1. Chapter13 :Metallization
GILAN UNIVERSITY
ELECTRICAL ENGINEERING COLLEGE
Device fabrication I:
Chapter13 :Metallization
By:
Moein Moshtaq Ebadghorbandoost
Keyvan alireza zad

2. What is the Metallization?
Introduction
What is the Metallization?
Fabrication of circuits iclude 2 part:
1- the active and passive part.
This is called the front end of the line (FEOL).
2- metal systems back end of the line (BEOL)

3. Objectives
List the requirements of a material for use as a chip surface conductor.
Draw cross sections of single and a two layer metal schemes.
Introduce three materials used in the metallization
All about sputtering , evaporation and CVD methods.
Describe the principle and operation of vacuum pumps.

4. Conductors—Single-Level Metal
In the MSI era metallization was relatively straightforward :
1 _Wafer with doped region
2_patterning : open contacts
contacts
3_layering : conducting layer
10000 to Å by(sputtering , cvd, evaporation)
4_ patterning: metal mask
Leads , Metal lines or interconnects
5_heat treatment ( alloying)

5. feature of metal system
convenient current
Connection and adhesion between sio2 and metal
Purification
Prevent against unwelcome rust
A Level without hole

6. Conductors_ Multilevel Metal Schemes
Increasing the chip density Decreasing the available space on the wafer for wiring
Multilevel Metallization

7. Conductors_ Multilevel Metal Schemes
Example:
creating a CMOS Inverter on the wafer
PMOS
NMOS
thin oxide

9. Salicide

10. Silicon nitride

23. Multilevel Metal Schemes

24. Conductivity Metal Bulk resistivity (mW*cm) Silver (Ag) 1.6
Copper (Cu)
1.7
Gold (Au)
2.2
Aluminum (Al)
2.8
Tungsten (W)
5.3
Molybdenum (Mo)

25. Conductors_ Aluminum From an electrical conduction standpoint,
aluminum is less conductive than copper and gold.
Prior to the development of VLSI-level circuits,
the primary metallization material was pure aluminum.
resistance
Melting point
Aluminum
2.7 mWcm
660 ºC
Copper
1.7 mWcm
1084 ºC

26. Conductors_ Aluminum
Copper, if used as a direct replacement for aluminum, has a high contact resistance
Aluminum emerged as the preferred metal because:
1- has a low enough resistivity (2.7 μΩ-cm) and good current-carrying density.
2-has superior adhesion to silicon dioxide
3- is available in high purity
4-has a naturally low contact resistance with silicon
5- is relatively easy to pattern with conventional photolithography processes.
6- Aluminum sources are purified to 5 to 6 “nines” of purity( )

27. Conductors_ Aluminum Problems in Aluminum:
one) AL and Si Alloy at 577 C
(Start at 455 C the convenient temperature for good junction)
But Aluminum spikes punctuate doped junction and make short contact

28. Conductors_ Aluminum
Real photo

29. Conductors_ Aluminum TiW and TiN are used Barrier Layer
(we saw in multilevel schematic)
Al with 1 to 2% Si
(not reliable)
TiW and TiN are used
Two Solution ways:

30. Conductors_ Aluminum Two)Electromigration
In longe distance path(vlsi,ulsi),Metal interconnections in ICs are operated in high current densities and under this situation metal movement causes void and metal pileup or hillocks.
Atoms are under electrical field and thermal gradiant concurrently and move in itself .

31. Solution: Al alloy with 0.5 to 4 % Cu or 0.1 to 0.5 % titanium
Conductors_ Aluminum
Solution: Al alloy with 0.5 to 4 % Cu or 0.1 to 0.5 % titanium
Al alloy with Si and Cu can use to solve both problems

32. Conductors_ Aluminum
Overall effectiveness of a metal system is governed
by the resistivity, length, thickness, and total contact resistance of all the metal-wafer interconnects.
In a simple aluminum system,there are two contacts: the silicon/aluminum interconnect and the aluminum interconnect/ wire.
In a ULSI circuit with multilevel metal layers, barrier layers, plug fills, polysilicon gates and conductors,
and other intermediate conductive layers, the number of connections becomes very large. The addition of all the individual contact resistances can dominate the conductivity of the metal system.