by Dennis Crouch
The Federal Circuit has affirmed the PTAB finding of obviousness in an inter partes review (IPR) challenge to an integrated circuit manufacturing patent, even while disagreeing with the Board’s claim construction. HD Silicon Solutions LLC v. Microchip Technology Inc., No. 2023-1397 (Fed. Cir. Feb. 6, 2025). The decision is fairly narrow – offering a situation where the lower tribunal’s errors can be harmless when alternative grounds support the ultimate conclusion.
The case centers on U.S. Patent 6,774,033, which describes methods for forming local interconnect layers in integrated circuits. Local interconnects provide the relatively short electrical pathways between transistors within a chip. The key claim limitation at issue requires “depositing a second film . . . comprising tungsten” as part of a metal stack structure. The claims themselves are surprisingly simple. Claim 1 is listed below:
1. A method of forming a local interconnect layer in an integrated circuit, the method comprising:
depositing a first film over an oxide layer, the first film comprising titanium nitride; and
depositing a second film over the first film, the second film comprising tungsten, the first film and the second film forming a metal stack of the local interconnect layer.
The patent document describes benefits of the tungsten layer in a local interconnect structure, including its chemical stability that prevents resist poisoning, ability to be probed for electrical testing, and its relatively low resistivity that enables good electrical performance while maintaining a thin overall stack height. When combined with titanium nitride in a metal stack, these properties create a more reliable and manufacturable interconnect structure.
The dispute began formally in October 2020 when HD Silicon Solutions sued Microchip in the Western District of Texas. Microchip responded in June 2021 by filing an inter partes review petition challenging all claims of the ‘033 patent based primarily on U.S. Patent 5,847,463 (“Trivedi”). While that IPR was pending, Microchip successfully moved to transfer the district court litigation to the Northern District of California as a more convenient forum for the accused infringer. That case was stayed pending outcome of the IPR.
Broad Claim Construction: The key claim construction dispute centered on the meaning of “comprising tungsten” in the challenged claims. HD Silicon (the patent owner) argued for a narrow construction that would limit the term to elemental tungsten, pointing to how the patent specification consistently uses precise language when referring to compound materials. For example, when the patent discusses titanium nitride, it explicitly names both elements. In contrast, when discussing tungsten layers, the specification simply refers to “tungsten” without mentioning other elements. Microchip advocated for a broader construction – under the BRI standard previously used by the IPR panels – that would encompass any form of tungsten, including both elemental tungsten and tungsten compounds. To support this position, Microchip relied heavily on the word “comprising,” which typically signals an open-ended scope in patent claims. The Board sided with Microchip, adopting the broader construction and ultimately relied upon that construction to find for the patent challenger.
On appeal, the Federal Circuit panel rejected the Board’s claim construction — agreeing with the patentee regarding the patentee’s careful use of language to distinguish between compounds (like “titanium nitride”) and elemental form. The court emphasized that while “comprising tungsten” allows for impurities from the manufacturing process, this does not extend to tungsten compounds.
However, this claim construction error proved harmless. The court found substantial evidence supported the Board’s alternative finding that Trivedi disclosed both tungsten silicide and elemental tungsten layers, with either capable of rendering the claims obvious. As the court noted, “Trivedi itself teaches that either tungsten silicide or elemental tungsten may be used to form the [second] local interconnect layer.” I pulled up the prior art patent and indeed found statements about the layer being a “material selected from the group consisting of tungsten, tungsten silicide …”
Motivation to Combine and claim scope: We know that under the analogous arts test, the scope of relevant prior art directly depends upon the scope of the claims. In addition, though the scope of the claims can impact the motivation to combine references. By arguing for a narrower construction limited to elemental tungsten, it seems that HD Silicon was also hoping to exclude some additional references from the obviousness analysis. Again, this was unsuccessful for the patentee.
Several dependent claims added limitations about deposition techniques, film thicknesses, and etchant types. The Board had found a motivation to combine the central reference with others teaching these elements – based upon KSR’s “predictable combination of known features.” In its decision, the Federal Circuit noted that motivation to combine is a factual finding reviewed with deference on appeal — ultimately recognizing that the Board had provided a “thorough and well-reasoned explanations” for the motivation to combine – particularly relying upon Microchip’s expert witness testimony.
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Sarah Brooks of Venable LLP argued for HD Silicon, while Brett Schuman of Goodwin Procter LLP represented Microchip. The case was heard by Judges Lourie, Stoll, and Cunningham.