Note: Descriptions are shown in the official language in which they were submitted.
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501-175
: PROCESS AND PROGRAMMABLE COMPUT~R CONTROLLED
SYSTEM FOR ~LECTROL~SS COPPER PLATING_
1. Field of tbe In,vention.
This invention relates to electroless copper plat-
ing baths~and improved copper plated substrates, and ismore particularly concerned wi~h a process and a program-
mable;computer controlled system for electroless copper
plating and with substrates coated with copper using said
~ syst~m.
:~ 10 2. Back~ound of the Invq,n,tion.,
Electroless copper plating of substrates such a~ :
printed circuit:boards and the like is a well known
process. Electrole~s copper plating baths contain a
sourc~:o~ cupric ions, generally coppex sulfate, together
with a reducing agent such ae formaldehyde, a complexing
agent for the cupric ion o~ which ethylenediamine tetra
acetic acid (EDTA) ie typical, sodium hydroxide to
:con~rol pH, and various additives such as surfactant~ and
stabilizing agents. ~ sta~ilizing agent which is common-
20 ly used is a ~ource o~ cyanide ions such as sodiumcyanide.
Underkofler et al U.S. Patent 3,844,799 teach
that, by maintaining the cyanide ion concentration of an
electroles~ copper pla~ing bath between 0.0002 and 0.0004
molar, and at the same time maintaining the temperature
of th~ bath between 70C. and 80C., it is poéeible j,
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to improve the propertie~ of the depo~ited copper layer.
The range of concentration of cyanide ion called for by
this teaching is sufficiently broad to permit variations
of up to 100 percent.
No details are given in the above patent as to how
the concentration of cyanide ions was monitored in order
to make additions to keep the concentration in the
required range. A common method of analysis for cyanide
ions employs a cyanide ion specific electrode and
measure~ the rate at which silver metal is removed from a
membrane in contact with the solution containing the
cyanide ion. The accuracy of this method is affected by
the presence of ions which may be present in the bath as
additives or impurities. ~or example, the presence of
ions such as sulfide, halide, thiocyanate or any ion
which form~ an insoluble complex with silver, can cause
significant interference with an accurate determination
of the actual cyanide concentration, particularly where
the latter is at a level as low as 0.0002 molar. Other
method~ of analysis include ion chromatography, polarog-
raphy, colorimetric, kjeldahl, and amperometric titra-
tion. In general these methods lack the required
accuracy andior are slow, i.e. re~quire significant time
to carry out thereby causing significant delay in correct-
ing a deficiency of cyanide ion in the bath.
This invention is ba~ed on the finding that if thep~, cupric ion and reducing agent concentration~ are
maintained at constant levels in accordance with norm~l
practice in the art, the rate at which the cyanide ion is
consumed is directly related to the temperature at which
the ba~h is maintained. The ra~e o~ consumption may be
di~ferent in baths having different ~ompo~itions, but, in
the case of any given bath wherein the above named para-
meter~ are maintained con~tant, the rate of cyanide ion
consumption at a given temperature is directly related to
said temperature and is reproducible, i~e. the rate at a
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given temperature sho~s ~ub~tantially the same pattern in
all haths which have the same chemical compo~ition.
Accordingly, on the basis of the foregoing find-
ings, for a given bath at a given temperature it is
possible to predict how much cyanide ion m~st be added to
the bath at any given elapsed time in order to restore
the cyanide ion concentration in the bath to the initial
desired level. Therefore, once the relationship of rate
of cyanide ion consumption at a chosen bath temperature
has been determined, it becomes unnecessary to do routine
cyanide analysis of that bath, or any other bath of the
same chemical composi~ion, during the operating life of
the bath in order to determine how much cyanide ion must
be added at any giYen time to maintain a desired cyanide
ion concentration.
Based on the above findings r it has become
possible in accordance with the invention to develop a
sys~em in which the cyanide ion concentration in an
electroless copper plating bath can be maintained substan-
tlally constant over a prolonged working period withoutrecourse to repeated analysis for cyanide ion in the
bath. In consequence, significant variation in cyanide
ion content, which has been a feature of prior sy~tems
due to the time cequired for analysi of ali~uots and
resulting delay in correcting the cyanide ion concentra-
tion after a deficiency is determined, has been
eliminated. Fuc~her, ag will be de~cribed in detail
below, the above findings have made po6sible the develop-
ment of an automated system to maintain the desired level
of cyanid~ ion concentration in an electroless copper
plating bathO
It has also been found that the copper layer
depo~ited on a subatrate in an electroles~ plating bath
operated in accordance with the in~ention exhibits
markedly improved physical proper~ies compared to a
copper layer deposited using the same bath but operated
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~ithout the benefit of the presen~ inventionO Likewi~e,
: the stability and plating rate of the bath are far more
- uniform leading to potentially longer bath life and a
more consistent productO
SUMMARY OF T~E INVENTION
The invention, in its broadest aspect, comprises a
: process for the electroless deposition of copper onto a
substrate surface from an electroless copper depositing
bath comprising cupric ions, a reducing agent, a complex-
ing agent and a stabilizing agent comprising cyanide
ions, which process comprises the step~ of:
(a) establi6hing, for fixed values of cupric ion
concentration, reducing agent concentration, temperature
and pH of the bath, a characteristic relationship between
cyanide ion con~umption in the bath and elapsed time of
operation of the bath;
~b~ substantially uniformly maintaining the bath
at the fixed value of temperature
~c) sub~tantially uniformly maintaining the
concentration of cupric ion, the concen~ration of reduc-
ing agent and the pH of the bath at the fixed values
thereo~; and
~d) adding to the bath over the course of copper
depo~ition therefrom an amount of cyanide ion su~icient
to æubstantially replace the cyanide ion consumed in the
bath based on the characteristic relationship established
in (a) above, the additions being carried out at time
intervals'su~ficient to sub~tantially uniformly maintain ~.
the cyanide ion concentration in the bath at the predeter
mined level over the cour e of copper depo~ition from the
bath.
The invention, in a particular aspect, comprises a
computer controlled sy~tem for carrying out ~he procesE
of the inventionl said system comprising in combination:
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an electroles~ copper plating bath compris~
ing cupric ions, a reducing agent, a complexing agent and
a stabilizing agent comprising cyanide ions, said bath
also comprising heat control means for adjusting the
temperature thereof;
storage means containing cyanide ion feed-
stock and pump means associated wi~h said storage mean~
for delivering cyanide ions via a feed line to said
plating bath;
programmable computer means for monitoring
the temperature of said plating bath at a predetermined
level and for controlling the addition of predetermined
or calculated amounts of cyanide ion at predetermined
intervals, said computer means including:
(a) means for producing base reference
signals characteristic of a predetermined bath
temperature:
(b) sensing means for producing signals
characteristic of the tempera~ure of said bath,
comparator means producing signals responsive to a
difference between ~aid signals so produced and
said base reference signalE~, and means sensitiva
to said comparator response to cause said heat
control means in said bath to make corrective
adiustment to the temperature of said bath; and
(c) means for producing at predetermined
time intervals signals characteristic of predeter-
mined or calculated amounts of cyanide feedstock
to be added to said bath at said predetermined
time intervals and means sen~itive to said signals
to activ~te said pump means to deliver each of
said predetermined or calculated amounts at each
of said inter~als.
The invention also comprises a method of electro-
le~ deposition of copper on a substrate using the above
recited process and system and the improved stability and
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rate uniformity of the bath and properties of the copper
layer so deposited.
; BRIEF DESCRIPTION OF THE DRAWINGS
FIGURE 1 show~ a plot of loss of cyanide ion
versus temperature of a typical electroless copper
plating bath.
FIGURE 2 shows a schematic rendering of a typical
computerized control system in accordance with the inven-
tion~
FIGURE 3 show~ in schematic form a breakdown of
the components of the computer means shown in FIG~R~ 2.
DETAILED DESCRIPTION OF T~E INVENTION :
The cyanide ion CN- ha~ an exceptional ability
to complex and sequester monovalent copper, so much so
that it can even strip a chelating agent such as ~DTA
from a monovalent copper-EDTA complex. The resultant
cuprous cyanide (CuCN) has such s'cability that it behaves
as an inert 6ubstance in the plating bath. The cyanide
ion reacts with monovalent copper so ~a~t that it will
20 often prevent other deR~abilizing reactions from occur-
: ring in an electroles~ copper plating bath. However, the
cyanide ion will also react with divalent copper in any
one of a number o~ ways which result in consumptiorl of
cyanide ion but do not csntribute to stabilizing of the
plating bath. The following are typical of these unde~ir-
able reactions with divalent copper, the cyanide being
u~ed in the form of potas~ium cyanide for purpo~es of
illu~;tration .
CuCN + 2 KCN~ ~ Cu(CN~3= + 2R+
CutCN)3= + KCN~ ~ Cu~CN)4= + K+
Cu++ ~ 2 KCN ~Cu~CN)2 + 2 K+
2 Cu(CN)2 - ~ 2 CuCN ~ 2 CN-
3 Cu(CN)2 ---~ CuCN.Cu(CN)2 + 3 CN-
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The above reactions and other related ones are the
primary cau~e of consumption of cyanide in the electro-
lesfi plating bathO The ra~e of consumption is related to
the concentration of monovalent and divalent copper, the
concentration of cyanide ion, the p~ of the bath, the
reducing agent (formaldehyde) concentration. and the bath
temperature. If the concentration of copper ion~ and
reducing agent and the p~ of the bath are each maintained
at a constant level during the operating life of the bath
as is the normal practice commercially, the rate of
con~umption is dependent only on the concentration of
cyanide ion and the temperature of the bath. A~ set
forth above, it ha~ been found that, or a bath having a
given compo~ition, ther~ is a direct relation between
rate o~ consumption and the tempera~ure of the bath.
Figure 1 shows a plot of lo~s of cyanide ion in
one hour versu~ bath temperature for a typical electro-
le~ plating bath having ~he following composition.
Part~ bv We iaht
2Q . cupric chloride5O25 -
formaldehyde 2.40
EDTA 20.00
s~dium cyanide0.0017
sodium hydroxideS.00
~urfactant 0,10
water to make 1000.00
The analyses for cyanide con~ent o~ the bath at
the variou~ time~ and temperature8 were carried out ulng
ion specific eleGtrode~ and by acidi~ication of aliquot~
followed by ~team di~tillation of hydrogen cyanide.
collection of the distillate in 8tandard alkali ~olutlon
and back-titration with 6tandard acid to determine the
amount of hydrogen cyanide 60 liberated.
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As noted previou~ly, the curve sho~n in Figure 1
is unique to a bath having the particular composition
shown above and is consistently shown by all baths having
that particular compo~ition. A similar but different
plot will be obtained from a bath having a different
composition.
Based on the data shown in Figure 1, a mathemati-
cal e~uation is derived from which loss of cyanide ion at
given in~ervals of time can be calculated for any
particular temperature of the bath from which the data
shown in the Figure wafi derived. This mathematical
equation is then used a~ the ba~is for determining the
amount and frequency with which cyanide ion mu~t be added ~ :`
to the bath in order to sub~tantially maintain the
cyanide ion concentration at a predetermined level
without the need to do any further analysis of ali~uots
of the ~ath in order to control the level of cyanide ion.
The addition of the cyanide ion at the requisite
intervals of time in accordance with the inventi~n can be
carried out manually. Ho~ever, in a particular and
~referred embodiment, the required additions are carried
out automa~ically using a computer controlled sy~tem.
Reference i8 made to Figure 2 which showfi in
schematic form a control ~ystem in accordance with the
invention. Electroless copper plating bath (2) is
pro~ided with temperatare con~roller t~)~ temperature
sensor probe6 (6) and (8), and a ~eed line (10) through
which cyanide ion feedgtock can be delivered from ~torage
vessel ~12) when pump ~14) is activated. The temperature
oP bath ~2) i8 maintained at a predetermined level by a
heater control system shown overall as (18). The various
components of the computer sy~tem (16) are illustrated
schematically in Figure 3 as will be discussed further
hereinafter. The control of bath temperature is effected
a~ follow~. At predetermined intervals signals character-
istic of the temperature of the bath ~2) are fed by
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sensor (6) to the heater control wherein a comparator
means compares tbe signals so received to ba~e reference
signals characteristic of the predetermined temperature
at which it is desired to maintain bath (2). If a
difference between the siynal~ received from ~ensor (Ç)
and the base reference signal6 is detected by the
oomparator, the latter generate~ signals characteristic
of the necessary adjustment to be made. These corrective
signals are u~ed by the heater control sy~tem (18) which
activates temperature control means (4) to allow the
necessary correction to be made to the bath temperature,
either up or down aR the case may be. Temperature sensor
(8) produces signalæ characteristic of the bath tempera-
ture and the~e signals are used to monitor the heater
control sy~tem (18) or proper operation. The above
described process of monitoring the temperature of the
bath is carried out continuously at very short intervals,
generally of about one ~econd, although the frequency of
monitoring can vary over a wi~e range and is not critical
to ~uccessful operation of the sy~tem.
In addition to monitoring the temperature of the
bath, the computer system (16) is also programmed to make
additions of cyanide ion at predetermined intervals of
time based on the mathematical re:Lationship between the
loæs of cyanide ion versus time for the operating life of
the particular bath at the operat:ing temperature which is
b~ing employed and monitored as described above. Timer
means incorporated in the computer sys~em cauEes signal~
- to be generated at predetermined periods of time which
signals are characteri#tic of the particular amount of
cyanide ion which i~ to be added at any particular period
of time. The signals 50 generated are fed to sensor
mean~ associated with pump ~14) to cause said pump to be
activated for a length of time sufficient to deliver the
predetermined or calculated amount of cyanide ion to bath
~2) via feed line (10~. Fluid ~low sensor means (20)
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located in the feed line (lO) between pump (14) and bath
~2) is activated by movement of fluid in the feed line
and generates signals which are fed to the computer (16)
and serve to confirm that pump (14) is delivering cyanide
ion feedstock when activated. The frequency with which
the additions of predetermined or calculated amounts of
cyanide ion are made can vary over a wide range. Advanta- -
geously, the additions are programmed to be made at inter-
vals of the order of o.nl to 120 minutes and preferably
of the order of about 1 minute to about 60 minutes.
Advantageou~ly, the temperature at which the bath
(2~ is maintained con~tant in accordance with the
invention is in the ranye of about 25C~ to about
75C. and, preferably, is within the ran~e of about
30C. to about 60C. However, the bath temperature
employed in the process o~ the invention is not critical
and the above range~ are given for purpo~es of illustra~
tion only. The optimum temperature in any giYen in~tance
can be determined by a proces~ o~ trial and error.
The level of concentration at which the cyanide
ion ~present typically in the form of the pota~sium or
~odium salt) i8 maintained i8 advantageously in th~ ranye
of about 0.1 to about lO0 ppm and, preferably, in the
range of about 0.5 to about 50 ppm~ However, the~e
concentration ranges are given for purposes of illustra-
tion only and are not limiting and the process of the
invention can be carried out with concentrations above or
below these ranges.
The system of the invention ha~ an additional
advantageou~ feature which centers around the significant
and unexpecte~ finding that the rate o~ lo~ of cyanide
ion i~ directly proportional to the actual concentration
of cyanide ion, i.e. the greater the concentration the
greater the rate of consumption. If the amount of
cyanide ion added at a given time is greater or les~ than
the amount actually needed at that par~icular time to
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maintain the de~ired level, the error i~ quickly correct-
ed because of the above factor. Thus, if the amount
added is in exces~ of that required, the increased level
of cyanide causes the rate of consumption to increa~e
accordingly and thereby quickly overcome the error.
~onversely, if the amount added is less than actually
required, the rate of consumption is lower than that at
the higher desired level of cyanide ion and the error is
again quickly corrected. This self-correcting effect is
; 10 an added and unexpected advantage in terms of the overall
control achie~ed by the system of the present invention.
Figure 3 shows in schematic form a breakdown of
the various components o~ the computer system shown
overall as 116) in Figure 2, the functioning of which
~ystem has been discussed above. As shown in Figure 3,
microproceæsor (22) serves to receive, proces~ and
transmit appropriate 6ignals characteristic of a variety
of functions. Timer means ~24) provide~ ~ignals to micro-
processor ~22) at preset intervals which ~ignals are
characteristic o~ the times at which one or other func-
tion of the system is to be activated. The R~M/ROM
component ~26) contains the so~tware for operation of the
system. The temperature monitoring component i8 prefer~
ably a cold junction compen~ated analog/digital convert-
ing temperature control system with track and holdfeatures WhiCh receives signals from the temperature
sensor ~8) lsee Figure 2~, converts these si~nals from
analog to digital mode, ~ran6mits the same to micro-
pro~e6sor (22~ for calculation o$ a temperature databa~e
u~d to determine cyanide confiumption. ~he~e signals are
also compared with base reference signal~ characteristic
of the desired temperature leYel ~nd moni~ors the proper
operation of the heater control system (18)~
Computer (22~ also generates signals, at predeter-
mined time intervals which are transmitted to pump
control component (30), which signals are cha~acteristic
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of the time component for which pump (14) mu~t be
activated to deliver ~he amount of cyanide ion from
storage vessel (12~ to bath (2) which amount has been
predetermined as required by the bath at the time in
gue6tion to maintain the desired cyanide level in the
bath. The component (30) is provided with the necessary
logic to transmit the appropriate signal to the operating
mechanism of the pump.
Unit (32) of the sys$em comprises a standard
terminal keyboard for accessing the system and video
display monitor to permit an operator to set up a program
for the overall sys~em, to make any necessary changes to
additions thereto and to monitor actual operating parame-
ters of the system during operation thereof. In an
al~ernative embodiment the overall system is entirely
preprogrammed and need only respond to alarm conditions
such as purnp failur2, heater failure, empty feedstock
reser~oir, and the like.
The electronic components necessary to carry out
the various ~unctions described above in re~pect of the
s~stems shown and discu ed in regard to Figure~ 2 and 3
~are well known and commercially available. A detailed
discus6ion of the circuit~ involved in producing the
afore~aid functions i8 accordingly omitted. Illustrative
of commercially available components which can be
employed in the computer ~y~tem dlescribed above are shown
below. It is to be noted that the~e are given for
purpo6es of exempllfication and the invention is not
limited to the use o~ the~ particular pieces of equip-
ment either individually or in combination.
Illustrative of commercially available timer units
is that available under the name DIGI-TIMER from SSAC
Inc. Represen~ative of microprocessor units is the mch
18 available from Wintek Corporation and for the ROM/RAM
component the mcm 2814p and mcm 6206p units available
from Motorola Semiconductors. A ~ypical display unit i~
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that available from Digital Equipment Corporation under
the name VT 420 and a typical terminal keyboard is the
RS232tRS423 COM-PAC available from Grayhill Inc. Repre-
sentative of temperature control system components is the
unit available from Protec Inc. as model dl5d, and repre-
sentative of pump control system components is the unit
available from Crydom Company as a series 6 module.
Programmable computer controlled means for monitor-
ing and adjusting the levels of copper ion, formaldehyde,
pH and other parameters of an electroless copper plating
bath are already known and commonly used in the art and
will not be described in detail herein. Such computer
means generally include means for producing base refer-
ence signals characteristic of a predetermined level of
the parameters to be monitored, sensing means for produc-
ing signals characteristic of the actual levels of each
parameter in the plating bath, comparator means producing
signals responsive to any difference between the signals
so produced and the base reference signals, and means
sensitive to said comparative response to indicate the
need for any necessary adjustment to the actual levels of
the various parameters in the bath. Such means may be
incorporated into the system of the invention if desired
or may be maintained separate therefrom.
The control process of the invention, when
employed in the electroless deposition of copper on a
substrate such as those employed in the manufacture of
printed circuit board, gives ~ise to a deposited copper
layer which has siynificantly improved properties
compared with layers deposited by prior methods.
Illustratively, two identical substrates comprising epoxy
fiberglass reinforced sheets having copper foil laminated
on one side thereof were subjected to electroless copper
plating. In the case of both substrates, the coating was
carried out using a bath having the following-
composition:
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Parts by Weight
copper sulfate pentahydrate 9.83
formaldehyde 1~90
EDTA 20.00
: 5 sodium cyanide 0.002
60dium hydroxide 2.B0
surfactant 0.10
additive~ 0~02
water ~o make 1000.00
The bath was maintained at 60C. In the case of one
substrate (A) the deposition was carried out u~ing a
~: computer control system of the invention. A layer of
: thickness 1.0 mil. was depo~ited. In the ca~e of the
other sub~rate (B~ the depo~ition to the same thickness
: 15 was carried out without benefit of a control sy~tem of
: - the invention and the cyanide level was maintained in the
conventional manner with the cyanide in the caus$ic soda
component wbich was added to the bath a~ required.
Plating times were o the order of 16 hour~ in both
cases. The following proper~ie~ were then determined ~or
the copper layer on each ~ubstrate after removal of the
reinforcing layer by dissolution in concentrated sulfuric
acid.
Ly Substrate A B
Ten~ile ~trength 51~000 psi 39,000 psi
% elongation 10 7
: It;i~ to be understood that ~he above description
ana exemplification ha~ been given for purpo~es of
illu~tration only and i8 not to be con~idered a~ limit- -
ing. Variou~ modifications wh.ich will be readily
apparent to one skilled in the art can be made without
departing from the ~cope o ~he inven~ion which latter i~
defined only by the claim~ which follow.
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