Martin P.M. Handbook of Deposition Technologies for Films and Coatings, Third Edition: Science, Applications and Technology
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Atomic Layer Deposition 381
Recently, different alkylsilyl compounds have been studied for tellurium and selenium
precursors. It has been demonstrated that metal chlorides react with (Et
3
Si)
2
Te and (Et
3
Si)
2
Se,
producing metal tellurides and selenides, respectively. For example, the Sb
2
Te
3
and GeTe
processes using SbCl
3
, GeCl
2
·C
4
H
8
O
2
and (Et
3
Si)
2
Te as precursors were combined to produce
GST films [29].
Surface-limiting growth can also be applied to the deposition of polymer thin films. This
polymer ALD is usually referred to as molecular layer deposition (MLD) [70]. For example,
the surface-limiting growth of polymer thin films has been demonstrated for poly(p-phenylene
terephthalamide) [71], polyimides [72], and Nylon 66 [73]. Since these depositions can be
carried out using similar coating tools to those used in conventional ALD, the preparation of
inorganic–organic hybrid materials and nanolaminates is straightforward [74].
8.5 Applications
The main industrial application of ALD is in the manufacture of semiconductor devices, both
memory devices and microprocessors. TFEL displays (Figure 8.12) have been manufactured
for over 20 years using ALD technology. The read heads of hard disk drives are enabled by
ALD. The silver jewelry industry has selected ALD coatings as a way of preventing tarnishing
of silver products. In addition to its established industrial uses, ALD is being investigated for a
large number of other production applications.
8.5.1 Displays
ALD technology was developed initially to enable TFEL display production. Production
started in Finland in the mid-1980s and several million TFEL displays have been produced to
Figure 8.12: Thin film electroluminescent (TFEL) displays. (Courtesy of Planar Systems.)
382 Chapter 8
Figure 8.13: P400 series batch mode (TFEL) manufacturing ALD systems. (Courtesy of Planar
Systems.)
date. In addition to the active dielectric–phosphor–dielectric stack (Figure 8.6), ALD
technology is used to deposit a pinhole-free ion barrier layer on a soda lime glass substrate and
as a passivation layer on top of the device structure [8]. Production is based on efficient
batch-mode ALD systems (Figure 8.13). Since the end of 1990s, ALD technology has been
used in Japan by Denso Corporation to produce transparent TFEL displays for automotive and
other applications (Figure 8.14).
The high quality and yield of ALD oxide materials over large areas, as well as high
conformality, are key advantages when considering and investigating ALD technology for
TFT (Thin Film Electroluminescent) liquid crystal display (LCD) applications [75], as well as
for microelectromechanical (MEMS)-based display applications [76].
Several research centers are investigating the use of an ALD barrier and passivation layers in
the development of flexible organic light-emitting device (OLED) displays. ALD films enhance
the barrier properties of plastic substrates at thickness levels of only tens of nanometers
[9, 77]. ALD coatings can also be used to enhance the properties of barriers made with other
coating methods, such as plasma-enhanced chemical vapor deposition (PECVD) [78].
Atomic Layer Deposition 383
Figure 8.14: Transparent car dashboard TFEL displays for automotive applications. (Courtesy of
Denso Corporation.)
8.5.2 Integrated Circuits
Because of the high precision of ultrathin films, and the low defect density and extreme
conformality of ALD films, ALD has been adapted by semiconductor companies in the
manufacture of IC products. Development work on semiconductor memory and
microprocessor applications started in the 1990s, and ALD is now an established thin film
coating technology in the semiconductor industry. The development of new material solutions
for semiconductor applications is a demanding task. In addition to the materials themselves,
surface pretreatments, interfaces, impurities, and postprocessing conditions all have roles.
ALD is a powerful technology for this development work, as surface chemistry has an
important role in the selection and optimization of materials, processes, and process conditions
for electronic devices [79].
The high conformality of ALD films has been one of the main drivers to investigate and adapt
ALD for semiconductor memory applications. Cylinder/stack architecture or deep trenches,
with aspect ratios of 50:1 or more, are easily coated with ALD dielectrics [6, 80]. ALD has
now been used for dynamic random access memory (DRAM) production since 2005, and is
enabling further scaling down to smaller feature size and trenches with a 100:1 aspect
ratio [14].
384 Chapter 8
Scaling down of the thickness of SiO
2
-based gate oxides and feature sizes has enabled the
semiconductor industry to follow Moore’s law, that the number of transistors on a chip will
double about every two years. Semiconductor industry has kept this pace for nearly 40 years.
In the 1990s semiconductor companies started the search for new materials and processes to
replace the SiO
2
-based gate oxide with a high-k material [12, 13]. ALD technology was also
introduced in the Sematech roadmap for the first time in 1999 [81]. After extensive work and
evaluation of several different materials and material combinations, Hf-oxide-based gate
oxides made with ALD were introduced into production for the first time, and products at
45 nm node were produced by Intel at the end of 2007 [15, 16].
Production ALD systems for front-end semiconductor manufacture are typically either
300 mm single-wafer systems (Figure 8.15) or 300 mm wafer semi-batch systems.
In addition to the mainstream use of ALD for memory cell and transistor manufacturing, ALD
is being widely investigated for back-end processes. Examples are ALD-based barrier, e.g.
TaN, and copper seed layer, e.g. Ru, materials to meet the requirements of shrinking
Figure 8.15: Pulsar 3000, a 300 mm single-wafer ALD production tool for manufacturing of
semiconductor devices. (Courtesy of ASM International.)
Atomic Layer Deposition 385
interconnection dimensions [82]. An additional challenge is matching of the barrier and seed
layer processes with the low-k processes and materials.
The semiconductor industry recognized the importance of ALD technology for semiconductor
applications at Semicon Europe 2004, when the European SEMI Award 2004 was awarded to
Dr Tuomo Suntola for the development of ALD technology for semiconductor applications.
8.5.3 Hard Disk Drives
Smaller critical dimensions and more challenging topographies of magnetic recording heads
have led to a need to replace conventional physical vapor deposition (PVD) coating technology
with a more conformal coating method. ALD was selected as a new method to enable further
scaling down. An additional benefit of ALD was the high quality of ultrathin insulating layers
with low pinhole density [17, 83]. ALD AlOx has been used in read heads for several years,
and ALD thin films and nanolaminates are being investigated for other applications in the
read/write head process flow, such as trench filling, tunneling barriers, and encapsulation.
8.5.4 Functional and Protective Coatings on Parts
The use of ALD technology for functional and protective coatings on parts is still for the most
part in an early phase of investigations. Possible applications range from chemical- and
corrosion-resistant coatings to the enhancement of the mechanical properties of parts.
Examples of enhancements of mechanical properties investigated for MEMS applications are
solid lubricants [84] and tribological coatings [85]. The high surface conformality of ALD
enables reliable coating of complex parts as well, including the inside of tubes and cavities.
One of the production uses of protective ALD coatings on parts is the nSILVER
®
ALD barrier
layer, which prevents tarnishing of silver objects, such as jewelry, collectible coins, and special
mirrors. Tarnishing of silver is mostly caused by airborne sulfur, which reacts with the surface
of a silver object and generates black silver sulfide. A thin, transparent, and pinhole-free
nSILVER ALD coating provides sufficient protection against tarnishing. The high surface
conformality of ALD coatings enables cost-efficient coating of a large number of parts using
simple processing chamber and rack configurations (Figure 8.16).
8.5.5 Photovoltaics
ALD technology and coatings provide potential for conversion and cost-efficiency
improvements in multiple photovoltaic applications. The use of ALD coatings has been
investigated for several photovoltaic technologies, including crystalline silicon solar cells, thin
film solar cells, and organic solar cells.
386 Chapter 8
Figure 8.16: Loading of silver jewelry into ALD processing chamber for nSILVER
®
coating.
(Courtesy of Lapponia Jewelry.)
ALD coatings for back-surface passivation of thin silicon solar cell wafers have been
investigated and the initial results have been encouraging [86, 87]. ALD Al
2
O
3
appears to be
an ideal candidate for high-quality surface passivation of p-type silicon and solar cells
application, owing to the negative fixed charge at the interface with silicon. This leads to low
surface recombination velocities, with a potential for conversion efficiency improvement.
Thin film solar cells are another field of active development work using ALD technology as an
option for efficiency improvements. ALD-based In
2
S
3
[88], Zn(O,S), and (Zn,Mg)O [89] have
been investigated as a buffer layer in Cu(In,Ga)Se
2
thin film solar cells.
ALD has been used to deposit blocking layers by atmospheric pressure systems to reduce
recombination reactions in flexible dye-sensitized solar cells (DSSCs) [90]. The encapsulation
of flexible organic solar cells presents similar challenges to the encapsulation of flexible
Atomic Layer Deposition 387
OLED displays. Pinhole-free ALD coatings have been considered and investigated as one
potential solution [91].
8.5.6 Optical Components
The benefits of ALD – high conformality, precision, and repeatability – are valuable in optical
applications [92, 93]. The optical quality of several ALD films is high, and ALD enables new
artificial optical materials to be developed by combining two or more different materials in
nanoscale (Figure 8.7), for new product opportunities. Examples of ALD materials for optical
coatings are SiO
2
,Al
2
O
3
,TiO
2
, ZnO, ZnO:Al, ZrO
2
, HfO
2
,Ta
2
O
5
, ZnS, and Al
2
O
3
:Er.
ALD can be used to make high-quality interference filters, e.g. infrared cut-off filters, and
dielectric mirrors, e.g. heat reflectors, on highly curved and structured surfaces. ALD also
provides a pinhole-free protective layer for the part to be coated. In situ double-sided coating
of flat substrates is also straightforward with ALD.
One ideal ALD application is trench filling and planarization of diffractive optical parts with
conformal ALD coating [7]. The mechanical durability of an optical element with trenches
could be enhanced considerably, while maintaining the optical and functional quality. The
conformality and precision of ALD coatings can also be utilized for making optical micro- and
nano-sized lens arrays with a high fill factor, through conformal growth of dielectric
monolayers on to prepatterned templates using ALD [94]. In addition, it is possible to coat the
surfaces of tubular objects with highly uniform films using ALD. Owing to the high surface
conformality, ALD is a powerful technology for the development of photonic crystals [95].
Doped waveguides have also been investigated using ALD technology [96, 97].
8.5.7 Other Applications
MEMS devices are a natural application for highly conformal, functional ALD coatings
[84, 85]. ALD can also be used to make or modify yttria-stabilized zirconia (YSZ) and other
layers for solid oxide fuel cells [98, 99] and to make fuel cell catalysts [100]. ALD has been
used to develop catalyst supports for producing new families of catalytic materials by coating
porous silica and alumina structures with ALD [101]. Novel sunblocking materials can be
fabricated by coating ultraviolet-absorbing films, e.g. ZnO, using ALD technology on primary
particles used by the cosmetics industry [102]. ALD coatings have also been used to form
homogeneously dispersed nano-sized titanium dioxide (TiO
2
) particles within a polymer
matrix, to replace traditionally melt-compounded nanofillers [103]. The ALD technique has
also been introduced for improving the crack resistance of glass. Suppressing the crack
resistance is most likely to be based on the filling of Griffith-like flaws by 10–20 nm thick
Al
2
O
3
or SiO
2
[104].
388 Chapter 8
Since ALD produces highly conformal thin films it can also be utilized quite straightforwardly
for tailoring surface properties in biotechnology applications. Interactions between biological
and inorganic interfaces are surface-related phenomena, so ALD is well suited for making
these thin surface modifications. Typically tens of nanometers are enough to alter the surface
properties. Deposition of photocatalytic TiO
2
films has been studied by ALD. Typically,
activation of TiO
2
is carried out by ultraviolet light [105–107] but careful doping by H
2
S
activation can also be carried out by visible light [108]. Recently, ALD-deposited Al
2
O
3
[109],
TiNOx [110], and hydroxyapatite-like films [111] have been studied for biomedical
interactions on different surfaces.
8.6 Summary
ALD technology was developed over 30 years ago, and it has been used for the industrial
production of TFEL displays for more than 20 years. Since the semiconductor industry
adapted ALD as a mainstream technology during the past few years, ALD has become clearly
visible and now ALD technology is being developed and adapted for a large number of other
industrial applications, ranging from nanotechnology coatings on particles to large-area
coatings for photovoltaic applications. At the same time ALD material and process research
continues actively throughout the world, providing new capabilities for the industry.
Acknowledgments
The authors thank Mr Jarmo Maula and Mr Jarmo Skarp for their discussions and
contributions.
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