sem magnification calibration. magnification errors proper calibration of the sem scans...

SEM Magnification Calibration

Magnification Errors• Proper calibration of the SEM scans

(magnification) is primary to metrology.• SEM Magnification requires calibration

– Standards needed

• Instrument has inherent systematic problems– kV compensation– Working distance compensation

• Operator Errors– Working distance

Magnification Calibration

• SEM magnification calibration is generally based on the measurement of the pitch (displacement) between two structures.– Pitch will be discussed later

• All traceable SEM magnification calibration is based on the measurement of a pitch.

SEM Magnification Calibration

• Procedures vary with instrument manufacturer.

• All require user supplied calibration sample.

• Most common sample for laboratory instruments is a copper grid.

• NIST SRM 484 is a sample with accurate pitch dimensions for SEM magnification calibration.

SEM Instrumentation

• Vast array of instruments in the field.

• Performance and capabilities vary substantially.– lab instruments– production line instruments

• Universally useful NIST sample preferred because of time and cost involved.

SEM Instrumentation

INSTRUMENT CLASS

RESOLUTION HIGHEST USEFUL MAGNIFICATION

RANGE In-lens FESEM

.4-.8 nm 1,000,000x+

Post-lens FESEM

.8- 1.2 nm 500,000x+

LaB6 Systems 4.0 - 5.0 nm 100,000x

Tungsten Systems

5.0+ nm 50-100,000x

Independent of Accelerating Voltage

Calibration Based on Pitch

Pitch vs. Width

Magnification Calibration

• Pitch measurements are considered to be self-compensating.

• Width measurements are NOT self compensating.

• There currently is NO accurate or traceable standard for the width of a line or a structure.

Pitch Measurements

• Electron beam modeling is not needed for pitch measurements or calculations based on pitch

• Pitch Measurements are self compensating

• But - the rules must be followed.– The edges measured must be the same

Resiliency of the Standard

High kV Low kV

SEM Magnification Calibration Samples

• Instrument Manufacturer– Hitachi– Biorad

• In-house standards– Company standards laboratory

• NIST Traceable Standards

NIST Traceable Standards

• SRM 484– Traditional SEM magnification standard

• RM 8090• RM 8820• Reference material currently available• ~$400-500

• MRS-2/MRS-3 (Geller MicroAnalytical)

• VLSI Standards

Traceable SEM StandardsSRM 484

• NIST certified standard• Electro-deposited gold and

nickel layers, cross sectioned and polished.

• Pitch is certified using a metrology SEM.

• Certified spacing*: 0.5, 1.0, 2.0, 5.0, 10.0, 30.0 and 50.0 micrometers.– *newer issues may vary from

these figures

SRM 484• Developed before the

emphasis on low keV SEM operation

• Much thicker than a semiconductor wafer.

• Does not easily fit in contemporary wafer inspection instruments.

• Suitable for many SEM applications

SRM 484

SRM 484

Traceable SEM StandardsMRS-5

• NIST traceable standard commercially available.

• Accessory structures for distortion measurements and astigmatism correction are present.

RM 8820 SEM Magnification Calibration Artifact• Metrology reference artifact - PolySi Chip

and Wafer

• Made with 193 nm phase shifting, 6 inch mask

• The design combines NIST and many leading IC manufacturing companies experts’ dimensional metrology patterns

• A very large variety of patterns:

– Isolated and dense lines and spaces

– Varying line width, space width, pitch

– Various contact holes

– Optical and SEM alignment and navigation patterns

– Scatterometry: optical and x-ray

– Line edge roughness

– Geometry distortion patterns

– Many other patterns

– Phase shifting and resolution enhancement patterns

• Grounded and electrically floating patterns

MTP will be presenting a paper on this standard during this Conference!!

• NIST patterns dedicated to– Optical metrology – SEM metrology– Optical scatterometry– X-ray scatterometry

• Optical metrology section– Based on the NIST SRM 2059 design– Linewidth model comparison features– Special, “noisy” scatterometry patterns– Binary and phase shifting patterns

• SEM linewidth metrology– Patterns sized for mask

measurements– Patterns sized for wafer measurements– Grounded and electrically floating

patterns– 70 nm to 1000 nm patterns– 4x 70 nm to 4 x 1000 nm patterns

• X-ray scatterometry– 4x and 1x dense structures

• Size 1500 mm by 1500 mm• Pitch patterns 1500 mm to 140

nm• Isolated and dense lines 1 mm to

70 nm• Vertical and horizontal structures• Beam focusing, navigation and

distortion measurement patterns• SEM, SPM, optical,

scatterometry and line scale interferometry

• Grounded structures – less charging

SEM pitch calibration metrology patterns

New SEM Magnification Calibration Artifact

New SEM Magnification Calibration Artifact

Pitch patterns

1500 mm to 140 nm

Isolated and dense lines 1 mm to 70 nm

Vertical and horizontal structures

Beam focusing, navigation and distortion measurement patterns

SEM pitch calibration metrology patterns