April 30, 2010 / Vol. 8, Supplement / CHINESE OPTICS LETTERS 53
Atomic layer deposition for industrial optical coatings
Jarmo Maula
Beneq Oy, Vantaa, Finland
E-mail: jarmo.maula@beneq.com
Received October 31, 2009
Atomic layer deposition (ALD) is under active research for it s many emerging applications. In the op-
tical field, ALD opens fundamentally new research paths, providing extreme conformality and capacity
to engineer new materials. ALD enables coatings which are undoable or difficult through physical vapor
deposition (PVD), like films inside tubes, highly repeatable sub-nanometer thick films, double-sided de-
position, large-area accurate coatings, and th ree-dimensional coatings that req uire extreme conformality.
We describe ALD from the practical and manufacturing viewpoint.
OCIS codes: 310.1860, 160.1245, 160.4236, 350.4238, 220.4241.
doi: 10.3788/COL201008S1.0053.
1. Introduction
Atomic layer deposition (ALD) is often used in nan-
otechnology and semiconductor applications. In these
applications, the films are often thin, and the deposi-
tion is done by single wafer or cluster tools. This may
be initially unattractive for people searching for optical
coating solutions. ALD was invented to solve the in-
dustrial problem of producing good ZnS films for thin
film electroluminescent (TFEL) displays
[1]
. In fact, the
origins of ALD are found in batch processing for films
with thickness over 1 µm, which fits well with industrial
optical a pplications. The production of TFEL displays
started in the mid-1980s and is currently operated in
Finland by Planar Systems, Inc.
[2]
Since then, TFEL has
been the main ALD user for 25 years. The semiconductor
industry found ALD during the 1990s. Intel started ALD
production for gate oxides in December of 2007. Now,
the state-of-the-art processors, memories, and hard disk
drives use ALD.
ALD is not directly competing with other coating tech-
nologies. ALD is an enabling technology that provides
coating and material features which are not pos sible us-
ing other technologies. Coatings in batches, large areas,
complex three-dimensio nal (3D) parts, double-sided, new
coating materials, and material structures ar e the main
uses of ALD in the optical field. Beyond the semiconduc-
tor industry, ALD is not yet very widely used, enabling
good patenting possibilities and making unauthorized
process duplication difficult.
The driving forces for the development of ALD optical
applications can be roughly categorized as fo llows: 1)
coating of objects that are difficult or impossible using
other methods; 2) engineering of novel optical materials;
3) auxiliary process to provide or improve the crucial fea-
tures in film stacks; 4) coating by batches and of large
areas becaus e the ALD process is highly repeatable.
2. ALD in short
ALD is a chemical vapor depositio n (CVD) method
using surface reactions. Deposition typically uses two
chemicals, called pr ecursors, meeting only at the s urface.
And it is surface controlled, self-limiting, confor mal, and
pinhole fr ee.
The typical ALD sequence (see Fig. 1) starts as the
AlCl
3
dose arrives and chemisorbed in molecules on the
surface, filling all the surface sites; then the purge phase
removes the AlCl
3
vap or; subsequently the H
2
O dose
arrives and oxidizes AlCl
3
, resulting in a solid Al
2
O
3
film and a by-product which is derived in the form of
HCl gas. The final purge phase removes both the H
2
O
and HCl vapor. To make more layers, the sequence is
repeated with the same or other precursors. The layer by
layer implies tha t the e xcessive dosing of AlCl
3
or H
2
O
does not and cannot increase the deposition rate. This
self-limiting featur e also provides extreme repeatability.
2.1 Features of ALD film growth
The following are the features of the ALD film growth
as follows. 1) Chemisorptions: Precursors make strong
chemical bonding. However, the temperature should be
high enough to enable reaction be tween pr e cursors and
low enough to prevent precursors decompositio n. 2) Sat-
uration: Self-limiting reactions eliminate the need for
precise dosing. Small precursor overdosing guarantees
that a ll surface sites are filled. 3) Surface controlled
reactions: Deposition reactions on the surface enable ex -
treme conformality into deep trenches and 3D structures.
All surface sites need to be exposed for precursors, and
this may affect the process design. 4) Sequential: High
repeatability and digital- like growth provide high accu-
racy. Also, sufficient purging is needed between pulses,
and good flow dynamics is required to ensure rapid gas
changes.
2.2 Typical deposition conditions
For successful atomic layer deposition, certain condi-
tions must concur 1 ) Pressure is in the range of 0.1−10
Fig. 1. Typical ALD deposition sequence.
1671-7694/2010/S10053-06
c
2010 Chinese Opt ics Letters