charge trapping, breakdown and hot carrier degradation. It was found that pre-oxidation prior to depositing an oxide results in a composite structure that greatly reduces the defect density by mismatching pores and weak spots in each film. The mobility behavior of these films was also found to be improved over as-deposited oxides. Post-deposition oxidation in O₂ and N₂O was also found to improve the mobility characteristics. Additionally, post-annealing in N₂ O was effective in improving the reliability of deposited oxides. These N₂O annealed films had low interface trap densities, improved high field mobility, very low charge trapping characteristics and enhanced resistance to hot carrier induced interface state generation. These improvements are attributed to 1) the presence of nitrogen at the interface and 2) to the reduction of nitrogen and hydrogen concentrations in the bulk of the oxide. The role of atomic oxygen during the post-anneal in N₂O is discussed along with differences in annealing ambients     

Interface properties of NO-annealed N2O-grown oxynitride

The oxide/Si interface properties of gate dielectric prepared by annealing N₂O-grown oxide in an NO ambient are intensively investigated and compared to those of O₂-grown oxide with the same annealing conditions. Hot-carrier stressings show that the former has a harder oxide/Si interface and near-interface oxide than the latter. As confirmed by SIMS analysis, this is associated with a higher nitrogen peak concentration near the oxide/Si interface and a larger total nitrogen content in the former, both arising from the initial oxidation in N₂O instead of O₂     

Influence of annealing ambient on GaAs oxide prepared by the liquid phase method

The characteristics of GaAs native oxides prepared by the liquid phase chemical-enhanced oxidation technique annealed at various ambiences including N₂, O₂, and mixture of N₂ (85%) and H₂ (15%) are investigated. The annealing temperatures range from 300 to 700 °C. The shrinkage of oxide film thickness, the increase of refractive index, the decrease of surface roughness, the enhancement of breakdown field strength and the reduction of leakage current have been obtained for all annealing conditions, except for annealing in the ambient atmosphere of N₂ or O₂ at temperature of 700 °C. It is found that annealed oxide films exhibited better thermal stability in an atmosphere of N₂/H₂ up to an annealing temperature of 700 °C. This is due to the existence of H atoms in the oxide films as demonstrated by SIMS depth profiles.     

Electrical properties of MOSFET's with N2O-nitridedLPCVD SiO2 gate dielectrics

Electrical properties of MOSFETs with gate dielectrics of low-pressure chemical-vapor-deposited (LPCVD) SiO₂ nitrided in N₂O ambient are compared to those with control thermal gate oxide. N₂O nitridation of CVD oxide, combines the advantages of interfacial oxynitride growth and the defectless nature of CVD oxide. As a result, devices with N₂O-nitrided CVD oxide show considerably enhanced performance (higher effective electron mobility), improved reliability (reduced charge trapping, interface state generation, and transconductance degradation), and better time-dependent dielectric breakdown (TDDB) properties (t_BD ) compared to devices with control thermal oxide     

Time-dependent dielectric breakdown characteristics ofN2O oxide under dynamic stressing

Time-dependent dielectric breakdown (TDDB) characteristics of MOS capacitors with thin (120-Å) N₂O gate oxide under dynamic unipolar and bipolar stress have been studied and compared to those with control thermal gate oxide of identical thickness. Results show that N₂O oxide has significant improvement in t _BD (2×under-V_g unipolar stress, 20×under+V_g unipolar stress, and 10×under bipolar stress). The improvement of t_BD in N₂O oxide is attributed to the suppressed electron trapping and enhanced hole detrapping due to the nitrogen incorporation at the SiO₂/Si interface     

AC hot-carrier effects in MOSFETs with furnaceN2O-nitrided gate oxides

AC hot-carrier effects in n-MOSFETs with thin (~85 Å) N₂O-nitrided gate oxides have been studied and compared with control devices with gate oxides grown in O₂. Results show that furnace N₂O-nitrided oxide devices exhibit significantly reduced AC-stress-induced degradation. In addition, they show weaker dependences of device degradation on applied gate pulse frequency and pulse width. Results suggest that the improved AC-hot-carrier immunity of the N₂O-nitrided oxide device may be due to the significantly suppressed interface state generation and neutral electron trap generation during stressing