Film casting technology for aluminium oxide ceramic substrates

. The paper presents the technology of aluminum oxide ceramic substrates formation by film casting using "KEKO" line (Slovenia). Components of binder (dispersion phase) including polymeric bonding agent, plasticizer, dispersant, solvent are presented. The following binder components were used: polymeric binder - polyvinyl butyral, plasticizer - dibutyl phthalate, dispersant - fish oil and solvent - mixture of toluene and ethanol. The preparation regimes for the slurry, casting and lamination of the ceramic strip are given. According to the thermal analysis (DSC and TG) of raw corundum substrate the main mass of the removed binder occurs in the temperature range 240-450oC, the observed exo-effects in this temperature range are caused by successive processes of decomposition and removal of binder components. Based on the results of thermal analysis the regime of organic component removal from the "raw blanks-substrates" which provides for heating to a temperature of 500oC with a given rate of heating and cooling is determined. The technology of film-casting provides obtaining raw billet-substrate with a given density and the necessary technological strength. We experimentally set the firing mode of corundum substrates in vacuum (0.1-1.0 Pa) at a speed of temperature 100оС/h to 1450оС with an exposure time of 2 hours, then at a speed of 60оС/h to a maximum temperature of 1620-1660оС with an exposure time of 4 hours. The substrates were cooled at a rate of 100oC/h. The structure of the annealed substrates is represented by corundum crystals with sizes predominantly not more than 10 microns, intergrain porosity not more than 0.5%. Sintered products have high density and minimum number of defects.


Introduction
Ceramic substrates based on α-Al2O3 are widely used in modern technology and due to the set of electrophysical and technical-economic parameters are the most demanded for the manufacture of hybrid integrated circuits (HIC), power semiconductor devices (PSD) of microwave range.
To form ceramic substrates of a certain thickness the different methods are used: pressing, rolling, extrusion method, but modern technology gives preference to film casting, for which various companies developed technological lines.[1][2][3][4][5][6] The most popular line in world practice is the Keko line (Slovenia).
The use of the technology of ceramic strip casting on the base has a number of advantages over traditional technologies of ceramic substrates production: the productivity of the technological equipment is dozens of times greater than that of the equipment for isostatic pressing, injection molding and casting of thermoplastic slips; energy consumption is several times less; complex and labour-intensive machining processes (planar and contour grinding etc.) are almost completely eliminated; the film casting procedure makes it possible to produce substrates from 100 µm to 1.5 mm thick, to obtain multilayer housings; group production of substrates in 165x165 mm cards makes it possible to carry out automated control of basic parameters and substrate rejection.At use of the given method of formation of ceramic substrates the important scientific and technical problems at this stage of technology are: development of composition of a slurry with selection of components compatible on casting properties, establishment of regimes of preparation of a slurry, casting, laminating, drying, removal of a binder from a raw blank and firing of products.When developing the composition of polymeric binder with optimal complex of rheological properties it is necessary to ensure the ability to disperse ceramic powder in the system, high quality of casting suspension and easy burnout of organics.The binder composition should include both polymeric binder and plasticizer, dispersant, surfactant and other functional additives [2][3][4][5][6][7][8][9] The aim of this work is to develop a technology for the production of corundum substrates by film casting using the "KEKO" line (Slovenia).
Research methods: X-ray phase and thermal analysis, optical, electron microscopy, laser granulometry.
In this study, the following components were tested in the selection of bond composition: Polymeric organic bond in the form of polyvinyl butyral (PVB), polyvinyl alcohol (PVA), polyvinyl acetate (PVA); plasticizer in the form of dibutyl phthalate (DBP), dioctyl phthalate (DOP), polyethylene glycol (PEG) and glycerol solvents -acetone, ethyl alcohol, toluene, azeotropic mixtures -toluene/ethyl alcohol, ortho-xylene/ethyl alcohol, methyl ethyl ketone (MEK)/toluene, MEK/ethyl alcohol, MEK/isopropyl alcohol.The criteria for the selection of binder components were: bond burn-up on heating (slow and residue-free); molecular weight; polymer chain structure; miscibility with ceramic powder; compatibility with dispersant, solvent and plasticizer; viscosity; low cost.The raw ceramic tape was a polymer matrix with large amounts of ceramic powder.The bond used gave the ceramic tape flexibility, strength, ductility, smoothness and hardness.
The plasticising additive must meet the following requirements: compatibility with the bond polymer; high boiling point and low vapour pressure; high plasticising efficiency of the tape; chemical and thermal stability.The properties of a number of plasticising additives are shown in Table 1.Comparing the basic properties (Table 1) of the most popular bonds (PVB, PVS, PVA) and experimental results of testing them in the preparation of slurries based on technical alumina, preference was given to polyvinylbutyrate (PVB) of brand PVB 98.
Table 2 shows the properties of commonly used plasticizers of DBF and DOF brands.
Comparing the properties of these plasticisers and taking into account the results of previous studies, dibutyl phthalate (DBP) of Santicizer S261A (Germany) was used as a plasticiser.
The dispersant plays a key role in maximising the concentration of mineral particles in the slurry at low viscosity suspensions.Fish oil is one of the most common dispersants in film casting.It binds powder particles and organic bonding agents by means of lipophilic and hydrophilic groups, preventing their aggregation.This type of dispersant has been used in binder slurry.
The organic bonding solvent is only required in the initial step of preparing the ceramic sheet material.The solvent makes it possible to mix organic binder components faster and obtain the desired viscosity for better homogenization with ceramic filler powders, to give the slurry the necessary flowability and thus the possibility of forming ceramic tapes.Various substances can be used as solvents, including ethanol, toluene, methyl ethyl ketone, xylene, etc., while it is advisable to use a binary solvent (Table 2).Based on the analysis of properties and preliminary tests, a toluene/ethanol mixture at a ratio of 1:2 was chosen as the solvent.
Thus, taking into account the properties of the different types and the experimental selection of the components ratio, the following components were included in the organic slurry: polyvinyl butyral, dibutyl phthalate, fish oil and a toluene/ethanol mixture.The ratio between the mineral (modified alumina) and organic constituents in the slurry was 4:1.
The process of moulding and production of ceramic substrate blanks by casting on a base included several technological operations: preparation of the slurry by mixing organic and inorganic components; casting of ceramic tape on a KEKO line (Slovenia); drying of ceramic tape; cutting into sheets, forming and laminating stacks; removal of bond from blanks.
The quality of the slurry was monitored by measuring flowability, density, stability, gelling and other parameters.The slurry was degassed.The properties of the slurry, cast tape and casting conditions are shown in Table 3. Ceramic tape was casted on the base by casting machine type CAM-M3520H (KEKO, Slovenia) at ambient temperature 23÷25°C; humidity 40-60%.
According to the technological scheme the plastic film after removal of solvent (together with Mylar ribbon-carrier) was cut into sheets of 200x200 mm.The film thickness was regulated by the knife gap, amount of solvent and quantity of used mineral substance.When forming the stack, the ceramic film sheets were separated from the tape and assembled into a pile (stack) of the desired thickness.When assembling the stack, an additional visual inspection of the film defects was carried out "through the light".The assembled stack was placed on a metal plate, evacuated in a polyethylene bag and then laminated to form a monolithic ceramic structure.The assembled stack in the vacuum bag was placed in a chamber that was filled with a liquid (most often a mixture of water and glycerine).The control parameters for the operation were temperature, pressure and exposure time.The lamination temperature was set according to the type of organic phases in the sheets (bond and plasticizers), the number of layers (ceramic sheets) and the stack thickness.The pressing pressure was varied from 35 to 45 MPa at a temperature of 65ºC.
Cutting of stacks was carried out according to the corresponding program on the machine of automatic cutting СМ-15 for receiving blanks of the set sizes and taking into account the subsequent shrinkage during baking.Before cutting, the stack was heated on a thermal table to a temperature of 35-40 °C.
Optimization of the bond removal regime should ensure that the integrity of the product is maintained, that incipient defects in the form of micro-cracks, pores, cavities are avoided and that the bond itself is removed as much as possible [10].The bond removal mode has been predicted on the basis of complex thermal analysis data (DSC and TG).According to the results of thermal analysis (DSC and TG) of the crude corundum substrate billet (Fig. 1) physico-chemical processes occurring during heating are divided into two main stages: up to 238°C an increase in sample mass (~6%) occurs, which is probably associated with the oxidative process of bonding components, with further heating up to 1000°C binder is removed (total losses are ~16%), with the bulk (~12%) of the removed bond occurs in the temperature range 240-450°C.A further monotonic decrease in sample mass is traceable up to a maximum heating temperature of 1000°C.On the DSC curve in the temperature interval 200-500°С three mutually overlapping exoeffects with maximums at 317,8; 360,4; 430,8°С and heat effects 1,29; 1,14; 1,01 μV/mg are clearly observed.The temperature intervals determining the main losses, the TG curve (binder removal) and the manifestation of exo-effects practically coincide and indicate the reflection of the same processes on the DSC and TG curves.Along with the pronounced heat effects in the temperature range of 230-2700C, a blurred exo-effect is evident.Comparing the results of thermogram of raw ceramic blanks with the properties of binder components, we can conclude that the first exo-effect in the range of 230-2600C is due to the decomposition and removal of polyvinylbutyrate (PVB), the second exo-effect at 317-3250C -dibutylphthalate (DBP), the third (350-3600C) -fish oil and at 430-4700C -polyethylene glycol (PEG) and carbon.
On the basis of results of thermal analysis of raw billets moulded by casting-on-base technology, binder removal process is mainly completed at heating up to 500°C, the most critical temperature interval, within which the main processes of binder component removal take place, is the interval from 220 to 450°C.
According to the obtained thermal analysis data the following regime of organic component removal from the raw corundum ceramics blanks was established: heating up to 1500C is performed at the rate of 40C/min, then slow rise in temperature from 150C to 2500C at the rate of 10C/min and holding for 30 minutes at 2500C.In the 250-5000C interval, the temperature is increased at the same rate of 10C/min.To remove residual carbon at the maximum temperature of 5000C, an exothermic soaking for 1 hour was carried out, followed by uniform cooling at a rate of 10C/minute to 2500C to prevent deformation of the substrates.Further cooling was carried out in the free cooling mode (Figure 2).When the binder was removed, the wafers were stacked in a single row on refractory tooling.After removal of the binder, the blanks were heat-treated in air at 1100ºC to give them technological strength before being loaded for the final firing.
Annealing of substrates was carried out in vacuum furnaces of SNEV type at maximum temperatures 1550, 1580, 1620, 1700ºС.Vacuum during the firing process corresponded to 0.1-1.0Pa.Temperature increase up to 1450°C was carried out at a rate of 120°C/h.To ensure uniform heating of the products throughout the volume and to avoid the formation of a layer preventing the removal of pores, the samples were seasoned for 2 hours at 1450 ° C, followed by raising the temperature (60 ° C / h) to a maximum.The exposure time at the maximum sintering temperature was 4 hours.The substrates were cooled at a rate of 100°C/hour (Figure 2).After grinding and polishing operation to achieve surface cleanliness with Rz no more than 0.1 µm, the substrate samples were subjected to polished surface quality analysis (presence of cavities, open pores, scratches).The method of calculating the number of surface defects of polished corundum substrates was carried out using the ImageJ software with a Biolam-M microscope with a 9x0.2 objective (field of view 0.183 mm2) and a DCM 130E video eyepiece, 1.3 MP.The essence of the method consists in the fact that photos of polished surface are processed in the computer program ImageJ and converted into black-and-white format, where all defects (pores, shells and scratches) are painted in black.The area of the black elements is then calculated.The calculation result is presented as a percentage of the area of all defects compared to the total surface area of the examined surface.Ceramic sintering processes during firing are influenced by many regime parameters [11], including the methods of setting the substrates in the furnace zone.Table 4 shows the characteristics of substrates fired in the same mode with different methods of setting.The preferred method is to place the substrates in the furnace zone in a single row.For samples of substrates fired at 1620ºC, surface imaging was carried out to analyse the microstructure of the resulting ceramics, for which the polished surface of the substrate was previously heat-treated in air at 1350ºC.X-ray phase and structural analysis showed that the structure of alumina-based ceramics Almatis CT 3000 SG or Nabalox 713-10 NM (Germany) consists of corundum crystals mostly up to 10 μm in size and 0.5% intergranular porosity (Fig. 4).The physical and technical properties of corundum ceramics are shown in Table 5. Thermal linear expansion coefficient between 20-900 о С, х10 -6 , 1/ о С 7. Thus, corundum substrates produced by casting on film with high-dispersed alumina CT 3000 SG Almatis or Nabalox 713-10 NM and annealed in vacuum at temperatures 1620-1660oC have a high level of physical and technical properties.The fired ceramics are characterized by a fine-grained structure, high strength, small defect sizes (less than 1 µm) and their minimal amount on the polished surface (0.5%).The achieved level of quality of corundum substrates meets the requirements of modern electronic engineering and ensures their use in the creation of various electronic systems and devices in the form of hybrid film integrated circuits (HFICs), integrated circuits (ICs), printed circuit boards (PCBs), etc.

Table 1 .
Characteristics of polymer bonds

Table 2 .
Characteristics of solvents

Table 3 .
Slurry properties and casting modes for ceramic tape

Table 4 .
Characteristics of substrates burnt with different methods

Table 5 .
Properties of corundum ceramics