Stopped technique

This technique with two indicator electrodes, proposed in 1956 by Dubois and Walisch146, is an intermediate between bipotentiometry and biamperometry, because neither the current nor the potential across the electrodes of the same metal are kept strictly constant. The authors opinion, e.g., in the titration of iodide with bromate in hydrochloric acid, that the AE value does not matter much, has been contradicted by Kies (ref. 141a, p. 18). As a method of alkalimetry or acidimetry it cannot be preferred, like any other dead-stop technique to the usual glass electrode methods147. Nevertheless, the fact that the apparatus permits a choice of adjustment towards constant current or constant potential can be useful, but then the method approaches either bipotentiometry or biamperometry. 

Aqueous electrolytes of high pH etch silicon even at open circuit potential (OCP) conditions. The etch rate can be enhanced or decreased by application of anodic or cathodic potentials respectively, as discussed in Section 4.5. The use of electrolytes of high pH in electrochemical applications is limited and mainly in the field of etch-stop techniques. At low pH silicon is quite inert because under anodic potentials a thin passivating oxide film is formed. This oxide film can only be dissolved if HF is present. The dissolution rate of bulk Si in HF at OCP, however, is negligible and an anodic bias is required for dissolution. These special properties of HF account for its prominent position among all electrolytes for silicon. Because most of the electrochemistry reported in the following chapters refers to HF electrolytes, they will be discussed in detail. 

The easiest way to have different parts of the electrode surface under different bias is to disconnect them by an insulator. This method is elucidated by an experiment in which an electrochemical etch-stop technique has been used to localize defects in an array of trench capacitors. In a perfect capacitor the polysilicon in the trench is insulated from the substrate whereas it is connected in a defect capacitor, as shown in Fig. 4.15 a. If an anodic bias is applied the bulk silicon and the polysilicon in the defect trench will be etched, while the other trenches are not etched if an aqueous HF electrolyte is used, as shown in Fig. 4.15b. The reverse is true for a KOH electrolyte, because the only polysilicon electrode in the defect trench is passivated by an anodic oxide, as shown in Fig. 4.15 c. 

Small leakage currents or a transistor-like action of the junction are sufficient to generate a small current that may cause undesired passivation. This can be circumvented by application of an additional potential to the etching layer, shown by the broken line in Fig. 4.16 a. This electrochemical etch-stop technique is favorable compared to the conventional chemical p+ etch stop in alkaline solutions, because it does not require high doping densities. This etch stop has mainly been apphed for manufacturing thin silicon membranes [Ge5, Pa7, Kll] used for example in pressure sensors [Hil]. 

A prerequisite for all etch-stop techniques discussed so far is an electrical connection to an external power supply. However, if the potential required for passivation in alkaline solutions is below 1 V, it can be generated by an internal galvanic cell, for example by a gold-silicon element [As4, Xil]. An internal galvanic cell can also be realized by a p-n junction illuminated in the etchant, as discussed in the next section. Internal cells eliminate the need for external contacts and make this technique suitable for simple batch fabrication. 

An initially flat silicon electrode surface will develop a surface topography if the photocurrent varies locally. This variation can be caused by a lateral variation of the recombination rate or by a lateral variation of the illumination intensity. The photoelectrochemical etching of a silicon electrode is related to the etch-stop techniques discussed in Chapter 3. While different etching rates for different areas of the electrode may be obtained by electrical insulation or by a different doping density, the etch rate may also be altered by a difference in illumination intensity. Basically four photoelectrochemical etching modes are possible for homogeneously doped substrates ... 

Sensors for measurements of physical parameters such as pressure, rotation or acceleration are commonly based on elongation or vibration of membranes, cantilevers or other proof masses. The electrochemical processes used to achieve these micromechanical structures are commonly etch-stop techniques, as discussed in Section 4.5, or sacrificial layer techniques, discussed in Section 10.7. 

Another possibihty to improve the temperature homogeneity is to introduce an additional polysiHcon plate in the membrane center. The thermal conductivity of polysilicon is lower than that of crystalline siHcon but much higher than the thermal conductivity of the dielectric layers, so that the heat conduction across the heated area is increased. Such an additional plate constitutes a heat spreader that can be realized without the use of an electrochemical etch stop technique. Although this device was not fabricated, simulations were performed in order to quantify the possible improvement of the temperature homogeneity. The simulation results of such a microhotplate are plotted in Fig. 4.9. The abbreviations Si to S4 denote the simulated temperatures at the characteristic locations of the temperature sensors. At the location T2, the simulated relative temperature difference is 5%, which corresponds to a temperature gradient of 0.15 °C/pm at 300 °C. 

For a given type and orientation, the etch rate of silicon in alkaline solutions is largely independent of doping concentration up to a concentration of about 1019 cm-3 [33, 80]. At a doping level of about 2 x 1019 cm 3, the etch rate of boron doped silicon drastically decreases with increasing dopant concentration [33, 86,144]. Reduction by as much as 3 orders of magnitude can be obtained by varying the boron concentration from about 1019 to above 102° cm-3. This feature has been widely used as an etch-stop technique for the fabrication of silicon microstructures. 

No matter what etchant is employed, in order to achieve high predictability and reproducibility in the micromachining process, the etching step must be combined with some technique for stopping it automatically. Even the most casual examination of the present micromachining literature reveals that virtually all reported micromachined structures rely on at least one etch-stop technique. In its most comprehensive definition, an etch-stop process is any method that allows for selective removal of a specific material to produce a predefined relief. 

The most suitable, successful etch-stop techniques are electrochemical etch stop (ECES), achieved by reverse biasing a pn junction [12, 15], and high boron dose etch stop [7, 16], which is explained below. Other etch-stop techniques have been reported [17], but they have not yet been used in mass production. A detailed review of etch-stop techniques for micromachining is given in [18]. 

S.D. Collins, Etch stop techniques for micromachining, J. Electrochem. Soc. 

There are a number of other etch stop techniques that are commonly used. The electrochemical diode technique utilizes a lightly doped p-n junction as the etch stop by applying a bias between the wafer and the etch solution. An... 

Process Physical Dimension Range/ Aspect Ratio Materials Etch Stop Techniques Through-put Cost... 

Two methods which are quite widely applicable to many local anaesthetics are non-aqueous titration (perchloric acid to crystal violet, p. 792) and titration with sodium nitrite (0 5 g in 75 ml water and 10 ml hydrochloric acid, titrating with O IM nitrite and determining the end-point using the dead-stop technique, p. 867). A few materials such as benzocaine may be determined by both methods but in most cases either one or the other is applicable. Lignocaine hydrochloride, for example, gives a satisfactory end-point by the non-aqueous method but cannot be determined by titration with nitrite procaine hydrochloride on the other hand is satisfactorily titrated wdth nitrite but gives rise to a precipitate during titration in non-aqueous medium which obscures the end-point. Certain compounds such as amethocaine hydrochloride cannot be determined by either method. 

Dissolve about 0 25 g in 30 ml of a mixture of equal volumes of dilute hydrochloric acid and water and titrate electrometrically with 0 1 M sodium nitrite using the dead-stop technique (see p. 867). 1 ml 0-1M = 0 01242 g C12H12O2N2S. 

Aminosalicylic acid and its salts may be determined either by bromin-ation or by titration with nitrite. The acid itself, official in the t/.5.P., is determined by titration with nitrite, the end-point being detected externally with starch-iodide paste. A more satisfactory method is to determine the end-point electrometrically by the dead-stop technique (see p. 867). A suitable method is as follows ... 

Dissolve 0 5 g in 75 ml of water in a beaker and add 10 ml of concentrated hydrochloric acid. Then add 1 g of potassium bromide and stir mechanically until dissolved. Titrate slowly with 0 1 M sodium nitrite, running the solution in below the surface of the liquid in the beaker and stirring continuously determine the end-point electrometrically ( dead-stop technique). 1 ml O IM = 0 01531 g C7H7O3N. 

Note 2—Pre-set end point indicated with polarized electrodes provides a detection technique similar to the dead stop technique specified in previous versions of this test method. 

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