THIN FLEXIBLE POLYMER SUBSTRATES COATED BY THICK FILMS IN ROLL-TO-ROLL VACUUM

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1 ARCOTRONICS INDUSTRIES SpA Via San Lorenzo, Sasso Marconi (BO) Italy Tel. (+39) Fax (+39) THIN FLEXIBLE POLYMER SUBSTRATES COATED BY THICK FILMS IN ROLL-TO-ROLL VACUUM COATERS Company: Arcotronics Industries Author: Nicola Magriotis Date: September 1 st 2004 ABSTRACT Vacuum coating is one of the most efficient technologies available for the deposition of layers of various materials on polymer substrates. In the industrial environment there is the constant demand of increasing the thickness of the layers that can be deposited under vacuum while decreasing the thickness of the substrate. On the other hand the deposition of thick coating layers on thin polymer films is a very challenging task because of the extremely high thermal load induced into the substrate by the process. This presentation illustrates the results of an experimental and theoretical investigation about the possibility of depositing thick copper and aluminium layers of good quality on thin plastic webs by thermal resistive evaporation. A set of tests have been carried on inside a vacuum web coater especially designed and built for the development of new products. A new web coating thermal model has been elaborated in order to predict the temperature profile of the substrate along the process. Theoretical and experimental results have been favorably compared under different operating conditions. INTRODUCTION An increasing number of application requires thin substrates carrying a thick coating layer of metal or electrical conductive materials in general. The major objective is the manufacturing of low resistance conductors that are now usually produced with processes that don t take place in vacuum. The possibility of manufacturing these products in a vacuum web coater could open new sceneries and markets in the world of vacuum metallized flexible substrates. On the other hand this application represents an extremely difficult challenge whose key of success depends on several factors. Among them you can find: first, the ability of handling thermal sensitive films during vacuum coating processes under very severe thermal conditions; second, the need to build a relatively dense and compact coated structure; third, the need to avoid delamination between the thick coated layer and the substrate or inside the deposit itself. The purpose of this paper is to illustrate the theoretical and experimental job in this field that has been carrying out by Arcotronics. In particular the critical combination of a very thin substrate with a very thick coating layer has been investigated both theoretically and experimentally in order to highlight the thermal behaviour of the substrate. From the theoretical point of view an analytical model of the web has been formulated to predict the thermal behavior of the film under the very high rates of heat flow associated with metal condensation and thermal radiation. From the practical point of view several experiments have been conducted in a laboratory roll-to-roll web coater designed and built by Arcotronics. The trials have been run in order to validate the theoretical web thermal model and to verify in practice the results on the final product. Finally the experimental web coater has been used to run some pilot pre-productions of material. In this way it has been followed the standard practice of driving and helping customers in the acquisition of new machines for innovative products reducing financial risks. The products investigated have been a few micrometers thick polymer webs with copper or aluminum coating layers of some hundreds nanometers of thickness. Copper and aluminum have been deposited by using standard thermal evaporation with ceramic boats. Several quality tests have been conducted on the final product. Among them it is worth mentioning peel-off tests for adhesion, measurements of surface energy, measurements of the thickness of deposited layers, and microscope observations. DESCRIPTION OF THE LABORATORY MACHINE The vacuum web coater used for the trials is a roll-to-roll laboratory machine designed and manufactured by Arcotronics and located in the facilities of Sasso Marconi. The general configuration of the machine is shown in Figure 1. It is basically a versatile two-drums, 350 mm web width metallizer with several flanges on the external walls that can accomodate different equipment in order to test different vacuum coating processes. Among them you can have: pre and post web-treaters (plasma treaters most of the time), an In- Line Pattern Printing unit, an oil evaporator for free-margin tracing, resistive evaporation with ceramic boats, sputter deposition with a planar rectangular magnetron. Cap. soc. Euro ,00 i.v. - R.E.A. Bologna Cod. Fisc. ed Iscr. Registro Imprese Bologna P.IVA IT Manuscript for AIMCAL Fall Conference 2004.doc 1/02/03

Figure 1. Web path inside the laboratory web-coater. The ceramic boats evaporation unit equipped with the metal wire-feeding system is placed under the first process drum.

The working pressure of 4 10-4 mbar is reached in a few minutes and it is safely maintained.

2 Figure 1. Web path inside the laboratory web-coater. The ceramic boats evaporation unit equipped with the metal wire-feeding system is placed under the first process drum. A system of mechanical pumps (one rotary and two roots blowers), a diffusion pump and a cryopump creates the vacuum in the corresponding machine area. The working pressure of mbar is reached in a few minutes and it is safely maintained. A magnetron for sputter deposition has been located in a stainless steel chamber served by a dedicated turbomolecular pump. The vacuum separation between the sputtering area and the rest of the chamber is particularly accurate so that the sputtering area is allowed to reach a vacuum of the order of 10-6 mbar extremely quickly during the pre-cleaning stage. Process pressure is then increased to the mbar range by the immission of a controlled flow of gas. Both the pre-cleaning stage and the sputter deposition process can be carried on without influencing the vacuum level of the rest of the metallizer chamber. Furthermore cross-contamination is almost totally avoided. Indipendent monitoring of the vacuum levels is made possible by using two pirani gauges and two penning gauges (two of them in the sputtering area, the other two in the evaporation area). The web tension is monitored and controlled by three couples of load cells. Five independent motors drive the two process drums, the tension roller, the unwinder reel and rewinder reel. The web speed can be selected between two different ranges: a low-range (0,5-25 m/min) and a high-range ( m/min). The accuracy of the rewinding process is extremely high with the chance to select between the lay-on mode with contact pressure adjustment (last roll in direct contact with the rewinding reel) or the gap-mode (gap existing Figure 2. View of lab-machine interior: evaporation box and sputtering area. Figure 2bis. Winding system of lab-machine

3 between the last roll and the rewinding rell). These two alternatives can be selected in order to obtain the best rewinding accuracy on the final reel depending on the material processed. THEORETICAL MODEL Introduction to the Web Thermal Model The deposition of a metal by thermal evaporation is always associated with a certain amount of heat load. This is due to the heat of radiation coming from the hot crucibles and the heat of condensation of the metal after its first contact with the cooled web. Generally speaking the major contribute comes from the condensation process. As this value is directly related with the total amount of material that is being deposited on the substrate you can easily understand that thick deposition are very challenging tasks for heat sensitive materials such as thin plastic films. In order to realize good process performance the temperature of the substrate around the process drum has to be kept under control. Thermal transmission between web and drum is obtained in two different ways: thermal conduction through the points of contact between drum and web (due to surface microroughness web and drum contact areas are limited to the peaks of the roughness) plus thermal convection due to the water vapour trapped inside the microcavities created by web and drum microroughness. drum conduction web convection Figure 3. Mechanics of heat transfer between web and drum When handling high outgassing substrates like plastic films the majority of the heat transfer is based upon the water vapour convection due to the high outgassing process taking place from the back side of the substrate. To predict the thermal behavior of the film during the metallization process a zero-order and a first-order analytical model have been created. Both models have several factors in common. First, they are both one-dimensional since the film thickness is several orders of magnitude smaller than the other two dimensions (width and length). In this way axial and transverse conduction effects are neglected. Second, both models take into account the main parameters of the process such as the web speed, the deposition rate and the radiation load. The only difference between the two models is that the zero-order one disregards the effect of thermal capacity of the substrate. Consequently, thermal equilibrium is assumed to be reached instantaneously and, at any position along the line, local heat transfer processes are considered steady. On the contrary the first-order model takes into account the thermal capacity of the substrate and the ensuing heat transfer processes are considered to depend on time, i.e. on position along the line. Brief overview of the Zero-Order Model This model work under the following assumptions: the one-dimensional heat transfer processes are assumed to be steady; the drum temperature is considered to be equal to the temperature t f of the refrigerant fluid; the total temperature difference t=t 1 -t f is unknown but is related to the total specific heat flux by the relationship: t q = R (1) where R is the total specific thermal resistance (sum of the two conduction resistances of the coating layer and the substrate, and the thermal resistance between web and drum), see Figure 4. The total specific heat flux is the sum of deposition and thermal radiation contributions: q = q d + q r (2) with:

4 v ( h hs q d = m v ) & (3) m & where v is the specific mass flow rate of vapor condensed on the web surface, hv is the enthalpy of the vapor at the crucible temperature, and h s is the enthalpy of the deposited metal at the web temperature. The radiation heat load: q r = ε s ε c F σ T 4 c T 4 ( ) (4) where ε s is the emissivity of the deposited metal, ε c is the emissivity of the crucible, F is a view factor, σ is the Stefan-Boltzmann constant, and T c and T are the absolute temperatures of the crucible and the web respectively. The parameter that drives all the process is the effective heat trasmission coefficient α through the gap between film and drum. This parameter is obtained by suitably combining the two parallel heat flow contributions of conduction through the contact zones and of convection through the gap. α α s +α g (5) where α s is the conduction transmission coefficient and α g is the convection transmission coefficient. The first one depends on the average contact pressure and, consequently, on the web tension. A good fit with experimental results has been obtained by setting α s equal to the constant value of 100 W/m 2 K. The convection coefficient has been estimated as: α g = λ g s g + 2l (6) where s g is the average gap thickness and l is the mean free path of the molecules (that depends on the saturation pressure of the vapor). Again a good fit with the experimental results has been obtained by evaluating the pressure at the temperature t f of the cold wall. Figure 4. Heat flow model. 1: coating layer; 2: web substrate; 3: interface between web and drum.

5 Brief overview of the First-Order Model In comparison to the zero-order model, in the first-order model the only additional assumption concerns the web thermal capacity which is not neglected any more. Figure 5. Schematization of the element energy balance. In Figure 5 you can see the energy fluxes connected to a single element of the substrate. The law that drives the process is the following energy balance for the element dx at the representative temperature t (averaged across the web thickness): ρ csut()+ x q d x = ρ csut( x+ dx)+α( t t f )dx (7) Since it is tx+ ( dx)=t()+ x dt dx (8) by simple algebric manipulations we obtain the governing differential equation ρ csu dt dx = q α ( t t f ) (9) Equation (9) is a first-order differential equation whose solution gives you the value of the temperature of the single element of web. THEORETICAL ANALYSIS AND PRACTICAL TESTS The first result of the analytical and experimental process is that for any given thickness of the coating layer the lowest web speed minimizes thermal stresses. The same quantity of material can be deposited at different web speeds by suitably adjusting the speed of the metal wire. Therefore a set of practical trials have been carried out with the support of the thermal model. The result is that the maximum temperature reached by the substrate during the metallization process increases as web speed increases. As an example an average thickness of the coating layer of 500 nm can be obtained by using the two different combinations of wire and web speeds reported in Table 1, with suitably identical settings of all the other machine parameters.

6 Table 1. Comparison of two different situations both producing a coated layer of 500 nm. Substrate Coating material Wire speed Web speed PET 19 µm copper 31 cm/min 4,6 m/min PET 19 µm copper 21 cm/min 3,1 m/min The two resulting temperature profiles are shown in Figure 6 and Figure 7 respectively. Figure 6. Test run at 4,6 m/min of web speed and 31 cm/min of wire speed t [ C] x [mm] Figure 7. Test run at 3,1 m/min of web speed and 21 cm/min of wire speed. Figures 6 and 7 describes the temperature of the web around the process drum. The axial coordinate x indicates web path distance around the coating drum (x=0 at the entrance point; metallization takes place in the space interval between the two dashed vertical lines). Looking at the two graphics it is evident that the substrate is subjected to lower thermal stress when the web speed is lower. Results of the experimental trials are alligned with the theoretical findings. During tests at higher speed it is necessary to increase the pressure of contact between the drum and the substrate in order to obtain the same quality of the final product. This can be made either mechanically or electrically. The first option consists in increasing the machine direction web tension between the drum and the friction roll whereas the second option consists in creating an electrostatic force between the web and the drum (BIAS system). On the other hand if trials at higher line speeds were performed using the same web tension suitable for lower speed occasional wrinkles would be observed through the metallizer window. These wrinkles would be generated by the excess of head load that is not removed by the chilled drum. These kind of defects cannot be eliminated after the process is over. The second result of the tests is that the web temperature profile t(x) doesn t practically depend on the web thickness provided that the global heat transmission coefficient α between drum and web doesn t change. On the other hand any small alteration of α

7 induces great variations in web-temperature profiles. Figures 8 and 9 respectively show a comparison of temperature profiles obtained for α = 100 W/m 2 K and α = 75 W/m 2 K. t [ C] x [mm] Figure 8. Substrate 10 µm thick, web speed 3,1 m/min, copper wire speed 21 cm/min, α = 100 W/m 2 K t [ C] x [mm] Figure 9. Substrate 10 µm thick, web speed 3,1 m/min, copper wire speed 21 cm/min, α = 75 W/m 2 K. When changing material thickness it is possible to keep the same final quality of the product as long as the pressure between the web and the drum is maintained the same. The explanation is that α depends mainly on the contact pressure, i.e., on the absolute web tension or BIAS voltage. As an example you can consider the comparison between a 19 µm thick material and a 10 µm thick material. 2 As shown in Table 2 the specific tension has to increase from 18 to 34,2 N/mm in order to have the same absolute tension of 342 N/m. This will allow you to have the same pressure between the drum and the web. Table 2. Absolute tension and specific tension. Substrate Absolute tension Specific tension PET 19 µm 342 N/m 18 N/mm 2 PET 10 µm 342 N/m 34,2 N/mm 2 PET 10 µm 180 N/m 18 N/mm 2 CONCLUSIONS This paper has illustrated one of the jobs carried out by Arcotronics in the development of vacuum deposited thick layers of metals (like Cu or Al) on thin flexible polymer substrates. The experimental trials have been run on Arcotronics lab-machine whereas the

8 theoretical analysis has been conducted on the basis of an analytical model specifically developed. Major results have shown that a deposition of 0,5 µm is possible on 10 µm thick PET by optimizing the heat exchange from the web to the chilled process roll. The parameters that mostly influence the good quality of the process are the web speed and the pressure of contact between film and drum. Any artificial way to increase the heat transfer to the drum gives you the chance to increase the thickness of the final deposition since the main obstacles are all connected to the thermal load affecting the substrate. The good results reached in this field are to be considered as a first step towards more complex products like patterned thick metallized films. Nicola Magriotis ARCOTRONICS INDUSTRIES S.p.a. Via S.Lorenzo, Sasso Marconi (BO) ITALY tel fax N.Magriotis@arcotronics.com

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