Surface Damage in Silicon Devices E. Fretwurst University of Hamburg Institute for Experimental Physics 4. Detector Workshop of the Helmholtz Alliance “Physics at the Terascale” E. Fretwurst, Uni-Hamburg 4th Detector Workshop of the Helmholtz Alliance, March 15-th 2011
Outline • Introduction • Properties of SiO 2 and SiO 2 -Si interface • Experimental Techniques • Radiation Damage - MOS and Gate-Controlled Diodes - Strip sensors - MOSFET 2 E. Fretwurst, Uni-Hamburg 4th Detector Workshop of the Helmholtz Alliance, March 15-th 2011
Introduction • What means surface damage? Damage effects induced in silicon-oxide layers grown on silicon wafers and at the SiO 2 -Si interface by ionizing radiation (charged particles, X-rays) • Where one has to take into account? - Silicon tracker in HEP Collider-Experiments (LHC, S-LHC, ILC,…), damage effects in sensors and electronics - Silicon Detector-Arrays in X-ray Free Electron Laser (XFEL) experiments – sensors and electronics - Space experiments • Typical dose values in different areas S-LHC: ~ 4.2 MGy at r = 4 cm for an integrated luminosity of 2500 fb -1 XFEL: up to 1 GGy in about 3 years of continuous operation 3 E. Fretwurst, Uni-Hamburg 4th Detector Workshop of the Helmholtz Alliance, March 15-th 2011
Typical Devices under Study MOS test-field Strip Sensor CMS Pixel sensor AGIPD readout chip in 130 nm IBM CMOS N-channel MOSFET 4 E. Fretwurst, Uni-Hamburg 4th Detector Workshop of the Helmholtz Alliance, March 15-th 2011
Properties of thermally grown SiO 2 Property Value Density 2.27 g/cm³ Dielectric constant 3.4 (dry), 3.9 (H 2 0 ambient) Refractive index 1.46 5 - 10 × 10 6 V/cm Dielectric strength Energy gap 8.8 eV 5 × 10 -7 cm/K Linear expansion coeff. 10 -3 J/(kgK) Specific heat 5 E. Fretwurst, Uni-Hamburg 4th Detector Workshop of the Helmholtz Alliance, March 15-th 2011
Defects/Impurities in SiO 2 Mobile Ionic Charge Q m affected early stage MOS structures, not an issue today Oxide Trapped Charge Q ot defects in the SiO 2 network, but difficult to communicate with free carriers Fixed Oxide Charge Q f due to the hole trapping, ~ nm from interface, highly disordered region Interface trap Q it due to dangling Si-O bonds with energy states in the forbidden band 6 E. Fretwurst, Uni-Hamburg 4th Detector Workshop of the Helmholtz Alliance, March 15-th 2011
SiO 2 -Si interface Structural imperfections between Si bulk and SiO 2 layer � interface states D it Example for structural model of (100) and (111) Si interface D it represent a P b center on (111) Si surface (detected by ESR): continuum of states in interface trivalent Si atom with dangling bond aimed into a vacancy in the oxide the band gap and is given in units (eVcm 2 ) -1 P b0 and P b1 on (100) Si surface: chemically identical to Pb center but different configurations D.K. Schroder, Semiconductor Material and Device Characterization, Jon Wiley & Sons, Inc., 2006 7 E. Fretwurst, Uni-Hamburg 4th Detector Workshop of the Helmholtz Alliance, March 15-th 2011
Classification of Interface Traps Capture/emission of charge carriers � Schockley-Read-Hall statistics E C shallow acceptors E F electrons E i + deep + donors holes E F shallow E V acceptors negatively charged Shallow traps � “fast” traps, responsible if below E F , otherwise neutral for frequency dependence of MOS C-V donors positively charged if Deep traps � generation/recombination above E F , otherwise neutral centers, responsible for surface current 8 E. Fretwurst, Uni-Hamburg 4th Detector Workshop of the Helmholtz Alliance, March 15-th 2011
Summary Oxide - Interface Charges � Mobile oxide charge Q m : positive ions e.g. Na + ( negligible ) � Trapped oxide charge Q ot : defects in SiO 2 network (+ or -) � Fixed oxide charge Q f : traps near to the interface (trapped holes, Q f positive) � Interface-trapped charge Q it : interface states with acceptor- or donor-character, occupation with electrons/holes depends on Fermi-level E F at the interface 9 E. Fretwurst, Uni-Hamburg 4th Detector Workshop of the Helmholtz Alliance, March 15-th 2011
Experimental Techniques � MOS capacitor Capacitance-Voltage characteristics ( C-V ) at different frequencies � information: flat band voltage V FB , Q f (N f ), Q it (N it ) Thermally Dielectric Relaxation Current ( TDRC ) for different bias voltage � information: D it (E t ) distribution in the band gap Other techniques not presented here: Conductance method G( ω ), quasi-static C-V, Deep Level Transient Spectroscopy (DLTS), Electron Spin Resonance (ESR or EPR) � Gate controlled – Diode Current-Gate Voltage characteristics for different junction bias voltage � information: surface recombination velocity S 0 or D it at mid gap 10 E. Fretwurst, Uni-Hamburg 4th Detector Workshop of the Helmholtz Alliance, March 15-th 2011
MOS Capacitor (ideal) n-type silicon C ox C MOS V G Al gate SiO 2 accumulation C FB LCR meter ~ depletion V n-type silicon high frequency C inv à inversion V G � 0 ⋅ C C C ox = ox Si C + MOS C C C Si ox Si depletion inversion accumulation (Si space charge region) (holes) (electrons) 11 E. Fretwurst, Uni-Hamburg 4th Detector Workshop of the Helmholtz Alliance, March 15-th 2011
MOS Capacitor (real) 1.8E-10 ε = = Q f,ox 1.6E-10 ox Accumulation: C C + + + + + + + + + + + + f = 10 kHz MOS ox t ● ● ● ● ● ● ● ● ● ● ● ● e - acc. 1.4E-10 ox 1.2E-10 layer ⋅ CMOS [F] C C 1.0E-10 = ox FB , S Flat band: C + MOS , FB C C 8.0E-11 ox FB , S + + + + + + + + + + + + 6.0E-11 ε ε Δ V FB k T C FB = = S B S 4.0E-11 C L D FB , S 2 L q N 2.0E-11 0 D D 0.0E+00 L D = Debey length , N D = Donor concentration -16 -14 -12 -10 -8 -6 -4 -2 0 V G [V] V FB Q − Δ = > f , ox Flat band voltage shift V , Q 0 FB f , ox C ox + + + + + + + + + + + + ⋅ ε ε C C depletion width w = = ∝ ox D S S Depletion: C , C + MOS , D D C C w V ox D ⋅ ε C C = = Deep inversion: ox inv S C , C inversion layer (holes) + MOS , inv inv C C w + + + + + + + + + + + + ox inv max w max 1 / 2 ⎛ ⎞ ln( N / n ) ⎜ ⎟ ∝ D i w , ⎜ ⎟ max ⎝ ⎠ N D = n intrinsic carrier concentrat ion i 12 E. Fretwurst, Uni-Hamburg 4th Detector Workshop of the Helmholtz Alliance, March 15-th 2011
Gate Controlled Diode Surface current density J s due to deep interface states N it V diode = -12 V V gate gate p+ n bulk -2.0E-11 accumulation -2.5E-11 holes depletion Is [A] -3.0E-11 I s holes -3.5E-11 inversion I / A = -4.0E-11 s gate Surface recombination velocity: S -16 -14 -12 -10 -8 -6 -4 -2 0 ⋅ 0 q n 0 i VG [V] 13 E. Fretwurst, Uni-Hamburg 4th Detector Workshop of the Helmholtz Alliance, March 15-th 2011
Thermally Dielectric Relaxation Current TDRC V G vacuum Cooling down: V G1 ≥ 0 V Electron accumulation MOS Interface traps filled with electrons At T = 30 K: V G2 < 0 V, depletion Heating up with TDRC [pA] constant rate � trapped electrons will be emitted, depending on D it (E t ) and T � I TDRC (T) T [K] 14 E. Fretwurst, Uni-Hamburg 4th Detector Workshop of the Helmholtz Alliance, March 15-th 2011
Radiation Damage Basic effects induced by ionizing radiation (X-rays, charged particles) depends on E-field in SiO 2 (1) e-h pair creation (2) hole transport - most e-h pairs recombine - electrons escape - holes are much slower and get finally trapped (3) hole trapping near SiO 2 -Si interface Q f (4) Build up of interface states N it SiO 2 µ e ≈ 20 cm 2 /(Vs) µ h ≈ 5×10 -5 cm 2 /(Vs) T.R. Oldham, Ionizing Radiation Effects in MOS Oxides, World Scientific, 1999 15 E. Fretwurst, Uni-Hamburg 4th Detector Workshop of the Helmholtz Alliance, March 15-th 2011
Unrecombined holes Buildup of N f,ox Δ N f,ox = D κ g f y f t,h t ox D = total dose κ g = e-h pair density per dose unit f y = fractional e-h yield f t,h = hole trapping efficiency t ox = oxide thickness Buildup of Δ V FB Δ V FB ∝ t ox Δ N f,ox ∝ t² ox H.J. Barnaby, IEEE TNS 53, NO.6, 3103, 2006 16 E. Fretwurst, Uni-Hamburg 4th Detector Workshop of the Helmholtz Alliance, March 15-th 2011
X-ray irradiation at DESY DORIS III COL1 COL2 DUI X-ray X-ray energy spectrum Energy spectrum of photons: • Typical energy: 12 keV • Flux density: 1.08 × 10 14 /(s ·mm 2 ) Beam profile: • Beam spot: 4 mm × 6 mm Dose rate: • Beam centre: 200 kGy/s • 2D scan: 500 kGy/scan Beam profile at beamline F4 17 E. Fretwurst, Uni-Hamburg 4th Detector Workshop of the Helmholtz Alliance, March 15-th 2011
Flat Band Voltage Shift Test field MOS - Δ V FB Flat band voltage shift: � Buildup of fixed oxide charge Q f and interface charge Q it with dose Q donor acceptor Q Q − Δ = + − f it it V � Q f > 0, trapped holes, shift to more negative V G FB C C C ox ox ox � Q it > 0, if interface states donors � larger V FB shift � Q it < 0, if interface states acceptors � less V FB shift � C-V stretch out caused by Q it (depends on D it - distribution in the band gap and the surface potential 18 E. Fretwurst, Uni-Hamburg 4th Detector Workshop of the Helmholtz Alliance, March 15-th 2011
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