Thermoreflectance Imaging: Key Applications

October 10, 2022by John Gaskins

Thermoreflectance is an optical thermal metrology technique rooted in the fact that the reflectivity of materials is temperature dependent. Thus, a measurement of a material’s reflectivity can be used to measure temperature changes. Thermoreflectance imaging can be used to create thermal maps, predict failure points for microelectronic devices, detect subsurface defects, and record accurate thermal property measurements of materials from nano-, to micro-, to macro-scales.

In this blog, we explain thermoreflectance imaging, how it works, and why its capabilities are so valuable for applications such as semiconductor manufacturing.

What is Thermoreflectance Imaging?

Thermoreflectance imaging is a non-contact, laser-based method for measuring thermal properties. This technique takes advantage of the relationship between temperature and reflectivity. By recording the change in intensity of a reflected laser off the surface of a sample, local temperatures can be readily calculated. A key advantage of the use of laser sources to monitor reflectivity is the ability to focus the area of interrogation on micrometer areal length scales (based on the diffraction limit) and nanometer depth length scales (based on Beer’s Law) to provide localized voxels for thermal property measurements during thermoreflectance imaging.  Thus, thermoreflectance measurements taken at various points across a material’s surface allow for the generation of a map of temperature gradients and hotspots with length scales on the order of hot spot generation volumes typical in a wide array of devices.

Compared to traditional thermal metrology techniques which utilize physical probes, thermoreflectance imaging techniques can offer:

  1. A non-contact method capable of recording accurate temperature data without touching the relevant material’s surface. This benefit is crucial for sensitive materials such as thin films (we explain why below).
  2. A high, micron-scale areal and nano-scale depth spatial resolution, is essential for working with materials such as thin films.
  3. The ability to accurately measure a wide range of temperatures.
  4. For certain thermoreflectance techniques, the ability to identify subsurface defects.

Important Applications for Thermoreflectance Imaging

The benefits of thermoreflectance imaging techniques are critical for meeting the metrology challenges associated with semiconductor materials used in nanoelectronics, barrier coatings used for hypersonic/aerospace applications, materials and technologies for energy harvesting, nuclear power generation, and ongoing materials research for high power 5G and 6G applications. All device components generate heat in operation, and understanding their thermal properties is critical for reducing thermal failure, accurately specifying operating limits, and designing adequate cooling solutions. While thermal stress testing can help filter out defective devices, accelerated temperature cycling does not provide precise data on thermal properties or a thermal image (we explore this topic in more depth in our blog on reducing thermal failure ).

The map of temperature gradients generated by thermoreflectance imaging may be used to infer thermal resistance/conductivity for materials that would be challenging to measure using traditional techniques. For example, the high-resolution, non-contact nature of thermoreflectance imaging is particularly well suited when applied to thin films, which are increasingly commonplace in micro- and nano-scale electronics components. Traditional thermal measurement techniques use heat sources or sensors that are often orders of magnitude larger than the thickness of the films themselves, resulting in inaccurate measurements. Because a given film’s thermal properties are dependent on its specific thickness and processing conditions, its thermal properties cannot simply be referenced from a textbook. We take a deeper look at measuring thermal conductivity for thin films in our blog here.

Steady-State versus Transient Thermoreflectance Imaging

Thermoreflectance imaging techniques can be categorized as either steady-state or transient.

Transient methods rely on the heat diffusion equation, with volumetric heat capacity and thermal conductivity coupled through thermal effusivity or thermal diffusivity, depending on the time and length scales of the measurement. Examples of transient thermoreflectance techniques include laser flash, time-domain thermoreflectance (TDTR), and frequency-domain thermoreflectance (FDTR). While certain transient methods have proven their ability to provide robust thermal property measurements for both bulk and thin film materials, they are generally expensive, require greater operator expertise, and inherently require prior knowledge of either volumetric heat capacity or thermal conductivity in order to calculate the other.

Steady-state methods allow for direct measurements of thermal conductivity via Fourier’s Law. Traditionally, however, steady-state methods were designed for bulk materials, requiring relatively large sensor sizes and material sample areas, and have also been slow, and poorly suited to high-throughput industries like semiconductor manufacturing. Laser Thermal’s proprietary thermoreflectance technique, SSTR-F (Steady State Thermoreflectance in Fiber Optics), addresses these limitations and offers the benefits of both steady state and transient testing. SSTR-F combines the operational simplicity of steady-state testing with faster cycle times, the ability to achieve greater penetration depths than transient techniques, and the elimination of undue influence from sensor contact.

Learn More About SSTR-F: Steady State Thermoreflectance in Fiber Optics

SSTR-F is a non-contact laser-based pump-probe technique that can measure the thermal conductivity and interfacial thermal resistance of thin films.  Laser Thermal’s patented design of the SSTR-F tool allows for turnkey, fully automated thermal conductivity measurements, with high throughput batch testing that is ideal for screening thermal resistance changes in materials. SSTR-F provides accurate (+/- 10 %), repeatable (+/- 0.5 %), and reproducible (+/- 1.0 %) measurements of the thermal resistances of thin films and interfaces.

If you’re interested in learning more about SSTR-F, how it works, and how it can help provide more precise thermal property measurements for challenging materials such as semiconductors, our video playlist is a great place to start. Or, you can find a selection of results generated using SSTR-F in our database here.

John Gaskins

As Co-Founder and CEO of Laser Thermal, John has conducted research characterizing the mechanical, optical and thermal properties of materials for almost two decades. John received his Ph.D. in Mechanical and Aerospace Engineering from the University of Virginia in 2013.  His work developed methodologies for testing the size dependent properties in thin film structures.  After his Ph.D. John joined Patrick Hopkins’ group extending his Ph.D. work focused on mechanical property characterization to optical and thermal properties. John is leading Laser Thermal’s development of SSTR-F to provide accessible small-scale thermal conductivity measurements to industrial and academic partners.