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Large-area surface-plasmon polariton interference lithography Xiaowei Guo, Jinglei Du, and Yongkang Guo Department of Physics, Sichuan University, Chengdu 610064, China
Jun Yao Department of Physics, Durham University, Durham DL1 3DJ, UK Received April 27, 2006; revised June 20, 2006; accepted June 21, 2006; posted June 23, 2006 (Doc. ID 70265); published August 9, 2006 Large-area surface-plasmon polariton (SPP) interference lithography is presented, which uses an attenuated total reflection-coupling mode to excite the interference of the SPPs. The interference of the SPPs causes a highly directional intensity range in a finite depth of the electric field, which is good for noncontact. Finite-difference time-domain simulations of the interference on a thin resist layer show that broad-beam illumination with a p-polarized light at a wavelength of 441 nm can produce features as small as 60 nm with high contrast, smaller than / 7. Our results illustrate the potential for patterning periodic structures over large areas at low cost. © 2006 Optical Society of America OCIS codes: 240.6680, 240.6690, 260.3160, 260.3910.
The continuing size reduction of integrated circuits to nanometer-scale dimensions requires the development of new lithographic techniques. It becomes increasingly complex and costly to use the established method of optical projection lithography at the short optical wavelengths. Interference lithography, which has some advantages such as practically unlimited depth of focus and large exposure fields,1,2 can achieve an ultimate resolution for a certain wavelength without complicated and expensive high numerical aperture optics, although it is common for making periodic patterns. Evanescent wave interference lithography3,4 (EIL) further improves the resolution, but is limited by low transmission, short expose depth, and low contrast. The presence of surface-plasmon polaritons (SPPs) makes it possible to solve the above problems for their characteristics of transmission enhancement in the near field.5 SPPs are waves that propagate along the surface of a conductor, usually a metal. Using SPPs, the surfaceplasmon (SP) resonant interference nanolithography technique6 (SPRINT) and SP interference nanolithography7 (SPIN) successfully fabricated a sub-100 nm interference pattern. However, both techniques are suitable for only small-area interference and need the fabrication of a mask grating with a very fine period. In this Letter, we discuss a simple but practical scheme, the attenuated total reflection (ATR)coupling mode, for exciting SPPs interference, which avoids fabricating the mask grating and can be applied to large-area fabrication. This physical situation is modeled as in Fig. 1, which is characterized by the Kretschmann configuration often used in a surface-plasmon resonance (SPR) sensing system.8 The uppermost layer is an isosceles triangle or semispherical prism with a high refractive index, then a thin metal film is coated on the bottom surface of the prism, and last they are brought into intimate con0146-9592/06/172613-3/$15.00
tact with a thin resist covered on a substrate. The momentum transfer of an optical wave to the resultant SPP is enhanced by ATR coupling. The SPP is associated with an electromagnetic wave, the field vector of which reaches its maxima at the interface between the metal and resist layers and decays evanescently into both media. The dispersion relation for the SPP at the interface can be expressed as9 kSPP = ko关md/共m + d兲兴1/2 ,
共1兲
where kSPP and ko are the wave vectors of the SPP and the incident light in free space m and d are the dielectric constants of the frequency-dependent metal film and resist, respectively. The wave vector of the SPP can become significantly large when the real part of m approaches −d, so employing the SPP for lithography may dramatically increase the pattern resolution. When SPR occurs, the wave vector kx of the incident optical wave projected on the plane parallel to the surface of the metal film must equal kSPP. The relation between kx and kSPP is kx = konpsin i = kSPP ,
共2兲
where np is the refractive index of the prism and i is the incident angle of the optical wave. If the reso-
Fig. 1. (Color online) Schematic of the LSPPIL process. © 2006 Optical Society of America
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Fig. 2. Electric field distribution of the interference patterns for the coated and uncoated prism configurations under the conditions: the incidence wavelength = 441 nm, TM polarization; the incidence angle i = 59.9°; the thickness of the silver film h = 30 nm.
nance frequency falls within the sensitivity range of a photoresist, the field enhancement can be used to locally expose a thin resist layer. When two mutually coherent plane waves with p polarizations are incident on the base of the prism in the vicinity of the resonance angle, there arise multiple counterparts about the SPP everywhere on the interface, which indicates that large-area surfaceplasmon polariton interference lithography (LSPPIL) is essentially different from SPRINT and SPIN. As a result, interference fringes are formed in the resist. Obviously, the technique can be applied to large-area interference if the prism is large enough. It is worth pointing out that the pattern period is fixed once the wavelength of the light source and the materials is chosen. The behaviors of the SPP interference fringes are analyzed below based on a finite-difference timedomain (FDTD) technique. The perfectly matched layer method10 is applied in our numerical model. The results in an uncoated prism configuration under the same conditions are also presented for comparison. The refractive indices for the prism and resist used in the simulation are 1.94325 and 1.53, respectively. The metal material is silver having a dielectric constant given by −8.9170+ 0.2320i at a wavelength of 441 nm. Figure 2 shows the electric field distributions of the interference pattern at the incidence angle of 59.9°. Figures 2(a) and 2(b), respectively, represent the Ex and Ez components of the SPPs’ interference. The thickness of the silver film is 30 nm. The origin of decay length is at the lower surface of the silver film. The electric field distribution indicates that a photoresist layer in the near field of the silver film is index matched to its supporting substrate, which is experimentally possible with appropriate choices of materials. The period ⌳ of the interference pattern is approximately 120 nm, say the critical dimension of
60 nm, smaller than / 7. The wavelength of the SPP is 2⌳ = 240 nm in good agreement with the theoretical value 247.6 nm. Figures 2(c) and 2(d) are corresponding to the evanescent wave interference. The origin is located in the lower surface of the prism. Although it would provide the same resolution,11,12 the decay length is much shorter. The simulated results show the intensity transmission of the former is 94%, whereas the latter only has a value of 17%. Such high transmission in the coated configuration provides a promise for the interference fringes having long exposure depth and high contrast. As with the SPIN, for both interference cases, the fringes of Ex and Ez components have a half-period shift in space due to their phase differences of / 2. The shift has a strong impact on the intensity contrast of the interference pattern, which will be discussed in detail later. Figure 3(a) illustrates the evolution of normalized intensities for both Ex and Ez components with the distance from the origin in the coated and uncoated cases. 兩E兩2 / 兩E0兩2 is the normalized squared electric field intensity in the near field of the metal film and determines the exposure profile in the photoresist,13 where 兩E0兩2 is the intensity of the incident radiation. For both the coated and uncoated cases, all Ex and Ez components exponentially decay away from the origin, and normalized intensities of the Ez components are much larger than those of the Ex components. Therefore the total intensity is dominated by the Ez components, whose intensity value in the coated case is far beyond that in the uncoated case. The intensity value is greatly improved by a factor of ⬃23 owing to the enhancement of the SPP in the near field. Given the fact that the typical threshold value of intensity 0.3 is adopted by conventional lithography, the effective exposed depth is approximately 250 nm for SPP interference, almost four times the theoretical decay length 1 / 兩kz 兩 = / 2 兩 冑共Ag + d兲 / 共d兲2 兩 = 76.9 nm, while the evanescent wave interference obtains only 60 nm. In this sense, hard contact is not needed for SPP interference during exposure. A layer of refractiveindex-matched liquid is only necessary to be added
Fig. 3. (a) Normalized intensities of the electrical components and (b) the intensity contrast of the Ez components as a function of the distance from the origin in coated and uncoated cases.
September 1, 2006 / Vol. 31, No. 17 / OPTICS LETTERS
on the bottom side of the silver film as a protection layer in the experiment. Since the photoresist is sensitive to the total optical field intensity, and there is a half-period shift in space between the interference fringes of Ex and Ez components, the ratio t of Ez / Ex must be high (or low) enough to provide sufficient intensity contrast. For the coated case, the simulated result at approximately 2.236 is larger than the theoretical prediction −kx / kz = −i冑兩Ag 兩 / d = 1.952, which is the contribution of multiple SPPs from neighboring points of the interference area. The intensity contrast defined by V = 共t2 − 1兲 / 共t2 + 1兲 as a function of the distance from the origin for both cases is shown in Fig. 3(b). The contrast of the SPP interference is approximately 0.75, which is sufficiently high for lithography purposes. For the uncoated case, the contrast has a value of 0.5 in the range of 50 nm near the origin, then rapidly falls down and approaches zero at approximately 100 nm. Thus the evanescent wave interference is difficult to be realized in the resist. Further analysis reveals that the normalized intensity and contrast of the SPP interference pattern are affected by the thickness of the silver film and the incidence angle. Figure 4(a) shows the normalized intensity and contrast as a function of the film thickness at y = 100 nm. There is an optimal thickness 30 nm over (or under), at which the intensity decreases rapidly in coincidence with the results in the SPR sensing system.14 It is interesting that the contrast decreases with the increase in film thickness. The contrast enhancement will drop due to the excitation of multipolar oscillations in the metal film when a large thickness is used. To investigate the effect of the angle, we fix one of the interfering beams and slightly adjust the angle of the other in view of the alignment errors in the experiment, and the results are plotted in Fig. 4(b). It has been found that
Fig. 4. Normalized intensity and contrast of the fringes as a function of (a) the silver thickness and (b) the angle deviation at y = 100 nm.
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the angle deviation within ±1° makes small influences on the contrast at y = 100 nm. The contrast curve is almost symmetric and the amplitude of variation is 3.8%, while for the normalized intensity, it is relatively large and reaches 10.7% but is still very small compared with the variation in the sensing system.15,16 The phenomena can be ascribed to more energy transfer to the SPPs under the interference condition resulting in the stability enhancement. This offers a relatively large tolerance for the arrangement of the experimental setup. In conclusion, we have described that a nanolithography technique, LSPPIL, can be attained with the interference of the SPPs excited by optical frequency. It has several advantages, including high transmission, high contrast, soft contact, and large-area fabrication by using a simple configuration. This technique will offer great promise for patterning nanometer structures and printing easily onto substrates with a small fluctuation surface. The authors thank Kest Nicholas for useful discussions. This research was supported by the National Natural Science Foundation of China (60376021) and the Institute of Optics and Electronics, CAS. X. Guo’s e-mail address is
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