For a cyclic approach most similar to a reverse ALD scheme, the process window to achieve a truly self-limited atomic layer etch ALE process is identified and the limitations as a functi
Trang 1b ZEON Chemicals L.P., Louisville, Kentucky 40211, USA
c University of Maryland, College Park, Maryland 20742-2115, USA
d Plasma Physics Division, Naval Research Laboratory, Washington, DC 20375-5320, USA
The ability to achieve atomic layer etch precision is reviewed in detail for a variety of material sets and implementation methods.
For a cyclic approach most similar to a reverse ALD scheme, the process window to achieve a truly self-limited atomic layer etch (ALE) process is identified and the limitations as a function of controlling the adsorption step, the irradiation energy, and the reaction process are examined Alternative approaches, namely processes to enable pseudo-ALE precision, are then introduced and results from their application investigated Most of the recent work in plasma process development can be characterized by three fundamental approaches to atomic layer etching Lastly, recent developments employing reactant flux control are briefly introduced, which have shown to provide a self-limited process that is able to exhibit high selectivity and pattern fidelity The key feature of this novel method may be the ability to combine advances from the other atomic layer etch approaches.
© The Author(s) 2015 Published by ECS This is an open access article distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives 4.0 License (CC BY-NC-ND, http://creativecommons.org/licenses/by-nc-nd/4.0/ ), which permits non-commercial reuse, distribution, and reproduction in any medium, provided the original work is not changed in any way and is properly cited For permission for commercial reuse, please email: oa@electrochem.org [DOI: 10.1149/2.0101506jss ] All rights reserved.
Manuscript submitted December 22, 2014; revised manuscript received March 2, 2015 Published April 1, 2015.This paper is part
of the JSS Focus Issue on Atomic Layer Etching and Cleaning.
Over the course of the past few years, the semiconductor industry
has continued to invent and innovate profoundly to adhere to Moore’s
Law1 and Dennard scaling.2From the introduction of new materials
to the transition to non-planar FinFET devices, each new generation
of technology bears considerable challenges for device fabrication
Looking at the 10 nm technology node and beyond, this issue is
com-pounded by the fact that scaling places inadvertent demands on etch
processes, requiring control in single nanometer scale dimensions
This ever continuing trend to shrink device sizes coupled with the
ad-vent of novel materials, multi-component materials or even nanoscale
materials, is driving the need for the ultimate etch solution: etching
with atomic layer precision.
Feasibility for atomic layer etching (ALE) has been proposed and
demonstrated on a lab scale for some time Dating back to the early
1990s, researchers first recognized the ability to control etch processes
down to the atomic scale for III-V materials Meguro et al3used
al-ternating cycles of a pulsed chemical etchant (Cl2) and an energetic
electron beam to demonstrate ‘digital etching’ of GaAs substrates
With this approach, self-limiting etch rates of as little as∼1 Å/cycle
(equivalent to 1/3 of a monolayer per cycle) were reported.3In a
differ-ent approach (derived from molecular beam epitaxy), Tsang et al also
showed sub-monolayer precision GaAs removal by injecting AsCl3
directly onto a heated sample surface in a chemical beam epitaxy
chamber.4This approach demonstrated not only the ability to control
a flux driven process down to the Angstrom level, but also the ability
to modify interface properties and/or change surface morphology For
Si-based etch systems, Matsuura demonstrated feasibility for ALE
using a 4-step process including alternating cycles of chlorine (gas
only), followed by exposure to an Ar+ion beam with an ion energy
chosen to ensure self-limiting mono-layer removal.5Similarly, Sakaue
et al also demonstrated ALE capability for silicon, however their
ap-proach utilized alternating cycles of a fluorine containing downstream
plasma (CF4/O2, NF3/N2or F2/He) at cryogenically cooled substrate
temperatures followed by Ar+ion irradiation.6
While each of these approaches demonstrates merit and
signifi-cance for atomic layer etching, the adoption of ALE had not garnered
z E-mail: suengelm@us.ibm.com
significant interest until now In the recent past, the focus of plasma etch equipment has been on the ability to minimize damage and/or increase etch pattern fidelity by means of either tailoring ion en-ergy distributions or reducing the average electron temperature of the plasma For example, early work by Sobolewski7and Wendt8 demon-strated the ability to modify ion energy distribution functions via plasma frequency or pulsed DC bias modulation to enable ion extrac-tion with either narrow or broad energy distribuextrac-tions This work was further expanded upon with the exploration of multi-frequency drive electrodes,9 , 10 the inclusion of DC superposition [DCS]11 , 12 and/or more elaborate plasma pulsing combinations of both source and/or bias powers to reduce ion energy and/or enable charge balance.13 – 15
Novel plasma sources which produce a more self-confined, high den-sity plasma with significantly reduced electron temperature at the wafer surface have also been evaluated.16 – 18 Furthermore, these re-cent concepts have been successfully commercialized and shown to reduce plasma damage and/or improve pattern fidelity.19 – 22Ironically,
it is these same hardware enhancements that have set the stage to en-able processing capability for atomic scale precision Plasma pulsing, low electron temperature plasmas, pulsed gas flows etc., which are now incorporated into state of the art plasma processing toolsets are envi-sioned to be key advances to potentially enable ALE in a large scale production setting However, a unified methodology and/or system to achieve high volume manufacturing with atomic scale precision has yet to be defined and/or developed for a variety of Si-based material systems (Si, SiO2, Si3N4, SiCOH)
In this paper, we will review the main methods under consideration
to achieve self-limiting atomic scale etching precision and focus on the role that chemistry, both surface chemistry and gas phase chemistry, will play We found four differing approaches to atomic layer etch, which are reviewed in detail
Approach to Atomic Layer Precision
In order to achieve atomic scale sensitivity, the essential require-ment is limitation However, the method to achieve that self-limited behavior can vary and a number of approaches have been proposed and even demonstrated, as mentioned above The first and foremost method under consideration for implementation of ALE is
Trang 2Fig 1 Schematic of the cyclic four ALE process elements Depicted species are: substrate atom (blue), adsorbent species (white), reactive species (red “R”) and
inert atom (purple)
that of a cyclical process, analogue to that of atomic layer
deposi-tion, which provides atomic scale precision based on surface-limited
reactions Described in detail for a chlorine-based etch system by
Economou23 – 25and more recently by Kanarik,26 , 27 this process has
four separate reoccurring process steps These are: (a) reactant
ad-sorption, (b) excess reactant evacuation, (c) energy-induced surface
reactions, and (d) reaction product evacuation The multi-step process
is illustrated in Fig.1 Here, steps (b) and (d) are rate limiting ‘purge’
steps, as they are dependent on the residence time of the chamber,
while steps (a) and (c) are the key surface-limited steps
For the adsorption step (a), the adsorbed species (white) is
ex-pected to form a monolayer on the to-be-etched substrate (blue), after
which no further adsorption occurs After evacuating the excess
ad-sorbent species (Fig.1b), the critical etch step is depicted in Fig.1c
While the approach varies in detail, in general, species ‘R’ (this could
be ions,3 , 28energetic neutrals44 , 46or others4 , 29) interact with the
ad-sorbed monolayer and the top layer of the substrate to form volatile
species Neutral species not participating in the reaction (purple) may
be present in case of plasma exposure The cycle is then completed by
purging the system of all species (Fig.1d) In case of the reaction step
(c), the process is expected to be chosen such that the reaction only
occurs when an adsorbed species is present at the surface This ideal
behavior is illustrated in Figure2in which both the adsorption step
(Fig.2a) and the reaction step (Fig.2b) are designed to be self-limited
at a maximum rate equivalent to 1 monolayer (ML) per cycle The
key thing to note here is that this self-limiting nature enables one to
Fig 2 Illustration of ideal ALE process showing etch rate per cycle vs the
(a) adsorption step time and (b) reaction step time.
overcome local variations of deposition/etch rates caused by chamber geometry, power distribution, flow distribution and others, yielding enhanced uniformity across feature size and across the wafer
Challenge for implementation.— While the ideal ALE process has
been demonstrated and shown to greatly reduce iso-dense loading for silicon etch,6there are a number of challenges that must be addressed
to enable ALE for a multitude of material systems The greatest chal-lenge associated with this cyclical, reaction-limited ALE approach is the ability to control the adsorption and reaction portions of the pro-cess Typically, a uniform adsorption layer is formed on all materials present on the substrate The reaction step is then tailored such that the energy input is above the threshold energy for reaction on a target material but below the threshold energy that would be needed to etch any other material If we are to illustrate the etch rate of an ideal ALE process as a function of reaction step irradiation energy, we would obtain a process window for self-limitation as depicted in Figure3 Here the ALE process window is bound by the lower energy E1, which is the energy threshold required to initiate reaction of the ad-sorbent with the target substrate material, and upper energy E2, which indicates the threshold energy for initiating reaction with any other material present on the substrate (e.g hardmask) In this regime, etch rates on the order of monolayer(s) per cycle should be observed up
to an energy of E2 Outside of this process window, four different
Fig 3 Illustration of the ALE window along with four zones of non-ideal
layer-by-layer etch behavior.
Trang 3Fig 4 Schematic pictures of etch selectivity mechanism and possible challenges for two material systems with (a) selective adsorption and (b) selective etch
reaction Depicted species are: substrate atom A (blue), more etch resistant substrate atom B (green), adsorbent species (white), reactive species (red “R”) and damaged substrate atom A (orange).
‘zones’, dependent on irradiation energy and adsorbent flux, illustrate
the causes for loss of self-limiting behavior The first two zones
con-sidered are above energy E2, but below the energy needed to initiate
substrate sputtering Es The first zone (Zone 1) occurs in a region
in which both the adsorption and the irradiation energy are greater
than desired and the self-limiting nature of the process is lost This is
because enough radical adsorption and irradiation energy are present
to react with and volatilize all materials on the substrate surface,
los-ing selectivity to any mask material or stop layer(s) In certain cases,
there may even be enough physical and chemical species with
suffi-cient energy and selectivity present to impact the substrate material
to a greater depth and remove multiple monolayers of material within
any given cycle In the lower zone (Zone 2), there remains a
self-limited amount of adsorption but an excess amount of energy, such
that modification to the adsorption layer is possible This could then
result in either the removal of the adsorbed material (without reacting
with the substrate) or modification of the adsorption layer
composi-tion, which could impact and/or compromise the ability to provide
the stoichiometric removal of the target material The experimental
validation and verification of such behavior may be challenging
On the other side of the spectrum at low irradiation energy, we
have two additional zones above E0, the minimum energy possible
in the system One case (Zone 3) occurs when the irradiation energy
is below the threshold for reaction, but the adsorption remains
self-limited In this zone, irradiated species are unable to remove the
adsorbed monolayer (ML) Thus, if we were to continue the cyclic
process,we could wind up in a regime in which net deposition occurs
over multiple cycles, possibly resulting in a thick polymer coating
Finally, in the last region (Zone 4), the irradiation energy remains
below the threshold for reaction, but the adsorption is no longer
self-limited In this case, the adsorption layer chemistry can change in
a manner that was not tuned correctly for a given material and the
system can shift into a process space in which spontaneous chemical
etching of either the adsorbed layer, or any of the substrate materials
commences
To complicate matters, the ability to tailor the energy to the level
of precision required for single ML reaction control is not
straight-forward, especially for the variety of material systems commonly
used As an example, conventional ICP plasmas may have an intrinsic
plasma potential that produces an ion energy which is greater than the
threshold for self-limitation, E2, for some materials Thus, regardless
of other controls, the process window is exceeded and either
selectiv-ity loss or surface modification can immediately occur In this regard,
alternative plasma sources capable of producing low ion energies are
attractive Electron bean generated plasmas, for example, have an
in-trinsically low electron temperature (0.3 eV< Te< 1 eV) and so ion
energies at adjacent surfaces are also very low.30Alternatively, other
factors such as UV and VUV photons that drive photo-assisted
pro-cesses may also provide uncontrolled energy into the reaction, causing
sub-threshold etch behavior and/or substrate damage, compromising
the ability to achieve self-limited ALE.31 – 34
Adsorption process control.— For the adsorption step, the ideal
process would include not only self-limiting adsorption but also se-lectivity That is, the adsorbed layer would only form on the target material This is illustrated in Figure 4a, where the reactant would only deposit on Material A, while it did not deposit on Material B Thus, in the follow-on reaction step the etch proceeds only where the adsorption occurred, selectively removing material A Concomitantly,
if one were to envision the other side of the process regime, adsorption could be tailored such that it occurs with one specific stoichiometry
on target material A, while having a different (more reaction resistant) stoichiometry on any other material present on the substrate surface This is illustrated in Figure4b With this in mind, we recognize that a significant effort should be put into chemistry development for achiev-ing such selective deposition If one could tailor the process conditions such that dissociation pathways with specifically designed deposition precursors are obtained, then selective volatilization may become fea-sible In particular, the impact of different plasma source types and configurations on plasma dissociation will be of great importance for further advancement For conventional discharges, electron-impact driven species generation favors those species with the lowest thresh-old and so not only will radical formation be high, it may be difficult
to control relative radical densities.35However, alternative approaches
to plasma generation, such as electron beam generated plasmas, could offer additional paths for maintaining precursor molecule chemistry, where in depth characterization of the dissociation and radical gen-eration pathways have the potential to produce significantly different chemistry as compared to more standard discharge plasma sources used today.36If the dissociation pathways of the precursor molecule are understood well, reverse engineering of etch mechanisms based on deposition chemistry37seems feasible even under plasma conditions
A wider process range in terms of process temperature and through-put may be expected for the plasma enhanced ALE, similar to Atomic Layer deposition,38and its plasma-based counterpart PE-ALD.39
Reaction process control.— Regardless of energy, reactive
species-induced damage is another issue in itself because there are multiple pathways to which it can be created In one instance, damage could occur from the interaction of the reacted species with surface sites that have already been removed (in a given cycle) Illustrated in Figure4b,
a reacted species complex may inadvertently re-react with exposed substrate material (in an area which the topmost layer has already been volatilized, shown as orange) during evacuation As an exam-ple for a silicon etch system, Athavale and Economou24 completed molecular dynamics simulations of a 4-step ALE Cl2-based process and discovered that the most prominent volatilized reaction products removed from the substrate surface were unsaturated silicon chloride species; SiCl and SiCl2,in order of yield This could then mean that under certain conditions, SiCl and SiCl2 liberated from the surface could undergo gas-phase collisions and/or recombination events that produce additional atomic chlorine, potentially leading to unintended reactions on the substrate surface Alternatively, SiCl and SiCl2could
Trang 4scavenge other Cl species at adjacent surface sites to form more stable
SiCl4, potentially leading to areas of unetched silicon
This is further complicated in the case of more complex etch
systems such as SiO2and Si3N4, where more than one atomic species
need to be volatilized from the surface to remove a monolayer of
material For instance, if CF4is used as an etch gas for SiO2etching,
a plasma-based adsorption step would most likely be employed and
lead to the deposition of a thin fluorocarbon reaction layer atop the
substrate Subsequently, the reaction step would then have to enable
removal of both Si and O atoms by volatilization of the complex
fluorocarbon layer However, since fluorocarbon layers usually take
the form of CF, CF2and/or CF3 groups,40 – 43it is possible that more
than one reaction layer is required to volatilize silicon and oxygen,
assuming the highest yielding volatilized species are SiF4and CO2
Recent Developments for Advancing Atomic Scale Precision
Etching
In addition to the approaches proposed above to enable a true
self-limited atomic layer etch process, other methods are simultaneously
being developed to enhance existing etch processes such that they can
achieve atomic scale precision These methods include processes to
enable pseudo-ALE precision by improving reaction control, energy
control and/or flux control in a non-typical approach The merits and
advantages of each, along with associated disadvantages are listed in
TableI
Most approaches to “reverse ALD” have already been reviewed
earlier One key issue remains is that applications of the methodology
to complimentary materials (e.g Silicon oxide, nitride etc.) of Silicon
(or III-V materials) have been not demonstrated so far It appears that
these systems work well for one specific set of materials, however the
performance is not easily transferred to related materials or different
material systems
Reaction chemistry control.— Some processes with atomic layer
precision are already being used in manufacturing today Typically
they are wet chemistries (e.g dilute HF, BOE, KOH, etc.), for which chemical reactions are very understood and well controlled Typi-cally they are of isotropic nature, but very well controlled Similarly, some work has shown to mirror this methodology to carefully react
a material under dry etch conditions, which then, in turn, can be re-moved by a second step (e.g Cu/Ag/Au or SiN etching) These 2 step processes typically yield anisotropic profiles, as one of the steps
is anisotropic in nature Additionally fluoro(hydro)carbon plasmas have been used quite extensively in manufacturing for many years Much understanding to the mechanism for etching silicon oxide was discussed by Zhang,44Standaert,41and others.45Typically, oxide ex-hibits a lower etching threshold than silicon nitride or silicon due to the thickness differences of a fluorocarbon reaction layer, leading to desired selectivites for back-end-of-line (BEOL) processing Based
on these findings, selective dry etch chemistries have been further de-veloped However, applicability to ALE is not so straightforward since the ideal ALE process regime confines the reaction layer to a thick-ness appropriate to achieve single monolayer removal of the substrate material As such, a slight modification of the process towards atomic layer processing is necessary Illustrated in Figure5, standard plasma processes are operated in the ‘selective etch’ window rather than the condition of “infinite selectivity” due to the need for polymer manage-ment in the etch chamber during processing However, with a switch
to a cyclic plasma process for ALE precision, the process window can be shifted to lower ion energies (see region labelled “Equasi-ALE”) This is because excessive polymer deposition is no longer necessary,
as the cyclic nature allows the “reset” of the substrate by removal of the polymer layer during steps 2–4 in the ALE cycle, and significantly reduced deposition should occur on the chamber walls, rather than depositing layer upon layer in case of the continuous plasma
Energy control.— The sensitivity of ALE processes to the
en-ergy at which they interact has already been discussed earlier It is clear that low energy processes are necessary, though the path which provides the most improvement is not While the requirements for en-ergy control will depend greatly on the material system to be etched,
Table I Comparison of experimental approaches in the area of atomic layer etch.
ALE (reverse ALD)
Reaction Chemistry Control – quasi ALE Energy Control – quasi ALE Flux-Control – quasi ALE Method Depositing adsorption layer,
controlled reaction of adsorption layer with substrate, cyclic 4 step process; complementary technique to established ALD processes
Extremely selective etchant attacks only material A, while Material B is left unaltered;
reaction chemistry is key element
Carefully tailoring plasma properties to simplify plasma-surface interactions (novel plasma approaches);
advanced plasma physics is key element
Continuous plasma during all cyclic process steps, depositing plasma layer and reacting layer energetically at top surface
Advantage High precision, true self
limitation, uniformity
Isotropic etch precision, crystallographic precision (some systems), batch process capability (wet systems), use of existing (manufacturing) toolset possible
Simplified plasma-surface interactions, fast process, wide process temperature range
Wide process temperature range, faster process time, ability to implement new approaches from all other ALE methods, synergy with PE-ALD learning Disadvantage Throughput, physical wafer
movement (some systems), process temperature limitations
Anisotropic etch capability (some systems), process temperature limitations, wafer scale uniformity, physical wafer movement
Complex plasma physics (not well understood at this point), Wafer scale uniformity
Plasma damage, true self-limitation deposit not known at this point, design of precursor gas needs understanding of ALE plasma decomposition
Examples Si ALE by Cl 2 and Ar
(neutral beam or ions), InP ALE by Cl 2 and neutral beam, GaAs by Br 2
Highly selective wet etches (dilute HF, BOE, KOH etc.), highly selective dry etch chemistries, modified layer + selective removal systems (Cu, Ag, Au, SiN etching)
Pulsed plasma, Ion Energy Distribution Function (IEDF) tailoring, negative ion extraction, very low substrate temperature plasma processing, very low electron temperature plasma processing
Cyclic etch of oxide by fluorocarbon/Ar plasma discharges
Trang 5Fig 5 Schematic representation of deposition/etch rate vs ion kinetic energy
in conventional plasma processes containing polymerizing etch gases such as
fluorocarbons.
surface interactions for the most challenging group of materials
(monolayer materials) deserves a careful look The most promising
of these kinds of materials are carbon materials, namely graphene
and carbon nanotubes.46Such materials require the utmost control of
plasma damage, as any excessive damage will result in the
destruc-tion of the material Some groups have reported encouraging results in
the past.47For example, low electron temperature plasmas have been
previously used to functionalize graphene materials.48
Other approaches in this methodology of carefully limiting
uncon-trolled plasma species interactions with the substrate are the previously
mentioned recent advances in reactor technology Plasma pulsing, low
electron temperature discharges, cryogenic substrate processing or Ion
Energy Distribution Function (IEDF) tailoring may provide means to
approach atomic layer etching by limiting the interactions of ions to
the atomic scale
Reactant flux control.— While enhanced energy control and
reac-tion chemistry control schemes may show promise to attain atomic
scale precision, one of the most attractive approaches currently under
investigation is via employing flux control In this technique, an inert
plasma is maintained continuously throughout the process, below the
energy threshold for etch As can be seen in Figure6, a fluorocarbon
polymer chemistry is then introduced via gas pulsing, to provide the
reactant adsorption Subsequently, once the gas pulse has concluded
and a time delay (>residence time of the adsorbent gas within the
chamber) has passed, then bias power to the substrate is introduced
to the inert chemistry plasma (here Ar), to provide enough energy to
initiate reaction of the adsorbent with the substrate surface below The
advantage of this approach is that instabilities and variability
associ-ated with pulsing the plasma source and the subsequent evacuation
steps are reduced and/or eliminated Furthermore, this process offers
maximum flexibility for the processing of multiple materials, since the
inert chemistry and reactive feedgas chemistry can be tuned together
to achieve the desired feedgas dissociation and radical generation for
selective deposition
Fig 6 Schematic picture of flux controlled ALE process based on continuous
plasma exposure.
strate temperatures and the ability to achieve true growth saturation Particle fluxes from the plasma have to be understood and managed well in order to yield true atomic layer etching The most obvious example here would be that if one yields perfect monolayer behavior
at the feature bottom (which enables the etching), feature sidewall conditions will also need to be accounted for a true atomic layer etch process (i.e no excessive deposition) Furthermore, better un-derstanding of plasma gas phase chemistry and the dissociation and fragmentation of precursor molecules will also be needed to identify proper precursor selection A look into the ALD world shows that the use of quartz crystal microbalance was instrumental in analyzing ALD systems.49Similarly, spectroscopic ellipsometry has been applied to identify process mechanisms for ALD.50In the realm of ALE, it will
be just as important to have and utilize in-situ methods to characterize etch mechanisms The only current application of such techniques to ALE was demonstrated by Metzler et al.66 by use of in-situ single wavelength ellipsometry Detailed surface analysis techniques such
as x-ray photoelectron spectroscopy, FTIR-spectroscopy (both on the substrate surface and in the gas phase of the plasma) and others will
be crucial to this effort
Conclusions/Outlook
For some systems, true self-limited ALE processes have been demonstrated and may already be ready for evaluation and imple-mentation on a manufacturing scale For example, etching results of silicon by Cl2 ALE look very close to the ideal scenarios depicted
in Fig.2(compare to Ref.44) Initial results for other materials show great promise for future ALE processes, but have yet to achieve the desired self-limited atomic scale precision Very promising results have been obtained using a reactant flux limited approach.66Current target applications appear to be applications with a high degree of aspect ratio dependence and/or overetch requirements Such possible applications could include self-aligned contact, spacer, or others The main challenge will be to identify applications that allow non-perfect ALE conditions to deliver key advantages in order to maintain re-search efforts to eventually yield true self-limited ALE on many more systems
Acknowledgments
Special thanks go to B Harrison, J.M Papalia, S Holmes, D Farmer, K Uppireddi, E Kamnang, G Tulevski, D Neumayer, A Bol, E Sikorski, P Bouvron, W Price, Y Zhang and N Fuller S.J Han, J Chang and Y Zhu are thanked for contributions to parts of this work The authors would also like to thank the MRL manage-ment for support of this work S.E furthermore specifically thanks L Lecordier and A Pyzyna for fruitful discussions and a close look into the world of ALD and PE-ALD This work was partially supported by the Naval Research Laboratory Base Program S.E., D.M and E.A.J gratefully acknowledge support by the National Science Foundation for research under award CBET-1134273 which led to part of the work described here, and also thank G S Oehrlein who serves as PI of this award
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