ISBN: 978-2-35500-006-5 Megasonic Enhanced Electrodeposition

Jens Kaufmann1, Marc P.Y. Desmulliez1, Dennis Price2 [1] MicroSystems Engineering Centre (MISEC), School of Engineering & Physical Science,
Heriot Watt University, Edinburgh, EH14 4AS, United Kingdom

[2] Merlin Circuit Technology LTD,Harwarden Industrial ParkManor Lane,
Deeside, Flintshire,West Wales, CH5 3QZ, United Kingdom

Abstract A novel way of filling high aspect ratio vertical interconnection (microvias) is presented. High frequency acoustic streaming at megasonic frequencies enables the decrease of the Nernst-diffusion layer down to the sub-micron range, allowing thereby conformal electrodeposition in deep grooves. Higher throughput and better control over the deposition properties are therefore possible for the manufacturing of interconnections and metal-based MEMS.

I.
INTRODUCTION The increasing consumer demand for faster, lighter and smarter electronic devices calls for enhanced system integration and packaging technologies. Key to the increasing density of electronic components is the introduction of the high density interconnection (HDI) technology in printed circuit boards (PCB), resulting in multilayer technology and increasing amounts of electrical connections that need to be handled. Microvias are elements that are particularly important as they allow the reduction of the footprint of electronic components through the redistribution of interconnects in the underlying layers. Microvias are formed by mechanical drilling or laser ablation of the PCB material and subsequent electroplating is used to fill the cavity to render them electrically conductive. The microvias need however a conductive seed layer on the side walls to allow the electrodeposition of metal. The technologies used in that respect are mainly autocatalytic plating processes or direct metallisation[1].

To allow further integration and miniaturization, it is necessary to form microvias with an aspect ratio (height over diameter hole ratio) greater than 1:1. This current limit arises because of the difficult hydrodynamic conditions and current crowding effect at the mouth of the hole; both conditions attenuate the convection in the electrolytic solution near the surface of the substrate. This lack of effective agitation reduces the ion concentration in the solution within the immediate proximity of the microvia, increases the Nernst diffusion layer and limits therefore the deposition rate of the metal [2]. To overcome this limitation, extensive research utilising a variety of additives and current waveforms was performed. The additives used are mainly large organic inhibitors, and small complex builders that accelerate the deposition at the bottom of the via cavity. This is a very complex method, which needs a carefully controlled solution. However, super conformal plating of deep trenches and holes can be achieved, as exampled by the damascene process metallisation. The use of this process in high volume consumer electronics would nevertheless be problematic because of the high parameter variants found in the usually large volumes of solution used in PCM manufacturing. This

Fig. 1 Megasonic generator.

Fig. 2 Megasonic transducer of 4 by 4 inch

500 W Electrical power at 1MHz

active area 9-11 April 2008 © EDA Publishing/DTIP 2008

ISBN: 978-2-35500-006-5 paper proposes therefore to use megasonic agitation to reduce the diffusion layer and achieve optimal electrodeposition for microvias with aspect ratio larger than 1:1. II. MEGASONIC AGITATION This paper describes a new method to enhance the capability and compatibility of metallisation processes based on megasonic assisted copper electrodeposition. The primary focus of this article concerns the filling of high aspect ratio microvias. However, this technology is transferable to the manufacture of various high aspect ratio metal MEMS structures as well as the development of photoresist in deep trenches. A high frequency acoustic streaming of frequency f allows the modification of the near surface hydrodynamics. The electrical potential of the ox-red reaction is governed by the activity of the ion near the electrode surface, according to the Nernst equation: d OZ e a a F z RT E E Re 0 ln +

(1)

Increasing the activity, a, and therefore the concentration, c, near the surface of the substrate, results in a decreased overpotential.

( ) + ⋅

= Z M M c a Z γ

(2)

δ 0c dx dc =

(3) More precisely, the Nernst diffusion layer which, in normal conditions, is governed by the velocity of the medium stream over the solid surface, as in (4), see [3], depends on 1/f1/2 in the presence of an acoustic field, as in
(5).

2 1 16 . 0      

Ux v ic Hydrodynam δ

(4)

2 1 2      

ω δ v acoustic

(5) The h

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