Devices fabrication

Device fabrication requires the successive deposition of a series of layers of metal, polymer and insulators, with appropriate patterning in the case of the MISFET for the formation of the source and drain areas. We use simple metal evaporation, of gold as an ohmic contact and, typically, aluminium as a blocking contact. Thus, to fabricate a Schottky barrier diode, we deposit a bottom layer of gold onto the substrate, coat this with the polyacetylene layer, and put the low work function metal such as aluminium or indium on top of this. The Schottky barrier diode is obviously very sensitive to the presence of pin-holes in the polymer layer, and we have therefore aimed to keep the thic ess of the polyacetylene layer sufficiently high so that the chances of this failure mode are reduced to a low level. We have worked typically with polymer layer thicknesses of around 1 xm. 

We have made extensive use of organic insulator layers in MIS devices. The interest here is, firstly, to establish the compatability of the polyacetylene with the insulator as shown in section 6, we find that the polyacetylene layer is better ordered when formed on an organic layer than when fcamed on silicon dioxide. Secondly, we were keen to be able to fabricate structures which are optically transparent over die whole region of interest for polyacetylene (up to 2.5 eV) those fabricated on silicon substrates are limited by the silicon band gap (1.1 eV). We have used principally polyimide and PMMA, though the maximum values of EfFb are considerably lower than obtained with silicon dioxide. Polyimide has the advantage that, after spin-coating from solution, the film is heat-treated to render it insoluble, and it is not.affect d by further treatment with solvents such as those used for the Durham precursor. In contrast, PMMA remains soluble after deposition, and devices were fabricated with PPMA applied on top of the previously-converted polyacetylene film. 

The polyimide precursor solution (II supplied by Brewer Science Inc.) was spin-coated onto a thin and semitransparent (300 nm) aluminium film previously deposited onto a spectrosil substrate. Imidisation was carried out using the prescribed heat treatment, to give a polyimide film of thickness 360 nm. Polyacetylene was then deposited on top in the 

Polyacetylene is very sensitive to oxygen, and we have therefore taken steps to ensure that, where possible, fabrication is performed in an oxygen-free environment. We perform all fabrication steps involving the polyacetylene inside a glove-box which is maintained with oxygen and water levels of less than 5 ppm. The glove box contains, therefore, the spin-coater, temperature-controlled ovens, and tiie metals evaporator (though for some of the earlier work on the Schottky diode structures, the sample was transfened rapidly in air to a metal evaporator, and pumped for 12 hours prior to evaporation of the top contact). 

The electronic structure of polyacetylene is very sensitive to the presence of conformational defects on the chains, and there are significant differences between the properties of polyacetylene prepared as unoriented and as stretch-oriented films. Stretch-oriented films show properties similar to those of good-quality Shirakawa polyacetylene, with the peak in the interband n-n optical absorption at around 1.9 eV [41-43]. In contrast, Ae band-gap in the unoriented material is raised, with the peak in the interband -absorption at about 2.3 eV [44-46], as seen in the optical absorption spectra shown in figure 4. There are concomitant increases in the frequencies of the Raman-active vibrational modes [44]. 

Individual 2 cm chips were diced using a Disco DAD-321 (Disco Corporation, Japan) dicing saw. Dicing lines were included on the electrode layer. 

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Rimini E 1995 Ion Implantation Basics to Device Fabrication (Boston, MA Kiuwer)... 

The apphcation of a high electric field across a thin conjugated polymer film has shown the materials to be electroluminescent (216—218). Until recentiy the development of electroluminescent displays has been confined to the use of inorganic semiconductors and a limited number of small molecule dyes as the emitter materials. Expansion to the broad array of conjugated polymers available gives advantages in control of emission frequency (color) and facihty in device fabrication as a result of the ease of processibiUty of soluble polymers (see Chromogenic materials,electrochromic). 

Although the majority of NAA applications have been in the area of bulk analysis, some specialized uses need to be mentioned. One such unique application is the measurement of phosphorus in thin films (about 5000 A) of phosphosilicate (PSG) or borophosphosilicate (BPSG) glasses used in VLSI device fabrication. In this case,... 

Figure 15-18. Schematic cross-section of a bilayer device fabricated from MEH-PPV and...

Figure 15-26. Schematic illustration of a device fabricated lroin a single layer of an interpenetrating donor-acceptor (conjugated polymer/CWi) network.

These model compounds can also be used in device fabrication, since thin films of appropriate thickness can be obtained by sublimation and subsequent deposition onto a substrate in vacuum. Electrical as well as optical properties of such devices have turned out to be strongly dependent on both the molecular packing within the crystallites and the polycrystalline morphology. Understanding and control of this aspect is one of the current scientific challenges. 

An important consideration in the sequence of semiconductor devices fabrication is the so-called thermal budget, a measure of both the CVD temperature and the time at that temperature for any given CVD operation. As a rule, the thermal budget becomes lower the farther away a given step is from the original surface of the silicon wafer. This restriction is the result of the temperature limitations of the already deposited materials. 

From a reaction engineering viewpoint, semiconductor device fabrication is a sequence of semibatch reactions interspersed with mass transfer steps such as polymer dissolution and physical vapor deposition (e.g., vacuum metallizing and sputtering). Similar sequences are used to manufacture still experimental devices known as NEMS (for nanoelectromechanical systems). 

The electrocatalytic oxidation of methanol has been widely investigated for exploitation in the so-called direct methanol fuel cell (DMFC). The most likely type of DMFC to be commercialized in the near future seems to be the polymer electrolyte membrane DMFC using proton exchange membrane, a special form of low-temperature fuel cell based on PEM technology. In this cell, methanol (a liquid fuel available at low cost, easily handled, stored, and transported) is dissolved in an acid electrolyte and burned directly by air to carbon dioxide. The prominence of the DMFCs with respect to safety, simple device fabrication, and low cost has rendered them promising candidates for applications ranging from portable power sources to secondary cells for prospective electric vehicles. Notwithstanding, DMFCs were... 

Alivisatos and coworkers reported on the realization of an electrode structure scaled down to the level of a single Au nanocluster [24]. They combined optical lithography and angle evaporation techniques (see previous discussion of SET-device fabrication) to define a narrow gap of a few nanometers between two Au leads on a Si substrate. The Au leads were functionalized with hexane-1,6-dithiol, which binds linearly to the Au surface. 5.8 nm Au nanoclusters were immobilized from solution between the leads via the free dithiol end, which faces the solution. Slight current steps in the I U) characteristic at 77K were reflected by the resulting device (see Figure 8). By curve fitting to classical Coulomb blockade models, the resistances are 32 MQ and 2 G 2, respectively, and the junction... 

On the way to more reliability in device fabrication, Kronholz et al. reported on the reproducible fabrication of protected metal nanoelectrodes on silicon chips with <30nm gap width and their electrochemical characterization [33]. For the fabrication of the chips, an optical lithography step and two electron-beam steps are combined (Figure 18). 

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