Specific Process Knowledge/Etch/Etching of Silicon/Si etch using ASE

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The Bosch process: Etching of silicon

The Bosch process uses alternation between an etch cycle and a passivation cycle. Introducing a passivation step in an etch process is very beneficial for the control of the angle of the sidewalls in the etch process because it allows us to cover them with a protective layer that suppresses the isotropic etching. Combined with the high plasma density in the ICP chamber, the excellent sidewall control enables us to etch high aspect ratio structures in silicon with very high etch rates.

In the case of the silicon etching on the ASE, an etch phase with SF₆ and O₂ alternates with a passivation phase with C₄F₈.

Quality control procedure on the ASE

Two recipes have been optimized for the ASE. Their specification is on a 10 % etch load wafer with trenches.

• Shallolr: The shallow etch process will etch a 2 µm opening down to make a 20 µm trench.
• Deepetch: The deep etch process will etch a 50 µm opening down to make a 300 µm trench.

The standardization procedure on the ASE covers these two etches.

Quality Control (QC) for ASE

The QC procedure for ASE The newest QC data for ASE QC Recipe: Shallolr (2 µm trench) Common parameters Multiplexed parameters Parameter Setting Parameter Etch Passivation Temperature 10oC SF6 Flow 260 sccm 0 sccm No. of cycles 31 O2 Flow 26 sccm 0 sccm Process time 5:56 mins C4F8 Flow 0 sccm 120 sccm APC mode manual RF coil 2800 W 1000 W APC setting 86.8 % RF Platen 16 W 0 W Cycle time 6.5 s 5 s QC limits ASE Etch rate in Si 3 - 4 µm/min Non-uniformity 5 % QC Recipe: Deepetch (50 µm trench) Common parameters Multiplexed parameters Parameter Setting Parameter Etch Passivation Temperature 20oC SF6 Flow 230 sccm 0 sccm No. of cycles 250 O2 Flow 23 sccm 0 sccm Process time 54:10 mins C4F8 Flow 0 sccm 120 sccm APC mode manual RF coil 2800 W 1000 W APC setting 87.7 % RF Platen 19 W 0 W Cycle time 8 s 5 s QC limits ASE Etch rate in Si 4 - 6 µm/min Non-uniformity 5 %

Recipes on the ASE

Shallolr

The shallolr recipe is designed to etch features (with sizes above 1 µm) in silicon down to a depth that ranges from a few microns to some 50 microns. (If you need to etch deeper use Deepetch or more shallow, see Nanoetches.) It is specified to etch a 2 µm wide trench down to a depth of 20 µm on a wafer that has a global/local etch opening density of 10%.

The recipe is given above.

The process runs for 31 cycles (5:56 mins). The fact that it's Bosch process is clear from the scallops on the sidewalls - one should be able to count 31 of them.

• Standardization images of the shallolr recipe
• 

  The profile of a 2 µm trench

• 

  The profile of a 50 µm trench

The process is designed to reach 20 µm down in a 2 µm trench but as is clear from the image of the corresponding 50 µm trench, this one is etched deeper. The reason is the so called Aspect Ratio Dependent Etching or ARDE: See below.

Deepetch

The deepetch recipe is designed to etch features (with sizes 2 µm) in silicon down to a depth that ranges from some 50 microns to hundreds of microns. (If you need to etch less, use shallow or Nanoetches.) It is specified to etch a 50 µm wide trench down to a depth of 300 µm on a wafer that has a global/local etch density of 10%.

• Standardization images of the deepetch recipe
• 

  The profile of a 2 µm trench

• 

  The profile of a 50 µm trench

As is clear from the two images ARDE also plays a role in this case: The 2 µm trench (widened to about 5-6 µm because of undercut/underetching) is only etched 150 µm.

Iso

The recipe Iso is the same recipe as Deepetch but without the passivation steps.
It has been tested once by Filip Sandborg-Olsen @nanotech.
He etched with 100% load for 10min. He got an etch rate of 5.51µm/min

Process development

Etch of nano sized structures

• See pxnano2 and comparison with nanotech on the Pegasus: Specific Process Knowledge/Etch/DRIE-Pegasus/nanoetch/nano142-pxnano2

Older work: Three different examples of etch are shown here. The masking material was zep520A (80 nm). By BGHE@danchip

Vertical sidewalls	Low ARDE	Positive tappered side walls
Etch cycle SF6 flow [sccm]:50 C4F8 flow [sccm]:100 Pressure [mTorr] 20 Coil power [W]: 350 Platen power [W]: 30 Cycle Time [s]: 5 Dep. Cycle C4F8 flow [sccm]: 100 Pressure [mTorr]: 20 Coil power [W]: 500 Platen power [W]: 0 Cycle Time [s]: 3 Temperature [Deg. C] 20 Results: Etch rate of 100nm lines: 146nm/min @etch time: 1:36min.	Etch cycle SF6 flow [sccm]:50 C4F8 flow [sccm]:100 Pressure [mTorr] 10 Coil power [W]: 350 Platen power [W]: 30 Cycle Time [s]: 5 Dep. Cycle C4F8 flow [sccm]: 100 Pressure [mTorr]: 10 Coil power [W]: 500 Platen power [W]: 0 Cycle Time [s]: 3 Temperature [Deg. C] -10 Results: Etch rate of 100nm lines: 131nm/min @etch time: 1:36min.	Etch cycle SF6 flow [sccm]:50 C4F8 flow [sccm]:100 Pressure [mTorr] 10 Coil power [W]: 350 Platen power [W]: 10 Cycle Time [s]: 5 Dep. Cycle C4F8 flow [sccm]: 100 Pressure [mTorr]: 10 Coil power [W]: 500 Platen power [W]: 0 Cycle Time [s]: 3 Temperature [Deg. C] 20 Results: Etch rate of 100nm lines: 209nm/min @etch time: 2:56min.

Etching Si without back side cooling

Etching in an ICP as the ASE without backside cooling normally results in heating up the sample to more than 100 degrees Celsius. This can be problematic especially when using resist as a masking material.

There can be many reasons for not applying cooling to the sample. Among these are samples that need to be processed on a carrier due to sensitive backsides, sticky backsides or structures on the backside, small samples, membranes that can break, bowing wafers or something else.

To meet these needs we have developed a silicon etch in the RIE mode where the coil power is not being used. The coil creates a high density plasma and that has a higher plasma temperature than a low density plasma created by the platen power in RIE mode.

This recipe is intended to only for low etch depths that are not very critical with regards to eg. the sidewall profile. For deep etching of silicon you will need to use the bosch process and the coil power.

Wafer ID	Resist before etch	Platen power	Pressure	Flow rate SF6	Flow rate O2	Flow rate Ar	T	Process time	Comment	Results	CD change	Profile angles	Etch rate in Si	Etch rate in resist	Selectivity (resist:Si)	Etch rate in SiO2
ASE RIE9	3750nm	100W	80mTorr	40sccm	27sccm	0	20	30min	HBC off, resist not burned		.	.	443.3 nm/min	91.7 nm/min	1:4.8	?
ASE RIE8	3750nm	100W	80mTorr	40sccm	27sccm	0	20	4min	HBC off		.	.	440 nm/min	nm/min		?	}