Forming a Prescribed Pattern on a Semiconductor Device Layer

Abstract: This invention provides a method in which a photoresist layer having a prescribed pattern (33a, 33b) is formed on a substrate (30, 31) to be etched. The substrate is immersed into a predeter­ mined solution to form a layer (35) on the substrate except on those portions where the photoresist layer is formed. The photoresist layer is removed, and the substrate is etched using the remaining layer as a mask.

Description: This invention concerns a method for forming a prescribed pattern of, e.g., a wiring layer by etching, on a semiconductor device. Conventionally, for etching a semiconductor substrate, a Reactive Ion Etching ( RIE ) is utilized using a photoresist pattern formed on the semiconductor substrate as a mask. FIG.1 is a diagram for explaining a problem which is present in a conventional process for forming a pattern of submicrometer dimensions using a conventional exposure and development technique. Namely, in the case where the width between the adjacent pattern is relatively wide, e.g. 1.0 µm, a desired pattern may be obtained by the conventional technique. However, in the case where the width is 0.8 µm (e.g., between the photoresist pattern 12 and 13 ), some undeveloped photoresist remains on the substrate 10 , since the bottom portion of the photoresist is not sufficiently exposed even if the upper portion is clearly patterned. Thus, it is difficult to use the photoresist pattern 12 and 13 as a mask in a subsequent etching process. FIGS.2A and 2B are diagrams for explaining a problem in a conventional process for forming a prescribed pattern of a high reflective metal layer 2 1 in a recessed portion 24 of a semiconductor substrate 20 . In the conventional process, a material of high reflectivity, such as a tungsten silicide 21, is formed. A photoresist layer 23 is then formed thereon (Fig. 2A). Next, an exposure and a development are carried out to form a photoresist pattern 23a and 23b. By this process, a desired pattern 23a and 23b of photoresist may be formed on a flat surface of the substrate 20. However, at the level difference portion 24, the reflected light from the angled surface 22 of the metal layer 21 exposes the photoresist 23 in the recess. Thus, the desired pattern illustrated by the dotted line 23c cannot be obtained, and a photoresist pattern 23d may result instead. Therefore, it is also difficult to use the pattern 23d as a mask to etch the metal layer 21 accurately. It is known from U.S. Patent No. 4599137 to provide a method, as defined in the pre-characterizing portion of Claim 1, of forming a resist micropattern in a semiconductor device manufacturing process. Further, U. S. Patent No. 4624 749 discloses the use of a metal (gold) as an interconnect layer, formed by gold-electroplating selectively over a photoresist pattern which is complementary to the desired gold interconnect pattern. An object of the present invention is to provide an improved method which results in a desired accurate photoresist pattern, even if the width between the adjacent portions of the pattern is of the submicrometer order. To achieve the object, this invention provides a method as defined in Claim 1 for placing a pattern in a recessed portion of the substrate. The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention, and, together with the description, serve to explain the principles of the invention. Referring now to the drawings, like reference characters designate like or corresponding parts throughout the several views. Of the drawings: FIG. 1 is a cross sectional view for explaining a problem in a conventional process. FIGS. 2A and 2B are drawings for explaining another conventional process and the problem inherent therein. FIGS. 3A to 3D are cross sectional views for explaining an embodiment of the present invention. FIGS. 4A to 4D, FIGS. 5A to 5E, FIGS. 6A to 6D and FIGS 7A to 7D are cross sectional views of structures not made using the present invention but included for background information. Referring now to the drawings, the present invention will be explained in detail. FIGS. 3A to 3D are cross sectional views for explaining an embodiment of the present invention, which allows formation of a wiring pattern at a recessed portion of a semiconductor substrate. At first, a semiconductor substrate 30 having a recessed portion or a dent 34 is prepared. Then, a tungsten silicide layer 31 to be patterned is formed on the substrate 30 . Next, a positive photoresist layer, e.g., an OFPR (manufactured by Tokyo Oka Co.), is formed over the entire surface, and a photoresist pattern 33a and 33b covering a wall 32 at the dent 34 is formed using a conventional exposure and development technique. ( FIG.3A ) Next, the substrate 30 is immersed into a silicofluoride aqueous solution. In this step, a silicon oxide (SiO2) layer 35 of about 100 nm is selectively formed at the area where the tungsten silicide (WSi) layer 31 is exposed, since the H2SiF6 and H2O react to precipitate SiO2. To selectively form the SiO2 layer, the surface of the photoresist layer 33a and 33b are subjected to, e.g., an oxygen plasma, to change the surface thereof from a hydrophilic condition (having, e.g., -OH radical as an end radical) to a hydrophobic condition (having, e.g., -O radical as an end radical), preferably. ( FIG.3B ) This process is referred to as a hydrophobic treatment hereafter. Then, the photoresist pattern 33a and 33b are removed by, e.g., an ashing.( FIG.3C ) Next, an anisotropic etching, such as a RIE , is carried out to form a desired WSi wiring layer 31a , 31b and 31c using the SiO2 layer 35 as a mask. ( FIG.3D ) Then, an etching for removing the SiO2 layer 35 follows thereafter. (not shown) According to this process, the wall 32 at the dent 34 is not exposed. Thus, the problem due to the light reflection from the surface 32 can be prevented. Furthermore, the etching selectively between the WSi and the Si02 is relatively high. Therefore, it is possible to obtain a desired pattern of WSi wiring 31a on the substrate 30, even if the substrate 30 includes a recessed portion, since the SiO2 layer 35 having a relatively low etching rate is used as a mask to etch the WSi layer 31. It is, of course, possible to use other materials for the wiring layer, as one of ordinary skill in this art will realise. The following description is included as background, even though the examples do not embody the invention. FIGS. 4A to 4D are cross sectional views. On a semiconductor substrate 40, a thermal oxide layer 41 is formed. On the oxide layer 41, an aluminium layer (A1) 42 of about 800 nm to be etched is formed by, e.g., sputtering. Then, a patterned photoresist pattern 43 of about 1200 nm is formed on the A1 layer 42 using a conventional exposure and development technique. Next, a glass layer 44 is formed by a Spin on Glass (SOG) method following a baking treatment. (FIG. 4A) Then, a conventional etching process is carried out to remove the relatively thin glass layer formed on the photoresist 43, and to leave a portion of the relatively thick glass layer 44 formed where the photoresist layer 43 is not formed. Next, a hydrophobic treatment is carried out. Then, the substrate is immersed into a silicofluoride aqueous solution to form a SiO2 pattern 45 of about 200 nm on the glass layer 44, selectively. (FIG 4B) Next, the photoresist pattern 43 is removed by, e. g., a wet etching. (FIG. 4C) Then, an anisotropic etching, e.g., a RIE, is carried out to form a desired Al wiring layer 42a and 42b using the SiO2 pattern 45 as a mask. ( FIG.4D ) Then, an etching for removing the SiO2 layer 45 and the glass layer 44 is carried out (not shown). In this process, the etching selectivity between the Al layer and the SiO2 layer is about 10, and is relatively large compared with that of between the Al layer and the photoresist layer, namely about 2. Thus, the selectivity of the etching can be improved. Furthermore, the remaining Al pattern 42a and 42b is determined by the SiO2 layer 45 . The area where the SiO2 layer is formed is determined by the remaining photoresist layer 43 . Thus, the narrower the photoresist pattern 43 becomes, the wider the Al pattern 42a and 42b becomes. It is easy to make the remaining photoresist pattern narrower by controlling the exposing condition, e.g. by an over exposure, since the photoresist is a positive photoresist. This means that the width between the adjacent patterns can be made narrower, and the resolution can be improved. In this process, the glass layer 44 serves to protect the Al layer 42 from etching during the immersion of the substrate into the solution for precipitating the SiO2 layer 42 . FIGS.5A to 5E are drawings of a further example. On a semiconductor substrate 50 , a thermal oxide layer 51 is formed. On the oxide layer 51 , a aluminium layer (Al) 52 of about 800 nm to be etched is formed by, e.g., a sputtering. Then, a glass layer 53 is formed by a Spin on Glass method following a baking treatment. Then, a patterned photoresist layer 54a , 54b and 54c is formed using a conventional exposure and development technique. ( FIG.5A ) After a hydrophobic treatment of the photoresist pattern 54, the substrate is immersed into a silicofluoride aqueous solution to form a SiO2 layer 55 . ( FIG.5B ) Then, the resist layer 54 is removed. ( FIG.5C ) Next, an etching, e.g., a wet etching, is carried out to etch the glass layer 53 using the SiO2 layer 55 as a mask. ( FIG.5D ) Then, an anisotropic etching, e.g., a RIE etching, is carried out to form a desired Al wiring layer 52a and 52b using the SiO2 layer 55 as a mask. ( FIG.5E ) Then, the glass layer 53 and the SiO2 layer 55 are removed (not shown). Using this process, a desired wiring pattern can be achieved, as in the example illustrated in FIGS.4A to 4D . FIGS.6A to 6D are drawings for explaining a further example of forming a contact hole in a semiconductor device. On a semiconductor substrate 60 , an insulating layer 61 is formed. Then, a polysilicon layer 62 of about 30 nm is formed on the insulating layer by, e.g., a low pressure CVD method. Next, a patterned photoresist layer 63 is formed on the polysilicon layer 62 using a conventional exposure and development technique. ( FIG.6A ) Then, a hydrophobic treatment of the photoresist layer 63 is carried out. Next, the substrate is immersed into a solution of palladium chloride (PbCl2) to form a palladium layer 64 of less than 10 nm using a non-electric plating method. Next, the substrate is immersed into a mixed solution of nickel sulfate (NiSO4) and hypophosphite to form a nickel layer 65 of about 100 nm. ( FIG.6B ) Next, the photoresist layer is removed by, e.g., an ashing treatment using an oxygen plasma. FIG.6C ) Then, an anisotropic etching, e.g., a RIE, is carried out to form a contact hole 66 using the nickel layer 65 as a mask. ( FIG.6D ) Next, the nickel layer 65 and the palladium layer 64 are removed by a wet etching using a mixed etchant of a hydrochloric acid, a nitric acid and an acetic acid. The polysilicon layer 62 may either be removed by a wet etching using an organic alkali solution, e.g., a choline or a dry etching using a fluorine as an active radical, or it may be oxidized and left as it is. In this example, the size of the contact hole 66 is determined by the remaining photoresist pattern 63 . Thus, by narrowing the size of the photoresist layer 63 using, e.g., an over exposure, it is possible to form a resist pattern having a narrower width than the resolution limit. Thus, it is possible to form a narrower contact hole than when the conventional etching process is used. Furthermore, the etching rate of the nickel layer 65 is extremely smaller than that of the insulating layer 61 . Thus, a desired fine contact hole can be formed easily. The palladium layer 64 provides a good adhesion between the polysilicon layer 62 and the nickel layer 65 . FIGS.7A to 7D are drawings for explaining a further example of oxidizing a semiconductor substrate 70 . On a semiconductor substrate 70 , a thermal insulating layer 71 is formed. Then, a silicon nitride layer 72 is formed on the insulating layer 71 . Next, on the silicon nitride layer 72 , a patterned photoresist layer 73 is formed. ( FIG.7A ) After a hydrophobic treatment of the photoresist layer 73 , the substrate is immersed into the same solution, in the same way, as the embodiment of FIGS.3A to 3D , to form a SiO2 layer 74 . Next, the photoresist layer 73 is removed by, e.g., ashing using an oxygen plasma. ( FIG.7C ) Then, the silicon nitride layer 72 is selectively etched by using the SiO2 layer 74 as a mask. An oxidizing treatment is then carried out to oxidize the substrate 70 to form a device separating oxide layer 75 , using the silicon nitride layer 72 selectively remaining under the SiO2 layer 74 as a mask layer. ( FIG.7D ) Next, an etching treatment, e.g., a Chemical Dry Etching ( CDE ) is carried out to remove the SiO2 layer 74 and the silicon nitride layer 72 . In the examples of Figures 3, 4, 5 and 7, a SiO2 layer formed by a precipitation is used as a mask layer. However, it is possible to use a nickel layer formed by a non-electric plating process, as used in Fig. 6, as a mask layer, and vice versa. Thus, in each example, either a SiO2 layer or a nickel layer may be used as the mask layer. Furthermore, it is possible to etch the semiconductor substrate instead of the wiring layer and the insulating layer. Moreover, it is possible to utilize the present invention to form a pattern on an optical disk for storing information. In the aforementioned examples, a photoresist layer is used for the pattern formation. However, any film layer can be used as long as the layer permits little or no precipitation of substances thereon during immersion.

Claims: A method of forming a prescribed pattern on a layer of a semiconductor device, comprising the steps of: preparing a substrate (30) having a first main surface; forming a first layer (31) on the first main surface; forming a second layer (33) on the first layer; selectively removing the second layer to form a first patterned layer; immersing the substrate having the first patterned layer into a predetermined solution to form a third layer (35) selectively on the exposed portions of the first layer; removing the first patterned layer; and etching the first layer using the third layer (35) as a mask; characterized in that the substrate (30) has a recessed portion (34) in its first main surface; the first layer (31) is formed over the entire surface of the recessed portion (34) so as to follow its profile such that the surface of the first layer (31) has a corresponding recess with a base and sides (32); the first patterned layer (33a, 33b) covers the sides (32) but not the base of the said recess in the first layer (31); and so that the remainder of the first layer (31) after the said etching includes a pattern (31b) in the base of the recessed portion (34). A method according to Claim 1, in which the first layer (31) is a metal layer, and the second layer (33) is a photoresist layer, whereby the remainder of the first layer (31) constitutes a metal interconnect layer for the semiconductor device layer. A method according to Claim 1 or 2, wherein the third layer (35) is a glass layer. A method according to Claim 3, wherein the third layer (35) is formed by a Spin on Glass method. A method according to any preceding claim, further comprising the step of forming a fourth layer (53) as the upper layer of the first layer (52) before the formation of the second layer (54), so that the fourth layer is etched using the third layer (55) as a mask. A method according to any preceding claim, wherein the second layer (33) is formed of hydrophilic material, the method comprising the step of converting the second layer (33) to a hydrophobic material before the immersion of the substrate.

Inventors: OKUMURA KATSUYA INTELLECTUAL P, WATANABE TOHRU INTELLECTUAL PR, WATASE MASAMI INTELLECTUAL PRO, OKUMURA, KATSUYA INTELLECTUAL PROPERTY DIVISION, WATANABE, TOHRU INTELLECTUAL PROPERTY DIVISION, WATASE, MASAMI INTELLECTUAL PROPERTY DIVISION
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1989
Main classification: H01L21
IPC codes: H01L21