Integrated Circuits and Materials

Electro-optical terahertz pulse reflectometry (EOTPR) is a technique used for characterizing the properties of materials, particularly semiconductor structures, at terahertz frequencies. Terahertz radiation lies in the electromagnetic spectrum between microwave and infrared radiation, typically ranging from about 0.1 to 10 terahertz (THz).

Classic TDR systems use an electrical pulse generator and oscilloscope to measure the reflected electrical signals as they travel through a transmission line or cable. The core mechanism is the reflection of an electrical pulse due to impedance discontinuities, not optical signals. However, in advanced variations of TDR, such as Electro-Optical Terahertz Pulsed Reflectometry (EOTPR), lasers can be included in the setup. EOTPR uses a laser source to generate high-speed optical pulses, which are converted into electrical signals. These signals are transmitted into the device under test (DUT) and the reflected signals are analyzed. Therefore, EOTPR combines optical and electrical techniques to generate and detect terahertz pulses. That is, the basic setup involves generating short optical pulses using a mode-locked laser, typically a femtosecond laser, and then converting these optical pulses into terahertz radiation using a photoconductive antenna or other terahertz emitter. The terahertz pulses are then directed towards the sample under investigation. This approach provides higher resolution compared to traditional TDR, which is especially useful for modern semiconductor packaging and 3D structures like through-silicon vias (TSVs)​.

Figure 2014 shows the schematic diagram of an EOTPR system. The EOTPR system consists of several critical components that work together to isolate defects in semiconductor devices. The system begins with a laser source, which generates a high-speed optical pulse. This pulse is directed to a transmitter photoconductive switch (PCS), which converts the optical signal into an electrical transient signal. The generated electrical signal then travels through a high-bandwidth TDR probe into the Device Under Test (DUT). Similar to conventional TDR, the traveling electrical signal carries continuity information of the structures within the DUT. As the signal reflects back, it can be reconstructed and analyzed, providing insights into the continuity and integrity of the internal structures of the device. The EOTPR system offers a significant advantage over conventional TDR by achieving higher resolution, capable of isolating defects down to less than 10 μm.

When the terahertz pulses interact with the sample, they are partially reflected back. The reflected pulses are then detected using a photoconductive antenna or other terahertz detector. By analyzing the time delay and intensity of the reflected pulses, researchers can extract information about the electrical properties, such as carrier concentration and mobility, as well as the structural properties, such as thickness and composition, of the sample. 

EOTPR has applications in various fields including semiconductor device characterization, material science, and non-destructive testing. It offers advantages such as high sensitivity, non-contact measurement, and the ability to probe materials that are opaque to visible and infrared light. 

[1] Jiann Min Chin, Vinod Narang, Xiaole Zhao, Meng Yeow Tay, Angeline Phoa, Venkat Ravikumar, Lwin Hnin Ei, Soon Huat Lim, Chea Wei Teo, Syahirah Zulkifli, Mei Chyn Ong, Ming Chuan Tan, Fault isolation in semiconductor product, process, physical and package failure analysis: Importance and overview, Microelectronics Reliability 51, 1440–1448, 2011.
[2] Diagram Courtesy of Teraview Limited.