Yau - 1SMT

Photolithography: Alignment and Exposure

Photolithography: Alignment and Exposure

Yau - 2SMT

ObjectivesAfter studying the material in this chapter, you will be able to:

1. Explain the purpose of alignment and exposure in photolithography.

2. Describe the properties of light and exposure sources important for optical lithography.

3. State and explain the critical aspects of optics for optical lithography.

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

5. Discuss each of the five equipment eras for alignment and exposure.

6. Describe reticles, explain how they are manufactured and discuss their use in microlithography.

7. Discuss the optical enhancement techniques for sub-wavelength lithography.

8. Explain how alignment is achieved in lithography.

Yau - 3SMT

Eight Basic Steps of Photolithography

Step ChapterCovered

1. Vapor prime 132. Spin coat 133. Soft bake 134. Alignment and exposure 145. Post-exposure bake 156. Develop 157. Hard bake 158. Develop inspect 15

Table 14.1

Yau - 4SMT

Three Functions of the Wafer Stepper

1. Focus and align the quartz plate reticle(that has the patterns) to the wafer surface.

2. Reproduce a high-resolution reticle image on the wafer through exposure ofphotoresist.

3. Produce an adequate quantity of acceptable wafers per unit time to meet production requirements.

Yau - 5SMT

Reticle Pattern Transfer to Resist

Single field exposure, includes: focus, align, expose, step, and repeat process

UV light source

Reticle (may contain one or more die in the reticle field)

Shutter

Wafer stage controls position of wafer in X, Y, Z, θ)

Projection lens (reduces the size of reticle field for presentation to the wafer surface)

Shutter is closed during focus and alignment and removed during wafer exposure

Alignment laser

Figure 14.1

Yau - 6SMT

Layout and Dimensions of Reticle Patterns

4) Poly gate etch1) STI etch 2) P-well implant 3) N-well implant

8) Metal etch5) N+ S/D implant 6) P+ S/D implant 7) Oxide contact etch

Top view

1

23

45

7

6

8 Cross section

Resulting layers

Figure 14.2

Yau - 7SMT

Optical Lithography

Light• Interference of Light Waves

– Optical Filters

• Electromagnetic Spectrum

Yau - 8SMT

Light Wavelength and Frequency

λ

λ = vf

Laser

v = velocity of light, 3 ’ 108 m/secf = frequency in Hertz (cycles per second)λ = wavelength, the physical length of one

cycle of a frequency, expressed in meters

Figure 14.3

Yau - 9SMT

Wave Interference

A

B

A+B

Waves in phase Waves out of phase

Constructive Destructive

Figure 14.4

Yau - 10SMT

Optical Filtration

Secondary reflections (interference)

Coating 1 (non-reflecting)

Coating 3

Glass

Coating 2

Reflected wavelengths

Transmitted wavelength

Broadband light

Figure 14.5

Yau - 11SMT

Ultraviolet Spectrum

λ (nm)

7004 550 600 65050045040035030025020015010050

Ultraviolet spectrum Visible spectrum

Mercury lampExcimer laser

Photolithography light sources

ghi365 40524819313 436157126

Violet RedBlue Green Yellow OrangeMid-UVEUV DUVVUV

Figure 14.6

Yau - 12SMT

Optical Lithography

Exposure Sources• Mercury Arc Lamp• Excimer Laser

– Spatial Coherence

• Exposure Control

Yau - 13SMT

Emission Spectrum of Typical High Pressure Mercury Arc Lamp

120

100

80

60

40

20

0

200 300 400 500 600Wavelength (nm)

Rel

ativ

e In

tens

ity (%

) h-line405 nm

g-line436 nm

i-line365 nm

DUV248 nm

Emission spectrum of high-intensity mercury lamp

Mercury lamp spectrum used with permission from USHIO Specialty Lighting Products

Figure 14.7

Yau - 14SMT

Mercury Arc Lamp Intensity Peaks

UV Light Wavelength (nm) Descriptor CD Resolution (µm)

436 g-line 0.5405 h-line 0.4365 i-line 0.35248 Deep UV

(DUV)0.25

Table 14.2

Yau - 15SMT

Spectral Emission Intensity of 248 nm Excimer Laser vs. Mercury Lamp

100

80

60

40

20

0

Rel

ativ

e In

tens

ity (%

)

KrF laser

280210 240 260220

Wavelength (nm)

Hg lamp

Figure 14.8

Yau - 16SMT

Excessive Resist Absorption of Incident Light

Photoresist (after develop)

Substrate

Sloping profile

Figure 14.9

Yau - 17SMT

Excimer Laser Sources for Semiconductor Photolithography

Material Wavelength (nm)

Max.Output

(mJ/pulse)

Frequency(pulses/sec)

PulseLength (ns)

CD Resolution(µm)

KrF 248 300 – 1500 500 25 ≤ 0.25

ArF 193 175 – 300 400 15 ≤ 0.18

F2 157 6 10 20 ≤ 0.15

Table 14.3

Yau - 18SMT

Spatial Coherence

Incoherent light sourceof a single wavelength

Slit

Coherent cylindrical wave front

Two coherent cylindrical wave fronts

Interference patterns

Two slitsclosely spaced

Black box illuminator

Figure 14.10

Yau - 19SMT

Optical Lithography

Optics• Reflection of Light• Refraction of Light• Lens• Diffraction• Numerical Aperture, NA• Antireflective Coating

Yau - 20SMT

Law of Reflection

θi θrIncident light Reflected light

Law of Reflection: θi = θr

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

Figure 14.11

Yau - 21SMT

Application of Mirrors

Used with permission from Canon USA

Mask

Flat mirror

Ellipsoidal mirror

Flat mirror

Illuminator for a simple aligner

Figure 14.12

Yau - 22SMT

Refraction of Light Based on Two Mediums

• Snell’s Law: sin θi = n sin θr

• Index of refraction, n = sin θi / sin θr

θθair (n ≅ 1.0)

glass (n = 1.5)

fast medium

slow medium

θθ

air (n ≅ 1.0)

glass (n = 1.5)

fast medium

slow medium

Figure 14.13

Yau - 23SMT

Absolute Index of Refraction for Select Materials

Material Index of Refraction (n)

Air 1.000293

Water 1.33

Fused Silica (AmorphousQuartz)

1.458

Diamond 2.419

Table 14.4

Yau - 24SMT

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, θ)

Projection optics

Optical focus sensor

Interferometer mirrorX-drive motor

Y-drive motor

θ-Z drive stage

Vacuum chuck

Wafer stage assembly

Light sensorIll

umin

ator a

ssem

bly

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

Yau - 25SMT

Converging Lens with Focal Point

OF F’ S’S

f

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

Real image

Object

Figure 14.15

Yau - 26SMT

Diverging Lens with Focal Point

OF F’ S’S

Virtual image

Object

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

Figure 14.16

Yau - 27SMT

Laser-Induced Lens Compaction

Compacted area of lens

Figure 14.17

Yau - 28SMT

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

Figure 14.18

Yau - 29SMT

Diffraction in a Reticle Pattern

Slit

Diffracted light rays

Plane light wave

Figure 14.19

Yau - 30SMT

Lens Capturing Diffracted Light

UV

0

12

3

4

12

3

4

Lens

Quartz

Chrome Diffraction patterns

Mask

Figure 14.20

Yau - 31SMT

Effect of Numerical Aperture on Imaging

Exposure light

Lens NA

Pinhole masks

Image results

Diffracted light

Good

Bad

Poor

Figure 14.21

Yau - 32SMT

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

Yau - 33SMT

Photoresist Reflective Notching Due to Light Reflections

Polysilicon

Substrate

STISTI

UV exposure light

Mask

Exposed photoresist

Unexposed photoresist

Notched photoresist

Edgediffraction

Surfacereflection

Figure 14.22

Yau - 34SMT

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

Figure 14.23

Yau - 35SMT

Effect of Standing Waves in Photoresist

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

Photo 14.1

Yau - 36SMT

Antireflective Coating to Prevent Standing Waves

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

Incident wave Antireflective coating

PhotoresistFilm

Substrate

Figure 14.24

Yau - 37SMT

Light Suppression with Bottom Antireflective Coating

BARCPolysilicon

Substrate

STISTI

UV exposure light

Mask

Exposed photoresist

Unexposed photoresist

Figure 14.25

Yau - 38SMT

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)

Figure 14.26

Yau - 39SMT

Top Antireflective Coating

Incident light

Photoresist

Resist-substrate reflections

Substrate

Incident light

Photoresist

Substrate reflection

Substrate

Top antireflective coating absorbs substrate reflections.

Figure 14.27

Yau - 40SMT

Optical Lithography

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

– Surface Planarity

Yau - 41SMT

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.

Figure 14.28

Yau - 42SMT

Calculating Resolution for a given λ, NA and k

Lens, NA

Wafer

Mask

Illuminator, λ

R

k = 0.6

λ ΝΑ 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 λNAR =

Figure 14.29

Yau - 43SMT

Depth of Focus (DOF)

+

-

Photoresist

Film

Depth of focusCenter of focusCenter of focus

Lens

Figure 14.30

Yau - 44SMT

Resolution Versus Depth of Focus for Varying NA

λ 2(NA)2DOF =

Photoresist

Film

Depth of focusDepth of focusCenter of focus

++

--Lens, NA

Wafer

Mask

Illuminator, λ

DOF

λ ΝΑ 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

Figure 14.31

Yau - 45SMT

Photolithograhy Equipment

• Contact Aligner• Proximity Aligner• Scanning Projection Aligner (scanner)• Step-and-Repeat Aligner (stepper)• Step-and-Scan System

Yau - 46SMT

Contact/Proximity Aligner System

Illuminator

Alignment scope (split

vision)Mask

Wafer

Vacuum chuck

Mask stage (X, Y , Z , θ)

Wafer stage (X, Y, Z, θ)

Mercury arc lamp

Used with permission from Canon USA, Figure 14.32

Yau - 47SMT

Edge Diffraction and Surface Reflectivity on Proximity Aligner

UV

Mask

Diffraction of light on edges results in reflections from underside of mask causing undesirable resist exposure.

UV exposure light

Substrate

Resist

Diffracted and reflected light

Gap

Mask

Substrate

Figure 14.33

Yau - 48SMT

Scanning Projection Aligner

Redrawn and used with permission from Silicon Valley Group Lithography

Mask

Wafer

Mercury arc lamp

Illuminator assembly

Projection optics assembly

Scan direction

Exposure light(narrow slit of UV

gradually scans entire mask field onto wafer)

Figure 14.34

Yau - 49SMT

Step-and-Repeat Aligner (Stepper)

Used with permission from Canon USA, FPA-3000 i5 (original drawing by FG2, Austin, TX)

Figure 14.35

Yau - 50SMT

Stepper Exposure Field

UV light

Reticle field size20 mm × 15mm,4 die per field

5:1 reduction lens

Wafer

Image exposure on wafer 1/5 of reticle field4 mm × 3 mm,4 die per exposure

Serpentine stepping pattern

Figure 14.36

Yau - 51SMT

Wafer Exposure Field for Step-and-Scan

5:1 lens

UVUV

Step and ScanImage Field

Scan

StepperImage Field

(single exposure)

4:1 lens

ReticleReticle Scan

Scan

Wafer WaferStepping direction

Redrawn and used with permission from ASM LithographyFigure 14.37

Yau - 52SMT

Step and Scan Exposure SystemIlluminator optics

Beam line

Excimer laser (193 nm ArF )

Operator console

4:1 Reduction lensNA = 0.45 to 　

0.6

Wafer transport system

Reticle stage

Auto-alignment system

Wafer stage

Reticle library (SMIF pod interface)

Used with permission from ASML, PAS 5500/900Figure 14.38

Yau - 53SMT

Reticles

• Comparison of Reticle Versus Mask• Reticle Materials• Reticle Reduction and Size• Reticle Fabrication• Sources of Reticle Damage

Yau - 54SMT

Comparison of Reticle Versus Mask

ParameterReticle

(Pattern for Step-and-RepeatExposure)

Mask(Pattern for 1:1 Mask-Wafer Transfer)

Critical Dimension

Easier to pattern submicrondimensions on wafer due to largerpattern size on reticle (e.g., 4:1,5:1).

Difficult to pattern submicron dimensions onmask and wafer without reduction optics.

Exposure Field Small exposure field that requiresstep-and-repeat process. Exposure field is entire wafer.

Mask TechnologyOptical reduction permits largerreticle dimensions – easier toprint.

Mask has same critical dimensions as wafer –more difficult to print.

Throughput Requires sophisticated automationto step-and-repeat across wafer.

Potentially higher (not always true if equipmentis not automated).

Die alignment & focus Adjusts for individual diealignment & focus.

Global wafer alignment, but no individual diealignment & focus.

Defect density

Improved yield but no reticledefect permitted. Reticle defectsare repeated for each fieldexposure.

Defects are not repeated multiple times on awafer.

Surface flatness

Stepper compensates during initialglobal pre-alignmentmeasurements or during die-by-die exposures.

No compensation, except for overall global focusand alignment.

Table 14.6

Yau - 55SMT

Photolithography Reticle

Photograph courtesy of Advanced Micro Devices

Photo 14.2

Yau - 56SMT

Comparison of Reticle Reduction Versus Exposure Field

Field size on reticle

Projection lens

Exposure field on wafer

Lens Type 10:1 5:1 4:1 1:1

Reticle FieldSize (mm)

100 100 100 100 100 100 30 30

Exposure Fieldon Wafer (mm)

10 10 20 20 25 25 30 30

Die per Field ofExposure(assume 5mm 5mm die size)

4 16 25 36

Figure 14.39

Yau - 57SMT

Principles of Electron Beam Lithography

Work chamber, load chamber,vibration isolation, ion pump, and vacuum system

Work Station

Transfer and drive

Stage position control

Height deflection

and dynamic

correction

Vacuum control module

TFE electron source control

Electron beam controlServo control

Operator console

9-track magnetic tape drive

Control computer

Super flash HTM

Printer/Plotter

Electronics Console

Data direct computer

Height detection assembly

Work table

Beam control and deflection

TFE electron beam column

Automatic loading chamberWork stage

Dynamic corrections

Utility Console

Cassette clamping control

Temperature control

Coolant flow

control

Air and nitrogen control

Water chiller

Backing pump

Roughing pump

Used with permission from Etec Systems, Inc., MEBES 4500 SystemsFigure 14.40

Yau - 58SMT

Pellicle on a Reticle

The particle on the pellicle surface is outside of optical focal range.

Antireflective coatings Pellicle film

Chrome patternDepth of focus

Mask material

Reticle

Pellicle film

Frame

Chrome pattern

Figure 14.41

Yau - 59SMT

Optical Enhancement Techniques

• Phase-Shift Mask (PSM)• Optical Proximity Correction (OPC)• Off-Axis Illumination • Bias

Yau - 60SMT

Phase-Shifting Mask

b) APSMa) BIM c) Rim PSM

Chrome Absortive phase shifters

Rim phase shifters

Blockers

-1

+10

-1

+10

+10

Intensity on wafer

Electric field on

mask

Electric field on

wafer

Reprinted from the January 1992 edition of Solid State Technology, copyright 1992 by PennWell Publishing Company

Figure 14.42

Yau - 61SMT

Optical Proximity Effects

Rounded corners

Nonuniform CDs Shortened lines

Figure 14.43

Yau - 62SMT

Off-Axis Illumination

A

B- A+

B+B+A-A-

BOff-axis illumination

(b)

Conventional illumination (on-axis)

- Order diffraction

+ Order diffraction

(a)

Pinhole mask

Projection optics

Wafer

Figure 14.44

Yau - 63SMT

Serifs to Minimize Rounding of Contact Corners

(b) Corrected with feature biasing

(a) Uncorrected design (c) Feature assisting technique

Figure 14.45

Yau - 64SMT

Alignment

• Baseline Compensation• Overlay Accuracy• Alignment Marks• Types of Alignment

Yau - 65SMT

Overlay Budget

-X +X

+Y

-Y∆X

−∆Y

Shift in registration

-X +X

+Y

-Y

Wafer patternReticle pattern

Perfect overlay accuracy

Figure 14.46

Yau - 66SMT

Grid of Exposure Fields on Wafer

1 2 3 4

11 12 13 14 15 16

10 9 8 7 6 5

23 24 25 26 27 28

22 21 20 19 18 17

32 31 30 29

Start

Stop

Figure 14.47

Yau - 67SMT

Step-and-Repeat Alignment System

Used with permission from Canon USA, FPA-2000 i1

Figure 14.48

Yau - 68SMT

Alignment Marks

2nd Mask

1st Mask

2nd mask layer

1st mask layer

RA, Reticle alignment marks, L/RGA, Wafer global alignment marks, L/RFA, Wafer fine alignment marks, L/R

+ +++

RAL

RAR+ GA

+ FAL

+ FAR

+ GAR

+ GAL

Notch, coarse alignment

FAL

FAR

FAL/R +

+FAL/R +

For 2nd mask

+ From 1st mask

{

Figure 14.49

Yau - 69SMT

On-Axis Versus Off-Axis Alignment System

Alignmentlaser (633 nm)

Optical fiber

Video

Off-axisalignment unit

Alignment BLC fiducial

Off-Axis Alignment System

Alignmentlaser (633 nm)

Microscope objectives for video camera

Projection optics

Wafer stage

Alignment BLC fiducial

Reticle

On-Axis Alignment System

Used with permission from Canon USA, redrawn after FPA-2000i1 schematics

Figure 14.50

Yau - 70SMT

Environmental Conditions

• Temperature• Humidity• Vibration• Atmospheric Pressure• Particle Contamination

Yau - 71SMT

Comparison of Photo Tools(Refer to Table 14.7)

• Models by Manufacturer• Wavelength (nm)• Type of Aligner• Illumination Type• Exposure Field Size (mm)• Resolution (µm)• Overlay Accuracy (nm)

Yau - 72SMT

Step-and-Scan Aligner

Photograph courtesy of Silicon Valley Group Lithography, Micrascan II

Photo 14.3