Guozhong Cao. Nanostructures & Nanomaterials: Synthesis, Properties & Applications

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276

Nanostructures and Nanomaterials

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Chapter

Nanostructures Fabricated by

Physical Techniques

7.1.

Introduction

In the previous chapters, we have discussed various routes for the synthesis

and fabrication

a variety of nanomaterials; however, the synthesis routes

applied have been focused mainly on the chemical methods approaches. In

this chapter, we will discuss a different approach: fabrication of nanoscale

structures with various physical techniques. Compared to the general chem-

ical fabrication and processing methods, physical fabrication techniques for

producing nanostructures are derived mainly from the techniques applied for

the fabrication of microstructures in semiconductor industry. Particularly the

hndamentals and basic approaches are mostly based on microfabrications.

In this chapter, the following techniques for the fabrication of nanostructures

and nanopatterns are discussed:

(1)

Lithographic techniques

(a) Photolithography

(b) Phase shifting optical lithography

(d) X-ray lithography

(e) Focused ion beam lithography

Neutral atomic beam lithography

277

278

Nanostructures and Nanomaterials

(2)

Nanomanipulation and nanolithography

(a) Scanning tunneling microscopy (STM)

(b) Atomic force microscopy

(AFM)

(NSOM)

(d) Nanomanipulation

(e) Nanolithography

(3)

Soft lithography

(a) Microcontact printing

(b) Molding

(d) Dip-pen nanolithography

(4)

Self-assembly of nanoparticles or nanowires

(a) Capillary force induced assembly

(b) Dispersion interaction assisted assembly

(d) Electric-field assisted assembly

(e) Covalently linked assembly

Gravitational field assisted assembly

(8)

Template assisted assembly

(5)

Other methods for microfabrication

(a) LIGA

(b) Laser direct writing

Although all the above-mentioned processes are discussed in this chapter, not

all methods have the same capability in fabricating nanoscaled structures. In

addition, the hndamentals of various fabrication processes differ signifi-

cantly from each other. Each method offers some advantages over other tech-

niques, but suffers from other limitations and drawbacks. No attempt has

been made to exhaustively list all the methods developed in the literature or

the technical details for fabricating nanostructured devices. Similar to previ-

ous chapters, the attention has been focused mainly on the fundamental con-

cepts and general technical approaches. However, more detailed discussion

has been devoted to

SPM

based nanomanipulation and nanolithography, not

only because the processes are relatively new, but also because they are truly

capable of fabricating nanometer scaled structures and devices.

7.2. Lithography

Lithography is also often referred to as photoengraving, and is the process

of transferring a pattern into a reactive polymer film, termed as resist,

Nunostructures

Fabricated

by Physical Techniques

279

which will subsequently be used to replicate that pattern into an underly-

ing thin film or Many techniques of lithography have been

developed in the last half a century with various lens systems and expo-

sure radiation sources including photons, X-rays, electrons, ions and neu-

tral atoms. In spite of different exposure radiation sources used in various

lithographic methods and instrumental details, they all share the same

general technical approaches and are based on similar fundamentals.

Photolithography is the most widely used technique in microelectronic

fabrication, particularly for mass production of integrated circuit.*

7.2.1.

Photolithography

Typical photolithographic process consists

producing a mask carrying

the requisite pattern information and subsequently transferring that

pattern, using some optical technique into a photoactive polymer or pho-

toresist

(or

simply resist). There are two basic photolithographic

approaches:

(i)

shadow printing, which can be hrther divided into contact

printing (or contact-mode printing) and proximity printing, and (ii) pro-

jection printing. The terms “printing” and “photolithography” are used

interchangeably in the literature.

Figure

7.1

outlines the basic steps of the photolithographic process, in

which the resist material is applied as a thin coating over some base and

subsequently exposed in an image-wise fashion through a mask, such that

light strikes selected areas of the resist material. The exposed resist is then

subjected to a development step. Depending on the chemical nature of the

resist material, the exposed areas may be rendered more soluble in some

developing solvent than the unexposed areas, thereby producing a positive

tone image of the mask. Conversely, the exposed areas may be rendered

less soluble, producing a negative tone image of the mask. The effect

this process is to produce a three-dimensional relief image in the resist

material that is a replication of the opaque and transparent areas of the

mask. The areas of resist that remain following the imaging and develop-

ing processes are used to mask the underlying substrate for subsequent

etching or other image transfer steps. The resist material resists the etchant

and prevents it from attacking the underlying substrate in those areas

where it remains in place after development. Following the etching

process, the resist is removed by stripping to produce a positive or nega-

tive tone relief image in the underlying substrate.

Diffraction sets the limit

the maximum resolution or the minimum

size of the individual elements by photolithography, which can be obtained.

280

Nanostructures and Nanomaterials

Resist

Base

llllll

--Mask

Positive

Coat

Expose

Develop

Etch

Strip

Fig.

7.1.

Schematic representation

the photolithographic process sequences, in which

images in the mask are transferred to the underlying substrate surface.

Diffraction refers to the apparent deviation of light from rectilinear prop-

agation as it passes an obstacle such as an opaque edge and the phenom-

enon of diffraction can be understood qualitatively as follows. According

to geometrical optics, if

opaque object is placed between a point light

source and a screen, the edge of the object will cast a sharp shadow on the

screen.

light will reach the screen at points within the geometrical

shadow, whereas outside the shadow the screen will be uniformly illumi-

nated. In reality, the shadow cast by the edge is diffuse, consisting of alter-

nate bright and dark bands that extend into the geometrical shadow. This

apparent bending of light around the edge is referred to as diffraction, and

the resulting intensity distribution is called a diffraction pattern.

Obviously diffraction causes the image

a perfectly delineated edge

become blurred or diffused at the resist surface. The theoretical resolution

capability

shadow photolithography with a mask consisting of equal

lines and spaces of width

is given by:

where

is the grating period

(1/2b

is the fundamental spatial frequency

v),

the gap width maintained between the mask and the photoresist sur-

face,

the wavelength of the exposing radiation and

the photoresist

thickness. For hard contact printing,

is equal to

and from the equation,

Nanostruchtres Fabricated

Physical Techniques

the maximum resolution for 400nm wavelength light and a

pm thick

resist film will be slightly less than

km.

In contact-mode photolithography, the mask and wafer are in intimate

contact, and thus this method can transfer a mask pattern into a photoresist

with almost

100%

accuracy and provides the highest resolution. Other pho-

tolithographic techniques can approach but not exceed its resolution capa-

bilities. However, the maximum resolution is seldom achieved because of

dust on substrates and non-uniformity of the thickness of the photoresist

and the substrate. Such problems can be avoided in proximity printing, in

which, a gap between the mask and the wafer is introduced. However,

increasing the gap degrades the resolution by expanding the penumbral

region caused by diffraction. The difficulties in proximity printing include

the control of a small and very constant space between the mask and wafer,

which can be achieved only with extremely flat wafers and masks.

Projection printing differs from shadow printing. In projection printing

techniques, lens elements are used to focus the mask image onto a wafer

substrate, which is separated from the mask by many centimeters. Because

of lens imperfections and diffraction considerations, projection techniques

generally have lower resolution capability than that provided by shadow

printing. The resolution limit in conventional projection photolithography

determined largely by the well-known Rayleigh’s equation. The resolu-

tion, i.e. the minimum resolvable feature,

and the corresponding depth

focus

(DOF)

are given by the following6:

(7.3)

Here

is the exposure wavelength,

and

are constants that depend on the

specific resist material, process technology and image-formation techniques

used, and

is the numerical aperture of the optical system and is defined as

nsin0

(7.4)

where

is the index of refraction in image space and is usually equal to

(air or vacuum), and

is the maximum cone angle of the exposure light

beam. The diffraction limit is a very basic law

physics directly related

Heisenberg’s uncertainty relation. It restricts any conventional imaging

process

a resolution of approximately

A/2.

Conventional photolithogra-

phy is capable of fabricating features of

200

nm and above?

obtain higher resolutions, shorter wavelength light and lens systems

with larger numerical apertures should be used. In general, the minimum

282

Nanostructures and Nanomaterials

feature size that can be obtained is almost the same as or slightly smaller

than the wavelength of light used for the exposure, when a relatively larger

numerical aperture (typically

>0.5)

is used. In such high

lens systems,

the depth of focus becomes very small and

the exposure process

becomes sensitive to slight variations in the thickness and absolute posi-

tion of the resist layer.8

Deep Ultra-Violet lithography

(DUV)

based on exposure at wave-

lengths below

300

nm, presents far more difficult technical challenges.

Classical

sources have lower output power in the DUY Excimer lasers

can provide

to 20 watts of power at any one of several wavelengths in

the DUV: Of particular interest are the KrCl and

KrF

excimer lasers,

which have outputs at

222

and 249 nm, respectively. High intensity,

microwave powered emission sources provide substantially higher DUV

output than classical electrode discharge mercury lamps9 Light sources

with shorter wavelengths exploited for optical lithography include:

KrF

excimer laser with a wavelength of 249 nm, ArF excimer laser of 193 nm,

F2 excimer laser of 157 nm. With DW, Optical lithography allows one to

obtain patterns with

minimal size of -lOOnm.lo~*' Extreme

(EW)

lithography with wavelengths in the range of

11-13

nm has

also

been

explored for fabricating features with even smaller dimensions and is

strong candidate for achieving dimensions of 70nm and

be lo^.'^*'^

However,

EUV

lithography meets other problems. The adsorption of light

in this wavelength regime is very strong, and therefore, refractive lens

systems cannot be used. The reflectivity from reflective mirrors is rather

low and, thus, the number of reflective mirrors should be kept

low as

possible, not more than six. In addition, an extremely high precision

metrology system is required to make this technique practically viable.I2

Experimentally it is found that when the width of the slit

narrower

than the wavelength, the radiation spreads out or is diffracted. Two edges

very close together constitute a slit from which very distinct diffraction

patterns are produced when illuminated with monochromatic light. The

particular intensity distribution observed depends on the distance between

the slit and the screen. For a short distance, the diffraction is Fresnel dif-

fraction, which is the case in shadow printing. For a large distance such as

in projection printing, the diffraction is Fraunhofer diffraction.

In addition to conventional photoresist polymers, Langmuir-Blodgett

films and self-assembled monolayers have been used as resist in photoli-

thography.I4,l5 In such applications, photochemical oxidation, cross-link-

ing, or generation of reactive groups are used to transfer patterns from the

mask to the

mono layer^.'^^'^

Nanostructures Fabricated by Physical Techniques

283

7.2.2.

Phase-shifting photolithography

Phase-shifting photolithography was first developed by Levenson

al.

In this method, a transparent mask induces abrupt changes of the phase

of the light

used

for exposure, and cause optical attenuation at desired loca-

tions. These phase masks, also known as phase shifters, have produced

futures of

100

nm in photore~ist.'~~~ Figure

7.2

schematically illustrates

the principles of phase-shifting lithography.

clear

film,

i.e. a phase shifter

or a phase mask, whose thickness is

A/2(n-

is placed on a photoresist

with conformal contact, the phase angle

the exposure light passing

through the

film

shifts by the amount of

to the incident light arriving at

the surface of the photoresist. Here

the wavelength of the exposure

light and

the index of refraction of the phase mask. Because the light

phase angle

between

the phase shifter and the photoresist is inverted, the

electric field at the phase shifter edge is

the intensity of the exposure

light at the surface of the photoresist would

zero.

image having zero

intensity can

formed about the edge of the phase shifter. Phase masks

can be used in both projection and contact-mode photolithographic tech-

niques. For a phase-shifting contact-mode photolithography, there are

two

possible approaches to increase the resolution: (i) reducing the wavelength

of the source of exposure light and (ii) increasing the index of refraction

Amplitude

at mask

at wafer

Amp'itude

Intensity

at wafer

Fig.

7.2.

Schematic illustrating the principles

phase shifting lithography, which utilizes

the optical phase change at the phase shifter edge.

Nanostructures and Nanomaterials

Fig.

7.3. Parallel lines formed in photoresist using near field contact-mode photolithogra-

phy have widths on the order of

100

nm and are

-300

nm in height as imaged

(A) AFM

and (B) SEM. [J.A. Rogers,

K.E.

Paul, R.J. Jackman, and G.M. Whitesides,

Vac.

Sci.

Technol.

B16,

(1998).]

the photoresist. The achievable photolithographic resolution is roughly of

-Ah/4n,

where

is the wavelength of the exposure light and

is the refrac-

tive index of the photoresist. Although contact-mode photolithography

with a phase-shifting mask has a higher resolution, conformal contact

between the phase-shifting mask and the photoresist on wafer is difficult to

achieve, due to the presence of dust, non-uniformities in the thickness of

the photoresist, and bowing of the mask or the substrate. However, by intro-

ducing elastomeric phase-shifting masks, conformal contact can be rela-

tively easily achieved and feature lines as narrow as 50nm have been

generated.2’,22 The resolution achieved corresponds approximately to A/5.

An improved approach to conformal near field photolithography is to use

masks constructed from

‘‘soft”

organic elastomeric

polymer^.^^-^^

Figure

7.3

shows a pattern created using such a contact-mode phase-shifting photolith-

ographic process.25

7.2.3.

Electron beam lithography

A finely focused beam of electrons can be deflected accurately and precisely

over a surface. When the surface is coated with a radiation sensitive poly-

meric material, the electron beam can be used to write patterns of very high

resolution.2629. The first experimental electron beam writing systems were

designed to take advantage of the high resolution capabilities in the late

Nunostructures Fabricated by Physical Techniques

285

sixties.30 Electron beams can be focused to a few nanometers in diameter and

rapidly deflected either electromagnetically or electrostatically. Electrons

possess both particle and wave properties; however, their wavelength is on

the order of a few tenths of angstrom, and therefore their resolution is not

limited by diffraction considerations. Resolution of electron beam lithogra-

phy is, however, limited by forward scattering of the electrons in the resist

layer and back scattering from the underlying substrate. Nevertheless, elec-

tron beam lithography is the most powerhl tool for the fabrication of feath-

ers as small as

3-5

nm.31332

When an electron beam enters a polymer film or any solid material,

loses energy via elastic and inelastic collisions known collectively as elec-

tron scattering. Elastic collisions result only in a change of direction of the

electrons, whereas inelastic collisions lead to energy

loss.

These scatter-

ing processes lead to a broadening of the beam, i.e. the electrons spread

out as they penetrate the solid producing

transverse or lateral electron

flux normal to the incident beam direction, and cause exposure of the

resist at points remote from the point of initial electron incidence, which

in turn results in developed resist images wider than expected. The mag-

nitude of electron scattering depends on the atomic number and density of

both the resist and substrate as well as the velocity of the electrons or the

accelerating voltage.

Exposure of the resist by the forward and backscattered electrons

depends on the beam energy, film thickness and substrate atomic number.

the beam energy increases, the energy loss per unit path length and scat-

tering cross-sections decreases. Thus the lateral transport

the forward

scattered electrons and the energy dissipated per electron decrease while

the lateral extent of the backscattered electrons increases due to the

increased electron range.

the resist film thickness increases, the cumu-

lative effect of the small angle collisions by the forward scattered electrons

increases. Thus the area exposed by the scattered electrons at the resist-

substrate interface is larger in thick films than in thin films. Proper expo-

sure requires that the electron range in the polymer film be greater than the

film thickness in order to ensure exposure of the resist at the interface.

the substrate atomic number increases, the electron reflection coefficient

increases which in turn increases the backscattered contribution.

Electron beam systems can be conveniently considered in two broad

categories: those using scanned, focused electron beams which expose the

wafer in serial fashion, and those projecting an entire pattern simultane-

ously onto a wafer. Scanning beam systems can be further divided into

Gausian or round beam systems and shaped beam systems. All scanning

beam systems have four typical subsystems: (i) electron source (gun),

Guozhong Cao. Nanostructures &amp; Nanomaterials: Synthesis, Properties &amp; Applications

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Guozhong Cao. Nanostructures & Nanomaterials: Synthesis, Properties & Applications