Etch rate silicon

A number of different fluoride salts have been used as electrolytes for electropolishing silicon. Not only must the fluoride salt be sufficiently water soluble but the anode reaction product, the corresponding fiuosilicate, must also be readily soluble in water. Potassium fluoride is highly water soluble but the fiuosilicate is not. At a critical anodic etch rate silicon will passivate in KF solutions due to the precipitation of SiFg on the surface. Ammonium fluoride and ammonium fiuosilicate are both sufficiently water soluble. The acids HF and SiFg are even more water soluble. 

A fonn of anisotropic etching that is of some importance is that of orientation-dependent etching, where one particular crystal face is etched at a faster rate than another crystal face. A connnonly used orientation-dependent wet etch for silicon surfaces is a mixture of KOH in water and isopropanol. At approximately 350 K, this etchant has an etch rate of 0.6 pm min for the Si(lOO) plane, 0.1 pm min for the Si(l 10) plane and 0.006 pm miiG for the Si(l 11) plane [24]. These different etch rates can be exploited to yield anisotropically etched surfaces. 

Dry etching is a commonly used teclmique for creating highly anisotropic, patterned surfaces. The interaction of gas phase etchants with surfaces is of fundamental interest to understanding such phenomena as undercutting and the dependence of etch rate on surface structure. Many surface science studies aim to understand these interactions at an atomic level, and the next section will explore what is known about the etching of silicon surfaces. 

Ion-Assisted Processes An alternative use of ion beams generated from low cost sources is to assist particular chemical reactions, or vapour deposition. An example here is in etching processes (Figure 16). The simultaneous use of an argon beam with XeFp gas compared with the use of either separately, to etch silicon produces an etch rate of a factor of at least fourteen. The use of ion beams can also increase the directionality (23) of the process (Figure 17). Examples are given in Table IV of how ion bombardment during film formation modifies the final film. 

Figure 16. The influence of Ar ion beams on the etch rate of silicon by XeF2 ...

Etch rate and homogeneity and anisotropic characteristics are the predominant factors in determining the resulting micro system device properties. Temperature and concentration of the KOH solution as well as the doping concentration of the silicon material have the largest impact on these properties and have to be thoroughly controlled. 

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. 

Steady-State Oxide Thickness. The steady-state etching rate (R = S/M) does not contain any of the kinetic parameters thus it does not contain any information about the kinetics of the oxidation process. In contrast, the steady-state oxide thickness is determined by the kinetics of the transport and oxidation processes thus one can learn about these processes by studying the steady-state oxide thickness. The silicon material balance (Eq. 9)... 

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. 

All standard cleaning processes for silicon wafers are performed in water-based solutions, with the exception of acetone or (isopropyl alcohol, IPA) treatments, which are mainly used to remove resist or other organic contaminants. The most common cleaning procedure for silicon wafers in electronic device manufacturing is the deionized (DI) water rinse. This and other common cleaning solutions for silicon, such as the SCI, the SC2 [Kel], the SPM [Ko7] and the HF dip do remove silicon from the wafer surface, but at very low rates. The etch rate of a cleaning solution is usually well below 1 nm min-1. 

The etch rate of highly p-doped silicon (> 1019 cm4) is found to be reduced by orders of magnitude compared to the etch rate of low doped or highly doped n-type silicon of (100) orientation. This effect is shown in Fig. 2.3 for KOH note that the etch rate is significantly affected by the amount of dissolved oxygen... 

In the temperature regime shown the etch rate of (110) silicon is roughly 1.5 times that... 

Fig. 2.3 Etch rate of (100) bulk silicon in 1 M KOH of low and high DOC at 70°C as a function of p-type dopant concentration. The DOC of the solution was controlled by bubbling with pure 02 or N2.

Etching of silicon in alkaline solutions occurs under evolution of hydrogen with a ratio of two molecules H2 per dissolved Si atom. This ratio is found to be reduced under positive bias [Pa6] or by addition of oxidizing agents like H202 [Sc6], If the anodic bias is increased beyond the passivation potential (PP), the dissolution rate is reduced by orders of magnitude. 

Highly p-doped layers can also be used as masking layers. If the p-type doping level of silicon substrates is high enough to cause degeneracy (NA > 1019 cm-3), a decrease in etch rate with doping density is observed in all alkaline solutions inde-... 

Silicon etch rates in alkaline solutions commonly increase monotonically with temperature. For KOH, for example, the etch rate r can be calculated according to ... 

Silicon is stable in acidic solutions that do not contain fluoride because the silicon surface is passivated by a native oxide. If only H F is present in an aqueous solution the etch rate remains low, showing values below 0.1 rim miri 1 on single crystalline silicon depending on the OFT concentration [Hu2]. This low etch rate... 

The etch rate is further increased if H202 is added to the solution, as shown in Fig. 2.5 b. At such low rates the reaction is controlled by the kinetics of the reaction at the interface and not by diffusion in the solution. This etching solution is therefore found to be perfect to remove micro- and mesoporous silicon selectively from a bulk silicon substrate or to increase the diameter of meso- or macropores in an well-controlled, isotropic manner [Sa3],... 

Masking is required for many micromechanical applications. While Si3N4 is only suitable for a small etching depth because of its significant etch rate in HF, noble metals like gold are sufficient mask materials. In contrast to alkaline etchants, organic materials like certain resists or even some adhesive tapes are well suited to protect the silicon surface in isotropic etchants. 

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