Patent Application
20130330936
The present invention concerns a method of deposition of Al2O3/SiO2 and Si3N4/Al2O3/SiO2 stacks, from aluminium and silicon precursors, useful for the deposition of thin films in photovoltaic technologies, in particular for solar cells.
The photovoltaic effect is known since the end of the 19th century. The principle consists in converting light energy into electricity. In the current context where shortages in fossil energy are expected by the end of the century, this is a promising solution to produce clean and renewable energy. One of the reasons for the slow development of photovoltaic electricity up to now is its lack of competitiveness compared to the traditional solutions such as coal, fossil fuels or nuclear based electricity. So the contribution of solar electricity as one significant component of the future energy mix is bounded to the capability to reduce further the cost per watt peak. To reach this goal, reduction of the manufacturing costs and improvement of cell's efficiency are two solutions that must be explored in parallel.
Reduction of the manufacturing costs is addressed for example with thinner wafers usage to limit the impact of silicon price on the overall cell's cost and in general with reduced raw materials consumption, including chemicals used during each step of the manufacturing. This manufacturing cost decrease is often driven by manufacturing tools providers (the OEM—Original Equipment Manufacturers) and by material suppliers.
Improvement of photovoltaic cell's efficiency requires innovation often driven by R&D laboratories. For example, there is significant R&D work carried out by academics on passivation phenomenon. This may contribute to the enhancement of the photovoltaic cell's performance.
SiO2 is known in semiconductor and photovoltaic industries to be a passivation material leading to a strong reduction in surface recombination. High quality SiO2 layer is grown by wet thermal oxidation at 900° C. or dry oxidation at 850° C.-1000° C. under oxygen. These high temperatures are generally not compatible with photovoltaic devices manufacturing. Therefore, alternative methods were developed such as Chemical Vapor Deposition of SiO2 from TEOS (Tetraethoxysilane) with O2. But one of the drawbacks of CVD is the difficulty to control the thickness and consequently the resulting inhomogeneity of the film. Another disadvantage is the relatively poor passivation of CVD SiO2. For these reasons Atomic Layer Deposition (ALD) is preferred as it allows achieving deposition of homogeneous layer, showing good passivation properties.
Whatever the deposition method, activation of the passivation capabilities of an as-deposited SiO2 layer, an annealing step must be performed under hydrogen at 850° C. If this annealing step is not carried out under hydrogen, structural defect will be reduced but the surface recombination velocity (SRV) will not decrease as massive hydrogen activation and consequently hydrogen diffusion is required to achieve significant dangling bonds passivation at the surface of silicon. This hydrogen can of course come from the film itself but the hydrogen is mainly supplied by the N2—H2 atmosphere. If the annealing temperature is over 900° C. a loss of hydrogen from the surface can happen and therefore be detrimental to the passivation properties of the silicon oxide layer. Also, even though this phenomenon is reversible thanks to another annealing, a natural loss of hydrogen can happen and induce a decrease of the SRV with time and therefore harm the passivation capabilities of the layer.
The conversion efficiency of a device is increased if the probability of hole-electron pairs to recombine at the surface or in the bulk of the silicon is reduced: the lower the number of defects into the material the higher the probability that charge carriers are collected. The recombination takes place on the front side of the solar cell as well as on the backside. In fact, hydrogen radicals are integrated into the film during deposition. The annealing step is performed under a nitrogen atmosphere with an appropriate hydrogen concentration to obtain a more pronounced driving force for the hydrogen to passivate the dandling bond. A hydrogen desorption phenomenon is increased with the annealing temperature but it is also observed at room temperature: it explains the decrease of the SiO2 layer's passivation properties. Hydrogen is therefore a key player and its chemical passivation capability is known.
SiO2 has passivation capabilities but, due to the drawbacks discussed above, Al2O3 passivation is now considered. As for SiO2 layers, recent studies of Al2O3 deposition demonstrate that the layer is naturally enriched with hydrogen during deposition. Al2O3 contains a reasonable level of hydrogen and therefore it is not strictly necessary to add H2 to the N2.
As for SiO2, hydrogen in the layer will chemically passivate the dangling bonds at the surface of the interface and in the bulk of the silicon. Contrary to SiO2, no hydrogen desorption is observed and therefore one can believe that the efficiency of the chemical passivation will not decrease with time. Consequently, Al2O3 capability to perform passivation can be higher than the SiO2 one.
So there is a need for a layer having a very efficient passivation for n-type and p-type substrates.
The present invention concerns a method of forming an Al2O3/SiO2 stack comprising successively the steps of:
According to other embodiments, the invention concerns:
Use of the stack as defined above for the passivation of photovoltaic devices, in particular for solar cells.
In the present invention, the as-deposited SiO2 layer has high hydrogen content: the higher the amount of hydrogen in the silicon precursor the higher the content of hydrogen in the layer. Al2O3 is used as a diffusion barrier for hydrogen and to transfer the hydrogen radicals from the alumina layer to the SiO2 layer during the annealing step. Thanks to the presence of the Al2O3 layer, the hydrogen atoms in the SiO2 are also better confined. In this case, the annealing step can be performed without hydrogen. Moreover, the thickness of the SiO2 layer is used to reduce the field effect passivation of Al2O3 that is not appropriate for n-type substrate. So, the stack is a good solution for an efficient passivation of n-type substrates and can be used for p-type substrates as well without significant increase in the surface recombination velocity.
Nevertheless, a very efficient stack results from the usage of the most appropriate combination of precursors.
The inventors of the present invention found that the precursors used in the method of the invention provide an appropriately high hydrogen concentration in the layers to feed a chemical equilibrium which effectively transfers hydrogen to the Si interface to passivate the dangling bonds. Moreover, another advantage of the invention is the use of the same oxidizer for the two precursors (during steps c) and f)) allowing an easier industrial usage.
The inventors have found that this combination of precursors will lead to a hydrogen-rich Al2O3/SiO2/Si stack with a low level of metallic contamination.
Thanks to this level of hydrogen, the stack has good chemical passivation capabilities. Another benefit of the invention is the usage of an ALD method, allowing a precise control of the SiO2 and Al2O3 layers' thicknesses: It is clearly an advantage to be able to grow a layer with a homogeneous thickness whatever the roughness of the substrate.
Those skilled in the art will recognize that this novel combination of precursors is not solely limited to the deposition of a back surface passivation stack for multi-crystalline and monocrystalline silicon wafer based photovoltaic solar cell but its benefit could be applied to other various applications where a passivation layer is used.
In one embodiment, the SiO2 layer is deposited first and then an Al2O3 capping layer is deposited. If necessary a new bilayer Al2O3/SiO2 can be deposited. The deposition of the bilayer can be repeated several times if necessary.
Deposition of a Bilayer Al2O3/SiO2 on Si from H2Si(NEt2)2 and Al(CH3)3.
The SiO2 layer is deposited on an n-type silicon substrate by PEALD. Oxygen plasma is used as a reactant in combination with H2Si(NEt2)2. The silicon precursor is stored in a stainless steel canister heated at 50° C. The precursor is vapor drawn. The substrate temperature is regulated at 150° C. The precursor is first introduced into the reactor (50 ms pulse). Oxygen is introduced continuously in the reactor as well as argon (this silicon precursor does not react with oxygen). After a 2 s purge sequence, a plasma is activated for 4 s. This sequence is followed by a new 2 s purge sequence. The pressure in the reactor is ˜0.2 Pa.
These conditions are compatible with a self-limited 1.1 Å/cycle growth.
The Al2O3 layer is deposited on the previously deposited SiO2 layer from trimethylaluminum (TMA) and oxygen plasma. TMA has a high vapor pressure and therefore the vapor is drawn into the reactor. The precursor is introduced into the reactor with a 10 ms duration pulse. Oxygen is introduced continuously in the reactor as well as argon. A first 10 ms TMA pulse is introduced into the reactor followed by a 2 s purge sequence. A plasma is then activated for 4 s and followed by a new 2 s purge sequence. A growth rate of 1 Å/cycle is achieved.
Several types of stacks are deposited on several substrates. SiO2 layers have a thickness between 1 nm and 15 nm. The Al2O3 layer thickness remains the same (˜30 nm). The stack is then annealed at 400° C. in an atmosphere of nitrogen. The duration of this annealing step is only 10 min. The surface recombination varies between 1 and 10 cm/s for this thickness range.
From this example, we can prove that the use of TMA and SiH2(NEt2)2, processed with the same oxidizer, for the deposition of a Al2O3/SiO2 stack leads to a very efficient passivation.
This type of combination can be easily used in ALD equipments such as standard ALD reactor or in-line spatial ALD reactor.
Deposition of a Triple Stack System Si3N4/Al2O3/SiO2 on Si from H2Si(NEt2)2 and Al(CH3)3.
The SiO2 layer is deposited on a n-type silicon substrate by PEALD. Oxygen plasma is used as a reactant in combination with H2Si(NEt2)2. The silicon precursor is stored in a stainless steel canister heated at 40° C. The carrier gas is argon. The substrate temperature is regulated at 150° C. The precursor is first introduced into the reactor (50 ms pulse). Oxygen is introduced continuously in the reactor as well as argon (this silicon precursor does not react with oxygen). After a 2 s purge sequence, a plasma is activated for 4 s. This sequence is followed by a new 2 s purge sequence. The pressure in the reactor is ˜0.2 Pa. These conditions are compatible with a self-limited 1.1 Å/cycle growth.
The Al2O3 layer is deposited on the previously deposited SiO2 layer from trimethylaluminum (TMA) and oxygen plasma. TMA has a high vapor pressure and therefore the vapor is drawn into the reactor. The precursor is introduced into the reactor with a 10 ms duration pulse. Oxygen is introduced continuously in the reactor as well as argon. A first 10 ms TMA pulse is introduced into the reactor followed by a 2 s purge sequence. A plasma is then activated for 4 s and followed by a new 2 s purge sequence. A growth rate of 1 Å/cycle is achieved.
A Si3N4 layer is then deposited by PEALD on Al2O3 from H2Si(NEt2)2 and NH3 plasma. The silicon precursor is stored in a stainless steel canister heated at 40° C. The carrier gas is argon. The substrate temperature is regulated at 150° C. The precursor is first introduced into the reactor (0.5 s pulse). NH3 is introduced continuously in the reactor. After a 2 s purge sequence, a plasma is activated for 4 s. This sequence is followed by a new 2 s purge sequence. The pressure in the reactor is ˜10.2 Pa.
This four steps cycle is repeated several times.
A triple stack system Si3N4/Al2O3/SiO2 is achieved.
Deposition of a Bilayer Al2O3/SiO2 on Si from H2Si(NEt2)2 and Al(Me)2(OiPr).
The SiO2 layer is deposited on an n-type silicon substrate by PEALD. Oxygen plasma is used as a reactant in combination with H2Si(NEt2)2. The silicon precursor is stored in a stainless steel canister heated at 50° C. The precursor is vapor drawn. The substrate temperature is regulated at 150° C. The precursor is first introduced into the reactor (50 ms pulse). Oxygen is introduced continuously in the reactor as well as argon (this silicon precursor does not react with oxygen). After a 2 s purge sequence, a plasma is activated for 4 s. This sequence is followed by a new 2 s purge sequence. The pressure in the reactor is ˜0.2 Pa.
These conditions are compatible with a self-limited 1.1 Å/cycle growth.
The Al2O3 layer is deposited on the previously deposited SiO2 layer from Al(Me)2(OiPr) and oxygen plasma. Al(Me)2(OiPr) has a high vapor pressure and therefore the vapor is drawn into the reactor. The precursor is introduced into the reactor with a 10 ms duration pulse. Oxygen is introduced continuously in the reactor as well as argon. A first 10 ms Al(Me)2(OiPr) pulse is introduced into the reactor followed by a 2 s purge sequence. A plasma is then activated for 4 s and followed by a new 2 s purge sequence. A growth rate of 1 Å/cycle is achieved.
Several types of stacks are deposited on several substrates. SiO2 layers have a thickness between 1 nm and 15 nm. The Al2O3 layer thickness remains the same (˜30 nm). The stack is then annealed at 400° C. in an atmosphere of nitrogen. The duration of this annealing step is only 10 min. The surface recombination varies between 1 and 10 cm/s for this thickness range.
From this example, we can prove that the use of Al(Me)2(OiPr) and SiH2(NEt2)2, processed with the same oxidizer, for the deposition of a Al2O3/SiO2 stack leads to a very efficient passivation.
This type of combination can be easily used in ALD equipments such as standard ALD reactor or in-line spatial ALD reactor.
Deposition of a Triple Stack System Si3N4/Al2O3/SiO2 on Si from H2Si(NEt2)2 and Al(Me)2(OiPr).
The SiO2 layer is deposited on a n-type silicon substrate by PEALD. Oxygen plasma is used as a reactant in combination with H2Si(NEt2)2. The silicon precursor is stored in a stainless steel canister heated at 40° C. The carrier gas is argon. The substrate temperature is regulated at 150° C. The precursor is first introduced into the reactor (50 ms pulse). Oxygen is introduced continuously in the reactor as well as argon (this silicon precursor does not react with oxygen). After a 2 s purge sequence, a plasma is activated for 4 s. This sequence is followed by a new 2 s purge sequence. The pressure in the reactor is ˜0.2 Pa. These conditions are compatible with a self-limited 1.1 Å/cycle growth.
The Al2O3 layer is deposited on the previously deposited SiO2 layer from Al(Me)2(OiPr) and oxygen plasma. Al(Me)2(OiPr) has a high vapor pressure and therefore the vapor is drawn into the reactor. The precursor is introduced into the reactor with a 10 ms duration pulse. Oxygen is introduced continuously in the reactor as well as argon. A first 10 ms Al(Me)2(OiPr) pulse is introduced into the reactor followed by a 2 s purge sequence. A plasma is then activated for 4 s and followed by a new 2 s purge sequence. A growth rate of 1 Å/cycle is achieved.
A Si3N4 layer is then deposited by PEALD on Al2O3 from H2Si(NEt2)2 and NH3 plasma. The silicon precursor is stored in a stainless steel canister heated at 40° C. The carrier gas is argon. The substrate temperature is regulated at 150° C. The precursor is first introduced into the reactor (0.5 s pulse). NH3 is introduced continuously in the reactor. After a 2 s purge sequence, a plasma is activated for 4 s. This sequence is followed by a new 2 s purge sequence. The pressure in the reactor is ˜10.2 Pa.
This four steps cycle is repeated several times.
A triple stack system Si3N4/Al2O3/SiO2 is achieved.
Deposition of a Stack System Si3N4/SiO2 on Si from H2Si(NEt2)2
The SiO2 layer is deposited on a n-type silicon substrate by PEALD. Oxygen plasma is used as a reactant in combination with H2Si(NEt2)2. The silicon precursor is stored in a stainless steel canister heated at 40° C. The carrier gas is argon. The substrate temperature is regulated at 150° C. The precursor is first introduced into the reactor (50 ms pulse). Oxygen is introduced continuously in the reactor as well as argon (this silicon precursor does not react with oxygen). After a 2 s purge sequence, a plasma is activated for 4 s. This sequence is followed by a new 2 s purge sequence. The pressure in the reactor is ˜0.2 Pa. These conditions are compatible with a self-limited 1.1 Å/cycle growth.
A Si3N4 layer is then deposited by PEALD on SiO2 from H2Si(NEt2)2 and NH3 plasma. The silicon precursor is stored in a stainless steel canister heated at 40° C. The carrier gas is argon. The substrate temperature is regulated at 150° C. The precursor is first introduced into the reactor (0.5 s pulse). NH3 is introduced continuously in the reactor. After a 2 s purge sequence, a plasma is activated for 4 s. This sequence is followed by a new 2 s purge sequence. The pressure in the reactor is ˜10.2 Pa.
This four steps cycle is repeated several times.
A stack system Si3N4/SiO2 is achieved.
While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the appended claims. The present invention may suitably comprise, consist or consist essentially of the elements disclosed and may be practiced in the absence of an element not disclosed. Furthermore, if there is language referring to order, such as first and second, it should be understood in an exemplary sense and not in a limiting sense. For example, it can be recognized by those skilled in the art that certain steps can be combined into a single step.
The singular forms “a”, “an” and “the” include plural referents, unless the context clearly dictates otherwise.
“Comprising” in a claim is an open transitional term which means the subsequently identified claim elements are a nonexclusive listing (i.e., anything else may be additionally included and remain within the scope of “comprising”). “Comprising” as used herein may be replaced by the more limited transitional terms “consisting essentially of” and “consisting of” unless otherwise indicated herein.
“Providing” in a claim is defined to mean furnishing, supplying, making available, or preparing something. The step may be performed by any actor in the absence of express language in the claim to the contrary.
Optional or optionally means that the subsequently described event or circumstances may or may not occur. The description includes instances where the event or circumstance occurs and instances where it does not occur.
Ranges may be expressed herein as from about one particular value, and/or to about another particular value. When such a range is expressed, it is to be understood that another embodiment is from the one particular value and/or to the other particular value, along with all combinations within said range.
All references identified herein are each hereby incorporated by reference into this application in their entireties, as well as for the specific information for which each is cited. It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. Thus, the present invention is not intended to be limited to the specific embodiments in the examples given above.
| Number | Date | Country | Kind |
|---|---|---|---|
| 11305114.8 | Feb 2011 | EP | regional |
| 11305115.5 | Feb 2011 | EP | regional |
This application is a 371 of International PCT Application no. PCT/EP2011/072970 filed Dec. 15, 2011, which claims priority to European Application Nos. 11305115.5 filed Feb. 7, 2011 and 11305114.8 filed Feb. 7, 2011, the entire contents of which are incorporated herein by reference.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2011/072970 | 12/15/2011 | WO | 00 | 8/7/2013 |
