Chemical Vapor Deposition written by S' SIVARAM Chaper 8 CVD of Conductors

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Chemical Vapor Depositionwritten by S.
SIVARAMChaper 8 CVD of Conductors
Semiconductor Nano Application Lab.

• Shin, Jong Hoon

Sungkyunkwan University
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Semiconductor Nano Application Lab.
Introduction 8.1 General requirements for
conductors in microelectronics 8.2. Other
property requirement 8.3. CVD of Copper 8.3.1.
Growth chemistry 8.4. CVD of Aluminum 8.4.1.
Growth chemistry 8.4.2. Structure and Properties
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8.1 General requirements for conductors in
microelectronics
? Electrical conductance is paramount. ? Bulk
value of the materials resistivity. ?
r resistance l length of the
wire c capacitance
td signal propagation time
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8.2. Other property requirement
? Gate level conductors - Conductors such as
transition metal silicides are used directly atop
the transistor gate, since they are deposited on
a layer of polysilicon - They need to be
fine-grained and isotropic so that property
variations on certain axes are prevented. ?
Contact level conductors - Conductors such as
Ti and TiN are used in direct contact to the
underlying semiconductor in a source or drain
contact. Metals such as tungsten are deposited
over these layers to smooth out harsh
topographies created by the contact hole.
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? Interconnect level - Interconnection between
devices is accomplished by very low resistivity
conductors such as aluminum alloys. This level
may be repeated in second-, third- or
fourth-level metallization over the first layer.
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8.3. CVD of Copper
? Metallic copper has a bulk resistivity of
1.6µ?m, almost 50 better than aluminum. ? It
still has the following shortcomings a.
Lack of volatile copper fluorides and copper
chlorides creates plasma etching problems. b.
Copper readily diffuses through SiO2. c.
Diffusion of copper to transistor region could be
detrimental to device performance. d.
Manufacturable CVD processes for the deposition
of copper have not succeeded in achieving bulk
resistivity values alongside other required
properties. Hence the overwhelming advantage of
copper metallization is lost when trying to
obtain good step coverage.
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8.3.1. Growth chemistry
? Requirement of copper - An obvious
requirement for CVD precursors is volatility.
- Safety is another requirement. Many precursors
are very toxic - Stability is also important.
- Control of surface decomposition
characteristics. - Ease of decomposition,
where by-products desorb cleanly without leaving
residues.
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? Organometallic source are currently the most
popular precursors of copper CVD. - Among
organometallic compounds, Cu(hfac)2 has received
a lot of attention. This compound has high
volatility, high solubility, good thermal
stability, and low toxicity. ? The growth rate
is nearly independent of the concentration of
Cu(hfac)2. ? The deposited films show a strong
dependence of composition on temperature. ?
Nucleation of the copper is not uniform on all
surface.
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8.4. CVD of Aluminum
? Metallic aluminum and its alloys are
universally used in metallization in VLSI
circuits. ? Strong points of Aluminum - low
resistivity. - good chemical stability. -
several aluminum halides are volatile. -
Improvement of diffusion characteristics through
allying is easy to achieve through sputtering. ?
Weak point of Aluminum - The one major
drawback of sputtered aluminum is its poor step
coverage. - Poor electromigration
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8.4.1. Growth chemistry
? The most common mode of CVD growth for aluminum
is through the pyrolysis of triisobutyl
aluminum(TIBAL). - TIBAL, the smallest
blanched-chain alkyl, is expected to provide the
lowest carbon content in the deposited film.
- It also has a high vapor pressure, which is
helpful in precursor transport to the reactor,
and requires a moderate deposition temperature.
- The pyrolysis reaction occurs as follows
Diisobutyl aluminum hydride(DIBAL) in turn
decomposes to form aluminum hydride.
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Aluminum hydride then decomposes to form
aluminum, releasing hydrogen
- Complete pyrolysis takes place at over 120?.
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8.4.2. Structure and Properties
? Sputtered aluminum films currently used in VLSI
metallization often use as much as 4 wt copper
as a dopant to improve electromigration and
stress migration performances. ? A bamboo-like
structure is preferred.
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? This is the structure we would like to obtain
from the CVD process, along with the promised
step coverage improvements. ? Other constraints
include film purity. - nitrogen and carbon
degrade reliability performance. ? The
overwhelming problem with CVD films has been the
degree of difficulty in controlling nucleation
and film roughness. ? All have been difficult to
achieve with CVD aluminum films.
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