Article pubs.acs.org/JPCC
The Cut-Off Phenomenon Effect on ZrO2 Growth Using Remote Plasma-Enhanced Atomic Layer Deposition Zheng Chen,† Haoran Wang,† Pengpeng Xiong,† Ping Chen,† Huiying Li,‡ Yunfei Liu,‡ and Yu Duan*,†,§ †
State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun, 130012, JilinChina ‡ Computer Science and Technology Department, Jilin University, Changchun 130012, Jilin, China § College of Science, Changchun University of Science and Technology, Changchun, 130012, Jilin, China ABSTRACT: Remote plasma-enhanced atomic layer deposition (R-PEALD) of zirconium dioxide (ZrO2) experiments were conducted using tetrakis(dimethylamino)zirconium (TDMAZ) with O2 plasma as the oxidant. The reaction mechanisms of ZrO2 were studied using in situ quartz crystal microbalance (QCM) and in situ quadrupole mass spectroscopy (QMS). QMS revealed typical combustion byproducts such as CO2, CO, NO, and H2O during the O2 plasma process. In addition, Fourier transform infrared spectroscopy (FTIR) measurements were used to identify the bonds present in the thin film at different deposition temperatures. In our previous work, it was found that an increase in temperature resulted in a reduction of impurities in thin films. The influence of the deposition temperature on several possible surface reaction characteristics of the plasma process was studied. Such characteristics included composition of the film and growth per cycle. In particular, it was first demonstrated that the −CN group gave rise to cutoff phenomenon at high temperature. Several reaction pathways were accordingly established. The present work initiates a new way of achieving controlled growth properties of ZrO2 thin films.
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INTRODUCTION Compared to other techniques, atomic layer deposition (ALD) is promising from the perspective of deposition rate control down to the Ångstrom scale and for conformality.1−3 In the ALD process, the precursors are alternately pulsed into the reactor and separated by purging with an inert carrier gas. The surface is successively saturated with monolayers of adsorbed or reacted precursor molecules. This results in a self-limiting growth mechanism. Thus, the thickness of the grown film is accurately controlled by the number of ALD cycles.4 The reactions at each step of the ALD cycle are often complex. Understanding such mechanisms helps to control and optimize the growth processes and to advance the development of new ALD processes and precursors. The remote plasma-enhanced ALD (R-PEALD) technique provides growth characteristics that are similar to those of typical thermal ALD. It also enables rapid deposition using O2 plasma instead of water vapor because the approaches yield the highest quality films and minimize thermal damage to the weak substrate during the deposition process.5 The ZrO2 film is a promising material used in many applications such as corrosion barriers, 6,7 high-k gate dielectrics,8,9 and resistive memory.10 To fabricate the necessary pinhole-free and dense thin film, ALD and PEALD have been widely used for ZrO2 thin film deposition.11,12 The ZrO2 films produced using the PEALD method are polycrystalline and have rough surfaces.13 Because of the relatively high boiling © XXXX American Chemical Society
temperature of tetrakis(dimethylamino)zirconium (TDMAZ)and the particular ALD window temperature,1 it is possible to study the effects of deposition temperature on the properties of the nanostructured ZrO2 thin films prepared using R-PEALD. In this study, we used Fourier transform infrared spectroscopy (FTIR) to analyze the thin film components at different deposition temperatures. The FTIR analytical method is common for investigating ALD reaction mechanisms. The infrared absorption spectrum of the sample within a certain range and the absorbance values are directly proportional to thickness and concentration, so the absorbance spectrum can be used for quantitative analysis. Therefore, FTIR is a widely used technique for analyzing thin films in different deposition environments and with different precursors. We found that the thin film components changed with a change in temperature. Interestingly, it generates −CN group formation in the plasma process at high temperature. This is a significant interruption for halting growth in the high temperature ALD process.
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EXPERIMENTAL SECTION The ZrO2 films were deposited by a R-PEALD (kemicro-150A) system. The chamber pressure was 0.15 Torr. TDMAZ and O2 Received: January 8, 2017 Revised: February 9, 2017 Published: February 9, 2017 A
DOI: 10.1021/acs.jpcc.7b00211 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C plasma were used as the precursors of Zr and O, respectively. TDMAZ was maintained at 85 °C. The R-PEALD cycle consisted of a 0.15 s dose of TDMAZ, a 150 s purge using 50 sccm high purity argon, and a 20 s 100 W remote O2 plasma exposure using 15 sccm O2 followed by a 60 s argon purge. The present gas species in the reactor during the PEALD process was investigated using an in situ quadrupole mass spectrometer (QMS) at 1 × 10−5 Torr. The ZrO2 thin films were deposited on Si substrates for atomic force microscopy (AFM) and on KBr tabletting for FTIR analysis. The Si substrates were successively cleaned ultrasonically in acetone and ethyl alcohol for 5 min at 40 °C. They were then rinsed using deionized water. After cleaning, the substrates were dried and put into the deposition chamber of the R-PEALD system. FTIR was performed using a VERTEX 70 (Bruker) with resolution of 4 cm−1. Samples for FTIR spectroscopy were grown on KBr tabletting with 100 cycles. The surface roughness of the test samples was measured using atomic force microscopy (AFM).
Figure 2. Illustration of the reaction in the O2 plasma process.
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RESULTS AND DISCUSSION Well-established R-PEALD chemistry was used to deposit ZrO2 using TDMAZ and O2 plasma. A self-limiting surface reaction was observed during the growth process of our report. The in situ QMS analysis of the R-PEALD ZrO2 revealed a combustion-like reaction in the plasma process. Figure 1
Figure 3. Thickness of ZrO2 measurement for three PEALD cycles of the TDMAZ−O2 plasma process at 80 °C by QCM.
increase because the QCM are influenced by O2 plasma. The following purge, the mass stabilized after a slight decrease due to the disappearance of the plasma impact. However, the height difference (Δd) maintain a relatively stable value, which implies that there is a self-limiting and controllable thickness of the surface reaction. To investigate the effect of temperature on the growth rate, the thin films deposited in the temperature range of 80 °C-250 °C by QCM measurement were plotted in Figure 4. The growth rate of ZrO2 was controlled at the atomic scale during the process, using the values 1.714, 1.5, 2.18, 2.17, and 1.24 Å/ cyc at 80, 120, 160, 200, and 250 °C, respectively. The film uniformity of the ZrO2 films was found to be more stable at
Figure 1. QMS signals for important byproducts of the ZrO2 deposition process.
shows the signals investigated with QMS and the corresponding fragments in the complete cycle. During the O2 plasma process, the molecular weights were observed at m/z = 44, m/z = 43, m/z = 30, m/z = 28, and m/z = 18, which respectively correspond to CO2, NH(CH3)2, NO, CO, and H2O. These are typical combustion byproducts in O2 plasma processes. The signal originating from fragments of the organometallic precursor (m/z = 43) was observed after the TDMAZ dose. This indicates an adsorption of the precursor without highspeed reaction of its ligand with an −OH group on the surface. Figure 2 displays the complex combustion reaction on the surface. The observed signals of QMS indicate that an extreme case of molecular or stoichiometric adsorption of TDMAZ followed by a combustion-like reaction may occur.14 Figure 3 shows the QCM mass change for the three cycles of the deposition process. First, the mass increased during the TDMAZ pulse, inducing adsorption of the precursor molecules on the surface. The O2 plasma process causes a significant
Figure 4. Growth rate and surface roughness of R-PEALD ZrO2 thin films as a function of substrate temperature. B
DOI: 10.1021/acs.jpcc.7b00211 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C
from the remaining −N(CH3)2. The reports also found that the “steric-hindrance effect” caused by ligand resistance might lead to a decrease in the growth rate. Ritala and Morozov proposed how the amount of the reaction source covering the entire substrate surface determined the maximum growth rate, which meant the reaction source reached a maximum amount when the reaction source physically adsorbed onto the surface of the substrate.18−20 In fact, the reaction source was chemically adsorbed onto the surface because of the ligand exchange with the chemical groups on the substrate surface. Ylilammi proposed a model in which the number of source chemical reactions determined the maximum growth rate when the chemical group, after ligand exchange, fully covered the surface.21 Siimon and Puurunen et al. proposed that the size of the reaction source ligand determined the saturation of the chemical reaction and the growth rate.22,23 The experimental data showed that, with a change in growth temperature, the growth rate was about 15% ∼ 30% of the monolayer adsorption.24 The absorbance peak at 780 cm−1 corresponds to the Zr− O−Zr stretching vibrations created by the chemical bond, which is also responsible for the additional Zr−O stretch observed at 424 cm−1. Between 80 and 200 °C, we observed that the content of Zr−O and Zr−O−Zr bonds relative to the content of −CH3 was rising. It is noteworthy that there was an observed higher reactivity with an increase in the growth temperature. Such a difference in the thickness of the thin film could have resulted from a different growth rate at a different temperature with the same cycle number. Therefore, the absorbance intensity of all of the peaks had a greater change on the revealed growth more fully with increaseing temperature. Interestingly, beginning at 200 °C, the −CN bond at 2170 cm−1 appeared and began to increase. However, the content of Zr−O and Zr−O−Zr bonds relative to the content of − CH3 at 1384 cm−1 decreased. Earlier studies of ALD with SiO2 observed vibrational features such as Si−NCH2.25 The subsequent H2O exposure was determined to react with N(CH3)2 or NCH2 surface species and to leave a surface hydroxyl at 550 °C. It has also been observed that the decomposition of Ti(N(CH3)2)4 forms CN and CH2 features above 200 °C.26−29 The decomposition pathway that produces the CC vibrational feature is not known. However, the H2CCH2 reaction products were observed during chemical vapor decomposition using Ta(N(CH3)2)5.30 In our study, we found that there are a large number of −CN bonds at 250 °C, and at the same time, the Zr−O−Zr stretching vibration peak ceased to increasing trend. Therefore, we speculate that the −CN bond inhibits the formation and results in decreasing in the growth rate. Figure 6 depicts possible adsorption and reaction steps at the ALD window and at higher temperature. The possible surface reactions during the O2 plasma exposure are given at the ALD window (Figure 6a) and eq A. However, part of dimethyl amine generated −CN, which inhibited the continued growth of the thin film, is shown in Figure 6b and eq B. In order to further explore the characteristics of the plasma process, FTIR was used to compare the H2O based-ZrO2 and the O2 plasma based-ZrO2 at 250 °C (Figure 7). We have not found that − CN existed in H2O based-ZrO2 thin films. We speculate that this is a cutoff phenomenon usually at high temperature. The phenomenon hinders the chain structure at a high temperature. Therefore, selection and design of the precursor have an important influence.
higher deposition temperatures. At a deposition temperature of 80 °C, the roughness is clearly higher than that at other temperatures. In this experiment, the growth rate outside the window temperature was lower than that at the window temperature. In previous reports, the adsorption of the precursor was incomplete at lower temperature and decomposed at higher temperature. Growth rate was more stable between 160−200 °C, and at a higher growth rate, the deposition temperature varied in the range of the ALD window temperature. However, at 250 °C, the growth rate decreased significantly due to the decomposition of the precursor and due to desorption. Extensive research regarding temperature effects on growth rate have been reported, and the effect of temperature on the growth rate is often described by a concept knows as an ALD window. The ALD window has been defined as a range in which the growth rate remains constant, and ideally there is complete adsorption.15 Outside the ALD window, the growth rate may be higher due to precursor condensation at deposition temperatures that are too low, whereas limited growth may result from insufficient reactivity (at deposition temperatures that are too low) or desorption of the precursor at deposition temperatures that are too high.4,15 However, in this work, there is a different phenomenon in the R-PEALD process at high temperature. The reaction of TDMAZ with O2 plasma was investigated in the temperature range of 80 °C-300 °C. The ZrO2 thin films were deposited on KBr tableting, and the chemical bonds were studied using FTIR analysis. For clarity in presentation, the FTIR absorbance spectrum from 350 to 2250 cm−1 is shown in Figure 5. Most of
Figure 5. Different FTIR spectra for ZrO2 film deposited by R-PEALD on KBr tabletting.
the prominent absorbance peaks were observed at different temperatures. Sharp absorbance peaks were observed at 424 and 780 cm−1.16,17 The positive absorbance peak at 780 cm−1 corresponds to the Zr−O−Zr stretching vibrations resulting from the chemical bond between TDMAZ and the O2 plasma. An additional absorbance peak corresponding to the Zr−O stretch was observed at 424 cm−1, indicating successful deposition of ZrO2 on the KBr surface. Peak height and peak area can be used for quantitative analysis of infrared spectra. Specifically, it can be used to analyze how thin films change with respect to temperature. We found that a certain amount of −CH3 thin film remained in the ZrO 2 thin film. The peak corresponding to the −CH 3 stretching vibration was observed at 1384 cm−1 resulting C
DOI: 10.1021/acs.jpcc.7b00211 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C −Zr−N(CH3)2 + O → −Zr−CN + CO2 + NO + CO + H 2O
(B)
The surface morphological characteristics of the ZrO2 films deposited directly on the clean Si substrate were investigated using a scanned area of 1 × 1um2 by AFM. Figure 8 shows the 3D and 2D surface morphologies of 50 nm-ZrO2 thin film at 80, 160, and 250 °C. The surface of the ZrO2 thin film deposited by R-PEALD at 80 °C was rough. Figure 8a1 and Figure 8a2 look like undulating mountains. The roughness of ZrO2 in 80 °C (RMS = 3.18 nm) is higher than that at the high temperature (RMS = 1.82 nm and 1.92 at 160 and 250 °C, respectively). As shown in Figure 8, parts b1, b2, c1, and c2, the surface morphology is more like a peak and is smoother at 250 °C. These observations indicate that the deposition of ZrO2 produced condensation of the precursor and incomplete reaction at low temperature, resulting in an aggregation effect. Therefore, the preparation of ZrO2 as an organic electronic encapsulation layer, needs to be further optimized within the deposition process. With an increase in temperature, the production of −CN and, to some extent, the steric-hindrance effect inhibited the growth of the ZrO2 thin film.
Figure 6. Reaction mechanism of R-PEALD ZrO2 at (a) window temperature (160 °C) and (b) high temperature (250 °C).
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CONCLUSION In conclusion, we analyzed the deposited ZrO2 thin films by RPEALD using TDMAZ with O2 plasma as the oxidant. On the basis of the QMS analysis, the primary reaction byproducts in R-PEALD (CO, CO2, NO, H2O) and the related fragments during the O2 plasma process were observed. This indicated a combustion-like reaction in the plasma process on the surface. Reaction mechanisms of ZrO2 thin films were studied via FTIR at different growth temperatures. In addition to the sterichindrance effect, precursor condensation and incomplete reaction resulted in an aggregation effect at low deposition temperature. At the high deposition temperature, formation of the −CN bond resulted from the increase in reactivity. This phenomenon can also explain the decrease in the growth rate at
Figure 7. Different FTIR spectra for O2 plasma based-ZrO2 and H2O based-ZrO2 thin films deposited by R-PEALD at 250 °C.
−Zr−N(CH3)2 + O → −Zr−OH + CO2 + NO + CO + H 2O
(A)
Figure 8. Atomic force microscope (AFM) 3D-images (a1−c1) and 2D-images (a2−c2) of ZrO2 thin films at deposition temperatures of 80, 160, and 250 °C, respectively. D
DOI: 10.1021/acs.jpcc.7b00211 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C high deposition temperature because the −CN bond cutoff phenomenon inhibit ZrO2 thin film grown at the chain. This phenomenon needs further study to assess the influence of other precursors, where growth temperature is crucial considering the reaction stability, to offer chances for depositing high quality thin films by ALD technology.
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AUTHOR INFORMATION
Corresponding Author
*(Y.D.) E-mail:
[email protected]. ORCID
Yu Duan: 0000-0002-2155-7188 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This study was supported by the International Science & Technology Cooperation Program of China (2014DFG12390), the National High Technology Research and Development Program of China (Grant No. 2011AA03A110), the National Key Research Program of China (Grant No. 2016YFB0401001), the National Natural Science Foundation of China (Grant Nos. 61675088, 61275024, 61377026, 61274002, and 61275033), the Scientific and Technological Developing Scheme of Jilin Province (Grant Nos. 20140101204JC, 20130206020GX, 20140520071JH, and 20130102009JC), the Scientific and Technological Developing Scheme of Changchun (Grant No. 13GH02), and the Opened Fund of the State Key Laboratory on Integrated Optoelectronics (No. IOSKL2012KF01). Prof. Duan want to thank Dr. M. Mazeeo and Prof. G. Gigli for useful discussion.
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