Characteristics and Mechanism of Cu Films Fabricated at Room Temperature by Aerosol Deposition
© Lee et al. 2016
Received: 20 January 2016
Accepted: 16 March 2016
Published: 24 March 2016
We were successful in growing a dense Cu film on Al2O3 substrates at room temperature using an aerosol deposition (AD) method. The characteristics of Cu films were investigated through electrical resistivity and X-ray photoelectron spectroscopy (XPS). The resistivity of Cu films was low (9.2–12.5 μΩ cm), but it was five to seven times higher than that of bulk copper. The deterioration of the resistivity indicates that a Cu2O phase with CuO occurs due to a particle-to-particle collision. Moreover, the growth of Cu films was investigated by observing their microstructures. At the initial stage in the AD process, the impacted particles were flattened and deformed on a rough Al2O3 substrate. The continuous collision of impacted particles leads to the densification of deposited coating layers due to the plastic deformation of particles. The bonding between the Cu particles and the rough Al2O3 substrate was explained in terms of the adhesive properties on the surface roughness of Al2O3 substrates. It was revealed that the roughness of substrates was considerably associated with the mechanical interlocking between Cu particles and rough Al2O3 substrate.
The rapid growth of wireless communications as well as the use of ubiquitous technology has led to a dramatic increase of interest in the design and fabrication of miniaturized RF/microwave devices . In particular, planar devices such as patch antennas and planar band-pass filters have attracted attention for their high frequency integrated circuits [2–4]. Therefore, the study of metallic materials and metallization processes is necessary to meet the strong need for planar device technology. Copper is a desirable material for connecting circuit elements of integrated circuits with submicron features because of their low resistivity, high thermal conductivity, and low coefficient of resistance [5, 6]. As techniques for Cu deposition, electroless deposition, electroplating, and chemical vapor deposition have usually been used as metallization processes. However, these processes have some environmental problems, such as the toxic wastewater and chemicals generated from the manufacturing and rinsing processes [7–10]. To solve these problems, an environmentally friendly and dry metallization process with no chemical solutions is required.
The aerosol deposition (AD) process as an environmentally friendly, simple, and dry metallization process was recently proposed as an alternative process for electrolyte and electroplating. Moreover, studies for RF devices such as capacitors and filters have been reported because dense ceramic films can be deposited at room temperature using AD process [11–16]. AD is the powder spray-coating method of using a thin/thick film under low vacuum conditions with micron-sized particles [17–20]. The fine particles are ejected through the nozzle and collide with the substrate with high kinetic energy, forming a dense metal film without external heating [21, 22].
In this study, we showed that dense and thick Cu films can be fabricated at room temperature by AD, and the properties of Cu films were investigated in terms of electronic properties and the characterization of the chemical state of copper in the form of Cu and copper oxides (Cu2O and CuO) distributed on the surface of two different oxide supports. Moreover, the growth process of Cu films was examined by observing the initial growth stages of Cu films deposited on Al2O3 substrates. The adhesion strength of grown Cu films on rough Al2O3 substrates was verified from a tensile pull-off test. To explain the correlation between the adhesion and the mechanical interlocking between Cu films and Al2O3 substrates, the adhesive strength was analyzed depending on the surface roughness of the Al2O3 substrates.
Cu films were fabricated on the Al2O3 substrates using an AD method, using Cu powder with a primary particle size of ~2 μm (Nippon Atomized Metal Powders co., Ltd., Japan). The prepared powder was mixed with a carrier gas, and an aerosol was then formed in the aerosol chamber. The aerosol flow was transported through a tube to a slit nozzle (10 mm × 0.04 mm) in a deposition chamber, which was evacuated using a rotary pump with a mechanical booster. The aerosol jet was ejected from the slit nozzle due to the differential pressure between the aerosol chamber and the deposition chamber. Helium gas was used as a carrier gas at a flow rate of 8 L/min. The details of the equipment for the AD process can be found elsewhere . The accelerated Cu particles collided with the prepared Al2O3 substrates, and Cu films then grew at room temperature.
To observe the growth of the aerosol-deposited Cu films from the initial growth stages, the substrates were scanned in steps. By controlling the concentration of the aerosol, individual Cu-coating layers form on the Al2O3 substrates for a scanning number from 2 scans to 20 scans. As the number of scans increased, the formation of Cu-coating layers was observed using field-emission scanning electron microscopy (FE-SEM; S-3700, HITACHI Ltd., Japan) at 10 kV. The samples were covered with platinum to perform these measurements. The crystal structure was examined using an X-ray diffractometer (XRD; X’Pert PRO, PANalytical, USA) by using Cu Kα radiation (λ = 1.5406 Å). Patterns were collected in a 2θ interval of 20°–80° with increments of 0.02° (2θ). The resistivity of Cu films was measured using a four-point probe system (Mitsubishi Chemical Corporation, Loresta-GP MCP-T6 00, Japan). The surface composition of the Cu films was analyzed by X-ray photoelectron spectroscopy (XPS; PHI 5000 VersaProbe™, Ulvac-PHI). High-resolution XPS conditions were fixed to a constant analysis energy mode, with 58.7 eV of pass energy and a monochromatic Al source. The adhesive strength between the aerosol-deposited Cu film and the Al2O3 substrate was measured using a universal testing machine (DUT-300CM, Daekyung engineering Corp., Korea) at a loading speed of 5 mm/min. A force-displacement curve was derived from the tensile test, and the adhesive strength F of the Cu coatings was estimated, using the following equation; F = |f max|/A, where f max is the measured peak load value of the breaking force and A is the area that was peeled off the Cu film.
Results and Discussion
Substrate Dependency on Properties of Cu Films
Growth Process of Cu Films
Cu Films Grown on Al2O3 Substrates
In this research, we confirmed that the AD process, with merits such as high deposition rate, capability for room-temperature processing, and low resistivity, can be used as a metallization process for microwave devices. Moreover, it was explained that the growth of Cu films was closely related with the plastic deformation of particles and the mechanical interlocking between particles and rough substrate. In a future work, we expect to investigate the complementary electrical properties of Cu films through the pre-treatment process of powders and the post-annealing process of coating layers.
Dense Cu films with a thickness of ~8 μm were deposited on Al2O3 substrates at room temperature using an AD process. The Cu films had a low electrical resistivity of 9.2–12.5 μΩ cm, but this was five to seven times larger than that of bulk copper. The causes of the increase in the resistivity were explained through XPS analysis. It was revealed that the generation of Cu2O and CuO oxides in Cu films affects the electrical resistivity. Based on the XRD analysis, it was also explained that the increase in FWHM of Cu films is due to the internal strain during the collision of Cu particles. To observe the particle behaviors when Cu particles move to the Al2O3 substrates, the initial growth stage of Cu films were investigated in steps. It was confirmed that the bonding between Al2O3 substrates and Cu particles strongly depends on the plastic deformation and mechanical interlocking. Moreover, the effect of substrate roughness on the resistivity, the adhesive strength, and the surface roughness of Cu films were investigated to understand the mechanical interlocking between Al2O3 substrates and Cu particles. The rough Al2O3 substrates showed a superior adhesive strength to the smooth Al2O3 substrates.
This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (No. 2013R1A1A2A10011202).
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