Silicon Surface Conditions and Cleaning Procedures

The surface condition of a silicon crystal depends on the way the surface was prepared. Only a silicon crystal that is cleaved in ultra high vacuum (UHV) exhibits a surface free of other elements. However, on an atomistic scale this surface does not look like the surface of a diamond lattice as we might expect from macroscopic models. If such simple surfaces existed, each surface silicon atom would carry one or two free bonds. This high density of free bonds corresponds to a high surface energy and the surface relaxes to a thermodynamically more favorable state. Therefore, the surface of a real silicon crystal is either free of other elements but reconstructed, or a perfect crystal plane but passivated with other elements. The first case can be studied for silicon crystals cleaved in UHV [Sc4], while unreconstructed silicon (100) [Pi2, Ar5, Th9] or (111) [Hi9, Ha2, Bi5] surfaces have so far only been reported for a termination of surface bonds by hydrogen. 

Under ambient atmospheric conditions a native oxide is formed on cleaved Si surfaces. The properties of native and chemical oxides are discussed in Section 5.2. The well-defined surface conditions produced by wet processes like rinsing and cleaning procedures will be discussed below. 

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. 

Different chemical treatments for silicon can be categorized depending on the condition of the Si surface after the clean. The two basic surface conditions for a silicon surface are hydrophobic and hydrophilic. 

A hydrophobic Si surface condition is also observed after alkaline treatments, like CMP at a pH of about 11 or after etching in alkaline solutions, as shown in Fig. 2.1b. Hydrophobic Si surfaces are very susceptible to hydrocarbon contamination, for example from the ambient atmosphere or from hydrocarbon films floating on a liquid. To avoid the latter case, water rinses and HF dips are often performed in an overflow wet bench. 

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