Structuring Methods of Plastic Substrates for Electroplating Applications
Authors: Bengsch Sebastian , Rechel Mathias , Asadi Esmail , Wurz Marc Christopher 
Published: February 2020

New approaches of packaging of integrated circuits show the need of flexible and cheap materials. Plastics like PET, PMMA and PC offer possibilities regarding flexibility and mass production possibilities in terms of roll to roll production for foil technology based substrates, as well the use of injection moulding for more complex three-dimensional housings, circuit and even sensor /actor arrays. [1] [2] [3] At the Institute of Micro Production Technology at the Leibniz Universität Hannover, we develop a new manufacturing strategy to produce prior structured wafer-substrates which can be used for electroplating processes without the need of photolithography. Important process steps are injection moulding, sputter deposition, electroplating, chemical mechanical polishing and micro milling. These manufacturing processes are not in the need of clean room technology and abstain from any lithography processes. Therefore this idea can be interesting for low cost production systems which neglect expensive clean room technology and expensive substrate materials without functionalized properties.
At the IMPT structures were formed by an injection moulding process and followed by sputter deposition and electroplating to analyse the feasibility of common micro production technology in combination with plastics. After these process steps, CMP is performed and afterwards the structures will be covered by a following injection moulding or spin coating process. The flow of the manufacturing process can be seen in figure 1. Further developments will show the combination of these manufacturing processes with the use of a micro-milling machine to realise a transformer structure with an integrated electroplated magnetic core. Transformer-like structures are complex to manufacture by common micro production technologies due to the limited 3D-capability of electroplating technologies and the need of a closed magnetic core combined with helical copper coils [4]. This new combination of well-known fabrication processes and easy-to-handle substrate materials can offer a cost effective, reliable alternative kind of packaging of integrated circuit technology.
At first structure size and height feasibility of the used injection moulding machine of the type BOY 55EV™ was performed. Structures of 100 µm height could easily be achieved and as well as structure sizes down to 50 µm diameter. Polycarbonate (PC) as a substrate material can be sputter deposited with Cu and NiFe up to 200 nm using a 50 nm adhesion layer of Cr (Figure 2). Temperatures during the sputter deposition process are critical and should not exceed 140°C as there will be the mechanical stability limit of the used PC. Electroplating was performed up to 5 µm on the first examined substrates. Mechanical dicing was used to create micrographs to analyse the electroplated structures and to examine the abrasive behaviour of the used polycarbonate. The CMP process is performed and should use slurry which does not solve or attack the polycarbonate. A neutral environment should be given. Figure 3 shows copper coated PC test structures after CMP process. The analysis of the process steps show feasibility of manufacturing of sensor arrays as well as transformer set-ups. This idea might open a new low cost fabrication process independent of clean room technology.
[1] Alp Yaradanakul, Ali Yildiz, Donald P. Butler, Zeynep Celik-Butler, Fabrication of Micromachined Devices on Flexible Substrates, IEEE Emerging Technology Symposium, 2001
[2] Hung-Wei Yeha, Chih-Lung Tsengb, Po-Jui Chiang, Jau-Ji Joub, Nai-Hsiang Sunc a, Deposition of Homo-Multilayer Indium Tin Oxide Films on PET Substrates by Roll-to-Roll Sputtering, Next-Generation Electronics (ISNE) International Symposium, 2013
[3] Mitko Tanevski, Francesco Merli, Alexis Boegli, Anja K. Skrivervik, Jean-François Zürcher, Pierre-André Farine, RoSe: A Subgigahertz Wireless Sensor Platform With Housing-Integrated Overmolded Antenna, Transactions on Instruments and Measurements Vol. 61, Issue 11, 2012
[4] Enrico Macrelli, Ningning Wang, Saibal Roy, Michael Hayes, Rudi Paolo Paganelli, Marco Tartagni, Aldo Romani, Design and Fabrication of a 315 µH Bondwire Micro-Transformer for Ultra-Low Voltage Energy Harvesting, Design, Automation & Test in Europe Conference & Exhibition, 2014

Figure 1