Ss

HD

a

ARR1A

KSTbTSUS

1

tgttflmtpr7bhatocmbesb

0d

J. of Supercritical Fluids 66 (2012) 59– 65

Contents lists available at SciVerse ScienceDirect

The Journal of Supercritical Fluids

jou rn al h om epage: www.elsev ier .com/ locate /supf lu

olubility of titanium diisopropoxide bis(dipivaloylmethanate) complex inupercritical carbon dioxide and its effect on supercritical fluid deposition process

iroshi Uchida ∗, Kazuyuki Sekino, Yuma Hayakawa, Seiichiro Kodaepartment of Materials and Life Sciences, Sophia University, 7-1 Kioi-cho, Chiyoda-ku, Tokyo 102-8554, Japan

r t i c l e i n f o

rticle history:eceived 30 July 2011eceived in revised form9 December 2011ccepted 20 December 2011

eywords:

a b s t r a c t

The solubility of a titanium (Ti) complex, titanium diisopropoxide bis(dipivaloylmethanate) [Ti(Oi-Pr)2(dpm)2], in supercritical carbon dioxide (scCO2) fluid was measured under various fluid temperaturesand pressures to clarify its effect on supercritical fluid deposition (SCFD) for titanium dioxide (TiO2) films,by means of the extraction method with 2-propanol as the extracting solvent and UV–vis spectroscopicanalysis. The solubility which is expressed by molar ratio of Ti complex to CO2, �, decreased with thefluid temperature, Tf, from 40 to 60 ◦C, while it increased rapidly with the fluid pressure, Pf, from 6 to10 MPa and then increased gradually with P from 10 to 20 MPa. The behavior was analyzed by Chrastil’s

upercritical carbon dioxideitanium diisopropoxideis(dipivaloylmethanate)itanium dioxideolubility

f

equation model. The deposition rate of TiO2 films by SCFD was understood to be proportional to thedifference in the mass concentration of Ti complex, S, between in the bulk transport medium and vicinityof the substrate surface.

© 2011 Elsevier B.V. All rights reserved.

V–vis spectrumupercritical fluid deposition

. Introduction

Supercritical fluid deposition (SCFD) is an attractive techniqueo fabricate inorganic thin films applicable to constructing inte-rated circuits for electronic devices. This technique includes theransportation and the deposition processes in supercritical fluids,hat is, the dissolution of organometallic precursors in supercriticaluids to supply them to the substrate surface, being followed by theaterial deposition by chemical and/or physical processes to form

hin solid films on the substrate [1–23]. These processes typicallyroceed under supercritical carbon dioxide (scCO2) because of itselatively moderate critical temperature and pressure (31.1 ◦C and.38 MPa [24,25]) as well as the favorable solubility and diffusionehaviors of various organometallic species. Many researchersave noted SCFD because the above mentioned properties enablettractive material deposition with some advantageous charac-eristics, e.g., diverse material syntheses using various types ofrganometallic precursors, conformal deposition on small andomplicated geometry such as trenches and holes with sizes of sub-icrons, and chemical promotion of the film-deposition reactions

y additive agents such as oxidants/reductants and entrainers,

tc. SCFD could be a versatile film-deposition technique to resolveome problems inherent in traditional film-deposition techniquesased on gas- or liquid-phase reaction, such as chemical vapor

∗ Corresponding author. Tel.: +81 3 3238 3375; fax: +81 3 3238 3361.E-mail address: uchidah@sophia.ac.jp (H. Uchida).

896-8446/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.supflu.2011.12.023

deposition (CVD), physical vapor deposition (PVD) and chemicalsolution deposition (CSD).

In previous works by many researchers, thin films of vari-ous metals [1–13] and metal oxides [14–23] have been fabricatedby SCFD successfully. SCFD of metal films for electrodes of inte-grated circuits was reported first ahead to other materials. Forexample, SCFD of Cu films has been studied for its deposition mech-anism [2–7], effect of precursor species [2,3] and reductant addition[4–6], and possibilities of gap-filling and step-coverage on com-plicated surface features [2,5–7]. Other metals such as Ni [2,8,9]and Ru [5,10,11] have also been studied to achieve flat or con-formal film deposition by SCFD, as well as Pt, Pd, Au, Rh [12],Ag [13] and Co [8] for flat film deposition. On the other hand,SCFD for metal-oxide films recently developed for various appli-cations such as insulators, dielectric capacitors, and some otherswith electronic and/or optic functionalities. Aggressive studies forSCFD of SiO2 films, basic components of integrated circuits andmicro-electromechanical systems (MEMSs), have been conductedfor achieving low-temperature material synthesis [14] and high-level step coverage on complicated features of nano-eave andtrenches with high aspect ratios [15,16]. The large-area 3D capac-itors with metal-oxide-silicon (MOS) structure was fabricated ontrenched silicon wafer by SCFD of SiO2 and Cu in the latest report[16]. Thin films of some metal oxides, Al2O3, MnOx, RuOx, Y2O3

[17], HfO2, CeO2, Ta2O5, Nb2O5, Bi2O3, TiO2 [18], ZrO2 [17,18], andZnO [19], were fabricated by SCFD using a batch-type reactionapparatus and various organometallic precursors where somestudies achieved conformal step coverage on trenched substrates

6 rcritical Fluids 66 (2012) 59– 65

sdwtcomsSmfwvi

aasnstmmpcfpoTsomdcrasflssStci

c(mirpTPihewvRwmHrdts

O

O(CH3)3C

(CH3)3CO

OC(CH3)3

C(CH3)3

O

O

Ti

CH(CH )

CH(CH3)2

0 H. Uchida et al. / J. of Supe

uccessfully. Additionally, our group has reported SCFD of titaniumioxide (TiO2) films using a vertical flow-type reaction apparatus,hich resulted in the deposition of crystalline anatase TiO2 films at

he substrate temperature as low as 100 ◦C [20–22]. Conformal stepoverage was achieved on trenched substrate with feature sizesf approximately 1.0 �m. An advanced research focused on binaryetal oxides, such as Sr Ti O system, has also proceeded for con-

tructing high-K capacitor for dynamic random access memories byCFD [23]. Furthermore, the SCFD technique have achieved confor-al material deposition on other kind of substrates with complexed

eatures such as particles [26,27] and mesoporous materials [28],hich would contribute greatly to nanocomposite synthesis for

arious applications of catalytic, electronic, energy and medicalssues.

For almost all SCFD processes, organometallic compounds suchs metal complexes (e.g., ˇ-diketone and diene complexes, etc.)nd alkoxides were used as precursors because of their relevantolubilities for scCO2. In general, ionic salts, such as metal chloride,itrates, sulfates, carbonates and so on, are unfavorable for materialynthesis in scCO2 because they hardly dissolve in CO2 fluid dueo non-polarity of O C O molecules. Typically, nonpolar organic

olecules (e.g., aromatics, nonaromatic keto-/carboxylic acids, andany other molecules including biological/pharmaceutical com-

ounds) are soluble in scCO2 fluid, as well as organometallicompounds with relatively small polarity. A well-organized reviewor the solubility of solids in sub- and supercritical fluids wasublished by Skerget et al. [29], which reported various types ofrganometallic compounds usable as precursor sources for SCFD.he review also mentioned relevant correlation models of theolubilities in supercritical fluids, and then denoted that manyrganometallic compounds can follow an empirical density-basedodel proposed by Chrastil [30]. For example, solubilities for ˇ-

iketonate complexes of Cr, Co, Cu, Pt and Ru in scCO2 [31–35] wereorrelated to the Chrastil’s equation, which simply represents theelationship between the solubility of the solute in a certain solventnd the density of the solvent. The solubilities of the complexes incCO2 at a certain fluid temperature, Tf, generally increase with theuid pressure of scCO2, Pf, up to approximately 20 MPa and thenaturate, which is coincident well with the change in the fluid den-ity of scCO2, �f [22,23]. These findings are important for designingCFD process which includes the dissolution and precipitation ofhe organometallic compounds, as well as for designing some otherhemical processes based on supercritical fluids such as materialnfusion and metal etching based on complex extraction [36–43].

In the present work, the solubility of a titanium ˇ-diketonateomplex, titanium diisopropoxide bis(dipibaloylmethanate)Ti[OCH(CH3)2]2(C9H11O2)2, denoted as Ti(Oi-Pr)2(dpm)2), was

easured in scCO2 under various fluid conditions to evaluatets effect on SCFD of TiO2 films. Ti(Oi-Pr)2(dpm)2 is presentlyecognized as a major organometallic precursor for various SCFDrocess of TiO2 films, as well as a precursor for CVD processes ofiO2 and related multi-component oxides such as (Ba,Sr)TiO3 andb(Zr,Ti)O3 [44–47]. These materials are of great value for manyndustrial applications such as optical coatings, photocatalysts,igh-K dielectrics, ferro- and piezoelectric components in variouslectronic devices. The present authors revealed in the previousorks [22] that the deposition rate of TiO2 films by SCFD, Rdepo.,

aried by the fluid condition of scCO2, i.e., Tf and Pf. The lowerdepo. realized conformal step coverage with uniform film thicknesshereas higher Rdepo. caused choking of the inlet to prevent theaterial deposition on the bottom surface of the trench structure.ere, we recognized that the deposition rate would be closely

elated to the change in solubility of the organometallic precursoruring the film-deposition process in scCO2. Detailed data forhe solubility of Ti(Oi-Pr)2(dpm)2 in scCO2 are therefore requiredtrongly for estimating the deposition mechanism of TiO2 films

3 2

Fig. 1. Illustration of molecular structure of Ti(Oi-Pr)2(dpm)2 complex.

by SCFD, although it has never been reported in earlier works.The present work determines the solubility of Ti(Oi-Pr)2(dpm)2in scCO2 using a flow-type extraction apparatus under variousscCO2 conditions. These data would be helpful for many otherprocesses of material syntheses based on scCO2 such as RESS andsupercritical infusion, not only for SCFD.

2. Methods

2.1. Chemicals

Ti ˇ-diketonate complex, Ti(Oi-Pr)2(dpm)2, was obtained fromKojundo Chemical Laboratory Co. (Saitama, Japan) with nominalpurity of 99.99%. Fig. 1 shows a schematic molecular formulaof Ti(Oi-Pr)2(dpm)2. The complex was provided as solid at roomtemperature in the form of white powders with particle sizes of50–100 �m. It was used as-received without further purification.Spectroscopic grade 2-propanol obtained from Dojindo LaboratoryCo. (Kumamoto, Japan) was used as an extraction solvent. LiquidCO2 with nominal purity >99.5% obtained from Yoshida Koatsu Co.was used as the scCO2 source.

2.2. Experimental apparatus and procedure

A flow-type extraction apparatus was used for determining thesolubility of Ti(Oi-Pr)2(dpm)2 in scCO2 under various fluid condi-tions. Fig. 2 shows a schematic diagram of the apparatus used inthe present work. The apparatus consisted of three systems, i.e.,fluid delivery, dissolution and separation systems. The fluid deliv-ery system supplied liquid CO2 using a CO2 delivery pump (SCF-Get,JASCO Co.) to obtain scCO2 with various fluid conditions in sub-sequent systems. The dissolution system consisted of precursorstorage cells including cylindrical in-line filters with mesh sizesof 0.5 �m (Swagelok Co.) to keep the solid complex and allow thepassage of dissolved complex in scCO2. The separation system wasarranged to collect the complex dissolved in scCO2 by extractingusing 2-propanol which was supplied from a liquid delivery pump(PU-2086 plus, JASCO Co.) and then to separate the resulting extract,i.e., the complex dissolved in 2-propanol from the CO2 gas flowafter passing through a back-pressure regulating valve (SCF-Bpq,JASCO Co.). All the systems were connected with standard 1/8 inchstainless-steel tubes. The systems of fluid delivery and dissolutionwere heated by flexible resistance heaters to keep a desired fluidtemperature. The fluid pressure of CO2 was kept at a desired valueby controlling the back-pressure regulating valve.

The complex was placed in the precursor storage cell followedby purging all the system using gaseous CO2. The flow of CO2 fluidwith a desired temperature and pressure was led to a bypass tubein the dissolution system, and then switched to the precursor stor-

age cell to obtain saturated dissolution of the complex in scCO2.The CO2 fluid including the complex was subsequently mixed withthe flow of 2-propanol solvent to extract the complex in scCO2.The extract was collected after the separation system to determine

H. Uchida et al. / J. of Supercritical Fluids 66 (2012) 59– 65 61

CO2

Gas

Extract(Liquid)

CO2-deliverypump

Liquid-deliverypump

Separator

Back-pressurecontrolling valve

Solvent

Check valve

Fluid-delivery System

rati

Dissolution System

Precursor cell(Organometallic)

determ

tTtTso

o

FT

Sepa

Fig. 2. Flow-type extraction apparatus used for

he molar concentration of the complex in the extracted solution.he fluid temperature and pressure of CO2, Tf and Pf, were con-rolled at desired values in 40–60 ◦C and 6–20 MPa, respectively.he CO2 fluid was supplied using a liquid delivery pump at a con-tant flow rate of 0.5 cm3 min−1 to keep the saturated dissolution

f the complex in scCO2.

The solubility of Ti(Oi-Pr)2(dpm)2, expressed by the molar ratiof Ti(Oi-Pr)2(dpm)2 to CO2, �, was calculated from the amount

Wavelength / nm

200 300

0.50

1.50

0250 350

Abs

orba

nce

/

a

1.00

ig. 3. (a) Typical UV–vis spectrum of reference Ti(Oi-Pr)2(dpm)2/2-propanol solution wi(Oi-Pr)2(dpm)2 in 2-propanol solvent measured at wavelengths of 235 and 330 nm (eac

on System

ining the solubility of metal complex in scCO2.

of the complex included in the extracted solution and the CO2supplied in the apparatus. The amount of the Ti(Oi-Pr)2(dpm)2complex was calculated from the molar concentration of thecomplex in the extracted solution, which was determined bymeans of the calibration curve method using a UV–vis spectrom-

eter (MD-2018, JASCO Co.). Fig. 3 shows the UV–vis spectrumof a Ti(Oi-Pr)2(dpm)2/2-propanol solution with molar concen-tration of 5.00 × 10−5 mol dm−3 (in Fig. 3(a)), together with the

Concentration / 10-5 mol dm-3

2.00

0.50

1.50

00

330 nm235 nm

Abs

orba

nce

/

b

1.00

4.00

ith the molar concentration of 5.00 × 10−5 mol dm−3, and (b) Standard curves ofh plot; n = 4).

62 H. Uchida et al. / J. of Supercritical Fluids 66 (2012) 59– 65

200 300 400Wavenumber / nm

Abs

orba

nce

(Arb

. uni

t)

20 MPa15 MPa12 MPa10 MPa

9 MPa8 MPa7 MPa6 MPa

Faa

cFtaa[a(t3r

3

3

PpetttdmacˇTd

vmrPrao�no

10 20

0.5

1.0

0155

Fluid pressure, Pf / MPa

Mol

ar ra

tio, μ

/ 10

-2

40oC45oC50oC60oC

1.5

Fig. 5. Molar ratio of Ti(Oi-Pr)2(dpm)2 and CO2, �, as a function of CO2 fluid pressure,

ig. 4. UV–vis spectra of Ti(Oi-Pr)2(dpm)2 in extracted solution obtained from scCO2

t a constant temperature (Tf = 40 ◦C) and various pressures (Pf = 6, 7, 8, 9, 10, 12, 15nd 20 MPa).

alibration curves of Ti(Oi-Pr)2(dpm)2 in 2-propanol solvent (inig. 3(b)). The spectrum for Ti(Oi-Pr)2(dpm)2 in 2-propanol solu-ion possessed two absorption peaks at wavelengths of 235nd 330 nm, ascribed to the transition between d-orbitals in Titom and �–�* transition in ˇ-diketonate ligand, respectively43,46]. The molar absorption coefficients of Ti(Oi-Pr)2(dpm)2t 235 and 330 nm were calculated to be(2.0 ± 0.12) × 104 and1.0 ± 0.07) × 104 dm3 mol−1 cm−1, respectively, from the calibra-ion curves. In the present work, the absorbance measured at30 nm was mainly used for the calibration, though the sameesults were obtained from the measurement at 235 nm.

. Results and discussion

.1. Solubility of Ti(Oi-Pr)2(dpm)2 complex

Typical UV–vis spectra of the extracts containing Ti(Oi-r)2(dpm)2 obtained at a constant temperature and variousressures (Tf = 40 ◦C, Pf = 6–20 MPa) were shown in Fig. 4. All thextracts exhibited similar shapes of the spectrum with two absorp-ion peaks around 235 and 330 nm which were coincident withhat observed in the reference 2-propanol solution without scCO2reatment. No decomposition or hydrolysis occurred during theissolution in scCO2 fluid considering that no dissociated dpmolecule was detected by the UV absorption at around 280 nm. The

bsorbance at the absorption peaks increased with Pf gradually, asonfirmed in the measurement for other metal complexes such as-diketonate complexes of Cr, Co, Cu, Pt and Ru in scCO2 [31–35].he same behavior was confirmed for other extracts obtained atifferent temperatures, 45, 50 and 60 ◦C.

The solubilities of Ti(Oi-Pr)2(dpm)2 complex in scCO2 underarious fluid conditions (Tf = 40–60 ◦C, Pf = 6–20 MPa) were deter-ined from the UV–vis measurements. Fig. 5 shows the

elationship between the molar ratio, �, and the fluid pressure,f, with different fluid temperature, Tf. We recognize that theseesults were obtained under saturated conditions through somedditional experiments with different CO2 flow rates and amount

f complex loaded. The solubility of Ti(Oi-Pr)2(dpm)2 expressed by

was of the order of 10−3, which was slightly smaller than that ofaphthalene (10−2 to 10−1) [48] but significantly larger than thosef metal complexes of Cr, Co, Cu, and Pt (10−5 to 10−3) [31–35]. The

Pf , with various fluid temperatures, Tf . The � value with a standard deviation (n = 4)at the fluid condision of 60 ◦C and 8 MPa was (2.5 ± 0.2) × 10−4, while at the conditionof 40 ◦C and 20 MPa was (1.1 ± 0.04) × 10−2.

smallest value of solubility � with a standard deviation (n = 4) was(2.5 ± 0.2) × 10−4 at the fluid condition of 60 ◦C and 8 MPa, whilethe largest was (1.1 ± 0.04) × 10−2 at the condition of 40 ◦C and20 MPa. The � value increased rapidly with Pf from 6 to 10 MPa andthen increased gradually with Pf from 10 to 20 MPa at a constantTf ranging from 40 to 60 ◦C, similarly to other metal complexes.On the other hand, the � value decreased with Tf from 40 to 60 ◦Cunder a constant Pf ranging from 6 to 20 MPa. The temperaturedependence of � value was partially coincident with some organicmolecules such as naphthalene and anthracene [48]. However, itwas different from other metal complexes [31–35] where typicallythe solubility increased with the fluid temperature. The differencewould be derived from the different affinity between the solute,Ti(Oi-Pr)2(dpm)2, and scCO2 fluid, compared with the cases of othermetal complexes or organic molecules.

For further discussion, the empirical density-based model pro-posed by Chrastil [29,30] was applied for analyzing the solubility ofTi(Oi-Pr)2(dpm)2 in scCO2. The model is expressed as the followingequation:

log S = k log �f + ˛

Tf+ (1)

where S is the mass concentration of solute in scCO2, k, the associa-tion number, ˛, the constants related to sum of the heat of solvationand vaporization and ˇ, the molecular weights of solute and sol-vent. A linear relation is expected between log S and log �f at aconstant fluid temperature. The � value can be converted to theS value as follows:

� = mTi

mCO2

(2.1)

S = mTi × MTi

mCO2 × MCO2 ÷ �f= � × MTi

MCO2

× �f (2.2)

where mTi and mCO2 were the mole number of Ti(Oi-Pr)2(dpm)2 andCO2. MTi and MCO2 , are the molecular weight of Ti(Oi-Pr)2(dpm)2and CO2, respectively. The equations include one approximationthat the density of complex-scCO2 solution is equal to that of purescCO2 fluid, �CO2 . It would be adequate when the concentration of

the solution is significantly low.

Fig. 6 shows double logarithmic plots of the mass concentra-tion of Ti(Oi-Pr)2(dpm)2 in scCO2, S, and the fluid density of CO2,�f, to be analyzed by the density-based model. The linear relation

H. Uchida et al. / J. of Supercritical Fluids 66 (2012) 59– 65 63

log (Fluid density, ρf / kg m-3)

log

(Con

cent

ratio

n, S

/ kg

m-3

)40oC45oC50oC60oC

2.0 2.5 3.0–1.0

0

1.0

2.0

Ffl

w5Flnahrevedeff

3

iccitai

(Athnptp

TFP

Fluid-delivery nozzle

Substrate

Thin film

scCO2 flow(containing Ti complex)

Substrate heater

Temp.

low (Tf)

high (Tsub.)

Solubility

high

low

tive � and S dependences on the fluid temperature (confirmed inFigs. 5 and 6) cause the supersaturation during the Ti(Oi-Pr)2(dpm)2precursor supply from the delivery nozzle to the substrate surfacewith the higher temperature. We assume that the supersaturation

depo

. / n

m m

in-1

10

ig. 6. Double logarithmic plot of mass concentration of Ti(Oi-Pr)2(dpm)2 in scCO2

uid, S, and CO2 fluid density, �f .

as confirmed between log S and log �f at a constant Tf of 40, 45,0 and 60 ◦C, respectively, which was coincident well with Eq. (1).itting parameters of these relations, k and (˛/Tf) + values calcu-ated by least square approximation, are summarized in Table 1. Aearly constant association number k of ∼2 was obtained from thepproximation, irrespective to the fluid temperature. On the otherand, the value (˛/Tf) + decreases with increasing Tf although theesult for 45 and 50 ◦C could be somewhat overlapped due to thexperimental scattering. This result suggests that is a positivealue and the total process of solvation and vaporization could bexothermic. The feature in the temperature dependence of � valueiscussed in Fig. 5 would be ascribed to the parameter ˛, i.e., thexothermic heat of solvation and vaporization in scCO2 is expectedor Ti(Oi-Pr)2(dpm)2 and naphthalene while the endothermic heator many other metal complexes.

.2. Film-deposition process in scCO2

The film deposition process using Ti(Oi-Pr)2(dpm)2 dissolvedn scCO2 was reported in our previous work [22], where we dis-ussed that the deposition rate on the substrate surface, Rdepo., isontrolled by the supersaturation of the Ti(Oi-Pr)2(dpm)2 precursorn the fluid located close to the surface. Therefore, the supersatura-ion which could be an origin of the precipitation was estimated in

semi-quantitative manner based on the solubility data obtainedn the present work.

The vertical flow-type reaction apparatus used in our worksFig. 1 in Ref. [22]) possessed the reaction cell for film deposition.

schematic image for the configuration of the reaction cell is illus-rated in Fig. 7. The reaction cell was equipped with a flat substrateeater to control the substrate temperature and a fluid-delivery

ozzle faced to the substrate surface to supply Ti(Oi-Pr)2(dpm)2recursor dissolved in scCO2. Prior to the film deposition, the reac-ion cell was filled with scCO2 with a controlled temperature andressure, Tf and Pf, followed by the heating up of the substrate to

able 1itting parameters obtained from Chrastil’s relation for solubility of Ti(Oi-r)2(dpm)2 in scCO2.

Tf k ˛(Tf/ K) + ˇ

40 ◦C (313.15 K) 1.78 −3.2045 ◦C (318.15 K) 1.93 −3.7950 ◦C (323.15 K) 1.69 −3.3160 ◦C (333.15 K) 1.94 −4.50

Fig. 7. Configuration of SCFD reaction cell in vertical flow-type apparatus(schematic, upside down).

a desired temperature, Tsub. The Ti(Oi-Pr)2(dpm)2 precursor dis-solved in scCO2 at the same Tf and Pf condition was then suppliedfrom the precursor storage through the delivery nozzle to the sub-strate surface at Tsub. There, the driving force for the supersaturationto form the precipitation is considered to be the temperature dif-ference between the delivery nozzle and the substrate surface,corresponding to Tsub.–Tf. The Tf was controlled in the range of40–60 ◦C depending on the experiment, while Tsub. was fixed typi-cally at 100 ◦C.

The deposition rare Rdepo. for the SCFD using Ti(Oi-Pr)2(dpm)2in the previous work [22] is summarized in Table 2 together withthe deposition conditions, Tf, Pf, �f and Tsub., where the S values ofTi(Oi-Pr)2(dpm)2 at respective fluid condition calculated from theexperimental data in this work were attached in order to discussthe supersaturation. The origin of the supersaturation is recognizedas the difference in the mass concentration of the Ti(Oi-Pr)2(dpm)2precursor in scCO2 between the delivery nozzle and the substratesurface, caused by the temperature difference Tsub.–Tf. The nega-

Concentration, S / kg m-3

Dep

ositi

on ra

te, R

0 50 1000

5

Fig. 8. Relationship between deposition rate of TiO2 films by SCFD, Rdepo., and massconcentration of Ti(Oi-Pr)2(dpm)2 in scCO2, S, under various CO2 fluid conditions.[22].

64 H. Uchida et al. / J. of Supercritical Fluids 66 (2012) 59– 65

Table 2Deposition conditions, deposition rate, and solubility of SCFD for TiO2 films using Ti(Oi-Pr)2(dpm)2 precursor [22].

Code Tf (◦C) Pf (MPa) �f (kg m−3) Tsub. (◦C) Rdepo. (nm min−1) S (kg m−3)

a 40 8.0 278 100 1.41 1.68 × 101

b 50 8.0 219 100 0.56 3.49 × 100

c 60 8.0 192 100 0.33 8.06 × 10−1

d 40 9.0 486 100 5.72 4.56 × 101

0

iiahvtcp

jswRnpcotweigtsepc

sastTcmvw

4

tmmsTtPpttnwTv

[

[

[

[

[

e 50 9.0 285

f 40 10.0 629

g 60 10.0 290

s eventually controlled by the concentration of Ti(Oi-Pr)2(dpm)2n scCO2 around the delivery nozzle with the fluid condition of Tfnd Pf, because scCO2 near the substrate surface with Tsub. = 100 ◦Cardly dissolves the Ti(Oi-Pr)2(dpm)2 solute and therefore the Salue around the substrate is estimated be almost zero. The rela-ionship between Rdepo. and the S value at the respective fluidondition is demonstrated in Fig. 8. The deposition rate was directlyroportional to the S value estimated from the present work.

In the latest report [22], we discussed that the boundary of theet from the nozzle was wide enough to almost cover the total sub-trate area and the impinging flow rate on the substrate surfaceas negligibly small. This is quite different from the conventionalESS or similar material processing processes [49,50]. The fieldear the substrate surface is quiescent and the principal trans-ort mechanism is molecular process, admitting that the diffusionoefficient of the solute and also the thermal diffusivity are in therder or more than 10−8 m2 s−1 [51]. Eventually, as one example,he diffusion coefficient of naphthalene in CO2 under 60 ◦C, 13 MPaas reported to be 2.2 × 10−8 m2 s−1, and the thermal diffusivity

stimated from the data of the literature [24] at 60 ◦C, 10 MPas 4.1 × 10−8 m2 s−1. The degree of the supersaturation increaseradually towards the substrate surface and we can expect thathe precipitation is initiated mostly heterogeneously on the sub-trate surface to yield an apparently homogeneous film. The actualxperiments would include the decomposition of Ti(Oi-Pr)2(dpm)2recipitation in simultaneous and/or subsequent processes to formrystalline TiO2 films which was confirmed in the previous work.

The deposition mechanism of Ti(Oi-Pr)2(dpm)2 in scCO2 wasignificantly different from those of other metal complexes suchs Cu and Ru which is based on the selective absorption of theubstrates with conductive or catalytic surfaces being followed byhe thermal decomposition to form metal films [1,3–5]. SCFD usingi(Oi-Pr)2(dpm)2 has less selectivity for the surface conductivity oratalytic activity of substrates because the deposition is mainly pro-oted by the supersaturation as suggested in this work. It enables

ersatile film deposition irrespective to the substrate materials, asas confirmed in [14–16,18].

. Conclusions

The solubility of Ti(Oi-Pr)2(dpm)2 in scCO2 was measured underhe fluid temperatures and pressures by means of the extraction

ethod using a vertical flow-type apparatus and UV–vis spectro-etric analysis. The molar ratio of Ti(Oi-Pr)2(dpm)2 and CO2 in

cCO2 fluid was of the order of 10−2 under the fluid condition off = 40–60 ◦C and Pf = 6–20 MPa, which is significantly higher thanhose for other metal complexes. The dissolution behavior of Ti(Oi-r)2(dpm)2 was explained by empirical the density-based modelroposed by Chrastil, which indicated that the mass concentra-ion of Ti(Oi-Pr)2(dpm)2 in scCO2 fluid was directly proportionalo the fluid density varied by changing the fluid pressure. The

egative temperature dependence of the solubility was confirmedithout thermal decomposition of the complex, suggesting that

i(Oi-Pr)2(dpm)2 had different behaviors for the solvation and theaporization compared to other complexes.

[

[

100 0.93 5.29 × 10100 8.78 7.24 × 101

100 1.04 1.84 × 100

The solubility data was utilized for understanding the depositionmechanism of TiO2 films by SCFD using Ti(Oi-Pr)2(dpm)2 precursor.The deposition rate of TiO2 films was proportional to the differencein the mass concentration of the Ti complex between in the bulktransport medium and vicinity of the substrate surface. The relationsuggests that the deposition rate is controlled by the supersatura-tion of the Ti(Oi-Pr)2(dpm)2 precursor in scCO2 fluid located closeto the surface. We also expect that many other phenomena con-cerning SCFD of TiO2 films using Ti(Oi-Pr)2(dpm)2 can be analyzedby the solubility data obtained in the present work.

Acknowledgement

This work was financially supported by a Grant-in Aid for YoungScientists (B), No. 21760593, from the Ministry of Education, Cul-ture, Sports, Science and Technology, Japan.

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