The Expanding Thermal Plasma (ETP) technique

Basics

The Expanding Thermal Plasma (ETP) setup consists of a high-pressure plasma source and a low-pressure chamber. The plasma source is a cascaded arc. It is operated at high flows (tens of sccs/several slm) of non-depositing gases (Ar, Ar-H₂, Ar-N₂, H₂, N₂, etc.) leading to pressures of 0.4 bar (300 Torr) when the plasma is ignited. In this plasma source reactive species are created that can be used for downstream precursor gas dissociation for, e.g., plasma deposition of materials or surface treatment. The discharge in the cascaded arc is current controlled by a dc power supply and the power dissipated is typically within the 2-5 kW range. The plasma in the arc is thermal with an electron density of ~10²² m^-3 (10¹⁶ cm^-3) and with an electron and heavy particle temperature of both ~1 eV. The high heavy particle temperature leads to almost full dissociation of molecular gases when these are injected in the arc. The plasma emanates from the cascaded arc source through a nozzle and expands into the deposition chamber which is typically at a pressure of ~0.2 mbar (~0.15 Torr). Due to the large difference in pressure between the arc and the chamber, the plasma is accelerated leading to a supersonic expansion. At a few centimeters from the arc outlet there is a stationary shock, after which the plasma expands subsonically. In the supersonic expansion ans shock, the electron temperature is reduced to ~0.1-0.3 eV and the electron density to ~10¹⁷-10¹⁹ m^-3 (10¹¹-10¹³ cm^-3). The directed velocity after the shock is typically 1000 m/s. Depending on the gas mixtures used, the cascaded arc can deliver large flows of several types of reactive ionic and atomic species such as Ar+, H, N, etc.

Figure and picture of the Expanding Thermal Plasma setup.

The picture is of the CASCADE system used for the deposition of thin film a-Si:H solar cells and has no coils for the creation of a magnetic field. Typical dimensions of the low-pressure chamber are a length of ~60-80 cm and a diameter of 32-50 cm. An injection ring for precursor gases is located at ~5 cm from the cascaded arc outlet while a substrate holder is typically placed 35-50 cm from the outlet. During processing the system is pumped by a stack of roots blowers (pumping capacity 500-1500 m³/hr), and overnight it is pump by a turbo pump reaching a typical base-pressure of ~10^-6 mbar (~10^-6 mTorr).

The supersonic expansion region of the expanding thermal plasma for an Ar-H2 plasma. At approximately 5 cm from the arc outlet the precursor injection ring is visible.

The cascaded arc plasma source

The cascaded arc is a dc plasma source operated at non-depositing gases and at high pressures (typically ~400 mbar or ~300 Torr). It is current controlled at 25-90 A with a corresponding operating voltage of 70-250 V, depending on the gas mixture used. The discharge is sustained in a narrow plasma channel (diameter typically 3-4 mm) between three cathodes and a grounded anode. The electrically insulated cascaded plates lead to a gradual potential drop. All parts are of copper, except for the cathode tips (tungsten with 2% lanthanum), the PVC and boronnitride spacers between the cascaded plates, and the boronnitride cover of the cathode feedthrough. The arc is vacuum-sealed with O-rings and the cathode, anode, and plates are all water-cooled. The length of a cascaded arc with ten plates is typically 10 cm.

Figure and picture of a cascaded arc plasma source

In the figure only one cathode is shown for clarity. The arc in the picture has its plasma channel in vertical direction. It is disconnected from the low-pressure chamber and it has only 4 cascaded plates.

Plasma deposition with the ETP technique

One of the main applications of the ETP technique so far is plasma deposition of thin films of several materials. This is usually done by injecting precursor gases by means of an injection ring 5 cm from the cascaded arc outlet. The injected precursor gas is ionized and dissociated by the reactive species emanating from the plasma source. Deposition can subsequently take place by the growth precursors at a temperature-controlled yoke and substrate holder (-50 (C - 500 (C). The substrates are accurately temperature-controlled by means of a He back flow. The substrates and substrate holder are loaded into the deposition chamber from a loadlock system by means of a magnetic transfer arm. Currently, 4 ETP setups are available in our group, of which 2 are used for deposition (Depo 1 and Depo 2). Two other ETP deposition setups are also directly used by our group, one in cooperation with DIMES at the Delft University of Technology (the CASCADE system) and one at the physics department of TNO in Eindhoven (TNO-TPD). Furthermore, some ETP setups are currently used in industry. An overview of the materials deposited by the ETP technique.

Copyright 2001 Contents Erwin Kessels / Layout Alquin Stevens

(Met dank aan de ETP-Groep van de TUE)