Development of technology for deposition of thick copper layers onto ceramic substrates applied in power electronics

The basic element of the design of a power module is a metallized ceramic substrate. In this work, the formation of metallization coatings by the method of thermal transfer of metallization pastes (Mo-Mn-Si + binder) for alumina and aluminum nitride ceramics was carried out. The fixing of the metallization coating on the ceramic substrate was performed by firing at a temperature of 1320 ° C. The subsequent deposition of the copper layer was carried out by the method of cold gas-dynamic spraying (CGDS) followed by annealing of the deposited coating. For high-quality adhesion, the optimum annealing temperature was 1000 ° C.


Introduction
Cold gas dynamic spraying (CGDS) is a relatively new modification of cold spraying techniques that uses converging-diverging (De Laval) nozzle at a supersonic velocity to accelerate different solid powders towards a substrate on which they are plastically deformed. This deformation results in adhesion to the surface. CGDS is one of the innovative cold spraying processes with fast-growing scientific interests and industrial applications in the fields of aerospace, automotive and biotechnology. Cold spray research and development efforts have doubled during the last decade and along with new industry applications and novel demands provide both a strong body of knowledge and market pull to identify and address these roadblocks. [1,2]. Due to the high strain rate deformation of particles in (CGDS), in situ investigation is challenging. Metallurgical bonding is one of the main adhesion mechanisms of particles during coating buildup [3]. The properties of the kinetically deposited coating layer are significantly affected by the microstructure of the coating. The most powerful influencing factors in microstructural evolution of kinetic-sprayed coating layers are instant generation of thermal energy and high-strain, high-strain-rate plastic deformation at the moment of particle impact [4]. Heat treatment of the 316 L austenitic steel coating improves its mechanical properties [5]. In [6] the microstructure of the coating obtained by cold gas-dynamic spraying was investigated. A Cu-Al2O3-Zn powder blend was sprayed onto a copper substrate to restore a worn copper contact wire. The coating thickness was 1-2.5 mm. Improved adhesion strength was achieved through substrate surface preprocessing with coarse Al2O3 particles.
To obtain the pattern of an electronic power module, ceramic substrates should be metallized. Therefore, conducting layers, over 300 µm thick, are deposited by different techniques to form multilevel metallization [7]. While in production and operation, the metallized structures of power modules are exposed to thermal and mechanical stresses.
The research objective is to optimize the technique of thick copper layers deposition onto ceramic substrates used in power electronics.
After preliminary experimental studies of gas dynamic cold spray technique (GDCS), we made a choice of PMС-1 copper powder (GOST 4960-2009) as the most appropriate, affordable and cheap.
The basic element of the power module structure is considered to be a metallized ceramic substrate with the power semiconductor crystal, which is used for implementing two main functions: firstly, for electrical isolation of conductor buses patterned on one or both sides; secondly, for the conductance of heat emitted by the active elements of the electronic power module to heat radiators. Besides their high heat conductivity, the substrates of power modules must be very strong, heat-and chemically resistant. In this regard, we use the substrates made from different grades of aluminum oxide ceramics and aluminum nitride ceramics providing high dissipation capacity. Based on the properties analysis carried out for ceramic materials applied by DBC technology abroad [8,9], we come to the conclusion that aluminum oxide ceramics with the content of aluminum oxide exceeding 95% is more frequently used, but aluminum-nitride ceramics with the content of nitride oxide, which is more than 98%, is suitable for the circuits with high specific dissipation capacity. Taking into account their main characteristics, aluminum-oxide ceramic substrates for DBC boards made by СЕТС (China) are comparable with ВК96 substrates produced by JSC NEVZ-Ceramics (Russia). However, their characteristics are inferior to the ones of ВК100 ceramics produced by JSC NEVZ-Ceramics (Russia), where the content of the basic substance is equal to 99.7% (in contrast to VК-96 ceramics with 96% content of the base material).
The substrates are produced by slip casting technique followed by annealing of aluminum-oxide ceramics at 1650 °С and aluminum-nitride ceramics at 1850 °С.

Experimental
Metallized coatings (MC) formation was tested by the heat transfer of two metallization pastes compositions for aluminum-oxide and aluminum-nitride ceramics.
Surface preparation is considered to be one of the main stages of metallized coating formation on ceramics. Ceramic substrates had been mechanically polished before metallization to obtain alignment and surface roughness of Ra = 0.15 µm.
MC bonding on the ceramic substrates was achieved via its annealing. In this regard, nitrogen-hydrogen through-and pusher-type furnaces were used. The furnaces consist of 5 mullite muffles, which are 90 cm long. The muffles are located in series to provide a continuous channel with 3 temperature ranges. Annealing was carried out with 30 minutes exposure at 1320 °С.
The GDCS technique is based on acceleration of 1-150 µm particles with a supersonic gas flow up to the speed of 500-1200 m/s. The particles colliding with an obstacle tend to bond on it without melting [14,15]. Meanwhile, the substrates are not strongly affected by temperatures.
Sputtering was carried out on VK-96 aluminum oxide substrates with the dimensions of 30290.3 mm, and with a Mo-Mn-Si sublayer being 10-20 μm thick. PМС-1 copper powder was used for sputtering.
Formation of metallized coatings from copper powders was carried out according to typical GDCS diagrams with the use of a planar contracting-expanding nozzle with 3.053.05 mm critical cross section and 9.53.05 mm exit geometry. The rate of powder consumption from a dispenser was set to 0.1 g/s. The distance of sputtering was equal to 30 mm; the nozzle scanning velocity against the substrate varied from 5 to 50 mm/sec. Air was chosen as a carrier and working gas. The deposition was conducted on the GDCS ITAM SB RAS test installation. The substrates were split into two batches after sputtering. Then annealing was carried out in the hydrogen medium at different temperatures to determine the optimal thermal mode.

Results and discussion
Since the products obtained are operated in air, the resistance of the coatings in aggressive media (acids or base solutions) was not determined. It is also known that semiconductor devices with these products are operated at low temperatures (not exceeding 125 °C). For this reason, the thermal stability of coatings was not studied. The most important performance characteristics of coatings are adhesion resistance and low electrical resistivity.
The key parameters were determined after annealing as follows: the measured values of adhesion and intrinsic resistance were compared with the same parameters for DBC-substrates produced in Germany and China (Table 1). 1000 °С appeared to be the optimum annealing temperature for adhesion. The best results are peculiar to DBCsubstrates with the lowest resistance, which is close to the resistance of pure copper. The substrates with thick copper layers sputtered by the GDCS technique are characterized by the key parameters close to the values of DBCsubstrates, despite the use of copper powder to obtain the copper coatings.

Conclusions
The technology of applying a two-layer metallization coating on ceramic substrates made of nitride and aluminum oxide was developed. Initially, a layer of molybdenummanganese-silicon was deposited on the surface of the substrates by burning in a nitrogen-hydrogen medium for 30 minutes at a temperature of 1320 °C. At the second stage, a layer of copper was deposited by the CGDS method with a flat Laval nozzle. The working gas was air. After deposition, annealing was carried out in hydrogen atmosphere. The optimal annealing temperature was 1000 °C. The obtained coatings are characterized by a stable adhesive bond of the copper coating with the base (the adhesion value exceeds 60 MPa) and low electrical resistance (at the level of 3·10 -6 Ohm·cm).

Supplementary materials
No supplementary materials are available.

Funding
The work was funded by the State task of Ministry of Science and Higher Education of Russia (project no. FSUN-2020-0008).