Wrinkled hard skins on polymers created by focused ion beam

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   Fig. 1.

   Wrinkle patterns on PDMS surface induced by FIB irradiation. (A) Selected surface areas of the PDMS are exposed to FIB as shown schematically. (B) The wrinkles appear only on the areas of the PDMS exposed to FIB. The SEM picture shows the boundary of a 200 × 200-μm region exposed to FIB with the fluence of ≈10¹³ ions per cm² in a single scan mode. The wrinkles on the exposed area are 1D with the wavelength of ≈460 nm, while the unexposed surface remains flat. (C) The morphology of the wrinkled skin depends on the ion fluence, which can be controlled by varying the relative velocity of the substrate and the beam. The SEM image shows a wrinkled pattern generated along a straight path with width of 4.0 μm with two different ion fluences: 1.8 × 10¹⁴ (Right) and 9 × 10¹⁵ (Left) ions per cm². The surface of the PDMS outside this path remains flat. (D) The wrinkle pattern can be generated along selected paths with specified width by controlling the relative motion of the substrate and ion beam. Here the pattern follows an S-shaped path with a width of 6 μm, created at a constant ion fluence, 5.0 × 10¹⁵ ions per cm². Inset shows a circular path with the same width created at the same ion fluence. (E) Islands of buckled stiff skins on the PDMS created by controlling the ion beam spot diameter and ion beam spot spacing. Each island has the diameter of ≈500 nm and is a smooth buckle with peak height of ≈35 nm. The spacing between the islands is ≈1.5 μm. The applied ion fluence (averaged over the whole area of the PDMS) is 7 × 10¹² ions per cm². (Scale bars: 10 μm.)

   
   
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   Fig. 2.

   The surface morphology of the wrinkle patterns induced by FIB depends primarily on the applied ion fluence. (A) SEM images of the surface morphology of the PDMS exposed to FIB in a single scan mode (N = 1) with various ion beam fluences, F (ions per cm²). (B) SEM images of the surface morphology created by N scans each with fluence, F = 2.0 × 10¹³ ions per cm². (Top) SEM image shows the surface morphology after the first scan. AFM examination of the surface shows that the surface has a wavelength of ≈470 nm. (Middle and Bottom) SEM images of the surface after N = 15 (Middle) and 20 (Bottom) scans (with accumulated fluence, N × F, of 3.0 × 10¹⁴ and 4.0 × 10¹⁴ ions per cm², respectively) reveals complex patterns of the surface with a hierarchical nature. (Scale bars: 10 μm.)

   
   
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   Fig. 3.

   Examination of the surface topology and chemical composition of the PDMS exposed to FIB through depth. (A) (Upper) Morphology of PDMS surface exposed to FIB with fluence of 5.0 × 10¹³ ions per cm² obtained by using AFM. (Scale bar: 3 μm.) (Lower) The normal deflection of the surface along the path identified in Upper by the dashed line. (B) Amplitude of the hierarchical wrinkles formed upon exposure of the PDMS surface to FIB with various fleunces. The primary wrinkles appear at the fluence of ≈10¹³ ions per cm². The characteristic wavelengths associated with the surface morphology are calculated by FFT of the surface undulations obtained by AFM. (C) Average compressive strain in the stiff skin formed by exposure of the PDMS surface area as a function of ion fluence. The compressive strain is calculated by averaging the strain along at least five directions for each morphology studied (see Materials and Methods). (D) AES analysis of the PDMS surface exposed to FIB with ion fluence of 1.1 × 10¹³ ions per cm². Because of exposure to the FIB, the concentration of C and O were changed at the first 25 nm of the PDMS from that of the original PDMS, whereas the Si component remains almost constant through the depth. Atomic concentrations of Si, O, and C become close to the pure PDMS after depth of 25 nm from the surface with average ratio of O/Si atomic concentration ≈0.97.

   
   
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   Fig. 4.

   Various morphologies of the wrinkled skins induced by controlling the relative motion of the ion beam and PDMS. (A) By moving the PDMS stage during the FIB irradiation, long channel patterns can be produced. (B) SEM images of the regions of the PDMS exposed to FIB showing various surface morphologies created by changing the width from 20 μm (top image) to 4 μm (bottom image). The PDMS stage has a constant velocity of 500 nm/s and the beam current is 1 pA. The ion fluence varies from 7.2 × 10¹³ to 1.8 × 10¹⁵ ions per cm² (top to bottom images). (C) Surface morphologies created by varying the moving speed of the PDMS stage from 50 to 500 nm/s, while the width of the exposed area is kept constant as 4 μm. The beam current is constant at 1 pA such that the fluence varies from 1.8 × 10¹⁵ to 1.8 × 10¹⁶ ions per cm². Note that the overall shape of the patterns also can be controlled as shown in Fig. 1 D. (Scale bars: 10 μm.)