Min-seog Choi

Zoom lens design using liquid lens for laparoscope

Traditional laparoscopic optical systems consisting of about 30 lenses have low optical magnification. To magnify tissue during surgical operations, one must change from one laparoscope to another or use a magnifying adapter between the laparoscope and the sensor. Our work focuses on how to change the sag of a liquid lens while zooming from 1 × zoom, to 2 × , and 4 × in an optical design for a laparoscope. The design includes several lenses and two liquid lenses with variable focal lengths. A pair of laparoscopes for 3-D stereoscopy is placed within a tube 11 mm in diameter. The predicted depth resolution of tissue is 0.5 mm without interpolation at 4 × zoom.

Microelectrofluidic iris for variable aperture

This paper presents a variable aperture design based on the microelectrofluidic technology which integrates electrowetting and microfluidics. The proposed microelectrofluidic iris (MEFI) consists of two immiscible fluids and two connected surface channels formed by three transparent plates and two spacers between them. In the initial state, the confined aqueous ring makes two fluidic interfaces, on which the Laplace pressure is same, in the hydrophobic surface channels. When a certain voltage is applied between the dielectric-coated control electrode beneath the three-phase contact line (TCL) and the reference electrode for grounding the aqueous, the contact angle changes on the activated control electrode. At high voltage over the threshold, the induced positive pressure difference makes the TCLs on the 1st channel advance to the center and the aperture narrow. If there is no potential difference between the control and reference electrodes, the pressure difference becomes negative. It makes the TCLs on the 1st channel recede and the aperture widen to the initial state. It is expected that the proposed MEFI is able to be widely used because of its fast response, circular aperture, digital operation, high aperture ratio, and possibility to be miniaturized for variable aperture.

Variable aperture controlled by microelectrofluidic iris

This Letter presents an adaptive liquid iris based on microelectrofluidic technology with experimental results. In the microelectrofluidic iris (MEFI), the electrostatic force generated by electrowetting in a surface channel unbalances the Laplace pressure acting on two fluidic interfaces between air and a light-absorbing liquid in two connected surface channels in a chamber. Then, the changed net pressure makes the iris aperture of the liquid diaphragm adjustable. The present MEFI was designed to have a tunable range from 4.2 to 0.85 mm in diameter and a tuning ratio of 80%. The MEFI was fabricated with a transparent electrode patterned on three glass plates and two channel spacers. Concerning the optical and interfacial properties of the MEFI for its operation, an aqueous near-infrared dye used in optical coherence tomography (OCT) was forced into a ring shape as the driving liquid in the hydrophobic chamber. By switching the segmented concentric control electrodes in steps, digital operation of the MEFI was successfully observed with clear aperture stops. The measured turnaround speed was 80 mm/s, which is significantly higher than that for other comparable adaptive liquid irises. Due to a scalable aperture range with fast response, the concept of MEFI is expected to be widely applied in various optical systems that require high-quality imaging, as well as in real-time diagnostic OCT.

Wafer-level low temperature bonding with Au-In system

Wafer bonding at low temperature is an essential process for next generation MEMS & Sensor packaging. Optoelectronic devices, such as image sensor module and laser diode integrated circuit, need low bonding temperature, high re-melting temperature, high thermal conductivity, and stress-relaxed structure in many cases. Eutectic Au-In system was developed as a replacement of previous Au-Sn system for specific systems require bonding temperature lower than 200degC. Bonding temperature of developed Au-In system was set at 180degC, which was 100degC lower than that of Au-Sn system. Though polymer materials has been used for low temperature bonding, out-gassing and volume shrinkage during the bonding process often degraded bonding quality and accurate alignment between the wafers. Clean packaging with accurate alignment was achieved with eutectic Au-In bonding which also possessed high re-melting temperature over 450degC. Majority of the deposited metallizations to construct the system was converted to intermetallic compounds (AuIn & AuIn2) after bonding reaction. Peak temperature and duration time were varied to investigate optimum condition of wafer-level bonding and diced separate dies are used for X-ray inspection, microstructural observation of the cross-section, and shear test. The results showed that bonding parameters critically affected mechanical reliability of the bonded joint. Failure through the solder layer (unreacted pure In) resulted in higher shear strength, while clear separation between the wafer and under bump metallization (UBM) revealed low bond strength. Re-melting temperature of Au-In system was measured using TMA and the result showed that it was closely related with melting phenomena of pre-formed intermetallic compounds such as AuIn and gamma phases. The wafer-level bonding with Au-In system showed good feasibility for MEMS & sensor packagings that require low temperature bonding with high quality.

Low Temperature, Wafer Level Au-In Bonding for ISM Packaging

A low process temperature, hermetic and reliable wafer level packaging (WLP) technology is required for image sensor module (ISM) packaging. Eutectic bonding is regarded as one of the most common used methods for WLP. Au-Sn metallization system has been applied as a wafer level bonding technology in many applications, but it still has process temperature around 300degC which is not applicable for temperature sensitive materials contained device wafer like ISM. In this paper, a fluxless Au-In solder system with Au, In multilayer metallizations has been developed and fabrication process is also presented, the metallization is achieved using e-beam evaporation, test vehicle was then prepared for bonding quality evaluation. Bonding process is performed at 180degC with static force for a relatively long dwelling time of 30minutes in N2 ambience, finally a void free joint is formed. Microstructure observation reveals a combination of different Au-In intermetallic compounds AuIn2 and AuIn at the interface. Shear strength around 20MPa could be obtained for as-bonded samples, and a remelting temperature over 300degC is confirmed using thermomechanical analysis (TMA) test. Real time helium leak rate test are performed to check hermeticity of the package, samples are also subjected to pressure cooker test (PCT) for evaluation of bonding performance after reliability test

Varifocal liquid lens based on microelectrofluidic technology

This Letter presents a tunable liquid lens based on microelectrofluidic technology. In the microelectrofluidic lens (MEFL), electrowetting in the hydrophobic surface channel induces the Laplace pressure difference between two fluidic interfaces on the lens aperture and the surface channel. Then, the pressure difference makes the lens curvature tunable. In spite of the contact angle saturation, the narrow surface channel increases the Laplace pressure to have a wide range of optical power variation in the MEFL. The magnitude of the applied voltage determines the lens curvature in the analog mode MEFL. Digital operation is also possible when the control electrodes of the MEFL are patterned to have an array. The lens aperture and maximum surface channel diameter were designed to 3.2 mm and 6.4 mm, respectively, with a channel height of 0.2 mm for an optical power range between +210 and -30 D. By switching the control electrodes, the averaged transit time in steps and turnaround time were as low as 2.4 ms and 16.5 ms, respectively, in good agreement with the simulation results. It is expected that the proposed MEFL may be widely used with advantages of wide variation of the optical power with fast and precise controllability in a digital manner.

Adaptive optical probe design for optical coherence tomography and microscopy using tunable optics.

We present a tunable, adaptive optical imaging probe for multimodal imaging such as optical coherence tomography and microscopy. The probe is compatible with forward-looking scanning laser imaging devices such as an endoscope. The lens configuration includes a tunable iris and two varifocal lenses, both driven by microelectrofluidics, as well as several conventional fixed focus lenses. The modulation transfer function and spot size in the focal plane is evaluated, and we show using optical simulations that there are three possible imaging modes with different transverse resolutions and focal depths.

Four zoom lens design for 3D laparoscope by using liquid lens

Laparoscopic lens module that is capable of zooming is presented. The lens module has a high magnification and a high resolution such as four zoom and 2M pixels full HD image. The lens module consists of two lens sets to get 3-D images. Each lens module has several lenses less than conventional laparoscope but has 8 lenses and two liquid lenses. The total length of module is 19 mm long and the diameter is less than 5 mm. The separated distance of two lens center is 5 mm and two lens modules are inserted into the 11mm diameter laparoscope. The lens module is designed by Code V™ by using the 2M pixels CMOS sensor that the pixel size is 1.75 μm. The merit of this fluidic lens design is being convertible between a convex and concave shape. The effective focal length of zoom-out and zoom-in modes is 3.24 mm and 12.94 mm respectively. The modulation transfer function of zoom-out and zoom-in modes is 40% and 30% at 140 lp/mm frequency. We have a diffraction of element at near stop to improve image resolution. Also the resolution of zoom-in mode is improved by using liquid iris. The F-number of a two modes is 4.4 and 5.8 and the optical distortion is 10% and 0.5%. It is expected that the z-direction resolution by this laparoscope is less than 2 mm

Microelectrofluidic lens for variable curvature

This paper presents a tunable liquid lens based on microelectrofluidic technology which integrates electrowetting and microfluidics. In the novel microelectrofluidic lens (MEFL), electrowetting in the hydrophobic surface channel induces the Laplace pressure difference between two fluidic interfaces on the lens aperture and the surface channel. Then, the pressure difference makes the lens curvature tunable. The previous electrowetting lens in which the contact angle changes at the side wall has a certain limitation of the curvature variation because of the contact angle saturation. Although the contact angle saturation also appears in the surface channel of the MEFL, the low surface channel increases the Laplace pressure and it makes the MEFL to have full variation of the optical power possible. The magnitude of the applied voltage determines the lens curvature in the analog mode MEFL as well as the electrowetting lens. Digital operation is also possible when the control electrodes of the MEFL are patterned to have an array. It is expected that the proposed MEFL is able to be widely used because of its full variation of the optical power without the use of oil and digital operation with fast response.

Reliability Assessment of MEMS Gyroscope Sensor

Reliability of MEMS devices is receiving more attention as they are heading towards commercial production. In particular are the reliability and long-term stability of wafer level vacuum packaged MEMS gyroscope sensors subjected to cyclic mechanical stresses at high frequencies. In this study, we carried out several reliability tests such as environmental storage, fatigue, shock, and vibration, and we investigated the failure mechanisms of the anodically bonded vacuum gyroscope sensors. It was found that successful vacuum packaging could be achieved through reducing outgassing inside the cavity by deposition of titanium as well as by pre-taking process. The current gyroscope structure is found to be safe from fatigue failure for 1000 hours of operation test. The gyroscope sensor survives the drop and vibration tests without any damage, indicating robustness of the sensor. The reliability test results presented in this study demonstrate that MEMS gyroscope sensor is very close to commercialization.