Ion Flux Uniformity Research Articles

Two‐dimensional sp2‐carbon‐linked covalent organic framework for large‐capacity and long‐life Na metal batteries

AbstractNa metal batteries are regarded as an encouraging route for energy‐dense and low‐cost battery systems. However, the unstable and irreversible Na plating/stripping, caused by the uncontrolled dendritic Na growth, prevents their practical applications. Herein, a two‐dimensional sp2‐carbon‐linked covalent organic framework (cyano‐sp2c‐COF) is adopted as seeding/hosting coating layer for a highly stable interface with long cycling life, large capacity, and high Na utilization. Benefit from the features of a fully π‐conjugated structure and well‐defined cyano groups, cyano‐sp2c‐COF with superior sodiophilicity and small interface resistance can reduce the nucleation barrier, enable Na ion flux uniformity, and enhance interface stability. Ultimately, the system achieves a low nucleation overpotential of only 10 mV, a remarkable average Coulombic efficiency of 99.7% maintained over 500 cycles in half cells, and exceptional interfacial durability of 8500 h with a high accumulated capacity of 8.5 Ah cm−2 in symmetric cells. Furthermore, the symmetric cells also present a steady cycling, even increasing the depth of discharge up to 90%. As proof, full cells demonstrate a long lifespan enduring 2700 cycles with tiny capacity decay, providing valuable insights into the long‐life Na batteries.

Control of the spatial distribution of ion flux in dual inductively coupled plasmas

The effects of external discharge parameters (applied RF power, operating pressure, and gas flow rate) on ion flux uniformity are investigated in a dual inductively coupled plasma. The ion fluxes and electron temperatures are measured using a Langmuir probe floated by a DC blocking capacitor based on the floating harmonic method. The RF power applied to the top antenna focuses on the total ion flux control throughout the chamber, while the power introduced to the bottom antenna can control the ion flux uniformity in the diffusion chamber. At high pressures, the local electron heating in the vicinity of the bottom antennas results in local maximum ionization, thereby increasing the ion flux near the chamber wall. Furthermore, the ion flux uniformity also can be affected by the gas flow rate and pressure because they are associated with the gas residence time and the electron heating region. A physical understanding of the effects of external discharge parameters on plasma uniformity is useful for optimizing plasma processes.

Modeling of low pressure plasma sources for microelectronics fabrication

Chemically reactive plasmas operating in the 1 mTorr–10 Torr pressure range are widely used for thin film processing in the semiconductor industry. Plasma modeling has come to play an important role in the design of these plasma processing systems. A number of 3-dimensional (3D) fluid and hybrid plasma modeling examples are used to illustrate the role of computational investigations in design of plasma processing hardware for applications such as ion implantation, deposition, and etching. A model for a rectangular inductively coupled plasma (ICP) source is described, which is employed as an ion source for ion implantation. It is shown that gas pressure strongly influences ion flux uniformity, which is determined by the balance between the location of plasma production and diffusion. The effect of chamber dimensions on plasma uniformity in a rectangular capacitively coupled plasma (CCP) is examined using an electromagnetic plasma model. Due to high pressure and small gap in this system, plasma uniformity is found to be primarily determined by the electric field profile in the sheath/pre-sheath region. A 3D model is utilized to investigate the confinement properties of a mesh in a cylindrical CCP. Results highlight the role of hole topology and size on the formation of localized hot-spots. A 3D electromagnetic plasma model for a cylindrical ICP is used to study inductive versus capacitive power coupling and how placement of ground return wires influences it. Finally, a 3D hybrid plasma model for an electron beam generated magnetized plasma is used to understand the role of reactor geometry on plasma uniformity in the presence of E × B drift.

Improving the quality of barrier/seed interface by optimizing physical vapor deposition of Cu Film in hollow cathode magnetron

The quality of physical vapor deposition (PVD) grown barrier/seed interface in Cu interconnect metallization was significantly improved by enhancing Cu nucleation on the Ta barrier surface. This was accomplished through filtering of nonenergetic species from the deposition flux, increasing the fraction of Cu ions, improving metal ion flux uniformity, and minimizing gas ion bombardment of the growing film. The self-sputtering ability of Cu was combined with a magnetically confined high-density plasma in the Novellus hollow cathode magnetron (HCM®) PVD source. Spatial profiles of plasma density and temperature, as well as ion flux, metal ion fraction, and ion energy, were measured by planar Langmuir probes, quartz crystal microbalances, and gridded energy analyzers, all located at the wafer level. Multiple criteria, such as seed step coverage and roughness, the seed layer’s resistance to agglomeration, and its stability in the plating bath, have been used to evaluate interface quality. As a result, a new and improved Cu PVD process which demonstrates superior stability during subsequent process steps and ensures successful electrofill performance with a more than 50 % reduction in minimal requirement of field thickness as well as sidewall thickness has been developed.

Investigation of the ion dose non-uniformity caused by sheath-lens focusing effect on silicon wafers

The ion dose non-uniformity induced on the wafer surface by modal and discrete focusing effects is investigated for different plasma densities and implantation parameters. Measured impact radius agrees well with values obtained by simulation. The optical pattern observed on the wafer surface is correlated with the ion dose by FT-IR measurements. The applicability of a previously proposed vertical ring is demonstrated, the ring being able to considerably improve the ion flux uniformity by shifting the discrete focusing effect out of the wafer surface and reducing the modal focusing. Experiments are performed in an inductively coupled plasma produced in hydrogen.

Controlling the ion flux on substrates of different geometry by sheath-lens focusing effect

It is shown that a three-dimensional plasma sheath lens that forms on biased electrodes interfacing insulators exhibits novel characteristics such as ion focusing on desired locations, controllability of ion flux uniformity, formation of passive surfaces and applicability to plasma diagnostics. The ion flux profile on substrates of different geometry is obtained by three dimensional simulations of potential distribution and ion trajectory, while experiments are realized in electropositive and electronegative DC and ICP discharges.

Particle modelling of inductively-coupled argon plasmas with wafer biasing

The characteristics of axisymmetrical inductively-coupled argon plasmas with wafer biasing are examined using the particle-in-cell Monte Carlo method. The simulation is performed on the condition that gas pressure is fixed at 5 mTorr and power deposition is at 200 W. The effects of bias frequency and bias voltage on the distributions of flux and energy of the ions that are incident onto the wafer are investigated. The bias frequency and bias voltage are changed from 2 MHz to 13.56 MHz and 100 V to 300 V, respectively. The plasma structure is largely influenced by the wafer biasing. The ion energy distribution strongly depends on the bias frequency, bias voltage and sheath thickness. Decreasing the bias frequency results in the improvement of the uniformity of ion flux. These results are consistent with the previous theory.

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.

Ion flux nonuniformities in large-area high-frequency capacitive discharges

Strong nonuniformities of plasma production are expected in capacitive discharges if the excitation wavelength becomes comparable to the reactor size (standing-wave effect) and/or if the plasma skin depth becomes comparable to the plate separation (skin effect) [M. A. Lieberman et al., Plasma Sources Sci. Technol. 11, 283 (2002)]. Ion flux uniformity measurements were carried out in a large-area square (40 cm×40 cm) capacitive discharge driven at frequencies between 13.56 MHz and 81.36 MHz in argon gas at 150 mTorr. At 13.56 MHz, the ion flux was uniform to ±5%. At 60 MHz (and above) and at low rf power, the standing-wave effect was seen (maximum of the ion flux at the center), in good quantitative agreement with theory. At higher rf power, maxima of the ion flux were observed at the edges, due either to the skin effect or to other edge effects.

Ion flux uniformity in inductively coupled plasma sources

The spatial distribution of the ion flux at the substrate or work surface in inductively coupled plasma source is studied experimentally and theoretically. The theoretical study is based on a two-dimensional fluid model and the well-known global model. The spatial profiles of the electron density and temperature, as well as the ion flux density, are obtained. The theoretical results are compared with the experimental data obtained using a plate probe. It is found that at high pressures the temperature dependence of the electron-neutral collision rate can be important. For argon, maximum uniformity of the ion flux at the work surface occurs for p∼20 mTorr, with the induction coils located on the cylindrical sidewall of the discharge chamber.

Two-dimensional plasma reactor simulation with self-consistent coupling of gas flow with plasma transport

A two-dimensional (2-D) fluid simulation applicable to high density plasma reactors was developed which couples gas flow with plasma transport in a self-consistent manner. Modified boundary conditions were used at the reactor walls to accommodate transport at low pressures. A modular approach circumvented the disparity in time scales and a quasi-neutral plasma assumption overcame the disparity in length scales. This approach allowed for a simulation tool that can execute rapidly on a desktop computer, and is, therefore, suitable for Technology Computer-Aided Design (TCAD). Simulation results for an electron cyclotron resonance (ECR) plasma system showed good agreement with experimental data on electron density and temperature as a function of plasma power and pressure. Also, the gas pressure drop in the reactor was predicted correctly by the simulation. Further simulation studies showed the effect of power and pressure on ion flux uniformity on a wafer substrate.

Investigation of electron source and ion flux uniformity in high plasma density inductively coupled etching tools using two-dimensional modeling

Inductively coupled plasma (ICP) sources are being developed as reactors for high plasma density (1011–1012 cm−3), low-pressure (<10–20 mTorr) etching of semiconductors and metals for microelectronics fabrication. Transformer coupled plasmas (TCPs) are one variant of ICP etching tools which use a flat spiral coil having a rectangular cross section powered at radio frequencies (rf) to produce a dense plasma in a cylindrical plasma chamber. Capacitive rf biasing of the substrate may also be used to independently control ion energies incident on the wafer. The uniformity of generating the plasma and the uniformity of the flux of reactants to the substrate are functions of the geometry and placement of the coil; and of the materials used in the construction of the chamber. In this article, we use results from a two-dimensional model to investigate design issues in TCPs for etching. We parametrize the number of turns and locations of the coil; and material properties of the reactor. We find that at low pressure, designs which produce ionization predominantly at larger radii near the edge of the wafer produce more uniform ion fluxes to the substrate. This results from a ‘‘converging’’ ion flux which compensates for losses to lateral surfaces. Careful attention must be paid to metal structures in the vicinity of the coils which restrict the azimuthal electrical field. This situation results in reduced power deposition at large radii, which can be compensated by over sizing the coil or by using auxiliary solenoidal coils. The plasma and neutral transport, dominated by diffusion, treats the advective flow from the gas inlets and pump port as local sources and sinks which are rapidly volume averaged.

Modeling an inductively coupled plasma source

Inductively coupled plasma (ICP) sources are becoming increasingly popular for etching submicrometer features, soft-cleans for physical vapor deposition cluster tools, and plasma-enhanced chemical-vapor deposition. Here, we report on a discharge model for argon for the ICP which combines an electromagnetic field distribution model with an ambipolar diffusion model. The model provides a self-consistent solution for the spatial distribution of the charged species in the bulk plasma, which permits the spatial distribution of the ion flux to the wafer surface to be calculated. The variations of the peak electron concentration, the ion flux, and its uniformity at the wafer have been simulated as a function of the rf power, antenna geometry, and chamber geometry. The electron number density increases linearly with rf power in accordance with experimental observations. The ion flux uniformity is primarily dependent on the diameter of the antenna and is only slightly dependent on the number of turns. The uniformity generally is relatively invariant with rf power above a threshold power level. Contouring the antenna or the dielectric separating the antenna from the plasma is useful for improving ion flux uniformity. The minimum thickness of the dielectric cap is constrained by the maximum allowable sheath potential. The induced electric field at the wafer can cause substantial wafer heating at high antenna power levels. Increasing the wafer to antenna spacing mitigates this problem without sacrificing the ion flux and its uniformity.

Modeling and Simulation of High Density Plasmas

We present a model of an ECR plasma in a cylindrical (z, r) geometry with resonant zone near the substrate surface. The model combines a hybrid (particle ion-fluid electron) treatment of plasma transport with an ad-hoc model of microwave power deposition. The model is used to test the effect of the radial profile of microwave power on the spatial profiles of plasma parameters: plasma density, plasma potential and electron temperature. It is shown that for uniformity of ion flux at the substrate surface, microwave power must be peaked off axis.