Electro-optical terahertz pulse reflectometry (EOTPR), an electro-optical system driven by an ultrafast laser source, was introduced recently to isolate faults and detect defects in advanced IC packages. The steadily growing complexity of integrated-circuit (IC) technology is pushing the available inspection and fault-analysis tools to their limits. Among the nondestructive methods, time-domain reflectometry (TDR) is considered to be an exceptionally fast method for fault detection in electronic packages. The EOTPR system provides 10µm in distance accuracy that can be used to localize package-level open and short failures non-destructively. Today’s TDR systems are generally all electronic and use a step or pulse generator and a high-bandwidth oscilloscope as the main components. The electromagnetic signal is transmitted through a high-frequency cable and a probe tip to the device under test (DUT). Every discontinuity at the transmission path within the DUT causes a part of the injected signal to be reflected. By monitoring these reflections in the time domain, it is possible to detect structural defects and distinguish functional from defective structures. Terahertz technology enables high-resolution fault isolation in advanced semiconductor packages and 3D imaging in integrated circuit devices. An EOTPR system was used to determine the location of open and short failures and to identify impedance variations in a series of package substrates. The experimental results demonstrate the higher accuracy of the EOTPR system in determining the distance to defect compared to traditional time-domain reflectometry systems.
The first femtosecond-laser-drive optoelectronic TDR system for fault isolation was recently introduced by Intel (Santa Clara, CA). With this system, a substantial improvement over all electronic systems was achieved with τrise= 5.7ps. However, a main bottleneck within this system, which prevents utilizing the full benefits of optoelectronic sub-picosecond-range switching speeds is brought about due to the waveguide and probe components that are required for signal transmission between the optoelectronic pulse-generation and detected devices and the DUT. Frequencies above 110 GHz are not transmitted by these components, and hence the majority of resolution power is lost within the TDR system. Nagel et al.. Opt. Expr…19, 12509-12514 (2011) and using Terahertz reflectometry imaging to find faults in silicon chips have recently demonstrated this bottleneck can now be effectively eliminated by novel micro-machined probe tips applied for broadband photoconductive (PC) pulse generation and detection immediately at the DUT. A TDR signal rise time of τrise = 1.1ps has been achieved by this advanced optoelectronic system. In contrast to earlier configurations, the probes can also be used in a contact-free mode through capacitive probe/wave guide coupling.