Abstract:
A process for forming a dielectric isolation structure on a silicon substrate includes forming at least one trench in the substrate, performing a high-temperature treatment in an oxidizing environment to form a first liner layer of silicon dioxide on the walls and the bottom of the trench, and performing a silicon dioxide deposition treatment to form a second liner layer on the first liner layer. A silicon nitride deposition treatment is also performed to form a third liner layer on the second liner layer. The trench is filled with isolating material.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a process for the formation of a dielectric insulation structure in a semiconductor device.  
       BACKGROUND OF THE INVENTION  
       [0002]     For the fabrication of integrated circuits having geometries of less than 0.5 μm it is usual to employ a technique, known as STI (Shallow Trench Isolation) for isolating the various parts of an integrated circuit from each other. This technique is briefly described below with reference to  FIGS. 1A  to  1 F and  2 A to  2 D, which show a section through a part of a silicon slice in the initial fabrication phases of an integrated circuit.  
         [0003]     A substrate of monocrystalline silicon  10  is oxidized at a high temperature to obtain a layer  11  of silicon dioxide. A layer  12  of silicon nitride is then deposited on the oxide layer  11  and a photoresist layer  13  is deposited and treated to form a pattern that masks some of the areas of the underlying nitride layer, while leaving others uncovered. By means of an anisotropic attack, usually a plasma attack, the parts of the nitride layer  12  that have been left uncovered are then removed, together with the underlying oxide layer  11 . Even the substrate layer is attacked down to a predetermined depth (typically 250-300 nm) to obtain a plurality of grooves or trenches  14 . Thereafter, the remainder of the photoresist layer  13  is removed.  
         [0004]     To recuperate the damage induced in the silicon by the plasma attack and to form an interface that will facilitate the adhesion of the filler oxide to be subsequently deposited, the substrate is subjected to a high-temperature oxidation phase. On the walls of the trenches there is thus formed a thin layer (15-25 nm) of silicon dioxide  15  ( FIG. 2A ). But the oxidation process causes surface stresses in the vicinity of the upper and lower corners of the trenches, and these induce defects in the crystalline structure of the silicon (e.g., dislocations). This effect makes itself more strongly felt as the size of the devices that have to be isolated becomes smaller.  
         [0005]     These stresses are reduced by depositing a nitride layer  16  ( FIG. 2B ) on the oxide lining layer  15 . For the sake of simplicity, the layers  15  and  16  have not been shown in  FIGS. 1A  to  1 F, and can be seen only in  FIGS. 2A  to  2 D. Silicon dioxide  17  is then deposited ( FIGS. 1B and 2C ) by a process of the APCVD (Atmospheric Pressure Chemical Vapor Deposition) type, for example, to fill the trenches. The substrate modified in this manner is then subjected to a heat treatment (typically at about 1000° C. for 10-30 minutes to render the oxide  17  denser and then ( FIG. 1C ) to a planarization treatment by chemical-mechanical polishing to remove the excess oxide layer  17  by using the underlying nitride layer  12  as a stop layer.  
         [0006]     Referring now to  FIGS. 1D and 2D , the nitride layer  12  and the oxide layer  11  are removed by appropriate wet attacks. During the attack on the oxide, the filler oxide  17  of the trenches is made substantially level with the front surface of the silicon substrate  10 . In this phase, nevertheless, some small grooves  18  are formed in the oxide  17  along the edges of the trenches  14 . This is brought about by the fact that the attack solution used to remove the oxide layer  11 , usually HF, attacks the filler oxide  17 , which is deposited oxide, more rapidly than the oxide of the layer  11 , which is thermal oxide. The small grooves  18  are also due to the fact that the attack is isotropic and therefore acts also laterally on the filler oxide  17 .  
         [0007]     As is shown in  FIG. 2D , at the end of the attack on the oxide there remain parts in relief  19  within the grooves  18 . These parts in relief  19  are made up of the edges of the nitride layers  16  that are part of the trench lining and are not attacked by the solution with which the oxide is attacked. These parts in relief  19  may cause defects of a morphological and electrical nature because they perform an undesired screening action during the subsequent attacks with the consequent formation of spurious structural elements caused by material residues. If these effects are to be attenuated, the process parameters of the attack operations have to be calibrated with great precision. Nevertheless, for example, in the case of an integrated circuit containing a memory with polysilicon floating gate cells, electrical failures due to short circuits between the memory cells caused by polycrystalline silicon residues are very probable. Consequently, use of the isolation structure described above implies relatively low production yields.  
       SUMMARY OF THE INVENTION  
       [0008]     An object of the present invention is to propose a process that will make it possible to form dielectric isolation structures that do not provoke or, at least, diminish the defects described above, especially crystallographic defects.  
         [0009]     This goal is attained by realizing the process defined and characterized in general terms in the first claim hereinbelow. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]     The invention will be more readily understood from the detailed description of two embodiments of the process, which are described by way of example and are not to be regarded as limiting in any way. The description makes reference to the attached drawings, of which:  
         [0011]      FIGS. 1A  to  1 F show a section through a portion of an isolation structure in accordance with the prior art;  
         [0012]      FIGS. 2A  to  2 D show a section through a portion of an isolation structure in accordance with the prior art in which there can be seen some details not shown in  FIGS. 1A  to  1 F;  
         [0013]      FIGS. 3A  to  3 D show a section through a portion of an isolation structure formed by the process in accordance with the present invention; and  
         [0014]      FIGS. 4A  to  4 E show a section through a portion of an isolation structure formed by another embodiment of the process in accordance with the present invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0015]     Referring to  FIGS. 3A  to  3 D, wherein the portions equal to those of  FIGS. 2A  to  2 D are indicated by the same reference numbers, the process in accordance with the invention differs from the known process described above by virtue of the fact that, following the formation of the layer  15  lining the trenches  14  by high-temperature oxidation of the silicon, a silicon dioxide deposition treatment is performed, for example, by a process of the APCVD type. On the first thermal oxide layer  15  there is thus formed a second deposited oxide layer  20 .  
         [0016]     The process then continues, just like the known process, with the deposition of a silicon nitride layer  16 , the deposition of silicon dioxide  17  to fill the trenches  14 , the planarization and the removal of the surface nitride and oxide layers, respectively,  12  and  11 . Even in this case some grooves will be formed, indicated by  18 ′ in  FIG. 3D , along the edges of the trenches. Nevertheless, due to the thickening of the oxide lining, the nitride layer is sufficiently distant from the silicon walls of the trenches to assure that the grooves will extend only between the edge of the trench and the nitride layer  16 , so that the edge of the nitride layer does not remain within the groove as in the known process.  
         [0017]     The screening action described above in connection with the known process does not take place because the nitride layer  16  does not form parts in relief. At the same time, the nitride layer  16  efficiently performs its screening action with respect to the oxidizing species, which in the course of the fabrication process could arrive at the silicon of the trench walls and thus give rise to crystallographic defects. Naturally, the process parameters, and therefore the thicknesses of the layers, have to be chosen in a manner known to persons skilled in the art to assure that the overall thickness of the oxide lining of the trenches will be sufficient to insure this effect.  
         [0018]     By way of general orientation, an isolation structure formed in accordance with the invention may be characterized by the following dimensions. The mean width of the trenches  14  is between 180 nm and 70 nm. The depth of the trenches  14  is between 350 nm and 100 nm. The thickness of the first lining layer  15  is between 30 nm and 5 nm. The thickness of the second lining layer  20  is between 50 nm and 5 nm. The thickness of the nitride layer  16  is between 15 nm and 3 nm.  
         [0019]     A particularly advantageous application of the process in accordance with the invention concerns the isolation of a memory formed by cells having gate electrodes self-aligned with the active areas adjacent to the trenches.  
         [0020]      FIGS. 4A  to  4 E, wherein portions equal to the corresponding portions of  FIGS. 3A  to  3 D are indicated by the same reference numbers, show a portion of a monocrystalline silicon substrate  10  containing a trench  14  of an isolation structure obtained by a process in accordance with the invention. The process envisages high-temperature oxidation of the surface of the substrate  10  to obtain a thin layer (10 nm)  30  of silicon dioxide to form the so-called tunnel dielectric of the memory cells, the deposition of a layer  31  of polycrystalline silicon to form the floating gate electrodes of the cells, the deposition of a thin layer (15 nm)  32  of silicon dioxide and the deposition of a stop layer  33  of silicon nitride. The process continues with operations, similar to those described in connection with  FIGS. 1A  to  1 F and  FIGS. 3A  to  3 D, for the definition of the areas where the trenches are to be formed and for carrying out the removal of the corresponding material.  
         [0021]     At the end of the material removal one thus obtains a cavity that forms the trench  14 , which extends into the silicon substrate  10 , and an aperture across the superposed layers  30  to  33  that combines with the trench and forms its entrance. In this case, once again, the process then envisages the formation of a first lining layer  15  of thermal oxide, a second lining layer  20  of deposited oxide ( FIG. 4A ) and a silicon nitride layer  16  ( FIG. 4B ), deposition of silicon dioxide  17  ( FIG. 4C ) to fill the trenches and, lastly, planarization. The nitride layer  33 , which forms the stop layer of the planarization operation, is then removed by a wet attack together with the underlying oxide layer  32  and also a part of the filler oxide (17+16+15).  
         [0022]     In this phase the filler oxide is attacked down to a level lower than that of the polycrystalline silicon  31  so that the floating gate electrode has part of its side uncovered, as can be seen in  FIG. 4D . Subsequently there is formed a composite layer  34  ( FIG. 4E ) to isolate the floating gate electrodes from the control gate electrodes (which will be formed later). This is achieved by a means of subsequent deposition of a first oxide layer, an intermediate silicon nitride layer and a second oxide layer, the so-called ONO (Oxide-Nitride-Oxide) dielectric, which makes it possible to seal the side with nitride already present on the side of the floating gate electrodes of the memory cells. This assures optimal electrical isolation of the cells and optimal capacitative coupling between the floating gate electrodes and the silicon substrate.  
         [0023]     The process described above makes it possible to form a memory (of the NAND or NOR type, Stand Alone or Embedded) and a circuit portion on the same silicon substrate with the possibility of integrating the standard isolation with a nitride lining isolation either only in the memory cells, or only in the circuit part, or in both memory cells and circuit part. This implies considerable advantages in terms of degrees of freedom of the overall process and in terms of yield. The advantage for the cell is given by the improvement of the capacitative coupling and the sealing of the gate, together with the elimination or drastic reduction of the dislocations. The advantage for the circuit part is represented by the elimination or drastic reduction of the dislocations.  
         [0024]     According to two variations of the process described in relation to  FIGS. 4A  to  4 E, the lining of the trenches to isolate the memory cells from each other may also be carried out, rather than by forming two oxide layers (one thermal, the other deposited) and a nitride layer, by forming a single oxide layer by deposition and then subjecting this layer to nitriding or by forming an oxide layer by deposition and a nitride layer. These variations do not consent the simultaneous formation of the isolation structure of the memory and the corresponding structure of the circuit part when the latter has to have an isolation of the type described by  FIGS. 3A  to  3 D. The two isolation structures will in this case be formed partly by distinct operations, utilizing an appropriate masking, and partly by common operations, i.e., the operations of planarization and the operations of wet attack.