FYS0460 / FYSZ460 Advanced Laboratory Exercise Electron Beam Lithography

1. FYS0460 / FYSZ460Advanced Laboratory ExerciseElectron Beam Lithography Terhi Hongisto terhi.hongisto@jyu.fi YN240

2. The Objective of the Laboratory Exercise To give an introduction to • The operation of a Scanning Electron Microscope (SEM) • Electron Beam Lithography (EBL) • Working in laboratory and in cleanroom conditions

3. About electron microscopes • The light used in optical microscopes is ‘substituted’ with very thin beam of electrons → smaller wave lenght → better resolution SEM: conducting samples TEM: (transmission electron microscope) the samples have to be thin enough for the electrons to transmitt • Operated in vacuum → less scattering of the electrons →only for solid state samples (e.g. biological samples have to be dryed and coated) Kuva: SEM/TEM/Optical

4. About the interaction of the electron beam and the sample • Different interactions → depending both on the sample and the detector different kind of information obtained →information from the different layers of the sample →different kind of detectors needed Kuva: vuorovaikutukset

5. About the interaction of the electron beam and the sample • Secondary electrons number of SEs strongly dependent on the incident angle, low energy → surface topography • Backscattered electrons energy much higher, the number and the scattering direction of BEs determined by the incident angle and the atomic number of the substanses composing the specimen surface

6. Magnification • Electron beam scanned over the sample • Information emitted from each scanned point • The signal from the detector (e.g. the number of SEs) amplified and fed into CRT (cathode-ray tube, nowadays often computer screen) • On the CRT the brightness is controlled according to the signal stength as a function of the position of electron beam on the sample Kuva: Kuvan muodostus

7. Generation of Electron beam • Filament (W) heated by current (thermionic emission source) • Emitted electrons accelerated by high voltage • e-beam focused by magnetic lenses • Beam scanned over the sample by deflection coils • Scattered (transmitted) electrons detected Kuva: Column

8. Detectors SE: collected by “post acceleration voltage” applied to scintillator → shadowless illumination image formed BE: semiconductor detector, image formed by the electrons emitted towards the detector → one-side illumination image formed

9. Features of the SEMs + high resolution + high contrast + high depth of field/focus, large focal depth - charging up effects - aberrations (chromatic, ) - astigmatism - sensitivity to vibrations etc.external factors • Kuva: glomerulus

10. Lithography UV lithography most commonly used in in industry -fast, parallel exposure -suitable for mass production -commercial products: line width 90 nm (65 nm ?), highly complicated & specified optics required EBL in research -smaller line width -slow(er), serial exposure -flexible -industrial application: mask making for UV lithography

11. Lithography 2 layer resist: PMMA, PMMA-MAA (=co-polymer)

12. Resists 2 tasks - to react to radiation - to protect the surface 3 components - film forming - sensitive to the radiation - solvent Positive vs. negative resist

13. Exposure Induces change in the resist • Optical • Mask required • Parallel • Limiting factors • wave length, diffraction in mask/resist • EBL • Serial, point by point • Limiting factors • resist Dose, cross linking, acceleration voltage, proximity effect (inter-/ intra-)

14. Developing • Chip immersed to the developer chemical • Exposed resist with smaller molec. Weight dissolves more readily • Undercut profile Other process steps: • Metal coating/deposition • Lift-off

15. Practical issues • Study material Invitation to the SEM World: ”Dictionary” Error on page 2 • Design Cad • Report • Schedule for the practical part