Rohm Semiconductor USA LLC v. MaxPower Semiconductor Inc

1. Case Identification

• Case #: IPR2020-01675
• Patent #: 8,076,719
• Filed: September 23, 2020
• Petitioner(s): Rohm Semiconductor USA, LLC
• Patent Owner(s): MaxPower Semiconductor Inc.
• Challenged Claims: 1, 5, and 6

2. Patent Overview

• Title: Trench MOSFET with Compensating Doped Zones
• Brief Description: The ’719 patent relates to trench metal-oxide-semiconductor field-effect transistors (MOSFETs). The purported invention aims to improve device performance by introducing a "deep compensated zone"—a doped region of opposite conductivity type—located at least partially beneath the device's recessed field plate (RFP) trenches.

3. Grounds for Unpatentability

Ground 1: Claims 1, 5, and 6 are obvious over Kocon in view of Hshieh

• Prior Art Relied Upon: Kocon (Patent 6,534,828) and Hshieh (WO 97/33320 A1).
• Core Argument for this Ground:
  • Prior Art Mapping: Petitioner argued that Kocon discloses a trench MOSFET with all major elements of claim 1, including a "deep well" diffusion region (the claimed "first additional diffusion component") located beneath an inactive gate structure that functions as a recessed field plate (RFP). This deep well, of a second conductivity type, is taught to improve breakdown voltage characteristics. Petitioner asserted Hshieh teaches adding a "second additional diffusion component" of a first conductivity type within the semiconductor layer specifically to reduce the device's on-resistance. Kocon was also argued to teach the limitations of dependent claims 5 (sufficient concentration to counterdope) and 6 (use of an epitaxial layer).
  • Motivation to Combine: A Person of Ordinary Skill in the Art (POSITA) would combine Kocon's structure for improving breakdown voltage with Hshieh's teachings on reducing on-resistance. Both are well-known, critical performance metrics for MOSFETs, and improving them simultaneously was a common goal in the field.
  • Expectation of Success: The combination involved applying a known technique for reducing resistance (Hshieh) to a known MOSFET structure (Kocon), which would have resulted in a predictable improvement in device performance.

Ground 2: Claims 1, 5, and 6 are obvious over Takahashi in view of Hshieh

• Prior Art Relied Upon: Takahashi (Patent 5,864,159) and Hshieh (WO 97/33320 A1).

• Core Argument for this Ground:

  • Prior Art Mapping: Petitioner contended that Takahashi discloses a trench MOSFET designed to prevent breakdown voltage reduction while maintaining low on-resistance. This design includes a p-type doped region (the "first additional diffusion component") situated beneath a "dummy trench" that serves as an RFP. This structure meets most limitations of claim 1. As in the first ground, Hshieh supplies the teaching of a "second additional diffusion component" (an N-doped region) to further reduce on-resistance. The combination of Takahashi and Hshieh was argued to render claims 5 and 6 obvious as well.
  • Motivation to Combine: A POSITA seeking to further enhance the performance of Takahashi's MOSFET would be motivated to incorporate Hshieh's method for reducing on-resistance. Takahashi's stated goal was to keep on-resistance low, and Hshieh provided a known method for achieving even lower resistance.
  • Expectation of Success: A POSITA would have reasonably expected that adding Hshieh's additional doping region to Takahashi's established structure would successfully and predictably lower the overall on-resistance without disrupting the device's function.

• Additional Grounds: Petitioner asserted an additional obviousness challenge against claims 1, 5, and 6 over Osawa in view of Hshieh and further in view of Kocon, relying on similar theories that Osawa taught a diffusion component under an RFP trench that could be combined with the teachings of Hshieh and Kocon to achieve improved performance.

4. Key Claim Construction Positions

• "Capacitively Coupled": Petitioner proposed this term be construed to mean two regions configured to have a static potential difference applied between them, thereby inducing a change in carrier concentrations at the semiconductor interface.
• "Recessed Field Plate": Petitioner proposed this term be construed as "conducting or semi-conducting material in a trench other than an active gate."
• "Diffusion Component": Petitioner proposed this term be construed as "a doped region of semi-conducting material."

These constructions were argued to align with the plain and ordinary meaning in the art and were critical to mapping the claim limitations onto the structures disclosed in the prior art references.

5. Arguments Regarding Discretionary Denial

• Petitioner argued that discretionary denial under 35 U.S.C. §325(d) would be inappropriate. The petition contended that while some references (Kocon, Hshieh, Takahashi) were cited in an Information Disclosure Statement, there is no evidence the Examiner substantively considered them.
• The central argument was that the Examiner made a material error during prosecution by repeatedly misconstruing the claims. Specifically, the Examiner appeared to believe the claims required doping beneath the active gate trench, rather than the claimed RFP trench. This fundamental misunderstanding allegedly caused the Examiner to overlook the direct applicability of the asserted prior art, which teaches doping under RFPs or equivalent non-gate trenches.
• Petitioner also asserted that denial under 35 U.S.C. §314(a) was not warranted because no related litigation was close to trial.

6. Relief Requested

• Petitioner requests institution of an inter partes review (IPR) and cancellation of claims 1, 5, and 6 of Patent 8,076,719 as unpatentable.