Daniel J. Hoffman

The impact of frequency mixing on sheath properties: Ion energy distribution and Vdc∕Vrf interaction

A dual frequency rf sheath is analyzed using a simple rf sheath model to study the interaction between the two driving rf currents and their effect on sheath parameters. A symmetric rf discharge with defined electron density and dc sheath potential is modeled using a sharp boundary sheath approximation. Three results of this study are reported: (1) reproduction of trends in ion energy distribution functions predicted and measured in previous studies, (2) a frequency-mixing-dependent relationship between the dc sheath potential and applied rf potential, and (3) an additional asymmetry in the ion energy distribution function generated by the intermodulation components resulting from the nonlinear sheath.

Combined rf and Transport Effects in Magnetized Capacitive Discharges

Magnetic fields can be used to enhance the performance and operational envelope of rf capacitive discharges for semiconductor processing. Antennas in magnetized experimental fusion devices can experience similar rf processes that lead to surface erosion and degraded antenna performance. Two-dimensional modeling is needed to understand the combined effects of production and transport in these plasmas; however, magnetized plasma is a complicated medium because of tensor rf conductivity, anisotropic transport, and the fact that rf power alone sustains the plasma. In this paper, we give results from a model originally derived for studies of magnetized fusion and helicon discharges that has been adapted to capacitive discharges and compare the results with experimental data. The two-dimensional model combines the effects of the magnetic field on the plasma’s rf properties and the bulk transport of plasma, including a sheath layer with finite thickness at the boundaries. A collisionless sheath model uses the rf fields in the sheath region, along with the density at the interface between the bulk plasma and the sheath, to determine the sheath thickness and to estimate the rectified dc potential. The driven rf fields are resolved inside the sheath region by including resistive dissipation caused by ion acceleration. These results are iterated with a model for transport of the bulk plasma to produce a global model of the sheath voltages and bulk rf plasma heating. The results at various iterative steps help isolate magnetic field effects that are caused by modification of the plasma’s rf response from transport effects that are caused by the reduced electron mobility perpendicular to the magnetic field. The magnetic field can enhance confinement for some pressure regimes and magnetic configurations. More importantly, the magnetic field can restrict the motion of electrons that are heated by the rf, localizing the nonequilibrium distribution of electron energy and reducing the electron transport across magnetic field lines. Changes in the plasma rf response can also play a role in the behavior of the discharge by further localizing the rf power deposition in the plasma.

Frequency optimization for capacitively coupled plasma source

Design of an all-in-one (main etch, PR ash and clean) dielectric etch chamber requires independent control of plasma generation from ion energy. Plasma simulation has been performed for a capacitively coupled discharge to study frequency effect on electron density, power deposition, and dissociation fraction. Simulation results demonstrate that plasma production efficiency enhances with increase in frequency while energy of the bombarding ions diminishes. A very high frequency source has been developed to generate high density plasma while radio frequency bias has been used to control ion energy. As illustrated, the etch rate for a dual damascene trench etch process increases, while damage due to ion bombardment is reduced. The dissociation fraction is well behaved to provide necessary corner protection. High-frequency source was used to achieve better performance for dual damascene trench etch process.

Plasma Simulation for High Frequency Plasma Source Design and Process Development

Summary form only given. Plasma simulation has been performed for a capacitively coupled discharge to study frequency effect on electron density, power deposition, and dissociation fraction. Simulation results demonstrated that plasma production efficiency improves with an increase in frequency while ion bombardment energy diminishes. At very high frequency, plasma generation can be controlled independent of ion energy. A very high frequency source has been developed that generates high-density plasma while RF bias is used to control ion energy. Process data showed that the etch rate for a dual damascene trench etch process increases while damage from ion bombardment is reduced. Simulation of plasma clean demonstrated higher species flux at lower pressure and higher source power. Process data confirmed that plasma cleaning is more efficient under these conditions. Thus, plasma simulation focused the selection of source frequency and helped optimize hardware and process design to make possible an effective all-in-one sequence of main etch, ash, and clean

Capacitive Systems for Dielectric Plasma Etch

Two and three frequency capacitive systems are being used to generate weakly ionized plasma in Ar/O/CxFy chemistries at the millitorr pressure range. One or two of the frequencies are generally used to accelerate ions (by sheath rectification) while the third is generally used to independently raise plasma density to levels sufficient for etching. The choice of frequencies is based on plasma impedances, which then yield rf voltages that can either consume power by creating a DC plasma sheath or consume power by creating plasma density (when sheath power is minimized). Within the two frequencies that create sheaths, the choice of ion energy spreads is determined by ion sheath transit time relative to an rf cycle. Technology challenges arising from the required plasma creation include significant intermodulation, very high cross talk between generators, and the avoidance of arcing through on any of the surfaces in contact with the plasma (including the gas injection system). The etch chamber is designed such that all generators are directly linked on a single coupling point or the plasma directly connects one launcher to another. We discuss and analyze different frequency ranges and their impact on chamber design.

Flow Simulation: Advanced Dielectric Etch Equipment Design and Process Development

With shrinkage in device size, use of new materials in multiple layers, and larger wafer size, control of process uniformity across the wafer becomes crucial in semiconductor manufacturing. Flow simulation has been used to design advanced dielectric etch equipment to achieve better flow uniformity. The ability to tune flow uniformity helped us to achieve various desired process characteristics, such as CD bias, profile, and etch stop across the wafer

Plasma confinement in a capacitively coupled VHF plasma reactor

Summary form only given. In order to minimize chamber contamination, to reduce cleaning time and cost, and to minimize process drift it is crucial to confine plasma in the process chamber preventing plasma penetrating to the downstream chamber. Plasma can be confined using physical confinement ring and/or by impedance confinement. In order to quantify the confinement level, we define the density ratio as the ratio of maximum plasma density below the confinement ring to that in the process chamber. The lower the density ratio, the better the plasma confinement. In order to design plasma confinement ring, plasma simulation has been performed to analyze the effectiveness of various confinement ring designs for a very high frequency capacitively coupled reactor. A confinement ring also increases chamber flow resistance adversely affecting the process window. Flow simulation has been performed to calculate pressure on wafer plane for different confinement ring gap widths. The confinement ring design is optimized not only to confine plasma but also to reduce flow resistance. To further improve the plasma confinement an innovative concept of impedance confinement has been analyzed using plasma simulation. An impedance parameter has been defined, and the density ratio is calculated. The impedance parameter has been optimized so as to achieve highly confined plasma. The impedance parameter can be controlled to increase plasma confinement during etch process, and to decrease plasma confinement during cleaning, as necessary

Effect of Magnetic Field on Plasma Density Distribution

Summary form only given. A numerical study of the fluid model for a cylindrical weakly ionized quasi-neutral plasma in an axial magnetic field is presented. The model takes into account inertia, ionization and frictional forces for ions and electrons for arbitrary magnitudes of the axial magnetic field. Accounting for both, ion inertia and nonlinear ion friction force, which is due to the ion charge exchange process, makes the model applicable for a wide range of discharge plasma conditions from near-collisionless to diffusion-dominated regime. The eigenvalue of the problem (the ionization frequency) and the plasma space distribution are found for a wide range of the magnetic parameter be=R/r e as well as electron and ion collisional parameters ce =nenR/vs and ci=R/li. Here R is the plasma radius, re is the electron cyclotron radius, nen is the electron-neutral collision frequency and v s is the ion sound speed. The behavior of plasma parameters and effect of plasma density distribution under the action of the magnetic field for a wide range of magnetic and collision parameters is discussed