Lithography

Micro-nano lithography is the process by which a pattern is written or transferred to a substrate. The process can be physical in nature, utilising a ‘stamp’ to press structures into a softer material, or chemical using light, ions or electron energy to write into photo and electron sensitive resists. This is necessary for the creation of the nanoscale components featured in nearly all modern technologies. MCN offers several different lithographic techniques and instruments, which are briefly explained below.

Focused ion beam lithography

The process

High-resolution Focused Ion Beam (FIB) lithography has significant advantages as a single-step, nanoscale prototyping method that requires neither mask nor resist. It is capable of:

• Subtractive lithography in which atoms are locally milled away by physical sputtering with sub-10nm resolution
• Additive lithography in which materials are locally deposited with sub-10nm resolution
• Local ion implantation for fabrication of an etching mask for subsequent pattern transfer
• Direct material modification by ion-induced mixing.

The uses

FIB lithography is a very versatile technique with a wide range of applications including

• Advanced materials development/characterisation,
• Resist-free, high-resolution patterning of nanostructures,
• Very precise, cross-sectional analysis of samples
• Sample preparation for transmission electron microscopy (TEM) and for atom probe analysis.

The instrument

MCN’s FIB tool is the FEI Helios NanoLab 600 Dual Beam FIB-SEM. It features an ultra-high resolution Scanning Electron Microscope (SEM) and Focused Ion Beam (FIB) with various gas injection systems, including platinum deposition, selective carbon milling and metal-enhanced etching.

It also has a Genesis EDX detector to analyse the atomic composition of samples as well as a STEM detector, allowing the SEM to image very thin samples in transmission. This offers a great opportunity to acquire TEM comparable images with all the advantages of the SEM.

The FIB-SEM has been fitted with additional accessories - the Kleindiek Micromanipulator, which is an insertable probe that allows in-situ physical sample manipulation and characterisation at the nanoscale, and the Kleindiek Nanotechnik Lift-Out Shuttle, which provides in-situ lift-out sample preparation techniques for TEM and atom probe inspection. It features a high-resolution micro-gripper for easy transport and assembly of micro-sized objects.

Electron beam lithography

The process

Electron Beam Lithography (EBL) allows users to write patterns with extremely high resolution, smaller than 10nm in size. It makes use of a highly energetic, tightly focused electron beam, which is scanned over a sample coated with an electron-sensitive resist. The electron beam scans the image according to a pattern defined on a CAD file. The sample is then developed in an appropriate solvent which reveals the structures defined into the resist. This acts as a mold for subsequent pattern transfer techniques such as dry etching or
metal lift-off.

The uses

Due to the high-resolution nature of the technique, EBL has a vast range of applications including, but not limited to, nano-electronics, photonics, plasmonics, nano-fluidics, MEMS, x-ray and neutron optics.

The instrument

MCN’s EBL tool is the Raith EBPG5000plusES, the only one of its kind in Australia. The tool is capable of writing four and six inch masks and wafers with a resolution of up to 10nm. It features 100kV acceleration voltage, automated aperture change and operation, 1mm main field size and <20nm accuracy.

Hot embossing

The process

Hot embossing is a pattern-transfer technique, involving the application of pressure and heat to a polymeric or resist-coated substrate, placed in contact with a master mold. This allows the relief features on the mold to be transferred faithfully. Hot embossing achieves fast patterning at a resolution of 50nm.

The uses

This technique addresses a wide range of applications, from polymer-based lab-on-chip systems, where imprinting is performed on thick polymers substrates, for the fabrication of sub 50nm features for bio-sensing or data recording applications, as well as microfluidics, MEMS, optoelectronics, packaging and SOI production.

The instrument

MCN’s hot embossing tool is the EVG520IS.

Nano imprint lithography

The process

Nano imprint lithography involves a pattern-transfer technique similar to hot embossing, though it is achieved by using a UV curable resist and subjecting the mold to UV light, rather than heat and pressure.

The uses

This is useful in the creation of patterned media, optics, and lab on a chip systems.

The instruments

Nanoimprint Lithography at MCN is performed on either the EVG 520IS or the EVG6200.

PDMS soft lithography

The process

Soft lithography is a process by which rigid master mold structures, typically silicon/SU8, are replicated in an elastomeric material known as polydimethylsiloxane (PDMS). Silicon is exposed to the desired pattern using photoresist, after which the patterned structure is treated with an anti-stick coating and liquid PDMS is poured over it. Once cured, "daughter" devices can be removed from the mold whose features match that of the master.

The uses

Useful for its flexibility, PDMS is ideal in applications involving surfaces which are not flat. PDMS can also be used in microfluidic applications, where rapid prototyping is important. It is also biocompatible and oxygen permeable making it useful in a broad range of bio-applications, and it is optically transparent.

The lab

MCN has a dedicated PDMS laboratory where staff use Sylgard 184 PDMS.

Direct write lithography

The process

Direct write lithography, or maskless lithography, involves exposing a substrate coated in a photosensitive resist to UV light which directly exposes a desired image into the resist film. In the case of a chrome-coated plate, the pattern can be etched to create a photomask.

The uses

Direct write lithography is popular due to the flexibility and cost-effectiveness associated with being unrestricted by a substrate geometry. This is useful in R&D where it is desirable to test many iterations before proceeding with creation of a photo mask.

The instrument

MCN’S direct write lithography tools are the Heidelberg MLA-150 (with Autoloader) &  the Intelligent Micropatterning (IMP) SF100 XPRESS.  The MLA-150 has resolution down to 600nm, and then IMP system features three resolutions – 1µm, 2µm, 5µm.  The write area size is variable.

Photolithography

The process

Photolithography is a process used to transfer a pattern from a mask to a photosensitive resist-coated substrate. The photoresist is then developed in a solution to remove the unwanted material, after which the substrate can move onto the next process such as metal deposition or etching.

The uses

Photolithography is central to most micro- and nanofabrication applications including microfluidics and cantilever fabrication. The minimum feature sized achieved at MCN using mask lithography is 1µm with a subsequent metal lift-off process.

The instruments

MCN features four photolithography tools – the EVG 6200 Mask Aligner, the SUSS MA6, ABM UV Flood Light Source and the PhableR-100C. The EVG6200 and SUSS MA6 allow for aligning multi layer features using multiple masks. This means that multiple layers can be built up on a single device by using appropriate alignment markers in the mask design. The ABM UV Flood Light Source allows for fast patterning at a lower resolution. The PhableR-100C uses DUV for patterning at high resolution below 300nm pitch and exposures are non-contact, protecting wafers and masks from physical contact and contamination.

Thermal Scanning Probe Lithography

The process

Thermal Scanning Probe Lithography is a direct-write lithography process that uses a thermal cantilever. The cantilever tip, heated to 1000ºC, sublimates local PPA resist. The same tip also provides in-situ AFM, providing real time characterisation.

The uses

Thermal Scanning Probe Lithography's rapid 3D nanofabrication is used for nanostructures with nanophotonics, nanooptics, nanoelectronics, and plasmonics applications.

The Instrument

MCN houses the NanoFrazor Explore, which provides sub-15nm resolution and 3D resolution less than 2nm.

Spin coating and wafer development

The process

A crucial element in any lithographic process, coating wafers with photoresist and subsequently developing them, allows for the mold features to be faithfully replicated on the wafers.

The instruments

MCN has both manual and automatic spin coaters, the latter offering users options for both high throughput and single wafer runs while utilising a fully automated and precise robotic system. The Pico Track PCT-150RRE is capable of running wafer sizes of 4 inch and 6 inch diameter with no change-over requirements. Comprised of numerous spin modules, the system can adapt to virtually all photolithography processes with precision and produce patterned wafers with extremely high uniformity. The track system automates all steps continuously including adhesion promoter, pre-baking, resist coating, edge-bead removal (EBR), development, and post-baking.

In addition to the Pico Track, MCN has a stand-alone automated developer system for enhanced photolithography processing. Capable of running 2 inch to 6 inch wafer sizes, this new developer system can be used by itself, or in-conjunction with the Pico Track to provide more resist development options.

Critical Point Drying

The Process

Critical point drying is a process used to dry Micro-electromechanical Systems (MEMs). Forces from the changing of liquid to gaseous phase in traditional drying techniques can destroy a device. The use of supercritical fluid bypasses this phase change, protecting the device in the drying process.

The Instrument

MCN's critical point dryer is the Quorum K850WM. The dryer is optimised for MEM drying and can hold 4 and 6-inch wafers. The dryer can also be used for SEM preparation.

Automated wetbench suite

The instrument:

The automated wetbench suite is comprised of three custom made instruments - a manual wet processing deck, a semi-automated chemical cleaning station and an alcohol vapour dryer.

The manual wet processing deck is equipped with five chemical baths for small batch processing of substrates up to six inches in diameter. The tool is configured specifically for silicon wet etching and targeted metal removal.

The semi-automated chemical cleaning station is enclosed, exhausted and tailored to applications where process control, repeatability, throughput and safety are required. This tool will perform hazardous substrate cleaning tasks using a rotary robot to access wet chemical processing baths for silicon dioxide etching, removing contaminants and controlled rinsing. An accessory to this, the ultrasonic acetone de-scum station, will also be available for the bulk removal of photoresists and high performance photolithography lift-off for submicron lithographic features.

The vapour dryer creates an alcohol vapour above the substrate surface, creating the differential surface tension for drying without any need for heaters or rotation. The system is well-suited to drying applications using planar substrates.