It can be tricky to evaluate written description support under 35 U.S.C. § 112 for negative claim limitations since the support may amount to the absence of a feature from an invention that is described positively with respect to what it includes, as opposed to what it does not.  The Court of Appeals of the Federal Circuit provided some clarity on this issue in Inphi Corp. v. Netlist, Inc., which is discussed below along with some practice tips in view of the decision.

The Federal Circuit held in Inphi Corp. v. Netlist, Inc. that describing alternative features, without identifying the comparative advantages or disadvantages of each feature, could constitute a “reason to exclude” to satisfy the written description requirement under 35 U.S.C. § 112 for a negative claim limitation under the Santarus standard, which states that “[n]egative claim limitations are adequately supported when the specification describes a reason to exclude the relevant limitation.”

Case Discussion

Netlist sued Inphi for infringement of U.S. Patent No. 7,532,537 (“the ’537 Patent”) in September 2009. Inphi filed a request for inter partes reexamination with the U.S. Patent and Trademark Office in June 2010. After the patent examiner rejected some of the claims as obvious, Netlist amended independent claim 1, in relevant part, by adding the negative claim limitation “the chip selects of the first and second number of chip selects are DDR chip selects that are not CAS, RAS, or bank address signals.”  The examiner then withdrew the rejection.

Inphi appealed the examiner’s withdrawal of the rejection to the Patent Trial and Appeal Board (“PTAB”). The PTAB affirmed the examiner’s decision. In August 2014, the PTAB denied Inphi’s request for rehearing of the PTAB’s decision affirming the examiner.

At the heart of this dispute is what constitutes “a reason to exclude” a relevant limitation in a patent claim – whether it requires identifying advantages or disadvantages of each alternative feature, as argued by Inphi, or requires no more than proper description of alternative features, as argued by Netlist. Finding substantial evidence from three parts of the specification of the ’537 Patent, the Court affirmed the PTAB’s ruling and decided the case in favor of Netlist.  The Federal Circuit highlighted three aspects of the ’537 Patent’s specification as providing a proper description of alternative features:

  1. The ’537 Patent incorporates by reference a JEDEC standards publication that specifies that DDR signals (including CAS, RAS, and bank address signals) are distinct from each other.
  2. Table 2 of the ’537 Patent provides separate columns for CS, CAS, and RAS signals, indicating that each signal is distinguishable.
  3. Figure 9A of the ’537 Patent displays chip select signals (CS0, CS1), command signals (including CAS and RAS signals), and bank address signals (BA0-BAm) on different signal lines to differentiate each signal.

As stated by the Court, this does not mean “that in all cases a patentee may arbitrarily dissect its invention by amending the claims to avoid the prior art.” Instead, per Santarus, there must be adequate support to introduce negative claim limitations into claims, and the support is adequate if the specification properly describes alternative features. Articulation of advantages and disadvantages of each feature is not required according to the Court.

Practice Tips

  • As possible, avoid directly foreclosing in the specification features that may later be added by amendment as negative claim limitations. If the specification leaves no room for argument that some features should be excluded while some should not, claims including the negative claim limitation would likely still be invalid.
  • List features/subgroups in the specification that are envisioned as maybe being subject to prior art challenges as examples in the specification. As indicated by the Inphi decision, listing alternative features or subgroups within a group may suggest that each feature or subgroup is distinct from each other and can be the basis for introducing a negative claim limitation.
  • When adding a negative claim limitation by amendment, carefully consider whether the application properly describes alternative features. It may be beneficial for the negative claim limitation to not be added to every independent claim, and instead secure patentability of some independent claims on grounds other than the negative claim limitation, to protect some claims of the patent from later being challenged under 112 due to the negative claim limitation.
  • Keep calm and carry on. It is not the end of a patent with a negative claim limitation if there is no description of alternative features in the specification. As long as the specification satisfies the written description requirement of 112 there can be “a reason to exclude” certain limitations.