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Decoupling of processes in coating systems by pumped buffers
Vacuum/volume 38/numbers Printed in Great Britain
Decoupling by pumped H Weisweiler,
8-l
O/pages
677 to 681 /I 988
0042-207X/88$3.00+.00 Pergamon Press plc
of processes buffers
W Buschbeck
and W Schwarz.
in coating
Leybold
systems
AG, PO Box 7555, D-6450
Hanau,
FRG
ln order to separate processes in coating systems where substrates are transported by carriers, vacuum locks and lock chambers are mainly used at present. As will be shown in this paper process decoupling can also be achieved by the use of pumped buffers. Besides excellent separation, pumped buffers furthermore allow a continuous coating operation and a maximum of productivity. In this report measured and calculated separation features will be presented concerning a buffer system consisting of three pumped chambers. Several cases are regarded, the system with and without carriers, with moved and static carriers. It is shown that without carriers only 1/20,000 of the pressure changes of the regarded coating-chamber could be detected in the second coating chamber separated by the buffer. In the case of moving carriers it will be shown that less than 11 lo6 of the pressure change occurs in the second coating chamber.
1. Introduction In industrial scale coating systems, besides high productivity and hence high coating speed, often low background pressures as well as reactive and nonreactive processes are required simultaneously in one system. Thus decoupling of the processes is necessary. Presently used systems need lock valves between the process chambers yielding excellent process separation. On the other hand for the transport of the substrate from one process chamber to another both processes have to be shut down or a transfer chamber between the process chambers has to be applied. Furthermore, many reactive sputtering processes are very sensitive to changes in pressure’.’ and pumping speed. Hence, for high productivity a continuous carrier transport and constant process conditions are highly desirable. Applying dynamically pumped buffer chambers between the process chambers results not only in a continuous operation of the deposition processes but also in an excellent gas separation of the process chambers. In this paper we discuss measured and computed results of the cross-contamination of the gases from one process chamber to the other for an industrial in-line coating system with buffer consisting of 3 pumped chambers and 4 slits. Investigations have been performed for a system with and without carriers, for static and moving carriers.
In order to quantify the cross-contamination of the processes the ratios of pressure changes can be regarded The separation factor T between the chambers i and i+ 1 is defined by the ratio of a pressure rise in chamber i causing a pressure increase in chamber i + 1.
‘=Ap(i+l)
In Figure 1 the investigated buffer-system with adjacent chambers (sputtering chambers) is shown.
I
Buffer
coating-
Sputtering chamber 2
1111-.-.-.___._._._.-.-.r 1 _._._._._.-.-.-.T -I? T I
2000
3. Calculation of the cross-contamination
449
2. The buffer-system
Sputtering chamber
The buffer consists of pumped buffer sections connected by slits or tunnels. With respect to low conductances the size of the tunnels should be as long as possible and so small that the carriers with the substrates to be coated can just pass through. The lower the conductances of the slits determined by their dimensions and the higher the pumping speed the better the separation features of the buffer compartment. Using several buffer sections the separation features increase, as will be shown later, proportionally to the product of the ratios of the pumping speed and the slit conductance of each section. The measured pumping speed S of the pumps at the three buffer sections was 460 I s-l, at sputtering chamber 1 S = 2000 1 s- ’ and S = 3000 I s- ’ at sputtering chamber 2. The slit size was about 450 x 175 x 9 mm3 corresponding to a calculated3 molecular conductance L = 73 1 s- ‘.
I
I s-l
3x4601s-’
Figure 1. Coating system with 2 sputtering consisting of three pumped swtions.
3000 chambers
I s-’ and
the buffer
where i is the chamber and p(i) > p(i + 1). For good separation, that is, a small gas flow from chamber i to chamber i + 1, a high value of T is required. In order to calculate T a steady state gas balance for the chambers is performed. In Figure 2 the gas flux is shown schematically for a single buffer section. The gas load into section i consists of the flux from the adjacent chamber i - 1 (Q (i- 1, i)) and Qw from outgassing and leaks. The amount of gas removed from the chamber (Qs+Q(i, i+ I)) is determined by the pumping speed S (Qs = Sp(i)) of the pump and the pressure difference to the next chamber. 677
H Weisweiler
et al: Decoupling
of processes
in coating
systems
Chamber
pressurc
in buff‘er section
p(bu(I))
= Ap(spl)+B
I is
and p(bu(i
+ 1)) = Ap(bu(i))
+ B.
As there is only one slit in the second coating chamber the pressure p(sp2) according to balance equation (3) is given bq p(sp2)
= Jp(bu(n))
+ B
0 = Throughput L
: Conductwty
S = Pumping
where speed
Figure 2. Gas flows of a pumped buffer section
s = S(sp2). Solving the equation
The flux balance yields the pressure p(i) in the chamber i depending on the pressures p(i- 1) and p(i+ 1) in the adjacent chambers, the conductances L(i) and L(i + 1) as well as Qw :
p(i-l)L(i)+p(i+l)L(i+l)+Qw = S(i)+L(i)+L(i+
p(i)
’
1)
For a buffer with n buffer sections denoted by bu(l). . . bu(n) between two process chambers spl and sp2 equation (3) has to be applied recurrently yielding :
of the separation
T = -S(sp2) ’ S(bu(i)) rI L (sp2) i = , L @u(i)) ’
(5)
It should be noted that these estimates are conservative. If L(i) is a significant value in comparison to S(i) (see equation (2)) larger separation factors are obtained. All L(i) are pressure independent (molecular pressure range) and equal just as the pumping speeds in the buffer sections. Assuming QM?in all chambers is the same equation (2) then gives p(i)
= @(i-
I)+p(i+ s+2L
l))L+Q~v
(6)
Defining A=-
L Sf2L
where 678
and B=-
Q)v
(6.1)
si-2L
l/A is the separation
factor
(8) is obtained. The above equation allows the calculation of with and without carriers, The conductance L according to the geometry. Calculating T for the empty system (L = equation (5) gives T = 10,282 and from equation is obtained. These data confirm that the rule of (5) results in a conservative estimate.
T for the system has to be varied 73 1 s ‘) using (8) T = 23,3 15 thumb equation
(4)
’
Comparing equation (4) and the definition tactor (equation (1)) we obtain for T:
= 7-P(SP2)
(2)
L (9 Ai> = Ai - 1)s(i).
L (SP2) fi L(bu(i)) ~ = P(SPI)----S(sp2) , = , WMi))
(7)
=f@(spI)).
For the system investigated P(SPl)
In many cases, however, some simplifying assumptions can be made. For stages with large separation factors the pumping effect of the adjacent chamber can be neglected. Assuming further that Qw is small compared to the flux through the slits and p(i + 1) <
P(SP2)
P(SP2)
system yields the desired function
for the buffer sections.
the
4. Comparison of measured and computed separation factors 4.1. Measurements without carriers. In the following the results of the pressure measurements and thus the experimentally found separation factors will be compared with the computations according to equation (8). The pressures in sputtering chamber 1 were varied up to 10. ’ mbar, which is slightly above usual maximum process pressures. In Figure 3 the separation factors are shown for adjacent chambers. When the pressure changes are small at low absolute pressures as is the case for bu3 and sp2, measuring errors lead to large variations of the ratios of the pressure changes, that is, considerable uncertainty concerning T for various pressures (see T(pl) Figure 3). The separation factors computed according to equation (6.1) and the values using the measured pressures show good agreement. The measured value for the buffer chambers is between 6 and IO; the computed value is T = 8.3. The measured separation factor for buffer section 3 (bu3) and the second coating chamber (sp2) ranges from 30 to 50. The calculated value is 42. I,
H Weisweiler
et al: Decoupling
of
processes
in coating
systems
10-Z bu I bu 2 +bu3 FAsp 2
C3
T
V
x
IO
A 0
II 10-s
I 2
3
pl
4
5
6
7
8
9 ,0-z
[mbarl
PI
Figure 3. Separation factor between the coating (sp) and buffer (bu) sections vs the pressure pl in coating chamber 1.
Figure 5. Pressure in buffer and sputtering in coating
The separation factor for the whole system without carriers is shown in Figure 4. T was found to be about 20,000, i.e. pressure changes in chamber 1 of 1 x 10e2 cause rises of 5 x lo-’ mbar in coating chamber 2. Equation (8) gives a separation factor T = 23,300 which is in good agreement with the measurement. It should be noted that, as can be seen by equation (8) slight errors concerning the values of pumping speed and conductance are multiplied. For example, using pumping speeds of 500 I s- ’ for the buffer chambers, 3000 1s- ’ in the sputtering chamber and 70 1 s-’ for the slit leads to T = 32,500 which is about 50% higher. Thus, pumping speeds and slit conductances should be precisely known to get results close to reality. 4.2. Separation factors with static carriers. The coating system was filled with plate shaped carriers (5 mm thickness) having a distance of 5-10 mm between each other. Figure 5 shows the resulting pressure in coating chamber 2 vs the pressure in chamber 1, where the gas was fed in. The pressure rise in sputtering chamber 2 is considerably lower in comparison to the empty system. The separation factors determined by the measured pressure changes are shown in Figures 6 and 7. The separation factors between the buffer sections varied between 20 and 70. The maximum value was obtained for buffer chamber 3 and sputtering chamber 2 and amounted to 80.
r
chamber
chamber
2 vs the pressure
pl
1 (with static carriers).
I03 o spl-> bul -x bul-> bu2 -A bu2 - > bu3 V bu3-
> Sp2
IO’ 7
V
*
6
T
V
; A
A
A
xx
A
a A
I1
I *
to-3
3
pl
Figure 6. Separation ,. secuons
vs the pressure
4
56
7
8
9 ,o-’
[mbarl
factor between the coating (sp) and buffer (bu) pl in coating chamber I (with static carriers).
108
IO' __-__-
____
-- ____
-_ _____
_________
f
pl tmbarl 0
@D____________
---------0o.-.
[mbarl
---.
n.~.~.n.o.~._._.-..-.-
factor between the coating chamber 1 and coating 2 vs the pressure pl in coating chamber 1 (with static carriers).
Figure 7. Separation chamber
0
T
IO“
o ’
0
.---
Measurements s-500 Is/L.70 Is-’ s =460 I s-1L -73 I r’
pl
Figure 4. Calculated chamber 1 (without
I
Cmbarl
and measured separation factors between coating and coating chamber 2 vs the pressure pl in coating chamber carriers).
In order to analyse the influence of the spacing between the carrier and the slit wall on the cross-contamination several computations have been done varying the spacings. Although the wall-carrier spacings only vary as little as 1 mm (to both sides of the carrier) the resulting conductances are very different affecting the corresponding calculated separation factors. For spacings of 2.5 mm to each carrier side (2 x 2.5 mm) and 3.5 mm (2 x 3.5 mm), respectively a computed separation 679
H Weisweiler
et al: Decoupling
of processes
in coating systems
factor of 5.47 x IOh and 5.9 x IO” is obtained. The value using the measured pressures is T = I .5 x 106. Comparing the results obtained by computations and measurements several a’spects have to be taken into consideration. Spacings between the carriers positioned in the buffer cause different conductances in comparison to a long plate completely filling up the coating-system. which was assumed for the computation. Furthermore a symmetric positioned flat plate was supposed. In reality the carriers were inclined slightly perpendicular to the transport direction also influencing the effective conductance. Comparing the results of measurement and calculation, at least for pressures below 10e2 mbar there is sufficient correspondence. At higher pressures the absolute error can be considerable, since, besides uncertainties concerning the geometry, the conductance becomes pressure dependent. Regarding the calculated separation factors it should be noted that mainly because of the above mentioned uncertainities in the geometry and hence in the conductances the calculated separation factors only give a rough estimate. 4.3. Cross-contamination with moving carriers. For investigations of the net gas flow between the coating chambers when moving the carriers, the plates were transported with 6 mm s- ’ through the coating system. In Figure 8 the pressures occurring for all the chambers vs time are shown. The light barrier signal allows a correlation of the pressure fluctuations to the carrier position. The pressure in chamber 1 was set to about 3 x 1O-3 mbar. The pressure variations in sputtering chamber 1 caused by the moving carrier were smaller than 5 x lo-’ mbar. A clear correlation to the carrier movement could not be found.
3
It is evident that there were significant pressure variations in buffer chamber 1 (bul) of about 2 x 10-j mbar at an average pressure of 8 x 10-j mbar and in buffer chamber 2 (bu2) of about 1 x IO-‘mbar with an average pressure of 3 x 10m6 mbar. The separation factor is about 20. This is comparable to the results with static carriers (see Figure 3). The pressure changes occur when the separating gap between two carriers reaches the chambers. In particular in buffer chamber I the carrier and gap can be clearly identified in the measured pressure curve. When the carrier does not fill up the slit completely the gas flow must be higher as previously mentioned because of the augmented effective conductance of the slit. Assuming the pressure pl in coating chamber 1 to be time constant because of the high pumping speed, the gas flow balance for buffer chamber 1 can be expressed as (see equation (6))
s+2L, P(SP1)
= -------p(bul)o L,
p(sp1)
= y3hp(bul)o
where Lg is the conductance with carrier completely in the slit, and LB is the conductance with carrier not completely in the slit P(SPl)
= P(SPl)o
= p(spl
)a.
Lg is equal to the value measured in the static case. The unknown La can be obtained from the above equations and is
where La = 11.6 I sm’ (2 mm spacing to both sides of the plate within the slit). With the pressure values of buffer chamber 1 (bul in Figure
I x m3
8)
-L-L (2 5
3 51 x 1om6
p(bul)o
= 7 x lO_‘mbar
p(bul)@
= 9 x lo- ’ mbar
and S = 4601~‘. La can be calculated
L,
:
= 15ls- ‘.
Thus, the conductance is about 30% higher which corresponds to a 22% lower separation f:dctor between chamber 1 and buffer section I. With this result we can verify the assumption used above of p(sp1) to be constant. In order to quantify p in sputtering chamber I (sp I) we use the calculated change of conductance of the slit. The balance for sputtering chamber I for both cases gives p(spl)o/p(spl)o
= ;-;-i-Q 0
Figure 8. Pressure changes with moving 680
carriers
(6
mm s ‘)
where .? = 2000 I sm’ (chamber
I)
H Weisweiler
et al: Decoupling
of processes
in coating systems
Inserting the conductances the pressure change in coating chamber 1 caused by the moving carrier is only 0.2% according to 0.5 x lo-’ mbar. So the assumption ofp(sp1) being constant is sufficiently accurate. In buffer section 3 the pressure shows variations of about lo-’ mbar at an average pressure of 1.5 x 10m6 mbar showing no significant correlation to the carrier position. The pressure changes in sputter section 2 are very small, that is, < 5 x 10m9 mbar at an average pressure of 1.3 x lo-’ mbar. As in the case of static carriers the separation factor of the system with moving carriers is about 1.2 x 106. 5. Conclusions The decoupling features of a pumped buffer system between two coating chambers have been investigated. The buffer consisted of three pumped buffer chambers and 4 connecting slits. Flat plates similar to the carriers for the transport of the substrates were moved through the coating system. The ratio of pressure changes in a chamber and the resulting pressure changes in the adjacent chamber is a measure of processdecoupling and is denoted as separation factor T. The separation of two chambers depends on the ratioT(i) of
the pumping speed at the adjacent chamber and the effective conductance of the connecting slit L including the effect of the carrier. Taking several pumped chambers the entire separation factor T is determined by the product of all ratios T(i). In the investigated system, without carriers T = 20,000 that is, l/20,000 of the pressure change in coating chamber 1 occurs in coating chamber 2. Moving the carriers through the system the separation factor from chamber to chamber is about 20-50 so the pressure variations occurring in the buffering chambers show corresponding values. The entire separation factor is T > 106, thus a pressure change < low9 mbar occurs in the second coating chamber when the pressure is changed by 10e3 mbar in chamber 1. T is constant for pressure levels in the coating chambers up to some 10e3 mbar, that is, for typical operating pressures for coating processes. References
‘R McMahon, J AfFinito and R R Parsons, J Vuc Sci Tech&, 20, 376 (1982). *E Schultheiss, G Brguer, W Dicken, H-P D Shieh, S Miiller and P Wirtz, Solid Slate Technol, 3, 107 (1988). ‘J Kieser and IMGrundner, Proc VIII Int Vat Cong, Suppl Rev, Le Vide, 201, 376 (1980).
681
Decoupling by pumped H Weisweiler,
8-l
O/pages
677 to 681 /I 988
0042-207X/88$3.00+.00 Pergamon Press plc
of processes buffers
W Buschbeck
and W Schwarz.
in coating
Leybold
systems
AG, PO Box 7555, D-6450
Hanau,
FRG
ln order to separate processes in coating systems where substrates are transported by carriers, vacuum locks and lock chambers are mainly used at present. As will be shown in this paper process decoupling can also be achieved by the use of pumped buffers. Besides excellent separation, pumped buffers furthermore allow a continuous coating operation and a maximum of productivity. In this report measured and calculated separation features will be presented concerning a buffer system consisting of three pumped chambers. Several cases are regarded, the system with and without carriers, with moved and static carriers. It is shown that without carriers only 1/20,000 of the pressure changes of the regarded coating-chamber could be detected in the second coating chamber separated by the buffer. In the case of moving carriers it will be shown that less than 11 lo6 of the pressure change occurs in the second coating chamber.
1. Introduction In industrial scale coating systems, besides high productivity and hence high coating speed, often low background pressures as well as reactive and nonreactive processes are required simultaneously in one system. Thus decoupling of the processes is necessary. Presently used systems need lock valves between the process chambers yielding excellent process separation. On the other hand for the transport of the substrate from one process chamber to another both processes have to be shut down or a transfer chamber between the process chambers has to be applied. Furthermore, many reactive sputtering processes are very sensitive to changes in pressure’.’ and pumping speed. Hence, for high productivity a continuous carrier transport and constant process conditions are highly desirable. Applying dynamically pumped buffer chambers between the process chambers results not only in a continuous operation of the deposition processes but also in an excellent gas separation of the process chambers. In this paper we discuss measured and computed results of the cross-contamination of the gases from one process chamber to the other for an industrial in-line coating system with buffer consisting of 3 pumped chambers and 4 slits. Investigations have been performed for a system with and without carriers, for static and moving carriers.
In order to quantify the cross-contamination of the processes the ratios of pressure changes can be regarded The separation factor T between the chambers i and i+ 1 is defined by the ratio of a pressure rise in chamber i causing a pressure increase in chamber i + 1.
‘=Ap(i+l)
In Figure 1 the investigated buffer-system with adjacent chambers (sputtering chambers) is shown.
I
Buffer
coating-
Sputtering chamber 2
1111-.-.-.___._._._.-.-.r 1 _._._._._.-.-.-.T -I? T I
2000
3. Calculation of the cross-contamination
449
2. The buffer-system
Sputtering chamber
The buffer consists of pumped buffer sections connected by slits or tunnels. With respect to low conductances the size of the tunnels should be as long as possible and so small that the carriers with the substrates to be coated can just pass through. The lower the conductances of the slits determined by their dimensions and the higher the pumping speed the better the separation features of the buffer compartment. Using several buffer sections the separation features increase, as will be shown later, proportionally to the product of the ratios of the pumping speed and the slit conductance of each section. The measured pumping speed S of the pumps at the three buffer sections was 460 I s-l, at sputtering chamber 1 S = 2000 1 s- ’ and S = 3000 I s- ’ at sputtering chamber 2. The slit size was about 450 x 175 x 9 mm3 corresponding to a calculated3 molecular conductance L = 73 1 s- ‘.
I
I s-l
3x4601s-’
Figure 1. Coating system with 2 sputtering consisting of three pumped swtions.
3000 chambers
I s-’ and
the buffer
where i is the chamber and p(i) > p(i + 1). For good separation, that is, a small gas flow from chamber i to chamber i + 1, a high value of T is required. In order to calculate T a steady state gas balance for the chambers is performed. In Figure 2 the gas flux is shown schematically for a single buffer section. The gas load into section i consists of the flux from the adjacent chamber i - 1 (Q (i- 1, i)) and Qw from outgassing and leaks. The amount of gas removed from the chamber (Qs+Q(i, i+ I)) is determined by the pumping speed S (Qs = Sp(i)) of the pump and the pressure difference to the next chamber. 677
H Weisweiler
et al: Decoupling
of processes
in coating
systems
Chamber
pressurc
in buff‘er section
p(bu(I))
= Ap(spl)+B
I is
and p(bu(i
+ 1)) = Ap(bu(i))
+ B.
As there is only one slit in the second coating chamber the pressure p(sp2) according to balance equation (3) is given bq p(sp2)
= Jp(bu(n))
+ B
0 = Throughput L
: Conductwty
S = Pumping
where speed
Figure 2. Gas flows of a pumped buffer section
s = S(sp2). Solving the equation
The flux balance yields the pressure p(i) in the chamber i depending on the pressures p(i- 1) and p(i+ 1) in the adjacent chambers, the conductances L(i) and L(i + 1) as well as Qw :
p(i-l)L(i)+p(i+l)L(i+l)+Qw = S(i)+L(i)+L(i+
p(i)
’
1)
For a buffer with n buffer sections denoted by bu(l). . . bu(n) between two process chambers spl and sp2 equation (3) has to be applied recurrently yielding :
of the separation
T = -S(sp2) ’ S(bu(i)) rI L (sp2) i = , L @u(i)) ’
(5)
It should be noted that these estimates are conservative. If L(i) is a significant value in comparison to S(i) (see equation (2)) larger separation factors are obtained. All L(i) are pressure independent (molecular pressure range) and equal just as the pumping speeds in the buffer sections. Assuming QM?in all chambers is the same equation (2) then gives p(i)
= @(i-
I)+p(i+ s+2L
l))L+Q~v
(6)
Defining A=-
L Sf2L
where 678
and B=-
Q)v
(6.1)
si-2L
l/A is the separation
factor
(8) is obtained. The above equation allows the calculation of with and without carriers, The conductance L according to the geometry. Calculating T for the empty system (L = equation (5) gives T = 10,282 and from equation is obtained. These data confirm that the rule of (5) results in a conservative estimate.
T for the system has to be varied 73 1 s ‘) using (8) T = 23,3 15 thumb equation
(4)
’
Comparing equation (4) and the definition tactor (equation (1)) we obtain for T:
= 7-P(SP2)
(2)
L (9 Ai> = Ai - 1)s(i).
L (SP2) fi L(bu(i)) ~ = P(SPI)----S(sp2) , = , WMi))
(7)
=f@(spI)).
For the system investigated P(SPl)
In many cases, however, some simplifying assumptions can be made. For stages with large separation factors the pumping effect of the adjacent chamber can be neglected. Assuming further that Qw is small compared to the flux through the slits and p(i + 1) <
P(SP2)
P(SP2)
system yields the desired function
for the buffer sections.
the
4. Comparison of measured and computed separation factors 4.1. Measurements without carriers. In the following the results of the pressure measurements and thus the experimentally found separation factors will be compared with the computations according to equation (8). The pressures in sputtering chamber 1 were varied up to 10. ’ mbar, which is slightly above usual maximum process pressures. In Figure 3 the separation factors are shown for adjacent chambers. When the pressure changes are small at low absolute pressures as is the case for bu3 and sp2, measuring errors lead to large variations of the ratios of the pressure changes, that is, considerable uncertainty concerning T for various pressures (see T(pl) Figure 3). The separation factors computed according to equation (6.1) and the values using the measured pressures show good agreement. The measured value for the buffer chambers is between 6 and IO; the computed value is T = 8.3. The measured separation factor for buffer section 3 (bu3) and the second coating chamber (sp2) ranges from 30 to 50. The calculated value is 42. I,
H Weisweiler
et al: Decoupling
of
processes
in coating
systems
10-Z bu I bu 2 +bu3 FAsp 2
C3
T
V
x
IO
A 0
II 10-s
I 2
3
pl
4
5
6
7
8
9 ,0-z
[mbarl
PI
Figure 3. Separation factor between the coating (sp) and buffer (bu) sections vs the pressure pl in coating chamber 1.
Figure 5. Pressure in buffer and sputtering in coating
The separation factor for the whole system without carriers is shown in Figure 4. T was found to be about 20,000, i.e. pressure changes in chamber 1 of 1 x 10e2 cause rises of 5 x lo-’ mbar in coating chamber 2. Equation (8) gives a separation factor T = 23,300 which is in good agreement with the measurement. It should be noted that, as can be seen by equation (8) slight errors concerning the values of pumping speed and conductance are multiplied. For example, using pumping speeds of 500 I s- ’ for the buffer chambers, 3000 1s- ’ in the sputtering chamber and 70 1 s-’ for the slit leads to T = 32,500 which is about 50% higher. Thus, pumping speeds and slit conductances should be precisely known to get results close to reality. 4.2. Separation factors with static carriers. The coating system was filled with plate shaped carriers (5 mm thickness) having a distance of 5-10 mm between each other. Figure 5 shows the resulting pressure in coating chamber 2 vs the pressure in chamber 1, where the gas was fed in. The pressure rise in sputtering chamber 2 is considerably lower in comparison to the empty system. The separation factors determined by the measured pressure changes are shown in Figures 6 and 7. The separation factors between the buffer sections varied between 20 and 70. The maximum value was obtained for buffer chamber 3 and sputtering chamber 2 and amounted to 80.
r
chamber
chamber
2 vs the pressure
pl
1 (with static carriers).
I03 o spl-> bul -x bul-> bu2 -A bu2 - > bu3 V bu3-
> Sp2
IO’ 7
V
*
6
T
V
; A
A
A
xx
A
a A
I1
I *
to-3
3
pl
Figure 6. Separation ,. secuons
vs the pressure
4
56
7
8
9 ,o-’
[mbarl
factor between the coating (sp) and buffer (bu) pl in coating chamber I (with static carriers).
108
IO' __-__-
____
-- ____
-_ _____
_________
f
pl tmbarl 0
@D____________
---------0o.-.
[mbarl
---.
n.~.~.n.o.~._._.-..-.-
factor between the coating chamber 1 and coating 2 vs the pressure pl in coating chamber 1 (with static carriers).
Figure 7. Separation chamber
0
T
IO“
o ’
0
.---
Measurements s-500 Is/L.70 Is-’ s =460 I s-1L -73 I r’
pl
Figure 4. Calculated chamber 1 (without
I
Cmbarl
and measured separation factors between coating and coating chamber 2 vs the pressure pl in coating chamber carriers).
In order to analyse the influence of the spacing between the carrier and the slit wall on the cross-contamination several computations have been done varying the spacings. Although the wall-carrier spacings only vary as little as 1 mm (to both sides of the carrier) the resulting conductances are very different affecting the corresponding calculated separation factors. For spacings of 2.5 mm to each carrier side (2 x 2.5 mm) and 3.5 mm (2 x 3.5 mm), respectively a computed separation 679
H Weisweiler
et al: Decoupling
of processes
in coating systems
factor of 5.47 x IOh and 5.9 x IO” is obtained. The value using the measured pressures is T = I .5 x 106. Comparing the results obtained by computations and measurements several a’spects have to be taken into consideration. Spacings between the carriers positioned in the buffer cause different conductances in comparison to a long plate completely filling up the coating-system. which was assumed for the computation. Furthermore a symmetric positioned flat plate was supposed. In reality the carriers were inclined slightly perpendicular to the transport direction also influencing the effective conductance. Comparing the results of measurement and calculation, at least for pressures below 10e2 mbar there is sufficient correspondence. At higher pressures the absolute error can be considerable, since, besides uncertainties concerning the geometry, the conductance becomes pressure dependent. Regarding the calculated separation factors it should be noted that mainly because of the above mentioned uncertainities in the geometry and hence in the conductances the calculated separation factors only give a rough estimate. 4.3. Cross-contamination with moving carriers. For investigations of the net gas flow between the coating chambers when moving the carriers, the plates were transported with 6 mm s- ’ through the coating system. In Figure 8 the pressures occurring for all the chambers vs time are shown. The light barrier signal allows a correlation of the pressure fluctuations to the carrier position. The pressure in chamber 1 was set to about 3 x 1O-3 mbar. The pressure variations in sputtering chamber 1 caused by the moving carrier were smaller than 5 x lo-’ mbar. A clear correlation to the carrier movement could not be found.
3
It is evident that there were significant pressure variations in buffer chamber 1 (bul) of about 2 x 10-j mbar at an average pressure of 8 x 10-j mbar and in buffer chamber 2 (bu2) of about 1 x IO-‘mbar with an average pressure of 3 x 10m6 mbar. The separation factor is about 20. This is comparable to the results with static carriers (see Figure 3). The pressure changes occur when the separating gap between two carriers reaches the chambers. In particular in buffer chamber I the carrier and gap can be clearly identified in the measured pressure curve. When the carrier does not fill up the slit completely the gas flow must be higher as previously mentioned because of the augmented effective conductance of the slit. Assuming the pressure pl in coating chamber 1 to be time constant because of the high pumping speed, the gas flow balance for buffer chamber 1 can be expressed as (see equation (6))
s+2L, P(SP1)
= -------p(bul)o L,
p(sp1)
= y3hp(bul)o
where Lg is the conductance with carrier completely in the slit, and LB is the conductance with carrier not completely in the slit P(SPl)
= P(SPl)o
= p(spl
)a.
Lg is equal to the value measured in the static case. The unknown La can be obtained from the above equations and is
where La = 11.6 I sm’ (2 mm spacing to both sides of the plate within the slit). With the pressure values of buffer chamber 1 (bul in Figure
I x m3
8)
-L-L (2 5
3 51 x 1om6
p(bul)o
= 7 x lO_‘mbar
p(bul)@
= 9 x lo- ’ mbar
and S = 4601~‘. La can be calculated
L,
:
= 15ls- ‘.
Thus, the conductance is about 30% higher which corresponds to a 22% lower separation f:dctor between chamber 1 and buffer section I. With this result we can verify the assumption used above of p(sp1) to be constant. In order to quantify p in sputtering chamber I (sp I) we use the calculated change of conductance of the slit. The balance for sputtering chamber I for both cases gives p(spl)o/p(spl)o
= ;-;-i-Q 0
Figure 8. Pressure changes with moving 680
carriers
(6
mm s ‘)
where .? = 2000 I sm’ (chamber
I)
H Weisweiler
et al: Decoupling
of processes
in coating systems
Inserting the conductances the pressure change in coating chamber 1 caused by the moving carrier is only 0.2% according to 0.5 x lo-’ mbar. So the assumption ofp(sp1) being constant is sufficiently accurate. In buffer section 3 the pressure shows variations of about lo-’ mbar at an average pressure of 1.5 x 10m6 mbar showing no significant correlation to the carrier position. The pressure changes in sputter section 2 are very small, that is, < 5 x 10m9 mbar at an average pressure of 1.3 x lo-’ mbar. As in the case of static carriers the separation factor of the system with moving carriers is about 1.2 x 106. 5. Conclusions The decoupling features of a pumped buffer system between two coating chambers have been investigated. The buffer consisted of three pumped buffer chambers and 4 connecting slits. Flat plates similar to the carriers for the transport of the substrates were moved through the coating system. The ratio of pressure changes in a chamber and the resulting pressure changes in the adjacent chamber is a measure of processdecoupling and is denoted as separation factor T. The separation of two chambers depends on the ratioT(i) of
the pumping speed at the adjacent chamber and the effective conductance of the connecting slit L including the effect of the carrier. Taking several pumped chambers the entire separation factor T is determined by the product of all ratios T(i). In the investigated system, without carriers T = 20,000 that is, l/20,000 of the pressure change in coating chamber 1 occurs in coating chamber 2. Moving the carriers through the system the separation factor from chamber to chamber is about 20-50 so the pressure variations occurring in the buffering chambers show corresponding values. The entire separation factor is T > 106, thus a pressure change < low9 mbar occurs in the second coating chamber when the pressure is changed by 10e3 mbar in chamber 1. T is constant for pressure levels in the coating chambers up to some 10e3 mbar, that is, for typical operating pressures for coating processes. References
‘R McMahon, J AfFinito and R R Parsons, J Vuc Sci Tech&, 20, 376 (1982). *E Schultheiss, G Brguer, W Dicken, H-P D Shieh, S Miiller and P Wirtz, Solid Slate Technol, 3, 107 (1988). ‘J Kieser and IMGrundner, Proc VIII Int Vat Cong, Suppl Rev, Le Vide, 201, 376 (1980).
681