Calibration of diffractive micromirror arrays for microscopy applications

Abstract

We report on our investigation to precisely actuate diffractive micromirror arrays (MMA) with an accuracy of /100. The test samples consist of analog, torsional MEMS arrays with 65 536 (256x256) mirror elements. These light modulators were developed for structured illumination purposes to be applied as programmable mask for life science and semiconductor microscopy application. Main part of the work relies on the well known characterization of MEMS mirrors with profilometry to automatically measure and approximate the MMA actuation state with high resolution. Examples illustrate the potential of this strategy to control the tilt state of many thousand micromirrors within the accuracy range of the characterization tool. In a dynamic range between 0 and >250 nm the MMA deflection has been precisely adjusted for final MMA application in the deep-UV - VIS - NIR spectral range. The optical properties of calibrated MMAs are tested in a laser measurement setup. After MMA calib ration an increased homogeneity and improved image contrast are demonstrated for various illumination patterns

Spatially resolved scatter measurement of diffractive micromirror arrays

“…The experiment is based upon an existing characterization setup for diffractive MMAs (Fig. 5) [20][21][22]. Five laser sources provide different illumination wavelengths from the deep ultraviolet to the near infrared (NIR): (i) a KrF excimer laser "MLI-1000LC" by MLase AG at 248 nm (10 W average power, 1 kHz repetition rate, 10 ns pulse width, 3 mm × 6 mm beam exit); (ii) three customer-specific laser diode modules "51nano" by Schäfter Kirchhoff GmbH at 405, 680, and 830 nm and all modules have 1 mW maximum power, temperature and power stabilized, single-mode fiber exit, and collimator with Gaussian beam profile, diameter 2 mm.…”

“…After extracting the individual mirror deflections with a pattern matching algorithm, the relation between address voltage and deflection for each mirror in the array is described individually by a polynomial function. By utilizing these characteristic curves the deflections can be set with an accuracy of a few nanometers [20,21], which has been found to, however, still provide good potential for contrast optimization.…”

“…From an SLM developer's perspective, two strategic directions are pursued to efficiently achieve a high phase accuracy. On the one hand, the technology is continuously improved by process innovations [17][18][19] and, on the other hand, precise characterization techniques are developed to better understand and optimize the individual device performance through calibration [20,21]. Because the surface topography is the most important factor for the optical performance of an MMA, optical profiling techniques such as white-light interferometric (WLI) microscopy are classical candidates for a variety of MMA inspection, characterization, and optimization tasks.…”

“…Because the surface topography is the most important factor for the optical performance of an MMA, optical profiling techniques such as white-light interferometric (WLI) microscopy are classical candidates for a variety of MMA inspection, characterization, and optimization tasks. For example, a WLI calibration routine provides precise control of the micromirror deflections with an uncertainty of only a few nanometers corresponding to about λ∕100 for visible light [20]. The ultimate goal of all these characterizations and calibration efforts is to deliver the SLM with the best possible phase accuracy to support an out-of-the-box utilization with minimal or no application-specific calibration.…”

Spatially resolved contrast measurement of diffractive micromirror arrays

“…Secondly, as complementary approach, the careful qualification and individual optimization of each single MEMS device may support highest optical performance. For example, a profilometric calibration of each micromirror's deflection characteristics is routinely performed to achieve a deflection accuracy as low as a few nanometers [8]. We study a novel characterization approach for diffractive MMAs that relies on intensity-based measures rather than interferometric principles.…”