High-radiance emitters with integral microlens

This invention deals with the fabrication of radiation emitting diodes having a small diameter shaped integral microlens formed by etching. Initially an oxide layer is deposited on the backside of the processed slice; then a ring pattern is opened in the oxide. An etch is used to form a ring groove with a mesa in the center. The center oxide dot over the mesa is removed and the etching continued to round off the edges of the mesa and to form a smooth shaped structure. Various shapes and diameters may be achieved with different ring dimensions and with different etch times.

FIELD OF THE INVENTION 
This invention relates to radiation emitting diodes and more particular 
small-area high-radiance emitters with integral microlens. 
BACKGROUND OF THE INVENTION 
Two principal factors which limit the efficiency of infrared emitting 
diodes are the absorption of the generative radiation of the semiconductor 
material before emission and, the total internal reflection at the 
semiconductor-air interface. In GaAs flat devices, the absorption can be 
minimized by the use of thin transparent layers of high bandgap energy 
GaAlAs. The high refractive index of GaAs (n.sub.s =3.6) compared to that 
of air (n.sub.a =1.0) results in a large refraction of rays at the 
semiconductor-air interface for rays which are not normal to the 
interface. Total internal reflection of rays occurs for angles greater 
than the critical angle (O.sub.s).sub.c defined by sin 
(.theta..sub.s).sub.c =n.sub.a /n.sub.s. Thus, only rays within a cone 
with a half angle of 16.1 degrees can be emitted through the top of the 
flat emitter. This corresponds to 2 percent of the total generated 
radiation, assuming no rays are reflected from the backside of the 
emitter. Shaped emitters can be used to eliminate total internal 
reflections. For a hemispherical emitter with a small junction diameter, 
all rays are incident approximately normal to the semiconductor-air 
interface and total internal reflection losses are eliminated. Ideally, a 
factor of 13 increase in radiant intensity (W/sr) can be achieved by use 
of a hemispherical emitter instead of a flat emitter. A disadvantage of 
the normal GaAs 18 mil diameter homostructure hemispherical emitter is 
that the increased path length leads to increased absorption and thus the 
lower values of radiance. GaAlAs 18-mil diameter hemispherical emitters 
with graded AlAs composition to reduce absorption have been fabricated in 
the past. However, very thick GaAlAs epitaxial layers are required (9-10 
mils). A technique for the growth of very thick GaAlAs epitaxial layers 
has been developed but the technique is not reproducible and is not 
compatible with multi-layer double heterostructure junctions which have 
high radiances. 
One approach to eliminate the requirement for thick transparent GaAlAs 
epitaxial layers is to greatly reduce the diameter of the hemisphere. To 
do this directly is not practical because of the difficulty of handling 
emitters with diameters less than 10-15 mils in diameter and because of a 
need of a minimum spacing between the N and P contacts on the base of the 
hemisphere to prevent formation of shorts during solder mounting of the 
devices on silicon submounts. 
A more practical approach for achieving smaller diameter shaped emitting 
surfaces is to use an etching process to form an integral lens in a larger 
size device chip. The process for forming the shaped emitting surface is 
part of the new invention. 
A previous approach for an integral lens structure has been described in 
the article "Integral Lens Coupled LED" by F. D. King and A. J. 
Springthorp, published in Electronic Materials Number 4, Page 243, 1975. 
The approach was to etch an array of small hemispherical holes in the GaAs 
substrate before growth of a multi-layer GaAlAs structure. The epitaxial 
growth fills in the etched holes. During device processing, a preferential 
etch (etches GaAs but not GaAlAs) was used to remove localized regions of 
the GaAs substrate from the backside of the slice and to leave a GaAlAs 
microlens structure. However, only a slight increase in optical output 
power and device performance was achieved. The limitation of this approach 
is that the hemispherical lenses are small compared to the junction size, 
because of the difficulty of filling in very deep holes by liquid phase 
epitaxial techniques. For optimum improvement with a hemispherical shape, 
the junction should be small compared to the lens size. This can be 
accomplished by the process described in this invention. 
SUMMARY OF THE INVENTION 
In order to provide an efficient, high radiance device which may be coupled 
to an optical fiber, an emitting diode is formed which has a lens integral 
therewith for increasing the total emitted radiation and for focusing the 
emitted radiation at the end of the fiber. The lens is relatively large 
compared to the emitting junction and it is slightly recessed in the 
semiconductor wafer. The lens is formed using an oxide mask and etching 
the semiconductor wafer. Etching is accomplished to two stages wherein a 
mask with a circular opening is used. The center portion of the circle is 
also mask so that a mesa type structure is formed. Then the central 
covering is removed to shape the mesa to form the lens. 
The emitting device may be further enhanced by plating around the lens to 
form a reflecting surface.

DESCRIPTION OF THE INVENTION 
Illustrated in FIG. 1 is an emitter having an integral lens coupled to a 
fiber optic. The emitter is fabricated from a semiconductor material 10 in 
which a P-N junction 13 has been formed, at which radiation is generated 
when the device is operated at forward currents. The P-N junction may be 
formed by diffusing directly into a GaAs substrate or by growing one or 
more layers of GaAs and/or GaAlAs on the substrate and then diffusing into 
one or more layers to form a buried junction emitter device as described 
in copending application Ser. No. 618,978 filed Oct. 2, 1975, now U.S. 
Pat. No. 4,037,241. The radiation from the emitting junction is collected 
and radiated through a lens 11, which is made from a portion of the 
semiconductor material 10. This optical radiation is then picked up by the 
optical fiber 12 and transmitted elsewhere. Contacts 14 which interconnect 
with the semiconductor material and the junction region 13 are formed on 
the opposite side of the device from which radiation is emitted. 
A process for making the lens integral with the emitter structure is 
described as follows with reference to FIGS. 2a and 2b of the drawings. 
Initially an oxide layer 21 is deposited on the backside of a processed 
slice. This processed slice will already have constructed thereon many 
emitting devices. The PN junctions 24 in the emitting devices may be made 
by diffusing directly into a GaAs substrate or by growing one or more 
layers of material on the substrate and then diffusing into one or more 
layers to form a buried junction structure. 
A ring pattern 22 is opened in the oxide and an isotropic etch is used to 
form a ring groove 23 with a mesa 27 in the center. The center oxide over 
the mesa is then removed and etching is continued as illustrated in FIG. 
2b until the top portion of the mesa is rounded off and a smooth shaped 
microlens structure 25 is obtained. Various shapes and diameters of the 
lens structure may be achieved with different ring dimensions and 
different etch times. 
The method described above is suitable for providing a lens structure which 
is large compared with the emitting junction size. Maximum efficiency can 
be maintained for such a structure as long as the hemisphere diameter is 
at least 3.6 times the junction diameter. For a 2-mil diameter junction 
the minimum hemisphere diameter would have to be 7.2 mils. 
In FIGS. 3a through 3c, the basic concept illustrated in FIGS. 2a and 2b is 
extended to the fabrication of an integral lens with the addition of a 
reflector. In FIG. 3a is illustrated a semiconductor slice 30 in which a 
PN junction 35 and ohmic contacts 36 have been formed. A hole has 
initially been etched in the semiconductor slice through a circular 
opening in the oxide 31. Then a circular dot of oxide 32 is added and 
additional etching is performed to form a ring grove 33 with a mesa 34 in 
the center. Thereafter, the center oxide dot 32 over the top of the mesa 
is removed and etching is continued to form the smooth shaped microlens 
37. Upon completion of the microlens a metallic deposition 38 may be made 
which covers the walls of the etch recess with the exception of the 
microlens, as illustrated in FIG. 3c. The metallic coating may be any 
highly reflective metal. However, it has been found that gold forms an 
excellent reflector since it does not oxidize and retains its reflective 
surface. 
Even though specific embodiments have been disclosed and described, it is 
to be understood that various modifications, changes and alterations may 
be made without departing from the spirit and scope of the invention as 
defined by the appended claims.