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5990-4853EN Quantitative Mechanical Measurements at the Nano-Scale Using the DCMII - Application Note c20141022 [4]


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Keysight Technologies
Quantitative Mechanical Measurements
at the Nano-Scale Using the DCM II
Application Note

Introduction                                 respectively. The range and resolution
                                             in force are 30mN and 3nN, respectively.
Feature miniaturization, especially in the   Because the DCM II has a resonant fre-
electronics industry, demands knowledge      quency of about 120Hz, measurements
of mechanical properties on the scale        of force and displacement are insensitive
of nanometers. Instrumented indenta-         to environmental noise which occurs at
tion facilitates such testing, because the   lower frequencies. The DCM II can be
area of the contact impression does not      used in combination with a variety of
have to be measured visually, but can        indenter tips including Berkovich,
be inferred solely from the relationship     cube-corner, and sphero-conical.
between applied force and consequen-         Changing from one tip to another
tial penetration of the indenter into the    takes just a few minutes.                    Figure 1. The Keysight DCM II.
testing surface 1. Instrumented indenta-
tion has been used since the 1980's to
make measurements at the sub-micron          Experimental Method
scale, but recent developments allow         Prior to testing, the shape of the diamond
quantitative determination of mechani-       indenter was "calibrated" by performing
cal properties using indents of just a few   55 indents on a reference material, fused
nanometers deep. This article addresses      silica. Then seven different materials,
special considerations for such testing,     including the fused silica, were tested
and reports results for seven different      using the force-time algorithm shown in
materials tested with the DCM II.            Figure 2. All materials were tested to the
                                             same peak force of 50N. The materials
The DCM II, shown in Figure 1, is an         tested were polycarbonate, Pyrex, fused
optional high-resolution actuating           silica, single-crystal aluminum, silicon     Figure 2. Force-time algorithm for
transducer for the Keysight Technologies,    (111), nickel, and sapphire. Because         indentation tests.
Inc. G200 Nano Indenter. The DCM II          these materials are of varying hardness,
may be used in addition to or instead of     the indentation depths resulting from
the standard indentation head. If both       the applied force of 50N varied. The
heads are included on a single system,       deepest indents of about 100nm were
transition from one head to the other is     achieved on the polycarbonate, while
entirely software controlled; the user       the shallowest indents of less than 7nm
doesn't have to make any adjustments         were achieved on the sapphire. Fifteen
to hardware, controllers, or calibrations.   indents were performed on each sample
The range and resolution in displace-        using the DCM II itted with a diamond
ment (travel) are 70m and 0.0002nm,          Berkovich indenter. Force and displace-
02 | Keysight | Quantitative Mechanical Measurements at the Nano-Scale Using the DCM II - Application Note


ment measurements were acquired at                        Results and Discussion                                relationship between the distance from the
a rate of 12.5kHz, averaged in a buffer                                                                         apex of the diamond, d, and the cross-sec-
and reported at a rate of 100Hz. Data                     Calibration                                           tional area at that distance, A, is called the
were analyzed according to the method                     The process for determining the precise               "area function". For a perfect Berkovich
prescribed by an international standard                   shape of the indenter is automated within             indenter, the area function is
for instrumented indentation testing, ISO                 the NanoSuite software. The following dis-
14577 2, which in turn draws heavily upon                 cussion should not intimidate new users;
the landmark article by Warren Oliver                                                                           However, when making nanometer-scale
                                                          it is only intended to explain what is done
and George Pharr 1. Average modulus and                                                                         indents, imperfections at the apex of the
                                                          and why.
standard deviation were computed using                                                                          diamond demand that the area function
all 15 tests. If a particular test yielded a                                                                    be determined more precisely. We do this
                                                          The data used to "calibrate" the shape of
measure of modulus that was different                                                                           by calculating contact depth and contact
                                                          the tip are shown in Figure 3 in the form of
from the average value by more than two                                                                         area, assuming a value for the reduced
                                                          stiffness squared divided by applied force
standard deviations, the result for that test                                                                   modulus of the material. Therefore, each
                                                          (S2/P) as a function of displacement into
was discarded, and the remaining results                                                                        indent on fused silica yields an ordered
                                                          the test surface, h. We begin by looking
were averaged again.                                                                                            pair (hc, A) with contact depth (hc) calcu-
                                                          at the data in this way, because S2/P is
                                                                                                                lated as
                                                          directly proportional to reduced modulus
The main focus of this work was the deter-                squared divided by hardness (Er2/H), but is
mination of quantitative mechanical prop-                                                                                                    ,
                                                          independent of contact area (A):
erties at the scale of nanometers. How-                                                                         and A calculated as
ever, when used in combination with the
NanoVision option, the DCM II becomes a                                                                                                          .
proilometer, capable of generating topo-
logical images with excellent dimensional
accuracy. In this work, a grid for verifying              If the machine is working well, then we               Figure 4 shows these (hc, A) data. To
the dimensional accuracy of atomic-force                  expect S2/P to be constant with increas-              determine the reined area function, we
microscopes was scanned. The grid has                     ing penetration, having a value of 700GPa             curve it this data to the functional form
periodic steps; the steps have a height of                6 50GPa. (As the displacement into the                suggested by Oliver and Pharr 1:
19nm and a period of 3 microns. A square                  surface decreases to zero, the contact
area of 6.5m on a side was scanned using                  becomes increasingly Hertzian, and we
a scanning force of 1.0N; the result-                     expect S2/P to increase exponentially, be-
ing scan was used to select a site for an                 cause for a Hertzian contact, the param-              If we only use the irst two terms of this
indentation test. Following this test, the                eter S2/P goes as h-1/2.) Since the trace of          expression to it the data, the best-it
same area was scanned again to reveal the                 S2/P meets our expectations, we proceed               coeficient of the second term can be used
residual indentation impression.                          to use this data to determine the pre-                to determine the radius of the tip. By this
                                                          cise shape of the diamond indenter. The               analysis, the radius of the diamond tip was




Figure 3. Stiffness squared divided by force (S2 /P) vs. displacement                  Figure 4. Fused silica data (hc, A) together with 5-term area function.
(h). For fully plastic contacts, acceptable range for fused silica is
700GPa 6 50GPa.
03 | Keysight | Quantitative Mechanical Measurements at the Nano-Scale Using the DCM II - Application Note




Figure 5. Young's modulus measured using DCM II at 50 N. Error bars                     Figure 6. Three consecutive indents on sapphire (E = 400 GPa). Loading
represent 1son n/15 measurements. Solid line indicates unity; i.e. per-                 and unloading curves coincide, indicating that the indents are completely
fect agreement between measured and nominal.                                            elastic.


determined to be 52nm. However, two                       from crystalline elastic constants for the             to the test method that was employed. As
terms rarely provide a it that is suficiently             direction normal to the testing surface                a polymer, polycarbonate manifests some
good for making nanometer-scale mea-                      [4-7]. For polycarbonate, the nominal value            viscoelasticity. Thus, obtaining the contact
surements. The it shown in Figure 4 is a                  is that reported on the website "engineer-             stiffness from the slope of the unloading
ive-term it. For hc < 100nm, the maxi-                    ingtoolbox.com".                                       curve likely results in a stiffness that is too
mum difference between this it and the                                                                           high, because the indenter continues to
data is about 4%.                                         Overall, the agreement between nominal                 move into the material even as the force
                                                          and measured values is excellent, espe-                is reduced. It would be better to measure
Young's modulus                                           cially given the scale of testing. For the             the modulus of this material with our
The results for Young's modulus are sum-                  Pyrex, fused silica, nickel, and sapphire,             continuous stiffness measurement option
marized in Table 1. The fourth column                     measured values for Young's modulus were               (CSM). For the single-crystal aluminum,
of this table gives the number of tests                   within 1 standard deviation of the nomi-               the measured value of 59.866.9 GPa is low
(out of 15) that were used in the calcula-                nal value. Two sets of 15 indents were                 relative to the nominal value of 70.0 GPa.
tion of results. On all but three materials,              performed on the fused silica, because it              However, we have no reason to question
all 15 tests were used. In Figure 5, the                  is standard practice to test this reference            the accuracy of the measured value. One
measured values for Young's modulus                       material before and after the materials of             possible explanation for the discrep-
are plotted against nominal values, with                  interest. The average maximum inden-                   ancy between the measured and nominal
the ideal plotted as a solid line. For fused              tation depths achieved for both sets of                values may be a surface oxide layer. For
silica and Pyrex, the nominal values are                  indents on fused silica agreed to 0.005nm!             the (111) silicon, the measured value of
what we measured sonically in-house 3.                                                                           186.3611.6 GPa was slightly high, rela-
For sapphire, aluminum, silicon (111), and                For polycarbonate, the measured value of               tive to the nominal value of 168.9 GPa. We
sapphire, the nominal values for Young's                  3.3560.08 GPa is high relative to the nomi-            chose (111) silicon for this testing, because
modulus are theoretical values calculated                 nal value of 2.6 GPa. This is probably due             it has the same Young's modulus perpen-


 Material                 Poisson's ratio        hmax           n valid tests         EIT             s(E IT )   Nominal E        Ref. for nominal E

                          



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