The use of Highly Ionized Pulsed Plasmas for the Synthesis of Advanced Thin Films and Nanoparticles †

Pulsed plasma processes open up the possibility of using very high plasma densities and modulated deposition in the synthesis of thin films and nanoparticles. The high plasma densities lead to a high degree of ionization of the source material, which creates new possibilities for surface engineering. Ions can, in contrast to atoms, be easily controlled with regard to their energy and direction, which is beneficial for thin film growth. Furthermore, ions can also increase the trapping probability of material on nanoparticles growing in the gas phase. The pulsed sputter ejection of source material also has other consequences: the material in the plasma and the material arrival on the growth surface will fluctuate strongly resulting in high level of supersaturation during pulse-on time. In this paper, an overview of the generation and properties of highly ionized pulsed plasmas is given. In addition, the use and importance of these types of discharges in the fields of thin-film and nanoparticle growth are also summarized.


Introduction
The processing of materials and surfaces is today very important in many industrial branches.This includes thinfilm deposition to functionalize surfaces with different properties such as wear resistance, color changes, corrosive protection, catalytic behavior, or electrical and optical properties.The properties of thin films can further be improved by, for instance, the incorporation of nanoparticles in solar cell applications (Atwater and Polman, 2010).Nanoparticles are also of great interest in catalysis (Cuenya, 2010) due to their large surface-to-volume ratio or plasmonics (Garcia, 2011), where the plasmon frequency is a function of the size of the nanoparticle.
For fabricating thin films or nanoparticles, the material that condenses onto the surface or into nanoparticles can be provided by different means-e.g.thermic evaporation, arc evaporation or pulsed laser evaporation.Another method is to utilize a plasma discharge where the vapor is removed from a target (source) by sputtering the material, which is the focus of the present article.A plasma consists of ions, electrons and neutral atoms.Plasmas used for material processing typically have a low ionization degree, which is the fraction of the density of ions to the sum of the ion and neutral density.The target material is sputtered by ions that collide with the surface, which leads to the removal of surface atoms.The sputtered material is subsequently transported out into the bulk plasma and eventually condenses on all surfaces.For thin-film deposition, a magnetic field configuration is commonly used in a way that the plasma is confined in front of the target to enhance the sputtering process (Window and Savvides, 1986).This technique is called magnetron sputtering.
For improving the properties and structure of deposited thin films, one needs to control, for instance, the energy and the deposition rate of atoms and ions that condense on the substrate (coated object).The motion of sputtered atoms is governed by ballistic transport as well as diffusion, but ions can be manipulated by electric or magnetic fields, which allow the energy of the ions reaching the substrate surface to be tailored.It is therefore desirable to have a high ionization degree of the vapor material.A way of increasing the ionization degree is to increase the plasma density, which can be achieved by increasing the power supplied to the sputter target.The maximum average power possible is defined by the melting temperature and heat conductivity of the target material.Very high plasma densities can be achieved-without exceeding the maximum average power-by supplying the power in short, high-power pulses with a low duty factor.A technique that utilizes this approach to reach very high ionization degrees of the sputtered material is high-power impulse magnetron sputtering (HiPIMS) (Kouznetsov et al., 1999, Lundin andSarakinos, 2012).
The same approach has also recently been applied to synthesize nanoparticles (Pilch et al., 2013).A high ionization degree is beneficial because the collection probability of the sputtered material-i.e.ions and atoms-on a nanoparticle is a function of the collection cross-section.For atoms, the cross-section is given by the geometric cross-section of the nanoparticle but for ions, the collection cross-section can be much larger.The reason for this is that nanoparticles attain a negative charge which is defined by the equilibrium of electron and ion currents to the nanoparticle (floating condition), and this increases the ion flux of positively charged ions onto the nanoparticle.Two consequences follow in the case of negatively charged nanoparticles.First, the collection probability of positively charged ions becomes larger than the collection probability of atoms and this leads to a faster growth, which is the reason of having a plasma with a high ionization degree.Second, the agglomeration of nanoparticles in the gas phase is inherently prevented when the nanoparticles are negatively charged.
In this contribution, results from both the deposition of thin film and the synthesis of nanoparticles using highpower pulses are presented.In Section 2 an overview on the pulsed plasma is given.Thin-film deposition and its applications using HiPIMS is presented in Section 3, and details on nanoparticles using a pulsed hollow cathode are presented in Section 4. A summary of the article is given in Section 5.

High-power pulsed plasmas
There are many methods for pulsed plasma processing and it is therefore appropriate to briefly discuss the most common techniques.We will thereby come to understand what distinguishes high-power pulsed plasmas (HiPP), discussed in the present article, from other types of pulsed plasmas.
The introduction of pulsed magnetron sputtering in the mid-90s boosted the deposition of dielectrics such as alumina, titania and silica (Kelly and Bradley, 2009).The reason is that DC discharges applied to cathodes using source materials with poor conductivity charge up the cathode surface positively during the discharge.This is due to the loss of electrons as the bombarding ions are neutralized at the cathode, and thus eventually extinguish the discharge or generate detrimental arc discharges.With the use of AC discharges, it is possible to neutralize the positive charge accumulated during one half-cycle by electron bombardment during the following half-cycle (Chapman, 1980).The most common operation constitutes asymmetric bipolar discharges, where the voltage U d (t) during pulse-off is reversed (positive) to approximately 10% or less of the magnitude of U d (t) during pulse-on at frequencies in the range 20-350 kHz and a duty cycle of about 50% (Kelly and Bradley, 2009).The technique is often referred to as mid-frequency pulsed DC sputtering.
Another pulsed sputtering technique is radio frequency (RF) sputtering.Here, a high-frequency (13.56 MHz) power is applied to achieve sputtering of the target material.This will also allow sputtering of isolators.However, the deposition rates for sputtering dielectrics are typically lower compared to mid-frequency pulsed DC sputtering (Glöß et al., 2005).
In HiPP discharges, the power is applied to the cathode in unipolar pulses at a low duty factor (< 10%) and low frequency (< 10 kHz), leading to peak cathode power densities of the order of several kW cm -2 while keeping the average target power density low enough to avoid heat damage to the cathode.In many ways, HiPP processes are similar to the above-described pulsed DC discharges, but they operate at a lower duty factor and considerably higher instantaneous pulse power.This means that setting up a system is fairly straightforward in the sense that it only involves changing the power supply.
An early description of the HiPP technology for material processing was provided by Kouznetsov et al. in 1999, where they used magnetron sputtering for thin-film deposition.The technique has since been commonly referred to as high-power impulse magnetron sputtering (HiPIMS) or sometimes high-power pulsed magnetron sputtering (HPPMS).It is also in this domain where we find a more strict definition distinguishing this technique from other pulsed plasma processes (Anders, 2011): HiPIMS is pulsed magnetron sputtering, where the peak power exceeds the time-averaged power by typically two orders of magnitude.This is very different from what one finds in, for example DC, pulsed DC or RF discharges, and has a dramatic influence on the plasma characteristics and hence the sputtered material, as will become apparent in the next section.
Recently, HiPP technology using the same power supplies as in HiPIMS has been investigated in the case of hollow cathodes for plasma-enhanced chemical vapor deposition (PECVD) of thin films (Pedersen et al., 2012) as well as for nanoparticle synthesis (Pilch et al., 2013).The deposition of thin films and the synthesis of nanoparticles depend on the pulse parameters: frequency, pulse width, and current, as well as other process parameters, such as operating gas pressure.The principles behind the plasma generation remain the same.Finally, a typical discharge pulse is seen in Fig. 1.

Pulsed plasmas for generating highly ionized material fluxes
In glow discharge processes such as magnetron sputtering, it is often difficult to achieve a large fraction of ionized sputtered material reaching the substrate (Christou and Barber, 2000, Rossnagel and Hopwood, 1994, Konstantinidis et al., 2004).When the deposition flux consists of more ions than neutrals, the process is referred to as ionized physical vapor deposition (PVD) or IPVD (Helmersson et al., 2006).
Commonly used IPVD techniques are inductively coupled discharges (ICP) where the plasma density is increased by inserting an antenna in the plasma and feeding it with RF-power (Hopwood, 2000), but also the industrially widespread method of cathodic arc evaporation (Johnson, 1991), which in some cases also operates in pulsed mode (Rosén et al., 2007).A drawback with arc evaporation is the formation of macro-particles through explosions caused by overheated spots on the sources.The HiPP technology also belongs to the IPVD group.
The high instantaneous power densities applied to the cathode used for HiPP plasma generation result in a strong increase of charge carriers in front of the cathode during the discharge pulse.As an example, in a HiPIMS discharge, the electron density in the region close to the cathode target surface is on the order of 10 18 -10 19 m -3 (Gudmundsson et al., 2009, Bohlmark et al., 2005).For an electron density around 10 19 m -3 , the ionization mean free path of a sputtered metal atom is about 1 cm, while for an electron density of 10 17 m -3 , commonly observed in a steady-state DC magnetron sputtering (DCMS) discharge, the ionization mean free path is approximately 50 cm for typical discharge conditions (Gudmundsson, 2010).Thus, given the high electron density in the HiPP discharge a significant fraction of the sputtered material is ionized, which has also been reported in a great number of publications [see Gudmundsson et al. (2012) and references therein].In Fig. 2, we schematically illustrate the sputter ejection of neutral material atoms by ions from the volume plasma.The sputtered particles are then transported out into the deposition chamber and may undergo an ionizing collision depending on the plasma conditions.A fraction of the sputtered material eventually reaches the substrate, either as neutrals or as ions.
Worth pointing out is that although the ionized flux fraction typically reaches > 50% (IPVD), there are, however, substantial differences in the degree of ionization of the sputtered material depending on the target material, ranging from only a few percent to almost fully ionized (DeKoven et al., 2003, Bohlmark et al., 2005).The reason for this behavior is mainly related to the fact that the ionization potential is material-dependent and for commonly used metals is in the range E IP = 5.99 eV (Al) to E IP = 11.26eV (C), meaning that Al is more easily ionized than C.
In the plasma discharge model by Samuelsson et al. (2010), these trends are investigated in more detail in the case of HiPP discharges by HiPIMS.As a reference, for direct current discharges such as DCMS, the degree of ionization of sputtered material is found to be 5% or less independent of the target material used (Hopwood, 2000).

Thin film growth by high-power impulse magnetron sputtering
The use of ionized deposition fluxes when growing thin Fig. 1 A typical discharge voltage-current plot in a HiPIMS discharge.This discharge was generated using a 6" circular magnetron equipped with a Cu target operated at 1.33 Pa Ar pressure.The peak current density is about 1.6 A cm -2 and the peak power density is approximately 1.5 kW cm -2 averaged over the entire cathode area.The time-averaged power density is 5.5 W cm -2 .films opens up several opportunities.It includes guiding the deposition material to use the ions for substrate pretreatment, and using the ions for generating effects on the growing film through self-ion-bombardment. Below, we go through some of the most attractive features.
Guiding the material flux is a great advantage for coating complex-shaped surfaces and for trench filling applications (Kouznetsov et al., 1999).On the microscopic scale, the electric field present in the sheath between the plasma and the substrate can be used.Since this field is perpendicular to the substrate surface, it will guide the ions to arrive with an angle close to the surface normal.This minimizes self-shadowing-a mechanism that promotes surface roughening-and smoother and denser films can be grown.This was demonstrated by Alami et al. (2005) by growing films on the wall inside of a 10-mm-wide and 10-mm-deep trench, see Fig. 3.The use of an ionized flux for deposition made a dramatic difference, going from a low-density tilted columnar structure when mainly neutral atoms were deposited by DCMS, to a dense microstructure with columnar grains parallel to the substrate normal when the deposition material was ionized by HiPIMS.Similar results have been demonstrated for cutting tool edges, where good film quality is achieved on both sides of the edge when using HiPIMS (Bobzin et al., 2009).
For very narrow trenches, where the plasma cannot penetrate into the small structures, the situation is different.
Here, the flux of deposition ions will be channeled down the trench and an improved bottom coverage can be achieved.In this case, sidewall coverage can be achieved by increasing the substrate bias-allowing the incoming ions to accelerate to higher energies and sputter material of the deposited coating at the entrance and at the bottom of the trench.The sputtered material can then be re-deposited on the side walls.This has been elegantly demonstrated and simulated by Hamaguchi and Rossnagel (Hamaguchi and Rossnagel, 1995).On a large scale, the ionized deposition fluxes can be guided and directed by using magnetic fields.An example of this is illustrated by Bohlmark et al. (2006b) where they focus the deposition flux towards the substrate position.The guiding can also be used for filtering away neutrals to increase the degree of ionization of the deposition material (Kouznetsov, 2004).
Ion bombardment during thin film growth is of great importance when it comes to controlling the microstructure of the film.The ions generated in HiPP processes have been found to have energy distributions shifted towards higher energies than are expected from the commonly accepted Thompson sputter energy distribution (Thompson, 1968, Bohlmark et al., 2006a).From ion energy measurements in HiPIMS discharges, it has been reported that the average ion energy is around 20 eV without using any substrate bias (Lundin et al., 2008).As it turns out, ion energies in the range 20-30 eV have been shown to have a densifying effect on thin films (Eriksson et al., 2006), since the ion flux enhances the surface mobility of the film-forming species on the growth surface and thereby reduces film porosity.Several research groups have demonstrated that HiPIMS results in denser films [see Samuelsson et al. (2010) and references therein].Ion-bombardment is also often used in non-IPVD techniques.However, in this case the bombarding species are ions of the inert sputtering gas.For reasons not yet fully understood, bombarding the growth surface with the film-forming species is more efficient than using inert gas ions.The former also minimizes the incorporation of gas into the film.
The ion bombardment of the growing film can also be used for phase-tailoring.Alami et al. (2007) demonstrated that it was possible to grow thermodynamically stable bcc-Ta.Commonly during film growth, the more brittle tetragonal-Ta phase is formed.A similar case is the one published by Aiempanakit et al. (2011).They found that TiO 2 films with anatase and rutile phases can be controllably grown at room temperature using different HiPIMS conditions.X-ray diffraction patterns of the TiO 2 films where the change from the anatase to the rutile phase can be seen in Fig. 4. A third example is Al 2 O 3 that can form a number of phases, but only one, the corundum phase, is thermodynamically stable and desired in many wearprotection applications (Selinder et al., 2009).However, for the corundum phase to form, usually very high substrate temperatures of up to 1000°C are needed.This limits the type of substrates that can be used and thereby the number of applications.Ion bombardment during growth reduces the temperature needed to grow this phase and with HiPIMS, it is possible to grow the corundum phase at a growth temperature of 575°C (Wallin et al., 2008).

Nanoparticle Synthesis
There is a wide variety of gas-phase techniques to synthesize nanoparticles, e.g.flame synthesis (Wegner and Pratsinis, 2000), thermal evaporation (Granqvist and Buhrman, 1976), and plasma synthesis (Vollath, 2007;Binns, 2001).The technique presented in this article is a plasma-based technique based on high-power pulses similar to what is used in the HiPIMS discharge.HiPP discharges for nanoparticle synthesis have not been studied in detail; however, Straňák et al. (2011) used a pulsed magnetron technique where the size of the clusters could be varied, and Werner et al. (2011) synthesized nanoparticles with different shapes using a DC magnetron with long pulse-off times.
To investigate the effect of high-power pulses and a high ionization degree on the nanoparticle synthesis, a hollow cathode was used as a sputter target which has been shown to be suitable for nanoparticle synthesis (Ishii et al., 1999).A sketch of the set-up is shown in Fig. 5 and more details of the experimental procedure can be found in Pilch et al. (2013).The nanoparticle synthesis was carried out in a stainless steel vacuum chamber in an argon atmosphere at a pressure of 106 Pa.The process gas was routed through the hollow cathode at a flow rate of 60 sccm.Typical pulse parameters used for the nanoparticle synthesis were: frequency f = 125 to 1300 Hz, peak current I H = 3 to 20 A, and pulse width t PW = 10 to 100 μs.
With the present set-up, it was possible to synthesize nanoparticles by sputtering different metals: Cu, Ag, Ti, Mo, and Zr.Examples of SEM micrographs of Ag and Cu nanoparticles are shown in Fig. 6.In both examples, a wide spread in sizes is found and two size populations were synthesized.The Ag nanoparticles have a diameter of around 60 nm and the shape of the nanoparticles is facetted; even cubic nanoparticles were found.Nanoparticles smaller than 5 nm (not seen in the image) were also synthesized.The Cu nanoparticles are spherical and two size populations were found, one with a diameter of 20 nm and the other of around 10 nm.TEM micrographs of the Cu nanoparticles are shown in Fig. 7.
To control the characteristics of nanoparticles, both their shape and size need to be controlled.For Ag it has been shown that nanoparticles with different shapes can be synthesized.Similar results for cubic Cu nanoparticles were also found.However, further control is needed to be able to synthesize nanoparticles with a specific shape.The change in size of Cu nanoparticles by adjusting the pulse parameters was studied in detail with the present set-up (Pilch et al., 2013).In Fig. 8, the variation of the nanoparticle diameter is shown for (a) varying peak current and (b) varying frequency (at constant average power).The two size populations shown in Fig. 7(b)-note that this is not the same data as shown in Fig. 8-can be avoided by choosing appropriate pulse parameters.By increasing the peak current, the nanoparticles with the smaller size become less and finally disappear.The nanoparticle size can also be changed when the frequency is varied and the average power is kept constant, see Fig. 8 power constant when the frequency was increased, the energy per pulse was lowered by decreasing the peak current.The possibility of synthesizing nanoparticles with different sizes at constant average powers allows one to optimize the nanoparticle synthesis with respect to power consumption.

(b). To keep the average
The increased control of the growth of the nanoparticles comes from the interaction and timing of the pulses, and the amount of ions also plays an important role for the growth of nanoparticles.The properties and dynamics of the discharge were modeled by Hasan et al. (2013) for an Al target.It was found that the sputtered atoms become almost fully ionized with an ionization degree of 85% at the end of the pulse.To transfer this result from Al to Cu, which has a slightly higher ionization potential, the results from modeling the ionization degree for HiPIMS discharges can be used for comparison (Samuelsson et al., 2010).Assuming the same ratio of the ionization degrees for Al (49% for HiPIMS) and Cu (27% for HiPIMS), a high ionization of about 50% for Cu is still reasonable.The high ionization degree is required to be able to effectively utilize the charge of the nanoparticles to collect ions and enhance the growth speed of nanoparticles due to the larger collection cross-section of ions (Pilch et al., 2013).
During each pulse it was found that a cloud of ions and neutrals of the sputtered species emanates from the hollow cathode.The expansion and diffusion of the ion and the atom cloud diverge because the motion of ions is governed by ambipolar diffusion, which is a faster process than the diffusion of neutrals in the background gas.Due to the periodical ejection of material and the pulsing of the discharge, four mechanisms that affect the growth have been proposed (Pilch et al., 2013): pulse strength, neutral overlap, ion overlap, and discharge overlap.
The pulse strength is a measure of the density of the sputtered material that is available for the growth of nanoparticles.The sputtered material that is blown out of the hollow cathode condenses into small clusters, which may coalesce into larger clusters depending on the density of the small clusters.Consecutive pulses provide more material to the region outside the hollow cathode.The ejected neutral clouds can overlap and merge whereas the ejected ion cloud flows, due to ambipolar diffusion, through the growth region providing material for nanoparticle growth.The ion overlap becomes particularly important when the nanoparticles attain a net negative charge.The charged nanoparticles will repel each other and coalescence and agglomeration of nanoparticles are prevented.Hence the nanoparticles grow by accretion of atoms and ions, whereas the process of collecting ionized material is more efficient than the collection of atoms.The higher collection probability of ions is due to a larger cross-collection of ions which is given by (Pilch et al., 2013): with the geometric collection cross-section for atoms σ 0 = πr 2 , r being the radius of a nanoparticle, the electron temperature T e , ion temperature T i , and a factor k that is a function of electron and ion temperature and ion mass.
For a temperature ratio of T e / T i ≈ 100, the collection cross section for ions is about two orders of magnitude larger than the geometric collection cross-section for atoms.The dependency of the collection cross-section of ions on the electron temperature is affected by the proposed discharge overlap.In the growth region outside the hollow cathode, a plasma is generated between the hollow cathode and the anode ring which leads to electron heating and possibly also ionization.A higher electron temperature will increase the collection cross-section of ions, and ionization will increase the density of ions in the growth region.Both effects can enhance the growth of nanoparticles by ions.
The increase in the size of nanoparticles when changing the peak current can be explained by an increase in pulse strength which leads to a higher density of sputtered material available for growth.In the case of varying the frequency but keeping the average power constant by reducing the pulse strength (i.e. the peak current), it can be assumed that the average density remains constant when increasing the frequency, though the density of sputtered material ejected during each pulse becomes less.Thus, the size increase can be ascribed to the overlapping of consecutive pulses.From the presented results it is not possible to deduce which one of the overlaps-neutral, ion or discharge overlap-is the most important one.
Comparing the pulsed process to a DC discharge, it was found that the pulse process can provide more tools to control the properties of the plasma environment for the growth of nanoparticles.The high power density leads to ejection of a cloud with a high density of sputtered material, which favors the nucleation of nanoparticles.Short, high-power pulses increase the ionization of the sputtered material substantially compared to a DC discharge.This is beneficial when the growth of nanoparticles is limited to the accretion of atoms and ions.

Summary and Conclusions
High-power pulses have been shown to be able to generate plasmas with a high degree of ionized material.The ionized material can be guided by electric and magnetic fields and the energy can be controlled by, for instance, the substrate bias.The thin-film deposition by ions rather than by atoms has shown to be capable of improving film properties such as higher density or better coverage of trench walls.
For the synthesis of nanoparticles, it was shown that the pulsing allows the sizes of nanoparticles to be controlled.The mechanism of size increase can be ascribed to the pulsing which causes a periodical ejection of source material.Nanoparticles of different metals were synthesized and in the case of Ag, it was shown that different shapes can be synthesized.This offers the possibility of obtaining a shape-selected growth by further manipulation of the growth zone-e.g. by applying an additional cooling source to quench the growth.

Fig. 2
Fig. 2 Schematic of the sputtering process based on the target material pathway model by Christie (Christie, 2005).The letters G and M stand for gas and metal, respectively.Adapted from Lundin (Lundin, The HiPIMS process, 2010).

Fig. 3
Fig. 3 Cross-sectional scanning electron microscope (SEM) images of Ta films grown by (a) HiPIMS and (b) DCMS on a Si substrate clamped on the side of a trench with an area of 1 cm 2 and a depth of 2 cm.The HiPIMS-deposited film is dense with columns growing perpendicular to the Ta/Si interface.The DCMSdeposited films have a porous microstructure with columns inclined toward the flux direction.[Reprinted with permission from Alami et al. (2005).Copyright 2005, AVS.]

Fig. 8
Fig. 8 Variation of the size of copper nanoparticles by changing a pulse parameter: (a) peak current (energy per pulse) and (b) frequency at constant average power.[Reprinted with permission from Pilch et al. (2013).Copyright 2013, AIP Publishing LLC.]