Fluorocarbon reactions

the surface. This type of reaction is in terms of adsorption terminology referred to as chemisorption. Physi- and chemisorption are macroscopically distinguished by the binding energy, which is commonly lower than 80-100 kJ/mol for physisorption.

The overall mass flux balance is

Γincident= Γadsorbing+ Γbs, (1.37) where bs stands for directly backscattered particles³. Mass loss from the polymer due to sputtering or chemical release of molecules has to be con-sidered as well:

Γadsorbing= Γpolymer+ Γreleased. (1.38) Here, Γ_polymer is the part of Γ_adsorbing which results in the formation of stably bond polymer. It is calculated from the experimental deposition rate. Those particles which are released from the deposit during the polymer formation and reenter the gas phase are forming the flux Γreleased.

calculated by the Troe formalism [12] with parameters given in the cited literature.

CF + F−→^M CF2 (R 1)

with∆H⁰=-516.8 kJ mol⁻¹ andk= 6.2×10⁻¹⁵cm³s⁻¹[13]

CF + F2

−→M CF2+ F (R 2)

with∆H⁰=-359.0 kJ mol⁻¹ andk= 3.9×10⁻¹²cm³s⁻¹[4]

CF + CF₂−→^M C₂F₃ (R 3) with∆H⁰=-271.6 kJ mol⁻¹ andk= 1×10⁻¹²cm³s⁻¹ [13]

CF + CF₃−−→^M? 2CF₂ (R 4) with ∆H⁰ =-161.8 kJ mol⁻¹ and k ≈ 10⁻¹¹−10⁻¹⁰cm³s⁻¹ (estimate in [4]).

The next species in this series isCF2(difluorocarbene) which is discussed separately in the subsequent section. The series of single carbon radicals is completed by CF3, which is also highly reactive. It is a precursor of e.g.CF₄,C₂F₅,C₂F₆. CF₄is formed by

CF3+ F−→^M CF4 (R 5)

withk2= 1.7×10⁻¹²cm³s⁻¹[14]. The reaction rate was found to be pres-sure dependent according to the literature. The reaction is highly exother-mic (∆H⁰=-544.5 kJ mol⁻¹). C₂F₅results from

CF₂+ CF₃−→C₂F₅ (R 6)

with a chemical rate ofk= 8×10⁻¹³ cm³s⁻¹and∆H⁰=-232.5 kJ mol⁻¹. Finally,C2F6is produced e.g. by

CF₃+ CF₃−→^M C₂F₆ (R 7)

with∆H⁰=-407.8 kJ mol⁻¹ andk= 2.1×10⁻¹²cm³s⁻¹[14].

1.3.3.2 Difluorocarbene

Carbene chemistry is an interesting field in organic chemistry due to the reaction paths of these molecules. They play an important role in the

chemistry of fluorocarbon plasmas and are often termed to be connected to oligomerization and polymer growth.

Carbenes are molecules in which the name-giving carbon atom possesses two single bonds and two unbound valence electrons. The binding partners can be atoms or molecules. Carbenes can possess very different reactivities, especially when the electronic ground states are compared. This is basically due to the fact that the two electrons can either be unpaired in a triplet state (spins parallel) or paired in one common orbital as a spin singlet state (antiparallel spins) [16]. Whether a spin triplet or a singlet is the energeti-cally favored ground state is determined by the substituents. For example, ground state CH2 is a spin triplet whereas CF2 is a spin singlet. Besides the electronegativity of the substituent, also the size of the substituent is a deciding factor. The geometry of the carbenes differs as well, it is obvious that the triplet state is a sp³ hybrid and the singlet state forms a sp².

Though sometimes mistakenly stated, ground stateCF2is therefore not a (bi-)radical at all. Nevertheless, singlets can undergo reactions as well and therefore, concentrations chemically diminish as well: as stated in the liter-ature, difluorocarbene (singlet) is a reactive molecule [17, 18]. Besides high-rate reactions with radicals, it also adds e.g. to methyl substituted olefins (=alkenes) [19] as well as to perfluoroolefins. Chain extension reactions by addition of CF2 to unterminated polymer chains are of high relevance for oligomer formation as found theoretically by Lauet al.for(CF2)nchains [8].

The reaction enthalpy perCF2addition was constantly -203.8 kJ/mol, inde-pendent of the chain length. This was attributed to a lack of interaction of the chain with the bond formation occurring at the chain end. Such, chain extension by addition to unterminated molecules is of general relevance and one of the key processes in oligomer formation. Another important reaction is dimerization, which leads to the development of the (1,2)-biradicaloid

·CF₂−CF₂· [20]. This molecule can further undergo extension reactions with difluorocarbene [8], or dimerize to tetrafluoroethlyene (CF₂= CF₂).

The latter molecule was found to by highly stable, such that dimerization represents a terminal reaction under many conditions. The dimerization rate has been reported to be merely pressure dependent [17, 21, 22].

The dimerization rate coefficient is

CF2+ CF2−→C2F4 (R 8)

with a chemical rate ofk= 4×10⁻¹⁴cm³s⁻¹[13, 23] in low pressure plasmas, and∆H⁰=-291.9 kJ mol⁻¹in general.

1.3.3.3 Ion-chemical reactions

In addition to the above radical-dominated chemistry, ion chemistry is tak-ing place in discharges as well.

Cationic-anionic recombination can lead to the formation of reactive neutral species. In addition, ion-neutral reactions may take place. There-fore, several mechanisms play a role: besides collision-induced dissociation (CID), dissociative charge transfer (DCT) is of great importance, but also electron detachment (ED) can lead to fragmentation.

Some calculated reaction rate data according to Fontet al.[24] are listed in Table 1.4 for reaction channels involvingCFx-neutrals.

Table 1.4: Ion reaction channels and rates

reaction k₂

CF⁺₂ + CF3−→CF⁺₃ + CF2 1.48×10⁻⁹ CF⁺₂ + CF−→CF⁺₃ + C 2.06×10⁻⁹ CF⁺₂ + C−→CF⁺+ CF 1.04×10⁻⁹ CF⁺+ CF3−→CF⁺₃ + CF 1.71×10⁻⁹ C⁺+ CF₃−→CF⁺₂ + CF 2.48×10⁻⁹ C⁺+ CF−→CF⁺+ C 3.18×10⁻⁹ F⁺+ CF3−→CF⁺₂ + F2 2.09×10⁻⁹ F⁺+ CF₂−→CF⁺+ F₂ 2.28×10⁻⁹ F⁺+ CF−→C⁺+ F2 2.71×10⁻⁹ C2F⁺₄ + F⁻−→CF + CF2+ F2 8.20×10⁻⁸ C3F⁺₅ + F⁻ −→C2F4+ CF2 8.00×10⁻⁸ CF⁺₃ + F⁻−→CF₂+ F₂ 8.70×10⁻⁸ CF⁺₂ + F⁻−→CF + F₂ 9.10×10⁻⁸ CF⁺+ F⁻ −→CF + F 9.80×10⁻⁸ Rate coefficients are given in cm³s⁻¹.

Experimental data on ion-neutral reactions are only available at several stages of phenomenological observation up to tentative cross sections, see e.g. [24–26]. Especially the abundance and composition of neutrals has been, up to the authors knowledge, not yet been determined experimentally.

The relevance of these gas-phase processes to the present work is dif-ficult to estimate. In the very most models, ion-neutral reactions are not considered. The error by doing so may be low, as the positive-negative ion recombination rate coefficients are commonly larger by almost two orders of magnitude. Due to the lack of experimental confirmation of the exact

reaction routes, the above ion-neutral processes were not considered in this study as well. Ion-ion recombination is well known [27, 28] and included in the chemical modeling in Section 3.3.

1.3.3.4 Ion-surface interactions

At the electrodes and walls, chemical rates of neutral reactions like recombi-nation, oligomerization, and polymerization can be enhanced due to sticking or consumption of excessive heat of reaction. Reactions of adsorbed species are affected and promoted by ions: Neutralization processes at the elec-trodes can be of importance for the production of gaseous neutrals [29].

Ions can further enhance etching or polymer deposition. Besides the ion energy, this depends mainly on the electrode material or, in case of poly-mer deposit on the electrodes, the particular cross sections for ion-impact induced "activation"⁴ or dissociation. The enhancement of polymer depo-sition is mainly due to the creation of radical sites ("dangling bonds") [30].

1.3.3.5 Additional chemical reactions

In addition to the previous processes, argon metastables can induce disso-ciation as well. The rate coefficients are aroundk= 4−8×10⁻⁵cm³/s⁻¹ [31]. Argon metastable densities can be quite large and this dissociation channel can become important.

Neutral-neutral collision-induced dissociation is possible as well. A the-oretical study on argon collisions with CF4 resulted in significant dissocia-tion processes with a threshold ofEkin= 6 eV[32]. Fast neutrals can result e.g. from ion-ion recombination.

4Term often used for creation of radical sites on surfaces.

Chapter 2

Experimental setup and measurements

2.1 Plasma reactor and measurements of the