chapter13 :metallization gilan university electrical engineering college device fabrication i: 1/33...
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Chapter13 :Metallization
GILAN UNIVERSITY
ELECTRICAL ENGINEERING COLLEGE
Device fabrication I:
1/33
By:
Moein Moshtaq Ebadghorbandoost
Keyvan alireza zad
Introduction
Fabrication of circuits iclude 2 part:
1- the active and passive part.
This is called the front end of the line (FEOL).
What is the Metallization?
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2- metal systems back end of the line (BEOL)
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.
Objectives
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•Conductors—Single-Level Metal
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In the MSI era metallization was relatively straightforward :
1 _Wafer with doped region
2_patterning : open contacts
contacts
3_layering : conducting layer
10000 to 15000 Å by(sputtering , cvd, evaporation)
4_ patterning: metal mask
Leads , Metal lines or interconnects
5_heat treatment ( alloying)
feature of metal system convenient current
Connection and adhesion between sio2 and metal
Purification
Prevent against unwelcome rust
A Level without hole
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•Conductors_ Multilevel Metal Schemes
Increasing the chip density Decreasing the available space on the wafer for wiring
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Multilevel
Metallization
•Conductors_ Multilevel Metal Schemes
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Example:
creating a CMOS Inverter on the wafer
PMOS
NMOSthin oxide
8/
9
Salicide
11
12
13
14
15
16
17
18
19
20
21
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•Multilevel Metal Schemes
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•Conductivity
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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) 5.3
•Conductors_ Aluminum
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Prior to the development of VLSI-level circuits,
the primary metallization material was pure aluminum.
From an electrical conduction standpoint,
aluminum is less conductive than copper and gold.
resistance
Melting point
Aluminum
2.7 mWcm
660 ºC
Copper 1.7 mWcm
1084 ºC
•Conductors_ Aluminum
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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(99.99999)
•Conductors_ Aluminum
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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
•Conductors_ Aluminum
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Real photo
•Conductors_ Aluminum
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Two Solution ways:
Barrier Layer
(we saw in multilevel schematic)
Al with 1 to 2% Si
(not reliable)
TiW and TiN are used
•Conductors_ Aluminum
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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 .
•Conductors_ Aluminum
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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
•Conductors_ Aluminum
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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.