Micro/Nanosystems Technology - Technische Fakultät€¦ · (absorber pattern metal, Cr typ....

Micro / Nanosystems TechnologyWagner / Meyners 1

Micro/Nanosystems Technology

Dr. Dirk Meyners

Prof. Wagner


UV - Lithography

Outline

- Lithography – Overview

- UV-Lithography

- Resolution Enhancement Techniques

- Electron Beam Lithography

- Patterning with Focused Ion Beam


Lithography - Overview

*

Thin Films

Implant

Diffusion Etch

Test/Sort

Polish

LithoPatterned

wafer

Lithography is at the Center of the Wafer Fabrication Process


Moore‘s Law

• Duplication of the complexity (number of circuits/transistors on a

chip) of integrated circuits every 2 years (exponentional growth)

[R. Zengerle, Mikrosystemtechnik, Lecture Notes]


Moore‘s Law

https://ourworldindata.org/wp-content/uploads/2013/05/Transistor-Count-over-time.png



• Lithography is used to produce 2 1/2-D images using radiation

sensitive resist and controlled exposure to radiation.

• The most widely used form of lithography is photolithography using

light sensitive resist.

• UV-Lithography

• X-ray lithography

• Electron Beam Lithography

• Ion Beam Lithography

• Wave length λ of radiation limits achievable resolution. (resolution of

e- and ion-beam lithography limit by scattering)

De Broglie:mv

h



Ten Basic Steps of

Lithography

1. Surface Preparation

2. Resist Application

3. Soft Bake

4. Alignment & Exposure

5. Develop

6. Hard Bake

7. Inspection

8. Etch

9. Resist Removal

10.Final Inspection



Basic Steps of Lithography

Image source:

M. Madou, Lecture Notes, 2008




Coat with resist

Expose

Develop

Transfer pattern

Strip resist

Etch Doping Deposit film

Resist

Substrate

Radiation

Mask

LIFT-OFF

Image source: R. Zengerle, lecture notes, Mikrosystemtechnik


• Alternative scheme:

Hard masks

(1) Structure transfer to

a deposited layer on

the substrate

(2) Patterned layer is

used as hard mask in

a second etching step

[R. Zengerle, Mikrosystemtechnik, lecture notes]





• A resist is a radiation sensitive polymer.

• patterned selectively

• resistant to following processing

Negative resist: Prints a pattern that is opposite of the pattern that is

on the mask.

Positive resist: Prints a pattern that is the same as the pattern on the

mask.

Image source:

R. Zengerle, lecture notes,

Mikrosystemtechnik



• Negative lithography

Island

silicon substrate

oxide

photoresist

Window

Areas exposed to light become polymerized and resist the develop chemical.

Resulting pattern after the resist is developed.

photoresist

oxide

silicon substrate

Ultraviolet Light

Exposed area

of photoresist

Shadow on

photoresist

Chrome island

on glass mask

Image source:




• Positive lithography

silicon substrate

oxide

photoresist

Island

Window

Areas exposed to light become soluble.

Resulting pattern after the resist is developed.

Shadow on

photoresist

Exposed area

of photoresist

Chrome island

on glass mask

photoresist

silicon substrate

oxide

Ultraviolet Light

Image source:




• During exposure with UV-light the photo active compound

DiazoNaphtoQuinone- (DNQ-) sulfonate (left) separates a N2

molecule (middle), and converts into indene carboxylic acid (right)

requiring H2O. Compared to unexposed DNQsulfonate, the

carboxylic acid yields a resist development rate (alkaline solubility)

several orders of magnitude higher.

• quantum efficiency ≈ 20 .. 30 %

• backbone molecules (viscosity)

Example: (DNQ-) Reaction (positive)

λ λ

[www.microchemicals.eu]


UV - Lithography

Outline

- Lithography – Overview

- UV-Lithography

- Resolution Enhancement Techniques

- Electron Beam Lithography

- Patterning with Focused Ion Beam


1. Surface Preparation by Wafer Priming (HMDS)

• Adhesion of the resist is often insufficient on Si or SiO2

• HMDS as adhesion promoter

HMDS• Dehydration bake in enclosed

chamber with exhaust

• Clean and dry wafer surface

(hydrophobic)

• Hexamethyldisilazane (HMDS)

• Temp ~ 200 - 250°C

• Time ~ 60 sec

[M. Madou, Fundamentals of Microfabrication, Lecture Notes]

UV - Lithography


• unpolar methyl groups form

hydrophobic surface with

corresponding resist

wettability and adhesion

HMDS

(hexamethyldisilazane)

[M. Madou, Fundamentals

of Microfabrication, Lecture

Notes]

UV - Lithography


2. Resist Application

• Wafer or substrate are held

onto vacuum chuck

• Dispense few milliliters of

photoresist

• Slow spin ~ 500 rpm

• Ramp up to ~ 3000 - 5000

rpm

• Quality issues:

– thickness

– uniformity

– particles & defects

vacuum chuck

spindleto vacuum

pump

photoresist dispenser

UV - Lithography


UV - Lithography

• Resist thickness T depends on:

– Spin speed

– Solution concentration

– Molecular weight (measured by intrinsic viscosity)

• In the equation for T, K is a calibration constant, C the polymer

concentration in grams per 100 ml solution, h the intrinsic viscosity,

and w the number of rotations per minute (rpm)

• The equation can be used to predict the thickness of the resist that

can be spun for various molecular weights and solution

concentrations of a given polymer and solvent system

w

hcKT


Spin Coater

• Spin Coater OPTIspin ST22P

– Substrate size up to 8“

– Chucks for 4“, 6“, 8“ and

pieces

– spin speed up to 10,000 rpm

• Hot Plate

– HMDS Adhesion promoter

(C6H19NSi2)

– Temperature up to 200°

UV - Lithography


UV - Lithography

• Spin Coating

• Spray Coating

Image source:



UV - Lithography

• Partial evaporation of resist solvents

• Improves adhesion

• Improves uniformity

• Improves etch resistance

• Improves linewidth control

• Optimizes energy absorbance

characteristics of resist

3. Softbake


UV - Lithography

• Transfers the mask image to the resist-coated wafer

• Activates photo-sensitive components of photoresist

• Quality issues: – linewidth resolution– overlay accuracy– particles & defects

UV Light Source

Mask

Resist

4. Alignment & Exposure


Masks for optical Lithography:

Drawing with CAD

• Example for a mask

structure for microinductors

• Design the layout with

CAD-programms

UV - Lithography


• Quartz glass plate with

structured chromium layer

(absorber pattern metal, Cr

typ. thickness: 100 nm)

• Typical costs:

– 30 €/cm2 for structures

> 5 µm

– 75 €/cm2 for structures

1 - 5 µm

– masks for 4“ wafer

approx. 400 - 2.500 €


Masks for optical Lithography:

UV - Lithography


• deposit resist on chromium

coated quartz plate

• structure resist with

electron beam lithography

• develop resist

• wet etching of chromium

• resist removal


Fabrication of masks:

UV - Lithography


UV - Lithography

[M. Madou, Fundamentals of Microfabrication, Lecture Notes]

Shadow Projection

Printing techniques:


• Mask is pressed to the wafer/resist

→ in contact without gap

• (hard/soft) contact mode / vacuum

mode

+ structures in sub-μm range are

possible

+ minor failures at the pattern

transfer

– contamination of the mask

– defects by dirt particles


UV - Lithography

Contact printing:


• No direct contact between mask

and wafer/resist → proximity gap of

~ 30µm

+ less stress on the mask

+ higher lifetime of the mask

+ higher throughput

– lower resolution


UV - Lithography

Proximity printing:


• Downsizing of the mask structure by

objectives

• Step-by-step projection of the mask

structure on the wafer

• only one functional unit (e.g. chip) on the

wafer

+ enlarged masks easier in production:

• better to control

• cheaper production of masks (only

1 chip on the mask)

– equipment is expensive (lenses)

– low throughput

mirror

light source

filter

condenser lens

mask

objective

field of exposure

movable sample table

(step and repeat)


UV - Lithography

Projection printing:


• emission spectrum of a mask aligner with Hg light source contains

three lines

h-, i-line resist

g-, h-, i-line resist

Wavelength

Inte

nsity


• the absorption

spectrum of the

photoresists is

matched to this Hg

emission spectrum.

• distinguish between

broadband sensitive

(g-, h-,and i-line)

photoresists, and

resists with a

absorption spectrum

more narrow

UV - Lithography

Ligth Source/Sensitivity of Resists:


UV - LithographyResolution in contact and proximity

printing:

The theoretical resolution R is

equal to the minimum resolved

dimension with a grating mask

(bmin for a line or a space).

It is limited by diffraction to:

22

3min

zsbR

bmin : half grating period

s: gap between mask

and photoresist

surface

z: resist thickness


UV - Lithography

Resolution in contact and proximity

printing:

Contact printing:

Typical values for a conventional system:

22

30

zRs

al)(theoretic 70

1

line)-(h 400

µm. R

µmz

nm

Proximity printing:

Typical values for a conventional system:

sRzs 2

3

al)(theoretic 4

20

line)-(h 400

µm R

µms

nm


• Resolution is influenced by:

– wavelength (diffraction)

– gaps between resist and mask

UV - Lithography

• Reasons for a gap between resist and chromium layer of the mask:

– particles in the resist caused by either insufficient cleanroom

conditions, contaminated substrates, or expired photoresist

– bubbles in the resist film caused during dispensing, or an

insufficient delay time after refilling/diluting/moving the resist

– mask contamination by particles, or resist (previous exposures)

– rough, structured or curved (strained) substrates

– an edge bead, or a mask attached upsidedown


Contributors to Non-Rectangular Wafer Pattern

• Lithography process variations:

– Lens aberration, misalignment, defocus, overexposure

• Sub-wavelength non-ideal optical effects due to

– Diffraction

• Those effects result in wafer pattern distortion:

– Line-end shortening, corner rounding, line-edge roughness


theore

tical m

ax.

resolu

tion (μ

m)

Light intensity distribution in a photo resist

film (cross section) with a - from top to

bottom- increasing gap between mask and

resist film.


UV - Lithography

Resolution: gaps between resist and mask

resist thickness not considered


• Resolution is influenced by:

– wavelength (diffraction)

– gaps between resist and mask

– defects in the mask

– bleaching of the photoresist

– contrast

– optical substrate properties

absorp

tion

coeff

(1/

μm

)


UV - Lithography


d/d

oof

the d

evelo

ped r

esis

t film

Log10 (exposure dose)

• The slope of the decay in the

contrast curve defines the

contrast

(remaining layer thickness d in

relation to the starting thickness d0)

UV - Lithography

Contrast:

high contrast

low contrast

DC (dose to clear) depends on

resist thickness, developer

concentration and development

time.


• UV-reflecting substrates (e.g.

metallized) increase the

absorbed light dose in the resist

near the substrate

• UV-transmissive substrates

(e.g. quartz, glass, thick SiO2 on

Si, transparent polymers)

laterally guide light along the

substrate, cause reflections

from the chuck and reduce the

lateral resolution


UV - Lithography

Resolution: optical substrate properties


• wafer stage in a coventional mask aligner


UV - Lithography

Alignment:


• Pattern transfer using

several masks demands

the alignment with sub-µm

accuracy

– performance in a mask

aligner

– substrate is movable in

x, y and θ

– first mask aligned at the

flat

large scale production:

fully automatic alignment

and exposure


UV - Lithography

Alignment:


large scale production:

fully automatic alignment

and exposure

– subsequent masks are

aligned by means of

alignment markers


UV - Lithography

Alignment:


=

first mask second mask


UV - Lithography

Alignment:


mask 1 mask 2 mask 3

double exposure 1 + 2 multiple exposure 1 + 2 + 3


UV - Lithography

Alignment:


UV - Lithography

5 . Develop

Soluble areas of the resist are

dissolved by developer

chemical

• Visible patterns appear on wafer

– windows

– islands

• Quality issues:– line resolution– uniformity– particles & defects

vacuum chuck

spindle

developerdispenser

to vacuum pump


UV - Lithography

Photoresist profiles:

– Undercut (LIFT-OFF)

– Vertical

– Overcut Dose : High

Developer: Low

Dose : Medium

Developer: Moderate

Dose : Low

Developer: Dominant


substrate

resist

metal

Aim: Structuring metal without etch process

• structure the photoresist

• deposit metal (e.g. Pt) on the whole surface

• resolve the photoresist

– metal on resist is removed

– metal on substrate remains

• Inversion of the edge profile by image reversal technique enables the lift-off process


UV - Lithography

Lift-Off process:


• Double exposure of resist and chemical modification in between

→ positv resist behaves like high resolution negativ resist

solubleinsoluble

still

insoluble

solubleflood exposure

crosslinking

→ insoluble

1. Exposure 2. Temperature (~90°)

3. Exposure 4. Develop


UV - Lithography

Image reversal:


• Inversion of the edge profile

unexposed

exposed

Image Reversal

Standard Lithography


UV - Lithography

Image reversal:


• example for different

resists for different

applications and

thickness


UV - Lithography

Photoresists:


• epoxy-based negativ resist

• layer thickness up to ~500µm (up to ~2mm

with several layers on top of each other)

• aspect ratio of 1:40 (high contrast of SU-8)

• high chemical resistance → application as

etching mask


–high volume shrinkage after postbake

–high stresses in large-scale structures

–handling more critical than for standard resists

UV - Lithography

SU-8 Technology:


UV - Lithography

6. Hard bake

• Evaporate remaining resist

• Improve adhesion

• Higher temperature than soft

bake


UV - Lithography

8. After inspection the pattern transfer is

performed, e.g. by etching

• Selective removal of upper layer of wafer through windows in resist:

subtractive• Two basic methods:

– wet acid etch

• Au with KI/I2• Si with HF

– dry plasma etch

• Reactive Ion Etching (RIE)

• Ion Beam Etching (IBE)• Quality issues:

– defects and particles– step height– selectivity

Alternatively• Adding materials (additive)• Two main techniques:

– Sputtering– evaporation

• Lift-Off

PlasmaPlasma

CF4CF4


UV - Lithography

9. Resist removal (strip)

• No need for resist in the following

preparation step

• Two common methods:

– wet removal with solvent (e.g.

aceton or N-methyl-2-pyrrolidone

(NMP))

– dry plasma etching with O2-

plasma

• Followed by wet clean to remove

remaining resist and strip byproducts

O2O2

PlasmaPlasma


UV - Lithography

10. Final inspection

• Resist has been completely removed

• Pattern on wafer matches mask pattern (positive resist)

• Quality issues:

– defects

– particles (cleanroom class)

– step height

– overlay accuracy

– critical dimensions

Date post:	27-Aug-2020
Category:	Documents
Upload:	others
View:	0 times
Download:	0 times

Micro/Nanosystems Technology - Technische Fakultät€¦ · (absorber pattern metal, Cr typ....

Documents