Papers by Author: Sung Hoon Choa

Abstract: In this paper, a low temperature hermetic wafer level packaging scheme for the RFMEMS devices is presented. For hermetic sealing, Au-Sn multilayer metallization with a square loop of 70 %m in width is performed. The size of the MEMS package is 1mm × 1mm × 700 %m. The shear strength and hermeticity of the package satisfy the requirements of MIL-STD-883F. The total insertion loss for the packaging is 0.075 dB at 2 GHz.

Abstract: In MEMS, packaging induced stress or stress induced structure deformation becomes increasing concerns since it directly affects the performance of the device. The conventional MEMS SOI (silicon-on-insulator) gyroscope, packaged using the anodic bonding at the wafer level and EMC (epoxy molding compound) molding, has a deformation of MEMS structure caused by thermal expansion mismatch. Therefore we propose a packaged SiOG (Silicon On Glass) process technology and more robust spring design.

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 thermal properties of thin films, such as thermal conductivity and diffusivity, are important in design and analysis of MEMS (micro electro mechanical systems), particularly in microscale thermal systems and high-power electronic/optoelectronic devices. In the present study, the thermal conductivity and diffusivity of a variety of thin film materials, which are commonly used in MEMS applications, are measured. The samples include Au, Sn, Mo, Al/Ti alloy, AlN, and SiC. The Au sample is deposited by the e-beam evaporation technique while the rest of the metallic samples are deposited by sputtering processes. The AlN and SiC films are also prepared by sputtering processes. In the experiment, the thermal diffusivities of metallic thin films are measured by two independent methods — the AC calorimetric method and photothermal mirage technique. The thermal conductivities of dielectric thin films are measured by the 3 omega technique. The results show that the thermal transport properties of some of the films are significantly smaller than those of the same material in bulk form. Especially, the AlN and SiC thin films exhibit pronounced thermal conductivity reduction because of the size effect. The electrical conductivities of the metallic thin films are measured as well. The results for Au and Sn are consistent with the thermal conductivity, confirming the Wiedmann-Franz law. However, Al/Ti and Mo thin films show considerable deviation from the law. The results are analyzed based on the XRD (X-Ray diffraction) and AFM (Atomic Force Microscope) measurement.

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: In this paper, mechanical reliability issues of copper through-wafer interconnection are investigated numerically and experimentally. Several factors which could induce via hole cracking failure are investigated such as thermal expansion mismatch, via etch profile, copper diffusion phenomenon, and cleaning process. Improvement methods are also suggested.

Abstract: In this paper, deformation behavior of the MEMS gyroscope package subjected to temperature change is investigated using a high-sensitivity moiré interferometry. Temperature dependent analyses of warpage and extension/contraction of the package are presented. Detailed global and local deformations of the package by temperature change are investigated and its effect on the frequency shift of the MEMS gyroscope is studied.

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 a novel experimental method to investigate the strength of material, Al-3%Ti, which is commonly used in RF(radio frequency) microelectromechanical systems(MEMS) switch. The experimental method involves the development of a new tensile loading system. The new tensile loading system 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. A voice coil of audio speaker is used as the actuator of the loading system. And new specimen was designed and fabricated to easily manipulate, align and grip a thin-film for a tensile and fatigue test. The material used in this study was Al-3%Ti thin film, which was used in RF switch. The thickness and width of the thin film of specimen are 1.1µm and 480µm, respectively. 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. Tensile tests were performed on 5 specimens. The ultimate strength of Al-3%Ti was 144MPa.

Abstract: We carry out reliablity tests and investigate the failure mechanisms for wafer level vacuum packaged (WLVP) MEMS resonator using an accelerated degradation test (ADT). ADT is also used to estimate the mean life-time (or failure) of WLVP. The main failure mechanism of WLVP is found to be outgassing inside the package. Failure distribution of log-logistic is well fitted with the failure data from ADT, and Arrhenius model is used as a stress-life model because the main stress parameter is temperature. Acceleration factor (AF) for temperature levels is calculated and used to evaluate mean life-time of the WLVP after redesign. Finally the accuracy of ADT model is examined by verification test. The successful WLVP is achieved by reducing outgassing through the deposition of titanium coating inside the package as a getter material.