low-temperature deposition of films from tetrakis(dimethylamido)titanium and ammonia

Ž .Thin Solid Films 323 1998 10–17

Low-temperature deposition of films fromž /tetrakis dimethylamido titanium and ammonia

Alan Berry ), Robert Mowery, Noel H. Turner, Larry Seitzman 1, Derren Dunn 2,Harold Ladouceur

Chemistry DiÕision, NaÕal Research Laboratory, Washington, DC 20375, USA

Received 17 June 1997; accepted 16 October 1997

Abstract

Ž .We report the chemical vapor deposition CVD and characterization of thin films grown from the reaction ofŽ . Ž Ž . .tetrakis dimethylamido titanium Ti NMe , TDMAT with NH at 423 K. These films, deposited on a variety of substrates, are2 4 3

amorphous except for a small number of embedded nanocrystallites. The films exhibit peeling at thicknesses greater than ca. 200 nm onall substrates and show no visible evidence of etching after exposure to concentrated hydrochloric or nitric acid for 5 min. X-ray

Ž .photoelectron spectroscopy XPS results yield a NrTi ratio of 0.89 after normalizing the data to that of a TiN standard. The OrTi ratioŽ .varies from 0.26 to 0.66, and the C content is less than 1 at.%. Transmission Fourier transform infrared FTIR spectroscopy of films

deposited on Si gives spectra consistent with the presence of N–H bonds. q 1998 Elsevier Science S.A. All rights reserved.

Ž . Ž .Keywords: Tetrakis dimethylamido titanium TDMAT ; Chemical vapor deposition; Nanocrystallite

1. Introduction

A considerable amount of work has been reported onthe deposition of protective coatings on various substrates,

w xmostly at temperatures of 473 K and above 1 . Transitionmetal nitrides in particular have been studied extensivelyand found to have good wear- and corrosion-resistant

w xproperties 2 . Physical vapor-deposition techniques suchas sputtering and ion implantation produce high qualityfilms at low temperatures but are limited to line-of-sight

Ž .processes. Chemical vapor deposition CVD , on the otherhand, typically produces good conformal coverage of ir-regularly shaped objects but at higher temperatures.

There have been numerous reports of the CVD of metaln i t r id e f i lm s f r o m th e r e a c t io n s o f

Ž . Ž .tetrakis dialkylamido metal IV compounds with ammo-w xnia at temperatures between 473–723 K 3–12 . In com-

) Corresponding author.1 Caterpillar, Technical Center E, PO Box 1875, Peoria, IL 61656-1875,

USA.2 Department of Materials Science and Engineering, University of

Virginia, Charlottesville, VA 22903-2442, USA.

parison to similar reactions of metal halides with ammonia,metal amides offer the advantages of lower depositiontemperatures, the absence of residual amounts of halogenthat contribute to corrosion problems, and the lack ofparasitic acid–base reactions that can produce adducts andsalts in reactor cold zones. Conversely, the sensitivity ofthe amides to oxygen and water and the presence oforganic groups can lead to increased levels of oxygen andcarbon in the films. Moreover, rapid reactions of amidessuch as TDMAT with ammonia, even at ambient tempera-

w xtures 13 , can result in high growth rates, incompletereactions, and significant amounts of hydrogen in filmsw x12 .

Fix et al. have reported the deposition of nitride filmsfrom transition metal amide–ammonia reactions at low

Ž .substrate temperatures 423–723 K in a horizontal, hot-w xwall, atmospheric pressure reactor 12 . Titanium nitride

films were smooth, nonporous, and pinhole free withthicknesses up to 200 nm and had NrTi ratios of 1.05–1.15with less than 2–3 at.% carbon and oxygen. Although thehydrogen content was approximately 33 at.%, the filmswere not air-sensitive and were stable to common inor-ganic acids. More recent work describing films deposited

0040-6090r98r$19.00 q 1998 Elsevier Science S.A. All rights reserved.Ž .PII S0040-6090 97 00963-2

( )A. Berry et al.rThin Solid Films 323 1998 10–17 11

at 463 K in a vertical, cold-wall configuration reportedw xOrTi ratios of 0.3–0.5 4 .

In this paper, we describe in detail the deposition andcharacterization of films from the reaction of TDMATwith ammonia at 423 K and atmospheric pressure. Ourobjective was to evaluate these films as corrosion-resistantcoatings for substrates that could not tolerate temperaturesabove 423 K.

2. Experimental

A cold-wall, inverted, vertical CVD reactor made froma quick-connect flange on a Pyrex-to-metal seal as shownin Fig. 1 was used in this work. The reactants weredelivered via concentric inlet tubes to minimize premature

Ž .reaction. TDMAT Strem is a liquid at ambient tempera-ture and was stored in a Pyrex bubbler wrapped withheating tape; the bubbler was connected to the reactor bystainless steel flexible tubing also wrapped with heatingtape. In a typical experiment the bubbler temperature wasset at 333–338 K, where the vapor pressure of the TD-

ŽMAT is 133 Pa M. Houston, The Schumacher Corp.,.personal communication , and the transport line was heated

Žto 378 K. Flow rates for the carrier gases He Research. ŽGrade, Air Products and N Ultrapure Carrier Grade, Air2

. Ž .Products and the reactant NH ULSI, Matheson were3

monitored by mass flow controllers; the controller usedwith NH was equipped with a Kalrez seal. Typical flow3

Ž .rates were 3.6 cubic centimeters per minute ccm NH , 283

ccm of carrier gas mixed with NH , and 54 ccm of carrier3

gas through the bubbler. Under these conditions, 2.6=y3 y1 Ž10 mmol min of TDMAT mol fractions7.4=y4 . y1 y1 Ž10 and 1.5=10 mmol min of NH mol fraction3

y2 .s4.2=10 were transported to the reactor. Substrateswere mounted on a copper block attached to a 3r8-inchstainless steel tube inserted through a compression fitting

Fig. 1. Diagram of experimental CVD apparatus.

in the flange. This allowed the block to be positioned atdifferent distances from the precursor inlet tubes; in mostexperiments this was 2.5 cm. A cartridge heater wasinserted through the tube into the block and controlled by aVariac. Temperatures were measured by a Chromel–Alumel thermocouple attached to the block.

Silicon substrates were first washed in detergent andrinsed in triply distilled water. This was followed by an

Ž .etching process consisting of a immersion in an ultra-sonic bath for 5 min in a 1:1 mixture of 30% H O and2 2

Ž .concentrated H SO , b rinsing in triply distilled water,2 4Ž . Ž .c sonication in 50% HF, d rinsing in triply distilled

Ž .water, and e drying under a stream of nitrogen. Metalsubstrates were washed in detergent, rinsed in triply dis-tilled water, and sonicated separately in methanol andtricholoroethylene before drying under nitrogen.

Ž 2 .After cleaning, the substrates approximately 1 cmwere mounted on the copper block, the reactor assembled,and evacuated. The gas delivery lines, reactor surface, andblock were evacuated to approximately 27 Pa and heatedto temperatures above those used in the deposition experi-ments before pressurizing with the carrier gas.

XPS data were collected on a Surface Science Instru-ments Model 100-03 system operating at a base pressureof approximately 4=10y7 Pa and using Al K a radia-1

tion. An electron beam charge neutralizer with a nominalpotential of about 2 V was used to minimize chargingeffects. Depth profiles of the samples were obtained with a

Ždifferentially pumped sputter gun Physical Electronic. y6Model 04-303 . The main chamber pressure was 3=10

Pa, and 3 keV Arq rastered over an area of approximatelyŽ .2 mm were used. Scanning electron microscopy SEM

was carried out using a Hitachi S-800 microscope. Trans-mission electron micrographs and selected area electrondiffraction patterns were obtained using an Hitachi H9000high resolution transmission electron microscope. Fixed-incidence-angle X-ray diffraction measurements were madeusing a Rigaku DrMax B diffractometer. Film thicknesseswere measured with an Alpha-Step 250 profilometer.Transmission infrared spectra were collected on a Nicoletmodel 760 FTIR spectrometer using both HgCdTerB andInSb detectors.

3. Results

Ž .The reaction between Ti NMe and NH produced2 4 3

films containing Ti, N, O, and C on Si, Cu, Al, and Tasubstrates at 423 K and 1 atm pressure. For thicknessesless than approximately 200 nm, the films exhibited goodadherence, as measured by the Scotch tape test, and wereconformal to the substrate surface. A variation in filmcolor was attributed to interference patterns caused bydifferences in thickness rather than composition. Exposureto a drop of concentrated hydrochloric or nitric acid for 5

( )A. Berry et al.rThin Solid Films 323 1998 10–1712

min at ambient conditions produced no visible etching ofthe film. Growth rates were rapid and varied from 10 to 30nm miny1 depending on the distance of the substrate fromthe precursor inlet tubes and the ratio of the NH and3

TDMAT concentrations. A distance of 2.5 cm provided acompromise between the loss of intermediate species dueto diffusion at longer substrate–TDMAT inlet distancesand increased reaction rates due to higher temperatures atthe end of the precursor tube when it was closer to theheated substrate. Ammonia concentrations higher than 5vol% in the reactor resulted in the formation of greater

amounts of an unanalyzed brown solid at the end of theTDMAT precursor tube and a decrease in the depositionrate.

Fig. 2a shows a scanning electron micrograph of thesurface of a film deposited on Si. At the maximum resolu-

Ž .tion of the micrograph 50 nm , the surface was feature-less. Films thicker than approximately 200 nm were dullgray and exhibited cracking and peeling on all substratessimilar to that shown in Fig. 2b and c for Si and Cu.

Fixed-incidence angle X-ray diffraction showed thefilms to be amorphous. Cross-sectional transmission elec-

Ž . Ž . Ž .Fig. 2. Scanning electron micrographs of a thin film on Si; b thick film on Si; c thick film on Cu.


Fig. 3. Cross-sectional TEM of film on Si.

Ž .tron microscopy TEM results, seen in Fig. 3, revealed anamorphous matrix containing a small number of nanocrys-tals about 8 nm in size. Fig. 4 is a selected-area electrondiffraction pattern of the area marked in Fig. 3. Two

Fig. 4. Selected-area electron diffraction pattern of area marked in Fig. 3.

primary reflections are seen; the inner is due to diffractionŽ . Ž .from 200 planes, and the outer from 220 planes of fcc

crystallites, which is consistent with the presence of TiNw x14 .

XPS survey spectra of two CVD samples deposited at423 K and one at 623 K and a standard sample of TiNobtained from magnetron sputtered Ti in N and character-2

w xized as described in Ref. 15 are shown in Fig. 5. All weresputtered with Arq for 60 s at an approximate rate of 0.1nm sy1 to remove surface contaminants. Peaks corre-

Ž . Žsponding to those for Ti 5s 562–563 ev , 2p 460–4611r2. Ž . Ž . Ž .eV , 2p 455 eV , 3s 60 eV , and 3p 34–35 eV ; N 1s3r2

Ž . Ž . w x396 eV ; and O 1s 530 eV are observed 16 . The depthprofile results for these samples are shown in Fig. 6.Atomic percent ratios were calculated after each 6-s sput-tering interval by measuring the area under each peakusing a program supplied with the instrument; this normal-ized the areas for differences in collection time and spec-trometer transmission factor, and applied the appropriate

w xScofield sensitivity factors 17 . For Ti, only the area ofthe 2p band centered at 455 eV was measured. This3r2

was complicated by the proximity of the Ti 2p peak,1r2

which resulted in an asymmetrically-shaped band as shownin Fig. 7. In all samples, carbon decreased to less than 1at.% after 12 s of sputtering. In the standard and thesample deposited at 623 K, the oxygen content was alsoreduced to less than 1 at.% with additional sputtering for


Ž . Ž . Ž .Fig. 5. XPS survey spectra of a standard sample of TiN prepared from magnetron sputtered Ti in N on Ni substrate; b film a1 deposited at 423 K; c2Ž .film a2 deposited at 423 K; d film deposited at 623 K.

36 and 30 s, respectively. The nitrogen and titanium levelsreached reasonably constant values after the initial 30 s ofsputtering.

Transmission FTIR spectra of films deposited on Si at423 and 623 K were recorded using a clean Si substrate asa reference, and the data are plotted as absorbance values

Ž .Fig. 6. XPS depth profile results for a standard sample of TiN preparedŽ .from magnetron sputtered Ti in N on Ni substrate; b film a1 deposited2

Ž . Ž .at 1508C; c film a2 deposited at 1508C; d film deposited at 3508C.

from 3500 to 1750 cmy1 in Fig. 8. Intensities have notbeen corrected for differences in film thicknesses.

4. Discussion

Smooth, adherent films approximately 200 nm thickcontaining Ti, N, O, and C have been deposited on sub-strates by the reaction of TDMAT with NH at 423 K and3

1 atm pressure. The absence of features in micrographs atthe 50-nm scale indicates dense, fine grain films, thatexhibit considerable cracking and peeling at thicknessesgreater than about 200 nm. The breakup pattern on Si

Ž .indicates the films are in tensile stress Fig. 2b , whereasthe buckling observed on Cu suggests the stress is com-

Ž . w xpressive Fig. 2c 18 . Initially, it was believed that thestress in our films might be due to differences in the

Ž . Žcoefficients of linear expansion a for the substrates Si,Ty6 y1 y6 y1. w x3=10 8C and Cu, 16.6=10 8C 19 and what

was expected to be the major component of the film, TiNŽ y6 y1. w x8.1=10 8C 20 . However, thicker films exhibitedpeeling during growth when the film-substrate system wasin thermal equilibrium, suggesting that a mismatch ofthermal properties, whatever the film composition, was nota major factor.

Intrinsic film stress is associated with a number offactors, including the degree of crystallinity, or lack thereof,and the presence of impurities. Previous reports of rapidthermal annealing at temperatures in excess of 973 K ofTiN films deposited at or below 723 K using TDMAT andammonia describe a significant decrease in tensile stress

w xthat might be due largely to recrystallization 21 . Similarconclusions have been reached in Raman spectroscopy


Ž . Ž . q Ž .Fig. 7. Ti 2p and 2p XPS peaks from films deposited at a 423 K before sputtering; b 423 K after 6 s Ar sputtering; c 623 K before sputtering;1r2 3r2Ž . qd 3508C after 6 s Ar sputtering.

studies on the thermally-induced crystallization of amor-phous TiO films deposited by the sol–gel process, which2

suggest that stress relaxation occurs during crystallizationw xin these systems 22 . The implication is that amorphous

films have a greater amount of intrinsic stress than thosethat are crystalline. Similarly, we propose that the largelyamorphous nature of materials obtained from low-tempera-ture deposition in this reaction system is a major contribu-tor to their intrinsic film stress. Fixed-incidence-angleX-ray diffraction data show no crystallinity in the filmsdeposited at 423 K. Cross-sectional TEM results are con-

Fig. 8. Transmission FTIR spectra of films deposited at 423 K and 623 K.

sistent with the X-ray data and indicate the material is anamorphous matrix containing a small amount of nanocrys-talline material whose electron diffraction pattern is consis-tent with the fcc pattern for TiN. However, the smallcrystallite sizes precluded measurement of lattice dis-tances, thus making it impossible to exclude the presenceof interstitial oxygen in the lattice. The predominantlyamorphous character of the film is not surprising in viewof the rapid rate of reaction between TDMAT and NH 3

and the low substrate temperature. The large second-orderrate constant of 1.2=10y16 cm3 moly1 sy1 at 297 K for

Ž Ž .. w xthe gas-phase transamination reaction Eq. 1 13 and theapparent high sticking

Ti NMe qNH ™Ti NH NMe qHNMe 1Ž . Ž . Ž . Ž .2 3 2 2 24 3

w xcoefficient of the Ti intermediates 10 , contribute to a highgrowth rate, which often favors the formation of amor-phous material. The low substrate temperature further hin-ders completion of the reaction to form TiN and surfacediffusion of deposited species, both of which would favorcrystallinity. Similar types of films have been reported in

w xthe 423–723 K range 12 .Impurities in a film may also contribute to intrinsic

w xstress 18 . XPS data show a significant amount of O infilms deposited at 423 K compared to those grown at 623K. Oxygen levels in the former range from 10 to 23 at.%for an OrTi ratio of 0.26 to 0.66, whereas in the latter theO is less than 1 at.% as seen in the depth profiles in Fig. 6.The source of O is believed to be small amounts of H O2

andror O in the carrier gases and possibly NH since we2 3

chose not to incur the additional expense of gas purifiers inour system. Evidence of impurities in the carrier gasescomes from the observation of a solid deposit in the dip


tube of the TDMAT bubbler. The maximum H O levels in2

the He and NH are given as 0.2 molar ppm and 2.03Ž .ppm v , respectively, and the corresponding O levels are2

Ž .1.0 molar ppm and 0.5 ppm v . Since H O and O in the2 2

He passed through the bubbler are effectively scrubbed byreacting with TDMAT and the O-containing products, e.g.,TiO , are expected to be nonvolatile, only the He flowx

mixed with NH and the NH are contributing H O and3 3 2

O to the system. The amounts of H O and O calculated2 2 2

to be transported to the reactor from the He and NH 3

under these conditions are 5.2=10y7 mmol miny1 and1.2=10y6 mmol miny1, respectively. Together this cor-responds to 0.07% of the flow of 2.6=10y3 mmol miny1

of TDMAT. Since transition metal amides show a roughcorrelation of reactivity with pK for protic compoundsaw x23 , the dominant reaction might be expected to be that ofTDMAT with the more acidic H O rather than with NH .2 3

It is possible that the difference in reaction rates for H O2

and NH decreases with increasing temperature and that3

ammonolysis is favored over hydrolysis at higher tempera-tures in the presence of excess NH , although to our3

knowledge there is no kinetic data available to supportthis. A similar argument could be proposed for the effectof O in this system.2

XPS data for the remaining elements of Ti, N, and Cprovide additional information about the nature of the

Ž .films. The low levels -1 at.% of C found in samplesdeposited at 423 K indicate that most of the NMe groups2

have been replaced and few remain in the deposited mate-rial. Very weak peaks were observed at 285–286 eV fororganic and 282 eV for carbidic C, respectively. The N 1speak at 396 eV is consistent with that for a nitride speciesw x Ž .16 . In the Ti 2p region 455 eV of the sputtered3r2

samples shown in Fig. 7b and d, the spectra are diffuse,indicating the existence of Ti atoms in more than oneenvironment. Several species could give rise to this effect,including TiN or an oxynitride such as TiO N in thex y

Ž . Ž .nanocrystalline phase and Ti NH or TiO NH in thex x y

amorphous material. Application of curve-fitting tech-niques to these spectra would be difficult due to theinstrument resolution under which the data were obtainedand the spectral baseline removal method chosen. More-over, the rate at which the samples adsorb gases present inthe vacuum chamber would make results from longer datacollection times suspect.

In order to obtain meaningful NrTi ratios from the XPSdepth profiles of the CVD samples, we have comparedtheir values to that of a standard sample of TiN analyzedin the same way. We believe it is necessary to do this sincethe measured NrTi ratio of the standard sample is1.30r1.00 instead of the expected value of 1r1. This isprobably the result of uncertainties associated with integra-tion of the asymmetric Ti 2p peak, calculation of3r2

sensitivity factors, and preferential sputtering. Normaliza-tions of the NrTi values of the CVD samples were doneby dividing the measured values by 1.30, which reduced

them from 1.16 and 1.14 for the 423 K samples to 0.89and 0.88, respectively. The corresponding NrTi ratio forthe 623 K sample was reduced from 1.46 to 1.12.

Previous work has reported the high percentage of HŽ . w x)33 at.% in films grown at low temperatures 12 . Thetransmission infrared results from our samples confirm thepresence of H bonded to N in the films. The weak band at3240 cmy1 in material deposited at both temperatures isconsistent with an absorption due to N–H stretching vibra-

w xtions 24 . The exact nature of the absorbing species isuncertain, but it is likely that a mixture of NH and NH2

groups is present. Although the presence of only one bandŽ .suggests an imide Ti5NH , the possibility of a second,

weaker absorption due to a symmetric vibration involvingtwo N–H bonds cannot be ruled out. In precipitates iso-lated from reactions of metal amides with NH in hydro-3

carbon solutions at, or below, ambient temperatures, theobservation of two bands at ca. 3300 cmy1 is reported to

w xbe consistent with the presence of NH groups 25 . It isxŽ .further interesting to note that h -C H Ti NMe under-5 5 5 2 3

goes ammonolysis in toluene at 363 K during 16 h to giveŽ . Ž .good yields of h -C H Ti m -N , which contains no5 5 5 4 3 4

w xN–H bonds 26 . This suggests that removal of H atoms inour films is kinetically, rather than thermodynamically,controlled at these temperatures.

Infrared spectra of these films have also revealed thepresence of bands near 2000 cmy1. In the spectrum of thesample grown at 423 K, the band at 2023 cmy1 is ofcomparable intensity to that at 3240 cmy1. However, forthe sample deposited at 623 K, the band at 2011 cmy1 isconsiderably more intense than that at 3240 cmy1. A bandat 2000 cmy1 has been assigned to a terminal Ti–H stretch

wŽ . x Ž . w xfor the compound C H TiH m-H , 27 . thus raising5 5 2 2

the possibility that the bands at 2011 and 2023 cmy1

originated from Ti–H bonds in the sample. However, thespectrum of a film deposited at 623 K with ND also3

contains a band in this region and shows no evidence of aband in the 1400–1500 cmy1 region as might be expectedfor a Ti–D species based on calculations for a diatomicisotopic shift. Consequently, the bands at 2011 and 2023cmy1 remain unidentified at this time.

5. Conclusions

We have deposited thin films from the reaction ofTDMAT with NH at 423 K that are promising candidates3

for protective coatings on temperature-sensitive substrates.At thicknesses less than approximately 200 nm, the filmsare smooth, adherent, and resistant to attack by concen-trated hydrochloric and nitric acids. Thicker films exhibitcracking and peeling, which is believed to be the result ofintrinsic stress introduced primarily by the lack of crys-tallinity found in an amorphous matrix with small amountsof nanocrystallites. Oxygen levels of 10–23 at.% are be-


lieved to be the result of trace levels of H O and O in the2 2

carrier gases and NH used without additional purifiers.3

Although undesirable for some applications, it did notadversely affect the corrosion-resistant properties of thesefilms.

Acknowledgements

The authors gratefully acknowledge the Office of NavalResearch and the Naval Research Laboratory for financialsupport and the reviewer for many helpful comments.

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low-temperature deposition of films from tetrakis(dimethylamido)titanium and ammonia

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