Surface conductivity dependent dynamic behaviour of an ultrafine atmospheric pressure plasma jet for microscale surface processing

Highlights

• Spatio-temporal behaviors of capillary APPJs are studied for various substrates.

• Plasma irradiation area depended on the substrate conductivity and permittivity.

• Surface irradiation area was significantly broadened in polymer-like substrate.

• Effect of applying a substrate bias on the APPJ irradiation area was investigated.

Abstract

An experimental study on the dynamic behaviour of microcapillary atmospheric pressure plasma jets (APPJs) with 5 μm tip size for surfaces of different conductivity is reported. Electrical and spatio-temporal characteristics of the APPJs are monitored using high voltage probe, current monitor and high speed intensified charge couple device camera. From these experimental results, we presented a simple model to understand the electrical discharge characteristics of the capillary APPJs with double electrodes, and estimated the velocity of the ionization fronts in the jet and the electron density to be 3.5–4.2 km/s and 2–7 × 10¹⁷ m⁻³. By analyzing the dynamics of the microcapillary APPJs for different substrate materials, it was found that the surface irradiation area strongly depended on the substrate conductivity and permittivity, especially in the case of polymer-like substrate, surface irradiation area was significantly broadened probably due to the repelling behaviour of the plasma jets from the accumulated electrical charges on the polymer surface. The effect of applying a substrate bias in the range from −900 V to +900 V on the plasma irradiation onto the substrates was also investigated. From the knowledge of the present results, it is helpful for choosing the substrate materials for microscale surface modification.

Introduction

Plasma technology is an effective technique for surface modification of materials because it is a simple, fast and induces minimal damage to the surface [1]. Nano or microscale surface modification is crucial in various applications including nano or microelectronics, and nanotechnology [2], [3], [4]. However, for nano or microscale surface modification, conventionally low-pressure plasmas driven by RF or microwaves are employed. In these plasmas generally physical masks are utilized that makes the process cumbersome, expensive and time consuming for ultrafine modification [5].

The use of atmospheric pressure plasma is indispensable for surface modification without physical mask of a variety of material [6], [7], [8]. Recently, we reported an ultrafine atmospheric pressure plasma jet (APPJ) with 100 nm–1 μm tip size for nano and microscale surface modification on different substrate materials, such as photoresist [9], [10], [11], polymers [10], carbon nanotubes [12], [13], and protein film [14]. While undertaking the research, it was recognized that a systematic study of the interaction of the ultrafine APPJ with different substrate materials would be useful to know the degree of surface modification on these materials based upon their surface conductivity. Specifically, to use APPJs for surface modification, it is necessary to understand the discharge characteristics of the APPJ having an ultrafine capillary aperture with 0.1–1 μm, and the extent of surface interaction with the substrate material so that surface functionalization with different radicals for use in biological and other applications may be realized [10], [11], [12], [13], [14], [15], [16], [17], [18]. The capillary tip size is one or two order less than the Debye length, which is ∼2.35–47.0 μm for a typical APPJ plasma having an electron density of 10¹⁷–10¹⁹ m³ and electron temperature of 1–4 eV. It has been reported that different substrate materials have a pronounced influence on APPJ characteristics, such as voltage, current, plume shape and colour [19]. Recently, extensive numerical studies on the interactions between APPJs and various materials have been carried out to understand the phenomena on the plasma surface interaction with various conductive and dielectric materials [20], [21]. These studies help us understand some of the phenomena between APPJ and surface of various materials.

However, no systematic experimental investigation of the dynamic behaviour of the microcapillary APPJ with commonly encountered research-grade substrate materials for microscale surface modification has been carried out so far. Therefore, in this work, the dynamic behaviour of the APPJ interacting with different substrate materials including current-voltage characteristics are investigated by using a high speed intensified charge coupled device (ICCD). From these results we obtain the characteristics of the current flowing through the ground line and the substrate line, including the spatial extent of the interaction with the different materials having various conductivity and permittivity interacting with the microcapillary APPJ plasmas.