U.S. patent application number 13/065949 was filed with the patent office on 2011-11-10 for boric acid replenishment in electroplating baths.
This patent application is currently assigned to ECI Technology, Inc.. Invention is credited to Peter Bratin, Eugene Shalyt.
Application Number | 20110272289 13/065949 |
Document ID | / |
Family ID | 44901224 |
Filed Date | 2011-11-10 |
United States Patent
Application |
20110272289 |
Kind Code |
A1 |
Shalyt; Eugene ; et
al. |
November 10, 2011 |
Boric acid replenishment in electroplating baths
Abstract
Boric acid is replenished in an electroplating bath via a
replenishment solution comprising boric acid dissolved in pure
water, in which the solubility at room temperature is comparable to
that in the plating bath at operating temperature. The
replenishment solution may be used to replace all or part of the
water lost by evaporation. An automated device may be used to
replenish boric acid in the electroplating bath.
Inventors: |
Shalyt; Eugene; (Washington
Township, NJ) ; Bratin; Peter; (Flushing,
NY) |
Assignee: |
ECI Technology, Inc.
Totowa
NJ
|
Family ID: |
44901224 |
Appl. No.: |
13/065949 |
Filed: |
April 2, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61395290 |
May 10, 2010 |
|
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|
Current U.S.
Class: |
205/271 ;
204/275.1 |
Current CPC
Class: |
C25D 21/14 20130101 |
Class at
Publication: |
205/271 ;
204/275.1 |
International
Class: |
C25D 21/14 20060101
C25D021/14; C25D 3/12 20060101 C25D003/12 |
Claims
1. A method of replenishing boric acid in a plating bath,
comprising the steps of: providing a replenishment solution
comprising a predetermined concentration of boric acid dissolved in
substantially pure water; and adding a predetermined volume of the
replenishment solution to the plating bath so as to maintain the
concentration of boric acid in the plating bath substantially at a
target boric acid concentration.
2. The method of claim 1, wherein the plating bath is a nickel
plating bath operated in the temperature range from 30 to
65.degree. C.
3. The method of claim 1, wherein the predetermined concentration
of boric acid in the replenishment solution is in the 30-45 g/L
range.
4. The method of claim 1, wherein the predetermined concentration
of boric acid in the replenishment solution is equal to or greater
than the target boric acid concentration in the plating bath.
5. A method of replenishing boric acid in a plating bath,
comprising the steps of: providing a replenishment solution
comprising a predetermined concentration of boric acid dissolved in
substantially pure water; and flowing the replenishment solution
from a replenishment solution reservoir into the plating bath at a
predetermined flow rate so as to maintain the concentration of
boric acid in the plating bath substantially at a target boric acid
concentration.
6. The method of claim 5, wherein the plating bath is a nickel
plating bath operated in the temperature range from 30 to
65.degree. C.
7. The method of claim 5, wherein the predetermined concentration
of boric acid in the replenishment solution is in the 30-45 g/L
range.
8. The method of claim 5, wherein the predetermined concentration
of boric acid in the replenishment solution is equal to or greater
than the target boric acid concentration in the plating bath.
9. An apparatus for replenishing boric acid in a plating bath,
comprising: a reservoir containing a replenishment solution
comprising a predetermined concentration of boric acid dissolved in
substantially pure water; a means of transferring a predetermined
volume of the replenishment solution from the reservoir to the
plating bath; and a computing device having a memory element with a
stored algorithm operative to effect, via appropriate interfacing,
transfer of the predetermined volume of the replenishment solution
from the reservoir to the plating bath so as to maintain the
concentration of boric acid in the plating bath substantially at a
target boric acid concentration.
10. The apparatus of claim 9, wherein the computing device is
further operative to determine the predetermined volume of the
replenishment solution to be added to the plating bath based an
analysis of the boric acid concentration in the plating bath.
11. The apparatus of claim 9, wherein the computing device is
further operative to determine the predetermined volume of the
replenishment solution to be added to the plating bath based on a
volume of water lost from the plating bath by evaporation.
12. The apparatus of claim 9, further comprising: a bath level
indicator.
13. The apparatus of claim 9, further comprising: a means of adding
a predetermined volume of substantially pure water to the plating
bath.
14. The apparatus of claim 13, wherein the computing device is
further operative to determine the predetermined volume of
substantially pure water to be added to the plating bath based on
the predetermined volume of the replenishment solution and a volume
of water lost from the plating bath by evaporation.
15. The apparatus of claim 9, wherein the memory element is
selected from the group consisting of computer hard drive,
microprocessor chip, read-only memory (ROM) chip, programmable
read-only memory (PROM) chip, magnetic storage device, computer
disk (CD), digital video disk (DVD), and combinations thereof.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority to U.S. Provisional
Application No. 61/395,290 filed May 10, 2010, which has the same
inventors.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This invention is concerned with control of electroplating
processes, and in particular with replenishing buffering agents in
electroplating baths.
[0004] 2. Description of the Related Art
[0005] Nickel electroplating baths are widely used in industry to
fabricate a wide variety of products ranging from microelectronic
integrated circuit (IC) chips to rocket engines. Sulfamate baths
typically comprise 300-450 g/L nickel sulfamate
[Ni(SO.sub.3NH.sub.2).sub.2], 0-30 g/L nickel chloride
[NiCl.sub.2.6H.sub.2O] and 30-45 g/L boric acid [H.sub.3BO.sub.3],
and are operated at a pH in the 3.5-5.0 range and a temperature in
the 30-60.degree. C. range. Watts nickel baths typically comprise
225-400 g/L nickel sulfate [NiSO.sub.4. 6H.sub.2O], 30-60 g/L
nickel chloride and 30- 45 g/L boric acid, and are operated at a pH
in the 2-4.5 range and a temperature in the 45-65.degree. C. range.
Either type of nickel bath may comprise nickel bromide instead of
nickel chloride. Boric acid functions as a buffering agent to
prevent pH excursions at the cathode surface that can degrade the
quality of the electrodeposit. Note that the hydrogen evolution
side reaction during the nickel electrodeposition process consumes
protons and tends to raise the pH of the solution near the cathode,
which can result in formation of nickel hydroxide particulates.
Both types of nickel electroplating baths may also contain wetting
agents and/or organic additives.
[0006] Replenishment of boric acid lost from the bath via dragout
with the plated parts, chemical/electrochemical decomposition and
precipitation associated with water evaporation presents a problem
since the solubility of boric acid in the bath at room temperature
is only about 30 g/L whereas the preferred boric acid concentration
is typically 30-45 g/L. The lower solubility in the bath at room
temperature generally precludes use of bath concentrates for boric
acid replenishment. Furthermore, boric acid dissolves very slowly
in the plating bath even at elevated temperatures so that
replenishment by addition of solid boric acid to the bath tends to
disrupt production and may introduce very small boric acid
particles that can degrade the deposit properties.
[0007] One currently-used approach to addressing the problem of
boric acid replenishment is to hang one or more cloth bags
containing boric acid in the bath so that boric acid is
continuously leached into the bath. A difficulty in this case is
that the relatively coarse cloth mesh size needed to provide a
sufficient leach rate may allow finer boric acid particles
generated during boric acid dissolution to enter the bath. This
approach also does not provide adequate control of the
replenishment rate.
SUMMARY OF THE INVENTION
[0008] The invention provides a method and an apparatus for
replenishing boric acid in an electroplating bath, a sulfamate
nickel or a Watts nickel bath, for example. In the method of the
invention, boric acid is dissolved in substantially pure water to
provide a replenishment solution that is added to the
electroplating bath to compensate for boric acid losses due to
dragout, precipitation resulting from water evaporation, and
chemical and electrochemical decomposition, for example. Since the
solubility of boric acid in pure water at room temperature is more
than 45 g/L, the boric acid concentration in the electroplating
bath can readily be maintained at even 45 g/L via addition of the
replenishment solution to compensate for water lost by
evaporation.
DETAILED DESCRIPTION OF THE INVENTION
[0009] Technical terms used in this document are generally known to
those skilled in the art. The terms "electroplating" and "plating"
are equivalent.
[0010] The invention provides a method and an apparatus for
replenishing boric acid in an electroplating bath. The invention is
particularly useful for control of plating baths that operate at an
elevated temperature, sulfamate nickel baths, for example, which
are typically operated in the temperature range from 30 to
60.degree. C.
[0011] In one embodiment, the method of the invention for
replenishing boric acid in an electroplating bath, comprises the
steps of: providing a replenishment solution comprising a
predetermined concentration of boric acid dissolved in
substantially pure water; and adding a predetermined volume of the
replenishment solution to the plating bath so as to maintain the
concentration of boric acid in the plating bath substantially at a
target boric acid concentration. In the simplest case, the
replenishment solution is used to compensate for both dragout of
the plating bath and water lost by evaporation so that the
predetermined volume of the replenishment solution added to the
plating bath is equivalent to the decrease in the volume of the
plating bath volume during a given time interval. In this case,
sufficient replenishment solution is added periodically,
continuously or intermittently to maintain the plating bath level
substantially constant, based on an index mark or a bath level
indicator, for example. The predetermined volume of the
replenishment solution to be added to the plating bath may also be
determined based on analysis of the boric acid concentration in the
plating bath, measurement of the liquid level in the plating bath
tank, or preferably both. Alternatively, the predetermined volume
of the replenishment solution to be added to the plating bath may
be estimated based on product throughput (amp-hours of charge)
during a given time interval, water evaporation rate for the
particular plating bath tank, or preferably both. The boric acid
replenishment solution may be used to compensate for all or part of
the water lost from the plating bath by evaporation.
[0012] The predetermined .concentration of boric acid in the
replenishment solution may be any concentration consistent with the
replenishment scheme used but will typically be in the 30-45 g/L
range. It may be advantageous for the concentration of boric acid
in the replenishment solution to be equal to or greater than the
target boric acid concentration in the plating bath.
[0013] In another embodiment, the method of the invention for
replenishing boric acid in an electroplating bath, comprises the
steps of: providing a replenishment solution comprising a
predetermined concentration of boric acid dissolved in
substantially pure water; and flowing the replenishment solution
from a replenishment solution reservoir into the plating bath at a
predetermined flow rate so as to maintain the concentration of
boric acid in the plating bath substantially at a target boric acid
concentration. In the simplest case, the replenishment solution is
used to compensate for both dragout of the plating bath and water
lost by evaporation so that the predetermined flow rate of the
replenishment solution, which may be variable or intermittent, is
sufficient to maintain the plating bath level substantially
constant. The predetermined flow rate of the replenishment solution
may also be determined based on analysis of the boric acid
concentration in the plating bath, measurement of the liquid level
in the plating bath tank, or preferably both. Alternatively, the
predetermined flow rate of the replenishment solution may be
estimated based on product throughput rate (amp-hours of charge),
water evaporation rate for the particular plating bath tank, or
preferably both. The boric acid replenishment solution may be used
to compensate for all or part of the water lost from the plating
bath by evaporation.
[0014] The apparatus of the invention for replenishing boric acid
in a plating bath, comprises: a reservoir containing a
replenishment solution comprising a predetermined concentration of
boric acid dissolved in substantially pure water; a means of
transferring a predetermined volume of the replenishment solution
from the reservoir to the plating bath; and a computing device
having a memory element with a stored algorithm operative to
effect, via appropriate interfacing, transfer of the predetermined
volume of the replenishment solution from the reservoir to the
plating bath so as to maintain the concentration of boric acid in
the plating bath substantially at a target boric acid
concentration. The predetermined volume of the replenishment
solution is preferably transferred from the reservoir to the
plating bath via a metering pump, but could be transferred
manually, or by any other suitable means. Preferably, the computing
device is further operative to determine the predetermined volume
of the replenishment solution to be added to the plating bath based
an analysis of the boric acid concentration in the plating bath.
The boric acid analysis may be performed manually or automatically
using methods and equipment that are known in the art.
[0015] The apparatus of the invention for replenishing boric acid
in a plating bath, may further comprise: a means of adding a
predetermined volume of substantially pure water to the plating
bath. In this case, the computing device preferably is further
operative to determine the predetermined volume of substantially
pure water to be added to the plating bath so as to maintain the
concentration of boric acid in the plating bath substantially at
the target boric acid concentration. The computing device may
determine the predetermined volume of substantially pure water to
be added to the plating bath based on the predetermined volume of
the replenishment solution added to the plating bath and a volume
of water lost from the plating bath by evaporation during a given
time interval.
[0016] The apparatus of the invention for replenishing boric acid
in a plating bath, may further comprise: a liquid level indicator,
whereby the volume of the plating bath lost during a given time
interval may be determined. The liquid level indicator may also be
used to provide feedback for maintaining the bath volume at a
predetermined value. Suitable mechanical, electrical and optical
liquid level indicators are known in the art.
[0017] The memory element may comprise any suitable type, including
a computer hard drive, a microprocessor chip, a read-only memory
(ROM) chip, a programmable read-only memory (PROM) chip, a magnetic
storage device, a computer disk (CD), a digital video disk (DVD),
and combinations thereof. The memory element may be separate or be
included in the computing device.
DESCRIPTION OF A PREFERRED EMBODIMENT
[0018] A typical sulfamate nickel bath for plating semiconductor
wafers comprises 30-45 g/L boric acid and operates at 50.degree.
C., and loses more water to evaporation (up to 15 liters/day) than
to dragout (typically 0.5-1.0 L for 100 wafers/day). By using a
replenishment solution comprising 45 g/L boric acid to replace all
or a portion of the water lost to evaporation, all boric acid
losses, including those due to precipitation and decomposition, can
readily be compensated so as to maintain the boric acid
concentration in the bath at a target value. Boric acid is
preferably replenished using a combination of relatively
concentrated replenishment solution and substantially pure water,
or a less concentrated replenishment solution.
[0019] As an example, consider a nickel plating bath that is
operated at 50.degree. C. and 45 g/L boric acid and is used to
plate 100 semiconductor wafers per day. Each wafer typically drags
in a volume of rinse water equal to the volume of plating bath that
it drags out so that the effect of dragout on the total volume of
the plating bath is negligible. For a typical dragin/dragout rate
of 10 mL per wafer and a throughput of 100 wafers per day (dragout
volume of 1000 mL per day), 45 g of boric acid are lost per day to
dragout and must be replenished without increasing the total volume
of the plating bath. This can be accomplished according to the
invention by adding 1000 mL per day of a replenishment solution
comprising 45 g/L boric acid to partially compensate for 1000 mL of
the water lost to evaporation, which is typically 3000 mL per day.
The remaining 2000 mL per day of evaporated water may be
replenished by addition de-ionized water and/or concentrates for
replenishment of other plating bath constituents depleted by
dragout.
* * * * *