Photolithography – II ( Part 3 ) - NTNUfolk.ntnu.no/jonathrg/fag/TFE4180/slides/Ch14...

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TFE4180 Semiconductor Manufacturing Technology, Photolithography - II

Photolithography – II ( Part 3 )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

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

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

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

4. Explain how alignment is achieved in lithography.

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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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Photolithography Exposure Equipment

• Contact Aligner: 1960s, CD > 0.4 m (CD > 5 m), mask, 1:1 • Proximity Aligner: 1970s, CD > 2 m, mask, 1:1 • Scanning Projection Aligner (scanner): 1980s, CD > 1 m,

mask, 1:1 • Step-and-Repeat Aligner (stepper): 1990s, CD > 0.25 m,

reticle, 5:1 (4:1) • Step-and-Scan System: 2000s, CD < 0.25 m, reticle, 4:1 (5:1)

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Contact Aligner System

Illuminator

Alignment scope (split

vision)Mask

Wafer

Vacuum chuck

Mask stage (X, Y , Z ,

Wafer stage (X, Y, Z,

Mercury arc lamp

Figure 14.32 Quirk & Serda

Positives:

• Avoids diffraction problems

Negetives:

• Particle contamination Mask must be replaced after 5-25 exposures

•Operator-controlledmanual alignment Variable repeatability

•”Overlay accuracy” problems with increased disc size

1960s, CD > 5 m (now: 0.4 m),mask, 1:1

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Lens Capturing Diffracted Light

UV

0

12

3

4

12

3

4

Lens

Quartz

Chrome Diffraction patterns

Mask

Figure 14.20 Quirk and Serda

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Proximity Aligner System

Illuminator

Alignment scope (split

vision)Mask

Wafer

Vacuum chuck

Mask stage (X, Y , Z ,

Wafer stage (X, Y, Z,

Mercury arc lamp


Positives:

• Avoids contamination

Negetives:

• Mask- resist gap = 2.5 – 25 m Diffraction

• Operator- controlled manual alignmentvariable repeatability

• ”Overlay accuracy” problems with increased disc size

1970s, CD > 2 m, mask, 1:1

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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


2.5 – 25 μm

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Scanning Projection Aligner

Redrawn and used with permission from Silicon Valley Group Lithography

Mask

Wafer

Mercury arc lamp

Illuminator assembly

Scan direction

Exposure light(narrow slit of UV

gradually scans entire mask field

onto wafer)

1980s, CD > 1 m, mask, 1:1


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Step-and-Repeat Aligner (Stepper)

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, q)

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


1990s, CD > 0.25 m, reticle, 5:1

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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


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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 chuckWafer stage assembly

Light sensor

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

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Step-and-Repeat Aligner (Stepper)

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


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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 Quirk & Serda

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Step-and-Scan 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 systemWafer stage

Reticle library (SMIF pod

interface)

Used with permission from ASML, PAS 5500/900


2000s, CD < 0.25 m, reticle, 4:1

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Step-and-Scan Aligner

Photograph courtesy of Silicon Valley Group Lithography, Micrascan II

Photo 14.3 Quirk & Serda

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Step-and-Scan Aligner

Modern Step-and-Scan System:

http://www.asml.com/asml/show.do?ctx=6720&rid=36951

Little History:

http://www.asml.com/asml/show.do?ctx=33254&year=2006-present

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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

Step-and-Scan Technology in 2011: NA = 0.85 – 1.35 (immersion lithography)

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Reticles

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

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Reticle Vs. MaskParameter

Reticle (Pattern for Step-and-Repeat

Exposure)

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

Critical Dimension

Easier to pattern submicron dimensions on wafer due to larger pattern size on reticle (e.g., 4:1, 5:1).

Difficult to pattern submicron dimensions on mask and wafer without reduction optics.

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

Mask Technology Optical reduction permits larger reticle dimensions – easier to print.

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

Throughput Requires sophisticated automation to step-and-repeat across wafer.

Potentially higher (not always true if equipment is not automated).

Die alignment & focus Adjusts for individual die alignment & focus.

Global wafer alignment, but no individual die alignment & focus.

Defect density

Improved yield but no reticle defect permitted. Reticle defects are repeated for each field exposure.

Defects are not repeated multiple times on a wafer.

Surface flatness

Stepper compensates during initial global pre-alignment measurements or during die-by-die exposures.

No compensation, except for overall global focus and alignment.

Table 14.6 Quirk & Serda

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Reticles

• Comparison of Reticle Versus Mask• Reticle Materials

- Cr pattern (opaque) on fused silica (high optical transmission and low thermal expansion)- chrome thickness < 1000 Å- Sometimes 200 Å of CrO ARC on the Cr

• Reticle Reduction and Size• Reticle Fabrication• Sources of Reticle Damage

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Reticles


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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


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Reticles

• Comparison of Reticle Versus Mask• Reticle Materials• Reticle Reduction and Size• Reticle Fabrication (e-beam lithography p. 396)• Sources of Reticle Damage

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Reticles


- dropping the reticle- surface scratching- electrostatic discharge (ESD)- particles of dirt

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Reticles

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

Antireflective coatings Pellicle filmChrome patternDepth of focus

Mask material

Reticle

Pellicle film

Frame

Chrome pattern


5 – 10 mm separation

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Photolithography Reticle

Photograph courtesy of Advanced Micro Devices

Photo 14.2 Quirk & Serda

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Optical Enhancement Techniques

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

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Phase-Shift Maskb) 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


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Optical Proximity Effects

Rounded corners

Nonuniform CDs Shortened lines


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Serifs to Minimize Rounding of Contact Corners

(b) Corrected with feature biasing

(a) Uncorrected design (c) Feature assisting technique


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Off-Axis IlluminationA

B- A+

B+B+A-A-

BOff-axis illumination

(b)

Conventional illumination (on-axis)

- Order diffractio

n

+ Order diffraction

(a)

Pinhole mask

Projection optics

Wafer


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Alignment

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

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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 chuckWafer stage assembly

Light sensor

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

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Alignment


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Overlay Budget

-X +X

+Y

-YX

Y

Shift in registration

-X +X

+Y

-Y

Wafer patternReticle pattern

Perfect overlay accuracy

Figure 14.46 Quirk & SerdaTypically 1/3 of CD

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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


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Alignment


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On-Axis vs. 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

schematicsFigure 14.50 Quirk & Serda

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Step-and-Repeat Alignment System

Used with permission from Canon USA, FPA-2000 i1


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Environmental Conditions

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

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Photolithography – II ( Part 3 ) - NTNUfolk.ntnu.no/jonathrg/fag/TFE4180/slides/Ch14...

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