L-NESS

Laboratory for Epitaxial Nanostructures on Silicon and Spintronics

Affiliated Institutions

Nanoscale Device Group

Facilities

Electron beam lithography

The ever-growing demand for increasing the integration scale of digital electronics has prompted a search for novel devices and fabrication techniques at the nanometer scale. State of the art ArF deep ultra violet (DUV) lithography is able to deliver transistors with the gate pitch as small as 54 nm at the 10 nm node (Intel, 2017). Electron beam lithography (EBL) is an indispensable technique for making the nanoelectronic devices on an even smaller scale (Fig. 1).

Fig. 1. The narrowest metallic line that can be patterned by our old EBL system is ~ 11 nm wide. The line consists of 3 nm of chromium and 10 nm of gold. The line was patterned on a standard SiO2/Si substrate using 200 nm thick PMMA.

In this technique, a highly focused electron beam is scanned over a surface covered by a resist sensitive to electron radiation (e.g., polymethyl-methacrylate - PMMA). Exposed areas become more (positive resist) or less (negative resist) soluble in organic solvents (e.g., methyl isobutyl ketone - MIBK), which are used to develop the exposed pattern. The pattern is transferred to the substrate by etching (e.g., reactive ion etching) or lift-off (e.g., metallization).

While the resolution of DUV lithography is mostly limited by the wavelength of the ArF excimer laser, diffraction is not the limiting factor in EBL due to a very small electron wavelength (less than 0.01 nm at 20 keV). The resolution of EBL is mostly limited by imperfections in the electron optics (aberrations limit the minimum electron beam spot size to ~ 1 nm) and electron scattering in resist and backscattering from the substrate. These effects limit the smallest feature size to ~ 10 nm after lift-off.

EBL is not suitable for a large scale production because electron beam exposure is a serial process (i.e., it is much slower than DUV lithography). On the other hand, EBL is a fabrication tool of choice in nanoelectronics and low-dimensional physics. It allows researchers to fabricate and investigate very small devices which might be used in the future. The main advantage of EBL in research setting is its direct write capability, i.e., no need for making a mask as required by a DUV lithography. EBL is used industrially in mask making for DUV lithography.

The work on EBL in Como started in May 2005 with the installation of a scanning electron microscope (SEM) with a lithography attachment. The group is currently equipped with two EBL systems:

  1. Raith eLINE. This EBL system is fully integrated by Raith and shown in Fig. 2. The main features are: Schottky field-emission gun, acceleration voltage up to 30 kV, high-speed pattern generator (20 MHz), laser interferometer stage with a stitching error < 20 nm and 100 x 100 mm horizontal and 30 mm vertical travel range under full interferometric control, alignment error < 20 nm, automated laser height sensing (working distance error < 5 µm), traxx module for stitching error free continuous writing mode for elongated paths (fixed-beam moving stage mode - FBMS), 3D module, and manual load-lock.
  2. Fig. 2. Our new Raith eLINE system. The EBL resolution limit is ~ 6 nm, corresponding to the width of the lines patterned in a thin HSQ resist at 30 kV.

  3. Philips XL30 SFEG SEM with a Raith Elphy Quantum lithography attachment (6 MHz pattern generator) and a Scanservice beam blanker. This is our original EBL system (shown in Fig. 3) which is mainly used as a backup.

Fig. 3. Our old EBL system. Parts: 1. Column 2. Specimen chamber 3. Secondary electron detector 4. Scanservice final aperture with integrated beam blanker 5. CCD camera 6. High tension valve 7. Liquid N2 tank (for X-ray detector) 8. Keithley pico-ammeter (to measure beam current) 9. Beam blanker power supply 10. Vacuum pumps and high tension circuitry 11. Electronics (printed circuits boards) 12. Power supply 13. Lithography PC (Raith Elphy Quantum) 14. SEM PC with integrated X-ray analyzer.

E-beam evaporators

The group is equipped with two e-beam evaporators. One evaporator is outside of the clean room and used only for large wafers. Our group mostly use the other evaporator which is located in the clean room and used for chips.

Probe stations

The electrical characterizations of our graphene electronic devices are performed in two FormFactor probe stations (e.g., Summit 11000) both by dc and real-time high-frequency measurements. The dc measurements are based on Keithley SMUs (five 2611B and one 2636B), while Keysight 4 GHz and 16 GHz scopes are used for real-time measurements.

Optical lithography

The state-of-the-art ArF DUV lithography is out of reach of most research groups and is commercially used by big semiconductor companies to fabricate integrated circuits. The research groups usually use much less expensive optical lithography in which longer wavelengths (typically, i-line at 365 nm) are used to expose photoresists. This increases the smallest feature size to ~ 500 nm. Our group does not use optical lithography so much because the minimum feature size in our optical process is ~ 1 µm (compare to the smallest feature size of ~ 6 nm of our eLINE EBL system) which is unsuitable for the nanofabrication of short gate graphene field-effect transistors. Photoresists are also more detrimental to electrical properties of graphene than PMMA.

Our group is equipped with a Karl Suss MA56 mask aligner (contact and proximity printing, 4” wafers, 5” masks, i-line, 20 mWcm-2). The aligner is mainly used to pattern SiGe heterostructures (e.g., into Hall bars on which nanostructures are overlaid by EBL).

Atomic force microscopy

The group is equipped with a Veeco Innova atomic force microscope. The microscope is used to scan graphene devices and quantum dots grown on pre-patterned Si substrates.

He cryostat

The group is equipped with a Cryogenic cryogen free (i.e., a closed cycle liquid helium-4) magnet system to 7.5 T with an integrated variable temperature insert. The lowest temperature which can be reached by this cryostat is ~ 1.4 K. The cryostat is connected to a transport measurement system comprised of digital source meters, multi meters, preamplifiers, function generators, an oscilloscope, and an acquisition board. This system is used for the characterization of fundamental investigations of nanodevices (devices for realistic applications are investigated in the probe stations in air ambient).

Clean room

We have a 100 < class < 1000 clean room in which all device processing is done. The clean room is equipped with ovens, spin-coaters, wet benches, ultrasonic baths, a plasma asher, and an optical microscope. A mask aligner is also inside the clean room. Our Raith eLINE system is located in a separate clean-room of a similar class.

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Last updated: 23rd April 2018