1. Plasma-Gas-Condensation Deposition of Nb Clusters to Obtain Giant Permittivity
9 Jan 2013
Jennifer DeCerbo Materials Engineer AFRL/RQQE

2. Program Goals
Overall: Use the giant permittivity (ε ~1010) of Nb12 clusters to enhance the permittivity of a dielectric film in order to produce ultrahigh energy storage devices
Year 1: Demonstrate preliminary formation of Nb clusters using procured cluster deposition source
Year 2: Optimize process inputs (sputtering power, gas flow rates, condensation length) for desired cluster size and distribution
Year 3: Investigate the approach of incorporating small size Nb clusters into a thermally stable, high resistance dielectric (ex. carbon nitride or anodized alumina) to develop high energy storage materials

3. Why Nb Clusters? Unique Properties
Inert nature of specific formations (Nb12)
Ferroelectric effect (dipole moments)
Potential for giant permittivity when embedded in dielectrics (Gor’kov-Eliashberg effect)
Potential material for ultrahigh energy storage devices
Novel high-k dielectric for energy storage, filtering, and signaling applications

4. First Phase Goal
Study the effects of process conditions on cluster formation to develop a better understanding of:
Mechanisms of the nanoparticle formation
How to control nanoparticle diameter and size dispersion
Parameters:
Argon Flow Rate
Source Current
Aggregation Length

5. The Deposition System In-house developed software
Controls gas flow rates
and source voltage
Monitors temperature and
pressure
Mantis Nanogen50 Deposition System
2” sputter source
4 mm/ 5 mm/ 6 mm apertures
amu mass filter
mm aggregation length
1-15 nm cluster size
5-175 sccm flow rate (Ar)
2 gas inlets
Liquid cooled cluster aggregation zone

6. Cluster Formation Mechanism
Cluster formation is a two step process
Initial cluster nucleation
Cluster growth
Metallic Nanocluster growth theory
Quesnel, et. al., J. Appl. Phys. 107, , 2010
Metallic vapor temperature must drop below a critical value for stable clusters to nucleate
Quantified as intensity of peak in mass spectrum
Cluster size depends the length traversed in the growth/aggregation zone after nucleation
Quantified as position of peak in mass spectrum

7. A Unique Feature of Nb Clusters
A dual cluster distribution has been observed under certain conditions, which is a unique feature of Nb clusters – not reported by many studies in literature on metals other than Nb
Nb mass spectra profiles varying the Ar flow rate
Source current = 200 mA
Aggregation length = 35 mm
Some lines have been offset upward for display clarity
The broader peak has a varying location, typically ranging from 5–10 nm diameter
The second peak at ~1 nm diameter appears at flow rates > 120 sccm

8. A Unique Feature of Nb Clusters (cont.)
We consider the peak at ~1 nm diameter to be stable and structured clusters, rather than simple condensation products, for two reasons:
The cluster size remains constant at various experimental conditions.
In a particular experiment with a moderate source current and a high Ar flow rate, the peak intensity was observed to decrease as the aggregation length changes from 0 to 50 mm, which is different from the larger clusters. It appears to suggest a different formation mechanism than the simple condensation process, i.e., one that requires a minimum temperature to overcome the reaction barrier.

9. Other Results
Atomic gas temperature is key to cluster nucleation and growth
There is a critical temperature above which clusters are unstable and will not nucleate
We can alter cluster size and intensity by manipulating the impact of processes inputs on the temperature
There is a range for Ar flow rate within which we can control the cluster size as expected by the mean of varying the aggregation length

10. Argon Flow Rate Dependence (source current=200mA, aggregation length=35mm)
Nucleation
Increased Ar density from increased flow enhances frequency of 3-body collisions assisting in the nucleation of clusters
Above a threshold, further increasing Ar flow rate begins to collimate the plasma, reducing heat transfer with the chamber walls, and therefore reducing nucleation
Cluster Growth
Below a critical level, the Ar concentration is too low to effectively conduct heat to the chamber walls
Above a flow threshold, the plasma begins to collimate, reducing heat transfer with the chamber walls, reducing nucleation and creating a shorter growth region

11. Source Current Dependence (Ar flow rate=80sccm, aggregation length=100mm)
Nucleation
Increased current increases the atomic metal species, facilitating nucleation
Above a threshold, further increasing current raises the temperature too high for efficient nucleation
Cluster Growth
At lower currents, the temperature profile facilitates nucleation to occur earlier, allowing longer time for cluster growth
At higher currents, the temperature profile reaches a steady state, resulting in a constant cluster size distribution

12. Aggregation Length Dependence (Ar flow rate=40sccm, source current=200mA)
Nucleation
As the length increases, the nucleation zone is enlarged and more clusters are formed
Above a threshold, the effect of diffusion-and-quench of clusters dominates
Cluster Growth
As the length increases, the enlarged nucleation zone results in both increase of the cluster size and the size dispersion

13. For larger clusters (red) For smaller clusters (blue)
Aggregation Length Dependence (Ar flow rate=150sccm, source current=200mA)
For larger clusters (red)
At short lengths, there is a narrow region in which nucleation can occur, resulting in a smaller number of clusters
As the length increases, the nucleation zone is enlarged
Above a threshold, the effect of diffusion-and-quench of clusters dominates
For smaller clusters (blue)
At longer lengths, the intensity follows the same trend as the larger clusters
At lengths below 50 mm, the intensity decreases as the length increases, suggesting a different formation mechanism than the larger clusters

14. Future Work
Having gained knowledge of the cluster source and the nucleation mechanisms, we will move on to the study of the formation of a special cluster, Nb12. It requires a hardware modification, in particular of that related to the mass filter, for the purpose of accurately detecting the mass of the clusters in the mass range of amu
Perform surface deposition and analysis to study the effects of the cluster size and size dispersion on the properties of the modified surface

15. Personnel Name Degree Discipline Air Force Personnel Jennifer DeCerbo
M.S.
Chemistry
Biswa Ganguly
Ph.D.
Physics
Contractors
Kevin Bray
Chemical Engineering
William Lanter
B.S.
Charles Jiao
Physical Chemist