CONTROL OF ETCH RATE RATIO OF SIO2/PHOTORESIST FOR QUARTZ PLANARIZATION ETCH BACK PROCESS

Abstract: A method of controlling the etch rate ratio of SiO₂ photoresist (PR) in a quartz planarization etch back process involves etching with a gaseous mixture containing CF₄ and either CHF₃ or C x F y with x > 1 or O₂. The preferred SiO₂ PR ratio of 1.2 = 0.1 is obtained by either adding CHF₃ to decrease the etch rate of the PR or by adding O₂ to increase the etch rate of the PR.

Description: The invention relates to a semiconductor etch back process and more particularly to a method of controlling the etch rate ratio of SiO₂/photoresist in a quartz planarization etch back process. In large scale integration (LSI) logic products metallurgy structures are positioned on several levels. In order to accurately interconnect these multilevel metallurgy structures, it is necessary to planarize each metallurgical structure level. As a result, obtaining a planar topography in multilevel metallization structures is a problem. One way to remove the topography in order to planarize a particular metallization level, is by the etch back process. The etch back process requires an etch rate ratio of SiO₂/photoresist to be 1.2 ± 0.1. The present etching techniques prior to this invention have not been able to achieve the desired etch back ratio at high etch rates and with tight tolerances. U.S. patent 4,324,611 describes an etching process for etching silicon dioxide and/or silicon nitride using a primary etching gas selected from the group consisting of C₂F₆, CF₄, C₃F₈ , C₄F₁₀, C₄F₈, and combinations thereof and a secondary gas selected from the group consisting of CHF₃, hydrogen, CF₄ plus hydrogen, and combinations thereof. The chamber pressure is on the order of 1.33 to 4 mbar when the primary gas is added. The addition of the secondary gas adds an additional 0.67 to 2 mbar of pressure to the system. A third gas, such as helium, is added at a pressure of the order of 6.65 mbar, when it is desired to prevent photoresist breakdown. This patent does not suggest a planarization process involving organic polymeric materials such as photoresist and silicon dioxide. Other U.S. patents 3,940,506 and 4,344,816, suggest the use of combinations of CF₄ and CHF₃ gases but again not for the same purpose and not at the same pressures and composition in the gaseous reactive plasma. UK patent application GB-A-2 119 166 relates to an integrated circuit planarizing process which comprises 1. depositing a first dielectric layer being comprised of silicon dioxide or nitride over first metal conductors; 2. depositing and polymerizing a layer of negative isoprene resist over the surface of the dielectric layer; 3. etching the surface of the isoprene and dielectric layers to a predetermined thickness over the metal conductors; 4. cleaning the etched surface and depositing a second dielectric layer being comprised of silicon dioxide or nitride over the surface. In this process, the negative isoprene resist layer is etched in a plasma of about 32 to 50 % oxygen in a gas of CF₄. This oxygen level provides uniform etching and similar etch rates between the dielectric layer and the negative isoprene resist layer. The primary object of this invention is to provide an improved quartz planarization etch back process and in particular to provide a method of controlling the etch rate ratio of SiO₂/photoresist (PR). These and other objects are accomplished by an etching method that involves contacting at a pressure of not less than 0.67 mbar the surface which includes both silicon dioxide and an organic polymer material with a reactive etching plasma which contains CF₄ and a gas taken from the group consisting of CHF₃, C x F y with x > 1 to cause the structure formed by said etching step to be substantially planar and provide an etch rate ratio of 1.2 ± 0.1. The preferred SiO₂/PR ratio of 1 is obtained by adding CHF₃ to decrease the etch rate of the PR or by adding O₂ to increase the etch rate of the PR. In one preferred embodiment using a single wafer reactor to etch SiO₂ covered with a commercial photoresist (AZ) a gaseous mixture containing 97.5 % CF₄, 2.5 % CHF₃ and 0 % oxygen provided an etch rate ratio of 1.096 whereas a gaseous mixture containing 97.0 % CF₄, 3 % CHF₃ and 0 % oxygen yields an etch rate ratio of 1.196. The advantages of the invention will be apparent from the following detailed description, reference being made to the following drawings in which a specific embodiment of the invention is shown. FIGURE 1 is a cross-sectional view of the SiO₂/photoresist system to be etched in accordance with this invention; FIGURE 2 is a cross-sectional view showing the planarized structure obtained in accordance with the etching process of this invention. The method of controlling the etch rate ratio of SiO₂/photoresist in a quartz planar etch back process includes the step of etching with a gaseous mixture containing CF₄ and either CHF₃ or C x F y with x > 1 or O₂. As shown in Fig. 1, topographical features 10, for example a metal such as copper, aluminum and the like, are covered with a rough substrate such as SiO₂ 12A. The purpose of this invention is to planarize the surface of the SiO₂ 12 that it is substantially planar or flat between the metal features 10 as shown in Fig. 2. The structure shown in Fig. 2 is obtained with a process in accordance with this invention that involves etching a layer 14 of a photoresist material, i.e. an organic polymer material, which is positioned over the SiO₂ layer 12 as shown in Fig. 1. It is desired to etch uniformly through the resist layer 14 and the SiO₂ layer 12A with an etch rate ratio of 1.2 ± 0.1 such that metal just sticks out of the SiO₂ surface. The etch rate ratio is defined as the etch rate of the oxide, SiO₂, over the etch rate of the photoresist. When a calculated etch rate ratio of 1.2 ± 0.1 is obtained, then a planar structure such as shown in Fig. 2 is produced. Since the etch rate of different photoresists varies significantly, it is difficult to find one set of conditions that is suitable for more than one photoresist. In addition, the etch rates of the SiO₂ and photoresist vary significantly depending upon the apparatus and equipment used. In accordance with this invention an etch rate ratio of 1.2 ± 0.1 is obtained by etching the structure shown in Fig. 1 with a gaseous mixture at a pressure of not less than 0.67 mbar containing CF₄ and either CHF₃ or C x F y with x > 1 or oxygen. The composition of the gaseous mixture depends upon the etching characteristics of the photoresist material used in layer 14. The composition of the gaseous mixture contains an etching gas, a polymerizing gas and, if necessary, an oxidizing gas to achieve a balance between surface etching, gas phase polymerization and surface oxidation reactions. The CF₄ gas is the primary gas for etching SiO₂. The CHF₃ or C x F y with x > 1 gas is primarily added to decrease the etch rate of the photoresist. The CHF₃ or C x F y with x > 1 gas is a polymerizing gas for the control of gas phase polymerization. The C x F y with x > 1 gas is unsaturated. Examples of such a gas are C₂F₄ and C₂F₆. The O₂ gas is used primarily to increase the etch rate of the resist. The oxygen gas is for controlling surface oxidation of the polymer that is deposited by the second gas (CHF₃). The combination of these gases provides a wide latitude in achieving etch rate ratios so that the planarization objectives can be easily met even with varying photoresist properties. The following examples illustrate the effect of CHF₃ on the etch rate ratio.

Claims: Method for uniformly etching a surface which includes both silicon dioxide and an organic polymer material comprising the steps of contacting said surface at a pressure of not less than 0.67 mbar with a reactive etching plasma which contains CF₄ and a gas taken from the group consisting of CHF₃, C x F y with x > 1, to cause the structure formed by said etching step to be substantially planar and provide an etch rate ratio of 1.2 ± 0.1. Method as described in claim 2 whereby said etching plasma contains CHF₃. Method as described in claim 2 whereby the gaseous mixture contains 0.5 to 5.0 % CHF₃.

Inventors: CHEN, LEE, MATHAD, GANGADHARA SWAMI
Applicants: INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1985
Main classification: H01L21
IPC codes: H01L21