| dc.description.abstract | Thermoreflectance (TR) imaging is a nonintrusive temperature measurement method which can obtain temperature distribution on target surface with high spatial and temperature resolution. Three-dimensional (3D) TR imaging utilizing light field camera with 4.5 micrometer lateral resolution and 2.8 micrometer depth resolution is achieved in this study. With the capability of resolving both light direction and intensity, directional TR signal from tilted or curved surfaces can be measured. Steady–state surface temperature measurements of 3D microscale electronic devices, including 808 nm laser diode and 25 micrometer gold electric wire, are conducted. The obtained thermal images show that 3D TR imaging can estimate temperature distributions of microscale 3D surfaces that cannot be achieved with traditional two-dimensional (2D) TR imaging.
In the TR imaging experiment, thermal expansion and its associated sample movement, which leads to non-physical temperature values on thermal maps, has been noticed. As a result, a two-wavelength thermoreflectance (2WTR) imaging technique is developed to conduct steady-state temperature measurement of miniature electronic devices. After spatially uniform illumination is achieved (±0.8%), 2WTR imaging obtains temperature information directly from heated target under operation. Therefore, 2WTR is not affected by movement of a heated target due to thermal expansion. Temperature distribution of a microscale gold resistor with 100 nm thickness under steady-state operation are measured via 2WTR (470 and 530 nm), which is challenging to be obtained by single wavelength TR considering the effect of thermal expansion.
A 3D projection system is developed with a microlens array (MLA) and a spatial light modulator (SLM) to perform light field 3D projection. The system collects light beams from multiple SLM pixels onto small voxels of a virtual 3D structure. Algorithm to render SLM pixel value maps is developed using ray tracing method. The projected 3D virtual structure is optically compressed 16X and delivered to an SU-8 photoresist layer for light field 3D photolithography. The fabricated structures have controlled depth change which is not possible in traditional 2D single photon photolithography. Also, the light field 3D photolithography technique has the ability to pattern computer designed microscale 3D structures in a large area with fast speed. | |
