Method of Manufacturing a Semiconductor Device

1. Field of the Invention

The present invention is directed to a method of manufacturing a semiconductor device, and more particularly to a novel method of forming minute and fine patterns by a novel sequence of etchings while avoiding the problems encountered in conventional etchings.

2. Description of the Prior Art

Methods for the manufacture of semiconductor devices are being continuously developed, and improved photolithograhic processes for patterning the substrate surface are fundamentally important. As is generally understood, conventional wet etching techniques using chemicals appear to have reached a limit for precision, and extremely minute patterns are now formed by means of dry etching instead. However, very sharp edges and steps are produced by the dry etching which is now practiced.

Technology for the manufacture of integrated circuits is reviewed these days for the purpose of increasing integration density, so that characteristics such as faster speed of operation, lower dissipation of power and so forth may be improved. While patterns formed on the substrate surface tend to become finely miniaturized, there is a new trend to fabricate integrated circuits in multi-layer structures. As the number of layers increase, from three to four, for example, the steps and differences in height between the levels of adjacent layers increase, resulting in differences in the thickness of layers grown on top of these others, such as in the wiring layers and insulation layers. If the wiring layer becomes very thin at a step, breaking of the wiring layer is likely to happen, and if the insulation layer becomes very thin at a step, wiring layers may easily be short-circuited.

Conventional wet etching which cannot produce fine patterns, has proved to be advantageous compared to dry etching when problems resulting from formation of sharp steps are taken into consideration.

In order to produce fine patterns in a wiring layer for example, a dry etching as is practiced recently becomes necessary. For avoiding steps or edges with sharp corners where the wiring or insulation layer is etched or cut through, a conventional wet etching has been found to be advantageous. But, to obtain a tapered or inclined side of the wiring or insulation layer contradicts the purpose of forming fine patterns. Yet, in the fabrication of densely packed integrated circuits, these two must be attained simultaneously.

The object of forming fine patterns has been partially accomplished by miniaturization of windows for electrodes and via holes formed in an insulation film on a semiconductor substrate. FIG. 1 schematically illustrates in cross-section a via hole formed by an isotropic etching in a conventional wet etching device or a barrel type dry etching device. A via hole is formed through an insulation layer 4, of phosphosilicate-glass (PSG) for example, reaching to a wiring layer 3, of say aluminum or polycrystalline silicon, formed on an insulation film 2 grown on a semiconductor substrate 1 such as of silicon. The via hole has an opening which is larger than the window in a photoresist film 5 used as a mask. This indicates that an accurate and fine via hole cannot be formed by the known isotropic etching technique.

A similar problem is also experienced in forming wiring patterns by a conventional isotropic etching. FIG. 2 shows schematically in cross-section wiring patterns 3 formed on an insulating film 2 grown on a semiconductor substrate 1 of say silicon by an isotropic etching according to known art using a photoresist film 5 as a mask. The width of the upper surface of the wiring layer 3 is smaller than the width of the photoresist film 5 used as a mask.

In order to overcome these problems, a planar type plasma etching device was selected to avoid the side etchings inevitable in isotropic etching. If a via hole is etched into the structure of FIG. 1 using 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 FIG. 3. The opening of the via hole thus formed is substantially equal to the opening of the photoresist film 5 used as a mask.

However, when the photoresist film 5 is removed and a wiring layer 6 of aluminum for example is patterned on the insulation layer 4, the wiring layer 6 covering the insulation layer 4 has thin portions 7 at the step, as shown in FIG. 4, because the edges of the via hole are sharp.

FIG. 5 illustrates schematically in cross-section two wiring layers fabricated in accordance with conventional techniques. The wiring layer 3 on the insulation film 2 was formed by a planar type plasma etching device and the wiring layer 6 was patterned on the insulation layer 4. It is seen that the wiring layer 6 may be broken at steps 7. This is due to the sharp or sudden steps formed by anisotropic dry etching carried out when the wiring layers 3 were patterned.

The present invention concerns a novel method of forming accurately a window for an electrode or a via hole of very small size, which comprises the following steps: a first insulation film of silicon dioxide (SiO.sub.2) for example is formed on a semiconductor substrate, a wiring layer of aluminum is formed on the insulation film, and a 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 film. A window is cut through the thin film by a known method to prepare for etching of the PSG insulation layer using the thin film of photoresist as a mask. The etching is first an isotropic etching until about one half the depth of the PSG layer is etched, then an anisotropic etching is carried out in the direction of the depth of the layer, that is in a direction perpendicular to the plane of the substrate, until the aluminum wiring layer is exposed. Both the isotropic etching and the anisotropic etching may be carried out for instance in a planar type plasma etching device.

Thus, in the present invention, a process is proposed for resolving the two conflicting objects of forming minute patterns while avoiding defects found in the prior art by producing tapering or inclined sides by a combination of isotropic and anisotropic etching. The method of the present invention may be applied to whatever materials may be etched by any known etching technique, and there may be as many layers as are required. If both the isotropic etching and the anisotropic etching are carried out by a dry etching process, the entire process may be carried out in the same apparatus. This is done by properly selecting the reactive gas and the conditions for each dry etching, such as the amount of reactive gas, the gas pressure, the applied radio frequency power, etc.

It is therefore a general object of the invention to offer a method in the patterning of a layer on a semiconductor substrate comprising the steps of forming a thin film having a predetermined pattern on the 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 another underlying layer is exposed, the sides of the etched layer being tapered or inclined, it being understood that an isotropic etching may be effected by a wet etching or a dry etching, and that an anisotropic etching can only be realized by a dry etching.

It is another object of the invention to present a method wherein said isotropic and anisotropic etchings are carried out in the same apparatus by properly selecting the reactive gas and conditions of dry etching such as the amount of gas, the gas pressure and the applied radio frequency power.

The above and further objects and novel features of the invention will appear more fully from the following detailed description when read in connection with the accompanying drawings. It is to be expressly understood, however, that the drawings are not intended as a definition of the invention but are for the purpose of illustration only.

FIG. 1 is a cross-sectional schematic view of a semiconductor device according to the prior art in which an insulation layer on a wiring layer formed on an insulation film grown on a semiconductor substrate is etched by an isotropic etching.

FIG. 2 is a cross-sectional schematic view of a wiring layer etched by a known isotropic etching.

FIG. 3 is a schematic view in cross-section of a structure similar to that shown in FIG. 1, but showing an insulation layer etched by an anisotropic etching.

FIG. 4 is a cross-sectional schematic view of a wiring layer formed on the insulation layer shown in FIG. 3.

FIG. 5 is a view similar to FIG. 4, but showing wiring layers formed by anisotropic etching of a wiring layer over which are formed an insulation layer and another wiring layer.

FIGS. 6 to 10 inclusive are cross-sectional schematic views illustrating the steps of the method of the present invention.

FIGS. 11 and 12 are cross-sectional schematic views of a planar type plasma etching device and a reactive sputter device respectively.

In patterning a film or layer formed on a semiconductor substrate according to the method of the present invention, etching of the film is partly done by an isotropic etching, then an anisotropic etching is carried out until the patterning is completed. Isotropic etching may be performed by a wet etching or a dry etching while anisotropic etching in the direction of the depth of the film, that is in the direction perpendicular to the surface of the semiconductor substrate, can only be realized by a dry etching. Thus, the method of the present invention may be carried out such as by the following three examples of combinations of etchings:

Other combinations are possible, as indicated in the following.

In FIGS. 11 and 12, there are shown schematically in cross-section a planar type plasma etching device E and a reactive sputter etching device or a reactive ion etching device F respectively, both of which can produce the anisotropic etching of the present invention. In either device, semiconductor substrates 10 are placed on an electrode A. Radio frequency power of for instance 13.56 MHz generated by a power source D is applied between the electrode A and an oppositely arranged electrode B. Reactive gas supplied from G (control tap and flow meter) is introduced through an inlet C, and turns into plasma by the applied r.f. power, and the substrates 10 are etched.

An embodiment of the invention will now be explained referring to FIGS. 6 to 10 inclusive which show schematically in cross-section the steps to carry out the method of the present invention. In FIG. 6 is shown a polycrystalline silicon film 13, with a thickness of say 5,000 A, which has been grown on an insulation film 12 of silicon dioxide, for example, that was previously grown on a semiconductor substrate 11. The film 13 is coated with a thin film 15 of a positive photographic emulsion or photoresist by a spinning and baking process, the film being shown in a condition after patterning. Using the film 15 as a mask, an isotropic etching was carried out in a planar type device E (FIG. 11) until a thickness of say 2,500 A is etched away from the film 13 of polycrystalline silicon (FIG. 7). In this embodiment, CF.sub.4 gas mixed with 5% O.sub.2 was selected as the reactive gas, and the etching conditions included an applied radio frequency power of 1 KW, a gas pressure of 1 Torr, and an etching time of 2.5 minutes. By virtue of isotropic etching, side etchings progressed sideways as shown in FIG. 7, and openings at the upper part of the polycrystalline silicon film 13 were made larger than the opening of the resist film 15. It was also found that by selecting CF.sub.4 gas mixed with C.sub.2 F.sub.5 Cl instead of 5% O.sub.2, and by maintaining the gas pressure at 2 Torr, the thickness of 2,500 A was etched away from the film 13 in 20 seconds using the same radio frequency power.

Next, in the same apparatus, an anisotropic etching was carried out to etch away the remaining thickness of the film 13 (FIG. 8). The selected reactive gas was CCl.sub.4, and the etching conditions were an r.f. power of 1 KW, a gas pressure of 0.2 Torr and an etching time of 30 seconds. Vertical sides separated by the same distance as the opening of the resist mask were produced by this etching as shown in FIG. 8.

This process just described corresponds to combination (c) mentioned above. The remaining thickness after the isotropic etching may also be etched away by anisotropic etching under other conditions as follows, using again for instance r.f. power of 1 KW:

Thus, an isotropic etching is carried out and in the same apparatus an anisotropic etching is subsequently done. A minutely fine pattern having tapered or inclined sides as shown in FIG. 8 is produced as a result. Thereafter, the resist film 15 used as a mask is washed or removed by an organic solvent, and an insulation film 14 of PSG for example is grown on the entire surface as shown in FIG. 9. The insulating film 14 and a wiring layer 16 thereon have uniform thickness devoid of thin parts.

The kind of gas selected plays an important role in etchings carried out in the device shown, with various types of etchants being most effective for different types of etching of different materials. The following tables indicate various types of isotropic and anisotropic etching processes for use in the present invention, with the materials to be etched on the left and the etchants or families of etchants along with appropriate pressure ranges on the right, and wherein the abbreviations employed in the tables include single crystal silicon S-Si, polycrystalline silicon Poly Si, chemical vapor deposition CVD, thermally grown TG, and phospho-silicate-glass PSG.

In the embodiments described above, polycrystalline silicon was etched according to the method of the present invention. As is seen from the tabulations above, films of thermally grown silicon dioxide or grown by chemical vapor deposition, phospho-silicate-glass (PSG) or single crystal silicon may similarly be etched by merely changing etching times, etc., and the present invention is not limited to the etching of polycrystalline silicon alone. Also with materials other than polycrystalline silicon, both isotropic and anisotropic etchings may be carried out in the same apparatus by selecting a suitable gas and etching condition.

For example, referring to FIG. 10 which shows another embodiment of the present invention, a film 21 of SiO.sub.2 is grown on a layer 20. Another film 22 of Si.sub.3 N.sub.4 is formed on the film 21, and the photoresist film 23 to be used as a mask is formed and patterned on the film 22. It is desired that the opening of this structure where the structure is in contact with the layer 20 be equal to the opening of the resist mask 23. At the same time, in order to avoid formation of steps as seen in FIG. 3, it is desired that the sides of the structure be tapered or inclined as seen in FIG. 9.

This object may be obtained to various degrees as follows. The Si.sub.3 N.sub.4 film 22 may be etched only partly as shown by the line 24. Or the Si.sub.3 N.sub.4 film 22 may be more completely etched as shown by the dotted line 25. Or the Si.sub.3 N.sub.4 film 22 may be further laterally etched along with part of the SiO.sub.2 film 21, as shown by the chain line 26. Thus the isotropic etching may remove any substantial amount of a layer being etched to be effective for providing the desired taper according to the present invention. For whichever alternative shown in FIG. 10 is desired, the isotropic etching may be carried out in a planar type plasma etching device using for example a mixed gas of CF.sub.4 +O.sub.2 as shown in Table I.3. This may then be followed by an anisotropic etching using CF.sub.4 gas under the pressure of 0.1-0.2 Torr or less, as per Table II.1. Care must be taken when two layers are isotropically etched that the etching rate of the top material must be larger than that of the lower material. Otherwise, the layer of the top material (Si.sub.3 N.sub.4 in this embodiment) will overhang the lower layer (SiO.sub.2 in this case).

Where the material to be etched is aluminum, isotropic and anisotropic etchings generally cannot be carried out in a single apparatus. An aluminum film or layer requires that the isotropic etching first be carried out by wet etching, because such etching cannot be realized by dry etching. This isotropic etching is followed by an anisotropic etching in a planar type plasma etching apparatus, for instance using a chloride compound gas such as CCl.sub.4, PCl.sub.3 or BCl.sub.3, or a mixture of these, or said mixture mixed with O.sub.2 or CF.sub.4, or said mixture mixed with an inactive gas such as argon, helium and the like, etc. This applies also to aluminum alloys such as Al-Cu, Al-Si or Al-Cu-Si. The etching of aluminum according to the present invention thus corresponds to the combination (a) mentioned above.

It will be understood that the method of dry etching suitable for formation of fine patterns according to the present invention enables making tapered or inclined patterns of wiring and insulation layers with more even thicknesses, thus, for instance, preventing breakdowns and short-circuiting, which contributes substantially to improving the reliability of semiconductor devices such as in integrated circuits.