Bonded wafer and method of manufacturing it

A bonded wafer comprising a filmy bond wafer, a base wafer, and an intermediate silicon dioxide layer, wherein the periphery of the bond wafer is etched; this bonded wafer is made by: subjecting the bond wafer to an oxidation treatment to form an oxide film over it; joining the two wafers in a manner such that the oxide film-covered face of the bond wafer is put on the base wafer to thereby sandwich the oxide film between the wafers; heating the combined wafers to thereby create a bonding strength between the two wafers; grinding the exposed face of the bond; etching the periphery of the bond wafer to remove the portion which is not in contact with the base wafer; and polishing the exposed face of the bond wafer until it becomes a thin film.

BACKGROUND OF THE INVENTION 
(1) Field of the Invention 
This invention relates to a bonded wafer consisting of two wafers joined 
together, and to a method of making such bonded wafers. 
(2) Description of the Prior Art 
As a method for forming a single crystal semiconductor film over a 
dielectric substrate, there has been known, of old, a technology which is 
characterized by epitaxially growing a single crystal silicon film or the 
like over a single crystal sapphire substrate. In this technology, 
however, because there exists a disagreement in lattice constant between 
the dielectric substrate and the silicon single crystal which is grown in 
vapor phase, numerous crystal defects occur in the silicon vapor-phase 
grown layer, and for this reason this technology is of no practical use. 
Another conventional technology of providing a single crystal film on a 
silicon substrate is characterized by first forming a thermally oxidized 
oxide film across the surface of the silicon substrate, laying a 
polycrystal or amorphous silicon film over this oxide film, and applying 
to this an energy beam such as an electron beam or a laser beam in a 
manner such that the beam spot thereon is shifted along lines all in one 
and the same direction to thereby melt the silicon film, and then cooling 
and solidifying the film to obtain a monolithic single crystal film. 
Now, the technology of changing the silicon polycrystal film over the oxide 
film into the single crystal film by means of the laser beam or the like 
is disclosed, for example, in Japanese Patent Kokoku 62-34716. According 
to this publication, a single crystal projection is monolithically formed 
at a corner of the periphery of a single crystal silicon substrate, and 
with this projection as the seed crystal, single crystallization of the 
polycrystal film is effected. Although locallized growth of a single 
crystal is possible owing to an interaction between the single crystal 
projection and the molten silicon oxide film, it is difficult to obtain a 
silicon single crystal film of practical use. 
Under the circumstances, bonded wafers having a Si-on-insulation structure 
(SOI structure) have come to draw attention of scientists in the field. An 
SOI bonded wafer is made from two semiconductor silicon mirror wafers in 
the following manner. (Incidentally, a mirror wafer is a wafer at least 
one of whose two faces has specular gloss.) At least one of the wafers is 
subjected to an oxidation treatment so that an oxide film is formed over 
at least one face of the wafer which face has specular gloss. Then, the 
two wafers are joined together with the specular-glossy faces meeting each 
other; hence the oxide layer is sandwiched between the wafers. The joined 
wafers are heated at an appropriate temperature until sufficient bonding 
strength is created between the wafers. The external face of at least one 
of the wafers is ground, and thereafter polished until it becomes a thin 
film. This wafer is called "the bond wafer". The other wafer incidentally 
is called "the base wafer". 
PROBLEMS THE INVENTION SEEKS TO SOLVE 
Referring to FIG. 8, when two semiconductor mirror wafers 101, 102 are 
joined together face to face, as described above, the periphery portions 
101a, 102a of the wafers are usually so shaped that they tend to fail to 
come in contact with each other, as shown at (a) in FIG. 8. In the next 
step, when the bond wafer 101 is ground, the periphery portion 101a of the 
bond wafer 101 cracks and chips off the wafer body, as shown at (b) in 
FIG. 8. As a result, the bond wafer 101 has indented edges, and when seen 
from above, it has an irregular circumference (periphery contour) as 
exaggeratedly shown at (d) in FIG. 8. Incidentally, in FIG. 8, the drawing 
at (a) is a cross section of the wafers 101 and 102 including the 
periphery portions after they are joined together; the drawing at (b) is a 
cross section of the wafers 101 and 102 after the bond wafer 101 is 
ground; the drawing at (c) is a top view of the wafers 101 and 102 after 
the bond wafer 101 is ground. 
If the periphery portion of the bond wafer 101 has indented edges and 
irregular contour, the edges crack and scatter as the bond wafer 101 is 
polished later, and the polished surface of the bond wafer 101 is 
contaminated and scarred with the particles scattered. 
The present invention was made in view of the above-stated problem as well 
as the desire for improving product efficiency; therefore, it is an object 
of the invention to provide a bonded wafer, and a method for manufacturing 
it, in which the periphery of the bond wafer is entirely in contact with 
the base wafer so that the bond wafer does not chip off as it is polished, 
and hence the bond wafer is not attacked by the harmful wafer particles. 
SUMMARY OF THE INVENTION 
In order to attain the objects of the invention, the inventors propose a 
bonded wafer comprising two mirror wafers, namely a filmy bond wafer and a 
base wafer, and an intermediate oxide layer which is interposed between 
mirror faces of the two wafers, characterized in that the periphery of the 
filmy bond wafer is etched. 
The inventors further propose a bonded wafer comprising two silicon single 
crystal mirror wafers, namely a filmy bond wafer and a base wafer, and an 
intermediate silicon dioxide layer which is interposed between mirror 
faces of the two mirror wafers, characterized in that the periphery of the 
filmy bond wafer is etched in a manner such that the contour of the filmy 
bond wafer is formed entirely within that of the base wafer when viewed 
from the side of the bond wafer. 
Another aspect of the invention is to propose a method for manufacturing a 
bonded wafer comprising the steps of: subjecting a first mirror wafer to 
an oxidation treatment to thereby form an oxide film over a mirror face of 
the first mirror wafer; joining the first mirror wafer with a second 
mirror wafer in a manner such that the oxide film-covered face of the 
first mirror wafer is put on a mirror face of the second mirror wafer to 
thereby sandwich the oxide film between the two wafers; heating the 
combined wafers at a predetermined temperature to thereby create a bonding 
strength between the two wafers; grinding the exposed face of the first 
mirror wafer; and polishing the exposed face of the first mirror wafer to 
thereby reduce the first mirror wafer to a thin film; the method being 
characterized by further including the steps, after grinding the first 
wafer, of masking all of the exposed face of the first wafer except for a 
periphery portion and at least the exposed face of the second wafer with a 
corrosion resistant film, stooping the combined wafers in an etching 
liquid for a predetermined period of time to thereby etch and remove that 
periphery portion of the first mirror wafer which is not masked, and 
removing the corrosion resistant film. 
In a preferred embodiment, the corrosion resistant film is made of teflon 
or polyethylene. 
Or more preferably, all the exposed surface of the second wafer is masked 
with a corrosion resistant film made of a wax or a high molecular organic 
compound. 
Still more preferably, an even number of pairs of wafers similar to the 
bonded pair of wafers described above are stacked together in a row such 
that like wafers are juxtaposed together, and that the both extremities of 
this wafer stack are occupied by second wafers, and then this stack of 
wafers is altogether steeped in the etching liquid. 
EFFECTS OF THE INVENTION 
According to the present invention, before the polishing step, that portion 
of the periphery of the bond wafer (first mirror wafer) which is not 
masked by the protective film and which includes the portion that is not 
in contact with the base wafer (second wafer) is completely etched and 
removed from the bond wafer so that the remaining bond wafer contains 
little portion which is not in contact with the base wafer, so that no 
particle is chipped and scattered from the bond wafer as it is polished in 
the next step. As a result, the bond wafer is not contaminated with or 
scarred by such particles so that the yield of the tips obtained from the 
bonded wafer is improved.

AN EMBODIMENT OF THE INVENTION 
With reference to the attached drawings, an embodiment of the invention 
will be explained. 
FIG. 1 schematically shows respective cross sections of wafers and 
illustrates in the order of (a), (b), and (c) how a bonded wafer is 
fabricated. At (a), two single crystal silicon mirror wafers 1, 2 are 
prepared. The one of reference numeral 1 is a bond wafer whose upper face, 
as viewed in FIG. 1, is to be ground and polished and thereby made into 
the device formation face. The other wafer of reference numeral 2 is a 
base wafer, which is employed mainly for a protective purpose, that is, 
for securing mechanical strength of the final bonded wafer when the bond 
wafer 1 is ground to a fragile thin film. The bond wafer 1 is subjected to 
a thermal oxidation treatment whereby a thin film 3 of silicon dioxide is 
formed throughout the lower face of the bond wafer 1 which has specular 
gloss. 
At the next stage (b), the two wafers 1 and 2 are put together in a manner 
such that the oxide film 3 is sandwiched between the two wafers, as shown, 
and heated to a predetermined temperature to thereby bind them 
permanently. Then, at the stage (c), the upper face of the bond wafer 1 is 
ground until the thickness of the bond wafer 1 reaches a predetermined 
value t1 (the hatched portion of the bond wafer 1 is ground off). 
Now, as explained earlier, some of the peripheral portion of the bond wafer 
1 is so shaped that it fails to come in contact with the base wafer 2 when 
the two wafers 1, 2 are joined face to face; consequently, when the bond 
wafer is ground from the external face, that part of the peripheral 
portion which is not in contact with the base wafer 2 chips off, as shown 
at (a) in FIG. 2, and the bond wafer 1 will have an irregular 
circumference (peripheral contour) as shown at (b) in FIG. 2. 
Next, as shown at (a) and (b) in FIG. 2, masking patches 4 and 5 which have 
diameters smaller than the diameters of the bond wafer 1 and base wafer 2, 
respectively, are pasted over the wafers 1 and 2, respectively. Now, the 
entire portion of the bond wafer 1 which is covered with the masking patch 
4 is in contact with the base wafer 2. That peripheral portion of the bond 
wafer 1 which is not sheltered by the masking patch 4 includes all the 
edges which did not chip off during the grinding operation and are not in 
contact with the base wafer 3 and therefore still remain to be chipped off 
during the subsequent polishing remain operation if they there. 
Incidentally, the masking patch may be made of teflon, polyethylene or the 
like. Also, in place of such masking patch, it is possible to employ a 
highly corrosion resistant wax, or other films made of high molecular 
organic compound. 
Thereafter, the both wafers 1, 2 are steeped in an etching liquid such as 
mixed acid (of hydrofluoric acid and nitric acid), potassium hydroxide, 
and sodium hydroxide for a predetermined period of time, preferably about 
two minutes, whereby that peripheral portions of the bond wafers 1, 2 
which are not covered with the masking patches 4, 5 as well as the 
periphery of the oxide film 3 are etched and dissolved into the etching 
liquid. As a result, as shown at (a) and (b) in FIG. 3, the irregular 
edges of the bond wafer 1, which are not sheltered by the masking patch 4, 
are completely removed from the periphery of the bond wafer 1. 
Incidentally, since the etching of the oxide film 3 proceeds more slowly 
than that of the wafers 1, 2, the peripheral portion of the oxide film 3 
are left exposed along the periphery of the bond wafer 1 [ref. (b) in FIG. 
3]. 
After that peripheral portion of the bond wafer 1 which is unsheltered by 
the masking patch 4 is etched and completely removed, the remaining main 
body of the bond wafer 1, which is sheltered by the masking patch 4, 
includes no portion which is not in contact with the base wafer 2; 
therefore, the bond wafer 1 whose contour is now defined by and along the 
contour of the masking patch 4 is entirely contacted by the base wafer 2. 
After the etching, the masking patches 4, 5 are peeled off the wafers 1, 2, 
as shown in FIG. 4. Then, the exposed face of the bond wafer 1 is polished 
till the thickness thereof becomes a predetermined value t2 (e.g., about 3 
.mu.m); the hatched portion of the bond wafer 1, as shown at (a) in FIG. 
5, is thus polished off. Thus, a bonded wafer 6 whose periphery is etched 
is obtained. 
Since the periphery of the bond wafer 1 is without that portion which is 
not in contact with the base wafer 2, the periphery of the bond wafer 1 
does not crack and scatter as the upper face of the bond wafer 1 is 
polished. Therefore, no particle is chipped off and thus the upper face of 
the bond wafer 1 is not contaminated or scarred. 
The bonded wafer 6 thus obtained will have a configuration such that the 
periphery of the base wafer 2 extends beyond the periphery of the bond 
wafer 1, and this lends to the bonded wafer 6 various advantages. For 
example, in a heat treatment, it is possible to place the bonded wafer 6 
such that only the base wafer 2 is directly contacted by the boat groove, 
so that only the base wafer 2 suffers slippage from the boat groove and, 
therefore, the bond wafer 1 is safe from the damaging effect of the 
slippage. Also, in a cleaning treatment, when the bonded wafer 6 is placed 
in a wafer basket, only the periphery of the base wafer 2 touches the 
wafer basket so that the bond wafer 1 is not contaminated with particles 
or scarred. 
Incidentally, although in the above embodiment the masking patch 5 is 
pasted on the lower face of the base wafer 2 [ref. (a) in FIG. 8], it is 
possible, as an alternative measure, to cover all the surface of the base 
wafer 2 except for its bonding interface with a protective film 7 such as 
an oxide film and a wax, as shown in FIG. 6, whereby it becomes 
unnecessary to paste the masking patch 5. 
FIG. 7 shows a procedure of etching a plurality of bonded wafers at a time. 
An even number of bonded wafers are prepared each consisting of the base 
wafer 2 covered with a protective film and the bond wafer 1 covered with 
the masking patch 4. These bonded wafers are stacked together in a manner 
such that the base wafers 2, 2 are put together back to back and the bond 
wafers 1, 1 are put together via the masking patches 4, 4 and the 
extremities of this wafer sandwich are occupied by the base wafers 2, 2, 
as shown in FIG. 7. Then, keeping the bonded wafers together, the sandwich 
of the wafers is steeped in an etching liquid 9 contained in a container 8 
whereby the peripheries of all the bond wafers 1 are simultaneously 
etched. This way the production efficiency of the bonded wafer is 
multiplied. This procedure can also be employed in the case where the base 
wafers are covered with the patches 5 instead of being entirely covered 
with the protective film.