Thermal Probe Lithography

Thermal Probe Lithography (TPL) is a form of Scanning Probe Lithography used to transcribe patterns or images to a surface at the micro or nanoscale. TPL uses a micro cantilever probe with a sharp tip, usually with a radius of curvature <50 nm,^[1] that can be heated to temperatures upwards of 700°C.^[2] The heated tip can be used to evaporate solvents from a resist or to decompose and remove a mask substrate. Both of these methods result in polymer masks. A thermal probe is also useful for depositing special inks, a process known as thermal Dip Pen Lithography.

Certain substrate materials such as PMMA can be indented instead of decomposed. Due to the inherent metrological properties of an AFM, these indents can be read by the movement of the scanning tip, and even deleted (by reheating the surface near the indentation). This forms the basis for a thermal data storage system.

Types and Methods

Thermal Probe Lithography has several important advantages over other forms of lithography. TPL can pattern much smaller features than other forms of lithography down to 10 nm, it is cheaper than many other lithography methods, and it is capable of performing surface metrology immediately following patterning. Optical systems (Photolithography) have a diffraction limit. Scanning Probe methods are not limited by diffraction. Optical systems are exponentially more expensive to achieve smaller resolutions, making them often impractical at nanometer scales. The Thermal Probe method is slower than optical systems, however there is the possibility to create large arrays of probes. This would dramatically increase writing speed. Another advantage of TPL is that an AFM cantilever can also perform surface metrology, allowing the patterned surface to be imaged immediately after patterning. This is helpful because it eliminates an essential separate inspection of the device often done with Scanning Electron Microscopy. This separate inspection requires additional tooling, time, and money.

TPL also has advantages over other forms of Scanning Probe Lithography Such as Dip Pen Lithography, or mechanical deformation (scratching). These methods require the probe tip to be removed from the substrate in order to stop writing. Thermal Probe Lithography offers the advantage that the heat can be turned off while the tip remains in contact with the surface, and the writing will stop. This gives TPL a significant speed advantage. This works because such a small tip has a time constant in the micro to milliseconds,^[2] allowing for rapid heating and cooling of the tip. Not having to remove the probe from the substrate offers a time advantage over other Scanning Probe methods. Distinct but closely related to TPL is Thermochemical Nanolithography (TCNL)

There are two approaches to patterning using Thermal Probe Lithography: Positive and Negative. These can be thought of as photolithography positive and negative exposures with the heating element being equivalent to UV exposure.

Negative

In certain resists, a heated probe will locally evaporate solvents in the resist.^[3] The evaporated areas can then resist a dilute developer, which removes the remaining resist and allows for subsequent processing. Lines as small as 2 μm were created using standard evaporated resists, however using an e-beam resist created lines as narrow as 100 nm.^[3]

Positive

A special form of polycarbonate has been used as a TPL mask.^[4] The polycarbonate must be crosslinked before lithography, preventing it from melting and running. At a temperature above 400°C, the polymer will decompose nearly completely.^[4] Lines as narrow as 10 nm have been drawn this way. It has also been shown that this polycarbonate performs with a comparable selectivity to other mask materials,^[4] this is important for subsequent processing such as Reactive Ion Etching (RIE).

Thermal polymer decomposition

The breaking of covalent bonds (such as in the polycarbonate process above) requires large amounts of energy, and is therefore not ideal for high patterning speeds. Special resists have been developed that are linked by hydrogen bonds. These resists require much less energy to dissolve, and can therefore be patterned faster. Speeds of up to 5*10⁴ μm²/hr have been demonstrated; sufficient for rapid prototyping.^[5] In addition, by controlling the duration and the temperature of the heat pulse, the depth of the tip into the resist can be controlled. Through this hydrogen bond cutting method, complex 3D geometries can be created. In one work, a nanoscale version of the Swiss mountain the Matterhorn was created in a resist. It was later transferred to the substrate by RIE.^[5]

Thermal Dip Pen Lithography

A heated probe tip version of Dip Pen Lithography has also been demonstrated, thermal Dip Pen Lithography (tDPL), to deposit nanoparticles.^[6] Semiconductor, magnetic, metallic, or optically active nanoparticles can be written to a substrate via this method. The particles are suspended in a PMMA or equivalent polymer matrix, and heated by the probe tip until they begin to flow. The probe tip acts as a nano-pen, and can pattern nanoparticles into a programmed structure. Depending on the size of the nanoparticles, resolutions of 78-400 nm were attained. An O₂ plasma etch can be used to remove the PMMA matrix, and in the case of Iron Oxide nanoparticles, further reduce the resolution of lines to 10 nm.^[6] Advantages unique to tDPL are that it is a maskless additive process that can achieve very narrow resolutions, it can also easily write many types of nanoparticles without requiring special solution preparation techniques. However there are limitations to this method. The nanoparticles must be smaller than the radius of gyration of the polymer, in the case of PMMA this is about 6 nm. Additionally, as nanoparticles increase in size viscosity increases, slowing the process. For a pure polymer deposition speeds of 200 μm/s are achievable. Adding nanoparticles reduces speeds to 2 μm/s, but is still faster than regular Dip Pen Lithography.^[6]

Thermal Memory

In the case of TPL, the polymer was often crosslinked before writing to ensure total decomposition instead of melting or running.But an interesting applications of certain non crosslinked polymers has been described.Non crosslinked polymers retain a low Glass Temperature, around 120°C for PMMA^[7] and if the tip is heated to above the glass temperature, it leaves a small indentation. Indentations have been made at 3 nm lateral resolution.^[8] By heating the probe immediately next to an indentation, the polymer will remelt and fill in the indentation, erasing it. After writing, the probe tip can be used to read the indentations. If each indentation is treated as one bit then a storage density of .9 Tb/in² has theoretically been achieved.^[8]

Thermal writing and erasing of a bit

The Millipede designed by IBM, is an example of a thermal memory storage device being researched.^[9] In this example, a large array of AFM probes are able to read and write in parallel, attempting to make the process as fast and data dense as current magnetic storage hard drives.

Probe Cantilevers

Resistive Heating

The AFM cantilever is generally made from a silicon wafer using traditional bulk and surface micromachining processes. The cantilever is heavily doped in the cantilever arms, and lightly in the tip to produce a resistive heater where the largest fraction of heat is dissipated in the tip. Such a small tip can heat and cool very fast; an average tip in contact with polycarbonate has a time constant of .35 ms.^[2]

Tip Wear and Hardness

Because these tips are very sharp, and in constant contact with materials that are often as hard or harder than silicon, the tips tend to wear. Since manufacturing processes will require high throughput, large arrays of cantilever tips will be required. If these tips wear easily or quickly the whole process becomes inefficient. Harder tips have been researched. Ultrananocrystalline diamond coated tips have been developed.^[1] The application of ultrananocrystalline diamond is done by hot filament Chemical vapor deposition and can be scaled to produce many probe tips simultaneously. Ultrananocrystalline diamond has been tested for wear resistance, and has proven very resilient.^[1] These tips show promise as very long lasting probes in large scale manufacturing with large arrays of probes.

Applications

Although Thermal Probe Lithography is still largely research-based, there are emerging applications. TPL has the potential to become a major player in lithography processes because it has several unique advantages such as very small-scale writing capability, and deposition of various nanoparticles. Although TPL is not currently fast enough for batch manufacturing, it can be scaled with large arrays of probes. The software required to run coordinated large probe arrays will be complex, as will the hardware required to keep probes aligned with the substrate and each other. The transition of TPL from a research tool to a manufacturing tool is a challenge of systems engineering.

TPL research has also shown the possibility for high-density data storage. Thermal memory storage is currently years of research behind current magnetic memory. If it can be proven that a theoretical advantage exists, it may be possible for this type of memory to become commercialized.

New Developments

A recently discovered material, Graphene offers very high mobilities, resulting in transistors that can operate in the Gigahertz range. The zero band gap leads to excessive leakage in many applications. Graphene oxide provides a band gap of greater than .5 eV, but becomes highly conductive upon reduction. Among the several ways to reduce graphene oxide is a heated thermal probe. At temperatures of 100-250°C, reduction of graphene oxide occurs.^[10] Nanoribbons of graphene oxide can be patterned from 12 nm to 20 μm, and large probe arrays could make this technology commercially viable for graphene electronics in the future.

References

See also

• Nanolithography
• Dip-Pen Nanolithography
• Photolithography
• Scanning Probe Lithography
• Atomic Force Microscopy