Comparative study of ALD SiO2 thin films for optical applications

: We have investigated the suitability of atomic layer deposition (ALD) for SiO 2 optical coatings and applied it to broadband antireflective multilayers in combination with HfO 2 as the high refractive index material. SiO 2 thin films were successfully grown using tris[dimethylamino]silane (3DMAS), bis[diethylamino]silane (BDEAS) with plasma activated oxygen as precursors, and the AP-LTO 330 precursor with ozone, respectively. The amorphous SiO 2 films show very low optical losses within a spectral range of 200 nm to 1100 nm. Laser calorimetric measurements show absorption losses of 300 nm thick SiO 2 films of about 1.5 parts per million at a wavelength of 1064 nm. The films are optically


Introduction
One of the challenges for using dielectric coatings in optical multilayer stacks is the realization of low absorption and low scattering losses. Furthermore, films have to be prepared with a precise control of optical thickness combined with a uniform thickness distribution. Commonly, thin films applied in precision optics are deposited by physical vapor deposition (PVD) [1][2][3][4]. However, this method requires great technical efforts to meet strict tolerances, e.g. in film thickness, and in situ film growth monitoring is often necessary to ensure good reproducibility. Atomic layer deposition (ALD) is an alternative and very promising method that inherently offers a high reproducibility, large-area thickness uniformity and conformal coating on structured substrates with high aspect ratio [5,6]. ALD is a special form of chemical vapor deposition (CVD), in which the gaseous precursors are exposed to the substrate sequentially and independently from each other. The precursor pulses are separated by purging cycles. Due to this cycle-based process the reaction does not take place in the gas phase, but is only surface limited. Since the substrate surface has a limited number of reaction surface sites, the reaction saturates after a certain precursor exposure. The surface-controlled ALD growth mechanism enables an excellent scalability of film thickness within a sub-nanometer range as well as high repeatability without in situ monitoring. Using atomic layer deposition, the resulting film thickness is precisely controlled by the number of ALD cycles.
ALD optical coatings were introduced using Al 2 O 3 as low refractive index material and ZnS [5], HfO 2 [7] or TiO 2 [6,8] as high refractive index material. However, highly precise optics require a higher refractive index contrast between the dielectrics. Silicon dioxide is one of the most applied low-index materials in optical interference coatings. Thin SiO 2 films deposited by ALD are widely studied for application in microelectronics such as gate oxide films in semiconductor devices [9, 10]. In semiconductor industry very thin continuous films of typically less than 4 nm are required [11]. For that reason, the growth during the early stages of deposition is critical for applications in semiconductor devices. In optical applications conversely, nucleation plays a minor role, since rather thick layers are needed (up to several hundred nm). To fulfill the stringent requirements on thickness control a linear and reproducible film growth as enabled by ALD is demanded.
Deposited dielectric ALD films are typically amorphous when they are prepared at low substrate temperatures (< 250°C) [12]. The formation of an amorphous film is accompanied by low surface roughening, for that reason low scattering losses can be expected. Furthermore, optically homogeneous films without refractive index gradients are expected, since ideally in every ALD cycle the same reaction takes place.
There is an evident lack of investigation on the optical properties, e.g. transmittance or optical homogeneity, of relatively thick SiO 2 ALD films. Before applying silicon dioxide films in multilayer stacks, fundamental questions on optical film quality have to be investigated. We present a thorough investigation of optical properties of silicon dioxide thin films synthesized by atomic layer deposition using common Si-precursors, namely 3DMAS [11, 13], BDEAS [14-16] and the AP-LTO ® 330 precursor [17][18][19]. The optical properties (refractive index, optical losses, and vacuum-to-air shift) and film stresses obtained in four different SiO 2 ALD processes are compared. To confirm the possibility of utilizing SiO 2 in optical coatings, a broad band antireflection coating was designed and successfully produced using SiO 2 and HfO 2 as low and high refractive index material, respectively.

Atomic layer deposition
ALD depositions of SiO 2 were carried out at substrate temperatures of 200°C (unless noted otherwise) using four different processes, see also Table 1: (1) SiO 2 thin films were grown from tris [dimethylamino] silane (3DMAS) and plasma activated oxygen. The films were deposited in a commercial Oxford Instruments open load ALD system (OpAL) equipped with an inductively coupled plasma (ICP) RF generator, operating at 13.56 MHz. The pulsing time of 3DMAS, which was heated to 30°C, was 400 ms. A "hold precursor" step of 4 s after the 3DMAS pulse was introduced to ensure surface-saturated reactions. The excess 3DMAS and byproducts were purged for 4 s. Oxygen plasma was pulsed for 3 s, where the oxygen flow rate was set to 50 sccm and the plasma power to 300 W. This step was followed by 4 s of purging. The process pressure was approximately 400 mTorr. A detailed description of film growth experiments was reported earlier [20].
(2) Furthermore, SiO 2 films were deposited using bis [diethylamino] silane (BDEAS) as silicon source in an Oxford Instruments FlexAl reactor, see also [15]. Oxygen plasma with a plasma power of 200 W was used as oxidizing agent. The pressure during the O 2 -plasma was 25 mTorr. The pulsing sequence was: 125 ms BDEAS pulse followed by a hold precursor step of 2 s (closed chamber), 5 s purge, 5 s plasma pulse and 2 s purge. The process pressure was approximately 14 mTorr. In the following, this process is hereafter referred to as BDEAS-I.
(3) SiO 2 films from BDEAS and O 2 -Plasma were additionally deposited in a Sentech SI ALD LL system. During the oxygen plasma pulse the plasma power was set to 200 W and the oxygen flow rate to 200 sccm. The ALD cycle consists of 140 ms BDEAS pulse, 1860 ms purge, 1000 ms oxygen plasma pulse and 1000 ms purge. The chamber pressure was 150 mTorr. This process is hereafter referred to as BDEAS-II.
(4) Depositions using the AP-LTO ® 330 precursor [17] were carried out in combination with O 3 as the oxygen source. A Picosun Oy Sunale R200 ALD tool was used in a single wafer showerhead mode with optional stop-flow set-up. The AP-LTO ® 330 precursor was kept at 20°C and pulsed for 100 ms by its own vapor pressure followed by a hold precursor step of 10 s. The excess precursor and by-products were purged for 5 s. After the subsequent O 3 pulse of 2 s the reactant was trapped in the chamber for 10 s before purging the chamber for 8 s. A double-sided broadband antireflective coating was deposited in an OpAL reactor using SiO 2 from 3DMAS and HfO 2 from tetrakis[dimethylamido] hafnium (TDMAH). For HfO 2 thin films the TDMAH was pulsed for 400 ms and subsequently purged for 8 s, an oxygen plasma pulse of 5 s was used to oxidize the film followed by 15 s purging.

Characterization
The film growth behavior of SiO 2 films from 3DMAS was investigated on Si (100) substrates. In addition the growth behavior was tested on dielectric sublayers, for this purpose we used Si substrates pre-coated with thin Ta 2 O 5 layers (by PVD) due to availability. The film thickness was determined by X-ray reflectometry (XRR) using a D8 advance type (Bruker AXS) X-ray diffractometer. Film morphology and crystal structure were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD) measurement, respectively.
For characterization of optical properties SiO 2 films with a thickness of about 300 nm have been grown using the afore-mentioned ALD processes on fused silica (Lithosil® Q1), BK7 and B270 substrates. The optical performance of the SiO 2 layers, i.e. transparency and optical homogeneity, was examined by evaluating the transmittance and reflectance spectra that were recorded in a wavelength range from 200 nm to 1100 nm using a PerkinElmer Lambda 950 spectrophotometer equipped with a self-developed VN-attachment [21]. The measurement accuracy of the spectrophotometer is ± 0.2% in the UV and ± 0.1% in the VIS. From these spectra, the optical losses and the refractive indices n were determined. Additional absorption measurements with detection limits of a few hundred ppb (parts per billion) were done using laser calorimetric measurements at a wavelength of 1064 nm [22].
Vacuum-to-air-shift (VAS) was calculated from the shift of the maximum of transmission spectra taken under ambient and under high vacuum conditions (less than 10 −5 mbar) [23]. The density of silica films that were deposited from 3DMAS at different substrate temperatures was identified from XRR spectra. Film stress measurements were performed using a TENCOR ® FLX 2320 for 300 nm thick SiO 2 coatings that were deposited on 3" double-sided polished silicon wafers. For all ALD processes a linear film growth rate of SiO 2 thin films is verified by the linear relation of performed ALD cycles and resulting film thickness. Figure 1 shows exemplarily the thickness calibration curve of SiO 2 deposited using 3DMAS. At 200°C substrate temperature the determined growth rate is 0.98 Å/cycle, that is comparable to observed growth rates using BDEAS-I (1.14 Å/cycle), BDEAS-II (1.23 Å/cycle) and the AP-LTO ® 330 precursor (1.85 Å/cycle). Deposition of SiO 2 from 3DMAS at 100°C results in a slightly increased growth rate of 1.13 Å/cycle. A higher growth rate at lower deposition temperatures was also found for BDEAS-I. This has been attributed to decreased precursor chemisorption at higher temperatures due to a lower -OH density after the plasma step [15].

SiO 2 single layers
ALD nucleation and growth is generally affected by different substrate materials [24,25]. For application of ALD SiO 2 layers in optical multilayer stacks, the linear film growth needs to be demonstrated also on dielectric coatings. Figure 1 shows that SiO 2 films have the same constant growth behavior on Ta 2 O 5 pre-coated Si substrates. In conclusion, the thickness of SiO 2 layers in e.g. dielectric layer stacks can be controlled easily by counting ALD cycles. Slightly deviating growth behavior of SiO 2 on Si and the dielectric sub-layer can be attributed to a substrate depending delay in thin film nucleation in the initial stages of the growth [26].
The crystallinity of the SiO 2 thin films is shown exemplarily for the layers deposited with the 3DMAS precursor (see Fig. 2). The single diffraction peak in the XRD pattern is attributed to the silicon substrate, suggesting that the SiO 2 layer is amorphous. This assumption is confirmed with SEM micrographs that show a dense SiO 2 thin film without visible crystallites. Intensity losses of transmitted light due to scattering or absorption are referred to as optical losses. Fused silica substrates were used for investigations of optical losses due to their low absorption down to the UV spectral range. Optical losses are subsequently calculated from transmittance T and reflectance R measurements of deposited SiO 2 thin films (100% -T -R). Figure 3 depicts the losses of SiO 2 coated and uncoated fused silica substrates. The optical losses of the prepared samples using the 3DMAS, BDEAS-I, BDEAS-II or the AP-LTO®330 precursor are in good agreement with those from the bare substrate in a wavelength range from 200 nm to 1100 nm. The deposited silicon oxide films show no significant absorption or scattering losses in this range. Laser calorimetric measurements revealed that absorption losses of samples with 300 nm SiO 2 films prepared using 3DMAS are less than 3.9 parts per million (ppm) at 1064 nm. The bare fused silica substrates have absorption losses of ca. 2.4 ppm. The extinction coefficient of the deposited SiO 2 corresponds to k ≈0.4·10 −6 . These SiO 2 layers with very low absorption losses might be considered for applications in laser optics. The deposited films from 3DMAS and BDEAS are optically homogeneous, confirmed by the matching spectra of the coated sample and the bare fused silica substrate in all half-wave positions (not shown). If there would be a small inhomogeneity in the samples prepared with the AP-LTO ® 330 precursor this cannot be evaluated reliably with the measurement technique used in this study since the refractive index contrast between the uncoated substrate and the SiO 2 is extremely low. Therefore, the oscillations in the reflectance and transmittance spectra cannot be resolved with the given measurement accuracy. The refractive index of this layer is nearly identical with that of the substrate material. MPa. An evaluation of film stress values over a period of 20 days has shown that this stress relaxed towards tensile stress with time when the sample was stored under ambient conditions (see Fig. 4). For comparison, stress values after long term are about 40 MPa, 10 MPa and 20 MPa, for SiO 2 films deposited using 3DMAS, BDEAS-I and the AP-LTO ® 330 precursor, respectively. Films prepared from 3DMAS at a deposition temperature of 100°C have tensile stress of about 110 MPa after long term. ALD is known to produce dense, pinhole-free layers [27]. However, the stress value relaxation indicates that the film is not highly densified and exhibits open pores [28]. If the pores are filled with water under ambient conditions, the film composition can change. As a result the effective refractive index is increased due to infiltration of water and hence the transmittance spectra of the film will be influenced. This effect can be measured as a vacuum to air shift (VAS).
The transmittance spectra of SiO 2 films, synthesized with 3DMAS, BDEAS-I and BDEAS-II, measured under ambient conditions and vacuum conditions show that the maximum positions of the spectra are shifted considerably (Fig. 5(a)). The films exhibit a negative vacuum-to-air shift of −2.1%, −1.8% and −1.3%, respectively, which can be attributed to a decrease in film refractive index under vacuum conditions [23]. Only the film deposited using the AP-LTO 330 precursor shows nearly no vacuum-to-air shift, see Fig. 5(a).
The occurring vacuum-to-air-shift (VAS) is a drawback for application in extreme environments, as for instance in space applications. The changed refractive index of the SiO 2 layers under vacuum conditions would alter the performance of the optical system. It should be noted that the substrate temperature during deposition, which was varied between 100°C and 300°C, has no influence on the density of the SiO 2 films deposited with 3DMAS and consequently the VAS could not be reduced by changing the substrate temperature (not shown). Considering the example of SiO 2 prepared using 3DMAS, we present two possibilities to reduce the shift. First, we adjusted the oxygen plasma parameter, and second, an alumina sealing layer was applied. The material properties of thin films prepared by PEALD depend on the radical flux as well as on the flux and energy of the ions during the plasma step [29-32]. The flux of the radicals and ions increase with higher plasma powers and the dose also increases with a longer plasma time. The ion energy is mostly determined by the pressure, i.e. the ion energy will be higher at lower pressures. Denser films are expected by increasing the plasma power, decreasing the pressure (using a lower oxygen flow rate) and by applying a longer plasma pulse time, since the surface will be exposed to a higher reactivity. In the developed process, the plasma power is already at its system-dependent maximum value of 300 W. The oxygen flow rate is reduced from 50 sccm to 10 sccm. Additionally, the plasma pulse time is increased to 8 s. With these process modifications the shift can be drastically reduced to ≤-0.5%, see Fig. 5(b). A reduced pressure is required for higher ion energy. Consequently, this process needs an additional 10 s pump-down time before the plasma ignition to provide a low and stable oxygen pressure during the plasma pulse. The lengthened cycle time leads to lower deposition rates (~9 nm/h instead of ~27 nm/h).
Another and more effective way to prevent the VAS is to seal the porous SiO 2 layer with an ultra-thin Al 2 O 3 film that prevents water penetration. Aluminum oxide has excellent sealing properties and it is widely used as protective or barrier layer against moisture [12,[33][34][35]. The sealing layer should be as thin as possible to have the lowest possible influence on the refractive index of the overall layer system. ALD is a preferable method to deposit such very thin layers of Al 2 O 3 . Aluminum oxide films were deposited using trimethylaluminium (TMA) and oxygen plasma with a plasma power of 300 W and an oxygen flow rate of 50 sccm. The pulsing time was 50 ms for TMA and 3 s for the O 2 -plasma. Deposited 300 nm SiO 2 films were coated with 1.5 nm and 3 nm Al 2 O 3 films. The VAS cannot be prevented by a 1.5 nm sealing layer. In contrast, an Al 2 O 3 layer of 3 nm is sufficient to prevent water molecules to fill pores of the SiO 2 sub-layer as the layer system shows no vacuum-to-air shift under the tentative conditions (see Fig. 5(b)). This result was verified in several reproduction runs. The determined refractive index under ambient conditions of SiO 2 layers deposited using 3DMAS, BDEAS and the AP-LTO ® 330 precursor is illustrated in Fig. 6 and listed in Table 2. Deviation of refractive indices originates among others from different film densities. The SiO 2 film obtained with 3DMAS has the lowest refractive index that correlates well to the highest measured VAS, which is due to higher porosity compared to the other precursors.

Broad band antireflection coating
The possibility to apply atomic layer deposited SiO 2 films in optical coatings was demonstrated with an 8-layer broadband antireflection coating (BBAR) using SiO 2 and HfO 2 as low and high refractive index material, respectively. The broad band antireflection coating was designed using commercially available software FilmWizard TM (SCI). The total thickness of the designed BBAR (Table 3) is 391 nm. The design includes a 3 nm thin Al 2 O 3 sealing layer to protect the coating from water penetration. Due to its low thickness this extra sealing layer has barely an influence on the spectral transmittance of the BBAR and the multilayer has been optimized to taking into account the top Al 2 O 3 . High refractive index N-SF8 glass substrates of 1 mm thickness were used as substrate material. The influence of the reflection at the substrate's backside surface is suppressed by a double-sided coating. HfO 2 thin films prepared at 100°C substrate temperature show better mechanical properties compared to HfO 2 thin films deposited at 200°C, hence the BBAR is deposited at a fixed substrate temperature of 100°C. The required number of ALD cycles to reach the particular layer thicknesses of the design (see Table 3) are calculated from the linear equation coefficients taken from the corresponding thickness calibration curves. The doublesided coating is accomplished side by side. To prevent backside coating, the substrates were placed in home-made substrate holders. Figure 7 shows the simulated and the measured transmittance spectra of the substrates with a double-sided BBAR coating. With this doublesided BBAR coating an average transmittance of about 97% can be reached in a spectral range from 390 -1100 nm. Repeated measurement of the transmittance spectra 6 months after deposition shows a good long term stability of the BBAR. As seen in Fig. 8 the transmission spectra of the BBAR double-sided coated N-SF8 sample do not shift (even 6 months after deposition) if the measurement is done under air or vacuum conditions. This implies that the additional 3 nm Al 2 O 3 thin film is sufficient to suppress the vacuum-to-air shift. Theoretical modelling has shown that a failure of the protective Al 2 O 3 sealing layer and subsequently a change of the optical properties of the top SiO 2 layer due to VAS would result in a lower transmittance of about 0.5% in the whole spectral range.

Conclusion
This comprehensive study on the optical properties of atomic layer deposited SiO 2 shows that various precursors and different ALD tools can be applied to produce silicon dioxide thin films with a high optical quality. We have demonstrated the optical performance of SiO 2 films deposited using 3DMAS, AP-LTO ® 330 and BDEAS. The grown SiO 2 thin films have low absorption or low scattering losses within a spectral range from 200 nm to 1100 nm. The optical losses are well below 0.5% in a spectral range from 230 nm to 1100 nm. Spectrophotometric measurements reveal a high optical homogeneity of deposited silicon dioxide films. Conclusively, using atomic layer deposition we were able to realize low-loss films with a good thickness control, making ALD an excellent technology for depositing optical elements. In this contribution, we demonstrated the application of ALD SiO 2 films as low-refractive index material in a BBAR with an average transmittance of 97% in a wavelength range from 390 -1100 nm. The rather long deposition times can be shortened by simultaneous double-sided coating. For application in extreme environment it is worthwhile to check other precursor combinations to reduce the vacuum-to-air shift of SiO 2 thin films.
Regarding the results of this work, a wide range of potential optical application especially in the UV spectral range is attributed to SiO 2 ALD films such as low index material in dichroic mirrors, anti-reflection coating and optical band pass filter. Due to the conformity of ALD thin films prospectively functionalized optical coatings can be deposited on highly curved lenses and on diffractive optical elements.