- Email: [email protected]
A new approach to controlling metal deposition during the pulse plating process
A N e w Approach to Controlling Metal D e p o s i t i o n During the Pulse Plating Process by M. Gladstein and H. Guterman Dept. of Electrical and Computer Engineering, Ben-Gurion University of the Negev, Beer-Sheva, Israel; Email, [email protected],ac.il he technology of pulse plating has been widely used and has been seen as a means of improving the distribution properties, 1-3 the quality of a deposit, 4-6 and of increasing solution efficiency and plating rate. TMAlthough pulse plating has long been used in the plating industry, it has been far from user friendly and much remains to be done to make it so. One of the ibasic requirements of a pulse electroplating process is to provide the required metal deposit weight or average thickness with a high degree of accuracy. If the real deposit weight or thickness of the plating parts were more than that required by the user, the quantity of the covering metal and power consumption increases. If the real weight or thickness is less than that required, it is necessary to repeat the process. In both cases the cost of electroplating increases. By providing a high degree of accuracy of the average deposit thickness we improve the quality of multilayer printed wiring boards and the accuracy of the required sizes of small plated parts in the electronics industry. 9,1° The real deposit weight during industrial plating processes is d e t e r m i n e d by an a m p e r e - m i n u t e meter, which m e a s u r e s the total c u r r e n t passing through the bath. The total current is not wholly effective in depositing the metal; a portion of the current is usually taken up by hydrogen evolution and hence it is necessary to take into account the current efficiency, which is a ratio of the current expended on metal deposition to the total current passing through the bath. 11 Only when the current efficiency is constant during a plating process, and is known, can we d e t e r m i n e the m e t a l deposit weight or the average deposit thickness with high accuracy by use of an ampere-minute meter, the socalled coulometric readout. 12 Some investigations in pulse plating suggest the possibility of calculating the current density utilized for metal deposition and the current density taken up by hydrogen evolution.13,14 After that one can find out the current efficiency. Unfortunately, the calculations for pulse plating processes with changing cathodic surface during plating give only a rough estimate of current efficiency. The current efficiency is known to be a function of
T
26
the current density. 15,16 When the current density is changed the current efficiency is changed, too. During some pulse plating processes, such as chip carrier plating, 17 the plating of high-density printed wiring boards, ]8,m and the plating of small parts in a barrel bath, the c u r r e n t density is changed because of the varying area of the cathodic surface. When a barrel in a bath is revolved the conductivity between small parts in the barrel and the total area of the cathodic surface changes randomly; thus, the current density also varies. The known systems for current density stabilization with an immersed current density sensor cannot work in the barrel bath. It should be noted that the current density in the bath during each pulse affects the deposit composition and its properties and is widely regarded as a useful control parameter. 2°,21 E S T I M A T I O N A N D C O N T R O L OF M E T A L D E P O S I T I O N
Current density stabilization can be realized by the stabilization of the relative potential difference across the cathode-solution interface Vdl u n d e r a given set of fixed conditions (temperature, electrolyte composition, barrel revolution velocity). If the area of the cathodic surface is increased, for example, the c u r r e n t density would be less t h a n required and Vd! would decrease. We can increase the power supply voltage to increase the relative potential difference across the cathode-solution interface and then the c u r r e n t density will be increased to its required value. Thus, to gain a constant value of the current efficiency we have to stabilize Vd]. For its stabilization we have to measure Vdl. In pulse plating this measurement may be carried out simply by means of a measuring electrode placed near the barrel; it is not necessary to use systems of compensation of the voltage drop across the solution resistance. We m e a s u r e Vdl i m m e d i a t e l y after the plating pulse when the bath current is zero and the cathodic double-layer capacitance has been charged. When the potential difference has been m e a s u r e d , we can change the a m p l i t u d e of the power supply voltage pulse to make Vdl equal to the reference. Barrel baths in pulse plating are usually supplied from a current source with constant plating pulse Metal Finishing
amplitude and frequency and variable pulse width. The equivalent circuit for the pulse plating process is shown in Figure 1, where Cdl A and Cdl are doublelayer capacitances of the anode and cathode accordingly, R F A and R F are faradaic resistances of the anode and cathode accordingly, Rma is the solution resistance between anode and measuring electrode, R s is the solution resistance between cathode and measuring electrode, Io is the current of the applied c u r r e n t source, and Vmc(t) is the voltage between cathode and measuring electrode. Plots of the current Io and the voltage Vmc(t) in steady state are shown in Figure 2. Time ton is the width of plating pulse and toff is the length of time between pulses, T is the period of the plating pulse. T = ton + toffDuring the plating pulse Vmc(t) consists of the voltage drop over the solution resistance between cathode and measuring electrode - Vrs and the voltage actually applied to the cathode interface -Vd](t):
I,, __.ITfV~(t)dt ToJ R~
(3)
The average value of Vdl(t) over a plating pulse period T is,
V.. 1 !Va(t)dt=
t(t)dt+
,(Oat]
using a simple RC model, Eq. (4) can be written as,
V.,~= +[ J[V,,.(max)(l-e'~')+
V=(O)e~"-V.ldt+IVo=ac-'~ (it] t=
where
(5)
I
V~, (t) = V= (t) + V,,
V. = Io (t)R,
(1)
During toff, when the current passing through the bath is zero, we believe Vms(t) is equal to ~elative potential difference across the cathode-solution interface Vd](t). Ven d is the relative potential difference across the cathode-solution interface measured immediately after the plating pulse, when the voltage drop over the solution resistance is equal to zero but the cathodic double-layer capacitance retains its charge as at the end of the plating pulse since the voltage across a capacitance cannot change instantaneously. Vav is the average value of Vd](t) over a plating pulse period T. We can estimate Vav by m e a n s ofVen d and change it by changing ton. As seen in Figure 3, when plating pulse width is increased, Ven d is increased, Vav is also increased, and vice versa (ton 2 > ton 1 Vend 2 > Vend 1 and Vav 2 > Vav 1). By changing ton we can stabilize Vav and, thereby, stabilize current density when the area of the cathode surface is changed. The plating current flowing through R F (see Fig. 1)
Vdl(0) is the relative potential difference across the cathode-solution interface at the beginning of the plating pulse, ton and ~offand are the constant time during the plating and off period, respectively. Given ton <
Va, ---
41-
28
Io ,,
I,,G,sccu ntl L..... source
I
|
[
........................................................ ....... ii:........ v
Rr^
....... ....... .......
Rr
(2)
where I c is the current flowing through the doublelayer capacitance Cdl. The average plating current might be calculated
as:
(6)
where, 5 = ton~. By using Eq. (6) into Eq. (3), the following approximation might be attained, 1 Tf V=(t)d t = --V.v ----kV~5 (7) IF.,='~'o~' RF RF
I F is,
IF(t ) = Io(t)-Ic(t) = V=(t) RF
(4)
t~
Cdl A
A __
ii
~mfl
Call C lr
J
Figure 1. The equivalent circuit for the pulse plating process. Metal Finishing
where k is a parameter that might be obtained by experimental calibration. As is seen from Eq.(7) the faradaic current might be controlled by altering the duty cycle (6).
Ml I.5
Jo 0.75
A control system for metal electrodeposition based on the above-mentioned method was designed and tested on gold plating chip carriers. The root-mean square error of the metal deposit weight required was half of the error of a system without overpotential stabilization.22 The system is readily adjusted and it is possible to determine V,, with help of the value Vendwhen the characteristic time constant of the change of V, during the plating pulse -7, is more than pulse width -t,,, or in other words, when the pulse width is narrow. Unfortunately, when z,, is St,, we cannot estimate the average value of V,,(t) using the value Vend. Thus, for two different pulse widths tonI and ton2 (see Fig. 4) the values of V, are different but the values of Vend are about the same. In this case we shall estimate the average value by the expression:
’
(8)
VP,= $ jlV,,dt 0
Now to stabilize the current density we will stabilize V,,. To define V,, we have to know V,, during the plating pulse. As noted above, during-the plating
2.5
v
end2:
V end1
Vav2
I
V avl 0.5
t-4 3
3.5 0
0.5
to nl
I Gm2
I*’
2
time [ms] Figure 2. Waveforms of the source current between cathode and measuring electrode.
August 2002
and the voltage
Figure 3. Effect of pulse width more than ton.
on average value of V,,, zon is
29
-%nc, vdl Kvl
kc IYI
2.4
-vend
V av2 1.5
V avl 1
to* time
time [ms]
[ms]
Figure 4. Effect of pulse width on average value ofV,I, than t,,.
q,, is less
pulse we can measure V,, only. To define V,, during the plating pulse we have to define Vrs and subtract it from V,,. Using Eq. (11, V pl =$V~wV,P
(9)
0
V,,
(curve 1) and V,, (curve 2) are shown in Figure 5, where V,,,_s, is the voltage across the double-layer cathode capacitance directly before the start of the plating pulse. V+,_,, does not contain I,R, because the bath current is zero. Vdl(+oj is the voltage across the double-layer cathode capacitance at the start of the plating pulse.
VW+o) = Vdl,,
(10)
since the voltage across a capacitance cannot change instantaneously. After the plating pulse is applied,
Vnrc(+o)=
Yfl(+o, + vra
(11)
- Y&.0, + v,AJj)dt
(12)
and, Pl,O)
30
T
Figure 5. Determination pulse.
of V,, at the beginning
of the plating
We can measure Vdl(_ojand store it in memory. Then we measure the voltage between the measuring electrode and cathode at the beginning of the plating pulse - Vmc(+oJ.Subtracting V,,,.,, from Vmc(+ojwe get V,, and store it in memory during the calculation of the integral (12) for one plating pulse. The block diagram of the proposed system for current density stabilization is shown in Figure 6. A, C, and ME are anode, cathode, and measuring electrode respectively. The current is measured by the cathodic current sensor (CCS). A clock circuit (CC) supplies the timing pulses for the synchronization of the pulse-width modulator (PWM), the sample and hold circuits SHC 1 and SHC 2, and the analog-digital calculator (Cal). The PWM provides current pulses feeding the plating bath, and their width depends on the value of the DC voltage supplied from the comparator (Corn). SHC 1 samples the voltages of the measuring electrode (ME) V,,(_o,, Vmc(+oj and supplies the estimated V,, to the analog-digital calculator that computes the value of V,,. SHC 2 samples V,, at t,, and holds it between plating pulses to compare with the reference voltage E, at the Corn. If the current density is less than required, V,, will
Metal Finishing
Plating
.,
Figure 6. Block diagram of the system for current density stabilization,
be less and Vp] will be less than the reference Er, the output voltage of the SHC 2 will fall, the output voltage of the comparator will increase, the width of the voltage pulse from the PWM will grow and Vp] will
also increase. As the voltage of the cathodic doublelayer capacitance approaches E r the current density will increase up to its required value. If the current density is more than that required, Vp] will be more than the reference Er, the width of the voltage pulse will fall, Vd] will decrease, and the current density will approach the required value; thus, the current density and the current efficiency have been stabilized. The value of the current efficiency is experimentally defined and is entered into the ampere-minute meter (AMM). Now the AMM will show the quantity of electricity employed on metal deposition; thus, the required metal deposit weight or average thickness is determined with a high degree of accuracy by using a coulometric readout. By means of this system the pulse plating process can be controlled to obtain the metal deposit weight required. The plating processing is initiated by providing the required quantity of electric charge, which must flow through the bath to obtain the desired weight to the AMM. Then, the system is switched on. The plating is finished when the real quantity of electricity counted by the AMM equals the entered value. There is another way to control a plating process. By changing Er, we can change the current density to provide the weight required in a specific time.
% 30
90
,
f
11
25
20
t-
15
1o-
-¢:
.=
10
30~
20
0 -3,5
-2.s
-1.s
~.s
0.s
~,5
~.s
3.s
Error in ¢let~ositgold wight in grams Figure 7. N u m b e r of experiments versus error in deposited gold weight.
August 2002
0
O.S
1
LS ~
2
2.6
3
3,S
v*kJe c~ e r ~ r In Wan~
Figure 8. C u m u l a t i v e distribution function versus absolute value of the error,
31
This is i m p o r t a n t w h e n s e v e r a l b a r r e l b a t h s are placed in the same b a t h plating line. EXPERIMENTAL
The proposed s y s t e m was applied to pulse plating of gold in a barrel b a t h u n d e r a given set of fixed conditions (temperature, electrolyte composition, barrel r o t a t i o n velocity) in a n acid electrolyte. A total of 329 pulse-plating experiments was carried out using the above-mentioned s y s t e m in the barrel bath. The results were compared to 329 pulse-plating experim e n t s ( s t a n d a r d set) performed u n d e r similar conditions b u t w i t h o u t c u r r e n t d e n s i t y stabilization. The weight of the gold deposit required in the experi m e n t was 35 grams. The results of the experiments ( n u m b e r of experiments as a function of the error of the gold deposited) are shown in Figure 7. It's easy to see t h a t t h e s t a n d a r d set (graph 2) p r e s e n t s a h i g h e r variability t h a n t h a t controlled by the proposed s y s t e m . The s t a n d a r d d e v i a t i o n was 1.555 grams, a n d 0.815 g r a m for the s t a n d a r d a n d controlled sets, respectively. The analysis of the cumulative distribution functions 23 as a f u n c t i o n of the a b s o l u t e v a l u e of t h e error is shown in Figure 8. It can be seen t h a t in 90% of the experiments controlled by the proposed s y s t e m the error was less t h a n 1.0 gram. For comparison, only 57% of the experiments from the stand a r d set were w i t h i n this range. The smoothness of the surface of the deposit was selectively investigated by the scanning electron microscope. The results of the experiments showed t h a t the deposits made using the above-mentioned system have a smoother surface t h a n the deposits made using the system without overpotential stabilization. CONCLUSIONS
The s t a b i l i z a t i o n of t h e o v e r p o t e n t i a l across t h e cathode-solution interface provides: • IncTeased a~uracy of the required metal deposit weight or average thickness • Improved properties and quality of metal deposition • Increased a~xtracy for obtaining the quantities required of metals from their alloys • Savings in metal deposited and reduced power consumption • Increased productivity especially for a bath plating line. The m e t h o d of overpotential stabilization depends on t h e ratio b e t w e e n t h e r a t e of o v e r p o t e n t i a l change d u r i n g the plating pulse a n d the w i d t h of the plating pulse. S y s t e m s w i t h such an overpotential stabilization August 2002
m e t h o d are a d a p t a b l e to rack b a t h s a n d other plating cell configurations w h e n there are reasons for c u r r e n t d e n s i t y c h a n g e s d u r i n g t h e pulse p l a t i n g process. REFERENCES
1. Kristof, P. and M. Pritzker, Plating and Surface Finishing~ 85(11):237-40; 1998 2. Watanabe, H. et al., Journal of the Electrochemical Society, 146(2): 574-9; 1999 3. Aroyo,M. et al., Plating and Surface Finishing, 85(9): 92-7; 1998. 4. Roy,S. and D. Landolt, Journal of Applied Electrochemistry, 27(3):299~07; 1997 5. Yin, I'L,Surface and Coating Technology, 88(1-3 ):162-4; 1997 6. Aroyo,M. et al, Plating and Surface Finishing, 82(11): 53-7; 1995 7. Varadarajan, D., et al.,Journal of the ElectrochemicalSociety, 147(9): 3382-3392; 2000 8. Gill,W. et al, Journal of the Electrochemical Society, 148(4): C289-296; 2001 9. Hung, S. et al, Microelectronics Reliability, 41(5):677-687; 2001 10. Wilson,J. et al, IEEE Transactions on ElectronicsPackaging Manufacturing, 23(2): 132-136; 2000 11. Gileadi, E. ,"Electrode Kinetics,"p. 154,VCH Publishers Inc., New York; 1993 12. Baldwin, P.,Metal Finishing, 93(1):24; 1995 13. Ruffoni, t~ and D. Landolt, J. Electrochim.Acta, 33, (l O):12811289; 1988 14. Bradley, P. and D. Landolt, J. Electrochimica Acta, 45(7): 1077-87; 1999 15. Raub, E. and I~ Muller,"Fundamentals of Metal Deposition," p. 204, Elsevier Publishing Co. Inc., New York; 1967 16. Kwak, S. et al., Journal of the Electrochemical Society, 143(9):2770-2776; 1996 17. Landau, U., "Copper Metallization of Semiconductor Interconnects--Issues and Prospects," Cell-Design,Western Reserve University, Cleveland; 2001 18. Taylor,E. et al, Plating and Surface Finishing, 87(12): 68-73; 2000 19. Abe, S. et al., Transactions of the Instituute of Metal Finishing, 76:12-15, Partl; 1998 20. Hadian, S. and D. Gabe, Metal Finishing, 98(6):116-132; 2000 21. Puippe, J. and F. Leaman, '~heory and Practice of th3]se Plating." American Electroplaters and Surface Finishers Society,Orlando, Fla.; 1986 22. Gladstein, M. and H. Guterman, Plating and Surface Finishin~ 88(3):78-79; 2001 23. Montgomery, D. and G. Runger, "Applied Statistics and Probability for Engineers," p. 109, John Wiley & Sons Inc., New York; 1994 W
33
T
26
the current density. 15,16 When the current density is changed the current efficiency is changed, too. During some pulse plating processes, such as chip carrier plating, 17 the plating of high-density printed wiring boards, ]8,m and the plating of small parts in a barrel bath, the c u r r e n t density is changed because of the varying area of the cathodic surface. When a barrel in a bath is revolved the conductivity between small parts in the barrel and the total area of the cathodic surface changes randomly; thus, the current density also varies. The known systems for current density stabilization with an immersed current density sensor cannot work in the barrel bath. It should be noted that the current density in the bath during each pulse affects the deposit composition and its properties and is widely regarded as a useful control parameter. 2°,21 E S T I M A T I O N A N D C O N T R O L OF M E T A L D E P O S I T I O N
Current density stabilization can be realized by the stabilization of the relative potential difference across the cathode-solution interface Vdl u n d e r a given set of fixed conditions (temperature, electrolyte composition, barrel revolution velocity). If the area of the cathodic surface is increased, for example, the c u r r e n t density would be less t h a n required and Vd! would decrease. We can increase the power supply voltage to increase the relative potential difference across the cathode-solution interface and then the c u r r e n t density will be increased to its required value. Thus, to gain a constant value of the current efficiency we have to stabilize Vd]. For its stabilization we have to measure Vdl. In pulse plating this measurement may be carried out simply by means of a measuring electrode placed near the barrel; it is not necessary to use systems of compensation of the voltage drop across the solution resistance. We m e a s u r e Vdl i m m e d i a t e l y after the plating pulse when the bath current is zero and the cathodic double-layer capacitance has been charged. When the potential difference has been m e a s u r e d , we can change the a m p l i t u d e of the power supply voltage pulse to make Vdl equal to the reference. Barrel baths in pulse plating are usually supplied from a current source with constant plating pulse Metal Finishing
amplitude and frequency and variable pulse width. The equivalent circuit for the pulse plating process is shown in Figure 1, where Cdl A and Cdl are doublelayer capacitances of the anode and cathode accordingly, R F A and R F are faradaic resistances of the anode and cathode accordingly, Rma is the solution resistance between anode and measuring electrode, R s is the solution resistance between cathode and measuring electrode, Io is the current of the applied c u r r e n t source, and Vmc(t) is the voltage between cathode and measuring electrode. Plots of the current Io and the voltage Vmc(t) in steady state are shown in Figure 2. Time ton is the width of plating pulse and toff is the length of time between pulses, T is the period of the plating pulse. T = ton + toffDuring the plating pulse Vmc(t) consists of the voltage drop over the solution resistance between cathode and measuring electrode - Vrs and the voltage actually applied to the cathode interface -Vd](t):
I,, __.ITfV~(t)dt ToJ R~
(3)
The average value of Vdl(t) over a plating pulse period T is,
V.. 1 !Va(t)dt=
t(t)dt+
,(Oat]
using a simple RC model, Eq. (4) can be written as,
V.,~= +[ J[V,,.(max)(l-e'~')+
V=(O)e~"-V.ldt+IVo=ac-'~ (it] t=
where
(5)
I
V~, (t) = V= (t) + V,,
V. = Io (t)R,
(1)
During toff, when the current passing through the bath is zero, we believe Vms(t) is equal to ~elative potential difference across the cathode-solution interface Vd](t). Ven d is the relative potential difference across the cathode-solution interface measured immediately after the plating pulse, when the voltage drop over the solution resistance is equal to zero but the cathodic double-layer capacitance retains its charge as at the end of the plating pulse since the voltage across a capacitance cannot change instantaneously. Vav is the average value of Vd](t) over a plating pulse period T. We can estimate Vav by m e a n s ofVen d and change it by changing ton. As seen in Figure 3, when plating pulse width is increased, Ven d is increased, Vav is also increased, and vice versa (ton 2 > ton 1 Vend 2 > Vend 1 and Vav 2 > Vav 1). By changing ton we can stabilize Vav and, thereby, stabilize current density when the area of the cathode surface is changed. The plating current flowing through R F (see Fig. 1)
Vdl(0) is the relative potential difference across the cathode-solution interface at the beginning of the plating pulse, ton and ~offand are the constant time during the plating and off period, respectively. Given ton <
Va, ---
41-
28
Io ,,
I,,G,sccu ntl L..... source
I
|
[
........................................................ ....... ii:........ v
Rr^
....... ....... .......
Rr
(2)
where I c is the current flowing through the doublelayer capacitance Cdl. The average plating current might be calculated
as:
(6)
where, 5 = ton~. By using Eq. (6) into Eq. (3), the following approximation might be attained, 1 Tf V=(t)d t = --V.v ----kV~5 (7) IF.,='~'o~' RF RF
I F is,
IF(t ) = Io(t)-Ic(t) = V=(t) RF
(4)
t~
Cdl A
A __
ii
~mfl
Call C lr
J
Figure 1. The equivalent circuit for the pulse plating process. Metal Finishing
where k is a parameter that might be obtained by experimental calibration. As is seen from Eq.(7) the faradaic current might be controlled by altering the duty cycle (6).
Ml I.5
Jo 0.75
A control system for metal electrodeposition based on the above-mentioned method was designed and tested on gold plating chip carriers. The root-mean square error of the metal deposit weight required was half of the error of a system without overpotential stabilization.22 The system is readily adjusted and it is possible to determine V,, with help of the value Vendwhen the characteristic time constant of the change of V, during the plating pulse -7, is more than pulse width -t,,, or in other words, when the pulse width is narrow. Unfortunately, when z,, is St,, we cannot estimate the average value of V,,(t) using the value Vend. Thus, for two different pulse widths tonI and ton2 (see Fig. 4) the values of V, are different but the values of Vend are about the same. In this case we shall estimate the average value by the expression:
’
(8)
VP,= $ jlV,,dt 0
Now to stabilize the current density we will stabilize V,,. To define V,, we have to know V,, during the plating pulse. As noted above, during-the plating
2.5
v
end2:
V end1
Vav2
I
V avl 0.5
t-4 3
3.5 0
0.5
to nl
I Gm2
I*’
2
time [ms] Figure 2. Waveforms of the source current between cathode and measuring electrode.
August 2002
and the voltage
Figure 3. Effect of pulse width more than ton.
on average value of V,,, zon is
29
-%nc, vdl Kvl
kc IYI
2.4
-vend
V av2 1.5
V avl 1
to* time
time [ms]
[ms]
Figure 4. Effect of pulse width on average value ofV,I, than t,,.
q,, is less
pulse we can measure V,, only. To define V,, during the plating pulse we have to define Vrs and subtract it from V,,. Using Eq. (11, V pl =$V~wV,P
(9)
0
V,,
(curve 1) and V,, (curve 2) are shown in Figure 5, where V,,,_s, is the voltage across the double-layer cathode capacitance directly before the start of the plating pulse. V+,_,, does not contain I,R, because the bath current is zero. Vdl(+oj is the voltage across the double-layer cathode capacitance at the start of the plating pulse.
VW+o) = Vdl,,
(10)
since the voltage across a capacitance cannot change instantaneously. After the plating pulse is applied,
Vnrc(+o)=
Yfl(+o, + vra
(11)
- Y&.0, + v,AJj)dt
(12)
and, Pl,O)
30
T
Figure 5. Determination pulse.
of V,, at the beginning
of the plating
We can measure Vdl(_ojand store it in memory. Then we measure the voltage between the measuring electrode and cathode at the beginning of the plating pulse - Vmc(+oJ.Subtracting V,,,.,, from Vmc(+ojwe get V,, and store it in memory during the calculation of the integral (12) for one plating pulse. The block diagram of the proposed system for current density stabilization is shown in Figure 6. A, C, and ME are anode, cathode, and measuring electrode respectively. The current is measured by the cathodic current sensor (CCS). A clock circuit (CC) supplies the timing pulses for the synchronization of the pulse-width modulator (PWM), the sample and hold circuits SHC 1 and SHC 2, and the analog-digital calculator (Cal). The PWM provides current pulses feeding the plating bath, and their width depends on the value of the DC voltage supplied from the comparator (Corn). SHC 1 samples the voltages of the measuring electrode (ME) V,,(_o,, Vmc(+oj and supplies the estimated V,, to the analog-digital calculator that computes the value of V,,. SHC 2 samples V,, at t,, and holds it between plating pulses to compare with the reference voltage E, at the Corn. If the current density is less than required, V,, will
Metal Finishing
Plating
.,
Figure 6. Block diagram of the system for current density stabilization,
be less and Vp] will be less than the reference Er, the output voltage of the SHC 2 will fall, the output voltage of the comparator will increase, the width of the voltage pulse from the PWM will grow and Vp] will
also increase. As the voltage of the cathodic doublelayer capacitance approaches E r the current density will increase up to its required value. If the current density is more than that required, Vp] will be more than the reference Er, the width of the voltage pulse will fall, Vd] will decrease, and the current density will approach the required value; thus, the current density and the current efficiency have been stabilized. The value of the current efficiency is experimentally defined and is entered into the ampere-minute meter (AMM). Now the AMM will show the quantity of electricity employed on metal deposition; thus, the required metal deposit weight or average thickness is determined with a high degree of accuracy by using a coulometric readout. By means of this system the pulse plating process can be controlled to obtain the metal deposit weight required. The plating processing is initiated by providing the required quantity of electric charge, which must flow through the bath to obtain the desired weight to the AMM. Then, the system is switched on. The plating is finished when the real quantity of electricity counted by the AMM equals the entered value. There is another way to control a plating process. By changing Er, we can change the current density to provide the weight required in a specific time.
% 30
90
,
f
11
25
20
t-
15
1o-
-¢:
.=
10
30~
20
0 -3,5
-2.s
-1.s
~.s
0.s
~,5
~.s
3.s
Error in ¢let~ositgold wight in grams Figure 7. N u m b e r of experiments versus error in deposited gold weight.
August 2002
0
O.S
1
LS ~
2
2.6
3
3,S
v*kJe c~ e r ~ r In Wan~
Figure 8. C u m u l a t i v e distribution function versus absolute value of the error,
31
This is i m p o r t a n t w h e n s e v e r a l b a r r e l b a t h s are placed in the same b a t h plating line. EXPERIMENTAL
The proposed s y s t e m was applied to pulse plating of gold in a barrel b a t h u n d e r a given set of fixed conditions (temperature, electrolyte composition, barrel r o t a t i o n velocity) in a n acid electrolyte. A total of 329 pulse-plating experiments was carried out using the above-mentioned s y s t e m in the barrel bath. The results were compared to 329 pulse-plating experim e n t s ( s t a n d a r d set) performed u n d e r similar conditions b u t w i t h o u t c u r r e n t d e n s i t y stabilization. The weight of the gold deposit required in the experi m e n t was 35 grams. The results of the experiments ( n u m b e r of experiments as a function of the error of the gold deposited) are shown in Figure 7. It's easy to see t h a t t h e s t a n d a r d set (graph 2) p r e s e n t s a h i g h e r variability t h a n t h a t controlled by the proposed s y s t e m . The s t a n d a r d d e v i a t i o n was 1.555 grams, a n d 0.815 g r a m for the s t a n d a r d a n d controlled sets, respectively. The analysis of the cumulative distribution functions 23 as a f u n c t i o n of the a b s o l u t e v a l u e of t h e error is shown in Figure 8. It can be seen t h a t in 90% of the experiments controlled by the proposed s y s t e m the error was less t h a n 1.0 gram. For comparison, only 57% of the experiments from the stand a r d set were w i t h i n this range. The smoothness of the surface of the deposit was selectively investigated by the scanning electron microscope. The results of the experiments showed t h a t the deposits made using the above-mentioned system have a smoother surface t h a n the deposits made using the system without overpotential stabilization. CONCLUSIONS
The s t a b i l i z a t i o n of t h e o v e r p o t e n t i a l across t h e cathode-solution interface provides: • IncTeased a~uracy of the required metal deposit weight or average thickness • Improved properties and quality of metal deposition • Increased a~xtracy for obtaining the quantities required of metals from their alloys • Savings in metal deposited and reduced power consumption • Increased productivity especially for a bath plating line. The m e t h o d of overpotential stabilization depends on t h e ratio b e t w e e n t h e r a t e of o v e r p o t e n t i a l change d u r i n g the plating pulse a n d the w i d t h of the plating pulse. S y s t e m s w i t h such an overpotential stabilization August 2002
m e t h o d are a d a p t a b l e to rack b a t h s a n d other plating cell configurations w h e n there are reasons for c u r r e n t d e n s i t y c h a n g e s d u r i n g t h e pulse p l a t i n g process. REFERENCES
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