A cross-correlation technique for measuring the very short-term (milliseconds to seconds) properties of stable oscillators is described. Time-dependent functions representing signals from two separate oscillators are led to a function multiplier where the instantaneous product of the functions is made. The oscillators are either set to a given phase relation or allowed a small relative drift so that a slow beat frequency is observed. Short-term fluctuations superimposed upon the slow beat signal from the multiplier output will represent the instantaneous phase difference between the oscillators when the inputs are in quadrature. When the inputs are in and out of phase, the fluctuations represent amplitude fluctuations. The time averaging function is determined by a filter having arectangular pass band from nearly zero frequency to a cutoff frequency vC. The mean square frequency deviation measured in a bandwidth wC is obtained by differentiating, filtering, squaring, and averaging the signal from the function multiplier data being taken when the input signals are in quadrature. Mean square averages of amplitude and phase averaged over various bandwidths wC may be obtained by bypassing the differentiator. Sample data from measurements on hydrogen masers are presented, and the effect of thermal noise is seen to be the major factor limiting the short-term frequency stability of the signals.

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We present optical emission spectroscopy (OES) as a technique for process optimization of the etch step during deep reactive ion etching of silicon. For specific process steps, the spectrum of optical plasma emission is investigated. Two specific wavelengths are identified (fluorine at 703.8 nm and CS compounds at 257.6 nm), which significantly change intensity during the etch step. Their intensity drop is used for the recognition of the passivation layer breakthrough. Thus, the net silicon etch time can be measured. This time can be used for process optimization. A structural analysis of the passivation layer shows its fragmentation during its breakthrough. The plasma–surface interaction and their correlation with the plasma emission are described. Within an application example, the passivation breakthrough is investigated in detail. For different process regimes, the residues of the fragmented passivation layer are analyzed by scanning electron microscopy. Residue densities of 14–38 µm−2 are fabricated. For silicon grass generation, the OES technique offers a versatile tool for the process optimization of the mask generating process within the first cycles.

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