Tutorial #110
George Riley
January 2011
Introduction
An automated wire-bonding process creates metal bond wires less than one micrometer (micron) in diameter, with bond footprints less than three microns diameter. The electrical and mechanical qualities of deposited copper wires are superior to solder connections.
The ohmic copper connections show breakdown current densities in excess of 1011 A/m2, more than five orders of magnitude higher than solder. Bond pull tests show strengths above 39 MPa, more than 4 times the MIL-STD-883G requirement.
Process Description
This direct-writing process uses a micropipette dispensing nozzle that confines electrolytic solutions to the liquid meniscus formed between the nozzle and the conducting substrate. The nozzle, shaped by focused ion beam milling, may range from several microns down to 100 nm in diameter.
When the nozzle is brought close to the conductive substrate surface, the electrolytic fluid bridges the gap from the nozzle to the surface, forming a stable, sustainable liquid meniscus. Applying an electric potential between the nozzle and substrate initiates metal plating within the meniscus.
Drawing the nozzle away under precise control as metal deposition proceeds extends the growth front of deposited metal to create a wire. Figure 1 illustrates how the nozzle may extend a wire vertically from the surface.
The wire diameter is determined by the size and shape of the meniscus, which in turn depends upon the nozzle, the properties of the fluid and interfaces, the withdrawal speed, and the growth rate.
Figure 1.Schematic view of meniscus-confined electrodeposition, with the fluid meniscus of the electrolyte (arrow) extending to the substrate.
Creating wire bonds requires horizontal bridging to the second bond pad by changing the direction of wire growth. This is enabled by making a side opening in the nozzle, as shown in Figure 2A. Figure 2B shows wires deposited in a variety of angles by the same cut nozzle.
Figure 2A. SEM photo of a side-cut nozzle, diameter approximately 3 microns.
Figure 2B. Angled wires created by a single cut nozzle.
Bridging and creating the second bond combines both changing the deposition angle and mechanically moving the wire. The steps are shown in Figure 3.
The free-standing wire is first plated vertically upwards from the conductive substrate. At the desired height, the deposition angle is changed from vertical to horizontal, and the wire is extended sideways.
Deposition is suspended while the horizontal wire is mechanically bent down to the substrate. At that point the meniscus envelopes both the wire and a portion of the substrate surface, and deposition resumes, creating the second bond.
Figure 3. Steps in bridging and forming the second bond on a connecting wire.
Figure 4A shows twenty sub-micron diameter copper bridging wires deposited in a fan-out from a 50µm by 50µm substrate. Figure 4B is a close-up of the bridging wires, showing consistent, uniform wires with small second bonds.
Figure 4A. Twenty deposited wires in a fan-out from a simulated chip.
Figure 4B Close-up photo showing uniformity of bridging wires.
Figure 5A shows multi-layered interconnections by copper wires bonded onto three progressive 5µm high steps. Wire diameters here approximate 800nm, with 3µm bonds. Figure 5B shows overlapping wires deposited on the same step geometry.
Figure 5A. Sub-micron diameter single bond wires to multi-level pads. (Scale is 10 microns long).
Figure 5B. Sub-micron diameter overlapping bond wires to multi-level pads. (Scale is 10 microns long).
Extensions, Applications, and Savings
Copper and platinum wire bridges have been demonstrated and tested. Many other metals and alloys with readily-available, inexpensive electrolyte solutions should be suitable, including noble metals, magnetics, and metal alloys.
For volume production, both shortening the nozzle taper and increasing the electrolyte concentration would increase the deposition rate of the wires from the 0.25µm/sec laboratory rate. An array of multiple nozzles may also be used.
Meniscus-confined 3D electrodeposition uses common electrolytes at room temperature in ambient air, providing lower-cost, less complicated sub-micron fabrication than old approaches such as e-beam, focused ion beam deposition, or colloidal ink writing.
Altering the deposition mechanics and controls allows creating sub-micron interconnections and complex structures. These may be for new nanoscale applications, or may be added to existing designs to increase connection density and reduce costs compared to the old methods.
FOR MORE INFORMATION:
Jie Hu and Min-Feng Yu, “Meniscus-Confined Three-Dimensional Electrodeposition for Direct Writing of Wire Bonds,” SCIENCE Vol. 329, 16 July 2010, pp 313 – 316.
CONTACT
David C. Dlesk Nanofab3D Inc. www.nanofab3d.com
978-621-9870 ddlesk@nanofab3d.com