We report on an array of atomic force microscopes (AFM) based on a simple optical set-up using heterodyne detection. The deflection of AFM cantilevers is given by the path differences between the reference and the measuring wave in a Michelson interferometer. A matrix of micro-lenses is placed just above the cantilevers, in such a way that the deflected light from each cantilever is collected by one micro-lens. Both the micro-lenses and the cantilever chips are previously glued to increase the robustness of the system. The interference between the light from each micro-lenses and the
reference light is selected by a diaphragm and subsequently detected by a photodetector. This procedure is repeated for each cantilever. In order to validate our instrument we measure the profile of a binary grating having a step height of 19.66 nm. By a piezoelectric platform a lateral range of 10 μm was scanned with a speed of 1 μm/s and an integration time of 10 ms, which leads to a lateral resolution of 10 nm. The profiles measured by the cantilevers are in good agreement with the profile of the sample grating.
White light scanning interference microscopy, with its high axial resolution, is particularly useful for the rapid 3D characterization of MOEMS micro-systems. Although this technique can be used for submicron critical dimension measurement on micron high microelectronic structures, recent tests using a standard system have revealed errors of up to 3 μm in the measurement of lateral position of deep square steps. Thus the 2 μm wide, 75 μm deep teeth of an electrostatic comb structure in a FT MOEMS spectrometer were measured to be nearly 7 μm wide using a Mirau interference objective with the aperture diaphragm of the illumination system fully open in white light. Tests under different conditions show that the error is greatest for the Mirau objective, with the aperture diaphragm fully open at longer wavelengths. In addition, the location of the centre of such structures can vary by up to 1 μm depending on the degree of reference mirror tilt. Investigations of the XZ images of square steps have revealed the presence of "ghost" fringes resulting from diffraction and the conical illumination used. The errors in edge position can be reduced using a Linnik type objective with the aperture diaphragm closed down using shorter wavelength light.
We present a lamellar grating interferometer realized with MEMS technology. It is used as time-scanning Fourier transform spectrometer. The motion is carried out by an electrostatic comb drive actuator fabricated by silicon micromachining, particularly by silicon-on-insulator technology. We have measured the spectrum of an extended white light source with a resolution of 1.2 nm at a wavelength of 436 nm, and of 13 nm at 1544 nm. The wavelength accuracy is better than 0.5 nm and the inspected wavelength range extends from 380 nm to 1700 nm. The optical path difference maximum is 226 μm and is limited by the mechanical instability of the actuator. The dimension of the device is 7 mm x 8 mm x 0.5 mm. The device includes two individual lamellar grating spectrometers operated by the same actuator, allowing the immediate calibration of the optical path difference.
We present several devices using different spectroscopic concepts. First, we show the successive steps and improvements in connection with the Michelson interferometer which we have already realized, in particular the use of fibers to bring in and collimate the light. A possible method to obtain micro-optical elements that are suitable for integration on the interferometer chip is proposed. Then, we present a lamellar grating interferometer, an array of commutable slits to realize a Hadamard transformer, and a movable curved diffraction grating. All of these devices have been realized by silicon micro-machining, more particularly with silicon-on-insulator (SOI) technology.
We present a miniaturized Fourier transform (FT) spectrometer based on silicon micromachined. The FTS is a Michelson interferometer with one scanning mirror. The motion of the mirror is carried out by a n electrostatic comb drive actuator. The mirror displacement is 39 micrometers and its reproducibility is +/- 13 nm, which leads to a resolution better than 10 nm in the visible wavelength range. A new design of this chip has been realized in order to integrate an input fiber, a collimating lens system as well as a beam splitting plate. This new design allows to undertake spectroscopy with white light. The limitation of light collimation and the effect of the size of the source have been studied by numerical simulations.
Optical MEMS is a challenging new field that combines micro- optics with micro-mechanics in order to build compact systems. In this paper we present a miniaturized Fourier transform spectrometer (FTS) fabricated on silicon. The FTS is a Michelson interferometer with one scanning mirror. The motion of the mirror is carried out by a new type of electrostatic comb drive actuator. The mirror is designed to be linear with respect to the applied voltage. Experimentally, we have measured a mirror displacement of 38.5 micrometer corresponding to a maximum optical path difference of 77 micrometer. The applied voltage was plus or minus 10 V and the non-linearity of the driving system is plus or minus 0.25 micrometer. A method is presented to correct the spectrum in order to get rid of the non-linearity. The measured resolution of the spectrometer after the phase correction is 16 nm at a wavelength of 633 nm.
In this paper we present a miniaturized Fourier Transform spectrometer based on a Michelson interferometer with a scanning mirror. The motion of the mirror is carried out by a new type of electrostatic comb drive actuator. The displacement of the mirror is linear with the applied voltage. Experimental results are presented.