Specific Process Knowledge/Etch/DRIE-Pegasus/Parameters

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A short description of the hardware on the Pegasus and the other dry etchers

Below are short descriptions of some process parameters on the dry etchers. By default, the descriptions apply to the Pegasus but the other dry etchers are very similar.

Hardware

The inside of the process chamber on the Pegasus may be slightly modified.

1. Spacers : The distance to the plasma source may be changed by using different spacers.
2. 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.

Acronym	Funnel	Baffle	Spacer
LF+B100	Long	with baffle	100 mm
LF-B100	Long	no baffle	100 mm
SF-B5	Short	no baffle	5 mm

Platen power

There is a separate low frequency (LF) RF generator that makes it possible to avoid notching at buried insulator layers. The notation

Pressure

In some cases the pressure in a recipe may be so low (the density of gas molecules is too low to sustain a plasma) or high (the density of gas molecules is so high that dissociating them into reactive species induced by the RF field produces a pressure increase that is out of the baratron operating range) that it becomes impossible to ignite the plasma. If so, one can include a plasma strike by briefly adjusting to a pressure where igniting the plasma is possible and then once it is striken, adjusting the pressure to the right setting. Hence, the notation 10 (3@15) mTorr means that the process pressure is 10 mTorr with a 3 second strike at 15 mTorr.

Ramping (>)

Most process parameters on the DRIE-Pegasus may be ramped during a process step. The notation 140>90 in a pressure setting indicates that from the initial value of 140 mTorr the pressure is decreased linearly to an end value of 90 mTorr in the process step. The ramp rate (mTorr/min) therefore depends on the duration of the step.

Forward power versus Load power

In order for the RF power supplied by the generators to be absorped by the plasma, the impedance of the plasma must match the impedance of the RF network. The impedance of the plasma relies on the gas composition, the pressure and the power and is therefore beyond direct control of the instrument. Between the RF generators and the coil, two capacitors called Load and Tune form a matching network that enables us to change the impedance of the RF network. The tuning of Load and Tune is usually done automatically by the software. If the two impedances are not similar, the RF power will be absorped by the RF network (RF generator and capacitors) instead of the plasma - this power is labelled the Reflected power and it must be minimized. The generators has two modes:

• Forward power The generator delivers the power according to the recipe. If 10 % of the power is reflected, the real value of the power supplied to the plasma will be 10 % less.
• Load power The RF power lost as reflected power is compensated for automatically. If 10 % of the forward power is lost, the input is automatically increased.