Structural devices
Structural devices

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Structural devices

8.4.2 Closed-loop control

Very often in the manufacture of microsystems, the etching steps are meant to remove a layer of material entirely from within the areas defined by the mask pattern. This offers the opportunity to detect the moment the etch is complete by spotting a change in composition of the reaction products from the etching. This is particularly useful in dry etching, where very small numbers of molecules can be detected using optical or mass-spectrometry-based residual gas analysis. Gaseous reaction products are quickly transported through the volume of the chamber, and so can be detected by these systems seconds after the layer that produced them is reached by the etchant.

End-point detection can sometimes also be done optically, directly on the substrate. In wet etching, for instance, the transparency of the wafer to infrared light can be monitored (though this, like the open-loop methods, requires a calibration run first to establish the relationship between material thickness and light transmission).

Table 5 Parameters affecting wet and dry etch rates

Parameter Wet etching Dry etching
Temperature of etchant Chemical reaction rates generally increase with temperature. Small changes in temperature can dramatically change the etch rate. The analogous quantity to etchant temperature in dry etching is the ion kinetic energy. This in turn is affected by gas pressure and bias voltage.
Temperature of target material High thermal mass of etchant and good heat transfer to target material generally means this is the same as the etchant temperature. Bombarding surfaces with high-energy ions results in heating. Poor thermal contact to the wafer means temperature control can easily be lost. Wafer temperature affects surface transport, evaporation and redeposition of reagents and reaction products. In turn, this affects both etch rate and etch profile.
Composition of etchant Important sources of uncertainty in the composition of the reagents include: accuracy of measurement of volumes in mixtures; changes in composition during storage; and local variation in composition during the etching process as reagents become consumed. This is especially so in deep, narrow holes. Composition of gas mixtures is controlled by flow rates through gas lines. Accuracy of metering directly affects the gas composition. Local variations in gas composition during etching occur when there is variation in the proportion of the surface area being etched.
Homogeneity Local variations in temperature and reagent composition can be greatly reduced by agitation of the liquid. This may be done using magnetic stirrers or bubblers. If a reaction product is a gas, bubbles can adhere to surfaces, preventing further attack by the etchant. Agitation is important in removing these quickly. The three-dimensional shape of the electric and magnetic fields in the etching chamber causes variations in the ion energy and hence the etch rate. Even the step caused by the edge of the wafer on the electrode can cause a zone on the wafer where the field, and hence the etch rate, is different.
Surface state Silicon spontaneously oxidises at room temperature, forming a layer up to 2 nm in thickness. This layer can delay the start of a silicon etch in an unpredictable way. Good practice therefore includes an HF dip to restore a pristine surface immediately before the etch is started. This issue affects dry etching equally.

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