Hyung-jae Shin

Chemical, optical and tribological characterization of perfluoropolymer films as an anti-stiction layer in micro-mirror arrays

Abstract Perfluoropolymer (PP) thin films were deposited on aluminum (Al) and tetraethylorthosilicate (TEOS) oxide surfaces by thermal vapor-phase (VP) deposition to prevent the stiction of micro-mirrors during their touchdown operation on TEOS pads. The fluorocarbon (FC) chemicals from the 3M Co. and perfluorodecanoic acid (PFDA) were used as precursors for the film deposition. Static contact angles (SCA) of the PP films on dry-cleaned Al and TEOS were measured and found to be larger than 110°. Dynamic contact angle (DCA) analysis of the films showed the poor surface coverage and poor homogeneity of the films. However, very low surface energies, less than 15 and 11 dynes/cm, were calculated from the contact angle measurements on Al and TEOS, respectively, after film deposition. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) analysis showed the presence of CF, CF 2 , CF 3 , and C–C stretching bands on PP films on Al. Optical characterization of the films, using variable angle spectroscopic ellipsometry (VASE), was carried out to determine the thickness of the films. Micro-tribological behavior was investigated by atomic and lateral force microscopy (AFM/LFM) as a function of the applied load. The AFM/LFM analysis showed that there were higher friction forces and a poorer coverage for films deposited on Al than for films deposited on TEOS. The reliability of the films was evaluated by the actual operation of the micro-mirror arrays at 3 kHz in a dry N 2 environment. The micro-mirrors were movable without stiction occurring, even after 2×10 8 touchdown cycles, in the presence of FC surface films.

A laser-based 2-dimesional angular deflection measurement system for tilting microplates

A laser-based system that is capable of measuring the angular deflection of the microplate in two-dimension is developed. The system consists of a HeNe laser, lenses, beamsplitters, objective lens, CCD camera and position sensitive photodiode. The operation of the system is based on the measurement of the displacement of a HeNe laser beam reflecting off the surface of the microplate using position sensitive photodiode. The angular resolution is estimated at 0.2° and the response time of the system is as short as 4 μs. This system can measure the static and dynamic angular response and resonance frequency of microplates, such as spatial light modulators and micromirrors, in two orthogonal directions.

Fabrication and deflection measurement of micromirrors supported by an S-shape girder

Micromirrors supported by S-shape girders were fabricated and their angular deflections were measured using a laser- based system. A micromirror consists of a 50 micrometers X 50 micrometers aluminum plate, posts and an S-shape girder. Two electrodes were deposited on two corners of the substrate beneath the mirror plate. 50 X 50 micromirror array were fabricated using the Al-MEMS process. The electrostatic force caused by the voltage difference between the mirror plate and one of the electrodes causes the plate to tilt under the girder touches substrate. Bias voltage of the mirror plate is between 25 approximately 35 V and signal pulse voltage on the electrodes is 5 V. A laser-based system capable of real-time two-dimensional measurements of the angular deflection of the micromirror was developed. The operation of the system is based on measuring the displacement of a HeNe laser beam reflecting off the micromirror. The resonance frequency of the micromirror is 50 kHz when the girder touches the substrate and it is 25 kHz when the micromirror goes back to flat position, since the moving mass is about twice of the former case. The measurement results also revealed that the micromirror slants to the other direction even after the girder touches the substrate.

Investigation of stress in aluminum thin film for MEMS applications

Single layer of aluminum film was sputter deposited on to (100) oriented 4 inch silicon wafer to study effect of film thickness, D.C. power and sputtering gas pressure on the film stress. The as-deposited stress appeared to be increasing as film thickness increases and argon pressure decreases. Thermal stress originated from difference in CTE and temperature variation during and after sputtering seems to be a main factor in room temperature sputter deposited aluminum films. From observation of temperature-stress behavior, it was found that the pure aluminum film has an elastic modulus of 56GPA and compressive yield strength of -100MPA. The yield strength was improved to about -175MPA by alloying with 3wt.%Ti. Titanium alloying also proved to be useful in extending linear elastic region before plastic deformation occurs. However, it was hard to determine the stress level with buckling phenomena of ring/beam microstructures because of imperfections such as stress gradient and thermal deformation. In stead, those diagnostic microstructures could be applied to give an information on whether a plastic deformation was introduced or not in a structure of specific dimension.

Automatic test equipment for the micromirror array

Test equipment for the development and mass production of micromirror array have been devised. Test equipment for the static and dynamic response of a single micromirror consists of HeNe laser, lenses, XY stage, CCD camera, position- sensitive photodiode and PC. It can be used to measure reflectance, tilt angle--input voltage relation, response time and resonant frequency, in the developing stage. It can also check the lifetime and uniformity of mirror quality over the wafer. Test equipment for the evaluation of micromirror array consists of CCD camera, lenses, XY stage, video signal processor and PC. It can classify the error- state of micromirror, generate statistical data and map of the position of abnormal micromirrors. The test results are shown on the monitor as a map that shows the error state, position and statistical data. It takes about 90 seconds to evaluate 50 X 50 micromirror array.

Young's modulus measurement of aluminum thin film with cantilever structure

Micromachined cantilever structures are commonly used for measuring mechanical properties of thin film materials in MEMS. The application of conventional cantilever theory in experiment raises severe problem. The deformation of the supporting post and flange is produced by the applied electrostatic force and lead to more reduced measurement value than real Youngí»s modulus of thin film materials. In order to determine Youngí»s modulus of aluminum thin film robustly and reproducibly, the modified cantilever structure is proposed. Two measurement methods, which are cantilever tip deflection measurement and resonant frequency measurement, are used for confirming the reliability of the proposed cantilever structure as well. Measured results indicate that the proposed measurement scheme provides useful and credible Youngí»s modulus value for thin film materials with sub-micron thickness. The proved validation of the proposed scheme makes sure that in addition to Youngí»s modulus of aluminum thin film, that of other thin film materials which are aluminum alloy, metal, and so forth, can be extracted easily and clearly.

From optical MEMS to photonic crystal

This paper presents the emergence of photonic crystals as significant optomechatronics components, following optical MEMS. It is predicted that, in the coming years, optical MEMS and photonic crystals may go through dynamic interactions leading to synergy as well as competition. First, we present the Structured Defect Photonic Crystal (SDPCTM) devised by the authors for providing the freedom of designing photonic bandgap structures, such that the application of photonic crystals be greatly extended. Then, we present the applications of optical MEMS and photonic crystals to displays and telecommunications. It is shown that many of the applications that optical MEMS can contribute to telecommunications and displays may be implemented by photonic crystals.

Conformal Coating Process of Anti-sticking Thin Film Using C4F8 and Ar Plasma without Additional Equipment

The relatively conformal fluorocarbon deposition process on aluminum surface was developed to eliminate the sticking problem in MEMS applications such as micromirror. The process was carried out in a capacitively-coupled plasma reactor with octafluorocyclobutane (C4F8) gas. The process parameters were optimized for maximum conformal quality. Dynamic contact angle measurement and AFM/LFM analysis on deposited film showed low surface free energy and homogeneous coverage resulting in high hydrophobic characteristics. Ellipsometry analysis and reflectance measurement revealed high enough transparency to be used in optical applications.

Design and simulation of a micromirror array for a projection TV

The design of a micromirror for a projection TV is investigated. A static structural analysis is performed to give an optimal shape of the micromirror using the FEM commercial package, ANSYS. A solid modeling is created, and mapped meshes are applied to it in order to satisfy a symmetric condition. A stress analysis shows that maximum stress does not exceed an allowable stress, which is the yield strength. A modal analysis is also executed to find the approximate natural frequencies with different design parameters. The result can be utilized to see which design parameter is strongly dominant. The micromirror was fabricated by Samsung Electronics. Dynamic deflection experiments confirm the results of the simulation.

A proposal of vertical spring as a mechanical element for MEMS and fabrication of vertical spring using simple shadow evaporation process [and micromirror application]

We proposed a vertical spring as a mechanical element for effective use of device area in micromachines. The experiments over shadow evaporation process showed that the shape of trench-hole cross-section is the most important parameter for a flat spring fabrication. To demonstrate the usefulness of the vertical spring, we adapted the vertical spring to micromirror array, and the fabricated micromirror operated successfully over 200 million cycles without any mechanical problems.