Method for fabricating a spring structure

#This invention generally relates to stress-engineered metal films, and more particularly to photo lithographically patterned micro-spring structures formed from stress-engineered metal films.

#Photo lithographically patterned spring structures (sometimes referred to as “microsprings”) have been developed, for example, to produce low cost probe cards, and to provide electrical connections between integrated circuits. A typical spring structure includes a spring metal finger having an anchor portion secured to a substrate, and a free (cantilevered) portion extending from the anchored portion over 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 free portion of the spring metal finger bends away from the substrate when the release material located under the free portion 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 free portion 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 have observed that conventional spring structures develop contact resistances that are detrimental to signal transmissions when the spring structures are used as conductors. The spring metal (e.g., Mo, MoCr, NiZr) is typically chosen for its ability to retain large amounts of internal stress. These materials typically oxidize in air, a phenomenon that can interfere with their ability to make electrical contact, for example, with the contact pad of an integrated circuit when used in a probe card. The spring metal materials can also gall to the contact pad, which is typically aluminum. Once the galled aluminum oxidizes, the contact resistance between the contact pad and the spring metal structure increases. One proposed approach to reducing contact resistance is to passivate the spring metal before etching and release. However, the passivating material tends to resist bending of the spring metal finger after release, and provides minimal coverage along the front edge at the tip, thereby allowing direct contact with the spring metal that can result in increased contact resistance.

#These and other features, aspects and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings, where:

#FIG. 1 is a plan view showing a spring structure according to a first embodiment of the present invention;

#FIG. 2 is a cross-sectional side view of the spring structure taken along section line 2—2 of FIG. 1, and the spring structure contacting a separate integrated circuit;

#FIGS. 1, 2 and 3 show a spring structure 100 according to a first embodiment of the present invention. FIG. 1 is a plan view of spring structure 100, FIG. 2 is a cross-sectional side view taken along section line 2—2 of FIG. 1, and FIG. 3 is a perspective view with a cut-away section indicated by section line 3—3 in FIG. 1.

#Spring structure 100 generally includes a substrate 101, a release material portion 110, and a spring metal finger 120. Substrate 101 (e.g., glass) includes an optional conductor 105 that can take several forms (i.e., a metal trace deposited on the substrate, as shown, or a buried conductor accessed by an opening in a passivation layer (not shown)). When present, conductor 105 may provide electrical connection between electronic components of an integrated circuit and spring structure 100. Alternatively, if substrate 101 is printed circuit board, printed wiring board, silicon device, or interposer, then conductor 105 may be an exposed portion of conducting material that is electrically connected to redistribution traces, through substrate vias, solder bumps, solder balls, mounted electrical components, integrated passive components, or interconnect pads. Release material portion 110 is formed on an upper surface of substrate 101 such that it contacts conductor 105 (if present). Spring metal finger 120 includes an anchor portion 122 and a free (i.e., cantilevered) portion 125. Anchor portion 122 is attached to release material portion 110 (i.e., such that release material portion 110 is located between anchor portion 122 and substrate 101). Free portion 125, which includes a tip 125-T, extends from anchor portion 122 over substrate 101, and includes an upper (first) surface 126 and an opposing lower (second) surface 127 that define a thickness T1 in the range of 0.1 and 10 microns (see FIG. 2). Note that edges 128 of free portion 125 extend between upper surface 126 and lower surface 127.

#Similar to prior art spring structures, spring metal finger 120 is etched from a stress-engineered metal film that is formed by DC magnetron sputtering one or more metals using gas (e.g., Argon) pressure variations in the sputter environment during film growth in accordance with known techniques. By carefully selecting the metals and/or processing parameters, sputtered metal films can be used to form tightly curved spring metal fingers, or very stiff spring metal fingers, but not both simultaneously because increasing the film thickness (which is necessary to increase stiffness) also increases the radius of the resulting spring metal finger. Further, the internal stress of the stress-engineered metal film cannot be increased arbitrarily because of material limitations.