Plasmonic Nanolithography: A Review
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- Xie, Z., Yu, W., Wang, T. et al. Plasmonics (2011) 6: 565. doi:10.1007/s11468-011-9237-0
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Surface plasmon polaritons (SPPs) has attracted great attention in the last decade and recently it has been successfully applied to nanolithography due to its ability of beyond diffraction limit. This article reviews the recent development in plasmonic nanolithography, which is considered as one of the most remarkable technology for next-generation nanolithography. Nanolithography experiments were highlighted on the basis of SPPs effect. Three types of plasmonic nanolithography methods: contact nanolithography, planar lens imaging nanolithography, and direct writing nanolithography were reviewed in detail, and their advantages and shortages are analyzed and compared, respectively. Finally, the development trend of plasmonic nanolithography is suggested.
KeywordsPlasmonic nanolithographyContact nanolithographyPlanar lens imaging nanolithography
Lithography technique has always been considered as a mainstream of fabrication methods in semiconductor industry for its high throughput and cost effective in comparison to electron beam lithography  and focused ion beam writing over the past several decades. Higher throughput, lower cost, higher resolution, and simplification of system configuration are the targets we always pursue. Various types of nanolithography techniques have been explored before, such as electron beam lithography, nanoimprint lithography , dip-pen lithography [3, 4], and others. For electron beam lithography, the smallest resolution of less than 10 nm has been demonstrated, but the throughput of this technique is rather low so that it is mainly used for the fabrication of masks rather than mass production. Nanoimprint lithography, with the resolution of less than 10 nm and a high throughput, can be used in mass production. However, as a replication method, there are still some issues for nanoimprint lithography to address. One of the issues is that the residual resist layer after imprinting can arise which may limit its application . Dip-pen lithography has the same disadvantage of low throughput as electron beam lithography. Apart from the aforementioned techniques, photolithography is another important technique for nanolithography. The conventional photolithography techniques for nanolithography include optical projection lithography (193 immersion lithography) [6–8], X-ray lithography , extreme ultraviolet lithography , zone-plate-array lithography [11–16], and so on. The optical projection lithography is mostly used in industry due to its high throughput. But with the requirement for smaller feature sizes, conventional optical projection lithography technique cannot meet the resolution requirement anymore because it is diffraction limited. Conventionally, the resolution of optical projection lithography is enhanced by means of reducing the illumination wavelength or increasing the numerical aperture (NA), which brings too much complexities and escalating cost. X-ray lithography can have a high throughput and has demonstrated a 50 nm resolution, but the X-ray lithography system is rather expensive, which urges us to cast about for a lithography system with lower cost to substitute it . Extreme ultraviolet lithography can also obtain a high resolution, but the high cost of light source and complexity of the optical exposure system limit its application in industry for mass production use. As to the zone-plate-array lithography, it is a novel schematic method to be able to expose arbitrary patterns with relative fast speed. However, its resolution is still diffraction limited.
Near-field optical lithography provides a new route to beat the diffraction limit and could achieve a resolution without the limitation in theory. Recently, many sorts of near-field lithography systems have been reported [18–22]. Conventional near-field lithography has achieved sub-50 nm resolution using special masks such as light-coupling mask or phase-shift mask . But one of the main shortcomings of near-field lithography is that the transmittance of the light is extremely low. For the apertures on the mask smaller than illumination wavelength, the amount of the light reaching resist is rather low because most of the light has been diffracted and scattered away. This leads to a considerably long exposure time and low image contrast.
Generally, plasmonic nano-photolithography can be categorized into three types in terms of the exposure methods: namely contact nanolithography, planar lens imaging nanolithography, and direct writing nanolithography, respectively. In the following section, we will try to review the plasmonic nanolithography experiments as many as possible by dividing all these experiments into the three types techniques mentioned above.
Plasmonic Contact Lithography
Plasmonic contact lithography is a modified form of evanescent near-field optical lithography designed to improve subwavelength image quality . In this method, the photoresist is exposed by SPPs which is originated from the metal mask. As the SPPs can only propagate tens of nanometers beneath the metal film, the intimate contact between the mask and photoresist film is necessary. Many efforts have been devoted to the exploitation of this technique.
Shao et al. exploited a surface plasmon-assisted nanolithography system similar to the one discussed above , whereas a UV lamp is employed to provide light irradiation. The photomask is made from a 70-nm thick titanium film with the patterns of grating structures and ring apertures. The mask is intimately in contact with the photoresist without the spacing layer. An 80-nm thick titanium shield is added in between the photoresist and the substrate to focus the light intensity in the photoresist so as to achieve nanoscale patterning with high density and absorb the light as it reaches the substrate. In this experiment, both the number of apertures and the periodicity are the critical factors affecting the lithography results. The grating with a period of 400 nm was transferred fairly well and the pattern obtained in the resist has a height of ∼35 nm. However, the pattern that has a single aperture was not transferred to the resist, which indicates that the performance of the mask mainly depends on design of the aperture shape and aperture size in the mask.
Zayats and Smolyaninov proposed a new method to achieve strongly enhanced transmittance of an individual subwavelength aperture . They demonstrated that the optical transmission of an individual subwavelength aperture in a multi-layered metal film was shown to be strongly enhanced compared with that of a homogeneous single-layered metal film due to the light coupling to surface plasmon excited by a periodically structured film. This method enhances optical transmission in up to two orders of magnitude in comparison to an aperture with the same size perforated in a homogeneous film. Moreover, the multi-layered metallic film can be conventionally prepared with well-developed thin-film deposition techniques. Such structures are robust and are easy to fabricate due to the absence of the fabrication of complex periodic structure required by conventional single-metal-film plasmonic mask. This proposal can be used in lithography and it may deliver a better lithography result. The application in lithography is going to be demonstrated experimentally. Similarly, a photolithography scheme using a designed metal-dielectric multilayer was proposed to generate subwavelength feature size . Generally speaking, the patterns fabricated in photoresist are identical to the patterns in mask for the scheme. However, numerical experiments show that a much smaller feature size could be obtained such as for a 400-nm period grating structure in a mask, a 67-nm period grating structure can be fabricated in photoresist.
The utilization of an array of holes in noble metals shows transmission enhancement with the aid of the resonant excitation of surface plasmon, while the spatial resolution is somehow sacrificed by the period of the holes array . Recent research showed that sharp-ridged apertures in metal, such as bowtie apertures or antennas, C-, H-shaped apertures and others, may realize a better result. The bowtie antenna was an important aperture type and was proposed firstly by Grober et al. and used as a near-field optical probe with high transmission efficiency at microwave frequency . Subsequently, a lot of efforts have been devoted to the research of the bowtie structure. And the applied frequency was extended to visible and even UV range. Highly confined hot spots with enhanced intensity were observed in the near-field of bowtie structures due to the localized surface plasmon resonance. It has been applied in many domains due to the enhanced transmittance of bowtie structures, such as high efficiency excitation of plasmonic waveguides , high-harmonic generation, and so on . Recently, it has been successfully applied in nanolithography as a novel method to improve the resolution.
Recently, Ueno et al. explored a new plasmonic lithography method using the localized field of nanogaps . The mask consisting of pairs of rectangular gold nanoblocks which are separated by nanogaps fabricated on glass substrate. In this experiment, a femtosecond laser (Tsunami, Spectra Physics) with a central wavelength of λL = 800 nm, a pulse length of τP = 100 fs, and a repetition rate of f = 82 MHz were employed as the illumination source. The beam is normally incident with respect to the substrate (x–y plane) and is linearly polarized along the diagonal of the blocks.
From above, we can conclude that the bowtie structure-based lithography and the nanogap-assisted lithography technique have a better resolution compared to the mask with holes array. Therefore, these two types of techniques may have a brighter future.
In addition, it is worth to mention that some researchers have tried to use spherical or nonspherical particles as masks for nanopatterning as well [24, 40]. In this case, the hole size and lattice period could be tuned independently.
Although the transmittance of plasmonic nanostructure has been greatly enhanced, plasmon damping originated from intrinsic metal absorption limits the achievable aspect ratio of the fabricated structures. To obtain higher field depth, K. Sathiyamoorthy proposed a novel concept of employing the dye medium to enhance plasmon propagation by compensating intrinsic loss associated with metal . In this proposal, polymethylmethacrylate (PMMA) doped with the dye material was employed as the mask substrate. Simulation results show that a 14.5-fold of field enhancement in the PMMA/dye medium can be obtained compared to that of the bare PMMA. In addition, Kid et al. exploited metal nanoparticle arrays for near-field optical lithography and spot sizes ranging from 30 to 80 nm with exposure depth ranging from 12 to 45 nm have been achieved using broad beam illumination with visible light and standard resist .
Plasmonic contact lithography has the advantages of high resolution and high throughput. But the mask used in this technique is rather expensive and the fabrication process is very complex. In addition, the intimate contact between the mask and substrate will result in the contamination of the mask and thus reduce lifetime of the mask. These intrinsic shortcomings will constrain its applications in mass production of industry.
Planar Lens Imaging Nanolithography
Many experiments have been implemented to confirm this prediction. Melville demonstrated optical imaging through a thin planar Ag layer with a thickness of 120 nm using a light source with a wavelength of 341 nm in 2004 . Feature size as small as 350 nm (with 700 nm period) was well resolved. Subwavelength imaging was not demonstrated because the Ag lens is not thin enough. In 2005, a modified system with a thinner lens with a thickness of 50 nm was demonstrated to be able to resolve a subwavelength image . Where, the superlens structure consists of 25 nm PMMA/50 nm Ag/10 nm SiO2. A 350 W mercury lamp with a 365 nm wavelength, which gives an intensity of 6.7 mW/cm2 was employed as the light source. The substrate is a 1-in. diameter silicon wafer. The image of a grating with a period of down to 145 nm has been successfully resolved. Numerical simulation shows that the smallest resolution that can be achieved using this method is 40 nm. A comparison between 50 nm single- and double-layer lens with two 30-nm layers was made by Melville in 2005 . Gratings with 170 nm period have been resolved for the double-layer lens. The results demonstrated that double-layer lens has a shorter exposure time. The enhancement of the transmission has been achieved through the double-layer stack despite the increase in total thickness. The resolution limit for the double layer is no less than a single layer with the same total Ag thickness. However, double-layer lens does not always lead to better imaging performance because of two reasons. One is that the lens has a specific resonance and the feature of the image will be distorted at spatial frequencies out of resonance. The other is that the double-layer lens has an increasing attenuation of the DC component of transmitted images, which reduces the image fidelity, particularly for dark-line features .
Lately, Zhong Shi proposed a method using a 193 nm Al film-based superlens with index-matching layer. Their simulation indicated that 20-nm resolution can be resolved by the introduction of an “index matching layer” between metal layer and the dielectric layer [66, 67]. The proposed 193-nm superlens may provide an alternative way to reach the 22-nm lithography node. In addition, Xu et al. proposed a metal-cladding superlens to effectively localize the surface plasmons for projecting deep subwavelength patterns . In this configuration, an additional Ag layer was inserted between the photoresist and the substrate. With this special configuration, both the intensity contrast of the interference pattern and the image can be remarkably improved.
For planar lens nanolithogrpahy, the light source is an important factor affecting the experimental results. A comparison about the illumination light with broadband and narrowband has been made experimentally by Blaikie. The results show that broadband light source offers the advantage of shorter exposure time and higher throughput while the depth of the pattern in the resist and the degree of line edge roughness are almost the same as that obtained by narrowband source. However, the resolution of the former is not good as that of the latter due to the dispersive nature of the broadband light source which will result in the aberrations of the planar lens . Therefore, the choice of light source is crucial for planar lens lithography experiment.
The near-field planar lens lithography relaxes the rigid request for intimate contact between mask and substrate to a certain extent. Therefore, damage to the mask due to the intimate contact can be reduced. But the superiority of the technique is still restricted because the image plane is located in the proximity of the superlens (normally the working distance is a few tens of nanometers or even less). Hence, the far-field superlens which can form subdiffraction limit image in far field of the superlens was proposed and experimentally demonstrated by Liu et al. . The far-field superlens consists of a conventional superlens and a nanoscale coupler. The coupler can convert evanescent wave into propagating wave by shifting the incident field wave vector into various diffraction orders and selectively enhance the evanescent waves from the object . In their experiment, an optical microscope (Zeiss Axiovert mat 200, 100× oil immersion objective, NA = 1.4) was used to illuminate the object with a 377-nm wavelength light source, combining with the far-field superlens, 50-nm width lines separated by 70 nm has been recorded by CCD. The experiment has successfully demonstrated the imaging ability of the far-field superlens, showing the potential to be applied to nanolithography.
Planar lens lithography has the advantage over the contact lithography at some aspects. However, it is still mask-based nanolithographpy technology. Therefore, it has all the disadvantages brought by the masks. In the next section, we will introduce the maskless nanolithography method based on plasmonics, i.e., plasmonic direct writing nanolithography method.
Plasmonic Direct Writing Nanolithography
In addition, plasmonic interference lithography, which is an important part of plasmonic part, is another maskless lithography technique. Conventional plasmonic interference lithography scheme created on the basis of attenuated total reflection-coupling mode has three important components: a high refractive index prism, a metal layer, and the photoresist coated on substrate. A high refractive index is preferred because the higher refractive index of a prism leads to a smaller critical resonance angle. And it was demonstrated that the finer experimental results are easy to achieve at smaller incident angle . Using this type of lithography system, 1D patterns using two-beam interference and 2D patterns using four-beam interference have been demonstrated [101–111]. The interferential beams are generally constituted from the first order diffraction light generated from the grating diffraction. In the conventional interference lithography, the photoresist and metal layer must be in contact closely. Therefore, the surface of the metal film and resist film are easily damaged or polluted. Furthermore, oxidation of the metal film is another problem due to exposure in air. To overcome these disadvantages, He et al. proposed and experimentally demonstrated a new approach based on backside-exposure technique . The structure of this system composes a prism, matching fluid layer and glass substrate, Ag film and resist layer. This new structure not only prevents the damage and pollution but also eliminates the high refractive index prism from the system. Using this method, interference fringes with feature size below 65 nm were experimentally obtained. The throughput of interference lithography is quite high and the image quality is acceptable. But it is applicable only to periodic and quasiperiodic patterns, which limits its applications .
Finally, we supplemented a quantum effect-based new optical method which can realize the subwavelength lithography . The method is similar to the traditional lithography but adding a critical step before dissociating the chemical bound of the photoresist. The subwavelength pattern is achieved by inducing the multi-Rabi oscillation between the two atomic levels. The proposed method does not require multiphoton absorption and the entanglement of photons. Initially, the molecules are in the ground state. Then two laser pulses with different frequencies were sequentially turned on. The first laser pulse, whose frequency is resonant with the energy difference between the two ground states, will induce Rabi oscillations between these two states. After that, the second laser pulse was turned on, and it will only dissociate the molecules that are in the excited states but not those in the ground states. The molecules that are dissociated will change their chemical properties, especially the solubility. The resulting patterns of the photoresist will thus depend on the spatial distribution of the excited state induced by the first laser pulse.
It is worthy to point out that as an alternative lithography technique, extreme-ultraviolet lithography at 13.5 nm is expected to be possibly introduced in high-volume semiconductor chip production over the next 3 years. Research is now underway to investigate sub-10 nm light sources that could support lithography over the coming decades . An extension of this technology could be an alternative option for next-generation sub-13.5-nm lithography. Many of the technical issues, such as target regeneration and high-repetition rate operation, are already solved. However, improving the low conversion efficiency (about 0.5% for 2.48 nm) remains a critical point. Moreover, high expenditure is a drawback issue in comparison to plasmonic nanolithography technique.
Comparison Between Different Plasmonic Lithography Methods
The three nanolithography methods have been reviewed in details in above sections. Below gives a comparison of three different plasmonic lithography methods.
Generally speaking, the plasmonic contact lithography can get a higher resolution using masks with ridged shape apertures. Although large-area lithography can be realized through the step and repeat exposure and the production is relatively high, it still cannot satisfy the industrial requirement. In addition, the fabrication of conformable masks is complex and the cost is rather high. Intimate contact, which is assured by the external force sometimes, could cause the severe damage on the mask and substrate. The complexity in conjunction with the rigid exposure requirement of intimate contact between mask and substrate limits its further applications in many aspects. Researches on the new kind masks, such as elastic masks, can be done to consummate this method. Furthermore, the environment should be maintained rigidly clean in the process of lithography since any particles may affect the experiment result and even destroy the experiment.
In comparison to the contact lithography, the planar lens lithography relaxes the complexity of process control without the requirement of intimate contact between the mask and substrate. Thus, the masks and substrates can be prevented from being destroyed. But the superiority is restrained by the limited working distance (a few tens of nanometers or even less). Therefore, the research on the superlens with larger working distance should be made in future. In addition, this method is relatively underdeveloped compared with the contact lithography. It is commonly known that the resolution of this method is inferior to plasmonic contact lithography. Researches should be made to enhance the resolution.
Plasmonic direct writing nanolithography is considered as the most promising technique for its flexibility and low cost. In addition, it is a maskless lithography method. Therefore, it avoids all the complexity aroused by the masks. Arbitrary patterns with the resolution as small as 50 nm (λ/8, λ = 405 nm) has been demonstrated by using this method. In addition, plasmonic direct writing lithography can be implemented in air and there is no such rigid environment requirement as the plasmonic contact lithography. Without the requirement of vacuum environment, the experiments are relatively easy to implement and it is a great superiority of this method. Forming arbitrary patterns and working in normal environment are the two superiorities of this method over former two lithography techniques. One of issues of the direct writing method is the low throughput. However, to expose the spinning substrate with flying plasmonic lens has the potential to achieve a throughput that is two to five orders of magnitude higher than other maskless lithography and thus to be able to satisfy the industrial requirement. However, issues for flying plasmonic nanolithography like pattern data management, lithography linewidth control, pattern overlay, and resist defect reduction are still need to be addressed to apply this technology for nanomanufacturing industry.
In this article, we reviewed a technology of plasmonic nanolithography in three catalogs: contact nanolithography, planar lens nanolithography, and direct writing nanolithography. Some experiments about the three kinds of lithography techniques are presented and the comparison between them is also made. From the analysis presented above, we can conclude that the higher resolution is the unique superiority of plasmonic lithography. And further studies need to be done to address the issues of plasmonic nanolithography like lower throughput, pattern relay, and others to introduce the technology into the real nanomanufacturing industry. In summary, plasmonic nanolithography is one of the most promising techniques for next generation nanolithography and it has the potential to be applied in mass production.
This work is supported by the National Natural Science Foundation of China with grant numbers of 90923036, 609770410, 60877021, and 61077010. The financial support from the 100 Talents Program of Chinese Academy of Sciences is acknowledged as well.