FIELD OF THE INVENTION This invention relates to materials having controlled porosity and more particularly to dielectric materials having controlled porosity.

BACKGROUND OF THE INVENTION

When considering a material for electrical or electronic purposes, the dielectric constant is often critical. The penchant for miniaturization places demands on potential dielectric materials. For example, substrate materials with low dielectric constants are needed to minimize crosstalk and input impedances. With microwave and digital circuits becoming more commonplace, propagation delays can be the performance limiting factor. Such delay, among other factors, is a function of the dielectric constant of the substrate material. There is increasing need for materials of very low dielectric constant to meet the ever increasing demands of the electronics industry. The predicted increase in microwave use and increasing speed of discrete devices, like GaAs based components, will increase the need for ultra-low dielectric constant materials.

A reduction in the dielectric constant can be achieved by incorporating porosity in the dielectric material. There are presently several approaches for introducing porosity into a dielectric material. The addition of hollow silica microspheres (Kellerman, David, "The Development and Characterization of a Low Dielectric Constant Thick Film Material", IEEE, 569-5503, 316-327 (1987)) and glass foaming agents (U.S. Patent No. 3,874,861) are two of the more popular techniques. However, both techniques have drawbacks. Although pore size is controllable by the former technique, the volume

percentage of porosity is limited by the substantial wall thickness of the microsphereε. This limit in porosity in turn limits the ability to achieve ultra-low dielectric constants.

The latter technique can produce a large amount of porosity; however, pore size and concentration are not controllable. As a result, the material does not exhibit a controllable pore structure. Furthermore, pore linkage is common. Pore linkage allows moisture to be incorporated throughout the material resulting in an increase in dielectric constant as well as electrical losses. Also, shorts can occur from water absorption or from conductor material being drawn into the linked pores by capillary action. It would be advantageous to have a low dielectric constant material having a closed pores structure of controlled size.

SUMMARY OF THE INVENTION The present invention provides a dielectric material and method of making that material in which the porosity is accurately controlled. This technique allows a material with a desired . dielectric constant to be fabricated.

According to the present invention, porosity is achieved by first adding a gas generating material to a glass or glass-ceramic matrix. The dielectric matrix including 'the gas .generating material is fired at a temperature which causes the matrix to become viscoelastic and completely encapsulate the gas generating material. The matrix with the gas generating material is subsequently subjected to a temperature at or above the reaction temperature of the gas generating material. At these temperatures, the gas generating material undergoes a reaction producing gas which is trapped in the matrix. Upon cooling, the pore structure is retained. The size and number of pores can be controlled through the

regulation of the composition and viscosity of the matrix, concentration and part.icle size of the gas generating material, and the subsequent heat treatment of the sample. The present invention is particularly useful in conjunction with the production of hybrid circuits. This utility is manifested in a variety of fields such as communications, data processing, consumer electronics, medical electronics, transportation electronics and industrial and testing electronics. By providing the ability to achieve ultra-low dielectric constant material, the present invention helps attain the higher signal propagation speed required by modern digital circuits. The ability to provide ultra-low dielectric constant material will be useful in the fabrication of microwave circuits.

Another important aspect of the invention is the formation of pores at a temperature which is higher than the original firing temperature. This feature allows porous interlayer material to be produced at any stage of multilayer fabrication providing improved uniformity and reproducibility of multilayer substrates. Although hermeticity can be lost from excessive pore formation, the present process for controlling pore formation overcomes moisture problems associated with pore linkage.

DESCRIPTION OF THE DRAWINGS The invention will be more fully understood from the following solely exemplary detailed description taken in conjunction with the accompanying drawings in which: FIG. 1 is a block diagram of a process according to the invention;

FIG. 2 is a micrograph of the dielectric material of the present invention deposited as a layer on a ceramic substrate taken at a magnification of 300X showing the pore density of a sample heated to a peak temperature of 920°C; and

FIG. 3 is a micrograph of the dielectric material of the present invention deposited as a layer on a ceramic substrate taken at a magnification of 300X showing the pore density of a sample heated to a peak temperature of 960°C.

DETAILED DESCRIPTION OF THE INVENTION

The method of the present invention is depicted in

FIG. 1. In this method, a gas generating material is added to a dielectric matrix. The matrix and heat treatment are chosen such ^' that the gas generating material is encapsulated by the viscoelastic matrix before the onset of the gas liberating reaction. The latter part of the heat treatment initiates a chemical reaction which releases gas to form the desired porosity. A preferred gas generating material has limited or slow solubility in the molten matrix and remains stable during encapsulation. Since the size and concentration of pores affects the dielectric constant of the material, the present invention provides several ways of producing materials with a desired dielectric constant.

Suitable gas generating materials are those which fit the ahove procedure. Candidate materials include oxides, peroxides, halides, hydrates, sulfates, sulfides, carbonates, nitrides and nitrate compounds of metals and metal ions. The resultant pore structure can. be adjusted by varying the concentration or size of the gas generating additive. A higher concentration of gas generating material will result in an increase in the concentration of pores. Particle size affects the number and size of the pores. Since gas is evolved from each particle, decreasing the particle size of a fixed amount of additive increases the number of pores. Pore size, on the other hand, is influenced by the volume of gas liberated. This volume of gas is proportional to particle size. Particle size is essentially continuous from chemically synthesized

colloidal material to conventionally ground material separated mechanically.

The amount of porosity and size of the pores can also be controlled by adjusting the heat treatment of the material. At higher temperatures, increased amounts of porosity are formed as a result of the decreased viscosity of the matrix and increased pressure of the liberated gas. FIGS. 2 and 3 show a cross-sectional view of dielectric material deposited as a layer on a ceramic substrate and subsequently heated at 920°C and 960°C, respectively. The material heated to 960°C shows a greater concentration of pores than that heated to 920°C. A higher subsequent firing temperature increases the size of the individual pores, as seen in FIG. 3. In those situations when linked pores are desirable, linked porosity may be achieved in accordance with the present invention by further increasing the temperature and or time parameters. Materials with controllable open or interconnected porosity are receiving attention as coatings for selective protection on gas sensors, for increasing the surface area of sensor films, for partial filtration of gas mixtures and modeling oil recovery from the earth's porous layers.

The glass or dielectric composition is important as it fixes the temperature at which the gas generating species is encapsulated and influences the temperature at which the gas generation initiates. It also affects the gas generator solubility and the matrix viscosity versus temperature relationship. Best porosity control is achieved with slow or limited solubility. Glass viscosity, determined by composition and temperature, limits pore size and pore coalescence. High viscosity also restricts the rate at which gas bubbles can rise to the surface and dissipate. The following examples will serve to illustrate the present invention.

EXAMPLE #1 A dielectric matrix was prepared by combining a commercially available glass with a dielectric filler and processing to an average particle size of about 2 microns. A gas generating material Al_(S0 ₄ )_,, in powder form was added to the matrix. The additive was processed to an average particle size of approximately 4 microns. The wt% of each material in the final composition was as follows:

MATERIAL WT%

Asahi 1380 glass 78.0 fillers 20.0

A1 ₂ (S0 ₄ ) ₃ 2.0

The above composition was fabricated into a thick film paste and separated into first, second, and third individual samples. The first sample was fired in an IR (infrared) belt furnace at a peak temperature of 750°C for ιo minutes with a 45 minute total cycle. The second sample was heated in an IR belt furnace to 850°C for 2.5 minutes with a 10 minute ^" total cycle. The third sample was heated in a belt furnace at a peak temperature of 850°C for 10 minutes with a 45 minute total cycle. The dielectric properties of samples referred to in this and subsequent examples were measured on 5mm x 5mm capacitors having a thickness between 30 and 60 microns. A fritless ,gold conductor was used as the electrode material. Dielectric constant measurements were performed at a frequency of 1 Mhz.

A microscopic analysis of the first sample showed no porosity. The dielectric constant of this sample was found to be 6.9. Similar analysis showed that the second sample had slight porosity and a measured dielectric constant of 6.50. Microscopic analysis of the third sample showed an increase in porosity and a measured dielectric constant of 6.15.

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EXAMPLE #2

An additional sample was prepared by removing the fillers from EXAMPLE #1. The wt of the remaining materials in the final composition was as follows:

MATERIAL WT%

Asahi 1380 glass 98.0 A1 ₂ (S0 ₄ ) ₃ 2.0

The above composition was fabricated into a thick film paste format and separated into first and second individual samples. The first sample was fired in an IR belt furnace at a peak temperature of 725°C for 10 minutes with a 45 minute total cycle. The second sample was fired in a belt furnace at a peak temperature of 850°C for 10 minutes with a 45 minute total cycle.

A microscopic analysis of the first sample showed no porosity and a dielectric constant of 6.96. The behavior of this sample was very similar to that of the corresponding sample of EXAMPLE #1. Similar analysis of the second sample showed substantial porosity and a dielectric constant of 4.96. A large decrease .in dielectric constant was achieved by lowering the matrix viscosity through filler removal.

EXAMPLE #3

A dielectric matrix was prepared from a mixture of a borosilicate glass and ceramic fillers. Each material was obtained commercially and processed according to known methods to reduce the average particle size to about 2 microns. A gas generating material, Co„0., in powder form was added to the matrix. The additive was processed to an average particle size of about 0.25 microns. The wt% of the final composition was as follows: MATERIAL WTS

ESL 130 glass 60.0 fillers 39.0

^Cθ 3°4 1.0

The above composition was fabricated into a thick film paste format and separated into first and second individual samples. The samples were fired in a belt furnace at a peak temperature of 850°C for 10 minutes with _a 45 minute, total cycle. The first sample was then subsequently heated in an IR belt furnace at a peak temperature of 920°C for 10 minutes with a 45 minute total cycle. The second sample was subsequently heated in an IR belt furnace at a peak temperature of 960°C for 10 minutes with a 45 minute total cycle.

Cross-sections of the resulting porous dielectric

• material are shown in FIGS. 2 and 3. The sample subsequently heated at a peak temperature of 960°C shows a higher concentration of porosity than the sample processed at a peak temperature of 920°C.

The dielectric constant of the material subsequently heated to a peak temperature of 920°C was found to be 3.56. The dielectric constant of the sample processed at a peak temperature of 960°C was measured to be 3.0. As expected, the sample fired at a higher temperature had a lower dielectric constant than the sample fired at the lower temperature.

EXAMPLE #4 A dielectric matrix was prepared from a commercially available borosilicate glass by processing the glass to an average particle size of about 2 microns. A gas 5 generating material, A1 ₂ (S0 ₄ ) ₃ , in powder form was added to the matrix. The wt% of each material in the final composition was as follows:

MATERIAL WT%

10 ESL 130 glass 98.0

A1 ₂ (S0 ₄ ) ₃ 2.0

The above composition was fabricated into a thick film paste format and separated into first, second, and 15 third samples. The first sample was fired in an IR belt furnace at a peak temperature of 750°C with a 45 minute total cycle. The second sample was fired in a belt furnace at a peak temperature of 850°C for 10 minutes with a 45 minute total cycle. The third sample was fired in a

20 belt furnace at a peak temperature of 930°C for 10 minutes in a 45 minute total cycle.

A microscopic analysis of the first sample showed no porosity. The second sample showed slight porosity and the third sample showed a significant increase in porosity.

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EXAMPLE #5 A gas generating material, PbCl_, in powder form, replaced the gas generating material, A1 ₇ (S0 ₄ )„, in the material of EXAMPLE #4. The gas-generating additive *-'*-' was processed to an average particle size of approximately 16 microns. The wt% of each material in the final composition was also the same as Example #4.

The above composition was fabricated into a thick film paste format and separated into first, second, and 5 third samples. The first sample was fired in an IR belt

furnace at a peak temperature of 750°C for 10 minutes with a 45 minute total cycle. The second sample as heated in an IR belt furnace at a peak temperature of 850°C for 10 minutes with a 45 minute total cycle. The third sample was heated in an IR belt furnace at a peak temperature of 930°C for 2.5 minutes with a 10 minute total cycle.

A microscopic analysis of the first sample showed no porosity. The second sample showed slight porosity and the third sample showed a significant increase in porosity.

EXAMPLE #6 A gas generating material, CuO, in powder form, replaced the gas generating material, Al (S0 ₄ )_, in the material of EXAMPLE #4. The gas-generating additive was processed to an average particle size of approximately 8 micron. The wt% of each material in the final composition was also the same as Example #4.

The above composition was fabricated into a thick film paste format and separated into first and second samples. The first sample was fired in an IR belt furnace at a peak temperature of 750°C for 10 minutes with a 45 minute total cycle. The second sample was fired in an IR belt furnace at a peak temperature of 850°C for 2.5 minutes with a 10 minute total cycle. A microscopic analysis of the first sample showed no porosity. The second sample showed slight porosity.

EXAMPLE #7

A gas generating material, Mn_0_, in powder form, replaced the gas generating material,

A1 ₂ S0 ₄ ) ₃ , in the material of EXAMPLE #4. The gas-generating additive was - processed to an average particle size of approximately 4 microns. The wt% of each material in the final composition was also the same as Example #4.

The above composition was fabricated into a thick film paste format and separated into first, second, and third samples. The first sample was fired in an IR belt furnace at a peak temperature of 750°C for 10 minutes with a 45 minute total cycle. The second sample was fired in a belt furnace at a peak temperature of 850°C for 10 minutes with a 45 minute cycle. The third sample was heated in a belt furnace at a peak temperature of 930°C for 10 minutes with a 45 minute total cycle.. A microscopic analysis of the first sample showed no porosity. The second sample showed the porosity to be slight. The third sample showed the concentration of porosity to be higher than the second sample.

EXAMPLE #8

A dielectric matrix was prepared from a commercially available calcium - barium silicate glass and processed to an average particle size of about 2 microns. A gas generating material, Co»0., in powder form was added to the matrix. The wt% of each material in the final composition of the matrix was as follows: MATERIAL WT%

NEG GA-13 glass 98.0 ^Co 3°4 ² '°

The above composition was fabricated into a thick film paste format and separated into first, second, third, fourth, and fifth samples. The first sample was fired in an IR belt furnace at a peak temperature of 800°C for 10 minutes with a 45 minute total cycle. The second sample was fired in an IR belt furnace at a peak temperature of 825°C for 10 minutes with a 45 minute total cycle. The third sample was fired in a belt furnace at a peak temperature of 850°C for 10 minutes with a 45 minute total cycle. The fourth sample was heated in a belt furnace at

a peak temperature of 930°C for 10 minutes with a 45 minute total cycle. The fifth sample was heated in a belt furnace at a peak temperature of 980°C for 10 minutes with a 45 minute total cycle. A microscopic analysis of the first sample showed no porosity. The second sample showed slight porosity. The third sample showed the concentration of porosity to be considerable. The fourth sample showed the concentration of porosity to be higher than the third sample. Finally, the fifth sample showed the concentration of porosity to be excessive. When comparing the gas generating behavior of this system with the system in EXAMPLE #3, one finds the present system produces porosity at a lower temperature. This can be attributed to the different chemical and physical enviroments encountered by the encapsulated gas generating additive.

EXAMPLE #9

A dielectric matrix was prepared from a commercially available calcium - alumino silicate glass and processed to an average particle size of about 2 microns. A gas generating material, ^W0 ₃ r i ⁿ powder form was added to the matrix. The additive was processed to an average particle size of approximately 4 microns. The wt% of each material in the final composition of the matrix was as follows:

MATERIAL . WTjs.

NEG GA-33 glass 98.0 W0 ₃ 2.0

The above composition was fabricated into a thick film paste format and separated into first and second samples. The first sample was fired in a belt furnace at a peak temperature of 980°C for 10 minutes with a 45 minute total cycle. The second sample was fired in a box

furnace with 5 minutes at a peak temperature of 1250°C.

The heating and cooling rate was approximately 50°C/minute.

A microscopic analysis of the first sample showed no porosity. An analysis of the second sample exhibited slight porosity.

EXAMPLE #10 A dielectric matrix was prepared from a commercially available high lead glass and processed to an average particle size of about 2 microns. A gas generating material, kaolin, in powder form was added to the matrix. The additive was processed to an average particle size of approximately 1 micron. The wt% of each material in the final composition of the matrix was as follows: MATERIAL WT%

Corning 7570 glass 98.0 kaolin 2.0

The above composition was fabricated into a thick film paste format and separated into first and second samples. The first sample was fired in an IR belt furnace at a peak temperature of 450°C for 10 minutes with a 45 minute total cycle. The second sample was fired in an IR belt furnace at a peak temperature of 550°C for minutes with a 45 minute total cycle.

A microscopic analysis of the first sample showed no porosity. An analysis of the second sample exhibited a significant concentration of porosity.

EXAMPLE #11

A dielectric matrix was prepared from a commercially available glass and processed to an average particle size of about 2 microns. A gas generating material, BaO_, in powder form was added to the matrix. The additive was processed to an average particle size of approximately 6.5 micron. The wt% of each material in the final composition of the matrix was as follows:

MATERIAL WT%

Ferro 3467 glass 98.0

Ba0 ₂ ^' 2.0

The above composition was fabricated into a thick film paste format and separated into first, second and third samples. The first sample was fired in an IR belt furnace at a peak temperature of 750°C for 10 minutes with a 45 minute total cycle. The second sample was fired in a belt furnace at a peak temperature of 850°C for 10 minutes with a 45 minute total cycle. The third sample was fired in a belt furnace at a peak temperature of 930°C for 10 minutes with a 45 minute total cycle.

A microscopic analysis of the first sample showed no porosity. An analysis of the second sample exhibited a slight concentration of porosity. The third sample also exhibited slight porosity.

The invention is not to be limited by what has been particularly shown and described, except as indicated by the appended claims.