U.S. patent application number 09/735991 was filed with the patent office on 2001-11-15 for reduction of plasma charge-induced damage in microfabricated devices.
Invention is credited to Cerrina, Francesco, Cismaru, Cristian, Shohet, J. Leon.
Application Number | 20010041375 09/735991 |
Document ID | / |
Family ID | 22619502 |
Filed Date | 2001-11-15 |
United States Patent
Application |
20010041375 |
Kind Code |
A1 |
Shohet, J. Leon ; et
al. |
November 15, 2001 |
Reduction of plasma charge-induced damage in microfabricated
devices
Abstract
A method and apparatus for reducing plasma-induced charging
damage in a semiconducting device are provided. The method includes
exposing an article having a dielectric material susceptible to
plasma-induced charging, to vacuum-ultraviolet (VUV) radiation of
an energy greater than the bandgap energy of the dielectric
material during or after plasma processing of the device. The
plasma-induced charge is conducted from, or recombined at, the
charging site.
Inventors: |
Shohet, J. Leon; (Madison,
WI) ; Cismaru, Cristian; (Tustin, CA) ;
Cerrina, Francesco; (Madison, WI) |
Correspondence
Address: |
Mary Rose Scozzafava, Ph.D.
Clark & Elbing LLP
176 Federal Street
Boston
MA
02110
US
|
Family ID: |
22619502 |
Appl. No.: |
09/735991 |
Filed: |
December 13, 2000 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60170340 |
Dec 13, 1999 |
|
|
|
Current U.S.
Class: |
438/4 ; 118/723R;
438/485; 438/513; 438/710; 438/788; 438/792 |
Current CPC
Class: |
H01L 21/67069 20130101;
H01J 37/32339 20130101; H01J 2237/0206 20130101 |
Class at
Publication: |
438/4 ; 438/485;
438/513; 438/710; 438/788; 438/792; 118/723.00R |
International
Class: |
H01L 021/00; C23C
016/00 |
Goverment Interests
[0002] The United States Government may have certain rights in the
invention under National Science Foundation Grant No. EEC-8721545.
Claims
What is claimed is:
1. A method for reducing plasma-induced charging damage in an
article, comprising: exposing an article comprising a dielectric
material, said dielectric material susceptible to plasma-induced
charging, to vacuum-ultraviolet (VUV) radiation of an energy
greater than the bandgap energy of the dielectric material during
or after plasma processing of the article, whereby plasma-induced
charging is reduced.
2. The method of claim 1, further comprising: conducting the
plasma-induced charge from the charging site.
3. The method of claim 2, wherein the step of conducting the
plasma-induced charge from the charging site includes establishing
plasma conditions under which charge is conducted to the
plasma.
4. The method of claim 2, wherein the step of conducting the
plasma-induced charge from the charging site comprises electrically
connecting the charging site to ground.
5. The method of claim 1, wherein the plasma-induced charging is
reduced by establishing VUV radiation exposure conditions under
which charge recombination takes place at the charging site.
6. The method of claim 1, wherein the article comprises a
dielectric material in contact with a conductive surface.
7. The method of claim 1, wherein a plasma is generated in a plasma
processing chamber containing the article, said plasma being a
source of VUV radiation.
8. The method of claim 7, wherein the source of VUV radiation is an
argon or oxygen plasma.
9. The method of claim 7, wherein a secondary gas is introduced
into a processing plasma, said secondary gas forming a plasma
emitting VUV radiation.
10. The method of claim 1, wherein the step of exposing the article
to VUV radiation occurs after a plasma-process is complete.
11. The method of claim 1, wherein the step of exposing the article
to VUV radiation occurs during plasma-processing.
12. The method of claim 1, wherein the step of exposing the article
to VUV radiation alternates with plasma-processing of the
article.
13. The method of claim 1, wherein the VUV radiation source
provides VUV radiation of an energy and/or flux density sufficient
to conduct charge from the charging site.
14. The method of claim 11, wherein the plasma has a VUV photon
flux of greater than or equal to about 1.times.10.sup.13
photons/cm.sup.2-s.
15. The method of claim 11, wherein the plasma has a VUV photon
flux of greater than or equal to about 1 mW/cm.sup.2.
16. The method of claim 7, wherein the VUV radiation is introduced
into the plasma chamber separately from the processing plasma.
17. The method of claim 11, wherein VUV radiation is introduced
using a glass capillary array.
18. The method of claim 1, wherein a selected portion of the
article surface is exposed to VUV radiation.
19. The method of claim 18, wherein exposure of a selected portion
of the article surface is accomplished by masking the surface.
20. The method of claim 18, wherein exposure of a selected portion
of the article surface is accomplished by VUV radiation exposure
using glass capillary array of a selected size and shape.
21. An apparatus for reducing plasma-induced charging damage in an
article, comprising: a plasma processing chamber for housing an
article; means for generating a plasma; a source of
vacuum-ultraviolet (VUV) radiation, said VUV radiation of an energy
greater than or equal to the bandgap energy of an article to be
plasma processed.
22. The apparatus of claim 21, further comprising conducting means
for conducting plasma-induced charge from the article.
23. The article of claim 21, wherein a plasma is generated in a
plasma processing chamber, said plasma being a source of VUV
radiation.
24. The article of claim 23, wherein the source of VUV radiation is
an argon or oxygen plasma.
25. The article of claim 21, wherein a secondary gas is introduced
into the plasma chamber, said secondary gas forming a plasma
emitting VUV radiation.
26. The article of claim 21, wherein the plasma is a pulsed
plasma.
27. The article of claim 21, wherein the VUV radiation source
provides VUV radiation of an energy and/or flux density sufficient
to conduct charge from the charging site.
28. The article of claim 21, wherein the VUV radiation source is
separate from the plasma chamber.
29. The method of claim 28, wherein VUV radiation is introduced
from the VUV source into the plasma chamber using a glass capillary
array.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority under 35 U.S.C.
.sctn.119(e) to co-pending U.S. application Ser. No. 60/170,340,
filed Dec. 13, 1999, entitled "Vacuum Ultraviolet Used to Minimize
Processing Damage," the contents of which are hereby incorporated
by reference.
BACKGROUND OF THE INVENTION
[0003] The present invention is related to minimizing device damage
during plasma-assisted microfabrication. In particular, it relates
to reducing charging damage of dielectrics during microfabrication
processing.
[0004] The tendency in the art of fabricating integrated circuit
(IC) devices is to achieve ever-higher integration (smaller
dimensions). During fabrication, various materials are deposited
and/or removed in different layers to build the desired integrated
circuit. Typically, conductive layers are separated from one
another by dielectric materials, e.g., SiO.sub.2, and the like.
Because semiconductor ICs are fabricated as multilayer structures,
there is a common need to interconnect features on one layer with
IC features on another. To form such interconnections between IC
features, etching through the dielectric materials down to the
underlying conducting feature, creates high-aspect ratio channels,
typically.
[0005] In these instances, low-pressure, high-density plasma
typically is used to micro-etch the IC device. When using a plasma
in an etch or deposition process, a glow discharge produces a
chemically reactive species from a relatively inert molecular gas,
which then reacts with the material and either deposits on or
etches the material. Any volatile by-products are then removed from
the surface. The use of plasma in etching and deposition provides
directionality, low temperature and processing convenience.
[0006] In such plasma-assisted IC structure patterning, the plasma
is often biased with respect to the semiconductor wafer (substrate)
to produce desired processing effects. As a result, an excess of
positively or negatively charged particles may deposit on the
substrate. If this excess deposits on dielectric material, it is
likely that the net charge will continue to accumulate and produce
an electric field across the dielectric, which can cause damage to
the dielectric. Such charging damage may arise due to potential
differences between floating gates and the silicon substrate due to
plasma non-uniformities. In addition, a layer covered with resist
or insulating film with apertures having a high aspect ratio is
susceptible to a type of plasma charging damage known as
electron-shading damage.
[0007] It is believed that electron-shading damage is caused by the
difference in behavior between electrons and ions. In general, the
semiconductor substrate and the plasma experience a bias potential
(electric field) such that the positively charged ions are
accelerated towards the substrate, whereas the electrons are
decelerated in the electric field. As a result, the velocity of the
positive ions becomes very large in the direction towards the
substrate and ions are nearly vertically incident to the substrate
on contact. In contrast, electrons have much larger angle of
incidence to the substrate. This is illustrated in FIG. 1.
[0008] Where the conductive layer to be etched has an insulating or
dielectric pattern thereon that surrounds the conductive surface,
the electrons approaching in an oblique angle are shaded by the
dielectric material and can be trapped in the walls of the channel.
Ions of normal incidence, i.e., predominantly positive ions, are
not shaded by the insulating pattern and are injected into the
conductive layer below. This results in an overflow of positive
charges into the conductive surface, as illustrated in FIG. 1.
[0009] As etching continues, electrons thus captured on the
sidewalls of the dielectric layer serve to form an electric field
that farther repels incoming electrons. In contrast, the positively
charged ions are accelerated by the electric field into the channel
and onto the conductive surface below. This augments and
exacerbates the charge difference between the top and walls of the
trench, which can result in increased potential for damage.
[0010] Charging damage may arise in numerous ways. One common way
occurs when tunneling currents pass through a gate-insulating film
in connection with the conductive layer in order to discharge the
accumulated charge, as shown in FIG. 1. When the gate-insulating
film is thick, the tunneling current is negligible. However, with
the higher integration of semiconductor devices, the thickness of
the gate oxide films becomes smaller and tunneling current passes
more easily and the quality and lifetime of the oxide is
degraded.
[0011] As a result, dielectric charging can cause permanent damage
to the devices being processed. In addition, dielectric charging
can result in the degradation of processing properties of the
plasma system, such as enhanced notching, trenching and sidewall
bowing. Indeed, it has been the consensus among both the scientific
and the industrial communities that plasma-process-induced damage
is a growing concern in the microfabrication community. This
phenomenon is now widely recognized as an important factor limiting
yield and device reliability in microfabrication.
SUMMARY OF THE INVENTION
[0012] The present invention provides a method for the reduction of
plasma charge-induced damage in microfabricated devices by reducing
the extent of dielectric charging during plasma processing.
[0013] In one aspect of the invention, the method includes exposing
an article containing a dielectric material that is susceptible to
plasma-induced charging to vacuum-ultraviolet (VUV) radiation of an
energy greater than the bandgap energy of the dielectric material
during or after plasma processing of the article. The temporary
increase in conductivity of the dielectric surface due to VUV
exposure permits the plasma-induced charge to be conducted from the
charging site, or alternatively, prevents charge accumulation in
the first instance by allowing charge recombination at the charging
site.
[0014] In one embodiment of the invention, a method for reducing
plasma-induced charging damage in an article includes exposing an
article comprising a dielectric material susceptible to
plasma-induced charging, to vacuum-ultraviolet (VUV) radiation of
an energy greater than the bandgap energy of the dielectric
material during or after plasma processing of the article, whereby
plasma-induced charging is reduced. The plasma-induced charge is
conducted from, or recombined at, the charging site.
[0015] In another embodiment, the plasma-induced charge is
conducted from the charging site, which may include establishing
plasma conditions under which charge is conducted to the plasma or
electrically connecting the charging site to ground.
[0016] In another embodiment of the invention, the plasma-induced
charging is reduced by establishing VUV radiation exposure
conditions under which charge recombination takes place at the
charging site.
[0017] In other embodiments, the article comprises a dielectric
material in contact with a conductive surface.
[0018] In still other embodiments, a plasma is generated in a
plasma processing chamber containing the article, for which the
plasma is a source of VUV radiation. In particular, the source of
VUV radiation is an argon or oxygen plasma, or a secondary gas is
introduced into a processing plasma, and the secondary gas forms a
plasma emitting VUV radiation.
[0019] In other embodiments of the invention, the step of exposing
the article to VUV radiation occurs after a plasma process is
complete, or during plasma processing, or alternates with
plasma-processing of the article.
[0020] In yet other embodiments of the invention, the VUV radiation
source provides VUV radiation of an energy and/or flux density
sufficient to conduct charge from the charging site, or the plasma
has a VUV photon flux of greater than or equal to about
1.times.10.sup.13 photons/cm.sup.2-s, or the plasma has a VUV
photon flux of greater than or equal to about 1 mW/cm.sup.2.
[0021] In other embodiments, the VUV radiation is introduced into
the plasma chamber separately from the processing plasma, such as
by using a glass capillary array.
[0022] In another embodiment, a selected portion of the article
surface is exposed to VUV radiation, which may be accomplished by
masking the surface, or by exposure using glass capillary array of
a selected size and shape.
[0023] In another aspect of the invention, an apparatus for
reducing plasma-induced charging damage in an article is provided,
which includes a plasma processing chamber for housing an article;
means for generating a plasma; a source of vacuum-ultraviolet (VUV)
radiation, in which the VUV radiation is of an energy greater than
or equal to the bandgap energy of an article to be plasma
processed.
[0024] In other embodiments, the apparatus includes conducting
means for conducting plasma-induced charge from the article.
[0025] In still other embodiments, a plasma is generated in a
plasma processing chamber, for which the plasma is a source of VUV
radiation, and for example, the source of VUV radiation is an argon
or oxygen plasma, or a secondary gas is introduced into the plasma
chamber that forms a plasma emitting VUV radiation.
[0026] In other embodiments, the plasma is a pulsed plasma. In
still other embodiments, the VUV radiation source provides VUV
radiation of an energy and/or flux density sufficient to conduct
charge from the charging site, and for example, the VUV radiation
source is separate from the plasma chamber, or the VUV radiation is
introduced from the VUV source into the plasma chamber using a
glass capillary array.
[0027] The method of VUV-induced charge reduction is applicable to
any process in which charge build-up occurs on a dielectric
surface. Exemplary processes include metal and oxide
plasma-assisted etching and plasma-assisted deposition of oxides,
plasma doping, ion implantation, and plasma ashing.
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] This invention is described with reference to the following
figures, which are presented for the purpose of illustration and
are in no way limiting of the invention, and in which:
[0029] FIG. 1 is a schematic illustration of charging and
discharging in an integrated circuit device;
[0030] FIG. 2 is a schematic illustration of the proposed mechanism
of the invention showing reduction of plasma-induced charging with
vacuum-ultraviolet radiation;
[0031] FIG. 3 is a cross-sectional illustration of an exemplary
plasma chamber used in the practice of the invention;
[0032] FIG. 4 is a schematic illustration of a glass capillary
array used to separate VUV production from the plasma processing
region;
[0033] FIG. 5 is a schematic illustration of an electron cyclotron
resonance (ECR) plasma etching system;
[0034] FIG. 6 is (A) a diagram of a test structure used in Example
1 and (B) the associated measurement circuitry;
[0035] FIG. 7 is an equivalent electrical circuit for the
measurement setup of FIG. 6;
[0036] FIG. 8 is (A) an exemplary I-V measurement on the 690
Angstrom thick oxide-covered Al pad and (B) a Langmuir probe
measurement on the uncovered Al pad for an O.sub.2 plasma at 1000 W
microwave power and 2 mTorr neutral pressure;
[0037] FIG. 9 is a plot of VUV-induced current density (solid
lines) and fitted values (dashed lines) as a function of applied
voltage across the oxide layer for layer thicknesses of 57 nm
(.diamond-solid.), 69 nm (.tangle-solidup.) and 295 nm
(.cndot.);
[0038] FIG. 10 is a plot of the normalized net conduction current
as a function of incident photon energy for an aluminum oxide
layer; and
[0039] FIG. 11 is surface potential scan of a wafer showing surface
potentials (A) after positive and negative charging, but before VUV
exposure and (B) the positively charged wafer after VUV
exposure.
DETAILED DESCRIPTION
[0040] The present invention demonstrates that dielectrics, such as
SiO.sub.2 and alumina, become temporarily conductive under direct
plasma VUV exposure. The conductive oxide provides a path for safe
discharging of charged surfaces, and may be especially useful for
high-aspect ratio structures that charge up due to the
electron-shading effect. By understanding the effects of both
plasma charging and plasma radiation on the properties of the
dielectric, changes in plasma processing conditions and methods are
proposed to minimize processing damage to microfabricated
devices.
[0041] According to the invention, a method for the reduction of
dielectric charging in a device is accomplished by exposure of the
device to vacuum-ultraviolet (VUV) radiation of an energy greater
than the bandgap of the dielectric, either during or after plasma
processing of the device. VUV radiation typically has a photon
energy range between 9 eV and 25 eV; however, the energy spectrum
depends upon the source of the radiation. For example, an argon
plasma produces a large amount of VUV energy of about 12 eV, while
an oxygen plasma has a large amount of VUV photon energy of about 9
eV. While not bound to any particular mode of operation, it is
believed that a dielectric material may act as a photoconductor
under these circumstances. Absorption of incident radiation of the
proper energy results in electron-hole pair formation, and current
flow is possible under an applied electric field.
[0042] This is illustrated schematically in FIG. 2, which shows a
conventional gated integrated circuit 200, illustrating the
electron shading effect during high aspect ratio etching of
poly-silicon with a hardmask of SiO.sub.2. As previously discussed,
the conductive bottom 201 of the trench will charge more positively
than the top sidewall 202 of the structure because of preferential
impingement of the more anisotropic ions 203 onto the bottom of the
trench. Most often, the conductive bottom is connected to a
transistor gate 204, and this induces a voltage stress across the
gate oxide. Charge build-up on surfaces of the device can result in
deleterious discharging currents 205. In addition local electric
fields determined by this differential charging may significantly
perturb ion transport in the trench, thus reducing both ion energy
and the ion flux arriving at the bottom of the surface.
[0043] Irradiation of the device with VUV radiation 206 temporarily
increases the dielectric conductivity, permitting non-damaging
discharge currents 207, 208. Discharge may arise through
recombination of charges, such as at surface currents 207 along the
sidewalls of the etched feature and through charge returned to the
plasma through bulk currents 208. In other embodiments, a current
drain, i.e., a conducting path to ground, may be included in the
device to remove charge once the dielectric conductivity increases
and current flow is established. It is reasonable to assume that,
if a conducting path were placed on the edge of the VUV exposed
region to ground, the charge would drain off as long as the VUV was
turned on, thus providing a means to discharge the accumulated
charge or to discharge it as soon as it accumulates on the
dielectric.
[0044] FIG. 3 shows a plasma etching system 100 useful in the
practice of the invention in accordance with one embodiment of the
invention. The process chamber 101 includes a bottom electrode 102
and a top electrode 104, which also acts as a shower head for
allowing input gas 108 to enter the processing chamber 101. In this
manner, the plasma is located directly above a substrate containing
the devices to be processed 110 that is placed on top of the bottom
electrode 102. In operation, the processing chamber 101 may exhaust
processing gases through a pumping system 112 that leads to an
appropriate storage or exhaust of waste products (not shown). In
operation, the device 110 is exposed to a plasma cloud that may
result in charging of the device surfaces.
[0045] By way of example, the invention may be practiced in a
number of other suitably arranged processing chambers that produce
and/or deliver energy to the plasma. Examples of this include, but
are not limited to, capacitively coupled electrode plates, electron
cyclotron resonance (ECR) microwave plasma sources, and inductively
coupled RF sources, such as helicon, helical resonators, and
transformer coupled plasma (TCP).
[0046] Conductivity increases in the dielectric material can be
achieved by use of a strongly VUV-emitting plasma. The VUV-emitting
plasma may be used in combination with the other plasmas of the
plasma process. For example, it may be combined with the processing
plasma so that the device is exposed to VUV radiation during the
plasma process itself This may be accomplished by adding a
secondary gas that has a high VUV emission to the plasma-forming
gases. Exemplary gases having a high VUV emission are argon and
oxygen. The preferred secondary gas for any particular application
depends upon the particular dielectric materials of the substrate,
since bandgap energies vary. By way of example, SiO.sub.2, a common
insulating mask used in the semiconductor industry, has a bandgap
of about 9 eV. Other dielectrics used in semiconductor device
manufacturing include silicon nitride (Si.sub.xN.sub.y) having a
bandgap energy of about 6 eV and aluminum oxide (Al.sub.2O.sub.3)
having a bandgap energy of about 8.3 eV.
[0047] In practice of the invention, the flux and/or photon density
of VUV radiation to the charging surface should be sufficient to
increase the conductivity of the surface and/or its bulk so that
discharging currents may form. While conventional processing
plasmas may generate a small level of VUV emission, it is
considered inadequate to discharge the plasma-induced charge
build-up in many devices. Conventional plasma provides photon flux
in the VUV range of about 0.01 mW/cm.sup.2. In preferred
embodiments, the photon flux is greater than 0.01 mW/cm.sup.2 and
is preferably greater than or equal to about 1 mW/cm.sup.2 or about
1.times.10.sup.13 photons/cm.sup.2-s. The minimum oxide
conductivity needed to effectively discharge the dielectric depends
on how much charge is present and how much time is available for
discharge. Ideally, conductivity is sufficient that the charge
decays as fast as it builds up, so the photon flux should be
comparable with the plasma (charged particle) flux. In other
embodiments of the invention, the device may be exposed to a plasma
produced with an enhanced VUV emitting gas alone, either at the end
of the processing step or in alternation with the processing gas
plasma. By way of example, the VUV emitting gas plasma may be
provided as a series of sequential pulsed discharges, with
alternating pulses of processing gas plasma. Pulsing of the plasma
system may take place by alternately puffing in measured amounts of
processing gas and VUV-producing gas and separate plasma rf or
microwave sources could also be alternately pulsed to specifically
excite each particular gas. Apparatus and method of producing a
pulsed plasma are known.
[0048] Alternatively, the device may be exposed to VUV-emission
after conventional plasma processing. Preferably, exposure occurs
during or immediately after plasma-processing to allow charge
dissipation before damage to the device can occur.
[0049] In other embodiments, VUV irradiation of the device may be
provided in a separate processing system. In some applications, it
may be preferably to produce the VUV photons in a location remote
from the location of plasma processing in order to avoid altering
the plasma processing conditions. It also may be preferred to
separate the relatively higher pressure plasma process from the
low-pressure environment ideally suited to generation of VUV
emissions. Traditional techniques for separating different pressure
regions while transmitting VUV radiation include thin film windows
and differentially pumped slits. For wavelengths between 800
Angstroms and 1050 Angstroms, thin films of indium may be used and
at longer wavelengths, LiF is suitable. However for the wavelengths
used in this invention, such windows are not normally usable.
[0050] A preferred embodiment includes the use of glass capillary
arrays to provide a rugged, large aperture window for VUV emission.
The flexible capillary glass tube array 400 is shown schematically
in FIG. 4, and can be used to effectively separate the VUV source
region from that of the plasma processing region. The array may be
arranged to enter the plasma chamber 404 above the processing
plasma 406, as is shown in FIG. 4. Each capillary tube 402 is a
hollow glass waveguide and functions to guide the VUV photons into
the plasma chamber 404. The arrays can be constructed to provide
large light conductance while restricting the pressure differential
between the VUV source 406 and the plasma chamber 404. A VUV
capillary array 400 also provides added flexibility in the exposure
of the device to VUV emission as the array may be oriented to cover
the entire surface of the device or to focus on only portions of
the device, for example, those regions considered most susceptible
to plasma-induced charge damage. Typically, the VUV may be produced
in a separate plasma discharge chamber specifically designed to
generate enhanced VUV or it may be produced directly in the
capillary arrays themselves (capillary discharge). When using a
capillary array, a source of VUV may be a small, low-pressure,
high-power, high-density inductively coupled plasma, running at the
conventional 13.56 MHz.
[0051] By separating the VUV photon production region from the
plasma processing region, each process can be separately
controlled, thereby providing controllable VUV emission during
plasma processing. Alternatively, VUV treatment may alternate with
plasma processing or may be used after plasma processing
completion. In addition to offering the ability to isolate the two
processes from each other, this will permit integration of the VUV
source with existing plasma processing systems, which offers
significant cost advantages. It is believed that many commercially
available plasma processing systems can be easily modified to
include a capillary array. If the processing plasma deposits
material on the capillary array, a shutter can be used to open the
array to the substrate only during periods when the plasma is off.
Suitable plasma processing systems include Applied Materials'
Centura and eMAX platforms, LAM's 9000 Series etchers, and the
like.
[0052] The method of the invention may be used with any insulating
or dielectric material. Exemplary materials include, but are not
limited to organic and inorganic resist materials used in
patterning during plasma etching, and in particular, those
dielectric materials used in the microfabrication and semiconductor
industries. While the discharging capabilities of the invention
have been discussed in light of processing of high aspect ratio
features in the semiconductor industry, it will be appreciated that
the method may be readily used for any plasma application where
charge buildup may occur. For example, the method may find
application in plasma sputter deposition processes where dielectric
materials are deposited, and in particular, in alumina sputter
deposition on magnetic recording heads, and also in plasma doping
and ion implantation processes.
[0053] The invention is described with reference to the following
examples, which are presented for the purpose of illustration and
are not intended to be limiting of the invention.
EXAMPLE 1
[0054] This example measures the in situ electrical properties of
SiO.sub.2 films and demonstrates an increased conductivity of these
layers during plasma exposure.
[0055] The method was implemented in an
electron-cyclotron-resonance (ECR) plasma-etching system shown in
FIG. 5. The system incorporated a 1.5 kW microwave plasma source
and a pair of magnets arranged in a vertical magnetic-mirror
configuration. The test structures used for these measurements were
placed on a wafer stage that is located 19 cm below the
electron-cyclotron-resonance region. For this work, the neutral
pressure and microwave power were held constant at 2 mTorr and 1000
W, respectively. Either argon or oxygen was used as the feed gas.
Under these conditions, plasmas with densities in the 10.sup.11 to
10.sup.12 cm.sup.-3 range were obtained. No wafer bias was
applied.
[0056] Device Preparation.
[0057] The test structure and its electrical circuitry is shown in
FIG. 6. This test structure and its associated measurements
provided the current-voltage relationship between the current
passing through the thin oxide layer and the voltage drop across
it, even though a direct measurement of the voltage drop is not
made. The measurements provide a good measurement of bulk
conductivity, regardless of oxide thickness.
[0058] With reference to FIGS. 6A-6B, the test structure was
prepared as follows. A 5.8.times.5.8 mm Al pad 50 was deposited on
top of 1 .mu.m thick thermally grown silicon oxide 52 on an Si
wafer 54 (n-type, 1 .OMEGA.-cm). The Al pad is covered with a thin
layer 56 of PECVD-deposited (plasma-enhanced chemical vapor
deposition) SiO.sub.2 of varying thicknesses, which is the layer to
be tested. For the purposes of this example, three separate sets of
test structures of this kind were built (on separate wafers), with
PECVD oxide thicknesses (as measured after deposition) of 570, 690
and 2925 .ANG.. The nonuniformity of each thickness was less than
5% across the surface of the wafers. The equivalent electric
circuit for the measurement setup in FIG. 6B is shown in FIG.
7.
[0059] Measurement of the J-V Characteristics and Oxide
Conductivity of Thin SiO.sub.2 Layers Directly Exposed to Plasma
Radiation.
[0060] The J.sub.ox vs E.sub.ox relationship was obtained through
the dielectric layer by making the following measurements, and
applying the extracted measurements to the equation, 1 J ox = C t
ox E ox [ A / cm 2 ] (eq 1)
[0061] First, a measurement of the current I as a function of
V.sub.s was made between -10 and 10V in the setup of FIGS. 6 and 7.
One example of this measurement is shown in FIG. 8A. Next, a
Langmuir probe measurement using the uncovered pad is made so as to
extract the electron temperature T.sub.e and the electron current
I.sub.e. The electron current is extracted as a function of the
pad's floating potential V.sub.f. An example of the Langmuir probe
measurement is given in FIG. 8B. To relate the electron current
I.sub.e to the source voltage V.sub.s as in the above equation, a
measurement of V.sub.f during the sweep of V.sub.s was also made.
The measurement of V.sub.s vs V.sub.f was made using a high-voltage
probe with an impedance of 10 M.OMEGA. connected to the uncovered
Al pad. Thus, the I-V relationship for the oxide layer was
determined; and the J.sub.ox-E.sub.ox relationship was determined
by subsequently dividing the current I by the pad area and the
oxide voltage drop V.sub.ox by the oxide thickness.
[0062] Based on these measurements, measurements of the J-V
characteristics of thin SiO.sub.2 layers were made during exposure
to argon and oxygen plasmas at 1000 W microwave power and 2 mTorr
neutral pressure. Examples of these measurements are shown in FIG.
9 for different oxide thicknesses of 57 nm (.diamond-solid.), 69 nm
(.tangle-solidup.) and 295 nm (.cndot.), which show the measured
current density across the oxide layer during exposure to an Ar
plasma.
[0063] The measured data can be fitted to a simple photoconductor
model to determine the conductivity of SiO.sub.2 layers under
exposure to plasma radiation. The model has been described
previously (Cismara & Shohet, Journal of Applied Physics.
88(4):1742 (August 2000) and U.S. Ser. No. 60/170,340 entitled
"Vacuum Ultraviolet Used to Minimize Processing Damage," which are
hereby incorporated by reference. This model is based on the
current density expression: 2 - J ox = C t ox 2 V ox ( V ox / t ox
) , (eq 2)
[0064] where .phi. is the ionizing radiation flux with energies
higher than the energy band gap of SiO.sub.2 (approximately 9 eV),
E.sub.ox is the electric field strength across the oxide layer
(E.sub.ox=V.sub.ox/t.sub.ox), t.sub.ox is the oxide thickness and C
is the fitting parameter. The radiation flux was measured to be
3.6.times.10.sup.14 photons/cm.sup.2 for argon and
5.1.times.10.sup.13 photons/cm.sup.2 for oxygen in the ECR etcher,
at 2 mTorr and 1000 W microwave power.
[0065] This leads to a conductivity that depends on the applied
electric field according to the following expression: 3 ox = C t ox
2 E ox [ - 1 cm - 1 ] . (eq 3)
[0066] The values of the parameters C and .xi. were determined for
both argon and oxygen by fitting the experimental data to the model
defined in eq. 2, which are shown as dashed lines. The standard
error of the fitting parameters C and .xi. was less than 3%.
Similar measurements were made for oxygen plasma under similar
conditions. Values of the fitting parameters for argon and oxygen
plasma are reported in Table 1.
1 Feed gas C (cm.sup.2-s-.OMEGA..sup.-1) .xi. (cm V.sup.-1) Ar 9.6
.times. 10.sup.-31 4.7 .times. 10.sup.-7 O.sub.2 8.9 .times.
10.sup.-30 8 .times. 10.sup.-7
[0067] Based on these measurements, oxide conductivity was
determined for an SiO.sub.2 layer of varying thickness upon
exposure to Ar plasma. The measurements of the oxygen conductivity
resulted in values in the range of 10.sup.-10 to 10.sup.-9
.OMEGA..sup.-1-cm.sup.-1 during argon and oxygen exposure,
respectively. The higher oxide conductivity induced during exposure
to oxygen plasma is due to the stronger emission of this plasma
near the SiO.sub.2 bandgap energy (ca. 9 eV). In contrast, argon
plasma has most of its VUV emission at higher energies (ca. 12 eV).
The lower energies photons emitted by the oxygen plasma will
penetrate more deeply into the SiO.sub.2 layer, inducing the
generation of electron-hole pairs through the oxide, while the
higher energy photons emitted by the argon plasma will penetrate
only a few hundred angstroms and will generate electron-hole pairs
in this outermost layer only. Therefore it is expected that the
conductivity of the oxide layers or more than a few hundreds of
Angstroms thick will be higher under an oxygen plasma than under an
argon plasma exposure.
[0068] Measurement of the J-V Characteristics and Oxide
Conductivity of Thin SiO.sub.2 Layers Before and After Plasma
Exposure.
[0069] The oxide conductivity measured in situ during plasma
exposure was compared to the conductivity measured before and after
the exposure, under complete darkness. To make these measurements
after the plasma exposure, another aluminum layer was added on top
of the PECVD oxide layer by magnetron sputtering, followed by a 20
minute metal anneal step at 425.degree. C. in forming gas. The I-V
curves recorded between the upper and lower Al layers before and
after plasma exposure showed conductivities three orders of
magnitude lower than that measured under plasma exposure. These
results demonstrate that oxide conductivity can be made to increase
dramatically under exposure to plasma containing significant levels
of VUV radiation.
EXAMPLE 2
[0070] This example measures the in situ electrical properties of
another dielectric material, Al.sub.2O.sub.3, and demonstrates an
increased conductivity of these layers during plasma exposure. This
demonstrates that the observed increase in conductivity during VUV
exposure may be generally applied to a variety of dielectric
materials.
[0071] Monochromatic synchrotron VUV radiation was used having the
same photon energy range as a conventional plasma. The advantage of
using synchrotron radiation is the ability to determine the
dependence of measured quantities on the wavelength of incoming
radiation. Oxide-coated wafers were exposed to synchrotron
radiation at the Synchrotron Radiation Center (SRC) at the
University of Wisconsin-Madison. The monochromatic light was
supplied by the Aladdin synchrotron storage ring, passing through a
VUV monochromator. The electron beam used to generate the
synchrotron radiation had a current of up to 250 mA, at an energy
of 800 MeV. The VUV monochromator that was connected to the
beamline is a normal-incidence monochromator, in a Seya-Namioka
mount, with an output energy range of 4 eV to 30 eV, and a bandpass
of 3 .ANG..
[0072] An oxide layer was deposited on the wafer substrate by
reactive sputtering and was 3000 Angstroms thick. The oxide-coated
samples were mounted in a vacuum chamber coupled to the beamline
monochromator. The mounting unit was set so as to have normal
incidence of VUV beam on the surface of the wafer. The
monochromatic synchrotron light was focused on the sample with a
spot of dimension 5 mm.times.15 mm. During the measurements, the
chamber was evacuated to pressures in the 10.sup.-8 Torr range.
[0073] The current through the oxide sample was measured at a fixed
bias voltage, 45 V, as the synchrotron light energy was varied
between 5 and 20 eV. The dark current was measured by closing off
the light source, but making the same sweep of energy to make sure
that there were no light leaks or other coupling to the circuit. In
all cases, the dark current was at least one order of magnitude
less than the conduction current. The net oxide current (measured
current less dark current) was normalized to a constant electron
beam current (200 mA) and the photon flux at 15 eV. The normalized
net conduction current as a function of incident photon energy is
shown in FIG. 10. No significant increase in conduction current
took place until the VUV photon energy approached the bandgap
energy of the aluminum oxide at around 8.32 eV.
[0074] In order to test the effect of VUV radiation on depletion of
charge, the samples were charged by placing them in a non-reactive
argon inductively coupled plasma (ICP) for several minutes. The ICP
power was 1 kW and two different sample bias conditions were used
so as to produce a net positive or negative charge in the oxide.
The former was achieved with 150 W RF bias and the latter with a
zero bias. FIG. 11A shows a scan across the 11.5 cm sample using
the CPD (contact potential difference) technique that was made
before exposure to the VUV under both sample biases.
[0075] Since most plasma processing occurs with a positive charge
being placed on the wafer, a CPD scan of a positively charged wafer
was made after exposure to VUV, which is shown in FIG. 11B. Three
VUV exposures were made on the sample at 15 eV for 5, 30 and 300
seconds. The first exposure was made at the exact center of the
sample, while the remaining two were displaced above and below the
center by about 10 mm. Two CPD scans at right angles to each other
show a dramatic change in the charge profiles. There was a
significant decrease in the charge at the center of the sample and
an increase in charge towards the outside of the wafer. The
temporary increase in conductivity in the center made this region
an equipotential and charge flowed outward from the central region
of exposure to the outside, pushing other charge ahead of it and
resulting in a charge pile-up at the wafer edges. When VUV exposure
is ended, the conductivity returns to its pre-exposure state, thus
trapping the charge as shown in FIG. 11B. It is reasonable to
assume that, if a conducting path were placed on the edge of the
VUV exposed region to ground, the charge would drain off as long as
the VUV was turned on, thus providing a means to discharge the
accumulated charge.
Other Embodiments
[0076] All publications and patent applications mentioned in this
specification are herein incorporated by reference to the same
extent as if each independent publication or patent application was
specifically and individually indicated to be incorporated by
reference.
* * * * *