Silicon electrolytic etching

Electrolytic corrosion (EC) test, 9 790 Electrolytic etching, of silicon,... 

If the hole concent ration in the semiconductor is relatively low, as in low resistivity n-type germanium or silicon, the available holes in the surface region are used up at low current densities and the etch rate is slow. The anodic current under these conditions can be increased by providing additional holes at the surface. Holes produced as a result of illuminating the semiconductor give uniform electrolytic etching on n-type semiconductors. Germanium is electro-lytically etched in several electrolytes while silicon can only be dissolved anodically in fluoride solutions. A thick film of amorphous silicon forms on silicon anodes in acid fluoride solutions below a critical current density. 

Germanium and silicon are electrolytically etched at about the same rate, about 3x10" 5 cm 3/coulomb. Thus at a current density of 500 ma/crn, Ge and Si are dissolved at the rate of about 1.7x10 cm/sec (0.0004 in /min). In order to electrolytically etch n-type semiconductors at a reasonable rate, some means must be found to increase the hole concentration at... 

Whereas germanium may be electrolytically etched in a large number of electrolytes, silicon has only been dissolved anodically in fluoride solutions. Strong alkaline solutions chemically attack silicon, forming a soluble silicate and hydrogen gas, and the rate of attack increases rapidly with temperature. However if a piece of silicon is made anodic in a hot strong alkaline solution such as IN KOH, the chemical attack stops when the anode potential is greater than a critical value. 

This critical current density decreases linearly with a decrease in the HF concentration. Wang (30) also found that silicon was readily electropolished in dilute aqueous HF solutions. Schmidt and co-workers (12,31) have electropolished small areas of both n- and p-type silicon using a jet of an aqueous electrolyte containing 8 gm NaF + either 5 cc 48% HF or 40 gm NH4F per liter of solution. To electrolytically etch n-type silicon rapidly they have had to illuminate the surface with an intense light as described above. 

Very little has been published on the electrolytic etching of semiconducting materials other than germanium and silicon. There probably have been many unpublished small experiments carried out to determine a suitable electropolishing process for many of the intermetallic semiconductors. Uhlir (36) for example, found that a largely nonaqueous HF solution suitable for electropolishing silicon would also electropolish GaSb. 

Electrolytic etching has been used to reveal p-n junctions (43) as well as to remove n- or p-type material preferentially from diodes and transistors (28). These processes make use of the rectifying barrier of p-n junctions as well as the hole depletion effect at the surface of n-type germanium and silicon. 

Silicon exhibits a diverse range of electrochemical phenomena, such as current oscillation, anisotropic etching, formation of porous silicon, etc. Each of these phenomena has extremely rich details that are governed by complex relationships between structures and properties of silicon electrodes on the one hand and between properties and experimental conditions on the other. The silicon/electrolyte interface is a complex system in which a great many variables are interacting with each other in a great many ways." ... 

The complexity of the system implies that many phenomena are not directly explainable by the basic theories of semiconductor electrochemistry. The basic theories are developed for idealized situations, but the electrode behavior of a specific system is almost always deviated from the idealized situations in many different ways. Also, the complex details of each phenomenon are associated with all the processes at the silicon/electrolyte interface from a macro scale to the atomic scale such that the rich details are lost when simplifications are made in developing theories. Additionally, most theories are developed based on the data that are from a limited domain in the multidimensional space of numerous variables. As a result, in general such theories are valid only within this domain of the variable space but are inconsistent with the data outside this domain. In fact, the specific theories developed by different research groups on the various phenomena of silicon electrodes are often inconsistent with each other. In this respect, this book had the opportunity to have the space and scope to assemble the data and to review the discrete theories in a global perspective. In a number of cases, this exercise resulted in more complete physical schemes for the mechanisms of the electrode phenomena, such as current oscillation, growth of anodic oxide, anisotropic etching, and formation of porous silicon. 

P. H. Beilin and W. K. Zwicker, Observation of surface defects in electrolytically etched silicon by infrared microscopy, J. Appl. Phys. 42, 1216, 1971. 

Figure 84. Pressureless-sintered silicon carbide doped with boron carbide. Electrolytically etched, BF. Grain face etching. Color etching reveals different grain orientations.

Walker, D.E.Y., 1968. The electrolytic etching of self-bonded silicon carbide. Prakt. Metallogr. 376. 

Electrochemical etching is one way of controlling the etch rate and determine a clear etch stop layer when bulk micromachining Silicon. In this case, the wafer is used as anode in an HF-Electrolyte. Sufficiently high currents lead to oxidation of the silicon. The resulting oxide which is dissolved by the HF-solution. Since lowly doped silicon material is not exhibiting a notable etch rate, it can be used as an etch stop. 

Aqueous electrolytes of high pH etch silicon even at open circuit potential (OCP) conditions. The etch rate can be enhanced or decreased by application of anodic or cathodic potentials respectively, as discussed in Section 4.5. The use of electrolytes of high pH in electrochemical applications is limited and mainly in the field of etch-stop techniques. At low pH silicon is quite inert because under anodic potentials a thin passivating oxide film is formed. This oxide film can only be dissolved if HF is present. The dissolution rate of bulk Si in HF at OCP, however, is negligible and an anodic bias is required for dissolution. These special properties of HF account for its prominent position among all electrolytes for silicon. Because most of the electrochemistry reported in the following chapters refers to HF electrolytes, they will be discussed in detail. 

A passivating oxide is formed under sufficiently anodic potentials in HF, too. However, there are decisive differences to the case of alkaline and fluoride-free acidic electrolytes. For the latter electrolyte the steady-state current density prior to passivation is zero and it is below 1 mA cnT2 for alkaline ones, while it ranges from mA cm-2 to A cm-2 in HF. Furthermore, in HF silicon oxide formation does not lead to passivation, because the anodic oxide is readily etched in HF. This gives rise to an anodic I-V curve specific to HF, it shows two current maxima and two minima and an oscillatory regime, as for example shown in Fig. 4.7. 

In contrast to acidic electrolytes, chemical dissolution of a silicon electrode proceeds already at OCP in alkaline electrolytes. For cathodic potentials chemical dissolution competes with cathodic reactions, this commonly leads to a reduced dissolution rate and the formation of a slush layer under certain conditions [Pa2]. For potentials slightly anodic of OCP, electrochemical dissolution accompanies the chemical one and the dissolution rate is thereby enhanced [Pa6]. For anodic potentials above the passivation potential (PP), the formation of an anodic oxide, as in the case of acidic electrolytes, is observed. Such oxides show a much lower dissolution rate in alkaline solutions than the silicon substrate. As a result the electrode surface becomes passivated and the current density decreases to small values that correspond to the oxide etch rate. That the current density peaks at PP in Fig. 3.4 are in fact connected with the growth of a passivating oxide is proved using in situ ellipsometry [Pa2]. Passivation is independent of the type of cation. Organic compounds like hydrazin [Sul], for example, show a behavior similar to inorganic ones, like KOH [Pa8]. Because of the presence of a passivating oxide the current peak at PP is not observed for a reverse potential scan. 

Fig. 4.12 Current-potential and etch rate-potential curves of p-type and n-type silicon electrodes in an electrolyte composed of 6 M HN03 and 6 M HF. Redrawn from [Kol4].

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. 

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. 

Fig. 5.10 Voltage-time curve (solid line) for an n -type silicon electrode (3 mficm) anodized with a constant current density of 6.25 mA crrf2 for t> 0 (sample at OCP for t<0) in 0.3 mol kg- NH4F (pH = 3.5). The thickness of the anodic oxide was measured by ellipsometry (open circles, broken line fitted as a guide to the eye). The etch rate of the anodic oxide in the electrolyte was measured (values above arrows) at different...

The term etching refers to the dissolution processes at OCP of silicon samples immersed in an electrolyte solution. The technique has been extensively explored for its useful applications in the fabrication of electronic devices, surface polishing, and micromachining. For example, it is widely used for the production of cantilevers for the AFM technology. 

---