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You are here: Home / Equipment / Dry Etching / Lam Research 9400 TCP Poly Etcher, lampoly (clean) / Isotropic silicon etching

Isotropic silicon etching

Lampoly is frequently used for shallow, anisotropic etching of poly and single-crystal silicon. Here, we present a recipe for isotropic etching of these materials based on SF6/Cl2 chemistry.

Background

This recipe was originally developed to etch sharp tips in single-crystal silicon for scanning probes. However, the ability to etch silicon isotropically in lampoly, a tool known for high uniformity and repeatability, likely has broad applications in the lab member community.

 

Recipe

The isotropic etch recipe is located on the hard drive of lampoly under recipe 80. A backup copy is stored under recipe 180. The details of the recipe are presented in the table below. You may change the time in step 3, but do not save over the recipe.

Step
1 2 3 4 5


stabilization,
gas adjust
clamp wafer
main etch
unclamp wafer
end
 Pressure  mT 13 13
13
13

 RF Top
 W

450

 RF Bottom
 W

50

Gap
cm
5.9
5.9 5.9 5.9 5.9
Cl2
sccm
10
10
10


SF6
sccm
75
75
75


HBr
sccm




O2 (20%)
sccm




CF4
sccm




He
sccm


50

C2F6
sccm




O2
sccm




N2
sccm




He clamp
T

6
6


Completion

stab
stab
time
time
end
Time
s
20
20
50
10

 

Characterization


Mask Design

The mask used to characterize the recipe consists of an array of circles and squares of increasing size (Fig. 1). This pattern was transferred into a silicon dioxide hard mask ~4800 A thick. When subjected to the isotropic etch process, silicon under the oxide features is undercut and is eventually reduced to a sharp point.

 

Lampoly iso mask

Fig. 1. Mask pattern used to characterize isotropic etch recipe. Digitized data is clear. Feature dimensions range from 4 um to 12 um in steps of 2 um.

 

Process Performance

All etches were terminated after 50 s to prevent the oxide caps from falling off. Sidewall profiles and etch depths were measured on sample cross-sections with sem4160. Oxide thickness before and after the etch was measured with Nanospec.

 

Etch time
s
50
Lateral etch
um
1.48
Vertical etch
um
2.19
Lateral:Vertical

0.68:1
Vertical etch rate
um/min
2.63
Si:SiO2 selectivity

11.1:1

 

SEM images

 

Lampoly iso both

Fig. 2. Silicon tip precursors formed under a square mask (foreground) and circular mask (background). Square edge length and circle diameter are 4 um.

 

Lampoly iso circle

Fig. 3. High magnification image of precursor under circular mask.

 

Lampoly iso square

Fig. 4. High magnification image of precursor under square mask.

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