Design and Development of Microstereolithography (MSL) System and Its Applications in Microfabrication of Polymer and Ceramic Structures
In the present era where MEMS (Micro Electro-Mechanical Systems) technology is in¬evitable from the perspective of applications in non-silicon based micro-devices (such as biosensors, microfluidics, microvalves etc.), it is imperative to develop different micro¬fabrication technologies which are simple in operation, have low operational cost and high versatility in terms of incorporating different materials. The microfabrication tech¬nologies (e.g: bulk micromachining, surface micromachining, X-ray LIGA (lithoqraphie galvanoformung abformung) etc.), which exist commercially are mostly limited to sili¬con based technologies. They are either constrained in fabricating complex geometry in micro dimension or have high operational cost. Microstereolithography (MSL) is one such rapid prototyping technique, which can satisfy the above requirements to a larger extent. MSL h8B evolved in the l8Bt decade from conventional stereolithographic (SLA) technique, which involves the free-form microfabrication of a UV sensitive liquid resin layer by layer photo-polymerization process, when it is exposed to UV irradiation accord¬ing to the predefined CAD (Computer Aided Design). However, this technique is not limited to polymer microfabrication and it h8B an immense potential to fabricate com¬plex 3D structures of ceramics in micro dimension. In this thesis, the primary focus is on developing an in house built scanning b8Bed MSL system indigenously and to explore the possibility of micro fabrication of different materials (from polymer to ceramics involving different routes. In addition, polymer micro cantilever h8B been fabricated using this technique and its application to surface stress me8Burement h8B been demonstrated. The thesis comprises of eight chapters. The following section describes the summary of the individual chapters. Chapter 1 describes the introduction and background literature of this technol¬ogy. A brief review on MSL technology developed by various research groups and their achievements h8B been listed. Since photopolymerizable resin is the primary material to fabricate micro dimensional structures, the rate of photopolymerization is an impor¬tant phenomena which requires an attention before choosing the photopolymerizable resin. Further, this chapter also describes the photoinitiation principles and the type of photo initiators (PI) which help to photopolymerize the resin in order to fabricate micro dimensional polymer structures. In addition, this chapter also gives a glimpse of applications of this technology in fabrication of micro cantilever b8Bed sensors. The later part of the chapter focused on the microfabrication of ceramic from colloidal and met¬alorganic routes in brief. In Chapter 2, the design of the in house built MSL system and its working princi¬ples including various optical issues have been addressed. Several research groups have attempted to optimize photopolymerization parameters to incre8Be the throughput of the scanning b8Bed MSL systems through modified beam scanning techniques. Efforts in reducing the curing line width in order to get low feature size have been implemented through high numerical aperture (NA) optical setups. However, the intensity contour symmetry and the depth of field of focus have led to grossly non-vertical and non-uniform curing profiles. The focus of the work h8B been to exploit the rich potential of photoreactor scanning system in achieving desired fabrication modalities (minimum curing width, uniform depth profile, and vertical curing profile) even with a reduced NA optical setup and a single movable stage. The present study tries to manipulate to its advantage the effect of optimized lower photoinitiator (PI) concentration ([c]) in reduc¬ing the minimum curing width to 10-15 jJm, even with the higher spot size (21.4 jJm) rv through a judiciously chosen gmonomer UPIi' system. In this chapter, two different cl8BS of multifunctional acrylates (1,6 Hexane diol diacrylate (HDDA) and Trimethylol propanetriacrylate (TMPTA)) and one monofunctional methacrylate (methyl mathacry¬late (MMA)) have been chosen to explore their fabricability in micro dimensions using this MSL technology, by varying the various operational parameters including the type and the concentration of the PI. Chapter 3 deals with the application of this technology in micro cantilever based sensors. Microcantilever based sensors have been explored for several decades for their application in bio-molecular or explosive detection, chemical sensing etc. Due to the adsorption of molecular species on the cantilever surface, differential surface stress gen¬erates between the top and bottom surface of the cantilever. Depending on the type of stress (tensile or compressive) generated, the cantilever bends accordingly. The, novel diffraction based deflection method has been proposed in order to measure the deflection profile accurately for low dimensional structures. To prove this method, a dual mi¬crocantilever structure with sufficiently low gap (100 f.lm) has been fabricated using the developed MSL set up, such that diffraction occurs during transillumination by spherical wavefronts. Among the two micro cantilevers one was fabricated bent with a specific di¬mension with respect to the other. The cantilever material was chosen as poly HDDA for its low elastic modulus in order to achieve high sensitivity. From the obtained diffraction pattern, the bent profile of the each cross section of one cantilever corresponding to the other has been measured. This proposition will enable to measure surface stress at each cross section of the cantilever depending on the adsorbed analyte molecule adsorption. In Chapter 4, an effort has been made to improve the thermal, thermo mechanical and mechanical properties of the cantilever material (poly HDDA). The sensitivity of a micro cantilever depends precisely on fabrication and material aspects. The former de¬pends on the aspect ratio of the structure and can be controlled by fabrication parameters whereas the latter is inherently limited by the choice of the material. The properties of the material which impact the applicability are elastic modulus, Poisson's ratio, thermal expansion and thermal stability. Hence, these properties are studied for poly HDDA. However, the properties are not completely satisfactory for only poly HDDA (PHDDA) since, PHDDA will fail for high surface stress measurement (>275 mN/m). Hence, it h8B been copolymerized with MMA with an intention to improve the above mentioned properties and to determine the best composition for the micro cantilever application. It is observed by Finite Element Analysis (FEM) that Phpm5050 (HDDA:MMA(50:50)) composition shows optimum sensitivity when reliability is concerned for me8Buring high surface stress (275 mN/m). Chapter 5 bridges Chapter 2 and Chapter 6. Chapter 2 highlights the polymer mi¬crofabrication where8B, Chapter 6 deals with the microfabrication of ceramics. In order to fabricate ceramic micro objects by MSL, ceramic particles need to be blended with a photopolymerizable monomer followed by l8Ber induced photopolymerization . Under l8Ber irradiation, the monomer gets cured and traps the ceramic particles. Thus near net shape of green ceramic structures are 0 btained. After achieving the near net shape, it is important to remove the polymer, which acts 8B the binder for the green ceramic body. This debinding should be diffusion controlled so 8B to achieve defect free micro ceramics. Here two multifunctional monomers (HDDA and TMPTA) have been chosen 8B a b8Be monomer for fabricating ceramics. Therefore it is essential to understand the debinding mechanism of these polymers. However, (HDDA) h8B high shrinkage upon polymeriza¬tion with low rate of polymerization kinetics and low viscosity where8B the properties of (TMPTA) are exactly opposite. Hence, in order to optimize these properties, copoly¬merization of HDDA and TMPTA h8B been carried out for different compositions and their thermal properties have been investigated to understand the degradation mech¬anism. This chapter deals with the mechanism of thermal degradation by model free kinetic methods with an intention to determine the optimum composition of HDDA and TMPTA copolymer, to used 8B the b8Be monomer material for ceramic microfabrication. Besides, the debinding strategy is also discussed b8Bed on the degradation profile of the optimum composition. TH20S0(TMPTA: HDDA(20:S0)) is found to be the ideal com¬position to fabricate ceramic micro-component by MSL since its degradation is diffusion controlled in N 2 atmosphere. Chapter 6 describes the methodology of microfabrication of ceramics by the de-veloped MSL technique. A colloidal approach has been adopted to fabricate ceramics in micro-dimensions. Two different ceramics have been chosen, which have potential applications in structural (alumina) and functional (Lead Iron Niobate (PFN))aspects. Before fabricating ceramic micro-objects, ceramic particles need to be blended in the monomer suspension in the presence of dispersant at an optimum solids loading. Opti¬mization of solids loading is important in view of low dimensional shrinkage after sin¬tering. However, lower loading leads to higher shrinkage whereas higher loading would increase the viscosity of the suspension and make the suspension inconvenient to deal with. Hence, rheological studies have been carried out to optimize the solids loading and dispersant concentration. 40 vol% alumina and 35 vol% PFN are found to be the highest achievable solids loading for the chosen monomer (TH2080) composition. This chapter also describes the limitation involved in ceramic microfabrication depending on their scattering factors during laser irradiation. The chapter demonstrates the fabrica¬tion methodology of several complex ceramic(alumina and PFN) micro-objects by the in house built MSL instrument. Chapter 7 investigates the possibility of microfabrication of ceramics from metalor¬ganic precursor. In this route, titanium metal-organic (Ti-n butoxide) precursor has been chosen which is stabilized by the addition of chelating monomer (2-( methacryloyloxy) ethyl acetoacetate). Following this, the crosslinker and photoinitiators have been added to form Ti photoresist which is coated on top of the bare silicon substrate by spin coating to achieve specific thickness. The coated silicon wafer by the above photoresist has been patterned by selectively exposure in the MSL setup. The cured patterns are washed and heat treated at high temperature in order to 0 btain the net shape of the Ti02 pattern of polycrystalline rutile phase. It is observed this route is advantageous in terms of reduc¬ing curing dimension (curing width 14 f.lm) than the colloidal route (curing width more than 80 f.lm ) of fabrication of ceramics where the scattering factor greatly influences the dimensions of the feature size. The key findings and future aspects are summarized in the Chapter 8. The work reported in this thesis has been carried out by the candidate as part of the Ph.D. programme. He hopes that this would constitute a worthwhile contribution towards developing an MSL technique and its aspects in micro fabrication of polymer and ceramic structures of any complex shape and its possible applications in microdevices.