Fehmi Civitci

A 35-

A thermo-mechanical MEMS detector with 35-μm pixel pitch is designed, fabricated, and characterized. This fabricated design has one of the smallest pixel sizes among the IR thermo-mechanical MEMS sensors in the literature. The working principle of the MEMS detector is based on the bimaterial effect that creates a deflection when exposed to IR radiation in the 8-12-μm waveband. The nanometer level out of plane mechanical motion is observed in response to IR heating of the pixel, which is detected by a diffraction grating-based optical readout. Performance of MEMS sensor arrays with optical readout have been limited by a large DC bias that accompanies a small AC signal. We developed a novel optical setup to reduce the DC term and the related noise using an AC-coupled detection scheme. Detailed noise characterization of the pixel and the readout system is reported in this paper. The noise equivalent temperature difference of our detector is measured as 216 mK using f/0.86 lens with the AC-coupled optical readout. Finally, we obtained a thermal image using a single MEMS pixel combined with a scanning configuration. Despite the reduced pixel size, the measured noise levels are comparable to the state-of-the-art thermo-mechanical IR sensors.

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We proposed a novel two-wavelength optical readout for grating based thermo-mechanical MEMS arrays by using two color LEDS butt-coupled to a multi-mode fiber, which provides spatial auto-registration of two different color images on a single camera. The -1st diffraction orders of the different colors are simultaneously monitored with different channels (R and G) of the RGB camera. The sensitivity of the MEMS array is improved by using the channel data with higher sensitivity for each MEMS detector under a thermal load.

Pitch IR Thermo-Mechanical MEMS Sensor With AC-Coupled Optical Readout

A low-cost confocal endoscope was developed consisting of a 3D printed laser scanner, a lens, and a housing. The developed tool, mainly made out of low cost polymer offers a disposable use. The scanner unit is overall 10x10mm and electromagnetically actuated in 2-dimensions using a magnet that is attached to the 3D printed scanner and an external miniaturized coil. Using 3D printer’s fabrication advantages the first two vibration modes of the scanner were tailored as out-of-plane displacement and torsion. The scanner employs lissajous scan, with 190 Hz and 340 Hz scan frequencies in the orthogonal directions and we were able to achieve ± 5° scan angles, respectively, with ~ 100 mA drive current. The lens which has 6-mm diameter and 10-mm focal length is 3D printed with Veroclear material and then polished in order to reach optical quality surface. Profilometer (Dektak) measurements indicate only x2 increase in rms roughness, with respect to a commercial glass lens having identical size and focal length.

Simultaneous two-wavelength readout for thermo-mechanical MEMS detectors

This letter demonstrates a novel prism-based optical-readout, which uses a single prism to detect the incoming TM polarized wave just below the critical angle. The method is used with a 35-μm-pixel pitch MEMS thermal sensor, whose inclination angle changes with the absorbed infrared (IR) radiation that results in an increase in the reflectivity at the prisms glass-air interface. We compared this approach with the conventional knife-edge method. Noise equivalent temperature difference for a single sensor was measured as 200 mK for knife-edge method, and 154 mK for the proposed critical angle approach. Our approach shows a significant improvement for the sensitivity of the IR sensor. Both methods utilize an AC-coupled readout method for a single MEMS pixel using a photodetector, which responds only to changes in the scene. This method can be scaled to achieve smart pixel cameras for read sensor arrays with low-noise and high-dynamic range.

Towards 3D printed confocal endoscopy

This paper demonstrates a 35-μm pixel pitch MEMS thermal sensor array with optical-readout. We implemented an AC-coupled readout method for a single MEMS pixel using a photodetector, which responds only to changes in the scene. AC-coupled readout substantially reduced the optical noise by eliminating the large DC beam. This method can be scaled to achieve smart pixel cameras for read sensor arrays with low-noise and high dynamic range.

A Prism-Based Optical Readout Method for MEMS Bimaterial Infrared Sensors

We developed two types of cantilever-based biosensors for portable diagnostics applications. One sensor is based on MEMS cantilever chip mounted in a microfluidic channel and the other sensor is based on a movable optical fiber placed across a microfluidic channel. Both types of sensors were aimed at direct mechanical measurement of coagulation time in a disposable cartridge using plasma or whole blood samples. There are several similarities and also some important differences between the MEMS based and the optical fiber based solutions. The aim of this paper is to provide a comparison between the two solutions and the results. For both types of sensors, actuation of the cantilever or the moving fiber is achieved using an electro coil and the readout is optical. Since both the actuation and sensing are remote, no electrical connections are required for the cartridge. Therefore it is possible to build low cost disposable cartridges. The reader unit for the cartridge contains light sources, photodetectors, the electro coil, a heater, analog electronics, and a microprocessor. The reader unit has different optical interfaces for the cartridges that have MEMS cantilevers and moving fibers. MEMS based platform has better sensitivity but optomechanical alignment is a challenge and measurements with whole blood were not possible due to high scattering of light by the red blood cells. Fiber sensor based platform has relaxed optomechanical tolerances, ease of manufacturing, and it allows measurements in whole blood. Both sensors were tested using control plasma samples for activated-Partial-Thromboplastin-Time (aPTT) measurements. Control plasma test results matched with the manufacturer’s datasheet. Optical fiber based system was tested for aPTT tests with human whole blood samples and the proposed platform provided repeatable test results making the system method of choice for portable diagnostics.

MEMS bimaterial IR sensor array with AC-coupled optical readout

Piezoelectric actuated fiber-scanners have often been employed in optical imaging of tissues, owing to their compact size, low cost, and high resolution that is accompanied by high frame-rates. Typically having a circular cross-section, the dynamics of the scan pattern is determined by the fiber geometry and material properties. Having circular symmetry, a conventional fiber results in coupling between its orthogonal mechanical modes, as the stiffness along both orthogonal directions (x, y) are theoretically identical. Here, we utilize the mechanical asymmetry of polarization-maintaining fibers to break the circular symmetry and thus mitigate the warping effects in the scan pattern that is encountered in conventional fibers. Through simulations and experiments we observe distinct resonance frequencies difference (28 Hz, which is ~6 times the FWHM of the frequency response) for the polarization maintaining fiber, whereas only a few Hz of difference is observed for the conventional fiber resonance frequencies between orthogonal directions that lead to a warped scan pattern. In return, in-resonance scanning of the polarization maintaining fiber produces a clean Lissajous pattern with a wide field of view. The proposed methodology is superior with respect to other studies, as it requires no extra components to be integrated to either the actuator or the fiber itself. Furthermore, it inherently enables polarization dependent imaging modalities without any extra component in the imaging path.

Disposable cartridge biosensor platform for portable diagnostics

3D-printed dynamic structures have arisen as a lower cost and easier to fabricate alternative to miniaturized sensor and actuator technologies. Here, we investigate the reliability of a selected 3D-printed laser scanner, which was initially designed for miniaturized confocal imaging, having 1 x 1 cm2 footprint. The scan-line, 1st resonant frequency and quality factor of 3 devices were monitored for 100,000,000 (hundred million) cycles, and an average deviation of <6% was observed for all three parameters under investigation, for the devices under test. We conclude that 3D printed dynamic structures are promising candidates for a variety of applications, including optomedical imaging applications that demand disposable and low-cost scanning technologies.

Unwarped Lissajous Scanning With Polarization Maintaining Fibers

We propose a novel fiber sensor utilizing a thermomechanical MEMS element at the fiber tip. Owing to its Parylene/Titanium bimaterial structure, the MEMS membrane exhibits an out-of plane displacement with changing temperature. Together with the MEMS element, the embedded diffraction grating forms an in-line interferometer, from which the displacement as well as the temperature can be deduced. The fabricated detector is placed at the single-mode fiber output that is collimated via a graded index lens. This novel architecture allows for integrating MEMS detectors on standard optical fibers, and easy substitution of the MEMS detector element to alter the measurement range and the response time of the sensor.Temperature and time-constant measurements are provided and verified with reference measurements, revealing better than 20 mK temperature sensitivity and 2.5 msec response time, using low-cost laser source and photodetectors.

Reliability of 3D-printed dynamic scanners

We propose a novel fiber sensor utilizing a thermomechanical MEMS element at the fiber tip. Owing to its Parylene/Titanium bimaterial structure, the MEMS membrane exhibits an out-of-plane displacement with changing temperature. Together with the MEMS element, the embedded diffraction grating forms an in-line interferometer, from which the displacement as well as the temperature can be deduced. The fabricated detector is placed at the single-mode fiber output that is collimated via a graded index lens. This novel architecture allows for integrating MEMS detectors on standard optical fibers, and easy substitution of the MEMS detector element to alter the measurement range and the response time of the sensor. Temperature and time-constant measurements are provided and verified with reference measurements, revealing a temperature sensitivity better than 20 mK and 2.5-ms response time, using low-cost laser source and photodetectors.