Boron nitride film and process for preparing same

A boron nitride (BN) film is disclosed having excellent electrical insulating and heat conduction characteristics and capable of acting as seeds for epitaxially growing a semiconductor film thereon which has a crystallizability sufficient to incorporate a semiconductor element therein. The BN film is formed by the growth on a substrate in a manner to be preferentially orientated to a predetermined axis thereof. A process for preparing such film is also disclosed.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a boron nitride film and a process for preparing 
the same, and more particularly to a boron nitride film of excellent 
electrical insulating and heat conduction characteristics formed on a 
substrate in a manner to be preferentially oriented to a predetermined 
axis thereof which is suitable for use as an intermediate insulating layer 
in a three-dimensional semiconductor device to allow semiconductor layers 
each having a semiconductor element incorporated thereinto to be 
three-dimensionally laminated to accomplish the high density packaging of 
the semiconductor device or as an intermediate insulating layer in a 
semiconductor device for large power. 
2. Description of the Prior Art 
Recently, researches have been made in the three-dimensional packaging of a 
semiconductor device wherein semiconductor chips and/or layers are 
superposed to form one semiconductor element, with the high integration of 
a semiconductor element. However, it is required to solve various problems 
resulting from the structural features of a semiconductor in order to 
realize such three-dimensional packaging. 
One of the problems is that it is difficult to form, on an intermediate 
insulating layer insulating interlayer a semiconductor device, a 
semiconductor layer having a crystallizability sufficient to incorporate a 
semiconductor element therein. When an insulating layer has been deposited 
on a semiconductor base plate of which the surface has roughness formed 
thereon due to a semiconductor element which has been already incorporated 
into the semiconductor base plate, it is impossible to grow a 
semiconductor layer of a good crystallizability on the insulating layer 
because the surface of the insulating layer exhibits an irregular state. 
Another important problem is how to attain insulation between semiconductor 
layers of a semiconductor device and how to improve heat dissipation of 
the device. An electrical insulator is generally inferior in heat 
conduction characteristics, and electrical insulating properties and heat 
dissipation characteristics are contrary to each other. Thus, if both 
electrical insulating properties and heat dissipation characteristics of a 
semiconductor device are improved giving cheerful consideration to the 
selection of materials, the manufacturing of a semiconductor device and 
the like, the high integration and high density packaging of a 
semiconductor device can be significantly promoted. 
The inventors proposed a beryllium oxide (BeO) and a method of preparing 
the same, and a semiconductor device using the BeO film in view of the 
foregoing problems of the prior art while taking notice of the fact that 
the use of a BeO film having excellent electric insulating properties and 
heat conduction characteristics as an insulating layer between 
semiconductor layers of a semiconductor device and the growth of at least 
one of the semiconductor layers on the BeO film allow the semiconductor 
layer to have a crystallizability sufficient to incorporate a 
semiconductor element thereinto. However, the prior art is often 
encountered with difficulty in the formation of such BeO film. 
Now, attention is paid to boron nitride (hereinafter referred to as "BN") 
in view of the fact that it has the most excellent electrical insulating 
properties and heat conduction characteristics next to BeO. A BN film has 
been conventionally formed on a semiconductor layer according to a 
chemical vacuum deposition (CVD) process or a sputtering process. However, 
all the so-formed BN films are amorphous. Although, the film has a thermal 
conductivity substantially equal to stainless steel, it makes the 
epitaxial growth of a semiconductor layer thereon having a 
crystallizability sufficient to incorporate a semiconductor element 
thereinto substantially impossible. 
The present invention has been made in view of the foregoing disadvantages 
of the prior art. 
Accordingly, it is an object of the present invention to provide a BN film 
of excellent electrical insulating properties and heat conduction 
characteristics which is capable of exhibiting thermal conductivity about 
three times as much as the conventional amorphous BN film, and acting as 
an orientation film utilized as seeds for epitaxially growing a 
semiconductor layer thereon even if the BN film has been formed on an 
amorphous substrate or a substrate having roughness formed thereon. 
It is another object of the present invention to provide a BN film which is 
capable of being used as an insulating layer between semiconductor layers 
of a three-dimensional semiconductor device to significantly promote the 
high integration and high density packaging of the device. 
It is a further object of the present invention to provide a BN film which 
is capable of being used as an insulating layer between a heat dissipation 
plate and a semiconductor layer in a semiconductor device for large power 
to provide the device with excellent heat dissipation characteristics 
while keeping high electrical insulating properties. 
It is still a further object of the present invention to provide a process 
for preparing a BN film having such excellent advantages as described 
above. 
In accordance with the present invention, there is provided a boron nitride 
film grown on the surface of a substrate in a manner to be preferentially 
orientated to a predetermined axis thereof. In accordance with another 
aspect of the present invention, there is provided a process for preparing 
a boron nitride film comprising the steps of heating boride charged in a 
closed-type crucible having at least one injection nozzle to form boron 
vapor; ejecting said boron vapor through said nozzle into a nitrogen 
atmosphere having a pressure below 10.sup.-2 Torr and ionizing at least a 
part thereof; and impinging said ionized boron vapor on the surface of a 
substrate together with nitrogen to form a boron nitride film on said 
substrate.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Now, the present invention will be described hereinafter by way of example 
with reference to the accompanying drawings. 
First, an example of an apparatus will be described with reference to FIG. 
1 which is adapted to prepare a BN film of the present invention according 
to an ionized cluster beam deposition process (hereinafter referred to as 
"ICB") process. 
The apparatus shown in FIG. 1 includes a closed-type crucible 1 having at 
least one injection nozzle 2 of about 0.5 to 2.0 mm in diameter formed at 
the upper portion thereof. The nozzle 2 is desirably formed to have an 
aspect ratio less than 1 by rendering the dimension along the axial 
direction thereof smaller than the diameter. The crucible 1 is charged 
with boron oxide (B.sub.2 O.sub.3) 3 in the form of powder, flake or 
pellet. 
The apparatus also includes a heating device 4 provided around the crucible 
1. In the example illustrated in FIG. 1, the heating device 4 is the 
electron impact type which comprises a coil filament arranged around the 
crucible 1 and adapted to emit electrons when heated, and a power source 
(not shown) for applying positive potential to the crucible 1 to 
accelerate electrons emitted from the filament, so that electrons are 
permitted to impinge on the outer surface of the crucible 1 to heat it. 
Alternatively, the crucible 1 may be heated by resistance heating, 
radiation heating or a combination thereof. The resistance heating is 
applicable when the crucible 1 is formed by a conductive material, and is 
carried out by providing the upper and lower portions of the crucible 1 
with specific terminals and flowing a large current into the crucible via 
the terminals under a low voltage, to thereby heat the entire crucible 1. 
The radiation heating is attained by arranging a heater around the 
crucible 1 to heat it. 
In addition, the apparatus includes a heat shielding plate 5 and an 
ionization chamber 6. The ionization chamber 6 comprises a mesh-like anode 
7 formed into a cylindrical shape, a rectangular cylindrical shape or a 
parallel-plate shape so as to surround a path for B.sub.2 O.sub.3 vapor 
described hereinafter and an electron emitting filament 8 and a shield 
plate 9 which are arranged around the anode 7. 
Further, the apparatus includes a holder 10 for holding thereon a substrate 
11 on which a BN film is to be deposited and a shutter 12 serving to 
prevent the impingement of boron (B) or oxygen (O) onto the substrate 11 
when the impingement is not desired. The substrate 11 may be a 
monocrystalline semiconductor substrate such as, for example, 
monocrystalline silicon, a metal substrate, an amorphous insulating 
substrate such as a glass substrate, or the like, as desired. 
Reference numeral 13 designates a gas supply pipe provided with at least 
one nozzle 14. In the illustrated example, the pipe 13 is disposed to 
allow the nozzle 14 to be arranged in proximity to the injection nozzle 2 
of the crucible 1. Alternatively, it may be provided in a manner to 
position the nozzle 14 adjacent to the substrate 11. 
Between the substrate 11 and the ionization chamber 6, an acceleration 
electrode 15 may be arranged as desired. The acceleration electrode 15 is 
applied thereto potential negative with respect to the crucible 1 from a 
power source (not shown) to accelerate B.sub.2 O.sub.3 vapor ionized in a 
manner as described hereinafter. 
Furthermore, the apparatus may have a heating means (not shown) arranged 
adjacent to the substrate 11 as required which acts to heat the substrate. 
The apparatus constructed as described above is received in a vacuum casing 
(not shown) and supported therein by a suitable support means. The vacuum 
casing is connected to an evacuation apparatus (not shown) so that a high 
vacuum atmosphere of 10.sup.-6 Torr or less is formed therein. The vacuum 
casing is also adapted to have a reactive gas G such as forming gas 
introduced therein through the gas supply pipe 13. The forming gas 
introduced in the casing may be, for example, a mixed gas consisting of 90 
parts of nitrogen gas and 10 parts of hydrogen gas. 
Now, a BN film of the present invention will be described which is prepared 
using the apparatus described above. 
First, the crucible 1 is charged with boron oxide (B.sub.2 O.sub.3) 3 
formed into a suitable shape and the vacuum casing is evacuated to a 
pressure of 10.sup.-6 Torr or less. Then, a mixture G of nitrogen gas and 
hydrogen gas mixed with a ratio of 90:10 is introduced in the vacuum 
casing through the gas supply pipe 13 to keep it at a pressure below 
10.sup.-2 Torr, preferably as low as 3.5.times.10.sup.-4 Torr. 
Then, the heating device 4 is actuated to heat boron oxide 3 in the 
crucible at a temperature between 1350.degree. C. and 1450.degree. C. to 
form vapor 3a of the boron oxide, which is ejected through the injection 
nozzle 2 of the crucible 1 to the exterior of the crucible 1 having a 
nitrogen atmosphere due to the difference in pressure between the interior 
and exterior of the crucible. 
The ejected vapor 3a has kinetic energy imparted thereto which corresponds 
to its injection velocity, to thereby form a vapor stream 3b flowing 
toward the substrate and vapor-like boron (B) forms clusters comprising a 
large aggregate of boron atoms loosely coupled together by van der Waals 
forces by utilizing a supercooled state attained due to the adiabatic 
expansion at the time of the ejection from the nozzle. The formation of 
such clusters contributes to an improvement in the ionization efficiency 
in the ionization chamber 6 and the formation of a BN film 16 of high 
quality on the substrate 11. 
The vapor stream 3b having kinetic energy imparted thereto as described 
above enters the ionization chamber 6, in which at least a part of the 
vapor is ionized. More particularly, in the ionization chamber 6, 
electrons emitted from the filament 8 heated to a high temperature are 
accelerated by voltage of 100 to 1000 V applied between the filament 8 and 
the meshy anode 7 to impinge on the vapor stream 3b to allow at least a 
part of the vapor to be ionized. In this instance, when the vapor stream 
3b ejected from the nozzle 2 forms clusters, one of atoms forming the 
cluster is ionized due to the impingement of electrons thereon to form a 
cluster ion. Also, a part of the mixed gas G ejected from the nozzle 14 
reaches the ionization chamber to be ionized therein due to electron 
impingement. 
The ionized clusters are transported toward the substrate 11 together with 
neutral clusters and the mixed gas G introduced in the path of clusters, 
and are permitted to impinge on the surface of the substrate 11 when the 
shutter 12 is opened, so that the clusters are separated into the 
individual atoms due to a surface migration effect to allow the atoms to 
diffuse on the surface of the substrate, to thereby form a BN film 16 on 
the substrate 11. After such deposition, the BN film 16 thus formed is 
subjected to annealing at a temperature of about 350.degree. C. in a 
vacuum atmosphere for 1 hour to remove moisture from the BN film 16 which 
is formed by a reaction of hydrogen contained in the mixed gas G with 
oxygen of the boron oxide. 
The vapor stream 3b impinging on the substrate 11, as described above, is 
ionized at least partially while it passes through the ionization chamber 
6, and the electrical field of the ion effectively acts on the initial 
stage of the film formation on the substrate 11. More specifically, the 
electrical field of the ion promotes the formation of seeds of crystal 
growth and further effectively acts on the coalescence of atoms into an 
island region about the so-formed seeds. In addition, such ionization has 
another effect of promoting the reaction between B and N to allow the 
formation of a BN film having a stoichemical composition and a good 
crystallizability. 
Further, the vapor stream 3b is adapted to impinge on the substrate 11 with 
kinetic energy imparted thereto at the time of the ejection. Thus, the 
formed BN film 16 has an excellent quality, because such incident energy 
contributes to the formation of a BN film which has a good adhesion with 
respect to the substrate 11, the increase in packing density of the BN 
film and the improvement in crystallizability of the BN film. This is for 
the reason that the film 16 is preferentially orientated to a 
predetermined axis thereof or the C-axis even if the substrate 11 has 
roughness somewhat formed on the surface thereof or exhibits an amorphous 
property, and when the substrate 11 is monocrystalline, the epitaxial 
growth of BN is accomplished on the substrate 11 to form the film 16 of a 
monocrystal regulated by the crystallographic axis of the substrate 11. 
Furthermore, the adjustment of acceleration voltage applied to the anode 7 
to vary an ionization current derived from the filament 8 allows the 
degree of ionization of the vapor stream 3b to be controlled, so that the 
BN film 16 deposited on the substrate 11 may be further controlled in 
various properties such as crystallizability. 
The embodiment illustrated, as described above, is adapted to eject the 
vapor 3a to the exterior of the crucible 1 utilizing the difference in 
pressure between the interior and exterior of the crucible to impart 
kinetic energy to the vapor 3a. However, the vapor stream 3b may be 
accelerated by electrical field after at least a part thereof is ionized, 
so that additional kinetic energy may be imparted to the vapor stream 3b 
to further carry out the control and/or improvement of crystallizability 
of the BN film 16. 
More particularly, the application of voltage negative with respect to the 
crucible 1, for example, the application of acceleration voltage of about 
0 to 10 kV to the acceleration electrode 15 accelerates particles of the 
vapor stream 3b ionized during the passing through the ionization chamber 
to permit kinetic energy to be imparted to the ionized particle, so that 
the energy may effectively act to improve and/or control the 
crystallizability of the film 16 at the time of the deposition to 
contribute to the formation of the film 16 having a further excellent 
quality. In particular, such construction, when a monocrystalline 
substrate such as a silicon monocrystalline substrate is used as the 
substrate 11, exhibits an excellent advantage of effectively controlling 
crystallizability of the BN film 16 by varying acceleration voltage to be 
applied to the acceleration electrode 15 in view of the relationship with 
the crystal face of the substrate 11. Also, it is a matter of course that 
the energy applied to the particles ionized by the electrical field 
effectively serves to improve adhesion of the film with respect to the 
substrate 11. 
Alternatively, when the holder 10 is formed of a conductive material, such 
acceleration voltage may be applied between the holder 10 and the crucible 
1 without independently providing the acceleration electrode 15. 
The BN film formed in the manner as described above is adapted to 
effectively act as an intermediate insulating layer on insulating 
interlayer in a semiconductor device of a three-dimensional structure. 
Some semiconductor devices into which a BN film of the present invention 
is incorporated in various manners to act as an insulating interlayer will 
be described with reference to FIGS. 2 to 7. 
FIG. 2 shows a first semiconductor device of such type, which is formed as 
described hereinafter. First, a monocrystalline semiconductor such as a 
monocrystalline Si, GaP, GaAs or the like is deposited on an insulating 
substrate 101 to form a lower semiconductor layer 102. Then, a 
semiconductor element is incorporated into the lower semiconductor layer 
102 utilizing conventional semiconductor element forming techniques widely 
known in the art to form a base 110, which is mounted on the holder 10 of 
the apparatus shown in FIG. 1. 
Then, a BN film 116 acting as an insulating layer is deposited on the lower 
semiconductor layer 102 according to the ICB process using the apparatus 
of FIG. 1 operated under the deposition conditions that acceleration 
voltage Va applied to the acceleration electrode 15 and electronic current 
Ie for ionization are set at 0.5 kV and 100 mA, respectively. The BN film 
116, as described above, is grown in a manner such that the 
crystallographic axis C.sub.1 is preferentially orientated to the C-axis, 
even when a semiconductor element is incorporated into the surface portion 
of the lower semiconductor layer 102 to the degree sufficient to lose the 
flatness of the surface. Thus, a three-dimensional film semiconductor 
device of the multilayer type can be prepared by epitaxially growing an 
upper semiconductor layer 103 of a monocrystal on the BN film 116 
according to an ion beam depositing process utilizing such orientation of 
the BN film and then incorporating a semiconductor element into the upper 
semiconductor layer 103. 
Alternatively, the semiconductor device shown in FIG. 2 may be prepared in 
a manner such that a material such as a rock salt which is easy to 
dissolve in a solvent such as water and form a cleavage plane is used as a 
material for a substrate 101. A lower semiconductor layer 102, a BN film 
116 and an upper semiconductor layer 103 are then deposited on the base 
plate 101 in order according to a suitable procedure such as an ion beam 
depositing process, an ICB process or the like and the base plate 101 is 
dissolved to expose the lower surface of the lower semiconductor layer 102 
through which a semiconductor element is to be incorporated into the layer 
102. In this instance, the BN film 116 is formed as a monocrystalline film 
because it is regulated by the crystallographic axis of the lower 
semiconductor layer 102 to be epitaxially grown. 
Also, in the first semiconductor device of FIG. 2, the upper semiconductor 
layer 103 is formed by growing a monocrystalline film of Si. 
Alternatively, the layer 103 may be formed of GaAs, GaSb, GaP, InSb or the 
like. 
A second semiconductor device in which a BN film according to the present 
invention is used as an intermediate insulating layer is shown in FIG. 3, 
which is prepared by first depositing a first BN film 216 acting as an 
insulating layer on a substrate 201 of an amorphous material such as 
glass, ceramic or the like according to the ICB process under the same 
deposition conditions as the first semiconductor device, as shown in FIG. 
3, wherein the crystallographic axis C.sub.2 of the BN film 216 is 
preferentially orientated along the C-axis (002) Then, on the first BN 
film 216 is deposited a lower semiconductor layer 202 of monocrystalline 
Si according to the ICB process which is regulated by the crystallographic 
axis of the BN film 216 to be epitaxially grown. Subsequently, a second BN 
film 226 is formed on the upper surface of the lower semiconductor layer 
202 which is regulated by the crystallographic axis of the lower 
semiconductor layer 202, and then an upper semiconductor layer 203 of, for 
example, monocrystalline GaAs is deposited on the second BN film 226, 
according to the ICB process, which is regulated by the crystallographic 
axis of the film 226 to be epitaxially grown. 
Thus, it should be noted that the second semiconductor device has an 
advantage that a desired material may be used for the base plate 201 
irrespective of its insulating properties or conductivity. 
FIG. 4 shows a third semiconductor device having a BN film of the present 
invention used therein as an insulating layer. The semiconductor device 
comprises a lower semiconductor layer 302 formed of a monocrystalline Si 
semiconductor wafer, a BN film 316 acting as an insulating layer which is 
deposited on the (111) plane of the monocrystalline Si in the 
substantially same manner as the first and second semiconductor devices, 
and an upper semiconductor layer 303 of a Si film deposited on the upper 
surface of the BN film 316 which is regulated by the crystallographic axis 
of the BN film 316 to be epitaxially grown. 
FIG. 5 shows a fourth semiconductor device wherein a BN film according to 
the present invention is interposed as an insulating layer between 
semiconductor layers. The semiconductor device of FIG. 5 is prepared in 
the following manner. First, a lower semiconductor layer 402 is formed of 
a monocrystalline Si semiconductor wafer in the substantially same manner 
as the device shown in FIG. 4, the layer 402 acting as a substrate. Then, 
a first BN film 416 serving as an insulating layer is deposited on the 
lower semiconductor layer 402 in a manner such that it is regulated by the 
crystallographic axis of the lower semiconductor layer 402. On the first 
BN film 416 is then formed a first upper semiconductor layer 403 of a Si 
film due to epitaxial growth, on which a second BN film 426 also acting as 
an insulating layer is deposited falling under the regulation of 
crystallographic axis of the first upper semiconductor layer 403. Finally, 
a second upper semiconductor layer 404 is deposited on the second BN film 
426 which is regulated by the crystallographic axis of the second BN film 
426 to be epitaxially grown. 
It will be noted that the semiconductor device shown in FIG. 5 is adapted 
to substantially improve the integration to a level sufficient to carry 
out the high density packaging because there are provided three 
monocrytalline semiconductor layers into each of which a semiconductor 
element is adapted to be incorporated. Also, the semiconductor device has 
an advantage that because the BN films 416 and 426 each are effectively 
preferentially orientated to the C-axis even with respect to a 
semiconductor layer having roughness somewhat formed on the surface 
thereof due to the incorporation of a semiconductor element thereinto, the 
formation or incorporation of a semiconductor element may be carried out 
with respect to either surface of each semiconductor layer 403 or 404. 
FIG. 6 shows a fifth semiconductor device including a BN film of the 
present invention acting as an insulating layer, which has been prepared 
in view of the foregoing. More particularly, in the device, a Si device 
such as an integrated circuit or the like is incorporated into the upper 
surface side 502a of a monocrystalline Si semiconductor wafer to form a 
lower semiconductor layer 502, and a BN film 516 of the present invention 
is deposited on the lower semiconductor layer 502 in such a manner as 
described above. Further, on the BN film 516 is deposited an upper 
semiconductor layer 503 of a monocrystalline Si. It is of course that in 
the device an integrated circuit or the like may be incorporated into the 
upper semiconductor layer 503. 
The high integration of a semiconductor integrated circuit in recent years 
requires to give a careful consideration to an improvement in heat 
dissipation of semiconductor devices such as a semiconductor element for 
large power, for example, a power transistor, a linear integrated circuit 
for an audio equipment and the like. Such an improvement in heat 
dissipation characteristics of a semiconductor device renders a further 
increase in integration of the device possible. 
FIG. 7 illustrates a sixth semiconductor device having a BN film of the 
present invention provided therein, which has been prepared in view of the 
foregoing while taking notice of the fact that the heat dissipation 
characteristics of a semiconductor device which is highly integrated and 
packaged with high density can be significantly improved by depositing a 
metallic film for heat dissipation such as an A1 film on a BN film of the 
present invention to more effectively improve an excellent heat 
conductivity of the BN film to efficiently discharge heat through the 
metallic film to the exterior or carry out shielding between multi-layer 
elements by means of the metallic film, depositing a second BN film of the 
present invention on the metallic film, and forming another semiconductor 
layer on the second BN film. 
More particularly, the semiconductor device of FIG. 7 comprises a lower 
semiconductor layer 602 having a semiconductor element incorporated 
thereinto in the substantially same manner as the device of FIG. 6, a 
first BN film 616 deposited on the lower semiconductor layer 602 to act as 
an insulating layer, a heat dissipation plate 605 of a metallic or 
magnetic material deposited on the first BN film 616, a second BN film 626 
deposited on the heat dissipation plate 605 to act as an insulating layer, 
and an upper semiconductor layer 604 formed on the second BN film 626. The 
upper semiconductor layer 604 is formed therein with an integrated circuit 
or the like, and the electrical connection between the upper and lower 
semiconductor layers 604 and 602 is carried out by means of wirings 607 
inserted through small through-holes 606 formed at the heat dissipation 
plate 605. 
In the semiconductor device of FIG. 7, the crystalline BN films 616 and 626 
have not only excellent heat conduction characteristics but an anisotropy 
in heat conduction characteristics. The thermal conductivity of each BN 
film in the direction parallel to the crystallographic axis is about three 
times as much as that in the perpendicular to the axis, so that the 
semiconductor device may exhibit an effect of enhancing heat dissipation 
from the upper and lower semiconductor layers 604 and 602 to the heat 
dissipation plate 605. This results in a semiconductor device for large 
power such as a power transistor or a semiconductor device packaged with 
high density which is provided therein with such crystalline BN film 
acting as an insulating layer carrying out heat dissipation with good 
efficiency. In this instance, the formation of heat dissipation plate 605 
by a metallic material exhibits an electrostatic shielding effect, whereas 
that by a magnetic material provides a magnetic shielding effect. 
In each of the semiconductor devices described above, it is most convenient 
that the formation of BN film or semiconductor layer of good quality is 
carried out according to the ICB process proposed by the inventors. The 
ICB process effectively exhibits a surface migration effect that clusters 
separate into individual atoms upon impingement on a substrate to diffuse 
on the surface of the substrate, an effect of effectively acting on the 
formation or coalescence of seeds of crystal growth at the initial stage 
at which ions contained in clusters or atoms form a film, a surface 
cleaning effect due to kinetic energy imparted to a vapor stream, an ion 
implantation effect and the like, resulting in a film being formed which 
has good quality, high adhesion to a substrate and a good flatness from 
the viewpoint of crystallography. Also, the ICB process has a high degree 
of freedom with respect to film deposition conditions such as, for 
example, the difference in pressure between the interior and exterior of a 
crucible, ionization in an ionization chamber and a degree of such 
ionization, an electrical field for acceleration and the like, so that the 
control of crystal structure of a film to be formed may be facilitated to 
permit a monocrystalline film of good quality to be prepared depending 
upon desired characteristics. 
Also, in the devices shown in FIGS. 2 to 7, a temperature of the substrate 
at the time of forming the BN film can be set as desired between a room 
temperature and 500.degree. to 600.degree. C. Thus, there is no fear of 
causing impurity concentration of the semiconductor element which has been 
already incorporated in the semiconductor layer to be inbalanced. This is 
helpful to form a three-dimensional semiconductor device. 
Now, results of experiments on a BN film according to the present invention 
will be described with reference to FIGS. 8 to 10. 
FIG. 8 shows a result of X-ray diffraction of a BN film formed on a 
substrate of a monocrystalline Si semiconductor layer in the substantially 
same manner as described above according to the ICB process under the 
conditions that acceleration voltage Va across an acceleration electrode 
and electronic current for ionization Ie are set at 0.5 kV and 100 mA, 
respectively, wherein the axis of abscissas indicates a reflection angle 
of X-ray (2.theta.) 
As shown in FIG. 8, a strong reflection peak appears from the direction of 
the C-axis (002) of the BN film having a wurtzite structure at the 
position where the X-ray reflection angle (2.theta.) is 12.2.degree.. This 
clearly indicates that the formed BN film is not amorphous but is 
preferentially orientated to the specified crystallographic axis along the 
C-axis (002). 
Also, a result (not shown) of another X-ray diffraction on a BN film formed 
on an amorphous substrate of glass indicates that a BN film can be 
effectively formed due to preferential orientation to the C-axis (002) 
also on glass which is amorphous, although a reflection peak from the 
C-axis (002) is weak as compared with that of a BN film formed on the 
above-mentioned monocrystalline Si semiconductor layer under the same 
conditions. 
Further, a misfit in lattice constant between the monocrystalline Si 
semiconductor layer and the BN film is as small as 5.9%. Thus, it will be 
noted that when Si is grown on the BN film preferentially orientated to 
the C-axis, it is substantially completely and readily epitaxially grown 
utilizing the crystallographic axis of the BN film as seeds of crystal 
growth to form a monocrystalline Si film sufficient to incorporate a 
semiconductor element thereinto. 
The determination of infrared reflectance spectral characteristics of a BN 
film formed on a glass substrate (amorphous) under the same disposition 
conditions as in FIG. 8 shows the appearance of a peak due to a stretching 
vibration of the bond between nitrogen atom and boron atom at a wavenumber 
of 1380 cm.sup.-1 as well as that due to a stretching vibration of the 
bond between oxygen atom and hydrogen atom or between nitrogen atom and 
hydrogen atom. The peak has a relatively small half band width. These 
facts clearly indicate the formation of a BN film of good quality from the 
viewpoint of crystallography although it somewhat contains hydrogen, 
oxygen or a combination thereof. 
FIG. 9 shows electrical conductivity of a BN film measured near a room 
temperature (T=300 K.) which was formed on a substrate under the same 
deposition conditions as in FIG. 8, wherein the axis of abscissas 
indicates temperature (1000/T) and the axis of ordinates indicates 
conductivity. 
As shown in FIG. 9, the BN film has an extremely low conductivity of about 
10.sup.-9 .OMEGA..sup.-1 .multidot.cm.sup.-1 to 10.sup.-11 .OMEGA..sup.-1 
.multidot.cm.sup.-1, and this clearly shows that it exhibits excellent 
electrical insulating properties. Accordingly, the use of such BN film in 
a manner to be interposed between layers of such an integration-settled 
semiconductor IC device as shown in FIGS. 2 to 7 permits the layers to be 
positively insulated from each other. 
Further, thermal conductivity of a BN film of the C-axis preferential 
orientation formed on an amorphous substrate of glass under the same 
deposition conditions as in FIGS. 8 and 9 was measured utilizing a thermal 
pulse method. In the experiment, it was difficult to measure conductivity 
of the BN film itself because it is of the order of micron in thickness, 
however, the BN film deposited on the glass substrate was measured to have 
conductivity of 0.116 W/cm.deg in the areal direction or the direction 
perpendicular to the growth axis, which is substantially equal to 
conductivity of stainless steel. As described above, in the crystalline BN 
film, the conductivity along the growth axis is about three times as much 
as that in the direction perpendicular thereto. 
Accordingly, as shown in FIG. 10, when a semiconductor layer 100 is 
deposited thereon a BN film in such a manner to be orientated to a 
predetermined axis or the C-axis along the heat dissipation direction of 
the layer 100 as in the semiconductor device of FIG. 7 and further a heat 
dissipation plate 800 is formed on the BN film 700, heat dissipation is 
effectively carried out because heat generated by the semiconductor layer 
600 is efficiently transmitted through the BN film 700 to the heat 
dissipation plate 800. This enhances the heat dissipation of a 
semiconductor device for large power such as a power transistor and 
realizes the integration and high density packaging of a semiconductor 
device. 
The BN film and the process for preparing the same according to the present 
invention are not restricted to such specific forms as described 
hereinbefore. For example, a material to be charged in the crucible may be 
boron sulfide. Alternatively, N.sub.2 gas or NH.sub.3 gas may be used as a 
reactive gas to be introduced in the vacuum casing. 
As can be seen from the foregoing, the BN film of the present invention is 
adapted to be formed in a manner to be preferentially orientated to a 
predetermined axis thereof on the surface of a substrate. The present BN 
film, when a substrate is crystalline such as a monocrystalline Si 
semiconductor layer, is epitaxially grown along the crystallographic axis 
of the layer, whereas, when a substrate is amorphous such as glass, it is 
grown along the C-axis (002). 
Thus, the BN film according to the present invention is capable of serving 
as seeds for allowing a semiconductor layer to be epitaxially grown 
thereon, into which a semiconductor element is incorporated, and being 
used as an intermediate insulating layer for a three-dimensional 
semiconductor device and the like even in the form of a thin layer because 
of its high electrical insulating properties so that it may promote the 
high integration and high density packaging of a three-dimensional 
semiconductor device. Also, the BN film of the invention contributes to 
the manufacture of a semiconductor device because it can use inexpensive 
glass as a substrate. 
In addition, in the BN film according to the present invention, the thermal 
conductivity along a predetermined axis thereof is about three times as 
much as a prior art amorphous BN film. Accordingly, the interposition of 
the present BN film between a semiconductor layer and a heat dissipation 
plate formed of metal or the like and acting as a heat sink provides an 
excellent heat dissipation capacity while keeping high electrical 
insulating properties. This effectively eliminates inconvenience such as 
insulation of the whole heat dissipation plate in a semiconductor device 
for large power generating a large amount of heat, to thereby carry out 
high density packaging of the semiconductor device. 
Further, the positioning of the present BN film between a semiconductor 
layer and a heat dissipation plate in a semiconductor device to carry out 
heat conduction along, in particular, a predetermined axis or the C-axis 
of the BN film exhibits a more effective heat dissipation effect. 
Furthermore, the process for preparing a BN film according to the present 
invention is practiced in a manner to heat and vaporize boride charged in 
a closed-type crucible having at least one injection nozzle, eject the 
vapor of boron through the nozzle into a nitrogen atmosphere below 
10.sup.-2 Torr in pressure to ionize at least a part thereof, and impinge 
the ionized vapor on the surface of a substrate together with nitrogen. 
Thus, it facilitates the preparation of a BN film and the present process 
can form a BN film at a relatively low temperature so that a BN film easy 
to be handled may be formed which has a high adhesion with respect to a 
substrate and a good surface flatness and never adversely affects the 
impurity profile of a semiconductor element which has been already 
incorporated into a semiconductor device. 
As many apparently widely different embodiments of this invention may be 
made without departing from the spirit and scope thereof, it is to be 
understood that the invention is not limited to the specific embodiments 
thereof except as defined in the appended claims.