U.S. patent application number 12/966844 was filed with the patent office on 2011-06-16 for atomic layer etching with pulsed plasmas.
This patent application is currently assigned to UNIVERSITY OF HOUSTON. Invention is credited to Vincent M. DONNELLY, Demetre J. ECONOMOU.
Application Number | 20110139748 12/966844 |
Document ID | / |
Family ID | 44121419 |
Filed Date | 2011-06-16 |
United States Patent
Application |
20110139748 |
Kind Code |
A1 |
DONNELLY; Vincent M. ; et
al. |
June 16, 2011 |
ATOMIC LAYER ETCHING WITH PULSED PLASMAS
Abstract
A system and method for rapid atomic layer etching (ALET)
including a pulsed plasma source, with a spiral coil electrode, a
cooled Faraday shield, a counter electrode disposed at the top of
the tube, a gas inlet and a reaction chamber including a substrate
support and a boundary electrode. The method includes positioning
an etchable substrate in a plasma etching chamber, forming a
product layer on the surface of the substrate, removing a portion
of the product layer by pulsing a plasma source, then repeating the
steps of forming a product layer and removing a portion of the
product layer to form an etched substrate.
Inventors: |
DONNELLY; Vincent M.;
(Houston, TX) ; ECONOMOU; Demetre J.; (Houston,
TX) |
Assignee: |
UNIVERSITY OF HOUSTON
Houston
TX
|
Family ID: |
44121419 |
Appl. No.: |
12/966844 |
Filed: |
December 13, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61286572 |
Dec 15, 2009 |
|
|
|
Current U.S.
Class: |
216/37 ;
156/345.33; 216/67 |
Current CPC
Class: |
H01J 37/32146 20130101;
H01J 37/32174 20130101; H01J 37/32045 20130101; H01L 21/30655
20130101; H01J 37/32036 20130101; H01J 37/32082 20130101 |
Class at
Publication: |
216/37 ;
156/345.33; 216/67 |
International
Class: |
C23F 1/00 20060101
C23F001/00; C23F 1/08 20060101 C23F001/08 |
Goverment Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] The U.S. Government has a paid-up license in this invention
and the right in limited circumstances to require the patent owner
to license others on reasonable terms as provided for by the terms
of Grant Nos. DE-P502-09ER09-01 awarded by the U.S. Department of
Energy and CBET-0903426 awarded by the National Science Foundation.
Claims
1. A system comprising: a pulsed plasma source, comprising: a
spiral coil electrode disposed around a chamber; an inlet disposed
in the tube and in fluid communication with a process gas supply;
and a reaction chamber in fluid communication with the pulsed
plasma source comprising: a substrate support; and a boundary
electrode.
2. The system of claim 1, wherein the spiral coil electrode is
coupled to a pulse generator, wherein the pulse generator
comprises: at least one radio frequency function generator; and an
impedance-matching network.
3. The system of claim 1, further comprising a counter electrode
disposed proximal to the top of the chamber and at least partially
extending into the chamber.
4. The system of claim 3, wherein the counter electrode is disposed
vertically opposite the substrate support.
5. The system of claim 1, wherein the inlet is connected to a gas
source chosen from the group consisting of oxygen, oxygenated
gases, noble gases, halogens, halogenated gases, nitrogen,
hydrogen, oxygen, and combinations thereof.
6. The system of claim 1, wherein the boundary electrode is
disposed approximately horizontally adjacent to the substrate
support within the reaction chamber.
7. The system of claim 1, wherein the substrate support comprises a
pulsed electrode.
8. A method for etching a substrate, comprising: introducing a feed
gas into a plasma chamber, the feed gas comprising a mixture of
inert gas and reactant gas; disposing the substrate in the plasma
chamber; generating a plasma from the feed gas, the plasma
containing reactants and ions; saturating a substrate surface with
the reactants to form a product layer, the product layer comprising
a monolayer of the reactant species and a first monolayer atoms of
the substrate; and removing the product layer by exposing the
product layer to the ions.
9. The method according to the claim 8, wherein the generating and
the saturating occurs during a first period and the removing occurs
during a second period.
10. The method according to claim 9, wherein the plasma source is
applied with a first RF power level during a first portion of the
first period.
11. The method according to claim 10, wherein the plasma source is
turned off during a second portion of the first period.
12. The method according to claim 10, wherein the plasma source is
applied with RF power pulses during the second period, the RF power
pulses having a second RF power level greater than the first RF
power level.
13. The method according to claim 12, wherein the electrode is
applied with bias pulses during the second period, wherein the RF
power pulses and the bias pulses applied during the second period
alternate sequentially, and wherein at least one of the bias pulses
is removed from at least one of the RF pulses by about 10
.mu.s.
14. The method according to claim 13, wherein the electrode is
applied with positive bias pulses during the second period, wherein
the electrode provides positive bias pulses to the plasma.
15. The method according to claim 13, wherein the electrode is
applied with negative bias pulses during the second period, and
wherein the electrode is electrically coupled to the substrate to
provide the negative bias pulses to the substrate.
16. The method according to claim 8, wherein the removing the
product layer is performed by increasing potential difference
between the plasma and the substrate so as to direct the ions from
the plasma toward the substrate.
17. The method according to claim 16, wherein the increasing the
potential difference is performed by at least one of applying
positive voltage to the plasma and applying negative voltage to the
substrate.
18. The method according to claim 8, wherein the feed gas is
continuously introduced into the chamber.
19. A method for processing a substrate comprising: directing ions
from plasma afterglow toward a substrate surface saturated with a
first substance.
20. The method according to claim 19, further comprising: removing
the first substance and a monolayer of substrate atoms with the
ions.
21. The method according to claim 20, wherein the first substance
comprise reactant species.
22. The method according to claim 19, further comprising: providing
pulsed RF power to the plasma, wherein the directing the ions is
performed between the RF pulses.
23. The method according to claim 19, wherein the directing the
ions is performed by providing bias pulses to an electrode near the
substrate, wherein the bias pulses applied to the electrode and the
RF power pulses applied to the plasma source alternate
sequentially.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims benefit under 35 USC .sctn.119 of
U.S. provisional application No. 61/286,572 filed Dec. 15, 2009,
entitled "Atomic Layer Etching with Pulsed Plasmas" which is hereby
incorporated herein by reference in its entirety for all
purposes.
BACKGROUND OF THE INVENTION
[0003] 1. Field of the Invention
[0004] The present disclosure relates to a nanofabrication process.
More specifically, the present disclosure relates to new cyclic
process for etching a solid surface with atomic layer
precision.
[0005] 2. Background of the Invention
[0006] Atomic layer deposition (ALD) is a nanofabrication process
that has become an important method for the growth of high
dielectric constant materials, also known as "high-k materials,"
for replacement of silicon oxides (SiO.sub.2) as the gate
dielectric in metal-oxide-semiconductor field-effect-transistors
(MOSFETs). Atomic layer etching (ALET), also known as "digital
etching" has developed as an alternate process to ALD. ALET was
first reported for gallium arsenide (GaAs) etching with alternating
chlorine gas (Cl.sub.2) adsorption and electron beam etching. With
the development of these techniques, additional research explored
the possibility of ion bombardment to effect ALET of silicon, but
the necessary period for each etch cycle exceeds the acceptable
limits even at the laboratory scale.
[0007] A complete cycle of the traditional approach to atomic layer
etching (ALET) consists of four steps. First, the chemisorption
step, including clean substrate exposure to a reactant gas to
facilitate the adsorption of the gas onto the surface. Second,
excess Cl.sub.2 gas is purged with an inert gas flow to avoid
etching by a gas-phase reactant in the subsequent step. Third, the
reaction step, such as chemical sputtering, is affected between the
adsorbed gas and the underlying solid reaction, often via inert gas
plasma. Ideally, this process is also self-limiting; ions react
only with substrate atoms bonded to the chemisorbed gas. Once the
chlorinated layer is removed, further etching by physical
sputtering of the substrate must not occur or be sufficiently
limited. Finally, the evacuation of the reaction chamber exhausts
the etching products. If the periods of chemisorption in the first
step and the etching third step are for sufficiently extended
durations, the etching rate approaches one atomic layer per cycle,
where the atomic layer thickness is that of the chlorinated layer,
but not necessarily one monolayer of the substrate. Additionally,
if the substrate surface remains nearly-atomically smooth during
repeated ALET cycling, it is possible to achieve the ideal
condition of removal of substantially one monolayer of the
substrate per cycle.
[0008] However, the achievement of nearly atomic monolayer,
substrate removal with traditional ALET processes requires a very
long etching cycle, approaching and exceeding 150 seconds per
cycle. Further, traditional ALET processes include additional
limitations. First, gas pulsing is a disadvantage, exacerbated by
the fact that chemisorption gases, such as Cl.sub.2, have a long
residence time on the chamber walls and require long pumping
periods before the inert gas plasma is ignited. This makes the
etching rate very slow, even for the times required to etch very
thin films. Second, the etching rate per cycle may not necessarily
be constant or controllable. Specifically, the ion bombardment
induced roughening can cause the saturated layer thickness to
increase with cycle number, and accelerate the etching rate with
each cycle number.
[0009] Moore's law and the continued development of semiconductors
predict that devices in future integrated circuits will be as small
as one atomic layer thick and less than several atomic layers wide.
Present plasma etching processes are too coarse to achieve such
precise pattern transfer and can damage underlying layers of the
substrate. In particular, traditional plasma etching techniques do
not have the level of control that is needed for precise patterning
of sub-20 nm structures and the current atomic layer etching with
pulsed gases is too slow to be practical for large volume
manufacturing of future integrated circuits. Additionally, the
current techniques require an excess of precursor raw materials,
such as chlorine gas, which represents potential cost reductions
for finding more efficient processes.
[0010] Therefore, a novel method is needed if atomic layer etching
is to overcome the problems of slow etch cycle time, substrate
damage, poor resolution, and inefficient operation, thus enabling
the use of plasma etching to fabricate future nanodevices
incorporating quantum dots and/or wires, self-assembled films, and
other sensitive components with atomic layer resolution with
improved cost efficiency.
BRIEF SUMMARY
[0011] A system according to one embodiment of the disclosure
comprising a pulsed plasma source, comprising: spiral coil
electrode disposed around a tube; a Faraday shield disposed between
the tube and the spiral coil electrode and cooled by a fluid flow;
a counter electrode disposed at the top of the tube and at least
partially extending into the tube; a gas inlet disposed in the tube
and in fluid communication with a process gas supply; and a
reaction chamber in fluid communication with the pulsed plasma
source comprising: a substrate support; and a boundary
electrode.
[0012] A method for etching a substrate according to one embodiment
of the disclosure, comprising introducing a feed gas comprising a
mixture of inert gas and reactant gas, into a plasma chamber;
disposing the substrate in the plasma chamber; generating a plasma
containing reactants and ions from the feed gas; saturating a
substrate surface with the reactants to form a product layer
comprising a monolayer of the reactant species and a first
monolayer atoms of the substrate; and removing the product layer by
exposing the product layer to the ions.
[0013] A method for processing a substrate according to one
embodiment of the disclosure comprising, directing ions from plasma
afterglow toward a substrate surface saturated with a first
substance. And in certain embodiments, removing the first substance
and a monolayer of substrate atoms with the ions.
[0014] The foregoing has outlined rather broadly the features and
technical advantages of the invention in order that the detailed
description of the invention that follows may be better understood.
Additional features and advantages of the invention will be
described hereinafter that form the subject of the claims of the
invention.
BRIEF DESCRIPTION OF THE FIGURES
[0015] For a detailed description of the preferred embodiments of
the invention, reference will now be made to the accompanying
drawings in which:
[0016] FIG. 1 illustrates traditional Atomic Layer Etching (ALET)
process.
[0017] FIG. 2 illustrates an exemplary ALET process according to
one embodiment of the present disclosure.
[0018] FIG. 3 illustrates an exemplary ALET system according to one
embodiment of the disclosure.
[0019] FIG. 4 illustrates another exemplary ALET process according
to another embodiment of the disclosure.
[0020] FIG. 5 illustrates another exemplary ALET process according
to another embodiment of the present disclosure.
[0021] FIG. 6 illustrates another exemplary ALET system according
to another embodiment of the disclosure.
[0022] FIG. 7 illustrates measured ion energy distributions (IED)
obtained by applying DC voltages of 30, 50, 70 and 100 V to a
boundary electrode in the afterglow of a pulsed plasma.
[0023] FIG. 8 illustrates simulated ion energy distributions (IED)
obtained by applying DC voltages of 30, 50, 70 and 100 V to a
boundary electrode in the afterglow period of a pulsed plasma.
[0024] FIG. 9 illustrates ion and electron densities as a function
of vertical position along the discharge tube axis.
[0025] FIG. 10 illustrates a simulated SiCl and SiBr laser-induced
fluorescence above a Si substrate after laser-induced thermal
desorption.
[0026] FIG. 11 illustrates IEDs at a fixed pressure for different
DC bias applied continuously at the boundary electrode.
[0027] FIG. 12 illustrates resolved Langmuir probe measurements of
electron temperature for different pressures.
[0028] FIG. 13 illustrates normalized IEDs with DC bias applied
continuous on the boundary electrode.
[0029] FIG. 14 illustrates IEDs at different pressures under pulsed
plasmas conditions.
[0030] FIG. 15 illustrates IEDs with a synchronous DC bias boundary
electrode pulses at different times during the afterglow of pulsed
plasma, and where (a) is graph for bias starting in the early
afterglow and (b) is the graph for bias starting in the late
afterglow.
[0031] FIG. 16 illustrates graphs of IEDs with a synchronous DC
bias boundary electrode pulses at the same time during the
afterglow of pulsed plasma, and where (a) is the graph for the bias
duration .DELTA.tb=50 microseconds and (b) is the graph for the
bias duration .DELTA.tb=15 microseconds.
[0032] FIG. 17 illustrates graphs of IEDs with a synchronous DC
bias during the afterglow of a pulsed plasma for different plasma
modulation frequencies, where (a) is the graph for bias duration
.DELTA.tb=50 .mu.s and (b) the graph of normalized IEDs with
FWHM.
[0033] FIG. 18 illustrates the graph of IEDs with a synchronous DC
bias boundary electrode pulses during the afterglow of pulsed
plasma for different duty cycles.
DETAILED DESCRIPTION
[0034] Conventional Atomic Layer Etching: As shown in FIG. 1,
traditional atomic layer etching (ALET) process may include four
stages: exposing a substrate, such as silicon (Si), to a reactant
gas such as chlorine (Cl); purging the excess reactant gas from the
chamber; exposing the adsorbed reactant gas to an energetic flux
such as a plasma; and exhausting the chamber of etching products,
such as silicon chloride radicals (SiCl.sub.x), where x is between
about 0 and about 4.
[0035] The first step comprises a chemisorption step (1). A clean
substrate, typically comprising silicon, is exposed to a reactant
gas, such as chlorine (Cl.sub.2). The reactant gas adsorption is
self-limiting as the chemisorption stops when all available surface
sites are occupied. The reactant gas flow is only activated during
this chemisorption step. The second step (2) is necessary to remove
the excess reactant gas that may be in proximity to the substrate
or the substrate surface and prevent temporary deposition on the
chamber walls. More specifically, purging of excess reactant gas
(Cl.sub.2) may avoid spontaneous etching by gas-phase reactant
released from the walls in the subsequent etching step (3).
Spontaneous etching caused by excess or lingering reactant gas
eliminates the possibility of monolayer precision. In the third
step (3) the surface of the substrate is exposed to an energetic
flux, such as ions, electrons, or fast neutrals often via inert gas
plasma, such as inductively coupled plasma (ICP) to effect the
reaction between the adsorbed gas and the underlying solid. The
reaction or chemical sputtering is also self-limiting, because the
ions react only with substrate atoms bonded to the chemisorbed gas.
Once the chemisorbed layer is removed, additional etching of the
substrate is not desirable to maintain approximately single atomic
layer etching resolution. Finally, the chamber is evacuated to
remove the etching products and any substrate-reactant gas radicals
that may be present.
[0036] It is noted that this traditional ALET process requires a
very long etching cycle that is, for example, about 150 seconds (s)
per cycle. Further, extending the periods of chemisorption (1) and
etching (3), the etching rate approaches one atomic layer per cycle
but at the expense of increased cycle times and decreased process
efficiency. If the substrate surface remains at or nearly
atomically smooth during repeated ALET cycling, it is possible to
achieve the ideal condition of removal of substantially one
monolayer of the substrate per cycle. However, if the process
overly extended, the atomic layer thickness is that of the
chlorinated layer, and not necessarily one monolayer of the
substrate, thereby at least partially failing the objective of
ALET.
[0037] Novel ALET Overview: In the present disclosure, several
exemplary embodiments of the techniques and systems for ALET
process are disclosed. For the purposes of clarity and simplicity,
the disclosure is made focusing on one or more specific exemplary
systems and one or more specific particular techniques. Those
skilled in the art will recognize that the embodiments are
exemplary only. The disclosure is not to be limited in scope by the
specific embodiments described herein. Indeed, other various
embodiments of and modification to the present disclosure, in
addition to those described herein, will be apparent to those of
ordinary skill in the art.
[0038] The system and method for new ALET process may be
pulsed-plasma and pulsed-electrode bias voltages based process. In
embodiments, the system may include a plasma source such as ICP
source, capacitively coupled plasma (CCP) source, or helicon
source. In certain embodiments, the plasma source is an ICP source.
The plasma source may be provided with DC or radio-frequency (RF)
power in a continuous or pulsed current. In some embodiments, at
least one electrode is disposed near the substrate or immersed in
the plasma. In some instances, the ICP pulsing system has at least
one radio-frequency (RF) power generator for creating rapid RF
plasma pulses. In additional instances, the rapid ALET system
comprises electrodes positioned in the reaction chamber for biasing
the chamber, biasing the chamber wall, and biasing the plasma. In
alternate configurations the plasma pulsing system comprises a
secondary or auxiliary plasma source to aid in the stabilization of
the ICP during pulsing.
[0039] Further, the new ALET process presents a potential means for
obviating the traditional ALET rate limiting steps, in a
non-limiting example, gas pulsing. In some embodiments, the new
ALET process method may comprise two stages: adsorption stage and
etching stage. In some instances, the process may utilize
switchable electrical pulses to the ICP source and electrodes
positioned in the reaction chamber to control the chemisorption and
etching. For example, the electrodes may apply bias voltages in the
plasma in an approximately synchronous manner with the plasma
pulses. Differential control of the plasma pulses and electrode
bias voltages may permit fine control of the ion energy
distribution impingent upon the substrate. In further instances,
the new ALET process uses reduced amounts of process and reactant
gases, which may be understood to be toxic and corrosive. Compared
to the traditional etching methods, this provides reduced costs for
gases, improved safety, and improving environmental implications
for the process.
[0040] Novel ALET process: Referring to FIG. 2, there is shown ALET
process 200 according to an embodiment of the present disclosure.
The top portion of the figure illustrates ALET process, whereas the
bottom portion of the figure illustrates the process parameter. As
illustrated in the figure, ALET process comprises two stages:
adsorption stage 212 and etching stage 252. During the adsorption
stage 212, a substrate may be exposed to adsorbate such that the
adsorbate may adsorb onto the surface of the substrate. In some
embodiments, the adsorbate may be a reactant. In instances, the
adsorbate may comprise dissociated reactant atoms or dissociated
reactant molecules having unpaired electrons or dangling bonds. The
reactant may comprise, without limitation, halogens, fluorine (F),
chlorine (Cl), bromine (Br), or iodine (I). In certain embodiments,
the reactant may be disassociated chlorine (Cl) atoms that are
derived from chlorine reactant gas (Cl.sub.2). Without limitation
by theory, a skilled artisan may recognize that other halogens,
halogenated species, or other reactants may also be used in the
adsorbate. In alternate embodiments, the intact or un-dissociated
reactant may also be used as the adsorbate on the substrate.
Further, a skilled artisan may recognize that the term "gas"
includes vapor generated from a substance in solid or liquid state
at room temperature or at standard temperature and pressure,
without limitation.
[0041] The adsorbate may be obtained by generating plasma
containing the reactants. In certain instances, inert gas may be
ionized along with the reactant. Without limitation, the resulting
plasma may contain reactants, reactant gas ions, and inert gas
ions. In embodiments, argon (Ar) is used as the inert gas.
Additionally, a skilled artisan will recognize that any noble gas
species or other inert gas species may also be used.
[0042] In embodiments, if the reactant is ionized with the inert
gas, the concentration of the reactant gas, may be between about
0.01% and about 20% by volume; alternatively, the reactant gas
concentration may be between about 0.01% and about 15%; and in
certain instances, the reactant gas concentration may be between
about 0.01% and about 10% by volume of the combined gas. In certain
embodiments, the reactant gas may comprise a concentration of less
than about 1% by volume. Without limitation, the plasma generated
may primarily comprise Ar species and a small portion of Cl
reactant gas species.
[0043] In embodiments, the plasma source is used to generate the
reactant. Non-limiting exemplary plasma sources may include,
inductively coupled plasma (ICP) sources, capacitively coupled
plasma (CCP) sources, or helicon sources. In certain embodiments,
the plasma source is an ICP source. In instances, the ICP source
may RF powered during the adsorption stage 212.
[0044] In embodiments, the plasma source is not powered through the
entire adsorption stage 212. In instances, the RF power applied to
the plasma source may be lowered during the latter portion of the
adsorption stage 212. In non-limiting examples, the plasma source
may be RF powered during the beginning portion of the adsorption
stage 212, as illustrated in FIG. 2. Further, during the latter
portion of the stage 212, lower power may be applied to the plasma
source, or the plasma source may be turned OFF to provide
afterglow. Alternatively, the plasma source may be powered
continuously throughout the entire adsorption stage 212.
[0045] Without limitation by theory, the adsorption process may
occur as described herein. A substrate comprising a clean surface,
without a passivating layer may include unpaired electron or
dangling bonds. In instances, reactants from the plasma near the
substrate surface may then easily bond with the dangling bonds of
the surface, such as through chemisorption, to form a product
layer. In instances, the product layer may comprise a monolayer of
the reactants and a monolayer of the substrate atoms that are
associated. In instances, the Cl reactants are adsorbed onto the
surface of an exemplary silicon (Si) substrate to form a product
layer comprising SiCl.sub.x. Further, in certain instances, the
product layer may comprise a monolayer of reactant species Cl atoms
and a monolayer of Si atoms. Adsorption may continue until the
substrate surface is saturated with the reactants. Without
limitation, saturation is achieved when substantially all available
substrate surface-sites, such as unpaired electrons or dangling
bonds, are occupied or associated with the reactants. As may be
understood by a skilled artisan, in certain instances a portion of
the substrate surface is not covered with the reactants. For
example, a portion of the substrate surface may contain a
passivating layer, such as but not limited to an oxide layer. In
non-limiting examples, the passivating layer may not contain
available sites, available unpaired electrons or dangling bonds,
and as such is not covered with the reactants. In certain
instances, the substrate surface is at least partially covered with
chemisorbed reactants in the product layer and at least partially
covered with a passivating layer.
[0046] In embodiments, during the adsorption stage 212, the
reactant gas ions and/or inert gas ions may be present in the
plasma, such that the substrate surface comprising a product layer
is exposed to the ions. In instances, the energy of the ions
bombarding the substrate may be selectively controlled to avoid or
minimize undesired etching, physical or chemical sputtering. For
example, the energy required by Cl ions to etch Si may be about
10-25 eV, whereas the energy required by Ar ions to cause
sputtering may be about 30-60 eV. In certain embodiments, the
energy of the ions bombarding the substrate during the adsorption
stage 412 may be controlled to be about 10 eV or less. The ion
energy may be controlled by, for example, providing an
electrostatic shielding (e.g. Faraday shield) of the plasma source
and/or performing the process under relatively high pressure in
order to minimize undesired etching, physical or chemical
sputtering. Moreover, Cl reactant atoms do not etch p-type or
moderately doped n-type Si at room temperature, requiring thermal
control of the process.
[0047] In embodiments, after completion of the adsorption stage
212, the etching stage 252 may be performed. During this etching
stage 252, ions may bombard the substrate to remove the product
layer. In certain embodiments, the ions comprise positively charged
ions or negatively charged ions. In instances, positively charged
ions are used to remove the product layer. As understood by a
skilled artisan, the energy of the ions bombarding the substrate
during the etching stage 252 may preferably be above the threshold
for chemically-assisted sputtering but below the threshold for
physical sputtering. The ions with selected energy may be directed
toward the substrate by controlling the potential difference
between the plasma and the substrate. To direct positive ions
toward the substrate, the potential difference between may be
increased by increasing the plasma potential relative to the
substrate potential, decreasing the substrate potential relative to
the plasma, or both. To direct negative ions, the potential
difference between may be increased by decreasing the plasma
potential relative to the substrate potential, increasing the
substrate potential relative to the plasma, or both. Positive or
negative, DC or RF bias may be applied to the plasma and/or the
substrate during the etching stage 252. In addition, continuous
bias may be provided to the plasma and/or the substrate as shown in
FIG. 2. Alternatively, a series of pulsed bias may be provided as
shown in FIG. 4.
[0048] In certain embodiments, the plasma source may be RF powered
during the etching stage 252, as shown in FIG. 4. In instances, the
plasma source may be provided with pulsed RF power, where each RF
power pulse is provided between the bias pulses noted above. For
example, a series of pulsed RF power may be applied to the plasma
source during the etching stage 252 and a series of pulsed DC or RF
bias may be applied to the plasma and/or the substrate. Each bias
pulse may be provided between the RF power pulses. Alternatively,
the bias pulse is between about 1 .mu.s and about 20 .mu.s;
alternatively about 10 .mu.s into the afterglow of each plasma
source pulse.
[0049] In instances, by selectively increasing the potential
difference between the plasma and the substrate, the product layer
comprising the chlorinated product layer in the certain embodiment
described here, may be removed. In the process, the monolayer of
the substrate atoms associated with the product may be removed from
the substrate concurrently. Additionally, the adsorption stage 212
and the etching stage 252 may be repeated to remove additional
layers of the substrate atoms one layer at a time.
[0050] Novel ALET system: Referring now to FIG. 3, there is shown
an exemplary ALET system 300 according to one embodiment of the
present disclosure. ALET system 300 according to one embodiment of
the present disclosure may comprise a plasma chamber 326 having top
wall 328, bottom wall 330, and side wall 332. ALET system 300 may
also comprise a plasma source 302, a shield 304 interposed between
the plasma chamber 326 and the plasma source 302, a substrate
support 306, a boundary electrode 308, a counter-electrode 310, and
an inlet 312. The plasma source 302 may be coupled to a pulsing
system 314. The substrate support 306, meanwhile, may be coupled to
a support system 316. The support system 316 may be a power supply
capable of providing continuous or pulsed DC or RF bias to the
substrate support 306. Alternatively, the support system 316 may
simply be a ground or a component connected to ground. The boundary
electrode 308 may be coupled to a first voltage system 318. The
counter-electrode 310 may be coupled to a second voltage system
320.
[0051] In embodiments, the ALET system may additionally comprise a
pump 124 coupled to the plasma chamber 126. In certain
configurations of ALET system 300, at least one cooling conduit 336
may be included. In other configurations, the substrate support 306
may comprise a differential pumping conduit 334. In alternate
configurations, the plasma chamber top 328 may comprise a
counter-electrode 110 and the gas inlet 112. In further alternate
configurations, the system 300 may further comprise an auxiliary
plasma chamber 350 coupled to the plasma chamber 326. An auxiliary
plasma source 352 may be disposed near the auxiliary plasma chamber
350.
[0052] In embodiments, the plasma source 302 and the auxiliary
plasma source 352 may be any type of plasma source known to those
skilled in the art, including an ICP source, CCP source, helicon
source and heat source, without limitation. In certain embodiments,
the plasma source 302 may be ICP source 302. ICP source 302 may be
a planar or a cylindrical ICP source 302 comprising a planar or
helical coil. Alternatively, the ICP source may have other
geometry. The portion of the plasma chamber 326 and/or the
auxiliary chamber 350 adjacent to the plasma source 302 and/or the
auxiliary plasma source 352 may be made out of dielectric material
such as, for example, quartz or alumina. For example, at least a
portion of the plasma chamber 326 and the auxiliary plasma chamber
352 or entire the plasma chamber 326 and the auxiliary plasma
chamber 352 may be made out of dielectric material. In certain
instances, the ICP source 302 comprises a spiral coil electrode
disposed around an alumina or other dielectric discharge tube. In
further instances, the ICP source comprises a three-coil spiral
electrode.
[0053] The shield 304 may comprise a Faraday shield. In
embodiments, the Faraday shield comprises any conducting material
suitable for preventing external interference with the ICP source
302. In instances, the shield 304 may comprise copper. In certain
instances, the shield 304 may be configured to prevent capacitive
coupling between the coil of the ICP source 302 and the plasma it
generates. Alternatively, the shield 304 is configured to prevent
any electrostatic signals from exiting the plasma chamber 326.
[0054] The substrate support 306 comprises a support for a
semiconductor during etching. In embodiments, the substrate support
306 comprises an electrode. In some instances, the substrate
support 306 is a ground electrode. In certain instances, the
substrate support 306 comprises a bias electrode, configured to
generate and maintain a bias voltage in response to an RF
electromagnetic-field or direct current (DC) pulsing. In further
embodiments, the substrate support 306 enters the plasma chamber
326 via the bottom 330 of the plasma chamber 326. In instances, the
substrate support 306 supports the substrate 301 at or in proximity
to the bottom 330 of the plasma chamber 326.
[0055] The boundary electrode 308 comprises an electric conducting
material disposed proximal to the substrate support 306. In some
embodiments, the boundary electrode 306 may be disposed
concentrically around the substrate support 306 near the bottom 330
of the plasma chamber 326. In instances, the boundary electrode 308
is configured to apply bias in response to an RF or DC signal
applied to the plasma source 350, the auxiliary plasma source 302,
and/or the counter electrode 302.
[0056] The counter-electrode 310 may comprise an electrical
conducting material disposed vertically opposite from the substrate
support 306. In embodiments, the counter-electrode 310 is disposed
opposite from the boundary electrode 308 in the chamber 326. In
some instances, the counter-electrode 310 is applied with a bias
voltage in response to a RF or DC signal applied to the plasma
source 302, the auxiliary plasma source 352, and the boundary
electrode 308. In certain instances, the counter-electrode 310
generates a bias voltage or a pulsed bias voltage that is opposite
to the bias voltage of the boundary electrode 308.
[0057] The inlet 312 comprises a gas conduit into the chamber 126.
In embodiments, the inlet 112 is proximal to the top of the chamber
126 or through the top 128 or the chamber 126. Without limitation,
the inlet 312 may introduce inert gas and reactant gas into the
plasma chamber 326. In instances, the inlet 312 provides heated
gases to the chamber 326 and the plasma source 302. In certain
instances, the inlet 312 may introduce non-ionized process and
reactant gases to the chamber 326 and the plasma source 302.
Alternatively, the inlet 312 is in communication with at least one
auxiliary plasma source 350, for introducing at least partially
ionized process and reactant gases to chamber 326 and the plasma
source 302.
[0058] The plasma source 302 may be coupled to a pulsing system
314. In embodiments, the pulsing system 314 comprises at least one
power supply capable of providing pulsed or continuous RF and/or DC
signal to the plasma source 302. In some instances, the pulsing
system 314 may comprise at least one RF or DC power supply and an
electric power amplifier. In some other instances, the pulsing
system 314 may comprise a plurality of RF or DC power supply and
power amplifiers. The pulsing system 314 may be coupled to the
plasma source 302 via an impedance-matching (e.g. L-type) network.
The pulsing system 314 is further configurable to provide
electrical power at any frequency to plasma source 302. In
instances, the pulsing system 315 is configured to cut or remove
power from the plasma source 302 in periodic pulses. In certain
instances, the RF or DC power supply may provide the plasma source
302 with a square wave function between zero volts and a
predetermined high voltage and at a predetermined frequency. As may
be understood by those skilled in the art, removing or altering the
RF electric current through the coil removes or enhances the
formation of plasma.
[0059] The substrate support 306 is coupled to a support system
316. In embodiments, the support system 316 comprises an electric
circuit including substrate support 306. In instances, the support
system 316 is a grounded electrode. In certain instances, the
support system 316 comprises an RF function generator or a DC
source. The support system 316 is configured for producing a bias
voltage at the substrate support 106 in response to electrical
pulses from the RF function generator or a DC source. In certain
configurations, the support system 316 receives an RF or DC current
from the pulsing system 314 as the bias voltage at substrate
support 305. Further, the bias voltage of the substrate support 316
may be pulsed in coordination with other electrodes in the system
300.
[0060] The boundary electrode 308 is coupled to a first voltage
system 318. In embodiments, the first voltage system 318 comprises
an electric circuit including the boundary electrode 318. In
instances, the first voltage system 318 is an electric ground, a RF
function generator, or a DC source. In certain instances, the first
voltage system 318 is configured for producing a bias voltage at
the boundary electrode 308 in response to DC source. In certain
configurations, the first voltage system 318 receives an RF or DC
current from the pulsing system 314, as the bias voltage at the
boundary electrode 308. Further, the bias voltage of the boundary
electrode 308 may be pulsed in coordination with other electrodes
in the system 300.
[0061] The counter-electrode 310 is coupled to a second voltage
system 320. In embodiments, the first voltage system 318 comprises
an electric circuit including the counter-electrode 310. In
instances, the second voltage system 320 is an electric ground, an
RF function generator, or a DC source. In certain instances, the
second voltage system 320 is configured for producing a bias
voltage at the counter-electrode 310 in response to DC source. In
certain configurations, the first second voltage system 320
receives an RF or DC current from the pulsing system 314, as the
bias voltage at the counter-electrode 310. Further, the bias
voltage of the counter-electrode 310 may be pulsed in coordination
with other electrodes in the system 300.
[0062] The gas inlet 312 is fluidly connected to a gas source 322.
In embodiments, the gas source 322 comprises process gas and
reactant gas mixture for introduction to the plasma source 302. In
instances, the process gas comprises any inert gas that will be
ionized to form plasma at the plasma source 302. In certain
instances, the process gas comprises a noble gas, nitrogen,
hydrogen, oxygen, oxygenated gases, or combinations thereof without
limitation. The reactant gas comprises any gas that will be
chemisorbed by the substrate 301 after partial ionization at the
plasma source 302. In certain instances, the reactant gas comprises
a halogen, a halocarbon, a halide, or other halogenated gases
without limitation. In further instances, the process gas and the
reactant gas may be any gases suitable for ALET. In embodiments,
the gas source comprises a concentration of the process gas of
greater than about 90% by volume; alternatively, greater than about
95% by volume; and in certain instances, the gas source has a
concentration of process gas that is greater then about 99% by
volume.
[0063] The thermal conduit 336 is configured to alter the
temperature of the gas in the system. In embodiments, the cooling
conduit may be any conduit in thermal contact with the system 100
and configured for carrying a cooling liquid or gas. In instances,
the cooling conduit 136 is in thermal communication with the
cylindrical wall 332, and the shield 304. In embodiments, the
cooling conduit 336 is disposed in thermal communication with a
flange such as chamber bottom 330, which couples the cylindrical
wall 332 and the shield 304.
[0064] The pump 324 may be any pump configured to reduce gas
pressure in a reaction chamber 326 to about 1 mTorr. In embodiments
the pump 324 is configured to lower and maintain the pressure in
the plasma chamber 326 to between about 1 mTorr and about 500
mTorr; alternatively between about 5 mTorr and about 250 mTorr; and
alternatively, between about 10 mTorr and about 100 mTorr. In
certain instances, the pump 324 operates a pressure between about
10 mTorr and about 75 mTorr in the chamber 326. In instances, the
pump 324 comprises at least one vacuum pump. In embodiments, the
pump 324 comprises a turbo vacuum pump and a dry pump. Without
limitation by theory, the pump 324 may be configured to operate
within any range of pressures in order to evacuate the chamber of
ionized gases, etched products, and other gaseous contaminants.
[0065] Alternate ALET process: Referring again to FIG. 4, there is
shown an alternate exemplary method for controlling the ALET
process according to another embodiment of the present disclosure.
FIG. 4 illustrates the timing sequence of RF/DC power/voltage
signals applied various components of the ALET system shown for
example in FIG. 3. In embodiments, the signals may be used to
control plasma physics and chemistry during ALET process.
[0066] Referring briefly to FIG. 3, the plasma source 302 is
applied with RF power for approximately 1 second during the etching
stage, as in stage 202 in FIG. 2, to provide reactants (e.g., Cl
atoms), to form a chemisorbed layer. In embodiments, the plasma
source is applied with RF power throughout the entire adsorption
stage. As described herein previously, the plasma may source may be
applied with RF power during the beginning portion of the
adsorption stage and powered down during the latter portion of the
stage. In certain embodiments, the plasma in the plasma chamber 326
may be ignited by the tail-end of a low-power, auxiliary plasma
generated in the auxiliary plasma chamber 350. During ignition of
the plasma, the ion bombardment energy may be sufficiently low
(<10 eV) to prevent any etching to occur. During the etching
stage, as in stage 252 in FIG. 2, a pulsed ICP period of
approximately 0.5 s removes the chemisorbed layer (e.g.
SiCl.sub.x). Pulsing the plasma source power, as a square wave
modulation of 13.56 MHz applied RF voltage, has several benefits
described hereinafter.
[0067] First, the electron energy distribution function (EEDF)
cools rapidly during the first several .mu.s of the power OFF
portions of the cycle in the afterglow, without a substantial loss
of plasma density, for example over a typical about 100 .mu.s OFF
time. The resulting lower energy time-averaged EEDF offers some
level of control of the degree of dissociation of the feed gases.
Second, during most of the about 100 .mu.s afterglow period, a
mono-energetic ion flux to the substrate can be generated, as
recently demonstrated in this laboratory. In this example, a pulse
of positive DC voltage may be applied to the boundary electrode,
raising the plasma potential and pushing positive ions toward the
surfaces substrate with lower potential. Thus, a grounded substrate
is bombarded with ions with energy equal to V.sub.DC1, as shown in
FIGS. 7 and 8. Since control of the ion energy distribution is
critical to effect chemical sputtering of the chemisorbed
halogenated layer, without physical sputtering of the underlying
substrate, this method of obtaining extremely narrow IEDs, and thus
extreme selectivity, is an effective means to achieve ALET with
monolayer accuracy. This pulsed-main-ICP with synchronous
pulsed-immersed-electrode-bias-voltage period is long enough (e.g.,
0.5 seconds) to sputter away the halogenated etch product layer.
However, those of ordinary skill in the art with recognize that
negative DC or RF voltage may be applied to the substrate, for
example, via the substrate support. In the process, the substrate
potential may be lowered to attract the positive ions.
[0068] Net positive ion bombardment can cause a positive charge to
build up on the substrate. However, after the boundary voltage
pulse returns to zero, and the plasma has had a chance to approach
its natural V.sub.p, any charged surfaces with potentials above
ground are the first to receive an excess electron flux over the
positive ion flux, bringing their potential back to the floating
potential, which is near ground potential. To accelerate positive
charge neutralization, a large negative DC bias may be applied to,
for example, the counter-electrode 310, while a continuous wave ICP
power is ON. This negative voltage may have no effect on V.sub.p.
However, the resulting high energy ion bombardment of the
counter-electrode 310 may generate secondary electrons that are
accelerated to the full sheath potential. These high energy
"ballistic" electrons may have a low scattering cross section and
bombard the substrate at nearly normal incidence, compensating
positive charge at the bottom of even high aspect ratio insulating
structures. The ballistic electrons can also have beneficial
effects on the bulk plasma, such as the enhanced plasma density and
lower bulk T.sub.e.
[0069] Alternatively, for insulating substrates, application of
synchronous pulsed RF voltage to the substrate electrode in the
afterglow period would result in a negative self-bias, and
energetic positive ion bombardment of the substrate. Depending on
plasma density and applied frequency, ion energies on a RF biased
substrate 301 can be peaked at the average sheath potential or
double peaked. The resulting ion energy distributions are normally
too wide to achieve the extreme selectivity required by ALET.
Application of very high frequency (100 MHz) bias can narrow the
IEDs, but the width of the IED depends on ion mass, making IED
control very difficult in mixed gas plasmas. Narrow ion energy
distributions could possibly be obtained with tailored bias pulses.
For conducting substrates, synchronous pulsed DC negative bias can
be applied directly on the substrate support electrode during the
afterglow, and nearly mono-energetic ion bombardment can be
achieved at any desired energy, the same way as in the case of the
boundary voltage described above.
[0070] Referring now to FIG. 5, a process flow diagram of the ALET
process is shown. As illustrated, the method 500 generally
comprises two stages: adsorption stage 502 and etching stage 550.
As may be understood, within each stage may comprise one or more
steps or incremental steps that when conducted sequentially or
synchronously accomplish the method 500. In other words, although
FIG. 5 illustrates the steps being performed sequentially, the
steps may be performed simultaneously, or at least some portions of
the steps may be performed simultaneously. As illustrated in FIG.
5, the adsorption stage 502 may comprises substrate positioning
step 504, reactants forming step 510, and reactant adsorption step
520. Meanwhile, the etching stage 550 may comprise a potential
difference increasing step 570. As noted above, the potential
difference between the plasma and the substrate may be increased by
applying RF or DC voltage to the plasma or the substrate.
Optionally, the etching stage 550 may also comprise substrate
charge neutralization step 552, and plasma pulsing step 560, and
etched product removing step 580. As noted above, charge
neutralization step 552 may be performed by biasing the counter
electrode. The present ALET process 500 may be considerably faster
than conventional ALET process. More specifically, after the
substrate positioning step 504, the remaining adsorption steps 520
may require a time between about 0.01 s and about 10 seconds;
alternatively, between about 0.1 second and about 5 seconds; and in
embodiments between about 0.5 second and about 1.5 seconds.
Additionally, the etching stage 550 may require a time between
about 0.01 second and about 10 seconds; alternatively, between
about 0.1 second and about 5 seconds; and in embodiments between
about 0.2 second and about 1 second. After the etch product
removing step 580, the stages or steps may be repeated in entirety
or partially until a desired etch depth is reached. In certain
instances, charge neutralization step 552, and plasma source
pulsing step 560, and the potential difference increasing step 570
may be performed simultaneously, or in the alternative,
synchronously.
[0071] More specifically, the adsorption stage 502 may comprises
the steps in the disclosed rapid ALET process suitable for
adsorbing reactants on the substrate. The first step in the stage
comprises the substrate positioning step 504 where the substrate is
positioned in a chamber. In some embodiments, the substrate is
mounted a substrate support. In certain instances, the substrate
support may be an electrode.
[0072] When the substrate is positioned in the chamber, the
pressure in the chamber may be reduced. In embodiments, the
pressure, during the ALET process, is maintained between about 1
mTorr and about 500 mTorr; alternatively between about 5 mTorr and
about 250 mTorr; and alternatively, between about 10 mTorr and
about 100 mTorr. In certain instances, the pressure is maintained
between about 10 mTorr and about 75 mTorr during the substrate
positioning step 504 and maintained there throughout the entire
ALET process. In further instances, the pressure may be altered to
provide IED control at any time throughout the novel ALET Process.
As may be understood by those skilled in the art increase in
pressure in the reaction chamber may correlate to an increase in
gas particles and radicals. Without being limited by theory,
increased pressure may decrease the peak energy of ions and broaden
the IED, and vice versa.
[0073] During the reactants forming step 510, feed gas may be
introduced into the chamber. In some embodiments, the feed gas may
comprise inert gas and reactant gas. Without limitation by theory,
the reactant gas may comprise, a reactive species, when ionized. In
the present embodiment, the reactant gas may comprise Cl.sub.2.
However, those skilled in the art may recognize that other reactant
gas such as other halogen containing gas may also be used.
Meanwhile, the inert gas may comprise Ar. However, those skilled in
the art may recognize that other inert gas may also be used. In the
present embodiment, the inert gas may have higher concentration by
volume than the reactant gas. In some instances, the reactant gas
may comprise a concentration by volume between about 0.01% and
about 20%; alternatively, between about 0.01% and about 15%; and
alternatively between about 0.01% and about 10% of the mixed gas.
In alternate instances, the reactant gas may comprise a
concentration anywhere greater than about 0% and below about 5% by
volume of the mixed gases.
[0074] The feed gas containing the reactant gas and the inert gas
may be ionized by the plasma source to form plasma containing,
among others, reactants, reactant gas ions, and inert gas ions. As
noted above, various types of plasma source may be used. In certain
embodiments, the feed gas may be heated to a temperature of greater
than about 200K; alternatively, to a temperature greater than about
400K. In certain instances, the gas stream is subjected to further
RF electromagnetic fields. Components of this plasma including
combinations of excited states of species, radicals, ions,
electrons, and photons are injected in to the etching chamber. The
partially ionized reactant gases are pulled directionally toward or
away from the substrate in response to the charge bias within the
chamber.
[0075] During the reactant adsorption step 520, the reactants are
adsorbed or chemisorbed onto the surface of the substrate. In
embodiments, a voltage bias within the chamber may attract the
ionized reactant gases to the substrate. As the substrate has a
limited number of surface sites to adsorb the reactants, such as
unpaired electrons or dangling bonds. The reactants will continue
to adsorb onto the substrate surface until the end of the
adsorption stage, when all available surface sites or dangling
bonds on the substrate are occupied with the reactants. As a
result, a product layer comprising a monolayer of the reactant
atoms and a monolayer of underlying substrate atoms may form.
During the reactant adsorption step 520, the plasma and the ions
are maintained at low energy (e.g. 10 eV or less) to avoid or
minimize etching during the reactant adsorption step 520.
[0076] After completion of the adsorption stage 502, etching stage
550 may be performed. As noted above, the etching stage 550 may
comprises the potential difference increasing step 570. During this
step, the potential difference between the plasma and the substrate
is increased such that ions from the plasma may bombard the
substrate at a desired energy range. For example, the ion energy
may be chosen that is below the physical sputtering threshold but
above the threshold for chemically-assisted sputtering. As noted
above, the potential difference may be increased by applying DC or
RF voltage to the plasma, substrate, or both. In addition, the
applied voltage may be continuous (as shown in FIG. 2) or pulsed
(as shown in FIG. 4). If pulsed voltage is applied, RF pulses may
be applied to the plasma source, between the voltage pulses. In
some embodiments, applying RF pulses may comprise subjecting plasma
source (e.g. ICP source) to a periodic square wave function, where
the square wave extends from zero power to a pre-determined power.
Without limitation by theory, the pre-determined high voltage is
capable of creating ions with enough ionic energy to remove the
product layer. In certain instances, ions with this energy
establish the lower ionic energy limit for an IED. Conversely, it
may be understood that the pre-determined high voltage is capable
of creating ions with a lower ionic energy that does not damage the
substrate. In certain instances, ions with this energy establish
the upper ionic energy limit for an IED. More specifically, the
high voltage pulsing for the ICP plasma is chosen during the plasma
pulsing step 560 such that the IED falls entirely with these
parameters.
[0077] During the optional plasma pulsing step 560, a square wave
function may pulse the plasma for between about 1 microsecond and
about 500 microseconds; alternatively between about 10 microseconds
and about 250 microseconds; and in certain instances, the plasma is
pulsed for between about 25 microseconds and about 100
microseconds. Further, the square wave function pulses the plasma
to about zero voltage for between about 10 microseconds and about
750 microseconds; alternatively between about 50 microseconds and
about 500 microseconds; and alternatively between about 100
microseconds and about 250 microseconds. When the plasma is pulsed
to about zero power, an afterglow of ions remains. Without
limitation by theory, the afterglow contains ions that are within
the IED needed to remove the product layer.
[0078] Optionally, in the optional charge neutralization step 552,
the counter-electrode may be applied with negative bias voltage. In
some instances, the counter-electrode may be applied with negative
voltages attract positively charged ions into the
counter-electrode. The bombardment of the positively charged ions
into the counter-electrode may generate high energy secondary
electrons which may bombard the substrate at nearly normal
incidence. In addition, the secondary electrons may enhance the
plasma density and lower bulk electron temperature T.sub.e.
[0079] Between the pulses applied to the plasma, the boundary
electrode may be applied with positive voltage pulses. In certain
instances, a square wave function DC pulses the boundary electrode
to positively-charged voltage bias for between about 10
microseconds and about 750 microseconds; alternatively between
about 50 microseconds and about 500 microseconds; and alternatively
between about 100 microseconds and about 250 microseconds. In
instances, the positively-charged voltage bias is present only when
the high voltage plasma pulse is not. Alternatively, the
positively-charged voltage bias is present for the complete
duration of the etching the product layer 250.
[0080] In certain instances, the substrate support may be grounded,
powered with RF, DC or a combination thereof. In embodiments, the
substrate stage may be pulsed commensurate with the boundary
electrode. Additionally, as certain substrates may have different
conductivities, pulsing the substrate support bias provides
additional means of controlling the IED as described previously for
any electrode in the system. More specifically, the substrate
support could be applied with negative DC voltage. Alternatively, a
high frequency RF pulse or tailored DC pulse to the substrate
support in the case of an insulating substrate or under other
selected conditions.
[0081] ALET Pulsing: As noted above, the optional plasma pulsing
during the etching stage 550 provides the ability to control the
disassociation of the feed gases and IED. Providing the plasma
pulsing during etching stage 550 may also reduce the angular
distribution of ions impacting the substrate. Under collision-less
conditions, the angular spread is given by Equation 1:
.theta..sub.IAD.apprxeq.arc tan( {square root over (T.sub.e/2V)})
Eq. 1
For a sheath voltage, V=V.sub.sh=50 V and T.sub.e=0.3 eV, the
angular spread .theta..sub.IAD=3.degree.. This small angular
spread, comparable to conventional plasma etching at much higher
ion energies, is very desirable for obtaining consistent deep
etching through multiple atomic layers, as well as minimizing ion
energy transfer to the sidewalls of features from glancing angle
collisions and sidewall damage.
[0082] It may be understood by a skilled artisan that all
discussions of charge, ionization, electromagnetic potential are
merely exemplary, and that any discussion of the state of matter in
one embodiment is equally applicable to opposite states. More
specifically, while some non-limiting examples describe the
relationship between a negatively charged ion and an electrode, one
of skill in the art will recognize that the interaction between a
positively charged ion and an electrode would follow similar
properties.
[0083] While preferred embodiments of the invention have been shown
and described, modifications thereof can be made by one skilled in
the art without departing from the spirit and teachings of the
invention. The embodiments described herein are exemplary only, and
are not intended to be limiting. Many variations and modifications
of the invention disclosed herein are possible and are within the
scope of the invention. Where numerical ranges or limitations are
expressly stated, such express ranges or limitations should be
understood to include iterative ranges or limitations of like
magnitude falling within the expressly stated ranges or limitations
(e.g., from about 1 to about 10 includes, 2, 3, 4, etc.; greater
than 0.10 includes 0.11, 0.12, 0.13, and so forth). Use of the term
"optionally" with respect to any element of a claim is intended to
mean that the subject element is required, or alternatively, is not
required. Both alternatives are intended to be within the scope of
the claim. Use of broader terms such as comprises, includes,
having, etc. should be understood to provide support for narrower
terms such as consisting of, consisting essentially of, comprised
substantially of, and the like. Accordingly, the scope of
protection is not limited by the description set out above but is
only limited by the claims which follow, that scope including all
equivalents of the subject matter of the claims. Each and every
claim is incorporated into the specification as an embodiment of
the present invention. Thus, the claims are a further description
and are an addition to the preferred embodiments of the present
invention. The discussion of a reference in the Description of
Related Art is not an admission that it is prior art to the present
invention, especially any reference that may have a publication
date after the priority date of this application. The disclosures
of all patents, patent applications, and publications cited herein
are hereby incorporated by reference, to the extent they provide
exemplary, procedural or other details supplementary to those set
forth herein.
[0084] To further illustrate various illustrative embodiments of
the present invention, the following examples are provided:
EXAMPLES
[0085] Experimental apparatus: FIGS. 3 and 6 show schematics of the
experimental apparatus used in this study. The inductively coupled
plasma (ICP) was ignited by a 3-turn spiral coil in a 17.8 cm long,
8.6 cm inside diameter alumina tube. A copper Faraday shield
prevented capacitive coupling between the coil and the plasma. The
discharge tube was connected to a cubical stainless steel (SS)
chamber through an adaptor flange. A water channel in that flange
served to cool the Faraday shield and prevent overheating of the
discharge tube. The system was pumped by a 300 l/s turbo pump
backed by a dry pump. Pressure was measured by a MKS 629
capacitance manometer mounted downstream of the plasma at 0.1 Torr
full scale setting. A calibration experiment, without plasma,
showed that the pressure at the discharge region was about 30-40%
higher than that measured at the position of the pressure gauge.
Pressures reported below are all calibrated values and refer to the
plasma region.
[0086] A stainless steel electrode comprised the top electrode of
the plasma source. The top electrode had three coaxial cylindrical
SS rings welded to the electrode to increase the total surface area
to about 300 cm.sup.2 and minimize sputtered metal from coating the
chamber. The large surface area was found to be necessary during
Langmuir probe measurements when the probe was biased close to
V.sub.P. A large grounded surface was then required to supply an
adequate electron current, preventing an artificial increase of
V.sub.P. Argon gas, with a high purity, 99.999% was fed into the
discharge tube through a 1-mm diameter hole at the center of the
top electrode. Plasma power at 13.56 MHz was supplied using a
function generator (HEWLETT PACKARD.RTM. Model 3325A) feeding a
power amplifier (ENI Model A-500). The output of the amplified was
connected to the coil via an L-type matching network. Forward and
reflected powers were monitored by in-line Bird meters placed
before the matching network. For typical continuous wave (cw) 300 W
Argon plasma at 14 mTorr, the reflected power was 1-2 W. The actual
power dissipated in the plasma is somewhat lower than the net power
delivered to the matching box due to power losses. For pulsed
plasma operation, the RF pulse was amplitude-modulated by another
function generator (BNC Model 645). Waveforms were monitored using
a four-channel oscilloscope (TEKTRONIX.RTM. Model TDS 2024B). Base
case conditions for pulsed plasma experiments were 120 W
time-average forward power, 8 W reflected power, 10 kHz power
modulation frequency, 20% duty cycle, 14 mTorr pressure, and 40
standard cubic centimeters per minute (sccm) argon gas flow rate.
The applied modulation frequency and duty cycle resulted in 20
.mu.s (microsecond) plasma ON (active glow) time and 80 .mu.s
plasma OFF (afterglow) time, during the 100 .mu.s period of a
pulse.
[0087] Experimental operation overview: FIGS. 2 and 4 show examples
of a timing sequence that is used to control plasma physics and
chemistry. First, an approximately 1 s (second) continuous-wave
main RF ICP is ignited by the tail-end of a low power auxiliary
plasma, and provides reactants (e.g., Cl) to form a chemisorbed
layer. During this time, the ion bombardment energy is too low
(<10 eV) for any etching to occur. Next, a pulsed ICP period of
typically .about.0.5 s removes the chemisorbed layer (e.g.
SiCl.sub.x). Pulsing the main RF-ICP plasma source power (e.g.,
square wave modulation of 13.56 MHz applied RF voltage) has several
benefits. First, the electron energy distribution function (EEDF)
cools rapidly during the first several .mu.s of the power OFF
portions of the cycle (in the "afterglow"), without a substantial
loss of plasma density (over a typical about 100 .mu.s OFF time).
The resulting lower energy time-averaged EEDF offers some level of
control of the degree of dissociation of the feed gases. Second,
during most of the about 100 .mu.s afterglow period, a
mono-energetic ion flux to the substrate can be generated, as
recently demonstrated in this laboratory. In this example, a pulse
of positive DC voltage is applied to the boundary electrode,
raising the plasma potential and "pushing" positive ions to
surfaces of lower potential. Thus, a grounded substrate is
bombarded with ions with energy equal to V.sub.DC1, as shown in
FIGS. 7 and 8. Since control of the ion energy distribution is
critical to effect chemical sputtering of the chemisorbed
halogenated layer, without physical sputtering of the underlying
substrate, this method of obtaining extremely narrow IEDs, and thus
extreme selectivity, is an effective means to achieve ALET with
monolayer accuracy. This pulsed-main-ICP with synchronous
pulsed-immersed-electrode-bias-voltage period is long enough (e.g.,
0.5 seconds) to sputter away the halogenated etch product
layer.
[0088] Net positive ion bombardment can cause a positive charge to
build up on insulating substrates. However, after the boundary
voltage pulse returns to zero, and the plasma has had a chance to
approach its natural V.sub.p, any charged surfaces with potentials
above ground are the first to receive an excess electron flux over
the positive ion flux, bringing their potential back to the
floating potential, which is near ground potential. To accelerate
positive charge neutralization, a large negative DC bias may be
applied to the counter-electrode, while a continuous wave ICP power
is ON, as in FIGS. 3, 5, and 6. This negative voltage has no effect
on V.sub.p. However, the resulting high energy ion bombardment of
the counter-electrode generates secondary electrons that are
accelerated to the full sheath potential. These high energy
"ballistic" electrons have a low scattering cross section and
bombard the substrate at nearly normal incidence, compensating
positive charge at the bottom of even high aspect ratio insulating
structures. The ballistic electrons can also have beneficial
effects on the bulk plasma, such as the enhanced plasma density and
lower bulk T.sub.e.
[0089] Alternatively, for insulating substrates, application of
synchronous pulsed RF voltage to the substrate electrode in the
afterglow period would result in a negative self-bias, and
energetic positive ion bombardment of the substrate. Depending on
plasma density and applied frequency, ion energies on a RF biased
substrate can be peaked at the average sheath potential or double
peaked. The resulting ion energy distributions are normally too
wide to achieve the extreme selectivity required by ALET.
Application of very high frequency (100 MHz) bias can narrow the
IEDs, but the width of the IED depends on ion mass, making IED
control very difficult in mixed gas plasmas. Narrow ion energy
distributions could possibly be obtained with tailored bias pulses.
For conducting substrates, synchronous pulsed DC negative bias can
be applied directly on the substrate support electrode during the
afterglow, and nearly mono-energetic ion bombardment can be
achieved at any desired energy, the same way as in the case of the
boundary voltage described above.
[0090] The ALET steps in the simplest configuration are illustrated
in an example in FIG. 2, using Si etching with Cl.sub.2 in Ar
(argon) as an example. In Step 1 (lasting typically one second),
the sample is exposed to a continuous-wave RF inductively-coupled
plasma with the substrate at ground potential. The plasma is mostly
inert gas with a very small amount (<1%) of Cl.sub.2. With
electrostatic shielding of the inductive source and a relatively
high pressure, the energy of ions impacting the substrate would be
less than the chemical sputtering threshold, so no etching would
occur during step 1. The Cl atoms do not etch p-type or moderately
doped n-type Si at room temperature. Cl atoms from dissociation of
Cl.sub.2 in the feed gas would allow a saturated layer of
chlorinated products (e.g., SiCl.sub.x for Si etching) to form in
about one second.
[0091] In step 2, lasting about 0.5 s, a pulsed main ICP would be
used and positive DC bias pulses would be applied synchronously to
the boundary electrode about 10 .mu.s into the afterglow of each
main ICP pulse, to chemically sputter the product layer.
Alternatively, the bias in Step 2 could be a negative DC voltage
applied to the (conducting) substrate electrode or a high frequency
RF pulse or tailored pulse to the (insulating) substrate electrode
under selected conditions. This step would be monitored by optical
emission from etch products, providing fundamental information on
chemical sputtering yields and a means of controlling the process.
An etching rate of one monolayer in one to several seconds, i.e.
quite practical for nanometer scale structures in future devices
and much faster than conventional atomic layer etching based on
pulsed gas and purge schemes.
[0092] During the etching step, ion energy is chosen that is below
the physical sputtering threshold but above the threshold for
chemically-assisted sputtering. This regime provides very high
selectivity combined with minimum damage, since etching would stop
(self-limiting) after the chemisorbed layer of etch products is
chemically sputtered away. Threshold values for Si are typically
10-25 eV under a variety of conditions.
[0093] Langmuir probe: A Langmuir probe (Smart Probe, Scientific
Systems) was used to measure ion and electron densities (ni and
ne), plasma potentials (VP), floating potentials, and electron
energy probability functions (EEPF). The probe tip had a diameter
of 0.19 mm and an exposed length of 40 mm. A compensation electrode
and RF chokes minimized distortion of the current-voltage (I-V)
characteristic due to oscillations of the plasma potential. This
was not an issue in the present system where, due to the Faraday
shield, peak-to-peak plasma potential oscillations were only 1-2
volts. The probe was movable along the discharge tube axis to
obtain spatially resolved measurements. Fast data acquisition
electronics enabled averaging of 100 s of I-V characteristics (at a
given location and for given plasma conditions) to reduce noise.
The current-voltage (I-V) characteristics were interpreted using
the software supplied by the manufacturer. This analysis relies on
Laframboise's orbital motion-limited (OML) theory for a
collision-less sheath. In the ion current region of the I-V, at
larger negative voltages on the probe, collisions in the sheath
(especially at higher pressures), will cause the ion current to be
attenuated. Hence the analysis will underestimate the ion number
density in the plasma. Since positive ion densities were extracted
from the ion saturation regime of the I-V characteristic by
applying voltages in the range .about.0 to -50V, the positive ion
densities were increasingly underestimated at pressures of
.about.10 mTorr and above. The probe was also operated in a
"boxcar" mode to measure time-resolved plasma characteristics
during pulsed plasma operation.
[0094] Retarding field energy analyzer: A retarding field energy
analyzer (RFEA) was constructed to measure the energy distribution
of ions passing through a grid on the grounded substrate stage. The
RFEA was made of a stack of three nickel grids and a stainless
steel current collector plate spaced 3 mm apart, as shown in FIG. 6
insert. The top grid having 50% open with square holes 18 mm on a
side, was attached to a grounded SS plate with a 0.3 mm pinhole in
contact with the plasma. This grid prevented the plasma sheath from
molding over the pinhole. The middle and bottom grids were each 85%
open with square holes 293, mm on a side. The middle grid was
biased with -30 V to repel electrons from the plasma, while the
bottom grid was biased with a saw-tooth ramp voltage and served as
an energy discriminator to measure the ion energy distribution
(IED). A current amplifier (KEITHLEY.RTM. model 427) was used to
measure the ion current on the collector plate. A 20 Hz ramp
voltage was applied to the discriminator grid using a pulse
generator and a power amplifier (AVTECH AVR-3-PS-P-UHF and
AV-112AH-PS). The experiment was controlled through a LabVIEW
(NATIONAL INSTRUMENTS.RTM.) program. Noise was reduced by averaging
5000 I-V characteristics resulting in "smooth" IEDs. The RFEA was
differentially pumped by a 210 l/s turbo pump to minimize
ion-neutral collisions in the analyzer. The pressure in the
analyzer was estimated to be two orders of magnitude lower than the
pressure in the discharge tube, resulting in collision-less ion
flow. The energy resolution of the RFEA was estimated using the
formula: .about.DE/E=2%.
[0095] FIG. 9 shows ion and electron densities as a function of
vertical position along the discharge tube axis, measured by the
Langmuir probe (LP), for different pressures. Charge density
reaches a maximum around the middle of the coil and increases with
pressure. A maximum ion density of 1.5.times.1012/cm3 is reached
for a pressure of 50 mTorr. As mentioned above, ion-neutral
collisions in the probe sheath will cause the positive ion density
to be increasingly underestimated at higher pressures; hence the
positive ion density could substantially exceed the value recorded
at 50 mTorr. The electron and ion density are nearly equal for
pressures of 3, 7, and 14 mTorr. For 28 mTorr, and especially near
the center at 50 mTorr, the electron density was lower than the
corresponding ion density. This was attributed to the fact that as
the probe was biased near V.sub.P, a large electron current was
drawn out of the plasma. Apparently, the grounded surface of the
boundary electrode in contact with the plasma was not high enough
to compensate for the electron loss at these high densities. The
Langmuir probe has a reference electrode that senses this imposed
shift in V.sub.P and corrects for it, but only up to the point that
the maximum positive voltage on the probe is reached before the
correct V.sub.P is observed. V.sub.P and Te measured by the
Langmuir probe at z=170 mm are shown in parenthesis next to each
corresponding pressure. With the Langmuir probe removed, the RFEA
was positioned at z=170 mm. IEDs measured without any applied bias,
for cw plasmas at 300 W power and pressures of 7 to 50 mTorr, were
single peaked at energies nearly equal to VP, as measured with the
Langmuir probe.
[0096] Optical emission spectroscopy for time-resolved detection of
etching products: Optical emission spectroscopy can be used to
monitor the time-dependence of etching products chemically
sputtered from the surface during the energetic ion flux pulses.
For Si ALET with chlorine, we anticipate that the emission from Si,
SiCl and SiCl.sub.2 products will be observed, as was found in
pulsed laser-induced thermal desorption in Cl.sub.2 plasmas. (Si
and SiBr emissions were also found in HBr plasmas). For GaN
etching, strong emission is expected from Ga and GaCl. If N.sub.2
is a primary product of GaN etching, then it can be easily detected
in the plasma via N.sub.2 optical emission. In addition to
laser-induced fluorescence excited by a resonance between the laser
frequency and excited states of SiCl and SiBr, emissions from all
these species are excited by electron impact of etch products
(either primary or secondary, after electron-impact dissociation),
and can be observed in the region close to the substrate surface.
Emission from e.g., SiCl provides a measure of the chemical
sputtering yield as a function of the instantaneous Cl coverage, as
well as the total amount of material removed per ion pulse. This
measurement can be used to control the etching rate in real time
(e.g., the ion pulse durations could be adjusted to obtain a
constant etching rate). Optical emission actinometry can be to
measure absolute Cl densities, as demonstrated previously in
several ICP systems.
[0097] In-situ laser-induced thermal desorption (LITD): In selected
experiments, laser-induced thermal desorption are used to monitor
instantaneous coverage of Cl, Br and perhaps other surface species.
This method can detect 1% of monolayer coverage with a time
resolution of 10 ns (the laser pulse width) as the substrate is
etching in a plasma as in FIG. 10. Each laser pulse up to 80 or
5000 pulses/s with the available lasers rapidly heats the surface,
resulting in a thermal desorption of typically half the Si-halide
(Cl or Br) layer formed in the plasma. The surface can thus be
probed as a function of time during the chemisorption step and well
as during the etching step.
[0098] In-situ XPS and in-situ AFM/STM surface roughness
measurements: After plasma exposure, samples are transferred under
vacuum to an ultrahigh vacuum chamber and analyzed by XPS.
Angle-resolved measurements is carried out to measure the depth of
penetration of reactants such as Cl and Br and also to obtain a
depth profile of Si-mono, di- and tri-halides, and the ".ident.Si."
moiety, a Si with 3 bonds to Si and 1 dangling bond. On masked
samples, electron shadowing is used to characterize the sidewalls
that are exposed to glancing angle ion bombardment. These methods
have been used with this system to characterize the surface after
Si etching in Cl.sub.2 and HBr plasmas. In-situ characterization of
the sidewalls is particularly important in the case of GaN. For
this material, XPS provide a wealth of information regarding any
changes in surface stoichiometry as a function of ALET process
parameters. An in-situ AFM-STM instrument allows atomic resolution
measurements on processed surfaces without exposure to the
atmosphere. Since rapid ALET offers atomic layer accuracy, it is
important to avoid even sub-monolayer coverage by atmospheric
contaminants, which can distort the experimental findings. These
measurements will aid in identifying process parameters that
minimize surface roughness after repeated ALET cycling, leading to
etching with accuracy down to one monolayer per cycle.
[0099] Effect of continuous DC bias on the boundary electrode: FIG.
11 shows IEDs for 14 mTorr, 300 W, and cw-Ar plasmas for different
values of DC bias, applied continuously to the boundary electrode.
The values of VP measured by the Langmuir probe at the location of
the RFEA for each DC bias voltage are shown in FIG. 11 by a
vertical dashed line. The measured V.sub.P values are in excellent
agreement with the peak energies of the IED. For positive values of
the DC bias, V.sub.P is raised, shifting the IED to higher
energies. For negative DC bias, there is an initial small drop in
V.sub.P, but it saturates as the applied bias becomes more
negative. When compared with measurements without DC bias, the peak
of the IED shifts by 3, 7, and 11 eV for applied DC bias of 4, 8,
and 12 V, respectively. The 1 V difference between the applied bias
and the peak ion energy is probably due to a slight gradient of
V.sub.P. When a negative DC bias is applied, the shift in the peak
ion energy saturates at 4 V lower than without bias. The shift in
V.sub.P with the application of a DC bias on the boundary electrode
is readily understood. A positive bias drains electrons from the
plasma raising V.sub.P so that all but the highest energy electrons
remain confined in the plasma. With the application of a small
negative bias (less than a few T.sub.e) V.sub.P becomes less
positive as electron current to the boundary electrode is cut off.
Larger negative bias on the boundary electrode causes negligible
change in the ion current, hardly affecting V.sub.P. The ion
current saturates at large enough negative bias, assuming there is
no perturbation of the plasma density or T.sub.e.
[0100] Pulsed Plasmas: To obtain nearly mono-energetic ion
bombardment it may be desirable to reduce the energy spread of ions
entering the sheath, as well as maintain a constant sheath
potential. Since RF oscillations of the plasma potential are
eliminated by the Faraday shield, the spread in the energy of ions
entering the sheath scales with T.sub.e. Hence, lowering T.sub.e
should reduce the energy spread. T.sub.e can be lowered by
modulating the plasma power, such as pulsed plasma. When a DC bias
is applied to the boundary electrode under these conditions, ions
can be accelerated to a desired energy with a narrow energy spread.
FIG. 12 shows time resolved Langmuir probe measurements of electron
temperature for different pressures. For a given pressure, the
T.sub.e increases rapidly after the plasma is turned ON,
overshoots, and then reaches a quasi steady-state value. The
steady-state T.sub.e decreases with increasing pressure, as
expected. After the plasma is turned OFF, T.sub.e decreases at a
progressively slower rate longer into the afterglow. In addition,
T.sub.e decays faster at lower pressure. In Ar plasmas, diffusion
to the walls is the dominant cooling mechanism during the afterglow
for electrons with energies below the lowest excited state (the
.sup.3P.sub.2 metastable state at 11.55 eV). Lower pressure results
in faster diffusion rates, and therefore a faster decay of Te in
the afterglow.
[0101] Continuous DC bias on the boundary electrode: FIG. 13 shows
IEDs under pulsed plasma conditions, when a DC bias was
continuously applied to the boundary electrode. For each value of
the DC bias, the IED has two peaks. The broader peaks at higher
energy correspond to ions bombarding the substrate when the plasma
is ON. The shape and energy of these peaks are nearly identical to
those observed in the cw plasma shown in FIG. 11. The sharper peaks
at lower energy correspond to ions bombarding the substrate during
the afterglow. The mean energy of these peaks corresponds to the
applied DC bias. In the afterglow, V.sub.P reaches a very low value
in the absence of DC bias. When a positive DC bias is applied, the
plasma potential is approximately equal to that DC bias. The width
of the IED is much smaller in the afterglow because of the rapid
quenching of electron energy (or T.sub.e). Similar results have
shown a nearly mono-energetic IED by applying a DC bias in the
afterglow of a pulsed capacitively-coupled plasma.
[0102] Synchronous pulsed DC bias on the boundary electrode: While
the above approach creates a narrow and tunable IED, it also leaves
a broad and not well-controlled population of ions that enter the
sheath during the plasma-ON portion of the cycle. One can reduce
the energy of these ions below the threshold for most ion-assisted
surface reactions by turning off the DC bias voltage during the
plasma-ON periods. Results hereinafter are reported from pulsed
plasma operation with a synchronous, pulsed positive DC bias
applied to the boundary electrode at specified times during the
afterglow.
[0103] Effect of Pressure: The IEDs measured by applying a
synchronous bias of +24.4 VDC in the afterglow, during the time
window .DELTA.tb=45-95 ms, for different values of pressure are
shown in FIG. 14. The sharp peaks at .about.22-23 V correspond to
the DC bias, while the broader peaks at lower energy arise from the
plasma ON portion of the cycle. The broader peaks shift to lower
energy as pressure increases, due to a concomitant decrease in
T.sub.e as in FIG. 9 and hence V.sub.P. The most important aspect
of the two-peaked IEDs shown in FIG. 14 is that the spacing between
a broad peak and the corresponding sharp peak can be varied by
varying the DC bias and reactor pressure. Such control is critical
for achieving very high selectivity of etching a film relative to
the underlying substrate. The pressure can be chosen so that the
low energy peak produces no etching. The DC bias can be chosen such
that the high energy peak lies between the thresholds of etching
the film and etching the substrate, assuming there is sufficient
separation between these two thresholds. The fraction of ions under
each peak can also be optimized by varying the duty cycle of the
pulsed plasma and/or the length of time in the afterglow during
which the DC bias is applied, as discussed next.
[0104] Effect of bias timing in the afterglow: IEDs in the
afterglow were also measured with a synchronous DC bias (+24.4 V)
applied to the boundary electrode for different start times (tb)
and time windows (.DELTA.tb). The pulsed plasma was generated with
120 W average power at 10 kHz and 20% duty cycle, 14 mTorr, and 40
sccm Ar flow rate. IEDs with the DC bias applied in the early
afterglow and late afterglow are shown in FIG. 15(a) and (b),
respectively. In FIG. 15(a) biasing starts at progressively later
times in the afterglow and ends 60 ms into the pulse, or 40 ms into
the afterglow, thus .DELTA.tb varies from 18 to 38 ms. As in FIG.
14, the higher energy peaks correspond to the applied bias, whereas
the lower energy peaks correspond to V.sub.P without bias. When
biasing starts at tb=22 ms, only 2 ms after plasma turn OFF,
T.sub.e is still high as in FIG. 12 resulting in a broader width of
the respective high energy peak. As tb is delayed further into the
afterglow, T.sub.e decreases and so does the width of the higher
energy peaks of the IED. In FIG. 15(b) biasing starts deep into the
afterglow when T.sub.e changes little with time, as in FIG. 12.
Therefore, the width of the IED is hardly affected by the biasing
starting time tb. In both FIG. 15(a) and (b), the collected ion
current is larger as .DELTA.tb increases.
[0105] In FIG. 16 the bias starting time tb was varied while
keeping a constant .DELTA.tb of 50 .mu.s or 15 .mu.s. The average
power into the pulsed plasma was 120 W. When the biasing window is
long, 50 .mu.s, compared to the T.sub.e decay time, .about.10
.mu.s, the biasing starting time hardly affects the ion energy
distribution as send in FIG. 16(a). This is because the average
T.sub.e over these bias windows is low and roughly equal. When
.DELTA.tb is short, 15 .mu.s, however, a biasing starting time in
the early afterglow (tb=20 .mu.s) results in a broad IED peak as
shown in FIG. 16b. The width of the IED diminishes progressively,
as tb is shifted to later times in the afterglow. Again, the width
of the IED correlates with T.sub.e during the corresponding biasing
window.
[0106] Further experiments were conducted varying the plasma power
modulation frequency (5, 7.5, and 10 kHz) while keeping a constant
.DELTA.tb=50 .mu.s as shown in FIG. 17. The pulsed plasma was
generated at 14 mTorr Ar pressure with a 20% duty cycle, and an
average power of 120 W. As the modulation frequency decreases,
keeping the same duty cycle, the duration of both the active glow
and the afterglow increase. In this case, tb was 145 .mu.s, 75
.mu.s, and 45 .mu.s for modulation frequency of 5 kHz, 7.5 kHz and
10 kHz, respectively. For all three modulation frequencies, the low
energy peaks are nearly identical because the duration of the
active glow is long compared to the decay time for T.sub.e and
therefore V.sub.P. On the other hand, the higher energy peak
becomes narrower and smaller as the modulation frequency decreases
because the plasma decays for a longer period at a lower modulation
frequency, resulting in a lower T.sub.e. The narrowing of the FWHM
of the peak with decreasing modulation frequency is more clearly
shown by the normalized curves of FIG. 17(b).
[0107] The IEDs for 14 mTorr Ar pulsed plasmas at two different
duty cycles (20 and 50%) are shown in FIG. 18. A synchronous DC
bias of +24.4 V was applied from 70 to 98 .mu.s in the afterglow.
The average power was 120 W and 280 W for 20% and 50% duty cycle,
respectively at 10 kHz modulation frequency. The area under the
peaks is higher for the longer duty cycle. The higher energy peak
has a smaller width for 20% duty cycle since the plasma decays for
longer time resulting in a lower T.sub.e and V.sub.P. Unlike the
20% duty cycle case, T.sub.e is still considerably high during the
application of bias for the 50% duty cycle, as shown in FIG. 12,
resulting in a residual V.sub.P is as high as 3.7 V, compared to
only 1.9V for the 20% duty cycle. This difference in the residual
V.sub.P explains the different widths of the respective IEDs in
FIG. 18. The area under the peak of the IED is proportional to the
ion charge collected during the biasing window. This charge was
estimated using the Bohm flux of ions J.sub.o=e ns uB (where ns is
the ion density at the sheath edge and uB is the Bohm velocity),
and the known biasing times. Using the measured ion density nb
(ns=0.6 nb) and electron temperature averaged over the duration of
the bias, the estimated ion charge was indeed found to be
proportional to the area under the respective peaks of FIGS.
15-18.
[0108] Energy spread of the IED: The full width at half maximum
(FWHM) of the peaks corresponding to the applied DC bias increases
with pressure from 1.7 eV at 7 mTorr to 2.5 eV at 50 mTorr in FIG.
14. These peaks are much tighter than those of the ions from the
active glow with no bias, though still broader than the energy
resolution of the RFEA. The latter was estimated to be DE/E
.about.2%, or a FWHM of 0.5 eV for E=25 V. Collisions in the
differentially pumped RFEA can be ignored, since the local pressure
was about two orders of magnitude lower that the discharge
pressure, making the ion mean free path, .about.15 cm corresponding
to the highest plasma pressure used, much longer than the analyzer
length of .about.1 cm. Some ion-neutral collisions do occur in the
sheath. These could contribute to the "tail" of the IED to the left
of the peaks at higher pressures, but are not expected to be the
major cause of the observed widths of 1.7 to 2.5 eV in the
afterglow. For instance, the ion mean free path at 14 mTorr is
about .lamda..sub.i=0.2 cm which is a factor of 10 larger than the
sheath width .about.250 .mu.m, estimated from the Child law. This
results in an ion collision probability
P.sub.c=1-exp(-s/.lamda..sub.i) of .about.10%. Note that the plasma
density increases strongly with pressure, causing the sheath width
to decrease, counteracting the decrease in mean free path with
pressure. Ion-neutral collisions in the pre-sheath can contribute
significantly to the spread of the IED. Depending on the ion
collision affects, the FWHM of the IED can be several T.sub.e.
* * * * *