1.1.1 History and Applications
Because of its relatively low cost and low pollution, the excimer ultraviolet (EUV) lamp has found wide applications since its debut in a form of dielectric barrier in xenon gas in 1988 [Eliasson and Kogelschatz, 1988]. These include, for example, surface cleaning [Kane et al., 2004], plasma display [Park et al., 2007], LCD backlighting [Shiga et al., 2001], water quality purification [Safta, 2004], material deposition [Buck et al., 1998] and material processing [Zhang and Boyd, 1996] in the semiconductor industry. EUV lamp emission is caused by the excimer falling from excited states to the ground state. An excimer (originally short for excited dimer) is an unstable dimeric with a lifetime of about 10 ns. The important features of an excimer include: emission with narrow wavelength bandwidth, high efficiency and free radiation direction. Two typical examples are the formation of Xe or *2 XeCl * excimer complexes. The EUV lamp has a dielectric barrier-type discharge which is a high-pressure non-equilibrium transient discharge. A primary function of the dielectric barrier is that it can suppress the occurrence of arc discharge, thereby preventing
damage to the metallic electrodes.
Because the plasma physics and chemistry inside the EUV lamp are very complex and difficult to measure, its design still heavily depends on the trial-and-error method that is both time-consuming and costly. Thus, detailed simulation of excimer discharge physics and chemistry could become a viable method for understanding the complicated plasma physics inside the lamp which, in turn, would lead to a better design.
1.1.2 Dielectric Barrier Discharges
The dielectric barrier discharge is a very high-pressure non-equilibrium transient discharge which usually consists of many tiny parallel current filaments showed in Figure 1.1 [Kogelschatz et al., 1999]. The barrier plays a role in producing
high-energy electrons and suppressing arc discharges if micro-filament discharge has started. A primary feature of the lamp is that the dielectric barrier suppresses the occurrence of arc discharges and damage to the metallic electrode. Dielectric barrier discharge excimer lamps were originally driven by an alternating voltage, usually a sine wave. Under special conditions also homogenous discharges can be obtained.
Most of the applications are operated up to now with filamentary discharges; but homogeneous discharges are coming up and open new perspectives.
In the filamentary mode the four typical stages are shown in Figure 1.2
[Chirokov et al., 2005]. If the electric field in the gas gap is sufficiently high to initiate avalanches, the breakdown starts with the Townsend phase. Next, a streamer occurs and a conducting channel-the filament-is formed. Charges are then transferred through the channel and accumulate at the dielectric surface. The voltage across the filament is finally suppressed and the discharge dies out. Group of local processes in the discharge gap initiated by avalanche and developed until electron current termination usually called microdischarge.
1.1.3 Breakdown Mechanism
The breakdown mechanism is based on the streamer theory, originally developed by Loeb [1960], Raether [1964] and Meek [1978]. A simplified conceptual development of a streamer begins with a single electron in an electric field. In the electric field the electron gains enough energy to ionize an atom or molecule, resulting in subsequent ionization and then an electron avalanche. This electron avalanche creates a strong positive field or streamer head within a small volume, which propagates across the gap, followed by a thin plasma channel (streamer). The streamer is a highly non-equilibrium plasma that is in a formative phase, with a small
“head” that has a high space-charge limited field, and produces significant ionization through the presence of energetic electrons. The small head is followed by a “tail”
with much lower electric field [Raizer, 1991]. Once the streamers bridge the gap
between the anode and the cathode, the plasma channel will become highly conductive and the discharge, an array of streamers, typically collapses to a single conducting channel.
1.1.4 Overview of Excimers
An excimer (originally short for excited dimer) is an unstable dimeric or heterodimeric molecule formed from two species, at least one of which is in an electronic excited state. The lifetime of an excimer is very short, on the order of nanoseconds. The feature of excimer include single wavelength, high efficiency and free radiation direction. The generation of excimer results from the reaction mechanism like ionization or excitation and collision process in the discharge.
Emission of UV is due to the process that excimer falls from excited states to ground state. Excimer formation requires frequent three-body collisions and is therefore favored in a high-pressure discharge.
In general, excimer lamps utilize high-pressure rare-gas or rare-gas/halogen mixtures. Two typical examples are the formation of Xe or *2 XeCl excimer * complexes. Firstly, the formation of Xe is formed essentially from neutral excited *2 atoms (which Xe is the working gas). This process takes place in microdischarge channels and consists of three consecutive stages:
e−+Xe→e−+Xe* (1.1)
* *
Xe +2Xe→Xe2+Xe (1.2)
*2
Xe →2Xe+hν (172 nm) (1.3) Secondly, the formation of XeCl* is formed mainly via recombination of ions with impurity in the working gas. A typical process consists of the following reaction steps:
e−+Xe→e−+Xe++e − (1.4)
e−+Cl2 →Cl−+Cl (1.5) Xe++Cl−→XeCl* (1.6) XeCl*→Xe Cl UV radiation (308 nm)+ + (1.7)
Other examples of excimer complexes may include, but not limit to, the rare-gas dimmers Ar radiating at 126nm, 2* Kr at 146 nm, the halogen dimmers 2* Cl at 259 *2
nm, Br at 289 nm, 2* I at 324 nm and the rare gas/halogen complexes *2 ArCl at 175 *
nm, KrBr at 207 nm, * KrCl at 222 nm, * XeI at 253 nm, * XeBr at 238 nm, * XeCl at 308 nm [Gellert and Kogelschatz, 1991; Zang and Boyd, 1998]. *