4.3.2 Examples

The following figures depict several etch steps as they occur during a topography process simulation.

To incorporate an electronic circuit (e. g.  a CPU) on a die, adjoining devices must be electrically isolated from each other. One isolation technique is to use the so-called local oxidation of silicon (LOCOS), that results in the well known "birds peak" structure. A more recent technique to achieve the isolation is the so-called shallow trench isolation (STI). This process is in principle performed in two major steps, where the first step is a highly selective and anisotropic etch step. The second step fills the trenches with oxide. Fig. 4.15 and Fig. 4.16 depict the simulation of an etching step as it is used in the STI formation. The right part of Fig. 4.17 illustrates an example for isotropic deposition as it is used to fill an STI trench with oxide. The left picture of Fig. 4.17 depicts an anisotropic deposition of oxide.

Figure 4.15: Selective anisotropic etching process. The left picture portrays the input Wafer structure as it is present to the etch simulator. The colors of the regions indicate the material type. The blue colored material is a resist material and acts as a mask. The thin gray colored layer depicts an oxide layer, the brown material is nitride. The green material at the bottom is silicon. The right picture depicts the result of the highly selective anisotropic etching process which stopped at the oxide layer.
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Figure 4.16: The left picture shows the "Detail 1" of Fig. 4.15. The under etching of nitride and oxide which takes place due to the imperfect anisotropy of the etching process is visible. The different etching rates that apply to silicon, oxide, and nitride are apparent. In the right picture " Detail 2" of Fig. 4.15 is shown. The polygonal silicon surface is clearly visible.
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Figure 4.17: Fill of STI trenches. On the left side the result of an anisotropic deposition simulation is shown. The right side depicts the isotropic deposition as used to fill the isolation trenches with oxide.