Richard J. Wilks

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.

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.

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.

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.

Polysilicon surface micromachined structural entities with continuous hinges and microrivets for assembling three-dimensional MEMS devices

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 microrivets with flexible barbs that readily flex to facilitate their joining and assembly. Because the 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 microrivet designs have been micromachined to facilitate the latching process, including a simple arrowhead design, a high-aspect ratio arrowhead variant, and a rivet-like structure with a hemispherical shaped cap and a flexible split shank.