This paper reviews the current state-of-the-art of several diagnostic techniques for characterizing, monitoring, and optimizing the properties of process plasmas. The plasma parameters of plasma configurations which are useful for materials processing, such as dc and rf glow discharges, are similar to the parameters of the boundary plasmas in magnetic fusion devices. As a result, much of the large development effort over the last decade on plasma diagnostics for boundary plasmas has direct application in process plasma systems. As particular examples, the authors discuss recent developments of: (1) electrostatic probes for electron density, temperature and power flow measurements; (2) laser fluorescence spectroscopy for ion density and flux measurements; and (3) quantitative plasma mass spectroscopy using quadrupole mass filters.

We report the development of two new sources for the production of intense pulsed jets of atomic and molecular free radicals. The present work applies these sources to the generation of the widely used semiconductor etchant, CF2, in each case characterizing the yield and internal energy distribution of this diradical by laser induced fluorescence. The methods employed modify a commercial high-vacuum compatible pulsed molecular beam valve to accept either a glow discharge or pyrolysis nozzle. The source compound for both systems is hexafluoropropylene oxide (HFPO). In each case measured yields of CF2 are high: For pyrolysis we find intensities as great as 5×1017 sr−1sec−1 representing nearly complete conversion of HFPO to CF2 + CF3CFO. The discharge source yields about an order of magnitude less CF2 intensity in a shorter beam-pulse which also contains ions and other atomic and molecular radicals. The vibrational energy distribution of the CF2 varies with source conditions, with the hottest CF2 produced by the pyrolysis source at high temperature.

As part of an effort to determine the feasibility of carbothermic reduction of alumina in plasmas, spatial distributions of temperature, velocity and species in the plume of an argon radio-frequency plasma with alumina and carbon additions were estimated using a two dimensional theoretical model of the flow field and a thermochemical model based on equilibrium conditions. Experimentally determined boundary conditions were input to the models. Using a 30 kW inductively coupled plasma, experimental measurements were made to allow comparison with predictions. A practical process for carbothermic reduction will require prevention of back reactions, either by separation of the aluminum species or by some kind of quenching of the process. The significance of the experimental results and comparisons with predictions are discussed.

Spatial distributions of plasma variables (concentration, temperature, and velocity) were calculated for alumina and carbon species injected in an argon thermal plasma. The plasma behavior was modeled from arc initiation through free plume expansion by combining a plasma code with a plume code. The model assumed a direct current, constrictor type, arc heater. This research, supported by the U.S. Bureau of Mines under the Strategic and Critical Materials Program, is examining the feasibility of using thermal plasmas for ore reduction. The process investigated in this paper is the carbothermic reduction of alumina.

A one-dimensional fluid model is presented for time-dependent ion concentrations in chlorine containing discharges. The ion formation rate is determined from experimental plasma-induced emission intensities. The local field is estimated from spectrally resolved laser-induced fluorescence data obtained in similar, BCl3 discharges. Quantitative agreement with experiment is obtained implying that the basic assumptions in the model are valid: (1) ground state ions in the sheath are formed predominantly by electron-impact ionization; (2) Cl2+ ion motion is mobilitylimited by charge exchange collisions with Cl2 neutrals; (3) the degree of Cl2 dissociation is ∼. 65% at a power density of 1.8 W cm−3; (4) the ion concentration near the electrode is spatially uniform throughout most of the rf cycle; and (5) there are two times during a low frequency rf cycle when ions experience a strong extraction force.

Semi-empirical models like the one described here should be useful in computational optimization of plasma processes such as etching and deposition, which are in wicre-spread use throughout the microelectronics industry.

We report quantitative measurements of the concentration of atomic oxygen in RF plasmas determined by two-photon laser induced fluorescence. The results are compared with concurrent measurements of spatially resolved plasma induced optical emission. These measurements establish: (1) the O atom concentration as a function of plasma parameters, (2) a semi-quantitative relationship between O* emission intensity normalized by Ar* in-tensity and O atom concentration, and (3) an understanding of the mechanisms for pro-duction of excited atoms in the plasma.

Optical emission spectroscopy (OES) has proven to be a valuable tool in the development and production of state-of-the-art semiconductor devices. Application to the plasma etching of a variety of materials necessary for integrated circuit fabrication is discussed, with particular emphasis placed on etch endpoint analysis. The utility of OES techniques in monitoring photolithographic processes is also presented.

Intensities of CH optical emission and electrical properties of methane rf discharges as a function of pressure are presented and discussed. The results are consistent with a model in which the properties of the discharge are dominated by secondary electrons traversing the gap between the electrodes.

Two processes of great importance in the semiconductor industry are vapor deposition and plasma etching. This paper presents a review of laser techniques for spectroscopic characterization of the gas phase species involved in these processes. Band strength and other spectroscopic data for selected molecules are used to give estimates of the detection sensitivity in vibrational and electronic bands. Preliminary results are given from work presently in progress in our laboratory on the detection of such species. The discussion includes examples of the application of these techniques to a number of laboratory deposition and etching devices.

In order to control the quality and properties of hydrogenated amor-phous silicon (a–Si:H) semiconductors produced by discharge deposition, we would like to control the gas species produced in the discharge and to understand how these deposit and influence the film properties. One would like to separately consider the gas processes that yield the mix of species arriving at the surface, and then the surface processes. I will somewhat follow that separation here, but it appears that many of the species found in the gas may, in fact, be released from the surface. Thus, the gas species appear to result from gas and surface reactions, and a sequential separation is actually not possible. Similarly, the study of the surface deposition process requires forming various radical species as well as an a–Si:H surface, which inevitably will involve the gas processes. Thus we really have coupled problems.

A capacitively coupled, rf glow discharge of silane in argon was studied with laser light scattering to determine the spatial concentration of small particles. Multi-wavelength scattering profiles have been obtained and are being analyzed to obtain size distributions as a function of spatial location. Very sharply defined particle zones can be found under some plasma conditions that are spatially related to the silicon atom profiles. We will report our results and attempt to qualitatively describe how these zones relate to plasma chemistry and film deposition processes.

A capacitively coupled rf glow discharge of silane in argon was studied to determine the spatial concentration of silicon atoms. Laserinduced fluorescence was used to determine the ground state concentration profiles. The fluorescence profiles clearly show the sharp boundaries of the sheath regions. The dc bias voltage, silane mole fractions, flow rates, and chamber pressure were all varied to establish the sensitivity of the silane profiles. The existing theory of sheath formation is used to qualitatively understand the existence of sharp spatial boundaries and the sensitivity of the anode sheath region to plasma chemistry.

This is a progress report on modelling the first of two aspects of plasma assisted chemical processes; it is shown here that by maintaining a constant plasma geometry, observable plasma parameters can be made to determine ion assisted surface chemistry uniquely because the mean energy of ions bombarding the reacting surfaces are thus also uniquely determined. Furthermore, the theory (1) given here sets the stage for another, given elsewhere (2), whicn shows that the resulting processes (i.e., reactive ion or plasma assisted etching and deposition) follow the same “rules”, as thermally driven cnei.lical vapor transport (CVT) for a fixed mean ion energy. Together, these theories permit a considerably more orderly understanding and application of plasma etch/deposition processes than has oeen previously possible.

This paper reviews our results of modulated ion beam studies of the ion-assisted etching of Si. It is shown that the experimental data of the Si(C12, Ar+) reactive system can be described by a model based upon an ion-bombardment induced amorphousness of the Si substrate and the formation of a mixed surface region of several atomic layers of chlorine, argon and silicon. It is also shown that the model is in general agreement with the experimental data of the Si(XeF2, Ar+) and C(H, Ar+) systems.

The ion etching of Si, SiO2 and Mo by fluorocarbon ions was studied using real time Auger electron spectroscopy. The carbon in the CF3+ ions was found to react rapidly with SiO2 to form volatile products, resulting in a very low carbon concentration on the SiO2 surface. Both Si and Mo formed carbides when bombarded with CF3+ ions. This resulted in a high carbon concentration on these surfaces.

Reactive sputter etching of SiO2 with CHF3-O2 plasmas has been investigated in a parallel plate reactor by combining etch rate measurements with concurrent determination of ion densities (using a Langmuir probe) and the composition of neutral plasma species (using a mass spectrometer). Etch rates are found to follow the ion density and to be fairly independent of the plasma chemistry under most experimental conditions. Moreover, a comparison of reactive sputter etching and reactive ion beam etching of SiO2 with CHF3 and CF4 shows that etch yields per incoming ion are essentially independent of the flux of neutral radicals to the substrate. This strongly suggests as the dominant etch mechanism for SiO2 direct reactive ion etching, where ions themselves are the main reactants in the etch reaction. Measured values of etch yields are consistent with this picture.

Etch rates of heavily doped silicon films (n- and p-type) and undoped polysilicon film were studied during plasma etching and also during reactive ion etching in a CF4/O2 plasma. The etch rate of undoped Si was lower than the n+ Si etch rate, but higher than the p+ Si etch rate, when the RF inductive heating by the eddy current was minimized by using thermal backing to the water-cooled electrode. The thermal activation energy for spontaneous chemical etching was measured to be 0.10 eV, independent of the doping characteristics. This doping effect may be explained by the opposite polarity of the space charge present in the depletion layer of n+ Si and p+ Si during reactive plasma etching.

The reaction of XeF2 with the Si(100) surface was studied by modulated (10–1000 Hz) molecular beam-mass spectrometry in the temperature range 300–1300K and equivalent XeF2 pressure of 5×10−6 to 10−4 Torr. Simultaneous bombardment of the reacting surface by Ar+ was used to determine the extent of ion-enhancement of the reaction.

In the absence of the ion beam, the main reaction product was SiF4, which was formed with a reaction probability of ˜5×10−2 at room temperature. In the presence of the ion beam three products, SiF4, SiF2 and F2 (or F), were detected with formation probabilities of ˜1×10−1, 6×10−2 and 7×10−2 respectively. Increasing surface temperature reduced the ion-enhanced reactivity.

The interaction of fluorine with a three-fold open site of the Si(111) surface has been examined using a cluster model. The site considered is taken as representative of one where penetration of the surface may occur. New analyses are used to show that the interaction is ionic with F having essentially a full negative charge for all distances near the surface, both above and below. The bonding is shown to be almost entirely electrostatic between the polarized charges of the positive Si surface and F-. The F-Si binding energy is estimated taking proper account of the electron affinity of F and the ionization potential, work function, of Si. When this is done, it is shown that there is no barrier for the penetration of the Si surface by F; hence, this penetration should not be a rate limiting step in the F-Si etching reaction.

An understanding of etching reactions in a plasma environment requires a knowledge of: (1) the types of gas phase particles which react at the surface, (2) the etch products formed, and (3) the processes which lead from reactants to products. Experimental data relavant to these topics are reviewed in this paper. A conceptual framework for understanding the etching reaction is reviewed and it is shown that the experimental data presently available is consistent with this framework. The influence of ion bombardment on etching reactions is extensively discussed.