Single-crystal – oriented-crystal – and epitaxy growth processes; – Processes of growth from liquid or supercritical state – Liquid phase epitaxial growth
Reexamination Certificate
2001-12-20
2004-10-12
Norton, Nadine G. (Department: 1765)
Single-crystal, oriented-crystal, and epitaxy growth processes;
Processes of growth from liquid or supercritical state
Liquid phase epitaxial growth
Reexamination Certificate
active
06802900
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to liquid phase growth methods and liquid phase growth apparatus and, more particularly, is suitably applicable to liquid phase growth methods and liquid phase growth apparatus of an immersion type in which a substrate of wafer size is held by a jig and immersed into a solution containing a growth material.
2. Related Background Art
Global environments are becoming worse because of emission of earth-warming gases such as carbon dioxide, nitrogen oxides, etc. from combustion of petroleum in thermal power generation, combustion of gasoline in automotive engines, and so on. In addition, there is future concern about a drain of crude oil and attention is thus being drawn toward power generation with solar cells as clean energy sources.
Since thin-film crystal silicon (Si) solar cells have a thin power-generating layer and are made using a small amount of the source material of Si, there is a perspective of cost reduction of the thin-film crystal Si solar cells. Since the power-generating layer is made of crystal Si, higher conversion efficiency and less deterioration can be expected as compared with solar cells of amorphous Si and others. Further, since the thin-film crystal Si solar cells can be bent to some extent, they can be used in a bonded state to curved surfaces of automotive bodies, household electrical appliances, roof tiles, and so on.
For substantiating the thin-film crystal Si solar cells, Japanese Patent Application Laid-Open No. 8-213645 discloses separation of thin films of monocrystalline Si, making use of epitaxial layers on a porous Si layer.
FIG. 16
is a cross-sectional view showing a method of forming a thin-film Si solar cell, described in Japanese Patent Application Laid-Open No. 8-213645. In the figure, numeral
101
designates an Si wafer,
102
a porous Si layer,
103
a p
+
type Si layer,
104
a p
−
type Si layer,
105
an n
+
type Si layer,
106
a protective film,
109
and
111
adhesive layers, and
110
and
112
jigs. In the process of producing the solar cell of
FIG. 16
, the porous Si layer
102
is made by anodization over the surface of Si wafer
101
. After that, the p
+
type Si layer
103
is epitaxially grown on the porous Si layer
102
, and the p
−
type Si layer
104
and n
+
type Si layer
105
are further grown thereon. Then, the protective film
106
is formed thereon. The adhesive layers
111
,
109
are then laid over the protective film
106
and over the Si wafer
101
, respectively, to be bonded to the jigs
112
,
110
. Thereafter, pulling forces P are exerted on the respective jigs
110
,
112
to separate the Si wafer
101
from the epitaxially grown Si layers (
103
,
104
,
105
) across the porous Si layer
102
. Then, the solar cell is formed in the epitaxially grown Si layers (
103
,
104
,
105
), while the Si wafer
101
is again subjected to similar steps, thereby reducing the cost.
Japanese Patent Application Laid-Open No. 5-283722 discloses growth of epitaxial Si layers on the porous Si layer by the liquid phase growth method. An Sn melt is used as a solvent, and Si is preliminarily dissolved into the Sn melt to saturate therein, prior to the growth. Then, the melt is slowly cooled, and at a certain level of supersaturation a porous surface of a wafer is dipped into the Sn melt to grow an epitaxial Si layer on the porous surface.
Japanese Patent Application Laid-Open No. 5-17284 discloses an immersion type liquid phase growth apparatus of compound semiconductor and a holding jig.
FIG. 17
is a cross-sectional view of this liquid phase growth apparatus. In the figure, numeral
81
designates a wafer holder,
82
a wafer,
83
a crucible,
84
a solution,
85
a quartz reactor tube,
886
a gas introducing tube,
887
a gas exhaust tube,
88
a heater, and
89
a dummy wafer. In this immersion type liquid phase growth apparatus, the wafer holder
81
holding the wafer
82
and dummy wafer
89
is moved down (in the direction A), the wafer
82
is immersed into the solution
84
in which a growth material is dissolved. The solution
84
is retained in the crucible
83
and the crucible
83
is placed in the quartz reactor tube
85
which maintains the interior in an atmospheric (or ambient) gas (reducing gas or inert gas) by means of the gas introducing tube
86
and gas exhaust tube
87
. The heater
88
is provided for control of temperature of the system. The temperature of the solution
84
is lowered by decreasing the temperature of the heater
88
, whereby the growth material is precipitated from the solution
84
onto the wafer
82
to grow in liquid phase. The immersion type liquid phase growth apparatus can be constructed in smaller size as the growth apparatus for liquid phase growth on wafers of the same size than the liquid phase growth apparatus of the slide boat type and the liquid injection type. The immersion type liquid phase growth apparatus is also convenient for mass production, because a lot of wafers can be set on the holder.
Japanese Patent Application Laid-Open No. 57-76821 also discloses an immersion type liquid phase growth method.
FIG. 18A
is a perspective view of a wafer holder disclosed in Japanese Patent Application Laid-Open No. 57-76821. Numeral
123
designates an arm,
124
an umbrella-like plate,
125
wafers,
126
a through hole, and
122
a cylindrical member. This wafer holder can be loaded with six substrates on the umbrella-like plate
124
.
FIG. 18B
is a plan view of this wafer holder.
FIG. 18C
shows a crucible
128
into which the wafer holder shown in
FIGS. 18A and 18B
is immersed, and a solution
129
is filled in the crucible
128
. A heater
130
is placed around the crucible
128
and an auxiliary heater
131
projecting upright is located in the center and in the lower part of the crucible
128
. Namely, the crucible
128
is recessed toward the interior around the auxiliary heater
131
and the auxiliary heater
131
is placed inside the recess. Since this auxiliary heater
131
prevents the central part of the solution
129
from becoming lower in temperature than the peripheral part, the solution
129
is wholly kept at even growth temperature. It is disclosed that this arrangement can thus implement the uniform liquid phase growth.
In the case of the immersion type liquid phase growth apparatus disclosed in Japanese Patent Application Laid-Open No. 5-17284 described referring to
FIG. 17
, when the wafer size is large for liquid phase growth over a large area, a deposited film becomes thick in the peripheral part of the wafer but thin in the central part.
FIG. 19
is a cross-sectional view showing the growth of the deposited film. In the figure, numeral
82
denotes a wafer and
90
an epitaxial film deposited on the wafer
82
. As illustrated, for the large wafer size, the deposited film
90
becomes thin in the central part of the wafer but thick in the peripheral part. This phenomenon becomes noticeable, particularly, when the wafer size is not less than five inches. A conceivable reason for it is that the outside part of the solution
84
is close enough to the atmospheric gas to reduce the temperature of the outside part of the solution according to the cooling of the system whereas the central part of the solution
84
is far from the atmospheric gas and is thus cooled with a lag behind the temperature reduction of the outside part. Namely, a temperature change is less since a growth start in the central part of the wafer than in the peripheral part. For this reason, a deposition amount of the deposited film
90
is considered to be smaller in the central part of the wafer than in the peripheral part.
This is also the case in the liquid phase growth method disclosed in Japanese Patent Application Laid-Open No. 57-76821, and the temperature is less unlikely to be reduced in the central part of the crucible than in the peripheral part, because there is no mechanism of intentionally cooling the solution in the crucible. For this reas
Iwane Masaaki
Mizutani Masaki
Nakagawa Katsumi
Nishida Shoji
Saito Tetsuro
Anderson Matthew
Norton Nadine G.
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