PROCESS FOR DEFINING VIAS THROUGH SILICON NITRIDE AND POLYIMIDE

Abstract: A process for defining vias through a polyimide and silicon nitride layer: 1. Providing a substrate having a first layer of silicon nitride and a second layer of polyimide; 2. Depositing a layer of photoresist capable of producing negatively sloped walls; 3. Lithographically defining a pattern of vias in the photoresist; 4. Developing the photoresist to produce a pattern of vias having negatively sloped walls; 5. Etching the polyimide layer using the developed photoresist layer as an etch mask with a CF₄/O₂ gas mixture; and 6. Etching the silicon nitride layer using the etched polyimide layer as an etch mask with a CF₄/H₂ gas mixture.

Description: This invention is related to a process for defining vias through polyimide and silicon nitride layers for making circuit interconnections in an integrated circuit. In the fabrication of very large scale integrated circuits (VLSI), it is sometimes necessary to make circuit interconnections between layers of metallization which are separated by one or more layers of insulating material. In the case of an integrated circuit as shown in Fig. 1 in which contact metal layer 10 is sequentially covered by a layer 12 of silicon nitride and then a layer 14 of polyimide, it is necessary to define vias 16 through both of these layers to make circuit interconnections with additional metal layers that are subsequently deposited. It is known in the art to define vias using the following process steps: 1) apply photoresist layer; 2) lithographically define the via pattern; and 3) etch the underlying layer with an appropriate wet or dry etchant. When necessary to define vias through two layers, the above process steps are repeated for the second layer. The problem, however, is that with the tolerances inherent in current technologies, it is very difficult to align the vias in each of the respective layers. To ensure that the vias do coincide to the extent necessary for adequate metallization, it is necessary to make the vias in each layer significantly larger than what would ordinarily be necessary as seen in Fig. 1. In VLSI applications, this would consume a large amount of real estate on a semiconductor chip. IBM Technical Disclosure Bulletin, Vol. 27, No. 11, April 1985, pp. 6783-84, VIA HOLE ETCH PROCESS THROUGH A POLYIMIDE/NITRIDE COMPOSITE LAYER describes a self-aligned process that allows the etching of via holes in a composite layer of polyimide/nitride by RIE. As the polyimide thickness in the areas of the individual openings is not homogeneous the exposed polyimide is subjected to RIE in a CF₄ atmosphere which is stopped when a thin polyimide layer is left at the base of a first opening in the areas with the minimum polyimide layer thickness to be removed. The RIE process is continued in an oxygen atmosphere until all polyimide in the first opening and all other openings has been removed. Next, the Si₃N₄ layer is etched in a CF₄ atmosphere. In this process there is no etch stop for the first CF₄ RIE step, and the etch bias in oxygen atmosphere cannot be neglected so that an increase in via hole dimension is obtained. Microelectronic Engineering 4(1986) 207-219, M. Deschler et al, OPTIMIZATION OF VIA HOLE PLASMA ETCHING IN POLYIMIDE FOR OVERLAY INTERCONNECTIONS relates to the etching of polyimide layers in a CF₄/O₂ high-pressure plasma to generate V-shaped grooves. According to this prior art optimized via hole opening conditions are obtained by etching in a plasma with a CF₄ concentration around 5 % in oxygen. It is an object of this invention to provide an improved process for defining vias through a silicon nitride and polyimide layer such that the vias are self-aligned so that no unnecessary chip area is used. In this process only one masking and lithographic step is required, thereby reducing the numver of process steps necessary. In accordance with these objects, the following process can be used to simultaneously define vias through a polyimide and silicon nitride layer: 1. Provide a substrate having a first layer of silicon nitride and a second layer of polyimide; 2. Deposit a layer of photoresist capable of producing negatively sloped profiles; 3. Lithographically define a pattern of vias in the photoresist; 4. Develop the photoresist to produce a pattern of vias having negatively sloped profiles; 5. Etch the polyimide layer using the developed photoresist layer as an etch mask with a CF₄/O₂ gas mixture; and 6. Etch the silicon nitride layer using the etched polyimide layer as an etch mask with a CF₄/H₂ gas mixture. The foregoing and other advantages of the invention will be more fully understood with reference to the description of the best mode and the drawing wherein: Fig. 1 is a cross-sectional view of a semiconductor chip showing a prior art method of defining vias through two insulating layers. Figs. 2A-E are cross-sectional views of a semiconductor chip showing the process steps for defining vias through two insulating layers according to this invention. A cross-sectional view of a portion of a semiconductor chip is shown schematically in Fig. 2A. Substrate 20, represents that portion of the semiconductor chip that has been processed prior to the deposition of metal, or other types of device interconnect lines. Already deposited on substrate 20 are the first level metallurgy 22. The metal chosen for first level metallurgy 22 is not critical to this invention and can be any metal that is commonly used for this purpose. It is very common to design VLSI circuits in which several levels of interconnect metallurgy are required. To eliminate short circuits between metal levels, insulation or passivation layers are deposited between the metal levels. In this best mode, a thin conformal layer of silicon nitride 24 approximately 0.2-1.0 microns thick is first deposited over substrate 20 and metal lines 22. The method used to deposit the silicon nitride can include chemical vapor deposition, or any other technique commonly used for this purpose. On top of silicon nitride layer 24, a planarizing layer of polyimide 26 is deposited. The polyimide can be spun or sprayed on as is well-known in the art with a thickness in the range of 0.4-2.5 microns. In order to make electrical contact between first level metal 22 and a subsequent level of metal (not shown) it is necessary to define a via through silicon nitride layer 24 and polyimide layer 26 with a profile as shown by dotted lines. The gradual slope of the via is necessary in order to ensure continuous metal coverage. The first step in the via definition process is the deposition of a layer of photoresist 28 as seen in Fig. 2B. The thickness of the photoresist layer 28 is preferably in the range 1.0 to 2.5 microns. The choice of photoresist is likewise not critical but should be one that is capable of producing an image having a negative profile as shown in Fig. 2C, and is relatively etch resistant. A positive photoresist with an additive such as imidazole has been used with excellent results. Still looking at Fig. 2C, the photoresist layer 28 is exposed to radiation through a mask (not shown) as conventionally done, and then developed, leaving the structure as shown in Fig. 2C. Using the developed photoresist layer 28 as an etch mask, the polyimide layer 26 is reactive ion etched with the following process conditions: Power density - 0.10 watts per centimeter square CF₄ mass flow rate - 6-10 sccm O₂ mass flow rate - 85-100 sccm Pressure - 33.25-53.2 µbar, (3.325-5.32 Pa). The resulting structure is shown in Fig. 2D. The via through silicon nitride layer 24 is next defined with another reactive ion etch process with the following parameters: Power density - 0.25 watts per centimeter square CF₄ mass flow rate - 80-83 sccm H₂ mass flow rate - 15-19 sccm Pressure - 26.6-46.55 µbar, (2.66-4.655 Pa). The final structure after resist strip is shown in Fig. 2E. Due to the negligible organic etch rate with the CF₄/H₂ process conditions, the dimensions and the profile of the polyimide are maintained. The final structure is now ready for the deposition of a second level of metal.

Claims: Process for defining vias through a silicon nitride layer (24) and a polyimide layer (26) with said layers being arranged on an integrated circuit chip having a plurality of semiconductor devices and at least one level of metal conductors (22), said process comprising the following steps in the following order: depositing a layer of silicon nitride (24) over said semiconductor devices, said silicon nitride layer having a thickness in the range 0.2 to 1.0 micrometers; depositing a layer of polyimide (26) over said silicon nitride layer (24), said polyimide layer having a thickness in the range 0.4 to 2.5 micrometers; depositing a layer of photoresist (28) capable of forming sidewalls having a negative profile over said polyimide layer (26); patternwise exposing said photoresist layer (28) to radiation; developing the exposed photoresist layer (28) to form an image having negatively sloped sidewalls; etching the polyimide layer (26) with a mixture of CF₄ and O₂ using the patterned photoresist layer (28) as an etch mask; and etching the silicon nitride layer (24) with a mixture of CF₄ and H₂, using the etched polyimide layer (26) as an etch mask. Process as claimed in claim 1, wherein the photoresist is a positive photoresist with an imidazole additive.

Inventors: NANDA, MADAN MOHAN, PETERMAN, STEVEN LOUIS, STANASOLOVICH, DAVID
Applicants: INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1987
Main classification: H01L21
IPC codes: H01L21, H01L23