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

Performance of the e-beam writers at DTU Danchip

Equipment		JEOL JBX-9500FSZ	Raith Elphy
Performance	Resolution	~5 nm beam diameter, ~10 nm lines obtained in 50 nm thick resist (CSAR)	~70 nm lines obtained in 50 nm thick resist (CSAR)
	Maximum writing area without stitching	1mm x 1mm	2mm x 2mma
Process parameter range	E-beam voltage	100 kV	5-25 kV
	Scanning speed	100 MHz	6 MHz
	Min. electron beam size	4 nm	Not measured
	Min. step size	1 nm	1 nma
	Beam current range	0.1nA to 60nA in normal conditions	0.01 to 5 nAa
	Dose range	0.001 - 100000µC/cm2	0.000001 µC/cm2 to infinitya,b
Samples	Batch size	Wafer cassettes: 6 x 2" wafers 2 x 4" wafers 1 x 6" wafer Special wafer cassette with slit openings of 20 mm (position A), 12 mm (position B), 8 mm (position C) and 4 mm (position D).	Only chips, max dimension about 20 x 20 mm. (If at least one dimension is less than 23 mm, bigger chips (e.g. 40 x 20) may be allowed - contact Danchip)
	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

^a The Raith tool with the attached LEO SEM allows ample freedom in the choice of most parameters. The limits indicated here refer to best cases and may be further constrained by your current selection of microscope and patterning parameters.

^b For all applicable purposes.

E-beam resists

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E-beam resists in the cleanroom

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

• 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. For CSAR, ZEP, mr EBL, and most PMMAs, it is anisole. If in doubt which solvent your e-beam resist contains, read the MSDS of the resist to be found here.
• If you need to dilute the resist, find a measurement beaker and clean it thorughly 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 anisole 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 trainning, 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.

E-beam resists and Process flow

The table describes the e-beam resist used in the cleanroom for standard e-beam exposure. Some of resists are not provided by DTU Danchip and some are not yet approved for common use in the cleanroom and are currently being tested. If you wish to test some of these resists or other resists, please contact Lithography.

Standard DTU Danchip resists purchased and tested by DTU Danchip:

Resist	Polarity	Manufacturer	Comments	Technical reports	Spin Coater	Thinner	Developer	Rinse	Remover	Process flows (in docx-format)
CSAR	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
ZEP520A (not supplied by Danchip anymore)	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

Non-standard DTU Danchip resists not purchased by DTU Danchip:

Resist	Polarity	Manufacturer	Comments	Technical reports	Spinner	Thinner	Developer	Rinse	Remover	Process flows (in docx-format)
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

If the resist is not adhering properly to the substrate, the coating looks like full of pinholes and the edge is not covered, try to remove the resist, dip 30s. in BHF, rinse 5 mins and spin dry, the spin coat again.

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 Danchip. 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 Danchip, 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