Etching of silicon

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. 

On the atomic level, etching is composed of several steps diflfiision of the etch molecules to the surface, adsorption to the surface, subsequent reaction with the surface and, finally, removal of the reaction products. The third step, that of reaction between the etchant and the surface, is of considerable interest to the understanding of surface reactions on an atomic scale. In recent years, STM has given considerable insight into the nature of etching reactions at surfaces. The following discussion will focus on the etching of silicon surfaces [28]. 

As an example, we look at tire etching of silicon in a CF plasma in more detail. Flat Si wafers are typically etched using quasi-one-dimensional homogeneous capacitively or inductively coupled RF-plasmas. The important process in tire bulk plasma is tire fonnation of fluorine atoms in collisions of CF molecules witli tire plasma electrons... 

Houle F A 1987 Dynamics of SiF, desorption during etching of silicon by XeF2 J. Chem Phys. 87 1866-72... 

Mizutani T, Dale C J, Chu W K and Mayer T M 1985 Surface modification in plasma-assisted etching of silicon Nucl. Instrum. Methods B 7 825-30... 

Yarmoff J A and McFeely F R 1988 Effect of sample doping level during etching of silicon by fluorine atoms Phys. Rev. B 38 2057-62... 

Barone M E and Graves D B 1995 Molecular dynamics simulations of direct reactive ion etching of silicon by fluorine and chlorine J. Appi. Phys. 78 6604-15... 

Hwang G S, Anderson C M, Gordon M J, Moore T A, Minton T K and Glapis K P 1996 Gas-surface dynamics and profile evolution during etching of silicon Phys. Rev. Lett. 77 3049-51... 

Houle F A 1989 Photochemical etching of silicon the Influence of photogenerated charge carriers Phys. Rev. B 39 10 120-32... 

Veprek S and Sarott F A 1982 Electron-impact-induced anisotropic etching of silicon by hydrogen Plasma Chem. Plasma Proc. 2 233-46... 

FIGURE 11.11 Etching of silicon wafer to create interconnections (a) desired profile achievable by ion bombardment (b) profile obtained by chemical etching. 

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

Plasma etching, of silicon, 22 492 Plasma FPDs, 22 259 Plasma fractionation, 12 128-159 economic aspects of, 12 147-150 health, safety, and environmental factors related to, 12 153... 

To treat all the different wet processes for silicon wafers developed in the last five decades exhaustively would make up a book of its own. However, a few basic aspects are important, because chemical etching of silicon is closely related to the electrochemical behavior of Si electrodes, especially to the OCP condition. A brief overview of the most common etching and cleaning solutions will be given, with emphasis on the electrochemical aspects. 

The high selectivity of wet etchants for different materials, e.g. Al, Si, SiOz and Si3N4, is indispensable in semiconductor manufacturing today. The combination of photolithographic patterning and anisotropic as well as isotropic etching of silicon led to a multitude of applications in the fabrication of microelectromechanical systems (MEMS). 

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. 

Figure 6. Example of ion-assisted gas-surface chemistry in the etching of silicon with Xep2. The Xep2 flow is 2 x 10 moles/sec and the argon energy and current are 450 eV and 2.5 ijlA, respectively. (Reproduced with permission from Ref. 31J...

C.I. Silicon and Poly silicon. The isotropic etching of silicon (Si) and polycrystalline silicon (Poly-Si) by atomic fluorine (F) is probably the most completely understood of all etch processes, particularly for the cases in which F atoms are produced in discharges of F2 (36) and CF4/O2 (42). Fluorine atoms etch (100) Si at a rate (A/min) given by (36) ... 

In reactive etching of silicon a patterned film is selectively etched by reacting with a gas such as chlorine... 

In the positive branch of the i/V graph, anodic dissolution process will remove material from silicon crystals. The conditions for optimal etching of silicon have been extensively explored for micromachining or surface polishing in the fabrication of electronic devices. Most generally, the etch rate of silicon in HE solutions is isotropic among the various crystalKne orientations. The etch rate of silicon at room temperature at the open-circuit potential (OCP) is very low, on the order of 10 nm s , which is equivalent to 100 nA cm , in aqueous HE solutions. 

Recent investigations in the field of plasma etching have almost universally found that surfaces subjected to ion bombardment (e.g., the target surface in Fig. 1) react (i.e., etch) much more rapidly than those which are held near plasma potential. Examples of this type have been reported by Hosokawa et al. for the halocarbon etching of silicon and by Holland and Ojha for the oxygen plasma-etching of carbon. Several systems exhibiting this type of behavior have also been observed in our laboratory . 

Similar conclusions can be drawn from the data shown in Fig. 27 where the partial pressure of SiF4 is monitored mass spectrometrically as an Si sample is rotated into a jet of XeFj gas with and without ion bombardment. The ion-enhanced etching of silicon is indicated by the much larger evolution of SiF4 when the sample is exposed... 

Electrolytic (anodic) etching of silicon occurs in HF solutions (5). Using concentrated HF, the silicon is dissolved in the divalent state ... 

Modern computer chips are made by chemical etching of silicon wafers, using "masks" to produce the tiny circuitry. 

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