WO2018091888A1

WO2018091888A1 - Method and apparatus for applying atomic hydrogen to an object

Info

Publication number: WO2018091888A1
Application number: PCT/GB2017/053437
Authority: WO
Inventors: Peter WILSHAW; Phillip HAMER; Sebastian BONILLA; Gabrielle BOURRET-SICOTTE; George MARTINS
Original assignee: Oxford University Innovation Ltd
Current assignee: Oxford University Innovation Ltd
Priority date: 2016-11-15
Filing date: 2017-11-15
Publication date: 2018-05-24
Anticipated expiration: 2019-05-15

Abstract

Methods and apparatus for applying atomic hydrogen to an object are disclosed. In one arrangement, the method comprises passing atomic hydrogen through a membrane and onto a portion of an object that is spaced apart from the membrane. The membrane comprises a solid material and the atomic hydrogen passes through the solid material.

Description

METHOD AND APPARATUS FOR APPLYING ATOMIC HYDROGEN TO AN OBJECT

The present invention relates to methods and apparatus for applying atomic hydrogen (i.e. neutral, hydrogen radicals H) to an object, for example to electrically passivate the object or otherwise alter the electrical properties of the object by treated the object with atomic hydrogen.

It is known that atomic hydrogen can be used to passivate surface and bulk defects and impurities in crystalline silicon. For traditional screen printed solar cells, hydrogen can be incorporated into dielectrics such as SiNx or AlOx which are deposited on the surface of the solar cell. Passivation can be achieved by annealing the dielectrics at high temperature to release the hydrogen. Not all solar cell architectures are compatible with such high temperature processing however.

An alternative approach is to anneal in an environment containing molecular hydrogen, such as forming gas (a mixture of hydrogen and an inert gas such as nitrogen and/or argon, sometimes referred to as "dissociated ammonia atmosphere"). Forming gas anneals have historically been one of the most common methods of introducing hydrogen, particularly for passivation of surfaces.

However, molecular hydrogen is less effective than atomic hydrogen at binding to defects at interfaces and in the silicon bulk.

A further alternative approach is to expose the device to an atomic hydrogen source such as a hydrogen plasma (also referred to as a hydrogen generating plasma). Hydrogen plasmas have three potential drawbacks: 1) if charged ions from the plasma impinge on dielectric surfaces they can lead to undesirable charging of the surfaces and electrostatic breakdown; 2) high kinetic energy species (of any sort) from the plasma can damage objects that they hit; and 3) hard UV from the plasma can damage objects that it reaches. Using a remote plasma reduces these effects because fewer charged ions may reach the device being treated, the kinetic energy of species reaching the device may be reduced, and UV photons will be largely prevented from reaching the device. It is still expected, however, that some charged ions will reach the device. It has been found that such charged ions can still damage devices, even at relatively low kinetic energies, and they have even been observed to etch the surface of silicon.

It is an object of the present invention to provide apparatus and methods for applying atomic hydrogen to an object that at least partially address one or more of the problems with the prior art mentioned above.

According to an aspect of the invention, there is provided a method for applying atomic hydrogen to an object, comprising: passing atomic hydrogen through a membrane and onto a portion of an object that is spaced apart from the membrane, wherein the membrane comprises a solid material and the atomic hydrogen passes through the solid material.

The inventors have recognised that significant amounts of atomic hydrogen can be made to pass through a membrane and that this effect can be used to improve the application of atomic hydrogen to an object. The membrane can take various compositions and forms (e.g. thicknesses) as long as the atomic hydrogen can pass through the solid material of the membrane.

Where the atomic hydrogen is provided in such a way that hydrogen ions are produced, the membrane can electrically neutralise all or portion of the ions before they reach the object, thereby reducing or preventing damage to the object from ions.

Where the atomic hydrogen is provided in a high kinetic energy form on one side of the membrane, the membrane will lower the kinetic energy of the atomic hydrogen as the atomic hydrogen passes through the membrane. This effect may be referred to as thermalization. Lowering the kinetic energy of the atomic hydrogen received by the object reduces damage to the object.

Where the atomic hydrogen is provided in such a way that UV photons are produced, the membrane can prevent or reduce the proportion of the UV photons that reach the object. Damage by UV photons is therefore prevented or reduced. In an embodiment the membrane is substantially opaque to UV radiation.

In an embodiment, the membrane comprises at least one continuous layer of solid material spanning all of a region through which the atomic hydrogen passes before reaching the object. In an embodiment, the membrane comprises a metallic layer, preferably a continuous layer spanning all of a region through which the atomic hydrogen passes before reaching the object. The metallic nature of the layer may assist with neutralising ions and is desirable when atomic hydrogen is provided by a plasma in a region directly adjacent to the membrane (e.g. via radio frequency discharge). In an embodiment the membrane is maintained at a constant electrical potential, optionally ground.

In an embodiment, the membrane seals a first region from a second region. In various such embodiments the first region contains atomic hydrogen, hydrogen ions, or molecular hydrogen, and the second region contains the object to be treated. The first region can therefore be maintained at a different pressure to the second region (e.g. a lower pressure). This means the atomic hydrogen can be generated in an environment at low pressures, which may be desirable for example where the atomic hydrogen is generated using a radio frequency discharge to generate a plasma, without necessarily keeping the object at a similar level of low pressure (although it may still be desirable to maintain the object in an environment that is below atmospheric pressure to reduce losses of atomic hydrogen by recombination events in the region between the membrane and the object).

In an embodiment, the membrane comprises a catalytic layer. The catalytic layer is configured to catalyse a conversion between molecular hydrogen and atomic hydrogen at a surface of the catalytic layer. This allows the atomic hydrogen applied to the object to originate, at least partly, from molecular hydrogen provided on the other side of the membrane. This approach may be desirable because it does not require formation of a plasma or any other method of forming atomic hydrogen in the gas phase. All that is required is an ¾ containing gas (or another gas capable of providing atomic hydrogen at the surface of the membrane). Furthermore, no vacuum is needed in the region on the side of the membrane opposite to the object, in contrast to when a plasma is created in this region.

In an embodiment the catalytic layer is treated to suppress the catalysis of the conversion between molecular hydrogen and atomic hydrogen on an object side of the catalytic layer and is not treated on the other side of the catalytic layer. In this way the catalytic layer can desirably promote conversion of molecular hydrogen to atomic hydrogen on the side of the membrane opposite to the object (thereby increasing the amount of atomic hydrogen passing through the membrane), while not contributing to any conversion of the atomic hydrogen back to molecular hydrogen on the object side of the membrane (which may reduce the amount of atomic hydrogen reaching the object). In an alternative approach the catalytic layer is covered by a support layer on an object side of the catalytic layer and the catalytic layer forms an outer surface of the membrane on the side of the membrane opposite to the object. The support layer also acts to reduce undesirable conversion of atomic hydrogen to molecular hydrogen on the object side of the membrane while also providing mechanical support to the membrane. This approach facilitates use of catalytic layer materials which are relatively expensive, such as palladium or palladium alloys, because the catalytic layer can be thinner. In other embodiments, a catalytic layer is provided that is treated on both sides to suppress the catalysis of the conversion between molecular hydrogen and atomic hydrogen. This reduces loss as molecular hydrogen from both surfaces. This approach would also prevent hydrogen from going into the membrane if the hydrogen is in molecular form, while having no effect on atomic hydrogen (e.g. from a plasma) entering the membrane.

In accordance with an alternative aspect of the invention, there is provided an apparatus for applying atomic hydrogen to an object, comprising: a chamber, wherein the chamber contains atomic hydrogen, hydrogen ions or molecular hydrogen, or the chamber is configured to generate a plasma in the chamber; a membrane; and an object support configured to support the object, wherein: the membrane is configured to allow atomic hydrogen to pass through the membrane from the chamber to the other side of the membrane, wherein the membrane comprises a solid material and the atomic hydrogen passes through the solid material; and the object support is configured such that an object present on the object support will receive the atomic hydrogen after the atomic hydrogen has passed through the membrane.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which corresponding reference symbols indicate corresponding parts, and in which:

Figure 1 is a schematic side sectional view of a membrane for use in applying atomic hydrogen to an object, a support structure for the membrane, and an example object;

Figure 2 is a top view of the membrane and support structure of Figure 1 ;

Figure 3 schematically depicts an apparatus for applying atomic hydrogen to an object using a radio frequency discharge to provide a hydrogen generating plasma; Figure 4 schematically depicts an alternative apparatus in which plural objects to be treated can be brought successively into position beneath a membrane sealing a chamber containing molecular hydrogen;

Figure 5 depicts a further alternative apparatus of the type shown in Figure 4 except that the chamber contains electrodes for generating a plasma in the chamber;

Figure 6 depict a membrane above an object to be treated, wherein the membrane comprises a catalytic layer that has been treated on a side facing the object to suppress the catalytic activity on that side;

Figure 7 depicts a further membrane above an object to be treated, wherein the membrane comprises a catalytic layer and a support layer;

Figure 8 is a graph showing measured average active boron concentration in the first 3-7 nm of a silicon object (sample) with a uniform doping concentration of 1.1 ^χ 10¹⁷ boron/cm³ after exposure to an ammonia plasma for 20 minutes at 473 K - samples were exposed directly to the plasma ("No Leaf) or under a membrane with an oxide present ("Oxide Present"), with the oxide removed ("No Oxide Leaf) or with the oxide removed and subsequent exposure to FbS ("Poisoned Leaf);

Figure 9 is a graph showing depth profiles of active boron concentration as measured using capacitance -voltage techniques after exposure of an object (silicon wafer) to atomic hydrogen provided according to an embodiment at temperatures between 423 and 523 K; the insert is a table showing integrated [H-B] and corresponding penetration depth;

Figure 10 is a graph showing effective minority carrier lifetimes as a function of minority carrier density for a ΙΩαη 200um n-type FZ Si sample with a 10 nm thermal oxide, with atomic hydrogen being provided according to an embodiment at 380C for 45 minutes each side; and

Figure 11 is a graph showing effective minority carrier lifetimes as a function of minority carrier density in the case where the membrane comprises a catalytic layer poisoned on both sides.

Figures 1 and 2 depict an example membrane 2 for use in a method of applying atomic hydrogen to an object 4. The method comprises passing atomic hydrogen through the membrane 2 and onto a portion of the object 4. The object 4 is spaced apart from the membrane 2. The portion of the object 4 that receives the atomic hydrogen is thus not in contact with the membrane 2. The atomic hydrogen leaves a surface 11 of the membrane 4 and travels across an open space (e.g. a space not containing any solid or liquid matter, such as a gaseous or vacuum region) before reaching the object 4. The object 4 receives the atomic hydrogen after the atomic hydrogen has passed through the membrane 2 (i.e. through the solid material of the membrane itself). The membrane 2 spans continuously across a region through which the atomic hydrogen passes. The atomic hydrogen thus passes through the solid material of the membrane 2 and not through open gaps in the membrane 2.

The object 4 is spaced apart from the membrane 2 by a distance that is small enough that a significant proportion of the atomic hydrogen leaving the membrane 2 reaches the object 4. In various embodiments, a maximum spacing between the portion of the object 4 that receives the atomic hydrogen and the membrane 2 is less than 5mm, optionally less than 2mm, optionally less than lmm, optionally less than 500 microns. The distance will generally need to be smaller where the object is held in a relatively high pressure environment (such as at atmospheric pressure) because collisions with gas particles will reduce the average distance travelled by atomic hydrogen leaving the membrane 2 before recombination events cause conversion to molecular hydrogen. Reducing the pressure around the object 4 will enable the distance to be increased. Even relatively small changes in pressure will enable the distance to be increased significantly, so it is possible to achieve gains in this area (if needed) without providing a full vacuum system. Whether or not the distance needs to be increased will depend on the particular practical constraints of the scenario in which the method is being used and the nature of the objects being treated (e.g. how convenient it is to position the objects very close to the membrane). In any given application the distance can be varied until satisfactory performance is obtained.

The method can be applied efficiently while keeping the object 4 at temperatures below 800 degrees C, optionally below 600 degrees C, optionally below 400 degrees C, optionally below 300 degrees C, optionally below 200 degrees C. In the detailed example discussed below, the method is shown to be effective even down to 150 degrees C. These temperatures are significantly lower than temperatures needed for some prior art forming gas anneals, thereby providing greater freedom for processing objects 4 that are sensitive to high temperatures.

The method is configured so that the amount of atomic hydrogen applied to the object 4 is such as to cause a significant change in an electrical property of the object 4, such as minority carrier lifetime. This is achieved, at least partially, by controlling the distance between the object 4 and the membrane 2 so that a sufficient amount of the atomic hydrogen reaches the object 4. In example embodiments the amount of atomic hydrogen received by the object 4 is effective to perform one or more of the following in the object 4: passivate surface defects, passivate bulk defects, passivate impurities, act as a dopant that alters the electrical conductivity of the object 4, act as a defect that alters the electrical conductivity of the object 4.

Methods according to embodiments can be applied to a wide range of objects, including for example one or more of the following: a semiconductor device, a photovoltaic device, a metal oxide (e.g. to improve the properties of a transparent conducting layer such as zinc oxide), and an optical sensor.

As indicated in Figure 1, atomic hydrogen passes through the membrane 2 from a first region 8 to a second region 10. Some of the atomic hydrogen may form molecular hydrogen on leaving the membrane 2 but some of the atomic hydrogen will leave the membrane 2 as atomic hydrogen. The atomic hydrogen passing through the membrane 2 is derived from a source of hydrogen provided in the first region 8. The source of hydrogen may take various forms.

In one class of embodiments, the source of hydrogen comprises atomic hydrogen or hydrogen ions. The atomic hydrogen or hydrogen ions may be provided by techniques known in the art, for example via generation of a direct or remote hydrogen plasma, for example using a radio frequency discharge.

In another class of embodiments, the source of hydrogen comprises molecular hydrogen. The molecular hydrogen is converted to atomic hydrogen at one surface 9 of the membrane 2 (the upper surface in Figure 1). The atomic hydrogen then passes through the bulk of the membrane 2 and at least a portion of the atomic hydrogen leaves an opposite surface 11 of the membrane 2 (the lower surface in Figure 1) as atomic hydrogen and reaches the object 4.

The composition and thickness of the membrane 2 are not particularly limited as long as the atomic hydrogen can pass through the membrane 2 efficiently and the membrane 2 is sufficiently robust (or sufficiently well supported) to support itself and/or any pressure differentials that may be applied across the membrane 2. The membrane 2 may comprise a single layer or multiple layers of different compositions. As described below, the membrane 2 may comprise a metallic layer and/or a catalytic layer (which may be one and the same layer or different layers). Additionally or alternatively, the membrane 2 may comprise a non-metallic layer. The non-metallic layer may comprise PTFE or another material that is substantially inert with respect to atomic hydrogen. The non-metallic layer may act as a support for a metallic layer or catalytic layer (which may for example be formed as a coating on the non-metallic layer, for example by evaporation of a metal onto the non- metallic layer).

In an embodiment, the membrane 2 comprises at least one layer formed from a material which allows the concentration of atomic hydrogen within the material to reach levels in use which provide a required flux of atomic hydrogen through the membrane 2 (e.g. a material having sufficiently high hydrogen solubility and/or diffusivity).

In an embodiment, the membrane 2 comprises a metallic layer. The metallic layer can comprise any suitable metal or alloy. In particular embodiments, the metallic layer may comprise, or consist essentially of, or consist of, one or more of the following in any combination: palladium, nickel, niobium, silver, titanium, vanadium, iron, tantalum, an alloy of vanadium and gallium (or at least comprising vanadium and gallium), and an alloy of nickel and silver (or at least comprising nickel and silver).

In an embodiment, examples of which are shown in Figures 6 and 7, the membrane 2 comprises a catalytic layer 30. The catalytic layer 30 and the metallic layer mentioned above may be the same layer or may be different layers. The catalytic layer 30 catalyses a conversion between molecular hydrogen and atomic hydrogen at a surface of the catalytic layer 30. The catalytic layer 30 may comprise a continuous layer of a material that provides the catalytic function (such as a continuous layer of palladium or nickel or an alloy containing palladium and/or nickel). Alternatively the catalytic layer 30 may be provided by doping a material that does not having any significant intrinsic catalytic behaviour (with respect to conversion between molecular hydrogen and atomic hydrogen) with a material (such as Pd) that does have such catalytic behaviour. The catalytic layer 30 may comprise palladium or an alloy of palladium, optionally an alloy of palladium with one or more of the following: silver, copper, yttrium, gold, ruthenium and indium. Providing palladium in alloy form can reduce cost by making it possible to use less palladium and still provide a mechanically robust membrane 2. Additionally, the alloying can improve the longevity of the membrane 2 by reducing the extent to which the structure of the membrane 2 is weakened over time by cumulative damage from the atomic hydrogen passing through the palladium.

It is known in the art that palladium has the above catalytic behaviour. The behaviour has been exploited extensively in the area of hydrogen purification, where hydrogen is passed through a palladium based membrane to separate the hydrogen from other gases. Purified molecular hydrogen is collected from the other side. There has been no recognition in the art, however, that atomic hydrogen might also be present on the other side of the membrane. Indeed, due to the fact that the catalytic behaviour apparently favours release of hydrogen from the membrane via recombinative desorption (in which atomic hydrogen recombines to form molecular hydrogen prior to release of the hydrogen), it is surprising that atomic hydrogen can in fact be released in significant quantities, as the inventors have found.

In an embodiment, the efficiency of the release of atomic hydrogen can be improved by treating the catalytic layer 30 to suppress the catalysis of the conversion between molecular hydrogen and atomic hydrogen. In the case wherein the catalytic layer 30 comprises palladium or an alloy of palladium, the suppression can be achieved for example by contamination 32 using sulphur (as mentioned in the detailed example below) or carbon.

In the case where atomic hydrogen or hydrogen ions are provided in the first region 8, it may be desirable for the treatment of the catalytic layer 30 to be applied on both sides of the catalytic layer 30 because the conversion of molecular hydrogen to atomic hydrogen is not then needed on either side of the membrane 2. Treating both sides reduces or prevents recombinative desorption from both sides, thereby helping to maintain a high concentration of atomic hydrogen within the membrane, which in turn favours a high rate of release of atomic hydrogen towards the object 4. Alternatively, the membrane 2 may be formed from materials which do not significantly catalyse conversion between molecular hydrogen and atomic hydrogen on their surfaces, or materials which at least have a weaker catalytic behaviour than Pd with respect to the conversion between molecular hydrogen and atomic hydrogen. Thus, the membrane 2 may be configured such that either or both sides of the outer surfaces of the membrane 2 have substantially no catalytic activity in respect of conversion between molecular hydrogen and atomic hydrogen at the surface. In various embodiments, the membrane 2 is configured for example to contain substantially no Pd or less Pd than would be necessary to confer any significant catalytic effect with respect to conversion between molecular hydrogen and atomic hydrogen. Materials which are known for use as hydrogenation catalysts should be avoided for such embodiments for example. On the other hand, as depicted in Figure 6, in the case where molecular hydrogen is provided in the first region 8 the treatment of the catalytic layer 30 may be applied on the object side 41 of the catalytic layer 30 and not on the hydrogen source side 39 of the catalytic layer 30. In this way the catalysis promotes conversion of molecular hydrogen to atomic hydrogen on the hydrogen source side 39 of the catalytic layer 30, thereby increasing the supply of atomic hydrogen in the bulk of the membrane 2, but does not inhibit (or inhibits less) the release of atomic hydrogen from the object side 41 of the catalytic layer 30.

In an alternative embodiment, as depicted in Figure 7, the catalytic layer 30 is covered by a support layer 34 on the object side 41 of the catalytic layer 30 and the catalytic layer 30 forms an outer surface of the membrane 2 on the side 9 of the membrane 2 opposite to the object 4 (provided by the hydrogen source side 39 of the catalytic layer 30). Without wishing to be bound by theory it is thought that the covering of the object side 41 of the catalytic layer 30 also suppresses or removes catalytically enhanced conversion of atomic hydrogen to molecular hydrogen on the object side 41 of the catalytic layer 30. Additionally, the support layer 34 provides mechanical robustness to the membrane 2. The catalytic layer 30 can therefore be made thinner, which may be desirable for example where the catalytic layer 30 is formed from expensive material (such as palladium).

In various embodiments, one example of which is depicted in Figures 1 and 2, the membrane 2 is additionally or alternatively supported by a support structure 6. The support structure 6 has a plurality of openings 7 though which the atomic hydrogen can pass without passing through the support structure 6. The atomic hydrogen can therefore be made to pass through the membrane 2 but not the support structure 6. In an embodiment, the support structure 6 comprises or consists of a porous layer or structure. The provision of a support structure 6 improves the robustness of the membrane 2 and makes it possible for the membrane 2 to be thinner. The support structure 6 is particularly desirable where a pressure differential is to be maintained across the membrane 2 (see below).

In various embodiments, examples of which are shown in Figures 1-5, the membrane 2 seals a first region 8 from a second region 10. The first region 8 contains the source of hydrogen (i.e. atomic hydrogen, hydrogen ions, or molecular hydrogen). The second region 10 contains the object 4 to be treated. A pressure differential may be maintained across the membrane 2 while the object 4 is receiving the atomic hydrogen. In various embodiments, particularly where the first region 8 contains a hydrogen plasma, the pressure in the first region 8 will be lower than a pressure in the second region 10.

In an embodiment, the membrane 2 is substantially opaque to UV radiation, e.g. with a transmittance of less than 1%, optionally less than 0.1%. This protects the object 4 from potential damage from UV radiation and is relatively easily achieved with a wide range of materials.

In an embodiment, the membrane 2 is maintained at a constant electrical potential, optionally by connecting the membrane 2 to ground so that the membrane 2 is maintained at ground potential. Maintaining the membrane 2 at constant potential or ground potential is used primarily where a plasma is being created in the first region 8 using high electric fields, where an electrical field may be generated between the membrane 2 and another electrode.

The above methods may be implemented using various apparatus configurations. Examples are discussed below with reference to Figures 3-5. In each example, a chamber 14 is provided that defines a first region 8. The chamber 14 is configured to contain, or contains, atomic hydrogen, hydrogen ions or molecular hydrogen. In the examples of Figures 3 and 5, the chamber 14 is configured so that a hydrogen plasma can be generated in the chamber 14 (e.g. by being provided with electrodes 12, 16 to generate a radio frequency discharge, one of which (12) may be connected to the membrane 2 and the other (16) driven to generate the discharge, a vacuum system 28 to generate an appropriate level of vacuum, and a gas supply system 26 to supply suitable gases for the plasma).

In each embodiment the apparatus 1 further comprises a membrane 2. The membrane 2 may be configured in any of the ways described above. The membrane 2 allows atomic hydrogen to pass through the membrane 2 from the chamber 14 to an opposite side of the membrane 2 (from the first region 8 to the second region 10). An object support 12 is configured such that an object 4 present on the object support 12 will receive the atomic hydrogen after the atomic hydrogen has passed through the membrane 2.

In the example of Figure 3, the membrane 2, object 4 and object support 12 are all provided within a common chamber 14. The membrane 2 seals the first region 8 within the chamber 14 from a second region 10 containing the object 4. Both of the first region 8 and the second region 10 are thus located within the same chamber 14 in this particular embodiment.

In the embodiment of Figure 3, the apparatus 1 comprises a power source 18 (e.g. a microwave source configured to drive at 13.56 MHz) connected to a top electrode 16 (which may be provided as a grid). The object support 12 is grounded via ground connection 24 and acts as a bottom electrode. The object support 12 may also be heated to control the temperature of the object 4 and the membrane 2 during treatment of the object 4 with the atomic hydrogen passing through the membrane 2. A hydrogen plasma is created in the first region 8 by supplying a suitable gas 20, such as Nt¼ or t¼, and applying an electric field between the top electrode 16 and the object support 12. A pressure in the first region 8 is controlled by creating an evacuation flow 22. In one embodiment, the pressure in the first region 8 is controlled to be about 650 mTorr and the power source is configured to provide 100W of microwave power.

Figures 4 and 5 depict alternative configurations in which the object 4 is held outside of the chamber 14 containing the first region 8. In the particular examples shown, plural objects 4 are provided on an object support 12 that is configured to bring the objects 4 one at a time to a position beneath the membrane 2 at which each object 4 can be treated with atomic hydrogen. The object support 12 in these examples may thus be configured to operate in the manner of a conveyor belt or similar. In the example of Figure 4, the first region 8 is configured to contain molecular hydrogen (provided via flow 20 from gas supply system 26). The flow 20 may comprise any mixture of gases in which molecular hydrogen is present. Unlike where a plasma is created in the first region 8, in the embodiment of Figure 4 the first region 8 does not have to be kept under vacuum, although in the embodiment shown a vacuum system 28 is provided to control the gas composition and pressure in the first region 8. The absence of a need to keep the first region 8 under vacuum makes the process potentially simpler and/or more economic. Furthermore, it is possible to use the apparatus 1 with a smaller or no pressure differential across the membrane 2, which provides greater design freedom for the membrane 2 (e.g. it does not need to be as strong). In embodiments of this type, the pressure differential may also be such that the pressure in the first region 8 is higher than the pressure in the second region 10. A temperature controller 25 may be provided to control the temperature within the first region 8 and/or of the membrane 2. The membrane 2 is electrically grounded in this embodiment via ground connection 24.

The example of Figure 5 is the same as Figure 4 except that a power source 18 and electrode 16 are provided. The power source 18 and electrode 16 may configured in substantially the same way as the corresponding elements discussed above with reference to Figure 3. The power source 18 and electrode 16 can be used to generate a hydrogen plasma in the first region 8, thereby providing atomic hydrogen and/or hydrogen ions in the first region 8.

Detailed Example

The following describes a specific example demonstrating the effectiveness of applying atomic hydrogen using a method according to an embodiment.

Using an arrangement of the type shown in Figure 3, a membrane 2 comprising a 100 nm layer of palladium, hereafter referred to as palladium leaf, was provided. The membrane 2 was held in place by an aluminium holder and introduced between a hydrogen generating plasma in the first region 8, which in this example was ammonia, and the object 4 to be passivated in the second region 10. The object 4 was a silicon wafer. Atomic hydrogen has been shown to be freely absorbed into the palladium bulk at relatively high concentrations when compared to other Group VIII elements.

Hydrogen diffuses through the palladium leaf (membrane 2) in atomic form and is released from the opposite surface. While the literature focusses on the release of molecular hydrogen from palladium surfaces, the inventors have found that atomic hydrogen is also released to a sufficient extent that the atomic hydrogen can be used to treat the object 4 (e.g. to achieve passivation).

The membrane 2 acts to protect the object 4 by absorbing the hard UV generated in the plasma. Additionally the membrane 2 is electrically grounded which neutralizes any charged hydrogen passing through the membrane 2. Finally, hydrogen passing through the membrane 2 will be thermalized such that the energy on leaving the membrane 2 is reduced. The concentration of atomic hydrogen incorporated into the object (silicon wafer) was observed through the electrical deactivation of boron in 0.175 Ω.αη p-type silicon. The increased resistivity of the deactivated region is then inferred using capacitance -voltage techniques which provides a lower limit for the total amount of hydrogen present.

It was found that the amount of hydrogen released from the membrane 2 depended on how the surface of the membrane 2 was prepared.

Figure 8 presents averaged active boron concentrations in the first 3-7 nm of the object (silicon wafer) with a range of different surface preparations, clearly demonstrating significant deactivation of boron acceptor. The lower quantity of atomic hydrogen released by the as-received leaf is attributed to the presence of a surface oxide that acts as an effective barrier to atomic hydrogen. When this oxide is removed through a 90 minute exposure to atomic hydrogen from the plasma, subsequent treatment of the object 4 by passing the atomic hydrogen through the membrane 2 achieves greatly increased release of atomic hydrogen. After the removal of the oxide a leaf was contaminated using H2S, which is known to suppress the catalytic properties of Pd for splitting and reforming hydrogen molecules. Not only was the flux of atomic hydrogen observed not reduced by this process but it appears that suppressing the parallel process for molecular formation at the palladium surface increases the amount of atomic hydrogen released.

Figure 9 presents the active boron concentration as a function of depth for wafers subjected to processes by a method of an embodiment with an oxide removed leaf at temperatures between 423 and 523 K. The vast majority of B is deactivated at low temperatures and with increasing temperature the trapping of hydrogen at boron acceptors becomes weaker. Figure 9 also shows that the total amount of hydrogen bonded to boron is increasing strongly with temperature in this range. It is not presently clear whether the increase in H with temperature is due to an increase in the supply of H or due to an increase in the incorporation of hydrogen from the silicon surface into the bulk.

Figure 10 demonstrates the effectiveness of the method for increasing minority carrier lifetimes. The method can be referred to as Shielded Hydrogen Passivation (SHP) and is indicated as "SHP" in the graph. The method was performed using a membrane 2 comprising a Pd leaf with no oxide on 1 Q.cm n-type FZ Si passivated with a lOnm thermal oxide. In order to obtain the best field effect passivation, the samples were corona charged as per R. S. Bonilla, F. Woodcock, P. R.

Wilshaw, "Very low surface recombination velocity in n-type c-Si using extrinsic field effect passivation," J. Appl. Phys. 116, 054102 (2014). Four curves are shown: 1) "No Treatment" (neither SHP nor corona); 2) "SHP" (SHP only, no corona); 3) "Corona" (Corona only, no SHP); and 4) "SHP + Corona" (SHP and corona both applied). It can be observed that the SHP treatment provides a clear improvement in minority carrier lifetimes, regardless of whether or not the corona processing is used (the values in curve 4 are higher than the values in curve 3, and the values in curve 2 are higher than the values in curve 1). Figure 11 demonstrates the effectiveness of the method in the case where the membrane 2 comprises a catalytic layer that has been treated to suppress the catalysis of the conversion between molecular hydrogen and atomic hydrogen on both sides of the catalytic layer. In this particular example, the catalytic layer was a palladium layer and the suppression of the catalysis was achieved by poisoning with sulphur. Three curves are shown: 1) "No Treatment" (neither SHP nor corona); 2) "SHP" (SHP only, no corona); and 3) "SHP + Corona" (SHP and corona both applied). Comparing curves 2 and 3 of Figure 11 with curves 2 and 4 of Figure 10 shows that the suppression of catalysis (poisoning) provides a further significant improvement in minority carrier lifetimes. The positive effect of the poisoning is seen more clearly in Figure 11 in comparison with Figure 8 because in the passivation of B technique used in Figure 8, the amount of passivated B is not linear with the H concentration above the specimen surface and becomes less sensitive the more H is produced.

The observation of atomic hydrogen permeating through the membrane 2 opens up a number of exciting new possibilities. The novel techniques disclosed allow damage free introduction of atomic hydrogen and do not require the objects (samples) to be treated to be in the same chamber as the plasma to receive effective passivation. This technique could therefore provide a low cost alternative to forming gas anneals and remote plasma exposures. The H levels seen are suitable for excellent passivation.

Claims

1. A method for applying atomic hydrogen to an object, comprising:

passing atomic hydrogen through a membrane and onto a portion of an object that is spaced apart from the membrane, wherein the membrane comprises a solid material and the atomic hydrogen passes through the solid material.

2. The method of claim 1, wherein the atomic hydrogen passes through the membrane from a first region to a second region, wherein the first region contains atomic hydrogen or hydrogen ions and the second region comprises the object.

3. The method of claim 2, further comprising providing a plasma in the first region to provide the atomic hydrogen or hydrogen ions.

4. The method of any preceding claim, wherein at least a portion of the atomic hydrogen is generated from molecular hydrogen at a surface of the membrane before the atomic hydrogen passes through the membrane and onto the object.

5. The method of any preceding claim, wherein an amount of atomic hydrogen received by the object is effective to perform one or more of the following in the object: passivate surface defects, passivate bulk defects, passivate impurities, act as a dopant that alters the electrical conductivity of the object, act as a defect that alters the electrical conductivity of the object.

6. The method of any preceding claim, wherein a maximum spacing between the portion of the object receiving the atomic hydrogen and the membrane is less than 5mm.

7. The method of any preceding claim, wherein the membrane seals a first region from a second region, wherein the first region contains atomic hydrogen, hydrogen ions, or molecular hydrogen, and the second region contains the object.

8. The method of claim 7, wherein a pressure differential is maintained across the membrane while the object is receiving the atomic hydrogen.

9. The method of claim 8, wherein the pressure in the first region is lower than a pressure in the second region.

10. The method of any preceding claim, wherein the membrane comprises at least one continuous layer of solid material spanning all of a region through which the atomic hydrogen passes before reaching the object.

11. The method of any preceding claim, wherein the membrane comprises a metallic layer.

12. The method of claim 11, wherein the metallic layer is a continuous layer spanning all of a region through which the atomic hydrogen passes before reaching the object.

13. The method of claim 12, wherein the metallic layer comprises one or more of the following in any combination: palladium, nickel, niobium, silver, titanium, vanadium, iron, tantalum, an alloy comprising vanadium and gallium, and an alloy comprising nickel and silver.

14. The method of any preceding claim, wherein the membrane comprises a catalytic layer, the catalytic layer being configured to catalyse a conversion between molecular hydrogen and atomic hydrogen at a surface of the catalytic layer.

15. The method of claim 13 or 14, wherein the catalytic layer comprises palladium or an alloy of palladium, optionally an alloy of palladium with one or more of the following: silver, copper, yttrium, gold, ruthenium and indium.

16. The method of claim 15, wherein the catalytic layer is treated to suppress the catalysis of the conversion between molecular hydrogen and atomic hydrogen.

17. The method of claim 16, where the treatment of the catalytic layer is applied on an object side of the catalytic layer and is not applied on the other side of the catalytic layer.

18. The method of claim 16, wherein the treatment of the catalytic layer is applied to both sides of the catalytic layer.

19. The method of any of claims 14-18, wherein the catalytic layer is covered by a support layer on an object side of the catalytic layer and the catalytic layer forms an outer surface of the membrane on the side of the membrane opposite to the object.

20. The method of any preceding claim, wherein the membrane is substantially opaque to UV radiation.

21. The method of any preceding claim, wherein the membrane is maintained at a constant electrical potential, optionally ground.

22. The method of any preceding claim, wherein the membrane is supported by a support structure having a plurality of openings though which the atomic hydrogen can pass without passing through the support structure.

23. The method of claim 22, wherein the support structure comprises a porous layer.

24. The method of any preceding claim, wherein the object comprises one or more of the following: a semiconductor device, a photovoltaic device, a metal oxide, and an optical sensor.

25. The method of any preceding claim, wherein the atomic hydrogen is applied while maintaining the temperature of the object below 800 degrees C.

26. An apparatus for applying atomic hydrogen to an object, comprising:

a chamber, wherein the chamber contains atomic hydrogen, hydrogen ions or molecular hydrogen, or the chamber is configured to generate a plasma in the chamber;

a membrane; and

an object support configured to support the object, wherein:

the membrane is configured to allow atomic hydrogen to pass through the membrane from the chamber to the other side of the membrane, wherein the membrane comprises a solid material and the atomic hydrogen passes through the solid material; and

the object support is configured such that an object present on the object support will receive the atomic hydrogen after the atomic hydrogen has passed through the membrane.

27. The apparatus of claim 26, wherein the membrane seals a first region from a second region, wherein the first region contains atomic hydrogen, hydrogen ions, or molecular hydrogen, and the second region contains the object.

28. The apparatus of claim 26 or 27, wherein the membrane comprises a metallic layer.

29. The apparatus of any of claims 26-28, wherein the membrane comprises a catalytic layer, the catalytic layer being configured to catalyse a conversion between molecular hydrogen and atomic hydrogen at a surface of the catalytic layer.

30. The apparatus of claim 29, wherein the catalytic layer is treated to suppress the catalysis of the conversion between molecular hydrogen and atomic hydrogen, and the treatment of the catalytic layer is applied on an object side of the catalytic layer and is not applied on the other side of the catalytic layer.

31. The apparatus of claim 29, wherein the catalytic layer is treated to suppress the catalysis of the conversion between molecular hydrogen and atomic hydrogen, and the treatment of the catalytic layer is applied on both sides of the catalytic layer.

32. The apparatus of any of claims 29-31, wherein the catalytic layer is covered by a support layer on an object side of the catalytic layer and the catalytic layer forms an outer surface of the membrane on the side of the membrane opposite to the object.

33. The apparatus of any of claims 26-32, wherein the membrane is supported by a support structure having a plurality of openings though which the atomic hydrogen can pass without passing through the support structure.

34. The apparatus of claim 33, wherein the support structure comprises a porous layer.