**2.7 Fabrication and experiment**

342 Recent Advances in Nanofabrication Techniques and Applications

part of the photon sieve, which the ratio of pinhole diameter to underlying zone width is

Fig. 6. Simulated pinholes distribution and PSF of a photon sieve.

(a) (b)

Fig. 7. Pinholes distribution of an apodized X-rays photon sieve.

The diffraction efficiency scales as the square of the transmission area. A photon sieve transmits only 15~30% of the incident light because of the Gaussian apodization whereas an amplitude-zone plate has a transmission of 50%. Therefore, the first-order diffraction efficiency of a photon sieve is lower than that of a zone plate of equal *NA* by a factor of 10. However, the photon sieve is an attractive alternative to conventional zone plates for the Xrays focusing and imaging elements in the situation using the brilliant X-rays from synchrotron light sources as illumination, the diffraction efficiency is not a very important consideration but the side-lobes suppression and the fabrication ease are relatively more

The process of designing an amplitude-photon sieve is shown as follows. Firstly, the operating wavelength, diameter and focal length of photon sieve are given according to the purpose of imaging or focusing. Secondly, the number of corresponding Fresnel zones is calculated. Lastly, the optimum radius and coordinate of each pinhole in each corresponding zone are determined, and then the data of all pinholes are stored in the same

chosen as 1.5.

important.

file in order to create the photon sieve.

In order to verify the feasibility of the above methods, we fabricated a Gaussian apodized amplitude-photon sieve (633nm design wavelength, 175mm focal length, 30mm pupil diameter, 0.08 *NA*) on chrome-coated quartz plate using laser-beam lithographic techniques. The resolution image was recorded at 627nm peak-wavelength and 20nm spectral halfwidth produced by a light-emitting diode. Figure 8 demonstrates the pinholes distribution of the central part of the Gaussian apodized amplitude-photon sieve and the ratio of pinhole diameter to underlying zone width is chosen as 1.5.

Fig. 8. Scanning electron microscope images of an amplitude-photon sieve. (a)Central portion, (b)the outermost portion.

For the purpose of testing the imaging property, a beam of even illumination was produced by a light-emitting diode (LED, with peak-wavelength 627nm and spectral half-width 20nm) and a ground-glass diffuser. The beam transmitted through a WT1005-62 resolution test target was then incident upon the photon sieve. The resolution images were recorded by an intensified charge coupled device (ICCD). Figure 9 shows that good agreement between experimental and theoretical results concerning reduction of side-lobes but the resolution of 5.9um obtained is somewhat lower than the expected resolution limit of 4.8um because of the wavelength difference and chromatic aberration.

Fig. 9. Resolution test target imaged with an apodized photon sieve at 627 nm.

Emerging Maskless Nanolithography Based on Novel Diffraction Gratings 345

Materials Parameters Thickness

*i nm* 40nm

1 3 *TimeStep N* 2 2 (5)

Substrate Refractive index n=1.6@365nm 100nm

Mask Refractive index n=2.9@365nm 40nm Spacing Refractive index n=1.52@365nm 40nm

Photoresist Refractive index n=1.7@365nm 100nm

In figure 10, the spacing layer with refractive index of n=1.517 (@365nm) acts as the following two functions: 1) to match the surface plasmon polaritons resonating conditions; 2) to protect the surface of silver slab. As for the image recording, we chose the negative

In order to explore the potential valid imaging distance between mask and photoresist, a repeat of spacing layer and silver slab layer was followed after the first silver slab. The sample was exposed in i-line light that shone from the substrate side. For the convenience of description, we defined D1 as the distance between mask and silver slab, and D2 as the

In order to save calculating time and PC resources, we computed the distribution of optical near-field intensity by the 2-D FDTD method. We chose cell size of X\*Z=2×1(nm), which is much smaller than both the exposure light wavelength and the mask's feature size. The time

N is the total cells of computing area. We chose 3500, by which the amplitude of electric field already became steady. The distance of 40nm comes from the spacing layer. At this situation, it is a kind of ideal condition, because mask and spacing layer had a hard contact. The surface plasmons polaritons of two interfaces between silver slab and its surroundings can magnify the evanescent waves that carried the detailed information of object. When D1=40nm, we calculated the following 4 conditions: D2=0nm, 20nm, 40nm, 60nm. Figure 11 show the distribution of electromagnetic (abbreviated to EM afterwards in the paper) field

It was found that the image of mask can be clear resolved by the method. Figure 11(a) shows better result, however, figure 11(b)-(d) showed worse results due to the exponential decay of the evanescent waves came from the exit side of the interface between silver slab and photoresist. The strong contrast of EM field may come from the edge effect of the evanescent waves. We chose the 10nm cross-section of photoresist layer to compare the imaging result of silver slab. When D1=40nm and D2 varied from 0nm to 60nm, the distribution of optical field in the section was shown in Figure 11. When D2 changed from 0nm to 60nm, the amplitude reduced to about a half under the same condition, but the high contrast of lines still can be clearly observed in figure 12. The amplitudes of lines were relatively uniform when D2=20nm and 60nm compared with D2=0nm. If the parameters of photoresist were

Silver Permittivity 2.4 0.2488@ 365 *Ag* 

Table 1. Physical parameters of the materials in Fig. 10.

distance between silver slab and photoresist.

step, according to Courant condition, should be:

**3.2 When silver slab separated 40nm from mask (i.e. D1=40nm)** 

photoresist of i-line.

respectively.

In brief, the experiment mentioned above demonstrates the feasibility of using photon sieves as the focusing optical element. We believe in that PSAL will be used as one of the promising tools in the nanometric device and special IC areas for the purpose of high resolution regardless of low yield.
