Microlithography

ESS4810 LectureFall 2010

Moore’s Law

• The observation made in 1965 that the number of transistors per square inch on integrated circuits had doubled every year since ICs were invented

• Moore predicted that this trend would continue for the foreseeable future

• In subsequent years, the pace slowed down a bit, but data density has doubled approximately every 18 months

Moore’s Law

Moore’s Law

1.5 mm

Microlithography

• Photolithography– Process– Resolution– Depth of focus

• Photoresist• Lift-off process• Other processes

Photolithography

• Transfer patterns from mask to wafer surface by light

• From mask to photoresist– Exposure– Development

• From photoresist to wafer surface– Wet or dry etching

Steps of Photolithography

• 1 Clean wafer• 2 Coat with photoresist• 3 Soft bake photoresist• 4 Align mask with wafer• 5 Expose pattern on photoresist• 6 Develop photoresist• 7 Hard bake photoresist• 8 Etch pattern on wafer• 9 Remove photoresist

Photoresist Application

• Photoresist (light-sensitive)• Clean and dry surface to ensure

good photoresist adhesion• Adhesion promoter• Applied in liquid form• The wafer is held on a vacuum

chuck and then spun at high speed to produce a thin uniformlayer

Adhesion Promoter

• Hexamethyldisilazane• Vapor priming• Liquid priming• Monolayer• One side bonds with

wafer surface• The other side bonds

with photoresist

HMDS

Baking

• Soft– 80 ~ 90 ºC for 10 to 30 minutes– Improve adhesion and remove

solvent from photoresist• Hard

– 120 ~ 180 ºC for 20 to 30 minutes– Harden the photoresist and improve

adhesion to substrate

Photoresist Removal

• Requirements– Complete removal without residues – No undesired etching

• Wet– Liquid resist strippers

• Dry– Plasma– Oxidizing (burning, ashing)

Pattern Generation

• Design - Mask - Wafer

Contact Printing

• Resolution (R)– 0.5 μm

• Mask plate is easily damaged or accumulates defects

Photoresist

Mask

Proximity Printing

• Resolution (R)– 1 μm– k(λg)1/2

Photoresist

Maskg

Projection Printing• Resolution (R)

– 0.2 μm (deep UV)• Trade-off

– Optics complicated and expensive

Photoresist

Mask

Lens

Image Comparison

Diffraction

• Diffraction can be thought of simply as the “bending” of light when it passes through an aperture

• The light that passes through the aperture carries with it the information on the size and shape of that aperture

• This information spreads out in space because of diffraction and it must all be collected to convey perfect information about the aperture to the resist on the wafer

Diffraction

• Because of its finite size, the focusing lens collects only part of the total diffraction pattern

• The light diffracted to wider anglescarries the information about the finer details of the aperture

• It is those details that are lost first when a lens of finite size is used to collect and focus the light

Image Degradation by Lens

Resolution

• How close together can A and B be and still be resolved in the image plane?

• The central maximums of each point image lie at the first minima of the adjacent point image

Resolution

• Numerical aperture (NA) of lens• K1: a constant between 0.25 and 1,

depending on optics, resist, and process latitude

Definition of Line & Space

Bragg Condition

X

X

Depth of Focus

Resolution vs DOF

• Requirements (1) and (2) need a compromise between λ and NA !

Excimer Laser Stepper

Photon Sources

Photon Sources

Photon Sources

Positive photoresist Negative photoresist

Photoresist

• Positive– Polymer (MW~5000)– Photoactive inhibitor (20%)– Volatile solvents– Inhibitor looses N2 => alkali soluble

acid– Develops by “etching” - no swelling

Positive Photoresist

Photoresist

• Negative– Polymer (MW~65000)– Light sensitive additive promotes

cross-linking– Volatile solvents– Light breaks N-N => crosslink chains– Sensitive, hard, swelling during

develop

Negative Photoresist

Comparison

• Positive– Higher resolution– Aqueous-based solvents– More popular

• Negative– More sensitive– Less expensive– Better chemical resistance– More tolerant of developing conditions

Overlay Errors

Thermal run-in/run-out Errors

• change of mask and wafer temp.

• coefficient of thermal expansion of mask and Si

Characterization

Example

• Center has only translation error

Example

• Run-out error: 0.2• Rotational error: 0.5, CCW

Lift-Off Process

• Allows definition of pattern on the wafer surface without etching

• Hard to etch metals• Lifted off in

selected areas by dissolving underlying resist

X-ray Lithography

X-ray Lithography

Mask Making• the most complicated and

challenging part of x-ray lithography

Electron Beam Lithography

Angstroms for V in Volts

30 kV e-beam> λ = 0.07 Å

Scanning Methods

Proximity Effect

• Electron scattering• Compensation

required

Nanoimprint Lithography

Nanoimprint Lithography

AFM Lithography

AFM Lithography

Immersion Lithography• Using an immersion fluid between the wafer and

the lens substantially changes the light path• First, it enhances depth of focus (DOF) for a

given Numerical Aperture• Second, immersion allows lens designs with

Numerical Apertures significantly larger than 1.0, therefore allowing improved resolution

nwater = 1.33nair = 1The higher-index medium couples

higher spatial frequency to the resist

NA = n sinθ

Smaller angle in the coupling medium is less sensitive to longitudinal displacement of the wafer

2NADOF

λ∝

Immersion Lithography