78 CHINESE OPTICS LETTERS / Vol. 8, Supplement / April 30, 2010
Effect of process conditions on the abrasion resistanc e of
broadband AR films prepared by electron-beam evaporation
Hua Shen (
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1
, Rihong Zhu (
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1∗
, Qing Wang (
)
1
, and Linhua Xu (
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2
1
School of Electronic Engineering and Photo-electric Technology, Nanjing University of Science and Technology,
Nanjing 210094, China
2
School of Science, Nanjing University of Science and Technology, Nanjing 210094, China
∗
E-mail: edward bayun@163.com
Received Novemb er 25, 2009
Until now, there are few rep orts on the effect of process conditions on abrasion resistance, which is the
most important mechanical property of optical films. Broadband antireflective (AR) films composed of
SiO
2
and TiO
2
, whose bands are from 620 to 860 nm and whose reflectivity is less than 0.5%, are prepared
by electron-beam evaporation (EBE) at different temperatures, ion beam energies, and cooldown times.
The structural properties of the films are investigated by atomic force microscopy, including the surface
roughness, crystallinity, shape, u niformity, and compactness of the grain. The abrasion resistance of
the samples is tested according to MIL-C-48497A4.5.5.1 standard. We discuss the relationship between
abrasion resistance and the structural properties produced under different process conditions, such as
preparation temperature, energy of the ion beam, and cooldown time. Grain shape and surface roughness
are indicated to codetermine the abrasion resistance of the film. Further, the AR film with triangular grain
and moderate roughness shows good abrasion resistance.
OCIS codes: 240.0310.
doi: 10.3788/COL201008S1.0078.
Antireflective (AR) films are used mostly in all kinds of
optical films, especially the broadband AR film, which is
the most important because transmissive optical compo-
nents are in great demand. With the advent of optical
high technology, which is evident in inertial confinement
fusion and the development o f the huge astronomical
telescope, a s well as the need to s atisfy the stringent de-
mands of using the optical system for a long time in the
field, it is necessary to pay attention to enhancing the
mechanical properties of films, including abrasion resis-
tance. In recent years, many scholars have researched on
the improvement of the abrasion resistance of films
[1−10]
.
However, most papers focused on superhard films such as
TiN and TiC
[1,2]
or on the improvement of preparation
methods to obtain single-layer films with high abrasion
resistance, such as ZrO
[3,4]
2
, TiO
[5]
2
, and MgF
[6]
2
.
At present, there are several methods to prepare broad-
band AR films, such as the electron- beam evaporation
(EBE) method, radio-frequency magnetron sputtering,
and sol-gel. Some scholars have examined the abrasion
resistance of multilayer AR films, the center of which
includes the sol-gel
[7]
and sputtering
[8]
. It is rar e to find
reports on the abrasion resistance of AR films prepared
by EBE, which is the main method in preparing AR
films in the coating industry. Ther e fore, it is necessary
to examine the effect of process conditions on the abra-
sion resistance of broadband AR films prepared by EB E.
In this letter, we focus on the effect of different temper-
atures, ion b eam energies, and cooldown times on the
abrasion resistance of broadband AR films composed of
SiO
2
and TiO
2
prepared by EBE with ion beam assis-
tance (IBA).
Four-layer broadband AR films composed of SiO
2
and
TiO
2
were grown on a one-side polishing K9 (1-mm
thickness) substrate by using the EBE method with IBA
(PMC90S, P rotech Korea Co. Ltd). The film’s band
was from 620 to 860 nm, and the reflectivity was less
than 0.5%. The pur ities of the deposited sources for
SiO
2
was 99.99% and for TiO
2
was 99.99%. Before the
deposition, TiO
2
was premelted in order to remove its
impure gas content, which could ensure the absence of
splashing and the steady rate of evaporation in the pro-
cess of deposition. Firstly, the K9 substrate was cleaned
by ultrasonic w ith acetone and alcohol, on which the
lying oxidation layer was removed by hydrofluoric acid,
then it was cleaned again by deionized water. Finally,
the substrate was dried in nitrogen gas ambient before
the deposition. The purpose was to make the multilayer
uniform. The substrate was placed on a uniformly ro-
tating (40 rpm) workpiece shelf whose distance from the
evaporation source was 1.5 m. In the process of deposi-
tion, the evaporation rates of TiO
2
(0.5 nm/s) and SiO
2
(1 nm/s) were controlled accura tely by a silicon crystal
oscillation monitor (IC5), and an O
+
2
ion beam with
high energy produced by O
2
(99.999 9% purity) and an
end-hall ion source assisted in the preparation.
The experiments were divided into three groups, and
the seven kinds of samples were finished. The first group
of samples was prepared at different temp e ratures, which
were 200, 300, and 400
◦
C. The else deposition conditions
were as follows: the chamber was evacuated to a base
pressure of 2.2×10
−2
Pa, the flow of O
2
was at a rate
of about 30 sccm, the anode voltage of the ion source
was 130 V, and the cool time after the deposition was
20 min. The second g roup of samples was prepared at
different anode voltages of the ion source, which were
120, 130, and 140 V. The else deposition conditions were
as follows: the chamber was evacuated to a base pr e s-
sure of 2.2×10
−2
Pa, the flow of O
2
was at a ra te about
30 sc cm, the deposition temper ature was 300
◦
C and the
1671-7694/2010/S10078-05
c
2010 Chinese Optics Letters