STS Pegasus 4/Processes/LNF Recipe 1

LNF Recipe 1 is the base silicon etch for the pegasus tools. It has a high selectivity, moderately fast etch rate and a clean profile.

About this Process
Recipe 1 for 20 min, 2 µm Trench
Process Details
Equipment	STS Pegasus 4
Technology	DRIE
Material	Si
Mask Materials	PR, SiO2
Gases Used	SF6, C4F8, O2
Date Created	Jan 1, 2009
Authored By	SPTS, Brian Vanderelzen, Kevin Owen

Contents

Procedure

LNF recipe 1 has 2 main process (DRIE) steps. For recipes shorter than 20 minutes, set the first DRIE step to the desired length. For recipes longer than 20 minutes, set the first DRIE step to 20:00 and the second DRIE step to the remaining time.

Run LNF Recipe 1 for up to 20 minutes with the 1 minute automated clean recipe, longer etches should be run with the 3 minute automated clean recipe.

Parameters

Parameter	Dep	Etch
		Boost	Main
ICP Power	2000 W	2800 W
Bias Power	0 W	60 W
Bias Pulsing	N/A	40 Hz - 80%
Pressure	24 mTorr	30 mTorr
C4F8 Flow	250 sccm	30-0 sccm (ramped)
SF6 Flow	0 sccm	390 sccm
O2 Flow	0 sccm	39 sccm
Time	2.0 s	2.6 s
Chuck Temperature	20°C

Capabilities

This process, as is true with all Bosch processes, varies depending on feature size and density, due to RIE lag. The below table contains typical results for 2 µm, 10 µm, and 100 µm trenches and the following sections expand on these results. It is strongly recommended that the process is tested with any new mask.

Parameter	2 µm Trench	10 µm Trench	100 µm Trench
Etch Rate	3 µm/min	4.5 µm/min	5.6 µm/min
Undercut	160 nm	200 nm	200 nm
Scallop height	370 nm	390 nm	370 nm
Scallop depth	70 nm	50 nm
Sidewall angle	89.6°	89.7°

Etch Rate

The etch rate will vary with the size of the trench being etched. Larger trenches tend to etch faster, this trend can be seen below.

Mask Selectivity

• Photoresist Etch Rate
  • SPR 220: 500 Å/min
• SiO₂ Etch Rate
  • Thermal oxide/HTO: 166 Å/min
  • LTO: 178 Å/min
  • PECVD: unknown
• Silicon nitride Etch Rate
  • LPCVD Nitride: 356 Å/min
  • LPCVD Low Stress Nitride: 291 Å/min

Sidewall Profile

Mask undercut, scalloping, and sidewall angle are shown below.

Trench

Running recipe 1 for 20 minutes with 3 µm of SPR 220 on a silicon wafer with our standard DRIE characterization mask can produce a profile like this.

• 

  Recipe 1 for 20 min, 2 µm Trench

• 

  Recipe 1 for 20 min, 10 µm Trench

• 

  Recipe 1 for 20 min, 100 µm Trench

Limitations

LNF Recipe 1 is known to work under a range of conditions. Outside of these conditions, etch performance is not guaranteed. Etch rates will vary depending on feature size, etch length, open area, etc. We strongly recommend testing the process prior to running device wafers.

One common problem seen when running outside of these conditions is grassing. This occurs when the C4F8/SF6 ratio is unbalanced causing micromasking. This creates tiny pillars of un-etched Si and teflon on the surface of the feature. An example of grassing can be seen below.

Grass (brown area) on perimeter of wafer

Etch Length

• Generally 30 sec - 20 minutes is known to work well.
• Recipe 1 is known to work well up to 20 min of etching. The process is tested for 20 min monthly to verify performance. Longer etches, particularly >30 min will likely cause undesirable results.
• Etches longer than 20 minutes should use the second DRIE step for any time past 20 minutes. This removes the C₄F₈ from the etch step to help reduce grassing.

Feature Size

• Tested on features from 1 μm - 100 μm.
• The standard characterization pattern used to qualify Recipe 1 has open lines, trenches, and grating patterns from 1 μm - 100 μm. Larger and smaller features have not been tested and may not provide the expected results.

Aspect ratio

• Aspect ratio limitations vary depending on feature size. For instance, 2 μm features have an aspect ratio limit of around 50:1. For larger features, aspect ratio is typically limited to etch length (see above).

Open area

• Very large features and masks with total open area >20% are known to degrade etch performance and should be tested prior to running device wafers. For masks with more than 20% open area Recipe 3 may provide better results.

Multi-step etching

• Multi-step processing (i.e. running two or more etches with overlapping features) with the Bosch process is known to cause undesirable results. Please contact the tool engineers if you wish to run such a process so we can help reduce or eliminate issues.

Qualification

Every third Friday Recipe 1 is run for 20 minutes on a Si wafer with our standard DRIE characterization mask, with 3 µm of SPR 220. This is performed after the weekly clean, and a 100 µm trench is measured in 5 locations with the Zygo NewView 5000. This etch rate is reported in the chart below.