Field of Invention
This invention relates to a device for improving layer thickness uniformity In spinning process for manufacturing optical discs. Particularly this invention relates to a double walled nozzle used in the spinning process to regulate the cooling air pressure in the inner peripheral area of an optical recording medium during spinning process, thereby Improving the layer thickness uniformity.
Bacicground of the Invention
Spin coating process is prevalently used in coating the optical storage medium where a transparent resin substrate /disc with flat or curved surface having a center hole is placed on a spindle holding the center hole of the substrate horizontally and a nozzle is used for dispensing a pigment solution from the upper part of the substrate . The pigment solution so dropped is spread on the surface of the substrate by rotation of the spindle, to form a pigment recording layer. This process is generally dependent on several factors such as dispense radius, substrate hold position, substrate rotation speed and substrate rotation speed vs. time profile, dispense volume, dispense pressure, time and dispense station design etc. which are known to persons skilled in the art.
Though the associated parameters value will differ significantly from industrial application to application, for disc manufacturing processes, these values are typically in the range of:
o Substrate rotation speed - 0 to 9000 rpm o Dispense Time - 0 to 2.5 sec. o Dispense Volume - 0 to 25 cc.

) Dispense Pressure - 0 to 2 kg/cm^2
Dispense Cup / Station design - varies a lot based on Line integrator.
Due to inherent process of spinning, the layer thickness formed on the substrate by this process varies from point to point. In certain application e.g. in CD/DVD manufacturing, the delta in layer thickness from point to point is very sensitive and critical for good disc quality and hence basic spinning set-up requires additional efforts, ideas and designs to improve layer uniformity. Also, due to inherent centrifugal force being present in any spinning process, the layer thickness tends to be lesser at inner peripheral area than to the outer peripheral area. In other words, layer thickness increases from inner to outer locations when measured with respect to center of the disc substrate.
Various methods have been tried to alleviate this problem Japanese Patent Application 2002-327955 discloses a method wherein an ultraviolet ray curable resin is dispensed from a nozzle to the light transmissive layer forming surface of a substrate , and applied on the full surface thereof by spin coating. Air is blown through an air blower nozzle from the backside of the substrate to the outer peripheral part thereof, and then ultraviolet rays are emitted to form an ultraviolet ray cured resin. Thus, by setting the pressure at a spot more outside than the outer peripheral end of the substratel as negative pressure and removing the swell of the outer peripheral part of the substrate formed by the spin-coating
It is also known to use cold air near the inner peripheral area. This results in to higher viscosity of the coating material near inner periphery , to increase layer thickness near the Inner area,.
Another solution is to use two or multi step dispensing along with local hardening techniques to increase thickness at the inner peripheral locations.

However, none of the above techniques are satisfactory so far as the coating layer uniformity on the disc substrate is concerned. Therefore, there is a constant need for an improved device which can alleviate the above drawbacks.
Objects of the Invention
It is an object of the invention to provide a device for controlling the cold air flow near the inner periphery of the optical disc substrate for better uniformity of the coating layers.
It is another object of the Invention to provide a novel nozzle for controlling the cold air flow near the inner periphery of the optical disc substrate for better uniformity of the coating layers.
It is yet another object of the invention is to provide a method for creating uniform coating layers on the optical disc substrate.
It is a further object of the invention to provide an improved process for manufacturing optical discs with better uniformity of coating layers.
Summary of the Invention
The above and other objects of the Invention are achieved by providing a device for improving layer thickness uniformity, in spinning process for manufacturing optical discs, including a spin coating process assembly having central disc holding chuck or spindle , wherein said assembly has a double walled air nozzle placed near the Inner periphery of the disc substrate and the chuck or spindle, to cause increase in the viscosity of glue/coating material at the Inner periphery of the disc substrate, thereby causing to prevent, spreading of glue/coating material towards the chuck/spindle due to centrifugal force, resulting In increase in thickness at the Inner radius of the disc.
In accordance with preferred embodiments of the device in accordance with the instant invention :
—said nozzle is of cylindrical shape.

--said nozzle is of frustoconical shape.
—said nozzle comprises two separate air passages to cause supply of higher volume of cooling air, in the vicinity of the inner periphery of the optical disc substrate.
—the aperture of the nozzle has an opening diameter of 25mm.
The present invention also provides a method for improving layer thickness
uniformity in spinning process for manufacturing optical disc including,
designing an assembly having central disc holding chuck or spindle and a double walled air nozzle,
placing said nozzle near the inner periphery of the disc substrate and the chuck
or spindle and blowing cool air through said nozzle, in the vicinity of the inner
periphery of the optical disc substrate,
thereby, increasing the total air volume blown through said nozzle for the
purpose of cooling the system and simultaneously, keeping the air pressure low
to prevent spreading of glue/coating material towards the chuck/spindle due to
centrifugal force, resulting in improving the uniformity of the coating layer on the
optical disc substrate .
Brief Description of the Accompanying Drawings
The invention is further elaborated with the help of the accompanying drawings, which are by way of illustration and understanding and not by way of any limitation, wherein :

Fig 1 shows a device comprising a spin coating process assembly with a conventional cold air nozzle placed near the inner periphery of the disc and the spindle.
Fig 2. is a cross sectional view of the nozzle specially developed for controlling of cooling air pressure in the spin -coating processes according to the invention.
Fig3 is a perspective view of the inventive nozzle of the invention
Fig 4 is a plan view of the opening diameter of the inventive nozzle according to the invention
Fig 5 is a graph depicting the relation between Thickness of coating layer and Radius of the disc manufactured using the state of the art spin coating process.
Fig 6 is a graph depicting the relation between Thickness of coating layer and Radius of the disc manufactured using the improved spin coating process using the inventive device for controlling air pressure.
Detailed Description of the Invention
Throughout the specification the term optical disc is used which includes but not limited to CD/DVD/CD-R/DVD-R and the like.
As shown in figure 1, a single nozzle is used to supply cooling air near the inner periphery of the optical disc substrate.
Mechanisms as described above, when used to increase layer thickness at the inner periphery of the disc to improve the layer uniformity, lead to splashes and dispensed coating material going in the to central disc holding chuck or spindle . As these coating materials keep spreading over into the central disc holding chuck /spindle, this leads to pick and place automation errors which in turn leads to alarms / warning signals on the Line and ultimately to stoppage of the Line for manually cleaning the chuck / spindle mechanism. Frequent stoppage of these high

index lines leads to different kinds of quality related problems in the discs under process as well as results in to lesser numbers of good discs coming out from the Line.
The present invention discloses a novel idea which aims to increase the viscosity of glue/coating material at the inner periphery of the disc. Higher viscosity glue will run away lesser towards the chuck/spindle due to centrifugal force, hence the thickness will increase at the inner radius. To increase the glue /coating material viscosity at the inner periphery of the disc, temperature of the glue/coating material at the inner periphery of the disc must be reduced. To reduce local temperature of glue/coating material in the inner periphery area, at present normal cold air nozzle is used . The critical parameter in the process is the volume of normal/cold air which is required to reduce the temperature of the glue/coating material to Increase viscosity. A single nozzle which Is used at present, to get higher air volume, quite high air pressure has to be maintained ,which causes splashes and glue/coating material running over to the chuck/spindle.
To alleviate this problem, a double walled nozzle has been invented . A cross sectional view of the double walled nozzle is shown in fig 2 . It can be seen from the said figure 2 that there are two separate air passages leading to supply of higher volume of cooling air in the vicinity of the inner periphery of the optical disc substrate. This design of the air nozzle increases the total air volume, but at the same time keeps air pressure low so that splashes etc. can be avoided which in turn helps in improving the uniformity of the coating layer on the optical disc substrate .
Figure 3 shows a perspective view of the inventive nozzle.
Preferably, the double walled nozzle may be cylindrical or frustoconicaI/funnel shaped.
It will be readily understood by a person skilled in the art that the dimension of the aperture of the mouth of the nozzle as shown in figure 4 can vary to suitably fit into the line assembly of the spin coating process.

Figure 5 and 6 are comparative examples of the uniformity of coated layers on the optical disc manufactured according to the state of the art vis-a vis using the inventive process and device
The graph of figure 5 shows the relation between thickness of coating layer in micron and radius of the optical disc in mm when manufactured using the existing spin coating process, whereas fig 6 illustrates the same relationship according to the present invention.
The same process parameters have been used to carry out the experiments . The exemplary parameters are (i) air temperature maintained at 15 degree centigrade and spinning speed is maintained at standard. It is also to be noted that the existing nozzle has an air aperture (opening diameter) of 2mm whereas the opening diameter of the double walled nozzle is 25mm. The two separate air passages lead to supply of higher volume of cooling air in the vicinity of the inner periphery of the optical disc substrate. This design of the air nozzle increases the total air volume, but at the same time keeps air pressure low so that splashes etc. can be avoided which in turn helps in improving the uniformity of the coating layer on the optical disc substrate .
Therefore , it is to be readily understood that to maintain the same volume of cooling air flow, the air pressure in the inventive nozzle is much lesser than the existing nozzle thereby reducing the incidence of running out of coating material towards the chuck/spindle.
A comparative analysis of graph shows that the plateau region of graph 6 is more significant than that of fig 5 evidencing superior uniformity of the coating layer in the inventive process.
The invention has been described with the help of non-limiting exemplary embodiments. However, it must be understood that various modifications and improvements is possible without departing from the scope and spirit of the invention disclosed hereinabove and claimed in the claims hereinafter.

We claim:
1. A device for improving layer thickness uniformity, In spinning process for manufacturing optical discs, Including a spin coating process assembly having central disc holding chuck or spindle , wherein said assembly has a double walled air nozzle placed near the inner periphery of the disc substrate and the chuck or spindle, to cause increase In the viscosity of glue/coating material at the Inner periphery of the disc substrate, thereby causing to prevent, spreading of glue/coating material towards the chuck/splndle due to centrifugal force, resulting In uniformity in thickness of the coating layer on the optical disc substrate.
2. A device as claimed In claim 1, wherein said nozzle Is of cylindrical shape.
3. A device as claimed in clalml, wherein said nozzle Is of frustoconical shape.
4. A device as claimed In any preceding claim, wherein said nozzle comprises two
separate air passages to cause supply of higher volume of cooling air. In the
vicinity of the Inner periphery of the optical disc substrate.
5. A device as claimed in any preceding claim wherein the aperture of the nozzle
has an opening diameter of 25mm.
6. A method for Improving layer thickness uniformity in spinning process for
manufacturing optical disc including,
designing an assembly having central disc holding chuck or spindle and a double
walled air nozzle,
placing said nozzle near the inner periphery of the disc substrate and the chuck or spindle and blowing cool air through said nozzle, in the vicinity of the inner periphery of the optical disc substrate,
thereby. Increasing the total air volume blown through said nozzle for the

purpose of cooling the system and simultaneously, keeping the air pressure low to prevent spreading of glue/coating material towards the chuck/spindle due to centrifugal force, resulting in improving the uniformity of the coating layer on the optical disc substrate.