HK1119153A - Combined etching and doping media for silicon dioxide layers and subjacent silicon - Google Patents

Description

Combined etch and doping media for silicon dioxide layer and underlying silicon layer

The invention relates firstly to an HF/fluoride-free etching and doping medium which is suitable both for etching silicon dioxide layers and for doping the lower silicon layer. Secondly, the invention relates to methods of using these media.

Prior art and objects of the invention

The term "solar cell" hereinafter refers to monocrystalline and polycrystalline silicon solar cells, irrespective of the type of solar cell based on other materials.

The mode of action of solar cells is based on the photoelectric effect, i.e. on the conversion of photon energy into electrical energy.

For silicon solar cells, a pre-doped silicon base material (usually p-doped Si) is doped for this purpose with opposite charge carriers having different names (for example, phosphorus doping results in n⁺Conducting), i.e., creating a p-n junction.

When photon energy is input into a solar cell (sunlight), charge carriers form on such a p-n junction, which leads to a broadening of the space charge region and a rise in the voltage. By providing contacts for the solar cell, the voltage is drawn off and the solar current is conducted to the user.

A typical process simplification for manufacturing solar cells includes:

structuring of the front side of a p-doped Si wafer

2.n⁺Doping (doping with phosphorus in general)

Etching of PSG (phosphosilicate glass)

4. Passivation/anti-reflection coating of silicon dioxide or silicon nitride layer

5. Front and back side metallization

In order to further understand the present invention, it is necessary to describe step 4 in more detail.

In some earlier production methods, SiO generated via heat₂Formation of a layer to obtain a passivation layer [1]]. The mode of action is to saturate and deactivate most of the unsaturated bonds on the Si surface. The defect density and recombination rate are therefore reduced. By mixing SiO₂The layer thickness is set to about 100 nm (λ/4 rule), additionally producing a antireflection surface.

The literature also discloses the use of TiO₂Layer coating, which, although exhibiting only a small passivation effect, significantly contributes to the reflection reduction of solar cells due to the higher refractive index [2 ]]。

The use of a silicon nitride layer as passivation layer has proven to be particularly advantageous in practice for the production of high-efficiency solar cells. Excellent passivation properties are known in semiconductor technology, for example as barrier or passivation layers in integrated circuits, FETs, capacitors, etc.

According to the state of the art, coating single-crystal or polycrystalline solar cells with silicon nitride constitutes the best surface passivation and antireflection method. Which is simultaneously used for mass production in more recent production lines.

In order to increase the efficiency, it has been demonstrated in laboratory experiments that the region below the emitter-side contact is doped to a higher extent than the enclosed n⁺Region (i.e. n is carried out with phosphorus)⁺⁺Diffusion) is advantageous. These structures are called selective or two-stage emitters [7]. At n⁺The doping in the region is about 10¹⁸cm^-3At n is⁺⁺The doping in the region is about 10²⁰cm^-3. Efficiencies of up to 24% are achieved in laboratory scale with such high efficiency solar cells.

In order to obtain these regions doped to different degrees to produce selective emitters, a number of methods are described in the literature, all of which are based on structuring steps. In this regard, photolithography is typically used to orient SiO₂Openings are made in the layer-this prevents gas phase doping of the underlying silicon, thus facilitating local doping. Such doping is typically with POCl₃Or pH₃In the gas phase. The doping window is typically etched into the silicon dioxide with hydrofluoric acid or buffered hydrofluoric acid.

These methods are still in the laboratory stage due to the very complex and expensive procedures.

Another method is based on removing n by etching with an etching mixture comprising hydrofluoric acid and nitric acid while masking the contact areas behind⁺⁺And (4) a region. This method itself cannot be implemented in practice because of its complicated procedure.

A common feature of all these methods is that the passivation layer must be locally opened using hydrofluoric acid or a hydrofluoride salt. Furthermore, the doping must be carried out in the gas phase after the washing and drying steps.

SiO is etched with the aid of an etching paste, as described in DE 10101926.2 or PCT/EP 01/03317₂Or the silicon nitride layer is partially opened and then treated with, for example, POCl₃Doping in the gas phase appears to be a significant advantage over these methods.

DE 10150040 a1 describes a combined etching and doping medium which is capable of etching a silicon nitride layer and doping the silicon under the silicon nitride in the etched-out region in a subsequent optional step.

So-called LOCOS methods are known in semiconductor technology, wherein silicon nitride is selectively etched using hot phosphoric acid in the presence of silicon dioxide at a temperature of 150-. This is quite clearly depicted in The form of graphs by w.van Gelder and v.e.hauser [ "The etching of Silicon Nitride in Phosphoric acid using Silicon Dioxide as a Mask" j.electrochem.s.114(8), 869(1967) ]:

examining the graph, it can be seen that by extrapolating the line segments representing the etch rates of silicon nitride and silicon dioxide in the graph, the etch rate of silicon dioxide should be the same as that of silicon nitride at about 270 ℃ (extrapolating the intersection of the line segments).

In practical applications, even at temperatures of 300 ℃, it has hitherto not been possible to achieve sufficiently high etching rates of silicon dioxide layers-which rates make etching media based on phosphoric acid or salts thereof useful for photovoltaic applications. In contrast, a silicon nitride layer made by the PE-CVD method with a thickness of 70 nm can be completely etched in less than 60 seconds at 300 c using the same etching medium.

It is therefore an object of the present invention to provide suitable etching media by means of which the silicon dioxide layer of a solar cell can be selectively etched at a high etching rate.It is therefore another object of the present invention to provide a method of selectively etching a silicon dioxide layer to fabricate a selective emitter structure in a solar cell that facilitates directional phosphorus doping in addition to etching to fabricate n⁺⁺And (4) a region.

Description of the invention

The object of the invention is achieved by the methods of etching silicon dioxide passivation and antireflection layers on solar cells according to claims 1 and 2 and the specific embodiments thereof according to claims 3 to 7. The method is carried out with the aid of an etching medium which contains phosphoric acid or a salt thereof, is applied in a single process step over the entire surface or selectively over the surface region to be etched, and is activated by the action of heat. The invention therefore also relates to a silicon surface treated by this method, and also to the resulting solar cell.

The object of the invention is achieved in particular by the following etching media: in the etching medium, phosphoric acid in various forms or suitable phosphates or compounds which decompose on heating to the corresponding phosphoric acid serve as etching components and optionally as doping components.

Detailed Description

The invention relates to a method for etching silicon dioxide passivation and antireflection layers on solar cells, wherein an etching medium containing phosphoric acid or a salt thereof is applied in a single process step over the entire surface or selectively over the surface region to be etched. In the method according to the invention, the silicon substrate with the etching medium is heated to 350 to 400 ℃ over the entire surface or locally for 30 to 120 seconds, and optionally for additional n⁺⁺Doped and then heated to > 800 ℃ for 1 to 60 minutes. It has been found that heating to 800 to 1050 ℃ gives good doping results. Printable paste-like etching media are particularly suitable for carrying out the method according to the invention. The etching medium used can be applied by spraying, spin coating, dipping or by screen printing, stencil printing, embossing, pad printing or ink-jet printing, depending on the consistencyAnd (4) application. The subsequent heating can be carried out on hot plates, in convection ovens, by IR radiation, UV radiation or microwaves. The local heating can be carried out using a laser, in particular an IR laser for heating to > 800 ℃. According to the invention, the claimed method can be used for producing solar cells with two-stage emitters.

The invention relates in particular to etching media for etching inorganic passivation and antireflection layers on solar cells, comprising phosphoric acid in various forms or suitable phosphates or compounds which decompose on heating to the corresponding phosphoric acid, which serve as etching or doping components in the process.

As active components, these can be orthophosphoric acid, metaphosphoric acid or pyrophosphoric acid, and/or phosphorus metaphosphoric oxide, or mixtures thereof, which serve both as etching components and as doping components. The etching medium used can also be an etching paste comprising one or more ammonium phosphates and/or phosphoric monoesters or diesters which liberate etching phosphoric acid by the input of thermal energy. Furthermore, the paste-like etching medium according to the invention comprises at least one etching and doping component, a solvent, a thickener and optionally additives, such as antifoams, thixotropic agents, flow control agents, deaerators and adhesion promoters. Etching media having a corresponding composition are particularly suitable for n of silicon⁺⁺Doped and can be used for the manufacture of solar cells with two-stage emitters and can be advantageously used in the above-described method. Accordingly, the invention also relates to solar cells produced in such a method using the etching medium according to the invention.

Although phosphoric acid itself and/or salts thereof have not proved suitable as etching component for silicon dioxide layers under the customary etching conditions, it has now been found, surprisingly by experiments, that orthophosphoric acid, metaphosphoric acid, pyrophosphoric acid or salts thereof, in particular from the group consisting of (NH)₄)₂HP₄And NH₄H₂PO₄And other compounds which upon thermal decomposition yield one of these compounds, are capable of etching and completely removing a silicon nitride layer 120 nm in layer thickness at a sufficiently high etch rate at temperatures above 350 c.

Furthermore, the partial etching may be performed by selective application, such as screen printing, ink-jet printing, or other methods, and heating the coated silicon substrate over the entire surface. This can also be achieved by full coating, e.g. spin coating, spray coating or other methods, followed by local heating, e.g. using IR laser heating.

Typical wet chemical processes for etching silicon dioxide are based on aqueous or buffered hydrofluoric acid solutions, such as NH₄HF₂Or KHF₂And (3) solution. These materials have high toxicity. Furthermore, during the etching of silicon dioxide, a volatile and also quite toxic gas SiF is formed₄。

In contrast, phosphoric acid and salts thereof, especially ammonium dihydrogen phosphate NH₄H₂PO₄And diammonium hydrogen phosphate (NH)₄)₂HPO₄Are substantially acceptable in toxicological and ecological aspects. Thus, a particular advantage of this etching method is that silicon dioxide can be etched without the use of toxic and expensive to handle hydrofluoric or hydrofluoride acids. In addition, the start and end of the etch can be simply controlled by the time and duration of the thermal excitation. A particular advantage is that n can be generated in a second process step following it by selectively imprinting phosphoric acid (orthophosphoric acid, metaphosphoric acid or pyrophosphoric acid) or salts thereof or compounds which can release them by, for example, thermal excitation⁺⁺Doping (which is necessary in the case of selective emitters). Such doping methods are known to the person skilled in the art and can be, for example, [7 ]]The process as described in (1).

The invention thus relates to the fabrication of selective emitter structures using a HF/fluoride free, printable, etched and doped combination medium.

The etching media used are HF/fluoride-free, easily processable inorganic acids or salts thereof and/or mixtures thereof, which may be in solution or paste form.

If the etching media are to be applied over the entire surface, they can be applied by those skilled in the artApplication is carried out by methods known to the expert, for example spin coating, spraying or dipping. In a second step, the phosphoric acid is excited by local heating (for example by means of a laser) to etch the silicon dioxide layer. In a further step, e.g. for n⁺⁺As necessary for doping, the phosphoric acid is locally heated to a temperature > 800 ℃, preferably to a temperature of about 900 ℃, using a second stronger laser. At this time, it is particularly advantageous to use an IR laser whose wavelength is not absorbed by the lower silicon, thereby preventing crystal defects. By implementing the method skillfully, the etching and doping processes can be performed in a single step according to the present invention. The surrounding silicon dioxide that has not yet been etched away advantageously acts as a diffusion barrier during the doping operation and prevents doping of the unetched regions.

However, it appears to be more advantageous to selectively coat the silicon substrate with the paste-like mixture containing phosphoric acid than to coat over the entire surface. This can be done by printing methods known to the person skilled in the art, such as screen printing, stencil printing, embossing or pad printing. After application, the substrate is heated in a later step to initiate the silicon dioxide etch. This can be done on a hot plate or by IR radiation or by other methods of heating the substrate (microwave, heat transfer oven) known to those skilled in the art. A temperature range of 350 to 400 c is advantageous for etching. The etch duration for a silicon dioxide layer with a thickness of 120 nm at 350 c is approximately 60 seconds. In the next step, e.g. heat n for phosphoric acid⁺⁺As necessary for doping, the substrate can be heated at a temperature of > 800 ℃ for 1 to 40 minutes immediately after the etching step. The mode of diffusion of phosphorus into silicon can be controlled by duration and temperature in a manner known to those skilled in the art.

By skillfully performing the method, the etching and the doping can be performed next to each other in a single process step.

Not only is the selective application more advantageous in terms of material consumption, but the yield of the substrate to be etched is also significantly faster than can be achieved using a laser to write surfaces in series.

Ink-jet printing applications (non-contact processes) different from the above-described printing processes should be mentioned as a further variant. The heated substrate can be directly printed and etched. By performing the method skillfully, it is also possible to etch and dope simultaneously under these conditions.

If the etching media used are paste-like, they can be applied over the entire surface or selectively to the areas to be etched.

Since the etchant in the paste acts simultaneously as a doping component, it can also act as a doping source in the actual etching step as described above.

For this purpose, the solar cell is subjected to a temperature of 800 ℃ ÷ 1050 ℃, during which the doping components present in the paste diffuse into the contact region and dope it. All other paste components are volatile at these temperatures and burn off without residue.

Thus, the paste can be applied in a single process step to the desired areas of the surface to be etched. A highly automated technique particularly suited to transferring paste onto a surface is printing. In particular, screen printing, stencil printing, embossing (and pad printing are methods known to those skilled in the art for this purpose.

A particular advantage of using the etching and doping medium of the invention is that all masking and photolithography steps normally necessary with wet chemical etching or gas phase selective doping are superfluous, as are cleaning operations.

The etching paste of the present invention has the following composition:

a. etching and optional doping Components

b. Solvent(s)

c. Thickening agent

d. Optionally additives, e.g. antifoams, thixotropic agents, flow-control agents, deaerators, tackifiers

The etching action of the proposed etching pastes is based on acidic components which are active by temperature excitation. The groupDerived from phosphoric acid (orthophosphoric acid, metaphosphoric acid or pyrophosphoric acid) and its salt, preferably selected from (NH)₄)₂HPO₄And NH₄H₂PO₄The ammonium salt of (1).

The etching component is present in a concentration of 1 to 80 wt. -%, based on the total weight of the etching paste. The etch and removal rate of silicon dioxide is significantly affected by the concentration of the etch component.

It has been found in experiments that the etch rate of phosphoric acid can be further increased by adding a strong oxidizing component, such as nitric acid or nitrate. Thus, such components may optionally be added to the etching medium of the present invention if the desired etching process is facilitated.

The proportion of solvent may be from 20 to 80% by weight, based on the total weight of the etching paste. Suitable solvents may be pure inorganic or organic solvents or mixtures thereof, for example water, mono-and/or polyhydric alcohols, ethers, in particular ethylene glycol monobutyl ether, triethylene glycol monomethyl ether, [2, 2-butoxy (ethoxy) ] -ethyl acetate.

In order to set a viscosity range which is specific and essential for the printability of the etching agent (i.e. for the formation of a printable paste), the proportion of thickener necessary is 1 to 20% by weight, based on the total weight of the etching paste.

The non-Newtonian behaviour of the etching pastes is achieved by network-forming thickeners which have a swelling action in the liquid phase and can be varied as a function of the desired application area. Useful thickeners are organic or inorganic products or mixtures thereof:

cellulose/cellulose derivatives, e.g. ethyl cellulose, hydroxypropyl cellulose or hydroxyethyl cellulose or sodium carboxymethyl cellulose

Starch/starch derivatives, e.g. sodium carboxymethyl starch (vivastar ®), anionic heteropolysaccharides

Acrylate (Borchigel ®)

Polymers, e.g. polyvinyl alcohol (Mowiol ®), polyvinyl pyrrolidone (PVP)

Highly disperse silicic acids, e.g. Aerosil ®

With regard to the use of cellulose/cellulose derivatives and other thickeners, it should be noted that only derivatives having sufficient adhesion to the substrate surface while preventing diffusion of the etching medium and facilitating accurate printing of extremely fine lines and structures can be used. Thus, for example, xanthan derivatives have been found to be unusable for the purposes of the present invention.

Unlike organic thickeners, inorganic thickeners, such as highly dispersed silicic acid, also remain on the substrate during subsequent doping steps at temperatures > 800 ℃ and can therefore be used to adjust the doped glass properties. These two types of thickeners, organic and inorganic, can also be combined with one another in the etching medium as desired, so that different compositions can be selected depending on the application.

Additives having advantageous properties for the desired use are antifoams, thixotropic agents, flow control/anti-flow agents, deaerators and tackifiers. These can advantageously influence the printability of the etching pastes.

In order to achieve high efficiency in solar cells, it is important that all raw materials used for preparing the etching paste have sufficient purity. In particular, readily diffusible elements such as copper, iron and sodium, which significantly shorten the lifetime of the support in silicon, should be present at concentrations of < 200 ppb.

According to the invention, these novel etching and doping pastes can be used in the solar cell industry for the production of photovoltaic elements, such as solar cells or photodiodes. In particular, the paste of the invention can be used in a two-step process for the manufacture of emitter structures.

[1] W.Wettling, Phys.Bl.12(1997), page 1197-1202

[2]J.Horzel，J.Slufzik，J.Nijs，R.Mertens，Proc.26^thIEEE PVSC, (1997), pp.139-42

[3] M.Schnell，R.Lüdemann，S.Sch*fer，Proc.16^th EU PVSEC, (2000), pages 1482-85

[4]D.S.Ruby，P.Yang，S.Zaidi，S.Brueck，M.Roy，S.Narayanan，Proc.2^ndWorld Conference and inhibition on PVSEC, (1998), pages 1460-63

[5] US 6,091,021(2000)，D.S.Ruby，W.K.Schubert，J.M.Gee，S.H.Zaidi

[6]US 5,871,591(1999)，D.S.Ruby，J.M.Gee，W.K.Schubert

[7]EP 0229915(1986)，M.Bock，K.Heymann，H.-J.Middeke，D.Tenbrink

Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the preceding description to its fullest extent. The preferred embodiments and examples are therefore to be regarded as merely illustrative disclosures which are in no way limiting.

The complete disclosure of all applications, patents and publications mentioned above and below and of the corresponding application DE 102005032807.5 filed on 7.12.2005 are hereby incorporated by reference into the present application.

For better understanding and for the sake of illustration, the following examples are given which are within the scope of protection of the invention but are not suitable to limit the invention to these examples.

Example 1:

preparation and composition of the paste

6 g of Aerosil 200(Degussa-Huels AG) were stirred into 100 g of 85% orthophosphoric acid (Merck Art.1.00573). The resulting paste was stirred for an additional 20 minutes using a paddle stirrer.

Example 2:

preparation and composition of the paste

Stir 3 wt.% PVP K90 to 48.5 wt.% H₃PO₄(85%) and 48.5% by weight of 1-methyl-2-pyrrolidone. The resulting paste was stirred for an additional 20 minutes using a paddle stirrer.

The etching paste prepared in the above manner was printed using a 120T-type polyester screen on a commercial screen printer. The cloth picture shown in figure 1 was placed on a screen and transferred to a substrate. The substrate used was a polycrystalline solar cell of size 100 x 100 square mm with a full area silicon dioxide passivation layer. Immediately after printing, the substrate was heated on a hot plate at 300 ℃ for 100 seconds. The silicon dioxide layer was seen to be completely etched through after only about 60 seconds. The substrate was then placed in a diffusion furnace containing atmospheric air at 850 ℃ for 30 minutes.

After removal of the phosphor glass layer, the conductivity was measured by means of a 4-spot sample, and approximately 10 of the areas could be determined²⁰cm^-3High local phosphorus doping.

Claims (14)

1. Method for etching silicon dioxide passivation and antireflection layers on solar cells, characterized in that an etching medium containing phosphoric acid or a salt thereof is applied in a single process step over the entire surface or selectively over the surface area to be etched.

2. Method according to claim 1, characterized in that the silicon substrate with the etching medium is heated to a temperature of 350 to 400 ℃ over the entire surface or locally for 30 to 120 seconds, and optionally for additional n⁺⁺Doped and then heated to a temperature of > 800 ℃, in particular 800 to 1050 ℃ for 1 to 60 minutes.

3. A method according to claims 1-2, characterized in that a printable paste-like etching medium is used.

4. Method according to claims 1-2, characterized in that the etching medium is applied according to consistency by spraying, spin coating, dipping or by screen printing, stencil printing, embossing, pad printing or ink jet printing.

5. Method according to claims 1-4, characterized in that the heating is performed on electric hot plates, in a convection oven, by IR radiation, UV radiation or microwaves.

6. Method according to claims 1-4, characterized in that the local heating is performed using a laser, in particular an IR laser for heating to > 800 ℃.

7. The method according to claims 1-6 for manufacturing solar cells with two-stage emitters.

8. Etching medium for etching inorganic passivation and antireflection layers on solar cells, comprising orthophosphoric acid, metaphosphoric acid or pyrophosphoric acid, and/or phosphorus metaphosphoric oxide, or mixtures thereof, as active components, which act both as etching component and as doping component.

9. Etching medium according to claim 8, comprising one or more ammonium salts of phosphoric acid and/or phosphoric monoesters or diesters which liberate etching phosphoric acid by the input of thermal energy.

10. Etching medium according to claims 8 to 9, which is paste-like and comprises at least one etching and doping component, a solvent, a thickener, and optionally additives, such as antifoams, thixotropic agents, flow control agents, deaerators and adhesion promoters.

11. Use of an etching medium according to claims 8 to 10 for n of silicon⁺⁺And (4) doping.

12. Use of an etching medium according to claims 8 to 10 for the production of solar cells with two-stage emitters.

13. Use of an etching medium according to claims 8 to 10 in a method according to claims 1 to 7.

14. Solar cell made by a method according to one or more of claims 1-7.