If contam- ination of the silicon sample is an issue, the nitrile O-rings may be replaced by vi- nylidene fluoride-hexafluoropropylene Viton O-rings . It is important to provide a good ohmic contact to a semiconductor like silicon. The ohmic contact is especially critical for open cell designs, like the immersion cell, because it is exposed to HF vapors from the electrolyte, which are corrosive.
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Platinum or gold are inert contact materials with respect to HF, however some kind of spring or clip is needed to press the noble metal to the sample. Metals commonly used to make springs, like stainless steel or brass, are found to corrode rapidly in HF vapor. Tungsten shows a better performance, but a more elegant way to solve this problem is to use the elastic properties of the sample holder material itself to effect a non-metallic clip, as shown in Fig.
For sample contacts that are not directly exposed to HF, other metals, e. Such contacts, however, should be easily exchangeable in case of corrosion.
For moderate currents, needle tip contacts at the edge of the wafer are useful such contacts are commercially available as spring contact probes for electronic testing of PCBs . The elasticity of the PVC body is used to clamp the Pt wire contact to the wafer. If a power supply is connected to the elec- trodes, the cell is ready for operation.
This simple set-up has several advantages. It is a clean way of sample preparation, because the sample is not in contact with an O- ring and the area contaminated by the contact can easily be removed by cleaving it off. This is advantageous if subsequent high-temperature processing of the sample is desirable. The flexibility of immersion cell designs is shown in Fig. An inevitable property of this cell concept is a current flow along the strip. This causes an inhomogeneous potential distribution along the stripe due to ohmic losses, especially for low doped substrates.
Porous layers, as a result, often show a thickness gradient along the stripe.
The potential drop along the strip can usually be neglected for silicon samples of a sufficiently high conductivity or for small ano- dization currents. If, however, the transformation of the whole thickness of a strip into mesoporous silicon is desired, a slight beveling of the strip or an immersion scanning technique is required, even in the case of highly doped silicon [Ba4, Ju2]. Another drawback of the immersion cell concept is that the active area is badly defined, because of the meniscus formed at the electrolyte-air interface. The form of the meniscus greatly depends on whether the sample is hydrophilic or hydro- phobic, which again is a function of applied potential.
This problem can be cir- cumvented, if the active area of the sample is defined by a window in an inert layer, for example resist or CVD nitride, which is fully immersed into the electro- lyte, as shown in Fig. This set-up is useful for measure- ments of transient electrode processes like anodic oxide growth during electrochemical oscillations. Stress is induced by the growth of anodic films. After [Le4]. Counter Electrode The simple immersion cell design is most suitable for applications for which the current densities involved are very low, such as anodic oxidation.
In this case ohmic losses in the substrate become negligible, even for moderate doping densi- ties and large samples, like whole wafers. Position and geometry of the counter electrode, however, become important, because the oxide thickness is sensitive to spacing of the electrodes. Large counter cathodes of the same size and shape as the oxidized wafer can be realized by two highly doped wafers, which avoids ex- pensive platinum sheets or meshes.
The slits of a standard wafer container pro- vides an easy way of positioning of the wafers sufficiently accurately to produce homogeneous anodic oxides. Holes in the top of the container allow for contact- ing. Figure 1. A more sophisticated cell for anodic oxide formation is de- scribed in the literature [Ba13].
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If the wafer is not fixed in the cell, a mechanical wafer support is advisable. The ohmic contact can be an integral part of such a sample fixture, as shown in Fig. During formation of mesoporous silicon on highly doped substrates at low bias It is re- markable that mesoporous silicon formation takes place under the contact, too, without significant degradation of the contact properties. These drawbacks are overcome if the sample area exposed to the electrolyte is de- fined by an O-ring seal. The backside of the sample is accessible in this case and can be used to realize an ohmic contact.
Now the current flows normal to the sample surface, which reduces ohmic losses significantly and leads to a homoge- neous current density distribution. However, at a distance of about the wafer thickness from the O-ring the current flow is not normal to the surface and the current density is therefore slightly enhanced there.
This effect has been found to be responsible for thickness inhomogeneities of porous layers [Kr3].
To reduce such inhomogeneities the O-ring should be of a diameter in excess of a centi- meter and its section thickness as small as possible. The sample has to be pressed against the O-ring in order to seal the cell. This can be done by various means, as shown in Fig. Simple fixtures use the weight of the upper part of the cell or screws to press the O-ring against the sili- con sample.
A fixture using magnets is advantageous if fast handling is required [Ch14]. For whole wafers pneumatic pistons or a vacuum seal [Ba13] are prefer- able. Note that the use of a vacuum chuck, as shown in Fig. An advantage of the set-ups shown in Fig. The option of electrolyte agitation, however, is limited. The upper right figure on the front cover of this book shows the top view of a simple O-ring cell according to Fig. This not only avoids a potential source of contami- nation but also establishes a transparent contact. A disadvantage of this arrange- ment is that the potential of the wafer is not known.
The double cell shown in Fig.
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The two cells are separated by the wafer carrier, which has to fit so tightly into the set-up that leakage currents be- come negligible compared to the current across the wafer. O-rings can be used if leakage currents are not acceptable, as is the case for anodic oxidation. Tight seal- ing of double O-ring cells for whole wafers requires pneumatic elements or an evacuable recess as shown in Fig. Such a double cell designed for ELYMAT measurements of mm wafers with a sophisticated sample holder is shown on the upper left of the front cover of this book.
The electrical contacts 1. The O-ring can be pressed against the sample a by the weight of the upper part of the cell, b by screws, c by magnets, d, e by vacuum or f by pneumatic pistons. These designs can be ex- tended to double O-ring cells: this requires g a vertical sample position or h a closed cell. In order to produce significant currents across moderately doped wafers the re- verse biased junction has to be illuminated. Hence the anode for the case of p- type substrates or the cathode for the case of n-type substrates should be made of a platinum mesh to be sufficiently transparent.
A special O-ring cell design is needed for in situ infrared IR vibrational charac- terization of an electrochemical interface. The absorption of one monolayer i. A set-up as shown in Fig. The best compromise in terms of sensitivity often leads to about ten reflections [Oz2]. Note that no ohmic contact to the silicon wafer is necessary. Illumination is needed for moder- ately doped samples, to generate a current in the reversely biased junction.
After [La5]. Electrolyte 1. A standard set-up developed to study kinetic electrode processes is the rotating disc electrode . The electrode is a small flat disc set in a vertical axle.
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The hydrodynamic flow pattern at the disc depends on rotation speed and can be calculated. An additional ring electrode set at a different potential provides information about reaction products such as, for example, hydrogen. However, because this set-up is designed to study kinetic pro- cesses and is usually equipped with a platinum disc, it becomes inconvenient if silicon samples of different geometries have to be mounted. It can remove bubbles from the electrode and it allows for better temperature control.
Electrochemistry of Silicon: Instrumentation, Science, Materials and Applications
A magnetic or mechanical stir- rer can be integrated in open cell designs, as shown in Fig. However, high flow rates can only be obtained with closed cell designs, as shown in Fig. Poly- tetrafluoroethylene membrane pumps  and non-metallic valves , as com- monly used for pumping of HF in wafer fabs, are sufficient to provide good circula- tion.
Peristaltic pumps are not advisable because of their relatively low flow rates. In order to produce a homogeneous flow of electrolyte from the intermittent pumping action a partly air-filled reservoir is added for damping.