Many patent attorneys have a visceral, disapproving reaction to negative claim limitations -- elements that specify what a claim does not cover.  While a line of Federal Circuit cases has established that negative limitations are acceptable in some situations, there still is a wide gap between claiming "a binary value that is not zero" versus "a kitchen utensil that is not a fork."  In a recent decision, a Federal Circuit panel has shed more light on when such limitations can be employed without violating the written description requirement.

Inphi filed a request for inter partes reexamination in the U.S. Patent and Trademark Office (USPTO) of Netlist's U.S. Patent No. 7,532,537.  During these proceedings, Netlist amended the claims to overcome cited art, and in doing so introduced a negative limitation to an independent claim.  The examiner allowed the claims.

Inphi then appealed to the USPTO's Patent Trial and Appeal Board (PTAB), alleging that the negative limitation resulted in the claim being invalid for lack of written description in the specification under 35 U.S.C. § 112, paragraph 1.  The PTAB affirmed the examiner's allowance, and Inphi appealed to the Federal Circuit.

Claim 1 of the '537 patent recites:

A memory module comprising:
    a plurality of memory devices, each memory device having a corresponding load; and
    a circuit electrically coupled to the plurality of memory devices and configured to be electrically coupled to a memory controller of a computer system, the circuit selectively isolating one or more of the loads of the memory devices from the computer system, the circuit comprising logic which translates between a system memory domain of the computer system and a physical memory domain of the memory module, wherein the system memory domain is compatible with a first number of chip selects, and the physical memory domain is compatible with a second number of chip selects equal to twice the first number of chip selects, wherein the plurality of memory devices comprises double-data rate (DDR) dynamic random-access memory (DRAM) devices and 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 italicized portion of claim 1 contains the negative limitation at issue, which excludes three specific types of signals.  Chip selects are typically control lines that select one or more chips out of several attached to a bus, and may be used to refer generically to the signals carried thereon.

DRAM stores data in banks of rows and columns, analogous to a spreadsheet.  Thus, each unit where data is stored can be addressed by a row and a column.  CAS stands for column address strobe, a signal that specifies a column address.  Similarly, RAS stands for row address strobe, a signal that specifies a row address.  The combination of CAS and RAS signals allows each unit in a bank of the DRAM matrix to be addressed.  Bank address signals, on the other hand, specify which of the banks is being addressed.

Inphi contended that the negative limitation was not supported by the specification of the '537 patent, and thus constitutes new matter.  In support of this notion, Inphi cited a 2012 Federal Circuit case, Santarus, Inc. v. Par Pharm., Inc., which held that "[n]egative claim limitations are adequately supported when the specification describes a reason to exclude the relevant limitation."

The PTAB identified "three parts of the specification of the '537 patent upon which it relied in finding that the negative claim limitation was reasonably supported."  In a nutshell, these parts, according to the PTAB, distinguish between chip select lines in general and CAS, RAS and bank address signals.  As a result, the '537 patent implicitly supports embodiments in which a chip select is not carrying CAS, RAS, or bank address signals.

In its review, the Federal Circuit stated that "[w]hether a patent claim satisfies the written description requirement of 35 U.S.C. § 112, paragraph 1 depends on whether the description clearly allows persons of ordinary skill in the art to recognize that the inventor invented what is claimed."  Inphi argued that a "reason to exclude" in Santarus "requires something more than properly describing alternative features of the patented invention."  Netlist took the position that "the written description requirement is satisfied when alternative features are properly described."  The Court framed the issue as "whether properly describing alternative features -- without articulating advantages or disadvantages of each feature -- can constitute a reason to exclude under the standard articulated in Santarus."

The Court held that "reason to exclude" is satisfied by an express description of the disadvantages of the excluded features, or an express description of the advantages of the claimed invention without these features.  However, such descriptions are not necessary to meet the requirements of § 112 paragraph 1 for negative claim limitations.  Doing so, in the Court's view, would improperly "articulate a new and heightened standard for negative claim limitations."  As such, a "reason to exclude" can be provided by "properly describing alternative features of the patented invention."  The "reason to exclude" of Santarus is sufficient, but not necessary.

The Court warned, however, "[t]hat is not to say that in all cases, a patentee may arbitrarily dissect its invention by amending the claims in order to avoid the prior art."  For instance, if the specification prohibits a particular negative claim limitation, addition of that limitation will render the claim invalid.

Once again, the Federal Circuit has explained how careful specification drafting can allow the patentee more claiming flexibility down the road.  For instance, if three features of a claim, A, B, and C, are optional, one can state in the specification that zero or more of A, B, and C may be present, in any combination or permutation.  As a result, a negative claim limitation with any of A, B, and/or C can be added during prosecution or post-grant proceedings.

Inphi Corp. v. Netlist, Inc. (Fed. Cir. 2015)
Panel:  Circuit Judges O'Malley, Reyna, and Chen
Opinion by Circuit Judge O'Malley