Oxide Etch Research Articles

PVA-Al sol film derived superior anti-oxidation layer on the surface of AISI304 steel

In this research, the polyvinyl alcohol (PVA) modified Al sol is coated on the AlSl304 steel to resist the oxidation etching by espousing to the air at 900℃. Compared with the substrate, the coated sample demonstrates the superior oxidation resistance indicated by the decreasing mass gain. Meanwhile, the generated oxide layer exhibits the features of dense, continuous and no flaking, which suppresses the inward diffusion of the O atoms. In the virtue of the PVA-Al sol film, the beneficial anti-oxidation (Cr, Al)2O3 and the Fe3O4 phase are derived and the general detrimental Fe-rich hematite phase is avoided.

Chemical mechanism of oxidative etching of ruthenium: Insights into continuous versus self-limiting conditions

Ruthenium (Ru) has emerged as a promising material for microelectronic applications that require precise modification and patterning of thin films at the nanoscale. However, the susceptibility of Ru to oxidative etching has raised challenges in achieving the desired profile of Ru thin films. In this study, we investigated the mechanisms and conditions required for oxidative etching of Ru using density functional theory (DFT) calculations. Our calculations examined the thermodynamics and kinetics of several pathways toward the formation of gaseous RuO4, including oxidations of bulk Ru and RuO2, from both surfaces of which surface reactions were considered. Our findings suggest that etching reactions under O3 exposure are more favored on a Ru surface than on RuO2. Experimental results obtained from etching of Ru using O3 further support our computational predictions, indicating that oxide formation leads to a reduction in the etching rate and that quasi-self-limiting etching conditions can be achieved towards oxidative ALE of Ru. Our study may contribute to the fundamental understanding of Ru surface phenomena and provides insights into the development of effective ALE techniques for Ru.

Comparison of BCl3, TiCl4, and SOCl2 chlorinating agents for atomic layer etching of TiO2 and ZrO2 using tungsten hexafluoride

Recent advances in the semiconductor industry have created an exigency for processes that allow to deposit and etch material in conformal matter in three-dimensional devices. While conformal deposition is achieved using atomic layer deposition (ALD), conformal etching can be accomplished by thermal atomic layer etching (ALE) which, like ALD, proceeds via a binary sequence of self-limiting reactions. This study explores ALE of TiO2 and ZrO2 using WF6 as a fluorinating agent, and BCl3, TiCl4, or SOCl2 as a co-reactant. The effect of co-reactant chemistry was studied using atomic force microscopy, in situ ellipsometry, and in vacuo Auger electron spectroscopy measurements along with thermodynamic modeling. All three co-reactants exhibited saturation and etch rates increasing with temperature. At 170 °C, TiO2 can be etched using WF6 with BCl3, TiCl4, or SOCl2, and the etching proceeds at 0.24, 0.18, and 0.20 nm/cycle, respectively. At 325 °C, ZrO2 ALE can occur using these same reactants, proceeding at 0.96, 0.74, and 0.13 nm/cycle, respectively. A higher temperature is needed for ZrO2 ALE versus TiO2 because the ZrCl4 product is less volatile than the corresponding TiCl4. During ZrO2 and TiO2 etching using BCl3 or TiCl4, boron oxide or titanium oxide intermediate layers, respectively, were formed on the surface, and they were subsequently removed by WF6. In contrast, for ALE of TiO2 using SOCl2, a similar intermediate layer is not observed. This study broadens the understanding of co-etchants role during thermal ALE and expands the range of reactants that can be used for vapor etching of metal oxides.

Bottom growth strategy for high areal capacity rechargeable aluminum batteries

Among all practical methods to build the high specific energy battery, increasing the areal loading of active materials is a promising choice. However, increasing the areal loading of the active material will inevitably increase the areal deposition amount of aluminum. If aluminum undergoes uneven deposition, dendrite growth is an inevitable problem with the increase of aluminum deposits. In this paper, an anode structure with bottom growth mode (2P-Al2O3/Al) are proposed through anodic oxidation and laser etching technology, which can provide an inverted electric field strength (Ebottom>Etop) to induce the directional deposition of Al. We have grown a dense alumina layer on the surface and pores of the 2P-Al2O3/Al electrode, which can form a stable electrode/electrolyte interface as an SEI layer. The in-situ deposition image of the electrode indicates that aluminum preferentially grows at the bottom of the electrode, and the deposition process of aluminum is very uniform. In addition, the electrode exhibits excellent cycling stability in both half and full batteries. Even under the large areal special capacity of 20 mAh·cm−2 without presenting significant interfacial degradation and early short circuit, it can operate cycle over 900 h, which is more than 3 times that of planer aluminum anode. This technology opens a new platform for designing highly safe and stable long-term energy storage system.

Electron-enhanced SiO2 atomic layer deposition at 35 °C using disilane and ozone or water as reactants

Electrons can enhance SiO2 atomic layer deposition (ALD) at low temperatures using disilane (Si2H6) and either ozone (O3/O2) or water (H2O) as reactants. SiO2 electron-enhanced ALD (EE-ALD) was demonstrated at 35 °C by exposing the sample to sequential electron, oxygen reactant, and Si2H6 exposures. The reaction sequence was electron beam exposure for 3 s, purge for 5 s, O3/O2 or H2O exposure at 0.5–1.0 Torr for 3 s, purge for 10 s, Si2H6 exposure at 100 mTorr for 1 s, and purge for 15 s. The electron exposure was an electron current of ∼150 mA for 3 s. The electrons were produced by a hollow cathode plasma electron source typically operating with a grid bias of ≈−300 V. These electrons could irradiate a sample area of ∼2 × 2 cm2. In situ spectroscopic ellipsometry measurements determined that SiO2 EE-ALD films nucleated rapidly and deposited linearly versus number of EE-ALD cycles. The SiO2 EE-ALD growth rate was 0.89 Å/cycle using O3/O2 and 0.88 Å/cycle using H2O. The SiO2 growth rate was also self-limiting at higher electron and Si2H6 exposures. In addition, SiO2 EE-ALD films were grown by changing the reaction sequence or codosing the electrons with the oxygen reactant. The SiO2 EE-ALD films could be grown on conducting silicon wafers or insulating SiO2 films. SiO2 EE-ALD is believed to be possible on insulating SiO2 films because the secondary electron yield for SiO2 at electron energies of ∼100–300 eV is greater than unity. Under these conditions, the SiO2 film charges positive during electron exposure and then pulls back secondary electrons to maintain charge neutrality. The SiO2 EE-ALD films had properties that were comparable with thermal SiO2 oxides. The refractive indices of the SiO2 EE-ALD films were similar at n = 1.44 ± 0.02 for various process conditions and equivalent to the refractive index of a wet thermal SiO2 oxide film. In addition, all the SiO2 EE-ALD films yielded etch rates in dilute buffered oxide etch solution that were only slightly higher than the etch rate of a thermal SiO2 oxide film. SiO2 EE-ALD should be useful to deposit high-quality SiO2 films for various applications at low temperatures.

Two-dimensional boron nanosheets for selective enrichment and detection of cis-diol compounds by surface-assisted laser desorption/ionization time-of-flight mass spectrometry

Surface-assisted laser desorption/ionization time-of-flight mass spectrometry (SALDI-TOF MS) is an effective method for detecting of low-mass molecules. In this study, two-dimensional boron nanosheets (2DBs) were fabricated through thermal oxidation etching and coupling liquid exfoliation technologies, and applied as a matrix and selective sorbent for detecting cis-diol compounds by SALDI-TOF MS. The outstanding nanostructure and boric acid active sites of 2DBs endow them with sensitivity for cis-diol compound detection, excellent selectivity, and low background interference for complex samples. The specific in-situ enrichment faculty of the 2DBs as a matrix was investigated by SALDI-TOF MS using glucose, arabinose, and lactose as model analytes. In the presence of 100 -fold more interfering substances, the 2DBs showed high selectivity against cis-diol compounds, and exhibited a better sensitivity and a reduced limit of detection through enrichment treatment than graphene oxide matrices. The linearity, limit of detection (LOD), reproducibility, and accuracy of the method were evaluated under optimized conditions. The results showed that the linear relationships of six saccharides remained in the range of 0.05–0.6 mM with a correlation coefficient r ≥0.98. The LODs of six saccharides were 1 nM (glucose, lactose, mannose, fructose) and 10 nM (galactose, arabinose). Sample-to-sample (n = 6) with relative standard deviations (RSDs) of 3.2% to 8.1% were observed. Recoveries (n = 5) of 87.9–104.6% were obtained at three spiked levels in the milk samples. The proposed strategy promoted the development of a matrix for use with SALDI-TOF MS detection, in which the UV absorption properties and enrichment capabilities of 2DBs were combined.

Bi-Cu Electrolytes with Aminocarboxylate Chelators for Reversible Metal Electrodeposition at High pH for Dynamic Windows

Dynamic windows, which possess electronically tunable light transmission, increase both the energy efficiency and aesthetics of spaces such as buildings and automobiles. Although reversible metal electrodeposition affords a promising approach to constructing high-performing dynamic windows, the acidic nature of the aqueous electrolytes frequently used in these windows has prevented their commercialization due to tin-doped indium oxide (ITO) etching. In this manuscript, we design neutral and alkaline electrolytes that support the reversible electrodeposition of Bi and Cu at rates comparable to existing acidic electrolytes. In these electrolytes, Bi3+ and Cu2+ are solubilized by using aminocarboxylate chelating ligands. By evaluating a series of ligands with varying denticities, we demonstrate that N-(2-hydroxyethyl)ethylenedianmine-N,N’,N’-triacetic acid (ED3A-OH) provides the optimal metal ion binding strength that enhances solubility while simultaneously supporting rapid metal electrodeposition. These results allow us to design alkaline ED3A-OH electrolytes that are compatible with ITO even after four weeks of immersion at 85 °C. This manuscript thus demonstrates that chelating ligands can be utilized to design alkaline reversible metal electrodeposition electrolytes that support dynamic windows with robust shelf lives.

Morphological features of synthetic diamond microcrystals subjected to oxidative etching

Morphological features of synthetic diamond microcrystals subjected to oxidative etching

Enhancing graphene's resistance to oxidative corrosion by positive charge doping: A DFT view

Although graphene has been widely used as a protective coating for many applications, the failure mechanism and the strategy to improve its resistance to the chemical etching is largely unknown. We herein have comparatively studied the generally oxidative corrosion process of neutral and positive-charge-doping (PCD) graphene via DFT (density functional theory) simulations. It is found that the positive-charge-doping (PCD) generally enhances the protecting ability of graphene for substrates from reactive environments. The C-network breakdown of graphene due to oxidative etching goes through three stages: the formation of oxidative corrosion crystal nucleus (OCCN), subsequent formation and expansion of oxidative lined defect (OLD), then the generation and extension of oxidative corrosion crack (OCC). Comparing to the neutral or negative charge doping, PCD decreases the electronic reactivity and thus suppresses the oxygen atoms diffusing on graphene of the rate-limiting step, especially in the stage of OCCN nucleation. Our work indicates a possible strategy to improve the resistance to oxidative corrosion (ROC) of graphene via positive charge doping. This protection enhancement of graphene can be achieved taking advantages of the electron capture of the neighboring substrates, reactive environments or external gate voltage in applications.

Holey MXene nanosheets intimately coupled with ultrathin Ni–Fe layered double hydroxides for boosted hydrogen and oxygen evolution reactions

Although Ni–Fe layered double hydroxides (LDH) are considered as efficient noble metal-free electrocatalysts for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), their overall electrocatalytic performance still needs to be significantly enhanced to satisfy the requirements of practical water splitting systems. Herein, we demonstrate the reasonable design and controllable fabrication of holey Ti3C2Tx MXene nanosheets intimately coupled with ultrathin Ni–Fe LDHs (LDH/H–Ti3C2Tx) through a combined in situ oxidative etching and hydrothermal assembly strategy. Such a newly-developed nanoarchitecture with abundant in-plane holes not only creates a sophisticated three-dimensional (3D) ion diffusion model to shorten the diffusion path of reactants, but also exposes a large number of extra catalytically active sites as well as guarantees a high charge transfer rate, which enable to dramatically accelerate the electrochemical HER and OER kinetics. As a consequence, the resulting LDH/H–Ti3C2Tx catalyst affords exceptional HER and OER performance with low overpotentials, small Tafel plots and excellent cycling stability, far outperforming the bare Ni–Fe LDHs and Ti3C2Tx catalysts. This work is expected to open up a new avenue to the confined growth of active nanomaterials on holey MXene nanosheets and promote their applications in the energy-conversion and -storage fields.

Polyimide-On-Silicon 2D Piezoelectric Micromachined Ultrasound Transducer (PMUT) Array

This paper presents a fully addressable 8 × 8 two-dimensional (2D) rigid piezoelectric micromachined ultrasonic transducer (PMUT) array. The PMUTs were fabricated on a standard silicon wafer, resulting in a low-cost solution for ultrasound imaging. A polyimide layer is used as the passive layer in the PMUT membranes on top of the active piezoelectric layer. The PMUT membranes are realized by backside deep reactive ion etching (DRIE) with an oxide etch stop. The polyimide passive layer enables high resonance frequencies that can be easily tuned by controlling the thickness of the polyimide. The fabricated PMUT with 6 µm polyimide thickness showed a 3.2 MHz in-air frequency with a 3 nm/V sensitivity. The PMUT has shown an effective coupling coefficient of 14% as calculated from the impedance analysis. An approximately 1% interelement crosstalk between the PMUT elements in one array is observed, which is at least a five-fold reduction compared to the state of the art. A pressure response of 40 Pa/V at 5 mm was measured underwater using a hydrophone while exciting a single PMUT element. A single-pulse response captured using the hydrophone suggested a 70% −6 dB fractional bandwidth for the 1.7 MHz center frequency. The demonstrated results have the potential to enable imaging and sensing applications in shallow-depth regions, subject to some optimization.

Cation-deficient T-Nb2O5/graphene Hybrids synthesized via chemical oxidative etching of MXene for advanced lithium-ion capacitors

Orthorhombic niobium pentoxide (T-Nb2O5) is widely acknowledged as a fast pseudocapacitive material. Nevertheless, its application is hindered by the narrow voltage window (1–3 V vs. Li/Li+) that arises from irreversible phase transformation and sluggish kinetics during deep lithiation. Herein, we demonstrate a unique method for introducing Nb vacancies in T-Nb2O5 nanoparticles via amine-assisted oxidative etching of Nb2CTx MXene, providing extra storage sites and improving structural flexibility by introducing cationic defects. Subsequently reduced graphene oxide (rGO) is employed as substrate to disperse T-Nb2O5 nanoparticles and construct T-Nb2O5/rGO nanohybrids. Multiple characterizations and computational simulations demonstrate that the resulting T-Nb2O5/rGO hybrid anode exhibits rapid and stable multi-electron transfer lithium storage. Owing to the enrichment of Nb vacancies and nanoparticle morphology, even when voltage window of 0.01–3 V (vs. Li/Li+) is extended, T-Nb2O5 exhibits a pseudocapacitive mechanism and integrity of partial crystal structure; effectively tackling the structural collapse and sluggish kinetics of T-Nb2O5. Consequently, the T-Nb2O5/rGO anode shows a superior rate capacity (148 mAh/g at 10 A/g) and cycling stability (3000 cycles at 5 A/g). Remarkably, the assembled lithium-ion capacitors achieve a high energy density of 123.7 Wh/kg, a power density of 22.5 kW/kg, and a capacity retention of 83.6% after 20,000 cycles.

Vapor etching to avoid micro-masking by gas-bubbles in wet release of MEMS

We report on the formation of gas bubbles during the release of MEMS devices using buffered oxide etch. Several approaches to mitigate the problem are proposed and tested together with a qualitative study of the phenomenon. The chemical reaction behind such phenomenon and the influence of defects and topography is discussed. Finally, a comparison with the HF-vapor release technique is shown.

Enhancing moisture-electric power generation through in situ incorporation of carbon dots into polyelectrolyte membrane

The electric generation performance of traditional polyelectrolyte membrane with moisture usually fall far behind expectations due to its limited hydrophilic groups and poor directional transport capacity of protons. To this end, we introduce the suspension of carbon dots (CDs) prepared by selective oxidation etching of coal pitch with the mixture of formic acid and H2O2 under ambient temperature into the aqueous solution of polyvinyl alcohol (PVA) and poly(4-styrensulfonic acid) (PSS) to fabricate a novel hybrid membrane. This membrane (1.00 cm2) continuously delivers a voltage output of 0.83 V with a short-circuit current of 5.20 μA under proper moist conditions over a prolonged period, outperforming the power generation performance of the pristine membrane and many current moisture-enabled electric generation (MEG) devices. Moreover, CDs endow the polymer membrane with more superior mechanical properties, in favor of processing and scale integration for higher power output. With excess hydrophilic groups provided by as-prepared CDs’ suspension, the denser hydrogen-bond network can be formed in the membrane and facilitates water adsorption and the formation of a higher proton concentration gradient. CDs-induced bridging interactions between PSS chains pave a fast directional transport pathway for moveable protons, resulting in a remarkable power output. Accordingly, this work offers a facile and economic method for remoulding traditional polyelectrolyte membrane to conduct highly efficient MEG for practical applications.

Gas-phase etching mechanism of silicon oxide by a mixture of hydrogen fluoride and ammonium fluoride: A density functional theory study

We report the selective etching mechanism of silicon oxide using a mixture of hydrogen fluoride (HF) and NH4F gases. A damage-free selective removal of native oxide has been used in semiconductor manufacturing by forming and removing the ammonium fluorosilicate [(NH4)2SiF6] salt layer. A downstream plasma of NF3/NH3 or a gas-phase mixture of HF and NH4F was used to form (NH4)2SiF6. We modeled and simulated the fluorination of silicon oxide and the salt formation by density functional theory calculation. First, we simulated the successive fluorination of silicon oxide using SiO2 slab models. The fluorination reactions of SiO2 surfaces by the mixture produced a volatile SiF4 molecule or a surface anion of –OSiF4−* with an NH4+ cation with low activation energies. Unlike HF, NH4F produced surface salt species consisting of a surface anion and an ammonium cation. Next, we simulated the (NH4)2SiF6 formation from the two reaction products on fluorinated SiO2 surfaces. (NH4)2SiF6 can be formed exothermally with low activation energies (0.27 or 0.30 eV). Finally, we compared silicon with SiO2 to demonstrate the inherently selective etching of silicon oxide. The fluorination reactions of silicon by the mixture showed the activation energies significantly higher than the SiO2 cases, 1.22–1.56 eV by HF and 1.94–2.46 eV by NH4F due to the less stable transition state geometries. Therefore, the selective salt formation on silicon oxide, not on silicon, is expected in near-room temperature processing, which enables selective etching of silicon oxide.

Fluorine-rich graphene quantum dots by selective oxidative cutting of hydroxy fluorographene and their application for sensing of Fe (III) ions

Nucleophilic substitutions of fluorine moieties on fluorographite by hydroxyl groups are carried out by a strategy in which the parallel reductive defluorinations are minimised. Mild sonochemical exfoliation of the resulting product in water provides an aqueous dispersion of hydroxylated fluorographene (HFG) in which C-F bonds are largely semi-ionic in character. Upon subsequent hydrothermal treatment with hydrogen peroxide, additional fluorinations may take place because of adsorbed fluoride ions. Further, oxidative etching results in the formation of blue fluorescent fluorine-rich small sized (1- 2.5 nm) graphene quantum dots (FGQDs) with excellent solubility and high stability. The prepared FGQDs is shown to be a good sensing platform for the detection of Fe3+ions in aqueous solution. A good linear response is obtained for Fe3+ concentrations over a range of 0–90 µM with a limit of detection of 1.93 µM.

Deep multilevel wet etching of fused silica glass microstructures in BOE solution

Fused silica glass is a material of choice for micromechanical, microfluidic, and optical devices due to its chemical resistance, optical, electrical, and mechanical performance. Wet etching is the key method for fabricating of such microdevices. Protective mask integrity is a big challenge due extremely aggressive properties of etching solution. Here, we propose multilevel microstructures fabrication route based on fused silica deep etching through a stepped mask. First, we provide an analysis of a fused silica dissolution mechanism in buffered oxide etching (BOE) solution and calculate the main fluoride fractions like {HF}_{2}^{-}, {F}^{-}, {(HF)}_{2} as a function of pH and NH4F:HF ratio. Then, we experimentally investigate the influence of BOE composition (1:1–14:1) on the mask resistance, etch rate and profile isotropy during deep etching through a metal/photoresist mask. Finally, we demonstrate a high-quality multilevel over-200 μm etching process with the rate up to 3 μm/min, which could be of a great interest for advanced microdevices with flexure suspensions, inertial masses, microchannels, and through-wafer holes.

Electrolyte effects on the shape-controlled synthesis of Pt nanocrystals by electrochemical square-wave potential method

Surface structures or shapes of metal nanocrystals (NCs) greatly affect their catalytic performance. Electrochemical square-wave potential (SWP) method shows great advantages in the shape-controlled synthesis of Pt-group metal nanocrystals with high reactive high-index facets, due to the ease in controlling the growth and oxidation (or etching) by potentials. The potential conditions have been investigated substantially for the SWP method previously. However, the effects of electrolytes on the shapes of NCs are rarely explored. Herein, we investigated the effect of electrolyte (H2SO4 and Na2SO4) on the shape evolution of Pt NCs prepared by electrochemical SWP method. We found that high-index faceted Pt NCs, including {hk0}-faceted tetrahexahedral (THH) and {hkk}-faceted trapezohedral (TPH) Pt NCs can be obtained in 0.1 M H2SO4 in broad potential window, while mainly low-index {111}-faceted octahedral Pt NCs are formed in 0.1 M Na2SO4 electrolyte. Through evaluating the effects of cations, anions, and solution pH value, we observed that the pH value plays a key role in shape evolution. Low acidity in Na2SO4 electrolyte enhances the oxygen adsorption on Pt on the one hand, and promotes the hydrolysis of PtCl62− that decreases the growth rate of Pt NCs on the other hand. These two factors enhance the oxidative etching effect at upper potential limit of SWP, which can remove low-coordinated step atoms, and form flat low-index facets. This study shows insight into the electrolyte effects on electrochemical shape-controlled synthesis of Pt NCs, and helps to understand the growth mechanism.

Colorimetric sensor for Cr (VI) by oxidative etching of gold nanotetrapods at room temperature

Hexavalent chromium (Cr(VI)) is highly carcinogenic and mutagenic, which is seriously harmful to human health. Hence, it is important to create a probe that can detect Cr(VI) effectively. In this work, gold nanotetrapods (Au NTPs) were applied in colorimetric detection for the first time. Based on the oxidative etching principle of Cr(VI) on Au NTPs, a sensitive and multicolor response detection method for Cr(VI) was established. The oxidative etching of Au NTPs by Cr(VI) resulted in the blue shift of plasmon resonance absorption peak of Au NTPs with the change of morphology. As the etching progress processed, Au NTPs solution exhibited obvious color changes from gray-green to blue-violet and then to pink. This multicolor response design is very convenient for naked-eye detection. The limit of detection (LOD) of Cr(VI) is 3 nM for the naked eyes and 0.5 nM for UV–vis spectrum, both of which are lower than the toxicity level of Cr(VI) (0.2 μM) set by United States Environmental Protection Agency. This sensing method exhibits good linearity between the wavelength shift and Cr(VI) concentration in the range of 0.5 nM to 8 nM. The detection results of Cr(VI) in actual environmental samples demonstrate that the Au NTPs colorimetric probe (Au-N-Probe) is expected to be applied to the detection of Cr(VI) in water environmental samples such as lake water and industrial wastewater.

Applications and mechanisms of anisotropic two-step Si3N4 etching with hydrogen plasma conditioning

The ability to precisely form Si3N4 spacers is critical to the success of dynamic random-access memory and NAND (NOT AND) flash memory technology development. In this study, we investigated the mechanisms and process windows of an innovative two-step nitride (Si3N4) etch consisting of H2 plasma processing in an inductively coupled plasma chamber followed by either buffered oxide etch (BOE, a mix of NH4F and HF solution) wet clean or in situ NF3 plasma etch. We obtained layer–by-layer removal with each layer’s removal capable of more than 10 nm. We revealed that H can penetrate more than 20 nm deep into the nitride film to transform pristine Si3N4 into SiON after air exposure, which can be subsequently removed by BOE wet clean. The H2 and BOE steps do not need to run back-to-back; the modified SiON layer is stable enough to sustain elevated temperature processing and can be removed by BOE later down-the-line integration. We also demonstrated that using NF3 plasma can have highly selective etch of nitride over oxide due to the incubation time difference between these two types of films. It takes much longer time to initiate the chemical reaction for oxide compared with nitride. Critically, the role of H2 is not the key for high selectivity; instead, it provides an etch directionality and shortens the incubation time for both nitride and oxide.