Patent Application
20050118349
This invention relates to layered structures, particularly layered diamond structures.
For certain diamond applications, it is necessary to have two or more layers of diamond which have different properties in atomic contact with each other. One example is in electronics where a layered structures can be used to make a device, for example as described in WO 01/18882 A1
One method of making a layered diamond structure is by using ion implantation. Ions may be implanted into diamond to create an n-type or p-type semi-conducting layer on top of a layer with a different property. This method has the disadvantage that damage to the diamond occurs during ion implantation which can result in permanent degradation in important properties such as carrier lifetimes and mobilities. Further, the types and concentration of ions which can be successfully implanted in diamond are limited and the process often requires complex post implantation annealing.
Although chemical vapour deposition (CVD) provides a method of synthesising epi-layers, i.e. an epitaxially grown layer, to a desired thickness, the CVD process enables only a few very specific defects and impurities to be incorporated into the layer. This is best illustrated with the following example. A boron doped diamond layer can be grown onto a high purity single crystal substrate using a CVD process known in the art. The substrate might be processed from a natural diamond or diamond synthesised by CVD or high pressure high temperature (HPHT) methods. This will produce a two layer structure. Using conventional terminology this diamond structure will have pi-properties (i.e. properties exhibited by a sharp p-type to intrinsic semiconductor interface). Many of the typical two layer device structures that might be produced in this way require one of the layers to be very thin (<20 μm). For instance a 10 μm thick boron doped diamond layer on a 500 μm thick high purity diamond layer. A structure of this form, where precise thicknesses and sharp interfaces are necessary, requires considerable control of the synthesis process and/or careful mechanical processing of the structure following growth.
A CVD process is conducive to the synthesis of thin epi layers but has the disadvantage that only layers containing certain dopants can be synthesised. For example it is well known that HPHT synthesis provides a method of incorporating nickel, cobalt and nitrogen into the diamond in high concentrations (>5 parts per million (ppm) carbon atoms) but to date this has not been possible using CVD methods. Thus, to produce a diamond structure that consists of a thin epi-layer (<20 μm) containing nickel and a thicker boron doped layer (>100 μm) it would be necessary to take a suitably prepared substrate containing Ni with a thickness typically >200 μm (for ease of handling and processing) and then synthesise, using a CVD method, an overlayer (>100 μm) which contains the required boron concentration. Following growth, considerable care would then be needed with mechanical processing to finish with a structure which consists of, for example, a 10 μm Ni doped layer and a 100 μm B doped layer where the thickness tolerances are better than about 2 μm.
U.S. Pat. No. 5,587,210 describes a method of separating a CVD diamond layer from a diamond substrate. The method includes the steps of ion implanting a diamond substrate, thus creating a damaged layer of non-diamond carbon below the surface of the substrate through which ion implantation occurred, growing diamond on the surface of the substrate through which the ion implantation occurred, and electrochemically etching the diamond substrate to remove the damaged layer. The resulting product is a free standing CVD layer having a thin, i.e less than 1000 nm, layer of diamond bonded to a surface thereof.
The CVD diamond layer which is grown on the diamond substrate is pure CVD diamond. There is no suggestion that the CVD diamond layer should be doped or otherwise treated to change its electronic or other properties.
This US patent also suggests that the diamond substrate can be doped by ion implantation with suitable atoms to create n-type and p-type semi-conductors. When ion implanting a diamond substrate to create such semi-conductor properties, the implanted region will vary considerably in its dopant content. The region of highest and most uniform dopant concentration will lie below the surface through which ion implantation occurred. The region adjacent the surface through which the ion implantation occurred will contain little or no dopant and of non-uniform concentration. Thus, on either side of the interface between the substrate and the CVD diamond layer the material will be essentially pure diamond.
Furthermore, ion implantation doping is always associated with lattice damage due to the ion implantation, which substantially reduces the benefit obtained from the dopant in that it adversely modifies the electronic properties of the doped layer.
According to the present invention, there is provided a process of making a product which comprises at least two layers in contact with each other, each layer being of a wide-band gap material and each layer differing from the other layer in at least one property, including the steps of:
The ion implantation should be carried out with ions which allow deep penetration into the substrate, creating the damaged layer substantially below the surface through which the ion implantation occurs. The ions suitable to achieve this are typically ions of low atomic mass, preferably an atomic mass less than 21 and more preferably an atomic mass less than 13. Examples of suitable ions are helium and hydrogen ions. The ions for the ion implantation are preferably of high energy, e.g. have an energy exceeding 5 keV. The precise depth of the damaged layer can be accurately controlled by manipulating the energy and type (i.e. mass) of the implanted ions. Typically, the ion implantation dose will exceed 1×1015 cm−2.
Generally, the damaged layer will lie at a depth of 0.05 to 200 μm, typically 0.3 to 10 μm, below the surface through which ion implantation occurred.
It is preferred that the region of the substrate between the surface through which ion implantation occurs and the damaged layer is substantially free of ion implantation doping damage.
The wide-band gap material may be silicon carbide, gallium nitride or the like and is preferably diamond.
Generally, the layers will differ from each other in the characteristic which provides the layers with different electrical properties. The product may comprise only two layers in contact with each other, or more than two layers. When the product consists of more than two layers, adjacent layers, in contact with each other, will have different characteristics. The interface between adjacent layers defines a sharp and well-defined interface between two regions having different properties. This is an important feature, particularly when the layered product is to be used in an electronic application.
The surface through which the ion implantation occurs may be planar or non-planar. Thus, the interface between adjacent layers may also be planar or non-planar. When non-planar, the profile may be designed to provide a specific useful feature for a device which includes the layered product as a component.
The substrate may be natural or synthetic diamond, particularly CVD diamond. The layer of grown wide-gap material may be CVD diamond or doped CVD diamond.
In one particular form of the invention, the region of the substrate adjacent to the surface through which ion implantation occurred is uniformly doped. The dopant may be selected from nitrogen, boron, nickel, cobalt, iron, phosphorus, sulphur or other elements which can occupy a lattice position, substitutional or otherwise, and provide the region with useful properties, particularly electronic properties.
The substrate and layer of grown wide-gap material may have the same thickness or differ in thickness. Generally, the layers will differ in thickness.
The process of the invention minimises excessive complicated post growth processing and enables structures that contain thin layers of diamond with properties very different to a second thicker layer to be synthesised. These structures have, for example, use in electronic applications.
The drawing illustrates, as FIGS. 1(a) to 1(c), schematically the steps in an embodiment of the invention.
The invention will now be illustrated with reference to the accompanying drawing. Referring to
The diamond substrate 10 may be natural, or synthesised by chemical vapour deposition (CVD) or by high pressure high temperature (HPHT) techniques. This diamond will have characteristic electronic properties associated with some specific incorporated defect. Selection of this diamond substrate from any source of diamond provides for the broadest possible range of dopants, impurities or defects within the diamond with which to tailor its properties. The diamond substrate surface may be flat, for example a polished surface, or it may be curved or have non-planar features such as trenches or raised features which may subsequently form elements of, for example, an electronic device structure. This latter possibility arises because the nature of ion implantation is to allow penetration of the ions down to a given depth, independent of the macroscopic variation in height of the substrate. This provides for device geometries that are not easily achieved in diamond by any other means. In one example of the invention, the dopant in the substrate may be present from growth of the diamond, e.g. nickel, cobalt or iron. As the dopant is present from growth of the substrate, the substrate diamond is free of the ion damage that would be associated with ion implantation doping, and the uniformity of the dopant is that of the original synthesis technique not the very non-uniform doping profile associated with ion implantation.
An epitaxial diamond layer 20 of different properties is then grown by CVD on the surface 12 of the substrate 10 (
The diamond substrate is then severed along region 18, by immersing the product into an acid etch, annealing or using appropriate electrochemical etching. The resulting product (
Implantation damage in the released layer 22 is generally low, since ion damage is low until the ion energy is almost exhausted which occurs as it reaches the damage layer 18. However, when using a substrate with a planar (preferably polished) surface, it is possible to reduce further the effect of this ion damage by implanting to a greater depth than is required (say to 5 μm), and after release removing a portion of the thickness of the released layer 22 by polishing, to leave a thinner final layer 22 (say 3 μm). This may be advantageous because the portion of diamond remaining had only higher energy ions traversing it, with proportionately lower ion damage, and the relatively heavily damaged region adjacent to the damaged region 18 is then wholly removed.
The process can be repeated more than once. For example bi-layer comprising the thin top layer 22 on a thicker layer 20 formed according to the invention can be further implanted through surface 26 of layer 22 into layer 20 to provide a damaged layer in layer 20. A thick CVD diamond layer is grown on surface 26 of layer 22 and then the sample severed along the implantation damaged layer. The result is a three layer structure, comprising the thin layer 22 sandwiched between a thin portion of the layer 20 and the new CVD diamond layer.
A high purity diamond substrate produced using a CVD method known in the art with thickness 600 μm, is first implanted with 2 MeV oxygen ions to a dose of 1×1017 cm−2. A thick (300 μm) boron doped single crystal CVD layer which has, as measured by SIMS, 2×1019 B atoms/cm−3 is grown on a surface of this substrate. Following growth the layered product is electrochemically etched to produce two samples: (i) a high purity diamond layer that can be reused and (ii) a two layer product consisting of a 1 μm high purity diamond layer and a 300 μm boron doped diamond layer in contact with the high purity diamond layer. This two layer product has an electronic application.
A boron doped (1×1019 cm−3) diamond substrate prepared using a CVD method with thickness 620 μm is first implanted with 2 MeV hydrogen ions to a dose of 1×1019 cm−2. A thick (300 μm) high purity single crystal CVD diamond layer is grown on to a surface of this substrate. Following growth the layered product is electrochemically etched to produce two samples: (i) a boron doped diamond plate which can be reused, and (ii) a two layer product consisting of a 10 μm boron doped diamond layer and a 300 μm high purity diamond layer. This two layer product has an electronic application.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0127263.2 | Nov 2001 | GB | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/IB02/04723 | 11/12/2002 | WO |
