i-coat, the (HPPMS) HIPIMS-based coating solution

Solution engineering Model

Coating problem solving demands deep and broad knowledge of the state-of-the-art coating technologies, as well as a capability to innovate. INI Coatings uses conventional PVD technologies as well as the newly developed HIPIMS technology (see below) to improve the existing coating products but also to provide new PVD coating solutions, inicoat.

inicoat is a coatings-family produced by INI's HIPIMS technology. They are charcterized by their unique properties listed in the table below, and qualitatively compared to existing conventional deposition techniques:

Properties HIPIMS DC Magnetron sputtering Arc evaporation
Plasma density Very high Low Very high
lonization degree (Target atoms) up to 90 % ~ 1 % up to 100 %
Process ambient temperature (°C) > 60 > 150 > 250
Deposition from insulating target Possible Possible Not possible
Deposition rate (reactive) High Low Very high
Deposition rate (metallic) Low High Very high
Adhesion to a substrate Very good Average Very good
Coating density Very high High Very high
Surface roughness Very low Low High
Homogeneity an non-flat surfaces Good Low Low
Target utilization High Low Very high

HIPIMS/HPPMS

HIPIMS (also known as HPPMS) is the new-commer in the High-Tec coating market, and is proven to give the user new tools to produce coatings with better adhesion and density, and smoother surfaces.

HIPIMS, or High power pulsed magnetron sputtering is a UBM technique. UBM stands for unbalanced magnetron and describes the magnetic field configuratioon behind the sputter target.

Characteristic for HIPIMS is that the power is supplied to the sputter target in short pulses of ~ 100 µs at low frequencies (typically in the 10s or 100s of Hz). As a result a high (electron) density plasma is produced leading to a large increase of the ionization probability of the sputtered material (see table above). This, in turn, affects not only the properties of the plasma but also those of the coatings grown in these conditions. For mor details please check our INI Library.

HIPIMS can also be used to coat materials and complex-shaped parts that, up to now, were not possible to treat with PVD.

For more than 10 years our scientists have been working and developping the HIPIMS technology. We have shown that HIPIMS could be integrated in small coating systems for R&D purposes as well as in larger ones for industrial purposes.

The following is a short review on the HIPIMS technology.


Content

  1. Introduction
  2. HIPIMS: the principle
  3. HIPIMS: new coating solutions and state of the art engineering
    1. Coating densification and surface smoothening
    2. Deposition on complex-shaped substrates
    3. Deposition on temperature sensitive substrates
    4. Electrical and optical properties modification by HIPIMS
  4. Summary
  5. References

1  Introduction

High power pulsed magnetron sputtering (HPPMS/HIPIMS) is a further development of the magnetron sputtering deposition techniques, especially fitted for demanding industrial applications. It is a novel ionized physical vapour deposition technique where high peak power unipolar pulses are applied to the target (cathode) during the sputtering process and resulting in a high ionization fraction of the sputtered atoms. The high ion-to-neutral ratio in HIPIMS has been shown to enable the deposition of ultra-dense and smooth metallic and compound films, to allow for phase tailoring of the growing coating, and to lead to an  enhancement of the coating’s optical and electrical properties. Other areas where HIPIMS has been shown to be advantageous compared to conventional techniques are: the enhancement of the adhesion of coatings, and deposition in holes and vias especially for sub-micron electrical and electronic applications. In the present paper, some of the advantages of the HIPIMS approach are reviewed by describing the coating growth mechanisms.

2 HIPIMS: the principle

The HIPIMS plasma is generated by applying a short pulse of an on-time typically of some 100 µs to a metal or a compound sputtering target. The pulse duty on-time is kept below 5 % while the frequency is commonly lower than 2 kHz. The figure below shows the time evolution of the pulsed current and voltage, where it is seen that the target current starts to increase as the voltage of 800 V is applied to the target. The target current reaches its maximum value of 1000 A at the end of the pulse, corresponding to a peak target power of 0.8 MW.

A common feature of the HIPIMS discharges is the high ionization fraction of the sputtered species. This is a result of the high electron impact ionization rates, due to the high plasma (electron) densities. A number of studies has shown that the ionization fraction in HIPIMS is a function of the target material and the peak target power. In particular, the ionization has been found to be low for species with a low electron impact ionization cross-section (si) and a high ionization potential (IPA), such as C (4.5 %), while it is high for species with a high si and a low IPA values, such as Ti (90 %) and Cu (70%).

3 HIPIMS: new coating solutions and state of the art engineering

3.1 Coating densification and surface smoothening

High power pulsed magnetron sputtering (HIPIMS) has been used over the last years for the deposition of elemental and compound films. The high fraction of ionized sputtered material during HIPIMS has been utilized in order to tailor and improve the properties of the growing films with respect to those films deposited by conventional sputtering techniques. We showed in earlier works that Ta films deposited by HIPIMS exhibited an ultra-dense microstructure and a very smooth surface, as opposed to films grown by dc magnetron sputtering (dcMS). Similar effects were observed not only for metallic but also for compound films. Earlier works showed that TiO2 films deposited from a ceramic TiO1.8 target using HIPIMS and dcMS  and found that the HIPIMS-films exhibited significantly smoother surfaces and higher densities than the dcMS ones, which was also the case for the reactively grown TiO2 films.

3. 2 Deposition on complex shaped substrates

Deposition on complex-shaped substrates such as cutting inserts or drills is of high importance to manufacturers and technologists alike. Apart from increased density and surface smoothness, one of the important implications of using HIPIMS for complex-shaped substrate is the much higher deposition rate achieved on surfaces not parallel to the surface of the target. This was observed in inserts that were coated with TiAlN, and where the deposition rate on the rake and the flank faces were compared. It was concluded that the deposition rate was almost equal on both faces. The HIPIMS technique was shown thus to prolong the life time of the cutting insert.

3. 3 Deposition on temperature sensitive substrates

One of the advantages related to HIPIMS is the “cool plasma” that reaches the substrate. This is a direct result of the increased gas phase collision frequency of the sputtered and the sputtering species, and has been shown to result in ion and electron temperatures close to room temperature in the substrate vicinity. This is also possible with other sputtering techniques if the substrates are placed far enough from the target which would result in bad coating quality and adhesion. The average temperature at the substrate could also be decraesed by having cooling “breaks” in between deposition periods, which would result in a long process time and more incorporation of residual species in the coating leading to a deterioration of its properties. The "cool" HIPIMS plasma allows for deposition of high density and good-adhesive coatings on temperature-sensitive substrates such as low-temperature steels and plastics thanks to it high-intensity low-energy ion flux at the substrate, and provides therefore new coating solutions for new applications.

3. 4 Electrical and optical properties modification by HIPIMS

The effect of HIPIMS on the electrical properties of thin metallic films was demonstrated in a number of works. Li et al. investigated the resistivity of thin Ag films deposited by HIPIMS. They showed that resistivity of films deposited by HIPIMS was lower than that achieved by dcMS, when the film thickness was below 15 nm. Similar results were reported by Sarakinos et al. who studied the microstructure, the surface topography and the electrical properties of silver films grown by HIPIMS at different peak target currents and by dcMS. They showed further, that the electrical resistivity of ultra thin Ag films was determined by the film density and the topographic characteristics of the film surface. These features were, in turn, a function of the peak target current, i.e. of the bombardment conditions of the growing films.

Particular effort has been put on the tailoring of the optical properties of transparent transition metal oxides by HIPIMS. Davis et al. studied the growth and the properties of TiO2 films deposited by HIPIMS and dcMS from a metallic Ti target and they found that the HIPIMS-grown films exhibit higher refractive index than the dcMS ones. Similar results were reported by Sarakinos et al. when TiO2 films were deposited from a ceramic TiO1.8 target. Glocker et al. deposited ZrO2 and Ta2O5 films both by HIPIMS and mid-frequency pulsed magnetron sputtering (MFPMS). In this case the MFPMS-grown films were found to exhibit the highest refractive indices. Finally, Konstantinidis et al. suggested that ultra-smooth and dense ZnO HIPIMS-grown films could result in higher reflectance and thus, better efficiency of Ag-based multilayer stacks use in low-emissivity windows.

4 Summay

The relatively new high ionization physical vapour deposition technique “high power pulsed magnetron sputtering” (HPPMS/HIPIMS) is reviewed. The main advantage of using HIPIMS is that it provides high plasma densities and high ionization of the sputtered material. The use of HIPIMS allows thus, for a better control of the energetic bombardment of the substrate. This makes it possible to tailor the phase composition, the microstructure and morphology, the elemental composition, and subsequently the properties and functionality of grown coatings on substrates ranging from plastics and ceramics to metals and semi-conductors.

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