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NC200U: Application Data
Over the past decade, nanocluster research has been a topic of intense activity(1). The large amount of academic and industrial interest has arisen due to the novel electronic, optical, chemical and magnetic properties that nanoclusters possess. Current research in this wide and exciting field ranges from fundamental studies and catalysis, LED devices, to film formation by energetic cluster impact (ECI). In the next decade it is likely that clusters will be used in nanodevices, optical data storage, magnetic data storage, and in the development of new materials.

Principle of Operation of the NC200U Nanocluster Source
The NC200U is a nanocluster source designed for use in an ultra high vacuum environment. A magnetron discharge is used to generate the clusters. Inside a liquid nitrogencooled aggregation tube, a rare gas, typically argon or helium, cools and sweeps the atoms and clusters from the aggregation region towards an aperture. The cluster size can be varied by adjusting several parameters such as the power supplied to the magnetron, the aperture size, the rate of rare gas flow, type of rare gas(es) being used, temperature of the aggregation region and distance between the magnetron and the aperture.
  • A 2" magnetron designed specifically for high pressure operation is used to provide the sputtered species. DC power can be used for sputtering metals, magnetic materials and semiconductors. The magnetron has been designed specifically for high operating pressure of up to 1 mbar in the aggregation region and for a high sputter rate. Water-cooled rare earth magnets are positioned behind the magnetron target . Standard 2" targets can be used.
  • The separation between the magnetron and the aperture can be adjusted using a linear motion-drive. This allows the user to vary the residence time of the clusters in the aggregation region, and hence the cluster size.
  • Liquid nitrogen can be used to cool the aggregation tube. This has the effect of stabilising the deposition and can also reduce the mean cluster size.
  • T-piece is provided for differential pumping. A number of apertures of different sizes are provided to suit the customer's deposition system.
  • A compatible addition to the NC200U source is our quadrupole mass filter (the QMF200) for high resolution cluster mass measurement and filtering.
1. Metallic clusters
The NC200U Nanocluster source can be used to produce metallic clusters with a high ionised content (>30% for Cu). The ionised clusters can be manipulated electrostatically and also accelerated onto surfaces to form highly adherent films(2) or to fill contact holes(3).

Cu deposition rates of up to 0.5nm/s can be achieved with the NC200U source. A Cu target, 5mm thick, has an approximate lifetime of 2kWhr using Ar as the sputtering gas. The target lifetime will depend on the sputtering rate of the material. Typical power during operation is 50 Watts and Ar gas flows range between 2 and 30sccm.

The NC200U source is capable of producing very small clusters containing a few atoms up to large clusters of greater than 12nm in diameter. The graphs below show two mass distribution spectra of Cu clusters from the NC200U source at two sets of typical operating parameters. The data was taken using the QMF200 quadrupole mass filter.

There are a number of parameters that can be varied to alter the cluster size using the NC200U cluster source. For the magnetron-based source these are:
  • Magnetron power
  • Ar flow rate
  • Adding He to the aggregation region, and the He flow rate.
  • Aggregation region temperature
  • Aperture size.
  • Aggregation length.

The relationship between the cluster size and these parameters can be quite complex. Figure 2 shows the effect of magnetron power on the mean cluster mass with different Ar flows.

Figure 1: Cu Cluster mass distributions

Figure 2: Cluster mass vs magnetron power for different Ar flows
2. Magnetic clusters
Fe, Co and Ni clusters have the potential for high density magnetic storage and more recently as catalysts for the growth of carbon nanotubes. The NC200U source is capable of producing clusters of these magnetic materials with high deposition rates.

It is necessary to use thin targets for such magnetic materials (typically 2mm thick) in order to achieve sputtering. In the extreme case of Fe, which has a very high permeability, ideally a target thickness of less than 2mm should be used.

The tapping mode AFM images (Fig3) show 5nm thick films of clusters of magnetic materials deposited onto oxidised Si substrates using the NC200U source. The clusters are observed as bright near-circular features and the mean cluster size in each case is approximately 10nm. By decreasing source parameters such as the aggregation length and the magnetron power the size of the clusters can be reduced significantly.

Figure 3: Co (left) and Ni (right) clusters on oxidised Si substrates

3. Semiconductor clusters
Si clusters have attracted considerable attention recently, not only because they luminesce, but also as potential building blocks in future nanodevices. The magnetron based NC200U nanocluster source is capable of producing Si nanoclusters with a variety of sizes with deposition rates of up to 10nm/min. The Si cluster beam contains ~25% ionised clusters which allows electrostatic manipulation and also the growth of strongly adherent films. Figure 4 shows a film of Si nanoclusters deposited on a hydrogen-terminated Si substrate using the magnetronbased NC200U source. The image was taken using an AFM in tapping mode.

Figure 4: Si clusters on a hydrogen terminated Si surface (250nm2)
Figures 5 and 6 show TEM images of the film. An amorphous–like structure can be observed. The zoomed image (Fig6) shows the individual clusters more clearly.

Figure 5 : TEM image of the Si cluster film

Figure 5: Zoomed TEM image of cluster film

4. Compound clusters
Compound clusters can be produced by adding other gases to the aggregation region. For example, TiN clusters can be formed by sputtering a Ti target with N2 as an aggregation gas.

1. For a review on the subject see for example: Moriarty P, Rep. Prog. Phys. 64 297-381 (2001) 2. Haberland H et al. J.Vac.Sci.Technol.A 10 3266 (1992). 3. Haberland H et al. J.Vac.Sci.Technol A 12 2925 (1994).