METHOD OF MANUFACTURING A SEMICONDUCTOR DEVICE HAVING A PATTERNED MULTI-LAYER STRUCTURE

Abstract: A layer 13, of poly crystal silicon for example, is etched using a thin photoresist film 15 as a mask. The layer 13 is first etched part way through by isotropic etching, then the remainder of the layer 13 is etched through by anisotropic etching in the direction of thickness of the layer 13. The side walls of a window formed in layer 13 in this way are tapered or inclined so that when an insulation layer 14 and a wiring layer 16 are subsequently formed, there is no danger of shorting of wiring layer 16 to poly crystal silicon layer 13 due to thinning of insulation layer 14 at the edges of the window. The isotropic etching and the anisotropic etching can in many cases be carried out in one and the same dry etching apparatus.

Description: The present invention relates to a method of manufacturing a semiconductor device. In the manufacture of semiconductor devices, which is being developed and improved continuously, photolithographic processes for patterning a substrate surface are fundamentally important. As is generally understood, conventional wet etching techniques using chemicals appear to have reached a limit of their precision, and extremely minute patterns are now formed by means of dry etching instead. However, very sharp edges or steps are produced by dry etching as presently practiced. These days, the technology for the manufacture of integrated circuits is under review for the purpose of increasing integrated circuit packing density so that the characteristics of integrated circuits may be improved, for example to provide faster speed of operation, lower dissipation of power and so forth. While patterns formed on a substrate surface have tended to become finely miniaturised, there is a new trend towards the fabrication of integrated circuit with multi-layer structures. As the number of layers increases from three to four, for example, steps or differences between levels of adjacent layers increase, and this results in differences of thickness of layers grown on top of other layers, for example wiring layers and insulation layers. If a wiring layer is formed in such a way that it is very thin at steps in an underlying layer, a break in the wiring layer is likely to appear, and if an insulation layer is formed in -such a way that it is very thin at steps in an underlying layer, a wiring layer over the insulation layer may easily be short-circuited. Compared to dry etching conventional wet etching, which could not produce such fine patterns, has proved to be advantageous from the point of view of problems resulting from formation of sharp steps. In order to produce a finely patterned wiring layer for example, dry etching as practiced recently becomes necessary. For avoiding steps or edges with sharp corners where a wiring or insulation layer is etched or cut through, conventional wet etching has been found to be advantageous. To obtain tapered or inclined sides for a wiring or insulation layer contradicts the purpose of forming fine patterns. Yet, in the fabrication of densely packed integrated circuits, these two qualities must be provided simultaneously. The formation of fine patterns has been partially accomplished by miniaturisation of windows for electrodes and via holes (through holes) formed through an insulation layer on a semiconductor substrate. Referring to Figure 1 of the accompanying drawings which schematically illustrates in cross-section a through or via hole formed by an isotropic etching process carried out by means of a conventional wet etching or a barrel type etching device, a via hole through an insulation layer 4, of phospho-silicate glass (PSG) for example, reaching through to a wiring layer 3 of aluminium or poly crystal silicon formed on an insulation layer 2 grown on a semiconductor (silicon) substrate 1 has an opening which is larger than that in a photoresist layer or film 5 used as a mask in formation of the hole. This indicates that an accurate and fine via hole cannot be formed by such a conventional isotropic etching technique. A similar problem is also experienced when forming wiring patterns by conventional isotropic etching. Figure 2 of the accompanying drawings illustrates schematically and in cross-section wiring patterns 3 formed on an insulating layer 2 grown on a semiconductor (silicon) substrate 1 by conventional isotropic etching using a photoresist layer or film 5 as a mask. The width of the upper surfaces of the wiring layer 3 parts is smaller than the width of the photoresist film 5 parts used as a mask, although it was expected that the former should be equal to the latter. In order to overcome these problems, a planar type plasma etching device has been selected for use to avoid side etchings inevitable in isotropic etching. If a via hole is etched into the structure of Figure 1 using such a planar type plasma etching device, the insulation layer 4 of PSG is etched in a direction vertical to the surface of the substrate 1 as shown in Figure 3 of the accompanying drawings, which is a schematic cross-sectional view. The opening of the via hole thus formed is substantially equal to the opening of the photoresist film 5 used as a mask. However, with a via hole formed by planar type plasma etching, when the photoresist film 5 is removed and wiring 6, of aluminium for example, patterned on the insulation layer 4, the wiring 6 covers the insulation layer 4 only thinly at those places indicated by reference numeral 7 in Figure 4 of the accompanying drawings, which is also a schematic cross-sectional view, because edges of the via hole are sharp. Figure 5 of the accompanying drawings illustrates schematically in cross-section two wiring layers fabricated in accordance with a conventional technique. In the illustrated arrangement of wiring layers, in which wiring layer 3 is formed on insulation layer 2, employing a planar type plasma etching device, and a wiring layer 6 is patterned on insulation layer 4, the wiring layer 6 may be broken at step 7. This is due to sharp or sudden steps formed by anisotropic dry etching carried out when wiring layers 3 were patterned. Accordingly to the present invention there is provided a method of manufacturing a semiconductor device in accordance with claim 1. The present invention provides a method of forming minute and fine patterns by a sequence of etchings, which avoids problems encountered in conventional etchings. An embodiment of the present invention provides a method of forming accurately a window for an electrode or a via hole of very small size, for example, which comprises the following steps: a first insulation layer of silicon dioxide (SiO₂) for example is formed on a semiconductor substrate, a wiring layer of aluminium is formed on the first insulation layer, and another (second) insulation layer, of phospho-silicate glass (PSG) for example, is formed on the wiring layer. A thin film of positive photoresist to be used as a mask is formed on the second insulation layer. A window is cut through the thin film by known means to prepare for etching of the PSG insulation layer using the thin film of photoresist as a mask. Firstly, an isotropic etching is effected to etch through a part of, e.g. about one half the depth of, the PSG insulation layer, then an anisotropic etching is carried out in the direction of the depth of the PSG layer, that is in a direction perpendicular to the plane of the substrate, until the aluminium wiring layer under the PSG layer is exposed. The isotropic etching and the anisotropic etching may both be carried out by means of a planar type plasma etching device. An embodiment of the present invention provides a process wherein two conflicting ends (the formation of minute patterns and the avoidance of defects found in the prior art) are attained by producing tapering or inclined sides in an etched layer by a combination of isotropic etching and anisotropic etching. The method of the present invention may be applied to any materials that may be etched away by any known etching techniques. There may be as many layers as are required, and if both isotropic etching and anisotropic etching are carried out by dry etching, both dry etchings may be carried out in the same apparatus. This can be provided by properly selecting reactive gases and conditions for dry etching such as the amount of reactive gas, the gas pressure and applied radio frequency power. The present invention provides a general method for the patterning of a layer on a semiconductor substrate comprising the steps of forming a thin film having a predetermined pattern on a layer to be etched, etching the layer partly by an isotropic etching using the thin film as a mask, then etching the layer in the direction of its depth by an anisotropic etching until an underlying layer appears. The sides of the layer etched have a tapered or inclined configuration. It will be understood that an isotropic etching may be effected by a wet etching or a dry etching, and an anisotropic etching can only be realised by a dry etching. The present invention also provides a method wherein the isotropic and anisotropic etchings are carried out in the same apparatus by properly selecting reactive gases and conditions for dry etching, such as the amount of gas, gas pressure and applied radio frequency power. Reference is made, by way of example, to the accompanying drawings, in which:- Figure 1, as mentioned above, is a cross-sectional schematic view of a semiconductor device structure according to the prior art in which an insulation layer on a wiring layer formed on an insulation film grown on a semiconductor substrate has been etched by an isotropic etching; Figure 2, as mentioned above, is a cross-sectional schematic view of wiring layers etched by an isotropic etching according to the prior art; Figure 3, as mentioned above, is a schematic view in cross-section of a structure similar to that shown in Figure 1, but showing an insulation layer etched by an anisotropic etching; Figure 4, as mentioned above, is a cross-sectional schematic view of a wiring layer formed on the insulation layer shown in Figure 3; Figure 5, as mentioned above, is a view similar to Figure 4, but showing wiring layers formed after an anisotropic etching and on which are formed an insulation layer and another wiring layer, Figures 6 to 8 inclusive are respective cross-sectional schematic views illustrating steps in a method embodying the present invention; Figure 9 is a cross-sectional schematic view illustrating manufacturing steps following the steps of Figures 6 to 8; Figure 10 is a cross-sectional schematic view illustrating further embodiments of the present invention; and Figures 11 and 12 are cross-sectional schematic views of a planar type plasma etching device and a reactive sputter etching device respectively. When patterning a film or layer formed (directly or indirectly) on a semiconductor substrate according to a method embodying the present invention, etching of the layer is partly effected by an isotropic etchingp then an anisotropic etching is carried out until the patterning is complete. Isotropic etching may be effected by means of a wet etching process or a dry etching process, whilst anisotropic etching in the direction of the depth of the film or layer, or in a direction perpendicular to the surface of the semiconductor substrate, can only be realzed by a dry etching process. Thus, in order to carry out embodiments of the present invention, possible combinations of different etching processes are as follows:- Any of these combinations may be selected to meet various requirements as will be described hereinafter. In Figure 11 and 12, there are shown schematically and in cross-section a planar type plasma etching device E and a reactive sputter etching device or a reactive ion etching device E respectively. In each of these devices, semiconductor substrates 10 are placed on an electrode A. Radio frequency power, at a frequency of 13.56 MHz for example, generated by a power source D is applied between the electrode A and an oppositely arranged electrode B. Reactive gas supplied from F (control tap and flow meter) is introduced into device E through an inlet C, and is turned into plasma by the applied r.f. power, and substrates 10 are etched. A method embodying the present invention will now be explained with reference to Figures 6 to 8 inclusive which show schematically in cross-section structures produced in the steps involved in the embodiment. Figure 6 shows a poly crystal silicon layer 13, thickness 500 nm (5,000A °), which has been grown on an insulation layer 12 of silicon dioxide, for example, that has already been grown on a semiconductor substrate 11. The layer 13 is coated with a thin film 15 of positive photographic emulsion, or photoresist, by a spinning and baking process, the film 15 being shown in a condition after patterning thereof. Using the film 15 as a mask, an isotropic etching is carried out until a thickness of 250 nm (2,500 A °) has been etched away from the layer 13 of poly crystal silicon (Figure 7). In this embodiment, CF₄ gas mixed with 5% O₂ is selected as reactive gas, and etching conditions were as follows:- radio frequency power 1 KW, gas pressure 133.3 Pa (1 Torr), and etching time 2.5 minutes. In this isotropic etching, side etchings progress sideways as shown in Figure 7, and an opening formed in the upper part of poly crystal silicon layer 13 is larger than the corresponding opening in the resist film 15. It was also found that one alternative to the reactive gas and etching conditions mentioned above is the use of CF₄ gas mixed with C₂F₅Cl, instead of 5% O₂, and by maintaining the gas pressure at 266.6 Pa (2 Torr), the thickness of 250 nm (2,500 A °) was etched away from the layer 13 in 20 seconds using the same radio frequency power. Next, in the same apparatus, an anisotropic etching is carried out to etch away the remaining thickness of 250 nm (2,500 A °) of the layer 13 (Figure 8). In this embodiment the selected reactive gas is CCl₄, and the etching conditions are as follows:- r.f. power 1 KW, gas pressure 26.7 Pa (0.2 Torr) and etching time 30 seconds. Vertically cut down sides separated by the same distance as the opening in the resist mask (film 15) are produced by this etching. (Figure 8, and combination (c) mentioned above). The remaining thickness of 250 nm (2,500 A °) of layer 13 may alternatively be etched away by anisotropic etching under any of the conditions as follows: (r.f. power applied, 1 KW in each case). Thus, in the described embodiment, an isotropic etching is carried out in one apparatus, and in this same apparatus, an anisotropic etching is subsequently effected. A minutely fine pattern wherein side walls are tapered or inclined as shown in Figure 8 is produced as a result. Thereafter, the resist film 15 used as a mask is washed away or removed by an organic solvent, and an insulation layer 14 of PSG for example is grown over the entire surface thus exposed, as shown in Figure 9. The layer 14 and a wiring layer 16 formed thereon have substantially uniform thicknesses respectively devoid of parts which cover only thinly the underlying layer or film. The kind of gases selected for use play an important role in etchings, for example as carried out in apparatus as shown. In the tables below various possibilities which can be employed for etching various materials in embodiments of the present invention are given. The tables give information relating to isotropic etching processes and anisotropic etching processes which can be employed in embodiments of the invention. In the embodiments described with reference to Figures 6 to 9, poly crystal silicon was etched. However, as will be understood from the tables above, layers or films of thermally grown silicon dioxide, silicon dioxide formed by chemical vapour deposition, phospho-silicate glass (PSG) or single crystal silicon may similarly be etched merely by changing etching times, and therefore the invention is not limited to the use of poly crystal silicon alone. Also with materials other than poly crystal silicon, both isotropic and anisotropic etchings may be carried out in the same apparatus by selecting suitable gas and etching conditions as will be understood from the above Tables 1-3 and 2-1 for example. For example, referring to Figure 10 which illustrates another embodiment of the invention, a layer 21 of SiO₂ is grown on a layer 20 already formed. Another layer 22 of Si₃N₄ is formed on the layer 21, and a photoresist film 23 to be used as a mask is formed and patterned on the layer 22. It is desired that the opening of a hole through the composite layer comprising layers 21 and 22 where that hole is in contact with layer 20 be equal to the corresponding opening formed in the resist mask 23. At the same time, in order to avoid formation of steps as seen in Figure 3, it is desired that the sides of the hole in the structures be tapered or inclined as seen in Figure 9. There are three modes of attaining the desired object. The first is to etch Si₃N₄ layer 22 partly as shown by continuous line 24 in Figure 10. The second mode is to etch away Si₃N₄ layer 22 completely as shown by the broken line 25. The third is not only to etch away Si₃N₄ layer 22 completely, but also to etch SiO₂ layer 21 partly as shown by chain line 26. In each mode, an isotropic etching is carried out in a planar type plasma etching device using a mixed gas of CF₄+O₂ for example (see Table 1-3). This is followed by an anisotropic etching using CF₄ gas under a pressure of 13.3 Pa (0.1 Torr) or less (see Table 2-1). Care must be taken in the third mode mentioned above in that the etching rate of the material that is etched first must be larger than that of the material that will be etched later. Otherwise, the first material (Si₃N₄ in Figure 10) will hang over the second material (SiO₂). Where the material to be etched is aluminium, isotropic and anisotropic etchings cannot at present be carried out in one and the same apparatus (see Tables 1-1 and 2-1 ). With an aluminium film or layer, an isotropic etching is first carried out by a wet etching process because a dry etching process cannot be used at present. This isotropic etching is followed by an anisotropic etching in a planar type plasma etching apparatus, using a chloride compound gas such as CCl₄, PCl₃ or BCl₃, or a mixture of these, or such a mixture mixed with O₂ or CF₄, or such a mixture mixed with inactive gas such as argon, helium and the like. This applies also to the etching of aluminium alloys such as Al-Cu, Al-Si, or Al-Cu-Si. (This corresponds to combination (a) mentioned above). It will be understood that the process of dry etching suitable for formation of fine patterns employed in a method according to the invention which also enables tapered or inclined patterns to be formed such as to prevent breaking down and short-circuiting of wiring layers, to offer a substantial contribution towards improving reliability of semiconductor devices such as integrated circuits. Thus, an embodiment of this invention provides a method of manufacturing a semiconductor device having a multi-layer structure, which method comprises the steps of patterning, in accordance with a predetermined pattern, a thin film of photoresist formed on a film to be etched which has been formed (directly or indirectly i.e. with intervening layers) on a semiconductor substrate, etching the film to be etched partly through by isotropic etching using the patterned film as a mask, then etching the film to be etched completely through by an anisotropic etching in the direction of its depth, and forming in the etched film a pattern having tapered or inclined sides. The isotropic and anisotropic etchings may be carried out in the same apparatus by changing reactive gases employed for etching and changing etching conditions such as the amount of gas, gas pressure and radio frequency power to be applied.

Claims: 1. A method of manufacturing a semiconductor device having a multi-layer structure, in which a first layer is formed to a predetermined pattern over a second layer, which second layer is to be etched and is formed above a semiconductor substrate of the device, and the second layer is etched through using the first layer as a mask, characterised in that the second layer is etched part way through by isotropic etching, and then the remainder of the second layer is etched through by anisotropic etching in the direction of thickness of the second layer, using the first layer as a mask, to form an overning therethrough, the first layer is then removed and a third layer is formed on the second layer. 2. A method according to claim 1 wherein said third layer is an insulating layer. 3. A method according to claim 2 wherein a conductive layer is formed a said third layer. 4. A method as claimed in claim 1, wherein the second layer is of a material listed in Tables 1-1 to 2-2 hereunder, the isotropic etching employs an etchant (and etching conditions) indicated as appropriate to that material in any one of Tables 1-1 to 1-4, and the anisotropic etching employs an etchant (and etching conditions) indicated as appropriate to that material in any one of Tables 2-1 and 2-2. 5. A method as claimed in claim 1, wherein the anisotropic etching is effected by a dry etching being either planar type plasma etching, reactive sputter etching or reactive ion etching. 6. A method as claimed in claim 5, wherein the isotropic and anisotropic etchings are carried out in one and the same dry etching apparatus, reactive gases employed and/or etching conditions such as amount of gas and gas pressure employed in the apparatus being changed after the isotropic etching to provide for the anisotropic etching. 7. A method as claimed in claim 6, wherein the second layer is of single crystal silicon or of poly crystal silicon or of Si₃N₄, and is first etched part way through by isotropic etching using as reactive gas either CF₄, or CF₄+O₂, or C₂F₅Cl+CF₄, and is then etched completely through by anisotropic etching using as reactive gas any of CCl₄, PCl₃, BCl₃, SiCl₄, CF₄, CF₄+O₂, C₂F₅Cl, C₂F₅Cl +CF₄, or such a chloride compound gas with O₂ or CF₄, or a mixture of such chloride compound gases, or such a mixture with O₂ or CF₄, or SF₆, or CF₆, in one and the same planar type plasma etching apparatus. 8. A method as claimed in claim 6, wherein the second layer is of phospho-silicate glass, or Si₃N₄ or silicon dioxide and is first etched part way through by isotropic etching using as reactive gas C₂F₅Cl+CF₄, or CF₄ or CF₄+O₂, and is then etched completely by anisotropic etching using as reactive gas CF₄ or CF₄+H₂, or CHF₃, or C₂F₆, or C₃F₈, or C₄F₈, in one and the same planar type plasma etching apparatus. 9. A method as claimed in claim 5, wherein the second layer is of aluminium or aluminium alloy and is first etched part way through by a wet etching process using a solution of H₃PO₄ as etchant, and then etched completely through by anisotropic etching using as reactive gas either CCl₄, or PCl₃, or BCl₃, or a mixture of these, or such a mixture mixed with O₂ or CF₄, or such a mixture mixed with an inactive gas such as Ar, He in a planar type plasma etching apparatus. 10. A method as claimed in claim 1, in which the second layer is a composite layer comprising two component layers one over the other, the said one component layer having a higher etching rate than the said other component layer. 11. A method as claimed in claim 10 wherein the said one component layer is of Si₃N₄ and the said other component layer is of SiO₂. 12. A method as claimed in any preceding claim, wherein the first layer is a thin film, for example of photoresist, and the second layer is etched half way through by the isotropic etching.

Inventors: SUGISHIMA, KENJI, TAKADA, TADAKAZU
Applicants: FUJITSU LIMITED
Application year: 1980
Main classification: H01L21
IPC codes: H01L21