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Vapor deposition of crystalline porous solids

Periodic Reporting for period 2 - VAPORE (Vapor deposition of crystalline porous solids)

Reporting period: 2018-06-01 to 2019-11-30

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 will open up the path to integrate MOFs in microelectronics by developing a solvent-free chemical vapor deposition (CVD) route for MOF films. MOF-CVD will be 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 will start from a proof-of-concept case and will be 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 will be illustrated in proof-of-concept sensors.

In summary, by growing porous crystalline films from the vapor phase for the first time, VAPORE implements molecular self-assembly as a scalable tool to fabricate highly controlled nanopores. In doing so, the project will enable cross-fertilization between the worlds of nanoscale chemistry and microelectronics, two previously incompatible fields.
The progress of the project is in line with the description of action. The progress per task package (that can be shared at this moment) is summarized below.

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 vapour 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 to deposit and characterize MOF thin films is under development. 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 vapour phase reactivity of precursors other than ZnO has been evaluated and led to the successful formation of MOF layers. The focus will continue to be on elements that (1) form stable MOF materials, (2) can in some form be vapour deposited and (3) are compatible with a microelectronics environment. Specifically, different precursor classes have been investigated for divalent metal ions other than Zn(II) and for higher valence metal ions. As a complementary strategy, defect engineering of ZnO has been explored and was confirmed to lead to the behaviour hypothesized in the proposal. In addition, MOF materials with more than one organic linker in the framework have been successfully deposited as thin films. To further explore this latter route, a method has been developed to compare the vapour pressure of organic linkers in a semi-quantitative way and thus obtain information essential to develop a deposition process.

Pillar 3 – Control. Experiments have been performed to control the roughness and the conformal growth of the deposited MOF films.

Integration. 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. In the remainder of the project we expect to demonstrate the integration of various MOF films in these and other types of devices.
The progress of the project is in line with the description of action. The progress per task package (that can be shared at this moment) is summarized below.

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. These results have been submitted for publication. The instrument platform to deposit and characterize MOF thin films is under development. Our activities in this area yielded the most complete toolbox for of porosity measurements on MOF thin films described so far. These results will be published shortly.

Pillar 2 – Scope. The MOF-CVD process has been expanded to several other MOF materials. A demonstration for Cu(II) carboxylate materials has been published. Several other concepts are under preparation for publication or have been submitted already. The scope expansion of the MOF-CVD process will continue.

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. Recent results indicate that at least for certain cases we can even further improve our process and bring it to the same level as what is currently available for more standard materials in a cleanroom setting. The conformality of the process improvements has been verified using high aspect ratio micropillar arrays.

Integration. 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. In the remainder of the project we expect to demonstrate the integration of various MOF films in these and other types of devices.