US20070057522A1

US20070057522A1 - Tool to pick up microparts

Info

Publication number: US20070057522A1
Application number: US11/351,749
Authority: US
Inventors: Christpher Keller
Original assignee: Individual
Current assignee: Individual
Priority date: 2005-02-14
Filing date: 2006-02-10
Publication date: 2007-03-15

Abstract

This is a tool for use in picking up small, or even microscopic, objects and placing them where they are wanted. Applications include the assembly of micromachines, and scientific collection of particle specimens. The various embodiments use one or more of the following quantities to control the pick-up force of the tool: electrostatic potential, capillary (surface tension) force using one or more liquids, and heat.

Description

The priority date for this application is Feb. 14, 2005, the filing date for provisional patent application No. 60/652,833. This provisional patent application is also enclosed for reference.

DESCRIPTION OF THE INVENTION

FIG. 1 shows an embodiment of the invention while in use picking up a mems (MicroElectroMechanical System) micromirror (5). Conventional tools are not able to pick up these mirrors because the mirrors are too small. Conventional tools are too stiff and can inadvertently apply too much force to the mirror, damaging it or even breaking it. A suitable tool must be small enough to not prevent the operator or vision system from seeing both the mirror and the target site where it is supposed to go. A suitable tool should have low enough stiffness that it can safely push on the mirror with appropriate force to seat it in the target spot that it is to be assembled to. In FIG. 1 the handle (3) is large enough for people to manipulate and mount into a micropositioning stage. The electrostatic pick-up tool (1) is mounted at the end of the handle (3). The user will never touch the silicon pick-up tool itself because it is too easily broken if touched directly by humans. The 3 contact fingers (11) of the pickup tool have been brought into contact with the mirror, and the voltage applied. Here the mirror is seen as it is being carried by the tool to the pedestal that the mirror is to be mounted on.
FIG. 2 shows a close up of the pick-up tool of FIG. 1 holding the mirror above the pedestal.
FIG. 3 shows a plan view of the mirror being carried by the pick-up tool.
FIG. 4A shows a side view of the entire system. A micromirror is sitting on a conductive chuck. The handle of the electrostatic pick-up tool is mounted on positioning stages that move in the x, y, and z directions. The microscopic object to be picked up, and the tool can be observed through the microscope. The chuck is electrically grounded. The pick-up tool is also electrically grounded when it is not in the process of picking up or holding the object. After the pick-up fingers of the tool are brought near to the object, the switch is thrown to apply the power supply voltage to the pick-up tool. As shown in FIG. 4B the tool is held at a potential (voltage V) with respect to ground by virtue of its connection to the power supply (or battery) (15). Depending on what is being picked up, the applied voltage may range from near zero to over 100 volts. A current limiting resistor R (17) with a value in the range from 1 megohm to 1 gigohm will prevent large current flow in case a low resistance path to ground occurs. Note that there is an ionizing source (19) (such as the polonium alpha source “nucleospot” from NRD, INC) (optional, but recommended during the release of the part from the pick-up tool) to nullify any trapped charges that may occur, and eliminate unwanted stray electric fields.
Note that the x, y, z positioners are mounted on top of a pitch/roll stage. The pitch/roll stage enables the user to make the plane containing the contacting surfaces of the three pickup fingers parallel to the plane desired for the final installed mirror position. The order of the stacking of the stages is arbitrary, the particular order shown here is just by way of example.
The sequence of events in using the device is:

1. move the pickup tool's three finger contact surfaces to lightly contact the object to be picked up. It is not always necessary for the fingers to actually contact the object because when the voltage is turned on the object will be pulled into contact with the fingers.
2. turn on the voltage. The voltage value should be set high enough to provide enough attractive force to prevent the object from moving (slipping or falling off) with respect to the tool when acted on by the ambient forces that may be present in the workspace.
3. run the positioning stages so the tool and object move away from the initial location to the target location.
4. the direction of approach of the object to the target location should be appropriate for it to enter into any locating features that hold it in the assembly in the proper position.
5. the appropriate force should be applied by the tool pushing on the object (by moving the handle with the positioning stages) to seat the object into the assembly. If the object is retained by latches in the assembly, more force may be needed to get past the entrance of the latches. If glue is used, then it should only be necessary to touch the object to the glue and then capillary forces should pull the object in to its desired final position (this requires the viscosity of the glue to be low enough (may apply heat if needed) and the glue must wet the surface of the object.
6. Once the object is seated, throw the switch to disconnect the pick-up tool from the voltage source, and connect it to electrical ground.
7. move the tool away from the object (which is now held in the assembly)

For completeness FIG. 4C and 4D show connections to the tool using a battery. In FIG. 4C the switch (21) is connecting the tool to ground to eliminate electrostatic forces at the tool.
In FIG. 4D the switch is connecting the tool to the battery so that it can induce dipoles in objects and pick them up.
FIGS. 5A-5M show the phenomena involved in the physical mechanisms for creating the pickup force between the tool and the object.
FIGS. 5A-5D concern electrostatic forces. In FIG. 5A no voltage is applied to the handle. The distribution of electrical charges throughout the conductive bodies is random there is no separation of positive and negative charges. Therefore there are no electrostatic fields or forces anywhere. In FIG. 5B a voltage is applied to the conductive handle. In this case the applied voltage is negative so an excess of electrons appears over the whole surface of the conductive handle. (the applied voltage could be positive, in which case change all the minus signs into plus signs, and plus signs into minus signs to show the resulting charge distributions). There is a greater concentration of electrons adjacent to the silicon pick-up tool because the silicon is conductive and electrons are repelled away from the handle. This leaves an excess of positive charge in the silicon near the handle. This positive charge stabilizes the greater concentration of electrons in the adjacent volume of the handle. Similarly, the electrons in the silicon tool can be stabilized near the object to be picked up because they induce a positive charge in the object there. The voltage applied to the handle has induced a dipole charge distribution in the pickup tool. In turn, the pick-up tool has induced a dipole charge distribution in the object to be picked up. Notice that no current flows through the pick-up tool because it is coated with silicon dioxide (or other nonconductive material such as silicon nitride). The silicon dioxide layer should be as thin as possible to maximize the electrostatic force available for picking up the object. For small objects and low voltage applications, 20 angstroms of silicon dioxide will be sufficient. For higher voltages (e.g., 100 volts) there should be about 100 angstroms of oxide. The oxide can be up to about 2 microns thick if desired, and the tool will still be useful.
FIG. 5C shows a tool which has had the oxide removed where it contacts the handle (typically silver epoxy is used to connect the tool to the handle mechanically and electrically). Therefore electrons can flow from the handle to the tool, and vice versa. Note that no current can flow from the tool to the object picked up because there is an electrically insulating oxide layer at the end of the tool that contacts the object.
When the object has been put in the desired assembly, the voltage to the handle is turned off, the handle is electrically connected to ground, and the excess electrons flow out of the handle. All charge concentrations disappear, there are no longer any electrostatic fields or forces. Now the force between the object and the pickup tool is less than the force between the object and the assembly. Therefore when the tool is taken away, the object remains in its place in the assembly.
FIGS. 5D-5J show the sequence of events in using capillary forces to pick and place a micro object. FIG. 5D is a side view showing a pick-up finger tip approaching a reservoir of liquid. In FIG. 5E the finger tip is dipped into the liquid. The liquid forms a meniscus because it wets the surface of the finger and is pulled up the side of the finger by the surface tension force. In FIG. 5F the finger has been lifted out of the reservoir and it retains a small volume of the liquid on the flat contact surface at the end of the finger tip. FIG. 5G shows a side view of the finger tip with liquid on it approaching the surface of the object to be picked up. FIG. 5H shows the liquid bridge formed between the finger tip and the object to be picked up. Notice that there is no solid-to-solid contact. The tensile stress in the liquid due to the weight of the hanging object is less than the capillary force that can be provided by the surface tension of the liquid. This is useful for picking up objects with delicate surfaces that would be damaged by contact with a solid tool. FIG. 5I shows the tool bringing the object to the target assembly site. Note that the target site has been treated with liquid (maybe the same or different than the liquid on the pick-up tool). When the object contacts the liquid on the target site, that liquid pulls with a greater total force than the liquid on the tool, and pulls the object into the kinematic locating features that define the desired position. The liquid in the assembly site can pull more strongly than liquid on the tool because of geometry of the tool and target site, and/or because of the chemical composition of the two liquids. Ways of increasing holding force by virtue of geometry include making the wetted area larger where you want greater holding force. The force is the rate of change of the energy (in this case surface energy as liquid surface area is created or reduced) with respect to distance moved. FIG. 5J shows the system after the tool has been moved away from the object which is now more strongly held by the target site. Note that some liquid may be left by the tool on the object.
FIGS. 5K-M show the use of heat to evaporate the liquid. This reduces the force between the object and the tool, so a smaller target area is needed to hold the object in the desired assembly site. FIG. 5K shows a side view of the step where both the tool and the target site are holding the object by liquid wetting. In FIG. 5L heat is applied to the tool to make the liquid evaporate faster than the liquid of the target site thereby reducing the gripping force of the tool. In FIG. 5M the liquid between the tool and the object is completely evaporated. Then the force between the tool and the object is zero. This makes it easy to remove the tool without disturbing the position of the object. Another way to accomplish this is to use a different liquid for the target site (one with a lower vapor pressure that evaporates more slowly) than the liquid on the tool.
FIGS. 6A and 6B show a pick-up tool design that can provide any desired compliance in x, y, and z direction by designing it with appropriate lengths, widths, and thicknesses of the three beams that go to the three pick-up fingers. FIG. 6A shows a perspective view. FIG. 6B shows a plan view.
FIG. 7 shows a two fingered embodiment. This design is easier to make, and is good for use in situations that only have significant moments about two axes. Other than that, the principles of operation are the same as previously described for the 3 fingered geometry. As in all designs, the contact area of the finger tips is a variable that must be designed to provide the desired pick up force at a given applied voltage (or with a given volume of a particular liquid). In general a larger contact area can provide a larger pick-up force.
FIGS. 8A and 8B show an embodiment for picking up cylinders. The angles of the contacting surfaces are set to be tangent to cylinder surface and provide a kinematic locating feature (as a V-groove). FIG. 9 shows a long cylinder held by a tool that is assembled with two of the units shown in FIG. 8. This assembly can hold a cylinder against the action of external moments and also allows compliance in any direction as needed to prevent excessive forces.
FIG. 10 shows a tool for picking up square or rectangular cross section beams.
FIG. 11 shows a pick-up tool that is attached to a handle by very low stiffness suspension. A high vertical force can be applied to the geometric center of the pick-up tool by the hemispherical contact on the rigid load beam. Because the plane of the pick-up fingers can rotate as needed about the hemispherical contact, that means that the orientation of the load beam, and handle can be off from the desired orientation with respect to the target site assembly, and it can still work because the tool just holds the part and allows the target site geometry to determine the final orientation of the part.
FIG. 12 shows an embodiment of a pick-up tool in which three pick-up fingers rigidly held by the inner triangular frame. This rigid frame is floating in a compliant suspension that allows it to move as needed to orient the object being placed into its geometric locating features at the assembly site. This enables precision placement even if the handle holding the pick-up tool is oriented slightly wrong with respect to the target site. The compliance of the suspension allows the geometry of the target site to determine the final orientation of the object being placed. Note that there are three bond pads that allow the flow of current to heat up the suspension beams, triangular frame, and pick-up fingers. Also, a voltage can be applied for the electrostatic pick-up function.
FIG. 13 shows a plan view of the pick-up finger contact surfaces and the supporting beams and suspension. FIG. 14 shows a perspective view.
FIGS. 15-21 show various other embodiments as actually laid out in the photolithography mask. Note that FIG. 21 just has 1 pick-up finger.
The embodiment of FIG. 22 does not have much compliance (i.e., it has high stiffness), but is has a simpler circuit for heating the pick-up fingers to evaporate liquid. Of course it can also be used as an electrostatic pick-up tool too. FIG. 23 shows a perspective view. FIG. 24 shows a close up of the pick-up fingers.
FIGS. 25 and 26 shows a tool for picking up spheres. It has three critical edges that have the geometry that makes them tangent to a spherical surface of a size in the predetermined range of interest. When a voltage is applied to the tool, a conductive sphere that comes near these three edges will have electrostatic dipoles induced and the sphere will be pulled to the tool. Like all of the tools in this document (when used for electrostatic pick up) the areas that contact the object to be picked up are coated with an electrically insulating material, such as silicon dioxide. The electrically insulating material is not needed if the tools are used only with liquid for the capillary (surface tension) pick up method.
FIGS. 27A and 27B show a resonating beam pick up tool. In FIG. 27A it is shown as-etched with the break away tethers needed to maintain relative position of the parts prior to assembly. In FIG. 27B the parts have been glued to a handle and the tethers broken and removed. In operation, the electrodes 45 and 47 are driven with applied voltage at the resonant frequency of the beam 43 to make the beam vibrate. The tip (49) is held very close to the surface of interest that has particles that are to be picked up. A voltage is applied to the beam (43) and therefore also tip (49) so that when the tip scans over a particle, the particle is attracted to the tip. The mass of the particle will change the resonant frequency of the vibrating beam. The driver circuit that keeps the beam in resonance will decrease the applied frequency. The controlling computer will then know that a particle was found, where it was found (since the same computer controls the positioning stages, and its mass. FIG. 28 shows the whole system.
FIG. 29 shows an assembled pickup tool. The handle makes an angle alpha (typically 30 degrees) with the horizontal. The Fingers 55 and 57 are shown holding a horizontal plate (59). The endpoints of the 3 fingers that contact the object to be picked up define a plane.
Method of Fabrication:
While there are many variations in possible methods of manufacture, the following is an example of a preferred method using the tools that are available to me at this time.

1. clean silicon 4inch wafers, p-type, boron doped, 0.02 to 10 ohm-centimeter resistivity, (100) orietation
2. grow 1 micron silicon dioxide in oxygen with steam at 1100 Celsius
3. spin on photoresist and pattern photolithographically to define the pick-up fingers (mask 1), hard bake the resist
4. remove the exposed oxide by plasma etching, leaving only the oxide that is protected by overlying photoresist.
5. spin on photoresist and pattern photolithographically to define the supporting silicon beams (mask 2). Hard bake the resist.
6. etch trenches to desired depth (for example 20 microns) deep into the exposed silicon. Use a deep silicon anisotropic etch process such as the Bosh process, or Alcatel (cryogenic chuck) process.
7. Remove the photoresist using acetone, water rinse, piranha (H₂SO₄, H₂O₂), water rinse, dry, oxygen plasma
8. etch the exposed silicon for the desired height of the pick-up fingers (e.g., 50 microns). Use a deep silicon anisotropic etch process such as the Bosh process, or Alcatel (cryogenic chuck) process.
9. clean the wafer
10. grow one micron of silicon dioxide at 1100 Celsius in oxygen with steam.
11. remove the oxide from the backside of the wafer, leaving a 3 mm ring of oxide at the OD edge.
12. submerge the wafer in 25% TMAH (tetramethyl ammonium hydroxide) at 60 degrees Celsius.
13. when the wafer has thinned to the point where the bottoms of the plasma etched trenches are exposed (can see through oxide membranes left where the bottoms of the trenches were prior to TMAH etching).
14. rinse wafer in water. Submerge wafer in HF (hydrofluoric acid) to remove all oxide.
15. grow 1 micron silicon dioxide at 1100 C in oxygen with steam. This is to remove sharp stress concentrators.
16. remove all oxide using HF, rinse with water
17. grow 0.05 microns of silicon dioxide at 1000 C in dry oxygen
18. at this point, individual tools are held on the wafer by breakaway silicon tethers. Grip a tool with microtweezers and pull to break the tethers. Place the tool electrical bond pads into small blobs of silver epoxy that have been placed on the bond pads of the handle structure. Cure the silver epoxy. The electrostatic pick-up tool is now ready for use. Note that if DC electrical connection between the handle and the silicon is needed, small sacrificial cantilevers (similar to the tethers) in the bond pad area can be broken off to reveal silicon that can be contacted by the silver epoxy.
Other Comments:

The tool does not have to be silicon. It can be made of any electrically conductive material such as plated nickel, or other metal. A thin layer of a nonconductive material must be applied at the contact surfaces of the pick-up finger tips to prevent electron flow between the object and the tool. The non conductive layer may be a polymer, ceramic, metal oxide, glass, or combination of these, as a continuous film or as discrete particles.
For the electrostatic force to be created, charges have to be able to move in the micro object to be picked up. Electrons or ions need to move, so the object can be an electronic conductor or an ionic conductor. For nonconductors, surface films of water will be important, so relative humidity must be controlled. Surface charges can move with sufficient humidity, so even nonconductive objects con be picked up.
For most tools and most micro objects the applied voltage to the tool will be about 100 volts, although the applied voltage will commonly range from 25 volts when less force is needed to 200 volts when greater force is needed. For any applied voltage there will be a resulting electrostatic force of attraction, so in practice, one simply turns up the voltage until the force is enough for the job at hand. The current needed is very small and very brief. Once the electrons have moved as dictated by the geometry of the tool and object and the applied voltage, current flow stops (goes to zero). A current limiting resistor of 1 Megohm or greater is used in series with the tool handle to prevent dangerously high currents in case a grounded metal object is contacted by an uninsulated spot on the handle or tool.
The use of liquids on the tool to provide surface tension forces to pick up objects eliminates conductivity requirements. Note that the electrostatic method can be used alone, or the liquid surface force method can be used alone, or the two methods can be used at the same time in desired proportion so that each supplies some fraction of the needed force.
The liquids used need to be free of particles, and free of nonvolatile residues. They should be filtered to remove particles, and distilled to remove nonvolatile components. They can also be purified by sublimation, recrystallization, and other known methods of purification. After the liquid is used and has evaporated, no residue should be left on the parts.
After the tool finger tips are touched to the liquid reservoir to pick up the desired volume, the liquid should not evaporate too fast before it can be used to pick up the part. At room temperature in normal conditions an example of a liquid with useful properties is hexadecane. At lower temperatures, or higher speed operations, or in an enclosed chamber with ambient partial pressure maintained of the liquid's vapor phase, more volatile liquids can be used, such as pentadecane, tetradecane, tridecane, dodecane, undecane, decane, nonane, octane, heptane, hexane, pentane, to name a few.
One liquid can be used on the pick-up tool, and another lower vapor pressure liquid can be used on the target site. It is necessary for the target site to be stickier than the tool. This can be done by having more wetted area to stick to, or have a more viscous liquid, or a liquid that wets the surfaces with greater energy per unit area.
A material can be used that is solid for part of the operation. The apparatus may be cooled and/or the micro pick-up tool heated when needed. A reservoir of a solid like cosane, docosane, tetracosane, etc, at room temperature may be touched by a pick-up tool having an electrically heated beam that heats the contacting finger tips and melts enough material to coat the finger tips when it is removed. After moving the coated tips away from the solid reservoir, the tips can be brought into contact with the object to be picked up. Now the current to the heated beam can be decreased, or turned off, to let the material cool and freeze. The solid material will provide a greater gripping force than the liquid. The object can be moved to where it is wanted. The heater current can be turned back on to melt and even evaporate the material between the finger tips and transported object. That reduces the gripping force so that the object will not be disturbed from its position as the pick-up tool is taken away.
The material can be one that sublimes. Under normal room conditions, naphthalene, paradichlorobenzene, and cyclododecane are examples of materials that will sublime. These can be used in the target site, first in liquid form (heated) to pull the parts into place by surface tension forces, and then cooler in solid form to hold parts together for a long time while some other assembly, or process, is done. Later, after the material is not wanted anymore, it simply sublimes.
At the target site, it is often useful to use a liquid glue that solidifies after assembly and remains permanently part of the structure.
Fabrication: note that silicon tools can also be conveniently made using SOI (silicon on insulator) wafers.
FIG. 29A shows a perspective view of a pick up tool holding a part. As shown in the side view FIG. 29B, this tool is designed so that the plane of its body makes an angle a with the plane of the object to be picked up. The lengths of the fingers away from the plane of the body of the tool are set to contact the object when oriented at angle α.

Claims

1. an electrostatic pickup tool for picking up parts weighing less than 0.0001 newtons, said electrostatic pick up tool having at least one finger such that:

a. said finger having a distal end and a proximal end

b. the material of the finger is electrically conductive, and the surface of the distal end is covered with an electrically nonconducting layer so that the part to be picked up is contacted only by nonconductive material

c. the material of the finger is mechanically elastic, and the geometry of the finger renders it mechanically compliant, so that the contact forces that can be applied to the part to be pick up fall within a predetermined range from 0 newtons to less than 0.01 newtons

d. the proximal end of the finger is attached to a handle which provides mechanical connection to positioning stages, and electrical connection to a voltage source.

2. the electrostatic pick up tool of claim 1 in which the material of the finger is single crystal silicon, and the nonconductive layer at the distal end is one of the following: silicon dioxide, silicon nitride, an organic polymer (such as parylene)

3. the electrostatic pick up tool of claim 2 having 2 fingers

4. the electrostatic pick up tool of claim 2 having three noncolinear fingers

5. the electrostatic pick up tool of claim 2 having more than three noncolinear noncoplanar fingers

6. the electrostatic pick up tool of claim 1 having a v shaped distal end to contact a cylindrical object

7. the electrostatic pick up tool of claim 3 having a v shaped distal end at each finger aligned to contact a cylindrical object

8. a capillary pickup tool for picking up parts weighing less than 0.0001 newtons, said capillary pick up tool having at least one finger such that:

a. said finger having a distal end and a proximal end

b. the surface of the distal end is to be coated with a liquid layer so that the part to be picked up is contacted by liquid

c. the material of the finger is mechanically elastic, and the geometry of the finger renders it compliant, so that the contact forces that can be applied to the part fall within a predetermined range from 0 newtons to less than 0.01 newtons

d. the proximal end of the finger is attached to a handle which provides mechanical connection to positioning stages

9. the capillary pick up tool of claim 8 in which the material of the finger is single crystal silicon, and the liquid is one of, or a combination of: hexadecane, ethylene glycol, water, hydrocarbon oil,

10. the capillary pick up tool of claim 9 having 2 fingers

11. the capillary pick up tool of claim 9 having three noncolinear fingers

12. the capillary pick up tool of claim 9 having more than three noncolinear noncoplanar fingers

13. the capillary pick up tool of claim 8 having a v shaped distal end to contact a cylindrical object

14. the capillary pick up tool of claim 10 having a v shaped distal end at each finger aligned to contact a cylindrical object