Effect of discharge conditions and cathode identity on charged particle populations in the negative glow region of a simple diode glow discharge

S ecmchimiia

Am

Vol.

46B. No. 6/l, pp. 983-1CQO.1991

0%4-8547/91 s3.00 + .txl 0 1991 Pergamon Press pk.

F&ted in Great B&in.

Effect of discharge conditions and cathode identity on charged particle populations in the negative glow region of a simple diode glow discharge* DUENCHENGFANG and R. KENNETHMARCUS? Department

of Chemistry, Howard L. Hunter Chemical Laboratories, SC 29634-1905, U.S.A.

Clemson University, Clemson,

(Received 20 August 1990; accepted 9 Janum-y 1991) Abstract-Plasma properties of electron temperature, average electron energy, electron energy distribution function, and electron and positive ion number densities in the negative glow region are determined for a range of discharge conditions for various cathode identities (materials). The results show that the so-called “slow” electrons are heavily populated, with depletion of higher energy electrons, above 1.2 eV. This depletion seems to be caused by atomic excitation collisions. The results also show that the electron temperature and average electron energy are controlled mainly bv “fast” electrons. The work function of the cathode plays an important role in the emission of secondary electrons but does not affect significantly the bulk plasma excitation conditions.

1.

INTRODUCTION

GLOW discharge plasmas have been attracting increased attention as atomizers, excitation sources, or ion sources for atomic spectroscopy [l-5]. The sources are characterized by their efficiency in generating atomic populations from solid, conductive samples. The combination of the sputtering process, which has certain advantages over thermal volatilization methods [3, 61, and the diffusion of atoms into the collision-rich negative glow region makes the devices useful for subsequent analyses by atomic spectrometries. One key aspect in the development of quantitation schemes for bulk elemental analysis is the determination of the roles that sample matrix identity and discharge conditions have in the plasma excitation/ionization conditions. Preliminary studies in this laboratory and results found in the literature suggest that excitation conditions, as indicated by excitation temperatures, are fairly insensitive to sample matrix identity [6] or discharge conditions in various discharge sources [7-101. Of course, such results are not unexpected because the glow discharge environment is non-Maxwellian in nature, i.e. glow discharges are non-LTE systems, and as such excitation temperatures are meaningless. Since charged particle collisions dominate excitation processes and the creation of important metastable discharge gas species, glow discharges can be more completely characterized by studying electron and ion populations. The properties of the negative glow region can be characterized in terms of the energies of charged particles and their densities. The electron temperature and electron number density have been measured by means of optical emission measurements in order to compare the excitation conditions between different cathodes. However, those optical methods, which either determine the electron temperature by Doppler broadening [ll] or the electron density by Ha line broadening [ll, 121, are not adequate in the diagnostics of glow discharge plasmas due to the fact that those techniques are only applicable to high density plasmas with electron densities larger than -1014 cms3, whereas the electron densities in glow discharge plasmas are in the order of 1O1”cmm3.

* This article was published in the Special Boumans Festschrift Issue of Spectrochimicu Acta. t Author to whom correspondence should be addressed. 983

984

DUENCHENG FANGand R. K. MARCUS

Three electron groups are believed to exist in the negative glow region [13]: (I) secondary electrons emitted from the cathode surface (so-called fast electrons), which gain kinetic energy through the cathode dark space and attain electron temperatures of -2&25 eV and densities on the order of lo6 cme3; (II) secondary electrons due to gas phase ionization, with electron temperatures of -2-10 eV and number densities in the range of lo’-108 cmT3; and (III) (ultimate or slow electrons) electrons from either group I or group II which have experienced several elastic and inelastic collisions with particles in the plasma and have electron temperatures of only 0.05-0.6 eV and densities in the range of 1Og-1O11cme3. Langmuir probes are the easiest and most accurate devices to make localized measurements for characterizing the plasma excitation/ionization conditions in terms of electron temperature (T,), electron energy distribution function (EEDF), and the densities of electrons (n,) and positive ions (ni) [l&22]. A preliminary study [23] has shown that a single cylindrical Langmuir probe is suitable for diagnosing planar diode glow discharge devices. Though the floating (double) probe technique [24] is another method to determine the electron temperature and plasma density without disturbing the plasma, the electron temperature measurements contain only those fast electrons in the tail of the EEDF, not those in the bulk of the distribution [25]. Additionally, the EEDF itself can only be determined by employing a “single” probe device. We present here results obtained in the sputtering of different cathode matrices (including metals and compacted samples), at various discharge currents, at a discharge pressure of 2 (2.66 hPa) torr argon, employing a computer controlled Langmuir probe system [26]. It is hoped that the information derived from this study is helpful in identifying the source of spectroscopic matrix effects apart from sputter rate differences. In particular, the possible role of the cathode type on T,, EEDF and charged particle densities is related to the work function of the sample material. Knowledge of these spectrochemical matrix effects is important to the further development of glow discharge spectroscopies. 2.

EXPERIMENTAL

2.1. Discharge source and Langmuir probe construction The discharge source and the construction of the Langmuir probe assembly have been described in detail in previous reports [23, 271. The glow discharge sputtering device has a simple, diode configuration consisting of a stainless steel, six-way cross chamber acting as the anode with the sample pressed into a water cooled cathode holder which is encased in an insulating glass ceramic sleeve. The holder serves as an efficient means of supplying the discharge voltage and restricting the plasma to the cathode surface. The Langmuir probe consists of an 0.025cm diameter tungsten wire with an 0.3-cm length (bent to be parallel to the cathode surface) exposed to the discharge. The wire is electrically shielded by two concentric glass sleeves. The tungsten wire is connected to a computer controlled voltage driving device. The cathode-to-probe distance may be varied by use of an 11 threads/cm assembly on the mounting flange. 2.2. Computer controlled voltage driving device A computer controlled voltage driving system has been constructed for the Langmuir probe and evaluated in terms of the effect of scanning rate of voltage applied and sputtered atom deposition [26]. The applied voltage onto the single cylinder Langmuir probe is controlled by a Macintosh SE computer via an eight-bit Motorola M68HCll microcontroller. The voltage applied onto the plasma is in the range of 210 V with a stepping voltage of 0.07 V and a slew rate of 40 V/s. The applied voltage and the corresponding current through the probe are recorded by a Nicolet 3091 digital oscilloscope. The reproducibility of the I-V curve in this system for three or four cycles results in a deviation in the determined properties, i.e. the electron temperature, plasma potential, and electron and ion number densities, of less than 3%. The cathode to cathode variation in the determined plasma properties is less than 10%. 2.3. Sample preparation and measurement of probe I-V characteristics The cathodes are sputtered as 0.635cm diameter, O-572-cm thick disks. Each sample is

Effect of discharge conditions on charged particle populations

985

polished with a water-proof silicon carbide paper (4000 grit) employing dry methanol as a lubricant. The disks are then rinsed in dry methanol and air dried. The compacted sample, Cu/FelOs/AlzOJ, is a mixture of 90% (w/w) copper powder (325 mesh, SPEX Industries, Inc., Edison, NJ), 1% FeZ03 (SPEX), and 9% A1203 (SPEX), all of which were rated above 99.9% pure. The total weight of the sample was 1.50 g. The mixture was mixed by a Wig-L-Bug (Crescent Dental Manufacturing Co., Lyons, Ill.) device for 10 min and then pressed under a pressure of 7000 psi for 1 min. Argon is employed as the discharge gas with the pressure maintained at 2 torr in this study. For the axial profile studies, the samples are sputtered at both 5 and 8 mA, with the sampling position varied from 5.4 to 18 mm away from the cathode surface. In order to understand the dependence of interested properties upon the discharge conditions, various cathode samples are sputtered at currents ranging from 5-10 mA. The sampling position in this study of discharge condition and cathode matrix effects is fixed at 5.4 mm, a position where the plasma is quite luminous and the probe disturbs the plasma least. The probe I-V characteristics are taken when the discharge is stable at the set current, i.e. the variation of discharge voltage is less than 1% over 3 min. Owing to the effect of sputtered atom deposition on the probe surface [26], the probe is set at the farthest distance, 2 cm, from the cathode surface during the pre-sputtering period, which is 5-10 min. The discharge conditions and cathode properties are listed in Table 1. The breakdown voltage, V,, is determined from the plot of discharge voltage (V) vs current square (mA2) and the reduced power is the product of discharge current and reduced voltage (WO = I x (V-VJ). 2.4. Analysis of probe I-V characteristic Four to six Z-V probe characteristics are analyzed for each probe measurement. Each Z-V characteristic is analyzed with several criteria using a computer program written in C language and compiled by THINK C (Symantec, Bedford, MA) software. The I-V analysis procedure in this study is similar to that employed previously [23]. The slope of the straight line in the electron retardation region (used to calculate the electron temperature, T,) is determined simply by a linear least squares technique, but is very sensitive to the range of chosen I-V points. Only those points with electron currents larger than 0.1 mA and less than 25% of electron current at plasma potential (IO), and yielding a correlation

Table 1. The discharge parameters of current I, (mA), voltage (v) volt, reduced power W,, (W), sputtered area A (cm2), work function @ (eV) of cathodes, and experimentally determined breakdown voltage V,, (V) for the examined cathodes Cathode

I

5mA

6mA

7mA

8mA

9mA

10mA

A

@

V,

cu

V w,

493 0.197

513 0.357

533 0.556

560 0.852

590 1.23

614 1.60

0.76

4.65

454

PI”

V W‘,

488 0.091 484 0.188

499 0.176 499 0.315

516 0.324 520 0.515

525 0.442 542 0.764

539 0.623 567 1.08

552 0.823 595 1.49

1.28

4.65

470

0.92

4.620

447

492 0.257

514 0.441

538 0.682

568 1.02

605 1.48

643 2.02

0.73

4.618

441

544

569 0.152

581 0.261

581 0.299

584 0.363

594 0.503

1.25

0.002 456 0.354

455 0.419

457 0.503

463 0.622

480 0.853

506 1.21

0.75

4.60

420

W,, Ni

V W,,

505 0.277

531 0.488

561 0.780

595 1.16

637 1.69

673 2.23

0.84

5.15

450

Ti

V w,, V W‘,

555 537 0.517

549 -

551 -

532 -

525 -

531 -

0.90

4.33

-

572 0.831

620 1.31

682 1.99

752 2.87

826 3.93

0.71

91% CulZn

V W<>

87% CufZn

V W‘,

CdFe/Alt MO

SS303$

V W,, V

* Cu cathode used for sputtered surface area effect. t[90% Cu: 1% Fe,O, : 9% Al,O,] compact sample. *Stainless steel 303, Fe:CR:Ni = 70:17:13. 8 Calculated from the Cu/Zn composition ratio. The work function of Zn is 4.33 eV.

-

-

544

434

DUENCHENG FANGand R. K. MARCUS

986

coefficient larger than 0.99 for the determined line were used to determine T,. To prevent distortion of the electron energy distribution function, no smoothing technique was applied to the measured Z-V characteristics. With the system employed here, the electron populations at higher energy consist of both positive and negative values owing to the detection limitation set by noise in the electronic circuit. Since the actual population of high energy electrons is not accurately known, the average electron energy ((6)) is determined from those electron populations of energies less than 5 eV. In order to establish whether this energy boundary setting affects the calculated results, energy boundaries from 2.5 to 5.0 eV were compared to the determined averaged electron energy at various discharge currents for a 87:13 (Cu:Zn) brass alloy. As shown in Fig. 1, the boundary energy does not change the relative (6) values at the various currents, but does change the absolute values. As would be expected, the (2) values are higher at the higher boundary energy settings. Therefore, an upper energy limit of 5 eV has been chosen to prevent bias against those EEDFs having appreciable populations in the high energy regime. The range of (c) values, across the boundary conditions, presents only a small increase (40%) for the discharge conditions shown here.

Electron number densities are determined from the corrected electron current measured at a determined plasma potential [23]. That is to say that contributions from ions to the total probe current at that potential have been cancelled by subtraction. Positive ion number densities were calculated

based on the current drained from the plasma in the ion saturation

region of the

Z-V curve [23].

3. RESULTS AND DISCUSSION 3.1. Axial distribution of plasma properties

Owing to the inhomogeneity of the glow discharge plasma, the sampling position is critical for any information detected by atomic spectrometries [28, 291. Variations in the plasma properties along the cathode axis should be characterized first, along with discharge condition dependencies, so that spatial biasing of the data can be minimized. A series of copper cathodes, including pure copper and brass alloys, are used for the axial profiles of the plasma properties with the cathodes sputtered at 5 and 8 mA. By choosing these two discharge conditions, the determined properties can be compared within and outside the negative glow region. About half of the discharge chamber, -1.3 cm, is filled with the negative glow at a current of 5 mA, while the whole chamber, -2.0 cm, is occupied by the negative glow at 8 mA. In addition, in a 2 torr Ar discharge gas environment, the 5-mA discharge is close to operating in the “normal” glow discharge regime for some cathodes, as can be seen in the small changes in discharge voltage between 5 and 6 mA. The visible cathode dark space is about 0.2 cm thick for all cathodes under both discharge conditions. 3.1.1. Electron temperature. The electron temperature data for pure copper and brass alloys at both 5 and 8 mA are shown in Fig. 2. At 5 mA, except in the 87:13 Cu/Zn brass alloy, electron temperature decreases (at least 22%) continually as the distance is increased from 5.4 to 18 mm. At 8 mA, the profiles for all three cathodes

0 2sev . 3.0ev 0 4.0ev l 5.0ev

4

5

6

7

9

9

10

11

Dlrchrrga curnnt, mA

Fig. 1. Average electron energies for a 87:13 brass alloy as determined by different highenergy boundaries in the EEDF.

Effect of discharge conditions on charged particle populations

5

7

9

11

13

15

17

--o-

Cu5mA

e

91%Cu5mA

v

87%Cu5mA

--+-P -

Cu8mA 91%CuamA 87%Cu8mA

987

19

Distance, mm

Fig. 2. Axial profile of electron temperature

for copper series cathodes 1 eV = 1160.5 K.

are quite similar and constant, with the absolute value decreasing as the Zn composition is increased. The different profiles observed for the electron temperatures at the two currents are probably the result of the differences in size of the negative glow. Electron temperature seems fairly constant within the negative glow region, but decreases beyond it. LEMPERIERE et al. [22] measured the axial electron temperature profile in a planar diode argon glow discharge device at lower pressures, 40-120 mtorr, with a titanium Langmuir probe. They found results similar to those presented here for the 8 mA case, the electron temperature staying fairly constant at -0.28 eV in the negative glow region. A possible reason for the lower electron temperatures observed at 8 mA compared to 5 mA will be discussed in detail in Section 32.1. At 8 mA, for those examined cathodes, T, values seem to decrease in the order Cu > 91% Cu:Zn > 87% Cu:Zn. This trend is also observed at other discharge currents as listed in Table 2. The lower electron temperature for the cathode with a higher Zn content may be explained by the different excitation energies of Cu and Zn atomic levels and the increasing contribution of electrons produced by either electron impact or Penning ionization of the respective atoms as shown in Eqn (1). Ar& (11.55-11.72 eV) + M”+M+

+ Ar” + e- (x eV).

(1)

The electron temperatures measured by a single Langmuir probe are representative of the slow electron group (group III) which has experienced several elastic and inelastic collisions with particles in the plasma [23]. The strongest emission lines for Cu are 324.7 (3.82 eV) and 327.4 nm (3.79 eV) and for Zn 213.8 (5.80 eV) and 307.6 nm (4.03 eV). Since the excitation energy for Zn is higher than for Cu, any excess (residual) kinetic energy will be higher for an electron which excites Cu resonance states than those of Zn atoms, i.e. leaving more high energy electrons. Additionally, the collisional population of the higher-lying Zn levels is a more efficient energy loss mechanism for high energy (fast) electrons. Both of these mechanisms support the electron temperature data presented here. Equation (2) depicts how the EEDFs would be shifted to lower energies by the presence of sputtered analyte atoms (relative to a plasma without analyte atoms): he (&) 01 ncu

2

f

A&vi

(Ed

+

eon

2

A&vi

(&J

(21

I

where be (EJ is the change in the EEDF (density) as a function of electron energy, ncu and nZn are the relative atom populations, AEi is the excitation energy for each particular analyte excitation level, and oi (E,) the cross section for the electron impact excitation of each state at the possible electron energies. The contribution of group II (secondary) electrons, from the Penning ionization of Cu and Zn, will also tend to affect the observed T,. Because the ionization potential M(B)443:6/7-U

988

DUENCHENG FANGand R. K. MARCUS

of Cu atoms (7.73 eV) is lower than that of Zn (9.39 eV), the contribution from emitted electrons to the electron population will be at higher energies. The decreasing T, profile along the axis at 5 mA may indicate that the supply of electrons from Penning ionization, with kinetic energies from 2 to 4 eV, is reduced, and is not sufficient to compensate the energy loss from the elastic and inelastic conditions, which becomes more extensive farther from the cathode surface. A spatial variation of the extent of the Penning mechanism could be verified by simultaneous atomic absorption and mass spectrometric measurements. The energy content of secondary electrons from electron impact ionization of Cu and Zn would also be reduced in the latter case. 3.1.2. Average electron energy and EElIF. Figures 3a and 3b show the axial profiles of the average electron energy for the 5 and 8 mA discharge currents, respectively. The spatial profiles are quite similar to those of the electron temperature, i.e. at 5 mA, (E) decreases with increased distance and at 8 mA, (E) is constant throughout the negative glow region. However, the random variation observed for (E) is not as severe as seen in the T, data in Fig. 2. One possible reason for the larger variation in T, is the high sensitivity of that calculation to deviations from linearity of the generated lines (In I,-V in the retardation region) as discussed in Section 2.4. As will be discussed in detail in Section 3.2.2, another reason, is that the electrons in the high energy tail of the EEDF (which are dominant in the T, calculation) are able to excite moderate energy gap transitions, but the bulk electrons (which make up the majority of the ~pulation) are much less efficient in those excitations and are thus not lost, to an appreciable extent, from the distribution. It is interesting to note that the electron temperatures and average electron energies (to a lesser extent) at 5 mA are greater than those of the higher current, even though the discharge voltages are higher at the higher current. This phenomenon will be discussed in detail in Section 3.2.2. So long as the measurement is taken within the negative glow region, the electron energy distribution does not change signifi~ntly with distance. However, the total population and the energy of maximum population will shift to lower values once they are outside the negative glow region, as shown in the profile of (E) and 12, in this study and the previous report [23]. 3.1.3. @ectron number density. The axial profile of electron number density is illustrated h Fig. 4. Once again, the electron number density is fairly constant within the negative glow region as are T, and (E). The electron density in the 8 mA discharge is approximately twice that of the 5 mA discharge at all of the sampling positions, as might have been expected intuitively based on the nearly doubled current value. The shape of the latter plots (5 mA) once again establishes the fact that the farthest sampling point (18 mm) is toward the exterior of the negative glow region in the 5 mA plasma. 3.1.4. Positive ion number deans. The axial profiles of the positive ion number densities are seen to be very different from the axial profile of T,, (E), and n,. The ion number density decreases logarithmically with the distance into the negative glow, as shown in Fig. 5. AST~N [30] and LEMPERIERE et al. [22] reported similar results. DUCKWORTHand MARCUS (311 have observed these sorts of profiles for sputtered and discharge gas ions in rf glow discharge mass spectrometry. Though consistent in experiments, the result of decreasing density along the axis still cannot be fully explained from the standpoints of the population of Ar metastable atoms and the theory of electron-ion recombination [23]. The ion number density increases as discharge current is increased from 5 to 8 mA. This is probably the result of the combination of the increased production of sputtered atoms (having lower ionization potentials than the argon gas) and the number and energy of secondary electrons emitted from the cathode accompanied with the increase in discharge voltage at 8 mA [23]. 3.2. E#ect of cathode identity on plasma properties One of the important issues facing the increased application of glow discharge devices is the role that cathode identity has on the excitation conditions in the plasma; more s~cifically, what the ~ssibilities are for single-standard analysis. The results presented

989

Effect of discharge conditions on charged particle populations

-cU ----ct-

91vocu 6PhCu

0.6 -

5

?

9

11

13

15

17

19

Distance, mm

b. 1.0 -CU ,,.

0.6-W

-

Ql%Cu

w%cu

0.6 -

0.4

;

. 1 . I

5

7

. I - I - 1 .

Q

11

13

15

1 . I 17

IQ

Distance, mm

Fig. 3. Axial profile of average electron energy at (a) 5 mA and (b) 8 mA for copper series cathodes.

iii::+==== t,

8

I”

;;pee\

v v I v v I

o!.,.,.

I

5

7

9

-

11

I

-

13

I

i5

-

I

17

-I

Cu5mA 9l%Cu 5mA 87%Cu 5mA Cu8mA 91% Cu 8mA 87%Cu8mA

19

Dlrtancs, mm

Fig. 4. Axial profile of electron number density for copper series cathodes.

cc it ---t -

o!.,.,. 5

I.

7

Q

I.

13 11 Dlrtance, mm

CuBmA Ql%Cu5mA 67% Cu 5mA CuBmA 91% Cu 8mA 67%Cu6mA

,‘I’

15

17

’

Fig. 5. Axial profile of ion number density for copper series cathodes.

DUENCHENG FANGand R. K. Mmxs

990

above were obtained for the family of copper matrices in order to establish appropriate sampling conditions for the more thorough analysis of matrix effects over a wide range of materials. The variation in the plasma characteristics between the samples was not emphasized, though the results indicate that the matrix effects in this limited system are minimal. The results actually corroborate the previous finding of consistent (albeit invalid) excitation temperature measurements across the brass family [6]. The more detailed “matrix effect” study was performed at a fixed sampling position (5.4 mm), covering a range of discharge currents. The 5.4-mm position was chosen because it is the most luminous region of the plasma, where emission measurements would be performed. Discharge current was varied in these studies so that, at the very least, trends in changes in the charged particle populations could be discerned. The results of these determinations are listed in Tables 2 and 3 for all examined cathodes. As can be seen, a wide variety of matrices were investigated, including a metal oxide powder mixture compacted in a copper host matrix. The Cn/Fe20,/Al,0, disk was sampled as being representative of nonconducting sample types such as cements and geological materials. In addition, another copper sample (designated [Cu]) was studied to evaluate the possible role that cathode area has on the plasma properties. 3.2.1. Electron ~e~~e~~~~~e.As presented in Table 2, the electron temperature decreases steadily as discharge current increases for all of the matrices except the compacted Cu/Fe203/A1203 and titanium cathodes, which show decreases albeit not monotonically. The decrease of electron temperature as discharge current increases may again be explained by the contribution of electrons produced by either Penning ionization of sputtered atoms or electron impact ionization of sputtered and argon atoms in the negative glow region. The electrons produced by ionization have quite small kinetic energy, especially relative to the secondary (sputtering) electrons which have gained kinetic energy in the potential fall of the cathode dark space, The ionization contribution is more significant at the higher discharge voltages, which give more kinetic energy to electrons for electron impact ionization and for exciting the Ar atoms to the metastable state. Therefore, lower electron temperatures might be expected at the higher discharge voltages based on increased ionization. Previous studies established

Table 2. The determined electron temperature

T, (eV) and average electron energy

(e} (eV) for various cathodes Cathode*

5mA

6mA

7mA

8mA

9mA

CU

0.389 0.939

0.330 0.887

0.304 0.840

0.286 0.832

0.275 0.g4Q

0.285 0.861

Ful

0.400 0.900

0.354 0.811

0.339 0.797

0.327 0.806

0.298 0.807

0.282 0.838

91% WZn

0.358 0.867

0.308 0.821

0.284 0.827

0.283 0.852

0.276 0.865

0.267 0.866

87% CulZn

0.332 0.820

0.287 0.716

0.273 0.744

0.265 0.764

0.250 0.790

0.260 0.870

C&Fe/AI

0.425 1.25

0.395 1.06

0.378 1.00

0.385 0.99

0.397 1.01

0.326 1.01

MO

0.333 0.862

0.299 0.826

0.278 0.778

0.275 0.820

0.264 0.825

0.260 0.797

Ni

0.280 0.684

0.252 0.609

0.232 0.585

0.228 0.545

0.215 0.538

0.205 0.536

Ti

0.375 0.922

0.362 0.832

0.336 0.836

0.301 0.775

0.276 0.831

0.284 0.836

ss303

0,312 0.808

0.276 0.764

0.247 0.779

0.227 0.773

0.233 0.816

0.235 0.813

* See footnotes of Table 1 for explanation.

10 mA

991

Effect of discharge conditions on charged particle populations Table

3. The determined electron number density n, (lOlocrn3) and positive ion number densityn, (lO’[’cmm3),for various cathodes

Cathode*

5mA

6mA

7mA

8mA

9mA

10 mA

cu

5.77 12.00

7.19 13.10

9.07 14.99

10.37 17.80

12.10 20.65

13.60 23.60

w

4.93 8.67

4.80 10.16

5.36 11.23

6.88 12.43

7.91 13.45

9.04 17.10

91% CulZn

4.86 9.60

6.81 10.90

8.98 13.70

11.10 14.70

13.60 17.70

15.80 20.70

87% CulZn

4.61 12.00

6.35 13.50

7.35 16.30

9.36 18.60

11.30 21.40

16.00 25.40

CuiFelAl

5.04 12.31

6.03 13.43

7.49 14.77

8.66 17.90

9.48 20.00

11.43 39.83

MO

5.67 13.35

6.58 14.80

7.90 16.95

10.40 20.93

12.46 24.30

14.22 27.55

Ni

6.58 15.72

8.64 17.11

9.49 19.43

11.89 21.34

14.25 25.55

15.89 28.23

Ti

5.22 11.20

5.43 13.00

7.42 15.20

8.14 17.20

10.64 18.80

11.74 21.80

ss303

5.61> 12.45

8.26 15.38

10.44 18.20

11.25 22.92

13.78 26.08

14.25 29.98

* See footnotes of Table 1 for explanation.

a linear relationship between discharge voltage and cathodic sputtering rates [27] which would be a source of atoms with lower ionization potentials than the argon gas. Figure 6 is a plot of T, as a function of discharge voltage. All of the cathodes, except the compacted sample, show the same trend, the higher the discharge voltage, the lower the electron temperature. The matrix effect (spread) shown in the T, vs V relationship can be normalized to some extent by relating the electron temperature to reduced voltage, which is the difference between discharge voltage and breakdown voltage (V-V,), as shown in Fig. 7. The fact that the curves converge only minimally, indirectly indicates that electron temperature is affected more significantly by the group II (secondary from ionization) electrons than group I (secondary from cathode) electrons. The breakdown voltage is the threshold voltage for secondary electron emission so that the reduced voltage relates to excess energy in sputtering and electron production. Secondary electron yields by Ar+ bombardment are more dependent on the cathode identity than the Ar+ kinetic energy at energies up to 1 keV [32], which is well above the potentials employed

QJ 91%Cu

87%cu 1c4 MO

E

ii 0

W

CulFelAl ss 303 Ni

0.25 0.20 400

500

600

700

a00

Dischargevoltage,V Fig. 6. Effect of discharge voltage on electron temperature for various cathode matrices.

992

DUENCHENG FANG and R. K. MARCUS

_ : 6z ; z 5 iiL 2 w

040

Cal 91%

035

-

030

cu

07%cu WJI MO Cu/Fe/Al ss 303

-

0.25

NI

0.20 0

100 Reduced

Fig. 7. Electron temperature

200 voltage,

300

400

V

as a function of reduced voltage for various cathode matrices.

here. In addition, the secondary electrons can attain the full energy of the potential fall in the cathode dark space [33], not only the partial energy associated with reduced voltage. Therefore, if electron temperature is mainly determined by group I electrons, there should be no matrix effect seen at the same voltages or reduced voltages in Figs 6 and 7, respectively. The remaining spread of values in the latter case indicates that the differences in T, are due to the identity of the (sputtered) atoms in the gas phase, i.e. group II electrons, because the reduced voltage variable should tend to normalize the sputtering rates [27]. More explicit delineation between sputtering rates and T, should be investigated in future studies. 3.2.2. Average electron energy and EEDF. Previous studies in this laboratory have suggested that the observed optical spectroscopy in glow discharges is probably more related to the energy distribution function and average electron energy, rather than electron temperature which is an ‘equilibrium’ quantity. The electron energy distribution functions for 8 and 10 mA for various cathodes are shown in Figs 8a and 8b, respectively. In the constant-current operating mode, the EEDFs for the cathodes are very similar. The electron populations show a little bit of mismatch from cathode to cathode which may be attributed to slight differences in the sputtered area of the cathodes as will be discussed in Section 3.2.3. Though the work function of each cathode is different, so long as the discharge current is held constant, the differences in work function should be compensated for through the discharge voltage, i.e. the higher work function cathode would have higher voltage at the same discharge current. Therefore, if the discharge were operated in a constant voltage mode, there might be greater differences in the EEDFs for various cathodes, i.e. the low work function cathodes will have larger electron populations. As with the other data presented to this point, there do not appear to be substantive differences in the EEDFs for the alloys studied here. The average electron energies (calculated from the electron populations within the 5 eV energy boundary) are listed in Table 2. All of the cathodes show a high average electron energy at the lowest discharge current (5 mA) with a sharp drop, and gradual rise with current as shown in Fig. 9. It is interesting to note that the average energy values converge at higher discharge currents to a relative spread of only -10% at the highest current. This spread is within the sample-to-sample deviations for the analysis of the same alloy, indicating that there is probably no significant difference in the values. The implication is that the excitation conditions of the bulk electron populations for these cathodes are taken into account, and would be near-identical for a given discharge current. The initial drop in the average electron energies is born out in the EEDFs as shown in Fig. 10 for 87% Cu:Zn brass sputtered at 5 and 6 mA. The distribution indicates that more electrons are populated above 1 eV at 5 mA than 6 mA. The electron populations then increase and shift slightly to higher energy values as the discharge current is increased. The abnormality in the profile is not affected by the chosen energy

Effect of discharge

conditionson chargedparticle populations

5

0 . 0

07%Cu 9l%Cu Q

:

l

3

+

Ni ss303

Q

0.0

0.5

1.0

1.5

2.0

2.5

993

3.0

Electron energy, eV

07% cu 91%Cu Q Ni 55303

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Electron energy, eV

Fig. 8. Electron energy distribution

functions for various cathode matrices sputtered 8 mA and (b) 10 mA.

at (a)

cu 91%Cu 87% cu PJI MO

ss 303 Ti B ; P

0 75 070 4

q

I 5

0 I 6

1 7

I 8

I 9

I 10

1’

Discharge current, mA

Fig. 9. Effect of discharge current on average electron energy for various cathode matrices.

boundary (as discussed in Section 2.4) sputtered surface area, or cathode identities. This anomaly seems to suggest that the discharge under study here may be in some sort of transition, possibly between normal and abnormal operation or that there may be a slight spatial change as the plasma “settles in”. In an effort to understand the origins of electron energy abnormality, plots of average electron energy vs discharge voltage, reduced voltage, and reduced power were examined. Those plots do not deviate in principle from the relationship shown in Fig. 9. The conductivity of the cathode does not affect the abnormality in the (E) data either, as shown in Fig. 11. Although the energy value spread (EEDF) for the compacted sample is about two times larger than for the pure Cu cathode, the trends and average energies are fairly comparable. This spread may be due to some effect of

DUENCHEN~FANGand R. K. MARCUS

l

3.0

5mA 6mA 8mA 1OmA

0 l 0

2.0 1.o

0.0 0.0

0.5

1.0

2.0

1.5

2.5

3.0

3.5

4.0

Electronenergy, eV

Fig. 10. Electron energy dist~bution function as a function of the discharge current for a 87% CuiZn brass alloy.

>

0.95

al I 6 %

0.90 -

1.3

L;

1.2

Q/Fe/Al

z

= 5 ‘i; u 0 0 F ?a P

> P) I 6 k

0,=_

1.1

z-

5 i; ar a!

1.o

0.80 -

0.75-f-.,.,*,=,.,.1. 4 5 6

0.9 7

8

9

10

%I t f *

I1

DIecharge current, mA

Fig.

11. Comparison

of the average electron Cu/FezO,/Al,O,

energy for pure copper cathodes.

and compacted

oxygen, which is sputtered from the cathode as oxides and dissociated in the negative glow region to form oxygen atoms in the plasma 1281. In this case, the quenching of metastables removes an electron energy loss channel, namely, that of electron ionization of argon metastables. The differences in the energy spreads between alloys and compacted samples, of various composition, may be quite enlightening and warrants further study. If there is a substantial contribution of group II electrons in the determined T, and (c), the abnormality could be explained by a change in the excitation/ionization conditions at the different discharge currents. At low discharge currents the discharge voltage, which gives the kinetic energy to group I electrons, is so low that the extent of ionization does not produce subsequent fast electrons which further excite sputtered species, and thus shift the bulk electron populations to lower energy values. By increasing the discharge current, the energy of fast electrons is increased (probably by introduction of sputtered species of low ionization potential) thus favoring the excitation as reflected at 5 and 6 mA in Fig. 10. Increasing the discharge current (voltage) further, increases group I produ~ion (with higher energies) and thus more group II electrons are created through ionization. These electrons will be in the 2-4 eV range and thus increases the average electron energy. Conversely, while the average energy increases, the electron temperature is seen to drop, which indicates that the EEDF is experiencing increased high energy tail depletion. The inconsistency between electron temperature and average electron energy indicates the existence of a non-Maxwellian distribution of group III electrons as discussed previousty [23]. According to the two- and three-election group models for low-pressure

Effect of discharge

conditionson chargedparticle populations

995

gas discharges developed by VRIENS [34, 351, Tt represents the electrons in the high energy tail of the EEDF, while T, represents the bulk electrons below the first inelastic threshold (lowest energy required for electronic excitation) in the plasma and was related to (e) [34]. (B ecause of the methods used in the calculations here, we maintain the use of T, to describe tail electrons.) Applying this model to experimental results for a pure copper cathode sputtered at 8 mA, two different Maxwell-Boltzmann distributions are derived by introducing the electrqn temperature, 0.286 eV, and average electron energy, 0.832 eV, into the equation as kT. Figure 12 shows these two Maxwell-Boltzmann distributions along with the experimentally determined EEDF. The first inelastic threshold is about 1.2 eV, which is fairly reasonable considering the electronic excitation of the atomic species (Cu and Ar) in this plasma. Also, as expected by the model, T, describes the high energy tail of EEDF, >1.2 eV, and the rest of electrons are represented by (E). By comparing the three distributions in the figure, the depletion of the high energy tail of the EEDF is obvious. This depletion is due to the energy loss by atomic excitation. From Fig. 9 and the values of T, in Table 2, the contribution of group II electrons and the products of collision by those electrons is important in understanding their resulting effect on T, and (E) and should be further studied by comparing atomic emission characteristics under various discharge conditions. 3.2.3. Electron number density. The electron number density, n,, for various cathodes has been determined for discharge currents from 5 to 10 mA at 2 torr. Table 3 shows the determined electron (and ion) number densities as a function of discharge current. As would be expected, the electron number density increases as the discharge current is increased. However, the relationship between electron number density and discharge current is not linear, but has a slightly exponential curvature. This indicates that electrons in the negative glow region originate not only from the emission of secondary electrons from the sputtered cathode surface, but also from the subsequent ionization of the discharge gas and sputtered atoms; a multiplicative effect. If n, is determined only by the work function of the cathode (i.e. the ease of electron ejection), the relative values of IZ, for the different cathodes should remain the same as the discharge is current changed. As seen in Table 3, the relative values of n, for each of the cathodes are not constant as the discharge current changes. Therefore the ionization conditions, as indicated by ne, do not appreciably depend on the work function of the cathode. Though each cathode has been intentionally cut to have the same dimensions, the sputtering area is somewhat different from sample to sample owing to poor machining reproducibility in either the cathode samples or the ceramic caps of the sample holder. Should the determined 12, be corrected for differences in the (ejection) surface area

10 l

8

A l

5 4

6

l

a

0.0

0.5

0.0

1.0

1.5

Electron

0.4

2.0

0.8

2.5

1.2

3.0

1,.6 2.0

3.5

4.0

energy, eV

Fig. 12. MaxwelCBoltzmann distribution functions determined by (A) electron temperature (0.286 eV), (B) average electron energy (0.832 eV) and (C) experimentally determined EEDF. Inserted is the full distribution of A. Copper cathode; 2 torr. 8 mA.

DUENCHENG FANGand R. K. MARCUS

996

(what we term the flwr density), as shown in Fig. 13, only the results of [Cu] (pure copper with a larger sputtered surface area), the compacted sample, and Ti cathodes are separated from the other matrices. The discharge voltages for these three cathodes stand out as lower than others for these reasons: large sputtered area of [Cu], large sputtered area and presence of oxygen in the sample causing a higher breakdown voltage for Cu/Fe20&A1203, or the intrinsic oxygen gettering nature of titanium which leads to fouling and creation of a dielectric. layer. Though the electron number density was found to be roughly linearly related to the discharge voltage for the copper series cathodes, the remainder of the examined cathodes show a non-linear relationship between electron number density and discharge voltage. In particular, if the breakdown voltage (which is related to cathode surface area) for each cathode was significant in the electron population density, a plot of n, vs reduced voltage should converge the curves to compensate for the cathode differences described above. However, as shown in Fig. 14, there is no appreciable relationship between the energetic effects of cathode surface area and ultimate electron density. Introducing the effects of discharge current through the reduced power function as shown in Fig. 15, produces more uniform responses. In the plot of the electron number density per unit sputtered surface vs reduced power, a relationship between the power delivered to the plasma and the number density per unit of group I electron source (target area) is established. Finally, the separated set of values for [Cu] in Fig. 15 can be brought into line by introducing a surface area component in the power delivery through the reduced power density (W/cm*) function, as shown in Fig. 16. It can be concluded from this figure that the electron number density for all determined cathodes is a function of reduced power per unit sputtered surface area. Figure 16 gives an

ill 91% Cu 87% cu

z!

.I

d

4

5

8

A

: A

e 0

6

7

;

.

b

*

6

9

WI Cu/Fe/Al MO ss 303 Ti Ni

10

11

Discharge current, mA Fig. 13. Electron number density (cm-‘) per unit sputtered surface area (cm-*) as a function of discharge current for various cathode matrices.

16

1

0

I

*.s

OJ 91% cu 87% cu s-w MO ss 303 Ni

14 12 10 0 6 4

0

100

200

300

400

Rrsdueedvoltage, V

Fig. 14. Effect of reduced voltage on electron number density for various cathode matrices.

Effect of discharge conditions on charged particle populations In

k 0 0 ‘j

997

25 20

cu 91% cu

3-

15

87% Cu ICul

; u

lo

I =2

5

b/Fe/Al MO ss 303 Ni

(Ip 2

0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Reducedpower,W Fig. 15. Electron number density per unit sputtered surface area as a function of reduced power for various cathode matrices.

cu 91%

cu

87% cu

PJI MO Cu/Fe/Al ss 303 Ni

0

1 Reduced

2

3 power

4 density,

5

6

Wlcm2

Fig. 16. Electron number density per unit sputtered surface area as a function of reduced power density for various cathode matrices.

overall indication that the relationship between n, per unit area and the reduced power density is an exponential function. A physical interpretation of these somewhat unusual units is that the electron density per unit of emitting surface area is a function of the power density delivered to the cathode surface. In terms of a glow discharge analysis, these parameters are fairly easily controlled, particularly in Grimm-type source geometries, where the exposed cathode surface area is restricted. The plot also demonstrates that the “matrix effect” in electron number density is minimal, with variations at the higher powers being ~20%) relative. Figure 17 summarizes the parametric dependencies of the electron number densities for the sputtering of a MO cathode (for a fixed surface area). The figure indicates that the electron number density is more dependent on discharge voltage than on discharge current, particularly in low power operation. The reduced power combines the effect of discharge current and voltage and shows a response more like that of discharge voltage. The voltage dependencies seen here support the operation of the following overall mechanism. Discharge voltage determines the group I electron energies such that high voltages produce higher energy (cathodic) secondary electrons. High energy group I electrons are more efficient for ionization thus producing group II electrons. The group II electron populations will be higher for high discharge voltages at the same overall discharge current. In addition, the excess energy of these electrons will be a reflection

998

DUENCHENG FANG and R. K. MARCUS 16

,

-I 4

5

6

6

7

9

10

11

Discharge current, mA I

450

.

I

460

-

I

470

’

1

460

1

-

490

.

1

=

f

500

510

1.2

1.4

Discharge voltage, V

072

014

0.6 I.

0.6 8.

1.0 I.

I.

I

Reduced power, W Fig. 17. Comparison

of the effect of discharge current, voltage number density, MO cathode.

and reduced

power

on electron

of the discharge voltage, i.e. the initial group I energies. It is the group II electron population which makes up the group III (bulk) population sampled by the probe. This model applies not only to the electron impact ionization of discharge gas atoms, but also to the ionization of sputtered species via a Penning mechanism. Large electron populations, with relatively high energies, will be more efficient at metastable atom production, and thus faster electrons (group III) will be created by Penning ionization. 3.2.4. Zen number density. The determined positive ion number densities are listed in Table 3. Ion number densities increase with discharge current, but as happens with the electron number density data, the relationship is not linear. Since the ion number density has a strong spatial dependency (it decreases dramatically with axial distance) and the sampling position in this study is fixed at 5.4 mm, comparisons of the ion number densities at various dicharge conditions are very tenuous. However, Fig. 18 presents a linear relationship between ion number density and the discharge voltage at the 5.4 mm position. The ion number densities in the MO and compact Cu/Fe203/A1203 cathodes deviate from the other cathodes. This might indicate that the probe was contaminated. Such a contamination will not result in erroneous values in the determined T,, (E), and n, values. The effect of probe contamination (sputtered atom deposition) has been demonstrated previously with this Langmuir probe system [26]. More thorough mapping of ion populations, combining mass spectrometry and the Langmuir probe system, are required to gain a better understanding of the distribution and any relationships to electron energies distributions.

4.

CONCLUSIONS

The computer controlled Langmuir probe system is very useful in identifying the parametric effects on different electron groups existing in the negative glow region. The group III electrons can be described by a two-electron group model. The electron temperatures determined by a single cylindrical Langmuir probe represent the high energy region of the electron energy distribution function with the average electron energy representing the lower-energy majority of electrons in the negative glow region. The depletion of high energy electrons in the overall energy distribution function is indicated by the fact that determined electron temperature is lower than the average electron energy for a given set of conditions. At this point in the Langmuir probe investigations in this laboratory, a few generalities

Effect of discharge conditions on charged particle populations

999

Cal

91% cu 87% cu ICul MO

Ni ss 303 Cu!Fe/Al

5! 400

500

600

700

800

Discharge voltage, V

Fig. 18. Effect of discharge voltage on ion number density for various cathode matrices.

can be made about the electron populations. First, discharge voltage controls the energy of the group I electrons, and thus the subsequent energy of the group II electrons created through electron impact and Penning ionization collisions. The result of increased voltage is a multiplicative effect on the electron density, beyond those increases expected for increased discharge current. Second, the production of high energy electrons (a function of discharge voltage) has the net effect of lowering the electron temperature in the negative glow region by virtue of the fact that those electrons have more energy loss pathways, depleting the high energy tail. Third, the average electron energies show parametric dependencies that are nearly opposite those characterizing the electron temperature, reflecting the non-Maxwellian nature of glow discharge plasmas. These values, though, appear to be an accurate reflection of the cumulative energetics of the plasma. Fourth, the electron densities as noted above are more a function of the subsequent voltage increases when discharge current is increased. The flux density of the electrons is most closely related to the reduced power (power density), having similar values across the range of parameters and matrices examined here. Finally, ion number densities show a definite dependence on discharge voltage, i.e. group I electron energies. This dependence is born out further in the spatial distribution, which is maximized at distances close to the cathode surface. Such a set of dependences can support either an electron impact or Penning ionization mechanism. The most relevant results of these studies are the comparisons of charged particle populations resulting from the sputtering of various cathode matrices, i.e. the matrix effects. Throughout all of the data presented here, even on the simplest levels, no dramatic matrix effects are observed. Spectroscopically, the two most important figures of merit are probably the average electron energy and the electron number density. The data presented here for a simple constant current mode of operation (Fig. 9) show a relative range of values of -10% for the alloy samples. The electron densities (flux density) at fixed reduced power densities vary by only -15% (Fig. 16). While the relationships here may seem somewhat abstract, in the most common application of glow discharge devices, Grimm-type emission systems, the exposed cathode area and power are strictly regulated. Therefore, one should expect very few spectroscopic matrix effects in these systems. Certainly those effects noted here are no worse than in any other spectroscopic sources. Future studies in this laboratory will focus on the expansion of the Langmuir probe methodology to the characterization of radio frequency (rf) glow discharge devices [29]. The rf plasmas allow for the direct atomization of oxide materials, which as seen here, may have some unique electron characteristics. Of particular interest will be multi-faceted studies employing the Langmuir probe, mass spectrometry, and Ar metastable atom density measurements through atomic absorption spectrophotometry. Acknowledgement-This material is based upon work supported by the National Science Foundation under Grant number CHE-8901788.

1ooo

DUENCHENGFANG and

R. K. MARCUS

REFERENCES K. R. Hess and R. K. Marcus, Spectroscopy 2(9), 24 (1987). K. Ohls, Fresenius Z. Anal. Chem. 327, 111 (1987). K. Wagatsuma and K. Hirokawa, Anal. Chem. 56, 412 (1984). B. W. Smith, N. Omenetto and J. D. Winefordner, Spectrochim.Actu 29B, 1389 (1T4). , [5] W. W. Harrison, K. R. Hess, R. K. Marcus and F. L. King, And. Chem. 58, 341A (1986). [6] D. Fang and R. K. Marcus, J. Anal. Atom. Spectrom. 3, 873 (1988). [7] J. M. Brackett, J. C. Mitchell and T. J. Vickers, Appl. Spectrosc. 38, 136 (1984). [8] D. M. Mehs and J. M. Niemczyk, Appl. Spectrosc. 35, 66 (1981). [9] K. W. Busch and T. J. Vickers, Spectrochim. Actu UIB, 85 (1973). [lo] P. E. Walters, T. L. Chester and J. D. Winefordner, Appl. Spectrosc. 31, 1 (1977). [ll] N. P. Ferreira, H. G. C. Human and L. R. P. Butler, Spectrochim. Acta 35B, 287 (1980). [12] S. R. Goode and J. P. Deavor, Spectrochim. Actu 39B, 813 (1984). [13] W. Stern, Beitr. Plasmuphys. 9, 59 (1968). [14] J. D. Swift and M. J. R. Schwar, Electrical Probe for Plusma Diagnostics. Iliffe Books, London (1970). [15] F. F. Chen, in PlasmaDiagnosticTechniques, Eds R. H. Huddlestone and S. L. Leonard. Academic Press, New York (1965). [16] J. G. Laframboise, University of Toronto, Institute for Aerospace Studies Report No. 100 (1966). [17] T. I. Cox, V. G. I. Deshmukh, D. A. 0. Hope, A. J. Hydes, N. St J. Braithwaite and N. M. P. Benjamin, J. Phys. D20, 820 (1987). [18] D. A. 0. Hope, T. I. Cox and V. G. I. Deshmukh, Vacuum 37, 275 (1987). [19] T. Uckan, Rev. Sci. Instrum. 59, 198 (1989). [20] P.-Q. Lu, P.-Z. Gong, T.-Z. Lin and R. S. Houk, Spectrochim. Acta 43B, 273 (1988). [21] E. Eser, R. E. Oghvie and K. A. Taylor, J. Vat. Sci. Technol. 15, 199 (1978). [22] G. Lemperiere, J. M. Poitevin and C. Fournier, .I. Phys. Dll, 293 (1978). [23] D. Fang and R. K. Marcus, Spectrochim. Actu (in press). [24] D. M. Mehs and T. M. Niemczyk, Appl. Spectrosc. 32, 269 (1978). [25] F. F. Chen, Research Report IPPJ-750 (1985). [26] D. Fang, R. Williams and R. K. Marcus, J. Atom. And. Spectrom. 5, 569 (1990). [27] D. Fang and R. K. Marcus, Spectrochim. Acta 43B, 1451 (1988). (281 R. K. Marcus and M. R. Winchester, paper presented at the 16th Annual Meeting of the Federation of Analytical Chemistry and Spectroscopy Societies, Chicago, IL, 2-6 October 1989. [29] D. C. Duckworth and R. K. Marcus, Anal. Chem. 61, 1879 (1989). [30] F. W. Aston, Proc. R. Sot. A. 84, 526 (1911). [31] D. C. Duckworth and R. K. Marcus, Paper presented at the 16th Annual Meeting of the Federation of Analytical Chemistry and Spectroscopy Societies, Chicago, IL, 2-6 October 1989. [32] B. Chapman, Glow Discharge Processes. Wiley-Interscience, New York (1980). [33] K. Wiesemann, Phys. Lett. 29A, 691 (1969). [34] W. L. Morgan and L. Vriens, J. Appl. Phys. 51, hO0 (1980). [35] L. Vriens, J. Appl. Phys. 45, 1191 (1974). [l] [2] [3] [4]