Lecture 18 –Optical Lithography 3 - Resist

EECS 598-002 Winter 2006Nanophotonics and Nano-scale Fabrication

P.C.Ku

2EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku

Photoresist

The most commonly used photoresist nowadays is a two-component system which consists of the novolac resin and the diazonaphthoquinone photoactive compound (PAC; a dissolution inhibitor).

novalac PAC

1. Inert to photochemistry2. For film-forming3. For adhesion4. For chemical resistance5. For thermal resistance

1. Responsible for photochemistry2. Quantum efficiency:

# of molecules transformed# of photons absorbed

φ =

Not the total intensity!

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Chemically amplified DUV resist

Since the quantum efficiency of a regular resist is ~ 100 fold smaller at the DUV wavelength, chemical amplification is used.Instead of optically destroying the dissolution inhibitors, chemically amplified DUV resist creates acid by photochemical reactions. Acid then destroys the inhibitor at an elevated temperature (PEB). Each acid molecule can trigger the catalysis event to destroy several inhibitors

amplification.

Ref: H. Ito, “Chemical amplification resists,” IBM J. R&D, 41 (1997)

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Resist process overviewAdhesion promotionor BARC coating

Resist coating

Softbake (SB)

Exposure

Post-exposure bake (PEB)

Resist development

Rinse and dry

Metrology (CD & REG)

bad good

EtchResist/BARC removal

TEBR

OEBR

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Resist process environment control

Resist process happens in a controlled environment (track).

From TEL

Exposuremodule

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Adhesion promotion

The most common adhesion promoter is HMDS (hexamethylenedisiloxane). Applied in vapor phase (100-160。C.

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Spin coating

KCtβ γ

α

ηω

=Resist thickness

ω = spin angular speedC = resist concentrationη = molecular weight

1. Low spin speed results in a thicker resist toward the edgeof the wafer

2. High spin speed results in opposite thickness profile as well as random variation in thickness due to thermal effects.

3. Best spinning speed 2000 ~ 4000 rpm.

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Edge bead removal (EBR)

The resist on the edge of the wafer is often removed (EBR) to reduce potential contamination sources and help the vacuum chuck to hold the wafer.TEBR: use chemicals to remove the edge beadOEBR: use optical source to expose the edge bead

resist

Thickness variation and spill over to the back

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Softbake (SB)

Purpose: To remove the residual solvent and anneal the stress. Note some resists do require some solvent to remain in order to improve the photochemical reaction speed.

From Introduction toMicrolithography

10EECS 598-002 Nanophotonics and Nanoscale Fabrication by P.C.Ku

Exposure (Dill’s model)

Concentration of dissolution inhibitor = M(z,t)

( , )( , )

( , ) ( , ) ( , )

z

AM z t BI z t eM z t I z t M z t C

t

α

α−

= +

=∂

= −∂

As M(z,t) reduces, the absorptionwill be bleached.

It is very important for bleaching to happen. Otherwise,there will be no light that can reach the bottom of the resist to clear. This is the main reason that photoresistused for G- or I-line does not work well at the DUVwavelength.

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Resist characteristics

log(dose)

Normalized resist thicknessafter development

1

0

Dose to clear

The slops determinesthe contrast and dosesensitivity.

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Swing curve

Swing curve plots the “dose to clear” vs film (e.g. resist) thickness.

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Post exposure bake (PEB)

PEB is different from the “hardbake” before an etching process.PEB is used to induce the diffusion of photogeneratedcompound in order to smooth out the interference effect on the resist profile.

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PEB temperature vs CD

From Future-Fab International, issue 12.

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PEB delay vs resist profile

Reactions with airborne molecules, such as ammonia or amines,neutralize acid near the top of the resist film, forming a mushroom cap.

Reactions with certain substrates, such as TiN, neutralize acid nearthe bottom of the resist, forming a foot.

From Future-Fab International,issue 12.

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Development time vs resist profile

From Introduction toMicrolithography

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-0.80 -0.40 -0.00 0.40 0.80

0.000

0.100

0.200

0.300

0.400

Focus Offset (µm)

Imag

e W

idth

(µm

)

Long Exposure

Short Exposure

Focus-Exposure Matrix (FEM)

- Focus window- Exposure latitude

Bossung plot:

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Metrology

Registration: overlay measurementCD

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Resist profile by top-down measurement

LER (line edge roughness)

Sloped profile

X-section

Top-down SEM