GB2059292A - Growing silicon films on substrates - Google Patents
Growing silicon films on substrates Download PDFInfo
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- GB2059292A GB2059292A GB8029976A GB8029976A GB2059292A GB 2059292 A GB2059292 A GB 2059292A GB 8029976 A GB8029976 A GB 8029976A GB 8029976 A GB8029976 A GB 8029976A GB 2059292 A GB2059292 A GB 2059292A
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- 239000000758 substrate Substances 0.000 title claims abstract description 139
- XUIMIQQOPSSXEZ-UHFFFAOYSA-N Silicon Chemical compound [Si] XUIMIQQOPSSXEZ-UHFFFAOYSA-N 0.000 title claims abstract description 126
- 229910052710 silicon Inorganic materials 0.000 title claims abstract description 126
- 239000010703 silicon Substances 0.000 title claims abstract description 126
- 238000000034 method Methods 0.000 claims abstract description 24
- 230000008023 solidification Effects 0.000 claims abstract description 19
- 238000007711 solidification Methods 0.000 claims abstract description 19
- 230000008014 freezing Effects 0.000 claims abstract description 7
- 238000007710 freezing Methods 0.000 claims abstract description 7
- 239000000919 ceramic Substances 0.000 claims description 23
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical group [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 claims description 20
- 229910052799 carbon Inorganic materials 0.000 claims description 20
- 238000001816 cooling Methods 0.000 claims description 11
- 239000011248 coating agent Substances 0.000 claims description 10
- 238000000576 coating method Methods 0.000 claims description 10
- 230000005499 meniscus Effects 0.000 claims description 10
- 239000000155 melt Substances 0.000 claims description 9
- 239000007788 liquid Substances 0.000 claims description 6
- 229910021420 polycrystalline silicon Inorganic materials 0.000 claims description 6
- 230000001154 acute effect Effects 0.000 claims description 5
- 238000010438 heat treatment Methods 0.000 claims description 2
- 239000007787 solid Substances 0.000 abstract description 29
- 239000012535 impurity Substances 0.000 abstract description 8
- 230000005669 field effect Effects 0.000 abstract 1
- 238000003618 dip coating Methods 0.000 description 8
- 230000004927 fusion Effects 0.000 description 7
- 230000001419 dependent effect Effects 0.000 description 4
- 238000005204 segregation Methods 0.000 description 4
- 238000010586 diagram Methods 0.000 description 3
- 230000005855 radiation Effects 0.000 description 3
- 238000004458 analytical method Methods 0.000 description 2
- 238000013459 approach Methods 0.000 description 2
- 239000002019 doping agent Substances 0.000 description 2
- 230000017525 heat dissipation Effects 0.000 description 2
- 238000002076 thermal analysis method Methods 0.000 description 2
- 101100332641 Caenorhabditis elegans eat-4 gene Proteins 0.000 description 1
- XAGFODPZIPBFFR-UHFFFAOYSA-N aluminium Chemical compound [Al] XAGFODPZIPBFFR-UHFFFAOYSA-N 0.000 description 1
- 229910052782 aluminium Inorganic materials 0.000 description 1
- 238000011109 contamination Methods 0.000 description 1
- 238000002425 crystallisation Methods 0.000 description 1
- 230000008025 crystallization Effects 0.000 description 1
- 230000000694 effects Effects 0.000 description 1
- 230000004907 flux Effects 0.000 description 1
- 230000014509 gene expression Effects 0.000 description 1
- 229910052734 helium Inorganic materials 0.000 description 1
- 239000001307 helium Substances 0.000 description 1
- SWQJXJOGLNCZEY-UHFFFAOYSA-N helium atom Chemical compound [He] SWQJXJOGLNCZEY-UHFFFAOYSA-N 0.000 description 1
- 230000006911 nucleation Effects 0.000 description 1
- 238000010899 nucleation Methods 0.000 description 1
- 230000000750 progressive effect Effects 0.000 description 1
Classifications
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
- H01L31/18—Processes or apparatus specially adapted for the manufacture or treatment of these devices or of parts thereof
-
- C—CHEMISTRY; METALLURGY
- C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
- C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
- C04B41/00—After-treatment of mortars, concrete, artificial stone or ceramics; Treatment of natural stone
- C04B41/45—Coating or impregnating, e.g. injection in masonry, partial coating of green or fired ceramics, organic coating compositions for adhering together two concrete elements
- C04B41/50—Coating or impregnating, e.g. injection in masonry, partial coating of green or fired ceramics, organic coating compositions for adhering together two concrete elements with inorganic materials
-
- C—CHEMISTRY; METALLURGY
- C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
- C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
- C04B41/00—After-treatment of mortars, concrete, artificial stone or ceramics; Treatment of natural stone
- C04B41/80—After-treatment of mortars, concrete, artificial stone or ceramics; Treatment of natural stone of only ceramics
- C04B41/81—Coating or impregnation
- C04B41/85—Coating or impregnation with inorganic materials
-
- C—CHEMISTRY; METALLURGY
- C30—CRYSTAL GROWTH
- C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
- C30B15/00—Single-crystal growth by pulling from a melt, e.g. Czochralski method
-
- C—CHEMISTRY; METALLURGY
- C30—CRYSTAL GROWTH
- C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
- C30B15/00—Single-crystal growth by pulling from a melt, e.g. Czochralski method
- C30B15/007—Pulling on a substrate
-
- C—CHEMISTRY; METALLURGY
- C30—CRYSTAL GROWTH
- C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
- C30B29/00—Single crystals or homogeneous polycrystalline material with defined structure characterised by the material or by their shape
- C30B29/60—Single crystals or homogeneous polycrystalline material with defined structure characterised by the material or by their shape characterised by shape
- C30B29/605—Products containing multiple oriented crystallites, e.g. columnar crystallites
Abstract
The present invention is an improvement to the method of growing silicon films on a substrate by bringing the substrate in contact with molten silicon. The improved growth technique may be classified as an asymmetric mode of growth of silicon on the substrate and is characterized by the substrate being maintained at a higher or a lower temperature than the solidification of silicon in the area of the substrate where the silicon layer growth is taking place, that is in the area of the liquid-solid interface. With a higher substrate temperature the liquid-solid interface is tilted nearly parallel to the substrate surface but inclined at a re- entrant angle, so that the leading edge of the crystalline front is away from the substrate. This provides several advantages including increased growth speed, a nonhomogeneous doping of the silicon layer, that is an impurity concentration gradient and results in a high-low junction at the back surface and gives the back surface field effect. With a lower substrate temperature, say 5-10 DEG C below the freezing temperature of silicon, the liquid-solid interface is tilted nearly parallel to the substrate surface but inclined at a re-entrant angle so that the leading edge of the crystalline front is on the substrate. This provides an advantage of increased growth speed.
Description
SPECIFICATION
Growth technique for silicon-on-ceramic
The present invention is related to the art of growing silicon films on ceramic substrates by bringing the substrate in contact with molten silicon. The present invention is broadly related to U.S. patents 4,112,135, 4,128,680 and 4,137,355, where there is described apparatus and method of coating ceramic bodies or sheets with molten silicon to prepare large area, thin sheets of large grain polycrystalline silicon on inexpensive ceramic substrate for use in solar cell panels and the like. This is referred to as silicon-on-ceramic or supported growth. In that method the side of the ceramic sheet or the area to be coated with silicon is first coated with a layer of carbon.It is taught in the above patents that when a ceramic (which normally is not wet by molten silicon) is first coated with a layer of carbon on a surface to be silicon coated, the carbon coated surface will then be wet by molten silicon, and by contacting the carbon coated ceramic substrate with molten silicon, a silicon coating will be formed thereon.
According to the present invention, there is provided a method of growing a layer of polyscrystalline silicon on a substrate from a source of molten silicon comprising the steps effecting relative movement between the substrate and the molten silicon whereby contact is made between the molten silicon and the substrate surface causing a silicon layer to grow on said surface; and heating or cooling the substrate from the non-coated side in the region of silicon solidification to provide a unidirectional heat flow through the substrate and the silicon layer whereby the solid-liquid interface of the growing silicon layer subtends a small acute angle with the coated substrate surface. By the term small acute angle we mean an angle less then 10 .
Embodiments of the invention will now be described by way of example only, with reference to the accompanying drawings, in which:
Figure 1 is a drawing of prior art vertical supported symmetrical silicon growth;
Figure la is an enlargement of a portion of Figure 1;
Figure 2 shows diagrammatically the basic concept of the method of the present invention involving asymmetric hot substrate growth;
Figures 2a and 2b are enlargements of a selected portion of Figure 2 to enhance the description;
Figure 3 shows one embodiment of the present invention involving asymmetrical polycrystalline silicon layer hot substrate growth by non-vertical dip coating;
Figure 4 shows another embodiment of the present invention involving asymmetrical silicon layer hot substrate growth;;
Figure 5 shows still another embodiment of the present invention involving asymmetrical silicon layer hot substrate growth;
Figures 6 and 7 show further embodiments of the present invention involving asymmetrical silicon layer cold substrate growth;
Figure 8 is a drawing of liquid-solid-vapor configuration in the vicinity of the trijunction between the three phases; and,
Figure 9 shows the advance of the solid-liquid interface.
In the prior art relating to coating large grain silicon-on-ceramic from the melt there is taught the method of contacting a carbon coated surface of a ceramic substrate with molten silicon to form a silicon coating on the carbon coated ceramic surface. One specific method of contacting the ceramic substrate with molten silicon described in these prior art patents is by dip coating. Referring now to Figure 1 which shows a prior art process for providing a supported growth of large grain polycrystalline silicon on a ceramic substrate, there is shown a substrate of ceramic 10 having a surface 11 which has been carbon coated (the carbon coated surface will be wet by molten silicon 13). The substrate 10 has been dipped into molten silicon and is being withdrawn upwardly at a rate V.On the carbon coated surface 11 is silicon coating 12 grows as the substrate is withdrawn from the silicon melt. The molten silicon is drawn up into a meniscus 13a at the carbon coated surface 11 and the crystallization of the silicon onto the substrate occurs in this meniscus at a liquid-solid interface 14.
Figure la is an enlargement of the portion of Figure 1 in the area of the liquid-solid interface 14 and also shows heat flow patterns from the solid silicon 12 by the use of a series of arrows. Figure 1 a shows that the solidification takes place at a liquid-solid interface 14 in which the interface angle is close to 90 to the surface 11. This 90 case is called the symmetric mode of growth. In this mode the radiation environment is essentially symmetric and thus heat is removed from the silicon approximately equally on both sides of the silicon sheets, and the growth occurs in the symmetric mode.
Referring now to Figure 2, molten silicon is brought into contact with a moving ceramic substrate that is inclined at an angle 05. The substrate 10 is kept hot during the growing of the silicon coating so that the liquid-solid interface 14 is tilted to be inclined at a small acute angle Oj (nearly parallel) to the as-grown silicon surface 1. This is shown in more detail in the enlargement of Figure 2a.
An advantage of this geometry with respect to speed is that the growth interface is growing at a velocity v which is much lower than the pull rate v. This is possible because the growth surface is much larger than the layer crossectional area.
An advantage of this asymmetric hot substrate growth concept with respect to undesired impurities is that at the lower growth velocity the segregation coefficient will effectively reject the impurities, pushing them toward the ceramic interface where they will not affect solar cell performance.
A further advantage of this concept is that the desired dopant may be chosen to have a reasonably low segregation coefficient. For example, to dope p-type, aluminum can be used with a segregation coefficient to 10-3. In Czochralski growth this type of impurity is undesirable because the impurities are rejected by the solid, so that the concentration in the liquid keeps changing, giving a nonhomogenious doping.
In a solar cell this nonhomogeneous doping can be used to advantage. This advantage relates to the natural impurity segregation. The top as-grown surface turns out to be very lightly doped, and, as dopant atoms are rejected towards the ceramic, the concentration increases. This results in a high-low junction at the back surface, and gives the BSF (back surface field) effect which is known to increase solar cell VOCT Jsc and efficiency, and provides a low resistance back contact to the base region.
A useful feature of this approach is that by keeping the angle Hj small, the impurities, both desired and undesired do not have a chance to diffuse into the bulk of the melt. This minimizes progressive contamination of the melt.
Another useful advantage to pulling at an angle such as described lies in the fact that heat conduction is basically perpendicular to the isotherms. The isotherms are parallel to the liquid-solid interface as is shown in more detail in Figure 2b. Thus the heat need be conducted essentially only through the thickness of the silicon. As can be seen in Figure 2b the angle Oc (which is the angle between the liquid-solid interface and the surface 1 of the solid silicon) is less than the angle 05. The angles 0, and 8i preferably should be much smaller than that represented in the drawing. This improved heat dissipation makes possible an increased growth speed.
Several embodiments of the asymmetric growth technique are shown in the next three figures of the drawing. Referring now to Figure 3 there is disclosed a non-vertical dip coating embodiment in which ceramic substrate 10 is immersed in molten silicon. On withdrawing the substrate from the silicon melt (by transport means shown in block diagram form) the direction of the velocity may be along the length of the substrate as shown by V or may be vertical as shown v'. An asymmetric growth occurs on the carbon coated substrate surface 11 providing a polycrystalline silicon layer 12 having the properties described in Figures 2, 2a and 2b.The asymmetric liquid-solid interface 14 can occur in this embodiment because the substrate in the region of interface 14 is maintained hot from beneath by still being immersed in the molten silicon so that the heat flow from the silicon layer is basically upwardly from the upper surface of the silicon layer 12. It is also to be understood that the same asymmetric growth concept may be implemented in a vertical dip coating configuration by having a substrate heater located near the back of the emerging substrate and cooling shoes located near the carbonized front side of the substrate.
Referring now to Figure 4, another embodiment of the asymmetrical growth technique is shown in which the substrate 10 with carbon coated surface 11 facing downwardly is supported and moved by transport apparatus 22, (and with the direction of the velocity indicated by an arrow) over the top surface of the molten silicon. The silicon melt is shown in an elongated crucible 15 containing a protruding meniscus of the molten silicon. The meniscus of silicon makes contact with surface 11 of the moving substrate to grow the silicon layer 12 on the substrate.In this embodiment a substrate heater 16 is shown as being positioned above the substrate 10 and extending to a point near the solidification interface to provide a hot substrate at the area of liquid-solid interface 14 so that the heat of fusion flows downwardly from the silicon as indicated by the triple arrows. Thus with the exposed surface of the silicon layer being at the lowest temperature, solidification begins at the exposed silicon surface and proceeds back toward the substrate.
As in dip-coating, the thickness of the silicon layer achieved in the inclined horizontal coating procedure of
Figure 4 is dependent upon the rate at which the heat of fusion is extracted from the layer and is dependent upon the velocity at which the layer is growing. If thermal conditions within the solidifiction zone are such that the liquid-solid interface lies in a place perpendicular to the surface of the substrate (as described in
Figure 1a, the growth rateof the crystalline layer and throughput rate of the coated layer are identical. The angle of the solidification front can, however, as herein described, be tilted toward the substrate, by altering thermal conditions in the heat flow from the two sides of the silicon layer during growth.Heater 16 provides this altered thermal condition so that the latent heat of fusion is removed by conduction away from the substrate (hot substrate growth). Whenever the plane of this front is substantially away from the normal to the substrate, the throughput rate with be substantially greater than the crystalline growth rate.
Referring now to Figure 5 there is disclosed still another embodiment of the asymmetric growth technique in which the ceramic substrate 10 with carbon coated surface 11 facing upwardly is supported and moved, bytransport apparatus shown in block diagram form, beneath a crucible 17 dispensing molten silicon through a slit onto the surface of the substrate. Again in this embodiment to incorporate a large asymmetry in the heat flow from the two sides of the silicon layer during growth, the substrate is maintained hot in the region where silicon solidification is occuring. This is accomplished by heater 20 underneath substrate 10 and which heater extends laterally to the right in the drawing to a point below substrate 10 which is opposite where solidification of the silicon is occuring. Thus as shown by the triple arrows the latent heat of fusion is removed by conduction away from the substrate, a large asymmetry in the heat flow from the two sides of the silicon layer during growth is again provided.
Although the above description of asymmetric growth has been stated in terms of the silicon layer on a ceramic substrate it will be understood that other suitable substrates may also be used, for example, a carbon substrate.
Referring now to Figures 6 and 7 the basic concept of "cold substrate" growth of sheet silicon is shown. By the use of the term "cold substrate" herein is meant that the temperature of the substrate in the region where the silicon film is growing is below the freezing point of silicon by a few degrees, say 5 to 1 0'C so that the molten silicon will rapidly freeze as it contacts the colder substrate. Thus in Figure 6 molte silicon is brought into contact with a moving ceramic substrate 110 that is inclined at an angle as shown. The substrate 110 is cooled below the temperature of the melt at the area of and during the growing of the silicon coating so that the liquid-solid interface 114 is nearly parallel to the substrate, but inclined at a small acute angle to the substrate surface so the first liquid to solidify solidifies on the substrate.The silicon layer is homogeneous, that is, the silicon grows from the previously solidified silicon.
An important advantage of cold substrate growth is the minimized contact time between the substrate and the melt. If the substrate temperature where the substrate first contacts the melt is below the freezing point of silicon it will cause the silicon to freeze immediately. This will cause the liquid-solid interface at the very tip of the solid silicon to curve toward the substrate. If this curvature is not excessive, no heterogeneous nucleation will occur, so that large grains will result. The contact time in this case approaches zero, so that in the limit, no impurities can enter the melt from the substrate except by diffusing through the solidified silicon. Thus higher purity silicon can be produced, leading to higher efficiency solar cells.It should be pointed out, that even though the Liquid-Solid Interface (LSI) at the tip curves toward the substrate, most of the LSI is at the cold substrate angle, as defined by Eq. (6) below.
An advantage of this geometry with respect to speed is that the growth interface is growing at a velocity V which is much lower than the pull rate V. This is possible because the growth surface is much larger than the layer crossectional area.
A related advantage of assymetric growth such as described lies in the fact that the latent heat of fusion can be removed more readily. Heat conduction is basically perpendicular to the isotherms. The isotherms I are parallel to the liquid-solid interface as is shown in Figure 6. Thus the heat need be conducted essentially through the thickness of the silicon to the substrate. This improved heat dissipation makes possible an increased growth speed. The angle the substrate makes with the surface of the melt and the angle the isotherm makes with the substrate both preferably should be smaller than that represented in Figure 6.
In the embodiment of Figure 6 a non-vertical dip-coating technique is employed in which the ceramic substrate 110 is immersed in molten silicon. In withdrawing the substrate from the silicon melt, by transport means 122 shown in block diagram form, the direction of the velocity may be along the length of the substrate as shown by V or may be vertical as shown V'. An asymmetric growth occurs on the carbon-coated substrate surface 111 providing a polycrystalline silicon layer 112. The asymmetric liquid-solid interface 114' can occur in this embodiment because the substrate in the region of interface 114' is cooled from above by cooling means 118 so that the heat flow (latent heat of freezing) from the silicon layer is basically upwardly to and through the substrate to the cooling means, such as a cooling shoe.It is also to be understood that the same cold substrate asymmetric growth concept may be implemented in a vertical dip coating configuration by having a substrate cooling shoe located near the back side of the emerging substrate. This cooling may be by radiation or convection to a cooler object. Forced convection such as a jet of helium may be utilized if desired.
Referring now to Figure 7, another embodiment of cold substrate asymmetrical growth is shown in which the substrate 110 with carbon-coated surface 111 facing downwardly is supported and moved, by transport means 122 shown, (and with the direction of the velocity V indicated by an arrow) over the top surface of the molten silicon. The silicon melt is shown in an elongated crucible 115 containing a protruding meniscus of the molten silicon. The meniscus of silicon makes contact with surface 111 of the moving substrate to grow the silicon layer 112 on the substrate.In this embodiment a substrate cooling is shown as occurring above the substrate 110 and extending to a point near the solidification interface to provide a cold substrate at the area of liquid-solid interface 114' so that the latent heat of freezing flows upwardly from the silicon to and through the substrate as indicated by the upward arrows above the substrate. Thus with the interface surface of the silicon layer adjacent the cold substrate being at the lowest temperature, solidification begins at the silicon interface surface and proceeds away from the substrate.
As in dip-coating, the thickness of the silicon layer achieved in the inclined horizontal coating procedure of
Figure 7 is dependent upon the rate at which the heat of fusion is extracted from the layer and is dependent upon the velocity at which the layer is growing. If thermal conditions within the solidification zone are such that the liquid-solid interface lies in a plane perpendicular to the surface of the substrate (as described in
Figure 1a), the growth rate of the crystalline layer and throughput rate of the coated layer are identical. The angle of the solidification front can, however, as herein described, be tilted to be nearly parallel to the substrate, by altering thermal conditions in the solidification zone to incorporate large asymmetry in the heat flow from the two sides of the silicon layer during growth.Heater 116 together with cooling means such as a cooling shoe 118 provides this altered thermal condition so that the latent heat of fusion is removed by conduction to the substrate (cold substrate growth). Whenever the plane of this front is substantially away from the normal to the substrate, the throughput rate will be substantially greater than the crystalline growth rate.
Analytical Consideration of Asymmetric Growth
The purpose of this section is to present a simplified thermal analysis which shows that the asymmetrical "hotsubstrate" or "cold substrate" types of growth technique is quite distinctfrom the usual symmetrictype growth. The analysis is based on the following assumptions: 1. The liquid-solid interface (LSI) is an isotherm. The temperature of the isotherm is defined to be TF, the
freezing temperature.
2. The LSI is essentially planar, having a large radius of curvature (R > 1 cm). There could be a small
region (on the order of microns) nearthetrijunction where curvature can exist as illustrated in Figure 8.
3. The pulling velocityv (with which the growing layer is withdrawn from melt in the region of interest for
sheet growth) is greater than 0.05 cm/sec.
4. The heat flux density JL in the liquid is small compared to that in the solid J5.
5. Heat transfer from the free surface is dominated by radiation according to the Stefan - Boltzman law:
The first part of the thermal analysis argument is to derive the boundary condition at the liquid-solid interface This condition is well known, but is derived here for continuity of the analysis.
Figure 9 shows the advance of the liquid-solid interface during growth. In a time tithe solid advances a distance vt. The volume solidified in time t is (vt 1 w) sin Bi, where w is the width of the sheet (perpendicular to thefigure) The heat energy liberated is (pLvt1 w)- sin Bi. This heat must be equal to the heat carried away by conduction, (J, - JL) 1wt.Equating the two expressions gives J#=JL + pLvsin 0. (1) In the highi speed limit JL will be negligible compared to J5 (assumption 4 above), giving 5 = pLV sin 0i. (2)
Since the liquidsolid-interface is an isotherm (assumption 1) the heat flow is perpendicular to it The
horizontal component is JL COS O#, which must equal the heat loss at the boundary: EoT4F, by assumption 5.
Thus:
pLv sin Bi cos #j = EaT4. (3)
The value of Or is then given by
With a value of V = 0.05 cm/sec. (assumption 3) the numerical value of 5 is about 0.1, so that
sin (3i cos Oi = 2- sin 2Oj = < 0.1 (5)
This equation for Or has two solutions: Oj = 900 ~ and Bi = , corresponding to vertical type growth and hot substrate growth, respectively.
5 ~ 6 Hot Symmetrical-type growth
= 90' - 5 Symmetrical-type growth (6) When the value of 8i is greater than 90 , heat flows toward the substrate. If sic is used to denote scaT4 Lv this equation for Bi has two solutions.0; = 90 ~bc and Bi = 180 ~bc, corresponding to vertical type growth and cold substrate growth, respectively: < 3 ~ 180 ~#c Cold substrate-type growth (7)
90 #c Symmetrical-type growth Symmetrical type growth would take place at values less than the limit given because heat would be removed from both sides of the silicon. In equation (7) #c is understood to be in radians. For example, 6 = .1 radians corresponds to 6 = 5.7 , and #j = =74.3 would be the angle of the liquid-solid interface at the trijunction.
Claims (8)
1. A method of growing a layer of polycrystalline silicon on a substrate from a source of molten silicon comprising the steps effecting relative movement between the substrate and the molten silicon whereby contact is made between the molten silicon and the substrate surface causing a silicon layer to grow on said surface; and heating or cooling the substrate from the non-coated side in the region of silicon solidification to provide a unidirectional heat flow through the substrate and the silicon layer whereby the solid-liquid interface of the growing silicon layer subtends a small acute angle with the coating substrate surface.
2. The method of Claim 1, wherein the substrate is a ceramic and wherein said substrate surface is carbon coated.
3. The method of Claim 1 or 2, wherein the region of silicon solidfication is heated to provide unidirectional heat flow from the heated substrate to the silicon layer whereby solidification of the silicon layer commences at the outer surface thereof and progresses inwardly toward the substate.
4. The method of Claim 2 or 3, including the steps of immersing the substrate in molten silicon; removing the substrate from the melt in a non vertical position with the carbon coated surface of the substrate facing upwardly; growing a silicon layer on the emerging carbon coated surface of the substrate; and wherein the substrate is heated from the non-coated side by way of the still immersed lower face of the substrate opposite the area where solidification of silicon is occuring.
5. The method according to Claim 2, 3 or 4, wherein an elongated crucible contains the molten silicon and the level in the crucible is maintained such that the silicon surface rises up from the crucible surface in a convex meniscus, wherein the substrate is supported with the carbon coated surface facing downwardly, and wherein the downwardly facing surface of the substrate is traversed over and across the surface of the molten silicon meniscus with the surface making contact with the silicon meniscus as the substrate traverses to grow a silicon layer as the carbonized surface emerges from contact with the silicon meniscus.
6. The method of Claim 2, wherein a crucible has a slit in the bottom thereof for releasing molten silicon downwardly therefrom, wherein the substrate is traversed under and across the area of the slit to receive molten silicon on the upper surface of said substrate in passing under said slit to grow a silicon layer thereon, wherein a heater means is located below the substrate and opposite the area at which the silicon solidification is occurring, and wherein a cooled area in front of said growing silicon area is provided in front of said growing silicon layer.
7. The method of Claim 1 or 2, wherein the region of silicon solidification is cooled to a temperature lower than the freezing point of silicon to provide unidirectional heat flow from the silicon layer to the cooled substrate whereby solidification of the silicon layer commences at the substrate surface and progresses outwardly.
8. A silicon layer on a substrate when made in accordance with any one of the preceding claims.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US06/079,844 US4252861A (en) | 1979-09-28 | 1979-09-28 | Growth technique for silicon-on-ceramic |
| US06/095,729 US4251570A (en) | 1979-11-19 | 1979-11-19 | Cold substrate growth technique for silicon-on-ceramic |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| GB2059292A true GB2059292A (en) | 1981-04-23 |
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ID=26762491
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB8029976A Withdrawn GB2059292A (en) | 1979-09-28 | 1980-09-17 | Growing silicon films on substrates |
Country Status (3)
| Country | Link |
|---|---|
| DE (1) | DE3036104A1 (en) |
| FR (1) | FR2466856A1 (en) |
| GB (1) | GB2059292A (en) |
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| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE3231267A1 (en) * | 1982-08-23 | 1984-02-23 | Siemens AG, 1000 Berlin und 8000 München | METHOD AND DEVICE FOR ASYMMETRICALLY COATING A BAND-SHAPED CARRIER BODY WITH SILICON FOR THE PROCESSING OF SOLAR CELLS |
| DE3231268A1 (en) * | 1982-08-23 | 1984-02-23 | Siemens AG, 1000 Berlin und 8000 München | METHOD AND DEVICE FOR ASYMMETRICALLY COATING A BAND-SHAPED CARRIER BODY WITH SILICON FOR THE PROCESSING OF SOLAR CELLS |
| DE3231326A1 (en) * | 1982-08-23 | 1984-02-23 | Siemens AG, 1000 Berlin und 8000 München | DEVICE FOR PRODUCING LARGE-SCALE, BAND-SHAPED SILICON BODIES FOR SOLAR CELLS |
| DE3240245A1 (en) * | 1982-10-29 | 1984-05-03 | Siemens AG, 1000 Berlin und 8000 München | DEVICE FOR PRODUCING TAPE-SHAPED SILICON BODIES FOR SOLAR CELLS |
Family Cites Families (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CA1081558A (en) * | 1975-10-24 | 1980-07-15 | Joseph D. Heaps | Method for dip-coating ceramic with molten silicon |
| FR2401696A1 (en) * | 1977-08-31 | 1979-03-30 | Ugine Kuhlmann | METHOD FOR DEPOSITING CRYSTALLINE SILICON IN THIN FILMS ON GRAPHITE SUBSTRATES |
| US4169739A (en) * | 1978-04-12 | 1979-10-02 | Semix, Incorporated | Method of making silicon-impregnated foraminous sheet by partial immersion and capillary action |
| DE2850805C2 (en) * | 1978-11-23 | 1986-08-28 | Siemens AG, 1000 Berlin und 8000 München | Process for the production of disk-shaped or ribbon-shaped silicon crystals with a columnar structure for solar cells |
-
1980
- 1980-09-17 GB GB8029976A patent/GB2059292A/en not_active Withdrawn
- 1980-09-25 DE DE19803036104 patent/DE3036104A1/en not_active Withdrawn
- 1980-09-25 FR FR8020607A patent/FR2466856A1/en not_active Withdrawn
Also Published As
| Publication number | Publication date |
|---|---|
| FR2466856A1 (en) | 1981-04-10 |
| DE3036104A1 (en) | 1981-04-16 |
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