New cover for the fabrication of analog-digital integrated circuits on 6" and 8" wafers

Honorary Medal for Early Completion of the Cleanroom Ceiling 01/2012

The X-FAB Semiconductor Foundries AG at the Dresden location manufactures analog-digital integrated circuits on 6" and 8" wafers on behalf of customers. The cleanroom with a filter ceiling of ISO 4 cleanliness class according to EN-ISO 14644 was commissioned in 1984. Since that time, the filters in the cleanroom ceiling H14 DIN-EN1822 have not been replaced. After approximately 25 years, the filter pressure loss had increased significantly, and the filter tightness became increasingly problematic when adjustments to the walls and pressure chambers were required.

The primary systems (outside air systems) with a volume flow share of about 10% and the secondary systems (recirculation systems) with a volume flow of 1,400,000 m³/h supply 1,600 m² of filter ceiling over several pressure chambers in overpressure. The tightness was achieved at that time with flat gaskets and potting with epoxy resin on separating agents. An exchange of the filter frames in the existing steel structure was therefore not possible. For this reason, the option was considered to break out the old filters and leave the overpressure ceiling with all its disadvantages. The new filter ceiling had to be hung 50 cm below the old filter frames and connected airtight to all existing walls.

The special requirements for the new filter ceiling were:
- Cleanliness class "at rest" ISO 4 as an overpressure ceiling, gas-tight
- Conversion in existing structures with technological equipment
- New filters U15 DIN-EN1822 with boron-free filter paper
- Conversion in multiple sections under production conditions
- Gas-tight ceiling connection to existing walls and supports
- Sealing of existing walls in the overpressure ceiling area
- Partitioning in the intermediate ceiling area
- Incorporation of a building expansion joint into the airtight ceiling
- Shortest construction time in shift operation

The solution was implemented with WISAG's own node cleanroom ceiling system. For the complex wall connection, a sequence of 3 to 4 seals was used as a solid and permanently elastic joint. The first construction phase as a test area was a small section of 36 m². Various sealing methods for overpressure ceilings were tested here, and the reliability of the sealing systems was demonstrated through leak and cleanliness class measurements. The use of filters with test certificates was of course standard.

The completion of the new cleanroom ceiling with a basic area of approximately 1,600 m² within 3 years in 10 construction phases was finished at the beginning of 2012. As a result, the cleanroom class now achieves ISO 3.

Furthermore, outside air systems are to be retrofitted with H13 particulate filters to reduce outdoor particle dependence on the new cleanroom filter ceiling. The pressure loss of the supply air filters has decreased despite increasing the separation efficiency from 250 Pa with the old filters to 60 Pa with the new filters.

Remarkably, the increase in filter pressure loss within 25 years is "only" 250 Pa, which can be attributed to cleanroom-appropriate clothing, good staff training, and an average displacement velocity of about 0.3 – 0.35 m/s. Filter service lives of now more than 20 years and significant energy savings should be achieved again with this solution.

Through good division of labor and continuous improvements in workflows, the retrofit times per m² were reduced at each section. As a token of appreciation, the client presented a certificate of honor in the form of a 200 mm wafer disc.