UGIM 2016

Research Service CentersHerbert Wertheim College of Engineering

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Ion Beam Lithography: faster writing strategies for features between 150nm and 1um

Brent P. Gila, Andes Trucco, David Hays

Located in sunny Gainesville, FL (100 miles north of Disney World)

https://nrf.aux.eng.ufl.edu/

UGIM 2016

Research Service CentersHerbert Wertheim College of Engineering

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Patterning strategies are to separate features based upon size to minimize exposure time.

Lithography processing gap:

Affordable lithographic processes for average university facilities are contact aligners (and projection aligners) and SEM pattern generator attachments. These cover >1um and <1um respectively. Optical exposures are parallel and fast, electron beam exposures are serial and slow.

Ion beam lithography can provide gap coverage between the 200nm and 1um feature range. This is a serial process like electron beam lithography. However the ion exposure required for the resists are 10 times lower, so the writing time is greatly reduced compared to electron beam lithography.

Ion beam lithography overview

UGIM 2016

Research Service CentersHerbert Wertheim College of Engineering

– 100 mm laser interferometer stage

– NanoFIBTM ion column

– 20 MHz, 16 bit digital pattern generator– dedicated software for lithography and

nano engineering

• Ion beam centric with high resolutionsmall WD, compact optics

• Normal incidenceprecise and stable navigation

• High beam stability

Raith ionLINE System Overview

• 15-40keV, 0.5pA-1nA, current stability <1%/hr• Multi-species ion sources available• Currently Au-Si

60nm stitch and overlay, 1nm min step

UGIM 2016

Research Service CentersHerbert Wertheim College of Engineering

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Ion beam lithography overview

Ga+Au+

30kV ion simulated in PMMA by SRIM

Si+

Resists under go chain scission in e-beam lithography.Due to a large mass, ions create more chain scission events compared to the same

dose of electrons.Typical exposures required for ion beam lithography are 2-10 uC/cm2 compared to 100-300uC/cm2 for electron beam lithography.

UGIM 2016

Research Service CentersHerbert Wertheim College of Engineering

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Ion beam lithography overview

Si+ ion simulated in PMMA by SRIM

Depth of resist exposure is dependent upon ion kV, but high kV does not lead to higher aspect ratio structures, only deeper exposures for thicker resists. Ion scattering in PMMA remains large.

20kV

30kV

40kV

100nm resist

UGIM 2016

Research Service CentersHerbert Wertheim College of Engineering

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Ion beam lithography overviewSimulations of incident ions only provide part of the total damage picture. When the recoiled atoms are include in the simulation, the cascade damage is much larger and thus the large minimum resolution for the ion beam exposure.

40kV Si+ 40kV Si+HCO

UGIM 2016

Research Service CentersHerbert Wertheim College of Engineering

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Ion beam lithography overview

Si ion beam exposure in 300nm PMMA has the same profile as the simulation.

To ensure proper clearing dose and not implant/damage the substrate, must us a sacrificial stopping layer under the PMMA.

Typical stopping layer employed is 200nm LOR.

UGIM 2016

Research Service CentersHerbert Wertheim College of Engineering

1.0mm/sec

0.1mm/sec 0.25mm/sec

Ga+ FBMS PMMA resist exposures

Patterning 50um PMMA over 300nm LOR with 6pA Ga+ 30kV,

Increasing stage speed to lower exposure dose.Very easy to overexpose and crosslink.

0.5mm/sec

UGIM 2016

Research Service CentersHerbert Wertheim College of Engineering

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Si+ dose PMMA resist exposures40kV, 2uC/cm2, 100nm PMMA on 200nm LOR

200nm design 400nm design 600nm design

800nm design 1000nm design

UGIM 2016

Research Service CentersHerbert Wertheim College of Engineering

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2uC/cm2 4uC/cm2 6uC/cm2

8uC/cm2 10uC/cm2 15uC/cm2

Si+ area dose PMMA resist exposures

UGIM 2016

Research Service CentersHerbert Wertheim College of Engineering

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Si+ area dose PMMA resist exposuresOver exposures to PMMA do not lead to milling, but severe crosslinking.

65uC/cm2 70uC/cm2

UGIM 2016

Research Service CentersHerbert Wertheim College of Engineering

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Au+ area dose PMMA resist exposures

10uC/cm2

8uC/cm26uC/cm24uC/cm2

20uC/cm2

Onset of PMMA crosslinking noticed at 6uC/cm2.

Complete crosslinking at 20uC/cm2.

UGIM 2016

Research Service CentersHerbert Wertheim College of Engineering

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Additional resist exposuresExtend process to use conventional/commonly available resists.

Use thinned nLOF (an AZ negative resist) to create features.

Si+ 40 kV, 2uC/cm2 , very low dose required to expose the resist.

UGIM 2016

Research Service CentersHerbert Wertheim College of Engineering

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Summary of ion beam lithography

The use of ion beams to expose resist provides:• 10x increase in write speed over electron beam exposures due to higher

polymer chain scission (cascade damage)• Correct exposure is ion dose and ion species dependent• Use of standard resists and known processes (positive and negative)• 3rd party companies are now supporting attachments for FIBs• Meets the need of patterning 200nm to 1000nm features at a fast rate.

HIGH-THROUGHPUT FOCUSED-ION-BEAM LITHOGRAPHY USING DOUBLY CHARGED IONS; N. Garraud et al., presented at ENRI 2013