Final Lesson

• ESD

• Summary VLSI Technologies

ESDThe gate oxide in CMOS transistors is extremely thin (100 Å or less). This leaves the gate oxide of the I/O cell input transistors susceptible to breakdown from static electricity ( electrostatic discharge , or ESD ). ESD arises when we or machines handle the package leads (like the shock I sometimes get when I touch a doorknob after walking across the carpet at work). Sometimes this problem is called electrical overstress (EOS) since most ESD-related failures are caused not by gate-oxide breakdown, but by the thermal stress (melting) that occurs when the n -channel transistor in an output driver overheats (melts) due to the large current that can flow in the drain diffusion connected to a pad during an ESD event.To protect the I/O cells from ESD, the input pads are normally tied to device structures that clamp the input voltage to below the gate breakdown voltage (which can be as low as 10 V with a 100 Å gate oxide). Some I/O cells use transistors with a special ESD implant that increases breakdown voltage and provides protection. I/O driver transistors can also use elongated drain structures (ladder structures) and large drain-to-gate spacing to help limit current, but in a salicide process that lowers the drain resistance this is difficult. One solution is to mask the I/O cells during the salicide step. Another solution is to use pnpn and npnp diffusion structures called silicon-controlled rectifiers (SCRs) to clamp voltages and divert current to protect the I/O circuits from ESD.

ESD Cont.

There are several ways to model the capability of an I/O cell to withstand EOS. The human-body model ( HBM ) represents ESD by a 100 pF capacitor discharging through a 1.5 kohm resistor (this is an International Electro technical Committee, IEC, specification). Typical voltages generated by the human body are in the range of 2–4 kV, and we often see an I/O pad cell rated by the voltage it can withstand using the HBM. The machine model ( MM ) represents an ESD event generated by automated machine handlers. Typical MM parameters use a 200 pF capacitor (typically charged to 200 V) discharged through a 25 ohm resistor, corresponding to a peak initial current of nearly 10 A. The charge-device model ( CDM , also called device charge–discharge) represents the problem when an IC package is charged, in a shipping tube for example, and then grounded. If the maximum charge on a package is 3 nC (a typical measured figure) and the package capacitance to ground is 1.5 pF, we can simulate this event by charging a 1.5 pF capacitor to 2 kV and discharging it through a 1 ohm resistor.

VLSI Technologies

CMOS: Integration,Memory, Power diss, Input Impedance (Composition),Cost

Bulk: Logic, Mixed Signal, Analog, Embedded (DRAM,Flash,EEProm)

SOI: Lower Voltage/Power dissipation , Less Substrate Noise, Smaller Layout, Eliminates latch up&High energy implanter

HV/LV: Standard , Up to 100V/5V

BiCMOS: Integration, Speed,Memory, Power diss,

Input Impedance, Drive, Noise

Bipolar: Speed, Drive, Noise, Cost

Non Silicon: SiGe (Emerging: BiCMOS: Integration ) ,

GaAs (BJT, better Ft)

HomoJunction & HetroJunction Transistors (HBT better: Emitter efficiency,lower Base Res. >> Freq. response, temp range)