Silver CVD by using (COD)(hfac)Ag

The CVD of silver [1] is based on a heat-induced decomposition of the ligand-stabilized silver β-dikentonate precursor (1,5-cyclooctadiene)(1,1,1,5,5,5-hexafluoro-acetylacetonato)silver(I) (in short terms: (COD)(hfac)Ag), to pure silver, metal-organic, and organic byproducts.

• The chemical reaction is given by
  
  1. (COD)(hfac)Ag^_(gaseous) → (hfac)Ag^_(adsorbed) + (COD)^_(gaseous)
  2. 2(hfac)Ag^(adsorbed) → Ag^(solid) + Ag(hfac)₂^(gaseous)

The Process

In order to avoid contamination by organic byproducts, it is recommended to use a cyclic process. Each CVD cycle begins with the sublimation (1) of the solid metal-organic precursor at 65°C at low pressure (<0.5mbar) which fills the whole reaction chamber with (COD)(hfac)Ag due to gas diffusion. The sublimation temperature must not be chosen too high as, otherwise, the metal-organic substance decomposes mainly in the crucible before being adsorbed at the substrate surface.

After approximately 45 minutes, enough precursor molecules are adsorbed to initiate the thermal decomposition (2). Here, the substrate temperature influences the kinetics of the chemical reaction. If the temperature is below 150°C, the metal film predominantly suffers from organic contamination. If it is higher than 170°C, the surface mobility of the silver atoms increases such that the films tend to become more granular which deteriorates the optical properties. From our experience, 160°C–165°C are the decomposition temperatures giving the best compromise.

The decomposition step takes 10 minutes and results in the deposition of a 3 nm-thick silver film. To further improve the quality of the coatings, it is useful activate the dielectric surface of the template by applying air plasma (e.g., PlasmaPrep5 by Gala Instrumente GmbH) for 15 minutes to the sample before starting the metallization. Moreover, the metal-organic precursor, being sensitive to oxygen, must be kept under nitrogen atmosphere as long as possible before evacuation!

During the last step, the reactor chamber is evacuated. While silver remains on top of the template, the metal-organic byproducts are pumped out of the chamber. Repeating this cycle 10 to 15 times results in an electrically connected metal coating with a thickness of about 40 nm. During the whole process, the walls of the reaction chamber are heated to 110°C to avoid condensation of the precursor.

Self-made chemical vapor deposition (CVD) system used to deposit isotropic silver coatings. (a) Overview of the whole setup. From left to right: (i) Control tower containing all controlling devices, i.e., the power supply, outer-chamber heaters, inner-chamber heaters, and the gas-flow controller. (ii) Reaction chamber, valves and particle trap. (iii) PC for controlling all devices inside the tower (i). The automation of the CVD enables the control of the heating temperatures and a temporal control of the cycles. (b) Detailed view of the reaction chamber showing the precursor lock at the underside and the sample lock at the upper side of the reaction chamber. (c) Schematic of the reactor’s line system.

Recommendations for Silver CVD Setup

1. To build up your own silver CVD (see e.g. the picture above), it is recommended to automate the setup components (e.g., by using National Instruments' Labview) since the whole process might take 12h (depending on the intended film thickness). Moreover, the process becomes more reproducible since all relevant experimental parameters (i.e., the substrate / precursor temperature and the chamber pressure) can be monitored.
2. Importantly, all CVD chamber parts have to be made of chemically compatible or highly inert materials (stainless steel, nitride ceramics, poly(tetrafluoroethylene), etc.) in order to prevent reactions with the organic byproducts. Especially, standard copper gaskets should be avoided since there exists a connatural precusor (hfac)(COD)Cu which might be produced inside the chamber during silver deposition.
3. The reactor chamber must be cleaned after each and every CVD process to reduce contamination by remaining organic molecules. Otherwise, the optical quality of the films will become worse.

References:

1. M. S. Rill, Three-Dimensional Photonic Metamaterials by Direct Laser Writing and Advanced Metallization Techniques, EVA Star: urn:nbn:de:swb:90-186141, ISBN: 978-383 811 8888 (2010).

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