WIRING FORMING METHOD FOR SEMICONDUCTOR DEVICE

Abstract: Grooves and holes of high aspect ratio are filled completely and uniformly. After forming connection holes (3) and wiring grooves (4) in a silicon oxide film (2) which is formed on a silicon substrate (1), a TiN film (5) is formed over the entire surface of the semiconductor substrate and a Ti film (6) is formed on the region except for the connection holes (3) and the wiring grooves (4). Then, in a state where the connection holes (3) and the wiring groove (4) are dipped in a plating solution, a plating treatment is carried out under a deposition overvoltage higher than the deposition overvoltage of TiN to copper and lower than the deposition overvoltage of Ti to copper. Since plating is thus applied only to the portion where the TiN film (5) is exposed, namely, only to the portion of the connection holes (3) and the wiring grooves (4), the connection holes (3) and the wiring grooves (4) are filled with copper and when they are polished by a chemical and mechanical polishing method to form wirings, satisfactory copper wiring which are uniform and well filled can be obtained.

Description: TECHINICAL FIELD The present invention concerns a method for forming wirings made of a metal material in connection holes or wiring grooves formed on a semiconductor substrate for conducting connections between wirings and semiconductor substrates and, more in particular, it enables complete and uniform filling of the metal material into connection holes or wiring grooves without complicating a production process. BACKGROUND ART Wirings in integrated circuits such as LSIs have been generally formed by fabricating films of aluminum (Al) series alloys deposited by a sputtering method by using, for example, photolithography or dry etching. However, along with increasing degree of integration in semiconductor integrated circuits, diameters for connection holes or diameters for wiring grooves apertured for conducting connection between wirings and semiconductor substrates have become finer and an aspect ratio of them has been increased more and more. Therefore, it is difficult to form wirings at a predetermined thickness as far as the inside of the connection holes by the sputtering method to result in a problem of lowering the step coverage, increasing the resistance of contact hole wirings and deteriorating electro-migration durability. In order to avoid them, a method of forming wirings to a vertical wiring portion in connection holes by a CVD method (chemical vapor deposition method) using tungsten (W) is adopted in LSIs under a design rule below half-micrometre. However, in the existent method of forming wirings by the CVD method using tungsten, since the resistance of tungsten is high, it is necessary to leave a tungsten film formed on the entire surface of a substrate only in the connection holes while removing other portions by a dry etching method or chemical and mechanical polishing method (CMP) and form wirings of an aluminum series alloy again. Therefore, this gives rise to a problem of making the wiring forming step lengthy to increase the cost. On the other hand, as the size for the wirings has become finer, it has been desired for wiring materials of high electro-migration durability and low resistance, and a film deposition method and a wiring fabrication method have now been under study for using copper capable of satisfying such a demand for the wiring material. However, since a film of copper can not be deposited uniformly in a connection hole by a usual sputtering method, a method of depositing the copper film by a CVD method has been studied, but it still leaves a subject such as for the development of organic metal compounds of copper as a starting material and the development for CVD apparatus. Further, while an electrolytic plating method for copper has also been studied, electrodeposition proceeds at the periphery outside of the fine connection holes or wiring grooves to at first close inlets for the connection holes or wiring grooves, which causes a problem of difficulty for filing copper in the fine connection holes without forming voids. Further, it is difficult to fabricate copper by dry etching and it has been proposed, for example, a DUAL DAMASCENE method of forming contact holes and wiring grooves to an insulation film, depositing copper for the entire surface and then removing excess copper by a chemical and mechanical polishing method thereby properly forming wirings having vertical connection portions. Development for the technique of properly filling and depositing a copper film into hole or groove portions of high aspect ratio such as connection hole or wiring grooves has also been desired with an aim of applying to this method. In view of the above, the present invention has been accomplished taking notice of the foregoing not yet solved problem. It is an object of the invention to provide a selective plating method capable of completely, and uniformly filling groove or hole portions of high aspect ratio and a wiring forming method for a semiconductor device using the same. DISCLOSURE OF THE INVENTION In order to attain the foregoing object, the present invention provides a method of forming wirings for a semiconductor device, which comprises the steps of providing a semiconductor substrate having an insulation film formed thereon; forming channels having an aspect ratio from 1 to 5 and a width of 1.0 µm or less in the insulation film, forming a first conductor layer by a CVD process on the surface of the insulation film including the inner surface of the channels, then forming a second conductor layer by a sputtering process on the surface of the first conductor layer except for the inner surface the channels, the first conductor layer and the second conductor layer having respective surface potentials such that the voltage for depositing a copper series metal from a plating solution on to the second conductor layer - the second overvoltage - is higher than the voltage for depositing a copper series metal from a plating solution on to the first conductor layer - the first overvoltage); subsequently, dipping at least the channels into the plating solution and applying a plating treatment at a voltage higher than the first overvoltage and lower than the second overvoltage to deposit the copper series metal material on the inner surface of the channels , and polishing the deposited copper series metal material to remove unnecessary portions whereby the copper series deposited metal forms wirings located in the channels. That is, the first conductor layer having a first deposition voltage is formed on the inner surface of the wiring channels such as connection holes or wiring grooves formed in the insulation film deposited on the semiconductor substrate by a CVD method, and then the second conductor layer having a second deposition voltage is formed thereon so as to cover the surface of the first conductor layer except for the inner surface of the channels by a sputtering method. Then, they are dipped in the plating solution such that at least the channels are immersed in the plating solution, and a plating treatment is applied at a deposition voltage higher than the first voltage and lower than the second voltage. Since the deposition voltage is a voltage required for depositing the copper series metal material in the plating solution at a predetermined rate, the metal material is not deposited on a portion in which the second conductor layer is exposed, whereas chemical reactions are taken place between the metal material in the plating solution and the first conductor layer to deposit the metal material and apply copper plating to the portion in which the first conductor layer is exposed. That is, since copper plating is applied only to the portion in which the first conductor layer is exposed, namely, only to the inner surface of the channels intended to be applied with plating, to result in a copper-filled state, a wiring consisting of copper filled in the channels can be obtained by polishing the copper. In this case, the second conductor layer is formed by a highly anisotropic deposition method. This is done for forming the second conductor layer to a region except for a region to be plated, namely, only to a region except for the inner surface of the channels and the formation of the second conductor layer in the wiring channels is avoided. In the invention, each wiring channel has an aspect ratio of 1 or greater and 5 or less and a width of 1.0 µm or less. This is so defined for avoiding the formation of the second conductor layer on the inner surface of the channels upon forming the second conductor layer by a sputtering method. Further, in a case of applying to a semiconductor device of a multi-layered wiring structure, plating is applied to form wirings and then wirings are conducted by forming a metal film further thereon. When wirings are formed on an insulator, a thin electrically conductive film is generally put between them with an aim, for example, of improving adhesion and electro-migration durability. In this case, since the first conductor layer is formed over the surface of the semiconductor substrate, the step of forming wirings can be shortened by utilizing the layer as the thin electrically conductive film. Further, the deposited metal material is preferably polished by a chemical and mechanical polishing method by which the wirings can be formed easily and accurately. Further, when making the first conductor layer with titanium nitride, the second conductor layer with titanium and the final plating with the copper series metal material, copper wirings uniformly filled in the channels can be formed by forming a titanium nitride film to the inner surface of the channels such as connection holes or wiring grooves, forming a titanium film so as to cover the surface of the titanium nitride film except for the wiring channels and then dipping them into a plating solution. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a fragmentary cross sectional view showing a portion of a wiring forming step in an embodiment of the present invention. Fig. 2 is a fragmentary cross sectional view showing a portion of a wiring forming step in an embodiment of the present invention. Fig. 3 shows an example of another embodiment of the present invention. Fig. 4 shows an example of another example of the present invention. BEST MODE FOR PRACTISING THE INVENTION The mode of practising the present invention is to be explained. Generally, in electrolytic plating of a metal, an electrode potential is set to a potential which is negative by an overvoltage than a thermodynamic equilibrium potential required for reducing to deposit metal ions in the plating solution. In electroless plating, a reducing agent having a reducing power (oxidation/reduction potential) higher than an equilibrium potential required for depositing metal ions is used. Deposition by reduction of metal ions does not proceed at a substantial rate even when the thermodynamic equilibrium electrode potential or an oxidant providing the oxidation/reduction potential is used; an overvoltage is required for a substantial reaction to proceed, which is different depending on the kind of the underlying metal. That is, there is a phenomenon that an electrode potential or oxidation/reduction potential of the reducing agent (hereinafter referred to as a deposition overvoltage) in excess of the thermodynamical equilibrium potential is required and is different depending on the kind of the underlying metal. When copper film is deposited by using substrates formed with underlying metals of various deposition overvoltages by utilizing the above described phenomenon, it has been found a phenomenon that copper deposits only to the portion in which metal of low deposition overvoltage is exposed and, thus, a selective copper plating method has been found. That is, for depositing copper only in the wiring grooves or the connection holes formed in the insulation film, a first underlying metal having a lower deposition overvoltage as the first deposition overvoltage is selected and deposited as a film on a portion to be deposited with copper, a second underlying metal having a deposition overvoltage higher than the first deposition overvoltage as a second deposition overvoltage is selected and deposited as a film to a portion not intended to be deposited with copper, and a voltage higher than the first deposition overvoltage and lower than the second deposition overvoltage is set as an electrode potential relative to a counter-electrode in a case of electrolytic plating, whereas a oxidation/reduction potential of a reducing agent is set higher than the first deposition overvoltage and lower than the second deposition overvoltage in a case of electroless plating. Therefore, copper is deposited to form a metal film only to a portion in which the first underlying metal is exposed. Accordingly, plating is conducted till copper is deposited as far as the upper surface of the wiring grooves and the connection holes, to provide a state in which a metal film of the deposited copper is filled in the wiring grooves and the contact holes. Accordingly, wirings of copper filled in the wiring grooves and the connection holes can be obtained by polishing the deposited copper by a chemical and mechanical polishing method to remove unnecessary portion. When wirings of aluminum or copper are formed on the insulation film, dense thin electrically conductive films such as of titanium nitride (TiN) are formed generally at the boundary between the insulation film and the wirings with various purposes such as prevention of diffusion, improvement of adhesion and improvement of electro-migration durability. Since such films have higher resistance compared with aluminum or copper, it is not preferred to make them unnecessarily thick. In the electrolytic plating, electric current can be supplied through the underlying metal such as titanium nitride formed as a film on the entire surface of the insulation film. Accordingly, when titanium nitride is used as the first underlying metal having the first deposition overvoltage as the deposition overvoltage and deposited as a film on the entire surface of the insulation film, and then a film of metal such as titanium having a high deposition overvoltage is formed thinly by a highly anisotropic deposition method being sputtering to a portion not intended to apply plating, namely, a region except for the side wall and the bottom of the connection holes and the wiring grooves, a copper film is not formed to the titanium deposited portion and copper can be deposited substantially only to the portion not deposited with titanium, namely, only to the side wall and the bottom of the connection holes and the wiring grooves. Accordingly, in a state of forming the wirings in the connection holes or the wiring grooves formed in the insulation film, since the film of titanium nitride is deposited on the surface of the semiconductor substrate, the titanium nitride film can be utilized as the thin electrically conductive film for the improvement of the adhesion and improvement of the electro-migration durability, so that it is not necessary to form an additional thin electrically conductive film, for example, in a case of applying wirings in the multi-layered wiring structure, and the wiring forming step can be saved. The second underlying metal having the second deposited overvoltage as the deposition overvoltage is deposited as a film by the sputtering method, but the metal of high deposition overvoltage adheres to the bottom of the wiring grooves and the connection holes if the width of the wiring grooves and the diameter of the connection holes are large. Therefore, the width of the grooves and the diameter of the connection holes are 1.0 µm or less, and the aspect ratio as the width to depth ratio in the wiring grooves and the connection holes is from 1 to 5. If necessary, preferred deposition can be applied by providing a cleaning step before electrodeposition by electrolytic plating. Accordingly, since the metal film of copper is formed so as to fill the wiring grooves and the like by applying plating described above, voids are not formed in the metal film and a uniform copper film can be obtained. Further, wirings can be formed effectively even in fine wiring grooves or connection holes and, in addition, copper having high electro-migration durability and low resistance can be used as the wiring material, so that the method can be applied even to fine wiring grooves or suitable to connection holes and it is preferred in a case of forming fine wirings such as the connection portions between a silicon substrate and wirings, or between wiring layers in a multi-layered wiring structure. Furthermore, since fine wirings can be formed, the size of the semiconductor substrate can be reduced. Further, selective deposition is possible also by a method of masking a portion not intended to deposit copper, namely, a portion not intended to apply plating with an insulation film and this method is customary. In this method, however, adhesion of the insulation film, even in a slight amount, to the inside of the wiring grooves increases the resistance to bring about a problem. In addition, since the insulation film for the unnecessary portion has to be removed completely after deposition, it is not efficient to make the step of forming wirings lengthy. In the above described embodiment, since the substrate has only to be dipped into the plating solution after deposition of titanium nitride and titanium films, wirings of high performance can be conducted more shortly in the wiring forming step. The conductors of different deposition overvoltages can be easily determined experimentally in accordance with the kind of metal to be deposited. For example, if the metal to be deposited is copper, copper (CU), zinc (Zn) or gold (Au) can be used for the conductor of low deposition overvoltage and, on the other hand, iron (Fe), nickel (Ni), cobalt (Co), titanium (Ti) and tungsten (W) can be used for the conductor of high deposition overvoltage. Further, since the difference of the deposition overvoltage is relative, optional materials may be selected. Further, if an insulator is used instead of a metal of high deposition overvoltage, since an insulation film adhered thinly to the inside of the connection holes and the wiring grooves gives a high resistance, this is not appropriate, and use of a conductor having low resistance although having high deposition overvoltage is preferred. Then, the above described embodiment is to be explained concretely with reference to examples. As shown in Fig. 1(a), a 15 cm (6 inch) substrate 1 of n-type Si (100) single crystal having a resistance of 2 Ωcm was used as a semiconductor substrate and a silicon oxide insulation film 2 was formed to a thickness of 2.0 µm on the substrate by a plasma CVD method. Then, a connection hole 3 and a wiring groove 4 were formed in the silicon oxide insulation film 2 by using a photolithographic method and a dry etching method (Fig. 1 (b)). The connection hole 3 had a 0.6 µm diameter and a 1.0 µm depth and the wiring groove 4 had a 0.8 µm width and a 1.0 µm depth. The connection hole 3 and the wiring groove 4 correspond to the wiring channel. Then, a titanium nitride film (first conductor layer) 5 having a deposition overvoltage of about -100 mV was deposited to a thickness of 0.1 µm over the entire surface of the semiconductor substrate 1 by a CVD method (Fig. 1(c)). The deposition overvoltage of titanium nitride is a value based on a saturation calomel electrode. Then, titanium (second conductor layer) 6 having a deposition overvoltage of about -300 mV was sputtered to a thickness of 0.03 µm on a flat surface, that is, a portion except for the connection hole 3 and the wiring groove 4 (Fig. 2(a)). For the method of sputtering the titanium film, a usual sputtering method, a sputtering method using a collimator, a long distance sputtering method or the like is adopted. The deposition overvoltage of titanium is a value based on the saturation calomel electrode. The specimen (material to be plated) was subjected to constant current electrolysis in a plating solution using copper sulfate (cupric metal material) using a copper electrode as a counter electrode, at a current of 50 mA for 10 min. The potential difference between both of the electrodes was about 200 mV. In this process, if the current density is increased, electrodeposition is completed within a shorter period of time but, since this increases the electrode potential and the selectivity due to the difference of overvoltage is lost, it is necessary to select the current density or the electrode potential within a range capable of developing the selectivity. Then, after water washing, when this specimen was observed by an optical microscope, it was confirmed that copper was deposited only to the portion for the wiring groove and the connection hole but copper was not adhered to a flat portion, namely, a portion in which titanium 6 was deposited (Fig. 2 (b)). Then, the specimen was polished by a chemical and mechanical polishing method to remove excessively deposited copper thereby forming wirings 7 consisting of copper (Fig. 2(c)). After forming the wirings 7, when a cross section was observed by a scattering type electron microscope, it could be confirmed that the wirings 7 were formed in a state where copper was filled in the connection hole 3 and the wiring groove 4. In the above described embodiment, although explanations have been made to a case of removing only the copper deposited excessively as shown in Fig. 2(c), the specimen may also be polished by using the chemical and mechanical polishing method to remove excessively deposited copper, titanium and titanium nitride as shown in Fig. 3(a) and, subsequently, applying molding, for example. Alternatively, after removing the excessively deposited copper, titanium and titanium nitride as shown in Fig. 3(a), the silicon oxide insulation film 8 is formed again and, subsequently, a connection hole 9 and a wiring groove 10 are formed in the silicon oxide insulation film 8 as shown in Fig. 3(b) by repeating the procedures described above. Then, after depositing a titanium nitride film 11 to the entire surface of the specimen, a titanium film is deposited to a portion except for the connection hole 9 and the wiring groove 10, then plating is applied under the same conditions as described above, excess copper and the like are removed by polishing, thereby forming wirings 12 in the connection hole 9 and the wiring groove 10 to form multi-layered wiring. Further, in the above described embodiment, although the explanations have been made to a case of forming the wirings 7 in the silicon oxide insulation film 2 but this is not restrictive. For example, as shown in Fig. 4, after forming only the connection hole 3 in a silicon oxide insulation film 2, a titanium nitride film 14 is deposited on the entire surface of the specimen, further, a titanium film 15 is formed to a region except for the connection hole 3, and then plating is applied to form wirings 7 in the connection hole 3 by deposition of copper. Then, excess copper is removed by polishing, a wiring film 17 is deposited further thereon, and the wiring film 17, the titanium film 15 and the titanium nitride film 14 are patterned together to form wirings. In a case of forming wirings on an insulation material, an electrically conductive thin film is generally put between them with an aim of improving the adhesion and improving the electro-migration durability. As shown in Fig. 4, the titanium nitride film 14 and the titanium film 15 are left upon polishing the deposited copper and the wiring film 17 is deposited on the titanium film 15, so that the titanium nitride film 14 can be utilized as the thin electrically conductive film to shorten the step of forming the wirings.

Claims: A method of forming wirings for a semiconductor device, which comprises the steps of providing a semiconductor substrate (1) having an insulation film (2) formed thereon; forming channels (4) having an aspect ratio from 1 to 5 and a width of 1.0 µm or less in the insulation film (2), forming a first conductor layer (5) by a CVD process on the surface of the insulation film including the inner surface of the channels (4), then forming a second conductor layer by a sputtering process on the surface of the first conductor layer (5) except for the inner surface the channels (4), the first conductor layer (5) and the second conductor layer (6) having respective surface potentials such that the voltage for depositing a copper series metal from a plating solution on to the second conductor layer - the second overvoltage - is higher than the voltage for depositing a copper series metal from a plating solution on to the first conductor layer - the first overvoltage; subsequently, dipping at least the channels (4) into the plating solution and applying a plating treatment at a voltage higher than the first overvoltage and lower than the second overvoltage to deposit the copper series metal material on the inner surface of the channels (4), and polishing the deposited copper series metal material to remove unnecessary portions whereby the copper series deposited metal forms wirings located in the channels (4). A method as claimed in claim 1, which is applied to a semiconductor device of a multi-layered wiring structure. A method as claimed in claim 1 or 2, wherein the first conductor layer (5) is made of titanium nitride, the second conductor layer (6) is made of titanium and the metal material (7) is made of a copper series metal material.

Inventors: KUNO TOYOHIKO, TONOMURA SHOICHIRO, KUNO, TOYOHIKO, TONOMURA, SHOICHIRO
Applicants: ASAHI CHEMICAL IND, ASAHI KASEI KABUSHIKI KAISHA
Application year: 1998
Main classification: H01L21
IPC codes: H01L23, H01L21