May_EDFA_Digital

edfas.org ELECTRONIC DEV ICE FA I LURE ANALYSIS | VOLUME 23 NO . 2 6 and high bandwidth path for communication between the interposeddies. The interposeddiesmay be fabricated by different processes and can even be completely differ- ent in the way they function. The dies may include but are not limited to FPGA, memory units, MEMS sensors, transceiver modules, power management blocks, and crypto module. OPTICAL ATTACK TECHNIQUES This sectiondescribes different types of optical attacks. All optical attacks are based on a well-known approach, the photons with wavelengths in near-infrared (NIR) can pass through the silicon substrate. Therefore, NIR photons can be actively injected or passively observed through the chip backside for probing the logic gates or stimulatedeviceparameters for exposing the assets stored in the chip. Optical inspection/attack techniques can be categorized into three major classes depending on the stimulation method. PHOTON EMISSION ANALYSIS (PEA) PEA is primarily developed for functional analysis and fault localization on silicon die without any external stimuli. During IC operation, hot-carrier luminescence coincides with the switching activities of the logic gates, which are directly related to their respective logic states. The current carriers gain kinetic energywhen theMOSFET transistor’s operation region switches to a saturated state. Then, the energy of the carriers is released in the form of emittedphotons at the drain edge of the transistor, i.e., the pinch-off region of the transistor’s space-charge region. In PEA, photons are collected with a detector and a 2D image mapping the locations of the emitted photons is produced for analysis (see Fig. 3a). In addition, temporal information of the signal propagating through the chip can be detected if PEA is incorporated with picosecond image circuit analysis (PICA). [2,15] The photon emission (PE) intensity depends on the applied voltage and switching frequency of the transistor. (a) Because hot-carrier luminescence is best detected when switching activity occurs in transistors with high electron mobility, PE is more prominent in n-type transistors than p-type transistors. [2,16] Such data-dependency in emission can be a source of side-channel information for an adver- sary to extract. ELECTRO-OPTICAL/LASER-VOLTAGE TECHNIQUES Electro-optical techniques are an active approach for optically probing the transistor state through two well-known methods–electro-optical probing (EOP) and electro-optical frequency mapping (EOFM). [6,17] A laser stimulus is focused on a transistor. The laser gets reflected fromthe interface of the silicon substrate andactive region of the MOSFET. The reflected laser beam amplitude and phase get modulated by the electric field at the drain of the transistor. The amplitude of the reflected beam can probe the logic gates state through EOP analysis. Though EOFM and EOP use the same effect of reflected beam modulation, EOFM collects data in the frequency domain, compared to EOP, where data is collected in the time domain. Another difference between EOFM and EOP is that the focus of the prior one is not bound to a single transistor, rather transistors in a region-of-interest (ROI) operating at a frequency-of-interest (T activity frequency in Fig. 3b). The reflected beam from the ROI is filtered with a narrow bandpass filter whose center frequency is set to frequency-of-interest. The filtered signal is then fed to a computer for generating a 2D activity mapping of the ROI and the nodes operating at the frequency-of-interest. LASER FAULT INJECTION (LFI) Unlike PEA and EOFM/EOP, LFI allows an adversary to perturb the logic state of the circuit. IR laser is applied from the chip backside in the laser stimulationmethod to induce perturbation in the circuitry (see Fig. 3c). A laser with higher or equal photon energy than silicon bandgap energy (1.1eV), i.e., a laser with a wavelength less than Fig. 3 (a) Detector captures the photons emitted during switching of N-type MOSFET and generates a 2D mapping of activity. (b) Optically probing a transistor operating at T activity frequency using EFOM/EOP. (c) Device parameters are observed during laser stimulation technique. Depending on the application, laser directed for a specific transistor (for LFI) or a region of interest (for thermal laser stimulation). (b) (c)