**3.1 Nanolithography method**

The exposure method of near-field nanolithography that was proposed by this paper is illustrated in Figure 10. A UV transparent substrate with refractive index of n = 1.6 (@365nm) is used for supporting the mask. The object layer with line width of 60nm and pitch of 120nm, which is made of Cr with refractive index of n=2.924 (@365nm), acts as the function of mask in exposure. The air gap comes from the vacuum contact between mask and silver layer, which can be viewed as a kind of practical nanolithography technique.

Fig. 10. Sketch of Nanolithography Model.

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

**3. Nanolithography in the evanescent near field by using nano-filmed noble** 

The sharpness of object can't be resolved by conventional lens due to the limitation by the wavelength of illumination light. J. B. Pendry had predicted that a slab of negative refractive index material has the power to focus all Fourier components of a 2D image. The super resolution of the negative index materials using silver layer, which was called 'superlens', can reconstruct the image of a pattern with line width of 40nm (Fang, et al., 2005). They made mask, silver slab and photoresist integrity in Fang's experiment, which likes the traditional contact exposure of lithography. It is not practical in real application by Fang's method of nanolithography because each wafer needs its respective mask. The experiments of super resolution using silver slab was reported, and the line width with one fifth of illumination wavelength can be successfully resolved by the silver slab

Though Blaikie's experiment made mask and silver integrity, they separated photoresist from silver slab. This kind of configuration still had limitation in practical application. In order to investigate the influence of distance between mask and noble metal slab on imaging, we designed a separated 'superlens' with silver slab 100nm away from mask. We analyzed the distribution of optical field by Finite Difference Time Domain (FDTD). The

The exposure method of near-field nanolithography that was proposed by this paper is illustrated in Figure 10. A UV transparent substrate with refractive index of n = 1.6 (@365nm) is used for supporting the mask. The object layer with line width of 60nm and pitch of 120nm, which is made of Cr with refractive index of n=2.924 (@365nm), acts as the function of mask in exposure. The air gap comes from the vacuum contact between mask and silver layer, which can be viewed as a kind of practical nanolithography technique.

results show that the images of object can be reconstructed by the structure.

resolution regardless of low yield.

**metal layers** 

(Blaikie et al., 2006).

**3.1 Nanolithography method** 

Fig. 10. Sketch of Nanolithography Model.


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

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 photoresist of i-line.

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 distance between silver slab and photoresist.

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

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 step, according to Courant condition, should be:

$$TimeStep = 2\sqrt{2}N^{1/3} \tag{5}$$

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 respectively.

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

Emerging Maskless Nanolithography Based on Novel Diffraction Gratings 347

Fig. 13. The final comparison when D1=60nm while D2=0, 20, 40 and 60nm.

Fig. 14. The final comparison when D1=80nm while D2=0, 20, 40 and 60nm.

The evanescent waves can not propagate to a long distance due to its exponential attenuation. The intensity of evanescent waves decays with a characteristic length *Z0* :

> <sup>0</sup> 2 2 <sup>1</sup> *<sup>t</sup>* sin

incidence angle of light from optically denser media to optically thinner media; where

In order to explore the potential imaging property of silver slab, we increased D1 as much as possible. Considering the evanescent waves may diminish when D1=100nm, so we calculated the condition of D2=0nm only, the distribution of electromagnetic filed was shown in figure 15. It was found that the image of mask still can be resolved clearly in photoresist layer with good uniformity of imaged lines. We investigated the distribution of optical field in the 10nm cross-section of photoresist layer. The distribution of optical filed in the transverse section of photoresist was shown in figure 16. The image of mask can be resolved with high contrast. With proper choice of exposure condition and materials, the information of mask can be transferred to photoresist layer, and the image of mask can be

In brief, the image of mask can be transferred to the photoresist layer by the enhancement function of surface plasmon polaritons in silver slab. We calculated the 2D distribution of electromagnetic field in our model; the results showed that the image of mask with feature

*k n* 

is the wavelength of incident light. Theoretically, *Z0* can be 100nm by

(6)

1 is the

*<sup>n</sup> <sup>Z</sup>*

where *nnn* 2 1 is the relative refractive index of two surrounding media;

**3.4 When silver slab separated 100nm from mask (i.e. D1=100nm)** 

<sup>2</sup> 2 *<sup>t</sup> k n* , 

calculation.

reconstructed by the silver slab layer.

under better control, the lines of images will be more uniform. However, compare with the condition of D2=0nm, the depth of lines in photoresist will be shallower at the same exposure condition.

Fig. 11. The distribution of EM field in the model. Photoresist layer lies (a) between Z=0.31μm and Z=0.41μm, (b) between Z=0.33μm and Z=0.43μm, (c) between Z=0.35μm and Z=0.45μm, (d) between Z=0.7μm and Z=0.47μm.

Fig. 12. The distribution of optical field in the 10 nm cross-section of photoresist when D1=40nm.

#### **3.3 When silver slab separated 60nm and 80nm from mask (i.e. D1=60nm, 80nm)**

When D1=60nm, 80nm, we calculated 4 conditions respectively for each D1. We still chose the 10nm cross-section of photoresist layer to investigate the optical field. In order to show the clear comparison results, we give the final comparison of the amplitude instead of EM distribution figures for each condition of different D2. Figure 13 and figure 14 showed the result respectively when silver slab separated 60nm and 80nm from mask.

It was found that there came out extra fringes in figure13 and figure 14. This kind of phenomena may be caused by the strong interference effects among evanescent waves. The image of mask still can be resolved in photoresist layer by proper choice of materials and exposure conditions. These results showed a bad conformity between mask and recorded image in photoresist layer. On the other hand, it gave us a hint to realize better resolution of optical lithography by reasonably using the interference effect.

under better control, the lines of images will be more uniform. However, compare with the condition of D2=0nm, the depth of lines in photoresist will be shallower at the same

(a) (b)

(c) (d)

Fig. 11. The distribution of EM field in the model. Photoresist layer lies (a) between

Fig. 12. The distribution of optical field in the 10 nm cross-section of photoresist when

**3.3 When silver slab separated 60nm and 80nm from mask (i.e. D1=60nm, 80nm)**  When D1=60nm, 80nm, we calculated 4 conditions respectively for each D1. We still chose the 10nm cross-section of photoresist layer to investigate the optical field. In order to show the clear comparison results, we give the final comparison of the amplitude instead of EM distribution figures for each condition of different D2. Figure 13 and figure 14 showed the

It was found that there came out extra fringes in figure13 and figure 14. This kind of phenomena may be caused by the strong interference effects among evanescent waves. The image of mask still can be resolved in photoresist layer by proper choice of materials and exposure conditions. These results showed a bad conformity between mask and recorded image in photoresist layer. On the other hand, it gave us a hint to realize better resolution of

result respectively when silver slab separated 60nm and 80nm from mask.

optical lithography by reasonably using the interference effect.

Z=0.45μm, (d) between Z=0.7μm and Z=0.47μm.

Z=0.31μm and Z=0.41μm, (b) between Z=0.33μm and Z=0.43μm, (c) between Z=0.35μm and

exposure condition.

D1=40nm.

Fig. 13. The final comparison when D1=60nm while D2=0, 20, 40 and 60nm.

Fig. 14. The final comparison when D1=80nm while D2=0, 20, 40 and 60nm.

#### **3.4 When silver slab separated 100nm from mask (i.e. D1=100nm)**

The evanescent waves can not propagate to a long distance due to its exponential attenuation. The intensity of evanescent waves decays with a characteristic length *Z0* :

$$Z\_0 = \frac{n}{k\_t \sqrt{\sin^2 \theta\_1 - n^2}}\tag{6}$$

where *nnn* 2 1 is the relative refractive index of two surrounding media; 1 is the incidence angle of light from optically denser media to optically thinner media; where <sup>2</sup> 2 *<sup>t</sup> k n* , is the wavelength of incident light. Theoretically, *Z0* can be 100nm by calculation.

In order to explore the potential imaging property of silver slab, we increased D1 as much as possible. Considering the evanescent waves may diminish when D1=100nm, so we calculated the condition of D2=0nm only, the distribution of electromagnetic filed was shown in figure 15. It was found that the image of mask still can be resolved clearly in photoresist layer with good uniformity of imaged lines. We investigated the distribution of optical field in the 10nm cross-section of photoresist layer. The distribution of optical filed in the transverse section of photoresist was shown in figure 16. The image of mask can be resolved with high contrast. With proper choice of exposure condition and materials, the information of mask can be transferred to photoresist layer, and the image of mask can be reconstructed by the silver slab layer.

In brief, the image of mask can be transferred to the photoresist layer by the enhancement function of surface plasmon polaritons in silver slab. We calculated the 2D distribution of electromagnetic field in our model; the results showed that the image of mask with feature

Emerging Maskless Nanolithography Based on Novel Diffraction Gratings 349

The characteristics of synchrotron radiation light, resolution limits and depth of focus of the lithographic system are discussed. The design and fabrication of photon sieve are illustrated with a low-numerical-aperture amplitude-photon sieve fabricated on a chrome-coated quartz plate by means of laser-beam lithographic process, which minimum size of pinhole was 5.6um. The PSF of photon sieve in terms of side-lobes strength and main lobe width may be controlled by utilizing the apodization window function. The focusing performance of the photon sieve operating at wavelength of 632.8nm was simulated and tested. In combination with the synchrotron light sources, the photon sieve array X-ray maskless nanolithography is a promising tool in the nanometric device and special IC areas for the

The second focuses on the evanescent near field in nano-filmed noble metals. Subdiffraction-limited feature size can be resolved by using i-line illumination exposure. Compared with the model of original superlens, we separated the superlens 100nm away from the mask, under the illumination of i-line light, the initial simulation shows that the sub-diffraction-limited feature as small as 60nm line width with 120nm pitch can be clearly resolved without hard contact between mask and nano-filmed noble metal. By proper design of the materials and the parameters of nano-filmed layers, better resolution can be

In brief, a plasmonic structure for imaging and super focusing is a new approach besides the concept of negative refractive index. It is possible to realize imaging resolution beyond diffraction limit with a certain working distance within several wavelengths range. To realize this target, one of technical challenges is that how to transfer the high spatial frequency near-field signals from evanescent wave to propagation wave. The other challenge is that how to amplify the near-field evanescent wave from conventional ~200 nm

This work was supported by the grants from the Guangdong Natural Science Foundation (S2011040000711), the Key Lab of Robotics & Intelligent System, Guangdong Province (2009A060800016), the Guangdong & Chinese Academy of Sciences Cooperation Project (2009B091300160), National Natural Science Foundation of China (60904031; 60776029), Shenzhen Science & Technology Research Funds (2009-203), the Main Direction Program of the Knowledge Innovation of Chinese Academy of Sciences (KGCX2-YW-166), and the Scientific Research Foundation for the Returned Overseas Chinese Scholars of Ministry of

Blaikie, R.J., Melville, D.O.S. & Alkaisi, M. M. (2006). Super-resolution near-field lithography

Cao, Q., & Jahns, J. (2002). Focusing analysis of the pinhole photon sieve: individual far-field model. *J. Opt. Soc. Am. A*, Vol.19, No.12, pp.2387-2393, ISSN 0740-3232 Cao, Q., & Jahns, J. (2003). Nonparaxial model for the focusing of high-numerical-aperture photon sieves. *J. Opt. Soc. Am. A*, Vol.20, No.6, pp. 1005-1012, ISSN 0740-3232

using silver lens: a review of recent developments. *Microelectronic Engineering*,

purpose of high resolution regardless of low yield.

*m* or even several wavelengths in free space.

Vol.83, No.4-9, pp.723-729, ISSN 0167-9317

realized.

to be ~ 1

**5. Acknowledgment** 

Education, China (2008-890).

**6. References** 

size of 60nm line width can be resolved in photoresist layer when silver slab separated 100nm from mask. By proper design and choice of material, nanolithography with better resolution can be realized by the very function of silver slab, and this technique will be a possible alternative nanolithography technique for the next generation lithography.

Fig. 15. The distribution of electromagnetic field in the model (Both X and Z is in unit of μm. Photoresist layer lies between Z=0.37μm and Z=0.47μm).

Fig. 16. The distribution of optical field in the 10nm cross-section of photoresist when silver slab separated 100nm from mask.

### **4. Conclusion**

Two types of maskless lithography are discussed in this chapter. The first uses an array of high-numerical-aperture photon sieves as focusing elements in a scanning X-ray maskless nanolithography system. The system operating at wavelength of 0.5~2nm synchrotron light sources radiated, each of a large array of photon sieves focuses incident X-ray into a diffraction-limited on-axis nanoscale spot on the substrate coated photoresist. The X-ray intensity of each spot is modulated by means of a spatial light modulator. Patterns of arbitrary geometry are exposed and written in a dot matrix fashion while the substrate on a stepping stage is precisely driven in two dimensions according to the computer program. The characteristics of synchrotron radiation light, resolution limits and depth of focus of the lithographic system are discussed. The design and fabrication of photon sieve are illustrated with a low-numerical-aperture amplitude-photon sieve fabricated on a chrome-coated quartz plate by means of laser-beam lithographic process, which minimum size of pinhole was 5.6um. The PSF of photon sieve in terms of side-lobes strength and main lobe width may be controlled by utilizing the apodization window function. The focusing performance of the photon sieve operating at wavelength of 632.8nm was simulated and tested. In combination with the synchrotron light sources, the photon sieve array X-ray maskless nanolithography is a promising tool in the nanometric device and special IC areas for the purpose of high resolution regardless of low yield.

The second focuses on the evanescent near field in nano-filmed noble metals. Subdiffraction-limited feature size can be resolved by using i-line illumination exposure. Compared with the model of original superlens, we separated the superlens 100nm away from the mask, under the illumination of i-line light, the initial simulation shows that the sub-diffraction-limited feature as small as 60nm line width with 120nm pitch can be clearly resolved without hard contact between mask and nano-filmed noble metal. By proper design of the materials and the parameters of nano-filmed layers, better resolution can be realized.

In brief, a plasmonic structure for imaging and super focusing is a new approach besides the concept of negative refractive index. It is possible to realize imaging resolution beyond diffraction limit with a certain working distance within several wavelengths range. To realize this target, one of technical challenges is that how to transfer the high spatial frequency near-field signals from evanescent wave to propagation wave. The other challenge is that how to amplify the near-field evanescent wave from conventional ~200 nm to be ~ 1*m* or even several wavelengths in free space.
