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3.1.2 Masks

The next step is applying patterned layers onto the wafer. As already indicated in the introduction above, this can be done with geometric primitives. For squares and circles the side length or the diameter has to be specified, for rectangles and ellipses both length and width or a pair of diameters are needed. Moreover the primitive can be positioned by shifting the two lateral coordinates of its center and the thickness of the mask has to be specified. As an example, Fig. 3.2 shows a circular pattern shifted away from the center of the simulation domain.

Figure 3.2: Circular pattern at an arbitrary position on the wafer.

By repeated calls of the masking operation, the primitives can be combined to form more complex structures. This is very helpful for the generation of simple masks. If very complex patterns are to be generated, combining the primitives gets confusing. How layout and lithography tools can then be used as alternative will be introduced later in Chapter 4. For the moment, however, let's concentrate on the basics of the solid modeling tool.

Fig. 3.3 shows how four circles -- three thereof are depicted on the left hand side of the figure -- plus a rectangle form a smoothened H-like pattern. In the following the pattern will be inverted. Therefor the structure will be filled with a different material up to a certain level and the first material will be stripped.

Figure 3.3: Combined geometric primitives forming simple masks.

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