Measuring the Thermal Conductivity of Buried Sub-Surface Materials and Substrates

SSTR offers the penetration ability needed to measure thermal conductivities for structures buried too deep for traditional methods.

The Challenge

Measuring the thermal conductivity of sub-surface buried substrates is of significant practical interests. However, this remains challenging with traditional pump–probe spectroscopies due to their limited thermal penetration depths.

The Solution

Because SSTR operates in a steady state regime, the measurement volume over which SSTR measures the thermal conductivity can penetrate under the surface to depths as large as the laser spot size. This capability allows SSTR to measure the thermal conductivity of materials beneath the surface based on tuning the pump and probe laser spot size.

Here, we demonstrate this by measuring the thermal conductivity of substrates that are buried 100 nm to a few microns under the surface below dielectric films. Specifically, here we generate measurements for SiO2, GaN substrates under 2.05 microns of GaN films, and sapphire substrates under 2 microns of AlN.

For estimating the uncertainty of SSTR measurements of a buried substrate a priori, sensitivity calculations provide the best means. Thus, detailed sensitivity calculations are provided to guide future measurements. Due to the steady-state nature of SSTR, it can measure the thermal conductivity of buried substrates that are traditionally challenging by transient pump–probe techniques, exemplified by measuring three control samples.

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Nanoscale to bulk

Thermal conductivity of nanoscale thin films

SSTR is ideal for your thin film thermal conductivity testing needs.
In addition to measurements of bulk wafers and substrates (source 1, source 2), SSTR has the unprecedented spatial resolution to measure the thermal conductivity of thin films.
For example, SSTR can measure the thermal conductivity of single nanometer thick ALD grown thin films or micron thick AlN films (source).

SSTR measurements on ALD grown ultrathin film amorphous gate dielectrics (compared to TDTR) (source).

SSTR measurements of AlN crystalline thin films.2 SSTR data (temperature rise vs. power) on right for AlN film and silicon wafer control.

Validated results

SSTR-F measures the thermal conductivity of Corning Pyroceram 9606 and Corning high purity fused silica (HPFS) 7980 to within 5% of their published values.
Additional validation was provided by measuring samples from the same piece of material using the Transient Plane Source technique.
Pyroceram is no longer available from NIST but was measured on similar material to the material that was validated as a Certified Reference Material.
Corning HPFS was procured from a distributor of HPFS 7980 wafers.

	Literature Value	TPS Measurement	SSTR-F Measurement
Pyroceram 9606	4.07 +/- .26	3.95	3.95 +/- 0.2
HPFS 7980	1.38	1.39	1.39 +/- 0.09

Unparalleled reliability

Thermal conductivity measurements: Accuracy +/- 10%, Repeatability +/- 0.5%, Reproducibility +/- 1%
Accurate thermal conductivity measurements of materials in agreement with literature values
With exceptional instrumentation reliability and repeatability, SSTR-F offers unprecedented measurement stability on a daily basis for any user, exemplified by the SSTR-F data shown below. Spread in randomly sampled spots are sample related, not measurement driven.

Source

20 SSTR-F measurements at same spot

20 SSTR-F measurements at random spots

Turnkey testing

Our patented fiber-optic design enables turnkey, automated testing of thermal conductivity.
High-throughput SSTR-F testing of multiple samples facilitates, for example, screening the thermal conductivity of different thin films processed under different conditions
Defects, stresses, compositional differences, and changes in density and microstructure that occur up during film growth result in changes in thermal conductivity, easily and rapidly identified vis SSTR-F

SSTR-F data on 4 different 200 nm dielectric films on silicon with thermal conductivity ranging from ~0.35 W/m/K up to 1.8 W/m/K. SSTR-F allows for easy, rapid and automated identification of different temperature rises based on the thermal conductivity of the different films.