Adjustable plating cell for uniform bump plating of semiconductor wafers

An apparatus plates metal bumps of uniform height on one surface of a semiconductor wafer (32). A plating tank (12) contains the plating solution. The plating solution is filtered (16) and pumped (14) through an inlet (22) to an anode plate (24) within plating cell (20). The anode plate has a solid center area to block direct in-line passage of the plating solution, and concentric rings of openings closer to its perimeter to pass the plating solution. The distance between the inlet and the anode plate is adjustable with supports to create a uniform flow of the plating solution to the surface of the semiconductor wafer for uniform plating of the array of metal bumps (30). The plating cell contains an adjustable sidewall extension (26) to set the proper distance between the anode plate and the semiconductor wafer.

BACKGROUND OF THE INVENTION 
The present invention relates in general to semiconductor device 
fabrication and, more particularly, to uniform plating of metal bumps on 
the surface of a semiconductor wafer. 
With recent advances in semiconductor fabrication technology, transistors 
in the submicron range have allowed a very large number of transistors to 
be formed on one semiconductor device. With the large number of 
transistors and corresponding circuit nodes, the semiconductor device 
often requires a large number of electrical contacts to pass electrical 
signals to and from the device. 
The electrical contacts are commonly formed by using pin grid arrays (PGA), 
or ball grid arrays (BGA). The array of contacts may be formed by plating 
an array of metal bumps on the semiconductor device while still in wafer 
form. Ideally, the plating process to form the metals bumps should produce 
metal bumps of uniform height and flat-end surface in order to make good 
bonding to a device package, such as flip-chip or tape automated bonding 
(TAB). The plating process should avoid over-plating and flow effects 
which can cause shorts between the metal bumps. 
In the prior art, for example as described in U.S. Pat. No. 5,000,827, the 
plating process may occur in a plating container having an open top and an 
inlet at the bottom. The semiconductor wafer is suspended over the top of 
the plating container. Predetermined sites of the semiconductor wafer 
where the metal bumps are to be formed are patterned by a photolithography 
process to mask all areas but the plating sites. A round metal anode plate 
with an array of openings evenly spaced about its entire surface, or in 
the form of a mesh, is placed typically at the bottom of the plating 
container over the plating solution inlet. A voltage source is applied 
between the metal anode plate and the semiconductor wafer. The plating 
solution circulates through the inlet and anode plate throughout the 
container whereby the plating solution is deposited at the predetermined 
sites on the semiconductor wafer by an electroplating process. 
A uniform flow is important during the electroplating process to provide 
even deposition of the metal ions from the plating solution onto the 
plating sites. Unfortunately, the prior art arrangement does not 
consistently result in the desired uniform height of the metal bumps in 
part because of the non-uniform flow of plating solution in and around the 
semiconductor wafer. The placement of openings in the anode plate and its 
position at the bottom of the plating container cause a turbulence in the 
plating container resulting in the uneven flow of plating solution. 
As the plating solution enters the plating container, it flows directly 
through the mesh network of the anode. The plating solution flow rate is 
greater nearer to the center of the inner cavity than it is around the 
walls of the container. That is, the plating solution flows straight up 
through the center of the container, strikes the semiconductor wafer, 
flows laterally toward the walls, and then circulates back down the walls 
toward the bottom of the contain causing the turbulent flow. The flow rate 
near the center of semiconductor wafer is thus greater than in the 
peripheral region as the plating solution velocity slows down. Thus, the 
non-uniform flow rate of the plating solution as it flows laterally across 
the plating sites results in non-uniform height in metal bumps. 
Furthermore, the prior art plating process has problems with shorts 
between the closely positioned metals bumps. The manufacturing defects 
result in rejection of defective semiconductor wafers and subsequent 
increase in manufacturing cost. 
Hence, a need exists for a plating process that produces isolated bumps of 
uniform height.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, a plating apparatus 10 is shown for fabricating metal 
bumps on a semiconductor wafer or substrate by an electroplating process. 
An electroplating solution 11 contained in plating tank 12 is drawn 
through return pipe 13 by pump 14. The plating solution is pumped through 
filter 16 to remove solid particulate contaminants. Flow meter 18 monitors 
the flow rate of the plating solution. The plating solution is delivered 
into the inner cavity of plating cell 20 by way of inlet 22. The plating 
solution flows through anode plate 24 and circulates in and about plating 
cell 20 in a uniform manner. The plating solution spills over the top edge 
of plating cell 20 back into plating tank 12. Plating tank 12 acts as a 
reservoir for excess plating solution. It is important that the excess 
plating solution maintain a minimum concentration of ions which are 
continuously removed during the plating process. 
To understand the plating process, plating sites must first be formed in 
specific locations on semiconductor wafer 32. The plating sites are formed 
by a photolithography process where a non-conductive photoresistive 
material is patterned to mask all areas but the plating sites so as to 
allow only specific areas to be plated. Metal bumps 30 are formed on the 
plating sites by passing an electrical current through the electroplating 
solution which is composed of dissolved ions of the material to be plated. 
Some examples of the plating material is indium, copper, tin, and gold. 
Conductor 34 of power supply 36 is coupled to semiconductor wafer 32 for 
providing a first power supply potential to negatively charge the wafer. 
Conductor 38 of power supply 36 is coupled to anode plate 24 for providing 
a second power supply potential to positively charge the anode plate. The 
potential difference between conductors 34 and 38 is approximately 1.2 
volts. Power supply 36 causes a current (electrons) to flow taking 
positively charged metal ions from anode plate 24 to the negatively 
charged plating sites on semiconductor wafer 32 to by way of electrical 
conduction through the plating solution. The electrons are transferred to 
positively charged metal ions at the plating sites by reduction and become 
neutral. Metal ions in the plating solution are thus reduced to a metallic 
state and deposited at the plating sites of semiconductor wafer 32 to form 
metal bumps 32 by a process of electrodeposition, or plating. 
In one method, the transfer of electrons from the anode surface is called 
oxidation. The anode material itself oxidizes and dissolves away as the 
metal becomes ionic. Anode plate 24 is made of the same material as that 
to be plated. Portions of anode plate 24 are corroded away by the plating 
solution to maintain the correct solution concentration. The ionic metal 
from anode plate 24 conducts away to the plating sites of semiconductor 
wafer 32 where it is reduced to the metallic state as metal bumps 30. The 
oxidation method is very practical in that the concentration of metal ions 
remains substantially constant, so the plating solution requires little 
maintenance. 
In some cases such as plating of precious metals, e.g. gold, where it is 
too expensive to use a corrodible anode plate. In this case, anode plate 
24 is made of a conducting material such as titanium or stainless steel, 
chosen for its inertness to the plating solution. Again, the positively 
charged metal ions are attracted to the negatively charged plating sites 
by way of electrical conduction through the plating solution. The 
electrons are transferred to positively charged metal ions at the plating 
sites by reduction and become neutral. The reduced metal atoms are 
deposited on the plating sites to form metal bumps 30. The metal in the 
plating solution must be periodically replaced. Solution concentration 
must be maintained by periodic analysis and specified additions of metal 
ions. The plating solution has a precious metal concentration of say 1.5 
troy ounces per gallon. 
In an alternate embodiment, conductive polymers may be electroplated to 
metal bumps 30. The polymers are reduced at anode plate 24. In this case, 
semiconductor wafer 32 operates as an anode and anode plate 24 functions 
as the cathode. 
The metal ions travel through the solution at a speed relative to the 
chemical and physical properties of the solution. The attraction of the 
negatively charged cathode increases the speed of ions near the 
semiconductor wafer. Eventually there becomes regions in the plating 
solution near the point of deposition that are depleted of metal ions for 
deposition. Typically the depletion is renewed by agitation. In the 
present embodiment, the replenishment is accomplished by forcing the 
plating solution through the cell towards the semiconductor wafer. 
Allowing the plating solution to flow through anode plate 24 improves the 
efficiency by keeping the ions in a more coherent path. The amount of 
metal to be deposited can be very closely approximated by use of Faraday's 
law which relates the weight of the metal to deposited to the amount of 
current supplied and the duration of time in which the current is on. 
Exact thicknesses are determined from the metal density and optimized 
experimentally. 
The placement of metal atoms during plating is also effected by the flow 
path of the solution. The deposition has a tendency to follow the path the 
solution takes as it moves across semiconductor wafer 32, detrimentally 
effecting the shape of the bumps as they grow. Computer modeling is 
employed to determine the positions of the various parts of plating cell 
20 which give an optimum plating solution flow path. It was determined 
that anode plate 24 may be raised a certain distance, say 0.5 inches above 
inlet 22 in plating cell 20 to create a regulation zone between inlet 22 
and anode plate 24 that regulates the dispersion and spread of the path of 
the solution and thus creates a more even solution velocity through 
plating cell 20 towards semiconductor wafer 32. 
A part of the present invention, anode plate 24 rests on anode supports 28 
to position anode plate 24 a predetermined distance from inlet 22. The 
plating solution passes through openings in anode plate 24 creating a flow 
regulation or dispersion zone between inlet 22 and anode plate 24. The 
proper placement of anode plate 24 by way of anode supports 28 achieves a 
uniform flow of the plating solution about the plating sites of 
semiconductor wafer 32 where metal bumps 30 are formed. The uniform flow 
of plating solution during the electroplating process creates 
substantially uniform height in metal bumps 30 while minimizing electrical 
shorts between the metal bumps. The metals bumps should be of uniform 
height and flat-end surface in order to make good bonding to a device 
package, such as flip-chip or tape automated bonding. 
A variety of plating solutions may be used and typically operate optimally 
at different flow rates depending on application and chemistry. Because of 
the possible variations in process, the regulation or dispersion zone 
within the cavity of plating cell 20 must be adjustable. The distance 
between inlet 22 and anode plate 24 is determined by computer simulation 
modeling to optimize the flow path for a given parameters of plating 
solution, plating cell volume, plating solution velocity, and anode 
design. Anode plate 24 is placed at a variety of positions for a given set 
of parameters. The computer simulation monitors flow rate and adjusts the 
distance between inlet 22 and anode plate 24 until an optimally uniform 
flow of the plating solution within plating cell 20 is achieved. For 
example, for a cylindrical plating cell having a volume of 570 milliliters 
with a plating solution containing indium and with a pump speed of four 
liter/minute, the proper distance between inlet 22 and anode plate 24 has 
been determined to be 0.5 inches. 
The adjustment is accomplished by changing the height of the 
regulation/dispersion zone, that is the space between inlet 22 and anode 
plate 24 using anode supports 28 of specific heights. Anode supports 28 
may be a ring that fits inside plating cell 20 having a diameter slightly 
less than the diameter of plating cell 20 to allow ease of insertion or 
removal. The fit must be sufficiently snug so as to remain in place and 
provide support for anode plate 24. Anode supports 28 are made of 
compressible and chemically resistant material such as polypropylene 
plastic or teflon. The anode support ring can be either of the exact 
height required, or several thinner ones can be stacked together to 
achieve the desired height and thereby optimize the flow regulation. 
As the height of anode plate 24 within plating cell 20 increases, the 
distance between anode plate 24 and semiconductor wafer 32 decreases. To 
provide sufficient distance between anode plate 24 and semiconductor wafer 
32, plating cell extension 26 is added onto the sidewalls of plating cell 
20. The height of plating cell extension 26 is selected to optimize the 
uniform flow of the plating solution. For example, a certain volume of 
plating solution is required in the plating cell to alleviate depletion of 
metal ions. Since the diameter of the semiconductor wafer dictates the 
diameter of the plating cell, volume can only be increased by increasing 
the length of the cell. Alternately, the cell length must be increased to 
change the flow pattern. Therefore, if the anode height is raised to 
increase the regulation zone, the plating cell itself must be extended. 
In general, the distance between anode plate 24 and semiconductor wafer 32 
is determined by computer modeling to optimize the flow path for a given 
plating solution. For example, the distances between the anode plate and 
the semiconductor wafer in fountain-style platers is typically selected at 
2.5 inches. 
It is further important to note that the plating cell with solution is part 
of the electrical circuit formed by the cathode area, anode area and 
solution resistance within the plating cell determine the total resistance 
of the cell. The solution resistance is itself a function of the 
conductivity of the solution, and the length and diameter of the solution 
within the cell. The conductivity of the solution is fixed by the 
requirements of the process chemistry. Every plating process has a 
specific voltage limit (electrode reduction potential) which must be met 
in order for plating to occur. If the cell potential is too low, plating 
will be very inefficient or probably not occur. If the potential is too 
high, unwanted side reactions may occur which could be detrimental to the 
process. 
By Ohm's Law, it can be seen that in order to maintain a specific plating 
voltage, at a specified current, resistance is the only condition 
(parameter) which can be changed. Within the total resistance of the 
plating cell, the only factor which can be easily changed is the distance 
from the anode plate to the semiconductor wafer, or cell length by use of 
plating cell extension 26. Extensions should be of the same material as 
the anode support and plating cell. They are fabricated so as to stack on 
top of each other and the main body of the plating cell. 
Turning to FIG. 2, anode plate 24 is shown in further detail. The center of 
anode plate 24 is solid to block the direct path of plating solution as it 
flows from inlet 22 into plating cell 20. Anode plate 24 includes 
concentric rings of openings close to its perimeter to pass the plating 
solution. One set of openings are placed in locations around anode plate 
24 approximately half the distance between the center and its edge, and 
another set nearer to the perimeter. The openings can cut as round holes, 
or semi-circular slots, but the center should be solid to provide a 
barrier over the plating inlet. For a four inch diameter anode plate, the 
openings are each 0.25 inches in diameter. The spacing between openings of 
the outer concentric ring is approximately 0.25 inches. The spacing 
between openings of the inner concentric ring is approximately 0.5 inches. 
In contrast to the prior art as described in the background, the present 
invention blocks the direct flow of plating solution entering plating cell 
20 by way of inlet 22. The plating solution is deflected from its center 
path in line with inlet 22 and forced to flow toward the perimeter of 
anode plate 24 where it exits through the anode openings into the cavity 
of the plating cell. The plating solution then flows up toward 
semiconductor wafer 32 with a uniform flow rate in all region of the 
plating cell. When the plating solution reaches the semiconductor wafer, 
it has a uniform flow rate at each plating site. The electroplating thus 
creates metal bumps of uniform height while minimizing electrical shorts. 
The combination of blocking direct flow of the plating solution with 
proper placement of the anode plate from the inlet achieves the desired 
uniform flow of plating solution in and about the plating sites of the 
semiconductor wafer. 
The present invention is especially useful in plating metals such as indium 
as the metal bumps on the semiconductor wafer 32. Indium is a metal known 
for difficulty in obtaining uniform bumps, prior to this method plated 
indium bumps tended to be much thicker on the sides corresponding the 
direction of flow, creating non-uniform processing and electrical shorts. 
In another embodiment, a combination of conductive inert anode material 
with an inert device in the shape described in FIG. 2 could also be used 
in certain applications such as precious metals. In conjunction with the 
anode height of the regulation zone, the position of the openings in the 
anode plate also effect on the solution flow path. 
By now it should be appreciated that the present invention provides for the 
fabrication of electrically conducting bumps used in manufacture of 
semiconductor devices. The apparatus is used to control and ensure uniform 
shape and height of electroplated bumps plated across the semiconductor 
wafer or substrate. The anode plate is selected with one set of openings 
half way between its center and perimeter and another set nearer to the 
perimeter. The center of the anode plate is solid center to block direct 
flow of the plating solution from the inlet into the plating cell. By 
properly adjusting the relative positions between the solution inlet 
plane, the anode plate, and semiconductor wafer, depending on the plating 
material selected, uniform metal bumps can be formed on the plating sites 
of the semiconductor wafer. 
While specific embodiments of the present invention have been shown and 
described, further modifications and improvements will occur to those 
skilled in the art. It is understood that the invention is not limited to 
the particular forms shown and it is intended for the appended claims to 
cover all modifications which do not depart from the spirit and scope of 
this invention.