BS IEC 62951-5:2019 pdf free download – Semiconductor devices – Flexible and stretchable semiconductor devices
4.1 General
Thermoreflectance is one of non-contact optical thermal characterization techniques that relies on the change of refractive index of materials as a function of temperature. Depending on their thickness, flexible semiconductor materials can be categorized as either substrate or thin-film. For substrate materials, their optical reflectance values change linearly with temperature. However, optical reflectance values of thin-film materials show highly non-linear behaviours as shown in Figure 1 . For thin-films, non-linear optical reflectance is also strongly dependent on the sample thickness as shown in Figure 2. Such a non-linearity requires reflectance measurements at multiple wavelengths. Once optical reflectance values at one or more wavelengths are calibrated at various temperatures, thermal characterization is enabled.
4.2 Test apparatus
In case of substrate materials, thermoreflectance signals at a given wavelength tend to change linearly with increasing temperature. Thin-film materials, however, exhibit highly non- linear thermoreflectance behaviours as temperature increases. Therefore, single wavelength optical probing is necessary and sufficient for substrate flexible semiconductor materials and at least dual wavelength probing is required for thin-film materials. Thermoreflectance ratio at different wavelengths is still non-linear with temperature but can provide an acceptable match with the theoretical estimation and more precise temperature measurements. Figure 3 and Figure 4 show schematics of thermoreflectance thermometry with one laser source used for calibration and measurement (for example, wavelength, λ = 633 nm herein but other wavelength can be used), respectively. For calibration, heating or cooling blocks with fixed temperature are used. The substrate is joule heated with DC or AC (simple periodic sine) power supplies for actual measurements.
A sample specimen is mounted between two blocks with constant temperature which are located on a fixed jig and a motorized linear stage, respectively. In general, the temperature blocks are running during calibration (reflectance vs. temperature at a given wavelength). Figure 7 shows typical results with a silicon thin-film during calibration where wavelength dependent non-linear behaviours can be seen. By taking the reflectance ratio as shown in Figure 8, a calibration curve for a thin-film can be obtained. The thickness of each thin-film sample is not necessarily measured.
For actual measurements, the sample specimen is joule-heated by using either DC or AC (simple periodic sine) powers in general. For highly resistive samples, heating methods other than joule heating shall be used. As shown in Figure 9, the single-axis motorized stage enables either bending or stretching for flexible or stretchable semiconductor materials, the dual-axis (xy) motorized stages are used for sample scanning for obtaining an average temperature, and the goniometer rotates the sample to maintain normal incidence of the probing laser.