METHOD OF FORMING A SUB-MICROMETER TRENCH STRUCTURE ON A SEMICONDUCTOR SUBSTRATE

Abstract: In the process of sidewall image transfer, a vertical step is etched in some material and then a conformal layer of some other material is deposited over the step. By reactive ion etching the conformal material can be anisotropically etched which results in a sidewall spacer of the second material on the vertical surfaces of the step material. By removing the step material, the free standing spacer can then be used as a mask. One area in which improvement is desired is in the selectivity of the etch of the spacer to the material immediately below it. Because of the limited number of materials and reactive ion etching gases it is difficult to avoid an etch in the underlying layer as the sidewall spacer is formed. A suitable etch stop is employed beneath the step material to avoid the problem. Because of the usual techno­ logy, the spacer material is plasma deposited silicon nitride and the step material is photoresist. Polysilicon, aluminum or similar metal is employed as an etch stop, since it is not affected by a CF₄ based gas which is used to form the spacer.

Description: This invention is also directed to making a deep dielectric isolation trenches having sub-micrometer width in a silicon body. A number of techniques are known to increase the performance and integration density of integrated circuits. Considerable increases of integration density have recently been achieved mainly by decreasing photolithographic defect densities. The use of electron and X-ray exposure methods instead of the hitherto used light radiation provides higher optical resolution. Techniques to reach very narrow line widths in the 1 µm range and less by extending conventional lithographic processes while avoiding the cost-intensive techniques of electron and X-ray lithography are also known. With the technique of plasma or reactive ion etching for etching metals, semiconductive and dielectric materials further developments have occurred in the direction of very narrow widths, and consequently toward increased performance and integration density. A number of publications and patents describe the so-called sidewall technology by means of which structures in the sub-micrometer range can be made. In IEEE Electron Device Letters, Vol. EDL2, No. 1, January 1981, pp. 4 to 6, a method is described where vertical (anisotropic) dry etching methods are applied for making MOSFETs which are defined by an edge and have dimensions in the sub-micrometer range. The technology described permits the production of physical channel lengths of the MOSFETs in the 0.1 to 0.15 µm range. U.S. Patent 4,358,340 describes a method of making sub-micrometer devices, but without using the sub-micrometer lithography, where a conductive thin film with dimensions in the sub-micrometer range is deposited across a vertical step between adjacent surfaces of an isolation, and subsequently vertically etched until there only remains that part of the conductive film which is adjacent the vertical step. The remaining isolation not covered by the conductor is removed, thus obtaining a gate of an MOS field effect transistor with a width in the sub-micrometer range which equals the layer thickness of the originally applied thin film. U.S. Patent 4,209,349 describes a method of forming very small mask openings for making semiconductor circuit arrangements. According to this method, first insulator regions are formed on a substrate so that substantially horizontal as well as substantially vertical surfaces are obtained. A second insulator layer is applied thereon of a material different from that of the first insulator layer, and it is subjected to a reactive ion etching process in such a manner that the horizontal regions of the second insulator layer are removed, with merely very narrow regions of this layer remaining on the vertical surface regions of the first insulator layer, and the respective regions of the substrate, respectively. Subsequently, the exposed substrate regions are thermally oxidized, and for finally forming the desired mask openings the regions of the second insulating layer there are removed. By means of this method, minimum dimensions smaller than those of hitherto applied photolithographic methods are to be obtained. In the process of U.S. Patent 4,209,349, the first insulator layer (silicon dioxide) determines the position and thickness of the mask (column 2, line 62). All openings in the insulator layer are made by standard photolithography and etching techniques (column 3, line 21) according to which no vertical sidewalls can be made. In the method as disclosed by the invention, however, a polymeric layer determines position and thickness of the mask. In the method of the U.S. patent, only hot processes are applied, e.g., the chemical vapor deposition of silicon nitride at approximately 1000°C (column 5, line 69), whereas in the method as disclosed by the invention so-called cold processes at less than 300°C are performed permitting a more universal application of the method. With the method of the U.S. patent, masks with a layer thickness of less than 0.5 µm can be made which are not suitable as masks, e.g., for etching deep trenches. With the method as disclosed by this patent, however, masks with a thickness of 2 to 3 µm can be made that can be used for etching 4 to 5 µm deep trenches in a silicon substrate. From thermal oxidation (column 4, line 64 of the U.S. patent), there results an asymmetrical mask with the bird's beak problem being involved which originates from the forming of a non-planar silicon dioxide on the trench surface, so that the mask cannot be used for trench etching also for that reason. The mask made in accordance with the invention has strictly vertical sidewalls and is of a symmetrical structure. A feature common to all these known methods is that for making structures with vertical sidewalls and dimensions in the submicrometer range materials as polysilicon, silicon nitride or silicon dioxide are used which are all deposited at higher temperatures. Problems associated with high temperature fabrication are well documented in this field. Another problem in the process of sidewall image transfer is the difficulty of avoiding etching into the underlying layer 7 upon which a spacer 5 is formed. This can be explained by referring to Figures 1 and 2. Basically, as illustrated in Figure 1A, the vertical step is first etched into some material 4 and then as illustrated in Figure 1B, a conformal layer of another material, a spacer material 5 is deposited over the first material 4. By the use of reactive ion etching (RIE), etching of the spacer material 5 is done anisotropically. This results in a leftover spacer 5 of a second material formed against the edge of the step material 4. Such is illustrated in Figure 1C. The step material 4 is then removed as illustrated in Figure 1D such that only the spacer 5, now free standing remains. This is illustrated in Figure 1D. The free standing spacer 5 can then be used as a mask to etch away an underlying layer 7 as illustrated in Figure 1E. This technique, while broadly demonstrated, has problems in terms of the selectivity of the etch of the spacer to the material immediately below it. There are a limited number of materials and RIE etch gases and therefore it is difficult to avoid etching into the underlying layer as the spacer is formed. Figure 2A illustrates an idea or technique where etching of the spacer material stops at the material beneath it. In Figure 2A, the dotted line indicates the original thickness of the spacer material 5 with the arrows indicative of the ideal depth of etch. Such ideally, therefore, conforms to that illustrated in Figure 1C. In fact, however, the etch, usually CF₄, causes a step or offset 8 from one side of the spacer 5 to the other side in the underlying layer 7. Such is illustrated in Figure 2B. The step 8 occurs because one side of the spacer 5 is covered by the step material 4, but the other side is exposed to the etching. The problem is exacerbated by the need for longer over-etch times as nonuniformity of the RIE system and spacer material 5 deposition system occurs. When the step material 4 is removed, the resulting structure is illustrated in Figure 2C. Thus, when the step material 4 is removed, the offset 8 will be propagated into whatever layer is beneath the underlying material 7. This occurs as the underlying material 7 is etched with one side being thicker than other other. The offset 8 is illustrated in Figure 2D. For purposes of illustration, example is the fabrication of an FET. The underlying layer 7 is SiO₂ with polysilicon 1 directly beneath it. Disposed beneath the polysilicon layer is a thin, approximately 14 nm gate oxide 9. Given the known technology as one side of the polysilicon 1 completes the etch cycle, the other side does not. Thus, over-etching begins to cutoff the gate oxide 9. The gate oxide 9 is etched through, the silicon 10 beneath is etched quickly. The result is an asymmetry in the device. Such is illustrated in Figures 2E and 2F. Technology that addresses this problem is represented by U.S. Patents 4,432,132 and 4,449,287. In neither, however, is the problem of avoiding the deep or offset effectively dealt with. Other technology directed more broadly to sub-micron technology is represented by U.S. Patent 4,093,503, 4,354,896, and 4,385,975. While generally of interest, those references also do not solve the problem set forth and explained relative to Figure 2. From IBM Technical Disclosure Bulletin, Vol. 26, No. 9, February 1984, pages 4587 - 4589, a method of forming a submicrometer mask structure using sidewall image transfer techniques is known. It consists of plasma depositing a Si₃N₄ sidewall against a standard resist edge on top of a silicon substrate having a silicon dioxide layer, a polysilicon layer and a second silicon dioxide layer. A reactive ion etching of the SI₃N₄ layer produces a Si₃N₄ stud of a very well controlled width. Then the resist pattern is removed. The stud sits on a layer of SiO₂, which really acts as a mask because the sidewall dimension is transferred to it by RIE before the polysilicon layer is etched. DE-A-3 016 736 discusses the etching of silicon dioxide films and discloses the use of an aluminum layer as an etch stop when etching SiO₂. Given deficiencies in the prior art, it is an object of this invention to provide the necessary selectivity of the etch of the spacer to the material which is immediately below it. In accordance with this invention, an etch stop is placed beneath the step material. Typically, the spacer material is a plasma deposited silicon nitride and the step material is a photoresist. In accordance with this invention, aluminum or a similar metal is used as an etch stop since it will be unaffected by the etchant which is used to form the spacer. It is another object of this invention to form very narrow openings, trenches, by first forming a sub-micron spacer over a metallic layer. When the combination is placed in an oxidizing environment (such as by anodization or exposure to O₂ plasma), the exposed metallic layer forms a metallic oxide skin and with the removal of the spacer, etching of the wafer will etch very narrow lines in the exposed metallic layer, making it useful as an etch mask. This invention will be described in greater detail by referring to the attached drawings and description of the preferred embodiments which follows. Figures 1A through 1E illustrate schematically the process of sidewall image transfer; Figures 2A through 2F illustrate the process of sidewall image transfer wherein etching in the underlying layer as the spacer forms occurs; Figures 3A through 3B illustrate a method which avoids etching in the underlying layer as the spacer is formed; and Figures 4A through 4D illustrate the preferred embodiment employing the technique of forming narrow sub-micron trenches utilizing this invention. Referring now to Figures 3A through 3B, a method is illustrated which avoids etching in the underlying layer 7 as the spacer 5 is being formed. Broadly, this method employs a suitable etch stop 3 beneath the step material 4. Generally, the etch stop 3 may be either aluminum or a similar metal. The spacer material 5 is a plasma Si x N y O Z , typically Si₃N₄ and the step material 4 is a photoresist. Such is illustrated in Figure 3A. With those materials, in this example aluminum provides a suitable etch stop 3 since it is not affected by the etchant, typically CF₄ which is used to form the spacer 5. The aluminum layer 3 illustrated in Figure 3A is very thin and typically not more than 150 nm. As illustrated in Figure 3A, aluminum 3 is deposited on top of a SiO₂ layer 2 which in actuality is the mask for the polysilicon 1 below it. In accordance with this technology, the oxide layer 2 is patterned by the spacer 5. As illustrated in Figure 3B, spacer material 5 is etched away; however, the aluminum 3 will not etch in CF₄. Thus, the RIE technique to form the spacer 5 will not offset the two sides of the spacer 5 given the presence of the aluminum etch stop 3. Referring now to Figures 4A through 4D, the technique of forming thin trenches 6 of the sub-micron range will be described. As set forth herein, in accordance with this invention, a material which does not form a volatile reaction product with the constituents of the plasma will not be etched in that plasma. Thus, a fluorine-containing plasma such as AlF₃ is a potential product of Al. It is nonvolatile and thus will not etch. There are numerous materials which can be used as an etch-stop in this manner. For example, chromium could be used in the case of both F or Cl containing plasma. It is noted, however, that O₂ must be absent. Other materials such as MgF₂ and CoF₂ may also be utilized. With this general methodology, it is possible to form a sidewall structure as an intermediate step in the ultimate formation of narrow trenches of the sub-micron range. In Figure 4A, the sidewall structure as described relative to Figure 3 is first formed over Al 3 or a similar metal. The composite of the spacer 5 and the metal layer 3B is then exposed for a short time to an oxidizing environment such as in anodization, leaving the underlying Al 3A unaffected. Such is illustrated in Figure 4B. A "skin" of Al₂O₃ 3B is formed except where the spacer 5 is present. Removal of the spacer is then accomplished by plasma or chemical removal techniques. Typically, hot H₃PO₄ can be used at a controlled temperature to remove only the spacer 5 and not the Al₂O₃ skin 3B. Such is illustrated in Figure 4C. The result is the now exposed Al 3A together with a Al₂O₃ skin 3B. Etching of the wafer, typically by a known Al etch will then etch narrow lines patterned on the exposed unpassivated Al 3A. Thus, the passivated Al layer 3A is useful as an etch mask and can be used to form a trench 6. The Al₂O₃ 3B and underlying Al 3A is unaffected but can be substantially removed by chemical techniques known in the art. It is apparent that modifications of this invention can be practiced without departing from the essential scope thereof.

Claims: Method of forming a sub-micrometer trench structure on a semiconductor substrate (1) comprising: a) forming an insulative layer (2) on a major surface of said substrate; b) forming a metallic stop layer (3) on said insulative layer (2), wherein the metallic stop layer (3) is a metal able to be passivated in an oxidizing environment; c) forming a step material layer (4) providing steps at selected areas on said metallic stop layer (3); d) covering said metallic stop layer (3) and said step material (4) with a spacer material (5); e) removing by selective etching, portions of said spacer material (5) to define a vertical spacer at said step locations, said trench structure to be created in the region under said vertical spacer, said selective etching being positively stopped by said stop layer (3); and f) removing said step material (4) to define a free-standing spacer (5) characterized by the following steps: g) subsequently exposing said spacer (5) and said metallic stop layer (3) to an oxidizing environment to form a passivated layer of metallic stop material (3B) except under that spacer (5); h) removing said spacer (5) to expose remaining unpassivated stop material (3A); and i) etching said substrate (1) using said passivated stop material (3B) as a mask to form narrow trench patterns by removing said remaining unpassivated metallic stop material (3A). Method of claim 1, wherein said insulative layer (2) is silicon dioxide. Method of claim 1, wherein said metallic stop layer (3) is aluminum and said oxidizing environment is anodization. Method of claim 1, wherein said step material is a photoresist. Method of claim 1, wherein said spacer material (5) is plasma deposited Si X N Y O Z , preferably Si₃N₄, and said step of removing said spacer (5) is by hot H₃PO₄ etch. Method of claim 1, wherein said selective etching of said spacer material (5) is CF₄ etching.

Inventors: OGURA, SEIKI, RISEMAN, JACOB, ROVEDO, NIVO, SCHULZ, RONALD NORMAN
Applicants: INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1986
Main classification: H01L21
IPC codes: H01L21, G03F1