Herbert H. Sawin

Characterization of a Time Multiplexed Inductively Coupled Plasma Etcher

We report the experimentally obtained response surfaces of silicon etching rate, aspect ratio dependent etching (ARDE), photoresist etching rate, and anisotropy parameter in a time multiplexed inductively coupled plasma etcher. The data were collected whi le varying eight etching variables. The relevance of electrode power, pressure, and gas flow rates is presented and has been found t o agree with observations reported in the literature. The observed behavior presented in this report serves as a tool to locate a nd optimize operating conditions to etch high aspect ratio structures. The performance of this deep reactive ion etcher allows the tai loring of silicon etching rates in excess of 4 mm/min with anisotropic profiles, nonuniformities of less than 4% across the wafer, and ARDE control with a depth variation of less than 1 mm for trenches of dissimilar width. Furthermore it is possible to prescribe the slope of etched trenches from positive to reentrant.

The Gaseous Electronics Conference radio‐frequency reference cell: A defined parallel‐plate radio‐frequency system for experimental and theoretical studies of plasma‐processing discharges

A “reference cell” for generating radio-frequency (rf) glow discharges in gases at a frequency of 13.56 MHz is described. The reference cell provides an experimental platform for comparing plasma measurements carried out in a common reactor geometry by different experimental groups, thereby enhancing the transfer of knowledge and insight gained in rf discharge studies. The results of performing ostensibly identical measurements on six of these cells in five different laboratories are analyzed and discussed. Measurements were made of plasma voltage and current characteristics for discharges in pure argon at specified values of applied voltages, gas pressures, and gas flow rates. Data are presented on relevant electrical quantities derived from Fourier analysis of the voltage and current wave forms. Amplitudes, phase shifts, self-bias voltages, and power dissipation were measured. Each of the cells was characterized in terms of its measured internal reactive components. Comparing results from different cells provides an indication of the degree of precision needed to define the electrical configuration and operating parameters in order to achieve identical performance at various laboratories. The results show, for example, that the external circuit, including the reactive components of the rf power source, can significantly influence the discharge. Results obtained in reference cells with identical rf power sources demonstrate that considerable progress has been made in developing a phenomenological understanding of the conditions needed to obtain reproducible discharge conditions in independent reference cells.

Charging of pattern features during plasma etching

The localized charging of a rectangular trench during the plasma etching of a perfectly insulating surface was modeled assuming an isotropic electron flux and monodirectional ion bombardment. The field set up by the localized charging acts to deflect arriving ions, modifying the ion flux densities within the feature, and thus, etching rates. Preliminary simulations indicate that this may be important in the shaping of etching profiles.

Phenomenological modeling of ion‐enhanced surface kinetics in fluorine‐based plasma etching

A multiple beam apparatus has been constructed to facilitate the study of ion‐enhanced fluorine chemistry on undoped polysilicon and silicon dioxide surfaces by allowing the fluxes of fluorine (F) atoms and argon (Ar+) ions to be independently varied over several orders of magnitude. The chemical nature of the etching surfaces has been investigated following the vacuum transfer of the sample dies to an adjoining x‐ray photoelectron spectroscopy facility. The etching ‘‘enhancement’’ effect of normally incident Ar+ ions has been quantified over a wide range of ion energy through the use of Kaufman and electron cyclotron resonance‐type ion sources. The increase in per ion etching yield of fluorine saturated silicon and silicon dioxide surfaces with increasing ion energy (Eion) was found to scale as (Eion1/2−Eth1/2), where Eth is the etching threshold energy for the process. Simple near‐surface site occupation models have been proposed for the quantification of the ion‐enhanced etching kinetics in these syste...

Ion bombardment in rf plasmas

Ion bombardment energy and angle distributions have been measured in an argon plasma. The measured ion angle distribution at 10 mTorr shows that 30% of the ions have incident angles greater than 10° from the surface normal. However, ions with large incident angles have much lower energies than those incident perpendicular to the surface. At 500 mTorr a very large fraction of the ions have large incident angles, and the average energies of these ions are relatively independent of incident angle. Monte Carlo simulations of the sheath kinetics predict the trends shown in the experimental data for ion energy and angle distributions. Fine structure in the ion energy distribution was observed below 50 mTorr and is shown to be caused by charge‐exchange collisions in the sheath. The average ion energy in a symmetric parallel plate system is linearly related to the voltage applied across the electrodes for measured plasma pressures up to 500 mTorr.

Continuum modeling of argon radio frequency glow discharges

A continuum model has been developed which successfully describes the concentration, movement, and energetics of charged particles within a rf discharge. This model includes continuity equations for all charged particles, Poisson’s equation to determine the local electric fields, and an electron energy balance to determine the ionization and energy‐loss rates. All input parameters (diffusivity, mobility, ionization, and energy‐loss rate) were defined using reported values determined in dc field experiments.

Kinetic study of low energy ion-enhanced polysilicon etching using Cl, Cl2, and Cl+ beam scattering

The chlorine ion-enhanced etching yield of polysilicon in the low ion energy regime was characterized as a function of Cl ion energy, ion flux, neutral-to-ion flux ratio, and the ion impingement angle by utilizing Cl+, Cl, and Cl2 beam scattering. The chlorine ionic, atomic, and molecular fluxes were controlled independently over more than an order of magnitude and at flux levels within an order of magnitude of that typically used in high density plasma processes. The etching yield increased with the increase of Cl/Cl+ flux ratio but gradually saturated at higher flux ratios as the surface became saturated with chlorine. The ion energy dependence was a linear function of (Eion1/2−Eth1/2), where the threshold energy Eth was found to be approximately 10 eV. With Cl+ ion bombardment, the etching yield of Cl was two to three times higher than that of Cl2 at higher flux ratios. The angular dependence of ion-enhanced etching yield was also measured. The etching yield was reduced by approximately 30% and 50% whe...

Continuum modeling of radio‐frequency glow discharges. I. Theory and results for electropositive and electronegative gases

A self‐consistent continuum (fluid) model for a radio‐frequency discharge is presented. The model is one dimensional, incorporates an electron energy balance, and is valid for both electropositive and electronegative discharges. A connection of the fluid model with the underlying physics is presented: issues such as the derivation of the fluid equations from moments of the Boltzmann equation, the closure of the set of moments, and the fundamental assumptions behind the fluid equations are discussed. A detailed set of results for an electropositive and an electronegative discharge is presented, and contrasted. The sustaining mechanisms and the electrical characteristics of the two discharges are also discussed. Comparison with experimental data of spatially and temporally resolved plasma induced emission is successfully done.

Kinetic study of low energy argon ion-enhanced plasma etching of polysilicon with atomic/molecular chlorine

Surface kinetics of ion-enhanced chlorine plasma etching in the low ion energy regime was studied by utilizing Ar+ for ion bombardment and Cl and Cl2 as reactants. The argon ion and chlorine atom (molecular) fluxes were controlled independently over more than an order of magnitude and at flux levels within an order of magnitude of that typically used in high density plasma processes. The ion-enhanced etching yield was characterized as a function of Ar ion energy, ion flux, neutral-to-ion flux ratio, and the ion incident angle. Possible reaction pathways are proposed and reduced into a two-parameter model which is useable in a profile simulator. The etching yield increases with the increase of flux ratio but gradually saturates at higher flux ratios as the ion flux limits the etching yield. The ion energy dependence was found to scale linearly with (Eion1/2−Eth1/2), where the threshold energy Eth is found to be 16 eV. The etching yield of Cl is found to be similar to that of Cl2 at flux ratios below 10, bu...

Ion energy and angular distributions in inductively coupled radio frequency discharges in argon

We report measurements of the energies and angular distributions of positive ions in an inductively coupled argon plasma in a Gaseous Electronics Conference Reference Cell. Use of two separate ion detectors allowed measurement of ion energies and fluxes as a function of position as well as ion angular distributions on the discharge centerline. The inductive drive on our system produced high plasma densities (up to 1012/cm3 electron densities) and relatively stable plasma potentials. As a result, ion energy distributions typically consisted of a single feature well separated from zero energy. Mean ion energy was independent of rf power and varied inversely with pressure, decreasing from 29 to 12 eV as pressure increased from 2.4 to 50 mTorr. The half‐widths of the ion angular distributions in these experiments varied from 5° to 9°, or equivalently, the transverse temperatures varied from 0.18 to 0.29 eV with the distributions broadening as either pressure or rf power was increased.