[text]

[text]
Chapter 10
New Technologies and Materials
Written by A. Tipek and P. Ripka, Czech Technical University
and E. Hulicius, Institute of Physics, AVCR
with contribution from A. Hospodková and P. Neužil
11.1.
Introduction - MEMS
MEMS is an abbreviation for “Micro Electro-Mechanical System“ [1]. These
devices feature the integration of mechanical elements, sensors, actuators and
operating electronics on a common silicon substrate with the use of microfabrication
technology [2].
While the electronic circuits (either analog or digital) are fabricated using
integrated circuit process sequences (CMOS, BiPolar or BiCMOS processes), the
micromechanical part is made by micromachining. During the micromachining
techniques selectively etch away parts of the silicon wafer or add new structural
layers to form the required mechanical and electromechanical devices [3].
Microelectronic integrated circuits (ICs) handle the signal processing, while
micromachined parts serve as sensors and actuators which allows the microsystems
to sense and control the environment.
Components of MEMS are:




MicroSensors
MicroActuators
MicroElectronics
MicroStructures
Since MEMS devices are manufactured by batch fabrication techniques, similar
to ICs, unprecedented levels of functionality, reliability, and sophistication can be
placed on a small silicon chip at a relatively low cost. With thin films, the
photolithographic fabrication procedures make it possible to build extremely small,
1
high precision mechanical structures using the same processes that have been
developed for electronic circuits [4].
MEMS promises to revolutionise nearly every product category by bringing
together silicon-based microelectronics with micromachining technology, thereby
making possible the realisation of a complete system-on-a-chip [5].
MEMS technology is enabling new discoveries in science and engineering such
as the Polymerase Chain Reaction (PCR) microsystems for DNA amplification and
identification, introducing new technologies as in the micromachined Atomic Force
Microscopes (AFM), Scanning Processing Microscopes (SPM) and Scanning
Tunnelling Microscopes (STM), biochips for detection of hazardous chemical and
biological agents, and microsystems for high-throughput drug screening and
selection.
Examples of MEMS devices, which we meet everyday, are inkjet-printer
cartridges, accelerometers that deploy car airbags and miniature robots [6].
The successful production of MEMS needs the development of appropriate
fabrication processes in four major areas:




micromachining
microfabrication
micromechanics
microelectronics
The conventional silicon planar microelectronics technology has been adapted to
the processing of both passive and active components. The passive material is one
that does not play an essential role in the sensing mechanism (e.g. SiO2 insulating
layer in a pressure sensor) in contrast to an active material, which does (e.g. metal
oxide layer in a chemical sensor).
The basic MEMS processes are:











oxidation
diffusion
LPCVD (low-pressure chemical vapour deposition)
Photolithography
Epitaxy
Sputtering
Micromachining Processes
Bulk Micromachining
Surface Micromachining
Wafer Bonding
Deep Silicon RIE (reactive ion etching)
2



LIGA (lithography, electroforming, moulding)
Micromoulding
etc.
MEMS devices are extremely small (e.g. electrically driven motors are smaller
than the diameter of a human hair), but MEMS technology is not only characterised
by the size [3]. Also, MEMS do not only include products based on silicon, even
though silicon possesses excellent material properties (e.g. the strength-to-weight
ratio for silicon is higher than for many other engineering materials). MEMS is a
manufacturing technology; a new way of making complex electromechanical
systems using batch fabrication techniques similar to the integrated circuits [7].
MEMS have several advantages: in a classical sensor-actuator electronic
systems, in which the sensors and actuators are the most costly and unreliable parts.
In comparison, MEMS allows these complex electromechanical systems to be
manufactured using batch fabrication techniques and therefore leading to a
substantial decrease in the cost with increased reliability [8].
One example of the advantages of MEMS is the accelerometers for crash air-bag
deployment systems in automobiles. The conventional system uses bulky
accelerometers made of discrete components mounted in the front of the car with the
separate electronics near the air bag and costs over $50. MEMS have made it
possible to integrate the accelerometer and electronics onto a single silicon chip at a
cost of $5 to $10. These MEMS accelerometers are much smaller, more functional,
lighter, and more reliable [3].
The microsensors produced using the silicon process have been developed since
1980, but for a long time they were just laboratory curiosity. Circuits for
preamplifier and logic elements have been integrated with transducers and used to
make an intelligent chip element, called an intelligent sensor or smart sensor.
Microsensors with moveable parts have been developed since 1985 and have
become the first applications of micromechanical parts in the industrial field.
11.2.
Materials
Materials used in electronics can play an active or passive role. Very often one
material can play both roles.
11.2.1. Passive Materials
Passive materials could be described as materials, which are only used to
provide either mechanical structure or electrical connection [9]. The following
3
Tables 11.1 and 11.2 summarize some examples of the physical properties of
several materials [10] that determine their use in applications [11]. Some of these
materials can be used as active as well as passive materials, mainly silicon and
gallium arsenide.
3
Density [kg/m ]
Melting point [C]
Thermal conductivity [W/m/K]
Dielectric constant
Young’s modulus [GPa]
Forbidden gap (300°C) [eV]
Si
GaAs
SiO2
Si3N4
GaN
2 330
1 414
168
11.7
190
1.12
5 316
1 238
47
12
1.427
2 200
1 600
6.5, 11
4.5, 4.3
380
3 100
19
7.5
380
6 150
2 500130
4
3.2
Tab. 11.1 Physical properties of non-metallic materials
3
Density [kg/m ]
Melting point [C]
Thermal conductivity [W/m/K]
Work function [eV]
Young’s modulus [GPa]
Al
Au
Cr
Ti
2 699
660
19 320
1 064
7 194
1 875
4 508
1 660
236
4.3
70
319
5.1
78
97
4.5
279
22
4.3
40
Tab. 11.2 Physical properties of metallic materials (often used in the passive role)
More details see in [12] or [13].
11.2.2. Active Materials
These materials are essential to the sensing process used in various types of
microsensors [9], such as photosensitive, piezoelectric, magnetoresistive,
chemoresistive films.
Nowadays there exists a wide range of functional materials currently used in
microsensors and these often take the form of thin or thick films and play an active
role in the sensing system. Some of them can be deposited using IC-compatible
deposition techniques (CVD or LPCVD) but others need special techniques such as
electrochemical deposition as in the case of conducting polymers [11]. The
properties of some active materials are given in Tab. 11.3.
4
Thermal
Pt
Radiation
Ge
Mechanical
Quartz AT-cut
Magnetic
Fe-pure
Chemical
SnO2
Density
[kg/cm3]
Melting point
[C]
Electrical
conductivity
[103 S/cm]
Thermal conductivity
[W/m/K]
21 470
1 769
9x104
72
5 323
937
3x10-4
67
1 544
1 880
5.1
2.8
7 874
1 535
105
449
6 950
1 360
low
-
Tab. 11.3 Some properties of active materials
For more details see [12] or [13].
11.2.3. Silicon
Silicon makes up to 26% of the Earth’s crust by weight. Elemental silicon is not
found in nature, but occurs in compounds like oxides and silicates. Silicon is
prepared by heating silica and carbon in an electric furnace, using carbon electrodes.
Silicon is under normal conditions a relatively inert element, but it is attacked by
halogens and dilute alkali [14].
Silicon is abundant, relatively inexpensive and exhibits a number of physical
properties, which are useful for sensor application [4].
However, a major problem with silicon is that many of its characteristics are
temperature dependant. Silicon does not display the piezoelectric effect and it is not
ferromagnetic. Silicon has also no efficient photo- or electro-luminescent properties
with the exception of porous or some nanocrystalline forms of silicon, which have
not yet found any industrial applications in this field. This is the reason for its
limited role in the field of optoelectronic active sources. Although silicon does not
display the desired effect, it is possible to deposit layers of materials with the
desired properties on the silicon substrate [15].
Single-Crystalline Silicon
Single-crystalline silicon is the most widely used semiconducting material. It is a
brittle material, yielding catastrophically rather than deforming plastically [4]. But
Young’s modulus of silicon is similar to stainless steel and is above that of quartz
5
and most types of glass. It is the basic material for the electronic industry. This
material may be produced with high purity and quality (containing very few
structural defects).
The silicon is cleaned by zonal melting (it is the way to remove lot of
impurities). Single crystals of silicon are then mostly prepared by cooling of the
melt using the Czochralski method.
Polysilicon
Polycrystalline layers may be formed by vacuum deposition onto an oxidised
silicon wafer with an oxide thickness of about 0.1 m. Polysilicon structures may be
doped with boron or other elements by ion implantation or other techniques to reach
the required conductivity. Even if the boron concentration is very high, the
resistivity of the polysilicon layers is always higher than that of a single-crystalline
material. The resistance change of the polysilicon with temperature is not linear. The
temperature coefficient of the resistance may be changed over a wide range, positive
or negative, through selective doping. The temperature sensitivity and the resistance
of undoped polysilicon is substantially higher than that of Single-crystalline silicon.
For some specific doping concentrations, the resistance may become insensitive to
temperature variation.
Polysilicon resistors are capable of reaching as high a level of long-term stability
as can be expected from resistors in Single-crystalline silicon, since surface effects
play only a secondary role in the device characteristics [4].
11.2.4. Other Semiconductors
There is a wide range of compound semiconductors that combine atoms from
columns III and V, II and VI or IV and VI of the periodic table. The importance of
compound semiconductors is the possibility of combining semiconductors from the
same family (for instance III/V) to prepare heterostructures with unique properties.
Only two compound semiconductors are presented in this chapter: GaAs that is the
most important and widely used compound semiconductor and InSb for its use in
magnetic sensors.
Gallium Arsenide (GaAs)
GaAs is a compound semiconductor combining group III and V elements from
the same row as the classical group IV semiconductor, germanium. GaAs has a
density of 5 317.4 kg.cm-3 at room temperature and crystallises into the zinc blende
structure. A shift in valence charge from gallium to arsenic atoms produces a mixed
ionic/covalent bond compared to the covalent bond of germanium and silicon,
which increases the melting point (1 260 oC) but decreases hardness.
6
The most significant attribute is the electronic band structure of GaAs, which
determines the major electrical and optical properties. Firstly, the optical absorption
and luminescence across the band gap do not require the participation of momentum
conserving phonons. This means that efficient luminescence is achievable for GaAs
unlike Si and Ge. Secondly, the effective mass of electrons is substantially lower for
GaAs than for Si (compare 0.3m0 for Si to 0.067m0 for GaAs), so faster electronic
devices are achievable in GaAs. Thirdly, since the forbidden energy gap for GaAs
(1.42 eV) is higher than that for Si (1.08 eV) superior device isolation is potentially
available for GaAs.
GaAs is dominantly used in heterostructures combining other ternary compound
semiconductors with wider band gaps such as AlGaAs, or lower band gaps such as
InGaAs.
Indium Antimonide (InSb)
InSb is useful for magnetic sensing devices such as Hall effect sensors and
magnetic resistors. InSb magnetoresistors are used as magnetic position sensors in
automotive applications such as crankshaft and camshaft sensors for engine control.
The sensitivity of magnetoresistors is proportional to the square of the electron
mobility. Thus the very large room temperature electron mobility of InSb is an
advantage for these sensors. The narrow energy gap (0.18 eV) makes the intrinsic
electron density high. Since the device operating temperature may be 200 0C in
some applications, InSb is normally n-type doped to stabilise the electron density.
InSb is also used for infrared imaging.
11.2.5. Plastics
Plastics are synthetic materials. They are made from monomers which consist of
one chemical unit. The long chains of repeating units (ethylene) form polymers
(polyethylene). In the same way e.g., polystyrene is formed from styrene monomers.
Polymers consist of carbon atoms in combination with only seven elements hydrogen (H), nitrogen (N), oxygen (O), fluorine (F), silicon (Si), sulphur (S) and
chlorine (Cl). The combinations of these elements create thousands of various
plastics.
The combination of the atoms must correspond to the rules of joining them with
other atoms. Each atom has a limited capacity of chemical bonds, if the compound
should be stable. Polymers a also used as detectors of radiation, chemical sensors
and othe sensing applications.
7
Thermoplastics
Heavier molecules are created by adding more carbon and hydrogen to a chain,
the step increase is 14 (one carbon + two hydrogens). For example: ethane gas
(C2H6) is heavier than methane gas (contains an additional carbon and two
additional hydrogens, molecular weight is 30). Pentane C5H12 is too heavy to be a
gas and it is a liquid at room temperature. Further additions of CH2 groups make
progressively heavier liquids until C18H38 – this is not a liquid but is a solid paraffin wax. If we reach a molecular weight of 1 402 (C100H202), the material is
tough and is called a low molecular weight polyethylene - the simplest of the
thermoplastics. Further addition of CH2 groups increases the toughness of the
material and we get medium and high-molecular weight polyethylene [4].
Polyethylene - the simplest polymer- is reasonably transparent in the mid- and far
infrared spectral ranges and therefore is used for fabrication of infrared windows
and lenses.
The long chains are formed by heat, pressure and by using a catalysts. This
process is called polymerisation. The chain length (molecular weight) determines
many properties of a plastic - toughness, creep resistance, stress-crack resistance,
melt temperature, melt viscosity, difficulty of processing, etc. These polymers are
called thermoplastic polymers (heat-mouldable).
If we pack the chains closer to one another we get denser polyethylene. These
plastics have crystal structures. Crystallised areas are stiffer and stronger. These
polymers are more difficult to process, because they have higher and sharper
melting temperatures. The crystalline thermoplastics abruptly transform into lowviscosity liquids, while amorphous thermoplastics soften gradually.
Examples:
Amorphous polymers include polystyrene, polycarbonate, polysulfone, etc.
Crystalline plastics include polyethylene, polypropylene, nylon, acetal, etc.
Thermosets
Thermosets are another type of plastic. The polymerization – curing - is
performed in two steps: 1 - material manufacturing 2 - moulding.
Example: phenolic compounds are liquefied under pressure during the moulding
process and a cross-linking reaction between molecular chains take place. After it
has been moulded, a thermoset plastic has all its molecules interconnected with
strong chemical bonds, which are not reversible by heating.
Thermoset plastics resist higher temperatures and provide greater dimensional
stability. Thermoplastics offer higher impact strength than thermosets. They are also
easier processed and allow more complex designs.
8
The useful thermoplastics in sensor-related applications are : Alkyd, Alkyl,
Epoxy, Phenolic and Polyester.
A Copolymer is a polymer formed in a polymerization reaction with two different
monomers.
Plastics are electrical insulators, but often we require them to behave as
conductors. In order to make them conductive we may either use lamination of the
metal foil, metallisation (e.g. for shielding purposes) or we can mix plastics with
conductive additives (graphite, metal fibres).
Piezoelectric plastics are made from PVF2 and PVDF (crystalline materials).
Initially, they do not have piezoelectric properties and they must be processed using
high voltages or by corona discharge. These plastic films are used in some
applications instead of ceramics, because they have better flexibility, stability
against mechanical stress and they can be formed into any desirable shape [4].
11.2.6. Metals
Ferromagnetic metals (steel, iron, manganese, nickel and some alloys) are used
in magnetic sensors, which are described in Chapter 10. Ferromagnetic metals are
also used for magnetic shielding. Nonferromagnetic metals such as Copper,
Aluminum and certain alloys such as some stainless steels have relative permeability
close to 1.
When selecting a metal for the sensor design, we must take into the account not
only the physical properties but also its mechanical processing. Example: copper has
excellent thermal and electrical properties, but it is difficult to machine. An
alternative compromise in this case is very often aluminium.
11.2.7. Ceramics
Ceramics are crystalline materials which are very useful in sensor fabrication.
The main common properties are structural strength, thermal stability, low weight,
resistance to many chemicals, ability to bond with other materials, excellent
electrical insulating properties. Another advantage of ceramics is that they mostly do
not react with oxygen and thus do not create oxides.
Several metal carbides and nitrides belong to ceramics. Boron carbides and
nitrides and aluminium nitrides (which have excellent heat transfer) are most
commonly used. Silicon carbide has a high dielectric constant, so that it is ideal for
designing capacitive sensors. Ceramics are usually hard, therefore they requires
9
special processing techniques. Various shapes of ceramics substrates are fabricated
by scribing, machining, and drilling with the use of computer-controlled CO2 lasers.
Ceramics for the sensor substrates are available from many manufacturers in
thicknesses ranging from 0.1-10 mm [4].
11.2.8. Glass
Glass is an amorphous solid material made by fusing usually silica with basic
oxide. Although its atoms does not form a crystalline structure, its atomic
arrangement is rather dense. Glass is transparent material available in many colours.
It is hard and resistant to most chemicals (except hydrofluoric acid). A lot of glasses
are based on a silicates and are composed of three major components - silica (SiO2),
lime (CaCO3) and sodium carbonate (Na2CO3).
Nonsilicate glasses include phosphate glass (resistant to hydrofluoric acid), heat
absorbing glass (made with FeO), glass based on oxides of aluminium, vanadium,
germanium and other types of metal. Example: Borosilicate glass is massively
resistant to thermal shocks due to its low thermal expansion and is used for the
fabrication of optical mirrors. Lead-alkali glass (lead glass) contains lead monoxide
(PbO), which increase the index of reflection and it is a better electrical insulator. It
is used for the construction of optical windows, prisms and as a shield against
nuclear radiation.
Light-sensitive glass forms another group. Photochromatic glass darkens when
exposed to ultraviolet radiation and clears when the UV is removed and/or the glass
is heated. The photochromatic material may keep its colour (at room temperature)
from a few minutes to a week or longer [4].
11.3.
Silicon Planar IC Technology
Microsensor processing has similar requirements as the current microelectronic
technology. The basic processing steps in silicon planar IC (integrated circuit)
technology often form the basis of microsensor technology. Conventional silicon
planar IC technologies are subsequently modified to include some additional
processing steps.
The monolithic fabrication processes can be divided into two basic types known
as Bipolar and CMOS. MOS is one of the most common IC technologies presently
used in microsensors [16].
The silicon planar IC fabrication generally involves all of the following
processes:
10






crystal growth and epitaxy
oxidation and film deposition
diffusion or implantation of dopants
lithography and etching
metallisation and wire bonding
testing and encapsulation
The existence of an oxide is very important: SiO2, whose preparation is simple,
and which is suitable for lithography and possesses high electrical resistivity.
Therefore, most ICs and microsensors are produced whenever possible using silicon
rather than gallium arsenide or other semiconductors. For the majority of IC
structures it is necessary to grow thin epitaxial layers, which have better
crystallographic quality in comparison with the bulk material [17].
11.3.1. The Substrate - the Crystal Growth
There are two main techniques for bulk silicon crystal growth: Czochralski
crystal pulling and floating zone process. Czochralski growth is used for the growth
of larger diameter crystals (300 mm diameter crystals are already industrially used)
and for doped Si crystals.
The advantage of the float zone crystals is purity not only with respect to
dopants but also as far as nondoping impurities such as carbon, oxygen, heavy
metals and others are concerned. By applying multi-pass zone melting the purity can
be even enhanced. This method is not suitable for the growth of doped crystals. The
largest diameter, which can be grown by this method, is about 100 mm, because of
the stability problems arising from the melted zone under gravity conditions.
11.3.2.
Diffusion and Ion Implantation
MOS transistors are generally made from conducting or semiinsulating silicon
wafers with layers that have been doped with n or p - type materials [11]. Controlled
amounts of dopants are inserted into the wafer by thermal diffusion, ion
implantation or during epitaxial growth.
The procedure of thermal diffusion of n-type materials is following. The wafers
are placed in a furnace and an inert gas containing the required dopant (e.g. AsH3 or
PH3) is passed over them. The p-type diffusion can be achieved by passing an inert
gas carrying, for instance, B2H6. (Note: AsH3 and PH3 are very toxic, in fact AsH3 is
the most toxic gas ever used in planar technology and they are typically replaced
with less harmful ways of placing the same element into the silicon substrate.
11
Arsenic is implanted from a solid source and phosphorus is doped in a furnace using
POCl3. Besides that B is doped using a solid source.)
An alternative method to thermal doping is ion implantation. The charged ions
of the desired dopant are accelerated to energies in the range of 10 to 1000 keV and
are fired at the surface. The technique is now commonly used by penetrating As, P,
and B to a depth of 0.5, 1, and 2 m at 1000 keV in silicon.
Ion implantation at 10 keV gives very little yield on the ion source so this energy
is used only very rarely, minimum reasonable energy is about 40 – 50 keV. On the
opposite side of the spectrum, typical implantation does not allow higher energy
than 200 keV. Besides the elements listed, BF2+ is also very common. After ion
implantation the material should be annealed.
Doped layers can be prepared directly by epitaxial growth. By this way it is
possible to dope layers homogeneously or with defined doping profile and at the
exact doping levels.
11.3.3. Oxidation
The oxide layer is formed by placing the wafers into a furnace containing
oxygen at 1100C. Oxygen reacts with silicon and diffuses through the growing
SiO2 layer [11].
The oxide films are used to prevent re-doping of areas with different types of
doping materials, also form an electrically insulated region on the semiconductor
device and is used for surface passivation.
11.3.4. Lithography and Etching
Lithography is a process of image transfer of a geometric pattern from a mask
onto a thin layer of material - called a resist. It is used in classical planar processes,
but it also is the principal mechanism for pattern definition in micromachining. The
name resist stands for a radiation - sensitive material. Firstly, a resist is usually
either spin coated or sprayed onto the silicon wafer. The next procedure is to place
correctly the mask onto the resist. Secondly, in optical lithography, ultraviolet (UV)
radiation is used to change the solubility of the photo resist in a given solvent. The
positive photo resist becomes more soluble after exposure to the UV light. The
negative photo resists become less soluble due to a polymerization process.
Washing in an organic solvent (typically 3.9% solution of tetra methyl
ammonium hydroxide) dissolves the uncured photo resists. The exposed SiO2 is
then etched away by HF solution or “buffered HF” (or BOE) – mixture of NH4H
12
with HF. Then the remaining polymerised resist is burned off. (The next procedure
in processing the MOSFET transistor is the formation of the gate by thermal
oxidation - the thickness of the layer is typically 0.1 m. This procedure does not
belong to lithography and etching.)
Few years ago was in optical lithography (  0.4 m) the minimum line-width
or resolution is determined by the shadow printing technique- the projection
printing. This printing has typically a resolution of 1 – 5 m. This resolution could
be improved to 0.3 – 0.5 m through the use of stronger UV light and better optics.
Sub-micron resolutions may be achieved through the use of electrons, X-rays or ion
beams [11]. The I-line stepper (UV at 365 nm) for 0.8 m can print up to 0.4 m.
High NA I-line stepper scan go even below that.
Nowadays, ArF and F2 sources emitting at 248 nm and 193 nm respectively, are
used. Off-axis optics can give resolution under 200 nm [37].
A photoresist may also be used as a template for patterning material deposited
after lithography (Fig. 11.19b). The resist is subsequently etched away, and the
material deposited on the resist is "lifted off".
11.3.5. Deposition of Materials
The forming of the gate electrode is a typical example of deposition. The silane
is pyrolysed to produce the polysilicon layer. The polysilicon layer can be doped by
the use of dopant gases during its formation or by diffusion or ion implantation. The
higher reliability of the polysilicon than the aluminium is the reason why we use it
in the construction of the gate. The polysilicon gate serves as a mask against source
and drain implant.
The technology of deposition is described in more detail in chapter 11.4
Deposition Technologies.
11.3.6.
Metallisation and Wire Bonding
At the end of processing the MOSFET, another oxide layer is formed and the
window for metallisation is exposed to lithography. The metal is then deposited by
either physical vapour deposition – evaporation, chemical vapour deposition, or
sputtering (1 m). The metal, mostly Al or Au, forms the ohmic contacts to the
source, drain and gate electrode.
The final wafer is then diced up. A saw or diamond scribing is used and the
Integrated Circuit is mounted into the package. The ultrasonic welding of thin Al or
Au wire or ribbons makes the electrical connections between the pads (ohmic
13
contacts) and package terminals. Interesting fact is that the crucial influence on the
reliability of the whole IC depends on the wire-bonding. [11]
11.3.7.
Passivation and Encapsulation
The final IC chip must be protected from the atmosphere. The sensing areas are
often covered up by the photo resist or by a silicon nitride layer. Silicon nitrate can
be deposited by LPCVD or CVD and acts as a firm barrier against water.
The film thickness for IC is usually limited to less than 0.2 m, because thicker
layers cause thermally-induced stresses.
The last step is the encapsulation of the IC in the following manner, e.g., sealed
in a plastic resin or hermetically sealed in a metal case. This process is highly
desirable, because it protects the silicon device from the surrounding environment.
In the case of MEMS, it is not always required to isolate the silicon structure from
the atmosphere. The atmosphere could transmit the measured quantity [11].
11.4.
Deposition Technologies
11.4.1. Introduction
One of the basic steps in MEMS processing is to deposit thin and thick films of
material, which provides the sensing surface with the required properties. For
example the sensitivity to thermal radiation is given by coating with nichrome. The
thick films are used to construct pressure sensors [18] or microphones, where the
membranes have to be produced. The following processes allow the fabrication of
films to have a thickness anywhere between a few nanometers and about 100
micrometers. The film can be locally etched using lithography and wet chemical
etching processes. Dry physical etching and laser processing can also be used [19].
Depositions processes are:
1. CHEMICAL deposition
- chemical Vapour Deposition (CVD)
- epitaxy : liquid, vapour, molecular (rare in MEMS)
- electrodeposition
- thermal oxidation (see 11.3.3)
These processes are based on the creation of solid materials directly by chemical
reactions with gas and/or liquid compounds or from the substrate material.
14
2. PHYSICAL deposition
- physical Vapour Deposition (PVD): Evaporation or sputtering
- casting
- spray coating, screen printing
- laser ablation
In these processes, the deposited material is physically placed onto the substrate,
with no classical chemical reaction, which forms the material on the substrate [3],
[4], [17].
11.4.2. Chemical Reactions
Chemical Vapour Deposition (CVD)
The substrate is placed inside the reactor, into which a number of gases are
supplied. The basic principle is that a chemical reaction takes place between the
source gases. This reaction creates a solid material, which is deposited on free
surfaces inside the reactor.
The CVD system is used to process thin films with good uniformity. This
technology allows to deposit a variety of materials, but some of them are less
popular, because of hazardous by-products formed during the process. But industry
nowadays prefers this technology.
One of the simplified forms of CVD processes is illustrated in Fig. 11.5. The
substrates or wafers are positioned on a stationary or rotating table whose
temperature is elevated up to the required level by heating elements. There are three
reasons for this - a) oxides are decomposed and evaporated from the wafer surface,
b) surface is smoothed, often at the atomic scale and c) the source gases are
thermally decomposed, which is necessary for the layer growth. The top cover of
the chamber has an inlet for the carrier gas, which can be added with various
precursors and dopants. These additives, while being carried over the heated surface
of the substrate, form a layer. The gas mixture flows from the distribution cone over
the top surface of the wafers and exits through the exhaust gas outlets.
15
gas inlet
deposition chamber
distribution cone
gass flow
gas outlet
gas outlet
substrate holder
heating element
Fig. 11.5 Simplified structure of a reactor chamber – after [4]
Epitaxy
Epitaxial growth is used not only for layer deposition, but also because this is the
way to prepare heterostructures of the highest quality from different materials. This
technology is a special case of CVD (Chemical Vapour Deposition) processes. It
consists of depositing atoms of the desired material on the substrate (e.g.
semiconductor crystal - silicon, gallium arsenide) with the same crystallographic
characteristic as the substrate. This concerns mainly crystallographic structures and
orientation of the axes. In particular it is possible to produce almost perfect singlecrystalline films on single-crystalline substrates, if the lattice constants of the two
materials are very close to each other. Film grown on polycrystalline or amorphous
substrate will also be amorphous or polycrystalline.
The most important epitaxially growth is Vapour Phase Epitaxy (VPE). In this
process, a number of gases are introduced into an induction-heated reactor where
only the substrate is heated. The temperature of the substrate typically must be high
because of oxides decomposition and evaporation, smoothing of the surface at an
atomic level as well as for the decomposition of precursors.
This technology is primarily used for deposition of silicon and is widely used for
producing Silicon On Insulator (SOI) substrates. The advantage of epitaxy is the
high growth rate of material - it allows the formation of films with a thickness
ranging from 1 m to >100 m. Some processes require high temperature
exposure of the substrate, others do not demand significant heating of the substrate.
Some processes can be used to perform selective deposition, depending on the
surface of the substrate [17]. The typical vapour phase epitaxial reactor is shown in
the schema in Fig. 11.8.
16
RF Inductive heating coil
Vent
Gas inlet
Graphite susceptor
Fig. 11.8 Typical "cold-wall" vapor phase epitaxial reactor – after [3]
Electrodeposition (Electroplating)
This process is restricted to electrically conductive materials. The process
is used to make films of metals such as copper, gold and nickel (the thickness 1 m
to 100 m). The deposition is best controlled when used with an external electrical
potential, but it requires electrical contact to the substrate. The typical setup for
electroplating is shown in Fig. 11.7.
DC voltage source
Electrical connector
Contanier
Electrolyte solution
Wafer
Counter electrode
Wafer holder
Fig. 11.7 Typical setup for electrodeposition – after [3]
The substrate is placed in a liquid solution (electrolyte) and an electrical
potential is applied between a conducting area on the substrate and a counter
electrode (usually platinum) in the liquid. The chemical process forms a layer of
17
material on the substrate. The various types of gases are very often generated at the
counter electrode [20].
Electroless plating does not require any external electrical potential and contact
with the substrate during processing. These processes use chemical solutions in
which deposition takes place spontaneously on any surface.The disadvantage of this
fabrication is that it is more difficult to control the film thickness and uniformity. [3]
11.4.3. Physical Reactions
Physical Vapor Deposition (PVD)
PVD comprises technologies for deposition of metallic as well as dielectric
films. It is more common than CVD for deposition of metals (lower process risk,
cheaper), even if the quality of the films is inferior - the higher resistivity for metals,
more defects and traps for insulators. The choice of deposition method is in many
cases arbitrary and depends on which technology is available for the specific
material at a given moment. Two main techniques for PVD are evaporation and
sputtering.
- Evaporation
The main principle of this PVD technique is that metal can be converted
into the gaseous form and then deposited on the surface of the sample. The substrate
is placed inside the vacuum chamber (usually 10-6 to 10-7 Torr). The block (source)
of the material, to be deposited is also located in the chamber. It is heated so that it
evaporates. For some materials (Cr, Ti) sublimation temperatures are lower than the
melting temperatures. The vacuum is required to allow the molecules to evaporate
and move freely in the chamber. They subsequently condense on all surfaces. All
evaporation technologies use this principle. The methods differ in the way the
source material is heated and evaporated.
The two popular evaporation technologies are e-beam and resistive
evaporation. In e-beam evaporation, an electron beam is focused on the surface of
the source material and causes local heating and subsequent evaporation. In resistive
evaporation, a tungsten boat, containing the source material, is heated electrically.
The film thickness is determined by the evaporation time and the vapour
pressure of the metal. The evaporation, just like all vacuum deposition processes,
produces layers with large residual stresses and therefore these techniques are
mostly used for depositing of thin layers.
18
Very often it is important to keep the substrate at some elevated temperature, in
order to evaporate moisture from its surface as well to remove some oxides or other
surface impurities.
The resistive evaporation is shown in Fig. 11.10.
rotor
holder (heated)
sample
mask
shutter
material for
evaporation
vacuum chamber
crucible
heater
vacuum
pump
Fig. 11.10 The evaporation - deposition of thin metal film in a vacuum chamber –
after [4]
- Sputtering
The source material in this PVD technology is subjected to lower
temperature compared to evaporation. The substrate is placed in a vacuum chamber
(about 2.10-6 to 5.10-6 Torr) with the source material (denoted as the target or the
cathode) and an inert gas (e.g. argon, helium ) of low pressure. A gas plasma is
ignited using an a.c. or d.c. high voltage power source. The gas becomes ionised.
The target-cathode is connected to this voltage. The sample wafer is attached to the
anode at some distance from the cathode. In some cases, when the non-conductive
substrate is used, the wafer need not be connected to the electrode and it is sufficient
to put the wafer between anode and cathode. The ions are accelerated against the
target. The kinetic energy of the bombarding ions is sufficiently high to free some
atoms from the target surface. The source material, now in a vapor form, condenses
on all surfaces including the substrate [3]. This principle of sputtering is common
for all sputtering technologies. The differences are typically in the methods ion
bombardment of the target [4].
The advantage of this technology is better uniformity, especially if a
magnetic field is introduced into the chamber. The field allows improved flow of
atoms towards the anode. Since this method does not require high target
temperature, theoretically any material, including an organic material, can be
sputtered. The process of sputtering can be extended to the sputtering of more than
19
one target at the same time (co-sputtering,). For example - sputtering nichrome (Ni
and Cr) [17].
The sputtering process is shown in the schematic Fig. 11.12 .
power supply
+
cathode (target)
ion induced
secondary emission
Ar or He
+
bombarding ion
sputtered atom
sample
electron induced
secondary emission
vacuum chamber
anode
Fig. 11.12 The sputtering process in a vacuum chamber – after [4]
Casting
In the casting process, the material to be deposited is dissolved in a volatile
liquid solvent. When the solvent is evaporated, a thin layer of the material remains
on the substrate [3]. The differences in this process are in the way the material is
transported onto the substrate. The most widely used are transmission by spraying
and spinning.
The thickness of the casting layer on the substrate depends on the solubility
of the deposited material and can be in the range from a single monolayer of
molecules/atoms (adhesion promotion) to tens of micrometers (0.1 to 50 m). The
control of the film thickness depends on exact conditions, but the thickness can be
uniform within +/- 10% in a wide range [4].
This process is often used for polymer, polyimide and other organic
materials. The casting method is also used for transferring the photoresists to the
substrate in the photolithography process and is an integral part of the
photolithographic technique [17]. This technique is often used for fabrication of
humidity and chemical sensors.
20
Spray coating
Thermal spray coating is used for metal deposition. The coating material is
fed into the flame where it melts. The melted metal is atomized by a high-velocity
stream of air or other gas. When the stream reaches the target, atoms are bonded to
the surface. This technology may replace traditional plating which often has serious
pollution problems as the electrolytes contain toxical chemicals such as cyanide.
Low-temperature spray coating is used to deposit paints. One of the
applications is in the production of thermal radiation sensors. The deposition layer,
(the surface of the sensor) is processed by covering it with a coating having a high
infrared emissivity. The coating must be very thin to have a good thermal
conductivity and a very small thermal capacity. However the available organic
materials have low thermal conductivity and cannot be effectively deposited with
thicknesses less than 10 m. This characteristic influences the sensor response [17].
Screen Printing
This process has been used for many years as a cheap way to make hybrid
circuits in electronics. The simplified explanation of the technique consists of the
preparation of an ink paste using suitable organic solvents. The paste is then
squeezed through a fine gauze mask and forms a 25 to 100 m film in the desired
areas. The film is then dried by heat treatment to form a conductive layer. The
lateral resolution is only about 100 m but the printing cost makes this technique
commercially viable for low volume low-cost electronic circuits [11].
For example, platinum electrodes have been printed and used in electrochemical
and bioelectrochemical sensors.
Laser Ablation
Laser ablation in general is removing of the material from the surface by laser
beam. Localized heating causes the material to evaporate. This technology is also
used for film deposition: in this case the target is heated by pulsed laser and the
substrate is positioned in the ablation plasma plume. This processed is made in
vacuum or in the low pressure background gases. Laser ablation allows to deposit
complex materials (including organic) from the target to the substrate. High
deposition rates are achievable, however the thickness is usually not perfectly even.
11.5.
Etching Processes
Thin films (the deposition methods have been described in the previous chapter)
may have shape by using masks during the deposition process. However the
achievable complexity of shapes and accuracy is limited. In order to form a
functional MEMS structure it is necessary to selectively remove parts of the film.
21
This is caused surface micromachining. Compared to that, bulk micromachining
only removes the material from the wafer. The most popular technique for surface
and bulk micromachining is etching
Actually, there exists two main groups of etching processes:
A. Wet Etching (micromachining), where the material not required is
dissolved, when the wafer is put into a chemical solution
B. Dry Etching (dry micromachining), where the material is sputtered or
dissolved by reactive ions or a vapour phase etchant
11.5.1. Wet Etching/Micromachining
This simple etching method uses liquid etchant to dissolve the material. A
mask should be made of material which will not dissolve during the process time.
The first etch solutions developed provided isotropic etching, the etch rate
was independent of crystal orientation. Generally, isotropic etchants consist of a
mixture of nitric, hydrofluoric and acetic acids.
Some single crystal materials (such as silicon) exhibit anisotropic etching
in certain chemicals. The anisotropic etching means that the etching rates are
different in different (crystallographic) directions. An example could be that the
<111> crystal plane side walls appear when etching a hole into a silicon wafer with
an <100> exposed plane in a chemical such as KOH [21]. The result is a pyramidshaped hole in contrast to a hole with rounded side walls when using an isotropic
etchant. This principle is illustrated in Fig. 11.15.
Fig. 11.15 Schematic results of anisotropic and isotropic wet etching– after [3]
Anisotropic Etch-Stops
In the construction of microstructures it is often necessary to ensure that the
etching stops at a predetermined position, to control the shape of the structure. The
22
main idea of the stops is that etch rates of alkaline anisotropic etchants strongly
depend on doping of silicon by boron. The disadvantage is that a high level of boron
doping is needed, which produces a residual tensile strain in silicon. This limitation
can be overcome by using of Electro-Chemical Etch stops. An anodic contact is
made to the n-Si. The potential is held at a level higher than the passivation potential
(0.6 V). The p-Si is then chemically etched until the n-Si layer is reached. At this
point the cell current increases and the etching process is stopped. This process can
be well controlled to 1 m accuracy [11].
Anisotropic wet etching is one of the main subtractive bulk microfabrication
(micromachining) techniques. If the orientation of the substrate plane is the
limitation of the application, more complicated and expensive dry etching should be
used. [17].
11.5.2. Dry Etching/Micromachining
Dry etching can be in general more precisely controlled than wet etching techniques.
The disadvantage is the process requires more complex instrumentation including a
vacuum chamber.
The methods of dry etching include
a.
b.
c.
reactive ion etching (RIE)
sputter etching
vapour phase etching
In RIE, the substrate is placed inside a reactor chamber filled by several gases.
The plasma is ignited in the gas mixture by an RF power source. In the plasma the
gas molecules are dissociated into ions. The ions are accelerated towards and react
at the surface of the material we want to etch. The output of the reaction is a
formation of another gaseous substance. This is the chemical part of reactive ion
etching and of course there is also a physical part. If the ions have sufficiently high
energy, they can knock atoms out of the material to be etched without a chemical
reaction. The etching by chemical reaction is very often isotropic and the physical
reaction may be anisotropic. By changing the balance between these two types of
etching, we can form side walls that have shapes from rounded to vertical. Fig. 16
shows the typical RIE system [22].
The deep RIE (DRIE) is a special subclass of RIE. The side walls obtained with
DRIE are almost vertical to a depth of hundreds of microns. In this process, two
different gas compositions are alternated in the working reactor. The first gas
composition creates a polymer on the surface of the substrate, and the second gas
composition etches the substrate. The polymer is immediately sputtered away by the
physical part of the etching. Only the horizontal surface is etched, not the side walls.
23
The polymer is dissolved very slowly in the chemical part of the etching and
therefore this chemical etching builds up the sidewalls. As a result, the etching
aspect ratio of 50 to 1 can be achieved. This process easily allows etching
completely through a silicon substrate. The etching rates are 3-4 times higher than
with wet etching.
An easier method of dry etching other than RIE is sputter etching (RIE without
reactive ions). The substrate is subjected to the ion bombardment to remove
material. Maximum etch rates are of the order of m/min over a wafer-sized area.
[11]. The disadvantage of this technique is the lack of the etch stop layer
The next easiest method of dry etching other than RIE is Vapour phase etching.
The material to be etched is dissociated at the surface in a chemical reaction with the
gas molecules. The two most popular processes are silicon dioxide etching using
hydrogen fluoride and silicon etching using xenon difluoride. Both methods are
isotropic dry etching processes.
Wafers
RF signal
Insulator
Upper electrode
PLASMA
Lower electrode
Wafer holder
Diffuser nossles
Gas
Pump
Gas
Fig. 11.16 Typical scheme of the parallel-plate reactive ion etching system – after
[3]
The technology of dry etching is more expensive compared to wet etching.
If fine resolution in the thin film structures is required or in the construction of
MEMS where deep etching in the substrate with vertical side walls is present, dry
etching should be used.
24
11.6.
3-D microfabrication techniques
The conventional silicon processes allows to build two-dimensional structures,
as the deposition thickness is limited to about 10 m. The 3-D processes extend this
limitation to more thick structures. e fabrication of MEMS to truly threedimensional structures.
11.6.1.
LIGA
The term LIGA originated in Germany and stands for Lithography,
Galvanoformung, Abformung (lithography, electroforming, moulding). LIGA
allows to built high aspect ration structures, i.e. very high and narrow shapes.
The LIGA process begins by generating a photoresist pattern by shortwavelength X-ray lithography on a conductive substrate. A heavy metal is used for
the mask and some kind of polymethyl methacrylate is used for the resist.
After removing the exposed resist, spaces between the resist are electroplated.
The created metal shape can be directly used, but in LIGA process it is used as a
tool for plastics molding. After curing, the mould is removed, leaving behind
microreplicas of the original pattern. A lot of different materials are compatible with
this process e.g. a number of polymers, some metals and even some ceramic
materials.. This technology allows to create a large numbers of low-cost
microstructures. The disadvantage is the need for a short-wavelength, highly
collimated X-ray source, typically a synchrotron orbital radiation (SOR) instrument.
The SLIGA (sacrificial layered LIGA) technique, gains another degree of design
freedom by combining LIGA with the sacrificial layers. It is useful for making small
gear linkages, or other released parts that can be assembled on a separate LIGA
structure or used in more traditional products.
The LIGA process can be combined with the silicon process and this allows a
novel application for the fabrication of micro turbines, gear trains and threedimensional structures – filters, fluid-logic, and conduits [29], [17].
25
11.6.2. Laser Assisted Etching (LAE)
This process belongs to the group of contactless subtracting methods of
fabrication. Electron beam or laser light could be used for the fabrication of the
MEMS. If we scan these beams in three dimensions, we can process complicated
three-dimensional microparts. The precision of the process is mostly decided by the
wavelength of the beam. Photolithography or electron beam lithography use beams
with a shorter wavelength. LAE - Laser Assisted Etching is one of the methods
using laser light.
LAE uses a photo-reactive or a thermal reactive process. The etching of the
appropriate material (semi-conducting materials, metals, ceramics, high polymers
etc.) in an etchant is obtained using a laser beam. LAE does not use patterning
masks.
The disadvantage or the limit of the precision of the classical laser fabrication is
the strong evaporation or melting caused by the high temperatures involved. LAE
works at lower temperatures. LAE is based on laser irradiation in a liquid with
etchant (or in gas) which gives rise to reactive ions or plasma which change the
material into a soluble or vaporised form. The output power of the laser used in this
method is mostly very low. The laser is used only for activating an opto- or thermochemical reaction and not for abrasion, burning or evaporation off at high
temperature [29].
Proton beam micromachining (PBM) is a novel technique for the
production of high aspect-ratio three dimensional microcomponents. PBM is a direct
write process in which a focused beam of MeV protons is scanned in a predetermined pattern over a suitable resist material and the latent image formed is
subsequently chemically developed [31].
11.6.3. Photo-Forming and Stereo Lithography
Photo-forming is a universal optical forming process. This technique includes
plain lithography and stereo lithography.
Plain lithography technique uses a layer of photoresist. This process is often
used in metal etching and silicon planar process. Even if the method is simple, it can
be easily used to fabricate three-dimensional structures by multiple layers of
photoresist. Exposition of the resist can be made with the use of a photo mask or a
micro-optical fibre for irradiation. [29].
Stereo lithography
26
This technique is based on scanning of the ultraviolet laser beam on photopolymerising solution. The photopolymer quickly solidifies when the laser hits its
surface. Once the complete layer is built, the support plate is slightly lowered and
another layer is made. The result is a three-dimensional structure.
The process is illustrated in Fig. 11.30.
Excimer Laser
He-Cd Laser or
Ar Ion Laser
XY
Scanner
Elevator
3D Structure
Photoreactive Resin Bath
Fig. 11.30 The Macro-Photo Forming process - after [29]
This technique is used for rapid prototyping of large parts: in this case the
layer thickness is typically 0.2 mm and the production time is several hours.
In the microscale the model is fabricated from top to bottom or inversely.
Microstereolithography process allows to build objects with a size of 5 m,
achievable resolution is 1 m . The bending pipes, cords, micro-coil springs, combs,
microturbine etc. have been made using this fabrication method [29].
11.6.4. Microelectrodischarging (MEDM and WEDG)
Conventional precision machining is needed to construct fine units using metals,
ceramics, alloys, and bulk silicon. The fine holes, gears, turbines of micro-size have
been produced by MicroElectric Discharge Machining (MEDM) and Wire
ElectroDicharge Grinding (WEDG). The principle of the three axis NC (numeric
control) microelectrodischarge machining is described in Fig. 11.32.
27
The discharge energy in a micro domain must be reduced to a low value 10-7 J.
The resulting capacitance (C) should be reduced to less than 10 pF. The machine
has been constructed to minimise the use of metallic mechanical components to
reduce the stray capacitance [35].
This method allows fabrication of electrode work pieces with complicated
shapes, holes to a depth of about one tenth of the wafer. The WEDG was used to
make a fine electrode 4.3 m in diameter and 50 m in length. For example, a threeaxis numeric control machining MEDM system was used to fabricate a micro-air
turbine and inserted into metal catheter of external diameter 2.2 m and rotating at
1000 rpm (Matsushita Research Institute Inc., Tokyo).
Microelectrodischarge Machining (MEDM)
R
Mandrel
V
C
Stray Capacitance
C´=C1+C2+C3
Electrode
C1
Workpiece
C2
Work Tank
C3
XY Positioning Table
Fig. 11.32 The MEDM process - after [29]
11.6.5. Microdrip Fabrication
The main idea is to build micro objects from the micro droplet. It is the same
technology used for thousands of years in nature by wasps, bees, and termites to
construct theirs homes.
Wax (heated to around 90C to melt) was used at the beginning of the
development of this technology. The wax was ejected through an adapted ink-jet
head, and the droplets had a diameter of around 50 m. The next step was to use the
materials at higher melting temperatures. Some newly developed systems use
28
photopolymers which are cured by UV light. This process is low-temperature and
allows similar resolution and precision as stereo lithography.
11.6.6. Manufacturing using Scanning Probe Microscopes and
Electron Microscopes
This method allows fabrication of three-dimensional microstructures. The main
instrument for this method is the Scanning Electron Microscope (SEM) with the
vacuum chamber used as a working table. A lithographic etching instrument called a
multi-face Fast Atom Beam (FAB), robot hands with four rotational and three
translational degrees of freedom and many mechanical tools such as diggers,
tweezers, blow pipes, scrapers, and sticking tools are placed on the table.
The University of Tokyo was one of the first laboratories, which developed this
complicated instrument. Fig. 11.35 and 11.36 shows tools for microscope
manufacturing. [29].
Stick
Tweeze
Dig
Blow off
Scape
Fig. 11.35 Various tools of microscope manufacturing - after [29]
11.6.7. Handling of Micro Particles with Laser Tweezers
The tiny forces are generated by the absorption, refraction or reflection of
light by a dielectric material. Several miliwatts of power produced by a strong laser
light generates a force of only a few piconewtons. This value of force is sufficient in
the microscopic domain. We must be aware of the fact that radiation pressure could
play a significant role in the handling of micro parts.
The engineers of AT&T Laboratories [36] proposed in 1980 a laser-based
optical trap for microscopic particles. The living material (viruses, bacteria, yeast,
and protozoa) can be non-invasively manipulated with this device. The invention
makes use of the so-called “gradient force” that appears in a light gradient when a
29
transparent material with a refractive index different from that of the surrounding
medium is placed in it. The same principle is used in optical tweezers,
micromanipulators, tensiometers etc.
Another principle: The fluctuating dipoles are induced, when the light
passes through polarizable material such as dielectrics. These dipoles interact with
the electromagnetic field gradient and produce the force, which directs
microparticles towards brighter regions. When we direct parallel laser light onto a
microsphere from above, the light is bent because the sphere acts as a lens. If the
intensity profile of the incoming beam is uniform, the reaction forces on the left and
right hand sides of the sphere cancel and there is no net sideways component. When
the light field is not uniform (gradient), an imbalance in the reaction forces is
created and the object is pulled towards the bright side. These forces have been used
in optical tweezers. The sharply focused light is required for these type of tweezers.
Focusing a laser through a microscope objective can achieve the sharp focus. Diode
laser in the near-infrared region (780-950 nm) makes these devices practical for the
low-power use. Laser tweezers are relatively free of creep, backlash, and hysteresis.
The laser tweezers are used in the assembly of micro-particles and to
measure the dynamics of movement of micro mechanical particles [29].
11.6.8.
Atomic manipulation
The finest possible tool for device assembly is provided by scanning probe
microscopes. The invention of their first representative, the scanning tunneling
microscope (STM) invented by G. Binnig and H. Rohrer in 1981 gave us a tool
capable of imaging and manipulating single atoms [38]. Eye-catching images
assembled by moving atoms by the STM tip appeared soon after and caught the
imagination of a wide public.
Since the seminal discovery of the STM, the field of nanotechnology has
expanded into many directions:

a large number of probe microscopes was developed which use various
interaction between the sharp scanning tip and a specimen: atomic force
microscopy (AFM), electric field microscopy (EFM), magnetic force
(MFM), near-field scanning optical microscopy (NSOM) etc.

Advanced probe designs use local heating, additional gate electrodes and
sensitization by attached biological molecules [39].

Multiple probes and/or probe arrays are investigated: arrays of probes in so
called ‘millipede’ or ‘nanodrive’ project are used to store and read
information with much higher density than the present hard drives [40],
possibly reaching the atomic scale memory [41].
30

Special molecules are designed and synthesized which could be used as
building blocks for constructing functional nanostructures. An example
may be ‘lander’ molecule named because of its similarity to the lander
modules used in space exploration [42].
Probe with an additional electrode next to the tunneling tip of the STM can be
used as a gate for the nanometer sized field effect study of metallic nanoclusters
[43].
11.7.
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31
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www.protron-mikrotechnik.de
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