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6CCVD Research

Thermal stress modelling of diamond on GaN/III-Nitride membranes

At a Glance

Section titled “At a Glance”
MetadataDetails
Publication Date2020-11-27
JournalCarbon
AuthorsJerome A. Cuenca, Matthew D. Smith, D. Field, Fabien Massabuau, Soumen Mandal
InstitutionsUniversity of Strathclyde, University of Glasgow
Citations43
AnalysisFull AI Review Included

Executive Summary

Section titled “Executive Summary”

This study investigates the thermal stress and deformation (bow) induced when integrating polycrystalline CVD diamond directly onto GaN/III-N membranes, proposing a bonding-free heat spreading solution for high-power electronics.

  • Core Challenge Identified: High-temperature CVD growth combined with the Coefficient of Thermal Expansion (CTE) mismatch between GaN/III-N, Si, and diamond induces significant thermal stresses and large membrane bow upon cooling.
  • Modeling Accuracy: Simplified analytical (Stoney) models proved inadequate, underestimating the bow by a factor of 3-4, demonstrating the critical need for Finite Element Modeling (FEM) that accounts for the stresses originating from the surrounding Si frame border.
  • Stress Validation: Experimental Raman spectroscopy confirmed that the residual tensile stress in the GaN layers of small membranes (0.5 mm and 2 mm) was approximately 1.0 ±0.2 GPa, which is safely below the GaN tensile strength limit (4 to 7.5 GPa).
  • Deformation Limit: Membrane bow was measured up to 58 ”m for the 5 mm sample, a deformation level that severely challenges standard device fabrication techniques like contact lithography.
  • Lock-in Mechanism: FEM demonstrated that a CVD diamond layer as thin as 1 ”m is sufficient to lock the membrane into the highly bowed state achieved during the initial heating phase, due to diamond’s extreme stiffness.
  • Interface Discovery: Cross-sectional analysis (STEM/EDX) revealed an unexpected thin layer of SiO2 at the diamond/AlN interface, potentially formed by Si redeposition during plasma pre-treatment, which may facilitate carbide bonding to the N-polar face.
  • Proposed Solution: To realize a viable platform, the GaN/III-N membrane must be sufficiently pre-stressed in the opposite direction prior to diamond growth, ensuring the final structure is flat at room temperature.

Technical Specifications

Section titled “Technical Specifications”
ParameterValueUnitContext
GaN Tensile Strength (Reported)4 to 7.5GPaMechanical limit of GaN at room temperature.
Measured GaN Tensile Stress1.0 ±0.2GPaResidual stress in 0.5 mm and 2 mm membranes post-CVD.
Maximum Measured Membrane Bow58”mNet bow for the 5 mm diameter membrane.
CVD Growth Temperature (Tg)720 to 750°CMeasured by pyrometer during MPCVD process.
CVD Forward Power5.5kWMicrowave plasma assisted CVD (MPCVD).
CVD Pressure110 to 120TorrGrowth pressure range.
Methane Concentration (CH4)3%In H2 gas mixture.
Final Diamond Thickness38 to 57”mAchieved after 19 hours of growth.
Diamond Young’s Modulus (EDia)1000GPaValue used in numerical modeling.
GaN E2(high) Peak Shift (0.5 mm)-2.8 ± 0.7cm-1Corresponds to measured tensile stress.
Si Handle Thickness (tSi)75”mBase model thickness.
GaN Layer Thickness (tGaN)2”mBase model thickness.
III-N Layer Thickness (tIII-N)3”mBase model thickness (lumped AlN/AlGaN).
Diamond Thermal Conductivity2270W/mKSingle crystal diamond reference value.
GaN Thermal Conductivity253W/mKReference value.

Key Methodologies

Section titled “Key Methodologies”

The investigation combined analytical modeling, numerical simulation, and experimental fabrication/characterization to understand thermal stress in the GaN-on-diamond membrane system.

  1. Membrane Fabrication:

    • Starting Material: GaN/III-N heterostructure grown on a Si wafer, diced into 15 mm x 15 mm squares.
    • Si Etch: High-power Inductively Coupled Plasma (ICP) etch (900 W) from the Si side to thin the wafer.
    • Membrane Patterning: Photolithography followed by a lower power ICP etch (600 W) to fully remove Si in circular regions (0.5 mm, 2 mm, 5 mm diameter), exposing the N-polar AlN layer.

  2. Diamond Seeding and Pre-treatment:

    • Plasma Pre-treatment: N2/H2 microwave plasma (1.5 kW, 20 Torr) was applied to the exposed GaN/III-N membranes to control seeding density and modify the AlN surface.
    • Seeding: A nano-diamond colloid solution was pipetted onto the exposed membrane and Si border (non-ultrasonic method used to prevent membrane damage).

  3. CVD Diamond Growth:

    • Method: Microwave Plasma Assisted Chemical Vapour Deposition (MPCVD) using a Carat systems CTS6U reactor.
    • Recipe: 5.5 kW forward power, 110-120 Torr pressure, 3% CH4 in H2 gas mixture.
    • Temperature Control: Growth temperature was maintained at 720-750 °C, monitored by a dual-wavelength pyrometer.

  4. Numerical Modeling (FEM):

    • Software: COMSOL Multiphysics (Thermal Stress module).
    • Process Steps: A four-step simulation was used: 1) Initial heating to Tg (determining initial bow), 2) Diamond layer deformation to fit the bowed structure, 3) Isolated cooling of the diamond layer (stress-free deposition assumption), and 4) Integration of cooled diamond with the initial membrane structure to find final stress and bow.
    • Refinement: Model accuracy was significantly improved by introducing a linear spatial temperature gradient (T(x,y)) across the sample, counteracting the central bow predicted by uniform temperature models.

  5. Characterization:

    • Stress Measurement: Raman spectroscopy (514 nm laser) was used to measure the shift of the GaN E2(high) peak, correlating the shift to residual biaxial tensile stress (using a 2.9 cm-1 GPa-1 relation).
    • Deformation Measurement: Surface profilometry was used to quantify the membrane bow (z displacement) before and after diamond growth.
    • Interface Analysis: High Angle Annular Dark Field Scanning Transmission Electron Microscopy (HAADF-STEM) and Energy Dispersive X-ray Spectroscopy (EDX) were used to analyze the diamond/III-N interface, confirming the presence of a thin SiO2 layer.

Commercial Applications

Section titled “Commercial Applications”

This research directly addresses a critical manufacturing bottleneck—thermal management—for high-performance GaN devices by developing a robust, bonding-free integration method for diamond heat spreaders.

  • High Power RF and Microwave Devices: GaN High Electron Mobility Transistors (HEMTs) are primary targets. Diamond heat spreaders are essential to mitigate self-heating, enabling higher power density and improved reliability in 5G/6G infrastructure and radar systems.
  • Advanced Thermal Management: The technology provides a pathway for integrating ultra-high thermal conductivity materials (diamond, >2000 W/mK) directly onto semiconductor stacks without relying on complex and thermally resistive wafer bonding processes.
  • Micro-Electro-Mechanical Systems (MEMS): The fabrication of robust, large-area GaN/III-N membranes is relevant for various MEMS applications, including pressure sensors and photonic devices, where controlled mechanical deformation is crucial.
  • Heterogeneous Integration: The methodology for managing extreme thermal stress during high-temperature deposition is applicable to other material systems requiring the integration of stiff, high-CTE-mismatch films onto thin, fragile semiconductor membranes.

Tech Support

Section titled “Tech Support”

Original Source

Section titled “Original Source”

References

Section titled “References”
  1. 2014 - “GaN-on-diamond: a brief history,” in 2014 lester eastman Conference on high performance devices (LEC)
  2. 2019 - Thermal conductivity of GaN, 71GaN, and SiC from 150 K to 850 K [Crossref]
  3. 2014 - Low thermal resistance GaN-on-diamond transistors characterized by three-dimensional Raman thermography mapping [Crossref]
  4. 1993 - Thermal conductivity of isotopically modified single crystal diamond [Crossref]
  5. 2011 - AlGaN/GaN high-electron mobility transistors with low thermal resistance grown on single-crystal diamond (111) substrates by metalorganic vapor-phase epitaxy [Crossref]
  6. 2012 - Reduced self-heating in AlGaN/GaN HEMTs using nanocrystalline diamond heat-spreading films [Crossref]
  7. 2017 - Thermal characterization of polycrystalline diamond thin film heat spreaders grown on GaN HEMTs [Crossref]
  8. 2015 - Thermal management of hotspots using diamond heat spreader on Si microcooler for GaN devices [Crossref]
  9. 2018 - Transfer of AlGaN/GaN RF-devices onto diamond substrates via van der Waals bonding [Crossref]

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