OXIDE AND INTERFACE TRAPPED CHARGES, OXIDE THICKNESS

1. CHAPTER 6 OXIDE AND INTERFACE TRAPPED CHARGES, OXIDE THICKNESS

2. 6.1 INTRODUCTION

3. INTRODUCTION Charges and their location for thermally oxidized silicon. • Interface trapped charge (Qit, Nit, Dit) • Fixed oxide charge (Qf, Nf) • Oxide trapped charge (Qot, Not) • Mobile oxide charge (Qm, Nm) “Deal triangle” showing the reversibility of heat treatment effects on Qf.

4. 6.2 FIXED, OXIDE TRAPPED, AND MOBILE OXIDE CHARGE

5. Cross section and potential band diagram of an MOS capacitor.

6. Capacitance-Voltage Curves Qs=Qp+Qb+Qn+Qit

7. Capacitances of an MOS capacitor for various bias conditions as discussed in the text.

8. In order for the inversion charge to be able to respond, Jscr = qniW/τg≦ Jd = CdVg/dt W in μm, tox in nm, τg in μs

9. is the dimensionless semiconductor surface electric field. Us=φs/kT, UF=qφF/kT = ±1 is the intrinsic Debye length

11. dd stands for deep depletion

12. Low-frequency (lf), high-frequency (hf), and deep-depletion (dd) normalized SiO2-Si capacitance-voltage curves of an MOS-C; (a) p-substrate NA= 1017 cm-3, (b) n-substrate ND = 1017 cm-3, tox= 10nm, T=300K.

13. (a) (b) • Effect of sweep direction on the hf MOS-C capacitance on an p-substrate, • entire C-VG curve, (b) enlarged portion of (a) showing the dc sweep • direction; f=1 MHz.

14. Flatband Voltage There is a built-in potential at epi-sub. junction normalized CFB

15. CFB/COX versus NA as a function of tox for the SiO2 -Si system at T=300K.

16. Schematic illustration of an MOS-C with finite gate doping density, showing gate depletion for positive gate voltage.

17. Low-frequency and high-frequency capacitance-voltage curves for various n+ polysilicon gate doping densities. The lowest Chf curve is for ND (gate) =1018 cm-3. Substrate NA =1016 cm-3, tox =10nm.

18. Capacitance Measurement for RG<<1 and (ωRC)2<<RG From the in-phase and out of phase component G and C can be determined. Simplified capacitance measuring circuit.

19. (a) (b) Block diagram of circuits to measure the current and charge of an MOS capacitor.

20. Low Frequency : Current-Voltage Low Frequency : Charge-Voltage CF is the feedback capacitance.

21. Ideal (line) and experimental (point) MOS-C curves. NA =5×1016 cm-3, tox=20nm, T=300K, CFB/Cox=0.77.

22. Fixed Charge

23. Gate-Semiconductor Work Function Difference Potential band diagram of a metal-oxide-semiconductor system at flatband.

24. Potential band diagram of (a) n+ polysilicon-p substrate, and (b) p+ polysilicon-n substrate at flatband.

26. Oxide Trapped Charge Flatband voltage of polysilicon-SiO2-Si MOS devices as a function of oxide thickness.

27. Work function difference as a function of doping density for polysilicon-SiO2 MOS devices.

29. Mobile Charge Drift time for Na, Li, K, and Cu for an oxide electric field of 106 V/cm and tox =100 nm.

31. C-VG curves illustrating the effect of mobile charge motion.

32. CIf and Chf measured at T=250OC. The mobile charge density is determined from the area between the two curves.

33. 6.3 INTERFACE TRAPPED CHARGE

34. Low-Frequency (Quasi-static) Method • Semiconductor band diagram illustrating the effect of interface traps; (a) V=0, (b) V>0, (c) V<0. Electron-occupied interface traps are indicated by the small horizontal heavy lines and unoccupied traps by the light lines

35. (a) (c) Theoretical ideal (Dit=0) and Dit ≠0 (a) hf , (b) If and (c) experimental lf C-V curves. (b)

36. CS=Cb+Cn

37. High- and low-frequency C-VG curves showing the offset △C/Cox due to interface traps.

38. Interface trapped charge density from the hf curve and the offset △C/Cox.

39. Conductance Method (a) MOS-C with interface trap time constant τit=RitCit , (b) simplified circuit of (a), (c) measured circuit, (d) including series rs resistance and tunnel conductance Gt.

41. Gp/ω versus ω for a single level, a continuum and experimental data. For all curves: Dit =1.9×109 cm-2 eV-1, τit=7×10-5s.

42. Interface trapped charge density versus energy from the quasi-static and conductance methods. (a) (111) n-Si, (b) (100) n-Si.

43. High-Frequency Methods Terman method: Gray-Brown method: The hf capacitance is measured as a function of temperature.

44. Charge Pumping Method Circuit diagram and energy bands for charge pumping measurement. The figures are explained in the text.

45. (c) (d) (e) (f)

47. Bilevel chare pumping waveforms.

48. Dit=7×109cm-2eV-1 MOSFET Qcp versus frequency

49. Trilevel charge pumping waveform and corresponding band diagrams.

50. (a) Icp as a function of tstep showing τe at the point where Icp begins to saturate. (b) insulator trap density versus insulator depth from the insulator/Si interface for Al2O3 and SiO2.