Electrolysis: processes – compositions used therein – and methods – Electrolytic coating – Agitating or moving electrolyte during coating
Reexamination Certificate
2000-10-06
2003-02-11
Wong, Edna (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic coating
Agitating or moving electrolyte during coating
C205S149000, C205S157000
Reexamination Certificate
active
06517698
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to electroplating, and in particular relates to the electroplating of integrated circuit wafers.
BACKGROUND OF THE INVENTION
Electroplating is a common technique for applying metal to a surface, and is often employed in the construction of integrated circuits on silicon wafers. One circumstance in which electroplating is particularly important is in the manufacture of integrated circuits that are to be employed in flip-chip assembly or direct chip attach (DCA). In contrast to the manufacture of standard integrated circuits, where integrated circuit chips are coupled to lead frames by way of wire bonding and then encased within encapsulate, integrated circuits to be used in DCA have leads formed by protrusions placed directly atop the integrated circuits themselves. These integrated circuits are then implemented in circuit boards by connecting the top surfaces of the integrated circuits directly to the circuit boards, where the electrical connections between the integrated circuits and the circuit boards are formed by the protrusions.
The formation of such protrusions atop DCA integrated circuits typically requires two steps of electroplating. First, the integrated circuits are electroplated with copper atop the silicon die containing the integrated circuits. Second, the integrated circuits are electroplated with solder atop the copper. During attachment of the DCA integrated circuits to circuit boards or substrates, the solder is heated and then cooled such that the solder melts and then resolidifies to electrically couple the integrated circuits to the circuit boards or substrates. To guarantee proper connection of the DCA integrated circuits to the circuit boards, the various electroplated protrusions must be formed to have equal heights to within a few microns. If the protrusions are too short, bad connections can result between the DCA integrated circuits and the circuit boards due to a lack of connective material. If the protrusions are too tall, the protrusions may include excessive amounts of solder which can result in the creation of unwanted short circuits.
Although the manufacture of DCA integrated circuits is one significant example of the importance of electroplating, electroplating is also important in a variety of other circumstances, including the creation of interconnecting copper wiring within integrated circuits. Additionally, electroplating of a variety of other metals aside from copper and solder is commonly performed, including electroplating of gold, silver and tin. Further, electroplating is employed in a variety of manufacturing and other circumstances besides the electroplating of integrated circuit wafers.
Referring to
FIG. 1
, conventional electroplating of an integrated circuit wafer
10
, such as a wafer for DCA integrated circuits, is performed by placing the wafer at a first end
14
of an electroplating chamber
12
having a cavity
16
. The integrated circuit wafer
10
is positioned with the top of the wafer facing the inside of the cavity
16
. In the embodiment shown, the electroplating chamber
12
supports the wafer with several fingers (not shown) that are spaced around the first end
14
. In other embodiments, the wafer is supported proximate to the first end
14
of the electroplating chamber
12
by a supporting device (not shown).
The electroplating chamber
12
includes a single channel
20
at a second end
18
of the electroplating chamber, and is filled with liquid. During electroplating of the integrated circuit wafer
10
with a particular metal (e.g., copper or solder), a jet of liquid solution such as sulfuric acid, in which the metal is dissolved, is sprayed through the channel
20
toward the integrated circuit wafer
10
. For a typical integrated circuit wafer
10
having a diameter of 8 inches, the jet of liquid solution may have a diameter of 1.3 inches at the channel
20
and diverge to a diameter of about 3 inches by the time it reaches the integrated circuit wafer. Inside chamber
12
is an anode
21
.
A voltage differential (e.g., 3 Volts) is applied between the integrated circuit wafer
10
, which is a cathode, through the liquid solution from the anode
21
, such that a portion of the metal dissolved within the solution is reduced onto the cathode (in this case, the wafer). Upon striking the integrated circuit wafer
10
, the jet of liquid solution radiates outward towards the outer edges of the integrated circuit wafer. Additional amounts of the metal come out of solution and adhere to the integrated circuit wafer
10
as the liquid solution travels along the surface of the wafer toward the outer edges of the wafer.
The solution then escapes the electroplating chamber
12
by flowing between the fingers (not shown) that are supporting the wafer, or in alternate embodiments where a separate supporting device is employed to support the wafer, between the wafer and a lip of the first end
14
of the electroplating chamber. The flow rate of the liquid solution during electroplating of the integrated circuit wafer
10
typically approaches 5-6 gallons per minute. The shape of the electroplating chamber
12
is typically cylindrical to minimize turbulence of the liquid within the electroplating chamber, and matches the shape of the integrated circuit wafer
10
, which is typically circular.
While this conventional system for electroplating integrated circuit wafers results in metal being deposited on the integrated circuit wafers, the system has a significant drawback in that it produces electroplating that is uneven across the wafers. As shown in
FIG. 2
, a typical distribution of electroplated metal across an integrated circuit wafer can vary from 1.6 microns of deposited metal near the center of the wafer to only about 1 micron of deposited metal near the edges of the wafer. Such an uneven distribution of electroplated metal is undesirable. As discussed, in the particular case of DCA integrated circuits, for example, uneven plating results in protrusions that are of uneven height, which in turn increases the difficulty of attaching and electrically coupling the DCA integrated circuits to circuit boards.
In order to provide a more even distribution of metal than is provided by this conventional electroplating system, certain modifications to the conventional electroplating system have been implemented. In one such modified electroplating system, the integrated circuit wafer is mechanically rotated relative to the electroplating chamber so that the liquid solution sprayed towards the wafer, upon reaching the wafer, spirals with reference to the wafer as it radiates outward to the edges of the electroplating chamber. That is, while the liquid solution radiates outward from the center of the wafer along straight paths, the liquid solution appears to spiral outward with respect to the integrated circuit wafer because the wafer is rotating. A second modified electroplating system mechanically rotates a nozzle (e.g., in place of the channel
20
) from which the jet of liquid solution emanates and thereby produces a rotating jet of liquid solution, such that the liquid solution similarly spirals outward relative to the integrated circuit wafer upon reaching the wafer.
These modified systems have the benefit that, because the wafer and the liquid solution are rotating relative to one another, the liquid solution flows across a longer path along the surface of the wafer as the liquid radiates to the outer edges of the wafer. Consequently, greater amounts of metal are deposited along the outer portions of the wafer than is the case with the electroplating system having a stationary wafer and simple channel (instead of a jet nozzle), and so the overall variation in the amount of metal deposited at different points on the wafer is decreased. Further, because of the greater contact between the liquid solution and the integrated circuit wafer, bubbles are less likely to be produced in the electroplated material deposited on the wafer.
Although these modified electropl
Johnson Timothy L.
Mitchell Douglas G.
Molla Jaynal Abedin
Clingan, Jr. James L.
Motorola Inc.
Wong Edna
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