Field of the invention

[001] The present invention relates to the development of ultra-low thermal expansivity high temperature co-fired ceramic (HTCC) substrates with low dielectric loss for highly integrated monolithic millimeter-wave integrated circuits (MMICs) used in high temperature environments. The present substrate is advantageous over commercially available HTCC substrates in terms of cost of production, dielectric, thermal, and mechanical properties.

Background of the invention

[002] The rapid developments in the microelectronic industry have generated a high demand for low loss microwave ceramics due to their reliability, integrability and excellent dielectric properties. Telecommunication industry today requires high volume efficiency without degradation of electrical performance of the devices. In the case of electronic substrate with low dielectric packaging applications, the relative permittivity should be less than 10 which is ideal for MMICs. The speed of the microwave signal within the dielectric is inversely proportional to square root of its relative permittivity. Reducing the permittivity, especially in large area high-speed chip will reduce cross talk, propagation delay time, noise, power dissipation that also subsequently increase the signal speed. Important characteristics required for a dielectric substrate for packaging applications are (i) low relative permittivity (to increase signal speed), (ii) low dielectric loss (for selectivity), (iii) high thermal conductivity (to dissipate heat), (iv) low or matching coefficient of thermal expansion (CTE) to that of the materials attached to it, and (iv) low temperature coefficient of relative permittivity as outlined by Sebastian in Dielectric Materials for Wireless Communications Oxford Elsevier Publishers, 2008.

[003] HTCC based components have covered a wide range of application fields in the past. Examples are electronic components (multilayer capacitors, multilayer actuators and R-C filters), 3-D multichip modules (engineering management systems, gear box- control in automotive applications, high frequency applications in aerospace, medical engineering), intelligent 3-D packages (Si-chip and MEMS packaging) as well as lab- on-chip systems. Traditionally, HTCC alumina substrates and packages have been commonly used in microelectronic packages for decades. This is because of their excellent properties, cost and easy manufacturing process. They can also be used to fabricate complex micro components, suitable for high temperature applications. Even though the applications of HTCC at microwave frequencies are increasing, a wide spread use of HTCC has been limited possibly due to the underexplored electrical performance capabilities of material systems.

In a typical co-firing process for both high and low temperature technology (HTCC and LTCC), a colloidal slurry is formed from ceramic particles and organic binders. It is then casted in to solid sheet which is often referred to as green tape because of its unfired state. Typical costs for moderate production volume associated with green tapes are $ 0.06 and $ 0.11 per square inch for HTCC and LTCC respectively as reported Doane et. al. Multi Chip Module Technologies and Alternatives: The Basics, Springer 1993. The HTCC cost estimation shows that out of 100 %, the material cost is around 41 % and all the other classification of cost such as labour 14 %, capital equipment 31 %, overhead 8 % and others 6 %. HTCC substrate cost estimation shows that 41 % of the total cost is due to the material used for substrate fabrication alone.

Alumina (AI ₂ O ₃ ) is the most commonly used HTCC ceramic substrate due to its electrical, mechanical and economic advantages. The common raw material for alumina production, bauxite is composed primarily of one or more aluminium hydroxide compounds, plus silica, iron and titanium dioxide as the main impurities. It is used to produce aluminium oxide through the Bayer chemical process. On a world-wide average, 4 to 5 tonnes of bauxite are needed to produce two tonnes of alumina as described by EAA, AA, JAA, ABAL, Alcan, Metallstatistics 2009.

[004] As outlined above, the processing of high purity alumina is time consuming and costly as compared to naturally occurring materials. On the other hand the proposed material, zircon, is a co-product / by product of the processing of heavy mineral beach sands, which are primarily extracted for titanium, ilmenite and tin minerals. The major end uses of zircon are refractory purpose, foundry sands, and ceramic opacification. Indian Rare Earths Ltd. (IRE) was the eighth largest producer of zircon in the world from its mine at Chavara in Kerala. IRE produced 22,000 tons of zircon in 2000 as revealed by Mineral Sands Report 2001c. The major advantages of mineral is that it resists metal penetration and burn-in, ensures dimensional accuracy, machinability, high thermal conductivity, low thermal expansion, erosion resistance, longer life, high thermal and mechanical stability etc. as reported by DuPont 1991. Thus, as evident from the above reports, that zircon mineral possess more attraction in terms of cost production and availability as compared to the present HTCC materials in the markets. The preliminary reports on its dimensional, thermal and mechanical stability will suggest that it can be developed as a better alternative material for HTCC substrates for MCM-C fabrications.

[005] The prior art discloses a number of applications of zircon as a refractory material suitable for use in contact with molten glass as described in US Patent No. 2553265 and US Patent No. 3347687. Another report reveals that the mechanical rupture modulus of zircon will be improved with the addition of iron chromate ore to it as described in the US Patent No. 3791834. Reference may also be made to the US Patent No. 5407873, where zircon bricks are isostatically pressed and densely sintered and used for glass melting facilities, especially for boron containing glasses. A few isolated research has been done in the prior art, on the dielectric and thermal properties of ZrSiO ₄ ceramics but not intended to the preferable application as an HTCC substrate. Reference may be made to dielectric and thermal expansion properties of ZrSiO ₄ ceramics as revealed by Varghese et. al. Materials Letters, vol. 65, 1092, 2011. It has ε _r of 10.5, tanδ of 0.002 at 1 MHz and ε _r of 7.4, tanδ of 0.0006 at 5.15 GHz. The relative permittivity of ZrSiO ₄ ceramics exhibited constant values within operating temperatures range of -20 to 70 °C. The ceramic also exhibited a negative coefficient of thermal expansion of -2.5 ppm/°C in the temperature range 30-600 °C and hence possess an additional advantage suitable for tuning the thermal expansion of materials having a positive CTE.

[006] As evident from the above reference, ZrSiO ₄ ceramic is one of the best candidates for microelectronic substrates in terms of cost effectiveness, with excellent dielectric and thermal properties. However there was no prior art on the fabrication of this material as HTCC substrates for multi-chip modules used in the high temperature environment.

[007] Objectives of the invention

1) The main objective of the present invention is to develop an ultra-low CTE HTCC substrate for microwave device applications.

2) Another objective is to process the mineral ZrSiO ₄ sand into fine powder suitable for tape casting.

3) Another objective is the formulation of optimal slurry composition for ZrSiO ₄ green tape preparation.

4) Another objective is to optimize the binder burn out stages of the developed green ZrSiO ₄ tape.

5) Another objective of the present invention is to optimize the sintering condition for best of microwave dielectric properties.

6) Another objective of the present invention is to optimize the post firing shrinkage and shelf life of the developed HTCC ZrSiO ₄ tapes. 7) Another objective of the present invention is to investigate the mechanical properties of green and sintered HTCC ZrSiO ₄ substrates.

8) Another objective of the present invention is to study the surface properties of sintered ZrSiO ₄ substrates.

Brief description of the accompanying drawings

[008] These and other features, aspects, and advantages of the present invention which will become better understood with reference to the following description, claims and accompanying drawings were

FIG.l Particle size distribution of processed mineral ZrSiO ₄ sand and its microstructure

FIG.2 Rheological behavior of optimized composition of ZrSiO ₄ mineral for tape casting

FIG.3 Microstructure of sintered HTCC ZrSiO ₄ ceramic substrates

FIG.4 Surface roughness of developed HTCC ZrSiO ₄ ceramic substrates

FIG.5 Aging studies of developed HTCC ZrSiO ₄ ceramic substrates

Statement of Invention:

[009] The present invention relates to the development of ultra-low CTE HTCC ZrSiO ₄ ceramic substrates used for multi-chip module in high temperature environment. The novel HTCC substrate is indigenously developed for microelectronic applications. The development of HTCC ZrSiO ₄ ceramic substrate includes the formulation of tape casting slurry composition, tape casting, lamination, binder burnout and sintering. The microwave dielectric properties of sintered HTCC ZrSiO ₄ ceramic tape are measured both at 5 and 15 GHz frequencies. The present disclosure also relates to the mechanical and thermal properties of developed substrates. Shrinkage, surface roughness and aging of the developed substrates are also investigated and optimized.

[010] In a principal embodiment among others of the present invention is a zircon based mineral, in particular, with the chemical formula ZrSiO ₄ having a purity of 97.4 %.

[011] In yet another embodiment among others of the present invention illustrates the formulation of the processed ZrSiO ₄ mineral based tape casting slurry with suitable organic vehicle.

[012] In one embodiment, the present disclosure relates to a thermo-laminated multilayered zircon based high temperature co-fired ceramic (HTCC) tape for microelectronic applications comprising zircon containing slurry including 50-70 wt. % of a filler mineral powder having average particle size of 540 nm well dispersed in 10- 30 wt. % of an organic solvent; 1-10 wt. % of an organic binder, and 0.5-5 wt. % of two types of plasticizers.

[013] In another embodiment of the present investigation, is the organic solvent preferably comprises of 10.0-30.0 wt. % solvent which is selected from a group consisting of anhydrous xylene, ethanol, toluene, methyl ethyl ketone and mixtures thereof, 50-70 wt. % processed ZrSiO ₄ filler, 1.0-10.0 wt. % of the organic binder which is selected from the group consisting of polyvinyl butyral, polyvinyl alcohol, polyvinyl acetate, polyacrylonitrile, polyethyleneimine, poly methyl methacrylate, vinyl chloride-acetate and mixtures thereof and 0.5-5.0 wt. % of two types of plasticizers which are selected from a group consisting of butyl benzyl phthalate, diisooctyl phthalate, diethyl phthalate, dimethyl adipate, monomethyl adipate, dioctyl adipate, dibutyl sebacate, dibutyl maleate, triethyl citrate, acetyl triethyl citrate, tributyl citrate, acetyl tributyl citrate, trioctyl citrate, acetyl trioctyl citrate, trihexyl citrate, trimethyl citrate, alkyl sulphonic acid phenyl ester, polyethylene glycol and mixtures thereof respectively.

[014] In another embodiment of the present disclosure, the organic solvent is selected from a group consisting of anhydrous xylene, ethanol, toluene, methyl ethyl ketone and mixtures thereof.

[015] In another embodiment of the present disclosure, the organic binder which is selected from the group consisting of polyvinyl butyral, polyvinyl alcohol, polyvinyl acetate, polyacrylonitrile, polyethyleneimine, poly methyl methacrylate, vinyl chloride- acetate and mixtures thereof.

[016] In another embodiment of the present disclosure, two types of plasticizers are selected from a group consisting of butyl benzyl phthalate, diisooctyl phthalate, diethyl phthalate, dimethyl adipate, monomethyl adipate, dioctyl adipate, dibutyl sebacate, dibutyl maleate, triethyl citrate, acetyl triethyl citrate, tributyl citrate, acetyl tributyl citrate, trioctyl citrate, acetyl trioctyl citrate, trihexyl citrate, trimethyl citrate, alkyl sulphonic acid phenyl ester, polyethylene glycol and mixtures thereof. [017] In one embodiment, the present disclosure relates to a thermo-laminated multilayered zircon based high temperature co-fired ceramic (HTCC) tape for microelectronic applications comprising zircon containing slurry including 50-70 wt. % of ZrSiO ₄ filler mineral powder having average particle size of 540 nm well dispersed in 10-30 wt. % of an organic solvent selected from a group consisting of anhydrous xylene, ethanol, toluene, methyl ethyl ketone and mixtures thereof; 1-10 wt. % of an organic binder selected from the group consisting of polyvinyl butyral, polyvinyl alcohol, polyvinyl acetate, polyacrylonitrile, polyethyleneimine, poly methyl methacrylate, vinyl chloride-acetate and mixtures thereof, and 0.5-5 wt. % of two types of plasticizers individually selected from a group consisting of butyl benzyl phthalate, diisooctyl phthalate, diethyl phthalate, dimethyl adipate, monomethyl adipate, dioctyl adipate, dibutyl sebacate, dibutyl maleate, triethyl citrate, acetyl triethyl citrate, tributyl citrate, acetyl tributyl citrate, trioctyl citrate, acetyl trioctyl citrate, trihexyl citrate, trimethyl citrate, alkyl sulphonic acid phenyl ester, polyethylene glycol and mixtures thereof.

[018] In another embodiment of the present disclosure, the HTCC tape has microwave dielectric properties at 5 and 15 GHz have relative permittivity (k) ε _r = 3.1-10.1 (5 GHz), Dielectric loss (tanδ) = 4-5x10 ^-4 (5 GHz), Relative permittivity (k) ε _r = 2.9-9.9 (15 GHz), Dielectric loss (tanδ) = 6-9x10 ^-4 (15 GHz); has thermal properties as Thermal conductivity: 10-16 W/mK, coefficent of thermal expansion: ±2 ppm/°C; has mechanical properties as tensile strength: 14-20 MPa, flexural strength: 130-150 MPa; has low thermal shrinkage and aging of microwave dielectric properties as X shrinkage ranges 5-10 %, Y shrinkage ranges 5-10 %, Z shrinkage less than 3-8 %; has good surface topography as Average surface roughness (Sa) = 100 nm, Root mean square roughness (Sq) = 140 nm, Surface skewness (Ssk) = -0.6876, Coefficient of kurtosis (Sku) = 3.5164.

[019] The present disclosure also relates to a process of preparing a low cost thermally stable thermo-laminated multilayered zircon based high temperature co-fired ceramic (HTCC) tape developed for microelectronic applications comprising: (a) obtaining mineral sand with zircon as the major component, with trace amounts of rutile, monazite, quartz, sillimanite; (b) reduction in size of the mineral sand by ball milling and tape casting of zircon mineral by developing a stable colloidal slurry; and (c) fabrication of HTCC substrate using the tape casted zircon (green tape).

[020] In yet another embodiment of the present invention, typical thixotropic behavior of the final slurry composition is revealed.

[021] In yet another embodiment of the present invention, crack free green tapes are casted using finely ground and sieved ZrSiO ₄ mineral.

[022] In yet another embodiment of the present invention, HTCC substrates sintered at 1400-1700 °C show 88-95 % densification.

[023] In yet another embodiment of the present invention, sintered HTCC substrates show total thermal shrinkage < 10 % with X and Y shrinkage ranging from 5-10 % and Z shrinkage ranging from 3-8 % at the optimized sintering temperature. [024] In yet another embodiment of the present invention, microwave dielectric properties of sintered samples exhibit relative permittivity of 7-10 and dielectric loss of 2-6x10 ^-4 at frequencies of 5 and 15 GHz respectively.

[025] In yet another embodiment of the present invention, thermal properties of sintered ZrSiO ₄ substrates show a low coefficient of thermal expansion (CTE) of ±2 ppm/°C and thermal conductivity in the range of 10-16 W/mK.

[026] In yet another embodiment of the present invention, mechanical properties of green tape in the casting direction shows a tensile strength of 0.1-0.4 MPa while that of sintered ZrSiO ₄ HTCC substrates show quite high strength in the range of 14-20 MPa. Flexural strength of sintered ZrSiO ₄ substrates is in the range of 130-150 MPa.

[027] In yet another embodiment of the present invention, the average surface roughness of the sintered unpolished HTCC ZrSiO ₄ substrate is around 100 nm.

Detailed description of the invention

[028] In order to facilitate a better understanding of the invention, a detailed description of the preferred embodiments of the present invention will now be explained with reference to the accompanying drawings. It needs to be understood that the disclosed embodiments are merely exemplary of the invention, which may be embodied in various forms. Therefore, the details disclosed herein are not to be interpreted as limiting but merely as the basis for the claims and as a basis for teaching one skilled in the art of how to make or use the invention. [029] In one exemplary method according to the present invention for making low loss HTCC ceramic substrate zircon is based on zirconium silicate (ZrSiO ₄ , zircon mineral, IRE Ltd, India). Zircon mineral sand from IRE is having the size greater than 300 micron and purity 96-98 %. According to the mineralogical analysis of IRE, the trace impurities present in the zircon mineral sand was rutile 0.20-0.80 %, monazite 0.20- 0.70 %, trace amount of quartz 0.00-0.20 %, 1.50-2.50 % of sillimanite and 0.00-0.40 % others. In one exemplary elemental analysis of zircon sand revealed that ZrO ₂ (+ trace amounts of HfO ₂ ) (64.50 %), SiO ₂ (32.10 %), TiO ₂ (0.70 %), Fe ₂ 0 ₃ (0.30 %) and P ₂ 0 ₅ (0.10 %) is the approximate chemical composition. In order to reduce the particle size of the zircon mineral sand, the material was ball milled for 48 hrs with yittria stabilized zirconia balls using distilled water as the milling medium. After ball milling, the dispersed slurry was sieved using 25 micron mesh size nylon sieve and dried overnight in a hot air oven at 60 °C. Particle size analysis was carried out using Malvern particle size analyzer (Zetasizer Nanoseries: ZEN 3600, Malvern Worcestershire, UK).

[030] A doctor blade tape casting machine (Casting Machine, Keko Equipment, Slovenia) was used for the development of ZrSiO ₄ based HTCC tapes in the present disclosure. The high purity processed ZrSiO ₄ mineral sand was used as ceramic filler, while suitable solvents, dispersants, binders, and plasticizers detailed in one of the embodiments, comprises the vehicle. Once the tape is cast, it will be allowed to dry either naturally or using hot air circulation depending on the solvents used in the tape casting slurry. The dried tapes will be used for further characterization.

[031] The images of the green and sintered tape were recorded with digital camera (Sony, 10x optical zoom, 16 Megapixel). The suspended ZrSiO ₄ particles were lifted on carbon coated copper grid, prepared ceramic thin bodies and the grids were dried on a filter paper under an infrared lamp, followed by inspection in the HR-TEM(FEI Tecnai G2 30S-TWIN, FEI Company, Hillsboro, OR) operated at 300 kV. The microstructures of all the developed samples were studied using scanning electron microscopy (CARL ZEISS, EV018 ESEM) at different magnifications. The surface roughness of the developed samples was measured using atomic force microscope (AFM) (NTEGRA, NT-MDT, Russia) operating in tapping mode. Micro-fabricated SiN cantilever tip with resonant frequency 300 kHz, curvature radius 10 nm and a force constant 3.08-37.6 Nm ^-1 were used in AFM. The image scan size of 10 x 10 μm and scan rate of lHz was fixed for measurement. The microwave dielectric properties of ZrSiO ₄ based green and sintered HTCC substrate were measured in a split post dielectric resonator (SPDR) operating at 5.15 and 15.15 GHz using Vector Network analyzer (E5071C, Agilent Technologies, Santa Clara, CA). In this technique the total uncertainty of relative permittivity does not exceed 0.5 % and is possible to resolve dielectric loss tangents to approximately 5x10 ^-5 .

[032] The coefficient of thermal expansion was measured using a push rod dilatometer (Model DIL 402 PC) with accuracy < 1% made by NETZSCH, Germany. The dilatometer used an alumina tube and push rod. The thermal expansion tests were performed according to the specifications of ASTM E228 Test Method. The sintered samples of diameter 8 mm and height 10 mm were used to measure the coefficient of thermal expansion (CTE) in the temperature range of 30-600 °C.

[033] The thermal conductivity is computed from the thermal diffusivity (mm /s), specific heat (J/g K) and density (g/cm ) which was measured using a Flashline 2000 Thermal Diffusivity System, made by Anter Corporation, Pittsburgh, PA. The flash method used a high speed xenon discharge pulse source directed to the top face of the specimen to increase the temperature of the specimen by ΔΤ as a function of time. The specimens used for thermal diffusivity testing were in the form of a disc, with a diameter of 12.6 mm and a thickness of 2 mm. Specimen preparation involves ensuring the smoothness and flatness of surfaces using a 400 grit SiC grinding paper, followed by coating both sides of the sample with carbon for thermal contacts to avoid reflection of the xenon discharge light beam. The error limit of thermal property measurement is ± 1 %.

[034] In the present disclosure, the flexural and tensile properties of the green and sintered HTCC substrates were measured using a Universal Testing Machine (Hounsfield, H5K-S UTM, Redhill, UK).

[035] The objectives and advantages of this invention will become clearer by a careful study of the following examples that are given by the way of illustration and therefore should not interpret to limit the scope of the invention. EXAMPLE-1

[036] An embodiment of the present invention is given in this example that illustrates the pre processing of zircon sand for tape casting. To reduce the particle size of the zircon mineral sand, the material was ball milled for 48 hrs with yittria stabilized zirconia balls and distilled water as the milling medium. After ball milling, the dispersed slurry was sieved using 25 micron mesh size nylon cloth and dried it in hot air oven. The result of particle size analysis shows that the milled powder has an average particle size of 540 nm as shown in the figure 1 (a) and the raw correlation data in figure 1 (b) supports the accuracy of measurement results. Zircon mineral sand photograph is shown in figure 1 (c). Transmission electron microstructure of processed zircon mineral is depicted in figure 1 (d). It is clear that the average particle size of the processed mineral zircon shows good agreement with that of the TEM microstructure. It is evident that the processed zircon underwent large mechanical stress during the particle size reduction from zircon sand to powder. Milling process may affect the particle lattice orientation which can lead to the defective grain growth. It is clear that the processed zircon powder shows irregular grain structure without any specific morphology, which is believed to be resulted from the stress due to milling.

EXAMPLE-2

[037] An embodiment of the present invention is given in this example illustrating the phase purity of processed zircon sand. Zircon has a tetragonal structure that belongs to the 141/amd space group. According to the ICDD file, ZrSiO ₄ has the unit cell parameters a = 6.573 A, c = 5.963 A and possess an X-ray density of 4.65 g/cm .

EXAMPLE 3

[038] An embodiment of the present invention is given in this example illustrating the optimized zircon slurry composition, its rheology, photographs of casted zircon tape and also its mechanical properties. It is important to note that for effective tape casting, a well dispersed and stable ceramic slurry is essential. Figure 2 describes the pseudoplastic behavior of the optimized zircon slurry composition. Obviously, the viscosity of well dispersed slurry will be low due to the presence of interparticulate fluid layer which offers increased mobility to the particles. Evidently, the slurry shows a typical pseudoplastic behavior. For ideal tape casting slurry, pseudoplastic behavior is essential so that the viscosity will decrease while passing through the blade gap due to the shear force and the viscosity will increase immediately beyond the blade, thereby preventing the unwanted flow after casting. Table 1 represents the mechanical properties of green tape before processing. Single layer green tape having thickness ranging from 0.07-0.1 mm exhibits tensile strength in the range of 0.1-0.4 MPa while that of laminated tapes having thickness ranging from 0.3-0.4 mm show an improvement in the tensile strength of 0.7-1.0 MPa. This mechanical strength of the green tape is well above the threshold mechanical strength to be qualified for HTCC substrate tapes. EXAMPLE 4

[039] An embodiment of the present invention is given in this example illustrating the sintering profile of the developed zircon tape. The green laminated tape contains several organic additives, and in order to eliminate the organic additives and polymeric matrix, a controlled heating is required. To control the binder burn out process, thick slab of macro porous high temperature brick (porous setter) are used during the co- firing process. Cooling rate of the furnace is also important to yield crack free sintered zircon tapes. In the present invention, very low cooling time of about 0.4 °C/minutes is given to reach at room temperature.

EXAMPLE 5

[040] An embodiment of the present invention is given in this example illustrating the microstructure of sintered laminates of developed zircon HTCC substrates. Figure 3 (a & b) shows the surface microstructure of sintered HTCC zircon substrates at different magnifications. Figure 3 (c & d) shows the cross section of sintered HTCC zircon substrates at lower and higher magnifications. It should be noted that substantial grain growth may occur during sintering process and hence the surface morphology of the sintered tape (figure 3 (a & b)) shows well packed grains with fractional porosity. In the SEM image recorded around the cross section of thermo-laminated stack (figure 3 (c & d)), no interphase boundary of the multilayer stack is visible within the resolution accuracy limits of SEM. This means that under the conditions of thermo-lamination, the polymer in the matrix softens and subsequently the layers diffuse one into another making the laminate homogeneous. After closely examining the microstructure of the cross section of the sintered stack, one can conclude that the stack densified to form a coherent body

EXAMPLE 6

[041] An embodiment of the present invention is given in this example illustrating the densification, microwave dielectric properties and shrinkage behavior of developed HTCC zircon substrates. Densification is found to increase with increase in sintering temperature and optimized sintering temperature is found to be in the range of 1600- 1700 °C as shown in Table 2. The densification increases from 80 % to 95 % as a function of temperature and the maximum densification is achieved at above 1600 °C. Sintering temperature is optimized based on densification and microwave dielectric properties. The variation of microwave dielectric properties of HTCC zircon substrate as a function of temperature also showed a similar trend as densification. The microwave dielectric properties of the HTCC substrate is very promising (relative permittivity (ε _r ) ranging from 3.1-10.1, 2.9-9.9 and dielectric loss tangent (tanδ) in the ranges of 4-5x10 ^-4 , 6-9x10 ^-4 respectively at 5 and 15 GHz). The thermal shrinkage during sintering of HTCC zircon substrate is shown in the Table 3. The optimally developed zircon substrate shows the percentage average shrinkage in X and Y direction of about 5-10 % while that of Z direction is less than 3-8 % with standard deviation of 2 %.

EXAMPLE 7 [042] An embodiment of the present invention is given in this example illustrating the average surface roughness, root mean square (RMS) roughness, surface skewness and surface kurtosis parameters of developed HTCC zircon substrate. The AFM image of the zircon substrate is shown in figure 4 which shows an average surface roughness (Sa) of 100, as depicted in the inset table. The prominently obvious features of the surface of the zircon substrate in 2-D and 3-D surface geometry are shown in figure 4. RMS deviations of surface, Sq is nearly 140 nm and the kurtosis of the topography height distribution (Sku) is nearly 3.5 which indicate the surface of zircon substrate needs further planarization, since the kurtosis of a well spread distribution should be greater than 3. The nonuniform nature of the surface of HTCC zircon tape with mountains and valleys is obvious in figure 4. Skewness of topographic height distribution (Ssk) is defined as the measure of asymmetry of the surface deviations about a reference plane. The Ssk of zircon substrate is -0.687 where the negative value of skewness generally indicates that the surface distribution has a longer tail at the lower side of the reference plane.

EXAMPLE 8

[043] An embodiment of the present invention is given in this example illustrating the mechanical, thermal and ageing studies of the developed HTCC zircon substrate which is depicted in Table 4 and Figure 5. The tensile strength, flexural strength, thermal conductivity and coefficient of thermal expansion are equally important in the stage of different device development process in microelectronics. Generally the refractory materials are having moderately high thermal and mechanical properties. The developed substrates show tensile strength in the range of 14-20 MPa and flexural strength of around the range of 130-150 MPa. The lower value of flexural strength is typical of silicates. The zircon substrate also possesses good thermal and mechanical properties after sintered at best densification temperature. The developed HTCC zircon substrate shows ultra-low thermal expansion value in the rage of ±2 ppm/°C and it has thermal conductivity in the range of 10-16 W/mK. The newly developed zircon HTCC substrate is showing excellent mechanical and thermal properties, as compared to the commercial alumina based HTCC substrates. Figure 5 shows the ageing studies of developed HTCC zircon substrates up to 60 days (2 months). Effect of aging on room temperature microwave dielectric properties of developed HTCC zircon substrate at 5 and 15 GHz depicts that there is only marginal variation in the relative permittivity during the studied period. In the case of dielectric loss, little variation is observed as a function of time, which is well within the error limit associated with dielectric loss measurement using SPDR.

Summary of the invention

[044] Accordingly, the present invention is an article that comprises a zircon containing substrate that provides a low cost preparation of ultra-low CTE and low dielectric loss high temperature co-fired ceramic (HTCC) substrates for highly integrated monolithic millimeter- wave integrated circuits (MMICs) utilized in high temperature environment. In another aspect, the invention relates to a method of forming an article, preferably at 5 and 15 GHz of frequencies. The ZrSi04 has ultra low coefficient of thermal expansion of ±2 ppm/°C and thermal conductivity in the range 10-16 W/m K. The average roughness (Sa = 100 nm), root mean square (RMS) roughness (Sq = 140 nm), surface skewness (Ssk = -0.6876) and surface kurtosis (Sku = 3.5164) parameters are used to analyze the surface morphology of the developed HTCC zircon substrate. The newly developed substrates show the XY shrinkage in the range of 5-10 % and Z shrinkage in the range of 3-8 %. It also showed a tensile strength of 14-20 MPa while flexural strength in the range 130-150 MPa as depicted in the results. This HTCC zircon substrate is advantageous over currently available HTCC substrates in terms of production cost, dielectric and thermal properties.

Table 2

Table 3 [045] The main advantages of the present invention are: 1) Zircon based HTCC substrate was fabricated for microelectronic applications. 2) Raw material zircon as such is available in beach sand and no need of purification and chemical processing.

3) Developed HTCC zircon substrate is more advantageous over commercially available HTCC substrates in terms of cost effectiveness, dielectric and thermal properties.

4) The ultra-low CTE is the highlight of the newly developed zircon HTCC substrate, whereas that for HTCC alumina >7 ppm/°C.

5) Stable zircon slurry shows a characteristic pseudoplastic nature which is advantageous for the development of crack-free HTCC zircon substrate.

6) Green zircon tape shows sufficient mechanical stability that is advantageous for further processing.

7) HTCC zircon substrates show low (less than 10 %) thermal shrinkage behavior at XY and Z direction which are advantageous for substrates operating in high temperature environments.

8) Developed HTCC substrates show microwave dielectric properties exceeding to or at least comparable to that of HTCC alumina.

9) Developed HTCC substrates show good surface characteristics.

10) Developed HTCC substrates show good mechanical stability and comparable thermal conductivity to that of existing HTCC substrates.

1 1) Developed HTCC zircon substrates show stable microwave dielectric properties over studied period of two months.