Diagnostics of an r.f. sputtering glow discharge - correlation between atomic absorption and mass spectrometry

129 Inrcrnarionat JOIUIU! of Mass Speczromctry :& Elsevia Scientifi, Publishing Company,

and Ion Pl~ysics, 17 (1975)

Amsterdam

- Printed

129438

in The Nctheriands

DIAGNOSTICS OF AN R.F. SPUI-IERING GLOW DISCHARGE CORRELATION BETWEEN ATOMIC ABSORPTION AND MASS SPECTROMETRY

E. W. ECKSTEIN”, IBM

(First

J. W. COBUREi AND

Rcscarch Laboraror?_

rccciwd

9 September

San Jose.

ERIC KAY

Cali/unia

1974; in final form

(U.SA.)

3 December

1974)

ABSTRMX

rn a neon r-f_ sputtering glow discharge with a copper target, the Cut ion current, measured by glow discharge mass spectrometj, is shown to be proportional to the product of the densities of the neutral Cu atoms and the neon metastable atoms, both determined by atomic absorption spectroscopy_ This resuIt supports the premise that Penning ionization is the dominant process for the ionization of trace species in this type of glow discharge and also provides confidence in the reliability of mass spectrometric diagnostics of a glow discharge. A discussion is presented of the relationship between the sputtering rate of the copper tarset and the neutral Cu atom density in the discharge as the discharge pressure is vzlried_

IXI-RODUCTION

ous

Two techniques which are often used to monitor the concentration of varispxies in a glow discharge environment are mass spectrometry [l-3] and

atomic absorption spectroscopy [4-Y]. Mass spcctrometric sampling of a glow discharge is accomplished by extracting a beam of the discharge species through a small aperture in the wall of the vessel containing the discharge. In most instances difGcrentia1 pumpin g of the volume containing the mass spectrometer is required in order that the mean free path for cqllisions with background gas is longer than the flight path through the mass spectrometer. Ionic species can be extracted directly from the discharge and detected with high efficiency whereas the + P-t

addrm:

Max-Plan&-Tnstitut

fiir Phannphys%

Garching

bei Miinchen.

West Germany_

130 requirement that the neutral particle be ionized after extraction (usually by eiectron impact ionization) greatly decreases the -sensitivity for observing neutral specks relative to ions. Atomic absorption spectroscopy, on the other hand, does not require that spcies be extracted from the discharge. Provided suitab!e sources of characteristic radiation are available, the concentration of both excited and ground state, neutral and ionic, atomic species can be determined_ In the work to be ,nz_mrted here, these two techniques have been combined in a study of the species sputtered from a Cu target in a neon r-F glow dischargeThe sputtered Cu atoms which are ionized in the glow discharge can be observed with high sensitivity mass spectrometrically and this constitutes the basis of an elemental composition profiling technique which we have discussed elsewhere [S]_ it should he emphasized that positive sputtered Cue ions (i.e., Cu atoms which are ionized by the sputtering process itself) canno: reach the detection system because of the large negative self-bias on the Cu target [9]_ Furthermore, evidence has beenpresented [IO] indicating that the dominant process for ionizing sputtered

species in the glow discharge is the Penning ionization process: Cut-

IVem+Cu*

f Nefe

where Nem is a mckstabfe

neon atom with 16.62 or 16-72 eV of eiccrronic energy. Whereas Cu* ions coming directly from the glow discharge can be seen easily, neutral Cu atoms have not been observed mass spectrometrEcaiiy in our apparatus.

is that only a very small fraction of the neutral Cu atoms which pass lhrough the sampling orifice is ionized by the ionization chamber of the mass spectrometer. The opposite situation applies for atomic absorption spectroscopy in thar this technique is sensitive to the density of absorbing species in the discharge volume primarily and. since the neutral Cu atom density is greater than the Cu’ The reason

ion density, Cu atoms arc more easily observed- There are no Cu* optical r’ines (i-e_, Cu (If) characteristic radiation) in the wavelength range available in this study (3000 ( /, < ci5CKlA) but efGorts to detect Eu* ions by atomic absorption during the sptitering of EuO were unsuccessful e\zn though several Eu(lC) lines lie in the acce&ble wavelength range_ However atomic absorption methods can be applied easily to measure the density of neutral copper atoms in the glow discharge. In addition the density of metastable neon atoms can be determined by atomic absorption spectroscopyIf Penning ionization is the domina:lt mechanism for ionization of Cu atoms in the glow dischargt then the densr.y of Cu’ ions should be proportional to the product of the densities of the Cu neutral atoms and the metastable neon atoms (i-e-, Cu* cc Cu - Nem)_ The combination of mass spectrometry (to measure Cu’) and atomic absorption spectroscopy (to measure Cu and Ne”) provides a way of investigating the extent to which this proportionality (Cu* cc Cu - Ne”) holds as discharge parameters are varied_

131 E-YPERIMENTAL

The mass spectrometric detection system has been discussed previously [S]_ The geometry of the discharge region is shown in Fig. I_ The glow discharge is established by capacitively applying a 13~5MHz r-f. voltage to the Cu target electrode. The 0.3-mm diameter sampling aperture leading to the mass spectometer

Fig_ I_ Geometry of gIow discharge region.

is located directly beneath the Cu target and the light beam for the atomic absorption spectroscopy passes through the discharge midway between the Cu target and the electrically grounded sampling aperture. The diameter of the beam from the hollow cathode lamp is 2-3 mm and the path length through the discharge is ca. 7 cm. A copper hoilow cathode lamp fiI!ed with neon is used as the light source. The light beam is modulated at 270 Hz by a mechanical chopper to eliminate-interference from the glow discharge radiation_ The optical detection system consists of a 0.3-m monochromator (Hifger-Watts Model D330), aphotomu1tipIier tube (RCA IP28) and a lock-in amplifier (Dynatrac Model 39!A)_

RESULTS

AND

DISCUSSION

Vurialion of r-f_ power

The first part of this work consists of measuring simultaneously the density of Cu neutral atoms, Ne metastabie atoms and Cu* ions as the r-f. power to the discharge is varied at a constant neon gas pressure_ The independent

variable which is

tocharacterize this power variation is the d-c. self-bias voltage on the target electrode, vd_c_p which can be measured directly. This quantity is a more fundamental measure of the discharge intensity than is the r-f_ power to the system because the latter includes various coupling losses which are characteristic of the experimental used

configuration_

Figure 2 is a plot of the measured optical absorption A as a function

two lines which are absorbed by ground state Cu (I) atoms (3174 A lplF 3217 A - ‘S, + ‘Pz) and for two fines which arc absorbed by + 2p5(‘Pg)3p[5g. the 3P2 mctastable state or Ne (I) [MO? A - 2p5(‘P;)3s A = 2p3(‘~;)3s + 2p’(‘P<)3p[Sjz], J = 21. The density of J = 3; 63x neon atoms in the ‘PO mrt~table state was measured in this study zlnd is approsimately O-3 times the density of neon atoms in the 3P2 state for the parameter spa* covered. Therefore. since only rehrtivc intensities are important in this work, the density of neon atoms in the ‘Pz state will bc used to represent the total density of neon metastabte atoms EM?_

ofFL.,_for 2s1 +

In order to obtain

the: absorption

coefficient-path

length product. X-J, from

of the emission iinc from the hollow cathode lamp is similar to that of the absorption line in the glow discharge_ This is cquivalcnt to assuming that the temperature of the neutrcll gas in the hollow cathode lamp and the glow discharge are similar. In view of the similarities in the two types of discharges, this is a reasonable assumption provided the absorption is not too large_ Therefore the lines with lower absorption (i-e_, 6334 A for rNem and 3274 A for Cu) have been used to determine the respcctivc atom densities_ Stimulated emission from the upper level involved in the atomic the absorption,

absorption

A, we have assumed that the shape and width

transition has k:n

neglected b,x.wse

of the relatively low density of

atoms in the upper state and because the solid angle subtended at the discharge by the optical detection system is small_ For these conditions k,lcan be obtained from tabulations [I I] of k,,l vs_ A tiz = 1).

133 This variation of r-f. power cxpximent was ptrformed at a discharge pressure of 20 mtorr of neon. When higher neon pressures were used the absorption of the 354 A Cu line at high r-f_ powers was so large that the assumptions made in our interpretation of tht absorption data are no longer vaiid

Fig_ 3_ Cu+ absorplion

(ma

spectrome~rir).

Cu

and

XL-

faromic

absorption)

vr‘rsus

Fd_,_ (using

rhe

ciaIa of Fig. 2).

Ths densities of Cu neutral atoms and Ne metastablcs obtained in this way versus V,_,_ are plotted in Fig_ 3_ The absolute densities of the Cu and Ne” have been dctcrmined from [I I]: ‘\r

=

i$x

-;c-) _“. nZ

A\-n(6-67 _ IO-'/;-I;)-'ko

wh crc the Doppler broadening AI-,, = (SkT In I?/i’~l)~_ A discharge :emperat w-e T of 400 K has bxn used and the oscillator strengths fin have b,oen obtained from published tabulations 112, 131. In the above equation N (cm-‘) is the density of absorbing particks, k, (cm-‘) is the measured absorption coefficient, i (cm) is the wavelength of the characteristic radiation and M (g) is the mass of the spzciesof interest_TheCu’ signal obtained from the massspectrometer is also plotted in FI,. -0 3_ In order to determine to what extent the Cu’ signal is proportional to the product Cu - Ne”‘, Cu+ is plotted versus Cu - Ne” in Fig. 3 and it can b= seen that Cu f is proportionai to Cu - Ne” for a considerable range of values of th=se quantities. This linear relationship provic@ support for the premise that Penning ionization is the dominant ionization process for trace

134

Fig- 4. Cu+ versus Cu - NC”. Thcsc vahacs vrxrc obttincd by varying the r-f. powcr at a neon pressure of 20 mtorr- The data are the same as in Figs_ 2 and 3-

species

in this type of discharge_

Also

this result inspires

confidence

in the mass

spcctrometric samplingprocedurewhich can be fraught with difficulties [ 1,14-16 1. The reason for the departure from linearity at high sputter etch rates in Fig- 4 is not known but this may be an indication either the assumptions made in the interpretation of the atomic absorption data are not valid when the absorption is hugez or that electron impact ionization of the Cu atoms becomes significant as the dischxge becomes more intense. Vaiiztion

of &chrge

gas pressure

The second part of this study is concerned with ascertaining the extent to which Cu’ is proportional to Cu - Ne” as the neon gas pressure in the discharge is varied at a constant r-f_ excitation power_ These quantities were measured as outlined in the previous section and Cu’ is plotted versus the product of Cu - Ne”’ on Fig_ 5 for a constant r-f_ power of 50 W (V’_, ca_ 320 V)_ As can be seen from this Fig, Cu* is proportional to Cu - Ne” at higher discharge gas pressures but the proportionality does not appear to hold as well at iowpressuresTwo effects occur in the discharge as the pressure is varied which may influence the data in Fig_ 5 First of all, as the neon pressure is decreased, the thickness of the cathode fall region in front of the Cu target increases, eventually extending outward to encompass a major portion of the discharge volume including the region sampIed by the atomic absorption beam This wili cause a decrease in the observed Cu* sigual since all Cu* ions formed in the cathode fall are returned to the Cu target electrode by the prevailing electric field However, the

fig 5_ Cu’ versus Cu - I%?‘_ These vahxs ~wrc obtained by l rying Constant r-f_ power of 50 W (V,_,_ cam320 V).

density

of Cu neutral

atoms

as measured

by atomic

the neon gas pressure at a

absorption

spectroscopy

is

nor afkcted by this changing potential configurationA second factor which may be significant is that as the pressure decreases the extent to which the Cu atoms diffuse radially away from the main discharge volume also decreases_ Consequently the Cu* ion signal which is proportional to the flux of species incident on the sampling aperture ~41 increase as the pressure is decreased_ However the Cu neutral density, as measured by atomic absorption spectroscopy, will not increase as rapidly b-muse a11 Cu atoms in the light beam contribute to the signal even though they may have travelled some distance radially. These two effkcts influence the data of Fig.5 in opposite ways: the expanding cathode fall region will decrease Cu* with respect to neutral Cu as the pressure is decreased whereas the Cu atom radial diffusion will result in an increase of Cu+ relative to neutral Cu as the pressure is decreased_ If Fig. 5 is to be interpreted using these arguments it is necessary to conclude that the cathode fali effiit dominates for P < 25 mtorr whereas the t-a&al diffusion efkct is significant in the pressure range 25 < P c 50 mtorrRelation to sputier etch rate

Although it has not been n ecessary to be concerned with the sputter etch rate in the preceding discussion, it is of interest to consider the relationship between the Cu neutral density, as measured by atomic absorption spectrometry, and the sputter etch rate- In the case of the r-f. power dependence the situation is straightforward in that the%putter etch rate should be proportional to the Cu

I36 atom concentration_ However, in the case of the pressure dependence, the situation is complicated by the fact that as the pressure increases, the average residence time of a copper atom in the discharge will also increase proportional to pressure if the mean free path between collisions, /I, is very much less than the total path length. L, traversed by a Cu atom- That is, the Cu atom experiences many collisions with the Ne gas atoms and consequently travels a much longer distance than the straight line distance through the discharge_ When the Cu atoms experience few collisions the residence time is independent of pressure_ The consequence of the influence of pressure on the time a Cu atom spends in the discharge volume is that, if the sputter etch rate is constant as the pressurc increases, the ccpper atom density should increase linearly with pressure above some pressure PO_At pressures below PO the requirement that 11 << f. is not adequately satisfied The Cu density as measured by atomic absorption is plotted versus pressure in Fig. 6 and the curve is linear with pressure up to about SO mtorr. in this Fig., PO = 8 mtorr corresponding to A ca_ 0.6 cm_ The interelectrode spacin_e is ca_ 4 cm_ The dcparture from linearity of Fig_ 6 for pressures above SO mtorr is attributed primarily

1.

PO r

r/

08

I

so

100

150

zw

250

to the fact that the sputtering rate at constant r-f. power decreases as the pressure increases 117, I8]_ Some support for this contention is provided by Fig- 7 where Cuf(P-P,), which should be proportional to the sputter etch rate, is plotted versus P and compared with some measured sputter etch rates. These sputter etch rates were determined by sputtering several Co films about I100 k%thick and 1 cm’ in area in the neon r-f_ discharge and using the mass spectrometric Co+ signal to determine the time required to remove each Co film completelyIt should bt emphas&d that these megjured sputter etch rates are removal rates and not

137

Neon P~~SLW(miIlirorr) Fig. 7. CU/
rapid&

rates- One wouid with pressure

b-muse

espect

that deposition

rates would

decrease

more

of lateral scattering of the sputtered Cu atoms

in

the discharge_ These sputter

etch rates were determined in two separate experiments using two r-f. power levels close to the level used when the atomic absorption dktta were recorded_ Results from each of the two experiments were normalizezd at one point (indicated by arrows in Ftg. 7) in order to correct for the small changes in r-f_ power from experiment to experiment and to enable a comparison of Co sputtering rate data with glow discharge diagnostics taken with a Cu target- The agreement between Cu/(P-PO) and the measured removal rate is satisfactory considering the scatter in the data_ This agreement supports the premise that. at a constant sputter etch rate, the Cu atom density is proportiona to the discharge pressure-

It has been shown that the CU+ signal as measured by mass spectrometric sampling of an r-f- neon glow discharge is proportional to the product of Cu - Nem, both measured by atomic absorption spetroscopy, over an estended range of r-f_ powers and neon gas pressures- The verification of this proportionality further supports the premise [lo]

that Pennin g ionization

is the dominant

process for the

ionization of trace species in the discharge volume and also provides in the reliability of mass spectrometric sampling of a glow discharge nostic procedure-

confidence as a diag-

138 Whereas

the Cu density

should

be a reliable measure of sputtering rate at is proportional to the sputtering removal rate if the pressure is an experimental variable in the pressure range most often encountered in glow discharge sputtering-

constant pressure, it is shown that Cuj(P-PO)

The authors are very appreciative of the assistance of R-W_ Sadowski who recorded most of the experimental data. We would aiso like to thank H-F_ Winters and G. Sfodzian for helpl‘ui discussions-

REFERENCES

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a

Skrent_rBhents. NBS Monograph 53, 1962 14 D_ K. Bohrne and J_ M_ GolDdings. I_ AppJ_ P~J-L 37 (1966) 4261_ 25 D_ Smilh and I_ C_ Plumb. J’_ Ph_rs_D 6 (1973) 14-11. 16 I_ B- Hasted. AmPn- Mass Spcrrrvnr., 6 (1974) 90117 J- L- Voatn ;md J_ J_ OINcitL Jr_. RCA Rer_. 29 (1968) 149IS C- R D_ Pricstlsnd and S_ D. Herscc, Vacuarm,22 (1972) S9-