Hsiharng Yang

A low-temperature wafer bonding technique using patternable materials

In this paper we present a silicon wafer bonding technique for 3D microstructures using MEMS process technology. Photo-definable material with patternable characteristics served as the bonding layer between the silicon wafers. A bonding process was developed and several types of photo-definable material were tested for bonding strength and pattern spatial resolution. The results indicated that SU-8 is the best material with a bonding strength of up to 213 kg cm−2 (20.6 MPa), and a spatial resolution of 10 μm, at a layer thickness of up to 100 μm. The low-temperature bonding technique that is presented is particularly suitable for microstructure and microelectronics integration involved in MEMS packaging.

High fill-factor microlens array mold insert fabrication using a thermal reflow process

An extreme high fill-factor microlens array mold insert in photoresist fabrication using a thermal reflow process is presented. The experimental results proved that a square microlens array could be produced without a peripheral gap. A square microlens array with an extreme high fill-factor (almost 100%) was successfully fabricated. In this experiment, square photoresist columns were formed on a silicon substrate using a lithographic process. The square pattern was laid out in an ortho-square on a polyethylene terephthalate (PET) based mask. Precise temperature and time control was used during the thermal reflow process. The square microlens array was formed from the uniformly flowing melted photoresist. The photoresist column surface transforms into a spherical profile due to the surface tension effect. The error was within ±8% between the fabricated microlens characteristics and the theoretical model used to predict the photoresist column thickness and actual thickness. This model is feasible for fabricating various sized high fill-factor square microlens arrays.

Hexagonal microlens array modeling and fabrication using a thermal reflow process

A mathematical model for designing and fabricating a hexagonal microlens array using a thermal reflow process was developed in this study. The experimental results proved that a hexagonal microlens array could be produced without a gap at each microlens periphery. A hexagonal microlens array with a higher fill factor was successfully produced. In this experiment, hexagonal photoresist columns were formed onto a silicon substrate made using a lithographic process. The hexagonal pattern was laid out in an ortho-triangle on a PET (polyethylene terephthalate)-based mask. Using precise temperature and time control during the thermal reflow process, a hexagonal microlens array with lateral honeycomb geometry was formed from the melted photoresist flowing outward simultaneously and uniformly. The surface tension effect transformed the photoresist column surface into a spherical profile. The error in the fabricated microlens characteristics was within ±3% between two theoretical models used to predict the photoresist column thickness and actual thickness. This model is feasible for fabricating various sized hexagonal microlens arrays.

Mechanical properties and tensile fatigue of graphene nanoplatelets reinforced polymer nanocomposites

Graphene nanoplatelets (GNPs) are novel nanofillers possessing attractive characteristics, including robust compatibility with most polymers, high absolute strength, and cost effectiveness. In this study, GNPs were used to reinforce epoxy composite and epoxy/carbon fiber composite laminates to enhance their mechanical properties. The mechanical properties of GNPs/epoxy nanocomposite, such as ultimate tensile strength and flexure properties, were investigated. The fatigue life of epoxy/carbon fiber composite laminate with GPs-added 0.25 wt% was increased over that of neat laminates at all levels of cyclic stress. Consequently, significant improvement in the mechanical properties of ultimate tensile strength, flexure, and fatigue life was attained for these epoxy resin composites and carbon fiber-reinforced epoxy composite laminates.

Ultra-fine machining tool/molds by LIGA technology

Machining tools based on the excimer laser and x-ray lithography to make ultra-fine machining tool/molds are described in this paper. The lower high-aspect ratio resist molds are fabricated using the KrF excimer laser. The higher aspect-ratio resist molds are made using x-ray lithography. Both low and high aspect-ratio resist molds are then converted into metallic structures using electroforming. The NiCo/SiC microcomposite electroforming with low internal stress (~0 kg mm-2) and high hardness (>Hv500) shows its feasibility as mold materials. An example of 2 mm thick integrated circuit (IC) packaging leadframe patterns using x-ray micromachining is illustrated to prove its feasible application. On the technical side, micro-structures with a high aspect ratio of 30 were developed using a graphite membrane based x-ray mask.

Micro-ball lens array modeling and fabrication using thermal reflow in two polymer layers

This paper presents a mathematical model to design and fabricate micro-ball lens array using thermal reflow in two polymer layers. The experimental results showed that micro-ball lens arrays were fabricated and integrated onto a planar substrate. Two polymer layers were coated onto a silicon substrate. The upper layer was a photoresist. The lower layer was a polyimide material. The polyimide was expected to form a pedestal to sustain the ball lens after the heat reflow process. Once the patterned polymer is heated above its glass transition temperature, the melting polymer surface will change into a spherical profile for minimizing its surface energy. A successful micro-ball array was formed in the photoresist through the different glass transition temperatures between two polymer materials. The interactive force between two material interfaces caused by surface tension causes the upper profile to form a spherical profile. This also forms the polyimide pedestal into a trapezoid with arc sides. The error in the fabricated micro-ball lens characteristics was 8% between the theoretical models used to predict the photoresist pattern thickness. This model is feasible for fabricating various sized micro-ball lens arrays.

Improvement of thickness uniformity in nickel electroforming for the LIGA process

Abstract An effective method to improve thickness uniformity in nickel electroforming for the LIGA process to produce metallic microstructures is described. The deposited metal distribution is naturally a concave shape over the whole area of the plating base proved by experiments. The edge thickness of the plating area is usually twice or higher than the center of the plating area. A secondary cathode (guard ring) was applied to improve the electroform uniformity. The experimental result of a Joule–Thomson micro-cooler proved its feasibility. The thickness ratio of edge to center of a 500 μm sample decreased from 2.5 to 1.4. The thickness uniformity improvement is achieved without a loss in plating rate in the center of the features.

Realization of fabricating microlens array in mass production

Micro-electro-mechanical system technology offers a wide number of applications for the military, industrial, and consumer markets. The miniaturization of components is a common objective for all studies. Refractive microlens array with density 400 lenses per cm2 are fabricated in three minutes by using hot embossing. The higher accuracy and lower cost of microlens fabrication methods are needed to meet the rapid growth of commercial devices. Higher density of microlens is achievable by a higher density mesh mold. Microlens diameters of 250-380 micrometers are shaped with various molds. Focal lengths of 185-225 micrometers are obtained by changing compression pressure and working temperature which are discovered in this experiment. Molding temperature effects on the surface tension in lens material as explored is essential. From accuracy of microlens arrays are less than Rt 0.1 micrometers that adapt to the form accuracy of the lenses. This article describes a mass fabricating method for microlens array by using hot embossing and the experimental results show its feasibility for practice.

Modeling and fabrication of a piezoelectric vibration-induced micro power generator

Abstract In this paper, theoretical and experimental study on a piezoelectric vibration‐induced micro power generator that can convert mechanical vibration energy into electrical energy is presented. The mechanical‐electrical energy conversion mechanism is a voltage between two capacitors, which belong to the mechanical and the piezoelectric equivalent circuits, respectively. To verify the theoretical analysis, two clusters of transducer structures are fabricated. Piezoelectric lead zirconate titanate (PZT) material is chosen to make the energy conversion transducer. The desired shape of the piezoelectric generator with its resonance frequency in accordance with the ambient vibration source is designed by finite element analysis (FEA). Experimental results show that the maximum output voltages are generated at the first mode resonance frequencies of the structure. The overall conversion efficiency is measured to be 33%. The experimental results coincide with the theoretical analysis.

Excimer laser-induced formation of metallic microstructures by electroless copper plating

Micro-patterns created by the excimer laser and activated by reactants for electroless copper plating are described in this paper. The generated micro-patterns are transformed into copper patterns on the substrate and copper microstructures are formed. This method simplifies the manufacturing process of making circuits on boards compared with the conventional lithography process of forming copper patterns on the substrate. Micro-patterns generated by the excimer laser cause changes of surface electric properties and activation selectively. A chemical reaction through these activated areas may deposit metal, such as copper. The KrF excimer laser not only provides simple and fast machining patterns, but also uses its high-energy density to drill holes and circuits directly. Palladium ions are added as mediators in the electroless plating solution to enable a continuous electroless copper deposition. According to the experiment of excimer laser-assisted electroless copper plating, the procedures of pretreatment and post-cleaning are the key factors that resulted in excellent selective plating. The samples were pretreated by sodium dodecyl sulfate (SDS) and post-cleaned by acetone and diluted nitric acid resulting in distinct micro-patterns. The deposition area is confined to the excimer laser-ablated portion resulting in good selective plating.