Martin P.M. Handbook of Deposition Technologies for Films and Coatings, Third Edition: Science, Applications and Technology
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Chemical Vapor Deposition 361
7.11 Outlook
CVD offers many advantages in thin film deposition and has given birth to a wide variety of
related technologies. With the use of new precursors and plasma enhancement deposition
temperature can be lowered considerably. By lowering total pressure, extremely sharp
interfaces with respect to chemical composition and topography can be obtained. Atmospheric
pressure CVD is attractive for many applications with its high deposition rates and resulting
short process times. Since CVD processes are based on interfacial chemistry, they are sensitive
to contamination and loadlock systems must be used to keep the contamination level low.
Selective deposition opens fascinating prospects for the future, not only for microelectronic
applications but also for materials science in general, and for engineering of interfaces and
artificial materials.
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Chemical Vapor Deposition 363
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CHAPTER 8
Atomic Layer Deposition
Arto Pakkala and Matti Putkonen
Beneq Oy, Vantaa, Finland
8.1 Introduction and Background 364
8.2 Principle of Atomic Layer Deposition 365
8.3 Advantages of Atomic Layer Deposition 366
8.3.1 Conformality 366
8.3.2 Pinhole-free Coatings 369
8.3.3 Repeatability of Coatings 369
8.3.4 Scalability 370
8.3.5 Ultrathin Films 371
8.3.6 Artificial Materials 374
8.4 Atomic Layer Deposition Precursors, Processes, and Materials 375
8.4.1 ALD Precursors in General 376
8.4.2 Non-Metal Precursors 376
8.4.3 Metal Precursors 377
8.4.4 Novel ALD Processes 379
8.5 Applications 381
8.5.1 Displays 381
8.5.2 Integrated Circuits 383
8.5.3 Hard Disk Drives 385
8.5.4 Functional and Protective Coatings on Parts 385
8.5.5 Photovoltaics 385
8.5.6 Optical Components 387
8.5.7 Other Applications 387
8.6 Summary 388
8.1 Introduction and Background
Atomic layer deposition (ALD) is a chemical gas phase thin film deposition method based on
sequential, self-saturating surface reactions [1–5]. Two or more precursor chemicals, each
containing different elements of the materials being deposited, are introduced to the substrate
surface separately, one at a time. Each precursor saturates the surface forming a monolayer of
material. Most of the commercial ALD deposition systems are based on using inert carrier gas,
Copyright © 2010 Peter M. Martin. Published by Elsevier Inc.
All rights reserved.
364
Atomic Layer Deposition 365
e.g. nitrogen, at about 1 mbar. Short precursor chemical pulses are injected into the flowing
carrier gas, separated by short purge or evacuation periods. A defined amount, typically a
monolayer of about 1
˚
A, of the thin film material is grown during each ALD cycle (pulse of
precursor 1 – purge – pulse of precursor 2 – purge). Typical cycle times are from one second to
several seconds. Process temperatures range from room temperature to over 500
◦
C. Most
ALD processes operate between 200
◦
C and 400
◦
C. ALD processes have been developed for
oxides, nitrides, carbides, fluorides, certain metals, II–VI and III–V compounds, and recently
also for the first organic materials.
The ALD method was developed for commercial use in Finland in the mid-1970s by Dr
Tuomo Suntola and co-workers [1–3]. ALD was initially developed to enable production of
thin film electroluminescent (TFEL) flat panel displays, and is currently also used for multiple
other industrial applications, including semiconductor device manufacturing. In addition,
ALD-related research is conducted in a large number of universities, research centers, and
companies.
8.2 Principle of Atomic Layer Deposition
ALD thin film deposition method utilizes highly reactive precursor chemicals, each containing
different elements of the material being deposited, by introducing each precursor chemical
separately into the reaction zone. In this way uncontrolled gas-phase reactions are prevented,
and film growth takes place through controlled and self-saturating surface reactions. This film
growth mechanism leads to extremely conformal and uniform material layers. In addition, film
growth is quite insensitive to process parameters other than choice of the precursor chemicals
and process temperature.
One ALD cycle is illustrated in Figure 8.1. In this example Al
2
O
3
is grown from AlCl
3
and
H
2
O. First, a short pulse of AlCl
3
is introduced into the flowing carrier gas and an
Al-containing monolayer is formed on the surface through a self-terminating reaction. Excess
AlCl
3
is purged away during a short purge period, when only the inert carrier gas is flowing.
H
2
O is introduced next and it reacts with the Al-containing monolayer on the surface forming
a monolayer of Al
2
O
3
. Finally, the ALD cycle is completed with a second purge period to
remove the excess H
2
O and the reaction by-products. The typical film growth rate is about 1
˚
A
Figure 8.1: Schematic illustration of one ALD cycle.
366 Chapter 8
Table 8.1: Advantages of ALD
Extreme surface conformality Unique conformality; for nanotubes, nanowires, particles,
diffractive optics, DRAM, 3D parts
Pinhole-free films ‘Bottom up’ growth of ALD is naturally pinhole free;
excellent for passivation, barriers, and insulators
Repeatability Robust in production; without in situ feedback
Scalability Processes based on robust ALD precursor chemistries
straightforward to scale up for production
Ultrathin, dense, smooth films One atomic layer at a time; gate oxides, tunneling
insulators, e.g. for hard disc drives
Artificial materials Digital control of ALD provides a way to create artificial
materials; enables R&D innovations
per cycle, somewhat less than a full monolayer, primarily owing to ‘shadowing’ of the ligands
of the precursor chemicals. The desired film thickness is achieved by repeating the ALD cycle
digitally. Typical cycle times range from less than one second to several seconds, depending on
the ALD system and process design. Film thicknesses range typically from nanometers to
several micrometers.
8.3 Advantages of Atomic Layer Deposition
The advantages of ALD, listed in Table 8.1, are based on the surface-controlled and
self-saturating film growth principle. This list provides guidance when coating method options
are selected for a specific application. When one or more of these advantages are important for
the application, ALD is a viable candidate to provide the coating solution, from research to
industrial-scale implementation.
8.3.1 Conformality
Extreme surface conformality is a unique benefit of ALD technology. The conformality of
various thin film coating methods is illustrated in Figure 8.2. The surface-controlled film
Figure 8.2: Comparison of thin film coating methods.
Atomic Layer Deposition 367
Figure 8.3: High-resolution SEM images of a cross-section of a deep trench with 20 nm HfO
2
deposited by a (CpMe)
2
HfMe
2
/O
3
process at 450
C. Trench aspect ratio is 50:1 with 6.2 m
depth and 120 nm opening. The upper images show the top part of the trenches, those below
show the middle and the bottom at a depth of 6.2 m [6].
growth of ALD provides highly uniform coatings over surfaces of three-dimensional (3D)
parts, vias, deep trenches, porous materials, and particles. The use of ALD to coat the
trenches of semiconductor memory devices is one example (Figure 8.3) [6]. ALD is also
ideally suited for trench filling planarization coatings, for example on diffractive optics parts
(Figure 8.4) [7].
368 Chapter 8
Figure 8.4: Trench filling of optical parts with conformal ALD coating [7].
Atomic Layer Deposition 369
Figure 8.5: Water vapor transmission rate of ALD Al
2
O
3
vs film thickness [9].
8.3.2 Pinhole-free Coatings
As a result of the ‘bottom–up’ film growth mechanism most ALD coatings are naturally
pinhole free. This feature is valuable in barrier and passivation applications. ALD also enables
high-quality, high-yield insulators over large areas. The TFEL displays are high electrical field
devices and ALD-based insulators have been a key factor in the high reliability of the products
and for the high production yield [2, 8]. In addition, ALD technology is widely investigated
as an option for oxygen and humidity barriers for organic and flexible electronics devices
(Figure 8.5). Major improvements in the barrier properties are already achieved in the 10 nm
film thickness range [9].
8.3.3 Repeatability of Coatings
High repeatability of ALD processes is based on the surface-controlled and self-saturating film
growth mechanism. Selection of robust and proven ALD precursor chemistries is also a key
factor for production repeatability and precision. Most of the ALD processes are operated in
production without in situ control or tuning, and the processes run automatically, without the
need for continuous operator attendance.
ALD technology enables a powerful platform to evaluate and optimize materials and material
combinations to provide industrial coating solutions. Development of a challenging, new
coating solution usually takes some time, and the end result is often a tailored and optimized
stack of materials. After verification of the solution, it is, however, quite straightforward to
scale up and implement the development result in production, owing to the high repeatability
of ALD.
370 Chapter 8
Figure 8.6: Cross-section of TFEL display structure. (Courtesy of Planar Systems and Beneq Oy.)
As an example, the TFEL display structure (Figure 8.6) is a stack of two nanolaminate
insulators (Al
2
O
3
/TiO
2
) and the phosphor layer (ZnS:Mn). Precision of individual layers is in
the nanometer range, and the total thickness of the stack is over 1 m. The stack is processed
in batch mode and in single pump-down, using six precursor chemicals (AlCl
3
, TiCl
4
,H
2
O,
ZnCl
2
, MnCl
2
, and H
2
S), and the processes are operated automatically without operator
attendance (see Figure 8.13). Production of these TFEL displays began in the mid-1980s and
is currently operated in Finland by Planar Systems Inc.
Modified TiO
2
is another example of the high precision and repeatability of ALD technology.
The high index of TiO
2
with very low absorption is achieved by modifying crystalline TiO
2
with subnanometer-scale intermediate layers [10] (Figure 8.7). The high repeatability of ALD
enables reliable processing of this new material on an industrial scale, without in situ
control.
8.3.4 Scalability
The surface-controlled and self-saturating film growth mechanism, and selection of robust and
proven ALD precursor chemistries, enable efficient scalability of the ALD processes. The first
flow type ALD system was based on a 100 mm main vacuum tube [3, 4]. The first production
ALD systems were based on a 250 mm main vacuum tube, followed by a 400 mm tube batch
version, and an 800 mm tube version, P800, for large batches and large parts (Figure 8.8). The
400 mm tube P400 series batch ALD systems have been the workhorses of the TFEL display