HFSS Etch Factor

Substrate manufacturing does not create exact geometry. The actual geometry of substrate manufacturing often results in different thicknesses in the dimensions of each geometry within a layer — and that difference is represented by an “etch” or “etch factor”. HFSS geometry allows for the presence of an etch factor in the design and modeling of different substrate layers. The specific etch factor that applies to a particular layer is usually available on the manufacturer. But in general, the etch factor is calculated by taking the ratio of the etch depth to the undercut, where the undercut is defined as being the part of the wafer that is etched away under the photoresist:

etch_factor = layer_thickness / (bottom_dimension - top_dimension) / 2

• When the bottom dimension of the stackup is larger than the top, the etch factor is positive.
• When the top dimension is larger than the bottom, the etch factor is negative.

For example, with a positive etch factor, a transmission line with a 2mm nominal width are 2mm wide on the bottom and narrower at the top. While the same transmission line with a negative etch factor are 2mm wide on the top and narrower at the bottom. An etch factor of 0 means no etch, and the top and bottom dimensions are the same. Also, very large etch factors — both positive and negative — result in little or no etch.

In HFSS, only signal layers can have an etch factor, dielectric and negative signal layers do not. HFSS supports an etch ratio that is referred to as the HFSS Etch Factor, and this etch is used when the mesh is created for an HFSS simulation. Note, however, the etch is not applied to geometry drawings in the 3D Layout view.

For more information see the Edit Layers window.