Ultrafast Spectroscopy to Advance Microelectronics

Continued advancement in microelectronics, including analog and digital electronics, power electronics, optics and photonics, and micromechanics for memory, processing, sensing, and communications as defined by the OSTP “National Strategy on Microelectronics Research,” requires knowledge of material properties such as conductivity, dielectric permittivity, and thermal conductivity. As devices are improved and enter more extreme regimes of frequency, bandwidth, voltage, and power handling, these material properties must be understood in ranges where they have often not been measured. For example, rapidly varying electronic signals are affected by the dielectric function at high frequencies where they are poorly understood. In addition, modern heterogeneous integrated devices combine multiple functions in one package and can include many interfaces that affect device performance. For example, thermal spread in complex multi-leveled packaged environments involves the flow of heat across nanoscale interfaces and propagates to orders of magnitude larger areas. This program aims to produce trusted spectroscopic reference data on material properties relevant to advanced microelectronics and integrated optics. A secondary goal is to develop novel techniques for measuring these properties more precisely and with sufficient spatial resolution to allow measurements on heterogeneous devices.

Properties of interest include conductivity spectra in the THz range, mobility, and electro-optic/nonlinear optical coefficients. Conventional conductivity techniques require making ohmic electrical contact to samples, which is often difficult in novel materials and cannot provide THz frequency dependent data. THz spectroscopic approaches do not require contacts and measure conductivity in a frequency range that is directly relevant to future high frequency electronics (beyond 5G). Electro-optic coefficients are needed for optoelectronic applications necessary for fiber-based data transmission such as modulators in hybrid optoelectronic platforms. Nonlinear optical coefficients are also needed to optimize integrated optical devices and laser-based material processing techniques, such as laser-writing of optical waveguides in glass substrates for optical interconnects. These measurands are all relatively difficult to measure accurately and our results will impact industry by serving as standard measurements and demonstrating best practices.

The program develops and utilizes advanced ultrafast spectroscopic techniques for metrology. Optical pump, THz probe spectroscopy is used to measure carrier conductivity, mobility and decay mechanisms of semiconductor materials without placing contacts on the material. Phase-sensitive pump-probe spectroscopy is used to measure electro-optic and nonlinear coefficients over a wide spectral range in optoelectronic materials. Optical two-dimensional coherent spectroscopy is used to fully characterize a material's nonlinear response, including dynamics, loss mechanisms and coherent processes relevant to all-optical devices.