cmi etchers

C. Hibert, EPFL-CMI CMI-Comlab revue, june 4th, 2002

Dry etching in MEMS Dry etching in MEMS fabricationfabrication

by Cyrille Hibertby Cyrille Hibert

in charge of etching activities in charge of etching activities in CMI clean roomin CMI clean room

É C O L E P O L Y T E C H N I Q U EF É D É R A L E D E L A U S A N N E


CMI etchersCMI etchers

Alcatel 601E STS Multiplex ICP

Other etcher manufacturers for MEMS processing: Oxford, Unaxis, AKT (Applied Material).


ICP reactorsICP reactors

Basic of ICP reactors Plasma density and ions energy are decoupled


Complementarity of the two ICP etchers Complementarity of the two ICP etchers in CMIin CMI

Alcatel 601E STS Multiplex

chuck Mechanical clamping Electrostatic clamping

Chemistry and material to be etched

Fluorine: Si (anisotropic, isotropic), Si3N4,

CxFy: SiO2(thin film).

Cl: metal (Al, Ti, Pt) and others Si, Saphir, AlN,

O2: Polymer,

CxFy: SiO2 (deep).


Si etchingSi etching

1) Deep anisotropic etching:• Bosch process,• Room T continuous process,• Cryogenic process.

2) Thin film etching.

3) Isotropic etching. Interdigit structure etching on SOI wafer using A601E.


Basic on Bosch processBasic on Bosch process

SF6 plasma

C4F8 plasma

SF6 plasma

SiF4F+ions

thin fluoro-carbon polymer film (passivation)

Si

Si

Simasque

ions

Si etching using Bosch process - scalloping effect (on

A601E)


Bosch process on A601EBosch process on A601E

State of the art at CMI:

• Anisotropy at 90° (vertical sidewall),• Etching uniformity (2 % to 5 %),• Selectivity Si:SiO2 (1:200 to 400) et Si:RP (1:100 to 200),• Etching rate: 6 to 12 um/min (loading effect + ARDE),• Sidewall roughness (actual process developpment),• Notching (hardware modification + process developpment).

Under control

In developpment


Sidewall roughness at the top of a deep anisotropic etching of Si (Bosch process on A601E) as a function of pulse duration:

(a) SF6/C4F8 = 7s/2s (b) SF6/C4F8 = 3s/1s.

(a) (b)

Bosch process: sidewall Bosch process: sidewall roughnessroughness


Bosch process: notching effectBosch process: notching effect

Si

Si

SiO2

Si

Si

SiO2

notchingx min etching

x min + overetch timeEtching through a Si wafer and

stop on SiO2 (A601E)

SiO2

Si

380 um

notching


Room T continuous processRoom T continuous process SF6 + C4F8

plasmaSiF4

CxFy+F+ions

thin fluoro-carbon polymer film

20 °CSi

mask

RIB waveguide on SOI wafer etch in A601E (optosimox project)

• very good anisotropy, • low roughness,• low etch rate,• well suited for low depth (<5 um).


Cryogenic processCryogenic process

SF6 + O2 plasma

SiF4

O+F+ions

Ultra thin layer of SiO2

- 110 °C

Limitation of spontaneous chemical reaction and improvement of O sticking

Si

masque

15 m 10 m20 m25 m30 m5 m

70 m

87 m

100

m

102

m

105

m

96 m

4 inches in diameter, Si load 25 % , 40 min, 2/3 um/min

Etching of different trenches width in bulk Si (A601E).

• No polymer contamination (reactor, substrate),• Low sidewall roughness (20 nm P to P),• BUT sensible process and not so flexible than Bosch process!


Anisotropic etching of thin Si filmAnisotropic etching of thin Si film

20 nm SiO2

100 nm Poly-Si

Stop on 20 nm gate oxide

Cryogenic process is highly selective on SiO2

(A601E)

Chlorine chemistry is highly selective on SiO2

(STS Multiplex ICP)

PR

polySiSiO2

Si


Si isotropic etchingSi isotropic etching

Isotropic Si etching(A601E).

aSi

SiO2

Al membrane

• Undercut etch rate can reach 7 um/min (for 1 um aSi),• Selectivity Si:SiO2 > 1000,• lateral aspect ratio > 200.

Largely used process for metal membranes releasing,More efficient dry release compare to polymer sacrficial layer,

Carateristics:


Deep SiO2 etching (1)Deep SiO2 etching (1)

CxFy plasma

SiF4, SiF2

COx, COF2

C-F + F+ ions

Fluorcarbon polymer deposition on sidewall

20 °CSiO2

maskFluorocarbon interfaceon SiO2 surface.

Bulk fused silica etching (40 um depth) on STS Multiplex ICP

Key parameters: mask material, ions flux and energy (pressure, rf source power, DC bias), C/F ratio (chemistry).


Deep SiO2 etching (2)Deep SiO2 etching (2)

Discussion:

• Anisotropy (vertical sidewall),• Masque material (PR, aSi, Al, Cr, Ni…),• Selectivity SiO2:mask (C/F, pressure, DCbias),• Reactor contamination (hardware problem), • Etch rate (till 1 um/min),• Roughness and slope sidewall,• Increase the aspect ratio.

Under control

challenges


Deep polymer etchingDeep polymer etchingO2

plasma COx

O + ions

Passivation layerformed by the

redeposition of sputtered material

20 °C thick polymer layer mask

hold substrate

6 um polyimide etching on STS Multiplex ICP

- Mask (PR, SiO2, Al, Pt),- ER: 1 um/min.


Metal etchingMetal etching

AlSi etching using Cl2/BCl3 chemistry (on STS Multiplex ICP)

• selectivity Al:RP 2:1,• ER: 0.2 to 0.5 um/min.

Pt etching using Cl2/Ar chemistry (on STS Multiplex ICP)

• selectivity Pt:RP 1:8,• ER: 30 nm/min.


ConclusionConclusionCMI etching process evolution:• Maintaining existing processes (Si, SiO2, Si3N4, Polymer, Al, Pt, Ti, AlN, Saphir),• Deep Si etching : sidewall roughness.

Equipements evolution:• A601E Upgrade for notching control (Si etching),• Etcher dedicated to silice:

- At the present time done on the 2 ICP not dedicated for this,- Increased ask for deep silica etching (microchannel, waveguide, holes),- Exclusive equipement (new internal/external pojects).

cmi etchers

Documents

decoupledcmicomlab revue

a601ecmicomlab revue

si anisotropic

dry etching

film etching

abbosch process

sensible process

si wafer