Metal-organic frameworks (MOF) materials have chemical versatility and high porosity, and have great potential in the fields of electronics and photonics. For example, its high porosity results in a low dielectric constant (low-k), which is very suitable for use as a high-performance insulator in future electronic devices. However, in order to be applied to electronic equipment and other fields, compared with existing silicon-based materials, the most important challenge for MOF materials is to develop sub-micron-level patterning processes, so as to realize the direct integration of MOF materials into solid-state devices. However, the conventional MOF patterning technology has low resolution and blurred pattern edges, which greatly limits its application in microelectronic devices.
In view of this, Rob Ameloot's research group at the University of Leuven in Belgium has developed a technology that does not require photoresist and uses X-rays or electron beams to directly lithography MOF materials. This technology avoids etching damage and contamination, and preserves the porosity and crystallinity of the patterned MOF material. The high-quality patterns produced by lithography achieve accuracy below 50nm. More importantly, the X-ray and electron beam lithography methods used are compatible with the existing microfabrication and nanofabrication processes, which will greatly promote the integration and application of MOF materials in micro-devices. The research was published in the latest issue of "Nature Materials" with a paper entitled "Direct X-ray and electron-beam lithography of halogenated zeolitic imidazolate frameworks". The first author is Tu Min.
The conventional photolithography process requires the use of photoresist, which is one of the key materials for micro-pattern processing in microelectronics. Photoresist is an organic compound that changes its solubility in the developing solution after being exposed to light of a specific wavelength, allowing selective removal of exposed (or unexposed) areas.
However, this method can only be applied to dense MOF films. For microporous MOF materials, pollution and porosity loss will inevitably occur during the etching process. Some scholars have reported some strategies to pattern MOF in the early stage. In 2016, Andrea Cattoni and others further improved the accuracy to 200 nm. However, this is far from sufficient compared to the current 7 nm or even 5 nm lithography accuracy of silicon-based materials. Moreover, the pattern quality of the above-mentioned technology is not good, and is not compatible with the current nano processing technology.
Rob Ameloot et al. are ingenious, using the halogenated zeolite imidazolate framework (ZIF), which is a MOF material that is soluble under radiation, to achieve selective dissolution of exposed areas during X-ray or electron beam lithography. .
Direct MOF patterning through XRL and EBL
Simple said than done. To illustrate this concept, Rob Ameloot et al. conducted a detailed study on the ZIF-71 film of the X-ray mask (Figure 1a). The initial ZIF-71 sample is a polycrystalline film obtained by the gas phase transformation of a ZnO layer. X-ray irradiation can cause partial overlap of structural and chemical modifications. At lower doses, the crystallinity of the material gradually disappears and turns into an amorphous state. Further increase the dose of X-rays to completely dissolve the remaining materials. 1H and 13C nuclear magnetic resonance, X-ray photoelectron spectroscopy and elemental analysis indicate that the decomposition of ZIF-71 may be caused by the attack and destruction of the imidazole ring and the Zn-N coordination bond by the Cl radical induced by X-rays. Although amorphization occurred during the X-ray or electron beam irradiation, the solubility of the unhalogenated ZIF material did not increase, further emphasizing the key role of halogenated ligands.
Direct lithography of MOF materials
This radiation-induced solubility change provides the possibility for the patterning of MOF materials. As shown in Figure 1b, Rob Ameloot et al. used this X-ray lithography technology to successfully etch a ZIF-8_dcIm micron-level single crystal with the assistance of a mask. This hexagonal hole (mask) cuts the entire crystal with a "cookie cutter" method. Importantly, the X-ray diffraction characterization showed that the single crystal properties of the crystal did not change. This direct X-ray patterning method can produce crystals with arbitrary shapes or layered structure porosity.
X-ray patterning of MOF single crystal
In order to further improve the resolution, Rob Ameloot et al. used electron beams instead of X-rays and successfully fabricated a sub-50 nanometer hole array in the ZIF-71 film, which is the smallest feature size reported so far in MOF materials. In order to maintain the porosity of the MOF material itself, Rob Ameloot et al. used polymers to fill the pores.
Electron beam lithography to achieve accuracy up to 30 nm
Finally, Rob Ameloot et al. prepared a ZIF-71 film with periodic micropatterns and used it as a response diffraction grating for vapor detection. After the holes in the MOF material absorb the vapor, the refractive index will increase, which will cause the diffraction efficiency to change, and then the vapor can be detected by the camera.
Undoubtedly, this work has achieved an industrially feasible patterning technology for MOF materials and promoted the integration process of MOF materials in miniature solid-state devices. Combined with the vapor deposition technology related to MOF materials in the early stage, it will make it a reality to use MOF materials as low-k materials in electronic devices.
However, in terms of technology, although the MOF material itself can be used as a photoresist, X-ray and electron beam lithography techniques are still inefficient in industrial mass production. Therefore, it is necessary to use the most advanced high-throughput technology (such as extreme ultraviolet lithography) to further explore nano-level MOF patterning technology. In addition, the resolution also needs to be further improved, which requires optimizing the uniformity of the polycrystalline MOF film to improve the roughness of the pattern edge. In addition, MOF materials also have potential applications in photoresists for three-dimensional shaping (for example, through two-photon lithography).
Another problem to be solved is how to extend this method to other MOF materials. The presence of halogen atoms on the imidazole linking group is the key to achieving solubility changes in ZIF. However, the author predicts that the presence of the halogenated linker is not enough to enable direct lithography with other MOF materials. To solve this problem, in-situ mechanism studies related to amorphization, free radical formation and chemical decomposition are also needed.
This work will stimulate new research on the radiation chemistry of MOF materials, a relatively unexplored area that may lead to a large number of applications.
Full paper link:
https://www.nature.com/articles/s41563-020-00827-x