referring now to the figures of the drawing in detail and first , particularly , to fig1 a thereof , there is shown a semiconductor structure including a semiconductor substrate 1 with a trench opening 3 for a shallow trench . to fabricate the structure illustrated in fig1 a , the semiconductor substrate 1 is provided with a thin oxide layer and / or pad nitride layer 11 . the pad nitride layer serves as a polishing stop layer in a subsequent method step . then , a trench opening 3 for a shallow trench is etched , resulting in the formation of the semiconductor structure illustrated in fig1 a . fig1 b shows that when deep trenches with a high aspect ratio are filled , cavities , known as voids 9 , which are generally arranged in the center of the cross section of the trench filling , can form in the interior of the trench filling . these voids 9 are formed during the deposition of the layer 4 that fills the trench . the layer 4 generally consists of an insulating material 14 . these voids 9 also cannot be completely eliminated by liquefying the insulating material 14 at high temperatures . if the layer 4 , which is initially deposited on the entire substrate surface , is removed at the surface , as shown in fig1 c , by using a chemical mechanical polishing operation in which a polishing pad 20 grinds down the layer 4 as far as the pad nitride layer 11 , then some of these voids 9 will be opened up . since these voids 9 often extend over relatively great distances in a trench , they can lead to short circuits , for example , if a conductive material 15 is deposited on the structure illustrated in fig1 c and this material fills up voids which are open at the surface . all conventional efforts have been aimed at either precluding the formation of voids from the outset or eliminating this formation of voids before the conductive material is deposited . however , these efforts have not been completely successful . according to the invention , as illustrated in fig1 d , a second electrically insulating material 16 is grown onto the first electrically insulating material 14 , with the result that a new surface is formed on the first insulating material 14 , and this new surface is free of voids . the second insulating material 16 covers the surface of the first insulating material 14 and also voids 9 which have been opened up by polishing and open out into this surface . these voids are closed up by the second electrically insulating material and therefore can no longer be filled with the conductive material . the deposition of the second electrically insulating material 16 preferably takes place selectively . for example , in fig1 d no material 16 is grown onto the pad nitride layer 11 . consequently , a further planarization operation is not required . this further planarization operation would open up voids that have previously been closed up . bpsg is preferably used as the first electrically insulating material 14 ; on account of its doping , it can be liquefied relatively easily , in order for voids that are present to be at least partially expelled again . silicon oxide is preferably grown on as the second material . the second material 16 , which is grown onto the first insulating material 14 , can be thermally compacted by a subsequent conditioning step t , in order to achieve even better sealing of the surface of the trench filling 14 . the method illustrated in fig1 a to 1e can be used to produce - shallow trenches 3 which are used for the electrical isolation of adjacent semiconductor structures on the substrate surface . finally , an electrically conductive material 15 is deposited on the semiconductor structure that has been covered with the second electrically insulating material 16 that was grown on using the inventive method , as illustrated in fig1 e . in the process , the new , void - free surface that has been formed by the second electrically insulating material 16 prevents the conductive material 15 from penetrating into the voids 9 in the first insulating material 14 that were opened up by polishing . the invention is furthermore suitable for producing and sealing a dielectric that has been deposited directly on and between gate layer stacks . in particular , inner walls of contact holes that have been etched into a dielectric can be sealed using the inventive method . as shown in fig2 a , a plurality of layers including polysilicon 17 , tungsten silicide 18 and a silicon nitride 19 are deposited on the gate oxide layer 2 of a semiconductor substrate 1 . the silicon nitride layer serves as a hard - mask layer for patterning word lines 5 . this layer sequence corresponds to a typical gate layer structure . the layers 17 , 18 , 19 are then patterned , so that the structure illustrated in fig2 b is formed . on the gate oxide layer 2 of the semiconductor substrate 1 , fig2 b shows two adjacent word lines 5 which consist of the layers 17 , 18 and 19 . between adjacent word lines 5 there is a recess 3 which is filled with a first insulating material 14 up to above the height of the word lines 5 , as illustrated in fig2 c . a thin silicon nitride layer 11 was deposited on the structure obtained in this way , and then the material 14 , which is typically bpsg , is applied to this thin silicon nitride layer 11 . the material 14 is used to fill the recesses 3 between the gate layer stacks . on account of the high aspect ratio of the recesses 3 , voids 9 are often formed , and as illustrated in fig2 e , these voids 9 may extend over great distances in the center of the cross section of the recesses 3 , parallel to the word lines 5 . to eliminate the voids 9 as much as possible , the structure is subjected to a heat treatment after the bpsg layer 14 has been deposited . in accordance with fig2 d , contact holes 8 are etched into the deposited dielectric 14 and then open up the voids 9 , which run in channel form , in regions . in particular , if there are a plurality of contact holes 8 at a certain distance from one another between two word lines 5 , the voids can be short - circuited if a metallic filling 15 is deposited in the contact hole openings 8 , and the metallic filling 15 penetrates into the passages of the voids 9 running parallel to the word lines 5 . fig2 e illustrates a plan view , from above , looking at the section plane a — a of the semiconductor structure illustrated in fig2 d . between the word lines 5 , which run vertically in fig2 e , which are located below the section plane , and which are therefore shown in dashed lines , there are two contact hole openings 8 . the two contact hole openings 8 have been etched into the surface of the semiconductor structure and extend as far as the base of the recesses 3 , i . e . as far as the underside of the first electrically insulating material 14 . these contact holes can be used to make contact with the semiconductor substrate 1 . on account of the formation of voids 9 , which may extend over great distances parallel to the word lines 5 in the interior of the recess 3 , the side walls 7 of the contact holes 8 are partially interrupted . if the contact hole openings 8 are filled with an electrically conductive material 15 , this material can creep through the voids 9 to the closest contact hole 8 and thus cause short circuits in the integrated semiconductor circuit . to prevent this , according to the invention , as illustrated in fig2 e on the basis of the lower contact hole 8 , the first electrically insulating material 14 is covered with a second electrically insulating material 16 by a selective growth process . the second electrically insulating material 16 covers the surface of the semiconductor structure illustrated in fig2 d outside the contact hole openings 8 , and also covers the side walls 7 in the interior of the contact holes , as illustrated with reference to the lower contact hole 8 . in the process , voids 9 which open out in the side walls 7 are also closed up at the surface , since the second electrically insulating material 16 is also grown on the inner wall of the voids 9 and closes up these voids at least in the vicinity of the side walls 7 . as a result , the voids 9 are sealed and can no longer be filled with an electrically conductive material . the view taken through the section line b — b , running through the semiconductor structure illustrated in fig2 e , is additionally illustrated in fig2 f . in this perspective , the word lines 5 run from the left to the right in front of and behind the plane of the drawing . between them is the section illustrated through the recess 3 into which the first material 14 has been deposited . two contact hole openings 8 have subsequently been etched into the material 14 . the contact holes 8 extend as far as the next layer down , for example as far as the gate oxide 2 or as far as the surface of a semiconductor substrate 1 . according to the invention , the second insulating material 16 is deposited on the surfaces of the semiconductor structure , which is formed by the first insulating material 14 , so that the upper side of the covering layer 14 and also the side walls 7 of the contact holes 8 are covered with the second conductive material 16 and sealed . in particular , the voids which are formed between adjacent contact holes 8 are closed off at the surface from the side wall 7 of the corresponding contact hole 8 . fig2 f shows two different alternate embodiments with regard to the etching of the contact hole 8 . the left - hand half of fig2 f illustrates a nitride layer 11 , which has also been etched , i . e . removed , during the contact hole etching . the gate oxide 2 below it then forms the base of the contact hole . if the growth process of the second insulating material 16 is not selective with respect to the material 2 at the base of the contact hole 8 , the second electrically insulating material 16 is likewise deposited there . therefore , it then additionally has to be removed by means of an anisotropic etch , which is indicated by the arrow 22 and is carried out , for example , by using a rie ( reactive ion etching ) process , in order then to allow for the introduction of the metallic contact hole filling , for example , tungsten . the contact hole filling fills the contact hole opening as far as the line 15 indicated in dashed form . in another , more advantageous embodiment , which is illustrated on the right - hand side of fig2 f , the nitride layer 11 is not initially removed during the contact hole etching . during the subsequent selective deposition of the second insulating material 16 , the selectivity of this deposition process is also utilized at the base of the contact hole 8 , with the result that the layer of the second insulating material 16 is formed only at the side walls 7 and not at the base of the contact hole 8 . consequently , only the barrier layer 11 and the gate oxide 2 then have to be etched in order to make contact with the semiconductor substrate 1 through the contact hole filling 15 . in this embodiment , there is no additional layer of the material 16 , which would also have to be removed , formed at the base of the contact hole 8 . the selective growth of the second insulating material 16 preferably takes place using a selox process , a selective silicon oxide deposition process that enables silicon oxide to be deposited on base layers , which contain silicon oxide , selectively with respect to silicon nitride , titanium nitride or metals , for example . the selox process is suitable , in particular , for deposition on bpsg , i . e . doped silicon oxide . by contrast , the second insulating material 16 , i . e . silicon oxide , is practically not deposited on nitride layers . the selox process is known per se and is carried out in a sacvd ( sub - atmospheric chemical vapor deposition ) chamber at 20 to 760 torr . at temperatures of between 400 and 600 ° c ., a gas mixture of ozone and teos ( tetraethyl orthosilicate ) with a mixing ratio of at least 5 : 1 is deposited . the deposition process is isotropic and leads to conformal deposition . according to the invention , the selox process is used for providing an exclusively external coverage of a trench filling that has already been introduced . the selox oxide produced can be compacted by a subsequent heat treatment . a hdp - cvd ( high density plasma cvd ) process , which is carried out at atmospheric pressures of between 1 and 10 mtorr and a plasma density of approximately 10 11 electrons / cm 3 , is particularly suitable for applying the first insulating material 14 , generally bpsg . furthermore , it is possible to use deposition processes which are currently customary , such as apcvd ( atmospheric pressure ), sacvd ( sub - atmospheric pressure ), and pecvd ( plasma enhanced cvd ), in which case , on account of the inventive sealing of layers that have been deposited and etched back , even deposition processes for the deposition of the first electrically insulating material , which have tended to lead to the formation of voids , can be used in a much more versatile way , since the inventive sealing of voids which are opened up by polishing eliminates short circuits .