Plasma Reactive Ion Etching Research Articles

Radical surface interactions in industrial silicon plasma etch reactors

Silicon etching in Cl2-based plasmas is an important step for the fabrication of IC circuits but the plasma surface interactions involved in this process remain poorly understood. Based on the developments in plasma and reactor wall diagnostics, this paper reviews the recent progress in the understanding of radicals' interactions with surfaces during silicon etching processes. X-ray photoelectron spectroscopy analysis of the reactor walls shows that during Si etching in Cl2/O2 plasmas, the initial Al2O3 chamber walls are coated with a thin SiOCl layer. Broadband absorption spectroscopy with UV light emitting diodes is used to measure the densities of SiClX radicals (X = 0–2) and Cl2 molecules in steady state plasmas running with the chamber walls coated with different materials. To estimate the surface sticking/recombination probability of these radicals on different surfaces, we have performed time-resolved absorption measurements in the afterglow of pulsed discharges. Our work, in agreement with previous results, shows that the Cl2/Cl density ratio in the discharge is driven mainly by the chemical nature of the chamber walls explaining why process drifts are often observed in Cl2/O2 plasmas. The recombination coefficient of Cl atoms on SiOCl surfaces is about 0.007, while it is about 0.1 on clean walls (AlF3). Based on these results, we discuss the best strategy leading to reproducible process control, the present strategy being a systematic reactor cleaning/conditioning between wafers. The SiOCl layer deposition mechanism is then discussed in detail. The sticking coefficient of SiCl on this surface is near unity, while SiCl2 appears to be weakly reactive toward it. Therefore, SiCl (and SiCl+ ions) are the main vectors of Si deposition on the reactor walls, where their subsequent oxidization by O atoms leads to the formation of a SiOCl deposit. Furthermore, we show that SiCl reaction in the plasma volume with Cl2, through the exchange reaction SiCl + Cl2 → SiCl2 + Cl, is very important for the control of radical densities. Finally, time-resolved measurements of the gas temperature by laser-absorption Doppler spectroscopy show that in the afterglow of pulsed discharges the transport of particles is affected by the convection phenomenon. The very fast electron temperature cooling in the early afterglow generates a pressure gradient in the reactor which induces a rapid gas movement from the cold regions of the reactor toward the reactor center, where the plasma was generated. This leads to an increase in the density of stable molecules in the volume previously occupied by the plasma.

Ga+ beam lithography for nanoscale silicon reactive ion etching

By using a dry etch chemistry which relies on the highly preferential etching of silicon, overthat of gallium (Ga), we show resist-free fabrication of precision, high aspect rationanostructures and microstructures in silicon using a focused ion beam (FIB) and aninductively coupled plasma reactive ion etcher (ICP-RIE). Silicon etch masks are patterned viaGa + ion implantation in a FIB and then anisotropically etched in an ICP-RIE using fluorinatedetch chemistries. We determine the critical areal density of the implanted Ga layerin silicon required to achieve a desired etch depth for both a Pseudo Bosch (SF6/C4F8) and cryogenicfluorine (SF6/O2) silicon etching. High fidelity nanoscale structures down to 30 nm and high aspect ratiostructures of 17:1 are demonstrated. Since etch masks may be patterned on unevensurfaces, we utilize this lithography to create multilayer structures in silicon. The linearselectivity versus implanted Ga density enables grayscale lithography. Limitson the ultimate resolution and selectivity of Ga lithography are also discussed.

Void nucleation at a sequentially plasma-activated silicon/silicon bonded interface

Two 4 inch silicon wafers were directly bonded using a sequentially plasma-activated bonding method (i.e. O2 reactive ion etching (RIE) plasma followed by N2 microwave (MW) radicals) at room temperature. The bonded wafers were annealed from 200 to 900 °C in order to explore the nucleation of voids at the interface. The plasma-induced void nucleation was dominated by O2 RIE power over O2 RIE activation time. The thermal-induced void nucleation occurred preferentially at the plasma-induced defect sites. The nucleation of void density was quantitatively determined and explained using high-resolution transmission electron microscopy observations. The electron energy loss spectroscopy results revealed the existence of silicon oxide at the bonded interface. The reduction in bonding strength after annealing at high temperature is correlated to the increase in void density. The contact angle and surface roughness of the sequentially plasma-treated surfaces have been observed to explain the nucleation of voids and the reduction of bonding strength. The plasma-induced defect sites such as nanopores and craters have been identified using an atomic force microscope.

Diamond double-sided micro-lenses and reflection gratings

Double-sided micro-lenses have been successfully fabricated on natural diamond substrates using photoresist reflow method followed by Inductively Coupled Plasma (ICP) etching. The micro-lenses were fabricated on both sides of the substrates which were aligned to each other. The optical properties of these lenses were characterised by a laser scanning transmission/reflection confocal microscope. It has been confirmed by the confocal microscopy technique that the laser beam was focused twice as it shone through the substrates. The focal length was calculated based on refraction formula at the spherical surface which has been derived from Fermat’s principle. The top focused point was found lying within the substrates while the bottom focused point lying outside of the substrates. Both focal lengths of the top and bottom micro-lenses agree well with the theoretical calculation based on the refraction formula. This demonstrates the functionality of the double-sided micro-lenses. In addition, highly smooth diffraction gratings (surface rms roughness of 1 nm) with near-vertical sidewalls (85 ± 5°) have also been successfully fabricated on the HPHT diamond substrate utilising SiO 2 as mask and Reactive Ion Etching (RIE) and ICP plasma etching. The optical functionality of the diamond reflection grating has been confirmed by the reflection measurement.

공정가스와 RF 주파수에 따른 웨이퍼 표면 텍스쳐 처리 공정에서 저반사율에 관한 연구

일반적으로 결정질 실리콘 태양전지에서 표면에 텍스쳐링(texturing)하는 것은 알칼리 또는 산성 같은 화학용액을 사용하고 있다. 그러나 실리콘 부족으로 실리콘의 양의 감소로 인하여 웨이퍼 두께가 감소하고 있는 추세에 일반적으로 사용하고 있는 습식 텍스쳐링 방법에서 화학용액에 의한 많은 양의 실리콘이 소모되고 있어 웨이퍼의 파손이 심각한 문제에 직면하고 있다. 그리하여 습식 텍스쳐링 방법보다는 플라즈마로 텍스쳐링할 수 있는 건식 텍스쳐링 방법인 RIE (reactive ion etching) 기법이 대두되고 있다. 그리고 습식 텍스쳐링으로는 결정질 실리콘 태양전지의 반사율을 10% 이하로는 낮출 수가 없다. 다결정 실리콘 웨이퍼 표면에 텍스쳐링을 하기 위하여 125 mm 웨이퍼 144개를 수용할 수 있는 대규모 플라즈마 RIE 장비를 개발하였다. 반사율을 4% 이하로 낮추기 위하여 공정가스는 <TEX>$Cl_2$</TEX>, <TEX>$SF_6$</TEX>, <TEX>$O_2$</TEX>를 기반으로 RIE 텍스쳐링을 하였고 텍스쳐링의 모양은 공정가스, 공정시간, RF 주파수 등에 의해 조절이 가능하였다. 본 연구에서 RIE 공정을 통하여 16.1%의 변환효율을 얻었으며, RF 주파수가 텍스쳐링의 모양에 미치는 영향을 살펴보았다. Conventional surface texturing in crystalline silicon solar cell have been use wet texturing by Alkali or Acid solution. But conventional wet texturing has the serious issue of wafer breakage by large consumption of wafer in wet solution and can not obtain the reflectance below 10% in multi crystalline silicon. Therefore it is focusing on RIE texturing, one method of dry etching. We developed large scale plasma RIE (Reactive Ion Etching) equipment which can accommodate 144 wafers (125 mm) in tray in order to provide surface texturing on the silicon wafer surface. Reflectance was controllable from 3% to 20% in crystalline silicon depending on the texture shape and height. We have achieved excellent reflectance below 4% on the weighted average (300~1,100 nm) in multi crystalline silicon using plasma texturing with gas mixture ratio such as <TEX>$SF_6$</TEX>, <TEX>$Cl_2$</TEX>, and <TEX>$O_2$</TEX>. The texture shape and height on the silicon wafer surface have an effect on gas chemistry, etching time, RF frequency, and so on. Excellent conversion efficiency of 16.1% is obtained in multi crystalline silicon by RIE process. In order to know the influence of RF frequency with 2 MHz and 13.56 MHz, texturing shape and conversion efficiency are compared and discussed mutually using RIE technology.

Comprehensive investigation of sequential plasma activated Si/Si bonded interfaces fornano-integration on the wafer scale

The sequentially plasma activated bonding of silicon wafers has been investigated tofacilitate the development of chemical free, room temperature and spontaneous bondingrequired for nanostructure integration on the wafer scale. The contact angle of the surfaceand the electrical and nanostructural behavior of the interface have been studied.The contact angle measurements show that the sequentially plasma (reactiveion etching plasma followed by microwave radicals) treated surfaces offer highlyreactive and hydrophilic surfaces. These highly reactive surfaces allow spontaneousintegration at the nanometer scale without any chemicals, external pressure or heating.Electrical characteristics show that the current transportation across the nanobondedinterface is dependent on the plasma parameters. High resolution transmissionelectron microscopy results confirm nanometer scale bonding which is needed for theintegration of nanostructures. The findings can be applied in spontaneous integration ofnanostructures such as nanowires/nanotubes/quantum dots on the wafer scale.

Study of SF6 and SF6/O2 plasmas in a hollow cathode reactive ion etching reactor using Langmuir probe and optical emission spectroscopy techniques

In this work, electrical and optical studies of SF6 and SF6/O2 plasmas generated in a hollow cathode reactive ion etching reactor were performed using the Langmuir probe and optical emission spectroscopy techniques, respectively. We carried out an investigation aimed at understanding the influence of radio-frequency power, gas pressure and O2 gas mixing ratio on plasma parameters, namely electron temperature, electron density and electronegativity, and also atomic fluorine density. The results indicate an increase of up to one order of magnitude in electron density and atomic fluorine in the overall gas volume when compared with a conventional reactive ion etching plasma generated under the same operation conditions.

Patterning of Cu Films by a Two-Step Plasma Etching Process at Low Temperature

Primarily due to the inability to form volatile etch products for copper (Cu) plasma etching at temperatures , no effective subtractive etch process for Cu has been reported. We have developed a low temperature , reactive plasma etch process for Cu films in an attempt to mitigate the size effect in electrical resistivity, a critical limitation to future integrated circuit device generations. As previously proposed by a thermochemical analysis of the Cu–Cl–H system, the plasma etch process is executed in two steps. In the first step, Cu films are exposed to a plasma to preferentially form , which is believed to be volatilized as by exposure to a plasma in the second step. Cu films (100 nm thick) with as a mask have been patterned by this two-step etching process in an inductively coupled plasma system. Experimental parameters such as power applied to the substrate electrode, gas flow rate, and plasma treatment time have been investigated to assess their effect on pattern quality. The addition of argon to the plasma improved Cu pattern fidelity and gave reasonable etch rates. Possible etch mechanisms for this low temperature process are discussed.

Modeling of the plasma chemistry and plasma–surface interactions in reactive plasmas

In this paper, an overview is given of modeling activities going on in our research group, for describing the plasma chemistry and plasma–surface interactions in reactive plasmas. The plasma chemistry is calculated by a fluid approach or by hybrid Monte Carlo (MC)–fluid modeling. An example of both is illustrated in the first part of the paper. The example of fluid modeling is given for a dielectric barrier discharge (DBD) in CH4/O2, to describe the partial oxidation of CH4 into value-added chemicals. The example of hybrid MC–fluid modeling concerns an inductively coupled plasma (ICP) etch reactor in Ar/Cl2/O2, including also the description of the etch process. The second part of the paper deals with the treatment of plasma–surface interactions on the atomic level, with molecular dynamics (MD) simulations or a combination of MD and MC simulations.

Thermally tunable lithium-niobate long-period waveguide grating filter fabricated by reactive ion etching

We fabricate a long-period grating band-rejection filter with a lithium niobate (LiNbO(3)) channel waveguide, where the grating is formed on the waveguide surface by plasma reactive ion etching. The filter shows three distinct thermal tuning regimes. Below approximately 55 degrees C, the center wavelength of the rejection band decreases linearly with an increase in the temperature at a high sensitivity (-1.4 nm/ degrees C). The temperature sensitivity drops abruptly to a negligible value at approximately 55 degrees C and stays low up to approximately 70 degrees C, above which it jumps abruptly to a large positive value (1.6 nm/ degrees C). We attribute these peculiar tuning characteristics to the formation of different crystal phases in the core and cladding areas of the LiNbO(3) waveguide and the abrupt transitions of the thermo-optic coefficients of some crystal phases. The grating could be developed into various kinds of integrated-optic filters and switches for application in optical signal processing.

Double Photonic Quasi-Crystal Structure Effect on GaN-Based Vertical-Injection Light-Emitting Diodes

GaN-based thin-film vertical-injection light-emitting diodes (VLEDs) with a double 12-fold photonic quasi-crystal (PQC) structure formed using nanoimprint lithography (NIL) and inductively coupled plasma reactive-ion etching (ICP-RIE) are fabricated and presented. At a driving current of 20 mA and with a chip size of 350 ×350 µm2, our thin-film LED with a double-12-fold-PQC structure gave a light output power of 40.5 mW, which is an increase of 77% when compared with the output power of a VLED without a PQC structure at a peak wavelength of 460 nm. In addition, the corresponding light radiation patterns show a narrow beam shape due to the strong guided light extraction on the n-GaN surface and the reflected light effect of the PQC structure formed in the vertical direction on the p-GaN surface.

Fine surface processing of LiNbO3 single crystals by maskless etching using NF3 system gas plasma RIE

AbstractThe possibility of deep etching by plasma reactive ion etching (RIE) without an etching‐mask (maskless) for −Z and +Z parts formed on the same surface of a partially polarization reversed LiNbO3 single crystal polarized in the direction of the c‐axis is investigated. A NF3/H2 gas mixture was used. The etching rates and depths and the profiles of the etched surfaces were evaluated by atomic force microscopy (AFM) and optical microscopy. The etching rate for the −Z surface was larger than that for the +Z surface. Extension of the +Z domain by partial polarization reversal was observed. Applying the high voltage quickly for partial polarization reversal, the area of the +Z domain was extended compared with the result obtained by applying the voltage slowly. An apparent step at the boundary between −Z and +Z parts formed on the same surface was observed. Using a NF3/H2 gas mixture, the segments were removed efficiently. It is concluded that RIE etching using a NF3/H2 gas mixture is suitable for processing of LiNbO3 crystal surfaces without an etching mask, in contrast with a CF4/H2 gas mixture. © 2010 Wiley Periodicals, Inc. Electron Comm Jpn, 93(2): 39–49, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecj.10168

Scaling of the quantum Hall plateau-plateau transition in graphene

The integer quantum Hall effect in two-dimensional electron systems 2DESs is caused by localized states in the tails of individual Landau levels which give rise to quantized plateaus in the Hall resistance. The states in the center of the Landau levels are extended; their wave functions are delocalized. Their delocalization is governed by a localization length, which decays exponentially away from the Landau level centers 1,2 with a universal critical scaling exponent related to this decay. 3 In this Rapid Communication we investigate the scaling behavior of the quantum Hall plateau-plateau transitions in the recently discovered new type of 2DES, graphene. 4,5 When changing the carrier concentration n at a constant field, the peak width of the longitudinal conductivity for higher Landau levels N 1 and the inverse slope of the Hall conductivity scale as T. Our experimentally measured scaling exponent = 0.40 0.02 is consistent with universal scaling theory. 1–3,6–8 The transition through the zeroth Landau level, however, shows no clear scaling behavior which we explain by a different scaling mechanism governed by a temperature independent intrinsic length scale. Our sample was made by micromechanical exfoliation of natural graphite and subsequently contacted by gold contacts and patterned into a 1-m-wide Hall bar by electron-beam lithography and reactive plasma etching. 9 The structure was deposited on a 300 nm Si/ SiO2 substrate thereby forming a graphene ambipolar field effect transistor. Prior to the measurements the sample was annealed at 400 K, placing its charge neutrality point CNP at zero gate voltage with a mobility of = 1.0 m 2 Vs �1 .

Fabrication of large-area gallium arsenide nanowires using silicon dioxide nanoparticle mask

Large-area GaAs nanowires are fabricated using SiO2 nanoparticles as the etching mask. SiO2 nanoparticle monolayer is spin coated on the GaAs substrate. To obtain a uniform monolayer of SiO2 nanoparticles across the substrate, raised temperature, adequate solution concentration, and the substrate treated with a solvent for interface activation are required. With the monolayer of SiO2 nanoparticles as the etching mask, the GaAs substrate is etched by induced-coupled plasma reactive ion etcher (ICP-RIE) to form GaAs nanowires with a high aspect ratio. The diameter and length of GaAs nanowires are 70nm and 1.2μm, respectively. The diameter and length of GaAs nanowires can be controlled by the size of SiO2 nanoparticles and etching time of ICP-RIE.

The effect of discharge conditions of ICP etching reactor on plasma parameters

This study investigated the inductively coupled plasma etching reactor and RF coils developed by North Microelectronic Corporation. Full three dimensional simulations were made at different discharge conditions. The simulations examined and compared the distribution and non-uniformity of several plasma parameters at a fixed position upon the wafer at different pressures and coil currents. These parameters included electron density, electron temperature and power deposition. The results demonstrate that the electron density, power deposition and uniformity increase with either higher pressure or stronger coil currents, while the electron temperature decreases at this condition. Coil number increase can reduce the non-uniformity of parameters in the spatial distribution. The linear relationship between power deposition and electron density does not always exist. The comparison between simulation results and experiment results is also presented in the paper.

Photoresists comparative analysis using soft X-ray synchrotron radiation and time-of-flight mass spectrometry

Positive photoresists are widely used in lithographic process for the fabrication of relief components. When exposed to UV radiation they suffer chemical reactions modifying their chemical and physical properties. Aiming to follow molecular modifications among two different photoresists unexposed and previously exposed to ultraviolet light we have employed spectroscopic techniques coupled with mass spectrometry in the study of the AZ-1518 and AZ-4620 photoresists. The photon stimulated ion desorption (PSID) technique following the S K-edge NEXAFS spectrum was employed at the brazilian synchrotron light source (LNLS), during single-bunch operation and using time-of-flight mass spectrometry (TOF-MS) for ion analysis. NEXAFS and PSID mass spectra on both AZ-1518 and AZ-4620 photoresists (unexposed and exposed) were obtained and relative desorption ion yield curves determined for the main fragments as a function of the photon energy. They present marked different PSID spectra. Fragments related to the photochemical decomposition of the AZ-1518 photoresist could be clearly identified differently from the AZ-4620. Studies on the hardness of both photoresists were performed using O 2 plasma reactive ion etching (RIE) technique, analyzed by scanning electron microscopy (SEM) and used to explain different desorption yields in the PSID spectra. These results show that the PSID technique is adequate to investigate structural changes in molecular level in different unexposed and exposed photoresists, which is crucial for improving our knowledge about the breakup process.

Role of Heating on Plasma-Activated Silicon Wafers Bonding

This paper reports on a comparative study of silicon wafer bonding using reactive ion etching (RIE) vs sequential plasma-activated bonding (SPAB). The study shows the measurement of silicon surface roughness and the investigation of heating influences on the bonding strength and microstructures of silicon∕silicon bonded interfaces as a function of the plasma processing parameters such as plasma time and gas pressure. In SPAB, the surfaces were activated using nitrogen radicals after treatment with RIE plasma for . The surface roughness created via RIE plasma is higher than that of the nitrogen radical. In both methods, although high strength bonding of silicon∕silicon interfaces was achieved before heating, bonding strength was reduced after heating except for the specimens activated for 10 and heated at in the RIE method. This reduction may be attributed to the growing number of voids generated across the bonded interface. High resolution transmission electron microscope observations showed a silicon oxide interfacial layer in the SPAB-processed silicon∕silicon interface, which is thicker than that of the RIE-processed interface without heating. After heating (at for in air), the thicknesses of the interfacial oxide layers were increased for both processes. The increased oxide layer thicknesses after heating are a result of the addition of thermally activated oxygen from water absorbed by the silicon bulk wafers and oxygen intrinsic to bulk silicon.

Self-organized vertically aligned single-crystal silicon nanostructures with controlled shape and aspect ratio by reactive plasma etching

The formation of vertically aligned single-crystalline silicon nanostructures via “self-organized” maskless etching in Ar+H2 plasmas is studied. The shape and aspect ratio can be effectively controlled by the reactive plasma composition. In the optimum parameter space, single-crystalline pyramid-like nanostructures are produced; otherwise, nanocones and nanodots are formed. This generic nanostructure formation approach does not involve any external material deposition. It is based on a concurrent sputtering, etching, hydrogen termination, and atom/radical redeposition and can be applied to other nanomaterials.

Inductively Coupled Pulsed Plasmas in the Presence of Synchronous Pulsed Substrate Bias for Robust, Reliable, and Fine Conductor Etching

Inductively coupled pulsed plasmas in the presence of synchronous pulsed substrate bias are characterized in a commercial plasma etching reactor for conductor etching. The synchronous pulsed plasma characteristics are evaluated through the following: 1) Ar-based Langmuir probe diagnostics; 2) Ar/Cl2 plasma modeling utilizing the hybrid plasma equipment model and the Monte Carlo feature model for the investigation of feature profile evolutions; 3) basic etching characteristics such as average etch rate and uniformity; 4) sub-50-nm Dynamic Random Access Memory (DRAM) basic etching performance and profile control; and 5) charge damage evaluation. It is demonstrated that one can control the etching uniformity and profile in advanced gate etching, and reduce the leakage current by varying the synchronous pulsed plasma parameters. Moreover, it is shown that synchronous pulsing has the promise of significantly reducing the electron shading effects compared with source pulsing mode and continuous-wave mode. The synchronous pulsed plasma paves the way to a wider window of operating conditions, which allows new plasma etching processes to address the large number of challenges emerging in the 45-nm and below technologies.

Effect of sidewall passivation in BCl3∕N2 inductively coupled plasma etching of two-dimensional GaAs photonic crystals

The effect of surface passivation in BCl3∕N2 inductively coupled plasma reactive-ion etching of GaAs-based photonic crystals (PhCs) was investigated. It is shown that sidewall passivation is crucial for achieving cylindrical, vertical PhC holes, where the exact shape of the hole is controlled via the N2 content in the plasma composition. The achieved quality of PhC membrane cavities was established by optical characterization of such cavities incorporating site-controlled quantum wires as integrated light source.