Enhanced electrodeposition for the filling of micro-vias
Jones, Thomas David Arthur
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This thesis investigated the introduction of megasound (MS) (1MHz) acoustic technology as an enhanced agitation method of an electrolyte solution for the electrochemical deposition of copper (Cu), used in electroplating processes. The thesis, carried out at Merlin Circuit Technology Ltd, studied the possibility of improving processing capabilities for use in Printed Circuit Board (PCB) industrial manufacture. Prior laboratory experiments demonstrated increased metallisation of vertical interconnect access (via) features in a Printed Circuit Board (PCB), which, if applied within manufacturing, would enable increased connectivity throughout a PCB and result in cost savings. PCB manufacturing quality after MS-assisted Cu electroplating was assessed by measurements of the topography of the electrodeposits, using scanning electron microscopy and white-light interferometry. Cu plating rate changes were also measured on the surface of the PCB and inside the vias. After plating Cu with MS-assistance, the macro and microscale surface composition was demonstrated to alter due to the direct influence of the acoustic waves. Systematic characteristic of the surface was conducted by varying the settings of the acoustic transducer device as well as the process parameters including electrical current distribution, bath additive chemistry and solution temperature. MS processing was shown to produce unique Cu artefacts. Their deleterious formation was demonstrated to be influenced by acoustic standing waves and microbubble formations at the electrolyte solution/PCB interface. Causes of these artefacts, microfluidic streaming and cavitation, were also observed and controlled to reduce the creation of these artefacts. MS plating Cu down through-hole via (THV) and blind-via (BV) interconnects was shown to produce measureable benefits. These include, for THVs, a 700 % increase of Cu plating deposit thickness within a 175 μm diameter, depth-to-width aspect ratio (ar) of 5.7:1, compared with processing under no-agitation conditions. For BVs, a 60 % average increase in Cu deposition in 150 μm and 200 μm, ar 1:1, was demonstrated against plating under standard manufacturing conditions - bubble agitation and panel movement.