Thursday, 15 November 2012

spin coating

Spin coating is used for many applications where relatively flatsubstrates or objects are coated with thin layers of material. The materialto be made into the coating must be dissolved or dispersed into a solvent of some kind and this coating solution is then deposited onto the surface andspun-off to leave a uniform layer for subsequent processing stages andultimate use. The spin coating method relating to the present invention comprises the steps of discharging a coating liquid from the tip of a nozzle and dropping onto a surface of a rotating substrate to form a uniform coating film on the surface of the substrate, characterized in that the distance between the tip of the nozzle and the surface of the substrate during discharging the coating liquid is 2.5 mm or less. It is an objective of the present invention to reduce or prevent incorporation of bubbles into a coating film formed by spin coating a surface of a substrate with a coating liquid having a high viscosity at a relatively high discharging velocity. The ability to deposit and tailor reliable semiconducting films (with a particular recent emphasis on ultrathin systems) is indispensable for contemporary solid-state electronics. The search for thin-film semiconductors that provide simultaneously high carrier mobility and convenient solution-based deposition is also an important research direction,
 , with the resulting expectations of new technologies (such as flexible or wearable computers, large-area high-resolution displays and electronic paper) and lower-cost device fabrication. Here we demonstrate a technique for spin coating ultrathin (50 Å), crystalline and continuous metal chalcogenide films, based on the low-temperature decomposition of highly soluble hydrazinium precursors. We fabricate thin-film field-effect transistors (TFTs) based on semiconducting SnS2-xSex films, which exhibit n-type transport, large current densities (>105 A cm-2) and mobilities greater than 10 cm2 V-1 s-1—an order of magnitude higher than previously reported values for spin-coated semiconductors. The spin-coating technique is expected to be applicable to a range of metal chalcogenides, particularly those based on main group metals, as well as for the fabrication of a variety of thin-film-based devices (for example, solar cells, thermoelectrics and memory devices). The image at the right, from Millsaps and Pohlhausen, [J. Aeronautical Sci., (1952) 120-126] shows a schematic of the ideal airflow field above an infinitely large spinning disk. At the surface of the disk there is a “no-slip” condition so the contacting air must be exactly co-rotating — hence the flow vectors pointing essentially tangentially to any point at a given radius (and proportional to the distance from the center). At moderate distances from the surface a centripetal acceleration must be provided by the viscous effects; this condition is thus maintained only when some outward radial air flow is also occurring. This outward flow is balanced by some minor downdraft over the entire wafer. This is a steady state configuration and does not include inertial effects included in the “spin-up” stages. This air flow pattern also only hold true so long as the flow is laminar. A “boundary layer” of uniform thickness thus exists over the entire surface area of the spinning wafer: it is through this boundary layer that evaporating solvent must diffuse. Because the boundary layer is constant in thickness over the wafer then the evaporation rate as a function of position is predicted to also be constant. The steady flow field described above is limited to cases where the flow is laminar and where it is “steady”. In fact, except for very large wafers, most spinning conditions DO satisfy the constraint of having laminar flow. However, there can be un-steady oscillating instabilities in the boundary layer near the surface of the wafer.
 These form spiral shaped waves or rolls that are called “Ekman” spirals. Wahal, et al [Applied Physics Letters 62 (1993) 2584-6] have experimentally observed Ekman spirals (shown in the figure at right) for nominally laminar conditions in spin coating. They claim that these instabilities can lead to coating thickness variations, but have not explained WHY that would be the case. Stage One: The first stage is the deposition of the coating fluid onto the wafer or substrate.
done using a nozzle that pours the coating solution out, or it could be sprayed onto the surface, etc. Usually this dispense stage provides a substantial excess of coating solution compared to the amount that will ultimately be required in the final coating thickness. For many solutions it is often beneficial to dispense through a sub micron sized filter to eliminate particles that could lead to flaws. Another potentially important issue is whether the solution wets the surface completely during this dispense stage. If not, then incomplete coverage can result.
Picture Stage Two: The second stage is when the substrate is accelerated up to its final, desired, rotation speed. This stage is usually characterized by aggressive fluid expulsion from the wafer surface by the rotational motion. Because of the initial depth of fluid on the wafer surface, spiral vortices may briefly be present during this stage; these would form as a result of the twisting motion caused by the inertia that the top of the fluid layer exerts while the wafer below rotates faster and faster. Eventually, the fluid is thin enough to be completely co-rotating with the wafer and any evidence of fluid thickness differences is gone. Ultimately, the wafer reaches its desired speed and the fluid is thin enough that the viscous shear drag exactly balances the rotational accelerations.
Picture Stage Three: The third stage is when the substrate is spinning at a constant rate and fluid viscous forces dominate fluid thinning behavior. This stage is characterized by gradual fluid thinning. Fluid thinning is generally quite uniform, though with solutions containing volatile solvents, it is often possible to see interference colors “spinning off”, and doing so progressively more slowly as the coating thickness is reduced. Edge effects are often seen because the fluid flows uniformly outward, but must form droplets at the edge to be flung off. Thus, depending on the surface tension, viscosity, rotation rate, etc., there may be a small bead of coating thickness difference around the rim of the final wafer. Mathematical treatments of the flow behavior show that if the liquid exhibits Newtonian viscosity (i.e. is linear) and if the fluid thickness is initially uniform across the wafer (albeit rather thick), then the fluid thickness profile at any following time will also be uniform — leading to a uniform final coating .