MEMS Cantilevers in Photonics

Detecting low energy photons, such as photons in the long-wave infrared range, is a technically challenging proposition using naturally occurring materials. In order to address this challenge, we herein demonstrate a micro-bolometer featuring an integrated metamaterial absorber (MA), which takes advantage of the resonant absorption and frequency selective properties of the MA. Importantly, our micro-bolometer exhibits polarization insensitivity and high absorption due to a novel metal-insulator-metal (MIM) absorber design, operating at 8-12 µm wavelength. The metamaterial structures we report herein feature an interconnected design, optimized towards their application to micro-bolometer-based, long-wave infrared detection. The micro-bolometers were fabricated using a combination of conventional photolithography and electron beam lithography (EBL), the latter owing to the small feature sizes within the design. The absorption response was designed using the coupled mode theory (CMT) and the finite integration technique, with the fabricated devices characterized using Fourier-transform infrared spectroscopy (FTIR). The metamaterial-based micro-bolometer exhibits a responsivity of approximately 198 V/W over the 8-12 µm wavelength regime, detectivity of ~ 0.6 × 109 Jones, thermal response time of ~ 3.3 ms, and a noise equivalent temperature difference (NETD) of ~33 mK under 1mA biasing current at room-temperature and atmosphere pressure. The ultimate detectivity and NETD are limited by Johnson noise and heat loss with thermal convection through air; however, further optimization could be achieved by reducing the thermal conductivity via vacuum packaging. Under vacuum conditions, the detectivity may be increased in excess of two-fold, to ~ 1.5 × 109 Jones. Finally, an infrared image of a soldering iron was generated using a single-pixel imaging process, serving as proof-of-concept of this detection platform. The results presented in this work pave the road towards high-efficiency and frequency-selective detection in the long-wave infrared range through the integration of infrared MAs with micro-bolometers.

A typical metamaterial perfect absorber (MPA) is comprised of a metamaterial layer, a dielectric spacer, and a ground plane. The conventional spacer material is usually a lossy dielectric with little-dispersion for the purpose of easing the design and optimization procedure of the MPA. In this paper, we present the design, fabrication, and characterization of metamaterial perfect absorbers with a highly dispersive spacer, which is compatible with functional microelectromechanical systems. The measured dispersive permittivity of a silicon nitride thin film is used in modeling the absorption response of MPAs with rigorous coupled wave analysis. Different designs of MPA structures are fabricated and characterized. Spectroscopy data shows two perfect absorption peaks in wavelengths ranging from 8 µm to 20 µm, which supports the theoretical calculation and numerical simulation. The dispersion of silicon nitride enables the shared resonant modes of the two peak wavelengths and decreases the wavelength shift led by variations in structural parameters. We demonstrate that the use of dispersive dielectric materials in MPAs potentiates various functional devices.

The application and commercialization of microelectromechanical system (MEMS) devices suffer from reliability problems due to the structural inelastic deformation during device operation. Nanocoatings have been demonstrated to be promising solutions for suppressing creep and stress relaxation in bilayer MEMS devices. However, the micro/nano-mechanics within and/or between microcantilevers and coatings are not fully understood, especially when temperature, time, and geometric and material nonlinearities play significant roles in the thermomechanical responses. In this study, the thermomechanical behavior of alumina-coated/uncoated Au/SiN_x bilayer microcantilevers was characterized by using thermal cycling and isothermal holding tests. Finite element analysis with power-law creep was used to simulate the mechanical behavior of microcantilevers during isothermal holding. To better understand the stress evolution and the mechanism of inelastic deformation, scanning electron microscopy and atomic force microscopy was employed to explore the grain growth and grain boundary grooving after isothermal holding at various temperatures of 100 °C, 150 °C and 200 °C. The methods and results presented in this paper are useful for the fundamental understanding of many similar bilayer microcantilever-based MEMS devices.

Bimaterial microcantilevers are used in numerous applications in microelectromechanical systems (MEMS) for thermal, mechanical, optical, tribological and biological functionalities. Unfortunately, the residual stress-induced curvature and combined effects of creep and stress relaxation in the thin film significantly compromises the performance of these structures. To fully understand the themomechanical deformation and microstructural evolution of such microcantilevers, SiN_x/Al bilayer cantilever beams were studied in this work. These microcantilevers were heated and subsequently cooled for five cycles between room temperature and 250 °C, with the peak temperature in each successive cycle increased in increments of 25 °C using a custom-built micro-heating stage. The in situ curvature change was monitored using an interferometric microscope. The general behavior of the bimaterial microcantilever beams can be characterized by linear thermoelastic regimes with (dκ/dT)_ave = 0.079 mm⁻¹ °C⁻¹ and inelastic regimes. After thermal cycling with a maximum temperature of 225 °C, upon returning to room temperature, the bimaterial microcantilever beams were flattened and the curvature decreased by 99%. The thermoelastic deformation during thermal cycling was well described by the Kirchhoff plate theory. Deformation of bimaterial microcantilevers during long-term isothermal holding was studied at temperatures of 100 °C, 125 °C and 150 °C with a holding period of 70 h. The curvature of bimaterial microcantilever beams decreased more for higher holding temperatures. Finite element analysis (FEA) with power-law creep in Al was used to simulate the creep and stress relaxation and thus the curvature change of the bimaterial microcantilever beams. The microstructure evolutions due to isothermal holding in SiN_x/Al microcantilevers were studied using an atomic force microscope (AFM). The grain growth in both the vertical and lateral directions was present due to isothermal holding. As the isothermal holding temperature increased, the surface roughness of the film increased with more prominent grain structures.

Uncooled microcantilever-based infrared (IR) detectors have recently gained interest due to their low noise equivalent temperature difference (NETD), while concurrently maintaining low costs. These properties have made them available for a wider range of applications. However, the curvature induced by residual strain mismatch severely compromises the device’s performance. Therefore, to meet performance and reliability requirements, it is important to fully understand the deformation of IR detectors. In this study, bimaterial (SiN_x/Al) microcantilever-based IR detectors were fabricated using surface micromachining with polyimide as a sacrificial layer. Thermo-mechanical deformation mechanisms were studied through the use of thermal cycling. A temperature chamber with accurate temperature control and an interferometer microscope were adopted in this study for thermal cycling and full-field curvature measurements. It was found that thermal cycling reduced the residual strain mismatch within the bimaterial structure and thus flattened the microcantilever-based IR detectors. Specifically, thermal cycling with a maximum temperature of 295 °C resulted in a 97% decrease in curvature of the microcantilever-based IR detectors upon return to room temperature. The thermoelastic deformation of the IR detectors was modeled using both finite element method (FEM) and analytical methods. A modified analytical solution based on plate theory was established to describe the thermoelastic mechanical responses by using a correction factor derived from FEM. Although in the current study Al and SiN_x were chosen for the application of microcantilever-based IR detectors, the general experimental protocol and modeling approach can be applied to describe thermoelastic mechanical responses of bimaterial devices with different materials. Toward the end of this paper, we studied the correction factors in the modified analytical solution while varying parameters such as Young’s modulus ratio, thickness ratio and coefficient of thermal expansion (CTE) mismatch to investigate the influences of these parameters.

This paper reports the recent progress on the development of double-cantilever infrared (IR) detectors, including the fabrication, the post-process curvature control, and also the first-time demonstration of thermal detection using capacitive-based IR focal plane arrays (FPAs). In this work, simplified double-cantilever IR FPAs based on bimaterial SiN_x/Al and Al/SiNx cantilevers are fabricated using a surface micromachining module with polyimide as the sacrificial material. Thermal-cycling experiments of both 200 nm-thick Ebeam Al and 200 nm-thick PECVD SiN_x films reveal that the residual stresses in IR materials can be significantly modified by thermal annealing. Therefore, an engineering approach to flattening IR FPAs is developed by using rapid thermal annealing (RTA). This article also demonstrates the thermal detection of cantilever IR FPAs using commercialized weak capacitance readout IC.

Bimaterial SiN_x/Al infrared cantilever structures are always initially curved because of the imbalanced residual stress in the two layers. Their performance and functionality are therefore significantly decreased. A thorough study of the residual stress (strain) has then become a key issue in the development of bimaterial SiN_x/Al cantilever structures. In the curvature-based approach to the film stress, the residual strain is derived from the measured curvature based on certain assumptions on the distribution of the residual strain in the thickness direction. Previous models for a bimaterial cantilever structure, however, are not sufficient to characterize the residual strain in bimaterial SiN_x/Al infrared structures. The main goal of this paper is to investigate gradient residual strain in bimaterial SiN_x/Al infrared structures. To achieve this goal, the relationship between the residual strain and bending curvature is developed with the assumption that the residual strain in each layer is linearly distributed rather than uniform throughout the thickness. The profile of the gradient strain is then derived from the curvatures measured during the continuous etching of the top-most SiN_x in the bimaterial cantilevers. The derived residual strain can then be inverted to predict curvature change further in the etching process. This paper demonstrates that a linear assumption of the residual strain yields a stronger agreement with the measured data in comparison to previously used models. In addition, several factors that may affect measurement accuracy are discussed at the end of the paper.

Multilayered structures are widely used as sensing or actuating components in MEMS devices. Since the thin films of multilayered structures are always subject to residual stresses, it is important to model the relation between these residual stresses and the resultant elastic deformation. The main purpose of this paper is to explore two different approaches to addressing this issue when the residual stress in each thin film is not necessarily uniform throughout the thickness. These two approaches are first briefly introduced and then used to arrive at identical solutions for a monolayer cantilever and a bilayer cantilever, both with arbitrary residual strain distributions throughout the thickness. The analytical formulas for a bilayer cantilever are further verified by the numerical simulation of a special case. After the discussion on the errors induced by assuming the gradient residual strains in the bilayer cantilevers are uniform, the relation between the bending plane and the neutral plane in bilayer cantilevers is also explored. Finally, we present an approach to characterizing residual stresses in thin films by using micromachined bilayer cantilevers in conjunction with the theory developed in this paper.

This paper reports an approach to eliminating stress-induced curvature in microcantilever-based infrared focal plane arrays (FPAs). Using a combination of argon ion beam machining and rapid thermal annealing (RTA), we successfully modified curvatures of free-standing SiN_x/Al bimaterial FPAs. The SiN_x/Al FPAs were fabricated using a surface micromachining technique with polyimide as a sacrificial material. The as-fabricated FPAs were concavely curved because of the imbalanced residual stresses in the two materials. To modify the FPAs curvature, first, Ar ions with energies of 500 eV was used, which sputter etched PECVD SiN_x at a rate of 4 nm/min, and 20 min of ion beam machining reduced the FPAs curvature from –1.92 to –0.96 mm⁻¹. Then based on the investigation on the thermomechanical behavior of both the e-beam Al and PECVD SiN_x films during the thermal cycling, RTA was proposed to further modify the FPAs curvature. It is found that 5 min of RTA at 375 °C resulted in flat FPAs with acceptable curvatures (<0.10 mm⁻¹).

Using the Stoney formula and its modifications, curvature-based techniques are gaining increasingly widespread application in evaluating the stress in a film on a substrate. In principle, the formula applies only when the stress is uniform throughout the film thickness. The main purpose of this paper is to extend the Stoney formula when the residual strain in the film is no longer uniform, but dependent on the z position. To achieve this goal, a general theory was introduced for the elastic deformation of an arbitrary, multilayered system. By practicing this general theory, we used a polynomial function to describe the gradient stress in a film, and contributions by different elements of the polynomial to both the curvature and the bending strain were derived. A finite element simulation for a typical film–substrate structure was then carried out, leading to the verification of the theory developed in this paper. In the discussion section, we explored the relation between the surface curvature and the bending curvature as well as the difference between the stress in the constrained planar state and that in the relaxed state. In addition, the accuracy of the simplified formula, using thin film approximation, was evaluated. Finally, a SiN_x-Al MEMS structure was studied by using the formula in this paper.