Rohm Semiconductor USA LLC v. MaxPower Semiconductor Inc

1. Case Identification

• Case #: IPR2020-01676
• Patent #: 8,466,025
• Filed: September 23, 2020
• Petitioner(s): Rohm Semiconductor USA, LLC
• Patent Owner(s): MaxPower Semiconductor Inc.
• Challenged Claims: 1, 2, 6, 7

2. Patent Overview

• Title: Semiconductor device structure with recessed field plates and method of operation and fabrication
• Brief Description: The ’025 patent describes trench metal-oxide-silicon field-effect transistors (MOSFETs) and methods for their operation and fabrication. The technology focuses on improving MOSFET performance by introducing doped diffusion components of an opposite conductivity type adjacent to and at least partially beneath non-gate trenches, also known as recessed field plates (RFPs), to reduce punchthrough and lower on-resistance without degrading breakdown voltage.

3. Grounds for Unpatentability

Ground 1: Obviousness over Kocon - Claims 1, 2, and 6 are obvious over Kocon.

• Prior Art Relied Upon: Kocon (Patent 6,534,828).
• Core Argument for this Ground:
  • Prior Art Mapping: Petitioner argued that Kocon, which was cited in an Information Disclosure Statement (IDS) but not substantively considered during prosecution, teaches all limitations of claims 1, 2, and 6. Kocon’s "inactive gate structure" located in a second trench was alleged to be the claimed "recessed field plate" (RFP). Crucially, Kocon’s "deep well region 35," a p-type diffusion, was identified as the claimed "diffusion component... lying at least partially directly beneath" the RFP trench. Petitioner asserted that Kocon explicitly teaches this deep well region improves breakdown voltage characteristics, thereby avoiding the claimed "punchthrough" and reducing depletion spreading in the OFF state. For dependent claim 2, Kocon's p-type body layer extending to the deep well was argued to meet the limitation of a dopant concentration layer extending from a source layer.
  • Motivation to Combine (for §103 grounds): This ground is based on a single reference. Petitioner argued a person of ordinary skill in the art (POSITA) would find it obvious to implement Kocon’s teachings, as Kocon itself describes the benefits of its structure for improving MOSFET performance.
  • Expectation of Success (for §103 grounds): A POSITA would have a high expectation of success as Kocon provides a detailed disclosure of the structure and its resulting benefits.

Ground 2: Obviousness over Osawa in view of Kocon - Claims 1, 2, and 6 are obvious over Osawa in view of Kocon.

• Prior Art Relied Upon: Osawa (Patent 6,501,129) and Kocon (Patent 6,534,828).
• Core Argument for this Ground:
  • Prior Art Mapping: Petitioner argued Osawa, which was not considered during prosecution, discloses a trench MOSFET that meets the core limitations of the claims. Osawa’s "contact trenches" filled with a conductive film were equated to the claimed RFPs. Osawa further discloses a p-type impurity "diffusion region 9" formed in the periphery of the contact trench’s side wall and bottom. Petitioner contended this structure meets the limitation of a diffusion component beneath the RFP. Osawa teaches that this diffusion region relaxes the electric field to "improve the breakdown voltage," which Petitioner argued is synonymous with avoiding punchthrough.
  • Motivation to Combine (for §103 grounds): A POSITA would combine Osawa with Kocon because both references address the same fundamental problems in trench MOSFETs: improving breakdown voltage and reducing on-resistance. Petitioner argued Kocon’s detailed explanation of the benefits of a deep well under an RFP would motivate a POSITA to apply its teachings to optimize the performance of Osawa's similar structure.
  • Expectation of Success (for §103 grounds): Success would be expected because both patents operate in the same technical field and describe analogous structures, making the integration of their teachings predictable.

Ground 3: Obviousness over Kocon in view of Hshieh - Claim 7 is obvious over Kocon in view of Hshieh.

• Prior Art Relied Upon: Kocon (Patent 6,534,828) and Hshieh (WO 97/33320).

• Core Argument for this Ground:

  • Prior Art Mapping: This ground targets fabrication process claim 7. Petitioner asserted Kocon teaches the foundational steps, including forming a trench MOSFET with an insulated gate trench (active gate), a second trench with an RFP (inactive gate), and an additional dose of acceptor dopants below the second trench (the p-type deep well). To meet the final limitation of providing an "additional dose of donor dopant atoms... to thereby reduce the on-resistance," Petitioner pointed to Hshieh. Hshieh explicitly teaches providing an "N doped region 58" that underlies the gate trench to create a more heavily doped area, which "advantageously further reduces the on-resistance."
  • Motivation to Combine (for §103 grounds): A POSITA would combine the references to achieve improved MOSFET performance, a well-known goal in the art. Kocon itself notes that its structure allows for heavier doping to reduce on-resistance. A POSITA seeking to implement this benefit would have looked to known techniques, such as the additional donor doping taught by Hshieh, to achieve the desired reduction in on-resistance.
  • Expectation of Success (for §103 grounds): The combination was argued to be a predictable application of a known technique (Hshieh's doping for on-resistance reduction) to a known device structure (Kocon's MOSFET) to achieve a known result.

• Additional Grounds: Petitioner asserted an additional obviousness challenge for claim 7 based on Osawa in view of Kocon and Hshieh, relying on similar theories of combination.

4. Key Claim Construction Positions

• "Recessed Field Plate": Petitioner proposed this term be construed as a "conducting or semi-conducting material in a trench other than an active gate." This construction was argued to be critical for mapping the term to structures in the prior art like Kocon’s "inactive gate structures" and Osawa’s "contact trenches," which are not active gates but perform a field-shaping function.
• "Diffusion Component": Petitioner proposed this term be construed as "a doped region of semi-conducting material." This construction supports the argument that the "deep well region" in Kocon and the "impurity diffusion layer" in Osawa, which are formed by doping, meet this limitation.

5. Arguments Regarding Discretionary Denial

• Petitioner argued against discretionary denial under 35 U.S.C. §325(d), asserting that the petition raises new, non-cumulative arguments that were not before the Examiner. It was argued that the Examiner committed a material error by failing to substantively consider the key teachings of the asserted prior art. Specifically, Osawa was never cited during prosecution. Kocon and Hshieh, though listed in an IDS, were never used as the basis for a rejection, and there is no evidence the Examiner considered their disclosure of the critical limitation—doping beneath an RFP trench—which was the ultimate basis for allowance of the challenged claims. Petitioner also noted that denial under §314(a) was inappropriate as the petition was not successive and there was no related litigation close to trial.

6. Relief Requested

• Petitioner requests institution of an inter partes review and cancellation of claims 1, 2, 6, and 7 of the ’025 patent as unpatentable.