Electrochemical Oxidation, Threading Dislocations and the Reliability of GaN HEMTs

Transcription

1 Electrochemical Oxidation, Threading Dislocations and the Reliability of GaN HEMTs Carl V. Thompson 1,3 Dept. of Materials Science and Engineering, M.I.T. Primary collaborators: Wardhana A. Sasangka 1, Govindo Syaranamual 1,2, Chee Lip Gan 1,2, Tomas Palacios 3, Feng Gao 3, Jesus Del Alamo 3, Jongwoo Joh 3, Prashanth Makaram 3 1 Singapore MIT Alliance for Research and Technology, 2 Nanyang Technological University, 3 M.I.T. 1

2 Outline Why HEMTs? Why GaN? Why GaN on Si? Physical degradation: pit formation Effects of dislocations SiN density passivation Modeling and reliability projections 2

3 GaN HEMT Structure SiN passivation GaN cap nucleation or buffer layer (AlN or AlN/GaN superlattice) source gate GaN Au Ni substrate (Al 2 O 3, SiC or Si) drain e.g. Au/Ni/Ti Al x Ga (1 x) N 2D electron gas 3

4 2D Electron Gas Al x Ga 1 x N spontaneous polarization and piezoelectric polarization due to misfit strain E c E F Ec Quantum well: 2D electron gas Al x Ga 1 x N GaN extrinsic GaN no impurity scattering in quantum well high electron mobility high switching frequencey 4

5 Materials Properties and Performance Si GaAs 4H SiC GaN Diamond Band Gap E g (ev) Breakdown Field E br (MV/cm) Electron Mobility cm 2 /Vs Saturation velocity v sat (10 7 cm/s) Johnson Figure of Merit (Bulk) 2000 (2DEG) Johnson Figure of Merit for Power Transistor Performance 2 (power frequency limits) after Mishra et al, Proc. IEEE 96, p287 (2008) 5

6 Current Target Markets Output Power Frequency on si power transistors french lab leti / 6

7 Performance Based on Materials Properties post/2018/02/25/the Case for Wide Bandgap Semiconductors 7

8 Cost GaN on SiC solid symbols Todays costs: dashed lines expected trends after Zhang, A. Dadgar and T. Palacios, J. Phys. D: Appl. Phys. 51, (2018) 8

9 Substrate and Threading Dislocation Density lattice misfit %* delta thermal expansion coefficient (x 10 6 )* approximate** threading dislocation density cm 2 GaN on sapphire ~ ** GaN on SiC ~10 7 *** GaN on Si ~10 8 to 10 9 **** GaN on Si has about 100x higher dislocation density than GaN on SiC * Kuramta et al, Fujitsu Sci. Tech. J. 342, p.191 (1998), ** HY Shih et al, Scientific Reports 5, (2015), *** Faleev et al, JAP 98 (2005), **** L Zhang et al,semicond. Sci. Technol (2017) 9

10 Two Components of I D,sat Degradation Permanent physical damage Current collapse due to filling of trap states Jungwoo Joh, Jesus A. del Alamo, et al, Microelectronics Reliability 51 (2011)

11 Physical Degradation: Pits and Cracks Cross Sectional TEM Images Correlation with Electrical Degradation Jungwoo Joh, Jesus A. del Alamo et al, Microelectronics Reliability 51 (2011) Uttiya Chowdhury, Jungwoo Joh, and Jesus A. del Alamo et al, EDL 29, 108 (2008)

12 Physical Degradation: De processing Stress devices. In this case in the off state for various V DG Chemically remove SiN passivation and gate/contact metals Inspect Using SEM and AFM. Increasing V DG causes an increase in the number and depths of the pits. P. Makaram, J. Joh, J.A. del Alamo, T. Palacios, and C.V. Thompson, Appl. Phys. Lett. 96, (2010). 12

13 Correlation of Electrical and Physical Degradation Electrical degradation Physical degradation P. Makaram, J. Joh, J.A. del Alamo, T. Palacios, and C.V. Thompson, Appl. Phys. Lett. 96, (2010). 13

14 Mechanism of Pit Formation: Accelerated by Electric Field V SD = 30V and V GD = 12V Ga/Al Oxide Reaction Product before deprocessing source edge particles V SD = 30V and V GS = +12V after deprocessing drain edge pits Pits form at gate edge with highest electric field Pits correlate with Ga /Al oxide particles on surface F. Gao, B. Lu, L. Li, S. Kaun, J.S. Speck, C.V. Thompson, and T. Palacios, Appl. Phys. Letts. 99, (2011). Feng Gao, B. Lu, L. Li, S. Kaun, J.S. Speck, C.V. Thompson, and T. Palacios, Appl. Phys. Letts. 99, (2011). 14

15 Mechanism of Pit Formation: Accelerated by the presence of water Strong temperature dependence air dry air T F. Gao, S.C. Tan, J.A. del Alamo, C.V. Thompson, and T. Palacios, IEEE Trans. Electron Devices 61, 444 (2014). 15

16 Model: Field and Water Assisted Electrochemical Oxidation 2Al x Ga 1 x N + 3H 2 O = xal 2 O 3 + (1 x)ga 2 O 3 + N 2 +3H 2. Reduction half reaction: 2H 2 O + 2e = H 2 + 2OH Decomposition and oxidation: 2Al x Ga 1 x N + 6h + = 2xAl (1 x)ga 3+ + N 2 and 2xAl (1 x)ga OH = xal 2 O 3 + (1 x)ga 2 O 3 + 3H 2 O Feng Gao, S.C. Tan, J.A. del Alamo, C.V. Thompson, and T. Palacios, IEEE Trans. Electron Devices 61, 444 (2014). 16

17 GaN on Si HEMTs W.A. Sasangka, G.J. Syaranamual, C.L. Gan, and C.V. Thompson, Proc. IRPS

18 Pits and Dislocations All pits have dislocations associated with them W.A. Sasangka, G.J. Syaranamual, C.L. Gan, and C.V. Thompson, Proc. IRPS

19 Pits and Dislocations TEM Cut Along Gate Edge, Early Pit Formation White Lines: Gate edges Yellow Lines: FIB slice for Cross Sectional TEM 19

20 Pits and Dislocations cross section along gate edge Dislocation density : Screw + mixed 2 x 10 9 /cm 2 Edge 4 x 10 9 /cm 2 # of pits with dislocations Screw = 8 out of 15 Edge = 4 out of 15 Mixed = 3 out of 15 W.A. Sasangka, G.J. Syaranamual, C.L. Gan, and C.V. Thompson, Proc. IRPS

21 Dislocation Motion During Stressing untested, dislocation etch tested, de processed tested, de processed, dislocation etch uniform dislocation distribution dislocation density higher at gate edge Dislocations move to gate edge W.A. Sasangka, G.J. Syaranamual, R. I. Made, C.L. Gan, and C.V. Thompson, AIP Advances 6, (2016). 21

22 Dislocation Motion During Stressing higher higher TD density at the gate edges center W.A. Sasangka, G.J. Syaranamual, R. I. Made, C.L. Gan, and C.V. Thompson, AIP Advances 6, (2016). 22

23 Dislocation Motion During Stressing center High overall residual stress. Piezoelectric stress causes peak stress and stress gradient at gate edge. W.A. Sasangka, G.J. Syaranamual, R. I. Made, C.L. Gan, and C.V. Thompson, AIP Advances 6, (2016). 23

24 Drain side Degradation of SiN Passivation Gate Gate Source side Drain side SiN degraded (lower film density than source side); XTEM and confirmed by EELS Increases diffusivity of oxidant (i.e. oxygen) from the ambient toward the GaN Constant reverse bias at 225 o C V G = 10 and 20 V, V D =20 V for 500 hrs. W.A. Sasangka, G.J. Syaranamual, Y. Gao, R.I. Made, C.L. Gan, and C.V. Thompson, Microelectronics Reliability 76 77, 287 (2017). 24

25 Degradation of SiN Degraded regions: Nano globes EELS scan along AA Globes contain oxygen, suggesting silicon oxide formation W.A. Sasangka, G.J. Syaranamual, Y. Gao, R.I. Made, C.L. Gan, and C.V. Thompson, Microelectronics Reliability 76 77, 287 (2017). 25

26 Cumulative Distribution Failure (%) Cumulative % Failed Failure Time Distribution for Low and low Passivation density SiNA, V G = -20 V high Passivation density SiN B, V G = -20 V low Passivation density SiNA, V G = -10 V high Passivation density SiN B, V G = -10 V High Density SiN Passivation B, VG = -10V Failure Time (h) Low density: 2.25 g/cm 3 High Density: 2.48 g/cm 3 High Density SiN: t 50 : increases by > 2X t 1 : increases by > 3X W.A. Sasangka, G.J. Syaranamual, Y. Gao, R.I. Made, C.L. Gan, and C.V. Thompson, Microelectronics Reliability 76 77, 287 (2017). 26

27 Electrical Degradation Two stages: fast and slow modes Fast mode similar for high and low density SiN Slow mode faster for low density SiN W.A. Sasangka, G.J. Syaranamual, Y. Gao, R.I. Made, C.L. Gan, and C.V. Thompson, Microelectronics Reliability 76 77, 287 (2017). 27

28 Physical Degradation: Model Electrochemical oxidation Pits nucleate and grow at screw dislocations Fast and slow modes Fast mode similar for high and low density SiN Slow mode faster for low density SiN SiN gate SiN gate In the initial fast mode, oxidant at interface diffuses to gate to supply oxidation reaction. In slow mode, oxidant diffuses through SiN to interface to gate edge See model details in: W.A. Sasangka et al, Microelectronics Reliability 76 77, 287 (2017). 28

29 Summary and Conclusions GaN HEMTs, for a wide range of high frequency and highpower applications. Failure due to trap states and physical degradation. Focusing on pitting at gate edges: Electrochemical oxidation at threading dislocations that can move to gate edge The SiN passivation layers degrade and their density affects the rate of pitting Trapped oxygen at the GaN/SiN interface contributes to failure This failure mechanism is complex and is not captured by a simple reliability model. 29