Floating Gate Devices are widely used in ROM based memories. In EEPROM, one such Floating gate MOS device FLOTOX is used. FLOTOX(Floating Gate Tunneling Oxide MOS) works on FN tunneling phenomenon. This presentation discusses the method for fabrication of such FLOTOX device(actually a EEPROM cell).

1. Fabrication of FLOTOX
(EEPROM Cell)
Sudhanshu Janwadkar, Teaching Asst, SVNIT, Surat
Lecture PPT: 24th Oct 2017

2. Cross-Section View

3. • The fabrication begins with the formation of
an oxide layer on a silicon substrate
• It is followed by the patterning of a
photoresist mask
• Ion implantation is carried out to form the
buried n+ regions of the EEPROM memory cell.
1
PR = Photoresist

4. • After the formation of the buried n+ regions,
a tunnel window opening is etched in the oxide
layer (utilizing a second photoresist mask).
2
Tunnelling Window

5. • A thin layer of tunnel oxide , approximately
80 Å thick, is then grown in the tunnel window
3

6. • Following the growth of the tunnel oxide, a
first layer of polysilicon is deposited and
doped to a desired conductivity.
4
Floating Gate (Poly)

7. • This is followed by formation of an
oxide/nitride/oxide (ONO) layer over the
first polysilicon layer.
5

8. • The ONO and underlying first polysilicon layer
are then masked and etched to define the
polysilicon floating gate of the memory cell
with an overlying ONO layer.
• Reoxidation and etchback results in the
formation of oxide sidewall spacers on the
edges of the floating gate and ONO.
6

9. • A second layer of polysilicon is then deposited and
doped to a desired concentration and then etched to
define a control gate of the memory cell and the gate
of the access transistor of the FLOTOX cell
7

10. • An N+ source/drain implant is then performed
to the source/drain regions of the access
transistor
8

11. • Finally, a layer of dielectric material is
formed and planarized and then etched to
form a contact opening to the N+ drain/bit
line. This is followed by formation of a metal
bit line structure, resulting in the FLOTOX
cell shown
9
DIELECTRIC

12. References
• Patent US 5856222 A