WO2009034358A2

WO2009034358A2 - Hydrogen trapping

Info

Publication number: WO2009034358A2
Application number: PCT/GB2008/003122
Authority: WO
Inventors: Robert William Mccullough; Harold Samuel Gamble; Mihaela Ghita
Original assignee: QUENN'S UNIVERSITY OF BELFAST
Current assignee: QUENN'S UNIVERSITY OF BELFAST
Priority date: 2007-09-12
Filing date: 2008-09-12
Publication date: 2009-03-19
Anticipated expiration: 2010-03-12
Also published as: GB0717718D0; WO2009034358A8; WO2009034358A3

Abstract

A method of trapping hydrogen in a material (1) which comprises an epitaxially-grown hydrogen-trapping substance forming a hydrogen- trapping region, the method comprising bombarding the material (1) with hydrogen atoms substantially all of which have an energy in the range of approximately 0.05 eV to approximately 0.1 eV, and allowing at least some of the hydrogen atoms to interact with the hydrogen-trapping region, effecting trapping of at least some of the hydrogen atoms in the hydrogen-trapping region of the material (1).

Description

Hydrogen Trapping

The invention relates to hydrogen trapping, particularly in the trapping of hydrogen in materials containing silicon.

In recent years, there has been considerable interest in the trapping of various substances in various materials. For example, investigations have been carried out into the bombardment of silicon with hydrogen ions, and the trapping of the hydrogen ions within the silicon. Such techniques have been used in the production of silicon on insulator material, used in, for example, silicon microcircuits. However, it is known that the bombardment of the silicon in this way, causes damage to the silicon material that requires rapid thermal annealing to restore the crystallinity of the material. This is undesirable, and improvements in the trapping of substances such as hydrogen are being sought.

According to the invention there is provided a method of trapping hydrogen in a material which comprises an epitaxially-grown hydrogen- trapping substance forming a hydrogen-trapping region, the method comprising bombarding the material with hydrogen atoms substantially all of which have an energy in the range of approximately 0.05 eV to approximately 0.1 eV, and allowing at least some of the hydrogen atoms to interact with the hydrogen-trapping region, effecting trapping of at least some of the hydrogen atoms in the hydrogen-trapping region of the material.

The use of hydrogen atoms having energies in this range, so-called thermal-energy hydrogen atoms, results in a relatively 'damage-free' process, which will not give rise to ionisation and nuclear stopping processes in the material, which are involved in bombardment with energetic hydrogen ions.

The method may comprise heating the material prior to bombardment with the hydrogen atoms. The material may be heated to approximately 500⁹C. The material may be heated for a time of approximately 1 hour. The material may be cooled after heating, for example the material be cooled or be allowed to cool to a temperature of approximately 400^sC.

The method may comprise bombarding the material with the hydrogen atoms for a period in the region of approximately 60min to approximately 220min.

The hydrogen atoms may be provided by an effusive source which emits an effusive beam of hydrogen atoms. The effusive beam of hydrogen atoms may comprise a beam of highly dissociated hydrogen molecules. The source may emit 656.3nm Balmer alpha series radiation. The source may emit hydrogen atoms having a concentration in the region of approximately 200x10¹⁸ atoms/cm² to approximately 1500x10¹⁸ atoms/cm².

The method may comprise heating the material during bombardment thereof with the hydrogen atoms. The material may be heated to a temperature in the range of approximately 200⁹C to approximately 500⁹C, preferably 400⁹C. The diffusion of the hydrogen atoms to the hydrogen- trapping region of the material may be enhanced by heating the material.

Allowing the hydrogen atoms to interact with the hydrogen-trapping region may comprise allowing the hydrogen atoms to diffuse to the hydrogen- trapping region. Allowing the hydrogen atoms to interact with the hydrogen-trapping region may comprise causing the hydrogen atoms to diffuse to the hydrogen-trapping region by providing a hydrogen concentration gradient at the surface of the material. Such a gradient may be established by providing a hydrogen concentration at the surface, for example of between approximately 2x10¹⁶ atoms/cm³ to approximately 2x10¹⁷ atoms/cm³.

The hydrogen-trapping region may comprise a hydrogen-trapping substance concentration in the form of a peak. The peak may have a maximum concentration of approximately 1.3x10¹⁹ atoms/cm³ up to

2.5x10²¹ atoms/cm³, for example 2x10²⁰ atoms/cm³. The peak may have a width in the region of approximately 50nm to approximately 100nm. The hydrogen-trapping substance may be boron.

Trapping hydrogen atoms in the hydrogen-trapping region of the material may cause formation of one or more platelets in the hydrogen-trapping region of the material. Formation of the one or more platelets may cause occurrence of cracks in the hydrogen-trapping region. The cracks may occur at a depth in the hydrogen-trapping region consistent with a depth of maximum concentration of the hydrogen-trapping substance in the hydrogen-trapping region. At least some of the cracks may occur in an orientation which is substantially parallel to a surface of the material. At least some of the cracks may form a line.

The method may comprise splitting the material at the hydrogen-trapping region. Splitting the material may be achieved by propagating cracks in the hydrogen-trapping region. Propagation of the cracks may be achieved by heating the material to a temperature in the range of approximately 400⁹C to approximately 550^eC. The material may comprise a plurality of layers. The material may comprise a first layer. The first layer may form a substrate. The first layer may comprise silicon. The material may comprise a second layer. The second layer may be epitaxially grown on the first layer. The second layer may contain the hydrogen-trapping substance and provide the hydrogen- trapping region in the material. The material may comprise a third layer. The third layer may be formed on the second layer. The third layer may be epitaxially grown on the second layer. The third layer may form a cap on the second layer. The third layer may comprise silicon. The third layer may have a thickness of approximately 45nm, or approximately 90nm, or approximately 180nm, or approximately 270nm, or approximately 500nm. The hydrogen-trapping region may therefore be provided at a depth of approximately 45nm, or approximately 90nm, or approximately 180nm, or approximately 270nm, or approximately 500nm in the material. The material may comprise one or more further layers, some of which may contain a hydrogen-trapping substance and provide a hydrogen-trapping region in the material.

An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawing, which is a schematic cross sectional representation of a material in which hydrogen has been trapped using the method according to the invention.

Referring to the figure, the material 1 comprises a first layer 3, a second layer 5 and, in this embodiment, a third layer 7. The first layer comprises silicon, and acts a substrate. The second layer 5 is epitaxially grown on the first layer 3. The second layer 5 comprises boron, which is a hydrogen-trapping substance. The third layer 7 is epitaxially grown on the second layer 5, and forms a cap on the second layer 5. The third layer 7 comprises silicon. The thickness of the third layer 7 may be determined by an end-use of the material. For example, the third layer 7 may have a thickness of any of approximately 45nm, approximately 90nm, approximately 180nm, approximately 270nm, approximately 500nm.

The boron is distributed in the second layer 5, such that the boron concentration in the layer forms a peak of width of approximately 10Onm and maximum concentration of approximately 2x10²⁰ atoms /cm³. Thus the boron concentration in the second layer 5 is very narrow, providing a sharp boron profile in the second layer 5. The third layer 7 is grown on the boron-containing second layer, and there is therefore little to no boron present in the third layer 7. Boron has a relatively high probability of interaction with hydrogen, and the boron in the second layer 5 of the material therefore forms a hydrogen-trapping region in the material 1. As the boron profile in the second layer 5 is sharp, and there is little or no boron the third layer 7, the hydrogen-trapping region of the material is well-defined and is provided at a depth in the material 1.

The boron is provided in the second layer 5 by epitaxially growth, rather than by prior art methods of diffusing boron through the third layer to the second layer. Defects in the silicon third layer 7 are therefore largely avoided. Changing the crystalline structure of the silicon third layer 7 to an amorphous structure is also avoided, and therefore no rapid thermal anneal process need be carried out on the material 1.

Prior to bombardment with hydrogen atoms, an exposed surface 9 of the silicon third layer 7 is treated to remove contaminants, such as dust, salts, skin oils, etc. The surface 9 is then treated to remove a silicon oxide layer therefrom. This is carried out by treating the surface with hydrofluoric (HF) acid. Removal of the silicon oxide layer can also be achieved by the bombardment of the hydrogen atoms. However, this takes considerably longer than removal using HF acid, and would therefore undesirably increase the time taken to carry out the hydrogen-trapping method.

The material 1 is then heated to a temperature of approximately 500^QC, and held at that temperature for approximately 1 hour. The material is then allowed to cool or is actively cooled to a temperature of approximately 400 ^QC.

The method then comprises bombarding the material 1 with hydrogen atoms having an energy in the range of approximately 0.05 eV to approximately 0.1 eV, i.e. thermal-energy hydrogen atoms. The hydrogen atoms are produced by an effusive source which emits an effusive beam of highly-dissociated hydrogen molecules, producing hydrogen atoms having a concentration in the region of approximately 200x10¹⁸ atoms/cm² to approximately 1500x10¹⁸ atoms/cm². The hydrogen atoms and molecules are caused to impinge on the exposed surface 9 of the third layer 7 of the material 1 , for a period in the region of approximately 60min to approximately 220min. The bombardment of the material 1 with the hydrogen atoms takes place at a temperature of approximately 400⁵C. This has been found to be the optimum temperature for trapping a desired amount of hydrogen in the material 1 in a desired trapping time.

The hydrogen atoms are then allowed to interact with the hydrogen- trapping region formed in the second layer 5. This is achieved by causing the hydrogen atoms to diffuse to the hydrogen-trapping region.

Bombardment of the surface 9 of the third layer 7 with the hydrogen atoms causes formation of a hydrogen concentration gradient in the material 1 , which decreases steeply from a region at and near the surface 9. The hydrogen atoms move within the material 1 is such a way as to nullify the concentration gradient, i.e. they move away from the surface 9 into the material 1.

As there are few defects present in the silicon third layer 7, the rate of diffusion of the hydrogen atoms through this layer is enhanced. In addition, as the concentration of boron in the material is confined to the second layer 5, diffusion of the hydrogen atoms is also enhanced. It has been found that the rate of diffusion of the hydrogen atoms to the hydrogen-trapping region is enhanced by heating the material 1. Thus during the bombardment and diffusion processes, the material 1 is heated to a temperature of approximately 400⁹C. As the hydrogen atom diffusion is enhanced by heating, effecting trapping of hydrogen atoms in the hydrogen-trapping region of the material 1 is enhanced by heating the material. Enhancement of diffusion of the hydrogen atoms away from the surface 9 substantially reduces clustering of hydrogen atoms near the surface 9 of the material 1. This, in turn, substantially reduces or eliminates deformation of the surface 9 by the formation of bubbles thereon. This is important if the surface 9 of the material 1 is subsequently to be bonded to another material, as damage at the surface reduces its flatness makes subsequent bonding difficult.

The hydrogen atoms diffusing through the third layer 7 have thermal energies. This results in a relatively 'damage free' diffusion process, with none of the silicon atom ionisation or displacement processes caused by diffusion of energetic hydrogen ions.

As the hydrogen atoms diffuse away from the surface 9 and through the third layer 7 of the material 1 , they diffuse into the hydrogen-trapping region of the second layer 5. Here they encounter a 'wall' of traps provided by the boron in the hydrogen-trapping region, and a large proportion of the hydrogen atoms are trapped in this region, by interaction with the boron. As the hydrogen-trapping region provided by the boron is well-defined, the hydrogen concentration within the material 1 is also well- defined.

Accumulation of hydrogen atoms in the boron hydrogen-trapping region of the material 1 gives rise to a high hydrogen pressure in the lattice of the hydrogen-trapping region and eventual hydrogen saturation in this region. This causes formation of platelets in the hydrogen-trapping region of the material 1. The hydrogen build-up occurs at a front edge of the boron profile, i.e. the edge close to the interface between the boron-containing second layer 5 and the silicon third layer 7. The platelets are substantially formed in an orientation which is substantially parallel to the surface 9 of the material, and form an approximately straight line essentially at the interface between the boron-containing second layer 5 and the silicon third layer 7. Formation of the platelets causes occurrence of cracks in the hydrogen-trapping region of the material 1 , at the interface between the boron-containing second layer 5 and the silicon third layer 7. The method then comprises causing the material 1 to split at this interface. This is achieved by heating the material 1 to a temperature of, for example, approximately 500³C, which causes propagation of the cracks. This separates the third layer 7 from the first and second layers 3, 5, creating a second exposed surface of the third layer. This surface may be attached to a further material.

The line formed by the cracks in the material 1 is straighter and more parallel to the surface of the material than lines provided in prior art materials where the hydrogen-trapping region is formed by diffusion of boron. The exposed surface of the third layer created by splitting the material 1 is therefore flatter, and there is less need for etching and polishing of the surface prior to attachment to another material.

Splitting the material 1 has important applications. For example, the third layer semiconductor elements may be placed on the surface 9 of the third layer 7 to form a semiconductor device. The first and second layers 3, 5 provide a substrate base for the third layer 7 allowing attachment of the elements to this layer. The third layer 7 may then be split from the remainder of the material 1 , to form a separate semiconductor device.

It will be appreciated that various modifications to the method and material described above may be made. For example, the concentration of the hydrogen atoms, the bombardment temperature and time, etc. may be changed. In the above method, the material was heated before bombardment to, inter alia, avoid the formation of bubbles on the surface 9 of the material 1. However, in some applications, the formation of bubbles may be desirable. The pre-heating step may then be omitted. Bubbles having a height of approximately 10nm and a width of a few microns have been then observed. The bubbles may form due to trapping of hydrogen by point defects in the third layer 7 of the material 1. (If the material is heated prior to bombardment and then cooled to the bombardment temperature, the defects are at least largely removed, and bubbles largely avoided). The frequency of occurrence and the height and diameter of the bubbles can be analysed, to obtain an indication of the amount of hydrogen trapped by point defects in the third layer 7 of the material 1. For example, the amount of hydrogen trapped in this layer may be estimated by obtaining a measure of an average bubble size and the number of bubbles per unit area of the surface 9.

Claims

1. A method of trapping hydrogen in a material which comprises an epitaxially-grown hydrogen-trapping substance forming a hydrogen- trapping region, the method comprising bombarding the material with hydrogen atoms substantially all of which have an energy in the range of approximately 0.05 eV to approximately 0.1 eV, and allowing at least some of the hydrogen atoms to interact with the hydrogen-trapping region, effecting trapping of at least some of the hydrogen atoms in the hydrogen- trapping region of the material.

2. A method according to claim 1 , comprising heating the material prior to bombardment with the hydrogen atoms.

3. A method according to claim 2, in which the material is heated to approximately 500^QC for a time of approximately 1 hour.

4. A method according to claim 2 or claim 3, in which the material is cooled after heating to a temperature of approximately 400⁹C.

5. A method according to any preceding claim, comprising bombarding the material with the hydrogen atoms for a period in the region of approximately 60min to approximately 220min.

6. A method according to any preceding claim, in which the hydrogen atoms are provided by an effusive source which emits an effusive beam of hydrogen atoms, having a concentration in the region of approximately 200x10¹⁸ atoms/cm² to approximately 1500x10¹⁸ atoms/cm².

7. A method according to any preceding claim, comprising heating the material during bombardment thereof with the hydrogen atoms.

8. A method according to claim 7, in which the material is heated to a temperature in the range of approximately 200⁹C to approximately 500^QC, preferably 400 ^QC.

9. A method according to any preceding claim, in which allowing the hydrogen atoms to interact with the hydrogen-trapping region comprises causing the hydrogen atoms to diffuse to the hydrogen-trapping region by providing a hydrogen concentration gradient at the surface of the material.

10. A method according to any preceding claim, in which the hydrogen- trapping region comprises a hydrogen-trapping substance concentration in the form of a peak.

11. A method according to claim 10, in which the peak has a maximum concentration of approximately 1.3x10¹⁹ atoms/cm³ up to 2.5x10²¹ atoms/cm³, and a width in the region of approximately 50nm to approximately 100nm.

12. A method according to any preceding claim, in which the hydrogen- trapping substance is boron.

13. A method according to any preceding claim, in which trapping hydrogen atoms in the hydrogen-trapping region of the material causes formation of one or more platelets in the hydrogen-trapping region of the material.

14. A method according to claim 13, in which formation of the one or more platelets causes the occurrence of cracks in the hydrogen-trapping region.

15. A method according to claim 14, in which the cracks occur at a depth in the hydrogen-trapping region consistent with a depth of maximum concentration of the hydrogen-trapping substance in the hydrogen- trapping region.

16. A method according to claim 14 or claim 15, in which at least some of the cracks form a line.

17. A method according to any of claims 14 to 16, comprising splitting the material at the hydrogen-trapping region.

18. A method according to claim 17, in which splitting the material is achieved by propagating cracks formed in the hydrogen-trapping region.

19. A method according to claim 18, in which propagation of the cracks is achieved by heating the material to a temperature in the range of approximately 400⁹C to approximately 550⁹C.

20. A method according to any preceding claim, in which the material comprises a first layer which forms a substrate and comprises silicon, a second layer epitaxially grown on the first layer which contains the hydrogen-trapping substance and provides the hydrogen-trapping region in the material, and a third layer formed on the second layer and forming a cap on the second layer.