Top-side device cooling methods, e.g., diamond heat sinks on top of the metal contacts, facilitate heat removal from hot spots near the gate, directly lowering local and global temperature rises. Because the top-side surface of modern devices is often complex and rarely flat, conformal nanocrystalline diamond (NCD) growth techniques are critical to maximize the contact area between the device and the heat sink. Weak adhesion at this interface will cripple thermal transport, acting as a large thermal resistance. As a result, thin adhesive metal layers are often deposited between the device and the heat sink. The effectiveness of heat sinks is directly dependent on the thermal boundary conductance of the metal/NCD interface. In this study, we seek to optimize thermal transport across Au/Cr/Pd/NCD stacks. Cr was chosen for its thermal merits. Excellent adhesive properties and high maximum phonon frequencies has enabled Cr to produce some of the largest thermal boundary conductance interfaces for materials like Diamond, Al~2~O~3~, and AlN. Despite this, Cr is predicted to react with NCD to form enthalpically favorable carbides, which will lower the thermal boundary conductance. We hypothesize that a thin non-reactive Pd layer can eliminate these reactions and preserve the metal/NCD thermal boundary conductance in heated conditions. Pd and then Cr layers are deposited as perpendicular wedges with thickness of 0 to 20 nm on a single NCD sample, allowing us to test all thickness combinations of Pd and Cr.  The thermal boundary conductance is measured using a non-destructive pump-probe optical technique, frequency domain thermoreflectance (FDTR). By scanning the FDTR across the sample surface, a 2D map of thermal boundary conductance is created for all Pd/Cr thickness combinations. A local maximum in thermal boundary conductance is observed for a specific Cr/Pd thickness combination, providing evidence that our hypothesis is correct. Using Tunneling Electron Microscopy (TEM), we inspect a lift-out sample taken from our original NCD results, at the location of the TBC local maximum. Using image filtering, we identify the structure of this metal interface and determine the thickness of each metallic and reacted-carbide layers. The sample was then annealed and remeasured multiple times. Changes in the 2D thermal boundary conductance map following each heating cycle is used to quantify the ability for thin Pd layers to improve the thermal stability of metal contacts like Cr. Since large temperature rises will alloy and chemically react metal films and interfaces, it is crucial that we evaluate the thermal performance of metal stacks throughout the entire device’s lifetime, not just when it’s first made.


Optimizing Top-Side Device Cooling via Metal/nanocrystalline Diamond Interfaces

Division / Sponsors: 
Aerospace, Bioengineering, Applied Mechanics, Design Engineering Division, Advanced Energy Systems, Manufacturing Engineering Division, Noise Control and Acoustics, Nondestructive Evaluation, Diagnosis, and Prognosis (NDPD), Nuclear Engineering Division, Process Industries Division, Fluids Engineering, Heat Transfer, Materials, Microelectromechanical Systems, Safety Engineering and Risk Analysis, Computers & Information in Engineering


Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2020/volume/84478

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