Low-Temperature Plasma-Enhanced Atomic Layer Deposition of SiO2 Using Carbon Dioxide
In this work, we report the successful growth of high-quality SiO2 films by low-temperature plasma-enhanced atomic layer deposition using an oxidant which is compatible with moisture/oxygen sensitive materials. The SiO2 films were grown at 90 °C using CO2 and Bis(tertiary-butylamino)silane as process precursors. Growth, chemical composition, density, optical properties, and residual stress of SiO2 films were investigated. SiO2 films having a saturated growth-per-cycle of ~ 1.15 Å/cycle showed a density of ~ 2.1 g/cm3, a refractive index of ~ 1.46 at a wavelength of 632 nm, and a low tensile residual stress of ~ 30 MPa. Furthermore, the films showed low impurity levels with bulk concentrations of ~ 2.4 and ~ 0.17 at. % for hydrogen and nitrogen, respectively, whereas the carbon content was found to be below the measurement limit of time-of-flight elastic recoil detection analysis. These results demonstrate that CO2 is a promising oxidizing precursor for moisture/oxygen sensitive materials related plasma-enhanced atomic layer deposition processes.
KeywordsCarbon dioxide Silicon dioxide ALD Plasma Radicals Oxidation
Atomic layer deposition
Attenuated total reflectance Fourier transform infrared spectroscopy
Glow-discharge optical emission spectroscopy
Plasma-enhanced atomic layer deposition
Plasma-enhanced chemical vapor deposition
Physical vapor deposition
Time-of-flight elastic recoil detection analysis
SiO2 is a widely used material for applications such as microelectronics [1, 2], microelectromechanical systems [3, 4], photovoltaics [5, 6], and optics [7, 8]. While SiO2 thin films can be grown by several methods such as thermal oxidation, plasma-enhanced chemical vapor deposition (PECVD), or physical vapor deposition (PVD), atomic layer deposition (ALD) offers the exceptional advantage of combining precise film thickness control, high uniformity, and conformality [9, 10, 11].
Many ALD processes, with various Si precursors (chlorosilanes or aminosilanes) and oxidants (H2O, H2O2, or O3), were developed for the growth of SiO2. Those processes usually require relatively high temperatures (> 150 °C) [12, 13, 14, 15, 16]. For processes compatible with thermally sensitive materials such as organic, biological, and polymeric materials, the catalyzed ALD [17, 18, 19] and plasma-enhanced atomic layer deposition (PEALD) [9, 20, 21, 22] have been used as an effective solution with process temperatures below 100 °C. However, the commonly used H2O and O2-based oxidants can lead to material degradation in the case of moisture/oxygen sensitive materials. Compared to H2O and O2, at low-temperature, CO2 is not chemically reactive. In this case, using CO2 as an oxidant can minimize the degradation of moisture/oxygen sensitive materials by avoiding unnecessary oxidization. Furthermore, CO2 was reported by King  to be a viable oxidizing agent for the growth of PEALD SiO2 films when using SiH4 as a Si precursor. However, the growth temperatures of those PEALD processes, which were in the range of 250–400 °C, are not compatible with high-temperature sensitive materials.
In this work, we report the development of a CO2-based PEALD process for SiO2 films at 90 °C. The dependence of the film growth on the process parameters (precursor pulse/purge time and plasma power) is investigated. We also report the chemical composition, structural and optical properties, and residual stress analysis of the films.
The main parameters of the PEALD process
Process temperature (°C)
Plasma power (W)
BTBAS pulse time (s)
BTBAS purge time (s)
CO2 plasma exposure time (s)
CO2 plasma purge time (s)
The thickness of PEALD SiO2 films was determined with a SENTECH SE400adv ellipsometer using a HeNe laser at a wavelength of 632.8 nm and at an incident angle of 70°. The growth-per-cycle (GPC) was calculated using the obtained film thickness divided by the number of ALD cycles. The deviation of the GPC was based on the non-uniformity of film thickness.
Chemical composition was measured by glow-discharge optical emission spectroscopy (GDOES), time-of-flight elastic recoil detection analysis (TOF-ERDA), and attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR). GDOES measurements were carried out on a Horiba GD-Profiler 2. A 4-mm-diameter anode and rf power of 35 W in the pulsed mode were used. The elemental intensities were reported as values integrated over the whole film thickness as described in Ref. . For TOF-ERDA measurements, 40 MeV energy Br ions obtained from a 5MV tandem accelerator were directed on the measured samples. The detection angle was 40°. ATR-FTIR measurements were done using a Thermo Electron Corporation Nicolet 380 ATR-FTIR spectrometer with a diamond crystal as an internal reflection element. The procedure included a background collection from the Si substrate and data collection from the samples. A 2-cm−1 resolution over the 800–4000 cm−1 wavenumber range was used.
X-ray reflectivity (XRR) analyses were performed with a Philips X’Pert Pro diffractometer using Cu-Kα1 radiation. The film density was acquired from the measured data by using an in-house-developed fitting software . An interfacial oxide layer between the silicon substrate and the PEALD SiO2 film was simulated as a part of the XRR fitting layer model. Using a PerkinElmer Lambda 900 spectrometer, transmittance spectrum of the PEALD SiO2 film was recorded in the 360–800 nm wavelength range following the growth on sapphire substrate. The refractive index (n) and extinction coefficient (k) were determined with Cauchy fitting from the transmittance spectrum. To ensure good fitting accuracy, for this measurement, 150-nm-thick SiO2 films were grown on sapphire substrates.
The residual stress of 50-nm-thick PEALD SiO2 films was determined with the wafer curvature method  and Stoney’s equation . The wafer curvature was measured before and after film growth with a TOHO FLX-2320-S tool. The wafers were scanned biaxially using a 120-mm scan length. Measured results were presented with maximum measurement uncertainty .
Results and Discussion
The effect of CO2 plasma purge time on the GPC is shown in Fig. 2b. As in the case of BTBAS purge time dependency, GPC values are found to remain constant when CO2 purge time is varied between 0.5 and 3 s. Thus, it can be concluded that the applied purge time of both precursors has a negligible impact on the GPC of our SiO2 thin films. This differs from an earlier reported PEALD process with SAM.24, one kindred aminosilanes of BTBAS, and O2 plasma , where purge steps with a purge time shorter than 2 s were found to have a significant effect on film growth. Here, the independence between our applied precursor purge time and the GPC could be assigned to the effective removal of residual precursors and byproducts which could partially benefit from the reaction chamber design using the cross-flow. Such configuration makes the gas exchange time between precursor pulses relatively short. Nevertheless, the stickiness of precursors cannot be ruled out. Based on the results shown in Fig. 2a, by using BTBAS pulse/purge time of 0.3 s/3 s and CO2 plasma exposure/purge time of 3 s/2 s, the highest deposition speed during the saturated growth is 50 nm/h. This implies that by applying a high plasma power and using BTBAS pulse/purge time of 0.1 s/0.5 s and CO2 plasma exposure/purge time of 3 s/0.5 s, a deposition speed of up to 100 nm/h is achievable.
Figure 5b shows ATR-FTIR spectrum measured on the same sample. The broad band features, located in the 3200–3800 cm−1 region, can be assigned to the O-H stretch of the Si–OH and water but the former is less likely [14, 31]. Another band, which is also typical of the Si–OH stretch , is visible at ~ 900 cm−1. The presence of –OH groups, which is consistent with TOF-ERDA results shown above, implies that combustion-like reactions, which involve the combustion of –NHtBu ligands and formation of –OH groups, dominate the oxidization step. A similar mechanism has been previously reported to take place during the growth of Al2O3 from trimethylaluminum and O2 plasma  and SiO2 from SAM.24 and O2 plasma . In addition to the –OH groups, the Si-O-Si bond stretching is detected around 1108 and 1226 cm−1 [14, 33] while the bond bending is seen at approximately 820 cm−1 [34, 35]. Note that compared to literature values [14, 34, 35], the Si-O-Si stretching frequency in this work is relatively high. This could be caused by the change of the Si-O bond length which can be influenced by the film residual stress. Jutarosaga et al. reported that the higher the compressive stress is, the lower the Si-O-Si stretching frequency is . The bands at ~ 970, 1301, and 1450 cm−1 are assigned to the CH3 rocking, CH3 symmetric deformation, and CH2 scissor, respectively . The finding of C-H surface groups is in line with the result of TOF-ERDA and is most likely due to the surface contamination.
Due to the uncertainty of the actual reaction products, the proposed surface reaction is purposely not balanced. To be able to fully determine this reaction, in-situ analyses during the film growth, such as by-product gas analyses, would be needed.
This work demonstrates the potential of CO2 as an oxidant for growing low-temperature PEALD SiO2 on moisture/oxygen sensitive materials. SiO2 films with low impurity levels and low tensile residual stress were grown at 90 °C by PEALD using CO2 and BTBAS as precursors. The films showed a saturated GPC of ~ 1.15 Å/cycle together with a density of ~ 2.1 g/cm3. This study also shows the possibility of reaching a saturated growth of the films with a very short ALD cycle time of about 4 s, which is considerably desirable for high throughput and therefore industrial applications.
Z.Z. would like to thank Dr. Emma Salmi, Dr. Iris Mack and Heli Seppänen for the warm discussions and supports. In addition, Elena Ostrovskaia is thanked for helping with the XRR measurements.
This work was partially financially supported by Tekes (“WAFER” project) and the Finnish Centre of Excellence in Atomic Layer Deposition (Reference No. 251220).
Availability of Data and Materials
The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.
ZZ conceived the idea and designed the experiments. ZZ, PS, OY, MDS, KM carried on the experiments, data analysis and interpretations. ZZ, CM, and SM contributed to the discussion and wrote the manuscript. HS and HL supervised the work. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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