For multilayered semiconductordevices, various nanostructureswith high aspect ratios, such as nanoplates, nanopillars, and nanowires,should be fabricated.6 Among the variousnanostructures, nanoplates with oxide or nitride/nitride/metal multilayersare one of the essential components in thin-film transistors sinceoxide, nitride, and metal layers are used as the gate dielectric,adhesion layer, and gate metal in metal/insulator/semiconductor (MIS)structures, respectively.4 For smallerchannel widths and higher stacks, the thickness of the thin filmsshould be decreased,7 which makes the thinfilms more vulnerable to various thermomechanical failures, such asbending, cracking, and, particularly, interfacial delamination.8,9 These factors eventually lead to the catastrophic failure of thedevices.4
Theresidual stress distribution in the multilayers during thedeposition process was calculated using plate bending theory.18,22 The distribution of thermomechanical-residual stress within themultilayered thin films was theoretically predictable through thefollowing calculations. Strain due to heating and cooling of a multilayersystem with n thin film is22
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In this study, external stress was imposedon the samples by fourdifferent methods: thermal annealing and quenching, laser irradiation,scratching, and four-point bending tests. For the thermal annealingand quenching test, the specimens in Table 1 were put into a thermal furnace at 800 Cfor 75 s. Since the annealing time was quite long, we could ignorethe thermal diffusion throughout the specimen. Then, the annealedsamples were quenched to room temperature using an aluminum heat sink.As ΔT in the thermal test is larger than thatof the deposition process (490 C), greater residual stressesthan those during the deposition were induced in the thin films.
Spin coating is a common technique for applying thin films to substrates. When a solution of a material and a solvent is spun at high speeds, the centripetal force and the surface tension of the liquid together create an even covering. After any remaining solvent has evaporated, spin coating results in a thin film ranging from a few nanometres to a few microns in thickness.
The use of spin coating in organic electronics and nanotechnology is widespread and has built upon many of the techniques used in other semiconductor industries. The relatively thin films and high uniformity required for effective device preparation, as well as the need for self-assembly and organisation to occur during the casting process, do however necessitate some differences in method.
Spin coating is extremely widely used and has a varied range of applications. The technique can be used to coat anything from small substrates measuring only a few millimetres squared, up to flat panel displays which might be a metre or more in diameter. It is used for coating substrates with everything from photoresistors, insulators, organic semiconductors, synthetic metals, nanomaterials, metal and metal oxide precursors, transparent conductive oxides and many more materials. In short, spin coating is ubiquitous throughout the semiconductor and nanotechnology R&D and industrial sectors.
As spin speeds decrease below 1000 rpm or with very viscous solutions it becomes increasingly difficult to get a high quality film. Therefore, if possible, it is generally recommended that the ink is reformulated (increased concentration or change in solvent) to allow spin coating above this speed. However, there are many situations in nanotechnology where this is either not desirable or not possible. For example, better crystallisation will happen at low speeds and some materials just do not have sufficient solubility to reach the desired thickness at 1000 rpm.
Here in the UK the ambient humidity in summer can swing quite dramatically from less than 20% to nearly 100% depending upon the weather. On times of very high humidity, typically during a rainstorm, films spun from aqueous solutions may still be wet after the normal 30 second spin duration and this can have a significant effect on device performance. As such, although the majority of work is done in temperature/humidity controlled environments it is advisable to keep a close eye on the ambient conditions and place a thermometer/hygrometer next to the spin coater.
However, for many materials an ink solution may not be stable and will re-form aggregates or crystallites over time, such as the below examples of P3HT films, PCBM crystallites and F8BT aggregates, all of which will form if an ink is left standing for long times (hours to days).
Many researchers will have faced issues coating films where the solute has poor or marginal solubility in the solvent used. This is especially common when using high molecular weight materials, or when trying to replace the solvent in an established system. Solutions with poor solubility can produce films with unwanted precipitation, especially as solvent evaporation drives the concentration above the solubility limit, leading to comets or particulates in the final film. Precipitation can be avoided by keeping solutions below the solubility limit of the material, but this may lead to deposited films being too thin.
Another method used to help improve film quality is the sonication of solutions with poor solubility. Although sonication itself does not increase the solubility limit of materials, it can improve the rate of dissolution, which can be a slow process for materials like high molecular weight polymers. In some cases, filtering a solution will improve film quality. However, this is not always recommended, as important solution components can sometimes be accidentally filtered out.
When highly-volatile solutions are used, they can easily drip out of pipettes during spin coating, thus requiring an adjustment of technique. This is due to the evaporation of solvent inside the pipette, which consequently increases the pressure within the tip. This is a problem as the solution will not be deposited in one continuous motion - resulting in uneven coating. The resulting issue can appear as swirls, where each droplet only partially covers an area of the substrate. For the most volatile solvents - typically those with boiling points below 50C - swirls can be seen even when using a continuous dispense technique. To counter this, either a static dispense technique or a larger volume of solution should be used.
Marangoni defects are not always seen when using high-volatility solutions. This is because where the solution also has particularly low or high viscosity, viscous forces dominate over the Marangoni flow. However, even if uniform films have been formed, fast evaporation can lead to unfavourably amorphous films. Thus, slowing the rate of evaporation is still important to yield better crystallinity.
Solutions with low volatility can also cause issues, mainly through long drying times. Using very long spin times to dry the solvent can lead to very thin films, as evaporation takes place so slowly - which means that more of the fluid is removed via flow thinning. To tackle this without impacting film thickness, a slower drying step can be introduced as a second stage - typically at a spin speed of approximately a quarter of the main speed. Low-volatility solvents can also cause more pronounced edge effects. This is because it takes a longer time for the solution to be thrown off from the edge of the substrate, therefore slow solvent evaporation leads to a thicker film around the edges compared to the centre.
Highly-viscous solutions can also present challenges, as they will be more resistant to deformation from shear forces during the spin coating process. This means that the outflow of solution from the substrate (as it reaches the desired spin speed) will be slower, and thinning of the solution during spinning will be reduced. This can lead to incomplete spreading of the solution across the surface of the substrate, which can sometimes be counteracted by static spin coating with a large amount of solution. Reduced thinning may also lead to undesirably thick films, thus requiring the use of lower solution concentrations.
For some solutions (e.g. colloidal solutions, polymer solutions, or solutions close to gelation), their behaviour will be significantly non-Newtonian. Newtonian solutions have a viscosity that does not change with force applied, meaning that shear stress and shear rate will scale linearly. In contrast, non-Newtonian solutions can change viscosity depending on the force applied, meaning shear rate responds to shear stress in a different way. These are known as 'shear-thinning' or 'shear-thickening' solutions, depending on whether or not the force applied decreases or increases the viscosity. Some solutions may also exhibit thixotropic or rheopectic behaviour, where the viscosity depends both on i) force applied, and ii) on how long it is applied for.
For these types of materials, final film thickness will not always be proportional to the inverse square of the spin speed - so film thickness can be difficult to predict, and final films are not always level. For a more in-depth explanation of the reasons behind this, please refer to the spin coating film thickness equations. Due to their diverse range of behaviours, non-Newtonian solutions can present a significant challenge when it comes to the deposition of highly-uniform films.
Thin film deposition techniques such as magnetron sputtering provide opportunities in this context since complex chemistries, including all metals and non-metals, are easily accessible by the design of the sputtering materials and the reactive gas atmosphere11,12,13. Moreover, a vast chemical composition space is accessible by combinatorial thin film deposition in which the geometric arrangement of the sputtering sources enables tailoring of a composition distribution on the substrate with more than 100 different individual chemical compositions being attainable per day within a single thin film growth experiment14,15. The lower boundary amounts for investigations into the composition-dependent phase formation of structural materials are on the order of ten grams for bulk processing and ten milligrams for thin films. Hence, given the same amount of material for both processing strategies, combinatorial thin film growth offers a factor of 105 higher efficiency than conventional bulk processing. The combinatorial approach has been e.g. successfully applied to the design of ultra-strong metallic glasses or low-cost alternatives to nickel-base superalloys for medium-to-high temperature structural applications16,17. 2ff7e9595c
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