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IR and UV laser-induced chemical vapor deposition: Chemical mechanism for a-Si:H and Cr (O,C) film formation
Acta, Vol. MA, No. 4, pp. 489 - 497, Spcctrochimica Printed in Great Britain.
0584 - 8539l90
1990.
$3.00
+ o.OLl
0 1990 Pergamon Press plc
IR AND UV LASER-INDUCED CHEMICAL VAPOR DEPOSITION: CHEMICAL MECHANISM FOR a-Si:H AND Cr (0,C) FILM FORMATION
Peter Hess Institute of Physical Chemistry, University of Heidelberg, Im Neuenheimer Feld 253, D-6900 Heidelberg, F.R.G.
Abstract The characteristic features of laser-induced chemical vapor deposition in the parallel and perpendicular laser beam /surface configurations are discussed. Low temperature chemical processing with directed and spatially local&d energy deposition in the system is investigated. Results obtained for the deposition of hytigenated amorphous silicon (a-Si:H) films in the andSi H~~~precursorsampresented.Asasecondexample, parallelconfigurationemployingC02andKrFlasersandSiH the growth of oxygen- andcarbon-containin chromium fdms%Jr(0.d from chromium hexacarbonyl as the precursor using cw and pulsed uv lasers is discussed. The ca emical pathways leading to film formation am investigated in detail.
1. General astwcts of laser-induced chemical vaDor detwsition 1.1. Introduction Laser-induced chemical vapor deposition (LICVD) is a rapidly expanding field. This is mainly due to the unique new possibilities in materials processing and microelectronics fabrication. There are several books available treating the subject in the form of monographs with emphasis on the fundamental understanding of deposition andetchingprocesses [ 11,[2] or in the form of review articles written by experts including also a discussion of the potential and advances in microfabrication [3], 141. Although the laser chemical processing technology with applications in the semiconductor industry is growing rapidly, there is no satisfactory understanding of the fundamental chemical processes involved. The reason is the complexity of the laser-driven chemical processes. Therefore, further technological advances depend on a better understanding of the spectroscopy, reaction dynamics and surface chemistry in the system under investigation. In this review the chemical aspects of laser CVD are discussed in detail. This includes laser excitation of the precursor molecule,primaryandsecondaryreactionsindu~eitherdirectly byphotonsorby heatinginthegasphaseandatthesurfaceand the crucial processes leading to film formation. After some generalremarks concerning laser chemical processing two examples will bepresented. The fmt example is laser deposmon of hydrogenated amorphous silicon (a-SkII). This material has many apphcations such asphotovoltaic solar cells, thin film transistors for liquid crystal displays, photoreceptors for electrophotography and laser printing,image sensors etc.. The second example treated in detail is uv laser deposition of chromium films from chromium hexacarbonyl. Photochemical deposition of refractory metals such asCr. MOand W from their hexacarbonyls has been studied carefully in mcent yeam and detailed results are available for chromium.
1.2 Laser excitation and chemical processing The efficiency of laser radiation to initiate and sustain a deposition process is determined by the interaction of the laser photons with the ambient molecules of the deposition system. Therefore, the laser wavelength and the absorption cross section of the ambient molecules must coincide. ‘Ibis is achieved by selecting appropriate percursor molecules for the type of laser troscopic and thermodynamic properties are to be employed. The choice of the precursor is very important because the responsible for the efficient activation of the depositron process and the cTee emical nature of the molecules influences the chemistry induced and thus the chemical and physical properties of the deposited film. For excitation of electronic states, laser light in the visible or ultraviolet spectral regionis used. Especially, uv laser light with photon energies comparable or larger than chemical binding energies is employed for photoexcitation and photodissociation. Laser-induced reactions due to electronic excitation occur either because of the enhanced reactivity of the precursor moleculesin the excited states or because of the reactive photofragments and radicalsgenerated by photodissociation. The efficiency of energy transferandrelaxationprocesseswill determinewhich partof the energyacquiredby the system from 489
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PETERHESS
photoexcitation can be us+ for chemical modification. Pulsed uv lasers may be best suited for direct photochemical processmg wrth mmrmal parhhonmgof the absorbedenergy into other degrees of freedom by energy exchange processes and negligible transport of energy away from the excited region. In theinfraredspectralregionespeciallyC0 lasersareemployedforvibrationalexcitation. Formanychemicalsystems, the ‘3 er than the energy of a single ir photon. Therefore, either multiple- hoton exactivation barriers for reactions arc much hrg citation is necessary to promote a reaction or the laser radiation is used as a spatially localized heat source to m dp uce a thermally activated chemical reaction. In the latter case the absorbed energy is thermal&xl locally. The spatial extent of the region that undergoes thermally enhanced processes depends on the spatial and temporal excitation characteristics and the thermal diffusivities of the deposition system.
1.3 Laser beam / surface configurations Two completely different relative arrangements of the exciting laser beam and substrate surface are possible: one where the beam travels parallel to the suface without touching it and one where the beam hits the surface, as shown in Fig. 1. In the parallel conf&uration the laser radiation directly excites or heats only the gas above the surface, and therefore the gas and surface temperatures may be controlled more or less independently. Thus, the processes occurring in tlte gas phase, namely the primary and secondary chemistry, and the processes taking place at the surface determining the film structure and composition may be synergistic. The substrate temperature is of crucial importance for the film properties, and therefore should be optimized according to the surface chemistry yielding the best solid material. No film growth is observed without laser radiation. Besides the photoexcitation and resulting reactions in the laser beam, the transportof activated species and reaction products from the localized light beam to the surface must be considered in a theoretical description of the processes involved. An additional transport process to be included might be surface diffusion of active species to a localized reactive spot Intheperpendicularlaserbeam/surfaceconfi former case at least nucleation will be mainly detemrined by the gas properties. However,energy deposition into the system may change during the stage of deposition, if film absorption takes place. In the second case of pure substrate absorption, the gas phase supplies the precursor molecules by diffusion for the photochemical and/or the photothermal processes initiated by the laser light at the surface. If both the gas and the solid phase absorb photons a series of processes must be considered. Not only excited molecules but also fragments or clusters may move by volume diffusion to the localized reactive spot on thesurface. Speciesadsorbedatthesurfaceareexposedtothelaserrsdiationandthusmayundergofurtherphotochemlcalor photothermal modification. This complex situation may arise also in cases where the substrate did not absorb initially. The ability to confine heatin to small regions or driving nonequilibrium chemistry on a highly localized scale in a heterogeneous system is one of f e attractive features of laser chemical processing.
1.4 Deposition conditions for laser processing An importantfeature of laserradiation is the high photon flux available in the ir to uv spectral region. Efficient laserexcitation can be used to enhance the sition rate or to lower the deposition temperature. Direct bond breaking with uv lasers, d”fp-power ir and uv lasers and spatial control of the heated zone allow low temperature promultiphoton dissociation with hig cessing with minimized heating of the whole deposition system. This low temperature capability of laser processing is imof fragile substrates and device portant from the point of view of energy consumption and is essential for mssing structures or temperature sensitive materials. Deposition at low temperatures often leads to a metastable amorphous network, where many properties of the deposited material vary with preparation conditions. The low surface mobdity of adatoms can lead to voids or result in columnar growth In this case a high-quality fdm can only be obtained by a subsequent thermal treatment. However, photoinduced pyrolytic or photolytic reactions of precursors may also lead to epitaxial growth at reduced temperatures, if suitable single crystals are selected as substrates [5]. Thefundamentalaspectsoflaser-inducedchemisayareverypoorlyun&rstoodinmostdepositionsystems.Thepictureofa confined photodissociation of precursor molecules in the gas phase and/or adsorbed state leading to direct deposition without additional reaction steps is too simple in most cases. Secondary pbotochemical and thermal reactions normally occur in the gas phase and at the substrate surface. In this complex chemical environment the chemical species responsible for nucleation may be different from those governing the main film growth process. At low surface temperatures incomplete drive-off of additional species adsorbed or chemisorbed at the surface often results in impurity incorporation. Thus, not only the morphology, but also the chemical composition of the film may change considerably in low-temperature deposition. If the surface of the growing film possesses catalytic activity the surface chemistry may be extensive and difficult to control. Even the strongest chemical bonds can be broken by heterogeneouscatalysisand only an understanding of the reaction processes on a microscopic level will allow an optimized processing. During nucleation and growth of the film, the optical and thermal properties of the deposition system normally change considerably. These changes are due to the formation of new chemical species with different spectroscopic properties and the growth of the film. The will affect the processes going on in the d sition system either directly, by new photochemical reactions occurring in x e gas phase or at the surface, or indirectly,eGr changes in the transient local heating. Due .tocontinuouschangesinthe~tionconditionsthelaser-drivenreactingsystem mayneverreach steady state. Infacfarigorous descriptiandthecomplicateddynamicalprocessesoccurrin betweenlaserinitiationofthechainofchemicalreactionsand the final building up of the condensed network of the film L not been achieved for any deposition system. Therefore., simplifying assumptions are always used in the development of models for specific systems.
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R and UV laser - induced chemical vapor deposition
2. Laser deposition of hydrogenated amorphous silicon 2.1 Introduction ~&e~ckmica! vapor&position of hydfogenatedamorphous silicon (a-Si:H) hasbeen studied with irand uvlasers &c~lllumtnationasrevlewedin[l]and[3].Thebestcharactenzationofthedeposltiod
film
beenachievedfortheparallelconfiguration.Inthefollowing,recentresultsw~bepresentedonlyforthis properties tzpen
can be controlled entirely by the photon flux, with negligible background latter configumtion, where the growth growth at substrate temperatures belowpEYs C even for Si2H6 as the precursor 161.
2.2 Laser excitation of SiH4 and Si2s
and chemical mechanisms
energy is. Si2H6 = SiH2 + SiH4 yielding again the radical SiH . However, other pathways are alsopossible [15] and, in fact, electronically excited SiH, for example,wasobsavedupon~FlaPerirradiation[13].TherearenoexpaimentalresultsconcemingtherelativeconcentrdtionsofSiH.SiH andSM3radicalsobtainedbyphotofragmeneationandreactio~inthegasphaseandtheirdirectconuibution to the forma%on of the solid network by subsequent surface reactions. As shown before, the channels with the lowest activation energy lead to the SiH,mdical in the case of ir laser pyrolysis and
process leads to clusters with inFor film deposition, homogeneous cleationisselectedinmostiranduvlaserdepositionexperiments.Asexpec~thiscriti~pressureislowerforSi H6asthe precursor [ ls].l’his similarity indicates that the higher silanes are mainly responsible for film formation at thes ul?ace in the cases of CO laser&H4 as well as ArF laser- Si H deposition. Recent measuremen of sticking probabilities for silicon hydrides un& r uhv dosmg conditions showed that 2dPnlane and uisilane are roughly 1f? times more reactive than silye on a cleanSi(lll)surfacebetween1~Cand~C[18].0nahydrogen-coveredSi(lll)surfacebothmolec~lesarel0 times more reactive than SiH [19]. If the growing film behaves in a similar way we have to assume that of the stable species reaching the surface thefii gher silanes are mainly responsible for film growth. This &position mechanism implies a higher growth when starting with Si2H6 as the precursor as observed experimentally [ 131.At surface temperatures between 200°C and 4tXl°C!employed in typical a-Si:H deposition experiments the film contains between about 30% and4% hydrogen, respectively. Therefore, the higher silanes adsorbed at the surface are further decomposed. The formation of Si-Si bonds and dissociation of Si-H bonds de nds mainly on the surface temperature and results in &sorption of hydrogen from the surface. These surface reactions fe eadina to a three- dimensional solid network are not the rate limitinn steps in the deposition Process r9.101. E(eV)
Laser
a-
Gas phase chemistry (hv,?) Surface chemistry (?)
/-Tzq
Laser Gas phase chemistry (hv,T) Surface chemistry (hv,T) 1
Substrate
1
Fig. 1 Scheme of the parallel and perpendicular con& gurationsforlaserprocessing.Theinductionofphotochemical “hv” and photothermal”T processes in the gas phase and at the surface is indicated.
Fig. 2 Schematic representation of the ir and uv bands of SiH4andSi2HgandC021aserandArFlaserexcitation.
492
PETERHess
2.3 Film composition and properties The irand uv deposition experiments indicate a correlation between the hydrogen content of the a-Si:H film and the surface temperature. As shown in Fig. 3, the results obtained by ir deposition agree very well, especially at higher surface temperatures [9,10], whereas uv deposition yields a much higher hydrogen content for the highest and a much lower hydrogen content for the lowest surface temperatures investigated 1131.
These polymeric structures with high hydrogen content can only be reabxed in a film with less than 25% hydrogen if a discontinuous distribution of hydrogen bonding occurs in the network, as can be seen in Fig. 5. In fact, a transmission electron mic~~hrevealeda~l~~~-~s~~f~a~~~~at~~C[ll].Inthisstructuretheislandsprobably consist of a netwcxk with relatively low hydrogen concentration, while the regions in between may contain high concentrations of hydrogen and even polysilanes, removing stress and defects. The deposited films were further characterixed by their optical and electrical properdes, which are of interest for photovolt& and other applications of the material [9,10,13,15]. The optical band gaps found for a-Si:H films deposited by the laser-SiH techniquevarybetween1.5and2.2eV, asshowninFig.6[9,10].Thebandgapvariationforftisgrownby the ?r Flase&$$ process, however, is much smaller, as can be seen in Fig. 6 [13, lS].This is reasonable, because the hydrogen conten o uv deposited films is higher at the highest and lower at the lowest substrate temperatures studied. The optical gap obviously increases with the hydrogen content of the film and its variation depends on the change in the hydrogen concentration. Another interesting film property studied for ir and uv laser deposition is the temperature dependence of the photoconductivity and dark conductivtty. Very good agreementis obtainedbetween the different ir measurements [9, lo]. However, the values measured for the photoconductivity and dark conductivity in the uv experiments at comparable deposition rates are 1-2 orders of ma itude higher [ 131.The ir results agree much better with the uv results obtained at a deposition rate a factor of 4 higher [ 1P1. This clearly demonstrates a pronounced dependence of the film properties on the deposition rate, which should be studied in more detail.
Substrate
temperature
T,(Z)
Fig. 3 Hydrogen content of a-Si:H films versus substrate temperature for ir deposition (-*-[9];- [lo]) and uv deposition( --- [ 133).
Fig. 4 FTIR transmission spectra of films grown at substrate temperatures of 250°C, 300°C. 35O’C and 4OOoc [ll].
493
IR and UV laser - induced chemical vapor deposition
SiH2
SiH
I I I I
a-5 :H
20%
40%
k& I I
2.4
SiHL
se
I
I n
I
I
I I
I I L
I e 8
s
1
80%
60%
2.0 -
Ef ; y
1.8-
0”
1.6-
H - Concentration
1
3 dimensional
_I_
I
2d
, Id
Substrate
I
Fig. 5 Chemistry in the silicon-hydrogen system. The
dimensionality of bonding is shown. The useful a-Si:H films is indicated.
region
Of
temperature
T,(X)
Fig. 6 Optical gap of a-Si:H films versus substrate temperature for rr deposition ( -*-[9];-[lo]) and uv deposition ( --- [ 131).
2.4 Conclusions Considezable progress hasbeenachievedin recent years concerning the fundamental understanding of ir and uv laser depo-
sition of a-Si:H films from silicon hydrides as precursors. The chemical mechanism is still not completely understood.In particular, more information is needed on the surface reactions leading to film growth. As discussed in detail, accurate controlofthe~ti~pocessispossiblewithlasers.Crucial&positionparameterssuchasthesurfacetemperatureshouldbe measured with higher accuracy. As shown in [20], an in situ measurement of this quantity is possible with an error off 1K. Other critical parameters such as the gas flow and pressure are already controlled very carefully in most experiments. Thea-SizHfdmsdepositedwithacw COZlaserandS~H4astheprecursorpossessmanysimilaritieswithfrlmsgrown witha pulsed ArF laser employing Si$Ie as the precursor. The chemical, optical and electrical film properties indicate, however, distinct differences which may not be due to inaccuracies in the determination of crucial deposition parameters. These differences probably originate from different chemical mechanisms developing from the two precursors SiH4 and Si a”er6: Theinfluenceofthekineticsofgarphaseandsurfacechemistryonthegrowthprocessandfilmpropertiesisnotwellun stood. This is one of the main challenges of laser processing technology to the chemistThe goal in a-Si:H deposition is the growthofaSi-Halloyinaone-stepprocess.Hydrtosaturatedanglingafl~g~~~d~ release strain in the amorphous network. The bonding configuration of hydrogen should be preferably SiH bonding to minimize defects in the three dimensional network. The hydrogen content should be somewhere between 320% depending on the application of the material, as shown in Fig. 5. This picture summarizes the chemistry of the silicon-hydrogen system. Several laser CVD processes have been developed in recent years that allow the deposition of material at higher substrate temperatures with properties needed for electronic applications. The reason for the differences in the material grown by the rX+ser-SiH andtheKrFlaser-Si2Hsprocessesisnotknown.Eitherthedifferentprecursorsorwavelengthsorbothmay be responsible.?lhe parallel laser deposrtion configuration allows excellent control of the deposition process and is ideally suited to mechanistic studies. A better understanding of the chemical pathways involved and their dependence on the deposition parameters will be a prerequisite for the production of high quality materials optimized for specific applications.
3. Laser deposition of chromium films 3.1 Overview Afterthefmtpaperin1981 [21l,showingthatuvlaserphotodepositionofrefractorymetalssuchasFe,CrandWispossible by dissociation of the metal carbonyls, a series of papers studied chromium deposition, due to the interest in hi h-purity metal films for both optical and microelectronic applications, such as coatings or mask repair or the production of metallic gates, contacts and interco~ects. Deposition was studied with substrates such as quartz or silicon held either parallel or perpendicular to the excimer laser beam f22.231. Only for normal incidence were adherent metallic films observe parallel illumination produced black particulate films. Therefore, the subsequent experiments were restricted to the perpendicular configuration. ItwasshownthatthehighestdepositionratescouldbeachievedbyaddingabuffergastotheprecursorCr(CO) withavapor pressureofabout0.2torratnxvntemperature[24].Acomparisonofdifferentexcimerlaserwavelengths(308nm.~8nm and 193mn) yielded the highest wtb rates for KrF laser radiation at 248 run [231.Pulsed tunable dye laser radiation was also employed to study the wave!? ngth dependence of deposition in the range 284-344 nm [251,however, the KrF laser has been used in most of the investigations published so far. Contradictory results were obtained for chromium deposition from chromium hexacarbonyl with 248 nm radiation in normal incidence for the dependence on deposition parameters and for film purity. According to [23] the film thickness and
494
kIER
HESS
thedepositionratewereafunctionof thenumberof laserpulsesonly, whereasin [261adccrease of film thickness on increasing repetition rate was found for a given number of laser pulses. All deposited chromium films were contaminated with carbon and oxygen. However, the chromium content reported varied between 92%. with 7% oxygen and about 1% carbon [22], and 50% with an oxygen to carbon (O/C) ratio decreasing with laser intensity [261. A considerably lower chromium content of 30%~38%and 70%-62% oxygen with no carbon was obtained for deposition with a cw frequency-doubled Ar+ laser at 257 nm [27]. The oxygen concentration of c 1 ppm in the buffer gas used was obviously sufficient tooxidize the growing film completely to Cr 03. The contamination of such films was studied in detail in [28]. Different mechanisms have been suggested for oxygen an3 carbon incorporation in the case of pulsed KrF laser deposition. These processes are direct gas phase multiphoton dissociation of CO molecules and dissociative chemisorption of CO at the growing metal sit, leading to oxygen and carbon atoms [23]. Another possibility is incomplete photolysis of adsorbed carbonyl, resY ting in CO incorporation into the condensed film [291. 3.2 Laser excitation of a(m)6 and chemical mechanism The uv spectrum of Cr(C0) exhibits two intense lines at 280 nm and 225 nm as shown in Fig. 7. Excitation of these electronic states corresponds& metal-to-&and charge transfer, whereby an electron is promoted from a molecular orbital localized on the chromium atom to an antibonding ti molecular orbital localized mainly on the carbonyl ligand [30]. There areseveralpathwaysby whichCratomscanbeformed, involvingtwo-and three-photonexcitationat248nm with SeVphoton energy, as indicated in Fig. 8. For completedissocuuion of Cr(CO)6 into Cr and 6 CO an energy of 6.49 eV and thus two photons are needed. Two distinct dissociation processes have been identified [31]. The first is direct multiphoton excitation of Cr(C0) to the dissociative continuum and the second is sequential absorption of photons involving intermediates Cr(C0) kr example is the loss of two ligands by single-photon absorption, leading to the intermediate Cr(C0) . The sequen&l process is extremely sensitive to buffer gas pressure while the pt$ir ocess is pressure independent [31?.The absorption coefficient at 248 run is relatively high with a value of 5.6 x lo- cm , and therefore an efficient dissociation of carbonyls is achieved at high laser fluences. Despite this fact, the contribution of chromium atoms to film growth may be E kV)
?
---
7-
. 11
Cr *
1
I_ c-
cr*
_A_! -t-
.-
Ar'
_____
-_------ Cr --CrlCOl 3
II
2
co2
Fig. 7 Schematic representation of the ir and uv+bands oAg$fyi6 and their eXCitatiOn with C02, Ar and
/
m m
m
1
’
Fig. 8 Energy situation for excitationof Cr(C0) with KrF laser radiation. Multiphoton excitation eo Cr atoms and Cr+ ions is indicated. D&o&tion of Cr(CO), (x= 5,6) by a second photon is shown.
relatively small. The addition reaction of Cr(CO), (x = 2-5) with CO is very fast and occurs within an order of magnitude of thegaskmeticcollisionrate [32].‘Iheassumptionofasmallinfluenceofchromiumatomsonthe&~sitionrateissupported by recent experiments using CO instead of a rare gas as the buffer gas, as shown in Fig. 9 [33]. In the.semeasurements the transmission of a He-Ne laser beam was employed for in situ detection of the growth rate as described m [34]. This allowed the separation of the nucleation time from the main phase of fdm growth. The nucleation time increased by less than a factor of two when the CO partial pressure was changed from zero to 200 torr in the deposition gas and the decrease of the deposition rate was v small (see Fig.9). This finding indicates that only chromium atoms and unsaturated chromium carbonyls located within“r a ew mean free path lengths from the surface have a chance to reach the surface before rccombmation. These highly active species may be responsible for nucleation, but cannot explain the measured pwth rate. This points to an important contribution of more stable carbonyl molecules and possibly clusters to the depositton process. Thuswould also imply that most of the CO groups are removed at the surface either by photolysis or by pyrolysis. The dependenceof the deposition rate on laser fluence, pulse repetitionrate, buffer gas pressure, gas flow rate and substrate temperature was studied for filmgrowth on the entrance window [34] and the exit window 1351of thedeposition cell. These results suggest that the kinetics is dominated by gas phase processes under the conditions employed. A simple gas phase diffusion model gives a reasonable description of most measured dependences in terms of a diffusion length WI. Thus, the surface reactions responsible for film growth, which may proceed predominantly during laser tlluminatron, were not rate limiting.
495
IR and UV laser - induced chemical vapor deposition
3.3 Film composition and properties The influence of buffer gases and pressureon the chemical composition of chromium films was investigated in detail by
X-ray photoelectron spectroscopy [36]. Films deposited using argon or helium as the buffer revealed a typical Cr concentration of about 60% and C and 0 contents of about 20% each, as shown in Fig. 10. In these films oxygen was bonded pmdominantyasCr203andcarbonwasbondedasCr3C2.Asecondcarboncompoundwasfound,whichmightbeinsertedCO. The O/C ratio, however, may vary with deposition conditions [34]. For hydrogen as the buffer gas the highest Cr content of 72% was obtained, connected with a reduction of the carbon concentration by about 105%1361.Figure 11 summarizes the results obtained for the composition of Cr(O,C) films. WithpulsedKrFlaserradiationafilmsizeinthecm2rangecanbeeasilyobtainedandthedepositionratemayapproach 100 A/s [37]. The film profde can be varied between hill-like shapes at low buffer gas pressures (about 80 mbar), mesa-like shapes at medium (200 mbar) and vulcanc&ke structures at high buffer gas pressures (800 mbar). It has been possible to improve the electrical conductivity of the films considerably. The value reported in [26J was 500 times less than that measured for bulk chromium. Improved results obtained recently were only 40 times less than the value for the bulk 1371.
3.4 Conclusions The deposition of Cr(0.c) fihns using Cr(CO)6 as the precursor has attracted increasing interest. In most experiments cw
frequency doubled Ar+laser radiationat 257 nm or pulsed KrF laser radiation at 248 nm was employed in the perpendicular ses.TheCrconcentrationinfilmsdepositedat257 con$uration.ThehighestCrcontentwasachievedwithexcimerlaser nm wrth cw laser radiation varied typically between 20% and 40% [!Y 81 (see Fig. 11). Forpulseduvlaserdepositionat248nmthefollowingscenariocanbeextractedfiom therecentdata.Thekineticsismainly determined by the diffusion of active species to the surface. Depending on the deposition conditions and the stage of film growth these can be Cr atoms, unsaturated or saturated cabonyls. However, larger carbonyl molecules such as polykemel chromium carbonyl complexes and clusters cannot be excluded for certain conditions as direct film precursors. Asaconsequence,thelasetpulseswillinducedecarbonylationpredominandyin tbeadsorbedandchemisorbedphase. This final deca&onylation effect, however. is not quantitative. Dissociation of CO molecules occurs on the surface, presumably by dissociativechemisor@on through a parallelx-bonded chemisorbed state as already detected on a Cr( 110) surface 1391. This impurity incorporahon process can easily explain why often an O/C ratio of 1 is found. The presence of other reachve molecules, such as oxygen, in the deposition gas may change the O/C ratio as described in [271.Oxygen not only oxidizes the chromium film to Cr 03, but also removes carbon, probably in the form of CO and /or CO .In a similar way the effect of hydrogen as the buff& gas could be the hydrogenation of carbon compounds or intermedia%s on the surface, leading to a reduction in the carbon content of the film, as shown in Fig. 12. This chemical method of diminishing impurities by a reducing agent seems to be more promising than heating during deposition, as indicated by the lower Cr content at higher substrate temperatures [36]. The importance of photodissociation processes at the surface is demonstrated by the ripples observed for pulsed and c w laser sition of pure refractory metals, can only be achieved on the basis of a better Cr deposition. The final goal, namely the microscopic understandmg of the surfaceTc emistry and photo processes involved. The original ideafordeposition of pure chromium films from Cr(CO)6 with uv lasers was the selective photodissociation of the weak metal ligand bond and quantitative desorption of the volatile stable CO ligand from the surface. This simple concept probably fails because the strong CO bond dissociates in an autocatalytic process on the growing film. The highly reactive0 andC atoms form Cr203 andCr39, which are incovted into the growing network. Only by a selective transformation into volatile compounds can this mcotporation of the unpurities be avoided. Inclusion of CO ligands seems to be a minor problem.
6-
C
50
100
150
CO partial pressure (mbor)
Fig.9DependenceofgrowthrateofaCr(O,C)filmon the CC$rarM pressure for a KrF laser fluence of 72 mJ/cm and a repetition rate of 20 Hz [33].
200
Sputter
time (h)
Fig. 10 Composition of Cr(O,C) films as determined by X-ray photoelectron spectroscopy as a function of sputter time [36].
F’ETER HESS
496
3
/
ml-
0' pulsed KrF
/
60 40 -
/*
/*
-Q-O-
cw A@
20 -
I 1980
1985
I I
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I 1990
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Fig. 11 Chromium content of Cr(0.C) tilms deposited by pulp KrF laser radiation [22,26,34.361 and by cw Ar laser radiation at 257 nm 127.28.381.
Fig. 12 ComparisionoftheKPSClslineofaCr(O,C) film deposited in (a) helium and (b) hydrogen after 2h sputtertime[36l.~esatelliteneat287eVmaybedue to CO incotporation, whereas the intense component at 283.2 eV can be attributed to the carbidic form.
Financialsupportofthis~kbytheGermanMinistryofResearchandTechnology(B~undercontractNo. and the Fonds der Chemischen Industrie is gratefully acknowledged.
13N53638
References [ll 121 131 [41 [5l @I
181 [91 [lOI 1111 r121 [131 1141 r.153
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IR and UV laser - induced chemical vapor deposition
U61 r171 WI WI PO1 Pll WI [=I WI ml WI ml WI WI r301 1311 1321 [331 1341 [35l [3fxl 1371 [3gl r391
491
M. Muraharaand K. Toyota, in Laser Processing and Diagnostics, B. Bauerle,Ed., Springer-Verlag, Berlin (1984). p 252 G. Moue and M. Suzuki. Chem. Phys. Lett. 122 (1985) 361 S.M. Gates, C.M. Greenlief, D.B. Beach and’RR. Kunz, Chem. Phys. L&t. 114 (1989) 505 S.M. Gates, B.A. Scott, D.B. Beach, R. Imbihl and J.E. Demuth.J. Vat. Sci. Technol. A, i (1987) 628 K. Hesch, P. Hess, H. Getzmannand C. Schmidt, Appl. Surf. Sci. s (1989) 81 DJ. Ehrlich, R.M. Osgood Jr. and T.F. Deutsch,J. Electrochem. Sot. lZ&(1981) 2039 R. Solar&i. P.K. Boyer and G.J. Collins, Appl. Phys. Lett. 41(1982) 1048 D.K. Flynn, J.I. Steinfeld and D.S. Sethi,J. Appl. Phys. 2 (1986) 3914 R. Solar&i, P.K. Boyer, J.E. Mahanand GJ. Collins, Appl. Phys. Lea. 3 (1981) 572 T.M. Mayer. GJ. Fisanick and T.S. Eichelberger,J. Appl. Phys. 3 (1982) 8462 H. Yokoyama, F. Uesugi, S. Kishida and K. Washio, Appl. Phys. A, 32 (1985) 25 N.S. Gluck. GJ. Wolga, C.E. Bartosch,W.Ho and Z. Ying, J. Appl. Phys. 6L (1987) 998 K.A. Singmaster,F.A. Houle and RJ. Wilson, Appl. Phys. Lett. a (1988) 1048 P.J.Love,R.T.Loda,P.R.LaRoe,A.K.GreenandV.Rehn,Mater.Res.Soc.Symp.Proc.Vol.2e(1984)101 H.B. Gray and N.A. Beach, J. Am. Chem. Sot. 85 (1%3) 2922 G.W. Tyndall and R.L. Jackson,J. Chem. Phys. 89 (1988) 1364 E. Weitz, J. Phys. Chem. X (1987) 3945 R. Nowak, L. Konstantinovand P. Hess, Mater. Res. Sot. Symp. Proc. Vol. 12e (1989)
L. Konstantinov,R. Nowak and P. Hess, Appl. Phys. A 4i! (1988) 171 R. Nowak, L. Konstantinovand P. Hess, Appl. Surf. Sci. 3 (1989) 177 R. Nowak and P. Hess, Appl. Surf. Sci.. to be published R. Nowak. Dissertation,Universityof Heidelberg(1989) K.A. Singmaster,F.A. Houle and RJ. Wilson, Mater.Res. Sot. Symp. Pnx. Vol. l3I (1989) 469 N.D. ShiM and TE. Ma&y, Phys. Rev. Lett.53 (1984) 2481
0584 - 8539l90
1990.
$3.00
+ o.OLl
0 1990 Pergamon Press plc
IR AND UV LASER-INDUCED CHEMICAL VAPOR DEPOSITION: CHEMICAL MECHANISM FOR a-Si:H AND Cr (0,C) FILM FORMATION
Peter Hess Institute of Physical Chemistry, University of Heidelberg, Im Neuenheimer Feld 253, D-6900 Heidelberg, F.R.G.
Abstract The characteristic features of laser-induced chemical vapor deposition in the parallel and perpendicular laser beam /surface configurations are discussed. Low temperature chemical processing with directed and spatially local&d energy deposition in the system is investigated. Results obtained for the deposition of hytigenated amorphous silicon (a-Si:H) films in the andSi H~~~precursorsampresented.Asasecondexample, parallelconfigurationemployingC02andKrFlasersandSiH the growth of oxygen- andcarbon-containin chromium fdms%Jr(0.d from chromium hexacarbonyl as the precursor using cw and pulsed uv lasers is discussed. The ca emical pathways leading to film formation am investigated in detail.
1. General astwcts of laser-induced chemical vaDor detwsition 1.1. Introduction Laser-induced chemical vapor deposition (LICVD) is a rapidly expanding field. This is mainly due to the unique new possibilities in materials processing and microelectronics fabrication. There are several books available treating the subject in the form of monographs with emphasis on the fundamental understanding of deposition andetchingprocesses [ 11,[2] or in the form of review articles written by experts including also a discussion of the potential and advances in microfabrication [3], 141. Although the laser chemical processing technology with applications in the semiconductor industry is growing rapidly, there is no satisfactory understanding of the fundamental chemical processes involved. The reason is the complexity of the laser-driven chemical processes. Therefore, further technological advances depend on a better understanding of the spectroscopy, reaction dynamics and surface chemistry in the system under investigation. In this review the chemical aspects of laser CVD are discussed in detail. This includes laser excitation of the precursor molecule,primaryandsecondaryreactionsindu~eitherdirectly byphotonsorby heatinginthegasphaseandatthesurfaceand the crucial processes leading to film formation. After some generalremarks concerning laser chemical processing two examples will bepresented. The fmt example is laser deposmon of hydrogenated amorphous silicon (a-SkII). This material has many apphcations such asphotovoltaic solar cells, thin film transistors for liquid crystal displays, photoreceptors for electrophotography and laser printing,image sensors etc.. The second example treated in detail is uv laser deposition of chromium films from chromium hexacarbonyl. Photochemical deposition of refractory metals such asCr. MOand W from their hexacarbonyls has been studied carefully in mcent yeam and detailed results are available for chromium.
1.2 Laser excitation and chemical processing The efficiency of laser radiation to initiate and sustain a deposition process is determined by the interaction of the laser photons with the ambient molecules of the deposition system. Therefore, the laser wavelength and the absorption cross section of the ambient molecules must coincide. ‘Ibis is achieved by selecting appropriate percursor molecules for the type of laser troscopic and thermodynamic properties are to be employed. The choice of the precursor is very important because the responsible for the efficient activation of the depositron process and the cTee emical nature of the molecules influences the chemistry induced and thus the chemical and physical properties of the deposited film. For excitation of electronic states, laser light in the visible or ultraviolet spectral regionis used. Especially, uv laser light with photon energies comparable or larger than chemical binding energies is employed for photoexcitation and photodissociation. Laser-induced reactions due to electronic excitation occur either because of the enhanced reactivity of the precursor moleculesin the excited states or because of the reactive photofragments and radicalsgenerated by photodissociation. The efficiency of energy transferandrelaxationprocesseswill determinewhich partof the energyacquiredby the system from 489
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PETERHESS
photoexcitation can be us+ for chemical modification. Pulsed uv lasers may be best suited for direct photochemical processmg wrth mmrmal parhhonmgof the absorbedenergy into other degrees of freedom by energy exchange processes and negligible transport of energy away from the excited region. In theinfraredspectralregionespeciallyC0 lasersareemployedforvibrationalexcitation. Formanychemicalsystems, the ‘3 er than the energy of a single ir photon. Therefore, either multiple- hoton exactivation barriers for reactions arc much hrg citation is necessary to promote a reaction or the laser radiation is used as a spatially localized heat source to m dp uce a thermally activated chemical reaction. In the latter case the absorbed energy is thermal&xl locally. The spatial extent of the region that undergoes thermally enhanced processes depends on the spatial and temporal excitation characteristics and the thermal diffusivities of the deposition system.
1.3 Laser beam / surface configurations Two completely different relative arrangements of the exciting laser beam and substrate surface are possible: one where the beam travels parallel to the suface without touching it and one where the beam hits the surface, as shown in Fig. 1. In the parallel conf&uration the laser radiation directly excites or heats only the gas above the surface, and therefore the gas and surface temperatures may be controlled more or less independently. Thus, the processes occurring in tlte gas phase, namely the primary and secondary chemistry, and the processes taking place at the surface determining the film structure and composition may be synergistic. The substrate temperature is of crucial importance for the film properties, and therefore should be optimized according to the surface chemistry yielding the best solid material. No film growth is observed without laser radiation. Besides the photoexcitation and resulting reactions in the laser beam, the transportof activated species and reaction products from the localized light beam to the surface must be considered in a theoretical description of the processes involved. An additional transport process to be included might be surface diffusion of active species to a localized reactive spot Intheperpendicularlaserbeam/surfaceconfi former case at least nucleation will be mainly detemrined by the gas properties. However,energy deposition into the system may change during the stage of deposition, if film absorption takes place. In the second case of pure substrate absorption, the gas phase supplies the precursor molecules by diffusion for the photochemical and/or the photothermal processes initiated by the laser light at the surface. If both the gas and the solid phase absorb photons a series of processes must be considered. Not only excited molecules but also fragments or clusters may move by volume diffusion to the localized reactive spot on thesurface. Speciesadsorbedatthesurfaceareexposedtothelaserrsdiationandthusmayundergofurtherphotochemlcalor photothermal modification. This complex situation may arise also in cases where the substrate did not absorb initially. The ability to confine heatin to small regions or driving nonequilibrium chemistry on a highly localized scale in a heterogeneous system is one of f e attractive features of laser chemical processing.
1.4 Deposition conditions for laser processing An importantfeature of laserradiation is the high photon flux available in the ir to uv spectral region. Efficient laserexcitation can be used to enhance the sition rate or to lower the deposition temperature. Direct bond breaking with uv lasers, d”fp-power ir and uv lasers and spatial control of the heated zone allow low temperature promultiphoton dissociation with hig cessing with minimized heating of the whole deposition system. This low temperature capability of laser processing is imof fragile substrates and device portant from the point of view of energy consumption and is essential for mssing structures or temperature sensitive materials. Deposition at low temperatures often leads to a metastable amorphous network, where many properties of the deposited material vary with preparation conditions. The low surface mobdity of adatoms can lead to voids or result in columnar growth In this case a high-quality fdm can only be obtained by a subsequent thermal treatment. However, photoinduced pyrolytic or photolytic reactions of precursors may also lead to epitaxial growth at reduced temperatures, if suitable single crystals are selected as substrates [5]. Thefundamentalaspectsoflaser-inducedchemisayareverypoorlyun&rstoodinmostdepositionsystems.Thepictureofa confined photodissociation of precursor molecules in the gas phase and/or adsorbed state leading to direct deposition without additional reaction steps is too simple in most cases. Secondary pbotochemical and thermal reactions normally occur in the gas phase and at the substrate surface. In this complex chemical environment the chemical species responsible for nucleation may be different from those governing the main film growth process. At low surface temperatures incomplete drive-off of additional species adsorbed or chemisorbed at the surface often results in impurity incorporation. Thus, not only the morphology, but also the chemical composition of the film may change considerably in low-temperature deposition. If the surface of the growing film possesses catalytic activity the surface chemistry may be extensive and difficult to control. Even the strongest chemical bonds can be broken by heterogeneouscatalysisand only an understanding of the reaction processes on a microscopic level will allow an optimized processing. During nucleation and growth of the film, the optical and thermal properties of the deposition system normally change considerably. These changes are due to the formation of new chemical species with different spectroscopic properties and the growth of the film. The will affect the processes going on in the d sition system either directly, by new photochemical reactions occurring in x e gas phase or at the surface, or indirectly,eGr changes in the transient local heating. Due .tocontinuouschangesinthe~tionconditionsthelaser-drivenreactingsystem mayneverreach steady state. Infacfarigorous descriptiandthecomplicateddynamicalprocessesoccurrin betweenlaserinitiationofthechainofchemicalreactionsand the final building up of the condensed network of the film L not been achieved for any deposition system. Therefore., simplifying assumptions are always used in the development of models for specific systems.
491
R and UV laser - induced chemical vapor deposition
2. Laser deposition of hydrogenated amorphous silicon 2.1 Introduction ~&e~ckmica! vapor&position of hydfogenatedamorphous silicon (a-Si:H) hasbeen studied with irand uvlasers &c~lllumtnationasrevlewedin[l]and[3].Thebestcharactenzationofthedeposltiod
film
beenachievedfortheparallelconfiguration.Inthefollowing,recentresultsw~bepresentedonlyforthis properties tzpen
can be controlled entirely by the photon flux, with negligible background latter configumtion, where the growth growth at substrate temperatures belowpEYs C even for Si2H6 as the precursor 161.
2.2 Laser excitation of SiH4 and Si2s
and chemical mechanisms
energy is. Si2H6 = SiH2 + SiH4 yielding again the radical SiH . However, other pathways are alsopossible [15] and, in fact, electronically excited SiH, for example,wasobsavedupon~FlaPerirradiation[13].TherearenoexpaimentalresultsconcemingtherelativeconcentrdtionsofSiH.SiH andSM3radicalsobtainedbyphotofragmeneationandreactio~inthegasphaseandtheirdirectconuibution to the forma%on of the solid network by subsequent surface reactions. As shown before, the channels with the lowest activation energy lead to the SiH,mdical in the case of ir laser pyrolysis and
process leads to clusters with inFor film deposition, homogeneous cleationisselectedinmostiranduvlaserdepositionexperiments.Asexpec~thiscriti~pressureislowerforSi H6asthe precursor [ ls].l’his similarity indicates that the higher silanes are mainly responsible for film formation at thes ul?ace in the cases of CO laser&H4 as well as ArF laser- Si H deposition. Recent measuremen of sticking probabilities for silicon hydrides un& r uhv dosmg conditions showed that 2dPnlane and uisilane are roughly 1f? times more reactive than silye on a cleanSi(lll)surfacebetween1~Cand~C[18].0nahydrogen-coveredSi(lll)surfacebothmolec~lesarel0 times more reactive than SiH [19]. If the growing film behaves in a similar way we have to assume that of the stable species reaching the surface thefii gher silanes are mainly responsible for film growth. This &position mechanism implies a higher growth when starting with Si2H6 as the precursor as observed experimentally [ 131.At surface temperatures between 200°C and 4tXl°C!employed in typical a-Si:H deposition experiments the film contains between about 30% and4% hydrogen, respectively. Therefore, the higher silanes adsorbed at the surface are further decomposed. The formation of Si-Si bonds and dissociation of Si-H bonds de nds mainly on the surface temperature and results in &sorption of hydrogen from the surface. These surface reactions fe eadina to a three- dimensional solid network are not the rate limitinn steps in the deposition Process r9.101. E(eV)
Laser
a-
Gas phase chemistry (hv,?) Surface chemistry (?)
/-Tzq
Laser Gas phase chemistry (hv,T) Surface chemistry (hv,T) 1
Substrate
1
Fig. 1 Scheme of the parallel and perpendicular con& gurationsforlaserprocessing.Theinductionofphotochemical “hv” and photothermal”T processes in the gas phase and at the surface is indicated.
Fig. 2 Schematic representation of the ir and uv bands of SiH4andSi2HgandC021aserandArFlaserexcitation.
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PETERHess
2.3 Film composition and properties The irand uv deposition experiments indicate a correlation between the hydrogen content of the a-Si:H film and the surface temperature. As shown in Fig. 3, the results obtained by ir deposition agree very well, especially at higher surface temperatures [9,10], whereas uv deposition yields a much higher hydrogen content for the highest and a much lower hydrogen content for the lowest surface temperatures investigated 1131.
These polymeric structures with high hydrogen content can only be reabxed in a film with less than 25% hydrogen if a discontinuous distribution of hydrogen bonding occurs in the network, as can be seen in Fig. 5. In fact, a transmission electron mic~~hrevealeda~l~~~-~s~~f~a~~~~at~~C[ll].Inthisstructuretheislandsprobably consist of a netwcxk with relatively low hydrogen concentration, while the regions in between may contain high concentrations of hydrogen and even polysilanes, removing stress and defects. The deposited films were further characterixed by their optical and electrical properdes, which are of interest for photovolt& and other applications of the material [9,10,13,15]. The optical band gaps found for a-Si:H films deposited by the laser-SiH techniquevarybetween1.5and2.2eV, asshowninFig.6[9,10].Thebandgapvariationforftisgrownby the ?r Flase&$$ process, however, is much smaller, as can be seen in Fig. 6 [13, lS].This is reasonable, because the hydrogen conten o uv deposited films is higher at the highest and lower at the lowest substrate temperatures studied. The optical gap obviously increases with the hydrogen content of the film and its variation depends on the change in the hydrogen concentration. Another interesting film property studied for ir and uv laser deposition is the temperature dependence of the photoconductivity and dark conductivtty. Very good agreementis obtainedbetween the different ir measurements [9, lo]. However, the values measured for the photoconductivity and dark conductivity in the uv experiments at comparable deposition rates are 1-2 orders of ma itude higher [ 131.The ir results agree much better with the uv results obtained at a deposition rate a factor of 4 higher [ 1P1. This clearly demonstrates a pronounced dependence of the film properties on the deposition rate, which should be studied in more detail.
Substrate
temperature
T,(Z)
Fig. 3 Hydrogen content of a-Si:H films versus substrate temperature for ir deposition (-*-[9];- [lo]) and uv deposition( --- [ 133).
Fig. 4 FTIR transmission spectra of films grown at substrate temperatures of 250°C, 300°C. 35O’C and 4OOoc [ll].
493
IR and UV laser - induced chemical vapor deposition
SiH2
SiH
I I I I
a-5 :H
20%
40%
k& I I
2.4
SiHL
se
I
I n
I
I
I I
I I L
I e 8
s
1
80%
60%
2.0 -
Ef ; y
1.8-
0”
1.6-
H - Concentration
1
3 dimensional
_I_
I
2d
, Id
Substrate
I
Fig. 5 Chemistry in the silicon-hydrogen system. The
dimensionality of bonding is shown. The useful a-Si:H films is indicated.
region
Of
temperature
T,(X)
Fig. 6 Optical gap of a-Si:H films versus substrate temperature for rr deposition ( -*-[9];-[lo]) and uv deposition ( --- [ 131).
2.4 Conclusions Considezable progress hasbeenachievedin recent years concerning the fundamental understanding of ir and uv laser depo-
sition of a-Si:H films from silicon hydrides as precursors. The chemical mechanism is still not completely understood.In particular, more information is needed on the surface reactions leading to film growth. As discussed in detail, accurate controlofthe~ti~pocessispossiblewithlasers.Crucial&positionparameterssuchasthesurfacetemperatureshouldbe measured with higher accuracy. As shown in [20], an in situ measurement of this quantity is possible with an error off 1K. Other critical parameters such as the gas flow and pressure are already controlled very carefully in most experiments. Thea-SizHfdmsdepositedwithacw COZlaserandS~H4astheprecursorpossessmanysimilaritieswithfrlmsgrown witha pulsed ArF laser employing Si$Ie as the precursor. The chemical, optical and electrical film properties indicate, however, distinct differences which may not be due to inaccuracies in the determination of crucial deposition parameters. These differences probably originate from different chemical mechanisms developing from the two precursors SiH4 and Si a”er6: Theinfluenceofthekineticsofgarphaseandsurfacechemistryonthegrowthprocessandfilmpropertiesisnotwellun stood. This is one of the main challenges of laser processing technology to the chemistThe goal in a-Si:H deposition is the growthofaSi-Halloyinaone-stepprocess.Hydrtosaturatedanglingafl~g~~~d~ release strain in the amorphous network. The bonding configuration of hydrogen should be preferably SiH bonding to minimize defects in the three dimensional network. The hydrogen content should be somewhere between 320% depending on the application of the material, as shown in Fig. 5. This picture summarizes the chemistry of the silicon-hydrogen system. Several laser CVD processes have been developed in recent years that allow the deposition of material at higher substrate temperatures with properties needed for electronic applications. The reason for the differences in the material grown by the rX+ser-SiH andtheKrFlaser-Si2Hsprocessesisnotknown.Eitherthedifferentprecursorsorwavelengthsorbothmay be responsible.?lhe parallel laser deposrtion configuration allows excellent control of the deposition process and is ideally suited to mechanistic studies. A better understanding of the chemical pathways involved and their dependence on the deposition parameters will be a prerequisite for the production of high quality materials optimized for specific applications.
3. Laser deposition of chromium films 3.1 Overview Afterthefmtpaperin1981 [21l,showingthatuvlaserphotodepositionofrefractorymetalssuchasFe,CrandWispossible by dissociation of the metal carbonyls, a series of papers studied chromium deposition, due to the interest in hi h-purity metal films for both optical and microelectronic applications, such as coatings or mask repair or the production of metallic gates, contacts and interco~ects. Deposition was studied with substrates such as quartz or silicon held either parallel or perpendicular to the excimer laser beam f22.231. Only for normal incidence were adherent metallic films observe parallel illumination produced black particulate films. Therefore, the subsequent experiments were restricted to the perpendicular configuration. ItwasshownthatthehighestdepositionratescouldbeachievedbyaddingabuffergastotheprecursorCr(CO) withavapor pressureofabout0.2torratnxvntemperature[24].Acomparisonofdifferentexcimerlaserwavelengths(308nm.~8nm and 193mn) yielded the highest wtb rates for KrF laser radiation at 248 run [231.Pulsed tunable dye laser radiation was also employed to study the wave!? ngth dependence of deposition in the range 284-344 nm [251,however, the KrF laser has been used in most of the investigations published so far. Contradictory results were obtained for chromium deposition from chromium hexacarbonyl with 248 nm radiation in normal incidence for the dependence on deposition parameters and for film purity. According to [23] the film thickness and
494
kIER
HESS
thedepositionratewereafunctionof thenumberof laserpulsesonly, whereasin [261adccrease of film thickness on increasing repetition rate was found for a given number of laser pulses. All deposited chromium films were contaminated with carbon and oxygen. However, the chromium content reported varied between 92%. with 7% oxygen and about 1% carbon [22], and 50% with an oxygen to carbon (O/C) ratio decreasing with laser intensity [261. A considerably lower chromium content of 30%~38%and 70%-62% oxygen with no carbon was obtained for deposition with a cw frequency-doubled Ar+ laser at 257 nm [27]. The oxygen concentration of c 1 ppm in the buffer gas used was obviously sufficient tooxidize the growing film completely to Cr 03. The contamination of such films was studied in detail in [28]. Different mechanisms have been suggested for oxygen an3 carbon incorporation in the case of pulsed KrF laser deposition. These processes are direct gas phase multiphoton dissociation of CO molecules and dissociative chemisorption of CO at the growing metal sit, leading to oxygen and carbon atoms [23]. Another possibility is incomplete photolysis of adsorbed carbonyl, resY ting in CO incorporation into the condensed film [291. 3.2 Laser excitation of a(m)6 and chemical mechanism The uv spectrum of Cr(C0) exhibits two intense lines at 280 nm and 225 nm as shown in Fig. 7. Excitation of these electronic states corresponds& metal-to-&and charge transfer, whereby an electron is promoted from a molecular orbital localized on the chromium atom to an antibonding ti molecular orbital localized mainly on the carbonyl ligand [30]. There areseveralpathwaysby whichCratomscanbeformed, involvingtwo-and three-photonexcitationat248nm with SeVphoton energy, as indicated in Fig. 8. For completedissocuuion of Cr(CO)6 into Cr and 6 CO an energy of 6.49 eV and thus two photons are needed. Two distinct dissociation processes have been identified [31]. The first is direct multiphoton excitation of Cr(C0) to the dissociative continuum and the second is sequential absorption of photons involving intermediates Cr(C0) kr example is the loss of two ligands by single-photon absorption, leading to the intermediate Cr(C0) . The sequen&l process is extremely sensitive to buffer gas pressure while the pt$ir ocess is pressure independent [31?.The absorption coefficient at 248 run is relatively high with a value of 5.6 x lo- cm , and therefore an efficient dissociation of carbonyls is achieved at high laser fluences. Despite this fact, the contribution of chromium atoms to film growth may be E kV)
?
---
7-
. 11
Cr *
1
I_ c-
cr*
_A_! -t-
.-
Ar'
_____
-_------ Cr --CrlCOl 3
II
2
co2
Fig. 7 Schematic representation of the ir and uv+bands oAg$fyi6 and their eXCitatiOn with C02, Ar and
/
m m
m
1
’
Fig. 8 Energy situation for excitationof Cr(C0) with KrF laser radiation. Multiphoton excitation eo Cr atoms and Cr+ ions is indicated. D&o&tion of Cr(CO), (x= 5,6) by a second photon is shown.
relatively small. The addition reaction of Cr(CO), (x = 2-5) with CO is very fast and occurs within an order of magnitude of thegaskmeticcollisionrate [32].‘Iheassumptionofasmallinfluenceofchromiumatomsonthe&~sitionrateissupported by recent experiments using CO instead of a rare gas as the buffer gas, as shown in Fig. 9 [33]. In the.semeasurements the transmission of a He-Ne laser beam was employed for in situ detection of the growth rate as described m [34]. This allowed the separation of the nucleation time from the main phase of fdm growth. The nucleation time increased by less than a factor of two when the CO partial pressure was changed from zero to 200 torr in the deposition gas and the decrease of the deposition rate was v small (see Fig.9). This finding indicates that only chromium atoms and unsaturated chromium carbonyls located within“r a ew mean free path lengths from the surface have a chance to reach the surface before rccombmation. These highly active species may be responsible for nucleation, but cannot explain the measured pwth rate. This points to an important contribution of more stable carbonyl molecules and possibly clusters to the depositton process. Thuswould also imply that most of the CO groups are removed at the surface either by photolysis or by pyrolysis. The dependenceof the deposition rate on laser fluence, pulse repetitionrate, buffer gas pressure, gas flow rate and substrate temperature was studied for filmgrowth on the entrance window [34] and the exit window 1351of thedeposition cell. These results suggest that the kinetics is dominated by gas phase processes under the conditions employed. A simple gas phase diffusion model gives a reasonable description of most measured dependences in terms of a diffusion length WI. Thus, the surface reactions responsible for film growth, which may proceed predominantly during laser tlluminatron, were not rate limiting.
495
IR and UV laser - induced chemical vapor deposition
3.3 Film composition and properties The influence of buffer gases and pressureon the chemical composition of chromium films was investigated in detail by
X-ray photoelectron spectroscopy [36]. Films deposited using argon or helium as the buffer revealed a typical Cr concentration of about 60% and C and 0 contents of about 20% each, as shown in Fig. 10. In these films oxygen was bonded pmdominantyasCr203andcarbonwasbondedasCr3C2.Asecondcarboncompoundwasfound,whichmightbeinsertedCO. The O/C ratio, however, may vary with deposition conditions [34]. For hydrogen as the buffer gas the highest Cr content of 72% was obtained, connected with a reduction of the carbon concentration by about 105%1361.Figure 11 summarizes the results obtained for the composition of Cr(O,C) films. WithpulsedKrFlaserradiationafilmsizeinthecm2rangecanbeeasilyobtainedandthedepositionratemayapproach 100 A/s [37]. The film profde can be varied between hill-like shapes at low buffer gas pressures (about 80 mbar), mesa-like shapes at medium (200 mbar) and vulcanc&ke structures at high buffer gas pressures (800 mbar). It has been possible to improve the electrical conductivity of the films considerably. The value reported in [26J was 500 times less than that measured for bulk chromium. Improved results obtained recently were only 40 times less than the value for the bulk 1371.
3.4 Conclusions The deposition of Cr(0.c) fihns using Cr(CO)6 as the precursor has attracted increasing interest. In most experiments cw
frequency doubled Ar+laser radiationat 257 nm or pulsed KrF laser radiation at 248 nm was employed in the perpendicular ses.TheCrconcentrationinfilmsdepositedat257 con$uration.ThehighestCrcontentwasachievedwithexcimerlaser nm wrth cw laser radiation varied typically between 20% and 40% [!Y 81 (see Fig. 11). Forpulseduvlaserdepositionat248nmthefollowingscenariocanbeextractedfiom therecentdata.Thekineticsismainly determined by the diffusion of active species to the surface. Depending on the deposition conditions and the stage of film growth these can be Cr atoms, unsaturated or saturated cabonyls. However, larger carbonyl molecules such as polykemel chromium carbonyl complexes and clusters cannot be excluded for certain conditions as direct film precursors. Asaconsequence,thelasetpulseswillinducedecarbonylationpredominandyin tbeadsorbedandchemisorbedphase. This final deca&onylation effect, however. is not quantitative. Dissociation of CO molecules occurs on the surface, presumably by dissociativechemisor@on through a parallelx-bonded chemisorbed state as already detected on a Cr( 110) surface 1391. This impurity incorporahon process can easily explain why often an O/C ratio of 1 is found. The presence of other reachve molecules, such as oxygen, in the deposition gas may change the O/C ratio as described in [271.Oxygen not only oxidizes the chromium film to Cr 03, but also removes carbon, probably in the form of CO and /or CO .In a similar way the effect of hydrogen as the buff& gas could be the hydrogenation of carbon compounds or intermedia%s on the surface, leading to a reduction in the carbon content of the film, as shown in Fig. 12. This chemical method of diminishing impurities by a reducing agent seems to be more promising than heating during deposition, as indicated by the lower Cr content at higher substrate temperatures [36]. The importance of photodissociation processes at the surface is demonstrated by the ripples observed for pulsed and c w laser sition of pure refractory metals, can only be achieved on the basis of a better Cr deposition. The final goal, namely the microscopic understandmg of the surfaceTc emistry and photo processes involved. The original ideafordeposition of pure chromium films from Cr(CO)6 with uv lasers was the selective photodissociation of the weak metal ligand bond and quantitative desorption of the volatile stable CO ligand from the surface. This simple concept probably fails because the strong CO bond dissociates in an autocatalytic process on the growing film. The highly reactive0 andC atoms form Cr203 andCr39, which are incovted into the growing network. Only by a selective transformation into volatile compounds can this mcotporation of the unpurities be avoided. Inclusion of CO ligands seems to be a minor problem.
6-
C
50
100
150
CO partial pressure (mbor)
Fig.9DependenceofgrowthrateofaCr(O,C)filmon the CC$rarM pressure for a KrF laser fluence of 72 mJ/cm and a repetition rate of 20 Hz [33].
200
Sputter
time (h)
Fig. 10 Composition of Cr(O,C) films as determined by X-ray photoelectron spectroscopy as a function of sputter time [36].
F’ETER HESS
496
3
/
ml-
0' pulsed KrF
/
60 40 -
/*
/*
-Q-O-
cw A@
20 -
I 1980
1985
I I
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I I
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Fig. 11 Chromium content of Cr(0.C) tilms deposited by pulp KrF laser radiation [22,26,34.361 and by cw Ar laser radiation at 257 nm 127.28.381.
Fig. 12 ComparisionoftheKPSClslineofaCr(O,C) film deposited in (a) helium and (b) hydrogen after 2h sputtertime[36l.~esatelliteneat287eVmaybedue to CO incotporation, whereas the intense component at 283.2 eV can be attributed to the carbidic form.
Financialsupportofthis~kbytheGermanMinistryofResearchandTechnology(B~undercontractNo. and the Fonds der Chemischen Industrie is gratefully acknowledged.
13N53638
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