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OXIDE AND INTERFACE TRAPPED CHARGES, OXIDE THICKNESS Yameng Bao Electron Physics Group 1.

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Presentation on theme: "OXIDE AND INTERFACE TRAPPED CHARGES, OXIDE THICKNESS Yameng Bao Electron Physics Group 1."— Presentation transcript:

1 OXIDE AND INTERFACE TRAPPED CHARGES, OXIDE THICKNESS Yameng Bao Electron Physics Group 1

2 Outline 1.Introduction 2. Fixed Oxide Trapped, and Mobile Oxide Charge 3. Interface Trapped Charge 4. Oxide Thickness 5. Strengths and Weaknesses 2

3 1.Introduction Capacitance-voltage and oxide thickness measurements must be more carefully interpreted for thin, leaky oxides Charges and defects in the oxide Variable Capacitance Insulation and passivation High dielectric constant Low leakage current and low tunnel current-lower power waste lower temperature of device Focus on SiO 2 -Si system 3

4 (1). Interface Trapped Charge(Q it,N it, D it ) (2). Fixed Oxide Charge(Q f, N f ) (3) Oxide Trapped Charge (Q ot, N ot ) (4) Mobile Oxide Charge (Q m,N m ) 4

5 Oxide Charges (1) Interface Trapped Charge(Q it ) Due to structural defects, oxidation-induced defects, metal impurities, or other defects caused by radiation or similar bond breaking processes Unlike xed charge or trapped charge interface trapped charge is in electrical communication with the underlying silicon Could be neutralized by low T H 2 or forming Gas(N 2 &H 2 ) 5

6 Oxide Charges (2) Fixed Oxide Charge(Q f )(near the interface) Coming from oxidation process Usually measure after Annealing to eliminate the effect of the interface trapped charge It depends on final oxidation temperature Always present in any cases 6

7 Oxide Charges (3) Oxide Trapped Charge(Q ot ) Due to the ionizing radiation, avalanche injection and so on Sometimes could be annealed by Low-T treatment but the neutral traps still remain (4) Mobile Oxide Charge(Q m ) Caused by Na +, Li +, K + and so on Chlorine atom may reduce this charge 7

8 2. Fixed Oxide Trapped, and Mobile Oxide Charge (1) Capacitance-voltage Curve 8 Q G is gate charge density V G is gate voltage Q G =-(Q s + Q it ) Q s is semiconductor charge density Q it is interface charge density V G = V FB + V ox + φ s V FB is flatband voltage V ox is oxide voltage φ s is surface potential Q S = Q p +Q b + Q n Q p is hole charge density, Q b is space-charge region bulk charge density Q n is electron charge density

9 2. Fixed Oxide Trapped, and Mobile Oxide Charge (1) Capacitance-voltage Curve 9 V < 0 V > 0 V >> 0 Accumulation Depletion Strong inversion For P type substrate

10 2. Fixed Oxide Trapped, and Mobile Oxide Charge (1) Capacitance-voltage Curve 10 For negative gate voltages Accumulation: 1) Big negative voltage Q p dominates. C p is short circuit

11 2. Fixed Oxide Trapped, and Mobile Oxide Charge (1) Capacitance-voltage Curve 11 Depletion Small negative voltage and small positive voltage Qb=-qN A W In week inversion C n begin to appear Strong inversion C n domains a) If the inversion charge could follow the HF-AC, C=C ox b) if the inversion could not follow, C=Cox+Cb

12 2. Fixed Oxide Trapped, and Mobile Oxide Charge (1) Capacitance-voltage Curve 12 When the dc bias voltage is changed rapidly with insufficient time for inversion charge generation, the deep-depletion curve results. Its high- or low- frequency semiconductor capacitance is C dd Effect of sweep direction and sweep rate on the hf MOS-C capacitance on p-substrate,

13 2. Fixed Oxide Trapped, and Mobile Oxide Charge (2) Flatband Voltage 13 The flatband voltage is determined by the metal-semiconductor work function difference φ MS and the various oxide charges through the relation Determine the V FB Metal-S work function different Fixed charge Interface trapped charge Charges in metal Charges in oxide

14 2. Fixed Oxide Trapped, and Mobile Oxide Charge (3) Capacitance Measurement 14 High Frequency: High-frequency C – V curves are typically measured at 10 kHz – 1 MHz. Using a phase sensitive detector, one can determine the conductance G or the capacitance C, knowing R and ω = 2πf

15 2. Fixed Oxide Trapped, and Mobile Oxide Charge (3) Capacitance Measurement 15 Low Frequency: Current-Voltage Low Frequency: Current-Voltage: The low-frequency capacitance of an MOS-C is usually not obtained by measuring the capacitance, but rather by measuring a current or a charge, because capacitance measurements at low frequencies are very noisy. Low F High F

16 2. Fixed Oxide Trapped, and Mobile Oxide Charge (3) Capacitance Measurement 16 Low Frequency: Current-Voltage and Charge-Voltage Q-V is more suitable for MOS measurement

17 2. Fixed Oxide Trapped, and Mobile Oxide Charge (4) Fixed Charge 17 a) The fixed charge is determined by comparing the flatband voltage shift of an experimental C – V curve with a theoretical curve and measure the voltage shift To determine Q f,one should eliminate or at least reduce the effects of all other oxide charges and reduce the interface trapped charge to as low a value as possible. Q it is reduced by annealing in a forming gas. b) Second method using differing t ox Plot V FB versus t ox with slope Q f /K ox ε 0 and intercept φ MS. This method, requires MOS capacitors with differing t ox.However, it is more accurate because it is independent of φ MS. K ox is semiconductor dielectric constant

18 2. Fixed Oxide Trapped, and Mobile Oxide Charge (5) Work function difference 18 φ MS depends on oxidation temperature, wafer orientation, interface trap density, and on the low temperature D it anneal (6) Oxide Trapped Charge(Q ot ) The distribution of Q ot must be known for proper interpretation of C –V curves. Trapped charge distributions are measured most commonly by the etch-off and the photo I –V methods A determination of the charge distribution in the oxide is tedious and therefore not routinely done. In the absence of such information, the Vfb shift due to charge injection is generally interpreted by assuming the charge is at the oxide-semiconductor interface using the expression

19 2. Fixed Oxide Trapped, and Mobile Oxide Charge (5) Mobile Charge 19 Mobile charge in SiO 2 is due primarily to the ionic impurities Na +, Li +, K +, and perhaps H +. Sodium is the dominant contaminant. Bias-Temperature Stress( BTS): Measured at 250C, under gate bias, measure CV then cool down to 25C, then measure CV, the Q m is determined by V fb shift. Triangular voltage sweep (TVS) method: C lf and C hf measured at T=250C, The Q m is determined from the area between the two curves

20 3. Interface trapped charge (1) Low frequency(Quasi-static) methods 20 Effect of D it on MOS-C capacitance-voltage curves. (a) Theoretical high-frequency,(b) theoretical low-frequency and (c) experimental low-frequency curves. Gate voltage stress generated interface traps This stretch-out is not the result of interface traps contributing excess capacitance, but rather it is the result of the C –V curve stretch-out along the gate voltage axis Interface traps do respond to the probe frequency at LF, and the curve distorts because the interface traps contribute interface trap capacitance C it and the curve stretches out along the voltage axis HF LF Experimental-LF

21 3. Interface trapped charge (1) Low frequency(Quasi-static) methods 21 ΔC/C ox =C lf /C ox C hf /C ox

22 3. Interface trapped charge (2) Conductance Method 22 The conductance is measured as a function of frequency and plotted as G P /ω versus ω. G P /ω has a maximum at ω =1/τ it and at that maximum D it =2G P /qω. we nd ω 2/τ it and D it =2.5G P /qω at the maximum. Hence we determine D it from the maximum G P /ω and determine τ it from ω at the peak conductance location on the ω-axis. One of the most sensitive methods to determine D it Interface trap densities of 10 9 cm 2 eV 1 and lower can be measured.

23 3. Interface trapped charge (3)High Frequency Method 23 Terman Method: In HF CV, interface traps do not respond to the ac probe frequency, they do respond to the slowly varying dc gate voltage and cause the hf C –V curve to stretch out along the gate voltage axis as interface trap occupancy changes with gate bias ΔV G = V G –V G (ideal) is the voltage shift of the experimental from the ideal curve, and V G the experimental gate voltage The method is generally considered to be useful for measuring interface trap densities of cm 2 eV 1 and above

24 3. Interface trapped charge (3)High Frequency Method 24 Gray-Brown and Jenq Method:, the C HF measured as a function of T. Reducing the T causes the Fermi level to shift towards the majority carrier band edge and the interface trap time constant τ it increases at lower T. Hence interface traps near the band edges should not respond to typical ac probe frequencies at low T whereas at room temperature they do respond. This method should extend the range of interface traps measurements to D it near the majority carrier band edge Compared with DLTS?

25 3. Interface trapped charge (4)Other Methods 25 1.Charge Pumping 2. MOSFET Sub-threshold Current method 3. DC-IV method 4. deep-level transient spectroscopy(DLTS) 3. DC-IV method 5. charge-coupled devices (CCD) 6. electron spin resonance (ESR)

26 3. Oxide thickness (1)Capacitance-Voltage(equivalent electrical thickness) 26 C –V, I –V, ellipsometry, transmission electron microscopy(TEM), X-ray photoelectron spectroscopy (XPS), medium energy ion scattering spectrometry (MEIS), nuclear reaction analysis (NRA), Rutherford backscattering (RBS), elastic backscattering spectrometry (EBS), secondary ion mass spectrometry (SIMS), grazing incidence X-ray reectometry (GIXRR), and neutron reectometry

27 3. Oxide thickness (2)Current-Voltage 27 The current owing through an insulator is either Fowler-Nordheim (FN) or direct tunnel current (a) V ox < qφ B (direct tunneling)(b) V ox > qφ B Fowler-Nordheim tunneling

28 3. Oxide thickness (3)Other methods 28 Ellipsometry: Suitable for oxides into the 1–2 nm regime. Variable angle, spectroscopic ellipsometry is especially suited for oxide thickness measurements Transmission Electron Microscopy is very precise and usable to very thin oxides, but sample preparation is tedious X-ray Photoelectron Spectroscopy

29 4. Strength and Weakness (1)Mobile Oxide Charge 29 Bias temperature stress method Requiring the measurement of a C –V at different Ts Total mobile charge density will be measured, No separation Triangular voltage sweep method Could differentiate different mobile charges, high sensitivity, fast Increasing oxide leakage current for thin film (2) Interface tapped charge(conductance and low frequency method Conductance method high sensitivity, majority carrier capture cross sections Limited surface potential range Quasi-static method(I-V/Q-V) Easy to measure, large surface potential range I-V the requirement for I-V, current is low For I-V and Q-V leakage current could be a big problem

30 4. Strength and Weakness (3)Oxide Thickness 30 MOS C –V measurements are most common. Leakage current make the result much difficult I-V used for thickness extraction Ellipsometry is mostly used for thickness, very sensitive to thin oxides XPS suitable for very thin oxide

31 5. Questions? 31 All the charges seems affect each other during the measurement. For thin oxide, the tunnel current or leakage current will effect the result. Real measurement is always not as simple as description in the book O(_)O


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