WO2008060148A2

WO2008060148A2 - Atomizer for atomizing a doping solution and a method for treating a substrate

Info

Publication number: WO2008060148A2
Application number: PCT/NL2007/050561
Authority: WO
Inventors: Cornelius Hendrik Kant; Martin Dinant Bijker
Original assignee: Otb Group B.V.
Priority date: 2006-11-13
Filing date: 2007-11-13
Publication date: 2008-05-22
Also published as: NL1032854C2; WO2008060148A3; TW200911382A

Abstract

Atomizer for atomizing a doping solution for treating a substrate therewith, wherein the atomizer comprises an atomizing element with a liquid compartment for receiving the doping solution to be atomized, wherein the atomizing element is an atomizing element functioning on the basis of an air flow. The invention further relates to a method for treating a substrate, wherein the method comprises the following steps: - providing a substrate; - atomizing a doping solution; - supplying the atomized doping solution to the substrate to treat the substrate therewith, wherein the doping solution is atomized by passing an air flow over the doping solution.

Description

Title: Atomizer for atomizing a doping solution and a method for treating a substrate

The invention relates to the atomization of a doping solution for treating a substrate therewith, where the atomizer comprises an atomizing element.

Such an atomizer is known and is used for atomizing a doping solution such as for instance a phosphorus or boron containing solution, for instance a phosphoric acid solution, in order to apply the phosphorus or boron containing solution onto a substrate. This concerns a solution in water. The known atomizer for applying a doping solution onto a substrate is a piezo-atomizer and is in fluid communication with a process chamber in which the substrate is provided. After the doping solution has been atomized, it flows from the atomizer into the process chamber. The atomized doping solution precipitates on the substrate, so that a liquid film is formed on the substrate.

The known piezo-atomizer is suitable for processing a phosphoric acid solution having a concentration of approximately 30% at a maximum. At a higher concentration of the phosphoric acid solution, the viscosity of the solution is so high that the atomizer is incapable of atomizing such a solution to form an atomized solution with a relatively small drop size, which is needed to obtain a qualitatively good phosphoric acid layer. As a consequence, a strongly diluted phosphoric acid solution, having a relatively low viscosity, is to be supplied to the atomizer. In this way, a relatively large amount of phosphoric acid solution is needed, which is unfavorable to the effectiveness of the atomization process.

Accordingly, the object of the invention is to provide an atomizer without the disadvantages described. More particularly, the invention contemplates an atomizer capable of atomizing a doping solution in an effective manner so as to apply a qualitatively good doping liquid layer onto a substrate.

To that end, the present invention provides an atomizer for atomizing a doping solution for treating a substrate therewith, wherein the atomizer comprises an atomizing element with a liquid compartment for receiving the doping solution to be atomized, wherein the atomizing element is an atomizing element functioning on the basis of an air flow.

Owing to atomization taking place with the aid of an atomizing element functioning on the basis of an air flow, a small drop size can be realized at a relatively high viscosity, and as a result the maximum concentration of the doping solution that can be processed is much greater. For instance, by an atomizer according to the invention, a doping solution, in particular a phosphoric acid solution, having a concentration of up to approximately 85% can be atomized. The drop size of the liquid atomized by the atomizer is relatively small, for instance in the order of magnitude of 5 μm or even smaller. This drop size is much smaller than a conventional drop size of 20 μm which is obtained through atomization by the known piezo-atomizers for phosphoric acid atomization. Owing to the present possibility of using a doping solution, in particular phosphoric acid solution, having a concentration of approximately 85% instead of a concentration of approximately 30% as is conventional, a smaller amount of doping solution is needed for a same quantity of doping liquid to be applied. Through these above-mentioned advantages, an effective manner of applying material is accomplished with such an atomizer.

A further disadvantage of a known atomizer is that it may comprise metal parts. Such metal parts can contaminate the substrate with metal atoms, which is unfavorable to the eventual quality of the product produced with that substrate. Therefore, it is particularly favorable when, according to a further elaboration of the invention, at least the atomizing element is of a plastic. Also, the known piezo-atomizer has a complicated construction and is therefore relatively costly. According to a further elaboration of the invention, the atomizing element is an existing atomizing element, for instance an atomizing element as disclosed in EP 0,627,266, which is an atomizing element for administering liquid medicines through inhalation. The atomizer with the atomizing element according to the invention is much cheaper than the known atomizer. The cost price of an atomizer according to the invention is negligible compared with that of the known atomizer, whilst the operation of the atomizer according to the invention as described earlier is an improvement over the known atomizer. The atomizing element from EP 0,627,266 is an atomizer based on the Venturi effect. It is self-evident that also any other existing atomizing element, utilizing a comparable manner of atomization, can be used in the atomizer according to the invention.

Applying a phosphorus or a boron containing layer may be desired for manufacturing a solar cell. According to a further elaboration of the invention, the substrate is then a silicon substrate. By applying a phosphorus or boron containing layer, a p-n junction is created in the silicon substrate, which junction is needed to obtain an electrical field in the substrate.

In a further elaboration of the invention, the atomizing element is provided with an air supply for supplying air to the doping solution, wherein the atomizing element is arranged for atomizing the doping solution by means of an air flow flowing over the doping solution. The doping solution atomized by means of the supplied air flow is transported to the substrate with the aid of the same air flow. Optionally, an additional air flow, generated by the Venturi effect, may be used for transporting the atomized doping solution. No other means of transport for displacing the atomized doping solution is needed, which is favorable to the simple construction of the atomizer. In a further elaboration of the invention, the atomizer comprises a liquid container, which is in fluid communication with the liquid compartment of the atomizing element, wherein the atomizer is arranged to provide a constant or continuous supply of doping solution to the liquid compartment. This is particularly favorable in view of the fact that a process for treating substrates in which the atomizer is used, is preferably a continuous or semicontinuous process. For that, it is desirable for the process not to be interrupted for the purpose of replenishing the liquid compartment when the liquid level of the doping solution is too low. By replenishing the liquid container timely, there is constantly an amount of doping solution present in the liquid compartment of the atomizing element.

Automatic replenishment of the liquid compartment can be accomplished, according to a further elaboration of the invention, in that the liquid compartment is provided at least partly in the liquid container of the atomizer, while the liquid compartment comprises an inlet opening which is situated below a liquid level in the liquid container. The doping solution flows from the liquid container via the inlet opening in the liquid compartment into the atomizing element. In another embodiment, the atomizing element may also be provided outside the liquid container, with a communication being provided between the liquid container and the liquid compartment for supplying the doping solution to the atomizing element. Constant or continuous supply should not be understood to mean exclusively a consistently equal supply. The supply may be variable in time. What is important is that liquid is at all times present in the liquid compartment and available for atomization.

For the continuity of the supply of the doping solution to the liquid compartment in the atomizing element, it is particularly favorable, according to a further elaboration of the invention, when the liquid level in the liquid container is regulable, in order to keep the inlet opening of the liquid compartment below the liquid level at all times. By keeping the liquid level in the liquid container above the inlet opening at all times, doping solution will always be present in the liquid compartment and be supplied. The liquid level in the liquid container can for instance be regulated by means of a sensor in the container, which is designed, in the event of too low a level, to communicate to a control that doping solution is to be supplied to the container. Such a sensor can also ensure that the liquid level in the container does not become unduly high, as a result of which the atomizing element could not function anymore.

In another embodiment of the invention, the atomizer may be provided with a regulating element for regulating a liquid level in the liquid compartment of the atomizing element. Such a regulating element in the atomizer ensures that when a liquid level in the liquid compartment threatens to fall below a desired level, doping solution is supplied to the liquid compartment. This may for instance proceed via a supply duct which has a first end connected to the liquid compartment and a second end connected to a doping solution supply. In such an embodiment, a separate liquid container in the atomizer is not needed. In a further elaboration of the invention, an outlet opening of the atomizer for letting out atomized doping solution is detachably connectible to an inlet opening of a process chamber in which the substrate is receivable. By virtue of such a construction of the atomizer, it is possible to uncouple the atomizer from the process chamber. In case of a (temporarily) modified process, the process chamber can be used for a different purpose, which is favorable to the flexibility and the costs of a total apparatus for treating substrates.

The invention further relates to a method for treating a substrate, wherein the method comprises the following steps: - providing a substrate;

- atomizing a doping solution;

- supplying the atomized doping solution to the substrate to treat the substrate therewith, characterized in that the doping solution is atomized by passing an air flow over the doping solution.

With such a method, corresponding advantages to those described above with reference to the atomizer according to the invention can be obtained.

In further elaboration of the invention, the method comprises at least one of the following steps:

- etching the substrate;

- diffusing doping atoms from the applied doping liquid layer into the substrate; - depositing coating on the substrate;

- applying at least one metal layer onto the substrate;

- laser ablating the substrate.

According to a still further elaboration of the method, the substrate may be a silicon substrate and the method is used for manufacturing a photovoltaic cell. As doping solution, for instance a phosphorus solution or a boron containing solution can be used. Such substances create a p-n junction in the silicon.

Further elaborations of the invention are described in the subclaims and will be further clarified hereinafter with reference to the drawing, wherein

Fig. 1 shows a schematic cross section of a process chamber and atomizer according to an embodiment of the invention;

Fig. 2 shows a cross section of the atomizer according to an embodiment of the invention; Fig. 3 shows a process diagram of a treatment process of a substrate according to an embodiment of the invention.

In the different figures, equal reference numerals refer to equal parts.

In Figure 1 a cross-sectional elevation of an atomizer 1 and a process chamber 2 according to an embodiment of the invention is represented. The atomizer 1 is arranged for atomizing a doping solution F to provide a substrate 3 with a film of doping liquid, such as for instance a phosphorus or boron containing layer of liquid. The substrate 3 is provided in the process chamber 2.

The atomizer 1 is provided with an atomizing element 4. The atomizing element 4 is an atomizing element functioning on the basis of an air flow and is partly placed in a liquid container 5. The atomizing element 4 is of a plastic and preferably comprises a minimal number of injection molded parts. The atomizing element 4 can be a generally commercially available element, for instance an atomizing element as described in EP 0,627,266. This atomizing element 4 works according to the Venturi effect. It is self-evident that other, similar atomizing elements based substantially on a same working principle could also be used.

The atomizing element 4 is in fluid communication with the process chamber 2 through a connection element 6 connected to an outlet opening 7 of the atomizer 1. The outlet opening 7 is detachably connected through the connection element 6 to a process chamber opening 8 of the process chamber 2. The atomized doping solution is introduced with the aid of an air flow A via the outlet opening 7 through the connection element 6 and via the process chamber opening 8 into the process chamber 2. The substrate 3 is so positioned in the process chamber 2 that both a bottom side 3a and a top side 3b of the substrate 3 can be reached by the atomized doping solution. In this way, it is possible to apply a film of for instance phosphoric acid onto the substrate 3 on at least one of the sides 3a, 3b. The substrate 3 can be a silicon substrate, needed for manufacturing for instance a solar cell.

The liquid container 5 of the atomizer 1 is arranged for receiving an amount of doping solution F. Preferably, the doping solution is a solution F, in particular a phosphoric acid solution F, having a strong concentration of at least 30% to maximally 85% of doping liquid, while the drop size of the atomized doping solution is relatively small, for instance 5 μm or less.

The atomizing element 4 comprises a liquid compartment 9, which is partly provided in the liquid container 5. The liquid container 5 and the liquid compartment 9 are in fluid communication by way of an inlet opening 10, which is situated below a liquid level 11 of the doping solution F.

Owing to the liquid level 11 being regulable, the liquid level 11 can be kept at such a level that the inlet opening 10 is always below the liquid level 11. This if of importance for a continuous presence of doping solution F in the liquid compartment 9. The liquid level 11 can for instance be regulated with the aid of a sensor (not shown) in the liquid container 5, which sensor may be arranged for detecting the approximation of a minimum desired liquid level and, on the basis thereof, driving a control for letting additional doping solution into the liquid container 5. In a same manner, such a sensor may indicate that the maximum liquid level has been reached, so that the liquid compartment 9 does not overfill, which could impede the operation of the atomizing element 4. The atomizing element 4 is further provided with an air supply 12 for supplying air to the doping solution in the atomizing element 4. The atomizing element 4 is arranged for atomizing the doping solution by means of this air flow S flowing over the doping solution. The atomizing element 4 and the operation thereof will be further described with reference to Fig. 2. In Fig. 2 a cross-sectional elevation of the atomizer 1 according to an embodiment of the invention is shown. In Fig. 2 it is clear to see that the liquid container 5 is in fluid communication with the liquid compartment 9 of the atomizing element 4.

The operation of the atomizer is based on an air flow S, which flows through the atomizing element 4. Via the air supply 12, air is supplied to the atomizing element 4. The air is compressed by means of a constriction 13 in the air supply 12, as a result of which the speed of the air flow S in the atomizing element 4 is increased. The air leaves the constriction 13 by way of an air opening 15 and then collides against a barrier 16, as a result of which the air flow changes direction. As a result, the air flows along a liquid reservoir 14, into which liquid is drawn from the liquid compartment 9 (in the direction of arrow V) by means of a reduced pressure which the flowing air creates above the reservoir 14. At the same time, a part of the doping solution is taken up from the reservoir 14 into the air flow S. The atomized doping solution is passed via a separation element 17 (in the direction of arrows B and C) out of the atomizing element 4, in the direction A to the process chamber (see Fig. 1). For transporting the atomized doping solution to the process chamber 2, the supplied first air flow S is used. Additionally, a second air flow P may be generated, which is supplied to the first air flow S. The air flow P is generated with the aid of the Venturi effect. Since owing to the first air flow S a reduced pressure prevails in the atomizing element 4, air is drawn in from outside via an inlet 18. This additional air flow P facilitates the supply of the atomized doping solution towards the substrate.

The separation element 17 is arranged to arrest drops in the atomized doping solution that are relatively large and hence undesired. The arrested drops flow down along the separation element 17 and eventually fall back into the liquid compartment 9. In this way, the large drops are prevented from ending up in the process chamber and adversely affecting the quality of the layer of doping liquid on the substrate 3.

For a further detailed description of the atomizing element, reference is made to the description of EP 0,627,266, which is hereby wholly incorporated by reference.

Fig. 3 represents a process diagram of a treatment process of a substrate 3, wherein one of the treatment steps is applying material onto the substrate 3 utilizing an atomizer 1. The treatment process that is represented shows the process steps for manufacturing a solar cell from a silicon substrate 3.

Presently, the different process steps will be described in succession. A substrate is provided, for instance via a load lock, to a first process chamber. In it, the substrate is first of all etched in process step SlOO, for instance to remove damages to the substrate 3, caused by sawing the substrate to size, as well as impurities on a surface 3a, 3b of the substrate 3. At the same time, a surface texture may be provided on the substrate surface. After this, the substrate 3 is led to a next process step SIlO, for instance in a next process chamber, this process step SIlO representing the application of a phosphorus or boron containing layer onto the substrate 3. Applying the layer of doping liquid is done with the aid of an atomizer 1 according to the invention, whereby the atomized doping solution is supplied to the substrate 3 with the aid of an air flow. In a next process step S 120 the substrate 3 is placed in a diffusion oven which causes the doping atoms to diffuse from the applied layer of doping liquid into the silicon. In process step S 130, a glass layer is removed from the substrate 3 by means of etching. This glass layer was formed in process step S120 during diffusion. The silicon substrate 3 is then displaced to a next process chamber, in which an SiNx antireflection coating is deposited on the substrate surface (process step S140), for instance by means of PECVD. In a next process step S150, a metal film is applied onto the substrate surfaces 3a, 3b with the aid of screen printing. This metallic layer is then burnt into the substrate in process step S 160. Finally, in process step S 170, the substrate is subjected to laser ablation to electrically insulate the emitter and collector at the edge of the substrate 3.

It will be clear that the invention is not limited to the exemplary embodiment described, but that various modifications are possible within the scope of the invention, as defined by the claims. Thus, it is clear that any generally commercially available atomizing element can be used in the atomizer according to the invention, as long as the atomizing element is an element based on air. Further, the liquid container and the liquid compartment may be provided at different relative positions and also the connection between the two may be realized in different ways. Also, arranging for the continuous presence of doping solution in the liquid compartment of the atomizing element may be realized in a different manner. To be considered here, by way of example, is a sensor provided in a supply of doping solution to the liquid compartment. In such a construction, a liquid container according to the invention is not always requisite.

Claims

1. An atomizer for atomizing a doping solution for treating a substrate therewith, wherein the atomizer comprises an atomizing element with a liquid compartment for receiving the doping solution to be atomized, wherein the atomizing element is an atomizing element functioning on the basis of an air flow.

2. An atomizer according to claim 1, wherein at least the atomizing element is of a plastic.

3. An atomizer according to any one of the preceding claims, wherein the atomizing element is an existing atomizing element, for instance an atomizing element as disclosed in EP 0,627,266.

4. An atomizer according to any one of the preceding claims, wherein the atomizing element is provided with an air supply for supplying air to the doping solution, wherein the atomizing element is arranged for atomizing the doping solution by means of an air flow flowing over the doping solution.

5. An atomizer according to any one of the preceding claims, wherein the atomizer comprises a liquid container, which is in fluid communication with the liquid compartment of the atomizing element of the atomizer, wherein the atomizer is arranged to provide a constant or continuous supply of doping solution to the liquid compartment.

6. An atomizer according to claim 5, wherein the liquid compartment is provided at least partly in the liquid container of the atomizer, wherein the liquid compartment comprises an inlet opening which is situated below a liquid level in the liquid container.

7. An atomizer according to claim 5 or 6, wherein the liquid level in the liquid container is regulable in order to keep the inlet opening of the liquid compartment below the liquid level at all times.

8. An atomizer according to any one of claims 1-4, wherein the atomizer is provided with a regulating element for regulating a liquid level in the liquid compartment of the atomizing element.

9. An atomizer according to any one of the preceding claims, wherein an outlet opening of the atomizer for letting out atomized doping solution is detachably connectible to an inlet opening of a process chamber in which the substrate is receivable.

10. A method for treating a substrate, wherein the method comprises the following steps:

- providing a substrate; - atomizing a doping solution;

11. A method according to claim 10, wherein the method comprises at least one of the following steps:

- etching the substrate;

- diffusing doping atoms from the applied doping liquid layer into the substrate; - depositing coating onto the substrate;

- applying at least one metal layer onto the substrate;

- laser ablating the substrate.

12. A method according to claim 10, wherein the substrate to be provided is a silicon substrate.

13. A method according to any one of claims 10-12, wherein with the method a photovoltaic cell is manufactured.

14. A method according to any one of claims 10-13, wherein the doping solution is a phosphorus solution.

15. A method according to any one of claims 10-13, wherein the doping solution is a boron containing solution.