Tor Paulin

Stroboscopic scanning white light interferometry at 2.7 MHz with 1.6 µm coherence length using a non-phosphor LED source.

Stroboscopic scanning white light interferometry (SSWLI) allows precise three dimensional (3D) measurements of oscillating samples. Commercial SSWLI devices feature limited pulsing frequency. To address this issue we built a 400-620 nm wideband 150 mW light source whose 1.6 µm wide interferogram is without side peaks. The source combines a non-phosphor white LED with a cyan LED. We measured a calibration artifact with 10 nm precision and obtained 40 nm precision when measuring the 3D profile of a capacitive micromachined ultrasonic transducer membrane operating at 2.72 MHz. This source is compatible with solid state technology.

Quasidynamic calibration of stroboscopic scanning white light interferometer with a transfer standard

Abstract. A stroboscopic scanning white light interferometer (SSWLI) can characterize both static features and motion in micro(nano)electromechanical system devices. SSWLI measurement results should be linked to the meter definition to be comparable and unambiguous. This traceability is achieved by careful error characterization and calibration of the interferometer. The main challenge in vertical scale calibration is to have a reference device with reproducible out-of-plane movement. A piezo-scanned flexure guided stage with capacitive sensor feedback was attached to a mirror and an Invar steel holder with a reference plane—forming a transfer standard that was calibrated by laser interferometry with 2.3 nm uncertainty. The moving mirror vertical position was then measured with the SSWLI, relative to the reference plane, between successive mirror position steppings. A light-emitting diode pulsed at 100 Hz with 0.5% duty cycle synchronized to the CCD camera and a halogen light source were used. Inside the scanned 14 μm range, the measured SSWLI scale amplification coefficient error was 0.12% with 4.5 nm repeatability of the steps. For SWLI measurements using a halogen lamp, the corresponding results were 0.05% and 6.7 nm. The presented methodology should permit accurate traceable calibration of the vertical scale of any SWLI.

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).

Non-phosphor white LED light source for interferometry

Solid state light sources are replacing a tungsten filament based bulbs in Scanning White Light Interferometers. White LEDs generate little heat, feature short switching times, and have long lifetimes. Phosphor-based white LEDs produce a wide spectrum but have two separate peaks which cause interferogram ringing. This makes measuring multi layered structures difficult and may degrade measurement precision even when measuring a single reflecting surface. Most non phosphor white LEDs exhibit a non Gaussian spectrum, but multi-LED based white LEDs can achieve switching times and stability similar to those of single color LEDs. By combining several LEDs and by controlling their input current independently it is possible to create almost an arbitrary spectrum. We designed a new light source by combining a non phosphor white LED (American Opto Plus LED, L-513NPWC- 15D) and single color LEDs. This allowed us to fill the spectral gap between the blue and yellow peaks of the non phosphor white LED. By controlling the input current of the LEDs individually a nearly Gaussian shaped spectrum was achieved. This wide continuous spectrum creates short interferograms (FWHM ~1.4 μm) without side peaks. To demonstrate the properties of this source we measured through a 5 μm thick polymer film. The well localized interference created by the source allows measuring both surfaces of thin films simultaneously. We were able to pulse the source at 5.4 MHz.

Static and (quasi)dynamic calibration of stroboscopic scanning white light interferometer

A scanning white light interferometer can characterize out of plane features and motion in M(N)EMS devices. Like any other form and displacement measuring instrument, the scanning interferometer results should be linked to the metre definition to be comparable and unambiguous. Traceability is built up by careful error characterization and calibration of the interferometer. The main challenge in this calibration is to have a reference device producing accurate and reproducible dynamic out-of-plane displacement when submitted to standard loads. We use a flat mirror attached to a piezoelectric transducer for static and (quasi)dynamic calibration of a stroboscopic scanning light interferometer. First we calibrated the piezo-scanned flexure guided transducer stage using a symmetric differential heterodyne laser interferometer developed at the Centre for Metrology and Accreditation (MIKES). The standard uncertainty of the piezo stage motion calibration was 3.0 nm. Then we used the piezo-stage as a transfer standard to calibrate our stroboscopic interferometer whose light source was pulsed at 200 Hz and 400 Hz with 0.5% duty cycle. We measured the static position and (quasi)dynamic motion of the attached mirror relative to a reference surface. This methodology permits calibrating the vertical scale of the stroboscopic scanning white light interferometer.

Sub-kHz traceable characterization of stroboscopic scanning white light interferometer

Scanning white light interferometry (SWLI) is an established methodology for non-destructive testing of MEMS/NEMS. In contrast to monochromatic interference microcopy SWLI can unambiguously resolve surfaces featuring tall vertical steps. Oscillating samples can be imaged using a stroboscopic SWLI (SSWLI) equipped with a pulsed light source. To measure static samples the lateral and vertical scales of the SSWLI can be calibrated using transfer standards with calibrated dimensions such as line scales, 2D gratings, gauge blocks, and step height standards. However, traceable dynamic characterization of SSWLI requires a transfer standard (TS) providing repeatable traceable periodic movement. A TS based on a piezo-scanned flexure guided stage with capacitive feedback was designed and manufactured. The trajectories of the stage motion for different amplitude and frequency settings were characterized to have ~2 nm standard uncertainty. Characterization was made using a symmetric differential heterodyne laser interferometer (SDHLI). The TS was first used to characterize quasidynamic measurements across the vertical range of the SSWLI, 100 μm. Dynamic measurement properties of the SSWLI were then characterized using a sinusoidal vertical trajectory with 2 μm nominal amplitude and 50 Hz frequency. The motion amplitude of the TS, 2038 nm, measured with the SSWLI was 6 nm smaller than the amplitude measured with SDHLI. The repeatability of SSWLI expressed as experimental standard deviation of the mean was 8.8 nm. The maximum deviation in instantaneous displacement and oscillation velocity were 49 nm and 27 μm/s, respectively. A traceable method to characterize the capacity of the SSWLI to perform dynamic measurements at sub-kHz frequencies was demonstrated.

LED driver for stroboscopic interferometry

Three different types of white light emitting diodes (LEDs) and three types of single color LEDs were tested as light sources for stroboscopic scanning white light interferometry (SSWLI) for dynamic (MEMS) characterization. Short, intense, light pulses and low duty cycle (< 10%) are required to freeze the motion of an oscillating sample. A custom designed LED driver was built utilizing a Metal-Core Printed Circuit Board. At the core of the circuit is a CMOS high speed high current gate driver (IXDD415SI). The LED pulser is compact (50×110 mm2), has good thermal resistivity (0.45 °C/W), wide bandwidth (~DC-10 MHz), and can drive single LEDs at 5A peak current (0.7% duty cycle at 1 MHz). The shortest measured electrical pulses were 6.2 ± 0.1 ns FDHM. The minimum measured Full Duration at Half Maximum (FDHM) of the optical pulse was 8.4 ± 0.1 ns using nonphosphor white LED and 32.1 ± 0.1 ns using white phosphor-converted LED (0.7 % duty cycle at 1 MHz in both cases). The minimum optical pulse FDHM for a single color blue/green LED was 6.4 ± 0.1 ns. The maximum intensity of these pulses was 630 ± 40 μW and 540 ± 30 μW, respectively. All types of white LEDs could be used for stroboscopic SWLI measurements at frequencies up to 2 MHz. For higher frequencies, non-phosphor white LEDs must be used together with a cyan LED to avoid ringing in the SWLI interferogram.

Ultrasound-enhanced electrospinning

Electrospinning is commonly used to produce polymeric nanofibers. Potential applications for such fibers include novel drug delivery systems, tissue engineering scaffolds, and filters. Electrospinning, however, has shortcomings such as needle clogging and limited ability to control the fiber-properties in a non-chemical manner. This study reports on an orifice-less technique that employs high-intensity focused ultrasound, i.e. ultrasound-enhanced electrospinning. Ultrasound bursts were used to generate a liquid protrusion with a Taylor cone from the surface of a polymer solution of polyethylene oxide. When the polymer was charged with a high negative voltage, nanofibers jetted off from the tip of the protrusion landed on an electrically grounded target held at a constant distance from the tip. Controlling the ultrasound characteristics permitted physical modification of the nanofiber topography at will without using supplemental chemical intervention. Possible applications of tailor-made fibers generated by ultrasound-enhanced electrospinning include pharmaceutical controlled-release applications and biomedical scaffolds with spatial gradients in fiber thickness and mechanical properties.

Delivery of Agents Into Articular Cartilage With Electric Spark-Induced Sound Waves

Localized delivery of drugs into articular cartilage (AC) may facilitate the development of novel therapies to treat osteoarthritis (OA). We investigated the potential of spark-gap-generated sound to deliver a drug surrogate, i.e. methylene blue (MB), into AC. In-vitro experiments exposed bovine AC samples to either simultaneous sonication and immersion in MB (Treatment 1; n = 10), immersion in MB after sonication (Control 1; n = 10), solely immersion in MB (Control 2; n = 10), or neither sonication nor immersion in MB (Control 3; n = 10). The sonication protocol consisted of 1000 spark-gap -generated pulses. Delivery of MB into AC was estimated from optical absorbance in transmission light microscopy. Optical absorbance was significantly greater in the treatment group up to 900 µm depth from AC surface as compared to all controls. Field emission scanning electron microscopy (FESEM), histological analysis, and digital densitometry (DD) of sonicated (n = 6) and non-sonicated (n = 6) samples showed no evidence of sonication-induced changes in AC. Consequently, spark-gap -generated sound may offer a solution for localized drug delivery into AC in a non-destructive fashion. Further research on this method may contribute to OA drug therapies.

Broadband phosphor conversion LED source for stroboscopic white light interferometry

We report on building a broadband LED light source for stroboscopic white light interferometry. We chose phosphor types, mass ratios, and encapsulant, to tailor the necessary emission spectrum. Based on known emission spectra, we mixed combinations of blue, cyan, yellow, and red down-conversion phosphors. The phosphor composite was excited with a modified UV LED (365 nm). UV provides primary excitation of blue phosphor BAM (BaMgAl10O17:Eu). The emission (≈ 450 nm) of the blue phosphor provides secondary excitation of longer wavelength phosphors (YAG (yttrium aluminum garnite), strontium-barium silicate, and sulfoselenide). The effective spectrums FWHM was 244±1.5 nm; spectral drop was 14%. The pulse width was 2.2 μs when the LED was driven with 14 A. We used the source for static MEMS measurements in a SWLI system. The obtained SWLI interferogram features 883 nm FWHM and low side lobes.