Substrate material

All microfabrication starts with a certain substrate. In some cases, the substrate merely provides a platform on which the devices will be built by layering. The layering constitutes an additive process. In some cases, part of the substrate is 

The most common substrates used in the semiconductor industry are Si and other compound semiconductor wafers such as gallium arsenide (GaAs). These wafers are produced in various sizes and cut along different crystalline planes. They are also doped with many types and concentrations of impurities that determine whether they are n- or p-type semiconductors. 

The crystalline structures and dopants of these semiconductor wafers are obviously critical factors in making semiconductor devices however, they may not be as relevant to the fabrication of microlenses. Nevertheless, semiconductor substrates are still important and useful for two reasons. First, most fabrication utilizes instruments developed for fabrication on these semiconductor wafers. Second, from a system integration view, microelectronics and optoelectronics made on semiconductor wafers will be needed at some point, whether for direct integration with the lenses or subsequent assembly. 

The main drawback of the semiconductor substrates is that they are not transparent in the range of visible light. This often limits aspects of the design and formation of lenses and other optical components because light transmission through substrates is not possible. 

Compared to Si and other semiconductor compound substrates, glass is transparent and thus serves as a good substrate candidate for fabrication of lenses and other optical components. Common glass substrates are wafers 

The purpose of the substrate is to effectively transport the generated electrons at the anode to the external circuit, finally arriving at the cathode for the reduction reactions, such as oxygen reduction and proton reduction. The requirements for the suitable substrates include high stability in the used electrolyte and low electrical resistance. Substrates also provide necessary support to anchor photocatalyst materials and facilitate electron collection. In the reported literatures, transparent conducting glass, titanium substrate, and carbon materials are the major substrates for the deposition of photocatalyst layers. 

In most cases, transparent conducting glasses, for example, FTO (fluorine-doped tin oxide) and ITO (indium-tin oxide), are preferred to be used as the substrate. The use of FTO and ITO glass 

Prior to the photocatalyst deposition, the transparent conducting glass is usually required to be cleaned in order to remove possible contaminants. A typical procedure of glass cleaning is described as follow the glass is ultrasonically cleaned in distilled water, ethanol, and acetone, and then naturally dried in air. The clean surface will ensure the successful coating of photocatalyst and enhance the adhesion between the transparent conductive oxide layer and the photocatalyst layer. 

Similar to transparent conducting glasses, a titanium substrate has to be cleaned before the growth of nano-structured Ti02 materials. A typical titanium pretreatment process can be found in published literatures (Liu et al., 201 la). In brief, it is first cleaned with acetone followed by absolute ethanol. Then it is washed with distilled water. Afterwards, it is boiled in 10 wt.% oxalic acid for 30 min to remove the potential oxide layer. Finally, it is ultrasonically cleaned in water for 15 min followed by air-drying in the atmosphere. 

Carbon materials, such as carbon cloth and carbon paper, are also good substrates for the deposition of photocatalysts, due to their low resistivity and cost. They are also widely used in the proton exchange membrane fuel cell (PEMFC), and are also commercially available at www.fuelcellstore.com. Carbon cloth/paper is supplied with an untreated surface or reinforced with PTFE. Since the photocatal5ftic reactions at the anode side involve three phases liquid electrol)4e, solid anode photocatalyst, and the produced gaseous CO2, carbon materials with hydrophilic surfaces are preferred in the fabrication of photoanodes. Compared to the previous two substrates, carbon material can be used directly without any pretreatment. 

In an effort to enhance diamond nucleation and to control film morphology, extensive work on the nucleation and early growth stages has been performed. As a result, technology problems associated with the nucleation of polycrystalline diamond films have been adequately addressed. A number of nucleation enhancement methods have been developed that enable the control of nucleation density over several orders of magnitude. Nucleation density has been increased from 10 cm on untreated substrates up to 10 cm on scratched or biased substrates. The effects of surface conditions on nucleation processes have been investigated to provide the guideline for the selection of optimum surface pretreatment methods. In this chapter, substrate materials, surface pretreatment methods and their influences on diamond nucleation are discussed. 

The chemical properties and surface conditions of substrate materials critically influence surface nucleation processes of diamond in 

The main reason for the existence of the incubation period frequently observed in diamond nucleation is the chemical interactions of the gas phase widi the substrate surface. Lux and Haubnerl l classified substrate materials into three major groups in terms of carbon-substrate interactions, as listed in Table 1. 

Little or no solubility or reaction Diamond, graphite, carbons, Cu, Ag, Au, Sn, Pb. etc. 

Although at present a complete picture is not yet available regarding which substrate materials favor diamond nucleation most, the following trends may be summarized from available experimental results and theoretical speculations  

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Besides the material based characteristics, the difference of density of the used particle/substrate combination is a very important criterion. The difference of density influences the contrast of the radiographic tests. Tungsten carbides were used as mechanically resistant particles and titanium based alloys as substrate. The substrate material is marked by an advantageous relation of strength to density. This material is often used in aeronautics, astronautics, and for modification of boundary layers. The density of tungsten carbide (15.7 g/cm ) is about 3.5 times higher than the density of titanium (4.45-4.6 g/cm ). 

The technological importance of thin films in snch areas as semicondnctor devices and sensors has led to a demand for mechanical property infonnation for these systems. Measuring the elastic modnlns for thin films is mnch harder than the corresponding measurement for bnlk samples, since the results obtained by traditional indentation methods are strongly perturbed by the properties of the substrate material. Additionally, the behaviour of the film under conditions of low load, which is necessary for the measnrement of thin-film properties, is strongly inflnenced by surface forces [75]. Since the force microscope is both sensitive to surface forces and has extremely high depth resolntion, it shows considerable promise as a teclnhqne for the mechanical characterization of thin films. 

III-V compound semiconductors with precisely controlled compositions and gaps can be prepared from several material systems. Representative III-V compounds are shown in tire gap-lattice constant plots of figure C2.16.3. The points representing binary semiconductors such as GaAs or InP are joined by lines indicating ternary and quaternary alloys. The special nature of tire binary compounds arises from tlieir availability as tire substrate material needed for epitaxial growtli of device stmctures. 

Figure C2.16.2 shows tire gap-lattice constant plots for tire III-V nitrides. These compounds can have eitlier tire WTirtzite or zincblende stmctures, witli tire wurtzite polytype having tire most interesting device applications. The large gaps of tliese materials make tliem particularly useful in tire preparation of LEDs and diode lasers emitting in tire blue part of tire visible spectmm. Unlike tire smaller-gap III-V compounds illustrated in figure C2.16.3 single crystals of tire nitride binaries of AIN, GaN and InN can be prepared only in very small sizes, too small for epitaxial growtli of device stmctures. Substrate materials such as sapphire and SiC are used instead.

Hybrid Circuits. The use of parylenes as a hybrid circuit coating is based on much the same rationale as its use in circuit boards. A significant distinction Hes in obtaining adhesion to the ceramic substrate material, the success of which determines the eventual performance of the coated part. Adhesion to the ceramic must be achieved using adhesion promoters, such as the organosilanes. 

Polymers are only marginally important in main memories of semiconductor technology, except for polymeric resist films used for chip production. For optical mass memories, however, they are important or even indispensable, being used as substrate material (in WORM, EOD) or for both substrate material and the memory layer (in CD-ROM). Peripheral uses of polymers in the manufacturing process of optical storage media are, eg, as binder for dye-in-polymer layers or as surfacing layers, protective overcoatings, uv-resist films, photopolymerization lacquers for repHcation, etc. 

High demands are placed on the substrate material of disk-shaped optical data storage devices regarding the optical, physical, chemical, mechanical, and thermal properties. In addition to these physical parameters, they have to meet special requirements regarding optical purity of the material, processing characteristics, and especially in mass production, economic characteristics (costs, processing). The question of recyclabiUty must also be tackled. 

The birefringence of substrate materials for optical data storage devices requires special attention, especially in the case of EOD(MOR) disks. Birefringence has no importance for glass substrates (glass does not exhibit any significant birefringence) and is only a subordinate factor for polymeric protective layers of aluminum substrates because of their reflective read/write technique. 

To efficiendy drive the development of improved substrate materials, the limiting values of birefringence have to be known this is especially tme for WORM and EOD(MOR) substrate disks. These limit values were laid down by the ANSI (American National Standard Institute) Technical Standard Committee (186—188). For 5.25 in. WORM disks, the ANSI document X 3 B 11/88-144 recommends a maximum LEP value of 9% this corresponds to an optical path difference perpendicular to the plane of the disk of not more than 80 nm/mm (double path). For 5.25 in. EOD(MOR) disks, more stringent conditions apply (ANSI-document X 3 B 11/88-049), which also allow calculation of the allowed range. 

With disk diameters above 5.25 in., all parameters, eg, water absorption and thermal expansion, become more critical which aggravates the expansion or warp of disks. If in the future disk rotation speeds have to be increased significantly to boost data transfer rates, higher demands will be placed on warp (tilt angle) and modulus to avoid creeping (ie, irreversible elongation in radial direction). A survey of the requirement profile for the substrate material of optical disks is given in Table 5 (182,186,187,189). 

Table 5. Requirements on Substrate Materials for Optical Memories...

Aluminum. Some manufacturers also have WORM disks above 5.25 in. on offer with aluminum as substrate material. Eor A1 the same advantages apply as for glass with the exception of a high coefficient of thermal expansion and lacking resistance to aggressive chemical vapors and Hquids. 

Although CD-modified BPA polycarbonate can be employed without problems for CD-DA and CD-ROM, the use as a substrate material for EOD(MOR) requites an optimum selection and meticulous adherence to production conditions to achieve the requited birefringence values of less than 20 nm/mm (184,193). 

Copolymers nd Blends of PC. Numerous co- and terpolymers as well as polymer blends of BPA-PC have been developed and their suitabihty as substrate materials for optical data storage media has been tested (Table 8) (195). From these products, three lines of development have been chosen for closer examination. 

Table 8. Substrate Materials for Optical Data Storage...

Copolymers and blends of BPA-PC and Modified PS. Theoretically, a blend or copolymer of 60 parts BPA-PC (positively birefringent) and 40 parts PS (negatively birefringent) should yield a product free of birefringence (Fig. 25) (207). In spite of modifications to PC to improve the compatibiUty, no blend could be produced which would be optically isotropic and thus suitable as a substrate material. The same holds tme for PC/PS-copolymers (208) in which... 

Poly(methyl methacrylate). PMMA offers distinct advantages over BPA-PC with respect to significandy lower birefringence, higher modulus, and lower costs, but has not been successhil as a material for audio CDs and CD-ROM as well as a substrate material for WORM and EOD disks because of its high water absorption (which makes it prone to warp) and its unsuitabiUty for metallising, and less so because of its low resistance to... 

Cyclic Polyolefins (GPO) and Gycloolefin Copolymers (GOG). Japanese and European companies are developing amorphous cycHc polyolefins as substrate materials for optical data storage (213—217). The materials are based on dicyclopentadiene and/or tetracyclododecene (10), where R = H, alkyl, or COOCH. Products are formed by Ziegler-Natta polymerization with addition of ethylene or propylene (11) or so-called metathesis polymerization and hydrogenation (12), (101,216). These products may stiU contain about 10% of the dicycHc stmcture (216). 

Table 9 compares the most important properties of substrate materials based on BPA-PC, PMMA, and CPO (three different products) (216,217). The future will prove if the current disadvantages of CPO against BPA-PC regarding warp, processibiUty (melt viscosity), and especially cost can be alleviated. CycHc polyolefins (CPO) and, especially cycloolefin copolymers (COC) (218) and blends of cycloolefin copolymers with suitable engineering plastics have the potential to be interesting materials for substrate disks for optical data storage. 

Table 9. Comparison of Characteristic Properties of Substrate Materials...

Table 10 compares the values of different experimental uv-curable cross-linked polymers with those of BPA-PC for the most important properties of substrate materials (220). In spite of this remarkable progress in the development of fast curing cross-linked polymers, BPA-PC and, to a small extent, glass are still the materials of choice for substrates for optical data storage. 

Other Polymers. Besides polycarbonates, poly(methyl methacrylate)s, cycfic polyolefins, and uv-curable cross-linked polymers, a host of other polymers have been examined for their suitabiUty as substrate materials for optical data storage, preferably compact disks, in the last years. These polymers have not gained commercial importance polystyrene (PS), poly(vinyl chloride) (PVC), cellulose acetobutyrate (CAB), bis(diallylpolycarbonate) (BDPC), poly(ethylene terephthalate) (PET), styrene—acrylonitrile copolymers (SAN), poly(vinyl acetate) (PVAC), and for substrates with high resistance to heat softening, polysulfones (PSU) and polyimides (PI). 

Fig. 26. Qualitative compatison of substrate materials for optical disks (187) An = birefringence IS = impact strength BM = bending modulus HDT = heat distortion temperature Met = metallizability WA = water absorption Proc = processibility. The materials are bisphenol A—polycarbonate (BPA-PC), copolymer (20 80) of BPA-PC and trimethylcyclohexane—polycarbonate (TMC-PC), poly(methyl methacrylate) (PMMA), uv-curable cross-linked polymer (uv-DM), cycHc polyolefins (CPO), and, for comparison, glass.

Of practical interest are detailed studies to influence the magnetooptical properties of RE-TM materials by the substrate material and the substrate adhesion of RE-TM layers by the selected deposition technique (226). Accordingly, measurements have been performed on glass, BPA-polycarbonate, and poly(ethylene terephthalate) (as a flexible substrate). 

Chemical Inhomogenities or Compositional Separation. Compositional separation at the grain boundaries influences the magnetic interactions of the individual grains. Deposition parameters such as temperature, substrate material, and the use of a seed layer play an important role. 

When an energetic ion penetrates a soHd, it undergoes a series of coUisions with the atoms and electrons in the target. In these coUisions the incident particle loses energy at a rate of a few to 100 eV pet nanometer, depending on the energy and mass of the ion as well as on the substrate material. 

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