Joseph J. Talghader

Curvature compensation in micromirrors with high-reflectivity optical coatings

High-reflectivity coatings on micromirrors are critical to reduce reflection losses and absorptive heating. Unfortunately, coating stress induces an unwanted curvature in micromirrors. This effect is much more serious than in bulk optics because the thin coatings are similar in thickness to the structural material of the mirror. This paper describes a method to apply coatings that simultaneously achieve high reflectivity and optical flatness. The design theory is based on one-dimensional (1-D) static analysis and achieves curvature compensation with only a single additional coating layer The technique is appropriate for any number of coating layers and includes both the elastic and plastic behaviors of the thin film layers. Plastic deformation is modeled using an empirically determined strain versus dielectric thickness curve. Experimental measurements of 200 /spl mu/m/spl times/200 /spl mu/m/spl times/3.5 /spl mu/m polysilicon plates show mirror flatness better than /spl lambda//10.

Thermal contact conductance of actuated interfaces

The thermal contact conductance (TCC) of microactuated mechanical interfaces has been characterized using an electronic technique, where micromachined test structures were heated with a current and the TCC was inferred from the change in resistance. For every device tested, the TCC was higher in vacuum than in air. This is in stark contrast to the behavior of bulk interfaces, and several experiments suggest that it may be the result of a decreased solid–solid contact area in air caused by the pressure of the interstitial gas. The average effective TCC of a polysilicon/nitride interface brought together by electrostatic actuation varies about values of 6.0×104 W/(K m2) in air and 9.5×104 W/(K m2) under vacuum for applied pressures of 1 MPa. These values are significantly higher than commonly reported for nonmetallic materials and probably reflect the very smooth surfaces of deposited thin films.

Thermally invariant dielectric coatings for micromirrors

Thermal expansion-induced curvature becomes a major effect in micromirrors as the mirror diameter exceeds 100 microm. Such mirrors are used for optical switching, scanning, and many other applications. By using multilayer coatings instead of a single metal reflector, one can use the mechanical properties of the multilayer to create mirrors with zero curvature across temperature. We demonstrate the fabrication of such thermally invariant mirrors using dielectric coatings. A semianalytic model based on free-plate elastic theory is developed that uses empirical parameters in place of the true thermal expansion coefficients of the coating materials. Micromirrors are demonstrated that maintain their design curvature to within lambda/60 for lambda = 633 nm across an operating range from 21 degrees C to 58 degrees C.

Thermal conductivity and refractive index of hafnia-alumina nanolaminates

Hafnia-alumina nanolaminates show improved smoothness and reduced crystallinity relative to pure hafnia in films formed by atomic layer deposition (ALD). However, typical nanolaminates also show reduced cross-plane thermal conductivity due to the much larger interface density relative to continuous films. We find that the interface thermal resistance in hafnia-alumina nanolaminates is very low and does not dominate the film thermal conductivity, which is 1.0 to 1.2 W/(m K) at room temperature in 100 nm thin films regardless of the interface density. Measured films had a number of interfaces ranging from 2 to 40, equivalent to interface spacing varying from about 40 to 2 nm. The degree of crystallinity of these films appears to have a much larger effect on thermal conductivity than that of interface density. Cryogenic measurements show good agreement with both the minimum thermal conductivity model for disordered solids and the diffuse mismatch model of interface resistance down to about 80 K before diverg...

Measurement of Rapid Temperature Profiles Using Thermoluminescent Microparticles

The thermal history of a material with initially filled trap states may be probed using thermoluminescence. Since luminescent microparticles are composed of robust oxides, they are viable candidates for sensing temperature under conditions where all other types of direct-contact sensors fail. Mg2SiO4: Tb, Co particles with two thermoluminescent peaks have been heated using micromachined heaters over a 232°C to 313°C range on time scales of less than 200 ms. The effect of maximum temperature during excitation on the intensity ratio of the two luminescent peaks has been compared with first-order kinetics theory and shown to match within an average error of 4.4%.

Coupled absorption filters for thermal detectors

A resonant absorption cavity that couples long-wavelength infrared (LWIR) light into a movable plate has been demonstrated for thermal detectors, especially microbolometers. Each device is continuously tunable over 8.7-11.1 microm by using electrostatic actuation with voltages from 0 to 42 V. The width of the resonance is relatively broad, approximately 1.5 microm, to match the large widths of many spectral features in the LWIR. At an actuation voltage of 45 V, the device switches into a broadband mode with an absorption width of 2.83 microm. This latter mode is used to enhance sensitivity in low-light situations in which little spectral information is present.

Design and characterization of adaptive microbolometers

We report the first practical results on design and characterization of adaptive microbolometers with a thermally tuned responsivity. Such devices are needed to simultaneously image scenes that contain objects at ambient and extremely hot temperatures. In the high detectivity state, the microbolometers are operated similar to standard commercial devices. In the low detectivity state, portions of the support beams are brought in contact with the substrate, which thermally shorts the devices on a pixel-by-pixel basis. This is the first time this concept has been practically implemented in a microbolometer device architecture as opposed to simple cantilever beams and plates. The maximum actuation voltage is set to 17 V, and the thermal conductances, responsivities and detectivities of a typical device can be switched more than an order of magnitude between 1.65 × 10−5 W K−1 and 2.99 × 10−4 W K−1, between 1.5 V W−1 and 0.2 V W−1 and between 1.8 × 106 cm Hz1/2 W−1 and 1.5 × 105 cm Hz1/2 W−1, respectively. This extends the dynamic range of the device more than an order of magnitude. The device pixel size is 100 µm ×100 µm.

Current-controlled curvature of coated micromirrors

Precise control of micromirror curvature is critical in many optical microsystems. Micromirrors with current-controlled curvature are demonstrated. The working principle is that resistive heating changes the temperature of the micromirrors and thermal expansion induces a controlled curvature whose magnitude is determined by coating design. For example, for wide focal-length tuning, the radius of curvature of a gold-coated mirror was tuned from 2.5 to 8.2 mm over a current-induced temperature range from 22 degrees to 72 degrees C. For fine focal-length tuning, the radius of curvature of a dielectric-coated (SiO2/Y2O3 lambda/4 pairs) mirror was tuned from -0.68 to -0.64 mm over a current-induced temperature range from 22 to 84 degrees C. These results should be readily extendable to mirror flattening or real-time adaptive shape control.

Control of thermal deformation in dielectric mirrors using mechanical design and atomic layer deposition

A mechanical design technique for optical coatings that simultaneously controls thermal deformation and optical reflectivity is reported. The method requires measurement of the refractive index and thermal stress of single films prior to the design. Atomic layer deposition was used for deposition because of the high repeatability of the film constants. An Al2O3/HfO2 distributed Bragg reflector was deposited with a predicted peak reflectivity of 87.9% at 542.4 nm and predicted edge deformation of -360 nm/K on a 10 cm silicon substrate. The measured peak reflectivity was 85.7% at 541.7 nm with an edge deformation of -346 nm/K.

Adjustable responsivity for thermal infrared detectors

With the recent interest in adaptive IR imaging, focal plane arrays are desired that can operate linearly over an enormous dynamic range. Unfortunately, large signals can cause thermal detectors to operate at temperatures significantly above their ambient resulting in intensity dependent performance or even device damage. In this letter, the responsivity of microbolometer devices is controlled using the detector and substrate as a simple electrostatic actuator. Microbolometers are demonstrated to switch between states that are over a factor of 50 apart in responsivity. The limits of the switching are theoretically separated by four to five orders of magnitude. In addition, intermediate values of responsivity can be obtained by designing devices in which the support beams snap down at lower voltage than the detector plate. Combining this idea with the pressure dependence of the thermal contact conductance, continuous thermal conductance tuning over a factor of 3 is demonstrated.