U.S. patent application number 09/908173 was filed with the patent office on 2003-01-23 for method of etching ferroelectric material.
Invention is credited to Gill, Douglas M., Jacobson, Dale Conrad.
Application Number | 20030015497 09/908173 |
Document ID | / |
Family ID | 25425319 |
Filed Date | 2003-01-23 |
United States Patent
Application |
20030015497 |
Kind Code |
A1 |
Gill, Douglas M. ; et
al. |
January 23, 2003 |
Method of etching ferroelectric material
Abstract
A method of etching a ferrolectric material is disclosed whereby
a single layer of ions is implanted into a surface of the film and
then, without first annealing the substrate, the material between
that surface and the layer of ion implantation is etched away. Such
a method has the benefit of being faster and much less costly as
compared to prior art methods. A single ion implantation of
sufficient energy causes a high level of electronic damage near the
surface of the material and a high level of crystalline damage at
the ion implant level. While it is well known that crystalline
damage greatly increases the etch rate of a ferroelectric material,
the inventors have discovered that the aforementioned electronic
damage also substantially increases the etch rate of the material.
Since damaged lithium niobate etches at a much faster rate then
undamaged lithium niobate, no annealing is necessary to create an
etch stop. Additionally, since there is sufficient damage, either
electronic or nuclear, continuously from the surface of the
material to the implant layer, multiple ion layer implantations are
not necessary.
Inventors: |
Gill, Douglas M.; (Hoboken,
NJ) ; Jacobson, Dale Conrad; (Allamuchy, NJ) |
Correspondence
Address: |
Docket Administrator (Room 3J-219)
Lucent Technologies Inc.
101 Crawfords Corner
Holmdel
NJ
07733-3030
US
|
Family ID: |
25425319 |
Appl. No.: |
09/908173 |
Filed: |
July 18, 2001 |
Current U.S.
Class: |
216/83 |
Current CPC
Class: |
G02B 6/136 20130101;
G02B 2006/12169 20130101; G02B 2006/12176 20130101; G02B 6/1347
20130101 |
Class at
Publication: |
216/83 |
International
Class: |
C23F 001/00 |
Claims
What is claimed is:
1. A method for use in etching structures in a substrate, said
method comprising the steps of: implanting a single layer of ions
into a substrate top surface using an energy sufficient to drive
the ions a predetermined depth into said substrate; and without
performing a preceding annealing step, etching with a chemical wet
etchant the substrate through said top surface of the substrate
until reaching the layer of ions at said predetermined depth from
said surface.
2. The method of claim 1 wherein said substrate is a lithium
niobate (LiNbO.sub.3) substrate.
3. The method of claim 1 wherein said top surface is the +z crystal
surface.
4. The method of claim 1 wherein said top surface is the -z crystal
surface.
5. The method of claim 1 further comprising, prior to said etching
step, implanting at least a second layer of ions into said
substrate top surface using an energy sufficient to drive the ions
to a level of minimum combined electronic and nuclear damage within
said substrate.
6. The method of claim 1 wherein at least one portion of said top
surface of said substrate is masked in such a way that the ions do
not penetrate into the substrate underlying said mask.
7. The method of claim 6 wherein said etching step comprises
exposing said top surface to a chemical wet etchant for a length of
time necessary to etch away at least a portion of the lithium
niobate to form a predetermined structure in said lithium
niobate.
8. The method of claim 7 wherein said structure is a ridge.
9. The method of claim 1 wherein said implanting step comprises
tilting the substrate in such a way that the ions form reentrant
angles between the base and the side walls of said substrate.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to etching structures into
ferroelectric materials.
BACKGROUND OF THE INVENTION
[0002] Lithium nobate (LiNbO.sub.3) and other ferroelectric
materials are often used as hosts for waveguides in various optical
devices such as optical switches, electro-optic modulators, and
optical wavelength conversion devices. In these applications, it
can be particularly advantageous to be able to form either a
relatively thin (i.e., <10 .mu.m) film layer of such material,
due to its strong optical confinement properties and large optical
nonlinearities, and/or to etch structures into the surface of the
ferroelectric material, such as ridges or gratings.
[0003] Many techniques have been used in the past to form suitable
waveguide structures in lithium niobate, such as thin film
waveguides and ridge waveguides. When forming such waveguides, the
lithium niobate is commonly etched, i.e., parts of the lithium
niobate are removed in order to form the desired structure.
Specific well-known etching techniques used in the past include ion
milling, plasma etching, and ion implant wet etching. Ion milling
and plasma etching both involve bombarding lithium niobate with
ions and either a) physically sputtering away portions of that
surface (i.e., ion milling), or b) causing a chemical reaction
between the ions and the surface of the material such that volatile
species are formed, resulting in etching of that surface (i.e.,
plasma etching). On the other hand, prior processes characterized
by ion implantation followed by wet etching formed waveguides by
bombarding either the -z crystal surface or the +z crystal surface
of the lithium niobate with a single implant of ions, for example
oxygen ions. Depending upon the structure to be created, various
areas of the bombarded surface could be masked to prevent ion
implantation in those areas. The depth at which the ions are
implanted is dependent upon the energy to which the ions are
accelerated and is typically <10 .mu.m. When accelerated to a
particular energy, the ions are implanted into the lithium niobate
at a particular depth, hereafter referred to as the implant layer,
that is proportional to that particular energy with little damage
to the crystalline structure of the lithium niobate between the
bombarded surface and that depth. For example, oxygen ions
implanted at an energy of 3.6 MeV have been found to produce an
implanted layer approximately 2 .mu.m below the surface.
[0004] While the majority of the damage to the crystalline
structure is near the implant layer in such implanting methods, it
has been noted that electronic damage may occur in the region
between the surface and that implant layer. To remedy this
electronic damage, prior processes annealed (heated) the
ion-implanted material. Such annealing also had the simultaneous
effect of creating, at the oxygen ion implant layer, a layer of
material that was highly resistant to etching, known as an "etch
stop." After this annealing, mechanical polishing and/or a chemical
wet etchant was applied to the -z side of the lithium niobate and
the lithium niobate between the -z surface and the ion-implant etch
stop was removed, leaving either a thin film or a more complex
structure formed as a result of the aforementioned masking.
[0005] Variations in the ion implant process to create a structure
in lithium niobate, for example a ridge waveguide, have been
developed. For example, the aforementioned plasma etching method
has been used in conjunction with a single oxygen ion implant step
with no annealing step between the implant step and the etching
step. Additionally, one prior art process known as crystal slicing
utilizes a single hydrogen or helium ion implant, as opposed to an
oxygen ion implant, followed by a rapid thermal annealing process.
Such rapid annealing is characterized by a rapid temperature
increase to a specific temperature (e.g., 800.degree. C.) followed
by a rapid decrease back to the starting temperature. Instead of
producing an etch stop at the ion implant layer, the damaged
material at that layer after the annealing is characterized by a
much faster etch rate than the surrounding material. By then
immersing the ion-implanted layer in a chemical etchant, the
damaged material is etched away, thereby undercutting the material
between the ion-implanted surface and the ion implant layer. That
material can then be readily removed thereby exposing the desired
structure in the ferroelectric material.
[0006] Another ion-implant method of creating a structure in
lithium niobate, for example a ridge waveguide, masks the ridge
portion of the lithium niobate and, instead of performing a single
ion implant, uses several ion implants at various depths to create
a relatively continuous volume of nuclear damage which, in turn,
results in significant damage to the crystalline structure of the
lithium niobate. Either surface (the +z or the -z crystal surface)
may be used in this process. The wet etchant is then applied to the
implanted surface to etch away the damaged material, leaving a
ridge of undamaged lithium niobate. This method has the advantage
of eliminating the need for annealing the lithium niobate after ion
implantation to reduce electronic damage. While annealing the
material after implantation was used in prior processes to form an
etch stop, such an etch stop is unnecessary when multiple layers of
crystalline damage are created. By knowing the etch rate of the
chemical when applied to the crystalline-damaged lithium niobate,
and emersing the material into the chemical etchant for a specific
time, a given etch depth can be achieved. Since undamaged lithium
niobate is etched at a much slower rate than is crystalline damaged
lithium niobate, it is not necessary when using these prior
processes to precisely control the etch time. Therefore, it would
appear that such a method of performing ion implants at multiple
levels without subsequent annealing, followed by chemical etching,
would have benefits over prior processes that use an intermediate
step of annealing. However, such multiple-level ion implants are
very costly and take a great deal of time to accomplish as compared
to single-layer implant methods.
SUMMARY OF THE INVENTION
[0007] The present inventors have discovered that various optical
waveguide structures may be formed in a ferroelectric material,
such as lithium niobate, by implanting only a single layer of ions
into a surface of the film and then, without first annealing the
substrate, etching away the material between that surface and the
single layer of ion implantation. Such a method has the benefit of
being faster and much less costly as compared to prior art
methods.
[0008] When ions are implanted into a ferroelectric material, two
different types of damage occur: nuclear and electronic. When ions
are implanted with sufficient energy into lithium niobate and other
ferrolectric materials, crystalline damage due to nuclear
interaction associated with this implantation is very low until the
ions begin to slow. At this point the ions begin to collide with
the atoms of the physical crystalline structure of the material and
cause a layer of nuclear damage at a depth dependent upon the
initial energy of the ion bombardment. This relationship is such
that the higher the initial energy of the ions, the deeper those
ions travel within the substrate prior to creating the layer of
nuclear damage.
[0009] While the crystalline damage is low until the energy of the
ions begins to dissipate, electronic damage is at its maximum near
the surface into which the ions are implanted. This electronic
damage gradually declines in magnitude as the energy of the ions
declines. Therefore, after ion implantation, the resultant
structure is characterized by a relatively high level of electronic
damage and low crystalline damage near the ion-implanted surface.
However, as one goes deeper into the substrate away from the
bombarded surface, the electronic damage decreases and the
crystalline damage increases until, at the implant layer, the
electronic damage is at a minimum and the crystalline damage is at
its maximum.
[0010] While it is well known that crystalline damage greatly
increases the etch rate of a ferroelectric material, the inventors
have discovered that the aforementioned electronic damage also
substantially increases the etch rate of the material. Thus, a
single implant layer of ions to a specific depth results in damage
that facilitates the etching process without requiring an
intermediate annealing step between the implant and etching steps,
and also obviates the need for multiple layers of ion
implantations.
BRIEF DESCRIPTION OF THE DRAWING
[0011] FIG. 1 shows a prior art method of producing a thin film
waveguide;
[0012] FIG. 2 shows the method of FIG. 1 used to produce a ridge
waveguide;
[0013] FIG. 3 shows a prior art method of producing a ridge
waveguide wherein multiple ion implants are made to cause multiple
layers of crystalline damage in the substrate;
[0014] FIG. 4 shows a ridge waveguide in cross section produced
using a prior art ion milling method;
[0015] FIG. 5 shows a ridge waveguide in cross section produced
using a prior art plasma etching method;
[0016] FIG. 6 shows a ridge waveguide in cross section produced
using ion implantation followed by chemical wet etching;
[0017] FIG. 7 shows the method of the present invention whereby a
single ion implant is made;
[0018] FIG. 8 shows a graph of the nuclear and the electronic
damage caused by the method of FIG. 7; and
[0019] FIG. 9 shows a ridge waveguide formed by the method of FIG.
7 when it is prematurely removed from the chemical wet etchant.
DETAILED DESCRIPTION OF THE INVENTION
[0020] FIG. 1 shows a prior art method of producing a thin film
waveguide. With reference to that figure, the +z crystal surface
101 of a ferroelectric film 100, here exemplified by lithium
niobate (LiNbO.sub.3), is bombarded with ions, for example oxygen
ions, shown as arrows 102, with a particular energy such that they
are implanted at level 103, which is a distance d from surface 101.
Level 103 is proportional to the level of energy with which the
ions are implanted into the lithium niobate. The bombarded material
is then heated (at 400.degree. C., for example) to partially
restore the etch rate of the implant region between surface 101 and
level 103 and the material at level 103 becomes an etch stop. This
etch stop is a layer of lithium niobate with a much higher
resistance to etching than the surrounding material. Next, the -z
surface 104 of the substrate is removed by mechanical and/or
chemical etching using a chemical etchant such as HNO.sub.3:HF
(2:1), or RCA1. During this etching, the lithium niobate between
the -z surface 104 and etch stop 103 is removed, leaving a slab
waveguide of lithium niobate of thickness d.
[0021] FIG. 2 shows how the above-described process is used in the
prior art to form a waveguide structure known as a ridge waveguide.
Referring to that figure, mask 205 is applied to the -z surface 201
of the lithium niobate 200 prior to ion bombardment. When bombarded
with ions 202, the ions are implanted at depth d below surface 201,
forming implant layer 203. The ions bombarding the masked portion
of surface 201, however, do not penetrate the surface of the
lithium niobate thus forming a region 206 of undisturbed material.
By then annealing the material an etch stop is formed at the
implant layer 203 such that a subsequent chemically etch will
remove material from surface 201, thereby forming ridge 206 on a
lithium niobate base.
[0022] FIG. 3 shows a prior art method of forming a ridge waveguide
through etching that does not require annealing to form an etch
stop at depth d.sub.N below the surface 301 of the lithium niobate
slab. Here, mask 305 is affixed over a portion of the surface 301
that will form the ridge 306. Surface 301 is then bombarded with
appropriate energies to implant ions at multiple depths
d.sub.1-d.sub.N from surface 301 down to depth d.sub.N to form
layer 303. Such multiple implantation depths are achieved by
varying the energy of the ions from low, to cause crystalline
damage near surface 301 at depth d.sub.1, to high, to cause damage
down to depth d.sub.N at layer 303. The effect of such multiple
implantations is to create a relatively continuous amount of
crystalline damage throughout the ion-bombarded material from
surface 301 down to layer 303, except below mask 305. Instead of
etching surface 304, mask 305 remains in place while surface 301 is
exposed to a chemical etchant. The crystalline damage in the
ion-implanted lithium niobate greatly increases the etch rate of
the material. By knowing the approximate rate of etching of the
chemical used on the damaged lithium niobate, the approximate etch
time required to reach layer 303 can be determined. A precise
timing of such etching is not necessary as the undamaged lithium
niobate between layer 303 and surface 304 is highly resistant to
etching relative to the crystalline-damaged lithium niobate.
Therefore, if left exposed to the chemical etchant for a period
modestly longer than necessary to remove the damaged material, very
little undamaged lithium niobate would be etched away. For example,
a typical rate of etching crystalline damaged lithium niobate using
HNO.sub.3:HF (2:1) is approximately 300 nanometers per minute. This
rate is merely exemplary as the etch rate greatly depends on the
magnitude of crystalline damage, the concentration of the etchant,
and the temperature of the etchant. The etch rate of undamaged
lithium niobate is approximately 10 nanometers per minute in
HNO.sub.3:HF (2:1). Therefore, using the above numbers, even if
left exposed to the chemical etchant for one minute longer than
necessary, only approximately 10 nanometers will be etched from the
undamaged material, which will have a negligible impact on the
ridge waveguide structure.
[0023] FIG. 4 shows an example of a ridge in cross section that
results from an alternative method of forming structures in
ferroelectric material, known as ion milling. In this method, ions
are not implanted into the ferroelectric material but, rather,
bombard the surface of the material with a lower energy such that
they impact the surface of the material and physically destroy
portions of that surface to remove material therefrom. This is
referred to as sputtering. Referring to FIG. 4, sides 402 of ridge
401 do not form a 90 degree angle with base 403 but, instead, form
an angle of approximately 80 degrees, which is typical of ridge
structures produced with ion milling. A 90 degree angle is
desireable for ridges used in electro-optic waveguide devices as it
facilitates concentration of the electrical field applied to such
waveguides and assists RF/optical velocity matching in the
interaction region of the electro-optic device
[0024] FIG. 5 shows an example of yet another method of producing a
ridge, shown here in cross section, known as plasma etching. In
this method, a gas is partially ionized, thereby forming ions and
radicals that are more chemically reactive with the lithium niobate
than the inital gas molecule. The ions are accelerated to the
substrate surface where, facilitated by the chemically reactive
radicals, they can react with that surface and sputter away
portions thereof. Fluorocarbon gases have been shown to be
effective in this process to etch ridges in lithium niobate. Once
again, however, the side walls 502 of ridge 501 form an angle of
less than 90 degrees with base 503, in this case approximately 70
degrees. This angle is typical of waveguides produced with this
method and, for the aformentioned reasons, is not as desirable as a
90 degree angle between the side walls and the base.
[0025] FIG. 6 shows an illustrative ridge 601 in cross section
produced using the ion implant etching process exemplified in FIGS.
1-3, and discussed above. Such a ridge is characterized by side
walls 602 that are at approximately the desirable 90 degree angles
from base 603. Such a ridge is also characterized by reentrant side
wall angles 604 where sides 602 meet base 603. Such side wall
angles can have the advantage in electro-optic devices of
beneficially further concentrating an electric field within the
waveguide. This advantage is directly related to the size of angle
604 such that, as angle 604 decreases (i.e., the side walls become
more reentrant), the concentration of the electric field
beneficially increases.
[0026] FIG. 7 shows the ion implant etch process of the present
invention whereby a single ion implantation 702 is performed on
lithium niobate 700 through surface 701 with sufficient energy to
penetrate to depth d to form a single implant layer 703 and
wherein, without performing an intermediate annealing step, the
ion-implanted lithium niobate 700 is then subjected to a chemical
wet etch. By masking the portion 707 of surface 701 over the region
that will form ridge 706, ions are prevented from penetrating this
region. Referring to FIG. 8, the result of a single ion implant to
depth d below surface 701 in FIG. 7 is graphically illustrated as
the electronic damage, represented by line 801, and nuclear
(crystalline) damage, represented by line 802, caused by this
implant. Axis 804 represents the electronic energy loss represented
by line 801 at a depth from the surface represented by axis 805.
High levels of electronic energy loss equate to high levels of
electronic damage within the lithium niobate structure. Thus, the
highest level of electronic damage is near the surface, and
declines according to graph 801 until it reaches a minimum at a
depth, in this case, of approximately 35000 .ANG.. Axis 803
represents the nuclear energy loss represented by line 802 at a
depth from the surface by axis 805. For a given implant ion, target
material and energy applied to ion implantation, the axes 803 and
804 have a relationship with each other. Nuclear energy loss occurs
as the ions interact with, and damage, the crystalline structure of
the lithium niobate. Such damage also varies with the depth into
the lithium niobate. However, in this case, a minimum level of
damage is incurred when the energy of the implanted ions is at a
maximum near the surface 701. As the depth increases and the ion
energy begins to dissipate, the ions begin to have a greater degree
of nuclear interaction with the crystalline structure of the
material. As such interaction increases, crystal damage increases
(i.e., the crystal structure is significantly disrupted). Finally,
as the ions reach their implant depth, the nuclear energy loss of
the ions greatly increases. The result is a peak level of
interaction, equating to a maximum level of disruption to the
crystalline structure at point 807 on line 802. The nuclear energy
loss drops off quickly after the ions slow upon reaching the
implant layer between -30000 .ANG. and -35000 .ANG.. Both
aforementioned types of damage, crystalline and electronic, enhance
the etch rate of lithium niobate in such a way that no intermediate
anealing step is required after the ions are implanted and before
the material is etched. Of special note in FIG. 8 is that total
structural damage to the lithium niobate, consisting of a
combination of crytalline and/or electronic damage, is at a minimum
at point 806.
[0027] FIG. 9 shows a ridge waveguide 901 on base 904 that
exemplifies the potential effect of a minimum energy-occurring at
point 806 in FIG. 8. The ridge profile of FIG. 9 results from
chemical wet etching lithium niobate that was implanted with ions
906 to achieve the electronic and nuclear damage represented by
FIG. 8. In FIG. 9, the etch rate along ridge side walls 902 of the
lithium niobate is increased by the electronic damage represented
by line 801 in FIG. 8. Thus the damaged lithium niobate is readily
etched away leaving side walls 902. When total damage to the
lithium niobate is at a minimum, however, represented by point 806
in FIG. 8, the etch rate is also at a corresponding minimum.
Therefore, if the material is removed from the chemical wet etchant
prematurely, proboscises 903 of lithium niobate may remain because
the etch rate at depth d, corresponding to the depth of point 806
in FIG. 8, is lower than that of the surrounding material that is
characterized by greater electronic and/or nuclear damage. As the
depth into the lithium niobate increases past the point of minimum
total damage 806 in FIG. 8, crystalline damage increases and,
correspondingly, the etch rate increases and forms side walls 905,
where crystalline damage is at its maximum, represented by point
807 in FIG. 8. Thus, for a given amount of time exposed to a
chemical wet etchant, the material at depth d will etch less than
surrounding areas, which will etch relatively quickly to form side
walls 902 and 905 before proboscis 903 is etched away. Therefore,
the minimum amount of etching time required to produce a ridge
waveguide using the process of the present invention is that time
necessary to etch away proboscis 903.
[0028] The foregoing merely illustrates the principles of the
invention. It will thus be appreciated that those skilled in the
art will be able to devise various arrangements that, although not
explicitly described or shown herein, embody the principles of the
invention and are within its spirit and scope. Furthermore, all
examples and conditional language recited herein are intended
expressly to be only for pedagogical purposes to aid the reader in
understanding the principles of the invention and are to be
construed as being without limitation to such specifically recited
examples and conditions. Moreover, all statements herein reciting
aspects and embodiments of the invention, as well as specific
examples thereof, are intended to encompass functional equivalents
thereof.
[0029] Diagrams herein represent conceptual views of bullk lithium
niobate, lithium niobate films and waveguides produced for use in
integrated optoelectronics. Diagrams of such ferroelectric films
and waveguides are not necessarily shown to scale but are, instead,
merely representative of possible physical arrangements of such
components. The lithium niobate films depicted in the diagrams
represent only one material suitable for use in waveguides. Many
other ferroelectric materials are suitable as a substitute for use
in the method shown in the diagrams.
[0030] Additionally, while in certain prior art methods etching was
optimal when accomplished from the -z crystal surface, the method
of the present invention does not depend on etching from any
specific surface. The electronic and crystalline damage caused by
the ion implantation sufficiently enhances the etch rate that even
etching the crystal from the +z surface is acceptable.
[0031] Other aspects of the disclosed embodiments of the present
invention are also merely illustrative in nature. For instance,
although the embodiment presented utilizes a single ion implant
followed by a chemical wet etch, the inventors realize that a
second implant at the level of minimum damage represented by point
806 in FIG. 8, could enhance the etch rate at that level, thus
minimizing the time necessary to eliminate the proboscis 903 of
FIG. 9 that may result from premature removal of the lithium
niobate from the chemical etchant. Additionally, as discussed in
the disclosed imbodiments, the inventors recognize that reentrant
side walls are advantageous to concentrate the electric field
within the waveguide and that such concentration beneficially
increases as the reentrant side wall angles decrease. One method of
decreasing these angles is to tilt the lithium niobate during the
ion implantation process in such a way that the ions penetrate
further into the side walls and, accordingly, decrease the angle
between the side walls and the base of the material. Any method of
increasing the reentrant side wall angles is intended to be
encompassed by the present invention. Finally, any method of using
a single ion implantation followed by a chemical wet etchant with
no prior annealing is intended to be encompassed by the present
invention.
* * * * *