Since many years, laser testing has proved is efficiency as a complementary technique to particle accelerator to evaluate in much detail the sensitivity of chips or system to different kinds of SEE. Laser pulses and heavy ions have in common the capability to create a very high density of electron-hole pairs during a very short time inside a very small volume. The charge deposition radius of a laser may be significantly higher than the one of the low energy heavy ions found in particle accelerators but it is closer to the one of high energy ions found in space. The laser can only reproduce the effects triggered by charge collection inside the semiconductor, it cannot induce structural damage such as displacement damage or charge trapping into oxides.
The laser test is more interesting than particle test because of its spatial and temporal resolutions. We can control the locations where the laser pulses arrive onto the device under test and we can also control the time when the pulse arrives inside a clocked test vector. This way the laser test can provide sensitive areas mapping and sensitive temporal window. Moreover laser testing can discover the different kinds of SEE depending on the location on the component whereas particle accelerator test may miss some.
Our team studies these types of SEEs :
- Single Event Upset (SEU) and multiple upsets (MBU or MCU): These effects occur inside digital devices and consist in changes of logical states. Figure 1 presents a comparison between a SEU cross section of commercial SRAM memories obtained in particle accelerator and with a pulsed laser.
Figure 1: Laser (a) and particle accelerator (b) SEU cross sections
- Single Event Transient (SET): This is the most basic SEE, it occurs in any devices but is mainly studied inside analog components. To type of SET are distinguished, the Analog SET (ASET) occurring in linear devices and the Digital SET (DSET) occurring in logical devices. Figure 2 presents an example of an ASET study of an operational amplifier. The cartography is an image of the amplitude of the ASET for strikes locations all over the chips.
Figure 2 : SET sensitivity testing of an LM6181 operational amplifier, SET cartography (a) and SET diagram (b).
- Single Event Latchup (SEL): This is the destructive effect of the CMOS technologies and IGBTs. Following the deposition of charges, a parasitic thyristor is activated. This activation induces a current strong enough to permanently modify the semiconductor along its way, resulting in the appearance of a short cut inside the device. Figure 3 presents an example of an SEL laser sensitive mapping of a commercial SRAM.
Figure 3: SEL mapping of a CYC1069 CMOS SRAM at different scales. Red points the occurrence of a SEL.
- Single Event Burnout (SEB): This is a destructive effect for the power components. The charge deposition triggers a cumulative event involving parasitic structures and avalanche mechanisms. This results to a melding of the semiconductor. Figure 4-b presents a laser mapping of the sensitive volume of a power MOSFET. The laser also allows testing the SEB sensitivity of high bandgap components such as SiC power diode.
Figure 4: a- Sensitive volume of a cell of an IRFP360 power MOSFET for Vds=400 V and b- SEB threshold voltage versus the laser pulse energy during the test of a 600 V SiC power diode
The ATLAS platform allows testing most of the technologies. Laser testing allows to test the hardness of radiation tolerant components and to identify the potential improvements to increase it.
If the metal coverage on the chip is too dense, the laser tests are performed by the backside of the device.
Two kinds of tests are available:
- Single Photon Absorption (SPA) test: The laser wavelength is chosen so that each photon has enough energy to create an electron-hole pair.
- Two Photon Absorption (TPA) test: In this mode two photons have to be simultaneously absorbed to create one electron-hole pair. This approach allows a better containment of created charges along the three spatial directions. It is used to image sensitive volumes such as the one presented in figure 4.