Note: Descriptions are shown in the official language in which they were submitted.
PHF 75-514C
~7~;Si~i .
The present invention relates to a method of
manufacturing semiconductor devices in which a layer of
semiconductor material is provided on a main surface of
a solid substrate by solidification of molten semicon-
ductor material on said substrate surface. The invention
further relates to an apparatus for use in carrying out
said method and semiconductor devices made by using
such a method.
The semiconductor material used so far was
monocrystalline and in general in the form of discs cut :
from rods obtained by the vertical drawing according to
the Czochralski method or by the method of floating zone
melting. Actually, the high cost-price limits the develop-
ment of dev;ces using said material9 specially devices -
which are to be used in large areas, for example for ~ ~:
the direct conversion of solar energy into electrical ~ - -
energy. For this purpose single crystal silicon slices
are used, but also other semiconductors of the III-V :
type and II-VI type, eventually with hetero-junctions
with other materials are known for said purpose.
Experience has proved that good conversion
, ... . .
efflciencies can also be obtained by using poly-
crystalline silicon. French patent specification No.
1,343,740 discloses cells which are manufactured com-
prising polycrystall;ne sil;con in graphite receptacles.
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p~Ir.75~5llic
29.1.1976
of vèry small dep-th. Cooling may be carried out progress-
ively in a horizontal direction. Polycrystalline ma-terial
may be obtained in this manner having grain sixes
sufficient to give photodiodes manu~actured from said
material a conversion efficiency of the order of 6%.
It is an object of the present invention to
! enable the manufacture of layers of semiconductor
~ material of large area ln a process which may be carried
out subs-tantially continuously. According to the in-
, '' 10 vention a method of manufacturing semiconductor devices
in which a layer of semiconductor material is provided
on a main surface of a solid substrate by solidification
of molten semiconductor material on saicl substrate sur-
face, is characterized in that a substant:ially floating
liquid mass of the semiconductor material is formed on
a substantially horizontal surface of a solid support,
, that said liquid mass is contacted with the said main
:' surface of the solid substrate onto which the layer of
semiconductor material is to be provided, the latter
: 20 main surface being wettable by the llquid semiconductor
, material and the solid substrate having the form of a
strip, and that said strip is moved in its longitudinal
direction along the liquid mass so that a liquid layer
.
of the semiconductor material is formed on said strip
and taken along wi-th it, said liquid layer being
i . :
. solidified substantially progFessively. It is found that
the method according to the lnvention can successfull,v
, ~ be used for manufacturing layers of~silic~on, which may
~ ; : in this relatively simple manner be obtained in poly-
:: .
: 30 crystalline form of suitable quality for the manufacture
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PIIF.75-51~1C
29.1.1976
47~ 5 5
of solar cells of acceptable conversion efficiencies not
withstanding the requiremen-t of heating at a temperature
of at least 1420C for obtaining liquid silicon. I~o~ever,
J the invention is not limi-ted to the use of silicon and
may, in principle be used in case of o-ther serniconductor
materials which can be melted, such as germanium and
-1 many compounds of the III-V t,vpe.
The term "substantially floating" with respect
`, to the liquid mass in this case means, that the liquid
mass is not fully enclosed laterally by solid walls.
The liquid mass may hang on the substantially horizontal
¦ surface at the underside of the support due to adhesive
forces in conjunction with cohesive forces. In that
, case the substrate will contact the liquid mass at a
~i 15 place lower than the substantially horizontal support
surface. A danger exists in that case that at least a
.
~ , major part of the volume of the molten mass may loosen
,,~ itself from the support and run along the substrate
, ~
,~. thus breaking the contact between the mass still hanging
j 20 on the substantially horizontal suppor-t surface and the
I subs-trate surface to be covered with the layer of semi-
l conductor material. This danger is a-t least reduced
when according to a preferred embodiment the liquid mass
of semiconductor material is positioned on top of a sub-
~,~ ~ 25 stantially horizontal surface constituting the upper
:~
,~ surface of the~support, -the liquid mass projecting above
, ~, . .
said support. The place of contact between the liquid
,
~; .
mass and the substrate for providing the layer of semi-
~^ conductor material will then be sltuated above the sub-
-
~, 30 stantially horizontal support surface or at least at
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P~IF.75.511~C
29.1.1976
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about the same level as said support surface.
The term "substantially hori~ontal" f`or the
surface of the support onto which the liquld mass is
provided, should be understood to mean that, if devi,ating
from absolutely horizontal, the liquid mass will not be
able to run off or drop off said surface and that, in
case of being the upper sur~ace of the support, the
liquid mass would have been run off for the major part
in case the only forces acting on -the molten mass would
have been gravitational forces. The molten mass may be
kept in place due to surface tension caused by cohesive
forces.
The support may comprise a high purity
,; refractory material at the substantially horizontal sur-
' 15 face, for exampIe high purity silicon oxide in the case
of silicon. Molten semiconductor material taken up by
, the substrate may be replenished by adding liquid semi-
conductor material by means of one or more capillary
`, channels in the support opening at the said substantially
, ~ ,
, horizontal surface said capillary channels receiving
;~, liquid semiconductor material from a molten pool con-
tacting the support at another place $han the substan-
tially horizontal surface. However, it is also possible
, .
~ to replenish said liquid mass by adding semiconductor
., , ~
, ~ ~ .
25~ material in powder form.
' ' According to a further preferred embodiment
:
the solid support consist of the, semiconductor material
~ . .
in solid form. This has the advantage that the material
of the support may have the same or even higher purity
than the~semiconductor material of the liquid mass itself.
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PHF.75.514C
29~1.1976
This liquid mass may be formed initially by melting an
end portion of the solid material of -the support. The
liquid mass may form a hanging drop or, according to a
preferred embodiment, i-t may substantially float on top
of the support. Suitable heating means may be used for
creating a substantially horizontal liquid solid inter-
face forming also the substantially horizontal surface
o~ the support for the liquid mass of semiconductor
material. In case of a support consisting of the semi-
conductor material itself replenishment of liquid serni-
conductor material removed from the mass by the substrate
may, according to a preferred embodiment,be realized by
progressively melting adjacent semiconductor material of
the support.
In principle it is possible to move the strip
along and in contact with the liquid mass on the sub-
stantially horizontal support su~face in any given
,~ direction. It may be moved horizontally at the area of
`'~ contact with the liquid mass. In the case of a molten
mas~ hanging at the lower end of the support the layer
~ of semiconductor material will be formed on the upper
; surface of the strip-shaped substrate. In the case of amolten mass~on top of the support the layer of semicon-
' i ~
ductor material will be formed on the lower surface of
the strip-shaped substrate. In the first case the amount
of material passing to the substrate may be rather
~' ~ abundant such that the layer is rather thick and
, . . . .
difficulties may arise in replenishing the liquid mass
:i ,:: :
on the support surface. In the second case care shoul~
be takèn to adjus-t the parameters such that/uneven thick-
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P~IF.75.511~C
29.1.197~)
~ ~r7~ 55
nesses are formed- due to the initial formation of
hanging drops at the substrate surface before solidi-
fication.
In principle also a downward movement of the
strip-shaped substrate may be used. However, care should
be taken for an even pick-up of molten semiconductor
material by the substrate surface in order to obviate
local accumulation of semiconductor material running
down over already solidlfied material.
I 10 According to a preferred embodiment the sub-
i strate is moved in an upward direction. In this way the
¦ chance o~ obtaining layers of uneven thicknesses is
¦ diminished as any momentarily increased amount Or liquid
j picked up by the substrate will flow back and may either
be added to a ne~t decreased amount of liquid picked
up in a subsequent moment from the somewhat reduced
liquid mass on the support surface, or may flow back to
said liquid mass.
The invention further relates to an apparatus
for use in carrying out
, J method according to the invention, said apparatus being
characterized in that it comprises a closed space having
inlet and outlet means for control of the gaseous at-
,
~ mosphere in said space, means for vertically placing a
;l ~25 longitudinal support for the subs-tantially floating
liquid mass of semiconductor material to be positioned
on top of said support, heating means for heating the
-
; liquid mass of semiconductor material and means for
passing a strip-shaped substrate in its longitudinal
directlon in close proxirnit-y along the upper portion of
` ~t - 7 -
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PI~F-75.514C
2~ . I . 1 976
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7~ 5i~;i
! the support for carrying the mass of molten sem-icon-
I ductor ma-terial such tha-t said strip-shaped substrate
contacts the liquid mass of the semiconductor material.
The apparatus may comprise means for adding
semiconductor material in powder form to the liquid
mass.
Preferably the apparatus ~omprises means to
vertically move the support for the liquid mass during
the formation of the layer of semiconductor ma-terial on -
the substrate surface. In this way the semiconductor
material for the molten mass may be properly brought
¦ in a position with respect to the heating means that the
'¦ molten mass is formed and that said mol-ten mass i9
properly contacted wi-th the substrate surface. In case
the support consists of the semiconductor materia~ said
support may be advanced in the direction of the liquid
mass in order to replenish said molten mass as already
mentioned herein.
The support may be given a variety
~, ~ Z0 of forms within the scope of the present invention. For
obtaining substantially broad semiconductor layers it
is advantageous to have a contact area between the strip-
shaped substrate and the molten mass which is substantially
large in a direction perpendicular to the direction of
movement of the strip at the area of contact with the
iquid mass. For this purpose, according to a preferred
"~ embodiment,~the substantlally horizontal surface of the
~ support of the liquid mass is bounded by a substantially
, :
stra]ght-lined edge, substantial~y according -to the
length of said substantially horizontal surface, the
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29.1.1976
~ ;76~
strip passing in close proximity of said edge. The sup-
port may to this end have the form of a par~llclepip~d.
Such a parallelepiped having t~ parallel edges of the
same length may enable the use of two substrates each
positioned in proximity of one of these parallel edges.
In order to limit contact between the liquid
mass and the support to the substantially horizontal
surface, specially in case sald liquid mass is positioned
on top of said suppor-t in which danger exist of flowing
o~er an edge of the substantially horizontal surface,
~ it is desired to use substantially sharp edges at the
j circumference of the substantially horizontal surface.
According to a preferred embodiment edges of the sub-
stantially horizontal surface of the body constitute edges
of substantially vertically positioned surfaces of the
support. For example, all surfaces bounding the sub-
~ stantially horizontal support surface may be positioned
`, ~ vertically. In case of à support of the semlconductor
material itself it has the furthe~ advantage that ln re-
plenishing the liquid mass by advancing the support in
; the directlon of said liquid mass the area and form of
the solid liquld interface and also of the liquid mass
itself may be kept constant.
~Acoording to a fur-ther preferred embodiment
high-frequency~induction heating is used for heating
:, ~ .
the liquld mass. In comparison with radiation heating in
a furnace, high-frequency induction heating may bet-ter
be llmlted to a desired area, l.e. to substantially the
Iiquid mass o~ semiconductor material it;self. In this
~ way not~only the danger of contamination of the semi-
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PIIF-75.5l4C
29.1.1976
conductor material by impurit:ies originating f`rom other
heated par-ts is decrcased, ~ut; also tlle liquid film ad-
hered to the moving substra-te is more quickly solidifiecl.
Moreover, the strip-shaped substrate, due to its form,
may be kep-t at a relatively low temperature before
contacting the liquid mass thus reducing the risk of
I contaminating the semiconductor material. Further, in
; the high frequency magnetic field forces may be e~erted
on the liquid mass which, in addition to surface tension,
aids in keeping said liquid mass in position on the sub-
,
stantially horizontal substrate surface.
In order to enable a substan-t:ially continuous
~'l process using strips of substantial length it is prefer-
able that said strips should be flexible. Such a strip
~'~ 15 may initially have been winded up on a reel such that it
`~ can be winded off during the process of providing the
semiconductor layer. In flexible condition it further
can better be guided along the liquid mass.
A condition is that at least the main surface
of the strip, onto which the laver of semiconduc-tor
material has -to be provided, can be we-tted by the liquid
semiconductor ma-terial. The wettable angle preferably
-;
should be not larger than 20C. In this respect the
term "we-ttable angle" with respect to a given liquid and
~25~ a glven sur~acs which is wettable by said liquld should
` be understood to~mean herein the angle between the
meniscus of the given liquid and the given surface a-t
the juncti~on between them when said surface is partly
inset in sald liquid in vertical posltion. As the
~ettability is larger the rising of the l:iquid agains-t
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PII~.75.51~1C
29.1.1976
the wall is larger and the we-ttable angle is smaller.
It is possible -to use several ma-terials for
the strip. The strip may comprise an insulating refrac-tory
material. According to a preferred embodiment the
electrically conducting refractory material may be used
as a material for the strip. Refractory metals may be
used as electrically conducting material for the s-trip.
Metals can be put in the form of flexible foils. Another
refractory material is elemental carbon. Carbon in the
form of graphite or in amorphous form may show suitable
wettability by molten silicon and is further electricall,v
conducting. It may be used in the form of a coating of
less wettable material. The substrate may also consist
entirely of carbon.
1 15 In case the material to be used in the strip
i is rather hard and brittle such that it is difficult to
1 use it as the main material of the strip in compact form
i:
~ such materials, i.e. carbon or ceramic materials, may be
,
used in fibrous form in order to obtain flexibility of
the strip. So carbon is available in fibrous form such
as graphite felt and carbon tissue. Such fibrous carbon
may in known manner be obt'ained by carbonization of
fibres of organic materials.
When the strip consists of electrically con-
~ ~ 25 ducting material it may be used afterwards as electric
,~ contact for the semiconductor devices subsequently made
, 1 ~ .
~, from'the layer of semiconductor material obtained.
j : ' ' ~:
At the area of contact ~ith the liquid mass of
semiconductor material on the substantially horizontal
surface of the support the strip used as a substrate
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PIIF.75.51llC
29.1.1976
~4~
for the layer of semiconductor materlal to be formed
may be used in a vertical pos:ition and preferably
moved upwards. For establish:ing contact bet~een the
liquid mass and the strip the liq~ld mass may b~ used
in an amount such that initially it p~otrudes already
`l laterally beyond the edge of the substantially horizontal
substrate surface. It is also possible to move the strip
towards the liquid mass in order to establ:ish contact
with it and that on returning the strip to its initial
l 10 position~free from the support the liquid mass is
il deformed due to adhesion to the substrate surface such
, that said contact is maintainedO
It has further been found that the thickness
~~ of the layer of semiconductor material formed on the
3i 15 substrate may, according to a further preferred embodi-
ment, be~ con~trolled by glving the strip a suitable
inclination at the~place of contact with the liquid mass.
To this end the app~Qratus ac~cordlng to the present in-
vention CompriSes~ means for adjusting the inclination
of the strip-shaped substrate at said place.
The invention will now be described in greater
detall~with reference to the accompanying drawings, in
gu,e l~-ho-s sch-m~tically ~apparatus Sor
25~ oarr~ylng~out~an~embodlment of the method~ according to
bhe invention~using two movable substrates.~ ;
.~ ~. , . .: ~
igure 2~is an lsometrlc projection on an
enl~rged s~oale~of a~part comprising a support in the
orm of~a~polycrystalline body positioned be~ween two
30~ movable~substrates and a high frequency;coil,
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PHF . 7 $ - 5 1 4 C
29. 1. 197G
6~i
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Figure 3 shows another apparatus for carrying
out another embodiment of the method according to the
invention.
Figure 1 shows a closed space 1 having metal
walls and comprising at least one inlet and one outlet,
2 and 3, respec-tively, permitting the creation of an
inert atmosphere. Placed vertically in the centre is a
body 4 of polycrystalline silicon which forms a support
for a liquid mass 5 of molten silicon which will be pro-
duced at the upper end 6 of the support 4 with the aid
i of heating means, for example comprising a high-frequency
¦ coil 7. The support 4 is held at the end of a vertical
bar 8 which enters the space 1 through an aperture 9.
Disposed on either side of the support 4 are substrates
in the form of strips 10 and 11 which are symmetric with
,
`, respect tb the support 4. The strips 10 and 11 which
,
are held by guiding~ means 12 enter the space I through
i apertures 13 and 14, respectively, and leave the upper
part of space 1 through the apertures 15 and 16.
: '
The said strips 10 and 11 are moved in the
direction of the arrows F by uslng driving means not
shown.
j; :
Figure 2 shows the po1ycrystalline silicon
body ~ wh1ch forms the support for the liquid mass 5
25~ which;is formed~by heating using the high-frequency coil
7. On either side of the support 4 are disposed strips
., :
lO and 11 which are symmetric with respect to the
support~4. The strips 10 and 11 are driven in the
d~rection of the arrows F by driving means not shown
~ 30 in the Fig~re.
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PHF.75.5l!lC
29.1.1976
The method is carried out as follows. The
upper end 6 of the silicon body 4 is h~a-ted so as to
form the liquid mass 5 of molten silicor1, on a sub-
stantially horizon-tal solid liquid interface forming -the
upper surface of the support. The support may consist
of pure silicon e-ventually suitably doped if desired.
As a resul-t of the near presence of the strips
10 and 11 manufactured from a material which can be
wetted by silicon, in the present case of carbon fibre,
the silicon of the zone 5 will wet the strips. The
~ strips are moved upwards in the direction of arrow F.
¦ The liquid silicon re-tained by the substrate
¦ strips lO and 11 is driven by the regular movement in
the directions of the a~rows F of the supports 10 and 11
and crystallizes i.n the form of layers 17 and 18,
- respectlve~ly, according as the silicon is moved away
from the heating means 7.
! In this manner continuous Silicon layers are
obtained on the suppor-t and are collected outside the
! :
metal space 1.
By this method silicon strips are obtained
having a thickness of the order of about 60 /um, a width
of 2 cms and a length of several centimetres on a
support9 consisting of fibrous carbon, driven at a
~25 ~ speed~of 20 cm per mln. The material was polycrystalline
with lateral~ graln sizes of the order of 300 /um. The
~average crystAl orientation was L21~ in the direction
of movement of the strip, and ( 111 ) in the plane of the
layer.
Figure 3 shows a space 21 comprising at least
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PllF. 75 . 5 1 llc
2 ~ . 1 . 1 9 7G
one inlet and one outlet 22 and 23, respectively, per~
mitting of creating an inert atmosphere. ~ suppor-t 24
consis-ting of a body o~ polycrystalline silicon is
placed vertically in the centre of the space, a molten
zone 25 being formed at -the upper end 26 of the rocl 24
by high frequency induction with the aid of HF-coil 27.
i The support 24 is held at the end of a vertical bar
28 which enters the space 21 through an aperture 29. On
either side of the support 2l~ are disposed substrates
in the form of strips 30 and 31 which are held by guiding
~ means 32; said strips 30j 31 enter the space 21 through
I
apertures 33 and 34, respectively, and leave at the
upper part of the space through the aperture 35 which
in this case is common since the two supports approach
¦ 15 each other. Suitable means may be employed for establish-
ing a desired inclination of the said strips, said means
including replaceable guides 36 and 37, permitting also
adjustment of said inclination in the course of forming
the layer. The strips 30, 31 are moved in the direction
of the arrows F' by driving means not shown ln F~igure 3.
In accordance as the inclina-tion of a strip
at the area of contacting the liquid mass of molten
silicon is further deviating from a vertical direction
i ~ the resulti~ng layer of silicon formed on the substrate
2~ is thick r
, e .
Experiments have been carried out in which the
! I
- angle of inclination of the strip-shaped substrate
relative to a vertical direction was increased
approximately~10, wherein the thickness of the silicon
.
layer on a suitabl~e support, for ex~mple graphite which
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PHF.75-511lc
29.1.197~
is considerably wettable, was increased from 10 microns
to 160 rnicrons.
Such results are given o.nly by way Or example
and do not constitute any limitation of the invention.
The layers. thus produced may further be worked
, in known manner for the manu~acture of solar cells
j having polycrystalline silicon or other semiconductor
devices, such as rectifiers. E~.periments have been
carried out with normal cylindrical rods but it is cl.ear
that broader layers may be obtained with silicon
supports having the form o~ a parallelepiped as shown in
Fig. 2. As an alternative normally available/zoYle rde~lned
! polycrystalline silicon rods may be sawn lengthwise
in two pieces each piece being used as a support
¦ 15 (pedestal) of a molten silicon mass on top of it9 the
substrate strlp~moving in proxlmity of the straight
; edge ~ormed due to the sawing operation.
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