Endpoint Detection System

Many endpoint detection systems, based on mechanisms, such as those based on reflected optical light [9], spindle motor current [10], pad temperature [11,12], have been used to resolve this problem, with limited success. Some systems may work with blank wafers or wafers with relatively low pattern density (at the STI level, for example), but for the PMD or ILD levels no useful results have been reported. The presence of a pattern at the PMD or ILD levels adds a great deal of complexity to the signals. Currently, use of an endpoint detection system to control the final post-CMP thickness is still a fertile topic for research and development. 

Christian, G. D. A Sensitive Amperometric Endpoint Detection System for Microcoulometric Titrations. Microchem. J. 9, 484 (1965). 

With any endpoint detection system several practical considerations are important for reliable results. For example, the indicator electrode should be placed in close proximity to the flow pattern from the burette, so that a degree of anticipation is provided to avoid overrunning the endpoint. Another important factor is that the indicator electrode be as inert and nonreactive as possible to avoid contamination and erratic response from attack by the titration solution. A third and frequently overlooked consideration is the makeup of the reference electrode and, in particular, its salt bridge. For example, a salt-bridge system that contains potassium chloride can cause extremely erratic behavior of any electrochemical system if the titrant solution contains perchlorate ion (because of the precipitation of potassium perchlorate at the salt-bridge titrant-solution interface). Likewise, a potassium chloride salt bridge in a potentiometric titra-... 

A specialized version of the potentiometric endpoint detection system is the use of two dissimilar metals as the electrode pair. For example, if a platinum... 

For this titration the dual-polarized amperometric endpoint detection system provides good sensitivity and rapid response. 

Protocols that enable the device to perform immuno-magnetic bead assays using AK bioluminescence as an endpoint detection system have also been developed, and are currently being evaluated. 

Assuming impurities can be satisfactorily pretitrated, the lower limit of the amount of sample that can be titrated is governed primarily by the sensitivity of the available endpoint detection system. Very small currents, such as 0.1 /xA, can be measured accurately (actually, currents smaller than 60 electrons per second have been measured and the time of electrogeneration can be measured accurately. With conventional amperometric and potentiometric endpoint indication, coulometric titrations in typical solution volumes cannot be accurately made at generating currents of less than about 100 A. 

An extensive database has demonstrated that many chemicals that are positive in this test also exhibit mutagenic activity in other tests. There are, however, examples of mutagenic substances, which are not detected by this test reasons for these shortcomings can be ascribed to the specific nature of the endpoint detected, differences in metabolic activation, or differences in bioavailability. On the other hand, factors which enhance the sensitivity of the bacterial reverse mutation test can lead to an overestimation of mutagenic activity. The bacterial reverse mutation test may not be appropriate for the evaluation of certain classes of chemicals for example, highly bactericidal compounds (e.g., certain antibiotics) and those which are thought (or known) to interfere specifically with the mammalian cell replication system (e.g., some topoisomerase inhibitors and some nucleoside analogues). In such cases, mammalian mutation tests may be more appropriate. 

In the iodimetric titration procedure, the combustion gases are bubbled through a diluent solution containing pyridine, methanol, and water. This solution is titrated with a titrant containing iodine in a pyridine, methanol, and water solution. In automated systems, the titrant is delivered automatically from a calibrated burette syringe and the endpoint detected amperometrically. The method is empirical, and standard reference materials with sulfur percentages in the range of the samples to be analyzed should be used to calibrate the instrument before use. Alternative formulations for the diluent and titrant may be used in this method to the extent that they can be demonstrated to yield equivalent results. 

Another specialized form of potentiometric endpoint detection is the use of dual-polarized electrodes, which consists of two metal pieces of electrode material, usually platinum, through which is imposed a small constant current, usually 2-10 /xA. The scheme of the electric circuit for this kind of titration is presented in Figure 4.1b. The differential potential created by the imposition of the ament is a function of the redox couples present in the titration solution. Examples of the resultant titration curve for three different systems are illustrated in Figure 4.3. In the case of two reversible couples, such as the titration of iron(II) with cerium(IV), curve a results in which there is little potential difference after initiation of the titration up to the equivalence point. Hie titration of arsenic(III) with iodine is representative of an irreversible couple that is titrated with a reversible system. Hence, prior to the equivalence point a large potential difference exists because the passage of current requires decomposition of the solvent for the cathode reaction (Figure 4.3b). Past the equivalence point the potential difference drops to zero because of the presence of both iodine and iodide ion. In contrast, when a reversible couple is titrated with an irreversible couple, the initial potential difference is equal to zero and the large potential difference appears after the equivalence point is reached. 

As with amperometric titrations, to have straight-line portions of the titration curve dilution corrections must be made because the response is directly dependent on the concentration of the ionic species. Also, the important data are taken before and after the equivalence point rather than precisely at the equivalence point. The general conditions for effective conductometric measurements of solutions are discussed in Chapter 5 and are directly applicable when the system is used as the endpoint detection method. A particularly complete review of the subject has been presented.8... 

Figure 4.8 Cell system for coulometric titration by a platinum generator electrode and an isolated auxiliary electrode system includes provision for potentiometric endpoint detection.

Because the generator electrodes must have a significant voltage applied across them to produce a constant current, the placement of the indicator electrodes (especially if a potentiometric detection system is to be used) is critical to avoid induced responses from the generator electrodes. Their placement should be adjusted such that both the indicator electrode and the reference electrode occupy positions on an equal potential contour. When dual-polarized amperometric electrodes are used, similar care is desirable in their placement to avoid interference from the electrolysis electrodes. These two considerations have prompted the use of visual or spectrophotometric endpoint detection in some applications of coulometric titrations. 

Coulometers for this type of work typically cost about 3000, which is roughly the price of a good potentiostat and integrator. Sampling systems, however, may double the price. These costs may be contrasted with the few hundred dollars needed for a constant-current supply for simple coulometric titrations (although some sort of endpoint detecting device is also usually needed). 

Standard methods Chloride is still frequently determined titrimetrically (argentimetry with potentio-metric endpoint detection). But chloride can also be determined in a flow-through system with photometric detection. Chloride releases thiocyanate from mercury(I) thiocyanate, which reacts with Fe(III) to form a colored complex. 

A typical setup of a working system for manual determinations of oxygen as given in Fig. 4-5 is convenient for standard oxygen titrations applying visual (starch) endpoint detection and may be converted into an automated titration unit by adding a detector and a computer plus interface. 

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