FYSZ460 Electron Beam Lithography Electron Beam Lithography FYSZ460 Advanced Laboratory Exercise Mikko Palosaari

FYSZ460 Electron Beam Lithography 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

FYSZ460 Electron Beam Lithography About electron microscopes First one developed in 1930s and the first commercial one in The light used in optical microscopes is substituted with very thin beam of electrons (0.4 nm-5 nm). First commercially produced 100 kV SEM by Siemens

FYSZ460 Electron Beam Lithography About electron microscopes With light microscopy the resolution is approximately limited with the so-called Abbe criteria due to diffraction of light With electrons the wavelength is so small that their wavelength does not limit imaging.

FYSZ460 Electron Beam Lithography About electron microscopes SEM: Conducting samples TEM (Transmission electron microscope): Samples have to be thin enough so that the electrons can transmit through. Operated in vacuum Less scattering of electrons Only for solid state samples, e.g. biological samples have to be dried and coated.

FYSZ460 Electron Beam Lithography

FYSZ460 Electron Beam Lithography Scales and magnifications

FYSZ460 Electron Beam Lithography Generation of Electron beam Electrons produced by electron gun (field emission, thermionic emission) Emitted electrons accelerated by high voltage (~ 10kV – 100 kV) e-beam focused by magnetic lenses Beam scanned over the sample by deflection coils Scattered (transmitted) electrons detected Courtesy of Iowa State Univ.

FYSZ460 Electron Beam Lithography About the interaction of the electron beam and the sample D epending 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.

FYSZ460 Electron Beam Lithography Interactions of the electron beam and the sample

FYSZ460 Electron Beam Lithography Interactions of the electron beam and the sample  Incident electron knocks out an inner shell electron of the target atom.  Outer shell electron fills the vacancy in the inner shell and this transition emits X-rays. These X-rays can be used to characterize the target material since every element has its unique characteristic X-rays. X-rays

FYSZ460 Electron Beam Lithography Interactions of the electron beam and the sample  Incident electron knocks out an inner shell electron of the target atom  Outer shell electron fills the vacancy in the inner shell and simultaneously another outer shell electron is emitted from the sample. These Auger electrons can also be used to characterize the target material. Auger Electrons

FYSZ460 Electron Beam Lithography Interactions of the electron beam and the sample  Cathodoluminescence occurs when electron beam promotes electrons from the valence band into the conduction band, leaving behind a hole.  When an electron and a hole recombine, it is possible for a photon to be emitted. This is called cathodoluminescence.  Only with non-metallic materials. Cathodoluminescence

FYSZ460 Electron Beam Lithography Interactions of the electron beam and the sample  Electron interacts with target material nucleus.  Energy of BEs much higher than that of SEs → information from deeper layers.  Number and scattering direction of BEs determined by the atomic number and the incident angle of the e-beam.  Can be used to detect composition difference. Backscattered Electrons

FYSZ460 Electron Beam Lithography Interactions of the electron beam and the sample  The number of SEs depends greatly on the incident angle of the electron beam to the specimen surface  SEs are low in energy -> emitted only from the surface  Most suitable signal for observing surface topography Secondary Electrons

FYSZ460 Electron Beam Lithography Image formation 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 strength as a function of the position of electron beam on the sample

FYSZ460 Electron Beam Lithography 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

FYSZ460 Electron Beam Lithography Sample preparation Surface must be clean SEM observes the surface layer of specimen Morphological construction must be maintained E.g. Biological samples must be dried and emulsion specimens may be frozen Specimen must not acquire an electrostatic charge Metal coating, low voltage observation or low vacuum may be used.

FYSZ460 Electron Beam Lithography Features of the SEM + High resolution + High contrast + High depth of field/focus, large focal depth - Charging up effects - Aberrations (Spherical, chromatic, diffractions) - Astigmatism - Sensitivity to vibrations etc. external factors -Vacuum

FYSZ460 Electron Beam Lithography High depth of field example. Different plant pollen.

FYSZ460 Electron Beam Lithography False color SEM images

FYSZ460 Electron Beam Lithography SEMs in NSC Raith e-LiNE EBL feature size < 20 nm SEM imaging resolution: <10 nm LEO 1430 EBL feature size ~ 200 nm SEM imaging resolution: <100 nm

FYSZ460 Electron Beam Lithography Electron beam lithography EBL used in research and in specialized tasks. Smaller line width than with UVL. Slow(er), serial exposure. Flexible. Industrial application: mask making for UV lithography.

FYSZ460 Electron Beam Lithography Lithography steps Substrate 2-layer resist: PMMA, P(MMA-MAA) 1. Resist Coating2. e-beam patterning3. Development4. Metal deposition5. Lift-off

FYSZ460 Electron Beam Lithography Resists Two tasks To react to radiation To protect the surface Three components Film forming Sensitive to radiation Solvent Positive vs. negative resist

FYSZ460 Electron Beam Lithography Exposure Optical -Mask required -Parallel -Limited by wavelength of light, diffraction and resist properties EBL –Serial, point by point –Not limited by wavelength of electrons –Limited by resist properties EBL exposure is governed with the formula: =dose ✕ exposed area beam current ✕ exposure time Total charge of incident electrons = step size

FYSZ460 Electron Beam Lithography 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

FYSZ460 Electron Beam Lithography Ultraviolet lithography UV lithography most commonly used in industry. Fast, parallel exposure. Suitable for mass production. Commercial products: line width 90 nm (less than 60nm ?), Highly complicated & specified optics required for the state of the art methods. With EUV even sub 20 nm line widths demonstrated.

FYSZ460 Electron Beam Lithography Practical issues Course website Study material –Invitation to the SEM World link at the website –JEOL Guide to Scanning Microscope Observation link at the website –Marc J. Madou: Fundamentals of Microfabrication the Science of Miniaturization 2nd ed. –Sami Franssila: Introduction to Microfabrication –Ivor Brodie and Julius J. Muray: The Physics of Micro/Nano –Fabrication Homework from website Design CAD file Schedule for the practical part Report (will be graded)

FYSZ460 Electron Beam Lithography CAD image

FYSZ460 Electron Beam Lithography Ready sample