Greenthread LLC v. Cirrus Logic Inc

I. Executive Summary and Procedural Information

• Parties & Counsel:
  • Plaintiff: Greenthread, LLC (Texas)
  • Defendant: Cirrus Logic, Inc. (Delaware)
  • Plaintiff’s Counsel: McKool Smith, P.C.
• Case Identification: 1:23-cv-00369, W.D. Tex., 06/16/2023
• Venue Allegations: Venue is based on Defendant’s principal place of business being in Austin, Texas, within the district. The complaint alleges that Defendant has a regular and established place of business in the district where it conducts research, development, testing, and sales, and has committed acts of infringement. The complaint includes a screenshot from a Cirrus Logic SEC filing stating its principal facilities in Austin, Texas are primarily occupied by research and development personnel and testing equipment.
• Core Dispute: Plaintiff alleges that Defendant’s semiconductor devices, including various integrated circuits, infringe a family of six patents related to the use of graded dopant regions to improve semiconductor performance.
• Technical Context: The technology involves manipulating the concentration of impurities (dopants) within a semiconductor substrate to create internal electric fields that guide charge carriers, thereby increasing device speed, efficiency, and reliability.
• Key Procedural History: The complaint notes that the asserted patents belong to a family previously litigated against other defendants, including Samsung and Intel. It highlights that courts in the Eastern District of Texas and the Western District of Texas/District of Oregon have previously adopted the Plaintiff’s proposed claim constructions for related patents, a fact which may influence claim construction proceedings in this case.

Case Timeline

Date	Event
2004-09-03	Priority Date for all Asserted Patents
2013-04-16	U.S. Patent No. 8,421,195 Issues
2015-11-17	U.S. Patent No. 9,190,502 Issues
2019-12-17	U.S. Patent No. 10,510,842 Issues
2020-08-04	U.S. Patent No. 10,734,481 Issues
2021-09-14	U.S. Patent No. 11,121,222 Issues
2022-04-26	U.S. Patent No. 11,316,014 Issues
2023-06-16	Complaint Filed

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

U.S. Patent No. 8,421,195 - "Semiconductor Devices with Graded Dopant Regions"

The Invention Explained

• Problem Addressed: The patent describes performance limitations in conventional semiconductor devices that use uniformly doped regions. Specifically, it notes that in memory and logic circuits, "spurious minority carriers" generated during operation can degrade performance, such as by reducing data retention (refresh) time in DRAMs or harming pixel quality in CMOS image sensors (’195 Patent, col. 2:56-66).
• The Patented Solution: The invention proposes creating a "graded dopant concentration" within the semiconductor substrate. This non-uniform doping creates a built-in "drift field" designed to sweep these unwanted minority carriers away from the active device regions on the surface and down into the substrate, where they can be harmlessly recombined. This is illustrated in Figure 5(b) of the patent, which shows a "graded dopant region to pull minority carriers from surface" ('195 Patent, Fig. 5(b); col. 4:56-64).
• Technical Importance: This technique offered a method to improve the reliability and performance of high-density integrated circuits by actively managing stray charge carriers, a significant issue in scaling devices to smaller sizes ('195 Patent, Abstract).

Key Claims at a Glance

• The complaint asserts at least one claim (Compl. ¶48). Independent claims 1 and 8 are available.
• Independent Claim 1 recites:
  • A CMOS Semiconductor device comprising: a surface layer; a substrate;
  • an active region including a source and a drain;
  • a single drift layer between the surface layer and substrate with a graded concentration of dopants, creating a "first static unidirectional electric drift field to aid the movement of minority carriers from said surface layer to said substrate"; and
  • at least one well region in the drift layer, also having a graded concentration of dopants creating a "second static unidirectional electric drift field" for the same purpose.
• The complaint implicitly reserves the right to assert other claims, including dependent claims.

U.S. Patent No. 9,190,502 - "Semiconductor Devices with Graded Dopant Regions"

The Invention Explained

• Problem Addressed: As a continuation, the ’502 Patent addresses the same fundamental problem as the '195 Patent: performance bottlenecks in semiconductor devices arising from suboptimal charge carrier transport in uniformly doped regions ('502 Patent, col. 1:24-34).
• The Patented Solution: The solution is also the use of graded dopant regions to create an aiding electric drift field. However, the claims of the '502 Patent are directed to a different function. Instead of sweeping unwanted carriers away from the surface, the invention described can be used to accelerate desired carriers toward the surface to, for example, decrease programming time in nonvolatile memory devices ('502 Patent, col. 4:49-59).
• Technical Importance: This application of graded doping provided a way to enhance different performance aspects, such as write speeds in flash memory, by actively pulling necessary charge carriers to the active region more quickly ('502 Patent, Abstract).

Key Claims at a Glance

• The complaint asserts at least one claim (Compl. ¶56). Independent claims 1 and 7 are available.
• Independent Claim 1 recites:
  • A semiconductor device comprising: a surface layer; a substrate;
  • an active region with a source and drain;
  • a single drift layer with a graded dopant concentration "generating a first static unidirectional electric drift field to aid the movement of minority carriers from said substrate to said surface layer"; and
  • at least one well region also having a graded dopant concentration "generating a second static unidirectional electric drift field" for the same purpose of moving carriers from the substrate to the surface layer.
• The complaint implicitly reserves the right to assert other claims.

U.S. Patent No. 10,510,842 - "Semiconductor Devices with Graded Dopant Regions"

• Technology Synopsis: This patent continues the family's focus on improving semiconductor device performance by replacing uniform dopant regions with graded ones. The created drift fields are described as improving performance in areas like digital logic, memory, and image sensors by better controlling carrier movement. (’842 Patent, Abstract; col. 2:15-27). (Compl. ¶21).
• Asserted Claims: At least one claim is asserted (Compl. ¶62). Independent claims are 1 and 9.
• Accused Features: The complaint alleges infringement based on the use of graded dopant regions in Defendant’s semiconductor products, such as amplifiers, converters, and other ICs (Compl. ¶41-42, 62).

U.S. Patent No. 10,734,481 - "Semiconductor Devices with Graded Dopant Regions"

• Technology Synopsis: This patent also discloses using graded dopant concentrations to create internal electric fields for performance enhancement. The invention is described as improving refresh time in DRAMs, decreasing programming time for nonvolatile memory, and improving visual quality for image ICs. (’481 Patent, Abstract). (Compl. ¶22).
• Asserted Claims: At least one claim is asserted (Compl. ¶70). Independent claims are 1 and 20.
• Accused Features: Infringement allegations target the same graded dopant technology within Cirrus Logic’s semiconductor device portfolio (Compl. ¶41-42, 70).

U.S. Patent No. 11,121,222 - "Semiconductor Devices with Graded Dopant Regions"

• Technology Synopsis: This patent describes a VLSI semiconductor device with graded dopant regions in both a drift layer and well regions. The purpose of the resulting drift fields is to aid carrier movement towards an area of the substrate with no active regions, thereby improving the performance of digital logic. (’222 Patent, Claim 1). (Compl. ¶23).
• Asserted Claims: At least one claim is asserted (Compl. ¶78). The patent contains numerous independent claims, including claims 1, 21, and 39.
• Accused Features: The accused features are the graded dopant regions allegedly present in Defendant’s semiconductor products (Compl. ¶41-42, 78).

U.S. Patent No. 11,316,014 - "Semiconductor Devices with Graded Dopant Regions"

• Technology Synopsis: This patent describes an electronic system incorporating a semiconductor device with graded dopant regions. The claims are directed at a system level, but the underlying technology remains the use of graded dopants to create drift fields that aid carrier movement away from the surface to improve the performance of digital logic. (’014 Patent, Claim 1). (Compl. ¶24).
• Asserted Claims: At least one claim is asserted (Compl. ¶86). Independent claims are 1 and 21.
• Accused Features: The complaint accuses electronic systems containing Cirrus Logic's semiconductor devices with graded dopant regions of infringement (Compl. ¶41-42, 88).

III. The Accused Instrumentality

Product Identification

The "Cirrus Logic Accused Products" are a broad category of semiconductor devices, including amplifiers, decoder and encoder integrated circuits (ICs), digital-to-analog converters, analog-to-digital converters, haptic drivers, and voice processor ICs (Compl. ¶9, 38). The complaint identifies an exemplary Cirrus Logic CLI1793B1 power management integrated circuit (“PIMIC”) as an infringing product (Compl. ¶41).

Functionality and Market Context

The Accused Products are described as semiconductor devices for mixed-signal and analog processing, used in applications such as boosted audio amplifiers, audio codecs, and haptics technology (Compl. ¶40). The complaint alleges Defendant derives 80% of its revenue from sales to Apple, Inc. (Compl. ¶14). It further alleges the products are sold through distributors in the United States, including Texas-based Mouser Electronics. The complaint includes a screenshot of the website for Texas-based distributor Mouser Electronics, which it alleges prominently features and sells the Accused Products (Compl. p. 6).

IV. Analysis of Infringement Allegations

The complaint does not include the claim chart exhibit (Exhibit 8) that it incorporates by reference, preventing a detailed, element-by-element analysis of the infringement allegations. The narrative theory is summarized below.

'195 Patent and '502 Patent Infringement Allegations

The complaint alleges that Cirrus Logic designs, manufactures, and sells semiconductor devices that "utilize regions with graded dopant concentrations" (Compl. ¶42). This general structure is alleged to meet each limitation of at least one claim of the '195 Patent and '502 Patent (Compl. ¶46, 54). The complaint states that a detailed mapping of the features of an exemplary accused product (CLI1793B1 PIMIC) to the patent claims is provided in Exhibit 8 (Compl. ¶41). Without this exhibit, it is unclear how Plaintiff specifically alleges the accused products create the "static unidirectional electric drift field" required by the claims of both patents, or how they meet the distinct directional requirements for carrier movement (away from the surface for the '195 Patent, towards the surface for the '502 Patent).

• Identified Points of Contention:
  • Evidentiary Question: The central dispute will likely be evidentiary. What technical evidence, such as from reverse engineering or internal documents obtained in discovery, will Greenthread present to prove that Cirrus Logic’s products actually contain the specific "graded dopant concentration" structures required by the patents? (Compl. ¶42).
  • Functional Question: A key technical question is whether any identified graded region in an accused product actually generates the "static unidirectional electric drift field" and "aid[s] carrier movement" in the specific direction required by an asserted claim (e.g., '195 Patent, Claim 1). The mere presence of a dopant gradient may not be sufficient without proof of the claimed function.

V. Key Claim Terms for Construction

"graded dopant concentration"

• Context and Importance: This term is the central structural element of the invention across all asserted patents. Its construction will determine the scope of infringing structures, specifically what type of non-uniform doping profile is covered.
• Intrinsic Evidence for Interpretation:
  • Evidence for a Broader Interpretation: The specification states the gradient can be "linear, quasi linear, exponential or complimentary error function," suggesting flexibility and covering any non-uniform profile that achieves the stated purpose ('195 Patent, col. 3:1-3).
  • Evidence for a Narrower Interpretation: A defendant might argue the term implies an intentional, functionally significant gradient designed to create a "suitable aiding drift electric field," not merely incidental process variations that may occur during manufacturing ('195 Patent, col. 3:3-5).

"static unidirectional electric drift field"

• Context and Importance: This term defines the functional result of the "graded dopant concentration." Proving infringement requires showing not just the structure but also that it produces this specific physical effect. Practitioners may focus on this term because it links the physical structure to a required function.
• Intrinsic Evidence for Interpretation:
  • Evidence for a Broader Interpretation: A plaintiff may argue the term simply requires a built-in field that, on the whole, pushes charge carriers in a single, consistent direction.
  • Evidence for a Narrower Interpretation: A defendant could argue that "static" and "unidirectional" require the field to be constant in time and point in a single, non-varying direction throughout the relevant device region, potentially excluding more complex fields that may exist in an actual device.

VI. Other Allegations

• Indirect Infringement: The complaint alleges inducement of infringement, stating that Cirrus Logic designs and markets the accused products for use in the United States and acts in concert with distributors and customers like Apple (Compl. ¶39, 49). The specific acts constituting inducement, such as instructions or user manuals, are not detailed.
• Willful Infringement: The complaint does not allege pre-suit knowledge of the patents. It alleges knowledge "at least through the service of the Complaint" (Compl. ¶39), which may support a claim for post-filing willful infringement. The prayer for relief seeks a finding that the case is exceptional, which is consistent with a willfulness claim (Compl. Prayer for Relief ¶D).

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

• A central issue will be evidentiary and factual: Can Greenthread produce specific, technical evidence from discovery or reverse engineering to demonstrate that Cirrus Logic's commercial products contain semiconductor structures with the "graded dopant concentration" and resulting "static unidirectional electric drift field" as claimed by the patents?
• A key legal and technical question will be one of functional scope: Assuming a graded dopant region is identified, does it perform the specific function of aiding carrier movement in the precise direction required by an asserted claim (e.g., away from the surface versus towards the surface), or is the gradient incidental and functionally unrelated to the claimed invention?
• A significant procedural question will be the impact of prior litigation: To what extent will the court adopt the claim construction rulings from Greenthread's prior cases against Intel and Samsung, potentially narrowing the scope of litigation and accelerating the case towards a resolution on infringement.