Mask alignment scheme for laterally and totally dielectrically isolated integrated circuits

A support material rectangle indicia is formed during the formation of the dielectrical isolation of starting material islands in one of said dielectrically isolated islands. An X indicia on a mask is positioned over the four corners of rectangular indicia to align the mask and the substrate.

BACKGROUND OF THE INVENTION 
The present invention relates generally to mask alignment schemes, and more 
specifically to a mask alignment scheme for forming devices in laterally 
dielectrically isolated islands. 
Devices in integrated circuits are formed by introducing inpurities into a 
surface of a substrate. This requires the use of a plurality of masks to 
form regions of different depths and impurity concentrations. 
Acceptability of the finally formed devices depends upon alignment of the 
diffused regions. Thus, there is great emphasis on schemes to align a mask 
to a substrate to assure proper alignment of the diffused regions. For 
junction isolated substrates, the first diffusion is from a top surface 
and consequently, all the diffusions may be aligned relative to the first 
diffusion. This technique may include positioning an indicia on a 
substrate and performing the first and subsequent diffusions using an 
indicia on the mask to align with the indicia on the substrate. 
For devices formed in dielectrically isolated islands of the type described 
in U.S. Pat. No. 3,865,649, the dielectrically isolated islands are formed 
from a first surface which subsequently becomes a buried surface and the 
devices are formed therein by diffusion into a second surface opposite the 
first surface which does not exist in the initial material. Thus, no 
initial mark may be formed which allows alignment of the diffused device 
regions to the dielectric isolation. Since it is unacceptable for diffused 
regions of the device to touch the dielectric isolation, more care and 
time have to be used to align the mask relative to the dielectric 
isolation. Since the islands have inwardly sloping sides, the surface area 
available for forming the diffused regions is a function of the amount of 
starting material removed. Thus, the size of the islands as perceived from 
the surface varies considerably as illustrated in FIGS. 1a and 1b. At 
present, the diffusions are aligned to the outside of the islands. In view 
of the variation in the size, the alignment in a possibly increased sized 
island will not assure alignment in the remaining islands on the wafer. 
The varied island size also results in a varying alignment tolerance. This 
causes difficulty in judging alignment visually. 
For devices formed in islands of the substrate having lateral dielectric 
isolation and a buried junction isolation according to U.S. Pat. No. 
3,979,237, the lateral dielectric isolation is formed in the same surface 
as the devices. Thus, there is a capability of providing an indicia on a 
common surface for alignment of diffusion. But as with the completely 
dielectric isolated islands the size of the island as perceived from the 
surface varies considerably. Thus alignment relative to the lateral 
dielectric isolation cannot be assured even using an indicia on the same 
surface for the formation of the lateral isolation and the devices. Thus 
there exists a need for an alignment technique to be used with laterally 
and totally dielectrically isolated wafers which assures alignment of the 
device formation mask with the originally formed islands. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide an alignment technique 
which can be used in laterally and totally dielectrically isolated 
islands. 
Another object of the present invention is to provide an improved alignment 
indicia between laterally or totally dielectrically isolated substrates 
and the subsequent masks. 
A further object of the present invention is to provide an alignment 
indicia on a substrate which assures subsequent alignment on a first 
surface relative to previous processings on a second opposite surface. 
A still further object is to provide an alignment scheme for laterally and 
totally dielectrically isolated islands which is independent of the size 
of the island. 
These and other objects of the present invention are attained by forming a 
rectangular fill material indicia interior to a starting material island 
exposed at the first surface during the formation of the dielectric 
isolation of the starting material islands from the first surface for 
lateral dielectric isolation and from a second surface for total 
dielectric isolation. The indicia rectangle is formed by etching the first 
or second surface through a rectangular aperture to produce a convergent 
etching. This assures the squareness of the rectangle corners. An X 
indicia on the first mask whose members have the angular relationship of 
the diagonals of the rectangular indicia of the substrate allows alignment 
of the mask relative to the substrate by positioning the members of the X 
on the four corners of the rectangular substrate indicia. The members of 
the X may be formed from two spaced parallel lines and the corner of the 
rectangular indicia may be positioned in the space between the parallel 
lines. 
Other objects, advantages and novel features of the present invention will 
become apparent from the following detailed description of the invention 
when considered in conjunction with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As depicted in FIG. 2, the process of the present invention begins with 
forming on a first surface 12 of a starting material 10 a masking layer 
14. In a typical example, the starting material is an N-type wafer of 
single crystal silicon having a thickness of approximately 18 mils, 
resistivity of approximately 5 ohm-centimeters and impurity concentration 
of approximately 1.times.10.sup.15 carriers per cubic centimeter. The 
starting material 10 is prepared or cut to have a planar surface of a 
crystal orientation in the [100] plane. These are but examples of the type 
of starting material, its depth and impurity concentration and other 
materials which may be used. The masking layer 14 may be an oxide layer of 
a thickness of approximately 6,000 ang stroms grown on the surface 12. A 
photoresist and oxide etch procedure is used to expose surface portions 16 
on surface 12 which defines the isolation regions and to expose a surface 
area 18 which will constitute the rectangular indicia. As is well known in 
this procedure, a photoresistant layer is deposited on the oxide mask; the 
photoresist is exposed to light in accordance with the desired pattern; 
then the pattern is developed to remove unexposed portions of the 
photoresist; and the exposed portions of the oxide layer is removed with a 
suitable etchant. This results in the mask illustrated in FIG. 2. 
A moat etch is performed to provide V-shaped moats in exposed areas 16 and 
a pyramid aperture in surface 18. The resulting structure is illustrated 
in FIG. 3. The oxide mask 14 is then removed and the exposed surfaces are 
cleaned. The wafer 10 with the moats and pyramid aperture therein is 
subjected to a thermal oxidation for a period sufficient to form a 10,000 
angstrom layer of oxide along top surface 12 and along the face of the 
moats and aperture. A polycrystalline support material, for example 
silicon, is deposited on the oxide 20 to over-fill the moat and form the 
support structure 22 as illustrated in FIG. 4. The second surface 24 
opposite the first surface is lapped and polished until the polishing 
plane intersects the etched isolation patterns and the pyramid. The 
starting material 10 is removed down to the dashed line 26 illustrated in 
FIG. 4. 
The wafer is inverted, as illustrated in FIG. 5, to have a top surface 
including a plurality of starting material surface regions 28 separated 
from each other by surface areas 30 of the support material 22. One of the 
starting material islands 28 includes a rectangular indicia surface area 
32 of support material 22 in the pyramid aperture. 
An enlarged view of the island including the rectangular indicia is 
illustrated in FIG. 6. As is noted from the view of FIG. 6, the outside 
edges of the starting material island 28 are substantially rounded off. 
More specifically, orthogonal edges 34 and 36 do not meet at a right 
angle, but are connected by angular segments 38 and 40. This results from 
the anisotropic etch of the starting material 10. In the etching of the 
exposed areas 16 from FIG. 2, the etchant dissolves material along the 
[1-1-1] plane to form the sloped edges. At the intersection of orthogonal 
moat patterns, the etching planes diverge which cause the etching along 
line 36 of FIG. 6 to etch the exposed edge of line 34 and produce the etch 
line 38 while the etching along line 34 of FIG. 6 etch the exposed edge of 
line 36 and produces the etched portion 40. The amount of rounding or the 
size of segments 38 and 40 is a function of the depth of the islands. As 
illustrated in FIG. 7, the etched surfaces 38 and 40 are triagular and 
thus diminish with the depth of the island. 
It should be noted that this rounding or side etching along the exterior of 
the island 28 is not produced on the interior convergent edges of surface 
18 of FIG. 2. This results because the edges of the orthogonal etching 
plane are not exposed to the etchant since they are converging versus 
diverging etch planes of the corners of the exterior moat pattern. Thus, 
the rectangular indicia pattern 32 has square corners whereas the island 
28 has a generally rectangular shape with rounded corners. 
The size of indicia 32 varies as does the size of the island 28 depending 
upon the depth of the original islands. Although this is true, the only 
variable that can be assured is that the indicia 32 is a rectangle having 
square corners whereas the corners of the rectangular island 28 may be 
generally rounded. Consequently, the mask to substrate alignment scheme is 
designed for the corners of the rectangle 32. 
As illustrated in FIG. 8, an indicia 42 is provided on the mask having a 
general X pattern. This X pattern is formed by the area between the 
plurality of adjacent sides of four orthogonally positioned triangles 44, 
46, 48 and 50 with adjacent apex. The four members or legs of the X 
pattern 52, 54, 56, and 58 are formed having the angular relationship of 
the diagonals of the rectangle 32. By aligning the members of the X 
pattern over the four corners of the rectangular indicia 32, the mask and 
substrate are aligned in the X and Y direction. 
Thus, a masking technique has been provided which does not require square 
corners on the starting material islands 28 and has a rectangle indicia 32 
which is formed during the formation of the moat regions in the starting 
materials. Consequently, an alignment for subsequent diffusion on a first 
surface relative to initial processing on a second surface is provided. 
As previously indicated, the concept of the present invention including 
converging etching to form a rectangular indicia to be used with an X for 
mask to substrate alignment may also be used for substrates having lateral 
dielectric isolation and a buried junction isolation as illustrated in 
U.S. Pat. No. 3,979,237 which is incorporated herein by reference. The 
modification of this proces is illustrated in FIGS. 9 through 12. A 
substrate 60 of, for example, a P-type single crystal silicon has a 
plurality of heavily doped N.sup.30 regions 62 formed in a surface 
thereof. A lightly doped N epitaxial layer 64 is then formed over the 
substrate 60. A thin film 68 of sufficient hardness to withstand normal 
polishing of the slice is deposited on the exposed surface of the 
epitaxial layer 64. An isolation pattern is defined in the hard film 68 by 
photoresist operation. After providing the holes in the film 68, isolation 
grooves 70 are etched through epitaxial layer 64 and into substrate 60. 
The resulting structure is illustrated in FIG. 9. 
A thin insulative layer 72 is formed on the surface of the grooves 70. The 
remainder of the grooves are then filled with a suitable material 74 for 
example polycrystalline silicon. The structure at this point is 
illustrated in FIG. 10. The excess polycrystalline material 74 and any 
insulative layer 72 that may have been formed on a planar surface of the 
thin hard film 68 is polished away by conventional machine polishing of 
the slice. This is followed by etching away the polishing stop film 68. 
This results in the structure of FIGS. 11 and 12 wherein the N.sup.- 
epitaxial layer 64 is divided into a plurality of segments surrounded 
laterally by a moat having an insulative layer 72 and polycrystalline fill 
74. 
The process described so far is identical to that of U.S. Pat. No. 
3,979,237. As in the previous example of the completely dielectrically 
isolated island of FIGS. 2 through 8, the moat regions produce N.sup.- 
epitaxial region 64 having rounded edges, because of the divergent etching 
at the corners. Thus these moat regions may not provide sufficient 
squareness to be used as the substrate indicia of a mask alignment 
technique. Thus, to incorporate the principles of the present invention, a 
diverging etch is needed to produce the square indicia. The modification 
of the process as illustrated in FIGS. 9 through 12 includes a square mask 
pattern 76 such that only converging etching occurs. As discussed 
previously, this assures the squareness of the corners. The resulting 
substrate indicia is illustrated in FIG. 12 as 76 having a rectangular 
polycrystalline filled area 74 enclosed by a dielectric layer 72 formed in 
the epitaxial layer 64. 
Although the rectangle 32 is illustrated as a square, obviously other 
rectangles may be used. The X pattern will assume the appropriate angular 
relationship corresponding to the diagonals of whatever rectangular 
pattern or shape is chosen for the rectangular indicia 32. 
Although the present alignment scheme was described as having a rectangle 
indicia on one element to be aligned with the X pattern with the other 
element wherein the X aligns along diagonals of the rectangle has been 
shown for dielectrically isolated islands with the rectangle indicia on 
the substrate and the X indicia on the mask, this scheme may be used to 
align any two other elements. 
From the preceding description of the preferred embodiments, it is evident 
that the objects of the invention are attained and although the invention 
has been described and illustrated in detail, it is to be clearly 
understood that the same is by way of illustration and example only and is 
not to be taken by way of limitation. The spirit and scope of this 
invention are limited only by the terms of the appended claims.