Qiu X.G. (Ed.) High Temperature Superconductors

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Deposition technologies, growth and properties of high-T

ﬁlms 11

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processes. In all cases, the actual process of deposition can be divided into three

steps which are sketched in Fig. 1.4:

1 The particle source can be a stoichiometric HTS compound (e.g. for sputtering

and PLD), an assembly of different components (e.g. for evaporation), a

mixture of organic molecules (e.g. for MOCVD) or a chemical solution (e.g.

for CSD or MOD).

2 The particle transport is initiated by phonons (evaporation), photons (PLD)

or ion bombardment (sputtering) in the case of PVD deposition. In these

processes the initial particle energy at the source, the scatter events between

particles and process gas, and the resulting energy of the particles impinging

on the substrates play an important role. This will be discussed in detail in the

section ‘Comparison of the different PVD techniques’, p. 18. For CVD, CSD

and MOD, evaporation and spray-, spin- or dip-coating deﬁne the particle

transport, respectively.

3 Finally, the nucleation, phase formation, and ﬁlm growth represents the most

important step of the deposition process, i.e., the actual formation of the HTS

ﬁlm. It strongly depends on all process parameters (e.g. substrate temperature,

process pressure, density of particles) and, obviously, on the conﬁguration of

particles (i.e., atoms, (organic) molecules, chemical solution).

Whereas processes (1) and (2) are reasonably well understood, the actual

nucleation, phase formation and ﬁlm growth is difﬁcult to analyse, theoretically

1.4 Schematic sketch of PVD, (MO)CVD and CSD/MOD technologies.

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as well as experimentally. Even simple parameters like the actual temperature at

the surface of the substrate are difﬁcult to determine. Therefore, this part of the

process is subject to theoretical models and experimental estimates. Nevertheless,

there is a reasonably good understanding of the basic principles.

In the following we will discuss the deposition in exactly these three different

steps, starting with the description of the different deposition processes used for

HTS thin ﬁlm deposition.

1.2.1 Physical vapour deposition techniques

Physical vapour deposition (PVD) involves purely physical processes, and

describes all methods to deposit ﬁlms via condensation of a vaporised form of the

material onto a surface. PVD deposition of HTS involves high-temperature

vacuum evaporation, pulsed lased deposition (PLD) or plasma sputter

bombardment (sputter technology).

Thermal co-evaporation and molecular beam epitaxy

The thermal evaporation is based on particle sources in the form of thermal boats

(typically resistively heated), e-guns, or Knudsen cells. As a consequence and in

contrast to sputter and PLD technologies, in evaporation or molecular beam

epitaxy (MBE) each material is supplied individually from metallic sources and

small gas pressures (especially of reactive components like oxygen) have to be

chosen.

In principle, this approach is more ﬂexible since the stoichiometry can be easily

changed. Examination of stoichiometric dependent effects of the superconducting

ﬁlm or the preparation of artiﬁcial superlattices seems to be easier with this

method. However, the difﬁculty lies in the fact that calibration and rate control is

rather complex and difﬁcult, especially since reactive gas (oxygen) is involved in

the deposition process of HTS material. Therefore, a very accurate rate control

for all components has to be guaranteed and the problem of low oxygen partial

pressures has to be solved.

First, by using active rate control utilising (collimated) quartz-crystal monitors

or atomic absorption monitors, metal stoichiometry control down to ~1%

repeatability seems to be achievable (Matijasevic et al., 1997). Second, the oxygen

background pressure at source should not exceed ~10

-4

mbar during deposition.

Therefore, reactive oxygen sources like atomic oxygen, ozone, or NO

can be

used to increase the oxidation efﬁciency and allow the formation of the

superconducting phase at a lower oxygen pressure. Alternatively, differentially

pumped evaporation devices have been developed that provide low pressure at the

source and high pressure at the substrate. This can among others, be established

by rotation of the substrate in a heater which is partially open facing the evaporation

sources (typically 650°C at 10

-5

mbar for YBCO) and partially closed forming an

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ﬁlms 13

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‘oxygen pocket’ with a 100 times higher oxygen partial pressure (Utz et al., 1997)

(see Fig. 1.5). Thus, in the closed part of the heater the oxygen partial pressure is

increased providing the oxygenation of each deposited layer. This method has

lately been adopted for the growth of high-quality large-area MgB

thin ﬁlms

≈ 38–39K) using the ‘heater pocket’ for the Mg-incorporation (Moeckly and

Ruby, 2006).

Alternatively, both problems are solved in differentially pumped MBE systems.

For instance, via atomic layer-by-layer MBE a variety of heterostructures based

on BSCCO compounds also including DyBaCuO, LaSrCuO and several titanates

have been prepared (Bozovic et al., 1994), ultrathin ﬁlms of La

2 – x

CuO

4 + y

have been grown via MBE (Jaccard et al., 1994), and novel superconductors have

been designed (Logvenov and Bozovic, 2008).

1.5 Schematic sketch of a thermal co-evaporation system (Utz et al.,

1997). An oxidation pocket encloses part of the heater. The substrate

is rotated continuously through the pocket, enabling intermittent

deposition and oxygenation and providing better uniformity during

evaporation.

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Pulsed laser deposition technique and Laser MBE

The PLD technology represents a special type of evaporation. Due to its ﬂexibility,

it has become an important technique to fabricate layers of novel compounds. It is

used for the deposition of HTS, ferroelectric and ferromagnetic oxide materials.

Film deposition by PLD is based on the irradiation of a single target by a focused

laser beam (e.g. excimer (

= 308 nm, 248 nm) or Nd:YAG (355 nm); energy

density 1–3 J/cm

/shot; frequency of several Hz). The laser beam removes

material from the target and this material is transferred to the substrate (Fig. 1.6).

During ablation a luminous cloud (plume) is formed along the normal of the

target. Due to the short wavelength, the photons of the laser beam interact only

with the free electrons of the target material. The subsequent electron–phonon

interaction leads to a sudden increase of the local temperature, surface or

subsurface vaporisation (depending on energy of the laser beam) and an explosive

removal of material. Laser-induced thermal evaporation or congruent PLD takes

place for lower and high (> 10

–10

W/cm

) energy densities, respectively. These

ablation processes can be explained in various models (see, for example, Morimoto

and Shimizu (1995)) as:

• shock wave caused by rapid surface evaporation,

• subsurface explosion caused by rapid evaporation-induced cooling of the

surface,

• formation of Knudsen layer due to collision of ejected atoms, and

• superheating of the surface by suppression due to the recoil pressure for

evaporated material.

PLD deposition has a number of advantages. In contrast to most other deposition

processes, the energy at the target can be controlled independent of the process

pressure and gas mixture. Thus, reactive processes can easily be conducted, and

stoichiometric deposition for high energy densities, high rates and high process

ﬂexibility are characteristic features of PLD. However, traditionally PLD is

limited to small areas (typically 1 cm

), the pulsed deposition has a large impact

upon the morphology of the sample and usually droplets of submicrometer size

(boulders) are formed on the surface of the deposited ﬁlm. Via off-axis PLD and

rotation of the substrate, HTS ﬁlms can be deposited on substrates up to 2" in

diameter; utilising rotation and translation in on-axis conﬁguration, ﬁlms have

been deposited even onto 3" wafer (Lorenz et al., 2003). Much effort has been

invested into eliminating the boulders on HTS-ﬁlm surfaces including optimising

the laser power (Dam et al., 1994) and masking the ablation plume.

Finally, sophisticated PLD systems have been developed for the deposition of

multilayer, complex or perfect ﬁlm systems by combining the advantages of PLD

and MBE (see for instance Blank et al., 2000). In the so-called Laser-PLD

(L-PLD) a ‘target carousel’ allows the subsequent deposition of different

components or systems and an in-situ characterisation such as low energy electron

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1.6 Schematic sketch of a Laser-MBE (a), and in-situ RHEED

characterisation of a continuous (b) and layer-by-layer (c) growth of

oxide ﬁlms via Laser-PLD according to Koster et al. (1999).

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diffraction (LEED) or reﬂection high energy diffraction (RHEED) allows a

perfectly controlled ﬁlm growth. Figure 1.6 shows a Laser-PLD and the layer-by-

layer growth of oxide ﬁlms.

Sputter techniques

Sputter deposition has the deserved reputation of being the technique for preparing

thin ﬁlms of alloys and complex materials for industrial application. It is based

on a discharge involving free ions and electrons in a gaseous atmosphere (see

Fig. 1.7). Three different kinds of discharges can be distinguished: arcs, glow

discharges and dark discharges. They can be distinguished by their luminescence

but also by their current-voltage characteristic, the current density and breakdown

voltage (Fig. 1.7c). These main characteristics depend on the geometry of the

electrodes and the vessel, the gas used, the electrode material.

1.7 Schematic sketches of the sputter deposition process: (a) plasma

generation and particle removal at the target, (b) sketch of a magnetron

sputter device for HTS deposition, (c) voltage-current characteristic

of the discharge, and (d) deposition rate as function of reactive gas

content during the sputter process.

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Dark discharge

Under the force of the electrical ﬁeld, ions and electrons migrate to the electrodes

producing a weak electric current. The current increases with increasing voltage in a

highly non-linear way. The region of exponential current increase (Townsend

discharge) ends in the electrical breakdown when the ions reaching the cathode start

to possess sufﬁcient energy to generate secondary electrons. The breakdown voltage

for a particular gas and electrode material depends on the product of the pressure and

the distance between the electrodes, it is expressed by Paschen’s law (Paschen, 1889).

Glow discharge

The glow discharge regime owes its name to the plasma gas emitting light. In this

regime a stable plasma is present. The secondary electrons will generate ions in

the plasma that are accelerated towards the target. At the target electrons and

particles are removed (Fig. 1.7a). The latter will form the ﬁlm at the substrate

placed opposite to the target (Fig. 1.7b). In the normal glow regime an increase of

the energy leads to an increase of the current due to increase of the plasma area. A

complete coverage of the target with the plasma is obtained in the abnormal

discharge. The abnormal discharge represents the regime of homogeneous sputter

deposition, and the energy of the particles can be modiﬁed by modifying the

potential at the cathode. The cathode fall potential increases rapidly with current,

and the dark space shrinks.

Arc discharge

The bombardment with ions ultimately heats the cathode causing thermionic

emission. Once the cathode is hot enough to emit electrons thermionically, the

discharge will change to the arc regime. Arcing will lead to destruction of the

target. In spectro chemistry arc discharges are used in spark OES and DC arc

spectroscopy.

Sputter deposition was the natural method to try when HTS materials were ﬁrst

discovered. However, the problem of large particle energies leading to defects and

resputtering (see next section) was already recognised in this class of materials

some 18 years before the discovery of YBCO for Ba(PbBi)O

deposition (Gilbert

et al., 1980) and has been thoroughly investigated since (Rossnagel and Cuomo,

1988). A method to avoid this problem is the thermalisation of the energetic

species (mainly the negatively charged oxygen that is accelerated in the electric

ﬁeld) either by working at very high pressure (which also provides a particularly

rich oxygen environment) or by ‘off-axis’ conﬁguration at a modestly high

sputtering pressure which forces all atoms emanating from the target to undergo a

few energy reducing collisions before approaching the substrate.

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Several different combinations and techniques using fully oxygenated

stoichiometric targets are established, i.e. off-axis techniques using a cylindrical

magnetron (Liu et al., 2000), facing magnetrons (Newman et al., 1991) or a single

magnetron (Eom et al., 1989), and on-axis techniques using non-magnetron

sputtering at very high pressure (Divin et al., 2008) or magnetron sputtering at

high pressure (Wördenweber et al., 2003, see Fig. 1.7b). Furthermore, the cathodes

can be charged with dc, rf (16.56 MHz) or mf (medium frequency ranging from

20–200 kHz) power or a combination of these (Schneider et al., 1997).

Generally, sputter techniques are usually highly reproducible. Furthermore, due

to the use of a single stoichiometric target the process is compatible with high

oxygen pressure, and calibration and rate control are readily obtained.

Homogeneous composition can be obtained over a rather large area (typically

1–2" for off-axis sputtering and 4" and more for on-axis sputtering). However,

drawbacks are the relatively small deposition rate (ranging between 0.1 nm/min

and 20 nm/min for non-magnetron sputtering and magnetron sputtering,

respectively), the presence of a plasma and high deposition pressures that hamper

the use of in-situ characterisation such as low energy electron diffraction (LEED)

or reﬂection high energy electron diffraction (RHEED), and the tendency of an

instable plasma for variation of the local oxygen partial pressure (Fig. 1.7d).

Comparison of the different PVD techniques

PVD technologies are vacuum technologies. The particles are removed from the

source via heating (evaporation and PLD) or ion bombardment (sputtering). The

resulting energy of the particles at the substrate is one of the most important

parameters that determines the nucleation and growth of resulting ﬁlm.

At the source the particle energy is described by the Thomson relation N(E)

∝E/(E + E

)

(Thompson, 1968, 1981) and the Maxwell-Boltzmann distribution

N(E) ∝E.exp(–E/kT

) for sputtering and evaporation, respectively. The resulting

average particle energy is given by ½ of the binding energy E

for sputtering and

the (local) temperature T

for evaporation, PLD, and CVD. Table 1.2 displays

typical initial (i.e., at the source) particle energies that are characteristic for the

different deposition technologies. On the one hand, the smallest energy is

Table 1.2 Typical energy of particles leaving the

‘source’ for the different PVD technologies

Typical particle energy (eV)

Sputtering 6–8

PLD 1–2

Evaporation 0.06–0.08

CVD 0.005–0.01

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associated with CVD followed by evaporation. This is one of the reasons for

using these processes for the fabrication of defect-free layers, e.g. in the

semiconductor industry. On the other hand, the largest particle energy is provided

in the sputter technologies. Due to the high particle energy, sputter deposited ﬁlms

have a good mechanical stability and adhesion, but they are usually loaded with

defects. This is of advantage for applications for which the mechanical properties

and ﬁlm adhesion play a major role, e.g. mechanical hardening, optical coating or

CD fabrication. However, in the case of structurally perfect HTS ﬁlms, defect

generation due to high energy represents a problem for sputtering as well as for

PLD. A solution to this problem is given by the so-called thermalisation of the

particles, i.e., the reduction of the particle energy due to interaction of the particles

with the process gas during the particle transport from the source to the substrate.

The transport process and, thus, the thermalisation of particles is simulated in

Hollmann et al., 2005 and illustrated in Plate I in the colour section between pages

244 and 245. The simulation is based on the Monte Carlo method for describing

the energy and angular distributions of particles leaving the source. The initial

angular distribution of particles is dN(

)/d

∝ cos

(

) with typically n = 1 for

evaporation and sputtering, n = 5–10 for PLD (explosion-like removal of material

in overcritical liquid state). The mean-free path of the particles can be evaluated.

The result is comparable with the text book expression for the mean free path for

atmosphere and room temperature

lit

≅ 0.063 mm/p, [1.1]

with p representing the pressure in mbar (see Plate I(a) in colour section).

In the simulation (Hollmann et al., 2005), the scatter events are described via a

quasi-hard-sphere (QHS) model for the interactions between atom particles.

Atoms leaving the target are characterised by their (randomised) initial energy E

and direction of motion. During the transport the atoms move straight over a

(randomised) distance

r before colliding with a process gas atom. The motion of

the gas atoms is chaotic, i.e. position and directions of motion of the colliding

atoms are randomised. The scattering process is repeated until the atom reaches

either a surface of the chamber or the substrate. The microscopic cross-section

qhs

of the elastic interaction of two particles within the framework of the obtained

QHS-model depends on the energy E

of their relative motion:

qhs

· r

min

), [1.2]

where r

min

represents the minimum distance between two atomic particles in case

of ‘head-on’ collision, i.e., impact parameter b = 0 (see colour Plate I(c)). Using

Born-Mayer interatomic interaction potential (Born and Mayer, 1932) for

interatomic distances r, the expression for r

min

is given by

[1.3]

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Here Z

and Z

represent the atomic numbers, and A

and b

the different

constants of the Born-Mayer expression (Petrov et al., 1999; Born and Mayer,

1932). Accordingly, the mean free path

qhs

of atomic particles in a gas medium

is given by:

[1.4]

where N is the number density of the gas atoms.

Plate I(a) shows the pressure dependence of

lit

and

qhs

. It demonstrates that

for ‘classical deposition techniques’ that work at low pressure, scatter processes

between the particles and the process gas need not be considered. However, for

the high-pressure processes scatter events have to be considered. The number of

scatter events per particle arriving at the substrate increase dramatically in the

pressure regime of 1 mbar which is the pressure regime for the deposition of HTS

ﬁlms via sputter and PLD (see Plate I(b)). As result of the scatter events:

– the particle energy is reduced (thermalisation),

– the deposition rate is reduced, and

– the stoichiometry is modiﬁed.

Whereas the ﬁrst effect is intended and necessary for the deposition of HTS

material, the latter two effects lead to problems for HTS deposition. For potential

industrial applications, the rate is of importance, and for large-area deposition

(where usually large target-substrate distances are necessary) the deviation of the

stoichiometry causes problems (see Plate I(d), Wördenweber et al., 2003).

Nevertheless, sputtering and PLD provide the possibility of inducing defects that

could be used to engineer mechanical as well as electronic properties (Einfeld

et al., 2001).

Finally, the different pressure regimes used for the different PVD technologies

automatically deﬁne the preparation regime in the pressure-temperature phase

diagram (Fig. 1.8). In order to provide a high mobility of the adsorbed adatoms at

the substrate, usually the highest temperature is chosen for which the YBCO

phase (1–2–3 phase) is still stable. This is typically the tetragonal phase

YBa

with y close to 6. The transition to the orthorhombic phase is

performed after deposition (see Fig. 1.8). Using activated oxygen or special

temperature-pressure, process steps are usually included in order to improve or

speed up the uptake of oxygen (Ockenfuss et al., 1995).

1.2.2 CVD and MOCVD methods

Attempts to use MOCVD to grow HTS ﬁlms began soon after the discovery of

YBCO in 1987 (Yamane et al., 1988). However, MOCVD of YBCO, an oxide

compound composed of multiple heavy-metal elements, turned out to be quite

difﬁcult. The lack of gas-phase or liquid-phase metalorganic precursors for the

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