Method of forming trench isolation having polishing step and method of manufacturing semiconductor device

Abstract: A method of forming trench isolation including a burying step of burying trenches by a deposition means for conducting etching and deposition simultaneously and a polishing step of flattening a burying material by polishing is conducted by disposing an isotropic etching step, a multi-layered etching stopper and a protrusion unifying structure. Polishing can be attained with satisfactory flatness uniformly or with no polishing residue even in a portion to be polished in which the etching stopper layer is distributed unevenly. The method can be applied to manufacture of a semiconductor device or the like.

Description: The present invention concerns a method of forming trench isolation having a polishing step and a method of manufacturing a semiconductor device having a polishing step. The present invention can be applied to the formation of trench isolation (trench type inter-device separation) in various kinds of electronic materials, a method of manufacturing various kinds of semiconductor devices having trench isolation, as well as a method of manufacturing various kinds of semiconductor devices having a recess burying step and a subsequent flattening and polishing step. Further, it can be utilized as a method of manufacturing a semiconductor device having a polishing step including a step of burying recesses defined with a plurality of protrusion patterns (that is, defined between each of protrusion patterns) by a burying material and a step of flattening the burying material formed on the protrusion patterns by polishing. Description of the Prior Art The polishing technique has a wide application use and it can be utilized, for example, for flattening unevenness resulting in a substrate such as a semiconductor substrate upon manufacturing a semiconductor device (refer, for example, to Japanese Patent Application JP-A 63 039 835). On the other hand, the capacitance of a device has been increased in the field of semiconductor devices and various kinds of techniques have been developed in order to increase the capacitance while minimizing a chip area as small as possible and, for example, a multi-layered wiring technique is indispensable therefor. In the multi-layered wiring technique, it is extremely important to flatten the underlying substrate in order to prevent disconnection of the multi-layred wiring. This is because unevenness on the underlying substrate, if any, will lead to occurrence of wire disconnection on a step caused by the unevenness (so-called step disconnection). In order to flatten the underlying substrate satisfactorily, flattening from the initial stage is important. For attaining the above-mentioned purpose, there has been considered, for example, a flat trench isolation. Trench isolation is a technique for inter-device isolation by burying an insulator to trenches formed on a semiconductor substrate and it is advantageous for higher degree integration since fine trenches can be formed. However, after burying the trenches, it is necessary to remove protrusions composed of a burying material deposited to the portions other than the trenches for attaining a flattened surface. The trench can be formed as a recess between two protrusion patterns but, when a burying material is buried in the recess (trench), since the burying material is also deposited on the protrusion patterns other than the trench to form a protruding portion, it has to be flattened. A method as shown in Fig. 3 has been known as a method of forming a flat trench isolation. In this method, as shown in Fig. 3 (a), a thin silicon oxide film 2 and a thin silicon nitride film 3 are formed on a semiconductor substrate 1, then trenches 41, 42 and 43 are formed by etching using a photolithographic step and, subsequently, an inner wall oxide film, that is, the silicon oxide layer 2 is formed by oxidation to provide a semiconductor substrate. Then, as shown in Fig. 3 (b), a burying material 5 is deposited in the trenches 41-43 by a deposition means, for example, CVD to obtain a structure as shown in the figure. In this case, the burying material 5 is deposited to a large thickness also to the portions other than the trenches 42 - 43 to result in protrusions 51. Accordingly, as shown in Fig. 3 (c), the protrusions 51 are removed to flatten by polishing. In a case where a silicon oxide is used as the burying material 5, a silicon nitride film 3 having a polishing speed lower than that may be used for instance as a stopper layer for polishing. Such a method is applied, in addition to the trench isolation process, also to other processes for forming flat interlayer insulation films such as formation of a trench capacitor accompanying trench burying, formation of trench contact (trench plug) or formation of a layer by a blanket W-CVD process. However, in a case where a wide recessed region (1) and a narrow protruding region (2) are formed as shown in Fig. 4(a), the following problem exists. When polishing is applied directly after burying the trenches 41 - 43, the burying material 52 (SiO 2 or the like) not removed completely remains in the central portion of the burying material 5 on the wide protruding region (1), and SiO 2 or the like which is the burying material 52 is raised to result in occurrence of particles when the stopper layer 2 (for example Si 3 N 4 ) is removed by hot phosphoric acid in the succeeding step. As a countermeasure for overcoming the problem, for instance, IBM has presented the following technique in IEDM in 1989 (IEDM 89, pp 61 - 64). That is, as shown in Fig. 5(a), a block resist 31 is formed in the recess of CVD-SiO 2 as the burying material 5, on which a resist coating film 3 is formed which are then etched back. Thus, a structure as shown in Fig. 5(b) is obtained. Then, it is flattened by polishing to obtain a flattened structure as shown in Fig. 5(c). However, in this method, if a patterning for the block resist is displaced to form a resist out of the recess as shown by reference numeral 31' in Fig. 5(d), no sufficient flatness can be obtained even if a resist coating film 3' is formed, so that the burying material 5 does not become flat as shown in Fig. 5(e) and, as a result, it is difficult to flatten by means of polishing. In addition, there is also the following problem. That is, the flattening technique by polishing involves a problem that the extent of polishing depends on the underlying pattern and sometimes it results in unevenness. Description will now be made to the problem with reference to Fig. 6. In Fig. 6, trenches 41 - 43 are formed as recesses between each of protrusion patterns 61 - 64. The protrusion patterns 61 - 64 function as a stopper upon polishing. As shown in Fig. 6, the density of the protrusion pattern 61 is small or course in the portion A of the figure in which the protrusion pattern 61 is present. In the portion B shown in the figure in which the protrusion patterns 62 - 64 are present, a ratio of the protrusion patterns in a unit area (the area ratio of the protrusion patterns) is great and, accordingly, the density of the protrusion pattern is large or dense. In the illustrated embodiment, since silicon nitride or the like as a polishing stopper layer is formed on the protrusion patterns 61 - 64, the area of the stopper layer 3 in the unit area is small and, accordingly, the density thereof is coarse in the illustrated portion A. On the other hand, since the area of the stopper layer 3 in the unit area is large, the density of the polishing stopper 3 is dense in the illustrated portion B shown in the figure. If there is unevenness of the ratio of the polishing stopper layer 3 (which exists each for the protrusion patterns 61 - 64), polishing tends to become uneven. For instance, in a peripheral circuit, if the area ratio per unit area of a protrusion pattern present there, the function of the polishing stopper layer is low (for example, in a case of the region A in Fig. 6). Since a polishing pressure is concentrated upon polishing to the protrusion pattern (stopper layer), the polishing rate is increased, so that mere selection ratio of the protrusion pattern (stopper layer) is insufficient and an isolation pattern 61 is scraped off as shown in Fig. 6(b) which no more provides an intended effect as the stopper layer. As a result, the region A is concaved, and uniform and satisfactory flattening can not be attained as shown in Fig. 6(b). Accordingly, it has been demanded for a technique capable of attaining satisfactory flattening by polishing also in a case where the distribution of the polishing stopper layer shows unevenness (for instance, in a case where there is an unevenness for the density of protrusion patterns and, accordingly, there is an unevenness for the ratio of the polishing stopper layer) and also for a portion in which the area ratio is low, that is, a portion of a circuit pattern in which the portion that functions as the polishing stopper is coarse. The EP-A-0424905 describes a semiconductor device manufacturing process comprising the steps of forming trenches in the surface of a substrate, forming an insulating film by bias ECRCVD over the surface of the substrate, etching the insulating film by lateral leveling etching to expand the width of grooves formed in portions of the insulating film formed in regions other than those corresponding to the trenches, masking the portions of the insulating film filling up the trenches and removing the portions of the insulating film formed in regions other than those corresponding to the trenches. An etching stop layer is formed over the surface of the substrate before forming the trenches and the insulating film, and the etching stop layer is removed by etching after removing the portions of the insulating film formed in the regions others than those corresponding to the trenches by etching with the portions of the insulating film filling up the trenches masked. With this process the surfaces of the portions of the insulating film filling up the trenches are finished flush with the surface of the substrate. In the EP-A-0341898 a method is disclosed for making a highly planarized integrated circuit structure having deposited oxide portions planarized to the level of adjacent portions of the integrated structure which method comprises; depositing, over an integrated circuit structure having first portions at a height higher than the remainder of the integrated circuit structure, a conformal oxide layer; depositing a layer of a planarizing material such as polysilicon over the conformal oxide layer; polishing the structure a first time to expose the highest portions of the underlying conformal oxide layer; etching the structure a first time with an etchant system capable of removing the conformal oxide preferentially to the planarizing material; further polishing the structure a second time to remove planarizing material left from the first etching step; and then optionally etching the remainder of the structure to remove any remaining planarizing material and the remaining conformal oxide over the raised portions of the underlying integrated circuit structure to provide the desired highly planarized structure. The US-A-4,836,885 describes a method of planarizing wide dielectric filled isolation trenches formed in the surface of a semiconductor surface. A self aligned mask is formed on the thick conformal layer of dielectric in the depressions over the wide trenches to protect the dielectric in those trenches from etching during planarization steps. The mask material is chosen to have etch characteristics different from the dielectric layer and a subsequent planarizing organic layer to allow selective etching of the mask material or dielectric without etching the other materials in the structure. It is an object of the present invention to overcome the foregoing problems in the prior art and provide a means capable of flattening without leaving a burying material on a wide (long) protrusion region thereby capable of forming trench isolation of satisfactory flatness, as well as a method of manufacturing a semiconductor device formed with such trench isolation. Another object of the present invention is to provide a method of manufacturing a semiconductor device having a polishing step of applying flattening after burying, wherein a satisfactory flattened shape can be formed even in a case where a distribution of a portion exhibiting the function of a polishing stopper on a portion to be polished has unevenness, also in a portion in which an area ratio of the stopper layer per unit area is low. The foregoing object can be attained by the present invention of claim 1. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1(a) - (f) shows steps in Example 1; Fig. 2(a) - (g) shows steps of a comparative example 2; Fig. 3(a) - (c) shows steps in the prior art; Fig. 4(a) - (b) shows problems in the prior art; Fig. 5(a) - (e) shows problems in the prior art; and Fig. 6(a) - (b) shows problems in the prior art. EXAMPLE Description will now be made to a preferred embodiment according to the present invention with reference to the drawings. It will be apparent that the present invention is not restricted to the following example. Example 1 In this example, the present invention was applied to a method of manufacturing a fine and integrated semi-conductor device formed with trench isolation. Fig. 1(a) - (f) shows the steps in this example. In this example, a plurality of protrusion patterns 61 - 63 each having a polishing stopper layer 3 in the upper portion were formed as shown in Fig. 1(a) and recesses 41 - 47 defined by the plurality of protrusion patterns were buried by a burying material 5 as shown in Fig. 1(b). After burying the recesses, a second polishing stopper layer 7 was formed at least on the burying material 5 in the recess of the portion in which the density of the polishing stopper layer 3 was coarse (wide recess 41) as shown in Fig. 1(c) and Fig. 1(d) (in this example, after forming the second polishing stopper layer 7 over the entire surface as in Fig. 1(c) and, subsequently, the second polishing stopper layer 7 was left only on the burying material 5 in the wide recess 41 by an etching step using a resist 8 as shown in Fig. 1(d)). Then, a step of flattening the burying material formed on the protrusion pattern with polishing was conducted. As a result, since the second polishing stopper layer 7 was formed on the burying material 5 in the wide recess 41 also to the portion in which the density of the polishing stopper layer 3 was coarse from the initial stage, no excess polishing proceeds even near the isolation protrusion pattern 61 and uniform and satisfactory flattening could be attained by polishing. Specifically, in this example, flattened trench isolation was formed by the following steps (1) - (6). (1) As shown in Fig. 1(a), after forming a thermally oxidized film as a silicon oxide layer 21, a poly-Si layer as an etching stopper layer 22 for a Si 3 N 4 layer which is a polishing stopper layer 3 and the Si 3 N 4 layer as the polishing stopper layer 3 on a semiconductor substrate 1 such as made of silicon, recesses 41 - 43 as trenches were formed by a RIE (reactive ion etching) step using a resist process. As the RIE conditions in this case, the following conditions were employed for instance. Gas system used: C 2 Cl 3 F 3 /SF 6 = 60/10 sccm Microwave power: 850 W RF power: 150 W Pressure: 1.33 Pa (2) Then, as shown in Fig. 1(b), a burying material 5 for burying the recesses 41 - 43 as the trenches was formed, for example, from an organic silicon compound (for example, TEOS) and ozone (0 3 ) by a CVD process such that the film thickness in the recess 41 of the trench was smaller than the depth of the trench by the thickness of the polishing stopper layer ( a in the figure). As the TEOS/O 3 CVD conditions, the following conditions were employed for instance. Gas system employed: TEOS/O 3 = 1000/2000 sccm Growing temperature: 390 °C Pressure: 1.20 x 10 4 Pa (3) Successively, as shown in Fig. 1(c), a silicon nitride film as a second polishing stopper layer was formed. The thickness of the silicon nitride film in this case was set such that the thickness of the silicon oxide film and the thickness of the silicon nitride film in the trench recess 41 (a+b in the figure) was made equal with the trench depth. As the conditions for forming the silicon nitride film, the following conditions were employed for instance. Gas system used: SiH 2 Cl 2 /NH 3 = 50/200 sccm Growing temperature: 760 °C Pressure: 70 Pa. In this case, as shown in Fig. 1(c), a resist pattern 8 was formed in order to remove the second polishing stopper layer 7 formed in the portions other than the wide recess 41 of the trench. (4) Then, an excess silicon nitride film was etched by isotropic etching. Thus, as shown in Fig. 1(d), the second polishing stopper layer 7 was left only on the burying material 5 in the wide recess 41. The etching was conducted in this case under the following etching conditions, for instance. Etching solution used: phosphoric acid Temperature: 150 °C (The selection ratio between the silicon nitride film and the silicon oxide film was about 50:1) (5) SiO 2 as the excess burying material 5 formed on the protrusion of the trench and the silicon nitride film as the second polishing stopper layer 7 left in the form of a protrusion on the trench recess 41 (indicated by a protrusion 31) were removed and flattened by polishing. Thus, a structure as shown in Fig. 1(e) was obtained. In this case, a customary device not shown was used as the polishing device. The polishing was conducted in this case under the following polishing conditions for instance. Number of rotation of polishing plate: 37 rpm Number of rotation of wafer holder specimen table: 17 rpm Polishing pressure: 5.5 x 10 3 Pa Slurry flow rate: 225 ml/min Pad temperature: 40 °C Slurry: Silica (0.025 - 0.03 um) KOH (pH ≦ 10.5) Water In the polishing step, since the selection ratio between the polishing stopper layers 3, 7 (Si 3 N 4 ) and the burying material 5 (SiO 2 ) was at SiO 2 /Si 3 N 4 = 4 - 6, and the area ratio of Si 3 N 4 as the stopper material per unit area was greater than a predetermined ratio irrespective of positions on the wafer, a sufficiently flattened shape could be obtained with no over polishing for the isolation pattern 61.Further, in flattening by the polishing method, it has been generally known that protrusion has much greater polishing rate than that of a flattened portion made of an identical material and, for instance, if a protrusion should occur, satisfactory flattened shape could be obtained. (6) Then, etching back was conducted entirely by magnetron RIE using poly-Si of the etching stopper layer 22 as a stopper. As the etching conditions in this case, the following conditions were employed, for instance. Gas system used: C 3 F 8 /O 2 = 45/15 sccm RF power: 1200 W Pressure: 2 Pa Finally, poly-Si as the etching stopper layer 22 and SiO 2 as the silicon oxide layer 21 were removed with KOH and hydrofluoric acid respectively. Thus, trench isolation as shown in Fig. 1(f) was completed. Example 2 This example is given for comparative purposes only, but does not form part of the present invention. In this example, the following process was applied to trench isolation. In Example 1, atmospheric CVD and vacuum CVD were used for forming a burying oxide film and a polishing stopper layer. In this example, a bias ECR-CVD process (this method is suitable as burying and flattening technique since etching and deposition are conducted simultaneously) was applied to the formation of the buried and flattened film and the second polishing stopper layer, thereby forming a polishing stopper layer 7 over the entire surface of a recess 41 as a trench. The following steps (1) - (7) were conducted. In the same manner as in Example 1, after forming a thermally oxidized film 21 as a silicon oxide layer, a poly-Si layer 22 as an etching stopper layer for a Si 3 N 4 layer as a stopper layer 3 for polishing and the Si 3 N 4 layer as the polishing stopper layer 3 on a semiconductor substrate 1 such as made of silicon trenches were formed by a RIE (reactive ion etching) step using a resist process. Then, a structure as shown in Fig. 2(a) was obtained. As the RIE conditions in this case, the following conditions were employed, for instance. Gas system used: C 2 Cl 3 F 3 /SF 6 = 60/10 sccm Microwave power: 850 W RF power: 150 W Pressure: 1.33 Pa (2) Then, a silicon oxide film was buried as a burying material 5 in trench recesses 41 - 43 by a bias ECR-CVD process having a high burying performance and providing a satisfactory buried and flattened shape. The buried film was formed such that the film thickness in the trench recesses 41 - 43 was less than the trench depth by the thickness of the polishing stopper layer 3 as shown in Fig. 2(b). As the bias ECR-CVD conditions in this case, the following conditions were employed, for instance. Gas system used: SiH 4 /N 2 O = 20/35 sccm Microwave power: 1000 W RF power: 500 W Magnetic flux density: 8.75 x 10 -2 T Pressure: 9.3 x 10 -2 Pa (3) Successively, a silicon nitride film as the polishing stopper layer 3 was formed by a bias ECR-CVD process. The silicon nitride film was formed to such a thickness as at least greater than the thickness of the polishing stopper layer 3 upon forming the trenches. Thus, a structure as shown in Fig. 2(c) was obtained. As the bias ECR-CVD conditions in this case, the following conditions were employed, for instance. Gas system used: SiH 4 /N 2 /Ar = 20/20/15 sccm Microwave power: 550 W RF power: 200 W Magnetic flux density: 8.75 x 10 -2 T Pressure: 2.0 x 10 -1 Pa Since the bias ECR-CVD process utilizes a simultaneous competing reaction between deposition and etching (sputter etching), it has dependency on angle of plane. Accordingly, the film was not formed in the regions other than those having a certain angle of plane relative to the horizontal plane of the substrate and provided a shape of the film as shown in Fig. 2(c). (4) As shown in Fig. 2(d). a resist 8 was formed by patterning on the portions other than the long trench protrusion (protrusions on the right and left in the figure) as shown in Fig. 2(d). (5) At first, silicon nitride films on the right and left in the figure as the second polishing stopper layer 7 formed on the protrusions of the long trenches were removed by isotropic etching using the resist 8. As etching conditions in this case, the following conditions were used for instance. Etching solution used: Phosphoric acid Temperature: 150 °C (selection ratio between the silicon nitride film and the silicon oxide film was about 50:1) Then, successively, the silicon oxide film as the excess burying material 5 on the long protrusion was removed by isotropic etching. As the etching conditions in this case, the following conditions were employed for instance. Etching solution used: HF:H 2 O = 1:40 Temperature: 150 °C (selection ratio between the SiO 2 film and the Si 3 N 4 film was about 8:1)In this case, the excess silicon oxide film on the long protrusion was underlying polishing stopper layers 3, 7. Etching could be applied with no problem at all till the Si 3 N 4 film was exposed. Subsequently, the resist 8 was removed. Thus, a structure as shown in Fig. 2(e) was obtained. (6) The excess silicon oxide film 52 in the form of protrusions formed in the preceding step (5) were removed by polishing. Since only the excess protruding silicon oxide film 52 was flattened, the polishing time could be shortened. Further, since the area ratio of the polishing stopper layers 3, 7 (Si 3 N 4 ) per unit area was greater than a predetermined ratio, irrespective of positions on the wafer, it was possible to obtain a sufficiently flattened shape without overpolishing the isolation pattern 61 (refer to Fig. 14(e)). Thus, a structure as shown in Fig. 2(f) could be obtained. (7) Then, etching back was applied for the entire surface by magnetron RIE under the conditions of SiO 2 and Si 3 N 4 selection ration at 1:1. As the etching back conditions in this case, the following conditions were employed, for instance. Gas system used: C 3 F 8 = 45/15 sccm RF power: 1200 W Pressure: 2 Pa Finally, poly-Si, SiO 2 forming the layer 22, etc. were removed by hot phosphoric acid and hydrofluoric acid respectively. Thus, trench isolation as shown in Fig. 2(g) was completed. It will be apparent that the present invention is not restricted only to the example 1 above. For instance, the polishing stopper layer may take various modes such that the material, processing conditions, etc. can be appropriately varied so long as the polishing stopper layer has a polishing rate lower than that of the burying material layer. According to the present invention, flattening can be attained without leaving the burying material on a wide (long) protrusion region, whereby it is possible to provide a means capable of forming trench isolation of satisfactory flatness, as well as a means for manufacturing a semiconductor device formed with such trench isolation. Further, according to the present invention, it is possible to provide a method of manufacturing a semiconductor device having a polishing step for conducting flattening after burying, wherein satisfactory flattened shape can be formed even in a portion in which the area ratio of a stopper layer per unit area is low in a case where there is unevenness in the distribution of a portion showing a function of a polishing stopper on the portion to be polished.

Claims: A method of manufacturing a semiconductor device from a semiconductor substrate (1) having a plurality of protrusion patterns (61 - 63) defining wide recess areas (41) and narrow recess areas (42, 43) and each having a first polishing stopper layer (3) in the upper portion thereof, the method comprising the following successive steps: a) burying the recess areas (41 - 43) with a burying material layer (5), the thickness of which is selected such that the upper surface of the burying material layer (5) in the wide recess areas (41) is aligned with the lower surface of the first polishing stopper layer (3) ; b) forming a second polishing stopper layer (7) on the burying material layer (5) in the wide recess areas (41) only with a thickness equal to the thickness of the first polishing stopper layer (3) ; c) flattening the surface by polishing. The method as defined in claim 1, wherein in step b) the second polishing stopper layer (7) was formed on the entire surface and then a resist pattern (8) was formed on the wide recess areas (41) in order to remove the second polishing stopper layer (7) in the portions other than the wide recess areas (41). The method as defined in claim 1, wherein the burying material (5) is silicon dioxide and the second polishing stopper (7) comprises silicon nitride. The method as defined in claim 1, wherein the burying material layer (5) is made of silicon dioxide and an atmospheric pressure CVD process is used for the formation of the burying material layer (5). The method as defined in claim 4, wherein the CVD silicon dioxide is formed by using an organic silicon gas.

Inventors: GOCHO TETSUO, HAYAKAWA HIDEAKI, GOCHO, TETSUO, HAYAKAWA, HIDEAKI
Applicants: SONY CORP, SONY CORPORATION
Application year: 1992
Main classification: H01L21
IPC codes: H01L21