Reactive Ion Etching (RIE) RF 13.56 ~ MHz

plasma

Parallel-Plate Reactor

wafers Sputtering

Plasma generates (1) Ions (2) Activated neutrals Enhance chemical reaction Professor Nathan Cheung, U.C. Berkeley

EE143 Lecture # 16

Remote Plasma Reactors Plasma Sources (1) Transformer Coupled Plasma (TCP) (2) Electron Cyclotron Resonance (ECR) Professor Nathan Cheung, U.C. Berkeley

e.g. quartz plasma

coils

wafers -bias

Pressure pump 1mTorr 10mTorr bias~ ≤ 1kV

EE143 Lecture # 16

• Synergism of ion bombardment AND chemical reaction give the high RIE rates. Professor Nathan Cheung, U.C. Berkeley

EE143 Lecture # 16

REMOVAL of surface film and DEPOSITION of plasma reaction products can occur simultaneously

Professor Nathan Cheung, U.C. Berkeley

EE143 Lecture # 16

RIE Etching Sequence gas flow

2

1

5

diffusion of reactant

diffusion of by product desorption 4

3 X

absorption

chemical reaction

gaseous by products

Substrate Professor Nathan Cheung, U.C. Berkeley

EE143 Lecture # 16

Volatility of Etching Product * Higher vapor pressure

⇒

higher volatility

e.g . Si + 4 F → SiF4 ↑ *

(high vapor pressure)

e.g . Cu + Cl → CuCl (low vapor pressure ) mask Metal

Professor Nathan Cheung, U.C. Berkeley

EE143 Lecture # 16

Vapor pressure of by-product has to be high.

P = P0 e

− ∆H v

kT

Example

P

Difficult to RIE Al-Cu alloy with high Cu content

1500oC AlCl3

CuCl

1~2% typical

[Al-Cu alloy] 200oC

1/T

Cl2 as etching gas. Professor Nathan Cheung, U.C. Berkeley

EE143 Lecture # 16

Professor Nathan Cheung, U.C. Berkeley

EE143 Lecture # 16

Examples Si

Use CF4 gas

CF4 → F + CF3 *

+

CF4 + e ⇔ CF3 + F + 2e *

Si + 4F → SiF4 ↑ *

F* are Fluorine atoms with electrons

Professor Nathan Cheung, U.C. Berkeley

EE143 Lecture # 16

Professor Nathan Cheung, U.C. Berkeley

EE143 Lecture # 16

Aluminum

CCl4 + e ⇔ CCl3 + +Cl + 2e *

Al + 3Cl → AlCl3 ↑ *

Photo Resist

C x H y Oz + O2

Professor Nathan Cheung, U.C. Berkeley

COx HOx

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

S=

Rate SiO2 Rate Si

S

Rates

P.R. SiO2 Si

SiO2 H2%

Si %H2 in (CF4+H2)

Reason:

F

*

+ H → ∴ SiF

Professor Nathan Cheung, U.C. Berkeley

4

HF ∴ F * content

↓

↓ EE143 Lecture # 16

Example Si etching in CF4+O2 mixture Rates 1

Reason:

Si

(1)O + CFx → COFx + F *

2

F * increases Si etching

rate

(2)Si + O2 → SiO2 ∴ rate↓

SiO2

%O2 in CF4

Poly-Si Oxide

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

R1 = A1e

− Q1

R2 = A2 e

kT

− Q2

R= etching rates A = proportional constants Q = activation energies

kT

R1 A1 − ( Q1 − Q2 ) kT ∴S = = e R2 A2 S

if Q1