Characterizing electric fields in semiconductor devices: effect of second-harmonic light interference.

Characterizing electric fields in semiconductor devices using electric field-induced second-harmonic generation (EFISHG) has opened new opportunities for an advanced device design. However, this new technique still has challenges due to the interference between background second-harmonic generation (SHG) and EFISHG generated light. We demonstrate that interference effects can effectively be eliminated during EFISHG measurements by focusing the laser from the transparent substrate side of a GaN PN diode, enabling straightforward quantitative electric field analysis, in contrast to PN junction interface side measurements. A model based on wave generation and propagation is proposed and highlights the incoherence between background SHG and EFISHG light. This incoherence may be attributed to the depth of focus of the incident laser and phase mismatch between incident and SHG light.

Simultaneous Measurement of Thermal Conductivity and Volumetric Heat Capacity of Thermal Interface Materials Using Thermoreflectance.

Thermal interface materials are crucial to minimize the thermal resistance between a semiconductor device and a heat sink, especially for high-power electronic devices, which are susceptible to self-heating-induced failures. The effectiveness of these interface materials depends on their low thermal contact resistance coupled with high thermal conductivity. Various characterization techniques are used to determine the thermal properties of the thermal interface materials. However, their bulk or free-standing thermal properties are typically assessed rather than their thermal performance when applied as a thin layer in real application. In this study, we introduce a low-frequency range frequency domain thermoreflectance method that can measure the effective thermal conductivity and volumetric heat capacity of thermal interface materials simultaneously in situ, illustrated on silver-filled thermal interface material samples, offering a distinct advantage over traditional techniques such as ASTM D5470. Monte Carlo fitting is used to quantify the thermal conductivities and heat capacities and their uncertainties, which are compared to a more efficient least-squares method.

Thermal Characterization of Metal-Diamond Composite Heat Spreaders Using Low-Frequency-Domain Thermoreflectance.

High thermal conductivity and an appropriate coefficient of thermal expansion are the key features of a perfect heat spreader for electronic device packaging, especially for applications with increased power density and the increasing demand for higher reliability and semiconductor device performance. For the past decade, metal-diamond composites have been thoroughly studied as a heat spreader, thanks to their high thermal conductivities and tailored coefficients of thermal expansion. While existing thermal characterization methods are good for quality control purposes, a more accurate method is needed to determine detailed thermal properties of these composite materials, especially if clad with metal. Low-frequency-range-domain thermoreflectance has been adopted to measure the thermal conductivity of a metal-diamond composite sandwiched between metal cladding layers. Due to this technique's low modulation frequencies, from 10 Hz to 10 kHz, multiple layers can be probed and measured at depths ranging from tens of micrometers to a few millimeters.

In situ Thermoreflectance Characterization of Thermal Resistance in Multilayer Electronics Packaging.

High-performance, high-reliability microelectronic devices are essential for many applications. Thermal management is required to ensure that the temperature of semiconductor devices remains in a safe operating range. Advanced materials, such as silver-sintered die attach (the bond layer between the semiconductor die and the heat sink) and metal-diamond composite heat sinks, are being developed for this purpose. These are typically multilayered structures, with individual layer thicknesses ranging from tens of micrometers to millimeters. The effective thermal conductivity of individual layers likely differs from their bulk values due to interface effects and potential material imperfections. A method is needed to characterize the thermal resistance of these structures at the design optimization stage to understand what effect non-idealities may have on the final packaged device temperature. We have adapted the frequency-domain thermoreflectance technique to measure at low frequencies, from 10 Hz to 10 kHz, enabling multiple layers to be probed at depths from tens of micrometers to millimeters, which is tailored to assess novel device packaging and heat sinks. This is demonstrated by measuring the thermal resistance of a sintered silver die attach.

Crystalline Interlayers for Reducing the Effective Thermal Boundary Resistance in GaN-on-Diamond.

Integrating diamond with GaN high electron mobility transistors (HEMTs) improves thermal management, ultimately increasing the reliability and performance of high-power high-frequency radio frequency amplifiers. Conventionally, an amorphous interlayer is used before growing polycrystalline diamond onto GaN in these devices. This layer contributes significantly to the effective thermal boundary resistance (TBReff) between the GaN HEMT and the diamond, reducing the benefit of the diamond heat spreader. Replacing the amorphous interlayer with a higher thermal conductivity crystalline material would reduce TBReff and help to enable the full potential of GaN-on-diamond devices. In this work, a crystalline Al0.32Ga0.68N interlayer has been integrated into a GaN/AlGaN HEMT device epitaxy. Two samples were studied, one with diamond grown directly on the AlGaN interlayer and another incorporating a thin crystalline SiC layer between AlGaN and diamond. The TBReff, measured using transient thermoreflectance, was improved for the sample with SiC (30 ± 5 m2 K GW-1) compared to the sample without (107 ± 44 m2 K GW-1). The reduced TBReff is thought to arise from improved adhesion between SiC and the diamond compared to the diamond directly on AlGaN because of an increased propensity for carbide bond formation between SiC and the diamond. The stronger carbide bonds aid transmission of phonons across the interface, improving heat transport.

Thick, Adherent Diamond Films on AlN with Low Thermal Barrier Resistance.

The growth of >100-µm-thick diamond layers adherent on aluminum nitride with low thermal boundary resistance between diamond and AlN is presented in this work. The thermal barrier resistance was found to be in the range of 16 m2·K/GW, which is a large improvement on the current state-of-the-art. While thick films failed to adhere on untreated AlN films, AlN films treated with hydrogen/nitrogen plasma retained the thick diamond layers. Clear differences in ζ-potential measurement confirm surface modification due to hydrogen/nitrogen plasma treatment. An increase in non-diamond carbon in the initial layers of diamond grown on pretreated AlN is seen by Raman spectroscopy. The presence of non-diamond carbon has minimal effect on the thermal barrier resistance. The surfaces studied with X-ray photoelectron spectroscopy revealed a clear distinction between pretreated and untreated samples. The surface aluminum goes from a nitrogen-rich environment to an oxygen-rich environment after pretreatment. A clean interface between diamond and AlN is seen by cross-sectional transmission electron microscopy.