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METROLOGY DEVELOPMENT FOR THERMAL CHALLENGES IN ADVANCED SEMICONDUCTOR PACKAGING

thesis
posted on 2024-09-24, 17:40 authored by Aalok Uday GaitondeAalok Uday Gaitonde

The high heat fluxes generated in electronic devices must be effectively diffused through the semiconductor substrate and packaging layers to avoid local, high-temperature “hotspots” that govern long-term device reliability. In particular, advanced semiconductor packaging trends toward thin form factor products increase the need for understanding and improving in-plane conduction heat spreading in anisotropic materials. Furthermore, predicting thermal transport in vertical stacks of thinned and bonded die hinges on accurately characterizing unknown buried interfacial thermal resistances. The design of semiconductor thermal packaging solutions is hence limited by the functionality and accuracy of metrology available for thermal properties characterization of engineered anisotropic heat spreading materials and buried interfaces. This work focuses on the development of two separate innovative metrology techniques for characterizing in-plane thermal properties of both isotropic and anisotropic materials, and the measurement of low thermal interfacial resistances embedded in stacks of semiconductor substrates.

In the first portion of this thesis, a new measurement technique is developed for characterizing the isotropic and anisotropic in-plane thermal properties of thin films and sheets, as an extension of the traditional Ångstrom method and other lock-in thermography techniques. The measurement leverages non-contact infrared temperature mapping to quantify the thermal response to laser-based periodic heating at the center of a suspended thin film sample. This novel data extraction method does not require precise knowledge of the boundary conditions. To validate the accuracy of this technique, numerical models are developed to generate transient temperature profiles for hypothetical anisotropic materials with known properties. The resultant temperature profiles are processed through a fitting algorithm to extract the in-plane thermal conductivities, without the knowledge of the input properties to the forward model. Across a wide range of in-plane thermal conductivities, these results agree well with the input values. The limits of accuracy of this technique are identified based on the experimental and sample parameters. Further, numerical simulations demonstrate the accuracy of this technique for materials with thermal conductivities from 0.1 to 1000 W m−1 K−1, and material thicknesses ranging from 0.1 to 10 mm. This technique effectively measures anisotropy ratios up to 1000:1. Data from multiple heating frequencies can be combined to fit for a single set of thermal properties (independent of frequency), which improves measurement sensitivity as the thermal penetration depth varies across frequencies. The post-processing algorithm filters out regions within the laser absorber and heat sink to eliminate regions in the sample domain with boundary effects. Based on these guidelines, experiments demonstrate the accuracy of this measurement technique for a wide range of known isotropic and anisotropic heat spreading materials across a thermal conductivity range of 0.3 to 700 W m−1 K−1, and in-plane anisotropy ratios of 30:1. These steps contribute towards standardization of this measurement technique, enabling the development and characterization of engineered heat spreading materials with desired anisotropic properties for various applications.

The second portion of this thesis focuses on characterization of thermal resistances across “buried” interfaces that are challenging to characterize in situ due to their low relative magnitude and embedded depth within a material stack. In particular, we target characterization of interfaces that are buried deeper than the thermal penetration depth of available transient measurement techniques, such as thermoreflectance, but have low thermal resistances that prohibit the use of steady-state techniques, such as the reference bar method, due to the very high temperature gradients that would be necessary resolve the resistances, among other sample preparation challenges. This work develops a technique for the non-destructive characterization of such deeply buried interfaces having thermal contact resistances of the order of 0.001 cm2K/W. Two different embodiments of the measurement approach are first assessed before down-selecting to a single experimental implementation. The working principle for both embodiments includes a combination of non-contact periodic heating and thermal sensing to measure the transient temperature response of a two-layer stack of materials with a bonded interface of unknown thermal resistance. The approaches aim to eliminate the preparation requirement of cutting samples to investigate their temperature in cross-section. In the first embodiment, the sample stack is heated periodically at the center of the sample, and cooled at the periphery, to create a radial temperature gradient. The second embodiment involves generating a one-dimensional temperature gradient across the stack by periodic heating of one face and steady cooling of the other face. The corresponding ing amplitude and phase delay of the temperature responses are used to fit for the thermal interfacial resistance, assuming a time-periodic solution for the heat diffusion equation for a system with periodic heating. Numerical models developed for both approaches simulate the transient temperature profiles across a two-layer bonded silicon stack of known thermal properties, and enable an assessment of both approaches. The one-dimensional (1D) gradient approach is found to have higher sensitivity and measurable signal compared to the radial spreading approach, at the same mean temperature of the sample.

Based on this 1D gradient concept, an experimental facility is developed, which includes a IR-transparent heat sink, laser-based heating, and two IR temperature sensors for noncontact temperature measurement of both sides of the sample. The unique IR transparent heat sink design allows for simultaneous cooling and non-contact temperature measurement of the bottom surface of the sample. An inverse fitting method is developed to extract the thermal resistances using the steady periodic temperature amplitude and phase delay across the thickness of the material. Thermal data generated using numerical simulations, along with the data fitting method, is first leveraged to validate the extracted thermal resistance values for two-layer material systems with an bonded interface, as well as for the thermal conductivity measurement of bulk materials without an interface. The data extraction process is shown to accurately extract thermal contact resistances on the order of 0.0001 cm2K/W in silicon-based packages for interfaces that are a few millimeters from the exposed surface. For bulk materials, this technique demonstrates accuracy in extracting the thermal conductivity of a wide range of materials ranging from thermal insulators to highly conductive materials, spanning a range of 0.1 to 2000 W m−1 K−1. Physical measurements of thermal conductivity of bulk silicon nitride and zinc oxide agree well with expected reference values, and these measurements also align well with data from independently performed experiments on the same materials using an established ASTM D5470 standard, thereby validating this new measurement technique experimentally. Two-layer dry-contact stacks of these two materials demonstrate the extraction of the thermal resistance across interfaces buried up to 2 mm from the exposed surface. This work contributes toward standardization of this technique for measurement of thermal resistances with low magnitudes and buried depths, which are commonly found in modern electronic packages, ranging from near-junction epitaxial semiconductor films to interconnect layers in emerging die-to-die and wafer hybrid bonding technologies.

Ultimately, these measurement techniques of in-plane thermal conductivity measurement of anisotropic materials and the interfacial contact resistance measurements across buried interfaces offer an important contribution to the area of thermal metrology, and advance the field of next-generation semiconductor packaging.

Funding

Cooling Technologies Research Center

Semiconductor Research Corporation

History

Degree Type

  • Doctor of Philosophy

Department

  • Mechanical Engineering

Campus location

  • West Lafayette

Advisor/Supervisor/Committee Chair

Amy Marconnet

Advisor/Supervisor/Committee co-chair

Justin Weibel

Additional Committee Member 2

Liang Pan

Additional Committee Member 3

Chelsea Davis