Reactive Ion Etching (RIE) - A-Tech Systematechsystem.co.kr/sub1/pdf/106.pdfReactive Ion Etching...
Transcript of Reactive Ion Etching (RIE) - A-Tech Systematechsystem.co.kr/sub1/pdf/106.pdfReactive Ion Etching...
Professor Nathan Cheung, U.C. Berkeley EE143 Lecture # 16
Reactive Ion Etching (RIE)
~ plasma
wafers
RF13.56MHz
Parallel-PlateReactor
Plasma generates (1) Ions(2) Activated neutrals
Enhance chemical reaction
Sputtering
Professor Nathan Cheung, U.C. Berkeley EE143 Lecture # 16
Remote Plasma Reactors
Plasma Sources(1) Transformer
CoupledPlasma (TCP)
(2) ElectronCyclotronResonance (ECR)
-bias
pump
e.g. quartz
coilsplasma
wafers
Pressure1mTorr 10mTorrbias~ 1kV≤
Professor Nathan Cheung, U.C. Berkeley EE143 Lecture # 16
• Synergism of ion bombardment AND chemical reactiongive the high RIE rates.
Professor Nathan Cheung, U.C. Berkeley EE143 Lecture # 16
REMOVAL ofsurface film and DEPOSITIONof plasma reactionproducts canoccur simultaneously
Professor Nathan Cheung, U.C. Berkeley EE143 Lecture # 16
RIE Etching Sequencegas flow
1
2 3
5
4X
diffusion of reactant
diffusion of by productdesorption
chemical reaction gaseous by productsabsorption
Substrate
Professor Nathan Cheung, U.C. Berkeley EE143 Lecture # 16
Volatility of Etching Product
* Higher vapor pressure higher volatility
)(..
4.. 4*
pressurevaporlowCuClClCuge
SiFFSige
→+↑→+
mask
Metal
⇒
(high vapor pressure)
Professor Nathan Cheung, U.C. Berkeley EE143 Lecture # 16
Vapor pressure of by-product has to be high.
kTH v
ePP∆−
= 0
1/T
P1500oC
AlCl3CuCl
[Al-Cu alloy]
Cl2 as etching gas.
200oC
1~2% typical
Example
Difficult to RIE Al-Cualloy with high Cu content
Professor Nathan Cheung, U.C. Berkeley EE143 Lecture # 16
Professor Nathan Cheung, U.C. Berkeley EE143 Lecture # 16
Examples
↑→+
++⇔+
+→+
4*
*34
3*
4
4
2
SiFFSi
eFCFeCF
CFFCF
Use CF4 gasSi
F* are Fluorine atoms with electrons
Professor Nathan Cheung, U.C. Berkeley EE143 Lecture # 16
Professor Nathan Cheung, U.C. Berkeley EE143 Lecture # 16
x
xzyx HO
COOOHC
AlClClAl
eClCCleCCl
2
3*
*34
3
2
+
↑→+
+++⇔+
Photo Resist
Aluminum
Professor Nathan Cheung, U.C. Berkeley EE143 Lecture # 16
How to Control Anisotropy ?1) ionic bombardment to damage expose surface.2) sidewall coating by inhibitor prevents sidewall etching.
Professor Nathan Cheung, U.C. Berkeley EE143 Lecture # 16
Sidewall passivation films Photoresist on top of Si
HCl/O2/BCl3 chemistry
Professor Nathan Cheung, U.C. Berkeley EE143 Lecture # 16
How to Control Selectivity ?
E.g. SiO2 etching in CF4+H2 plasma
SiO2
Si
Rates
%H2 in (CF4+H2)
SRate SiO
Rate Si= 2
H2%
P.R.
Si
SiO2
Reason:↓∴
↓∴→+
4
**
SiF
contentFHFHF
S
Professor Nathan Cheung, U.C. Berkeley EE143 Lecture # 16
Example Si etching in CF4+O2 mixture
12
Si
SiO2
Poly-Si
Oxide
Reason:
↓∴→+
+→+
rateSiOOSi
rateetchingSiincreasesF
FCOFCFO xx
22
*
)2(
*
)1(
%O2 in CF4
Rates
Professor Nathan Cheung, U.C. Berkeley EE143 Lecture # 16
Professor Nathan Cheung, U.C. Berkeley EE143 Lecture # 16
Effect of RIE process variables on etching characteristics
Professor Nathan Cheung, U.C. Berkeley EE143 Lecture # 16
Temperature Dependence of Selectivity
( )
R A e
R A e
SRR
AA
e
QkT
QkT
Q QkT
1 1
2 2
1
2
1
2
1
2
1 2
=
=
∴ = =
−
−
− −
77oK if Q1<Q2
1/T
S
R= etching ratesA = proportional constantsQ = activation energies
Professor Nathan Cheung, U.C. Berkeley EE143 Lecture # 16
Example: RIE of Aluminum Lines
P.R.
Al
BCl31
2 Cl2-based RIE
native Al2O3
3 Form oxide again (gently)
Al Al
* It is a three-step sequence :1) Remove native oxide with BCl32) Etch Al with Cl-based plasma3) Protect fresh Al surface with thin oxidation
Professor Nathan Cheung, U.C. Berkeley EE143 Lecture # 16
Example :Etching of Deep Trenches
~1µm
mask
ballooning
mask
mask erosion
trenching
by-productresidue
Si
“ideal” “problems”
Professor Nathan Cheung, U.C. Berkeley EE143 Lecture # 16
• (2) Ballooning:– Use chemistry with a good sidewall inhibitor.
• (3) Trenching– Use high pressure to increase ion.
• neutral scattering (less directional)
• Bottom Roughness– Increase P of byproduct.
Professor Nathan Cheung, U.C. Berkeley EE143 Lecture # 16
Solution: Multiple step RIE sequence
Professor Nathan Cheung, U.C. Berkeley EE143 Lecture # 16
robotto
handlewafers
EtchStep 1
EtchStep 2
EtchStep 3
wafer I/O
We need Cluster Tools
In general, an etching process can be several steps in sequence
Professor Nathan Cheung, U.C. Berkeley EE143 Lecture # 16
Local Loading Effect
Wsmall
Wlarge
More etchant consumptionLess etchant consumption
Professor Nathan Cheung, U.C. Berkeley EE143 Lecture # 16
RIE Lag* smaller trenches etch at a slower rate than larger trenches.
CCl2F2/O2 RIE
Professor Nathan Cheung, U.C. Berkeley EE143 Lecture # 16
XeF2 Etching of Si
* Dry, isotropic, vapor-phase etch XeF2 vapor pressure (~3.8 Torr at 25 °C)
2 XeF2 + Si àà 2 Xe (g)↑↑ + SiF4 (g) ↑↑
Advantages :•Highly selective to silicon with respect to Al, photoresist, and SiO2. •Isotropic, large structures can be undercut.•Fast ( ~10µm per hour)•Gas phase etching, no stiction between freed structure and substrate
Disadvantages: • No known etch stops for Si substrate
Professor Nathan Cheung, U.C. Berkeley EE143 Lecture # 16
Professor Nathan Cheung, U.C. Berkeley EE143 Lecture # 16
Photon-Assisted Etching