Detecting Sub-Surface Defect Layers

SSTR can be used to detect defective substrate layers at depths impossible to resolve for traditional thermal metrology techniques.

The Challenge

While high-frequency time-domain thermoreflectance measurements provide insight into the thermal properties of the near-surface preceding the longitudinal projected range depth, these approaches cannot resolve the material properties deeper than ~ 1 micron under the surface. Thus, TDTR cannot detect the end of range, and only probes the high-quality diamond near surface region.

The Solution

In this case, we used SSTR to detect the presence and measure the thermal conductivity of a highly defective and amorphous layer of carbon that was 7 microns beneath a diamond surface after ion irradiation (a depth that would otherwise be impossible to resolve).

Through measurements with a series of focusing objective lenses for the laser spot size, we find the thermal conductivity of the amorphous region to be approximately 1.4 W m-1 K-1, which is comparable to that measured for amorphous carbon films fabricated through other techniques. The key enabling capability is SSTR’s fully controllable thermal conductivity measurement depth, which is exemplified by greater sensitivity as temperature rises in SSTR as compared to TDTR (see  below).

Detecting Sub-Surface Defect Layers

Sensitivity analysis of the thermal properties of the implanted diamond by considering the sample as a four-layer system measured with SSTR (a) and TDTR (b). The subscripts refer to the corresponding layer in a top-down manner (for example, layer 1 refers to the Al transducer).

Differences in sensitivity to a particular parameter between the two techniques are attributed to differences in thermal penetration depth. The expected temperature rise of the material system in response to a periodic heat source with frequency of 400 Hz (c) and 8.8 MHz (d) displays the difference in the heating profile of SSTR and TDTR. Both calculations apply a pump and probe radius of 10 μm; while the temperature rise of an 8.8 MHz modulated heating event is primarily contained within the 80 nm transducer, a 400 Hz periodic heating event yields a 1/e thermal penetration depth capable of extending to the amorphous layer. The temperature profile at the center of the pump/probe radius is displayed in (e) and (f).

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