Process of producing semiconductor article

Details Process of producing semiconductor article Process of producing semiconductor article

2000-03-24

2002-04-23

Kunemund, Robert (Department: 1765)

Single-crystal, oriented-crystal, and epitaxy growth processes;

Forming from vapor or gaseous state

With decomposition of a precursor

C117S094000, C117S095000, C117S104000, C117S915000

Reexamination Certificate

active

06375738

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention generally relates to a process of producing a semiconductor article and, more particularly, it relates to a method of producing a semiconductor article in which a single-crystal substrate separated by the method can be reused.
2. Related Background Art
The technique of forming a single-crystal semiconductor layer on an insulator is known as Silicon on Insulator or Semiconductor on Insulator (hereinafter, totally referred to as “SOI”) technique and has been investigated by many researchers because devices utilizing the SOI technique have a number of advantages that cannot be obtained by bulk silicon substrates that are used for producing ordinary silicon integrated circuits. In short, the SOI technique provides the advantages of:
(1) easy dielectric isolation and feasibility to a high degree of integration;
(2) excellent radiation hardness;
(3) reduced floating capacitance and adaptability to high speed operation;
(4) capability of omitting a well step:
(5) prevention of latch-ups; and
(6) capability of producing fully depleted type field effect transistors as a result of realizing thin film formation.
Recently, T. Yonehara et al. have reported a bonded SOI that is excellent in film thickness uniformity and crystallinity and can be batch-processed (T. Yonehara et al., Appl. Phys. Lett. Vol. 64, 2108 (1994); U.S. Pat. No. 5,371,037; Japanese Patent Application Laid-Open No. 5-21338).
FIGS. 12A
,
12
B,
12
C,
12
D and
12
E are schematic sectional views showing the steps of a conventional method of producing a semiconductor article. As shown in
FIG. 12A
, a porous layer
2
is formed on a surface of an Si substrate
1
. As shown in
FIG. 12B
, a non-porous single-crystal Si layer
3
is epitaxially grown on the porous layer
2
. Then, an silicon oxide
5
is formed thereon. Next, as shown in
FIG. 12C
, the first substrate
1
and a second substrate
4
are bonded to each other.
Then, as shown in
FIG. 12D
, the first substrate
1
is made thin from the rear side by a technique such as grinding or the like to make the porous layer
2
exposed over the entire surface of the substrate. The exposed porous layer
2
is then etched and removed by a selective etching solution such as HF+H
2
O
2
or the like. Since the etch selectivity of the porous Si layer
2
relative to the bulk Si (non-porous single-crystal Si) can be made as high as 100,000, the non-porous single-crystal Si layer formed on the porous layer
2
can be left on the second substrate
4
to form an SOI substrate without reducing the thickness of the non-porous single-crystal Si layer (see FIG.
12
E).
However, a semiconductor substrate produced by way of the bonding process inevitably requires two wafers, one of which is substantially wasted away by grinding, polishing, etching, or the like.
In order to fully enjoy the advantages of the bonded SOI, there is a continuing need for a process that can produce a high quality SOI substrate with excellent reproducibility and enable the reuse of a prime wafer to attain resource saving and reduction of the production cost. Under such circumstances, Sakaguchi et al. have recently reported a process of reusing a first substrate to be wasted out in the bonding process (Japanese Patent Application Laid-Open No. 7-302889; U.S. Pat. No. 5,856,229).
The uniformity of resistivity of the CZ wafers as silicon substrates which are commercially available easily and inexpensive is relatively good in each wafer but is not satisfactory between wafers. For instance, there is generally used a specification with a variation of ±30-50% such as 0.01-0.02 &OHgr;cm. The specification with a range results from the controllability of variance of resistivity within an ingot. Therefore, when the acceptable value as to variance in resistivity between wafers is to be narrowed, the yield of wafers obtained from one ingot is decreased, thus resulting in an increase of the production cost.
On the other hand, in the steps of forming a porous silicon layer on a surface of a silicon substrate and separating a pair of substrates as bonded in the porous silicon layer, controlling the structure of the porous layer is important in two points.
A first point is the control of the crystallinity of the single-crystal layer as the SOI layer. The crystallinity is greatly influenced by the structure of the porous layer.
That is, silicon materials of different resistivities will form porous layers with different structures, which will affect the density of crystal defects introduced into the non-porous single-crystal layers to be formed thereon. For example, when a porous silicon layer is formed on a substrate doped with boron in about 10
18
/cm
3
, and an epitaxial silicon layer is formed thereon by the CVD method, a substrate with a resistivity of 0.013 &OHgr;cm and a substrate with a resistivity of 0.017 &OHgr;cm provide crystal defect densities of the epitaxial silicon layers as the SOI layers which are different from each other on the order of about 10
1
.
A second point is the control of the separability. A porous structure suitable for the separation is, for example, a porous layer with a high porosity. However, a high porosity means in most cases that the side walls of the pores are thin, and too high a porosity will results in peeling off at the porous layer prior to the separation, which poses a problem in the step stability.
Especially, the increase of the pore diameters of the porous material is complementary to the decrease of the pore wall thickness, and in case of a high porosity layer, the affection of the change in resistivity, i.e., the change in impurity concentration on the porosity is larger than that in case of a low porosity layer.
Japanese Patent Application Laid-Open Nos. 9-102594 and 10-200078 disclose a method of diffusing an impurity such as boron, etc. to form a uniform porous layer in a low-grade Si substrate with good reproducibility.
The method is described with reference to
FIGS. 13A
,
13
B,
13
C,
13
D, and
13
E.
As shown in
FIG. 13A
, a doped layer
11
is formed by diffusion or epitaxial growth on a surface of a silicon wafer
1
as a prime wafer.
As shown in
FIG. 13B
, the silicon substrate
1
having the doped layer
11
is subjected to anodization to form a porous region
2
. Since the pore formation is carried out such that the thickness t
2
of the porous region
2
is larger than the thickness t
1
of the doped layer
11
, the porous region
2
consists of two layers
12
and
13
having different porosities from each other.
As shown in
FIG. 13C
, a non-porous layer
3
is epitaxially grown on the porous region
2
, and an insulating layer
5
is formed on a surface thereof.
As shown in
FIG. 13D
, the insulating layer
5
is bonded to a support substrate
4
to make a bonded substrate.
As shown in
FIG. 13E
, an external force is applied so as to separate the bonded substrate to generate a crack in the porous region
2
, thus effecting separation into two members.
Removing the remaining porous layer
12
and smoothing the exposed surface provides an SOI substrate.
The separated silicon substrate
1
, when subjected to removal of the remaining porous layer
13
and smoothing of the exposed surface, can be reused as a silicon wafer (prime wafer)
1
not made porous.
However, since at least the porous layer
13
is obtained by making porous the surface of the original silicon substrate
1
, the silicon substrate
1
subjected to the removal of the remaining porous layer
13
is in thickness that the original substrate.
Further, when the impurity concentrations of the used silicon substrates themselves differ from one another, the obtained porous layers
13
are nonuniform ones.
When the separated substrate
1
is to be reused, it is preferable that the quality thereof is suitable for bonding as with the original substrate, and further that the quality of the non-porous single-crystal layer formed on the porous layer is at least not inferior to the quality of that formed on the original substra

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