Atomic layer deposition of semiconductor oxides on electric sail tethers

Thin Solid Films 621 (2017) 195–201

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Atomic layer deposition of semiconductor oxides on electric sail tethers Mehwish Hassan a, Laura Borgese b,c,⁎, Giuditta Montesanti b,d, Edoardo Bemporad b,d, Gianmario Cesarini e, Roberto Li Voti e, Laura E. Depero b,c a

Department of Sciences and Humanities, National University of Computer and Emerging Sciences, Islamabad, Pakistan Consorzio Interuniversitario Nazionale per la Scienza e Tecnologia dei Materiali (INSTM), via G. Giusti 9, 50121, Firenze, Italy c Dipartimento di Ingegneria Meccanica ed Industriale, Università di Brescia, INSTM UdR BRESCIA, via Branze 38, 25123 Brescia, Italy d Dipartimento di Ingegneria, Università degli Studi di Roma Tre, INSTM UdR Roma TRE, via della Vasca Navale 79/81, 00146, Roma, Italy e Dipartimento di Scienze di Base ed Applicate per l'Ingegneria, Sapienza Università Roma, Via A. Scarpa 16, 00161 Rome, Italy b

a r t i c l e

i n f o

Article history: Received 21 March 2016 Received in revised form 16 November 2016 Accepted 26 November 2016 Available online 1 December 2016 Keywords: ALD Tether TiO2 ZnO Mixed oxides

a b s t r a c t

• This study shows that thin coating layers of semiconductor oxides, deposited by atomic layer deposition, can be successfully used to improve the performance of tethers used in space technology. Thin layers of ZnO, TiO2 and Ti-Zn mixed oxide with 25 to 100 nm thickness were considered, and compared with native Al2O3 as reference. For this purpose, morphological, optical and electrochemical characterization was performed. As a result, deposited materials show very good adhesion and conformality. Coatings act as anti-reflective layers, increasing the absorbance of tethers. The presence of TiO2 thin layers and mixed oxide improves the corrosion resistance of tethers both in dark and under UV illumination conditions. Our results show that TiO2 has the best performance for space tether application followed by the mixed oxide.

© 2016 Elsevier B.V. All rights reserved.

1. Introduction The hoytether is a device used in space technology to perform attitude manoeuvres and re-entry operations, to create propulsion, or to generate energy [1]. In practice, the hoytether is a conducting wire that interacts with the surrounding plasma when a potential difference is applied to it. The hoytether taken into consideration in this study is the one developed in the frame of the “Electric sail propulsion technology” (ESAIL) project (FP7 project number 262733) as a propellantless thrust generating system [2–5]. The peculiar feature of this technology is to use many km-long electrically charged wires, which gain momentum from the electrons of the interplanetary plasma, in order to generate a low and continuous thrust in space. The ESAIL tether is made of aluminum wires, 25 or 50 μm diameter, generally used for bonding in the electronic industry [6]. The ESAIL tether is constituted by a main wire to which three wires are attached via ultrasonic bonding [7] (Fig. 1).

⁎ Corresponding author at: Dipartimento di Ingegneria Meccanica ed Industriale, Università di Brescia, via Branze 38, 25123 Brescia, Italy. E-mail address: [email protected] (L. Borgese).

http://dx.doi.org/10.1016/j.tsf.2016.11.044 0040-6090/© 2016 Elsevier B.V. All rights reserved.

ESAIL tethers' performance can be significantly improved by coating the bulk material. The main objectives are to increase absorbance and/ or the emissivity depending on the mission, reducing the risk of cold welding during launch, increasing the diffusive reflectance for optical visibility purposes. In addition, it is required to have resistance to harsh space environment, such as UV radiation and atomic oxygen. Indeed, synergic or separate interactions in the harsh aerospace environment can provoke corrosion, erosion, structure modification and surface roughening, thus degrading the optical, thermal, electrical and mechanical tether properties. In order to mitigate the effects for different types of tethers and external spacecraft parts, thin film coatings were applied. As known, in space, the only heat exchange mechanism is radiation. Consequently, contributions to the thermal tether equilibrium are the absorbed power from the Sun, function of the absorptivity α, and the radiated power by the tether, which is a function of the emissivity of the surface ε. Given the distance from the Sun, the Earth and other planets and bodies, the tether equilibrium temperature depends on the term ffiffiffiffiffiffiffiffiffi p 4 α=ε, which is associated with the coating material. For this purpose, α is generally measured over the visible spectrum and ε over the infrared spectrum. Assuming a constant value of the emissivity, higher absorbance coatings may be chosen for missions to the outer solar system, where the equilibrium temperature needs to be higher, while

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2. Experimental 2.1. Substrates

Fig. 1. SEM image of the ESAIL tether used in this work. (Courtesy of Henri Seppänen, University of Helsinki)

Aluminum wires with composition 99% Al and 1% Si, with 32 μm diameter and 4–6 cm length were used (Tanaka Electronics). Flat aluminum samples (series 4000, 4.4% Si), 10 × 10 mm wide and about 2 mm thick, were polished to obtain roughness average about 20 nm, comparable with the one of the bare tethers. The roughness average, Ra, of the tether was measured with a confocal profilometer (Leica DCM 3D) both on wires and flat samples. Sample preparation procedure included a degreasing step with acetone (Sigma-Aldrich) and a temperature controlled UV surface decontamination for 5 min at 50 °C (Novascan PSD-UVT). 2.2. ALD

coatings with lower values of the α/ε coefficient are to be selected for missions closer to the Sun. For ESAIL tethers, electrical coating resistivity is a parameter of great importance [8]. In order to facilitate the collection of electrons from the solar wind and reduce the risk of sparks due to charge buildup, the electrical resistivity should be minimized. The coating thickness should also be kept at the minimum both due to mass constraints and to enable the collection of electrons from the plasma. For these reasons, the thickness of an ESAIL tether insulating coating should be set in the nanometer range. Consequently, such dimensional constraint, in turn, has an impact on the selection of the coating material, since the emissivity of a thin film changes when it goes below a critical thickness level, in the order of tens or hundreds of nanometers depending on the material [9]. SiO2, Al2O3 and indium tin oxide (ITO) thin film coatings with thicknesses higher than 50 nm applied by sputtering or vapor deposition are widely used in aerospace applications as protective coatings [10–12]. SiO2 coating was Magnetron sputtered on Kapton® H polyimide used for solar array of International Space Station [13]. SiO2 and polytetrafluoroethylene Teflon® coatings, when deposited by cosputter deposition, are less prone to crack due to their ability to conform to flexure compression and substrate expansion. Painted coatings based on ZnO and Zn2TiO4, considered for their UV stability, have proven to maintain their thermo-optical properties in space environments, compared to zirconia, alumina, and silica pigments [14,15]. The reason for using TiO2 and ZnO is their high optical visibility, electrical conductivity and thermal stability compared to bare Al oxide. ZnO has strong UVA absorption [16–19] and TiO2 is known for its photocatalytic activity [20,21]. Ti and Zn mixed oxides may be considered new materials with intermediate properties. Indeed, high UV absorption profiles have been already reported [22–24]. In the frame of the ESAIL project, it was demonstrated that ceramic coatings deposited by atomic layer deposition (ALD), such as Al2O3, are beneficial in preventing cold tether welding, thus proving that this technology can be applied in aerospace. ALD is a modified form of chemical vapor deposition technique which produces highly conformal coatings that are precisely controlled at Angstrom due to self-limiting surface reactions and separate introduction of material precursors in a cyclical manner [24–26]. ALD coatings are without line of sight restrictions as in plasma enhanced vapor deposition, Pinhole free and without defects [27]. We have recently reported that it is possible to obtain different stoichiometries of Ti-Zn mixed oxides by tuning ALD parameters [28,29]. In this study, low thickness TiO2, ZnO and Ti-Zn mixed oxide coatings were deposited on ESAIL tethers by ALD at low temperature. Morphological, optical, and electrochemical characterizations were performed to evaluate the coating performances with respect to the final application.

TiO2, ZnO and TiZnO were deposited by ALD in the Savannah 100 flow reactor (Ultratech Cambridge Nanotech Inc.) at 90 °C. Titanium source was tetrakis(dimethylamido)titanium (TDMAT 99.999%; Sigma-Aldrich, Germany). Zinc source was diethylzinc (DEZ; 99.999%; Sigma-Aldrich, Germany). Oxygen source was ultrapure water (H2O conductivity 0.054 μS/cm) produced directly from tap water with a Direct-Q system (Millipore, Italy). Detailed description of deposition parameters and processing cycles for the studied materials have been already reported [29]. The ratio between deposition cycles of ZnO to TiO2 was 1 to 4. Nominal thickness of 25, 50 and 100 nm were selected. The processing cycles number was calculated on the basis of growth curves of TiO2, ZnO and Mixed oxide obtained measuring the coating thickness by means of X-ray reflectivity (Bruker D8 Advance) on Si wafer substrates. Samples were placed in the reaction chamber after cleaning and stick to the bottom with kapton tape. 2.3. Electrochemical tests Electrochemical measurements were performed at room temperature (25 °C) with a potentiostat (Voltalab PGZ 402 Radiometer Analytical) in the three electrode configuration. Saturated calomel electrode (SCE) was used as reference, and Pt wire as counter electrode. Electrolyte was NaCl solution 3.5 wt%. All measurements were recorded, both in dark and under UV illumination (256 nm). Tafel plots were obtained scanning the potential between 200 mV above and under the open circuit potential at 0.1 mV/s scan rate. The geometrical area of the studied samples was similar. 2.4. FIB measurements A Focused Ion Beam/Scanning Electrons Microscope (FIB/SEM) dualbeam instrument (FEI Helios NanoLab600) was used to observe the morphology of the coating. Cross-section of the coated tether was made using the instrument ion beam to measure the thickness. The tether was placed in the vacuum chamber of the FIB with its lateral surface in a position orthogonal to the ion gun, so that the non-flat surface seen from the gun was completely covered by the coating. Afterwards, platinum was first deposited in order to protect the surface of the sample from the subsequent milling. By using this precaution, the artifacts created during the milling were minimized. Then, the milling was performed using progressively decreasing values of ionic current (from 9 nA to 0.9 nA) in order to perform a fast and efficient removal of the material at the beginning, and to clean the surface at the end. Finally, the in-situ SEM with ionic source was used to take highmagnification images of the coating cross section. Using the ionic source instead of the electron source allows to obtain higher contrast between the crystallographic grains and to better observe the sample microstructure. Each image was calibrated using the software analySIS® (Soft


Imaging System GmbH). The average thickness was calculated from 5 measurements. 2.5. Optical measurements Reflectance spectra were obtained by using a standard spectrophotometric device (MiniSpectrometer TM-UV/VIS: C10082CAH (200 to 800 nm) HAMAMATSU). A Xe lamp (power 250 W) was used as optical source. The incidence angle of light was 45° with respect to the sample surface, and the reflected light was collected specularly at 45° by an optical fiber connected to the spectrum analyzer. A polarizing filter was used to discriminate the two polarizations components: vertical (p) and horizontal (s). Finally the sample spectrum was normalized to the reference Al mirror, according to a standard procedure for the reflectance measurements. 3. Results and discussion ESAIL aluminum tethers have complex geometry due to multiple wire bonding (Fig. 1). For this reason ALD is the best choice, as the unique technique that assures the complete coverage and conformality of the coating [28–30]. The following oxides are considered as tethers coating in this study: TiO2, ZnO and a mixed oxide containing Ti and Zn (TiZnO) in mole ratio 1.75 [20], selected on the basis of the observed electrical resistivity [29].

a

0.18 0.16 0.14 0.12 0.10 0.08 0.06 0.04

TDMAT

0.02 0.00

b

DEZ 0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

120 100 80 60 40

TiO2 ZnO TiZnO

20 0

0

500

1000

1500

Fig. 2. a) Saturation curves of precursors TDMAT (circles) and DEZ (squares). The thickness normalized per number of cycles is reported as a function of precursor pulse time. b) ALD growth curves for TiO2 (circles), ZnO (squares), and TiZnO (triangles). Thickness (nm) is reported as a function of the number of ALD cycles. Linear fitting curves and parameters are reported for TiO2 (dashed and dotted line), ZnO (dashed line), and TiZnO (dotted line) respectively.

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TiO2 and TiZnO grown in the selected conditions are amorphous. While, it was observed that ZnO is polycrystalline [30]. ALD growth plots of TiO2, ZnO and TiZnO are shown in Fig. 2. The linear fitting parameters and saturation curves, performed during recipe optimization, clearly indicate the ALD deposition mechanism. An inhibition period, negative intercepts is observed for the growth of all oxides, in particular for ZnO. FIB measurements are used to study morphology and thickness of coating layers deposited on tethers and flat aluminum substrates. Fig. 3 shows SEM images of FIB cross sections taken on uncoated tether, and tethers coated with 100 nm of TiO2, ZnO, and TiZnO respectively. Different features can be highlighted. The surface is rough and presents grooving (Fig. 3c), with repetitive patterns in the longitudinal direction of the wire. This is probably related to the extrusion process of wire production. This feature is still visible at the highest thicknesses, as it is expected due to the conformal features of ALD coatings. A layer is present on the uncoated tether surface (Fig. 3a) and underneath the coating (Fig. 3d). This layer is probably native Al2O3, naturally formed on the surface of aluminum. The accuracy of thickness measurements by FIB cross section and following SEM image analysis may be affected by the removal of some coating from surface. For this reason two tests are performed on the same sample, protecting the cross section with different materials. Platinum and carbon are compared as protective layer for measurements of tether coated with 25 nm of TiZnO, SEM images are reported in Fig. 4a and b respectively. In the case of platinum protective layer, which provides better results in terms of contrast, the presence of artifacts which could have led to inaccurate thickness results was observed (Fig. 4a). The average calculated thickness is not significantly different in the two cases, confirming that the measured thickness is comparable. Fig. 4b also shows that the ALD coating can effectively penetrate inside cracks. The comparison between coating thickness calculated from ALD growth curves and measured by coupled FIB-SEM on tethers sample is reported in Table 1. A good agreement is observed, highlighting the suitability of ALD for this application. The higher values found for tethers are probably due to the rounded geometry, and the high difficulty to create the cross section in perfect perpendicular conditions. It is well known that aluminum is a high reflecting metal in the whole visible and infrared range. As a consequence it absorbs less than 10% of the solar energy. The tether absorbance (A) can be increased depositing a thin coating layer of a material with lower reflectance than aluminum. Numerical calculations were performed to simulate the absorbance of Al2O3, ZnO, and TiO2 with different thicknesses, by using the optical properties (refractive indexes and absorption coefficients) found in the literature [31–34] It was not possible to simulate the response of TiZnO coating, due to the lack of information on the refractive index of the material in literature. Fig. 5 shows the results averaged over the solar spectrum, from 350 nm to 2 μm. For all the investigated oxides a maximum of absorbance lays in the thickness range 50–100 nm, giving a strong motivation to the experimental work presented herewith. Indeed, in this case thin layers act as anti-reflecting coatings, maximizing absorbance, thus affecting the equilibrium temperature of ESAIL tether in space. Other minor oscillations of the curves in Fig. 5, are due to interference effects [35,36] Both TiO2 and ZnO have higher absorbance than Al2O3, considered as reference. Assuming equal emissivity values, TiO2 and ZnO would absorb more power from the Sun, with respect to Al2O3, and they would be preferable for missions to the outer solar system. The optical behavior of the mixed oxide is expected to be similar to the one of TiO2 and ZnO, but little bibliographic information is available, so it was included in the study. Reflectance (R) spectra in the visible range were measured on flat Al samples coated with 100 nm of TiO2, 50 nm of ZnO, and 100 nm of TiZnO. These measurements were used to evaluate oxides thickness and/or refractive indexes, and to calculate absorbance values. Normalized reflectance data are shown in Fig. 6. The fitting of experimental data allow to estimate the thickness of deposited layers, if the refractive index of material is known. The thickness of TiO2 and ZnO extracted

198


TiO2 coating

Protective layer

Protective layer

Al2O3 layer

Al2O3 layer

Substrate

a

Substrate

b Protective layer

TiZnO coating

Al2O3 layer

Substrate

c

d

Fig. 3. SEM images of FIB cross section of uncoated tether (a), and tethers coated with 100 nm of TiO2 (b), ZnO (c), and TiZnO (d) respectively.

with this procedure are 115 nm and 60 nm, in comparison with the values of 102 (1) and 55 (7) obtained by FIB, respectively. A very good agreement is observed considering the different substrates and measurement techniques. Any thickness cannot be extracted for TiZnO due to the lacking of refractive index data in literature. Absorbance spectra calculated from reflectance measurements (A = 1 − R) are shown in Fig. 7. TiZnO has the highest absorbance in the infrared range, and it is similar to TiO2 in the violet range. While, ZnO

shows the best absorption performance in the wavelengths from 450 to 680 nm. Average absorbance over the whole solar spectrum, from 350 to 850 nm, gives increasing values from TiO2, ZnO, and TiZnO corresponding to 0.17, 0.22, and 0.23 respectively. It is worth noting that the optical scattering, not measured here, has been neglected [37]. As a consequence, absorbance values could be overestimated. In order to obtain a direct measurement of the absorbance spectrum, other techniques

Pt protective layer

C protective layer

Pt artefact

a

Correct measure of coating thickness

Crack filled with ALD deposited coating

b

Fig. 4. SEM image of FIB cross section of tether coated with 25 nm of TiZnO, protected with a layer of platinum (a) and carbon (b).


199

Table 1 Comparison of coating thickness calculated from ALD growth curves (2nd column) and measured by coupled FIB-SEM (3rd column) on tethers samples. The average and standard deviation (round brackets) of five measurements is reported. Thickness (nm) Coating

ALD-growth

FIB-SEM

TiO2

25 50 100 25 50 100 25 50 100

33.7 (2.3) 51.9 (3.3) 112.4 (5.2) 28.0 (2.4) 46.7 (3.7) 76.8 (9.6) 26.0 (2.7) 57.8 (4.1) 97.2 (7.6)

ZnO

TiZnO

could be applied in the near future, such as photo-thermal deflection or photo-acoustic spectroscopy [38–40]. Electrochemical tests are performed to evaluate the corrosion resistance of the studied materials in dark and under UV illumination. Polarization curves, of tethers samples, obtained in dark and under UV illumination, are shown in Fig. 8a and b respectively. Uncoated and coated tethers, with 50 nm of TiO2, ZnO, and TiZnO are measured. The corresponding values of corrosion potential (Ecorr) are reported in Table 2. The photo-electrochemical behavior of crystalline TiO2 and ZnO has been extensively studied [41–44]. Both these materials are n-type semiconductors oxides. It is known that UV illumination causes anodic shift of the equilibrium potential and raise current densities in such materials. A similar behavior was observed for Ti and Zn mixed oxides systems [45,46] However, both TiO2 and the mixed oxide deposited on tethers are amorphous, so it is difficult to predict their photo-active behavior. In dark, samples uncoated and coated with ZnO show similar corrosion potential, with slightly higher current values for the bare tether. Increasing corrosion potentials are observed for the samples coated with TiZnO and TiO2 respectively. However, current values are one order of magnitude higher for TiO2. Considering comparable geometrical areas of the studied electrodes, all the coated tethers show a significant decrease of corrosion current density, of about one order of magnitude for ZnO and TiO2 and even higher for the mixed oxide. Under UV illumination, bare tether shows the highest anodic shift, followed by TiZnO, TiO2, and ZnO. In terms of corrosion protection, TiO2 maintains the best performances. Results show significant modifications in the tethers behavior starting from very low coating thicknesses.

Fig. 6. Normalized reflectance at 45° polarization for ZnO (a), TiO2 (b), and TiZnO (c). Experimental data measured on 50 nm of ZnO (red dots), 100 nm of TiO2 (pink dots) and 100 nm of TiZnO (black dots) are reported. Two polarization conditions are considered for TiZnO, horizontal (s) and vertical (p) respectively. Simulated curves for three different thicknesses (solid, dashed, and dashed dotted lines) of ZnO and TiO2 are also reported.

4. Conclusion

Fig. 5. Absorbance of Al2O3 (blue), ZnO (red), and TiO2 (pink) averaged over the whole solar spectrum and calculated for normal incidence (dashed line) and over the whole solid angle of acceptance (solid line).

In this paper we report a study of ALD coating for space applications. ESAIL tethers made of aluminum are coated with TiO2, ZnO and Ti-Zn mixed oxides by low temperature ALD. Morphological, optical, and electrochemical characterization is performed to compare the behavior of thin oxide layers. Excellent conformal coatings are observed, highlighting the suitability of ALD as coating technique in this field of application. Electrochemical and optical measurements show that the studied oxide coatings may improve the performance of tethers, in harsh space environment conditions. Coatings thickness was measured both directly, using the coupled FIB-SEM technique, and indirectly, using reflectance measurements, obtaining coherent results. Reflectance measurements show anti-reflecting effect of thin layers, with absorbance values significantly changing with the coating material. Electrochemical tests performed in dark and UV show increased corrosion resistance of tethers coated with TiO2 and ZnO.

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M. Hassan et al. / Thin Solid Films 621 (2017) 195–201 Table 2 Corrosion potentials extracted from polarization curves obtained in dark (2nd column) and under UV illumination (3rd column). Ecorr (V vs SCE)

Fig. 7. Absorbance spectra calculated from reflectance measurements of 50 nm of ZnO (red dots), 100 nm of TiO2 (pink dots) and 100 nm of TiZnO (black dots).

Sample

Dark

UV

Uncoated 50 nm TiO2 50 nm ZnO 50 nm TiZnO

−1.04 −0.87 −1.05 −0.95

−0.85 −0.76 −0.95 −0.78

In conclusion, TiO2 coating outperforms all the other studied materials for space applications of ESAIL tethers. However, the performance of Ti and Zn mixed oxide is also interesting. Further studies will be needed to evaluate potential changes due to temperature and light conditions and target the material for the selected aerospace application. Acknowledgments

a

This study was supported by the agreement “Accordo di Collaborazione per la sperimentazione d'iniziative di sviluppo, valorizzazione del capitale umano e trasferimento dei risultati della ricerca con ricaduta diretta sul territorio lombardo” between Regione Lombardia (D71J12000520009) and INSTM “Consorzio Interuniversitario Nazionale per la Scienza e Tecnologia dei Materiali” signed the 13th November 2012 titled, upon the MALDIT Project “New Materials for medicine, Energy and aerospace through Atomic Layer Deposition based Integrated (multi-step) Technologies”. Authors acknowledge Pekka Janhunen for providing the tether material and technical expertise on the ESAIL project. References

b

Fig. 8. Polarization curves of tethers samples obtained in dark (a) and under UV illumination (b). Data for uncoated tethers (solid line), and tethers coated with 50 nm of TiO2 (dotted line), 50 nm of ZnO (short dashed line), and 50 nm of TiZnO (long dashed line) are shown.

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