semiconductor manufacturing technologyweng/courses/ic...semiconductor manufacturing technology...

© 2001 by Prentice HallSemiconductor Manufacturing Technologyby Michael Quirk and Julian Serda

Semiconductor Manufacturing Technology

Michael Quirk & Julian Serda© October 2001 by Prentice Hall

Chapter 14

Photolithography: Alignment and Exposure

Semiconductor Manufacturing Technology

Michael Quirk & Julian Serda© October 2001 by Prentice Hall

Chapter 14

Photolithography: Alignment and Exposure



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.


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


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


Optical Lithography

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


Wave Interference

A

B

A+B

Waves in phase Waves out of phase

Constructive Destructive

Figure 14.4


Optical Filtration

Secondary reflections (interference)

Coating 1 (non-reflecting)

Coating 3

Glass

Coating 2

Reflected wavelengths

Transmitted wavelength

Broadband light

Figure 14.5


Optical Lithography

Exposure Sources• Mercury Arc Lamp• Excimer Laser

– Spatial Coherence

• Exposure Control


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


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


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


Excessive Resist Absorption of Incident Light

Photoresist (after develop)

Substrate

Sloping profile

Figure 14.9


Optical Lithography

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


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


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


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


Diffraction in a Reticle Pattern

Slit

Diffracted light rays

Plane light wave

Figure 14.19


Lens Capturing Diffracted Light

UV

0

12

3

4

12

3

4

Lens

Quartz

Chrome Diffraction patterns

Mask

Figure 14.20


Effect of Numerical Aperture on Imaging

Figure 14.21

Exposure light

Lens NA

Pinhole masks

Image results

Diffracted light

Good

Bad

Poor


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


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


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


Effect of Standing Waves in Photoresist

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

Photo 14.1


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


Light Suppression with Bottom Antireflective Coating

BARCPolysilicon

Substrate

STISTI

UV exposure light

Mask

Exposed photoresist

Unexposed photoresist

Figure 14.25


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


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


Optical Lithography

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

– Surface Planarity


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


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


Depth of Focus (DOF)

+

-

Photoresist

Film

Depth of focusCenter of focusCenter of focus

Lens

Figure 14.30


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


Photolithograhy Equipment

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


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


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


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


Step-and-Repeat Aligner (Stepper)

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

Figure 14.35


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


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


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


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


Reticles

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


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


Photolithography Reticle

Photograph courtesy of Advanced Micro Devices

Photo 14.2


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


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


Optical Enhancement Techniques

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


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


Optical Proximity Effects

Rounded corners

Nonuniform CDs Shortened lines

Figure 14.43


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


Serifs to Minimize Rounding of Contact Corners

(b) Corrected with feature biasing

(a) Uncorrected design (c) Feature assisting technique

Figure 14.45


Alignment

• Overlay Accuracy• Alignment Marks


Overlay Budget

-X +X

+Y

-Y∆X

−∆Y

Shift in registration

-X +X

+Y

-Y

Wafer patternReticle pattern

Perfect overlay accuracy

Figure 14.46


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

semiconductor manufacturing technologyweng/courses/ic...semiconductor manufacturing technology...

Documents