Electricity: conductors and insulators – Conduits – cables or conductors – Preformed panel circuit arrangement
Reexamination Certificate
1999-10-29
2001-12-18
Gaffin, Jeffrey (Department: 2841)
Electricity: conductors and insulators
Conduits, cables or conductors
Preformed panel circuit arrangement
C174S255000, C174S256000, C361S750000, C361S751000, C428S209000, C428S901000
Reexamination Certificate
active
06331678
ABSTRACT:
FIELD OF THE INVENTION
The present invention is related to techniques for forming high temperature devices that have films of dissimilar thermal expansion coefficients adhered together, more specifically, for forming sensor devices which are used in high-temperature operation and which are formed by a method in which metal films are processed under high temperature.
BACKGROUND
The use of metal thin films and filaments is very common in measurement and sensing devices, such as flow and velocity sensors. In such measuring and sensing devices, often a hot conductor thin filament that can dissipate heat is held in a fluid stream. When the conductor filament loses heat to the fluid stream, its temperature falls. The resulting change in electrical resistance of the conductor filament associated with the fall in temperature can be measured and it would be indicative of the flow characteristics of the fluid stream. Often such conductor filaments are made from conductor thin films. For making the conductor filament (or thin film), a variety of metals can be used. For such measurement and sensing applications, platinum is preferred over other metallic substances because of its refractory nature as well as its high electrical resistance and temperature coefficient of resistance (TCR). It is also inert and can operate at a high temperature.
In some applications, because of the catalytic nature of platinum, to prevent unwanted chemical reaction, it is desirable to completely encapsulate the platinum in a thin dielectric film such as silicon nitride or silicon oxynitride. The dielectric also serves as a support material for the thin film platinum. However, because of the inert nature of platinum, it does not adhere well to most dielectric materials. Furthermore, the platinum thin film is made by deposition on a surface and during the subsequent fabrication of the measurement or sensing device, it may be necessary to anneal the deposited thin film at a high temperature. Annealing improves the material properties of thin film platinum. It typically reduces the resistance and increases the thermal coefficient of resistance of the thin film to close to the bulk value. The annealing typically takes place at temperatures at or above 800° C. At this high temperature, due to thermal expansion mismatch of the thin metal film with the substrate (i.e., the material supporting the thin metal film), high stress results in the film and at the interface. The stress can be so high as to cause the metal to totally come off (delaminate) or blister from the substrate. If a platinum thin film is used to form one of the layers of electrical contact pads, the poor adhesion of the platinum on the underlying dielectric material causes a weak link that can result in bond pad failure. On a metal film attached to a dielectric material having a different coefficient of expansion from that of the metal film, blistering and protrusions (hillocks) can form when the metal film experiences high temperature fluctuation. What is needed a technique for forming a secure adhesion of the conductor metal (especially platinum) on the dielectric material, and methods to reduce the interfacial stress of the conductor metal on the dielectric material so that delamination and blistering are prevented or greatly reduced.
Various groups have conducted investigation on improvement of adhesion of platinum to silicon nitride layers and reduction of stress by texturing the surface before platinum deposition. Documents of interest related to metal film adhesion to dielectric material in sensors include U.S. Pat. No. 4,501,144 (Higashi et al.), U.S. Pat. No. 4,683,159 (Bohrer et al.), U.S. Pat. No. 4,696,188 (Higashi), U.S. Pat. No. 4,891,977 (Johnson et al.), and U.S. Pat. No. 4,952,904 (Johnson et al.). Documents of interest related to hillock formation include U.S. Pat. No. 3,986,897 (McMillan et al.) and U.S. Pat. No. 4,012,756 (Chaudhari et al.).
SUMMARY
In the present invention, a micro-mesh structure is used to reduce the interfacial stress and improve the adhesion of a conductor metal to a dielectric material. In one aspect, a device having such a micro-mesh structure is provided and the method for making it is provided in the present invention. In an embodiment, such a device includes a component (for example, a bond pad, i.e., conductor pad) having a micro-mesh of conductor metal (sometimes referred to as “micro-mesh conductor metal” herein) contacting another conductor metal (which may be a metal different from the micro-mesh conductor metal, such as the conductor metal of a bond pad) to provide electrical communication to the micro-mesh. The second conductor metal, preferably without a micro-mesh structure of its own, adheres to the micro-mesh and the dielectric layer through the openings in the micro-mesh.
In a preferred mode, the present invention provides improved adhesion between the conductor metal and the dielectric material by using an adhesive layer sandwiched between the dielectric material and the micro-mesh conductor metal. This is especially useful in adhering platinum to a dielectric material such as silicon nitride. Further, in an embodiment, the micro-mesh conductor metal is sandwiched between two adhesive layers. Lines (or branches) in the micro-mesh have a cross-section in which the top adhesive layer (which will face a bond pad metal) has a smaller width than that of the micro-mesh conductor metal. In turn, the micro-mesh conductor metal has a smaller width than that of the bottom adhesive layer, which is nearest a supporting substrate. In this way, the bond pad (conductor pad) metal can has contact with the adhesive material and the micro-mesh conductor metal to maintain mechanical integrity and electrical communication.
The incorporation of the micro-mesh (for example, checkerboard) design gives good adhesion of the conductor metal (of the micro-mesh) to the dielectric material. Because the conductor metal and the dielectric material have thermal expansion coefficients that are quite different, temperature changes tend to put stress to delaminate the conductor metal from the dielectric material. Due to the good adhesion provided by the micro-mesh, the risk of delamination is reduced in the device under repeated cycles of high temperature operation. In preferred embodiments, a device according to the present invention is operable at a temperature of at least 250° C. or higher without the first electrical conductor delaminating or blistering from the dielectric layer. Further, this technique of utilizing a micro-mesh reduces the risk of delamination of platinum on silicon nitride when platinum on dielectric (such as silicon nitride) is annealed at high temperature, for example, as in forming and annealing a layer of silicon nitride to encapsulate the platinum. The micro-mesh design also enables devices utilizing a noble metal thin film electrical conductor layer to operate at high temperature in repeated cycles without delamination. The application of the micro-mesh of the present invention is particularly advantageous in integrated circuits using a flip-chip technique. In such circuits, after two components are flip-chip-bonded together, stress on one component will be transmitted to the bond pads of the other component, thus causing delamination of the bond pad conductor metal from the conductor underneath it if the adhesion force between them is not strong enough.
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Barth Phillip W.
Goedert Michel G.
Wang Tak Kui
Agilent Technologie,s Inc.
Gaffin Jeffrey
Patel I B
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