Plasma-induced Damage Research Articles

Preferential self-healing at grain boundaries in plasma-treated graphene.

Engineering of defects located in grains or at grain boundaries is central to the development of functional materials. Although there is a surge of interest in the formation, migration and annihilation of defects during ion and plasma irradiation of bulk materials, these processes are rarely assessed in low-dimensional materials and remain mostly unexplored spectroscopically at the micrometre scale due to experimental limitations. Here, we use a hyperspectral Raman imaging scheme providing high selectivity and diffraction-limited spatial resolution to examine plasma-induced damage in a polycrystalline graphene film. Measurements conducted before and after very low-energy (11-13 eV) ion bombardment show defect generation in graphene grains following a zero-dimensional defect curve, whereas domain boundaries tend to develop as one-dimensional defects. Damage generation is slower at grain boundaries than within the grains, a behaviour ascribed to preferential self-healing. This evidence of local defect migration and structural recovery in graphene sheds light on the complexity of chemical and physical processes at the grain boundaries of two-dimensional materials.

In situ surface analysis of an ion-energy-dependent chlorination layer on GaN during cyclic etching using Ar+ ions and Cl radicals

Gallium nitride (GaN) semiconductor devices must be fabricated using plasma etching with precise control of the etching depths and minimal plasma-induced damage on the atomic scale. A cyclic process comprising etchant adsorption and product removal may be suitable for this purpose but an understanding of the associated etching surface reactions is required. The present work examined the formation of a chlorinated layer based on Cl radical adsorption on a GaN surface in conjunction with Ar ion irradiation. This research employed beam experiments and in situ x-ray photoelectron spectroscopy. The results show that N atoms are preferentially desorbed during exposure to Ar ions to produce Ga-rich layers at depths of 0.8 and 1.1 nm at an Ar ion dosage on the order of 1016 cm−2 and ion energies of 116.0 and 212.6 eV, respectively. Subsequent exposure of the irradiated Ga-rich layer to Cl radicals removes some Ga atoms and produces a chlorinated layer over the GaN surface. This chlorinated layer has a thickness on the order of 1 nm following Cl radical dosages on the order of 1019 cm−2. This study of plasma-treated surfaces is expected to assist in developing means of controlling the etching depth during the atomic layer etching of GaN via Ar ion bombardment.

Effects of Ion Energy and Density on the Plasma Etching-Induced Surface Area, Edge Electrical Field, and Multivacancies in MoSe2 Nanosheets for Enhancement of the Hydrogen Evolution Reaction.

Plasma functionalization can increase the efficiency of MoSe2 in the hydrogen evolution reaction (HER) by providing multiple species but the interactions between the plasma and catalyst are not well understood. In this work, the effects of the ion energy and plasma density on the catalytic properties of MoSe2 nanosheets are studied. The through-holes resulting from plasma etching and multi-vacancies induced by plasma-induced damage enhance the HER efficiency as exemplified by a small overpotential of 148 mV at 10 mA cm-2 and Tafel slope of 51.6 mV dec-1 after the plasma treatment using a power of 20 W. The interactions between the plasma and catalyst during etching and vacancies generation are evaluated by plasma simulation. Finite element and first-principles density functional theory calculations are also conducted and the results are consistent with the experimental results, indicating that the improved HER catalytic activity stems from the enhanced electric field and more active sites on the catalyst, and reduced bandgap and adsorption energy arising from the etched through-holes and vacancies, respectively. The results convey new fundamental knowledge about the plasma effects and means to enhance the efficiency of catalysts in water splitting as well insights into the design of high-performance HER catalysts.

(Invited) Defect Engineering in Plasma-Treated Graphene Films

Engineering of defects located in-grain or at grain boundary is central to the development of functional materials and nanomaterials. While there is a recent surge of interest in the formation, migration, and annihilation of defects during ion and plasma irradiation of bulk (3D) materials, the detailed behavior in low-dimensional materials remains most unexplored and especially difficult to assess experimentally. A new hyperspectral Raman imaging scheme providing high selectivity and diffraction-limited spatial resolution is here adapted to examine plasma-induced damage in a polycrystalline graphene film grown by chemical vapor deposition on copper substrates and then transferred on silicon substrates. For experiments realized in nominally pure argon plasmas at low pressure, spatially resolved Raman conducted before and after each plasma treatment shows that the defect generation in graphene films exposed to very low-energy (11 eV) ion bombardment follows a 0D defect curve, while the domain boundaries tend to develop as 1D defects. Surprisingly and contrary to common expectations of plasma-surface interactions, damage generation at grain boundaries is slower than within the grains. Inspired by recent modeling studies, this behavior can be ascribed to a lattice reconstruction mechanism occurring preferentially at domain boundaries and induced by preferential atom migration and adatom-vacancy recombination. Further studies were realized to compare the impact of different plasma environments promoting either positive argon ions, metastable argon species, or VUV-photons on the damage formation dynamics. While most of the defect formation is due to knock-on collisions by 11-eV argon ions, the combination with VUV-photon or metastable atom irradiation is found to have a very different impact. In the former, the photons are mainly thought to clean the films from PMMA residues due to graphene transfer from copper to silicon substrates. On the other hand, in conditions with both ion and metastable atom irradiation, the surface de-excitation of the latter seem to greatly enhance the self-healing of the grain boundaries due to an increase of the local energy deposition. Finally, these experiments were used as building blocks to examine the formation of chemically doped graphene film in such plasmas using argon mixed with either traces of N- or B-bearing gases.

Plasma Etching in InAlN/GaN Hemt Technology

The experimental data on the plasma etching of Si3N4 for sub-100 nm gate fabrication for high electron mobility transistors (HEMTs) based on InAlN/GaN heterostructures are analyzed. The influence of the plasma etching process parameters on the plasma-induced damage to the InAlN/GaN heterostructure is considered. The possibility of formation of a 70 nm-length gate InAlN/GaN HEMT using low damage reactive ion etching of Si3N4 is demonstrated.

Performance improvement of Ge fin field-effect transistors by post-fin-fabrication annealing

We demonstrated post-fin-fabrication annealing (PFA) as a recovery process for suppressing plasma-induced damage (PID) in Ge fin field-effect-transistors (FinFETs) after the Ge fin fabrication. There are two key points in the PFA process. One is an annealing temperature (≥600 °C) higher than the recrystallization temperature of Ge for the curing of PID. The other is that a SiO2 film is capped on the Ge fin to prevent GeO desorption. Furthermore, we investigated the impact of PFA at 600 °C–800 °C on the electrical characteristics of Ge FinFETs. The PFA process improves the subthreshold slope and the on current in Ge n- and p-type FinFETs and reduces the off current. We found that the optimum PFA temperature is 600 °C in this experiment because of the minimum thermal expansion between Ge and SiO2.

Study on efficiency improvement of multi-crystalline silicon solar cell by removing by-product and plasma induced damage generated during reactive ion etching

Silicon solar cell texturing is the process of reducing the light reflection of the solar cells by changing their surface structure. Given that multi-crystalline silicon (mc-Si) wafers have grains with different orientations, their texturing process is more difficult to realize compared with those of monocrystalline silicon (mono-Si) wafers. There are two types of texturing processes: wet and dry texturing. Acidic solution-based wet-textured mc-Si samples have higher reflectance values than mono-Si wafers. However, the reactive ion etching (RIE)-texturing of mc-Si wafers has the advantage of decreasing reflectance to values below 10%. Despite this decrease in reflectance, RIE byproducts and plasma-induced damage bring about recombination that results in a decrease the passivation properties of the solar cell. In this study, the RIE byproducts as well as the plasma-induced damage of the RIE-textured mc-Si wafers were analyzed using SEM, STEM, and EDS analysis, while quasi-steady-state photoconductance (QSSPC) was used to confirm the optical characteristics of the RIE-textured mc-Si wafers. Additionally, HF treatment resulted in byproduct removal, and a relatively high implied Voc (673 mV) was confirmed within 3 min of HF treatment. The defect removal etching (DRE) process using KOH was applied to remove the lattice defect layer, and after 40 s of the DRE treatment, the implied Voc increased to ~680 mV. Thus, the RIE-textured mc-Si solar cell showed a power conversion efficiency of 19.6%, which was ~0.6% higher than that of acid-textured mc-Si solar cells based on the increase in short-circuit current (Isc).

Evaluation of residual defects created by plasma exposure of Si substrates using vertical and lateral pn junctions

Defect creation in both the vertical and lateral directions of Si substrates during plasma processing has become a critical problem in the fabrication of three-dimensional structural devices. In this study, the authors present pn junction structures that can be used to evaluate defects in both the vertical and lateral directions of a Si substrate. Samples with these pn junction structures were exposed to fluorocarbon plasma; after plasma exposure, a chemical dry etching process was employed to determine the influence of residual species on damaged layer formation, and capacitance–voltage measurements were conducted to detect the formation of defects in the Si substrate. The results confirmed that defects created by plasma exposure act as carrier trapping sites. Spectroscopic ellipsometry and time-of-flight secondary ion mass spectrometry revealed that the damaged layers were tens of nanometers thick, and cathodoluminescence analysis identified the presence of “latent” defects in the damaged layer even after furnace annealing. Moreover, current–voltage measurements of devices with different pn junction distances revealed that leakage current in both the vertical and lateral directions increased with decreasing pn junction distance. The experimental results of this study demonstrate that plasma-induced damage (PID) creates defects in both the vertical and lateral directions; lateral defects are assumed to be caused by the stochastic straggling of incident ions, which has been predicted by molecular dynamic simulations. The implementation of devices with lateral pn junctions is essential in improving the understanding of PID mechanisms and designing future electronic devices that are sensitive to latent defects.

Reduction of plasma-induced damage in n-type GaN by multistep-bias etching in inductively coupled plasma reactive ion etching

Plasma-induced damage was reduced by multistep-bias etching that involved a stepwise decrease of the etching bias power (Pbias) and subsequent annealing. The depth of damage at Pbias = 60 W was determined to be 60 nm from the capacitance–voltage characteristics of Ni/Al2O3/etched-GaN metal-oxide-semiconductor diodes. The damaged layer was removed by subsequent etching at Pbias = 5 W and 2.5 W. The residual and shallow damage induced by the low Pbias was then recovered by subsequent annealing at 400 °C. The multistep-bias etching of inductively coupled plasma reactive ion etching was thus confirmed to be effective for achieving a high etching rate with low damage.

Surface and interfacial study of atomic layer deposited Al2O3 on MoTe2 and WTe2

The atomic layer deposition (ALD) of high-k dielectrics could build an efficient barrier against moisture and O2 adsorption. Such a barrier is highly needed for MoTe2 and WTe2 transition metal dichalcogenides because of the poor structural stability and the fast oxidization in ambient air. In situ x-ray photoelectron spectroscopy and ex situ atomic force microscopy and scanning transmission electron microscopy were employed to report a comparative study between the growth of Al2O3 on MoTe2 and WTe2 by means of traditional thermal ALD and plasma-enhanced ALD (PEALD). Similar to what has been observed on other 2D materials such as MoS2 and Graphene, the thermal ALD results in an islanding growth of Al2O3 on MoTe2 due to the dearth of dangling bonds, whereas, a uniform coverage of Al2O3 on WTe2 is observed and likely contributed to the high concentration of intrinsic structural defects. The PEALD behavior is consistent between MoTe2 and WTe2 providing a conformal and linear growth rate (∼0.08 nm/cycle), which correlates with the creation of Te–O and metal-O nucleation sites. However, a thin layer of interfacial Mo or W oxides gradually forms, resulting from the plasma-induced damage in the topmost (1–2) layers. Attempts to enhance the Al2O3/MoTe2 interfacial quality by physically evaporating an Al2O3 seed layer are investigated as well. However, the evaporated Al2O3 process causes thermal damage on MoTe2, necessitating a more ‘gentle’ ALD technique for the surface passivation.

Novel Volatile Film for the Protection of Organo-Silicate Glass Dielectric Materials

In back-end-of-the-line (BEOL) patterning process, porous low-k dielectrics are degraded by plasma reactive species during etching and resist stripping –this is so-called plasma induced damage (PID). PID increases the k-value of dielectrics and inter-line capacitance, which causes resistive-capacitive (RC) delay. In this paper, the authors introduce two approaches to suppress the PID, using a novel volatile organic film. The first approach is an improved Post Porosity Plasma Protection (P4, or pore stuffing), and the second approach is based on the use of a temporary plug. In the case of pore stuffing, the filling ability and protection effect are evaluated on various blanket low-k dielectric materials. For the plug approach, its effectiveness is demonstrated in terms of electrical characteristics and reliability using a patterned interconnect test vehicle.

Simulation-aided design of very-high-frequency excited nitrogen plasma confinement using a shield plate

Nitrogen atoms are versatile for nitridation applications and do not lead to plasma-induced damage. Large-sized wafer processing demands a uniform supply of nitrogen atoms produced in a high-density very-high-frequency excited plasma of N2 without ammonia. The confinement of plasma through the use of a plasma shield plate (PSP) allows the samples to be separated in a downstream chamber. Generation and transport of N atoms were computationally simulated, and the PSP designs were implemented by PSP parameterization. The supply of high-density N radicals to the sample stage was optimally designed with sufficiently small holes and thin PSP to satisfy an aspect ratio of thickness-to-hole-diameter of less than 2.5.

Impact of NH3 plasma treatment for solution-processed indium oxide thin-film transistors with low thermal budget

The challenges associated with low-temperature solution-processed metal oxide thin-film transistors (TFTs) have hindered the realization of flexible oxide electronics at temperatures lower than 200 °C. NH3 plasma treatment is embarked upon as a route to effectively transform the chemical precursors to semiconducting metal oxides with high electrical quality. NH3 plasma-activated indium oxide (InOx) TFTs with saturated mobility 2.48 cm2V−1s−1 have been obtained at temperature as low as 150 °C. The low-temperature activation mechanism of NH3 plasma treatment for InOx films and the role of post-annealing in the fabrication process of InOx TFTs have been explored. Research shows that NH3 plasma treatment activates InOx films based on plasma bombardment, radicals doping and radical reactions; then, unreacted radicals and interstitial radicals were effectively removed by extra thermal energy from post-annealing. In comparison to thermal annealing, NH3 plasma treatment with moderate post-annealing is capable of achieving high degrees of condensation and densification reactions for InOx film and modulating the carrier concentration in a manner suitable for high-performance semiconducting behavior with low thermal budget. However, The InOx film after NH3 plasma treatment exhibited a non-uniform geometry due to the effect of plasma-induced damage in the local regions of InOx film, which restricts its commercial application. Some strategies were reported to eliminate the effect of plasma-induced damage by changing plasma conditions or pre-annealing temperature. The results demonstrated that changing plasma conditions by decreasing the plasma chamber temperature or the plasma power can eliminate plasma-induced damage, but attenuate the electrical performance of InOx TFT; nevertheless, changing pre-annealing temperature to 30 °C not only effectively eliminate plasma-induced damage, but also further improve the electrical performances of InOx TFT. These results provide a way for fabricating low-temperature oxide circuitry and supply an effective idea for avoiding plasma-induced damage.

(Invited) High-Performance GaN Hbts with Regrown Emitters

GaN-based hetero-junction bipolar transistors (HBTs) are suitable for power amplifiers because they have inherent advantages such as high current and power density, high linearity and normally off operation. However, low conductivity in the p-type base layer and plasma-induced dry-etching damage slow down the improvement of GaN HBTs. Although GaN-based HBTs demonstrated large current gain (β) and high current and power density by using p-type InGaN base [1], N2-incorporation dry-etching process, wet repairing and regrown base contact layer [2,3], etching damage also exists, which limits the maximum oscillation frequency (fmax) [4]. This essay reports an AlGaN/GaN heterojunction bipolar transistor (HBT) on sapphire with selective-area regrown (SAG) n-AlGaN emitter by metal-organic chemical vapor deposition (MOCVD). 1-μm n+-GaN, 0.5-μm undoped-GaN and 100-nm p-GaN are grown on sapphire in sequence by MOCVD. Then a 100-nm thick SiO2 mask layer is deposited on the wafer and patterned for SAG emitter. Then the wafer is reloaded to reactor to grow an n-AlGaN emitter layer at 980 ℃ and 50 torr. Hall measurements performed on p-GaN base layer indicate a hole concentration of 9.8×1017 /cm3 and mobility of 14.1 cm2/V.s. The SAG n-AlGaN emitter is 100-nm thick with the Al component of 7%. The electron concentration and mobility of the SAG n-AlGaN layer are 1.6×1018 /cm3 and 290 cm2/V s. Ohmic contact is achieved on the base contact with the calculated sheet resistance Rs of 238 KΩ/sq, and the specific contact resistance ρc of 2×10-3 Ω.cm2. As a result, the fabricated HBTs exhibit high β of 110, which is highest among reported values on GaN-based HBTs on sapphire substrates. The common-emitter I-V characteristics show a high J C of 22 kA/cm2 and power density of 220 kW/cm2 with a low Voffset <1V and Vknee<5V. These voltage are the lowest among reported values on GaN-based HBTs with comparable emitter areas, owing to the realization of base ohmic contact that decreases base resistance. The open-base breakdown voltage is 98 V with the leakage current of 1 μA, as a consequence the breakdown field is about 2.2 MV/cm. we also demonstrated RF performance of GaN HBTs with fT of 4 GHz at the collector voltage of 9 V, although the chip size is large. It is expected that high RF performance can be achieved by down scaling the device layout. These results for GaN HBTs/sapphire show that high performance III-N HBTs can be achieved with regrowth technology. In this talk, we will demonstrate that a large performance improvement can be obtained for RF HBT devices by the use of regrowth emitter. The current gain is increased with the emitter area decreasing. It is favorable for RF HBT devices, which are expected to achieve higher current gain, benefiting from small recombination current. By down scaling the device layout, an RF HBT is expected to be used in PA in the next generation communication.

Insights into different etching properties of continuous wave and atomic layer etching processes for SiO2 and Si3N4 films using voxel-slab model

This work describes the modeling of the surface reactions involved in atomic layer etching (ALE) of SiO2 and Si3N4 with a deposition step using C4F8/O2/Ar plasma and an Ar plasma etch step. In the etching step, the surface was assumed to consist of two layers: a C-F polymer layer and a reactive layer. The effects of residual F from the deposition step and F originating from the C-F polymer layer during the etch step and the influences of the O and N outfluxes generated from the reactive layer were considered, in terms of their effects on the etch rates of the SiO2 and Si3N4 films. Using a three-dimensional voxel-slab model that included the surface reactions described above, an analysis was performed based on the differences between the etching properties of continuous wave (CW) etching and ALE in the cases of blanket wafers and self-aligned contact layers from the viewpoints of numerical simulations. As a result of these analyses, it was found that the use of monoenergetic ion energy improves surface layer thickness controllability for both the polymer layer and the reactive layer and that quantitative control of time variations in both the C-F polymer layer thickness and the ion penetration depth is necessary for high selectivity of SiO2 over Si3N4 (SiO2/Si3N4) and for low plasma-induced damage on the Si3N4 film. Furthermore, in the authors’ simulations, a relatively high SiO2 etch rate was obtained for a modified quasi-ALE (43 nm/min) while maintaining high SiO2/Si3N4 selectivity (more than 100) after optimization of the C-F polymer layer thickness, the ion energy, and the ALE cycle time; this represents a solution in terms of the important issue of the very low etch rate of ALE. These simulation results indicate that accurate prediction of the surface reaction, further quantitative control of the plasma parameters, and optimization of the pattern layout design are necessary to realize higher ALE process performance for practical use in mass production.

Characterization of electric-field enhancement leading to circuit-layout dependent damage of low-k films when exposed to processing plasma

We investigated the effects of Cu-line layouts on plasma-induced radiation damage (PRD) to interline low-k dielectric films. We carried out a finite-element-method-based three-dimensional (3D) electromagnetic simulation (EMS), in which a Drude free-electron model was implemented for the dielectric function of Cu lines. The 3D EMS analysis revealed that the electric field in the low-k films was enhanced for specific Cu-line layouts exposed to N2 plasma irradiation, while no clear electric-field enhancement was observed for Ar plasma exposure. The specific optical emission lines produced in N2 plasmas are the root cause of damage creation in the low-k dielectric films with embedded Cu lines. The 3D EMS analysis predicted that the electric field was enhanced with decreasing the line and space (L/S) widths. The prediction implies that the Cu-line-layout dependent PRD should be more prominent as device dimensions shrink. We verified the predicted results experimentally using devices with low-k films between various L/S Cu lines, in addition to a blanket wafer—without Cu lines. We found that an increasing peak in the low-k dielectric constant appeared at a specific L/S width after N2 plasma exposure and no clear increase in the dielectric constant was observed after Ar plasma exposure. The obtained experimental results are consistent with the 3D EMS analysis. We propose a damage model, where the electric field enhancement plays an important role in the low-k dielectric films with embedded Cu lines. The proposed damage model is indispensable for minimizing PRD and designing Cu layouts in future devices.

Low-k integration: Gas screening for cryogenic etching and plasma damage mitigation

The integration of porous organo-silicate low-k materials has met a lot of technical challenges. One of the main issues is plasma-induced damage, occurring for all plasma steps involved during interconnects processing. In the present paper, we focus on porous SiOCH low-k damage mitigation using cryogenic temperature so as to enable micro-capillary condensation. The aim is to protect the porous low-k from plasma-induced damage and keep the k-value of the material unchanged, in order to limit the RC delay of interconnexion levels while shrinking the microchip dimension. The cryogenic temperature is used to condense a gas inside the porous low-k material. Then, the etching process is performed at the temperature of condensation in order to keep the condensate trapped inside the material during the etching. In the first part of this work, the condensation properties of several gases are screened, leading to a down selection of five gases. Then, their stability into the porous structure is evaluated at different temperature. Four of them are used for plasma damage mitigation comparison. Damage mitigation is effective and shows negligible damage for one of the gases at −50°C.

Observation and mitigation of RF-plasma-induced damage to III-nitrides grown by molecular beam epitaxy

In this work, radio-frequency (RF) plasma-induced damage to III-nitride surfaces and bulk defects is observed and mitigated. It is shown that for InN films, the surface is more sensitive to plasma-induced damage than GaN films, as observed via atomic force microscopy and reflection high energy electron diffraction. In order to isolate any possible plasma-induced damage, a growth window for InN is established, and temperature ranges are determined for other damaging effects which include roughening due to low adatom mobility, InN decomposition, and indium desorption. In situ plasma monitoring and optimization are accomplished with a combination of optical emission spectroscopy as well as a remote Langmuir probe. It is shown that by increasing the plasma nitrogen flow, the positive ion content increases; however, the ion acceleration potential reduces. Additionally, a reduced RF plasma power results in a reduction of atomic nitrogen species. These plasma species and energetic variations result in variations in the bulk unintentional background electron concentrations observed by room temperature Hall effect measurements of ∼1 μm thick InN films. By increasing the nitrogen flow from 2.5 to 7.5 sccm for a constant RF power of 350 W, the background electron concentration decreases by 74% from 1.36 × 1019 cm−3 to 3.54 × 1018 cm−3, while maintaining a smooth surface morphology. Additionally, photoluminescence spectra indicate optical emission energies shift from ∼0.81 to 0.71 eV (closer to the fundamental bandgap of InN) by limiting the damaging plasma species. Finally, conditions are presented to further minimize plasma-induced damage in III-nitride devices.

Laser plasma-induced damage characteristics of Ta2O5 films

As a material that exhibits a high refractive index and an excellent chemical stability, Ta2O5 is widely used in the microelectronics and chemical industries. In this study, we analyzed the effect of plasma shock waves on Ta2O5 thin films. The experimental results showed that the damage characteristics of the film include thermal ablation, surface cracks, and peeling off from the substrate owing to the repeated action of the pulse. Coating characterization before and after exposure to laser plasma was performed using UV-Vis spectroscopy, energy dispersive spectroscopy (EDS), Raman spectroscopy, and X-ray diffraction. With the increase in the light absorbed by the ablated products, the transmittance of the film decreased after damage, and the Raman spectrum and X-ray diffraction pattern of the sample indicated that the film changes from an amorphous to a crystalline structure owing to the action of the high-temperature and high-pressure plasma. The EDS analysis results indicated that the oxygen content in the film decreases because of the damage inflicted to its structure.

Observation of Plasma-Induced Damage in Bulk Germanium ${p}$ -Type FinFET Devices and Curing in High-Pressure Anneal

In this letter, evidence of plasma-induced damage (PID) occurring in p-type FinFET Germanium devices due to BEOL processing is reported. The ability of high-pressure anneal (HPA) (20 min at 450° with 20 atm pressure) to cure this damage is investigated by comparing different ambients (hydrogen and deuterium) with the reference case without HPA. The degradation and partial recovery during anneal of the threshold voltage, the transconductance, and the subthreshold slope are observed. Further, bias temperature instability stress is employed to evaluate the impact of PID in terms of reliability. It is found that the damage observed in the dc and reliability measurements is more efficiently cured by the H2 ambient than the D2 ambient.