A novel way of filling high aspect ratio vertical interconnection (microvias)
with an aspect ratio of >2:1 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 possible for the manufacturing of interconnections and metal-based MEMS.Comment: Submitted on behalf of EDA Publishing Association
  (http://irevues.inist.fr/handle/2042/16838

Desmulliez, Marc

Kaufmann, Jens Georg

Price, D.

English

arXiv

A novel way of filling high aspect ratio vertical interconnection (microvias) with an aspect ratio of >2:1 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 possible for the manufacturing of interconnections and metal-based MEMS

I-Revues

9-11 April 2008 
© EDA Publishing/DTIP 2008  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
Fz
RTEE
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 Mca 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
icHydrodynamδ   (4) 
  
2
1
2






=
ω
δ vacoustic   (5) 
The high frequency (above 500 KHz) not only reduces the 
thickness of the Nernst diffusion layer but avoids the 
creation of cavitations as traditionally experienced in 
ultrasonic agitation. These highly volatile collapsing bubbles 
inhibit the usage of acoustic streaming in the plating 
industry, due to the likelihood of board delamination. The 
use of acoustic streaming on the diffusion layer can be found 
in cleaning processes for micro parts. Furthermore the 
modelling of the megasonic induced flow in trench like 
cavities is also described in [4]  
II. EXPERIMENTAL WORK 
In order to achieve the filling of microvias of high aspect 
ratio, a full Design of Experiments (DOE) has been 
conducted to analyse the influence of megasonic agitation on 
the plating quality of the microvias. The considered 
parameters include the current waveform, the power of the 
transducer used and the included angle of radiation between 
the transducer and the PCB. These parameters were chosen 
due to previous work in microvia plating. This abstract 
focuses at this stage on the fundamental differences 
introduced by using direct current (DC), pulsed current (PP) 
and reverse pulsed current (RP) plating. The different power 
levels for the megasonic agitation range between 0W and 
500Watts of electrical power to distinguish between the 
normal agitation within the tank and the acoustic streaming. 
To change the angle between the transducer and the test 
vehicle (cathode) a custom carrier for the transducer was 
designed and manufactured. This carrier allows changing the 
radiation angle without changing the distance between the 
transducer and the centre of propagation. 
 
The transducer itself consists of a 4 by 4 inch wide PZT 
crystal plate contained in a laser welded container. The 
experiments were conducted in a 60 litres polypropylene 
tank. The electrolyte contains the standard components for 
 
      Fig. 3 Schematic of the transducer mounted   Fig. 4  Insufficient (left) or no filling (right) 
         on the carrier focussing the sound     of the microvias 
 towards the sample holder 
 
 
9-11 April 2008 
© EDA Publishing/DTIP 2008  ISBN: 978-2-35500-006-5 
copper deposition; 10% sulphuric acid 300 g/l copper 
sulphate, 0.1% HCL and 0.05% of organic brighteners.  
 
As measure of the quality of the microvias, several criteria 
were specified. The throwing power and the amount of 
deposited metal per unit time were measured. Finally, the 
highest achievable aspect ratio was recorded. The filling of 
the microvia was possible till an aspect ratio of >2:1 was 
reached. The filling profile of the holes shows a largely over 
proportional deposition at the bottom of the blind vias. 
Larger aspect ratio could not be achieved because of the lack 
of seed conductive layer deep into the microvia cavity. This 
result was obtained for 125 and 250 W of radiated power, at 
any angle and with a DC waveform. Pulse electroplating is 
not necessary. 
III. Conclusions 
These results demonstrate the world’s first application of 
megasonic agitation for the plating of microvias of aspect 
ratio greater than 1:1. The improvement in the deposition 
rate, as well as the quality of the deposits, make megasonic 
agitation a serious contender for the filling of deep aspect 
ratio microvias. The throwing power of the plating, the 
relation between the thicknesses on the mouth to the 
thickness on the bottom reached 5. This allows super 
conformal filling, as known from the damascene process in 
semiconductor manufacturing. This method does not require 
any special plating chemistry or optimization of waveform 
parameters. The effect of megasonic agitation on the 
crystalline structure needs however further investigation.  
 
REFERENCES 
 
 
[1]  J. H. Lau and C. Chang, An overview of microvia technology, 
Circuit World vol. 26, pp. 22-32, 2000. 
[2]  S. Castaldi, D. Fritz, and R. Schaeffer, Limits of copper plating in 
high aspect ratio microvias, Circuit World vol. 25, pp. 35-40, 1999. 
[3]  K. M. Yin, Duplex diffusion layer model for pulse with reverse 
plating, Surface and Coatings Technology, vol. 88, pp. 162-164, 
1997. 
[4]  R. H. Nilson and S. K. Griffiths, Enhanced Transport by Acoustic 
Streaming in Deep Trench-Like Cavities, Journal of The 
Electrochemical Society, vol. 149, pp. G286-G296, 2002 
  
 
Fig. 5 Completely filled via (left) using megasonic agitation (125 W) with an aspect ratio of 2:1, 
uniformly plated vias with insufficient seed layer (right) 


Megasonic Enhanced Electrodeposition

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

Jens Kaufmann

Marc P. Y. Desmulliez

Dennis Price

CiteSeerX

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. ©EDA Publishing/DTIP 2008.</p

Megasonic Enhanced Electrodeposition

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