Total Ionizing Dose Effect on Programmable Input Configurations

1
Total Ionizing Dose Effect on Programmable Input
Configurations

• J. J. Wang, R. Chan, G. Kuganesan, N. Charest, B.
  Cronquist
• Actel Corporation

2
Outline

• Total Ionizing Dose Testing
• Input Threshold TID Testing Data
• Annealing Effect
• Failure Analysis and Mechanism
• Lesson Learned and TID Hardening

3
Total Ionizing Dose Testing

• TM1019 Military Standard for TID testing (Fig 1)

Fig 1 TID testing flow
4
DUT and Irradiation

• 0.25µm CMOS technology
• Commercial off-shore foundry
• VCCI/VCCA 5V/2.5V
• TTL I/O configuration
• Defense Microelectronic Activity (DMEA)
• Co-60 Source
• Dose Rate 1 krad(Si)/min (5)
• Room temperature irradiation
• Static biased irradiation

Fig 2 Picture showing Gamma-ray irradiator
5
Parameter Measurement
Parameters Logic Design
1 Functionality All key architectural functions
2 ICC (ICCA/ICCI) DUT power supply
3 Input Threshold (VIL/VIH) Input buffers
4 Output Drive (VOL/VOH) Output buffers
5 Propagation Delay String of buffers, Clock to Q
6 Transition Characteristic D flip-flop output

• VIL defined as the start of low to high
  transition
• VIH defined as the start of high to low
  transition
• TTL trip point (average of VIL and VIH) 1.5V,
  CMOS 2.5V

6
Rad-induced Input Threshold Shift
Table 1 Lot B Pre- and Post-Irradiation VT (Net
0)
DUT Total Dose Pre-Irradiation Pre-Irradiation Post-Irradiation Post-Irradiation
DUT Total Dose VIL (V) VIH (V) VIL (V) VIH (V)
B1 100 krad 1.25 1.48 1.26 1.53
B2 100 krad 1.24 1.48 2.29 2.52
B3 100 krad 1.25 1.47 1.25 1.47
B4 100 krad 1.25 1.49 1.23 1.51
B5 100 krad 1.25 1.47 2.32 2.55

• Five (A, B, C, D, E in chronological order) lots
  from foundry X tested
• 2 lots (B, C) show VIL/VIH switching from TTL to
  CMOS
• The number of events increases with total
  accumulated dose and can be removed by annealing

Table 2 Lot C Pre- and Post-Irradiation VT (Net
0)
DUT Total Dose Pre-Irradiation Pre-Irradiation Post-Irradiation Post-Irradiation
DUT Total Dose VIL (V) VIH (V) VIL (V) VIH (V)
C1 60 krad 1.24 1.51 1.38 1.45
C2 60 krad 1.25 1.52 1.22 1.53
C3 60 krad 1.25 1.51 1.24 1.48
C4 100 krad 1.25 1.52 1.23 1.49
C5 100 krad 1.24 1.51 2.41 2.67
C6 100 krad 1.23 1.50 1.24 1.56
C7 100 krad 1.25 1.51 1.26 1.48
C8 100 krad 1.26 1.52 1.41 1.57
7
Post-Irradiation Input Threshold Switching from
TTL to CMOS

• Lot C is chosen for investigation
• More design nets are tested for post-irradiation
  input threshold
• Part to part and pin to pin dependence observed

Table 3 Lot C Post-Irradiation VT
DUT Total Dose Net 1 Net 1 Net 2 Net 2 Net 3 Net 3 Net 4 Net 4
DUT Total Dose VIL (V) VIH (V) VIL (V) VIH (V) VIL (V) VIH (V) VIL (V) VIH (V)
C1 60krad 1.43 1.46 1.38 1.44 1.39 1.46 NA NA
C2 60krad 1.39 1.5 1.41 1.47 1.41 1.49 1.21 1.56
C3 60krad 1.39 1.49 1.31 1.54 1.38 1.46 1.38 1.48
C4 100krad 2.59 2.63 1.42 1.48 2.57 2.64 1.4 1.55
C5 100krad 1.47 1.51 1.44 1.47 1.45 1.49 1.43 1.51
C6 100krad 1.39 1.53 1.35 1.49 1.38 1.5 1.4 1.53
C7 100krad 1.41 1.48 1.38 1.48 1.35 1.49 1.36 1.55
C8 100krad 1.42 1.52 1.43 1.49 1.42 1.49 1.41 1.49
8
Post-Irradiation Input Threshold Switching from
TTL to CMOS
Table 3 Lot C Post-Irradiation VT
DUT Total Dose Net 5 Net 5 Net 6 Net 6 Net 7 Net 7 Net 8 Net 8
DUT Total Dose VIL (V) VIH (V) VIL (V) VIH (V) VIL (V) VIH (V) VIL (V) VIH (V)
C1 60krad 1.39 1.45 1.38 1.46 1.38 1.45 1.39 1.45
C2 60krad 1.4 1.52 1.39 1.55 NA NA 1.38 1.44
C3 60krad 1.37 1.47 1.37 1.47 1.32 1.46 1.36 1.45
C4 100krad 1.37 1.49 1.43 1.49 1.41 1.49 2.61 2.66
C5 100krad 1.47 1.51 1.45 1.51 1.42 1.48 1.42 1.48
C6 100krad 1.39 1.49 NA NA 1.4 1.62 1.51 1.63
C7 100krad 1.38 1.45 1.43 1.51 1.31 1.56 1.4 1.48
C8 100krad 1.44 1.47 1.44 1.48 1.42 1.47 1.43 1.47
DUT Total Dose Net 9 Net 9 Net 10 Net 10 Net 11 Net 11
DUT Total Dose VIL (V) VIH (V) VIL (V) VIH (V) VIL (V) VIH (V)
C1 60krad 1.38 1.45 1.38 1.46 1.4 1.45
C2 60krad 1.38 1.45 1.22 1.58 1.41 1.49
C3 60krad 1.34 1.46 1.38 1.48 1.37 1.44
C4 100krad 1.41 1.47 1.38 1.54 1.36 1.46
C5 100krad 1.44 1.51 1.48 1.54 1.45 1.48
C6 100krad 1.37 1.49 1.37 1.57 1.37 1.48
C7 100krad 1.36 1.43 1.13 1.64 1.39 1.49
C8 100krad 1.42 1.49 1.43 1.5 1.42 1.48
9
Annealing Effect Experiment

• Five DUT from lot C are irradiated to 100 krad
  with a lower dose rate (1 krad/hr)
• No switching from TTL to CMOS observed

DUT Total Dose Net 0 Net 0 Net 1 Net 1 Net 2 Net 2 Net 3 Net 3 Net 4 Net 4 Net 5 Net 5
DUT Total Dose VIL (V) VIH (V) VIL (V) VIH (V) VIL (V) VIH (V) VIL (V) VIH (V) VIL (V) VIH (V) VIL (V) VIH (V)
C9 100krad 1.36 1.49 1.37 1.49 1.37 1.48 1.33 1.46 1.40 1.49 1.39 1.48
C10 100krad 1.36 1.47 1.42 1.52 1.40 1.48 1.38 1.49 1.43 1.51 1.41 1.50
C11 100krad 1.36 1.49 1.40 1.51 1.39 1.49 1.38 1.50 1.40 1.50 1.41 1.50
C12 100krad 1.36 1.61 1.39 1.50 1.38 1.48 1.35 1.48 1.43 1.52 1.33 1.51
C13 100krad 1.34 1.53 1.37 1.52 1.36 1.54 1.35 1.50 1.34 1.54 1.25 1.59
DUT Total Dose Net 6 Net 6 Net 7 Net 7 Net 8 Net 8 Net 9 Net 9 Net 10 Net 10 Net 11 Net 11
DUT Total Dose VIL (V) VIH (V) VIL (V) VIH (V) VIL (V) VIH (V) VIL (V) VIH (V) VIL (V) VIH (V) VIL (V) VIH (V)
C9 100krad 1.41 1.51 1.38 1.46 1.37 1.46 1.35 1.46 1.44 1.53 1.34 1.45
C10 100krad 1.41 1.51 1.38 1.47 1.39 1.49 1.37 1.49 1.42 1.52 1.38 1.46
C11 100krad 1.41 1.52 1.39 1.48 1.39 1.48 1.38 1.50 1.41 1.51 1.37 1.46
C12 100krad 1.41 1.52 1.38 1.48 1.20 1.59 1.35 1.53 1.43 1.53 1.38 1.47
C13 100krad 1.35 1.55 1.43 1.52 1.42 1.52 1.36 1.49 1.41 1.51 1.43 1.53
10
Focus Ion Beam Experiment

• The internal node that is suspected being pulled
  down by radiation-induced leakage is FIBed for
  microprobing
• However, the heat generated during the FIB
  process annealed the device and hence recovered
  the TTL input threshold from CMOS

11
Fabrication Process Dependence

• Foundry X show TTL to CMOS switching in 2 out of
  5 lots, more recent lots show no switching
• Foundry Y doesnt show TTL to CMOS switching in 3
  lots
• Variable material characteristics of the
  commercial foundry FOX (field oxide) determine
  the TID tolerance of this phenomenon

12
Configurable Input

• As shown in Figure below, a popular way to vary
  the input threshold is to change the strength of
  the pull-down by changing the turn-on number of
  NMOSFET pull-downs
• TTL (1.5V trip point) has more turn-on NMOSFET
  pull-downs than CMOS (2.5V trip point)

Fig 3 showing the simplified schematic of
configurable input
13
Failure Mechanism

• For testing the programmable switch, node X is
  holding high by a single weak pull-up for TTL
  configuration
• Radiation induced leakage in the NMOS pull-down
  device pulls node X down (after certain total
  dose) and switches the input configuration from
  TTL (trip point 1.5V) to CMOS (trip point 2.5V)

14
Physical Mechanism
The charge generation, transport and trapping in
a biased oxide layer. The primary effect in
sub-micron device is the hole trapping near the
Si/SiO2 interface.
15
Physical Mechanism
16
Physical Mechanism

• Total dose induced edge and field leakage

17
Lesson Learned and TID Hardening

• Accelerated testing overestimates the effects
  caused by radiation-induced field leakages
• Commercial foundries have variable FOX
  characteristics
• Weak pull-up is a weak spot for total dose effect
• Commercial design often is not perfectly
  radiation optimized due to time to market
  pressure
• Two design options
• Redesign the logic so there is no weak pull-up
• Re-layout the leaky NMOSFET to edgeless (shown
  below)