Creating lattice structures on a microscale is a complex process. The
“NanoScribe photonic professional GT” uses a 3D printing technology capable of printing in the sub-micron
range. This allows advantages such as rapid prototyping and complex structuring
for microfabrication. The generated structure can only be analyzed
with scanning electron microscopy (SEM) or similar technology due to its small
features.

1.1           
NanoScribe
structure fabrication process

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The fabrication process consists of several different steps. Not only the
desired structure must be designed in a CAD tool, the process parameters of the
printing highly depend on the structure and the used photoresist.

1.1.1       
CAD
structure modeling and preparation

The mathematical definition of a 3D structure in a compatible file format
is inevitable to use a 3D printer. This can either be done by defining a
structure directly in a file or using a CAD program like SolidWorks. The most
used file format for this is the STL file format. STL files describe the
triangulated surface of the structure by the cartesian coordinates of the
corner points and the triangle normal vectors. The resolution of the structure
is given by the size of the triangles. Big triangles will show poor resolution
while small triangles have a high resolution, but a bigger file size (figure
x).

Figure 1:   Impact
of triangle size to the resolution of a CAD model (a) CAD model (b) small
triangles (c) big triangles

A big file size is not a problem due to its binary nature and thus having
in general a small file size. Standardized STL files do not support colour
information, but this information is mostly irrelevant for 3D printing. STL
files were the first file format for 3D printing and still common because of
their simple structure and small file size. PROOF

The data from the STL file need to be prepared before they can be printed.
This process is called ‘slicing and hatching’. A program that comes along with
the 3D printer slices the structure from the STL file by intersecting the solid
with a series of parallel planes. These are the planes that are later printed.
Additionally, the program computes the intersection contour with the plan and
fills the whole contour with a line hatch. This way, there are closed contours,
which is essential for 3D printing. Hatching divides the several sliced planes
with a standardized pattern. The distance of these hatchlines defines how
hollow or dense the printed structure is (figure x). If the hatch distance is
small enough, the surface will be printed smoothly.

Figure 2: Examples for different distances
for hatch lines of a solid structure

A too small slicing distance can also lead to excessive heating of the
resist. This results in higher internal stress and can be cause of uncontrolled
polymerization (manual p79)

1.1.2       
Fabrication
process and parameters

Photoresists are used in photolithography to create structures. The desired
structure is generated within the photoresist under light exposure. Photoresists
are divided in two categories dependent on their behavior under light exposure:
negative and positive resists. Positive resist liquefies the exposed area,
while negative resist hardens under exposure. The used photoresists are
negative photoresists.

1.1.2.1  Two photon lithography

3D printing has been getting popular over the last couple of years. The
ability to manufacture complex structures with one simple tool had its impact
on industrial fabrication and prototyping. It also enables a lot of scientific
applications to be evaluated more quickly and on an affordable price scale. Two
photon lithography (2PL) allows to create structures with a resolution
down to 200 nm. 1

Figure 3:   3D
printing with 2PL. (a) deposition of photoresist (b,c) creating 3D
structure with focused beam (d) development

Two photon lithography is often described as Direct Laser Writing (DLW).
This is in general true, the difference is in the scale of the printed
structure. Conventional 3D printer don’t need 2PL, because the resolution of
the standard technology is enough. 2PL is an additive manufacturing method,
that is processed by polymerization through a focused femtosecond pulsed laser
beam. Due to the focused beam, the local intensity in the photoresist and
therefore the absorption is high. In the focus spot, the enhanced absorption
initiates the crosslinking in the photoresist. Outside the focus the
probability of two-photon absorption is low, which suppresses accumulation
effects and improves the resolution that can be achieved. 3D structures are
obtained by moving the laser focus through the material (Figure x). 2

1.1.2.2  Dip-In Laser Lithography DiLL

One common problem with photoresists is that their refractive index is not
aligned with the objective. Therefore, the resolution decreases. A technique to
get rid of this problem is Dip-In laser lithography (DiLL). To avoid the
additional refractive index of air or oil in between the photoresist and the
objective, it dips into the photoresist (figure x). Hence, the photoresist acts
as the immersion fluid and the photosensitive material at once. This has
several advantages: The photoresist doesn’t have to be index-matched,
objectives with a high numerical aperture (NA) can be used and the height
of the printed structure is not limited by the working distance.

Figure 4: Comparison between the conventional writing
configuration and Dip-In Laser lithography (DiLL)

DiLL is essential for the experiments of carbonization because it can is
able to print on opaque substrates such as silicon. With temperatures above
900°C glass substrates would melt in the furnace and this process would destroy
the printed structure. (in experimente- section verschieben!)

With DiLL, homogeneity is preserved along the optical axis. However, the
resolution still depends on the refractive index of the resist. Ideal focusing
is possible for n = 1.52,
which is the refractive index of the photoresist provided by the NanoScribe
company named IP-DIP.

 

1.1.3       
Process
parameters

Printing process

The final
step of the structure preparation includes choosing the hardware that the 3D
printer will use. The NanoScribe has a Galvo and a piezo drive mode for both
the xy- and the z-axis. The piezo drive is more precise. It has a positioning
accuracy in the range of 10nm within a writing area of 300 ?m and represents the standard
scanning mode. The Galvo drive is for high speed writing and its accuracy
depends on the used objective. Using the piezo drive for both axis results in a
significantly long print time. Depending on the structure, the timeframe can be
several days. Additionally, if structures are out of range, the print must be
processed in segments. Therefore, this mode is designed for printings
structures with smallest features around 100 nanometers in size. For bigger
structures, the Galvo drive is recommended.

Development
missing!

1.1.4       
SEM
analysis

Different SEMs for polymers and carbons!

A Scanning Electron Microscope is a tool for imaging structures that are
below the resolution of optical microscopes. By using a focused beam of electrons,
this method is able to get magnifications of up to 300.000 times. Depending on
the accelerating voltage, the material penetration of the electrons and the
transported information changes. The higher the voltage, the higher the
penetration. According to this, different kinds of electrons are emitted from
the material: Auger electrons, secondary electrons or backscattered electrons.
For topological information, secondary electrons are used.

Another point to consider is what kind of material to analyze. Polymers
tend to gas out while being under influence of the electron beam and thus cause
shrinkage of the material and pollution of the vacuum chamber of the
microscope. Furthermore, an increased number of electrons due to higher
accelerating voltage charges insulating samples, causing artefacts to occur.

2                 
Carbonization
of polymers

Carbon exists in many forms, but only two of
them occur naturally. These are graphite and diamond. Allotropes are elements
that exist in multiple forms in the same physical state. There are other
allotropes of carbon like fullerene, amorphous carbon and glassy carbon.

For one, the polymer that is carbonized has a
huge impact to the carbonization. Depending on how the compound reacts to
dehydrogenation, condensation, hydrogen transfer and outgassing, the stability
of the original structure is endangered. 3, p. 484 In general, volatile
organic compounds (VOC) are removed through carbonization. With the removed elements,
carbon polymerizes with itself and forms new bonds to other carbon elements.
The amount of carbon in the polymer in comparison to other compounds affects
the volume loss or shrinkage that occurs. Finally, the maximum temperature of
the pyrolysis controls the degree of carbonization. While pyrolyzing a polymer
compound, multiple carbonization processes take place at different temperature
regimes. In the 300-500°C temperature range, which is called the carbonization
regime, there is a rapid polymer weight loss due to outgassing oxygen. Above
500°C, hydrogen starts to split from carbon elements and the purity of the
carbon compound increases. A high purity glassy carbon is usually obtained
between 800-1200°C depending on the original polymer 4. Graphitic zones start
to form at higher temperatures. In the graphitization regime, which is located at
2500-3000°C, it is assumed that defects of carbon can be completely annealed as
they become mobile. (Buch
Jenkins!)

The photoresist IP-DIP contains 60–80% of Pentaerythritol triacrylate1,
which has a considerate amount of bonded oxygen. It has been seen that
structures out of IP-DIP shrink by up to 80%. This can be attributed to the
formation of CO2 and CO during the pyrolysis 5.

 

2.1           
Glassy
carbon

The carbonization with a maximum temperature of 900°C results in a
structure made out of glassy carbon. Glassy carbon is a form of carbon that
consists of interconnected graphene fragments.

1
2-(hydroxymethyl)-2-(1-oxoallyl)oxymethyl-1,3-propanediyl
diacrylate