X-ray and Proton Radiation Effects on 40 nm CMOS Physically Unclonable Function Devices

I would also like to thank AFRL and AFOSR through the Hi-REV program and the Defense Threat Reduction Agency through its Basic Research Program for supporting this project. Current from the input flows through the pFET selector and then through the parallel combination. The resistance of a broken nFET is much lower than that of a continuous nFET, so before and after proton irradiation almost all current flows through the broken nFET.

While the majority of current still flows through the broken nFET after proton irradiation, an increasing amount of leakage current flows through the intact nFET, which shares a common body connection with the broken nFET ..43 Fig.

Overview of Space Radiation Environments

Van Allen is credited with the discovery of the trapped proton and electron regions around the Earth. The tilt of the Earth's magnetic pole forms the geographic pole and the displacement of the magnetic field from the center causes a dip in the field over the South Atlantic, causing a bulge in the bottom of the inner belt. The trapped particles' levels and locations are highly dependent on particle energy, altitude, inclination and the activity level of the sun and are highly dynamic.

Transient particles penetrate the Earth's magnetosphere and they can reach spacecraft in near-Earth orbit and are particularly dangerous for satellites in polar, highly elliptical and geostationary orbits (GEO) [1]-[6].

Total Ionizing Dose Effects

1-2 Schematic of an n-channel MOSFET illustrating radiation-induced gate oxide charging: (a) normal operation and (b) after irradiation. Electrons are more mobile than holes [7]-[8] and are swept out of the oxide in a picosecond or less. The second process is the transport of holes to the Si/SiO2 interface, which causes a short-term recovery of the threshold voltage.

A third process is that a fraction of the transport holes fall into relatively deep long-lived trap states.

Fig. 1-2 Schematic of n-channel MOSFET illustrating radiation-induced charging of the gate oxide: (a) normal operation and (b) post-irradiation

Displacement Damage

The test structure of the unit cell of the BD-PUF is shown in Fig. This leads to the overall drop in the discharge current of the BD-PUF in Fig. Although this does not significantly affect the measured discharge current of the BD-PUF. BD-PUF in fig.

ID-VD curves of pFET selector and nFET load line as a function of conductance.

Physically Unclonable Function (PUF)

Information Security

For each instantiation of the primitive, a secure key can be generated, which is random, unique, and unpredictable. The key can be assigned, stored, and retrieved by the instantiation without being revealed. We need to develop physical techniques and primitives based on physical reasoning, which we can trust to withstand certain physical attacks and therefore provide certain physical security objectives.

How information security goals can be achieved through physical security and eventually physical roots of trust is shown in the figure.

Fig. 2-1 Relationships between information security, cryptography, physical security and physical roots of trust

PUF Concept

Types of PUFs

In practice, a high voltage is applied to the gates of the nFETs by activating the pFET compliance transistor. Less than 11% change in current ratio at 1.2 V was observed with low dose X-ray exposure, i.e., the BD-PUF stability is not significantly affected by X-ray irradiation. 4-4(a) shows ∆VBD as a function of flux and the percentage decrease of the BL current magnitude.

For the readout condition, the input voltage was 1.2 V and the remaining connections of the BD-PUF were grounded during irradiation, as shown in figure. For most applications, BD-PUFs are in the standby state for a significant period of time. of the operational time. 4-8(a) shows the measured electrical response of the BD-PUF before and after various proton exposures up to 3 x 1014 cm-2 fluence.

4-13 shows a schematic diagram of the current through the PUF after its formation stage and proton irradiation. Current from the input flows through the pFET selector and then through the parallel combination of the broken and unbroken nFETs. 4-10(a) shows that the current through the pFET selector decreases with proton fluence, as a result of radiation-induced build-up.

4-14 shows the ID-VD characteristics of the selector and the IG-VG curve (load line) of the broken nFET as a function of proton flow. The current through the pFET selector decreases after proton irradiation, as a result of irradiation-induced charge accumulation and the corresponding V-th negative shift.

Experimental Details of BD-PUFs

Device Information

3-1(a), and the unit cell is designed in an array configuration as shown in Fig. The unit cell of the examined BD-PUF consists of two minimally sized nFETs, each with shorted source and drain, and a pFET selector [32]. A shaping step is used to create the PUF unit by (random) breakdown of the gate dielectric in one of the two nFETs.

The high voltage applied to the nFET gate generates random defects in the gate oxide until a hard fault occurs. As soon as one of the nFETs experiences breakdown, the current through the destroyed oxide will create a voltage drop on the pFET selector, which now acts as a compliance FET in saturation mode [33], limiting the stress voltage and current. The breakdown path in the broken nFET will wear further under this condition in a current-limited manner [34].

However, the intact nFET will not accumulate additional damage in this phase due to.

DC Characteristics

Breakdown can be randomly generated on one of the nFETs. b) A 60-cell matrix fabricated using a commercial CMOS process. Breakdown in a BL nFET is represented by a logic "0;" conversely, if a failure occurs on the nFET, it is represented as a logic "1". After the design process, the current reading is taken by shifting the input voltage from 0V to 1.5V, with all other terminals grounded.

Fig. 3-2 HP4156 semiconductor parameter analyzer.

X-ray Irradiation

Proton Irradiation

Below a fluence of 3 x 1013 cm-2 there is no radiation-induced change in the input and BL currents. While the majority of the current still flows through the broken nFET, an increasing amount of leakage is observed in Fig. 4-11(b) after proton irradiation through the intact nFET, which shares a common body connection with the broken nFET.

4-3(b) and the increased off-state leakage of the intact nFET therefore suggest that a small percentage (~ . 0.1 to 1% in this case) of the total current flows through the body-to-S/ The D contacts of the intact nFET at the highest observed proton fluence. The voltage drop across the pFET selector increases as the fluence becomes larger, and the maximum voltage drop is 0.11 V at a fluence of 1014 cm-2, which leads to the observed drop in the discharge current of the BD-PUF. For the readout mode of BD-PUFs, the pFET selector is on during proton exposure and large current flows through the pFET selector and nFETs.

The leakage current through the uninterrupted nFET does not change significantly due probably to the large current through the. The leakage current through the uninterrupted nFET increases due to probably the same reason as the grounded condition we discussed previously. Because the non-ionizing energy loss of 1.8-MeV protons is much higher than that of the higher-energy protons that typically lead to the degradation in space systems, the equivalent displacement damage doses in this study are quite high compared to most realistic space- environments [ 36].

The breakdown current through the broken nFET decreases, and the leakage current through the unbroken nFET increases. The radiation response of the broken and unbroken nFETs indicates that a small percentage (~0.1 to 1% in this case) of the total current flows through the body-to-S/D contacts of the unbroken nFET at the highest observed proton fluence .

Fig. 3-6 Pelletron accelerator at Vanderbilt University.

Radiation Effects on BD-PUFs

X-ray Irradiation Response

One BD-PUF has three states in the circuits: grounded, read state, and standby state. Note that the current, which is quite small, is plotted in logarithm for visibility. For a fixed value of IBL, the corresponding input voltage increases as the fluence is higher.

The memory ratio (IBL/IBL-bar) is a key application parameter to distinguish between "0" or "1", it is extracted. Similar to what is observed for ∆VBD, the memory ratio between IBL and IBL-bar at 1.2 V decreases with fluence, then partially recovers at room temperature and finally returns to its original value after annealing. 4-4 (a) Change in input voltage when BL current is -0.2 μA and percent decrease in BL current at 1.2 V, and (b) memory ratio as a function of fluence and annealing time.

Keeping the word line voltage equal to the input voltage turns off the pFET selector. It should be noted that the BL current, which is quite small compared to current, is plotted on a log scale for visibility. 4-9 plots the memory ratio of current to BL current at 1.2 V decreases with fluence, then partially recovers at room temperature and finally returns to the original value after.

4-8 (a) Current readout of BD-PUF and (b) BL current before and after 1.8 MeV proton irradiation.

Fig. 4-1 Input/output currents vs. input voltage for (a) 20 cycles, demonstrating little cycle-to- cycle-to-cycle variation, and (b) a BD-PUF at different TID levels for 10-keV X-ray irradiation

Selector-transistor and Leakage Effects

Off-state leakage currents of both the broken and unbroken nFET increase as the fluence becomes larger. An obvious reason why off-state leakage currents may increase for either the broken or intact nFET is increased gate leakage current due to proton-induced defect formation [37], [38]. 4-12 show IG-VG characteristics for (a) broken and (b) unbroken nFET, and in neither case does the gate leakage current change significantly with proton irradiation.

Semi-log plots of ID – VG curves for (a) the interrupted nVET, and (b) the uninterrupted nVET as a function of fluency. IG – VG curves for (a) the interrupted nVET, and (b) the uninterrupted nVET as a function of fluency. While the majority of current still flows through the broken nFET after proton irradiation, an increasing amount of leakage current flows through the unbroken nFET, which shares a common body junction with the broken nFET.

For the bias condition with the pFET selector during irradiation, the source/input voltage is 1.2 V, the gate/WL voltage is 0 V, and the drain voltage is approximately 1.1 V according to the load line in Fig. For the standby condition of BD-PUFs during exposure, the pFET selector was biased with the source/input voltage of 1.2 V, the gate/WL voltage of 1.2 V, and the drain voltage of approximately 0 V. In addition, the voltage drop takes across the pFET selector as the flow increases, which also leads to the observed drop in breakdown current.

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