Case Studies

If you haven’t got time to read this article now, you can download it for later by clicking here.

Photolithography is an important step in semiconductor device fabrication. In photolithography, a circuit design is transferred to wafer through a pattern imaged onto a photo resist layer deposited on the wafer surface. The wafer then undergoes various etch and deposition processes before a new design is transferred to the wafer surface. This cyclical process continues, building up multiple layers of the semiconductor device.

The minimum feature that may be printed using photolithography is determined by the resolution limit W, which may be defined by the Raleigh equation as:

W= k1 l/ N A

Where k1 is the resolution factor, l is the wavelength of the exposing radiation and NA is the numerical aperture. In lithographic processes used in the manufacture of semiconductor devices, it is therefore advantageous to use radiation of very short wavelength in order to improve the optical resolution so that very small features in the device can be accurately reproduced, Monochromic visible light of various wavelengths have been used, and more recently radiation in the deep ultra violet (DUV) range has been used, including radiation at 193 nm as generated using an ArF excimer laser. The value of NA is determined by the acceptance angle (a) of the lens ands the index of refraction (n) of the medium surrounding the lens, and is given by the equation:

NA = sin a

For clean dry air (CDA), the value of n is 1 and so the physical limit to NA for a lithographic technique using CDA as medium between the lens and the wafer is 1 with the practical limit being currently around 0.9

Immersion photolithography is a known technique for improving optical resolution by increasing the value of NA. In this technique a liquid having a refractive index n > 1 is placed between the lower surface of the objective lens of a projection device and the upper surface of a wafer located on a moveable wafer stage. The liquid placed between the lens and the wafer should ideally, have a low optical absorption at 193 nm, be compatible with the lens material and the photo resist deposited on the wafer surface, and have good uniformity. These criteria are met with ultra-pure degassed water, which has a refractive index n = 1.44 the increased value of n, in comparison to technique where the medium between the lens CDA, increases the value of NA which in turn decreases the limit W, enabling smaller features to be reproduced.

If you haven’t got time to read this article now, you can download it for later by clicking here.

An un-envisaged process fault caused a quantity of insoluble and highly abrasive coarse ceramic fines and fuel cladding debris to enter a decanter vessel within the THORP reprocessing plant. Subsequent operation of existing equipment moved this very erosive product through the system. Ultimately, after several years operation, this caused an erosion failure of some in-cell pipe work which resulted in a temporary shut down of the THORP reprocessing facility.

Working with the site team as technical support the issues concerning the primary transfer device were resolved and the THORP reprocessing facility restarted ahead of schedule.

An alternative means of transfer needed to be developed to protect against another primary device failure.

The device would need to handle both fines and hulls (small pieces of cropped stainless steel tube that hold the fuel pellets), be resistant to erosion and would not become blocked with the anticipated process materials. The tip of the device had to be precisely located in the decanter vessel so that the debris would move into its sphere of influence . The only means of deploying the device was remotely via a vertical 9 m long, 100 mm diameter camera access pipe whose opening was situated in the basket handling cave above. Access into the basket handling cave was from a personnel access area via a roof plug, a further 11 m above the camera access pipe.

A liquid operated dredging ejector was developed that could be deployed through the camera access pipe. The drive fluid was taken to a series of peripheral nozzles coaxially positioned in the annulus surrounding the discharge pipe. The nozzles were arranged so that free access was provided for the entrained materials and there was no possibility for a blockage to occur. The design of the assembly ensured that during deployment its tip was guided to the optimum position in the decanter vessel. Operation of the decanter swirl wash system ensured that the fines were constantly introduced to the suction device. (schematic diag fig 1)

The control surfaces were manufactured in Boron Carbide to resist erosion. As this material is very hard it is difficult to produce the small tangential holes for the motive jets. This was overcome by casting the holes directly into the Boron carbide and finished using diamond tools.

A full scale unit and replica of the cell was constructed to validate both the performance and deployment. The prototype was exhaustively tested under varying conditions. The hydraulic performance was achieved with 95% of the fines and occasional hulls being transported to the separation tank.

During the design and validation phase a full risk analysis was carried out leading to approval being granted to manufacture actual plant items.

These were completed in line with the project schedule and made ready for deployment with full documentation.

If you haven’t got time to read this article now, you can download it for later by clicking here.

The First Generation Magnox Storage Pond has a large quantity of sludge on the pond base and in several process bays which has accumulated over many years of operation. The Office for Nuclear Regulation dictated that 95% of this should be removed in the next few years. The sludge properties are almost non-Newtonian in nature and therefore difficult to transport. The pond contains skips and other equipment making the deployment of any removal device difficult.

The task was to develop a transfer device that could be attached to an existing manipulator. This would then be deployed into the bays and used to remove the sludge from around the submerged process equipment.

A study was undertaken prior to the final design to model the fluid transport through the suction and discharge pipe work. This defined the hydraulic profile that the device would need to operate within along with any potential constraints. These would have to be understood to achieve a successful deployment. The consultation document was approved but is commercially sensitive.

The solution utilised liquid-liquid mixing technology but with some fundamental changes to cater for the sludge properties. The basic prototype unit is shown below (fig 2). The final design embodied fittings which attached the device to the deployment lance and a sludge mobilisation system.

The unit was tested with simulated sludge mixtures and proved very successful.