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High-rate deposition of thick aluminum coatings on plastic parts for electromagnetic shielding
Journal Pre-proof High-rate deposition of thick aluminum coatings on plastic parts for electromagnetic shielding
Jens-Peter Heinß, Fred Fietzke PII:
S0257-8972(19)31125-9
DOI:
https://doi.org/10.1016/j.surfcoat.2019.125134
Reference:
SCT 125134
To appear in:
Surface & Coatings Technology
Received date:
12 July 2019
Revised date:
1 November 2019
Accepted date:
3 November 2019
Please cite this article as: J.-P. Heinß and F. Fietzke, High-rate deposition of thick aluminum coatings on plastic parts for electromagnetic shielding, Surface & Coatings Technology (2018), https://doi.org/10.1016/j.surfcoat.2019.125134
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© 2018 Published by Elsevier.
Journal Pre-proof Manuscript cover page Surface and Coatings Technology Manuscript Draft for special issue of Society of Vacuum Coaters Annual Technical Conference 2019
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Abstract ID# at SVC-Conference 2019: 2019SVC48 Paper ID# at SVC-Conference 2019: TT 9 Manuscript Number: SURFCOAT-D-19-02249R2
Journal Pre-proof High-Rate Deposition of Thick Aluminum Coatings on Plastic Parts for Electromagnetic Shielding Jens-Peter Heinß, Fred Fietzke Fraunhofer Institute for Organic Electronics, Electron Beam and Plasma Technology FEP, Winterbergstrasse 28, 01277 Dresden, Germany *correspondence to: Jens-Peter Heinß, email: [email protected] phone: +49 351 2586-244 fax: +49 351 2586-55244 ___________________________________________________________________________ Abstract
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A new approach is presented for depositing electromagnetic-shielding layers on plastic
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components by PVD direct metallization for improved electromagnetic compatibility (EMC).
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Both the plasma pre-treatment and the coating steps were executed using parameters that
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allow short processing times. Plastic substrates (PC, ABS, and PLA) were exposed to an intense oxygen/argon plasma from a hollow-cathode arc-discharge source. Cross-cut tests
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revealed that a plasma pre-treatment time of one minute was sufficient to achieve excellent
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adhesion. It was possible to deposit a 5-µm aluminum layer onto these different types of plastics using electron-beam evaporation with an axial-beam gun.
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High-rate deposition can evidently overcome difficulties caused by overheating these synthetic materials. The coatings were applied at a deposition rate above 100 nm/s. Because of this high rate, electron-beam evaporation provided a considerably lower temperature increase for a given layer thickness in comparison to magnetron sputtering. This difference amounted to more than one order of magnitude. The evaporative coating process was also adapted to additively manufactured parts for the first time. The aluminum layers deposited provided the intended functional properties, in particular the electromagnetic shielding. The attenuation in magnetic near field configuration was determined to 44 dB at 3 GHz.
Journal Pre-proof Keywords: High Deposition Rate; Aluminum Thin Films; Electron Beam Evaporation; Thermal Load; Electromagnetic Compatibility Shielding.
Highlights Electron-beam evaporation of aluminum with deposition rate above 100 nm/s
Deposition of 5-µm thick aluminum coatings that adhere well on plastics
Low specific heat flux per unit thickness of the deposited layer
Attenuation in magnetic near field configuration of more than 40 dB at 3 GHz
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Journal Pre-proof 1. Introduction Synthetic materials are broadly used in the automobile industry, sanitary engineering, and consumer electronics. When suitably coated, these materials often exhibit a metallic appearance and additional functionalities such as scratch resistance, for example. The metallization is usually applied with chemical or electrochemical technologies [1]. Environmentally sustainable alternatives are gaining in importance through implementation of EU REACH legislation [2]. Physical vapor deposition (PVD) methods fulfill these
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expectations [3]. Thin decorative coatings can be provided by magnetron sputtering, in which
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the metal layer is embedded between two coats of lacquer.
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Shielding to achieve electromagnetic compatibility for internal electronic components exposed to low-frequency external interference requires materials having high magnetic
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permeability, such as nickel. At higher frequencies, the skin effect determines the amount of
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absorption by the housing’s wall material [4],[5]. The wall material itself can be conductive, or laminated with materials having high electrical conductivity and magnetic permeability
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[6],[7]. Both in combination lead to the well-known skin effect. The outer alternating
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electromagnetic field induces a voltage. Because of the electrical conductivity, a current is induced in the material, which in itself generates a magnetic field, that is opposite oriented to the causal field and compensates it [4]. Effective electromagnetic shielding in the gigahertz range requires metallic layers with a thickness of several microns. Evaporative deposition processes appear to be suitable for ensuring a high deposition rate. For example, electron-beam evaporation with axial-beam guns has been applied for the deposition of back contacts of solar cells. 20-µm aluminum layers were deposited on thin silicon wafers (185 µm) [8]. This study enabled us to explore and deduce the relationship between high deposition rate and heat input. It is known that conventional plastic parts exhibit temperature limits of approximately 80-100 °C and are subject to damage above that temperature.
Journal Pre-proof A further challenge is to achieve high-grade adhesion of thick coatings on plastic substrates. Often organic intermediate layers known as primers are applied. So a second question arises: can reliable metallization be achieved without this additional adhesive priming layer? Plasmabased pre-treatment is frequently utilized in the case of vacuum deposition methods. The impact of the ions promotes the formation of functional groups responsible for chemical-layer bonding [9]. As well, topographic modification of the substrate surface by the ions has been verified [10]. Both effects typically occur during plasma treatment of plastics [11],[12]. There
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is an additional technical requirement in industrial scenarios: to achieve high-rate deposition,
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the plasma pre-treatment needs to be executed in a comparably short period of time.
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Last but not least, the range of applications for additively manufactured parts has broadened significantly in recent years, so that PVD coating of such materials becomes a focus of
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increasing interest.
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Here we report on plasma pre-treatment of plastic substrates and the deposition of aluminum layers several microns thick using two different PVD methods – magnetron sputtering and
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electron-beam evaporation. The heat flux densities and the temperature limitations are
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analyzed. Electromagnetic attenuation properties of the deposited coatings are determined for high frequency range.
2. Experimental setup
The plasma pre-treatment and deposition experiments were divided into two groups. The first series was realized with the UNIVERSA sputter-coating equipment already described in [13],[14]. The device is equipped with four rectangular magnetron sources (target dimensions 512 mm x 128 mm) that are arranged in pairs at both side doors. Only one source was used in DC mode for the deposition experiments conducted at a power level of 8 kW. The out-gassing of plastic substrates was monitored with a PrismaPlus® QMA 200 quadrupole analyzer (Pfeiffer Vacuum). Taking scan measurements in a mass number range
Journal Pre-proof between 1 and 100 allows identification of the dominant gases. The plasma for the substrate pre-treatment was produced by a hollow-cathode arc-discharge source developed in-house [15]. Enhancement via magnetic field enables an argon-plasma density to be developed on the order of 1018 m-3 at a distance of 300 mm [16]. During the plasma pre-treatment of substrates, the hollow-cathode discharge is driven to 70 V with a discharge current of 100 A, corresponding to a discharge power of 7 kW. Electron-beam evaporation was carried out in the NOVELLA experimental facility (Fig. 1),
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comprising a load-lock chamber and a coating chamber separated by a plate valve. A versatile
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substrate transport system allows transfer of parts (maximum dimension Ø = 150 mm,
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L = 300 mm) between the chambers. Translational and rotational movements are possible with this transport system and can be combined. Both chambers are equipped with
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turbomolecular pumps (2300 l/s, Pfeiffer Vacuum). The base pressure for the experiments
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was 3.10-6 mbar. The load-lock chamber is 0.6 m x 0.9 m x 0.8 m and is made of stainless steel. A hollow-cathode arc-discharge source is mounted on a bottom flange and is used for
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plasma pre-treatment. The deposition chamber, also made of stainless steel, is
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1.2 m x 0.9 m x 1.1 m. An aluminum-oxide crucible (bath dimension: 100 mm x 100 mm) is located at the bottom of that chamber. The distance between the crucible and central position of a substrate in the coating chamber is 450 mm. A plasma-based axial-beam gun [17] with a maximum beam power of 120 kW is used for the electron-beam evaporation and is mounted at an oblique angle on top of the chamber. Different types of injection-molded plastics were used: polycarbonate (PC), acrylonitrile butadiene
styrene
(ABS),
and
polylactic
acid
(PLA).
The
substrate
size
was
100 mm x 100 mm x 2 mm. The additively manufactured substrates from the same types of plastic were 50 mm x 10 mm x 10 mm. 100 mm x 60 mm x 1 mm stainless steel sheets were used for individual measurements of temperature increase and determination of the heat flux. For the subsequent measurement of electromagnetic attenuation, junction boxes made from
Journal Pre-proof high-density polyethylene (85 mm x 85 mm x 60 mm) were internally coated. For the sputter-deposition experiments, the substrates were hung with wires, while in the evaporative-deposition experiments the substrates were fixed with a screw on a metallic holder. All substrates rotated during deposition. The substrate temperature was measured with chromel-alumel-thermocouples that were mounted on the rear side. The thickness of the aluminum layer deposited on steel substrates was established gravimetrically from mass difference and the aluminum density. The layer thickness of the
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plastic substrates was checked by Dektak profilometer measurements at shadowing edges
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prepared for this purpose. The same method was employed for measuring the thickness
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profile of the internally coated junction boxes. Shadowing edges were prepared for this purpose using a few glass sheets fixed inside the boxes.
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The surfaces of the plasma-treated polymer substrates were investigated by optical
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microscopy with differential interference contrast (Polyvar 2 Met, Reichert). Ion-polished cross-sections of the aluminum layers were prepared using an argon-ion beam
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preparation technique (Cross-Section Polisher, SM-0910, Jeol). The cross-sections were
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analyzed by field-emission scanning electron microscopy (FE-SEM, SU 8000, Hitachi) using channeling contrast and backscattered electrons for imaging the microstructure. The cross-cut tests were executed in accordance with DIN EN ISO 2409. The sheet resistance was determined with an FPP 5000 four-point probe (Veeco Instruments Inc.). The specific electrical resistance was deduced from this using the thickness of the aluminum layer. The magnetic attenuation of coated flat samples was measured with a laboratory setup in the frequency range between 0.1 and 3 GHz. The return loss was measured with a spectrum analyzer R3132 (ADVANTEST). The coated samples were positioned between a miniaturized transmitting and a receiving loop antenna. The attenuation, which is caused by the distance between the antennas, was determined by measuring an uncoated sample of the same
Journal Pre-proof thickness. The shielding effectiveness of an aluminum foil with a thickness of 3 µm (Goodfellow, AL000320) was measured as a reference. The electromagnetic attenuation of aluminum-coated junction boxes also was determined. The electric field strength for two junction-box configurations - grounded and ungrounded - was measured from 5 Hz to 32 kHz with an EFA 300 field analyzer (Narda Safety Test Solutions GmbH) located outside the box.
3. Results 3.1 Sample outgassing and plasma pretreatment
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The evacuation behavior of the vacuum chamber was studied after the additively
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manufactured plastic substrates were placed in the coating chamber. In comparison with the
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empty chamber, two effects were observed. First, an extension of evacuation time was needed
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(Fig. 2). With PLA and ABS substrates, 40 minutes were needed to achieve a pressure of 3.10-3 Pa, compared to 25 minutes without the substrates in the chamber. Second, the
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evacuation curves show two points, after 85 and120 minutes as indicated in Fig. 2 by arrows,
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after which the pressure drops faster. The out-gassing of the additively manufactured substrates in the chamber is nearly complete by these times. This out-gassing is responsible
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for the additional time necessary to achieve the pressure of 3.10-3 Pa as described above. Plasma pre-treatment using the hollow-cathode arc discharge source was first tested solely with argon. In this case the adhesion of the sputtered layers was insufficient. Next, a gas flow of 175 sccm of oxygen was introduced for the plasma pre-treatment in addition to the gas flow of 25 sccm of argon through the hollow-cathode device. After this oxygen/argon plasma pre-treatment, all subsequently deposited layers exhibit defect-free adhesion. The authors assume that the cause of the improved adhesion observed is due to changes in chemical bonding at the surface and/or coating/substrate boundary. The process heat leads to a linear increase of the substrate temperature with time (Fig. 3). The linear time dependence between 1.5 and 4.2 minutes is marked with a straight gray line for the “3-minute plasma pre-treatment” curve in Fig. 3. The temperature rise, checked for the three
Journal Pre-proof different treatment times, can be represented as 13 K/min. The assumed temperature limit for the plastics of 80 °C is not exceeded for plasma pre-treatment times of less than 4 minutes. Fig. 4 illustrates the topological alteration of the substrate. The polycarbonate surface (Fig. 4b) treated for 2 minutes exhibits an increased roughness in comparison to the untreated surface (Fig. 4a). This was confirmed by measurements with an atomic force microscope (AFM) (not presented here).
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3.2 Coating by magnetron sputtering
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Next, the plasma pre-treatment was followed by magnetron-sputter deposition of aluminum
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onto the substrates. The substrate was coated with a 200-nm aluminum layer during the 2-min deposition time (100 nm/min).
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Both process steps cause a temperature increase (Fig. 5). During the 2-min plasma pre-
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treatment, the temperature rises from 24 °C to 49 °C. The substrate cools down during the interruption time and the temperature decreases. The sputter deposition effects a temperature
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rise from 27 °C to 70 °C. The linear temperature increase with time is marked green in the
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diagram for the time interval of minute 20 to minute 22. A time-dependent increase of 19 K/min can be observed. The substrate undergoes final cool-down inside the vacuum chamber.
The negative slopes of the temperature-time curve are different for the two cool-down periods. Cooling after the plasma pre-treatment step is faster. This was determined to be a result of the differing surface states after the plasma pre-treatment and coating steps. Polycarbonate exhibits an emissivity of 0.9 [18] and aluminum less than 0.1 [19]. It is thought this difference is responsible for different amounts of thermal radiation and the different cooling speeds for the uncoated and coated states.
3.3 Coating by electron beam evaporation
Journal Pre-proof The experiments with electron-beam evaporation were executed in the NOVELLA facility. Deposition rates in the 100 nm/s range were confirmed for electron-beam power between 6 and 8 kW (Fig. 6). Aluminum layers with thicknesses in the range between 3 and 7 µm were deposited on different plastic substrates varying the beam power and the deposition time. The related temperature increase with time for a 45 s long-running deposition is presented in Fig. 7. The polycarbonate substrate temperature rises from 35 °C to a maximum of 70 °C. The time
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period over which the temperature rise occurs is longer than the deposition time itself. The
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deposition time of 45 s is defined by the duration of the period the shutter is open.
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The linear temperature increase with time is marked green in the diagram for the time interval from minute 1.6 until minute 2.4 (0.8 minutes total compared to 2.0 minutes total for sputter
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deposition). This range is characterized by a linear slope of 17 K/min. The subsequent
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apparent temperature increase after deposition is likely the result of thermal conduction processes in the plastic substrate material that delay the temperature measurement taken at the
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backside of the substrate. Finally, it can be stated that an aluminum layer several microns
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thick can be deposited using electron-beam evaporation without exceeding the relevant temperature limit of plastics.
For purposes of exploring the temperature delay and for comparing heat flux densities, a subsequent deposition of aluminum onto a 1-mm steel substrate was executed. The rate of temperature increase was determined to be 62 K/min (diagram not shown here). The temperature rises linearly with time during the entire deposition period. Immediately after deposition, the temperature curve shows a sharp transition into the cooling regime with decreasing temperatures. The thermal conductivity of the steel substrate material is high enough not to be the limiting factor for accurately measuring substrate temperature and calculating heating and cooling rates.
Journal Pre-proof 3.4 Layer characterization The aluminum coating shows a columnar microstructure (Fig. 8). In the bottom region a layered structure is visible, which corresponds to the substrate rotation during the deposition. In this region the crystallites have size of about 100 nm. The crystallites broaden in the layer growth direction and exhibit at the surface a size of about 500 nm (Fig. 9). At the top the columns feature a facet microstructure. The aluminum-oxide crucible employed causes a certain amount of porosity in the aluminum layers. This aspect has been investigated earlier
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by the comparison of different types of crucibles [20].
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The cross-cut test demonstrated that the oxygen/argon plasma of the hollow-cathode arc
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discharge produces well-adhering layers. A plasma pre-treatment period of less than one minute is sufficient to ensure defect-free adhesion of all the sputtered and evaporated layers
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tested on different types of plastics – flat PC and ABS substrates as well as the additively
peeling of layers occurred.
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manufactured PLA and ABS parts (Fig. 10). Inspection of the cut samples showed that no
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The specific electrical resistance was estimated for aluminum layers with different thicknesses
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(Fig. 11). Surprisingly, the primary data show an increase in measured resistance with increasing layer thickness. From Fig. 8 we deduce that the upper layer regions exhibit larger gaps between the aluminum crystallites that cause higher resistance. A specific electrical resistance of 1.5.10-7 Ω.m and corresponding a conductivity of 7.106 S/m were calculated for a 5-µm aluminum layer. The shielding effectiveness of a 3 µm thick aluminum foil and of two different aluminum layers was determined in the frequency range between 0.1 and 3 GHz (Fig. 12). The aluminum foil has the highest shielding effectiveness of all the three samples. This is due to the higher electrical conductivity of the compact aluminum in contrast to the layers. The curve shows fluctuations that may indicate reflections.
Journal Pre-proof The thicker aluminum layer (4.2 µm) exhibits a larger attenuation in comparison to the thinner layer (2.5 µm). The difference amounts to 5 … 8 dB in the tested frequency range. The shielding effect increases with frequency and achieves 37 respectively 44 dB at 3 GHz. After completing the evaporation experiments with flat substrates, the inner surfaces of junction boxes were coated. The rectangular cross-section of the boxes causes a specific distribution in layer thickness (Fig. 13). The maximum layer thickness is found at the center of the base. The minimum layer thickness was determined to be near the bottom edge of the
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side walls. This difference is caused in part by the different coating distances, but mainly by
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the angle between the direction normal to the substrate surface and the propagation direction
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of the vapor. The rotation of the junction boxes during deposition varies the angles of incidence steadily with time so that the layer distribution varies smoothly. Consequently, the
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minimum layer thickness is half of the maximum.
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The electric-field strength outside of the junction boxes was determined to be 50.5 V/m with the aluminum layer not connected to ground. The test of the shielding effect with the
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aluminum layer electrically connected to ground yielded an electric-field strength of
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0.54 V/m. This difference in field strengths corresponds to an attenuation of 39 dB of the applied low-frequency electromagnetic field (a few kHz).
4.
Discussion
4.1 Layer properties The structural characteristics of the aluminum layer are strongly influenced by the aluminumoxide crucible employed. In addition to the temperature, pressure and rate of deposition, the degree of contamination in the evaporative bath and residual gas pressure influences the layer structure. It is thought that the aluminum oxide of the crucible wall is slowly converted in the melting bath with time and is responsible for the porous layer structure observed. Dense aluminum layers with lower porosity and larger crystallite sizes have been achieved by
Journal Pre-proof evaporative deposition using a water-cooled copper crucible and also with other hot ceramic crucibles, such as boron nitride [20]. These previous results have confirmed that evaporation from ceramic crucibles can produce compact and dense crystalline layers. The present layer structures were realized with an effective and robust high-rate evaporation process. A change from the aluminum-oxide crucible to other crucible types can be made if this is necessary for technical or economic reasons. The estimated specific electrical resistance of the aluminum layer is higher by a factor of 6
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compared to bulk aluminum. However, reference should be made to published data for
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aluminum layers produced by evaporation from ceramic boron nitride and copper crucibles
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[8]. These data exceeded the specific electrical resistance of bulk aluminum by only 10 %. This is taken as an affirmation that crucible conditions define the layer properties, here the
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electrical layer resistance.
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In the following we analyze how the aluminum layers with the determined properties nevertheless produce the intended electromagnetic shielding. The “impedance concept”,
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developed by S.A. Schelkunoff [21], is frequently used for simulation and evaluation of
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electromagnetic shielding configurations. We use the mathematical expressions for magnetic attenuation in the near field configuration derived from C.R. Paul [22]: 𝑓𝑟 2 𝜎𝑟 𝑅 = 14,57 + 10 log ( ) 𝜇𝑟
(1)
with 𝑅 being the reflection loss, 𝑓 the frequency, 𝑟 the source/shield distance, 𝜎𝑟 the electrical conductivity related to copper, and 𝜇𝑟 the relative permeability. 𝐿
𝐴 = 20 log 𝑒 𝛿
(2)
with 𝐴 being the absorption loss, 𝐿 the layer thickness, and 𝛿 the skin depth. 𝑀 = 20 log(1 − 𝑒
−
2𝐿 𝛿)
with 𝑀 being the multiple reflection loss.
(3)
Journal Pre-proof The three terms together give the shielding effectiveness 𝑆 in decibel. A calculated attenuation curve is also shown in Fig. 12 based on the impedance concept with equations (1)(3) using only experimentally determined layer parameters (see Fig. 12). The calculated curve well describes the qualitative dependency: the strong reduction of attenuation at low frequencies and a comparable slope in the whole investigated frequency range. The absolute value for the calculated attenuation remains lower in comparison to the experimentally estimated one.
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As an example, for frequency 𝑓 = 1.5 𝐺𝐻𝑧 a closer look at the calculated, separate
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contributions should be illustrated: 𝑅 = 25 𝑑𝐵, 𝐴 = 4 𝑑𝐵, 𝑀 = −4 𝑑𝐵. The multiple-
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reflection causes an increased field which is transmitted through the barrier, and so causing a reduced shielding effectiveness. This is considered by a negative value, calculated from
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equation (3). In sum the shielding efficiency amounts to 𝑆 = 26 𝑑𝐵, in comparison to
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𝑆 = 32 𝑑𝐵 which was determined experimentally. Because the skin depth amounts to 5 µm, the absorbed fraction in the 2.5 µm thick layer is
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only small. On the other side, it becomes understandable that multiple reflections take place
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under these conditions. The dominant contribution is given by the reflection term. Another calculation yields a shielding efficiency of 𝑆 = 31 𝑑𝐵 for the thicker aluminum layer (4.2 µm), not shown in Fig. 12. This increase of 5 dB attenuation in comparison to the thinner layer is caused by the increase of absorption and the related reduction of multiple reflection. It can be concluded that the impedance concept in the near field configuration well describes the experimentally determined dependency of magnetic attenuation on the layer thickness. Once more, it should be indicated that the experimentally determined magnetic attenuation is higher than the calculated one according to the impedance concept. It is assumed that the real layer structure and its outer surface are responsible therefor and contribute to a higher amount of reflection. This means that the observed degree of porosity is acceptable, possibly even beneficial, for the electromagnetic shielding effectiveness.
Journal Pre-proof A comparison with shielding data from literature is given in Table 1. The examples were arranged in the order of increasing aluminum thickness. The associated measuring frequencies are not given in all references, the same is the case for the electric measuring configuration. This has to be taken into account for a detailed evaluation. The data from [27] were the only one in table 1 which were obtained in the magnetic near field configuration as we have used, otherwise the aluminum substrate type, the thickness and the measuring frequency are different there.
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Additional calculations based on the impedance concept according [22] and the available
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layer properties data were executed for further comparison with references in Table 1. We
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predict a shielding effectiveness of above 100 dB in the near field configuration of an electric source and above 70 dB in the far field configuration. These values can be compared with the
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data from reference [25] in table 1. Despite the deposited aluminum layers exhibiting some
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porosity, the shielding fulfills practical expectations. Kaden [30] deduced analytical solutions for different geometrical arrangements solving the
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Maxwell equations. A simple plate arrangement is used as a basic configuration for most
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evaluations in literature. Therefore, the magnetic attenuation 𝑎𝑚 is given by: 𝑎𝑚 = 20 ∙ log(cosh 𝑘𝑤 𝐿 + µ𝑟 𝑘𝑤 𝑥0 sinh 𝑘𝑤 𝐿)
(4)
with 𝐿 being the layer thickness, 𝑥0 the half of plate distance, µ𝑟 the relative magnetic permeability, 𝑘𝑤 the eddy current number. An approximate solution for the plate arrangement is also expressed in [30], which is used with the experimentally determined data in the following computation for the aluminum layers: Layer thickness: 𝐿 = 4.2 𝜇𝑚, electrical conductivity: 𝜎 = 7 ∙ 106 𝑆⁄𝑚 Further input values are the plate distance 2 ∙ 𝑥0 = 1 𝑐𝑚, and the frequency: 𝑓 = 1.5 𝐺𝐻𝑧. Magnetic attenuation of 𝑎𝑚 = 65 𝑑𝐵 is calculated from these data. This value implies an attractive level of electromagnetic shielding for sensors, components, etc. This can be taken as
Journal Pre-proof a confirmation for the high quality level of the aluminum coatings we realized with a highrate deposition method.
4.2 Heat flux relations From the estimated temperature increase during the sputtering, a maximum deposition time of 3 minutes can be deduced without exceeding the assumed temperature limit of 80 °C for the plastics. This maximum sputtering time would limit the layer thickness to 300 nm for a single
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run at the estimated sputtering rate. This layer thickness is too low for the intended shielding
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level.
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In all the three different process steps (plasma pre-treatment, magnetron sputtering, and electron-beam evaporation) the temperature increase is caused by the process heat flux to the
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substrate. So it is possible to determine the temperature increase ∆𝑇 from the incoming heat
∆𝑇 = 𝐾 ∙ 𝑞̇ ∙ 𝑡
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flux density 𝑞̇ according to the equation: (5)
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where 𝐾 = 1/(𝜌 ∙ 𝑐 ∙ 𝑑), 𝜌 is the density, 𝑐 the specific heat capacity, 𝑑 the thickness of the
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substrate, and 𝑡 the exposure time.
This linear dependency of ∆T on t in equation (5) was confirmed in the experiments for the three processes. The values for temperature increase with time have been ascertained. The calculated heat flux densities are summarized in Table 2. The heat was considered to have impinged on the substrate uniformly for plasma pre-treatment and sputter deposition.
The different processes are characterized by different heat flux densities. It is essential for each deposition process to consider the total amount of heat or the total temperature increase required to achieve the intended layer thickness. The transformation from the variable “time t” to the variable “layer thickness L” is given by the deposition rate R.
Journal Pre-proof 𝐿 =𝑅∙𝑡
(6)
Using equation (5) and (6) one obtains: 𝑞̇ ∆𝑇 = 𝐾 ∙ ⁄𝑅 ∙ 𝐿
(7)
𝑞̇ From equation (7), it follows that a low ⁄𝑅 quotient is the pivotal requirement for low final temperature of a given deposited layer. This illustrates that a deposition method with high
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deposition speed can lead to lower substrate warming for a given layer thickness.
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𝑞̇ 𝑞̇ The quotients ⁄𝑅 for the both coating processes are also listed in Table 2. The quotient ⁄𝑅 for the evaporation process is lower by a factor of 20. This comparison between magnetron
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sputtering and electron-beam evaporation makes it clear that high-rate evaporation is
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associated with considerably lower increase in temperature for the same layer thickness. In other words, the high-rate evaporative deposition can realize thicker coatings without
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exceeding critical temperature values.
5. Conclusions
The investigations presented demonstrate a fast aluminum metallization process for plastic parts. Both the plasma pre-treatment and the electron-beam evaporation steps were executed with parameters which allow processing in shorter periods of time compared to traditional sputtering techniques. First the substrates were exposed to an intense oxygen/argon plasma via a hollow-cathode arcdischarge source. A plasma pre-treatment time of one minute is sufficient to achieve complete and effective adhesion based on cross-cut tests. Additional intermediate primer layers are not needed. Next, it was possible to deposit a 5-µm aluminum layer onto different types of plastics using
Journal Pre-proof electron-beam evaporation. High-rate deposition with this method can avoid overheating of synthetic materials. Coatings were realized at deposition rates above 100 nm/s. Electron-beam evaporation offers a considerably lower temperature increase for a given layer thickness in comparison to magnetron sputtering, thus enabling thicker layers to be applied. The aluminum layers show a certain porosity which is likely caused by the use of aluminumoxide crucibles. Nevertheless, the layers provide the intended functional properties for effective EM shielding. The attenuation in the magnetic near field configuration amounts to
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44 dB at 3 GHz.
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The evaporative vacuum-coating process was also adapted to additively manufactured parts
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for the first time. These and other coatings will broaden the application of additively manufactured parts in future.
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The presented work is a demonstration of the capabilities of high-rate electron-beam
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evaporation.
Journal Pre-proof 6. Acknowledgements The authors would like to thank S. König and G. Langer from Langer EMV-Technik GmbH for the magnetic attenuation measurements, as well as Robin Lesche from Fraunhofer FEP for his assistance and commitment during all the coating experiments. The project was funded by the European Union and the State of Saxony (funding reference 100276002/3363).
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Comment: Please print the logo EFRE nearby acknowledgment!
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7. Conflict of Interest
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The authors declare that they have no conflict of interest concerning the content of this article.
Journal Pre-proof 8. List of references [1] R. Suchentrunk, Kunststoff-Metallisierung (Schriftenreihe Galvanotechnik), Eugen G. Leuze Verlag, 2007, 488 S. [2] Verordnung (EG) Nr. 1907/2006 des Europäischen Parlaments und des Rates vom 18. Dezember 2006 zur Registrierung, Bewertung, Zulassung und Beschränkung chemischer Stoffe (REACH) [3] K.W. Merz, H.A. Jehn (Hrsg.), Praxishandbuch moderne Beschichtungen, Carl Hanser Verlag, München, Wien, 2001, 251 S. [4] H.A. Wolfsperger, Elektromagnetische Abschirmung, Springer-Verlag Berlin Heidelberg, 2008, 506 S.
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[5] S. Getha, K. K. S. Kumar, C. R. K. Rao, M. Vijayan, D. C. Trivedi, EMI Shielding: Methods and Materials – A Review, Journal of Applied Polymer Science, Vol. 112, 20732086 (2009), doi: 10.1002/app.29812
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[6] B.D. Mottahed, S. Manoochehri, A Review of Research in Materials, Modelling and Simulation, Design Factors, Testing, and Measurements Related to Electromagnetic Interference Shielding, Polym.-Plast. Technol. Eng., 34(2), 1995, 271-346
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[7] B.C. Jackson, G. Shawhan, Current review of the performance characteristics of conductive coatings for EMI control, In: IEEE Int. Symp. Electromagnetic Compatibility, vol. 1, Piscataway, New Jersey, USA: IEEE, 1998, 567–72
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[8] J.-P. Heinß, Chr. Mader, A. Merkle, T. Brendemühl, R. Brendel, L. Ehlers, R. Meyer, „Inline high-rate deposition of aluminum onto RISE solar cells by electron beam technology“, 26th EUPVSEC Proceedings, 2011, 2121-2124 doi: 10.4229/26thEUPVSEC2011-2CV.2.47
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[9] K. de Bruyn, M. v. Stappen, H. de Deurwaerder, L. Rouxhet, J.P. Celis, Study of pretreatment methods for vacuum metallization of plastics, Surface & Coatings Technology, 163 –164 (2003) 710–715, doi: 10.1016/S0257-8972(02)00684-9
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[10] S.F. Ahmed, J. W. Yi, M.-W. Moon, Y.-J. Jang, B.-H. Park, S.-H. Lee, K.-R. Lee, The Morphology and Mechanical Properties of Polycarbonate / Acrylonitrile Butadiene Styrene Modified by Ar Ion Beam Irradiation, Plasma Process. Polym. 6 (2009), 860-865 doi: 10.1002/ppap.200900043 [11] H. Kupfer, G. Hecht, R. Ostwald, Ecologically important metallization processes for high-performance polymers, Surface & Coatings Technology 112 (1999) 379-383, doi: 10.1016/S0257-8972(98)00782-8 [12] C. Lambare, P.-Y. Tessier, F. Poncin-Epaillard, D. Debarnot, Plasma functionalization and etching for enhancing metal adhesion onto polymeric substrates, RSC Adv. 2015, 5, 62348-62357, doi: 10.1039/C5RA08844E [13] F. Fietzke, K. Goedicke, W. Hempel, The deposition of hard crystalline Al2O3 layers by means of bipolar pulsed magnetron sputtering, Surface & Coatings Technology, 8687(1996) 657-663, doi: 10.1016/S0257-8972(96)03075-7 [14] H. Klostermann, F. Fietzke, R. Labitzke, T. Modes, O. Zywitzki, Zr–Nb–N hard coatings deposited by high power pulsed sputtering using different pulse modes, Surface & Coatings Technology 204 (2009) 1076–1080, doi: 10.1016/j.surfcoat.2009.09.012 [15] F. Fietzke, H. Morgner, S. Günther, Magnetically enhanced hollow cathode - a new plasma source for high-rate deposition processes, Plasma Process. Polym., 6 (2009) S242246, doi: 10.1002/ppap.200930607
Journal Pre-proof [16] B. Zimmermann, F. Fietzke, W. Möller, Spatially resolved Langmuir probe measurements of a magnetically enhanced hollow cathode arc plasma, Surface & Coatings Technology 205 (2011) 393–396, doi: 10.1016/j.surfcoat.2011.03.125 [17] P. Feinaeugle, G. Mattausch, S. Schmidt, and F.-H. Roegner, “A New Generation of Plasma-Based Electron Beam Sources with High Power Density as a Novel Tool for High-Rate PVD”, SVC Tech. Con. Proc., pp. 202-209, 2012 [18] L. v.d. Tempel, Thermography of semi-transparent materials by a FLIR ThermaCAM SC3000 infrared camera, Philips Research Eindhoven, Technical Note PR-TN2004/00384, 06/2004, p.25 [19] C.-D. Wen, I. Mudwar, Experimental Investigation of Emissivity of Aluminum Alloys and Temperature Determination Using Multispectral Radiation Thermometry (MRT) Algorithms, Journal of Materials Engineering and Performance, Volume 11(5) October 2002, 551-562, doi: 10.1361/105994902770343818
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[20] J.-P. Heinß, H. Schlemm, F. Wünsch, “High-productive aluminum deposition of back contacts for hetero-junction solar cells by electron beam evaporation“, 32nd EUPVSEC Proceedings, 2016, 961-965, doi: 10.4229/EUPVSEC20162016-2BV.7.49
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[21] S.A. Schelkunoff, The Impedance Concept and Its Application to Problems of Reflection, Refraction, Shielding and Power Absorption, Bell System Technical Journal, 1938, Vol.17, No.1, 17-48
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[22] C.R. Paul, Introduction to electromagnetic compatibility, John Wiley & Sons, Inc., Hoboken, New Jersey, 2nd ed., 2006, 983 p.
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[23] D.M. Mihut, K. Lozano, S.C. Tidrow, H. Garcia, Electromagnetic interference shielding effectiveness of nanoreinforced polymer composites deposited with conductive metallic thin films, Thin Solid Films 520 (2012) 6547–6550, doi: 10.1016/j.tsf.2012.07.018
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[24] R.A. Weck, Thin-Film Shielding for Microcircuit Applications and a Useful Laboratory Tool for Plane-Wave Shielding Evaluations, IEEE TRANSACTIONS ON ELECTROMAGNETIC COMPATIBILITY, Vol. EMC-10, No.1, March 1968, 105-112
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[25] S. Yasufuku, Technical Progress of EMI Shielding Materials in Japan, IEEE Electrical Insulation Magazine, Nov./Dec. 1990, Vol.6, No.6, 21-30 [26] G. Blasek, Beschichtung von Kunststoffgehäusen zur Sicherung der elektromagnetischen Verträglichkeit elektronischer Geräte, Vakuum in Forschung und Praxis (1997), Nr.3, 187-195, doi: 10.1002/vipr.19970090306 [27] A. Eckersley, H-Field Shielding Effectiveness of Flame-Sprayed and Thin Solid Aluminum and Copper Sheets, IEEE TRANSACTIONS ON ELECTROMAGNETIC COMPATIBILITY, VOL. EMC-10, No.1, March 1968, 101-104 [28] R.W. Simpson, EMI shielding with flexible laminates, National Symposium on Electromagnetic Compatibility, IEEE 1984, 267-270 [29] T. Amato, D.J. Mis, and B.B. Wilard, Shielding Effectiveness Before and After the Effects of Environmental Stress on Metalized Plastics, IEEE TRANSACTIONS ON ELECTROMAGNETIC COMPATIBILITY, Vol.30, No.3, August 1988, 312-325 [30] H. Kaden, Wirbelströme und Schirmung in der Nachrichtentechnik, Springer-Verlag Berlin Heidelberg, 2. Aufl., 2006, 350 S.
Journal Pre-proof Tables Table 1. Reference data for aluminum shielding effectiveness.
layer layer layer layer layer foil foil layer plate
Thickness [ µm ] 0.1 0.5 2.5 – 5 6 5 - 10 15 - 40 50 130 – 500 ~ 1000
Shielding [ dB ] 33 - 41 60 40 - 70 60 - 80 60 – 80 30 - 38 59 20 – 42 100
Ref. [23] [24] [25] [26] [7] [27] [28] [27] [29]
of
Al type
Electron-beam Evaporation 0.38
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𝑊𝑐𝑚−2
0.04
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𝑛𝑚 𝑠−1
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𝑅
/
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𝑞̇
Magnetron Sputtering 0.07
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𝑞̇ / 𝑊 𝑐𝑚−2
Plasma Pretreament 0.06
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Table 2. Heat flux densities and their ratio to deposition speed for the three process steps.
0.002
Journal Pre-proof List of figure captions Figure 1. Diagram of the NOVELLA experimental facility.
Figure 2. Chamber evacuation with and without plastic substrates. An ion current of 1.5.10-8 A corresponds to 3.10-3 Pa. (AM-PLA: additively manufactured PLA substrate, AM-ABS: additively manufactured ABS substrate)
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Figure 3. Substrate temperature as function of time for different plasma pre-treatment exposures.
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Figure 4a. Untreated polycarbonate substrate surface.
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Figure 4b. Polycarbonate substrate after 2-minute oxygen/argon plasma pre-treatment and NiCr deposition.
Figure 5. Substrate temperature as function of time during plasma pre-treatment and magnetron sputter
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deposition.
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Figure 6. Deposition rate as function of electron beam power.
Figure 7. Substrate temperature as function of time during electron-beam evaporation.
Figure 8.
Cross-section of a 4.2-µm aluminum layer on additively manufactured PLA, evaporated from
aluminum-oxide crucible.
Figure 9. Surface of a 4.2-µm aluminum layer on additively manufactured PLA, evaporated from aluminumoxide crucible.
Figure 10. Cross-cut test of a 5-µm aluminum layer on additively manufactured ABS.
Figure 11. Specific electrical resistance as a function of layer thickness.
Journal Pre-proof Figure 12. Shielding effectiveness in magnetic near field configuration as a function of frequency for an aluminum foil (smoothed) and two aluminum layers of different thickness. The reference dashed line was calculated according to the impedance concept (used parameters: source/sample distance: 0.25 mm, aluminum layer thickness: 2.5 µm, electrical conductivity: 7.106 S/m).
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Figure 13. Layer thickness distribution inside a coated junction box.
Journal Pre-proof Tables Table 1. Reference data for aluminum shielding effectiveness.
layer layer layer layer layer foil foil layer plate
Thickness [ µm ] 0.1 0.5 2.5 – 5 6 5 - 10 15 - 40 50 130 – 500 ~ 1000
Shielding [ dB ] 33 - 41 60 40 - 70 60 - 80 60 – 80 30 - 38 59 20 – 42 100
Ref. [23] [24] [25] [26] [7] [27] [28] [27] [29]
of
Al type
Electron-beam Evaporation 0.38
re
𝑊𝑐𝑚−2
0.04
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𝑛𝑚 𝑠−1
na
𝑅
/
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𝑞̇
Magnetron Sputtering 0.07
-p
𝑞̇ / 𝑊 𝑐𝑚−2
Plasma Pretreament 0.06
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Table 2. Heat flux densities and their ratio to deposition speed for the three process steps.
0.002
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Journal Pre-proof
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
Figure 8
Figure 9
Figure 10
Figure 11
Figure 12
Figure 13
Jens-Peter Heinß, Fred Fietzke PII:
S0257-8972(19)31125-9
DOI:
https://doi.org/10.1016/j.surfcoat.2019.125134
Reference:
SCT 125134
To appear in:
Surface & Coatings Technology
Received date:
12 July 2019
Revised date:
1 November 2019
Accepted date:
3 November 2019
Please cite this article as: J.-P. Heinß and F. Fietzke, High-rate deposition of thick aluminum coatings on plastic parts for electromagnetic shielding, Surface & Coatings Technology (2018), https://doi.org/10.1016/j.surfcoat.2019.125134
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© 2018 Published by Elsevier.
Journal Pre-proof Manuscript cover page Surface and Coatings Technology Manuscript Draft for special issue of Society of Vacuum Coaters Annual Technical Conference 2019
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Abstract ID# at SVC-Conference 2019: 2019SVC48 Paper ID# at SVC-Conference 2019: TT 9 Manuscript Number: SURFCOAT-D-19-02249R2
Journal Pre-proof High-Rate Deposition of Thick Aluminum Coatings on Plastic Parts for Electromagnetic Shielding Jens-Peter Heinß, Fred Fietzke Fraunhofer Institute for Organic Electronics, Electron Beam and Plasma Technology FEP, Winterbergstrasse 28, 01277 Dresden, Germany *correspondence to: Jens-Peter Heinß, email: [email protected] phone: +49 351 2586-244 fax: +49 351 2586-55244 ___________________________________________________________________________ Abstract
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A new approach is presented for depositing electromagnetic-shielding layers on plastic
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components by PVD direct metallization for improved electromagnetic compatibility (EMC).
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Both the plasma pre-treatment and the coating steps were executed using parameters that
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allow short processing times. Plastic substrates (PC, ABS, and PLA) were exposed to an intense oxygen/argon plasma from a hollow-cathode arc-discharge source. Cross-cut tests
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revealed that a plasma pre-treatment time of one minute was sufficient to achieve excellent
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adhesion. It was possible to deposit a 5-µm aluminum layer onto these different types of plastics using electron-beam evaporation with an axial-beam gun.
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High-rate deposition can evidently overcome difficulties caused by overheating these synthetic materials. The coatings were applied at a deposition rate above 100 nm/s. Because of this high rate, electron-beam evaporation provided a considerably lower temperature increase for a given layer thickness in comparison to magnetron sputtering. This difference amounted to more than one order of magnitude. The evaporative coating process was also adapted to additively manufactured parts for the first time. The aluminum layers deposited provided the intended functional properties, in particular the electromagnetic shielding. The attenuation in magnetic near field configuration was determined to 44 dB at 3 GHz.
Journal Pre-proof Keywords: High Deposition Rate; Aluminum Thin Films; Electron Beam Evaporation; Thermal Load; Electromagnetic Compatibility Shielding.
Highlights Electron-beam evaporation of aluminum with deposition rate above 100 nm/s
Deposition of 5-µm thick aluminum coatings that adhere well on plastics
Low specific heat flux per unit thickness of the deposited layer
Attenuation in magnetic near field configuration of more than 40 dB at 3 GHz
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Journal Pre-proof 1. Introduction Synthetic materials are broadly used in the automobile industry, sanitary engineering, and consumer electronics. When suitably coated, these materials often exhibit a metallic appearance and additional functionalities such as scratch resistance, for example. The metallization is usually applied with chemical or electrochemical technologies [1]. Environmentally sustainable alternatives are gaining in importance through implementation of EU REACH legislation [2]. Physical vapor deposition (PVD) methods fulfill these
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expectations [3]. Thin decorative coatings can be provided by magnetron sputtering, in which
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the metal layer is embedded between two coats of lacquer.
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Shielding to achieve electromagnetic compatibility for internal electronic components exposed to low-frequency external interference requires materials having high magnetic
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permeability, such as nickel. At higher frequencies, the skin effect determines the amount of
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absorption by the housing’s wall material [4],[5]. The wall material itself can be conductive, or laminated with materials having high electrical conductivity and magnetic permeability
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[6],[7]. Both in combination lead to the well-known skin effect. The outer alternating
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electromagnetic field induces a voltage. Because of the electrical conductivity, a current is induced in the material, which in itself generates a magnetic field, that is opposite oriented to the causal field and compensates it [4]. Effective electromagnetic shielding in the gigahertz range requires metallic layers with a thickness of several microns. Evaporative deposition processes appear to be suitable for ensuring a high deposition rate. For example, electron-beam evaporation with axial-beam guns has been applied for the deposition of back contacts of solar cells. 20-µm aluminum layers were deposited on thin silicon wafers (185 µm) [8]. This study enabled us to explore and deduce the relationship between high deposition rate and heat input. It is known that conventional plastic parts exhibit temperature limits of approximately 80-100 °C and are subject to damage above that temperature.
Journal Pre-proof A further challenge is to achieve high-grade adhesion of thick coatings on plastic substrates. Often organic intermediate layers known as primers are applied. So a second question arises: can reliable metallization be achieved without this additional adhesive priming layer? Plasmabased pre-treatment is frequently utilized in the case of vacuum deposition methods. The impact of the ions promotes the formation of functional groups responsible for chemical-layer bonding [9]. As well, topographic modification of the substrate surface by the ions has been verified [10]. Both effects typically occur during plasma treatment of plastics [11],[12]. There
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is an additional technical requirement in industrial scenarios: to achieve high-rate deposition,
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the plasma pre-treatment needs to be executed in a comparably short period of time.
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Last but not least, the range of applications for additively manufactured parts has broadened significantly in recent years, so that PVD coating of such materials becomes a focus of
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increasing interest.
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Here we report on plasma pre-treatment of plastic substrates and the deposition of aluminum layers several microns thick using two different PVD methods – magnetron sputtering and
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electron-beam evaporation. The heat flux densities and the temperature limitations are
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analyzed. Electromagnetic attenuation properties of the deposited coatings are determined for high frequency range.
2. Experimental setup
The plasma pre-treatment and deposition experiments were divided into two groups. The first series was realized with the UNIVERSA sputter-coating equipment already described in [13],[14]. The device is equipped with four rectangular magnetron sources (target dimensions 512 mm x 128 mm) that are arranged in pairs at both side doors. Only one source was used in DC mode for the deposition experiments conducted at a power level of 8 kW. The out-gassing of plastic substrates was monitored with a PrismaPlus® QMA 200 quadrupole analyzer (Pfeiffer Vacuum). Taking scan measurements in a mass number range
Journal Pre-proof between 1 and 100 allows identification of the dominant gases. The plasma for the substrate pre-treatment was produced by a hollow-cathode arc-discharge source developed in-house [15]. Enhancement via magnetic field enables an argon-plasma density to be developed on the order of 1018 m-3 at a distance of 300 mm [16]. During the plasma pre-treatment of substrates, the hollow-cathode discharge is driven to 70 V with a discharge current of 100 A, corresponding to a discharge power of 7 kW. Electron-beam evaporation was carried out in the NOVELLA experimental facility (Fig. 1),
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comprising a load-lock chamber and a coating chamber separated by a plate valve. A versatile
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substrate transport system allows transfer of parts (maximum dimension Ø = 150 mm,
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L = 300 mm) between the chambers. Translational and rotational movements are possible with this transport system and can be combined. Both chambers are equipped with
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turbomolecular pumps (2300 l/s, Pfeiffer Vacuum). The base pressure for the experiments
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was 3.10-6 mbar. The load-lock chamber is 0.6 m x 0.9 m x 0.8 m and is made of stainless steel. A hollow-cathode arc-discharge source is mounted on a bottom flange and is used for
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plasma pre-treatment. The deposition chamber, also made of stainless steel, is
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1.2 m x 0.9 m x 1.1 m. An aluminum-oxide crucible (bath dimension: 100 mm x 100 mm) is located at the bottom of that chamber. The distance between the crucible and central position of a substrate in the coating chamber is 450 mm. A plasma-based axial-beam gun [17] with a maximum beam power of 120 kW is used for the electron-beam evaporation and is mounted at an oblique angle on top of the chamber. Different types of injection-molded plastics were used: polycarbonate (PC), acrylonitrile butadiene
styrene
(ABS),
and
polylactic
acid
(PLA).
The
substrate
size
was
100 mm x 100 mm x 2 mm. The additively manufactured substrates from the same types of plastic were 50 mm x 10 mm x 10 mm. 100 mm x 60 mm x 1 mm stainless steel sheets were used for individual measurements of temperature increase and determination of the heat flux. For the subsequent measurement of electromagnetic attenuation, junction boxes made from
Journal Pre-proof high-density polyethylene (85 mm x 85 mm x 60 mm) were internally coated. For the sputter-deposition experiments, the substrates were hung with wires, while in the evaporative-deposition experiments the substrates were fixed with a screw on a metallic holder. All substrates rotated during deposition. The substrate temperature was measured with chromel-alumel-thermocouples that were mounted on the rear side. The thickness of the aluminum layer deposited on steel substrates was established gravimetrically from mass difference and the aluminum density. The layer thickness of the
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plastic substrates was checked by Dektak profilometer measurements at shadowing edges
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prepared for this purpose. The same method was employed for measuring the thickness
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profile of the internally coated junction boxes. Shadowing edges were prepared for this purpose using a few glass sheets fixed inside the boxes.
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The surfaces of the plasma-treated polymer substrates were investigated by optical
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microscopy with differential interference contrast (Polyvar 2 Met, Reichert). Ion-polished cross-sections of the aluminum layers were prepared using an argon-ion beam
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preparation technique (Cross-Section Polisher, SM-0910, Jeol). The cross-sections were
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analyzed by field-emission scanning electron microscopy (FE-SEM, SU 8000, Hitachi) using channeling contrast and backscattered electrons for imaging the microstructure. The cross-cut tests were executed in accordance with DIN EN ISO 2409. The sheet resistance was determined with an FPP 5000 four-point probe (Veeco Instruments Inc.). The specific electrical resistance was deduced from this using the thickness of the aluminum layer. The magnetic attenuation of coated flat samples was measured with a laboratory setup in the frequency range between 0.1 and 3 GHz. The return loss was measured with a spectrum analyzer R3132 (ADVANTEST). The coated samples were positioned between a miniaturized transmitting and a receiving loop antenna. The attenuation, which is caused by the distance between the antennas, was determined by measuring an uncoated sample of the same
Journal Pre-proof thickness. The shielding effectiveness of an aluminum foil with a thickness of 3 µm (Goodfellow, AL000320) was measured as a reference. The electromagnetic attenuation of aluminum-coated junction boxes also was determined. The electric field strength for two junction-box configurations - grounded and ungrounded - was measured from 5 Hz to 32 kHz with an EFA 300 field analyzer (Narda Safety Test Solutions GmbH) located outside the box.
3. Results 3.1 Sample outgassing and plasma pretreatment
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The evacuation behavior of the vacuum chamber was studied after the additively
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manufactured plastic substrates were placed in the coating chamber. In comparison with the
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empty chamber, two effects were observed. First, an extension of evacuation time was needed
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(Fig. 2). With PLA and ABS substrates, 40 minutes were needed to achieve a pressure of 3.10-3 Pa, compared to 25 minutes without the substrates in the chamber. Second, the
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evacuation curves show two points, after 85 and120 minutes as indicated in Fig. 2 by arrows,
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after which the pressure drops faster. The out-gassing of the additively manufactured substrates in the chamber is nearly complete by these times. This out-gassing is responsible
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for the additional time necessary to achieve the pressure of 3.10-3 Pa as described above. Plasma pre-treatment using the hollow-cathode arc discharge source was first tested solely with argon. In this case the adhesion of the sputtered layers was insufficient. Next, a gas flow of 175 sccm of oxygen was introduced for the plasma pre-treatment in addition to the gas flow of 25 sccm of argon through the hollow-cathode device. After this oxygen/argon plasma pre-treatment, all subsequently deposited layers exhibit defect-free adhesion. The authors assume that the cause of the improved adhesion observed is due to changes in chemical bonding at the surface and/or coating/substrate boundary. The process heat leads to a linear increase of the substrate temperature with time (Fig. 3). The linear time dependence between 1.5 and 4.2 minutes is marked with a straight gray line for the “3-minute plasma pre-treatment” curve in Fig. 3. The temperature rise, checked for the three
Journal Pre-proof different treatment times, can be represented as 13 K/min. The assumed temperature limit for the plastics of 80 °C is not exceeded for plasma pre-treatment times of less than 4 minutes. Fig. 4 illustrates the topological alteration of the substrate. The polycarbonate surface (Fig. 4b) treated for 2 minutes exhibits an increased roughness in comparison to the untreated surface (Fig. 4a). This was confirmed by measurements with an atomic force microscope (AFM) (not presented here).
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3.2 Coating by magnetron sputtering
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Next, the plasma pre-treatment was followed by magnetron-sputter deposition of aluminum
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onto the substrates. The substrate was coated with a 200-nm aluminum layer during the 2-min deposition time (100 nm/min).
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Both process steps cause a temperature increase (Fig. 5). During the 2-min plasma pre-
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treatment, the temperature rises from 24 °C to 49 °C. The substrate cools down during the interruption time and the temperature decreases. The sputter deposition effects a temperature
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rise from 27 °C to 70 °C. The linear temperature increase with time is marked green in the
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diagram for the time interval of minute 20 to minute 22. A time-dependent increase of 19 K/min can be observed. The substrate undergoes final cool-down inside the vacuum chamber.
The negative slopes of the temperature-time curve are different for the two cool-down periods. Cooling after the plasma pre-treatment step is faster. This was determined to be a result of the differing surface states after the plasma pre-treatment and coating steps. Polycarbonate exhibits an emissivity of 0.9 [18] and aluminum less than 0.1 [19]. It is thought this difference is responsible for different amounts of thermal radiation and the different cooling speeds for the uncoated and coated states.
3.3 Coating by electron beam evaporation
Journal Pre-proof The experiments with electron-beam evaporation were executed in the NOVELLA facility. Deposition rates in the 100 nm/s range were confirmed for electron-beam power between 6 and 8 kW (Fig. 6). Aluminum layers with thicknesses in the range between 3 and 7 µm were deposited on different plastic substrates varying the beam power and the deposition time. The related temperature increase with time for a 45 s long-running deposition is presented in Fig. 7. The polycarbonate substrate temperature rises from 35 °C to a maximum of 70 °C. The time
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period over which the temperature rise occurs is longer than the deposition time itself. The
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deposition time of 45 s is defined by the duration of the period the shutter is open.
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The linear temperature increase with time is marked green in the diagram for the time interval from minute 1.6 until minute 2.4 (0.8 minutes total compared to 2.0 minutes total for sputter
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deposition). This range is characterized by a linear slope of 17 K/min. The subsequent
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apparent temperature increase after deposition is likely the result of thermal conduction processes in the plastic substrate material that delay the temperature measurement taken at the
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backside of the substrate. Finally, it can be stated that an aluminum layer several microns
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thick can be deposited using electron-beam evaporation without exceeding the relevant temperature limit of plastics.
For purposes of exploring the temperature delay and for comparing heat flux densities, a subsequent deposition of aluminum onto a 1-mm steel substrate was executed. The rate of temperature increase was determined to be 62 K/min (diagram not shown here). The temperature rises linearly with time during the entire deposition period. Immediately after deposition, the temperature curve shows a sharp transition into the cooling regime with decreasing temperatures. The thermal conductivity of the steel substrate material is high enough not to be the limiting factor for accurately measuring substrate temperature and calculating heating and cooling rates.
Journal Pre-proof 3.4 Layer characterization The aluminum coating shows a columnar microstructure (Fig. 8). In the bottom region a layered structure is visible, which corresponds to the substrate rotation during the deposition. In this region the crystallites have size of about 100 nm. The crystallites broaden in the layer growth direction and exhibit at the surface a size of about 500 nm (Fig. 9). At the top the columns feature a facet microstructure. The aluminum-oxide crucible employed causes a certain amount of porosity in the aluminum layers. This aspect has been investigated earlier
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by the comparison of different types of crucibles [20].
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The cross-cut test demonstrated that the oxygen/argon plasma of the hollow-cathode arc
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discharge produces well-adhering layers. A plasma pre-treatment period of less than one minute is sufficient to ensure defect-free adhesion of all the sputtered and evaporated layers
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tested on different types of plastics – flat PC and ABS substrates as well as the additively
peeling of layers occurred.
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manufactured PLA and ABS parts (Fig. 10). Inspection of the cut samples showed that no
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The specific electrical resistance was estimated for aluminum layers with different thicknesses
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(Fig. 11). Surprisingly, the primary data show an increase in measured resistance with increasing layer thickness. From Fig. 8 we deduce that the upper layer regions exhibit larger gaps between the aluminum crystallites that cause higher resistance. A specific electrical resistance of 1.5.10-7 Ω.m and corresponding a conductivity of 7.106 S/m were calculated for a 5-µm aluminum layer. The shielding effectiveness of a 3 µm thick aluminum foil and of two different aluminum layers was determined in the frequency range between 0.1 and 3 GHz (Fig. 12). The aluminum foil has the highest shielding effectiveness of all the three samples. This is due to the higher electrical conductivity of the compact aluminum in contrast to the layers. The curve shows fluctuations that may indicate reflections.
Journal Pre-proof The thicker aluminum layer (4.2 µm) exhibits a larger attenuation in comparison to the thinner layer (2.5 µm). The difference amounts to 5 … 8 dB in the tested frequency range. The shielding effect increases with frequency and achieves 37 respectively 44 dB at 3 GHz. After completing the evaporation experiments with flat substrates, the inner surfaces of junction boxes were coated. The rectangular cross-section of the boxes causes a specific distribution in layer thickness (Fig. 13). The maximum layer thickness is found at the center of the base. The minimum layer thickness was determined to be near the bottom edge of the
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side walls. This difference is caused in part by the different coating distances, but mainly by
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the angle between the direction normal to the substrate surface and the propagation direction
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of the vapor. The rotation of the junction boxes during deposition varies the angles of incidence steadily with time so that the layer distribution varies smoothly. Consequently, the
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minimum layer thickness is half of the maximum.
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The electric-field strength outside of the junction boxes was determined to be 50.5 V/m with the aluminum layer not connected to ground. The test of the shielding effect with the
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aluminum layer electrically connected to ground yielded an electric-field strength of
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0.54 V/m. This difference in field strengths corresponds to an attenuation of 39 dB of the applied low-frequency electromagnetic field (a few kHz).
4.
Discussion
4.1 Layer properties The structural characteristics of the aluminum layer are strongly influenced by the aluminumoxide crucible employed. In addition to the temperature, pressure and rate of deposition, the degree of contamination in the evaporative bath and residual gas pressure influences the layer structure. It is thought that the aluminum oxide of the crucible wall is slowly converted in the melting bath with time and is responsible for the porous layer structure observed. Dense aluminum layers with lower porosity and larger crystallite sizes have been achieved by
Journal Pre-proof evaporative deposition using a water-cooled copper crucible and also with other hot ceramic crucibles, such as boron nitride [20]. These previous results have confirmed that evaporation from ceramic crucibles can produce compact and dense crystalline layers. The present layer structures were realized with an effective and robust high-rate evaporation process. A change from the aluminum-oxide crucible to other crucible types can be made if this is necessary for technical or economic reasons. The estimated specific electrical resistance of the aluminum layer is higher by a factor of 6
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compared to bulk aluminum. However, reference should be made to published data for
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aluminum layers produced by evaporation from ceramic boron nitride and copper crucibles
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[8]. These data exceeded the specific electrical resistance of bulk aluminum by only 10 %. This is taken as an affirmation that crucible conditions define the layer properties, here the
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electrical layer resistance.
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In the following we analyze how the aluminum layers with the determined properties nevertheless produce the intended electromagnetic shielding. The “impedance concept”,
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developed by S.A. Schelkunoff [21], is frequently used for simulation and evaluation of
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electromagnetic shielding configurations. We use the mathematical expressions for magnetic attenuation in the near field configuration derived from C.R. Paul [22]: 𝑓𝑟 2 𝜎𝑟 𝑅 = 14,57 + 10 log ( ) 𝜇𝑟
(1)
with 𝑅 being the reflection loss, 𝑓 the frequency, 𝑟 the source/shield distance, 𝜎𝑟 the electrical conductivity related to copper, and 𝜇𝑟 the relative permeability. 𝐿
𝐴 = 20 log 𝑒 𝛿
(2)
with 𝐴 being the absorption loss, 𝐿 the layer thickness, and 𝛿 the skin depth. 𝑀 = 20 log(1 − 𝑒
−
2𝐿 𝛿)
with 𝑀 being the multiple reflection loss.
(3)
Journal Pre-proof The three terms together give the shielding effectiveness 𝑆 in decibel. A calculated attenuation curve is also shown in Fig. 12 based on the impedance concept with equations (1)(3) using only experimentally determined layer parameters (see Fig. 12). The calculated curve well describes the qualitative dependency: the strong reduction of attenuation at low frequencies and a comparable slope in the whole investigated frequency range. The absolute value for the calculated attenuation remains lower in comparison to the experimentally estimated one.
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As an example, for frequency 𝑓 = 1.5 𝐺𝐻𝑧 a closer look at the calculated, separate
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contributions should be illustrated: 𝑅 = 25 𝑑𝐵, 𝐴 = 4 𝑑𝐵, 𝑀 = −4 𝑑𝐵. The multiple-
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reflection causes an increased field which is transmitted through the barrier, and so causing a reduced shielding effectiveness. This is considered by a negative value, calculated from
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equation (3). In sum the shielding efficiency amounts to 𝑆 = 26 𝑑𝐵, in comparison to
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𝑆 = 32 𝑑𝐵 which was determined experimentally. Because the skin depth amounts to 5 µm, the absorbed fraction in the 2.5 µm thick layer is
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only small. On the other side, it becomes understandable that multiple reflections take place
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under these conditions. The dominant contribution is given by the reflection term. Another calculation yields a shielding efficiency of 𝑆 = 31 𝑑𝐵 for the thicker aluminum layer (4.2 µm), not shown in Fig. 12. This increase of 5 dB attenuation in comparison to the thinner layer is caused by the increase of absorption and the related reduction of multiple reflection. It can be concluded that the impedance concept in the near field configuration well describes the experimentally determined dependency of magnetic attenuation on the layer thickness. Once more, it should be indicated that the experimentally determined magnetic attenuation is higher than the calculated one according to the impedance concept. It is assumed that the real layer structure and its outer surface are responsible therefor and contribute to a higher amount of reflection. This means that the observed degree of porosity is acceptable, possibly even beneficial, for the electromagnetic shielding effectiveness.
Journal Pre-proof A comparison with shielding data from literature is given in Table 1. The examples were arranged in the order of increasing aluminum thickness. The associated measuring frequencies are not given in all references, the same is the case for the electric measuring configuration. This has to be taken into account for a detailed evaluation. The data from [27] were the only one in table 1 which were obtained in the magnetic near field configuration as we have used, otherwise the aluminum substrate type, the thickness and the measuring frequency are different there.
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Additional calculations based on the impedance concept according [22] and the available
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layer properties data were executed for further comparison with references in Table 1. We
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predict a shielding effectiveness of above 100 dB in the near field configuration of an electric source and above 70 dB in the far field configuration. These values can be compared with the
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data from reference [25] in table 1. Despite the deposited aluminum layers exhibiting some
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porosity, the shielding fulfills practical expectations. Kaden [30] deduced analytical solutions for different geometrical arrangements solving the
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Maxwell equations. A simple plate arrangement is used as a basic configuration for most
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evaluations in literature. Therefore, the magnetic attenuation 𝑎𝑚 is given by: 𝑎𝑚 = 20 ∙ log(cosh 𝑘𝑤 𝐿 + µ𝑟 𝑘𝑤 𝑥0 sinh 𝑘𝑤 𝐿)
(4)
with 𝐿 being the layer thickness, 𝑥0 the half of plate distance, µ𝑟 the relative magnetic permeability, 𝑘𝑤 the eddy current number. An approximate solution for the plate arrangement is also expressed in [30], which is used with the experimentally determined data in the following computation for the aluminum layers: Layer thickness: 𝐿 = 4.2 𝜇𝑚, electrical conductivity: 𝜎 = 7 ∙ 106 𝑆⁄𝑚 Further input values are the plate distance 2 ∙ 𝑥0 = 1 𝑐𝑚, and the frequency: 𝑓 = 1.5 𝐺𝐻𝑧. Magnetic attenuation of 𝑎𝑚 = 65 𝑑𝐵 is calculated from these data. This value implies an attractive level of electromagnetic shielding for sensors, components, etc. This can be taken as
Journal Pre-proof a confirmation for the high quality level of the aluminum coatings we realized with a highrate deposition method.
4.2 Heat flux relations From the estimated temperature increase during the sputtering, a maximum deposition time of 3 minutes can be deduced without exceeding the assumed temperature limit of 80 °C for the plastics. This maximum sputtering time would limit the layer thickness to 300 nm for a single
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run at the estimated sputtering rate. This layer thickness is too low for the intended shielding
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level.
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In all the three different process steps (plasma pre-treatment, magnetron sputtering, and electron-beam evaporation) the temperature increase is caused by the process heat flux to the
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substrate. So it is possible to determine the temperature increase ∆𝑇 from the incoming heat
∆𝑇 = 𝐾 ∙ 𝑞̇ ∙ 𝑡
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flux density 𝑞̇ according to the equation: (5)
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where 𝐾 = 1/(𝜌 ∙ 𝑐 ∙ 𝑑), 𝜌 is the density, 𝑐 the specific heat capacity, 𝑑 the thickness of the
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substrate, and 𝑡 the exposure time.
This linear dependency of ∆T on t in equation (5) was confirmed in the experiments for the three processes. The values for temperature increase with time have been ascertained. The calculated heat flux densities are summarized in Table 2. The heat was considered to have impinged on the substrate uniformly for plasma pre-treatment and sputter deposition.
The different processes are characterized by different heat flux densities. It is essential for each deposition process to consider the total amount of heat or the total temperature increase required to achieve the intended layer thickness. The transformation from the variable “time t” to the variable “layer thickness L” is given by the deposition rate R.
Journal Pre-proof 𝐿 =𝑅∙𝑡
(6)
Using equation (5) and (6) one obtains: 𝑞̇ ∆𝑇 = 𝐾 ∙ ⁄𝑅 ∙ 𝐿
(7)
𝑞̇ From equation (7), it follows that a low ⁄𝑅 quotient is the pivotal requirement for low final temperature of a given deposited layer. This illustrates that a deposition method with high
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deposition speed can lead to lower substrate warming for a given layer thickness.
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𝑞̇ 𝑞̇ The quotients ⁄𝑅 for the both coating processes are also listed in Table 2. The quotient ⁄𝑅 for the evaporation process is lower by a factor of 20. This comparison between magnetron
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sputtering and electron-beam evaporation makes it clear that high-rate evaporation is
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associated with considerably lower increase in temperature for the same layer thickness. In other words, the high-rate evaporative deposition can realize thicker coatings without
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exceeding critical temperature values.
5. Conclusions
The investigations presented demonstrate a fast aluminum metallization process for plastic parts. Both the plasma pre-treatment and the electron-beam evaporation steps were executed with parameters which allow processing in shorter periods of time compared to traditional sputtering techniques. First the substrates were exposed to an intense oxygen/argon plasma via a hollow-cathode arcdischarge source. A plasma pre-treatment time of one minute is sufficient to achieve complete and effective adhesion based on cross-cut tests. Additional intermediate primer layers are not needed. Next, it was possible to deposit a 5-µm aluminum layer onto different types of plastics using
Journal Pre-proof electron-beam evaporation. High-rate deposition with this method can avoid overheating of synthetic materials. Coatings were realized at deposition rates above 100 nm/s. Electron-beam evaporation offers a considerably lower temperature increase for a given layer thickness in comparison to magnetron sputtering, thus enabling thicker layers to be applied. The aluminum layers show a certain porosity which is likely caused by the use of aluminumoxide crucibles. Nevertheless, the layers provide the intended functional properties for effective EM shielding. The attenuation in the magnetic near field configuration amounts to
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44 dB at 3 GHz.
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The evaporative vacuum-coating process was also adapted to additively manufactured parts
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for the first time. These and other coatings will broaden the application of additively manufactured parts in future.
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The presented work is a demonstration of the capabilities of high-rate electron-beam
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evaporation.
Journal Pre-proof 6. Acknowledgements The authors would like to thank S. König and G. Langer from Langer EMV-Technik GmbH for the magnetic attenuation measurements, as well as Robin Lesche from Fraunhofer FEP for his assistance and commitment during all the coating experiments. The project was funded by the European Union and the State of Saxony (funding reference 100276002/3363).
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Comment: Please print the logo EFRE nearby acknowledgment!
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7. Conflict of Interest
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The authors declare that they have no conflict of interest concerning the content of this article.
Journal Pre-proof 8. List of references [1] R. Suchentrunk, Kunststoff-Metallisierung (Schriftenreihe Galvanotechnik), Eugen G. Leuze Verlag, 2007, 488 S. [2] Verordnung (EG) Nr. 1907/2006 des Europäischen Parlaments und des Rates vom 18. Dezember 2006 zur Registrierung, Bewertung, Zulassung und Beschränkung chemischer Stoffe (REACH) [3] K.W. Merz, H.A. Jehn (Hrsg.), Praxishandbuch moderne Beschichtungen, Carl Hanser Verlag, München, Wien, 2001, 251 S. [4] H.A. Wolfsperger, Elektromagnetische Abschirmung, Springer-Verlag Berlin Heidelberg, 2008, 506 S.
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[5] S. Getha, K. K. S. Kumar, C. R. K. Rao, M. Vijayan, D. C. Trivedi, EMI Shielding: Methods and Materials – A Review, Journal of Applied Polymer Science, Vol. 112, 20732086 (2009), doi: 10.1002/app.29812
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[6] B.D. Mottahed, S. Manoochehri, A Review of Research in Materials, Modelling and Simulation, Design Factors, Testing, and Measurements Related to Electromagnetic Interference Shielding, Polym.-Plast. Technol. Eng., 34(2), 1995, 271-346
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[7] B.C. Jackson, G. Shawhan, Current review of the performance characteristics of conductive coatings for EMI control, In: IEEE Int. Symp. Electromagnetic Compatibility, vol. 1, Piscataway, New Jersey, USA: IEEE, 1998, 567–72
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[8] J.-P. Heinß, Chr. Mader, A. Merkle, T. Brendemühl, R. Brendel, L. Ehlers, R. Meyer, „Inline high-rate deposition of aluminum onto RISE solar cells by electron beam technology“, 26th EUPVSEC Proceedings, 2011, 2121-2124 doi: 10.4229/26thEUPVSEC2011-2CV.2.47
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[9] K. de Bruyn, M. v. Stappen, H. de Deurwaerder, L. Rouxhet, J.P. Celis, Study of pretreatment methods for vacuum metallization of plastics, Surface & Coatings Technology, 163 –164 (2003) 710–715, doi: 10.1016/S0257-8972(02)00684-9
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[10] S.F. Ahmed, J. W. Yi, M.-W. Moon, Y.-J. Jang, B.-H. Park, S.-H. Lee, K.-R. Lee, The Morphology and Mechanical Properties of Polycarbonate / Acrylonitrile Butadiene Styrene Modified by Ar Ion Beam Irradiation, Plasma Process. Polym. 6 (2009), 860-865 doi: 10.1002/ppap.200900043 [11] H. Kupfer, G. Hecht, R. Ostwald, Ecologically important metallization processes for high-performance polymers, Surface & Coatings Technology 112 (1999) 379-383, doi: 10.1016/S0257-8972(98)00782-8 [12] C. Lambare, P.-Y. Tessier, F. Poncin-Epaillard, D. Debarnot, Plasma functionalization and etching for enhancing metal adhesion onto polymeric substrates, RSC Adv. 2015, 5, 62348-62357, doi: 10.1039/C5RA08844E [13] F. Fietzke, K. Goedicke, W. Hempel, The deposition of hard crystalline Al2O3 layers by means of bipolar pulsed magnetron sputtering, Surface & Coatings Technology, 8687(1996) 657-663, doi: 10.1016/S0257-8972(96)03075-7 [14] H. Klostermann, F. Fietzke, R. Labitzke, T. Modes, O. Zywitzki, Zr–Nb–N hard coatings deposited by high power pulsed sputtering using different pulse modes, Surface & Coatings Technology 204 (2009) 1076–1080, doi: 10.1016/j.surfcoat.2009.09.012 [15] F. Fietzke, H. Morgner, S. Günther, Magnetically enhanced hollow cathode - a new plasma source for high-rate deposition processes, Plasma Process. Polym., 6 (2009) S242246, doi: 10.1002/ppap.200930607
Journal Pre-proof [16] B. Zimmermann, F. Fietzke, W. Möller, Spatially resolved Langmuir probe measurements of a magnetically enhanced hollow cathode arc plasma, Surface & Coatings Technology 205 (2011) 393–396, doi: 10.1016/j.surfcoat.2011.03.125 [17] P. Feinaeugle, G. Mattausch, S. Schmidt, and F.-H. Roegner, “A New Generation of Plasma-Based Electron Beam Sources with High Power Density as a Novel Tool for High-Rate PVD”, SVC Tech. Con. Proc., pp. 202-209, 2012 [18] L. v.d. Tempel, Thermography of semi-transparent materials by a FLIR ThermaCAM SC3000 infrared camera, Philips Research Eindhoven, Technical Note PR-TN2004/00384, 06/2004, p.25 [19] C.-D. Wen, I. Mudwar, Experimental Investigation of Emissivity of Aluminum Alloys and Temperature Determination Using Multispectral Radiation Thermometry (MRT) Algorithms, Journal of Materials Engineering and Performance, Volume 11(5) October 2002, 551-562, doi: 10.1361/105994902770343818
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[20] J.-P. Heinß, H. Schlemm, F. Wünsch, “High-productive aluminum deposition of back contacts for hetero-junction solar cells by electron beam evaporation“, 32nd EUPVSEC Proceedings, 2016, 961-965, doi: 10.4229/EUPVSEC20162016-2BV.7.49
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[21] S.A. Schelkunoff, The Impedance Concept and Its Application to Problems of Reflection, Refraction, Shielding and Power Absorption, Bell System Technical Journal, 1938, Vol.17, No.1, 17-48
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[22] C.R. Paul, Introduction to electromagnetic compatibility, John Wiley & Sons, Inc., Hoboken, New Jersey, 2nd ed., 2006, 983 p.
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[23] D.M. Mihut, K. Lozano, S.C. Tidrow, H. Garcia, Electromagnetic interference shielding effectiveness of nanoreinforced polymer composites deposited with conductive metallic thin films, Thin Solid Films 520 (2012) 6547–6550, doi: 10.1016/j.tsf.2012.07.018
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[24] R.A. Weck, Thin-Film Shielding for Microcircuit Applications and a Useful Laboratory Tool for Plane-Wave Shielding Evaluations, IEEE TRANSACTIONS ON ELECTROMAGNETIC COMPATIBILITY, Vol. EMC-10, No.1, March 1968, 105-112
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[25] S. Yasufuku, Technical Progress of EMI Shielding Materials in Japan, IEEE Electrical Insulation Magazine, Nov./Dec. 1990, Vol.6, No.6, 21-30 [26] G. Blasek, Beschichtung von Kunststoffgehäusen zur Sicherung der elektromagnetischen Verträglichkeit elektronischer Geräte, Vakuum in Forschung und Praxis (1997), Nr.3, 187-195, doi: 10.1002/vipr.19970090306 [27] A. Eckersley, H-Field Shielding Effectiveness of Flame-Sprayed and Thin Solid Aluminum and Copper Sheets, IEEE TRANSACTIONS ON ELECTROMAGNETIC COMPATIBILITY, VOL. EMC-10, No.1, March 1968, 101-104 [28] R.W. Simpson, EMI shielding with flexible laminates, National Symposium on Electromagnetic Compatibility, IEEE 1984, 267-270 [29] T. Amato, D.J. Mis, and B.B. Wilard, Shielding Effectiveness Before and After the Effects of Environmental Stress on Metalized Plastics, IEEE TRANSACTIONS ON ELECTROMAGNETIC COMPATIBILITY, Vol.30, No.3, August 1988, 312-325 [30] H. Kaden, Wirbelströme und Schirmung in der Nachrichtentechnik, Springer-Verlag Berlin Heidelberg, 2. Aufl., 2006, 350 S.
Journal Pre-proof Tables Table 1. Reference data for aluminum shielding effectiveness.
layer layer layer layer layer foil foil layer plate
Thickness [ µm ] 0.1 0.5 2.5 – 5 6 5 - 10 15 - 40 50 130 – 500 ~ 1000
Shielding [ dB ] 33 - 41 60 40 - 70 60 - 80 60 – 80 30 - 38 59 20 – 42 100
Ref. [23] [24] [25] [26] [7] [27] [28] [27] [29]
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Al type
Electron-beam Evaporation 0.38
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𝑊𝑐𝑚−2
0.04
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𝑛𝑚 𝑠−1
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𝑅
/
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𝑞̇
Magnetron Sputtering 0.07
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𝑞̇ / 𝑊 𝑐𝑚−2
Plasma Pretreament 0.06
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Table 2. Heat flux densities and their ratio to deposition speed for the three process steps.
0.002
Journal Pre-proof List of figure captions Figure 1. Diagram of the NOVELLA experimental facility.
Figure 2. Chamber evacuation with and without plastic substrates. An ion current of 1.5.10-8 A corresponds to 3.10-3 Pa. (AM-PLA: additively manufactured PLA substrate, AM-ABS: additively manufactured ABS substrate)
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Figure 3. Substrate temperature as function of time for different plasma pre-treatment exposures.
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Figure 4a. Untreated polycarbonate substrate surface.
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Figure 4b. Polycarbonate substrate after 2-minute oxygen/argon plasma pre-treatment and NiCr deposition.
Figure 5. Substrate temperature as function of time during plasma pre-treatment and magnetron sputter
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deposition.
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Figure 6. Deposition rate as function of electron beam power.
Figure 7. Substrate temperature as function of time during electron-beam evaporation.
Figure 8.
Cross-section of a 4.2-µm aluminum layer on additively manufactured PLA, evaporated from
aluminum-oxide crucible.
Figure 9. Surface of a 4.2-µm aluminum layer on additively manufactured PLA, evaporated from aluminumoxide crucible.
Figure 10. Cross-cut test of a 5-µm aluminum layer on additively manufactured ABS.
Figure 11. Specific electrical resistance as a function of layer thickness.
Journal Pre-proof Figure 12. Shielding effectiveness in magnetic near field configuration as a function of frequency for an aluminum foil (smoothed) and two aluminum layers of different thickness. The reference dashed line was calculated according to the impedance concept (used parameters: source/sample distance: 0.25 mm, aluminum layer thickness: 2.5 µm, electrical conductivity: 7.106 S/m).
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Figure 13. Layer thickness distribution inside a coated junction box.
Journal Pre-proof Tables Table 1. Reference data for aluminum shielding effectiveness.
layer layer layer layer layer foil foil layer plate
Thickness [ µm ] 0.1 0.5 2.5 – 5 6 5 - 10 15 - 40 50 130 – 500 ~ 1000
Shielding [ dB ] 33 - 41 60 40 - 70 60 - 80 60 – 80 30 - 38 59 20 – 42 100
Ref. [23] [24] [25] [26] [7] [27] [28] [27] [29]
of
Al type
Electron-beam Evaporation 0.38
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𝑊𝑐𝑚−2
0.04
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𝑛𝑚 𝑠−1
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𝑅
/
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𝑞̇
Magnetron Sputtering 0.07
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𝑞̇ / 𝑊 𝑐𝑚−2
Plasma Pretreament 0.06
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Table 2. Heat flux densities and their ratio to deposition speed for the three process steps.
0.002
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Figure 1
Figure 2
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Figure 4
Figure 5
Figure 6
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Figure 12
Figure 13