ELSEVIER Spectrochimica Acta Part B 52 (1997) 567-578
SPECTROCHIMICA ACTA
PART B
Effect of discharge parameters on emission yields in a radio-frequency glow-discharge atomic-emission source
Mark Parker a, Matthew L. Hartenstein b, R. Kenneth Marcus b'*
~Spectrum Laboratories, P.O. Box 1578, Coebarn, VA 24230, USA bDepartment of Chemist~', Howard L. Hunter Chemical Laboratory, Clemson Universi~', Clemson, SC 29634-1905. USA
Keywords: Emission yield; Radio frequency glow discharge; Sputtering
1. Introduction
Glow discharge (GD) devices have been routinely used for bulk metals analysis since the introduction of the planar cathode design by Grimm in the late 1960s [1]. While bulk analysis is still the most active area of use, the possibility of performing depth-profile analysis with the GD is receiving more attention. In fact, the first applications of depth profiling by glow- discharge atomic-emission spectroscopy (GD-AES) were described in 1973 as Greene and Whelan
* Corresponding author.
analyzed GaAs thin films [2] and Belle and Johnson sputtered metal alloys [3]. The use of GD-AES for depth profiling has increased primarily due to its application in the steel and related industries, where protective metallic coatings which retard oxidation are essential. Introduction of radio frequency (rf) powered sources for GD analysis extends the potential application of bulk/layer analysis from metals to non- conductive materials [4]. With rf powering, the direct sputtering of glasses, ceramics, and polymers is pos- sible without matrix modification (impregnating with a metallic host), resulting in the capability of perform- ing nonconductive layer analysis.
568 M. Parker et al./Spectroehimica Acta Part B 52 (1997) 567-578
Since the GD (dc and rf) is a layer-by-layer sputter ablation (sampling) process, depth profiling may be viewed as simply repeating a bulk analysis many times over in succession. However, the simplicity is lost due to the fact that the GD is both the sample introduction (sputtering) and excitation source. Each process, sputtering and excitation, is dependent on the discharge parameters (current, voltage and pressure) resulting in complex and interwoven relationships. From an operational standpoint, two of these para- meters may be held constant while the third is forced to seek its own level to keep the other parameters constant. As an analysis proceeds through a layered sample of widely varying matrices, the dependent parameter will be forced to change with time (depth). This complexity produces results which may not be easily interpreted. Also, the externally monitored parameters (current, voltage and pressure) may not be reflective of the complex processes occur- ring in the discharge. Fundamental studies have provided much insight [5-15] but have not comple- tely unravelled all of the elaborate processes which dominate the GD.
Quantitative depth profiling has proven to be one of the strengths of the glow discharge. A number of qua- litative-to-quantitative depth profiling conversion pro- cedures for dc-GD-AES is available [ 16-20]. In each approach, corrections for changes in analyte emission intensity must be applied to account for variations in lamp conditions and sputtering rates as the analysis proceeds. The simplest algorithm involves the con- version of temporal data to depth by using the average sputter removal rate (/~m s ~). This requires deter- mination of the sputter rate after each analysis by either performing a weight-loss measurement (con- verting to depth via the material's density) or physi- cally determining the average crater bottom depth. The sputtering rates of subsequent samples are assumed to be constant by burning at carefully con- trolled, and known, 'standard' conditions. While applicable for a bulk analysis or a simple layered system, this approach may not be applicable for more complex systems as the overall sputter rates may not be indicative of the rates for each individual layer of the sample. This shortcoming was demon- strated by Nickel et al. in the analysis of oxide scales on Ni-based alloys [21]. Other approaches typically involve[ 16-20]:
1. correcting the intensities for voltage and current changes;
2. keeping the voltage and current constant during an analysis by varying the operating pressure (Ar flow rate); or
3. holding the pressure constant (assuming that cur- rent and voltage changes do not significantly affect the monitored intensities/emission yield).
Detailed studies comparing the various methods have been reported by Payling and co-workers [22,23] for the case of a dc-GD-AES system. Their findings reveal that good working curves are possible with each method and that both voltage and pressure are important parameters in producing good quantita- tive results (current was not tested).
The study presented here focuses on the role of glow-discharge parameters (power and pressure) on the emission yields obtained for a radio-frequency glow-discharge atomic-emission spectroscopy (rf- GD-AES) source in the analysis of both conductive and nonconductive sample types. This evaluation is a primary step in developing quantitative depth- profiling capabilities for rf-GD-AES. Also discussed here is a method of compensating for the reduced emission intensities observed in the analysis of non- conductive samples of various thickness through the use of the atomic emission of discharge gas species as an internal standard.
2. Experimental
The external sample mount rf-GD source has been described in a previous study so only a brief descrip- tion is given here [24]. The source body (anode) is constructed of stainless steel with the dimensions 8 cm x 6.5 cm x 6.5 cm. The flat sample is pressed against a Teflon o-ring (located on a demountable limiting orifice plate) by means of a pneumatically controlled piston. A ceramic spacer around the o-ring ensures a constant (0.14mm) anode-to-cathode spacing. Argon (99.999% purity) is introduced into the discharge cell through an MKS 1259C (Le Bourget, France) mass-flow controller. Pressure is monitored by means of an Alcatel model P13C Piranni gauge with an Alcatel model CN 111 controller. The discharge is powered by a modified
M. Parker et al./Spectrochimica Acta Part B 52 (1997) 567-578 569
Table 1 Optical transitions monitored by the JY RF-5000 polychromator and the respective elemental compositions in the NIST SRM 1250 Fe-Ni-Co high-temperature alloy and Macor test matrices
Element Wavelength (/nm) [X] in NIST 1250 [X] in Macor (/(wt %)) (/(wt %))
AI 396.152 0.99 21.5 Ar 404.442 - - Co 345.351 16.1 - Cr 425.433 0.077 - Cu 327.396 0.022 - Fe 371.994 40.5 - Mg 280.270 - 10 Mn 257.610 0.052 - Ni 341.477 37.78 - O 130.217 - 19.8 Si 288.158 0.097 25.5
Dressier rf power supply (13.56 MHz) with the dis- charge potential supplied to the sample through a water-cooled Cu block (piston). A magnesium fluor- ide window located opposite to the cathode surface is used for optical monitoring, termed end-on viewing. For the studies presented here a 4 mm i.d. anode was used, thus producing an - 4 - m m diameter sputtering area.
The detection system employed in this study is a 0.5 m Paschen-Runge polychromator (Jobin-Yvon RF 5000, Longjumeau, France) which is presently outfitted with 15 optical channels. The accessible ele- ments used in these studies, along with their respec- tive wavelengths, are given in Table 1. The source optical emission is focused onto a 2400 gr mm i ion-etched, holographic grating. The optical path of the spectrometer is nitrogen-purged and operates over the wavelength range 110-620 nm, with the practical spectral resolution for this instrument being - 0 . 0 1 n m . The instrument is controlled using Windows-based software on a 486/DX PC. Each channel (PMT) may be independently sampled at a rate of 2 kHz.
In order to fully utilize the advantages of the rf-GD, emission yields for both conductive and nonconduc- tive matrices, an NIST SRM 1250 F e - N i - C o high temperature alloy standard (Gaithersburg, MD, USA) and Macor (Coming Glass Works, Coming, NY, USA) were evaluated. Table 1 lists the elemental concentrations of the subject analytes in these two matrices. Prior to each analysis the respective samples were manually polished with 600 grit polishing paper
and wiped clean with a soft cloth. The sample was then mounted, the discharge cell evacuated, and argon discharge gas introduced to achieve the desired pres- sure. The plasma was then ignited at the desired power and impedance-matched, maintaining less than 0.5 W reflected power in all cases during the entire bum process. Optical data for all channels of interest were acquired simultaneously during the burn and stored for subsequent calculations. After reaching the predefined bum time, the sample was removed and the resultant crater immediately analyzed to deter- mine the sputtering rate (#m s -I) under the chosen discharge conditions. The average crater-bottom depths were determined with a Tencor P-10 (Moun- tain View, CA, USA) surface profiler system. The sample sputtering rates were calculated (average depth/sputtering time) based on the assumption that the removal rates were constant throughout the burn (a reasonable assumption for homogeneous bulk samples).
3. Theory
The glow discharge is somewhat unique in that the sputtering and excitation are both controlled by the plasma energetics inside the same source, yet the two processes are essentially independent of one another. This has proven to be an asset for the GD in that matrix effects from the sample are greatly reduced, i.e. the emission process is essentially matrix-independent. This results in a relatively simple
570 M. Parker et al./Spectrochimica Acta Part B 52 (1997) 567-578
expression (as used in the quantitative approach by Payling and Jones [19]) describing the observed emission intensity
Ix =kxcxSR (1)
where Ix is the measured intensity of element X, Cx represents the concentration of X in the sample matrix, and SR is the sputtering rate (usually expressed in/zm s-~). The SR is assumed to be directly related to the mass of sample removed, so that the determination of the crater depth is a valid measure of the SR for like samples. The variable kx may be incorporated as [17]:
kx =KxRx (2)
where Kx is the instrument detection efficiency and R x is the emission yield, i.e. the fraction of sputtered species (X) for which a photon is detected (Ix) at a specific wavelength (X). The Kx term may be found by calibration processes and is expected to be constant as long as instrumental changes (lenses, optical win- dows, gratings, etc.) do not occur over the course of the analysis. Therefore the matrix/condition depen- dent variable is the emission yield (Rx) term, which for the case of comparing samples of like concen- tration is simply the emission intensity divided by the sputtering rate:
Rx =Ix /SR (3)
Normalization of the observed intensity by the sputtering (sample introduction) rate produces an emission yield term that is controlled by the excitation processes occurring in the negative glow region and is, therefore, highly dependent on energetics of the rf- GD plasma.
While it has been verified for the case of dc power- ing, Eq. (1) is believed to be applicable to rf-GD-AES systems as well [25]. Also noteworthy, intensity as defined in Eq. (1) shows no direct dependence on voltage, current or pressure. Of the proposed expres- sions for R x appearing in recent years [ 17-19,26-28], each one contains voltage and/or current terms but show no explicit dependence on pressure. The pre- viously mentioned quantification schemes make dif- ferent assumptions as to the degree of the dependence of emission yield on these operating parameters. The intent of this study is to determine the degree of dependence of elemental emission yield on the
operating parameters (rf power and Ar pressure) of an rf-GD-AES source. Unlike the case of dc-powered devices, rf-GD analysis is typically performed by holding the operating pressure and power constant; the voltage and current may then each change in order to maintain a constant power.
Typically, the emission yield (Rx) is reported as a normalized value (R' x) since Eq. (2) shows that the emission and detection terms are separate, thus any 'units' may be included in the Kx term. In practice, the emission yield is normalized relative to that of a matrix element under 'reference' conditions. For example, the Fe 371.99 nm line is typically used in the many applications of GD-AES in the steel industry [29] and thus yields are defined at 'reference' voltages and currents as
R'x = Rx/RFe (4)
where R Fe designates the reference element. Practi- cally, R 'x is the slope of a calibration curve of the ratio of the analyte/standard element intensities (Ix~ IFe) versus the ratio of their concentrations (Cx/CF~) under controlled discharge conditions. The study pre- sented here is not intended to compare R x terms between the various quantitative schemes or even to compare R x values of one element to another. In addi- tion, typical 'reference' conditions set forth in dc stu- dies, for example 700 V and 60 mA, are not relevant in the rf case where voltages are typically much lower and currents higher [6], so that comparison with dc- GD sources is not practical either.
For the study here, R' x values for each element are reported relative to itself for that particular set of conditions, where the denominator value in Eq. (4) is replaced by the maximum emission yield value obtained for the element of interest during that particular study (changing power, pressure) as seen in Eq. (5).
R'x =Rx/Rx(max) (5)
Expressing the data in this fashion allows direct com- parison of the efficiency of analyte excitation over a range of discharge conditions. In the experiments described here each reported emission yield repre- sents three separate bums, with the analyte emission intensities and resultant crater depths (i.e. sputtering rates) for each used to calculate an Rx value. These values were averaged to yield a single emission yield
M. Parker et al./Spectrochimica Acta Part B 52 (1997) 567-578 571 12! 1.1
Fig. 1. Effect of applied rf power on the emission yields of elements in an alloy (NIST 1250) matrix (burn time = 5 min, source pressure =
10 tort).
for each set of discharge conditions. Variations in emission yields therefore represent the cumulative process of sample mounting, plasma atomization and excitation, emission detection, and the measure- ment of the resultant crater depth. Across the range of experiments reported here, the variations in emission- yield values averaged approximately 15% RSD, for each element/set of discharge conditions. This value is higher than expected on the basis of previous eva- luations of rf-GD sources [14,30,31]. The major source of error has since been determined to be due to thermal instabilities in the polychromator system. Even so, we believe that the basic trends seen here are correct.
4. Results and discussion
4.1. Effect of operating power
Operating power is a key parameter in depth- resolved analysis utilizing rf-GD devices. The applied if power is responsible for generating the acquired dc- bias voltage, and thus controls the energy of the bom- barding (sputtering) particles. Sputter rates have been shown in both dc- and if-powered GDs to be linearly
related to operating power (specifically power den- sity) [24,32,33]. Shown in Fig. 1 are the relative emission yields (Eq. (5)) obtained for analytes in NIST SRM 1250 high-temperature alloy over the 10-50 W power range at an operating pressure of 10 torr. As can be seen, the R 'x values do not seem to rely significantly on power as the average variation is 11% for all of the eight elements present. The exact cause for the decrease in the Si response is not known and is still under investigation. With the exception of Si, the data here indicate that power for the if plasma is primarily responsible for analyte supply (sputter- ing) and has little impact on plasma energetics. These findings are in line with those of Langmuir probe studies performed in an rf-GD source by Ye and Marcus [34]. They found that increases in input power did not appreciably effect the electron energy distribution function (EEDF), average electron energy ( < e > ), or the electron temperature (Te) for con- ductive or nonconductive samples. While the geome- try of that source was somewhat different from that of the one employed here, the same tendencies are expected to apply as the operating conditions are simi- lar. Most rf-GD analyses are performed in the 25- 40 W regime where little variance of emission yield with power is observed. This is an important fact,
572 M. Parker et al./Spectrochimica Acta Part B 52 (1997) 567-578
Fig. 2. Effect of applied rf power on the emission yields of elements in a nonconduct ive (Macor) matrix (burn t ime = 30 rain, source pressure =
4 torr, sample thickness = 0.84 mm).
which will be especially important in depth-profiling applications where selecting a single reference power (say 30 W) for all materials may not be practical.
Shown in Fig. 2 are the relative emission yields obtained for a nonconductive (Macor) sample over the 20-60 W power range at an operating pressure of 4 tort. As can be seen, the responses of the R'x values for the elements in the nonconductive sample are not as uniform as those of the alloy samples. First, the O (I) (130.2 nm) emission yield shows a dramatic decrease in the 20-40 W region with a subsequent levelling off at higher powers. This response is the opposite of the observed increase in sputtering rate for nonconductors under identical sets of conditions [24]. The response might suggest that this upper level of this particular transition is less efficiently populated as the power is increased. Based on earlier rf-GD-AES studies [30], though, a more likely reason is self- absorption. The O (I) 130.2 nm emission monitored here is resonant transition, having a large transition probability (A = 3.3 x 108 s-1)[35], which in the case of a high atomic population would make the optical emission prone to self-absorption. Given the fact that oxygen makes up > 50% (by mass) of the Macor sample, this would be a logical explanation of the observed trend.
The R'x values of the other elements in Macor reveal a trend more in keeping with that seen for the
elements in metallic samples. Above 30 W, the emis- sion yields appear relatively constant, with Mg show- ing a slight decrease with power. The low emission yields at 20 W are not unexpected as they are likely associated with inefficient power coupling (as seen in the very low sputtering rates) at this power. Low sput- tering rates lead to more deviation in the resultant crater profiles and thus add more uncertainty to the R' x values.
4.2. Effect of operating pressure
Emission yields are expected to be highly influ- enced by the operating pressure. Intuitively, this is reasonable since the discharge pressure (gas density) determines the electron/atom mean free paths, the dark space thickness, the diffusion rate of the sput- tered analyte atoms, and the Ar metastable population [6-9]. Each of these items has a direct influence on the energy associated within the GD plasma negative glow. Shown in Fig. 3 are the R'x values for elements in NIST 1250 high temperature alloy over the pressure range 2-15 torr at a constant operating power of 30 W. In contrast to the effect of power, operating pressure has a distinctive effect on the emission yields. The R'x values for each of the elements is shown to increase gradually from 2 to 6 torr, increase sharply up to 11 torr, and level off above 12 torr. Both
M. Parker et al./Spectrochimica Acta Part B 52 (1997) 567-578 573
Fig. 3. Effect of source pressure on the emission yields of elements in an alloy (NIST 1250) matrix (burn time = 5 min, rf power = 30 W).
the raw emission intensities for each element in the sample and the sputtering rate increase as the pressure is raised. The data suggest that the overall excitation efficiency of the plasma is increasing at a rate faster than the analyte supply function (sputtering). Thus, either the energetics for atomic excitation are more favourable or the electron-atom collision frequency is higher.
Comparisons of the data shown in Fig. 3 with the corresponding Langmuir probe experiments [34] are not easy to interpret; due in part to the nature of the plasma. As alluded to previously, pressure controls the thickness of the dark space and thus the cathode dark space/negative glow (CDS/NG) boundary region relative to the sample surface. In effect, Langmuir probe measurements at different pressures are essentially sampling different areas of the plasma so that < c > , EEDF, and Te measurements produce somewhat varied relationships [34]. However, increases in pressure do result in increases in both electron and ion-number density under all conditions for both nonconductive and conductive samples. Increasing the electron density should, in turn, increase the probability of electron-analyte collisions and, thus, atomic emission, supporting the observed results.
The R' x values of the elements in the nonconduc- tive matrix are shown in Fig. 4 over the 2 -7 torr
pressure range at an operating power of 40 W. As can be seen, the R 'x values increase steadily over the pressure range even though the sputtering rate maximizes at 4 torr and declines sharply above this pressure. Data were not collected above 7 torr as earlier sputtering studies indicated that these pressure ranges are not practical for rf-GD analysis of non- conductive samples. Again, consistent with Langmuir probe studies [34], the R 'x values are greatly affected by the operating pressure regardless of the sample type employed.
4.3. Effect of nonconductive sample thickness
Employing the rf powering scheme enables the GD to directly sputter-atomize nonconductive matrices with no prior matrix modification. This remains a primary advantage of rf- over dc-powered sources. However, little effort to date has gone into character- ization of nonconductive layers and nonconductive substrates for in-depth analysis. Previous studies in this laboratory have shown that the nonconductive sample thickness is a critical parameter in rf-GD analyses, revealing that power 'losses' are experi- enced through the nonconductive sample [31,36]. These power losses result in decreases both in the sputtering rates and in the observed analyte atomic emission signals. These observations are believed to
574 M. Parker et al./Spectrochimica Acta Part B 52 (1997) 567-578
1.00
0 .90
"0 0 .80
"~, 0.70
.~ 0.60 "¢ - 0 .50 , , l ib
® 0.40
m 0.30 , , - , • f S l , '
a
0.20 ,1~'g
0 .10
0 .00 I I I I I
2 3 4 5 6 7
Pressure (torr) Fig. 4. Effect of source pressure on the emission yields of elements in a nonconductive (Macor) matrix (burn time = 30 min, rf power = 40 W, sample thickness = 1.1 mm).
be related to the dielectr ic propert ies of the sample.
Parker and Marcus [31] have shown that variat ions in
a tomizat ion rates are not a critical issue as the loss
in signal (atomizat ion) can be compensa ted with the
de-bias value o f each sample. Previous studies have
also shown that a tomic emiss ion signals fo l low the
same trend as the sputtered crater depths (sputtering
Fig. 5. Effect of nonconductive sample thickness on the raw emission intensities of the sputtered analyte elements and Ar (I) emission (rf power = 40 W, source pressure = 4 torr).
M. Parker et al./Spectrochimica Acta Part B 52 (1997) 567-578 575
Fig. 6. Effect of nonconductive sample thickness on the emission
2 2 .5 3 3 .5 4
Sample thickness (mm)
yields of elements in Macor (rf power = 40 W, source pressure = 4 tort).
Illustrated in Fig. 5 is the effect of nonconductive sample thickness on the analyte emission intensi- ties for a 40 W, 4 torr plasma. The sample thick- nesses used here are typical for rf-GD-AES analysis ( < 4 mm). The responses for each element show an approximately 1/thickness dependence. Very differ- ent from the raw emission intensity responses, the R 'x values for each of the elements present in Macor remain fairly constant (as seen in Fig. 6) up to the sample with the greatest thickness (3.8 mm). Consistent with previous examples, the abnormal points occur where the sputtering rate is at its lowest (i.e. the depth measurement error is largest). These results are also supported by Langmuir probe measurements [34] where the electron temperature and energy are shown not to vary appreciably with nonconductive sample thickness.
It was observed throughout this study that the Ar (I) emission seems to follow the trend of the emission characteristics of the sputtered elements. This relationship may be used to aid in the quantification of nonconductors, or perhaps metallic samples of varying sputtering characteristics, by essentially utilizing the Ar (I) intensity as an 'internal standard'. The following discussion illustrates this point. As shown in Fig. 5, the emission signal for each of the elements follows a similar pattern. This 1/thickness
relationship has been observed in past studies [36], but the Ar emission in those studies could not be monitored simultaneously with the analyte species because a single detector (monochromator) system was employed. The decreases in Ar (I) emission inten- sity are likely due to the decreased electron number densities, as shown by Langmuir probe studies [34]. A plot of each element's emission intensity versus that of Ar (I) is shown in Fig. 7 for Macor samples of various thickness. Shown here is a near linear relationship between each element's emission and that of the Ar bath gas, although different slopes are observed for each.
The direct relationship between the analytes' and Ar (I) emission intensity points to the possibility of a correction factor based on the slope of each element's line intensity via an equation of the form
E c = E m + (m × u) (6)
where Ec is the corrected emission intensity for the element of interest, Em is the measured intensity for the element of interest, m is the slope of the relative response (from Fig. 7), and u is the Ar emission inten- sity deficit between that of the most intense Ar signal (thinnest sample) and each of the other samples. The m x u term of Eq. (6) provides a correction for the emission intensity based on the power losses observed
576 M. Parker et al./Speetrochimica Aeta Part B 52 (1997) 567-578
0.250 -
A > v
._,~' 0 . 2 0 0 - m C 0
C "-- 0.150
O E ¢) 0 . 1 0 0 o
"O
o ~ 0 . 0 5 0 ¢,j
0 .050
0 .000
A!
/
/
/ ¢
2'
/
/ I ) '
f Mg , f , - -
• - - - Si
i I I I I I
0.020 0.040 0.050 0.080 0.100 0.120 0.140
Ar intensity (V)
I
0 . 1 6 0
Fig. 7. Relationship between sputtered analyte emission intensity and Ar (I) emission intensity for Macor sample thickness from - I to --4 mm
(rf power = 40 W, source pressure = 4 torr).
with increasing sample thickness. This correction technique is similar to that utilized in previous atomic absorbance studies [31 ], with the Ar (I) emission sig- nal replacing the dc-bias voltage correction utilized in that study. Shown in Fig. 8 are the corrected emission signals for each of the Macor samples. As can be seen, the corrected emission intensity for each element results in a signal which is no longer affected by the sample thickness. This appears to be a promising
approach for the quantitative analysis of bulk noncon- ductors and in applications where depth-profiling of nonconductive layers is required. In principal, extra- polation to a sample thickness of '0 mm' would give the 'maximum' Ar emission for that set of conditions and should be comparable to that of metallic samples at the same power/pressure conditions. The case used here is obviously simplistic as each sample has the same composition, differing only in thickness, but
0.250
O ( J
I J . ° - . ~ . °
A 0.200
v
W ¢ 0 . 1 5 0 f,,
"O ~ O.lOO u o &,,
0 .050
0.000 I
1.0 1.5
A I . . - e
. . . - e . . . . . . . . . . e . - - ' " ~ ' " ~
Mg
Si
I I I I I
2.0 2.5 3.0 3.5 4.0
Sample thickness (mm)
Fig. 8. Corrected sputtered analyte emission intensities for various thickness Macor samples (rf power = 40 W, source pressure = 4 torr).
M. Parker et al./Spectrochimica Acta Part B 52 (1997) 567-578 577
the promise of such a correction can easily be envisioned.
5. Conclusions
Application of the rf-GD-AES source for accurate quantitative depth profiling will depend not only on correctly choosing the discharge parameters for the analysis but also on the manner in which the emission yield is determined. These studies have focused on the effects of operating parameters (power and pressure) on the relative emission yields present in the rf-GD- AES source for both conductive and nonconductive sample types. Operating power is shown to have only negligible effects on the emission yield in the 20- 40 W range typically employed for rf-GD-AES ana- lysis of metals. Operating pressure is shown to be a key parameter as it significantly affects the emission yields for all matrices studied here. This suggests that data-collection schemes employing variation of the operating pressure will adversely effect quantitative depth-resolved analyses. All observations in this study may be directly related to the plasma energetics, with relevant Langmuir probe studies tending to verify the observed trends in analyte emission yields presented here.
Analysis of nonconductive samples is a primary advantage of the rf-GD as no matrix modification is required. However, sputtering characteristics (rates) and signal intensities change with varying sample thickness and/or dielectric properties. Quantification must employ matrix-matched standards or a method of signal correction. Argon discharge gas emission signals are shown here to follow the same trends as those of sputtered analyte species. This allows calcu- lation of a compensation factor which is shown to account for losses in the elemental emission intensi- ties of sputtered species. The result is, in effect, a built in 'internal standard'.
Future studies will focus on the evaluation of layered and multi-layered samples, concentrating on the development of quantitative in-depth profiling algorithms. The next key step in this process is to evaluate the effect of matrix composition on the ele- mental emission yields. While different sample matrices will undoubtedly sputter at different rates, differences in dc bias voltages (at constant power
and pressure) may, in turn, cause differences in plasma excitation conditions (i.e. emission yields). In addition, emphasis will be placed on nonconductive layers and mixed, nonconductive/conductive, systems in order to take full advantage of the rf powering scheme. It is believed that the development of quanti- tative depth profiling capabilities will make rf-GD- AES a very powerful problem-solving tool for a wide range of application areas.
Acknowledgements
Financial support of the National Science Foundation under grant no. CHE-9420751 and Jobin-Yvon, Division of Instruments SA, are grate- fully acknowledged.
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