Rohm Semiconductor USA LLC v. MaxPower Semiconductor Inc

1. Case Identification

• Case #: IPR2020-01674
• Patent #: 7,843,004
• Filed: September 23, 2020
• Petitioner(s): Rohm Semiconductor USA, LLC
• Patent Owner(s): MaxPower Semiconductor, Inc.
• Challenged Claims: 1-27

2. Patent Overview

• Title: Power MOSFET with Recessed Field Plate
• Brief Description: The ’004 patent discloses a trench metal-oxide-semiconductor field-effect transistor (MOSFET) structure. The design incorporates a gate trench flanked by one or more recessed field plate (RFP) trenches to improve device performance, particularly by enhancing breakdown voltage characteristics.

3. Grounds for Unpatentability

Ground 1: Claims 1-27 are obvious over Girdhar in view of Zundel.

• Prior Art Relied Upon: Girdhar (Application # 2006/0060916) and Zundel (Application # 2006/0145247).
• Core Argument for this Ground:
  • Prior Art Mapping: Petitioner asserted that Girdhar, a reference considered during prosecution, discloses a trench MOSFET structure that meets nearly all limitations of the independent claims, including a gate trench positioned between two RFP trenches. However, Petitioner conceded, as the Examiner found, that Girdhar does not explicitly disclose the claimed "body contact region" which must, among other things, have a greater doping concentration and define a bottom junction depth deeper than the adjacent body region. To supply this missing element, Petitioner relied on Zundel, which explicitly teaches a trench MOSFET with a highly conductive body contact region designed to project deeper into the epitaxial layer to improve the device's breakdown properties and ensure a good electrical connection to the power supply.
  • Motivation to Combine: A Person of Ordinary Skill in the Art (POSITA) would combine Zundel's well-known body contact region with Girdhar's MOSFET structure to achieve predictable benefits. Zundel's stated purpose for its deeper body contact region—improving breakdown characteristics and avalanche strength—is the same benefit touted in the ’004 patent. Therefore, a POSITA would have been motivated to incorporate this known feature from Zundel into Girdhar's otherwise similar device to improve its performance in a predictable manner.
  • Expectation of Success: A POSITA would have had a reasonable expectation of success in this combination, as it involved applying a known technique (deep body contacts) to a known device type (trench MOSFETs) to achieve a well-understood and expected improvement in electrical characteristics.

Ground 2: Claims 7, 10, 13, 20, 23, and 26 are obvious over Girdhar in view of Zundel and Ninomiya.

• Prior Art Relied Upon: Girdhar (Application # 2006/0060916), Zundel (Application # 2006/0145247), and Ninomiya (Application # 2002/0187597).
• Core Argument for this Ground:
  • Prior Art Mapping: This ground addresses the limitation in the specified claims requiring "metal plugs which overlie said RFP electrodes, and also a metal layer overlying said metal plugs." Petitioner argued that Girdhar discloses a monolithic source contact that performs both functions. However, should the Patent Owner argue that the plugs and overlying layer must be distinct structures formed in separate steps, Ninomiya provides the motivation and method to do so. Ninomiya explicitly addresses the difficulty of filling small trench contact holes with a single sputtered metal layer and teaches a solution: first filling the holes with tungsten plugs and then depositing an overlying metal wiring layer.
  • Motivation to Combine: A POSITA would have been motivated to integrate Ninomiya’s two-step metallization process into the Girdhar/Zundel device. Ninomiya’s technique was a known solution to a common fabrication problem (inadequate filling of high-aspect-ratio contacts) and would allow for smaller, more compact device features. Applying this known fabrication technique to the Girdhar/Zundel MOSFET would have been a predictable design choice to improve manufacturing yield and device density.
  • Expectation of Success: The combination was a straightforward application of a known manufacturing process (tungsten plugs) to solve a known problem, and a POSITA would have expected it to function as intended without undue experimentation.

4. Key Claim Construction Positions

• "said body region, which is at least partly self-aligned to [at least one of] said RFP electrodes" (claims 1 and 15): Petitioner proposed this term should be construed to mean that the edge of the doped body contact region is defined by an existing structure (the RFP electrode) rather than a separate photolithography mask. This construction is based on the well-understood meaning of "self-aligned" in semiconductor fabrication, where an existing feature serves as an implantation mask.
• "recessed field plate": Petitioner argued this term should be construed as "conducting or semi-conducting material in a trench other than an active gate." This construction distinguishes the non-active RFP trenches from the active gate trench, consistent with the patent's disclosure and the general understanding in the art.

5. Key Technical Contentions (Beyond Claim Construction)

• Claims Not Entitled to Provisional Priority Date: A central contention of the petition was that the challenged claims are not entitled to the priority date of the '551 provisional application. Petitioner argued the provisional application lacks written description support for the key "body contact region" limitation, specifically the requirement that it defines a junction depth deeper than the adjacent body region. Because this limitation was added during prosecution to overcome prior art, its absence from the provisional invalidates the priority claim, making Girdhar and Zundel available as prior art under pre-AIA 35 U.S.C. §102(b).

6. Arguments Regarding Discretionary Denial

• Petitioner argued that discretionary denial under 35 U.S.C. §325(d) was inappropriate because the petition relied on new prior art and arguments not before the Examiner. Specifically, Zundel was never considered during prosecution and directly teaches the "body contact region" limitation that was the basis for allowance. Petitioner contended the Examiner materially erred by failing to find a reference disclosing this well-known feature. Further, Petitioner asserted that denial under §314(a) was unwarranted as there was no successive petition and no related litigation close to trial.

7. Relief Requested

• Petitioner requested institution of an inter partes review and cancellation of claims 1-27 of the ’004 patent as unpatentable.