Specific Process Knowledge/Lithography/EBeamLithography

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The JEOL JBX-9500 electron beam lithography system is a spot electron beam type lithography system designed for writing patterns with dimensions from nanometers to sub-micrometers. The minimum electron beam is around 5 nm, the maximum writitng field without stitching is 1 mm x 1 mm.

The machine is located in a class 10 cleanroom (E-2) with tight temperature and moisture control. The room must only be entered when the machines or equipment inside the room is intended to be used.

The user manual, technical information and contact information can be found in LabManager:

E-beam writer in LabManager

Performance of the e-beam writer

Purpose	pattern an electron sensitive resist	Mainly for pattering structures with minimum feature size between 12 nm - 1 µm
Performance	Resolution	Minimum electron-beam size: 12-30 nm depending on resist
	Maximum writing area without stitching	1mm x 1mm
Process parameter range	E-beam voltage	100kV
	Scanning speed	100MHz
	Min. electron beam size	5nm
	Min. step size	1nm
	Beam current range	0.1nA to 60nA in normal conditions (see available condition files here)
	Dose range	0.001µC/cm2 to 100000µC/cm2
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).
	Substrate material allowed	Silicon, quartz, pyrex, III-V materials Wafers with layers of silicon oxide or silicon (oxy)nitride Wafers with layers of metal

Getting started

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To request for an e-beam training session, contact e-beam@danchip.dtu.dk; a DTU Danchip personnel will hereafter provide a time slot. Users require at least 4 training sessions before being allowed full acccess to the machine. The first training will focus on file preparation and compilation alone.

Before you request for a training on the machine

1. It is crucial to have your pattern ready in GDSII format. Check your GDS-file by importing it in e.g. CleWin or L-edit. In order to reach the files from the computers inside the cleanroom it is recommended to either dropbox them or send them per email to yourself.
2. Create sdf and jdf-files:
   • download SuperEdi,
   • read the sdf and jdf-file manual from technical documents found here,
   • find templates of sdf and jdf files on the cleanroom drive in the folder E-beam sdf and jdf templates.
3. Assist a fully trained colleague of yours when she or he e-beam writes, gather as much knowledge about your e-beam run, i.e. which e-beam current, aperture and dose to use, which shot pitch (e.g. SHOT A,10).
4. Study the logbook for the e-beam writer: sheet 1 gives you an overview of which condition files (currents and apertures) have been in use recently by which user on which type of resist. On sheet 2 in this logbook you can find a writing time estimation program.
5. Study all 3 manuals for the machine, they can be found here

General Rules

For safety reasons, even fully trained users are only authorized to mount substrates into the e-beam cassettes, but not authorized to load the cassettes into the autoloader.

There are two daily loading sessions where cassettes will be loaded into the autoloader by trained DTU Danchip personel:

• 10:00
• 13:30

To use the e-beam writer, book the machine via LabManager, note number and type of substrate and which condition file to use in the description field in LabManager. Show up to the loading session before your exposure to mount your substrate and pre-align if necessary.

After your exposure, fully trained users can unload their cassettes from the autoloader, unmount their substrates and re-load an empty cassette into the autoloader. If you are prohibited to unmount your substrates before another user requires the cassette, you must accept that either the next user or DTU Danchip personel unmount your substrates.

Manuals

There are 3 manuals for the e-beam writer:

• A user manual describing the standard procedure when e-beam writing
• A jdf-, and sdf-file manual describing how to prepare sdf-, and jdf-files (found under 'Technical Documents')
• A BEAMER manual describing how to convert your pattern file (GDSII-format) to v30-format (found under 'Technical Documents')

E-beam resists and Process Flows

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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.

Resist	Polarity	Manufacturer	Comments	Technical reports	Spinner	Developer	Rinse	Remover	Process flows (in docx-format)
CSAR	Positive	AllResist	Standard positive resist, approved, very similar to ZEP520. Currently under test.	CSAR_62_and_process_chemicals.pdf‎, CSAR_62_Abstract_Allresist.pdf‎	Manual Spinner 1 (Laurell), Spin Coater Labspin	X AR 600-54/6, MIBK:IPA	IPA		Process_Flow_CSAR.docx‎
ZEP520A	Positive	ZEON	Positive resist	ZEP520A.pdf, ZEP520A spin curves on SSE Spinner	SSE, Manual Spinner 1 (Laurell), Spin Coater Labspin	ZED-N50/Hexyl Acetate,n-amyl acetate, oxylene. JJAP-51-06FC05.pdf‎, JVB001037.pdf‎	IPA	acetone/1165	Process_Flow_ZEP.docx, Process_Flow_ZEP_with_Al.docx‎
ZEP7000	Positive	ZEON	Not approved and not yet purchased. Low dose to clear, can be used for trilayer (PEC-free) resist-stack. Please contact Lithography for information.	ZEP7000.pdf	Manual Spinner 1 (Laurell), Spin Coater Labspin	ZED-500/Hexyl Acetate,n-amyl acetate, oxylene.	IPA	acetone/1165	Trilayer stack: Process_Flow_Trilayer_Ebeam_Resist.docx‎
PMMA	Positive		We have various types of PMMA in the cleanroom, none are provided by DTU Danchip. Please contact Lithography for information.		Manual Spinner 1 (Laurell), Spin Coater Labspin	MIBK:IPA (1:3), IPA:H2O	IPA	acetone/1165/Pirahna
Copolymer AR-P 617	Positive	AllResist	Approved, not tested yet. Used for trilayer (PEC-free) resist-stack or double-layer lift-off resist stack. Please contact Lithography for information.	AR_P617.pdf‎	Manual Spinner 1 (Laurell), Spin Coater Labspin	AR600-55, MIBK:IPA		acetone/1165	Trilayer stack: Process_Flow_Trilayer_Ebeam_Resist.docx‎
HSQ (XR-1541)	Negative	DOW Corning	Approved. Standard negative resist, mainly for III-V materials		Spin Coater Labspin	TMAH, AZ400K:H2O	H2O
ma-N 2403	Negative	Micro Resist	Approved. Standard negative resist.		Spin Coater Labspin	Ma-D333, TMAH, MIF726	H2O	acteone/O2 plasma
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	Manual Spinner 1 (Laurell), Spin Coater Labspin	AR 300-47, TMAH

Alignment of exposure to existing pattern on wafer

Recommended design of global mark: + shaped, 500-1000 microns in length, a few microns in width, clearly labeled.

A set of global marks are needed in order to align an exposure to an existing pattern on a wafer. These marks should be clearly visible in a 100keV SEM, i.e. preferably defined by Ti/Au or another 'heavy' metal. Etched global marks or global marks defined by a light metal as Al can be hard to locate manually as well as automatically.

The global marks should either be crosses or L-shaped and be quite large, i.e. 500 - 1000 microns in length. If the wafer contains a number of identical marks, the marks should be marked in order to identify the 'right' alignment marks (the scan width of the SEM is 1 mm x 1 mm).

The marks should be as narrow as possible.

Global marks formed as crosses with lengths of 1000 microns and 3-5 microns in width are recommended.

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 ready more about this function in the BEAMER manual in LabManager under Technical Documents 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 ZEP520 with Al on top can be found here Process_Flow_ZEP_with_Al.docx‎.