Post on 11-Jan-2016
Microfabrication
Nathaniel J. C. Libatique, Ph.D.nlibatique@gmail.com
Sze, Semiconductor Devices, John Wiley and Sons
Process StepsProcess Steps
• Start with polished wafers of chosen and crystal orientation• Films: epitaxial, thermal oxides, polysilicon, dielectrics, metals• Doping: via diffusion or ion implantation• Lithography: shadow masked or projection• Etching: Wet and Dry• Sequential Mask Transfer• Stepper Iteration
Sze, Semiconductor Devices, John Wiley and Sons
Wafer Wafer Die Die Device Device
IngredientsIngredients
• Clean Rooms• Exposure Techniques• Masks• Photoresist• Pattern Transfer• Etching
Clean Room TechnologyClean Room Technology
Sze, Semiconductor Devices, John Wiley and Sons
1.Pinholes2.Constriction
of I3.Short ckt
• Epitaxy: Dislocations
• Gate Oxide: Low VbRule of Thumb: particles greater than 1/10 of Lmin is disruptive. Lmin = 5 m requires < 0.5 micron dust particles
Clean Room TechnologyClean Room Technology• Dust count should be four orders of magnitude lower than ordinary room air.
• Class 100: 100 particles (half micron or greater) per cubic foot = 3500 particles per cubic meter
• If we expose a 125 mm wafer for 1 minute to a laminar flow air stream at 30 m/min, how many dust particles will land on the wafer in a class 10 clean room?
Sze, Semiconductor Devices, John Wiley and Sons
Particle EmissionParticle Emission
Sze, Semiconductor Devices, John Wiley and Sons
Clean Room ClassesClean Room Classes
Keep critical areas very smallKeep critical areas very small Separate working areasSeparate working areas Slight overpressure in white areasSlight overpressure in white areas Laminar flow boxes in poor air Laminar flow boxes in poor air
quality areasquality areas
DesignDesign
Comb StructureComb Structure
White area for wafer and chip processing
Ball Room StructureBall Room Structure
“HEPA filter” = high efficiency particulate air filter, Ceiling to floor laminar flows, Perforations in floor
Ceiling
Floor
ExposureExposure
Sze, Semiconductor Devices, John Wiley and Sons
GoalsGoals
ResolutionResolution RegistrationRegistration ThroughputThroughput
Yield and cost, complexity-function, power dissipation, speed
Shadow PrintingShadow Printing
llmm ~ ( ~ (g)g)1/21/2
the gap the gap g g includes the resist includes the resist layerlayer
= 0.4 um, = 0.4 um, gg = 50 um, 4 um = 50 um, 4 um = 0.25 um, = 0.25 um, gg = 15 um, 2 um = 15 um, 2 um Dust dimensions > g can Dust dimensions > g can
damage the mask!damage the mask!
Projection PrintingProjection Printing
Avoids mask damageAvoids mask damage To increase resolution To increase resolution image a image a
small portion at a timesmall portion at a time Large masks followed by 10:1 Large masks followed by 10:1
demag ordemag or 1:1 masks1:1 masks Tradeoff: defect free masks vs. Tradeoff: defect free masks vs.
simpler opticssimpler optics
Annular Field ScanAnnular Field Scan
Sze, Semiconductor Devices, John Wiley and Sons
Small-Field Raster ScanSmall-Field Raster Scan
Sze, Semiconductor Devices, John Wiley and Sons
Reduction Step and Reduction Step and RepeatRepeat
Sze, Semiconductor Devices, John Wiley and Sons
1:1 Step and Repeat1:1 Step and Repeat
Sze, Semiconductor Devices, John Wiley and Sons
Sze, Semiconductor Devices, John Wiley and Sons
Resolution and DOFResolution and DOF
http://en.wikipedia.org/wiki/F-number
f/# = f/Df/# = f/D
f/32f/5
D
f
D
f
CAD used to generate mask artworkSecondary chip sites for process evaluationas well as for alignment-registration
Mask defect density is a concern in mask fabrication
Yield vs Defect DensityYield vs Defect Density
Semicon’s Dirty Secret
Y ~ e-DA for one mask level
For multiple mask levels:Y ~ e-NDA
PhotolithographyPhotolithography
Response CurveResponse Curve
Vertical axis: % Remaining after Vertical axis: % Remaining after exposure and developmentexposure and development
Horizontal Axis: ExposureHorizontal Axis: Exposure
Solubility increases with exposure for a positive resist
ET
Completely soluble. Measure of sensitivity for +ve resist
100%
E1
Negative resist – cross linked polymers insoluble
Positive resist – exposed areas become soluble
ET = threshold energy, E1 drawn from tangent at ET (+ve)
Finite Solubility
Post-EtchPost-Etch
gamma = solubility with incremental energy increase, contrast ratio, sharpness
Negative resists: lower exposure times due Negative resists: lower exposure times due to higher sensitivity to higher sensitivity high throughput high throughput
Positive resists: does not swell significantly Positive resists: does not swell significantly unlike negative resists unlike negative resists high resolution high resolution
CRM Grovenor, Microelectronic Materials
SitesSites
http://jas.eng.buffalo.edu/education/fhttp://jas.eng.buffalo.edu/education/fab/NMOS/nmos.htmlab/NMOS/nmos.html
http://www.ecse.rpi.edu/~schubert/http://www.ecse.rpi.edu/~schubert/Course-ECSE-6290Course-ECSE-6290
http://www.nikon.com/about/technolhttp://www.nikon.com/about/technology/core/optical_u/evanescent_e/indogy/core/optical_u/evanescent_e/index.htmex.htm