OPTIMISATION DES PLANS DEXPRIENCE

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(No Transcript)
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SUMMARY

• Ion bombardment control,
• Thermal management,
• No Defects (pits or pillars) formation,
• Profile control
• Plasma repeatability
• Plasma uniformity
• Loading effect
• Optimum process conditions

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Ion Bombardment Control
Physical sputtering is due to cinetic energy of
positive ions which impinge the etched areas.
This leads to positive effects as ?? Anisotropic
profiles, But this also may lead to significant
damages as ?? Rough surface morphology (pits
formation) ?? Trenching ?? Bad selectivity
against mask and underlayers ?? Non
stoechiometric surfaces (impact on device
performance).
High density Plasma enhances chemical reaction
and decreases RF bias. This leads to better
etching efficiency while reducing the physical
part of the etch mechanism. In Corial systems, a
quartz plate (a few mm thick) covers the cathode.
This insulating cover also decreases the ion
energy and reduces excessive damages to the GaN
surface.
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Thermal Management
Temperature control of substrate to etch leads to
better process control. This leads to positive
effects as ?? Better etch selectivity, ?? Stable
etch rates as surface reaction kinetic is well
controlled. Negative effects are ?? High
temperature of substrate during etching leads to
excessive loss of N2. That leaves strong n-types
conductive regions which affect multi layer
epitaxy (Review of Dry Etching of GaN and Related
Materials). In Corial systems, the temperature
control of substrates during etching is performed
using helium backside cooling and mechanical
clamping.
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Profile Control
Device manufacturing may request vertical or
tapered profiles according to applications and
technologies. Process must enable low mask
erosion to achieve vertical profiles. Hard masks
as Si3N4, SiO2 and Ni are good candidates. Negati
ve slopes can be obtained with a specific cathode
design and particular process conditions. Control
led PR Mask erosion enables tapered to almost
vertical profiles (55 to 85). This process
feature depends upon size of the substrate.
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Plasma Repeatability
Plasma ignition has dramatic influence on final
GaN etching results. It is extremely important to
ensure high reproducibility of plasma
ignition. In Corial systems, the editable
multi-step processes warrants the repeatability
of plasma ignition and, thereby, stable etch
performance.
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Plasma Uniformity
In many cases, as the wafer size is 50 mm
diameter, batch loading is requested to achieve
high throughput. Consequently, good uniformity
accross the wafer and wafer to wafer is
required. In Corial ICP source, the large size of
the source (Ø270 mm) and a carefull design of the
coil terminations enable excellent GaN etch
uniformity.
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Loading Effect
In batch loading, the loading effect has to be
considered. The etch rate decreases with the
wafer size and the number of wafers.
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RECAP of GaN PROCESS

• Adequate adjustment of RF biasing versus ICP
  power to get smooth etched surfaces,
• Control of wafer temperature by helium assisted
  heat exchange to maintain resist and device
  integrity,
• Control of the etching profile by the flow rate
  of polymerizing gas (CH4) in the process recipe,
• Repeatability of the etching results by a well
  controlled plasma ignition using a multi-step
  approach,
• Uniform etching of a batch of seven 2 wafers
  thanks to the large size of the ICP source (Ø270
  mm).