Photolithography - Home | · • Standard photolithography uses UV light from mercury gas-discharge lamps that produce a broad spectrum which is filtered to select a single spectral

Providing micro and nano fabrication facilities for Australian researchers

Photolithography

UNSW ANFF NSW-Node

Providing micro and nano fabrication facilities for Australian researchers

• What is lithography – optical

• Considerations and limitations

• Future directions and alternatives

OVERVIEW

Providing micro and nano fabrication facilities for Australian researchers Providing micro and nano fabrication facilities for Australian researchers

LITHO GRAPHY

λίθος + γράφειν

'stone' + ‘to write’

WHAT IS LITHOGRAPHY?

PATTERN TRANSFER

http://en.wiktionary.org/wiki/%CE%BB%CE%AF%CE%B8%CE%BF%CF%82

http://en.wiktionary.org/wiki/%CE%B3%CF%81%CE%AC%CF%86%CE%B5%CE%B9%CE%BD


• For industry (e.g. chip manufacturing), photolithography has been

advanced for high volume, low cost fabrication:

• For research, standard photolithography is used for repeated, microscale

structures with sizes from 1-2 to 1000’s of microns with a standard UV lamp.

• For research, we go to EBL or laser patterning for smaller features which require

iterative development or rapid prototyping...

PHOTOLITHOGRAPHY – ROLE IN INDUSTRY

• 10 µm – 1971

• 3 µm – 1975

• 1.5 µm – 1982

• 1 µm – 1985

• 800 nm – 1989

• 600 nm – 1994

• 350 nm – 1995

• 250 nm – 1998

• 180 nm – 1999

• 130 nm– 2000

• 90 nm – 2002

• 65 nm – 2006

• 45 nm – 2008 (DOUBLE EXPOSURE)

• 32 nm – 2010 (Double + IL)

• 22 nm – 2011 (+ Introduction of 3D tri-gate)

• 16 nm – 2013 (Transition to Nanoelectronics)?

• 11 nm – 2015 (Nanoelectronics)?

INDUSTRY HALF-PITCH SIZES


WHAT IS PHOTOLITHOGRAPHY?

PATTERN TRANSFER WITH LIGHT


substrate

photomask

photoresist

Example with POSITIVE photoresist

EXAMPLE


UV light

substrate

photomask

photoresist


EXAMPLE


substrate

photoresist

developer


EXAMPLE


substrate

photoresist

ADDITIVE PROCESS

Metal deposition

Lift-off solvent


EXAMPLE


substrate

photoresist

SUBTRACTIVE PROCESS

etch process


EXAMPLE


PHOTOLITHOGRAPHY – TYPICAL PROCESS

FLOW AND PARAMETERS

Process

Flow


PHOTOLITHOGRAPHY – PRACTICAL

CONSIDERATIONS

EXPOSURE

• Different photoresists have different sensitivities to different wavelengths

of radiation

• Dose per unit volume is usually constant, but resist thickness and

differences in the exposure process can alter the dose required for good

pattern transfer. E.g. a reflective layer beneath the resist (metallic thin

film), interference effects from mask geometry in thicker films

Desired pattern

(i.e. positive resist/dark part of mask)

Over-exposed pattern Under-exposed pattern



CONSIDERATIONS

EXPOSURE

• Light is scattered and diffracted at the pattern edges

• Overexposed – too much resist is exposed

• Underexposed – too little resist is exposed

(positive resist)

• Uniformity across the wafer is affected by substrate topography (i.e. Bad

substrate to begin with, or underlying high aspect ratio features

Desired pattern Over-exposed pattern Under-exposed pattern

(i.e. positive resist/dark part of mask)



CONSIDERATIONS

Sloping walls

Deposited film is

mostly ripped off

Undercut walls

Pattern is deposited

correctly

Vertical walls

Some sidewall coating

causes thin rips and

tails (may short out)

• Resist pattern profile is important

• Desired shape depends on the type of process the mask is for

• In this example: lift-off of a deposited metallic thin film (i.e. metal gates)



CONSIDERATIONS

Too thick

Not enough clearance

below the mask opening

Correct thickness

Pattern is deposited

correctly

• Thickness of the resist w.r.t. Patterns (aspect ratio) is also important

• Too thick and there may not be enough clearance below the mask opening

• Too thin (similar to metal layer thickness) and the features merge with the mask

Too thin

Features merge with

metal ontop of mask


REGISTRATION

Features on wafer

Features on mask

Mask over wafer

Correct alignment


PHOTOLITHOGRAPHY CONSIDERATIONS

AND LIMITATIONS

• What kind of mask?

What projection method?

Contact or proximity masks,

projection/stepper

• Masks must be designed

and manufactured

(e.g. chrome on quartz)

• Sample or substrate must be

compatible with all the steps in

the process flow (e.g. application

of resists, various solvents and

developers, temperature processes, metals)

• There may be many steps required (e.g. modern CMOS fabrication requires

around 50 photolithography steps)

CONTACT PROXIMITY

PROJECTION


FACILITIES AVAILABLE

• Cleanroom (no dust or organic/inorganic contaminants,

controlled humidity and temperature)

• Fume cupboard (spinner, wet chemistry bench)

• Ovens/hotplates

• Mask aligner (alignment and UV exposure)

• Suss MA6: up to 6” wafers, 1µm resolution

and BSA

• Quintel Q6000: up to 4” wafers, 2µm

resolution

• Plasma asher (for descum)


Where can this be done? • ACT Node

• WA Node

• QLD Node

• NSW Node: UNSW

• SA Node

• OptoFab Node

• VIC Node

see www.anff.org.au

FACILITIES AVAILABLE

http://www.anff.org.au/


FUTURE DIRECTIONS AND

ALTERNATIVES

• Standard photolithography uses UV light from mercury gas-discharge lamps that produce a

broad spectrum which is filtered to select a single spectral line, e.g. 436 nm or 365 nm

• Advanced lithography uses "deep ultraviolet“ (DUV)

produced by excimer lasers e.g. krypton fluoride

produces a 248-nm spectral line, and argon fluoride,

a 193-nm line.

• Further advancements produce feature sizes below

50 nm using 193 nm DUV and liquid immersion techniques

- use a liquid with refractive index greater than 1

• One major (but complementary) alternative to photolithography:

direct write lithography

– Expose each feature in the pattern by writing it with a focussed beam rather than by a

flood exposure through a patterned mask

Documents

Photolithography - Home | · • Standard photolithography uses UV light from mercury gas-discharge lamps that produce a broad spectrum which is filtered to select a single spectral