Spin-coating and ellipsometry investigation of thin polymer films - PDF document

X-Ray and Neutron Science – International Student Summer Programme at ILL/ESRF Spin-coating and ellipsometry investigation of thin polymer films Report Albert Prause supervised by Aljoˇ sa Hafner Berlin, October 29, 2017

Report Contents Contents 1 Introduction 1 2 Chemicals and methods 2 2.1 Chemicals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 2.2 Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 2.2.1 Cleaning of wafers . . . . . . . . . . . . . . . . . . . . . . . . . 2 2.2.2 Spin-coating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 2.3 Ellipsometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 3 Results 4 3.1 Layer thickness of spin-coated films . . . . . . . . . . . . . . . . . . . . 4 3.1.1 Influence of concentration and molecular weight . . . . . . . . . 4 3.1.2 Homogeneity of polymer film . . . . . . . . . . . . . . . . . . . 5 3.2 Evaluation of scaling with viscosity . . . . . . . . . . . . . . . . . . . . 6 4 Discussion 9 5 Conclusion 9 References 10 Abbreviations 11 II

Report Introduction 1 Introduction Thin polymer films play an important role for industrial applications and scientific re- search. They are used in microelectronics for chip production or organic semiconductors as well as optical coatings for lenses, mirrors or glasses. [1] Thereby the film thickness of the layer is one major parameter to adjust physical properties and interactions. Addi- tionally well defined polymer layers are necessary for fundamental research, for example in the investigation of long-range van der Waals forces associated with stabilization and destabilization of surrounding layer. [2] For this, the controllability and reproducibility of the polymer layer are essential for the desired application. Preparation of the films can be done using different methods like spin, dip, flow or spray-coating or grafting methods. Especially spin-coating is widely used. It is a simple and cheap method for depositing polymers on solid substrates. Additionally spin-coating exhibits very good reproducibility and homogeneity in thickness. [1,3,4] The prediction of spin-coated layer thicknesses is a challenging task. Likewise theo- retical description of the spin-coating process involves mass transport phenomena and fluid dynamics whereby it becomes rapidly complex. For that reason the prediction of layer thickness is based on empirical models with experimental data as the basis. Although these models have experimental data as underlying basis. In addition spin- coating has several parameters which influence the resulting layer thickness. The main parameters are the polymer concentration c g , the angular velocity ω , solution viscosity η and evaporation rate of solvent e . In order to obtain the scaling behavior, many studies investigate the dependence of the layer thickness on these parameters. [4,5] In this work the main aspect was the investigation of the layer thickness as a function of concentration for different molecular weights of hydrogenated polystyrene (h-PS). We used four different molecular weights to cover almost three orders of magnitude. Secondly, we studied the reproducibility and homogeneity of the deposited layer and finally, the influence of viscosity was studied in dependence on the layer thickness in order to verify our experiments with the literature. The films were prepared by spin-coating and the thickness of the polymer film was measured via ellipsometry. 1

Report Chemicals and methods 2 Chemicals and methods 2.1 Chemicals The used polymer was h-PS with four different molecular weights M n (2.8, 21, 432, 1690 kg mol − 1 ) which correspondingly cover almost three orders of magnitude and a low polydispersity ( M w /M n between 1.04 and 1.07) a . For the preparation of polymer solutions toluene (99 %) was used as solvent. The substrates were silicon wafer (Si), cut along the (100) crystal plane, of different sizes and thicknesses. Cleaning of wafers was performed with MilliQ water (resistance > 18 MΩ cm), ethanol, acetone and chloroform. 2.2 Preparation 2.2.1 Cleaning of wafers Every wafer was cleaned with a multi-step procedure: (i) 15 min sonication in mild DECON90/water solution, (ii) 15 min sonication in ethanol, (iii) 15 min sonication in acetone, (iv) 15 min sonication in chloroform, (v) 15 min sonication in water. After the cleaning procedure the wafers were inspected and, if necessary, cleaned again from step (iii). Afterwards, they were dried with filtered nitrogen flow and stored in a plastic dish. Before spin-coating the wafer, the thickness of the SiO 2 layer on top was determined by ellipsometry. 2.2.2 Spin-coating Uniform films can be produced via spin-coating by spreading a fluid on a horizontal rotating substrate. Commonly the deposition is done under static conditions or with a low angular velocity. After deposition the substrate is accelerated to high angular velocities. During rotation the deposited liquid is thinned by ejecting excess liquid and evaporation of the solvent. [1] Therefore, the polymer has to be in liquid form. This is achieved by solubilising the polymer in an organic solvent. Preparation of polymer films was performed by spin-coating from solution of h-PS in toluene. At first the desired amount of polymer was dissolved in appropriate amount of toluene to get the desired concentration and mixed for a minimum of 72 h. The error of preparation of polymer solutions was investigated by weighing the vials directly after preparation and before using the solution for spin-coating. Also independently a M w /M n for h-PS with 432 kg mol − 1 was not available. 2

Report Chemicals and methods prepared solutions were used to figure out how accurate one can prepare a specific concentration. For spin-coating, we used a two step programme where the first step was set to 500 rpm angular velocity for 2 s. The second step was set to 3000 rpm for 50 s. During all preparations the spin-coating programme was hold constant. The spin-coating was performed with the described programme in two runs. The first run was carried out only with toluene and shortly afterward the second run with the desired polymer solution. Thereby the wafer was wet fully with solution either toluene or polymer solution. The thickness of the resulting polymer layer was evaluated by ellipsometry. 2.3 Ellipsometry The reflection of light on thin films involves a phase and amplitude shift between parallel and perpendicular polarization. This shift depends mainly on the thickness and the refractive index of the layer which is widely used to determine the thickness of the layer. To determine the phase shift one uses an ellipsometer. Hereby elliptically polarized light is introduced via a polarizer and a compensator, e.g. a λ/ 4 wave plate. The specific polarized light reflects on the film and passes through an analyzer. The resulting intensity of the reflected light is measured. Thereof the phase shift between parallel and perpendicular polarization can be counted back. With these information the film thickness and the refractive index can be calculated via Fresnel theory. [6,7] Ellipsometry measurements were performed on a variable angle phase modulation ellip- someter by Beaglehole Instruments. The instrument uses a He-Ne laser ( λ = 633 nm). For every measurement an angle scan between 35 ◦ and 73 ◦ in 2 ◦ steps was performed. The data evaluation was done with the instrument software in Igor Pro 6 (WaveMet- rics). For evaluation a multi-layer model was created with the layer sequence Si, SiO 2 , h-PS and air. The refractive indices of the materials for the used wavelength were included into the software ( n Si = 3 . 882, n SiO 2 = 1 . 457, n h-PS = 1 . 588 and n air = 1). The thickness of SiO 2 was fixed to the previously measured value which was obtained without the polymer film. With this parameters the evaluation of the h-PS film thick- nesses was performed whereby the initial value for the h-PS film had to be in the proper range. Error and homogeneity estimation of the layer thickness were conducted via measuring different spots on the same wafer. Furthermore independently spin-coated wafers with the same polymer concentration were measured. Also the instrumental accuracy was taken into consideration which is in the range of 1 Å. [8] 3