$1.2.$ Why do you need a barrier metallization in the formation of Al on Si ohmic contact?

The following process sequence is to be used to fabricate a simple LOCOS isolated poly

gate CMOS technology. Find and explain four $($there are five$)$ fatal errors that are

embedded in the sequence. Be sure to be specific about why the step is wrong. $($It may be

helpful to draw some cross sections of the transistors, but this is not required.$)$

Starting wafer: p$-$type, concentration $<1015$ cm$-3$

Initial oxidation $(1000-$ thermal oxide$)$

N$-$well mask

Implant n$-$well $($P$,1$x$1013$ cm$-2,60$ keV$)$

Strip resist

Etch oxide to dewet $(10$:$1$ HF$)$

N$-$well drive $(1100$C for $3$ hr in N$2,1$ hr in O$2)$

Strip all oxide $(10$:$1$ HF$)$

Pad oxidation $(400-$ thermal oxide$)$

Nitride deposition $(9000-$ LPCVD nitride$)$

Field mask lithography

Etch nitride

Strip resist

Field oxidation $(5000-$ wet thermal oxide$)$

Strip nitride

PMOS select mask

Implant PMOS devices for threshold voltage control

NMOS select mask

Implant NMOS devices for threshold voltage and punchthrough control Strip resist

Strip pad oxide

Gate poly deposition

Explain the purpose of the following steps in a fabrication process flow:

a$.$ Lightly doped drain implant

b$.$ Silicidation

c$.$ HALO implant

d$.$ Field implant before LOCOS isolation