Cleaning method for removal of contaminants

A cleaning method whereby a cleaning liquid used for cleaning in one cleaning stage is mixed after the one cleaning stage with a cleaning liquid used for cleaning in a rear cleaning stage. The flow rate of the one cleaning stage being greater than that of the cleaning liquid used for cleaning in the rear cleaning stage.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention will be described by reference to the drawings. 
FIG. 2 is a diagrammatic structural drawing to illustrate the structure of 
the cleaning apparatus of the present invention. In FIG. 2, reference 
numerals 101, 102, 103 designate a first cleaning liquid bath, a second 
cleaning liquid bath, and a third cleaning liquid bath, respectively; 104 
a substrate as a treated body subjected to cleaning; 105 a belt as 
carrying means for carrying the substrate 104; 106, 107, 108 shower heads 
for discharging or spraying respective cleaning liquids supplied thereto 
toward the substrate 104; 109, 116, 117, 118, 119 pipes for supplying 
respective cleaning liquids; 111, 114 pumps for feeding the respective 
cleaning liquids; 113 a converging device; and 110, 112, 115 a first 
treatment liquid, a second treatment liquid, and a third treatment liquid, 
respectively. 
Also in the cleaning apparatus shown in FIG. 2, the substrate 104 is 
successively carried from left to right in the drawing and is finally 
cleaned with a new cleaning water. A significant difference of the 
cleaning apparatus shown in FIG. 2 from the cleaning apparatus as 
described with FIG. 1 resides in the following point. 
In case of the cleaning apparatus shown in FIG. 2, flows of the cleaning 
liquids are greatly different from those in the cleaning apparatus shown 
in FIG. 1. 
A new cleaning liquid is supplied through the pipe 109 to the shower head 
108 to be jet-sprayed through the shower head 108 toward the substrate 104 
so as to clean the substrate 104. The cleaning liquid after cleaning of 
substrate 104 is stored as the primary treatment liquid 110 in the third 
cleaning liquid bath 103, and this primary treatment liquid 110 is fed 
through the pipe 116, one end of which is connected to the third cleaning 
liquid bath 103, to the shower head 107 by the pump 111 and is then 
jet-sprayed through the shower head 107 toward the substrate 104 to clean 
the substrate 104. After cleaning the substrate 104, the primary treatment 
liquid 110 is stored as the secondary treatment liquid 112 in the second 
cleaning liquid bath 102. The arrangement up to this point is the same as 
that in the cleaning apparatus described with FIG. 1. 
Next, the secondary treatment liquid 112 is fed through the pipe 118, 
connected to the second cleaning liquid bath 102, past the converging 
device 113, and through the pipe 117 to the shower head 106 by the pump 
114. The cleaning liquid supplied to the shower head 106 is jet-sprayed 
through the shower head 106 toward the substrate 104 to clean the 
substrate 104. After cleaning the substrate 104, the cleaning liquid is 
stored as the tertiary treatment liquid 115 in the first cleaning liquid 
bath 101. 
The pipe 119 is connected to the first cleaning liquid bath 101, which 
enables the tertiary treatment liquid 115 stored in the first cleaning 
liquid bath 101 to be supplied through the converging device 113 to the 
pipe 117. 
Namely, in the case of this drawing, the cleaning liquid supplied to the 
shower head 106 by the pump 114 is a mixture of the secondary treatment 
liquid 112 used to clean the substrate 104 twice with the tertiary 
treatment liquid 115 used to clean the substrates 104 three or more times. 
Such an arrangement enables the substrate 104 to be cleaned with the 
cleaning liquid jet-sprayed through the shower head 106 at a rate greater 
than the supply rate (V 1/min) of the newly supplied cleaning liquid. 
The capability of jet-spraying a greater amount of the cleaning liquid per 
unit time results in increasing an amount of the cleaning liquid per unit 
area, whereby the substrate can be cleaned more evenly and pollutants can 
be washed out under a higher pressure. 
Also, this apparatus requires simply increasing the capacity only of the 
final-stage pump 114, but does not always have to increase the performance 
of the all other pumps, thus realizing a lower cost cleaning apparatus. 
A simplest structure of the converging device 113 is a structure having 
openings allowing three-way communications of the cleaning liquid and 
opening to one space, for example a three way joint as shown in FIG. 3A, 
or a structure obtained by perforating a midway part of one pipe and 
connecting another pipe thereto. 
Of course, taking into consideration the cases where pipe resistances of 
the pipe 118 and pipe 119 or pressures on the cleaning liquids in these 
pipes are different from each other or the cases where a mixture ratio 
between the secondary treatment liquid and the tertiary treatment liquid 
is desired to change, the converging device 113 may be constructed in the 
structure shown in FIG. 3B or the structure as shown in FIG. 3C. 
In FIG. 3B, a projection 31 is provided in a flow path to form a flow 
resistance, thereby making the flow of liquid or pipe resistance 
controllable. For example, the controllability can be achieved by properly 
determining a projecting amount of the projection or angles of slant faces 
of the projection, differentiating an angle of left slope from an angle of 
right slope, or shifting the position of the projection horizontally. 
FIG. 3C shows an arrangement with a valve 32, in which the controllability 
is similarly realized by adjusting the position of the valve or an angle 
(rotation angle) of the valve. 
In case of the arrangement as shown in FIG. 2 where the cleaning liquid as 
the mixture of the secondary treatment liquid and the tertiary treatment 
liquid is fed by the pump, the simple structure as described above can be 
employed as the structure of the converging device without any practical 
problem. If it is necessary to prevent unprepared mixture of the secondary 
treatment liquid with the tertiary treatment liquid, a check valve is 
desirably provided on the way of the flow path, for example on the way of 
the pipes 118, 119. 
FIG. 2 shows an example in which one pump 114 supplies the cleaning liquid 
to the shower head 106 in a greater amount than an amount of the new 
cleaning liquid, but the pipe 118 and pipe 119 may be provided with 
respective pumps. In this case, the capacity of each pump can be made 
smaller, which enables the cleaning apparatus to be designed in a smaller 
scale. Also, the mixture ratio between the secondary treatment liquid and 
the tertiary treatment liquid can be adjusted by adjusting driving forces 
of the pumps, that is, discharge amounts of cleaning liquid. 
Of course, the converging device 113 is by no means limited to the 
above-mentioned structures, but may take a variety of forms. Instead of 
the structures having the three way openings as described above, the joint 
apparatus may be constructed in such a structure that a storage portion is 
provided as a separate part for storing the mixture of the treatment 
liquids and the mixture is supplied from the storage portion through a 
pipe to the shower head 106. 
There are no restrictions on the shape, the number of holes, the size, and 
the layout of the shower heads for jet-spraying the cleaning liquid as 
long as they can jet-spray the cleaning liquid toward the treated body 
(substrate) with efficiency. Thus preferred are a structure (as 
illustrated in the diagrammatic perspective view of FIG. 4A) in which a 
bottom surface of a circular cone is perforated with a lot of jet pores 
41, a structure (as illustrated in the diagrammatic perspective view of 
FIG. 4B) in which a pipelike member is perforated with a lot of jet pores 
41 along the lengthwise direction, and a structure (as illustrated in the 
diagrammatic cross section of FIG. 4C) in which an opening 41 is provided 
and a restricting member 43 is located at a position opposed thereto so as 
to let the cleaning liquid impinge on the restricting member 43, thereby 
scattering the cleaning liquid. Although not shown, the jet pores 41 shown 
in FIG. 4B may be modified as a slit. Of course, any other shapes and 
structures than the above structures may be employed, and thus, there are 
no restrictions on the structure etc. of the shower heads insofar as 
cleaning of treated body can be effectively performed. In FIGS. 4A to 4C, 
an arrow represents the direction of flow of the cleaning liquid. 
Upon cleaning, brushing may be carried out with a brush if necessary. 
As described, the present invention makes a part of a waste liquid from a 
lower cleaning bath join a waste liquid from an at least one upper stage 
cleaning bath to use the mixture as a cleaning liquid for the above lower 
cleaning bath, thereby enabling to independently increase an amount of the 
cleaning liquid that can be used at a bath for first cleaning of the 
substrate, for example at the first bath. Accordingly, it becomes possible 
that pollutants of the types difficult to clean can be removed with 
efficiency at the first bath without changing the total amount of the 
cleaning liquid used. 
Using the cleaning apparatus as described above, the present invention also 
is able to remove highly sticky contaminants, which were heretofore 
difficult to clean, for example deposits comprised of an etching paste or 
the like with a viscosity of not less than 10,000 mPa.cndot.s. 
Further, in the present invention, the flow rate W of the cleaning liquid 
used for the lower cleaning bath is arranged as variable between the flow 
rate in cleaning mode 1 with only highly pure cleaning water, i.e., the 
flow rate V, and the flow rate in cleaning mode 2 with addition of a waste 
water from a lower cleaning bath, i.e., the flow rate (V+V.sub.1), so that 
the two types of cleaning modes can be selectively used. In this case, 
cleaning with higher cleanliness becomes possible, because cleaning can be 
carried out depending upon circumstances, as against the cases where the 
flow rate is fixed at (V+V.sub.1). 
It is more preferred that a pollutant trap member such as a filter be 
interposed between a cleaning liquid bath and a pipe in order to remove 
the pollutant. 
The moving means for moving the substrate as a treated body is not limited 
to the above-mentioned belt, but it may be a moving means using an arm. If 
it is a beltlike member, it is preferably formed as mesh. 
The present invention can be applied to not only single treated bodies in a 
batch manner, but also a continuous body. 
&lt;Experiment&gt; 
Next, the cleaning apparatus shown in FIG. 2 was used to clean a stainless 
steel plate of 10 cm.sup.2 on which a paste with a viscosity of 30,000 
mPa.cndot.s was deposited as sticky pollutant. 
As the new cleaning liquid pure water was supplied at a flow rate of V 
1/min from the shower head 108 corresponding to the third cleaning liquid 
bath 103. Thus, the cleaning liquid from the shower head 107 was also 
supplied at the flow rate of V 1/min. 
A jet spray time of the cleaning liquid from each shower head was 5 
minutes. 
A jet spray rate of the cleaning liquid jet-sprayed from the shower head 
106 was determined for a mixture of the cleaning liquids between the flow 
rate of the secondary treatment liquid 112, V 1/min, and the flow rate of 
the tertiary treatment liquid 115, V.sub.1 1/min. Namely, the flow rate of 
the cleaning liquid jet-sprayed from the shower head 106 was V+V.sub.1 
1/min. Here, the flow rate was adjusted as V&gt;V.sub.1. 
&lt;Comparative Experiment&gt; 
The cleaning apparatus shown in FIG. 1 was used to clean a stainless steel 
plate of 10 cm.sup.2 on which a paste with a viscosity of 30,000 
mPa.cndot.s was deposited as sticky pollutant, similarly as in the above 
experiment. 
Pure water was also used as the newly supplied cleaning liquid, similarly 
as in the above experiment, and the flow rate thereof was V 1/min 
similarly as in the above experiment. Accordingly, the flow rate of the 
cleaning liquid jet-sprayed from each shower head was V 1/min. 
The other conditions were the same as those in the above experiment, 
including the shape of shower heads, the number, size, and layout of jet 
pores formed therein, and the distance between the shower heads and the 
stainless steel plate. 
For comparison's sake, a removal effect of pollutant was checked for a case 
where the flow rate of the cleaning liquid used for first cleaning was set 
smaller than V. 
FIG. 5 shows a relation between the amount of unremovable pollutant and the 
flow rate of cleaning liquid. The amount of unremovable pollutant Y was 
measured in the following manner. 
A stainless steel substrate as a treated body after cleaned was fully 
dried, and a weight y.sub.1 thereof was measured. In order to dissolve and 
remove the paste as unremoved pollutant on the substrate, the substrate 
after cleaned was next immersed in a 1:1 mixture solvent (temperature 
60.degree. C.) of xylene and IPA (isopropyl alcohol) for 10 minutes. 
After immersed, the substrate was next fully dried, and a weight y.sub.2 of 
the substrate was measured. A difference between the weights of the 
substrate before and after immersion, i.e. y.sub.1 -y.sub.2, was defined 
as an amount of unremovable pollutant Y. 
As seen from FIG. 5, the amount of unremovable pollutant Y tended to 
decrease with an increase of the water jet amount X from the shower head. 
However, it seemed that as to the sticky pollutant, the amount of 
unremovable pollutant Y had little relevance to the cleanliness of the 
cleaning water. 
Namely, it was found that the sticky pollutant deposited on a substrate not 
cleaned at all was able to be removed more as the amount of cleaning water 
increased. 
It was thus found out that the effect to remove the sticky pollutant from 
the substrate became higher when the water jet amount X of shower was set 
to (V+V.sub.1) than when the water jet amount X of shower was set to V. 
Also, the sticky pollutant can be removed more easily with a higher 
pressure of the cleaning liquid on the substrate. A pollutant soluble to 
the cleaning liquid or a pollutant with less stickiness can also be 
effectively removed by increasing the water jet amount of shower. 
It was thus confirmed that the total amount of supply water and waste water 
necessary in the experiment was the same as that in the comparative 
experiment. It was thus confirmed that the use of the path of cleaning 
water in the experiment was able to overcome the problems of increasing 
the supply demand, of pure water etc., increasing the disposal demand, and 
increasing the feed demand (pump capacity etc.) of cleaning water. 
Accordingly, the sticky pollutant can be effectively removed without 
increasing the cost for the apparatus. As a consequence, it was confirmed 
that the present invention was able to provide the cleaning apparatus 
permitting a high-yield fabrication line as maintaining the cost for final 
products. 
The flow rate of the cleaning liquid changes depending upon the shape of 
shower heads used, the size of substrates cleaned, the shape of substrates 
cleaned, a treatment number of cleaned substrates per unit time, and the 
cleanliness demanded. Specifically, for plate substrates, the flow rate V 
is preferably 2 to 20 1/min, more preferably about 5 to 10 1/min, and the 
above flow rate V.sub.1 is preferably 2 to 20 1/min, more preferably about 
5 to 20 1/min. 
The number of cleaning stages n does not have to be 3 as in FIG. 2, but n 
may be more than 3 for necessary cleaning. However, n should be an integer 
not less than 2 in the present invention. 
Further, it is preferred that preliminary washing be carried out before the 
above cleaning, using a low-grade cleaning liquid, for example such as tap 
water. 
In addition, the above description concerned mixing the first-stage 
treatment liquid with the second-stage treatment liquid out of the 
three-stage cleaning liquids, but a mixture of the second-stage treatment 
liquid with the third-stage treatment liquid may be further used as the 
second-stage cleaning liquid if necessary. 
In this manner, the present invention enables to remove pollutants of the 
types which were heretofore difficult to clean, such as highly sticky 
pollutants, and it is a matter of course that the invention demonstrates 
better effects to clean pollutants with low stickiness than the 
conventional methods. 
The following description concerns examples where the semiconductor devices 
are solar cells. 
EXAMPLE 1 
In this example a relation was checked between the viscosity P of pollutant 
with stickiness and the amount Q of unremovable pollutant remaining on the 
substrate after cleaned. A change of the viscosity P was achieved by 
changing the viscosity of the etching paste 307 as detailed later. Also, 
an evaluation method of the amount of unremovable pollutant Q was the same 
as that for the amount of unremovable pollutant Y as obtained in the above 
experiment. 
First described with FIGS. 6A to 6E is a method for producing a solar cell 
substrate cleaned and evaluated in this example. 
FIG. 6A is a diagrammatic plan view to show a state where the etching paste 
as detailed later is printed on a solar cell substrate, and FIG. 6B is a 
diagrammatic side view thereof. FIG. 6C is a diagrammatic cross section of 
the solar cell substrate shown in FIG. 6A and FIG. 6B. 
The solar cell substrate of FIG. 6C was a structure obtained by forming a 
back reflective layer 302, a non-single-crystal silicon layer 303, and a 
transparent conductive layer 304 in order on a surface of base 301 
comprised of stainless steel as a conductive material, by a film-forming 
method such as sputtering, CVD, etc. The stainless steel 301 had the 
thickness of 150 .mu.m. The back reflective layer 302 was provided for 
reusing incident light, which was a laminate structure of aluminum-silicon 
of a conductive material and zinc oxide, and the thickness thereof was 1 
.mu.m. The non-single-crystal silicon layer 303 was a subject to generate 
a photoelectromotive force, which was a laminate structure of p-type 
semiconductor layer, i-type semiconductor layer, n-type semiconductor 
layer, p-type semiconductor layer, i-type semiconductor layer, and n-type 
semiconductor layer in order from the bottom, and the thickness thereof 
was 1 .mu.m. The transparent conductive film 304 was provided for the 
purposes of anti-reflection and collection of electrons and was comprised 
of indium oxide herein, and the thickness thereof was 20 nm. 
Now, the solar cell substrate 305 having the structure shown in FIG. 6C is 
used after cut in a size as needed. When the substrate was cut, short 
circuit has frequently occurred between the transparent conductive film 
304 and the base 301 of the conductive material on the cut surface. This 
short circuit makes charges generated in the above non-single-crystal 
silicon layer 303 flow in the reverse direction or vanish, resulting in 
dropping the power generation efficiency of the above solar cell. 
Specifically speaking, the short-circuit resistance of the above solar 
cell decreases so as to fail to obtain a sufficient electromotive force. 
In order to prevent this short-circuit phenomenon, this example included a 
groove 304a formed by removing a part of the transparent conductive film 
304, as conceptually shown in FIG. 6D, so as to form an independent 
generation area 306 as shown in FIG. 6E. 
Specifically, the surface of the solar cell substrate 305 was coated with 
the etching paste 307 including FeCl.sub.3, AlCl.sub.3, etc. by the method 
of screen printing, etc., the coated substrate was then heated, and the 
above etching paste 307 was finally removed and cleaned to obtain the 
substrate in the state shown in FIG. 6E. For the use in screen printing as 
described above, the etching paste 307 was provided with a viscosity of 
not less than 10,000 mPa.cndot.s by adding a filler, a thickening agent, 
etc. to the above substance including FeCl.sub.3, AlCl.sub.3, etc. 
In this example, the viscosity of the etching paste 307 was changed in the 
range of 600 to 46,000 mPa.cndot.s to check an amount of unremovable 
pollutant remaining on the solar cell substrate after cleaned. 
FIG. 13 is a diagrammatic structural drawing to schematically illustrate a 
producing apparatus for producing the solar cell substrate in the present 
example. This apparatus was comprised of three steps of printing, drying, 
and cleaning, and carrying conveyors for moving the solar cell substrate 
were provided between the steps. 
The printing step is a step of using a printing apparatus 403 to form the 
above etching paste 307 in a desired pattern on the surface of the solar 
cell substrate 305 by a squeegee 410, using a screen 401. 
The drying step is a step of using an IR type tunnel drying furnace 405 to 
heat the solar cell substrate 305 after printed, at 150.degree. C. for one 
minute by an IR heater installed inside the drying furnace 405. This heat 
treatment removed only the part coated with the above etching paste 307 
from the transparent conductive film 304 of the substrate surface. 
The cleaning step is a step of, after cooling the solar cell substrate 305 
after down to the ordinary temperature, cleaning to remove the etching 
paste 307, using the cleaning apparatus 407 shown in FIG. 2. This cleaning 
apparatus 407 was one comprised of multiple baths connected in cascade 
connection, in which the first stage was arranged to make a part of waste 
water from the first cleaning liquid bath 101 join the waste water from 
the second cleaning liquid bath 102 to supply a strong jet flow, thereby 
removing the etching paste 307 from the surface of substrate 305. On this 
occasion, the rate of the cleaning water jet-sprayed from the first shower 
head 106 was set to (V+V.sub.1) of 20 1/min. At and after the second 
stage, the substrate surface was properly cleaned at the flow rate of 7 
1/min of cleaning water gradually becoming cleaner, finally obtaining a 
clean surface having almost zero removable pollutant. Finally, the solar 
cell substrate 305 after cleaned was subjected to a hydro-extracting 
treatment with hot air knife 408. 
Since the next step and steps after the next step are not directly related 
to this example, the detailed description thereof is omitted herein, but 
briefly described, the following steps are for forming electrodes for 
collecting electrons in the power generation area separated in the above 
steps. 
COMATIVE EXAMPLE 1 
As a comparative example, the substrate was cleaned with the cleaning 
liquid of the secondary treatment liquid from the second cleaning liquid 
bath 102 but without using the tertiary treatment liquid from the first 
cleaning liquid bath 101, while the cleaning liquid was jet-sprayed at the 
flow rate of 7 1/min from the first shower head 106. Namely, the amount of 
the cleaning liquid jet-sprayed from the first shower head 106 was set to 
be the same as the amount of waste liquid from the second cleaning liquid 
bath 102 one step upper. The other points were the same as those in 
Example 1. 
FIG. 7 shows results of evaluation of relations between the viscosity of 
sticky pollutant, i.e., the viscosity P of the etching paste 307, and the 
amount of unremovable pollutant Q remaining on the surface of the solar 
cell substrate 305 produced as described above. The evaluation results can 
be summarized as follows. 
(1) The amount of unremovable pollutant Q in Example 1 is smaller 
throughout the entire viscosity range than that in Comparative Example 1. 
Namely, it was confirmed that the surface of the solar cell substrate 305 
in Example 1 was cleaner. 
(2) It was confirmed that the amount of unremovable pollutant in Example 1 
was far smaller than that in Comparative Example 1 particularly in the 
range where the viscosity P of the etching paste 307 was not less than 
10,000 mPa.cndot.s. 
It can be thus said that the surface of the solar cell substrate 305 can be 
made clearer by the cleaning apparatus of the present invention. Since the 
viscosity P of the etching paste 307 can be relatively freely selected, 
etching pastes with particularly high viscosities can be used, which 
enabled to achieve finer etching lines, that is, finer and clearer 
patterns. 
EXAMPLE 2 
In this example, a substrate .alpha., in which a collector electrode was 
formed on the surface of the solar cell substrate after completion of 
cleaning and hydro-extraction with air knife as in Example 1, was again 
cleaned, and an amount of unremovable pollutant R remaining on the surface 
of the substrate a was evaluated. 
FIG. 8A and FIG. 8B are schematic drawings to show the structure of the 
collector electrode, wherein FIG. 8A is a diagrammatic plan view thereof 
and FIG. 8B a diagrammatic perspective view. In the drawings, reference 
numeral 600 designates the solar cell substrate, 601 a copper paste, 602 a 
solder, and 604 a soldering paste. 
FIG. 9A to FIG. 9D are schematic step diagrams to illustrate steps for 
forming the collector electrode in the structure shown in FIG. 8A and FIG. 
8B. 
(1) The copper paste 601 was screen-printed in width of 150 .mu.m and at 
pitch of 5 mm on an effective surface of the solar cell substrate 600, and 
was hardened at 150 .degree. C. in an IR drying furnace. (FIG. 9A) 
(2) The soldering paste 602 was screen-printed in width of 400 .mu.m and at 
pitch of 5 mm so as to fully cover the copper paste 601 in the above step 
(1). (FIG. 9B) 
(3) The substrate in the above step (2) was heated at 300.degree. C. in an 
IR hardening furnace. When the solder was melted in the IR hardening 
furnace, the transparent conductive film (not shown) of the outermost 
surface of the solar cell substrate 600 repelled the soldering paste 602, 
so that the soldering paste 602 was automatically self-aligned on the 
copper paste 601 to form the collector electrode of solder coated copper 
in width of 150 .mu.m. (FIG. 9C) 
(4) Further, the substrate in the above step (3) was cleaned to remove flux 
603, using the cleaning apparatus shown in FIG. 2. (FIG. 9D) 
The water jet amount of the cleaning liquid from the first shower head, 
that is, the rate of cleaning liquid in the first stage was (V+V.sub.1) of 
20 1/min. 
A purpose of this cleaning is to remove the flux 603 of solder formed on 
the both sides of the copper paste 601 present on the substrate 600, as 
shown in FIG. 9C. Since this flux 603 is acid, it could adversely affect 
devices in the future. Thus, it must be removed completely. The flow rate 
of the cleaning liquid in and after the second stage was 5 1/min. 
COMATIVE EXAMPLE 2 
As a comparative example, only the waste water of cleaning in the second 
stage was used as the cleaning liquid jet-sprayed from the first shower 
head, and the rate of the cleaning liquid was 5 1/min. The other points 
were the same as those in example 2. 
The amount of unremovable pollutant R remaining on the surface was checked 
using the substrate 600 formed in the above-stated manner. The above flux 
was cleanly removed without trace in Example 2, whereas the flux was not 
perfectly removed so as to leave trace in Comparative Example 2, failing 
to achieve a satisfactory result in respect of appearance as well. 
It was thus confirmed that the cleaning apparatus of the present invention 
was effective to remove sticky pollutant including the etching paste or 
the like and was also able to remove the soldering flux. 
EXAMPLE 3 
In this example, another soldering paste, in which silver particles in 
particle size of 1 .mu.m were mixed in proportion of 10 wt %, was used in 
place of the soldering paste in Example 2, and a substrate was obtained by 
forming the collector electrode on the surface of the solar cell 
substrate. FIG. 10A and FIG. 10B are schematic drawings to illustrate the 
structure of the above collector electrode, wherein FIG. 10A is a 
diagrammatic plan view and FIG. 10B a diagrammatic perspective view. In 
the drawings, reference numeral 700 designates a solar cell substrate, 701 
a copper paste, 702 a solder, 703 a flux, 704 silver particles, and 705 a 
soldering paste. 
FIG. 11A to FIG. 11D are schematic step diagrams to illustrate steps for 
forming the collector electrode in the structure shown in FIG. 10A and 
FIG. 10B. 
Also in the present example, similarly as in Example 2, the copper paste 
701 was printed and hardened on the substrate 700 (FIG. 11A), and then the 
soldering paste 705 containing dispersed silver particles 704 was 
screen-printed on the copper paste 701 thus hardened (FIG. 11B). 
Subsequently, the substrate was heated in the IR hardening furnace to melt 
the solder and thereafter to harden (FIG. 11C). Then the substrate 700 
lowered to the ordinary temperature was cleaned using the cleaning 
apparatus shown in FIG. 2 to remove the flux 703 or the like remaining on 
the surface of substrate 700 at the previous steps (FIG. 11D). 
In the present example, the substrate after completion of the above 
cleaning step was again cleaned, and then the amount of unremovable 
pollutant S remaining on the surface of the substrate was evaluated. Here, 
a reason of incorporating the silver particles 704 into the soldering 
paste 705 is to thicken the solder layer. When the solder layer is 
thickened, a cross section for an electric current to pass increases, 
thereby decreasing current losses due to resistance. 
The other points were the same as those in Example 2. 
For example, the rate of the cleaning liquid jet-sprayed from the first 
stage shower head in the cleaning apparatus was set to the same rate, 20 
1/min, as in Example 2. Also, the rate of cleaning liquid in and after the 
second stage was 6 1/min. Purposes of cleaning in the present example are 
to remove the flux 703 of solder formed on the both sides of the copper 
paste 701 present on the surface of the solar cell substrate 700 and to 
remove the silver particles 704 irregularly scattered on the transparent 
conductive layer (not shown) of the outermost surface of the solar cell 
substrate 700. Since this flux 703 is acid, it could adversely affect the 
devices in future. Thus, it must be completely removed by cleaning. 
Further, the silver particles 704 have a possibility of causing shunt due 
to migration of silver ions to grow in the semiconductor layer. Thus, the 
silver particles scattered must be perfectly removed by cleaning. 
COMATIVE EXAMPLE 3 
The rate of the cleaning liquid jet-sprayed from the first-stage shower 
head was set to 6 1/min, which was the same as in and after the second 
stage. Further, the cleaning liquid jet-sprayed from the first-stage 
shower head was only the waste water (secondary treatment liquid) from the 
second stage bath one stage upper. The other points were the same as in 
Example 3. 
For the substrates formed in the above manner, the amount of unremovable 
pollutant S remaining on the surface was checked. In Example 3, the flux 
was cleaned without trace and the silver particles scattered over the 
transparent conductive film were also removed. Comparative Example 3, 
however, failed to perfectly remove the flux and silver particles, and 
also failed to achieve a satisfactory result in respect of appearance. 
It was thus confirmed that the cleaning apparatus of the present invention 
was effective to remove sticky pollutant consisting of the etching paste 
etc. and was able to remove the flux of solder and silver particles 
scattered over the transparent conductive film. 
EXAMPLE 4 
In this example, the rate of the cleaning liquid jet-sprayed from the 
first-stage shower head was changed at least once between V (=7 1/min) and 
(V+V.sub.1) (=20 1/min), instead of the fixed rate of the cleaning liquid 
jet-sprayed from the first-stage shower head at (V+V.sub.1) (=20 1/min) in 
Example 1. The other points were the same as in Example 1. 
FIG. 12 is a graph to show a relation between the number of changes in 
amount of cleaning water jet-sprayed from the first shower within a 
cleaning time of five minutes and the cleanliness of substrate. Here, the 
rate of the cleaning liquid jet-sprayed from the first shower was first 
set at (V+V.sub.1) (=20 1/min). 
This example demonstrated that the cleanliness of substrate was able to be 
further improved by changing the rate of the cleaning liquid jet-sprayed 
from the first-stage shower head at least once. In particular, it was 
confirmed that higher cleanliness was able to be achieved by setting the 
number of changes to an odd number, that is, by setting the rate of the 
cleaning liquid jet-sprayed from the first-stage shower head to V (=7 
1/min) finally or immediately before moving to the second stage. Namely, 
it is understood that to decrease the rate of the cleaning liquid before 
the substrate moves to the next stage is effective for achieving higher 
cleanliness. 
EXAMPLE 5 
In this example, a solar cell substrate was immersed in an electrolyte to 
be electrochemically treated so as to eliminate defects present in the 
solar cell substrate and thereafter was cleaned, and then an amount of 
unremovable pollutant remaining on the surface of substrate was evaluated. 
The solar cell substrate similar to that in Example 1 was put in an 
electrolytic apparatus containing an aluminum chloride solution not shown, 
and a forward voltage was applied to treat the defects. 
Next, cleaning was carried out in the same manner as in Example 1. After 
cleaning, the substrate was subjected to hydro-extraction with air knife 
and then was dried in a hot air drying furnace not shown. 
The substrate after completion of cleaning and drying was put into an 
ultrasonic cleaning apparatus containing pure water not shown, and 
conductivities of liquids inside the ultrasonic apparatus before and after 
cleaning were compared. There was no change observed. 
This example confirmed that high cleanliness could be achieved by the 
present invention in cleaning of liquid with low stickiness as well. 
As described above, the present invention provides the cleaning apparatus 
and cleaning method that can be operated at low cost and depending upon a 
degree of contamination on the substrate. 
Further, the present invention provides the cleaning apparatus and cleaning 
method that can effectively remove pollutant with high stickiness. 
In addition, the present invention provides the cleaning apparatus and 
cleaning method that can also perform higher-cleanliness cleaning as well 
as removing the pollutant with high stickiness. 
It is noted that the present invention is by no means limited to the above 
method for producing the solar cell substrate, but can be generally 
applied to cleaning steps including those necessitating batch cleaning. 
Particularly, the present invention is effective for cleaning steps for 
removing sticky pollutant from substrate. 
Although the present invention was described assuming the cleaning 
apparatus comprised of three baths, the invention can also be applied of 
course to cleaning apparatus having a plurality of cleaning liquid baths, 
as described previously. 
Further, although the present invention was described assuming the cleaning 
system with water-base cleaning liquids, the invention can also be applied 
to cleaning systems with non-water base cleaning liquids (for example 
solvents such as acetone or alcohol). 
It is needless to mention that the present invention can be modified with 
necessity within the scope of the essence of the present invention.