3.155J/6.152J Lecture 10,
Lithography – Part 1
Prof,Martin A,Schmidt
Massachusetts Institute of Technology
10/8/2003
Outline
? The Lithographic Process
? Basic Process
? Definitions
? Fundamentals of Exposure
? Exposure Systems
? Resists
? Advanced Lithography
? Recommended reading
? Plummer,Chapter 5
? Other,Campbell,Chapter 7,8,9
Fall 2003 – M.A,Schmidt 3.155J/6.152J – Lecture 10 – Slide 2
IC Process
Circuit Design
Process Simulation
Device Simulation
Layout
BOXES
Mask
Shop
Design Rules
Wafers
Wafer
Fab
Fall 2003 – M.A,Schmidt 3.155J/6.152J – Lecture 10 – Slide 3
An idea
A file
Wafer Fab - Lithography
? Most Common Measure of Complexity
? # of Masks,Minimum Feature (examples)
? Approximately 50% of the Process Steps
Litho Etch Oxidation/Deposition Litho Etch Implant
Anneal Metal DepositionLitho Etch Litho Etch Sinter
Oxidation
? Drives Infrastructure
? Cleanliness
? Vibration
? Temperature and Humidity
Fall 2003 – M.A,Schmidt 3.155J/6.152J – Lecture 10 – Slide 4
Semiconductor Roadmap
Fall 2003 – M.A,Schmidt 3.155J/6.152J – Lecture 10 – Slide 5
Pattern Transfer Steps
Mask
Coat with
photoresist
Develop
Strip resist
Expose
Etch
*
*
Wet etch
Fall 2003 – M.A,Schmidt 3.155J/6.152J – Lecture 10 – Slide 6
Definitions
? Metrics
? Resolution
? Throughtput
? Registration (Alignment)
? Exposure Systems - UV
? Projection - Fraunhofer
? Proximity - Fresnel
? Contact - Fresnel
? Advanced
? DUV,E-Beam,X-Ray,Nano-imprint
? Resists
? Positive/Negative
? Contrast
? CMTF
Fall 2003 – M.A,Schmidt 3.155J/6.152J – Lecture 10 – Slide 7
Lithography Systems
Proximity
Contact Projection
Mask
Wafer
Fall 2003 – M.A,Schmidt 3.155J/6.152J – Lecture 10 – Slide 8
Contact/Proximity Printing
Fall 2003 – M.A,Schmidt 3.155J/6.152J – Lecture 10 – Slide 9
Contact/Proximity Printing
Fall 2003 – M.A,Schmidt 3.155J/6.152J – Lecture 10 – Slide 9
Applies when O < g < W
2
/O
Minimum resolvable feature = (Og)
1/2
Proximity Printing Limits
Minimum resolvable feature = (Og)
1/2
Gap = 20 Pm and Source = 436 nm
3.0 Pm
Fall 2003 – M.A,Schmidt 3.155J/6.152J – Lecture 10 – Slide 11
Projection Printing
Fall 2003 – M.A,Schmidt 3.155J/6.152J – Lecture 10 – Slide 12
Image from a Circular Opening
Note limit of d
Fall 2003 – M.A,Schmidt 3.155J/6.152J – Lecture 10 – Slide 13
Resolving Features
Fall 2003 – M.A,Schmidt 3.155J/6.152J – Lecture 10 – Slide 14
,When the
peak of one projection lands on
the first zero of the other,
Rayleigh Limit
Rayleigh Criterion
S,Wolf,Microchip Manufacturing,Lattice Press
Fall 2003 – M.A,Schmidt 3.155J/6.152J – Lecture 10 – Slide 15
D
R
n=1 (air)
The Rayleigh Criterion
R = 1.22Of/d = 1.22Of/n(2fsinD) = 0.61O/nsinD
NA = nsinD
Range from 0.16-0.76 )
R = 0.61O/NA = k
1
O/NA
(practical k
1
= 0.6-0.8)
Fall 2003 – M.A,Schmidt 3.155J/6.152J – Lecture 10 – Slide 16
Modulation Transfer Function (MTF)
Fall 2003 – M.A,Schmidt 3.155J/6.152J – Lecture 10 – Slide 17
Intensity Intensity
Modulation Transfer Function (MTF)
Mask
MTF =
I
max
-I
min
Distance
I
max
+ I
min
Wafer
I
max
I
min
Fall 2003 – M.A,Schmidt
Distance
3.155J/6.152J – Lecture 10 – Slide 18
MTF vs Feature Size
Fall 2003 – M.A,Schmidt 3.155J/6.152J – Lecture 10 – Slide 19
Depth of Focus
f
G
)
d
O/4 = G - Gcos)
Small ),
O/4 = G)
2
/2
) = sin ) = d/2f = NA
Depth of Focus = G = O/2(NA)
2
= k
2
O/(NA)
2
R = 0.61O/NA = k
1
O/NA
Fall 2003 – M.A,Schmidt 3.155J/6.152J – Lecture 10 – Slide 21
Resist Contrast
Dose = Intensity (W/cm
-2
) x time (s)
Positive Negative
Developed Thickness
Q
0
Q
f
Developed Thickness
Q
0
Q
f
Exposure Dose (log scale) Exposure Dose (log scale)
mJ cm
-2
mJ cm
-2
J = 1 / log
10
(Q
f
/Q
0
)
Fall 2003 – M.A,Schmidt 3.155J/6.152J – Lecture 10 – Slide 22
Ideal Exposure – Ideal Resist
Dose
Distance
Resist Thickness
Distance
Fall 2003 – M.A,Schmidt 3.155J/6.152J – Lecture 10 – Slide 23
Dose
Q
f
Real Exposure – Ideal Resist
Distance
Resist Thickness
Increase
Time
Fall 2003 – M.A,Schmidt 3.155J/6.152J – Lecture 10 – Slide 24
Dose
Q
f
Q
0
Real Exposure – Real Resist
Distance
Resist Thickness
Increase
Time
Decrease
Contrast
Fall 2003 – M.A,Schmidt 3.155J/6.152J – Lecture 10 – Slide 25
Decreasing ‘Pitch’
Dose
Q
f
Q
0
Dose
Distance Distance
Resist Thickness Resist Thickness
Distance
Fall 2003 – M.A,Schmidt 3.155J/6.152J – Lecture 10 – Slide 26
Critical Modulation Transfer Function (CMTF)
Developed Thickness
Q
0
Q
f
Q
CMTF =
f
–Q
0
10
J
–1
=
Q
f
+ Q
0
10
J
+ 1
If J = 3,CMTF = 0.37
J = 1 / log
10
(Q
f
/Q
0
)
If J = 2,CMTF = 0.52
Fall 2003 – M.A,Schmidt 3.155J/6.152J – Lecture 10 – Slide 28
Effect of Coherence on MTF
CMTF
CMTF
Fall 2003 – M.A,Schmidt 3.155J/6.152J – Lecture 10 – Slide 30