This value represents the degree of side etching. The degree of side etching is expressed as D/R, where D is the amount of etching in the depth direction, and R is the amount of etching in the horizontal direction (side etching) per side. This value is usually between 1.5 and 3.5. The larger the value, the higher the accuracy of the etching. The etching factor varies depending on the material, the type of etching solution, conditions, etc.