Coating processes – Coating by vapor – gas – or smoke – Plural coatings applied by vapor – gas – or smoke
Reexamination Certificate
2001-09-27
2004-12-14
Chen, Bret (Department: 1762)
Coating processes
Coating by vapor, gas, or smoke
Plural coatings applied by vapor, gas, or smoke
C427S250000, C204S192100
Reexamination Certificate
active
06830780
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to fabricating brazeable diamond products and components for the microelectronic industry that are used as heat spreaders, heat conductors and electrical insulators in electronic packages. More particularly, the product of this invention relates to a multilayer metallization structure that allows diamond to be attached into electronic packages using conventional brazing operations. Still more particularly, the products for this invention have applications involving thermal management of high power semiconductor devices by the use of diamond.
BACKGROUND OF THE INVENTION
Integration of high power electronic devices into electronic systems typically requires the construction of an electronic package. An electronic package is typically comprised of a metal flange onto which is attached an electronic device (“chip”). In some package designs, it is important to insure that the chip does not electrically short to the metal flange, a layer of electrically insulating material is positioned between the device and the flange. A window frame or lead frame may be attached around the device. This frame enables electrical connections via wire bonds or other techniques from the chip to the outside of the package. For environmental protection of the chip, a cap attached over the flange may seal the package. The electronic package is then attached to a heat sink, which may be a metal element in contact with a cooling medium such as air, fluorocarbon liquid, and the like.
High power devices such as those used in high speed computing, microwave and RF telecommunications, and the like typically use ceramic materials such as aluminum oxide, beryllium oxide or aluminum nitride as materials. These materials are cut to shape from sheets, or molded to shape, and are readily attached into electronic packages using conventional solders or brazes. As the power electronic devices are miniaturized, and their power output is increased, their operating temperature is dramatically increased. These ceramic materials have relatively low thermal conductivity compared to diamond (aluminum oxide=20 W/mK (watts/meter/° K), beryllium oxide=260 W/mK, aluminum nitride=170 W/mK). Therefore, they provide a relatively high thermal resistance between the chip and the flange, which ultimately limits the operating power and reliability of the chip.
Because of its properties, Chemical Vapor Deposition (“CVD”) diamond is the ultimate electrical insulator for high power device applications. CVD diamond has extremely high electrical resistivity, high breakdown voltage and high thermal conductivity of 1,000 to 2,000 W/mK, up to four times that of copper. Because of its high thermal conductivity, a diamond layer of significant thickness (typically at least 200 microns thick) also functions as a heat spreader, effectively spreading the heat from the localized area at the chip to a larger area at the flange and ultimate heat sink. However, the use of diamond as an electrical insulator in high power electronic devices has been limited because of the cost of manufacturing diamond components, and the inability to reliably attach diamond into device packages using established methods.
Recently, as the manufacturing cost of diamond has declined with improved CVD synthesis techniques, the demand for diamond components has increased. The need for CVD diamond in high power density applications is rapidly increasing as the package sizes decrease and package power increases. Therefore, it is likely that if a robust method for attaching diamond into electronic packages could be developed, diamond heat spreaders and components would be used in almost every application where enhanced thermal management is vital to prolong the life of microelectronic packages. This is especially true for computer chips, associated power supplies and high-frequency telecommunications.
The two most common joining technologies that are used in microelectronic packaging are soldering and brazing. Each of these methods requires metallization of the components and subsequent heating to perform the attachment. Soldering is typically defined as attachment in which requires at temperature of less than 500° C. to melt metal layers and join the components, while brazing requires a temperature greater than 500° C. to melt the metal layers and join the components.
Solderable metallizations to diamond, such as a multilayer structure of titanium, platinum, gold, followed by gold-tin eutectic solder are well-established. An advantage to the solderable metallizations for diamond is the low attachment temperature, which minimizes thermal stresses resulting from the difference in thermal expansion coefficient between diamond and most materials, especially metals. However, due to the tendency of void formation in soldering, intimate thermal contact may be limited for large areas without specialized bonding techniques.
Brazing is the preferred attachment method for high power electronic packages since the high temperatures involved in the brazing process, typically around 800° C. or greater, usually ensure a very good wetting of the braze material at the interface between two components. It is important that there are no voided areas at the component interfaces that are involved in thermal transfer, since any voids would increase the overall thermal resistance of the package.
The interfaces joined by brazing are usually between the insulator, i.e., diamond, and electrical lead frame or flanges. The electrical leads must be able to pass a peel test, and hence the adhesion between the diamond and the leads must be extremely good. Typically, a peel strength of 2.0 pounds minimum at 90° is required for leads of 0.15″ in width.
One of the established industrial methods of achieving interface wetting is the use of hydrogen in the atmosphere during the brazing process, since metal oxide formation is prevented. An eutectic alloy, such as Cu—Sil (72% silver, 28% copper), is commonly used for joining articles in brazing processes, but other brazeable materials such as pure copper and gold may be used as well. The Cu—Sil alloy melts at 780° C. and wets extremely well to nickel and copper surfaces in an atmosphere of nitrogen containing from 2% up to perhaps 75% hydrogen. Brazing temperatures for Cu—Sil are typically as high as 820° C.
To-date, it has been difficult to produce reliable brazeable metallization structures for diamond in microelectronic packages. These difficulties are due to the significant differences in the coefficient of thermal expansion between diamond and metals and also to the chemical nature of the metals comprising the metallization structure.
Many methods are currently known for fabricating diamond products suitable for electronic applications. However, they all have the disadvantage of not being able to withstand brazing temperatures, i.e., temperatures in the range of about 500 to 1,100° C.
Iacovangelo, et al., U.S. Pat. Nos. 5,324,987 and 5,500,248 describe the use of a diamond product for electronic applications, but which are not suitable for use at such brazing temperatures. One example of the disclosed product is a diamond having an adhesion-promoting material interposed between the diamond and the conductive metal, e.g. copper. The adhesion-promoting material is titanium or a titanium-tungsten alloy. It is believed that the metallization in such a product becomes delaminated when the temperature is increased to brazing temperatures in a standard hydrogen-nitrogen-containing brazing atmosphere because of hydride or nitride formation.
Iacovangelo, et al., U.S. Pat. No. 5,529,805 describe fabrication of brazeable diamond tool inserts using a substantially non-oxidizable nickel protective layer deposited onto a chromium metal layer bonded to the diamond component in a nitrogen-free atmosphere.
Iacovangelo, et al., U.S. Pat. No. 5,567,985 describe an electronic structure comprising a CVD diamond substrate, a tungsten-titanium bond layer, a silver compliant layer, a tungsten-copper layer and a go
Chen Bret
Gray Bruce D.
Kilpatrick & Stockton LLP
Morgan Chemical Products, Inc.
Russell Dean W.
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