Peter B. Allen

Thermally-actuated cantilever beam for achieving large in-plane mechanical deflections

Abstract The design, finite-element analysis, and experimental performance evaluation of a microelectromechanical systems (MEMS) device known as a thermally-actuated beam is presented. A MEMS polysilicon thermally-actuated beam is a device that uses resistive (Joule) heating to generate thermal expansion and movement. To be a useful MEMS device, a thermally-actuated beam needs to produce incremental in-plane mechanical beam tip deflections that span 0–10 μm, while generating force magnitudes on the order of 10 μN. The thermally-actuated beam design was accomplished with the L-Edit software program, and the devices were fabricated using the Multi-User Microelectromechanical Systems (MEMS) Process (MUMPs) foundry at the Microelectronics Center of North Carolina (MCNC). Finite-element modeling analysis was accomplished with the IntelliCAD computer program. This CAD software incorporates an MCNC fabrication process description file that generates a 3-D solid model of the thermal beam. The resulting thermo- and electro-mechanical finite-element analyses predicted beam tip deflections and forces consistent with experimental observations. For example, when the drive voltage was varied between 0 and 6.5 V DC (corresponding to currents spanning 0–4.5 mA), tip deflections on the order of 0–13 μm were observed and calculated. When the ‘hot’ arms temperature was modeled to be 200°C (Joule heating), the resulting beam tip deflection was calculated to be 4.55 μm. The resonant frequency associated with in-plane motion, without damping, was calculated to be 75.16 kHz. The average resonant frequency measured in ambient air was 69.73 kHz. The average tip force generated by the thermal beam was measured to be 8.5 μN. A relative measure of the reliability of the thermal beam was established to be greater than 3 million cycles when continuously operated with a 30 Hz, 3-volt amplitude square wave with a 1.5-V DC offset.

In-plane tip deflection and force achieved with asymmetrical polysilicon electrothermal microactuators

Abstract Several microactuator technologies have recently been investigated for positioning individual elements in large-scale microelectromechanical systems (MEMS). Electrostatic, magnetostatic, piezoelectric and thermal expansion are the most common modes of microactuator operation. This research focuses on the design and experimental characterization of two types of asymmetrical MEMS electrothermal microactuators. The motivation is to present a unified description of the behavior of the electrothermal microactuator so that it can be adapted to a variety of MEMS applications. Both MEMS polysilicon electrothermal microactuator design variants use resistive (Joule) heating to generate thermal expansion and movement. In a conventional electrothermal microactuator, the ‘hot’ arm is positioned parallel to a ‘cold’ arm, but because the ‘hot’ arm is narrower than the ‘cold’ arm, the electrical resistance of the ‘hot’ arm is higher. When an electric current passes through the microactuator (through the series connected electrical resistance of the ‘hot’ and ‘cold’ arms), the ‘hot’ arm is heated to a higher temperature than the ‘cold’ arm. This temperature increase causes the ‘hot’ arm to expand along its length, thus forcing the tip of the device to rotate about a mechanical flexure element. The new thermal actuator design eliminates the parasitic electrical resistance of the ‘cold’ arm by incorporating an additional ‘hot’ arm. The second ‘hot’ arm results in an improvement in electrical efficiency by providing an active return current path. Additionally, the ‘cold’ arm can have a narrower flexure than the flexure in a conventional single-‘hot’ arm device because it does not have to pass an electric current. The narrower flexure element manifests improved mechanical efficiency. Deflection and force measurements of both actuators as a function of applied electrical power have been presented in this work.

Thermally actuated microbeam for large in-plane mechanical deflections

The design, finite-element analysis, and experimental performance evaluation of a microelectromechanical systems (MEMS) device, known as a thermally actuated beam, is presented. The behavior of the thermal beam has been characterized so that it can be considered as an actuator in future MEMS applications. A MEMS polysilicon thermally actuated beam uses resistive (Joule) heating to generate thermal expansion and movement. To be a useful MEMS device, a thermally actuated beam will need to produce in-plane tip deflections that span 0–10 μm; while generating force magnitudes on the order of 10 μN. The thermally actuated beam design was accomplished with the L-Edit® software program. The devices were fabricated using the Multi-User Microelectromechanical Systems Process foundry service at the Microelectronics Center of North Carolina. The finite-element modeling analysis was accomplished with the IntelliCAD® computer program. These analyses predicted thermal beam tip deflections (0–13 μm) consistent with exper...

Single- and double-hot arm asymmetrical polysilicon surface micromachined electrothermal microactuators applied to realize a microengine

Abstract This research focuses on the design and experimental characterization of two types of MEMS asymmetrical electrothermal microactuators. The motivation is to present a description of the behavior of the electrothermal microactuator to facilitate its adaptation to a variety of MEMS applications. Both MEMS polysilicon electrothermal microactuator design variants use resistive (Joule) heating to generate thermal expansion and movement. In a conventional electrothermal microactuator, the ‘hot’ arm is positioned parallel to a ‘cold’ arm, but since the ‘hot’ arm is narrower than the ‘cold’ arm, the electrical resistance of the ‘hot’ arm is larger. When an electrical current passes through the microactuator (through the series connected electrical resistance of the ‘hot’ and ‘cold’ arms), the ‘hot’ arm is heated to a higher temperature than the ‘cold’ arm. This temperature increase causes the ‘hot’ arm to expand along its length, thus forcing the tip of the device to rotate about a mechanical flexure element. A new electrothermal actuator design eliminates the parasitic electrical resistance of the ‘cold’ arm by incorporating an additional ‘hot’ arm. The second ‘hot’ arm results in an improvement in electrical efficiency by providing an active return current path. Additionally, the ‘cold’ arm can now have a narrower flexure compared with the conventional single-‘hot’ arm device because it does not have to pass an electric current. A narrower flexure element manifests improved mechanical efficiency. Deflection and force measurements of both electrothermal actuators as a function of applied electrical power are presented. Also described is the practical integration of the electrothermal microactuators in a monolithic microengine that is capable of rotating a set of gears.

Implementation of micromirror arrays as optical binary switches and amplitude modulators

Abstract Five types of micromirror arrays were designed and fabricated using a three-level, polysilicon, surface micromachined, microelectromechanical systems (MEMS) process. The electrostatically deflectable micromirror designs included arrays of simple cantilever beams, torsion beams, tethered (piston-style) beams, circular membranes and oval membranes. The smallest micromirror element was the simple cantilever beam, measuring 50 μm square. The largest micromirror element was the oval membrane; it possessed an active optical surface that was 320 μm by 920 μm. Each of the remaining micromirror designs have gold-coated polysilicon optical surfaces with geometries between these two limits. Electrostatically induced vertical deflections on the order of 2.75 μm were achieved. The torsion beam micromirror design exhibits both in-plane and out-of-plane deflection. The other micromirror designs only manifest in-plane deflections. The modeling phase focused on the microdynamical behavior of the torsion beam micromirror. The IntelliCAD ® finite element analysis program was used to generate a plot of the micromirror’s deflection (d) versus applied direct current voltage (V). The data was least-squares fitted to the well-established V ∝ d 3/2 relationship. A resonant frequency analysis predicted an approximate switching speed of 6 μsec. The reliability (number of operational cycles) of each micromirror design, when operated with a rectified 60 Hz alternating current (ac) signal, was measured to exceed more than 1 million flexure events. Experimental evidence supporting the potential for using micromirrors as binary optical switches and amplitude modulators was also addressed.

Three-dimensional structures assembled from polysilicon surface micromachined components containing continuous hinges and microrivets

Abstract A new polysilicon surface micromachining technique for fabricating and assembling three-dimensional structures has been developed. Single-layer polysilicon elements and laminated polysilicon panels incorporating trapped-glass reinforcement ribs have been successfully fabricated on a silicon substrate with robust and continuous hinges that facilitate out-of-plane rotation and assembly. To realize a stable three-dimensional structure, one of the devices elevatable panel components is terminated with an array of open windows, and the mating rotatable element has a matched set of protruding arrowheads/microrivets with flexible barbs that readily flex to facilitate their joining and assembly. Because the arrowhead/microrivet barb tip-to-barb tip separation is larger than the opening in the mating window, the barbs flex inward as they pass through the open window and then expand to their original shape upon exiting the window, resulting in a permanently latched joint and a three-dimensional structure. Three novel arrowhead/microrivet designs have been micromachined to facilitate the latching process, including a simple arrowhead, a high-aspect ratio arrowhead, and a rivet-like structure with a hemispherical shaped cap and a flexible split shank. To minimize panel breakage after the sacrificial glass release etch process and to facilitate mechanical alignment during assembly, a network of sacrificial electrothermally-actuated mechanical links (‘fuses’) have been integrated into the MEMS structure designs.

Design and performance of a polysilicon surface micromachined microengine realized with arrays of asymmetrical electrothermal microactuators

Several microactuator technologies have recently been investigated for positioning individual elements in large-scale microelectromechanical systems (MEMS). Electrostatic, magnetostatic, piezoelectric and thermal expansion are the most common modes of microactuator operation. This research focuses on the design and experimental characterization of two types of asymmetrical MEMS electrothermal microactuators. The motivation is to present a unified description of the behavior of the electrothermal microactuator so that it can be adapted to a variety of MEMS applications.

Theoretical and experimental characterization of the in-plane tip force and deflection achieved with asymmetrical polysilicon electrothermal microactuators

Several microactuator technologies have recently been investigated for positioning individual elements in large-scale microelectromechanical systems (MEMS). Electrostatic, magnetostatic, piezoelectric and thermal expansion are the most common modes of microactuator operation. This research focuses on the design and experimental characterization of two types of asymmetrical MEMS electrothermal microactuators. The motivation is to present a unified description of the behavior of the electrothermal microactuator so that it can be adapted to a variety of MEMS applications. Both MEMS polysilicon electrothermal microactuator design variants use resistive (Joule) heating to generate thermal expansion and movement. In a conventional electrothermal microactuator, the hot arm is positioned parallel to a cold arm, but because the hot arm is narrower than the cold arm, the electrical resistance of the hot arm is higher. When an electric curren passes through the microactuator (through the series connected electrical resistance of the hot and cold arms), the hot arm is heated to a higher temperature than the cold arm. This temperature increase causes the hot arm to expand along its length, thus forcing the tip of the device to rotate about a mechanical flexure element. The new thermal actuator design eliminates the parasitic electrical resistance of the cold arm by incorporating an additional hot arm. The second hot arm results in an improvement in electrical efficiency by providing an active return current path. Additionally, the rotating cold arm can have a narrower flexure than the flexure in a conventional single-hot arm device because it does not have to pass an electric current. The narrower flexure element results in an improvement in mechanical efficiency. Deflection and force measurements of both actuators as a function of applied electrical power are presented.

Optical binary switch and amplitude modulator micromirror arrays

Five types of micromirror arrays were designed and fabricated using a three-level, polysilicon, surface micromachined, micro-electromechanical systems (MEMS) process. The electrostatically deflectable micromirror designs included arrays of simple cantilever beams, torsion beams, tethered (piston-style) beams, circular membranes, and oval membranes. The smallest micromirror element was the simple cantilever beam, measuring 50 micrometer square. The largest micromirror element was the oval membrane; it possessed an active optical surface that was 320 micrometer by 920 micrometer. Each of the remaining micromirror designs have gold-coated polysilicon optical surfaces with geometries between these two limits. Electrostatically induced vertical deflections on the order of 2.75 micrometer were achieved. The torsion beam micromirror design exhibits both in-plane and out-of-plane deflection. The other micromirror designs only manifest in-plane deflections. The modeling phase focused on the microdynamical behavior of the torsion beam micromirror. The IntelliCADR finite element analysis program was used to generate a plot of the micromirrors deflection (d) versus applied direct current voltage (V). The data was least-squares fitted to the well- established V varies direct as d3/2 relationship. A resonant frequency analysis predicted an approximate switching speed of 6 microseconds. The reliability (number of operational cycles) of each micromirror design, when operated with a rectified 60 Hz alternating current (ac) signal, was measured to exceed more than 1 million flexure events. Experimental evidence supporting the potential for using micromirrors as binary optical switches and amplitude modulators is also addressed.

Design and Performance of a Microengine Realized with Arrays of Asymmetrical Electrothermal Polysilicon Surface Micromachined Microactuators

This research focuses on the design and experimental characterization of two types of MEMS asymmetrical electrothermal microactuators. Both microactuator design variants use resistive (Joule) heating to generate thermal expansion and movement. Deflection and force measurements of both microactuators as a function of applied electrical power are presented. Also described is the practical integration of the electrothermal microactuators in a monolithic microengine that is capable of rotating a set of gears.