Two-dimensional simulation of polysilicon etching with chlorine ina high density plasma reactor

A two-dimensional fluid simulation of polysilicon etching with chlorine in an inductively-coupled high density plasma source is presented. A modular approach was used to couple in a self-consistent manner the disparate time scales of plasma and neutral species transport. This way, complex plasma chemical reactions (involving electrons, ions and neutrals) as well as surface chemistry can be included in the simulation, The power deposited into the plasma was calculated by an electromagnetics module which solves Maxwell's equations. The power deposition was used in the electron energy module to find the electron temperature and the rate coefficients of electron-impact reactions. These were in turn used as source terms in separate neutral and charged species transport modules. By iterating among the modules, a self-consistent solution was obtained. Quantities of interest, such as power deposition, species density and flux, and etch rate and uniformity were thus calculated, As power deposition was increased, the electron density increased linearly, the plasma became less electronegative, the degree of gas dissociation increased, and the plasma potential remained constant. The radial uniformity of the Cl atom flux was better than that of the ion flux. The reactivity of the wafer as compared to that of the surrounding electrode surface significantly affected the etch uniformity, despite the low pressure of 10 mtorr     

Energy transport in plasma etching of nanoporous dielectric materials

A Monte Carlo routine was developed to simulate the motion and energetics of ions in the pores of a xerogel material under plasma etching conditions. The simulation included the effects of an applied electric field and input conditions for the pore as a function of pressure and applied voltage in the plasma reactor. We were interested in the ion energy in a pore, the ion penetration depth and the effect of ion energy on etching.At low pressures the nanoporous material etches faster than dense silicon dioxide. This is to be expected given the decrease in density and increase in surface area that arises due to the porosity. However, as the pressure is increased, the etch rate decreases dramatically and, eventually, the dense oxide may etch faster than the porous material. CHF₃ was used as the etchant gas and, for this gas, we believe this behavior to be controlled by the ion energy and energy transport in the pores of the xerogel material. As the pressure in the plasma reactor is increased, the incoming ions switch over from etching activation to polymerisation activation. This agrees with the observed crossover in etch rate seen experimentally and with the cessation in etching as pressure is increased. The switch is affected by pore roughness and correlates with the average ion energy in the pore.     

Direct simulation Monte Carlo analysis of flows and etch rate in aninductively coupled plasma reactor

The direct simulation Monte Carlo (DSMC) method was employed to predict the etch rate distribution on Si wafer. The etchant is assumed to be Cl. The production rate of Cl due to electron impact was obtained separately by preprocessing an inductively coupled chlorine plasma by use of the particle-in-cell/Monte Carlo method. Under the condition of constant total pressure, the etch rate increases with the mass flow rate of source gas Cl₂. The density of the etch product SiCl₂ rapidly decreases with increasing the flow rate. The density of the etchant hardly depends on the flow rate. The recombination 2Cl→Cl₂ on the inner walls of etching apparatus has a large effect on the etch rate; recombination probability of 0.1 results in 50% reduction of the etch rate. The etch rate distribution becomes more uniform when the reaction probability at the wafer surface is reduced     

Diagnostics of Low-Pressure Oxygen RF Plasmas and the Mechanism for Polymer Etching: A Comparison of Reactive Sputter Etching and Magnetron Sputter Etching

We have compared low-pressure oxygen RF plasmas and the etching of photoresist in a reactive sputter etch reactor and in a magnetron etch reactor using Langmuir probe, optical emission actinometry, and mass spectrometry measurements. The Langmuir probe data allow the determination of the plasma ion density and electron temperature, and thus the ion flux onto the substrate. The optical data yield information on the presence of O atoms and O2+ ions. Stable reactant and product species are monitored with a mass spectrometer. The main difference between the two reactors is that in magnetron sputter etching (MSE), the ion flux to the substrate is about an order of magnitude higher, under comparable plasma conditions, than in reactive sputter etching (RSE). This accounts for the higher etch rate in MSE. However, the etch yield per ion is higher in RSE because of the higher ion energy. Etch rates correlate neither with the ion flux to the substrate nor with the density of O atoms in the plasma, but change in parallel with the consumption of reactant gas. We conclude that in etching a polymer in a low-pressure oxygen plasma, the main neutral reactant species are O2 molecules, and an important role of the ions is to remove reaction products from the substrate surface.     

Model for ion-initiated trench etching

We model the plasma etching of trenches by Langmuir kinetics for neutral molecules and bombarding ions. The parallel combination of an isotropic etch rate for the neutrals and an anisotropic etch rate for the ions gives an effective etch rate. The ion etch rate is proportional to the normal surface component of the ion energy flux. An approximate analytical expression for the composite etch rate offers a new approach to the computation of etch profiles for these mixed systems. Etch profiles are displayed for three cases: the nearly ion flux-limited regime, an intermediate case, and the nearly neutral-flux limited regime for the trench bottom. The numerical calculation of the etch profiles follows from the integration of three characteristic strip equations which are nonlinear first-order ordinary differential equations (ODE's)     

Dry etching of ITO by magnetic pole enhanced inductively coupled plasma for display and biosensing devices

The dry etching of indium tin oxide (ITO) layers deposited on glass substrates was investigated in a high density inductively coupled plasma (ICP) source. This innovative low pressure plasma source uses a magnetic core in order to concentrate the electromagnetic energy on the plasma and thus provides for higher plasma density and better uniformity. Different gas mixtures were tested containing mainly hydrogen, argon and methane. In Ar/H₂ mixtures and at constant bias voltage (−100 V), the etch rate shows a linear dependence with input power varying the same way as the ion density, which confirms the hypothesis that the etching process is mainly physical. In CH₄/H₂ mixtures, the etch rate goes through a maximum for 10% CH₄ indicating a participation of the radicals to the etching process. However, the etch rate remains quite low with this type of gas mixture (around 10 nm/min) because the etching mechanism appears to be competing with a deposition process. With CH₄/Ar mixtures, a similar feature appeared but the etch rate was much higher, reaching 130 nm/min at 10% of CH₄ in Ar. The increase in etch rate with the addition of a small quantity of methane indicates that the physical etching process is enhanced by a chemical mechanism. The etching process was monitored by optical emission spectroscopy that appeared to be a valuable tool for endpoint detection.     

Modification of plasma-etched profiles by sputtering

Two-dimensional etch profiles are modeled for plasma etching. The etch rate dependence on the angle of incidence of the bombarding ions on the etched surface has a sputtering-type yield. The etch profile is advanced in time by an evolution equation for an etch rate proportional to the modified ion energy flux. Approximate analytical expressions for the etch rates are derived as a product of the etch rates in the absence of the sputtering-type yield and a weighting factor that depends on the angle the ion drift velocity makes with the normal to the wafer surface. The weighting factor is determined from experimental measurements of the angular dependence of ion beam etching by sputtering. These etch rates are valid when the ratio of the ion drift speed to the ion thermal speed is large compared to one. The etching is modeled in the ion flux-limited regime for simplicity. The modifications of the shape of etch profiles of a long rectangular trench and a waveguide structure or strip are treated     

Influence of the ion charge sign on the stimulation of plasmochemical etching of silicon

The influence of ions with different charge signs on the stimulation of silicon etching under plasma conditions is studied. Fluorine radicals are produced in a glow discharge with a nonuniform pressure. A beam of positive or negative ions is created using a Penning ion source. The flow of fluorine radicals and the ion beam are superposed on a silicon surface placed in a high vacuum. Positive ions may be converted into fast neutrals via resonance charge exchange in the parent gas. It is shown that fast neutrals have the highest catalytic effect. The catalytic effect of positive ions is about two times less. Negative ions occupy the intermediate position. For the first time, it is found that some kinds of ions (e.g., molecular oxygen) do not accelerate, but rather decelerate the etching process; i.e., they behave as inhibitors.     

In situ monitoring of uniformity by end point signal during plasma etching

A theory describing the etch rate uniformity based on the end point signal during the cleaning phase of etching is formulated for the case where the influence of residence time of the etching products and the loading effect can be neglected. An etch rate distribution function representing the probability density that the layer is etched at a given etch rate is introduced to calculate the relative root mean square of the etch rate across the wafer. The reduction in the size of the etched surface or the reduction of the flux of etching product molecules leaving the surface during the cleaning phase is used to determine the etch rate distribution function. A numerical simulation of the end point signal was used to illustrate possible applications of the theory. It is also shown that the slope of the end point curve itself cannot be used to characterize the etching nonuniformity as it is generally supposed. The theory was formulated with the aim of finding a simplein situ method applicable to statistical process control in semiconductor manufacturing when the nonuniformity of an etch process is characterized by a number (so-called 3σ method). Dedicated to Prof. Jan Janča on the occasion of his 60th birthday. This work was supported by the Slovak Grant Agency (No. 1/2312/95).     

BEHAVIOR OF Ar PLASMA FORMED IN A HIGH DENSITY PLASMA SOURCE-AN ECR REACTOR

In order to develop the ultra-large scale integration(ULSI), low pressure and high density plasma apparatus are required for etching and deposit of thin films. To understand critical parameters such as the pressure, temperature, electrostatic potential and energy distribution of ions impacting on the wafer, it is necessary to understand how these parameters are influenced by the power input and neutral gas pressure. In the present work, a 2-D hybrid electron fluid-particle ion model has been developed to simulate one of the high density plasma sources-an Electron Cyclotron Resonance (ECR) plasma system with various pressures and power inputs in a non-uniform magnetic field. By means of numerical simulation, the energy distributions of argon ion impacting on the wafer are obtained and the plasma density, electron temperature and plasma electrostatic potential are plotted in 3-D. It is concluded that the plasma density depends mainly on both the power input and neutral gas pressure. However, the plasma potential and electron temperature can hardly be affected by the power input, they seem to be primarily dependent on the neutral gas pressure. The comparison shows that the simulation results are qualitatively in good agreement with the experiment measurements.     

Effect of reactor surface modification on the neutral gas temperature in a transformer-coupled toroidal plasma

Transformer-coupled toroidal plasma (TCTP) has found applications in versatile fields as a high-power-density plasma source. However, most research on TCTP has focused on plasma states in the TCTP, without any change in reactor structures. Therefore, in this paper, we present a study on the effects of inner-surface modification of a TCTP reactor on plasma conditions. Using two types of reactors with different inner surfaces, the silicon-wafer etch rate by fluorine atoms from dissociated NF₃ gas molecules is compared. Then a computer simulation, which derives various plasma parameters, is carried out. As a result, the TCTP reactor with a grooved inner surface shows a higher etch rate than the reactor with a normal inner surface. The computer simulation suggests the reason for this etch rate difference is that the reactor with the grooved inner surface structure has a higher neutral gas temperature than the reactor with the normal inner surface so that more pyrolytic dissociation, resulting in a higher fluorine atom density, is achieved. Our results may be useful for those fields that require a higher neutral temperature or a higher degree of pyrolysis.     

Argon-dominated plasma beam generated by filtered vacuum arc and its substrate etching

A new technique to etch a substrate as a pre-treatment prior to functional film deposition was developed using a filtered vacuum arc plasma. An Ar-dominated plasma beam was generated from filtered carbon arc plasma by introducing appropriate flow rate of Ar gas in a T-shape filtered arc deposition (T-FAD) system. The radiation spectra emitted from the filtered plasma beam in front of a substrate table were measured. The substrate was etched by the Ar-dominated plasma beam. The principal results are summarized as follows. At a high flow rate of Ar gas (50 ml/min), when the bias was applied to the substrate, the plasma was attracted toward the substrate table and the substrate was well etched without film formation on the substrate. Super hard alloy (WC), bearing steel (SUJ2), and Si wafer were etched by the Ar-dominated plasma beam. The etching rate was dependent on the kind of substrate. The roughness of the substrate increased, when the etching rate was high. A pulse bias etched the substrate without roughening the substrate surface excessively.     

Influence of dielectric materials on uniformity of large-area capacitively coupled plasmas for N_2/Ar discharges

The effect of the dielectric ring on the plasma radial uniformity is numerically investigated in the practical 450-mm capacitively coupled plasma reactor by a two-dimensional self-consistent fluid model. The simulations were performed for N2/Ar discharges at the pressure of 300 Pa, and the frequency of 13.56 MHz. In the practical plasma treatment process,the wafer is always surrounded by a dielectric ring, which is less studied. In this paper, the plasma characteristics are systematically investigated by changing the properties of the dielectric ring, i.e., the relative permittivity, the thickness and the length. The results indicate that the plasma parameters strongly depend on the properties of the dielectric ring. As the ratio of the thickness to the relative permittivity of the dielectric ring increases, the electric field at the wafer edge becomes weaker due to the stronger surface charging effect. This gives rise to the lower N_2~+ ion density, flux and N atom density at the wafer edge. Thus the homogeneous plasma density is obtained by selecting optimal dielectric ring relative permittivity and thickness. In addition, we also find that the length of the dielectric ring should be as short as possible to avoid the discontinuity of the dielectric materials, and thus obtain the large area uniform plasma.     

Low pressure plasma etching of silicon carbide

A low pressure etching of silicon carbide is qualitatively characterized by using a neural network. To construct a predictive model, the etch process was characterized by means of a 2⁵ full factorial experiment. Experimental factors that were varied include radio frequency (rf) source power, bias power, pressure, O₂ fraction, and gap between the plasma source and wafer. An additional 15 experiments were conducted to test the appropriateness of the trained model. An optimized etch rate model has a root mean-squared error of 12.78 nm/min. Model response surface behaviors were certified by actual measurements. Several noticeable features at lower pressure etching include a lower etch rate, inverse relationship between the source power level and the dc bias, and a smaller etch rate variation with the source power. The effect of the bias power on the etch rate or dc bias was affected little by the pressure level. Etch mechanisms for the gap variations were quite different depending on the bias powers. Several etching aspects useful for plasma control were revealed. PACS 52.75.R     

Nanometer-scale etching of CoFeB thin films using pulse-modulated high density plasma

Pulse-modulated inductively coupled plasma reactive ion etching of nanometer-scale patterned CoFeB thin films was performed in CH₄/O₂/Ar gas mixture. As the pulse on-off duty ratio decreased, the etch selectivity of CoFeB/TiN slightly increased and the etch profiles were improved. Moreover, the etch selectivity of the CoFeB films and the etch profiles were improved with the increase in the pulse frequency of the plasma. X-ray photoelectron spectroscopy revealed that during the pulse-modulated etching in the CH₄/O₂/Ar gas mixture, some polymeric layers were formed on the CoFeB films, which helped prevent the lateral etching and increased the etch selectivity. Consequently, nanometer-scale etching of CoFeB thin films patterned with TiN hard masks could be achieved using pulsed-modulated plasma in CH₄/O₂/Ar gas mixture.     

Parameter space map of an electron cyclotron resonance plasma in acompact chamber

Ion current density measurements were made in an electron cyclotron resonance (ECR) plasma reactor for both argon and oxygen discharges. Spatial changes in the ion current density were also recorded across the reactor diameter for changes in pressure and power. These measurements revealed a minimum in the ion current density on the reactor axis. This observation has been explained as a consequence of the shape of the ECR region, which, in turn, is dependent on the mode of coupling. Current density measurements were made as a function of reactor pressure and microwave power for two different axial locations in the system. A Langmuir probe was also used at these two locations to measure the electron temperature as a function of these process conditions. It was observed that the ion current density and/or plasma density measured downstream from the ECR zone, increased significantly in the low-pressure/high-microwave power region. Results from this region of the operating parameter space have not previously been reported. Further existing models do not predict this observed increase in plasma density or ion current density. It has been proposed that a rarefication of the gas in the ECR region, as a result of gas heating, has acted to increase the outward diffusion of electrons from the ECR zone and, thus, has increased the ambipolar diffusion of ions to the downstream location. This proposal has been partially validated by experimental results in which the ion energy was measured as a function of reactor pressure and gas flow rate. The shape of the oxygen parameter space map differs significantly from that for Ar. The principal reasons for these changes are a number of different inelastic electron scattering mechanisms which effect the transport electrons out of the ECR zone and through ambipolar diffusion also the transport of ions. The second factor is the production of negative ionic species which varies with reactor pressure and, thus, T_e     

Modern Applications of Plasma Etching and Patterning in Silicon Process Technology

Besides plasma etching of through-wafer interconnects in wafer stacks for vertical integration of chips, fabrication of platinum (Pt) electrodes with non-tapered sidewalls for the storage node in modern memories (DRAMs and FeRAMs) is one of the most challenging tasks of plasma process technology today. This paper describes the achievement of vertical integration of chips by plasma etching of high aspect ratio interchip vias. The etching processes for dielectrics, single crystal silicon, and the organic glue layer were all optimized for minimum reactive ion etching (RIE) lag i.e. for minimum decrease of etch rate with increasing etch depth. Furthermore the fabrication of perfect Pt electrodes for modern DRAMs and FeRAMs is reported. Vertical Pt profiles were achieved by plasma processing with resist mask. In this novel approach, the build-up of thin redepositions of Pt onto the sidewalls of the resist, obtained as a result of processing in pure Ar plasmas, is utilized to achieve a sidewall steepness of the patterned Pt film which is determined by the steepness of the pre-etch resist profile. After pattern transfer and resist stripping, the portion of the redepositions protruding above the fabricated storage node was completely removed by chemical mechanical polishing.     

圆筒电极对离子磁电加热的影响

高效的磁电加热不仅能够提高电子回旋共振(ECR)等离子体的离子温度,还能改善离子的径向和轴向分布,促进ECR等离子体在化学气相沉积金刚石膜刻蚀中的应用.将磁电加热系统中的圆环电极改进为圆筒电极,研究了圆筒电极对离子磁电加热的影响,对比了圆筒和圆环电极加热离子的区别.结果表明:在同一阳极偏压下,圆筒比圆环电极更有利于提高离子温度,圆筒电极加热时各径向位置的离子温度升高的幅度较大,其中圆筒电极内部的离子温度径向分布差异较大,而圆筒下游的离子温度径向分布比较均匀;磁电加热对离子密度的影响很小;采用圆筒电极加热时,有利于离子向轴向下游的输运,改善了离子的轴向均匀性.     

Neutral gas free molecular flow algorithm including ionization and walls for use in plasma simulations

Several plasma devices, including Hall and ion thrusters, operate by ionizing a low density neutral gas for which the mean free path between collisions of gas molecules is greater than typical device dimensions. In general, the discrete-particle algorithms used to calculate the neutral gas ignore velocity changes due to collisions between gas molecules. However, particle algorithms are a source of unphysical statistical noise that may detract from the study of the plasma physics, the prime purpose of most simulations. In this paper we present a new neutral gas algorithm for use in plasma simulation codes that exploits the fact that very few collisions change the velocity of neutral gas molecules. The algorithm assumes that the particle velocity distribution function for neutrals emitted from a given surface remains unchanged except for a scale factor that reflects the loss of neutrals to ionization. The sources of neutrals may be gas inlets, and isotropic, thermally accommodated, gas molecules coming off chamber surfaces including recombined ions. The algorithm is implemented in two dimensions (R–Z) with emitting surfaces represented as surfaces of revolution. The advantage of this algorithm over the conventional particle approach is the absence of statistical noise.     

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建立了包含两种正离子的碰撞等离子体鞘层的流体模型,通过四阶龙格-库塔法模拟了碰撞对含有两种正离子的等离子体鞘层中的离子密度和速度分布产生的影响。结果表明,对于两种正离子的等离子体来说,鞘层中无论哪种离子与中性粒子碰撞频率的增加,该种离子的密度和速度分布都将呈现波动变化,密度是先增加后降低,速度是先降低后增加;而另一种离子的密度和速度呈单调变化。鞘边正离子的含量越少,受自身与中性粒子的碰撞频率增加,鞘层中该种离子密度先增加后降低的变化位置就越远离等离子体的鞘层边界。同时发现该种离子密度分布受自身碰撞频率增加,降低的幅度变化非常小。另外发现电子碰撞器壁产生的二次电子发射系数对质量较轻的离子影响要大一些。