Photolithography – II ( Part 2 ) - NTNUfolk.ntnu.no/jonathrg/fag/TFE4180/slides/Ch14 Photolitography II... · 1 TFE4180 Semiconductor Manufacturing Technology, Photolithography

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TFE4180 Semiconductor Manufacturing Technology, Photolithography - II

Photolithography – II ( Part 2 )Chapter 14 : Semiconductor Manufacturing Technology by M. Quirk & J. Serda

Saroj Kumar Patra,Department of Electronics and Telecommunication,

Norwegian University of Science and Technology ( NTNU )

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Objectives of this Lecture

1. State and explain the critical aspects of optics for optical lithography.– Reflection of Light– Refraction of Light– Lens– Diffraction– Numerical Aperture, NA– Antireflective Coating

2. Explain resolution, describe its critical parameters, and discuss how it is calculated.

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Ten steps of Photolithography

10) Develop inspect7) Post-exposure bake (PEB)

8) Develop 9) Hard bake

UV Light

Mask

6) Alignmentand Exposure

Resist

4) Spin coat 5) Soft bake1-3) Vapor prime

HMDS

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Laws of Reflection

i rIncident light Reflected light

Law of Reflection: i r

The angle of incidence of a light wavefront with a plane mirror is equal to the angle of reflection.

Figure 14.11 Quirk & Serda

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Application of Mirrors

Used with permission from Canon USA

Mask

Flat mirror

Ellipsoidal mirror

Flat mirror

Illuminator for a simple aligner


Important for uniform illumination of the mask

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Refraction of Light based on two mediums

• Snell’s Law: sin i = n sin r

• Index of refraction, n = sin i / sin r

air (n 1.0)

glass (n 1.5)

fast medium

slow medium

air (n 1.0)

glass (n 1.5)

fast medium

slow medium


Speed of light: c = c0 / n

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Absolute Index of Refraction for selected materials

Material Index of Refraction (n)

Air 1.000293

Water 1.33

Fused Silica (AmorphousQuartz)

1.458

Diamond 2.419

Table 14.4 Quirk & Serda

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Converging Lens with Focal Point

OF F´ S´S

f

2f f = focal lengthF = focal pointS = 2fO = origin, center of lens

Real image

Object


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Diverging Lens with Focal Point

OF F´ S´S

Virtual image

Object

f = focal lengthF = focal pointS = 2fO = origin, center of lens


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Optical System of Lenses

Mercury lamp

Lamp position knob

Lamp monitor

Ellipsoidal mirror

Shutter

Fly’s eye lens

Flat mirror

Masking unitMirror

MirrorCollimator lens

Condenser lensCondenser lens

Optical filter

Fiber optics

Reticle

Reticle stage (X, Y, )

Projection optics

Optical focus sensorInterferometer mirror

X-drive motor

Y-drive motor

-Z drive stage

Vacuum chuck

Wafer stage assembly

Light sensor

Used with permission from Canon U.S.A., FPA-2000 i1 exposure systemFigure 14.14

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Lens Material

365 nm UV: glass (traditionally)

248 nm DUV: fused silica (less light absorption at DUV wavelengths)

193 nm DUV and 157 nm VUV: calcium fluoride (CaF2) which is more transparent at these wavelengths then fused silica

Absorption => loss in exposure power and induces heat in the optics, which leads to refractive indexchanges and imaging problems

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Laser-Induced Lens Compaction

=> reduced image quality

Compacted area of lens


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Interference Pattern from Light Diffraction at Small Opening

• Light travels in straight lines.• Diffraction occurs when light hits edges of objects.• Diffraction bands, or interference patterns, occur when light waves

pass through narrow slits.

Diffraction bands


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Diffraction in a Reticle Pattern

Slit

Diffracted light rays

Plane light wave


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Lens Capturing Diffracted Light

UV

0

12

3

4

12

3

4

Lens

Quartz

Chrome Diffraction patterns

Mask


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Numerical Aperture (NA)

For a lens, the NA is a measure of how much diffracted light the lens can accept and image by converging the diffracted light to a single point.

NA = (n) sin θm ≈ (n) (radius of lens) / (focal length of lens)

where, n = index of refraction of the image medium (n ≈ 1 for air)θm = angle between the optical principal axis and the

marginal ray at the edge of the lens

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Effect of Numerical Aperture on Imaging


Lens NA

Pinhole masks

Image results

Diffracted light

Good

Bad

Poor

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Typical NA Values for Photolithography Tools

Type of Equipment NA Value

Scanning Projection Aligner with mirrors (1970s technology) 0.25

Step-and-Repeat 0.60 – 0.68

Step-and-Scan 0.60 – 0.68

Table 14.5 Quirk & Serda

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Photoresist Reflective Notching Due to Light Reflections

Polysilicon

Substrate

STISTI

UV exposure light

Mask

Exposed photoresist

Unexposed photoresist

Notched photoresist

Edgediffraction

Surfacereflection


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Light Suppression (up to 99 %) with Bottom Antireflective Coating (BARC)

BARCPolysilicon

Substrate

STISTI

UV exposure light

Mask

Exposed photoresist

Unexposed photoresist


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Incident and Reflected Light Wave Interference in Photoresist

Standing waves cause nonuniform exposure along the thickness of the photoresist film.

Incident wave

Reflected wave

PhotoresistFilm

Substrate


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Effect of Standing Waves in Photoresist

Photograph courtesy of the Willson Research Group, University of Texas at Austin

Photo 14.1 Quirk & Serda

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Antireflective Coating to Prevent Standing Waves

The use of antireflective coatings, dyes, and filters can help prevent

interference.

Incident wave Antireflective coating 200 – 2000 Å

PhotoresistFilm

Substrate


≈ 1 μm

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BARC Phase-Shift Cancellation of Light

(A) Incident light

Photoresist

BARC (TiN)Aluminum

C and D cancel due to phase difference

(B) Top surface reflection

(C)(D)


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Top Antireflective Coating (TARC)

Incident light

Photoresist

Resist-substrate reflections

Substrate

Incident light

Photoresist

Substrate reflection

Substrate

Top antireflective coating absorbs substrate reflections.


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Antireflective Coatings (ARC)

• Organic ARC reduces reflection by absorbing light• Inorganic ARC (e.g. TiN) work by phase-shift cancellation• Organic ARC easier to remove than inorganic ARC (sometimes

left to become part of the device)

• BARC in general more effective than TARC

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Optical Lithography

Resolution• Calculating Resolution• Depth of Focus• Resolution Versus Depth of Focus

– Surface Planarity

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Resolution of Features

2.0

1.0

0.5

0.10.25

The dimensions of linewidths and spaces must be equal. As feature sizes decrease, it is more difficult to separate features from each other.


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Calculating Resolution for a given , NA and k

Lens, NA

Wafer

Mask

Illuminator,

R

k = 0.6

R365 nm 0.45 486 nm365 nm 0.60 365 nm193 nm 0.45 257 nm193 nm 0.60 193 nm

i-line

DUV

k NAR =


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Depth of Focus (DOF)

+

-

Photoresist

Film

Depth of focusCenter of focusCenter of focus

Lens


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Resolution Versus Depth of Focus for Varying NA

2(NA)2DOF =

Photoresist

Film

Depth of focusDepth of focusCenter of focus

++

--Lens, NA

Wafer

Mask

Illuminator,

DOF

R DOF365 nm 0.45 486 nm 901 nm365 nm 0.60 365 nm 507 nm193 nm 0.45 257 nm 476 nm193 nm 0.60 193 nm 268 nm

i-line

DUV


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Photolithography – II ( Part 2 ) - NTNUfolk.ntnu.no/jonathrg/fag/TFE4180/slides/Ch14 Photolitography II... · 1 TFE4180 Semiconductor Manufacturing Technology, Photolithography