Spaceflight system electronic devices must survive a wide range of radiation environments with various particle types including energetic protons, electrons, gamma rays, x-rays, and heavy ions. High-energy charged particles such as heavy ions can pass straight through a semiconductor material and interact with a charge-sensitive region, generating a significant amount of charge (electron-hole pairs) along their tracks. These excess charges can damage the device, and the response can range from temporary perturbations to permanent changes in the state or performance. These phenomena are called single event effects (SEE).
Before application in flight systems, electronic parts need to be qualified and tested for performance and radiation sensitivity. Typically, their susceptibility to SEE is tested by exposure to an ion beam from a particle accelerator. At such facilities, the device under test (DUT) is irradiated with large beams so there is no fine resolution to investigate particular regions of sensitivity on the parts. While it is the most reliable approach for radiation qualification, these evaluations are time consuming and costly. There is always a need for new cost-efficient strategies to complement accelerator testing: pulsed lasers provide such a solution.
Pulsed laser light can be utilized to simulate heavy ion effects with the advantage of being able to localize the sensitive region of an integrated circuit. Generally, a focused laser beam of approximately picosecond pulse duration is used to generate carrier density in the semiconductor device. During irradiation, the laser pulse is absorbed by the electronic medium with a wavelength selected accordingly by the user, and the laser energy can ionize and simulate SEE as would occur in space. With a tightly focused near infrared (NIR) laser beam, the beam waist of about a micrometer can be achieved, and additional scanning techniques are able to yield submicron resolution. This feature allows mapping of all of the sensitive regions of the studied device with fine resolution, unlike heavy ion experiments. The problematic regions can be precisely identified, and it provides a considerable amount of information about the circuit. In addition, the system allows flexibility for testing the device in different configurations in situ.
JPL has built and tested a pulsed laser facility with a mode-locked Ti:sapphire cavity pumped by a 5-W diode-pumped solid-state laser at 532 nm. A laser beam with a pulse width of about 2 picoseconds is tightly focused through a microscope objective onto the DUT. The Ti:sapphire has a wavelength range of 720–850 nm, with average power ranging from about 500m W to as high as 1 Wat 780 nm. The cavity outputs an 80-MHz repetition rate of pulses, which are precisely selected by an acoustic-optical modulator (AOM). The beam power can be attenuated to deliver calibrated energy to the DUT, typically chosen in the range of 1 to 500 pJ. When focused to a micron-scale waist, the electromagnetic radiation intensity ionizes the silicon medium to a penetration depth determined by the wavelength, and induces SEE within the device medium, simulating a heavy ion penetrating into the part.
The laser system utilizes the tunable pulsed source along with a HeNe laser continuous wave source at 635 nm for alignment guidance. It also incorporates a motorized two-axis stage to move the DUT and scan the area with resolution down to 100 nm. With automated computer control of the pulses, stage, and digital oscilloscope, the system can record the voltage transient responses as it steps through a pre-set scanning area, thereby generating a functional map of the device sensitivity with extremely fine resolution.
While it cannot fully replace heavy ion testing, this pulsed laser technology is a necessary complementary tool that helps the radiation hardness assurance flow when qualifying electronics for space applications. It also offers high-resolution imaging of the device sensitivity to localize problematic areas of the integrated circuits under test. This spatial and temporal information allows parts to be analyzed and issues to be diagnosed for research and development, leading to more robust and better-performing electronics for space applications.
This work was done by David C. Aveline, Philippe C. Adell, Gregory R. Allen, Steven M. Guertin, and Steven S. McClure of Caltech for NASA’s Jet Propulsion Laboratory. NPO-47254