Chapter 12Nanosphere Lithography

Martin Waegner

12.1 Introduction

The development of lithography was one of the driving forces in microfabricationtechnology. However, today the improvement of this technology is restrictive forfurther shrinking of microelectronic and micromechanical devices, because conven-tional lithography allows resolutions down to 100 nm. So there is a need for newermethods which allow the fabrication of structures in the sub-100 nm range. There-fore, a couple of new technologies were developed and are actually in use in researchand industry. Immersion lithography is an enhanced method of conventional photo-lithography where a liquid between the lens and the wafer is used. The liquid has abigger refracting index then air, hence the numerical aperture NA = n · sin α (α halfangle of incidence) is larger and the increased resolution achieves values of a few10 nm. E-beam lithography uses a beam of electrons to reduce the diffraction limitof photo-lithography and reaches structure sizes in the range of 20 nm. Identicallyto this technique, ions can be used instead of electrons which is called ion beamlithography. All these methods have good resolution for actual needs in microelec-tronics. But the tools have a couple of disadvantages. The throughput is quite low andthe equipment is expensive. Thus, newer, cheaper and especially faster technologiesarouse interest in research and development.

One of these methods is nanoimprinting as it is described in Chap. 11. Aneven more promising approach is the exploitation of nature-given mechanisms andproperties. These self-assembly methods are mostly cheap und fast. One examplewith high potential are diblock-copolymers, composed of two different polymerchains, are used to create self-assembled patterns. The two polymers are unable tophase-seperate and so they form ordered structures in the macroscopic length scale.Thereby size and shape can be controlled in manipulating the molecular weight and

M. Waegner (B)

Solid-State Electronics Laboratory, Technische Universität Dresden,01062 Dresden, Germanye-mail: martin.waegner@mailbox.tu-dresden.de

G. Gerlach and K.-J. Wolter (eds.), Bio and Nano Packaging Techniques 269for Electron Devices, DOI: 10.1007/978-3-642-28522-6_12,© Springer-Verlag Berlin Heidelberg 2012

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the composition of the two polymers. For instance, structures like spheres, cylindersand lamellas are feasible [23] and can be used as a mask for so-called block-copolymerlithography.

Fischer et al. [8] and Deckman et al. [6] were the first in the early 1980s whopresented a nanostructuring process based on colloidal particles as a mask. Recentadvances in colloidal chemistry have enabled the production of highly monodis-perse colloidal particles with good phase stability. Such colloidal dispersions showa long-range ordering caused by self-assembly. Consequently, they form 2D arraysof particles on a surface, which act in nanosphere lithography (NSL) as a mask foretching or deposition processes. This technology has several advantages.

• Colloidal dispersions are commercially available at relative low cost.• No complex equipment is required for the production of nanopatterns down to

10 nm.• Layers can be easily synthesized by dip-coating and spin-coating with very eco-

nomical consumption of material.• The feature scale can be easily tuned by size and alignment of the particles.• The shape of particles and structures can be changed by etching, annealing and

tilted evaporation.• Deposition of multilayers offers the possibility to make 3D structures like hemi-

spherical metal caps, sculptured colloids [4] and nanorings [2].• NSL is suitable for biomaterials and -devices, because surface treatment of the

particles is possible [26].

The development of this promising technology is still at the beginning. The followingsections describe the capabilities of this technique. At first, fabrication routes forcolloidal monolayers and perspective applications for deposited nanopatterns willbe introduced. Finally, particle layers as etching masks for patterning thin-layers andsurfaces with different shapes are presented.

12.2 Ordering of Nanospheres

Up-to-date research opened methods for the synthesis of monodispersed sphereswith a narrow size distribution. Accordingly a variety of materials for polymer par-ticles like polysterene (PS) or polymethylmetacrylat (PMMA) can be fabricated bysuspension, emulsion and dispersion. The particle size can be easily controlled bychanging the reaction conditions like concentration of monomers, reaction tempera-ture or pH-value. Inorganic particles are mostly synthesized via sol-gel routes. Thismethod offers only a limited influence on the initial size of the particles, but the sizecan be adapted very well through nucleation and subsequent growth [22].

Using monodispersed solutions as a mask is advantageous because the movementof colloidal particles is influenced by different forces [19]. Van der Waals forces,steric repulsions and Coulombic repulsions protect the stability of the dispersion andcan be described with the Derjaguin-Landau-Verver-Overbeek (DLVO) theory [25].

12 Nanosphere Lithography 271

Fig. 12.1 Precipitation tech-niques for close-packedmonolayers of nanospheres:a dip-coating,b electrophoretic deposition,c lift-up from an interface,and d spin-coating

(a) (b)

(c) (d)

Stability of the dispersions means that particles do not form agglomerations and,moreover, spheres do not seperate from the dispersion. For the final self-assemblyprocess the evaporation of water or organic dissolvers and the related forces play animportant role. Capillary forces influence highly the self-ordering of the nanosphereswhich leads to hexagonal closed-packages of nanospheres.

Figure 12.1 shows different routes for precipitation of monolayers. The very firstdeveloped method was dip-coating which is shown in Fig. 12.1a [7]. The substrateis dipped in a solution containing the nanoshperes. While the substrate is pulled-outa thin film of dispersion forms and covers the surface. The controlled movement andordering of the nanospheres starts when the thickness of the liquid layer around thespheres becomes smaller then the diameter of the spheres. This self-assembly processis forced by a convective flow which transports the particles, while the solution evap-orates. In conclusion the formed monolayers are polycrystalline with crystallites inthe range between a few nanometers and a few microns. Nevertheless, they showdefects like grain boundaries, dislocations, vacancies and variations in layer thick-ness. To achieve layers with less defects other, better controllable, more effective andmore qualitative methods are needed.

The electrophoretic synthesis is fast and accurate. The particles are forced in theirmovement by an electrical AC- or DC-field based on electrodynamic interactions[18, 21]. Nevertheless, the deposition of the particles on the substrate cannot beinfluenced directly. Due to the random deposition process, no larger areas with close-packed monolayers can be synthesized. As a result areas with single particles or withagglomerations occur.

Floating on a liquid surface is one of the most promising methods for laboratoryapplications (Fig. 12.1c) [20]. The nanospheres order on an liquid-gas interface ofmostly water and air. On this interface close-packed monolayers are formed by longrange repulsive forces between the spheres [1]. In particular in laboratory, the dis-persion is droped onto a substrate with very hydrophilic properties, e.g. especiallyprepared glass or silicon. Subsenquently, the solution spreads all over the substrateat once. Therby a first self-assembly process starts. Afterwards, the substrate withthe fine-distributed dispersion is lowered very slowly to a petri dish with ultra pure

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water. The nanospheres begin moving on the water surface and start forming anunordered monolayer. With the addition of 2% sodium dodecyl sulfate (SDS) solu-tion, which reduces the surface tension of water, large areas of well-ordered spheresare formed. These monolayers can now be lifted-up by using any substrate whichagain has a hydrophilic surface. Finally, the substrates have to be dried very slowly.A medium temperature and a high value of humidity should be used, providing asufficient slow evaporation process to avoid destruction of the monolayers. With thistechnique areas of more or less closed-packages up to several cm2 are achievable.Although a Langmuir-Blodgett technique is in principle usable [9], it cannot be usedfor bulk production up to now. Using repeated depositions multilayers can be formed.

A better approach for large-scale production is spin-coating, which is already inuse in standard wafer processes (Fig. 12.1d). Spin-coating is a fast layer depositionmethod and can be used both for small substrates and whole wafers. With regard tothe adhesion of the dispersion onto the surface, the substrates have to be hydrophilic.Otherwise, the dispersion forms drops and does not adhere to the surface. The den-sity of particles and similarly the number of layers can be controlled by tuning thespin parameters (speed, acceleration, period) and the dispersion parameters (particleconcentration, addition of solvents). Using this parameter spectrum, it is possibleto fabricate either closed-packages of mono- and multilayers as well as statisticallydistributed single particles with controllable density. Nevertheless, the quality of themonolayers does not reach that one of the lift-up from an interface.

Despite its premature state recent developments show the potential of NSL as alter-native for expensive conventional lithography. For example, Canpean et al. demon-strated a new method for convective coating which leads in a few of minutes toclose-packed layers over several cm2 [3].

12.3 Modification of Nanosphere Masks

For application reasons it might be good to control the shape and the size of thedeposited nanosphere layers. Particular nanodevices need other shapes like rings andrectangles instead of circular and triangel patterns from unmodified masks. Oftendevice properties do not only depend on particle size but also on their shape, forinstance in applications using local surface plasmon resonance (LSPR) [12] andsurface-enhanced Raman scattering (SERS) [24]. To overcome the limitation in shapevariety two different process modifications can be applied: (i) deposition with tiltedand rotated substrates (see Sect. 12.5) and (ii) modification of colloidal masks bypost-treatment.

Usual post-treatments are for example reactive ion etching (RIE), ion-millingand thermal annealing. Especially RIE is a versatile tool to control pattern sizes andshapes. As an illustration Fig. 12.2 shows a series of SEM pictures of colloidal masksmodified by RIE. The etching parameters were choosen for a maximum of isotropyto ensure a homogenous etch rate. Zhang et al. reported an empirical equation forthe reduction of the initial diameter D of PS nanospheres in oxygen-based RIE [28].

12 Nanosphere Lithography 273

Fig. 12.2 Modification of polystyrene (PS) nanosphere masks by RIE. The SEM pictures weremade after a 4 min, b 6 min, c 8 min, d 10 min and e 12 min etching time. The diameter of the spherecould be reduced from ∼830 nm down to ∼360 nm (e). The white bar has a size of 2 µm

Fig. 12.3 Modification ofcolloidal masks by RIE of abilayer in hcp orientation

D = D0 cos

(arcsin

(k · t

2D0

))(12.1)

Here t is the etching time and k a constant depending on the etching conditionslike power, pressure and gas mixture. The etching conditions should be chosen verycarefully because they have a big influence on the etching mechanism. Increasingpower and decreasing pressure lead to higher anisotropy of particle etching. Toachieve homogeneously distributed sphere diameters, the RIE process should bedominated by the chemical part of etching providing more isotropic conditions.Li et al. investigated the influence of different parameters, especially of the gasmixture, on the etching rates of PS nanospheres [16].

An even wider diversity of mask structures can be achieved with multilayer masks.Colloidal particles in multilayers form either face-centered cubic (fcc) or hexago-nal close-packed (hcp) crystal structures. Depending on the orientation relative tothe crystal, the number of layers and the RIE conditions, a vast number of differ-ent patterns like micro-channels, V-shaped grooves and pyramid pits are reachable[27]. Figure 12.3 shows a SEM picture of a bilayer colloidal mask modified by RIE.Triangular patterns form due to the shadowing effects of the hcp-oriented. In the caseof the fcc-orientation quadratic structures emerge.

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D d

a

a 3 3 11

3

D2

(12.2)

d1

3D (12.3)

F4

3D2(12.4)

Fig. 12.4 Modell of a closed-hexagonal package and equations for the prediction of particle sizea, distance d between the particles and the dots density F (afer [15])

Fig. 12.5 Schematic of the angle-resolved deposition process. a Samples viewed at 0◦. The inter-stices in the nanosphere mask are equally spaced and of equal size. b Sample viewed at 30◦. Theinterstices in the nanosphere mask follow a pattern including two different interparticle spacingvalues, and the interstitial area is smaller. c Sample viewed at 45◦. The interstices are now closedto line of sight deposition. Reprinted from [10] with permisson

12.4 Shadow Mask Technology for Deposition Processes

Nanosphere layers can be directly used as mask for the deposition of material byevaporation or sputtering. By choosing different initial sphere diameters the size ofthe resulting deposited patterns can be varied in the range from 20 to 1000 nm [10].To predict size and shape of the resulting patterns, a closed-hexagonal package ofspheres can be considered and the dimensions of the triangular-shaped material dotson the substrate can be calculated theoretically (Fig. 12.4). Li et al. calculated theparticle size a, the distance between the particles d and the dot density F as functionof initial sphere diameter D [15]. Experimental investigations showed a sufficientgood accordance between the calculated and the measured values with a really narrowsize distribution [11].

Nevertheless, all the deposited structures will have triangular shapes if the depo-sition of material is perpendicular to the substrate. For this purpose tilting eitherthe substrate or the evaporation source shows shadowing effects which lead to otherinteresting structures like cups, rods and wires [14]. Figure 12.5 illustrates the effectof tilted evaporation for the resulting patterns. Furthermore, the combination of aseries of evaporation steps with different tilting angels and maybe different materialsoffers the fabrication of a huge amount of different structures. Zhou et al. [29] andRetsch et al. [17] used a conjunction of evaporating and etching steps with differenttilted angels which resulted in 3D nanostructures (Sect. 12.5).

After etching the nanospheres can be removed by sonification with a appropriatedissolver. In the case of PS nanospheres, chloroform showed satisfying results [5].

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Fig. 12.6 Size-tunable colloidal lithography process using RIE and ion-milling. a Schematic ofthe whole structuring process. Reprinted from [13] with permisson. b AFM pictures of a thin layerstructured by NSL

12.5 Nanosphere Masks for Conventional Etching Processes

After dissolving the nanosheres the resulting structure shapes are defined by the gapsbetween the previous single spheres. This means that the nanosphere layer acted asa negative mask.

In the next step the structure of the mask has to be transfered to the substrate.In the case of a substrate coated with a thin-layer, the mask can be used for struc-turing this layer with nanopatterns. Dry etching processes with high anisotropy areusually applied. During the etching process the mask has to be stable, at least asstable as the layer which will be structured. So the combination of mask materialand thin-layer has to be chosen very carefully. PS spheres for example show an etch-ing rate which is approximately similar to that one of silicon. Otherwise, the maskwould be etched much faster than the thin layer or the surface. Ion-milling suitesvery well the requirements for structuring metal layers, due to its anisotropy andthe resulting almost perpendicular sidewalls. The resulting pattern of the substratecorresponds almost one-to-one to the etch mask. Figure 12.6 illustrates the completepattern transfer process including post-treatment for mask modification and showsan AFM picture of the resulting structures.

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