Transcript Applications - Dr. Gary Rubloff Research Group

Atomic Layer Deposition
ENMA465
14 May 2003
Nicole Harrison
Bryan Sadowski
Anne Samuel
Kunal Thaker
Presentation Outline
• ALD Theory and Processes
• Applications
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– Focus on ALD applications in gate
dielectrics
Equipment
Comparisons to other Processes
Current and Future Development
Questions
What is ALD?
• ALD (Atomic layer deposition) previously
known by the name ALE (Atomic layer
Epitaxy) was originated by T.Suntola in
Finland.
• Deposition method by which precursor
gases or vapors are alternately pulsed on
to the substrate surface.
• Precursor gases introduced on to the
substrate surface will chemisorb or surface
reaction takes place at the surface.
ALD
• Surface reactions on ALD are all selflimiting.
• Self-limiting characteristics of the process
steps is the foundation of ALD.
• Two fundamental self-limiting mechanisms
in ALD: CS-ALD and RS-ALD.
• CS-ALD: Chemisorption Saturation process
followed by exchange reaction.
• RS-ALD: Sequential surface chemical
reaction.
CS-ALD Process
•
Substrate is exposed to the first
molecular precursor. The first molecular
precursor is retained on the surface by
chemisorption.
•
Second precursor is introduced on to the
surface, which reacts on the surface of
the first precursor.
•
Exchange reaction takes place between
the two precursors and by-products are
formed.
•
Exchange reaction continues till the
second precursor reacts with first
precursor: self-limiting step.
•
Final reaction is ML2+AN2MA(film) + 2
LN
•
Sequence is repeated to grow films.
RS-ALD Process
•
Promoted by chemistry between
reactive surface and reactive
molecular precursor.
•
Deposition process is as a result of
the chemical reaction between
reactive molecular precursors and
substrate.
•
Substrate surface is made reactive
by certain surface groups. And first
precursor is introduced to this
surface.
•
The reaction now looks like this
AN + ML2  AML +NL(A)
reaction self-saturates until AN
groups are converted to AML groups.
RS-ALD Process
•
Followed by this reaction, the first precursor is removed by inert
gas purging prior to the introduction of second precursor.
•
Second precursor is introduced, which reacts with ML surface to
form
AML + AN2  MAN + NL(B)
– Self saturates
•
The surface now looks like initial surface (AN) and surface is now
ready for reaction (A). The repetition of ABAB…sequence will
deposit films.
•
During each half-reaction(A&B), surface functionality changes
from one surface species to another.
ALD Precursor
Requirements
• Must be volatile and thermally stable
• Preferrably liquids and gases
– Can be solids
• no sintering related problems
• Should Chemi-sorb on surface or react agressively with
surface groups and each other
– Short saturation time, good deposition rate, no gas phase
reactions
• Should not self-decompose
– Destorys self-limiting property
• Affects thickness, uniformity, and containation
• Should not etch, dissolute into film or substrate
– Prevents self-limiting film growth
• Examples of substrates
– Zn, Cd, S, Se, metal halides like AlCl3, TiCl4,TaCl5, Alkyls, bDiketonates etc [2].
Applications
• Transistor Gate
Dielectrics
• MEMS
• Opto-electronics
• Diffusion Barriers
• Flat- Panel displays
– Organic Light
Emitting Diodes
(OLED)
• Interconnect
Barriers
• Interconnect seed
layer
• DRAM and MRAM
dielectrics
• Embedded
capacitors
• All thin films
(<90 nm)
• Electromagnetic
recording heads
Applications
• Transistor gate dielectrics
– Smaller transistor sizes
• Smaller channel lengths
• Thinner gate oxide (limit
of ~2.3 nm)
• Larger leakage current
– Need for high-K dielectrics
for transistor gate dielectrics
• Will allow for the use of
thicker gate dielectrics
• Lower leakage current
• Lower power loss
Applications
• New processes are concurrently being
developed as new materials are being
tested
• ALD provides one of the best alternatives
for depositing these new materials
– Conformality
– Uniformity
– Compositional
Control
– Thickness control
Equipment
● Closed System Chambers
- The Reaction chamber walls
are such that they effect the
transport (trajectory, etc.) of
the precursors.
● Open System Chambers
- Dimensions of reaction
chamber do not interfere with
ALD process (very large
relative to wafer position).
● Semi-Closed System chambers
- A channel is used with the top and bottom surfaces consisting
of two separate wafers and the precursor gas is passed
through the center.
● Semi-Open System chambers
- Similar to a semi-closed system except one side is a wafer
and the other side is gas- limited.
Equipment
• CVD conversion to ALD
– The installation of a module: facilitates fast gas switching
• ALD specific equipment
– Wafer output
– Reaction volume
Basic
– Factory Automation
ALD
Mecha
– Consistency
nism
– Conformality
– Uniformity
ALD
Mode
Equipment
• Research into more efficient equipment design continues
– PEALD/ ALD reactor developed by Genitech
Corporation
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Hf[N(CH3)2]4 as the reactant
species with an O2 plasma
and H2O
Temperature in the range of
200-350C
Ar is the carrier and purge
gas
Allows for increase in the
choice of reactants
Better film quality
Increased deposition rates
Comparison of ALD to
CVD and PVD
•
Process description
– ALD
• atomic layer-by-layer deposition process where precursor
is introduced into system, allowed to react with substrate
surface until saturation, remaining by-products or
unreacted gases are purged, process is repeated to form
a monolayer/film
– CVD
• chemical reaction between gaseous precursors in gas
phase; solid product of reaction is deposited on surface of
substrate.
– PVD
• deposition process in gas phase where source material is
physically transferred in the vacuum to the substrate
without any chemical reactions being involved
Comparison Continued
• self-limiting: gas-surface reactions occur till
surface is saturated
• Precursors are introduced into system separately
• reactions occur at the surface
• precise thickness control
• 100% step coverage even at high aspect ratios
• Films are very uniform, smooth, and stoichiometric
Comparison Continued
• less contamination
• low pressures
– typically less than 10-6 torr
• Film thickness can be as low as 1nm
• produces amorphous films
• lower deposition temperatures
– as low as 180oC for HfO2
Comparison Continued
• slow deposition rates
because have to form
monolayer in two steps
– HfO2: dep. rate of
0.1nm/s @ 300oC
• Growth rate
– increases, reaches
a max, and
decreases slightly
as temperature is
increased
– increases with
number of cycles
Thin Solid Films, 427(1-2), p.147-151.
Current Developments
• Materials being used
– ZrO2 , Al2O3, HfO2
• All exhibit good qualities
– thickness control
– good conformity
– excellent step coverage
Current Developments
• High-k materials
– traps electrons between barrier gate and SiO2
– prevents burrowing of electrons through SiO2
• High barrier height
– energy required for electrons to pass from gate to
SiO2
material
formula
kox
silicon oxide
silicon nitride
Oxynitrides
aluminum oxide
tantalum pentoxide
hafnium oxide
zirconium oxide
barium strontium titanate (BST)
SiO2
Si3N4
SixNyOz
Al2O3
Ta2O5
HfO2
ZrO2
BaSrTiO3
3.9
7
4-7
9
25
30 - 40
25
300
Future Developments
• HfO2-Al2O3
laminate
– Being Developed
by Samsung
– combines good
qualities of both
– Reduced current
leakage
• due to high-k of
HfO2 and high
barrier height of
Al2O3
Future Developments
• SiN/SiO2 stack gate-dielectric
– SiO2 experiences soft-breakdown
• due to differences in thermal expansion, high
electric field, and strained Si-O bonds
–
–
–
–
low thermal budget (550oC)
lower leakage current
higher reliability
soft-breakdown free
• fewer wasted samples
• Reduces Boron penetration in SiO2
Future Developments
Future Developments
• Zr(t-OC4H9)4, commonly called
ZTB being used to replace ZrCl4
as a precursor when making
ZrO2
– Cl supposed to be purged once Zr
has been reacted
– sometimes reacts with substrate
– can cause damage to whole
system
References
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Leskela Markku, Ritala Mikko. " Atomic Layer Deposition (ALD): From Precursors to
Thin Film Structures". Thin Solid films, 2002 Vol 409. PP 138-139.
Ritala Mikko. "Fundamentals of ALD Chemistry" presented at AVS Topical Conference
on Atomic layer deposition, May 14-15. 2001 Monterey, California.
Sneh Ofer, Phelps Robert et.al. “ Thin film atomiclayer deposition equipment for
semiconductor processing” . Thin solid films, 2001 Vol 402. PP 248-261.
Scott Thompson, Portland Technology Development, Intel Corp. Paul Packan,
Technology Computer Aided Design, Intel Corp. Mark Sohr, Portland Technology
Development, Intel Corp. Intel Technology Journal. MOS Scaling: Transistor Challenges
for the 21st Century. 10 April
2003.<http://www.intel.com/technology/itj/q31998/articles/art_3.htm>.
Ho-Kyu Kang. Samsung Electronics Co., LTD. ALD high k materials for gate and
capacitor gate dielectrics. ALD Conference 2002.
ASM. The process of innovation. Pulsar. Single Wafer Atomic Layer CVD. 10 April
20003. <http://www.asm.com/prod_pulsar.asp>.
Moo-Han Corporation. ALD System. ELFRA 7000 Series. 10 April 2003.<http://www.moohan.com/eng/html/product_ald_2.html>.
Dae-Youn Kim, Kyung-Il Hong, Jeongseok Ha, Cheol-Min Chang, Seung-Woo Choi,
Hyung-Sang, Park, Yong-Min Yoo, Wonyong Koh and Choon-Soo Lee.Plasma Enhanced
Atomic Layer Deposition of HfO2.Genitech, Inc. ALD Conference 2002.
Ho-Kyu Kang, Samsung Electronics Co., LTD, San#24 Nongseo-Ri, Giheung-Eup, YonginCity, Gyeonggi-Do, Korea 449-711
Anri Nakajima and Shin Yokoyama, Research Center for Nanodevices and Systems,
Hiroshima University 1-4-2 Kagamiyama, Higashi-, Hiroshima, Hiroshima 739-8527,
Japan
Question and Answer