U.S. patent application number 09/888780 was filed with the patent office on 2002-01-17 for surface planarization of high temperature superconductors.
This patent application is currently assigned to Epion Corporation. Invention is credited to Chu, Wei-Kan, Wu, Judy Z..
Application Number | 20020006877 09/888780 |
Document ID | / |
Family ID | 26723413 |
Filed Date | 2002-01-17 |
United States Patent
Application |
20020006877 |
Kind Code |
A1 |
Chu, Wei-Kan ; et
al. |
January 17, 2002 |
Surface planarization of high temperature superconductors
Abstract
Planarizing High Temperature Superconductor (HTS) surfaces,
especially HTS thin film surfaces is crucial for HTS thin film
device processing. Disclosed is a method of surface planarization
for HTS film. The method includes first smoothing the HTS surface
by Gas Cluster Ion Beam bombardment, followed by annealing in
partial pressure of oxygen to regrow the damaged surface layer. A
rough HTS surface can be planarized down to a smoothness with a
standard deviation of one nanometer or better.
Inventors: |
Chu, Wei-Kan; (Pearland,
TX) ; Wu, Judy Z.; (Lawrence, KS) |
Correspondence
Address: |
PERKINS, SMITH & COHEN LLP
ONE BEACON STREET
30TH FLOOR
BOSTON
MA
02108
US
|
Assignee: |
Epion Corporation
|
Family ID: |
26723413 |
Appl. No.: |
09/888780 |
Filed: |
June 25, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09888780 |
Jun 25, 2001 |
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09073786 |
May 6, 1998 |
|
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6251835 |
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60045967 |
May 8, 1997 |
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Current U.S.
Class: |
505/411 |
Current CPC
Class: |
C30B 29/225 20130101;
C30B 31/22 20130101; C30B 29/22 20130101; C30B 33/00 20130101; B82Y
15/00 20130101; H01L 39/2464 20130101 |
Class at
Publication: |
505/411 |
International
Class: |
B05D 001/00; B23K
001/00 |
Claims
What is claimed is:
1. A method of processing a substrate comprising a high temperature
crystalline superconductor surface, having a surface roughness
R.sub.0, the method comprising: (a) contacting the surface of the
substrate with gas cluster ions to reduce both the surface
roughness and the crystallinity of the surface; and (b) processing
the surface of the substrate to restore the crystallinity of the
surface.
2. The method of claim 1, wherein said processing step comprises
thermally annealing the surface to restore crystallinity.
3. The method of claim 1, wherein said processing step comprises
removing a shall ow layer of the surface to expose underlying
material having higher crystallinity.
4. The method of claim 1, wherein the superconductor surface
comprises a YBCO type of superconductor.
5. The method of claim 1, wherein the contacting in step (a)
comprises bombarding with gas cluster ions of high energy.
6. The method of claim 1 further comprising: (c) depositing a
single crystal layer on the surface.
7. The method of claim 1, wherein said contacting step comprises
bombarding said surface with gas cluster ions comprising a cluster
in a range between 10 and 30 KeV with a total dose of 10.sup.14 to
2.times.10.sup.16 clusters/cm.sup.2.
8. The method of claim 1, wherein said annealing step comprises
annealing the surface under a partial oxygen atmosphere.
9. The method of claim 8, wherein said annealing is carried out at
approximately 800.degree. C. under oxygen partial pressure of
approximately 100 mTorr for approximately 30 minutes followed by an
approximately 450.degree. C. and 200 mTorr oxygen pressure for
approximately 30 minutes.
10. The method of claim 8, wherein said annealing is carried out at
approximately 870.degree. C., 660.degree. C., and 450.degree. C.
consecutively for approximately 35 minutes, each under flowing
oxygen gas.
11. The method of claim 6 further comprising: (d) processing said
single crystal layer to configure structures thereon.
12. The method of claim 11, wherein said second processing step
comprises smoothing said single crystal layer, and annealing the
smoothed single crystal layer.
13. A method of processing a substrate comprising: (a) depositing a
crystalline high temperature superconductor surface layer on said
substrate, said layer having a surface roughness RO; (b) bombarding
said surface layer with gas cluster ions to reduce the surface
roughness to R.sub.1, wherein R.sub.1 is less than R.sub.0, and to
reduce the crystallinity of the surface layer; and (c) processing
the surface layer of the substrate to restore the crystallinity of
the surface layer.
14. The method of claim 13, wherein said processing step comprises
thermally annealing the surface to restore crystallinity.
15. The method of claim 13, wherein said processing step comprises
removing a shallow layer of the surface to expose underlying
material having higher crystallinity.
16. The method of claim 13, wherein said surface layer comprises a
YBCO type of superconductor.
17. The method of claim 13 further comprising: (d) depositing a
single crystal layer on the surface.
18. The method of claim 17, wherein said single crystal layer
comprises an insulating layer.
19. The method of claim 17, wherein said single crystal layer
comprises a superconducting layer.
20. The method of claim 13, wherein said contacting step comprises
bombarding said surface with gas cluster ions comprising a cluster
in a range between 10 and 30 KeV with a total dose of 10.sup.14 to
2.times.10.sup.16 clusters/cm.sup.2.
21. The method of claim 14, wherein said annealing step comprises
annealing the surface in partial oxygen.
22. The method of claim 21, wherein said annealing is carried out
at approximately 800.degree. C. under oxygen partial pressure of
approximately 100 mTorr for approximately 30 minutes followed by an
approximately 450.degree. C. and 200 mTorr oxygen pressure for
approximately 30 minutes.
23. The method of claim 21, wherein said annealing is carried out
at approximately 870.degree. C., 660.degree. C., and 450.degree. C.
consecutively for approximately 35 minutes, each under flowing
oxygen gas.
24. The method of claim 17 further comprising: (e) processing said
single crystal layer to configure structures thereon.
Description
PRIORITY INFORMATION
[0001] This application claims priority from provisional
application Ser. No. 60/045,967 filed May 8, 1997.
BACKGROUND OF THE INVENTION
[0002] The invention relates to processing high temperature
superconductor surfaces.
[0003] High Temperature Superconductor (HTS) thin films are
intrinsically not smooth. HTS thin films deposition is made mostly
by four different processes: pulsed laser ablation, magnetron
sputtering, vapor phase evaporation, and metal organic chemical
vapor deposition (MOCVD). In order to promote the epitaxy of the
HTS film on a selected substrate which in turn provides the
template of the crystalline growth, substrates are usually kept at
an elevated temperature during HTS film deposition. Due to the
nucleation and growth, islands or mesas are formed on the
substrates to produce a rough surface. Typical peak to valley
surface height variation ranges from 10 nm to 30 nm, and interpeak
distance ranges from 0.3 micron to 0.6 micron.
[0004] For the fabrication of high temperature superconductor thin
film devices, especially digital circuits or HTS interconnects in
multi-chip modules (MCM), a smooth surface is crucial to the
reliability and yield of the fabrication process. A rough surface
is detrimental to the growth and performance of additional layer(s)
on top of the film and becomes a reliability and yield killer to
the large scale production of thin film devices.
[0005] There are many different ways to planarize a surface. For
example, a non-smooth surface can be mechanically polished by
lapping the surface with or without filler material. A rough
surface, or a surface with steps, lines or structures can be
planarized by first filling the surface with photoresist,
spin-on-glass, or some polymer, then etched back chemically, or
etched back by use of ion beam sputtering. A rough surface can also
be smoothed by ion beam sputtering of the surface at a glancing
angle within 4 degrees from the surface.
[0006] All existing methods have their problems. Mechanical
lapping, and filling subject the surface to different materials
which complicate the process, and detract from the yield in IC
processing. Glancing angle sputtering is slow and difficult to
control. Due to the above mentioned problems on existing
planarization methods, HTS thin film devices are generally
fabricated without surface smoothing procedures. As a consequence,
devices have reliability and yield problems. The absence of a new
method to planarize the HTS surface is impeding the progress of HTS
thin film applications to devices.
[0007] It has been demonstrated that the bombarding effects of gas
cluster ions on solid surfaces are quite different from those
produced by monomer ion beams. Monomer ion beam sputtering is a
well established technology in ion etching for surface layer
removal, and in ion implantation. During monomer ion sputtering,
the target atoms are removed layer by layer, a process that does
not significantly alter the surface morphology. However, gas
cluster ions contain hundreds or even thousands of atoms or
molecules. The total energy of the cluster ranges from a few keV to
a few tens of keV, which corresponds to several eV to several tens
of eV per constituent atom. When the gas cluster hits the target
surface, the collective motions of the cluster atoms during impact
play dominant roles in the process kinetics. Computer simulation
using molecular dynamics has shown the collective effect of gas
cluster ion beams (GCIB) gives shallower, but massive damage 10 and
lateral sputtering. Due to the GCIB lateral sputtering effect,
surfaces of Au, Pt, Cu, poly-Si, SiO.sub.2, Si.sub.3N.sub.4, Si,
and other materials, can be smoothed.
[0008] All conventional GCIB applications to date have involved
semiconductors and metals. Demonstrations of shallow ion
implantation, surface smoothing, high rate sputtering, and thin
film deposition have been made. All cases demonstrated so far are
simple systems involving single elements such as poly-Si, or
diamond, Au, Pt, Cu, or binary compounds such as SiO.sub.2 or
Si.sub.3N.sub.4. High temperature superconductors such as
Y.sub.1Ba.sub.2Cu.sub.3O.sub.7 (YBCO) involve 13 atoms per unit
cell of four different elements, and are very complicated as
compared to the simple mono-element and binary compounds.
SUMMARY OF THE INVENTION
[0009] The invention involves applying GCIB to planarize HTS
surfaces. Specifically, it is an object of the invention to provide
the application of GCIB on YBCQ surface by varying beam parameters,
characterizing the treated surfaces, and producing smooth surfaces.
In addition, the invention utilizes Rutherford Backscattering
Spectrometry in combination with channeling effect to monitor the
surface damage due to GCIB, and includes methods to repair the
damage and make the surface useful for device processing. The
invention also involves the regrowth of an epitaxial YBCO layer on
top of the smoothed and repaired surface.
[0010] The invention provides a method of surface planarization for
HTS films. The method includes first smoothing the HTS surface by
Gas Cluster Ion Beam Bombardment, followed by annealing in a
partial pressure of Oxygen to regrow the damaged surface layer. A
rough HTS surface can be planarized down to a smoothness with a
standard deviation of one nanometer or better.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 is a flow chart of the HTS thin film planarization
processing in accordance with the invention;
[0012] FIG. 2 is an atomic force microscope (AFM) picture of a YBCO
film deposited by pulsed laser ablation;
[0013] FIG. 3 is an enlarged view of a 1 micron square area of FIG.
2;
[0014] FIG. 4. is an AFM picture of a surface smoothed using an Ar
gas cluster;
[0015] FIG. 5 is an AFM picture of a surface irradiated with
2.times.10.sup.16 clusters/cm.sup.2 with nearly all mountains or
peaks flattened;
[0016] FIG. 6 is a graph showing the average YBCO thickness removed
versus doses of the GCIB for a 30 keV Ar cluster beam;
[0017] FIG. 7 is a graph showing the thickness removed versus total
energy of a cluster at 5.times.10.sup.15 clusters/cm.sup.2 in which
two measurements were made using two different samples for 20 keV
and 30 keV GCIB;
[0018] FIGS. 8A-C are graphs of surface peaks of the channeling
spectra indicating the amount of surface damage before GCIB
bombardment (FIG. 8A), after GCIB smoothing (FIG. 8B), and after
GCIB smoothing and annealing at 870.degree. C., 660.degree. C. and
450.degree. C. consecutively for 35 minutes each under flowing
oxygen gas (FIG. 8C); and
[0019] FIG. 9 is a graph showing the resistance versus temperature
curve for a YBCO film before and after planarization with an Ar
GCIB at 20 KeV, 2.times.10.sup.16 clusters/cm.sup.2, followed by
annealing at 800.degree. C.
DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS
[0020] FIG. 1 is a flow chart of the GCIB surface smoothing and
damage repair sequence in accordance with the invention. In step
100, deposition of HTS films on a substrate is done in a
conventional manner. Various HTS films, such as Y-, T1, Hg-HTS
cuprates can be deposited on single crystalline or
multi-crystalline substrates, such as SrTiO.sub.3, LaAlO.sub.3,
MgO, or other substrates with or without buffer layers.
[0021] Deposition methods such as pulsed laser ablation, magnetron
sputtering, vapor phase evaporation, and metal organic chemical
vapor deposition (MOCVD) can be used. Typically, to obtain films
with excellent superconducting properties requires a heated
substrate at elevated temperatures during the deposition process,
and/or a post heat treatment of the deposit. Due to the nucleation
and growth process, the surface of the film is not smooth.
[0022] A typical landscape of a YBCO HTS film produced by pulsed
laser ablation on a LaAlO.sub.3 substrate is shown in FIG. 2. FIG.
2 is an atomic force microscope (AFM) picture of a YBCO film
deposited by pulsed laser ablation. The area coverage is 10 microns
square with height scale of 400 nm. The surface smoothness standard
deviation Ra=13 nm for large areas, and =9 nm for smaller areas
that excluded the large peaks. It will be appreciated that the
picture is made by AFM, which gives an amplified height scale. Due
to the variation of growth speed along various crystalline
directions during deposition, islands or mesas are formed on the
substrates, and they merge into a complete layer with rough
surfaces.
[0023] FIG. 3 shows a smaller area 1 micrometer square with a peak
height of 40 nm and Ra=7 nm. Typical peak to valley surface height
variations range around 10 nm to 30 nm with surface smoothness
standard deviation of between 5 nm to 10 nm. Additional debris and
impurity phases cause additional non-uniformity as seen in FIG. 2.
Debris and impurity phases (peaks as high as 20 nm to 60 nm) can be
avoided by processing control. However, in the deposited film. the
surface is intrinsically not smooth such as shown in FIG. 2, and
enlarged in FIG. 3.
[0024] The method of the invention continues in FIG. 1 with a gas
cluster ion beam bombardment of the HTS surface in order to smooth
the rough surface (Step 102). GCIB bombardment requires the use of
a GCIB accelerator. Description of such equipment is provided in
Yamada et al., "Lateral Sputtering of Gas Cluster Ion Beams and its
Application to Atomic Level Surface Modification", Mat. Res. Soc.
Symp. Proc., vol. 396. pp. 149-154 (1996), incorporated herein by
reference. Briefly, the cluster beams of the gases, such as Ar, Ne,
N.sub.2, O.sub.2, CO.sub.2, SF.sub.6, etc. are formed by supersonic
expansion through a small nozzle into high vacuum. Collimated
neutral gas cluster beams are ionized by electron bombardment in a
multiple-stage high-vacuum chamber. Ionized cluster beams are
extracted and particular sizes of cluster ions can be selected by
an electrostatic lens system consisting of several electrostatic
field plates with small apertures. The mass selected cluster ions
are then electrostatically accelerated towards the target to have
total energies in a range of a few keV to 200 keV. Extensive
testing of gas cluster beam formation, cluster sizes, their
extraction, and acceleration, versus various machine parameters
such as inlet pressure, and focusing requirement are also
documented by Yamada and Matsuo.
[0025] After GCIB bombardment, the surface of the YBCO film can be
smoothed significantly. For example, FIG. 4 is an AFM picture of a
surface smoothed using Ar clusters (around 3000 atoms per cluster)
of 20 keV with a total dose of 5.times.10.sup.15 clusters/cm.sup.2.
The standard deviation of the surface smoothness improved from
Ra=13 nm (in FIG. 2) to 7 nm. If one only measures the flat area,
excluding the debris peaks, Ra is as smooth as 1.3 nm as shown in
FIG. 4.
[0026] FIG. 5 is an AFM picture of a surface smoothed with
2.times.10.sup.16 clusters/cm.sup.2, where nearly all mountains or
peaks are flattened. The surface smoothness standard deviation is
improved to 0.8 nm. In experiments testing the smoothing procedures
on ten samples and under various rough starting conditions,
improvement was obtained in all cases. The starting surfaces had
roughnesses ranging between 6 nm to 31 nm. After GCIB bombardment,
the roughnesses were reduced to 0.8 to 3 nm.
[0027] The GCIB smoothing result is caused by a collective effect
of lateral sputtering which fills up the valleys with material from
the high ground. Once the landscape becomes flat, the GCIB
bombardment removes the surface uniformly. The smoothing and
sputtering with Ar clusters at 10, 20 and 30 keV with dosages
between 10.sup.14 to 2.times.10.sup.16 clusters/cm.sup.2 has been
studied. The size of the Ar cluster in these studies is estimated
to be 3000 atoms. The size distribution of the Ar clusters is very
broad with most of the population between 1000 and 5000 Ar
atoms.
[0028] The average thickness of the layer removed is directly
proportional to the total amount of dosage used in the bombardment.
For example, at 10.sup.15 clusters/cm.sup.2, 20 keV Ar cluster
bombardment removes 22 nm of YBCO. At 10.sup.16 clusters/cm, (ten
times the previous exposure) 220 nm of YBCO is removed. FIG. 6 is a
graph showing the average YBCO thickness removed versus dose of the
GCIB for 30 keV Ar cluster beams.
[0029] The layer removal rate, or sputtering rate normalized with
dosages, is a strong function of energy. In the energy region
studied, the higher the energy, the higher the YBCO layer removal
rate. For example, at 5.times.10.sup.15 clusters/cm.sup.2, 10 keV
clusters remove 68 nm, 20 keV clusters remove 110 nm, and 30 keV
clusters remove 180 nm. Rutherford Backscattering Spectrometry
(RBS) is used to measure the YBCO layer thickness, before and after
the GCIB smoothing. The difference in layer thickness before and
after the GCIB bombardment yields the layer removal, and provides
the sputtering rate measurement. FIG. 7 is a graph showing the
thickness removed versus total energy of the cluster at
5.times.10.sup.15 clusters/cm.sup.2 in which two measurements were
made using two different samples for 20 keV, and 30 keV GCIB,
respectively, average values are also plotted between the two
measurements.
[0030] During the GCIB bombardment for surface removal and
smoothing, one of the undesirable effects is the damage underneath
the bombarded area, due to total amorphorization of the crystalline
structure. The damage formation has been quantified by RBS in
combination with channeling effect. Based on RBS channeling peak
spectra, the surface damage amounts to 15 to 20 nm. For a sample
not bombarded by GCIB, the channeling surface peak gives 3-6 nm of
surface damage as a background measurement. This is due to the
combination of top monolayer dechanneling, and to possible
degradation of the surface layer to moisture and carbon dioxide.
This background surface damage amounts to 3 to 6 unit cell
thicknesses. By subtracting the background channeling surface peak,
the surface damage due to GCIB treatment is determined to be 10 to
15 nm. This damage layer needs to be removed for further device
processing; otherwise, it will impede all superconducting
properties required for device performance.
[0031] The GCIB surface damaged layer can be removed by chemical
etching or by ordinary ion sputtering layer removal. Since the
surface is already smoothed within 1 nm, with an uniform damaged
surface layer of 10 to 15 nm, standard layer removal methods such
as etching or sputtering will not disturb the flatness of the layer
after this damage layer is removed. Another method to remove the
damaged layer is by annealing in a partial oxygen ambient
atmosphere.
[0032] Annealing (Step 104) is a preferred method for damage
removal, especially for an in-situ processing procedure. In an
in-situ processing case, thin films can be deposited, smoothed by
GCIB, then damage annealed all inside an environmentally controlled
chamber for device processing. In-situ processing will improve the
device fabrication yield significantly.
[0033] The effect of annealing of a GCIB damaged layer has been
studied by channeling in combination with RBS. Examples of
channeling spectra are given in FIGS. 8A-C. It has been found that
the damaged surface can be restored to its original state. Two
different annealing schedules have been carried out for recovering
the shallow surface damage.
[0034] One schedule involves annealing the GCIB damaged HTS film at
800.degree. C. under oxygen partial pressure of 100 mTorr for 30
minutes followed by a 450.degree. C. and 200 mTorr oxygen pressure
for another 30 minutes as a step during cooling down. The advantage
of this annealing schedule is that these conditions are exactly the
same as the film deposition procedure. A second annealing procedure
is to anneal the GCIB treated surface at 870.degree. C.,
660.degree. C. and 450.degree. C. consecutively for 35 minutes each
under flowing oxygen gas. FIGS. 8A-C are graphs of surface peaks of
the channeling spectra indicating the amount of surface damage
before GCIB bombardment (FIG. 8A), after GCIB smoothing (FIG. 8B),
and after GCIB smoothing, then annealing at 870.degree. C.,
660.degree. C. and 450.degree. C. consecutively for 35 minutes each
under flowing oxygen gas (FIG. 8C).
[0035] Both annealing methods produce excellent recovery of the
GCIB damage. However, the high temperature annealing with flowing
oxygen degrades the HTS film surface, if it has not been bombarded
with GCIB. From these studies, it has been concluded that there is
a very broad window for annealing of the GCIB damage. The important
point is to keep the temperature high enough so that the crystal
regrowth kinetics can keep the annealing time reasonably short. It
is also desirable to keep the temperature below crystal melting,
and below a second phase formation, and below chemical reaction
between the film and its substrate and surroundings. With respect
to oxygen partial pressure settings, it is preferable to have
sufficient oxygen to repair the surface oxygen deficiency due to
GCIB treatment, yet not too much oxygen to promote a second phase
formation.
[0036] GCIB bombarded and annealed samples have also been studied
for their superconductivity properties. FIG. 9 is a graph showing
the resistance versus temperature curve for a YBCO film before and
after GCIB planarization at 20 KeV, 2.times.10.sup.16
clusters/cm.sup.2, followed by annealing at 800.degree. C. A small
shift of the transition temperature is observed, and the change of
the normal state resistance is due to the thinning of the layer by
GCIB sputtering. One of the best tests to make sure that the GCIB
smoothing process and the annealing method work, is to grow
additional HTS layers on top of the surface which underwent the
smoothing and annealing process.
[0037] In one example, a pulsed laser ablated YBCO thin film is
deposited, smoothed with 20 keV, 5.times.10.sup.15
clusters/cm.sup.2, and annealed with the annealing schedules
described above. A YBCO film of 200 nm thickness is subsequently
deposited on top of the smoothed and annealed surface in order to
obtain homoepitaxial YBCO. The crystalline regrowth is verified by
channeling in combination with RBS. Although the newly deposited
film is single crystal, as expected the surface is rough as
verified by AFM studies.
[0038] Accordingly , the final step of the invention involves
additional processing for device fabrication (Step 106). If it is
desired to build structures on top of this new layer, the GCIB
smoothing needs to be repeated on the new layer. Smoothed
multilayer systems can be constructed by sequencing the deposition,
smoothing, annealing, and then repeating.
[0039] In summary, a GCIB smoothing technique has been demonstrated
which can be applied to complicated HTS film. The radiation damage
due to GCIB processing can be annealed out in ambient oxygen, and
the restored surface can be used for additional homo, or
hetero-epitaxial film growth. The disclosed process is essential to
form multilayers, such as Multi-Chip-Module (MCM), using HTS for
interconnections in semiconductor devices. It is equally important
to improve the yield and reliability for HTS junction IC devices,
as well as yield and reliability improvements for passive stripline
delay lines, antennae, and filters using HTS films.
[0040] The foregoing description has been set forth to illustrate
the invention and is not intended to be limiting. Since
modifications of the described embodiments incorporating the spirit
and substance of the invention may occur to persons skilled in the
art, the scope of the invention should be limited solely with
reference to the appended claims and equivalents thereof.
* * * * *