THIN FILM DEPOSITION

Introduction to thin film techniques



INTRODUCTION

Technobiochip has been using surface modification by thin film Langmuir-Blodgett (LB) and Langmuir-Shaeffer (LS) deposition technologies far several years. These techniques allow to deposit organic layers characterized by a highly-ordered molecular structure. The creation of an ordered molecular monolayer marks a great improvement in both chemical and biomolecular sensoristics, as the preferential orientation of the “active sites” of deposited molecules leads to an improvement in terms of signal following the analyte-sensor interaction.
Molecules deposited by these techniques are generally amphiphilic, that feature a hydrophilic portion, which lies on after being sprayed onto the aqueous surface, and a hydrophobic portion, which is in contact with air (Fig. 1). Examples of amphiphilic molecules are metallo-porphyrines, metallo-phtalocyanines, several polymers and biological molecules.


Figure 1: example of amphiphilic molecule(metallo-porphyrine).


EQUIPMENT

The instrument used for LB/LS deposition KSV 5000 consists of two thermostable teflon throughs and two balances with on top the Wilhelmy’s plates (Fig. 1).



TECHNIQUES

A) LANGMUIR-BLODGETT(LB) DEPOSITION
Thin films deposition by means of Langmuir-Blodgett technique consists in depositing a volume of an amphiphilic molecule in solution with known both concentration and MW onto the aqueous surface (usually ultrapure 12.8 M Ω/cm water) in one of the two KSV 5000 throughs, (Fig. 1A). After the solvent has evaporated, the aqueous surface is compressed by barriers (Fig. 1B), until it reaches an ordered surface monolayer (Fig. 1C); then, by immerging the substrate a few times into the aqueous surface and then taking it out, it is possible to deposit molecularly ordered layers onto it (Fig. 1D).

Figure 1: Langmuir-Blodgett’s tecnique scheme


By means of KSV 5000, thanks to the double troughs and balances, is possible to make ordered alternated depositions, and obtain films with different features (Fig. 2).

Figure 2: different types of depositions possible by Langmuir-Blodgett


B) LANGMUIR-SHAEFFER(LS) DEPOSITION
Similarly to LB, in this case a volume of an amphiphilic molecule in solution with known both concentration and MW is deposited (Fig. 3A). After solvent is evaporated, aqueous surface is compressed by barriers (Fig. 3B), until it reaches an ordered surface monolayer (Fig. 3C); then, by means of substrate contact with the aqueous surface, it is possible to deposit molecularly ordered layers onto it (Fig. 3D).


Figure 3: Langmuir-Shaeffer’s technique scheme

Compared with LB, this technique appears to be quicker, but shows some limitations, the most important is that allows one deposition type only (Fig. 4).


Figure 4: Langmuir-Shaeffer possible deposition


In order to have a homogeneous surface deposition, it is crucial to know the compression isotherm of the molecule before deposition (Fig. 5). As a matter of fact, by relying on the compression isotherm it is possible to calculate the target surface tension at which it is possible to deposit the amphiphilic molecule layers can be deposited.


Figure 5: example of compression isotherm


The monolayer formation is monitored by following the compression isotherm curve. As a matter of fact, as shown in Figure 5, from the right to the left, the curve goes parallely to the abscissa, without increment in the surface tension (molecularly surface disorder that mimics the gas phase)(A). More and more the barriers compress the surface, a little increment in the surface tension is observed (B), since for very little movements of barriers, a high increment is achieved due to the formation of an ordered monolayer on the top of the aqueous surface (C).
The next step is the deposition isotherm (Fig. 6), which allows the deposition of a certain number of molecules layers, by dipping and taking out the substrate from the aqueous surface at a constant target tension (Fig. 6A ), at controlled rate and depending on the percentage calculated real-time of the molecular transfer (Fig. 6B ).

Figure 6: example of deposition isotherm at constant target tension