Jackson M.J. Micro and Nanomanufacturing

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Principles of Micro- and Nanofabrication 51

Table 1.4. Wet etch rates for various micro- and nanomachined materials

Mate-

rial

etched

Si02

Si3N4

GaAs

Mask

material

Resist

Si02

Resist

Wet etcliant

H3PO4

HNO3/CH3CO

OH/H3PO4/H2

KOH

H2SO4/H2O2/

H2O

Etch

rate

(nm/min)

600

800

Comments

Used at 80*^C,

anisotropic.

Stops etching

on (111) plane

Anisotropic

etch

1.3 Conclusions

Micro- and nanofabrication is a challenge brought about by scal-

ing down the size of transistors so that microprocessors can run

faster and more efficiently. However, the development of these fab-

ricating techniques will have a positive effect of producing products

other than semiconductors and microprocessors. These techniques

have yet to be exploited by the automotive, aerospace, healthcare,

and many other industries that have yet to invent and manufacture

micro- and nanofabricated products.

1.4 Problems

Describe the materials used in the semiconductor indus-

try.

Explain the process of micromachining. Use a flowchart

to show your answer.

52 Micro- and Nanomanufacturing

Explain how deep trenches are machined in sihcon.

Illustrate how microcapacitors are manufactured.

Show how cantilever probes for AFM applications are

manufactured using deep reactive ion etching.

6. Describe and illustrate the basic chipmaking process.

What is soft lithography? How does soft lithography

overcome the problems encountered using optical lithog-

raphy?

8. Show how field effect transistors are manufactured using

soft lithographic techniques.

9. Explain the operation of a scanning tunneling micro-

scope.

10.

What is an atomic force microscope? Explain its modes

of operation.

11.

Explain how carbon nanotubes can be used to make field

effect transistors.

12.

Describe the properties of carbon nanotubes.

13.

How are carbon nanotubes used to make AFM probes?

14.

Show how a field effect transistor, variable area capaci-

tor, and electronic resonators are manufactured.

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Principles of Micro- and Nanofabrication 53

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Microfabrication Using X- ray

Lithography

2.1 Introduction

During the last decade there has been a rapid development in micro-

fabrication technology driven by the market need for low-cost con-

sumer products such as portable telecommunications equipment,

computers, and healthcare diagnostics. Much of the technology used

for these is based on production of silicon semiconductors and mi-

crochips. Interest in non-silicon-based technologies started to grow

back in the early 1980s with the development of a German fabrica-

tion process known as LIGA, an acronym for Lithography (LJtho-

graphie), electroplating (Galvanoformung), molding (Abformung)

[1,2].

It originated at the Karlsruhe Nuclear Research Laboratory in

Germany. Since then a number of groups, mainly in Germany and

the United States have been active in developing the process to make

precision microcomponents for a range of innovative products such

as microspectrometers, fiber-optic wave guides, micro-reactors and

microfluidic devices. A few of these have been manufactured on a

large scale and placed on the market. LIGA was often used to fabri-

cate the components, which were then integrated with others into the

end product. High costs are often associated with the integration

and packaging of the final product.

The most active groups developing and using the LIGA process

are the Sandia National Laboratories at Livermore, the Center for

Advanced Microstructures and Devices (CAMD) at the Louisiana

State University at Baton Rouge, in the United States, Institut fur

Mikrostrukturtechnik (FZK) Karlsruhe and Antwenderzentrum

BESSY, Berlin in Germany. In addition the work done in the United

Kingdom at Central Microstructure Facility at the CCLRC's Ruther-

ford Appleton Laboratory using the national synchrotron at Dares-

56 Micro- and Nanomanufacturing

bury as part of a European network program has advanced fabrica-

tion techniques in mask and resist development. A small start in

commercialization has been made by two companies in the United

States, Axsun and International Mezzo, which provides commercial

LIGA services.

Progress in commercialization has been slow owing to the ab-

sence of fast prototyping and large- scale manufacturing capabilities,

and the lack of established design rules and standards. This situation

was recently reviewed at a workshop at the COMS2004 conference

in Edmonton. The delegates decided to form an International LIGA

Interest Group to bring together major researchers, practitioners,

manufacturers, and users into an international network to provide

mechanisms for communication to solve the problems and be an in-

centive for commercialization.

Since 1980 numerous reports and papers on the development and

use of the LIGA process have been published and are too numerous

to be listed here. A more recent overview of the current status of this

process was given by Hruby at the HARMST Conference in 2003

[3],

and commercialization issues were reviewed recently by Tolfree

at COMS2004 [4] and Goettert at COMS2004 [5].

The latest market survey [6] indicated that a global market val-

ued in excess $40 billion is emerging for microproducts. The in-

creased interest in nanotechnology driven by the prospect of produc-

ing new non-silicon-based materials with unique properties, has

increased market estimates to over a $1 trillion. Micro-

nanomanufacturing is now a key value-added element in many sec-

tors of industry.

The boundaries between nanotechnology and microtechnology

may be blurred, but there is a degree of commonality in the tech-

niques and equipment involved in both - but they are, in essence and

in application, very different. It is at the nano not the micro scale

that the physical and chemical properties of materials change. Mi-

crofabrication is essentially a top-down technology but at the nano-

scale, either top-down or bottom-up techniques can be used, and the

latter are significantly different. Many products require a variety of

top-down processes for their manufacture. For example, the com-

mon CD has data pits about 500 nm wide and 125 nm deep formed

Microfabrication Using X- ray Lithography 57

in a plastic disc. The read-write heads are very precise mechanisms

that require a number of electro- mechanical processes.

Extensive papers and reviews on microfabrication technologies

have been published. Two examples are: "The Fundamentals of Mi-

crofabrication" by Madou 2002 [7], and 'Microfabrication using

Synchrotron Radiation" by Tolfree in 1998 [8]. These and others can

be found in the literature and on internet sites, cover most of the

relevant principles and issues associated with the development and

exploitation of the technologies. It was therefore decided to restrict

the content on this chapter to microfabrication using lithographic X-

ray techniques. This technique is known as deep X- ray lithography

(DXRL), is like all lithographic processes, ultimately limited in line-

width by the wavelength of the illuminating radiation. The conver-

sion from a 2-D pattern to a 3-D structure is dependent on a number

of factors, which are examined below.

There are multiple types of lithography, including UV, deep UV,

X- ray and electron-beam lithography. Currently, for non-silicon-

based materials, the highest precision can be achieved using DXRL

with very parallel, high-energy x- rays from a synchrotron radiation

source (SRS). It is the increased access provided by the large num-

ber (>80) of synchrotrons now operating world wide, coupled to

availability of low-solubility resists, thus reducing exposure time

that has encouraged a greater interest in DXRL. This technique still

has to find a wider community of users outside of research but it will

have a significant role to play in the range of tools and processes re-

quired to develop a micro-nanotechnology (MNT)-based industry.

Micro-nanotechnology (MNT) is pervasive and will have an im-

pact, sometimes disruptive, on almost every industry sector and

through the generation of new products and systems, on the society

in general. The universal use of the mobile telephone and ink-jet

printer are two well-known examples. The availability of a vast

range of new consumer and industrial products such as sensors, em-

bedded transducers and actuators, displays, health care diagnostics

etc.

will revolutionize the way people will live and work in the fu-

ture.

58 Micro-and Nanomanufacturing

2.2 X- ray Lithography

2.2.1.

History

International Business Machines (IBM) first combined electrode-

position and

X-

ray lithography in 1969. They made high-aspect ratio

metal structures by plating gold patterns of 20 jum in thickness in a

resist that had been exposed to x- rays. The IBM work was an ex-

tension of through-mask plating, also pioneered by IBM in 1969,

and was directed toward the fabrication of thin film magnetic re-

cording heads. A historical background of lithography was provided

by Cerrina [9].

The development of the LIGA process referred to above required

small slotted nozzles for uranium isotope separation [10] to be pro-

duced. Since then, the X- ray lithographic technique has been de-

veloped to fabricate a variety of microstructures in materials [11 -

19].

The potential of LIGA for the development of microsystems

was reviewed by Bacher [20]. Essentially, a three-step process, the

LIGA technique can be used to make 3D microstructures.

By adding molding techniques the broader implications of x- ray

lithography as a means of low-cost manufacturing of a wide variety

of microparts with unprecedented accuracy from various materials

can be realised. In Germany, x- ray lithography was originally de-

veloped outside of the semiconductor industry.

Early pioneering work in the use of synchrotron radiation for

microfabrication was carried out by Professor Guckel at the Univer-

sity of Wisconsin in the United States. This included use of the

LIGA technique to develop micromotors [21-24]. Guckel reposi-

tioned the field in light of semiconductor process capabilities and

brought it closer to standard manufacturing processes.

Microfabrication Using X- ray Lithography 59

2.3 Synchrotron Radiation (SR)

2.3.

General Characteristics

The radiation emitted by relativistic electrons when traversing a

magnetic field can be understood from purely classical electromag-

netic theory. Its properties can be expressed by basic equations that

are used in the design of synchrotron radiation sources [25-32]. A

basic introduction to synchrotron radiation sources is given by

Marks [33] and a general review that provides details on the subject

by Turner [34]. The power of the emitted radiation is inversely pro-

portional to the mass of the charged particle, so electrons yield use-

ful quantities of radiation in the visible and X- ray regions of the

electromagnetic spectrum.

Centripetal acceleration of highly relativistic charged particles in

a magnetic bending field results in the tangential emission of syn-

chrotron radiation over a wide spectrum at every point of the curved

particle trajectory. Considering only electrons, the emission pattern

is essentially determined by that of

single circulating electron.

With reference to Fig. 2.1, the radiation pattern emitted by rela-

tivistic electrons can be transformed into the laboratory reference

frame, resulting in its being compressed in a narrow forward cone,

tangential with respect to the electrons' circular path. This natural

collimation is an important characteristic property of synchrotron

radiation. As the electron beam sweeps out the curved path, a con-

tinuous fan of radiation results in the horizontal plane while the dis-

tribution in the vertical plane is highly coUimated.

The opening half-angle of the emission cone of radiation is

wavelength-dependent [35], its angular distribution can be approxi-

mated by a Gaussian distribution, the width of which is related to the

kinetic energy, E, and the rest energy

(mc^).

The natural divergence

dn is given by:

Sn =(mc^)/E (2.1)

60 Micro- and Nanomanufacturing

The divergence is an important parameter when considering the

use of synchrotron radiation sources for deep x- ray Uthography.

This has to be as low as possible but is limited to the practical values

obtainable for E, which are in the range (1.5-3) GeV for typical na-

tional sources, resulting in values for the natural divergence between

0.2 and 0.3 mrad in the x- ray region.

The continuous emission of radiation excites particle oscillations

that give rise to a finite extension of the particle beam and corre-

sponding angular deviations with respect to the ideal trajectory.

Since the direction of photon emission follows the instantaneous

particle direction, an additional angular width, dp^ results, which is

independent of the natural divergence and when added to the natural

divergence it forms the total angular width of the synchrotron radia-

tion and is given by:

Stot-(dn'

dp'y'' (2.2)

The electron beam emittance is determined by the particular de-

sign of the synchrotron but can be optimized to be similar in magni-

tude to the natural divergence. A typical value for stot is in the range

0.3-0.4 mrad that leads to a vertical intensity distribution of the

beam. At a distance of 10 m from the emission point and with a

typical beam width of 3-4 mm, the beam is seen as a broad radiation

fan in the horizontal direction. In calculating the above, the finite

width of the beam has been ignored but could be important at the lo-

cation of the lithography station on an external beam line on a syn-

chrotron. When the beam width is taken into account, the product of

beam size, W, and its angular width, dt, is given by the emittance, E:

E=WSt (2.3)

2.3.2 Spectral Characteristics

Owing to both longitudinal and transverse oscillations of the cir-

culating electrons, individual components in the frequency spectrum

become smeared out, resulting in a continuous spectrum of radiation