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Electrons, resist and substrates

Table of content

  1. Vistec EBPG5000 overview
  2. What is an electron beam (ebeam) lithography system?
  3. Electrons, resists and substrates
  4. Process Flow for Ebeam Lithography
  5. Data conversion
  6. Running the ebeam

Electrons, resist and substrates

The smallest thing you can write with the ebeam depends on a large number of factors. These are the spot size used, the type of resist used, the thickness of the resist, the density of the features and the substrate material.

When electrons are used to expose a pattern in resist it is not a simple process. Electrons enter the resist and hit the atoms of the resist, these will either forward scatter or back scatter. Backscattered electrons from the resist will leave the resist and, in general, do not contribute to the resist exposure, forward scattered electrons continue into the resist and contribute to the exposure. The thicker the resist the larger the forward scattering and the lower the resolution. High energy electrons (in our case 100kV) will go through the resist and deep into the substrate. Here they will again get scattered and will forward and backscatter. In this case the forward scattered electrons will be moving away from the resist and don’t contribute to the exposure, backscattered electrons from the substrate have a large contribution to the exposure. The higher the energy of the incoming electrons the deeper they will penetrate into the resist and hence the contribution to the resist exposure will be reduced. In the figure below you can see that going from 10kV to 20kV increases the penetration depth of the electrons from 1µm to around 6µm. At 100kV the penetration depth in Silicon is around 100µm.

penetration depth in Silicon

The smallest feature sizes that can be achieved are when the features are isolated from one another. As you make your features closer together the backscatter from the neighbouring features will all contribute to the exposure and it will become harder to find the correct dose to correctly expose all your features. This is call proximity effects. There are 2 main effects of this; inter-proximity and intra-proximity.

inter-proximity intra-proximity
inter-proximity intra-proximity

With inter-proximity when two features are close together the electrons from the exposure of on shape contributes to the dose of the neighbouring pattern. The larger and closer the features the worse this effect.

With intra-proximity the dose in the centre of the pattern is larger than at the edges, and especially the corners. This is simply a geometric effect as there are less electrons contributing to the dose in the corners of the shape.

The electrons need a path to ground. If you are using a conducting (or semi-conducting) substrate the contact with the holder is sufficient to provide a conducting path. If you are using an insulating substrate (fused glass, quartz) you will need to provide a conductive path for the electrons. This is normally done by evaporating a metal layer on top of the or underneath the resist. Aluminium or Chrome is are often good choices as they can often be easily be removed without effecting the resist, but you should check the chemical compatibility of your process with the removal procedure.