19-12-2012, 01:36 PM
CMOS fabrication process overview
CMOS fabrication.ppt (Size: 2.34 MB / Downloads: 62)
Complementary MOS fabrication
CMOS Technology depends on using both N-Type and P-Type devices on the same chip.
The two main technologies to do this task are:
P-Well (Will discuss the process steps involved with this technology)
The substrate is N-Type. The N-Channel device is built into a P-Type well within the parent N-Type substrate. The P-channel device is built directly on the substrate.
N-Well
The substrate is P-Type. The N-channel device is built directly on the substrate, while the P-channel device is built into a N-type well within the parent P-Type substrate.
Two more advanced technologies to do this task are:
Becoming more popular for sub-micron geometries where device performance and density must be pushed beyond the limits of the conventional p & n-well CMOS processes.
Twin Tub
Both an N-Well and a P-Well are manufactured on a lightly doped N-type substrate.
Silicon-on-Insulator (SOI) CMOS Process
SOI allows the creation of independent, completely isolated nMOS and pMOS transistors virtually side-by-side on an insulating substrate.
P-well on N-substrate
Steps :
N-type substrate
Oxidation, and mask (MASK 1) to create P-well (4-5m deep)
P-well doping
P-well acts as substrate for nMOS devices.
The two areas are electrically isolated using thick field oxide (and often
isolation implants [not shown here])
Twin-Tub (Twin-Well) CMOS Process
This technology provides the basis for separate optimization of the nMOS and pMOS transistors, thus making it possible for threshold voltage, body effect and the channel transconductance of both types of transistors to be tuned independently. Generally, the starting material is a n+ or p+ substrate, with a lightly doped epitaxial layer on top. This epitaxial layer provides the actual substrate on which the n-well and the p-well are formed. Since two independent doping steps are performed for the creation of the well regions, the dopant concentrations can be carefully optimized to produce the desired device characteristics. The Twin-Tub process is shown below.