reference will now be made in detail to the present preferred embodiments of the invention , examples of which are illustrated in the accompanying drawings . wherever possible , the same reference numbers are used in the drawings and the description to refer to the same or like parts . fig2 a through 2e are schematic , cross - sectional views showing the progression of manufacturing steps for producing a shallow trench isolation structure using a self - aligned mask according to one preferred embodiment of this invention . first , as shown in fig2 a , a semiconductor substrate 100 is provided . thereafter , a silicon nitride layer 102 is deposited over the substrate 100 . next , photolithographic and anisotropic etching operations of the silicon nitride layer 102 and the substrate 100 are carried out to form shallow trenches 104 between active regions 106 . due to layout considerations , different active regions require different degree of isolation . consequently , different shallow trenches 104 may have different dimensions . next , as shown in fig2 b , an insulation layer 108 , preferably a silicon dioxide layer , is formed over the substrate 100 using , for example , a chemical vapor deposition ( cvd ) method . the insulation layer not only covers the substrate 100 , but also fills all the trenches 104 . since the substrate 100 had a highly undulating cross - sectional profile due to the presence of those trenches 104 , the deposited silicon dioxide layer 108 over the substrate 100 also has a highly rugged contour . in other words , the upper surface of the silicon dioxide layer 108 has a rugged cross - sectional profile . subsequently , a mask layer 110 , preferably a silicon nitride layer , conformal to the surface of the insulation layer 108 is deposited . the mask layer 110 is slightly higher over the active regions 106 than over the shallow trench regions 104 . after that , an oxide layer 112 , preferably a silicon dioxide layer is deposited over the mask layer 110 . next , as shown in fig2 c , using the mask layer 110 in the active region 106 as a polishing stop layer , a portion of the oxide layer 112 is removed using , for example , a chemical - mechanical polishing ( cmp ) method . ultimately , the mask layer 112 within the active region 106 is exposed and a portion of the oxide layer 112 above the shallow trenches 104 is retained forming an oxide layer 112a . since the mask layer 110 is harder than the oxide layer 112 , the mask layer 110 above the active region 106 can protect the insulation layer 108 below . hence , no micro - scratches are formed on the insulation layer in the active regions 106 after the cmp operation . therefore , kink effect and the short - circuiting of devices can be prevented , thereby able to increase product yield . next , the exposed mask layer 110 is removed using , for example , a wet etching or a dry etching method , thereby retaining a mask layer 110a above the shallow trenches 104 as shown in fig2 d . thereafter , using the mask layer 110a as an etching stop layer , the oxide layer 112a and a portion of the insulation layer 108 is removed to expose the mask layer 110a . hence , a mask layer 110a and an insulation layer 108a are formed . one major aspect of this invention is the provision of this self - aligned mask layer 110a by depositing mask material over the active regions 106 and the trench regions 104 , which have different height levels . since conventional method of forming a reverse tone mask using photoresist is replaced , problems caused by photomask misalignment can be prevented . finally , as shown in fig2 e , the mask layer 110a is removed to expose the insulation layer 108a using , for example , a wet etching or a dry etching method . thereafter , a chemical - mechanical polishing ( cmp ) operation is again carried out to remove a portion of the insulation layer 108a above the silicon nitride layer 102 , thereby forming an insulation layer 108b . by replacing the conventional reverse tone mask by a silicon nitride ( sin ) mask , this invention is able to reduce the polishing time required in a cmp operation while maintaining all the advantages of using a reverse tone mask . because the polishing time is reduced , micro - scratches are formed not so readily . furthermore , the silicon nitride mask layer can produce a self - aligning effect when the insulation layer is etched , and so conventional mask misalignment problems are eliminated . hence , cost of production can be reduced and yield of device can be increased . in summary , the advantages of using a self - aligned mask for manufacturing shallow trench isolation structure include : 1 . the method can prevent a kink effect caused by mask misalignment in a conventional manufacturing process . therefore , device short - circuiting is minimized and the yield can be increased . 2 . by providing a self - aligned mask layer over the insulation layer , the method can prevent the formation of micro - scratches on the surface of an insulation layer above the active region when cmp operation is carried out . therefore , the kink effect is prevented and failure of device due to short - circuit is minimized . 3 . the method can reduce the polishing time of cmp operation considerably , hence production yield is higher and processing window for manufacturing the shallow trench structure is wider . 4 . the method reduced the over - polishing time and hence the degree of polishing in the insulation layer . therefore , the number of recess cavities and defects resulting from the cmp operation will be reduced . 5 . since the deposition of a reverse tone mask layer and a photolithographic operation is unnecessary , the process of forming the shallow trench isolation structure is simpler , which is good for lowering production cost . it will be apparent to those skilled in the art that various modification and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention . in view of the foregoing , it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents .