Mask thickness for selective implantation of dopants

eng         implantation energy in keV
pdos       percentage of implant dose penetrating through the mask
dop         type of the dopant, select Boron or Phosphorus


Silicon dioxide and photoresist are often used in MEMS processing to mask areas that do not need ions to be implanted. This mask layer is later removed before further processing is done. Sometimes ions are implanted through an oxide layer. Based on the energy of the implant and thickness of the oxide layer some portion of the dose may reach the silicon substrate. This design interface can be used to determine an approximate thickness of the mask to allow or deny a specific percentage of the dose to pass through the mask.

Silicon dioxide has similar stopping power like silicon to ion implantation of boron and Phosphorus. Phosphorus atom being bigger in size compared to boron will penetrate less deeper. Hence a thinner mask is required for masking Phosphorus. Photoresist is less effective in stopping ions and hence would require a thicker layer than oxide to mask ions with the same energy. A photoresist layer 1.9 times the oxide layer has been found to be effective to mask ions of the same energy in most cases. This could vary for different photoresist.

The plot shows the mask thickness of either oxide or photoresist required to mask a given percentage of implanted dose at the given energy of ion implant. At 50% dose penetration, the mask thickness would be approximately equal to the range of implant.


-Silicon dioxide is assumed to have similar stopping power like silicon.
-The photoresist layer 1.9 times the oxide layer is assumed to be as effective..
-The interaction with the background concentration of the substrate is not considered.
-The influence of the shape of the mask walls or the angle of ion implant are not considered.
-The true stopping power of either oxide or photoresist will depend on the individual properties of the mask.
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