CONTROLLING MATERIAL PROPERTIES OF THIN FILMS USING THE ELECTRIC FIELD IN PLASMA ENHANCED ALD

Interference coatings applied in refractive optics viz. narrow bandpass filters, hot/cold mirrors, edge filters etc. consist of several layers of low/high index material. The total thickness of this multilayer system can exceed up to several micrometers. The high performance of these coatings requires very good mechanical stability, thickness uniformity, adhesion, wear and scratch resistance, low optical losses and high stability in harsh environments [1]. Physical vapor deposition has been actively used for the development of these functional coatings; however, uniform and conformal coatings on such a complex shaped substrate is challenging. Atomic layer deposition (ALD) is a promising technology to realize uniform and conformal coatings with precise thickness control on an atomic scale. Thin films are grown layer by layer in a cyclic form and each monolayer is synthesized in a self-limiting manner.
In conventional thermal ALD, surface reactions are thermally activated by substrate heating; whereas in plasma enhanced ALD, highly energetic plasma species provide extra energy to enable surface reactions at low temperature. Additionally, ion energy in the PEALD process can be modified by applying/varying substrate-bias potential which in turn can tailor material properties [2-5]. Bombardment of these energetic ions can promote ion-induced effects like sub-surface implantation, desorption of adsorbed impurities, adatoms migration etc. [2] [3]. The material properties can be altered significantly by these effects.

Vivek_Bias_alumina_stress.jpg Vivek_Bias_silica_optical losses.jpg
Figure 1. a) Mechanical stress of alumina thin films deposited by PEALD process was modified from tensile to compressive on applying average bias voltage (-100 V). On further increasing the substrate bias, stress was found to be decreasing. Increase in compressive stress in the silica film was observed on applying substrate bias and remained unchanged on increasing bias. b) Optical losses in silica thin films at 220 nm reduced from 0.7 % (without bias) to 0.2 % (-100 V to -300 V) on applying an average bias voltage.(rights: IAP)


We have observed that the mechanical stress of the alumina and silica thin films has changed dramatically on applying substrate biasing as shown in fig. 1a. Substrate bias can also influence optical losses in the thin films as shown in fig. 1b.
Further, our aim is to synthesis PEALD coatings with improved mechanical properties by a profound understanding of structure and mechanisms of void formation due to defects, investigate film stress formation due to adhesion forces in the film and the interface.

References:

[1] S. Shestaeva, A. Bingel, P. Munzert, L. Ghazaryan, C. Patzig, A. Tünnermann,
     A. Szeghalmi, ꞌMechanical, structural, and optical properties of PEALD
     metallic oxides for optical applications, Appl. Opt. 41 (16), C47-C59 (2017).

[2] T. Takagi, Ion-surface interactions during thin film deposition,
      J. Vac. Sci. Technol. A 2 (2), 382-388 (1984).

[3] H.B. Profijt, M.C.M. van de Sanden, W.M.M. Kessels, Substrate-biasing during
     plasma-assisted atomic layer deposition to tailor metal-oxide thin film growth,
     J. Vac. Sci. Technol. A 31 (1), 01A106 (2013).

[4] S. Ratzsch, E.-B. Kley, A. Tünnermann, A. Szeghalmi, Inhibition of crystal
     growth during plasma enhanced atomic layer deposition by applying bIAS,
     Materials 8 (11), 7805-7812 (2015).

[5] T. Faraz, H.C.M. Knoops, M.A. Verheijen, C.A.A. van Helvoirt, S. Karwal,
     A. Sharma, V. Beladiya, A. Szeghalmi, D.M. Hausmann, J. Henri, M. Creatore,
     W.M.M. Kessels, Tuning material properties of oxides and nitrides by substrate
     biasing during plasma-enhanced atomic layer deposition on planar and 3d
     substrate topographies, ACS Appl. Mater. Interfaces 10 (15),
     13158-13180 (2018).

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