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