28 Mar 2013

Mesmoproc – Implementation of a Maskless Electrochemical Surface Modification Process

Martin Goosey, MTG Research Ltd, and Sudipta Roy, Royenface Ltd, Newcastle, UK discuss the implementation of a maskless electrochemical surface modification process for use in manufacturing

The conventional processes used to fabricate microscale interconnections, devices and components use photoresists and a range of developing, etching and stripping chemicals to produce the requisite shapes and features in a process known as photolithography.

In conventional production approaches, a photolithography stage is required to pattern each individual substrate. Currently, PCB boards (PCBs) are manufactured by using a copper coated board which is coated with a photosensitive etch resist material.

This is typically in the form of a so-called dry film resist, although liquid materials are also sometimes used. Although there are several approaches that can be used to form subsequent patterns and features etc, a commonly used method requires the resist to be cured by exposing it to ultra violet light through a mask.

A pattern is then formed in the resist by removing the material in the unexposed areas. This process is known as development. In PCB board applications, additional copper may then be plated on top of the thin copper coating as required. Once the copper has been plated, the resist is then removed entirely using a range of chemical stripping solutions. Next, the exposed copper is removed using a variety of etching solutions, which are typically very corrosive. This process, therefore, requires multiple steps to create a metallic pattern on a substrate and also involves many chemicals which are all removed.

conventional lithographic route to pattern generation

Figure 1: A conventional lithographic route to pattern generation

In addition, PCB manufacturing is carried out in tanks where the circuit board and counter-electrode are often placed at some distance from each other, which in some applications increases the energy requirements of the process and reduces the overall efficiency. This is partially due to the use of standard tank designs and efforts to incorporate electrolyte agitation schemes such as eductors and stirrers, which necessitate a large gap between the electrodes.

Many of the electrolytes used in PCB fabrication are strongly acidic corrosive agents that have associated health, safety and environmental concerns. PCB manufacturing, therefore, remains an expensive, multistep process involving specialised equipment and infrastructure which generates copious waste and that consumes large amounts of energy, thereby raising operational costs. Due to increasing cost and sustainability issues, as well as growing social and environmental concerns, many companies using photolithography are keen to implement more efficient alternative approaches. One such alternative is to use a maskless approach known as Enface which obviates the need for individual photolithography stages.

Enface Technology

The EnFACE, (Electrochemical Patterning by Flow and Chemistry) approach exploits the possibility of localising deposition and dissolution current on an unmasked substrate by optimising reactor design, the flow conditions and the electrochemistry of a system [1 - 4]. In this technique, instead of patterning the substrate, a pattern is defined on the counter electrode, which serves as an electrochemical tool, see the example shown in Figure 2. The substrate is a conducting material, such as a metal and the tool and substrate are placed in an electrochemical reactor, facing each other. A direct or pulsed current, pulsing at rates between 100 and 1000 Hz, is imposed through an external circuit. The substrate is etched selectively opposite the unmasked portions of the tool, thereby reproducing a replica of the resist pattern on the substrate.

The concept of Enface Technology

Figure 2: The concept of Enface Technology. A patterned tool is used to selectively plate or etch patterns on a substrate.

The advantage of the Enface process is that it can be used to transfer patterns onto numerous substrates using a single tool. Eliminating the requirement to perform a lithography step on each substrate means that there is the potential for significant cost and energy savings to be achieved. It has already been shown that up to 25 substrates can be etched using the same tool and standard positive working photoresists. In addition, Enface technology requires the use of dilute metal solutions, which can reduce the overall operating costs of the plating processes.

Plated and etched patterns in copper have been transferred from a tool to a substrate with sizes of up to 2 cm2 using electrolytes containing 0.1 M CuSO4 and water (without the use of any additional agents). Both linear and square patterns ranging between 10 µm and 200 µm have been plated or etched using the EnFACE technique, with higher resolution being achieved when a pulsing voltage was used [2, 5]. Figure 3 shows a linear pattern of copper achieved by pulse current deposition. More recently, nickel deposition has also been attempted, and patterns of 300 to 500 microns have been transferred [6].

Copper lines plated using Enface Technology in a flow system

Figure 3: Copper lines plated using Enface Technology in a flow system.

EnFACE technology for copper deposition and etching has been proven using a reactor where flow velocity could be controlled which enabled the removal of reaction by-products. The design was a flow-by cell based on a model previously constructed for plating wafers. The anode and cathode were mounted on the holders and the electrolyte was circulated upwards through the channel [2,5]. Nickel deposition experiments were performed in a laboratory scale tank type cell, without agitation [6]. There is thus scope to utilise the new technology with a range of metals.

The Mesmoproc Project and its Objectives

MESMOPROC is a European multi-partner development project that has recently commenced to introduce more efficient and cleaner production technology into the PCB and metal finishing industries. The project is aiming to enhance and introduce the ENFACE approach on a production scale to decrease the use of lithography, which will result in reduced chemical usage and generate less waste. Energy efficiency will also be improved.

The three year project is being supported via the European Commission’s CIP Eco-innovation scheme for ‘First Application and Market Replication Projects’. It has seven partners from across Europe, including three UK-based organisations, namely MTG Research Ltd, Royenface Ltd and Coventry University. The other partners are Pragoboard s.r.o. from the Czech Republic, Protection des Metaux S.A.S. (Promet) and International Project Management, Plating and Materials from France and EIPC Services B.V. from the Netherlands. Together, these organisations represent a broad cross section of the requisite industry supply chains and several of them have already worked together on other related projects.

The Mesmoproc project is further developing the Enface technology to plate in tank type cells where agitation is provided by ultrasound (U/S) agitation focused within the gap between the tool and substrate. Preliminary experiments at Newcastle University have already shown that U/S agitation can improve agitation within the narrow gap [7]. This focused agitation will allow scale up of the process over larger surface areas, comparable to those used for fabricating PCBs. On the other hand, the project is also exploring the possibility of using Enface technology to selectively metallise substrates with complex geometries, such as those that are typically encountered in the metal finishing industry.

The Mesmoproc technical development programme is investigating the effect of U/S, current and potential waveforms and electrolyte composition on pattern transfer. New laboratory-scale and pilot scale tank reactors, incorporating ultrasound agitation will be designed and constructed as the project moves towards industrial implementation.

Copper, nickel and other metals will be plated on substrates to optimise the technology for circuit board manufacture. Ultimately, pilot plant scale tanks will be constructed based on the findings from the laboratory scale system. The Mesmoproc project aims to implement the first industrial scale plants incorporating the new maskless electrochemical surface modification process within the project partners’ facilities in the metal finishing (Promet) and PCB (Pragoboard) manufacturing sectors. Key factors that will encourage the adoption and uptake of the developed technology are the increasing costs of raw materials, energy and treatment of waste (and ultimately the disposal of waste from site), which are all projected to continue to rise inexorably due to a combination of legislative demands in the instance of waste and escalating world demand, primarily from Eastern manufacturing areas.

There is well defined competitive requirement for this technology and an opportunity to achieve significant cost benefits in the reduction of direct manufacturing overheads in these strategically important sectors of European industry. The major outputs from the project will be industrial scale units matched to the needs of the targeted sector manufacturing plants. These plants will be used to collect detailed field trial data that will enable the true performance benefits to be accurately determined and further iterative improvements to be made to the equipment and processes. Based on the construction and operational data generated, another key objective will be to undertake techno-economic modelling to determine the overall environmental impact and benefits compared to traditional processes. The operation of the equipment in true industrial manufacturing environments with a range of processes will enable them to be fully assessed and demonstrated.

Europe needs to retain its know-how and leadership in PCB board and associated micro-fabrication technologies due to its extensive use in strategically important sectors such as electronics, aerospace, defence and transport. This type of manufacturing is currently under great pressure on two different fronts; the first being the intense competition from China and India, where wages are much lower, and the second being sustainability and environmental requirements to lower energy and resource consumption without compromising material compatibility and performance.

Summary

Conventional approaches to the fabrication of fine features in the PCB and metal finishing industries typically employ a multistage photolithography stage to process each individual substrate. This is both inefficient and wasteful of valuable materials. The Mesmoproc project is aiming to help the European PCB fabrication and metal finishing industries by developing and implementing a new novel maskless electrochemical surface modification process that decreases the use of lithography, Subsequent uptake of the technology is expected to provide environmental, sustainability, economic and societal benefits by virtue of the reductions in hazardous chemicals used and associated cost savings from reduced energy, waste treatment and waste generation/disposal. The new processes will also enable European companies in the PCB and metal finishing sectors, and their customers, to produce more competitive products via both increased process and assembly yields and to provide higher levels of quality in finished products. Further information about the Mesmoproc project is available from a dedicated website, www.mesmoproc.eu, and there will also be a wide range of publications and dissemination activities throughout the life of the project.

Acknowledgment The Mesmoproc project is supported by funding from the European Commission. The sole responsibility for the content of this article lies with the authors. It does not necessarily reflect the opinion of the European Communities. The European Commission is not responsible for any use that may be made of the information contained therein.

References:

1 S. Roy, “A Process for Manufacturing Micro- and Nano- Devices” US Patent 7776227 (July 2010)

2 I. Schoenenberger and S. Roy, Electrochim. Acta, 51, 809-819 (2005).

3 S. Roy, J. Phys. D: Appl. Phys. 40 (2007) R413-R426.

4 S. Nouraei and S. Roy, J. Electrochem. Soc., 155(2) D97-D103 (2008).

5 Qi-Bai Wu, T.A. Green, S. Roy, Electrochem. Comm. 13 (2011), 1229–1232.

6 T. Widayatno and S. Roy, GPE-3rd International Conference on Green Process Engineering, December 6-8, 2011, Kuala Lumpur.

7 S. Coleman and S. Roy, Interfinish 2012, 14-16 Nov 2012, Milano, Italy.

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About the author

Professor Martin Goosey (photo) has over 35 years experience in the electronics industry and is currently Industrial Director of the UK’s Innovative Electronics Manufacturing Research Centre (IeMRC). Martin’s work has, for the last twenty years, focussed on environmental issues and new legislation impacting the electronics and related industries. He works with numerous industrial, academic and government partner organisations and routinely participates in UK and European multi-partner research projects related to sustainable electronics manufacturing, as well as electronics reuse and related recycling technologies. He is a Chartered Scientist, a Chartered Chemist, a Fellow of the Royal Society of Chemistry, a Fellow of the Institute of Metal Finishing and a Fellow of the Institute of Materials. Martin is also currently Chairman of the Institute of Circuit Technology and also a Director of his own research company, MTG Research Ltd. MTG Research Ltd is the project co-ordinator of the European Commission funded multi-partner Mesmoproc project.

Sudipta Roy graduated from IIT, Delhi (India) with a degree in chemical engineering. She received MS and PhD from Tulane University in the USA (1986-1991) followed by post-doctoral studies at the Swiss Federal Institute of Technology, Lausanne (1991-94). She moved to UK with an academic appointment as a lecturer (1994), and progressed to a Readership (2000) and a Full Professor (2005). She is an expert in electrodeposition, particularly pulse plating where she has recently published a book. Her work has led to the development of novel electrochemical processes for gold deposition, and more recently a mask-less micro fabrication method leading to a University spin out company Royenface. She has had numerous collaborations with industry in the microfabrication and corrosion area and spent a secondment year at as a Royal Academy Fellow with IOC (2001/02).

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