lt 0 1 by weight of sn all the adhesives and frames in which the filter mats are installed should likewise contain lt 0 1 by weight of boron and phosphorus and less than 0 01 by weight of arsenic and aluminum and lt 0 1 by weight of tin it has been found that the preliminarily filtered air does not contain any dopant containing particles therefore conditions which were achieved in the prior art only after an outgassing phase of several months exist in the cleanroom the invention envisages a plurality of filter stages for deposition of particles of different size the airborne particle filters should achieve a deposition level of more than 99 for particles of size less than 0 2 u03bcm it has been found that this can be accomplished by a two stage preliminary filtration preliminary filter stage 1 provides a coarse dust filter of the g1 to g4 class for gt 10 u03bcm particles this consists of a synthetic material preferably polypropylene or polyester preliminary filter stage 2 provides a fine dust filter of the m5 or m6 or f7 to f9 class for 1 to 10 u03bcm particles this likewise consists of a synthetic material preferably polypropylene or polyester final filter stage 3 provides an airborne particle filter of the e10 to u17 class for lt 1 u03bcm particles the airborne particle filter also consists of a synthetic material preferably polypropylene or polyester also possible in principle is a two stage system composed of a fine dust filter of the m5 or m6 or f7 to f9 class for 1 to 10 u03bcm particles and an airborne particle filter of the e10 to u17 class for lt 1 u03bcm particles preference is given however to a two.