Chemistry: electrical and wave energy – Apparatus – Electrolytic
Reexamination Certificate
1998-08-31
2001-09-04
Gorgos, Kathryn (Department: 1741)
Chemistry: electrical and wave energy
Apparatus
Electrolytic
Reexamination Certificate
active
06284108
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to electrochemical plating. More specifically, the present invention is directed to a method and apparatus for increasing and controlling the flow of plating fluid to increase the rate of plating of a workpiece.
BACKGROUND OF THE INVENTION
Plating is the process of electrochemically depositing the layer onto a surface of a workpiece. In a typical plating process according to the prior art, a positively-charged element, the anode, is disposed in a plating fluid. A negatively-charged workpiece is also immersed in the fluid. The electric charge between the anode and the cathode creates ions in the plating fluid. These ions are then electrically attracted to the workpiece and are deposited on the surface.
The rate of ionic exchange at the surface of the workpiece can affect the quality of the plating. An increased ionic exchange rate can produce an improved plating grain structure. In addition, such increased ionic exchange rate promotes higher current densities. This results in faster plating and, therefore, a higher plating throughput.
A high ionic exchange rate can be promoted by continually refreshing the plating fluid at the surface of the workpiece. For example, a laminar fluid flow can be created by moving the plating fluid across the surface of the workpiece. However, a laminar fluid flow is relatively slow. The plating fluid is subject to the effects of friction at the surface of the workpiece. As this frictional force is increased, the plating fluid is slowed. At the molecular level, the plating fluid flow can be stopped. The ionic exchange rate is therefore decreased, and the plating process slowed. Thus, the ionic exchange rate produced by a laminar flow is limited.
It is well-known to use a turbulent plating fluid flow to provide a high ionic exchange rate. However, more energy is required to generate a turbulent fluid flow than a laminar flow. In addition, it is difficult to produce a uniform ionic exchange rate at each point on the surface by using a turbulent flow. Thus, a non-uniform coating will be formed over the surface. The maximum ionic exchange rate is therefore limited by the maximum amount of turbulent flow that permits the creation of a relatively uniform coating.
One prior art method for increasing turbulent flow is by circulating the plating fluid in the plating tank.
FIG. 1
is a top plan view of a dip tank plating system according to the prior art. In the Figure, three parallel rows
12
,
14
,
16
of in-line anode baskets
18
are disposed in a plating tank
10
. The plating tank holds a plating fluid (not shown). A cathodic workpiece
20
,
22
is immersed in the plating fluid, between the rows of anode baskets.
Spargers
24
,
26
,
28
are located, for example, at the bottom of the tank, such that spargers are positioned on both sides of the workpiece. The spargers release air bubbles to agitate the plating fluid. The resulting agitation can improve the plating efficiency of the system. However, one known problem with such system is that the air bubbles lower the density of the plating fluid. Each air bubble displaces the conductive plating fluid with an insulative air bubble. Furthermore, air bubbles can also increase the evaporation of the plating fluid. Thus, the rate and amount of air bubbles introduced into the tank must be balanced by the lowered density of plating fluid caused thereby
Another problem inherent to the dip tank system is that air bubbles can adhere to the surface of the workpiece during plating. An adhering air bubble can then detach from the surface, leaving a recessed portion in the plated surface of the workpiece. To produce a consistent, and even plating, it is important to constantly detach adhering air bubbles from the workpiece surface. The maximum plating efficiency of the prior art dip tank system is therefore limited by the ability of the system to detach adhering bubbles from the workpiece surface. Under ideal conditions, the prior dip tank can achieve a plating current density of approximately 10-150 amperes per square foot, with a typical plating current density of between 10-30 amperes per square foot.
The circulation plating system attempts to solve these recognized problems of dip tank plating systems.
FIG. 2
is a side sectional view of a circulation plating system
38
according to the prior art. Such circulation plating systems include the SER-DUCTOR™ Systems developed by Serfilco Ltd. of Northbrook, Ill.
In
FIG. 2
, a centrifugal pump (not shown) draws plating fluid
36
from a plating tank
34
and delivers this plating fluid back into the tank through a plurality of nozzles
32
. The plating fluid is thereby circulated within the plating tank.
However, one problem with a circulation plating system is achieving a constant circulation of plating fluid directed at all locations on a surface
31
of the workpiece
30
. Differing rates of circulation result in different ionic exchange rates across the surface, producing an uneven coating. For example, the plating fluid circulation
35
dispersed by the different nozzles could result in locations on the surface at which the ionic density is significantly greater, or significantly less than other locations. This is a significant disadvantage in plating devices that require extreme precision.
In the Serfilco system, the nozzles are generally not directed at the surface of the workpiece. Directing an inadequate amount of nozzles at the workpiece surface promotes an unequal distribution of ions at the surface. Thus, the plating current density is limited by the circulation rate which can be achieved by nozzles directed away from the workpiece surface. The Serfilco circulation plating system can achieve plating current densities that are as high as 2 times, and typically from 1.25 to 1.5 times greater than those achieved using a dip tank system.
The use of a plating fluid flow to achieve a higher ionic exchange rate is also known in the prior art. An example of such flow process is the fountain plating process of the International Business Machines Corporation (IBM) of Armonk, N.Y.
FIG. 3
is a side view of a portion of a fountain plating apparatus
44
according to the prior art.
In the fountain plating process, a vertical nozzle
46
directs a fountain of plating fluid
48
up towards the rotating workpiece
50
. The plating fluid contacts the surface
52
of the workpiece at a velocity sufficient to promote an increased ionic exchange. A plurality (not shown) of these fountains are used in the fountain plating system.
However, the vertical fluid stream used in the fountain plating process is subject to the effects of gravity. Gravity attracts the fluid stream, pulling the fluid downwards. Thus, the stream curves as it approaches the surface of the workpiece. This curvature of the fluid stream can result in a “dead spot”
54
at which there is a reduced fluid flow contacting the surface. The resulting unequal ionic distribution at the surface produces an uneven plating. The workpiece is rotated over the fountains to compensate for the uneven ionic distribution produced by the fluid streams. This procedure requires the additional use of a motor and a control system for the workpiece rotation.
Unfortunately, practical and effective techniques for plating particulate materials are not readily available. Such particulate materials include the particle interconnect material described in DiFrancesco, Method For Cold Bonding, U.S. Pat. No. 4,804,132 and DiFrancesco, Particle-Enhanced Joining of Metal Surfaces, U.S. Pat. No. 5,083,697. Particle interconnect material contains coated metal particles, which are formed of diamond, silicon carbide particles coated by metals such as nickel or copper. These particles range in size from approximately 3 &mgr;m to approximately 200 &mgr;m. Particle interconnect material is typically used to pattern regions of thermal, electrical, and mechanical conductivity or insulation.
The particles can be dispersed in a binder, such as an adhesive, as described in DiFrancesco, Patter
Dergosits & Noah LLP
DiFrancesco Louis
Gorgos Kathryn
Smith-Hicks Erica
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