Plasma Assisted Chemical Etching Research Articles

Influence of electrolytic plasma spatial distribution on nanoporous structure etching on 4H-SiC surface

Single-crystal SiC nanoporous structure has important applications in the field of micro electromechanical systems, chemical sensors and biomedical devices. However, SiC exhibits a strong chemical inertness due to the short interatomic distance and high binding energy, making it significantly challenging to fabricate nanoporous structure. Electrolytic plasma-assisted chemical etching (EPACE) based on electrochemical anodic oxidation and high-activity electrolytic plasma etching is an ideal method for SiC nanoporous structure etching. In which, the spatial distribution of electrolytic plasma has an important impact on EPACE performance, such as etching efficiency and stability. In order to analyze the impact mechanism, this paper studies EPACE in tool electrode mounting posture of vertical and horizontal types by visual observation, real-time recording of voltage-current waveforms during processing, and the morphology analysis of SiC nanoporous structure after etching. It can be found that in the vertical etching type, EPACE can proceed stably because the workpiece immersion depth is greater than the bubble accumulation thickness. However, in the horizontal etching type, bubbles accumulate seriously between the workpiece and the liquid surface, which may cause bubble cavitation and abnormal discharge, leading to damage to the etching surface. The etching current in the vertical etching type is about 1.3 times that of the horizontal etching type, indicating that the bubble mass transfer in the vertical etching type is faster and the etching impedance is smaller, which is beneficial to improve the etching efficiency and obtain a uniform nanoporous structure. Finally, a SiC nanoporous layer with a thickness of 10.6 μm was prepared at etching time t = 50 min in vertical etching type, and the etching efficiency was 212 nm/min. In addition, the prepared nanoporous structure (nanofiber) was connected to the SiC substrate, indicating that it has a good bonding strength.

Fabrication of Homogeneous Nanoporous Structure on 4H-/6H-SiC Wafer Surface via Efficient and Eco-Friendly Electrolytic Plasma-Assisted Chemical Etching.

Nanoporous single-crystal silicon carbide (SiC) is widely used in various applications such as protein dialysis, as a catalyst support, and in photoanodes for photoelectrochemical water splitting. However, the fabrication of nano-structured SiC is challenging owing to its extreme chemical and mechanical stability. This study demonstrates a highly-efficient, open-circuit electrolytic plasma-assisted chemical etching (EPACE) method without aggressive fluorine-containing reactants. The EPACE method enables the nano-structuring of SiC via a plasma-enveloped microtool traversing over the target material in an electrolyte bath. Through process design, EPACE readily produces a uniform nanoporous layer on a 4H-SiC wafer in KOH aqueous solution, with adjustable pore diameters in the range 40-130nm. Plasma diagnosis by optical emission spectrometry (OES) and surface microanalysis reveal that EPACE realizes a nanoporous structure by electrolytic plasma-assisted oxidation and subsequent thermochemical reduction of an oxide. An increase in voltage or a decrease in etch gap intensifies the plasma and improves the etching efficiency. The maximum etch rate and depth reach 540nm min-1 and 10µm, respectively, demonstrating the significant potential of the approach as a time-saving and sustainable nanofabrication method for industrial applications. Further, the effectiveness of the fabricated SiC nanoporous structure for application in photoelectrochemical water splitting is demonstrated.

Plasma Jet Machining

AbstractPlasma Jet Machining (PJM) is a surface figuring technology based on atmospheric plasma assisted chemical etching or deposition, respectively. In both cases a sub‐aperture plasma jet source is used combined with a CNC multi‐axes system for the processing of curved surfaces. It is under development for the surface figuring of a variety of optical materials by IOM for about 15 years. PJM is capable to figure deep aspheric or free‐form substrates with high material removal rate and high spatial resolution. Based on chemical reactions between plasma generated radicals and the surface PJM does not introduce any damage to the processed surface and sub‐surface region in contrast to abrasive techniques. Deterministic deposition of SiOx layers and subsequent proportional transfer using ion beams or polishing is another plasma jet based technique for surface figuring that extends the range of machinable materials. The article gives an overview on the current state of PJM development in IOM and shows examples of its application.

Heavy-Metal (Fe/Ni/Cu) Behavior in Ultrathin Bonded Silicon-On-Insulator (SOI) Wafers Evaluated Using Radioactive Isotope Tracers

The behavior of Fe, Ni and Cu in bonded silicon-on-insulator (SOI) wafers thinned down to 0.5 µm by plasma-assisted chemical etching (PACE) was investigated for the first time by the radioactive isotope tracer method, which can avoid the evaluation errors due to contamination during sample preparation or analysis. When ultrathin bonded SOI wafers without an intentional gettering site were contaminated with Fe or Ni from the surface, Fe and Ni did not diffuse into the substrate through the buried oxide (BOX) layer after annealing in N2(2%O2) ambient at 900°C and 700°C, respectively. Cu easily diffused into the substrate through the BOX layer after annealing at 700°C for 60 min, and was captured at the bonding interface. It was found that the behavior of Ni, which exhibits the same diffusivity in Si as does Cu, was quite different in ultrathin bonded SOI wafers from that in bulk Si wafers due to the BOX layer of the SOI structure.

New developments in silicon Czochralski crystal growth and wafer technology

An overview of recent progress in large-diameter crystal pulling and associated new wafer preparation technologies is given. A clear trend can be observed that, with increasing crystal diameter, the average oxygen concentration and the density of intrinsic grown in defects is reduced. Oxygen reduction and control is achieved by using magnetic fields, while bulk defect reduction requires sophisticated hot zone engineering. New growing methods currently under development result in new crystal types with a high density of interstitials in the outer area and a lower density of vacancies in the center. In addition, crystals are grown under near-equilibrium conditions so the super-saturation of the intrinsic point defects remains below the critical value required for clustering of the point defects into larger agglomerates . Geometry and light point defects/localized light scatter requirements for future wafer generations are becoming more stringent with each decrease in design rule. This leads to the necessity of novel wafer surface preparation methodologies such as advanced single side polishing, double side polishing and plasma assisted chemical etching with local wafer surface treatment. Finally, the trend toward low thermal budget integrated circuit processes leads to the necessity of having oxygen precipitation tuning and gate oxide integrity tuning, as well as the creation of a denuded zone by the silicon wafer manufacturer. New approaches to meet these requirements are annealing at high temperature and/or rapid thermal processing. The development of the new epitaxial wafer types is highlighted.

Study of HF Defects in Thin, Bonded Silicon-on-Insulator Dependent on Original Wafers

To study the origin of HF defects in thin, bonded silicon-on-insulator (SOI) wafers fabricated by the plasma assisted chemical etching (PACE) process, the dependence of HF defects on original wafers [wafers fabricated by Czochralski method (CZ wafers), hydrogen-annealed CZ wafers and epitaxial wafers] was investigated. It was shown that HF defect density was affected by the type of original wafer used, and no HF defect was detected when epitaxial wafers were used as bond wafers. HF defects were detected on 0.2 µm or thinner SOI wafers with CZ wafers. Crystal originated particles (COPs) at SOI and buried oxide (SOI/BOX) interface were found to be the main origin of HF defects by inspecting light point defects (LPDs) at the SOI/BOX interface.

Precise Measurement of Strain Induced by Local Oxidation in Thin Silicon Layers of Silicon-on-Insulator Structures

We used X-ray diffraction to quantitatively measure the strain induced by the local oxidation of silicon in thin-film silicon-on-insulator (SOI) wafers. In the samples of bonded SOI wafers thinned by plasma-assisted chemical etching, the reflection peaks of the top Si layer and the base Si substrate could be measured independently because the orientation of these lattice planes differs slightly. Moreover, the strain near the surface region could be measured because the top Si layer is only 200-nm thick. We show that the strain in the SOI wafers is more than one order of magnitude larger than that in the bulk wafers when the device region is fully isolated by the surrounding SiO2.

Effective KOH Etching Prior to Modified Secco Etching for Analyzing Defects in Thin Bonded Silicon on Insulator (SOI) Wafers

The modified Secco etching which was developed for detecting threading dislocations in separation by implanted oxygen (SIMOX) wafers showed etch pits with a density of 103–106/ cm2 when it was applied to bonded silicon on insulator (SOI) wafers thinner than 1 µ m produced by the plasma assisted chemical etching (PACE) process including touch polishing. When these pits were observed, a group of pits appeared as a scratch pattern. Also the density of pits was dependent on the remaining SOI thickness after the first diluted Secco etching in the modified Secco etching process. These results indicated that the density of defects had a certain distribution in the SOI thickness direction and that the defects were different from threading dislocations which reached buried oxides from the surface. In order to observe the distribution of defect density in the SOI depth direction, KOH etching was utilized prior to the modified Secco etching. Using this method, it was found that surface defects were predominant and bulk defects were also present in thin bonded SOI layers.

Basis of macroscopic and microscopic surface shaping and smoothing by plasma assisted chemical etching

This is a summary of the basis of macroscopic shaping (or figuring) and microscopic pattern transfer and microsmoothing (or polishing) by plasma assisted chemical etching (PACE). The ability of PACE to precisely shape surfaces without mechanical contact stems from the reliable etch rate map of a plasma assisted chemical etch tool by programmed scanning of the etch tool’s profile over the surface to be shaped. For a given shape modification and the plasma tool removal rate map this programmed tool dwell time is obtained by deconvolution using Fourier transforms. The transforms of these quantities also yield their spatial frequency characteristics and clarify PACE’s figuring behavior, but do not explain the observed polishing. Both polishing and micropattern transfer etch profiles are governed by a differential surface evolution equation, whose behavior is reviewed here. We also show why the PACE process is able to etch directionally, produces negligible subsurface damage and vertical etch rates to 100 μm/min or volume removal rates of ∼10 mm3/min for a 1 cm diam plasma due to the atypical plasma regime employed.

Comparison of the smoothing and shaping of optics by plasma-assisted chemical etching and ion milling using the surface evolution theory

Plasma-assisted chemical etching (PACE) and ion milling (IM), originally developed for microelectronic fabrication, can now be used to shape or figure and smooth or polish optical and other surfaces with the unique advantage of doing so without mechanical contact. Using a recent theory of the temporal evolution of surfaces that is due to arbitrary additive or subtractive microscopic processes, the author critically compares the predicted and observed smoothing of PACE and IM in this paper to provide insight into their smoothing and other behavior. PACE removes > 10 mm(3)/min and smooths surfaces of considerable roughness (> 10-µm rms). It has produced smoothed surfaces to < 1.0-nm rms without generating electronic subsurface damage. The IM removal rate is > 10(3) times slower, is used primarily for making corrections of less than a micrometer, intrinsically generates electronic subsurface damage, and furthermore requires starting with a smooth surface (≈ 1.0 nm) to maintain smoothness. Both PACE and IM remove mechanical subsurface damage. In agreement with observation, it is shown theoretically that the central distinction between PACE and IM stems from substantial differences in their intrinsic smoothing.

4668366 Optical figuring by plasma assisted chemical transport and etching apparatus therefor : Charles Zarowin assigned to The Perkin-Elmer Corporation

4668366 Optical figuring by plasma assisted chemical transport and etching apparatus therefor : Charles Zarowin assigned to The Perkin-Elmer Corporation

Plasma-assisted deposition processes: Theory, mechanisms and applications

The energetic electrons in electric discharge plasmas interact effectively with the electron systems of atoms and molecules and thereby impart to these plasmas the same remarkable catalytic properties that characterize the radiation chemistry environment. Plasmas offer the additional important attribute that the energy and flux of active species passing from the plasma volume to an adjacent surface can be controlled to a large extent by controlling the volume chemistry and the electrical potential of the surface relative to the plasma. Accordingly, preferred interactions can be promoted with the surfaces of substrates and growing coatings, and plasmas have come to play a significant role in a range of deposition processes including sputtering, ion plating, activated reactive evaporation and plasma-assisted chemical vapor deposition, etching, polymerization and nitriding. In this paper these applications are reviewed briefly and the analytical modeling of electric discharge plasmas is discussed with emphasis on the principal mechanisms that should be taken into account.