Papers by Author: Chang Seung Lee

Abstract: A new tensile tester using an electromagnetic-force actuator (voice coil) was developed to measure the mechanical characteristics of surface-micromachined thin film materials. The tester has a load cell with maximum capacity of 0.5N and a non-contact position measuring system based on the principle of capacitance micrometry with 0.1nm resolution for displacement measurement. The tester was applied for tensile testing of Al-3%Ti thin films with dimensions of 1000μm long, 50-480μm wide and 1.0 and 1.1 μm thick. The Al-3%Ti is commonly used in RF(radio frequency) microelectromechanical systems(MEMS) switch. The specimen with holes and bridges was designed for easy tensile test. The holes at center of grip end are able to make alignment and gripping easy. The bridges are to remove the side supports easily and extract specimen from wafer without sawing. It was found that the mean tensile strengths of Al-3%Ti are 140-380MPa, depending on the width of specimens and converging to a certain tensile strength as the width decreases.

Abstract: The accurate characterization of linear coefficient of thermal expansion (CTE) of thin films is vital for predicting the thermal stress, which often results in warpage and failure of a MEMS structure. In this paper, special emphasis is placed on the development of novel test method to extend an ISDG (Interferometric Strain/Displacement Gage) technique to the direct and accurate CTE measurement of MEMS materials, AlN and Au. The freestanding AlN and Au films are 1 μm thick and 5 mm wide. Strain is directly measured by a brand-new digital type ISDG with two Cr lines deposited on the specimen while heating a specimen in a furnace. The whole test system is verified first by measuring the CTE for the NIST’s SRM (Standard Reference Material) 736 (Cu) block. The measured CTE is 17.3 με/oC up to 167 oC, which agrees well with the NIST’s certified value. The CTE of Au is 25.4 ± 1.15 με/oC and that of AlN film is 3.77 ± 0.12 με/oC. The in-plane displacement resolution is about 5 nm at the best circumstances.

Abstract: This paper describes a novel test procedure, new structure of specimen easy to manipulate, align and grip a thin-film and test machine for a tensile and fatigue test. For the proposed specimen, the surroundings of the specimen including the side support strips except for six bridges are etched during the fabrication of the specimen, which in turn makes it possible to cut off easily the specimen from the wafer by minimizing a damage to the test film and also possible to produce the specimen in mass production. For the present specimen, a small hole is made at the grip end and using a small pin for setting the specimen onto the tester, the setting process and alignment of specimen is much easier, compared to the specimen proposed by Sharpe et al. To gain confidence in reliability of testing results, pre-test using the Al-3%Ti is performed, which is widely used in the RF switch and other MEMS devices. Tensile tests are performed, from which tensile strengths of the Al-3%Ti are measured as 343±16.22MPa at 200μm width and 1.1μm thickness.

Abstract: This paper presents high cycle fatigue properties of a Al-3%Ti thin film, used in a RF (radio-frequency) MEMS switch for a mobile phone. The thickness and width of the thin film of specimen are 1.1μm and 480.0μm, respectively. Tensile tests of five specimens are performed, from which the ultimate strength is found to be 144MPa. High cycle fatigue tests of six specimens are also performed, from which the fatigue strength coefficient and the fatigue strength exponent are found to be 336MPa and –0.1514, respectively.

Abstract: Mechanical property evaluation of micrometer-sized structures is necessary to help design reliable microelectromechanical systems (MEMS) devices. Most material properties are known to exhibit dependence on specimen size and such properties of microscale structures are not well characterized. This paper describes techniques developed for tensile testing of materials used in MEMS. Epi-polycrystalline silicon is currently the most widely used material, and its tensile strength has been measured as 1.52GPa. We have developed an axial testing machine for testing microscale specimen using electro-magnetic actuator. The field magnet and the moving coil taken from an audio-speaker were utilized as the components of the actuator. Structure of specimen was designed and manufactured for easy handling and alignment. In addition to the static tensile tests, new techniques and procedures for measuring strength are described.