Smooth Etched Surface Research Articles

Nonhalogen Dry Etching of Metal Carbide TiAlC by Low-Pressure N2/H2 Plasma at Room Temperature.

Ternary metal carbide TiAlC has been proposed as a metal gate material in logic semiconductor devices. It is a hard-to-etch material due to the low volatility of the etch byproducts. Here, a simple, highly controllable, and dry etching method for TiAlC has been first presented using nonhalogen N2/H2 plasmas at low pressure (several Pa) and 20 °C. A capacitively coupled plasma etcher was used to generate N2/H2 plasmas containing active species, such as N, NH, and H to modify the metal carbide surface. The etch rate of TiAlC was obtained at 3 nm/min by using the N2/H2 plasma, whereas no etching occurred with pure N2 plasma or pure H2 plasma under the same conditions. The surface roughness of the TiAlC film etched by N2/H2 plasma was controlled at the atomic level. A smooth etched surface was achieved with a root-mean-square roughness of 0.40 nm, comparable to the initial roughness of 0.44 nm. The plasma properties of the N2/H2 plasmas were diagnosed by using a high-resolution optical emission spectrometer, detecting the NH molecular line at 336 nm. The etching behavior and plasma-surface reaction between N2/H2 plasma and TiAlC were investigated by using in situ spectroscopic ellipsometry, in situ attenuated total reflectance-Fourier transform infrared spectrometry, and X-ray photoelectron spectroscopy. The findings indicate that the N-H, C-N, and Ti(Al)-N bonds form on the TiAlC surface etched by the N2/H2 plasmas. The mechanism for etching of TiAlC involving transformation reactions between inorganic materials (metal carbides) and inorganic etchants (N2/H2 plasma) to form volatile organic compounds such as methylated, methyl-aminated, and aminated metals is proposed. Nonhalogen or nonorganic compound etchants were used during the etching process. The study provides useful insights into microfabrication for large-scale integrated circuits.

Wet and dry etching of ultrawide bandgap LiGa5O8 and LiGaO2

Crystalline thin films of LiGa5O8 have recently been realized through epitaxial growth via mist-chemical vapor deposition. The single crystal, spinel cubic LiGa5O8 films show promising fundamental material properties and, therefore, make LiGa5O8 a potential enabling material for power electronics. In this work, chemical resistance and etch susceptibility were investigated for the first time on crystalline LiGa5O8 thin films with various wet chemistries. It was found that LiGa5O8 is very chemically resistive to acid solutions, with no apparent etching effects observed when placed in concentrated acid solutions of HCl, H2SO4, HF, or H3PO4 at room temperature. In contrast, orthorhombic (010) LiGaO2 shows effective etching in HCl solutions at varying dilution concentrations, with etch rates measured between 8.6 [1000:1 (DI water: HCl concentration)] and 6092 nm/min (37 wt. % HCl). The inductively coupled plasma reactive ion etching (ICP-RIE) of LiGa5O8 using BCl3/Ar and CF4/Ar/O2 gas chemistries was investigated. The etching rate and surface morphology of etched surfaces were examined as a function of RIE and ICP power. Using a CF4/Ar/O2 gas chemistry with an RIE power of 75 W and an ICP power of 300 W resulted in smooth etched planar surfaces while maintaining an etch rate of ∼24.6 nm/min. Similar dry etching studies were performed for LiGaO2. It was found that the BCl3/Ar gas chemistry was better suited for LiGaO2 etching, with similar surface morphology quality being obtained after etching as prior etching when a RIE power of 15 W and an ICP power of 400 W is utilized.

Fused silica microchannel fabrication with smooth surface and high etching selectivity

Channel fabrication technology has become increasingly important for microfluidic and nanofluidic devices. In particular, glass channels have high chemical and physical stability, high optical transparency, and ease of surface modification, so that there is increasing interest in glass microfluidic devices for chemical experiments in microfluidics and nanofluidics. For the fabrication of glass channels, especially those with a high aspect ratio (depth/width), lithography using a metal resist and dry etching have mainly been used. However, there are still issues involving the surface roughness of the etched channel and the low etching selectivity. In this study, a microchannel fabrication method with high etching selectivity that produces a smooth etched surface was developed. First, interference during dry etching by remaining Cr particles after the photolithography and Cr etching processes was assumed as the cause of the rough etched surface. Three different dry etching processes were introduced to verify this. In process 1 without removal of the Cr particles, the etched surface was not flat and had a 1 μm scale roughness. In process 2 where a cleaning process was included and high power etching was conducted, a smooth surface with a 1 nm scale roughness and a faster etching rate of 0.3 μm min−1 were obtained. For this high-power etching condition, the etching selectivity (fused silica/Cr) was relatively low at approximately 39–43. In process 3 with a cleaning process and low-power etching, although the etching rate was relatively low at 0.1 μm min−1, a smooth surface with 1 nm scale roughness (10 nm scale roughness deeper than 40 μm in the depth region) and a much higher etching selectivity of approximately 79–84 were obtained. The dry etching method presented in this study represents a significant contribution to microfluidics/nanofluidics for microchannel/nanochannel fabrication.

Thermal-cyclic atomic layer etching of cobalt with smooth etched surface by plasma oxidation and organometallization

Thermal-cyclic atomic layer etching of a Co film and a fine pattern with a smooth etched surface by plasma oxidation and organometallization is demonstrated. One cycle of the etching process consists of a two-temperature process. In the first step, plasma oxidation of Co is carried out at a low temperature of 25 °C to obtain a smooth Co oxide layer. In the second step, the Co oxide is organometallized with acetylacetone and desorbs at a high temperature of 210 °C that is suitable for organometallization and desorption. The etched amount per cycle is 0.6 nm, which is the same as the saturation amount of oxidation. A smooth etched surface is obtained after 20 cycles with a root mean square roughness of 0.53 nm, which is only slightly higher than the initial value of 0.34 nm. High etching selectivity over barrier metal of titanium-nitride was obtained. The two-step temperature cycle is crucial to achieve favorable etching with smooth etched surfaces.

Low-threshold AlGaN-based UVB VCSELs enabled by post-growth cavity detuning

The performance of vertical-cavity surface-emitting lasers (VCSELs) is strongly dependent on the spectral detuning between the gain peak and the resonance wavelength. Here, we use angle-resolved photoluminescence spectroscopy to investigate the emission properties of AlGaN-based VCSELs emitting in the ultraviolet-B spectral range with different detuning between the photoluminescence peak of the quantum-wells and the resonance wavelength. Accurate setting of the cavity length, and thereby the resonance wavelength, is accomplished by using doping-selective electrochemical etching of AlGaN sacrificial layers for substrate removal combined with deposition of dielectric spacer layers. By matching the resonance wavelength to the quantum-wells photoluminescence peak, a threshold power density of 0.4 MW/cm2 was achieved, and this was possible only for smooth etched surfaces with a root mean square roughness below 2 nm. These results demonstrate the importance of accurate cavity length control and surface smoothness to achieve low-threshold AlGaN-based ultraviolet VCSELs.

Selectively dry etched of p-GaN/InAlN heterostructures using BCI3-based plasma for normally-off HEMT technology

In this paper, an alternative selective dry etching of p-GaN over InAlN was studied as a function of the ICP source powers, RF chuck powers and process pressures by using inductively coupled plasma reactive ion etching (ICP RIE) system. A recipe using only BCI3-based plasma with a resulting selectivity 13.5 for p-GaN in respect to InAlN was demonstrated. Surface roughness measurements depending on the etching time was performed by atomic force microscope (AFM) measurement and showed that a smooth etched surface with the root-mean-square roughness of 0.45 nm for p-GaN and 0.37 nm for InAlN were achieved. Normally-off p-GaN/InAlN HEMT devices were fabricated and tested by using the BCI3-based plasma we developed.

Study on silicon removal property and surface smoothing phenomenon by moderate-pressure microwave hydrogen plasma

The Si removal property achieved by a localized microwave hydrogen plasma was investigated. The relationship between plasma generation parameters (input power, H2 gas flow rate, and initial surface roughness) and the removal property (etching rate and surface roughness) was revealed. Stress relief and removal of the affected Si layer by H2 plasma etching were also demonstrated. Moreover, this H2 plasma etching technique presents no etching-rate dependence on the crystal plane orientation. Therefore, even though a multi-crystalline Si sample was etched, a mirror surface could be revealed. In addition, a 100-μm-thick Si sample could be thinned to 5.7 μm by H2 plasma etching without breaking the sample. The surface roughness of the etched Si sample depends on the etching rate. A critical etching rate exists, which determines whether the etched Si surface becomes rough or smooth. When the etching rate is significantly lower than the critical etching rate, large surface roughness is generated owing to the formation of {111} platelet defects induced by H atoms in Si crystal. By contrast, when the etching rate is higher than the critical value, the roughness formation is suppressed, and the Si surface becomes smooth despite the high H flux supplied to the Si surface. The suppression of roughness formation is discussed by comparing the etching rate with the diffusion rate of H atoms into Si. It is important to maintain the Si etching rate higher than the H diffusion rate to obtain a smooth etched Si surface.

Low-Damage and Self-Limiting (Al)GaN Etching Process through Atomic Layer Etching Using O2 and BCl3 Plasma

This paper reports on the use of low-damage atomic layer etching (ALE) performed using O2 and BCl3 plasma for etching (Al)GaN. The proposed ALE process led to excellent self-limiting etch characteristics with a low direct current (DC) self-bias, which resulted in a high linearity between the etching depth and number of cycles. The etching damage was evaluated using several methods, including atomic force microscopy, photoluminescence (PL), and X-ray photoelectron spectroscopy, and the I–V properties of the recessed Schottky diodes were compared with those of digital etching performed using O2 plasma and HCl solution. The electrical characteristics of the recessed Schottky diode fabricated using the proposed ALE process were superior to those of the diodes fabricated using the conventional digital etching process. Moreover, the ALE process yielded a higher PL intensity and N/(Al + Ga) ratio of the etched AlGaN surface, along with a smoother etched surface.

Effect of concentration change of 0.1% triton added 25 wt% TMAH during fabrication of deep cavities with mesa structures in SOI wafer

Tetramethylammonium hydroxide (TMAH) is the widely used CMOS compatible anisotropic etchant in silicon wet bulk micromachining to fabricate MEMS structures. 25 wt% TMAH is commonly used because it provides the best etch selectivity between silicon and silicon dioxide with smooth etched surface morphology. If Triton X-100 is used as an additive, it results in significant decrease in the corner undercut while maintaining the good etch rate which is very useful to create mesa structures. This paper mainly focuses on the changes in etching characteristics of 0.1% Triton added 25 wt% TMAH solution when used to fabricate 380 μm deep cavities involving mesa structures in SOI wafer. We observed that the changes in etching characteristics are due to change in the concentration of etching solution which primarily arises because of loss of water content. By replenishing the etching solution with lost water content of same temperature, etching characteristics comparable to that of fresh solution have been obtained. The change in the etchant concentration during the course of etching has been studied. Further, corner undercut and etch rate of silicon have been investigated in both fresh etching solution and the solution replenished with lost water content. This study is especially useful for the control of etching characteristics like the etch rate and undercut during the fabrication of bulk-micromachined structures involving long duration of etching by replenishing the etching solution with the lost water content of same temperature at prefixed intervals of time during the course of etching to regain the etching properties of fresh solution.

Nanoscale etching of perovskite oxides for field effect transistor applications

The etching of epitaxially grown perovskite oxide BaSnO3 (BSO) and BaTiO3 (BTO) thin films is studied using Cl-based (BCl3/Ar) and F-based (CF4/Ar) plasma chemistries in an inductively coupled plasma reactive ion etching (ICP-RIE) system for the development of field effect transistors (FETs). It is found that the BCl3/Ar process has a time-independent and a higher etch rate and creates a smooth etched surface, while the etch rate of BSO and BTO in CF4/Ar plasma decreases with the etching time duration. For the BCl3/Ar etching process, the etch rate increases with both ion density and ion energy, suggesting the combination of chemical plasma etching and physical ion sputtering mechanisms. Using the Cl-based etching process, BaSnO3 and BaTiO3 heterojunction FETs are developed. The devices with a gate length of 1.5 μm have a saturation current density of 287.6 mA/mm, a maximum transconductance of gm = 91.3 mS/mm, an FET mobility of 45.3 cm2/V s, and a threshold voltage of −1.75 V. The etching processes developed in this work will enable further development of perovskite oxide heterostructure electronic devices.

Single crystal diamond blazed diffraction gratings and Fresnel microlens arrays with improved optical performance by high-resolution 3D laser lithography and pattern transfer by dry etching

We present a method to fabricate high precision 3D micro-optical components in single crystal diamond, and we demonstrate and experimentally characterize Fresnel microlens arrays and blazed diffraction gratings. The fabrication process is based on high-resolution 3D dip-in laser lithography and pattern transfer by chlorine-based reactive ion etching with a low diamond:photoresist selectivity (1:15) to obtain smooth etched surfaces with Ra < 2 nm. The fabricated microlenses have a focal length that deviates by 4% from the target value, while the blazed diffraction gratings have a maximum relative efficiency of 66% in the first order.

Electrochemical etching of AlGaN for the realization of thin-film devices

Heterogeneously integrated AlGaN epitaxial layers will be essential for future optical and electrical devices like thin-film flip-chip ultraviolet (UV) light-emitting diodes, UV vertical-cavity surface-emitting lasers, and high-electron mobility transistors on efficient heat sinks. Such AlGaN-membranes will also enable flexible and micromechanical devices. However, to develop a method to separate the AlGaN-device membranes from the substrate has proven to be challenging, in particular, for high-quality device materials, which require the use of a lattice-matched AlGaN sacrificial layer. We demonstrate an electrochemical etching method by which it is possible to achieve complete lateral etching of an AlGaN sacrificial layer with up to 50% Al-content. The influence of etching voltage and the Al-content of the sacrificial layer on the etching process is investigated. The etched N-polar surface shows the same macroscopic topography as that of the as-grown epitaxial structure, and the root-mean square roughness is 3.5 nm for 1 μm×1 μm scan areas. Separated device layers have a well-defined thickness and smooth etched surfaces. Transferred multi-quantum-well structures were fabricated and investigated by time-resolved photoluminescence measurements. The quantum wells showed no sign of degradation caused by the thin-film process.

POROUS SILICON FABRICATION PROCESS FOR OPTICAL REFLECTORS

We describe the use of porous silicon fabrication technique for fabricating non-manhattan structures in silicon using wet etching. The fabrication method is simple to set up, economical and produces smooth etched surface. A solid source diffusion of N++ in a P type wafer with low stress thermally grown silicon nitride is used as a masking layer. Comparison of porous silicon etches with wafers solid source diffusion and implanted diffusion is presented. The result show that areas where a solid source diffusion is used form an etch angle of 70-80°, however using an implanted diffusion the etch angle is closer to 90°. The selectivity of the etch during porous silicon fabrication using any of the above two as masking layer results in fabrication of high aspect ratio non-manhattan structures. These structures since are wet etched do not have surface roughness and can be used for optical applications.

Dry Etching of Metal Inserted Poly-Si Stack for Dual High-k and Dual Metal Gate Integration

Dry etching of AlN or TaN barrier layer/Mo stack (pMOS) and TaN (nMOS) in metal inserted poly-Si stacks structure for dual high-k and dual metal gate integration are investigated in detail. Firstly, in order to get a vertical profile, smooth etched surface and low Si loss, the optimal BCl3/O2/Ar plasma is developed for the etching of AlN/Mo stack. After that, both the optimal BCl3/Cl2/O2/Ar and BCl3/SF6/O2/Ar plasma are developed for the etching of TaN and TaN/Mo stack. Then, the TaN/Mo stack is chosen as the final pMOS structure since nMOS TaN metal gate will get a tapered profile under the optimal etching condition of AlN/Mo. Moreover, optimized BCl3/SF6/O2/Ar plasma is adopted for the etching of TaN and TaN/Mo stack simultaneously with both a vertical profile and a higher selectivity to the underlying layer. Finally, good electrical characteristics of CMOS device, obtained by utilizing the new developed technology, further prove that it is a practicable technology for the dual high-k and dual metal gate integration.

Investigation of the influence of ultrasonic stirring on mass transfer in the through-mask electrochemical micromachining process

Surface texturing is a widely accepted approach for friction reduction between mechanical components. Through-mask electrochemical micromachining is a simple and reliable process for metal surface texturing in which mass transport conditions have profound influence on final machined quality. An ultrasonic stirrer is usually adopted for mass transfer enhancement. However, understanding of the effects of ultrasonic stirring on mass transfer is limited, and is far from sufficient for developing guidelines for its practical application. In this work, the influences of ultrasonic stirring parameters on mass transfer have been investigated numerically and experimentally. With the numerical method, periodic pressure change in the electrolyte over time has been obtained, showing that ultrasonic stirring results in drastic transient pressure change in electrolyte fluid fields. Parameters related to ultrasonic frequency, vibration amplitude, and the depth of anode surface immersed in the electrolyte solution influence pressure amplitude. Validation experiments have been conducted and etched surface profile and morphology characterized, which show that the experimental observations are in agreement with numerical predictions. With the optimized mass transfer, well-defined micro-pits array of 30 μm and a smooth etched surface on tin-bronze substrate in large scale have been demonstrated.

Effect of Surfactant and Alcohol Additives on Etching Characteristics in Aqueous Potassium Hydroxide Solutions

Anisotropic wet etching is a simple and low cost technique for silicon bulk micromachining to fabricate microelectromechanical systems (MEMS) components. In this work, we have systematically studied the etching characteristics of Si{100}, Si{110}, and Si{111}) in various concentration of KOH (5-40 wt% in step of 5 wt%) without and with addition of surfactant Triton-X-100 and isopropyl alcohol (IPA). KOH solution with addition of IPA and Triton is termed as ternary solution. The addition of Triton and IPA affects the etching characteristics dramatically, especially the undercutting at convex corners on Si{100} surface and the etch rate of Si{110}. The undercutting at convex corners and the etch rate of Si{110} in ternary solution suppress to a significantly low level. Low undercutting is very useful for the formation of microstructure with protected corners such as mesa, while significantly low etch rate of Si{110} can be exploited to fabricate microstructure with 45º sidewall (or 45º micromirror). In addition, ternary solution with low concentration KOH provides smooth etched surface morphology.

Modified TMAH based etchant for improved etching characteristics on Si{1 0 0} wafer

Wet bulk micromachining is a popular technique for the fabrication of microstructures in research labs as well as in industry. However, increasing the throughput still remains an active area of research, and can be done by increasing the etching rate. Moreover, the release time of a freestanding structure can be reduced if the undercutting rate at convex corners can be improved. In this paper, we investigate a non-conventional etchant in the form of NH2OH added in 5 wt% tetramethylammonium hydroxide (TMAH) to determine its etching characteristics. Our analysis is focused on a Si{1 0 0} wafer as this is the most widely used in the fabrication of planer devices (e.g. complementary metal oxide semiconductors) and microelectromechanical systems (e.g. inertial sensors). We perform a systematic and parametric analysis with concentrations of NH2OH varying from 5% to 20% in step of 5%, all in 5 wt% TMAH, to obtain the optimum concentration for achieving improved etching characteristics including higher etch rate, undercutting at convex corners, and smooth etched surface morphology. Average surface roughness (Ra), etch depth, and undercutting length are measured using a 3D scanning laser microscope. Surface morphology of the etched Si{1 0 0} surface is examined using a scanning electron microscope. Our investigation has revealed a two-fold increment in the etch rate of a {1 0 0} surface with the addition of NH2OH in the TMAH solution. Additionally, the incorporation of NH2OH significantly improves the etched surface morphology and the undercutting at convex corners, which is highly desirable for the quick release of microstructures from the substrate. The results presented in this paper are extremely useful for engineering applications and will open a new direction of research for scientists in both academic and industrial laboratories.

Inductively-coupled-plasma reactive ion etching of single-crystal β-Ga2O3

Dry etching behavior of unintentionally-doped β-Ga2O3 has been studied in a BCl3/Ar chemistry using inductively-coupled-plasma reactive ion etching (ICP-RIE). The effects of various etch parameters like ICP and RIE powers, BCl3/Ar gas ratio and chamber pressure on etch rate are studied systematically. Higher ICP, RIE powers and lower pressure conditions are found to enhance the etch rate. A synergic etching mechanism between chemical and physical components is proposed and used to obtain fast Ga2O3 etch rates more than 160 nm/min, nearly-vertical sidewalls and smooth etched surfaces. The findings of this work will enable Ga2O3 vertical devices for power electronics.

Roughness effect for tunable wetting surfaces on metallic substrate

A facile process for controllable fabrication of wetting surfaces with variable hierarchical structures on metallic substrates is proposed in this study. This process, which combines the through-mask electrochemical micromachining with hydrothermal growth method, could be applied on all kinds of type and size of conductive metal. First, the anodic dissolution process is predicted using numerical simulation and experiments. The formulation of electrolyte and the etching conditions in through-mask electrochemical micromachining are optimized. Ordered microstructures and smooth etched surface in large scale are obtained using the optimized parameters. Moreover, a technology has been explored to obtain various styles of multi-level structures through an alignment system or combining with a hydrothermal method of growing ZnO nanorods. The wetting effects of the rough three-dimensional surfaces are evaluated using a contact angle system. Furthermore, the wetting and the preliminary friction reduction effects of the rough three-dimensional surfaces are evaluated using contact angle system.

Optimization of Reactive Ion Etching of Polycrystalline Diamond for MEMS Applications

It has been found that diamond columns can be formed unintentionally in reactive ion etching (RIE) with O2 plasma even without a precoated metal layer. The experimental results indicate that the existence of these diamond columns prevents the effective removal of polycrystalline diamond (poly-C), also known as microcrystalline diamond. A three-step sequential RIE of poly-C thin film in CF4 (or CF4/Ar), O2, and H2 plasmas is developed using lithographically patterned Al acting as a hard mask to achieve a smooth-etched surface after the removal of poly-C. This etching technique can remove a thick poly-C layer very effectively. For the first time, this letter eliminates RIE-related damage to underlying substrate (specific to RIE of poly-C) to optimize the technology for single- and multi-material MEMS made from poly-C. [2015-0141]