Atomic layer deposition of rare earth oxides: erbium oxide thin films from β-diketonate and ozone precursors

Journal of Alloys and Compounds 374 (2004) 124–128

Atomic layer deposition of rare earth oxides: erbium oxide thin films from ␤-diketonate and ozone precursors Jani Päiväsaari a , Matti Putkonen a , Timo Sajavaara b , Lauri Niinistö a,∗ a

Laboratory of Inorganic and Analytical Chemistry, Helsinki University of Technology, P.O. Box 6100, FIN-02015 HUT, Espoo, Finland b Accelerator Laboratory, University of Helsinki, P.O. Box 43, FIN-00014 Helsinki, Finland

Abstract Er2 O3 thin films were grown onto Si(1 0 0) and soda lime glass substrates by atomic layer deposition (ALD) from Er(thd)3 (thd = 2,2,6,6tetramethyl-3,5-heptanedione) and ozone precursors. Temperature range studied was 200–450 ◦ C where a region of constant growth rate for Er2 O3 (ALD window) was observed at 250–375 and 275–350 ◦ C on Si(1 0 0) and soda lime glass, respectively. Within the ALD window, the growth rates of Er2 O3 films deposited onto Si(1 0 0) and soda lime glass were 0.25 and 0.20 Å (cycle)−1 , respectively. Films were polycrystalline, cubic Er2 O3 , and their preferred orientation changed from (4 0 0) to (2 2 2) as the deposition temperature was raised above 325 ◦ C. Below 250 ◦ C, the films were amorphous. The surface morphology studies by AFM revealed that the films were very smooth (rms = 0.3–1.4 nm), when deposited within the ALD window. Time-of-flight elastic recoil detection (TOF-ERD) analyses were carried out to determine stoichiometry and impurity levels and they proved the films to be nearly stoichiometric Er2 O3 with some hydrogen, carbon and fluorine as impurities. Within the ALD window, the hydrogen, carbon and fluorine contents were in the order of 1.7–4.0, 0.5–1.8 and 0.7–1.7 at.%, respectively. © 2004 Elsevier B.V. All rights reserved. Keywords: Atomic layer deposition; Thin films; Rare earth oxides; Erbium oxide; ␤-Diketonate precursor

1. Introduction Rare earth (RE) oxides are an interesting group of materials with high application potential. They are thermodynamically very stable refractory materials [1]. The melting points of RE sesquioxides are high being in the range of 2230–2490 ◦ C [2]. Therefore, RE oxide thin films are of interest for protective and corrosion resistive coatings [3,4]. RE oxides have also other favourable properties, such as high refractive indices (1.91–1.98 for the C-type structure), [5] applicable in optics, e.g. as antireflection coatings [6]. C-type cubic lanthanide oxides have lattice constants between 10.39 and 11.08 Å (from Lu to Nd), [7] while the lattice parameter of Si(1 0 0) is 5.43 Å. Small lattice mismatch enables the growth of epitaxial lanthanide oxide films on Si(1 0 0), as has been demonstrated with Gd2 O3 [8]. The mismatch between cubic Er2 O3 (a = 10.55 Å) and two Si(1 0 0) unit cells is 2.9%. The continuous miniaturization of CMOS transistors is reaching its limits with the current silicon-based dielectric materials. Also, RE oxides

∗

Corresponding author. Tel.: +358-9-451-2600; fax: +358-9-462-373. E-mail address: [email protected] (L. Niinistö).

0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2003.11.149

are among the potential candidates for the new generation of gate oxides, [9] because of their high relative permittivity (εr = 12.4–14.8) [10] and stability. In addition, RE oxide films are expected to find applications in the passivation of III–V compound semiconductors as Gd2 O3 was reported to effectively passivate the GaAs surface [11]. There are a few papers dealing with the deposition of erbium oxide thin films. Nevertheless, several deposition methods of different type have been applied, such as sol–gel technique, [12] laser ablation, [13] electron beam evaporation, [14–16] pyrolysis, [17] chemical vapor deposition (CVD) [3,4,18] and its variant plasma-enhanced CVD (PECVD) [19,20]. In most of the CVD processes [3,4,18,19], the well-characterized, volatile and thermally stable ␤-diketonate-type chelate, Er(thd)3 (thd = 2,2,6,6-tetramethyl-3,5-heptanedione), [21–23] has been used as precursor. Atomic layer deposition (ALD), also known as atomic layer epitaxy (ALE) or atomic layer CVD (ALCVD), is an advanced CVD-type process for thin film growth, where the precursors are alternately pulsed onto the substrate. Inert gas purge is applied onto the substrates between the precursor pulses in order to avoid gas phase reactions. Under optimised conditions, ALD can be used to deposit high quality,

J. Päiväsaari et al. / Journal of Alloys and Compounds 374 (2004) 124–128

uniform and conformal thin films. Thickness control of thin films is accurate and facile, when the process is operated within the ALD window, i.e. the temperature range where the self-limiting growth mechanism takes place resulting in a constant growth rate. The ALD method and its applications have recently been reviewed in a general and comprehensive way, [24,25] as well as with a more limited scope focusing on RE oxides [26,27]. Since the early 1990s, ALD has been used to deposit thin films of several RE oxides, including Sc2 O3 , [28] Y2 O3 , [29,30] La2 O3 [31] and CeO2 [32,33]. Here, the ALD of Er2 O3 thin films is reported for the first time.

2. Experimental The ␤-diketonate chelate, Er(thd)3 was used as precursor, synthesized by the standard procedure, [34] followed by sublimation in vacuum to purify the product. Its volatility was determined by simultaneous TG/DTA measurements, carried out in nitrogen atmosphere under a reduced pressure (ca. 400 Pa). Seiko Instruments SSC/5200 TG/DTA thermobalance was used in the simultaneous TG/DTA thermoanalytical measurements. Mass spectra of the purified precursor were measured under UHV conditions in a JEOL DX 303/DA 5000 mass spectrometer using 70 eV ionisation potential. F-120 research-type ALD-reactor (ASM Microchemistry Ltd, Espoo, Finland) was used for the ALD of Er2 O3 . The deposition of thin films was studied in the temperature range of 200–450 ◦ C. O3 was used as the oxygen source, produced from >99.999% O2 in an ozone generator (Fischer Model 502), while nitrogen (>99.999%) was used as carrier and purge gas. N2 was separated from air in a nitrogen generator (Nitrox UHPN 3000-1). The pulsing times applied for Er(thd)3 and ozone were 1.0 and 2.5 s, respectively, i.e. long enough to obtain saturation of the substrate surface. Nitrogen purging time between the precursor pulses was 1.25–3.0 s. Er2 O3 thin films were deposited both on Si(1 0 0) (Okmetic, Espoo, Finland) and soda lime glass substrates, main focus being on silicon. Substrates were ultrasonically cleaned before the depositions, but the native oxide on Si(1 0 0) was not removed. The size of the substrates was 10 cm × 5 cm. Thicknesses of the Er2 O3 films were obtained by fitting the transmittance and reflectance spectra, [35] which were measured using a Hitachi U-2000 spectrophotometer. Crystallite orientations were determined in a Philips MPD 1880 X-ray diffraction instrument with Cu K␣ radiation. Nanoscope III atomic force microscope (Digital Instruments) was employed for the surface morphology measurements. AFM measurements were carried out in tapping mode with a scanning frequency of 1–2 Hz. Roughness was calculated as root-mean-square (rms) values. Elemental concentrations of Er2 O3 thin films were determined by time-of-flight elastic recoil detection analysis (TOF-ERDA) at the Accelerator Laboratory of the Univer-

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sity of Helsinki [36]. In this method, heavy ions are accelerated and projected into the sample. When high-energy ions hit the sample, elastic collisions result and recoils of the sample atoms are measured. Timing gates and charged particle detector were utilized to determine recoil velocity and energy, respectively, which enabled mass separation. For TOF-ERD analyses, a beam of 51 MeV 127 I10+ ions was used, obtained from a 5 MV tandem accelerator EGP-10-II. Samples were measured at an angle of 20◦ and the recoils were detected at 40◦ with respect to the incoming beam. The elemental content reported below were obtained directly from the total recoil yields using Rutherford scattering cross sections. Er2 O3 films were also analysed by Fourier transform infrared spectroscopy (FTIR) to get qualitative information of the carbon impurities. Transmission spectra were collected from the samples deposited onto Si(1 0 0) substrates. Nicolet Magna-IR 750 instrument equipped with deuterated triglysime sulfate (DTGS) detector was used in the FTIR measurements.

3. Results and discussion The evaporation temperature of the Er(thd)3 precursor was evaluated from the TG-curve (Fig. 1) and set at 130 ◦ C. The main Er-containing gas-phase species of Er(thd)3 was identified by mass spectrometry (Fig. 1, inset). The strongest peak in the mass spectrum at mass number m/z = 57 can be assigned to trimethyl radical [C(CH3 )3 ]+ . The other main peaks with mass numbers m/z = 659, 534 and 716, originate from the Er-containing fragments [Er(thd)3 -C(CH3 )3 ]+ , [Er(thd)2 ]+ and the molecular peak [Er(thd)3 ]+ , respectively. The mass spectra of Er(thd)3 closely resemble the mass spectra of other trivalent rare earth thd chelates, e.g. Sc(thd)3 , [28,37] Y(thd)3 [30,38]. In these previous studies, the metal-containing fragments with strongest intensity have been the same as in the present case. It should be noted, however, that these MS measurements are not directly comparable to the ALD experiments, because they were carried out under UHV conditions, while the pressure inside the ALD reactor was around 200–300 Pa. At first, the Er2 O3 growth rate, i.e. thickness increment during one cycle, was studied as a function of deposition temperature (Fig. 2). Substrate temperatures of 200–450 ◦ C were used. On silicon substrates, the Er2 O3 growth rate was observed to have a constant value of 0.25 Å (cycle)−1 at 250–375 ◦ C. Outside this region, the Er2 O3 growth rate decreased to 0.04 Å (cycle)−1 at 200 ◦ C and increased to 0.58 Å (cycle)−1 at 450 ◦ C. In the case of soda lime glass substrates, growth rate had a constant value of approximately 0.20 Å (cycle)−1 at 275–350 ◦ C. Outside this temperature range, the Er2 O3 growth rate showed similar behaviour as in the case of silicon substrates. Depositions carried out at 300 ◦ C, i.e. inside the ALD window, showed

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Fig. 1. The results of the thermogravimetric analysis of Er(thd)3 recorded under a reduced pressure (400–500 Pa) in a dynamic nitrogen flow (100 ml/min). The inset shows the ex situ mass spectra of Er(thd)3 collected under UHV conditions with a 70 eV ionisation potential. 125

100

Thickness / nm

that the thin film thickness is linearly dependent on the number of deposition cycles (Fig. 3). This is a characteristic feature of ALD and it provides an easy and accurate way to control the film thickness [24,25]. X-ray diffraction measurements showed that the films deposited above 250 ◦ C were polycrystalline, C-type Er2 O3 . Below 250 ◦ C the films were amorphous. On both substrate materials, the predominant orientation varied as the temperature was increased. Between 275 and 350 ◦ C, the (4 0 0) orientation was preferred, but also the (2 2 2) reflection was observed. Above 350 ◦ C, the (2 2 2) reflection was the strongest one. Also, minor intensity peaks (4 4 0) and (6 2 2) were observed within the temperature range studied (Fig. 4). As the thickness of Er2 O3 films deposited at 300 ◦ C was increased, the relative intensity of (4 0 0) reflection significantly increased. On Si(1 0 0) substrates, the widths of (4 0 0) peaks (FWHM) at 300 ◦ C were in the order of 0.24–0.28◦ .

75

50

Si

25

Soda lime glass 0 0

1000

2000

3000

4000

5000

Number of deposition cycles

Fig. 3. The thickness of Er2 O3 thin films grown at 300 ◦ C as a function of deposition cycles. The growth rate of Er2 O3 on Si(1 0 0) and soda lime glass substrates was 0.25 and 0.23 Å (cycle)−1 , respectively.

1.00

0.60

Relative intensity / %

Growth rate / Å (cycle)

-1

0.70

0.50 0.40 0.30 0.20 Si

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Soda lime glass

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250

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400

450

500

o

Temperature / C

0.80 222 400 440 622

0.60 0.40 0.20 0.00 250

300

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400

450

o

Deposition temperature / C

Fig. 2. The growth rate of Er2 O3 on Si(1 0 0) and soda lime glass substrates as a function of the substrate temperature. The distance between the leading and trailing edges is 5 cm.

Fig. 4. Relative intensities of selected XRD peaks on Si(1 0 0) substrate as a function of the deposition temperature.

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Fig. 5. AFM images of Er2 O3 thin films deposited onto Si(1 0 0) at (a) 250 ◦ C, (b) 300 ◦ C and (c) 400 ◦ C. Thicknesses of the films were 50 nm (a, b) and 70 nm (c). Image size: 2 ␮m × 2 ␮m.

studied being in the order of 0.5–1.8 and 0.7–1.7 at.%, respectively. Both carbon and fluorine contents of the films were at their highest, when the deposition temperature was at its lowest. Fluorine impurity is assumed to originate from the vacuum grease or Teflon gaskets used [30]. The same samples which were used in the TOF-ERD analyses were also analysed by FTIR to qualitatively identify the impurities. For the FTIR study, the Er2 O3 films used as samples were deposited onto Si(1 0 0) at 300–425 ◦ C. A major absorption peak at 553 cm−1 was found in each sample originating from Er2 O3 [5,39]. In addition, weak and broad absorption bands due to the asymmetric carbonate C–O stretching were observed in the range of 1450–1600 and 1350–1450 cm−1 with maxima at 1539 and 1416 cm−1 , respectively [39–41]. When the carbonate peak area was

10.0

0.60 0.55

8.0 Er:O H C F

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4.0

0.45

2.0

0.40 275

300

325

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375

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425

Impurity content / atom%

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Er:O ratio

Results from the XRD measurements are in agreement with the reports for the other ALD-grown RE oxides [28,30,31]. Amorphous or slightly polycrystalline structures are typically obtained in ALD due to the low deposition temperatures. Furthermore, the native SiO2 on Si(1 0 0) substrate hinders the forming of heteroepitaxial Er2 O3 film. AFM measurements were carried out in order to analyse the morphology of the Er2 O3 thin films deposited onto Si(1 0 0) substrates at 200–400 ◦ C (Fig. 5). Films deposited below 350 ◦ C proved to be very smooth and the rms-values of ca. 50 nm thick Er2 O3 films were only 0.3–1.4 nm. A slight increase in the roughness values was observed as the deposition temperature was increased. Above the ALD window, roughness of the Er2 O3 film was significantly higher and an rms-value of 5.6 nm was obtained for a 70 nm thick film deposited at 400 ◦ C. TOF-ERDA was used for elemental analysis of the films. The Er2 O3 films studied were deposited onto Si(1 0 0) at 300–425 ◦ C, i.e. at temperatures inside as well as above the ALD window. The Er:O atomic ratio was 0.59–0.63, which means that the films were nearly stoichiometric, the difference being within the limits of uncertainty. Films contained hydrogen, carbon and fluorine as impurities (Fig. 6). At temperatures inside the ALD window, hydrogen content decreased from 4.0 to 1.7 at.% as deposition temperature was increased from 300 to 375◦ C. When the deposition temperature was raised above 375 ◦ C, hydrogen levels were significantly higher or 5.0 at.%. The observed increase in hydrogen levels when the deposition temperature was raised above the ALD window is probably due to the partial decomposition of the Er(thd)3 precursor. Carbon and fluorine levels remained low throughout the temperature range

0.0 450

Temperature / oC

Fig. 6. Results of TOF-ERD analysis of the Er:O ratio and impurity levels in the Er2 O3 films deposited onto Si(1 0 0) as a function of the deposition temperature.

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normalized in relation to the film thickness, the intensities of carbonate peaks were found to decrease as the deposition temperature was increased. Thus, it appears that the carbon level in Er2 O3 films detected by TOF-ERDA is mainly due to a carbonate-type impurity, which would also explain the slight excess of oxygen in the films and the resulting deviation from ideal stoichiometry.

4. Conclusions Er2 O3 thin film deposition was studied at relative low temperatures, viz. 200–450 ◦ C. A plateau of constant growth rate, i.e. an ALD window, was achieved at 250–375 and at 275–350 ◦ C for silicon and soda lime glass substrates. Within the ALD window, the growth rates of Er2 O3 films deposited onto Si(1 0 0) and soda lime glass were 0.25 and 0.20 Å (cycle)−1 , respectively. The observed optimal deposition temperatures were lower than in the previously reported CVD processes for Er2 O3 , where the substrate temperatures have been around 600 ◦ C [3,4,18]; in plasma-assisted CVD the deposition temperature of 400 ◦ C has been reported [19,20]. Due to the self-limiting film growth mechanism, uniform and smooth Er2 O3 thin films could be deposited, when operated within the ALD window. According to the AFM measurements, the rms-values of Er2 O3 films on Si were around 0.3–1.4 nm. TOF-ERD analyses proved the films to be almost stoichiometric, the Er:O ratio being 0.59–0.63. Films contained low levels of hydrogen (1.7–4.0 at.%), carbon (0.8–1.8 at.%) and fluorine (0.7–1.7 at.%) as impurities. FTIR measurements showed that carbon is present in the films mainly as a carbonate-type impurity, which explains the oxygen excess and the stoichiometry deviating slightly from the ideal one. The crystallinity of the films was found to depend on the deposition temperature. Within the ALD window, the (4 0 0) orientation caused the strongest peak, but the relative intensity of the (2 2 2) reflection increased as the deposition temperature was increased.

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