Single-crystal – oriented-crystal – and epitaxy growth processes; – Forming from vapor or gaseous state
Reexamination Certificate
2000-09-18
2003-07-08
Kunemund, Robert (Department: 1765)
Single-crystal, oriented-crystal, and epitaxy growth processes;
Forming from vapor or gaseous state
C117S094000, C117S095000, C117S096000, C117S918000, C117S929000
Reexamination Certificate
active
06589333
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a method for manufacture of a suitable substrate for the subsequent growth of a single crystal diamond layer and for the manufacture of a single crystal diamond layer, to a substrate as well as to a carrier wafer and a diamond jewel.
Single crystal diamond layers are particularly desirable for applications in high temperature electronics. Diamond is a crystalline high-pressure phase of carbon which is meta-stable under normal conditions. The stable phase is graphite. In addition to naturally occurring diamonds, diamonds are also produced artificially by a high-pressure method. These diamonds are normally very small and are used for grinding purposes because of the hardness of the diamonds. For electronic applications of diamonds, it is principally thin, single crystal diamond layers that are of interest. High-temperature applications which are made possible by the high band gap of diamond of about 5 eV are at the center of interest. Diamond can in principle be deposited epitaxially onto single crystal diamond by means of chemical vapor deposition (CVD) in a corresponding hydrogen atmosphere. The presence of hydrogen serves in this connection for the preferential etching away of the stable equilibrium phase in the form of graphite, which is likewise deposited. For practical applications, the epitaxy of diamond layers on single crystal diamond crystals is not of great importance, because only very small single crystal diamond substrates are available and because large area substrates of other materials with similar lattice constants to diamond do not exist. In the case of microelectronics and optoelectronics there are, however, semiconductor wafers which are commercially available in part with a diameter of up to 30 cm. Since no large area single crystal diamond substrates are available, numerous efforts have been made to produce single crystal diamond layers on other easily available substrates. The best success hitherto has been achieved with (100) orientated silicon substrates on which strongly textured, likewise almost (100) orientated diamond layers can be deposited by means of suitable CVD methods using an electrical voltage at the silicon substrate. These diamond layers consist of individual single crystal diamond grains in the micron range, which are twisted and tilted relative to the silicon substrate, with the twisting and tilting angles lying in the order of magnitude of about 1°. So-called grain boundaries thereby arise at points at which individual diamond grains abut, and greatly impair the electronic characteristics of the diamond film. It would, in contrast, be desirable to avoid these grain boundaries in order to actually produce a single crystal diamond layer. These diamond layers are described in the article by X. Jiang et al., Appl. Phys. Lett. 62 (1993) 3438.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method for the manufacture of a substrate for the growth of single crystal diamond layers and also corresponding substrates which make it possible to produce extended single crystal diamond layers by epitaxy, so far as possible without disturbing grain boundaries, and diamonds built up on this for electronic and/or other purposes, such as industrial cutting and grinding processes, or in the form of diamond jewels.
The method of the invention for the manufacture of a suitable substrate for the subsequent growth of a single crystal diamond layer is characterized by the following steps:
a) selection of a substrate of a mono-crystalline material having a fixed lattice constant (a
Si
) or with a layer consisting of such a material,
b) manufacturing either a strained silicon layer with foreign material atoms incorporated at substitutional lattice sites on the monocrystalline material of the substrate,
c) transferring the strained layer into an at least partly relaxed state in which it adopts by relaxation and through the selected foreign atom concentration a lattice constant (a
Si(C
) which satisfies the condition
n.a
Si(C)
=m.a
D
where n and m are integers, preferably different integers, and a
D
is the lattice constant of diamond, with the relaxed layer forming the substrate, for example the substrate surface, for the epitaxial growth of the diamond layer.
In particular it is proposed that carbon atoms should be used for the foreign material atoms and n should be selected=2 and m=3.
The method of the invention is based on the fundamental realization that the almost epitaxial alignment of the diamond layers which can be grown on (100) silicon wafers is to be associated with the fact that the lattice constant of diamond a
D
has an almost rational relationship to the lattice constant of the silicon a
Si
, so that one can write
2
a
Si
≅a
D
The condition
2
a
Si
=3
a
D
(1)
is, however, not precisely satisfied. Furthermore, it has been speculated, in accordance with the invention, that if the corresponding relationship were precisely satisfied, one could expect direct epitaxial growth without misorientation and without grain boundaries.
In reality, 2a
Si
is approximately 1½% larger than 3a
D
. This is now seen as the reason why an adaptation arises with a corresponding faulty orientation (twisting and tilting) in the 1-degree region.
The basic concept of the present invention is that if one could reduce the lattice constant of silicon substrate by about 1%, so that 2a
Si
3a
D
, i.e. so that the condition (1) is precisely satisfied, the growth of single crystal diamond layers without substantial structuring should be possible on such a silicon substrate.
Furthermore, the invention recognizes that the desired reduction of the lattice constant of silicon by the incorporation of foreign material atoms on substitutional lattice sites of the crystalline silicon can be achieved. It is, for example, known that carbon as a group-IV-element can be electrically neutrally incorporated in crystalline silicon at substitutional lattice sites. Since carbon atoms are considerably smaller than silicon atoms, the incorporation of carbon leads to a volume reduction of the silicon crystal. In simplified manner, one can say that the volume of a silicon crystal reduces for each substitutionally incorporated carbon atom by an atomic volume &OHgr;Si of the silicon. This is explained in more detail in the article by U. Gösele in MRS-Proc. Vol. 59 (1986), pages 419 to 431.
Thus, a corresponding reduction of the average lattice constant a
Si(C)
results in dependence on the concentration C
c
of the incorporated carbon.
The relationship
a
SiC
(
C
c
)≅
a
Si
(1
−&agr;C
c
) (2)
applies approximately, with &agr; having the value of 6,9×10
−24
cm
−3
. From this it can be calculated that a carbon concentration of approximately 2×10
21
cm
−3
(corresponding to approximately 1,5%) would be necessary in order to largely accurately satisfy the relationship (1). Since the diamond deposition takes place at elevated temperatures in the range of 800° C., the different thermal expansion of diamond and silicon should also be taken into account, so that the relationship (1) applies at the deposition temperature and not necessarily at room temperature. The taking into account of the different coefficients of thermal expansion, however, only leads to a small modification of the carbon concentration that is required.
The solubility of carbon in silicon in thermal equilibrium is known and is extremely small (maximum about 10
17
cm
−3
) compared to the carbon concentration of approximately 2×10
21
cm
−3
required for the desired reduction in size of the lattice. It has, however, been shown that it is possible by means of both CVD processes and also by means of molecular beam epitaxy to grow carbon at these high concentrations (corresponding to a lattice contraction of ca. 2.5% and more) into epitaxial silicon layers in a meta-stable form, as can be found in the literature. In this connection reference
Gösele Ulrich
Plössl Andreas
Kunemund Robert
Max-Planck-Gesellschaft zur Foerderung der Wissenschaften e.V.
Townsend and Townsend / and Crew LLP
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