State-of-the-art high performance IR sensing and imaging systems utilize highly expensive photodetector technology, which requires exotic and toxic materials and cooling. Cost-effective alternatives, uncooled bolometer detectors, are widely used in commercial long-wave IR (LWIR) systems. Compared to the cooled detectors they are much slower and have approximately an order of magnitude lower detectivity in the LWIR. We present uncooled bolometer technology which is foreseen to be capable of narrowing the gap between the cooled and uncooled technologies. The proposed technology is based on ultra-thin silicon membranes, the thermal conductivity and electrical properties of which can be controlled by membrane thickness and doping, respectively. The thermal signal is transduced into electric voltage using thermocouple consisting of highly-doped n and p type Si beams. Reducing the thickness of the Si membrane improves the performance (i.e. sensitivity and speed) as thermal conductivity and thermal mass of Si membrane decreases with decreasing thickness. Based on experimental data we estimate the performance of these uncooled thermoelectric bolometers.
We have fabricated a micro-supercapacitor with porous silicon electrodes coated with TiN by atomic layer deposition technique. The coating provides an efficient surface passivation and high electrical conductivity of the electrodes, resulting in stable and almost ideal electrochemical double layer capacitor behavior with characteristics comparable to the best carbon based micro-supercapacitors. Stability of the supercapacitor is verified by performing 50 000 voltammetry cycles with high capacitance retention obtained. Silicon microfabrication techniques facilitate integration of both supercapacitor electrodes inside the silicon substrate and, in this work, such in-chip supercapacitor is demonstrated. This approach allows realization of very high capacitance per foot print area. The in-chip micro-supercapacitor can be integrated with energy harvesting elements and can be used in wearable and implantable microdevices.
Microfluidic devices allow experimentation in smaller space using small amounts of liquid, resulting in improved reaction rates, cheaper equipment, reduced amount of expensive reagents. Very precise channel shape measurements are needed to assure the designed flow pattern. Several 3D imaging devices provide the necessary precision but typically they cannot image inside closed devices. Hence it is difficult to measure the shape of a microfluidic channel without destroying it. We fabricated and investigated samples with different microchannels. Several types of microfluidic channels were prepared in silicon wafer with a subsequent covering by bonding glass wafer on top. Microchannels in polymer have been done using epoxy-type photoresist SU-8. The internal geometry of the channels was measured using a Scanning White Light Interferometer (SWLI) equipped with optics that compensates for the effects of the top glass of the channels. The geometry of the interior of the channels can be measured with a precision similar to surface layer SWLI measurements without destroying the channels.
Scanning White Light Interferometry (SWLI) provides high vertical precision for measuring step-like structures in
microelectromechanical systems (MEMS). The SWLI performance depends on its light source. A rapidly modulated
light source with a broad bandwidth inside the infrared (IR) region is necessary to measure layered MEMS that move.
Typical SWLI light sources - light emitting diodes (LEDs) and Halogen (HG) bulbs - fulfill only one of these
To overcome this shortcoming we equipped our SWLI setup with a supercontinuum (SC) light source produced by
Fiberware Gmbh (Ilum 100 USB II). We tested our setup by measuring in plane and out of plane oscillating thermal
bridges with visible light, as well as top and bottom surfaces of silicon structures using IR light. The wide SC spectrum
creates localized interferograms. This allowed us to measure top and bottom surfaces of a thin (4 μm) bridge. The
stroboscopically measured profiles of oscillating thermal bridges were comparable to those measured using a white LED.
The results of static measurements were similar to those achieved with an HG lamp.
We apply a hybrid light source with adjustable spectrum to Scanning White Light Interferometric MEMS device
characterization. The source combines light from a blue laser (409 nm), a fluorescent material (emission peak 521 nm),
amber LED (597 nm) and cyan LED (505 nm) to cover the visible wavelengths. The Gaussian spectrum of the light
source reduces interference ringing and improves surface localization, which is important when imaging diffuse surfaces
or layered structures. The new light source allows both stroboscopic illumination and spectrum shaping during a
measurement. Changing the illumination spectrum allows one to maximize the reflection from the measured surface -
compared to reflections from other surfaces - as a mean to improve signal-to-noise-ratio.
To validate the source we measured static MEMS samples featuring known step heights using the light source at three
different mean wavelengths (508 nm, 524 nm and 579 nm). The measured step heights (7.029 ± 0.045 μm,
7.002 ± 0.041 μm and 7.005 ± 0.056 μm) were close to those measured using a halogen lamp (7.025 ± 0.020 μm).
Interferograms without the side lobes typical for white LEDs were achieved. The FWHM of the interferogram of the
combined light source was (1.859 ± 0.008 μm).
Scanning White Light Interferometry (SWLI) allows surface characterization of MEMS components. With transparent
samples SWLI can image multiple stacked layers. However, since silicon is opaque to visible wavelengths, only the top
layer can be measured using visible light. We combined multiple infrared light emitting diodes (IR-LEDs) to achieve
adjustable IR illumination. This allows simultaneous measurement of top and bottom surface topographies of silicon
samples - such as MEMS membranes- using a SWLI equipped with an IR camera. This advances the state of the art of
the field of MEMS characterization by allowing looking under membranes of these devices during operation.
Scanning white light interferometry (SWLI) allows dynamic full-field 3D profiling of MEMS devices. With stroboscopic
illumination periodic out-of-plane oscillation can be characterized, but in-plane movement is unresolved. We combine
stroboscopic SWLI with image processing to concurrently characterize periodic out-of-plane and in-plane displacement.
A difference in frequency is induced between the sample excitation and stroboscopic illumination signals. The difference
frequency is chosen to allow recording the surface movement at video rate. The stroboscopic image is thus no longer
frozen in time, but moves at frequency equal to the difference in stroboscopic frequencies. This motion is captured with a
CCD camera. The surface velocity is extracted from the apparent motion using optical flow algorithms. For concept
validation we characterize the in-plane and out-of-plane movement of thermal microbridges fabricated on silicon-oninsulator
by deep reactive ion etching. The microbridge geometry was designed for in-plane movement with minor outof
Stroboscopic scanning white light interferometry is a method for dynamic nanometer range profilometry that is widely
applied for quality control in the MEMS industry. Monochromatic and phosphor coated (PC) white LEDs produce short
light pulses for stroboscopy. The time resolution of a stroboscopic setup depends on its capability to produce short light
pulses with duty cycles less than 5%. The peak wavelength and the spectral shape of PC white light diodes change with
duty cycle. The spectrum of a PC white light LED was measured using Czerny-Turner-type monochromator (Jobin Yvon
H 25) with an optical power meter (Ando AQ-1125). A custom made pulse amplifier drove the LED with a square wave
voltage at 120 Hz. The blue peak wavelength of the white diode was blue-shifted by 7 nm when the duty cycle was
reduced from 10% to 0.5%. The impact of the spectral change on the vertical resolution of the stroboscopic measurement
was characterized through simulating the change in measurement uncertainty. The results were applied to characterize
out-of-plane vibration of thermal MEMS bridges manufactured from SOI wafers. The simulated increase in
measurement uncertainty was 1 nm, when the spectrum shifted 10 nm towards blue. Noise from background vibration
obscured the effect of spectral shift. Although literature says that temperature increase shifts the spectrum of LED, and
although our simulations indicate the existence of such a shift, our experimental results indicate that the deletory effect is
negligible (it does not introduce bias or uncertainty to profiling measurement).
The investigations of photo luminescence spectra of the electrochemically produced porous GaAs layers, excited by continuous Ar laser radiation, were carried out. The chemical composition of the anodized p- and n-GaAs was analyzed by x-ray photoelectron spectroscopy. The GaAs surface morphology was examined by high-resolution transmission-electron microscopy and surface structure was investigated by electronograph EMR100 and Atomic Force Microscopy. It is established that increasing a duration and current density of etching changes the porosity of bulk GaAs and both Galium and arsenic oxides are formed on the sample surface Photo luminescence spectra of investigated porous surface consist of 'IR' and 'green' spectral structures. The 'IR' structure exhibits redshifts of its peak energies, and 'green' structure intensity is dependent on etching conditions. A possible reason of origin and changes in those spectra is discussed.
Possible and already realized applications of porous silicon (PS) layers in silicon solar cell technology are overviewed. Four main directions are marked for PS incorporation: (1) heterojunction formation, (2) light trapping, (3) antireflection coating, and (4) surface passivation. Standard PS preparation technique is discussed and a new one is proposed, applicable for large area devices. Optical and electric parameters of PS layers, obtained at different electrochemical etching conditions, are presented. Experimental results of PS action are compared with calculations for light trapping and antireflection coating effects. Detailed study of possible passivation effect is given both for mono- and multicrystalline silicon solar cells. The obtained results show good perspectives for PS application in cheap and efficient silicon solar cell production.