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Improved Method for Dopant Profiling by Force Microscopy

IP.com Disclosure Number: IPCOM000106352D
Original Publication Date: 1993-Oct-01
Included in the Prior Art Database: 2005-Mar-21
Document File: 2 page(s) / 98K

Publishing Venue

IBM

Related People

Abraham, DW: AUTHOR [+2]

Abstract

Disclosed is a new method for measuring dopant concentration by force microscopy which provides greater rejection of non-dopant signals and allows auto-calibration of the measured signal.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 52% of the total text.

Improved Method for Dopant Profiling by Force Microscopy

      Disclosed is a new method for measuring dopant concentration by
force microscopy which provides greater rejection of non-dopant
signals and allows auto-calibration of the measured signal.

      Dopant profiling has been demonstrated using the capacitive
force between a tip and samples to excite mechanical motion in the
cantilever supporting the tip.   The amplitude of excitation is due
to the capacitance, which in turn is in part determined by the
depletion of the sample due to an applied bias voltage between tip
and sample.
 Hence, measuring the induced motion amounts to a measure of the
depletion capacitance and thereby a measure of the dopant density in
the sample under the tip.  Successful profiling of dopant gratings
and FET structures has been accomplished in this manner [*].

      Unfortunately, several technical problems with this scheme make
both qualitative and quantitative  measurements difficult.
Specifically, a large background capacitance signal can make
detection of the dopant contribution difficult to spot.  Variation in
this background signal can occur if oxide of varying thickness is
present, for example, which can be confused with variation due to
dopant density.  Also, because the tip and sample change with each
measurement, and because different control electronics adjustment can
radically affect the measured capacitive motion, quantitative
measurements of dopant density are difficult.  A method is presented
whereby these problems can be alleviated.

      Conventional operation consists of operating the attractive
mode force microscope in the usual topographic mode, and in addition
providing an AC bias to the tip at a frequency omega sub1.  As shown
below, motion at 3 omega sub1 is due solely to depletion capacitance.
By measuring both 3 omega sub1 term and the 2 omega sub2 term, where
omega sub2 is a second bias frequency, the depletion capacitance can
be isolated from various parameters such as tip-sample spacing and
the like mentioned above.

      First consider the response of a tip biased with DC plus AC at
omega sub1, positioned over a metallic or dielectric sample.  In this
case, the induced motion is described by the following equation:

  z(t) eqsym 1 over <2 k>
  <<vardelta C> over <vardelta z> (V sub0 plus V sub1 e
  sup<i omega sub1 t>) sup2>reqno 1
which consists of oscillations at omega sub1 ' and ' 2 omega sub1 The
capacitive term is a constant (albeit of unknown value), and
therefore the tip motion occurs only at omega sub1 and 2 omega sub1.

      Next, consider the case in which thetip is placed over a
lightly doped semiconductor.  In this case, the tip AC voltage causes
the capacitance to change by depletin...