Rohm Semiconductor USA LLC v. MaxPower Semiconductor Inc

I. Executive Summary and Procedural Information

• Parties & Counsel:
  • Plaintiff: ROHM Semiconductor USA, LLC (Delaware)
  • Defendant: MaxPower Semiconductor, Inc. (California)
  • Plaintiff’s Counsel: Kramer Levin Naftalis & Frankel LLP
• Case Identification: 3:20-cv-06686, N.D. Cal., 09/23/2020
• Venue Allegations: Venue is alleged to be proper in the Northern District of California as Defendant MaxPower maintains its principal place of business in the district, and the events giving rise to the action, specifically MaxPower’s assertion of infringement, occurred within the district.
• Core Dispute: Plaintiff ROHM USA, the accused infringer, seeks a declaratory judgment from the court that its Silicon-Carbide MOSFET products do not infringe four patents owned by Defendant MaxPower related to the structure and fabrication of trench-gate power transistors.
• Technical Context: The technology concerns the design of power MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors), which are fundamental components for high-efficiency power conversion and management in a vast range of electronic devices.
• Key Procedural History: This declaratory judgment action was initiated by ROHM USA following a telephone call on September 10, 2020, during which MaxPower allegedly accused ROHM USA’s products of infringement and demanded a large license payment, thereby creating what ROHM USA asserts is an actual and justiciable controversy.

Case Timeline

Date	Event
2006-09-27	Earliest Priority Date for ’004 Patent
2008-02-14	Earliest Priority Date for ’719, ’025, and ’076 Patents
2010-11-30	’004 Patent Issued
2011-12-13	’719 Patent Issued
2013-06-18	’025 Patent Issued
2014-02-25	’076 Patent Issued
2020-09-10	MaxPower alleges infringement and demands license payment
2020-09-23	Complaint for Declaratory Judgment Filed

II. Technology and Patent(s)-in-Suit Analysis

U.S. Patent No. 7,843,004 - "Power MOSFET With Recessed Field Plate"

• Patent Identification: U.S. Patent No. 7,843,004, "Power MOSFET With Recessed Field Plate," issued November 30, 2010.

The Invention Explained

• Problem Addressed: The patent describes a fundamental trade-off in conventional power MOSFET design: increasing the density of transistor cells to lower on-state resistance (Ron) undesirably increases internal capacitances, which slows down switching speeds and increases power loss. Additionally, charge storage during certain operating modes (third-quadrant operation) results in a slow reverse-recovery (Qrr), further limiting performance at high frequencies (’004 Patent, col. 1:15-39).
• The Patented Solution: The invention is a trench-gate MOSFET architecture featuring a standard gate trench positioned between two "recessed field plate" (RFP) trenches. These trenches are described as self-aligned and of substantially equal depth, which simplifies manufacturing (’004 Patent, col. 2:60-65). The RFP contains an electrode that, when biased, helps to deplete the drift region to reduce electric fields and also provides an additional current path that reduces the undesirable reverse-recovery charge, enabling faster and more efficient switching (’004 Patent, Abstract; col. 5:1-11).
• Technical Importance: This architecture aimed to simultaneously achieve low on-resistance, low gate charge, and fast reverse recovery, a combination of features critical for improving the efficiency of next-generation power converters, such as those used in CPU voltage regulators (’004 Patent, col. 1:50-57).

Key Claims at a Glance

• The complaint seeks a declaration of non-infringement of "any claim" of the patent (Compl. ¶26). Independent claim 1 is representative of the core invention.
• Essential elements of independent claim 1 include:
  • A gate trench with a gate electrode, insulated by a first dielectric layer having a thicker bottom section and a thinner sidewall section.
  • First and second recessed field plate (RFP) trenches, between which the gate trench is located.
  • RFP electrodes within the RFP trenches, with an upper surface recessed below the upper surface of the gate electrode.
  • A "body contact region" that abuts the RFP trenches, is at least "partly self-aligned" to the RFP electrodes, and has a doping concentration and junction depth greater than the adjacent body region.
  • A drain-drift region adjacent to the body region.

U.S. Patent No. 8,076,719 - "Semiconductor Device Structures and Related Processes"

• Patent Identification: U.S. Patent No. 8,076,719, "Semiconductor Device Structures and Related Processes," issued December 13, 2011.

The Invention Explained

• Problem Addressed: Building on the RFP structure, this patent addresses the challenge of further improving the trade-off between on-resistance and breakdown voltage. The goal is to avoid device failure (punch-through) while enabling even lower on-resistance (’719 Patent, col. 3:1-10).
• The Patented Solution: The invention introduces a "deep compensated zone" via a compensating implant into the semiconductor material beneath the RFP trench (’719 Patent, col. 3:1-4). As illustrated in Figure 4a, this buried doped region (114) alters the electric field distribution when the device is off, reducing stress on the insulating layers and helping to prevent breakdown. This allows the primary current-carrying region (the drift region) to be more heavily doped, which in turn lowers the device’s on-resistance without sacrificing voltage-handling capability (’719 Patent, col. 6:1-11).
• Technical Importance: This technique provides designers with an additional method to optimize MOSFET performance beyond what is possible with the RFP structure alone, pushing the boundaries of power-switching efficiency.

Key Claims at a Glance

• The complaint seeks a declaration of non-infringement of "any claim" of the patent (Compl. ¶31). Independent claim 1 is representative.
• Essential elements of independent claim 1 include:
  • A device with a gate trench and recessed field plate (RFP) trenches.
  • A "first additional diffusion component" (the deep compensated zone) of a second conductivity type located at least partially beneath the RFP trenches.
  • A "second additional diffusion component" of a first conductivity type (an enhanced doping region) within the semiconductor layer.
  • The first component functions to reduce depletion spreading in the OFF state.
  • The second component functions to reduce on-resistance in the ON state.

U.S. Patent No. 8,466,025 - "Semiconductor Device Structures and Related Processes"

• Patent Identification: U.S. Patent No. 8,466,025, "Semiconductor Device Structures and Related Processes," issued June 18, 2013.
• Technology Synopsis: This patent claims the methods for manufacturing the advanced MOSFET structures described in the ’719 patent. A key problem is the precise formation of the deep compensated zone. The patented solution describes a sequence of fabrication steps, including etching the trenches, filling them with a temporary oxide, selectively removing the oxide from the bottom of the RFP trenches, and then implanting dopants through the exposed trench bottom to create the buried compensated zone (’025 Patent, col. 7:37-48; col. 9:8-14).
• Asserted Claims: Independent claims 1 and 7 are asserted (Compl. ¶36).
• Accused Features: The complaint alleges that MaxPower has asserted that ROHM USA’s process of making its SiC MOSFETs infringes the ’025 patent (Compl. ¶35).

U.S. Patent No. 8,659,076 - "Semiconductor Device Structures and Related Processes"

• Patent Identification: U.S. Patent No. 8,659,076, "Semiconductor Device Structures and Related Processes," issued February 25, 2014.
• Technology Synopsis: As a continuation of the same family, this patent claims the resulting structures of the MOSFETs with deep compensated zones, similar to the ’719 patent. The technical problem and patented solution are functionally identical to the ’719 patent: using a buried, doped region beneath the RFP trenches to reshape electric fields, thereby enabling a superior combination of low on-resistance and high breakdown voltage (’076 Patent, Abstract; col. 3:1-10).
• Asserted Claims: Independent claims 1, 2, 4, and 10 are asserted (Compl. ¶41).
• Accused Features: The complaint alleges that MaxPower has asserted that the structure of ROHM USA’s SiC MOSFETs infringes the ’076 patent (Compl. ¶40).

III. The Accused Instrumentality

• Product Identification: The accused instrumentalities are Plaintiff ROHM USA’s silicon-carbide MOSFETs ("SiC MOSFETs") (Compl. ¶12).
• Functionality and Market Context: The complaint describes the accused products as "innovative, high-quality electrical components, including semiconductors, integrated circuits, modules, and commercial products" developed by its parent company, ROHM Co., Ltd. (Compl. ¶¶10, 12). It further alleges that these products are supplied "in large volumes to the global market" (Compl. ¶11). The complaint does not provide specific technical details on the structure or operation of the SiC MOSFETs beyond their general classification.

IV. Analysis of Infringement Allegations

As this is a complaint for a declaratory judgment of non-infringement, it does not contain affirmative infringement allegations or claim charts mapping the accused products to the patent claims. The complaint makes only general statements that MaxPower has alleged infringement and that ROHM USA denies these allegations (Compl. ¶¶ 25-26, 30-31, 35-36, 40-41). Therefore, a claim chart summary cannot be constructed from the provided complaint.

No probative visual evidence provided in complaint.

• Identified Points of Contention: Based on the patent claims and the general nature of the accused products, several technical questions are likely to form the core of the dispute.
  • Scope Questions: A primary question may be whether the patent claims, which are described in the context of silicon-based devices, can be read to cover devices made from a different material, namely silicon carbide (SiC), as used in the accused products (Compl. ¶12).
  • Technical Questions: For the ’004 patent, a factual dispute may arise over whether the accused SiC MOSFETs incorporate the specific architecture of a gate trench located between two self-aligned RFP trenches with the claimed recessed electrode and deep, self-aligned body contacts. For the ’719, ’025, and ’076 patents, the central technical question will likely be whether the accused products are made with or contain a "deep compensated zone" beneath an RFP trench that performs the claimed function of shaping the electric field to improve device performance.

V. Key Claim Terms for Construction

• The Term: "a body contact region ... which is at least partly self-aligned to at least one of said RFP electrodes" (’004 Patent, Claim 1)
• Context and Importance: This term describes a key structural feature intended to simplify manufacturing and ensure proper device function. The degree to which a structure is "self-aligned" is often a point of contention. Practitioners may focus on this term because its construction will determine whether ROHM’s manufacturing process and the resulting physical layout of its transistors fall within the scope of the claim.
• Intrinsic Evidence for Interpretation:
  • Evidence for a Broader Interpretation: The use of the modifier "partly" suggests that the alignment does not need to be perfect, potentially broadening the scope to include structures with some degree of misalignment inherent in manufacturing processes.
  • Evidence for a Narrower Interpretation: The specification describes forming the gate and RFP trenches in the same processing step to achieve self-alignment (’004 Patent, col. 3:46-48). A court could be asked to limit the term to structures formed by such a simultaneous process, as depicted in embodiments like Figure 4B (element 52).
• The Term: "a first additional diffusion component of a second conductivity type lying at least partially beneath said respective second trenches" (’719 Patent, Claim 1)
• Context and Importance: This term recites the core inventive concept of the '719 patent family: the "deep compensated zone." The definition of what constitutes this "component" and its location "beneath" the trench is critical to the infringement analysis for three of the four asserted patents.
• Intrinsic Evidence for Interpretation:
  • Evidence for a Broader Interpretation: The patent states this component can be a "floating" island, connected to the source, or merged with the p-body, suggesting structural flexibility (’719 Patent, col. 4:9-18). The general term "diffusion component" could be argued to cover any appropriately doped region, regardless of its precise formation method.
  • Evidence for a Narrower Interpretation: The specification provides detailed descriptions and figures showing this zone (e.g., element 114 in Fig. 4a) being formed by specific processes, such as implantation through the bottom of the RFP trench (’719 Patent, col. 7:40-48). An accused infringer may argue the term should be limited to structures that are physically separate and perform the specific recited function of reducing voltage stress on the trench dielectric (’719 Patent, col. 6:35-38).

VI. Other Allegations

• Indirect Infringement: The complaint does not contain allegations of indirect infringement.
• Willful Infringement: The complaint does not contain allegations of willful infringement.

VII. Analyst’s Conclusion: Key Questions for the Case

This declaratory judgment action will likely center on three key questions for the court's determination:

• A foundational issue will be one of structural correspondence: Do ROHM USA’s silicon-carbide (SiC) MOSFETs physically embody the specific trench geometries, layered dielectrics, and "self-aligned" contact regions required by the claims of the ’004 patent, which were described in a silicon-based context?
• A critical evidentiary question for the ’719, ’025, and ’076 patents will be the existence of the "deep compensated zone." The case will likely turn on whether ROHM’s devices contain, or its processes create, a distinct doped region beneath the RFP trenches that performs the specific claimed functions of shaping electric fields and improving device performance.
• For the ’025 patent, the dispute will be one of process infringement: Does the evidence show that ROHM’s proprietary method for manufacturing its SiC MOSFETs practices the specific sequence of masking, etching, implantation, and thermal processing steps recited in the patent’s method claims?