ISSN 10637397, Russian Microelectronics, 2015, Vol. 44, No. 4, pp. 248–254. © Pleiades Publishing, Ltd., 2015.
Original Russian Text © S.V. Ishutkin, V.A. Kagadey, E.V. Erofeev, E.V. Anishchenko, 2015, published in Mikroelektronika, 2015, Vol. 44, No. 4, pp. 282–289.
248
1. INTRODUCTION
Copper (Cu) is now being used in siliconbased
microelectronics as the basic material for intercon
nects of integrated circuits (ICs) [1]. In GaAsbased
microelectronics, gold (Au) is traditionally used to
create interconnect metallization layers. Considering
that Cu possesses better electrophysical properties and
has a considerably lower cost, the development of the
Cubased metallization technology for GaAs mono
lithic microwave integrated circuits (MMICs) is of
high practical importance. Research in this direction
are of interest due to one of the basic trends in modern
microelectronics: heterointegration of semiconductor
materials. Using transistors based on various semicon
ductor materials (Si, SiGe, and
А
3
В
5
compound semi
conductors) in a single IC requires creating uniform
interconnects, where Cu is the optimal choice [1–5].
It is well known that there are certain problems in
using Cubased metallization for GaAs MMICs. In
GaAs, Cu is a fastdiffusing impurity of the
p
type
conductivity [6, 7]; it is easily oxidized [8] and actively
interacts with some substances traditionally employed
for manufacturing GaAs MMICs [9]. To prevent dif
fusion of Cu, layers of diffusion barriers (DBs) based
on refractory metals (
Та
, Mo, Pt, W, etc.) [1, 10–13]
and metal nitrides (TiN, TaN, and WN) [1, 14, 15] are
used. Positively,
Si
x
N
y
, which is compatible to Cu [1],
is employed in the GaAs MMIC technology as an
interlevel dielectric.
By analyzing the data presented in [9–16], it can be
concluded that only planar DBs are used in investiga
tions on developing Cubased metallization for GaAs
MMICs, while DBs on vertical (sidewall) surfaces of
copper conductors are not created. In [13], to form an
ohmic contact to the
p
+
layer of an InGaP/GaAs het
erobipolar transistor, a Pt/Ti/Pt/Cu thinfilm com
position was used, while interconnects were imple
mented based on Ti/Pt/Cu thin films. The thickness of
the planar Pt barrier layer was 60 nm. To protect the
copper conductor against oxidation, its surface was
passivated with a
Si
x
N
y
layer 40 nm thick. In [14], air
bridges for a GaAs pHEMT were formed by electro
chemical deposition of a Cu film. As a seed layer, the
WN
x
/Cu
thinfilm composition formed by sputtering
was used. In [15], thermal stability of the TaN/Cu/TaN
multilayer composition with two planar barriers was
investigated. All the three layers were formed through
sputtering (the second TaN layer was formed to pro
tect Cu against oxidation).
When manufacturing siliconbased ICs, the dama
scene process is used with the copper conductor being
surrounded all over its surface by layers of planar and
sidewall DBs. Conductive films (TiN/Ti and TaN/Ta)
and dielectric layers (in particular,
Si
3
N
4
) are used as
the barriers to ensure the reliability of ICs.
The fact that copper conductors in GaAs MMICs
have no sidewall DB considerably reduces their reli
ability and poses a problem of developing the technology
of their production. Sidewall DBs are difficult to form
Formation of CopperBased Interconnects for GaAs Monolithic
Microwave Integrated Circuits
S. V. Ishutkin
a
, V. A. Kagadey
b
, E. V. Erofeev
b
, and E. V. Anishchenko
b
a
Research Institute of Telecommunication Systems, Tomsk State University of Control Systems and Radioelectronics,
Tomsk, Russia
b
Micran Research and Production Company, Russia
email: ishsv@mail.ru
Received September 22, 2014
Abstract
—Technological solutions are presented for forming interconnect metallization of GaAs monolithic
microwave integrated circuits (MMICs) based on the W/Cu/WN
x
thinfilm composition. A peculiarity of
these solutions is that, for the first time, copper conductors with the surface completely surrounded by layers
of diffusion barriers are created using the liftoff method traditionally employed for producing GaAs MMICs.
Copper conductors 380 nm thick were deposited by Ebeam evaporation, while the layers of planar and side
wall diffusion barriers based on W and WN
x
were sputtered. For successful formation of the sidewall diffusion
barriers on the vertical surfaces of the copper conductors, the profile optimization of the photoresist mask was
carried out. Modes of removing the photoresist mask are presented that enable effective elimination of the
metallic “veils” formed on the vertical walls of the photoresist mask in the process of sputtering of diffusion
barriers. The interconnects W/Cu/WN
x
were integrated into the manufacturing process of GaAs microwave
lownoise amplifiers based on the pHEMT with 250 nm Tgates.
DOI:
10.1134/S1063739715030038
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