Specific Process Knowledge/Etch/DRIE-Pegasus/System-description: Difference between revisions
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== Description of the Bosch process at the DRIE-Pegasus == | == Description of the Bosch process at the DRIE-Pegasus == | ||
-- | <!-- revised 1/4-2020 by jmli --> | ||
<!-- revised | <!-- revised 28/4-2023 by jmli --> | ||
{{Author-jmli1}} | |||
This section applies to all Pegasus tools. For Pegasus 4, however, it is less important as dielectrics etches are continuous. | |||
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: | ||
*In the passivation cycle, a C<sub>4</sub>F<sub>8</sub> plasma is formed using the RF coil power only and a teflon-like coating is created on all surfaces thus protecting the sidewalls in the subsequent etch cycle. | *In the passivation cycle, a C<sub>4</sub>F<sub>8</sub> plasma is formed using the RF coil power only and a teflon-like coating is created on all surfaces thus protecting the sidewalls in the subsequent etch cycle. | ||
*In the etch cycle | *In the etch cycle | ||
**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) | **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. | **''then'' as the bottom of the structures are opened the etch of silicon itself starts. | ||
[[Image:boostdelay4b.jpg |400x400px|thumb|The etch cycle may be split into three parts, Boost, Delay and Main, where process parameters such as pressure, gas flows or RF powers have different values. The red curve shows the possible characteristics of the platen power.]] | [[Image:boostdelay4b.jpg |400x400px|thumb|The etch cycle may be split into three parts, Boost, Delay and Main, where process parameters such as pressure, gas flows or RF powers have different values. The red curve shows the possible characteristics of the platen power. {{image1}}]] | ||
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 | 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. | #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. | ||
#a higher pressure during the etch increases the density of reactive species and hence the etch rate. Since a high platen power is no longer necessary to drive the ion bombardement, lowering it will reduce the impact on the masking material thus improving the selectivity. | #a higher pressure during the etch increases the density of reactive species and hence the etch rate. Since a high platen power is no longer necessary to drive the ion bombardement, lowering it will reduce the impact on the masking material thus improving the selectivity. | ||
These conflicting demands are the same on the ASE. However, with hardware improvements on the DRIE-Pegasus such as | These conflicting demands are the same on the ASE. However, with hardware improvements on the DRIE-Pegasus tools such as | ||
*fast response digtal MFC's mounted on top of the process chamber itself to shorten the gas line | *fast response digtal MFC's mounted on top of the process chamber itself to shorten the gas line | ||
*fast APC valve | *fast APC valve | ||
*fast RF power supplies | *fast RF power supplies | ||
the etch and deposition cycles may be split into three separate phases, | the etch and deposition cycles may be split into three separate phases, labelled Delay, Boost and Main. Following the arguments from above, the third phase (Delay) may be thought of as a short delay that ensures a very low pressure (and thus extremely good ion directionality) before the ion bombardment. The standard etches on the Pegasus only make use of up to two phases. | ||
== Processing options on the Pegasus == | == Processing options on the Pegasus == | ||
This section applies to all Pegasus tools. | |||
The Pegasus has a lot of advanced processing options. | The Pegasus has a lot of advanced processing options. | ||
# '''Multiplexing''': As described above, the multiplexed Bosch process may have the etch or passivation cycle each split into three separate phases: Delay, Boost and Main. | # '''Multiplexing''': As described above, the multiplexed Bosch process may have the etch or passivation cycle each split into three separate phases: Delay, Boost and Main. The duration of each individual phase (delay, boost and main) for each individual process parameter (pressure, gases and RF generators) may be given any value thus enabling the phases to run asynchronously. This gives an infinite process parameter space... | ||
# '''Ramping''':Using the ramp option one can change process parameters linearly over the course of each processing step. | # '''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. | # '''Process steps''': Stich any number of processing steps with different parameters to make one continuous process. | ||
# '''Hardware''': The Pegasus has several hardware settings | # '''Hardware''': The Pegasus has several hardware settings that can be changed - you will, however, need a very strong argument to come through with a request for such a changed as it will also change processing conditions for all other processes. | ||
## ''Spacers '': The distance to the plasma source may be changed by using different spacers. | ## ''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. | ## ''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== | ||
This section applies to all Pegasus tools. | |||
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. | 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. | ||
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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. | 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. | ||
Click [[Specific Process Knowledge/Etch/DRIE-Pegasus/showerheadchange|HERE]] to see a comparison of some the etches before and after the change of the showerhead. | Click [[Specific Process Knowledge/Etch/DRIE-Pegasus/showerheadchange|'''HERE''']] to see a comparison of some the etches before and after the change of the showerhead. | ||
== | == RF Matching == | ||
=== | === RF matching in general === | ||
(This section contains material from the Dry Etch TPT lecture) | (This section contains material from the Dry Etch TPT lecture) | ||
Connecting a RF generator to an AC circuit in order to drive a plasma in a dry etch tool is not as trivial as one might expect. One cannot just connect an RF generator to a coil, put a certain power through and expect that a plasma generated will automatically absorb all energy. It is not possible: At RF frequencies cables become inductors so circuits must be carefully constructed. | Connecting a RF generator to an AC circuit in order to drive a plasma in a dry etch tool is not as trivial as one might expect. One cannot just connect an RF generator to a coil, put a certain power through and expect that a plasma generated will automatically absorb all energy. It is not possible: At RF frequencies cables become inductors so circuits must be carefully constructed. | ||
[[File:RF matching 1.jpg|500px]] | [[File:RF matching 1.jpg|500px|thumb]] | ||
Most RF power generators are designed to transfer power into a 50 ohm load. This means that the circuit, in addition to the coil that has a certain inductance and resistance will also require additional electrical components if the total 50 ohm impedance criteria is to be met (unless the inductance and resistance of the coil are carefully chosen and constant). If the impedance is off, part of the power will be dissipated elsewhere in the circuit as reflected power - for instance as localized heating of RF cables due to standing waves. One must therefore have a way of controlling the circuit impedance. | Most RF power generators are designed to transfer power into a 50 ohm load. This means that the circuit, in addition to the coil that has a certain inductance and resistance will also require additional electrical components if the total 50 ohm impedance criteria is to be met (unless the inductance and resistance of the coil are carefully chosen and constant). If the impedance is off, part of the power will be dissipated elsewhere in the circuit as reflected power - for instance as localized heating of RF cables due to standing waves. One must therefore have a way of controlling the circuit impedance. | ||
| Line 70: | Line 72: | ||
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: | 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: | ||
* 100 sccm | * 100 sccm O<sub>2</sub> | ||
* 1200 W coil power | * 1200 W coil power | ||
* 20 W platen power | * 20 W platen power | ||
| Line 82: | Line 84: | ||
'''What happens in a Bosch process where the plasma conditions change every few seconds?''' | '''What happens in a Bosch process where the plasma conditions change every few seconds?''' | ||
The situation during a Bosch process as described in the top of this page is very different from the continuous process with respect to RF matching. | |||
The situation during a Bosch process as described in the top of this page is very different from the continuous process with respect to RF matching. Here, | |||
* the pressure | |||
* the gas flow | |||
* the gas composition | |||
* 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. | |||
== The Picoscope == | |||
The process monitoring on Pegasus 1, 2 and 3 has been dramatically improved with the installation of the Picoscope. Click [[Specific Process Knowledge/Etch/DRIE-Pegasus/picoscope|'''HERE''']] to access a page on the Picoscope only. | |||
== The Claritas endpoint system == | |||
This section applies to Pegasus 1. Too bad that it is empty so far... | |||