Silicon wafer surface contamination

Vapor-phase decomposition and collection (Figs 4.16 to 4.18) is a standardized method of silicon wafer surface analysis [4.11]. The native oxide on wafer surfaces readily reacts with isothermally distilled HF vapor and forms small droplets on the hydrophobic wafer surface at room temperature [4.66]. These small droplets can be collected with a scanning droplet. The scanned, accumulated droplets finally contain all dissolved contamination in the scanning droplet. It must be dried on a concentrated spot (diameter approximately 150 pm) and measured against the blank droplet residue of the scanning solution [4.67-4.69]. VPD-TXRF has been carefully evaluated against standardized surface analytical methods. The user is advised to use reliable reference materials [4.70-4.72]. 

HIBS is the same as RBS, except that heavy ions are used instead of He++. It is an ion beam analysis tool patented by the Sandia Corporation of the USA, and was developed to enable the measurement of trace levels of surface contamination on silicon wafers. Metal contamination present in starting material is detrimental to devices, since it results in defects which limit wafer yields and impair circuit operation. 

F. Sugimoto and S. Okamura, Adsorption behavior of organic contaminants on a silicon wafer surface, J. Electrochem. Soc. 46, 2725, 1981. 

E. Hsu, H. G. Parks, R. Craigin, S. Tomooka, J. S. Ramberg, and R. K. Lowry, Deposition characteristics of metal contaminants from HF-based process solutions onto silicon wafer surfaces, J. Electrochem. Soc. 139, 3659, 1992. 

Neumann, C., Eichinger, P. (1991) Ultra-trace analysis of metaUic contaminations on silicon wafer surfaces by vapour phase decomposition/total reflection X-ray fluorescence (VPD/TXRF). Spectrochimica Acta Part B Atomic Spectroscopy, 46,1369-1377. 

Fucsko,J.,Tan, S. S-, and Balazs, M. K. (1993). Measurement of trace metallic contaminents on silicon wafer surfaces in native and dielectric silicon oxides by vapor-phase decomposition flow injection inductively coupled plasma-mass spectrometry. J. Electrochem. Soc. 140(4), 1105. 

The silicon wafer surface is routinely analyzed for many purposes. Before a hare wafer is processed, the surface must be checked for impurities. Later on in the process, the wafer surface is analyzed to check the thermal oxide layer or the thin layer after deposition. The wafer surface can also be used to monitor the contamination coming from atmosphere, cleaning baths or tools. 

Fig. 5. Hydrogen depth profile of a deuterated polystyrene PS(D) film deposited on a protonated polystyrene PS(H) film on top of a silicon wafer as obtained by l5N-nuclear reaction analysis ( 5N-NRA). The small hydrogen peak at the surface is due to contamination (probably water) of the surface. The sharp interface between PS(D) and PS(H) is smeared by the experimental resolution (approx. 10 nm at a depth of 80 nm) [57], The solid line is a guide for the eye...

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. 

Metallic contamination is mainly present on the wafer surface as adsorbed ions, oxides, hydroxides and salts. These species are generally instantaneously dissolved in acidic media, even in weak acids such as HF or citric acid. Furthermore HF-based chemistries are able to remove the contamination diffused into silicon oxides or nitrides by lift-off mechanism [23] even faster than a conventional SC2 [1]. 

The initial alloy coatings deposited on to polymers showed a tendency to exhibit pinhole defects most of which have since been attributed to the presence of dust particles. Polymer films have a propensity to accumulate an electrostatic charge that attracts fine particles. As an alternative, interim method to address the issue of defects in the Pd-Cu alloy membrane films, films were deposited onto smooth, silicon wafers. Particulate and other contaminants can be more readily controlled (i.e., minimized) on a silicon surface, in comparison to plastic, and is considerably smoother than plastic. In experiments using magnetron sputtering, we were able to produce defect-free Pd-Cu films on 12 in. diameter silicon and 4 in. square glass plates. A released film, 4 p.m thick, is shown in Fig. 11.4. Films as thin as 0.7 p.m have been produced in this way. 

---