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WHITE
PA P E R
Integrating high frequency capacitance measurement
for monitoring process variation of equivalent oxide thickness
of ultra-thin gate dielectrics
Yuegang Zhao
Keithley Instruments, Inc.
Introduction
As CMOS transistors have gotten smaller and smaller, so has the
thickness of their gate dielectrics. This presents a great challenge to traditional
capacitance measurement used to monitor dielectric thickness for process
variation. First, the relationship between the capacitance value in the inversion
or accumulation region of the capacitance-voltage (C-V) curve to the gate
oxide thickness is no longer simple. It's necessary to apply new models,
including quantum mechanics and polysilicon depletion effects, to determine
oxide thickness accurately from the C-V curve [1, 2]. Second, gate leakage
increases exponentially as thickness decreases due to tunneling of carriers
through the ultra-thin gate [3]. The gate capacitor becomes very lossy due to
high leakage, and the gate capacitance measurement shows roll-off effects in
both the inversion and accumulation regions of the C-V curve [4]. These roll-
off effects make it impossible for engineers to extract COX directly and use it
to monitor thickness variations in production. The roll-off behavior is also
dependant on the DC leakage of the gate. Therefore, even for two gate
dielectrics with the same physical thickness and area, the lower quality one
with higher gate leakage will show the greater roll-off in the C-V curve, which
makes it more difficult to monitor thickness variations.
Some roll-off effects in the C-V curve are device related [5, 6]. At high
frequency the two main factors are channel resistance and contact resistance.
These effects could be modeled by a different equivalent circuit model and
could be reduced by a new device layout. On the other hand, some of the roll-
offs in C-V measurement are related to non-optimized setups, including
cabling, connectors, and probe station setup [7]. The first part of the paper
provides a comprehensive overview of difficulties and precautions on C-V
measurement on ultra-thin gate dielectrics using LCR meters at high
frequencies (1