Page 94 - Nanotechnology an introduction

installations. What might be called the traditional route, that of scaling down processes familiar in macro and micro engineering, appears on the
extreme right of the diagram, Figure 8.1. Concerted incremental improvement in the entire manufacturing process transforms precision engineering
into ultraprecision engineering (Figure 1.2). The stiffness of the parts of mechanical devices used to shape objects is particularly important for
achieving precision. These processes are essentially subtractive: material is removed by grinding or etching.

8.1.1. Semiconductor Processing
This well-established industrial technology refers to the operations of sequentially modifying (e.g., oxidizing), depositing (additional layers on), and
removing (parts of) a substratum (e.g., silicon) over areas selected by exposing photoresist coating the working surface through a mask and then
dissolving away the unexposed resist (or the converse). Remaining resist either protects from etching or from fresh deposition. This works well at
the micrometer scale, and is used to fabricate very large-scale integrated circuits. Problems of scaling it down to produce features with lateral sizes
in the nanorange runs into the diffraction limit of the light used to create the mask (cf. equation 5.2), partly solved by using light of shorter
wavelengths or high energy electrons.
In contrast to the difficulties of nanoscaling lateral features, very high-quality thin films can be deposited with nanometer control perpendicular to the
plane of a substratum. These methods are grouped under the heading of physical vapor deposition (PVD). The material to be deposited is
evaporated from a reservoir or magnetron sputtered from a target. The most precise control is obtainable with molecular beam epitaxy (MBE),
developed at AT&T Bell Laboratories in the late 1960s: evaporated material is beamed onto the substratum under conditions of ultrahigh vacuum.
Deposition is typically very slow (several seconds to achieve 1 nm film thickness) and hence can be epitaxial. Ultrathin layers (of the order of a
nanometer) with atomically sharp interfaces can be deposited (Section 6.3.1).
Chemical vapor deposition (CVD) is similar to PVD, except that the precursor of the thin layer is a reactive gas or mixture of gases, and the
substratum is typically heated to accelerate chemical reaction to form a solid product deposited as a film. The decomposition can be enhanced
with a plasma (this typically allows the substratum to be maintained at a lower temperature than otherwise).
Related technologies are used to modify existing surfaces of materials, such as exposure to a plasma, and ion implantation, in which
electrostatically charged high-energy (typically 10–100 keV) ions are directed towards the surface, where they arrive with kinetic energies several
orders of magnitude higher than the binding energy of the host material, and become implanted in a surface layer that may be tens of nanometers
thick.
Physical and chemical vapor deposition processes are often able to yield structure at the nanoscale, or the lower end of the microscale, with some
structural control achievable via the deposition parameters. The structures obtained emerge from a particular combination of deposition
parameters and must be established experimentally. Theoretical understanding of structure and process is still very rudimentary, although attempts
are under way (such as the “structure zone model”).

8.1.2. Mismatched Epitaxy

In the epitaxial growth of semiconductors (see Section 8.1.1), when material B is evaporated onto a substrate of a different material A, three
analogous situations can arise:
• Frank–van der Merwe (B wets A, and a uniform layer is formed).
• Volmer–Weber (no wetting, hence islets distributed in size of B on A are formed).
• Stranski–Krastanov (B can wet A, but there is a slight lattice mismatch between A and B, and the accumulating strain energy is ultimately
sufficient to cause spontaneous dewetting, resulting in rather uniform islets of B, which relieves the strain—this explanation is plausible but there
is still much discussion regarding the mechanism).
One can therefore apply equation (3.22). If S > 0 we have the Frank–van der Merwe regime; the substratum is wet and layer-by-layer growth takes
place. If S < 0 we have the Volmer–Weber regime; there is no wetting and three-dimensional islands grow. Yet if S > 0 for the first layer at least, but
there is a geometric mismatch between the lattices of the substratum and the deposited layer, strain builds up in the latter, which is subsequently
relieved by the spontaneous formation of monodisperse islands (the Stranski–Krastanov regime); it can be thought of as frustrated wetting. Its main
application is for fabricating quantum dots for lasers (quantum dot lasers are a development of quantum well lasers; the carriers are confined in a
small volume and population inversion occurs more easily than in large volumes, leading to lower threshold currents for lasing, and the emission
wavelength can be readily tuned by simply varying the dimensions of the dot (or well), see Section 2.5). The main difficulty is to ensure that the dots
comprising a device are uniformly sized. If not, the density of states is smeared out and the behavior reverts to bulk-like. Initially, the quantum dots
were prepared by conventional semiconductor processing, but it was found to be very difficult to eliminate defects and impurities, whereas the
Stranski–Krastanov self-assembly process does not introduce these problems.

8.1.3. Electrostatic Spray Deposition (ESD)

Ceramic precursors are dissolved in a suitable solvent and mixed immediately prior to forcing through an orifice maintained at a high potential
difference with respect to the substratum. The liquid breaks up into electrostatically charged droplets, which are attracted both by gravity and the
Coulombic force to the substratum, which is typically heated to accelerate the reaction that forms the final material. For example, calcium nitrate
and phosphoric acid dissolved in butyl carbitol and atomized upon leaving the nozzle at a potential of 6–7 kV with respect to a titanium or silicon
substratum maintained at a few hundred °C about 30 mm below the nozzle create coatings of calcium phosphate with intricate and intriguing
nanostructures [104].

8.1.4. Felting

The fabrication of texture by felting has been known for centuries (in Europe, and much longer in China) in the guise of papermaking, reputed to
have been invented in ad 105 by Ts'ai Lun in China. It arrived in Europe, firstly in Italy, by the end of the 13th century, probably via Samarkand,