Page 162 - Sami Franssila Introduction to Microfabrication

Wafer Cleaning and Surface Preparation 141

– brush scrubbing 4. Calculate the daily (24 h) chemical and DI-water con-
– jet scrubbing sumption for an SPM-DIW-rinse-RCA1-DIWrinse-
– ultrasonic/megasonic. DHF-DIW rinse-RCA2-DIWrinse1-DIWrinse2 clean-
ing cycle when a tank for 25 wafers of 200 mm
In brush scrubbing, nylon or PVA brushes physically diameter is used. Assume a 4 h changing interval for
touch the wafer and brush away the particles. This RCA-cleans and 24 h bath life for SPM and DHF.
is effective especially when lots of particles or large 5. What happens to particle contamination in (a) wet
particles have been deposited on the wafer. Therefore, etching and (b) plasma etching?
brush scrubbing is often done after wafer scribing or 6. If we had an Olympic swimming pool full of
polishing steps. UPW, how many droplets of sweat can be dissolved
In jet scrubbing, high-pressure water is sprayed on the before Na + and Cl − exceed the specification level
wafer. The removal mechanism is similar to brush scrub- of 0.1 ppb?
bing but no physical contact with the wafer is needed.
Increasing pressure improves cleaning efficiency, but
electrostatic charging can damage thin films. REFERENCES AND RELATED READINGS
In sonic cleaning, shock waves supply localized
E. Grannemann: Film interface control in integrated processing
sound energy that helps in particle removal. Ultrasonic
systems, J. Vac. Sci. Technol., 12 (1994), 2741.
agitation (20–40 kHz) is also beneficial in wet removal T. Hattori (ed.): Ultraclean Surface Processing of Silicon
of photoresist. However, cavitation may damage the Wafers, Springer (1998).
wafers. Above 1 MHz, this is not an issue, and the W. Kern: The evolution of silicon wafer cleaning technology,
method is termed ‘megasonics’. Megasonic agitation J. Electrochem. Soc., 137 (1990), 1887.
improves particle removal even for very small particles, W. Kern (ed.): Handbook of Semiconductor Wafer Cleaning
<100 nm size. Technology, Noyes Publications (1993).
H. Kitajima & Y. Shiramizu: Requirements for contamination
control in the gigabit era, IEEE TSM, 10 (1997), 267.
12.8 EXERCISES S. Middleman & A.K. Hochberg: Process Engineering Anal-
ysis in Semiconductor Device Fabrication, McGraw-Hill
10
1. Translate surface iron contamination of 10 cm −2 (1993).
into a number of monolayers! T. Ohmi, et al.: Dependence of thin-oxide film quality on
2. If there is one monolayer coverage of organic surface microroughness, IEEE TED, 39 (1992), 537.
contamination on the wafer, how much is that H. Okorn-Schmidt: Characterization of silicon surface prepa-
2
counted as carbon atoms/cm ? ration processes for advanced gate dielectrics, IBM J. Res.
3. Area of an NMOS transistor with 1 µm minimum Dev., 43 (1999), 351.
linewidth is about the same as that of a red blood D.K. Schroder: Semiconductor Material and Device Charac-
nd
terization, 2
ed., John Wiley & Sons (1998).
cell, 5 × 8 µm. The source/drain areas are doped to S.M. Sze & J.C. Irvin: Resistivity, mobility and impurity levels
very high concentration, but the number of dopant in GaAs, Ge and Si at 300 K, Solid-State Electron., 11
◦
atoms is only 10 9 because of small area. What (1964), 599.
concentration will result if the blood cell decomposes F. Zhang, et al.: The removal of deformed submicron particles
on the transistor, releasing its phosphorus atoms and from silicon wafers by spin rinse and megasonics, J.
doping the silicon? Electron. Mater., 29 (2000), 199.