Etching current

A further improvement was introduced by Weiss and co-workers,35 who connected the circuit between the lamellae and the lower portion of the wire through an electrolyte in a conducting beaker. This avoids introducing any forces on the tungsten wire whilst retaining the automatic etching current... 

Figure 3.13 Three methods of chemically etching metal tips for STM. In (a) the current cut-off is manually or electronically triggered when the end of the etched wire falls the finite time delay inherent in this approach results in a blunting of the final tip as etching continues after separation, (b) This shows an adaptation in which the etching current is automatically cut off when the lower portion of the wire drops - it is the lower portion that is used as an STM tip. (c) This shows an improved design in which the etching current is fed to the lower portion of the tungsten wire through an electrolyte held in a conductive beaker. In this case the upper portion of the etched wire is kept.

Fig. 9.16 A sine wave modulation of the etching current produces an array of macropores with a corresponding modulation of diameter.

The experimentally observed parabolic increase in pore depth and linear decrease in concentration shown in Fig. 9.18 c indicate that Eq. (9.6) is valid [Le9]. The macropore growth rate decreases linearly with l according to Eq. (9.6). If, therefore, a constant pore diameter is desired for a macropore array, a decrease in etching current or illumination intensity, respectively, with time is required. 

Arrays with pore diameters d as small as about 0.3 pm have been achieved [Lel7]. The lower limit for the pore diameter is established by breakdown, according to Fig. 8.1b, which leads to light-independent pore growth and spiking. There seems to be no upper limit for the pore diameter, because the formation of 100 pm wide pores has been shown to be feasible [Kl3]. Array porosities may range from 0.01 to close to 1. The porosity, which is controlled by the etching current, determines the ratio between pore diameter and pitch of the pore pattern. 

The IR filter is realized by a PS layer with a modulation of porosity, which constitutes an interference filter as described in detail in the next section. The 30 pm thick porous layer is then released from the substrate by electropolishing, which is easily done in situ by increasing the etching current density above JPS. This process is commonly applied to form free-standing PS membranes and PS tubes [Tj 1], The internal strain between the Si3N4 layer used for masking and the porous layer lifts the filter up to its rest position, as shown in Fig. 10.10. The filter is suspended at two microactuator arms, which work as thermal bimorph actua-... 

Fig. 5.7 Time variation of titanium electrode etching current density for (a) Nitric acid/hydrofluoric acid electrolyte (b) Boric acid/nitric acid/hydrofluoric acid electrolyte.

Fig. 13.2. Dependence of tip radius of curvature with cutoff time. Scanning electron micrographs of tips with different etching-current cutoff time, (a) 600 ns, with an average radius of curvature 32 nm. (b) 140 ms, with an average radius of curvature 58 nm. (c) 640 ms, with an average radius of curvature lOOnm. (reproduced from ibe et al., 1990, with permission.)...

The most important parameter that affects the final shape of the tip end is the time for the etching current to cut off after the lower part drops off. The shorter the cutoff time, the etching current, the sharper the tip end, as shown in Fig. 13.2. To shorten the cutoff time, a simple electronic circuit is helpful. When the lower part of the wire drops off, the etching current suddenly drops. The electronic circuit senses the drop of the etching current and turns off the current completely through an electronic switch. An example of such a circuit is given in detail by Ibe et al. (1990). 

According to Jacquet (34) the above very low electro-etching current results from the fact that a viscous layer of high resistivity forms at the anode during electropolishing. 

Fig. 10. Apparatus used for cuinulative etching. Current densities are determined by the thickness of the Bakelite mask (B) and the size of the opening of the mask (M-sample, F-stainless steel forceps, S-stainless steel cathode) (25),...

The etching rate at room temperature is proportional to the Xep2 pressure, with rates as high as 12 nm s at 1.4x 10 Torr [42]. This etch rate corresponds to an equivalent etching current of about 7 mAcm on a (100) surface and assuming a dissolution valence of 2. 

In the pore formation regime, the dissolution current increases exponentially with applied potential for p-type silicon and heavily doped n-type silicon [61, 63-66]. The potential range over which this exponential behavior is observed is dependent on dopant concentration and HF concentration. The exponential current-potential curves are characterized by a slope of 60 mV (kT/q) on a plot of the logarithm of the etching current versus applied potential, as can be seen in Fig. 8 [65]. 

Fig. 13. Plot of the reciprocal of the time at which the pore front penetrates the wafer, as a function of etching current density [74].

Highly doped p-type silicon the pores propagate in the [100] direction and are polygonal in cross-section, with pore diameters in the range 10-100 nm depending on the etching current density. The pores are interconnected [79]. 

H. S. Park, T. S. Kang, and K. J. Kim, Influence of etching current density on visible electroluminescence from porous n-Si under cathodic bias, J. Electrochem. Soc. 146, 1991, 1999. 

C-face PEC Constant Voltage Etching Current Density vs. Time Curve... 

Figure 2.11 Pore density in the moderately doped SiC substrate vs etching current density...

The capacitance and the conductance of a porous layer depend on its porosity (i.e. the size and distribution of pores, which can vary from mesopores to micro- and nano-pores). Capacitance and conductance are also affected by controlling the etching current density, etching time, the concentration of the electrolyte, and the intensity and wavelength of the illumination (Baratto... 

Fig. 4 Pore density versus silicon electrode doping density for porous silicon layers of different geometries. Notice that macropores are essentially obtained on low to moderately doped substrates. The dashed line shows the pore density of a triangular pore pattern with a pore pitch equal to two times the SCR width for a 3 V applied bias. Note that only macropores on n-type substrates may show a pore spacing significantly exceeding this limit. The regime of stable macropore array formation on n-Si is indicated by a dot pattern. Doping type and etching current density (in mA/cm ) are indicated in the legend (After Lehmann 1993)...

Sample Substrate resistivity, O cm Electrolyte composition HFiCaHsOH Etching current density, mA/em ... 

Fig. 11 Pore size distributions for samples PS20 open rectangles) and PS60 open triangles) and for a sample obtained by using a sawtooth variation of the etching current density...

Barillaro G, Strambini LM (2010) Controlling macropore formation in patterned n-t3/pe silicon existence of a pitch-dependent etching current density lower bound. Electrochem Commun 12 1314-1317... 

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