Deposition of Thin Films on Solid Surfaces - NPTELnptel.ac.in/courses/103103033/module2/lecture5.pdf · Deposition of Thin Films on Solid Surfaces ... 2.5.1.2 Molecular beam epitaxy

NPTEL Chemical Engineering Interfacial Engineering Module 2: Lecture 5

Joint Initiative of IITs and IISc Funded by MHRD 1/22

Deposition of Thin Films on Solid Surfaces

Dr. Pallab Ghosh

Associate Professor

Department of Chemical Engineering

IIT Guwahati, Guwahati–781039

India



Table of Contents

Section/Subsection Page No. 2.5.1 Nucleation and film growth 3–20

2.5.1.1 Evaporation 5

2.5.1.2 Molecular beam epitaxy 7

2.5.1.3 Sputtering 9

2.5.1.4 Chemical vapor deposition 10

2.5.1.5 Atomic layer deposition 14

2.5.1.6 Electrochemical deposition 16

2.5.1.7 Langmuir–Blodgett films 16

Exercise 21

Suggested reading 22



2.5.1 Nucleation and film growth

Deposition of thin films has been studied for almost a century. Some of the

techniques developed during the past five decades are widely used in the industries. The

methods for depositing a film can be divided into two categories: (i) vapor-phase

deposition (e.g., chemical vapor deposition, evaporation, molecular beam epitaxy,

sputtering and atomic layer deposition), and (ii) liquid-based deposition (e.g.,

electrochemical deposition, chemical solution deposition, Langmuir–Blodgett films and

self-assembled monolayers).

The film deposition involves heterogeneous processes such as heterogeneous

chemical reactions, evaporation, adsorption and desorption on growth surfaces.

Growth of films of nanoscale thickness involves nucleation and growth on the

surface of the substrate. The nucleation step is very important because it governs

the crystallinity and microstructure of the film. There are three basic modes of

nucleation: (i) island or Volmer–Weber, (ii) layer or Frank–van der Merwe, and

(iii) island–layer or Stranski–Krastonov nucleation. These three modes of

nucleation are illustrated in Fig. 2.5.1.

Fig. 2.5.1 Schematic illustration of three basic modes of initial nucleation in the

film growth.

Island growth occurs when the species are bound to each other more strongly than

to the substrate. The islands subsequently merge and form a continuous film.

Metals on insulators, alkali halides, graphite and mica display this type of



mechanism during the initial film deposition. Subsequent growth results in the

islands to join to form a continuous film. The layer growth is the opposite of the

island growth. The species are bound more strongly to the substrate than to each

other. First a complete monolayer is formed and then the deposition of the second

layer begins. The epitaxial growth of single-crystal films is an important example

of layer growth. The island– layer growth mechanism is a combination of both

island and layer growths. Such a growth mode typically involves the stress, which

is developed during the formation of the nuclei or films.

The critical nucleus size r and the corresponding free energy barrier G are

given by the following equations.

2

3

2 sin 2cos 2

2 3cos cos

vf

vr

G

(2.5.1)

3

2

16 2 3cos cos

43

vf

v

GG

(2.5.2)

where is the contact angle. vf , fs and sv are the interfacial energies of

vapor–nucleus, nucleus–substrate and substrate–vapor interfaces, respectively,

and vG is the change in Gibbs free energy per unit volume of the solid phase.

For the layer growth, the deposit wets the substrate completely, and therefore, the

contact angle is zero. Thus, Young–Dupré equation becomes,

sv fs vf (2.5.3)

For island growth, the contact angle is larger than zero. According to Young–

Dupré equation, we have,

sv fs vf (2.5.4)

If the deposit does not wet the substrate at all, , and the nucleation is a

homogeneous nucleation.

The most important layer growth is the deposition of single crystal films through

either homoepitaxy (in which the depositing film has the same crystal structure

and chemical composition as that of the substrate), or heteroepitaxy (in which the

depositing film has a close matching crystal structure as that of the substrate).



Homoepitaxy is a simple extension of the substrate, and there is virtually no

interface between the substrate and the depositing film. Although the deposit has

a chemical composition different from that of the substrate, the growth species

prefers to bind to the substrate rather than to each other. Because of the difference

in chemical composition, the lattice constants of the deposit are most likely to

differ from those of the substrate. Such a difference can lead to the development

of stress in the deposit. Stress is one of the common reasons for the island–layer

growth.

Most of film deposition and processing are carried out in vacuo. In a gas phase,

the molecules are continuously in motion. They collide among themselves as well

as with the walls of the container. The mean distance traveled by molecules

between successive collisions is called the mean free path. It is inversely

proportional to pressure. Under typical film deposition and characterization

conditions, the gas molecules virtually collide only with the walls of the vacuum

chamber, i.e., there is no collision among the gas molecules.

The gas impingement flux in film deposition is a measure of the frequency with

which the gas molecules impinge on or collide with a surface. Only those

molecules which impinge on the growth surface are able to contribute to the

growth process. The number of gas molecules that strike a surface per unit time

and area is defined as the gas impingement flux, which is proportional to

P MT , where P is pressure, M is molecular weight and T is temperature.

In the physical vapor deposition processes (which are discussed below), the

growth species is transferred from a source (or target) and deposited on a

substrate to form a film. The process is essentially atomistic and no chemical

reaction is involved. Various methods have been developed for this transfer

process. The thickness of the deposit can vary from ~1 nm to several millimeters.

2.5.1.1 Evaporation

Evaporation is the simplest physical deposition method for preparation of thin

films. The evaporation system consists of an evaporation source that vaporizes

the desired material and the substrate. Both the source and the substrate are



placed in the vacuum chamber. The substrate faces the evaporation source. The

set-up is schematically shown in Fig. 2.5.2.

Fig. 2.5.2 A typical evaporation system depicting the source, substrate and the

vacuum chamber.

The substrate can be placed and heated as desired. The necessary vapor pressure

of the source material can be generated by simply heating the source as per the

requirements of the concentration of the growth species in the gas phase. The

equilibrium vapor pressure vP of an element can be estimated from the

Clausius–Clapeyron equation.

ln vH

P CRT

(2.5.5)

where H is the molar heat of evaporation, R is the gas constant, T is

temperature and C is a constant. The rate of evaporation is given by,

2A v hN P P

MRT

(2.5.6)

where is the coefficient of evaporation, AN is Avogadro’s number, hP is the

hydrostatic pressure acting on the source, vP is vapor pressure, M is the

molecular weight, R is gas constant and T is temperature.

Precautions need to be taken for pyrolysis, decomposition and dissociation of the

compound being heated. Deposition of thin films by evaporation is carried out at



very low pressures (1 108 – 0.1 Pa). The molecules in the vapor phase do not

collide with each other prior to the arrival at the growth surface because the mean

free path is very large as compared to the distance between the source and the

substrate. The transport of molecules from the source to the growth substrate is

straight-forward along the line of sight. Therefore, the conformal coverage is

relatively poor and it is difficult to obtain a uniform film over a large area. To

overcome this difficulty, multiple sources are used and the substrate is rotated.

Laser beams have been used to evaporate the material. Pulsed laser beams are

used where high power density is required. This process is known as laser

ablation. The composition of the vapor can be controlled precisely using this

technique. Laser ablation has been used for depositing metal oxides in

superconductor films.

2.5.1.2 Molecular beam epitaxy

Molecular beam epitaxy (MBE) can be considered as a more sophisticated

version of the evaporation technique. In MBE, the vacuum is very high such that

the pressure inside the reactor is of the order of 81 10 Pa. At this pressure, the

mean free path of the gas molecules far exceeds the distance between the source

and the target. Apart from the ultrahigh vacuum system, MBE usually consists of

real time structural and chemical characterization capability. Other analytical

instruments may also be attached to the deposition chamber such that the grown

films can be transferred to and from the growth chamber without exposing to the

ambient. The MBE set-up is expensive, and a typical one may cost ~US$ 1

million. A MBE facility is shown in Fig. 2.5.3.

In MBE, the evaporated atoms or molecules from one or more sources do not

interact with each other in the vapor phase under such a low pressure. Although

some gaseous sources are used in MBE, most molecular beams are generated by

heating the solid source material in effusion cells (known as Knudsen cells). The

material is raised to the desired temperature by resistive heating.



Fig. 2.5.3 Molecular beam epitaxy system to grow and characterize thin crystalline films of oxides and ceramics.

The molecules strike on the single-crystal substrate resulting in the formation of

the desired epitaxial film. The extremely clean environment, slow growth rate

(~ 103 10 m/s) and independent control of evaporation of the source material

ensures precise formation of the film. Ultrahigh vacuum environment ensures

absence of impurity or contamination. Thus, a highly pure film can be readily

obtained. Individually controlled evaporation of sources permits the precise

control of chemical composition of the deposit at any given time. The slow

growth rate ensures sufficient surface diffusion and relaxation so that the

formation of crystal defects is kept minimal. The main attributes of MBE are:

(i) A low growth temperaure (e.g., 800 K) that limits diffusion and maintains

hyperabrupt interfaces, which are very important in fabricating two-dimensional

nanostructures or multilayer structures such as quantum wells.

(ii) A slow growth rate that ensures a well controlled two dimensional growth at a

typical rate of 1 micrometer per hour. A very smooth surface is achievable by

controlling the growth at the monoatomic layer level.

(iii) A simple growth mechanism compared to other film growth techniques ensures

better understanding due to the ability of individually controlled evaporation of

sources.

(iv) A variety of in situ analysis capabilities provide important information for the

understanding and refinement of the process.



2.5.1.3 Sputtering

Sputtering involves use of energetic ions to knock molecules out from a target

which acts as one electrode and subsequently deposit them on the substrate that

acts as the second electrode. In a typical sputtering chamber, the source and

substrate electrodes face each other. An inert gas such as argon at low pressure

(~15 Pa) is used as the medium. When a high electric field (~10 kV/cm) is

applied to initiate the glow-discharge between the electrodes, free electrons are

accelerated by the electric field and ionize the argon atoms. The gas pressure

should not be too low, otherwise the electrons will simply strike the anode

without having gas-phase collision with the argon atoms. The Ar+ ions thus

generated strike the source electrode resulting in the ejection of neutral target

atoms. These atoms pass through the discharge and deposit on the substrate

electrode. In addition to the growth species (i.e., neutral atoms), other negatively

charged species under the electric field will also bombard and interact with the

surface of the substrate or grown film.

For the deposition of insulating films, an alternate electric field is applied to

generate plasma between two electrodes. Typical frequencies employed range

from 5 to 30 MHz. The key element in sputtering is that the target self-biases to a

negative potential and behaves like a DC target. Such a self-negative target bias is

a consequence of the fact that the electrons are considerably more mobile than

ions and have little difficulty in following the periodic change in the electric field.

To prevent simultaneous sputtering on the grown film or substrate, the sputter

target must be an insulator and be capacitively coupled to the generator. This

capacitor will have a low impedence and will allow the formation of a DC bias on

the electrodes.

The types of plasmas encountered in thin film processing systems are typiclly

formed by partially ionizing a gas at a pressure well below the atmospheric

pressure. These plasmas are very weakly ionized (the ionization fraction varies

between 51 10 0.1 ). The plasma based film processes differ from other film

deposition techniques such as evaporation, because the plasma process is not

thermal.



A comparison between evaporation and sputtering is given in Table 2.5.1.

Table 2.5.1 Comparison between evaporation and sputtering

Evaporation Sputtering

Pressure: 1 108 – 0.1 Pa Pressure: 10–15 Pa

Atoms in the evaporation chamber do not

collide with each other prior to arrival at

the growth surface

Atoms and molecules collide with each

other prior to arrival at the growth surface

The process occurs under equilibrium The sputtering process is not governed by

thermodynamic equilibrium

The growth surface is not activated The growth surface is continuously

bombarded by electrons, and thus it is

highly energetic

The evaporated films consist of large

grains

The sputtered films consist of smaller

grains with better adhesion to the substrates

2.5.1.4 Chemical vapor deposition

Chemical vapor deposition (CVD) involves chemically reacting a volatile

compound with other gases to produce a nonvolatile solid that deposits

atomistically on a suitably placed substrate. The CVD process is widely used in

the manufacture of solid-state microelectronic devices.

Both gas phase and surface chemical reactions are involved in CVD, and they are

intricately combined. The gas phase reactions become progressively important

with increasing temperature and partial pressure of the reactants. An extremely

high concentration of reactants will make the gas phase reactions predominant,

leading to homogeneous nucleation. For deposition of good quality films,

homogeneous nucleation should be avoided. The major types of chemical

reaction are: pyrolysis, oxidation, reduction and disproportionation. Some

examples of the reactions are given below.

1. Pyrolysis (or thermal decomposition):



4 2

4

SiH (g) Si(s) 2H g at 923 K

Ni CO (g) Ni(s) + 4CO (g) at 453 K

2. Oxidation:

4 2 2 2

3 2 2 5 2

SiH (g) O (g) SiO (s) + 2H (g) at 723 K

4PH (g) + 5O (g) 2P O (s) + 6H (g) at 723 K

3. Reduction:

4 2

6 2

SiCl (g) 2H (g) Si(s) + 4HCl(g) at 1473 K

WF (g) + 3H (g) W(s) + 6HF(g) at 573 K

4. Disproportionation:

2 42GeI (g) Ge(s) + GeI (g) at 573 K

The CVD process is versatile because for depositing a given film, many different

reactants or precursors may be used. From the same precursors and reactants,

different films can be obtained by varying the ratio of reactants and deposition

conditions. One of the important applications of CVD is to make diamond films

from gas phase deposition at low pressure.

Various types of CVD methods and reactors are used depending on the reactants,

reaction conditions and the forms of energy used to activate the reactions.

o When an organometallic compound is used as the precursor, the process is called

metalorganic CVD.

o When plasma is used to promote the reaction, the process is known as plasma-

enhanced CVD (PECVD).

o There are modified CVD processes such as low-pressure CVD (LPCVD), laser-

enhanced CVD and aerosol-assisted CVD (AACVD).

A set-up for CVD is shown in Fig. 2.5.4. Some of the most common types of

CVD reactors are horizontal, vertical, barrel and pan cake reactors. The reactors

used in the CVD processes can be classified into hot wall and cold wall types.

The hot wall reactors are usually tubular, and heating is accomplished by

surrounding the reactor with resistance elements. In cold wall reactors, the



substrates are directly heated inductively by graphite susceptors and the chamber

walls are cooled by air or water.

Fig. 2.5.4 Microwave plasma-enhanced chemical vapor deposition (PECVD) system for the growth of ultrananocrystalline diamond films.

LPCVD differs from conventional CVD in the low pressure used, viz. ~0.1 kPa.

The low pressure enhances the mass flux of gaseous reactants and products

through the boundary layer between the laminar gas stream and substrates. In

PECVD processing, plasma is sustained within chambers where simultaneous

CVD reactions occur. Typically, the plasma is excited by an electric field with

frequency ranging from 100 kHz to 40 MHz at very low pressures, or by

microwave with a frequency of 2.45 GHz. Often the microwave energy is coupled

to the natural resonant frequency of the plasma electrons in the presence of a

static magnetic field. Such plasma is referred to as electron cyclotron resonance

plasma. The introduction of plasma results in much enhanced deposition rates,

which permits the growth of films at relatively low substrate temperatures.

Laser has also been employed to enhance or assist the chemical reactions or

deposition. Two mechanisms are involved, viz. pyrolytic and photolytic

processes. In the pyroltic process, the laser heats the substrate to decompose gases

above it and enhances rates of chemical reactions, whereas in the photolytic

process, laser photons are used to directly dissociate the precursor molecules in

the gas phase. Aerosol assisted CVD is developed for the systems where no

gaseous precursors are available and the vapor pressures of liquid and solid



precursors are too low. In this process, liquid precursors are atomized to form tiny

droplets, which are dispersed in a carrier gas and delivered to the deposition

chamber. Inside the deposition chamber, the precursor droplets decompose, react

and grow films on the substrate.

In addition to the growth of thin films on a planar substrate, CVD methods have

been modified and developed to deposit solid phase from gaseous precursors on

highly porous substrates or inside porous media. Two important varieties of CVD

for these applications are electrochemical vapor deposition (EVD) and chemical

vapor infiltration (CVI). EVD has been explored for making gas-tight dense solid

electrolyte films on porous substrates. One of the most studied system is yttria-

stabilized zirconia films on porous alumina substrates for solid oxide fuel cell

applications and dense membranes.

In EVD process for growing solid oxide electrolyte films, a porous substrate

separates metal precursors and the oxygen source. Typically chlorides are used as

metal precursors, whereas water vapor, oxygen, or air, or a mixture of them is

used as the source of oxygen. Initially, the two reactants inter-diffuse in the

substrate pores and react with each other only when they concur to deposit the

corresponding solid oxides. When the deposition conditions are appropriately

controlled, the solid deposition can be located at the entrance of the pores on the

side facing metal precursors, and plug the pores. The location of the solid deposit

is mainly dependent on the diffusion rate of the reactants inside the pores as well

as the concentrations of the reactants in the deposition chamber.

Under typical deposition conditions, reactant molecules diffuse inside the pores

by Knudsen diffusion, and the diffusion rate is inversely proportional to the

square root of the molecular weight. Oxygen precursors diffuse much faster than

metal precursors, and consequently, the deposit occurs normally near the entrance

of pores facing the metal precursor chamber. If the solid deposit is an insulator,

the deposition by the CVD process stops when the pores are plugged by the

deposit, since no further direct reaction between the two reactants can occur.

However, for solid electrolytes, particularly ionic–eletronic mixed conductors, the

deposition would proceed further by means of EVD, and the film may grow on



the surface exposed to the metal precursor vapor. In this process, the oxygen or

water is reduced at the oxygen–film interface, and the oxygen transfers in the

film, as the oxygen vacancies diffuse in the opposite direction, and react with the

metal precursors at the film–metal interface to continuously form metal oxide.

Chemical vapor infiltration (CVI) involves the deposition of solid products onto a

porous medium. The primary focus of CVI is on the filling of voids in porous

graphite and fibrous mats to prepare carbon–carbon composites. Various CVI

techniques have been developed for infiltrating porous substrates with the main

goals to shorten the deposition time and to achieve homogeneous deposition.

Some of these techniques are:

o Isothermal and isobaric infiltration

o Thermal gradient infiltration

o Pressure gradient infiltration

o Forced flow infiltration

o Pulsed infiltration

o Plasma enhanced infiltration

Various hydrocarbons have been used as precursors for CVI. Typical deposition

temperatures range from 1000 to 1300 K. The deposition time ranges from 10 to

70 h. The rather long deposition time is due to the relatively low chemical

reactivity and gas diffusion into the porous media. The gas diffusion gets

progressivley slower with the increase in deposition of solid. To enhance gas

deposition, various techniques have been introduced such as forced flow, thermal

and pressure gradients.

2.5.1.5 Atomic layer deposition

Atomic layer deposition (ALD) is a unique method for depositing thin films. It is

also known as atomic layer epitaxy, atomic layer growth or atomic layer CVD. Its

most distinctive feature is that it has a self-limiting growth: each time only one

molecular layer can grow. Therefore, ALD offers a very good method for

depositing films having thickness in the nanometer range.



In a typical ALD process, the surface is first activated by chemical reaction, e.g.,

to deposit a titania film by ALD, the substrate is hydroxylated. When the

precursor is introduced in the deposition chamber, the molecules of the precursor

react with the surface species and forms bonds with the substrate. Since the

precursor molecules do not react with each other, a film of only a single

molecular thickness can be deposited. In the next step, the monolayer is activated

again by surface reaction and a layer of the same or a different precursor is

deposited on top of the layer of the previous precursor molecules. A few layers of

the same or different precursors can be deposited by this procedure.

The choice of appropriate precursors is a very important aspect of the ALD

process. A major disadvantage of the ALD method is the slow deposition rate.

The typical deposition rate is 0.2 nm per cycle (i.e., less than half a monolayer per

cycle) with the theoretical maximum of one monolayer per cycle.

Hausmann et al. (2002) have deposited layers of amorphous silicon dioxide and

aluminum oxide nanolaminates at rates of 12 nanometers per cycle, which is

equivalent to more than 32 monolayers per cycle. Vapors of trimethylaluminum

(Me3Al) and tris(t-butoxy)silanol [(ButO3SiOH] were supplied in alternating

pulses to the heated surface on which transparent, smooth films of alumina-doped

silica grew. To test whether the ALD reactions saturate by a self-limiting

mechanism, they deposited films on a silicon wafer in which deep, narrow holes

had previously been itched. After depositing four ALD cycles, the wafer was

cleaved and cross-sectional scanning electron micrographs were recorded.

The micrographs of the cross-sections of the top, middle and bottom parts of a

hole are shown in Fig. 2.5.5. The images show uniform, conformal coating

indicative of an ideal self-limiting ALD reaction.



Fig. 2.5.5 Cross-sections of holes 7 m deep and 0.1–0.2 m in diameter. Magnified images of the (a) top, (b) middle and (c) bottom parts of a hole coated

conformally with a uniform silica film 46 nm thick made by four ALD cycles (Hausmann et al., 2002) (reproduced by permission from The American

Association for the Advancement of Science, 2002).

2.5.1.6 Electrochemical deposition

Electrochemical deposition is a well-established method for thin film deposition.

It is a special kind of electrolysis resulting in the deposition of solid material on

an electrode. The electrochemical deposition process involves oriented deposition

of charged growth species (e.g., cations) through a solution when an external

electric field is applied, and reduction of the charged growth species at the

deposition surface which also serves as an electrode.

Generally this method is applicable to the electrically conductive materials such

as metals, alloys, semiconductors and conductive polymers. The electrochemical

deposition method is widely used for making metallic coatings, which is known

as electroplating. A somewhat similar method is electrophoretic deposition. It has

been explored for the deposition of ceramic and organoceramic materials from

colloidal dispersions. The material in this case need not be electrically

conductive. The colloid particles are stabilized by electrostatic double layer or

steric forces.

2.5.1.7 Langmuir–Blodgett films

Langmuir–Blodgett films (commonly known as LB films) are layers of

amphiphilic molecules transferred from a gas–liquid interface onto a solid

substrate. In 1920, Irving Langmuir introduced the technique for transferring a



floating monolayer to a solid surface by slowly raising the hydrophilic solid

through the liquid surface. In 1934, Katharine Blodgett announced the discovery

that sequential transfer of monolayer could be accomplished to build-up

multilayer films. In addition to the vertical deposition mode, Langmuir and

Schaefer (1938) suggested a horizontal deposition method by which the floating

monolayer can be transferred to a hydrophobic solid surface by allowing the

horizontal solid surface to touch the monolayer. Only one monolayer can be

deposited by this method.

The Langmuir film balance can be used for building up highly organized

multilayers. This is accomplished by successively dipping a solid substrate up and

down through the monolayer while simultaneously keeping the surface pressure

constant by a computer controlled feedback system between the electrobalance

measuring the surface pressure and the barrier-moving mechanism. In this way,

multilayer structures of hundreds of layers can be produced. The deposition

process is schematically shown in Fig. 2.5.6.

Fig. 2.5.6 Formation of Langmuir–Blodgett film.

Traditionally the deposition is carried out in the solid phase, where the surface

tension is very low so that the monolayer does not fall apart during the transfer to

the solid substrate. This also ensures the build-up of homogeneous multilayers.

The value of surface pressure, 0s (where 0 is the surface tension of

pure water and is the surface tension of the solution) that gives the best results

depends on the nature of the monolayer. It is usually established empirically.

However, monolayers can seldom be successfully deposited at 10s mN/m. At

40s mN/m, monolayers can collapse and the film-rigidity can pose problems.

When the solid substrate is hydrophilic (e.g., glass or SiO2), the first layer is



deposited by raising the solid substrate from the liquid subphase through the

monolayer. On the other hand, if the solid substrate is hydrophobic (e.g., silanized

SiO2), the first layer is deposited by lowering the substrate into the subphase

through the monolayer.

The quantity and the quality of the deposited monolayer on a solid sustrate is

measured by the transfer ratio . It is defined as,

decrease in area of Langmuir monolayer

area of the transferred film on the solid substrate (2.5.7)

If all the area lost from the floating monolayer is a result of deposition (rather

than loss through evaporation, dissolution or collapse) then 1 , which indicates

successful deposition, but it may not produce a well-ordered film. During vertical

dipping to deposit monolayers on hydrophilic solid surfaces, both the film and a

thin water layer are actually applied to the solid surface. Under certain conditions,

the substrate is visibly wet immediately after the transfer. This layer of water may

be expelled by either drainage or evaporation, leaving the monolayer on the solid

substrate.

The rate at which the water layer is removed is known as the speed of the

deposition. Langmuir introduced the term zipper angle to refer to the angle

formed by the water meniscus against the solid plate as it is withdrawn. If the

plate emerges wet, the zipper angle is zero. A large zipper angle 6 rad is

observed when the monolayer becomes tightly bound to the solid, expelling water

rapidly in a zipper-like action. The zipper angle is largest when the interaction

between the monolayer and the solid is large, and the term ‘reactive’ has been

used for such depositions. When the deposition of a monolayer takes place with

an intervening hydrous layer the zipper angle is almost zero, and such a

deposition is known as ‘nonreactive’ deposition.

There are three types of LB deposition which are designated as X, Y and Z. When

a solid plate is inserted in and out of a monolayer-covered liquid surface, it is

often found that once the first layer has been deposited, an additional layer is

deposited each time the plate enters or removed from the liquid. This two-way

deposition is called Y-type deposition. This is shown in Fig. 2.5.7.



Fig. 2.5.7 Langmuir–Blodgett films of X, Y and Z types.

The molecules in the deposited film are arranged head-to-head and tail-to-tail as

shown in the figure. Under certain conditions, a layer is deposited only as the

plate enters the liquid. This is known as X-type deposition in which the molecules

are arranged head-to-tail. The deposition is called Z-type if it only takes place as

the plate is withdrawn from the liquid. Intermediate structures are sometimes

observed for some LB multilayers. They are known as XY-type multilayers.

Surface pressure, dipping speed, properties of the solid substrate and the pH of

the subphase are some of the important factors that govern the LB films.

The commonest and most easily produced multilayers are those from Y-type

deposition. X-ray diffraction measurements have shown that the spacing of the

metal ions incorporated in the film during deposition is nearly twice the single-

layer thickness, confirming the head-to-head, tail-to-tail arrangement. The Y-type

films exhibit high contact angles for water 2 rad (i.e. they are

hydrophobic) which supports the structure shown in the figure that the outer

surface of the films is composed of the hydrophobic chain.

It might be expected from the deposition characteristics that the X-type films

prepared by the one-way mechanism would have a different structure to the Y-

films. Since deposition occurs only on the in-stroke, the outer surface should be

hydrophilic as shown in the figure. However, for some materials (e.g., fatty

acids), it has been found that these films are hydrophobic and give contact angles

similar to those on Y-films. In addition, the spacing between metal ions in

multilayers is the same whether the deposition is X- or Y-type. Therefore, it is

likely that the molecules in these X-films rearrange during or after deposition to

give a structure identical to that of the Y-type films.



Only a few examples of the Z-type films have been reported in the literature.

Substituted anthracene derivatives containing short chain carboxylic acid groups

and molecules possessing an -amino carboxylic acid head-group and two amide

groups along the chain have been reported to form Z-type multilayers under

suitable conditions. Arachidic and behenic acid also deposit Z-type films onto

freshly-cleaved mica surfaces when the subphase contains no added electrolyte at

pH < 5.

The Langmuir–Blodgett films have potential applications in optical and electronic

devices, which are similar to the thin films produced by molecular beam epitaxy

(MBE) or chemical vapor deposition (CVD). The organic thin films have very

bright future prospects because their richness of chemical functionality. The LB

films of phospholipids and proteins can be used for the development of

biosensors and biochemical probes.

The potential of the LB films for these applications is sensitive to the details of

their molecular packing. Also, these applications require that the layers have a

defect-free periodic structure. Defects in the LB films have been studied by

conventional surface analysis such as X-ray and electron diffraction. These

techniques, however, are not sensitive to defects such as the pinholes or tears

within the layers. The presence of such defects has restricted the practical

applications of the LB films.

Atomic force microscopy (AFM) has proved to be a nearly-ideal, non-destructive

and high resolution method to investigate the LB film structure and detect defects

at length scales from 0.1 nm to 10 m. A variety of LB films such as lipid and

protein films, polymer films, and specially functionalized molecular films have

been studied by AFM.



Exercise

Exercise 2.5.1: Answer the following questions clearly.

(a) Mention three methods for deposition of thin films on solid substrates.

(b) Describe the basic modes of nucleation for the development thin films on a

substrate.

(c) Explain how thin films can be deposited by evaporation.

(d) What is laser ablation?

(e) Explain molecular beam epitaxy.

(f) What is chemical vapor deposition?

(g) Explain atomic layer deposition. How can the speed of this method be enhanced?

(h) What is surface pressure? How is it related to surface tension?

(i) Explain X-, Y- and Z- types of Langmuir–Blodgett films.

(j) Give five applications of the Langmuir–Blodgett films.

Exercise 2.5.2: Explain the difference between (a) evaporation and sputtering, and (b)

chemical vapor deposition and atomic layer deposition.



Suggested reading

Textbooks

G. Cao, Nanostructures and Nanomaterials, Imperial College Press, London,

2004, Chapter 5.

P. Ghosh, Colloid and Interface Science, PHI Learning, New Delhi, 2009,

Chapters 8 & 11.

Reference books

S. M. Sze, Semiconductor Devices: Physics and Technology, Wiley, New York,

1985, Chapters 6 & 8.

M. Ohring, The Materials Science of Thin Films, Academic Press, San Diego,

CA, 1992, Chapter 5.

R. J. Stokes and D. F. Evans, Fundamentals of Interfacial Engineering, Wiley-

VCH, New York, 1997, Chapter 10.

Journal articles

B. P. Binks, Adv. Colloid Interface Sci., 34, 343 (1991).

D. Hausmann, J. Becker, S. Wang and R. G. Gordon, Science, 298, 402 (2002).

I. Langmuir, Trans. Faraday Soc., 15, 62 (1920).

I. Langmuir and V. J. Schaefer, J. Am. Chem. Soc., 60, 1351 (1938).

I. Langmuir, Science, 87, 493 (1938).

J. A. Zasadzinski, R. Viswanathan, L. Madsen, J. Garnaes, and D. K. Schwartz,

Science, 263, 1726 (1994).

K. B. Blodgett, J. Am. Chem. Soc., 56, 495 (1934).

K. B. Blodgett, J. Am. Chem. Soc., 57, 1007 (1935).

Documents

Deposition of Thin Films on Solid Surfaces - NPTELnptel.ac.in/courses/103103033/module2/lecture5.pdf · Deposition of Thin Films on Solid Surfaces ... 2.5.1.2 Molecular beam epitaxy