Thin Film Thermal Conductivity and Its Implications for Electronic Devices
Thin films, layers of material that can range from several micrometers to less than a single nanometer in thickness, create unique challenges for measuring basic thermal properties like thermal conductivity. In short, these materials are simply too thin to rely on traditional thermal measurement techniques. To make matters even worse, the thermal conductivity of thin films is strongly dependent on the film thickness and processing conditions, meaning one cannot simply “look up” the thermal conductivity of thin films in the back of a textbook.
Understanding these materials’ thermal properties, however, is vital across a broad range of thin film applications (particularly electronics components, which commonly operate under a sustained thermal load). In this blog, we explain why measuring the thermal conductivity of thin films is so difficult, and how a new approach to thermal metrology can help.
What is the effect of thickness on thermal conductivity?
Thermal conductivity describes a material’s ability to transfer or “conduct” heat. We take a deeper dive into the fundamentals of thermal conductivity in our blog here. To understand the fundamental reason why thickness impacts the thermal conductivity of thin films, we must take a nanoscopic viewpoint of heat transfer. In crystalline solids, energy is primarily conducted via electrons or phonons (quantized vibrations of the lattice). The ability of a material to move energy away from a hot spot is related to how far an electron or phonon can travel before it scatters, thus transferring its energy and momentum to another particle. This distance is called the mean free path.
Consider the analogy of driving your car on a highway. If no one else is on the road, then you can travel to your destination efficiently and on time (going the speed limit of course!). However, if it’s rush hour and there is gridlock, you will most likely need to stop and go and change lanes frequently to get from point A to point B. In this case, you are changing speed and direction often, and your net travel time is longer. This high traffic scenario is analogous to a material with a short mean free path: the electrons or phonons take longer to transfer energy across the material. Now, consider if you are driving on a highway that has major construction and reduces down to a 1 lane road. Your speed will reduce, which is due to the dimension of the highway reducing.
In thin films, defects and interfaces may arise during growth and heterogeneous integration that change the electron and phonon scattering events, resulting in drastically lower thermal conductivities in materials than their bulk counterparts. Therefore, to truly understand the thermal conductivity of a thin film in a device, one must measure the thermal conductivity of a film of the same thickness, grown under the same conditions.
What is the effect of thermal conductivity on thin-film performance?
Sustained thermal stress can drive a variety of different failure modes in these thin film materials, which must be carefully engineered to operate within specified thermal limitations. Our blog here takes you more detailed look at thermal failures in semiconductor devices(and why understanding thin-film thermal properties are crucial to preventing them).
Thermal conductivity is the inverse of thermal resistance, another crucial property for predicting semiconductor components’ ability to dissipate heat. A more conductive material has a lower thermal resistance, allowing heat to transfer out of semiconductor devices more efficiently to reduce thermal stress. We take a deeper look at the semiconductor testing process, how it works, and why it must employ effective thermal resistance measurement techniques, in our blog here. The unique properties of thin films discussed above, however, make measuring thermal conductivity/resistance more challenging than for more traditional materials.
How is thermal conductivity measured in thin films?
In most traditional applications, sensors such as thermocouples or resistive thermometers can be used to capture measurements of the temperature gradients or changing heat flux that result as heat is applied to a material (measurements which can be directly related to thermal conductivity via Fourier’s law or the heat equation). Thin film materials, however, are often substantially thinner than the dimensions of the heaters and sensors used to induce the temperature gradients and capture temperature changes, respectively. This fact renders these measurement platforms unsuitable for these applications. For example, the material in the thermal sensor itself will contribute more to the recorded thermal resistances and temperature changes than the thin film of interest!
Furthermore, the physical testing probes used in traditional measurement techniques are not amenable to the high-throughput screening techniques necessary in today’s semiconductor fabrication facilities. The accurate measurement of thermal resistance and thermal conductivity of thin films requires a different approach to thermal metrology, which must achieve the substantially higher temporal/spatial resolutions demanded by these nano-meter scale materials.
Optical measurement techniques can directly address these limitations. For example, Laser Thermal’s products and testing services are anchored in Steady-State Thermoreflectance in Fiber Optics (SSTR-F), a non-contact, laser-based pump-probe technique. This approach works by utilizing a laser to create a nano-to-microscale heating event on the surface of a sample, resulting in a steady-state temperature gradient in the sample. A secondary probe laser is then used to measure the temperature on the surface of the sample, which is driven by different steady-state temperature gradients established by varying pump powers. The spot laser used in this process can be tuned to be as small as a few microns 1/e2 diameter, which, given the nanoscale localized absorption of the laser, enables SSTR-F to be used effectively to measure in-plane and through-plane (cross-plane) thermal resistances—even for nanometer-scale thin films and atomically thin interfaces.
SSTR-F can provide accurate (+/- 10 %), repeatable (+/- 0.5 %) and reproducible (+/- 1.0 %) thermal conductivity measurements. This approach is also capable of fully automated, high-throughput measurements, which are crucial for the large volumes of testing required by the ever-growing semiconductor industry.
If you’re interested in seeing this technique in action, our video playlist here provides demonstrations of several relevant applications, including measuring the thermal conductivity of thin films. Or, if you have more specific questions, please reach out to our team using the button below.