Kalle Hanhijärvi

Scanning white-light interferometry with a supercontinuum source

A supercontinuum light source was incorporated into a custom-built scanning white-light interferometer. This light source based on a Nd:YAG pumped microstructured optical fiber exhibits 1.21+/-0.10 microm temporal coherence length. The device operation was validated by characterizing the step height on a microelectromechanical system component. The measured step height- 7.027+/-0.020 microm-agreed with results obtained by employing traditional light sources: a halogen lamp and a white light-emitting diode. The new light source features high output intensity of 20-35 mW, which is beneficial when measuring low-reflectivity samples. As the supercontinuum light source may be modulated at frequencies exceeding 10 MHz, it holds potential for stroboscopic dynamic measurements.

Stroboscopic supercontinuum white-light interferometer for MEMS characterization

We used a supercontinuum-based scanning white-light interferometer to characterize the oscillation of a MEMS device. The output of a commercially available supercontinuum light source (FiberWare Ilum II USB) was modulated to achieve stroboscopic operation. By synchronizing the modulation frequency of the source to the sample oscillation, dynamic 3-D profile measurements were recorded. These results were validated against those obtained with a white light LED setup. The measured maximum deflection of a 400×25×4 μm(3) microbridge driven with 0-6.8 V sinusoidal voltage at 10 Hz was 1.42±0.03 μm (supercontinuum), which agreed with the LED measurement. The method shows promise for characterization of high-frequency MEMS devices.

Effect of LED spectral shift on vertical resolution in stroboscopic white light interferometry

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).

Hybrid light source for scanning white light interferometry-based MEMS quality control

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).

Scratch resistance of plasticized hydroxypropyl methylcellulose (HPMC) films intended for tablet coatings

Scratch resistance (SR) of externally plasticized hydroxypropyl methylcellulose (HPMC) films intended for tablet film coatings was studied. Special attention was paid to effects of short-term aging and ultraviolet (UV) treatment on the SR properties of these films. Controlled scratching of the films was performed with a Lloyd LRX materials tester featuring a spherical steel tip. Scratch surface profiles were measured by scanning white light interferometry (SWLI). The influence of using an external plasticizer on the SR was studied by comparing scratch dimensions in non-plasticized films to samples plasticized either with glycerol or polyethylene glycol (PEG) 400. The study demonstrates that both the amount and type of plasticizer influences the SR of aqueous HPMC films. It also shows that SWLI can quantitatively evaluate the effect of plasticizer content and aging on the SR of pharmaceutical films. This knowledge could be used to optimize pharmaceutical film coating formulations.

Nondestructive Inspection of Buried Channels and Cavities in Silicon

Microelectromechanical systems and microfluidic devices feature buried channels, cavities, and other embedded microstructures. These features are usually examined by breaking the wafer and by imaging the revealed cross section. We propose a nondestructive technique to evaluate the shape and surface quality of buried microchannels with submicrometer resolution. The technique relies on infrared light interferometry. We employed the technique to nondestructively examine channels and cavities through a silicon roof. With the proposed technique, we can quantitatively examine the size and shape of microchannels that are hidden to visible light.

Through-Silicon Stroboscopic Characterization of an Oscillating MEMS Thermal Actuator Using Supercontinuum Interferometry

We measured the surface profile of the hidden face of a thermally actuated oscillating 4-μm-thick silicon microelectromechanical system (MEMS) bridge. To do this, we employed a stroboscopically synchronized supercontinuum light source incorporated into a scanning low-coherence interferometer. The instrument exploited the near-infrared part (1.1-1.7 μm) of the emitted spectrum and a camera sensitive to near infrared. The MEMS bridge was driven with 6.8-V sinusoidal voltage at 10 Hz, which resulted in oscillation amplitudes of 1.50 ± 0.07 μm and 1.35 ± 0.07 μm for the top and bottom surfaces, respectively. We believe this technique opens up new possibilities for validating simulation effort as well as for qualifying new device designs.

Nondestructive static and dynamic MEMS characterization using supercontinuum scanning white light interferometry

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 requirements. 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.

Stroboscopic White Light Interferometry for Dynamic Characterization of Capacitive Pressure Sensors

Scanning white light interferometry (SWLI) is a well‐established method for accurate static out‐of‐plane 3‐D profiling of micromechanical devices. Periodic displacement can be measured using stroboscopic illumination synchronized to an arbitrary phase angle of the sample oscillation. We modified an existing SWLI setup for dynamic MEMS characterization. A two‐channel function generator drives the sample and the stroboscopic illumination, and controls the inter‐channel phase. A phosphorous white light LED or single‐color LED is used as light source. Currently our shortest stroboscopic pulse is 50 ns. We measured the out‐of‐plane displacement of a thermal microbridge fabricated on SOI wafer with 20 nm accuracy. The microbridge was driven with sinusoidal and square wave signals (1 Hz ‐ 960 Hz). The stroboscopic duty cycles were 0.1% ‐ 5%. We also characterize the vibration modes of the membrane of a capacitive pressure sensor. Stroboscopic SWLI is useful for nanoscale profile measurements of periodic oscillat...

Single Molecule Studies on the RNA Polymerase QDE-1 by Optical Tweezers

Cellular regulatory mechanisms which rely on small dsRNA molecules (RNA silencing) are major (new) discoveries in biology.In many eukaryotic organisms silencing is achieved post-transcriptionally through pathways where dsRNA is synthesized by RNA-dependent RNA polymerases (RdRPs) and processed into 21-25 nucleotide long RNA molecules.Here we study QDE-1, an RdRP involved in the quelling (RNA silencing) pathway of Neurospora crassa. This filamentous fungus displays noticeable genomic stability, which has been attributed to quelling and other silencing mechanisms. Recently it was shown that QDE-1 is more active as a DNA-dependent RNA polymerase (DdRP) than as an RdRp and that it is also involved in DNA damage response (1,2). Recombinant QDE-1 displays five different enzymatic activities in vitro (3). The structural and biochemical data on this enzyme is extensive (1-5).We are interested on the basic biophysical parameters of QDE-1 action. The approaches taken rely on single molecule assays using high resolution optical tweezers, combined with fluorescence imaging. An optically levitated ‘dumbbell’ assay is used: the nucleic acid (NA) construct features one or two biotinylated ends that tether two different microspheres. The protein of interest is attached to the microsphere or directly to the NA tether. Since in our double-trap instrument one trap is stable whereas the other mobile, we can manipulate the tethers, detect changes in tether length and stiffness, and apply different forces and simultaneously observe the mobility of the fluorescently labelled template or protein.1. Lee, H-C et al. (2009). Nature 459(7244), 274-277.2. Lee, H-C et al. (2010). PLoS Biol. 8 (10), e1000469.3. Aalto et al. (2010). The Journal of Biological Chemistry, 285, 29367-29374.4. Salgado et al. (2006). PLoS. Biol. 4 (12), e434.5. Makeyev E.V. & Bamford D.H. (2002). Mol Cell 10(6), 1417-1427.