Optical Diagnostics and Imaging - Applied Optics Fundamentals and Device Applications - page 300

6.5 “PostOffice”Profiling
This system overcomes some of the limitations of the CV technique. The
system was originally developed at the British Post Office Research Centre
by Gyles Webster [7] to profile carrier concentration in III-V materials. The
semiconductor sample is loaded onto a PTFE electrochemical cell where a
small area (typically 0.1 or 0.01 cm 2 ) is defined and used to form an electrode.
A DC voltage is applied to produce a bias which causes the formation of a
Schottky barrier at the material surface; simultaneously, continuous electro-
etching takes place under illumination. Capacitance and voltage are peri-
odically monitored, in situ, as the etch progresses, in automatic fashion. A
continuous profile is obtained, and a plot of log (carrier concentration versus
depth) is drawn.

6.6 SpreadingResistanceProfiling
The two-point spreading resistance measurement can be used to map the
resistivity of a structure as a function of depth. This can provide information
concerning the degree of implant damage produced as well as uniformity
and abruptness of transition regions. This is in addition to the standard four-
point probe measurement of resistivity shown in Figure 6.7.
The spreading resistivity measurement is made with two probes contact-
ing a surface of the material. At least one of the probes is so sharp as to
contact the surface with a small circular area of radius r . Applying a volt-
age across the electrodes causes a flow of current, whose value is restricted
by the resistance of the sharp contact. (Often, both contacts are sharp.) This
resistance, called the spreading resistance, is caused by the concentration of
the current density in the vicinity of the sharp contact and is thus localized
at that point of measurement. Its value is given by

ρ
π

R = (6.11)

2 r

where ρ is the resistivity (Ω-cm) in the neighborhood of the probe.
Figure 6.8 shows schematically how the spreading resistance measure-
ment serves to map the carrier concentration in implanted samples. After
exposure the samples are cut into test specimens which are cleaned and then
lapped to provide an angle of about 5.7° (tangent = 0.1). This angle provides
a 10:1 distance magnification in the measurement. The measurements start
with both electrodes positioned on the line where the lapped surface meets
the original surface at an angle of about 174.3°. After providing a value of the