Gas Cluster Ion Beam (GCIB)

1. Meeting report 11.14.2007 Gas Cluster Ion Beam (GCIB) Inkyu Eu Univ.of Michigan Ann Arbor Mechanical Engineering

2. Overview 1.What is the GCIB? 2.The GCIB equipment 3.Velocity of cluster 4.Charge to mass ratio and low-energy atomic interactions

3. 1.What is the GCIB? -GCIB: Gas cluster ion beam -Using electrically charged cluster ions consisting of a few hundreds to a few thousands of atoms or molecules of gas-eous materials. -When an cluster ion impacts upon a surface, it interacts nearly simultaneously with many target atoms and deposits high energy density into a very small volume of the target material. Fig.1.Illustration of monomer and cluster ion beam concept I.Yamada and N.Toyoda(2006)

4. 1.What is the GCIB? -Result in highly non-linear sputtering and implantation effects which are fundamentally different from monomer ion impacts. Fig.2.Characteristics ofGCIB-solid surface interaction and their industrial application fields. I.Yamada (1999)

5. 2.The GCIB equipment Fig.4.Schematic of gas cluster ion beam system. Fig. 3. Schematic configuration of GCIB processor. Allen Kirkpatrick(2003)

6. 2.The GCIB equipment Many different gases: Ar, O2, N2 and mixtures of these gases with various reactive components such as CF4 or SF6. Neutral clusters: Formed by expansion of the source gas at high pressure through a supersonic nozzle into vacuum. Second vacuum stage: Clusters are ionized by electron bombardment and accelerated to high potential (A few kilovolts to a few tens of kilovolts) Magnetic filtering : Eliminate monomer ion contamination results in a beam comprised only of cluster ions (Size distribution ranging from a few hundred to several thousand atoms) Neutralizer: Injects low energy electrons into the beam so as to minimize space charge blow-up and to prevent charge build-up on nonconductive targets. Allen Kirkpatrick(2003)

7. 2.The GCIB Experiment 15 -1 10 ion/cm2 doses of 20 keV Cluster ions -Cluster ions: Pure Ar, Ar with a few percent added CF4 and Ar with a few percent added SF6. -Rate of surface removal due to cluster ion bombardment increased by an order of magnitude or more when a small amount of halogen compound was added to the Ar source gas. + Fig.4. Etch rate comparisons. Allen Kirkpatrick(2003)

8. 3.Velocity of cluster 1.Mach Num(M): The ratio of translational velocity to sound velocity 2.The ejected vapor attained supersonic translational velocities. 3.Mach Num of the vapor increased with increasing source temperature. 4.As nozzle size increased, the velocity increased. (Indicating the enhanced conversion of thermal energy to streaming KE as expected for gas expansion.) Fig.6.Translational velocity of Ag clusters measured using the time-of-flight method. The broken lines indicate translational velocities attained in isentropic expansions at the Mach numbers indicated. I.Yamada and G.H.Takaoka(1993)

9. 4.Charge to mass ratio and low-energy atomic interactions The effect of a very low charge to mass ratio -Cluster ions containing up to several thousands of atoms typically become only singly or doubly ionized. (Doubly & highly charged cluster ions can be broken up by Coulomb repulsion) Low-energy individual atomic interactions -Clusters have a kinetic energy (K.E.) proportional to the cluster mass. -The K.E. can be measured by ionizing the clusters and measuring the current on a collecting electrode as a function of the retarding potential. -The K.E. per atom: 0.1~0.2eV Fig. 1. Schematic of electrode configuration of an ionizer and a size separator in the liquid cluster ion beam system. I.Yamada and N.Toyoda(2006)

10. Meeting report 11.14.2007 Cluster beam deposition Inkyu Eu Univ.of Michigan Ann Arbor Mechanical Engineering Fig.1.Carbon film obtained by a hard mask method with CBD

11. Overview 1.What is the CBD? 2.Supersonic expansions 3.Supersonic expansions experiment 4.Comparisons

12. 1.What is the CBD? • Cluster Beam Deposition • Method that deposits neutral nanoparticles as a type of cluster on substrate • High degree of purity and exact nanoparticle size control Fig. 2. Schematic showing the deposition of patterned films by placing a hard mask into the particle beam at some distance from the substrate. K.Wegner and P.Piseri(2006)

13. 2.Supersonic expansions Fig.3. Schematic representation of the supersonic beam apparatus for the deposition of the cluster beam P. Milani and P. Piseri(2001)

14. 2.Supersonic expansions 7 The first chamber : Cluster source, Base pressure (1 10 Torr) The second chamber: The supersonic cluster beam enters (2 mm of electroformed skimmer diameter), Sample holder, a quartz microbalance for beam intensity monitoring (Fast ionization gauge for time of flight measurements of the velocity distribution of particles in the beam), Background pressure(1 10 Torr) The third chamber: Linear time of flight mass spectrometer, which is placed collinear to the beam axis in order to achieve the best transmission. + 7 + P. Milani and P. Piseri(2001)

15. 3.Supersonic expansions experiment Carbon thin films growing at high deposition rates. Fig. 4.(A,B). Cluster-assembled films. Scanning electron microscopy (SEM) micrographs taken with different magnifications of the surface and of the section of a cluster-assembled carbon film. (C) Granular structure based on clumps of spherical aggregates with typical diameter of few tens of nanometers. Atomic force microscopy (AFM) image of the topography of the film surface. E. Barborini and P. Piseri(1999)

16. 4.Comparisons Gas cluster ion Cluster beam deposition • The exploitation of nanoparticle inertial • properties • -Supersonic CBD favors the manipulation • and positioning of nanoparticles • Higher intensity and high growth rates • Cluster beams can be used to grow • nanostructured thin films where the • original cluster structure is preserved • after the deposition. • -The use of supersonic expansions may • improve the deposition rate and favor a • better control on cluster mass distribution The effect of a very low charge to mass ratio -Cluster ions containing up to several thousands of atoms typically become only singly or doubly ionized. (Doubly & highly charged cluster ions can be broken up by Coulomb repulsion) Low-energy individual atomic interactions -Clusters have a kinetic energy (K.E.) proportional to the cluster mass. -The K.E. can be measured by ionizing the clusters and measuring the current on a collecting electrode as a function of the retarding potential. -The K.E. per atom: 0.1~0.2eV K.Wegner and P.Piseri(2006)

17. The End

18. 1.How can we choose proper size of clusters for this experiment? Retarding voltage -Energy filter and distribution corresponding to the cluster size distribution -The kinetic energy of a neutral cluster is proportional to its size, so cluster ion that has a kinetic energy larger than the retarding potential is measured as an ion current. -Retarding potential exceeds the acceleration voltage, low-energy particles such as monomers and small clusters are repelled by it. -Large cluster ions with high energy pass through it. Fig. 1. Schematic of electrode configuration of an ionizer and a size separator in the liquid cluster ion beam system.

19. 1.How can we choose proper size of clusters for this experiment? The electron voltage for ionization (Ve) was 200 V, and the electron current for ionization (Ie) was 200 mA. The extraction voltage (Vext) was 1 kV, and the acceleration voltage (Va) was 5 kV. A cluster ion beam contains many monomer ions, and it decreases rapidly at a retarding voltage of 0V. the cluster ion current measured at positive retarding voltages increases with the increase in vapor pressure, and an ion current of a few hundreds of nA is obtained. Based on the retarding spectrum at the positive retarding voltages, the energy can be converted to cluster size using the assumption that each ethanol molecule has an energy of 284 meV. Retarding spectrum for an ethanol ion beam

20. 1.How can we choose proper size of clusters for this experiment? the cluster size is distributed between a few hundreds and a few thousands, and the intensity of ethanol clusters increases with the increase in vapor pressure. With regards to the mass resolution, which is defined principally by the uniformity of the potential at the ionizing point and the retarding electrode, the energy resolution of a typical retarding electrode is about 10 eV, and it corresponds to a size of 35 molecules per cluster. Although the mass resolution is quite low in this method, ethanol cluster ions with a size larger than a few hundreds are obtained. ethanolcluster size distribution measured at ionization conditions of Ve = 200V and Ie =200mA as a parameter of ethanol vapor pressure.

21. 4.Charateristics of Cluster ion beam The effect of a very low charge to mass ratio. Cluster ions containing up to several thousands of atoms typically become only singly or doubly ionized. Consequently, a cluster ion beam at any given current density can transport up to thousands of times more atoms than a monomer ion beam at the same current density. Another advantage of GCIB processes is that they involve essentially low-energy individual atomic interactions even when the total energy of the clusters is high. Since the kinetic energy of each atom in a cluster ion is equal to the total energy of the cluster divided by the number of atoms comprising the cluster, cluster ion beams inherently produce low-energy irradiation effects. As an example, within a 20 keV cluster ion consisting of 2000 atoms, each of the individual atoms has energy of only 10 eV. While, due to space charge effects, it is exceptionally difficult to transport monomer ion beams at energies as low as 10 eV,equivalently low energy ion beams can be realized by using cluster ion beams at relatively high acceleration voltages.

22. 4.Charateristics of Cluster beam deposition Supersonic CBD favors the manipulation and positioning of nanoparticles by the exploitation of nanoparticle inertial properties. Cluster beam deposition (CBD) has great potential for the production of nanostructured and nanocomposite films although several technical limitations have hampered its use as one of the bottom-up approaches to the synthesis of nanomaterials [20]. Supersonic expansions have several advantages for cluster manipulation over effusive beams that make this approach very powerful for the deposition of nanostructured films and the coupling with microfabrication techniques. This is due to the fact that supersonic CBD favours the manipulation and positioning of nanoparticles by the exploitation of nanoparticle inertial properties [22, 23]. Many theoretical and experimental approaches have been developed to solve the problem of neutral nanoparticle manipulation in the gas phase