Photon Factory Activity Report 2014 #32 (2015) B

BL-15A2/2014G169

Depth Dependence of Lattice Deformation of Cylindrical Microdomain ofPolystyrene-b-poly(2-vinyl pyridine) Thin Film Revealed by GISAXS using Low
Energy X-ray
Katsuhiro Yamamoto* and Itsuki Saito
Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan
1 Introduction
Block copolymers (BCPs) are composed of two or
more polymers connected by covalent bond. They canform a few tens nanometer scale periodic structure, whichis called micro-phase separated structure, such as
lamellae, cylinders and spheres, etc. In case of thin film,cylindrical and lamellar microdomains are preferentiallyaligned parallel to a substrate when one component of theblock copolymer segregates the surface or the substrate tominimize the free energy at the surface and interface. Theparallel orientation of the domains was analyzed byneutron reflectivity measurement (NR), TOF secondary
ion mass spectroscopy (TOF-SIMS), X-ray photoelectron
spectroscopy (XPS), and grazing incident SAXS(GISAXS). NR, TOF-SIMS, and XPS can allow us toreveal depth profiling of the microphase separated
structures. On the other hand, GISAXS using hard X-ray
is unsuited for the depth profiling because of penetration
depth of the X-ray abruptly increases up to a thousand of
nanometers at the critical angle. Below the critical angle,
the penetration depth is a few nanometers.
Wide range 2.1 – 15keV of energy of X-ray at BL15A2
can be available. Using relatively low energy of X-rayfor example 2.4 keV, the penetration depth is suited for
depth profiling of the typical phase separated structurebecause of the shallower penetration depth even near thecritical angle and mechanically ease control of theincident X-ray.1,2
In this study, we performed GISAXS measurementwith low energy X-ray (2.4keV) in order to investigate
depth profiling of lattice parameters of the parallel
cylindrical structures in polystyrene-b-poly(2-vinylpyridiene) (S2VP) thin film.
2 Experiment
GISAXS measurement with soft X-ray was performed
at BL15A2 in Photon Factory, KEK, Tsukuba in Japan.
This beam line offers a wide energy range from 2.1 to15keV. In this research, the experiment was conducted at
2.4keV corresponding to wavelength, 5.16Å. Sample todetector distance (SDD) was about 830 mm. Pilatus 2M
specified for use in vacuum was used as a detector of 2D
scattering pattern. Exposure time was 300sec. GISAXS
measurements with hard X-ray were performed (atBL10C in Photon Factory and) BL03XU in SPring-8,Harima, Japan. SDD and wavelength were 2 m and 1.0Å,respectively.
Block copolymer S2PV thin film was prepared by spincasting on a silicon substrate (3000 rpm, 30s) fromtoluene solution (10 wt%). After drying the thin film inair, the S2VP thin film was thermally annealed at 443 K
for 48 h to obtain parallel cylindrical microdoimains.
3 Results and Discussion
GISAXS measurement using soft X-ray was performed
with various incident angles. Figure 1 shows typical
GISAXS pattern at incident angle over the critical angleof the X-ray. Many scattering peaks can be obtained andassigned as hexagonally packed cylinders that wereoriented parallel to the surface of the substrate. Figure 2shows 1D scattering profiles along qz direction vertically
cut at qy = 0.26 nm-1 with various incident angles. Here,the second order reflections (qz = 0.6 – 0.7 nm-1) wereused for precise analysis of the lattice parameters of thecylindrical microdomains because of missing the firstorder peaks due to detector gaps. Scattering peaks became
broad with a decrease in the penetration depth. This
broadening can be explained with the reduction ofmeasured region. That is, when the penetration depth of
the X-ray is small, the number of reflected planes
decreases.

Fig. 1: GISAXS pattern and illustration of paralleloriented cylinder domains.

Photon Factory Activity Report 2014 #32 (2015) B

Full width at half maximum (FWHM) of the scatteringpeaks depends on the number of the layers according tothe Laue function.
( )=exp(∙ )=sin[( + 1) ∙ ⁄2]sin[ ∙ ⁄2]

where N is the number of the (10) planes, Dcyl is theperiod of (10) plane. Herein, Dcyl was 20 nm that was thespacing obtained from the bulk sample. Taking intoconsideration an attenuation of the X-ray, the Laue
function can be expressed as follows;( )=1 2+⁄2]sin[( + 1)
exp − 2 3
⁄2]
sin[Λ

where L is the penetration depth.
Experimentally obtained FWHM was plotted against
incident angles with open red circles in Figure 3.
Simulated FWHM using above Laue function was also
indicated with black marks and solid line. Experimental
FWHM was in good agreement with the simulated ones.
When incident angle is beyond the critical angle, thelattice parameters can be calculated using DWBA. Lattice
parameters of the cylindrical microdomains at eachpenetration depth were determined at various depth of thethin film. The cylinder lattices normal to the substrate andalong lateral direction were found to be narrowed andelongated as compared with the value in the bulk state of
S2VP-26k, respectively. Moreover, this deformation wasrelaxed as the depth was shallow (near the surface) exceptthat the parameters in the lateral direction were almost
unchanged as shown in Figure 3. This lattice relaxation
behavior was associated with the mobility of polymer
chains in the thin film, i.e., a higher mobility wasexpected near the surface, in contrast the mobility(diffusion) along the lateral direction was suppressed near
the interface. In conclusion, a low energy GISAXS canprovide precise analysis of depth dependence of thestructure.

Fig. 2: 1D GISAXS profiles along qz direction vertically
cut at qy ~ 0.26 nm-1.

Fig. 3: Depth dependent of the lattice parameters.
References[1] H. Okuda et al., J. Appl. Cryst. 44, 380 (2011).
[2] J. Wernecke et al., Macromolecules 47, 5719 (2014).
* yamamoto.katsuhiro@nitech.ac.jp