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8 Low-k Materials as Interlayer Dielectrics (ILD)

8 Low-k Materials as Interlayer Dielectrics (ILD)

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9 Nanotechnology for Computers, Memories, and Hard Disks

where ρ is the metal resistivity, ε0 the vacuum permittivity, k is the relative dielectric

constant of the interlayer dielectric (ILD), P is the metal line pitch (sum of line width

and line spacing), T is the metal thickness, and L is the metal line length. Therefore,

the device speed, increased through smaller feature sizes, will become overshadowed by this interconnect delay [9.117, 9.118]. The International Technology

Roadmap for semiconductors (ITRS) predicts the necessary (maximum) k value of

the ILD, and the estimated RC delay, for the various line width (DRAM half-pitch)

“technology nodes”, according to the expected year of production (see Fig. 9.41).

Fig. 9.41 The ITRS (2004 Update) industrial roadmap for the semiconductor industry, showing the planned decrease in device spacing (DRAM half-pitch), the expected increase in the RC

delay, and the required k values of the interlayer dielectric (ILD), indicating the extent to which

manufacturable solutions are known. (Reprinted with permission from [9.118]. © 2006 Elsevier)


Low-k Materials as Interlayer Dielectrics (ILD)


Future low-k materials for ILDs [9.119] are porous materials such as periodic mesoporous organosilicas (PMOs; see [9.118, 9.120]) which are produced

by template-based synthesis making use of bridged-organic silsesquioxane (SSQ)

precursors to incorporate organic groups directly into the pore or channel walls

(Fig. 9.42a). These PMOs have, however, polar silanol groups with OH bonds on the

channel walls to be hydrophilic. The ingress of water with its high dielectric constant

(k ∼ 80) must be suppressed because this would cause the effective k to increase dramatically. This can be induced by thermal “hydrophobization” (Fig. 9.42b), where

a proton from the hydrophilic hydroxyl group of a silanol (Si–OH) is transferred to

a bridging group to break one Si–CH2 –Si bridge, thus creating a hydrophobic surface with terminal organic groups. This treatment causes k to decrease below 2.0 for

the methenesilica, ethenesilica, and three-ring PMOs (see [9.118]). The film porosity – and thereby k – can be controlled by the template (surfactant)/precursor ratio

R (see Fig. 9.42c) achieving values of k ∼ 1.75 as required for ultralow-k ILDs.

The mechanical properties of low-k dielectric materials are of importance for circuit


Fig. 9.42 (a) Transmission electron micrograph of periodic mesoporous organosilica (PMO)

showing a 2D hexagonal structure of a ∼ 4 nm channel spacing. (b) Illustration of the thermally

activated chemical reaction that transforms an initial bridging organic group (left) to a terminal

organic group (right), as a result of proton transfer from a silanol group in close proximity. (c)

Decrease of the dielectric constant k for three-ring PMO films of increasing porosity as determined

by the surfactant (CTACI)/precursor (Si-organo-of-silica) molar ratio R. The temperatures of the

thermal treatments are given in centigrades. (Reprinted with permission from [9.118]. © 2008

American Institute of Physics)


9 Nanotechnology for Computers, Memories, and Hard Disks

9.9 Summary

The field of computer and data storage development with the fabrication of transistors or of data bits in storage media, respectively, is of particular importance

for the application of nanoscience in computer industry with current revenues of

US$ ∼ 200 billion annually. According to Moore’s law the computing power – and

in a similar fashion the data storage density – doubles about every 18 months due to

a shrinkage of transistor sizes on an integrated circuit or of data bits to the nanometer scale. Due to physical limitations in the downscaling of transistors and data

bits, novel strategies for the fabrication of these components have to be developed.

Carbon nanotubes and graphene nanoribbons may provide us with future transistor

technologies. Flash memories and magnetoresistive memories exhibit considerable

downscaling potentials. With the implementation of nanoscale magnetic domain

wall racetrack memories, the possibility of simplifying computers is opened by

reducing the number of memory storage technologies. The storage capacity of magnetic hard disks could be dramatically increased by the use of GMR and TMR read

heads based on nanoscale multilayers. For ultrahigh-density integration of computer

components the development of high-dielectric constant (high k) and low-dielectric

constant (low k) materials is indispensable.



























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9 Nanotechnology for Computers, Memories, and Hard Disks


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Chapter 10

Nanochemistry – From Supramolecular

Chemistry to Chemistry on the Nanoscale,

Catalysis, Renewable Energy, Batteries,

and Environmental Protection

Chemistry plays an important role in the synthesis of nanostructures (see Chap. 3

and [10.1]). In the present section, the main features of supramolecular chemistry

and of inorganic hollow clusters will be outlined with a subsequent discussion of

chemical reactions on the nanoscale and catalysis. Furthermore, nanochemistry and

nanoscience appear to be of importance for future renewable energy production,

battery development, and environmental protection.

10.1 Supramolecular Chemistry

Supramolecular chemistry [10.2–10.6] or – as sometimes called – supramolecular

science is located at the meeting point provided by the design and investigation of

organized, informed, and functional supramolecular architectures. As a bottom-up

approach, supramolecular chemistry is concerned with the next step in increasing complexity beyond the molecule toward the supramolecule and organized

polymolecular systems on a nanometric size scale, held together by non-covalent

interactions. This type of molecular interactions forms the basis of the highly specific recognition, reaction, transport, and regulation processes that occur in biology.

Due to weak intermolecular bonds, supramolecules are in general thermodynamically less stable, kinetically more labile, and dynamically more flexible so that a

type of “soft chemistry” emerges. Binding of a substrate σ to a receptor ρ yields the

supramolecule σ ρ and involves recognition, transformation, and translocation. The

association and organization of functional supramolecules may lead to molecular

and supramolecular devices.

According to a consideration of Lehn [10.3] there was in the beginning the Big

Bang and physics reigned. Then chemistry came along at milder temperatures; particles formed atoms; these united to give more and more complex molecules, which

in turn associated into aggregates and membranes, defining primitive cells out of

which life emerged. In this sense chemistry is settled between the laws of physics

and the rules of life.

H.-E. Schaefer, Nanoscience, DOI 10.1007/978-3-642-10559-3_10,

C Springer-Verlag Berlin Heidelberg 2010





However, not only nanoscaled organic entities are available but also nanometersized inorganic molecules, approaching the size range of bacteria, can be synthesized [10.7] which will be briefly discussed at the end of this section.

10.1.1 Architecture in Supramolecular Chemistry

Receptors are often mesomolecules, i.e., molecules with sizes intermediate between

small organic molecules and large molecules of macromolecular chemistry.

Cyclic and non-cyclic elements can be combined (Fig. 10.1). Some trivial

names, which characterize their functions and activities, have been given to these

supramolecules: crown ether, cryptands, cavitands, carcerands. Spherical recognition cryptates of metal cations represent the simplest recognition process of

spherical substrates, e.g., metal cations. In Fig. 10.1c, a natural macrocyclic

molecule with antibiotic properties, such as valinomycin, is shown with a K+ ion

included in the macrocyclic cavity. By employing a receptor molecule with an

appropriate size of hollow volume, a particular type of ion can be selected from

a mixture of these ions by the formation of a cryptate compound [10.3]. This selectivity is of importance for electrical stimulation in nerve cells which is based on

changes of the gradient in concentration of Na+ and K+ ions as well as of Ca2+ ions

[10.8] in channels across membranes (see Sect. 11.5). Therefore, there should be

molecules available in these membranes which are capable of differentiating these

simple ions by the difference in diameter of about 0.06 nm.

Whereas macrocycles define a 2D hole, bicycles define a 3D hole which can be

highly stabilized by alkali or alkali earth cations or protons giving rise to spherical

Fig. 10.1 (a) Macrocylic and (b) cylindrical macrobicyclic molecular structure. (c) Recognition

of a K+ ion included in a macrocyclic cavity. (d) Tetrahedral recognition of a NH+

4 cation by a

tricyclic cryptand. (Reprinted with permission from [10.2]. © 1995 Wiley-VCH)


Supramolecular Chemistry


Fig. 10.2 (a) Nanosized supramolecular guest–host container molecule (carcerand). (b)

Recognition of neutral molecules by hydrogen bonding in a cleft. (Reprinted with permission from

[10.2]. © 1995 Wiley-VCH)

recognition. Such a 3D molecule shows a strong “cryptate effect” (buried atom)

characterized by high stability and selectivity, slow exchange, efficient shielding

of the bound ion from environment supplying, e.g., transition metal recognition.

Tetrahedral recognition of the NH+

4 cation is achieved by a tricyclic cryptand

(Fig. 10.1d).

The supramolecular structures can get larger and larger as in the case of the

guest–host container molecules (carcerand in Fig. 10.2a with a diameter of about

2.8 nm). They make the protection of highly reactive species possible. For example,

highly reactive white phosphorus (P4 tetrahedra) can be encapsulated in supramolecular complexes where the phosphorus becomes inert [10.9]. Recognition of neutral

molecules may occur by hydrogen bonding in, e.g., a cleft (Fig. 10.2b). Cyclodextrin Encaging

Cyclodextrins are cyclic shell molecules (Fig. 10.3) with a diameter of ∼0.8 nm.

They can be produced by enzymatic decomposition of starch. The molecule has a

hydrophilic outer surface and a hydrophobic interior and exhibits typical host–guest

complexing properties. Here, the driving force for incorporation of a hydrophobic

guest molecule is a reduction of the total energy when the guest replaces water

molecules. Pharmaceutical applications are due to a reduction of the irritation of the

stomach lining when the drug is cyclodextrin encapsulated. The sensitivity of cosmetics, e.g., the anti-aging drug retinol, which can reduce the depth of wrinkles

can be reduced by a cyclodextrin coating (see Fig. 10.3b). In food technology

cyclodextrin may be used for removing unpleasant tastes by enclosing the aldehyde

molecules released from the packaging material.




Fig. 10.3 (a) γ-Cyclodextrin ring. (b) Two cyclodextrin rings can enclose a retinol molecule for

protection of this vitamin from oxygen and light. (Reprinted with permission from [10.10]. © 2005

Wiley-VCH) Supramolecular Nanostructures at Metal Surfaces

The mastery of the non-covalent bond is the leitmotif in supramolecular chemistry. This issue can be directly addressed by the deposition of molecular building

blocks on a substrate and the study of the mutual molecular interaction and the

interaction with, e.g., a well-characterized metallic surface on a molecular level.

The supramolecular patterning of carboxylic acids such as 4-[trans-2-(pyrid-4-ylvinyl)]-benzoic acid (PVBA) and trimesic acid (1,3,5-benzenetricarboxylic acid)

(TMA) on metal surfaces are shown in Fig. 10.4. The head-to-tail hydrogen bonding of the rod-like species PVBA with supramolecular chiral H-bonded twin chains

yields 1D nanogratings on Ag(111). The threefold symmetry of TMA accounts for

the formation of H-bonded honeycomb networks on Cu(100) at low temperatures.

10.1.2 Supramolecular Materials

Organic supramolecular materials are of interest because of their interaction

and recognition capabilities. Examples are organic nanotubes formed by selfassembly of cyclic peptide units [10.12]. Networks of self-assembled actin filaments

play a basic role in the physicochemical behavior of cells and vesicles [10.13].

Technologies resorting to self-organization processes should be able to bypass

nanofabrication procedures by making use of the spontaneous formation of the

desired suprastructures and devices from instructed and functional building blocks.

In the future, supramolecular devices may be organized by recognition-directed

self-assembly (see Sect. 1.5) into well-defined architectures with novel properties.

Components and molecular devices such as molecular wires, channels, resistors,

rectifiers, diodes, and photosensitive elements might be assembled into nanocircuits.

Artificial transmembrane ion channels. Tubes and channels are ubiquitous in

living systems. Constructed from various proteins these conduits are responsible for numerous biological functions such as ion flow, signal transduction, and

molecular transport. Artificial transmembrane ion channels were constructed from


Supramolecular Chemistry


Fig. 10.4 (a) STM image of the formation of a regular 1D supramolecular PVBA nanograting

by H-bond-mediated self-assembly and connection by the molecular end groups on the Ag(111)

surface extended over micrometer domains. Lower panel: Schematic of the supramolecular chiral PVBA twin chains with OH. . .N and weak lateral CH. . .OC hydrogen bonds indicated.

(b) STM image of TMA on Cu(100) taken at 205 K. In the islands decorating the step edges, a honeycomb motif is resolved which is associated with the extensively H-bonded 2D TMA networks.

Lower panel: Schematic model for the honeycomb TMA structure with threefold dimerization of

self-complementary carboxyl moieties. (Reprinted with permission from [10.11]. © 2003 Springer


cyclic peptide structures containing alternating D- and L-aminoacids, where D and L

refer to the amino acid chirality. These cyclic subunits adopt a flat ring conformation, allowing them to stack on top of one another to form a hydrogen-bonded,

hollow tubular structure (Fig. 10.5). The rings are composed of the eight-residue

cyclic peptide cyclo[-(TRP-D-Leu)3 -Gln-D-Leu-] with a pore diameter of 0.75 nm.






Fig. 10.5 (a) The chemical structure of the peptide subunit in a flat ring-shaped conformation.

(b) Peptide subunits shown in a self-assembled tubular configuration (horizontal) embedded in a

lipid bilayer membrane (vertical). The representation emphasizes the antiparallel ring stacking,

the presence of inter-subunit hydrogen-bonding and side chain–lipid interactions. (Reprinted with

permission from [10.14]. © 1994 Nature Publishing Group)

The incorporation of the tubules into lipid bilayers was demonstrated by specific

Fourier transform infrared (FTIR) spectroscopy. In addition, the synthetic tubules

showed ion transport activity with rates exceeding 107 ions/s, similar to biological channels such as gramicidin A [10.14, 10.15]. Supramolecular nanotubes have

been shown to exhibit selective antibacterial activity in mice by increasing the

permeability of bacterial membranes [10.16].

Supramolecular polymers. Polymers can respond to changes in temperature, solvent, or the presence of signal chemicals by reversible assembly/disassembly (see

[10.17]). Scaffolds, such as those that constitute the cellular skeleton, are formed

only where and when they are required, and they are dissembled in small building

blocks when their task has been fulfilled. As opposed to a purely macromolecular approach, such a modular, supramolecular strategy allows a fast and efficient

response to changing needs in the cellular cycle.

Whereas in macromolecular polymers [10.18] a large number of repeating units

are covalently linked into long chains, in supramolecular polymers monomers are

held together by noncovalent interactions [10.1, 10.2] such as directional and versatile hydrogen (H) bonds. Very stable complexes can be obtained when quadruple

H-bonding units are employed as in 2-ureido-4[1H]-pyrimidinones (UPy; Fig. 10.6)

dimerized in toluene.

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