MEMS-based micro and nano grippers with two-axis force sensors

This application claims the benefit of Canadian Patent No. 2,551,191, filed 23 Jun. 2006.

The present invention relates to micro and nanosystems, and micro and nanotechnology.

Intelligent manipulation (e.g., grasping/gripping) of micro- and nanometer-sized objects requires the use of miniaturized microgrippers with integrated force sensors. Currently, micro- and nanomanipulation typically relies purely on visual feedback either from an optical microscope or an electron microscope. The lack of force feedback at the microNewton and nanoNewton level severely limits intelligent micro- and nanomanipulation.

Besides miniaturization and electrical control, microgrippers must be capable of providing multi-axis force feedback to satisfy the following requirements: (i) to protect the microgripper and detect the contact between the microgripper and the object to be manipulated; and (ii) to provide gripping force feedback during grasping to obtain secured grasping while protecting the object to be grasped.

The vast majority of existing microgrippers lack force feedback due to the difficulty of integrating force sensors with microgrippers. The lack of force feedback does not permit force-controlled manipulation and easily causes breakage of microgrippers and damage to the object to be manipulated.

A recently reported electrothermally driven microgripper design is integrated with a single-axis piezoresistive force sensor that is only capable of measuring gripping forces. (See K. Molhave and O. Hansen, “Electrothermally actuated microgrippers with integrated force-feedback,” J. of Micromechanics and Microengineering, 15(6), pp. 1265-1270, 2005.) However, the gripping force sensing resolution is somewhat poor, on the order of milli-Newton that is orders of magnitude worse than what micro-nanomanipulation requires.

A recent paper reports a design of an electrostatically driven microgripper with a single-axis capacitive force sensor that is only capable of measuring gripping forces. (See F. Beyeler, D. J. Bell, B. J. Nelson, Yu Sun, A. Neild, S. Oberti, and J. Dual, “Design of a micro-gripper and an ultrasonic manipulator for handling micron sized objects,” IEEE/RSJ International Conference on Intelligent Robots and Systems, Beijing, China, October, 2006.) Due to the limitation of electrostatic actuation (low force output, small displacements, and high driving voltage), the microgripper design is only capable of grasping objects of a small range of sizes.

The lack of force sensing capabilities along a second-axis down to nanoNewton in existing designs does not allow for the protection of microgrippers and the detection of contact between the microgripper and object to be manipulated. What is needed is design and microfabrication of microgrippers that are capable of grasping micro and nano objects of a large range of sizes and having two-axis force sensing capabilities.

In one aspect, the present invention employs MEMS (microelectromechanical systems) technology in the design and microfabrication of microgrippers that are capable of grasping micro and nano objects of a large range of sizes and two-axis force sensing capabilities.

In an embodiment of the present invention, integrated, single-chip, batch microfabricated MEMS devices are disclosed that are electrothermally-driven microgrippers with integrated dual-axis force sensing capabilities. The gripping motion is produced by an actuator, such as a bent-beam actuator. The bent-beam actuator requires little power and is capable of producing a large range of gripping forces and displacements. Integrated force sensors along the in-plane x and y directions using transverse differential capacitive comb drives enable the measurement of gripping forces as well as the forces applied at the end of microgripper arms along the normal direction, both with a resolution down to nanoNewton.

In another aspect, a microfabrication process for a microgripper is provided.

This microgripper design features two-axis force sensing capabilities suitable for use in intelligent micro and nanomanipulation. Additionally, the employment of bent-beam electrothermal microactuators permits the grasping of objects of a large range of sizes.

A detailed description of one or more embodiments is provided herein below by way of example only and with reference to the following drawings, in which:

FIG. 1 illustrates a microgripper with integrated dual-axis capacitive force sensors;

FIG. 2 is a cross sectional view of the microgripper corresponding to FIG. 1 along axis A-A;

FIG. 3 is a cross sectional view of the microgripper corresponding to FIG. 1 along axis B-B; and

FIG. 4 illustrates microfabrication steps for the construction of a microgripper.

In the drawings, one or more embodiments of the present invention are illustrated by way of example. It is to be expressly understood that the description and drawings are only for the purpose of illustration and as an aid to understanding, and are not intended as a definition of the limits of the present invention.

In an embodiment of the present invention, an electrothermally actuated microgripper comprises four parts, as illustrated in FIG. 1 : (i) electrothermal microactuator D to drive gripper arm G 1 ; (ii) driving arm G 1 and sensing arm G 2 used together to grasp micro/nano objects; (iii) linear beam flexures F 1 , F 2 , F 3 , F 4 , and F 5 used to transform forces into displacements; and (iv) pairs of capacitor plates forming capacitors Cx 1 , Cx 2 , Cy 1 , and Cy 2 to transform displacements into capacitance changes.

In this case, the electrothermal microactuator D is a bent-beam microactuator. However, it should be understood that other types of electrothermal actuators are possible and within the scope of the present invention, such as U-beam electrothermal actuators or electrostatic actuators, for example. It should also be understood that piezoresistive force sensors could be used instead of capacitive force sensors.

Electrothermal bent-beam microactuator D produces forces to deflect the microgripper arm G 1 through flexure F 3 . When electrothermal forces are produced by applying voltages/current between electrodes E 1 and E 2 , the translational movement of F 3 is converted into a rotational movement of the driving arm G 1 . The displacement and driving force from a single bent-beam of the electrothermal microactuator are

U = αΔ ⁢ ⁢ Tl d ⁢ sin ⁢ ⁢ θ ( sin 2 ⁢ θ + cos 2 ⁢ θ ⁢ 12 ⁢ ⁢ I d A d ⁢ I d 2 ) F d = αΔ ⁢ ⁢ TEA d ⁢ cos ⁢ ⁢ θ ( sin 2 ⁢ θ ⁢ ⁢ A d ⁢ l d 2 12 ⁢ I d + cos 2 ⁢ θ )

where α is the coefficient of thermal expansion of the beam material, E is Young's modulus, I d is the beam length, A d is the beam cross sectional area, I d is the moment of inertia, and θ is the bending angle of bent beams. The bending of flexure F 1 increases the reaction force of G 1 ; however, this contribution from the bending of flexure F 1 is trivial and thus, can be neglected. The displacement of the end of G 1 is amplified by an amplification factor from the displacement of the bent-beam microactuator.

The second microgripper arm G 2 is supported by flexures F 2 and F 5 and is connected to the capacitive force sensor Cy 1 and Cy 2 . G 2 transmits gripping forces to the movable capacitor plates of the transverse comb drive Cy 1 and Cy 2 that together form a differential comb drive. As a gripping force F g is applied, flexure F 5 is deformed and the capacitance change of Cy 1 and Cy 2 can be measured through electrodes E 3 , E 4 , and E 5 . The bending force of flexure F 2 converts the rotational motion of G 2 into a translational displacement that is small and can be neglected.

F g = 4 ⁢ Etw g 3 l g 3 ⁢ x

where l is the length of the flexures F 5 , t is the out-of-plane thickness, w g is the in-plane width, and x is the deflection. The capacitance C for each comb drive is

C = ɛ 0 ⁢ nA d

where ∈ 0 is the dielectric constant, n the number of capacitor plate pairs, A is the overlapping area, and d is the gap distance. Changing the gap d instead of the overlapping area A provides a high change of capacitance for a small displacement Δd and thus increases the resolution of force sensing.

When the microgripper approaches a micro object, it is difficult to detect from pure visual feedback the contact between the microgripper arms and the substrate. Thus, x-directional (i.e. longitudinal) force feedback is necessary for contact detection and to avoid the breakage of the microgripper. In addition, the x-directional force feedback can also be used to measure the tensile strength or adhesion force, such as biological cells sticking on a substrate by gripping and pulling. As flexures F 3 and F 4 are deflected, capacitance changes Cx 1 and Cx 2 are measured through electrodes E 6 , E 7 and E 8 . Cx 1 and Cx 2 together form a differential comb drive.

Flexures F 1 and F 2 are designed to be deformed in the y-direction and translate forces in the y-direction (i.e. lateral direction). In contrast, flexure F 3 is designed to be deformed in the x-direction and translate forces in the x-direction. Besides serving as flexures, F 4 and F 5 are also used for electrical signal routing. Flexure F 5 must be such designed that it has a high enough stiffness in the x-direction to protect Cy 1 and Cy 2 from the rotational motion of gripper arms G 1 and G 2 .

In order to obtain a linear relationship between force/displacement and capacitance/voltage changes, differential comb drive structures are used for Cx and Cy

Cx ⁢ ⁢ 1 = ɛ 0 ⁢ A x d 1 - x + ɛ 0 ⁢ A x d 2 + x , Cx ⁢ ⁢ 2 = ɛ 0 ⁢ A x d 1 + x + ɛ 0 ⁢ A x d 2 - x Cy ⁢ ⁢ 1 = ɛ 0 ⁢ A y d 1 - y + ɛ 0 ⁢ A y d 2 + y , Cy ⁢ ⁢ 2 = ɛ 0 ⁢ A y d 1 + y + ɛ 0 ⁢ A y d 2 - y

where d 1 is the smaller gap and d 2 is the larger gab between two capacitive plates along the x and y-direction, x is the deflection in x-direction, y is the deflection in the y-direction, and Ax and Ay are the overlapping area. The capacitor plates of the capacitors Cx 1 and Cx 2 are oriented parallel to the xz-plane. The capacitor plates of the capacitor Cy 1 and Cy 2 are oriented parallel to the yz-plane. To determine the gripping force F g , the deflection of flexure F 5 in the y-direction is measured via Cy 1 and Cy 2 . To determine the force Fx to the normal direction of microgripper arms, Cx 1 and Cx 2 are measured.

The length of microgripper arm L can be larger than 3.0 mm for the purpose of operating in an aqueous environment, such as for handling biological cells. To make the arms mechanically connected and electrically insulated, the gripper arms are preferably connected using the handle layer of an SOL (Silicon on Insulator) wafer as shown in FIGS. 2 and 3 . The buried oxide layer, device layer, and handle layer are used together to form the structure and achieve electrical signal routing.

FIG. 4 shows a microfabrication process of the microgripper, as an example. According to application needs, an SOL wafer having a 200-500 μm thick handle layer, 1-2 μm thick SiO 2 and 0.5-300 μm thick device layer can be chosen, as an example. A total of 4 photolithography masks are required to construct the microgrippers.

In particular, the specific steps as illustrated include:

A) SiO 2 is deposited on the handle layer of SOI wafer. B) SiO 2 is patterned to form DRIE (Deep Reactive Ion Etching) etch mask (mask 1 ). C) Center part of photo resist is removed (mask 2 ) and handle layer of the wafer is etched up to 50 μm forming the structure for electrical insulation and mechanical connection. D) Center part of SiO 2 is etched. E) Handle layer of the wafer is etched again up to handle layer thickness minus up to half the thickness of the handle layer, e.g., 50 μm. F) Buried oxide layer is etched. G) Ohmic contacts are formed by e-beam evaporation and patterned by lift-off (mask 3 ). H) Device layer of wafer is etched to form the structural elements thereof, including in this case gripper arms, flexures, bent-beam actuators, and comb drives (mask 4 ), for the microgripper of this example.

Note that changing the tethering spring dimensions and capacitance readout circuits can allow devices in accordance with the present invention to resolve forces down to pico-Newtons. Force resolution at this level enable a larger range of applications, particularly in nano device assembly and biophysics studies in which individual molecules are manipulated and characterized.

It should be understood that the present invention is the first of its kind in terms of actuation range for grasping a range of micro-nano objects and sensing forces along two axes. The stumbling block in this area has been the monolithic integration of both actuators and force sensors, but is achieved by the present invention. The present invention also provides for the novel de-coupling of force sensing along two axes.

It will be appreciated by those skilled in the art that other variations of the one or more embodiments described herein are possible and may be practised without departing from the scope of the present invention.