Rokas Sakalys

Polycarbonate as an Elasto-Plastic Material Model for Simulation of the Microstructure Hot Imprint Process

The thermal imprint process of polymer micro-patterning is widely applied in areas such as manufacturing of optical parts, solar energy, bio-mechanical devices and chemical chips. Polycarbonate (PC), as an amorphous polymer, is often used in thermoforming processes because of its good replication characteristics. In order to obtain replicas of the best quality, the imprint parameters (e.g., pressure, temperature, time, etc.) must be determined. Therefore finite element model of the hot imprint process of lamellar periodical microstructure into PC has been created using COMSOL Multiphysics. The mathematical model of the hot imprint process includes three steps: heating, imprinting and demolding. The material properties of amorphous PC strongly depend on the imprint temperature and loading pressure. Polycarbonate was modelled as an elasto-plastic material, since it was analyzed below the glass transition temperature. The hot imprint model was solved using the heat transfer and the solid stress-strain application modes with thermal contact problem between the mold and polycarbonate. It was used for the evaluation of temperature and stress distributions in the polycarbonate during the hot imprint process. The quality of the replica, by means of lands filling ratio, was determined as well.

Design and fabrication of piezoelectric nanocomposite structures for microdevice applications

Abstract. We present a new group of piezoelectric nanocomposite thin films based on integrating piezoelectric material poly(vinylidene fluoride) and nanoparticles of barium titanate in a matrix of an organic polymer poly(methyl methacrylate). Implementation of piezoelectric properties in designed new nanocomposites allows us not only to increase the sensitivity and functionality of the overall system, where this material is used, but also to expand the application fields in sensing and actuating systems. Results implied that new nanostructures fabricated by nanoimprint lithography exhibit good piezoelectric, surface, and mechanical properties and allow independent control of tribological properties. Formed nanocomposite systems were integrated in designing optical components employed in medicine for sensing applications.

Influence of binding material of PZT coating on microresonator's electrical and mechanical properties

Microresonators are fundamental components integrated in hosts of MEMS applications: covering the automotive sector, the telecommunication industry, electronic equipment for surface/material characterization and motion sensing, and etc. The aim of this paper is to investigate the mechanical and electrical properties of PZT film fabricated with three binding materials: polyvinyl butyral (PVB), polymethyl methacrylate (PMMA) and polystyrene (PS) and to evaluate applicability in control of microresonators Q factor. Micro particles of PZT powder were mixed with 20% solution of PVB, PMMA and PS in benzyl alcohol. For investigation of mechanical and electrical properties multilayer cantilevers were made. Obtained PZT and polymer paste was screen printed on copper (thickness 40 μm) using polyester monofilament screen meshes (layer thickness 50 μm) and dried for 30 min at 100°C. Electric dipoles of the PZT particles in composite material were aligned using high voltage generator (5 kV) and a custom–made holder. Electric field was held for 30 min. Surfaces of the applied films were investigated by Atomic Force Microscope NanoWizard(R)3 NanoScience. Dynamic and electrical characteristics of the multilayer were investigated using laser triangular displacement sensor LK-G3000. The measured vibration amplitude and generated electrical potential was collected with USB oscilloscope PicoScope 3424. As the results showed, these cantilevers were able to transform mechanical strain energy into electric potential and, v.v. However, roughness of PZT coatings with PMMA and PS were higher, what could be the reason of the worse quality of the top electrode. However, the main advantage of the created composite piezoelectric material is the possibility to apply it on any uniform or non-uniform vibrating surface and to transform low frequency vibrations into electricity.

Development of complex 3D microstructures based on computer generated holography and their usage for biomedical applications

The main focus of the paper is the development of technological route of the production of complex 3D microstructure, from designing it by the method of computer generated holography till its physical 3D patterning by exploiting the process of electron beam lithography and thermal replication which is used for biomedical application. A phase data of a complex 3D microstructure was generated by using Gerchberg-Saxton algorithm which later was used to produce a computer generated hologram. Physical implementation of microstructure was done using a single layer polymethyl methacrylate (PMMA) as a basis for 3D microstructure, which was exposed using e-beam lithography system e-Line and replicated, using high frequency vibration. Manufactured 3D microstructure is used for designing micro sensor for biomedical applications.

Vibration Based Microstructure Replication and Analysis

The main focus of the paper is the development of technological process for the production of complex 3D microstructure, from designing it by using computer generated holography to its physical 3D patterning by exploiting the process of electron beam lithography. The logo image was chosen for numerical generation, which was performed by using Gerchberg–Saxton algorithm for computer generated hologram design. Physical implementation of microstructure was performed by using a single layer polymethyl methacrylate (PMMA) as a basis for 3D microstructure, which was exposed by using e-beam lithography system e-LINEplus. After production, verification of 3D microstructure is performed by exposing it under the laser beam and qualitative analysis is performed by using atomic force microscope. Finally, improvement of mass production of complex microstructures, designed by using computer generated holography, is presented.

Analysis of the Influence of High-Frequency Excitation Into Quality of the Replicated Microstructure

In the present research, surface relief diffraction gratings were fabricated and investigated. The purpose of this research was to determine the collection of parameters, which influence the diffraction efficiency most positively. For replication process ultrasonic thermal embossing was selected with different manufacturing regimes (time, pressure, and temperature). Diffraction efficiencies of periodical microstructures were measured experimentally. The results have shown increase of periodical microstructure quality with the help of high-frequency oscillations during manufacturing. Combination of pressing time, pressure, temperature, and vibrations improved the efficiency of replication process.

Microstructures replication using high frequency excitation

Diffraction efficiency of grating, created by hot imprint process on the surface of polycarbonate is one of the parameters, which determines the quality of microstructure. Microstructures are replicated by using hot imprint process with and without high frequency excitation and during the quality investigation, diffraction efficiencies were measured on purpose to find microstructure of best possible optical quality, as well determine whether high frequency excitation and other process parameters during the process affect this parameter. Process parameters include: temperature, excitation frequency, force of mechanical load and duration of hot imprint process, the purpose is to determine the collection of parameters, which influences the diffraction efficiency most positively. The novel vibro active pad is proposed for microstructures replication. The main dynamical characteristics of the vibropad are presented in the paper.

Novel composite piezoelectric material for energy harvesting applications

Past few decades were concentrated on researches related to effective energy harvesting applied in modern technologies, MEMS or MOEMS systems. There are many methods for harvesting energy as, for example, usage of electromagnetic devices, but most dramatic changes were noticed in the usage of piezoelectric materials in small scale devices. Major limitation faced was too small generated power by piezoelectric materials or high resonant frequencies of such smallscale harvesters. In this research, novel composite piezoelectric material was created by mixing PZT powder with 20% solution of polyvinyl butyral in benzyl alcohol. Obtained paste was screen printed on copper foil using 325 mesh stainless steel screen and dried for 30 min at 100 °C. Polyvinyl butyral ensures good adhesion and flexibility of a new material at the conditions that requires strong binding. Five types of a composite piezoelectric material with different concentrations of PZT (40%, 50%, 60%, 70% and 80 %) were produced. As the results showed, these harvesters were able to transform mechanical strain energy into electric potential and, v.v. In experimental setup, electromagnetic shaker was used to excite energy harvester that is fixed in the custom-built clamp, while generated electric potential were registered with USB oscilloscope PICO 3424. The designed devices generate up to 80 μV at 50 Hz excitation. This property can be applied to power microsystem devices or to use them in portable electronics and wireless sensors. However, the main advantage of the created composite piezoelectric material is possibility to apply it on any uniform or nonuniform vibrating surface and to transform low frequency vibrations into electricity.

Investigation of Dependency of Microstructure Quality on Vibration Mode

The quality of microstructure, created on polycarbonate (PC), by using hot imprint process and changing vibration parameters is investigated in this paper, in order to reveal the best option and further successfully apply this process in research and industry. Earlier it was revealed, that high frequency excitation, with usage of vibroactive pad, made from aluminum and piezoceramic (PZT-4) positively affects the quality of microstructure. Now, when working regimes are known it is necessary to find out which of them influences the quality of created microstructure best. Thus the experiment, when holding all parameters of the process constant, and only changing vibration modes, which are already known, is carried out. After the experiment qualitative analysis, using optical microscope is performed.