Planar View of Structural Degradation in GaN HEMT: Voltage, Time and Temperature Dependence

Transcription

1 Planar View of Structural Degradation in GaN HEMT: Voltage, Time and Temperature Dependence Jungwoo Joh 1, Prashanth Makaram 2 Carl V. Thompson 2 and Jesús A. del Alamo 1 1 Microsystems Technology Laboratories, 2 Material Processing Center Massachusetts Institute of Technology, Cambridge, MA, USA ( ) Acknowledgements: ARL (DARPA WBGS program) ONR (DRIFT MURI) TriQuint Semiconductor 1

2 Introduction GaN HEMT Reliability: big concern RF power degradation I D decrease, R D increase, I G increase, V T change Goal: understand degradation mechanism (db) RF stress GHz,V D =28 V I DQ =150 ma/mm -0.6 P in =23 dbm -0.8 P out =33.7 dbm -1 P out Time (hr) 2

3 High Voltage Degradation in GaN HEMTs S AlGaN GaN G V GS = 10 V 1.2 OFF-state, V GS =-10 V 1.E+01 V E 1.E+00 V DS D 2DEG Joh, EDL 2008 I Dmax /I Dmax (0 0), R/R(0) I Goff R D R S I Dmax V DGstress (V) I D, R D, and I G start to degrade beyond critical voltage (V crit) (+ trapping behavior current collapse) Common physical origin in I D and I G degradation V crit 1.E-01 1.E-02 1.E-03 1.E-04 1E05 1.E-05 1.E-06 I Dmax : V DS =5 V, V GS =2 V I Goff : V DS =0.1 V, V GS = 5 V 3 I Goff (A A/mm)

4 Permanent vs. Trapping Degradation (0) I Dmax/I Dmax stress V DS =0 V GS = 30 V total (apparent) degradation recovery I Dmax : V DS =5 V, V GS =2 V Time (min) permanent degradation trapping degradation 88 days recovery Two components: Native traps Stress induced traps Separated by current collapse measurements: 1s V DS =0, V GS = 10V diagnostic pulse 13 % permanent degradation + 15 % trapping degradation Joh, ICNS

5 Material Degradation around V crit V DG =0 V V DG =16 V~V crit SiN V DG =25 V V DG =37 V Dmax Degradation (%) Permanent I Joh, ROCS Pit depth (nm) Initial dimple followed by deeper pitandcrack crack. Good correlation between pit depth and I Dmax degradationd How do these defects extend along the device width? 5

6 Plan View Approach Limitations of TEM: Costly Extremely local This work: (details in Makaram, APL 2010) Removal of SiN passivation and gate SiN passivation: HF etch Contact tand gate metals: tl aqua regia Surface cleaning: piranha solution Plan view imaging through SEM and AFM 6

7 SiN and Gate Removal Source Gate Drain Unstressed (high T storage) Stressed (> V crit ) 7

8 Voltage Acceleration OFF state step stress: V GS = 7 V V DS stepped from 5 to 8, 12, 35, 50 V (1 min/1 V step) T base =150 C Detailed device characterization: DC device parameters: I Dmax, R S, R D, V T Trap characterization: current collapse Removal of passivation and gate metal SEM and AFM plan view imaging 8

9 Voltage Acceleration OFF state step stress, V GS = 7 V, T base =150 C 200 nm 200 nm Unstressed V DG =15 V V DG =19 V DG DG (V crit ) 200 nm V DG =42 V 200 nm V DG =57 V 200 nm Initial continuous groove formation Deeper pit formation along the groove 9

10 Pit Cross Section Area nm) Averag ge Pit Depth ( Source Gate Drain x ( m) Avera ge Pit Area (n m 2 ) V DGstress =57 V 0 Cross section averaged over 1 µm Stress Voltage V DGstress (V) Pit formation is voltage accelerated. Drain side pit area also shows critical behavior. 10

11 Correlation between Electrical and Structural Degradation Perman nent I Dmax Deg gradation (%) Post St tress Current Collapse (%) Average Defect Area (nm 2 ) Average Defect Area (nm 2 ) Good correlation between electrical degradation and pit area 11

12 Cross section Gt Gate SiN Degradation Process 1. Below and around V crit : Groove formation in GN GaN cap Plan view AlGaN GaN 2. Beyond V crit : Pitformation in AlGaN barrier 3. Pit growth (to AlGaN/GaN interface) and merge Consistent behavior Complimentary techniques 12

13 Time Evolution Electrical Degradation ation (%) %) Dmax Degrada t collapse (% ermanent I D Curren Pe V DS =0, V GS = 40 V, T base =150 C Current V collapse I Dmax degradationd Stress Time (s) S AlGaN GaN Current collapse tends to saturate. Permanent degradation keeps increasing. G V GS D 2DEG 13

14 Time Evolution SEM V DS =0, V GS = 40 V, T base =150 C 10s 100s 1ks 10ks G A T E 0.5µm Very fast groove formation (10 s) on both sides. Pit density/size increase with time. 14

15 Time Evolution AFM Cross section averaged over 5 µm. 0.5 Gate 0 10s 100s Depth (nm) µm 1ks 10ks Aver rage Pit Area ( nm 2 ) x( ( m) Stress Time (s) Average pit area (~pit volume) ~ t 1/4 15

16 Electrical vs. Structural Degradation Pe ermanent I Dmax Degra dation (%) Post stress Current Co ollapse (%) Current Collapse I Dmax degradation Average Pit Area (nm 2 ) Good correlation between electrical and structural degradation. 16

17 Temperature Dependence V DS =0, V GS = 10 to 50 V (1min/1V step) 25C 25C 75C 125C 175C G A T E 02µm 0.2µm 5 Linear defect density (~1/degradation rate) is thermally activated with E a ~0.11 ev Ln (Line Dens ity) E a =0.11 ev /kT (ev 1 ) 17

18 Degradation Mechanisms Groove formation: fast process Field induced oxidation? Electrochemical etching? Pit formation: slow process E field driven (Little current is needed) Thermally activated Field/stress induced diffusion of material away from gate? In any event, mass transport is involved. 18

19 Summary Developed eeopeda simple pepocess process for plan view pa assessment of structural degradation Evolution of structural damage: Below V crit : shallow continuous groove formation at gate edge Above V crit : local lpit formation along the groove Pits grow with V stress and time and merge Pit formation is thermally activated. Field/stress induced mass transport is involved in GaN HEMT degradation 19