Cao Z. (Ed.) Thin Film Growth: Physics, materials science and applications

Подождите немного. Документ загружается.

116 Thin ﬁlm growth

Forker, R. & Fritz, T. (2009) Optical differential reectance spectroscopy of ultrathin

epitaxial organic lms. Physical Chemistry Chemical Physics, 11, 2142–2155.

Forrest, S. R. (1997) Ultrathin organic lms grown by organic molecular beam deposition

and related techniques. Chemical Reviews, 97, 1793–1896.

frenken, J. w. M., Toennies, J. P. & wöll, c. (1988) Self-diffusion at a melting surface

observed by He scattering. Physical Review Letters, 60, 1727–1730.

fuoss, P. H., Kisker, d. w., lamelas, f. J., Stephenson, G. B., imperatori, P. & Brennan,

S. (1992) Time-resolved x-ray scattering studies of layer-by-layer epitaxial growth.

Physical Review Letters, 69, 2791–2794.

Gerlach, a., Schreiber, f., Sellner, S., dosch, H., Vartanyants, i. a., cowie, B. c. c., lee,

T.-l. & Zegenhagen, J. (2005) adsorption-induced distortion of f16cuPc on cu(111)

and ag(111): an X-ray standing wave study. Physical Review B, 71, 205425.

Gerlach, a., Sellner, S., Schreiber, f., Koch, n. & Zegenhagen, J. (2007) Substrate

dependent bonding distances of PTcda – a comparative X-ray standing wave study

on cu(111) and ag(111). Physical Review B, 75, 045401.

Haftel, M. i. (2001) ehrlich–Schwoebel effect for vacancies: low-index faces of silver.

Physical Review B, 64, 125415.

Hartell, a. d., Tok, e. S. & Zhang, J. (2001) The development of raS and rHeed

as in situ probes to monitor dopant segregation in GS-MBe on Si(001). Journal of

Crystal Growth, 227, 729–734.

Heinemeyer, U. (2009) Optical properties of organic semiconductor thin lms: static

spectra and real-time growth studies. Phd thesis, Universität Tübingen.

Heinemeyer, U., Broch, K., Hinderhofer, a., Kytka, M., Scholz, r., Gerlach, a. & Schreiber,

F. (2010) Real-time changes in the optical spectrum of organic semiconducting lms

and their thickness regimes during growth. Physical Review Letters, 104, 257401.

Hinch, B. J., lock, a., Madden, H. H., Toennies, J. P. & witte, G. (1990) Helium-atom

scattering investigation of facetting of the al stepped (332) surface. Physical Review

B, 42, 1547.

Hinderhofer, a., Gerlach, a., Kowarik, S., Zontone, f., Krug, J. & Schreiber, f. (2010)

Smoothing and coherent structure formation in organic-organic heterostructure growth.

EPL (Europhysics Letters), 91, 56002.

Hlawacek, G., Puschnig, P., frank, P., winkler, a., ambrosch-draxl, c. & Teichert, c.

(2008) Characterization of step-edge barriers in organic thin-lm growth. Science,

321, 108–111.

Hofer, w. a., foster, a. S. & Shluger, a. l. (2003) Theories of scanning probe microscopes

at the atomic scale. Reviews of Modern Physics, 75, 1287–1331.

Hosokai, T., Gerlach, a., Hinderhofer, a., frank, c., ligorio, G., Heinemeyer, U.,

Vorobiev, a. & Schreiber, f. (2010) Simultaneous in situ measurements of x-ray

reectivity and optical spectroscopy during organic semiconductor thin lm growth.

Applied Physics Letters, 97, 063301.

Hyde, M. e., Jacobs, r. & compton, r. G. (2002) In situ afM studies of metal deposition.

The Journal of Physical Chemistry B, 106, 11075–11080.

Jardine, a. P., ellis, J. & allison, w. (2002) Quasi-elastic helium-atom scattering from

surfaces: experiment and interpretation. Journal of Physics: Condensed Matter, 14,

6173.

Kern, K., niehus, H., Schatz, a., Zeppenfeld, P., Goerge, J., & comsa, G. (1991) long-

range spatial self-organization in the adsorbate-induced restructuring of surfaces:

cu{100}-(2 ¥ 1)o. Physical Review Letters, 67, 855–858.

Kneppe, M., dorna, V., Kohstall, P., Kot, e. & Köhler, U. (2000) a kinetic scanning

ThinFilm-Zexian-05.indd 116 7/1/11 9:40:52 AM



117Modelling thin ﬁlm deposition processes

tunneling microscopy study of iron silicide growth on Si(113). Surface Science,

454–456, 802–806.

Koch, n., Gerlach, a., duhm, S., Glowatzki, H., Heimel, G., Vollmer, a., Sakamoto, Y.,

Suzuki, T., Zegenhagen, J., rabe, J. P. & Schreiber, f. (2008) adsorption-induced

intramolecular dipole: correlating molecular conformation and interface electronic

structure. Journal of the American Chemical Society, 130, 7300–7304.

Korte, U. & Maksym, P. A. (1997) Role of the step density in reection high-energy

electron diffraction: questioning the step density model. Physical Review Letters,

78, 2381.

Kowarik, S., Gerlach, a., Sellner, S., Schreiber, f., cavalcanti, l. & Konovalov, o.

(2006) real-time observation of structural and orientational transitions during growth

of organic thin lms. Physical Review Letters, 96, 125504.

Kowarik, S., Gerlach, a., leitenberger, w., Hu, J., witte, G., wöll, c., Pietsch, U. &

Schreiber, F. (2007) Energy-dispersive X-ray reectivity and GID for real-time growth

studies of pentacene thin lms. Thin Solid Films, 515, 5606–5610.

Kowarik, S., Gerlach, a., Sellner, S., cavalcanti, l., Konovalov, o. & Schreiber,

f. (2009a) real-time X-ray diffraction measurements of structural dynamics and

polymorphism in diindenoperylene growth. Applied Physics A – Materials Science

& Processing, 95, 233–239.

Kowarik, S., Gerlach, a., Sellner, S., cavalcanti, l. & Schreiber, f. (2009b) dewetting

of an organic semiconductor thin lm observed in real-time. Advanced Engineering

Materials, 11, 291–294.

Kowarik, S., Gerlach, a., Skoda, M. w. a., Sellner, S. & Schreiber, f. (2009c) real-time

studies of thin lm growth: measurement and analysis of X-ray growth oscillations

beyond the anti-Bragg point. European Physical Journal – Special Topics, 167,

11–18.

Krause, B., durr, a. c., Schreiber, f., dosch, H. & Seeck, o. H. (2004a) late growth

stages and post-growth diffusion in organic epitaxy: PTcda on ag(111). Surface

Science, 572, 385–395.

Krause, B., Schreiber, f., dosch, H., Pimpinelli, a. & Seeck, o. H. (2004b) Temperature

dependence of the 2d–3d transition in the growth of PTcda on ag(111): a real-time

X-ray and kinetic Monte carlo study. Europhysics Letters, 65, 372–378.

Krug, J. (1997) origins of scale invariance in growth processes. Advances in Physics,

46, 139–282.

Krug, J. (2004) Power laws in surface physics: the deep, the shallow and the useful.

Physica A, 340, 647–655.

Krug, K., Stettner, J. & Magnussen, o. M. (2006) In situ surface X-ray diffraction studies

of homoepitaxial electrochemical growth on au(100). Physical Review Letters, 96,

246101–4.

lazzari, r., renaud, G., revenant, c., Jupille, J. & Borensztein, Y. (2009) adhesion

of growing nanoparticles at a glance: surface differential reectivity spectroscopy

and grazing incidence small angle X-ray scattering. Physical Review B, 79, 125428.

lippmaa, M., nakagawa, n., Kawasaki, M., ohashi, S. & Koinuma, H. (2000) Growth

mode mapping of SrTio

epitaxy. Applied Physics Letters, 76, 2439–2441.

lüth, H. (2001) Solid Surfaces, Interfaces and Thin Films. Berlin: Springer-Verlag.

Markov, i. V. (2004) Crystal Growth for Beginners, 2nd edn. Hackensack, nJ: world

Scientic Press.

Mcintyre, J. D. E. & Aspnes, D. E. (1971) Differential reection spectroscopy of very

thin surface lms. Surface Science, 24, 417–434.

ThinFilm-Zexian-05.indd 117 7/1/11 9:40:53 AM



118 Thin ﬁlm growth

Mcrae, e. G. (1967) Self-consistent multiple-scattering approach to the interpretation

of low-energy electron diffraction. Surface Science, 8, 14–34.

Mercurio, G., Mcnellis, e. r., Martin, i., Hagen, S., leyssner, f., Soubatch, S., Meyer,

J., wolf, M., Tegeder, P., Tautz, f. S. & reuter, K. (2010) Structure and energetics of

azobenzene on ag(111): benchmarking semiempirical dispersion correction approaches.

Physical Review Letters, 104, 036102.

Meyer Zu Heringdorf, f. J., reuter, M. c. & Tromp, r. M. (2001) Growth dynamics of

pentacene thin lms. Nature, 412, 517–520.

Michely, T. & Krug, J. (2004) Islands, Mounds and Atoms: Patterns and Processes in

Crystal Growth Far From Equilibrium. Heidelberg: Springer.

Murty, M. V. r., curcic, T., Judy, a., cooper, B. H., woll, a. r., Brock, J. d., Kycia,

S. & Headrick, r. l. (1998) X-ray scattering study of the surface morphology of

au(111) during ar+ ion irradiation. Physical Review Letters, 80, 4713–4716.

oura, K., lifshits, V. G., Saranin, a. a., Zotov, a. V. & Katayama, M. (2003) Surface

Science. Heidelberg: Springer.

Parratt, L. G. (1954) Surface studies of solids by total reection of x-rays. Physical

Review, 95, 359–369.

Pimpinelli, a. & Villain, J. (1998) Physics of Crystal Growth. cambridge: cambridge

University Press.

Poelsema, B. & comsa, G. (1989) Scattering of Thermal Energy Atoms from Disordered

Surfaces Berlin/Heidelberg: Springer.

Poelsema, B., Becker, a. f., rosenfeld, G., Kunkel, r., nagel, n., Verheij, l. K. &

comsa, G. (1992) on the shape of the in-phase TeaS oscillations during epitaxial

growth of Pt(111). Surface Science, 272, 269–275.

rabe, J. P. & Buchholz, S. (1991) direct observation of molecular-structure and dynamics

at the interface between a solid wall and an organic solution by scanning tunneling

microscopy. Physical Review Letters, 66, 2096–2099.

Ratsch, C. & Schefer, M. (1998) Density-functional theory calculations of hopping rates

of surface diffusion. Physical Review B, 58, 13163.

ritley, K. a., Krause, B., Schreiber, f. & dosch, H. (2001) a portable ultrahigh vacuum

organic molecular beam deposition system for in situ X-ray diffraction measurements.

Review of Scientic Instruments, 72, 1453–1457.

rost, M. J. (2007) In situ real-time observation of thin lm deposition: roughening,

zeno effect, grain boundary crossing barrier, and steering. Physical Review Letters,

99, 266101.

rusponi, S., Boragno, c. & Valbusa, U. (1997) ripple structure on ag(110) surface

induced by ion sputtering. Physical Review Letters, 78, 2795–2798.

Santoro, G., franchini, a., Bortolani, V., Harten, U., Toennies, J. P. & wöll, c. (1987)

inelastic scattering of helium atoms from the phonons of the reconstructed au(111)

surface. Surface Science, 183, 180–188.

Schlettwein, d., Graaf, H., Meyer, J. P., oekermann, T. & Jaeger, n. i. (1999) Molecular

interactions in thin lms of hexadecauorophthalocyaninatozinc (F16PcZn) as compared

to islands of n,n¢-dimethylperylene-3,4,9,10-biscarboximide (MePTcdi). The Journal

of Physical Chemistry B, 103, 3078–3086.

Schreiber, f. (2000) Structure and growth of self-assembling monolayers. Progress in

Surface Science, 65, 151–257.

Schreiber, F. (2004) Organic molecular beam deposition: growth studies beyond the rst

monolayer. Physica Status Solidi A – Applied Research, 201, 1037–1054.

Schreiber, f., eberhardt, a., leung, T. Y. B., Schwartz, P., wetterer, S. M., lavrich, d.

ThinFilm-Zexian-05.indd 118 7/1/11 9:40:53 AM



119Modelling thin ﬁlm deposition processes

J., Berman, l., fenter, P., eisenberger, P. & Scoles, G. (1998) adsorption mechanisms,

structures, and growth regimes of an archetypal self-assembling system: decanethiol

on au.111. Physical Review B, 57, 12476.

Schwoebel, r. l. & Shipsey, e. J. (1966) Step motion on crystal surfaces. Journal of

Applied Physics, 37, 3682–3686.

Scoles, G., Bassi, d., Buck, U. & lainé, d. (1988) Atomic and Molecular Beam Methods.

oxford: oxford University Press.

Seidel, c., Poppensieker, J. & fuchs, H. (1998) real-time monitoring of phase transitions

of vacuum deposited organic lms by molecular beam deposition LEED. Surface

Science, 408, 223–231.

Seydel, T., Madsen, a., Sprung, M., Tolan, M., Grübel, G. & Press, w. (2003) Setup

for in situ surface investigations of the liquid/glass transition with (coherent) x rays.

Review of Scientic Instruments, 74, 4033–4040.

Shehu, a., Quiroga, S. d., d’angelo, P., albonetti, c., Borgatti, f., Murgia, M., Scorzoni,

a., Stoliar, P. & Biscarini, f. (2010) layered distribution of charge carriers in organic

thin lm transistors. Physical Review Letters, 104, 246602.

Shin, B. & aziz, M. J. (2007) Modeling rHeed intensity oscillations in multilayer

epitaxy: determination of the ehrlich–Schwoebel barrier in Ge(001) homoepitaxy.

Physical Review B, 76, 165408.

Stadler, c., Hansen, S., Kroger, i., Kumpf, c. & Umbach, e. (2009) Tuning intermolecular

interaction in long-range-ordered submonolayer organic lms. Nature Physics, 5,

153–158.

Stephenson, G. B., eastman, J. a., Thompson, c., auciello, o., Thompson, l. J.,

Munkholm, a., fini, P., denbaars, S. P. & Speck, J. S. (1999) observation of growth

modes during metal-organic chemical vapor deposition of Gan. Applied Physics

Letters, 74, 3326–3328.

Teng, H. H., fenter, P., cheng, l. w. & Sturchio, n. c. (2001) resolving orthoclase

dissolution processes with atomic force microscopy and X-ray reectivity. Geochimica

et Cosmochimica Acta, 65, 3459–3474.

Tischler, J. Z., eres, G., larson, B. c., rouleau, c. M., Zschack, P. & lowndes, d.

H. (2006) nonequilibrium interlayer transport in pulsed laser deposition. Physical

Review Letters, 96, 226104.

Tromov, V. I. & Mokerov, V. G. (2002) Epitaxial growth kinetics in the presence of

an ehrlich–Schwoebel barrier: comparative analysis of different models. Materials

Science and Engineering B, 89, 420–425.

Van der Vegt, H. a., alvarez, J., Torrelles, X., ferrer, S. & Vlieg, e. (1995) indium-

induced layer-by-layer growth and suppression of twin formation in the homoepitaxial

growth of cu(111). Physical Review B, 52, 17443–17448.

Venables, J. (2000) Introduction to Surface and Thin Film Processes. cambridge:

cambridge University Press.

Venables, J. a., Spiller, G. d. T. & Hanbücken, M. (1984) nucleation and growth of

thin lms. Reports on Progress in Physics, 47, 399–459.

Weightman, P., Martin, D. S., Cole, R. J. & Farrell, T. (2005) Reection anisotropy

spectroscopy. Reports on Progress in Physics, 68, 1251.

weschke, e., Schüßler-langeheine, c., Meier, r., Kaindl, G., Sutter, c., abernathy, d. &

Grübel, G. (1997) q dependence of the growth-oscillation period of x-ray reectivity

in heteroepitaxy: Ho/w(110). Physical Review Letters, 79, 3954–3957.

williamson, M. J., Tromp, r. M., Vereecken, P. M., Hull, r. & ross, f. M. (2003)

dynamic microscopy of nanoscale cluster growth at the solid-liquid interface. Nat

Mater, 2, 532–536.

ThinFilm-Zexian-05.indd 119 7/1/11 9:40:53 AM



120 Thin ﬁlm growth

witte, G. & wöll, c. (1995) external vibrations of hydrocarbons on cu(100). The Journal

of Chemical Physics, 103, 5860–5863.

witte, G. & wöll, c. (2004) Growth of aromatic molecules on solid substrates for

applications in organic electronics. Journal of Materials Research, 19, 1889–1916.

woll, a. r., Headrick, r. l., Kycia, S. & Brock, J. d. (1999) Gan nucleation and

growth on sapphire(0001): incorporation and interlayer transport. Physical Review

Letters, 83, 4349–4352.

Xu, H., Yang, Y.-W. & Engel, T. (1991) Quantication of defects in epitaxial metal

lm growth: a helium diffraction investigation of the Cu/W(110) system. Surface

Science, 255, 73–90.

Yamane, H., Gerlach, a., duhm, S., Tanaka, Y., Hosokai, T., Mi, Y. Y., Zegenhagen,

J., Koch, N., Seki, K. & Schreiber, F. (2010) Site-specic geometric and electronic

relaxations at organic-metal interfaces. Physical Review Letters, 105, 046103.

Yim, S. & Jones, T. S. (2006) anomalous scaling behavior and Surface roughening in

molecular thin-lm deposition. Physical Review B, 73, 161305(r).

Zangwill, a. (1988) Physics at Surfaces, cambridge: cambridge University Press.

Zhang, X. n., Barrena, e., de oteyza, d. G. & dosch, H. (2007) Transition from layer-

by-layer to rapid roughening in the growth of diP on Sio

. Surface Science, 601,

2420–2425.

Zhang, X., Barrena, e., Goswami, d., de oteyza, d. G., weis, c. & dosch, H. (2009)

Evidence for a layer-dependent Ehrlich–Schwoebel barrier in organic thin lm growth.

Physical Review Letters, 103, 136101.

Zheng, H., Smith, r. K., Jun, Y.-w., Kisielowski, c., dahmen, U. & alivisatos, a.

P. (2009) observation of single colloidal platinum nanocrystal growth trajectories.

Science, 324, 1309–1312.

ThinFilm-Zexian-05.indd 120 7/1/11 9:40:53 AM



123

Silicon nanostructured films grown on

templated surfaces by oblique angle

deposition

D. Ye, Virginia Commonwealth University, USA and

T.-M. L U, Rensselaer Polytechnic Institute, USA

Abstract: We observed that the deposition of silicon (Si) nanostructured thin

lms on a patterned substrate with oblique angle incidence ux gives rise to

a phenomenon referred to as ‘fan-out’ growth, which entails the overgrowth

of the structures along the direction perpendicular to the incident deposition

ux. We concluded in our Monte Carlo simulations that the side-sticking of

incoming particles to the previously deposited particles is the cause of ‘fan-

out’ growth. We have designed two methods to overcome fan-out growth:

‘two-phase’ rotation and ‘swing’ rotation. By these methods, uniform slanted

Si nanorods and square nanosprings were fabricated.

Key words: oblique angle deposition, fan-out growth, ballistic sticking,

Monte Carlo simulation, nanostructured thin lm, substrate rotation.

6.1 Introduction

The morphological evolution and pattern formation in growing surfaces have

been the subject of intense study using a framework of statistical mechanics

in the past decades (Barabasi and Stanley, 1995; Meakin, 1998; Marsili et al.,

1996). Several theoretical growth models have been proposed to describe

the deposition processes and scaling behaviors of a growing surface with the

atoms depositing from an orthogonal ux on a smooth substrate (Barabasi

and Stanley, 1995; Kardar et al., 1989; Tong and Williams, 1994). On the

other hand, there has been little effort made to understand the process of

atoms striking the surface from non-normal directions (Meakin and Krug,

1992; Barabasi and Stanley, 1995). This process, referred to as oblique angle

deposition, can result in a porous thin lm whose porosity greatly depends

on the incident angle of the atoms.

In 1995, Robbie and Brett systematically studied the oblique angle deposition

with substrate rotation method, which they called ‘glancing angle deposition’

(GLAD), at an incident angle close to 90∞ with respect to the substrate

normal in experiments (Robbie et al., 1995a,b). The substrate rotation was

driven through a stepper motor and a computer programmed to control the

motion of the motor. Since then, this technique has been demonstrated as

ThinFilm-Zexian-06.indd 123 7/1/11 9:41:14 AM



124 Thin ﬁlm growth

a powerful tool to create complex nanostructured thin lms (Robbie et al.,

1996; Hawkeye and Brett, 2007). Figure 6.1(a) shows a common system used

for oblique angle deposition with the freedom of substrate rotation through a

stepper motor. A large distance between the substrate and source is necessary

to grow high quality structured thin lm in oblique angle deposition. For

some systems, a collimator or a nozzle can be used to minimize the angular

spread of the deposition ux. The source materials can be evaporated at

sufciently low base pressures by methods such as thermal evaporation,

electron-beam evaporation, pulsed laser ablation, and sputtering. either

single elements or compounds can be deposited in oblique angle deposition

with a suitable evaporation method. As the incident atoms approaching the

surface at an incident angle q, as shown in Fig. 6.1(a), they will deposit on

the extruding part of the surface. The lower parts of the surface are shadowed

by the neighboring structures. This is called the ‘global shadowing effect’,

as illustrated in the inset of Fig. 6.1. After landing on the surface, the atoms

may diffuse across the surface to an energetically favorable site. As discussed

above, the substrates can be placed in the chamber and locked in a xed

position, or can be rotated in a designed motion pattern. For both cases, a

columnar thin lm is the result of the competition between shadowing effect

and surface diffusion. The difference is that tilted columns will grow if the

substrate is xed in one position, while vertical columns will grow if the

substrate is rotated with sufcient speed, as shown in Fig. 6.1(b).

Substrate

Motor

Vapor

Shadowing effect

Incident vapor

Fix substrate

Rotating substrate

Incident vapor

Source

(a) (b)

6.1 (a) Schematic of a common oblique angle deposition setup.

The substrate can be rotated by a stepper motor. The shadowing

effect is depicted in the inset. (b) Nanorod structures resulting from

shadowing effect in oblique angle deposition. In the ﬁxed substrate

case, tilted nanorods will grow; in the rotating substrate case,

vertical nanorods can be deposited.

ThinFilm-Zexian-06.indd 124 7/1/11 9:41:14 AM



125Silicon nanostructured ﬁ lms grown on templated surfaces

In the case of no substrate rotation in oblique angle deposition, the columns

tend to incline toward the incident  ux to a tilt angle b, which is measured

from the substrate normal. There is an empirical rule known as the ‘tangent

rule’ describing the relationship of b and q as (Nieuwenheuzen and Haanstra,

1966; Leamy et al., 1980):

tan

[6.1]

The origin of the tangent rule result is generally believed to be the limited

surface diffusion and the shadowing of atoms on the surface caused by the

 nite size of the atoms (Ablemann and Lodder, 1997). This tangent rule is

not universal, however, because the tilt angles also depend on the deposition

conditions and the material properties (Nakhodkin and Shaldervan, 1972;

Hashimoto et al., 1982; Geszti et al., 1986). Even for the same material, the

tangent rule will not hold for all incident angles. For larger incident angles

(generally q > 60∞), another empirical rule called the ‘cosine rule’ may give

better  tting of the experimental data (Trait et al., 1993; Tang et al., 2003).

The cosine rule can be written as:

–

1 –

csin

[6.2]

Experimentally, the tilt angle of cobalt columns we obtained follows the trend

of the cosine rule, but has smaller values than the calculated ones using the

cosine rule (Tang et al., 2003).

The columnar structures can be generated in computer simulations based

on the ballistic deposition model. In a (2+1)-dimensional (3D) ballistic

deposition model, the simulated columnar structures have two correlation

lengths x

and x

where the x lies in the plane of substrate normal and the

incident direction of the atoms and y is perpendicular to it (Meakin and

Krug, 1990, 1992). The growth of the correlation lengths as a function of

time s follows a power law. The power law for x

and x

can be written as

ss ss

ss~~ss

[6.3]

where s

= 1/3 and s

= 2/3, which are obtained from simulations by Meakin

and Krug (1992). The anisotropy of the exponents in the x and y directions

is reasonable in that the shadowing effect in the x direction is more severe

than that in the y direction.

Unlike the case just discussed above, there has been limited research done

to date that studies the growth dynamics in oblique angle deposition with

substrate rotation. Smy et al. (2000) developed a 3D Monte Carlo simulation

to study the growth mechanisms including shadowing effect, surface diffusion,

and substrate rotation in oblique angle deposition with substrate rotation.

ThinFilm-Zexian-06.indd 125 7/1/11 9:41:15 AM



126 Thin ﬁlm growth

This 3D Monte Carlo simulation is based on a grid of size N × N × L (N =

L = 500), and a periodic boundary condition is employed (Smy et al., 2000).

The cubic particle approaches the system at a given oblique angle until it is

deposited on the surface. After the deposition, this same particle is chosen to

diffuse to a length of several lattice units (Dew et al., 1992). After the sharp

edges of the lms are smoothed out, a variety of nanostructures including

springs, vertical rods, and zigzags can be obtained and depicted very well by

the simulations. Suzuki and Taga (2001) used a similar model to study the

oblique angle deposition with substrate rotation. This model is also based on

a simple cubic lattice and allows ballistic sticking. Ballistic sticking means

a moving particle can stick to any other particles deposited on the surface

if the distance between the two particles is close enough. The diffusion of

the newly deposited particle is similar to a random walk in the Suzuki and

Taga model.

For some applications, e.g., in the preparation of photonic crystals

(Joannopoulos et al., 1995), the arrangement and the position of the

nanostructures must be controlled. Other important applications include

micro-electro-mechanical systems, nano-magnet arrays, and high frequency

resonators. Pre-patterned seeds on a substrate are necessary in order to

fabricate periodic, uniform, and well-separated 3D nanostructures in oblique

angle deposition. During deposition, the incoming particles only deposit on

the top of the seeds exposed to the ux. One would therefore expect that

well-aligned 3D nanostructures could be created that maintain the period of

the initial nanoseed arrays. However, we have discovered that if the substrate

is xed in one position, a ‘fan-out’ structure will quickly form (Ye et al.,

2004; Lu et al., 2005). The fan-like structure will continue to grow laterally

in the direction perpendicular to the incident ux and eventually contact

neighboring nanostructures. This is not desirable if one wants to create an

array of equally separated nanostructures. In this chapter, the main issues

to be addressed are as follows: (1) the origin of the ‘fan-out’ growth when

periodic seed arrays are used in oblique angle deposition; (2) a method that

can be used to eliminate the ‘fan-out’ growth. We will study the ‘fan-out’

growth by on-lattice Monte Carlo simulations including the mechanisms of

ballistic aggregation, surface diffusion, shadowing effect, steering effect,

and angular spreading of deposition ux. Experimentally, we will introduce

three methods of substrate rotation that can be used to control the ‘fan-out’

growth and generate uniform nanostructures on the periodic seed arrays.

6.2 Preparation of templated surface for oblique

angle deposition

There are many viable techniques for the preparation of templated surfaces

consisting of nano-sized seeds, such as photolithography and electron

ThinFilm-Zexian-06.indd 126 7/1/11 9:41:15 AM



127Silicon nanostructured ﬁlms grown on templated surfaces

beam direct writing, or e-beam lithography. In photolithography, a layer

of photoresist is deposited by spin-coating. When exposed to light with a

patterned mask, the exposed photoresist changes its chemical structures.

After development, the exposed areas are either dissolved away or stay on

the substrate depending on the property of the photoresist. The former case

uses what is known as positive photoresist and requires an inverting step to

have seeds protrude out of the substrate. A more advanced technique used

to fabricate seeds is electron beam direct write, or e-beam lithography. In

e-beam lithography, the e-beam resist is exposed to an extensive electron ux

that changes its chemical structures. The advantages of e-beam lithography

are: (1) the feature size of individual seeds can be < 100 nm in diameter;

and (2) it is a mask-less process. The disadvantage of this technique is the

time it takes to write the pattern, making large-scale production almost

impossible. Typically, the whole process takes several hours in e-beam

lithography. Most recently, e-beam lithography has been used to provide a

mold for nanoimprint lithography. Similarly, patterned seeds with nanometer

feature size are available using deep UV photolithography, but at a much

added cost.

In Fig. 6.2, the templated surfaces were patterned using deep UV

photolithography and e-beam lithography, respectively. The processing steps

to produce this kind of seed template by deep UV photolithography are as

follows. First, a layer of 1000 nm thick SiO

is grown on a at Si (100)

substrate. Second, arrays of holes about 120 nm in diameter are patterned in

the SiO

layer by deep UV photolithography. The holes were arranged either

in a square lattice with a ~1000 nm lattice constant or in a triangular lattice

with a ~750 nm lattice constant. Third, tungsten (W) is used to ll in the

holes by means of chemical vapor deposition (CVD). The thickness of the W

lm is about 360 nm. Finally, the surface with excess W is then removed by

chemical mechanical polishing until the W in the holes is exposed. At this

point, the cylindrical W seeds, also called W-pillars, are still buried inside

the SiO

matrix. Before oblique angle deposition, the SiO

matrix is removed

by reactive ion-etching, exposing the W seeds, as shown in Fig. 6.2(a) and

(b). The height of the W seeds was measured to be 360 nm from SeM cross-

sectional views. The oxide layer can also be removed using commercially

available buffered oxide etch, which is a wet-chemical process. In e-beam

lithography, a layer of 270 nm thick hydrogensilsesquioxane (HSQ) is used

as the resist for e-beam direct writing inside a Zeiss Supra-55 scanning

electron microscopy (SeM) with a Nabity pattern generator. The electron

beam was accelerated by a 30 kV accelerating voltage and focused by a 7.5

mm aperture. This electron beam was used to expose a square lattice of 100

nm pillars or 50 nm pillars with a 750 nm lattice constant in the HSQ resist.

After the exposure, the HSQ resist was baked at 160∞C on a hot plate for

60 seconds and developed in a 25 wt% tetramethyl-ammonium-hydroxide

ThinFilm-Zexian-06.indd 127 7/1/11 9:41:15 AM

