chapter13 :metallization gilan university electrical engineering college device fabrication i: 1/33...

Chapter13 :Metallization

GILAN UNIVERSITY

ELECTRICAL ENGINEERING COLLEGE

Device fabrication I:

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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

10

Silicon nitride

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.