This paper presents a review of Ion Beam Technology. In this review the main applications and advantages of using on Beam technology for deposition processes when compared to technology such as plasma or evaporation (PVD) will be presented. Firstly, the technique of ion beam generation will be explained. Beneficial applications of ion beam technology will also be explained in detail.
Equipment and Technology
An ion beam source is a plasma source having a set of grids, which allows a stream of ions to be extracted. The ion beam source has the following three major components:
- Discharge chamber,
- The grids and
- The neutraliser.
Ions are generated in the discharge chamber by subjecting a gas such as Argon to an RF field. The gas is injected into an alumina or quartz chamber having an RF powered coil antenna surrounding it.
Free electrons are excited by the RF field until they have sufficient energy to break gas atoms into ions and electrons; this phenomenon is known as “inductive coupling”. The gas is then subjected to ionization and plasma is established. The antenna’s end-to-end RF voltage can reach high values.
The effect of the end-to-end RF voltage on the ions can be an electrostatic force that will create highly energised ions. Although this effect will make the ion source easy to start, these ions will erode the ion source by sputtering hence damaging it and forming contamination in the process; this phenomenon is called as “capacitive coupling”.
In order to suppress capacitive coupling in the Oxford Instruments ion source an electrostatic shield is placed inside the quartz chamber to allow only the RF magnetic component to transfer energy to the gas atoms.
The electrostatic shield suppresses the electric field, generated across the coil of the RF antenna, from entering the ion source. It also enables break up of any continuous conducting coating from depositing on the interior of the quartz plasma tube which could screen and reduce the plasma generation efficiency of the RF power.
The role of the grids is mainly to accelerate ions to a high velocity. The grid sets are made of two or three grids as shown in Figure 1a. The grids have a defined hole pattern with a number of apertures. It is the combination of all the individual beamlets that form the beam.
Based on the desired application, inter-grid separation and grid curvature are also important, for example based on the target size to be sputtered or the deposition rate. The Oxford Instruments ion source provides a low temperature plasma with slow (cold) ions (<1eV) that can be extracted from the ion source through the grids with a specifically defined energy, and which do not cause any considerable erosion of the grid structure.
While considering a three grid system, a specific applied potential difference or voltage across the grids offers the driving force for the ions.
“Screen grid” is the inner grid, which is in contact with the plasma that sets the beam voltage or energy. This is set at a positive potential with respect to the ground. Once through the screen grid holes, a negative potential relative to the ground and thus far more negative relative to the screen grid is used to accelerate the ions. The total potential difference signifies the extraction voltage for the beam. The “decelerator” grid is normally grounded and helps beam collimation, suppresses electron back-streaming and minimises redeposition of sputtered material back onto the accelerator grid and inside the source. This in turn maximizes the period between down times for grid cleaning and also makes grids cleaning easier. The final energy of the extracted ions is equal to the set beam energy (VB) as shown in Figure 1b.
Figure 1. A three grid beam formation structure a) Ion acceleration b) Ion deceleration
The neutraliser is basically an electron source that balances the ion charge in the beam so as to minimise space-charge effects allowing beam divergence through mutual repulsion of the ions and in order to prevent charging of the illuminated target or wafer. Normally, more electrons are emitted from the neutraliser than ions from the source, however these do not directly combine with the ion stream to form neutral atoms.
Beam divergence is based on several parameters that include VB(beam voltage), IB(beam Current), VA(accelerator voltage), IN(neutraliser current), etc. and is also impacted by gas scattering based on the chamber pressure, due to which the chamber pressure is kept as low as possible.
Dual Ion Beam Sputtering Chamber
The basic Dual Ion Beam Sputtering (DIBS) chamber set up, as shown in Figure 2 below, consists of a deposition source that precisely focuses a neutralised ion beam onto a target with minimal overspill so as to avoid contamination of depositing films. Using this, materials such as Au, Cr, Ti, Pt for metal tracks, magnetic materials such as Fe, Co, Ni, etc. or dielectrics such as SiO2, Al2O3, etc. can be deposited.
Figure 2. Schematic view of an Ionfab system
The chamber has an assist/etch source that can fulfill the following functions:
- It can be used to etch (or ion mill) the substrate
- It can provide “assistance” to the deposition process by bombarding the depositing film with energetic ions that can enhance or alter the film properties or stoichiometry by physical and/or chemical effects
- It can also be used as a low-energy pre-clean of the substrate before deposition. Sometimes, this source is used without grids as a plasma source of ‘thermal’ activated radicals for chemical modification of the depositing material while reducing physical bombardment of the substrate.
The Ion fabrication tool can also be supplied with only one or other of the above ion sources, either for deposition where assist or etch is not required for the process, or as an etching/milling/surface modification tool where no deposition is required.
Some of the most common materials deposited are oxides such as Al2O3, Ta2O5, SiO2 and TiO2 (usually from Al2O3, Ta, Si, SiO2 and Ti targets and with O2 added to the process gas. Indeed, O2 can be introduced either into the chamber directly or through the deposition and/or the assist source and this enables stoichiometric dielectrics to be deposited either from a stochiometric dielectric target, where oxygen depletion during sputtering is replaced, or from a metal target in reactive mode where the sputtered metal atoms are oxidized at some point, which could be on the target, during transit to the substrate or on the substrate if an oxygen-bearing assist beam or plasma is used.
The second source can also be used for substrate pre-clean to, for example, achieve improved adhesion of the films or remove native oxides, or as a physical assist during deposition to further increase the density of the films. The same can be done for nitride deposition, e.g. Si3N4 using a Si3N4 target and N2 in the chamber or assist source. Other, more ‘exotic’, materials that include MgF2, LaF3, Nb2O5, ZrO3, Y2O3, HfO2 YF3 etc., may also be deposited by assisted and/or reactive ion beam sputtering and the list includes such material as VOX that needs highly accurate control of process gas ratios to allow specific thermo-electrical properties to be attained for sensitive thermal imaging applications.
The tool also enables rotation of the substrate and inclination of the same relative to the sputter flux direction allowing further 'tuning' of film growth/characteristics and step coverage control for deposition onto surface topology. Deposition rates will be lesser than evaporation rates, but this does allows much more control with a highly reproducible and predictable deposition rate enabling very precise thickness control just by timing. The material is also sputtered and deposited in a much lower temperature environment than evaporation. The actual substrate temperature can thus be kept low during processing using the helium fed back-side cooling capability.
Ion beam deposition operates in a much lower pressure environment in the range of 10-4 Torr or less than standard magnetron sputtering, so the inclusion of any sputter gas such as Ar in the film is a lesser problem as is true for evaporation. The mean free path of ions and sputtered material is increased considerably, which also inhibits thermalization of sputtered material as well, resulting in depositing atom kinetic energies normally between 1 to 100 eV much higher than, for example, in the case of evaporated atoms. Since substrate preparation and/or film stress are usually the cause of problems in adhesion for thicker films, ion beam deposition can offer both surface pre-clean and film stress control by the second ion source.
The deposited film qualities may be classified into optical and mechanical categories:
Optical properties of a thin film have the following qualities:
- Transmittance, which is associated with dispersive values and homogeneity
- Absorption, which is associated with transparency properties
- Scatter, which is associated with surface roughness and volume defects
Oxford Instruments Plasma Technology's dedicated ion beam optical coaters give good scatter loss results thanks to smooth film deposition. Figure 3 below shows some examples of single oxide layers of SiO2 and TiO2 on Si wafer. A surface roughness of 0.22nm rms for Si3N4 deposited on a Si wafer has also been measured.
Figure 3. Surface roughness evaluation measurements by AFM
In Figure 4 below is shown Ta2O5 deposition over three consecutive runs measured over an 8” wafer deposited on a Si wafer. Figure 5 shows the corresponding refractive index repeatability obtained over the same three consecutive deposition runs. Figure 6 shows an example of Si3N4 deposition uniformity over 100mm on Si wafer with ±0.1% refractive index uniformity. Figure 7 shows an example of SiO2 deposition uniformity over 200mm on Si wafer with better than ±0.1% refractive index uniformity with 5mm edge exclusion. It can be observed that the different profile of the curve compared to Ta2O5 deposition is linked with the different platen positioning and beam parameters that affect beam divergence. SiO2.
Figure 4. Ta2O5 deposition uniformity over three consecutive runs on 200mm Si wafer
Figure 5. Repeatable dispersion for Ta2O5 refractive index over three consecutive runs
Figure 6. Si3N4 deposition uniformity over 100mm on Si wafer
Figure 7. SiO2 deposition uniformity over 200mm on Si wafer
Some examples of where Oxford Instruments Plasma Technology's tools are being used include:
- Laser Bar Coating for Individual Bars on Both Facets
- Single Cavity Filter
- Three Cavities Mirror
- Ring Laser Gyroscope (RLG)
These are outlined in more detail in the following sections.
Laser Bar Coating for Individual Bars on Both Facets
- Dual wavelength anti-reflection (AR) coating parameters with an eight layer Ta2O5/SiO2 coating
- Transmission @ 532nm: 99.815%
- Transmission @ 1064nm : 99.390%
Figure 8. Anti Reflection coating (AR) coating with 8 layers coating Ta2O5/SiO2.
Single Cavity Filter
The theoretical transmittance is shown in Figure 9 as calculated with MacLeod together with the as-deposited multilayer scan measured with a spectrophotometer
- Peak Insertion loss –0.08dB
- FWHM = 2.021nm
- Centre Wavelength: 1553.4 nm,
- 40 QW’s
Figure 9. Single cavity transmittance
Three Cavities Mirror
Figure 10 below shows insertion loss scan versus wavelength.
- Centre Wavelength 1549.8nm (ITU = 1549.72nm)
- Pass Band Bandwidth (@ - 0.5dB) = 1.07nm
- Stop Band Bandwidth (@ - 25dB) = 2.7nm
- Insertion loss (@1549.7nm:193.45THz) = -0.086dB
Figure 10. Insertion loss scan versus wavelength for a three cavities mirror
Ring Laser Gyroscope (RLG)
Figure 11 below shows transmission scan of a mirror designed for 633nm at 45°.
- Mirror loss <60ppm
- Uniformity <±0.0005
- Surface Roughness <1Å
Figure 11. Transmission scan of a mirror designed for 633nm at 45°
The list of applications is extensive with only a few examples having been shown. Many types of multilayer coatings are possible and based on the type of coating, throughput and quality required, various means can be provided for monitoring and controlling their growth such as quartz crystal monitors or in situ optical monitoring.
The main benefits offered by ion beam deposition are listed below:
- High surface quality
- Dense smooth films
- Minimal scattering
- Minimal optical losses
- High run-to-run process repeatability
- Excellent uniformity
- Maximum flexibility
- Wide Range of applications
This information has been sourced, reviewed and adapted from materials provided by Oxford Instruments Plasma Technology.
For more information on this source, please visit Oxford Instruments Plasma Technology.