Stock material or miscellaneous articles – Structurally defined web or sheet – Discontinuous or differential coating – impregnation or bond
Reexamination Certificate
2002-04-09
2004-04-20
Jones, Deborah (Department: 1775)
Stock material or miscellaneous articles
Structurally defined web or sheet
Discontinuous or differential coating, impregnation or bond
C428S209000, C428S432000, C428S433000, C428S689000, C428S698000, C428S702000, C428S704000, C428S434000
Reexamination Certificate
active
06723420
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a method for the fabrication of thick film passive electrical components on diamond substrates for the microelectronics industry. More particularly, the method of this invention relates to depositing a thick film system of pastes onto diamond, and after firing, allowing the diamond to be incorporated into electronic packages using conventional joining techniques.
BACKGROUND OF THE INVENTION
Integration of high power electronic devices into electronic systems typically requires the construction of an electronic package comprised of active and passive components. Since the manufacturing costs for microelectronic parts must be low, inexpensive fabrication methods are required. Many microelectronic components are manufactured by the methods of thick film technology, which typically utilize screen-printing techniques to deposit a thick film paste in a pattern to define the circuit element. The printing is followed by a high temperature treatment, i.e. firing operation, in a belt furnace, at up to about 900° C., which sinters and stabilizes the paste components.
A passive circuit element consists of the passive material and a conductor material, which establishes its connection to the other elements of the circuit. Usually the conductor material is deposited and fired, followed next by the deposition and firing of the passive element material. An overcoat material may also be used to environmentally protect the passive material. In the case of a resistor element, for example, typically the conductor material is first printed and fired. The resistive material is then printed such that a portion of the printed resistive material is overlaid onto the terminating ends of the conductor, then fired. An overcoat, i.e. passivation layer, is printed over the resistive material and fired in a final step to environmentally protect the resistive material. Thus, the fabrication of a resistor element typically requires a series of three steps, each consisting of printing and firing of a thick film paste for the conductor, resistor, and overcoat. The sintering processes require a high temperature cycle, and each of the materials that comprise the parts of a resistor must, therefore, be compatible at these high temperatures during processing. A collection of compatible thick film pastes that are used in the fabrication of a particular circuit element is referred to as a “thick film paste system.”
The choice of the substrate material used for the mechanical support of the circuit elements is dependent upon the application. In high performance circuits used in telecommunications networks, the requirements for operation at high power and at high frequencies are increasing. In such a case, the substrates must have relatively high thermal conductivity and excellent dielectric properties. Synthetic diamond such as chemical vapor deposited diamond (“CVD diamond”) typically has a thermal conductivity that is many times greater than other conventional substrate materials currently used in the manufacture of thick film circuitry, such as aluminum oxide and beryllium oxide. Diamond also has a dielectric constant and loss tangent within the acceptable limits required for high frequency applications. Thus, diamond substrates have the potential to accommodate the demand for increasing power density and operational frequency requirements for high performance circuits.
High power devices, such as those used in high speed computing, microwave and RF telecommunications, typically use conventional ceramic materials such as beryllium oxide or aluminum nitride as substrates for thermal management. These materials are cut to shape from sheets and electronic circuits are fabricated onto the surfaces by the screen-printing methods discussed above. These conventional ceramic substrate materials are stable in the high-temperature oxygen-containing environment, e.g. air, required for sintering commercially available thick film pastes. Diamond, however, deteriorates at temperatures in excess of about 600° C. in air. At typical temperatures and times required for complete sintering of prior art thick film materials in air, a diamond substrate is disintegrated.
There are a variety of thick film pastes commercially available that are formulated for sintering in a non-oxidizing ambient, e.g. nitrogen. These pastes are in the minority of commercially available thick film systems for manufacturing circuit elements. Similarly, there are resistor formulations and passivation overcoats that may be fired in nitrogen. These thick film pastes usually contain a glass constituent that does not reduce in a non-oxidizing atmosphere and becomes fluidic at about 900° C. The glass wets to the substrate, resulting in a sintered microstructure that possesses robust adhesion and cohesion. Conductor and resistor thick film paste formulations are made by adding constituents that are more conductive than the glass, which lower the resistivity of the sintered structure to a desired specification. In these cases, the glass also wets to the additional constituent, retaining cohesion and adhesion. A commercially available thick film conductor paste based on a mixture of copper and glass powders, for example, has been formulated for firing in nitrogen at about 900° C.
In addition to the limited selection of thick film pastes, the chemistry of the constituents are generally designed for bonding to oxide substrates, such as beryllium oxide. This particular material has dominated the microelectronics market for thermal management applications because of its relatively high thermal conductivity compared to aluminum oxide, and relatively low cost. Beryllium oxide, however, is considered to be an environmentally unsafe material and will be phased out of usage in the near future. Alumina is inexpensive, but has a relatively low thermal conductivity, and is not used for high-performance thermal management applications.
Recently, as the manufacturing cost of diamond has declined due to improved CVD diamond synthesis techniques, the demand for diamond components has increased. The need for CVD diamond in high power density applications is rapidly increasing as package sizes decrease and package power increases. Sheets of diamond substrates have recently become commercially available with the lateral dimensions and flatness that are amenable to the screen-printing techniques discussed above. As a result, a thick film paste system for fabricating microelectronic circuits on the surface of diamond substrates, diamond heat spreaders and components would be useful in many applications where enhanced thermal management is vital to the performance of microelectronic packages. Therefore, there is a commercial need for improved thick film paste materials and manufacturing processes for application of these paste materials to enable the use of diamond substrates for high-power, high frequency electronic circuit elements.
To-date, techniques for production of reliable thick film structures on diamond for microelectronic packages using commercially available thick film pastes have not been known. Generally, thick film formulations that work well with oxide-based substrates do not work well for diamond substrates. These difficulties are generally due to one or more of four factors:
(1) the requirement to sinter the pastes in an oxygen-containing environment which is incompatible with diamond,
(2) the lack of adhesion of available thick film formulations to diamond due to significant differences in the coefficient of thermal expansion (CTE) between diamond and constituents of most commercially available thick film pastes, which were originally developed for use on substrates with a significantly higher CTE that that of diamond, such as beryllium oxide or aluminum oxide,
(3) the lack of adhesion of available thick film formulations to diamond due to the lack of chemical interaction or bonding, between diamond and the constituents in most commercially available thick film pastes, which were developed for oxide substrates, and/or
(4
Gray Bruce D.
Jones Deborah
Kilpatrick & Stockton LLP
Morgan Chemical Products, Inc.
Russell Dean W.
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