Time dependent breakdown of ultrathin gate oxide

Time dependent dielectric breakdown (TDDB) of ultrathin gate oxide (<40 Å) was measured for a wide range of oxide fields (3.4<|E_ox|<10.3 MV/cm) at various temperatures (100⩽T⩽342°C). It was found that TDDB of ultrathin oxide follows the E model. It was also found that TDDB t₅₀ starts deviating from the 1/E model for fields below 7.2 MV/cm. Below 4.8 MV/cm, TDDB t₅₀ of intrinsic oxide increased above the value predicted by the E model obtained for fields >4.8 MV/cm. The TDDB activation energy for this type of gate oxide was found to have linear dependence on oxide field. In addition, we found that γ (the field acceleration parameter) decreases with increasing temperature. Furthermore, it was found that testing at high temperatures (up to 342°C) and low electric field values did not introduce new gate oxide failure mechanism. It is also shown that TDDB data obtained at very high temperature (342°C) and low fields can be used to generate TDDB model at lower temperatures and low fields. Our results (an enthalpy of activation of 1.98 eV and dipole moment of 12.3 eÅ) are in complete agreement with previous results by McPherson and Mogul. Additionally, it was found that TDDB is exponentially dependent on the gate voltage     

Field and temperature dependence of TDDB of ultrathin gate oxide

The results of an investigation of time-dependent breakdown (TDDB) of intrinsic ultrathin gate oxide are presented for a wide range of oxide fields 4.6ox<10.4 MV/cm at elevated temperatures. It was found that TDDB of ultrathin oxide follows the E model down to 4.6 MV/cm. The data show that TDDB t₅₀ starts deviating from the 1/E model for fields below 7.2 MV/cm. The data also show that the TDDB activation energy for this type of gate oxide is linearly dependent on oxide field. In addition, we show that the field acceleration parameter γ decreases as temperature increases     

Field Acceleration Model for TDDB: Still a Valid Tool to Study the Reliability of Thick SiO2-Based Dielectric Layers?

The aim of this paper is to investigate the reliability of thick oxides that are dedicated to the power integrated device fabrication. The field dependence of defect-related time-dependent dielectric breakdown (TDDB) mode over a wide range of oxide thickness T_OX and electric field E, using multiple wafer fabrication lots, is investigated. TDDB tests are conducted under constant current injection using structures with different areas. For that, we have applied a new electric field model based on a 1/E model (reciprocal field dependence) that is proposed recently in the literature. We show that when the dielectric thickness increases, this electric field acceleration model gives an erroneous prediction of the long-term reliability. We conclude that it is too early to give a generalized law of dielectric to predict their reliability without taking into account the influence of thicknesses. Consequently, the 1/E model may therefore have to be revised.     

Electric field dependence of TDDB activation energy in ultrathin oxides

A study of the electric field dependence of the TDDB activation energy is presented for 12 nm down to 4.7 nm thin oxides. It is shown that the TDDB activation energy depends linearly on the stress electric field and that this behavior depends strongly on the oxide thickness. Moreover, a relationship between the TDDB activation energy attenuation per MV/cm and the oxide thickness has been found. As will be demonstrated, these results are of great importance for the rigorous estimation of the oxide lifetime of both present and future technologies.     

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     

Stress voltage polarity dependence of thermally grown thin gateoxide wearout

Gate oxide wearout for thermally grown 57-190-A SiO₂ films in a polycrystalline silicon-SiO₂-Si structure prepared on n-type and p-type wafers was studied by examining time-dependent dielectric breakdown (TDDB) under 1-mA/cm² constant current with positive and negative voltages at 250°C. TDDB lifetimes for positive voltage stress are more than one order longer than those for negative voltage stress. TDDB lifetimes depend on oxide thickness, that is, they increase for positive voltage stress and decreases for negative voltage stress with decreasing oxide thickness. They also depend on whether the oxide films are prepared on n-type or p-type wafers. After the positive voltage TDDB stress, negative charges are predominantly produced in the oxide layer, and the electric field at the cathode in the oxide film slightly decreases. On the contrary, after the negative voltage TDDB stress, positive charges are predominantly produced at the cathode in the oxide layer and the electric field at the cathode is built up, resulting in an increase in Fowler-Nordheim tunnel current flowing though the oxide film     

On the optimum thickness to test dielectric reliability, in an integrated technology of power devices

This paper deals with the extensive characterization of dielectric films with thicknesses from 20 to 65 nm. Thick dielectric reliability has been investigated with time dependent dielectric breakdown (TDDB). TDDB tests are conducted under constant current injection. Assuming that the logarithm of the median time-to-failure is described by a linear electric field dependence, a generalized empirical law for the long-term reliability of the dielectric is proposed. This law takes into account the applied electric field and the dielectric thickness. This reliability law is available for dielectric thicknesses greater than 10 nm. A procedure to test dielectrics of various thicknesses is given in order to predict their reliability in power integrated devices.     

Electrical conduction and dielectric breakdown in aluminum oxideinsulators on silicon

Leakage currents and dielectric breakdown were studied in MIS capacitors of metal-aluminum oxide-silicon. The aluminum oxide was produced by thermally oxidizing AlN at 800-1160°C under dry O₂ conditions. The AlN films were deposited by RF magnetron sputtering on p-type Si (100) substrates. Thermal oxidation produced Al ₂O₃ with a thickness and structure that depended on the process time and temperature. The MIS capacitors exhibited the charge regimes of accumulation, depletion, and inversion on the Si semiconductor surface. The best electrical properties were obtained when all of the AlN was fully oxidized to Al₂O₃ with no residual AlN. The MIS flatband voltage was near 0 V, the net oxide trapped charge density, Q_0x, was less than 10¹¹ cm ^-2, and the interface trap density, D_it, was less than 10¹¹ cm^-2 eV^-1, At an oxide electric field of 0.3 MV/cm, the leakage current density was less than 10^-7 A cm^-2, with a resistivity greater than 10 ¹² Ω-cm. The critical field for dielectric breakdown ranged from 4 to 5 MV/cm. The temperature dependence of the current versus electric field indicated that the conduction mechanism was Frenkel-Poole emission, which has the property that higher temperatures reduce the current. This may be important for the reliability of circuits operating under extreme conditions. The dielectric constant ranged from 3 to 9. The excellent electronic quality of aluminum oxide may be attractive for field effect transistor applications     

Time-dependent dielectric breakdown of SiO2 films in a wide electric field range

We have performed time dependent dielectric breakdown measurement of SiO₂ films in the electric field (E_OX) range 7–13.5 MV/cm and evaluated the electric field dependence of intrinsic lifetime, using both area and temperature dependences of oxide lifetime. We have evaluated the electric field dependence of time to breakdown (t_BD) below 125°C, because the activation energy of intrinsic lifetime changes at 125°C t_BD of 7.1 and 9.6 nm oxides is not proportional to exp(E_OX) but proportional to exp(1/E_OX). This suggests that the breakdown mechanism of 9.6 and 7.1 nm oxides is the same and adheres to the anode hole injection model. However, the breakdown mechanism of 4.0 nm oxides is not the same as that of 7.1 and 9.6 nm oxides. The slope of log(t_BD) versus 1/E_OX plot in 4.0 nm oxide increases with decreasing oxide fields. The intrinsic lifetime in the positive gate bias decreases with increasing oxide thicknesses in the range of electric fields employed in the present experiment.     

Gate-oxide breakdown accelerated by large drain current inn-channel MOSFET's

A study of the time-dependent dielectric breakdown (TDDB) of thin gate oxides in small n-channel MOSFETs operated beyond punchthrough is discussed. Catastrophic gate-oxide breakdown is accelerated when holes generated by the large drain current are injected into the gate oxide. More specifically, the gate-oxide breakdown in a MOSFET (gate length=1.0 μm, gate width-15 μm) occurs in ~100 s at an applied gate oxide field of ~5.2 MV/cm during the high drain current stress, while it occurs in ~100 s at an applied gate oxide field of ~10.7 MV/cm during a conventional time-dependent dielectric breakdown (TDDB) test. The results indicate that the gate oxide lifetime is much shorter in MOSFETs when there is hot-hole injection than that expected using the conventional TDDB method     

Mechanism of oxide thickness and temperature dependent current conduction in n+-polySi/SiO2/p-Si structures — a new analysis

The conduction mechanism of gate leakage current through thermally grown silicon dioxide (SiO₂) films on (100) p-type silicon has been investigated in detail under negative bias on the degenerately doped n-type polysilicon (n⁺-polySi) gate. The analysis utilizes the measured gate current density J_G at high oxide fields E_ox in 5.4 to 12 nm thick SiO₂ films between 25 and 300℃. The leakage current measured up to 300℃ was due to Fowler–Nordheim (FN) tunneling of electrons from the accumulated n⁺-polySi gate in conjunction with Poole Frenkel (PF) emission of trapped-electrons from the electron traps located at energy levels ranging from 0.6 to 1.12 eV (depending on the oxide thickness) below the SiO₂ conduction band (CB). It was observed that PF emission current I_PF dominates FN electron tunneling current I_FN at oxide electric fields E_ox between 6 and 10 MV/cm and throughout the temperature range studied here. Understanding of the mechanism of leakage current conduction through SiO₂ films plays a crucial role in simulation of time-dependent dielectric breakdown (TDDB) of metaloxide–semiconductor (MOS) devices and to precisely predict the normal operating field or applied gate voltage for lifetime projection of the MOS integrated circuits.     

Lifetime modeling of intrinsic gate oxide breakdown at high temperature

High resolution time-dependent dielectric breakdown tests are carried out on 7.2 nm gate oxide capacitors (n-type) in the electric field range 8.3–13.2 MV/cm at high temperatures (160–240 °C). It is proven that even at these high temperatures log(t_BD) is proportional to 1/E_OX and the time-to-breakdown mechanism matches the anode hole injection (AHI) model (1/E_OX model). In addition it is presented that the TDDB activation energy E_a for this type of gate oxide has linear dependence on stress electric oxide field.     

Dominant factors in TDDB degradation of Cu interconnects

The field acceleration factor (/spl gamma/) for the E-model of time-dependent dielectric breakdown (TDDB) in various Cu interconnect structures has been studied. The /spl gamma/ for pSiCN structures is larger than that of pSiN structures and independent of the kind of interlayer dielectric material or other processes used to make it. The relationship between the breakdown electric field strength (E/sub BD/) and the TDDB lifetime has been investigated. It has been demonstrated that the TDDB lifetime can be predicted from experimentally measured E/sub BD/ and /spl gamma/. An E/sub BD/ of at least 4.2 MV/cm is necessary to assure ten-year reliability under 0.2 MV/cm operation. Moreover, the important factors influencing the TDDB lifetime for Cu interconnects have been discussed. These include the Cu chemical-mechanical polishing (CMP), the post-CMP annealing, line edge roughness, and fine line effect.     

Temperature and electric field characteristics of time-dependentdielectric breakdown for silicon dioxide and reoxidized-nitrided oxides

TDDB characteristics of 150 Å reoxidized nitrided oxide (ONO) gate dielectrics were examined at temperatures from 77 K to 400 K. These ONO films were processed with different conditions of rapid thermal nitridation (RTN) and rapid thermal re-oxidation (RTO). Optimized ONO films show better Q_bd performance while maintaining a similar temperature and electric field dependence compared to SiO₂. The low temperature activation energy for ONO and SiO₂ is found to be strongly temperature dependent, and the charge to breakdown, Q_bd, is closely related to the electron trap generation/trapping rate rather than the amount of hole trapping for high field stress. To further verify the effect of hole trapping on TDDB, X-ray irradiation was applied to wafers at different process steps. The results clearly show that the amount of hole trapping does not correlate with the charge to breakdown     

Time-dependent breakdown of ultra-thin SiO2 gatedielectrics under pulsed biased stress

Ultra-thin SiO₂ films (t_ox~2.0 nm) were stressed under DC, unipolar, and bipolar pulsed bias conditions up to a pulse repetition frequency of 50 kHz. The time-to-breakdown (t_BD ), the number of defects at breakdown (N_BD), and the number of defects generated inside the oxide as a function of stress time were monitored during each stress condition. Oxide lifetime under unipolar pulsed bias is similar to that under DC conditions; however lifetime under bipolar pulsed bias is significantly improved and exhibits a dependence on pulse repetition frequency. The observation of a lifetime increase under bipolar pulsed bias for the oxide thickness and voltage range used in this study suggests that a different physical mechanism may be responsible for the lifetime increase from that assumed in earlier studies for thicker films     

Oxide thickness dependence of nitridation effects on TDDB characteristics

Wet oxide thicknesses dependence of nitridation effects on electrical characteristics, charge trapping properties and TDDB (Time Dependent Dielectric Breakdown) characteristics have been investigated. It is found that the difference of conduction current between the wet and nitrided wet oxide increases with increasing oxide thickness both for negative and positive bias to the gate until constant current stress is applied. After the stress, with decreasing oxide thickness both in wet and nitrided wet oxide leakage current increases. Up to 60 Å no difference was observed between the wet and nitrided wet oxide but at 50 Å nitrided wet oxide has less increase of current comparing to the wet oxide for the same stress. In wet oxide with increasing stress current density initial hole trap decreases but electron trap increases whereas in nitrided wet oxide has less initial hole trap and also electron trap is less comparing to the wet oxide. Both in wet and nitrided wet oxide for negative bias stress, time to 50 % breakdown decreases with decreasing thickness but at 50 Å a turn-around effect was observed due to nitridation i.e., the 50 % breakdown time is greater for nitrided wet oxide comparing to the wet oxide. On the contrary, for positive bias stress 50 % breakdown time increases with decreasing oxide thickness both in wet and nitrided wet oxide. For positive bias also a turn-around effect is observed at 50 Å i.e., 50% breakdown time is less in nitrided wet oxide comparing to the wet oxide. The improved reliability of nitrided wet oxide at the thin region of 50 Å seems to be due to the increase of more Si---N bond to the interface of oxide and Si comparing to the thick oxide of above 60 Å for the same nitridation conditions.     

Electrical characteristics of highly reliable ultrathin hafniumoxide gate dielectric

Electrical and reliability properties of ultrathin HfO₂ have been investigated. Pt electroded MOS capacitors with HfO₂ gate dielectric (physical thickness ~45-135 Å and equivalent oxide thickness ~13.5-25 Å) were fabricated. HfO₂ was deposited using reactive sputtering of a Hf target with O₂ modulation technique. The leakage current of the 45 Å HfO₂ sample was about 1×10^-4 A/cm ² at +1.0 V with a breakdown field ~8.5 MV/cm. Hysteresis was <100 mV after 500°C annealing in N₂ ambient and there was no significant frequency dispersion of capacitance (<1%/dec.). It was also found that HfO₂ exhibits negligible charge trapping and excellent TDDB characteristics with more than ten years lifetime even at V_DD=2.0 V     

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     

A new empirical extrapolation method for time-dependent dielectric breakdown reliability projections of thin SiO2 and nitride–oxide dielectrics

The results of an investigation of time-dependent dielectric breakdown (TDDB) of thin gate oxide and nitride–oxide (N–O) films are presented for a wide range of fields and temperatures. It was found that TDDB of both gate oxide and N–O films followed a power-law dependence of mean value of average leakage current (I_avg). An empirical extrapolation model using average leakage current as a major parameter was proposed based on experimental results. This proposed lifetime model has been successful to predict dielectric reliability. It could continuously fit the entire breakdown data from both wafer level and module level stress. The extrapolation from wafer level data to module data was excellent. The power of current versus TDDB showed exponential dependence on oxide thickness. This proposed TDDB projection methodology also worked for N–O films with an abrupt current increase in the I–V curve at a certain voltage well below the breakdown voltage, while the conventional models clearly failed to fit all data from this region. The observation of TDDB dependence of the current may open a new window for oxide lifetime projections and provide some insights into the nature of oxide breakdown and its implications for reliability studies.     

Study of MOS gate dielectric breakdown due to drain avalanche breakdown

This work reports the effects of drain impact ionization injection on the gate dielectric breakdown. Results show that due to the high energy hot carrier injection, the gate oxide can break down twice at a low oxide electric field (<1.2 MV/cm). The first breakdown occurs simultaneously with the drain avalanche breakdown whereas the second breakdown occurs beyond the drain breakdown. It is further identified that the first gate oxide breakdown is governed by the thermionic emission of hot electrons at low oxide fields (<1.0 MV/cm) and by the scattering processes at higher oxide fields. The second breakdown is attributed to the Fowler–Nordheim (F–N) tunneling.