Electronic Supplementary Information (ESI) for
Nanodomain Analysis with Cluster SIMS: Application to the
Characterization of Macromolecular Brush Architecture
Fan Yanga, Sangho Choa,b,c,d, Guorong Suna,b,c,d, Stanislav V. Verkhoturova, Karen L. Wooleya,b,c,d,
Emile A. Schweikerta*
a
Department of Chemistry, Texas A&M University, College Station, TX 77842
b
Artie McFerrin Department of Chemical Engineering, Texas A&M University, College Station, TX 77843
c
Department of Materials Science and Engineering, Texas A&M University, College Station, TX 77843
d
Laboratory for Synthetic-Biologic Interactions, Texas A&M University, College Station, TX, 77842
Fluorine depth profiles of SIMS for DBT thin films.
The secondary ion mass spectrometer SIMS CAMECA 4F (Material Characterization Facility,
TAMU) has been used for the depth profiling.
The analyzed surface was sputtered by the 14.5 keV Cs+ beam with a current of 6 nA. The
diameter of the beam was ~1 µm; the raster was 500×500 µm2 for the DBT sample. The angle of
incidence of the beam was 26o.
Measuring the profile of a thin (~10 nm), low density organic film is a challenging task. First,
the sputtering process damages the film via ion-atom collisions leading to the subsequent diffusion
of recoiled/displaced atoms toward the substrate. Second, implantation of the Cs atoms, delivered
by the Cs+ beam, increases the ionization coefficients of the sputtered species as a function of the
increasing surface/interface Cs concentration. Usually, the equilibrium concentration of the Cs
atoms is obtained by the profiling of the first few tens nanometers of the analyzed film. This is not
a case here (the equilibrium cannot be reached at the time of profiling), since the expected
thickness of the organic layer is ~20 nm. The solution of the problems mentioned above is to
measure the profile as a ratio of intensities of F and CH2 ions.
The measured intensity of F ions, IF-(t), can be expressed as
𝐼𝐹− (𝑡) = 𝐼0 𝑘𝑌𝐹𝑜 (𝑐𝐹 )𝛼𝐹 (𝑐𝐶𝑠 )
(1)
1
where 𝐼0 is the Cs beam current (ions/s); 𝑘 is the transmission/detection efficiency of mass
spectrometer; 𝑌𝐹𝑜 (𝑐𝐹 ) is the sputtering yield of the F atoms (number of sputtered atoms per
projectile impact). The parameter 𝛼𝐹 (𝑐𝐶𝑠 ) denotes an ionization probability of F atoms. The
ionization probability is a function of the concentration of Cs atoms in the topmost layer of the
sputtered film, 𝑐𝐶𝑠 . A widely accepted mechanism of the negative ionization of the sputtered atoms
is the tunneling of electron from the surface into the emitted atom.[1] For this mechanism, the
ionization probability is a function of the atom electron affinity, A, and the surface work
function, 𝜑:
𝛼𝐹 (𝑐𝐶𝑠 ) ≈ 𝑒 −(𝜑−𝐴𝐹)/𝜀𝑜
(2)
where the parameter 𝜀𝑜 presents the “typical excitation energy due to atom motion”.[2] This
parameter is independent of moderate (1-2 eV) changes in 𝜑 and 𝐴. (AF =3.4 eV; ACH2 =0.6 eV).
Hence, we obtain:
𝛼𝐹 (𝑐𝐶𝑠 )
𝛼𝐶𝐻2 (𝑐𝐶𝑠 )
= e(𝐴𝐹−𝐴𝐶𝐻2 )⁄𝜀𝑜
(3)
The ratio from equation (3) does not depend on the work function, φ. Thus, the variation of φ,
stimulated by the Cs implantation, does not affect the ratio of intensities:
𝐼𝐹− (𝑡)
−
𝐼𝐶𝐻2 (𝑡)
=
𝑌𝐹𝑜 (𝑐𝐹 )
𝑜 (𝑐 )
𝑌𝐶𝐻
𝐹
e(𝐴𝐹 −𝐴𝐶𝐻2 )⁄𝜀𝑜
(4)
2
Additionally, to reduce the effect of accumulation of Cs, we deposited on the top of the organic
film a thin film (~5nm) of Pt/Pd alloy. This film works as “an obstacle”, where the first, transition
processes of bombardment occur, including of initial accumulation of the Cs and diffusion of the
target atoms.[3] Taking in account the procedures mentioned above, we assume that the sputtering
yield is a linear function of the concentration of F and CH2. Thus, the ratio of concentrations is a
linear function of the ratio of intensities:
𝑐𝐹 (𝑡)
𝑐𝐶𝐻2
𝐼 − (𝑡)
= 𝛽 𝐼−𝐹
(𝑡)
𝐶𝐻2 (𝑡)
(5)
where the constant , 𝛽, reflected the differences between F and CH2 ions, which are their electron
affinities and detection efficiencies. The measured profiles are shown in Figures S1. For the Pt/Pd
film, to rescale the time of profiling into the profile depth, we use the sputtering rate (specified in
the figure captions), which was computed with the SRIM 2011.08 software.[4] For the molecular
film, we use the AFM measurement of the film thickness (~25nm).
2
The profile result shows that the relative intensity of F ions drops at the depth of ~3-7 nm. This
verifies that the fluorine enriched molecules are on the top of the film.
References
[1]
[2]
[3]
[4]
M. L. Yu, N. D. Lang, Nuclear Instruments and Methods in Physics Research Section B:
Beam Interactions with Materials and Atoms 1986, 14, 403.
N. Lang, J. Nørskov, Physica Scripta 1983, 1983, 15.
S. Harton, F. Stevie, H. Ade, Journal of Vacuum Science & Technology A 2006, 24, 362.
J. F. Ziegler, M. D. Ziegler, J. P. Biersack, Nuclear Instruments and Methods in Physics
Research Section B: Beam Interactions with Materials and Atoms 2010, 268, 1818.
Fig. S1. Depth profiling of the DBT sample. Sputtering rate is ~0.05 nm/s for the DBT film, and
~0.02 nm/s for the Si.
3
Fig. S2. Intensities of secondary ions as a function of sputtering time.
4