Buffered HF Solution Research Articles

Communication—A Powerful Method to Improve Dielectric/GaN Interface Properties: A Dummy SiO2 Process

We report a simple and effective method for improving dielectric/GaN interface properties. In the process, a 5 nm thick SiO2 layer was deposited onto a GaN(0001) substrate via plasma-enhanced atomic layer deposition, followed by annealing at 800 °C for 300 s under a flowing N2 atmosphere. The SiO2 layer was then removed using buffered HF solution, and Pt/Al2O3/GaN metal-oxide-semiconductor capacitors were fabricated on the substrate. Positive-bias stress tests revealed that the flat-band voltage shifts were substantially reduced for devices fabricated using this process, probably because of improved interface crystallinity. This method can also be applied to other dielectric/GaN systems.

Concentration, chemical bonding, and etching behavior of P and N at the SiO2/SiC(0001) interface

Phosphorous and nitrogen are electrically active species at the SiO2/SiC interface in SiC MOSFETs. We compare the concentration, chemical bonding, and etching behavior of P and N at the SiO2/SiC(0001) interface using photoemission, ion scattering, and secondary ion mass spectrometry. Both interfacial P and N are found to be resistant to buffered HF solution etching at the SiO2/SiC(0001) interface while both are completely removed from the SiO2/Si interface. The medium energy ion scattering results of etched phosphosilicate glass/SiC not only provide an accurate coverage but also indicate that both the passivating nitrogen and phosphorus are confined to within 0.5 nm of the interface. Angle resolved photoemission shows that P and N are likely situated in different chemical environments at the interface. We conclude that N is primarily bound to Si atoms at the interface while P is primarily bound to O and possibly to Si or C. Different interface passivating element coverages and bonding configurations on different SiC crystal faces are also discussed. The study provides insights into the mechanisms by which P and N passivate the SiO2/SiC(0001) interface and hence improve the performance of SiC MOSFETs.

Fabrication of a double-tapered probe with enhanced aspect ratio for near-field scanning optical microscopy

A double-tapered probe has widely been used for high-throughput NSOM, which has an infrared cutoff wavelength of around 0.8 μm. Some researchers often need a single-mode fiber probe with a visible wavelength cutoff for polarization-modulation NSOM and related techniques. Improvement in the aspect ratio of the probe is the most important issue to be addressed for apertured NSOM applications. This paper proposes a new NSOM fiber and a double-tapered probe with the protruding clad and single-mode waveguide. The fiber consists of Ge-SiO2 core, SiO2 clad, and F-SiO2 support and has a cutoff wavelength of 0.47 μm. The doping ratio profiles of Cl and F must be carefully controlled while fabricating the fiber preform rod. Furthermore, the double-tapered probe is optimized for the aspect ratio on the basis of geometric consideration. To fabricate the proposed probe, the new single-mode fiber was tapered by immersing it in buffered HF solutions. Owing to the protruding clad, the fabricated probe has an aspect ratio of 0.24. The aspect ratio is 1.8 times as high as the fabricated previous probe.

Redeposition in plasma-assisted atomic layer deposition: Silicon nitride film quality ruled by the gas residence time

The requirements on the material properties and growth control of silicon nitride (SiNx) spacer films in transistors are becoming ever more stringent as scaling of transistor structures continues. One method to deposit high-quality films with excellent control is atomic layer deposition (ALD). However, depositing SiNx by ALD has turned out to be very challenging. In this work, it is shown that the plasma gas residence time τ is a key parameter for the deposition of SiNx by plasma-assisted ALD and that this parameter can be linked to a so-called “redeposition effect”. This previously ignored effect, which takes place during the plasma step, is the dissociation of reaction products in the plasma and the subsequent redeposition of reaction-product fragments on the surface. For SiNx ALD using SiH2(NHtBu)2 as precursor and N2 plasma as reactant, the gas residence time τ was found to determine both SiNx film quality and the resulting growth per cycle. It is shown that redeposition can be minimized by using a short residence time resulting in high-quality films with a high wet-etch resistance (i.e., a wet-etch rate of 0.5 nm/min in buffered HF solution). Due to the fundamental nature of the redeposition effect, it is expected to play a role in many more plasma-assisted ALD processes.

Cleaning of pyrolytic hexagonal boron nitride surfaces

Hexagonal boron nitride (h‐BN) has recently garnered significant interest as a substrate and dielectric for two‐dimensional materials and devices based on graphene or transition metal dichalcogenides such as molybdenum disulfide (MoS2). As substrate surface impurities and defects can negatively impact the structure and properties of two‐dimensional materials, h‐BN surface preparation and cleaning are a critical consideration. In this regard, we have utilized X‐ray photoelectron spectroscopy to investigate the influence of several ex situ wet chemical and in situ thermal desorption cleaning procedures on pyrolytic h‐BN surfaces. Of the various wet chemistries investigated, a 10 : 1 buffered HF solution was found to produce surfaces with the lowest amount of oxygen and carbon contamination. Ultraviolet/ozone oxidation was found to be the most effective ex situ treatment for reducing carbon contamination. Annealing at 1050 °C in vacuum or 10−5 Torr NH3 was found to further reduce oxygen and carbon contamination to the XPS detection limits. Copyright © 2015 John Wiley & Sons, Ltd.

Atomic state and characterization of nitrogen at the SiC/SiO2 interface

We report on the concentration, chemical bonding, and etching behavior of N at the SiC(0001)/SiO2 interface using photoemission, ion scattering, and computational modeling. For standard NO processing of a SiC MOSFET, a sub-monolayer of nitrogen is found in a thin inter-layer between the substrate and the gate oxide (SiO2). Photoemission shows one main nitrogen related core-level peak with two broad, higher energy satellites. Comparison to theory indicates that the main peak is assigned to nitrogen bound with three silicon neighbors, with second nearest neighbors including carbon, nitrogen, and oxygen atoms. Surprisingly, N remains at the surface after the oxide was completely etched by a buffered HF solution. This is in striking contrast to the behavior of Si(100) undergoing the same etching process. We conclude that N is bound directly to the substrate SiC, or incorporated within the first layers of SiC, as opposed to bonding within the oxide network. These observations provide insights into the chemistry and function of N as an interface passivating additive in SiC MOSFETs.

Preparation of 24 Ternary Thin Film Materials Libraries on a Single Substrate in One Experiment for Irreversible High-Throughput Studies

For different areas of combinatorial materials science, it is desirable to have multiple materials libraries: especially for irreversible high-throughput studies, like, for example, corrosion resistance testing in different media or annealing of complete materials libraries at different temperatures. Therefore a new combinatorial sputter-deposition process was developed which yields 24 materials libraries in one experiment on a single substrate. It is discussed with the example of 24 Ti-Ni-Ag materials libraries. They are divided based on the composition coverage and orientation of composition gradient into two sets of 12 nearly identical materials libraries. Each materials library covers at least 30-40% of the complete ternary composition range. An acid etch test in buffered-HF solution was performed, illustrating the feasibility of our approach for destructive materials characterization. The results revealed that within the composition range of Ni < 30 at.%, the films were severely etched. The composition range which shows reversible martensitic transformations was confirmed to be outside this region. The high output of the present method makes it attractive for combinatorial studies requiring multiple materials libraries.

Fabrication of Micro-Cantilevers Using RF Magnetron Sputtered Silicon Carbide Films

In the present work, silicon carbide (SiC) films were deposited by RF magnetron sputtering process on Si (100) substrates for micro-cantilever fabrication. The films were deposited without external substrate heating using a ceramic SiC target at 10 mTorr sputtering pressure, 200 W power and 50 mm target-to-substrate spacing. X-ray diffraction pattern shows that the films were amorphous in nature. In order to investigate the chemical inertness, the SiC coated Si substrates were dipped in buffered HF (BHF) at room temperature and in 40% KOH solution at 80 °C for varying length of time. Atomic force microscope was used to investigate surface roughness and morphology of the films before and after chemical processing. Micro-cantilever beams of the SiC film were fabricated by a single mask process. The SiC film was patterned using reactive ion etching (RIE) in SF6-O2 plasma. Thermally evaporated Al film was used as a mask during RIE process. This process also resulted in the formation of convex corners which were exploited for anisotropic etching of Si under the SiC film. The SiC cantilever beams were released by anisotropic etching of Si in KOH at 80 °C without using additional masking material. Scanning electron microscopy was used to observe the fabricated SiC micro-cantilever beams. The morphology of the SiC film after prolonged exposure to KOH was observed to be similar to that of the as-deposited film. The RF magnetron sputtered SiC films were found to be highly inert in KOH and BHF solutions. Due to difficulty in micromachining of bulk SiC material and its high cost, the RF sputtered SiC films on Si can provide a low cost structural material in MEMS.

Interface Properties of the Two Step Oxide Layers by UV Light Excited Ozone Silicon Oxidation and Chemical Vapor Deposition (CVD)-SiO2 Film

SiO2 film was made on Si(100) by two-step film growth process at 100°C to improve the interface properties. The first part of the process is the direct Si oxidation using UV-excited ozone (O(1D)), which is generated by the UV irradiation to high concentrated (>90%) ozone gas. In the second part, O(1D) and Hexamethyldisilazane chemical vopor deposition (CVD)(O(1D)-CVD) process is used. The SiO2 film thickness of the direct Si oxidation for 10 min. in the two-step SiO2 film was estimated to be about 3.2 nm from the etching rate in change a buffered HF solution. Interface trap density Dit of the two-step SiO2 film with direct Si oxidation of 10 min. is almost equal to that of Si direct oxidation film alone. This result indicates that two-step oxidation process can be applied for the practical fabrication.

The Computer-Controlled Chemical Polishing Techniques for Precision Optics

The computer-controlled chemical polishing (CCCP) techniques based on the Marangoni effect have been developed to manufacture precision optics on polished fused silica surface. In this study, we present the Marangoni confined chemical-etching process in which the material removal on optical surfaces can be accomplished by etching with buffered HF solution. The process shows stable characteristics and good repeatability while the etching depth can be controlled in the order of ten nanometers. We also present the experimental results of this technology for fabrication of phase corrector and continuous phase plate. Results show that the CCCP’s deterministic sub-aperture-polishing characteristics make it possible to correct the surface error and imprint complex phase structure with spatial scale-length of several millimeters onto optical surface.

Creating microstructures on silicon wafers using UV‐crosslinked polystyrene thin films

AbstractWe report that UV‐crosslinked polystyrene (PS) thin films can be used as an inexpensive wet chemical etch resist to create microstructures on silicon wafers. When spin‐coated PS films were irradiated, the polymer undergoes UV‐induced crosslinking. Patterned PS films were successfully used as the protective layer withstanding chemical etching with the buffered HF solution. Removal of the polymer films by sonicating in water generated microstructures in silicon wafers. This simple and environmentally friendly procedure employs inexpensive commodity polymer and eliminates the use of organic solvents and harsh conditions in the subsequent development process. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers

Formation of AlN layer on (1 1 1)Al substrate by ammonia nitridation

AlN layers were formed on (1 1 1)Al substrates at a temperature below the melting point of Al using preheated ammonia in vacuum. The effect of the removal of the surface oxides on the substrate in the process was investigated. It was found that treatment using a buffered HF solution and subsequent annealing in vacuum was effective for obtaining a clean Al substrate surface. It was clarified that the removal of the surface oxides is required for the nitridation of Al substrates at a low temperature below the melting point of Al.

Formation of porous silicon on a non-conductive substrate and its use as a sacrificial layer

Porous silicon (PS) is an excellent material to be used as a sacrificial layer (SL) for the fabrication of membranes, bridges, cantilevers and other complicated micro-sensor structures. Major advantages of this material are its smooth surface and large area of pores that provide a high rate of PS SL removal. In this work, electrochemical and chemical etching processes of porous silicon formation from polycrystalline silicon deposited on insulating substrates have been studied. It was shown that a sufficiently thick (about 2 µm), uniform PS with a smooth surface and high etching rate in buffered HF solution can be made by both electrochemical and chemical etching from low resistive p-type poly-Si only. Finally, the obtained PS layers have been used as sacrificial layers in a fabrication process of microbolometer array.

Rapid fabrication of polymer microfluidic systems for the production of artificial lipid bilayers

A polymer microfluidic device has been fabricated using rapid prototyping techniques. The device was built to allow the formation and subsequent investigation of artificial bilayer lipid membranes (BLMs). A simple dry film photoresist stamp was used to hot emboss microfluidic channels into PMMA films. Laser micromachining was employed to form an aperture through the PMMA channels, across which the BLM was later formed. The dry film photoresist was also used as a simple etch mask for the deep etching of glass substrates in buffered HF solutions, which was used in this work for the production of glass embossing stamps. We show that bilayer films can be successfully produced across laser micromachined apertures in PMMA films.

Enhancement of p-GaN Conductivity Using PECVD SiO[sub x

A technique to enhance the hole concentration in activated Mg-doped p-type GaN epitaxial layers is described. The method consists of depositing a porous plasma-nhancedchemical vapor deposited SiOx layer on top of p-GaN after which the sample is heated to 950°C in nitrogen ambient for 1 min followed by the removal of the SiOx layer in a buffered HF solution. A significant improvement of the conductivity of the p-GaN layer has been obtained.

Effect of the hydrogen content in the optical properties and etching of silicon nitride films deposited by PECVD for uncooled microbolometers

AbstractWe have studied silicon nitride films a-SiN:H deposited at a substrate temperature of 350°C by means of the Low frequency (LF) PECVD from silane and nitrogen as stock gases. Film properties as hydrogen bonding and content, nitrogen content, refractive index and etch rate are reported and analyzed. Our deposited films show physical properties similar to those that are obtained deposition temperatures of 700°C by the low pressure chemical vapor deposition (LPCVD) technique. An investigation of bonding structures for the deposited films was performed, and quantitative results for hydrogen bonding based on Fourier Transform Infrared (FTIR) analysis are presented. It was observed that low hydrogen content in the films is in good correlation with low etch rate in 10% buffered HF solution, therefore these films present a material with good etch selectivity in respect to others materials (as phosphosilicate glass PSG, Al etc). Selectivity which makes these films very promising in surface micromachining for fabrication of sensors and device structures, e.g. microbolometers. Additionally, the Si-N bond at 830-840cm-1was analyzed because of its big absorption produced at 12μm; therefore these films can be used as absorber layers in uncooled microbolometres.

Novel post CMP cleaning using buffered HF solution and ozone water

Post chemical mechanical polishing (CMP) cleaning is a key process for copper (Cu) CMP in dual damascene interconnection technology. During the post CMP cleaning, it is an important issue to minimize organic and Cu contamination residues on the dielectric surface. This study proposed a novel post CMP cleaning using HAL buffer hydrofluoric (BHF) solution and ozone (O 3) water cleaning. The performance of the proposed cleaning technology was investigated and compared to conventional citric solution cleaning, which is currently used in post Cu CMP cleaning. From roughness, contamination residues and electrical characteristics, the proposed cleaning technology showed better performance than citric solution cleaning did. This excellent cleaning performance is attributed to the surface etching and contamination elimination effect of HAL BHF solution and O 3 water. Based on the experimental results, the proposed cleaning technology is feasible and superior to the conventional post CMP cleaning.

Etching characteristics of SiO 2 irradiated with focused ion beam

The etching characteristics of SiO 2 films, which were irradiated with Si 2+ focused ion beams, in a buffered HF solution were comprehensively investigated. Dependence of the etching rate on the dose was investigated. The rate increased with increasing dose (8 × 10 13–1 × 10 15 cm −2), however it decreased for doses exceeding a critical value (1 × 10 15 cm −2). The TRIM simulation indicated that irradiation with this critical value resulted in the concentration of vacancies at the Si site equal to that of Si atoms in SiO 2. These results can be explained by considering the enhancement of etching by vacancies and the retardation by implanted Si atoms. The retardation was confirmed experimentally.

Proposed Sample Structure for Marked Enlargement of Excimer-Laser-Induced Lateral Grain Growth in Si Thin Films

60-µm-long grains were grown laterally in a 50-nm-thick Si layer on thermally grown oxide by a single shot irradiation of an excimer-laser-light pulse. The origin of such a marked (about 10 times) enlargement of grains can be found in a newly introduced photosensitive capping layer on the Si layer, which has been heated uniformly to extremely high temperatures by absorbing a fraction of excimer-laser-light and gradually transfers the stored heat to the Si layer after the laser irradiation, resulting in the extension of the liquid-phase duration of the Si layer. Ultrathinning the Si layer can effectively reduce the lateral heat loss along the highly heat-conductive, liquid or crystallized Si layer, and, in turn, extend the liquid-phase duration further. SiON and SiOC films are good candidates for the photosensitive capping layer, although SiOC is preferable since it can be etched by a buffered HF solution and has an appropriate light absorption coefficient for a broad band of laser-light-wavelengths.

The etching behavior of APCVD PSG thin films used as sacrificial layers for surface micromachined resonant microstructures

Abstract The deposition and the configuration of the sacrificial (phosphosilicate glass (PSG)) and active layers (polysilicon) are one of the most sensitive processing steps in surface micromachining technique for resonant microstructures. Starting from the technological steps for the resonant microstructures manufacturing, a new solution in order to optimise these technological steps are proposed. During the doping processes of polysilicon (∼1000 °C) a significant densification of the PSG layer appears. This leads to a dramatical decrease of the PSG etch rate in buffered HF solution and a strong limitation in surface micromachining technique (with unwanted effects on resist mask and metallic layer integrity) is resulting. The decrease of the PSG etch rate is mainly due to the ex-diffusion of the phosphorus from PSG film. An original solution is proposed in order to minimize the decrease of the PSG etch rate: the replacement of the PSG film with a Si 3 N 4 /PSG/SiO 2 /Si 3 N 4 multilayer. The using of a thin Si 3 N 4 layer has the advantage of suppressing the ex-diffusion of P atoms and avoids the attack of HF on the polysilicon layer.