A good understanding of the etch process starts with understanding the initial mask profile, whether it is photoresist or hardmask. Important parameters of the mask are thickness and sidewall angle. If possible, perform an SEM inspection of the cross-section to determine the sidewall angles for different feature sizes for your etch step. For large features and non-critical etchs, it may only be necessary to measure the mask thickness.
Check the resist mask for descumming. In general, descumming will make the etch step more consistent from run to run by removing the organic residue left over from the development step. In some cases, this residue remains regardless of exposure and development times, and a descum step is necessary for the etch job. Examples of AZ resists on silicon dioxide are as follows:
To measure mask thickness, you can use a cross-sectional SEM and complementary software. Also, for transparent thin films, such as photoresists and dielectrics, you can use reflectometer or ellipsometer tools. The initial thickness of the mask is important because the result of the metal etching process is called selectivity, defined as the rate at which the material etches the mask. If you choose a process with poor selectivity, you won’t be able to etch very deep into your material, or you may need a thicker mask. On the other hand, for some processes, such as deep silicon etch, the selectivity can be very high, exceeding 100 to 1, with a properly chosen mask.
Etching tools are highly specialized, each with its own limitations for gases, materials, and sample size. It is important to select the proper sample for your process and ensure your device composite material limits. Using the wrong material during etching can ruin your own process and contaminate future tools. Restoring the chamber to pristine condition can take the entire etch chamber down for a full day to clean the contamination. Use the table below as a guide for tool selection. It’s never a bad idea to check with staff to make sure your samples and processes are compatible.
Most etch tools readily accept 4″ wafers and can be reconfigured to 2″ or 6″. However, irregular shapes, such as small pieces and square substrates, require carriers to hold them inside the chamber. The choice of carrier is as important as the choice of process chemistry. During the etch process, gases can interact with the support, producing by-products that can affect the sample. In addition, thermal conductivity has a great influence on the selectivity and etch rate of the mask. Although some tools provide alternate inert carriers, it is always a good idea to equip your process with your own carriers to prevent cross-contamination.