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Development of an in-house process recipe for MEMS application characterized by scanning microscope

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Development of an in-house process recipe for MEMS application characterized by scanning microscope
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Mali, Ankur H
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English
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x, 83 leaves : ; 28 cm

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Microelectromechanical systems ( lcsh )
Integrated circuits ( lcsh )
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Microelectromechanical systems ( fast )
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theses ( marcgt )
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Includes bibliographical references (leaves 81-83).
General Note:
Department of Electrical Engineering
Statement of Responsibility:
by Ankur H. Mali.

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|University of Colorado Denver
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Full Text
DEVELOPMENT OF AN IN-HOUSE PROCESS RECIPE FOR
MEMS APPLICATION CHARACTERIZED
BY SCANNING ELECTRON MICROSCOPE
By
Ankur II Mali
B.E., Sardar Patel University, 2006.
A thesis submitted to the
University of Colorado Denver
In partial fulfillment
Of the requirements for the degree of
Master of Science
Electrical Engineering
2009


I'his thesis for the Master of Science
degree by
Ankur H Mali
has been approved
by
Prof. Hamid Fardi
Prof. Manciiia-David Fernando
Prof. Titsa Papantoni
11/ 30 j
Date


Mali, Ankur H (MS, Electrical Engineering)
Development of an In-House Process Recipe for MEMS Application
Characterized by Scanning Electron Microscope
Thesis directed by Professor Hamid Fardi
ABSTRACT
The aim of this thesis is to develop a Micro-Electro-Mechanical Systems
(MEMS) recipe for use in silicon integrated circuits and systems.
Scanning Electrons Microscope (SEM) is used for the characterization of
MEMS recipe. The major portion of this thesis consists of assembling and
testing the operation of SEM after becomes operative. The development of
the MEMS recipe was important to test the functionally of the SEM,
where a MEMS device fabricated as a test structure.
SEM was donated by Tandberg Data Inc. in 2007. It took approximately
one and a half years to repair the SEM, a major part of my contribution to
this thesis in addition to the development of an experimental MEMS
recipe. We fabricated the silicon wafer test structure for the MEMS
application. We learned all the functions of the SEMs operation.
Eventually as part of the accomplishment for this project, we were able to
extract the v-groove image of the MEMS in silicon wafer. We observed
the anisotropic etching a feature pertained to MEMS application. We
measured the depth of the etched pattern on silicon wafer and found it in
line with the theoretical results.
This abstract accurately represents the content of the candidates thesis. I
recommend its publication.
Signed
Dr. Hamid Fardi


TABLE OF CONTENTS
Figures.........................................................vii
Tables............................................................x
Chapter
1. Introduction to SEM............................................1
1.1 Physics of SE-M...............................................3
1.1.1 Secondary Electrons.........................................6
1.1.2 Characteristics X-Rays......................................7
1.1.3 Backscattered Electrons.....................................9
1.2 Basic concepts in surface imaging & localized spectroscopy...10
1.3 Localization of the Probe....................................11
2. Assembly of SEM ..............................................13
2.1 Principles of Scanning Electron Microscopy (SEM).............13
2.1.1 Comparison of scanning electron microscope and the optical
microscope.......................................................14
2.1.2 Stigmator Apparatus........................................17
2.1.3 Numerical Aperture and Depth of Field......................17
2.1.4 Effect of accelerating Voltage.............................18
2.1.5 Resolution.................................................19
IV


2.2 Column assembly and its component systems..................20
2.2.1 Condenser Lens...........................................22
2.2.2 The Final Condenser Lens.................................23
2.2.3 Electron Gun.............................................25
2.2.4 Specimen Chamber.........................................28
2.2.5 Secondary Electron Detector..............................29
2.2.6 Vacuum System............................................34
3. Operation of SEM............................................39
3.1 Startup, Shutdown and Emergency Operation..................39
3.1.1 Startup..................................................39
3.1.2 Shutdown.................................................41
3.1.3 Emergency Operation......................................41
3.2 Specimen Exchange..........................................42
3.2.1 Loading the specimen holder with a specimen..............42
3.2.2 Specimen holder exchange.................................43
3.3 Image formation in the SEM.................................44
3.4 Specimen Preparation.......................................51
4. MEMS Fabrication............................................53
4.1 Bulk micromachining........................................55
v


4.1.1 Isotropic and anisotropic etching.............................55
5. Results and Applications.........................................71
5.1 Results.......................................................71
5.2 Applications...................................................78
6. Conclusion.......................................................80
References..........................................................81
vi


LIST OF FIGURES
Figure
1.1 Image of working SEM...................................2
1.2 The energy released due to inelastic scattering of the primary
electron beam as it interacts with the atoms in the sample which
results in the continuum radiation or Bremsstrahlung...5
1.3 The incident electron beam can release an electron from an inner
shell. The vacancy created may be filled by electrons from the
higher energy shell. The energy released is given off in the form of
X-rays or it can also cause the ejection of another electron (Auger
Electrons).....................................................8
1.4 Localization of the Probe Technique..........................11
2.1 X-Ray is produced when the electrons are fired from the electron
gun The column assembly for the SEM...........................21
2.2 The column assembly for the SEM .............................24
2.3 Electron gun configuration...................................26
2.4 Different methods of detecting electrons in SEM. (a) Secondary
electrons, (b) Backscattered Electrons, solid state detector, (c)
VII


k>J
Backscattered Electrons, scintillation counter and (d) Absorbed
electron current............................................30
2.5 Oil Rotary Pump............................................35
2.6 Diffusion Pump.............................................37
.1 Specimen Holder...........................................42
3.2 Specimen Chamber in SEM....................................44
3.3 Schematic Diagram of a Scanning Electron Microscope (SEM)
with secondary electrons forming an image on CRT........46
4.1 Isotropic Etching (a) without agitation (b) with agitation.56
4.2 Anisotropic etching of <100> exposing slow etching <111> planes
which form a 54.74 degree angle with a surface, (a) Formation of a
flat bottomed trench and (b) a cavity in the form of an inverted
pyramid......................................................57
4.3 Velocity of the spinner starts from 250 rpm with acceleration of
200 RPM/s and reach 3000 RPM within 7 seconds................60
4.4 Photoresist thickness versus the spin Speed.................61
4.5 DEKTAK graph indicates the depth of the pattern on the wafer
with photoresist and SiCT
VIII
66


4.6 Depending on the wide of the Pattern, the depth of the pattern is
determined to get v-grooves...................................67
4.7 The etching rate for 50% KOH solution........................69
5.1 The depth of the pattern after 5 minutes of etching..........72
5.2 The depth of the pattern after 12 minutes of etching.........73
5.3 The depth of the pattern taken after 15 minutes of etching...74
5.4 The image of the wafer taken under the SEM after 15 minutes of
KOH etching...................................................75
5.5 The image of wafer taken under the SEM after 30 minutes of KOH
etching.......................................................76
5.6 The depth of pattern measured on the DEKTAK instrument.......77
5.7 The pattern has a v-groove on the wafer taken from SEM.......78
IX


LIST OF TABLES
Tables
2.1 Comparison of SEM and OM.......................................15
3.1 Minimum resolvable distance (do) of a light microscope depends on
the wavelength (k), the refractive index (n) and the angle
(a).............................................................49
3.2 By changing the accelerating voltage for an electron microscope, the
electron velocity, wavelength and minimum resolvable distance will
also change accordingly
50


1. Introduction to Scanning Electron Microscope
(SEM)
Scanning Electron Microscope (SEM) is used as a characterization tool for
the development of biological-micro-manipulation system in SEM [1].
Several articles including ref [2] used SEM for quantitative measurement
with high time resolution of internal waveforms on MOS RAM's [2] and
"microrobot system for automatic nanohandling [3J. The Application of
the SEM to the development of semiconductor high-reliability products is
studied in [4],
The objective of this thesis is to develop an in-house process recipe for the
Micro-Electro-Mechanical Systems (MEMS) fabrication for the use of
silicon integrated circuit and systems. In this report, SEM is used for the
characterization of a MEMS process recipe developed at our laboratory. A
major portion of the thesis was to assemble and test the operation of SEM
after it was repaired.
SEM was donated to University of Colorado Denver by Tandberg Data
Inc., in 2007. At the time, this microscope was not in a working condition
and needed full assembly and repair and assembly. Figure 1.1 shows an
image of the w orking SEM after repair.
1


Figure 1.1 Image of working SEM
Through University of Colorado Denver funds, we ordered parts and
started assembling it. The process took approximately one and half years.
After setting the microscope into working condition, we started learning
how to operate it. Simultaneously we started developing an in-house
process recipe for MEMS application i.e. to achieve the v-groove on the
silicon wafer pattern. This process recipe is intrinsic to individual
laboratories, it was necessary to develop an in-house accurate process
2


model for silicon MEMS. Below, some basic explanation about the
internal mechanism of SEM will be provided.
1.1 Physics of SEM
The physics of SEM is outlined below to further clarify the operation of
SEM. X-rays are generated as a result of the ejection of an inner level
electron (low energy) by an energetic electron from an electron column.
The ejected electron is replaced by an electron from a higher energy shell.
The energy lost as it moves from a higher energy shell to a low energy
shell is released in the form of x-rays [5 J.
Each element of sample has many energy levels and therefore many
potential vacancy-filling mechanisms are carried out. As a consequence
even pure elements emit x-rays at high energy. Because the atomic
structure of each element is different, it follows that each element will
emit a different pattern of x-rays, due to Plancks equation:
he 12.4
X = or X -----
3


Where X is the wavelength of the radiation in angstroms, c is Speed of
light, h is Planck's constant and E is Energy of the radiation measured in
kilo-electron volts.
These x-rays can be analyzed either by wavelength dispersive methods or
by energy dispersive methods. The Oxford system [5] uses the energy
dispersive method. Energy dispersive systems use a semiconductor
detector first developed in the 1960's. Basically it is a single 15 mm2
crystal of silicon which has been treated with lithium (lithium drifted
silicon).
An X-ray photon first creates a charge pulse in a semiconductor detector;
the charge pulse is then converted into a voltage pulse whose amplitude
reflects the energy level of the detected x-ray. This voltage pulse is
converted into the digital signal which causes one count to be added to the
corresponding voltage channel of the multichannel analyzer.
4


Incident Electrons
electron
Figure 1.2 The energy released due to inelastic scattering of the primary
electron beam as it interacts with the atoms in the sample which results in the
continuum radiation or Bremsstrahlung.[5]
The primary electrons may be scattered inelastically by the coulomb field
of an atomic nucleus, thus giving up some or all of its energy as shown in
the Figure 1.2. This energy can be emitted in the form of x-radiation
named bremsstrahlung (German for "braking radiation'). Since the
primary electron can give up any amount of its energy, the energy
distribution of the emitted x-rays is continuous, thus the term x-ray
continuum.
5


1.1.1 Secondary Electrons
When an electron beam is interacting with a specimen surface, a different
reaction occurs. The various signals arising from the specimens surface
on incidence of the primary electron (PE) beam are shown in the Figure
1.3. A collision of electrons from the PE beam with surface of the
specimen results in the detachment of the so called Secondary Electrons
(SE). The number of SEs depends on the surface topography, the
accelerating voltage, and the atomic number of the surface elements. They
create the SE current, which is collected for imaging.
The primary electron interacts with the electron in the sample, ejecting
some kind of kinetic energy. If the ejected electron is weakly bound, it
typically emerges with only few eVs (<50) of energy and it is called
Secondary Electron. Since they have low energy, such electrons can only
escape from the sample if they are created near the surface.
6


1.1.2 Characteristic X-Rays
When as electron is ejected from an inner atomic shell by interaction with
a high electron beam, the result is creation of an ion in an excited state
(Figure 1.3). Through a relaxation process, this excited ion releases energy
to return to a normal ground state. The most likely process in most of the
cases is a series of transformations in each of which an electron from an
outer shell drops into a vacancy in an inner shell. Each drop results in
the loss of a specific amount of energy that equals the (quantum energy)
difference in energy between the vacant shell and the shell contributing
the electron. For the higher energy inner shell, this energy is released in
the form of x-rays. The energy of radiation uniquely indicates the element
of its origin, hence the term characteristic x-rays
The intensity of the characteristic radiation under a given excitation
condition is influenced by three factors:
1. The atomic number of both the emitting atom and the average
atomic number of the bulk sample. The emitting atom atomic
number determines both the ionization cross section and the
7


Characteristics X-ray
Figure 1.3 The incident electron beam can release an electron from an inner
shell. The vacancy created may be filled by electrons from the higher energy
shell. The energy released is given off in the form of X-rays or it can also cause
the ejection of another electron (Auger Electrons) [5].
fluorescent yield. The average atomic number of the sample affects
the energy lost due to other energy processes.
2. The probability that the emitted characteristic x-rays will be
absorbed before they emerge from the sample.
8


3. Secondary fluorescence, which may be a result of absorption. For
example, a high-energy x-rays characteristic of element A may be
absorbed by the atom of element B, thus stimulating lower energy
emission characteristics of the second element. The presence of
elements A and B in the same sample will therefore increase the
intensity of characteristic emission from element B and decrease it
from A (matrix effect).
1.1.3 Backscattered Electrons
If the primary electron interacts with the nucleus of a sample atom, it may
cause electron-scattering with little loss of energy. Some scattered
electrons are directed out of the sample, allowing ease of detections. The
backscattered electrons have more energy than secondary electrons and so
may escape with a greater depth from the sample. They do not depend on
the surface topographic information as the secondary electron does, but
depends on nucleus of the sample atom. As the size of the sample atom
nucleus increases, the number of backscattered electrons increases.
9


1.2 Basic concepts in surface imaging & localized
spectroscopy
Most surface spectroscopic techniques involve probing the surface by
exposing it to a tlux of particles" (hv, e~, A+...) while simultaneously
monitoring the response to this stimulation by, for example, measuring the
energy distribution of emitted electrons. In their most basic form, these
techniques collect information from a relative large surface area (~mm). In
most cases, however, there are variations of these techniques which permit
either,
1. Information to be collected only from a specific region of the
surface that is Localized Spectroscopy or
2. The spatial distribution of a property (e.g. simple topography or
elemental concentration) to be mapped that is Surface Imaging
(Surface Microscopy).
The requirement for both cases is spatial localization of the spectroscopic
technique. This may be activated in the following way.
10


1.3 Localization of the probe
Spatial localization of the spectroscopic technique can be achieved by
focusing the probe so that it interacts with only a small region of the
surface, as shown in the Figure 1.4. The ease with which this can be
accomplished and the spatial resolution (degree of localization) attainable
Figure 1.4 Localization of the Probe Technique [6],
depends primarily upon the nature of the probe charged particles (e.g.
electrons and ions) is easily focused; neutral particles (e.g. x-ray photons
and neutral atoms) are not readily focused.
11


With this localization of the probe into a small area, it is clearly
advantageous to collect as many as possible of the outgoing particles
whose detection completes the spectroscopic process in order to maximize
the signal-to-noise ratio.
12


2. Assembly of the SEM
Scanning electron microscopy (SEM) is a method for the high-resolution
imaging of surfaces. The SEM uses electrons for the imaging, much as a
optical microscope uses visible light. The advantages of SEM over optical
microscopy include greater magnification (up to 100,000 X) and much
greater depth of field.
An incident electron beam is raster-scanned across the samples surface,
and the resulting electrons emitted from the sample are collected to form
an image of the surface. Imaging is typically obtained using secondary
electrons for the best resolution of fine surface topographical features.
Alternatively, imaging with backscattered electrons gives contrast based
on atomic number to resolve the microscopic composition variations, as
well as, topographical information.
2.1 Principles of Scanning Electron Microscopy
(SEM)
The growing popularity of the scanning electron microscopes has
advanced to a state where an unskilled operator, who is unfamiliar with
13


the involved operating principles and the physical construction, can
operate the instrument with sufficient reliability and efficiency.
Even so, a rudimentary knowledge of how a microscope works, its
structure, etc. presents a distinct advantage. The following, therefore,
attempts to fill this need simply and concisely.
2.1.1 Comparison of scanning electron microscope
and the optical microscope [5]
The scanning electron microscope (SEM) and optical microscope (OM)
have the common function of rendering visible objects which are too small
for our naked eye. However, the difference begins with their illuminating
beams. The SEM uses electron beam with the wavelength of 0.06 A ~ OC,
while the OM uses natural light beam with the wavelength of 2,000 to
7,500 A. Other major differences are derived from this point on which are
summarized in Table 2.1.
Although the scanning electron microscope is restricted in operation and
14


Table 2.1: Comparison of SEM and OM [5]
Items SEM OM
Illuminating Electron Beam Light beam ( wavelength:
beam (wavelength: 0.06 A ~ OC) 2,000 to 7,500 A)
Medium Vacuum Atmosphere
Lens Electron lens Optical lens
Resolution Secondary Electron image: 60 A Visible region : 2,000 A Ultraviolet region: 1,000 A
Depth of field 30 pm (at lOOOx) About 0.1 pm
Magnification lOx to 180,000x lOx to 2,000x (by changing
(continuous) lenses)
Focusing Electrical Mechanical
Contrast Geometrical Shape, Absorption and reflection of
physical and chemical the light (color and
properties brightness)
Monitor Cathode Ray Tube (CRT) Direct observation or screen projection.
15


applications due to its illuminating beam and operating medium, it is
effective for obtaining a variety of information from specimens by
utilizing many available attachments. Among these, a X-ray spectrometer,
a backscattered electron detector, a transmitter electron detector,
heating/cooling/tensile stages and a semiconductor device specimen
holder can greatly contribute to research activities when they are used in
appropriate combinations.
On the other hand, SEM not only permits observations of very fine details
(high resolution), but also provides good focus over a wide range of
specimen surfaces (large depth of field). It also produces an image of
those specimens ranging from objects visible by the naked eye to several
tens of angstroms. Furthermore, according to the shape or composition of
the specimen, its contrast can be varied. In addition, there is little
restrictions about the specimen size and type, which simplifies specimen
preparation.
16


2.1.2 Stigmator Apparatus
Stigma means mark or spot in Greek. The Stigmator is a device that is
used to control any distortion in the roundness of the spot formed by the
electron probe scanned on the specimen. Since the spot on Cathode Ray
Tube (CRT) is round, it is important that secondary electrons, from the
corresponding point on the specimen, emanate from the round spot.
2.1.3 Numerical Aperture and Depth of Field
Numerical Aperture is a number that expresses the ability of a lens to
resolve fine detail in an object being observed. It can be used to control
the depth of the field in the specimen. Depth of field refers to the depth in
the specimen that appears to be in focus. Depth of field (Dtj is expressed
as
X
Df ~ AM7
Where X is the Wavelength of illumination, NA is the Numerical Aperture
17


From the above equation, it can be inferred that smaller aperture can
generate narrower beams, with small aperture angle. The length of narrow
beams affects the diameter of the beam (i.e. spot size) less than that of the
wider angled beams. Consequently, as the beam is scanned along the
contours of the specimen, the effect on the Spot size is less at various
levels, so that specimen will appear sharply in focus at various levels.
The distance from the specimen to the objective lens is called Working
Distance (WD). The depth of field is also affected by this distance. As the
working distance increases, the aperture angle decreases and as the
aperture angle decreases, the numerical aperture decreases. Consequently,
the depth of field increases as one increase the working distance. But this
increase in the depth of field is at the expense of resolution.
2.1.4 Effect of Accelerating Voltage
The accelerating Voltage used in the electron column of the microscope
influences both the spatial resolution of the x-ray signal and the efficiency
with which characteristics x-rays are excited from the sample. Higher
18


voltage produces higher energy electrons that penetrate more deeply into
the sample and spread more widely than the low energy electrons. This
result in degradation of the resolution, on the other hand, however it
improves the efficiency of the excitation. It is generally accepted that the
latter tradeoff is optimized at an overvoltage (the ratio of the accelerating
voltage to the energy of the excited line) of 2.5 to 3.
2.1.5 Resolution
Resolution is referred to as the ability of the instrument to display two
closely placed objects into two different entities rather than a single object
on CRT. The size of the final spot is related to the resolution of the SEM.
Smaller beam spot sizes permit better resolution. The final lens is used to
focus the size of the illuminating beam spot to match the magnification
used. Since the secondary electrons arising from the beam spot striking the
specimen are additively displayed as a spot of a fixed size (usually around
100 pm) on the CRT, the diameter of the beam spot on the specimen must
not exceed a certain size, therefore it is defined as
19


100 jum
Maximum Spot Size = -------------
Magnification
For example, if the working magnification is lOx, the beam spot size of
the specimen must not exceed 10 pm. A magnification of 100,000x would
require a beam spot size of 1 nm or less on the specimen. If the beam spot
size goes beyond this size, secondary electrons are generated from areas
outside of what is being summarized on the spot on the CRT. This results
in unsharp image, since extraneous information is present in the display
spot.
2.2 Column assembly and its component systems
Electrons leave the filament and are accelerated down the column through
a potential that can approach 40,000 volts. This gives the incident
electrons enough energy to eject electrons from the sample. An X-Ray is
produced when an outer shell electrons falls in the place the inner shell
electrons shown in Figure 2.1.
20


The SEM may be subdivided into a number of different component
systems that carry out various functions as shown in the Figure 2.2.
Among these systems, a lens system is involved in producing a small,
Bottom of
column
Hoctron accclersU
.mi- e-i -cy
Tuhgsten Filament
<
Figure 2.1 X-Ray is produced when the electrons are fired from the electron gun
[8],
focused spot of electrons that are then raster over a specimen surface by
means of a scan deflection system. A specimen stage is needed so that the
21


specimen may be inserted and situated relative to the beam. A secondary
electron
detector is used to collect the electrons and to generate a signal that is
processed by electronics and ultimately displayed on viewing and
recording monitors i.e. CRT. Modern SEMs also have the capability for
storing and processing digital images. A vacuum system is necessary to
remove air molecules that might impede the passage of the high energy
down the column as well as to permit the low energy secondary electrons
to travel to the detector.
2.2.1 Condenser Lens (CL)
The condenser lenses are necessary for resolving powers needed. The first
condenser lens (CL) begins the demagnification (i.e. decrease in spot size)
of the 50 pm focused of electrons formed in the area of the electron gun.
The amount of current running through the first condenser lens is
increased, the focal length of the lens becomes progressively shorter and
the focused spot of electrons becomes smaller. The overall effect of
22


increasing the strength of CL is to decrease the spot size, but with a loss of
electrons. This increases the resolution, but the overall signal (number of
secondary electrons) coming from the specimen will become weaker since
fewer beam electrons strike the specimen.
Apertures are placed in the lens to help decrease the spot size and to
reduce spherical aberration by excluding the more peripheral electrons.
Each of the condenser lenses behave in a similar manner and posses
apertures, some of which may be either in size and placement in the
column or which may be variable and adjustable using controls on the
column of the SEM as shown in the Figure 2.2.
2.2.2 The Final Condenser Lens
The final condenser lens, often called as objective lens, is the strongest
length in the SEM and does the final demagnification of the focused spot
electrons. This final or Third lens is used primarily to fine tune the spot
size without loss of electron beam and secondary to focus the image seen
on Cathode Ray Tube.
23


High tension cable
Figure 2.2 The column assembly for the SEM [9]
24


This final condenser lens usually contains two sets of deflected coils and a
stigmator. The deflected coil is connected to a scan generator to raster the
electron spot across the specimen. Rastering not only move the spot in a
straight line across the specimen, but also moves the spot down the
specimen as well ( i.e. it possesses both x and y movements). A change in
the magnification is achieved by varying the length of beam scanned on
the specimen versus the length displayed on the viewing screen. For
example, if the electron probe is scanned over a 10 mm distance on the
specimen and the length displayed on CRT is 10 cm, then we get
magnification to be lOx. Suppose that we have a smaller scanning length
of 1pm and CRT is displaying 10 cm, then we can say that magnification
is of 100,000x. In general magnification is given by
Length displayed on CRT
Magnification = -------;--------------------
Length scanned on Specimen
2.2.3 Electron Gun
Electron gun is essentially required in SEM for high brightness, a small
source area and high stability especially that of velocity of emitted
25


electrons. There are several types of electrons gun which meet these
requirement, which may vary in composition details, but are basically
composed of three electrodes, as shown in the Figure 8 (thermionic
emission electron gun).
The hot cathode Filament in the Figure is normally a hair-pin filament.
Considering the work function (WF), melting point, vapor pressure,
mechanical strength and so on, tungsten is normally selected for the
Filament Hea:inq Supply
Figure 2.3 Electron gun configuration [9]
26


filament material. Wehnelt Cylinder is equivalent to control grid in a
vacuum tube. The characteristics of the electron gun are determined by the
shape and position of filament, grid and anode and the relation between
their electric potentials. When a higher brightness and finer source is
necessary, a field emission electron gun may be used.
When a high negative potential from high voltage supply is applied to the
filament and a current from the filament heating supply flows through
filament, thermionic electrons are emitted from the tip of the filament and
are accelerated by the filament-anode plate potential. At the same time, a
voltage drop across the Bias Resistor Rb,as supplies the bias potential for
the Wehnelt. The electron gun used in JSM 820 SEM is designed to
minimize the diameter of the crossover and to obtain higher brightness.
However, position of the filament must be carefully adjusted to ensure
proper performance. The stability of the electron gun greatly depends on
negative feedback by self-biasing. However, the power source must be
properly regulated and the instrument must be designed to minimize the
possibility of minute discharging. Furthermore, the filament is heated
gradually; the quantity of emitted electrons reaches its saturation in
27


accordance with the space charge. This heat level must be maintained to
produce a stable electron beam. However, the excessive heating of the
filament is wasteful, and merely shortens the life of the filament.
2.2.4 Specimen Chamber
They are three basic specifications for any well-designed Specimen
chamber are good specimen or specimen holder stability, smooth stage
motion for the efficient selection of the field of view and quick, easy
specimen exchange. In the high performance SEM, the specimen chamber
must be completely isolated from external vibrations which can reduce the
resolution and the desired field of view must be precisely located.
Another important requirement of the specimen chamber is capability for
accommodating various attachments for expanded applications; for
example, X-ray spectrometers for quantitative and qualitative specimen
analysis, a backscattered electron detector for observation of composition
and topographic images, a transmitter electron detector for observing the
transmitted electrons images, a cathodoluminescence detector for
28


observing cathodoluminescent substance, and a semiconductor specimen
holder for obtaining images of potential distributions and electromotive
force in integrated circuits.
2.2.5 Secondary Electron Detector
The secondary electron detector unit for a scanning electron microscope is
destined particularly for working at pressures of order of 100 Pa in the
sample chamber. The secondary electrons detector unit is composed of
secondary electron scintillator detector placed behind the microporous
plate that is built into vacuum wall separating the intermediate chamber
and the electron optical column part. Here, the detector scintillator and the
microporous plate are located coaxially with the electron optical column,
along the electron beam goes. The scintillator is placed aside from the
electron optical axis, where the stream of secondary electrons is led into
the intermediate chamber through the hole in the lower throttling aperture
and impinges symmetrically the input surface of the microporous plate,
which is placed at the optical axis. The electrons passing across the
microporous plate are multiplied, and attracted by the scintillator biased
29


with a high voltage at the output side, which is finally detected. The active
input surface of the microporous plate is ring-shaped and coaxial with the
electron beam. In the central part it is limited by the hole made on the axis
of the microporous plate to let through the electron beam while its
Electron beam
Electron beam Scintillator
Specimen
Electron
tx
sm
Specimen
Amplifier
(C)
(d>
Figure 2.4 Different methods of detecting electrons in SEM. (a) Secondary
electrons, (b) Backscattered Electrons, solid state detector, (c) Backscattered
Electrons, scintillation counter and (d) Absorbed electron current [10].
30


maximum diameter is limited by the opening in the frame plate, which
also fixes the microporous plate in the frame body of the detector. The
substance of the invention consists in the fact, that the active input area of
the microporous plate is placed asymmetrically with regards to the axis of
the scanning electrons beam.
The advantages of the secondary electron detector unit for a scanning
electron microscope according to the invention are that the gas flow to the
high vacuum part can be reduced and the detector lifetime can be
prolonged while the sensitivity of the detector is preserved.
> Secondary Electrons
1. Secondary electrons are detected by scintillator -
photomultiplier system known as the Everhart-Thorny
detector as shown in the Figure 8.
2. The number of detected secondary electrons increases with
the tilt. About 20 to 40 degrees tilt towards the detector
may be necessary.
31


3. Topographic images obtained with secondary electrons
look remarkably like the image of the solid objects viewed
with light.
4. We find these topographic images easy to interpret.
V Backscattered Electrons (BSE)
The backscattered electron (BSE) moves so fast that it always travels
in a straight line. In order to form an image with BSE, a detector is
placed in the path of these electrons. All the elements have different
sized nuclei. As the size of the atom nucleus increases, the number of
BSE increases. Thus, I3SE can be used to get image that showed the
different elements present in a sample.
Topographical images may be obtained by using Backscattered
electrons which is detected with:
Scintillator light pipe photomultiplier (e.g. Robinson
detector) type detectors, known for their rapid response time as
shown in Figure 8.
32


Solid State detectors, with a disadvantage of relatively slow'
response time.
Backscattered electrons and X-rays from the specimens are also capable of
yielding composition information. The compositions of samples are
sensitive to backscattered coefficient (r|), which varies monotonically with
the atomic number.
The magnitude of the compositional or atomic contrast (C) from two
phases of backscattered coefficient r]l and r)2 is calculated as
£ Vl-V2
Vi
This method is not recommended for the phases with similar atomic
number, as the resolution will be very poor.
As the primary electron beam is scanned across the surface, electrons of a
wide range of energies will be emitted from the surface in the region
where the beam is incident. These electrons will include Backscattered
electrons and Auger electrons, but majority of the electrons are Secondary
33


electrons formed in multiple inelastic scattering processes. The secondary
electron current reaching the detector is recorded and the microscope
image consists of a plot of this current against probe position on the
surface. In the micrograph, the contrast arises from several mechanisms,
but first and foremost variation is from surface topography. Consequently,
the secondary electron micrograph is a direct image of a real surface
structure. The attainable resolution is limited by minimum spot size that
can be obtained with the incident electrons beam and thus it results in
scattering of this beam as it interacts with the substrate. With the modern
instruments, resolution of better than 5 nm is achievable. This is more than
adequate for imaging semiconductor device structures, but insufficient to
enable many supported metal catalysts to be studied in any details.
2.2.6 Vacuum System
Vacuum system is used to minimize the interference of extraneous
particles with the electron beam. In a scanning electron microscope
(SEM), vacuum pressure in the column should be below 10~5 Torr. If the
pressure in the column is high, then it can result in high tension discharges
34


and specimen contamination. To achieve the required pressure, the
scanning electron microscope is usually evacuated by oil rotary pump and
oil diffusion pump.
F. thrust outlet
valvr
Blatk
Sprion
Pump oil and
oil rexcrvoH
Figure 2.5 Oil Rotary Pump [5]
Figure 2.5 shows the structure of Oil rotary pump. The pump interior is
placed in a housing containing oil. This oil serves as a lubricant to the
pump and makes it airtight. When the rotor rotates in the direction of the
35


arrows, the air in one crescent chamber is compressed above atmospheric
pressure. As a result the air is pumped out through the exhaust port.
Simultaneously, air enters the crescent from the inlet port for the next
compression. So we can say that two pumping processes takes place with
each revolution of the rotor. Moreover, this two stage rotary pump also has
very less vibration and exhaust noise levels. However, the attainable
pressure by this pump is limited to about 10'3 Torr, which is not enough
for pumping and to maintain back-pressure in the diffusion pump.
Figure 2.6 shows the structure of an oil diffusion pump which is composed
of heater/boiler, water-cooled casing, jet chimney and a water-cooled
baffle to keep oil from back streaming. The oil is heated by the
boiler/heater which turns into a vapor that is led to the chimney and is
jetted from the nozzle at supersonic speed toward the wide low-pressure
space. By doing so, the gas molecules from the column diffuse into the oil
jet steam and the intermingled gas is compressed by the kinetic energy of
the jet flow and is transferred to the exhaust port. This jetted oil vapor is
condensed by the water-cooled casing and the condensed oil is drained
back into the boiler. The low-boiling point contents in the recovered oil
36


are removed by evaporation. Oil diffusion pump uses an auxiliary rotary
pump to provide the initial low pressure (below 10'1 Torr). The oil
o
diffusion pump can maintain the pressure of approximately 10" Torr and
the pressure can be further lowered by using traps.
1 Heater
2 Boiler
3 Pump body
4 Cooling coil
5 High vacuum
flange
6 Gas molecules
7 Vhpor jet
8 Backing vacuum
connection
B 1
'-Nozzles
0
o
o
0
c
o
O o~
6
Figure 2.6 Diffusion Pump [5]
A basic vacuum system is composed of an oil rotary pump (PR), an oil
diffusion pump (DP) and five manual valves. The pumping down
37


procedure starts with the valves V|, V2, V3 and leak valves LVi is closed
and LV2 is open.
Start
1. Close leak valve LV2 and turn on the RP switch
2. Open valve V2 and turn on the DP heater.
Evacuation
1. Close valve V2 and open V3 for roughing (down to ~ 10'2
Torr)
2. Close V3, and open V2 and V| to attain require pressure
down to ~ 10'5Torr.
Leaking and re-evacuation
1. Close Vi and open LV| to admit air into the column.
2. Close LV| and re-evacuate.
Shutdown
1. Close V i and turn off the DP heater.
2. Close V2, turn off the RP and open LV2.
38


3. Operation of SEM
3.1 Startup, Shutdown and Emergency Operation
The Operation of the SEM is the most important part. You have to be
carefully about many things before you start the SEM. Firstly, set an
appropriate nitrogen pressure from the valve of the nitrogen cylinder,
otherwise the roughing pump will not come on. Secondly, the water
flow pressure is also important. If the How is less, then it will fail to
cool down the diffusion pump and which will cause the sudden
shutdown of the SEM. If the flow is too high, then diffusion pump will
not reach its appropriate temperature and it will not turn the SEM on
and after one hour it will shutdown automatically. Mere are the
operations to prevent the sudden shutdown of SEM are outlined as
follows:
3.1.1 Startup
1. Open the valve of the nitrogen cylinder and verify that the
39


m
secondary pressure gauge of the cylinder should read at least 70
Psi.
2. Turn on the cooling water with an appropriate water pressure.
. Turn on the distribution board switch.
4. Turn on the power key to start. This starts the instrument and all
procedures required for high voltage application are automatically
carried out.
5. The following things appears on the operation and display panels:
Scanning mode PIC button lights up, Scanning speed
- TV button lights up, Magnification button lights up and
show you lOOx on CRT, WD indicator should show 15 on
CRT, and film number indicator reads according to the
photo number sw itch setting.
6. When the evacuation is complete and the column reaches its
specific pressure, the current/emission meter lights up in about 35
to 40 minutes. T his indicates that high vacuum pressure is being
applied and the SEM is ready for microscopy.
40


3.1.2 Shutdown
1. Verify that the column is under high vacuum pressure (by looking
at current meter) and the Accelerating voltage button is turned off.
2. Turn off the power key and wait for 3 to 4 minutes. (You may hear
the sound of nitrogen gas going in the column).
3. Turn off the distribution board switch.
4. Turn off the valve of nitrogen gas cylinder.
5. Do not close the water supply, until the Diffusion pump totally
cools down, for about couple of hours.
3.1.3 Emergency Operation
In the case of an emergency or sudden instrument shutdown, take the
following steps:
1. Turn off the distribution sw itch.
2. Turn off the power key.
3. Turn off the nitrogen gas cylinder.
41


Figure 3.1 Specimen Holder [5]
3.2 Specimen Exchange
3.2.1 Loading the specimen holder with a specimen
Mount the specimen on the stub which is fixed in the cylinder. This
cylinder is placed on the height adjust screw to adjust the height and fix
the specimen with the setscrew. Then mount the cylinder on the specimen
holder and fix the cylinder with the setscrew as shown in the Figure 3.1.
42


3.2.2 Specimen holder exchange
1. Turn off the Accel Voltage-on button on the 1st operation panel.
2. Set the tilt control to 0.
3. Turn the vent button on. The built-in lamp brightens and nitrogen
gas enters the specimen chamber. The specimen chamber pressure
will rise to atmospheric pressure within 3 to4 minutes.
4. Unfasten the stage clamps and pull out the specimen stage.
5. Insert the specimen holder in the holder mount as shown in the
Figure 3.2.
6. Push the stage back in the specimen chamber and fasten the
clamps.
7. Turn off the vent button. The built-in lamp dims out and diffusion
pump comes on to provide the specific pressure for SEM. After 5
to 6 minutes the current meter lamb lights up.
43


Specimen holder
Stage clamp
Holder mount
Figure 3.2 Specimen Chamber in SEM [5]
8. Make sure that the ALARM/AEM switch on the supplementary
panel is set on ALARM. Because as soon as the specimen or the
specimen holder touches the objective lens, SEM will start
ALARM indicating that something is wrong in the specimen
chamber.
3.2 Image Formation in the SEM
44


In the SEM, the image is formed and presented by a very fine electron
beam, which is focused on the surface of the specimen. The beam is
scanned over the specimen in a series of lines and frames called a raster,
just like the (much weaker) electron beam in an ordinary television. The
raster movement is accomplished by means of small coils of wire carrying
the controlling current (the scan coils). A schematic drawing of an electron
microscope is shown in Figure 3.3.
At any given moment, the specimen is bombarded with over a very small
area. Several things may happen to these electrons. They may be
elastically reflected from the specimen, with no loss of energy. They may
be absorbed by the specimen and give rise to secondary electrons of very
low energy, together with X-rays. They may also give rise to emission of
visible light (an effect known as cathodoluminescence) and to electric
current within the specimen. All these effects are used to produce an
image. By far the most common image formation is by the means of low
energy secondary electrons.
45


Figure 3.3 Schematic Diagram of a Scanning Electron Microscope (SEM) with
secondary electrons forming an image on CRT [9],
The secondary electrons are selectively attracted to a grid held at low
positive potential (50 volts) with respect to specimen. Behind the grid is a
disc which is held at about 10 kilovolts positive with respect to the
specimen. This disc consists of scintillant coated with a thin layer of
aluminum. The secondary electrons pass through the grid and strike the
46


disc, causing the emission of light from the scintillant. The light is led
down to a photomultiplier tube which converts the photons of light to
voltage. T he strength of the voltage depends on the number of secondary
electrons that are striking the disc. Thus, the secondary electrons produced
from a small area of the specimen gives rise to a voltage signal of the
particular strength. The voltage is led out of the microscope column to the
electric console, where it is processed and amplified to generate a point of
brightness on the CRT screen. An image is built up simply by scanning the
electron beam across the specimen in exact synchrony with the scan of the
electron beam in the CRT.
The SEM does not contain objective, intermediate and projector lenses to
magnify the image as in the optical microscope. Instead the magnification
results from the ratio of the area scanned on the specimen to the area of
the CRT screen. Increasing the magnification in an SEM is therefore
achieved by simply by scanning the electron beam over a smaller area of
the specimen.
47


The principle of an electron microscope is based on the light microscope
except that electrons are used instead of light. T he resolving power of any
microscope is given by Abbes equation and shown in Figure 2.4.
d0
0.61 X
n sin a
[nm]
Where dQ is minimum resolvable separation distance, n is refractive index
of the medium between the object and the objective lens, X is wavelength
of the light and a is half-angle subtended by the objective at the object.
Lower the minimum resolvable distance, higher the resolution of the
instrument. Therefore, the resolution is enhanced by a short wavelength of
the light, a high refractive index of the medium between the objective lens
and the object, and a small distance between the object and the lens. The
product n sin a is called the numerical aperture of the objective lens. As a
can never exceed 90, an objective in air can never resolve distances
smaller than 0.61 X. The limits of a light microscope are given in the Table
3.1. De Broglie2 showed that an electron has a dual character. It can be
48


regarded either as a moving charged single particle or as a radiation with a
distinct wavelength.
Table 3.1: Minimum resolvable distance (do) of a light microscope
depends on the wavelength (X), the refractive index (n) and the angle (a).
Wavelength (A.) Angle (a) Refractive index (n) Minimum resolving distance (do)
800 15 1.000a 1885
400 15 1.000a 942
800 30 1,000a 972
400 30 1.000a 488
800 60 1,000a 563
400 60 1,000a 281
800 60 1.516b 371
49


The relation between the two is given by
A = -- [nm]
mv
Where m is mass of the electron and v is velocity of the electron.
The velocity of electron depends on the voltage applied and can be used to
calculate the wavelength (A), and the minimum resolvable distance (do), as
shown in the Table 3.2. The advantage to use electrons as a light source
compared with visible light is obvious.
Table 3.2: By changing the accelerating voltage for an electron
microscope, the electron velocity, wavelength and minimum resolvable
distance will also change accordingly.
Accelerating Voltage (kV) Velocity (km/hr) X (nm) do (nm)
1 18,370 0.03876 0.0169
10 58460 0.0122 0.0053
0.0016 0.0016 0.0016 0.0016
50


3.4 Specimen Preparation
A wide variety of specimens may be viewed with scanning electron
microscope. Their preparative techniques may be equally diverse. This
section intends only to outline on very general terms some of the
procedures more commonly used for viewing specimens in the secondary
electron mode.
> Type of Specimen:
a. Those materials which are already hard and dry, such as
mineral specimens, bone, hair, scales, metal, etc
b. Those which may not be dry, but are fairly rigid and
resistant to distortions which result from desiccation. This
group might include some hard bodied insects (beetles,
flies), seeds, pollen grains, wood, etc.
c. Those samples which are soft and wet; this includes most
biological specimens. These needs to be dried carefully to
51


avoid distortion. They are likewise in need of chemical
fixation in order to preserve the structure.
Dehydration: For SEM, dehydration is an essential process; it
eliminates water from the specimen, typically by replacing it with
an organic solvent with a low surface tension, usually either
ethanol or acetone.
Drying: Air drying is useful for those specimens that can stand it.
The specimens are simply dumped out on a piece of lint free paper
and the solvent is allowed to evaporate.
52


4. MEMS FABRICATION
Micro-Electro-Mechanical Systems (MEMS) is the integration of
mechanical elements, sensors, actuators and electronics on a common
silicon substrate through microfabrication technology. While the
electronics are fabricated using the Integrated Circuit (1C) process, the
micromechanical components are fabricated using the compatible
micromachining processes thats electively etch away parts of the
silicon wafer to add new structural layers to form the mechanical and
electromechanical devices.
Today MEMS represent one of the most exciting areas in the field of
microelectronics activity. MEMS technology has brought together
innovations from different area of microelectronics only to develop
rapidly into the disciple of its own. Todays micro-machined systems
combine signal processing and computational capability of analog and
digital integrated circuit with a wide variety of non-electrical elements,
including pressure, temperature and chemical sensor, mechanical gears,
and actuators, 3D mirror structure, etc [11"].
53


MEMS structures are based upon our ability to sculpt or machine silicon
on a microelectronics scale. These micromachining technologies can be
broken into three groups: (i) Bulk micromachining, (ii) Surface
micromachining and (iii) High-aspect ratio electroplated structures.
The first, bulk micromachining was back in 1960s, which was used for
wet anisotropic etching to achieve various forms of trenches, grooves and
membranes in silicon wafer was developed. In 1970s, the advance bulk
micromachining was used to develop the impurity-dependent etch stops,
wafer dissolution processes, and wafer fusion bonding. During the next
two decades, surface micromachining came into the development, which
makes use of full lithography capability of IC processing. The application
of circuits to improve the sensor characteristics advanced greatly during
this time. In 1990s, processes capable of producing high-aspect-ratio
structures were developed using thick polymer molds and electroplating
and a variety of methods for merging MEMS fabrication with standard
CMOS and bipolar processes were demonstrated [11], 1 will concentrate
on bulk micromachining anisotropic etching.
54


4.1 Bulk micromachining
Bulk micromachining is the process of removing the selective region of
silicon substrate, ranging from a simple cavity formation to almost
complete dissolution of the wafer. Wet chemistry was used in original
micromachining processes, whereas vapor and plasma etching are in
widely used today [11].
4.1.1 Isotropic and anisotropic etching
Figure 4.1 is the case of isotropic etching. In this etching processes, the
etch progresses equally in all directions and will also undercut the
masking material at the surface. Agitation of the fluid during the etching
process makes more round features, but does not change the undercutting
at the surface.
In the case of anisotropic etching, it attacks certain crystal planes rapidly
than others and produce etched cavity with the tlat surfaces that intersect
each other at sharp well-defined angles. In case of silicon, the etching rate
of <100> and <110> planes are much higher that <111> plane. For
55


example, in the case of KOH solution, water and alcohol, the etch rate of
<100>, <110> and <111> is 40:30:1 [11],
7
7
(a)
(b)
Figure 4.1 Isotropic Etching (a) without agitation (b) with agitation [11],
These etching rates can create various cavity and grooves structures, as
shown in the Figure 4.2. At <100> silicon, the <111> plane intersect the
surface at an angle of 54.74, forming a v-groove and pyramidal cavities
56


bounded by these planes. The etching front is defined by the silicon
dioxide opening and processes vertically downward as the <100> plane is
(a)
<100
<1111-
Figure 4.2 Anisotropic etching of <100> exposing slow etching <111> planes
which form a 54.74 degree angle with a surface, (a) Formation of a flat
bottomed trench and (b) a cavity in the form of an inverted pyramid [11],
57


etched away, exposing the slow-etching <111> plane. If the etch is
permitted to continue, the <111> plane will intersect each other, forming
an inverted pyramid or a v-groove, as indicated in the Figure 15. The
depth of the flat-bottomed cavities is determined by the etch rate and
etching time. On the other hand, we can also say that the v-groove is a
self-terminating structure, because of the slow etch rate of <111> plane.
The depth of the v-groove depends on the width of the mask opening at
the surface. It is important to realize that the sidewalls are called <111>
plane only for an etchant. The actual angle of <111> plane will be
approximately 54.6 for 400:1 etch selectivity and 52 for a 20:1
selectivity.
The process to achieve this v-groove on the silicon wafer at our
laboratory is outlines as follows:
1. Wafer Cleaning
Fill up 3 squeeze bottles with DI water, acetone and alcohol
(isopropyl). Insert the 2 vacuum chuck on the motor shaft, place
the wafer on the chuck and turn on the spinner. Take acetone in
58


one hand and alcohol in another. As soon as the spinner starts
rotating squeeze the acetone on it and then squeeze alcohol on it as
soon as you stop squeezing acetone. Now squeeze DI water on it
and let the spinner spin the wafer for 40 seconds. We use acetone
and alcohol to get rid of abrasive particles, lint from wipers,
photoresist residue from the previous photolithography, dust, etc.
during this entire process, make sure to turn on the vent and the
station sash is always down in order to avoid dust or any other
particles to get on the surface of the wafer.
2. Pre-Baking (or Dehydration Bake)
We remove the wafer form the spinner chuck and place it on the
hot plate for 5 minutes at 95 C. The purpose of doing so is to let
all solvents to evaporate from the surface of the wafer. After pre-
baking the wafer, it needs to be cooled down for like 2 minutes
before the photoresist coating process starts.
3. Photoresist Coating
In order to get a uniform resist (S-1813) coating on the wafer, the
wafer placed on the spinner should have appropriate velocity.
59


From the previous experiments, we figured out that the velocity of
the spinner should start from 250 RPM with an acceleration of 200
rpm/s as shown in the Figure 4.3. When the wafer is on the
spinner, we pore a HDMI solution (positive photoresist) on the
wafer with a pipette and let it spin with the same velocity that we
set for photoresist coating.

Figure 4.3 Velocity of the spinner starts from 250 rpm with acceleration of 200
RPM/s and reach 3000 RPM within 7 seconds [5]
Thereafter the coating process follows. We start to pour lmL of the
solution on the center of the wafer with a pipette, while it starts
slowly to spin. Within 7 seconds it will reach the velocity of 3000
RPM and as a result the entire resist covers the wafer. We get a
60


thin layer of photoresist on it. The duration of this whole process
of coating the w afer is about 40 seconds.
SPIN SPEED (rpm)
Figure 4.4 Photoresist thickness versus the spin Speed [5]
From Figure 4.4. we can see the relationship between the spin
speed and the photoresist thickness and for 3000 RPM speed we
get 1.5 pm thickness of the photoresist (S-l 813) on the wafer.
4. Soft-Baking
Carefully remove the wafer from the spinner and place it on the hot
plate for 3 minutes at 95 C. It is important to soft-bake it because
61


it is better to evaporate the coating solvent for a better adhesion
before we expose it.
5. Mask Alignment (OAI) / Exposure
Turn on the vacuum through a red switch behind the table for N2.
Turn on the power supply for the UV light and wait for 2 to 3
minutes to get ready for the exposure. Place the wafer on the chuck
and place the mask on the top of the wafer. Be careful not to break
the mask itself, a glass plate with patterned emulsion of metal film
on one side. Mask alignment is one of the most important steps if it
comes to photolithography. The distance between the mask and
wafer is also important. Bring the wafer up until the knob is little
bit hard to turn i.e. wafer should be about 10 to 25 microns wide
close to mask. Align the mask with the wafer in order to get pattern
through exposure. Once the alignment is done, push the small
black button in the front of OAI and move the movable part under
the light exposure area. As soon as the wafer/mask is pushed under
the exposure area. UV light starts to expose for 30 seconds. Move
62


the movable part of OAI to its original position and carefully
remove the mask and wafer. You can see the pattern on the wafer.
6. Post-Baking
After exposing of the wafer is done, we remove the wafer from the
chuck and place the wafer on the hot plate at 110 C for 99
seconds. This step is important as we need to harden the
photoresist and to improve adhesion of the resist on the wafer.
7. Development
Pour DI water in one of the beakers and developer (MFim 24A)
in another beaker. Take the tweezers and grab the wafer from it.
Put it in the developer for 70 seconds and agitate it until you are
able to see the pattern on the wafer surface. If you keep the wafer
in the developer more than 70 seconds, it will start removing the
photoresist other than the exposed area. Then you have to start
everything from the scratch. Thereafter, rinse the wafer in the Dl
water. Place the wafer on the spinner. Turn on the vacuum and
then spinner. Take the squeeze bottle of Dl water and start pouring
63


on it. And wait until the spinner stops completely. Rinsing it with
DI water will stop the reaction of the developer chemical.
8. Hard-Baking
Place the wafer on the hot plate as soon as we remove it from the
spinner. We leave the wafer on it for 5 minutes at 110 C to harden
the remaining photoresist and improve adhesion to the substrate.
Therefore, the photoresist is ready to protect the Si02 layer during
etching process.
9. BOE Etching
BOE is a dangerous chemical. BOE consists of HF acid which
causes a burn hazardous in a sense that the discomfort from the
burn is not noticed for a long period of time after the exposure. It
burns the tissues with an internal injury without any initial burn
sensation. If exposed then immediately wash the exposed part with
water and let the lab instructor know of HE acid contact. Always
wear a lab coat, goggles, acid proof gloves on the top of normal
cleanroom gloves, and an acid face mask with the face shield down
are required.
64


After wearing all the protection, take out BOE in one beaker. Place
the wafer on Teflon coated wafer holder. The etch rate of BOE for
thermal Si02 is 1000 A/minute. By seeing the color of the wafer,
we can say that thickness of oxide layer is approximately 0.5 pm.
Firstly. 1 immerse the wafer into BOE for 8 minutes. Remove and
rinse the wafer with D1 water. Place the wafer on the spinner and
start it. Pour the DI water on it and keep it on the spinner for 40
seconds. Remove the wafer from spinner and inspect the wafer
under the microscope. The etch regions with exposed silicon
should appear to be silver metallic colored. By observing under
microscope, 1 found out that there was not enough etching done on
the wafer. Again 1 immersed the wafer in the BOE for another 2
mins and rinse the wafer with DI water. This time when observed
under microscope, 1 can see the silver metallic color in the exposed
region. In BOE etching, do not remove photoresist from wafer
unless you are certain that there is no oxide left on it. BOE etching
can also be done a little bit longer time if you are not sure whether
65


oxide layer is there or not, as it does not affect silicon layer.
Dektak was used to measure the depth of the
Figure 4.5 DEKTAK graph indicates the depth of the pattern on the wafer with
photoresist and Si02.
pattern on the wafer. From dektak, I found out that the exposed
pattern is now 24618 A (2.46 pm) deep, with photoresist and Si02
on the wafer as shown in the Figure 4.5. Therefore, the total BOE
etching time was 10 minutes.
66


Now remove the unwanted photoresist left on the wafer. Place the
wafer on the spinner and turn it on. Pour the acetone and alcohol
on it one after the other and pour some DI water to clean it. Keep it
on the spinner for 40 seconds.
10. KOH etching
KOH solutions are very corrosive, reactive to air and water. IPA
solution of KOH reduces the surface tension. If KOH solution falls
on your any part of body or exposed wash off that part
Figure 4.6 Depending on the wide of the Pattern, the depth of the
pattern is determined to get v-grooves
----- 7 urn ------!
Si02 mask
67


immediately with water and let the lab instructor know about it.
After removing the photoresist you can see the SiC>2 on the wafer
with the pattern with white or metallic color on it. Take the Teflon
coated holder and mount the wafer on it. Before dipping the wafer
into the KOH solution, we will measure the depth of the pattern so
that we can achieve v-groove on the wafer. This is shown in the
Figure 4.6.
From Figure 4.6, we can say that if the width of the pattern or
mask is 7 pm. Then the depth of the pattern is approximately V2 *
3.5 = 4.95 pm. We have the KOFI solution which has the etching
rates depends on the temperature as shown in the Figure 4.7.
50% KOH solution means it contains 50% of KOH solution and
remaining 50% is DI water and alcohol. 1 selected 85 C where the
etching rate is approximately 60 microns/hour i.e. 1 micron/min.
The etching rate of the silicon dioxide is 550 nm/hour at 85 C.
Since the etching rate of Si and Si02 is different, it will also help to
form a v-groove in the wafer, but it may also widen up the feature
of the pattern.
68


Figure 4.7 The etching rate for 50% KOH solution [12].
Take clean glassware for the KOll etching. Pour the solution in it. Set the
hot plate to 85 C. It will take like 10 minutes to reach 85 C. Keep the
glassware on the hot plate for around 25 to 30 minutes. Take the holder
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with wafer and dip it in the KOH solution. Once the wafer is completely in
the solution, you will start seeing the small bubbles coming out of the
pattern on the wafer. Keep agitating the wafer in the solution to obtain a
proper result. It took 40 minutes of etching to form a v-groove on the
wafer.
\
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5. Results and Application
KOH etching is done to form a cavity on the pattern. Basically, the etching
rate of <111> planes is very slow compared to <100> planes and this helps
to form v-grooves. So two <111> planes come together in the bottom of
the grooves and etching will be continue until they come together. The
angle between the <100> planes and the sidewall of the groove (< 111 >
planes) is 54.74. So this type of the etching is called Anisotropic Etching
because the etching does not proceed at the same rate in all directions.
5.1 Results
In KOH etching, several iterative steps were required between the etching
and SEM observation. We started the sample with 5 minutes of etching in
KOH solution.
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Figure 5.1 The depth of the pattern after 5 minutes of etching
To stop the etch we rinse the sample with DI wafer. Keep the water on the
spinner and turn it on and pour the DI water on it to remove the KOH
solution on it. look the wafer at DEKTAK instrument to measure the
depth of the pattern as shown in the Figure 5.1. We found out that the
depth of the pattern is 2.4 pm at this stage of etching, which was 50% of
our estimated depth i.e. 5 pm. Followed the same process as discussed
above and kept the wafer for 7 additonal minutes in the solution.
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Figure 5.2 The depth of the pattern after 12 minutes of etching
We again measured the depth of the pattern on DEKTAK. instrument. This
time, it was 4.6 pm deep into the wafer as shown in the Figure 5.2. this
process was repeated for 3 more minutes. Thereafter, the depth of the
pattern was 4.7 pm as shown in the Figure 5.3. But width of the pattern
was also increased during KOH etching.
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Figure 5.3 The depth of the pattern taken after 15 minutes of etching
During the characterization using SEM we found out that the pattern on
the wafer does not have v-groove but it had a trapezoidal pattern as shown
in the Figure.5.4.
From Figure 5.4, we found out that the depth of the pattern on the wafer is
approximately 5 pm and it can be said that more KOFI etching is needed.
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Figure 5.4 The image of the wafer taken under the SEM after 15 minutes of KOH
etching
Therefore, from the Figure 5.4, we needed to etch the wafer approximately
for an additional 15 minutes. This time we found the v-groove, but it still
had the flat bottom as shown in the figure 5.5.
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Figure 5.5 The image of wafer taken under the SEM after 30 minutes of KOH
etching.
After 10 additional minutes, the depth of the pattern was on DEKTAK
instrument and we found out that it was 15.13 pm as shown in the Figure
5.6.
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Figure 5.6 The depth of pattern measured on the DEKTAK instrument
Finally, Figure 5.7 shows the v-groove that is generated on the wafer
where <111> planes have come together. The depth of the pattern from the
SEM image is approximately 15 pm. For 40 minutes of etching at 85 C,
we were able to produce a 15 pm of depth where the width of the feature
is 24.6 pm. So we can also say that etching widen the feature of the
pattern.
77


5.e*
xi,100 10
' 3 k.
Figure 5.7 The pattern has a v-groove on the wafer taken from SEM
5.2 Application
The SEM is routinely used to generate high resolution images of shapes of
objects (SEI) and to show spatial variations in chemical compositions
(usually EDS, also BSE and CL). This instrument is also widely used to
identify phases based on qualitative chemical analysis and/or crystalline
78


structure. Precise measurement of very small features and objects down to
50 nm in size is also accomplished using SEM. Backscattered Electron
images can be used for rapid discrimination of phases in multiphase
samples. SEMs equipped with diffracted backscattered electron detectors
can be used to examine micro fabric and crystallographic orientation in
many materials.
One of the principles uses of SEM is to study the surface feature, or
topography of a sample. Other example for which SEM is useful is
microscopic feature measurement, fracture characterization,
microstructure studies, thin coating evaluations, surface contamination
examination and failure analysis.
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6. CONCLUSION
The main aim of my thesis is to reassemble and repair the SEM and to
create a fabrication process and recipe for silicon MEMS. The process
recipe is different for individual laboratories due to the various setup of a
laboratory including. It was necessary to develop an in-house accurate
process model for MEMS. From this process recipe, we obtain v-groove in
the silicon wafer pattern with the depth of 15.13 pm as expected. Finally,
from this experimental result, we have examined the operation of the
microscope. We have concluded that the SEM instrument is working
properly and the MEMS recipe development is also satisfactory.
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