microlithography - national tsing hua universitymx.nthu.edu.tw/~yucsu/4810/lec02.pdfmicrolithography...
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Microlithography
ESS4810 LectureFall 2010
Moore’s Law
• The observation made in 1965 that the number of transistors per square inch on integrated circuits had doubled every year since ICs were invented
• Moore predicted that this trend would continue for the foreseeable future
• In subsequent years, the pace slowed down a bit, but data density has doubled approximately every 18 months
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Moore’s Law
Moore’s Law
1.5 mm
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Microlithography
• Photolithography– Process– Resolution– Depth of focus
• Photoresist• Lift-off process• Other processes
Photolithography
• Transfer patterns from mask to wafer surface by light
• From mask to photoresist– Exposure– Development
• From photoresist to wafer surface– Wet or dry etching
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Steps of Photolithography
• 1 Clean wafer• 2 Coat with photoresist• 3 Soft bake photoresist• 4 Align mask with wafer• 5 Expose pattern on photoresist• 6 Develop photoresist• 7 Hard bake photoresist• 8 Etch pattern on wafer• 9 Remove photoresist
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Photoresist Application
• Photoresist (light-sensitive)• Clean and dry surface to ensure
good photoresist adhesion• Adhesion promoter• Applied in liquid form• The wafer is held on a vacuum
chuck and then spun at high speed to produce a thin uniformlayer
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Adhesion Promoter
• Hexamethyldisilazane• Vapor priming• Liquid priming• Monolayer• One side bonds with
wafer surface• The other side bonds
with photoresist
HMDS
Baking
• Soft– 80 ~ 90 ºC for 10 to 30 minutes– Improve adhesion and remove
solvent from photoresist• Hard
– 120 ~ 180 ºC for 20 to 30 minutes– Harden the photoresist and improve
adhesion to substrate
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Photoresist Removal
• Requirements– Complete removal without residues – No undesired etching
• Wet– Liquid resist strippers
• Dry– Plasma– Oxidizing (burning, ashing)
Pattern Generation
• Design - Mask - Wafer
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Contact Printing
• Resolution (R)– 0.5 μm
• Mask plate is easily damaged or accumulates defects
Photoresist
Mask
Proximity Printing
• Resolution (R)– 1 μm– k(λg)1/2
Photoresist
Maskg
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Projection Printing• Resolution (R)
– 0.2 μm (deep UV)• Trade-off
– Optics complicated and expensive
Photoresist
Mask
Lens
Image Comparison
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Diffraction
• Diffraction can be thought of simply as the “bending” of light when it passes through an aperture
• The light that passes through the aperture carries with it the information on the size and shape of that aperture
• This information spreads out in space because of diffraction and it must all be collected to convey perfect information about the aperture to the resist on the wafer
Diffraction
• Because of its finite size, the focusing lens collects only part of the total diffraction pattern
• The light diffracted to wider anglescarries the information about the finer details of the aperture
• It is those details that are lost first when a lens of finite size is used to collect and focus the light
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Image Degradation by Lens
Resolution
• How close together can A and B be and still be resolved in the image plane?
• The central maximums of each point image lie at the first minima of the adjacent point image
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Resolution
• Numerical aperture (NA) of lens• K1: a constant between 0.25 and 1,
depending on optics, resist, and process latitude
Definition of Line & Space
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Bragg Condition
X
X
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Depth of Focus
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Resolution vs DOF
• Requirements (1) and (2) need a compromise between λ and NA !
Excimer Laser Stepper
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Photon Sources
Photon Sources
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Photon Sources
Positive photoresist Negative photoresist
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Photoresist
• Positive– Polymer (MW~5000)– Photoactive inhibitor (20%)– Volatile solvents– Inhibitor looses N2 => alkali soluble
acid– Develops by “etching” - no swelling
Positive Photoresist
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Photoresist
• Negative– Polymer (MW~65000)– Light sensitive additive promotes
cross-linking– Volatile solvents– Light breaks N-N => crosslink chains– Sensitive, hard, swelling during
develop
Negative Photoresist
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Comparison
• Positive– Higher resolution– Aqueous-based solvents– More popular
• Negative– More sensitive– Less expensive– Better chemical resistance– More tolerant of developing conditions
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Overlay Errors
Thermal run-in/run-out Errors
• change of mask and wafer temp.
• coefficient of thermal expansion of mask and Si
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Characterization
Example
• Center has only translation error
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Example
• Run-out error: 0.2• Rotational error: 0.5, CCW
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Lift-Off Process
• Allows definition of pattern on the wafer surface without etching
• Hard to etch metals• Lifted off in
selected areas by dissolving underlying resist
X-ray Lithography
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X-ray Lithography
Mask Making• the most complicated and
challenging part of x-ray lithography
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Electron Beam Lithography
Angstroms for V in Volts
30 kV e-beam> λ = 0.07 Å
Scanning Methods
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Proximity Effect
• Electron scattering• Compensation
required
Nanoimprint Lithography
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Nanoimprint Lithography
AFM Lithography
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AFM Lithography
Immersion Lithography• Using an immersion fluid between the wafer and
the lens substantially changes the light path• First, it enhances depth of focus (DOF) for a
given Numerical Aperture• Second, immersion allows lens designs with
Numerical Apertures significantly larger than 1.0, therefore allowing improved resolution
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nwater = 1.33nair = 1The higher-index medium couples
higher spatial frequency to the resist
NA = n sinθ
Smaller angle in the coupling medium is less sensitive to longitudinal displacement of the wafer
2NADOF
λ∝
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Immersion Lithography
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