The document summarizes key aspects of microfabrication technology taught by Assoc. Prof. Dr. Mariya Aleksandrova at the Technical University of Sofia, Bulgaria. It outlines the teaching activities, which involve project-oriented education in microtechnology and nanoengineering. It describes several basic microfabrication processes, including thermal oxidation, doping, vacuum deposition, photolithography, etching, and thick film deposition techniques like electroplating and screen printing. Examples are provided to illustrate how these processes are used to fabricate microelectronic and microelectromechanical systems devices.

1. International Webinar on
Microfabrication Technology
by Assoc. Prof. Dr. Mariya Aleksandrova,
Department of Microelectronics,
Technical University of Sofia, Bulgaria
E-mail: m_aleksandrova@tu-sofia.bg
09.07.2020 at 2pm( IST)
1

2. 2
Outline:
• Who we are and where we are found.
• What is our teaching activity – particularly learning “Microtechnology
and nanoengineering” – project oriented education.
• Basic principles and processes related to the microfabrication
technology.
• COVID-19 challenges – distant learning thought a home-made
platform for distant learning.
• What is our research activity and how we involve our students in it.

3. 3
• Who we are and where we are found.
Bulgaria is located in the South-Eastern Europe Sofia is the capital city of Bulgaria,
situated in the West Bulgaria

4. 4
The Technical University of Sofia, is the largest technical university in
Bulgaria. Founded October 1945 as part of the Higher Technical School, it
is an independent institution since 1953 with 14 faculties in Sofia, three
departments and serval centres. The University is a leader in the field of
nanotechnologies, virtual engineering, energy efficiency, renewable
energy resources, engineering ecology and engineering design.

5. 5
Group “Materials science for micro-/nanoelectronics and thin films deposition”
Head of the group Dr. Mariya Aleksandrova (m_aleksandrova@tu-sofia.bg)
Fabrication and study of flexible and glass based organic and inorganic
electroluminescent and light-emitting diode devices (OLEDs)

6. 6
Fabrication and study of flexible
piezoelectric energy harvesting
based on new materials and new
approaches for nanostructuring.

7. 7
We are dealing with non-conventional approaches for nanopatterning and
nanostructuring, as well as with the conventional microtechnology for integrated
circuits fabrication and printed circuit boards fabrication

8. 8
Education in Microelectronics and Microtechnologies & Nanoengineering
Ist-3rd course – common electronic engineering subjects (mathematics, physics, analog
and digital devices, micropocessors, etc.)
4th course – 4 choices for specialty: Electronic systems, Medical electronics, Power
electronics and Microelectronics
Bachelor and master from Faculty of Electronic Engineering and Technologies,
specialty Electronics
7th sem.
-Microelectronic technologies (e)
-Microelectronic circuits (e)
-Materials science in microel. (e)
-Microel. engineering automati-
zation (e)
-Specialized practice (s)
-Course project (s + m)
8th sem.
-Surface mounting technologies (e)
-Microsystems technologies (o)* (oa)
-Integrated circuits design and
programming (o)* (oa)
-Micromodules for automotive
Industry (e)
-Pre-diploma project (s+m)
8th semester ends with diploma work defense.
Students get electronic signatures to verify their presence in class
during the semester.
(e) – exam; (s+m) – signature + mark; (oa) – ongoing assessment, no exam ; (o)* - means optional, or
you may select subject from another specialty.
• What is our teaching activity

9. 9
Education in Microelectronics and Microtechnologies & Nanoengineering
Bachelor and master from Faculty of Electronic Engineering and Technologies,
specialty Electronics or other Faculties with additional year of study
Ist
sem.
-CAD systems in microelectronics (e)
-Introduction in nanoelectronics (oa)
-Common engineering subjects –
Mathematical methods for signals
processing, Programming, Energy
Convertors, Projects design (e)
- Course work (s)
2nd
sem.
-Very large scale integrated circuits (e)
-Display devices (e)
-Functional microelectronics (e)
-Nanomaterials (o)*(oa)
-Thin films deposition methods (o)*(oa)
-Panning and analysis of the expe-
riment (o)* (oa)
-Course project (s+m)
(e) – exam; (s+m) – signature + mark; (oa) – ongoing assessment, no exam ; (o)* - means optional, or
you may select subject from another specialty.
3rd sem. – Diploma work preparation and defense
Students get electronic signatures to verify their presence in class during the semester.

10. 10
Education in Microelectronics and Microtechnologies & Nanoengineering
Master only from all engineering specialties in TU-Sofia (Telecommunications,
Mechatronic, Industrial Engineering, etc.) – relatively new, project-oriented specialty
established in 2014.
2nd
sem.
3rd sem. – Diploma work preparation and defense
-Nanomaterials
-Technologies in micro- and nanosystems
-Basic principles and applications of
micro- and nanosystems
-Nanocommunication networks
-Project design
- Optional between “Quantum physics”,
“Nanochemistry” or “Environmental
nanotechnology”
- Optional between “Reliability of
nanosystems”, “Micromechanics and
Nanotribology” or “Metrology and
Mechanical testing of microsystems”
- Optional between “Thin film
electronics”, “Microelectronic tech-
nologies for energy harvesting” or
“Bioelectronics”
- Optional between “3D modelling
and simulation of micro- and
nanosystems”, “Piezoelectric sen-
sors” or “CAD systems for design of
micro- and nanosystems”
Ist
sem.

11. 11
Project-oriented laboratory work? For example let see the subject Technologies for micro-
nanosystems
Exercise 1: Wafer cleaning, nanocoatings vacuum
deposition and nanocoatings characterization.
Exercise 2: Photolithographic patterning of the
nanocoatings deposited during Ex.1
Exercise 3: Etching of the nanocoatings unprotected
with the patterned photoresist during the Ex.2.
Exercise 4: Electroplating of some coatings etched
during the Ex.3.
Exercise 5: Deep silicon etching
………………………………
Step-by-step and process – by – process the students build their own
microsensor/microactuator/simple microcircuit
• Basic principles and processes related to the microfabrication technology.

12. 12
Thermal oxidation of silicon wafer. Doping of silicon wafer
The thermal oxidation is a process that uses oxidant to oxidize a bare silicon surface to
silicon dioxide at elevated temperatures. Silicon dioxide (SiO2) is an excellent isolator,
with a resistivity higher than 1016 Ω.cm with excellent thermal and mechanical stability
and for this reason it serves as protective mask for ion implantation and diffusion, and as
a undergate oxide in the Metal-Oxide-Semiconductor transistors and capacitors (MOS).
Although SiO2 films can be formed with chemical vapor deposition (CVD), reactive vacuum
sputtering, the thermal oxidation of bare silicon provides the best oxide quality in terms
of purity, density and insulation. However, thermal oxidation has some application
limitations. It requires presence of a silicon surface, and it must be conducted at relatively
high temperatures, that are usually higher than 800oC. The thermal oxidation can be
conducted in a dry or wet ambient, with oxygen only or water vapor enriched oxygen.
Schematic illustration of the furnace and photos of working furnace and
equipment with 4 sections for oxidation and doping and the control blocks

13. 13
Thermal oxidation of silicon wafer. Doping of silicon wafer
Dependence of the thickness of SiO2 on the time and
temperature at a dry (left) and wet oxidation (right).
One of the most typical applications
of the thermally grown SiO2 is
protection of the silicon wafer
during doping process which is local
introduction of dopants under the
silicon surface by diffusion process.

14. 14
vapor flux
substrate holder
substrate
current
controller
vacuum
chamber
to the vacuum
pump
water cooling
system
evaporator
vapor flux
substrate holder
substrate
current
controller
vacuum
chamber
to the vacuum
pump
water cooling
system
evaporator
materials for
evaporation
copper or graphite
pocket evaporator
water cooling
system
filament
10kV
accelerating
aperture
electron
beam
magnetic field
for e-beam
bending
substrate
melt
vaporized flux
materials for
evaporation
copper or graphite
pocket evaporator
water cooling
system
filament
10kV
accelerating
aperture
electron
beam
magnetic field
for e-beam
bending
substrate
melt
vaporized flux
Vacuum deposition of thin films – single component (metals) nanocoatings are produced
by thermal evaporation and alloys and other multicomponent (compounds) are produced
by electron beam evaporation. It is for metal interconnection in the integrated circuits.
Thermal evaporation E-beam evaporation

15. 15
Vacuum deposition of thin films – metal oxides and metal nitrides are produced by
RF reactive sputtering. It is for transparent conductive oxides, gas sensing, magnetic,
temperature, pressure and other sensing complex compounds.
-DC (or ~RF)
+DC (or ground)
substrate holder
(anode) substrate
thin film
plasma
cathode
water cooling
system
shield
Ar+ Ar+
inert gas
(Ar) ions
ejected
particle
target
Ar
Ar
Ar Ar
---
electron
-DC (or ~RF)
+DC (or ground)
substrate holder
(anode) substrate
thin film
plasma
cathode
water cooling
system
shield
Ar+Ar+ Ar+Ar+
inert gas
(Ar) ions
ejected
particle
target
Ar
Ar
Ar Ar
------
electron
Basic principle, high – vacuum equipment and parameters control at sputtering process

16. 16
Deposit-ion
method
Substrate
type
Deposition
rate
Substrate
temperature
Density,
Adhesion
Step
covera-ge
Materials Typical
applications
Thermal
Evapora-
tion
Glass,
quartz,
silicon,
ceramic,
flexible
<1 nm/min Lower than
50 oC if no
additional
heating is
supplied
Poor Poor if
planeta-
ry or
movable
holder is
not used
Single
component
coatings,
mostly
metals with
low melting
point
Metallization
of integrated
circuits;
adhesive
sublayers (Al,
Ag, Au, Ni, Cr,
Cu)
E-beam
Evapora-
tion
Glass,
quartz,
silicon,
ceramic,
flexible
~ 1 nm/min Lower than
100 oC if no
additional
heating is
supplied
Better that
thermal
evapora-
tion, but
worse than
sputter
Poor if
planeta-
ry or
movable
holder is
not used
Alloys;
complex
compound
(excluding
metal
oxides);
refractory
metals
NiCr chip
resistors; ZnS,
CdTe, InSb, Zn
Se
photosensi-
tive and
electrolumi-
nescent
semiconduc-
tors
Sputtering Glass,
quartz,
silicon,
ceramic,
flexible
(shorter
sputter
time)
> 10 nm/min Can reaches
100oC at
continuous
sputtering
Good Good Multicom-
ponent
semicon-
ductors and
dielectrics,
including
metal
oxides.
Transparent
conductive
oxides; high –
k dielectrics:
ITO, ZnO, SiO,
TiO2, Ta2O5
Comparative table of the features of the vacuum deposition processes

17. 17
Photolithographic patterning of coatings
Photolithographic patterning of a coating is the process of transferring of the geometrical
dimensions, shapes and positions of the microelectronic or micromechanical components
from the drawn topology (usually in specialized CAD system) into the substrate (wafer),
covered with certain functional film. The film is most often produced by some of the
vacuum deposition techniques. Film growth is not selective process in nature, so the
coating (film) cover entire surface area of the substrate. After that, by supplying
photolithographic sequence, the film is shaped according to the desired configuration
(schematic project), obtained by computer program.
Example image of glass photomask,
consisting part of integrated
circuit’s topology.

18. 18
Principle of projection stepper photolithography (left) and principle of mask alignment (right)
Simple sensor device with heater, requiring masks alignment
Photolithographic patterning of coatings

19. 19
substrate
film
photoresist
photomask
UV rays
negative
photoresist
positive
photoresist
substrate
film
photoresist
photomask
UV rays
negative
photoresist
positive
photoresist
Patterning effects for positive and negative
photoresist after UV exposure
Examples for patterning of coatings
with shapes typically used in MEMS
devices.
Photolithographic patterning of coatings

20. 20
Selective etching of nanocoatings – surface micromachining
Etching is a process of selectively removing given material unprotected from photoresist.
The etching must follows the edges of the patterned photoresistive mask and to not affect
the coating under this mask. There are two main types of etching: wet and dry. The wet
etching is chemical process of dissolving given target coating by dipping the wafer in
chemical solution (etcher), which is aggressive to this target material and doesn’t affects
the other coatings on the wafer. Dry etching is conducted in vacuum chamber, where ions
of inert gas sputter the coating and physically remove particles for the material, in similar
way like they are able to sputter the target disk material during deposition of thin films
Specifics of the etching process

21. 21
Anisotropic (deep) silicon etching – bulk micromachining
This process falls into the group of bulk micromachining processes, which means that
three-dimensional features are created into the bulk of crystalline (silicon) substrate. In
contrast, surface micromachined features are deposited layer by layer on the top of the
silicon substrate. Deep silicon etching can be also wet and dry like at surface
micromachining. Again the considerations for selection of either wet or dry process are
connected with the cost, equipment complexity, etch rate and precision.
Basic silicon building block and main crystal
planes in the silicon, with the Miller indices.
Left: dry etching of silicon – completely
anisotropic; right: wet etching of silicon –
partially anisotropic.
400 μm
upper aluminum
contact
movable silicon
capacitor’s electrode
100 μm
bottom aluminum
capacitor’s electrodeetched cavitydielectric film
titanium dioxide
silicon wafer
depoxy resin
S
d
S
C rTiOo 2

external pressure
d
S
C rTiOo 2

400 μm
upper aluminum
contact
movable silicon
capacitor’s electrode
100 μm
bottom aluminum
capacitor’s electrodeetched cavitydielectric film
titanium dioxide
silicon wafer
depoxy resin
S
400 μm
upper aluminum
contact
movable silicon
capacitor’s electrode
100 μm
bottom aluminum
capacitor’s electrodeetched cavitydielectric film
titanium dioxide
silicon wafer
depoxy resin
S
d
S
C rTiOo 2

external pressureexternal pressure
d
S
C rTiOo 2

Example of a pressure sensor and flexible silicon wafer

22. 22
Deposition of thick films – electroplating and screen printing
The purpose of thick-layer technology application is to obtain coatings with a thickness
over 1μm. This is necessary when better heat dissipation is required in the case of high
current density flowing through the electronic microsystem; or better wear resistance of
sliding surfaces in switching microcontacts is required; or corrosion resistance of parts of
sensors operating in harsh environments; or resistances and capacities that cannot be
provided by thin-film integral resistors and capacitors. Depending on whether the
coating is conductive, resistive, insulating, piezoelectric, magnetic, etc., and whether the
substrate’s surface is flat-parallel or has pre-patterned elements, there are varieties of
thick-film technologies. For example, for metal coatings, the method of electrochemical
growth (electroplating) on a preliminary deposited conductive film is preferred. When
using materials without electrical conductivity and the electrochemical process cannot
be realized, then screen printing technology (printing topological shapes through a screen
with fine apertures forming the desired configuration) is used.
Examples of electrochemically grown
contacts (left) and screen printed thick
meander type resistor (right).

23. 23
Schematic representation of the electroplating
and electroplating station.
Electroplating and screen printing
Principle of paste (or ink) screen printing – the
only one possibility for non-conductive thick
films deposition. The printer design is down.
Chip mounting and wire bonding on golden
electroplated pads in micrometric range

24. 24
Packaging and mounting processes
Wafer dicing by laser or diamond edge
Chip and wire bonding
with gold wire Chip encapsulation

25. 25
Packaging and mounting processes
Types of integrated circuits packages for low power electronics (left and middle);
hybrid IC for high power electronics (right) and soldering of integrated circuit on PCB (down).

26. 26
• COVID-19 challenges
During the pandemic we used a home-made
platform “E-management” developed by Prof.
Dr. Valentin Videkov and Assoc. Prof. Rossen
Radonov from our department, that covers
the standards involved in the commercial
systems, but has enriched functionality.
The new features are:
• Self-adapted questioner system
providing exam questions based on
the gap in the knowledge (this is
estimated by previous answers
provided during the example test),
thus stimulates the students to fill this
gap by additional study on the topic.
• Random inverting of the sense of the sentences during examination to avoid
answers memorizing and stimulation of thinking and answering.

27. 27
……. The new features are:
• If a student look in the fellow’s individual results, the system automatically penalize
him/her with score decrease. This can be set in the case when the lecturer or tutor
doesn’t allow group work, to provoke students to perform alone their individual
assignments, not taking the results from elsewhere .
• There is build in plagiarism check function, however, limited within the information
uploaded in the web-site – avoiding copy-paste images and graphs from fellows. It
also gives an information how many keys are knocked on the keyboards during the
students work in the system, which is also useful when there is a doubt of copy-
paste.

28. 28
The rest of resources are similar like in the other platforms – it is possible to upload video
demonstrators of the processes, quizzes, live chat with the teacher, etc.
Who answer first his/her questions for homework assignments and has greater than 70%
true answers gets bonus scores, thus stimulating the students to not forget of their
homework and in the same time to make efforts not just to register first, but reasonable
login.

29. 29
• What is our research activity and how we involve our students in it.
Currently 4 big projects are running in my group (2 national and 2 international) and 2
consortiums intercontinental projects are submitted pending assessment.
• Bi-lateral projects “Bulgaria – India” 2018, “Stable and High Sensitive Low
Dimensional Perovskite Photodetectors”, 2019-2021.
• Bi-lateral projects “Bulgaria – India” 2018, “Ultrahigh efficient lead-free perovskite
solar cells”, 2019-2021.
• Researcher and coordinator in project of National Science Fund in “Competition for
financial support of fundamental research”, entitled “Ferroelectric materials on
silicon for new sensor devices”, 2018-2021.
• Researcher and coordinator in project of National Science Fund in “Competition for
financial support of fundamental research”, entitled “Study of the piezoelectric
response of layered microgenerators on flexible substrates”, 2016-2020.
Our students prepare their course projects, diploma thesis and students conference
papers, working on the projects mostly with technical assistance. Thus, they are eligible
to apply for scholarships and grants provided from the European Union programmes for
Youth Education, Future young scientists development, etc. Thus they are paid during
their learning and they gain practical experience and knowledge beyond the curriculum
content, which is of help for their future engineering realization.

30. 30
Thank you for your attention!
Any collaborations in the mentioned
fields of study and learning/education are welcome!
Contact: m_aleksandrova@tu-sofia.bg
https://maleksandrova.wixsite.com/oled
Assoc. Prof. Dr. Mariya Aleksandrova