Microfabricated system for magnetic field generation and focusing
View Patent ↗A method of forming, in or on a Si substrate, planar micro-coils with coil windings of high aspect ratio (>3) and a wide variety of geometric shapes. The micro-coils may be formed on a Si substrate and be embedded in a dielectric, or they may be formed in trenches within a Si substrate. The micro-coils may have field enhancing ferromagnetic pillars rising above the micro-coil plane, formed at positions of maximum magnetic field strength and the micro-coils may also include magnetic layers formed beneath the substrate and contacting the pillars to form a substantially closed pathway for the magnetic flux. The substrate may be thinned to membrane proportions. These micro-coils produce strong magnetic fields with strong field gradients and can be used in a wide variety of processes that involve the exertion of strong magnetic forces at small distances or the creation of magnetic wells for trapping and manipulating small particles.
1. A micro-coil device for the generation, focusing and manipulation of magnetic fields and the exertion of magnetic forces on magnetizable particles thereby, comprising:
a substrate having an upper and a lower surface;
a micro-coil, formed of a plated conductor having an aspect ratio greater than 3/1, formed in a variety of geometrical shapes on the upper surface of the substrate or in a trench formed through said upper surface and said micro-coil having at least two ends electrically connected to bonding pads for making electrical connections to said micro-coil whereby a current can be introduced into said micro-coil to produce a magnetic field thereat;
a passivating oxide layer formed over said micro-coil;
a dielectric layer formed over said oxide layer; and
at least one pillar formed of ferromagnetic material having a high permeability, said pillar having an upper and lower end, said pillar being located at a position of maximum magnetic field strength generated by said micro-coil and the lower end of said pillar contacting a patterned metal base formed on said oxide and the upper end of said pillar extending through said dielectric layer.
2. The device of claim 1 wherein the substrate is a silicon wafer having plane, parallel upper and lower surfaces, the upper surface being covered by a first layer of oxide and the lower surface being covered by a second layer of oxide and wherein said micro-coil is formed within a trench formed through said first oxide layer and terminating within said wafer.
3. The device of claim 2 wherein said trench has a width of approximately 3 microns and a depth of approximately 10 microns to provide said micro-coil with an aspect ratio of approximately 10/3.
4. The device of claim 1 wherein said pillar is formed of the electroless plated ferromagnetic material NiCoP and said patterned metal base is a patterned bi-layer comprising a layer of Al formed on a layer of Ti.
5. The device of claim 1 wherein said dielectric layer is a layer of BCB formed to a thickness between approximately 2 and 5 microns.
6. The device of claim 1 , wherein said micro-coil is shaped as a curved spiral, a square spiral, a meander coil with rectangular loops, a pair of interdigitated meander coils, a pair of independent concentric circular spirals wherein an inner spiral is formed entirely within an outer spiral, a pair of interdigitated circular spirals, a meander coil with small rectangular loops formed within the larger loops, a meander coil with small circular or elliptical loops formed within the larger loops, a semi-spiral coil, an array of spirals having a common central axis, or as arrays of such shapes connected in series or in parallel.
7. The device of claim 1 wherein the bonding pads are formed as a patterned bilayer of Ti/Al formed over a Cu mesh of intersecting Cu filled trenches and are connected directly to said micro-coil ends or to conducting bridging leads that are connected to similar bonding-pads formed on said micro-coil ends.