1. Atomic Layer Deposition System
By: Henry Medina
Nanoelectronics Laboratory
National Tsing Hua University

2. ALD: Reference
Riikka L. Puurunen. “Surface chemistry of atomic layer deposition: A case study for the trimethylaluminum/water process”. JOURNAL OF APPLIED PHYSICS 97, (2005)
Cambridge Nanotech website for ALD

3. ALD: Outline First Part Second Part Third Part What is ALD process
Applications
Basic Characteristics of ALD
Second Part
Chemical adsorption Mechanism
Growth per cycle (GPC) Effects with T on GPC
Number of cycles vs. GPC
Third Part
Requirements for Precursors
Classes
Patterning of ALD growing layers (Techniques)

4. ALD
First Part
What is ALD process

5. ALD Process
ALD is a CVD technique suitable for inorganic material layer as oxides (high-k dielectrics), nitrides and some metals.
The success of ALD is to divide CVD process in half
Perfect for deposition of very thin layers of the size of a monolayer* coating complex shapes with high quality (good step coverage).
Definition: “Film deposition technique based on sequential use of self terminating* gas-solid reactions” **
*Definition of Monolayer and Self terminating reaction will be explained later
** Definition taken from the main reference

6. ALD
First Part
What is ALD process
Applications

7. ALD: Applications Semi & Nanoelectronics Optical Humidity Barriers
MEMS
Nanostructures
Chemical

8. ALD: Applications Semi & Nanoelectronics Gate Dielectrics
Intel has recently announced that their 45 nm generation processors will include a high-k HfO2 gate dielectric made by ALD. There are various reasons that ALD has become the method of choice
Unlike ALD, conventional evaporated films suffer from porosity and sputtering creates defects in the sensitive silicon surface layer.
In addition, ALD guarantees extremely uniform and reproducible thickness, low stress, growth on amorphous structure and low defect density.
Besides the industrial silicon platforms, ALD has proven essential to create gate dielectrics on device substrates without native oxides 8

9. ALD: Applications Semi & Nanoelectronics
Gate Dielectrics and Gate Electrodes
Gate Electrode (TiN by ALD)
High K dielectric (HfO2 by ALD)
TEM micrograph of 45nm Intel high-k and metal gate pMOS transistor. (Source: IEDM ) 9

10. WN metal barrier for Cu interconnects
Top
Bottom
• Prevents Cu diffusion into silicon
• Refractory nature
• Amorphous
• Acts as an adhesion promotor for Cu and Co
Cambridge NanoTech co-authored publication
ALD Tungsten nitride (WN)

11. Applications: Anti-reflection coatings
ALD good for AR coatings: large area precision thickness control and batch coating.
=> Graded index coatings posssible by varying the number of Al2O3/TiO2 low n/high n layers inside a nanolaminate stack
Zaitsu et al. Applied Phyics Letters, 80, 2442, 2002

12. Applications: Transparent conductors
ALD-ZnO transparent conductors advantages:
No costly indium as in ITO
Good optical transmission
Low resistivity (1 mOhmcm)
Large area uniformity
Very smooth films in contrast to ITO
Thin film transistors:
ALD of ZnO active matrix thin film
transistors possible as well.

13. Applications: humidity barriers
Water vapor transmission rate of 25 nm ALD Al2O3 better than 1 mm polymer encapsulation!
WVTR <10−5 g/m2 day demonstrated
Applied Physics Letters, 89,

14. Applications: Semiconductor memory
3D DRAM needs conformal coating
of high-k dielectric and metal electrode
C=kA/d: Al2O3, ZrO2, Ta2O5
DRAM crown
DRAM trench
High aspect ratio ALD
of Ta2O5 in vias of
170 nm dia, 7 microns
deep
100 nm
BST: Barium Strontium Titanate
PZT: Lead Zirconium Titanate (perovskite)
SBT: Strontium Bismuth Tantalate (layered perovskite)
100 nm
Samsung uses ALD for DRAM manufacture!
Hausmann et al.
Thin Solid films 2003.

15. Applications: Gate dielectrics on non-Si devices
Client: Prof. C.M. Marcus,
Harvard University.
(a) Schematic of finger gated devices. Mo gates (150
nm wide 10 nm thick) were defined lithographically on a Si/
SiO2 substrate and subsequently coated with 25 nm of HfO2 grown by low-temperature ALD. Nanotubes were grown across these local gates by CVD and contacted with Ti/Au electrodes. Not to scale.
(b) Atomic force micrograph of nanotubes grown across Mo finger gates and contacted (far left and far right) by Ti/Au leads. Note that one finger gate passes directly underneath the nanotube-metal contact. Arrows indicate the location of the nanotube. Finger gates are labeled as in the text.
Local gating of carbon nanotubes, Biercuk, Nano Letters 2003

16. Applications: Gate dielectrics on non-Si devices
Client: Nobel laureate Prof. Tsui,
Princeton University

17. Applications: Gate dielectrics on non-Si devices
Client: Prof. Ohno,
Tohoku University, Japan.

18. Applications: ALD lift-off technology
Client: C.M. Marcus, Harvard University.
Cambridge NanoTech co-authored publication,
Applied Physics Letters 2003.

19. Applications: ferromagnets
Nickel nanotubes grown in porous alumina, then alumina etched away
Client: K. Nielsch, Max Planck Germany
K. Nielsch, Max Planck Institute, 2006
Cambridge NanoTech Inc. Confidential

20. Applications: Porous structures
Client: K. Nielsch, Max Planck Germany
TiO2-Al2O3-TiO2 coaxial nanotubes grown with ALD inside porous alumina.
even coating inside aerogels has been demonstrated for gas sensing and catalytic applications
K. Nielsch, Max Planck Institute, 2006

21. Applications: Nanotube formation
Client: K. Nielsch, Max Planck Germany
Nature Materials Published online:
2 July 2006; doi: /nmat1673
even coating inside aerogels has been demonstrated for gas sensing and catalytic applications

22. Play-LD: coating of virus
Deposition of Al2O3 inside and around tubular shaped tobacco mozaic virus length 300 nm, OD 18 nm, ID 4 nm. Grown < 80C
Client: K. Nielsch, Max Planck Germany
Cambridge NanoTech Inc. Confidential

23. ALD First Part What is ALD process Applications
Basic Characteristics of ALD

24. ALD: Basic Characteristics
Steps:
Self-terminating reaction of the first reactant (Reactant A)
Purge or evacuation to remove non-reacted reactant and by products
Self-terminating reaction of the second reactant (Reactant B)
Purge
This is considered as one reaction cycle

25. ALD example cycle for Al2O3 deposition
2005 © All rights reserved
Cambridge NanoTech Inc.
Tri-methyl
aluminum
Al(CH3)3(g) C H Al O
Hydroxyl (OH)
from surface
adsorbed H2O
Methyl group
(CH3)
Substrate surface (e.g. Si)
In air H2O vapor is adsorbed on most surfaces, forming a hydroxyl group.
With silicon this forms: Si-O-H (s)
After placing the substrate in the reactor, Trimethyl Aluminum (TMA) is pulsed into the reaction chamber.
Cambridge NanoTech Inc. Confidential

26. ALD cycle for Al2O3
2005 © All rights reserved
Cambridge NanoTech Inc. C H Al O
Reaction of
TMA with OH
Methane reaction
product CH4
Substrate surface (e.g. Si)
Trimethyl Aluminum (TMA) reacts with the adsorbed hydroxyl groups,
producing methane as the reaction product
Al(CH3)3 (g) + : Si-O-H (s) :Si-O-Al(CH3)2 (s) + CH4
Cambridge NanoTech Inc. Confidential

27. ALD cycle for Al2O3
2005 © All rights reserved
Cambridge NanoTech Inc.
Methane reaction
product CH4
Excess TMA H H C C H H Al O
Substrate surface (e.g. Si)
Trimethyl Aluminum (TMA) reacts with the adsorbed hydroxyl groups,
until the surface is passivated. TMA does not react with itself, terminating the reaction to one layer. This causes the perfect uniformity of ALD.
The excess TMA is pumped away with the methane reaction product.
Cambridge NanoTech Inc. Confidential

28. Cambridge NanoTech Inc.
ALD cycle for Al2O3
2005 © All rights reserved
Cambridge NanoTech Inc.
H2O O H H H H C C H H Al O
After the TMA and methane reaction product is pumped away, water vapor (H2O) is pulsed into the reaction chamber.
Cambridge NanoTech Inc. Confidential

29. Cambridge NanoTech Inc.
ALD cycle for Al2O3
2005 © All rights reserved
Cambridge NanoTech Inc.
Methane reaction product
New hydroxyl group
Methane reaction
product H
Oxygen bridges O O Al Al Al O
H2O reacts with the dangling methyl groups on the new surface forming aluminum-oxygen (Al-O) bridges and hydroxyl surface groups, waiting for a new TMA pulse. Again metane is the reaction product.
2 H2O (g) + :Si-O-Al(CH3)2 (s) :Si-O-Al(OH)2 (s) + 2 CH4
Cambridge NanoTech Inc. Confidential

30. Cambridge NanoTech Inc.
ALD cycle for Al2O3
2005 © All rights reserved
Cambridge NanoTech Inc. H O O O Al Al Al O
The reaction product methane is pumped away. Excess H2O vapor does not react with the hydroxyl surface groups, again causing perfect passivation to one atomic layer.
Cambridge NanoTech Inc. Confidential

31. Two reaction steps in each cycle:
ALD cycle for Al2O3
2005 © All rights reserved
Cambridge NanoTech Inc. O H Al
One TMA and one H2O vapor pulse form one cycle. Here three cycles are shown, with approximately 1 Angstrom per cycle. Each cycle including pulsing and pumping takes e.g. 3 sec.
Two reaction steps in each cycle:
Al(CH3)3 (g) + :Al-O-H (s) :Al-O-Al(CH3)2 (s) + CH4
2 H2O (g) + :O-Al(CH3)2 (s) :Al-O-Al(OH)2 (s) + 2 CH4
Cambridge NanoTech Inc. Confidential

32. ALD: Basic Characteristics
The surface must be in a controlled state, e.g. heated
Parameters to be adjusted:
Reactants (precursors)
Substrate
Temperature

33. ALD First Part Second Part What is ALD process Applications
Basic Characteristics of ALD
Second Part
Chemical adsorption Mechanism

34. Chemical Adsorption Mechanism
Both have been seen in ALD applications

35. Chemical Adsorption Mechanism

36. Chemical Adsorption Mechanism
Self terminating Reaction
Both have been seen in ALD applications but Chemisorption by ligand exchange is preferred because is associated with (a)

37. Chemical Adsorption Mechanism
For ALD process ligand exchange is preferred
For ligand exchange saturation in the process is due to 2 factors:
(a) Steric Hidrance
(b) Number of Reactive
Surface Sites

38. ALD First Part Second Part What is ALD process Applications
Basic Characteristics of ALD
Second Part
Chemical adsorption Mechanism
Growth per cycle (GPC) Effects with T on GPC

39. Growth per Cycle (GPC)
Unit used in ALD system to describe different process.
Ideal would be growth one monolayer per cycle but this is not true due to steric hindrance

40. Growth per Cycle (GPC)
3 Models to describe the effect of steric hindrance on GPC

41. Growth per Cycle (GPC)
Model I: Just few processes can be assumed as model I ( GPC is around 25% of a monolayer)
Model II: Often used to model many ALD processes. (Gives growths of GPC under 20% of a monolayer)
Model III: Is applied only for some ideal ALD processes as AlMe3/H2O. Most optimistic (GPC around 30% of a monolayer)

42. Temperature vs. GPC
Basically depends on the surface and the reactant, an there are 4 possibilities

43. Temperature vs. GPC
Most of the papers said AlMe3/H2O for Al2O3 over SiO2 is rather insensitive to T, there are documentation from 80oC to 300oC so it’s assumed as model (b)
This is not completely true because T affect the adsorption time on surface so the cycle time per precursor should be adjusted to wait for the self terminating reaction*.
*On FAQ of ALD Savannah is possible to find some information related of how to fix this problem

44. ALD First Part Second Part What is ALD process Applications
Basic Characteristics of ALD
Second Part
Chemical adsorption Mechanism
Growth per cycle (GPC) Effects with T on GPC
Number of cycles vs. GPC

45. Number of Cycles vs. GPC
Due to the substrate there are 4 types:

46. ALD First Part Second Part Third Part What is ALD process Applications
Basic Characteristics of ALD
Second Part
Chemical adsorption Mechanism
Growth per cycle (GPC) Effects with T on GPC
Number of cycles vs. GPC
Third Part
Requirements for Precursors
Classes

47. Requirements for Precursor
Ligand precursor
To prepare the surface for next layer, and define the kind of material to growth i.e. H2O for oxides, N2 or NH3 for nitrides, etc.
Main Precursor (metallic precursor)
Highly reactive (usually this means volatile precursors), thermally stable, and full-fill the requirement for self terminating reaction

48. Ligand Precursor Nitrides
For Oxides
H2O this is the material preferred because of its physical properties
Easily decompose at low temperature by ligand exchange on H2 (Gas) and O attached to the substrate O2 O3
ROH (Alcohols with organic Chains)
Nitrides
NH3 N2
To growth pure materials the ligand precursor should be selected depending on the main precursor

49. Ligand precursor: Al2O3 from beer
Recent Cambridge NanoTech experiment:
replacement of H2O with beer + =
Cambridge NanoTech Inc. Confidential

50. Ligand Precursor: Al2O3 from beer
Al2O3 grown with H2O/TMA
Al2O3 grown with beer/TMA
Both same thickness. Beer LD is similar because the water vapor is distilled from the cylinder with beer, and thus pure H2O.This demonstrates how the intrinsic distillation of the vapor draw process in the system reduces the need for high purity precursors.
Cambridge NanoTech Inc. Confidential

51. Main Precursor Inorganic Elements
React with nonmetal compounds and hidrogen compounds
As elements don’t carry extra ligands*
Selective reactivity (just few elements)
* Free ligands are associated with impurities grown in the film

52. Main Precursor Inorganic Halides Variety of materials grown Reactive
Stable in Temperature
No extra ligands
Drawbacks: gaseous byproducts containing hydrogen nonmetal reactants e.g. HCl Corrosive etching the film and can be readsorb at the surface

53. Main Precursor Metal Organic Non direct metal carbon bounds
Organometallic

54. Main Precursor Metal Organic Alkoxides ß-diketanones Aminidates
Non direct metal carbon bounds
Alkoxides
ß-diketanones
Aminidates

55. Main Precursor Metal Organic Alkoxides Non direct metal carbon bounds
Decompose at low temperature, usually lower than 200oC
The decomposition produces the oxide already
If deposited on the surface the process lose the conformality
Alcohol as by-product, promote readsorption
The chains have high content of carbon and hydrogen

56. Main Precursor
Metal Organic
Non direct metal carbon bounds
Alkoxides

57. Main Precursor Metal Organic ß-diketanones
Non direct metal carbon bounds
ß-diketanones
Before Cyclopentadienyls were widely used to grow alkaline-earth metals
Bulky chains, causing a marked steric hindrance so GPC very low
As alkoxides the M-O ligand is difficult to remove or change for Nitrogen so is not suitable for nitrides

58. Main Precursor Metal Organic ß-diketanones
Non direct metal carbon bounds
ß-diketanones
Before Cyclopentadienyls were widely used to grow alkaline-earth metals
Bulky chains, causing a marked steric hindrance so GPC very low
As alkoxides the M-O ligand is difficult to remove or change for Nitrogen so is not suitable for nitrides

59. Main Precursor Metal Organic ß-diketanones Non direct metal
carbon bounds
ß-diketanones

60. Main Precursor Metal Organic Amidinates Non direct metal carbon bounds
New… from 2003 not widely studied
Decomposition at around 300oC
Seems to be self terminating process but GPC larger than a monolayer has been reported
Maybe for decomposition or readsorption

61. Main Precursor Metal Organic Non direct metal carbon bounds
Organometallic
Alkyls
Cyclopentadienyls

62. Main Precursor Metal Organic Alkyls: M-Cn-H2n+1 Organometallic
The reactivity of Alkyls reactant based is considered of the highest in ALD process e.g. AlMe3/H2O has a GPC of 30% of a monolayer at 300 oC

63. Main Precursor Alkyls: M-Cn-H2n+1
The reactivity of Alkyls reactant based is considered of the highest in ALD process e.g. AlMe3/H2O has a GPC of 30% of a monolayer at 300 oC

64. Main Precursor Metal Organic Cyclopentadienyls : M-5 Carbon ring
Organometallic
Cyclopentadienyls : M-5 Carbon ring
For deposition of pure metals as Ru (RuCp 2/O2). These reactants have been used since long time but gain popularity after 2000 however there are few information about the process.

65. Main Precursor Metal Organic Cyclopentadienyls : M-5 Carbon ring
For deposition of pure metals as Ru (RuCp 2/O2). These reactants have been used since long time but gain popularity after 2000 however there are few information about the process.

66. ALD First Part Second Part Third Part What is ALD process Applications
Basic Characteristics of ALD
Second Part
Chemical adsorption Mechanism
Growth per cycle (GPC) Effects with T on GPC
Number of cycles vs. GPC
Third Part
Requirements for Precursors
Classes
Patterning of ALD growing layers (Techniques)

67. Patterning of ALD Films
Photoresist
PMMA
Surface modification chemicals to become hydrophobic
Etching
On the other hand surface modification is also used to promote adhesion (hydrophilic)

68. Patterning of ALD Films
Photoresist*
Rhodium growth Rh(acac)3/O2.
Temperature too high for photoresist ??(300oC)
Additional the use of HMDS inhibit the film growth.
Film grown over Oxide
* K. J. Parka and G. N. Parsons. “Selective area atomic layer deposition of rhodium and effective work function characterization in capacitor structures”. Applied Physics Letters 89,

69. Patterning of ALD Films
PMMA*
Titanium Dioxide TiCl4/H2O and Ti(OCH(CH3)2)/H2O.
At 160oC
For TiCl4.
Cl React with PMMA, PMMA difficult to remove when more than 150 cycles are applied.
For Ti(OCH(CH3)2).
No reaction with PMMA, PMMA easy to remove by normal solvents.
There’s no specification about the substrate
* Ashwini Sinha, Dennis W. Hess, and Clifford L. Henderson. “Area selective atomic layer deposition of titanium dioxide: Effect of precursor chemistry”. J. Vac. Sci. Technol. B 24(6) Nov/Dec 2006

70. Patterning of ALD Films
Surface Modification*
APS
HMDS

71. Our ALD Results
APS treatment at 150oC

72. Our ALD Results
APS treatment at 150oC

73. Patterning of ALD Films
Surface Modification*
APS
Not good results but I never try to modify parameter
150oC too high for APS??
HMDS

74. Patterning of ALD Films
Surface Modification*
ODTS and Octadecene. Liquid and are baked for thermal reaction
For positive and negative patterning.
Surface treated before growth
* Rong Chen, Stacey F. Bent. “Chemistry for Positive Pattern Transfer Using Area-Selective Atomic Layer Deposition”. Adv. Mater. 2006, 18, p.1086–1090

75. Patterning of ALD Films
Surface Modification
Surface treated before growth to increase adhesion
For Si.
Piranha treatment (H2SO4/H2O2 7:3)
HF 2%
Note: Sample are immediately transfer to next process
Generate Hydride termination (Si-H).

76. Patterning of ALD Films
Etching
After coating the surface with ALD film, the excess can be removed by wet etching or dry etxhing
Wet etching: NaOH for Al2O3 has given good results