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Electrical insulation and breakdown properties of SiO2 and Al2O3 thin multilayer films deposited on stainless steel by physical vapor deposition
Thin Solid Films 595 (2015) 171–175
Contents lists available at ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Electrical insulation and breakdown properties of SiO2 and Al2O3 thin multilayer films deposited on stainless steel by physical vapor deposition Josu Martinez-Perdiguero a,b,⁎, Lucia Mendizabal b, Maria C. Morant-Miñana a,c, Irene Castro-Hurtado a,c, Aritz Juarros a,b, Rocío Ortiz a,d, Ainara Rodriguez a,c a
CIC microGUNE, Arrasate-Mondragón, Spain Micro-NanoFabrication Unit, IK4-Tekniker, Eibar, Spain CEIT-IK4 & Tecnun (University of Navarra), San Sebastián, Spain d Ultraprecision Processes Unit, IK4-Tekniker, Eibar, Spain b c
a r t i c l e
i n f o
Article history: Received 10 July 2015 Received in revised form 30 October 2015 Accepted 30 October 2015 Available online 3 November 2015 Keywords: Silicon oxide Alumina Multilayer Stainless steel Magnetron sputtering Breakdown field strength Leakage current
a b s t r a c t The electrical properties of dielectric thin layers deposited on conducting substrates still need to be thoroughly characterized for a wide variety of applications such as solar modules, flexible displays and sensor integration. In this work, thin dielectric films composed of layers and alternated multilayers of SiO2 and Al2O3 up to a total thickness of 3 μm have been deposited on flexible rough stainless steel substrates by means of reactive magnetron sputtering. Their electrical properties have been studied focusing on important parameters such as leakage current density and disruptive field strengths. Moreover, temperature annealing and bending effects have been quantified. It is concluded that the best electrical properties with this type of materials are achieved with multilayered structures. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Stainless steel is, apart from a ubiquitous material found in many parts, a very attractive material for the development of added value applications. For example, research on photovoltaic devices considers stainless steel as one of the best contenders for the substitution of the classical glass substrates to obtain improved flexible and lightweight monolithically integrated thin film solar modules [1,2]. Flexible display devices are also expected to benefit from the possible application of stainless steel in thin film transistor backplanes substituting the rigid and more expensive silicon [3,4]. Another important field where high performance dielectric layers are required is that of sensors. The direct integration of, e.g., strain, pressure or temperature gauges in metallic parts to operate in harsh environments requires high performance dielectric thin layers [5]. These layers should virtually not affect the measurement and possess the necessary chemical and electrical stabilities [6]. In all these applications, due to the inherent electrical conductivity of stainless steel, a high performance dielectric layer must be deposited to electrically isolate the functional layers from the substrate. Also, taking ⁎ Corresponding author at: IK4-Tekniker, Calle Iñaki Goenaga, 5 20600 Eibar, Gipuzkoa, Spain. Tel.: + 34 943206744. E-mail addresses: [email protected], [email protected] (J. Martinez-Perdiguero).
http://dx.doi.org/10.1016/j.tsf.2015.10.076 0040-6090/© 2015 Elsevier B.V. All rights reserved.
into account the high process temperatures reached at some device fabrication steps (up to 600 °C in the case of Cu(In,Ga)Se2 type solar cells [2]), this layer must hinder the diffusion of metallic impurities at the working temperatures. The surface of typical stainless steel substrates is noticeably rougher than that of glass or silicon and, to minimize processing costs and avoid polishing steps, this dielectric layer should effectively reduce the roughness inherent to steel substrates or parts to an acceptable extent while maintaining its properties. Only a few reports are found dealing with the deposition and characterization of dielectric materials over stainless steel substrates in the literature. SiOx is by far the most studied material for these purposes and barriers prepared on stainless steel by a wide variety of techniques such as ion beam assisted deposition [7,8], sol–gel deposition [7,9], plasma enhanced chemical vapor deposition [7,9,10], and flame-assisted chemical vapor deposition [11], have been characterized. ZnOx thin layers have also been deposited by DC magnetron sputtering for CIGS module fabrication strongly reducing iron diffusion [12]. Li et al. extended their research on SiOx with TaOx, TiOx and TaOx/SiOx barriers deposited by IBAD showing the better dielectric and resistance to fatigue failure properties of the latter compared to the formers [8]. Herz et al. also included the insulation properties of sputtered Al2O3 layers on stainless steel, titanium and Kovar substrates for CIGS module fabrication [9]. Sputtering techniques have been also employed to deposit Al2O3 and AlN insulating barrier layers and multilayers [13].
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Interestingly, Al2O3 was also thermally sprayed on steel and employed for the fabrication of strain sensors [14,15]. As can be seen, multilayers have been also used as insulator materials and it has been generally concluded that they reduce the possibility of defects, equilibrates the stress of the deposited layers, acts as a barrier resulting in a more stable device and prevents atomic diffusion or charge carrier transport. In this paper, we report on the electrical properties of SiO2 and Al2O3 dielectric barrier layers deposited over rough stainless steel substrates by reactive pulsed DC magnetron sputtering. Layers and multilayers of various thicknesses have been fabricated and thoroughly characterized. Noteworthily, the effect of temperature annealing has also been investigated.
2. Experimental details 2.1. Materials Widely-used AISI 304 (UNS S30400) stainless steel substrates were purchased from Unceta (Elgoibar, Spain) with 100 μm thickness and used as received. No polishing of the samples was carried out. The measured surface roughness was quantified as Ra = 201 nm and RRMS = 240 nm, the average and root mean squared roughness respectively. This stemmed mainly from rolling traces on the surface. A high power wirebound resistor (1.5 kΩ, 13 W) from Vishay Dale was employed in the electrical setup.
2.2. Dielectric thin film growth Studied barrier layers were deposited in a closed field unbalanced magnetron sputtering system designed at IK4-TEKNIKER and equipped with three rectangular cathodes 550 × 125 mm2 in size. Single SiO2 and Al2O3 layers were deposited by sputtering either Si or Al targets in Ar + O2 atmosphere by the pulsed dc magnetron sputtering technique (PMS). 1, 2 and 3 μm thick SiO2 and Al2O3 layers were grown by adjusting the deposition time for that purpose. The SiO2/Al2O3 multilayer coatings were grown by alternatively powering Si and Al targets by PMS in reactive atmosphere (Ar/O2). Alternate SiO2 and Al2O3 layers of 0.25 μm, 0.5 μm and 1 μm were deposited up to a total thickness of 2 μm. Substrates (AISI 304L coupons and Si wafers) were ultrasonically cleaned in acetone and ethanol for 10 min and mounted on a substrate holder at a distance of 70 mm from the target surfaces. A base pressure less than 5 mPa was achieved before all depositions. The substrate temperature reached 350 °C after 2 h of intentional heating by external sources. This temperature was maintained during deposition processes. Prior to barrier layers, a Ti layer (600 nm) was deposited for adhesion enhancement. The Si target was powered at 2 kW and Al target at 4 kW. The working pressure was 0.4 Pa. The Ar flow was kept constant at 70 sccm, while O2 flow was controlled via a Speedflo controller (Gencoa) in order to hold the processes in high rate metallic or transition mode. The required voltage for SiO2 and Al2O3 layers was set at 290 V and 375 V, respectively.
2.3. Structural characterization The morphology and microstructure of the samples were observed using a Carl Zeiss Ultra Plus field emission scanning electron microscope (SEM) with an accelerating voltage of 2 kV. Due to their cleaner profiles after fracturing, mainly layer barriers formed on the control silicon samples were characterized using this technique. Surface topography was characterized via optical profilometry with a Veeco Wyko NT1100 in vertical shift interference mode. Glow discharge optical emission spectroscopy (GDOES) was employed for analyzing the composition depth profile of the obtained barrier layers with a GDPROFILER 2 from Horiba Jobin Yvon.
2.4. Electrical characterization Square-shaped Cu contact pads were deposited to perform the electrical characterization. A shadow mask with an array of 36 squaredshaped holes (5 × 5 mm2) was fabricated with a picosecond pulsed laser micromachining system in 200 μm thick stainless steel film. The mask was brought into contact with the deposited barrier layers and 100 nm thick layer of Cu was sputtered in an unbalanced Gencoa (United Kingdom) PP150 magnetron sputtering system. The sputtering power density was 8.52 W/cm2 and the deposition rate 2 nm/s in Ar atmosphere at 0.7 Pa. The leakage current and dielectric breakdown fields were characterized with a dedicated setup (see Fig. 1). An Agilent N5771A DC power supply was used to apply a known voltage across the layer and the current was monitored by a Keithley 2700 multimeter with a 10 nA measurement resolution. Each measurement presented in this paper corresponds to an average of 100 current readings per voltage point obtained for 10 s. All measuring tools were automatically controlled via a developed Labview control program.
3. Results and discussion 3.1. Adhesion and morphology of SiO2 and Al2O3 layers and multilayers Adhesion layers are usually deposited immediately before the deposition of the dielectric layer in the same vacuum chamber. The role of the adhesion layer is to assure surface cleanliness of the substrate and reduce the differences of the thermal expansion coefficients of the film and the substrate [16]. After barrier deposition, qualitative adhesion tests were performed on all samples by means of 5 sequential tape pull tests. Initial coatings included thinner Ti adhesion interlayers (0.1 μm) that resulted in layer delamination probably due to the high internal stresses created during the deposition process. The thickness of this interlayer was increased to 0.6 μm to better accommodate the stresses stemming from the difference between the thermal expansion coefficients of the coatings and the steel substrate. This resulted in excellent adhesion as no detaching or cracks were observed in any sample. The physical vapor deposition processes were carried out as described in Section 2.2 and nine different barrier layers and multilayers of SiO2 and Al2O3 over unpolished stainless steel substrates were obtained for characterization: 1, 2 and 3 μm thick Al2O3 and SiO2 layers and 2 μm thick (total thickness) multilayered structures composed of 2, 4 and 8 alternated layers of these same materials. Characterization via GDOES of the samples was also carried out. As an example, the depth profile of the 0.5 μm thick Al2O3 and SiO2 multilayers (2 μm total barrier thickness) is shown in Fig. 2. Fig. 3a and b shows cross-sectional SEM micrographs of these 0.5 μm thick and 0.25 μm thick multilayers, respectively. Some distortions were observed at the interfaces but no pores or holes were present in any case. The typical columnar growth of Al2O3 is clearly observed. Both characterization methods support the finding that the layered structure is very wellformed. First, a characterization of the surface roughness was performed. The layers involving only SiO2 reflected almost perfectly the surface roughness of the stainless steel substrate (Ra = 201 nm and RRMS = 240), which contrasted with a 15–20% enhancement of these values for the pure Al2O3 samples. This fact was attributed to the columnar growth of the Al2O3 vs. the denser growth of SiO2. Similar values were found for the 1 μm thick Al2O3 and SiO2 multilayers (2 μm total barrier thickness). However, in the case of the 0.5 and 0.25 μm thick alternating layers, the final roughness was of Ra = 184 and 110 nm (RRMS = 228 and 132 nm) respectively which was possibly due to a better accommodation in the interlayers (see Fig. 3.)
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Fig. 1. Scheme and image of the electrical characterization test bench prepared for the dielectric barrier layers.
3.2. Electrical properties of the insulating layers For leakage current studies the thickness of the samples was fixed to 2 μm to enable a better comparison of the results. This thickness was considered appropriate taking into account the applications mentioned in the introduction. The set-up described in Section 2.4 and Fig. 1 was employed after the deposition of the 25 mm2 squared Cu contacts. The Cu contact was used as one electrode and the other was the substrate itself. Fig. 4 illustrates the leakage current densities up to 0.5 MV/cm for the deposited 2 μm thick SiO2 and Al2O3 layers and multilayers. Single SiO2 and Al2O3 layers showed the largest currents. The effect of stacking alternated SiO2 and Al2O3 layers resulted in a marked decrease of the leakage currents up to 2 orders of magnitude for the case of the structure with 8 layers. This could be due to the fact that the multilayered structure reduced the possibility of through pinholes, pores or defects because the different interfaces among layers stop their progression during growth. Relief of internal stresses taking place at these interfaces can also effectively hinder the formation of cracks. Interestingly the lower values are obtained for the structure formed by 0.25 μm layers. In general, insulating layers are influenced considerably by the substrate material and the surface roughness and less by variation of the sputter parameter. Since the substrates did not have any pretreatment, the results reveal that layer thickness plays the major role. It can be concluded
Fig. 2. Elemental composition depth profiles as obtained from GD-OES measurements for the 0.5 μm thick Al2O3 and SiO2 multilayers over a stainless steel substrate (2 μm total barrier thickness).
that, for constant film thicknesses (2 μm), the higher the number of layers, the better the insulation properties. In a further step, a high temperature annealing treatment was carried out. Again, 2 μm thick multilayered structures composed of 2, 4 and 8 Al2O3 and SiO2 alternated layers were fabricated. The samples were annealed inside a PEO 601 ATV furnace under an inert atmosphere with a constant flow of 2000 sccm. Argon was selected as the inert gas, because it does not react with the deposited film and will not produce a change in the composition of the material. The temperature was raised from room temperature to 500 °C at a heating rate of 8.3 °C/min and maintained for 1 h for the stabilization of the samples. Then, the temperature was decreased to room temperature at a cooling rate of 11 °C/min. As shown in Fig. 5, no cracks or defects appear after the high temperature annealing of the samples. The leakage current decreased at least one order of magnitude in the different multilayered
Fig. 3. Fractographic cross-sectional SEM images of a) Al2O3(0.5 μm)/SiO2(0.5 μm) × 2 (total thickness 2 μm) and b) Al2O3(0.25 μm)/SiO2(0.25 μm) × 4 (total thickness 2 μm) multilayers. As mentioned in the text a 0.6 μm thick Ti layer was deposited to enhance adhesion.
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Fig. 4. Leakage current density represented vs. electric field strength E of different fabricated SiO2 and Al2O3 layers and multilayers.
structures. In the case of the sample with 8 layers, the current was below 10−8 A (our experimental limit) up to an intensity of 0.3 MV/cm. Since crystallinity changes are not possible at these temperatures, the drop in currents was attributed to a better matching of the structure due to the thermal process. It is worth mentioning that further temperature annealing treatments up to 700 °C were performed and no stability deterioration was observed. This assures the use of the developed barriers in fabrication steps at high temperatures (such as, e.g., Cu(In,Ga)Se2 type solar cells). The breakdown field strength was measured for all samples applying voltages up to 300 V (the maximum voltage reachable with our equipment). The breakdown value was estimated as the voltage at which the current increased abruptly (see Fig. 6a). At least 6 samples were measured for each layer configuration and results averaged. Some samples did not reach breakdown at the maximum voltage and the value could not be estimated. However, this threshold value was considered appropriate for real applicability of the layers. As expected, the breakdown voltage increased with the layer thickness. Fig. 6b
Fig. 6. a) Examples of current vs. electric field strength curves showing the dielectric breakdown points for 2 μm thick layers. Breakdown electric field strengths for b) 1, 2 and 3 μm thick SiO2 and Al2O3 layers and c) 2 μm thick SiO2 and Al2O3 layers and multilayers. Asterisks indicate that the breakdown was not reached and must be larger than that value.
Fig. 5. SEM image of the SiO2 top surface of an annealed Al2O3(0.5 μm)/SiO2(0.5 μm) × 2 (total thickness 2 μm) multilayer. No cracks or defects were observed. For reference purposes, the inset shows, at the same scale, the uncoated stainless steel surface.
illustrates this trend for 1, 2 and 3 μm thick SiO2 and Al2O3 layers. The thickest layers did not reach breakdown at the employed voltages. Breakdown field strengths for the multilayered barriers were also tested. Fig. 6c compares the values obtained for the alternated multilayers with the pure Al2O3 and SiO2. As it can be seen the values
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to a bending radius of approximately 10 mm (see Fig. 8). This process did not provoke any observable change in the electrical properties of the barriers.
4. Conclusions In this work, different SiO2 and Al2O3 thin dielectric layers and multilayers have been fabricated on flexible unpolished stainless steel substrates via reactive magnetron sputtering. A thorough characterization of the electrical properties has been carried out including critical parameters such as leakage current densities and breakdown electric fields. The stacking of alternated thin layers of SiO2 and Al2O3 has resulted in improved properties achieving reductions up to two orders of magnitude in leakage current and large enhancement of the breakdown fields. The structural stabilization reached by a temperature annealing process has also been studied and quantified.
Acknowledgments
Fig. 7. Breakdown electric field strengths for the fabricated multilayers with and without annealing. The asterisk indicates that the breakdown was not reached and must be larger than that value.
This work was supported by the Basque Government under the Etortek Program (Grant No. IE13–360).
References
Fig. 8. Photograph of a fabricated dielectric barrier on a flexible stainless steel substrate during a bending test.
increased with respect to a single Al2O3 barrier but did not improve the breakdown of a single SiO2 layer. However, it must be noted that the leakage currents before breakdown were much lower for the multilayers and they showed a much more stable behavior. The effect of the above-mentioned annealing treatment was also quantified in terms of breakdown field strength. After the multilayered barriers were fabricated the annealing was carried out and the Cu contacts were deposited. As expected, due to the lower leakage current measured, breakdown values also increased (see Fig. 7). Interestingly, in the case of the sample composed of eight 0.25 μm thick Al2O3 and SiO2 layers, the improvement was more significant up to the point when no breakdown was observed. Again, this could be due to stabilization of the structure during the annealing process. The bending properties of the barrier were also qualitatively studied by manual bending of the substrates in both directions (50 cycles) down
[1] J.-M. Delgado-Sanchez, N. Guilera, L. Francesch, M.D. Alba, L. Lopez, E. Sanchez, Ceramic barrier layers for flexible thin film solar cells on metallic substrates: a laboratory scale study for process optimization and barrier layer properties, ACS Appl. Mater. Interfaces 6 (2014) 18543–18549. [2] A. Chirilă, S. Buecheler, F. Pianezzi, P. Bloesch, C. Gretener, A.R. Uhl, C. Fella, L. Kranz, J. Perrenoud, S. Seyrling, R. Verma, S. Nishiwaki, Y.E. Romanyuk, G. Bilger, A.N. Tiwari, Highly efficient Cu(In,Ga)Se2 solar cells grown on flexible polymer films, Nat. Mater. 10 (2011) 857. [3] Y. Chen, J. Au, P. Kazlas, A. Ritenour, H. Gates, M. McCreary, Electronic paper: flexible active-matrix electronic ink display, Nature 423 (2003) 136. [4] Y. Hong, G. Heiler, I.-C. Cheng, A. Kattamis, S. Wagner, Stainless steel foil substrates: robust, low-cost, flexible active-matrix backplane technology, Proceedings of IMID 2005, p. 892. [5] X. Li, X. Zhang, D. Werschmoeller, H. Choi, X. Cheng, Embedded micro/nano sensors for harsh engineering environments, IEEE Sensors (2008) 1273–1276. [6] T. Bravo, A. Tersalvi, A. Tosi, Comparison of SiOx and polyimide as a dielectric layer on stainless steel in thin-film pressure sensor manufacture, Sensors Actuators A Phys. 32 (1992) 611–615. [7] Y. Li, Z. Yu, J. Leng, D. Zhang, S. Chen, G. Jin, Electrical insulation and bending properties of SiOx barrier layers prepared on flexible stainless steel foils by different preparing methods, Thin Solid Films 519 (2011) 4234–4238. [8] Y. Li, Z. Yu, J. Leng, H. Wang, S. Chen, Y. Dong, G. Jin, Dielectric properties and resistance to fatigue failure of different barrier layers prepared on flexible stainless-steel foils by ion-beam assisted deposition, Thin Solid Films 520 (2012) 6556–6560. [9] K. Herz, F. Kessler, R. Wächter, M. Powalla, J. Schneider, A. Schulz, U. Schumacher, Dielectric barriers for flexible CIGS solar modules, Thin Solid Films 403–404 (2002) 384–389. [10] F. Kessler, D. Herrmann, M. Powalla, Approaches to flexible CIGS thin-film solar cells, Thin Solid Films 480–481 (2005) 491–498. [11] P. Evans, T. English, D. Hammond, M.E. Pemble, D.W. Sheel, The role of SiO2 barrier layers in determining the structure and photocatalytic activity of TiO2 films deposited on stainless steel, Appl. Catal. A Gen. 321 (2007) 140–146. [12] C. Y. Shi, Y. Sun, Q. He, F. Y. Li, J. C. Zhao, Cu(In,Ga)Se2 solar cells on stainless-steel substrates covered with ZnO diffusion barriers, Sol. Energy Mater. Sol. Cells 93, (2009) 654 656. [13] R. Djugum, Novel Fabrication Processes for Thin Film Vapour Deposited Strain Gauges on Mild Steel, Swinburne University Technology, 2006 (Dissertation). [14] R. Djugum, K.I. Jolic, A fabrication process for vacuum-deposited strain gauges on thermally sprayed Al2O3, J. Micromech. Microeng. 16 (2006) 457–462. [15] M. Fasching, F.B. Prinz, L.E. Weiss, Smart coatings: a technical note, J. Therm. Spray Technol. 4 (1995) 133–136. [16] S. Franssila, Introduction to Microfabrication, second ed. John Wiley & Sons, West Sussex, 2010.
Contents lists available at ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Electrical insulation and breakdown properties of SiO2 and Al2O3 thin multilayer films deposited on stainless steel by physical vapor deposition Josu Martinez-Perdiguero a,b,⁎, Lucia Mendizabal b, Maria C. Morant-Miñana a,c, Irene Castro-Hurtado a,c, Aritz Juarros a,b, Rocío Ortiz a,d, Ainara Rodriguez a,c a
CIC microGUNE, Arrasate-Mondragón, Spain Micro-NanoFabrication Unit, IK4-Tekniker, Eibar, Spain CEIT-IK4 & Tecnun (University of Navarra), San Sebastián, Spain d Ultraprecision Processes Unit, IK4-Tekniker, Eibar, Spain b c
a r t i c l e
i n f o
Article history: Received 10 July 2015 Received in revised form 30 October 2015 Accepted 30 October 2015 Available online 3 November 2015 Keywords: Silicon oxide Alumina Multilayer Stainless steel Magnetron sputtering Breakdown field strength Leakage current
a b s t r a c t The electrical properties of dielectric thin layers deposited on conducting substrates still need to be thoroughly characterized for a wide variety of applications such as solar modules, flexible displays and sensor integration. In this work, thin dielectric films composed of layers and alternated multilayers of SiO2 and Al2O3 up to a total thickness of 3 μm have been deposited on flexible rough stainless steel substrates by means of reactive magnetron sputtering. Their electrical properties have been studied focusing on important parameters such as leakage current density and disruptive field strengths. Moreover, temperature annealing and bending effects have been quantified. It is concluded that the best electrical properties with this type of materials are achieved with multilayered structures. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Stainless steel is, apart from a ubiquitous material found in many parts, a very attractive material for the development of added value applications. For example, research on photovoltaic devices considers stainless steel as one of the best contenders for the substitution of the classical glass substrates to obtain improved flexible and lightweight monolithically integrated thin film solar modules [1,2]. Flexible display devices are also expected to benefit from the possible application of stainless steel in thin film transistor backplanes substituting the rigid and more expensive silicon [3,4]. Another important field where high performance dielectric layers are required is that of sensors. The direct integration of, e.g., strain, pressure or temperature gauges in metallic parts to operate in harsh environments requires high performance dielectric thin layers [5]. These layers should virtually not affect the measurement and possess the necessary chemical and electrical stabilities [6]. In all these applications, due to the inherent electrical conductivity of stainless steel, a high performance dielectric layer must be deposited to electrically isolate the functional layers from the substrate. Also, taking ⁎ Corresponding author at: IK4-Tekniker, Calle Iñaki Goenaga, 5 20600 Eibar, Gipuzkoa, Spain. Tel.: + 34 943206744. E-mail addresses: [email protected], [email protected] (J. Martinez-Perdiguero).
http://dx.doi.org/10.1016/j.tsf.2015.10.076 0040-6090/© 2015 Elsevier B.V. All rights reserved.
into account the high process temperatures reached at some device fabrication steps (up to 600 °C in the case of Cu(In,Ga)Se2 type solar cells [2]), this layer must hinder the diffusion of metallic impurities at the working temperatures. The surface of typical stainless steel substrates is noticeably rougher than that of glass or silicon and, to minimize processing costs and avoid polishing steps, this dielectric layer should effectively reduce the roughness inherent to steel substrates or parts to an acceptable extent while maintaining its properties. Only a few reports are found dealing with the deposition and characterization of dielectric materials over stainless steel substrates in the literature. SiOx is by far the most studied material for these purposes and barriers prepared on stainless steel by a wide variety of techniques such as ion beam assisted deposition [7,8], sol–gel deposition [7,9], plasma enhanced chemical vapor deposition [7,9,10], and flame-assisted chemical vapor deposition [11], have been characterized. ZnOx thin layers have also been deposited by DC magnetron sputtering for CIGS module fabrication strongly reducing iron diffusion [12]. Li et al. extended their research on SiOx with TaOx, TiOx and TaOx/SiOx barriers deposited by IBAD showing the better dielectric and resistance to fatigue failure properties of the latter compared to the formers [8]. Herz et al. also included the insulation properties of sputtered Al2O3 layers on stainless steel, titanium and Kovar substrates for CIGS module fabrication [9]. Sputtering techniques have been also employed to deposit Al2O3 and AlN insulating barrier layers and multilayers [13].
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J. Martinez-Perdiguero et al. / Thin Solid Films 595 (2015) 171–175
Interestingly, Al2O3 was also thermally sprayed on steel and employed for the fabrication of strain sensors [14,15]. As can be seen, multilayers have been also used as insulator materials and it has been generally concluded that they reduce the possibility of defects, equilibrates the stress of the deposited layers, acts as a barrier resulting in a more stable device and prevents atomic diffusion or charge carrier transport. In this paper, we report on the electrical properties of SiO2 and Al2O3 dielectric barrier layers deposited over rough stainless steel substrates by reactive pulsed DC magnetron sputtering. Layers and multilayers of various thicknesses have been fabricated and thoroughly characterized. Noteworthily, the effect of temperature annealing has also been investigated.
2. Experimental details 2.1. Materials Widely-used AISI 304 (UNS S30400) stainless steel substrates were purchased from Unceta (Elgoibar, Spain) with 100 μm thickness and used as received. No polishing of the samples was carried out. The measured surface roughness was quantified as Ra = 201 nm and RRMS = 240 nm, the average and root mean squared roughness respectively. This stemmed mainly from rolling traces on the surface. A high power wirebound resistor (1.5 kΩ, 13 W) from Vishay Dale was employed in the electrical setup.
2.2. Dielectric thin film growth Studied barrier layers were deposited in a closed field unbalanced magnetron sputtering system designed at IK4-TEKNIKER and equipped with three rectangular cathodes 550 × 125 mm2 in size. Single SiO2 and Al2O3 layers were deposited by sputtering either Si or Al targets in Ar + O2 atmosphere by the pulsed dc magnetron sputtering technique (PMS). 1, 2 and 3 μm thick SiO2 and Al2O3 layers were grown by adjusting the deposition time for that purpose. The SiO2/Al2O3 multilayer coatings were grown by alternatively powering Si and Al targets by PMS in reactive atmosphere (Ar/O2). Alternate SiO2 and Al2O3 layers of 0.25 μm, 0.5 μm and 1 μm were deposited up to a total thickness of 2 μm. Substrates (AISI 304L coupons and Si wafers) were ultrasonically cleaned in acetone and ethanol for 10 min and mounted on a substrate holder at a distance of 70 mm from the target surfaces. A base pressure less than 5 mPa was achieved before all depositions. The substrate temperature reached 350 °C after 2 h of intentional heating by external sources. This temperature was maintained during deposition processes. Prior to barrier layers, a Ti layer (600 nm) was deposited for adhesion enhancement. The Si target was powered at 2 kW and Al target at 4 kW. The working pressure was 0.4 Pa. The Ar flow was kept constant at 70 sccm, while O2 flow was controlled via a Speedflo controller (Gencoa) in order to hold the processes in high rate metallic or transition mode. The required voltage for SiO2 and Al2O3 layers was set at 290 V and 375 V, respectively.
2.3. Structural characterization The morphology and microstructure of the samples were observed using a Carl Zeiss Ultra Plus field emission scanning electron microscope (SEM) with an accelerating voltage of 2 kV. Due to their cleaner profiles after fracturing, mainly layer barriers formed on the control silicon samples were characterized using this technique. Surface topography was characterized via optical profilometry with a Veeco Wyko NT1100 in vertical shift interference mode. Glow discharge optical emission spectroscopy (GDOES) was employed for analyzing the composition depth profile of the obtained barrier layers with a GDPROFILER 2 from Horiba Jobin Yvon.
2.4. Electrical characterization Square-shaped Cu contact pads were deposited to perform the electrical characterization. A shadow mask with an array of 36 squaredshaped holes (5 × 5 mm2) was fabricated with a picosecond pulsed laser micromachining system in 200 μm thick stainless steel film. The mask was brought into contact with the deposited barrier layers and 100 nm thick layer of Cu was sputtered in an unbalanced Gencoa (United Kingdom) PP150 magnetron sputtering system. The sputtering power density was 8.52 W/cm2 and the deposition rate 2 nm/s in Ar atmosphere at 0.7 Pa. The leakage current and dielectric breakdown fields were characterized with a dedicated setup (see Fig. 1). An Agilent N5771A DC power supply was used to apply a known voltage across the layer and the current was monitored by a Keithley 2700 multimeter with a 10 nA measurement resolution. Each measurement presented in this paper corresponds to an average of 100 current readings per voltage point obtained for 10 s. All measuring tools were automatically controlled via a developed Labview control program.
3. Results and discussion 3.1. Adhesion and morphology of SiO2 and Al2O3 layers and multilayers Adhesion layers are usually deposited immediately before the deposition of the dielectric layer in the same vacuum chamber. The role of the adhesion layer is to assure surface cleanliness of the substrate and reduce the differences of the thermal expansion coefficients of the film and the substrate [16]. After barrier deposition, qualitative adhesion tests were performed on all samples by means of 5 sequential tape pull tests. Initial coatings included thinner Ti adhesion interlayers (0.1 μm) that resulted in layer delamination probably due to the high internal stresses created during the deposition process. The thickness of this interlayer was increased to 0.6 μm to better accommodate the stresses stemming from the difference between the thermal expansion coefficients of the coatings and the steel substrate. This resulted in excellent adhesion as no detaching or cracks were observed in any sample. The physical vapor deposition processes were carried out as described in Section 2.2 and nine different barrier layers and multilayers of SiO2 and Al2O3 over unpolished stainless steel substrates were obtained for characterization: 1, 2 and 3 μm thick Al2O3 and SiO2 layers and 2 μm thick (total thickness) multilayered structures composed of 2, 4 and 8 alternated layers of these same materials. Characterization via GDOES of the samples was also carried out. As an example, the depth profile of the 0.5 μm thick Al2O3 and SiO2 multilayers (2 μm total barrier thickness) is shown in Fig. 2. Fig. 3a and b shows cross-sectional SEM micrographs of these 0.5 μm thick and 0.25 μm thick multilayers, respectively. Some distortions were observed at the interfaces but no pores or holes were present in any case. The typical columnar growth of Al2O3 is clearly observed. Both characterization methods support the finding that the layered structure is very wellformed. First, a characterization of the surface roughness was performed. The layers involving only SiO2 reflected almost perfectly the surface roughness of the stainless steel substrate (Ra = 201 nm and RRMS = 240), which contrasted with a 15–20% enhancement of these values for the pure Al2O3 samples. This fact was attributed to the columnar growth of the Al2O3 vs. the denser growth of SiO2. Similar values were found for the 1 μm thick Al2O3 and SiO2 multilayers (2 μm total barrier thickness). However, in the case of the 0.5 and 0.25 μm thick alternating layers, the final roughness was of Ra = 184 and 110 nm (RRMS = 228 and 132 nm) respectively which was possibly due to a better accommodation in the interlayers (see Fig. 3.)
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Fig. 1. Scheme and image of the electrical characterization test bench prepared for the dielectric barrier layers.
3.2. Electrical properties of the insulating layers For leakage current studies the thickness of the samples was fixed to 2 μm to enable a better comparison of the results. This thickness was considered appropriate taking into account the applications mentioned in the introduction. The set-up described in Section 2.4 and Fig. 1 was employed after the deposition of the 25 mm2 squared Cu contacts. The Cu contact was used as one electrode and the other was the substrate itself. Fig. 4 illustrates the leakage current densities up to 0.5 MV/cm for the deposited 2 μm thick SiO2 and Al2O3 layers and multilayers. Single SiO2 and Al2O3 layers showed the largest currents. The effect of stacking alternated SiO2 and Al2O3 layers resulted in a marked decrease of the leakage currents up to 2 orders of magnitude for the case of the structure with 8 layers. This could be due to the fact that the multilayered structure reduced the possibility of through pinholes, pores or defects because the different interfaces among layers stop their progression during growth. Relief of internal stresses taking place at these interfaces can also effectively hinder the formation of cracks. Interestingly the lower values are obtained for the structure formed by 0.25 μm layers. In general, insulating layers are influenced considerably by the substrate material and the surface roughness and less by variation of the sputter parameter. Since the substrates did not have any pretreatment, the results reveal that layer thickness plays the major role. It can be concluded
Fig. 2. Elemental composition depth profiles as obtained from GD-OES measurements for the 0.5 μm thick Al2O3 and SiO2 multilayers over a stainless steel substrate (2 μm total barrier thickness).
that, for constant film thicknesses (2 μm), the higher the number of layers, the better the insulation properties. In a further step, a high temperature annealing treatment was carried out. Again, 2 μm thick multilayered structures composed of 2, 4 and 8 Al2O3 and SiO2 alternated layers were fabricated. The samples were annealed inside a PEO 601 ATV furnace under an inert atmosphere with a constant flow of 2000 sccm. Argon was selected as the inert gas, because it does not react with the deposited film and will not produce a change in the composition of the material. The temperature was raised from room temperature to 500 °C at a heating rate of 8.3 °C/min and maintained for 1 h for the stabilization of the samples. Then, the temperature was decreased to room temperature at a cooling rate of 11 °C/min. As shown in Fig. 5, no cracks or defects appear after the high temperature annealing of the samples. The leakage current decreased at least one order of magnitude in the different multilayered
Fig. 3. Fractographic cross-sectional SEM images of a) Al2O3(0.5 μm)/SiO2(0.5 μm) × 2 (total thickness 2 μm) and b) Al2O3(0.25 μm)/SiO2(0.25 μm) × 4 (total thickness 2 μm) multilayers. As mentioned in the text a 0.6 μm thick Ti layer was deposited to enhance adhesion.
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Fig. 4. Leakage current density represented vs. electric field strength E of different fabricated SiO2 and Al2O3 layers and multilayers.
structures. In the case of the sample with 8 layers, the current was below 10−8 A (our experimental limit) up to an intensity of 0.3 MV/cm. Since crystallinity changes are not possible at these temperatures, the drop in currents was attributed to a better matching of the structure due to the thermal process. It is worth mentioning that further temperature annealing treatments up to 700 °C were performed and no stability deterioration was observed. This assures the use of the developed barriers in fabrication steps at high temperatures (such as, e.g., Cu(In,Ga)Se2 type solar cells). The breakdown field strength was measured for all samples applying voltages up to 300 V (the maximum voltage reachable with our equipment). The breakdown value was estimated as the voltage at which the current increased abruptly (see Fig. 6a). At least 6 samples were measured for each layer configuration and results averaged. Some samples did not reach breakdown at the maximum voltage and the value could not be estimated. However, this threshold value was considered appropriate for real applicability of the layers. As expected, the breakdown voltage increased with the layer thickness. Fig. 6b
Fig. 6. a) Examples of current vs. electric field strength curves showing the dielectric breakdown points for 2 μm thick layers. Breakdown electric field strengths for b) 1, 2 and 3 μm thick SiO2 and Al2O3 layers and c) 2 μm thick SiO2 and Al2O3 layers and multilayers. Asterisks indicate that the breakdown was not reached and must be larger than that value.
Fig. 5. SEM image of the SiO2 top surface of an annealed Al2O3(0.5 μm)/SiO2(0.5 μm) × 2 (total thickness 2 μm) multilayer. No cracks or defects were observed. For reference purposes, the inset shows, at the same scale, the uncoated stainless steel surface.
illustrates this trend for 1, 2 and 3 μm thick SiO2 and Al2O3 layers. The thickest layers did not reach breakdown at the employed voltages. Breakdown field strengths for the multilayered barriers were also tested. Fig. 6c compares the values obtained for the alternated multilayers with the pure Al2O3 and SiO2. As it can be seen the values
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to a bending radius of approximately 10 mm (see Fig. 8). This process did not provoke any observable change in the electrical properties of the barriers.
4. Conclusions In this work, different SiO2 and Al2O3 thin dielectric layers and multilayers have been fabricated on flexible unpolished stainless steel substrates via reactive magnetron sputtering. A thorough characterization of the electrical properties has been carried out including critical parameters such as leakage current densities and breakdown electric fields. The stacking of alternated thin layers of SiO2 and Al2O3 has resulted in improved properties achieving reductions up to two orders of magnitude in leakage current and large enhancement of the breakdown fields. The structural stabilization reached by a temperature annealing process has also been studied and quantified.
Acknowledgments
Fig. 7. Breakdown electric field strengths for the fabricated multilayers with and without annealing. The asterisk indicates that the breakdown was not reached and must be larger than that value.
This work was supported by the Basque Government under the Etortek Program (Grant No. IE13–360).
References
Fig. 8. Photograph of a fabricated dielectric barrier on a flexible stainless steel substrate during a bending test.
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