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Vapor deposition of crystalline porous solids

Periodic Reporting for period 4 - VAPORE (Vapor deposition of crystalline porous solids)

Reporting period: 2021-06-01 to 2021-11-30

Metal-organic frameworks (MOFs) are crystalline solids with highly regular pores in the nanometer range. The possibility to create a tailored nano-environment inside the MOF pores makes these materials high-potential candidates for integration with microelectronics, e.g. as sensor coatings, solid electrolytes, etc. However, current solvent-based methods for MOF film deposition, a key enabling step in device integration, are incompatible with microelectronics fabrication because of contamination and corrosion issues.

VAPORE opened up the path to integrate MOFs in microelectronics by developing a solvent-free chemical vapor deposition (CVD) route for MOF films. MOF-CVD is the first example of vapor-phase deposition of any type of microporous crystalline network solid and marks an important milestone in processing such materials. Development of the MOF-CVD technology platform started from a proof-of-concept case and was supported by the following pillars: (1) Insight in the process, (2) expansion of the materials scope and (3) fine-tuning process control. The potential of MOF-CVD coatings was illustrated in proof-of-concept sensors.

In summary, by growing porous crystalline films from the vapor phase for the first time, VAPORE implemented molecular self-assembly as a scalable tool to fabricate highly controlled nanopores. In doing so, the project enabled cross-fertilization between the worlds of nanoscale chemistry and microelectronics, two previously incompatible fields.
Pillar 1 – Understanding. The MOF material ZIF-8 was studied in depth as a model material. The hypothesis regarding the importance of water in the vapor phase deposition of this material was confirmed through a series of rigorous experiments. A standardized deposition protocol was developed in which the water content in the atmosphere above the reacting surface is precisely controlled. This protocol enables to isolate the importance of temperature control and uniformity in the deposition of high-quality films over large areas. In addition, as part of the ERC action, a state-of-the-art instrument platform has been developed to deposit and characterize MOF thin films. Extensive testing has been performed for different parts of this setup to determine the most suitable make-up of the platform.

Pillar 2 – Scope. The vapor phase reactivity of precursors other than ZnO has been evaluated and led to the successful formation of MOF layers. Specifically, different precursor classes have been investigated for divalent metal ions other than Zn(II) and for higher valence metal ions. The landscape of possible polymorphs for some MOFs can pose a challenge for controlling the outcome of their syntheses. We demonstrated the use of a template to control in the vapor-assisted formation of zeolitic imidazolate framework (ZIF) powders and thin films.

Pillar 3 – Control. Methods have been developed to control the roughness and the conformal growth of the deposited MOF films. Nanoscale patterning is a fundamental step in the implementation of MOFs in miniaturized solid-state devices. We demonstrated the resist-free, direct X-ray and electron-beam lithography of MOFs. The resulting high-quality patterns have excellent sub-50-nm resolution. This work was highlighted on the cover of Nature Materials (Jan 2021 issue). In addition, we developed routes for area-selective deposition of MOF precursors and established how the precursor-to-MOF transformation is influenced by the deposition method. We introduced positron annihilation lifetime spectroscopy (PALS) to obtain pore size information and depth profiling in MOF films and illustrated its complementarity to established methods.

Integration & proof-of-concept applications. The integration of high-quality MOF-CVD films as high-performance insulators was demonstrated. The properties of the MOF-CVD process and the deposited films are well-suited for the integration in future low-power processor chips. These results led to the ERC PoC grant ‘LO-KMOF’ (2019-2021). In addition, two types of photonic sensing devices have been demonstrated: single-layer surface plasmon sensors (using ellipsometry as a read-out), and multilayer photonic crystal structures (using spectrometry as a read-out method). In addition, a strategy was developed to fabricate sensors with a direct electrical readout through the integration of transistors. This route will be further explored in the ERC PoC grant ‘MOFFET’ that was granted in Feb 2022.
Pillar 1 – Understanding.
• A standardized deposition protocol was developed for ZIF-8 and has been demonstrated on 200 mm wafers. The pinhole-free nature of these films was established.
• A unique instrument platform to deposit and characterize MOF thin films has been developed.
• Our activities in this area yielded the most complete toolbox for of porosity measurements on MOF thin films described so far.

Pillar 2 – Scope.
• The MOF-CVD process has been expanded to several other MOF materials. Specifically, different precursor classes have been investigated for divalent metal ions other than Zn(II) and for higher valence metal ions.
• We demonstrated the use of a template to control in the vapor-assisted formation of zeolitic imidazolate framework (ZIF) powders and thin films. The templating principle was demonstrated for other MOFs as well.
• Defect engineering of ZnO has been explored and was confirmed to lead to the hypothesized enhanced reactivity.
• MOF materials with more than one organic linker in the framework have been successfully deposited as thin films.
• A method has been developed to compare the vapor pressure of organic linkers in a semi-quantitative way and thus obtain information essential to develop a deposition process.

Pillar 3 – Control.
• It was determined that certain additives dramatically influence the film roughness. This knowledge has enabled us to deposit MOF films of higher quality and over a larger area than reported thus far in the literature.
• We demonstrated the resist-free, direct X-ray and electron-beam lithography of MOFs. The resulting high-quality patterns have excellent sub-50-nm resolution.
• We developed routes for area-selective deposition of MOF precursors and established how the precursor-to-MOF transformation is influenced by the deposition method.
• We introduced positron annihilation lifetime spectroscopy (PALS) to obtain pore size information and depth profiling in MOF films and illustrated its complementarity to established methods.

Integration & proof-of-concept applications
• The integration of high-quality MOF-CVD films as high-performance insulators was demonstrated. The properties of the MOF-CVD process and the deposited films are well-suited for the integration in future low-power processor chips. These results led to the ERC PoC grant ‘LO-KMOF’ (2019-2021).
• Two types of photonic sensing devices have been demonstrated: single-layer surface plasmon sensors (using ellipsometry as a read-out), and multilayer photonic crystal structures (using spectrometry as a read-out method). Both types of devices require the smooth MOF layers developed in VAPORE.
• A strategy was developed to fabricate sensors with a direct electrical readout through the integration of transistors. This route will be further explored in the ERC PoC grant ‘MOFFET’ that was granted in Feb 2022.
Nature Materials cover 2021