Mission accomplished

Cleanroom Solutions Ltd has completed a £1.4 million cleanroom, as part of a £2 million plus project for Leica Microsystems Lithography Ltd in Cambridge.

Leica Microsystems designs, manufactures and supplies electron beam lithography tools, providing "leading edge" technology solutions in all areas of advanced lithography, both in the manufacturing roles of direct, write, and maskwriting, as well as the key enabling nano-lithography role for research and development. The latest Leica Microsystems VB6 vector beam series systems use the Gaussian beam – vector scan pattern generation. The tools are assembled and tested in individual bays over several months prior to customer acceptance and despatch. The cleanroom facility was designed to have several bays fed from a central corridor so that a number of tools can be assembled and tested at the same time. Each bay consists of a Class 1000 general area with a central Class 100 area protected on all four sides by overlapping PVC curtains to maintain the temperature and air flow within the bays as well as giving easy access for the assembly process. All plant items as well as materials of construction were selected to meet Leica's stringent magnetic field specification. The temperature in the bays is maintained at 200°C, ± 10°C with the inner curtained area at ± 0.10°C. This is achieved by using an air handling unit for each bay with extremely accurate sensors in the inner bay as well as a number of tight control trim features. Each bay is complete with its own service area where various gasses and services are kept.

Chemical process benches The facility includes a number of specialist cleanroom laboratory areas used for various aspects of the company's business. One area was supplied with "low extract" chemical process benches for both acid and solvent based processes. Low extract benches were used to minimise the amount of extracted air, therefore minimising the size of the fresh air makeup plant. The benches were custom-designed and made to meet Leica Microsystems' exact requirements and manufactured using fire retardant polypropylene with polypropylene worktops for the acid benches and stainless steel worktops and liners for the solvent handling benches. These are hooked up to the solvent and acid extract systems which extract to outside the building and discharge using high velocity discharge nozzles above the building roof line. The cleanroom consists of a number of "walk on" steel mezzanines that were designed to support all the main plant above the cleanrooms enabling the whole structure to be self-supporting. This includes the air handling units, controls, LTHW pipework, electrical trunking and process gases. The room fabric consists of modular fire rated cleanroom walls with clear fire-rated double glazed vision panels. Doors had to be non-metallic to avoid variations in the magnetic field around the machines and the door furniture is solid nylon for the same reasons. Also, all access doors are 3 metres high so that the tools can be moved in and out of the bays fully assembled. Metal ceilings were used throughout and include high frequency lighting which is designed for access from the room side. The units are sealed to maintain the integrity of the rooms during routine maintenance. Final filtration is through HEPA filter units located within the ceilings and again maintained from the room side.

Welded and bonded floors This project used fully welded, fully bonded anti-static vinyl sheet on the floor. The floors were then ESD tested by the manufacturers to prove adherence to specification. A fully equipped changing room including stepover benching, tacky mats, coverall hanging rails, wire shelving, mirrors, lockers and stainless steel bins was designed and built to support 50 staff. Due to the very close control of the assembly bays (±0.10°C) each bay has to use its own air handling system including separate controls, heating and cooling. All the air handling units are supplied with fresh air using a further air handling unit that pre-treats the fresh air supply. The fresh air is used to make up for the extracted air as well as provide air for pressurisation. All areas were designed to operate at a positive pressure, with the highest class areas set to the highest pressures so that air leakage is always from the cleaner areas. Air pressure is monitored using room mounted pressure sensors and controlled by inverter units on the supply air fans. The controls from each system are provided with a link so that they can be connected to the main BMS system. All temperature and humidity parameters are monitored and logged. The project was extremely fast track. The new building had to be completely refurbished inside and out and was only available to start 12 weeks before Leica Microsystems had to move in. The whole project was co-ordinated by Colin Davies, Leica Microsystems Facilities Manager who commented: "The original time-scale was tight, we selected contractors who felt they could achieve the end result and we felt were committed – we were not let down".