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Role of the MOCVD deposition conditions on physico-chemical properties of tetragonal ZrO2 thin films
Applied Surface Science 255 (2009) 8986–8994
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Role of the MOCVD deposition conditions on physico-chemical properties of tetragonal ZrO2 thin films K. Galicka-Fau a,*, C. Legros a, M. Andrieux a, M. Brunet b, J. Szade c, G. Garry d a
Univ. Paris Sud 11, LEMHE-ICMMO, CNRS UMR 8182, Baˆt. 410, F-91405 Orsay Cedex, France CNRS, LAAS, Universite´ de Toulouse, 7 av. du Colonel Roche, F-31077 Toulouse Cedex, France c University of Silesia, August Chełkowski Institute of Physics, Uniwersytecka 4, 40-007 Katowice, Poland d THALES Research & Technology France, RD 128, 91767 Palaiseau, France b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 7 April 2009 Received in revised form 14 May 2009 Accepted 16 June 2009 Available online 23 June 2009
High-k ZrO2 thin films suitable for microelectronics applications were deposited by DLI-MOCVD method on planar Si (1 0 0) and pores etched in Si (1 0 0). The effects of various experimental parameters such as temperature of substrates, injection frequency, concentration of the precursor and oxygen partial pressure in the reactive chamber, were investigated in order to produce a single tetragonal ZrO2 phase which exhibits, according to the literature, the best permittivity. Taking into account the crystal structure, microstructure and chemistry of the films, the expected phase was successfully deposited for high temperature of substrates, relatively high feeding rate and low oxygen partial pressure. Although the 3D coverage is actually not perfect in high aspect ratio pores, the electric properties of this sample are very promising with permittivity up to 27. Published by Elsevier B.V.
Keywords: DLI-MOCVD ZrO2 Tetragonal and monoclinic phases 3D High aspect ratio pores
1. Introduction ZrO2 thin films are very interesting materials because of their use as optical and heat resistant coatings, laser mirrors, oxygen ion conductors and sensors, buffer layers for growing superconductors, etc. [1–3]. Interest in ZrO2 thin films has increased recently due to the need to find a replacement for SiO2 (er = 3.9) as a gate dielectric of MOS transistors in the microelectronics industry. Pure ZrO2 films exist in metastable tetragonal, cubic or monoclinic phase [4–6]. The dielectric constant of tetragonal ZrO2 (er(t) = 37.7) is predicted to be much higher than that of monoclinic (er(m) = 19.7) zirconia [7], this suggests that the dielectric constant er is a strong function of the structural arrangement. Thus, deposition of ZrO2 in tetragonal phase is the most desirable to increase the capacitance density of integrated capacitors. Many different methods have been attempted to their synthesis as chemical vapor deposition (CVD), natural oxidation, electron evaporation, metal organic chemical vapor deposition (MOCVD) and other methods [4,5,8], but few of them deal with the effect of deposition conditions on the structural and chemical properties of the so-obtained films.
* Corresponding author. Tel.: +33 1 69 15 70 20; fax: +33 1 69 15 48 19. E-mail address: [email protected] (K. Galicka-Fau). 0169-4332/$ – see front matter . Published by Elsevier B.V. doi:10.1016/j.apsusc.2009.06.067
Liquid injection MOCVD is a useful technique for large scale deposition, but it becomes crucial to find a precursor/solvent/ experimental conditions arrangement that permits to deposit the expected tetragonal ZrO2, and avoids carbon contamination in the grown film, which is known to be a poison for electronic applications [9,10]. Literature data agree well with the fact that when as-deposited thin films, ZrO2 often present a mixture of monoclinic and tetragonal phases and in several studies it has been suggested that the tetragonal phase could be stabilized through its small crystallite size [4–6] but also through the existence of high compressive stresses [4– 6]. Many efforts are presently conducted [4–6,11] to succeed in realizing a highly stabilized tetragonal ZrO2 phase. The aim of this work is to study the role of the MOCVD deposition conditions in order to bring valuable information to grow the tetragonal zirconia on planar silicon (1 0 0) and 3D silicon (1 0 0) (containing pores) for high density MIS (Metal Insulator Semiconductor) capacitors as well as on planar platinum substrates for MIM (Metal Insulator Metal) capacitors. Several deposition parameters have been tested and gathered in four different groups of samples (so-called: groups 1–4). In each group only one deposition parameter was modified in order to clearly identify a relationship between the growing of the tetragonal ZrO2 phase, the chemistry of the layer and experimental conditions. This study is focused on structure, microstructure and chemical characterization to show that by controlling processing MOCVD
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Table 1 Deposition conditions of ZrO2 thin films.
Group 1 Temperature of substrate
Group 2 Concentration of precursor
Group 3 Frequency of injection
Group 4 Oxygen flow
Varied parameter
Samples
550 8C 500 8C 450 8C
12I 26I 25I
60 55 19
n = 2 Hz
0.03 mol/L 0.05 mol/L 0.1 mol/L
12I 36I 40I
60 55 19
Tsub = 550 8C D(O2, N2) = (0.1, 0.1) L/min n = 2 Hz
2 Hz 1 Hz 0.5 Hz
36I 41I 38I
112 59 125
Tsub = 550 8C D(O2, N2) = (0.1, 0.1) L/min c = 0.05 mol/L
0.5 L/min 0.1 L/min 0.05 L/min
4I 36I 39I
202 112 87
Tsub = 550 8C n = 2 Hz c = 0.05 mol/L
parameters, a suitable single phase tetragonal ZrO2, thin film can be obtained for electronic applications. 2. Experiment 2.1. Substrates All samples were deposited on four types of substrates in order to manage the different characterizations. Thus, the planar Si (1 0 0) and planar Si (1 0 0) double-side polished were prepared for the crystal structure, chemistry and microstructure studies. Planar Pt (1 0 0) and 3D high conductivity (20 mV. cm) Si (1 0 0) substrate [12] containing different pore networks were prepared for electrical characterizations. The 3D capacitor structures contained high aspect ratio pores network realized by DRIE (Deep Reactive Ion Etching) with the following dimensions: pore height and width varying from 10 to 40 mm and 2 to 8 mm, respectively. 2.2. Thin films elaboration All thin films were prepared by direct liquid injection MOCVD (DLI-MOCVD) method using the same precursor Zr2(OiPr)6(thd)2. A low pressure cold wall MOCVD industrial reactor was used to realize the films. The reaction chamber is a vertical quartz tube equipped with a rotative susceptor maintained at various temperatures of the substrates. The precursor dissolved in cyclohexane was injected in a furnace maintained at the furnace temperature (Tfurnace = 240 8C). So evaporated, the precursor was transported to the substrate using a nitrogen flow. Decomposition was promoted using both different temperatures and additive oxygen and nitrogen flows. Precise explanation of the experimental procedure and device was presented elsewhere [13]. Nine ZrO2 thin films were deposited under various deposition conditions. The sample parameters are summarized in Table 1. For each group, just one experimental parameter was varied, i.e., the temperature of substrate Tsub [8C], the concentration of the precursor c (mol/L), the frequency of the precursor injection n (Hz) and finally the oxygen gas flow D(O2) (L/min) for the groups 1, 2, 3 and 4, respectively. Moreover, for each group, three various values of the studied parameters were explored. For a given experimental batch, several substrates (for structural analysis, electrical characterizations and spectroscopic measurements) were simultaneously introduced in the reactor so that the obtained films were rigorously the same.
Thickness (nm)
Constant parameters
D(O2, N2) = (0.1, 0.1) L/min c = 0.03 mol/L
2.3. Thin films characterization The crystal structure of samples was verified by grazing incidence (28) X-ray diffraction (XRD) performed on a PANALYTICAL X’pert pro MRD diffractometer using CuKa radiation. All the patterns were realized with detector angle (2u) varying from 278 to 378 with a step of 0.058 and 144 s counting rate. Tetragonal zirconia ZrO2(t), cubic zirconia ZrO2(c) and monoclinic zirconia ZrO2(m) were identified using JCPDS files #17–923, #49–1642 and #37–1484, respectively. As it is difficult by XRD technique to clearly distinguish the differences between ZrO2(c) and ZrO2(t) phases due to the similarity of their patterns, it was decided to focus this study on the ZrO2(t) phase although it cannot be proved that ZrO2(c) phase exists. In order to ascertain this point, further experiments will be conducted to bring more information about nature of the ZrO2 phases. In particular two spectroscopic methods such as Raman [14] and FT-IR [15] could be used since several authors have shown that the three polymorphs of zirconia have an unambiguously typical spectra in the range of 200–700 cm 1. Microstructures of planar deposited ZrO2 thin films were studied using a Scanning Electronic Microscope equipped with a Field Emission Gun (FEG-SEM LEO 1525 GEMINI) and the 3D coverage was observed in cavities with a Hitachi S-4800 SEM. Thin films were also investigated by Fourier Transform InfraRed (FT-IR) using a Perkin Elmer 2000 FT-IR spectrophotometer in the range 4000–220 cm 1 to identify chemical species formed during the deposition of ZrO2 thin film. Each spectrum was obtained by averaging nine or more scans in order to obtain a good S/N ratio. X-ray Photoelectron Spectroscopy (XPS) measurements were carried out using the Physical Electronics PHI 5700 spectrometer, with an AlKa monochromatized radiation (1486.6 eV). The vacuum during the measurements was about 10 10 Torr. The survey spectra were obtained for the as-grown films and after selected cycles of Ar+ bombardment at the ion energy of 2 keV. The total sputtering time was 35 min. One can estimate from the XPS depth profile analysis, not shown in this paper, performed down to the Si substrate that after 35 min about 20 nm of the film was removed. The core level spectra of C1s, Zr3d and O1s were acquired at the resolution of about 0.35 eV. Secondary Ion Mass Spectrometry (SIMS) measurements were performed on thin films and bulk ZrO2 reference using a CAMECA IMS 3F SIMS in the following conditions: Cs+ as primary ions under 10 kV, 15 nA for the primary current, raster size of 125 mm and negative secondary ions. In a first depth profile measurement, only signals of Zr, O, C, H and Si elements were collected since no other
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Fig. 1. SEM-FEG micrographs of ZrO2 thin films deposited under various conditions: (a) sample 4I (Tsub = 550 8C, n = 2 Hz, D(O2) = 0.5 L/min, and c = 0.05 mol/L), (b) sample 36I (Tsub = 550 8C, n = 2 Hz, D(O2) = 0.1 L/min, and c = 0.05 mol/L), (c) sample 39I (Tsub = 550 8C, n = 2 Hz, D(O2) = 0.05 L/min, and c = 0.05 mol/L), (d) sample 25I (Tsub = 450 8C, n = 2 Hz, D(O2) = 0.1 L/min, and c = 0.03 mol/L), (e) sample 41I (Tsub = 550 8C, n = 1 Hz, D(O2) = 0.1 L/min, and c = 0.05 mol/L) and (f) sample 40I (Tsub = 550 8C, n = 2 Hz, D(O2) = 0.1 L/min, and c = 0.1 mol/L).
chemical element was detected. Thus, the depth profiles give evidence of the homogeneity of the as-deposited films. Then, in a second depth profile, measurement was stopped on the surface of the substrate to bring information about thickness of the film. The thicknesses of layers were evaluated by the sputtering rate and also determined thanks to the depth of final craters after SIMS analysis using TENCOR Alpha Step 500 surface profilometer with a resolution of 5 nm. 3. Results All thin films were characterized with various analysis methods but for clarity, only representative results showing the influence of the studied parameter will be discussed. Moreover, whatever the experimental conditions, thin films exhibit the same homogeneous microstructure more or less dense as shown in Fig. 1. Thin films were made of small crystallites with an average size of 10–30 nm that was estimated with SEM observations. This value is in good agreement with the crystallite size estimated by the Scherrer formula applied to the XRD peaks of the tetragonal and monoclinic phases presented in previous work [6] and with the literature data [5,16].
Fig. 2. Grazing incidence (28) X-ray diffraction patterns of as-deposited thin films as a function of the temperature of the substrates Tsub [8C] (samples 12I, 26I and 25I).
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Table 2 Carbon content C1s (%) and O1s/Zr3d ratio on the surface and in the depth of the films estimated from XPS measurements for 12I, 26I and 25I thin films (values ‘‘Inside film*’’ were obtained after 35 min of Ar+ sputtering). 12I
C1s(%) Ratio O1s/Zr3d
26I
25I
Surface
Inside film*
Surface
Inside film*
Surface
Inside film*
45.58 2.18
5.38 1.87
45.60 2.20
5.40 1.79
45.20 2.49
6.63 1.29
3.1. Influence of the temperature of substrates Tsub [8C] (group 1) Fig. 2 shows XRD diffraction patterns of zirconia thin films deposited at different temperatures of substrates. Depending on Tsub [8C] chosen during deposition, the films 12I and 26I, deposited at 550 and 500 8C, respectively, were crystallized in a mixture of both tetragonal and monoclinic phases whereas they were amorphous for lower temperature (25I). For crystallized films (12I and 26I), on the X-ray diffraction patterns recorded between 278 and 378, two intensive peaks: (1 1 1)t at 30.178 and (2 0 0)t, at 35.318 of the tetragonal phase are clearly observed. In addition, two relatively intensive diffraction lines ( 1 1 1)m at 29.38 and (1 1 1)m at 31.058 are belonged unambiguously to the monoclinic phase. It can be noticed that increasing the temperature of substrates from 500 to 550 8C enhances significantly the intensity of the expected tetragonal peaks (1 1 1)t and (2 0 0)t. FT-IR spectra, which are shown in Fig. 3a, clearly revealed that the strong absorption band located in the 400–800 cm 1 region is
Fig. 3. FT-IR spectra as–deposited zirconia thin films: (a) as a function of the temperature of the substrates Tsub [8C] (samples 12I, 26I and 25I) and (b) as a function of the oxygen gas flow D(O2) (L/min) (samples 36I and 39I).
typical of zirconium–oxygen (Zr–O) bonding and those located between 4000 and 600 cm 1 with a small spectral activity are associated with unreacted organic compounds in the films, which typically involve carbon–hydrogen bonding (C–H) between 600 and 700 cm 1, carbon–oxygen bonding (C–O and C5 5O) between 1300 and 1700 cm 1, –CH2 bonding between 2800 and 3000 cm 1, and oxygen–hydrogen (O–H) bonding between 3000 and 3800 cm 1. Depending on the substrate temperature, no significant changes were found except the absorption band at 480 cm 1 relative to Zr–O bonding. This band is well defined for films 12I and 26I, which were deposited at 550 8C and 500 8C, respectively, while for 25I (Tsub = 450 8C) a broadening of this Zr–O band is observed, which is an indication of the disordering of the structure. FT-IR spectra also showed that organic species were not well decomposed during the deposition of the film, leading to films containing carbon atoms which are harmful for electronic applications. Further XPS study was conducted to check if the carbon contamination corresponds only to organic species adsorbed on the surface of the film or contrary included in the depth of the film. The survey photoemission spectra collected for this group exhibited some surface contamination with carbon and oxygen, but no nitrogen that could unexpectedly appear during MOCVD deposition. The stoichiometry of the films (ratio O1s/Zr3d) as well as the atomic concentration of carbon estimated on the surface and in the depth of the films from XPS measurements are presented in Table 2. For surface spectra of C1s two bonds appeared: the most intensive one at about 286 eV caused by typical contamination by C–H species existing in air and the less intense at about 290 eV due to carbon–oxygen bonding. After 35 min of Ar+ sputtering, the carbon content in the depth of the film was the most important for the lower temperature of the substrate (equals 6.63%), while for two others temperatures it did not exceed 5.40% (Table 2). The stoichiometry of ZrO2 requires that ratio O1s/Zr3d equals 2. After
Fig. 4. Grazing incidence (28) X-ray diffraction patterns of as-deposited thin films: as a function of frequency n (Hz) (samples 36I, 41I and 38I) and as a function of the concentration of the precursor c (mol/L) (samples 36I and 40I).
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argon etching processes, this ratio was estimated to be: 1.29, 1.79 and 1.87 for 25I (450 8C), 26I (500 8C) and 12I (550 8C), respectively which suggests that Tsub = 450 8C is not high enough to elaborate the stoichiometric ZrO2 films. Thickness of these three films deduced from SIMS/profilometry analysis increases from 19 nm to 55 nm and 60 nm for 450 8C, 550 8C and 500 8C, respectively. 3.2. Influence of the precursor concentration c [mol/L] (group 2) Considering the above results, a temperature of substrates of 550 8C was selected and the precursor concentration was then varied for the deposition of ZrO2 thin films. Similarly to previous group, thin films presented also a mixture of both monoclinic and tetragonal phases manifested by the presence of the ( 1 1 1)m, (1 1 1)m and (1 1 1)t, (2 0 0)t, peaks, respectively (Fig. 4). Additionally, other monoclinic and/or tetragonal diffraction peaks: (0 0 2)t, (0 2 0)m and (2 0 0)m located at around 348 can be observed for the 36I film, as shown in Fig. 4. However, depending on precursor concentration c [mol/L] injected during the deposition, the contribution of the monoclinic phase can be varied. It is worth noting that no difference was observed between XRD patterns for the films 36I and 40I while a significant change of the intensity peaks was noted between 36I and 12I (see Fig. 2). Decreasing the precursor concentration from 0.1 mol/L and/ or 0.05–0.03 mol/L strongly increases the intensity of the (1 1 1)t representing the diffraction peak of the expected tetragonal crystal structure. And since the ratio between (2 0 0)t/(1 1 1)t differs significantly; 1.57 for sample 12I (c = 0.03 mol/L) and 8.05 for sample 36I (c = 0.05 mol/L) and/or 40I (c = 0.1 mol/L), a strongly textured thin film may be obtained likewise. The carbon contamination studied by FT-IR and XPS seems to be not sensitive to the concentration of the precursor. The percentage of the C1s analysis from the depth of the films is always close to 5%. However, the precursor concentration c [mol/L] seems to have an influence on the O1s/Zr3d ratio calculated from the depth of the films since this ratio equals 1.87 for film 12I and 2 for two others 36I and 40I. 3.3. Influence of the injection frequency n [Hz] (group 3) According to the above results, Tsub equals 550 8C and a medium concentration of precursor c = 0.05 mol/L was fixed and the feeding rate of the precursor through the injection frequency n [Hz] was modified during the process. Various frequencies of the precursor injection from n = 2 to n = 0.5 Hz were investigated. Independently on the frequency n [Hz], XRD patterns of all samples presented diffraction peaks belonging to both monoclinic and tetragonal phases as can be seen in Fig. 4. Therefore, the intensity of the monoclinic peaks ( 1 1 1)m and (1 1 1)m seems to be slightly influenced by the injection frequency. On the other hand, the intensity of the tetragonal peak (2 0 0)t changes significantly with injection frequency n [Hz]. For 36I film (2 Hz), the (2 0 0)t is the most intense peak whereas it is minimum for 41I film (1 Hz). It is also worth noticing that frequency of injection changes the (2 0 0)t/ (1 1 1)t intensity ratio which equals 8.05, 1.78 and 1.47 for 36I (2 Hz), 38I (0.5 Hz) and 41I (1 Hz) films, respectively. FT-IR spectra are very similar for all three films. The Zr–O bonding at 480 cm 1 is well defined for each thin film and absorption bands related to the carbon contamination are present in each film too. Nevertheless, the Zr–O bonding at 480 cm 1 is better defined for samples 41I and 36I which were deposited at high frequency. The XPS spectra confirmed the carbon contamination with the same content on the surface (2.3%) and onto the depths of the films (5%). In the same way, since for all three films the O1s/Zr3d ratio calculated from the depth of the films is always
Fig. 5. Grazing incidence (28) X-ray diffraction patterns of as-deposited thin films as a function of the oxygen gas flow D(O2) (L/min) (samples 4I, 36I and 39I).
close to 1.85, the stoichiometry of the films seems to be not influenced by the injection frequency n [Hz]. 3.4. Influence of the oxygen gas flow D(O2) [L/min] (group 4) As the best results were obtained for a higher feeding rate, the frequency was selected to be 2 Hz in the following experiments and we explored one of the most important parameter in the reactor, the oxygen gas flow. Fig. 5 presents the X-ray diffraction patterns of 4I, 36I and 39I films recorded between 278 and 378. In this range, a mixture of both monoclinic and tetragonal phase was also detected. Both intense diffraction peaks observed at about 30.178 and 35.318 correspond to the tetragonal diffraction lines (1 1 1)t, and (2 0 0)t, respectively. These two peaks are well defined for 4I and 39I, while for 36I, instead of the diffraction line (1 1 1)t, a broad peak was observed. Moreover, exclusively for 4I film, an additional diffraction peak attributed to monoclinic diffraction line (1 1 1)m appeared at about 31.058. It is worth noting that an oxygen rich atmosphere favors or enhances in the same time the contribution of the undesirable monoclinic crystal structure. FT-IR spectra of 36I and 39I films were also presented in Fig. 3b. For both films, the Zr–O bonding at 480 cm 1 is well defined, but seems to be narrower for the lower oxygen flow (39I) which could suggest a better defined ZrO2 crystal structure. Furthermore FT-IR spectra showed that a lowest spectral activity of carbon–hydrogen bonding (C–H), carbon–oxygen bonding (C–O) and oxygen– hydrogen (O–H) bonding was observed for 39I film. This result suggested that with a low oxygen gas flow the organic molecules could be removed completely. Fig. 6 presents the XPS spectra of 4I, 36I and 39I in the range 280–293 eV and Table 3 summarizes the atomic concentration of Zr, C and O collected on their surfaces and after 35 min of argon Ar+ etching. For surface spectra of C1s, for the three films, two peaks appeared: the most intensive one at about 286 eV was caused by typical contamination by carbon–oxygen and a less intense one at about 290 eV was due to volatile C–H species existing in air. Surfaces of all films seem to be contaminated by carbon approximately in the same way (Table 3) but when the weaker oxygen flow during deposition process, the smaller carbon contamination on the surface. The sputtered spectra of 39I and 36I presented no carbon contamination while for 4I a new peak at 282 eV could be observed in Fig. 6. This peak could correspond to
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Table 3 Atomic concentrations of: C1s, O1s and Zr3d on surfaces and in the depths of films for samples 4I, 36I and 39I (values ‘‘Inside film*’’ were obtained after 35 min of Ar+ sputtering). 4I
C1s(%) O1s(%) Zr3d(%) Ratio O1s/Zr3d
36I
39I
Surface
Inside film*
Surface
Inside film*
Surface
Inside film*
47.02 35.60 17.38 2.04
5.09 53.71 41.14 1.30
45.15 38.56 16.11 2.39
5.04 64.18 32.27 1.98
38.19 41.69 18.97 2.19
2.67 65.26 32.2 2.02
C–Zr bonding from zirconium carbide since in Zr3d core level spectra, exclusively for this 4I film, low energy bond at 180 eV is observed, probably due to the presence of a ZrC phase in this film. Generally, for all samples when spectra are collected from deeper layers of the films, the concentration of carbon becomes smaller. This regularity is the best defined for 39I film, i.e., for the lowest oxygen flow. Concerning the O1s/Zr3d ratio, it equals 1.30 for 4I, while for 36I and 39I values of 1.98 and 2.02 were calculated, respectively. It is worth noting that for the lowest oxygen gas flow the oxygen–zirconium ratio is close to 2 and is not a function of the studied depth. It is not the case of the 36I film since O1s/Zr3d ratio calculated on the surface equals 2.39, and calculated from the depth profile spectra is decreasing (from 2.39 to 1.79) with increasing of the studied depth. Fig. 7 exhibits SIMS depth profiles as a function of sputtering time (s) for the three films of this group. First of all, the thicknesses of the layers deduced from profilometry measurements varied from 202 nm, 112 nm to 87 nm, respectively for 4I, 36I and 39I films. Thus it could be concluded that the film thickness increases as oxygen flow increases. Focusing on the Zr and O intensity signals allow us to demonstrate the chemical homogeneity of the film. For 36I sample deposited at oxygen flow 0.1 L/min, the distributions of Zr and O species were not stable, as shown in Fig. 6. Contrary, for both: 4I and 39I samples, Zr and O signals stayed roughly at the same level all along the oxide layer sputtering indicating that these species are well distributed in the layers. 4. Discussion 4.1. On the chemistry, composition and carbon contamination of the as-deposited thin films To succeed in realizing ZrO2(t) single phase as suggested in introduction, the first step is to manage the experimental parameters in order to achieve a stoichiometric ZrO2 film (i.e., a film with a O/Zr ratio (XPS O1s/Zr3d ratio) close to 2. Results gathered in Tables 2 and 3 show that a good O/Zr ratio is found at the surface and inside the film (i.e., in the bulk of the film) for films deposited at 550 8C. Whereas the feeding rate (through injection frequency and precursor concentration) has no significant influence, oxygen flow rate parameter seems to modify the O/Zr ratio in the films. Indeed, for low oxygen flow (D(O2) = 0.05 L/min) that is used during deposition process, a small difference between O/Zr ratio at the surface and in the bulk of the film is obtained. For these experimental conditions (low oxygen flow and high temperature), the more stoichiometric ZrO2 film is performed. On the other hand, to realize a good dielectric material, a lesser amount of carbon contamination in the depth of the film should also be obtained [9,10]. Otherwise, carbon contamination would increase the leakage current in the film and makes it not suitable for electronic applications. Considering this parameter, whose data are gathered in Tables 2 and 3, and also in Fig. 6, one can conclude that the less carbon contain is found at higher temperature and lower oxygen flow rate. That is in good agreement with the literature as it is well known that a higher temperature would
Fig. 6. The XPS narrow-scan spectra for C1s obtained between 280 and 293 eV on surface and after 35 min of the Ar+ sputtering for samples 4I, 36I and 39I.
pyrolyse carbon into carbon dioxide in the reactor [17,18]. As the precursor self-contained oxygen in its molecular structure, it is not necessary to provide too much oxygen in the reactor for the combustion of carbon atoms. For higher oxygen partial pressure, the species activities are modified and the precursor is probably oxidized and broken up before arriving near the substrate (homogeneous gas phase reaction) [19]. As a consequence, the carbon content increases in the film because lots of radicals (carboxylates and hydroxides) would be adsorbed on the surface of the substrates. FT-IR results confirm this explanation as we can note in Fig. 3a and b that the carboxylate stretch band around 1500 cm 1 and hydroxide stretch band around 3400 cm 1 have lower activities for higher temperature and lower oxygen gas flow (i.e., oxygen partial pressure). One can wonder that the carbon contamination is located at the surface and/or in the bulk of the film. Such a surface carbon contamination has been already seen for SrZrO3 thin films deposition [9,10,19] and an extra thermal treatment was sufficient to remove the carboxylate and hydroxide species adsorbed on the surface of the films. Unfortunately, as the size of the crystallites of tetragonal phase is close to the critical size above which the tetragonal phase would transform into the monoclinic ones [5,6,16,20], it is not possible to proceed to an additional annealing that would increase the size of the crystallites by grain coarsening phenomena. As FT-IR gives information of the whole layer and XPS local information only, the regularity of the carbon, zirconium and oxygen contents all along the depth of the films have been also controlled with SIMS measurements. The SIMS profiles presented in Fig. 7 showed that stable species contents could be obtained at lower oxygen flow. Meanwhile, this figure shows that depending on the experimental conditions (temperature, frequency or concentration), the C, Zr and O signals
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carbon contamination and O/Zr profiles in the depth of the films are achieved for films deposited in the following conditions: Tsub = 550 8C, n = 2 Hz, c = 0.05 mol/L, and D(O2) = 0.05 L/mn. 4.2. On the crystallization of the thin films The crystal structure has a strong influence on the electronic properties of ZrO2 films. The permittivity varies between 19.7 and 37.7 for monoclinic and tetragonal phases, respectively [7]. It depends on the deposition conditions such as temperature of substrates, oxygen gas flow, precursor concentration and frequency of injection. As shown in Fig. 2, depending on the Tsub [8C], the as-deposited thin films were either crystallized (Tsub = 500 8C or/and 550 8C) or amorphous (Tsub = 450 8C). Most of the time, the as-deposited films are composed of tetragonal phase, with some contribution of the monoclinic one, depending on the experimental conditions. For the same reason that was discussed on the previous part (i.e., crystallite coarsening during heat treatment), it is not possible to post-anneal the films to crystallize them or to texture them. It is not so obvious to conclude using XRD technique whether or not it is a tetragonal or a cubic phase. Several authors [21,22] affirm that a cubic phase could be stabilized at room temperature if the crystallite size is small enough (typically several nanometers). But their studies deal with nanometric powders and as we focus our study on thin films and as the crystallite size measured on SEM pictures and calculated on XRD patterns was close to 50 nm, the discussion was conducted on the ZrO2(t) phase. Ultimately, the Tsub [8C] is the parameter which has a strong influence on the crystal structure of the as-deposited films. The other experimental parameters play a role through the texturation of the thin films. Indeed, the concentration of the precursor has strong influence on the (2 0 0)t/(1 1 1)t ratio calculated using XRD patterns. This ratio increases when concentration rises; it could be a good approach to develop (2 0 0)t single phase thin films. The (2 0 0)t/(1 1 1)t ratio is also very sensitive to the variation of the frequency n [Hz]. Decreasing the frequency of the injection n [Hz] from 2 to 1 Hz can significantly lower the (2 0 0)t/(1 1 1)t ratio between tetragonal peaks. Thereby, a preferable (1 1 1)t orientation can be selected. Further pole figures measurement would be conducted to make the light on this point. 4.3. On the conformal deposition on 3D substrates
Fig. 7. SIMS depth profiles of 1H, 12C, deposited thin films 4I, 36I and 39I.
18
O,
30
Si and
90
Zr chemical elements of as-
could fluctuate during the sputtering whereas the O/Zr signal could stay roughly at the same level. As a consequence, even if stable Zr and O signals as in the case of thin film 38I (injection frequency equals 0.5 Hz), a higher frequency (2 Hz) is preferred as the O/Zr ratio is almost constant. Moreover, for this higher injection frequency, the suitable O1s/Zr3d ratio is attained. The best compromise in terms of stoichiometric ZrO2 (XPS O/Zr ratio),
The 3D coverage was observed in cavities with different depths (from 10 to 40 mm) and widths (from 2 to 8 mm). Fig. 8a shows the schematic of the 3D silicon substrates and Fig. 8b–d present the SEM pictures of the measured thicknesses on the cross-section of sample 36I. For all samples, the deposition regimes in the pores were described in the same way through the film thickness inside the pores normalized to the top thickness versus the pore aspect ratio (depth/width). On the whole experimental conditions that were explored, only peculiar conditions could be exploited; the 3D coverage was very bad at temperature lower than 550 8C (group 1 is concerned in particular) and thus in this part, the most promising results obtained for the films deposited at 550 8C will be discussed. For group 3 (i.e., when injection frequency is varied) the ZrO2 thickness was measured inside the cavities at various depths. Fig. 9 shows the normalized thickness versus the pore aspect ratio for 36I, 41I and 38I. One can see that for all the frequencies, the thickness decreases down to 20–30% of its top value for aspect ratios above 2 with a slight improvement for low frequency (38I at n = 0.5 Hz). For the groups 2 and 4 (concentration of the precursor, oxygen gas flow), there was no major difference observed in the 3D coverage compared to the previous results.
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Fig. 8. The cross-sectional pictures of silicon pores with ZrO2 layer for sample 36I (Tsub = 550 8C, n = 2 Hz, D(O2) = 0.1 L/min, and c = 0.05 mol/L): (a) schematic representation of the 3D pores etched in the silicon substrates, (b) SEM general view of 40 mm high aspect ratio pores, (c) SEM detailed view of top structure, and (d) SEM detailed view at 10 mm deep.
These results prove that the MOCVD deposition conditions may have an influence on the 3D coverage. The temperature should be high enough to perform a good diffusion of reactive species. At a given temperature of 550 8C, the feeding rate (through the concentration of the precursor or the injection frequency) should partially modify the 3D coverage. This is probably due to a competition between two regimes in the reactive chamber: a surface reaction limited regime and a diffusion limited regime [12]. Unfortunately, it is not possible to manage experiment in our CVD device with total pressure below 100 Pa. If it could be possible, it would allow an increase in the mean length path of molecules and thus would favor the diffusion limited regime that is required for a suitable conformal deposition in high aspect ratio pores. The ZrO2 tetragonal phase was systematically deposited on planar Pt (1 0 0) substrates during the study. Unfortunately, the Pt evaporated layer was textured and stressed so that the ZrO2 films
presented micro-cracks. It was thus not possible to perform electrical characterizations on these samples. MIS capacitors were fabricated by depositing metallic contact (Ti/Au) on top of the films (100 mm 100 mm area) and on the back side of the silicon substrate. Electrical characterizations were then conducted by performing CV measurements with an impedance analyser Agilent 4294A and the results can be found in Ref. [12]. A relatively good permittivity for ZrO2 films presenting a tetragonal phase (such as 39I) was determined: it is equal to 27 in good agreement with other values found in the literature [23,24]. As it is lower than the theoretical value reported for tetragonal phase, it could be attributed first, to the presence of a native oxide (e = 3.9) at the interface between the silicon substrate and ZrO2 film, then to the texture of the film. It was predicted by Zhao and Vanderbilt [7] that the permittivity of the tetragonal phase would be degraded (down to 15) in the preferred z-axis orientation
Fig. 9. Normalized thickness of the ZrO2 layer versus the pores aspect ratio (depth/width).
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(perpendicular to the surface). Pole figures measurement on the film could help understanding this particular point. 5. Conclusion Influence of the experimental parameters on the deposition of ZrO2 was studied. The zirconia films were deposited by DLIMOCVD on planar Si (1 0 0) and pores etched in Si (1 0 0). As the tetragonal ZrO2 phase exhibits the higher permittivity (er(t) = 37.7), this metastable phase was sought for microelectronics applications. This study proved that variation of the deposition parameters (temperature of substrate, injection frequency, concentration of precursor and oxygen partial pressure in the reactive chamber) influence the structure of the film such as monoclinic/ tetragonal ratio, tetragonal (2 0 0)t orientation of the film but also the chemistry of the film. Among all the deposition parameters, we clearly demonstrated that the ones mentioned above were the predominant to manage the structure of the zirconia films and to control their purity (in terms of carbon contamination, carboxylates,. . .). This could be helpful in CVD process of others oxides films. In our case the expected tetragonal phase was successfully deposited for the higher (than 450 8C) temperature of substrates explored in the experimental plan, with oxygen gas flow D(O2) = 0.05 L/min, concentration of the precursor c = 0.1 mol/L and frequency of the injection n = 2 Hz. In such experimental conditions, a promising permittivity in agreement with the literature data was extracted from electric measurements. Acknowledgements The authors would like to thank I. Gallet, M. Herbst-Ghysel, F. Jomard and B. Servet, E. Scheid for their irreplaceable help with this work.
This work was funded by French National Research Agency (project ANR-06-JCJC-0081). References [1] K. Izumi, M. Murakami, T. Deguchi, A. Morita, J. Am. Ceram. Soc. 72 (1989) 1465– 1468. [2] M.A. Cameron, S.M. George, Thin Solid Films 348 (1999) 90–98. [3] A. Bastianini, G.A. Basttiston, R. Gerbasi, M. Porchia, S. Daolio, J. Phys. IV 5 (1995) 525–531. [4] B. Benali, M. Herbst-Ghysel, I. Gallet, A.M. Huntz, M. Andrieux, Appl. Surf. Sci. 253 (2006) 1222–1226. [5] Y. Shen, S. Shao, H. Yu, Z. Fan, H. He, J. Shao, Appl. Surf. Sci. 254 (2007) 552–556. [6] B. Benali, A.M. Huntz, M. Andrieux, M. Ignat, S. Poissonnet, Appl. Surf. Sci. 254 (2008) 5807–5813. [7] X. Zhao, D. Vanderbilt, Phys. Rev. B 65 (2002) 075105–75109. [8] A.P. Huang, Z.F. Di, R.K.Y. Fu, P.K. Chu, Surf. Coat. Technol. 201 (2007) 8282–8285. [9] P.G. Karlsson, J.H. Richter, J. Blomquist, P. Uvdal, T.M. Grehk, A. Sandell, Surf. Sci. 601 (2007) 1008–1018. [10] J.J. Yu, J.Y. Zhang, I.W. Boyd, Appl. Surf. Sci. 186 (2002) 190–194. [11] S. Zhao, F. Ma, Z. Song, K. Xu, Opt. Mater. 30 (6) (2008) 910–915. [12] M. Brunet, E. Scheid, K. Galicka-Fau, M. Andrieux, C. Legros, I. Gallet, M. Herbst, S. Schamm, Microelectron. Eng., doi:10.1016/j.mee.2009.01.034, in press. [13] M. Andrieux, C. Gasquerers, C. Legros, I. Gallet, M. Herbst-Ghysel, M. Condat, V.G. Kessler, G.A. Seisenbaeva, O. Heintz, S. Poissonnet, Appl. Surf. Sci. 253 (2007) 9091–9098. [14] L.D. Huy, P. Laffez, Ph. Daniel, A. Jouanneaux, N. Khoi, D. Sime´one, Mater. Sci. Eng. B 104 (2003) 163–168. [15] J. Si, Ch.H. Peng, S.B. Desu, Mater. Res. Soc. Symp. Proc. 250 (1992) 323–329. [16] T. Charska, A.H. King, Ch.C. Berndt, Mater. Sci. Eng. A 286 (2000) 169–178. [17] G. Stefanic, S. Music, A. Sekulic, Thermochim. Acta 273 (1996) 119–133. [18] C.M. Phillippi, K.S. Mazdiyasni, J. Am. Ceram. Soc. 54 (1971) 254–258. [19] K. Galicka-Fau, C. Legros, M. Andrieux, M. Herbst-Ghysel, I. Gallet, M. Condat, O. Durand, B. Servet, Thin Solid Films 516 (2008) 7967–7973. [20] A. Christensen, E.A. Carter, Phys. Rev. B 58 (1998) 8050–8064. [21] D.G. Lamas, A.M. Rosso, M.S. Anzorena, A. Fernandez, M.G. Bellino, M.D. Cabezas, N.E. Walsoe de Reca, A.F. Craievich, Scripta Mater. 55 (2006) 553–556. [22] S. Roy, J. Ghose, Mater. Res. Bull. 35 (2000) 1195–1203. [23] E.P. Gusev, E. Cartier, D.A. Buchanan, M. Gribelyuk, M. Copel, H. Okorn-Schmidt, C. D’Emic, Microelectron. Eng. 59 (2001) 341–349. [24] G.D. Wilk, R.M. Wallace, J.M. Anthony, J. Appl. Phys. 89 (2001) 5243–5275.
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Role of the MOCVD deposition conditions on physico-chemical properties of tetragonal ZrO2 thin films K. Galicka-Fau a,*, C. Legros a, M. Andrieux a, M. Brunet b, J. Szade c, G. Garry d a
Univ. Paris Sud 11, LEMHE-ICMMO, CNRS UMR 8182, Baˆt. 410, F-91405 Orsay Cedex, France CNRS, LAAS, Universite´ de Toulouse, 7 av. du Colonel Roche, F-31077 Toulouse Cedex, France c University of Silesia, August Chełkowski Institute of Physics, Uniwersytecka 4, 40-007 Katowice, Poland d THALES Research & Technology France, RD 128, 91767 Palaiseau, France b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 7 April 2009 Received in revised form 14 May 2009 Accepted 16 June 2009 Available online 23 June 2009
High-k ZrO2 thin films suitable for microelectronics applications were deposited by DLI-MOCVD method on planar Si (1 0 0) and pores etched in Si (1 0 0). The effects of various experimental parameters such as temperature of substrates, injection frequency, concentration of the precursor and oxygen partial pressure in the reactive chamber, were investigated in order to produce a single tetragonal ZrO2 phase which exhibits, according to the literature, the best permittivity. Taking into account the crystal structure, microstructure and chemistry of the films, the expected phase was successfully deposited for high temperature of substrates, relatively high feeding rate and low oxygen partial pressure. Although the 3D coverage is actually not perfect in high aspect ratio pores, the electric properties of this sample are very promising with permittivity up to 27. Published by Elsevier B.V.
Keywords: DLI-MOCVD ZrO2 Tetragonal and monoclinic phases 3D High aspect ratio pores
1. Introduction ZrO2 thin films are very interesting materials because of their use as optical and heat resistant coatings, laser mirrors, oxygen ion conductors and sensors, buffer layers for growing superconductors, etc. [1–3]. Interest in ZrO2 thin films has increased recently due to the need to find a replacement for SiO2 (er = 3.9) as a gate dielectric of MOS transistors in the microelectronics industry. Pure ZrO2 films exist in metastable tetragonal, cubic or monoclinic phase [4–6]. The dielectric constant of tetragonal ZrO2 (er(t) = 37.7) is predicted to be much higher than that of monoclinic (er(m) = 19.7) zirconia [7], this suggests that the dielectric constant er is a strong function of the structural arrangement. Thus, deposition of ZrO2 in tetragonal phase is the most desirable to increase the capacitance density of integrated capacitors. Many different methods have been attempted to their synthesis as chemical vapor deposition (CVD), natural oxidation, electron evaporation, metal organic chemical vapor deposition (MOCVD) and other methods [4,5,8], but few of them deal with the effect of deposition conditions on the structural and chemical properties of the so-obtained films.
* Corresponding author. Tel.: +33 1 69 15 70 20; fax: +33 1 69 15 48 19. E-mail address: [email protected] (K. Galicka-Fau). 0169-4332/$ – see front matter . Published by Elsevier B.V. doi:10.1016/j.apsusc.2009.06.067
Liquid injection MOCVD is a useful technique for large scale deposition, but it becomes crucial to find a precursor/solvent/ experimental conditions arrangement that permits to deposit the expected tetragonal ZrO2, and avoids carbon contamination in the grown film, which is known to be a poison for electronic applications [9,10]. Literature data agree well with the fact that when as-deposited thin films, ZrO2 often present a mixture of monoclinic and tetragonal phases and in several studies it has been suggested that the tetragonal phase could be stabilized through its small crystallite size [4–6] but also through the existence of high compressive stresses [4– 6]. Many efforts are presently conducted [4–6,11] to succeed in realizing a highly stabilized tetragonal ZrO2 phase. The aim of this work is to study the role of the MOCVD deposition conditions in order to bring valuable information to grow the tetragonal zirconia on planar silicon (1 0 0) and 3D silicon (1 0 0) (containing pores) for high density MIS (Metal Insulator Semiconductor) capacitors as well as on planar platinum substrates for MIM (Metal Insulator Metal) capacitors. Several deposition parameters have been tested and gathered in four different groups of samples (so-called: groups 1–4). In each group only one deposition parameter was modified in order to clearly identify a relationship between the growing of the tetragonal ZrO2 phase, the chemistry of the layer and experimental conditions. This study is focused on structure, microstructure and chemical characterization to show that by controlling processing MOCVD
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Table 1 Deposition conditions of ZrO2 thin films.
Group 1 Temperature of substrate
Group 2 Concentration of precursor
Group 3 Frequency of injection
Group 4 Oxygen flow
Varied parameter
Samples
550 8C 500 8C 450 8C
12I 26I 25I
60 55 19
n = 2 Hz
0.03 mol/L 0.05 mol/L 0.1 mol/L
12I 36I 40I
60 55 19
Tsub = 550 8C D(O2, N2) = (0.1, 0.1) L/min n = 2 Hz
2 Hz 1 Hz 0.5 Hz
36I 41I 38I
112 59 125
Tsub = 550 8C D(O2, N2) = (0.1, 0.1) L/min c = 0.05 mol/L
0.5 L/min 0.1 L/min 0.05 L/min
4I 36I 39I
202 112 87
Tsub = 550 8C n = 2 Hz c = 0.05 mol/L
parameters, a suitable single phase tetragonal ZrO2, thin film can be obtained for electronic applications. 2. Experiment 2.1. Substrates All samples were deposited on four types of substrates in order to manage the different characterizations. Thus, the planar Si (1 0 0) and planar Si (1 0 0) double-side polished were prepared for the crystal structure, chemistry and microstructure studies. Planar Pt (1 0 0) and 3D high conductivity (20 mV. cm) Si (1 0 0) substrate [12] containing different pore networks were prepared for electrical characterizations. The 3D capacitor structures contained high aspect ratio pores network realized by DRIE (Deep Reactive Ion Etching) with the following dimensions: pore height and width varying from 10 to 40 mm and 2 to 8 mm, respectively. 2.2. Thin films elaboration All thin films were prepared by direct liquid injection MOCVD (DLI-MOCVD) method using the same precursor Zr2(OiPr)6(thd)2. A low pressure cold wall MOCVD industrial reactor was used to realize the films. The reaction chamber is a vertical quartz tube equipped with a rotative susceptor maintained at various temperatures of the substrates. The precursor dissolved in cyclohexane was injected in a furnace maintained at the furnace temperature (Tfurnace = 240 8C). So evaporated, the precursor was transported to the substrate using a nitrogen flow. Decomposition was promoted using both different temperatures and additive oxygen and nitrogen flows. Precise explanation of the experimental procedure and device was presented elsewhere [13]. Nine ZrO2 thin films were deposited under various deposition conditions. The sample parameters are summarized in Table 1. For each group, just one experimental parameter was varied, i.e., the temperature of substrate Tsub [8C], the concentration of the precursor c (mol/L), the frequency of the precursor injection n (Hz) and finally the oxygen gas flow D(O2) (L/min) for the groups 1, 2, 3 and 4, respectively. Moreover, for each group, three various values of the studied parameters were explored. For a given experimental batch, several substrates (for structural analysis, electrical characterizations and spectroscopic measurements) were simultaneously introduced in the reactor so that the obtained films were rigorously the same.
Thickness (nm)
Constant parameters
D(O2, N2) = (0.1, 0.1) L/min c = 0.03 mol/L
2.3. Thin films characterization The crystal structure of samples was verified by grazing incidence (28) X-ray diffraction (XRD) performed on a PANALYTICAL X’pert pro MRD diffractometer using CuKa radiation. All the patterns were realized with detector angle (2u) varying from 278 to 378 with a step of 0.058 and 144 s counting rate. Tetragonal zirconia ZrO2(t), cubic zirconia ZrO2(c) and monoclinic zirconia ZrO2(m) were identified using JCPDS files #17–923, #49–1642 and #37–1484, respectively. As it is difficult by XRD technique to clearly distinguish the differences between ZrO2(c) and ZrO2(t) phases due to the similarity of their patterns, it was decided to focus this study on the ZrO2(t) phase although it cannot be proved that ZrO2(c) phase exists. In order to ascertain this point, further experiments will be conducted to bring more information about nature of the ZrO2 phases. In particular two spectroscopic methods such as Raman [14] and FT-IR [15] could be used since several authors have shown that the three polymorphs of zirconia have an unambiguously typical spectra in the range of 200–700 cm 1. Microstructures of planar deposited ZrO2 thin films were studied using a Scanning Electronic Microscope equipped with a Field Emission Gun (FEG-SEM LEO 1525 GEMINI) and the 3D coverage was observed in cavities with a Hitachi S-4800 SEM. Thin films were also investigated by Fourier Transform InfraRed (FT-IR) using a Perkin Elmer 2000 FT-IR spectrophotometer in the range 4000–220 cm 1 to identify chemical species formed during the deposition of ZrO2 thin film. Each spectrum was obtained by averaging nine or more scans in order to obtain a good S/N ratio. X-ray Photoelectron Spectroscopy (XPS) measurements were carried out using the Physical Electronics PHI 5700 spectrometer, with an AlKa monochromatized radiation (1486.6 eV). The vacuum during the measurements was about 10 10 Torr. The survey spectra were obtained for the as-grown films and after selected cycles of Ar+ bombardment at the ion energy of 2 keV. The total sputtering time was 35 min. One can estimate from the XPS depth profile analysis, not shown in this paper, performed down to the Si substrate that after 35 min about 20 nm of the film was removed. The core level spectra of C1s, Zr3d and O1s were acquired at the resolution of about 0.35 eV. Secondary Ion Mass Spectrometry (SIMS) measurements were performed on thin films and bulk ZrO2 reference using a CAMECA IMS 3F SIMS in the following conditions: Cs+ as primary ions under 10 kV, 15 nA for the primary current, raster size of 125 mm and negative secondary ions. In a first depth profile measurement, only signals of Zr, O, C, H and Si elements were collected since no other
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Fig. 1. SEM-FEG micrographs of ZrO2 thin films deposited under various conditions: (a) sample 4I (Tsub = 550 8C, n = 2 Hz, D(O2) = 0.5 L/min, and c = 0.05 mol/L), (b) sample 36I (Tsub = 550 8C, n = 2 Hz, D(O2) = 0.1 L/min, and c = 0.05 mol/L), (c) sample 39I (Tsub = 550 8C, n = 2 Hz, D(O2) = 0.05 L/min, and c = 0.05 mol/L), (d) sample 25I (Tsub = 450 8C, n = 2 Hz, D(O2) = 0.1 L/min, and c = 0.03 mol/L), (e) sample 41I (Tsub = 550 8C, n = 1 Hz, D(O2) = 0.1 L/min, and c = 0.05 mol/L) and (f) sample 40I (Tsub = 550 8C, n = 2 Hz, D(O2) = 0.1 L/min, and c = 0.1 mol/L).
chemical element was detected. Thus, the depth profiles give evidence of the homogeneity of the as-deposited films. Then, in a second depth profile, measurement was stopped on the surface of the substrate to bring information about thickness of the film. The thicknesses of layers were evaluated by the sputtering rate and also determined thanks to the depth of final craters after SIMS analysis using TENCOR Alpha Step 500 surface profilometer with a resolution of 5 nm. 3. Results All thin films were characterized with various analysis methods but for clarity, only representative results showing the influence of the studied parameter will be discussed. Moreover, whatever the experimental conditions, thin films exhibit the same homogeneous microstructure more or less dense as shown in Fig. 1. Thin films were made of small crystallites with an average size of 10–30 nm that was estimated with SEM observations. This value is in good agreement with the crystallite size estimated by the Scherrer formula applied to the XRD peaks of the tetragonal and monoclinic phases presented in previous work [6] and with the literature data [5,16].
Fig. 2. Grazing incidence (28) X-ray diffraction patterns of as-deposited thin films as a function of the temperature of the substrates Tsub [8C] (samples 12I, 26I and 25I).
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Table 2 Carbon content C1s (%) and O1s/Zr3d ratio on the surface and in the depth of the films estimated from XPS measurements for 12I, 26I and 25I thin films (values ‘‘Inside film*’’ were obtained after 35 min of Ar+ sputtering). 12I
C1s(%) Ratio O1s/Zr3d
26I
25I
Surface
Inside film*
Surface
Inside film*
Surface
Inside film*
45.58 2.18
5.38 1.87
45.60 2.20
5.40 1.79
45.20 2.49
6.63 1.29
3.1. Influence of the temperature of substrates Tsub [8C] (group 1) Fig. 2 shows XRD diffraction patterns of zirconia thin films deposited at different temperatures of substrates. Depending on Tsub [8C] chosen during deposition, the films 12I and 26I, deposited at 550 and 500 8C, respectively, were crystallized in a mixture of both tetragonal and monoclinic phases whereas they were amorphous for lower temperature (25I). For crystallized films (12I and 26I), on the X-ray diffraction patterns recorded between 278 and 378, two intensive peaks: (1 1 1)t at 30.178 and (2 0 0)t, at 35.318 of the tetragonal phase are clearly observed. In addition, two relatively intensive diffraction lines ( 1 1 1)m at 29.38 and (1 1 1)m at 31.058 are belonged unambiguously to the monoclinic phase. It can be noticed that increasing the temperature of substrates from 500 to 550 8C enhances significantly the intensity of the expected tetragonal peaks (1 1 1)t and (2 0 0)t. FT-IR spectra, which are shown in Fig. 3a, clearly revealed that the strong absorption band located in the 400–800 cm 1 region is
Fig. 3. FT-IR spectra as–deposited zirconia thin films: (a) as a function of the temperature of the substrates Tsub [8C] (samples 12I, 26I and 25I) and (b) as a function of the oxygen gas flow D(O2) (L/min) (samples 36I and 39I).
typical of zirconium–oxygen (Zr–O) bonding and those located between 4000 and 600 cm 1 with a small spectral activity are associated with unreacted organic compounds in the films, which typically involve carbon–hydrogen bonding (C–H) between 600 and 700 cm 1, carbon–oxygen bonding (C–O and C5 5O) between 1300 and 1700 cm 1, –CH2 bonding between 2800 and 3000 cm 1, and oxygen–hydrogen (O–H) bonding between 3000 and 3800 cm 1. Depending on the substrate temperature, no significant changes were found except the absorption band at 480 cm 1 relative to Zr–O bonding. This band is well defined for films 12I and 26I, which were deposited at 550 8C and 500 8C, respectively, while for 25I (Tsub = 450 8C) a broadening of this Zr–O band is observed, which is an indication of the disordering of the structure. FT-IR spectra also showed that organic species were not well decomposed during the deposition of the film, leading to films containing carbon atoms which are harmful for electronic applications. Further XPS study was conducted to check if the carbon contamination corresponds only to organic species adsorbed on the surface of the film or contrary included in the depth of the film. The survey photoemission spectra collected for this group exhibited some surface contamination with carbon and oxygen, but no nitrogen that could unexpectedly appear during MOCVD deposition. The stoichiometry of the films (ratio O1s/Zr3d) as well as the atomic concentration of carbon estimated on the surface and in the depth of the films from XPS measurements are presented in Table 2. For surface spectra of C1s two bonds appeared: the most intensive one at about 286 eV caused by typical contamination by C–H species existing in air and the less intense at about 290 eV due to carbon–oxygen bonding. After 35 min of Ar+ sputtering, the carbon content in the depth of the film was the most important for the lower temperature of the substrate (equals 6.63%), while for two others temperatures it did not exceed 5.40% (Table 2). The stoichiometry of ZrO2 requires that ratio O1s/Zr3d equals 2. After
Fig. 4. Grazing incidence (28) X-ray diffraction patterns of as-deposited thin films: as a function of frequency n (Hz) (samples 36I, 41I and 38I) and as a function of the concentration of the precursor c (mol/L) (samples 36I and 40I).
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argon etching processes, this ratio was estimated to be: 1.29, 1.79 and 1.87 for 25I (450 8C), 26I (500 8C) and 12I (550 8C), respectively which suggests that Tsub = 450 8C is not high enough to elaborate the stoichiometric ZrO2 films. Thickness of these three films deduced from SIMS/profilometry analysis increases from 19 nm to 55 nm and 60 nm for 450 8C, 550 8C and 500 8C, respectively. 3.2. Influence of the precursor concentration c [mol/L] (group 2) Considering the above results, a temperature of substrates of 550 8C was selected and the precursor concentration was then varied for the deposition of ZrO2 thin films. Similarly to previous group, thin films presented also a mixture of both monoclinic and tetragonal phases manifested by the presence of the ( 1 1 1)m, (1 1 1)m and (1 1 1)t, (2 0 0)t, peaks, respectively (Fig. 4). Additionally, other monoclinic and/or tetragonal diffraction peaks: (0 0 2)t, (0 2 0)m and (2 0 0)m located at around 348 can be observed for the 36I film, as shown in Fig. 4. However, depending on precursor concentration c [mol/L] injected during the deposition, the contribution of the monoclinic phase can be varied. It is worth noting that no difference was observed between XRD patterns for the films 36I and 40I while a significant change of the intensity peaks was noted between 36I and 12I (see Fig. 2). Decreasing the precursor concentration from 0.1 mol/L and/ or 0.05–0.03 mol/L strongly increases the intensity of the (1 1 1)t representing the diffraction peak of the expected tetragonal crystal structure. And since the ratio between (2 0 0)t/(1 1 1)t differs significantly; 1.57 for sample 12I (c = 0.03 mol/L) and 8.05 for sample 36I (c = 0.05 mol/L) and/or 40I (c = 0.1 mol/L), a strongly textured thin film may be obtained likewise. The carbon contamination studied by FT-IR and XPS seems to be not sensitive to the concentration of the precursor. The percentage of the C1s analysis from the depth of the films is always close to 5%. However, the precursor concentration c [mol/L] seems to have an influence on the O1s/Zr3d ratio calculated from the depth of the films since this ratio equals 1.87 for film 12I and 2 for two others 36I and 40I. 3.3. Influence of the injection frequency n [Hz] (group 3) According to the above results, Tsub equals 550 8C and a medium concentration of precursor c = 0.05 mol/L was fixed and the feeding rate of the precursor through the injection frequency n [Hz] was modified during the process. Various frequencies of the precursor injection from n = 2 to n = 0.5 Hz were investigated. Independently on the frequency n [Hz], XRD patterns of all samples presented diffraction peaks belonging to both monoclinic and tetragonal phases as can be seen in Fig. 4. Therefore, the intensity of the monoclinic peaks ( 1 1 1)m and (1 1 1)m seems to be slightly influenced by the injection frequency. On the other hand, the intensity of the tetragonal peak (2 0 0)t changes significantly with injection frequency n [Hz]. For 36I film (2 Hz), the (2 0 0)t is the most intense peak whereas it is minimum for 41I film (1 Hz). It is also worth noticing that frequency of injection changes the (2 0 0)t/ (1 1 1)t intensity ratio which equals 8.05, 1.78 and 1.47 for 36I (2 Hz), 38I (0.5 Hz) and 41I (1 Hz) films, respectively. FT-IR spectra are very similar for all three films. The Zr–O bonding at 480 cm 1 is well defined for each thin film and absorption bands related to the carbon contamination are present in each film too. Nevertheless, the Zr–O bonding at 480 cm 1 is better defined for samples 41I and 36I which were deposited at high frequency. The XPS spectra confirmed the carbon contamination with the same content on the surface (2.3%) and onto the depths of the films (5%). In the same way, since for all three films the O1s/Zr3d ratio calculated from the depth of the films is always
Fig. 5. Grazing incidence (28) X-ray diffraction patterns of as-deposited thin films as a function of the oxygen gas flow D(O2) (L/min) (samples 4I, 36I and 39I).
close to 1.85, the stoichiometry of the films seems to be not influenced by the injection frequency n [Hz]. 3.4. Influence of the oxygen gas flow D(O2) [L/min] (group 4) As the best results were obtained for a higher feeding rate, the frequency was selected to be 2 Hz in the following experiments and we explored one of the most important parameter in the reactor, the oxygen gas flow. Fig. 5 presents the X-ray diffraction patterns of 4I, 36I and 39I films recorded between 278 and 378. In this range, a mixture of both monoclinic and tetragonal phase was also detected. Both intense diffraction peaks observed at about 30.178 and 35.318 correspond to the tetragonal diffraction lines (1 1 1)t, and (2 0 0)t, respectively. These two peaks are well defined for 4I and 39I, while for 36I, instead of the diffraction line (1 1 1)t, a broad peak was observed. Moreover, exclusively for 4I film, an additional diffraction peak attributed to monoclinic diffraction line (1 1 1)m appeared at about 31.058. It is worth noting that an oxygen rich atmosphere favors or enhances in the same time the contribution of the undesirable monoclinic crystal structure. FT-IR spectra of 36I and 39I films were also presented in Fig. 3b. For both films, the Zr–O bonding at 480 cm 1 is well defined, but seems to be narrower for the lower oxygen flow (39I) which could suggest a better defined ZrO2 crystal structure. Furthermore FT-IR spectra showed that a lowest spectral activity of carbon–hydrogen bonding (C–H), carbon–oxygen bonding (C–O) and oxygen– hydrogen (O–H) bonding was observed for 39I film. This result suggested that with a low oxygen gas flow the organic molecules could be removed completely. Fig. 6 presents the XPS spectra of 4I, 36I and 39I in the range 280–293 eV and Table 3 summarizes the atomic concentration of Zr, C and O collected on their surfaces and after 35 min of argon Ar+ etching. For surface spectra of C1s, for the three films, two peaks appeared: the most intensive one at about 286 eV was caused by typical contamination by carbon–oxygen and a less intense one at about 290 eV was due to volatile C–H species existing in air. Surfaces of all films seem to be contaminated by carbon approximately in the same way (Table 3) but when the weaker oxygen flow during deposition process, the smaller carbon contamination on the surface. The sputtered spectra of 39I and 36I presented no carbon contamination while for 4I a new peak at 282 eV could be observed in Fig. 6. This peak could correspond to
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Table 3 Atomic concentrations of: C1s, O1s and Zr3d on surfaces and in the depths of films for samples 4I, 36I and 39I (values ‘‘Inside film*’’ were obtained after 35 min of Ar+ sputtering). 4I
C1s(%) O1s(%) Zr3d(%) Ratio O1s/Zr3d
36I
39I
Surface
Inside film*
Surface
Inside film*
Surface
Inside film*
47.02 35.60 17.38 2.04
5.09 53.71 41.14 1.30
45.15 38.56 16.11 2.39
5.04 64.18 32.27 1.98
38.19 41.69 18.97 2.19
2.67 65.26 32.2 2.02
C–Zr bonding from zirconium carbide since in Zr3d core level spectra, exclusively for this 4I film, low energy bond at 180 eV is observed, probably due to the presence of a ZrC phase in this film. Generally, for all samples when spectra are collected from deeper layers of the films, the concentration of carbon becomes smaller. This regularity is the best defined for 39I film, i.e., for the lowest oxygen flow. Concerning the O1s/Zr3d ratio, it equals 1.30 for 4I, while for 36I and 39I values of 1.98 and 2.02 were calculated, respectively. It is worth noting that for the lowest oxygen gas flow the oxygen–zirconium ratio is close to 2 and is not a function of the studied depth. It is not the case of the 36I film since O1s/Zr3d ratio calculated on the surface equals 2.39, and calculated from the depth profile spectra is decreasing (from 2.39 to 1.79) with increasing of the studied depth. Fig. 7 exhibits SIMS depth profiles as a function of sputtering time (s) for the three films of this group. First of all, the thicknesses of the layers deduced from profilometry measurements varied from 202 nm, 112 nm to 87 nm, respectively for 4I, 36I and 39I films. Thus it could be concluded that the film thickness increases as oxygen flow increases. Focusing on the Zr and O intensity signals allow us to demonstrate the chemical homogeneity of the film. For 36I sample deposited at oxygen flow 0.1 L/min, the distributions of Zr and O species were not stable, as shown in Fig. 6. Contrary, for both: 4I and 39I samples, Zr and O signals stayed roughly at the same level all along the oxide layer sputtering indicating that these species are well distributed in the layers. 4. Discussion 4.1. On the chemistry, composition and carbon contamination of the as-deposited thin films To succeed in realizing ZrO2(t) single phase as suggested in introduction, the first step is to manage the experimental parameters in order to achieve a stoichiometric ZrO2 film (i.e., a film with a O/Zr ratio (XPS O1s/Zr3d ratio) close to 2. Results gathered in Tables 2 and 3 show that a good O/Zr ratio is found at the surface and inside the film (i.e., in the bulk of the film) for films deposited at 550 8C. Whereas the feeding rate (through injection frequency and precursor concentration) has no significant influence, oxygen flow rate parameter seems to modify the O/Zr ratio in the films. Indeed, for low oxygen flow (D(O2) = 0.05 L/min) that is used during deposition process, a small difference between O/Zr ratio at the surface and in the bulk of the film is obtained. For these experimental conditions (low oxygen flow and high temperature), the more stoichiometric ZrO2 film is performed. On the other hand, to realize a good dielectric material, a lesser amount of carbon contamination in the depth of the film should also be obtained [9,10]. Otherwise, carbon contamination would increase the leakage current in the film and makes it not suitable for electronic applications. Considering this parameter, whose data are gathered in Tables 2 and 3, and also in Fig. 6, one can conclude that the less carbon contain is found at higher temperature and lower oxygen flow rate. That is in good agreement with the literature as it is well known that a higher temperature would
Fig. 6. The XPS narrow-scan spectra for C1s obtained between 280 and 293 eV on surface and after 35 min of the Ar+ sputtering for samples 4I, 36I and 39I.
pyrolyse carbon into carbon dioxide in the reactor [17,18]. As the precursor self-contained oxygen in its molecular structure, it is not necessary to provide too much oxygen in the reactor for the combustion of carbon atoms. For higher oxygen partial pressure, the species activities are modified and the precursor is probably oxidized and broken up before arriving near the substrate (homogeneous gas phase reaction) [19]. As a consequence, the carbon content increases in the film because lots of radicals (carboxylates and hydroxides) would be adsorbed on the surface of the substrates. FT-IR results confirm this explanation as we can note in Fig. 3a and b that the carboxylate stretch band around 1500 cm 1 and hydroxide stretch band around 3400 cm 1 have lower activities for higher temperature and lower oxygen gas flow (i.e., oxygen partial pressure). One can wonder that the carbon contamination is located at the surface and/or in the bulk of the film. Such a surface carbon contamination has been already seen for SrZrO3 thin films deposition [9,10,19] and an extra thermal treatment was sufficient to remove the carboxylate and hydroxide species adsorbed on the surface of the films. Unfortunately, as the size of the crystallites of tetragonal phase is close to the critical size above which the tetragonal phase would transform into the monoclinic ones [5,6,16,20], it is not possible to proceed to an additional annealing that would increase the size of the crystallites by grain coarsening phenomena. As FT-IR gives information of the whole layer and XPS local information only, the regularity of the carbon, zirconium and oxygen contents all along the depth of the films have been also controlled with SIMS measurements. The SIMS profiles presented in Fig. 7 showed that stable species contents could be obtained at lower oxygen flow. Meanwhile, this figure shows that depending on the experimental conditions (temperature, frequency or concentration), the C, Zr and O signals
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carbon contamination and O/Zr profiles in the depth of the films are achieved for films deposited in the following conditions: Tsub = 550 8C, n = 2 Hz, c = 0.05 mol/L, and D(O2) = 0.05 L/mn. 4.2. On the crystallization of the thin films The crystal structure has a strong influence on the electronic properties of ZrO2 films. The permittivity varies between 19.7 and 37.7 for monoclinic and tetragonal phases, respectively [7]. It depends on the deposition conditions such as temperature of substrates, oxygen gas flow, precursor concentration and frequency of injection. As shown in Fig. 2, depending on the Tsub [8C], the as-deposited thin films were either crystallized (Tsub = 500 8C or/and 550 8C) or amorphous (Tsub = 450 8C). Most of the time, the as-deposited films are composed of tetragonal phase, with some contribution of the monoclinic one, depending on the experimental conditions. For the same reason that was discussed on the previous part (i.e., crystallite coarsening during heat treatment), it is not possible to post-anneal the films to crystallize them or to texture them. It is not so obvious to conclude using XRD technique whether or not it is a tetragonal or a cubic phase. Several authors [21,22] affirm that a cubic phase could be stabilized at room temperature if the crystallite size is small enough (typically several nanometers). But their studies deal with nanometric powders and as we focus our study on thin films and as the crystallite size measured on SEM pictures and calculated on XRD patterns was close to 50 nm, the discussion was conducted on the ZrO2(t) phase. Ultimately, the Tsub [8C] is the parameter which has a strong influence on the crystal structure of the as-deposited films. The other experimental parameters play a role through the texturation of the thin films. Indeed, the concentration of the precursor has strong influence on the (2 0 0)t/(1 1 1)t ratio calculated using XRD patterns. This ratio increases when concentration rises; it could be a good approach to develop (2 0 0)t single phase thin films. The (2 0 0)t/(1 1 1)t ratio is also very sensitive to the variation of the frequency n [Hz]. Decreasing the frequency of the injection n [Hz] from 2 to 1 Hz can significantly lower the (2 0 0)t/(1 1 1)t ratio between tetragonal peaks. Thereby, a preferable (1 1 1)t orientation can be selected. Further pole figures measurement would be conducted to make the light on this point. 4.3. On the conformal deposition on 3D substrates
Fig. 7. SIMS depth profiles of 1H, 12C, deposited thin films 4I, 36I and 39I.
18
O,
30
Si and
90
Zr chemical elements of as-
could fluctuate during the sputtering whereas the O/Zr signal could stay roughly at the same level. As a consequence, even if stable Zr and O signals as in the case of thin film 38I (injection frequency equals 0.5 Hz), a higher frequency (2 Hz) is preferred as the O/Zr ratio is almost constant. Moreover, for this higher injection frequency, the suitable O1s/Zr3d ratio is attained. The best compromise in terms of stoichiometric ZrO2 (XPS O/Zr ratio),
The 3D coverage was observed in cavities with different depths (from 10 to 40 mm) and widths (from 2 to 8 mm). Fig. 8a shows the schematic of the 3D silicon substrates and Fig. 8b–d present the SEM pictures of the measured thicknesses on the cross-section of sample 36I. For all samples, the deposition regimes in the pores were described in the same way through the film thickness inside the pores normalized to the top thickness versus the pore aspect ratio (depth/width). On the whole experimental conditions that were explored, only peculiar conditions could be exploited; the 3D coverage was very bad at temperature lower than 550 8C (group 1 is concerned in particular) and thus in this part, the most promising results obtained for the films deposited at 550 8C will be discussed. For group 3 (i.e., when injection frequency is varied) the ZrO2 thickness was measured inside the cavities at various depths. Fig. 9 shows the normalized thickness versus the pore aspect ratio for 36I, 41I and 38I. One can see that for all the frequencies, the thickness decreases down to 20–30% of its top value for aspect ratios above 2 with a slight improvement for low frequency (38I at n = 0.5 Hz). For the groups 2 and 4 (concentration of the precursor, oxygen gas flow), there was no major difference observed in the 3D coverage compared to the previous results.
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Fig. 8. The cross-sectional pictures of silicon pores with ZrO2 layer for sample 36I (Tsub = 550 8C, n = 2 Hz, D(O2) = 0.1 L/min, and c = 0.05 mol/L): (a) schematic representation of the 3D pores etched in the silicon substrates, (b) SEM general view of 40 mm high aspect ratio pores, (c) SEM detailed view of top structure, and (d) SEM detailed view at 10 mm deep.
These results prove that the MOCVD deposition conditions may have an influence on the 3D coverage. The temperature should be high enough to perform a good diffusion of reactive species. At a given temperature of 550 8C, the feeding rate (through the concentration of the precursor or the injection frequency) should partially modify the 3D coverage. This is probably due to a competition between two regimes in the reactive chamber: a surface reaction limited regime and a diffusion limited regime [12]. Unfortunately, it is not possible to manage experiment in our CVD device with total pressure below 100 Pa. If it could be possible, it would allow an increase in the mean length path of molecules and thus would favor the diffusion limited regime that is required for a suitable conformal deposition in high aspect ratio pores. The ZrO2 tetragonal phase was systematically deposited on planar Pt (1 0 0) substrates during the study. Unfortunately, the Pt evaporated layer was textured and stressed so that the ZrO2 films
presented micro-cracks. It was thus not possible to perform electrical characterizations on these samples. MIS capacitors were fabricated by depositing metallic contact (Ti/Au) on top of the films (100 mm 100 mm area) and on the back side of the silicon substrate. Electrical characterizations were then conducted by performing CV measurements with an impedance analyser Agilent 4294A and the results can be found in Ref. [12]. A relatively good permittivity for ZrO2 films presenting a tetragonal phase (such as 39I) was determined: it is equal to 27 in good agreement with other values found in the literature [23,24]. As it is lower than the theoretical value reported for tetragonal phase, it could be attributed first, to the presence of a native oxide (e = 3.9) at the interface between the silicon substrate and ZrO2 film, then to the texture of the film. It was predicted by Zhao and Vanderbilt [7] that the permittivity of the tetragonal phase would be degraded (down to 15) in the preferred z-axis orientation
Fig. 9. Normalized thickness of the ZrO2 layer versus the pores aspect ratio (depth/width).
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(perpendicular to the surface). Pole figures measurement on the film could help understanding this particular point. 5. Conclusion Influence of the experimental parameters on the deposition of ZrO2 was studied. The zirconia films were deposited by DLIMOCVD on planar Si (1 0 0) and pores etched in Si (1 0 0). As the tetragonal ZrO2 phase exhibits the higher permittivity (er(t) = 37.7), this metastable phase was sought for microelectronics applications. This study proved that variation of the deposition parameters (temperature of substrate, injection frequency, concentration of precursor and oxygen partial pressure in the reactive chamber) influence the structure of the film such as monoclinic/ tetragonal ratio, tetragonal (2 0 0)t orientation of the film but also the chemistry of the film. Among all the deposition parameters, we clearly demonstrated that the ones mentioned above were the predominant to manage the structure of the zirconia films and to control their purity (in terms of carbon contamination, carboxylates,. . .). This could be helpful in CVD process of others oxides films. In our case the expected tetragonal phase was successfully deposited for the higher (than 450 8C) temperature of substrates explored in the experimental plan, with oxygen gas flow D(O2) = 0.05 L/min, concentration of the precursor c = 0.1 mol/L and frequency of the injection n = 2 Hz. In such experimental conditions, a promising permittivity in agreement with the literature data was extracted from electric measurements. Acknowledgements The authors would like to thank I. Gallet, M. Herbst-Ghysel, F. Jomard and B. Servet, E. Scheid for their irreplaceable help with this work.
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