referring to fig1 , an information recording apparatus of the present invention comprises a vacuum chamber 1 and an electron gun 2 . the bottom portion of the vacuum chamber 1 is carried on a foundation 3 with leg supports 4 and is connected to one or more vacuum pumps 5 . the top portion of the vacuum chamber 1 is fixedly connected to one side of a base or reference plate 6 . the electron gun 2 is attached to the opposite side of the base plate 6 and extends into the vacuum chamber 1 through an opening 7 in the base plate 6 . the electron gun 2 is of conventional design , emitting a beam of electrons in the vacuum chamber 1 . a hinge 8 is provided between the electron gun 2 and the base plate 6 , so that the electron gun 2 can be swung between a tilted maintenance position and an upright writing position . inside the vacuum chamber 1 , a substrate holder 9 is attached to the base plate 6 . the substrate holder 9 comprises a stationary holder part 9 a connected to the base plate , and a movable holder part 9 b which is guided in a linear path on the stationary part 9 a . the movable holder part 9 b carries a spin unit 10 . the spin unit 10 , which is partly arranged in a casing forming an atmospheric chamber 11 inside the movable holder part 9 b , comprises a spindle motor 12 , a spindle 13 ( indicated with dashed lines in fig1 ) and a substrate reception surface 14 adjoining the spindle 13 and facing a beam outlet 15 of the electron gun 2 . to minimize friction , the spindle 13 is supported on the spindle motor 12 by a pneumatic bearing ( not shown ), as is well known in the art . thus , the spindle motor 12 is adapted to effect controlled rotation of the reception surface 14 inside the vacuum chamber 1 . for ease of maintenance , the spin unit 10 is formed as a detachable unit . thus , the operator can simply disengage the unit 10 from the movable holder part 9 b and remove it through the opening 7 which is formed essentially in line with the unit 10 . further , the apparatus comprises a feed motor unit 16 which is connected to the movable holder part 9 b to effect a linear movement thereof . the feed motor unit 16 is preferably a high - precision linear motor , typically with a resolution of 2 nm or less , which is electrically connected to a feed control unit 17 . the linear motor 16 is attached to the base plate 6 and is connected to the movable holder part 9 b by means of a hollow rod 18 . the rod 18 extends through a hole 19 in the vacuum chamber wall . a simple seal , which accommodates the movement of the rod 18 , is provided by a flexible sleeve in the form of a bellows element 20 that surrounds the rod 18 and is sealingly connected to the linear motor 16 and the vacuum chamber wall . the open ends of the rod 18 communicates with the atmospheric surroundings and the atmospheric chamber 11 , respectively , so that a fluid path is established between the atmospheric chamber 11 and the surroundings . further , electrical cables ( not shown ) are led from a spin control unit 21 to the spindle motor 12 through the interior of the rod 18 . in another conceivable embodiment ( not shown ), the feed motor unit is enclosed in a low - vacuum chamber , wherein a vertical wall of the main vacuum chamber forms a partition between the low - vacuum chamber and the main vacuum chamber . the pressure in each chamber is controlled by a respective vacuum pump . the connecting rod extends from the feed motor unit to the substrate holder in the main vacuum chamber through a hole in the vertical wall , the dimensions of the hole being close to those of the rod , so that the leak rate from the low - vacuum chamber to the main vacuum chamber is kept low . in such a two - stage vacuum chamber construction the flexible sleeve element discussed above can be omitted . in operation , the vacuum chamber 1 is evacuated by means of the vacuum pump 5 , and a recording medium or substrate 30 ( fig3 ) is placed on the reception surface 14 inside the vacuum chamber 1 , optionally by an automatic loading unit ( not shown ). this automatic loading unit could also include an air - lock mechanism to allow for introduction of one or more substrates 30 from the outside of the vacuum chamber 1 . then , the spin unit 10 is actuated to rotate the substrate 30 , typically at a rate of 50 - 4000 rpm . subsequently , the electron gun 2 is controlled to focus a beam b of electrons ( fig3 ) on the substrate 30 to incorporate an information - containing structure therein . control of the writing operation can be effected by a computer ( not shown ) or the like . during the writing operation , the substrate 30 is moved helically relative to the beam by the spindle motor 12 rotating the spindle 13 and by the linear motor 16 translating the movable holder part 9 b , which is suspended from the base plate 6 by means of the stationary part 9 a . when the writing operation is completed , the rotation of the spindle 13 is interrupted and the finished master is removed from the reception surface 14 . fig2 illustrates a preferred embodiment of a seal structure provided between the spindle 13 and the movable holder part 9 b to prevent outside atmosphere from leaking into the vacuum chamber 1 . in the illustrated embodiment , the spindle 13 extends into the vacuum chamber 1 through a port 9 ′ in the movable holder part 9 b . a magnetic seal producing means 40 arranged at the port 9 ′ comprises two pole pieces 41 , 42 with a magnet 43 in between . thus , a closed magnetic circuit is formed so that a magnetic fluid 44 is generated and maintained in the gaps formed between the front ends of the pole pieces 41 , 42 and the spindle 13 . the spindle 13 is made of essentially nonmagnetic material , such a stainless steel , aluminum or brass . a sleeve 45 of magnetic sheet material , for example μ - metal or soft permeable steel , is attached to the outer periphery of the spindle 13 , for example by shrink fitting , adhesive bonding or any other suitable means . the sleeve 45 of magnetic material is aligned with the magnetic seal producing means 40 and is thereby confined within the holder part 9 b . this has been found to effectively prevent the generated magnetic fields from reaching the substrate on the receiving surface 14 . further , the combination of two sandwiched materials has also been found to improve the rigidity of the spindle 13 . thus , it is conceivable to provide a sleeve 45 extending further towards the spindle motor 12 . alternatively , not shown , the spindle could be integrally formed with a seal portion of magnetic material and a vacuum - side portion of essentially non - magnetic material . fig3 illustrates a further preferred embodiment of the apparatus according to the invention and shows a front portion of the electron gun 2 which as such is of conventional design . upstream of the beam outlet 15 , as seen in the direction of the beam path b , a backscatter detector 2 ′ is provided in a manner known per se . the output of the backscatter detector 2 ′ is typically used for preparatory optimization of the position of the electron gun 2 before the writing operation , by operating the electron gun 2 in a microscope mode . a distance sensor 50 in the shape of an annular ring is concentrically mounted with respect to the beam outlet 15 , such that the beam can pass the center of the sensor 50 . the distance sensor 50 is preferably of inductive or capacitive type and senses the distance to the substrate 30 on the receiving surface 14 . alternatively , the sensor could be of optical type . the distance sensor 50 allows for continuous monitoring of the distance to the rotating substrate 30 during the writing operation . based on the output of the distance sensor 50 , the electron gun 2 can be controlled in real time to accommodate for any small changes in distance occurring during the writing operation , for example by optimizing the focal length of a focussing device ( not shown ) in the electron gun 2 . such changes in distance could for example be due to dimensional changes in the apparatus or out - of - flatness or other irregularity of the substrate 30 . the design and location of the distance sensor 50 also allows the operator to use the backscatter detector 2 ′ without removing the distance sensor 50 . while the presently preferred embodiments of the invention have been shown and described above , it is to be understood that these disclosures are for the purpose of illustration and that various changes and modifications may be made without departing from the scope of the invention as set forth in the appended claims . for example , any suitable energy beam could be used for writing and / or editing information on the recording medium , e . g . any beam of charged particles such as ions or electrons . it is also to be understood that the base plate could be an integral part of the vacuum chamber , i . e . integrally formed with the vertical vacuum chamber walls .