Electrolysis: processes – compositions used therein – and methods – Electrolytic coating – Coating moving substrate
Reexamination Certificate
2000-06-09
2004-02-03
Valentine, Donald R. (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic coating
Coating moving substrate
C205S148000, C204S212000, C204S22400M, C204S241000, C204S275100, C118S052000, C427S425000, C427S430100
Reexamination Certificate
active
06685817
ABSTRACT:
BACKGROUND OF THE INVENTION
1). Field of the Invention
The present invention relates generally to a method and an apparatus for plating onto a substrate. More specifically, the present invention relates to a method and apparatus for controlling thickness of plating over a width of a substrate. In particular, the present invention relates to a method and apparatus for counteracting non-uniform plating over a width of a face of a substrate.
2). Discussion of Related Art
Plating techniques are widely known in the art and are used for a wide variety of purposes. In the microelectronic industry, for example, electroplating and electroless plating techniques are used for plating layers on a face of a substrate or for forming individual structures on substrates, such as on semiconductor wafers during the manufacture of integrated circuits, on semiconductor dies, on printed circuit boards, and on various other active and non-active electrical components. For example, the manufacture of an integrated circuit on a wafer typically involves the formation of a pattern of metal lines on the wafer. The metal lines may be plated within trenches formed in a surface of the wafer. The metal lines may be formed utilizing an electroplating technique wherein a voltage is applied to a metal shorting layer in a base of the trenches. Previously, metal was plated which filled the trenches and covered the wafer, whereafter the metal would be planarized to leave a layer of metal lines in the trenches only. In order to do away with a planarization step, techniques were then developed wherein only the trenches were filled with metal without covering all of the wafer. Such a technique requires uniform plating over the entire width of the wafer so as to ensure uniform metal line thicknesses over the width of the wafer. However, for various reasons some of which will be discussed hereinbelow, uniform plating of a layer over a width of a wafer is often not easily accomplished.
In another example, individual contact structures known as controlled collapse chip connection (C
4
) bumps are formed on a wafer after an integrated circuit is formed in the wafer. The C
4
bumps are used to mount the integrated circuit to a package substrate. The C
4
bumps are formed by applying a voltage to individual bond pads on the wafer. In order to ensure C
4
bumps of substantially equal height, as is required for mounting the integrated circuit to the package substrate, uniform plating over the width of the wafer is required. However, as mentioned, uniform plating over the width of the wafer may be difficult to obtain.
A number of factors may contribute to non-uniform plating over a face of a substrate. These factors may, for example, include the direction of flow of a plating solution over the face of the substrate, depletion of the plating solution, the positioning of an anode or a cathode which is used for plating, or current density variations over the face of the substrate, particularly current density edge effects around an edge of the substrate. Temperature is also a known factor which influences plating rate. Some of the influences that temperature may have on plating rate are discussed in more detail with respect to
FIGS. 5G and 5H
in the specification of U.S. patent application Ser. No. 08/452,255.
Typical problems associated with plating are discussed with reference to
FIGS. 1
,
2
and
3
.
FIG. 1
illustrates a typical plating system
20
which may be used for plating on a lower face
22
of a substrate
24
such as a wafer or a printed circuit board.
The plating system
20
in the illustrated example comprises a tank
26
, a substrate holder
28
, a pump
30
, and an electrical biasing device
32
.
The substrate holder
28
locates the substrate
24
over an upper opening of the tank
26
so that the lower surface
22
of the substrate
24
faces downwardly into the tank
26
. The tank
26
may be filled with a plating solution which contacts the lower face
22
of the substrate
24
.
The electrical biasing device
32
is used for creating a voltage potential between the substrate
24
and the plating solution in the tank
26
. By creating a voltage potential between the substrate
24
and the plating solution, a layer is plated on the lower face
22
of the substrate
24
.
Due to depletion of the plating solution near the lower face
22
of the substrate
24
, it may be necessary to continuously circulate the plating solution through the tank
26
. The pump
30
is used to circulate the plating solution through the tank and over the lower face
22
of the substrate. The pump
30
supplies plating solution to a nozzle
34
in the bottom of the tank
26
. The nozzle
34
then directs the plating solution upwardly and onto a central region
36
of the lower face
22
of the substrate
24
. The plating solution then flows concentrically outwardly over an outer region
38
of the lower face
22
of the substrate
24
, and then over an edge region
40
of the lower face
22
of the substrate
24
. The plating solution then exists through multiple holes
42
near an edge of the substrate
24
, from where the plating solution flows back to the pump
30
.
FIG. 2
illustrates a typical profile of a plating layer
44
which is formed on the lower face of the substrate
24
when using a system such as illustrated in FIG.
1
. The plating layer
44
is typically relatively thick on the central region
36
of the lower face
22
of the substrate
24
. The plating layer
44
then decreases in thickness towards the outer region
38
on the lower face
22
of the substrate
24
. The plating layer
44
then typically increases in thickness towards the edge region
40
of the lower face
22
of the substrate
24
. The plating layer
44
may vary by 50% or more in thickness, depending on the particular plating system and substrate characteristics. A number of factors contribute to variations in thickness of the plating layer
44
.
One factor which may contribute to a non-uniformity in thickness of the plating layer
44
deals with lack of agitation and subsequent replacement of the plating solution after the plating solution has become depleted. The lower face
22
of the substrate
24
is initially exposed to a concentration and chemical composition of the plating solution substantially the same as the plating solution throughout the tank
26
. Once plating is initiated the plating solution near the substrate starts losing molecules to the plating layer
44
being formed. The result of the loss of molecules is that a thin boundary layer of depleted plating solution forms on the lower face
22
of the substrate
24
. The effect of the boundary layer of depleted plating solution is that less plating occurs as what could be achieved with undepleted plating solution, thus reducing the rate of plating on the lower face
22
of the substrate
24
.
A further complication is that the boundary layer does not form uniformly over the lower face
22
of the substrate
24
due to differential agitation of the boundary layer, thus resulting in non-uniform rates of plating over the face of the substrate. Non-uniform formation of the plating layer may be attributed to a number of factors, one of which is differences in agitation of the plating solution over the lower face
22
of the substrate
24
.
For example,
FIG. 3
illustrates schematically how the plating solution flows concentrically outwardly over the lower face of the substrate
24
. The plating solution first flows through an area indicated by the small circle
46
A and then through an area indicated by the large circle
46
B. When flowing through the small area
46
A, the plating solution has a relatively high velocity, indicated by the arrow
48
A. However, due to the larger outer area
46
B, the plating solution has a velocity, indicated by the arrow
48
B, when flowing through the outer area
46
B which is less than the velocity
48
A when flowing through the inner area
46
A. The velocity
48
A may be sufficient to cause agitation of the plating solution after having become de
Burraston N. Kenneth
FormFactor Inc.
Merkadeau Stuart L.
Valentine Donald R.
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