**4. Effect of plasma process parameters on final layer properties**

Many parameters, such as the nature of the precursor, the precursor flow rate, the reactor chamber's pressure, the reaction time, the power and frequency of the discharge, the geometry and physical dimensions of the reactor, etc. are believed to influence the surface properties of the resulting thin film [67].

In this section, a summary of the effect of plasma process parameters on various aminedeposited film properties as elucidated by researchers in this field will be given. Process parameters will be discussed based on different studies, but one should always keep in mind that various results emerging from different studies are hinted by the possible role of the distinctive processes (different reactor, plasma media and plasma source) used in each case.

#### **4.1. Discharge power and deposition time effect**

Electrical power and treatment time have a large impact on the nature and concentration of the molecular species formed in the reactor, which in turn significantly influences the depo‐ sition rate and the final atomic composition of the deposited layers.

*Effect on deposition rate / film thickness:* Lucas et al. [61] studied the power variation (between 3 and 100 W) effect on the deposition rate of allylamine plasma polymers. They found that deposition rate increases with increasing power. The same effect was observed by Myung et al. [64] with power variation between 30 and 90 W. The thickness of the film increases with increasing plasma power since high power generates high plasma density which increases the deposition efficiency. However, in another study by Lejeune et al. [57], the increase of deposition rate with power was found to be valid only up to a certain limit. In fact, the deposition of a film relies on a dynamic equilibrium between a process of deposition and a process of sputtering by incoming particles from the plasma. At low power of deposition (P < Plimit), the first process dominates the equilibrium. The amount of low energetic particles arriving on the surface of the film increases and the rearrangement of these deposited particles is low. This growth mode favors the formation of a low density structure (low cross-linking degree) with a high growth rate. With the increase of the power, the incoming particles have a higher energy and can penetrate more deeply in the growing film. Structural reorganization processes such as cross-linking of the polymeric chains and re-sputtering phenomena can occur. When the power is high enough (P > Plimit), the second process of the dynamic equili‐ brium acts effectively on the deposition: due to the sputtering effect, the growth rate becomes constant and due to the bombardment effect, the density of the film increases (high crosslinking degree) (see figure 6). For Lejeune et al. the limiting power was 30 W, but as already mentioned, results from different sources can only be compared to a certain extent and this limiting power varies from one treatment to another by taking into account the plasma media, plasma source, plasma parameters and reactor geometry.

**3.4. Scanning Electron Microscopy (SEM)**

**3.5. Atomic Force Microscopy (AFM)**

well below 1 nm.

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roughness, Rrms) [44].

the resulting thin film [67].

**4.1. Discharge power and deposition time effect**

sition rate and the final atomic composition of the deposited layers.

morphology and the treatment effect on the sample surface.

SEM images the sample surface by scanning it with a high-energy beam of electrons. Depend‐ ing on the instrument, the resolution can fall somewhere between less than 1 nm and 20 nm. SEM has been used in different studies involving plasma treatments. For instance, in a study by Hamerli et al. [43], SEM images showed that allylamine plasma polymerization yields homogenous pinhole-free layers. In another study by Sanchis et al. [44], SEM images showed that nitrogen plasma treatment formed micro-cracks on the sample surface. In this way, SEM can be used on plasma treated samples to give information regarding the deposited film

AFM is a mechanical imaging instrument that measures the three-dimensional topography as well as physical properties of a surface with a sharpened probe. Typical AFM resolutions are

AFM can be used to study the way plasma polymers grow: AFM images have been used to show the surface morphology of the deposited films while varying the treatment time [66]. AFM analysis is useful, not only in a qualitative way but also for quantitative determinations, since it allows a 3D representation of the treated surface and quantifies the effects of the plasma-etching mechanism by calculation of the surface roughness (root-mean-squared

Many parameters, such as the nature of the precursor, the precursor flow rate, the reactor chamber's pressure, the reaction time, the power and frequency of the discharge, the geometry and physical dimensions of the reactor, etc. are believed to influence the surface properties of

In this section, a summary of the effect of plasma process parameters on various aminedeposited film properties as elucidated by researchers in this field will be given. Process parameters will be discussed based on different studies, but one should always keep in mind that various results emerging from different studies are hinted by the possible role of the distinctive processes (different reactor, plasma media and plasma source) used in each case.

Electrical power and treatment time have a large impact on the nature and concentration of the molecular species formed in the reactor, which in turn significantly influences the depo‐

*Effect on deposition rate / film thickness:* Lucas et al. [61] studied the power variation (between 3 and 100 W) effect on the deposition rate of allylamine plasma polymers. They found that

**4. Effect of plasma process parameters on final layer properties**

Myung et al. [64] also investigated the effect of treatment time on the deposition rate. They found that the thickness of the deposited layers increases with the increase of plasma poly‐ merization time. Martin et al. [68] investigated the synergistic effect of plasma power and deposition time on n-heptylamine plasma polymerized (HApp) film thicknesses using AFM step height measurements combined with a surface masking technique. The results showed a dramatic difference between conditions involving high power and long deposition (thickness average of 47 nm) to those involving low power and short deposition time (thickness average of 3 nm) (see figure 7).

*Effect on film's atomic composition:* Martin et al. [68] also studied the effect of power on the atomic composition of the layers using XPS analysis. High power yielded relatively lower surface concentration of nitrogen atoms than the use of low power: by varying the power from 80 W to 10 W nitrogen content increased by ca. 15%. This is due to the more successful breakdown of the monomer molecule achieved at higher power, yielding layers which present less nitrogen atoms. This behavior has also been observed by Shard et al. [35]. However, deposition time and time-power interaction do not have a significant effect on the atomic composition of the layers [68].

In the study conducted by Lucas et al. [61], the use of XPS coupled to derivatization reactions for allylamine plasma polymerization showed that %NH2 decreases with the increase of power. This was also observed by Lejeune et al. and Müller et al. [57, 69]. The effect of plasma on the retention of the precursor functional group also depends on the plasma mode. Basarir et al. [70] worked on plasma polymerization of allylamine using both CW and pulsed modes. Results showed that pulsed plasma polymerization further increased amine density. In fact,

**Figure 6.** Growth rate of plasma polymerized allylamine films as a function of power [93]

**Figure 7.** Square plot summarizing the influence of power and deposition time on the thickness of HApp layers [94]

in the pulsed mode, the mean power per precursor molecule is lower than that in the CW mode. In the off-time, dissociated monomers react with each other instead of continuing to dissociate. Moreover, the use of lower Pmean in pulsed plasma polymerization results in a higher retention of the primary amine functional group [18, 56].

By using IR spectroscopy on allylamine plasma deposited films, Myung et al. [64] noticed changes in film composition by varying input power between 30 and 90 W. High-power plasma led to a higher ratio of C≡N to CH than the ratio of NH to CH, thus to a recombination of amine functionalities into nitrile (C≡N) groups. This was also observed in two studies by Hamerli et al. [43, 47]. At high power, monomer fragmentation is accelerated and results in the formation of imine groups and nitrile groups. A high retention of amine groups is mainly favored by low input power (< 30 W). This has been confirmed by Wen-Juan et al. [71] where power was varied between 5 and 30 W. The increase of plasma power caused more formation of C=N groups which can be explained by the sufficient fragmentation of monomer at higher applied powers.

Furthermore, Myung et al. [64] correlated the contact angle to the plasma input power. The contact angle increases with increasing input power thus causing a decrease of the surface free energy i.e. a decrease of the surface hydrophilicity. The main factors which resulted in this effect were the loss of amine functional groups and the formation of cross-linked structures due to more fragmentation of allylamine by increasing input power. In another study by Sanchis et al. [44], treatment time effect on surface morphology was investigated via AFM imaging. Different surface morphologies with a slight increase in surface roughness were observed as exposure time increased.

As we can see, power has a great influence on the chemical and physical properties of the deposited films. The synthesis conditions for deposition of plasma polymer films with high functional group concentration are characterized by low powers thus leading to low precursor fragmentation. This will result in the reduction of the plasma polymer film cross-linking degree [56]. However, the control of the cross-linking degree is an important factor for the optimiza‐ tion of the plasma polymer film stability and mechanical and thermal properties [72]. There‐ fore, it is important to evaluate the cross-linking degree in addition to the plasma and polymer film chemistry in order to choose the appropriate power [70, 73].

### **4.2. Monomer flow rate effect**

Monomer flow rate is an important plasma process parameter that has been investigated in order to correlate it to the deposited film properties.

Hamerli et al. [43] and Basarir et al. [70] investigated monomer flow effect on amine function‐ ality retention. At constant power and treatment time, higher monomer flow rates yielded higher amine retention. The increased amine density with increased flow rates can be ex‐ plained by less dissociation of monomers, owing to the decreased plasma power for each molecule as the monomer flow rate increased.

Another study by Martin et al. [68] showed that the monomer flow rate does not influence the thickness of the deposited layers.

#### **4.3. Precursor type effect**

in the pulsed mode, the mean power per precursor molecule is lower than that in the CW mode. In the off-time, dissociated monomers react with each other instead of continuing to dissociate. Moreover, the use of lower Pmean in pulsed plasma polymerization results in a higher

**Figure 7.** Square plot summarizing the influence of power and deposition time on the thickness of HApp layers [94]

By using IR spectroscopy on allylamine plasma deposited films, Myung et al. [64] noticed changes in film composition by varying input power between 30 and 90 W. High-power plasma led to a higher ratio of C≡N to CH than the ratio of NH to CH, thus to a recombination of amine functionalities into nitrile (C≡N) groups. This was also observed in two studies by Hamerli et al. [43, 47]. At high power, monomer fragmentation is accelerated and results in the formation of imine groups and nitrile groups. A high retention of amine groups is mainly favored by low input power (< 30 W). This has been confirmed by Wen-Juan et al. [71] where

retention of the primary amine functional group [18, 56].

**Figure 6.** Growth rate of plasma polymerized allylamine films as a function of power [93]

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The precursor type is one of the most important factors in plasma treatments. For instance, in plasma polymerization, the use of a non-saturated monomer like allylamine is advantageous compared to saturated ones because in the former case less energy is needed for the polymer‐ ization process. In fact, the double bond in allylamine encourages a deposition by a combina‐ tion of plasma and conventional free-radical polymerization. Because of that, allylamine typically polymerizes at low energies. Due to less fragmentation, a higher amount of primary amines can be retained in the plasma polymer [34, 59]. Furthermore, each precursor is characterized by its own chemical structure and thus its particular bond breaking energies which influence to a great extent the selectivity of the fragmentation processes in the plasma. Therefore, depending on the kind of monomer used, various chemical compositions are obtained after each plasma treatment [18, 25, 55]. For example, Hamerli et al. [47] used ammonia and allylamine as plasma precursors. A higher amine concentration was found on the allylamine modified samples. Mangindaan et al. [60] used allylamine, propylamine and propargylamine (another unsaturated monomer) as precursors. XPS coupled to derivatization showed that allylamine incorporates the highest amount of amine functionalities into the corresponding thin films compared to those synthesized from the two other precursors.

The precursor type also influences the growth mode and thickness of the deposited layers. Michelmore et al. [66] noticed that films grown from n-heptylamine initially show "islandlike" growth before a continuous smooth film is formed. In contrast, films from allylamine grow smoothly from the very earliest stages. Moreover, it has been found that monomers containing double bonds polymerize faster in plasma than their saturated counterparts. Gancarz et al. [25] have investigated the plasma polymerization of n-butylamine and allyla‐ mine and observed that the deposited layers are much thicker for allylamine plasmas. This observation has also been confirmed in a study performed by Mangindaan et al. [60].
