Neutron reflectivity characterization of the diffuse layer at a metal-polymer interface

In microfabrication processes, ultra-thin metallized polymers are commonly prepared using physical vapor deposition of a metal film on the polymer surface. During the deposition process, it is often expected that the metal layer will defuse into the polymer surface, thereby creating a bonding/sandwich layer near the metal/polymer interface. In this work, we demonstrate the presence of a diffused metal layer and its thickness using neutron reflectometry [1] and also discuss the electrical conductivity and mechanical properties of these metallized polymers [2, 3].

The azobenzene containing photosensitive polymer PAZO (pol{1-[4-(3-carboxy-4-hydroxyphenylazo)benzenesulfonamido]-1,2-ethanediyl,sodiumsalt}) with molecular weight of M_n = 1.4×10⁴ g/mol (ca. 40 repeat units) is dissolved in a solvent mixture of 95 % methoxyethanol and 5 % ethyleneglycol. A micrometer thick polymer film is prepared by spin coating the PAZO solution with 250 mg/ml concentration onto a glass substrate (5×7 cm²) followed by the physical vapor deposition of 10 nm thin gold film on top. The samples are then used to obtain neutron reflectivity curves at the time of flight reflectometer REFSANS Instrument operated at the MLZ in Garching near Munich.

Due to the unique possibility it offers, we chose neutron reflectometry to study the diffused metal layer near the metal/polymer interface. The instrument was operated using an incident wavelength spectrum ranging from 2 to 10 Å with a Δλ/λ resolution of 3 %. The opening of the beam defining slits were selected so as to keep the total ΔQ/Q = 6 % and the reflectivity was measured using three consecutive overlapping measurements at incident angles 0.3°, 0.6° and 1.4° over the q range extending to 0.2 Å-1.

We performed neutron reflectivity measurements on a pure glass substrate, PAZO/glass and Au/PAZO/glass samples. To model the neutron reflectivty data, we used a matrix formalism in terms of dynamical scattering theory. The parameters of our samples were optimized using the Levenberg-Marquardt least square fitting procedure. For the pure glass sample, the parameters obtained are as follows: scattering length density (SLD) 3.565×10^-6 ± 1.014×10^-9 Å-2 and roughness σ = 8.003 ± 0.024 Å.

Since the PAZO/Glass and Au/PAZO/Glass samples were prepared on the same glass substrate, we used the values obtained in further simulations. For the PAZO/Glass sample, we found that the air/polymer interface has a square-root gradient profile and is characterized by the following parameters: SLD of PAZO film at z = 0 ρ_PAZO (z = 0) = 1.135×10^-6 ± 4.49×10^-9 Å^-2, gradient thickness 125.9 ± 1×0.5 Å, mean SLD and absorption SLD of PAZO = 2.245×10^-6 ± 2.557×10^-9 Å-2, ρ_PAZO^abs = 4.479×10^-9 ± 2.732×10^-11 Å^-2.

On depositing a 10 nm gold layer on the polymer surface, one would expect it to form a interdiffusion layer composed of two compounds, a polymer and Au and an averaged profile which would be a superposition of two profiles. In order to confirm this, the first 200 Å of the scattering length density profile including the Au layer and the interface between the gold layer and the PAZO film is split into 10 slices. The parameters of every slice were optimized independently for each slice with physical constrains on the slice thickness and SLD. The former could have values in the range from zero to 25 Å, defined by the maximum experimental scattering vector Qmax, and the latter was limited from the upper side by 5×10^-6 Å^-2, which is larger than the calculated value of the scattering length density of gold 4.5×10^-6 Å^-2. The result in figure 1 shows perfect agreement between the measured and fitted curves and clearly indicates an approximately 45 Å thin diffused Au layer with gradient into the PAZO surface from the SLD profile (inset).

To study the mechanical properties of ultra-thin metal films, we utilized the unique photosensitive properties of the PAZO material. Azo-modified photosensitive polymer film is a suitable candidate, offering the possibility of reshaping bulk polymers and thin films by light-irradiation while in the glassy state. Azo-benzene molecules undergo photo-isomerisation transitions from the trans to the cis configuration and mass transports during irradiation with light interference patterns resulting in the formation of surface relief grating (SRG). The metal film deposited on a photosensitive polymer creates a spontaneous bonding layer (fig. 2a) and gives the advantage of making it possible to translate the optomechanical strain induced in a photosensitive polymer to the metal film on top leading to crack formation under very localized stresses (fig. 2b). More details are provided in our recent reports [1, 2]. We also observed that when thin metal films are ruptured, up to 80 % of metal film width and 500 nm depth into the polymer has no significant influence on the film’s electrical conductivity, making the ultra-thin metallized polymers advantageous for applications in deformable electronics [3].