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Fabrication of Floating Gate MOS (FLOTOX)


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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).

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Fabrication of Floating Gate MOS (FLOTOX)

  1. 1. Fabrication of FLOTOX (EEPROM Cell) Sudhanshu Janwadkar, Teaching Asst, SVNIT, Surat Lecture PPT: 24th Oct 2017
  2. 2. Cross-Section View
  3. 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. 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. 5. • A thin layer of tunnel oxide , approximately 80 Å thick, is then grown in the tunnel window 3
  6. 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. 7. • This is followed by formation of an oxide/nitride/oxide (ONO) layer over the first polysilicon layer. 5
  8. 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. 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. 10. • An N+ source/drain implant is then performed to the source/drain regions of the access transistor 8
  11. 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. 12. References • Patent US 5856222 A