IEEE TRANSACTIONS ON EDUCATION, VOL. 40, NO. 3, AUGUST 1997 207

Plasma Etching and Plasma Physics Experiments forthe Undergraduate Microelectronics Course

Charles B. Fleddermann,Senior Member, IEEE

Abstract—Many universities offer lecture/laboratory courseson the processing of microelectronic circuits; these courses aredesigned to introduce electrical or chemical engineering studentsto the fundamentals of integrated circuit (IC) fabrication. Giventhe nearly universal adoption of plasma processing by the ICindustry, experiments with plasmas are a necessary addition tothis type of course. In this paper, a modified microelectronicslaboratory sequence will be described which incorporates twonew experiments. In the first experiment, students study the fun-damental nature of plasmas used for materials processing. Thisis followed by a second experiment designed to investigate theeffects of plasma reactor parameters on the resulting etch. Theseexperiments can be performed on minimally modified industrial-type plasma etching reactors. The experiments described maybe easily implemented at universities with a microelectronicsfabrication program or course, and may also be applicablefor training in an industrial setting. The goal of these twoexperiments is to give the student preparing for employment in anIC fabrication environment broad exposure to the fundamentalphysics of low-pressure plasmas, in addition to some knowledge ofthe impact of reactor settings on the quality of the resulting etch.

Index Terms—Integrated circuit fabrication, materials process-ing, plasma etching, plasma physics experiments.

I. INTRODUCTION

M ANY undergraduate curricula in electrical engineeringinclude a course in fabrication of integrated circuits

(IC’s), frequently including a laboratory component in whichstudents perform the basic steps of fabrication and testingof integrated circuits. Lab sessions generally cover diffusion,oxidation, metallization, measurement and testing, etc. Sinceplasma etching is now a standard process for pattern formationduring the fabrication of integrated circuits in the IC indus-try, a comprehensive sequence of experiments covering ICfabrication should include some training in plasma methods.In this paper, two experiments on semiconductor materialsprocessing using plasmas are proposed. Similar experimentshave been tested in the laboratory sequence for the senior-levelintegrated circuits course taught in the Electrical and ComputerEngineering Department at the University of New Mexico. Theobjectives of these experiments are to introduce the studentsto some of the basic ideas of plasma physics and gaseouselectronics, to give them hands-on experience in measuring

Manuscript received February 1995; revised June 5, 1997. This workwas supported in part by the National Science Foundation UndergraduateInstrumentation and Laboratory Improvement (ILI) program under Grant#USE-9151962, and in part by the University of New Mexico.

The author is with the University of New Mexico, Albuquerque, NM 87131-1356 USA.

Publisher Item Identifier S 0018-9359(97)06292-4.

fundamental properties of both inert and reactive plasmas, andto etch materials relevant to IC fabrication to determine theeffects of reactor parameters on the resulting quality of theetched material. The result of this comprehensive approach tostudying plasma etching of IC’s will be a knowledge of theprocesses which occur in a plasma, and an understanding of therelation between reactor settings and the resulting etch quality.

The plasma etching experiments described here are ap-propriate for a senior-level microelectronics course whichtypically consists of three hours of lecture per week, and anapproximately three hour laboratory session each week. Boththe lecture and laboratory should include work in diffusion,oxidation, mask making, etching, etc., while other topicsnot easily accessible to a university laboratory (such as ionimplantation) are discussed in the lectures. Of course, thelevel of coverage of plasma processing of materials in thelecture will need to be increased in order for students to fullyunderstand the implications of the laboratory experiments. Twolaboratory experiments designed to introduce students to basicplasma physics and plasma etching are described.

This paper begins with a brief introduction to some of thebasic plasma physics concepts that provide a background forlaboratory experiments in plasma etching. This is followedby a discussion of some of the important properties of anetching process and the resulting etched material. A detaileddescription of the actual experiments to be performed by thestudents as well as the context of these experiments withinthe laboratory and lecture course follows. Finally, suggestionswill be made for implementing this type of laboratory at otheruniversities as well as in an industrial setting.

II. THE PLASMA ENVIRONMENT, PLASMA

ETCHING, AND BASIC DIAGNOSTICS

In this section, a brief outline of the properties of a plasma,especially as they relate to plasma etching of IC’s is presented.These basic properties are taught through both the lectureand laboratory sections of the IC fabrication course. Manygood texts and journal articles relating to plasma processingof IC’s are available, such as [1] and [2]. In addition, basicconcepts relating to plasma etching and the properties ofetched materials are described. Finally, the basic theory ofLangmuir probe diagnostics, the plasma diagnostic techniqueused in the plasma etching experiments is described.

A. The Plasma Environment

A plasma, sometimes referred to as the “fourth state ofmatter,” is a collection of electrons, ions, and neutrals, which

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208 IEEE TRANSACTIONS ON EDUCATION, VOL. 40, NO. 3, AUGUST 1997

for plasma processing, is generated from a gas input to areactor. Many of the important processes in a plasma areinitiated by electron impact. For example, ionization whichis essential to maintaining a discharge is initiated by electronsthrough the following equation:

e Ar Ar e (1)

Electron impact also initiates excitation

e Ar Ar (2)

(Ar indicates an excited state of argon) which is ultimatelyresponsible for the glow that emanates from a discharge.Especially important for plasma etching is electron impactdissociation, described by

e CF CF F e (3)

This process is responsible for breaking up relatively inertinput gases, such as CFin the example above, into reactiveradicals (for example, atomic fluorine) which react with silicondioxide or silicon on the wafer. The byproduct of the surfacereaction is SiF, which is volatile at room temperature andis pumped away by the vacuum system. Photolithography isused to open selected areas of the wafer to the plasma, sothat structures such as holes in the oxide for implantation ordiffusion can be etched.

Since electrons initiate many of the important plasma pro-cesses, a good starting point for understanding plasmas isto measure the properties of the electrons. Two relativelyaccessible electron measurements are: electron density, whichindicates the number of electrons in a unit volume of plasma(and therefore the number that are available to participatein dissociation and ionization of the input gas); and electrontemperature, which is a measure of the average energy of theelectrons, which determines the electron’s ability to participatein the reactions described above. For etching, electron temper-ature will help determine how much dissociation of the inputmolecule takes place, how much reactive fluorine is produced,and therefore how fast etching will take place in the reactor.Fortunately, these two electron parameters can be measured ina fairly straightforward manner using a Langmuir probe, whichwill be discussed below. Electron energy and temperature arehighly dependent on reactor settings such as input power, gaspressure, gas mixture, etc., and therefore are useful for relatingthe reactor settings to the properties of the etched materials.Also of interest are comparisons between the properties ofinert gas plasmas (such as argon) and reactive plasmas (such asCF ). Halogen-containing plasmas such as those in CFdifferfrom inert-gas plasmas because of the formation of electron-attaching daughter products (for example, F radicals) whenCF is dissociated. The presence of negative ions and largedensities of attaching species alters the– characteristic ofLangmuir probes.

B. Plasma Etching

There are many important issues regarding materials etchingwhich are addressed in the plasma etching experiment. Theseinclude etch rate (how fast material is removed from the

Fig. 1. Schematic representation of a parallel-plate plasma etching reactorincluding the drive circuit.

wafer?), selectivity (how fast does one material etch relative toanother material?), anisotropy (how directional is the etch?),and damage to the unetched substrate. All of these are affectedby the choice of reactor settings. The etching experimentdiscussed below is designed so that students evaluate theeffects of reactor settings on some of these etching results.Time permitting, it is also instructive for students to comparethe properties of plasma-etched IC’s to those etched in aconventional wet-chemical solution.

A typical reactor configuration used for plasma etching isshown schematically in Fig. 1. This is a parallel-plate, RFreactor which is typical of those used for plasma etching inthe IC industry.1 The reactor consists of a vacuum chamberin which there are two parallel electrodes. The wafer to beetched is placed on one of the electrodes, a controlled flow ofthe etching gas is introduced into the chamber, and a plasmais struck by the application of an RF voltage to one of theplates. Since the impedance of the reactor is not 50 W (reactorimpedance is affected by the presence of the plasma and is afunction of the power absorbed by the plasma), a matchingnetwork is included so that the RF generator can be efficientlymatched to the reactor. The matching network reduces theamount of RF power reflected back to the generator. Typicalpressures for plasma etching of IC’s in a parallel-plate RFreactor are around 100 mtorr.

C. Langmuir Probe Diagnostics

The Langmuir probe is perhaps the most widely useddiagnostic tool for low-pressure plasmas such as those foundin etching reactors. In its most basic form, the Langmuir probeis a conducting surface which is placed in a plasma. Theprobe can be biased either negative or positive with respectto the plasma (see Fig. 2), and is capable of collecting currentdue to electron and ion fluxes from the plasma. When ametallic surface is placed in a plasma, a sheath forms around it(sheaths form around any object placed in the plasma includinginsulating materials). There is a flux of electrons and a flux ofions from the plasma which cross the sheath edge and impingeon the probe surface. By biasing the probe positive with respect

1This paper was first submitted in January, 1995; since then the IC industryhas largely shifted to the use of inductively coupled plasma (ICP) reactorsoperating at 10–20 mtorr rather than the parallel-plate systems described here.However, the experiments described in this paper are equally applicable tothe ICP or any other plasma reactor.

FLEDDERMANN: PLASMA ETCHING AND PLASMA PHYSICS EXPERIMENTS FOR THE UNDERGRADUATE MICROELECTRONICS COURSE 209

Fig. 2. Schematic diagram of Langmuir probe in the plasma environment.Also shown is the tuning and filtering for measurements in an RF reactor, thevariable voltage supply for setting probe voltage, and an ammeter for probecurrent measurement.

Fig. 3. Schematic representation of Langmuir probeI–V characteristicillustrating current saturation, and plasma and floating potential.

to the plasma, ion flux to the probe is suppressed and theprobe current is mostly due to electrons. Similarly, if the probeis biased negative with respect to the plasma, the electronflux is suppressed and the probe collects ions. This behavioris illustrated in Fig. 3, an idealized Langmuir probe–characteristic. For both positive and negative bias, the currentshould saturate because the current to the probe is limitedby the flux of charge carriers crossing the plasma–sheathinterface, which is relatively unaffected by the probe bias.The electron saturation current is higher than the ion saturationcurrent not because there are more electrons in the plasma (theelectron and ion density are roughly equal in the plasma), butrather because the electrons are much more mobile than theions.

Also evident on this graph are two important potentials:plasma potential and floating potential. The plasma is at themost positive potential in the reactor. When the probe isbiased above , ion flow to the probe is suppressed, andonly electrons will be collected as described above. For probevoltages moderately below , the ion flux is not completelysuppressed, and there is some ion current in addition to theelectron current. This leads to a reduction in the total currentmeasured. Thus is located at the inflection point of the

– characteristic.Any electrically isolated object placed in a plasma must

have no net current. Since there is always a flux of ionsand electrons to any surface, and since the electron flux willinitially be larger due to the higher mobility of electrons, thesurface must charge up to some potential that will suppress

electron flux and thus balance the electron and ion currents.The potential required to balance the current flow to an isolatedobject is the floating potential , and is at the point on the

– curve where no net current flows to the probe.Langmuir probes can be used to directly measure plasma

density and electron temperature [3], [4]. The magnitude ofthe ion saturation current can be related to the ion density inthe plasma by the following equation [4]:

(4)

where is the effective collecting area of the probe (takinginto account the surface area of the sheath),is the iondensity, and is the electron temperature (the method formeasuring is described below). In practice, the electronsaturation current is difficult to measure since for variousreasons it is difficult to saturate the electron current by raisingthe probe potential to large positive values. Unlike the–characteristic shown in Fig. 3, in real measurements the probecurrent for potentials above tends to increase rapidly. Forthis reason, it is much easier to use the ion saturation currentto measure ion density; since electrons and ions are made inpairs in the plasma, under most circumstances the ion densitymeasured will be approximately the same as the electrondensity in the plasma; in other words, .

In order to use (4), the electron temperature must beknown. is determined from the Langmuir probe curve byconsidering the region of the– characteristic between theplasma potential and floating potential. If the probe potential isset fairly close to the plasma potential, then the probe currentcan be described by [4]

(5)

where is the electron or ion density in the plasma andisthe potential applied to the probe. Taking the natural logarithmof both sides of the equation yields

(6)

where is a constant. Thus the electron temperature isrelated to the slope of a plot of the probe current as afunction of probe potential near the plasma potential. Thismeasurement is especially amenable to computer techniqueswhich automatically make the measurement, take the naturallogarithm, and calculate the slope to determine the electrontemperature.

III. A DDITIONAL LECTURES

In order for the students to fully understand the laboratoryexperiments on plasma etching, it is important to spendseveral classroom lectures during the course of the semesteron both topics covered in the laboratory: plasma physics andplasma etching. The lectures on plasma physics should includediscussions of basic plasma phenomena and plasma reactions(like those described above) including ionization, excitation,dissociation, recombination, and attachment. Of course, theexamples discussed should be based on gases relevant tosemiconductor processing such as CFor SF . Cross sectionsfor these processes should be discussed in general as a means

210 IEEE TRANSACTIONS ON EDUCATION, VOL. 40, NO. 3, AUGUST 1997

for establishing rates at which processes take place in a plasma.Since many of the processes important to plasma etching areinitiated by ion bombardment of surfaces, sheath formationand potential should be discussed since this is where theions get their energy. Mechanisms for sustaining dischargesshould be discussed in terms of electron production and lossmechanisms, leading to a discussion of breakdown and self-sustained discharges. Finally, some detailed discussion of thearchitecture of radio-frequency plasmas (the workhorse of thesemiconductor processing industry) and RF reactors should bediscussed.

Discussion of plasma properties should be followed by adiscussion of plasma etching technology. This section shouldinclude discussions on etch rate, anisotropy, selectivity, anddamage. A discussion of various gases for etching differentmaterials will give students a feel for the range of materialsthat can be etched and how the choice of gas can affectthe etch properties mentioned above. In particular, ashing(the removal of photoresist in an oxygen plasma) shouldbe covered. Other processes, such as sputtering and reactiveion etching (RIE), should also be discussed. Finally, the useof plasmas for deposition of materials and growth of thinfilms should be discussed with emphasis on plasma-enhancedchemical vapor deposition (PECVD), and various sputteringmethods (magnetron, ion beam, etc.).

IV. DETAILS OF EXPERIMENTS

Two experiments on plasma processing are proposed for thesenior-level microelectronics fabrication laboratory sequence.The first experiment studies the fundamental properties of aplasma using a Langmuir probe. In the second experiment,students study the effects of changes in reactor setting on theproperties of etched silicon and silicon dioxide.

A. Required Equipment

The experiments described here can be implemented onnearly any commercial or “home-made” plasma reactor. It isbeneficial from the student’s point of view to use a commercialreactor which will provide more experience on the types oftools used in industrial environments. Commercial reactorsare frequently available as used equipment from several ven-dors, or as donated equipment from IC manufacturers. Somechanges in laboratory facilities will be required to providepower to the reactor and to safely store and exhaust the gasesused in the reactor.

Only two modifications need to be made to commercial re-actors to make them suitable for performing these experiments.An appropriate port for inserting a Langmuir probe into theplasma region must be fabricated. Many commercial reactorsare fitted with laser-based endpoint detection schemes whichutilize a small hole drilled through one of the electrodes totransport the laser beam to the surface of the wafer beingetched. This hole is ideal for insertion of the Langmuir probe.It is essential that there also be an optical viewing port so thatstudents can observe the characteristics of the plasma and seehow these characteristics vary with changes in reactor controlsettings. Many commercial reactors already come with this

type of view port; those not so equipped can generally bemodified with minimal machining.

The diagnostic experiments are performed using a Langmuirprobe. These probes can be fabricated in any reasonablyequipped laboratory, and are easily customized to the particularreactor available. The probe itself consists of a small-diameterwire which is mostly enclosed in an insulating sheath exceptfor a very small portion which accesses the plasma. In itssimplest form, the electronics for probe control and readout,shown in Fig. 2, consist of a dc voltage supply capable ofapplying both positive and negative voltages (with respect tothe reactor ground) to the probe, and an ammeter. More sophis-ticated computer-controlled voltage sweeps with automatedcurrent readout are also easily implemented.

For the etching experiments, a stylus profilometer is requiredto measure etch rates. If possible, access to a scanning electronmicroscope to study selected samples for anisotropy is valu-able. Of course, this is too sophisticated a piece of equipmentfor the students in an undergraduate lab to handle; the samplescan be evaluated by the teaching assistant and results presentedto the class for analysis and discussion.

B. Experiment 1: Plasma Properties

The first experiment consists of a series of observations andmeasurements designed to enhance and confirm the plasmaphysics concepts that the students learn in the lectures. Theexperiment begins with qualitative observations of the plasma,such as observing the color of a discharge in the differentgases, observing the extent of the glow as pressure is varied,and visually observing changes in the plasma-sheath thicknessas reactor parameters are varied. The student can readilycouple these simple observations to basic plasma physics. Forexample, the color of the plasma is different in the gasesstudied because of the difference in electronic energy levels inthe atoms and molecules present. Students can easily grasp thisconcept without too much discussion of optical spectroscopy.

Quantitative measurements of plasma properties are madeusing a Langmuir probe. These probe diagnostics have beenthe standard plasma diagnostic tool for several decades, areeasily implemented, and are not difficult to interpret. (Aswith any diagnostic tool, there are many subtleties and pitfallsassociated with using Langmuir probes; however, for teachingbasic plasma processes, these problems can easily be ignored.)Since an RF plasma reactor is being used, a tuned Langmuirprobe must be used to eliminate RF interference in themeasurements [5]. The procedure for making these measure-ments is quite straightforward: the probe tip is immersedin the plasma and the probe potential is varied while thecurrent to the probe is measured as described above. Anactual – characteristic of a Langmuir probe in an argonplasma measured in a commercial reactor at the Universityof New Mexico is shown in Fig. 4. This is similar to theidealized – characteristic shown in Fig. 3, and also indicatestypical current values that can be expected from this type ofmeasurement.

The Langmuir probe is also used to determine electron tem-perature in the plasma. As described above, this measurement

FLEDDERMANN: PLASMA ETCHING AND PLASMA PHYSICS EXPERIMENTS FOR THE UNDERGRADUATE MICROELECTRONICS COURSE 211

Fig. 4. Measured Langmuir probeI–V characteristic for a commercialplasma-etch reactor.

uses the positively biased part of the– curve, up to theplasma potential where the probe current begins to saturate.The slope of the natural logarithm of the probe current versusvoltage applied to the probe is inversely proportional to theelectron temperature. Measurements of both electron densityand temperature can be performed by hand, or can be readilyautomated.

Since the electron density and temperature are functions ofthe reactor operating conditions, the students make Langmuirprobe measurements over a range of pressures, gases, powerinputs, etc. Of special interest is a comparison of theseparameters between an inert (argon) and a reactive (CF)plasma; processes such as electron attachment and dissociationgreatly affect the electron density and temperature and areimportant in reactive plasmas used for etching, but are notpresent in inert plasmas. Since the parameter space for thesemeasurements is very large and the lab time for each studentlimited, the Langmuir probe analysis of the plasmas can befacilitated by having each group perform measurements overan assigned subset of the parameter space, with data from allgroups provided to each student for analysis and report writing.

Student reports on these experiments include graphs of theprobe data, including plots of electron density and electrontemperature as a function of gas pressure and input powerfor both argon and CF. Of course, students who are familiarwith and have access to experimental design software packagesmay use this to more readily plot and analyze their data.In their reports, students describe changes in brightness andcolor of the plasma, changes in sheath thickness observedduring the changes in reactor parameters, and answer questionsregarding why these changes take place. (This analysis will bebased on what has been learned in the lecture.) Reports shouldalso describe differences in electron density and temperaturebetween Ar and CFplasmas, and descriptions of the originand significance of these differences.

C. Experiment 2: Etching

The second experiment involves actual plasma etchingof devices using CF. This involves etching silicon andoxidized silicon side-by-side over a wide range of reactorpower and gas pressure. If possible, measurements should be

Fig. 5. Graph of etch rate versus power in commercial reactor. Input poweris the measured incident power minus the reflected power.

made of the dc self-bias during the etch runs, and some runsshould be performed with no self-bias if possible. Results ofmeasurements of silicon etch rate in CFin the reactor at theUniversity of New Mexico are shown in Fig. 5. Since theparameter space is again very large, it is impossible in thecontext of one laboratory period for students to completelycharacterize the plasma etcher. Instead, the parameter space isdivided up and each group performs some subset of the etches,with all of the results made available to all of the groups. Itis also valuable for students to see a microscopic view of theetch results, so scanning electron microscope (SEM) picturesof selected samples should be taken by the instructor and madeavailable to students for their reports.

Reports should include discussions of how etch rate varieswith power and pressure. Students should also graph selectivityof silicon etch to silicon dioxide etch as a function of the reac-tor parameters. Using the SEM pictures provided, an estimateof anisotropy and the effects of reactor setting on anisotropyshould also be discussed. Finally, a comparison between theetch results and the Langmuir probe measurements on the CFplasma should be made; students should try to determine howthe basic plasma properties measured in experiment 1 affectthe final etch quality measured in experiment 2.

V. IMPLEMENTATION AT OTHER INSTITUTIONS

AND INDUSTRY

A. Incorporation into a One-SemesterMicroelectronics Course

The resources required to effect this change in the mi-croelectronics course will vary from institution to institutiondepending on the equipment that is already in place. Mostof the required equipment for making masks, applying pho-toresists, etc., will already exist in a microelectronics teachinglaboratory. Of course, a commercial plasma etching tool isrequired; the major requirements for the plasma etcher areease of use (especially important for undergraduate teaching),and visual access to the plasma. The Langmuir probe andassociated circuitry can be home-made, and can generally beinstalled on the reactor with only minor modifications required.

212 IEEE TRANSACTIONS ON EDUCATION, VOL. 40, NO. 3, AUGUST 1997

The probe measurements may be made by hand using a powersupply and ammeter, or can easily be automated by computer.

B. A Full-Semester Plasma Processing Course

The types of experiments outlined above can easily beexpanded into a series of lectures and experiments that en-compass a full semester course on plasma processing ofmaterials at either the undergraduate or graduate level. Thiscould include more extensive diagnostic experiments suchas microwave interferometry, mass spectrometry, and opti-cal spectroscopy. Other topics that could be covered (somerequiring more equipment) are photoresist ashing, sputtering,and plasma CVD. These topics are amenable to the approachdiscussed above of coupling fundamental studies with morepractical experiments.

C. Implementation in Industrial Environments

The experiments described here can also be adapted to anindustrial setting. Many of the industries participating in ICfabrication might benefit from giving technicians and engi-neers with plasma processing responsibilities hands-on trainingin basic plasma concepts and etching/deposition before be-coming responsible for actual production machines. Suchtraining can be tailored to an individual plant’s productionenvironment.

D. Safety

Of course safety is generally a concern in experimentalcourses in microelectronic fabrication. Experiments involvingplasma etching present some unique safety considerations thatshould also be mentioned here. Many of the byproducts ofplasma etching are hazardous, and special attention to gashandling procedures are essential. Even though some of thegases commonly used in plasma etching are relatively inert,gas safety is still essential since plasma-dissociated productsof the input gas may be hazardous and must be disposed ofsafely.

Most commercial plasma etchers utilize relatively highvoltage RF power supplies which present an obvious electricalhazard. Although these reactors are generally well-engineeredto ensure safety and to limit access to high voltages, caremust be taken when modifying the plasma reactor to per-form diagnostic measurements such as the Langmuir probeexperiments described above. Inadvertent shorting of the probeto electrodes or other high-potential surfaces can presentoperator-accessible high voltages during experimental mea-surements.

VI. CONCLUSION

A program designed to introduce the senior electrical en-gineering student to the fundamentals of plasma etching ofintegrated circuits in the context of a one-semester microelec-tronic fabrication course has been presented. The experimentsproposed are designed to acquaint students with basic plasmaphysics and plasma diagnostic techniques, to allow students toexplore the effects of reactor parameters on the etching processand the etched structures, and to compare the performance ofwet-chemical- and dry-etched circuits. Since plasma etching isthe standard process in industry for patterning IC’s, it seemsessential that these kinds of experiments be incorporated intothe curriculum.

ACKNOWLEDGMENT

The author wishes to thank W. Mays for his efforts inobtaining and maintaining much of the equipment used for thislaboratory, Intel Corporation for the donation of the etchingtool used in these experiments, and Prof. S. Hersee and Prof.D. Kendall who have taught the microelectronics processingclass at the University of New Mexico for their comments andsuggestions on implementing this program.

REFERENCES

[1] J. W. Coburn, “Plasma etching and reactive ion etching,” inAmericanVacuum Society Monograph Series, N. R. Whetten, Ed. New York:Amer. Inst. Physics, 1982.

[2] B. Chapman,Glow Discharge Processes, Sputtering and Plasma Etch-ing. New York: Wiley, 1980.

[3] B. Cherrington, “The use of electrostatic probes for plasma diagnos-tics—A review,” Plasma Chem. Plasma Process., vol. 2, pp. 113–140,1982.

[4] F. Chen, “Electric probes,” inPlasma Diagnostic Techniques, R. H.Huddlestone and S. L. Leonard, Eds. New York: Academic, 1965, pp.113–200.

[5] For example, A. P. Paranjpe, J. P. McVittie, and S. A. Self, “A tunedLangmuir probe for measurements in RF glow discharges,”J. Appl.Phys., vol. 67, pp. 6718–6727, June 1, 1990.

Charles B. Fleddermann (M’90–SM’96) received the B.S. degree in elec-trical engineering from the University of Notre Dame, Notre Dame, IN, in1977, and the M.S. and Ph.D. degrees in electrical engineering in 1980 and1985, respectively, from the University of Illinois at Urbana-Champaign.

He has been on the Electrical Engineering Faculty at the University ofNew Mexico (UNM), Albuquerque, since 1985, and has also held a jointappointment with Sandia National Laboratories and UNM. His researchinterests include plasma processing of electronic materials and diagnosticsof plasma processes.