COL 11(10), S10214(2013) CHINESE OPTICS LETTERS June 30, 2013
SiO
x
protective film thermally evaporated
downwards for 2-m level primary mirror
Wenjun Pei (
©©©
ddd
)
∗
and Hongxiang Liu (
444
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)
∗∗
Institute of Optics and Electronics, Chinese Academy of Sciences, Chengdu 610209, China
∗
Corresponding author: Peiwenjun1208@163.com;
∗∗
corresponding author: hxliu123@yahoo.com.cn
Received November 29, 2012; accepted January 8, 2013; posted online May 21, 2013
The service life of the large primary mirror with aluminum coating can be effectively prolonged by a
protective layer. The SiO
x
(1<x<2) protective material which is thermally evaporated from up to bottom is
studied. Environmental adaptability experiment, spectral measurement and micro-morphology analysis are
executed on bare SiO
x
coatings within different oxygen concentrations, and the repeatability verification is
implemented by testing the SiO
x
protected aluminum coatings. The results show that the SiO
x
coatings can
meet the protective qualification with in oxygen flow of 50 sccm (p= 5.6×10
−3
Pa). The fine compactness of
the coating has excellent moisture resistance with the average molecular spacing of 0.335 nm. In addition,
the average reflectivity of the SiO
x
protected aluminum coating is 90.47% in the band 400 —2 500 nm and
the coating is confirmed to have good environmental performance.
OCIS codes: 310.1515, 310.6870.
doi: 10.3788/COL201311.S10214.
Abo ut 5–10 nm alumina film which makes the reflective
coating a certain resistance to damage will be formed on
the surface of bare aluminum reflective coating in the
air
[1]
. However, in practical applications, this naturally
formed alumina protective film is fragile. In order to en-
hance the damage resistance and corr osion resistance to
extend the aluminum coating’s service life (generally 1–3
years
[2]
), the protective film with transparent and firm
performance is needful. More freq uent use of the pro-
tective materia ls are SiO, SiO
x
(SiO oxidation, 1<x<2),
and SiO
[3]
2
.
The 2 -m le vel primary mirror is mainly used in the
sp e ctral band of 400–2 500 nm. In the whole process
of the preparation, the mirror temperature maintains
at around room temperature. In view of the deforma-
tion and evaporation security, the mirror is placed in
the bottom of the vacuum chamber, being hold by 18
points flexible support, and the evap oration sources are
located above, evaporating from top to bo ttom. In ac-
cordance with these technical r e quirements, we s e lect the
SiO
x
(1<x<2) as the protective material and bring to
further study relying on the nation’s largest bell-type
high-vacuum coater.
The coater is insta lled in the peak of 3 200 m above
sea level near Lijiang C ity, and the ultimate vacuum can
meet 4.0×10
−5
Pa. The high vacuum sys tem equipped
with two Leybold COOVAL1800 cryogenic pumps is di-
vided into two parallel branches to exhaust. This design
not o nly can improve the pumping speed, obtaining the
desired evaporation pressure as s oon as possible (two
hours to reach 4.0×10
−4
Pa), but also can ensure the
reliability of equipment operation—when o ne branch has
problems, the other branch still meets the work demands.
The material of SiO
x
(1<x<2) is generated from the in-
sufficiently oxidized SiO, and it displays the performance
characteristics of SiO or SiO
2
subject to the deg ree of
oxidation. The SiO which has the sublimation physi-
cal pro perty can ensure the security when evaporating
from top to bottom. Evaporation sources are special
molybdenum evaporation boats, sealed by the three-tier
structure, and the above two holes with lids are con-
nected to two material stor e rooms, one hole below is
the material exit portal (Fig. 1). The whole evaporation
system consists of three evaporation boats, which are
distributed with the c e ntral ang le of 120
◦
in the same
ring, s e parately controlled by thre e transfor mers
[4,5]
.
The vacuum chamber diameter is 3 205 mm, and the
evap oration boats can be place d on the range of 0–2600
mm (without considering the influence of the glow dis-
charge device) as well as the vertical removable region
of 1 000—1 250 nm (net of about 280 mm from the mir-
ror surface to the support base yet). By means of the
simulation
[6]
, we have learned that the non-uniform can
meet the minimum value at the height of 1 250 mm and
the diameter of 2 120 mm. And all the results suggest
that the non-uniformity of the film is less than 17%
[4]
.
Fig. 1. SiO
x
evaporation sources.
Fig. 2. (Color online) Transmittance of the SiO
x
(1<x<2)
films with different oxygen flows.
1671-7694/2013/S10214(4) S10214-1
c
2013 Chinese Optics Letters