TAILORING OF VIA-HOLE SIDEWALL SLOPE IN AN INSULATING LAYER

Abstract: A method of selectively tailoring the slope of via hole sidewalls of insulating layers of integrated circuits. A first insulator layer (12) (in which the vias are to be formed) is covered by a removable layer (14), and the two layers are isotropically etched. By varying the thickness of the removable layer (14) with respect to that of the insulator layer (12), the slope of the via hole sidewalls can be controlled.

Description: The invention relates to a method of forming vias in an insulator layer. The general idea of forming holes in an insulator layer formed atop an integrated circuit device is well known. These holes are filled with a conductive material which establishes electrical contact between the underlying device and metallization on top of the insulation layer. More recently, multiple layers of these metallizations have been formed, using multiple layers of insulators to isolate the metallizations from one another except for preselected areas which are interconnected through the above-described vias. A problem is that of metal step coverage. If the slope of the sidewalls of the via holes in the insulator layer is too great, the metal layer may not cover the "step" created by the via. Such a discontinuity can cause an open line condition, thus leading to a defective chip. One way of addressing this problem is to reduce the sharp angle of the slope of the via hole sidewalls. The European Patent Application with the application number 85 100 407.7 relates to a method of forming via holes having rounded sidewall profiles. U.S. Patent 3 880 684 (issued 4/29/75 to Abe and assigned to Mitsubishi) teaches a process for etching a dual dielectric structure (consisting of dielectric layers having different etching characteristics) using a non-erodible mask. The dielectric materials, as well as the relative positions of the layers, are preselected in order to produce the desired via profile. U.S. Patent 4 369 090 (issued 1/18/83 to Wilson et al and assigned to Texas Instruments) teaches a method for forming sloped vias in polyimide insulators by starting with a partially cured film and selectively exposing it to curing agents to develop the desired etching characteristics in the film. U.S. Patent 4 378 383 (issued 3/29/83 to Moritz and assigned to assignee of the invention) teaches a method for forming vias in an insulating layer. A masking layer is used having overhanging walls which erode during etching. An article by J. J. Kelly et al, entitled "Electrochemical Aspects of the Beveling of Sputtered Permalloy Films", Electrochemical Science and Technology, June 1978, pp. 860-865, discusses the formation of a preselected edge profile in a base layer through the use of an upper layer having a greater etch rate. It is therefore an object of the invention as described in the claims to provide an improved method of selective ly tailoring the slope of the via hole sidewalls. This is achieved by overlaying the insulator layer, in which the via holes are formed, with a removable layer of variable thickness and similar etch characteristics as the insulating layer. The ratio of the thicknesses of the insulating layer and the removable sacrificial layer controls the slope of the via hole sidewalls. A non-erodible mask is formed on the sacrificial layer; this mask having etching characteristics which differ from those of the sacrificial layer. The desired via hole pattern is then formed on the non-erodible mask, and the sacrificial and insulating layers are etched through the non-erodible mask to thereby form vias in the insulating layer. These and other features will become more apparent upon a detailed description of the invention. Reference will be made to the accompanying drawings, in which: Figs. 1A-1D are cross-sectional views of the deposition and etching of the layers utilized in the present invention; and Figs. 2A-2E are cross-sectional views which illustrate the variation in via hole sidewall slope as a function of the thickness of the sacrificial layer. The present invention will now be described with reference to Figs. 1A-1D. Fig. 1A shows a cross section of the deposited layers prior to etching. A layer of polyimide 12 is formed by conventional techniques on a silicon substrate 10. The polyimide 12 is fully cured by subjecting the substrate to a baking step at 350°C. Note that the bare substrate 10 is given only by way of example; that is, the polyimide 12 can also be formed on a previously-deposited metallization layer, on a diffusion region, or any other structure present in conventional integrated circuits and the like. A removable layer 14 is then applied to the cured polyimide layer 12. The removable layer 14 can consist of any material which can be stripped or otherwise removed without attacking the underlying polyimide 12. In addition, the etch rates of layer 14 and the polyimide 12 should be similar. One such material is partially cured polyimide (i.e. polyimide which is baked at approximately 120°C for approximately 20 minutes). This material can be dissolved by an N-methyl pyrrolidinone (NMP) solvent without appreciably dissolve the underlying polyimide. On the other hand, the etch rates of the two polyimide layers are substantially similar. Another advantageous feature of uncured polyimide is that it does not reflow or otherwise deform during subsequent processing. As will be discussed in more detail below, the difference in the thicknesses of the two polyimide layers determines the slope of the sidewalls of the via produced in the polyimide layer 12. The polyimide layer 14 is then coated with a mask layer 16. The mask layer 16 can be made of any material (such as silicon or spun-on glasses like borosilicate glass or phosphosilicate glass) which will not be etched (i.e. is "non-erodible") in an oxygen (O₂) plasma gas but which will be etched in a tetrafluoromethane/oxygen (CF₄ and O₂) plasma gas. Preferably, the mask layer 16 is made of silicon which is deposited using low temperature (less than 90°C) vapor deposition techniques. The mask layer 16 should be thin, approximately 50 nm thick. At this thickness, the silicon will be thick enough to eliminate pinhole formation, while still being thin enough to be transparent (facilitating mask alignment). A layer of photoresist 18 is then coated onto the mask layer 16. The photoresist is masked, exposed and developed in KOH as per conventional techniques. Note that as an alternative to the silicon mask layer 16 and the photoresist 18, a single layer of silicon-rich photoresist (having a low etch rate in an O₂ plasma) may be used. Such a photoresist may be formed by exposing a standard photoresist to a hardening solution such as hexamethyldisilazane (HMDS). The photoresist will absorb the silicon present in these compounds, making the photo resist silicon-rich. As shown in Fig. 1B, the mask layer 16 is etched through the photoresist 18 in a CF₄ + O₂ anisotropic plasma etch. The structure is etched for a time period sufficient to transfer the pattern formed in the photoresist to the mask layer 16. During this process, the underlying removable layer 14 is not etched since the polyimide will not be etched appreciably in CF₄ + O₂ plasma. Then, as shown in Fig. 1C, the gas content of the etch gas is changed from CF₄ + O₂ to pure O₂, such that the removable layer 14 as well as the polyimide 12 are isotropically etched. The mask layer 16 protects the upper surface of the removable layer 14 during this etching step. Before the underlying layers are etched, the photoresist layer 18 is removed. This etch step can be performed using a plasma etch or a high pressure reactive ion etch. The profile of the apertures etched into the removable layer 14 and the polyimide layer 12 is a function of the etch temperature, time, etc. The ratio of the thicknesses of the two layers yields the controllable slope of the sidewalls of the via holes etched into the polyimide layer 12. This feature is illustrated more detailed in Figs. 2A-2E. As shown in Fig. 2A, the slope of the side walls of the via hole is approximately 60° when the removable layer 14 and the polyimide layer 12 are of the same thickness. The angle  (describing the slope) between the sidewall and the surface of the substrate is given by As shown in Fig. 2B, notice that the angle decreases to 53° when the thickness of the removable layer 14 equals 1.5x the thickness of the polyimide layer 12. As shown in Fig. 2C, the angle decreases to 48 when this ratio is 2:1; as shown in Fig. 2D, the angle is 41° when this ratio is 3:1; and, as shown in Fig. 2E, the angle is 37° when this ratio is 4:1. For each of the examples shown in Figs. 2A-2E, the overall profile of the aperture etched into layers 12 and 14 is the same. As the thickness of layer 14 increases with respect to layer 12, the sidewalls of the vias etched into polyimide layer 12 are defined to a greater extent by the bottom portion of the overall aperture profile. Thus, since the overall aperture profile has a decreasing slope at its bottom region, the via hole sidewall slope decreases as the thickness of layer 14 increases. After the aperture is completely formed in layers 14 and 12, the plasma etch gas is changed from pure O₂ back to a CF₄ + O₂ mixture, and the mask layer 16 is removed. Finally, as shown in Fig. 1D, the removable layer is removed by dissolving it in an N-methyl pyrrolidinone (NMP) solution, leaving the polyimide 12 with via holes having sidewalls of a predetermined slope. The via holes formed in the manner of the invention can be metallized using any one of the many known metallization processes. As previously mentioned, the removable layer 14 consists of a material such as polyimide cured at 120°C which can be removed by dissolving it without appreciably effecting the underlying polyimide layer 12. Prior to the NMP strip step, the removable layer 14 can be exposed to a short oxygen plasma in order to remove any impurities. This step enhances the strippability of the removable layer 14. Thus, as described above, the inventors have proposed a method of selectively tailoring the sidewall slope of via holes formed in an insulating layer. The invention enables a process designer to select the most advantageous sidewall slope for the metallization pattern to be employed. It is to be noted that the concepts of the invention can be extended to a wide range of via hole formation and metallization techniques. For example, these concepts can be applied to a structure wherein a silicon oxide layer performs the function of the polyimide layer 12, a silicon nitride layer performs the function of the removable layer 14, and a patterned aluminum layer performs the function of the non-erodible mask.

Claims: 1. A method of forming via holes and tailoring the sidewall slope of these via holes in an insulator layer (12), characterized by the steps of overlaying the insulator layer (12) with a removable layer (14) with at least similar etch characteristics as the insulator layer (12); overlaying the removable layer (14) with a patterned mask layer (16); etching via holes in the removable layer (14) and the insulator layer (12) through apertures in the patterned mask layer (16); removing the mask layer (16); removing the removable layer (14); whereby the sidewall slope of the via holes in the insulator layer (12) is determined by the ratio of the thicknesses of the insulator layer (12) and the removable layer (14). 2. The method of claim 1, characterized in that the insulator layer (12) consists of polyimide, preferably fully cured. 3. The method of claims 1 or 2, characterized in that the removable layer (14) consists of polyimide, preferably partially cured at approximately 120°C for approximately 20 minutes. 4. The method of one of the previous claims, characterized in that the via holes are etched isotropically in the removable layer (14) and in the insulator layer (12) by exposure to a first etchant, which does not etch the mask layer (16); and that the mask layer (16) is removed by exposure to a second etchant, which does not appreciably etch the removable layer (14); and that the removable layer (14) is stripped by exposure to a third etchant which does not appreciably etch the insulator layer (12). 5. The method of claim 4, characterized in that the first etchant is a plasma consisting of oxygen. 6. The method of claim 4, characterized in that the second etchant is a plasma consisting of tetrafluoromethane and oxygen. 7. The method of claim 4, characterized in that the third etchant is N-methyl-pyrrolidinone. 8. The method of one of the previous claims, wherein the apertures in the patterned mask layer (16) are formed by the steps of: forming on the non-patterned mask layer (16) a second non-patterned mask layer (18), preferably consisting of a photoresist; patterning the second mask layer (18) to expose regions of the non-patterned mask layer (16); and etching the non-patterned mask layer (16) through the second mask layer (18). 9. The method of one of the previous claims, characterized in that the mask layer (16) consists of a thin layer of a transparent material, preferably of a thin layer of a silicon-rich photoresist, or silicon, or a compound selected from the group consisting of borosilicate glass and phosphorsilicate glass. 10. The method of claim 8 or 9, characterized in that the second mask layer consists of a photoresist. 11. The method of claim 8, 9 or 10, characterized in that the second mask layer (18) is substantially removed during the isotropic etch of the removable layer (14) and the insulator (12).

Inventors: ABRAMS, ALLAN DAVID, BAUSMITH, ROBERT CROWELL, HOLLAND, KAREY LYNN, HOLLAND, STEVEN PAUL
Applicants: INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1985
Main classification: H01L21
IPC codes: H01L21