E SC 412 Nanotechnology: Materials, Infrastructure, and ...€¦ · DC Glow Discharge (Paschen...

E SC 412

Nanotechnology: Materials, Infrastructure, and Safety

Wook Jun Nam

Lecture 10 Outline

1. Wet Etching/Vapor Phase Etching

2. Dry Etching

DC/RF Plasma

Plasma Reactors

Materials/Gases

Etching Parameters

Bosch Process

Cryogenic Process

Copyright 2014 by Wook Jun Nam

Top Down Approach


Mask- the word “mask” is used in etching to mean a protective

layer (covering). Ideally a mask material is not etched at all.

Etch rate-how fast material is removed (usually in nm/sec)

Selectivity-how good an etching process is at attacking one

material and leaving another alone

Isotropic-etching which attacks a material equally in all

directions

Anisotropic-etching which attacks a material mainly in one

direction

Etching: Some Key Terminology


Isotropic / Anisotropic Etching

http://home.comcast.net/~dwdm2/MEMS_micromachining.html


Wet / Vapor Phase Etching


Wet Etching

• Advantages:

− Relatively simple, easy, fast, and economic (e.g., batch

process)

− High etch selectivity

− No physical damages on a substrate

• Disadvantages:

− Etch rate is not reproducible

− Usually Isotropic etching

− Chemical wastes


Wet Etching: Typical Materials / Etching

Chemicals

J. D. Plummer, M. Deal, and P. D. Griffin, Silicon VLSI Technology Fundamentals, Practices, and Modeling , Prentice Hall, 2000Copyright 2014 by Wook Jun Nam

Vapor Phase Etching (XeF2)

• Selectivity:

XeF2 shows very high

selectivity vs silicon to the

majority of semiconductor

materials (e.g., photoresist,

silicon dioxide (>1000:1),

silicon nitride (>100:1), and

aluminum).

• Isotropic etching

• Safety issues when

loading/unloading samples.

2XeF2 + Si SiF4 + Xe


http://www.xactix.com/xetch_e1.html

Vapor Phase Etching (XeF2)

• No release stiction

XeF2 etching is a dry process

so no drying is needed which

avoids the sticking issues that

often plague wet release

processes.

• Delicate structures are safely

released

Since XeF2 etching is a dry,

room temperature process

delicate structures can be

released. This is particularly

useful for releasing delicate

devices (e.g., micro-mirrors).


http://www.xactix.com/xetch_e1.html

DC / RF Plasma


Reactions in Plasma

very reactive radicals

very reactive radicals

photon generation:

plasma glow


DC Glow Discharge (Paschen Curve)

• When a high DC bias is

applied between two

electrodes in a gas, a

breakdown is occurred.

• Small pd: either too low

pressure or too close space

between the electrodes

electrons move across the

space with no or few collisions.

• Large pd: either too high

pressure or too big electrodes

space not enough energy

transfer by collisions.

small pd area large pd area

http://commons.wikimedia.org/wiki/File:PaschenCurve.jpg


• Electrons oscillate between the electrodes wit the AC

voltage. No need for electron emission from cathode

• Can sustain plasma at lower pressure than DC plasma.

• Can etch dielectrics as well as metals.

RF Plasma


• The smaller electrode has greater voltage drop.

• The anode should be bigger than the cathode : the anode

is usually connected to the chamber wall to increase the

area.

• The big anode area reduces Vp reduce the plasma

induced damage on the chamber wall.

RF Plasma (continued)

powered electrode

(cathode)

grounded electrode

(anode)VT = |VDC| + Vp


Plasma Reactors


• Powered electrode is

directly coupled to the

plasma.

• High electric field is formed

near the powered electrode.

• Power transfer efficiency is

relatively low, but very

uniform plasma generation.

• Applied Power (e.g., DC,

RF (13.56 MHz), VHF

(>30MHz)).

Capacitively Coupled Plasma (CCP)


• High etch rate requires high plasma densities (>1011/cm3)

• Higher process pressures higher plasma densities

short mean free path

less directional

• Different plasma systems are needed to generate HDP at

low pressure

− Inductively coupled plasma (ICP)

− Electron cyclotron resonance (ECR)

High Density (HD) Plasma


• HD plasma offers;

– Good etch selectivity

– High Etch rate

– Anisotropic etch profile

– Low plasma induced physical damages

– Good control in critical dimension (CD)

High Density (HD) Plasma (continued)


• Also called as transformer coupled plasma (TCP).

• Upper part of chamber: ceramic or quartz

• Source RF inductively couple with plasma (remote plasma)

RF source does not directly contact with plasma (no

contamination)

• Source RF generates plasma and controls ion density

(~1012 /cm3)

• Bias RF controls ion bombardment energy.

• Ion energy and density independently controlled.

ICP: Operation


ICP: Typical Tool Configuration


• An electron in a static and uniform magnetic field will

move in a circle.

• Applying an alternating electric field will results in a

cycloid. The frequency of this cyclotron motion is given by

• This is called electron cyclotron resonance frequency

• When the frequency of the electric field set to electron

resonance occur.

• For commonly used microwave frequency, 2.45 GHz, the

resonance condition is met B=875.

ECR: Operation


ECR: Typical Tool Configuration


• Long MFP, insufficient ionization collisions

• In a magnetic field, electron is forced to spin with very

small gyro-radius

• Electrons have to travel longer distance/more collisions

• Increasing plasma density at low pressure

• Magnetic field increasing electron density in sheath layer

• Less charge difference in sheath region

• Lower DC Bias

• Effects on ion bombardment

– increasing ion density

– reducing ion energy

Magnets/ Magnetic Field


• Ion bombardments generate large amount heat.

• High temperature can cause PR reticulation/low etch

selectivity.

• Need cool wafer to control temperature.

• Helium backside cooling is commonly used.

• Helium transfer heat from wafer to water cooled chuck.

Wafer Cooling


Helium

Clamp Ring

WaferSeal O-

ring

Water-cooled pedestal,

cathode, or chuck

Mechanical Chuck (Clamp Chuck)


• Helium needs to be pressurized

• Wafer has high pressure at backside because low chamber

pressure

• Need mechanisms to hold wafer

• Either mechanical clamp or E-chuck

• Clamp ring causes particles and shadowing effect

• E-chuck is rapidly replacing clamp ring

Electrostatic Chuck


Materials / Etching Gases


Materials & Etching Gases


Dry Etching: Processes at the Etched

Material Surface


Chemical/ Physical Etching


Anisotropic Etching


Anisotropic Etching: Inhibitors

J. D. Plummer, M. Deal, and P. D. Griffin, Silicon VLSI Technology Fundamentals, Practices, and Modeling , Prentice Hall, 2000

• H2 consumes F, and forms HF which does not contributes

for Si etching. The low concentration of F reduces the

chemical reaction to form SiF4, and slows down the etch

rate.


Anisotropic Etching (continued)

• Hydrogen consumes F.

• Too much addition of H2 will cause too slow etch rate.


Anisotropic Etching (continued)


Anisotropic Etching (continued) : ICP Si

Etching

All other etching conditions (e.g., rf power, etch time, process

pressure) are the same

Si

Cr

CF4: 30sccm, SF6: 20 sccm

80 sec etch time


80 sec etch time


Etching


80 sec etch time


80 sec etch time

All other etching conditions (e.g., rf power, etch time, process

pressure) are the same


Etching


80sec etch time


120 sec etch time

All other etching conditions (e.g., rf power, process pressure)

are the same

Macro-loading Effect

• Etch rate is decreased as the overall etch area is increased

Z. Cui, Nanofabrication: Principles, Capabilities, and Limits, Springer (2008)Copyright 2014 by Wook Jun Nam

Micro-loading Effect

• Micro-loading effect is caused by localized pattern density.

• Micro-loading effect is related with localized depletion of

reactive species or accumulation of etch by products.


Aspect Ratio Effect (Aperture Effect)

• The aspect ratio effect is strongly dependent on dimensions

of pattern.

• The etch rate for small features is slower than bigger ones.

• The mechanism for the effect is very complicated, and is

related with available reactive species and reaction

byproducts. Z. Cui, Nanofabrication: Principles, Capabilities, and Limits, Springer (2008)


Aspect Ratio Effect (Aperture Effect)

http://cmi.epfl.ch/etch/601E.php Copyright 2014 by Wook Jun Nam

Micro Trenching Effect

• Micro-trenching effect is a phenomenon that the etch rate near

the trench corner is faster than the center.

• The effect is caused by the impact of high energy ions at grazing

angles (> 80°) on the side walls then reflected to the bottom of

the trench.

• Both side wall slope angle and the incident angle of the ions can

significantly influence the resulting etch profile.


Micro Trenching Effect (continued)


Notching Effect (DRIE)

• The addition of etch stop layer is very helpful for removing

loading effects.

• The etch stop layer (e.g., SiO2) can cause a notching effect

as the layer is locally charged.


Bosch / Cryogenic Processing


Bosch Process: Deep Reactive Ion Etch

(DRIE)

• The Bosch process is used

for high aspect ratio etching

by alternating passivation

(C4F8 plasma) and etching

(SF6 plasma) cycles.


Bosch Process: Deep Reactive Ion Etch

(DRIE)

• The deposition of a passivation layer protects the side

walls from chemical etching during the subsequent etching

cycle.

• Directional etching caused by ion bombardment removes

the passivation layer at the bottom, so that the radicals are

able to attack the substrate.

http://www.iue.tuwien.ac.at/phd/ertl/node68.html


Bosch Process: Scalloping Issue

Lateral roughness due to the scalloping is about 150nm or more !

http://en.wikipedia.org/wiki/Deep_reactive-ion_etching


Sidewall roughness can be tuned little bit !:

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

(a) (b)

Bosch Process: Scalloping Issue

(continued)


• In cryogenic-DRIE, the wafer

is chilled to −110 °C (163 K).

• The low temperature slows

down the chemical reaction

that produces isotropic

etching.

• However, ions continue to

bombard upward-facing

surfaces and etch them away.

• This process produces

trenches with highly vertical

smooth sidewalls.

Cryogenic Process


• Very high selectivity over photoresist (to 100:1) and SiO2

masks (to 200:1)

• Simple and extremely clean plasma chemistry: SF6-O2

plasma (no fluorocarbons) instead of SF6-C4F8 plasma.

- almost no chamber cleaning

• The primary issues with cryo-DRIE is that the standard

masks on substrates crack under the extreme cold, plus

etch by-products have a tendency of depositing on the

nearest cold surface, i.e. the substrate or electrode.

Cryogenic Process (continued)


Lecture 10 Outline

1. Wet Etching/Vapor Phase Etching

2. Dry Etching

DC/RF Plasma

Plasma Reactors

Materials/Gases

Etching Parameters

Bosch Process

Cryogenic Process


E SC 412 Nanotechnology: Materials, Infrastructure, and ...€¦ · DC Glow Discharge (Paschen...

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