Thermal Transport and Mechanical Stress Mapping of a Compression Bonded GaN/Diamond Interface for Vertical Power Devices.

Bonding diamond to the back side of gallium nitride (GaN) electronics has been shown to improve thermal management in lateral devices; however, engineering challenges remain with the bonding process and characterizing the bond quality for vertical device architectures. Here, integration of these two materials is achieved by room-temperature compression bonding centimeter-scale GaN and a diamond die via an intermetallic bonding layer of Ti/Au. Recent attempts at GaN/diamond bonding have utilized a modified surface activation bonding (SAB) method, which requires Ar fast atom bombardment immediately followed by bonding within the same tool under ultrahigh vacuum (UHV) conditions. The method presented here does not require a dedicated SAB tool yet still achieves bonding via a room-temperature metal-metal compression process. Imaging of the buried interface and the total bonding area is achieved via transmission electron microscopy (TEM) and confocal acoustic scanning microscopy (C-SAM), respectively. The thermal transport quality of the bond is extracted from spatially resolved frequency-domain thermoreflectance (FDTR) with the bonded areas boasting a thermal boundary conductance of >100 MW/m2·K. Additionally, Raman maps of GaN near the GaN-diamond interface reveal a low level of compressive stress, <80 MPa, in well-bonded regions. FDTR and Raman were coutilized to map these buried interfaces and revealed some poor thermally bonded areas bordered by high-stress regions, highlighting the importance of spatial sampling for a complete picture of bond quality. Overall, this work demonstrates a novel method for thermal management in vertical GaN devices that maintains low intrinsic stresses while boasting high thermal boundary conductances.

Inversion for Thermal Properties with Frequency Domain Thermoreflectance.

3D integration of multiple microelectronic devices improves size, weight, and power while increasing the number of interconnections between components. One integration method involves the use of metal bump bonds to connect devices and components on a common interposer platform. Significant variations in the coefficient of thermal expansion in such systems lead to stresses that can cause thermomechanical and electrical failures. More advanced characterization and failure analysis techniques are necessary to assess the bond quality between components. Frequency domain thermoreflectance (FDTR) is a nondestructive, noncontact testing method used to determine thermal properties in a sample by fitting the phase lag between an applied heat flux and the surface temperature response. The typical use of FDTR data involves fitting for thermal properties in geometries with a high degree of symmetry. In this work, finite element method simulations are performed using high performance computing codes to facilitate the modeling of samples with arbitrary geometric complexity. A gradient-based optimization technique is also presented to determine unknown thermal properties in a discretized domain. Using experimental FDTR data from a GaN-diamond sample, thermal conductivity is then determined in an unknown layer to provide a spatial map of bond quality at various points in the sample.

Temperature sensitivity of scattering-type near-field nanoscopic imaging in the visible range.

Due to its superb imaging spatial resolution and spectroscopic viability, scattering-type scanning near-field optical microscopy (s-SNOM) has proven to be widely applicable for nanoscale surface imaging and characterization. However, limited works have investigated the sensitivity of the s-SNOM signal to sample temperature. This paper reports the sample temperature effect on the non-interferometric (self-homodyne) s-SNOM scheme at a visible wavelength (λ=638 nm). Our s-SNOM measurements for an arrayed vanadium/quartz sample demonstrate a monotonic decrease in signal intensity as sample temperature increases. As a result, s-SNOM imaging cannot distinguish quartz or vanadium when the sample is heated to ∼309 K: all signals are close to the root-mean-square noise of the detection scheme used for this study (i.e., 19 µV-rms). While further studies are required to better understand the underlying physics of such temperature dependence, the obtained results suggest that s-SNOM measurements should be carefully conducted to meet a constant sample temperature condition, particularly when a visible-spectrum laser is to be used as the light source.

Feedback control of local hotspot temperature using resistive on-substrate nanoheater/thermometer.

This article reports the active control of a local hotspot temperature for accurate nanoscale thermal transport measurement. To this end, we have fabricated resistive on-substrate nanoheater/thermometer (NH/T) devices that have a sensing area of ∼350 nm × 300 nm. Feedback-controlled temporal heating and cooling experiments of the NH/T device confirm that the feedback integral gain plays a dominant role in device's response time for various setpoint temperatures. To further verify the integration of the feedback controller with the NH/T devices, a local tip-induced cooling experiment is performed by scanning a silicon tip over the hotspot area in an atomic force microscope platform. By carefully optimizing the feedback gain and the tip scan speed, we can control the hotspot temperature with the accuracy of ∼±1 K for a broad range of setpoints from 325 K to 355 K. The obtained tip-substrate thermal conductance, including the effects of solid-solid conduction, water meniscus, air conduction, and near-field thermal radiation, is found to be a slightly increasing function of temperature in the range of 127 ± 25 to 179 ± 16 nW/K. Our work demonstrates the reliable controllability of a local hotspot temperature, which will allow the further improvement of various nanoscale thermal metrologies including scanning thermal microscopy and nanoscale thermometry.