U.S. patent application number 11/230292 was filed with the patent office on 2006-03-30 for process for etching trenches in an integrated optical device.
This patent application is currently assigned to STMicroelectronics S.r.l.. Invention is credited to Ilaria Gelmi, Giovanna Germani, Pietro Montanini, Marta Mottura.
Application Number | 20060068554 11/230292 |
Document ID | / |
Family ID | 36099758 |
Filed Date | 2006-03-30 |
United States Patent
Application |
20060068554 |
Kind Code |
A1 |
Montanini; Pietro ; et
al. |
March 30, 2006 |
Process for etching trenches in an integrated optical device
Abstract
The described process allows trenches to be etched in a
structure comprising a support substrate and a multilayer, formed
on the substrate, for the definition of wave guides of an
integrated optical device and comprises a selective plasma attack
in the multilayer through a masking structure that leaves uncovered
areas of the multilayer corresponding to the trenches to be etched.
Such a masking structure is obtained by forming a mask of metallic
material on the multilayer that leaves uncovered the areas
corresponding to the trenches to be etched and forming a mask of
non-metallic material, for example photoresist, on it that leaves
uncovered regions comprising at least part of the areas and an edge
portion of the mask of metallic material.
Inventors: |
Montanini; Pietro; (Milano,
IT) ; Germani; Giovanna; (Bareggio (Milano), IT)
; Gelmi; Ilaria; (Albiate (Milano), IT) ; Mottura;
Marta; (Milano, IT) |
Correspondence
Address: |
GRAYBEAL JACKSON HALEY LLP;Suite. 350
155-108th Avenue N.E.
Bellevue
WA
98004-5973
US
|
Assignee: |
STMicroelectronics S.r.l.
|
Family ID: |
36099758 |
Appl. No.: |
11/230292 |
Filed: |
September 19, 2005 |
Current U.S.
Class: |
438/296 ;
257/E21.035; 257/E21.036; 257/E21.252; 257/E21.257 |
Current CPC
Class: |
H01L 21/31144 20130101;
H01L 21/0332 20130101; H01L 21/0334 20130101; H01L 21/31116
20130101 |
Class at
Publication: |
438/296 |
International
Class: |
H01L 21/336 20060101
H01L021/336 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 17, 2004 |
IT |
RM2004A000445 |
Claims
1. Process for etching trenches in a structure comprising a support
substrate and a multilayer, formed on the substrate, comprising a
core layer and coating layers for the definition of wave guides of
an integrated optical device, the process comprising a selective
plasma attack in the multilayer through a masking structure that
leaves uncovered areas of the multilayer corresponding to the
trenches to be etched and being wherein the masking structure is
obtained with the following successive operations: forming a mask
of metallic material on the multilayer that leaves uncovered the
areas corresponding to the trenches to be etched and forming a mask
of non-metallic material that leaves uncovered regions comprising
at least part of the areas corresponding to the trenches to be
etched and an edge portion of the mask of metallic material.
2. Process for etching trenches according to claim 1, wherein the
operation of forming the mask of non-metallic material leaves
uncovered regions comprising the areas corresponding to the
trenches to be etched and an edge portion of the mask of metallic
material.
3. Process for etching trenches according to claim 1, wherein the
mask of metallic material leaves uncovered further areas of the
multilayer not corresponding to the trenches to be etched.
4. Process for etching trenches according to claim 1, wherein the
operation of formation of the mask of non-metallic material is
preceded by operations for the formation of other structures of the
integrated optical device.
5. Process for etching trenches according to claim 1, wherein the
non-metallic material comprises a photoresist.
6. Process for etching trenches according to claim 1, wherein the
attack comprises a low-density plasma attack.
7. Process for etching trenches according to claim 1, wherein the
attack comprises a high-density plasma attack.
8. A method, comprising: forming on a material a metallic first
mask that exposes a region of the material; forming on the first
mask a second mask that exposes a section of the region and a
portion of the first mask that is contiguous with the section of
the region; and forming a trench in the exposed section of the
material.
9. The method of claim 8, further comprising: forming on a
substrate a first optical layer having a first index of refraction;
forming on the first layer a second optical layer having a second
index of refraction; and forming on the second layer a third
optical layer having a third index of refraction, the first,
second, and third layers composing the material.
10. The method of claim 8, further comprising removing the first
and second masks after forming the trench.
11. The method of claim 8 wherein forming the first mask comprises
forming the first mask only within predetermined distance from a
perimeter of the region.
12. The method of claim 8 wherein forming the second mask comprises
forming the second mask such that the second mask exposes the
entire region of the material and exposes a portion of the first
mask that is contiguous with multiple sides of the region.
13. The method of claim 8 wherein forming the second mask comprises
forming the second mask such that the second mask exposes a section
that is smaller than the entire region of the material and exposes
only a portion of the first mask that is contiguous with fewer than
all sides of the section.
14. The method of claim 8 wherein forming the second mask comprises
forming the second mask such that the second mask exposes a section
that is smaller than the entire region of the material and exposes
only a portion of the first mask that is contiguous with a single
side of the section.
15. The method of claim 8 wherein forming the trench comprises
anisotropically etching the exposed section of the material.
16. The method of claim 8 wherein forming the trench comprises
plasma etching the exposed section of the material.
17. The method of claim 8 wherein forming the second mask comprises
forming the second mask from a non-metallic material.
18. A structure, comprising: an optical path having a surface and
through which an optical signal can propagate; and a trench
disposed in the path, having a first side wall that is
substantially perpendicular to the surface, and having a bottom
that is substantially flat and substantially parallel to the
surface.
19. The structure of claim 18 wherein the trench further comprises
a second side wall that is substantially perpendicular to the
surface of the optical path.
20. The processor of claim 18 wherein the trench further comprises
a second side wall that forms an angle other than 90.degree. with
the surface of the optical path.
21. The structure of claim 18 wherein the trench further comprises
a second side wall that is substantially perpendicular to the
surface of the optical path and that is substantially parallel to
the first side wall.
22. The structure of claim 18 wherein the optical path comprises: a
first optical layer having a first index of refraction; a second
optical layer disposed on the first optical layer and having a
second index of refraction; and a third optical layer disposed on
the second optical layer, having a third index of refraction, and
having a surface that forms the surface of optical path.
23. The structure of claim 22 wherein the first index of refraction
substantially equals the third index of refraction.
24. The structure of claim 22 wherein: the first index of
refraction substantially equals the third index of refraction; and
the second index of refraction is greater than the first and third
indices of refraction.
25. The structure of claim 22 wherein the trench is disposed at an
end of the optical path.
26. An integrated circuit, comprising: an optical path having a
surface and through which an optical signal can propagate; and a
trench disposed in the path, having a first side wall that is
substantially perpendicular to the surface, and having a bottom
that is substantially flat and substantially parallel to the
surface.
27. An electronic system, comprising: an integrated circuit having,
an optical path having a surface and through which an optical
signal can propagate, and a trench disposed in the path, having a
first side wall that is substantially perpendicular to the surface,
and having a bottom that is substantially flat and substantially
parallel to the surface.
Description
PRIORITY CLAIM
[0001] This application claims priority from Italian patent
application No. RM2004A000445, filed Sep. 17, 2005, which is
incorporated herein by reference.
TECHNICAL FIELD
[0002] The present invention refers generally to processes for
manufacturing integrated optical devices and, more specifically, to
a process for etching trenches in an integrated optical device.
BACKGROUND
[0003] Integrated optical devices are used, as known, in
communications networks that use light signals for the transmission
of information. The most common optical devices are modulators,
attenuators and interferometers.
[0004] FIG. 1 of the attached drawings shows an integrated optical
device 1 in an initial manufacturing step. In order to form the
represented structure, a first silicon dioxide (SiO.sub.2) layer 3
is formed on a silicon support substrate 2, usually by deposition,
said coating layer or lower cladding, doped with impurities of a
type and concentration such as to obtain a predetermined refraction
index n.sub.cladding. A layer 4 is deposited on the layer 3, said
core layer, again made from silicon dioxide, but doped with a type
and concentration of impurities different to those of the
underlying layer so that the refraction index of the core is
greater than the refraction index of the cladding
(n.sub.core>n.sub.cladding). The core layer is subjected to a
known photolithographic process, after which the plot of the cores
of the wave guides of the designed optical device is obtained. The
last step necessary for the formation of the optical circuit is the
deposition of a coating layer or upper cladding 5. Of course, this
layer covers the top and side of each track of the plot formed by
the core layer. The upper layer 5 is preferably made from silicon
dioxide doped with the same type and the same concentration of
impurities with which the lower coating layer is doped, so as to
have the same refraction index n.sub.cladding. Under the condition
n.sub.core>n.sub.cladding the light propagates substantially
inside the optical paths defined by the cores.
[0005] In a standard manufacturing process of an integrated optical
device the subsequent step is the formation of further structures 6
of the integrated optical device. An example of these structures is
shown in FIG. 2, which also shows a masking layer 7 with an opening
for the etching of a trench.
[0006] The last step of the process is the formation of trenches in
the multilayer consisting of the layers of oxide 3, 4 and 5.
[0007] A trench can have different functions: for example, it can
make a physical insulation, both from the optical point of view and
from the thermal point of view, between two optical paths that are
adjacent or in any case very close to each other, or else it can
thermally insulate one or more optical paths from heat sources
present on the device itself or it can be used to make an optical
switch. In this case the trench is filled with a medium the
characteristics of which, in terms of refraction index and
propagation constant, can be modified so as to be selectively equal
to or less than those of the cores of the wave guides of the
interrupted optical path. In this way it is possible to close or
open the path of the light signal.
[0008] The trench that one wishes to make typically must cut the
entire thickness of the multilayer of oxide that, as stated,
comprises three layers: the lower and upper coating layers 3 and 5,
both typically having a thickness of about 16 .mu.m, and the core
layer 4, on the other hand, having a thickness that can be between
4 .mu.m and 8 .mu.m. In total, therefore, the depth that typically
must be reached with the etch is about 36-40 .mu.m. Typically, for
these depth values (a few tens of .mu.m) a trench is considered
deep; on the other hand, for depths of a few .mu.m a trench is
considered not very deep, i.e., shallow.
[0009] For the etching of a trench, currently techniques are used
that foresee the deposition on the multilayer of a layer of a
material resistant to the attack of the oxide, the definition on it
of a mask 7, like the one shown in FIG. 2, which leaves uncovered
the area 8 of the multilayer corresponding to the mouth of the
trench, and an anisotropic attack for the removal of the oxide
through the non-masked areas. This step is typically carried out
with a dry attack through plasma (plasma dry etching). Plasma
techniques allow a highly anisotropic attack to be carried out,
i.e. having an attack speed in the vertical direction, or more
precisely perpendicular to the surface to be attacked, much greater
than the attack speed in the lateral direction. This allows
high-resolution images to be transferred even when the layer of
material to be removed has a high thickness. Currently, for the
attack through plasma both old-style machines that carry out
low-density plasma attacks, and modern-style machines that carry
out high-density plasma attacks, are used. The difference between
the two types of attack lies in the amount of species contained in
the plasma that react with the material to be removed. The
low-density plasma attack is substantially of the chemical type: a
chemical reaction between the chemical reactants contained in the
plasma and the material of the multilayer causes the removal of
material from the multilayer itself. The high-density plasma attack
can, on the other hand, be considered both chemical and physical:
combined with a chemical reaction there is an actual abrasion of
the oxide due to particles present in the plasma that bombard the
surface to be removed. In the same time period a high-density
plasma attack reaches a greater depth than a low-density plasma
attack. Old-style machines are more common and easier to find on
the market, and therefore they have a relatively low cost.
Modern-style machines, being technologically more advanced, have a
greater cost and are put onto the market by few suppliers.
[0010] As stated previously, the attack step of the multilayer
through plasma is preceded by a process for the definition of a
mask that leaves unprotected the areas in which the trenches are to
be etched. The mask typically must keep its profile as unaltered as
possible under the action of the plasma and typically must have
high selectivity.
[0011] If the material constituting the masking layer is such that
the profile of the mask during the attack step remains unaltered,
the attack shall be substantially vertical and shall allow the
desired resolution to be obtained. Here, resolution means the
measurement of the fidelity of transfer of the image from the mask
to the attacked material. The trenches shall thus have
substantially vertical walls, i.e. with inclination of at least
88.degree. with respect to the surface plane of the silicon
substrate.
[0012] The selectivity is the ratio between the attack speeds on
different materials: in this case it is the ratio between the
attack speed of the oxide of the multilayer and the attack speed of
the masking material. In common processes this ratio can be between
5:1 and 8:1. This means that the attack of the oxide is from 5 to 8
times quicker than the attack of the masking material. Ratios of
this magnitude are typical of masks of low selectivity. For greater
ratios (even of 10:1, 20:1) a mask is considered to have good
selectivity; for ratios of 100:1 a mask can be considered to have
infinite selectivity.
[0013] In standard processes the masks used are of photosensitive
organic polymeric material, commonly known as photoresist. This
type of material is not very resistant to the action of the attack
through plasma, be it high or low density so that it reacts
chemically with the plasma. In particular, during the attack step
it deteriorates considerably at the edges that define the
unprotected areas to be etched. For example, if in the structure of
FIG. 2 the masking layer 7 is considered as a photoresist layer,
this phenomenon of erosion of the edges, or "tip" effect,
progressively modifies the profile of the mask to such a point that
in the oxide a trench with vertical walls is not obtained but
rather, as represented in FIG. 3, an incision 9 with oblique or in
any case irregular walls. Typically, this drawback is limited by
increasing the thickness of the masking layer. Unfortunately,
current materials and equipment typically do not allow photoresist
thicknesses of any more than 6-7 .mu.m to be obtained. Moreover,
during the definition of a mask in a layer of photoresist of such a
thickness, it is often difficult to obtain sharp and regular
profiles of the edges, but usually the profile extends beyond the
desired limits. The result is a reduction in the area of the
multilayer exposed to the attack, for which reason the design
specifications relative to the size of the trenches may not be
respected.
[0014] As an alternative to organic masks, so-called hard masks can
be used, for example masks made from metallic material. A metal
mask 10, like the one represented in FIG. 4, ensures substantially
infinite selectivity (attack speed ratio 100:1) and greater
protection against the action of the plasma since the chemical
composition of the metal does not react with the chemical attack
agents. Therefore, the metal mask 10 keeps its profile
substantially unaltered during the attack.
[0015] In the case in which the area of the multilayer not
protected by the mask of metallic material is a very small
percentage of the area of the device (roughly less than 25%), a
large amount of metallic material is exposed to the attack. It
follows from this that numerous metallic particles are gradually
removed from the mask and find themselves free in the plasma
itself, for which reason they slow down the action of the plasma.
This effect, known as "resputtering", often does not allow the
attack to be completed up to the desired depth. Moreover, a large
amount of metal exposed directly to the plasma may become
electrically charged during the attack, and this can cause further
drawbacks from the electrical point of view. The trenches obtained,
like the trench 11 shown in FIG. 4, consequently have imperfections
on the side walls, especially in the deepest zone in which the
attack may not be completed.
SUMMARY
[0016] An embodiment of the present invention is a process for
etching deep trenches in the multilayer of oxide of an integrated
optical device that allows the drawbacks outlined above to be
avoided.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] The invention shall become clearer from the following
detailed description of two embodiments thereof given as an example
with reference to the attached drawings, in which the figures are
section views of structures of integrated optical devices in
different manufacturing stages. In particular:
[0018] FIG. 1 illustrates an initial step of the manufacturing
process of an integrated optical device according to the prior
art;
[0019] FIG. 2 shows a structure with a mask of non-metallic
material, which leaves unprotected an area corresponding to the
mouth of a trench, obtained in a step of a process according to the
prior art,
[0020] FIG. 3 illustrates the result of a plasma attack step on the
structure of FIG. 2 in a process according to the prior art,
[0021] FIG. 4 shows a structure with mask of metallic material
after a plasma attack step in a process according to the prior
art;
[0022] FIGS. 5, 6, 7 and 8 illustrate some intermediate steps of a
process according to an embodiment of the invention,
[0023] FIG. 9 shows the structure of an integrated optical device
with trench obtained with a process according to an embodiment of
the invention,
[0024] FIGS. 10 and 11 illustrate a variant of some intermediate
steps of the process according to an embodiment of the invention,
and
[0025] FIG. 12 shows the structure of an integrated optical device
with trench obtained with a process according to an embodiment of
the invention that includes the variant illustrated in FIGS. 10 and
11.
DETAILED DESCRIPTION
[0026] A process according to an embodiment of the invention
foresees, like in the prior art described with reference to FIG. 1,
the formation on a silicon substrate 2 of a multilayer of oxide
comprising two coating layers 3 and 5 and an intermediate core
layer 4. Before the deposition of the upper coating layer 5, the
intermediate layer 4 is subjected to a photolithographic process to
obtain the plot of the cores of the waves guides of the optical
circuit.
[0027] As shown in FIG. 5, one proceeds to the deposition of a
layer of metallic material and to the consequent definition of a
metallic mask 12, the profile of which is such as to leave
uncovered an area 13 of multilayer corresponding to a trench to be
etched with a width, for example, of 15 .mu.m. In particular, in
this embodiment of the invention, the metallic mask 12 is
substantially limited to framing the area 13 corresponding to the
trench with a metallic strip with a width, for example, of 20
.mu.m: therefore, on the multilayer there are other areas 14 not
protected by the metallic material and not corresponding to
trenches to be etched. Then, one proceeds to the steps of
deposition and definition of possible structures 15 of the
integrated optical device.
[0028] At this point, as shown in FIG. 6, a thick protective layer
of non-metallic material, for example photoresist, is deposited and
a mask 16 is defined in it. Preferably, the layer of photoresist is
as thick as possible, even up to the maximum technological limit of
about 7 .mu.m. The mask 16 leaves uncovered a region substantially
larger than the area 13 not protected by the metallic mask 12.
Indeed, as can be seen, such a region comprises the area 13 and an
edge portion of the metallic mask 12. This uncovered edge is at
least wide enough to avoid problems due to alignment tolerances and
irregularities of the profile of the mask 16. In particular, it
should be noted that, with a layer of photoresist of such a
thickness, a profile of the walls of the openings can be very
irregular, faceted or even curved, for which reason, without this
provision, there is the risk of the photoresist partially covering
the area 13 left uncovered by the metallic mask 12. Therefore, the
function of the mask of photoresist 16 is to cover most of the
metallic masking layer, the areas 14 of the multilayer left
uncovered by the metallic mask 12 and the structures 15 possibly
deposited.
[0029] The next operation is an anisotropic attack preferably with
high-density plasma. The "resputtering" effect shall be much less
pronounced than that of the known process since no metallic part
with the exception of the edge close to the area 13 is directly
exposed to the attack. As can be seen in FIG. 7, at the edges of
the openings of the mask of photoresist there will be the usual
erosion effect of the edges 17, whereas, thanks to the metallic
mask 12, the anisotropic attack shall proceed with high selectivity
managing to etch a trench 18 that completely crosses the multilayer
in its total depth keeping side walls vertical.
[0030] The last two steps of the process, after which the
structures represented in FIGS. 8 and 9 are obtained, are the
removal, with known techniques, of the mask of photoresist 16 and
of the metallic mask 12, respectively.
[0031] A variant of the process according to an embodiment of the
invention allows an asymmetrical trench to be etched into the
multilayer, i.e. a trench that has, in the section view of the
figures, on one side, a substantially vertical wall and on the
opposite side a wall that is inclined, for example by
50.degree.-60.degree., with respect to the plane of the multilayer.
As can be seen in FIG. 10, after the intermediate step of
definition of the mask of photoresist, the profile of the mask 16
leaves uncovered a portion of just one of the edges of the metallic
mask 12 visible in the figure, in this case the edge to the left of
the area 13 corresponding to the trench to be etched, whereas it
totally covers the opposite edge to the right of the area 13, and
in part such an area.
[0032] As can be seen in FIG. 11, the anisotropic attack shall
proceed with high selectivity etching a trench 19 that completely
crosses the multilayer in its total depth with the vertical side
wall 20 on the left whereas on the right it shall have an oblique
wall 21. The "resputtering" effect shall be much less pronounced
both than that of the known process and even than that of the
process described previously in conjunction with FIGS. 5-9 since
the uncovered area of metal is also less than that of the previous
example. The erosion effect of the edges (walls 22) is present, but
it does not change the result of the process as far as the vertical
wall 20 is concerned, as is also described for the previous case.
However, such an effect is of operative importance to obtain the
oblique wall 21.
[0033] The last two steps of the process are the removal, with
known techniques, of the mask of photoresist and of the metallic
mask, respectively, and they allow the structure of FIG. 12 to be
obtained.
[0034] As can easily be seen, the described processes allow
trenches with vertical walls to be etched in the oxide avoiding the
drawbacks of the prior art. In particular, these processes can be
implemented also proceeding to a low-density plasma attack, thus
allowing the use of old-style machines having low costs and greater
availability, as explained previously.
[0035] An integrated circuit (IC) may incorporate one or more of
the structures of FIGS. 5-12 and an electronic system, such as an
optical communication system, may incorporate one or more of this
type of IC.
* * * * *