Swelling of Poly(methyl methacrylate) Films in Different Solvent Vapor Environments: A Comparison of Solvents Used in Nanolithographic Processing Open Access

Rohrbach, Alan (Spring 2019)

Permanent URL: https://etd.library.emory.edu/concern/etds/db78td07g?locale=en


Polymer films are used extensively as sacrificial resist layers in the nanolithographic process. Their properties as resists are quantified by sensitivity, the minimum dose required to expose the resist, and contrast, a measure of how quickly the resist goes from completely unexposed to completely exposed. Existing work in the lithographic field determines sensitivity and contrast of polymer resist layers through the production of contrast curves, which are functions of exposure dose. However, this method makes it hard to isolate the effects of developer solvent conditions from other variables. Solvent conditions are very important to nanolithography since the development step of the process depends on finding and maintaining a delicate balance of solvent quality to achieve selective dissolution. In positive-tone resists, the solvent used must selectively dissolve away the short chain segments in the regions exposed by the electron beam while leaving undisturbed the unexposed regions. This work looks at the interaction of a widely used polymer resist with the solvent/non-solvent pair that is most often used in its lithographic processing, without the confounding factors of electron beam exposure.

We designed and fabricated a sample chamber for use on an ellipsometer to allow samples of polymer films to be exposed to a solvent vapor environment while being measured by spectroscopic ellipsometry. The sample chamber consists of an aluminum base with a solvent "moat" surrounding the sample itself and a 3D printed polypropylene lid with a viewing window, solvent filler hole, and openings for the beam path. Initial measurements were made using the sample chamber and samples of poly(methyl methacrylate) (PMMA) exposed to either isopropyl alcohol (IPA) or methyl isobutyl ketone (MIBK) vapor. We observed faster swelling in the PMMA films exposed to MIBK vapor than in the PMMA films exposed to IPA vapor, which agrees with our expectation that MIBK will behave as a solvent and IPA as a non-solvent.

Table of Contents

Chapter 1 Introduction and Background 1

1.1       Introduction to Electron Beam Lithography 1

1.1.1       Process of EBL 1

1.1.2        Sample Geometry 4

1.2       Relevant Properties of Polymers 5

1.2.1        Polymer Solubility in Solvents 7

1.3     Electron Dose on Polymers and Contrast Curves 8

1.3.1    Contrast Curves 8

1.3.2       Interpreting the Lithographic Process 10

1.4     Existing Research in Nanolithography 10

1.4.1        Effect of Molecular Weight on Sensitivity 11

1.4.2        Effect of Developer Temperature 12

1.4.3        Developer Type/Solvent Mixture 15

1.4.4       Dissolution Rate Studies 16

1.5       Goal of Thesis 16

Chapter 2 Experimental Methods and Development of Sample Chamber 18

2.1       Ellipsometry 18

2.2        Sample Preparation 21

2.3       Sample Chamber Design 22

2.4       Experimental Procedure 26

Chapter 3 Results and Discussion 27

3.1       Control for Ambient Index 27

3.2       PMMA + IPA 31

3.2.1       Alternate Fitting Method 33

3.3       PMMA + MIBK 36

3.4       Comparison of MIBK and IPA 37

Chapter 4 Summary and Conclusions 39

References 40

Appendix 43

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