METHOD FOR MANUFACTURING A SEMICONDUCTOR DEVICE HAVING WIRING LAYERS

Abstract: A semiconductor device includes a first wiring layer (22) whose thickness within a contact hole (24) is great in a low­ er portion thereof and is small in an upper portion thereof. Since the first wiring layer (22) is sufficiently thick at the pe­ riphery of the bottom surface of the contact hole (24), reac­ tion between a second wiring layer (23) formed on the first wiring layer (22) and a substrate (20) is effectively prevent­ ed. The first wiring layer (22) is formed by bias sputtering in which a bias voltage is applied to the substrate (20).

Description: BACKGROUND OF THE INVENTION The present invention relates to a method for manufacturing a semiconductor device, which has a highly reliable wiring in a contact hole portion. With a trend of higher density of integration of a semiconductor integrated circuit, connecting portions of a wiring (such as a connecting portion between a wiring and a semiconductor substrate, a connecting portion between an upper layer wiring and a lower layer wiring, etc.) formed for connection between elements and the like are extremely reduced in dimension. The above-described connection is made through a contact hole formed in an insulating film, but the ratio of depth to diameter (or narrower width) of the contact hole (hereinafter referred to as "aspect ratio") also gradually increases. As a result, there occurs a tendency in that a defective connection is liable to occur to deteriorate the reliability of the wiring. A proposal has been made to provide a dual layer construction of a wiring in order to prevent the reliability from being deteriorated as described above. Fig. 1a shows one example of such proposal, in which a wiring of a dual layer construction comprising a first wiring layer 12 and a second wiring layer 13 is shown (C.Y. Tiry, J. Vac. Sci. Technol., 21 (1) 1982, P14 - P18). The first wiring layer 12 of transition metal or compounds thereof such as TiN, TiW, W, Mo is provided to prevent occurrence of reaction between the second wiring layer 13 formed of alloy containing Al as a main component and a substrate 10 within a contact hole 14. However, since the first and second wiring layers 12, 13 are formed by vacuum evaporation or sputtering, the wiring layer 12 is extremely thin in a bottom end portion A of the contact hole 14 formed in an insulating film 11. The degree of such thinness is affected by the aspect ratio of the contact hole 14, method of forming the first wiring layer 12 and film thickness thereof. For example, where the aspect ratio of the contact hole is 1, the step coverage factor (the ratio of film thickness between a thick portion of a wiring layer and a thin portion thereof in the contact hole) is 0.1 to 0.5. If in the end portion A, the thickness of the first wiring layer 12 is reduced as described above, the effect of preventing the reaction between the second wiring layer 13 and the substrate 10 is lowered and the reliability of wiring is lowered. Further, not only the thickness of the first wiring layer 12 but also the thickness of the second wiring layer 13 become small in the end portion A for the same reason as described above, and therefore, the current capacity of the wiring (maximum current amount that may be applied to the wiring) is also lowered. Therefore, the above method is only successful in cases where the contact is not too narrow. JP-A-58-108738 for example describes that defects occurring at the upper edge of the contact hole in an aluminum wiring layer formed by vapour deposition can be effectively covered by a second layer of an aluminum alloy formed by a sputtering method. To solve the problem noted above, a method has been proposed wherein a first wiring layer 12' is formed inside a contact hole 14 by selective chemical vapor deposition (hereinafter referred to as "selective CVD") , as shown in Fig. 1b, (C.E. Miller, Solid State Technol., Dec. 1982, 85). According to this method, the first wiring layer 12' of transition metal such as W, Mo, etc. is selectively formed within the contact hole, and the contact hole 14 is embedded, after which a second wiring layer 13' is formed by sputtering or the like. As clearly shown in Fig. 1b, the contact hole 14 is almost completely embedded or filled up by the first wiring layer 12', and therefore, the step coverage of the second wiring layer 13' and current capacity are improved over those shown in Fig. 1a. However, as shown in Fig. 1b, since the side of the first wiring layer 12' formed by the selective CVD and the side of the insulating film 11 are not in close contact with each other but leave a gap therebetween, the thickness of the first wiring layer 12' becomes thin in the end portion A' similar to the case show in Fig. 1a, and the effect of preventing the reaction between the second wiring layer 13' and the substrate 10 is lowered, thus making it difficult to obtain a good wiring. Furthermore, three of the present inventors have proposed a method wherein a contact hole is filled with a first metal and thereafter a second metal layer is formed (see Japanese Patent Laid-Open No. 152192/82 i.e. JP-A-59042642). However, it has been desired to develop a simpler and more practical method. SUMMARY OF THE INVENTION It is an object of the present invention to provide a method for manufacturing a semiconductor device in which even if the dimension of a contact hole is made extremely small, a wiring having a high reliability is obtained, in overcoming those problems noted above with respect to prior art. This object is solved with the invention characterised in claim 1. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1a and 1b are respectively views showing a sectional construction of a conventional wiring; Figs. 2a and 2b are respectively views showing a sectional construction of different embodiments of the present invention; and Fig. 3 is a schematic view for explaining bias sputtering used in the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS As described above, in the present invention, a first wiring layer is provided to prevent a reaction between a semiconductor substrate at a bottom of a contact hole and a second wiring layer, and in the first wiring layer, a portion formed at the lower portion of the side wall of the contact hole is made greater in thickness than a portion formed on the upper portion of the side wall of the contact hole. More specifically, as in one example shown in Fig. 2a, an insulating film 21 having a contact hole 24, a first wiring layer 22 and a second wiring layer 23 are formed on a semiconductor substrate 20, and the first wiring layer 22 within the contact hole 24 has a thickness thicker in a lower portion thereof than that in an upper portion thereof. Therefore, as is clear from Fig. 2a, the thickness of the first wiring layer 22 in the lower end portion B of the contact hole 24 is extremely great to effectively prevent a contact between the second wiring layer 23 and the semiconductor substrate 20 and reaction therebetween at the lower end portion B. The wiring layer in which, within the contact hole, the thickness of the lower portion is great whereas the thickness of the upper portion is small, can be formed by a method called "bias sputtering". As is known, normal sputtering comprises a method of bombarding high speed ions (such as helium ions, neon ions etc.) against a source target formed of material forming a wiring, knocking out a material constituting the source target, accumulating the material knocked out of the source target on the substrate arranged opposite of the source target, thus forming a film formed of said material on the substrate. The bias sputtering used in the present invention is characterized in that independent voltages are applied to both source target and substrate target on which a semiconductor substrate is placed, respectively, as shown in C.Y. Ting, et al., J. Vac. Sci. Technol. 15 (3), May/June 1978, P.1105 - P.1112. (This method is called "bias sputtering" because not only a voltage is applied to a source target but also a bias voltage is applied to a substrate target.) Fig. 3 is a schematic view showing one example of such method. Independent power supplies (R.F. generators) 34, 35 are connected to an upper target (source target) 31 and a lower target (substrate target) 32, respectively, and a film formed of a material constituting the source target 31 is formed on a semiconductor substrate 33 placed on the substrate target 32. The present inventors have found that when a wiring layer is formed by the bias sputtering, the material is thickly accumulated on the lower portion of the side wall of the contact hole as shown in Fig. 2a. They have further found that the wiring layer formed by the bias sputtering is far more dense than the wiring formed by other methods such as normal sputtering or vacuum evaporation, and in addition, the adhesiveness with respect to the semiconductor and insulating film is far superior. The present invention has been achieved on the basis of the novel concept described above. At least the first wiring layer is formed by the bias sputtering whereby a wiring of extremely high reliability can be formed. Needless to say, the second wiring layer formed on the first wiring layer can be formed by various known methods such as normal sputtering, vacuum evaporation, etc. However, if the bias sputtering is used, rugged portions on the upper surface are advantageously reduced. Embodiment 1 As shown in Fig. 2a, an insulating film 21 (SiO₂) having a thickness of 1 µm is formed on a silicon substrate 20 by a known heat oxidizing method, and a square contact hole 24 having a side length of 1 µm is formed by a known photoetching method. By the bias sputtering, tungsten is accumulated in the contact hole 24 and on the insulating film 21 to form a first wiring layer 22 made of tungsten. When the bias sputtering is carried out under the condition of a resputtering rate of 50% to form the tungsten film 22 whose thickness in a flat portion is 0.2 µm, the thickness in the lower end portion B of the contact hole 24 is more than 0.3 µm. This thickness of 0.3 µm is four to five times the thickness in the case a tungsten film is formed by normal sputtering. Next, Al-Cu alloy (Cu 3%) having a thickness of 1 µm is formed under the same condition as before by the bias sputtering to form a second wiring layer 23. According to the above-described embodiment, the thickness of the first wiring layer 22 in the lower end portion B of the contact hole 24 is extremely great, that is, four to five times that of prior art, and therefore, any reaction between the second wiring layer 23 and the silicon substrate 20 is completely prevented and the life time of the wiring is improved by more than twice over the case where formation of the first wiring layer having a thickness of 0.2 µm is performed by normal sputtering. The aforesaid resputtering rate is represented by where a is the accumulated amount on the substrate when sputtering is carried out by not applying a voltage to the substrate electrode 32 but applying a voltage only to the source electrode 31, and b is the accumulated amount on the substrate when sputtering is carried out by applying a voltage to both substrate electrode 32 and source electrode 31. The resputtering rate can be adjusted to the desired value by varying the voltage (electric power) applied to the substrate electrode 32 and/or source electrode 31. While Fig. 3 shows the case where R.F. power supplies 34, 35 are connected to substrate electrode 32 and source electrode 31, respectively, it should be noted that a DC power supply can be connected to the source electrode 31. Embodiment 2 An insulating film 21 having a contact hole 24 is formed in the same manner as in embodiment 1. A TiN film 22b is formed by the bias sputtering under the condition of a resputtering rate of 80%. However, where the resputtering rate is so great as such, the surfaces of the substrate 20 and insulating film 21 are etched and fine particles caused by etching enter into the TiN film 22b and possibly deteriorate the purity of the TiN film 22b. Therefore, a TiN film 22a having a thickness of 0.05 µm is formed by the normal sputtering in advance and then the TiN film 22b is formed thereon by the bias sputtering so as to form a first wiring layer having a thickness of 0.2 µm in total. Thereafter, a second wiring layer 23 of Al-Cu alloy is formed in the same manner as in embodiment 1. It has been confirmed that as shown in Fig. 2b, the thus obtained wiring is great in thickness in the lower portion in the contact hole 24 to effectively prevent reaction between the second wiring layer 23 and substrate 20, thus providing higher reliability and longer life time than in the prior art. It will be noted that the first wiring layers 22a, 22b can be formed of a different material, for example, Ti can be used in place of TiN. Further, the material used for the film 22b may also be different from the material used for the film 22a, although the case has been shown in which both films consist of identical material. In either case, it will suffice that the combined thickness of both layers is greater in the lower portion than in the upper portion within the contact hole. It will be further noted that a silicon substrate is used in the above-described embodiment, but other semiconductor substrate, insulated substrate or metal substrate can be used. Moreover, while the case has been shown in which a contact hole is used to connect a substrate with a wiring layer, it will be noted that the contact hole is not limited to this purpose but can also be used to interconnect upper and lower wiring layers. Furthermore, while in the above-described embodiment, the case has been shown in which a first wiring layer is formed on the entire surface under a second wiring layer, it is possible that the first wiring layer is left only within the contact hole and is removed from the insulating film. For the second wiring layer, various alloys principally containing Al or Cu such as Al-Si alloy, Al-Mn alloy and others can be used in addition to Al, Al-Cu alloy, etc. Also, laminated films such as Al·/X/Y/Al·Z can be used for the second wiring layer. Here, X, Y, and Z can be noble metals other than Al, transition metals or alloys thereof, or nitride or silicate. Also, X, Y and Z can be of the same kind of metal or different kinds of material. For the first wiring layer, there can be used at least one of various kinds of transition metals such as Ti, W, Mo, Pt, Pd, etc., alloys of those transition metals such as TiW, and various compounds of said transition metals such as titanium nitride, tungsten silicide, etc. According to the present invention, the second wiring layer within the contact hole is preferably formed by the bias sputtering as described above, and a favorable result may be obtained if the resputtering rate is set to about 50 to 90%. If the resputtering rate is less than 50%, a cavity is sometimes formed in the filling layer. If the resputtering rate is above 90%, the accumulating speed of the film becomes extremely low. In the first wiring layer, filling is not made only by the first wiring layer but is formed on the side surface and bottom surface, and therefore, a good result may be obtained by the bias sputtering at a resputtering above about 20%. If the resputtering rate is less than about 20%, the thickness of the first wiring layer at the lower portion of the contact hole is sometimes insufficient, and therefore, it is preferable to set the resputtering rate above about 20%. A resputtering rate above 90% should be avoided since the accumulating speed is extremely low and the etching of the substrate and insulating film becomes conspicuous. Therefore, it is favorable to set the resputtering rate to about 20% to 90%.

Claims: A method for manufacturing a semiconductor device comprising the steps of forming an insulating film (21) on a base layer (20); forming a contact hole (24) in said insulating film (21); forming a first wiring layer (22) continuously covering the bottom and the side surface of said contact hole (24); and forming a second wiring layer (23) made of an Al film or an alloy film containing Al as a main component and extending from the interior of said contact hole (24) across said insulating film (21); characterized in that the formation of said first wiring layer (22) includes bias sputtering, and that said first wiring layer (22) comprises a transition metal, an alloy or a compound of said transition metal. The method of claim 1, wherein said alloy film contains Al-Cu alloy, Al-Si alloy and/or Al-Mn alloy. The method of claim 1 or 2, wherein said transition metal is Ti, W, Mo, Pt and/or Pd. The method of anyone of claims 1 to 3, wherein said bias sputtering is carried out at a resputtering rate of about 20 to 90 %. The method of claims 1 to 4, wherein said second wiring layer (23) is formed by vacuum evaporation, sputtering or bias sputtering. The method of claim 5, wherein said bias sputtering applied to form said second wiring layer (23) is carried out at a resputtering rate of 50 to 90 %. The method of any of claims 1 to 6, wherein the formation of said first wiring layer includes forming a first film (22a) by normal sputtering and thereupon a second film (22b) by bias sputtering.

Inventors: HONMA, YOSHIO, MORISAKI, HIROSHI, TSUNEKAWA, SUKEYOSHI, YOKOYAMA, NATSUKI
Applicants: HITACHI, LTD.
Application year: 1985
Main classification: H01L23
IPC codes: H01L23, H01L21