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Posted by Roger Brennan ( on December 18, 2009 at 19:19:09:

In Reply to: SIMS VS. SRP posted by Yunfei on December 16, 2009 at 07:10:09:

: Thank you very much for the former answer. I have another question. For the emitter profile, it is said SIMS give you the atom concentraion distribution along the depth, while SRP give you the electrically active concentraion. can you explain the difference between these two concentration. I will appreciate for directly communication by your email.

Spreading resistance profiling (SRP) determines resistivity and net carrier concentration is calculated from that. Carriers are the result of certain dopants (boron, phosphorus, antimony, and arsenic among others) being placed into the silicon lattice where silicon atoms used to be. The carriers are either electrons or "holes", the absence of an electron. The distribution of these carriers is what makes the electron device work.

SIMS determines the concentration of dopant atoms present. It will not tell you if the dopant atoms are "electrically active" (producing carriers). If you have a heavily doped phosphorus emitter, the carrier concentration at the surface is considerably lower than the total phosphorus concentration. Phosphorus atoms not electrically active can be from one perspective considered so much "dead weight". From another perspective, they may contribute the "gettering" of the device (removal of certain contaminates).

If at a given location, you have the same concentration of boron and phosphorus atoms (all electrically active), you will have zero carriers. The SRP will show this as a cusp in the resistivity data and a low point in the carrier concentration.

General rules of thumb:

SRP is less expensive.

SIMS has better spatial resolution for ultra-shallow structures (typically for structures less than 80nm deep). Still, there is a need to know how much of the concentration is electrically active. This is usually determined by a resistivity measurement with SRP being the premier depth profiling technique.

SRP has about 9 decades of dynamic range and this is especially useful for low concentrations.

For extremely deep structures, SRP is unquestionably the preferred choice. For extremely shallow structures, both SIMS and SRP should be tried and compared.

SRP is quite good for profiling silicon, somewhat less for germanium, and useful in certain cases for some compound semiconductors. SIMS is not limited to semiconductor applications

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