Post-CMP cleaning

To understand integration of post-CMP cleaners into CMP machines it is important to understand the seven basic rules that apply to post-CMP cleaning [57]. For an extended discussion of post-CMP cleaning, refer to Chapter 7. The Gotkis rules are  

Never allow slurry to dry on wafers. Strong bonds are formed when slurry dries, making the removal of the slurry nearly impossible. The bonds are formed between the abrasive, the abraded pad, the abraded material, and the wafer. The bond strength is such that even supplementary cleaning steps are insufficient to remove the slurry. 

Include a precleaning andjor buffing step as an integral part of the polish recipe. 

Rinse and buff immediately after the bulk CMP step. Rinsing removes the residual slurry and the polishing by-products including abraded film from the wafer, abraded polish pad material, and agglomerated slurry. Buffing removes most of the particles that are adhering to the surface as well as many mechanically embedded particles. 

Decreasing particle size means increased effort required to remove it. This rule arises from the smaller interaction cross sections for collision and momentum transfer. Furthermore, the electrostatic forces are stronger for smaller particles, and they diminish in proportion to 1/r as opposed to 1/r. Both of these factors lead to redeposition being a major source of small particles. Since mechanical action requires increasing amounts of work, chemical dissolution is more effective at removing small particles than is mechanical action. 

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Use any and all options for post-CMP cleaning. Keep particles from adhering more strongly than they already are, and remove them as quickly as possible. Once they are removed they must be transported away from the wafer surface as quickly and efficiently as possible to minimize redeposition. 

These rules basically state that post-CMP cleaning must be done as part of the CMP process, and it must be done as quickly as possible after beginning the CMP process. Rule 7 is driving the design of cleaners comprised of... 

Fig. 14. Cross-sectional views of post-CMP cleaning using double-sided roller brushes. The two brushes roll in opposite directions to keep the wafer pressed against two rollers. The rollers are used to rotate the wafer and are withdrawn to allow the wafer to pass once the wafer is clean (a) a single-brush configuration and (b) a multiple-brush configuration.

Fig. 15, Brushes used for post-CMP cleaning (a) roller brushes (Merocel Corp. web site, reprinted with permission) and (b) puck brushes (Rippey Corp. web site, reprinted with...

A. Jha, A. Gupta, S. Zhu, A. Blazev, M. Kason, S. Basak, Post-CMP Clean on integrated Dry-in/Dry-out Auriga C Tool, presented at the 3rd Annual Workshop on Chemical Mechanical Polishing, Lake Placid, NY, Aug. 16-19, 1998. 

M. Olesen, C. Franklin, A Single Wafer Non-Contact Post CMP Cleaning Technology, Third International Chemical-Mechanical Polish (CMP) for VLSI Multilevel Interconnection Conference, pp. 375-378, Feb. 19-20, 1998. 

Fumed silica and colloidal slurry with and without filtration have been evaluated. All slurries are KOH-based. All slurries have pH values greater than 10. A same polisher was used, and a same pad was used. However, to separate the effect from post-CMP cleaning, neither scrubbing nor chemistry was applied. The oxide wafers were only water-rinsed after polishing. The results are shown in Fig. 11. The slurry with fumed silica left more particles compared to that with colloidal silica. 

Filtration can reduce the particles adhered to the wafer, but only slightly. Filtration will, however, reduce scratch defects. Slurry particles can be more effectively cleaned if more elaborate post-CMP clean chemistry is used [8]. 

More detailed information is found in Chapter 7 (post-CMP clean) of this book. 

Aluminum is a strong reducing agent ( oai/af+ = —1-66 V), that is, it can be very easily oxidized in cleaning solutions. This behavior could represent an additional limitation of the use of aluminum in the case of damascene structures for which post-CMP cleaning is required. Nevertheless aluminum oxide (AljOj) constitutes a very well known passivating layer but is only stable in relatively neutral pH (4-8.5). 

Fig. 6. Schematic illustration of a photoassisted corrosion phenomenon during the post CMP cleaning of damascene interconnections. At the electrode connected to the p-side of the junction, the metal is corroded by the oxidation reaction M - M" + n.e , while the produced soluble M"" species diffuse to the other electrode where the opposite reaction can occur.

Scanning electron microscope (SEM) picture after a Cu post-CMP cleaning... 

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]. 

In conclusion, note that an acid step is always required at the end of the post-CMP cleaning process to remove the residual metallic contamination. 

Examples of Post-CMP Cleanings Performed by Scrubbing or by Wet Processes... 

Scrubber and wet benches are sometimes used one after the other in the same post-CMP cleaning, but this probably does not represent the most suitable solution. In the near future, the more rational way will be to integrate a scrubber or megasonic bath in the CMP tool to avoid handling wet wafers. 

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