Contamination Specifications, an Overall Perspective

Specifications on residual contamination have played a determining role in the development of cleaning processes. The intention of this contribution is to reflect on contamination specifications and show a potential route of how to proceed. One major platform for specifications is the ITRS [1], where a set of different contamination specifications are posted.

Currently, particle specifications are as follows. For 20 nm half pitch technology nodes the critical particle diameter is 10 nm. It should be noticed that a Si sphere with a diameter of 10 nm consist of only 25000 Si atoms.

The maximum surface concentration , is derived from the assumption that it should correspond to a yield number of 99.9 for an active area of 0.1 cm2. If one assumes that each particle with a size equal or larger than the critical value leads to one defect. This should leads to the following specification: particle density < 0.01/cm2. This specification is represented in Fig. 1 (as the a corner of the red-area). It should be noted that both specifications for diameter and particle density are (orthogonal in the sense) obtained in totally independent ways.

When specifying particles, one should realise that a particular size (and concentration) may only be a temporary embodiment of an amount of contamination.. Small particles can agglomerate to larger ones or alternatively large particles (e.g. flakes) can fragment into many smaller pieces. Through various processes.

If one assumes the total volume is maintained, it implies that contamination could evolve along the slanted lines (isochores) as indicated in the diagram of Fig. 1 This would mean that contamination situations that are within specification (green) could evolve to out-of specification conditions (red)even when the total amount (volume) of contamination remains constant.

Another specification that is used is the surface concentration of molecular dispersed O and C on the surface of 1e12/cm2. This specification has been added to the diagram, by using the Van der Waals diameter of the atoms for the horizontal coordinate.

One can see that the total contamination volume represented by the O- and C-specification is several orders of magnitude higher than that for the small particles. In other terms the amount of contamination allowed in particles is much smaller than the amount that can be tolerated as molecular contaminant on the surface.

Finally, fig 2 shows a possible alternative specification of the total amount of impurities on the surface (irrespective of its morphology).

References:

[1] ITRS 2013 edition, http://www.itrs.net/.