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-micronrange. This allows advantages such as rapid prototyping and complex structuringfor microfabrication. The generated structure can only be analyzedwith scanning electron microscopy (SEM) or similar technology due to its smallfeatures.1.1 NanoScribestructure fabrication processThe fabrication process consists of several different steps.
Not only thedesired structure must be designed in a CAD tool, the process parameters of theprinting highly depend on the structure and the used photoresist.1.1.1 CADstructure modeling and preparation The mathematical definition of a 3D structure in a compatible file formatis inevitable to use a 3D printer. This can either be done by defining astructure directly in a file or using a CAD program like SolidWorks. The mostused file format for this is the STL file format. STL files describe thetriangulated surface of the structure by the cartesian coordinates of thecorner points and the triangle normal vectors. The resolution of the structureis given by the size of the triangles.
Big triangles will show poor resolutionwhile small triangles have a high resolution, but a bigger file size (figurex). Figure 1: Impactof triangle size to the resolution of a CAD model (a) CAD model (b) smalltriangles (c) big trianglesA big file size is not a problem due to its binary nature and thus havingin general a small file size. Standardized STL files do not support colourinformation, but this information is mostly irrelevant for 3D printing. STLfiles were the first file format for 3D printing and still common because oftheir simple structure and small file size. PROOFThe 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 withthe 3D printer slices the structure from the STL file by intersecting the solidwith a series of parallel planes. These are the planes that are later printed.Additionally, the program computes the intersection contour with the plan andfills the whole contour with a line hatch.
This way, there are closed contours,which is essential for 3D printing. Hatching divides the several sliced planeswith a standardized pattern. The distance of these hatchlines defines howhollow or dense the printed structure is (figure x). If the hatch distance issmall enough, the surface will be printed smoothly.
Figure 2: Examples for different distancesfor hatch lines of a solid structureA too small slicing distance can also lead to excessive heating of theresist. This results in higher internal stress and can be cause of uncontrolledpolymerization (manual p79)1.1.2 Fabricationprocess and parametersPhotoresists are used in photolithography to create structures.
The desiredstructure is generated within the photoresist under light exposure. Photoresistsare 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 arenegative photoresists.1.1.
2.1 Two photon lithography3D printing has been getting popular over the last couple of years. Theability to manufacture complex structures with one simple tool had its impacton industrial fabrication and prototyping. It also enables a lot of scientificapplications to be evaluated more quickly and on an affordable price scale. Twophoton lithography (2PL) allows to create structures with a resolutiondown to 200 nm.
1 Figure 3: 3Dprinting with 2PL. (a) deposition of photoresist (b,c) creating 3Dstructure with focused beam (d) developmentTwo photon lithography is often described as Direct Laser Writing (DLW).This is in general true, the difference is in the scale of the printedstructure. Conventional 3D printer don’t need 2PL, because the resolution ofthe standard technology is enough.
2PL is an additive manufacturing method,that is processed by polymerization through a focused femtosecond pulsed laserbeam. Due to the focused beam, the local intensity in the photoresist andtherefore the absorption is high. In the focus spot, the enhanced absorptioninitiates the crosslinking in the photoresist. Outside the focus theprobability of two-photon absorption is low, which suppresses accumulationeffects and improves the resolution that can be achieved. 3D structures areobtained by moving the laser focus through the material (Figure x).
2 1.1.2.2 Dip-In Laser Lithography DiLLOne common problem with photoresists is that their refractive index is notaligned with the objective. Therefore, the resolution decreases.
A technique toget rid of this problem is Dip-In laser lithography (DiLL). To avoid theadditional refractive index of air or oil in between the photoresist and theobjective, it dips into the photoresist (figure x). Hence, the photoresist actsas the immersion fluid and the photosensitive material at once. This hasseveral advantages: The photoresist doesn’t have to be index-matched,objectives with a high numerical aperture (NA) can be used and the heightof the printed structure is not limited by the working distance.
Figure 4: Comparison between the conventional writingconfiguration and Dip-In Laser lithography (DiLL)DiLL is essential for the experiments of carbonization because it can isable to print on opaque substrates such as silicon. With temperatures above900°C glass substrates would melt in the furnace and this process would destroythe printed structure. (in experimente- section verschieben!) With DiLL, homogeneity is preserved along the optical axis. However, theresolution still depends on the refractive index of the resist. Ideal focusingis possible for n = 1.52,which is the refractive index of the photoresist provided by the NanoScribecompany named IP-DIP.
1.1.3 ProcessparametersPrinting processThe finalstep of the structure preparation includes choosing the hardware that the 3Dprinter will use.
The NanoScribe has a Galvo and a piezo drive mode for boththe xy- and the z-axis. The piezo drive is more precise. It has a positioningaccuracy in the range of 10nm within a writing area of 300 ?m and represents the standardscanning mode. The Galvo drive is for high speed writing and its accuracydepends on the used objective. Using the piezo drive for both axis results in asignificantly long print time. Depending on the structure, the timeframe can beseveral days.
Additionally, if structures are out of range, the print must beprocessed in segments. Therefore, this mode is designed for printingsstructures with smallest features around 100 nanometers in size. For biggerstructures, the Galvo drive is recommended. Developmentmissing!1.1.
4 SEManalysisDifferent SEMs for polymers and carbons! A Scanning Electron Microscope is a tool for imaging structures that arebelow 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 onthe accelerating voltage, the material penetration of the electrons and thetransported information changes. The higher the voltage, the higher thepenetration. According to this, different kinds of electrons are emitted fromthe 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. Polymerstend to gas out while being under influence of the electron beam and thus causeshrinkage of the material and pollution of the vacuum chamber of themicroscope. Furthermore, an increased number of electrons due to higheraccelerating voltage charges insulating samples, causing artefacts to occur. 2 Carbonizationof polymersCarbon exists in many forms, but only two ofthem occur naturally. These are graphite and diamond. Allotropes are elementsthat exist in multiple forms in the same physical state. There are otherallotropes of carbon like fullerene, amorphous carbon and glassy carbon.
For one, the polymer that is carbonized has ahuge impact to the carbonization. Depending on how the compound reacts todehydrogenation, condensation, hydrogen transfer and outgassing, the stabilityof the original structure is endangered. 3, p.
484 In general, volatileorganic 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 affectsthe volume loss or shrinkage that occurs. Finally, the maximum temperature ofthe pyrolysis controls the degree of carbonization. While pyrolyzing a polymercompound, multiple carbonization processes take place at different temperatureregimes. In the 300-500°C temperature range, which is called the carbonizationregime, there is a rapid polymer weight loss due to outgassing oxygen. Above500°C, hydrogen starts to split from carbon elements and the purity of thecarbon compound increases. A high purity glassy carbon is usually obtainedbetween 800-1200°C depending on the original polymer 4.
Graphitic zones startto form at higher temperatures. In the graphitization regime, which is located at2500-3000°C, it is assumed that defects of carbon can be completely annealed asthey become mobile. (BuchJenkins!) The photoresist IP-DIP contains 60–80% of Pentaerythritol triacrylate1,which has a considerate amount of bonded oxygen. It has been seen thatstructures out of IP-DIP shrink by up to 80%. This can be attributed to theformation of CO2 and CO during the pyrolysis 5.
2.1 GlassycarbonThe carbonization with a maximum temperature of 900°C results in astructure made out of glassy carbon. Glassy carbon is a form of carbon thatconsists of interconnected graphene fragments. 12-(hydroxymethyl)-2-(1-oxoallyl)oxymethyl-1,3-propanediyldiacrylate
