Micro/Nanolithography - People Micro/Nanolithography ©2012 Dale E. Ewbank page 3 DRAM LITHO FEATURE EXPOSURE TOOL YEAR SIZE WAVELENGTH COST 1985 1000 nm 436 nm $0.8 MILLION

Micro/Nanolithography

©2013 Dale E. Ewbank

page 1 Micro/Nanolithography

Dale E. Ewbank

[email protected]

unL081413_microe.ppt

OUTLINE

Micro/Nanolithography ©2009 Dale E. Ewbank

page 2

Micro/Nanolithography

Masks

Optical Lithography

Photoresist

Sensitivity

Processing

Exposure Tools

Advanced Processes

MICROLITHOGRAPHY


page 3

DRAM LITHO

FEATURE EXPOSURE TOOL

YEAR SIZE WAVELENGTH COST

1985 1000 nm 436 nm $0.8 MILLION

1988 750 436 $1.0

1991 500 365 $3.0

1994 350 365 $5.5

1997 250 248 $7.0

2003 110 248 $10

2006 65 193 $20-30

2009 45 193 or 193i $50-80

2012 22 193i double patterning $70-90

The production of ultra-small three dimensional relief images based on

exposure and subsequent development of a photon sensitive polymer

called photoresist.

MASKS


page 4

Reduction reticle with

pellicle: ASML, Canon,

Nikon, others

1X projection with pellicle:

Perkin Elmer Micralign

TRANSMISSION PROPERTIES OF

OPTICAL GLASS


page 5


page 6

Ultra Violet Radiation Ultra Violet Radiation 248 nm KrF laser 193 nm ArF laser

365 nm i-line

436 nm g-line

Modified from http://www.osram.co.at/_global/pdf/Professional/Display_Optic/Semiconductor_Medical/HBO-IC/hbo_ic.pdf

Optical Lithography


page 7

Illumination

System

Source

Condenser Lens

Mask

Objective Lens

Substrate

Illumination

System

Source

Condenser Lens

Mask

Substrate

Gap (z)

Proximity/Contact Projection NA = n sinq

q

DIFFRACTION


page 8

FRESNEL DIFFRACTION


page 9

Proximity/Contact System Resolution

Wmin ~ 0.7 ( Z)1/2

W = space opening width on the mask

= exposing wavelength

Z = gap (distance) from the mask to the wafer

Note that Z cannot go be zero because of resist thickness

RAYLEIGH CRITERIA


page 10

Projection System Resolution

P/2 = k1 / NA NA = n sinq

DOF = +/- k2 / (n sin2q) k2= 0.5

g-line i-line KrF KrF ArF ArF ArF

k1 0.61 0.61 0.50 0.35 0.43 0.40 0.26

[nm] 436 365 248 248 193 193i 193i

NA 0.28 0.50 0.50 0.70 0.93 1.20 1.35

P/2 [nm] 950 450 250 124 90 65 38

DOF[+/- nm] 2780 730 500 253 111 95 54

NEGATIVE AND POSITIVE PHOTORESISTS


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Photomask

Photoresist Film to be Patterned Substrate

Light

Negative Resist Positive Resist

Rendered Insoluble Rendered Soluble

After

Development

After Etch

and Strip

ONE AND TWO COMPONENT RESIST

HIERARCHY


page 12

Positive Resist

Components

Acrylates

(PMMA)

ONE TWO

Diazo naphtho quinone

Novolac

(Shipley 1400)

Acid Generator -- Onium salt

Polymer backbone -- Polystyrene

TWO

Chemically Amplified Resist

Components

NOVALAC RESIN


page 13

Novalac resins are soluble in organic solvents,

exhibit good film forming characteristics, are

capable of combining with orthoquinone diazide

sensitizers

CH2

OH

CH3

CH2

OH

CH3

n

PHOTOACTIVE COMPOUND (PAC)


page 14

Naphthoquinone

diazide (PAC)

N2

O

R

N2

O

R

+

Keto-carbene

and Nitrogen

Stable ketene

molecule

O

R

C

OH

R

C

O H

Base soluble

carboxylic acid

add H2O

hu

• •

POSITIVE NOVOLAC PHOTORESIST


page 15

Novolac Base Matrix Resin - 5-15% Sensitizer or Photoactive Compound (PAC) - 5-15% Casting Solvent - 60-80% Additives - 10% PAC - such as naphthoquinone diazide Solvents - such as EL (Ethyl Lactate) or PGMEA (Propylene Glycol Monomethyl Ether Acetate) Additives - such as adhesion promoters, surfactants, dyes, antioxidants, polymerization inhibitors Unexposed resists dissolves in an alkali developer at a rate of 2 nm/sec Partially exposed resists dissolves in an alkali developer at a rate of 200 nm/sec

SENSITIVITY


page 16

500 460 380 340 300 420

Wavelength (nm)

unexposed resist

exposed resist

difference

Ab

sorb

an

ce

0.2

0.4

0.6 Resist absorbs light

in the base resin and in

the PAC. The difference

is the more important

parameter

Deep ULTRA VIOLET RADIATION


page 17

CHEMICALLY AMPLIFIED RESIST


page 18

Poly(t-butoxycarbonyloxystyrene)

with onium salt cationic photoinitiator for acid generation

THICKNESS LOG DOSE


page 19

g

Dose to clear

The higher the slope, gamma, then the smaller the difference

needs to be between exposure in areas to be cleared and areas to

leave resist. That is the required aerial image modulation is

smaller.

MODULATION


page 20

Aerial image

I(x)

Wafer

Mask m(x)

Modulation = Imax - Imin

Imax + Imin

Imax

Imin

1

0

Actual

Ideal

PROCESSING


page 21

Substrate Cleaning

Coat track •Priming if needed

•BARC coat and bake

•Spin Coating Photoresist

•Soft-Bake (PAB)

•Top Coat and bake if needed

Stepper Exposure

Develop track •Post Exposure Bake (PEB)

•Develop

•Rinse

•Hard-Bake (PDB)

Etching

Stripping

SUBSTRATE CLEAN AND PRIME


page 22

Cleaning is done with a high pressure (2000 psi) water scrub A dehydration bake is typically done on a hot plate at 250 °C for 1 min. (Wafers are clean and dry just after removing from oxide growth furnace) HMDS (hexa-methyl-di-silizane), - Adhesion promoter or primer: Are commonly applied as a liquid or vapor. HMDS attaches to remaining OH molecules releasing ammonia gas and creating an organic-like surface improving adhesion Too much HMDS is detrimental to sensitivity and adhesion.

HMDS PRIME


page 23

SPIN COATING


page 24

Most spin coating is performed at spin speeds from 3000 to 6000 RPM

for 20 to 60 seconds, producing coating uniformities to +/- 100 Å

SOFT-BAKE


page 25

The main purpose is to reduce the solvents from a level

of 20 - 30% down to 4 - 7%. Baking in a convection oven

about 20 minutes is equivalent to hot plate baking for about 1 minute.

Hot Plate

Exhaust Photoresist

wafer

Forced Air Oven

Fan 90 TO 100 °C

EXPOSURE


page 26

E = I t

where E is exposure dose in mJ/cm2

I is irradiance in mW/cm2

t is exposure time in seconds

Humidity should be 40-44% because exposed PAC

requires water to convert to carboxylic acid

POST EXPOSURE BAKE


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Post exposure bake increases speed of resist Post exposure bake reduces standing wave effects Post exposure bake is require for chemically amplified and image reversal resists (100 to 115 °C for 1 min.)

DEVELOP AND RINSE


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Develop is done in an alkali solution such as NaOH or

KOH (Metal Containing Developers) Trace quantities

of these metals can cause transistor threshold voltage

shifts.

Metal Ion Free Developers are available (TMAH)

Developer Concentration and Temperature of

Developer are the most important parameters to

control.

HARD BAKE


page 29

Hard Bake is done at or slightly above the glass transition temperature. The resist is crosslinked (and is toughened prior to plasma etch). The resist flows some as shown below. Pinholes are filled. Improves adhesion also. No flow should occur at the substrate. Photo stabilization involves applying UV radiation and heat at 110 °C for dose of 1000 mJ/cm2 then ramping up the temperature to 150-200 °C to complete the photo stabilization process.

After Develop After Hard Bake

125 to 140 °C for 1 min.

ETCHING


page 30

Isotropic Etching - etches at equal rate in all directions Wet Chemical Etching - is isotropic Anisotropic Etching - etches faster vertically than horizontally Plasma Etching (Dry Etch or Reactive Ion Etching, RIE) – is either isotropic or anisotropic depending on ion energy and chemistry of etch.

Oxide wet etched

Photoresist

STRIPPING


page 31

Oxygen Plasma Ashing

Plasma Damage is Possible to sensitive gate oxide layers

Hot Sulfuric Acid and Hydrogen Peroxide

If underlying layers are not etched by these chemicals

Organic Solvents are available

GCA 6700 G-LINE Stepper


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Microlithography ©2011 Dale E. Ewbank

page 33

ASML 5500/200

NA = 0.48 to 0.60 variable = 0.35 to 0.85 variable With Variable Kohler, or Variable Annular illumination Resolution = K1 /NA = ~ 0.35µm for NA=0.6, =0.85 Depth of Focus = k2 /(NA)2

= > 1.0 µm for NA = 0.6 i-Line Stepper = 365 nm

22 x 27 mm Field Size

STEPPER


page 34

X Y

UV-Light Source

Diffuser

Condenser Lens, NAC

Filter

Reticle Alignment Motors Thru Lens Alignment

Detector

Illumination

Reticle

5X Reduction Lens, NA O

Auto Focus

Aperture Blades

Baseline

Stage Motors

Auto Leveling X-Y Stage

Reference Mirror

Alignment Microscope

and TV

Zeeman Two Frequency

Laser Interferometer

Fiducial Marks

STEP AND SCAN


page 35

ADVANCED PROCESSES


page 36

Tri-layer Process

Bi-layer Process

Lift-off Process

Reversal Process

Dyed Resists

Anti-reflective Coatings

Contrast Enhancement Materials

Chemically Amplified Resists

Silylation Process

TRI-LAYER


page 37

Substrate with Topology

Film to be Etched

Coat with Planarizing Layer Coat with Barrier Layer

Coat with Photoresist Image Photoresist

Etch Barrier Layer Reactive Ion Etch Planarizing Layer Etch Film

SILYLATION


page 38

Film to be Etched

Expose Desired Pattern

causing exposed areas

to become polymerized

Coat with Planarizing Layer

of Photoresist

After Reactive

Ion Etch

Etch Film

Polymerized Areas

Soak in HMDS Vapor

Note: HMDS is incorporated

into the non polymerized

areas only

Reactive Ion Etch in

Oxygen coverts silicon

into SiO2, Only

polymerized areas are

etched. Silicon

containing areas form

and in-situ mask

SiO2, 10% in Photoresist

LIFT-OFF


page 39

1. Create a reverse slope or

undercut resist edge profile 2. Deposit film by evaporation

3. Chemically strip

photoresist and lift off

film, leaving film in

desired pattern

Substrate

Photoresist Film

REVERSAL PROCESS


page 40

Coat

Photoresist

Substrate

Expose

Normal Process

No Extra

Processing

Reversal Process

Post Exposure Reversal Bake

Flood Expose After Development

After Development

REFERENCES

• Microlithography Science and Technology, Second Edition, edited by Kazuaki Suzuki and

Bruce W. Smith, CRC Press, 2007.

• Introduction to Micro/Nanolithography, Second Edition, Edited by Larry F. Thompson,

C.Grant Willson and Murrae J. Bowden, ACS Professional Reference Book, American

Chemical Society, Washington, DC 1994.

• Micro/Nanolithography, David Elliott, McGraw Hill Book Company, 1986.

• www.osram.co.at/_global/pdf/Professional/Display_Optic/Semiconductor_Medical/HBO-

IC/hbo_ic.pdf, accessed 03/13/2009.

• www.mellesgriot.com, accessed 06/10/2008.

• http://www.itrs.net/reports.html, accessed 03/14/2011.


page 41

Documents

Micro/Nanolithography - People Micro/Nanolithography ©2012 Dale E. Ewbank page 3 DRAM LITHO FEATURE EXPOSURE TOOL YEAR SIZE WAVELENGTH COST 1985 1000 nm 436 nm $0.8 MILLION