Surface and Coatings Technology 98 (1998) 848-850
Barrier layers on aluminium and gold by plasma polymerization C. Oehr, B. Schindler * Fraunlw!er-Institutfiir Grell:f!iichen- IIlld Biorerfahrellstecllllik (1GB). Nobelstrafle 12. D 70569 Stutt/?urt. Germany
Abstract Plasma procedures were used for the deposition of polymer barrier layers that are highly cross-linked as a consequence of the molecular fragmentation of the feed gases. Plasma polymers from vinyltrimethysilan (VTMS) as feed gas showed good protection against chemical and electrochemical attacks on gold (against 4 M H2S0 4 ), A combination of a plasma-VTMS primer covered by a plasma hydrocarbon layer showed good resistance against alkaline corrosion on aluminium. © 1998 Published by Elsevier Science S.A. Keyll'ords: Aluminium; Barrier layers; Gold: Plasma polymerization
1. Introduction Barrier layers on metals are of great importance for various purposes, e.g. as corrosion protection in the case of aluminium of zinc, or as a non-conductive barrier to provide electrochemical neutrality of noble metals such as gold. In all applications, good adhesion of the protective layers and the absence of pin-holes and cracks are crucial for the function. To maintain a permanent functioning, the barrier layer must withstand mechanical stress without forming crevices and must resist acidic or basic attacks and corrosive gases as oxygens. Two additional aspects of the importance of the quality of barrier layers shall be mentioned here, namely: • adsorption at the interface between barrier layer and substrate, and • adsortion of destructive reactants in the barrier layer. As both phenomena need adsorption sites, these shall be avoided as far as possible. In the first case, they might favour weak boundary layers; in the second case, they support diffusion of reactants to the interface. It is well known that the immediate pre-bond condi• tion of the metal surface greatly determines the durabil• ity of the adhesive bond. A great advantage of plasma deposition processes with respect to normal chemical processing is that adsorption layers like organics or water can be removed by plasma-etching of the substrate, and a direct bonding of the plasma polymer layer on a metal or metal oxide • Corresponding author. Tel: +49 71 I 9704140; Fax: +49 711 970 4200; e-mail: [email protected]
surface is achieved without any further intermediate contamination out of the atmosphere. The deposition process itself should be performed under conditions where the impact of functional groups (which favours diffusion) is minimized and high three• dimensional cross-linking of the plasma polymer is enhanced by high fragmentation of the plasma feed (i.e. high ratio of energy input to feed) until the layers become rigid enough. 2. Experimental and results The descriptions of the expcriments are presented for gold and aluminium in different sections.
2.1. Gold The gold electrodes are part of an open microstruc• tured electrochemical sensor and have been coatcd with a hydrophobic plasmapolymer. The electrodes are fabri• cated with the LlGA technique and are sketched in
Fig. I. This layer should coat only the top surface of the electrodes in order to prohibit leakage of the electrolyte out of the electrode assembly during the later usage by providing a non-wettable surface (with a high advancing contact angle). The walls of the hexagonal holes should not be coated because these inner surfaces have to be electrochemically active. Leakage of the electrolyte can also be avoided by reducing the aperture diameter of the LlGA structure.
0257-8972/98/$19.00 © 1998 Published by Elsevier Science SA All rights reserved. Pl/ S0257-8972(97)00171.0
C. Dehr, B. Schindler
Sur/lice and CoarinKs Teclllwlogr WI ( I'N8) 8-18 851J Table I Plasma process parameters for two layers Parameter
Fig, I. Section of the microstructured gold electrode (schematic) dis• tance between the holes: 10 30 Ilm.
As feed for the plasma polymerization hydrocarbons, silanes and siloxanes were chosen, in order to form gold/carbon bonding, which is considerably more stable than other gold compounds. Polymerization was carried out in a parallel plate reactor capacitively coupled to a 13.56-MHz power supply. The deposition process was optimized in order to obtain the higher stability of the plasma-polymerized layer against the electrolyte solution (4 M H 2S0 4 ) as well as the best adhesion of the plasma hydrocarbon, silane and siloxane films on gold electrodes. A survey of different precursors and process conditions is given below: Typical procedure for the deposition of plasma poly• mer layers: • cleaning of the gold surface of 02-plasma (20 Pa, 20 seem, 20 W); • deposition of the plasma polymer; • curing of the layer by saturation of reactive sites (see below) (cross-section of streaming channel (SC): 40 mm x 200 mm, length of SC: 100 mm electrode dimensions: 100 mm x 200 mm).
Pressure Gas flow Cross-section of stream Distance of the electrodes Power density Duration of the process Thickness of the layer Deposition rate
Plasma deposition I. Layer: VTMS feed gas
, Layer: C,H.H, feed gas
20 Pa 10 sccm 3.5 cm x 20 cm .'.5 cm 1.0Wcm 20 min 1.21lm 60 nm minI
80 Pa C,H.:H" 10:200 sccm 3.5 cm x 20 cm .'.5 cm I.OWcm 15min O. 16 1lm II nmmin l
• Precursors for plasma deposition: • ethylene; • vinyltrimethylsilane/VTMS, hexamethyldisiloxane/H M DSO (20 Pa. 10-30 seem, 20-50 W, 25-100 C substrate temperature, typical deposition rate: 35 nm min I- )
o Fig. 3. AFM picture of aluminium aftcr alkalim: ctching.
J6f1 2 fig. 2. SEM picture of a VTMS layer on a gold microstructure.
Fig. 4, AFM picturc of aluminium alkr alkinc I:lching + plasma VTMS polymcr primcr deposition + plasma C,II •. II, polymer deposition.
C. Oehr, B, Sclllndier / Surface and Coatings Technology 98 ( 1998) 848-850
. .a:lhllrg (wllh phcaphorlc or chromlc ecld) or chromldaLlpt1.lItc 8C1d ..
coal1ru or adh_lVa 10111
Plasm a procedure Plasma
(aU reactions In one vessel)
• Curing treatment after plasma procedure: • Grafting reactions with: ethylene, hexafluorpropene (10 5 Pa, I h) or vinyltrimethylsilane (vapour pressure at 25°C h r ). The best results were achieved by vinyltrimethylsilane plasma deposition and curing. These layers show a sufficient degree of stability against 4 M H 2S0 4 solution. They withstand a voltage of 8 V (without any measur• able current), and exhibit good adhesion during the electrochemical experiments as well as a high degree of hydrophobicity with a contact angle 9(adVancing/receeding) = 115 0/74 ° for VTMS layers. The contamination in the hexagonal channels caused by the penetration of plasma is negligible and was 'burned off' electrochemically by a short electrical pulse (Fig. 2).
2.2. Aluminium Aluminium for architectural purposes must be pro• tected against corrosion in the long term. The commer• cially available aluminium is covered by an undefined oxide/hydroxide layer on the surface and therefore requires alkaline etching with NaOH solution, followed by sulphuric acid treatment for the elimination of sodium ions and water rinsing before any coating pro• cedure is applied. Beyond that for all conventional coatings, a pretreat• ment (chromic- or phosphoric acid treatment or anodiza• tion of aluminium) is indispensable in order to form a stable conversion oxide/hydroxide layer as a basis. It is
also this layer itself that causes problems: in most cases, cohesion fractures are oberved in the conversion layers. Plasma procedures might overcome these obstacles. Direct deposition of plasma-hydrocarbon polymers on aluminium was realized in several experiments, and satisfactory adhesion could be observed. However, no sufficient resistance against alkaline attack could be achieved with' ultrathin layers. Since thicker layers (J.lIl1) of plasma-hydrocarbon polymers became very brittle, we returned to a plasma-deposited primer followed by a second deposition. Plasma-vinyltrimethylsilane poly• mer used as primer showed the best results. In a series of experiments, the best process parameters for the formation of plasma hydrocarbons upon the above-mentioned primer layer were investigated. In Table 1, the plasma process parameters are pre• sented for those layers that brought the best over all results for adhesion, scratch resistance and alkaline stability. In order to examine the anti-corrosive and barrier effect of the layers, a modified test method with NaOH for testing the alkaline stability was employed. Instead of the normal concentration of 0.2% NaOH in water, we used a 10% NaOH solution (weight/weight) and controlled the chemical nature of the contact area before and after the contact by ESCA-analysis. In this case, we did not observe any change in the chemical composi• tion after 2 h of NaOH contact, and therefore, we considered the alkaline stability to be sufficient. The change in roughness between the aluminium surface after alkaline etching and after covering with a barrier layer is demonstrated in the atomic force micro• graphs (AFM) (Figs. 3 and 4) ..