Specific Process Knowledge/Etch/DRIE-Pegasus/System-description: Difference between revisions

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== Description of the Bosch process at the DRIE-Pegasus ==

The DRIE-Pegasus tools take the well established Bosch process known from the [[Specific_Process_Knowledge/Etch/ASE_(Advanced_Silicon_Etch)|ASE]] a significant step further. In the ASE the Bosch process has two cycles, etch and passivation. During each cycle the process parameters are kept constant:  

**the ion bombardment driven by the platen power ''first'' removes the passivation layer on the surfaces directly exposed to the ions (i.e. horizontal surfaces)  

**''then'' as the bottom of the structures are opened the etch of silicon itself starts.  

Here, it is clear that one can distinguish two phases of the etch cycle; one where the ion bombardment removes the polymer and one where the actual etching of silicon takes place. Considering what process conditions are favorable we realize that

#the ion bombardment requires a low pressure in order for the ions to have a long mean free path and hence good directionality. Also, a high platen power is required to drive the ion bombardment.

== Processing options on the Pegasus ==

The Pegasus has a lot of advanced processing options.  

# '''Ramping''':Using the ramp option one can change process parameters linearly over the course of each processing step.

# '''Process steps''': Stich any number of processing steps with different parameters to make one continuous process.

## ''Spacers '': The distance to the plasma source may be changed by using different spacers.

## ''Baffle and funnel'': The funnel inside the chamber helps to focus the plasma/ions towards the electrode. They may be taken out but don't expect this option to be part of the parameters that you can change in your experiments.

== Modification of the showerhead==

It is clear that optimizing the Bosch processes on the Pegasus can be done in countless ways. Reducing the cycle time is a very convenient way of getting rid of, or at least reducing, scallops. The faster the process switches the smaller the scallops will be. However, at some point the hardware (MFS's, RF generators, APC valve) cannot keep up with the demand and the process will essentially be continuous causing the Bosch process to break down. In its original setup, the Pegasus had standard processes with cycle durations down to 2 seconds. The hardware on the Pegasus is indeed very fast and allows faster switching. The limiting factor in the Bosch process was identified by Henri Jansen - and we have taken steps to improve this as described below.

As stated, we believe that only switched processes will be affected by this change. Continuous processes such as [[Specific Process Knowledge/Etch/DRIE-Pegasus/processC|Process C]], [[Specific Process Knowledge/Etch/DRIE-Pegasus/nanoetch/nano142|Nano1.42]], isotropic etches, barc etches or the black silicon recipes are not believed to be noticeably affected.

=== RF matching in general ===

In the continuous processes (that means fixed parameters that do not change during the process) that are used in most dry etch tools the action of the impedance matching network is mostly seen in the beginning of the process when the plasma stabilizes. Once stabilized and a minimum reflected power has been established, the capacitors usually needs only minor adjustments. Below is a datalog of an oxygen cleaning process running with:

* 1200 W coil power

* 20 W platen power

* the RF powers

are changed every few seconds - hence requiring the capacitors to adjust to an entirely new situation. In other words, if not carefully tuned, the Bosch process will spend most of the time stabilizing the plasma.