Plasma Reactive Ion Etching Research Articles

Etching of high aspect ratio structures in Si using SF6/O2 plasma

We have investigated etching of deep (∼10 μm) submicron diameter holes with high aspect ratios (>10) using plasmas maintained in mixtures of SF6 and O2 gases. The etching experiments were conducted in a low-pressure (5–80 mTorr), high-density, inductively coupled plasma etching reactor with a planar coil. We have studied the effects of pressure, rf-bias voltage, and SF6-to-O2 gas ratio on the etch rate, selectivity, and feature profile using Si wafers patterned with 0.35–0.5 μm diameter holes in a SiO2 mask. Visualization of the profiles with scanning electron microscopy is used in conjunction with plasma diagnostics such as optical emission and mass spectroscopies to understand the key factors that control the anisotropy, selectivity, and etch rate. The F-to-ion flux ratio and F-to-O flux ratio are found to be the important plasma parameters that determine the etch rate and anisotropy. Increasing the SF6-to-O2 ratio in the feed gas increases the F-to-O ratio in the plasma. At high SF6-to-O2 ratio, the mask undercut is severe because sidewall passivation by O atoms cannot keep up with the chemical etching by F atoms. As the F-to-O ratio is decreased, effective sidewall passivation by O atoms results in nearly vertical sidewalls. A further reduction in the F-to-O ratio results in sidewalls that slope inwards toward the bottom of the feature.

Collisional effects on characteristics of asymmetric radio-frequency sheaths

For most reactive radio-frequency plasma etching processing, the plasma sheaths near two electrodes are asymmetric due to the powered electrode area being smaller than the grounded electrode area. Taking collisional effects of ions with neutrals in the sheaths into account, a selfconsistent dynamic model is proposed here to describe the characteristics of radio-frequency sheaths. Numerical results show the collisional effects on some physical quantities, such as instantaneous voltages on the electrodes, instantaneous thicknesses of the electron sheathes, spatial distributions of the ion density and the electric field in the sheaths, and instantaneous voltage difference between two electrodes.

The effects of nitrogen plasma on reactive-ion etching induced damage in GaN

Lightly doped n-GaN samples were etched with reactive ion etching (RIE) in BCl3/Cl2/Ar and BCl3/Cl2/N2 plasmas. Replacing the argon by nitrogen in the plasma chemistry resulted in better etch rates, and in addition reduced etch damage for relatively low values of the plasma power. By treating the samples in a nitrogen plasma following etching, we dramatically reduced surface damage, as determined from Schottky IV characteristics measurements. Specifically, the reverse breakdown voltages returned to 70% to that measured from control diode samples. Further, x-ray photoelectron spectroscopy analysis showed that the post-etch nitrogen step decreased the Ga/N ratio. These results suggest that much of the damage induced by RIE plasma etching comes from a nitrogen loss mechanism.

A detailed theoretical and experimental noise study in n-on-p Hg 0.68Cd 0.32Te photodiodes

Both the method enabling numerical modeling of fluctuation phenomena in semiconductor devices as well as experimental results of noise current in Hg 0.68Cd 0.32Te n-on-p photodiodes are presented. While the concept of the applied numerical method was derived earlier [J. Appl. Phys. 90 (3) (2001) 1318], it has been enhanced in this paper by taking into account the influence of temperature fluctuations. The method is based on the assumption that the fluctuations of the generation–recombination processes, carrier mobility, heat generation and heat transfer result in the fluctuations of carrier and ionized impurity concentrations as well as fluctuations of carrier motion and temperature. These, in turn, may be expressed by the fluctuations of electrostatic potential, quasi-Fermi levels and temperature. The fluctuations of the electrostatic potential, quasi-Fermi level fluctuations and temperature fluctuations were calculated by solving the set of “transport equations for fluctuations” in which the fluctuations of generation–recombination processes, heat transfer and mobility act as random source terms. The method was applied to a theoretical analysis of the noise current in n-on-p Hg 0.68Cd 0.32 photodiodes. The noise spectrum in selected HgCdTe photodiodes is calculated and the contribution of different noise sources is determined. Theoretical results are compared with the experimental data. The planar homojunction devices were formed on LPE grown vacancy doped HgCdTe using a reactive ion etching plasma induced conversion process [J. Electron. Mater. 29 (6) (2000) 841].

Experimental and theoretical study of two-dimensional ion flux uniformity at the wafer plane in inductively coupled plasmas

A two-dimensional array of planar Langmuir probes manufactured on a 200-mm-diameter silicon wafer was used to measure the spatial distribution of ion flux impinging on the wafer surface in various discharges of electropositive (Ar) and electronegative (SF/sub 6/ and Cl/sub 2/) gases maintained in an inductively coupled plasma etching reactor with a planar spiral coil. In conjunction with the experiments, a two-dimensional fluid model of the plasma was developed to capture the dependence of the ion flux uniformity on plasma operating parameters and reactor geometry through a set of dimensionless numbers which are the ratios of various time and length scales intrinsic to the discharge. These dimensionless ratios include reactor dimensions, the skin depth, the electron energy relaxation length, ion diffusion length, and ionization and attachment rates. The model provides a simple framework within which the spatial variation of ion flux in inductively coupled plasmas can be understood. The approach captures the dependence of ion flux uniformity on plasma operating variables such as pressure and feed gas composition.

Effects of Chamber Wall Conditions on Cl Concentration and Si Etch Rate Uniformity in Plasma Etching Reactors

Experimental measurements of the etch rate and ion flux distributions on the wafer are combined with modeling to elucidate the effects of reactor wall conditions on Cl concentration and polysilicon etch rate uniformity in an inductively coupled plasma etching reactor. The spatially averaged etch rate across the wafer increases with time as etch products react with residual oxygen in the chamber and coat the reactor walls with a thin layer of silicon oxychloride film. Chlorine concentration in the plasma and the Si etch rate increase due to lower recombination probability of Cl on this film as compared to the “clean” anodized aluminum wall surface. Etch rate is highest at the wafer center when the walls are maintained in the clean state. In contrast, the etch rate peaks at the wafer edges when the walls are coated with the silicon oxychloride film. The drift in etch rate and uniformity is primarily due to a drift in Cl concentration and its spatial distribution. As the reactor walls are coated, the etch rate distribution changes from a center-fast profile to an edge-fast profile due to a change in the dominant Cl depletion mechanism from wall recombination to recombination on the wafer surface. © 2003 The Electrochemical Society. All rights reserved.

Design of Domain Wall Displacement Detection Disk with Groove Recording for High Density and Wide Recording Power Tolerance

Domain wall displacement detection (DWDD) magneto-optical (MO) recording is the most useful method for achieving high linear density recording without using a higher numerical aperture (NA) objective lenses. Securing a wide recording power tolerance is very important to realize a high density optical storage system. From this point of view, we designed the DWDD disk with groove recording using blue laser and NA 0.60 optics. We optimized the heat sink structure and the displacement layer for groove recording, and we could improve the bit error rate with a track pitch of 0.44 µm and a high linear density of 0.07 µm/bit, which corresponds to 21 Gbit/inch2. Additionally, we developed groove substrates using reactive ion etching (RIE) and O2 plasma treatment. We found that the land shape was very important for the recording power tolerance, and we could obtain a wide recording power tolerance with a bit length of 0.09 µm. We confirmed that we could achieve higher density by using a narrower track pitch while maintaining sufficient recording power tolerance.

CF A 2Σ+–X 2Π and B 2Δ–X 2Π study by broadband absorption spectroscopy in a plasma etch reactor: Determination of transition probabilities, CF X 2Π concentrations, and gas temperatures

Broadband absorption spectroscopy was applied to study the CF A 2Σ+–X 2Π and B 2Δ–X 2Π transitions in a plasma etch reactor. We report a previously unobserved band, which is assigned as CF A 2Σ+–X 2Π (3,0). This band is significantly broadened by predissociation, and we estimate the average collision-free lifetime of the CF A 2Σ+ v′=3 level to be 0.30±0.08 ps. Experimental relative oscillator strength measurements, together with ab initio calculations, Rydberg–Klein–Rees-based wave functions and experimental lifetimes were used to calculate a full set of transition probabilities for the CF A 2Σ+–X 2Π and B 2Δ–X 2Π bands. The maximum observed number densities of CF X 2Π were ∼2×1013 cm−3 with sensitivity to measure to 1010 cm−3. The excited state and ground state temperatures were determined by comparing the spectra to simulations. The ground state rotational temperature was 450±30 K and the vibrational temperature was 850±80 K near the substrate surface. The CF B 2Δ excited state rotational temperatures are higher than those of the ground state. We show that this absorption technique is practical for determining gas temperatures and absolute concentrations in plasma etch reactors.

Two-dimensional ion flux distributions in inductively coupled plasmas: Effect of adding electronegative gases to Ar

A two-dimensional array of planar Langmuir probes built on a 200 mm diam silicon wafer was used to measure the radial and azimuthal variation of ion flux impinging on the wafer surface in Ar/SF6 and Ar/Cl2 discharges maintained in an inductively coupled plasma etching reactor. The spatial variation of ion flux in a pure Ar discharge is approximately radially symmetric and peaks at the center of the wafer for pressures between 10 and 60 mTorr. The spatially averaged ion flux in a pure Ar discharge increases with increasing pressure and the corresponding uniformity degrades with increasing pressure within the pressure range studied. Addition of small amounts of electronegative gases to an Ar discharge flattens the radial and azimuthal ion flux distribution and accentuates azimuthal variations due to subtle asymmetries in the reactor geometry such as pumping ports. At fixed power, pressure, and flow rate, the spatially averaged ion current density decreases with increasing mole fraction of the electronegative gases in the feed gas.

An on-wafer probe array for measuring two-dimensional ion flux distributions in plasma reactors

In plasma etching processes, the spatial distribution of ion flux across the wafer surface determines the uniformity and profile evolution when etching is ion limited. We have designed and built a two-dimensional array of planar Langmuir probes on a 200 mm diameter silicon wafer to measure the radial (r) and azimuthal (θ) variation of ion flux impinging on the wafer surface in plasma etching reactors. Herein we demonstrate the use of this probe array to obtain two-dimensional ion flux distributions in Ar, Cl2, and Cl2/HBr/He discharges in an inductively coupled plasma reactor. The results obtained using the probe array are in good agreement with Langmuir probe measurements but also reveal azimuthal asymmetries, due to irregularities in chamber geometry such as the pumping port and radio frequency coil configuration, that cannot be detected using radially movable Langmuir probes. The probe array can also be used to investigate the spatiotemporal fluctuations of the ion flux in the 1–100 Hz range.

Mechanical vibration assisted plasma etching for etch rate and anisotropy improvement

A novel plasma etching principle for etch rate and anisotropy improvement is proposed. A substrate vibration mechanism has been used to increase plasma collision energy and realize a better gas flow in the plasma etching reactor, as well as ultrasonic cleaning effects at the low vacuum. An reactive ion etching (RIE) system with an anode fixed to an ultrasonically vibrating support is used to demonstrate the principle. Two vibration frequencies of 28 and 38 kHz are applied to the substrate by changing vibration units, with vibration amplitudes of 13 and 9.3 μm, respectively. Etching experiments upon silicon substrates are performed by using SF 6 gas with and without vibration. Vibration effects on etch rate and anisotropy are examined by SEM micrographs at cleaved cross-section of the etching holes. At present, the system is confirmed to have a maximum etch rate improvement of 78% from its original without vibration.

Effect of H2 on the etch profile of InP/InGaAsP alloys in Cl2/Ar/H2 inductively coupled plasma reactive ion etching chemistries for photonic device fabrication

This study demonstrates etch profile engineering of InP, In1−xGaxAs1−yPy, and In0.53Ga0.47As heterostructures results from adding H2 to standard Cl2/Ar inductively coupled plasma-reactive ion etching chemistries. Etch rate curves of bulk InP, In1−xGaxAs1−yPy, and In0.53Ga0.47As show a general parabolic trend as a function of the H2 component of the Cl2/Ar/H2 ratio. Three distinct etching profiles of InP/InGaAsP layers were realized by varying the Cl2/Ar/H2 ratio. Highly anisotropic profiles result for Cl2/Ar/H2 ratios between 2/3/1 and 2/3/2. Waveguiding structures fabricated using this technology are presented with a loss as low as 2 dB/cm. An InP racetrack resonator with a quality factor (Q)>8000 is also presented.

Maintaining reproducible plasma reactor wall conditions: SF6 plasma cleaning of films deposited on chamber walls during Cl2/O2 plasma etching of Si

Silicon oxychloride films deposited on plasma etching reactor walls during the Cl2/O2 plasma etching of Si must be removed to return the reactor to a reproducible state prior to etching the next wafer. Using multiple surface and plasma diagnostics, we have investigated the removal of this silicon oxychloride film using an SF6 plasma. In particular, a diagnostic technique based on the principles of multiple total internal reflection Fourier transform infrared spectroscopy was used to monitor the films that formed on the reactor walls. The silicon oxychloride film etching proceeds by incorporation of F, which also abstracts and replaces the Cl atoms in the film. If the SF6 plasma is not maintained for a sufficiently long period to remove all the deposits, the F incorporated into the film leaches out into the gas phase during the subsequent etch processes. This residual F can have undesirable effects on the etching performance and the wafer-to-wafer reproducibility. The removal of the silicon oxychloride film from the reactor walls is inherently nonuniform and the end of the cleaning can be detected most easily by monitoring reactor averaged F and SiF emissions.

Direct Simulation Monte Carlo Analysis of Rarefied Gas Flow Structures and Ventilation of Etching Gas in Magneto-Microwave Plasma Etching Reactors

Gas flows in plasma etching reactors for semiconductor fabrication became a chief consideration in designing second-generation reactors with higher etching rates. An axisymmetrical model based on the direct simulation Monte Carlo method has been developed for analyzing rarefied gas flows in a vacuum chamber with the conditions of downstream pressure and gas flow rate. By using this simulator, rarefied gas flows with radicals and etch-products were calculated for microwave-plasma etching reactors. The results showed that the flow patterns in the plasma chamber strongly depend on the Knudsen number and the gas-supply structure. The ventilation of the etch-products in the plasma chamber was found to be improved both for higher Knudsen numbers and for gas-supply structures of the downward-flow type, as compared with those of the radial-flow or upward-flow types.

Development of a data-driven dynamic model for a plasma etching reactor

We compare several parameter identification methods for data-driven identification and validation of an empirical linear dynamic model for a helicon plasma reactor. The model relates easily measurable process variables to ellipsometry measurements from which the etch depth can be determined in real time. The potential use of such a model for process control is obvious. The model developed shows improvement over a neural network model developed in a previous study based upon the same data.

Fabrication of electrostatic micro-actuators for a hard disk drive application

Micro-actuators are developed for a hard disk drive application. The aim is to obtain a high density of data storage by positioning the read and write head element precisely. The desired specifications are a displacement of ±0.5 μm, operating voltage of 20 V and a structural resonance frequency of 15 kHz. We manufactured the metallic actuator by using thick photoresist lithography and nickel electroplating. The electrostatic gap between the mover and the stator has a high aspect ratio, in order to achieve low voltage operation. We deposited a copper layer by electroplating as a vertical sacrificial layer. Then we removed the copper layer at last process step to make the electrostatic gap with high aspect ratio. We utilize a photoresist which has a thickness of 10 μm as the first trial. We completed the process and operated the actuator successfully. We also developed a silicon actuator using the silicon deep etching and deposition technology. We use an Inductive Coupled Plasma-Reactive Ion Etching (ICP-RIE) machine for silicon etching and a Low Pressure Chemical Vapor Deposition (LPCVD) machine for deposition of the vertical sacrificial layer of SiO2 and the mover structure of polySi. We obtained an actuator which had thickness of 100 μm with the electrode gap having a aspect ratio of 50.

A coupled plasma and sheath model for high density reactors

We present a coupled plasma and collisionless sheath model for the simulation of high-density plasma processing reactors. Due to inefficiencies in numerical schemes and the resulting computational burden, a coupled multidimensional plasma and sheath simulation has not been possible for gas mixtures and high-density reactors of practical interest. In this work, we demonstrate that with a fully implicit algorithm and a refined computational mesh, a self-consistent simulation of a reactor including both the plasma and sheath is feasible. We discuss the details of the model equations, the importance of ion inertia, and the resulting sheath profiles for argon and chlorine plasmas. We find that at low operating pressures (10-30 mtorr), the charge separation occurs only within a 0.5-mm layer near the surface in a 300 mm inductively coupled plasma etch reactor. A unified simulation eliminates the use of offline or loosely coupled and oversimplified sheath models which generally leads to uncertainties in ion flux and sheath electrical properties.

Coatings Technology Handbook, Second Edition

Coatings Technology Handbook, Second Edition

Reactive ion etching of SiC using C2F6/O2 inductively coupled plasma

The inductively coupled plasma-reactive ion etching (ICP-RIE) of SiC single crystals using the C2F6/O2 gas mixture was investigated. It was observed that the etch rate increased as the ICP power and bias power increased. With increasing sample-coil distance, O2 concentration, and chamber pressure, the etch rate initially increased, reached a maximum, and then decreased. Mesas with smooth surfaces (roughness ≤1 nm) and vertical sidewalls (∼85°) were obtained at low bias conditions with a reasonable etch rate of about 100 nm/min. A maximum etch rate of 300 nm/min could be obtained by etching at high bias conditions (≥300 V), in which case rough surfaces and the trenched sidewall base were observed. The trenching effect could be suppressed by etching the samples on anodized Al plates, although mesas with sloped (60–70°) sidewalls were obtained. Results of various surface characterization indicated little contamination and damage on the etched SiC surfaces.

Modification of Al/Si interface and Schottky barrier height with chemical treatment

The influence of different chemical treatments on the electrical behaviour of n- and p-type Al/Si Schottky junctions was studied. A Schottky barrier height of 0.91 eV was achieved on p-type Si probably due to the unpinning of the Fermi-level at the Al/Si interface. This is one of the highest barrier height values reported so far for a solid-state Schottky junction prepared to p-Si. A doping level reduction was observed in the vicinity of the Si surface for wafers with native oxide and for those boiled in acetone or annealed in forming gas. It was observed unexpectedly that the reactive plasma etch used for the formation of mesa structures decreases the apparent Schottky barrier height. The relation between the sum of n- and p-type Schottky barrier heights and forbidden gap is discussed.