U.S. patent application number 09/725308 was filed with the patent office on 2002-07-25 for patterning of gan crystal films with ion beams and subsequent wet etching.
Invention is credited to Carosella, Carmine, Molnar, Bela, Schiestel, Stefanie.
Application Number | 20020096496 09/725308 |
Document ID | / |
Family ID | 24914012 |
Filed Date | 2002-07-25 |
United States Patent
Application |
20020096496 |
Kind Code |
A1 |
Molnar, Bela ; et
al. |
July 25, 2002 |
Patterning of GaN crystal films with ion beams and subsequent wet
etching
Abstract
The invention provides a method for etching gallium nitride
(GaN) comprising the steps of: providing a GaN film; imagewise
amorphizing a portion of the GaN film by ion implantation to form
an amorphized portion; and wet etching of the GaN film having an
amorphized portion to remove the amorphized portion. When the
imagewise amorphizing process can be done without a mask, such as
with a focused implantation ion beam, the process itself becomes
maskless.
Inventors: |
Molnar, Bela; (Alexandria,
VA) ; Schiestel, Stefanie; (Laurel, MD) ;
Carosella, Carmine; (Falls Church, VA) |
Correspondence
Address: |
Associate Counsel (Patents)
Code 1008.2
Naval Research Laboratory
Washington
DC
20375-5000
US
|
Family ID: |
24914012 |
Appl. No.: |
09/725308 |
Filed: |
November 29, 2000 |
Current U.S.
Class: |
216/87 ;
257/E21.032; 257/E21.22 |
Current CPC
Class: |
H01L 21/30612 20130101;
H01L 21/0279 20130101 |
Class at
Publication: |
216/87 |
International
Class: |
C23F 001/00 |
Claims
What is claimed is:
1. A method for etching gallium nitride (GaN) comprising: providing
a GaN film; imagewise amorphizing a portion of said GaN film by ion
implantation to form an amorphized portion; and wet etching of said
GaN film having an amorphized portion to remove said amorphized
portion.
2. The method of claim 1, wherein said GaN film is a wurzite GaN
single crystal film grown either by chemical vapor deposition (CVD)
or molecular beam epitaxy (MBE) on a c-plane oriented sapphire
substrate.
3. The method of claim 1, wherein said GaN film is between 1 and 15
micrometers thick.
4. The method of claim 1, wherein said GaN film is
semi-insulating.
5. The method of claim 1 wherein said imagewise amorphizing
comprises implanting said GaN film with either B.sup.+ or Ar.sup.+
ions.
6. The method of claim 1 wherein said imagewise amorphizing
comprises the use of a focused ion beam to form an image, such that
an imagewise mask is not required.
7. The method of claim 1 wherein, in said imagewise amorphizing,
said ion implantation is carried out at an ion energy between about
30 and 180 keV.
8. The method of claim 1 wherein, in said imagewise amorphizing,
said ion implantation is carried out with an ion dose from about
1.times.10.sup.14 to 5.times.10.sup.16 ions/cm.sup.2.
9. The method of claim 1 wherein, after said imagewise amorphizing,
said amorphized portion is thermally annealed.
10. The method of claim 9 wherein said amorphized portion is
covered by an encapsulant layer before being thermally
annealed.
11. The method of claim 10 wherein said encapsulant layer is an AlN
film.
12. The method of claim 1 wherein said wet etching comprises the
use of the photoresist developer that comprises an aqueous solution
of KOH or NaOH.
13. The method of claim 12 wherein said photoresist developer is
Az-400K.
14. The method of claim 1 wherein said wet etching is carried out
at a temperature between about room temperature and 80.degree.
C.
15. The method of claim 1 wherein, in said wet etching, an etching
rate of from about 5 to about 700 Angstroms/min is achieved.
16. The method of claim 1 wherein, in said wet etching, an etching
rate of from about 500 to about 700 Angstroms/min is achieved.
17. The method of claim 1 wherein, in said wet etching, the etching
depth substantially corresponds to the amorphized portion of said
GaN film.
18. A device fabricated in accordance with the method of claim 1.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a method for etching
nitride and, more particularly, to a wet-etching method for gallium
nitride (GaN) enhanced by an imagewise amorphizing step.
DESCRIPTION OF RELATED ART
[0002] The semiconductor GaN is currently the subject of interest
in optoelectronics and in high-power, high-temperature device
operations circuits. Specific GaN-based device technologies include
light emitting diodes (LEDs), laser diodes, and UV detectors on the
photonic side and, on the electronics side, microwave power and
ultrahigh power switches. With respect to electronic devices for
microwave power applications particularly, one of the main
improvements needed is low damage etching that maintains surface
stoichiometry.
[0003] AlN and Al-rich alloys can be wet etched in KOH at
temperatures of 50-100.degree. C. However, under normal conditions,
only molten salts such as KOH or NaOH at temperatures above about
250.degree. C. have been found to etch GaN at practical rates and,
even then, only in certain crystallographic planes. Table I below
shows a comparison of etching results for GaN with that obtained
with other substrates in a number of acid and basic solutions,
performed at room temperature (25.degree. C.) unless otherwise
noted.
1TABLE I Solution GaN InN AlN InAlN InGaN Oxalic acid O Lifts off
Lifts off Lifts off Lifts off (75.degree. C.) Nitric acid O Lifts
off Lifts off Lifts off Lifts off (75.degree. C.) Phosphoric O O
Oxide Oxide O acid (75.degree. C.) removed removed Hydrofluoric O
Lifts off O O Lifts off acid Sulfuric acid O Lifts off O O O
(75.degree. C.) Sodium O Lifts off Lifts off Lifts off Lifts off
hydroxide Potassium O Lifts off 22,650 O O hydroxide Angst/min
AZ400K O Lifts off 60-10,000 Com- O Photoresist Angst/min position
developer dependent (75.degree. C.) Nitric acid/ O Lifts off O O O
potassium triphosphate (75.degree. C.) Nitric/boric O Lifts off O O
Lifts off acid (75.degree. C.) Nitric/boric/ O Lifts off O O
Removes hydrogen oxide peroxide HCl/H.sub.2O.sub.2/ O Lifts off O
Lifts off Lifts off HNO.sub.3 Potassium O Oxide Oxide Oxide Oxide
tetraborate removal removal removal removal (75.degree. C.)
[0004] The difficulty of handling the very strong etching solutions
that are required to etch GaN and the inability to find masks that
will hold up to them has limited the application of wet etching in
GaN device technology. Pearton et al., "GaN: Processing, Defects,
and Devices", Journal of Applied Physics, 86 (1) at 1, 25-26
(1999).
[0005] Photochemical etching of n-GaN using 365 nm illumination of
KOH solutions near room temperature has been reported, and rates of
3000 Angstroms/min have been obtained for light intensities of 50
mW/cm.sup.2. Minsky et al., Appl. Phys. Lett., 68(11) at 1531
(1996); and C. Youtsey et al., Appl. Phys. Lett. 72(5), 560 (1998).
However, intrinsic and p-GaN do not etch even under these
conditions. And, when etching does occur, it is generally diffusion
limited and produces undesirably rough surfaces. Further, undercut
encroachment occurs in some small-scale features due to light
scattering and hole diffusion in the GaN itself. Pearton et al. at
25.
[0006] Due to the limited wet chemical etch results for the
group-III nitrides, such as GaN, a significant amount of effort has
been devoted to the development of dry etch processing. Dry etch
development was initially focused on mesa structures where high
etch rates, anisotropic profiles, smooth sidewalls, and equirate
etching of dissimilar materials were required. For example,
commercially available LEDs and laser facets for GaN-based laser
diodes have been patterned using reactive ion etch (RIE). However,
as interest in high power, high temperature electronics has
increased, etch requirements have expanded to include smooth
surface morphology, low plasma-induced damage, and selective
etching of one layer over another. Dry etch development has been
further complicated by the inert chemical nature and strong bond
energies of the group-III nitrides, as compared to other compound
semiconductors. GaN has a bond energy of 8.92 eV/atom, InN 7.72
eV/atom, and AlN 1N 11.52 eV/atom, as compared to GaAs, for
example, which has a bond energy of 6.52 eV/atom. Pearton et al. at
28.
[0007] Dry plasma etching has become the dominant patterning
technique for the group-III nitrides due to the shortcomings of wet
chemical etching. Plasma etching proceeds by either physical
sputtering, chemical reaction, or a combination of the two that is
often referred to as "ion-assisted plasma etching." Physical
sputtering involves the acceleration of energetic ions formed in
the plasma to the substrate surface at relatively high energies,
typically >200 eV. Due to the transfer of energy and momentum to
the substrate, substrate material is then ejected from the surface.
However, sputter mechanisms can result in significant damage, rough
surface morphology, trenching, poor selectivity and
nonstoichiometric surfaces, thus minimizing device performance.
Pearton et al. at 28-29.
[0008] Chemically dominated etch mechanisms rely on the formation
of reactive species in the plasma, which adsorb to the surface,
form volatile etch products, and then desorb from the surfaces.
While plasma-induced damage is minimized due to the lower ion
energies used, etch rates in the vertical and lateral direction are
often similar, resulting in the loss of critical dimensions.
Alternatively, ion-assisted plasma etching relies on both chemical
reactions and physical sputtering to yield acceptable anisotropic
profiles at reasonably high etch rates. By balancing the chemical
and physical components, high resolution features with minimal
damage can be realized. Pearton et al. at 29.
[0009] Reactive ion etching (RIE) uses both the chemical and
physical mechanisms to achieve fast etch rates and dimensional
control. RIE plasmas are typically generated by applying a radio
frequency (rf) power of 13.56 MHz between two parallel electrodes
in a reactive gas. RIE results have been obtained for SiCl.sub.4-,
HBr-, CHF.sub.3- and CCl.sub.2F.sub.2-based plasmas with etch rates
typically <600 Angstroms/min. Plasma damage can occur and
degrade both electrical and optical device performance, and the
amount of this damage can only be reduced by slowing etch rates,
which significantly limits critical dimensions. Related processes
include the use of high-density plasma etch systems, such as
electron cyclotron resonance (ECR), inductively coupled plasma
(ICP) and magnetron RIE (MRIE); chemically assisted ion beam
etching (CAIBE) and reactive ion beam etching (RIBE); and low
energy electron enhanced etching (LE4). Pearton et al. at
29-30.
[0010] In all of these, dry etch characteristics are dependent upon
plasma parameters, including pressure, ion energy, and plasma
density. Other factors include the substrate temperature and the
volatility of the etch products formed. Due to the complexity of
dry etch processes, redeposition, polymer formation, and gas-phase
kinetics can also influence the results. Pearton et al. at
31-33.
[0011] Etch profile and etched surface morphology can be critical
to post-etch processing steps, including the formation of metal
contacts, the deposition of interlevel dielectric and passivation
films, or epitaxial regrowth. Sidewall morphology is especially
critical in the formation of laser mesas for ridge waveguide
emitters or for buried planar devices. The vertical striations
often observed in the GaN sidewalls are often due to striations in
the photoresist mask that are transferred to the GaN feature during
the etch. While GaN sidewall morphology etched in an ECR using a
SiO.sub.2 mask can be improved, this involves optimizing the
lithography process used to pattern the SiO.sub.2 and often
requires a low temperature dielectric overcoat to protect the
resist sidewall during the etch. Rough etch morphology often
indicates a nonstoichiometric surface due to preferential removal
of either the group III or group-V species. Pearton et al. at
36-37.
[0012] Plasma-induced damage also often degrades the electrical and
optical properties of compound semiconductor devices.
Nonstoichiometric surfaces, created by preferential loss of the one
of the lattice constituents, may be attributed to higher volatility
of the respective etch products, leading to enrichment of the less
volatile species or preferential sputtering of the lighter element.
Thus, reliable and well-controlled patterning of group-III nitrides
can be achieved by a variety of dry etch platforms but, to obtain
very smooth, anisotropic pattern transfer, a wide range of factors
for different plasma etch platforms, chemistries and conditions
must be carefully balanced to produce a fast etch rate, high
resolution features, and low damage. Pearton et al. at 37-39.
Further, the dry etching process is also equipment intensive and
relatively expense. In view of the foregoing, it is an object of
the invention to provide a method for etching a GaN substrate that
takes advantage of the relative simplicity and the dimensional
accuracy of wet etching equipment and methods.
[0013] Ion implantation has been an accepted method for integrating
certain devices into circuits. Specifically, implant isolation has
been widely used in compound semiconductor devices for interdevice
isolation, such as in transistor circuits or to produce current
channeling, such as in lasers. The implantation process can
compensate the semiconductor layer either by damage or chemical
mechanism.
[0014] Typically, there is a minimum dose (dependent on the doping
level of the sample) required for the chemically active isolation
species to achieve thermally stable compensation. Sometimes,
however, doping of GaN by ion implantation requires an encapsulant
layer, such as AlN, to minimize GaN decomposition during high
temperature activation annealing. The use of an encapsulant layer
then necessitates the selective removal of this layer after the
annealing treatment. The photoresist developer, AZ-400 K, has acted
as such a selective wet etching agent for AlN over GaN.
[0015] N implantation (at doses of about 10.sup.12-10.sup.13
cm.sup.-3) effectively compensates both p- and n-type GaN. For
example, Zolper et al., U.S. Pat. No. 5,866,925 issued Feb. 2,
1999, discloses the implantation of GaN films to produce--and
p-type regions using Si.sup.+ and Mg.sup.+/P.sup.+ ions and
subsequent annealing to activate the implanted ions. For both--and
p-type doping, the resistance typically first increases with
annealing temperature, then reaches a maximum before demonstrating
a significant reduction in resistance after a 850.degree. C. anneal
for n-type and a 950.degree. C. anneal for p-type GaN. This
behavior is typical of implant-damage compensation. The
implantation-induced defects in GaN appear to be more thermally
stable than other III-V semiconductor materials, such as GaAs or
InP, where the damage levels begin to anneal out below 700.degree.
C. Further work is still being done to understand more precisely
the nature of implantation damage in GaN. Pearton et al. at 39.
[0016] The inventors have now discovered that GaN can, in fact, be
selectively etched with conventional wet etching solutions directly
after ion implantation, allowing for the use of etchant materials
that are compatible within the semiconductor device manufacturing
industry. Further, if ion implantation can itself be performed in
an imagewise manner, such as with a focused ion beam, no masking or
additional patterning steps may be required. Imagewise, as used
herein, is meant to describe a process commonly known and used in
semiconductor technology in which ion implantation is controlled
and directed to a predetermined location on the GaN film.
SUMMARY OF THE INVENTION
[0017] According to a first aspect of the present invention, there
is provided a method for etching gallium nitride (GaN) comprising
the steps of: providing a GaN film; imagewise amorphizing a portion
of the GaN film by ion implantation to form an amorphized portion;
and wet etching of the said GaN film having an amorphized portion
to remove the amorphized portion.
[0018] The GaN film is preferably a wurzite GaN single crystal
film, grown either by chemical vapor deposition (CVD) or molecular
beam epitaxy (MBE) on a c-plane oriented sapphire substrate, having
a thickness of between 1 and 15 micrometers. The GaN film is also
preferably semi-insulating.
[0019] The imagewise amorphizing preferably includes implanting the
GaN film with either B.sup.+ or Ar.sup.+ ions utilizing a focused
ion beam to form an image. In a preferred embodiment, the ion
implantation is carried out at an ion energy between about 30 and
180 keV with a ion dose from about 1.times.10.sup.14 to
5.times.10.sup.16 ions/cm.sup.2. The amorphized portion may be
thermally annealed after said imagewise amorphizing, and may also
be covered by an encapsulant layer, for example an AlN film, before
being thermally annealed.
[0020] The wet etching includes the use of the photoresist
developer that comprises an aqueous solution of KOH or NaOH. A
preferred commercially available photoresist developer is AZ-400K.
The wet etching is preferably carried out at a temperature between
about room temperature and 80.degree. C., with an etching rate of
from about 5 to about 700 Angstroms/min and a most preferred rate
of about 500 to about 700 Angstroms/min is achieved. The etching
depth substantially corresponds to the aniorphized portion of said
GaN film.
[0021] According to a second aspect of the invention, there is
provided a GaN device made by the method of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] The invention will be described in greater detail with
reference to the following figures, wherein:
[0023] FIG. 1 is a micrograph of a GaN substrate treated with the
method of the invention;
[0024] FIG. 2 is a profilometer of a GaN substrate treated with the
method of the invention;
[0025] FIG. 3A shows the etching profiles of GaN, implanted with
100 keV Ar, in AZ-400K and KOH, at room temperature and 80.degree.
C.; FIG. 3B shows the etching profiles of GaN implanted with 40 keV
Ar and at different ion doses in KOH at 80.degree. C.;
[0026] FIG. 4 shows the dependence of etching depth on etching time
for the implantation of 100 keV Ar ions at different ion doses;
[0027] FIG. 5 shows damage calculations for the implantation of 100
keV Ar ions at different ion doses;
[0028] FIG. 6 is a graphic representation of the dependence of
etching depth on etching time for different Ar ion energies;
[0029] FIG. 7 is a cross-sectional view illustrating the formation
of a buried etchable region; and
[0030] FIG. 8 is a cross-sectional view of a waveguide made from
the process of FIG. 7.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0031] The GaN film useful in the method of the invention comprises
GaN film crystalline material. Preferably, the GaN film is a
wurzite GaN single crystal film grown either by chemical vapor
deposition (CVD) or molecular beam epitaxy (MBE) on a c-plane
oriented sapphire substrate. Typically, the GaN used in the
invention is semi-insulating, MOCVD-grown material available from a
number of commercial suppliers.
[0032] The GaN film may be undoped, and thus highly resistive
semi-insulating or doped with various dopants, such as Si and Mg to
produce a conductive--or p-type area.
[0033] The thickness of the GaN film can vary widely, but typically
varies between about 1 and 15 micrometers. Preferably, the GaN film
is about 1-3 micrometers in thickness and is formed on a c-plane
sapphire substrate. However, any substrate that matches the desired
lattice parameters such as a thin interlayer of AlN may be
utilized. The surface roughness is preferably about 100
Angstroms.
[0034] Imagewise Amorphizing Step
[0035] The GaN film of the invention is then subjected to ion
implantation. Ion implantation induces damage in GaN proportional
to ion energy and ion dose, and the inventors have discovered that
the damaged area can be removed in a subsequent wet etching
process. The etching depth depends on the ion energy used, as well
as on the ion dose, and can be easily controlled. However, a
certain level of damage (about 5 displacements per atom) is
necessary to initiate the etching.
[0036] Useful types of implantation ions for this amorphizing step
include B.sup.+, Ar.sup.+, Si.sup.+, Ga.sup.+, Mg.sup.+ and
He.sup.+. However, the preferred ions for implantation are Si.sup.+
or Ar.sup.+ ions, or Ga.sup.+ for focused ion beam (FIB) systems.
The process of ion implantation and the use of a focused ion beam
system for this purpose are commonly available. Argon ion energies,
for example, vary widely but typically range from about 30 to about
400 keV, preferably from about 40 to about 180 keV. A particularly
preferred ion energy is from about 50 to about 100 keV. High energy
implantation of Si ions will be significantly higher, for example,
in the range of about 2-4 MeV.
[0037] The ion dose also varies widely but typically ranges from
about 1.times.10.sup.14 to about 5.times.10.sup.16 ions/cm.sup.2
for Ar ions, preferably ranging from about 2.times.10.sup.15 to
about 5.times.10.sup.16 ions/cm.sup.2. Ion dosages for Si ions are
about the same as for Ar ions.
[0038] The process of the invention is independent of
crystallographic directions in GaN, because the GaN is amorphized
by the ion beam. Further, although patterning of the GaN film is
almost always possible by ion implantation through a mask (followed
by etching), maskless patterning can also be achieved with a
focused ion beam, such as that provided by Micrion 2500. Further
still, implantation through masks offers the potential of
three-dimensional patterning of GaN.
[0039] Ion-implanted GaN films can be annealed after implantation
in a conventional tube furnace up to about 1000-1400.degree. C. for
about one minute in a N.sub.2/H.sub.2 gas mixture for ion
activation. However, any temperature that achieves activation,
which may be higher for p-type ions, can be utilized as long as the
ion is incorporated within the lattice structure.
[0040] Upon implantation, the implanted areas may show an elevation
without any etching step taking place. For example, such an
elevation may vary from about 500 to 1000 Angstroms, preferably
from about 800 to about 900 Angstroms.
[0041] Wet Etching Step
[0042] The wet etchant can be any one of a number of agents that do
not ordinarily etch GaN materials, for example, as shown above in
Table I. In a preferred embodiment, the wet etchant is either
approximately a 1M KOH solution or the photoresist developer
AZ-400K, which is a commercially available buffered KOH solution
commonly used in the semiconductor photolithographic process and to
remove AlN encapsulant layers on GaN crystalline films. However,
most preferably, ion-implanted GaN is selectively etched by
AZ-400K.
[0043] If ion implantation has resulted in a slight elevation, the
etching process first removes these elevated areas and then etches
down to a deeper depth. Observations of etching depth under various
ion-implanted conditions can be correlated with the number of
displacements per atoms (dpa) required for amorphization. A number
of about 4 dpa at the surface is typically necessary to initiate
the etching process. Therefore, etching behavior can be predicted
from calculations.
[0044] Etch rates as high as 5-700, preferably 500-700,
Angstroms/min. can be observed. Etching tends to take place in two
steps:
[0045] a. a rapid, linear removal of the damaged GaN to a depth
corresponding to the depth of the amorphous region, and then
[0046] b. a saturation of the etching depth.
[0047] Both the first linear etching as well as the final etching
depth depend on etching bath temperature, ion energy and ion dose.
Studies of etching solutions and etching bath temperatures show
that increasing etching bath temperatures also increases the
initial etch rate, as well as the final etching depth. For example,
while etching at 80.degree. C. may etch down to a depth of several
hundred Angstroms in the first minute, etching at room temperature
is customarily slower, for example, with the final etching depth
being reached in about ten minutes. Although temperatures can vary
widely, a range of from about room temperature to about 80.degree.
C. is preferred.
[0048] The thickness of the material that easily etches away
increases with ion energy and ion dose, and decreases with the ion
mass. Etching behavior is predictable from TRIM calculations and
measurement of the dpa needed to amorphize GaN. The depth of
etching depends primarily on the implantation parameters and
increases with increasing ion dose and ion energy. Accordingly, the
final etching depth can be correlated with the depth and degree of
damage induced by ion implantation. Typically, however,
ion-implanted regions etch to a depth of about 150 nm, while
unimplanted regions normally show no significant etching. Deep
etches can be achieved by multiple implants. For example, multiple
implants at varying energies produce etching to a total depth of
about 1.5 microns.
[0049] Annealing of highly amorphized samples up to 1000.degree. C.
for about one minute in a N.sub.2/H.sub.2 gas mixture does not
reduce the etch rate, but the etch rate is often reduced by
annealing at lower ion doses.
[0050] Feature sizes down to about 2.5-10.0 nm are observed when
using a focused ion beam. Further, the roughness of the etched
surface is typically at least as good as the original surface.
EXAMPLES
Example 1
[0051] A semi-insulating, MOCVD-grown GaN film was obtained from a
commercial supplier.
[0052] The GaN film was about 3 micrometers thick and formed on a
c-plane sapphire substrate. The GaN was implanted with argon ions
at 100 keV, 5.times.10.sup.16 ions/cm.sup.2, through a mask having
square holes, 20 micrometers on a side. The GaN was then exposed to
the photoresist etchant AZ-400K, a commercially available buffered
KOH solution, at 80.degree. C. The ion-implanted squares (A) etched
rapidly (45 nm/min.) to a depth of about 150 nm and are shown in
FIG. 1. The unimplanted region (B) in FIG. 1 showed no etching. The
line (C) in FIG. 1 describes the path of a profilometer tip that
was pulled across the same to measure the surface profile.
[0053] The profilometer results of the etching process are shown in
FIG. 2. The etching created a very abrupt step (E). Further, the
unetched regions F1 and the etched regions F2 showed similar source
roughness, indicating little change from the original GaN surface
roughness.
Example 2
[0054] Undoped GaN films on a c-plane sapphire substrate were
obtained from Emcore Corporation. The film thickness was about 2
micrometers, and the surface roughness about 100 Angstroms. These
GaN films were implanted with Ar ion with ion energies varying from
about 40 to 4000 keV and at ion doses ranging from about
2.times.10.sup.15 to about 5.times.10.sup.16 ions/cm.sup.2. High
energy implantation of Si ions (2 MeV and 2.times.10.sup.16
ions/cm.sup.2) was also done. All samples were covered with a
transmission electron microscope (TEM) grid to create convenient
implanted versus unimplanted areas. The implantation with a focused
ion beam was performed with a Micrion 2500, using Ga ions of 50 keV
and ion doses ranging from about 2.times.10.sup.15 to 10.sup.16
ions/cm.sup.2. After implantation, the samples were immersed either
in the photoresist developer AZ-400K or 1M KOH, both at room
temperature and at 80.degree. C. The surface profiles were taken
with a KLA TENCOR Profilometer P10.
[0055] The unimplanted GaN areas did not show any etching from
either AZ-400K or KOH at 80.degree. C. In FIG. 3A, the etching
behavior for both etchants are shown for an ion energy of 100 keV
and an ion dose of 5.times.10.sup.16 ions/cm.sup.2. At both room
temperature and 80.degree. C., no significant difference between
the two etchants was observed. The etched surface showed about the
same surface roughness of about 100 Angstroms as the unimplanted
GaN. The implanted areas of these samples showed an elevation of
about 850 Angstroms. Etching at 80.degree. C. removed these
elevated areas and etched down to a depth of about 700 Angstroms in
the first minute, which produced a removal of 1550 Angstroms in
total. Etching at room temperature was slower, with the removal of
the elevated areas requiring about 2.5 minutes and the final
etching depth being reached about 10 minutes. FIG. 3B shows GaN
implanted with 40 keV Ar and different ion doses in KOH at
80.degree. C.
[0056] The etch profile at 80.degree. C. seemed to saturate after
10 minutes and then slightly increased. This increase in etching
depth after 30 minutes of etch time was accompanied by the
formation of pores with a depth of about 800 Angstroms. The final
etching depth amounted to about 1600-1700 Angstroms, which is about
500 Angstroms larger that the damage range (1100 Angstroms)
calculated with SRIM 2000 for these implantation conditions. To
better understand this phenomenon, a cross section TEM image of a
sample implanted with a 100 keV Ar ions and an ion dose of
5.times.10.sup.16 ions/cm.sup.2 was taken. These TEM results showed
four different regions could be observed:
[0057] Region (a) is Pt layer, which is provided as part of the
sample preparation process;
[0058] Layer (b) with a depth between 100 and 130 nm represents the
implanted regions, where Ar bubbles or voids can be observed;
and
[0059] Region (c), a deeper damage band of about 100 nm shows the
same crystalline structure as the underlaying GaN, region (d), but
with a higher defect density.
[0060] For long exposure times (.gtoreq.30 minutes), etching
extends into the deeper damage band (c).
[0061] For samples implanted with 100 keV Ar and.gtoreq.10.sup.16
ions/cm.sup.2, only the formation of pores up to 1500 Angstroms
could be observed. For this lower ion dose, the degree of
amorphization of the GaN was not sufficient to remove the damaged
layer in the subsequent process. Liu et al., Phys. Rev. B, 57(4) at
2530 (1998), has reported a threshold ion dose of 6.times.10.sup.15
Ar ions/cm.sup.2 of 180 keV for the amorphization of GaN, which
corresponds to about 4-6 dpa. SRIM calculations for 100 keV Ar ions
revealed an ion dose of about 10.sup.16 ions/cm.sup.2 as
sufficient, regarding the magnitude of damage (18 dpa), but located
at about 300 Angstroms below the surface. At the surface (50
Angstroms), the calculated number of dpa amounts to only 3, not
sufficient to start etching of the whole implanted area. To keep
damage close to the surface, implantations were performed with 40
keV Ar ions at ion doses ranging from about 2.times.10.sup.15 to
about 2.times.10.sup.16 ions/cm.sup.2 (2-20 dpa). The etching
profiles of these samples are shown in FIG. 3B. For this ion
energy, etching was also observed for an ion dose of
5.times.10.sup.15 ions/cm.sup.2 (5 dpa), but not for an ion dose of
2.times.10.sup.15 ions/cm.sup.2, as expected from SRIM.
[0062] GaN implanted with a higher ion energies, where damage is
buried more deeply, is not expected to be etched unless the region
of implantation is connected by channels to the surface in contact
with the etchant. For an ion energy of 400 keV, the formation of
pores could still be observed but, for an ion energy of 2 MeV, no
etching at all took place. Thus, for practical applications,
multiple implants with different ion energies are needed if etching
depths of 1-2 microns are desired. In another sample, an etching
depth of 1.5 microns was obtained after implantation of 100 keV and
400 keV Ar ions and 2 MeV Si ions.
[0063] The above-described process can be utilized to form well
defined device structures. As shown in FIG. 7, for example, 2 MeV
Si ions can be used to produce a buried etchable regions 12 in a
GaN substrate 10. The regions 12 ranges in thickness from 0.5 to
1.5 microns beneath the surface of the GaN substrate 10. Using
multiple implant methods described above, channels 14 are provided
from the regions 12 to the surface of the GaN substrate 10. When
etching occurs, the material in the regions 12 and the material in
channels 14 are removed to leave a well defined device structure.
For example, as shown in FIG. 8, the well defined free-standing
device structure 16 remaining after the process is completed
functions as a waveguide.
Example 3
[0064] An implantation with a FIB was performed to determine
whether implantation followed by wet etching would hold for smaller
feature sizes and could be done maskless. Lines of 25 to 200 nm
beam diameter (FWHM) were implanted with doses ranging from about
2.times.10.sup.15 ions/cm.sup.2 to 10.sup.16 ions/cm.sup.2. For the
heavier Ga ions, the amorphization level was reached at lower ion
doses and, therefore, etching took place for an implantation dose
of about 2.times.10.sup.15 ions/cm.sup.2. This process of FIB
implantation followed by wet etching was significantly more rapid
than ion milling would have been.
Example 4
[0065] A variety of GaN films were used to study the influence of
growth method, thickness, carrier concentration and mobility on
etching properties. In the first step, the as-grown GaN films were
immersed in AZ-400K at room temperature and at 80.degree. C. Some
of the GaN films could be etched as grown without any further
treatment. Table 2 summarizes the etching results for the
different, as-grown GaN films.
2TABLE 2 Carrier Thickness Growth Conc. (micro- Mobility 1 Hour 1
Hour Type (1/cm.sup.3) .times. 10.sup.17 meters) (cm.sup.2/Vs)
25.degree. C. 80.degree. C. MOCVD 1.5 2 -15 No No MOCVD 2.5 1.6 +9
No No HVPE 0.7 16.0 -630 No No MOCVD 3.0 3.1 -300 No No MOCVD S.I.
3.0 No No MBE 20.0 2.0 -230 Yes Yes MBE 0.1 2.8 Yes Yes MBE 1.0 1.0
No No MBE 10.0 0.3 +5 Yes Yes
[0066] Table 2 shows that none of the MOCVD GaN films could be
etched. In contrast, 3 out of 4 MBE-grown GaN films studied showed
an etching in AZ-400K. There did not appear to be any influence of
film thickness, carrier concentration or mobility on etching
behavior.
[0067] However, damaging by ion implantation clearly promoted
etching. Some of the MOCVD GaN films were implanted with B and Ar
ions of different energies and at various ion doses. The first
implantation experiments were performed with Ar ions with a
constant ion energy of 100 keV, while the ion dose was varied
between 10.sup.15 and 5.times.10.sup.16 ions/cm.sup.2. The
implanted GaN layers turned brown with increasing ion dose and, for
the highest ion dose of 5.times.10.sup.16 ions/cm.sup.2, a dark
brown, metallic shiny layer was obtain. Four-point probe
resistivity measurements on a semi-insulating GaN film implanted
with 5.times.10.sup.16 Ar ions/cm.sup.2 showed a conductance of
about 100 (ohm-cm).sup.-1.
[0068] After ion implantation, the GaN films were immersed in
AZ-400K at 80.degree. C. FIG. 4 shows the etching depth for
different Ar ion doses and etching times. A linear etching was
observed during the first five minutes for the highest ion dose
(5.times.10.sup.16 ions/cm.sup.2, circles). The surface roughness
of the etched area was comparable to the one of the untreated
surface and amounts to .+-.100 Angstroms. After five minutes, the
etching profile seemed to saturate at about 1400 Angstroms, which
roughly corresponded to the damage region induced by ion
implantation, which was calculated/estimated by TRM. After about 60
minutes, pores started to develop at the etched surface, up to
about 1500 Angstroms in depth. Simultaneously with the development
of pores at the etched surface, the etching depth increased
slightly more. The etching behavior for the highest implantation
dose of 5.times.10.sup.16 ions/cm.sup.2 differed from the ones
implanted at lower doses. For all samples implanted with an ion
dose of.ltoreq.10.sup.16 ions/cm.sup.2, the etching of the
implanted area was accompanied by the presence of deep pores (about
500-1500 Angstroms). The etching depth (saturation) decreased with
decreasing ion dose and was about 1200 Angstroms for 10.sup.16
ions/cm.sup.2 and about 500 Angstroms for 5.times.10.sup.15
ions/cm.sup.2. No etching, except for pore formation, occurred for
an ion dose of about 10.sup.15 ions/cm.sup.2. Pores with a depth up
to about 1000 Angstroms were present after one hour; they increased
to about 1500 Angstroms after another hour in AZ-400K at 80.degree.
C.
[0069] Tan et al., Appl. Phy. Lett. 69(16), 2364 (1996), has
reported amorphization of GaN after implantation of 90 keV Si ions
at ion doses >10.sup.16 ions/cm.sup.2 even at the temperature of
liquid nitrogen. At room temperature, the amorphization dose is
expected to be higher. For example, Liu et al., Nucl. Instr. Meth.
B 148, 396 (1999), have reported that 3.times.10.sup.14 Ca.sup.+
ions/cm.sup.2 at 180 keV are necessary to initiate amorphization of
GaN at 77.degree. K, but 8.times.10.sup.14 ions/cm.sup.2 are needed
at room temperature.
[0070] It has also been reported that GaN is amorphized at
77.degree. K with 180 keV Ar ions at about 5-6 dpa. Liu et al.,
Phys. Rev. B 57(4), 2530 (1998). The number of dpa for the ion
energies and ion doses of these experiments were calculated using
TRIM. The results of these calculations for 100 keV Ar ions and ion
doses from 10.sup.15 to about 5.times.10.sup.16 ions/cm.sup.2 are
shown in FIG. 5. For the lowest ion dose, the number of dpa was
less than one. Therefore, for this ion dose, no etching of the
implanted GaN was observed. For the higher ion doses, the depth of
the damage region roughly corresponded to the saturation level of
the etching depth.
[0071] TRIM calculations for the implantation of 1.times.10.sup.16
B.sup.+ ions/cm.sup.2 with ion energies of 30 and 100 keV revealed
a number of dpa at the damage peak of less than 3, too small to
amorphize GaN. Experimental results for GaN implanted with B ions
at these doses and energies revealed no etching.
Example 5
[0072] In another set of experiments, the annealing influence at
900.degree. C. and 1000.degree. C. in N.sub.2/H.sub.2 gas on
implanted GaN films (100 keV Ar ions, 10.sup.15 to
5.times.10.sup.16 ions/cm.sup.2) was examined. The etch rate for
the sample implanted at 5.times.10.sup.16 ions/cm.sup.2 before and
after annealing was the same, and almost all of the etching was
completed in the first five minutes. In contrast, the annealing
slowed the etch rates for the lower implantation doses
(<10.sup.16 ions/cm.sup.2), and no saturation was observed.
There was also a delayed onset of etching in these samples. At
10.sup.16 ions/cm.sup.2, etching commenced at 30 minutes; at
5.times.10.sup.15 ions/cm.sup.2, etching only began after one hour
in AZ-400K at 80.degree. C. Again, no etching at all was observed
for the lowest implantation dose of 10.sup.15 ions/cm.sup.2. From
these data, it was concluded that some of the amorphizing damage
was recoverable at the lower doses, but no recovery was possible at
the highest dose.
Example 6
[0073] To investigate the effect of ion energy on etching depth, Ar
ions of 30 or 180 keV were implanted. The predicted damage range
for 30 keV is 400 Angstroms and, for 180 keV, 2000 Angstroms. The
ion doses were chosen to result in a comparable dpa. They were
1.6.times.10.sup.16 ions/cm.sup.2 for 30 keV and
5.5.times.10.sup.16 ions/cm.sup.2 for 180 keV. In FIG. 6, the
results of these etching experiments are presented. The saturation
etching depth for 30 keV was 600 Angstroms and, for 180 keV, 2000
Angstroms. For 30 keV, etching of the implanted layer was observed
after two minutes, with complete removal after five minutes. At 180
keV, the implanted layer started to etch after five minutes and was
completely removed after ten minutes.
[0074] Additional advantages and modifications will readily occur
to those skilled in the art. Therefore, the invention in its
broader aspects is not limited to the specific details and
representative embodiments shown and described herein. Accordingly,
various modifications may be made without departing from the spirit
or scope of the general inventive concept as defined by the
appended claims and their equivalents.
* * * * *