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Submicrometer modification of polymer surfaces with a surface force microscope
X. Jin and W. N. Unertl
Citation: Applied Physics Letters 61, 657 (1992); doi: 10.1063/1.107813
View online: http://dx.doi.org/10.1063/1.107813
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/61/6?ver=pdfcov
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Submicrometer modification
force microscope

of polymer surfaces with a surface

X. Jinal and W . N . Unertlb)
Laboratory for Surface Science and Technology, Sawyer Research Center, University of Maine, Orono,
Maine 04469

(Received 18 February 1992; accepted for publication 22 May 1992)
We have used the sharp tip of a surface force microscope to make modifications with
submicrometer dimensions on polymer surfaces. In this letter we show three examples: scribed
grooves with widths less than 120 nm, raised areas with heights up to 1 nm above the original
surface, and pits with depths of 6 nm. We also discuss possible sources of contrast in surface
force microscope images that are not due to height variations in the surface topography. Because
the surface force microscope can be used for both conducting and nonconducting materials, it
has an advantage over the higher resolution scanning tunneling microscope.
Controlled modifications of surface morphology are
important for fabrication of micromechanical and electronic devices with nanometer scale dimensions. Nanometer scale switches and lines have been fabricated using the
scanning tunneling microscope ( STM) . ’ However, since
the STM requires a current of electrons, its use is limited to
materials with electrical conductivity. This letter shows
that nonconducting polymer surfaces can be modified with
the tip of a scanning surface force microscope (SFM) . A
variety of structures, including scribed lines, pits, and
raised areas, have been machined on several types of polyimide. Polyimides were used because of their importance in
electronic packaging2*3 and liquid crystal displays.4
Figure 1 is a drawing of the SFM used.’ The tips are
S&N4 pyramids with apex radii less than 40 nm microfabricated on 0.2 mm long triangular cantilevers with force
constants of 0.37 N/m. The axis of the pyramidal tip is
tilted about 15” with respect to the surface normal. Cantilever deflections are measured using an optical lever and
tip-surface forces (loads) are calculated using Hooke’s
Law. Loads quoted in this letter are uncertain to at least
lo%-20%. Samples are mounted on a piezoelectric tube
scanner capable of xy-raster scanning over areas on the
sample surface as large as 10X 10 pm2. The total axial
extension (z-direction) was about 2.4 pm so that the maximum load that could be applied was about 900 nN. Surfaces were modified by rapidly extending the piezoelectric
tube in the z-direction (along the sample normal) to apply
a predetermined load to the tip and then moving it across
the surface either to make a single line or to x-y-raster scan
an area. Starting and modified surfaces of polyimides can
be imaged without detectable damage by xy-raster scanning the tip over the surface while the tip-to-sample spacing is varied to maintain zero applied load. All experiments
were carried out in an ambient air atmosphere. The principles of SFM are described in Ref. 6.
The polyimide samples were commerical foils 0.025
mm thick or spin-coated films several micrometers thick’
supported on silicon wafers. Figure 2(a) shows examples
‘IPresent address: Department of Materials Science and Engineering,
University of Pennsylvania, Philadelphia, PA 19104.
b)Author to whom correspondence should be addressed.

of grooves scribed on the surface of a Kapton-H foil.’ Each
groove was made by a single pass of the tip across the
surface from left to right with a load of about 490 nN. The
hook at the start of the longer lines is caused by nonlinear
response of the piezoelectric scanner during the rapid initial increase in applied load and by lateral motion of the tip
as the cantilever is bent. A magnified view of the shortest
groove is also shown along with a typical profile of the
groove cross section.g Each groove is a furrow with raised
edges. The relative volumes of the furrow and its raised
edges are nearly equal. Thus microplowing is the dominant
mechanism for groove formation rather than microcutting
or microcracking. lo As the force is increased, the grooves
become wider and deeper.” No grooves are formed for
applied loads less than the yield stress of the polyimide.
The narrowest grooves we have made on Kapton-W are
about 20 nm wide by 2 nm deep and require an applied
.load of about 100 nN.‘1112 Figure 2(b) shows an example
of writing on the Kapton-H surface with the tip.
The grooves formed by the SFM tip have the same
cross-sectional profiles as those formed by rubbing the surface with a cotton tipped swab [Fig. 2(c)]. Rubbed polyimide surfaces are used to align liquid crystal molecules in
display devices.4 SFM machining and imaging can be applied to study the micromechanics of the rubbing process

Cantilever

-’ J. PZT Scanner

FIG. 1. The scanning surface force microscope. The difference signal
(A-B) from the position sensitive photodiode PSPD measures lateral
shifts of the reflected laser light caused by deflection of the cantilever. The
PZT scanner controls the sample z-position using feedback from the
PSPD.

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RG. 2. (a) Grooves scribed on a Kapton-H surface. The load was 490 nN. To the upper right-hand side is a cross-sectional profile of the top groove;
this groove is 5.2 nm deep and 120 nm wide from ridge to ridge. (b) The University of Maine logo “UME” scribed with 110 nm wide grooves. (c) 175
nm wide surface grooves formed on Kapton-H by rubbing with a cotton tipped swab. All calibration bars are 1 pm long.

and the effects of rubbing parameters on the alignment of
liquid crystal overlayers.
Modified areas can also be created by raster scanning
the tip with an applied load. Both pits and raised areas can
be formed. Figure 3 shows an example of a raised area on
a spin-coated film of 6FDA-ODA. l3 Figure 3 (a), the starting surface, shows the typical fine structure of 6FDAODA. Figure 3 (b) shows the result of scanning a 3 x 3
,um” area with an applied load of 800 nN. The entire area
appears raised above the starting surface by about l-l.5
nm. However, the average roughness of the ‘kaised” surface is unchanged from the starting surface value of about
6 A. Examination of the fine structure in both images

(a)
(b)
FIG. 3. (a) Unmodified surface of 6FDA-ODA. (b) A 3 X 3 pm” raised
area with average height of 1.2 nm. The calibration bar is 1 pm and the
gray scale is 7.5 nm for both images.

shows that even small details of the surface morphology
are preserved in the raised area!
We do not understand the mechanism by which the
raised areas are formed. Viscoelastic effects may play a role
since raised areas appear to form only for loads estimated
to be smaller than the yield strength of the polymer. Another possible mechanism is a morphology change induced
by localized heating caused by rupture of the adhesive
bond between the tip and polymer. The measured adhesive
force between the Si3N4 tip and polyimides is in the range
of 40-120 nN.12 If all the energy is dissipated within one
tip radius of the surface, we estimate that the surface temperature can increase by up to several hundred kelvin.
Studies of macroscopic wear on polymer surfaces, carried
out at higher velocities and lower loads than used here,
demonstrate that heating is an important factor in surface
wear. I4
If the load exceeds the yield strength, pits are formed.
An example for a BPDA-PDA15 film is shown in Fig.
4(a). The pits typically have distinct ridges on each side
with the largest ridge at the top. The structure of the pit
bottom [Fig. 4(c)], is substantially rougher than the starting surface [Fig. 4(b)]. Similar results are obtained for
PMDA-ODA
films and Kapton-H foils for loads above
about 100 nN and pits up to 90 nm deep have been machined.12 Leung and GohI have also reported that pits can
be formed on polystyrene films. The roughness may be
related to the inhomogeneous nature of the polyimides.

Appl. Phys. Lett., Vol. 61, No. 6, 10 August 1992
658
X. Jin and W. N. Unertl
658
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Cal

::.::.
..:1i::

ICI
FIG. 4. (a) A 6 nm deep pit machined on BPDA-PDA. The dark bands
crossing the image horizontally near the upper and lower edges of the pit
are artifacts of the image acquisition system. Calibration bar is 1 pm long
and gray scale is 200 nm. (b) A close-up image of the unmodified
surface-average roughness is 0.6 nm. The calibration bar is 1 pm long
and the gray scale is 40 nm. (c) A close-up image of the pit bottomaverage roughness is 3.8 nm. The calibration bar and gray scale are the
same as in (b).

Polyimides are partially crystalline and this crystallinity
extends to the surface, at least for Kapton-H whose crystal&es are typically about 15 nm across.” As the tip plows
through the surface it will meet the least resistance in the
less dense, amorphous regions between crystallites where
the cohesive energy will be lowest. Further experiments to
test this mechanism are underway.
The starting surfaces of all the polyimides imaged with
SFM have a surface microstructure of irregular undulations with average roughness less than 1 nm and lateral
dimensions of several hundred nanometers.12 The elasticity
theory’* shows that lateral variations in surface energy are
one possible cause of these undulations. For the cantilevers
used by us, a change in surface energy by only 1 mJ/m2 (a
few percent) can change the contact force by 0.37 nN. A
related possibility is the intrinsic inhomogeneity of the
polyimide surfaces due to partial crystallinity. Variations

in both surface energy and Young’s modulus are to be
expected. These variations will change the contact area
which will in turn give the appearance in the image of a
change in applied load. Whether similar grainy surface
morphologies are observed” or not observed2’ on spincoated polyimide films imaged with STM is a matter of
controversy.
In summary, we have used the SFM to make submicrometer modifications to the surfaces of several types of
polyimide. These results demonstrate that the force microscope is a useful tool to create and then image surface
structures. This capability should be useful, for example, to
study the orientation induced in liquid crystal and evaporated polymer thin films” by prerubbed surfaces and for
study of the micromechanical properties of nonconducting
materials.
This work was supported by grants from the Air Force
Office of Scientific Research and the IBM Corporation.

‘C. F. @ate, Nature 352, 571 (1991), and references therein.
’PrincipIes of Electronic Packaging, D. P. Seraphim, R. C. Lasky, and C.
Y. Li, Eds. (McGraw-Hill,
New York, 1989).
3H. Satou, H. Suzuki, and D. Makino, in Polvimides, edited by D. Wilson, H. D. Stenzenberger, and P. M. Hergenrother (Chapman and Hall,
New York, 1990) pp. 227-251.
4N. A. Clark and S. T. Lagerwall, Appl. Phys. Lett. 36, 899 (1980).
5Model STM-SFM-BD2, Park Scientific Instruments, Sunnyvale, CA.
6D. Sarid, Scanning Force Microscopy (Oxford University Press, New
York, 1991).
‘Spin-coated samples were fabricated by R. C. White, Department of
Electrical Engineering, Columbia University, New York, NY 10027.
‘Kanton-H
is a nolvimide manufactured by DuPont and based on
PMDA-ODA
which is a polyimide derived from pyromellitic dianhydride and 4,4’-oxydianyIene. See also Ref. 3.
‘In measuring the groove profiles, we have neglected any artifacts due to
the finite size of the tip and changes in contact area due to changes in
surface curvature. These tip-convolution effects will not affect measurements of the width of a groove which are made between the tops of the
ridges, but they could result in an underestimate of the groove depth.
See also: D. Keller, Surf: Sci. 253, 353 (1991).
ioK . H . zum Gahr, Mfcrosrructure and Weur of Materials (Elsevier, Amsterdam, 1987), pp. 80-350.
“W. N. Unertl, X. Jin, and R. C. White, in Polyimides and Other HighTemperature Polymers, edited by M. J. M. Abadie and B. Sillion
(Elsevier Science, Amsterdam, 199 1) , pp. 427435.
“X. Jin, M. S. thesis, University of Maine, 1991.
i36FDA-ODA
is a nolvimide derived from hexafluorodianhydride and
4,4’- oxydianylenel
14M. K. Kar and S. Bahadur, in Wear of Materials 1977, edited by W. A.
Glaeser. K. C. Ludema. and S. K. Rhee (American Society Mechanical
Engineers, New York, 1977), p, 501.
“BPDA-PDA
is a polyimide derived from 3,3’,4,4’-biphenyl tetracarboxylic dianhydride and p-phenylene diamine.
160. M. Leung and M. C. Goh, Science 255, 64 (1992).
“B J Factor, T. P. Russell, and M. F. Toney, Phys. Rev. Lett. 66, 1181
(1991,.
“K. L. Johnson, K. Kendall, and A. D. Roberts, Proc. R. Sot. London,
Ser. A 324, 301 (1971).
19M. Suzuki, T. Maruno, F. Yamamoto, and K. Nagai, J. Vat. Sci. Technol. A 8, 631 ( 1990). The polyimide surfaces were coated with 10 nm
of Au prior to imaging.
2oR. C. White (private communication).
“J. Sakata and M. Mochizuki, Thin Solid Films 195, 175 (1991).
r--2----

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