S.A.S. Asif et al.: The influence of oxide and adsorbates on the nanomechanical response of silicon surfaces
phase shift as a function of frequency. The phase shift
plotted here is the measured phase shift due to electronics
(filters) and mechanical damping by the capacitance
plate. The important point to note in Fig. 1 is the large
phase shift at resonance. This implies that when the driv-
ing frequency is close to the resonance frequency in an
experiment, any change in dynamic compliance due to
tip surface interaction will result in a large phase shift.
For example, if the driving frequency is slightly less than
the resonance frequency (110 Hz) during tip-specimen
approach, shown as a dotted line in Fig. 1, an increasing
attractive force (positive force gradient) between the tip
and specimen surface will shift the resonance frequency
to lower frequency.10 This will lead to an increase in
dynamic compliance (or decrease in interaction stiffness)
and decrease in phase (Fig. 1). As the surface comes
closer, there will be repulsive interaction between the tip
and surface, leading to an increase in interaction stiffness
and increase in phase shift. Thus it is possible to observe
the tip-surface interaction (or the pre-contact response)
during approach to contact.
attractive interaction is negative (A-C, AЈ-CЈ) and the
repulsive interaction is positive (C-D, CЈ and beyond).
The tip experienced a maximum attractive force of
140 nN and interaction stiffness of −3 N/m. The maxi-
mum (BЈ) attractive interaction stiffness (the force gra-
dient at point B) is less than the spring stiffness of the
indenter (132 N/m), hence there is no mechanical
instability.
From the force curve measurement the surface en-
ergy of the sample can be calculated using the following
equation11
,
(1)
where F is the attractive force, ␥ is the surface energy,
and R ( 200 nm) is the radius of curvature of the tip
(assuming sphere on flat geometry). The calculated sur-
face energy of 60–80 mJ/m2 is in good agreement with
the surface energy of water (72 mJ/m2).11 This is ex-
pected as the experiments are conducted under ambient
conditions with a relative humidity of 54%.
As the distance between the sample and tip is reduced,
the repulsive interaction increases and the tip comes in
contact with the sample resulting in indentation (C-D).
The maximum load applied to the sample surface during
contact is 300 nN. At point D in Fig. 2, the sample di-
rection is reversed such that the sample moves away from
the indenter. During retraction, hysteresis in the force
and interaction stiffness curve can be seen in both contact
(indentation) and attractive regimes. Although hysteresis
can occur due to experimental artifacts such as piezo
creep and improper lock-in time constants, care was
taken to avoid these artifacts. In the contact regime, for a
maximum load of 300 nN (point D) the unloading is not
reversible and the deformation is not elastic. This type of
behavior was found for loads less than 500 nN and varied
for different samples and location. The variation could be
due to the localized deformation of the contaminant sur-
face layer, which is not uniformly covering the oxide
surface. In the force curve measurement, the hysteresis
during pull-off (attractive regime) is generally attributed
to adhesion.12 From Fig. 2, the calculated surface energy
is close to that of water. This suggests that the hysteresis
in the attractive regime is due to meniscus formation.
B. Force and interaction stiffness curve:
Pre- and apparent contact
Figure 2 shows the typical force and interaction stiff-
ness curves during approach and retraction for an as-
received Si surface with the native oxide layer intact
under ambient conditions (54% RH). The force curve
shown here is similar to the force curve measurement in
AFM.3 The interaction stiffness is a convolution of force
gradient and contact stiffness between the tip and sur-
face. More work is in progress for the complete interpre-
tation of these interaction stiffness curves. In general, the
force curves can be divided into three regimes: pre-
contact, apparent- or intermittent-contact, and elastic or
elasto-plastic contact regimes as shown in Fig. 2. The
C. Effect of humidity and surface chemistry
1. Hydrophilic surface
Hydrophilic Si surfaces were exposed to increasing
and then decreasing humidities between <2% and 80%.
Although many humidity experiments were conducted,
only the results for exposures from low to high to low
humidities are presented here.
Figure 3(a) shows the interaction stiffness curve dur-
ing approach and retraction for 2% RH (starting condi-
tion). As the sample approaches the tip, the tip is
FIG. 2. (ࡗ
) Force and (+) interaction stiffness curves during (←)
approach and (→) retraction.
548
J. Mater. Res., Vol. 15, No. 2, Feb 2000
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