Device isolation method for a semiconductor device

Abstract: In a device isolation method for a semiconductor device, after a pad oxide layer (52) and a nitride layer (54) are formed on a semiconductor substrate (50), the nitride layer located above a device isolation region is removed. An undercut (C) is formed under the nitride layer (54) by partially etching the pad oxide layer (52). After a first oxide layer (56) is formed on the exposed substrate and a polysilicon spacer (58) is formed on the sidewalls of the nitride layer, a void (V) is formed in the oxide layer under the nitride layer which is formed on the active region, by oxidizing the resultant structure in which the polysilicon spacer is formed, at a temperature at or above 950°C. Thus, good cell definition and stable device isolation can be realized, while solving a typical problem of a conventional LOCOS method by forming the void intentionally in the pad oxide layer thickened by bird's beak punch through.

Description: The present invention relates to a device isolation method of a semiconductor device, and more particularly, to an device isolation method of a semiconductor device for intentionally forming a void in a pad oxide layer thickened by a bird's beak punch through in a device isolation method using a local oxidation of silicon (LOCOS) process. According to recent trends toward high integration of semiconductor devices, research and development of an device isolation technology (a micro-technology) has been actively advanced. The formation of an device isolation region is an initial step of a semiconductor manufacturing process, and determines the size of an active region and defines the process margin of all subsequent manufacturing process steps. Isolation has been achieved by the LOCOS method. Figs. 1A-1D of the accompanying drawings show a method for forming a field oxide layer by a conventional LOCOS method. Referring to Fig. 1A, after a pad oxide layer 12 is formed on a semiconductor substrate 10, a nitride layer 14 is formed thereon. Referring to Fig. 1B, photoresist is coated and patterned on nitride layer 14 to form a photoresist pattern 16. Next, impurities of the same conductive type as the substrate are ion-implanted on the whole surface of the substrate, in order to form a channel stop region. Referring to Fig. 1C, after nitride layer 14 is etched using photoresist pattern 16 as an etching mask and the photoresist pattern is removed, a thermal oxidation process is performed such that a field oxide layer 18 is formed in the device isolation region of the substrate. At this time, a channel stop region 20 is formed below field oxide layer 18. Referring to Fig. 1D, device isolation is realized by removing nitride layer 14 and pad oxide layer 12. However, as known widely, in the LOCOS method, there is a serious problem resulted from bird's beak because oxygen permeates to the side surface of the pad oxide layer under the nitride layer used as an oxidation prevention mask, and thus the silicon under the nitride layer becomes oxidized. Further, with the high integration of elements, the cell interval in an active region becomes even smaller. In a conventional LOCOS technology as above, it is impossible to prevent a bird's beak punch-through phenomenon by which adjacently formed bird's beaks make contact with each other under the nitride layer. If cells are overlapped, the above phenomenon is especially serious at an end of the cells where the bird's beaks extend in three directions. Referring to Figs. 2 and 3A-3C of the accompanying drawings, in the "P" end of an overlapped cell, the bird's beaks contact each other by the bird's beak punch-through phenomenon, the pad oxide layer located under the nitride layer is thickened (Figs. 3A and 3C). The bird's beak phenomenon is not serious in the central portion of the cell (Fig. 3B) When the bird's beak punch-through phenomenon is generated, the definition of the active region becomes virtually impossible. Also, in order to form the active region, the oxide layer thickened by the bird's beak punch-through phenomenon must be removed. In order to remove the thickened oxide layer, an over-etching process needs to be performed. Here, however, due to the over-etching performed to the field oxide layer as well as to the thickened oxide film, element characteristics deteriorate or the actual device isolation is impossible (refer to Fig. 7A). EP-A-0 284 456 discloses a conventional method of forming field oxide. It is an object of the present invention to provide a device isolation method of a semiconductor device for realizing proper device isolation and cell definition, by using a bird's beak punch-through phenomenon. According to claim 1 of the present invention there is provided a device isolation method for a semiconductor device comprising the steps of: forming a pad oxide layer and a nitride layer on a semiconductor substrate; removing said nitride layer located over a device isolation region; forming an undercut region (C) under said nitride layer by partially etching said pad oxide layer; forming a first oxide layer on the exposed substrate with a thickness less than that of the pad oxide layer; forming a polysilicon spacer on the sidewalls of said nitride layer and filling said undercut region (C), and selectively oxidizing the resultant structure in which said polysilicon spacer is formed, whereby a void (V) is formed in a portion separating the pad oxide and the silicon substrate, said portion located under said nitride layer formed on an active region. Here, it is preferable that the selective oxidation process is executed at a temperature of 950°C or above, and a void is formed in an oxide layer located under the nitride layer which is formed on an active region. According to a preferable embodiment of the present invention, the first oxide layer has a thickness of 30-160Å (10Å = 1nm). On the other hand, the polysilicon spacer can be formed lower than the nitride layer by over-etching a polysilicon spacer, as necessary. According to claim 2 of the present invention there is provided a device isolation method for a semiconductor device comprising the steps of: forming a pad oxide layer and a nitride layer on a semiconductor substrate; removing said nitride layer located on a device isolation region; etching said substrate by using said nitride layer as an etching mask; forming an undercut region (C) under said nitride layer by partially etching said pad oxide layer; forming a first oxide layer on the exposed substrate with a thickness less than that of the pad oxide layer; forming a polysilicon spacer on the sidewalls of said nitride layer and of the etched portion of the substrate under said nitride layer said polysilicon filling said undercut region (C), and selectively oxidizing the resultant structure in which said polysilicon spacer is formed, whereby a void (V) is formed in a portion separating the pad oxide and the silicon substrate, said portion located under said nitride layer formed on an active region. According to a preferred embodiment, the substrate is etched by 200-1000Å, the selective oxide process is executed at high temperature greater than 950°C. Also, the polysilicon spacer is formed so that the polysilicon is filled in the undercut and the first oxide layer has a thickness of 30-160Å. On the other hand, the polysilicon spacer can be formed lower than the nitride layer by over-etching the polysilicon spacer, as necessary. A void is intentionally formed in the oxide layer thickened by the bird's beak punch-through so that the typical problem of a conventional LOCOS method can be solved and stable device isolation and proper cell definition can be realized. Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings in which: Figs. 1A-1D are cross-sectional views showing the sequential process steps for forming a field oxide layer by a conventional LOCOS method; Fig. 2 is a plan view of one example of a conventional cell array; Figs. 3A-3C are cross-sectional views taken along the lines A-A', B-B' and C-C' of Fig. 2; Fig. 4 is a plan view of an element cell array according to an embodiment of the present invention; Figs. 5A-5C are cross-sectional views taken along the lines A-A', B-B' and C-C' of Fig. 4; Figs. 6A-6G are cross-sectional views showing the sequential process steps according to an embodiment of a device isolation method of the present invention; Figs. 7A to 7E are cross-sectional views showing the sequential process steps according to a further embodiment of the device isolation method of the present invention; Figs. 8A and 8B are SEM photographs of a cell formed by a conventional method and one formed by a method according to an embodiment of the present invention, respectively, and Figs. 9A and 9B are each SEM photographs showing the shape of a void formed by the method of the present invention. One embodiment of an device isolation method according to the present invention will be explained in detail, with reference to Figs. 4 to 7E. In Fig. 4, a reference symbol 22 indicates a cell, 24 indicates an inactive region in which a field oxide layer is formed, and "P" indicates a portion in which cells are overlapped. Referring to Fig. 4, voids "V" are formed in both ends of overlapped cell 22. Figs. 5A-5C are for comparing a conventional device isolation method with the device isolation method according to the present invention. Here, reference numeral 50 indicates a semiconductor substrate, reference numeral 54 indicates a nitride layer, reference numeral 60 indicates a field oxide layer, reference character V indicates a void, and reference character b indicates the bump of the oxidation layer. Referring to Figs. 5A and 5C showing the cross section of A-A' and C-C' of the ends of the cell to which bird's beaks extend in three directions, the bird's beak punch-through phenomenon causes the bird's beaks to contact each other, and void V is formed in the thickened pad oxide layer located under the nitride layer. On the other hand, referring to Fig. 5B showing the B-B' cross section of the cell in which the bird's beaks extend in two directions, it can be seen that the bird's beak punch through phenomenon is not generated. Over-etching has been traditionally carried out in order to remove the oxide layer under the nitride layer thickened by bird's beak punch-through. Accordingly, the oxide layer below the nitride layer in a B-B' region (where the bird's beak punch through is not generated) became very thin, which deteriorated certain element characteristics. However, according to the embodiment of the present invention, since the void is formed in the oxide layer under the nitride layer where the punch through is generated, thereby to thin the resultant oxide layer, over-etching is not needed. Thus, the deterioration of the element characteristics and the device isolation are not generated. Figs. 6A to 6G are cross-sectional views showing the sequential process steps according to a first embodiment of a device isolation method of the present invention. Fig. 6A illustrates the step of forming a pad oxide layer 52 and nitride layer 54 on a semiconductor substrate 50. After pad oxide layer 52 of about 300Å is grown on semiconductor substrate 50 by a thermal oxidation method, the nitride layer of 1500-2500Å is stacked on pad oxide layer 52 by a low-pressure chemical vapor deposition (LPCVD) method. Fig. 6B illustrates the step of patterning nitride layer 54. After a photoresist is coated on the resultant structure in which the nitride layer is formed, a photoresist pattern (not shown) is formed by applying a mask pattern for defining an active region and an device isolation region. Next, nitride layer 54 over the device isolation region is removed and the active region is defined by using the photoresist pattern as an etching mask and etching the nitride layer. Fig. 6C illustrates the step of forming an undercut under nitride layer 54. Undercut C is formed under nitride layer 54 by partially etching pad oxide layer 52. At this time, it is preferable to use a wet etching in an etching process. Fig. 6D illustrates the step of forming a first oxide layer 56. First oxide layer 56 is formed thinner than the above pad oxide layer, for example, to a thickness of 30-160Å on the resultant structure in which undercut C is formed. Here, first oxide layer 56 is formed to prevent a defect generating in the silicon substrate by a stress applied to the silicon substrate at the time of a subsequent thermal oxidation process for forming the field oxide layer. Also, the first oxide layer has a uniform thickness over the whole surface of the isolation region and the adjustment of the thickness is easy. Therefore, it is possible to adjust the size of the bird's beak easily because the bird's beak is formed depending on the thickness of the pad oxide layer. However, if the thickness of the pad oxide layer located under the nitride layer becomes thinner by more than a certain thickness, say, one third that of the nitride layer, since a dislocation is caused in the silicon substrate and exerts a negative influence on the electrical characteristics of the device, there is a fundamental limitation to reduce the thickness of the pad oxide layer. Accordingly, the size of the bird's beak can be adjusted easily by forming the undercut and the first oxide layer having a uniform thickness which is thinner than the pad oxide layer. Fig. 6E illustrates the process forming a polysilicon spacer 58. After a polysilicon is deposited onto the whole surface of the resultant structure in which first oxide layer 56 is formed, spacer 58 is formed on the sidewalls of nitride layer 54 by anisotropically etching the deposited layer. At this time, polysilicon spacer 58 fills in the undercut region c under nitride layer 54. On the other hand, after the polysilicon is deposited and the spread thickness is etched as needed, 20-30% over-etching can be executed so that polysilicon spacer 58 is formed with a lower profile than the nitride layer. The polysilicon spacer 58 with a lower profile prevents problems caused by polysilicon bump (refer to 'b' of Fig. 6F) to be formed in subsequent oxide process. The problems includes the impossibility of subsequent processing caused by unetched bump and deterioration of electrical characteristics of the element caused by a thin field oxide layer resulting from over-etching to remove the bump. Fig. 6F illustrates the step of the thermal oxidation process. The oxidation process is carried out on the resultant structure in which the polysilicon spacer is formed, and a field oxide layer 60 is formed in the device isolation region. At this time, void V is formed in the pad oxide layer below nitride layer 54 (refer to SEM photographs of Figs. 8A and 8B). Here, the spacer formed on the sidewalls of the nitride layer is oxidized, and bump b of the oxide layer is formed. Especially, nitride layer 54 receives an upward force by a volume expansion generated during the oxidation of the polysilicon formed in the undercut region and on the sidewalls of nitride layer 54 in the beginning of the oxidation process. The longer the oxidation process is carried out, the stronger the force for lifting the nitride layer becomes. On the other hand, if the oxidation process is performed at high temperature, for example, 950-1150°C, the oxygen diffuses rapidly, and the bird's beak phenomenon is generated beneath the nitride layer. If the oxidation process is continued, the bird's beak punch-through phenomenon in which adjacent bird's beaks contact each other under the nitride layer is generated. At this time, while oxygen is supplied through the pad oxide layer in a conventional case, and the oxide layer is formed in between the nitride layer and the silicon substrate, the first oxide layer formed under the polysilicon spacer becomes the supply path of the oxygen in the case of the present invention. Since the first oxide layer is very thin, the oxygen cannot be supplied between the nitride layer and the substrate. Accordingly, if the oxidization continues, the nitride layer receives a stronger upward force. If the coupling force of the silicon substrate and the pad oxide layer is weaker than the force lifting the nitride layer, the pad oxide layer and the silicon substrate are separated. Conventionally, the oxygen is supplied before separation of oxide layer and silicon substrate, the oxide layer is formed under the pad oxide layer, and the separated portion is filled in by the volume expansion. However, in the case of the present invention, since the oxygen is not supplied well, the separated portion is not filled in. If the oxidation process is continued, void V is formed in the portion separating the pad oxide layer and the silicon substrate. The void increases in size as the oxidation process proceeds. Fig. 6G illustrates the step of forming field oxide layer 60. The cell is defined by removing nitride layer 54, etching back the oxide layer in which the void located under the nitride layer is formed, and forming field oxide layer 60 (refer to Fig. 8B). According to the first embodiment, since the void is formed intentionally in the pad oxide layer, the typical problem of the conventional LOCOS method can be solved. Figs. 7A to 7E are sequential process drawings depicting a second embodiment of device isolation method according to the present invention. The same reference numbers in Figs. 6A to 6G and Figs. 7A to 7E denote the same element. The second embodiment is carried out in the same manner as the first embodiment except that semiconductor substrate 50 is etched by a predetermined depth after the step (Fig. 6C) forming the undercut (C) is executed. Fig. 7A depicts the step of etching semiconductor substrate 50. Substrate 50 is etched by a predetermined thickness, for example, around 200-1000Å by using nitride layer 54 as an etching mask. Fig. 7B depicts the step of forming the first oxide layer 56. First oxide layer 56 is formed thinner than the pad oxide layer, for example, 30-160Å thick on the resultant structure like the first embodiment. Fig. 7C depicts the process forming polysilicon spacer 58. Spacer 58 is formed on the sidewall of nitride layer 54 and of the etched portion of the substrate under said nitride layer by anisotropically etching after polysilicon is coated on the whole surface of the resultant structure in which first oxide layer 56 is formed. At this time, polysilicon spacer 58 is formed to fill the undercut (C) located under nitride layer 54. On the other hand, if desired, the polysilicon may be over-etched by 20-30% to make the polysilicon spacer 58 lower than the nitride layer. Fig. 7D depicts the thermal oxidation process. The oxidation process is executed with respect to the resultant structure in which polysilicon spacer is formed, and field oxide layer 60 is formed in device isolation region. At this time, the void (V) is formed inside the pad oxide layer located under nitride layer 54 as in the first embodiment. Fig. 7E depicts the step of forming field oxide layer 60. The cell is defined by removing nitride layer 54, etching back the oxide layer in which the void is formed under the nitride layer, and forming field oxide layer 60. According to the second embodiment, since the void is formed intentionally, the typical problem of the conventional LOCOS method can be solved. Also, since the oxide process is executed after the silicon substrate of the device isolation region is etched by a predetermined thickness, the field oxide layer 60 is formed deeper than in the conventional method and a superior device isolation characteristic can be obtained. Referring to Fig. 8A which depicts a conventional cell, since the over-etching process is performed to remove the oxide layer thickened by the bird's beak punch-through phenomenon, even part of the field oxide layer is removed so that the definition of the cell is not clear. Referring to Fig. 8B which depicts a cell of the present invention, since there is no need to perform over-etching due to the void formed in the pad oxide layer located under the nitride layer, the definition of the cell is clear. Figs. 9A and 9B are cross sectional views showing the void shape taken along the lines A-A' and C-C' of Fig. 4, after the thermal oxidation process. Here, the void is formed in the pad oxide layer located under the nitride layer. According to the present invention, it is possible to adjust the size of the bird's beak easily by forming an undercut region under the nitride layer and the thin oxidation layer. The typical problems of a conventional LOCOS method are solved and good cell definition and stable device isolation can be realized by forming the void intentionally in the pad oxide layer thickened by the punch through.

Claims: A device isolation method for a semiconductor device comprising the steps of: forming a pad oxide layer (52) and a nitride layer (54) on a semiconductor substrate (50); removing said nitride layer located over a device isolation region; forming an undercut region (C) under said nitride layer (54) by partially etching said pad oxide layer (52); forming a first oxide layer (56) on the exposed substrate with a thickness less than that of the pad oxide layer; forming a polysilicon spacer (58) on the sidewalls of said nitride layer and filling said undercut region (C), and selectively oxidizing the resultant structure in which said polysilicon spacer is formed, whereby a void (V) is formed in a portion separating the pad oxide and the silicon substrate, said portion located under said nitride layer formed on an active region. A device isolation method for a semiconductor device according to claim 1 comprising the additional steps of: etching said substrate by using said nitride layer as an etching mask, after removing said nitride layer located on a device isolation region; and forming additionally a polysilicon spacer on the etched portion of the substrate under said nitride layer said polysilicon filling said undercut region (C). A device isolation method for a semiconductor device according to claim 2, wherein said substrate is etched by 20-100 nm (200-1000Å). A device isolation method for a semiconductor device according to any preceding claim, wherein said selective oxidation process is executed at a temperature of 950°C or above. A device isolation method for a semiconductor device according to any preceding claim, wherein said first oxide layer has a thickness of 3-16 nm (30-160Å). A device isolation method of a semiconductor device according to any preceding claim, wherein said polysilicon spacer (58) is formed lower than said nitride layer by over-etching a polysilicon layer at the time of forming said polysilicon spacer. A device isolation method for a semiconductor device according to claim 2, wherein a polysilicon layer is formed lower than said nitride layer by over-etching the polysilicon layer at the time of forming said polysilicon spacer (58).

Inventors: AHN DONG-HO, AHN SEONG-JOON, KIM YUN-GI, SHIN YU-GYUN, AHN, DONG-HO, AHN, SEONG-JOON, KIM, YUN-GI, SHIN, YU-GYUN
Applicants: SAMSUNG ELECTRONICS CO LTD, SAMSUNG ELECTRONICS CO., LTD.
Application year: 1995
Main classification: H01L21
IPC codes: H01L21