1. What is the HPPMS/MPP technique
2. MPP plasma characteristics
3. Advances in the MPP technique
3.1 High rate deposition
3.2 Dense structure and excellent adhesion
3.3 Uniform coverage
3.4 Think metal and tribological coatings
1. What is the HPPMS/MPP technique
For forty years magnetron sputtering has dominated many industries, while technological evolution has brought various other sputtering and chemical vapor deposition techniques into practice. The plasma generated in dc magnetron sputtering (dcMS) is characterized by low ion energies (1-10 eV) that are composed largely of gaseous ions with less than 5% metal ions. Cathodic arc evaporation (CAE) has been a primary competitor to sputtering over the past twenty years because of the high deposition rate and high metal ionization fraction (up to 90%) and high ion energy (10-200 eV). The intense plasma generated by CAE is its main advantage over magnetron sputtering, but the generation of macroparticle defects impairs the quality and properties of the deposited film, while the high ion energy can also lead to high stress in the coating and degrade the tribological properties.
In recent years, high power pulsed magnetron sputtering (HPPMS) (also known as high power impulse magnetron sputtering (HIPIMS)) [1-3] and modulated pulse power magnetron sputtering (MPP) techniques, a variation of HPPMS, have shown great advantages as compared to the conventional dcMS and pulsed dc magnetron sputtering (PMS) techniques [4-10]. By applying a large amount of peak power and current to a target (e.g. 0.1-3 kWcm-2) for a short period of time (e.g. in the µs to ms range), the HIPIMS and MPP techniques can produce a high degree of ionization of the target material and a high plasma density, and this high ion flux can be used to improve the structure and properties of the sputtered coating. Nevertheless by pulsing the power, the average target power remains as low as in the dcMS and PMS conditions to avoid overheating of the target.
Over the past 10 years, the development of HPPMS/HIPIMS has shown considerable potential in improving the quality of sputtered films in terms of the density, adhesion, conformal deposition and high performance. However, early HPPMS/HIPIMS technique showed a significantly decreased deposition rate as compared to continuous dc magnetron sputtering . The HPPMS to DCMS deposition rate ratios for Ti is in a range of 15-75%, for Cr is 29%, for Cu is 37-80%, for Al is 35%, for Ta is 20-40%, for Zr is 15%, for Al2O3 is of 25-31%.
The MPP magnetron sputtering technique aimed at overcoming the rate loss problem in HPPMS/HIPIMS to improve sputter deposition efficiency while still achieving a high degree of ionization of the sputtered material. The difference between the originally introduced version of HPPMS by Kouznetsov and co-workers  and the MPP technique [4-5]is basically in the magnitude, duration, and shape of the high power pulses. With the Kouznetsov technique, a single short high power pulse, on the order of 100 µs in duration is applied to the sputtering cathode, and the magnitude of the pulse is on the order of 1.0 to 3.0 kWcm-2 in order to achieve a high degree of ionization of the sputtered material. With the MPP technique, the pulse length can be as long as 3,000 µs, and the peak power is typically in the 100 to 800 kW range. The most important feature of the MPP technique is that the MPP pulse shape can be arbitrarily tailored into a multistep pulse  . Fig 1 shows the target power, voltage, and current waveforms during one typical MPP pulse period (the voltage values are negative). Unlike the simple one pulse shape in HPPMS, MPP generates a high density metal ion plasma by first producing a weakly ionized plasma followed by a transition to a strongly ionized plasma within one overall pulse. The weak ionization pulse segment is used to ignite an initial stable discharge with low power and current (steps 1 and 2 in Fig 1). Then a strong ionization pulse segment is generated with high power and current on the target as the main ionization stage (steps 3 and 4 in Fig 1). The utilization of the transition step from weakly ionized to strongly ionized plasma is critical in obtaining stable discharges for different target materials in that it can make the generation of the high power pulse discharge much easier as compared to the situation when the high voltage and current are suddenly applied to the target as in HIPIMS.
In MPP, the maximum pulse width can be in the range of 500-3000 µs and the maximum duty cycle is 28 %. The pulse frequency can be selected to be between 4 and 400 Hz. The MPP plasma generator is a switching power supply which can control the voltage ‘on’ time (ton) (the width of micro pulses) and the voltage ‘off’ time (toff) (the distance between micro pulses) within the micro pulse region (see the small insert graph on the right side in Fig 1). By manipulating the pulse width, the frequency, and ton and toff in the micro pulses, MPP can generate a pulsed plasma with controllable peak power up to 800 kW, a maximum average power of 20 kW, and a maximum peak current of 550 A on the target during the strongly ionizing segment. In addition, given the flexibility to create multiple micro pulse steps within one pulse and to adjust the power and current in each step, together with an arc suppression capability, stable deposition processes can be achieved for sputtering various film materials.
Fig. 1. Typical MPP pulse showing four steps of the target voltage, current and power evolutions during one modulated pulse (1500 ms overall pulse length in this example): 1) ignition of the weakly ionized plasma; 2) duration of the weakly ionized plasma; 3) transition stage from the weakly ionized plasma to the strongly ionized plasma; 4) duration of the strongly ionized plasma .
2. MPP plasma characteristics
2.1 Ion mass distributions (as compared to a dcMS plasma)
Recent plasma diagnostics performed in ACSEL on the MPP plasma using a Hiden EQP mass spectrometer have proved that the MPP plasma contains large percentages of metal ions [7-9]. Fig. 2 shows the positive ion mass distributions (IMD) measured using a Hiden electrostatic quadrupole plasma mass spectrometer (EQP) for dcMS and MPP discharges generated by sputtering a Cr target in pure Ar and Ar/N2 atmospheres at different average target powers (Pa).
As shown in Fig. 2a, the dc plasma exhibits the ion species of Cr+, Ar+, and their isotopes, and a very small number of Cr2+ and Ar2+ double charged ions at high powers. The intensities of the Ar+ ions obtained at all dc powers are much higher than the metal Cr+ ions, suggesting that the major ions in the dc discharge plasma are from the sputtering gas species while the degree of ionization of the metal target atoms is very low.
On the other hand, the MPP plasma obtained at similar Pa exhibits considerable increases in the peak intensities for all ion species (Ar+ , Cr+, Cr2+ and Ar2+), especially for the metal ion species (Cr+ and Cr2+) as compared to the dc plasma (Fig. 2b). The intensities of all ion species steadily increased as the Pa was increased. The intensities of the metal Cr+ peaks are higher than the Ar+ peaks, which suggests a significant increase in the metal ion species in the plasma as the target power and current were increased in the MPP conditions. In the Ar/N2 mixture, a wide variety of ion species including single, double, triple ionized Cr, Ar and N ions, N3+, N4+, CrN+, CrN2+ and CrAr+ ions were identified in the MPP plasma (Fig 2c).
Fig. 2. Ion mass distributions of (a) dc plasma and (b) MPP plasma, generated at different average target powers during sputtering a Cr target in pure Ar atmosphere .
2.2 Ion energy distributions (as compared to a dcMS and a PMS plasma)
A comparison of the ion energy distributions (IEDs) of metal Cr+ species within the discharged plasma generated using continuous dc, middle frequency pulsed dc (pulsed at 100 kHz and 60% duty cycle) and MPP techniques during Cr coating deposition in a closed magnetic field magnetron sputtering system is shown in Fig. 3 .
The IED of dcMS Cr+ ions (the black curve) exhibits a peak energy of 4 eV and a maximum ion energy of 15 eV in the tail (Fig 3a). The integrated ion fluxes in dcMS condition are low (Fig 3b). The numbers of Ar+ and N+ ions are much higher than that of Cr+ ions, suggesting that the major ion species in the dcMS plasma is from the gas, while the ionization degree of the metal target species is low. The IED of PMS Cr+ ions (the red curve) contains a wide range of energetic ions with a maximum energy distribution extended above 100 eV (as shown in the curve plotted with the logarithmic scale). An increase in the numbers of Cr+, Ar+ and N+ ions was identified in the PMS plasma as compared to the dcMS condition, as shown in Fig 3b. However, similar to the dcMS condition, the ionization degree of the target metal species is still low as compared to the gas ion species in the PMS plasma.
In contrast, the IEDs of MPP Cr+ ions (the blue and magenta curves) exhibit a peak ion energy of 4 eV with a short ion energy tail (<30 eV) (Fig 3a). Unlike in the PMS condition, no high energetic ions (>50 eV) can be observed in the MPP conditions. Nevertheless, the integrated ion fluxes of all ion species increased significantly as compared to the dcMS and PMS plasmas (Fig 3b). Moreover, the intensity of the Cr+ flux is higher than those of the gas ions. This observation confirmed that a large number of target metal atoms were ionized in the MPP plasma by the high peak power and current applied on the target. Similar observations of high ionization degree of the target materials in MPP plasmas have also been confirmed by other plasma diagnostic work for sputtering pure Ta and Cr material [9,10].
These low energy metallic ions were attracted towards the substrates by the negative substrate bias and therefore increasing the substrate ion current densities dramatically. Fig 3c shows the peak and mean substrate ion current densities (Ipeak and Imean) as a function of the negative substrate bias (Vb) during the MPP CrN film depositions. The saturation Ipeakand Imean are in the range of 250-280 mAcm-2 and 50-54 mAcm-2, respectively, as theVbwas larger than -50 V . Evidently, Imean as observed in the MPP deposition is at least 10-25 times higher than that in the conventional dcMS, which is about 2-5 mAcm-2 . The high substrate current densities observed in the MPP deposition confirmed that a significantly large number of ions (ion flux) arrived on the growing film.
Fig 3. (a) Comparison of the ion energy distributions (IED) of metal 52Cr+ species within the discharged plasma generated using conventional dc (black curve), pulsed dc (pulsed at 100 kHz and 60% duty cycle - red curve) and MPP discharges (blue curve) during Cr coating deposition in a closed magnetic field magnetron sputtering system; (b) Comparison of the integrate 52Cr+ ion flux (integrated areas under the IED curves in Fig 10a) for dc, pulsed dc and MPP systems [8,11].
A comparison of the light emission intensities of the dcMS and MPP plasma at similar average target powers during Cr sputtering is presented in Fig 4. The MPP plasma exhibits extremely higher light emission intensity (bright white color) than the dcMS plasma, indicating significant increased number densities of the excited species in the plasma. It also can be seen that the brightest part within the plasma extends far away from the target surface.
Fig. 4 Photos showing the comparison of the light emission intensities between dc and MPP plasma 
3. Advances in the MPP technique
The low ion energy and high ion flux characteristics of the MPP plasma are exactly the ideal conditions to tailor the microstructure and deposit fully dense films with a minimum incorporation of defects and residual stresses. Recent research at ACSEL has demonstrated the significantly improved microstructure and properties of metallic and compound films deposited by MPP compared to the films deposited by traditional sputtering techniques.
3.1. High rate deposition
It has been demonstrated that for some MPP depositions that it is possible to deposit films (e.g. Cu, graphite, Al, Ti, and Cr) at rates close to or greater than the rate for dcMS for the same average power [4-6]. Figs. 5a and 5b shows the comparison of the deposition rate of Cr and Al materials by the MPP technique as a function of Pa. The magnetic field strength was 350 G. The MPP rate to the dcMS rate gradually increased as the Pa was increased for both materials (blue curves). The MPP rate exceeds the DCMS rate at a Pa=4.4 kW for the Cr material and at a Pa=3.3 kW for the Al material . Moreover, recent work in ACSEL has shown that the MPP deposition rate can be further increased by changing the magnetic field strength in front of the target (Fig. 5c) . Recently, researchers from Hauzer Techno Coating BV have demonstrated that the MPP (also named HPPMS+ in Hauzer) rate is fully comparable with the rate achieved with arc evaporation for the TiAlN coating deposition . Moreover, the MPP TiAlN coatings exhibited longer life, tunable residual stress and improved properties compared to the industrial CAE TiAlN coatings. The high deposition rate in the MPP technique is a great advantage and improvement compared to traditional HIPIMS/HPPMS technique. This advantage is critical for the commercial production of thick, low stress coatings with cost reduction and energy saving.
Fig 5. Comparison of the deposition rate of (a) Cr and (b) Al deposited using DCMS and MPP techniques as a function of the average target power density .
3.2. Dense and high performance coatings
A comparison of the microstructure of the Cr film deposited by dcMS and MPP at similar deposition conditions is shown in Fig. 6. As shown in Fig. 6a, the dcMS Cr coating deposited at a 4 kW average target power exhibited extremely large grains (600-800 nm) with wide grain boundaries, due to the low ion bombardment from the dcMS plasma. On the other hand, the MPP Cr coating deposited at the same 4 kW average target power (150 kW peak power) exhibited much denser structure (Fig. 6b). The grain size of the MPP Cr coatings decreased significantly to less than 50 nm, as evident from the TEM image (Fig. 6b). The grain refinement in the MPP Cr coatings led to an increase in the hardness of the coatings from 4 GPa (DCMS) to 8-15 GPa (MPP) . The great improvements on the microstructure and mechanical of the MPP Cr coating are the results of the extensive metallic ion bombardment, as evident from the plasma diagnostics results (Fig. 3). Nevertheless, the low energy of the ions kept the residual stress low in the coatings (-0.7 to -1.7 GPa).
Fig 6. (a) Top view TEM image of a DCMS Cr coating desposited at a 4 kW average target power, and (b) TEM micrograph of Cr coatings deposited using MPP at a Pa=4 kW and Pp=150 kW during the pulses.
The MPP, dcMS and PMS techniques have been compared for the reactive sputtering CrN films . Fig 7 shows TEM micrographs of the CrN films deposited at a floating substrate bias (about -20 V) using dcMS, PMS and MPP techniques with the plasma conditions shown in Fig 3a. The CrN films deposited by dcMS and PMS exhibited long columnar grain growth with clear grain boundaries and porosities (Fig 7a and 7b). In contrast, the MPP CrN film deposited at a floating substrate bias showed a fully dense microstructure without clear grain boundaries and a decrease in the columnar grain size (Fig 7c). The MPP CrN film deposited at a floating substrate bias showed significant improved hardness (26 GPa) compared to the dcMS and PMS CrN films without a significantly increase in the residual stress (-1.2 GPa). A comparison of their wear resistance demonstrated that the MPP CrN film exhibited decreased wear depth and wear rate compared to the dcMS and PMS CrN films (Fig 7d) . Moreover, a comparison of their oxidation resistance confirmed that the MPP CrN film showed greatly improved oxidation resistance with an annealing temperature up to 900 oC, whereas the dcMS and PMS CrN films were severely oxidized, as shown in Fig 8. The great improved wear and oxidation resistance of the MPP films are attributed to the dense microstructure and low residual stress and defects in the film.
Fig 7. Cross-sectional TEM images of CrN coatings deposited under (a) DCMS, (b) PMS (100 kHz and 60% duty cycle) and (c) MPP (Pp=180 kW andIp=250 A) conditions at a floating substrate bias .
Fig 8. Cross-sectional SEM micrographs of CrN coatings deposited at similar conditions by (a) dcMS, (b) PMS and (c) MPP techniques after annealling in ambient air at 900 oC for one hour .
3.3. Uniform substrate coverage
Conformal coating deposition is important in precision cutting and forming tools, semiconductor chip and microelectronic device, magnetic disc, and biomedical implant fabrication. Nevertheless, it is a challenge to coat complex shaped substrate/components in most PVD processes due to the line-of-sight limitation. However, as the deposited species are largely metallic ions in the MPP/HPPMS plasma, it is possible to control the metal ion trajectory by biasing the substrate to improve the step coverage and achieve good deposition rate on the surface placed at an angle to the target. Early work by Alami et al  showed that the Ta thin films grown on a Si substrate placed along the wall of a 2 cm deep and 1 cm wide trench by HPPMS exhibited a denser microstructure and a higher deposition rate than the film grown by dc magnetron sputtering. Recently, ACSEL researchers have deposited an 80 µm alpha-Ta coating on the steel substrate placed with one side facing the target surface and the other side orthogonal to the target . As shown in Fig. 9, the thickness of the coating on the orthogonal side is 65 µm, which is of 81% of the thickness of the coating deposited on the side facing the target (80 µm). Considering the large thickness of the coating, the good homogeneity achieved by MPP is a direct consequence of the high ion fraction of sputtered species being controlled by the bias on the substrate.
Fig. 9 Optical images of an 80 mm thick a-Ta coating deposited using MPP showing a good homogeneity of the coating coverage on the side orthogonal to the target .
3.4. Thick metal and tribological coatings
It is highly desirable to deposit hard protective coatings at a high deposition rate to increase the cost effectiveness of the process and to increase the life and performance of the coating by applying a relatively thick coating (e.g. > 6 µm). Many other thick coating preparation techniques (e.g. electroplating, thermal plasma spraying, CAE, and chemical vapor deposition (CVD), etc) have good process stability and in many cases low costs, and they have been adapted for industrial production. Nevertheless, some of the above techniques such as electroplating are not environmental friendly, and the coatings can exhibit low density, poor adhesion, and poor properties that restrict their use for a limited number of applications. Magnetron sputtering, in the right application and practiced by one skilled in the art, can overcome many of the above listed disadvantages for the different coating techniques. However, compared to other PVD processes such as CAD, the sputter deposition rate usually is not as high as it is for CAD. In addition, the thickness of sputter deposited coatings is often limited by the thermal stresses generated by differences in the coefficient of thermal expansion (CTE) between the coating and the substrate material combined with the intrinsic stress that builds up in the coating during the deposition, which is directly affected by the ion energy and ion flux arriving at the substrate.
The traditional PMS and HPPMS/HiPIMS depositions showed a low deposition rate and contains a wide range of ion energy distributions (up to 100 eV) . The high energetic ion bombardment (>80 eV) can also induce residual stress and point/line defects in the growing film , which is detrimental to thick coating growth. In contrast, the MPP technique can achieve a high deposition rate (Fig 5) while at the same time achieving a high degree of ionization of the sputtered material with low ion energies (2-5 eV) (Figs 2 and 3). Thus it is practical to deposit thick coatings using the MPP technique by taking advantage of its high deposition rate and ideal low energy/high ion flux bombardment to achieve dense coatings with low stress and defect incorporations.
Recent ACSEL researchers have successfully deposited 55-100 µm thick CrN, Cr2N, Cr, Ta conformal coatings using MPP at high deposition rates ( e.g. 10 µm/hr for the CrN, 15 µm/hr for the Cr2N and 16 µm/hr for the Ta) [10,19]. These thick coatings showed dense microstructure and fine grain size, as shown in Fig. 10. The residual stresses of these thick coatings are low, in the range of -2 GPa to -5 GPa. On account of their dense microstructure, the thick MPP coatings also exhibited excellent adhesion, mechanical and tribological properties . Progressive scratch tests performed on the 55 µm CrN showed no coating failure at a maximum load of 100 N (Fig. 11a). Rockwell C indentation (150 kg load) on the 100 µm Ta coating deposited on AISI 304 stainless steel demonstrated its excellent adhesion strength and toughness in that no coating delimination and cracks can be seen along the indent circumference (Fig. 11b).
The excellent adhesion of the thick MPP coatings is attributed to the low residual stress in the thick coatings (Fig. 10) and the strong bonding force between the coating and the substrate, which was benefited from the enhanced ion bombardment from the highly ionized MPP plasma. The thick MPP CrN coating (>10 µm) exhibited significantly improved wear resistance compared to the thin films (<5 µm). Fig 11c show the COF and wear rate of the MPP thick CrN coatings sliding against a 5 mm Al2O3 ball at a 10 N load with a 200 rpm rotation speed for a travel length of 2000 meters. The COF values of the CrN coatings gradually increased from 0.38 to 0.56 as the coating thickness was increased. However, the coatings showed pronounced improvements in the wear resistance as the coating thickness increased above 5 µm. The 2.5 µm thin CrN coating exhibited a wear rate of 1.1x10-6mm3N-1m-1. The wear rate of the CrN coating decreased by a factor of 3 as the coating thickness was increased to 10 µm (3.5x10-7 mm3N-1m-1 ), and maintained in the low 4~5 x10-7mm3N-1m-1 range as the coating thickness was further increased.
Fig. 10 Cross-sectional SEM images of thick CrN, Cr2N and Ta coatings deposited by the MPP technique with high deposition rates: (a) a 20 µm CrN coating (10 µm/hr), (b) a 55 µm CrN coating, (c) a 55 µm Cr2N coating (15 µm/hr), and (d) a 100 µm Ta coating (16 µm/hr) [10,19].
Fig. 11 (a) Friction force versus applied load during the progressive load scratch tests performed on a 55 mm CrN, (b) SEM micrograph of the indent morphology after Rockwell C-Brale indentation of the 100 mm Ta coatings, and (c) COF and wear rate of the MPP sputtered CrN coatings with different coating thicknesses
There is a great need to develop high performance films and coatings that exhibit superior performance under harsh environments (oxidation and corrosion), coupled with high hardness, low coefficient of friction (COF), increased toughness and increased coating thickness. This range of multi-functional properties can be achieved by depositing the films from a highly ionized metal plasma vapor deposition (e.g. the MPP technique) of controlled low ion energy and high ion flux, that densifies the growing film, improves the adhesion, controls the microstructure and coupled with the potential for providing thicker coatings and increased conformality and deposition rate without detrimental increases in stress.
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