PROCESS FOR FORMING ISOLATION TRENCHES IN A SEMICONDUCTOR SUBSTRATE

Abstract: In a process for forming planar, dielectric filled, narrow and wide trenches in integrated circuit devices, following the formation of the trenches (2, 3) in the substrate (1), successive layers of conformal silicon nitride (15), conformal polysilicon (14), and relatively conformal CVD oxide (16) are formed to the relative depth of the trenches (2, 3). A photoresist mask (22) is then first selectively formed over the central regions of the wide trenches (3) and then used as a mask during the anisotropic etch of exposed oxide. The underlying polysilicon layer (14) serves as an oxide etchant stop, and also provides the material from which the next successive oxidation partially fills the previously etched regions with thermal silicon dioxide. A further layer of oxide is then formed by poly deposition and oxidation. The nitride layer (13) underlying the polysilicon layer prevents oxidation of the substrate. Fabrication is concluded with a planarization to the level of the active regions (8, 9, 11), including an etch of the nitride layer (13) over such active regions (8, 9, 11).

Description: PROCESS FOR FORMING ISOLATION TRENCHES IN A SEMICONDUCTOR SUBSTRATE Technical Field This invention relates to processes of the kind for forming dielectric material filled trenches between active regions of a semiconductor substrate. Background Art The creation in a semiconductor substrate of trenches having vertical walls and being uniformly filled with dielectric materials such as silicon dioxide, while providing trench surfaces which are substantially coplanar with the active regions, has proven to be more elusive than most practitioners originally contemplated. In particular, significant efforts have been expended to develop broadly applicable techniques for filling with dielectric trenches of varying width dimensions. Unfortunately, the techniques which grow silicon dioxide dielectric from the silicon of the substrate typically introduce stress problems in the final structure because of the volumetric increases associated with the conversion of the monocrystalline silicon to silicon dioxide, while the techniques which rely upon the use of deposited silicon dioxide to fill the trenches either form voids in narrow trenches, insufficiently fill with dielectric the wide trench regions, or produce a composite surfaced topography so uneven that planarizing techniques, including the use of viscous polymers, are incapable of producing a substantially planar substrate surface upon the conclusion of trench formation. One approach suggested for overcoming the various problems is set forth in U.S. Patent No. 4,445,967. According to the teaching therein, as illustrated in Figs. 1A-1F thereof, following the formation of the major trenches in the substrate, a dielectric layer is blanket deposited and then selectively masked by photoresist in the wide trench regions. An anisotropic etch of the exposed dielectric, and thereafter the subsequently exposed substrate, is followed, in order, by a second dielectric deposition and a planarization etch. With the focus of attention being directed to the filling of wide trench regions, no attempt is made to limit the formation of voids in the narrow trenches. If anything, the selective deepening of the trench regions adjacent the active devices would accentuate the likelihood of forming voids. The problems created by contaminents trapped in voids are well known. Disclosure of the Invention It is an object of the present invention to provide a process of the kind specified, adapted to provide dielectrically filled trenches of varying widths, without voids and with a substantially planar surface topography. Therefore, according to the present invention, there is provided a process for forming dielectric material filled trenches between active regions of a semiconductor substrate, characterized by the steps of: forming trenches of relatively narrow and wide widths in the substrate; forming a first conformal layer including an oxidation barrier material over the trenches and active regions; forming a second conformal layer over the oxidation barrier material, the second conformal layer being of a material which is resistant to etchants of a dielectric material and is convertible to a dielectric; depositing a third, substantially conformal, layer of a dielectric material; selectively removing dielectric material from the active regions, the narrow width trenches, and the perimeter regions of the wide width trenches; converting the material of the second conformal layer to a dielectric; filling the regions of selectively removed dielectric material with a further dielectric material to a level above 5 the level of the active regions; and planarizing the substrate to the level of the active regions. Brief Description of the Drawings Fig. 1 is a schematic cross-section of a substrate with active and trench regions covered by 10 the various deposited and patterned layers. Fig. 2 is a schematic cross-section of the substrate in Fig. 1 following an anisotropic etch of the exposed oxide. Fig. 3 is a schematic cross-section of the 15 substrate following thermal oxidation of* the polysilicon. Fig. 4 is a schematic cross-section of the substrate following further oxide formation. Fig. 5 is a schematic cross-section of the 20 substrate following planarization. Best Mode for Carrying Out the Invention Referring first to Fig. 1, a monocrystalline silicon substrate is first anisotropically etched in any one of numerous known manners to create narrow 25 trench regions 2 and wide trench regions 3, the trenches serving to separate active regions 8, 9 and 11 having respective active surfaces 4, 6 and 7. For purposes of this embodiment, narrow trenches are defined to be those which are less than or equal to 4 30. micrometers in width; all trenches wider than 4 micrometers are correspondingly defined as being wide. Substrate 1 is then subjected to a relatively brief thermal oxidation operation to form a PAD oxide layer 12. PAD oxide layer 12 provides a stress relief 35 interface between monocrystalline substrate 1 and the subsequently deposited silicon nitride layer 13. Preferably, the thermal oxidation is performed in a H2O and O2 gas ambient at a temperature at approximately 1050°C, at atmospheric pressure, and for a period of approximately 45 minutes, to create a substantially conformal thermal PAD oxide layer approximately 100 nanometers thick. Following the growth" of PAD oxide 12, the wafer is confor ally covered with a layer of silicon nitride 13, preferably deposited using a plasma enhanced CVD operation with a mixture of SUΪ2CI2 and NH3, while in a chamber having an ambient temperature of approximately 860°C at a pressure of approximately 0.25 torr for period of approximately 55 minutes, to form a layer approximately 100 nanometers thick. As will be more fully understood from the ensuing development, nitride layer 13 will serve as an oxidation barrier to mask the active regions and trenches during the conversion of subsequently formed polycrystalline or amorphous silicon layers to trench oxides. Again referring to Fig. 1, the substrate is next blanket covered by a conformal deposition of polycrystalline silicon layer 14. Preferably polysilicon (poly) layer 14 is formed in a CVD operation using SiI-4, at a temperature of approximately 620°C, at a pressure of approximately 0.26 torr and for a period of approximately 35 minutes, to form a layer 14 approximately 250 nanometers thick. Though it is preferred that poly layer 14 be intrinsic polycrystalline silicon, it is also feasible to deposit lightly doped polycrystalline silicon or amorphous silicon. As will be more fully appreciated later, poly layer 14 between nitride layer 13 and oxide layer 16, the formation of which is set forth hereinafter, serves two purposes. First, poly layer 14 serves as an etch stop for the plasma etch of oxide layer 16. In its second capacity, poly layer 14 provides the silicon which upon later conversion to silicon dioxide inhibits the formation of voids in the narrow trenches. The last layer formed on substrate 1 to create the cross-sectional structure depicted in Fig. 1 is plasma enhanced CVD silicon dioxide layer 16. Preferably, this is practiced by subjecting substrate 1 to reactants of SiI-4 and O2 at a pressure of approximately 0.25 Torr, at a temperature of approximately 425°C, and for a period of approximately 85 minutes, to form a layer approximately 2,000 nanometers thick on the wide horizontal surfaces, such as the center region of wide trench 3. As is known by those who routinely practicing in the art, though plasma enhanced CVD oxide is relatively conformal in its deposition it does exhibit some re-entry or cusping characteristics at relatively sharp corners, such as 17 and 18 in Fig. 1. Therefore, it is not unusual to have a premature closure of the oxide being deposited into a narrow trench. For example, see the closure at 19 in Fig. 1 and void 21 formed thereby. Note that the level of CVD oxide 16 in the central region of wide trench 3 approximates the thickness over active region surfaces 4, 6 and 7. To improve the likelihood of creating a substantially planar final structure, it is desirable to match the depth of wide trench 3 to the thickness of CVD oxide 16. Next, according to the preferred practice of the present invention, a photoresist is deposited and photolithographically processed to retain selected mask segments such as 22 over CVD oxide 16 in the central region of wide trench 3. Note the absence of resist over the active regions 8, 9 and 11, narrow trench region 2, and the perimeter regions of wide trench 3. Substrate 1 is then subjected to an anisotropic oxide etch 23 in the presence of photoresist mask 22 to remove substantially all unmasked CVD oxide 16. Preferably this is performed with a plasma of CHF3 and 02 at a pressure of approximately 0.03 Torr, a temperature of approximately 30°C, using approximately 1000 watts of power, and is continued for a period of approximately 50 minutes. Note that silicon layer 14, whether it is comprised of intrinsic poly, doped poly or amorphous silicon, serves as an effective oxide etchant stop. Following the etch of CVD oxide 16 with plasma 23 as described with reference to Fig. 1, and a conventional solvent strip of photoresist mask 22, one is left with the structure depicted in Fig. 2 of the drawings. With reference to this cross-section, note that CVD oxide 16 formerly under photoresist 22 (Fig. 1) remains intact, while elsewhere all that remains of oxide layer 16 are the small sidewall segments at locations 24, 25, 26 and 27. The later remain by virtue of the relative thickness of oxide layer 16 (Fig. 1) in the direction of the reactive ion etching. It is particularly noteworthy that poly layer 14 serves as a highly effective oxide etch stop, and as such, returns the narrow trench 2 and the etched perimeter regions 28 and 29 of wide trench 3 to nearly their original topography. Following the anisotropic etch which produces the cross-section depicted in Fig. 2, the substrate is subjected to another oxidizing ambient, preferably composed of H2O and O2, at an temperature of 11500C and a pressure of 1 atmosphere, for a period of approximately 320 minutes, to convert polysilicon layer 14 into silicon dioxide. A representative resultant structure is generally depicted in Fig. 3 of the drawings. Note that the nominal 2:1 volumetric increase during the conversion of the poly 14 (Fig. 2) to silicon dioxide produces a partial dielectric fill of the narrow trenches and perimeter regions, from the bottom and sides, to blend in with the residual CVD oxides formerly at 24, 25, 26 and 27 (Fig. 2). Undesired oxidation of silicon substrate 1 is prevented by the masking effect of nitride layer 13. Upon the conclusion of. the oxidation, the cross- section as depicted in Fig. 3 shows the presence of a trench oxide 33 rising to a level above the plane of active region surfaces 4, 6 and 7, and various moderately tapered recesses 31 and 32. Recesses 31 correspond to narrow trench regions 2, while recesses 32 are related to the perimeters of the wide trenches. Given the constrained shapes and relatively shallow depths of such recesses 31 and 32, it is now possible to fill all trench regions with oxide to a level suitable for planarization. To complete the oxide filling and planarization, the structure in Fig. 3 is preferably next subjected to a second CVD polysilicon conformal deposition, which deposition is to provide a polysilicon layer having nominal depth of approximately 250 nanometers following the formerly described technique. The deposition can be implemented by any one of various other conventionally utilized techniques, and can, as suggested earlier, encompass the use of lightly doped poly or amorphous silicon. Thereafter, the silicon is # converted to a silicon dioxide dielectric by thermal oxidation in progressing to the structure depicted in Fig. 4. Though it is also possible to fill recesses 31 and 32 directly with a CVD oxide, or any other readily planarizable dielectric, the dielectric properties of thermal oxide are commonly preferred. Note again that nitride layer 13 provides the oxidation barrier by which undesired conversion of silicon substrate 1 is inhibited. Fig. 4 shows the cross-section following the void free filling of all trenches and recesses to form a silicon dioxide 34 having a level materially above the plane of active region surfaces 4, 6 and 7. The structure depicted in Fig. 4 is then planarized following any one of various known techniques. Preferably, such final stages of planarization involves th§ use of a spin-on planarizing polymer 36 and an etch back using a plasma which etches polymer 36 and any exposed oxide 34 at substantially equal rates. The etch is continued until nitride layer 14 overlying active region surfaces 4, 6 and 7 is exposed. Thereafter, a 1:1 silicon nitride-to-silicon d ' ioxide etch is used to remove the silicon nitride cap over surfaces 4, 6 and 7, later being further modified to remove PAD oxide 12 and oxide 34 at equal rates. The concluding structure is illustrated in Fig. 5. Note that the completed structure in Fig. 5 includes both narrow 2 and wide 3 dielectric filled trenches separating active regions 8, 9 and 11, in which the trench and active region surfaces are substantially coplanar. Thereafter, active devices may be formed by utilizing any one of numerous commonly practiced techniques. The etch back from the structure in Fig. 4 is preferably performed using a planarizing polymer and an etching plasma composed of CHF3 and O2. at a temperature of approximately 50°C, a pressure of approximately 0.04 Torr, and using approximately 1000 watts of power. To ensure a 1:1 etch ratio in progressing through the various layers the gas composition is optimally varied by changing the O2 flow rate in four increments from 40 seem to 5 seem while maintaining the CHF3 flow rate at approximately 75 seem. It will no doubt now be appreciated that the present invention provides a trench oxide isolation fabrication process by which trenches of diverse widths can be readily filled and planarized using thermal oxide while substantially avoiding the inclusion of cavities in the narrow trenches. It should also be understood that the present process avoids critical fabrication operations and multiple masks, with the intent of improving manufacturing yield and reducing costs. In particular note the dual role of poly layer 14, first as an etch stop during the patterned removal of CVD oxide 16 and later as a source of silicon for the partial filling of the ■ narrow trenches and wide trench perimeters. The use of the poly as an etch stop also eliminates the need for elaborate end point detection during the oxide etch operation. In still another respect, note that the alignment of photoresist 22 is not critical, in that misalignments merely vary the widths of regions 28 and 29, which regions continue to exhibit the characteristics of a narrow trench and as such are subsequently filled by the conversion of polysilicon layer 14 and further oxide formation steps. In addition to the above benefits, it should be understood and appreciated that the dimensions of poly layer 14 and oxide layer 16 can be adjusted for the alignment tolerances associated with the formation of photoresist 22. The ability to refine such variables provides the user of the present invention with a significant degree of latitude in particularizing the features to the intended application.

Claims: CLAIMS : 1. A process for forming dielectric material filled trenches between active regions (8, 9, 11) of a semiconductor substrate (1), characterized by the steps of: forming trenches (2, 3) of relatively narrow and wide widths in the substrate; forming a first conformal layer (13) including an oxidation barrier material over the trenches (2, 3) and active regions (8, 9, 11); forming a second conformal layer (14) over the oxidation barrier material, the second conformal layer being of a material which is resistant to etchants of a dielectric material and is convertible to a dielectric; depositing a third, substantially conformal, layer (16) of a dielectric material; selectively removing dielectric material (16) from the active regions (8, 9, 11), the narrow width trenches (2), and the perimeter regions (28, 29) of the wide width trenches (3); converting the material of the second conformal layer (14) to a dielectric; filling the regions of selectively removed dielectric material with a further dielectric material to a level above the level of the active regions (8, 9, 11); and planarizing the substrate (1) to the level of the active regions (8, 9, 11). 2. A process according to claim 1, characterized in that the step of selectively removing dielectric material (16) includes the steps of: forming a mask (22) over the central regions of the wide width trenches (3); and anisotropically etching the dielectric material (16) in the presence of the mask (22). 3. A process according to claim 2, characterized in that the anisotropic etch of the dielectric material (16) is terminated following the removal of all vertically exposed dielectric material except nominal residuals situated within the inside corners of the narrow trenches (2) and the perimeter regions of the wide trenches (3). 4. A process according to claim 2 or claim 3, characterized in that the material of the second conformal layer (14) volumetrically expands upon conversion to a dielectric material and in so doing partially fills the narrow width trenches (2) and perimeter regions of the wide width trenches (3) with dielectric. 5. A process according to claim 4,_ characterized in that the step of converting the material of the second conformal layer (14) to a dielectric is effected by thermal oxidation. 6. A process according to claim 5, characterized in that the step of filling the regions of selectively removed dielectric material with further dielectric material includes that step of: forming a silicon layer; and thermally oxidizing the silicon layer. 7. A process according to claim 5, characterized in that the step of filling the regions of selectively removed dielectric material with further dielectric material includes depositing a layer of CVD silicon dioxide. 8. A process according to claim 6, characterized in that the oxidation barrier material (13) consists of silicon nitride, the second conformal layer (14) consists of polycrystalline or amorphous silicon, and the third, substantially conformal, layer (16) consists of CVD silicon dioxide. 9. A process according to claim 8, characterized in that the step of planarizing the substrate to a level of the active regions includes the steps of: forming a planarizing polymer layer (36) over the thermally oxidized silicon layer; etching the polymer (36) and thermally oxidized " silicon at corresponding rates to the plane of the active regions (8, 9, 11); and removing the silicon nitride layer (13) from over the active regions (8, 9, 0 11).

Inventors: KEISER ALAN ELLSWORTH, MUNDT RANDALL STEVEN, KEISER, ALAN, ELLSWORTH, MUNDT, RANDALL, STEVEN
Applicants: NCR CO, NCR CORPORATION
Application year: 1987
Main classification: H01L21
IPC codes: H01L21