Active solid-state devices (e.g. – transistors – solid-state diode – Lead frame – With stress relief
Reexamination Certificate
2001-10-12
2004-09-21
Flynn, Nathan J. (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Lead frame
With stress relief
C257S698000, C257S696000, C438S123000, C438S652000, C029S832000
Reexamination Certificate
active
06794737
ABSTRACT:
FIELD OF THE INVENTION
This invention generally relates to stress-engineered metal films, and more particularly to photo lithographically patterned spring structures formed from stress-engineered metal films.
BACKGROUND OF THE INVENTION
Photo lithographically patterned spring structures (sometimes referred to as “micro-springs”) have been developed, for example, to produce low cost probe cards, and to provide electrical connections between integrated circuits. A typical spring includes a spring metal finger having a flat anchor portion secured to a substrate, and a curved claw extending from the anchor portion and bending away from the substrate. The spring metal finger is formed from a stress-engineered metal film (i.e., a metal film fabricated such that its lower portions have a higher internal compressive stress than its upper portions) that is at least partially formed on a release material layer. The claw of the spring metal finger bends away from the substrate when the release material located under the claw is etched away. The internal stress gradient is produced in the spring metal by layering different metals having the desired stress characteristics, or using a single metal by altering the fabrication parameters. Such spring metal structures may be used in probe cards, for electrically bonding integrated circuits, circuit boards, and electrode arrays, and for producing other devices such as inductors, variable capacitors, and actuated mirrors. For example, when utilized in a probe card application, the tip of the claw is brought into contact with a contact pad formed on an integrated circuit, and signals are passed between the integrated circuit and test equipment via the probe card (i.e., using the spring metal structure as a conductor). Other examples of such spring structures are disclosed in U.S. Pat. No. 3,842,189 (Southgate) and U.S. Pat. No. 5,613,861 (Smith).
The present inventors recognized that most failures of spring structures (e.g., separation of the spring structure from an underlying substrate through delamination or peeling) occur a significant amount of time after fabrication. The present inventors believe these failures are caused at least in part by the internal stress gradient retained in the anchor portion of the spring metal finger. That is, although the internal stress is essentially relieved in the claw of the spring metal finger upon release, the internal stress is retained in the anchor portion of the spring metal finger, along with other “trace” or unreleased portions of the spring metal layer. Over time, this retained internal stress is believed to bend the edges of the anchor portion upward (i.e., away from the underlying substrate), thereby causing delamination or peeling that weakens the attachment of the spring metal finger to the substrate. It is essential that the unlifted anchor portion of the spring metal finger adheres to the substrate (i.e., that the anchor portion resists the internal stress tending to bend the edges of the anchor portion away from the substrate). Most probing and packaging applications require large amounts of contact force (~50-100 mg) between the claw tip and a contacted structure. The force scales quadratically with film thickness, but the peeling moment increases also.
One possible solution to the delamination/peeling problem is to use a spring material in which the stress is annealed out after release (i.e., after the claw of the spring metal finger is allowed to bend away from the substrate). However, this solution places other limitations on the material properties, such as a reduction in the total stress differental.
Another solution is to incorporate a ductile, dry etchable metal such as Aluminum (Al) or Titanium (Ti) as an interfacial release layer between the substrate and the finger metal. This approach has been demonstrated to improve adherence of the anchor portion to the substrate when the thickness and/or internal stress of the spring metal layer is relatively small, but is less effective as the thickness or the stress of the metal layer is increased.
What is needed is a spring structure that resists delamination and/or peeling, thereby improving the strength and durability of the spring structures.
SUMMARY OF THE INVENTION
In accordance with the present invention, the strength and durability of a spring structure is increased by providing a stress-balancing pad formed on the unlifted anchor portion of the spring metal finger, where the stress-balancing pad is formed with an internal stress gradient (and stress moment) that is opposite in sign to the internal stress gradient (and stress moment) of the spring metal finger. Specifically, in contrast to the spring metal finger, the stress-balancing pad is formed from a stress-engineered metal film fabricated such that portions furthest from the anchor portion have a higher internal compressive stress than portions closest to the anchor portion. This opposite internal stress gradient causes the stress-balancing pad to apply a downward force on the edges of the anchor portion, thereby resisting the delamination or peeling of the anchor portion that can result in separation from an underlying substrate. In one embodiment, the internal stress gradient (and moment) of the stress-balancing pad has a magnitude that is equal to or greater than the internal stress gradient (and moment) of the spring metal finger, thereby preventing delamination or peeling of the anchor portion by completely countering (nullifying) the internal stress (and moment) of the spring metal finger.
In accordance with an aspect of the present invention, the spring metal finger and the stress-balancing pad can be formed either from materials that have the same composition, or from materials that have different compositions. For example, both the spring metal finger and the stress-balancing pad can be formed from Mo or MoCr. The fabrication process is simplified when the same material is used for both layers because the number of targets in the deposition equipment is minimized. However, an etch stop layer (e.g., Cr or Ti) may be needed between the spring metal finger and the stress-balancing pad to prevent undesirable etching of the spring metal finger during the fabrication process. When different materials are used, it may be necessary to increase the number of deposition equipment targets, but the etch stop layer can be omitted when the two materials are selectively etchable. For example, a stress-balancing pad formed from Mo is selectively etched from a spring metal finger formed from MoCr using an anisotropic fluorine etch. Similarly, a stress-balancing pad formed from Ti solution hardened with Si (Ti:Si) is selectively removed from a spring metal finger formed from NiZr using a Ti etch. Note that the stress-balancing pad can be electrically conducting or non-conducting, but electrical conductivity of the stress-balancing pad beneficially improves the total conductance through the anchor portion of the spring metal finger, and through other trace structures formed on the substrate using the spring metal and stress-balancing layers.
In accordance with another aspect of the present invention, the spring structure further includes a support pad formed between the anchor portion of the spring metal finger and the substrate. When formed from a conductive material (e.g., Ti), the support pad may be utilized to conduct signals between the spring metal finger and a conductor formed on the substrate under the support pad. In one embodiment, the support pad is formed from a portion of the release material layer.
In accordance with yet another aspect of the present invention, a spring structure is fabricated by forming a spring metal island on a release material island, forming the stress-balancing pad over an anchor portion of the spring metal island, and then releasing the claw portion of the spring metal finger by removing an associated portion of the release material island.
In accordance with a first disclosed method, a release material layer, a spring metal layer, and a stress-balan
Fork David K.
Romano Linda T.
Bever Patrick T.
Bever Hoffman & Harms LLP
Flynn Nathan J.
Mandala Jr. Victor A.
Xerox Corporation
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