Semiconductor problems, surface

Structural information on the atomic arrangements at the early stage of formation of metal-metal, metal-semiconductor interfaces and semiconductor-semiconductor heterojunctions is needed along with the determination of the structure of the electron states in order to put on a complete experimental ground the discussion of the formation of solid-solid junctions. Amongst the structural tools that have been applied to the interface formation problem Surface-EXAFS is probably the best... 

However, this article is not intended to provide an exhaustive review of the voluminous literature on the application of surface analytical techniques to semiconductor problems. Numerous reviews have been published which have treated various aspects of these applications (1-jj). This article is intended to give an overview, drawing from more recent publications, of the ways in which surface analysis continues to play a vital role in the development and application of the numerous material technologies involved in semiconductor processes. In addition, the need for further development of surface techniques and a summary of the materials problem that do not lend themselves to the available analytical techniques are described. 

It is expected that the geometrical dimensions of IC devices will continue to decrease through the use of electron beam and x-ray lithography. Analysis of these small geometries presents additional challenges since a tradeoff exists between analysis area, and detection limits for the microbeam analysis techniques, AES and SIMS. The other surface analysis techniques of XPS and RBS already have very limited spatial resolution with respect to the current geometrical dimensions of IC s. The fabrication of denser and more complicated IC s also increases the value of each wafer which increases the need for additional process characterization and control. The increased application of surface analysis to semiconductor problems will provide a better understanding of these processes and will stimulate the further development of instrumental surface analysis techniques. 

Theoretical eflBciencies for the process are high ( 25 %) but, in practice, an order of magnitude lower is observed. A major problem with the process is the evolution of oxygen at the semiconductor anode surface. To date, very few materials of the required band-gap (1.5-2 eV) have proved suitable [36, 43]. 

The band-edge photoluminescence (PL) of K-CaSe has been shown to respond to the adsorption of a variety of analytes to the semiconductor s surface. This conceivably can be used for sensor development. However, one major problem is the issue of selectivity. Although Lewis bases and acids can be readily distinguished due to their differential effect on the electronic properties of the semiconductor, analyte-specific analysis is difficult to achieve. Ellis and co-workers [64] have examined the effect of imprinted polymer coating on the surface on the response of the semiconductor. Without the coating, the bare surface of CdSe responds to the adsorption of ammonia, mono-, di-, and trimethylamine with similar PL enhancement. However, upon coating the surface with ammonia imprinted poly(arylic acid) (PAA), CdSe only responded to the presence of ammonia, but not trimethylamine. On the other hand, CdSe coated with trimethylamine-imprinted polymer does not provide this selectivity, indicating the selectivity was mostly due to the size effect. 

One of the approaches to the use of nanoantennas in photodetection is to use a Schottky metal-semiconductor junction. An optical antenna forms flie metal part of the metal-dielectric contact at the semiconductor detector surface [320]. Photoexcitation generates hot electron-hole pairs by plasmon decay and the electrons are injected over the Schottky barrier, thus directly generating photocurrent. A problem with this approach is its low efficiency. 

The ISFET, developed from the fabrication techniques of semiconductor devices, is an important sensor device used in potentiometry. The main advantages are the extremely small size, solid-state structure and the ability to fabricate multi-ion sensors. More than 30 years ago, methods have been proposed to work with a differential arrangement, i.e. the integration of an ion-sensitive and an ion-insensitive structure, the later one working as the reference element (R(E)FET). The main problem is that semiconductor-modified surfaces required for R(E)FET are also not always in thermodynamical equilibrium with the test solution and can be sensitive to aggressive or interfering dissolved species or not well characterised aging phenomena. 

In spite of the fact that high-quality Schottky contacts are critical for ZnO device applications, there is little information about the Schottky contacts on ZnO to date. The chemical reactions between the metal and the semiconductor, the surface states, the contaminants, the defects in the surface layer, and the diffusion of the metal into the semiconductor are well known problems in the formation of Schottky contacts. In the case of ZnO, for instance, A1 is expected to produce the most... 

Like XPS, the application of AES has been very widespread, particularly in the earlier years of its existence more recently, the technique has been applied increasingly to those problem areas that need the high spatial resolution that AES can provide and XPS, currently, cannot. Because data acquisition in AES is faster than in XPS, it is also employed widely in routine quality control by surface analysis of random samples from production lines of for example, integrated circuits. In the semiconductor industry, in particular, SIMS is a competing method. Note that AES and XPS on the one hand and SIMS/SNMS on the other, both in depth-profiling mode, are complementary, the former gaining signal from the sputter-modified surface and the latter from the flux of sputtered particles. 

How can such problems be counterbalanced Since a large capacitance of a semiconductor/electrolyte junction will not negatively affect the PMC transient measurement, a large area electrode (nanostructured materials) should be selected to decrease the effective excess charge carrier concentration (excess carriers per surface area) in the interface. PMC transient measurements have been performed at a sensitized nanostructured Ti02 liquidjunction solar cell.40 With a 10-ns laser pulse excitation, only the slow decay processes can be studied. The very fast rise time cannot be resolved, but this should be the aim of picosecond studies. Such experiments are being prepared in our laboratory, but using nanostructured... 

Polymer films that are sensitive to light, x-rays, or electrons— known as photoresists—are nsed extensively to transfer the pattern of an electronic circuit onto a semiconductor surface. Such films must adhere to the semiconductor surface, cross-link or decompose on exposure to radiation, and nndergo development in a solvent to achieve pattern definition. Virtually all aspects of photoresist processing involve surface and interfacial phenomena, and there are many outstanding problems where these phenomena mnst be controlled. For example, the fabrication of multilayer circuits requires that photoresist films of about 1-pm thickness be laid down over a semiconductor surface that has already been patterned in preceding steps. 

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