Introduction to ebeam Lithography
Transcript of 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