introduction to ebeam lithography

Introduction to ebeam Lithography

History of Elionix ebeam Lithography

T-Gate with 3 Layers of Resists

5nm Resist Lines

600nn Hole Array (2um deep)

85nm Ring Array (200nm pitch) 3D Hologram Structure

What Can 100kV ebeam Do?

Application Example Using 100kV ebeam

Fabrication of Double Quantum DotsBy Xiao Mi @ Petta Group, Princeton University

ebeam System – Elionix ELS-F125

Elionix ELS-G100 ebeam Lithography System

Elionix ELS-F100 – Main Unit

State-of-the-art 100kV Elionix Electron Beam Lithography System

Elionix ELS-F100 – Upper Column

Column Section I

Ion Pump SIP1 ZrO2

W filament~1800 °CI ~ 2.3 A

Suppressor

Extractor

Acceleration(100kV)

Alignment CoilsAL1

Condensor Lens CL1

TFE gun

Column Section II

Ion Pump SIP2 Alignment CoilsAL2

Condensor Lens CL2

Limiting Aperture

Alignment CoilsAL3

Condensor Lens CL3 (focus)

Beam Blanker

Elionix ELS-F100 – Middle Column

Interferometer Stage

Position Accuracy: 0.3nm

Ion Pump SIP3

Alignment CoilsAL4

OL Apertures(60µm, 120µm, 240µm)

Objective LensCL4

Stage-XY (±5µm error)

WD(40 mm)

SE1

BSE

SE2

Isolation valve

Stage-Z (±0.1µm error)

HS(±0.1µm error)

Beam Deflector(20-bit DAC, 100MHz)

Elionix ELS-F100 – Lower Column / Chamber

Column Section III

33◦

Coordinates of ebeam Lithography System

v

u

Y

X

100 MHzBeam Deflector

Sample Coordinates

Stage Coordinates

V

UBeam Coordinates

100kVEbeam

6” Stage0.3nm Stage Position Precision

Field size: ≤ 1mm X 1mm(for 100kV acceleration)

Introduce sample tilt/rotationRotation tolertance <3.4 degree

Dynamic Correction of ebeam Focus/Stigmation

- Field CenterPerfect Focus/Stigmation

- Field EdgeDefocused/Astigmatic

Shape of the ebeam spot in a write fieldWithout dynamic correction

Fiel

d C

ente

r

Beam Deflector(Speed: 100MHz)

During exposure, the dynamic amplifier will help to correct the Focus/Stigmation when the beam is deflected.

Sample

ebeam

How to Expose a Pattern with ebeam?5

00

um

500um

1. A user pattern may be split intomultiple fields;

2. For Elionix ELS-F100 ebeam, thefield size can be set to 1mm,500um, 250um or 100um;

3. Within a write field, the stagedoes not move, only the ebeam isdeflected to expose the specificpattern inside;

4. Bigger field size means fewerstitching boundaries, but mayresult in worse stitching errors atan individual field boundary. Forexample, the stitching tolerancefor Elionix ELS-F100 is:

(i). 1mm field25nm stitching error;

(ii). 500um field15nm stitching error.

How to Expose a Pattern with ebeam?

1. A write field can be furtherdivided into subfields;

2. For Elionix ELS-F100 ebeam, awrite field is typically divided into20X20 subfields;

3. When exposing a subfield withina write field, the stage does notmove, the ebeam is deflected toexpose the specific pattern insidea subfield before moving ontothe next one;

4. Advantages of subfields in ebeamexposure –(i). less beam settling time;(ii). faster beam deflection;(iii). better beam position fidelity;

1. Disadvantage of subfields inebeam exposure – extra stitchingbetween subfields.

20

sub

fields

20 subfields

How to Expose a Pattern with ebeam?

1. Pattern within a subfield isfurther divided into trapezoids;

2. Each trapezoid is filled by anarray of individual ebeam shots;

3. The spot size of an ebeam shot isdetermined by the ebeamcurrent (e.g. 100pA ~2nm);

4. The minimal step size betweentwo neighboring ebeam shots isdefined by the user (e.g. when auser selects 500um field and10,000 dots number, the minimalstep size is set to 50nm);

5. Within a trapezoid, the ebeamwill continuously fill it withebeam shots; when moving fromone trapezoid to another, theebeam will be “blanked” first,then will be “unblanked” at thenew trapezoid to start exposure

20

sub

fields

20 subfields

How Is a Shape Filled by ebeam?

Raster Scan vs. Vector Scan

(e.g. Heidelberg DWL66+ Laser Writer) (e.g. Elionix ELS-G100 ebeam System)

• Particles have a wavelength (1924 de Broglie Ph.D. thesis)

l = h/p, p = momentum, h = 6.626 × 10–34 m2 kg s–1

• Electron wavelengths depend on its energy

A range of energies in e-beam lithography

Energy Wavelength

100 keV 0.037 Å

10 keV 0.122 Å

1 keV 0.387 Å

100 eV 1.23 Å

Chris Mack, Lecture 61 (ChE 323) E-Beam Lithography, Part 1.

How Small Can an Electron Beam Be?

De Broglie Wavelength of ebeam

How small one can “write” a patternwith an electron beam is ultimatelydefined by the tip size of the “pen”,i.e. the size of the electron beam.

Calculated ebeam Spot Size for Elionix ELS-F100

Schematic illustration of ebeam entering resist/substrate, creating (i) forward-scattered electrons; (ii) back-scattered electrons. The low-energy secondary electrons (a few eV) created in the process is responsible for the resist exposure.

Back-scattered electron

Forward-scattered electron

Effect of Interaction with Resis/Substrate on ebeam Spot Size

Thesis by Filiz Yesilkoy at UMD (2010)

Why Does Higher kV Lead to Smaller Feature Size?

At resist thicknesses above critical thickness, forward scattering limits resolution; whereas below the critical thickness, the only resolutionlimiter is the ebeam diameter (theoretically energy-independent, albeit resist development also matters). PhD thesis, Bryan M. Cord, MIT, June 2009

What is Ultimate ebeam Lithography Resolution Limit?

Beam energy (keV) α (um) β (um) η

5 1.33 [0.18] [0.74]

10 0.39 [0.60] [0.74]

20 0.12 2.0 0.74

50 0.024 9.5 0.74

100 0.007 31.2 0.74

𝑷 𝒓 = 𝟏 − 𝜷 𝐞𝐱𝐩 −𝒓𝟐

𝟐𝝈𝟐+ 𝜷𝐞𝐱𝐩(−

𝒓𝟐

𝟐𝝈𝑩𝟐 )

Forward scattering Backscattering

• Higher electron-beam accelerating voltage affects forward and backscatter ranges• Simple description: Double Gaussian fit to simulated spot energy distribution in the

resist/substrate (usually described by a PSF - Point Spread Function)

Understand the ebeam Proximity Effect

: range of forward scattering (in m) : range of backscattering (in m) : ratio of backscattering to forward scattering

Scattering probability varies as square of atomic number Z, and inversely as the incident kinetic energy.

Number of backscattered electrons is notdependent on energy, but its spatial distributionis. Proximity effects are “diluted” (spread overlarger area) at high energies.

Monte-Carlo Simulations of Electron Trajectory

Best contrast curve at -15oC.

Contrast curves for PMMA developed in 3:1 IPA:MIBK at various temperatures. The initial PMMA film thickness was 160 nm, and thedevelopment time was 60 seconds, except in the -40°C and -50°C cases (120 seconds) and the -60°C case (600 seconds), where longerdevelopment was necessary. PhD thesis, Bryan M. Cord, MIT, June 2009

Post Processing – Effect of Temperature – PMMA

Comparison of line edge roughness of ZEP-520 (40nm wide) Resist lines developed at Room Temperature (Top Right) ; at -4oC (Bottom Right)

Contrast curve for ZEP-520 at various temperatures

ZEP-520a resist lines – Developed @ room T

ZEP-520a resist lines – Developed @ -4oC

PhD thesis, Bryan M. Cord, MIT, June 2009

Post Processing – Effect of Temperature – ZEP-520

PhD thesis, Bryan M. Cord, MIT, June 2009

Note: The energy level of the ultrasonication is critical to the process – if it’s too high, the resist pattern can peel off; if it’s too low, its effectiveness in helping with the resist development may be limited

Post Processing – Effect of Sonication – PMMA

Clear Dose(Unit: µC/cm2)

120nm ZEP

ZEP-520 : Anisole = 1:1

42nm ZEP

ZEP-520 : Anisole = 1:3

10nm Spike Lines 1504 µC/cm2 1184 µC/cm2

20nm Spike Lines 800 µC/cm2 672 µC/cm2

30nm Spike Lines 640 µC/cm2 544 µC/cm2

40nm Spike Lines 576 µC/cm2 416 µC/cm2

50nm Spike Lines 544 µC/cm2 416 µC/cm2

60nm Spike Lines 544 µC/cm2 384 µC/cm2

70nm Spike Lines 544 µC/cm2 384 µC/cm2

80nm Spike Lines 544 µC/cm2 384 µC/cm2

90nm Spike Lines 544 µC/cm2 384 µC/cm2

Size-Dependent Dose for ZEP-520

Processing Note:Coating: Spin at 4500rpm for 60s, ramp 1000rpm, pre-bake at 175°C for 15min

Exposure: Elionix ELS-F125, 125kV, 100pA, 500um field, 2.5nm step size

Develop: 30s in ZED-N50 (chilled in ice-water bath for 15min), then 5s in IPA

Post Processing: post-bake at 95°C for 2min before SEM

50nm Spike Example

Clear Dose 416 µC/cm2

Review of Lithography Roadmap

ebeam

“Overview of Next Generation Lithography, Advanced Patterning, EUV and Self Assembly” by Mark Neisser & Stefan Wurm, SEMATECH 2013

Magnetic Lens vs. Electrostatic Lens

Magnetic Lens ElectroStatic Lens

Elionix ELS-G100 – Vacuum System

Why Use a Faraday Cup to Measure Beam Current?

ebeam Pattern Generator (PG)

A write field is divided into pixels by the PG

𝑀𝑖𝑛 𝑃𝑖𝑥𝑒𝑙 𝑆𝑖𝑧𝑒 = Field Size

Max Dot Number

(e.g. 500um field, 10,000 dot number Min Pixel Size = 50nm)

Fiel

d C

ente

r

Beam Deflector(with 20bit DAC)

For a 20bit DAC PG, the max number of dots in one line scan: Dot Number = 220 = 1,048,576 ~ 1 million

The total number of pixels in one write field:Number of Pixels = 106 𝑋 106 = 1012

ebeam

Charging Effect on ebeam Exposure

By Devin K. Brown, Georgia Tech

Here D1 and D0 is defined for BResist development curves:(a) Resist A is of higher

sensitivity than B.(b) A is of higher contrast than

B; C is negative resist.

substrate

thickness

Sensitivity:

• For positive resist: D1 value, or dose required to fully develop the resist to bottom, close to D1 value.

• For negative resist: dose that results in half resist thickness remaining after development.

Contrast : defined as slope of the development curve, the equation is shown below:

0110 /log

1

DD

negativepositive

Definition of Resist Development