Electrochemical Etch Stops

If the relevant literature is surveyed for the keywords etch stops and silicon, a confusing multiplicity of methods is found using different electrolytes, different bias and differently doped silicon substrates. This section does not aim to be a comprehensive review of all these techniques [Co2], but an introduction to the basic principles of electrochemical etch stops, which will be illustrated by a few typical examples. 

All electrochemical etch stops of silicon are based on the dissolution behavior discussed above and a method to have different parts of the electrode interface under different potentials. 

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]. 

The membranes of the microhotplates were released by anisotropic, wet-chemical etching in KOH. In order to fabricate defined Si-islands that serve as heat spreaders of the microhotplate, an electrochemical etch stop (ECE) technique using a 4-electrode configuration was applied [109]. ECE on fully processed CMOS wafers requires, that aU reticles on the wafers are electrically interconnected to provide distributed biasing to the n-well regions and the substrate from two contact pads [1 lOj. The formation of the contact pads and the reticle interconnection requires a special photolithographic process flow in the CMOS process, but no additional non-standard processes. 

The electrochemical etch-stop technology that produces the silicon island is rather complex, so that an etch stop directly on the dielectric layer would simplify the sensor fabrication (Sect. 4.1.2). The second device as presented in Fig. 4.6 was derived from the circular microhotplate design and features the same layout parameters of heaters and electrodes. It does, however, not feature any sihcon island. Due to the missing heat spreader, significant temperature gradients across the heated area are to be expected. Therefore, an array of temperature sensors was integrated on the hotplate to assess the temperature distribution. The temperature sensors (nominal resistance of 1 kfl) were placed in characteristic locations on the microhotplate, which were numbered Ti to T4. 

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. 

B. Kloeck, S.D. CoUins, N.F. de Rooij, and R.L. Smith. Study of electrochemical etch-stop for high-precision thickness control of silicon membranes , IEEE Transactions on Electron Devices 36 (1989), 663-669. 

L. Smith and A. Soderbarg, Electrochemical etch stop obtained by accumulation of free carriers without p-n junction, J. Electrochem. Soc. 140, 271, 1993. 

A. Gotz, J. Esteve, J. Bausells, S. Marco, J. Samitier, and J. R. Morante, Passivation analysis of micro-mechanical silicon structures obtained by electrochemical etch stop, Sensors Actuators A 37, 744, 1993. 

M. C. Acero, J. Esteve, C. Burrer, and A. Gotz, Electrochemical etch-stop characteristics of TMAHrlPA solutions, Sensors Actuators A 46, 22, 1997. 

B. Kloeck, S. D. Collins, N. F. De Rooij, and R. L. Smith, Study of electrochemical etch-stop for high-precision thickness control of silicon membranes, IEEE Trans. Electron Devices 36(4), 663, 1989. T. Ohmi, M. Miyashita, M. Itano, T. Imaoka, and I. Kawanabe, Dependence of thin-oxide film quality on surface microroughness, IEEE Trans. Electron Devices 39(3), 537, 1992. 

D. Eichner and W. von Munch, A two-step electrochemical etch-stop to produce freestanding bulk-micromachined structures, Sensors Actuators A 60, 103, 1997. 

P. J. French, M. Nagao, and M. Esashi, Electrochemical etch-stop in TMAH without externally applied... 

A planar substrate, such as silicon wafer, could be micromachined by a sequence of deposition and etching processes. This results in three-dimensional microstructures which can be implemented in cavities, grooves, holes, diaphragms, cantilever beams etc. The process referred to as silicon micromachining often employs anisotropic etchants such as potassium hydroxide and ethylene diamine pyrocatechol. The crystallographic orientation is important as the above-mentioned etchants show an etch-rate anisotropy. The ratio for the (100)-, (110)- and (111)- planes is typically 100 16 1. The technique of electrochemical etch stop could be applied for control of the microstructural dimensions. An alternative... 

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]. 

V. McNeil, A thin-film silicon microaccelerometer fabricated using electrochemical etch-stop and wafer bonding tech-... 

The most appropriate technique to make low-doped, single crystalline silicon membrane-type microstructures with well-controlled dimension and thickness is the electrochemical etch stop on a diffusion or an epitaxial layer as shown in Fig. 4. 

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