Atomic Layer Deposition of AlF3 Thin Films Using Halide Precursors

Department of Physics, University of Helsinki, P.O. Box 43, FI-00014 Helsinki, Finland. Chem. Mater. , 2015, 27 (2), pp 604–611. DOI: 10.1021/cm5042...
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Atomic Layer Deposition of AlF3 Thin Films Using Halide Precursors Miia Man̈ tymak̈ i,*,† Mikko J. Heikkila,̈ † Esa Puukilainen,† Kenichiro Mizohata,‡ Benoît Marchand,‡ Jyrki Raï san̈ en,‡ Mikko Ritala,† and Markku Leskela†̈ †

Laboratory of Inorganic Chemistry, Department of Chemistry, University of Helsinki, P.O. Box 55, FI-00014 Helsinki, Finland Department of Physics, University of Helsinki, P.O. Box 43, FI-00014 Helsinki, Finland

‡

ABSTRACT: Aluminum fluoride thin films have potential in both optic and lithium-ion battery applications. AlF3 thin films have mostly been deposited using physical vapor deposition methods. In this study, we present a new atomic layer deposition process for AlF3. Our method makes use of a halide−halide exchange reaction with AlCl3 and TiF4 as the precursors. With this new chemistry, thin films of AlF3 can be deposited at a temperature range of 160−340 °C. The films have been studied by UV−vis spectroscopy, field emission scanning electron microscopy, X-ray diffraction, X-ray reflectance, atomic force microscopy, timeof-flight elastic recoil detection analysis (ToF-ERDA), energy dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy. At 220 °C, the growth rate of the films is approximately 1.1 Å per cycle, and the refractive index is 1.36 (at 580 nm). The films show only small amounts of Cl and Ti impurities when deposited at high temperatures, as determined by ToF-ERDA. Surface oxidation of the films due to moisture in ambient air is observed.

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INTRODUCTION AlF3, like many other metal fluorides, is a material with a high band gap and low refractive index.1−3 Because of these properties, AlF3 thin films have many potential uses in optics in the ultraviolet wavelength range: AlF3 can be used, for example, as a polarizing mirror material2 and an antireflection coating material.4,5 AlF3 is also a suitable resist material for electron-beam (e-beam) lithography.6−8 During the e-beam irradiation, AlF3 experiences radiolysis with fluoride desorption and formation of lower valence Al species, including metallic aluminum.6,8 In addition, the lithium-ion battery community has shown a lot of interest in AlF3, which can be used as an artificial solid-electrolyte-interface layer to improve cathode rate capability and capacity retention.9−12 Previously, AlF3 was also demonstrated to show lithium-ion conductivities of the order of 10−6 S/cm when combined with LiF.13−16 Therefore, the combination of AlF3 and LiF could potentially be used as a solid electrolyte material in lithium-ion batteries. AlF3 thin films have been deposited with a variety of techniques, the most often used being thermal evaporation3,6,17,18 and sputtering.4,5,19,20 Both the optical and battery related applications of AlF3 often require thin and conformal films, which makes atomic layer deposition (ALD) an attractive method for AlF3 deposition. Fluorides can be deposited by ALD from a variety of metal precursors using HF as the fluorine source.21−23 Also AlF3 has already been deposited by ALD using as the precursors trimethylaluminum and HF, which was obtained from a HF-pyridine solution.22 However, HF is highly corrosive and, as such, dangerous to both the ALD equipment and its operator. Therefore, we have extensively explored safer fluorine precursors such as TiF4 and TaF5.24−30 These precursors are moderate vapor pressure solids and are © XXXX American Chemical Society

thus readily condensed from the process exhaust gases. All these fluoride ALD processes have utilized thd complexes as the metal precursor. A ligand exchange reaction takes place, which leads to the deposition of a metal fluoride (CaF2, MgF2, LaF3, YF3, LiF) and the formation of presumably volatile byproducts Ti(thd)4−xFx or Ta(thd)5−xFx. A downside with these metal containing fluorine precursors is that they can incorporate some metal impurities in the films especially at low deposition temperatures.26,27 Our ALD experiments with Al(thd)3 and TiF4 revealed that this precursor combination does not produce good quality AlF3. No film growth was observed on silicon and aluminum oxide. A film could be deposited onto LiF thin films at 300 °C, but these films contained large amounts of titanium impurities. Therefore, other means to deposit AlF3 by ALD needed to be studied. This paper introduces a new halide−halide exchange reaction for the atomic layer deposition of AlF3 with AlCl3 and TiF4 as the precursors.

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EXPERIMENTAL SECTION

Film Deposition. AlF3 thin films were deposited in an ASM Microchemistry F-120 hot-wall flow-type ALD reactor. The deposition temperature was varied between 160 and 340 °C. During depositions, the pressure inside the reactor was of the order of 5 mbar. Single crystalline Si(111) wafers cut into 5 cm × 5 cm pieces were used as substrates. AlCl3 (Alfa Aesar GmbH & Co., 99%) and TiF4 (Strem Chemicals Inc., 98%) were used as precursors. The precursors were evaporated inside the reactor from open glass boats at 80 and 135 °C, Received: November 18, 2014 Revised: December 16, 2014

A

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Chemistry of Materials respectively. The pulsing of the precursors was done by inert gas valving. Nitrogen (H2O ≤ 3 ppm, O2 ≤ 3 ppm) was used as the carrier and purging gas. Precursor pulse lengths were varied between 0.2 and 4.0 s. In the saturation tests, purge times were kept at 3.0 s. In all the other experiments, the purge times were 0.5 s longer than the preceding precursor pulse time. Film Characterization. UV−vis spectroscopy was used to evaluate the thickness and refractive index of the films. A Hitachi U2000 spectrophotometer was used in the measurements, and the thickness and refractive index values were determined from reflectance spectra by a fitting program developed by Ylilammi and Ranta-aho.31 The wavelength range was 370−1050 nm. For the thinnest films (up to ∼60 nm), thickness was determined with X-ray reflectivity (XRR) measurements with a PANalytical X’Pert Pro MPD X-ray diffractometer. Crystallization of the films was achieved by annealing in a tube furnace in N2 flow (purity 5.0). The annealing temperature was 575 °C with a heating rate of 5 °C per minute. The crystallinity of the films was studied by grazing incidence X-ray diffraction (XRD) measurements conducted with the PANalytical X’Pert Pro MPD X-ray diffractometer. The morphology of the films was studied by field effect scanning electron microscopy (FESEM) with a Hitachi S-4800 FESEM instrument and atomic force microscopy (AFM) using a MultiMode V equipped with NanoScope V controller (Bruker). For the FESEM imaging, the samples were coated with approximately 2 nm of Au/Pd by sputtering. In the AFM studies, the samples were imaged in tapping mode in air using a phosphorus-doped silicon probe (RTESP) delivered by Bruker. Image processing and data analysis were performed with the NanoScope software version 7.30. Both tapping mode topography and phase images were acquired. The composition of the films was studied both with energy dispersive X-ray spectroscopy (EDX) (Oxford INCA 350 energy spectrometer connected to the Hitachi S-4800 FESEM instrument) and time-of-flight elastic recoil detection analysis (ToF-ERDA). The ToF-ERDA measurements were performed with 50 MeV 127I and 40 MeV 79Br beams from the 5 MV EGP-10-II tandem accelerator at the University of Helsinki.32 The detection angle was 40°, and the sample was tilted 15° relative to the incident beam direction. X-ray photoelectron spectroscopy (XPS) was used to study the surface composition in more detail. Mg Kα X-rays were produced by Omicron DAR 400 source (75 W), and the photoelectrons were analyzed by an Argus spectrometer with a 20 eV pass energy. Electron suppression was used to minimize charging effects. The C 1s peak from adventitious carbon at 284.8 eV was used for energy calibration.33

Figure 1. Growth rate of AlF3 as a function of the deposition temperature. The black squares denote films deposited with 900 cycles. Four-hundred cycles were used at 320 and 340 °C. The pulsing sequence was 0.5 s pulse and 1 s purge for AlCl3, and 1 s pulse and 1.5 s purge for TiF4.

of over 1.5 Å/cycle are common for fluorides deposited with TiF4.24−26 It has been proposed26 that the high growth rates of many fluorides could be due to the special reaction mechanism of these processes, which is shown in Scheme 1 for the current AlF3 process. AlCl3 is expected to react with a surface covered with TiF4 groups to form AlF3 and volatile side products, most likely TiCl4 and TiFxCl4−x. On top of this freshly formed AlF3, more AlCl3 can adsorb until the surface saturates. In the second step, TiF4 is introduced to the reaction chamber and reacts with the AlCl3 covered surface formed during the previous pulse. Through this reaction, more AlF3 forms, with the same side products as in the previous step. Again, TiF4 can adsorb on the AlF3 surface until saturation is reached. Therefore, AlF3 can be formed in both steps of the ALD cycle as opposed to the normal reaction, where a monolayer (or less) is formed only after one full cycle. In addition, the dimeric structure of AlCl3 at low temperatures may also have an effect on the large growth rate. The decrease of the growth rate at higher temperatures has been suggested to originate from a decreased TiF4 adsorption density caused by the reversibility of the chemisorption.24,26 However, this has not been verified experimentally. For this halide−halide exchange process, the decrease in AlCl 3 adsorption density is also a possibility. Below 220 °C, the film thicknesses were determined by UV− vis measurements. With these measurements, both the film thickness and refractive index can be fitted. The refractive index (at 580 nm) decreases from 1.40 at 160 °C to 1.36 at 220 °C, most likely because of decreasing amounts of impurities in the films. These values correspond to refractive indices obtained for thick AlF3 films deposited by thermal evaporation3 and films deposited with ALD from TMA and HF.22 Thick films deposited at higher temperatures were too rough for the determination of the refractive index. Therefore, XRR was used for the thickness measurements at higher temperatures by using thinner films. The measurements indicated that the density of the films was somewhat lower than the bulk value of 3.1 g/cm3, being of the order of 2.8−2.9 g/cm3. A key requirement for any ALD process is the saturation of the growth rate with precursor pulses, or in other words, selflimiting growth.36 Self-limiting growth means that the density

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RESULTS AND DISCUSSION Film Deposition. Our novel AlF3 process makes use of a halide−halide exchange reaction with the net reaction proposed in eq 1: AlCl3(g) + 0.75TiF4 (g) → AlF3(s) + 0.75TiCl4(g)

(1)

The Gibbs free energy change for this reaction is negative for a large temperature range, being −232 kJ/mol at 200 °C, for example. The AlF3 growth rate as a function of deposition temperature is presented in Figure 1. Nine-hundred deposition cycles were applied at most temperatures. At low temperatures, the growth rate is exceptionally high for an ALD process. The growth rate drops rapidly as the temperature is increased, with no clear ALD window present. At 320 and 340 °C, thinner films were needed for the thickness measurements because of surface roughening and consequent scattering problems caused by strong crystallization in thicker films. Therefore, these films were deposited with fewer cycles (hollow squares in Figure 1). Although decreasing growth rates with increasing temperatures have been seen before in ALD, both with fluorides and other materials,24−27,34,35 the change from approximately 3 Å/cycle to 0.3 Å/cycle over the range of 140 °C is notable. Growth rates B

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Scheme 1. The Mechanism of the Atomic Layer Deposition of AlF3 Is Proposed to Take Place in Reaction and Adsorption Steps, which Leads to a “Double Monolayer” Type Growth Instead of the Normal Monolayer Type Often Seen in ALD

experiments. At 200 °C, the film uniformity visually decreased when the AlCl3 pulse time was made longer. Going from a 0.5 s AlCl3 pulse to a 4 s pulse at 200 °C increased the thickness difference between the edges of the 5 cm × 5 cm substrate from 7 to 14%. The film appeared thinner on the substrate areas closer to the AlCl3 inlet than close to the TiF4 inlet. This type of thickness profile could result from etching reactions caused by the aluminum precursor since the length of the TiF4 pulse seemed to have no clear effect on the film uniformity. This etching effect could be clearly seen also at 240 °C. When the AlCl3 pulse was made longer at this temperature, the film thickness decreased steadily. On the basis of these results, 0.5 s was deemed both sufficient and the upper limit for the AlCl3 pulse length. The etching could also be seen when the number of cycles applied was changed: larger cycle numbers resulted in higher relative nonuniformity. The mechanism of the etching reaction is, for the moment, unclear. However, some speculation can be made. It has been reported that the volatility of AlF3 is enhanced by the presence of AlCl3 vapor at temperatures between 923 and 983 °C.37 The mixed halides, AlF2Cl and AlFCl2, were deemed possible reaction products in the reported experiment (eq 2):

of reactive sites on the surface determines the growth rate per cycle, and higher growth rates are not achieved by increased precursor doses. Here, saturation was studied at 160, 200, and 240 °C (Figures 2 and 3). AlCl3 shows saturation already with a

Figure 2. Growth rate of AlF3 as a function of the AlCl3 pulse length at 160 (squares), 200 (circles), and 240 °C (triangles). The TiF4 pulse length was kept at 1 s, and purge times were 3 s.

m AlCl3(g) + [3 − m]AlF3(s) → 3AlCl mF3 − m(g)

(2)

where m = 1, 2. At the temperatures used in our ALD experiments, both of these mixed halide species have a more negative Gibbs free energy of formation than does AlCl3.38 Therefore, it might be possible that these mixed halides are formed during the AlCl3 pulse and lead to the etching of the AlF3 film, especially on the AlCl3 inlet side because of a larger AlCl3 dose. To further test this reaction, AlF3 films were exposed to AlCl3 vapor at different temperatures. It was found that especially at 240 °C the film was thinning during the AlCl3 exposure. We believe that better uniformity could be obtained by using a different reactor design. For example, in a showerhead-type reactor the substrate would receive a more uniform precursor dose than in the current cross-flow reactor. Figure 3 illustrates the saturation of the growth rate as a function of the TiF4 pulse length. At 160 °C, a 0.5 s TiF4 pulse is not long enough for proper saturation. This film was not completely uniform in the flow direction as opposed to the film deposited with a 1 s TiF4 pulse. The refractive index of the films (not shown) increased when longer TiF4 pulses were used, which possibly indicates a larger amount of impurities. At 200 °C, saturation is again seen with a 1 s pulse. The growth rate of AlF3 increases slightly when TiF4 pulse lengths longer than 2 s were used at this temperature. The same effect could also be seen in the saturation test made at 240 °C. A similar effect was

Figure 3. Growth rate of AlF3 as a function of the TiF4 pulse length at 160 (squares), 200 (circles), and 240 °C (triangles). The AlCl3 pulse length was kept at 0.5 s, and purge times were 3 s.

0.5 s pulse length at all temperatures. The small scatter in the growth rates can be explained with the high sensitivity of the growth rate to the growth temperature (Figure 1) and minor differences in the growth temperatures in the different C

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Chemistry of Materials previously seen in the deposition of LiF from Lithd and TiF4,30 where it was first assumed that the increasing growth rate, accompanied by an increasing refractive index, would be a result of an increasing amount of impurities. However, ERDA measurements revealed that the amount of impurities was not elevated in these LiF films. The origin of this increase in growth rate in both of these processes has thus not yet been uncovered. As a result of the self-limiting growth, deposition rates in ALD processes should not depend on the number of deposition cycles. The ability to easily tune the film thickness by selecting an appropriate number of deposition cycles has made ALD very attractive for a large number of applications, particularly within the semiconductor industry.39 Because of the lack of an ALD window, the AlF3 growth rate as a function of the number of cycles has been studied at a number of different temperatures. These results are collected in Figures 4 and 5. There are small

used is limited by the complications in thickness measurements caused by the crystallization and the consequent roughening and light scattering of the films. Also, deviations from the linear growth are larger at higher temperatures, which indicates a lessthan-perfect chemical reaction. Tests were also done on the repeatability of these results, and it was found that at temperatures higher than 240 °C, variations of the order of 10% do occur in the growth rate. As already mentioned, the relative nonuniformity seems to increase with the number of cycles. This might be one factor that affects the repeatability. For optical applications, the observed variation and nonuniformity can be detrimental. However, for lithium-ion battery applications, they are less of a concern. Film Characterization. The crystallinity of the films was studied with grazing incidence (GI)-XRD measurements. AlF3 is amorphous at the lowest deposition temperatures and shows first small signs of crystallization at 280 °C (Figure 6). At 340

Figure 4. AlF3 film thickness as a function of the number of cycles at 160 (squares) and 200 °C (circles). The R2 value for a linear fitting of the points is 0.992 for 160 °C (slope 3.0 Å/cycle) and 0.988 for 200 °C (slope 1.7 Å/cycle).

Figure 6. X-ray diffractograms of AlF3 films deposited at different temperatures. Samples in the range of 160−300 °C were deposited with 900 cycles, and those at 320 and 340 °C were deposited with 400 cycles.

°C, a film deposited with 400 cycles is amorphous to XRD (Figure 6), but a film deposited with 900 cycles is opaque and shows very good crystallinity (Figure 7). The phase in this film is hexagonal. Amorphous films can be crystallized by annealing in N2. Figure 7 also shows a diffractogram for one such film deposited at 200 °C and annealed at 575 °C for 3 h. In this case, the film is in the tetragonal phase. FESEM images corroborate the XRD results (Figures 8 and 9). As expected based on the XRD results, no structural features can be seen in the amorphous films deposited at low temperatures. At 240 °C, some very sparsely spaced globules could be seen on top of the film surface at high magnification (not visible in Figure 8). The number of the globules increases with deposition temperature. EDX measurements revealed that both the globules and the surrounding film contain Al and F. It is postulated that the globules form the crystalline component of the film and are embedded in an amorphous layer of AlF3. Similar morphology has been seen before in YF3 films deposited using Y(thd)3 and TiF4.27 Also there it was assumed that the globules formed the crystalline part of the film. For the AlF3 films, the highly crystalline sample deposited at 340 °C has a different morphology in the FESEM image and appears more like the atomic layer deposited fluorides previously re-

Figure 5. AlF3 film thickness as a function of the number of cycles at 240 (squares) and 280 °C (circles). The linearity of the growth rate is not as good at 280 °C as it is at 240 °C. The R2 value for a linear fitting of the points is 0.995 for 240 °C (slope 0.80 Å/cycle) and 0.966 for 280 °C (slope 0.44 Å/cycle).

variations in the growth rate at a given temperature. The small drop at 160 °C for the 1000 cycle film is likely at least in part a result of the higher measurement error for such a thick film. At the high temperature end, the number of cycles that can be D

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rms roughness of 11.6 nm. This high roughness is a result of the globules already seen in the FESEM images. The globules were measured to have a diameter of approximately 180−240 nm. Interestingly, the height of the globules was 70−80 nm, as measured from the surrounding smooth surface. This is much larger than the film thickness, which was determined by XRR. This particular film showed no clear indication of high roughness in the XRR measurement, probably because of the rather scarce occurrence of the globules. The XRR measurement resulted in a thickness of 35 nm, but if the total amount of material also present in the globules is taken into account by using basic geometrical calculations, the total average film thickness would be close to 60 nm. The globules are also clearly visible in the AFM phase image, which indicates that they have different physical properties compared to the surrounding smooth film. Such a difference can exist between the amorphous and crystalline forms of the same composition, for example. This supports our proposal that the globules represent the small crystalline component of the films seen in the X-ray diffractograms of Figure 6. The height of the globules, in turn, implies that crystalline AlF3 grows much faster than the surrounding amorphous phase. AlF3 films have been reported to absorb water.7 As a result, Al2O3 formation has been observed in thin films of AlF3 deposited by other methods.3 It should be noted, however, that despite reports of Al2O3 formation in AlF3 films, the Gibbs free energy change for the reaction of AlF3 with water is in fact positive and should thus be unfavorable. To study the

Figure 7. X-ray diffractograms of a hexagonal AlF3 film (upper) deposited at 340 °C using 900 cycles and a tetragonal film (lower) deposited at 200 °C using 600 cycles and annealed at 575 °C for 3 h.

ported.29,30 The size of the globules increases as the films are made thicker, as can be seen in Figure 9. The film morphology was also studied with AFM (Figure 10). The films deposited at a temperature range of 160−240 °C showed rms roughnesses of