Multilevel metallization for VLSI and method for forming the same

Abstract: An integrated circuit includes a substrate, a first level horizontal conductor formed on the substrate, an interlayer insulator formed to cover the first level conductor, a second level horizontal conductor formed on the interlayer insulator, and a vertical conductive pillar extending through the interlayer insulator for interconnecting the first level horizontal conductor and the second level horizontal conductor. The vertical conductive pillar has a side surface coplanar with a longitudinal side surface of the first level horizontal conductor at a position where the vertical conductive pillar is in electric contact with the first level horizontal conductor.

Description: The present invention relates to a multilevel metallization and a method for forming the same, and more specifically to an improved method of forming a pillar structure for interconnection between two different levels of wiring conductors. In the prior art, a vertical connection between two different levels of wiring conductors in a multilevel metallization has been effected by completely charging a conducting material into a through hole or via hole formed in an interlayer insulating layer, by vacuum evaporation or by sputtering. However, advanced integrated circuit technology, particularly, increased integration density of the integrated circuit has made it difficult to realize a stable interlayer interconnection by means of through holes. In order to overcome this problem, it has been proposed to previously form a stud-like connection member, often called a "pillar", on a lower level metallization so as to vertically protrude from the lower level metallization, whereby a good interlayer connection between the lower level metallization and an upper level metallization can be realized by the conductive pillar vertically protruding from the lower level metallization. Referring to Figures 1 and 2, there are diagrammatically shown a typical example of a conventional conductive pillar vertically protruding from the lower level metallization. As shown in Figure 1, a plurality of lower level conductors 12 are formed on a semiconductor substrate 10, and a conductive pillar 14 is formed on the lower level conductor 12 at a portion 16 where a through hole 18 is to be formed in an interlayer insulator (not shown) which is to be formed to cover the lower level conductors 12 for the purpose of interconnection between the lower level conductors 12 and an upper level conductor (not shown) to be formed on the interlayer insulator. As seen from Figure 2, however, in order to previously ensure a tolerance of alignment and to compensate for variation of dimensional accuracy in the semiconductor process, it has been necessary to widen the portion 16 of the lower level conductor 12 where the pillar 14 is formed, with the result that a conductor pattern pitch (line pitch) must be enlarged. For example, assuming that the width of each conductor 12 is "F" at a portion excluding the pillar formation portion 16 and "2F" at the pillar formation portion 16, and that a minimum space between each pair of adjacent conductors is "F", the conductor pattern pitch must be "3F". In other words, it is disadvantageous that the wiring density has been decreased. Recently, K. Haberle et at proposed in Proceedings of IEEE V-MIC Conference, 1988, pp117-124, that the wiring conductors and the pillars are formed of gold metallization so as to reduce the wiring pitch. Referring to Figures 3 and 4, there is diagrammatically shown a conductive pillar proposed by K. Haberle et at. As shown in Figure 4, a through hole 18A is formed to have a size larger than the width of the lower level conductor 12 formed on the substrate 10, and then, as shown in Figure 3, a pillar 14A is formed to straddle the lower level conductor 12 by means of a gold plating method. In this method, assuming that the width of each lower level conductor 12 is "F" and the width of the pillar 14A is "2F" in the direction perpendicular to the longitudinal direction of the lower level conductor 12, the wiring pitch (line pitch) can be made "2.5F", which is shorter than the example shown in Figures 1 and 2. However, due to possible misalignment of mask patterns and due to variations of the shape of the pillar, there is a danger of short-circuiting between adjacent conductors or another danger of increasing a wiring capacitance even if the adjacent conductors are not short-circuited. In other words, in order to avoid these dangers, the wiring pitch (line pitch) cannot be sufficiently reduced. It is an object of the present invention is to provide a method of forming an interlayer connection structure of the pillar type, which allows to reduce the wiring pitch as compared with the conventional methods. This object is achieved by a method as defined in claim 1; the dependent claims are related to further developments of the invention. In addition, in the method in accordance with the present invention for forming the interlayer connection structure, the lower level wiring conductor and the conductive pillar are formed within the groove formed in the deposited first photoresist layer. This means that the conductive pillar is formed in a self-alignment with the lower level wiring conductor by action of the groove formed in the deposited first photoresist layer. Therefore, the wiring pitch can be reduced to a limit determined by a photolithography. In other words, the wiring pitch can become free from a tolerance margin for misalignment and a dimensional error of semiconductor process, which were causes for increasing the wiring pitch in the prior art. The above and other objects, features and advantages of the present invention will be apparent from the following description of a preferred embodiment of the invention with reference to the accompanying drawings. Figure 1 is a diagrammatic perspective view of one example of the conventional conductive pillar vertically protruding from the lower level conductor in the multilevel metallization; Figure 2 is a wiring pattern diagram of the lower level conductor and the conductive pillar vertically protruding therefrom as shown in Figure 1; Figure 3 is a diagrammatic perspective view of another example of the conventional conductive pillar vertically protruding from the lower level conductor in the multilevel metallization; Figure 4 is a wiring pattern diagram of the lower level conductor and the conductive pillar vertically protruding therefrom as shown in Figure 3; Figure 5A to 5E are diagrammatic perspective views of an integrated circuit chip for illustrating one embodiment of the pillar type interlayer connection construction forming method in accordance with the present invention; and Figure 6 is a wiring pattern diagram of the lower level conductor and the conductive pillar vertically protruding therefrom formed in the method illustrated in Figures 5A to 5E. Referring to Figure 5A to 5E, there are shown diagrammatic perspective views of an integrated circuit chip for illustrating one embodiment of the pillar type interlayer connection construction forming method in accordance with the present invention. Firstly, in order to form a conductive underlayer which is used for forming necessary wiring conductors and vertical conductive pillars on a semiconductor substrate 20 by use of an Au (gold) electroplating method, a thin layer 22 of TiW and another thin layer 24 of Au are continuously deposited on a principal surface of the semiconductor substrate 20 by sputtering. Thereafter, a first photoresist 26 is deposited to cover the whole of the principal surface of the semiconductor substrate 20 and to have a thickness of 2.0µm, and the deposited first photoresist is patterned by photolithography to form a plurality of grooves 28 for a lower level conductor, so that the underlying Au thin layer 24 is exposed within the grooves 28. Then, as shown in Figure 5A, a first or lower level conductor 30 of Au having a thickness of 0.8µm is formed within each groove 28, namely deposited on the exposed underlying Au thin layer 24 within each groove 28, by means of the Au electroplating method using the conducting underlayer 24 as an electrode to be plated. This Au electroplating method is described in detail in the K. Haberle et al article referred to hereinbefore, and therefore, a detailed explanation thereof will be omitted. As shown in Figure 5B, a second photoresist 32 is deposited to cover the whole upper surface of the semiconductor substrate 20, with the portion of each groove 28 above the first or lower level conductor 30 being filled with the second photoresist 32. The deposited second photoresist 32 is patterned by a photolithography to form a plurality of openings 34 at predetermined positions where a conductive pillar is to be formed, so that the first or lower level conductor 30 is partially exposed. As shown in Figure 6, the size of the opening 34 in the direction perpendicular to the longitudinal direction of the first or lower level conductor 30 is larger than the width of the first or lower level conductor 30, namely, the width of the groove 28. Specifically, at a level above a horizontal boundary between the first photoresist 26 and the second photoresist 32, each opening 34 has the size larger than the width of the lower level conductor 30. On the other hand, at a level below the horizontal boundary between the first photoresist 26 and the second photoresist 32, each opening 34 has a size limited by the width of the groove 28, namely, the width of the lower level conductor 30. Accordingly, the relative positioning of the opening 34 to the first or lower level conductor 30 may be at a relatively low degree of accuracy, if there is assured that the full width of the lower level conductor 30 is exposed within the opening 34. In other words, the center of the opening is not necessarily in precise alignment to the longitudinal center axis of the lower level conductor 30. Here, it should be noted that, for the photolithography for the second photoresist 32, it is necessary to select a process which gives no damage to the shape of the first photoresist 26. For example, if the first and second photoresists 26 and 32 are of a positive type which contains a Novorak resin as a main component, the first photoresist 26 is cured within a plasma atmosphere, and thereafter, the second photoresist 32 is deposited. With this, the second photoresist 32 can be patterned to a desired shape without giving a substantial damage to the shape of the first photoresist 26. After the formation of the openings 34, Au is deposited by the Au electroplating method using the conducting underlayer 24 as an electrode to be plated, until a first pillar 36 of Au having a height of 0.8µm is grown on the lower level conductor 30 within the opening 34. The total height of the lower level conductor 30 and the first pillar 36 is 1.6µm, which is smaller than the thickness of the first photoresist 26. Therefore, a pair of opposite side surfaces of the first pillar 36 positioned in a longitudinal direction of the lower level conductor 30 are coplanar with a pair of corresponding opposite longitudinal side surfaces of the lower level conductor 30. In other words, although the opening 34 having the size larger than the width of the lower level conductor 30 is formed in the second photoresist 32, the first pillar 36 is formed in self-alignment with the lower level conductor 30. Thereafter, the first and second photoresists 26 and 32 are removed, and then, the exposed underlayer composed of the Au layer 24 and TiW layer 22 is removed by means of ion milling, as shown in Figure 5C. Then, as shown in Figure 5D, a silicone polyimide (PSI) layer 38 is deposited as an interlayer insulator on the whole surface of the substrate, and etched back by means of an entire etch-back process until a head portion 36A of the first pillar 36 is exposed above the silicone polyimide layer 38 so that the exposed portion has a height of 0.2µm. Thereafter, as shown in Figure 5E, a required number of second or upper level conductors 40 and second pillars 42 of Au protruding from the upper level conductors 40 are formed in the same manner as that explained above with reference to Figures 5A to 5D. The upper level conductors 40 are formed in a direction orthogonal to the lower level conductors 30, and some of the upper level conductors 40 are formed above and in electric contact with selected ones of the exposed heads 36A of the first pillars 36. The second pillars 42 are formed in self alignment with the upper level conductors 40. Therefore, the upper level conductors 40 can be formed with the wiring pitch (line pitch) of 2F, similarly to the lower level conductors 30. The process as mentioned above can be repeatedly performed. Therefore, a multilevel metallization can be realized with a desired number of levels and with a minimum wiring pitch (line pitch) allowed by the photolithography. Accordingly, a maximum wiring density can be obtained.

Claims: A method of forming an interlayer connection conductor in an integrated circuit, comprising the steps of: depositing a first photoresist layer (26) on a conducting layer (22, 24) formed on a semiconductor substrate (20) and patterning the deposited first photoresist layer (26) to form in said deposited first photoresist layer (26) a groove (28) exposing a portion of said conducting layer (22, 24) forming on the exposed portion of the underlying conducting layer (22, 24) within said groove (28) a first level wiring conductor (30) having a smaller thickness than the deposited first photoresist layer (26), forming a second photoresist layer (32) to cover said first photoresist layer (26) and the first level wiring conductor (30) within said groove (28), selectively removing only the second photoresist layer (32) at a predetermined position thereof where the first level wiring conductor (30) is to be interconnected with a second level wiring conductor (40), so as to form an opening (34) exposing a portion of the first wiring conductor (30), said opening (34) having first and second opening portions above and below the horizontal boundary between the first photoresist layer (26) and the second photoresist layer (32), respectively, said first opening portion having a width larger than the first wiring conductor (30) width, and said second opening portion having a width limited by the width of the groove (28), and forming a conductive pillar (36) within the second opening portion of the opening (34), so that the conductive pillar is formed in the groove in contact with said portion of the first level wiring conductor. The method claimed in Claim 1, wherein the first level wiring conductor (30) is formed by means of electroplating using the conducting layer (22, 24) as an electrode to be plated, and the conductive pillar (36) is formed by means of an electroplating using the conducting layer (22, 24) as the electrode to be plated. The method claimed in Claim 1, wherein the conductive pillar is formed so that the top surface of the conductive pillar (36) is lower than the upper surface level of the first photoresist layer (26). The method claimed in Claim 3, further including the steps of removing the first and second photoresist layers (26, 32), removing the thus exposed conducting layer (22, 24) and forming an insulating layer (38) to cover the whole substrate (20) but to allow a head portion (36A) of the conductive pillar (36) to be exposed from the insulating layer (38). A method claimed in Claim 4 further including the step of etching back the insulating layer (38) so that the head portion (36A) of the conductive pillar (36) is protruded from the insulating layer (38). A method claimed in Claim 5, further including the steps of forming: a second conducting layer on the insulating layer (38), depositing a third photoresist layer on the second conducting layer and patterning the deposited third photoresist layer to form a second groove for formation of a second level wiring conductor (40) therein, the second groove being positioned to cross the first level conductor (30) in a plan view and to selectively pass on the first conductive pillars (36), forming said second level wiring conductor (40) within said second groove by means of electroplating using the second conducting layer as an electrode to be plated so that second level wiring conductor (40) is selectively formed within said second groove in electrical contact with the first conductive pillars (36), the second wiring conductor (40) having a smaller thickness than the deposited third photoresist layer, forming a fourth photoresist layer to cover the third photoresist layer and the second level wiring conductor, within the second groove, selectively removing the fourth photoresist layer at a predetermined position thereof where the second level wiring conductor is to be interconnected with an upper level wiring conductor, so as to form a second opening exposing a portion of the second level wiring conductor, said opening having first and second opening portions above and below the horizontal boundary between the third and fourth photoresist layers, respectively, said first opening portion having a width larger than the second level wiring conductor width, and said second opening portion having a width limited by the width of the second groove, and forming a second conductive pillar (42) within the opening by means of an electroplating using the second conducting layer as an electrode to be plated, so that the second conductive pillar (42) is formed in the second groove above the second wiring conductor, and removing the third and fourth photoresist layers.

Inventors: KUDOH OSAMU, OKADA KENJI, SHIBA HIROSHI, KUDOH, OSAMU, OKADA, KENJI, SHIBA, HIROSHI
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1990
Main classification: H01L21
IPC codes: H01L21, H01L23