Low Etch Rate Research Articles

Subthreshold Erosion of an Organic Polymer Induced by Multiple Shots of an X-Ray Free-Electron Laser

Solids irradiated by energetic photons can be eroded in two modes, depending on the radiation intensity. High average, low-peak power sources, e.g., synchrotron radiation and high-order harmonics, induce desorption of the material at a low etch rate. In contrast, high-peak-power radiation from extreme ultraviolet and x-ray lasers usually causes a massive removal of the material even by a single shot. In this contribution, an effective material erosion is reported in PMMA exposed to multiple accumulated pulses generated by the free-electron x-ray-laser Linac Coherent Light Source (LCLS, tuned at a photon energy of 830 eV in this study, operated in Menlo Park at Stanford, CA, USA) at a fluence below the single-pulse ablation threshold. The effect is caused by polymer-chain scissions initiated by single photons carrying enough energy to break the $\mathrm{C}---\mathrm{C}$ bounds. High efficiency of the erosion is supposed to occur due to a correlation of the single-photon effects. The subthreshold damage exhibits a nonlinear dose dependence resulting from a competition between chain scissions and cross-linking processes. The cross-linking is proven by Raman spectroscopy of the irradiated polymer. Two theoretical models of the x-ray free-electron-laser-induced erosion are suggested, which provide an excellent agreement with the experimental results.

Development of Catalytic-CVD SiNx Passivation Process for AlGaN/GaN-on-Si HEMTs

We optimized a silicon nitride (SiNx) passivation process using a catalytic-chemical vapor deposition (Cat-CVD) system to suppress the current collapse phenomenon of AlGaN/GaN-on-Si high electron mobility transistors (HEMTs). The optimized Cat-CVD SiNx film exhibited a high film density of 2.7 g/cm3 with a low wet etch rate (buffered oxide etchant (BOE) 10:1) of 2 nm/min and a breakdown field of 8.2 MV/cm. The AlGaN/GaN-on-Si HEMT fabricated by the optimized Cat-CVD SiNx passivation process, which had a gate length of 1.5 μm and a source-to-drain distance of 6 μm, exhibited the maximum drain current density of 670 mA/mm and the maximum transconductance of 162 mS/mm with negligible hysteresis. We found that the optimized SiNx film had positive charges, which were responsible for suppressing the current collapse phenomenon.

Neon and helium focused ion beam etching of resist patterns

Helium ion microscopy has attracted many applications in imaging, nanofabrication and analysis. One important field of study in nanofabrication using ion beam is the milling or etching of materials using a helium or neon focused ion beam (FIB), with and without chemical gas assistance. In particular, the neon FIB has a relatively high sputtering rate with a lower probability of swelling and less re-deposition issues compared to a helium FIB. Here, both neon and helium FIB etchings are investigated for milling and repairing electron-beam lithography (EBL) defined hydrogen silsesquioxane (HSQ) and polymethyl methacrylate (PMMA) resist patterns. Different dosages of neon FIB etching result in distinct etching profiles. Using the appropriate doses, arrays of uniform gap with aspect ratio more than 20 can be achieved on HSQ nanostructures. The neon FIB etching has a resolution of 20 nm on HSQ patterns. With XeF2 assistance, neon FIB etching can be enhanced for etching depth by a factor of ∼1.2. Whereas, helium FIB can also etch thick HSQ patterns, with much lower etch rates. But with XeF2 assistance, helium FIB etching depth can be enhanced significantly by a factor of around 5. Furthermore, both helium and neon FIB etching methods have been employed to selectively remove residual particles in deep and narrow trenches without affecting the resist patterns. The chemical analysis of these residual particle composition and resist patterns can be also performed using helium ion microscopy coupled with secondary ion mass spectrometry (SIMS) using neon FIB. Besides, a neon FIB can also effectively etch PMMA patterns which are commonly used in nanofabrication and the unwanted connections can be etched away.

Etching characteristics of thin SiON films using a liquefied perfluorocarbon precursor of C6F12O with a low global warming potential

Perfluorocarbon gas is widely used in the semiconductor industry. However, perfluorocarbon has a negative effect on the global environment owing to its high global warming potential (GWP) value. An alternative solution is essential. Therefore, we evaluated the possibility of replacing conventional perfluorocarbon etching gases such as CHF3 with C6F12O, which has a low GWP and is in a liquid state at room temperature. In this study, silicon oxynitride (SiON) films were plasma-etched using inductively coupled CF4 + C6F12O + O2 mixed plasmas. Subsequently, the etching characteristics of the film, such as etching rate, etching profile, selectivity over Si, and photoresist, were investigated. A double Langmuir probe was used and optical emission spectroscopy was performed for plasma diagnostics. In addition, a contact angle goniometer and x-ray photoelectron spectroscope were used to confirm the change in the surface properties of the etched SiON film surface. Consequently, the etching characteristics of the C6F12O mixed plasma exhibited a lower etching rate, higher SiON/Si selectivity, lower plasma damage, and more vertical etched profiles than the conventional CHF3 mixed plasma. In addition, the C6F12O gas can be recovered in the liquid state, thereby decreasing global warming. These results confirmed that the C6F12O precursor can sufficiently replace the conventional etching gas.

Surface action mechanism and planarization effect of sarcosine as an auxiliary complexing agent in copper film chemical mechanical polishing

The planarization property of slurry is a crucial factor in evaluating the quality of the copper (Cu) film chemical mechanical polishing (CMP) process. Various chemical additives of amino acids have been widely used in semiconductor and microelectronics industries to develop novel CMP slurries owing to their pollution-free and high-performance characteristics. In this paper, sarcosine was served as a crucial chemical additive in Cu film slurries. Through electrochemical measurements, X-ray photoelectron spectroscopy, Ultraviolet–visible, and Raman spectra tests, it was revealed that sarcosine could react with cupric ions in a ratio of 4:1 to form a stable water-soluble Cu-sarcosine complex, which can accelerate the chemical dissolution of Cu surface during CMP process, and it was also found that the complexing effect was relatively weak. Moreover, various tests on Cu wafers showed that high Cu removal rate (RR), low Cu static etching rate (SER), and ideal surface quality could be realized when sarcosine was used as an auxiliary complexing agent in glycine-based Cu slurries. The purpose of such a study is to find an environmentally friendly chemical additive that can improve the performance of slurry in the manufacturing process of Cu film and other materials used in integrated circuits.

Simulation-based optimization of out-of-plane, variable-height, convoluted quartz micro needle arrays via single-step anisotropic wet etching

Variable-height, out-of-plane, convoluted microneedle arrays with high aspect ratio, tapered shafts, sharp tips, and smooth surfaces are required for applications where the microneedle heights must conform to a specific target geometry. Here, we show that it is possible to achieve such convoluted microneedle arrays on a Z-cut quartz substrate by a single wet etching step in saturated NH4HF2 at 80 °C. Due to the low etch rate of quartz, the procedure is based on carefully controlling the dimensions of every needle-like structure by properly dimensioning the masked area on top. In particular, we show that the mask dimensions for all the microneedles can be optimized via a combination of Level Set (LS) simulations of wet etching and an Evolutionary Algorithm (EA) to search the optimal dimensions. As an example, we fabricate two different microneedle arrays, where the needle heights follow two different spherical surfaces. Our experiments confirm that (i) the LS method correctly predicts the evolution of the etch front for microneedles etched on AT, BT and Z quartz cuts; and (ii) the LS + EA approach provides a suitable procedure to determine an optimal collection of mask sizes for the fabrication of convoluted microneedle arrays on the Z cut.

A comparative study of Cx(x = 4, 5, 7)F8 plasmas for dry etch processing

In this study, the SiO2 etch characteristics of perfluorocarbon such as C4F8, C5F8, and C7F8 were investigated using inductively coupled plasmas (ICPs) to study the effect of a high C/F ratio on the etch characteristics of SiO2 for the ICP. The SiO2 rates and etch selectivities over Si3N4 and amorphous carbon layer (ACL) were measured by using the mixtures of Cx(x = 4, 5, 7)F8/Ar/O2. The higher C/F ratio of perfluorocarbon showed lower SiO2 etch rate but exhibited higher etch selectivities over Si3N4 and ACL due to the higher C2 while keeping the similar F in the plasma as observed by optical emission spectroscopy and due to the thicker fluorocarbon layer with more carbon-rich fluorocarbon on the materials surface as observed by X-ray photoelectron spectroscopy. Especially, C7F8 is environmentally benign material because it not only has a relatively low global warming potential but also can be captured easily using a capture system (a liquid state at the room temperature). Therefore, C7F8 could be applicable as one of the next generation perfluorocarbon etching materials.

Ozone based high-temperature atomic layer deposition of SiO2 thin films

In this paper, atomic layer deposition of SiO2 thin films was investigated with Si2Cl6 and O3/O2 (400 g m−3). O3/O2 is not preferred for high-temperature (>400 °C) processes due to its lower decomposition temperature, especially in a furnace-type chamber. However, with Si oxidation test using a cold-wall chamber, we have demonstrated the reactivity of O3/O2 up to 800 °C in comparison with O2 and H2O. The ALD of SiO2 films was examined at deposition temperatures from 500 °C to 700 °C. The growth rate at 600 °C was saturated to 0.03 nm/cycle with Si2Cl6 exposure over 1.2 × 105 L. O3/O2 also showed ALD-like saturation behaviors for exposures over 2.4 × 106 L. The ALD films deposited at 600 °C exhibited relatively smooth surface roughness (<0.18 nm) and low wet etch rate (1.6 nm min−1, 500:1 HF) that are comparable with PECVD SiO2 deposited at 250 °C and LPCVD SiO2 deposited at 450 °C.

High Haze Ratio-Based ITO Films Prepared on Periodic Textured Glass Surfaces for Thin Film Solar Cells

Light-trapping phenomenon is limited due to non-uniform surface structure of transparent conducting oxide (TCO) films. The proper control of surface structure with uniform cauliflower TCO films may be appropriate for efficient light trapping. We report a light-trapping scheme of SF6/Ar plasma-based textured glass surfaces for high root-mean-square (RMS) roughness and haze ratio of ITO films. It was observed that the variation in Ar flow ratio in SF6/Ar plasma during the inductive coupled plasma-reactive ion etching (ICP-RIE) process was an important factor to improve the haze ratio of textured glass. The SF6/Ar plasma textured glass showed low etching rates due to the presence of various metal elements, such as Al, B, F, and Na. The ITO films were deposited on SF6/Ar plasma-textured glass substrates showed the high RMS roughness (433 nm) and haze ratio (67.8%) in the visible wavelength region. The change in surface structure has a negligible influence on the electrical properties of ITO films. The TCO films deposited on periodic textured glass surfaces with high RMS roughness and haze ratio are proposed for high-efficiency amorphous silicon (a-Si) thin-film solar cells.

Formulation of Sodium Hypochlorite Based Slurry for Copper-Cobalt Chemical Mechanical Planarization Process

Chemical mechanical planarization of interconnect metal copper and barrier metal cobalt using NaOCl based slurry is investigated in this study. The slurry consists of 2 wt% silica, 0.5 wt% NaOCl and 5 mM BTA as inhibitor. The formulated slurry gives a combination of low etch rates and comparatively fair removal rates along with selectivity of ∼1:1.006 at pH 9, which are desired to be used in semiconductor industry. A decrease in removal rates for both Cu and Co is observed as the pH regime changes from acidic to alkaline. At acidic pH, the passive layer if any formed on the surface becomes very unstable resulting in higher polish rates. However, at neutral and alkaline medium, the passivation layer comprising of Co (II) oxide/hydroxides is formed which subsequently decreases the polish rate. A highly stable and dense passivation layer of Co (III) oxides comprising of mostly Co3O4 and CoOOH is formed at the highly alkaline region explaining the reason behind lower removal rates. The variation in turntable speed and down force pressure does not have any significant impact on selectivity obtained at optimum conditions. XRD analysis reveals the formation of oxide on Co surface after etching at pH 9.

Reactive ion beam etching of piezoelectric ScAlN for bulk acoustic wave device applications

Etching piezoelectric Scandium Aluminum Nitride (ScAlN) films containing a high concentration of Scandium with controllable feature profile angle and smooth surface is needed for next-generation Bulk Acoustic Wave (BAW) RF filter applications. This paper reports the facile etching of ScAlN with Scandium concentration of up to 15% using Reactive Ion Beam Etching (RIBE) with a very smooth surface of less than 5 nm average roughness and profile angle between 60 to 80 degrees. Recent studies show that doping AlN with a high concentration of Sc increases the piezoelectric response by five times and the bandwidth of RF filters also improved. However, etching ScAlN with a high concentration of Sc using traditional RIE and ICP based methods are extremely difficult and often results in low etch rate and rough surface. We have developed RIBE processes using a mixture of Reactive Gas and Ar with highly controllable profile angle from 60 to 80 degrees, improved etch rate (~ 36nm/min) and selectivity to photoresist mask (0.7:1), in comparison to conventional IBE. This work has positive implications in fabricating ScAlN based BAW RF filters for next-generation mobile and wireless applications.

In situ and selective area etching of GaN by tertiarybutylchloride (TBCl)

In situ etching (ISE) of gallium nitride (GaN) can enable lateral and vertical junctions through selective area etching (SAE) and regrowth. We report the study of ISE and SAE of GaN using an organometallic precursor, tertiarybutylchloride (TBCl), in a metal-organic chemical vapor deposition reactor. Compared to the conventional etching in hydrogen, the use of TBCl allows in situ etching at a much lower temperature (&amp;lt;850 °C), likely due to a more reactive etchant and a more efficient desorption rate of the etching products. The TBCl etching is near equilibrium and can be significantly changed with the change of the NH3 flow rate. We also report initial results of SAE on SiO2 patterned GaN samples. An important finding is the need to control the desorption of the reaction products in order to achieve smooth surfaces. TBCl etching is crystallographically anisotropic with low etch rates on N-terminated facets. The use of TBCl and possibly other organometallic halogen precursors is expected to enable the design and implementation of III-nitride lateral junction devices that have not been possible.

A two-step acidic texturization procedure for the manufacture of low-reflective multi-crystalline silicon solar wafer

Texturization of multi-crystalline silicon wafers for photovoltaic application comprises the removal of the saw damage and shaping the topography of the bulk surface to create a surface with a low reflectivity, the so-called texture. Etching of multi-crystalline silicon wafers is usually carried out with acid mixtures consisting of hydrofluoric acid (HF), nitric acid (HNO3) and hexafluorosilicic acid (H2SiF6). The present study reveals that such acid mixtures diluted by water or modified by the addition of ammonia solution, NH3 (added as ammonium hydroxide solution, NH4OH) can create textures with a significantly increased surface area exceeding that obtained by standard etching mixtures by a factor of 2.5–3. This yields a significantly reduced reflectivity of the etched wafer surface. However, the addition of water or NH3 causes a very low etching rate, which makes such mixtures inapplicable for industrial application. To overcome this disadvantage, a two-step etching regime was developed to produce surface-enlarged solar wafers within a timespan typical for industrial production lines. This procedure comprises a first step of slow etching with a NH3-modified etching mixture to pre-shape the as-cut wafer surface. The second etching step is performed with a typical HF/HNO3/H2SiF6 etching mixture that finalizes the texturization. Electrical measurements made on solar cells produced from such etched wafer confirm the improved surface quality of the two-step etched wafer compared to the reference wafer.

Dielectric ALD with Hydrogen Peroxide: Comparative Study of Growth and Film Characteristics for Anhydrous H2O2, H2O2/H2o Mixtures, H2o and Ozone

ALD of dielectrics requires new precursor chemistries. Recent development efforts have focused on new Organometallic, Organosilicon and Organoaluminum precursors. Our research focus has been on oxidants, and specifically hydrogen peroxide reactivity. Due to this reactivity, hydrogen peroxide use may allow lower deposition temperatures and achieve distinct properties in the resulting film when compared to other oxidants. Our research study uses: Gas-phase hydrogen peroxide, delivered from an anhydrous, ampoule-based formulation by use of a membrane delivery system.High concentration H2O2/H2O delivery by in situ concentration methods and use of a membrane vaporizer as a gas generator. Initial results for ALD growth of ZrO2 from anhydrous H2O2 and CpZr(N(CH3)3) exhibit high quality growth of film at 260° C. Minimal saturation delay and a linear growth curve were observed. XPS and XRR were used to characterize ZrO2 composition, showing significant similarities to films grown using ozone. Subsequently, films grown using ALD and H2O2 were placed into MIMCAP structures, which had high k values measured at 35. This was a slight improvement over films grown with 20% ozone concentration which had high k values of 32. Novel Gas Generator Our approach involved development of a novel gas generator that delivers H2O2/H2O mixtures. A carrier gas is connected to this generator, which delivers up to 5% H2O2/21% H2O gas by volume from 30wt% H2O2 liquid solution (H2O/H2O2=4.2). This gas mixture enables SiO2 films to be grown at highly reduced temperature compared to water. Initial testing was done with tris(dimethylaminosilane) (N(CH3) 2) 3SiH and H2O2/H2O. SiO2 was deposited at temperatures at least 200° C lower with the hydrogen peroxide mixture than with water. Titanium dioxide ALD is of interest for several applications. The unique properties of this material make it an attractive candidate for advanced patterning applications. Here, low temperature deposition is required (100-250° C). Key required properties of the resultant film include film density and wet etch rate, where low wet etch rates are needed for selective etching of SiO2 vs TiO2. Moreover, high growth per ald cycle (GPC) is needed for increased manufacturing throughput. For TiO2 ALD, we began our study with tetrakis(dimethylamino)titanium (TDMAT) precursor. Our initial goal was to compare the growth and film properties of Ozone (O3) vs Water (H2O) vs H2O2/H2O mixtures. Initial results show that H2O2/H2O mixtures can be grown at growth rates (GPC) 10-20% higher than H2O and 20-50% higher than O3. More significantly, the H2O2/H2O mixtures generate films with wet etch rates (WER) much lower than the traditional oxidants. At 125° C , over 50% reduction in WER was observed for H2O2/H2O vs H2O and over 70% reduction was observed for H2O2/H2O vs O3. Additional comparision data will be reported on wet etch rates, refractive index and composition. Initial composition results by XPS show that residual carbon and nitrogen for the H2O2/H2O film grown at 125° C are non-detectable vs 1.1% carbon and 1.1% nitrogen for Ozone at 125° C. Compositional correlations will be made with respect to wet etch rates. Figure 1

Balancing ion parameters and fluorocarbon chemical reactants for SiO2 pattern transfer control using fluorocarbon-based atomic layer etching

In manufacturing, etch profiles play a significant role in device patterning. Here, the authors present a study of the evolution of etch profiles of nanopatterned silicon oxide using a chromium hard mask and a CHF3/Ar atomic layer etching in a conventional inductively coupled plasma tool. The authors show the effect of substrate electrode temperature, chamber pressure, and electrode forward power on the etch profile evolution of nanopatterned silicon oxide. Chamber pressure has an especially significant role, with lower pressure leading to lower etch rates and higher pattern fidelity. The authors also find that at higher electrode forward power, the physical component of etching increases and more anisotropic etching is achieved. By carefully tuning the process parameters, the authors are able to find the best conditions to achieve aspect-ratio independent etching and high fidelity patterning, with an average sidewall angle of 87° ± 1.5° and undercut values as low as 3.7 ± 0.5% for five trench sizes ranging from 150 to 30 nm. Furthermore, they provide some guidelines to understand the impact of plasma parameters on plasma ion distribution and thus on the atomic layer etching process.

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.

Plasma corrosion resistance of aluminosilicate glasses containing Ca, Y and B under fluorocarbon plasma with Ar+

In this study, the corrosion resistance of alumino-silicate glass containing Ca, Y and B is assessed against CF4/O2/Ar plasma, with the results compared to those of poly-crystalline and single-crystalline alumina. All of the alumino-silicate glasses containing Ca, Y and B were more resistant than sintered alumina to a plasma-etching gas. Glass samples with boron oxide were less corroded by plasma than those without B2O3, and their surfaces were contaminated by the CFx generated by the plasma reaction.CaO-Y2O3-Al2O3-SiO2 (CYAS) showed a lower etch rate than sapphire and retained a smooth surface microstructure, unlike sintered alumina. The plasma resistance improved with an increase in the calcium oxide content. The improved plasma resistance of CYAS glass is thought to be due to the high sublimation temperature of the calcium fluoride and the strong atomic binding energy of the calcium ions in the glass.

Silicon Deep Reactive Ion Etching with aluminum hard mask

Silicon Deep Reactive Ion Etching (DRIE) process using a multi-layer mask containing a sputtered Aluminum thin film is studied. Aluminum is a candidate for very high aspect ratio DRIE, due to tis very low etch rate, but its adoption is hindered by the presence of micromasking. Effects of adding aluminum as the middle layer in a stack of silicon oxides are studied: heat dissipation and catalyst contribution to plasma chemistry are identified. The novel assumption that micromasking is generated only in the first part of the DRIE etching is demonstrated. Based on this, the effect of adding a protective coating on top of the mask is studied. The protective coating preserves aluminum in the first part of the etching, to reach a given depth before the aluminum mask is exposed to the etching plasma. A Safe aspect ratio value is obtained, beyond which micromasking does not take place. This aspect ratio is shown to be a function of the etching geometry and feature size.

Photo-Enhanced Acid Chemical Etching of High-Quality Aluminum Nitride Grown by Metal-Organic Chemical Vapor Deposition

Photo-enhanced chemical (PEC) etch of wurtzite aluminum nitride (AlN) was investigated to overcome its low etch rate. Epitaxial AlN grown on Al2O3 substrate by metal-organic chemical vapor deposition method was immersed in phosphoric acid at different temperatures under 185 nm deep-UV excitation. An inverted pyramid structure with {10-1-1} faces initially appeared, after chemical etching, evolving into hexagonal columns with {1-100} m-planes. The PEC etch rate of m-plane AlN was six-fold higher than that of non-PEC one because the reaction was enhanced by the electron-hole pairs photo-generated by 185 nm wavelength. The decrease of the activation energy from 68.5 kJ/mol (non-PEC etch) to 48.1 kJ/mol (PEC etch) was extracted, implying a reaction-limited etching process. The PEC etch mechanism of wurtzite AlN in phosphoric acid was proposed, including the initiation and growth of the etch pit and the agglomeration of the hexagonal columns. Our results propose a facile method to control the surface morphology of AlN-based electronic and optoelectronic devices.

Characterization of 1, 2, 4-Triazole as Corrosion Inhibitor for Chemical Mechanical Polishing of Cobalt in H2O2 Based Acid Slurry

Cobalt has been applied as liners in current Cu interconnect and even as interconnect conductors in next-generation interconnect. In this work, effects of corrosion inhibitors 1, 2, 4-triazole (TAZ) on Co chemical corrosion, galvanic corrosion between Cu and Co, and Co polishing properties in the H2O2 based acidic slurry have been systematically investigated. Results show that 5 mM TAZ yields a low static etching and removal rate with a selectivity of about 1: 1 for Co to Cu, applicable for Co barrier/ Cu CMP, while 100 mM TAZ yields a high removal rate with same selectivity, applicable for Co via and trench CMP. It demonstrates that TAZ can effectively inhibit the galvanic corrosion between Co and Cu. Additionally, the adsorption behavior of TAZ on Co surface is both physisorption and chemisorption. In case of 5 mM TAZ, self-assembly TAZ molecules absorb on native Co oxide. In case of 100 mM TAZ, more soluble Co-TAZ complexes form, which reduces the stability of the passivation layer and results in pitting corrosion.