MICRO-STEREOLITHOGRAPHY
Authors : Ruchita Kulkarni, Kedar Malusare _____________________________________________________________________________

1. INTRODUCTION
1.1 Rapid prototyping Rapid Prototyping (RP) can be defined as a group of techniques used to quickly fabricate a scale model of a part or assembly using three-dimensional computer aided design (CAD) data. What is commonly considered to be the first RP technique, Stereolithography, was developed by 3D Systems of Valencia, CA, USA. The company was founded in 1986, and since then, a number of different RP techniques have become available. Rapid Prototyping has also been referred to as solid free-form manufacturing; computer automated manufacturing, and layered manufacturing. RP has obvious use as a vehicle for visualization. In addition, RP models can be used for testing, such as when an airfoil shape is put into a wind tunnel. RP models can be used to create male models for tooling, such as silicone rubber molds and investment casts. In some cases, the RP part can be the final part, but typically the RP material is not strong or accurate enough. When the RP material is suitable, highly convoluted shapes (including parts nested within parts) can be produced because of the nature of RP.

Fig 1. Rapid prototyping worldwide

There is a multitude of experimental RP methodologies either in development or used by small groups of individuals. They are listed as given below.     Stereolithography (SLA) Selective Laser Sintering (SLS®) Laminated Object Manufacturing (LOM™) Fused Deposition Modeling (FDM)

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Solid Ground Curing (SGC) Ink Jet printing techniques

1.2 Stereolithography The term stereolithography was coined in 1986 by Charles W. Hull. Stereolithography was defined as a method and apparatus for making solid objects by successively ‘printing’ thin layers of the ultraviolet curable material one on top of the other. Hull described a concentrated beam of ultraviolet light focused onto the surface of a vat filled with liquid photopolymer. The light beam draws the object onto the surface of the liquid layer by layer, causing polymerization orcrosslinking to give a solid. Because of the complexity of the process, it must be computer controlled. Stereolithography is a common rapid manufacturing and rapid prototyping technology for producing parts with high accuracy and good surface finish. Stereolithography is an additive fabrication process utilizing a vat of liquid UV-curable photopolymer "resin" and a UV laser to build parts a layer at a time. On each layer, the laser beam traces a part cross-section pattern on the surface of the liquid resin. Exposure to the UV laser light cures, or, solidifies the pattern traced on the resin and adheres it to the layer below. On this new liquid surface the subsequent layer pattern is traced, adhering to the previous layer. A complete 3-D part is formed by this process. After building, parts are cleaned of excess resin by immersion in a chemical bath and then cured in a UV oven. Stereolithography requires the use of support structures to attach the part to the elevator platform and to prevent certain geometry from not only deflecting due to gravity, but to also accurately hold the 2-D cross sections in place such that they resist lateral pressure from the re-coater blade. Supports are generated automatically during the preparation of 3-D CAD models for use on the stereolithography machine, although they may be manipulated manually. Supports must be removed from the finished product manually. The Stereolithography System overall arrangement:

Fig. 2: Schematic representation of the laboratory stereolithography apparatus

(1) UV Laser (4) Galvanometric mirrors (7) Reactor with photomaterial 1.3 Microstereolithography

(2) Acousto-optic shutter (5) Elevator (8) Computer

(3) Focusing lens (6) Scrapper

Microstereolithography is also called microphotoforming & was first introduced by Ikuta & Hirowatari in 1993. The resolution of microstereolithography is better than stereolithography. UV laser beam is focused down to a 1 to 2 μm-dia. Spot that solidifies a resin layer of 1 to 10 μm thickness whereas in stereolithography laser beam spot size & layer thickness are 100 to1000 μm.Microstereolithography allows to build small size, high resolution, 3D objects by superimposing a certain no of layers obtained by light-induced and space-resolved polymerization of liquid resin into solid polymer. Sub micron control of both x-y-z translation stage & small UV beam spot enables precise fabrication of complex 3D microstructures. µSL is an additive process in contrast to conventional subtractive micromachining & in principle compatibility with silicon technology. Different µSL systems have been developed in recent years to improve upon their precision & speed. Another research effort in µSL is the incorporation of broad spectrum of materials like polymer, metal, ceramic to create MEMS with new special functions. The difference between normal stereolithography (SL) and microstereolithography (µSL) is simply that the resolution of the process is lower. Just as with the larger SL machine, each layer image of the object is transferred to the digital mirror device (DMD) and projected into the liquid resin. The liquid resin cross-links and solidifies where it is illuminated. In preparation for the next layer the z-stage dips into the liquid resin to create the next 10μm layer on top of the already solid object. A UV laser beam is used as the light source, which is focused down to a 1 to 2μm-diameter spot. This tiny focused spot solidifies the resin layer of about 1 to 10μm in thickness. The x-y-z translation stages are controlled on a sub micron level.

Fig 3. Schematic drawing of the microstereolithography system; (a) the principles of microstereolithography and (b) the apparatus used in our microstereolithography system

1.4 UV-curable resin Properties: 1. Photosensitivity at the operating wavelength 2. Low viscosity to produce a smooth surface 3. High curing speed 4. Low shrinkage during polymerization 5. High absorption for low penetration of light

1.5 Types of resins 1. Epoxy Resins 2. Acrylateresins 3. HDDA (1,6 –Hexanedioldiacrylate) with 4% by wt. photoinitiator

1.6 Microstereolithography limitations 1. Smooth 3D surfaces difficult to produce; stepping effects will always be present 2. Mass production of several components is another challenge 3. Extremely small features difficult to produce

1.7 Metal and ceramic microstereolithography 1.7.1 Metal Microstereolithography A metal is a chemical element that is a good conductor of both electricity and heat and forms cations and ionic bonds with non-metals. In chemistry, a metal is an element, compound, or alloy characterized by high electrical conductivity. Metal microstereolithography is used in cases where electrical conductivity is of vital importance. Electrical conductivity is obtained by using a suspension in which photopolymerizing resins were mixed with metal powder. Copper powder consisting of particles 3 µm

in diameter is used for the metal suspension material, since it is both inexpensive and is the second highest electrically conductive metal, next to silver. A new photopolymer, IMS03 resin, was developed specially for this purpose. It was composed of monomers including 57 wt% 1,6-hexanediol-diacrylate (HDDA) and 10 wt% trimethylol propane triacrylate (TMPTA). To this, 29 wt% N-vinyl-2-pyrrolidone (NVP) was added to dilute the resin and 4 wt% dimethoxy phenyl acetophenone (DMPA) was used to initiate the photopolymerization. All of the monomers were manufactured by Sigma–Aldrich. The resin was mixed for 24 h at a temperature of 25 ˚C.

Fig. 4. The image of the product using copper and photopolymer (IMS03)

1.7.1.1 Sintering process After the fabrication process it was observed the products did not have the desired electrical conductivity while they were in the state of a green body, which refers to the mixed state of copper and resin, because there was insufficient contact between the particles of metal powder. The products were sintered to bring the copper powder particles closer together by burning the resin and deforming the surface of the metal particles. Because copper is oxidized by oxygen when it is sintered in air, atmospheric conditions are important in the sintering process. Any copper oxide on the copper surface obstructs contact between the particles of powder, preventing combination with other particles and resulting in a lack of electrical conductivity in the finished product. Therefore, the sintering process must be performed in a vacuum. To fabricate a good

quality structure requires an adequate temperature in the sintering process.

The temperature profile for the Sintering Process

Usually, the sintering temperature for copper is adjusted according to the particle size of the powder. In the sintering process, the cured resin is first removed from the structure. A previous paper reported the temperature (600˚C) required to remove HDDA monomer. After burning the cured resin, the sintering temperature was raised to 900˚C-1000˚C to sinter the copper powder. Fig. 5 gives the temperature profile used in the sintering process. Only pure copper remained at the end of the process. Fig. 6 shows images of the sintered products. Sintering caused a shrinkage of 22–28%, since the cured resin was burned, removing the space formally occupied by the resin. However, the structure did not distort. If the ratio of solid added to the suspension were increased, the shrinkage would be less than in the current results. The electrical resistivity of the sintered products was measured to be 200–300 nΩm, which is approximately 10 times greater than that of pure copper (17.2 nΩm) due to the porosity of the finished product. However, this value is still similar to the electrical resistivity of other metals. 1.7.2 Ceramic microstereolithography Ceramics encompass such a vast array of materials that a concise definition is almost impossible. However, one workable definition of ceramics is a refractory, inorganic, and nonmetallic material. Ceramics can be divided into two classes: traditional and advanced. Traditional ceramics include clay products, silicate glass and cement; while advanced ceramics consist of carbides (SiC), pure oxides (Al2O3), nitrides (Si3N4), non-silicate glasses and many others. Ceramics offer many advantages compared to other materials. They are harder and stiffer than steel; more heat and corrosion resistant than metals or polymers; less dense than most metals and their alloys; and their raw materials are both plentiful and inexpensive. Ceramic materials display a wide range of properties which facilitate their use in many different product areas.

The use of ceramic materials in microsystem technology is of particular interest when their good mechanical properties, their thermal and chemical resistance or special physical (dielectric or piezoelectric) properties qualify them for uses that are not covered by polymers or metals. The ceramic powder used is α-alumina (A-16SG obtained from Alcoa). The photosensitive resin is an acrylate monomer of low viscosity and low toxicity (Polyethyleneglycol 400 diacrylate, SR344 Cray Valley) in which was dissolved 5 wt.% (with respect to the mass of monomer) of an UV photoinitiator (2-2dimethoxy-2- phenylacetophenone, Fluka). A cross-condensation of the alumina particles with a silane (3-glycidoxypropyltrimethoxy- silane, Degussa) is undertaken to stabilize the formulations, this is obtained by the addition of 4 wt.% (with respect to the mass of alumina) of this component to the alumina powder. Similarly a variety of suspension may be obtained, the above being only one of the examples. Microcomponents in ceramic material can be advantageous compared to their plastic equivalents for their mechanical properties, but also for their refractory properties when dealing with high temperature micro-reaction technologies, or for their good biocompatibility for producing components in the biomedical domain. Where plastic components made by microstereolithography were mostly used for shape and appearance assessment, ceramic ones could be used as fully functional prototypes in many applications.

Fig 5. microcylinders made of alumina fabricated using ceramic microstereolithography

1.8 Applications of microstereolithography The microstereolithography process is used for manufacturing of complex 3D micro-components in polymer/ceramic composite materials. It can be applied in different fields, such as microrobotics, microrobotics or microfluidics. Microfluidics is important for all applications requiring the use of small amounts of fluid for example in microreactors. Microfabrication

processes have to allow the production of size and shape controlled structures such as channels and thin walls with features of a few µm upto 100µm. Also the fluids passing through or by these structures can be either acidic or basic solvents and may need high temperature environment. These requirements cannot be met sufficiently by polymeric materials so metallic, composite and ceramic materials are preferred for these applications. Microstereolithography has been successfully used to fabricate complex scaffolds which guide cells to form functional tissue. These tissue scaffolds are constructed using a biocompatible and biodegradable material.

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1.9 Need for analysis During fabrication the scattering of laser light is found to significantly influence the lateral resolution and curing depth. Several laws viz. Beers Law, Kubelka-Munk (KM) Theory were used study and describe the multiple light scattering. Beers Law does not explain how the light scattering influences the lateral resolution and the shape of the solidified parts. Kubelka-Munk (KM) Theory does not explain the geometrical distribution of the scattered light. Monte Carlo ray-tracing technique has been developed not only to investigate the light scattering effect on µSL of ceramic, but also to overcome the above limitations in using theoretical models Thus it can be concluded that Monte Carlo ray-tracing technique can be used to investigate the light scattering effect on µSL of ceramic.

References : 1. Micro-stereolithography of polymeric and ceramic microstructures - X. Zang, X.N. Jiang, C. Sun 2. Complex Ceramic-Polymer Composite Microparts Made by Microstereolithograpy Christophe Provin and Serge Monneret