Hollow-cathode sputtering source for the production of gas-phase metal atoms of the refractory elements

Volume

number 4

93.

CHEhllCAL PHYSICS LETTERS

HOLLOWCATHODE

SPU’ITERJNC SOURCE

FOR THE PRODUCTION

Recavcd

23 August

OF ~AS-PHASE

hfETAL ATOMS OF THE REFRACTORY

1982. m rmal form 21 Scptcmbcr

ELEMENTS

1982

A sunple. vcrutde hollowcnthodc sputtering system lor the producllon of gx-phase metal aloms 11~sbeen dcvdopcd The source has been used lo product garpham mctal-conlammg dlatomlc and polyalomlc molcculcs Chcmdummsccncc and lascratduced fluorcscencc studtcs have been used to charackrlzc the source and show lh~t II produces III@ dcnstttes ( 10’4/cm3) of aloms at P low tcmpcraturc (340 K)

atom-contammg

1. Introduction

molecules

vanced by the devclopmcnt A simple,

versnttle hollow-cathode

tem for lhc productton

sputtermg

sys-

of gas-phase metal atoms has

has been stgntficantly of rclatlvcly

turc flow systems [3] for thetr production. atonuc [4] and polyatomlc

rd-

low-tempcraBoth dt-

151 species IISVC been pro-

been developed. Initml studies have shown that the gas-phase reaction of Cu and Fe wtth SF6 and O-, re-

duced by thus techmque. The metal atoms for thcsc molecules have been unroduced mto the gas phase by

spectively give easily wsible dienulunltnesccncc front CuF and FeO. Laser-Induced fluorescence studlcs of

vaporiztng lhc metal of lnlercst in a cc’ra~n~ccructblc healed wth tungsten rcststancc we. Although 1111sIS

CuOH and CuOD, formed by the reactton of Cu atoms wtth Hz02 and DzOl respecttvely, indicdtc tllat the

smlple and works well for those me!als wluch have a slgmficant vapor prcssurc at tcmpcratures below 1000

molcculcs formed by thrs source are stgmlicantly cooler than those formed by thermal vaponzatlon sources

K, the thermal insulatton rcqmrcmcnts make 11 less than satisfactory for the more refractory elements. In addazm. molecules produces by thernaal vaporrz3tion are usually at a tcmperaturc clisractcristic of the iact3l-

Traditional

techniques

for the productmn

of Iugh-

temperature molecules are the heat p~pc oven the Ktng rurnace [Z] Botlt of these tcchniqucs relatively

large amounts

[I ] and require

of startmg matenaIls and pro-

atom source [6] We leave adapted a techmquc commonly

used rn

duce molecules at temperatures of IOOO- 1500 K for

atonuc

heat pipe ovens and 1500-2500 K for Kmg furnaces. The high temperatures associated with these sources

troscopy [8,9] tn order to product gas-phase metal atoms. A dc dlschargc. hollow-cathode sputtering

often lead to thermal

source with a flow of carrter gas lItrOugh the cathode

populatton

of excited

elcctromc

spectroscopy

[7] and matrix

isolntlon

spcc-

states, broad populatton distributions within an elecrronx stare, and severe Doppler broadenmg of the ob-

IS used to carry sputtcrcd metal atoms mto a reaction regton located B few ccntimcters from the discharge

served spectroscoptc Iransittons. All of the above facIors can complicate a spectroscopic analysis. In reccm

This system yields a large density

years, the spectroscopy of h&temperature,

cases, produces vlslble chemdummescence and a rela-

0 009-26

14/82/0000-0000/S

02.75

metal-

0 1982 North-Holland

of metal atoms

which when allowed 10 react with olhcr gases, in msny

343

CHEhilCAL PHYSKS

Votumu 93. number4

t~vciy cool population of molecules for laser-mduced ilt~o~se~ncc studtos. Htstoruxlly,

elccrrlcnl dlschargc sources have been

IO Dccrmbrr 1982

LETFEltS

GLASS OXIOllNT tlNc.DE

ELEerRlCI‘

FEED THRU fNS”L*TINC SLEEVE --,

AND

used cxtcnstveiy m molecular cmrssron spectroscopy [IO]. More rcccnrly, the development of a composttlon hollow-cathode source (1 I] proved to be a useful tool tn the analysts of the CuO green bands [ 121.It ts important to emphasize that dlc hollow-cathode sput-

tcrmg sourceciescnbcd here ISprimorlly used as a metal zttoms into the &as phnsc

~CORS of introductng,

SUPPORT AND ELECri?lCAL CUNNEC~ION CATHODE

All of the subsequent chenncsl reacttons which pro-

duce the molecuics of mtercst and the speclroscoplc observstrons take place m a regton enttrcly separate from the discharge regton

VACUUM

FLANGE

The tcdtnrque described here has several advantages over prcvlousiy used methods for producmg gas-phase metsl atoms

i-

SPUTTERING

AND CARR,ER

GAS

(1)

Bccausc the itoiio~v-~atitode source d&pates rel3trvcly sm311 omounls of power (typtcalty 4040 W), the thermal msulatton and cooimg requirements are nnnrmnl and the apparatus IS quite sin@. (2) Smce the atoms arc produced by sputtermg end not by thermal VaportLation, essenttoily any material whtch can be placed in the be used to produce atoms

cathode of the system can ’ (3) The relattvely low currents used tn the discharge (usu.& between 200 and JO0 mA) produce ncgligiblc rnsgnetrcfields tn the observation regton. Thus Zeeman broadentng, observed in n~tcro~vav~-opttcaf double-resonancestudies using the heated crucible system

1131are avorded. (4) Stnce the subsequent gas-phasereactton of the atoms 10 form molecules and the spectroscopic obser-

wtlons take place m a region several centimeters from the dtschoge, cool,

the molecules

produced

rtre relatively

controioverrhc chemistryof their production

ISobl~lned,and the s~ctroscopic observ~tlonsare free of overlappmg atomic emission. (5) The system ISboth mechantcally and electricaily wmpk. None of the paratneters related to the design arc crittcai to successfui operatton of the source

A schcmattc diagram of the hollow-cathode flow

system ISshown m fig. 1. A dc discharge in a carrier gas (usually argon) IS matntained between the anode 346

and cathode. The cathode ISa tapered %~p” made of the metal to bc sputtered The cathode cup IS =9 mm Internal dztmeter and 6

mm deep. The central bore

through the cathode support rod is !I mm in diameter

and provldcs a convement laser beam “dump” for Iasere~cttatton s~ctroscopy. The anode ISa simple loop of tungsten wlrc of approxtmately the same dtamcter as the cathode. The anode-cathode spacing is a few mtiitmeters The upper baffle reduces the amount ofstray dtscharge light m the reactton region and the lower baffle prevents arcing to the cathode support. At ;f system pressure of a few Torr,

the discharge

creepsinto the end of the cathodeand atomicline enttsston of the cathode material can be observed in the discharge. The flow rate of the earner gas ISa few tenths of a nuihmole per second. The system has been operated between 1.5 and 5 Torr total pressure. The discharge operates at currents between 50 and 400 nlA and at 200 V. In our expenments the entire dtscharge system IS containediu a IOcmdiameter statnless steel vacuum system with Four 5 cm dtameter cross arms in the reaEtton regron The cross arms are fitted with the appropriate windows, Icnses, oxtdant Feedthroughs and ~umptng hnes, All vacuum seals are the half groove

CtlEhllCAL

Volume 93. number 4

PHYSICS LEITIXS

O.ring type and the syslem 1s pumped by a mecliamcal MC”“In pump. A 6 5 mm outer diameter glass tube mounted on

The total number density m tllc mctastnble “D3,1

and 2Dsj3_ states was cstimatcd by lixmg a cw dye laser to the ZP,Iz-2D,,2 transilion and monitoring

one of the cross arms ~6 cm above the discharge

the IntensIly

brings the oxidant

ccncc 1hrough an ultravIolet

to the reactIon region Ch~nulu-

of the unrcsolvcd laser-Induced fluorcscutoff

filter. Taking InIo

minescence IS observed as a narrow vertical “flanic”dI-

account the branchmg ratio of IIIC 2P3,2 fluorcsccncc.

rectly above IIIC cathode along the dIrectIon

the laser Irradldnce,

ner gas flow. If the oxidant chcrmluminescence

of the car-

pressure IS Increased, the

moves lowards the dIschargc rc-

gion. In the absence of oxidant.

emission from cxcrted

argon atoms and weak atonuc emission from tilt calh.

ode matenal can be seen along the carncr gas flow path. It should be noted llia~ the oxidant IS. for Ihe most part, not mvolvcd In Ihc clcctncal dlschargc TIIC oxidant

IS Isolated

from the dlschargc

by the upper

baffle and by the net flow of the carncr small amount of the oxidant

gas. Only a

enters the dlschargc rc-

gion by diffusion. It IS difficult

to deternunc

accurately the number

the laser bandwidth

111at111c‘DJ,~ and “Ds/T SIBICSarc populalcd 111proportion to theu degencrkics gsvc cwualcs of rhc Io-

t,d Cu mctastsble 1013/cm3.

nurnbcr

Combmmg

mmcd population

density

this wItI

ratio puts tlIc lot.11 Cu atonuc den-

sity in the rcactlon zone at 1014-1015/~n~3 estimates

based on the intcnslty

ROW&

of CuF chcnulununcs-

cencc yield nunlbcr dcnsllws in the rcglon of 101J/cn13 or greater. In good agreomcnt with the atomic k.cr-IIIduced lluoresccncc

rcsuks

Eqxrnncnts

the role of IIK large Cu uictastablc

tive studies of hollow.carhode sys~cms [ 141 indlcatc that atomic densities m the range of 10”-10’4/cm3

cheni~luiiuncscencc

pressure and current flux

in thr idng’ of 101’-

the prcv~ously dctcr-

density of atoms produced by this source. QuantlIa-

can readily be achieved under condItIons

and Doppler

profde overlap. the frcqucncy factor, dcgcncracIcs, .md the volume of space sampled [I 51, and agam dssunuu$

to dctcrnunc In rllc

populdtlon

rC3c11on Wchanisnl

arc currcnUy

in propress

of sinular

In order to estimate tlIe

number density of atoms In the reactIon rcgIon and to determine the rclatlvc populailon of ground-state

3. Observation

of molecular

chemiluminescence

- CuF and Fe0

Cu 2S1,2 to mctastable Cu ‘Dv~ pernncnts were pcrformcd The relative population

plus ‘Ds,~ two exIn the abscncc of oxIdanI.

of ground-state

state Cu was cshmatcd by IkIng flashlamp-pumped

to mctastable-

a frcquencydoubled

dye laser to Ihe Cu 2P,,,-%t12

Fig 7 shows lhc

e”uss,on

sptc’nu,,

obscrwd

with J copper

cathode

resonance transItIon at 324.7 nm dnd recordmg the mtenslty of the Cu 2P~,2-2Dg,2 cnusslon at 5 IO 6 nm through a monochromator. The dye laser was then operated on Its fundamental and the Cu 2P,,2-‘D3,1 transItIon was excited at 570.0 nm and the emission InIensiIy

of the 2Pjp-2D5,?

through a monochromator tics. The Intensity

was agan

monitored

with an IdcntIcal set of op-

ratio of the IWO e\perinsnts,

after

taking Into account dcgeneraaes. laser Irradlance, laser bandwidth and Doppler profile overlap, transition strengths and frequency factors [I 51, and assuming that the 2D,p and 2D5,z states arc populated In proportIon to their degencracles, gives a population ratio of ground-state 100-300.

to metastablc-sratc Cu in the nngc of

The major

contributions

values arises from dlfliculties

beam area and in the determination In the reaction zone

to the spread m

m determining

the laser

of the laser power

when

The cmlssron IS observed as a bright, greemsh ll~mc WI~ICII IS easily vlsrblc with room lights on The spcrtrum was

SF, IS used as the oxidant

r,,‘.

‘

-

I-

A-.

,

CtIChlICALPHYSlCS

VU~UIIIC 93. number -I

rccorclod

wth

cathode

100 mA dlsclmrge current.

potential

difference

3 Torr. The !uonochromator /.tni sl!ts (0

detected

I nm spectral

w!th

t!on w!th multipl!cr

with

lndwidually resolved rovlbromc transitIons were found to have a Doppler width between 0.03 and

I00

and the signal !s

operntcd

has been appl!ed !c,

1~ systems ofCuF

A In-X

was operated

m combma-

No correction

= 0 scqucnces of the C In-x served cmlssmn

Au

!s,

to be direct

and

The ob-

c~~emt~unt!nCs-

cence from CuF nlolecules rcact!on

orCu

formed

alo!ns

fornicd !n the gas-phase and SF6 and not CuF molecules

m IIIC d!schnrge

and subscqucntly

Cl~el!!llun!!l!esccncc

from

the so.called

bands, whtcll contain

quantum

gives the populahon

orange

number(J)

The d!stnbutton

a rotatlonnl

for CuOH

ru atoms

iurthcr

mvcstrgatlon

or D-,0, of-&

hollow

ta!1011a1 sub-band

of CuOH

sho\v!ng

structure

analysis of the Ii@-rcsolutioii

and

of sputtered

rcspcctlvcly,

Fig. 3 IS a low-resolution

spectrum

of CuOH

has led to

cathode

sputter-

cw dyr laser euciwell-developed

ro-

A co~nplctc rotational dye laser cxcitatton

IS I”

temperature

from

search

cvcitcd

K a!!d

m the range 270-470

giving

K

spectra

levels of the ground

vlbratlonal

No such transItIons

state was performed.

al distnbutlon ture.

For CuOD

!nd!cat!ng

the range I S-20

for lascr-excilallon

fat-

d!str!but!on.

m the range 23-O-360

maxlmumJ

were observed.Thls

gas-phase rcxtlon

chJractcrlzat:on

mgsource tation

WIUI H207

temperature the

clcctron!c spectroscopy

specrroscoplc

Boltzmann. lies in the range J = 14-18,

An extenstve

arising

formed by IIIC

is very nearly

the mawmum

ture as compared to rhe spectra obtamcd wlth a ther!!!a1 cvaporatlon source for Fc atoms [IS]

A !eccnt

lines in the

were chosen for this relanve !ntens!ties,

Fe0

a rotntlonol

CuOD.

unoverlapped

line strength and frequency state total angular monientum

band structure

4. Laser evcilation

many

spcclra of both CuOH and CuOD, purpose A plot of tl~c normakcd

SIIOWS qtAt~tlVCI~~ a IWCII ICSS and IICIICC 3 lower tempera-

recorded

overlapped

temper-

dlvlded by appropriate tors. versus the lower

[ 171 IS observed

spectrunl

a translatlonal

K.

cxclted.

when 0, is used as an oxidant will1 an iron cathode (300 n!A, 200 V, 3 Torr Ar). The system

indlcatmg

m the range 300-420

An estlmnte of tllc rotatlonal temperatute may bc obtamed from the intensity d!stnbutlon of a branch of any sub-band. The Q branches of the (I, 0) sub-

to the spectrum

B !C-X

cm-1 (Mm),

0.030 ature

for photo-

can bc seen [16]

IS bchevcd

out and will be publIshed

elsewhere [l9].

and 580 n!n the well-developed

480

has been carried

prcssurc of

a photomultlpluzr

rcsponsc

Between

spectrum

1982

and at a total

resolution)

a p!coamn!ctcr.

2 IO V anode-

IO Dcccnlbcr

LklTERS

IS an !nd!cation

IS also characterized

that the wbratlonby a low tempera-

All of these results mdicate tht the molecules probcd by laser-induced fluorescence arc translationally, rotatlonally.

and vlbrnl!onally

near room temperature. tcmpcrature molecules a I!ollow-cathode analysis

relaxed

sputlcr!ng

of tlic spcclra

lo a temperature

The ab!l!ty to produce at such low temperatures source

highwith

is a great aid !n the

of high-temperature

molecules.

5. Conclusion The hollow-cathode sputtering source has shown itself to bc a s~mplc and versaMe tool for the productIon of gas-phase

atoms

of htgh melting

pomt

metals.

The

source produces a suffictently III& number density of metal atoms for n variety of spectroscopic applications Molecules atoms

produced

WIIII oxldanls

ture much

lower

I’lg 3. Lo\\-rerolul~on cw dye 13scr cwltatlon spectrum of CuOll produced III the rcxtlon of Cu + li202.Thu structure IS due to rotdtlonal sub-bands The rOtatlOnJl tcnlpcrature IS

The separation

near 300 K

from

ccules produced

by the reactlon are characterized

than the temperature using thermal of the molecular

the sputtering

region

of the sputtered by 3 tempcn-

of the same mol-

vapon’zation productlon

leads to greater

sources region control

over

Volume 93, number 4

CHELIICAL

the chcmstry. Overlappmg SVXI are also mmdcd.

spectra

due to atomic

cmis-

References ( 1]

M..Sshur;lr and

H.P Brolda, Cbcm. Phys Lclrcr=, 38

(1976) 234 (21 V.I Srdanov.md DS PCSIC,1 hlol Spcctry 90(1981) 27 131 J B. WCG, RS Bradlord, J D. Cvcrsol~ and C R Jones, Rev.Sct [J]

R.S.

lnslr

46(1975)

Ilndford,C.R.

78 (1979)

Reed)

191 J.K

BJtCS Jnd D M. Crucn, J Mel Spcctr). 78 (1979) 28-I If 01 G. Hrrzbcrg. The spccrrn dnd wnxurc nf wplc trrc

r.Id11’3ls(Curncl UlllV Press. ~~ili,L!J. 1971) D.W. Sclscr,cd . RCXIIW mxnlrd~dlcs III lhc ~,IF pl~~tc pxwr.trlon and obwrwrmn (Acadcnw Prw,. NW York. 1979). [ 1II R. B.w~r.ThL\tc de doctorat. Umvcrw! dc Lyon. Vdlcurbannc, rrJncc (I 974) [ 131 Y Lcfcbvrc, 0. Pmcbcmcl and R BJCII. Cm J PII)F 51 (1976)

La% South.ll

and J G. Ray,

735

[ I3 I PJ

Dom.~llw. T C Srclmk and 0 0 Ilxr15. J hlol Spcclrg 68 (I 977) 146.

164

Jonus.

and H.P.

Brolda, J Cbcm. Pbys 61. (1975) 2060. [Sj R r. Wormsbechcr, hl.Trhula. R.E. Ptnn.C. hhr~n~r and D.O. HWIS. J. hlol Spcclry.. IO bc pubbshcd 161 T.C. Srclmlc, P1r.D Tbcs~s, lJn~wr~~~~ o~Cablorn~a. Santa Darbara (I 978) 171 P HanWord and R.h!. Low, J. Phyb 014 (1981) LS. D.W. Duquclrc. S Sal111dnd J.C. La\! Icr, Phys Rev. A?4

(1981) 2847, 181 D.W. Green, G.T.

PHYSICS LETTERS

I. t1c.l

[ 161 R

H. Strcle and H P Rrolda. J Chcm Pb)c 69 (1978) 2300

Spcctry

157 [

191 A1 TrbulJ and D 0. tfarrrc. III prcl~.lr.lflon

313