Short-pulse terahertz waves technology used for chip circuit analysis

The new technology overcomes the technical obstacles and prohibitive cost of existing technologies, and will contribute significantly to the development and wider adoption of these leading-edge devices.

The popularity of smartphones and other mobile devices has driven enormous increases in wireless communications traffic, which now threatens to overwhelm the capacity of currently assigned frequencies. Hence, worldwide R&D efforts have begun to focus on the sub-terahertz band, a higher frequency range which has not been used for wireless communications to date.

In high-frequency device development, it is crucial to evaluate the frequency characteristics of the overall system, including active device gain and input and output impedance, as well as the board and connectors.

Part of this process is measurement of the reflection and transmission characteristics of the amplitude and phase of signals emitted, known as S-parameters or scattering parameters. However, existing network analysers can only measure frequency ranges up to 100GHz wide at one time, so when the signal characteristics of broader ranges must be evaluated, engineers have to repeatedly change the configuration of their equipment and measure again.

This causes extra work, longer measurement times, and discontinuities in measured data. Measurement costs also rise proportionately to these drawbacks.

This new technology promises to reduce these burdens significantly. It employs a femtosecond optical pulsed laser as a signal source, enabling one-pass measurement of S-parameters up to 1.5THz with a broadband optical/electrical switching probe. The benefits of these efficiency gains will accrue to users in terms of time, labor, and cost savings.

Although continued shrinks of semiconductor circuits have facilitated generations of smaller, faster consumer electronics, Moore’s law is in danger of hitting a technological wall. To circumvent the physical limits of miniaturization, chipmakers are developing 3D semiconductors with multiple layers of circuits in a single package.

However, wiring failure analysis is a major challenge in 3D chip development. With multiple boards stacked on top of each other, it is difficult to identify where wiring failures (open circuits, short-circuits, impedance mismatching) have occurred with X-ray inspection and other existing technologies. Generally, oscilloscope TDR (time domain reflectometry) and/or TDT (time domain transmissometry) is used to pinpoint these failures, but at these tiny geometries, extremely high spatial resolution is a must.   

Because Advantest’s new technology uses a femtosecond optical pulsed laser as a signal source, it achieves superior spatial resolution of less than 5μm and a maximum measurement range of 300mm. With a successful track record of usage in the company’s terahertz spectroscopic and imaging systems, Advantest’s femtosecond optical pulsed laser boasts extremely high resolution. Moreover, the new technology provides a mapping function which can pinpoint the location of wiring failures on the device’s CAD data, making it an optimal tool for finding flaws in extremely complex, high-density circuits.