14 - 19
Eight Steps of Photolithography
S te p 1 . V a p o r p r im e 2 . S p in c o a t 3 . S o ft b a ke 4 . A lig n m e n t a n d e x p o s u re 5 . P o s t-e x p o s u re b a k e (P E B ) 6 . D e v e lo p 7 . H a rd b a k e 8 . D e v e lo p in s p e c t C h a p te r 13 13 13 14 15 15 15 15
Areas exposed to light become crosslinked and resist the developer chemical. Island Exposed area of photoresist
Photoresist
Ultraviolet light Chrome island on glass mask
14 - 1
Objectives
• Basic concepts for photolithography, including process overview, critical dimension generations, light spectrum, resolution and process latitude. • Difference between negative and positive lithography. • Eight basic steps to photolithography. • Wafer surface preparation for photolithography. • Photoresist physical properties. • Applications of conventional i-line photoresist. • Deep UV resists • Photoresist application techniques • Soft bake processing
-12
-10
(nm)
157
193
248
365 i
405 436 h g
VUV DUV DUV
Common UV wavelengths used in optical lithography.
14 - 11
Light Sources
Decreasing feature sizes requires shorter λ.
Window
Shadow on photoresist
Photoresist Oxide Silicon substrate
Oxide Silicon substrate
Resulting pattern after the resist is developed.
14 - 16
Positive Lithography
14 - 6
Steps in Lithography Process
Lithography has three parts: (1) Light source, (2) Wafer exposure (3) Resist
14 - 7
14 - 8
Photomask and Reticle for Microlithography
Important Wavelengths for Photolithography Exposure
U V W a v e le n g t h (n m ) 436 405 365 248 193 157 W a v e le n g t h N am e g - lin e h - lin e i- lin e D eep U V (D U V ) D eep U V (D U V ) V acu u m U V (V U V ) U V E m issio n S o u r c e M e r c u r y a r c la m p M e r c u r y a r c la m p M e r c u r y a r c la m p M e r c u r y a r c la m p o r K r y p t o n F lu o r id e ( K r F ) e x c im e r la s e r A r g o n F lu o r id e ( A r F ) e x c im e r la se r F lu o r in e ( F 2 ) e x c im e r la se r
Table 13.1
14 - 13
Importance of Mask Overlay Accuracy
Top view of CMOS inverter
The masking layers determine the accuracy by which subsequent processes can be performed. The photoresist mask pattern prepares individual layers for proper placement, orientation, and size of structures to be etched or implanted. Small sizes and low tolerances do not provide much room for error.
Visible Microwaves
12
Gamma rays
X-rays
UV
Infrared
Radio waves
10
f (Hz) (m)
10 10
22
10 10
20
10 10
18
10 10
-8
16
10 10
-6
14
10 10
-4
10 10
-2
10 10 0
8
10 10 2
6
10 10 4
4
-14
14 - 4
Three Basic Exposure Methods
1:1 Exposure 1:1 Exposure ~5:1 Exposure
14 - 5
• Contact printing capable of high resolution but has unacceptable defect densities. May be
Ultraviolet light Areas exposed to light are dissolved. Chrome island on glass mask Shadow on photoresist
Island
photoresist Photoresist
Window
Exposed area of photoresist
• Positive Resist
– Mask image is same as wafer image – Exposed resist softens and is soluble – Developer removes exposed resist
14 - 15
Negative Lithography
14 - 17
Relationship Between Mask and Resist
Desired photoresist structure to be printed on wafer Island of photoresist
Substrate
Chrome Window
Quartz Island
1:1 Mask 4:1 Reticle
14 - 9
Three DΒιβλιοθήκη mensional Pattern in Photoresist
Linewidth Space Photoresist
Thickness
Substrate
14 - 10
Section of the Electromagnetic Spectrum
14 - 18
Clear Field and Dark Field Masks
Clear Field Mask Dark Field Mask
Simulation of metal interconnect lines (positive resist lithography)
Simulation of contact holes (positive resist lithography)
Test/Sort
Implant
14 - 3
Photolithography Concepts
• Patterning process – Photomask – Reticle • • • • • Critical dimension generations Light spectrum and wavelengths Resolution Overlay accuracy Process latitude
used in Development but not manufacturing. • Proximity printing cannot easily print features below a few mm in line width. Used in nano-technolgy. • Projection printing provides high resolution and low defect densities and dominates today. Typical projection systems use reduction optics (2X - 5X), step and repeat or step and scan. They print » 50 wafers/hour and cost $5 - 10M.