Thermomechanical noise for a MEMs-based infrared detector using null switching (US patent 7977635) depends on vibrational amplitude, since IR radiation is transduced to a change in the duty cycle of a repetitively closing switch. Equipartition theorem gives a maximum rms vibrational amplitude of 45 pm for the fabricated cantilever switch at its natural frequency. This gives a worst case timing uncertainty of 700 ns and an NEP of 2 pW/Sqrt[Hz].
A MEMS cantilever IR detector that repetitively lifts from the surface under the influence of a saw-tooth electrostatic force, where the contact duty cycle is a measure of the absorbed IR radiation, is analyzed. The design is comprised of three parallel conducting plates. Fixed buried and surface plates are held at opposite potential. A moveable cantilever is biased the same as the surface plate. Calculations based on energy methods with position-dependent capacity and electrostatic induction coefficients demonstrate the upward sign of the force on the cantilever and determine the force magnitude. 2D finite element method calculations of the local fields confirm the sign of the force and determine its distribution across the cantilever. The upward force is maximized when the surface plate is slightly larger than the other two. The electrostatic repulsion is compared with Casimir sticking force to determine the maximum useful contact area. MEMS devices were fabricated and the vertical displacement of the cantilever was observed in a number of experiments. The approach may be applied also to MEMS actuators and micromirrors.
We present performance calculations for a MEMS cantilever device for sensing heat input from convection or radiation. The cantilever deflects upwards under an electrostatic repulsive force from an applied periodic saw-tooth bias voltage, and returns to a null position as the bias decreases. Heat absorbed during the cycle causes the cantilever to deflect downwards, thus decreasing the time to return to the null position. In these calculations, the total deflection with respect to absorbed heat is determined and is described as a function of time. We present estimates of responsivity and noise.
Patterning of gold-black infrared absorbing films by stencil lithography and hardening by polymer infusion is reported. Gold black nano-structured films are deposited through a thin metal shadow mask in a thermal evaporator in ~400 mTorr pressure of inert gas, followed by ethyl cyanoacrylate fuming through the same mask to produce rugged IR absorptive patterns of ~100 micron scale dimensions. Infrared absorptivity is determined by transmission and reflectivity measurements using a Fourier spectrometer and infrared microscope. Results indicate that the optimized hardening process reduces the usual degradation of the absorptivity with age. This work has potential application to infrared array bolometers.
Removal of polyimides used as sacrificial layer in fabricating MEMS devices can be challenging after hardbaking, which may easily result by the end of multiple-step processing. We consider the specific commercial co-developable polyimide ProLift 100 (Brewer Science). Excessive heat hardens this material, so that during wet release in TMAH based solvents, intact sheets break free from the substrate, move around in the solution, and break delicate structures. On the other hand, dry reactive-ion etching of hard-baked ProLift is so slow, that MEMS structures are damaged from undesirably-prolonged physical bombardment by plasma ions. We found that blanket exposure to ultraviolet light allows rapid dry etch of the ProLift surrounding the desired structures without damaging them. Subsequent removal of ProLift from under the devices can then be safely performed using wet or dry etch. We demonstrate the approach on PECVD-grown silicon-oxide cantilevers of 100 micron × 100 micron area supported 2 microns above the substrate by ~100-micron-long 8-micron-wide oxide arms.
The convergence of silicon photonics and infrared plasmonics allows compact, chip-scale spectral sensors. We report on
the development of a compact mid-IR spectrometer based on a broad-band IR source, dielectric waveguides, a
transformer to convert between waveguide modes and surface plasmon polaritons (SPP), an interaction region where
analyte molecules are interrogated by SPPs, an array of ring resonators to disperse the light into spectral components,
and photodetectors. The mid-IR light source emits into a dielectric waveguide, leading to a region that allows coupling
of the incident photons into SPPs. The SPPs propagate along a functionalized metal surface within an interaction region.
Interactions between the propagating SPP and any analytes bound to the surface increase loss at those wavelengths that
correspond to the analyte vibrational modes. After a suitable propagation length the SPP will be coupled back into a
dielectric waveguide, where specific wavelength components will be out-coupled to detectors by an array of ring
resonators. We have selected a 3.4 micron LED as the IR source, based on both cost and performance. Initial
experiments with circular waveguides formed from GLSO glass include measurement of the loss per mm.
Electrodynamic simulations have been performed to inform the eventual Si taper design of the proposed
photonic/plasmonic transformer. The SPP propagation length necessary for a discernible change in the signal due to
absorption in the interaction region has been estimated to be on the order of 1 mm, well within the bounds of calculated
propagation lengths for SPPs on Au.