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Atomic layer deposition of metal fluorides through oxide chemistry

Matti Putkonen,*ab Adriana Szeghalmi,c Eckhard Pippelc and Mato Knezc

Received 26th April 2011, Accepted 15th July 2011

DOI: 10.1039/c1jm11825k

In this paper, we report on the atomic layer deposition of magnesium, calcium and lanthanum

fluorides utilising two different approaches with hexafluoroacetylacetonate as a fluorine source. The

first approach is based on the reaction of fluorinated metal precursors with a strong oxygen source. A

deposition rate of 0.3 �A per cycle was obtained when calcium hexafluoroacetylacetonate (Ca(hfac)2)

was used as a metal precursor and ozone as a second reactant at 300 �C. From Rutherford

backscattering spectroscopy (RBS) measurements, the film stoichiometry was determined to be

CaF2.17 with less than 5 atomic% of oxygen. The second and more feasible approach is based on

metal oxide formation from traditional nonfluorinated b-diketonate type metal precursors and an

oxygen source, followed by fluorination using hexafluoroacetylacetonate (Hhfac) and ozone. The

MgF2, CaF2 and LaF3 thin films prepared using this method showed refractive indices of 1.429, 1.472

and 1.687, respectively. The oxygen content of these films was below the detection limit of the RBS,

which is 2 at%.

Introduction

Traditionally, the atomic layer deposition (ALD) chemistry of

binary compounds relies on the chemisorption of the precursor

onto the surface and subsequent reaction with oxygen or

nitrogen containing compounds, such as H2O, O3 or NH3. This

concept has proved to be useful for metal oxides or nitrides,

because counter precursors are available. However, for metal

fluorides and phosphates the selection of reactive non-metal

precursors is very limited. In principle HF or PH3 would be

suitable to use as precursors, but due to their corrosive nature

and toxicity they are not widely used.1 Alternatively, ex situ

fluorination with application of CF4 plasma to ALD deposited

Al2O3 has been performed.2 We have previously shown that

phosphorus can be incorporated into carbonate films by

exchange reactions to produce hydroxyapatite-like thin films.3 In

this work we adopt a similar approach using exchange reactions

to convert metal oxides to metal fluorides during ALD

processing.

Typically, nonfluorinated metal b-diketonates are used as

ALD precursors although they are relatively inert and require

strong oxidizer such as ozone to produce metal oxide films. Their

use as precursors for nitride and sulfide films is not straightfor-

ward due to the already present metal–oxygen bond. Typical

aBeneq Oy, P.O. Box 262, FI-01511 Vantaa, Finland. E-mail: matti.putkonen@beneq.com; Fax: +358 9 7599 5310; Tel: +358 40 5203099bLaboratory of Inorganic Chemistry, Aalto University School of ChemicalTechnology, P.O. Box 16100, FI-00076 Aalto, Espoo, FinlandcMax-Planck-Institut f€ur Mikrostrukturphysik, Weinberg 2, D-06120Halle, Saale, Germany

This journal is ª The Royal Society of Chemistry 2011

examples are the formation of yttrium oxysulfide from the Y

(thd)3 + H2S,4 and indium sulfide from In(acac)3 + H2S.

5

In principle b-diketonates can also hydrolyze in the presence of

water at very high temperatures, but at the same time they

undergo thermal decomposition, thus limiting their use in H2O

based processes. Another problem associated with some b-

diketonates, such as Ca, Sr, and La, is the carbonate formation.

For example, films made by using Ca(thd)2 + O3 consist of

almost pure CaCO3.6 Fluorinated b-diketonate ligands are

known to be relatively high-vapor pressure precursors for

chemical vapor deposition (CVD) but their use for ALD

precursors is rather scarce.7 Although they are quite bulky, they

are reactive with strong oxidizers such as O3 or oxygen plasma.

Quite recently Pilvi et al. reported on the deposition of metal

fluorides (MgF2, CaF2, LaF3) with metal b-diketonates as metal

precursors and solid TiF4 or TaF5 as fluorine source.8,9 These

reactions proceed through a ligand exchange reaction between

the metal b-diketonate and the metal fluoride producing titanium

or tantalum b-diketonates as volatile byproducts. It is worth

noting that although oxygen coordinated b-diketonate ligands

are present, no significant amount of oxygen is present in the

films. However, small amounts of titanium or tantalum are

observed as impurities limiting the use of these films for deep UV

applications because of reduced transmittance.

In this report we have studied the deposition of metal fluorides

by two novel methods. The first approach is based on the

traditional metal oxide ALD chemistry with fluorinated metal

chelates, whereas fluorinated hydrocarbons were used as the

fluorine source in the second approach. Both of these concepts

are based on the exchange reactions of oxygen containing

precursors and adsorbed fluorine species at the surface.

J. Mater. Chem., 2011, 21, 14461–14465 | 14461

Fig. 1 Deposition rate of the Ca(hfac)2/O3 process as a function of

Ca(hfac)2 pulse time at 300 �C.

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Experimental

In the first approach Ca(hfac)2 (obtained from Volatec Oy)

and O3 were used as precursors ((hfac ¼ 1,1,1,5,5,5-hexa-

fluoroacetylacetonate). In the second approach Mg(thd)2, Ca

(thd)2 and La(thd)3 (thd¼ 2,2,6,6-tetramethyl-3,5-heptanedione)

were used as metal precursors and O3 as the oxygen source.

Hhfac (1,1,1,5,5,5-hexafluoroacetylacetonate) (Sigma-Aldrich)

was used as a fluorine source. The films were deposited on

a commercial TFS 200 or TFS 500 cross-flow ALD reactor

manufactured by Beneq Oy. The film depositions were carried

out onto Si(100) wafers measuring 100–200 mm in diameter. For

measuring the optical properties of the ALD films, also quartz

substrates were used. Ozone was used as the oxygen source and it

was generated from O2 (99.999%) in an ozone generator

(Wedeco). Nitrogen (>99.999%) was used as the carrier gas

without any additional purification.

The film thicknesses were measured by ellipsometry using

either a PLASMOS SD 2300 or a synchrotron beam at the

BESSY Helmholtz-Zentrum Berlin. Details of the experimental

setup at BESSY can be found in ref. 10. The measurements were

performed in the 9.5–3.5 eV energy region at an angle of

incidence of ca. 67.5� using a MgF2 Rochon polarizer. X-Ray

diffraction using Cu Ka radiation (Philips MPD 1880) was used

to characterize crystalline phases. TEM images of the represen-

tative samples were taken by using a Titan 80–300 TEM

from FEI.

The compositions of the selected thin film samples were

determined using Rutherford backscattering spectrometry (RBS)

with a 2 MeV 4He+ ion beam backscattered at 160�. The 2 mm

diameter beam was incident at 7� to the sample surface normal.

The SIMNRA simulation program11 was used to obtain the

elemental ratios by comparison of simulated and experimental

RBS spectra.

Fig. 2 Deposition rates of Ca(thd)2/O3 and Ca(thd)2/O3 + Hhfac/O3

processes.

Results and discussion

In the first approach we used fluorinated metal b-diketonates as

a metal source and ozone as the second precursor.

Initial experiments were carried out with Ca(hfac)2 and water

as precursors, but no film was obtained up to 300 �C, which is

probably due to the already expected low reactivity. By changing

the oxygen source to O3, a deposition rate of 0.3 �A per cycle was

obtained which is reasonable in view of the bulkiness of the

fluorinated b-diketonate precursor. The pulsing time of the

precursors did not affect the deposition rate if kept over 200 ms,

indicating an ALD type of growth (Fig. 1). With optimized

deposition parameters a max–min uniformity of 1.5% was

obtained over a 100 mm silicon wafer. The refractive index was

approximately 1.46 which is much lower than that expected for

CaO or CaCO3 thin films and closer to the refractive index of

1.43, reported for CaF2 thin films.

According to the RBS measurements, the film composition

was Ca 30 at%, F 65 at% and O <5 at%, indicating a stoichi-

ometry close to CaF2, with less than 5% of oxygen as impurity.

However, the oxygen impurity is the most probable reason for

the increased refractive index compared to the bulk values.

Typically O3 has been used as an oxygen source for b-diket-

onate precursors to deposit oxide films. Therefore it is rather

14462 | J. Mater. Chem., 2011, 21, 14461–14465

surprising that with fluorinated metal precursors the reaction

yields metal fluorides. Previously, fluorinated metal b-diketo-

nates have been utilized for the deposition of palladium12 and

copper,13,14 and recently Goldstein and George reported that

during the Pd(hfac)2 + formalin process the released Hhfac

ligand is chemisorbed to the surface, blocking the reactive sites.15

This is the main reason for the low deposition rate and rough-

ening of the Pd film since hfac cannot be easily removed from the

surface.

It seems that the ozone reacts with the chemisorbed metal

precursor and the released fluoride-containing hydrocarbons

further react with the metal oxide surface forming CaF2. In

addition, although O3 is a strong oxidizer, it seems that O3 is not

reactive enough to convert the metal fluoride to the corre-

sponding metal oxide. Bulk CaF2 is reported to be stable in an

oxygen atmosphere at these temperatures,16,17 although it is

reported that slight surface oxidation of CaF2 occurs even at

room temperature under normal pressure.18

Metal fluoride deposition by using fluorinated b-diketonates

together with ozone is a quite limited strategy to produce a large

variety of metal fluorides, since the commercial availability of

suitable fluorinated metal chelates is rather restricted. Therefore,

we tried a more practical approach to synthesize fluoride films,

which is based on traditional non-fluorinated metal precursors.

This journal is ª The Royal Society of Chemistry 2011

Fig. 3 Schematic representation of ALD processing of metal fluorides using Hhfac as a fluorine source during the (a) Ca(thd)2/O3 and (b) Hhfac/O3

cycle.

Fig. 4 RBS depth profile of the CaFx thin film.Fig. 5 Crystallinity of CaFx (a) and LaFx (b).

Fig. 6 CaF2.03 thin film surface morphology on the silicon substrate

measured by AFM from a 2 � 2 mm area, height axis 10 nm.

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In our second approach, we used nonfluorinated metal precur-

sors as metal sources and provided fluorine as a separate

precursor. Since hfac is known to strongly adsorb to surfaces,15

we studied the possibility to fluorinate the oxides during an ALD

cycle.

CaF2 films were deposited by first using Ca(thd)2 + O3 sub-

cycle for incorporating Ca species to the surface and subse-

quently applying a Hhfac + O3 cycle for fluorination. The

deposition rate of Ca(thd)2 + O3 + Hhfac + O3 was 0.4 �A per

cycle (Fig. 2). It seems that the Hhfac + O3 pulses do not affect

the deposition rate, since the films grown from Ca(thd)2 and O3

had a similar deposition rate of 0.41 �A per cycle at 250–350 �C.3,6

Similarly using Mg(thd)2 and La(thd)2 resulted in uniform thin

films with deposition rates of 0.38 and 0.49 �A per cycle, respec-

tively. The deposition rate of lanthanum fluoride is quite close to

the deposition rate of the La(thd)3/O3 process.19

Although the Hhfac ligand gets adsorbed to the surface, it

seems that it is not able to get desorbed in the form of volatile Ca

(hfac)2, since no decrease in the deposition rate was observed,

even if the Hhfac pulse time was increased. Typical Hhfac doses

of 0.35–0.4 mg per cycle resulted in uniform films with constant

refractive index over the entire substrate. Even if the Hhfac dose

was increased up to 15 mg per cycle, the deposition rate and

refractive index still remained constant, provided the purge times

This journal is ª The Royal Society of Chemistry 2011

were kept sufficiently long. On the other hand, if the ozone pulse

was omitted after the Hhfac, the deposition rate was significantly

reduced. Based on these observations, the following mechanism

can be suggested (Fig. 3). First, the ALD reaction proceeds by

J. Mater. Chem., 2011, 21, 14461–14465 | 14463

Fig. 7 CaF2.03 film deposited by using the Ca(thd)2 + O3 + Hhfac + O3

process onto silicon wafer with native oxide.

Fig. 8 Refractive indices and k-values of metal fluorides on quartz

substrates.

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producing the metal oxide or carbonate from M(thd)x/O3. In the

following phase, Hhfac is adsorbed to the surface. This adsorp-

tion seems to be surface controlled, since the film growth rate or

refractive index did not change with different Hhfac doses.

However, ozone must be introduced in order to decompose the

adsorbed Hhfac at the surface. Although this step is analogous to

the oxidation step in the Ca(hfac)2 + O3 process, the process with

separated calcium and fluorine sources resulted in a better stoi-

chiometric material and lower oxygen content. One possible

reason might be that in the one step process the amount of

fluorine is fixed with the calcium precursor, leading to a lower

fluoride content. The Hhfac pulse ensures the presence of excess

fluorine at the surface. According to the RBS measurements, the

compositions were 33 : 67 for Ca–F films and 29 : 71 for La–F

films, giving stoichiometries of CaF2.03 and LaF2.56. The oxygen

content was below the detection limit of the RBS (<2 at%)

(Fig. 4).

All deposited fluorine films were polycrystalline with

a preferred (111) orientation as determined by XRD (Fig. 5). The

CaFx films showed a strong (111) peak with a smaller (220) peak

only. From LaFx films other reflections such as (110) and (300)

were also observed. The crystallinity of the CaF2 films did not

change considerably as a function of deposition temperature

between 250 and 300 �C.According to AFM measurements, the roughness was depen-

dent on the film crystallinity. CaF2 and LaF3 films with a similar

thickness around 50 nm had roughness values around 5 nm

(Fig. 6). The increased roughness of ALD deposited fluorides has

also been observed earlier, when other precursor chemistries

have been applied.8,9

Increased roughness was also verified from the TEM micro-

graph (Fig. 7). Film crystallinity was also seen as there were

relatively large grains with a columnar structure on the deposited

films.

14464 | J. Mater. Chem., 2011, 21, 14461–14465

Fluorides are well known transparent materials in the ultra-

violet spectral region. Magnesium8 and calcium9 fluorides have

some of the lowest refractive indices with reported values around

1.4, and are therefore widely used in various optical structures. In

multilayer interference optics, a high refractive index counterpart

material is necessary. While numerous adequate oxides can be

found for the visible spectral range, only few materials have both

low absorption and high refractive index values in the ultraviolet

spectral range.20 Al2O3 thin films have a relatively high refractive

index of ca. 1.7, but their extinction coefficient rapidly increases

below a wavelength of 200 nm. The band gap of Al2O3 films

deposited by physical vapor deposition is ca. 6.41 eV corre-

sponding to the first interband transition. Hence, their use in

vacuum UV is limited. Alternatively, LaF3 thin films are highly

promising high refractive index coatings with application

potential down to a wavelength of ca. 150 nm.21

Two aspects are highly important to achieve deep and vacuum

UV optics for analytical chemistry or lithography applications,

for spectrometers or laser instruments. High quality coating films

must be produced and excellent thickness control for ultra-thin

layers must be guaranteed. The film quality of the multilayer

structure is determined by its purity, homogeneity, low rough-

ness, uniformity, etc. Small amounts of impurities in the films

could drastically increase the UV absorption of the optics. The

RBS data presented here demonstrate high purity fluorides and

the AFM measurements indicate relatively smooth films, which

are very promising for optical coatings. Concerning the thickness

control, the individual layer thicknesses get downscaled with

decreasing wavelengths.22 Atomic layer deposition is a promising

coating technology achieving excellent thickness control for

ultra-thin layers of only a few nanometre thicknesses.23,24

The optical properties of the fluoride thin films were investi-

gated by vacuum ultraviolet ellipsometry using synchrotron

radiation. The ellipsometry parameters J and D have been

modeled by the Cauchy model to determine the refractive index

and film thickness of the ALD coating. An EMA (effective

medium approximation) layer to account for film roughness was

included in the fitting procedure. The EMA layer thicknesses

were in the range of 2 to 8 nm corresponding to the roughness

measurements performed by AFM. The spectral range was

limited to achieve a good fit of the ellipsometry data taking into

account the low absorption wavelengths. The optical properties

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are summarized in Fig. 8. The refractive indices measured at 320

nm for MgFx, CaFx and LaFx films were 1.429, 1.472 and 1.687,

respectively.

Transmittance of CaFx films deposited on various substrates

was evaluated by recording transmittance spectra (Fig. 8). The

films deposited on quartz substrates were highly transparent

down to ca. 140 nm wavelength, with the transmittance being

limited by the substrate material.

Conclusions

We have demonstrated a new ALD concept for producing metal

fluoride films using either fluorinated metal precursors with

ozone or conventional metal precursors with fluorine compounds

and ozone. Ozone proved to be essential for the activation of the

hexafluoroacetylacetonate adsorbed to the surface. Using Hhfac

as a fluorine source increased the purity of the films by reducing

the oxygen content below the detection limit of the RBS. The

deposited films are highly uniform and polycrystalline with (111)

as the preferred orientation. The refractive indices of the mate-

rials have been determined by vacuum ellipsometryand the

transmittance measurements of the films deposited on quartz

substrates indicate their application for UV optics.

Acknowledgements

The authors are grateful to C. Cobet for support with the

ellipsometer equipment. Dr Timo Sajavaara is acknowledged for

performing the RBS measurements at University of Jyv€askyl€a.

Financial support within the German Federal Ministry of

Education and Research BMBF project FKZ 03X5507 and by

the Helmholtz Zentrum Berlin is highly acknowledged.

Notes and references

1 M. Ylilammi and T. Ranta-aho, J. Electrochem. Soc., 1994, 141, 1278.

This journal is ª The Royal Society of Chemistry 2011

2 K. M. Fan, C. S. Lai, H. K. Peng, S. J. Lin and C. Y. Lee, J.Electrochem. Soc., 2008, 155, G51.

3 M. Putkonen, T. Sajavaara, P. Rahkila, L. Xu, S. Cheng, L. Niinist€oand H. J. Whitlow, Thin Solid Films, 2009, 517, 5819.

4 K. Kukli, M. Peussa, L.-S. Johansson, E. Nyk€anen and L. Niinist€o,Mater. Sci. Forum, 1999, 315, 216.

5 S. K. Sarkar, J. Y. Kim, D. N. Goldstein, N. R. Neale, K. Zhu,M. Elliott, A. J. Frank and S. M. George, J. Phys. Chem. C, 2010,114, 8032.

6 O. Nilsen, H. Fjellv�ag and A. Kjekshus, Thin Solid Films, 2004, 450,240.

7 J. W. Elam, A. B. F. Martinson, M. J. Pellin and J. T. Hupp, Chem.Mater., 2006, 18, 3571.

8 T. Pilvi, T. Hatanp€a€a, E. Puukilainen, K. Arstila, M. Bischoff,U. Kaiser, N. Kaiser, M. Leskel€a and M. Ritala, J. Mater. Chem.,2007, 17, 5077.

9 T. Pilvi, K. Arstila, M. Leskel€a and M. Ritala, Chem. Mater., 2007,19, 3387.

10 R. L. Johnson, J. Barth, M. Cardona, D. Fuchs and A. M. Bradshaw,Rev. Sci. Instrum., 1989, 60, 2209.

11 SIMNRA Simulation Program, <http://www.rzg.mpg.de/�mam/>.12 J. W. Elam, A. Zinovev, C. Y. Han, H. H. Wang, U. Welp, J. N. Hryn

and M. J. Pellin, Thin Solid Films, 2006, 515, 1664.13 J. Huo and R. Solanki, J. Mater. Res., 2002, 17, 2394.14 D. Y. Moon, W. S. Kim, T. S. Kim, B. W. Kang and J. W. Park, J.

Korean Phys. Soc., 2009, 54, 1330.15 D. N. Goldstein and S. M. George, Appl. Phys. Lett., 2009, 95,

143106/1.16 W. Bontinck, Physica, 1958, 24, 650.17 D. L. Deadmore, J. S. Machin and A. W. Allen, J. Am. Ceram. Soc.,

1961, 44, 105.18 M. Reichling, M. Huisinga, S. Gogoll and C. Barth, Surf. Sci., 1999,

439, 181.19 M. Nieminen, M. Putkonen and L. Niinisto, Appl. Surf. Sci., 2001,

174, 155.20 J. H. Correia, A. R. Emadi and R. F. Wolffenbuttel, ECS Trans.,

2006, 4, 141.21 B. von Blackenhagen, D. Tanova and J. Ullmann, Appl. Opt., 2002,

41, 3137.22 M. Yang, A. Gatto and N. Kaiser, Appl. Opt., 2006, 45, 1359.23 D. Riihel€a, M. Ritala, R. Matero and M. Leskel€a, Thin Solid Films,

1996, 289, 250.24 A. Szeghalmi, M. Helgert, R. Brunner, F. Heyroth, U. Gosele and

M. Knez, Appl. Opt., 2009, 48, 1727.

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