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Transcript of Printable Electronics
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Printable Electronics
Final Report
Author: Supervisor:
Rana Azhar Shaheen Matti Mntysalo
Student # 217446
3rd May 2010
Department of Electronics
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II
ABSTRACT
The fabrication of different electronic devices includes the deposition of thin film
materials. Development in the fabrication techniques such as lithography, masking,
tools and photo resist materials brought the concept of miniaturization but with the
expense of cost and time. Rapid prototyping of electronic devices with the help of
CAD/CAM processes is gaining importance day by day, due to which lots of direct-
write technologies have been emerged to satisfy this need. They build structures with
different resolutions, writing speeds, three-dimensional processing, and operational
environment.
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III
CONTENT
ABSTRACT ................................................................................................................ IICONTENT ................................................................................................................. III1. Introduction ............................................................................................................ 1
1.1. Direct-Write technologies in Electronics......................................................... 11.2. Biomaterials ................................................................................................... 2
2. Direct-Write Technologies used for microelectronic manufacturing ........... ............. 32.1. Introduction .................................................................................................... 32.2. Micro-dispensers ............................................................................................ 32.3. Laser-Assisted Transfer .................................................................................. 42.4. Electrostatic-Assisted Transfer ....................................................................... 52.5. Jetting............................................................................................................. 5
3. Electrochemical Power Devices in Direct-Write Technologies .......... ...................... 63.1. Introduction .................................................................................................... 63.2. Classification of Electrical Power Devices ...................................................... 63.3. Direct-Write Layers ........................................................................................ 83.4. Battery and fuel cell applications using Direct-Write Layers......................... 10
4. Advanced Materials for Direct-Write Technologies ............................................... 124.1. Introduction .................................................................................................. 124.2. Existing Material Systems Direct-Write Technologies .............. .................... 124.3. Materials Requirements in Different Deposition Methods ....................... ...... 134.4. Super-Low-Fire Inks and Pastes ................................................................... 15
5. Ink-Jet Technique .................................................................................................. 175.1.
Continuous Mode Ink-jet Technology........................................................... 17
5.2. Demand Mode Ink-Jet Technology ............................................................... 185.3. Materials for Jetting Technology .................................................................. 185.4. Pattern Formation ......................................................................................... 185.5. Direct-Write Applications............................................................................. 19
6. Direct-Write Micromachining Using Laser............................................................ 22
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IV
6.1. Introduction .................................................................................................. 226.2. Laser-Matter Interaction ............................................................................... 226.3. Laser Micromachining .................................................................................. 236.4. Laser Micromachining Tools ........................................................................ 246.5. Applications of Laser Micromachining ......................................................... 25
7. Micrometer and Nanometer Pattern and Material Transfer Technologies ............. .. 267.1. Introduction .................................................................................................. 267.2. Applications of Pattern Transfer Technologies ............................................. 277.3. Different Lithography Techniques ................................................................ 277.4. Applications of Material Transfer Technologies ........................................... 287.5. Methods of Material Transfer Technologies ................................................. 28
8. References ............................................................................................................ 29
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1. Introduction
The fabrication of different electronic devices includes the deposition of thin film
materials. Development in the fabrication techniques such as lithography, masking,
tools and photo resist materials brought the concept of miniaturization but with the
expense of cost and time. Rapid prototyping of electronic devices with the help of
CAD/CAM processes is gaining importance day by day, due to which lots of direct-
write technologies have been emerged to satisfy this need. They build structures with
different resolutions, writing speeds, three-dimensional processing, and operational
environment. Different direct-write technologies are available in the market such as,
ink-jet printing, laser transfer techniques, laser vapor deposition (LCVD), matrix-
assisted pulsed-laser evaporation direct-write (MAPLE-DW) and micropen [1]. The
development in the direct-write technologies is growing due to the development of
different microlevel devices such as third generation sensors for medical applications
which require more miniaturization to decrease their size. Two major areas, electronics
and biomaterials are playing the key role in the development of direct-write prototyping
as the majority applications of direct-write technologies belong to these areas.
1.1. Direct-Write technologies in Electronics
Direct-write technologies can be used to reduce the cost with rapid prototyping of
different electronic devices e.g. biological and chemical sensors, to simplify the
manufacturing of printed circuit board at low costs and tissue engineering. Fig.1 shows
different applications using different direct-write techniques. Each application has
different manufacturing criteria depending on the method of direct-writing. Different
tools are needed to transfer the different materials, not a single tool can write all the
materials at the same time, so more than one tool is used to direct-write the material on
the substrate.
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Figure 1. Direct-write applications using different tools.[1]
The major challenges in direct-writing the materials on 3-D surfaces are low
temperature processing and high writing speeds. Today, direct-write techniques are able
to manufacture the passive components and interconnect between the components with
almost same performance as with thick-film techniques using different conductive and
dielectric materials. The available techniques for direct-writing the material on the
substrate as shown in fig. 1, differ from each other by the method they use to transfer
the material. Material development play very important role in the development of
direct-write systems. Starting materials commonly known as pastes or inks consist
of different powders, organic precursors, binders, vehicles, solvent, dispersants and
surfactants [1]. These materials are deposited at low processing temperatures to
manufacture conductors, dielectrics and resistors for passive components at low
temperature substrates.
1.2. Biomaterials
Advancement in the materials has driven the development in direct-writing of
electronic materials but advancements in transfer techniques developed the deposition
of biomaterials. There are many tools that have to ability to deposit any kind of material
on any substrate with micrometer scale accuracy. Two laser transfer techniques are
being used to deposit the biomaterials. Laser guided transfer technique is used to
deposit the neural living cells which can be further used for analysis. The deposition of
biomaterials can be done in three dimensional manner to manipulate physical structures
for analysis their behavior and diagnose different diseases.
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2. Direct-Write Technologies used for
microelectronic manufacturing
2.1. Introduction
Different significant properties such as three dimensional fabrication,
miniaturization, substrate versatility and rapid prototyping made the direct-write
technologies a very important candidate for the next generation electronic devices. The
recent trends in microelectronic industry are adopting the direct-write technologies
because they use compact system assemblies with very low weights. Rapid prototyping
is another main feature of these technologies which play important role in the
development of electronic industry. For continuous, non stop, long-term and reliable
supplying it is important to make some industry standard for these technologies which
can be used by multiple vendors. There are different kinds of technologies available in
the market which can be used to manufacture microelectronics with direct-write
techniques which are listed as follows:
1- Microdispensing
2- Laser-assisted transfer
3- Electrostatic assisted transfer
4- Jetting
2.2. Micro-dispensers
In the microelectronics industry different commercial micro-dispensing technologies
are available such as, rotary screw, needle valve and positive displacement piston. Cost
of these systems vary between $125K to $250K, require special clean room
environment with the dimensions of room approximately 9.3 2m of floor. Sensitivity of
direct-write systems for microelectronics is high as very small gauge dispensing tips are
used to get fine features. To dispense uniform structures, different parameters such as
viscosity and surface tension of starting materials, needle gauge and length play an
important role. The repeatability of the deposited materials and its volume depends on
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the dispensing technique used. The dimensions range of the deposited materials with
micro-dispensers are 25 - 300m wide and 1.3 - 51m thick [1]. The fabricated
electronic circuit by using micro-dispensing system is shown in fig. 1.
Figure 2. fabricated electronic circuitry with 40m width and thickness of 5m
using micro-dispensing system.
2.3. Laser-Assisted Transfer
The transfer of metals and other conductive and insulating materials can be possible
with the use of laser-assisted transfer systems from the donor films to substrates. In
microelectronics industry, these systems are being used in wafer fabrication and also intransferring of gold onto pin connections. Multiple laser processes such as transfer,
sintering, curing and annealing are being combined to develop a single system by using
a laser with different wavelengths. Higher power lasers to heat the small part of the
substrates for sintering and curing, are under development. The schematic of laser-
assisted transfer technology is shown in fig. 2.
The deposition of materials with 25m dimensions can be possible with the use a
laser-assisted technique. Some systems of laser-assisted transfer melt the metal
nanoparticles during the flight before hitting to the substrate.
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Figure 2. Schematic of laser assisted technology. [1]
2.4. Electrostatic-Assisted Transfer
Electrostatic-assisted transfer is based on xerographic printing [1]. Differentmaterial parameters are important in this type of transfer. Electrostatic-assisted transfer
technique is being used commercially for manufacturing of different scales of
microelectronics devices. They can deposit line widths of 30 m with 3.5 m spot size
at micro-scale. Scanning speed of a 10 cm wide scanner can be 500 rev/s . Micro-scale
manufacturing of a 10 cm x 10 cm substrate can be done in 16 sec.
2.5. Jetting
Jetting technology has the ability to deposit the materials at low temperatures. Some
ink-jetting techniques are used to process the inks thermally, with small range of
viscosities. The ink-jetting process is described in details in section 4. The modern
jetting techniques are using the piezoelectric materials to pressurize the ink to exit from
the orifice with applied voltage pulse. the viscosities of the inks used in jetting systems
should be high which can easily come out from the orifice. Different modes are used to
transfer the material onto the substrate, i.e. continuous mode and demand-on mode
jetting.
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3. Electrochemical Power Devices in Direct-
Write Technologies
3.1. Introduction
The devices like batteries, fuel cells and capacitors are the devices that stores or
produces electrical energy, these devices are known as electrochemical devices. Direct-
write deposition technologies can be used to improve the efficiency of electrochemical
devices. Materials can be deposited with direct-write tools with different tools at low
temperatures.
Power devices are considered as the back bone of the electronic industry. Batteries
are used for portable devices like, laptops, cell phones and MP3 players while
nonportable devices can be powered from the national grid distributed system. Fuel
cells are gaining importance in the distribution power generation due to the better
performance and high-reliability electrical power.
3.2. Classification of Electrical Power Devices
The electrical power devices are classified into different categories depending on the
method by which they generate energy, e-g. batteries and capacitors store chemical
energy and convert or produce electrical energy, fuel cells convert chemical energy into
electrical energy by using different fuels i.e. gasoline, natural gas or methanol. Power
devices are manufactured depending on the performance characteristics of the power
devices rather than the requirement of the device to which it is providing power. Hence
there is a strong need of such packaging structures in which power devices are
integrated into the package, deriving power to the device on demand. Direct-write
technologies specifically printed technology is the best candidate in this regard in which
the battery is printed into the device packaging.
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Figure 1. schematic representation of the power requiremts for devices as a function
of the size of the device [1].
Fuel cells
Fuel cells have several advantages over their mechanical counter parts in generating
electrical power. They create less noise, operate at low temperatures, avoid pollution
and have higher efficiency. Fuel cells convert a fuel such as gasoline, natural gas, or
hydrogen into electrical energy. There are five types of fuel cells; alkaline fuel cells
(AFC), phosphoric acid fuel cells (PAFC), the proton exchange membrane fuel cells
(PEMFC), molten carbonate fuel cells (MCFC) and solid oxide fuel cells (SOFC).
These technologies are suitable for different kind of applications like DMFCs are most
suitable for small portables. Among all of these, PEMFC suits best candidate for
different kind of big applications like, electric vehicles and large portables.
The range power production of these technologies varies with the used technology,
like DMFC provide power in the range of 0.01-300W which is best for small
applications, while PEMFCs produce high power in the range of 100-300,000 W and
SOFCs provide best alternative for low-power applications such as, MEMS and RF
tags. The MCFCs and SOFCs are based on high temperatures to produce electrical
power so they are not suitable to fabricate with direct-write technologies, while other
technologies like, DMFC, PEMFC, PAFC and AFC are related with low temperature
fuels known as electrocatalysts.
Batteries
Batteries convert chemical energy into electrical energy. They store energy in the
form of chemical energy and convert it into electrical energy on demand. There are two
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categories of batteries with respect to the demand of applications, nonchargeable and
rechargeable. Due to the increasing demand of performace and the very fast market-
adoption rates, the new trend is being developed in battery industry to develop the
application-specific batteries using custom-designs.
As the development in the products which depend on the batteries are getting high
speed, while the battery industry still based on the 20 year old theories. Due to this
lagging in technology development of batteries also become a hurdle in the
development of many electronic products. Also there are lots of applications and
devices (RF tags, powered greeting cards and magazines) that can become cheaply
available if the dimensions of batteries can be reduces (very thin with high power). The
reduction in size and weight are material and fabrication technology dependent, which
can be easily achieved by using printable material systems. The development of battery
industry bring the Li-polymer batteries which have the same energy density as of Li-ion
but it can be formed into different shapes and sizes to fulfill the application-specific
requirements.
Supercapacitors
Supercapacitors have capability to store much more energy than conventional
batteries on the same weight and size. They are a type of capacitors that stores energy
within two electrochemical parallel plates. The charge separation is high so the resultant
capacitance per unit area is also large. Supercapacitors can achieve high efficiency
performance if combined with a battery. They have no limit for charging and
discharging providing high discharge currents. They have identical structure to the
membrane electrode assembly (MEA) in a PEMFC.
3.3. Direct-Write Layers
The performance of batteries and fuel cells can be enhanced by improving the
materials of thinner and well composed layer structures, and this can be done by
depositing the patterned layers using the different direct-write deposition technologies.
The efficient use of materials for high volume manufacturing, in cases where the
materials can be extremely expensive is one advantage of using direct write layers. The
direct-write tools can also be efficiently used to control the deposition of material layers
to a surface over very short distances in different dimensions. Direct-write tools can be
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used to deposit the layers to provide the complex topographical structures. Patterned
layers are used to control the functionality of the application, which can be deposited by
using direct-write technologies.
PEMFCs
A PEMFC is composed of three sections as shown in figure 2, the fuel processor,
the power section of fuel cell stack and the power conditioner.
The fuel processor converts the fuel of the cell into hydrogen rich gas. The ideal
case is to convert the fuel into hydrogen and supply it to power section of PEMFC. This
hydrogen rich gas is then used in the fuel cell stack for further operation. MEA
(Membrane Electrode Assembly) is the place where the hydrogen gas from the fuel
processor and air is delivered to convert chemical energy into chemical energy by using
electrocatalysts as shown in figure 2(b). Each MEA unit provides up to 0.8 V and the
overall voltage is formed from the combination of number of MEA cells. The MEA cell
voltage and its current density are the two performance parameters which can be
represented by the curve, called polarization curve.
(a) (b)
Figure 2.[1] (a) schematic of PEMFC, (b) view of one MEA
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Metal-Air Batteries
There are different kinds of metal-air batteries which have the best and highest
power densities of all other battery power sources. Different metal can be used in the
combination of air to construct these batteries such as, Li, Fe, Al and Zn. Metal is used
at one terminal of the battery (anode) and life cycle of the battery depends on the mass
of metal used. The air is used on the other terminal of the battery (cathode) and the
discharge rate depends on this part of the battery. Metals used in the metal-air batteries
have limited scaling and life cycle due to which their use in portable devices is limited.
The critical part of the construction of these batteries is to print the layers especially
the electrode that compose air and the performance of the battery depends on these
layers.
The materials used in fuel cells and metal-air batteries are almost similar becausethese two technologies based on the similar structures. These materials are categorized
under three classes, (i) materials that can be processed at low temperatures, such as
silver to generate current collectors, (ii) electrocatalyst powders, which is normally used
in spray-based manufacturing, (iii) ion transport and hydrophobic control materials.
3.4. Battery and fuel cell applications using Direct-WriteLayers
The performance of the battery and fuel cell can be improved by using the layers
deposited by direct-write technologies. There are different layers such as current
collector and active layers which can be deposited using direct layer tools to improve
the performance of these layers.
Current Collector
Generally the current collector layers used in batteries are composed of nickel mesh,
which have some advantages such as its structural rigidity, and some disadvantages as
well such as its relatively large thickness and difficult to integrate into high-volumemanufacturing process. In this scenario, a direct-write current collector can be the best
alternative to this problem as the silver based grid structures can be deposited with
different direct write tools.
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Active Layers
The active layers are the layers which are deposited on the surface of the current
collector layer of metal-air batteries and fuel cells for the catalytic reaction of the gases.
Different kinds of performance parameters can be achieved if these layers are deposited
using direct-write technologies such as, deposition of controllable thickness of layers,
specific-placement of materials and the control of composition of different layers over
very small distances. Syringe dispenser is the best choice of direct-write tool to deposit
active layer because it has better control over these parameters.
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4. Advanced Materials for Direct-Write
Technologies
4.1. Introduction
Materials play a key role in the development of the direct-write technologies. The
goal of a direct write tool is to produce the rapid prototyping of the electronic devices
such as sensors, by fabricating the different materials which are dissimilar to each other,
but have same intrinsic properties to perform the desired function. The whole direct-
write prototyping process highly depends on these materials as they are the starting
point. The material used in direct-write technologies depends on the method of transfer.
The growing field of wireless devices increases the demand for mobile phones and
portable devices with high data transfer capability is revolutionizing the electronics
packaging and designing industry. Lots of new technologies have been emerged to
replace the old ones to meet the challenges in the industry. Flip chip technology is one
example of next generation interconnects technology which is the alternative of wire-
bonded packages. It provides the direct electrical connection between substrate and die.
Due to the lots of developments at huge pace in the electronics industry the new hot
topics for the research and development is to manufacturing the passive and active
components (resistors, capacitors, inductors, transformers, sensors, batteries) on three
dimensional substrates with the help of computer aided design or computer aided
manufacturing (CAD/CAM) tools.
4.2. Existing Material Systems Direct-Write Technologies
The performance of any direct-write technology mainly depends on the materialperformance after processing at low temperatures. The available source materials such
as thin film and thick film do not fulfill completely the required promising processing in
direct-write technologies under desired conditions. So, the new tool specific materials
needed to be developed that can be compatible with direct-write technologies under
specific thermal processing conditions. There are different material systems
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commercially available in the market such as, thin film, thick film and polymer thick
film, they are summarized in fig. 1.
Figure 1. overview of different technologies with their sizes and processing
temperatures.
Thick film like polymer thick film and inorganic thick film technologies are
compatible for the higher processing temperatures so unusable for organic substrates.
Inorganic thick-film technology include sintering of metal powders, metal oxide, glass
and other powders and need high processing temperatures. Also they have higher
feature sizes of about 100 microns as shown by fig. 1. Polymer thick-film also have the
problem of feature size but they can be processed at relatively low temperatures with the
expense of low quality and reliability problems. Thick film technologies are mainly
used in screen printing where the resolution is pretty higher than other existing
technologies. Thin-film material technology can be processed at low temperatures and
also have small feature sizes but with higher costs than other technologies.
4.3. Materials Requirements in Different Deposition MethodsThe available and emerging direct-write technologies (printing) need specific
requirements for the processing of materials under specific conditions. Some are
discussed here such as, ink jetting, plasma spray deposition, microsyringe dispensing,
matrix assisted pulsed laser evaporation-direct write (MAPLE-DW), laser guidance and
laser chemical vapor deposition (LCVD).
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Ink jetting is used in a wide range of printing techniques where the drop or column
of ink printed on a printing substrates. The drop-on-demand printing has some
advantages such as it is more compatible with digital technology, due to its printing in
discrete manners, its speed and scalability give options for high frequency printing with
multiple nozzles adjusted in array which makes it possible to use different types of inks
in a single printing process. Different techniques are used in jetting the ink and most
important is the use of piezoelectric materials for creating the pressure to eject the single
drop through the nozzle. When the thick layers ( > 10m) of metals, metal alloys,
ceramics, dielectrics and polymers are desired to deposit on a substrate then
thermal/plasma spray deposition process would be the best option. This technology has
wide range of applications such as, automotive components, gas turbine components,
paper processing, printing and biomedical parts. Thermal spray technology is becoming
the topic of research in different new technologies like, fabrication of meso-electronic
multi layers, sensor systems and embedded sensors. The most straight-forward method
for direct-writing the materials on the substrates is microsyringe dispensing technique.
The ink or paste is pressurized or forced through a small syringe to deposit on the
substrate to form the deposited pattern by moving the pen relative to the instructions
given through CAD/CAM processing. The deposited material is then dried or heated to
make it functional.
Modern technologies of direct-writing the materials onto the substrates include the
important role of lasers. One of the technique used for direct-writing using lasers is
matrix assisted pulsed laser evaporation which uses the laser to transfer the material
onto the substrate. Laser pulse acts as a transparent carrier in transferring the material.
While transferring, the material absorbs the energy of the laser which make it
evaporated at the interface causing the transfer of discrete packet of material towards
the substrate. Desired pattern of writing can be achieved by using a sequence of laser
pulses for material transfer while moving one or both pulses at a time. Laser guidance
and flow guidance technique is another use of laser in direct-writing the materials. The
generation of precursor materials is done in a aerosol generator from which the droplets
are deposited onto the desired substrate by using either laser guidance through a thin
fiber optic or flow guidance through an orifice. For the generation of precursor materials
in aerosol generator, the organic solutions of molecular precursors or suspension of
powders are used which is different than other delivery methods discussed in this
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section. Finally, the thermal processing of direct-writing using the laser is known as
laser chemical vapor deposition (LCVD). The small part of the substrate is heated up
using the laser and transfer the material in gaseous state to the substrate. This processing
is done at low temperatures by heating a small spatial part at one time by using a short
pulse of the laser.
4.4. Super-Low-Fire Inks and Pastes
The development in inks and pastes is being done to produce the materials for
conventional as well as direct-write technologies to deposit the components such as
conductors, resistors, batteries, capacitors and inductors. The previously discussed
thick-film and thin film approaches are compatible with the low temperatures. Also
metals and ceramic deposition occur at high temperatures which are not suitable for
organic substrates. To overcome these problems, the approach to combine the material
chemistry and laser processing is used, which can be shown in fig. 2.
Figure 2. Graphical representation of the general approach to combine the material
chemistries and laser processing
The formulation of ink or paste is the basic and starting point for any deposition
system. The ink formulation include the low viscosity formulation that can be used in
ink jetting technique, paste is a thick formulation than ink and can be used in screenprinting or tape casting. After depositing the inks or pastes to the substrate the drying
process is used to evaporate the liquid from the deposited paste or ink. Rapid drying at
high temperatures should be avoided. To speedup the drying process air flow drying can
be used. The formulation of inks or pastes for deposition of passive components with
direct-write tool depends on the molecular precursors. The precursor chemistry depends
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on whether an ink or a thick film paste and the direct write tool used. A typical mixture
for a paste would contain particles, a molecular precursor to the functional phase,
vehicle, binder and additives. Low-melting glass can be used as additives which has
many technological advantages such as good dielectric and structural strengths.
Conductors act as mainly as wiring to connect all the components to form the
functional circuit. They played an important role in the hybrid microelectronics. They
can be formulated in both low viscosity inks with or without solid particles and thick
film pastes with high solid particles. The determining parameters for the best suited
materials are low cost, compatibility with the substrates, conductivity, resistance and
attainable minimum feature size.
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5. Ink-Jet Technique
Ink-jet printing is a valuable technique for direct-writing the materials on organic
substrates at low temperatures with the drop size of material to be transferred ranges
from 15-200m at rates of 0-25,000 per second per droplet. There are different
applications of printing technique using piezoelectric dispensing, such as biomedical
reagents, liquid metals and optical polymers. Depositing materials using ink-jet
technique requires CAD information only and is data-driven with no mask or screens
required.
5.1. Continuous Mode Ink-jet Technology
In this technology a continuous stream of drops under pressure, ejected from an
orifice of diameter 50-80m due to the pressure created from some electromechanical
device. The drop forming comes into place when fluid passes under charged field when
comes out from orifice thus each drop acquires static charge. These charged drops
transfer to their desired positions on the substrate with the help of an electrostatic field
which is used to deflect the charged drops. This procedure is well described in fig. 1.
Figure 1. Schematic diagram of a continuous mode ink jet printing system
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5.2. Demand Mode Ink-Jet Technology
In this technology the drop formation depends on the applied voltage pulse to the
electromechanical device such as, piezoelectric material, which is generating pressure to
the fluid causing the production of a drop from the orifice. The name of this system
Drop on Demand is due to the nature of its working as the drop is generated when
desired. This phenomenon is described in fig. 2.
Figure 2. Schematic of a Drop on Demand printing system.
5.3. Materials for Jetting Technology
Jetting materials have some property requirements which should be taken intoaccount. The viscosity of the ink should not be very low as it can create some
difficulties in satellite formation and lack of acoustic damping. Similarly, very high
surface tensions of the inks can lead to difficulties but some materials are being
dispensed with very high surface tensions such as solders have surface tensions 6 times
that of distilled water. There is a range of material properties as mentioned above which
should not be exceeded, if particle suspensions are required then the density and size of
the particle size should not exceed the material properties range.
5.4. Pattern Formation
Pixel size and its selection is the basic thing in image formation, then the desired
pixels can be filled with the help of ink-jet dispenser. The development and
performance of ink-jet printing system for electronic assembly mainly depends on
whether the system is using aqueous ink or solder. As the rapid spreading of ink on
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some organic substrates such as polymer can cause the larger spot size than drop size
which may degrade the quality of printed lines. So for desired quality of resolution of
printed lines is required then spreading of ink on substrate should be control. Phase
change materials can be one alternative to this problem as the ink solidifies just after
printing, such as solders for electronic manufacturing. Phase change materials are good
for avoiding spreading but it creates a bump when a uniform layer is required instead of
a bump. This was the problem for the better resolution of the image as it distorted the
image by diffraction of projected light due to rounded shapes of individual bumps. To
cope with this problem, flattening of bumps into a smooth surface was used.
5.5. Direct-Write Applications
Solder jetting is widely used in electronics industry to deposit small solders for
interconnections in chip-packaging. Different methods are used to deposit solder bumps
such as ink-jet technology to produce metal spheres and balloons. Piezoelectric
actuators are used in demand mode jetting which is better than continuous mode jetting
for deposition of solder bumps.
Optical Microlenses
Direct-write technology is gaining importance in the field of photonics as well due
to the acceptance of optical technologies for transmission of higher data with lower
costs. The deposition of optical waveguides and refractive microlenses onto the optical
components using ink-jetting have higher thermal performance than conventional
photoresist materials used in photolithographic process. Different kinds of epoxies and
thermoplastics are being used in ink-jet printing tools. Ink-jet based printing can be used
to print different configurations of microlenses such as convex/plano hemispherical,
hemi-elliptical and square to convex-convex which are used in different optical
applications. Printing a microlens on the tip of a fiber can reduce the manufacturing cost
of optical fiber communication system, as this microlens will increase the acceptance
angle of the laser light which can be useful in increasing the sensitivity of alignment of
laser. Direct-write technologies can also be utilized in the development of medical
science by fabrication of multifunctional biochemical-sensors.
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Display Industry
Slim and smart display screens are gaining potential in display market and different
performance parameters are under research to reduce the cost and improve the color and
resolution performance. Ink-jet printing of display materials can be alternatively used in
display industry to reduce the cost as phosphors manufacturing is expensive. Deposition
of color light-emitting polymers in display screen by using ink-jet printing technique is
under research to improve the quality of the display screens.
Passive components
Passive components (resistors, capacitors and inductors) fabricated in different chips
cover the majority area of the chip or circuit board hence limiting the performance of
the device. Printed passive elements can be used alternatively to reduce the size and cost
of the electronic circuitries. Imbedded passives such as resistors are being fabricated on
the inner layers of circuit board with the use of direct-write techniques. Inductors and
capacitors can be printed using different layers of materials. Printing insulators as
dielectrics which are sandwiched between conductors as electrodes layers to form
capacitors and printing a coil of conductor on printed ferrite layer end up in an inductor.
(a)
(b)
Figure 3. (a) Printing process of a capacitor, (b) printing process of an inductor
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Bioactive Materials
Ink-jet printing is a hot topic for research in bioengineering field. Printing technique
can be used in depositing the DNA array for the analysis and synthesis of
oligonucleotides. This helps in greatly decreasing the number of fluids required for
synthesis of DNA. The synthesis of peptides are quite similar to that of DNA with little
bit more complexity due to the presence of amino acids.
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6. Direct-Write Micromachining Using Laser
6.1. Introduction
Laser plays a vital role in the development of electronics industry as it provides high
peak intensity energy source, higher spectral purity and spatial coherence, etc. Lasers
are used in drilling, cutting and welding at micro levels. Due to their small feature sizes,
they are widely used in direct-write micromachining technologies. The production
speeds of fabrication for large parts are much slower than other conventional methods
because laser micromachining or miniaturization process is a serial process but its
performance is much better than other mechanical systems for the small-scale
prototyping, throughput rates and customization. As described in fig. 1, the performance
and manufacturing speed, of laser systems gets higher to remove a blocklike feature
from stainless steel, when its size decreases below 100m as compared to other
mechanical systems performance which uses conventional drilling and milling.
Figure 1. Comparison of laser system with conventional mechanical system toremove a blocklike feature of different sizes. [1]
6.2. Laser-Matter Interaction
When the laser beam incident on the surface of the solid, different complex
phenomena may occur, due to that the photons of laser light can be absorbed into the
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surface, which also results in different photophysical or photochemical processes or
both, depending on the wavelength, energy and pulse of incident beam of laser. When
the electrons are excited from the absorption of incident photons, two kinds of
relaxation processes occur, i.e. localized and nonlocalized, depending on the
involvement of part or the whole lattice, respectively, in the excitation process.
Nonlocalized processes are further distributed into desorption, radiative recombination
and ablation processes.
Ablation
Ablation is the most important localized process in the laser micromachining
domain. When the electrons are photoexcited, they convert into kinetic energy of
nuclear motion which results in the ejection of some material from the surface of solid,
resulting in two different phenomena of laser-induced desorption and ablation.Desorption process has very little role in laser based direct-write technologies, while the
ablation process is highly used. Ablation process is shown in fig. 2.
Figure 2. Schematic diagram of ablation process for removal of material from the
surface of solid by using incident laser beam.
Best performance can be achieved by using small pulse duration of incident laser
beam. Extremely well-defined boundaries can be achieved by using right ablation
conditions to isolate the ablated regions from the non-exposed regions to laser beam to
avoid minimal heating.
6.3. Laser Micromachining
Different techniques are available for advanced material processing such as,
molecular beam epitaxy (MBE), chemical vapour deposition (CVD) and focused ion-
beam etching (FIB), but the most useful tool is laser micromachining technique for
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material processing. Laser micromachining can be used in different applications, i.e.
cutting, heat treatment, welding, etching, deposition, lithography, etc. The most
important laser parameters which define the performance of micromachining are listed
here, the spot size, the focus depth, and the laser beam intensity.
Different types of laser sources are used in laser micromachining applications such
as, Nd:YAG solid state lasers emitting pulses at 1.06m, 2CO gas lasers emitting pulses
at 10.6m and excimer lasers emitting pulses at 193nm. 2CO gas lasers are not used
efficiently in applications where very small feature sizes are required. There are other
laser sources also available like 2F lasers, which are now used in semiconductor
industry in photolithography process. To get better results from laser micromachining, it
is desirable to use laser matching with the substrate. For example, using laser
parameters such as wavelength, which should be completely compatible with the targetmaterial to get required results.
6.4. Laser Micromachining Tools
Laser micromachining composed of different stages and parts as shown in the fig. 3.
Figure 3. Schematic of a laser micromachining tool, showing basic elements [1].
The relative motion of the substrate material and the laser beam is used in the laser
micromachining tools used in direct-write technologies to process the material in
desired shapes. This relative motion should have high precision and resolution range to
produce the very small feature processing as the typical feature production range of
laser micromachining is 2-200m. this relative motion includes the motion of part,
motion of beam or both at the same time in some hybrid systems.
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Video imaging is the most important part of the laser microfabrication system as it
gives the monitoring of the process, alignment of features, location of the desired
material part and inspection of completed operations. Different configurations are used
in viewing the processing as shown in the fig. 4.
Figure 4. Different configurations of video imaging to monitor the material
processing [1].
Finally, the computer provides the feature of controlling the whole process by
moving the substrate or laser beam with the use of some user friendly software.
Computer improves the processing of different tasks which are at micron levels and
controlling of these tasks manually is not easy.
6.5. Applications of Laser Micromachining
Three-dimensional structures can be constructed with laser micromachining by
using specific conditions of ablation depths and highly absorbing substrates.
Micromachining of layered materials is another industrial application of laser
micromachining. Micromarking of very small components is the essential element in
the development of miniaturization of electronic, medical and optical devices.
Micrmarking can be achieved with high resolution by using laser micromachining.
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7. Micrometer and Nanometer Pattern and
Material Transfer Technologies
7.1. Introduction
Size of a body has many effects on its functionality and lots of advantages and
disadvantages are always taken into account related with the size of the device. In many
cases, cost of the device depends on its size. We can characterize the size scale of
different devices as shown in fig. 1.
Figure 1. Size scale of the human made objects.
Micrometer and nanometer devices are manufactured three-dimensionally, and
include lots of serial processes using thin-film technologies. The block diagram
showing the manufactured process of small-scale devices is shown in fig. 2.
Figure 2. Block diagram of a manufactured process of small-scale devices.
There are two types of transfer technologies such as, material transfer and pattern
transfer technologies. We will take a look to pattern transfer technologies and their
applications.
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7.2. Applications of Pattern Transfer Technologies
Pattern transfer technologies include lots of different areas of electronics. They are
used in manufacturing of well known integrated circuits. Manufacturing of MEMS
(Micro Electro Mechanical System) also include the transferring of different layers of
exact shapes and at exact locations. The developments in the printed circuit boards
brought the complexity by including the different layers to imbed the resistive and
capacitive components in them. The number of layers increases with the increase in the
complexity of electronics device, as 50 layers of conductive and insulating materials
have been patterned in the mother board of a PC.
As the space becoming crucial for many consumer electronics devices like cell
phone, the need of bonding the chips on the circuit boards without using packages
which occupy more space has arisen. This can be done by using the solder bumps withsame size and at same locations from each other to make the contact of the chip which
become a package on the circuit board. This process is done by lithography as the size
of the bumps is very small of submicron levels. Pattern transfer techniques are also used
in thin-film magnetic heads. Microsystems related to the fluid control and manipulate its
data are gaining importance. These systems include passive and active components.
They are also manufactured by using lithography processes.
7.3. Different Lithography TechniquesThere are different techniques available for lithography. Optical methods are used to
align the resists on wafers accurately which can decrease the cost of the chip
fabrication. Extreme ultraviolet lithography is used with the wavelengths of 13nm and
very small numerical aperture to improve the resolution. To improve the lithography
process x-ray lithography is used. X-ray lithography at 1nm required new sources,
optics, masks and resists. Electron beam direct-write lithography is used to produce the
finest masks and can be used in chip production. They require bright sources of
electrons. Ion beam lithography is used to repair the masks and not used for the
production processes.
In addition to all the conventional lithography techniques, there are some other
production techniques to transfer the pattern on the substrate. LIGA (Lithography
Galvanoformung Abformung) includes the sequential use of lithography,
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electrodeposition and molding. Very thick resists are used with additional processing
steps are used in LIGA. Thin resists with different masks are used in Lithography
induced self-construction (LISC).
7.4. Applications of Material Transfer Technologies
The pattern transfer technology applications depend on the characterization of the
used technology, whereas the material transfer technology uses programmable methods
that determine its applications. Pattern transfer technologies are used to manufacture the
products at very high rates, while material transfer technology has low productivity
rates, hence these techniques are suitable for rapid prototyping.
7.5. Methods of Material Transfer Technologies
Different old and new methods for bulk micromachining processes are common in
subtractive material transfer techniques. The deeply etching into the bulk of the material
is important. There are lots of methods to make high aspect ratio structures in different
materials. some use methods of wet chemical etching technique which is older method.
Other are also called plasma or dry processes. Other methods of subtractive material
transfer technique include electrochemical, electrodischarge and ultrasonic machining.
Next technique for material transferring is programmable subtractive technique. The
first ever programmable technique for removing the material from a work piece (or
substrate) was engraver. Different techniques are used in this subtractive technique.
Focused beams of laser can be used to remove the materials from the substrate by using
processes like melting of the area or plasma formation. The process depends on the
parameters of the incident layer beam, i.e. intensity of the laser beam, power density etc.
Different kinds of beams are used in this process depends on the required removal of the
material, electron, photon and ion beams are used. Another method to remove the
materials from the substrate is to use the fine hard powder propelled in a gas jet at
substrate. This method is known as abrasive jets.
For fixed pattern additive material transfer, many options are available such as
masked evaporation, screen printing and microcontact printing.
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8. References
[1] Direct-Write Technologies for Rapid Prototyping Applications by Alberto Piqu