Applied Physics A manuscript No.(will be inserted by the editor)

Advances in gas-mediated electron beam induced etching andrelated material processing techniques

Milos Toth

Received: July 5, 2014/ Accepted: Insert Date

Abstract Electron beam induced etching (EBIE) has

traditionally been used for top-down, direct-write, chem-

ical dry etching and iterative editing of materials. The

present article reviews recent advances in EBIE mod-

eling and emerging applications, with an emphasis on

use cases in which the approaches that have convention-

ally been used to realize EBIE are instead used for ma-

terial analysis, surface functionalization, or bottom-up

growth of nanostructured materials. Such applications

are used to highlight the shortcomings of existing quan-

titative EBIE models, and to identify physico-chemical

phenomena that must be accounted for in order to en-

able full exploitation and predictive modeling of EBIE

and related electron beam fabrication techniques.

Keywords: electron beam induced etching, direct-writenanofabrication, nanostructures, surface functionaliza-

tion, self-assembly, radiation effects, nanomaterials

PACS codes: 81.65.Cf, 81.07.-b, 79.60.Dp, 68.43.Mn,

68.43.-h, 82.30.Lp, 61.80.Fe, 61.80.-x, 81.16.Dn

1 Introduction

Gas-mediated electron beam induced etching (EBIE) is

a direct-write, subtractive nanofabrication technique in

which an electron beam and a precursor gas are used

to realize chemical dry etching with a spatial resolu-

tion of ∼ 10 nm [1–5]. EBIE is typically performed us-

ing electron microscopes that are equipped with gas

M. Toth*School of Physics and Advanced MaterialsUniversity of Technology, SydneyPO Box 123, Broadway, NSW 2007, AustraliaE-mail: Milos.Toth@uts.edu.au

injectors, and enable in-situ imaging and analysis of

the features fabricated by an electron beam. The tech-

nique is analogous to focused ion beam processing [6–

11], but avoids damage, staining and redeposition ar-

tifacts caused by ion bombardment. EBIE is realized

using gaseous precursors such as H2O, O2, NH3, XeF2,

Cl2 and SF6, which have been used to volatilize a wide

range of materials, including graphene [12], single [13]

and multi-walled [14] C nanotubes, amorphous carbon

[15–18], single crystal [19–22] and nano-crystalline [23]

diamond, Si, SiO2, Si3N4, Cr, Ti, TaN and photoresist

[24–38]. Historical overviews and reviews of the EBIE

technique and the underlying chemical pathways can

be found in references [1–5]. The present article is fo-

cused on recent advances in EBIE modeling methods,

and emerging applications of EBIE and related electron

beam material restructuring, fabrication and analysis

methods.

2 Mechanisms

EBIE precursor gases are injected into an electron mi-

croscope specimen chamber using one of two methods.

Either a capillary is positioned near the electron beam

impact point at the substrate surface, and used to in-

ject the gas into a chamber that is pumped continu-

ously using a high vacuum pumping system [39,40].

Alternatively, the entire vacuum chamber, or a sub-

chamber [18] is filled with a precursor gas, as is done

in environmental electron microscopy [41–44]. Ideally,

the precursor gas does not etch the substrate sponta-

neously1. Instead, the chemical reactions that give rise

to etching are driven by interactions between the inci-

1 This is, however, not true in some cases, such as whenXeF2 is used for etching of Si [45,46,31], TaN [28], or TaBN

2 Milos Toth

dent and emitted electrons, and surface-adsorbed pre-

cursor molecules (Fig. 1(a)). When an electron beam ir-

radiates a substrate, precursor molecule adsorbates are

consumed in the etch reaction, and the local surface

concentration decreases to a steady state value, typi-

cally within ∼ 1 millisecond [1]. The time-evolution of

adsorbate concentration at each point on the surface is

given by a competition between adsorbate consumption

in etching, desorption, and adsorbate replenishment.

The latter proceeds through adsorption from the gas

phase and diffusion along the substrate surface (Fig.

1(b)). The vertical etch rate (∂z/∂t) generally scales

with the product of the electron flux, f(x, y), and the

concentration of surface-adsorbed precursor molecules,

Na(x, y), at each point (x, y) on the substrate surface

(Fig. 1(c)). It can be calculated as a function of elec-

tron beam irradiation time (t) by solving differential

equations of the form [1,2]:

∂Na∂t

= Λ− kNa −∂Nα∂t

+D∇2Na, (1)

∂z

∂t= V

∂Nα∂t

= V σfNa, (2)

where a and α signify surface-adsorbed precursor molecules

and dissociation products, respectively; Λ, kNa and∂Nα

∂t are the adsorption, desorption and electron in-

duced dissociation rates (per unit area), and D is the

precursor adsorbate diffusion coefficient. The constant

V is the volume of a single molecule removed from the

substrate in the etch reaction, and σ is an electron scat-

tering cross-section for the activated process that leads

to volatilization2. Adsorption is usually assumed to pro-

ceed through a single physisorbed state (Fig. 2(a)), and

surface coverage is normally limited to 1 ML by the

Langmuir isotherm:

Λ = sF (1−Θ), (3)

where s is the sticking coefficient, F (x, y) is the gas

molecule flux at the substrate surface (given by the gas

pressure and temperature), and Θ(x, y, ) is the precur-

sor adsorbate coverage.

[27], where delocalized etching occurs spontaneously and theelectron beam is used to accelerate the local etch rate.2 It is usually assumed that electrons dissociate precursor

molecule adsorbates, thereby generating reactive fragmentswhich react with and volatilize the substrate [1]. Hence, σ isan ‘effective’ [23] cross-section for fragment generation. How-ever, in some cases, such as XeF2 EBIE of SiO2 [26] andXeF2 EBIE of Si3N4 [30], etching has been argued to pro-ceed through a cyclic process of electron induced removal ofO (or N) from the substrate surface, and spontaneous etch-ing of excess Si by XeF2. Such processes can be modeled bythe above equations provided that σ is taken to represent across-section for the electron induced restructuring step thatleads to the removal of O (or N) from the surface.

The above modeling approach is also applicable to

the related technique of electron beam induced depo-

sition (EBID) [1–5,47–49], and is often used in studies

of deposition and etch kinetics. Reaction rate kinetics

are of interest because they affect spatial resolution,

proximity effects, fabrication rates, composition and the

topography of nanostructures fabricated by EBIE and

EBID [50–53,49,54,23,55,16]. In recent years, the basic

model defined by Eqns. 1–2 has been used (or modified)

to account for the following phenomena:

– The gas pressure distribution inside the electron mi-

croscope specimen chamber [39,40,56,57], which gov-

erns the gas molecule flux F (x, y) across the sub-

strate surface. The precursor pressure can vary sub-

stantially across the surface region irradiated by the

electron beam, particularly when the precursor gas

is delivered into the vacuum chamber using a capil-

lary located near the beam impact point at the sur-

face. Pressure distributions are significant because

they causes the EBIE rate to vary across the sub-

strate surface through Eqn. 3.

– Precursor transport into high aspect ratio pits [55].

The gas flow conductance of a pit decreases with

increasing aspect ratio, and hence alters the replen-

ishment rate of precursor molecules consumed in

EBIE. The replenishment rate affects Na which de-

termines the etch rate through Eqn. 2. Etch pit con-

ductance can be the dominant, etch-rate-limiting

process when fabricating high aspect ratio pits. It

causes the etch rate to decrease as the etch pit grows

during EBIE, giving rise to a characteristic, sub-

linear dependence of etch pit depth on electron beam

processing time [55].

– The behavior of etch reaction products (α) at the

substrate surface [58]. The diffusion, desorption, and

electron induced re-dissociation of etch product molecules

can be modeled by setting up a differential equation

analogous to Eqn. 1 for each molecular species at the

substrate surface. Etch reaction product kinetics are

relevant if the molecules have a significant residence

time at the surface, or if multiple reaction steps are

needed to produce a volatile species that ultimately

desorbs from the surface. Electron induced dissoci-

ation of reaction products acts to reverse EBIE. It

can limit the etch rate, and can give rise to complex

dependencies of etch rate on electron flux [58].

– EBIE performed using a precursor gas mixture com-

prised of an etch precursor and a deposition pre-

cursor [16,54]. Mixtures are implemented by setting

up a differential equation for each molecular species

making up the gas, and can be used to simulate the

resulting dependencies of the etch (and deposition)

rate on electron flux. Mixtures play a role in EBIE

Advances in gas-mediated electron beam induced etching and related material processing techniques 3

when residual contaminants such as hydrocarbons

are present in the vacuum chamber and give rise to

unintended EBID that competes with etching [18,

29,16]. Mixtures can also be used to intentionally

modify and control reaction kinetics, and hence con-

trol the morphology [54] or composition [59–61,53,

62–66,49,67] of nanostructures fabricated by elec-

tron beam fabrication techniques.

– The potential well and energy barrier associated with

activated chemisorption [68], a type of adsorption

in which a gas molecule overcomes an energy bar-

rier and forms a chemical bond with a surface (Fig.

2(b)). Activated chemisorption alters the tempera-

ture dependence of the adsorbate concentration Na,

and enables electron beam chemical processing at el-

evated temperatures where the surface coverage of

physisorbed precursor molecules is negligible. This

has a number of benefits such as accelerated desorp-

tion of unwanted adsorbates (e.g. C-containing con-

taminants), and enables control over the species of

surface-adsorbed precursor molecules through par-

tial, thermal decomposition of the adsorbates.

– Spontaneous decomposition of precursor molecules

at the substrate surface, which can occur in paral-

lel with electron induced dissociation. This effect

has been modeled for the case of XeF2 [69] which

can fragment through a dissociative chemisorption

pathway, leading to fluorination of many surfaces

[45,46,70,71] at room temperature. The model used

to simulate the spontaneous and electron induced

dissociation of XeF2 [69] is a variant of a model

of activated chemisorption [68] that had been de-

veloped to describe the temperature-dependence of

EBID preformed using tetraethoxysilane (TEOS) as

the precursor gas.

– Chemically active (and inactive) surface sites that

enable (or inhibit) EBIE, and dynamic activation

of surface sites through electron beam restructuring

of the substrate [23]. Surface site activation can en-

able EBIE of materials that can not be etched in

their virgin, unmodified state. It can also alter etch

kinetics, and give rise to a characteristic super-linear

dependence of etch pit depth on electron beam pro-

cessing time.

The above continuum models of EBIE [16,23,54,

55,58,69] and analogous (continuum and Monte Carlo)

models of EBID have been used to simulate the time-

evolution of the geometries (and in some cases the com-

position [53]) of structures fabricated by these tech-

niques. The key limitations of existing models are that:

(i) the model input parameters are often not known for

precursor-substrate combinations of interest, (ii) changes

in sample geometry caused by EBIE (or EBID) and

their consequences for the electron flux profile, f(x, y),

have not been modeled realistically over large areas

and for long processing times, and (iii) all of the in-

dividual effects listed above have thus far been stud-

ied in isolation, and are yet to be consolidated into

a generic, predictive model of etching and deposition.

Furthermore, a number of physical and chemical pro-

cesses have, to date, not been incorporated explicitly in

published EBIE models. These include electron stim-

ulated desorption [72–74], knock-on damage and sput-

tering caused by high energy (& 100 keV) electrons and

other mechanisms through which an electron beam can

alter the substrate composition and nanostructure [75–

82], the use of actual (rather than effective [23]) cross-

sections for EBIE, realistic multi-step reaction path-

ways, surface roughening caused by EBIE (Fig. 3(a-d)),

electron beam induced heating [83], and the effects of

charging [84] caused by electron injection into insula-

tors or electrically isolated substrate regions.

3 Applications

Traditionally, EBIE has been used as a direct-write sub-

tractive nanofabrication technique (Fig. 3(a-h)). Sam-

ple applications include iterative editing of individual

nanostructures [85,19], repair of photolithographic masks

[27,28], fabrication of nanopores in membranes [30,35],

etching of 3D in-plane features in photoresist [37], and

re-shaping of tips used in scanning probe microscopy

[33,38]. EBIE has also been used to improve the purity

of materials grown by EBID. This application exploits

the fact that EBIE is a material-specific chemical etch

process. A gas mixture comprised of a deposition pre-cursor (e.g. Au(CH3)2(C5H7O2)) and an etch precur-

sor (e.g. H2O) is used to deposit a material such as Au

and simultaneously etch impurities (C) that are unin-

tentionally co-deposited during EBID. The gas mixing

method has been used to purify EBID-grown Au [62–

65], Pt [66], Fe3O4 [61] and SiO2 nanostructures [59,60,

86].

Vanhove et al. [87,31,88] have developed a material

characterization technique in which gaseous EBIE re-

action products are ionized by ultra-short laser pulses

(above the substrate surface) and analyzed by a mass

spectrometer. This innovative method enables depth-

resolved analysis of solids, and fundamental studies of

EBIE mechanisms that lead to volatilization.

Recently, precursors that are conventionally used for

EBIE have been used to achieve other forms of material

restructuring. This has been demonstrated most dra-

matically using XeF2, which can be used to realize three

distinct processes (Fig. 4(a-c)): electron beam induced

fluorination of surfaces [69,89], conventional EBIE, and

4 Milos Toth

growth of GaF3-containing microstructures [90]. Flu-

orination (Fig. 4(a)) has bee used to functionalize a

range of surfaces, and to develop a deposition process in

which F-terminated surface regions are used to catalyze

chemical reactions that initiate localized, room temper-

ature chemical vapor deposition [69]. The key, distin-

guishing aspect of this electron beam writing technique

(Fig. 4(a)) is that the function of the beam is to remove

surface-terminating molecules and replace them with a

chemisorbed species such as fluorine (as opposed to the

removal of bulk material from the substrate, as is done

in conventional EBIE, Fig. 4(b)). The technique is a

variant of similar methods used to fabricate chemically

active surface regions [91–97], and electron irradiation

methods used in surface chemistry studies of phenom-

ena such as electron induced oxidation [98].

Electron induced dissociation of XeF2 adsorbates

can also be used to fabricate GaF3-containing microstruc-

tures (Fig. 4(c)) through a spontaneous, chemically-

assisted structure formation mechanism driven by a

charged particle beam [90]. Specifically, a focused Ga+

ion beam is used to induce bottom-up growth of Ga-

filled, GaF3 microcapillaries by irradiating a GaN sub-

strate in the presence of XeF2 precursor gas. The GaF3

structures form as a result of electron induced decom-

position of XeF2 adsorbates on a surface that contains

excess Ga. The electrons that dissociate XeF2 are sec-

ondary electrons emitted from GaN as a result of ion

irradiation.

Finally, Lassiter et al. [99] have used electron beam

irradiation in a H2O environment to iteratively edit

the geometry and modify the plasmonic properties of

a single (gold shell – silica core) nanoparticle. In this

work, chemical etching was excluded as the underlying

mechanism which was ascribed to a form of ablation

assisted by heating and charging. This application il-

lustrates the potential of direct-write, electron beam

writing techniques for iterative editing of active, func-

tional nanostructures.

3.1 The gap between applications and present

modeling capability

The above applications highlight the need for ad-

vances in predictive, quantitative modeling of electron

(and ion) beam induced etching, deposition and restruc-

turing of solids in gaseous environments. For exam-

ple, EBIE models are needed to improve present un-

derstanding of surface roughening that occurs during

etching, and typically limits the spatial resolution and

geometries of sub-10 nm features fabricated by EBIE

[85] (see Fig. 3(a-d)). Roughening must be minimized

to improve EBIE resolution and the depth resolution of

the EBIE-based mass-spectroscopic analysis technique

developed by Vanhove et al. [88].

Furthermore, existing models can not simulate: (i)

heat- and charge-assisted restructuring processes such

as that proposed by Lassiter et al. [99], (ii) the growth

and size distribution of nanocrystallites grown by EBID

(which are known to depend on fabrication conditions

[79–82,67]), and (iii) the clustering and spatial distribu-

tion of impurities present in deposits that were grown

by conventional EBID or purified by gas mixtures that

give rise to simultaneous EBID and EBIE [59–66,86].

Predictive models of these phenomena will improve our

ability to tune material functionality by controlling com-

position, internal nanostructure and feature geometry

at the sub-10 nm scale.

Electron beam induced fluorination (Fig. 4(a)) has

been modeled in detail, but the process of spontaneous

deposition catalyzed by chemisorbed fluorine has not

been modeled [69]. Similarly, the spontaneous growth

of the Ga-filled microstructures shown in Fig. 3(i) and

4(c) was simulated by a mass transport model that

helped explain the self-organized structure formation

mechanism [90]. However, this new form of bottom-up

growth has not been incorporated into standard models

of particle beam processing (such as Eqns. 1-3). A uni-

fied model would improve present understanding of the

scope and applicability of chemically-assisted electron

and ion beam processing methods in terms of through-

put, resolution, feature placement accuracy, and the ge-

ometries, composition and complexity of materials that

can be fabricated by these techniques.

4 Outlook

Recent applications of electron beam writing techniques

have blurred the distinction between EBIE and related

deposition methods by demonstrating that precursors

which have conventionally been used for etching can

also be used to functionalize surfaces [69,89] and to

grow complex, non-planar microstructures [90]. These

developments, and other examples of surface activa-

tion [91–97] and nanostructure editing through non-

chemical pathways [99], highlight the wide diversity of

processes that can be exploited by electron beam pro-

cessing techniques. Furthermore, these recent applica-

tions illustrate the need for advances in predictive mod-

eling of electron (and ion) beam restructuring of mate-

rials in both inert and reactive environments. Models

published to date have been used to explain a range

of isolated etching, deposition and surface restructur-

ing phenomena. However, the models neglect numer-

ous physical and chemical mechanisms and can not de-

scribe effects such as surface roughening and the simul-

taneous evolution of surface topology, internal nanos-

Advances in gas-mediated electron beam induced etching and related material processing techniques 5

tructure and composition of materials fabricated or re-

structured by charged particle beams. Future work will

likely address these shortcomings, and lead to a better

understanding of the full potential of EBIE and related

material restructuring [69,90–97] and characterization

[87,31,88] techniques.

References

1. I. Utke, S. Moshkalev, P. Russell, Nanofabrication UsingFocused Ion and Electron Beams. Principles and Appli-cations (Oxford University Press, USA, 2012)

2. I. Utke, A. Goelzhaeuser, Angew Chem Int Edit 49(49),9328 (2010)

3. C.R. Arumainayagam, H.L. Lee, R.B. Nelson, D.R.Haines, R.P. Gunawardane, Surf. Sci. Rep. 65(1), 1(2010)

4. I. Utke, P. Hoffmann, J. Melngailis, J. Vac. Sci. Technol.B 26(4), 1197 (2008)

5. S.J. Randolph, J.D. Fowlkes, P.D. Rack, Cr Rev SolidState 31(3), 55 (2006)

6. C.S. Kim, S.H. Ahn, D.Y. Jang, Vaccum 86(8), 1014(2012)

7. R. Kometani, S. Ishihara, Sci. Technol. Adv. Mater.10(3), 034501 (2009)

8. S. Matsui, Y. Ochiai, Nanotechnology 7, 274 (1996)9. A.A. Tseng, J. Micromech. Microeng. 14(4), R15 (2004)

10. A.A. Tseng, Small 1(10), 924 (2005)11. F. Watt, A.A. Bettiol, J.A. Van Kan, E.J. Teo, M. Breese,

International Journal of Nanoscience 4(03), 269 (2005)12. S. Goler, V. Piazza, S. Roddaro, V. Pellegrini, F. Bel-

tram, P. Pingue, J. Appl. Phys. 110(6), 064308 (2011)13. C. Thiele, M. Engel, F. Hennrich, M.M. Kappes, K.P.

Johnsen, C.G. Frase, H.V. Loehneysen, R. Krupke, Appl.Phys. Lett. 99(17), 173105 (2011)

14. P.S. Spinney, D.G. Howitt, S.D. Collins, R.L. Smith,Nanotechnology 20(46), (2009)

15. D. Wang, Hoyle, P. C., J.R.A. Cleaver, G.A. Porkolab,N.C. Macdonald, J. Vac. Sci. Technol. B 13(5), 1984(1995)

16. M. Toth, C.J. Lobo, G. Hartigan, W.R. Knowles, J. Appl.Phys. 101(5), 054309 (2007)

17. H. Miyazoe, I. Utke, J. Michler, K. Terashima, Appl.Phys. Lett. 92(4), 043124 (2008)

18. C.J. Lobo, A. Martin, M.R. Phillips, M. Toth, Nanotech-nology 23(37), 375302 (2012)

19. A.A. Martin, M. Toth, I. Aharonovich, Sci. Rep. 4, 5022(2014)

20. P. Hoffmann, I. Utke, A. Perentes, T. Bret, C. Santschi,V. Apostolopoulos, Proc. SPIE 5925, 592506 (2005)

21. J. Niitsuma, X. Yuan, S. Koizumi, T. Sekiguchi, Jpn. J.Appl. Phys., Part 2 45(1–3), L71 (2006)

22. J. Taniguchi, I. Miyamoto, N. Ohno, K. Kantani, M. Ko-muro, H. Hiroshima, Jpn. J. Appl. Phys. 36, 7691 (1997)

23. A.A. Martin, M.R. Phillips, M. Toth, ACS Appl. Mater.Interfaces 5(16), 8002 (2013)

24. P.D. Rack, S. Randolph, Y. Deng, J. Fowlkes, Y. Choi,D.C. Joy, Appl. Phys. Lett. 82(14), 2326 (2003)

25. J.H. Wang, D.P. Griffis, R. Garcia, P.E. Russell, Semi-cond. Sci. Technol. 18(4), 199 (2003)

26. S.J. Randolph, J.D. Fowlkes, P.D. Rack, J. Appl. Phys.98(3), 034902 (2005)

27. T. Liang, E. Frendberg, B. Lieberman, A. Stivers, J. Vac.Sci. Technol. B 23(6), 3101 (2005)

28. M.G. Lassiter, T. Liang, P.D. Rack, Journal of Vac-uum Science and Technology B: Microelectronics andNanometer Structures 26(3), 963 (2008)

29. M. Toth, C.J. Lobo, M.J. Lysaght, A.E. Vladar, M.T.Postek, J. Appl. Phys. 106(3), 034306 (2009)

30. M. Yemini, B. Hadad, Y. Liebes, A. Goldner, N. Ashke-nasy, Nanotechnology 20(24), 245302 (2009)

31. N. Vanhove, P. Lievens, W. Vandervorst, Phys. Rev. B79(3), 035305 (2009)

32. N. Vanhove, P. Lievens, W. Vandervorst, J. Vac. Sci.Technol. B 28(6), 1206 (2010)

33. J.H. Noh, M. Nikiforov, S.V. Kalinin, A.A. Vertegel, P.D.Rack, Nanotechnology 21(36) (2010)

34. F.J. Schoenaker, R. Cordoba, R. Fernandez-Pacheco,C. Magen, O. Stephan, C. Zuriaga-Monroy, M.R. Ibarra,J.M. De Teresa, Nanotechnology 22(26), 265304 (2011)

35. Y. Liebes, B. Hadad, N. Ashkenasy, Nanotechnology22(28), 285303 (2011)

36. P. Roediger, H.D. Wanzenboeck, S. Waid, G. Hochleitner,E. Bertagnolli, Nanotechnology 22(23), (2011)

37. J.M. Perry, Z.D. Harms, S.C. Jacobson, Small 8(10), 1521(2012)

38. N.A. Roberts, J.H. Noh, M.G. Lassiter, S. Guo, S.V.Kalinin, P.D. Rack, Nanotechnology 23(14), 145301(2012)

39. V. Friedli, I. Utke, J. Phys. D 42(12), 125305 (2009)40. I. Utke, V. Friedli, S. Amorosi, J. Michler, P. Hoffmann,

Microelectronic Engineering 83(4-9), 1499 (2006)41. D.F. Parsons, Science 186, 407 (1974)42. G.D. Danilatos, Adv. Electron. El. Phys. 71, 109 (1988)43. M. Toth, W.R. Knowles, B.L. Thiel, Appl. Phys. Lett.

88(2), 023105 (2006)44. M. Toth, M. Uncovsky, W.R. Knowles, F.S. Baker, Appl.

Phys. Lett. 91(5), 053122 (2007)45. H.F. Winters, J.W. Coburn, Appl. Phys. Lett. 34(1), 70

(1979)46. Y.Y. Tu, T.J. Chuang, H.F. Winters, Phys. Rev. B 23(2),

823 (1981)47. W.F. van Dorp, C.W. Hagen, J. Appl. Phys. 104, 081301

(2008)48. A. Botman, J.J.L. Mulders, C.W. Hagen, Nanotechnol-

ogy 20(37), 372001 (2009)49. M. Huth, F. Porrati, C. Schwalb, M. Winhold, R. Sachser,

M. Dukic, J. Adams, G. Fantner, Beilstein J. Nanotech-nol. 3, 597 (2012)

50. J.D. Fowlkes, P.D. Rack, ACS Nano 4(3), 1619 (2010)51. D.A. Smith, J.D. Fowlkes, P.D. Rack, Nanotechnology

18(26) (2007)52. D.A. Smith, J.D. Fowlkes, P.D. Rack, Small 4(9), 1382

(2008)53. L. Bernau, M. Gabureac, R. Erni, I. Utke, Angew Chem

Int Edit 49(47), 8880 (2010)54. C.J. Lobo, M. Toth, R. Wagner, B.L. Thiel, M. Lysaght,

Nanotechnology 19(2), 025303 (2008)55. S. Randolph, M. Toth, J. Cullen, C. Chandler, C. Lobo,

Appl. Phys. Lett. 99(21), 213103 (2011)56. G.D. Danilatos, M.R. Phillips, J.V. Nailon, Micros. Mi-

croanal. 7(5), 397 (2001)57. G. Danilatos, J. Microsc.-Oxford 234, 26 (2009)58. M.G. Lassiter, P.D. Rack, Nanotechnology 19(45),

455306 (2008)59. A. Perentes, P. Hoffmann, J. Vac. Sci. Technol. B 25(6),

2233 (2007)60. A. Perentes, P. Hoffmann, Chem. Vapor Depos. 13(4),

176 (2007)61. M. Shimojo, M. Takeguchi, K. Furuya, Nanotechnology

17(15), 3637 (2006)

6 Milos Toth

62. A. Folch, J. Tejada, C.H. Peters, M.S. Wrighton, Appl.Phys. Lett. 66(16), 2080 (1995)

63. A. Folch, J. Servat, J. Esteve, J. Tejada, M. Seco, J. Vac.Sci. Technol. B 14(4), 2609 (1996)

64. K. Molhave, D.N. Madsen, A.M. Rasmussen, A. Carlsson,C.C. Appel, M. Brorson, C.J.H. Jacobsen, P. Boggild,Nano Lett. 3(11), 1499 (2003)

65. K. Molhave, D.N. Madsen, S. Dohn, P. Boggild, Nan-otechnology 15(8), 1047 (2004)

66. S. Wang, Y.M. Sun, Q. Wang, J.M. White, J. Vac. Sci.Technol. B 22(4), 1803 (2004)

67. F. Porrati, B. Kampken, A. Terfort, M. Huth, J. Appl.Phys. 113(5), 053707 (2013)

68. J. Bishop, C.J. Lobo, A. Martin, M. Ford, M.R. Phillips,M. Toth, Phys. Rev. Lett. 109, 146103 (2012)

69. S.J. Randolph, A. Botman, M. Toth, Particle 30(8), 672(2013)

70. M. Loudiana, A. Schmid, J. Dickinson, E. Ashley, Surf.Sci. 141, 409 (1984)

71. M. Hills, G. Arnold, Appl. Surf. Sci. 47(1), 77 (1991)72. R.D. Ramsier, J.T. Yates, Surf. Sci. Rep. 12(6-8), 243

(1991)73. W. Li, D.C. Joy, J. Vac. Sci. Technol. A 24(3), 431 (2006)74. W.F. van Dorp, T.W. Hansen, J.B. Wagner, J.T.M.

De Hosson, Beilstein J. Nanotechnol. 4, 474 (2013)75. F. Banhart, Rep. Prog. Phys. 62(8), 1181 (1999)76. R.F. Egerton, P. Li, M. Malac, Micron 35(6), 399 (2004)77. R.F. Egerton, R. McLeod, F. Wang, M. Malac, Ultrami-

croscopy 110(8), 991 (2010)78. A.V. Krasheninnikov, K. Nordlund, J. Appl. Phys.

107(7), 071301 (2010)79. A. Botman, C.W. Hagen, J. Li, B.L. Thiel, K.A. Dunn,

J.J.L. Mulders, S. Randolph, M. Toth, J. Vac. Sci. Tech-nol. B 27(6), 2759 (2009)

80. J. Li, M. Toth, V. Tileli, K.A. Dunn, C.J. Lobo, B.L.Thiel, Appl. Phys. Lett. 93(2), 023130 (2008)

81. J.T. Li, M. Toth, K.A. Dunn, B.L. Thiel, J. Appl. Phys.107(10), 103540 (2010)

82. F. Porrati, R. Sachser, C.H. Schwalb, A.S. Frangakis,M. Huth, J. Appl. Phys. 109(6) (2011)

83. S.J. Randolph, J.D. Fowlkes, P.D. Rack, J. Appl. Phys.97(12), 124312 (2005)

84. J. Cazaux, J. Appl. Phys. 95(2), 731 (2004)85. M. Toth, C.J. Lobo, W.R. Knowles, M.R. Phillips, M.T.

Postek, A.E. Vladar, Nano Lett. 7(2), 525 (2007)86. J. Bishop, M. Toth, M. Phillips, C. Lobo, Appl. Phys.

Lett. 101(21), 211605 (2012)87. N. Vanhove, P. Lievens, W. Vandervorst, Appl. Surf. Sci.

255(4), 1360 (2008)88. N. Vanhove, P. Lievens, W. Vandervorst, Surf. Interface

Anal. 43(1-2), 159 (2010)89. T. Shanley, A.A. Martin, I. Aharonovich, M. Toth, Appl.

Phys. Lett. in press (2014)90. A. Botman, A. Bahm, S. Randolph, M. Straw, M. Toth,

Phys. Rev. Lett. 111(13), 135503 (2013)91. T. Lukasczyk, M. Schirmer, H.P. Steinruck, H. Marbach,

Langmuir 25(19), 11930 (2009)92. M. Walz, M. Schirmer, F. Vollnhals, T. Lukasczyk,

H.P. Steinruck, H. Marbach, Angewandte Chemie-international Edition 49, 4669 (2010)

93. F. Vollnhals, P. Wintrich, M.M. Walz, H.P. Steinruck,H. Marbach, Langmuir 29(39), 12290 (2013)

94. F. Vollnhals, T. Woolcot, M.M. Walz, S. Seiler, H.P.Steinruck, G. Thornton, H. Marbach, The Journal ofPhysical Chemistry C 117(34), 17674 (2013)

95. M.M. Walz, F. Vollnhals, F. Rietzler, M. Schirmer, H.P.Steinruck, H. Marbach, Appl. Phys. Lett. 100(5), 053118(2012)

96. K. Muthukumar, H.O. Jeschke, R. Valentı, E. Begun,J. Schwenk, F. Porrati, M. Huth, Beilstein J. Nanotech-nol. 3, 546 (2012)

97. M.N. Khan, V. Tjong, A. Chilkoti, M. Zharnikov, AngewChem Int Edit 51(41), 10303 (2012)

98. H. Ebinger, J. Yates, Phys. Rev. B 57(3), 1976 (1998)99. J.B. Lassiter, M.W. Knight, N.A. Mirin, N.J. Halas, Nano

Lett. 9(12), 4326 (2009)

Advances in gas-mediated electron beam induced etching and related material processing techniques 7

Pote

ntia

l ene

rgy

Distance above the substrate surface

gas phase

physisorbed state

a)

Pote

ntia

l ene

rgy

Distance above the substrate surface

gas phasephysisorbed state

chemisorbed stateb)

activation barrier

Fig. 2 Potential energy diagrams for physisorption (a) andactivated chemisorption (b) used in models of gas-mediatedelectron beam induced processing [68,69].

Nanotechnology 23 (2012) 145301 N A Roberts et al

Figure 1. SEM images of the process steps of the standard conductive AFM tip. (a) As-received standard conductive AFM tip. (b) 30 nm Ptsputtered onto AFM tip. (c) 50 nm SiNx PECVD. (d) Electron beam induced etch with XeF2. (e) Electrochemical deposition of Au.(f) Schematic of the setup for electroplating of the isolated AFM tip. (g) Current measurement during electrochemical deposition. Thedeposition begins at the location labeled start and ends at the location labeled end.

The latter necessitates a reduced interaction region of the apexof the tip, and ideally a higher aspect ratio of the conicalpart of the probe [13]. One method for the size reduction orincreased aspect ratio is to attach a high aspect ratio material,such as carbon nanotube (CNT) or nanowire, at the probeapex [14]. In this paper, we demonstrate a synthesis route foran electrically shielded carbon nanotube scanning probe tipoffering high resolution and electrical isolation.

2. Standard insulated scanning probes

Illustrated in figure 1 is a sequence of scanning electronmicroscopy (SEM) images demonstrating the steps involvedin synthesizing an insulated (covered by insulator) orshielded (having additional external shield) scanning probetip. Figure 1(a) shows the as-received silicon scanning probeand 1(b) shows the tip after a 30 nm Pt conductive layerand etch stop layer was deposited (albeit thinner at the tip

due to geometric considerations of the nearly line-of-sitedeposition). The platinum sputtering conditions were 10 Wdc power (150 mm diameter target), 3 mTorr pressure andan approximate source to substrate distance of 7.5 cm.Figure 1(c) shows the tip after a 50 nm thick silicon nitridefilm was deposited (process conditions given below) andfigure 1(d) shows the tip after a selective focused electronbeam induced etching at the tip apex. For more informationon electron beam induced processing (EBIP) see [15–17].In addition to the listed reviews of EBIP, recent work byLobo et al [18] demonstrated sub-1 nm length scales with acombined electron beam induced etch (EBIE) and deposition(EBID) process via simulations.

The high uniformity of dielectric coating can bedemonstrated by the electroplating experiment illustratedin figure 1(e). Here, the shielded SPM tip is mountedin an Asylum Research MFP-3D AFM platform and isthen submerged in a droplet of gold electroplating solution

2

it is possible to perform a direct etching of graphene byinjecting oxygen gas in the SEM chamber during imaging atlow voltage (5 kV in this case) and exploiting the electronbeam in order to locally create ozone and oxygen radicals.36

We verified that the EBIE is particularly effective on SLGand we were able to monitor the etching process in real timeby observing how the contrast changed during a line scan onthe graphene flake. Figure 5(a) shows the SEM image of thegraphene flakes together with the vertical scan line (dashedred line) where the single-line lithographic process wasapplied. Figure 5(b) reports the SEM image of the SGL afterthe spatially resolved etching. Two of the three larger blistersalready shown in Fig. 4(a) were strongly modified by theetching and the graphene membrane relaxed onto the sur-face. One of the blisters was pierced by direct EBIE [Fig.5(c), red arrow]. In this case the result was an almost com-plete flattening of the suspended SLG membrane [note thechange in the dark image contrast between Fig. 5(a), 5(b),

and 5(c)]. Strain relaxation probably originates from dam-ages to the crystalline structure of graphene induced by oxy-gen. This interpretation was strengthened by the emergenceof a strong D peak in the Raman spectra [inset in Fig. 5(c)]performed after the spatially resolved etching, in the vicinityof the exposed areas.

Blisters in graphene produced by standard microme-chanical exfoliation (at much lower density compared to thepresent case) were observed and “bubbles” were alsoinduced in exfoliated graphene by chemical methods andproton irradiation, as described in Ref. 29. Similar results ongraphene or other single-layer materials obtained by PDMS-based transfer printing were reported: in those cases the pres-ence of blistering was not highlighted by the authors but isevident from the published AFM images.37 Recently, anothergroup demonstrated that gold and silver nanoparticles depos-ited on top of SiO2 substrates can create suspended graphenemembranes,38 as we observed in correspondence of the nano-scale debris present on our substrates.

We believe that a more precise control of the pressureapplied in the PDMS transfer-printing technique will favorthe production of graphene layers having a surface topogra-phy ranging continuously from “flat” to “blistered” shapes.

III. CONCLUSIONS

In conclusion, PDMS transfer-printing represents a veryuseful approach for the production of graphene from bulkgraphite with much reduced contamination with respect toother transfer-printing methods that rely on scotch tape, resinor PMMA-based materials. We believe that it constitutes astraightforward methodology to investigate strain-inducedeffects in single-layer graphene. Moreover further improve-ments and optimization of the PDMS transfer-printingmethod reported here may allow the controlled and orderedself-assembly of blistering and rippling on single graphenelayers on various substrates (e.g., as recently studied in sus-pended graphene membranes with thermally-generatedstrain),39,40 allowing a systematic investigation of the effectof blistering on the electronic and optical properties of gra-phene and their links and interactions with different substratespecies. Finally, in order to prove the nature of the blisterednanostructures, a spatially resolved direct etching of gra-phene was successfully demonstrated employing e-beaminduced ionization of O2 in a SEM vacuum chamber. Wethink that this technique could become a valuable tool forgraphene lithography once its impact on electronic and opti-cal properties of the resulting patterned nanostructure isproperly assessed.41

ACKNOWLEDGMENTS

We would thank Dr. Marco Cecchini for technicalassistance during the PDMS sample preparation andfunctionalization.

1K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V.Dubonos, I. V. Grigorieva, and A. A. Firsov, Science 306, 666 (2004).

2A. K. Geim and K. S. Novoselov, Nature Mater 6, 183 (2007).3Y. Zheng, Y. W. Tan, H. L. Stormer, and P. Kim, Nature 438, 201 (2005).

FIG. 5. (Color online) SEM images of the same SLG region analyzed inFigs. 3 and 4. (a) Large view of the sample: the dashed line is the one whereEBIE was performed. (b) SEM picture of the SLG after the spatially resolvedEBIE: the obtained cut has a width of 37 nm and some blisters disappeared,relaxing on the substrate. (c) The same region after EBIE pricking of the cen-tral blister (evidenced by the red arrow). Inset: Raman spectrum on the sameSLG after the EBIE process. Notice the appearance of the D peak.

064308-5 Goler et al. J. Appl. Phys. 110, 064308 (2011)

Downloaded 30 Oct 2011 to 138.25.78.25. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions

a) b) c) d)

e) f)

g)

500 nm

200 nm

EBIO gap formation. During the EBIO process, a constantsource-drain voltage VSD of maximum 1 V was applied andthe current continuously measured in intervals of 50 ms.

To cut the carbon nanotubes, an electron-beam line scanwas executed across the nanotube while injecting the oxy-gen. During line scans, the microscope magnification wasadjusted to be either 25 kX or 50 kX, yielding a line scanwidth of 4.57 lm and 2.29 lm, respectively. The primaryelectron-beam current used was !100 pA, which yields aline dose of !21.8 lC/m and !43.7 lC/m per second,respectively. Acceleration voltage of the primary electrons(PEs) was set to 10 kV. Scale-calibrated images were used toassess the reproducibility of gap sizes.

Figs. 1(b) and 1(c) show SEM images of an individualmSWNT on SiO2/Si and on Si3N4 after EBIO-induced gapformation. Both images were recorded with an in-lens detec-tor that is expected to be sensitive to SE1 and SE2 secondaryelectrons.14 For better visibility of the mSWNT on SiO2/Si,we used voltage-contrast scanning electron microscopy(VCSEM) to suppress the SEs of the SiO2/Si substrate.15

The required backgate voltage Vg"#10 V was applied withrespect to the source-drain electrodes only for imaging. Aneffect of Vg on the EBIO process was not observed.Although Fig. 1(c) was recorded without VCSEM, the mem-brane appears dark due to its limited SE2 generation.16 TheEBIO-induced gaps shown in Figs. 1(b) and 1(c) are !20 nmwide. This is a typical result since the gap size average over62 devices on SiO2/Si substrates is (19 6 5) nm, as shown inFig. 1(d). We should note that the formation of a gap hasalso been confirmed by topography measurements with anatomic force microscope (not shown). EBIO is not limited tothe cutting of individual nanotubes but can also be extendedto form gaps in devices with multiple nanotubes. Fig. 2(c)

shows such an example. With EBIO, gaps of similar size canbe produced in all of the nanotubes at the same relative posi-tion. Such a result cannot be obtained by current-inducedoxidation.17–19

The critical line dose nC that is required for gap forma-tion has been determined from in-situ conductance measure-ments. Fig. 2(a) shows the typical conductance reduction ofan mSWNT-device on SiO2/Si and Si3N4, during the EBIOprocess. The traces of the conductance G have been normal-ized to their initial values Gi, at the beginning of the EBIOprocess at t" 0. Gi is typically on the order of (40 kX)$1 af-ter current-annealing. G decreases with n roughly exponen-tially by two orders of magnitude, before a sudden drop byadditional 3-4 orders of magnitude down to the electron-beam-induced residual conductance is recorded< (10GX)$1. This last step indicates the formation of a gap in thenanotube. There are no plateaus in G which could indicate astep-wise introduction of defects. Typically, nC is on theorder of a few mC/m. Conductance traces for several thin-film devices during the EBIO process are shown in Fig. 2(b).The critical dose is comparable to or larger than the dose forsingle-tube devices, which is likely due to the presence ofoverlapping nanotubes in some of the thin-film devices.

The conductance drop remains irreversible even atVSD % 10 V. This behavior is in marked contrast to the con-tinuous electron-beam-induced metal-to-insulator transitions,which are caused by charges trapped in the substrate sur-face.13 It has been shown that such transitions require a linedose of !200 lC/m at 10 kV PE energy. During the EBIOprocess, such a small dose causes only a small reduction inG. We assume that charging is not important here since oxy-gen ions are compensating electron-beam-induced surfacecharges.

FIG. 1. (Color online) (a) Schematic of the EBIO setup: Primary-beam electrons are repeatedly scanned across an mSWNT in the presence of injected oxygen.The mSWNTs are wired to Pd electrodes and supported by SiO2/Si substrates or Si3N4 membranes (inset). Indicated are the trajectories of secondary (SE1,SE2) and BSE. (b) SE image of an mSWNT on SiO2 after gap formation, recorded with Vg"#10 V. (c) SE image of an mSWNT on a Si3N4 membrane aftergap formation. (d) Histogram of nanogap sizes of 62 devices on SiO2/Si substrates, fitted to a normal distribution with l" 19 nm and r" 5 nm.

FIG. 2. (Color online) (a) Normalized conductance G/Gi vs. line dose n recorded during the EBIO process for a single-tube device on SiO2/Si and on a Si3N4

membrane. (b) Normalized conductance G/Gi vs. line dose n recorded during the EBIO process for several thin-film devices on SiO2/Si. The spread in criticaldose is likely due to the presence of overlapping nanotubes in some of the thin-film devices. (c) SE image of an mSWNT thin-film device after gap formation.

173105-2 Thiele et al. Appl. Phys. Lett. 99, 173105 (2011)

Downloaded 15 Mar 2012 to 138.25.78.25. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions

Ga GaF3

GaN

h) i)

Fig. 3 (a-d) Four frames from a movie showing the formationof a nano-gap in a carbonaceous nanowire on a bulk SiO2 sub-strate by high resolution, H2O EBIE [85], (e) scanning probemicroscopy tip sculpted by XeF2 EBIE [38], (f) etch pit inchrome fabricated by XeF2 EBIE [16], (g) 37 nm gap etchedin graphene by O2 EBIE [12], (h) gap cut into a single-walledcarbon nanotube by O2 EBIE [13], (i) false-color electron im-age of a Ga-filled GaF3 microcapillary grown using XeF2 asshown in Fig. 4(c) [90].

8 Milos Toth

a) b) c) e- e- e-

precursor gas

adsorbates

substrate

x y

z

Fig. 1 General steps involved in electron beam induced etching: (a) electron beam irradiation, and emission of secondary andbackscattered electrons from the substrate, (b) consumption of adsorbates in the etch reaction, and precursor replenishmentthrough adsorption from the gas phase and diffusion along the substrate surface, (c) volatilization of the substrate in thevicinity of the electron beam.

a) b) e-

e-

c) Ga+

F-terminated surface

GaF3 sheath

Ga

GaN

e-

Fig. 4 Three forms of material processing driven by electron dissociation of XeF2 adsorbates: (a) fluorination of the substratesurface [69], (b) electron beam induced etching, and (c) fabrication of gallium-filled, GaF3 capillaries by ion bombardmentof GaN [90]. GaF3 forms due to XeF2 decomposition by secondary electrons which are emitted from GaN and irradiate thesidewall of the growing pillar. An electron image of such a microstructure is shown in Fig. 3(i)