1063-7397/04/3301- © 2004 MAIK “Nauka /Interperiodica”
Russian Microelectronics, Vol. 33, No. 1, 2004, pp. 7–12. Translated from Mikroelektronika, Vol. 33, No. 1, 2004, pp. 10–16.
Original Russian Text Copyright © 2004 by Smolin.
Aluminum is a major material for interconnections
in hybrid and monolithic integrated circuits (ICs). It
offers a resistivity as small as about
good adhesiveness to oxide insulators, ohmic contact to
silicon, a melting point as low as 933 K, high plasticity,
resistance to air oxidation and to radiation, etc. It is held
to be the most promising material for multilevel metal-
or low-permittivity dielectrics in
very-large-scale-integration (VLSI) circuits of mini-
mum feature size 0.25–0.35
m [1, 2].
The use of aluminum and its alloys in metallization
has been extensively studied and reviewed since the
1970s. The main problems that are still faced by alumi-
num as a wiring material are as follows :
(i) It lends itself to electroplating with difﬁculty.
(ii) It is susceptible to electromigration.
(iii) It is prone to galvanic corrosion.
(iv) It recrystallizes at fairly low temperatures, in
which case the ﬁlm develops hillocks.
(v) It is highly reactive with
at about 773 K.
MASS TRANSFER IN ALUMINUM THIN FILMS
At 673–773 K the diffusion coefﬁcient of silicon in
aluminum thin ﬁlms is an order of magnitude larger
than that in bulk aluminum, due to grain-boundary dif-
fusion. Dissolution of silicon is hindered by the native
oxide of aluminum .
Electromigration in aluminum thin ﬁlms has been
the subject of many studies [5, 6]. It was found that the
activation energy is 0.5–0.6 eV for temperatures up to
623 K; this indicates that the transport is by grain-
boundary diffusion, as the bulk diffusion has an activa-
tion energy of 1.4 eV.
Resistance to electromigration is enhanced by the
(i) using a coarsely crystalline material (for exam-
ple, ﬁlms of grain size 8
m show an endurance an
order of magnitude longer than those with a grain size
(ii) varying the temperature and deposition rate dur-
ing the process;
(iii) using multistep deposition, with oxide ﬁlms
formed at the interfaces;
(iv) passivation of the ﬁlm with a porous or contin-
uous oxide or with another dielectric to inhibit mass
(v) changing to a precision aluminum alloy contain-
ing copper or silicon.
The last method may increase the endurance by two
orders of magnitude. On the other hand, raising the Si
or Cu content by one percent increases the resistivity by
cm, respectively. The best
results are shown by materials with 0.5% Cu if grain
boundaries only cross the conducting track (bamboo
Under thermal treatment, aluminum ﬁlms often
develop hillocks due to diffusion at the ﬁlm–substrate
interface: near-surface grains are expelled by the result-
ant compressive stress as toothpaste is squeezed out of
the tube . The hillocks may short-circuit the inter-
level insulator or damage the contact photomask
Thus, the mass-transfer problem is best solved by
changing from pure aluminum to an alloy.
FUNDAMENTALS OF ALUMINUM
Aluminum ﬁlms are mostly deposited by physical
vapor deposition (PVD). These approaches fall into two
general classes: vacuum evaporation and plasma sput-
tering. In the former, electrical power is converted to
heat and this in turn changes into the mechanical energy
of particles incident on the substrate. In plasma sputter-
ing, electrical power is directly converted to the
mechanical energy; hence a higher degree of control
Vacuum evaporation using resistor heating was the
ﬁrst widely adopted method. However, it was soon
abandoned because the heater contaminated the ﬁlm
and the evaporation process was affected by decrease in
the mass of the material.
Making Al Metallization Patterns
V. K. Smolin
Research Institute of Measuring Systems, Nizhni Novgorod, Russia
Received November 11, 2002
—A retrospective overview is given of approaches to the deposition, modiﬁcation, and etching of alu-
minum layers in microelectronics.