Investigating Diblock Copolymer Surface Structures with Advanced Microscopy Techniques

Topics Covered

Introduction
Experimental Procedure
     Sample
     Sarfus Analysis
Results and Discussion
Conclusion
Advantages of Sarfus

Introduction

In the experiment discussed here the thin layer of diblock copolymer is formed by spin-coating from solution. The initial diblock copolymer is quite disorganized, meaning that the material is not microphase separated and so the PS and PMMA segments are intimately mixed. When the dibloc copolymer is heated, the diblock will slowly organize. Sarfus was used to investigate the surface structure of the diblock copolymer.

Experimental Procedure

Sample

A dibloc copolymer solution is prepared from commercial copolymer (from Polymer Source. Inc. Mn : PS(43500)-PMMA (21800)) and toluene by dissolution (7.5 mg/ml). The solution is spin-coated during 30s (3000 rpm/min).

Sarfus Analysis

Optical images are realized using Sarfus technology. This optical microscopy technique is based on the particular surface reflection properties of contrast-enhanced substrates called Surf (Nanolane, France). In this study, the topmost layer of the Surf substrates is SiO₂ ('Standard Surf'). Optical images are obtained on a LEICA DM4000 optical microscope and collected via a SONY 3CCD camera. The 2D images are treated with Sarfusoft (Nanolane software) and after calibration, 3D images are generated.

Results and Discussion

A dibloc PS-PMMA copolymer solution is firstly spin-coated on a standard Surf. The prepared layer is observed by Sarfus (a tweezer scratch is visible on the left part of the image). The surface appears to be flat (Ra ~ 2.2nm) and the mean thickness is about 73.1nm (Figures 1a & 1b). Some particles (Ø 5-10 µm) are also present on the surface.

Figure 1a. 2D Sarfus image of the spin-coated dibloc PS-PMMA copolymer (before annealing).

Figure 1b. Step height measurement of the spin-coated dibloc PS-PMMA copolymer (before annealing).

After annealing (1h at 180°C, under air), the copolymer surface displays orange skin aspect (Figure 2) due to copolymer structuration. By comparison with the characteristic dimensions of demixion domains (Figure 3), the larger structures observed are probably due to a dewetting phenomenon rather than to a demixion process.

Figure 2. Sarfus image of the annealed dibloc PS-PMMA copolymer layer (image scale: 152x118µm²).

Figure 3. AFM image of demixion structures on annealing dibloc PS-PMMA copolymer.

Profile section allows accessing the different thicknesses of layers, holes and peaks (Figure 4b). A residual layer of 4 nm is present probably due to molecules migration during annealing.

Figure 4a. Sarfus image of the annealed dibloc PS-PMMA copolymer layer (image scale: 76 x 59µm²).

Figure 4b. Section profile (following the dot line, red to green points) on the structured dibloc copolymer.

The green layer is about 28nm (31.5 - 4) whereas the peak and holes thicknesses are about 86 nm and 53 nm, respectively. In addition, Sarfusoft allows measuring the area percentage of surface structures. Thus the hole and peak area percentages are about 37.7% (h=53nm) and 62.3% (h=86nm), respectively (Figure 5). A perfect conservation of the matter before and after annealing since 0.38*53+0.62*86 ~ 73.1nm (initial layer thickness) is to be noted.

Figure 5. Area percentages of peak (red) and hole (yellow) structures.

Conclusion

We have demonstrated the ability of Sarfus to easily and rapidly analyze copolymer microphase structuration. Thanks to surface area determination, matter conservation is also demonstrated before and after annealing. In-situ microstructuration study could be done simply using a hot-plate on the microscope stage.

Advantages of Sarfus

The advantages of Sarfus include:

• Fast analyse of the surface (analyse duration: 2h)
• Field of view (from 60µm² to several mm²) for statistical results
• Non-invasive/non contact technique
• Ability to real-time study

Source: Nanolane

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