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28 April 2008 Effect of LED spectral shift on vertical resolution in stroboscopic white light interferometry
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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).
© (2008) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Kalle Hanhijärvi, Juha Aaltonen, Ivan Kassamakov, Kestutis Grigoras, Lauri Sainiemi, Sami Franssila, and Edward Hæggström "Effect of LED spectral shift on vertical resolution in stroboscopic white light interferometry", Proc. SPIE 7003, Optical Sensors 2008, 70031S (28 April 2008);

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