Effects of annealing on the molecular orientation in polytetraﬂuoroethylene thin ﬁlms Eiichi Kobayashi, a * Koji K. Okudaira b and Toshihiro Okajima a The effects of annealing on the molecular orientation in polytetraﬂuoroethylene (PTFE) thin ﬁlms evaporated on copper and silicon plates were studied using F K-edge near-edge X-ray absorption ﬁne structure (NEXAFS) spectroscopy. The intensity of the F 1 s ! s*(C-F) transition in the NEXAFS spectrum from a non-annealed PTFE thin ﬁlm on a copper plate at a glancing X-ray incidence angle was stronger than that at the normal incidence angle. For a thin ﬁlm annealed at ~250  C in a nitrogen atmosphere, the s*(C-F) peak intensity at the glancing angle was weaker than that at a normal angle. The F K-edge NEXAFS spectra of a thin ﬁlm annealed at ~350  C showed a weakening dependence on the X-ray incidence angle. This result indicates that the molecules in a PTFE thin ﬁlm are increasingly oriented normal to the substrate surface with increasing annealing temperature and are oriented randomly. Copyright © 2012 John Wiley & Sons, Ltd. Keywords: near-edge X-ray absorption ﬁne structure (NEXAFS) spectroscopy; polytetraﬂuoroethylene (PTFE); molecular orientation Introduction Polytetraﬂuoroethylene (PTFE) is an important material because of its low friction coefﬁcient, excellent chemical properties, electrical insulating properties, and thermal stability [1] . PTFE is stable up to 260  C, has a helical structure, and is known to occur in four crystalline forms [1] . PTFE thin ﬁlms are very attractive for electronic device applications; they are used as insulating barriers in spintro- nics [2] and as protective and insulation layers in microelectronics. Therefore, it is important to investigate the inﬂuence of heating of the thin ﬁlms on their characteristics. The effects of heat treatment on the adhesion and morphological properties of PTFE ﬁlms have been previously investigated [3] . It is recognized that the molecular orientation in thin ﬁlms is important for applications because it can signiﬁcantly affect the ﬁlm characteristics [4] . However, there have been few studies on the effects of annealing on the molecular orientation in thin ﬁlms of PTFE. Various techniques such as Fourier transform infrared spectros- copy [5] , optical second harmonic generation [6] , and angle resolved photoemission spectroscopy [7] have been used to investigate the molecular orientation in organic ﬁlms. Near-edge X-ray absorption ﬁne structure (NEXAFS) spectroscopy is also known to be a powerful tool for investigating the molecular orientation [8] . The polarization dependence of NEXAFS spectra for the carbon and ﬂuorine K-edge regions of PTFE at room temperature has previously been measured and peak assignments were proposed [9] . Nagayama et al. studied mechanical rubbing effects on PTFE using NEXAFS [10,11] . In the present study, the effects of annealing on the molecular orientation in PTFE thin ﬁlms were studied by F K-edge NEXAFS spectroscopy. Experimental Polytetraﬂuoroethylene thin ﬁlms were prepared by evaporating PTFE powder (Central Glass Co.) on polished copper plates and silicon wafers under a vacuum of about 5  10 -4 Pa at room temperature. The surfaces of the copper plates and silicon wafers were oxidized. The composition of the powder was n-C n F 2n+2 with n = 100–400, peaking at n = 170. The ﬁlm thickness and rate of evaporation were monitored by a quartz crystal oscillator. The ﬁlm thickness was about 5 nm. The ﬁlms were annealed at ~250 and ~350  C in a nitrogen atmosphere for 15min. Near-edge X-ray absorption ﬁne structure spectra were mea- sured at the soft X-ray beamline BL12 [12] of the SAGA Light Source (SAGA-LS). This beamline is equipped with a varied line spacing plane grating monochromator with a typical energy resolution (E/ΔE) of ~2500. The NEXAFS spectra were obtained by measuring the drain current from the sample. The NEXAFS probing depth for PTFE using the total electron yield mode in the F K-edge region is below 2 ~ 3 nm. The samples were rotated around the vertical axis to measure the polarization dependence of the NEXAFS spectra. The spot size of the X-rays was below ~5  2 mm on the surface at the normal incidence angle. Results and discussion The F K-edge NEXAFS spectra of PTFE on a copper plate as a function of the X-ray incidence angle θ (deﬁned in the inset) are shown in Fig. 1. All the spectra were normalized to the intensity at 750eV. As shown in the ﬁgure, the spectra are not affected in both NEXAFS and extended X-ray absorption ﬁne structure. Peak A is attributed to transitions from F 1 s to the unoccupied s*(C-F) [9] . In subsequent discussions, we assume that the C-F bonds are * Correspondence to: Eiichi Kobayashi Kyushu Synchrotron Light Research Center, 8–7 Yayoigaoka, Tosu, Saga 841–0005, Japan. E-mail: kobayashi@saga-ls.jp a Kyushu Synchrotron Light Research Center, 8-7 Yayoigaoka, Tosu, Saga, 841- 0005, Japan b Graduate School of Science and Technology, Chiba University, Chiba 263-8522, Japan Surf. Interface Anal. (2012) Copyright © 2012 John Wiley & Sons, Ltd. Research article Received: 19 January 2012 Accepted: 23 January 2012 Published online in Wiley Online Library (wileyonlinelibrary.com) DOI 10.1002/sia.4895