Specific Process Knowledge/Lithography/EBeamLithography

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ebl e-beam ebeam ebeam ebeam ebeam ebeam ebeam ebeam ebeam ebeam ebeam ebeam ebeam ebeam ebeam ebeam ebeam ebeam ebeam ebeam ebeam ebeam ebeam ebeam ebeam ebeam ebeam ebeam ebeam writer e-beamwriter ebeamwriter e-beamlithography

JEOL JBX-9500 E-beam writer positioned in room E-2

Introduction to E-beam lithography at DTU Nanolab

DTU Nanolab has two EBL exposure systems, a JEOL JBX-9500FSZ and a Raith eLINE Plus system. The two systems are very different and new users should consult the EBL team to dertermine which system is appropriate for a particular project or type of sample. The general specifications of the two tools are given in the table below and may serve as a guideline for choice of system to use, especially the pros and cons list at the end of the table.

Users can request training sessions on either of the two exposure systems by contacting e-beam@nanolab.dtu.dk. Please provide all relevant process information about your substrate/process in your inquiry.

Equipment		JEOL JBX-9500FSZ	Raith eLINE Plus
Performance	Resolution	~5 nm beam diameter, ~10 nm lines obtained in 50 nm thick resist (CSAR), 7 nm in HSQ	~35 nm lines obtained in 180 nm thick resist (CSAR)
	Maximum writing area without stitching	1mm x 1mm	1mm x 1mm
Process parameter range	E-beam voltage	100 kV	1-30 kV
	Scanning speed	100 MHz	20 MHz
	Min. electron beam size	4 nm	10 nm
	Min. step size	0.25 nm	1 nm
	Beam current range	0.22 nA to 100 nA	0.01 to 12 nA
	Dose range	0.001 - 100000µC/cm2	?
Samples	Batch size	Wafer cassettes: 6 x 2" wafers 2 x 4" wafers 1 x 6" wafer 1 x 8" wafer Special chip cassette with slit openings of 20 mm (position A), 12 mm (position B), 8 mm (position C) and 4 mm (position D).	Chips up to 75 x 75 mm 4" wafer holder 6" wafer holder (stage movement limited to central 100 x 100 mm region)
	Substrate material allowed	Silicon, quartz, pyrex, III-V materials Wafers with layers of silicon oxide or silicon (oxy)nitride Wafers with layers of metal	Silicon, quartz, pyrex, III-V materials Wafers with layers of silicon oxide or silicon (oxy)nitride Wafers with layers of metal
General considerations	Pros	100 kV Sub 10 nm resolution Automatic beam optimization High current and process speed Automatic sample exchange High level of programmability for automatic job execution EBL workhorse for large designs	Readily available More intuitive software Easier SEM mode alignment Build in SEM automation for post exposure process control 2D stacks (HBN/graphene) allowed without Al coating Excellent for small chips or small area design exposure
	Cons	Steep learning curve Availability - booking calendar is usually full 5 weeks ahead	Maximum 30 kV User dependent performance/beam optimization Minimum feature size >35 nm Difficult to handle design files >1 GB Slower writing speed

E-beam resists

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E-beam resists and process guidelines

DTU Nanolab offers a limited number of standard EBL resist for our users. Our standard resist and process guidelines are summarized below.

Resist	Polarity	Manufacturer	Comments	Technical reports	Spin Coater	Thinner	Developer	Rinse	Remover	Process flows (in docx-format)
CSAR AR-P 6200	Positive	AllResist	Standard positive resist, very similar to ZEP520.	Allresist_CSAR62_English.pdf‎,, CSAR_62_Abstract_Allresist.pdf‎	See table here	Anisole	AR-600-546, AR-600-548, N50, MIBK:IPA	IPA	AR-600-71, 1165 Remover	Process Flow CSAR.docx‎ Process Flow CSAR with ESPACER Process Flow CSAR with Al Process Flow LOR5A with CSAR
Medusa AR-N 8200	Negative	AllResist	Both e-beam and DUV sensitive resist.				AR 300-47	H2O
AR-N 7500	Negative	AllResist	Both e-beam, DUV and UV-sensitive resist. Currently being tested, contact Thomas Pedersen for information.	ARN7500.pdf‎	See table here	PGMEA	MIF726	H2O

It is possible to obtain permission to user other resists at DTU Nanolab, users must however provide these resists and possibly developers themselves. A non-exhaustive list of user supplied EBL resist used at DTU Nanolab and some process guidelines can be found in the table below.

Resist	Polarity	Manufacturer	Comments	Technical reports	Spin Coater	Thinner	Developer	Rinse	Remover	Process flows (in docx-format)
ZEP520A	Positive resist, contact Lithography if you plan to use this resist	ZEON	Positive resist	ZEP520A.pdf, ZEP520A spin curves on SSE Spinner	See table here	Anisole	ZED-N50/Hexyl Acetate,n-amyl acetate, oxylene. JJAP-51-06FC05.pdf‎, JVB001037.pdf‎	IPA	acetone/1165	Process_Flow_ZEP.docx
Copolymer AR-P 617	Positive	AllResist	Approved, not tested yet. Used for trilayer (PE-free) resist-stack or double-layer lift-off resist stack. Please contact Lithography for information.	AR_P617.pdf‎	See table here	PGME	AR 600-55, MIBK:IPA		acetone/1165	Trilayer stack: Process_Flow_Trilayer_Ebeam_Resist.docx‎
mr EBL 6000.1	Negative	MicroResist	Standard negative resist	mrEBL6000 processing Guidelines.pdf‎	See table here	Anisole	mr DEV	IPA	mr REM	Process_Flow_mrEBL6000.docx‎
HSQ (XR-1541)	Negative	DOW Corning	Approved. Standard negative resist	HSQ Dow Corning, MSDS HSQ	See table here		TMAH, AZ400K:H2O	H2O		process flow HSQ High resolution patterning with HSQ
AR-N 7520	Negative	AllResist	Both e-beam, DUV and UV-sensitive resist. Currently being tested, contact Peixiong Shi for information.	AR-N7500-7520.pdf‎	See table here	PGMEA	AR 300-47, TMAH	H2O
PMMA	Positive	AllResist	We have various types of PMMA in the cleanroom. Please contact Lithography for information.		See table here	Anisole	MIBK:IPA (1:3), IPA:H2O	IPA	acetone/1165/Pirahna
ZEP7000	Positive	ZEON	Not approved. Low dose to clear, can be used for trilayer (PEC-free) resist-stack. Please contact Lithography for information.	ZEP7000.pdf	See table here	Anisole	ZED-500/Hexyl Acetate,n-amyl acetate, oxylene.	IPA	acetone/1165	Trilayer stack: Process_Flow_Trilayer_Ebeam_Resist.docx‎

User resist bottles in the cleanroom

We recommend all groups or users to have their own bottle of e-beam resist inside the cleanroom. Please follow the guidelines below.

• Find a blue-capped glass bottle in the cupboard next to office 055 in 346 (outside the cleanroom).
• Bring the bottle inside gowning; clean it thoroughly on the outside with water or alcohol
• Bring the bottle to a fumehood inside the cleanroom; clean the bottle and the lid thoroughly on the inside with the main solvent of your resist, i.e. for CSAR use anisole. If in doubt which solvent your e-beam resist contains, consult the MSDS of the resist found here.
• If you need to dilute the resist, find a measurement beaker and clean it thoroughly in same solvent as your own bottle. For CSAR, ZEP, mr EBL, and anisole-based PMMA, you can use the measurement beaker in the box inside the fumehood in E-4.
• Let the bottle dry in the fumehood.
• Bring the (main) bottle of resist to the fumehood. Carefully unscrew the lid of the resist bottle. If necessary, wipe the thread of the resist bottle before you pour resist into your own bottle; dried resists may sit on the thread and be transferred into your bottle (or worse into the large resist bottle) when pouring.
• Clean all bottles on the outside with acetone or IPA, let them fume off in the fumehood. Clean the measurement beaker as well.
• Find a label to your resist bottle; bottles without labels will be removed from the cleanroom.
• Write name, Lotnumber, group and date on your bottle.

When spin coating e-beam resist, you should use a pipette to transfer resist from your bottle to the substrate. If you pour the resist directly from your bottle, you will leave resist in the thread that will soon dry out and leave particles in the resist. The disposable pipettes need to be thoroughly cleaned with a N2 gun before use (app. 20 s). After some practice, you can obtain particle-free 4" wafers if bottle and pipette (and spin coater) are properly cleaned.

Keep your resist bottles in up-right position, do not tilt or shake them too much, this can spread particles from the sidewall into the resist.

Development

There are many different developers for different E-beam resist, but since CSAR and ZEP520A are the most used at Nanolab, we have installed a semi automatic puddle developer: Developer E-beam in E-4.

To accommodate most users, this tool uses developers: AR 600-546 for development of CSAR 6200 resist series and ZED N-50 for ZEP520A resists and IPA as a rinsing step. Intermediate results indicate that using ZED N-50 for CSAR 6200 series resist will introduce an elevated amount of residues, and vice verva for AR 600 - 546 used on ZEP 520A, but it is inconclusive.
We therefore recommend to use:

• ZED N-50 for Zep520A
• AR 600-546 for CSAR 6200 series

The recipes are divided into different developer and development times.

The recipes 546 are short for AR 600-546 and N-50 is short for ZED N-50.
After the developer there is a time indication eg. 10s, 30s, 60s ect. Since this system is a spray nozzle that creates a puddle of developer on the substrate the spray is on for 10s. to cover any type of substrates, however this procedure starts the actual development, and the "idle" time is hence shortened accordingly. The system is setup so that 10s. in the Developer E-beam correlates to 10s. in a beaker, however all designs are different and larger or smaller structures may need different development times.

Cold development

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We do have the possibility of making cold development, to get more vertical and smoother sidewalls. We have a coolplate that can go down to -2°C, with a ramp of app. 3 °C/min.

Is it recommended to make a dose test, if one is to transfer from ordinary development to cold.

Procedure

1. Pour developer suited for your resist into a small blue cap bottle, and label it, remember to mark it with lot number, date and your name.
2. Leave this small bottle of developer in the fridge in Cx1 at least overnight to acclimatize.
3. Take the coolplate located in D3 (see picture) and connect it in Fumehood 10 (E-beam) place it on the sink-lid to keep a good airflow.
4. Hold left button on the front panel of the cool plate and set temperature with up/down arrows.
5. The cool plate can go down to -2°C, and the fridge is +5°C hence use a beaker and pour cold developer into it and place on the coolplate, leave it to acclimatize for the desired temperature.

Development time in cold development, should be subject to testing, depending on structure size and density, but is comparable with "normal" development times.

When developing at low temperatures, condensation can occur

Remember to clean up after you are done

Proximity Error Correction

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Even though the electron beam diameter is below 5 nm, the feature and pitch resolution in resist is limited by the forward and backward scattering of the electrons. The forward scattering depends on the electron acceleration voltage, the resist material and thickness. The backward scattering depends on the electron acceleration voltage and the substrate material [1], [2].

As the travel distance of backscattered electrons is fairly large, e-beam patterned structures will be influenced by adjacent e-beam patterned structures, i.e. a proximity effect. These proximity effects can be avoided either by simulating a proximity error correction (PEC) in BEAMER or by using the right stack of e-beam resist.

Proximity Error Correction (PEC) in BEAMER

BEAMER is endowed with a software that corrects for proximity errors in the e-beam exposure. You can read more about this function in the BEAMER manual here and in the BEAMER presentation here BEAMERPresentation.pdf‎.

The proximity error correction require a forward and a backward range parameter, alfa and beta, and a ratio of backscattered energy to the forward scattered energy, eta. As alfa depends on the electron acceleration voltage, which is constant at 100kV, alfa is in BEAMER fixed to 0.007. Help to find beta and eta can be found here.

Alternatively, a point-spread function can be used in BEAMER to calculate the optimised dose-variation.

Trilayer resist stack

As an alternative to PEC, a trilayer reists stack with a thin layer of thermally evaporated Ge can be used [3]. This reists stack has not yet been tested at DTU Nanolab. A process flow for this procedure can be found here Process_Flow_Trilayer_Ebeam_Resist.docx‎, but please contact Lithography before use.

Charging of non-conductive substrates

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All substrates are grounded to the cassette when properly loaded. In a non-conducting substrate, the accumulation of charges in the substrates will however destroy the e-beam patterning. To avoid this, a charge dissipating layer is added on top of the e-beam resist; this will provide a conducting layer for the electrons to escape, while high-energy electrons will pass through the layer to expose the resist.

If you wish to investigate the charge dissipation using other methods than below, please contact Lithography.

ESPACER

Espacer is a chemical that works as a discharging layer; it is spun onto the wafer on top of the resist and easily rinsed off the wafer after e-beam exposure. Visit this page for more information: Espacer

Aluminum coating

At DTU Nanolab, we recommend to use a thin (20 nm) layer of thermally evaporated aluminum on top of the e-beam resist. Preferably, the thickness of Al and the e-beam dose should be optimised to the features you wish to e-beam pattern [4]. A good starting point is 20 nm Al; from here dose and development can be optimised to reach the resolution and feature size required.

The process flow for a standard e-beam exposure on CSAR with Al on top can be found here Process Flow CSAR with Al.

Literature on E-beam Lithography

• Handbook of Microlithography, Micromachining, and Microfabrication, Volume 1: Microlithography, P. Rai-Choudhury (Editor), chapter 2 (p 139 – 250). Link to book can be found here: http://www.cnf.cornell.edu/cnf_spietoc.html
• Lithography, Wiley, 2011: Chapter 3, Electron Beam Lithography by Stefan Landis: http://onlinelibrary.wiley.com/doi/10.1002/9781118557662.ch3/summary