Method for manufacturing sharp waveguide branches in integrated optical circuits

A method for manufacturing branching-off or intersecting channel-shaped waveguides on or in a substrate, which substrate encloses a light-guiding layer, and on which substrate there is applied an auxiliary-mask layer having a thickness t, the method comprising steps of: PA1 applying a first mask pattern of a first mask material in a first mask position on the auxiliary-mask layer, the first mask pattern including a subpattern for defining a first channel-shaped waveguide; PA1 etching portions of the auxiliary-mask layer not covered by the first mask pattern using first etchants, the auxiliary-mask material being etched over a first etching depth d which is less than the thickness t; PA1 removing the first mask material of the first mask pattern; PA1 applying a second mask pattern of a second mask material in a second mask position which overlaps the position of the auxiliary-mask pattern at least in part, the second mask pattern including a subpattern for defining a second channel-shaped waveguide which makes an acute angle with the first waveguide; and PA1 a second etching step for etching portions of the auxiliary-mask material not covered by the second mask material of the second mask pattern using second etchants, the auxiliary-mask material being etched over a second etching depth t-d, wherein portions of the auxiliary-mask layer not covered by the second mask material and that were not covered by the first mask material are removed such that the surface of the substrate is reached.

A. BACKGROUND OF THE INVENTION 
The intention lies in the field of manufacturing integrated optical 
components. More in particular, it relates to a method for manufacturing 
branching-off or intersecting channel-shaped waveguides on or in a 
substrate, which substrate encloses a light-guiding layer, and on which 
substrate there is applied an auxiliary-mask layer, which method comprises 
the following steps: 
a first application step for applying a first mask pattern of a first mask 
material in a first mask position on the auxiliary-mask layer, which first 
mask pattern comprises a subpattern for the definition of a first 
channel-shaped waveguide, 
a first etching step for etching portions of the auxiliary-mask layer not 
covered by the first mask pattern with the help of first etchants, 
a removal step for removing the first mask material of the first mask 
pattern, with an auxiliary-mask pattern of auxiliary-mask material 
remaining on the substrate, 
a second application step for applying a second mask pattern of a second 
mask material in a second mask position which overlaps the position of the 
auxiliary-mask pattern at least in part, which second mask pattern 
comprises a subpattern for the definition of a second channel-shaped 
waveguide which makes an acute angle with the first waveguide, 
a second etching step for etching portions of the substrate not covered by 
mask material with the help of second etchants, 
an intermediate etching step carried out between the first and the second 
etching step with the help of third etchants. 
Such a method is disclosed in the European patent specification 
EP-A-0599394. In this known method, there is, first realized a composite 
mask pattern which consists of an auxiliary-mask pattern formed by the 
first mask pattern transferred to the auxiliary-mask material of the 
auxiliary-mask layer, and a second mask pattern of photosensitive mask 
material overlapping the auxiliary-mask pattern. Said composite mask 
pattern is then used in the second etching step, the actual etching step 
for etching the substrate. Said known method has the restriction that for 
a good result it is required that the materials of the auxiliary-mask 
layer and of the second mask pattern be sufficiently resistant to the 
etchants used in the second etching step. Such is not always feasible in 
practice. Thus, it has become apparent that an accepted photosensitive 
mask material, such as photoresist, is not sufficiently resistant under a 
dry-etching step, such as, e.g., with an oxygen plasm, with which the 
substrate of a semiconductor material, such as InP, is etched. 
B. SUMMARY OF THE INVENTION 
The object of the invention is to provide a method of the kind mentioned 
above, which does not have the restriction referred to. In this 
connection, it first realizes a complete mask pattern in an auxiliary-mask 
layer, which is then transferred to the substrate. For this purpose, the 
method of the kind referred to above according to the invention is a 
method wherein during the first etching step the auxiliary-mask material 
of the auxiliary-mask layer is etched over a first etching depth; wherein 
the intermediary etching step is carried out after the application of the 
second mask pattern, and wherein during the intermediate etching step the 
portions of the auxiliary-mask material not covered by the second mask 
material of the second mask pattern are etched over a second etching 
depth. The sharp vertex is therefore first realized in the auxiliary-mask 
layer and only then in the substrate. The advantage hereof is that the 
etching step in which the substrate material is etched, may optionally be 
carried out wet- or dry-chemically since, due to the absence of 
photosensitive mask material in the last etching step, the risk of 
underetching, which exists for wet-chemical etching, is much less. In 
addition, the etchants used in this connection are less selective. 
Further embodiments of the method according to the invention are summarized 
in the subclaims. 
The European patent specification referred to above is considered 
incorporated into the present application.

D. DESCRIPTION OF AN EXEMPLARY EMBODIMENT 
Channel-shaped waveguides in integrated optical circuits are mostly 
manufactured by means of etching ridge-type patterns in or on a substrate, 
mostly a layer stack, of suitable transparent materials. Many transparent 
materials applied are crystalline. Etching processes in such materials may 
be carried out wet-chemically and in the dry. In crystalline materials, a 
wet-chemical etching process progresses isotropically or by way of crystal 
planes, while a dry-etching process may always be carried out 
specifically. The method according to the invention is basically feasible 
both with wet-chemical and dry-etching processes, all this of course 
depending on, and due to, the substrate material applied and the waveguide 
patterns to be realized therein. The embodiment of the method described 
below is directed, by way of example only, at the application of 
dry-etching techniques to a III-V semiconductor material. The example 
relates to the manufacture of a ridge-type wave-guiding Y-junction with a 
sharp vertex, based on an InP substrate, with an RIE process (RIE=Reactive 
Ion Etching) being applied. 
FIG. 1 shows, in cross section, a layer stack to be processed, comprising a 
substrate 1 based on InP, on which there is applied a thin auxiliary-mask 
layer 2 of SiO.sub.2. The substrate 1 comprises a base layer 3 of InP, a 
light-guiding layer 4 of InGaAsP, and a top layer 5 of InP. The top layer 
5 is the layer which must be provided with a pattern of ridges defining 
the desired waveguide pattern, in this case a wave-guiding Y-junction, in 
the light-guiding layer. The auxiliary-mask layer 2 has a thickness t. 
FIG. 2 having subfigures FIGS. 2.1 to 2.5 inclusive, and FIG. 3 having 
subfigures FIGS. 3.1 to 3.5 inclusive, successively show various 
processing stages of the layer stack. Each subfigure FIG. 2.i (i=1, . . . 
,5) shows the layer stack in a plan view, while the corresponding 
subfigure 3.i shows the layer stack in a cross section along the line 
III.i--III.i. On the auxiliary-mask layer 2 there is applied, by means of 
a photolithographic process, a first mask pattern 6 in photoresist. Said 
first mask pattern 6 comprises a portion of a Y-shaped pattern, in this 
example the trunk 6.1 and a branch 6.2 thereof, which together form a 
pattern for a channel-shaped waveguide having a bend in point P. This 
stage--a first stage--is shown in FIG. 2.1 and FIG. 3.1. 
Subsequently, in a first etching step of dry-etching the auxiliary-mask 
layer 2, i.e., the portions of the auxiliary-mask layer 2 not covered by 
the first mask pattern 6, is etched down to a depth d. Then the 
photoresist of the first mask pattern 6 is removed. In the auxiliary-mask 
layer 2, however, there remain ridge-type elevations 2.1 and 2.2 as an 
impression of the first mask pattern 6. This stage--a second stage--is 
shown in FIG. 2.2 and FIG. 3.2. 
As the next step there is applied, on the substrate 1 by means of a second 
photolithographic process, a second mask pattern of photoresist, which 
partly overlaps the ridge-type elevations 2.1 and 2.2 as an impression of 
the first mask pattern 6 in the auxiliary-mask layer 2. The second mask 
pattern comprises a straight strip 7 for the definition of a straight 
channel-shaped conductor. The straight strip 7 near the bend P intersects, 
in an overlapping manner, with the ridge-type elevation 2.2 of the 
impression in the auxiliary-mask layer 2 at an acute angle. This stage is 
shown in FIG. 2.3 and FIG. 3.3. 
The auxiliary-mask layer 2, i.e., the portions of the auxiliary-mask layer 
2 not covered by the second mask pattern of photoresist, in this case the 
straight strip 7, is subsequently further etched, in a second etching 
step, which is preferably carried out with the same means as the first 
etching step, over a depth t-d, until a surface 1.4 of the substrate of 
InP is reached. Then the photoresist of the second mask pattern, in this 
case the straight strip 7, is removed. This stage--a third stage--is shown 
in FIG. 2.4 and FIG. 3.4. On the substrate surface 1.4, of the 
auxiliary-mask material of the original auxiliary-mask layer there have 
remained only strips 2.5, 2.6 and 2.7 of thickness t-d, as the respective 
impressions of the ridge-type elevations 2.1 and 2.2, and the straight 
strip 7. Only in a zone where, in the third stage, the straight strip 7 
has overlapped the ridge-type elevations 2.1 and 2.2, the auxiliary-mask 
material still has an additional elevation 2.8 of the original thickness 
t. The strips 2.5, 2.6 and 2.7 (including the additional elevation 2.8) 
form a Y-shaped mask pattern of exclusively the auxiliary-mask material, 
which in fact already shows a sharp vertex W. 
Said sharp vertex W is transferred in a next, third etching process, with 
the Y-shaped mask pattern being used as a mask, to the InP material of the 
substrate 1. Finally, the residues of the auxiliary-mask layer 2 are 
removed. This latter stage--a fifth stage--is shown in FIG. 2.5 and FIG. 
3.5. Here, the ridge of the trunk of the Y-shaped waveguide pattern is 
denoted by 1.1, the ridge of the first branch by 1.2, the ridge of the 
second branch by 1.3, and the etched top surface of the substrate by 1.5. 
Between the first branch 1.2 and the second branch 1.3, there has been 
produced a sharp vertex V. 
The mask patterns of photoresist are applied and removed again according to 
conventional photolithographic processes. The etching processes of the 
first and second etching steps, in which the auxiliary-mask material 
silicon dioxide (SiO.sub.2) is etched, may be carried out in an RIE 
process with fluor-containing gases such as CHF.sub.3. Dry-etching indium 
phosphide may be carried out in an RIE process with a CH.sub.4 /H.sub.2 
gas mixture. The silicon-dioxide residues may be removed with an HF 
solution or a CHF.sub.3 etching process. 
The auxiliary-mask layer of silicon dioxide may be applied to the substrate 
using various techniques, such as vapor deposition, sputtering, or using a 
PECVD (Plasma-enhanced Chemical Vapor Deposition) process. 
Instead of silicon dioxide, other dielectric materials, which are 
conventionally applied as mask materials in the event of integration 
techniques for manufacturing components based on III-V semiconductor 
materials, such as silicon nitride (Si.sub.3 N.sub.4) and silicon 
oxynitride, a hybrid of silicon nitride and silicon dioxide may be used. 
The thickness t of the auxiliary-mask layer and the etching depth d in the 
first etching step are not critical. The auxiliary-mask layer, e.g., has a 
thickness t=approx. 200 nm, which in the first etching process is etched 
away to roughly half. 
In lieu of one auxiliary-mask layer with a relatively large thickness 
(e.g., 200 nm), there may also be applied two auxiliary-mask layers, 
preferably of the same material and roughly equal in thickness (approx. 
100 nm). A first auxiliary-mask layer 8 of thickness d1 is applied to the 
substrate 1 prior to the application of the first mask pattern having 
portions 6.1 and 6.2 of photoresist. In the first etching process, etching 
is then carried out down to the surface of the substrate (therefore 
etching depth d1 is approx. 100 nm). A second auxiliary-mask layer 9 is 
applied after the removal of the first mask pattern, on the substrate 1 on 
top of residues 8.1 of the first auxiliary-mask layer 8. Subsequently, the 
second mask pattern of photoresist is applied, including the straight 
strip 7. FIG. 4 shows the layer stack in this processing stage. 
Thereafter, the method is continued using the subsequent steps described 
above. 
The third etching step may, instead of using dry etching, also be carried 
out using wet-chemical etchants. Suitable selective etching liquids for 
etching the indium-phosphide surface are, e.g., HCl:H.sub.3 PO.sub.4 or 
C.sub.3 H.sub.8 O.sub.3 :HCl.HClO.sub.4. 
The method described may be applied, by suitably dimensioning the mask 
patterns, for the manufacture of branching-off waveguide structures 
requiring sharp vertices, such as 3 dB splitters, asymmetrical 
Y-splitters, power couplers. For structures having multiple branches, such 
as 1.times.N and M.times.N splitters which are composed of Y-shaped 
branches, the method is also applicable: by always having, in each 
Y-shaped branch of such a composite structure, one waveguide branch 
defined by one and the same first mask pattern and the other waveguide 
branch by one and the same second mask pattern. 
A sharp vertex may also be obtained if the subpattern of the first mask 
pattern defines a complete Y-shaped waveguide structure, the subpattern of 
the second mask pattern is equal to the subpattern of the first mask 
pattern, and the subpattern of the second mask pattern is applied at a 
mask position which is displaced somewhat with respect to the mask 
position of the subpattern of the first mask pattern. The direction in 
which the displacement is/has been effected must be such that a situation 
corresponding to the stage shown in FIG. 2.3 may therewith occur in an 
area around the location where a sharp vertex must be produced. A 
processing stage of the layer stack after the application of the second 
mask pattern, with the first and the second mask pattern comprising an 
identical Y-shaped subpattern, is shown in FIG. 5. After the first etching 
step there have remained, in the auxiliary-mask layer 2, ridge-type 
elevations 2.9, 2.10 and 2.11 as an impression of the first mask pattern, 
which elevations form a Y-shaped relief 10 (broken-dashed-line pattern) in 
the auxiliary-mask material of the auxiliary-mask layer 2. The ridge-type 
elevations 2.9, 2.10 and 2.11 have respective ridge widths b1, b2 and b3. 
The ridge-type elevations 2.10 and 2.11 have an angle .PHI.. On top of the 
Y-shaped relief 10, there is applied the second mask pattern of 
photoresist. The second mask pattern comprises substrips 7.1, 7.2 and 7.3, 
which form a Y-shaped subpattern 11 of the same dimensions as the Y-shaped 
relief 10. The substrips 7.2 and 7.3 have the same angle .PHI. as the 
ridge-type elevations 2.10 and 2.11. The Y-shaped subpattern 11 lies in a 
somewhat displaced position on top of the Y-shaped relief 10. Said 
displaced position is such that an inner edge 12 of the ridge-type 
elevation 2.10 and an inner edge 13 of the substrip 7.3 intersect in a 
point Q. The Y-shaped subpattern 11 of the second mask pattern is 
preferably applied in a position displaced (according to a translation) 
over a spacing .DELTA.x along a direction perpendicular to a bisector s of 
the angle .PHI. between the ridge-type elevations 2.10 and 2.11. In order 
for the substrips 7.1, 7.2 and 7.3 to overlap the corresponding ridge-type 
elevations 2.9, 2.10 and 2.11 at least in part, the spacing .DELTA.x must 
be less than the smallest of the ridge widths b1, b2 and b3. 
Such a displacement variant of the method, with two mask patterns having 
identical Y-shaped subpatterns being applied under mutually displaced 
positions, is very suitable for the manufacture of sharp vertices in 
structures having multiple branches, such as 1.times.N and M.times.N 
splitters. A great advantage hereof is that it is not necessary to design 
two different mask patterns. When dimensioning the design, however, there 
must be taken into account the fact that the eventual widths of the 
channel-shaped waveguide patterns to be realized will be inclusive of the 
displacement.