Method and apparatus for etching integrated optoelectronic devices

Optoelectronic devices are produced as integrated chips which include elements to control optical signals and electronic elements to control the optical elements. An integrated chip includes optical wave guides with tapered ends to facilitate exit and entry of optical signals. The tapered ends are produced by photo-electrolytic etch using illumination of graded intensity, e.g. the penumbra of a shadow.

FIELD OF THE INVENTION 
This invention relates to integrated optoelectronic devices and in 
particular to the production of integrated devices which are suitable for 
processing telecommunications and other signals. 
BACKGROUND OF THE INVENTION 
The use of optical frequencies for the transmission of telecommunications 
signals has resulted in the development of many active components which 
operate at optical frequencies. Examples of such active components 
include, optical sources, optical detectors, modulators and switches. 
Although these components operate on optical signals, they are usually 
controlled electronically. Thus a device which contains one or more of the 
active elements must also provide paths for the optical (traffic) signals 
and paths of the electrical control signals. 
Thus the complete device performs electronic and optical functions and it 
is usual to employ the term "optoelectronic devices". 
It is particularly desirable to implement optoelectronic devices in 
integrated forms wherein the paths and the active elements are produced as 
layers, e.g. by epitaxial deposition and etching. In such structures the 
optical paths take the form of suitably configured strips of transmissive 
material, e.g. indium phosphide and gallium arsenide. It is, of course, 
necessary to couple the paths to the active devices and it has been 
established that tapered configurations are particularly effective for 
coupling an optical path to an active device. It will be appreciated that 
the tapers need to be accurately located and dimensioned. It is an object 
of this invention to facilitate the production of such tapers. 
It is well established to produce optical paths by irradiation controlled 
electrolytic etching. A paper by Bell Laboratories describes the 
production of lenses by such techniques and it states that arbitrary 
features can be produced by appropriate light patterns. U.S. Pat. No. 
4,415,414 describes the use of masks consisting of alternate opaque and 
clear rings to produce lenses. 
SUMMARY OF THE INVENTION 
According to this invention radiation including a penumbra is applied to 
the transmissive layer during etching whereby the graded intensity in the 
penumbra controls the rate of etching to produce a taper. 
The method described above is capable of producing a wide range of tapers, 
e.g. tapers which extend 5 to 1000 times the thickness of transmissive 
layers. In most applications the taper extends 20 to 150 times, especially 
60 to 70 times, the said thickness. The thickness is usually between 2 to 
10 .mu.m, e.g. 3 .mu.m. During the use of the method the penumbra is 
adjusted to extend over the region where the taper is required. 
The invention also includes apparatus for performing the method using a 
source of radiation and a screen outside the electrolytic cell. The screen 
is between the source and the cell in order to produce the penumbra.

DETAILED DESCRIPTION OF THE DRAWINGS 
FIGS. 1 and 2 illustrate a partially completed device, generally indicated 
by numeral 10, wherein the finished device includes optical wave guide 
paths. At the stage shown in FIGS. 1 and 2, the device has a transmissive 
layer, generally indicated by number 11, which has a uniform section 12 
and a tapered section 13 with an exit face 14. The uniform section 12 is 9 
.mu.m thick and the tapered section 13 tapers to 3 .mu.m over a distance 
of 200 .mu.m. Thus the angle of taper (which is exaggerated in the 
drawings) is about 1.degree. of arc. The process of the invention produces 
the configuration shown in FIGS. 1 and 2 from a complete transmissive 
layer 11 uniform over the whole surface. 
As illustrated in FIGS. 3 and 4 an electrolytic etch is carried out in a 
glass vessel 20 which contains an aqueous electrolyte 21 and the necessary 
electrodes. (As is conventional, aqueous solutions of ammonium hydroxide 
and ammonium tartrate are used to etch indium phosphide, gallium indium 
arsenide phosphide and gallium indium arsenide.) Three electrodes are used 
namely the device 22 to be etched which is connected as anode (also 
referenced as 22), a graphite rod which is connected as cathode 24 and 
(for control) a calomel reference electrode 28 is also included in the 
cell. 
The anode (i.e. device 22), the calomel electrode 28 and the cathode 24 are 
connected into a conventional external circuit 23 which provides the power 
for the electrolysis. (The external circuit 23, which is not separately 
illustrated, comprises a potentiostat to which the three electrodes 22, 24 
and 28 are connected. The potentiostat continually adjusts the 
electrolysis current so that the anode 22 keeps a pre-set potential 
relative to the calomel electrode 28 and hence to the electrolyte.) 
The electrolysis only proceeds under illumination which is provided by 
mercury lamp 25 focused on slit 26 (both of which are outside the vessel 
20). Screen 27 is positioned between the lamp 25 and anode 22. The window 
of vessel 20 is of good quality to keep scattering and distortion of the 
light to an acceptable level. 
The precise arrangement of the slit 26 and screen 27 is an important 
feature of the invention and this feature is more fully illustrated in 
FIG. 4. The slit 26 is adjusted to a width of 500 .mu.m and it is 
positioned about 5 cm from screen 27 which is about 2.5 cm from anode 22. 
The arrangement is such that the slit 26 illuminates the anode 22 and the 
screen 27 casts a shadow. There are three regions, i.e. AX which is in 
full shadow so that no etching occurs, BY which is in full illumination so 
that rapid etching occurs and AB which is a penumbra with graded intensity 
of illumination. Thus the rate of etch is slow at A increasing to fast at 
B whereby a taper is produced. It will be noticed that AB is half the 
width of the slit, i.e. 200 .mu.m. 
In general terms (using U for the distance between slit 26 and screen 27, V 
for the distance between screen 27 and anode 22, S for the width of the 
slit and P for the length of the penumbra): 
EQU P=(V/U)S 
Since it is simple to adjust U, V and S over a wide range of values, P is 
equally adjustable. It will also be appreciated that P represents length 
of the taper produced by the etching. 
This simple theory takes no account of diffraction which may be a defect at 
the relevant dimensions. An alternative formula, which assumes that the 
slit is a line source of zero width, i.e. it assumes S=0, gives: 
EQU Q=[(V/U)(V+U).lambda.].sup.0.5 
where U and V are as defined above, .lambda. is the wave length of the 
radiation and Q is the penumbra width by this formula, i.e. a penumbra 
caused entirely by diffraction. 
The table below compares calculated values by the two formulae with 
measurements, represented by M, based on etched tapers. 
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A B C 
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S (.mu.m) 500 500 500 
U (cm) 5 5 6 
V (cm) 5 2 1.8 
.lambda. (nm) 
436 436 436 
P (.mu.m) 500 200 150 
Q (.mu.m) 210 110 101 
M (.mu.m) 250 130 100 
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(The etched layers were n.sup.+ GaAs in the case of experiments A and B and 
n.sup.+ InP in the case of C). 
It appears that the simple theory may overestimate the length of the taper. 
However, it has been observed that the penumbra technique produces a 
linear taper which curves at both ends. The curves appear to facilitate 
the coupling. It appears that the Q values (based on refraction) relate to 
the linear portion of the taper only whereas the curved portions extend 
beyond both calculated values. 
The electrolysis is carried out under conventional conditions, e.g. with 
the anode 22 at -200 mv relative to the calomel electrode 28. Under these 
conditions the current is only slightly affected by voltage changes (at 
higher and lower voltages a small change of voltage causes a large change 
of current. Such conditions should be avoided.) 
The positions of the slit 26 and the screen 27 are easily adjusted by 
visual inspection so that the penumbra AB is correctly located. Magnifying 
viewers may be utilised if necessary. Visual inspection may also be used 
to confirm that the penumbra has the correct width. Measurements of the 
taper on a finished product can also be used to confirm correct adjustment 
of the slit and screen. 
It will be appreciated that the method of the invention is a convenient way 
of obtaining the configuration illustrated in FIGS. 1 and 2. Further 
(conventional) processing is then used to obtain a completed device. 
During this further processing photoresists may be used to modify the 
basic configuration shown in FIG. 1. For example portions of the layer 11 
may be removed to give a plurality of parallel paths. Also active elements 
may be produced in contact with exit surface 14. 
During use of the finished device optical signals are conveyed in layer 11. 
These signals meet the exit surface 14 at angles of incidence which 
facilitate transfer to the elements in contact therewith.