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6.152J/3.155J - Microelectronics Procssing Technology Lithography Lecture #1
Lithography Lecture #1
OUTLINE
• Overview of Lithography
• Optics of Lithography
–Metrics, Optics of Micro-Lithography, Aligners, Photomasks
• Photoresists
–Components of Photoresist, Metrics, Photoresist processing, Multi-layer
resist
• Advanced Lithography
–E-beam Lithography, Soft Lithography
Reading Assignment: Plummer, Chapter 5
Reference: Campbell, Chapters 7, 8 & 9
6.152J / 3.155J Spring Term 2005 Lecture 09 - Lithography I 1
Overview of Lithography
• Lithography is the process by which circuit or device patterns are
transferred from layout to Si wafers
ENERGY Patterned exposure of resist
to an energy source using a MASK +
ALIGNER mask to create an aerial
image of mask in the resist
Modified
PHOTORESIST
Un-Modified
WAFER
Un-Modified
After development, a PHOTORESIST
pattern of resist created
WAFER by the aerial image is
Modified PHOTORESIST left.
removed after development
6.152J / 3.155J Spring Term 2005 Lecture 09 - Lithography I 2
6.152J.ST05 Lecture 09 16.152J/3.155J - Microelectronics Procssing Technology Lithography Lecture #1
Typical Lithography Process I
Plummer Fig. 5-1
• Layout functional blocks (or use previous designs) and use software
tools help route or wire connections between functional blocks
• Tools check for design rule violations
• Circuit and system level simulation tools predict performance
• Information from design transferred to mask making machine and
pattern written on a mask blank using scanning electron or laser beam
6.152J / 3.155J Spring Term 2005 Lecture 09 - Lithography I 3
Typical Lithography Process II
Plummer Fig. 5-2
• Use mask to expose the resist using a photo aligner
– Creates an aerial image of mask pattern on the resist
• Resist is then developed removing (exposed or exposed regions)
• Resist is used to transfer mask pattern onto wafer
– Ion implantation, oxide tech, metal etch, silicon etch, etc
6.152J / 3.155J Spring Term 2005 Lecture 09 - Lithography I 4
6.152J.ST05 Lecture 09 2Each step can lose information,
distorting device pattern
6.152J/3.155J - Microelectronics Procssing Technology Lithography Lecture #1
Lithography as Information Flow
Design
Mask
Aerial Image
Real Image
Latent Image
Resist Image
Device Layer
Tim Brunner, IBM
6.152J / 3.155J Spring Term 2005 Lecture 09 - Lithography I 5
Typical Lithography Process Sequence
• Wafers spend about 40-
APPLY 50% of the time in photo
PHOTORESIST
bay
DEPOSIT/GROW PRE-BAKE • Majority of the steps in
NEW LAYER
any process sequence are MASK
ALIGN & EXPOSE RESIST STRIP followed by lithography
and etch/implant
DEVELOP ETCH
POST-BAKE
INSPECT &
MEASURE
PHOTOLITHOGRAPHY BAY
6.152J / 3.155J Spring Term 2005 Lecture 09 - Lithography I 6
6.152J.ST05 Lecture 09 36.152J/3.155J - Microelectronics Procssing Technology Lithography Lecture #1
Typical Pattern Transfer Steps
Coat with
photoresist
Expose
Mask
Develop
*Etch
Strip resist
* SchmidtWet etch
6.152J / 3.155J Spring Term 2005 Lecture 09 - Lithography I 7
Functional Components of Lithography
ENERGY
MASK +
ALIGNER
PHOTORESIST
WAFER
• Energy —cause (photo)chemical reactions that modify resist dissolution rate
• Mask —Pattern (or direct) energy to create an aerial image of mask in resist
• Aligner —Align mask to previous patterns on wafer (to a tolerance level)
• Resist —Transfer image from mask to wafer, After development Positive resist
reproduces the mask pattern, Negative resist reproduces inverse mask pattern
• Substrate —Has previous mask patterns
6.152J / 3.155J Spring Term 2005 Lecture 09 - Lithography I 8
6.152J.ST05 Lecture 09 46.152J/3.155J - Microelectronics Procssing Technology Lithography Lecture #1
Energy Sources
Waves or Particles
• Energy sources are required to modify the photoresist.
• The energy source is aerial imaged on the photoresist.
• The imaging can be done by scanning the energy beam or by masking the
energy beam.
• Bright sources are usually required for high throughput.
Wavelength Energy
Light UV 400 nm 3.1 eV
Deep UV 250 nm 4.96 eV hc
E = hν =
λX-Ray 0.5 nm 2480 eV
Particles Electrons 0.62 Å 20 keV
Ions 0.12 Å 100 keV
6.152J / 3.155J Spring Term 2005 Lecture 09 - Lithography I 9
Mask
Mask
Schmidt
• Block radiation where it is not wanted i.e. absorb
radiation
– Need opaque material at the desired wavelength
• Transmit radiation where it is needed
– Need material with high transmission at the desired
wavelength
• For Optical lithography, mask is
– Quartz glass (transparent) + Cr (opaque)
6.152J / 3.155J Spring Term 2005 Lecture 09 - Lithography I 10
6.152J.ST05 Lecture 09 56.152J/3.155J - Microelectronics Procssing Technology Lithography Lecture #1
Aligner
Schmidt
• Align pattern on mask to previous patterns on the wafer
• Exposure of photoresist to radiation pattern which is an
aerial image of mask
• Types of aligners
– Contact, Proximity & Projection
6.152J / 3.155J Spring Term 2005 Lecture 09 - Lithography I 11
Resist
• Viscous liquid which has a “solid” form when solvents are
driven out
• Spin coated on coated on surface to be patterned
• Exposure of resist to energy/radiation leads to (photo)
chemical reactions and changes the resist dissolution rate
in the developer
• Remaining resist is “rugged” enough to protect (mask)
underlying substrate during subsequent processing
6.152J / 3.155J Spring Term 2005 Lecture 09 - Lithography I 12
6.152J.ST05 Lecture 09 66.152J/3.155J - Microelectronics Procssing Technology Lithography Lecture #1
Semiconductor Roadmap
Minimum Feature Size
300
Isolated Lines
Dense Lines
250
200
150
100
50
0
1996 1998 2000 2002 2004 2006 2008 2010 2012 2014
Year
6.152J / 3.155J Spring Term 2005 Lecture 09 - Lithography I 13
Metrics of Lithography Systems
• Resolution(smallest dimension that can be printed)
– Determined by optical system, resist, etch process
– Critical Dimension (CD) control (3σ = 10% of mean)
• Registration (alignment 3σ=1/3 resolution)
– Determined by optical system and aligner
• Dimensional Control (device, die, wafer, lot uniformity)
– Determi, mask, resist, etch
process
• Throughput (how many wafers/hour)
– Determined by optical system, resist
6.152J / 3.155J Spring Term 2005 Lecture 09 - Lithography I 14
6.152J.ST05 Lecture 09 7
Feature Size, L(nm)6.152J/3.155J - Microelectronics Procssing Technology Lithography Lecture #1
Optical Aligners
Exposure Systems
Comparison of three exposure systems Light intensity profile for
three exposure systems
Contact Aligner: photoresist is in intimate contact with the chrome-side of the mask at the
time of exposure. Mask image:Resist image is 1:1, not limited by diffraction
Proximity Aligner: photoresist is not in intimate contact with mask⎯separated by a few
microns. Mask image:Resist image is 1:1. Limited by near field (Fresnel diffraction) diffraction
Projection Aligner: photoresist is not in intimate contact with mask ⎯mask image is
projected onto resist by lenses. Limited by far field (Fraunoffer diffraction) diffraction
6.152J / 3.155J Spring Term 2005 Lecture 09 - Lithography I 15
Optics Review
What is Diffraction?
• Diffraction is the spread of radiation into un-exposed
regions
– Near-field diffraction (Fresnel Diffraction)
– Far-field diffraction (Frauhoffer Diffraction)
6.152J / 3.155J Spring Term 2005 Lecture 09 - Lithography I 16
6.152J.ST05 Lecture 09 86.152J/3.155J - Microelectronics Procssing Technology Lithography Lecture #1
Contact Printing
W
Incident
Plane
Wave
Light Intensity
Mask Resist Wafer at Resist Surface
Aperture
• Contact Aligner—mask is in hard contact with resist
• not diffraction limited
6.152J / 3.155J Spring Term 2005 Lecture 09 - Lithography I 17
Proximity Printing
Near Field (Fresnel) Diffraction
2
W
λ < g <
λ
W = mask feature size
W ≈ λgmin
• Mask and wafer are separated by a small gap of 2-20 µm
• The resulting diffraction pattern has several features
– Intensity rises gradually near the edges producing some resist
exposure outside the mask edge
– Ringing in intensity distribution within the aperture
• As mask separation g increases, quality of image degrades
6.152J / 3.155J Spring Term 2005 Lecture 09 - Lithography I 18
6.152J.ST05 Lecture 09 96.152J/3.155J - Microelectronics Procssing Technology Lithography Lecture #1
Projection Printing
6.152J / 3.155J Spring Term 2005 Lecture 09 - Lithography I 19
Far Field (Frauhoffer) Diffraction
d
R
Rλ fλ
q = 1.22 = 1.22First minimum 1
2a d
• The analysis is for circular apertures with radius a, diameter d=2a.
• Observation of the light intensity at a distance R (usually at the focal
length f) shows the above circular diffraction pattern.
• The diffraction pattern has a diameter (of central maximum) equal to q .1
6.152J / 3.155J Spring Term 2005 Lecture 09 - Lithography I 20
6.152J.ST05 Lecture 09 10