ELECTROMIGRATION-RESISTANT LEAD-FREE SOLDER INTERCONNECT STRUCTURES
Embodiments of the invention include a lead-free solder interconnect structure and methods for making a lead-free interconnect structure. The structure includes a semiconductor substrate having a last metal layer, a copper pedestal attached to the last metal layer, a barrier layer attached to the copper pedestal, a barrier protection layer attached to the barrier layer, and a lead-free solder layer contacting at least one side of the copper pedestal.
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The present invention generally relates to semiconductor devices, and particularly to lead-free solder interconnect structures.
As semiconductor technology reduces in size, electromigration of metals in interconnect structures may occur. Electromigration refers to the gradual movement of metal atoms caused by momentum transferred to the atoms from electrons moving in the direction of an electric field. As electron momentum is proportional to current, increased current density increases electromigration. This movement of metal atoms may lead to early failure of the semiconductor device due to defects forming in the interconnect structure. Since decreasing the size of an interconnect structure may lead to an increase in current density, reducing the effects of electromigration in turn reduces the resulting defects in the interconnect structure in favor of increased device reliability.
SUMMARYThe present invention relates to lead-free solder interconnect structures featuring improved electromigration performance through the introduction of a source of available copper into the system. This available copper causes the formation of particular intermetallic compounds that, among other things, enhance the overall reliability of the interconnect structure.
A first embodiment of the invention may include a semiconductor substrate having a last metal layer, a copper pedestal attached to the last metal layer, a barrier layer on the copper pedestal, a barrier protection layer on the barrier layer, and a solder layer that directly contacts at least one side of the copper pedestal.
A second embodiment of the invention may include a method of constructing the first embodiment. The method includes providing a semiconductor substrate having a metal layer, forming a copper pedestal coupled to the metal layer, forming a barrier layer on the copper pedestal, forming a barrier protection layer on the barrier layer, forming a solder layer on the barrier protection layer, and pressing a mold over the solder layer so that the solder layer directly contacts at least one side of the copper pedestal.
A third embodiment of the invention may include a method of constructing the first embodiment. The method includes providing a semiconductor substrate having a metal layer, forming a copper pedestal coupled to the metal layer, forming a barrier layer on the copper pedestal, forming a barrier protection layer on the barrier layer, filling a mold with solder, and transferring the solder from the mold to the copper pedestal so that the solder layer directly contacts at least one side of the copper pedestal.
A fourth embodiment of the invention may include a method of constructing the first embodiment. The method includes providing a semiconductor substrate having a metal layer, forming a copper pedestal coupled to the metal layer, forming a barrier layer on the copper pedestal, forming a barrier protection layer on the barrier layer, forming a solder layer on the barrier protection layer so that the solder layer contacts at least one side of the copper pedestal, and reflowing the solder layer.
According to at least one embodiment, the utilization of available copper in a lead-free solder interconnect structure is described. Based on interconnect systems where the solder layer was electrically connected only to two nickel electromigration barrier layers, electromigration performance was hindered by the formation of nickel-rich intermetallic compounds at the surface of the nickel electromigration barrier layers. By introducing sources of copper into this system, the formation of these nickel-rich intermetallic compounds is reduced in favor of providing, for example, intermetallic compounds with greater copper concentrations. The preferable formation of these copper-rich intermetallic compounds preserves the integrity of the nickel electromigration barrier layers and thus enhances electromigration performance.
1. First EmbodimentSemiconductor substrate 110 includes a last metal layer 111. A copper pedestal 140 is connected to the semiconductor substrate 110 at a surface 112 of the last metal layer 111. The copper pedestal 140 is preferably 8 micrometers (μm) to 12 μm thick. In this embodiment, the copper pedestal 140 may be approximately 10 μm thick. The last metal layer 111 forms an electrical connection between the semiconductor substrate 110 and the copper pedestal 140. An electromigration barrier layer 150 is then connected to the top of the copper pedestal 140. The electromigration barrier layer 150 is preferably 1 μm to 3 μm thick. In this embodiment, the electromigration barrier layer 150 may be approximately 2 μm of nickel; other embodiments may use other materials as an electromigration barrier layer including, for example, iron, nickel-iron, or cobalt. A barrier protection layer 160 is then connected to the top of electromigration barrier layer 150. The barrier protection layer 160 is preferably 0.5 μm to 3 μm thick. In this embodiment, the barrier protection layer 160 is 1 μm thick and consists of copper. Upon reflow of the solder layer 130, some or all of the barrier protection layer 160 may be converted to intermetallic compounds. Collectively, the copper pedestal 140, the electromigration barrier layer 150, and the barrier protection layer 160 will be referred to as an assembled pedestal structure 200.
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The following sections outline three methods for fabricating the embodiments depicted in
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Due to surface tension, the solder layer 130 remains balled up on the surface of the barrier protection layer 160 and will not naturally or controllably contact the sides of the copper pedestal 140. Therefore, as shown in
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The third embodiment begins similarly to the second embodiment, with the semiconductor substrate 110 having the last metal layer 111, as shown in
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Next, the solder-filled mold 500 and semiconductor substrate 110 with assembled pedestal structure 200 are heated to reflow temperature and placed in a chemically reducing environment (not shown). In this embodiment, this environment is provided by about 5% formic acid in nitrogen, formed by bubbling nitrogen gas through a formic acid solution.
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A fourth embodiment utilizes a second photoresist step in lieu of the physical mold of the first and second method embodiment. This embodiment again begins with the semiconductor substrate 110 having the last metal layer 111, as shown in
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While the present invention has been particularly shown and described with respect to preferred embodiments, it will be understood by those skilled in the art that the foregoing and other changes in forms and details may be made without departing from the spirit and scope of the invention. It is therefore intended that the present invention not be limited to the exact forms and details described and illustrated but fall within the scope of the appended claims.
Claims
1. An interconnect structure comprising:
- a semiconductor substrate having a last metal layer;
- a copper pedestal having a first end coupled to the last metal layer, at least one side, and a second end;
- a barrier layer formed on the second end of the copper pedestal;
- a barrier protection layer formed on the barrier layer; and
- a lead-free solder layer, wherein one or more of the at least one side of the copper pedestal directly contacts the solder layer.
2. The structure of claim 1, wherein the barrier protection layer comprises copper.
3. The structure of claim 1, wherein the barrier protection layer comprises copper-containing intermetallic.
4. The structure of claim 1, wherein the barrier protection layer is 0.5 μm to 3μm thick.
5. The structure of claim 1, wherein the copper pedestal is 8 μm to 12 μm thick.
6. The structure of claim 1, wherein the barrier layer comprises a material selected from the group consisting of nickel, iron, nickel-iron, and cobalt.
7. The structure of claim 1, wherein the barrier layer 1 μm to 3 μm thick.
8. The structure of claim 1, wherein the solder layer comprises tin-silver.
9-23. (canceled)
Type: Application
Filed: Mar 23, 2012
Publication Date: Sep 26, 2013
Applicant: INTERNATIONAL BUSINESS MACHINES CORPORATION (ARMONK, NY)
Inventors: CHARLES L. ARVIN (HOPEWELL JUNCTION, NY), KENNETH BIRD (HOPEWELL JUNCTION, NY), CHARLES C. GOLDSMITH (HOPEWELL JUNCTION, NY), SUNG K. KANG (CHAPPAQUA, NY), MINHUA LU (YORKTOWN HEIGHTS, NY), CLARE JOHANNA MCCARTHY (HOPEWELL JUNCTION, NY), ERIC DANIEL PERFECTO (HOPEWELL JUNCTION, NY), SRINIVASA S.N. REDDY (LAGRANGEVILLE, NY), KRYSTYNA WALERIA SEMKOW (HOPEWELL JUNCTION, NY), THOMAS ANTHONY WASSICK (HOPEWELL JUNCTION, NY)
Application Number: 13/427,940
International Classification: H01L 23/495 (20060101); H01L 21/28 (20060101);