Heightened Written Description Requirement for Negative Claim Limitations?

Negative patent claim limitations were once disfavored by the USPTO.  This is because defining claim scope by exclusion was considered less precise relative to more traditional, positive claim scope recitations.  Over the years, the USPTO has been guided to accept negative limitations as in some cases it may be the only way to adequately claim a given invention.  Yet, in assessing written description support for such claim features, the USPTO explains that "the mere absence of a positive recitation in a patent specification is not a basis for exclusion consistent with written description requirements." MPEP 2173.05(i) 

In some cases, examiners have been known to require explicit support for a negative claim limitation. The degree of specification support necessary to support a negative claim limitation was analyzed last week by the Federal Circuit. 

Inphi Corporation (“Inphi”) filed a request for inter partes reexamination of Netlist, Inc.’s (“Netlist”) U.S. Patent No. 7,532,537 (“the ‘537 patent”) directed to computer system memory modules comprising memory chips.  Numerous claims were rejected as obvious.  In overcoming the rejection, Netlist amended its claims, narrowing them and adding a negative limitation.  The negative claim limitation excluded three particular types of memory chip selection signals.  

Inphi appealed to the Board arguing in pertinent part that Netlist’s negative claim limitation failed to satisfy the written description requirement.  The Board affirmed the Examiner’s final decision and denied Inphi’s subsequent request for rehearing. 

The question posed to the CAFC was whether Netlist’s negative claim limitation was supported by the specification or constituted impermissible new matter. That is, whether or not the disclosure of alternative implementations was enough, or whether a particular advantage disadvantage of one option to the exclusion of another, was required. 

The CAFC affirmed the Board, holding that simply describing alternative features without articulating advantages or disadvantages of each feature can support a negative limitation. (here)

The parties’ dispute centered on interpreting the key language in Santarus, Inc. v. Par Pharm, Inc.:

Negative claim limitations are adequately supported when the specification describes a reason to exclude the relevant limitation.  Such written description support need not rise to the level of disclaimer.  In fact, it is possible for the patentee to support both the inclusion and exclusion of the same material.

694 F.3d 1344, 1351 (Fed. Cir. 2012).  Inphi argued that the specification in Santarusclearly differentiated between an advantageous option and a disadvantageous alternative, and this was necessary to support a negative limitation directed to the disadvantageous alternative.  The ‘537 patent lacked such clear differentiation, but the Board pointed to three parts of the specification where the chip signaling alternatives were distinguished:  in a standards publication for memory devices that was incorporated by reference, in a table where these three types of chips were listed separately, and in a figure where these three types of chips were displayed on different signal lines.  The Court held that this constituted substantial evidence supporting the Board’s finding, emphasizing that “Santarus did not create a heightened written description standard for negative claim limitations and that properly described, alternative features are sufficient to satisfy the written description standard of § 112, paragraph 1 for negative claim limitations.”