Electrolysis: processes – compositions used therein – and methods – Electrolytic coating – Controlling current distribution within bath
Reexamination Certificate
2003-04-02
2003-12-30
King, Roy (Department: 1742)
Electrolysis: processes, compositions used therein, and methods
Electrolytic coating
Controlling current distribution within bath
C204S22400M, C204S229900, C204SDIG007, C205S123000, C205S148000
Reexamination Certificate
active
06669833
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to processes and apparatus for plating metals onto a workpiece and more particularly to processes and apparatus for relatively uniformly, and selectively plating of small features on a workpiece.
2. Description of Related Art
A serious problem in electroplating microscopic features non-uniformly dispersed on a large substrate of a workpiece is that the total current required for depositing the material onto these microscopic features is very small. There are no commercially available power supplies that can reliably deliver the required kind of small currents to the workpiece/substrate.
FIGS. 1A-1C
are schematic diagrams which illustrate a prior art type of plating system using a thief electrode ring
22
surrounding a workpiece
26
such as a silicon wafer to enhance the quality of plating of metal onto the workpiece
26
through a photoresist mask formed thereon (not shown) as is widely practiced in the art. In
FIG. 1A
which is a vertical elevational view, a plating tank
10
has a bottom
12
, left sidewall
14
and right sidewall
15
and a top
16
shown to be open.
FIG. 1B
shows the thief ring
22
and the substrate
26
isolated from the other elements seen in FIG.
1
A.
FIG. 1C
is a right side view of what is shown in FIG.
1
B. In the example shown here, the tank
10
, which is formed of a dielectric material, contains an electroplating bath
17
up to the level shown by line
18
. An anode
33
is located in the plating tank
10
near the right sidewall
15
. A positive voltage V
3
is applied to anode
33
by a connection wire
32
. The workpiece
26
, which is shown on the side of the tank
10
near the left sidewall
14
has a negative voltage (−V
1
) applied thereto by a connection wire
36
. There is a thief ring
22
, which surrounds the workpiece
26
is coplanar with the workpiece
26
. A second negative voltage (−V
2
) is applied to the thief ring
22
by a connection wire
34
. A space
24
is provided between the workpiece
26
and the thief ring
22
. The thief ring is adjusted in voltage to adjust the plating current to the workpiece
26
, but it is not possible to maintain an equal current density across the large surface of the substrate
26
which may be several inches wide.
As a result, the thickness of the material deposited on various features on the workpiece
26
can vary from workpiece to workpiece. This variation creates a very big quality control problem for the plating engineer who is required to deposit a desired thickness on all of the features of the workpieces
26
within narrow tolerances.
A second and more important problem in dealing with small features dispersed on a large substrate
26
is that the secondary current, and higher order currents, cause tremendous non-uniformity in the thickness of the deposited material from place to place across the workpiece
26
. This non-uniformity will vary depending upon the density of the features and also on the size of the features on the substrate
26
.
Prior art relating to cathodes in electroplating baths include the following patents.
U.S. Pat. No. 3,652,442 of Powers et al. for “Electroplating Cell Including Means to. Agitate Electrolyte in a Laminar Flow” describes a Horizontal Paddle Electroplating Cell (HPEC) in which a cathode in the form of an insulating board to which is affixed a conductive sheet or coating with a very smooth upper surface. The cathode is shown with its flat upper surface extending horizontally at the bottom of the cell lying on a conductive support block. The bath is agitated during plating by a base portion which moves continuously at a substantially uniform rate in a path back and forth along the length of the cathode and just above the surface thereof. The result is that the bath solution is homogenized on the surface of the cathode. Agitating means is provided including a motor connected by linkages to the base portion which causes a uniform laminar flow of the bath across the surface of the cathode without causing any measurable turbulence on the surface thereof. The agitating base, which is designed to cause minimal resistance to flow of the bath, is triangular in cross section with a blunted apex at an angle which permits flow thereover with minimal turbulence, while at its base which confronts the cathode the agitating base is flat so that the agitating caused by the agitating base caused the bath to flow over the base and to effect mixing with the bulk of the bath at the apex of the base by convection. As the mixture passes the apex, the laminar flow is restored. The system is used to plate magnetic metal alloys.
In U.S. Pat. No. 4,102,756 of Castellani et al. entitled “Nickel-Iron (80:20) Alloy Thin Film Electroplating Method and Electrochemical Treatment and Plating Apparatus”, which describes another HPEC for plating films to form batch-fabricated, magnetic bubble devices and magnetic recording thin film heads, in which the plating bath is maintained at a level at which the anode is immersed in the bath during electroplating of a magnetic recording device. The constituents of the bath are constantly replenished and bath temperature is controlled by recirculation from a reservoir where it is refreshed by dispensing acid, iron and preferably also Na, Saccharin, Na lauryl sulfate and/or Ni
++
if needed and constantly stirred by a horizontal reciprocating mixer otherwise referred to herein as a paddle, which travels back and forth horizontally above the surface of the cathode at an approximate distance of {fraction (1/32)} to ⅛ inch (79 mm to 318 mm) for providing agitation of the bath with minimal turbulence.
U.S. Pat. No. 5,516,412 of Andricacos et al. describes a “Vertical Paddle Plating Cell” (VPPC) which is a modification of the Castellani et al cell adapted for microplating metal onto a substrate an article which is a flat, circular wafer or substrate having a substantial number of individual IC chip patterns arranged suitably thereon. The microplating process may comprise electroplating or electroless plating process. As microplating techniques were being developed for manufacturing devices such as features with a trend to continuously smaller and smaller dimensions of integrated circuits (ICs) in the form of microscopic chips formed on a flat circular wafer or substrate, it became necessary to reorient the plating system to suspend the article being plated vertically to remove debris from the surface being plated. Furthermore, as metal ions are depleted from the electrolyte, the uniformity of the electrolyte is decreased and must be suitably corrected to avoid degradation of the electroplating process so use of the laminar flow type of paddles was required to assure uniformity of the composition of the plating bath at the microsufaces being microplated. Because of the very small areas being plated in the microplating process of forming microcircuits on IC devices, a thief electrode was added behind the article being plated extending beyond the periphery thereof to enhance performance. Moreover the clearance between the surface of the article (substrate) being plated and the laminar paddle was decreased by one or more orders of magnitude to 1 mm to 4 mm from the 79 mm to 318 mm of the above HPEC plating apparatus of Castellani et al., U.S. Pat. No. 4,102,756. Thus the plating cell was adapted for electroplating the exposed surface of an article that is supported vertically on a vertical rack. The rack includes a thief electrode laterally surrounding the article to define a cathode. The cell includes a reciprocating vertical paddle (of the kind described in the above Powers et al. patent) which includes two elongated, parallel prisms which have oppositely facing, parallel, flat bases with one of the bases being disposed parallel to and closely adjacent to the article or rack for parallel movement over the article supported therein, preferably skimming across the surface of the article being plated, about 4.0 mm therefrom. Since the surfac
Kaja Suryanarayana
Prasad Chandrika
Yu RongQing Roy
Blecker Ira D.
Jones II Graham S.
King Roy
Leader William T.
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