UNCLASSIFIED UNCLASSIFIED Measurement Considerations for Evaluating BTI Effects in SiC MOSFETs Daniel Habersat Aivars Lelis (TL), Ronald Green The Nation’s Premier Laboratory for Land Forces The Nation’s Premier Laboratory for Land Forces UNCLASSIFIED UNCLASSIFIED
UNCLASSIFIED Background Switching oxide trap, direct tunneling mechanism. SiO 2 SiC Defect is an E′ center (hole trap), associated with O E C vacancy Similar behavior as seen with ionizing radiation E FS on Si/SiO 2 . Presence of defect E V confirmed in SiC MOS Bias and temperature convert neutral Si-dimer to active E′ center Same as in Si/SiO 2 PBTI E FM ? Meta-stable high-temperature electron trap ? Counteracting mobile ion or polarization charge ? Indirect tunneling via interface traps The Nation’s Premier Laboratory for Land Forces UNCLASSIFIED
UNCLASSIFIED Typical PBS Response (simplified) • Representative pre- and pre-stress stress 1 post-stress I D - V GS 15 + stress Drain Current [mA] 2 1 2 • Charge-trapping-like 10 response: 5 • PBS shifts curves Vendor A positively 0 2.9 MV/s 3 3.5 4 4.5 • NBS shifts curves Gate-Source Bias [V] negatively 5.8 • Roughly linear-with-log Threshold Voltage [V] 5.6 time 5.4 5.2 Generally nonpermanent • 5 and reversible 4.8 Vendor B 4.6 Vendor A 4.4 4.2 … BUT: 10 ⁻⁶ 10 ⁻ ³ 10 ⁰ 10³ 10 ⁶ Stress Time [s] The Nation’s Premier Laboratory for Land Forces UNCLASSIFIED
UNCLASSIFIED Experimental: Measurement Considerations 20 V T drift results are highly Stress VGS [V] 15 10 dependent on the delay Pre-stress Recover 5 between stress and 0 Time measurement! V GS = +2 V V GS = +15 V 0.7 Data from Vendor A COTS 0.6 2 μ s measurement time 0.5 V T Drift [V] Increasing t S 0.4 Δ V T as measured 0.3 normally 0.2 0.1 2 s measurement time 0 10 -6 10 -4 10 -2 10 0 10 2 10 4 10 -6 10 -4 10 -2 10 0 10 2 10 4 t S [s] t M [s] Recovery during delay appears to be the dominant reason for differences between measurements. The Nation’s Premier Laboratory for Land Forces UNCLASSIFIED
UNCLASSIFIED HTGB Qualification Specifications AEC – Q101 Stress Test Qualification for Automotive Grade Discrete Semiconductors JEDEC JESD-22 A108E Reliability Test Methods for Packaged Devices MIL-STD-750 Test Methods for Semiconductors AEC Q101 – Rev D JESD-22 A108E “Electrical testing shall be completed as soon as possible and no longer than 96 hours after removal of bias from devices.” The Nation’s Premier Laboratory for Land Forces UNCLASSIFIED
UNCLASSIFIED Industry Needs and Existing HTGB Test Method for SiC MOSFETs Example of HTGB test procedure used by • Industry requires large-scale test manufacturers for qualification: systems for qualification testing and screening. • Devices stressed in parallel for 1000 • Many SiC manufacturers qualify hours. devices to Q101 automotive • Biased cool down to 25 ° C (1 min. bias standard. interruption allowed per JESD22 - A108C for moving devices to test area). AEC-Q101- Rev D1 Sept. 6, 2013 • Bias interruption times generally >> 1 minute in practice. • Devices can remain unbiased for up to 96 hrs. prior to electrical testing per JESD22 – A108C without any requirement to reapply bias stress. Type Test Description Condition • JESD22-A108C does have an additional stress requirement when devices have HTRB High 1000 hr. Temperature remained unbiased for longer than 96 Reverse Bias hrs. prior to electrical testing. HTGB High 1000 hr. Temperature Device • Serial testing leads to variation in Gate Bias interrupt/delay times High 1000 hr. HTOL Temperature Operating Life The Nation’s Premier Laboratory for Land Forces UNCLASSIFIED
UNCLASSIFIED Gate Biasing Sequence Stress Reapply Gate-Source Bias Interrupt (1) (2) (3) (4) Pre-stress Time [arb] The Nation’s Premier Laboratory for Land Forces UNCLASSIFIED
UNCLASSIFIED Sequential Charging of the Oxide Traps Trap Occupation Reapply Gate-Source Bias Stress Interrupt (1) (2) (3) (4) Measurement Static Prestress Oxide Bulk Interface Time [arb] Tunneling fronts move into • pre-stress interrupt reapply stress the oxide 1 3 4 15 + stress Drain Current [mA] 2 1 2 Bias determines resultant • interrupt 10 charge state 3 2 reapply • Time determines penetration 3 4 5 depth (log t ) Vendor A 0 2.9 MV/s Superposition of multiple • 3 3.5 4 4.5 fronts Gate-Source Bias [V] The Nation’s Premier Laboratory for Land Forces UNCLASSIFIED
UNCLASSIFIED Pre-stress Biasing Stress Reapply Gate-Source Bias Interrupt (1) (2) (3) (4) Pre-stress Pre-stress Time [arb] The Nation’s Premier Laboratory for Land Forces UNCLASSIFIED
UNCLASSIFIED Pre-stress bias (PBS) • Identical stressing 5.5 parameters Vendor B • Pre-stress time for roughly 5 the same time as the stress (2×10 4 s) Threshold Voltage [V] Three pre-stress biases: • 4.5 0V, -5V, and -10V (saturated by -15V) 0 V pre / +25 V stress 4 More negative biasing • – 5 V pre / +25 V stress – 10 V pre / +25 V stress causes: • Negative shift in original 3.5 V T • Reduced V T for first 100s or so 3 10 ⁻⁶ 10 ⁻ ³ 10 ⁰ 10³ Positive Bias Stress Time [s] The Nation’s Premier Laboratory for Land Forces UNCLASSIFIED
UNCLASSIFIED Pre-stress bias (NBS) 5.5 • Identical stressing parameters Vendor B • Pre-stress time for roughly 5 +10 V pre / – 15 V stress the same time as the stress +5 V pre / – 15 V stress (2×10 4 s) 0 V pre / – 15 V stress Threshold Voltage [V] Three pre-stress biases: • 4.5 0V, +5V, and +10V (no saturation seen to +25V) 4 More positive biasing • causes: 3.5 • Positive shift in original V T • No significant changes 3 10 ⁻⁶ 10 ⁻ ³ 10 ⁰ 10³ in post-NBS VT values Negative Bias Stress Time [s] The Nation’s Premier Laboratory for Land Forces UNCLASSIFIED
UNCLASSIFIED Pre-stress Bias (Drift) – 10 V pre / +25 V stress 0 V pre / – 15 V stress • Interpretation of results usually – 5 V pre / +25 V stress +5 V pre / – 15 V stress based on drift values 0 V pre / +25 V stress +10 V pre / – 15 V stress PBS Drift: 2 • Early times appear identical • 1.5 Vendor B Dispersion in final drift • values based on pre-bias 1 magnitude (larger pre-bias -> Threshold Voltage Drift [V] larger drift) 0.5 0V device seems to have • lower density of switching 0 oxide traps NBS Drift: • -0.5 • Drifts are parallel Magnitude of drift correlates • -1 with pre-bias magnitude • 0V device seems to have a -1.5 lower density of fast interface states -2 10 ⁻⁶ 10 ⁻ ³ 10 ⁰ 10³ Stress Time [s] The Nation’s Premier Laboratory for Land Forces UNCLASSIFIED
UNCLASSIFIED Sweep Speed Stress Reapply Gate-Source Bias Interrupt (1) (2) (3) (4) Sweep Speed Pre-stress Time [arb] The Nation’s Premier Laboratory for Land Forces UNCLASSIFIED
UNCLASSIFIED Sweep Speed (PBS) • Already know that faster 5 sweeps larger drift • With negative pre-stress bias, reference VT was 4.5 independent of slew rate Threshold Voltage [V] (???) 4 Faster sweep capture of • more near-interface states Vendor A Growth of VT (i.e. drift) is • 3.5 Slew Rate [V/s] roughly parallel for longer 2 x 10 ⁶ (5 µs) 2 x 10 ⁵ (50 µs) times 2 x 10 ⁴ (500 µs) 4 x 10³ (2.5 ms) 3 • As stress’ tunneling front moves deeper, measurement speed 2.5 matters less 10 ⁻⁶ 10 ⁻ ³ 10 ⁰ 10³ Stress Time [s] The Nation’s Premier Laboratory for Land Forces UNCLASSIFIED
UNCLASSIFIED Sweep Speed (NBS) • Contrast NBS to PBS: 5 Reference VT IS slew-rate • dependent Slew Rate [V/s] 4 x 10³ (2.5 ms) Drift rate (slope) is also slew- • 2 x 10 ⁴ (500 µs) 4.5 rate dependent 2 x 10 ⁵ (50 µs) 2 x 10 ⁶ (5 µs) • Why are the responses so Vendor A different between PBS and NBS? Threshold Voltage [V] • ¯\_( ツ )_/¯ 4 Under investigation • ? Combination of interactions between pre-stress bias, 3.5 stress bias, and portions of the IV sweep before and after the VT measurement 3 ? Initial response to NBS is very rapid, while PBS tends to be slower… ? PBS: (Np/nPp/nPp /…) 2.5 ? NBS: (Pn/pNn/pNn /…) 10 ⁻⁶ 10 ⁻ ³ 10 ⁰ 10³ Stress Time [s] The Nation’s Premier Laboratory for Land Forces UNCLASSIFIED
UNCLASSIFIED Interrupt Time Stress Reapply Gate-Source Bias (1) (2) (3) Interrupt Pre-stress Time [arb] The Nation’s Premier Laboratory for Land Forces UNCLASSIFIED
UNCLASSIFIED Interrupt Time Study the effects of • pre-stress interrupt stress interrupt time, for relatively 1 3 15 + stress Drain Current [mA] 2 “long” stresses 1 2 interrupt 10 3 2 Drift between the two I-V • curves immediately before 5 and after the bias Vendor A interruption 0 2.9 MV/s 3 3.5 4 4.5 With interrupt time << • Gate-Source Bias [V] stress time, we get a recovery in the VT drift that 0 Vendor A Threshold Voltage Drift is independent of stress 1.3 MV/s -0.1 Stress Time [s]: -0.2 time 3.2 x 10¹ -0.3 1.0 x 10² • Only affecting the traps -0.4 [V] 3.2 x 10² 1.0 x 10³ nearest to the interface -0.5 3.2 x 10³ -0.6 • Deeper traps remain -0.7 undisturbed -0.8 10 ⁻⁶ 10 ⁻ ³ 10 ⁰ 10³ Interrupt Time [s] The Nation’s Premier Laboratory for Land Forces UNCLASSIFIED
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