Esd

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Esd

  1. 1. ELE863/EE8501 VLSI SystemsElectrostatic Discharge Protection GATE BOND SOURCE PAD Fei Yuan, Ph.D, P.Eng. Department of Electrical & Computer Engineering Ryerson University Copyright c 2012 Copyright (c) F. Yuan (1)
  2. 2. OUTLINE• Introduction to ESD• Principle Sources of ESD in ICs• ESD Models• ESD Protection Mechanisms• ESD Protection Devices• ESD Protection Circuits Copyright (c) F. Yuan (2)
  3. 3. Introduction to ESD• What is ESD ? ⊲ ESD - Electro-Static Discharge. ⊲ ESD is a transient discharge of static charge that arises from either human handling or a machine contact. ⊲ Although ESD is the result of a static potential in a charged object, the energy dissipated and damages made are mainly due to the current flowing through ICs during discharge. ⊲ Most ESD damages are thermally initiated in the form of device / interconnect burn-out or oxide break-down. The basic phenomenon of ESD is that is a large amount of heat is generated in a localized volume significantly faster than it can be removed, leading to a temperature in excess of the materials’ safe operating limits. ⊲ ESD Damages pn-junction may melt. Gate oxide may have void formation. Metal interconnects & Vias may melt or vaporization, leading to shorts or opens. ⊲ Gate-oxide breakdown is another form of ESD damage. Copyright (c) F. Yuan (3)
  4. 4. Introduction to ESD (cont’d)• Why is ESD Critical ? ⊲ The aggressive decrease in physical dimensions and increase in doping in modern CMOS technology result in a significant decrease in gate-oxide thickness and pn-junction width −→ Require less energy and lower voltages to destroy MOS devices. 500 450 400 350 300 tox [A] 250 200 150 100 50 0 0.5 1 1.5 2 2.5 3 Lmin [µm] Figure 1: Scaling of gate oxide thickness of MOS transistors. Copyright (c) F. Yuan (4)
  5. 5. Introduction to ESD (cont’d) 0.8 0.7 0.6 Junction depth [µm] 0.5 0.4 0.3 0.2 0.1 0 0.5 1 1.5 2 2.5 3 L [µm] min Figure 2: Scaling of junction depth of MOS transistors. 20 V Student Version of MATLAB t1 Vox 15 Vt1, Vox [V] 10 5 20 40 60 80 100 Gate oxide thickness, t [A] oxFigure 3: Scaling of the breakdown voltage of gate oxide (Vox ) and the avalanche breakdown voltage(Vt1 ) of pn-junctions. ⊲ The level of ESD stress, however, does not scale down with the technology. Copyright (c) F. Yuan (5)
  6. 6. Principle Sources of ESD in ICs• Human Handling ⊲ A person walking on a synthetic floor can accumulated up to 20 kV. This voltage is discharged when the person touches an object that is sufficiently at ground. Charge exchange occurs between the person and the object in a very short time duration (10 ns - 100 ns). The charging current is approximately 1A - 10A, depending upon the time constant.• Test and Handling Systems ⊲ Equipment can accumulate static charge due to improper grounding. The charge is transmitted through ICs when it is picked up for placement in test sockets.• IC Itself is Charged During Transport / Contact With Charged Objects ⊲ ICs remain charged until they come into contact with a grounded surface (large metal plates /test sockets). Charge is discharged through the pins of ICs. Large currents in the internal interconnects can result in high voltage inside the devices which can cause damage to thin dielectrics and insulators. Copyright (c) F. Yuan (6)
  7. 7. ESD Models • Human Body Model (HBM) ⊲ HBM models the ESD of a human body. ⊲ Peak current ≈ 1.3A, rise time ≈10-30ns. Human body model R=1.5kW VC (0 - ) C=100pF DUTFigure 4: Equivalent circuit of the human body model of ESD. The switch closes upon an ESD event. Copyright (c) F. Yuan (7)
  8. 8. ESD Models (cont’d) • Machine Model (MM) ⊲ MM models the ESD of manufacturing / testing equipment. ⊲ Peak current ≈ 3.7A, rise time ≈15-30ns, bandwidth ≈ 12 MHz. Machine model VC (0 - ) DUT C=200pFFigure 5: Equivalent circuit of the machine model of ESD. The switch closes upon an ESD event. ⊲ ESD stress caused by charged machines is sever because of zero body resistance. Most ESD protection circuits can only protect HBM and MM. Copyright (c) F. Yuan (8)
  9. 9. ESD Models (cont’d) • Charge Device Model (CDM) ⊲ CDM models the ESD of charged integrated circuits. ⊲ Inductance in the model is mainly due to the inductance of bond wires. ⊲ Peak current ≈ 10A, rise time ≈1ns. ⊲ Gate oxide breakdown is the signature failure of CDM stress, in contrast to the thermal failure signature of HBM and MM stress. ⊲ CDM stress has the fastest transient and has the max. peak current. ⊲ CDM stress is the most difficult ESD stress to protect against. Charged device model R<10 W L<10 nH DUTFigure 6: Equivalent circuit of charged device model of ESD. The switch closes upon an ESD event. Copyright (c) F. Yuan (9)
  10. 10. ESD Protection Mechanisms (cont’d)• Current Limiting Characteristics of n-well Resistors• Impact Ionization• Avalanche Multiplication of pn-junctions• First Breakdown (Avalanche Breakdown) of nMOS• Second Breakdown (Thermal Breakdown) of nMOS Copyright (c) F. Yuan (10)
  11. 11. ESD Protection Mechanisms (cont’d)• Current Limiting Characteristics of n-well Resistors ⊲ Conductivity ⊲ Majority charge carriers in n-well resistors are free electrons. ⊲ At low voltages, the velocity of free electrons (majority charge carriers) and that of holes (minority charge carriers) in a n-well resistor are linearly proportional to the field intensity of the applied electric field −→ a linear resistor with a constant resistance. vn = µn E, vp = µp E, (1) where vn and vp are the velocity of free electrons and that of holes, respectively, µn and µp are the bulk mobility of free electrons and holes, respectively. ⊲ The total charge crossing a cross-section of area A per second is given by Q = (vnn + vp p)Aq, (2) where n and p are the concentration of free electrons and holes, respectively, and q is the charge of an electron. ⊲ The current density is obtained from Q J= = (nµn + pµp )qE = σE, (3) A Copyright (c) F. Yuan (11)
  12. 12. where σ = (nµn + pµp )q (4) is the conductivity.⊲ Current Density ⊲ For n-type, the concentration of free electronics is approximately the doping of the donors, i.e. n≈nn , where nn is the doping of donors, we have vn≈µn E. Consequently J≈Jn = (nnµn E)q = nn vn q = σE (5) J is linearly proportional to E or equivalently the voltage across the semiconductors. Copyright (c) F. Yuan (12)
  13. 13. ESD Protection Mechanisms (cont’d)⊲ Velocity Saturation ⊲ At high voltages, the velocity of free electrons saturates due to the increasing collision with silicon lattices. As a result, the current through n-well resistors remains nearly constant regardless of voltage increase Jsat = nn vn,satq (6) vn,sat ≈107cm/s, nn = 1017/cm3 −→ Jsat = 1.6×105A/cm2. vn Ohmic region Saturation region v sat (10 7 cm/s) 0 4 10 V/cm E Figure 7: Velocity saturation ⊲ n-well resistors exhibit a large resistance in the saturation region shown above. ⊲ n-well resistors in the saturation region can be used as current-limiting devices for ESD protection by limiting the amount of ESD discharging current that can pass through the devices. Copyright (c) F. Yuan (13)
  14. 14. ESD Protection Mechanisms (cont’d)• Avalanche Multiplication in pn-junctions ⊲ When a pn-junction is reverse biased, the reverse current (leakage current) is solely due to (i) the movement of thermally generated charge carriers in the depletion region and (ii) the diffusion of charge carriers in the neutral regions. ⊲ When the reverse biasing voltage exceeds Vsat = 105V/cm, the carriers in the depletion region can impart enough energy in the collision with the silicon lattices to generate electron-hole pairs, which become free charge carriers. These charge carriers are then accelerated, collide with the silicon lattices, and create more free charge carriers −→ Avalanche Multiplication. Strike the covalent bond and destroy it Free electron - Si Si Si - Holes Hole - Si Si Si Si Si Si Si Si Si Covalent bonds Si Si Covalent bond Si destroyed Figure 8: Avalanche multiplication Copyright (c) F. Yuan (14)
  15. 15. ESD Protection Mechanisms (cont’d)• Avalanche Breakdown of nMOS ⊲ Parasitic Lateral Bipolar Transistor in nMOS PAD G D S p+ n+ n+ Isub Rsub VB p-substrate Figure 9: Parasitic lateral BJT in nMOS transistors. ⊲ nMOS is used as ESD protection devices. ⊲ Gate is grounded to ensure nMOS is off under normal operation conditions. ⊲ Drain-substrate/source-substrate pn-junctions are reverse biased. ⊲ A parasitic lateral BJT as shown exists in the substrate. ⊲ Under normal operation conditions, this parasitic BJT is off because both the pn-junctions of the nMOS transistor are reverse biased −→ Isub = 0, −→ VB = 0. Copyright (c) F. Yuan (15)
  16. 16. ESD Protection Mechanisms (cont’d)⊲ Avalanche Breakdown of Drain pn-junction of nMOS ⊲ When VD increases, the electric field across the drain-substrate depletion region becomes high enough such that avalanche multiplication occurs at the drain-substrate junction −→ free electron-hole pairs are generated. ⊲ The generated free electrons go to the drain due to its high potential, whereas the generated holes go to substrate due to its low potential −→ It gives rise to Isub −→ The base potential VB = Rsub Isub is increased subsequently. ⊲ When VB is sufficiently high, the source-substrate pn-junction becomes forward biased −→ electrons in the source (emitter) are emitted to the substrate (base) −→ the parasitic lateral BJT is ON −→ Avalanche Breakdown of MOSFETs −→ Static charge on pads is discharged via the BJT to the ground. Current capacity is determined by the width of the MOSFETs (the width of the BJT). ⊲ The turn-on time of the parasitic BJT is determined by the base transit time (τb = 250ps for 1 µm channel length). ⊲ Note the fundamental differences between the parasitic BJT and normal BJTs. The base width of the parasitic BJT equals to the channel length of the nMOS transistor (in the range of µm), whereas that of a normal BJT is very small (in the range of ˚) −→ A This BJT has a small current gain as the emitter and collector have the same dimension. Copyright (c) F. Yuan (16)
  17. 17. ESD Protection Mechanisms (cont’d) ⊲ Snapback ID Thermal breakdown region (Vt2 , I t2 ) Thermal breakdown Snapback region (ESD protection operation region) Slope=1/R sh Avalanche breakdown Vsh (Vt1 , I t1 ) VDFigure 10: Avalanche breakdown, snap back, and thermal breakdown of nMOS transistors. ⊲ When the BJT is ON, more electrons flows from the source to the drain −→ ID ↑ and VD decreases sharply until the snapback holding voltage Vsp is reached. The snapback holding voltage is mainly across the drain-substrate pn-junction. The marginal increase of the drain voltage is due to voltage drop across the drain diffusion, source diffusion, and contact resistance. ⊲ During snapback, the resistance has positive temperature coefficients. This implies that if the current in any region increases, the temperature of the region will increase, thereby increasing the resistance, which encourages the current to flow elsewhere −→ current is conducted uniformly by all figures of nMOS transistors. Copyright (c) F. Yuan (17)
  18. 18. ESD Protection Mechanisms (cont’d)• Thermal Breakdown of nMOS ⊲ During snapback, the current of nMOS increases with the external voltage VDS . ⊲ If VDS continues to increase, the device enters the thermal breakdown −→ with the onset of the thermal breakdown, the resistance of the current path has negative temperature coefficients, encourages current to concentrates in certain localized fingers of nMOS and eventually destroys the fingers. ⊲ The concentration of ESD current into a few fingers indicates that no matter how many fingers are used, only a few will be activated in case of an ESD strike −→ ESD current capability of the device does not scale with its size. ⊲ To enhance the self-protection of ESD protection devices, the current must flow UNIFORMLY in all fingers of ESD protection devices. Copyright (c) F. Yuan (18)
  19. 19. ESD Protection Mechanisms (cont’d)• Avalanche Breakdown versus Thermal Breakdown ⊲ If the avalanche breakdown voltage of ESD protection devices is LESS THAN their thermal breakdown voltage, then the avalanche breakdown occurs before the thermal breakdown −→ ESD stress will be released by the avalanche breakdown and the devices will not enter the thermal breakdown. ⊲ If the avalanche breakdown voltage of ESD protection devices is GREATER THAN their thermal breakdown voltage, the avalanche breakdown will occur at a voltage higher than the thermal breakdown voltage. After the avalanche breakdown occurs, VDS will still higher than the thermal breakdown voltage −→ the devices will enter thermal breakdown and concentrate currents in a localized area due to negative temperature coefficients −→ device will be destroyed. ID ID Thermal breakdown Thermal breakdown Avalanche Avalanche breakdown breakdown Vsp Vt1 < Vt2 VD Vsp Vt2 < Vt1 VD (A) Avalanche breakdown voltage is less than (B) Avalanche breakdown voltage is greater then thermal breakdown voltage thermal breakdown voltage Figure 11: Avalanche breakdown/thermal breakdown voltages Copyright (c) F. Yuan (19)
  20. 20. ESD Protection Devices• n-well Resistors• Gate-Grounded nMOS Transistors (GGNMOS)• Gate-Coupled nMOS Transistors (GCNMOS)• Silicon Controlled Rectifiers (SCR)• Medium Voltage Triggered SCR Copyright (c) F. Yuan (20)
  21. 21. ESD Protection Devices (cont’d)• n-well Resistors M1 M1 M1 M1 SiO2 Poly SiO2 SiO2 N-well P-substrate P-substrate n+ Poly-resistor N-well resistor Figure 12: Poly and n-well resistors ⊲ Poly-resistors in general should not be used as ESD protection devices due to their poor heat dissipation capability (Poly resistors are isolated from the substrate by the SiO2 layer). Note that the heat generated by ICs is taken away via two paths (i) PADs/traces/pins and (ii) substrate/ground plate. ⊲ n-well resistors have a good contact with the substrate. They are used as primary current-limiting devices. ⊲ When a n-well resistor is used as the current-limiting device, together with a nMOS (primary ESD protection device), it is essential to make sure that the n-well resistor will not enter its thermal breakdown when the nMOS is in its avalanche breakdown. Copyright (c) F. Yuan (21)
  22. 22. ESD Protection Devices (cont’d) • Diodes n-well p+ n+ pn-junction Rn-well p-substrate p+/n-well diode (a) p+/n-well diodes Internal PAD circuits n+ p+ p-well n+/p-well diode R p-well p-substrate (b) n+/p-well diodesFigure 13: Diodes in CMOS. (a) p+ /n-well diodes; (b) n+ /p-well diodes; (c) ESD protection usingdiodes. ⊲ Two main types of diodes : n+ /p-well diodes and p+ /n-well diodes. p+ /n-well diodes have a pn-junction between the n-well and p-substrate whereas there is no isolation between the diode and the p-substrate in n+ /p-well diodes. ⊲ When forward-biased, diodes can sustain a large current with a small device dimension. ⊲ Diodes are widely used for ESD protection at radio frequencies due to the small junction capacitance, which has a less impact on the bandwidth of RF circuits. Copyright (c) F. Yuan (22)
  23. 23. ESD Protection Devices (cont’d) • Gate-Grounded nMOS Transistors PAD D G S DS SS n+ RD RS R sub p-substrateFigure 14: Drain contact-gate spacing (DS) and source contact-gate spacing (SS) of ESD protectionnMOS transistors. ⊲ During a ESD strike, the pn-junction at the drain undergoes an avalanche breakdown. Holes flow to the substrate, resulting in an increase in VB −→ parasitic BJT will be turned on −→ ESD current flows from the collector the (drain of nMOS) to the emitter (the source of nMOS that is connected to the ground) −→ ESD stress at the drain of the nMOS transistor (PAD) is released. ⊲ The dimensions of ESD nMOS should be large enough to handle large ESD currents −→ multiple fingers structure is used to implement ESD nMOS. ⊲ The main design parameters of nMOS are (i) channel length, (ii) drain contact-to-gate spacing, and (iii) device width. The source contact-to-gate spacing is not critical and is kept at its minimum design value. Copyright (c) F. Yuan (23)
  24. 24. i) The minimum channel length is good for efficient turn-on but thepunch-through limit will be reduced.ii) Drain contact-to-gate spacing affects the resistance of ballastresistors. For silicided processes, the minimum drain contact-to-gatespacing is used.iii) Device width determines the maximum current that the devicecan conduct. Copyright (c) F. Yuan (24)
  25. 25. ESD Protection Devices (cont’d) • Gate-Grounded nMOS Transistors (cont’d) n-well ballasting D resistor Contact/via n+ 1 2 3 4 G S Metal-2 Metal-1Figure 15: Lumped ballast resistors are added at the drains to ensure a uniform ESD current distri-bution among the fingers of ESD protection transistors. ⊲ During an avalanche breakdown, the current flowing through the drain increases. However, the positive temperature coefficient of the resistance of n+ -diffusion (called Ballast Resistors) at the drain prevents current from concentrating in a localized region −→ it forces the ESD current to flow into other fingers −→ uniform current distribution is achieved. ⊲ The effect of n+ -diffusion resistance is virtually eliminated in silicided CMOS processes because in these processes n+ is silicided and the resistance of silicided n+ is small (a few ohms). ⊲ To preserve the current-limiting ability of ballast resistors, explicit n-well ballast resistors at the drain are added. Note that the sheet resistance of n-well is much higher than that of n+ -diffusion. Copyright (c) F. Yuan (25)
  26. 26. ESD Protection Devices (cont’d)• Gate-Coupled nMOS Transistors ⊲ In most applications, the gate of ESD nMOS is grounded to ensure that the ESD nMOS will not cause any extra leakage at the pin during normal operation (Without ESD). ⊲ The avalanche breakdown voltage is reduced if the gate of nMOS is properly biased during a ESD strike. ⊲ The gate voltage helps reduce the width of the pn-junction at the drain −→ increase the electric field in the junction and lower the avalanche breakdown voltage of the junction. ⊲ The value of C and R must be such that (i) they have no effect on the operation of the circuit when there is no ESD stress, (ii) they must couple a sufficient voltage to the gate during a ESD strike such that the avalanche breakdown voltage of nMOS is effectively reduced. Copyright (c) F. Yuan (26)
  27. 27. ESD Protection Devices (cont’d)• Gate-Coupled nMOS Transistors (cont’d) Internal PAD V circuits DS C (Avalanche breakdown voltage) R VG ID Thermal breakdown Reduced avalanche breakdown Avalanche breakdown VD Vsp Figure 16: Gate-coupled nMOS ⊲ C behaves as a short-circuit for ESD strikes (high frequencies). ⊲ Gate voltage helps reduce the width of the pn-junction at the drain (connected to the pad) as the potential of the substrate underneath the gate oxide is increased −→ drain pn-junction will undergo an avalanche breakdown at a lower junction voltage −→ ESD protection occurs at a lower drain voltage (better ESD protection). Copyright (c) F. Yuan (27)
  28. 28. ESD Protection Devices (cont’d)⊲ Drawbacks of nMOS-based ESD Protection ⊲ Effective for non-silicided processes. Less effective for silicided processes. ⊲ Need additional ballast n-well resistors to increase ESD protection. ⊲ Need gate-coupling circuitry to improve ESD protection. Copyright (c) F. Yuan (28)
  29. 29. ESD Protection Devices (cont’d)• Silicon-Controlled Rectifiers (SCR) Internal PAD circuits pn-junction p+ n+ n+ p+ Rnwell T1 T2 n-well Rsub Ic2 p-substrate Figure 17: Silicon-controlled rectifiers (SCRs). ⊲ Under normal operation, the pn-junction between n-well and p-substrate is reverse biased and SCR has no effect on the operation of the protected circuits. ⊲ During a ESD strike, the pn-junction between n-well and substrate undergoes an avalanche breakdown −→ currents flow from n+ through Rnwell to the substrate −→ a sufficient voltage drop across Rnwell turns on T2 −→ a large current flows from p+-diffusion through Rsub to the ground −→ T1 turns on −→ T1 and T2 latch up to release ESD stress. ⊲ SCR has a high ESD breakdown voltage (≈40V with the latch-up time ≈1ns) as compared with that of nMOS because the breakdown voltage of n+/p-sub is lower than that of n-well/p-sub (large pn-junction width) −→ internal circuits might have already been destroyed even before ESD protection circuits are activated. ⊲ SCR is not affected by silicidation −→ attractive for modern CMOS processes where silicidation is common. Copyright (c) F. Yuan (29)
  30. 30. ESD Protection Devices (cont’d)• Medium-Voltage Triggered SCR ⊲ An additional n+ is added at the edge of the n-well to reduce the junction width. n+-diffusion Internal PAD is added circuits p+ n+ n+ n+ p+ Rnwell T1 T2 n-well Rsub Ic2 p-substrate Figure 18: Modified silicon-controlled rectifiers (MSCRs). ⊲ During an ESD strike, pn-junction in this region undergoes an avalanche breakdown at a LOWER voltage. ⊲ Breakdown voltage : 25V for 0.35µm CMOS (40V for conventional SCR). Copyright (c) F. Yuan (30)
  31. 31. ESD Protection Devices (cont’d)• Low-Voltage Triggered SCR ⊲ An additional n+ is added at the edge of the n-well to reduce the junction width. Grouded-gate Internal PAD is added circuits p+ n+ n+ n+ p+ R nwell T1 T2 n-well Rsub Ic2 p-substrate Figure 19: Low-voltage silicon-controlled rectifiers (LVTSCRs). ⊲ The added gate-grounded nMOS enters avalanche breakdown first. ⊲ Avalanche voltage is similar to gate-grounded nMOS (10V approximately). Copyright (c) F. Yuan (31)
  32. 32. ESD Protection Circuits• Requirements of ESD Protection Circuits : ⊲ Provide a low-impedance path from input pads to the ground during an ESD strike to release the static charge accumulated on the pads. ⊲ Clamp the voltage of the pads at a level that is below the dielectric breakdown voltage of thin transistors during an ESD strike. ⊲ Provide a very high impedance and a low capacitance during normal operation such that it has a little effect on the operation of the protected circuits. Copyright (c) F. Yuan (32)
  33. 33. ESD Protection Circuits• Basic Configuration Primary ESD Current Secondary ESD Internal PAD Elements limiting Elements circuits resistor Figure 20: Configuration of ESD protection circuits ⊲ Primary ESD protection elements shunt most of ESD currents. Primary ESD protection elements have large width and need more time to turn on. ⊲ Secondary ESD protection elements serve to limit the voltage at the circuit being protected until the primary ESD protection devices are fully operational. Secondary ESD protection devices have smaller width. ⊲ The effectiveness of the primary ESD protection devices is determined by the secondary protection stage. Note that due to the small dimensions, the secondary protection devices enter avalanche breakdown before the primary protection devices are activated. It is critical to ensure that the avalanche breakdown of the primary protection devices is activated before the secondary protection devices enter their thermal breakdown so that the secondary ESD protection devices will not be destroyed by ESD stress. ⊲ Current-limiting resistor has two functions (i) limit the current flowing into the internal circuits. (ii) withstand some ESD voltage so that the secondary protection circuit will not be damaged in an ESD strike. Copyright (c) F. Yuan (33)
  34. 34. ESD Protection Circuits (cont’d)• Basic Circuits R Internal PAD circuits Current-limiting resistor Primary ESD elements Secondary ESD elements Figure 21: Basic ESD protection circuits ⊲ Both nMOS and pMOS are used for positive and negative ESD strikes. ⊲ Under normal operation conditions, ESD devices are off −→ minimize the leakage current of these devices. Copyright (c) F. Yuan (34)
  35. 35. • ESD of 0.35µm CMOS Processes ⊲ In sub-micron CMOS, the onset of damage has been observed at between 1 kV and 2kV. ⊲ Design rules are set for 2 kV HBM and 200V MM. ⊲ Min. resistance of the isolation n-well resistor : 200 Ω. ⊲ Soft-pull is used to balance the breakdown voltage and the speed of I/O fingers under ESD zapping. Copyright (c) F. Yuan (35)
  36. 36. ESD Protection Circuits (cont’d) • Distributed ESD Protection Circuits ⊲ De-centralize a large capacitance into a set of small capacitances separated by inductors - a transmission line is constructed −→ capable of transmitting high-frequency signals. zo zo zo zo PAD Internal circuits z in zo zo zo zo PAD Internal circuits C C C CFigure 22: Top - Distributed ESD protection with equal sections; Bottom - small-signal equivalentcircuit. Copyright (c) F. Yuan (36)
  37. 37. ESD Protection Circuits (cont’d)• Multi-Finger Turn-On (MFT) ⊲ Thermal breakdown voltage of each finger of a large ESD protection transistor is made higher than the avalanche breakdown voltage of the finger. ⊲ Lumped resistors are added at source and drain. The one at drain functions as ballast resistors while the one at source sense the ESD current and generates a voltage that is applied to the gate of the adjacent finger −→ behave as a gate-coupled nMOS transistor −→ to initiate ESD protection earlier, better ESD protection. PAD Internal Rd Rd Rd Rd circuits Rs Rs Rs Rs Figure 23: Equivalent circuit of poly back-end ballast with segmentation. Copyright (c) F. Yuan (37)
  38. 38. ESD Protection Circuits (cont’d)• Soft-Ground-Gate nMOS MFT ⊲ Based on substrate pick-up technique −→ when an ESD strike occurs, the potential of substrate increases because ESD currents flow to the substrate. The gate potential increases subsequently −→ behaves as gate-coupled nMOS transistor that have a low avalanche breakdown voltage (better ESD protection). PAD Internal Rd Rd Rd Rd circuits Rs Rs Rs Rs Figure 24: Equivalent circuit of a soft-grounded-gate nMOS MFT. Copyright (c) F. Yuan (38)
  39. 39. ESD Protection Circuits (cont’d)• Domino nMOS MFT ⊲ Lumped resistors are added at source and drain. The one at drain functions as ballast resistors while the one at source sense the ESD current and generates a voltage that is applied to the gate of the adjacent finger −→ behave as a gate-coupled nMOS transistor. PAD Internal Rd Rd Rd Rd circuits Rs1 Rs1 Rs1 Rs1 Rs2 Rs2 Rs2 Rs2 Macro-ballasting resistors Figure 25: Equivalent circuit of domino nMOS MFT. Copyright (c) F. Yuan (39)
  40. 40. References⊲ For details on ESD protection and references, please read Chapter 8 of CMOS Current-Mode Circuits for Data Communications (Fei Yuan, Springer : New York, 2007). Copyright (c) F. Yuan (40)

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