Low-pressure Plasma Etching Research Articles

The inhibition of biodegradation on building limestone by plasma etching

Plasma etching is an innovative technique that has been recently applied in the cleaning of soiled archaeological objects. This research investigated the use of low-pressure plasma etching in cleaning microbial contaminations on an oolitic limestone from an UNESCO World Heritage listed monument: the Batalha Monastery in Central Portugal. The cleaning effect was assessed by FTIR, SEM, optical microscope, and cell viability index measurement. Experimental work suggests that plasma discharge can cause rupture in the microbial cell structures and is helpful in removing microorganisms from the surface of the stone. At the macroscopic level, detachment of microbial crust was also observed in plasma etched bio-deteriorated limestone surfaces. Furthermore, plasma etching can inhibit the microbial growth by decomposing and eliminating the sugar-containing compounds on the limestones, thus eliminating a major nutrient supply for microbial metabolism and reproduction. Plasma etching can therefore be regarded as a fast and eco-friendly conservation tool for stone heritage architecture to prevent/reduce the onset of bio-colonization and biodegradation.

Mechanism of oxygen and argon low pressure plasma etching on polyethylene (UHMWPE)

Different preferential sites for etching are observed for low pressure argon and oxygen plasma on ultra-high molecular weight polyethylene. Atomic force microscopy analysis showed an organized pattern after 1 min of oxygen plasma treatment. Roughness decreases after 3 min and then after 5 min increases again, when surface is completely damaged. A different etching pattern is observed after argon plasma treatment, where nanometric pits are formed with size increasing with treatment time. Infrared spectroscopy shows that, by analyzing the relative intensity of the different peaks, the different etching mechanisms are explained where oxygen plasma reduces coupling between parallel chains in the crystalline short axis direction and argon plasma suppresses hydrogen and induces surface reorientation of functional groups.

Atmospheric plasma etching of polymers: A palette of applications in cleaning/ashing, pattern formation, nanotexturing and superhydrophobic surface fabrication

Atmospheric Pressure Plasmas (APPs) are promising alternatives to their low-pressure counterparts for material surface treatment due to their potential for cost-effective continuous processing. Here we present for the first time to our knowledge a palette of applications of polymer etching showing the potential of APPs for cleaning/ashing, nanopattern formation, nanotexturing, and hierarchical superhydrophobic surface fabrication using the same plasma source configuration. First, we used a dielectric barrier discharge (DBD) plasma source to study etching/cleaning/ashing of different polymers such as PMMA films and AZ photoresist with lithographic patterns using He/O2 mixtures. We achieved high-rate, uniform etching, indicating the source potential for cleaning/ashing even in ambient air conditions. We have noticed that thick photoresist films are damaged after prolonged etching in air plasmas, a phenomenon resembling photoresist damage observed during low-pressure plasma etching. Second, we applied colloidal lithography with polystyrene (PS) nanoparticles followed by isotropic APP etching in order to form nanopatterns on both Silicon and Polymers. We observed that APP induces “cauliflower-like” nanotexturing of the PS spheres. This nanotexturing in combination with plasma etching of the underlying polymer and the spheres allow the fabrication of hierarchical surfaces. Optimization of plasma etching duration (i.e. hierarchical topography) followed by low-pressure plasma deposition of a low surface energy fluorocarbon (hydrophobic) coating led to the preparation of superhydrophobic surfaces with water contact angle 158° and hysteresis 9° (roll-off behavior).

Low-pressure plasma-etching of bulk polymer materials using gas mixture of CF4 and O2

In this study, we have proposed a low-pressure reactive ion etching of bulk polymer materials with a gas mixture of CF4 and O2, and have achieved precise fabrication of poly(methyl methacrylate) (PMMA) and perfluoroalkoxy (PFA) bulk polymer plates with high-aspect-ratio and narrow gap array structures, such as, pillar, frustum, or cone, on a nano/micro scale. The effects of the etching conditions on the shape and size of each pillar were evaluated by changing etching duration and the size/material of etching mask. The fabricated PMMA array structures indicate possibilities of optical waveguide and nanofiber array. PFA cone array structures showed super-hydrophobicity without any chemical treatments. Also, polystyrene-coated silica spheres were used as an etching mask for the pillar array structure formation to control the gap between pillars.

Feasibility of atomic layer etching of polymer material based on sequential O2 exposure and Ar low-pressure plasma-etching

We describe controlled, self-limited etching of a polystyrene polymer using a composite etching cycle consisting of sequential deposition of a thin reactive layer from precursors produced from a polymer-coated electrode within the etching chamber, modification using O2 exposure, and subsequent low-pressure Ar plasma etching, which removes the oxygen-modified deposited reactive layer along with ≈0.1 nm unmodified polymer. Deposition prevents net etching of the unmodified polymer during the etching step and enables self-limited etch rates of 0.1 nm/cycle.

Top-Down Fabrication of Ordered Mesoscopic PZT Dot Arrays by Natural Lithography

In this work, we describe the top-down fabrication of ordered mesoscopic, submicron Pb(Zr,Ti)O3 (PZT) dot arrays by means of natural lithography. A monolayer of a well-ordered hexagonal array of latex spheres with a diameter of less than 1 μm was deposited onto the PZT thin film. To adjust a desired sphere size in the range between 300 and 800 nm, the latex sphere monolayer was treated by low-pressure plasma etching in an argon-oxygen atmosphere at low RF power. Afterwards, the remaining template serves as a mask for PZT ion beam etching.

Going beyond the Surface: Revealing Complex Block Copolymer Morphologies with 3D Scanning Force Microscopy

We report on the quasi in situ scanning force microscopy nanotomography which proved to be a key method to effectively obtain a three-dimensional (3D) microdomain structure of a complex ABC triblock morphology. As an example, we studied polybutadiene-block-poly(2-vinyl pyridine)-block-poly(tert-butyl methacrylate) (BVT) thin triblock terpolymer films. We realized a controlled erosion of the material by using low-pressure plasma etching coupled to the scanning force microscope. The 3D reconstruction provides insights into the structural behavior in very thin volume elements revealing morphological details not accessible with other methods.

Single, aligned carbon nanotubes in 3D nanoscale architectures enabled by top-down andbottom-up manufacturable processes

We have developed manufacturable approaches for forming single, vertically aligned carbonnanotubes, where the tubes are centered precisely, and placed within a few hundred nm of1–1.5 µm deep trenches. These wafer-scale approaches were enabled by using chemically amplifiedresists and high density, low pressure plasma etching techniques to form the 3Dnanoscale architectures. The tube growth was performed using dc plasma-enhancedchemical vapor deposition (PECVD), and the materials used in the pre-fabricated 3Darchitectures were chemically and structurally compatible with the high temperature(700 °C) PECVD synthesis of our tubes, in an ammonia and acetylene ambient. Such scalable,high throughput top-down fabrication processes, when integrated with the bottom-up tubesynthesis techniques, should accelerate the development of plasma grown tubes for a widevariety of applications in electronics, such as nanoelectromechanical systems, interconnects,field emitters and sensors. Tube characteristics were also engineered to some extent, byadjusting the Ni catalyst thickness, as well as the pressure and plasma power duringgrowth.

Real-time studies of surface roughness development and reticulation mechanism of advanced photoresist materials during plasma processing

Surface roughness development of photoresist (PR) films during low pressure plasma etching has been studied using real-time laser light scattering from photoresist materials along with ellipsometric and atomic force microscopy (AFM) characterization. We show that evolution of the intensity of light scattered from a film surface can be used to study the development of surface roughness for a wide range of roughness starting from subnanometer to few hundred nanometers. Laser light scattering in combination with ellipsometry and AFM is also used to study the reticulation mechanism of 193 and 248 nm PRs during argon plasma processing. We employ a three-layer model (modified layer, rough layer, and bulk film) of the modified PR surface (193 and 248 nm PRs) to simulate and understand the behavior of ellipsometric Ψ-Δ trajectories. Bruggeman’s effective medium approximation is employed to study the roughness that develops on the surface after reticulation. When the glass transition temperature of the organic materials is reached during Ar plasma processing, the PR films reticulate and roughness develops rapidly. Roughness development is more pronounced for 248 nm PR than for 193 nm PR. Simulation of Ψ-Δ shows that the growth of roughness is accompanied by strong expansion in the materials, which is stronger for 248 nm PR than 193 nm PR. The leading factors responsible for reticulation are found to be compressive stress that develops in the modified surface layer as it is created along with strong molecular chain motion and expansion of the material when the temperature is increased past the glass transition temperature. Reticulation leads to a significantly different surface morphology for 248 nm PR as compared to 193 nm PR and can be related to differences in molecular structure and composition leading to different responses when a modified surface layer is formed by ion bombardment accompanying plasma etching.

Investigation of surface modifications of 193 and 248nm photoresist materials during low-pressure plasma etching

Plasma-based pattern transfer of lithographically produced nanoscale patterns in advanced photoresist materials is often accompanied by photoresist surface roughening and line edge roughening due to factors which are not well understood. We have studied the evolution of surface roughening in prototypical 193 and 248nm photoresist materials during plasma processing as a function of plasma operating parameters. We used real-time ellipsometry and mass spectrometry, along with atomic force microscopy, x-ray photoemission spectroscopy and time-of-flight secondary ion mass spectrometry in an effort to understand the morphological and chemical changes of the photoresist materials as a function of plasma–surface interactions parameters, e.g., maximum ion energy, total energy flux, and plasma chemistry, and photoresist material. A comparison of 248nm photoresist with 193nm photoresist shows that significantly more surface roughness is introduced in the 193nm photoresist for most plasma processing conditions investigated. We also find a dramatic dependence of surface roughening on the chemistry of the plasma process, e.g., for Ar–C4F8 a modified photoresist surface layer with an extent of about 50nm is produced in 193nm photoresist, whereas for C4F8 discharges the surface modification is much less for otherwise similar conditions. We show that one important reason for these differences may be ion-enhanced selective volatilization of carbonyl groups of the 193nm photoresist polymer backbone which is absent for the 248nm material, along with modulation of the ion-interaction with the photoresist material by fluorocarbon surface passivation.

Sharp edged silicon structures generated using atom lithography with metastable helium atoms

By combining atom lithography and plasma etching technology in a two-step process, we demonstrate the transfer of sharp edged structures into silicon with a depth of 580 nm and an inclination of better than 86°. A self-assembled monolayer resist deposited on a Au-coated Si surface is damaged by a beam of metastable helium atoms through a physical mask. A wet etching process removes Au in the damaged regions, resulting in an intermediate mask of patterned Au on Si. Low-pressure plasma etching is then used to transfer the pattern of the Au mask into the Si. This plasma etching process shows a selectivity greater than 19 with respect to the Au mask.

The dynamics of hot F atoms in low pressure plasma etching reactors

The time dependent distribution function of translationally energetic F atoms produced by electron impact dissociative collisions with CF 4 in a low pressure CF 4 plasma is determined from the solution of the Boltzmann equation. The hot F atom distribution function departs from equilibrium owing to the strength and energy distribution of the source atoms and is driven to equilibrium via F-CF 4 elastic collisions. The extent of the departure from equilibrium depends on the rate of production of hot F atoms by reaction and the rate of thermalization by elastic collisions. The time dependent hot F distribution function is determined from the Boltzmann equation for an initial non-equilibrium distribution, with and without a steady source. The differential elastic collision cross section for F-CF 4 collisions, that defines the collision operator in the Boltzmann equation, is calculated quantum mechanically. The phase shifts are determined with the WKB approximation for an appropriate Lennard-Jones 6–12 potential for the F-CF 4 interaction. The results of these calculations suggest that hot neutral F radicals may contribute to the etching process and be competitive with ion activated etching. The enhancement of reaction rates owing to energetic F atoms is also determined and compared with the corresponding thermal rates.

Flow and transport modeling of a low pressure plasma etching system

Plasma etching of aluminum–copper alloys is performed at high densities and low pressures (10 mTorr) in a modified parallel plate plasma etching system with a Cl2/BCl3 gas chemistry. In this study, flow modeling is used to characterize the etching environment. The flow distribution and transport of chemical species within the reactor is modeled by the commercially available fluid dynamic software package, fluent, to predict metal etch rate and uniformity patterns on 6 in. aluminum wafers. Flow velocity vectors and chemical species distributions are presented. Cl2 is considered as the primary etchant by the single step surface reaction 3Cl2+2Al=2AlCl3. [D. Smith and R. Bruce, J. Electrochem. Soc. 129, 2045 (1982).] A good fit is achieved between the measured etch pattern and the predicted pattern. The etching is determined to be primarily transport limited, with diffusion as the dominant transport mechanism. Theoretical comparisons diffusive to convective transport in this pressure region are made.

Low pressure plasma etching with magnetic confinement : T. D. Mantei and T. E. Wicker. Solid St. Technol. 263 (April 1985)

Low pressure plasma etching with magnetic confinement : T. D. Mantei and T. E. Wicker. Solid St. Technol. 263 (April 1985)