Wafer Cleaning Techniques

Wafer Cleaning Techniques Wafer cleaning affects postpolish particle and contamination levels, and therefore affects yield and reliability. Development of a CMP process must include the post-polish clean sequence to optimize yield and reliability. 

J. A. Amick, Cleanliness and the cleaning of silicon wafers. Solid State Technol. 33, November 1976. D. Tolliver, LSI wafer cleaning techniques, Solid State Technol. 33, November 1975. 

The old scrubber technique is in fact very attractive for post-CMP cleaning as the same mechanical effect is active for all the materials present at the surface (insulators, metal barriers). Doubled-sided scrubbers for cleaning the frontside and the backside of the wafer and lateral brushes to take care of the wafer side are now proposed on the market. Furthermore, the implementation of megasonic sprays in the scrubber can sometimes help for difficult cases. The major limitation is in terms of cost of ownership (COO) as a single-wafer process is involved. Indeed according to Witt et al. [17] who used the standard SEMATECH COO model, brush cleaning is more than three times more expensive than wet cleaning, which was confirmed by other economic studies [18]. 

The trace metal left on the surface of the wafer is mostly due to the slurry. The quality of the chemicals and abrasive used in the slurry is never perfect. An average oxide CMP slurry has trace metals in the range of 1-10 ppm per element. There are very few ultrahigh-purity slurries with trace metals below 10 ppb (actually under the metrology detection limit). What matters most is not the amount of trace metals in the slurry but the amount left on the surface after post-CMP cleaning. The most effective technique to remove the trace metals from the wafer surface is to perform a very thin etching of the oxide surface using dilute HF, for example. 

In all of the above discussion, the focus has been entirely on the local conditions where the point on the wafer, an abrasive particle and the pad meet. In practice, the system is substantially more complicated, with much effort directed toward producing a clean, uniform and manufacturable process. Thus the polishing machine usually produces a complex wafer motion, including wafer rotation, which averages polish conditions which vary with position across the wafer. In addition, the pad and the asperities created also have position dependence for which complex wafer paths and conditioning procedures are used. These useful and necessary techniques enhance the performance of the technology but also obscure the behavior of individual polishing events. 

A silicon wafer was cleaned with ethanol and coated with a thin layer (about 0.5 pm) of 2.8e (3% solution in THF) by the application of a spin-coater. Irradiation through a mask (or any other device which provides irradiated and unirradiated zones) with UV light. X-rays or electron beams generated a pattern which was developed by a short immersion into THF. The unchanged polymer was dissolved and the irradiated parts with the crosslinked polymer remained. (Principally, this imaging technique with a negative resist... 

Extended defects range from well characterized dislocations to grain boimdaries, interfaces, stacking faults, etch pits, D-defects, misfit dislocations (common in epitaxial growth), blisters induced by H or He implantation etc. Microscopic studies of such defects are veiy difficult, and crystal growers use years of experience and trial-and-error techniques to avoid or control them. Some extended defects can change in impredictable ways upon heat treatments. Others become gettering centres for transition metals, a phenomenon which can be desirable or not, but is always difficult to control. Extended defects are sometimes cleverly used. For example, the smart-cut process relies on the controlled implantation of H followed by heat treatments to create blisters. This allows a thin layer of clean material to be lifted from a bulk wafer [26]. 

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