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Micro/Nano Lithography

How to make 3D functional microstructures

Controlling the size of quantum dots produced in situ for multi-color luminescent polymers and silver particles can fabricate luminescent polymers and nanoscale metal structures.
20 January 2010, SPIE Newsroom. DOI: 10.1117/2.1201012.003300

Photolithography has played a critically important role in the development of the microelectronics industry. For instance, it is used to manufacture chips, where light-sensitive materials that resist etching or plating coat a surface. Some areas of the ‘photoresist’ are exposed to laser light, become soluble and are washed away. The chip surface is then either etched or plated to make circuit patterns.

As electronic circuitry becomes smaller and smaller, the diffraction limit requires short-wavelength light sources (i.e., deep UV of 193nm) for nanometer-resolution photolithography. However, in the last decade, an emerging photolithography technique called multiphoton processing (MPP) has overcome this limitation.1 Although MPP uses an ultrafast 800nm-wavelength laser, resolutions of as few as several tens of nanometers have been achieved.2,3 Furthermore, unlike traditional photolithography, MPP can fabricate different designs of 3D microstructures, which is vital for the development of microdevices, such as optical microswitches, microamplifiers, or metallic microcircuits. Until now, only standard polymer photoresists have been used in MPP. However, these polymers lack additional properties such as luminescence and conductivity, and so cannot be used as functional devices.

We developed ways to separately enhance the luminescence and conductivity of these materials. Our method fabricates luminescent quantum dots, in situ, dispersed in a photoresist. By tuning the cross-linking of the photoresist polymers, we control the size of the QDs and obtain multi-color luminescence. In a separate process, we fabricate the conductive metal nanostructures by MPP of diammine silver ions, using metal-growth-inhibiting surfactants to control the size of the resulting silver particles.

As luminescent materials, quantum dots (QDs) exhibit unique optical properties. However, they clump together and so it is difficult to disperse QDs into a high-viscosity photoresist. This aggregation makes the photoresist become opaque, which hinders the laser from focusing inside the photoresist and prevents us from fabricating 3D microstructures. To overcome this problem, we have proposed a method that combines MPP and in-situ synthesis of nanoparticles. In this way, we have successfully fabricated 3D photonic crystal titanium oxide/polymer nanocomposites.4 However, if we want to fabricate the luminescent microstructures with different colors, we need to control the QD size.

Our approach5 for controlling the size of in-situ synthesized cadmium sulfide (CdS) QDs within a polymer matrix is to tune the crosslinking density of the polymers obtained by MPP. The size of the in-situ synthesized CdS QDs within the polymer matrix is confined by the space available within the crosslinking polymer network. Our design of photopolymerizable resins comprises precursors of CdS QDs, monomers, and oligomers, along with the photoinitiator and the photosensitizer. We have verified that the polymer network density has a significant effect on the CdS QDs size by changing the crosslinker content in a series of photopolymerizable resins.

We have successfully fabricated multicolor microstructures via the combination of MPP and the size controlled in-situ synthesis of CdS QDs within 3D microstructured polymer matrices.5 First, we used MPP to fabricate 3D microstructures of polymers containing CdS precursors. Then, we placed the 3D microstructures inside a glass container to generate CdS QDs. Subsequently, we introduced hydrogen sulfide gas into the container and kept it there for over 48h for the in-situ synthesis of CdS QDs inside the microstructures.

We obtained scanning electron microscopy (SEM) and fluorescence images of multicolor luminescent 3D microstructures of CdS–polymer nanocomposites (see Figure 1:a and b). The same 3D microstructures could luminesce with different colors, because we made a variety of sizes of CdS QDs in the microstructures, and these showed different colors under UV light irradiation. Clearly, our way of combining 3D MPP and the in-situ synthesis of semiconductor QDs provides an effective method for the microfabrication of 3D structures from semiconductor–polymer nanocomposites.

Figure 1.Scanning electron microscopy (left) and fluorescence-microscopy (right) images of a 3D microbull (a) and a 3D microlizard (b) of cadmium sulfide (CdS)-polymer nanocomposites. It can be seen that the same 3D microstructures can luminesce with different colors.

We separately fabricated metallic structures by multiphoton photoreduction. We expect to use these in microelectronic devices and metamaterials, which require conductive microstructures. We used diammine silver ions as the silver resource. However, when we performed the photoreduction by MPP, the generated silver particles showed a wide size distribution from micrometers to nanometers. We used surfactants as a metal growth inhibitor to control the size of silver particles. We were able to obtain silver particles with the average size of 18nm,6 which makes the fabrication of nanoscale silver structures possible. The minimum size of the silver microstructures fabricated by the proposed method was much finer than the diffraction limit of light. The line width of the silver lines decreased with decreasing laser power, and at a laser power of 0.87mW, a 120nm line was obtained as shown in Figure 2.7 In this way, we also reduced the feature size of 3D structures to 180nm.

Figure 2.(a) Scanning electron microscope images of a silver line (width 120nm) and (b) silver-pyramid array fabricated by MPP photoreduction.

In summary, we have successfully used MPP to fabricate 3D microstructure functional materials, such as luminescent nanocomposites and noble metals. We are now working to apply these functional 3D microstructures in photonic micro/nanodevices. We hope to use the novel optical, electric and magnetic functions in micro/nanoscale functional devices and integrated circuits by developing functional materials.

This research was financially supported by the National Natural Science Foundation of China (Grant Nos. 50973126, 60907019, 51003113), the National Basic Research Program of China (2010CB934103), and the International Cooperation Program of the Chinese Ministry of Science and Technology (2008DFA02050, S2010GR0980).

Xuan-Ming Duan, Xian-Zi Dong, Zhen-Sheng Zhao
Key Laboratory of Functional Crystals and Laser Technology Technical Institute of Physics and Chemistry, Chinese Academy of Sciences
Beijing, China