Integrated circuit device with an improved interconnection line

Abstract: A semiconductor device including interconnection lines for connecting element regions is disclosed. Each of inter­ connection lines is comprised of a first layer consisting es­ sentially of aluminum, an alumina film formed on the first layer and a second layer containing silicon and deposited on the alumina film. Refractory metal silicide such as tungsten silicide, molybdenum silicide, titanium silicide, tantalum sili­ cide and chromium silicide is favorably employed as the second layer. Hillcock formation and electromigration are thus prevented or suppressed.

Description: BACKGROUND OF THE INVENTION The present invention relates to a semiconductor integrated circuit device (hereinafter referred to as an "IC"), and more particularly to aluminum interconnection lines in an IC. With advances in integrated circuit technology, circuit element dimensions are being scaled down and interconnection lines are being made fine, so that a high integration density IC is obtainable. Such an IC has a multilevel interconnection structure, and each level interconnection line is made of a low resistivity material, i.e. aluminium, in order to suppress a signal transmission delay. A problem occurring therein is formation of hillock which causes a short-circuit between lower and upper level aluminum interconnection lines. In order to prevent the hillock formation, it has been proposed that the surface of the aluminum interconnection line is covered with a layer of refractory metal such as titanium, molybdenum, etc., see for example US-A-4 531 144. Another problem of the aluminum interconnection line is electromigration of aluminum, which causes breaking down of fine interconnection lines. It has been found that the aluminum interconnection line covered with the refractory metal layer is poor in preventing the breaking-down due to electromigration. WO-A-8401471 discloses a semiconductor device according to the preamble of claim 1 in which an interconnection line consists of an aluminum layer and a refractory metal silicide layer thereon. DE-A-30 26 026, US-A-4 531 144, Applied Physics. vol. 52, no. 7, July 1981, pp. 4630-4639 and Japanese Journal of Applied Physics, vol. 12, no. 4, April 1973, pp. 518-522 describe interconnection lines made of an aluminum layer and an aluminum oxide layer formed thereon, wherein the latter document indicates that such a structure may suppress electromigration. However, such a structure requires a relatively thick aluminum oxide layer to prevent hillock formation such that the manufacturing process is complicated and the aluminum layer is thin which increases resistivity thereof. It is therefore an object of the invention to provide a semi-conductor device comprising an interconnection line which is not only free from hillock formation but also from electromigration. This object is achieved by a device as defined in claim 1; The dependent claims are related to further developments of the invention. The interconnection line according to the present invention can prevent hillock formation and suppress the electromigration. Accordingly, the present invention provides a high integration density IC with a high reliability. BRIEF DESCRIPTION OF THE DRAWINGS The above and other objects, advantages and features of the present invention will be more apparent from the following description taken in conjunction with the accompanying drawings, in which; Fig. 1A to Fig. ID are cross-sectional views in the respective manufacturing steps of a device according to an embodiment of the present invention; Fig. 2 shows "mean time to failure (MTTF)" values for interconnection lines of various contents; Fig. 3A to Fig. 3C are cross-sectional views in the respective manufacturing steps of a device according to another embodiment of the present invention; and Fig. 4 is a cross-sectional view of a still another embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS Referring to Fig. 1A, a substrate 1 of silicon is prepared in which a number of circuit elements having semiconductor regions or "element regions" such as transistors and resistors are formed (only one element region 2 is shown in order to avoid complexity of the drawing). The region 2 has an opposite conductivity type to the substrate 1. The surface of the silicon substrate 1 is covered with a silicon oxide layer 3 which is provided with a plurality of contact holes (only one contact hole 4 is shown in Fig. 1) to expose a part of an associated element region 2. The silicon substrate 1 with the above-mentioned structure is placed in a D.C. magnetron sputtering equipment to deposit an aluminum layer 5 of a thickness of about 1 µm which is in contact with the element region 2 through the contact hole 4 and with other element regions through the associated contact holes (not shown). The silicon substrate 1 is thereafter taken out of the sputtering equipment and then exposed to the atmosphere. As a result, an aluminum oxide (alumina) film 6 is formed on the surface of the aluminum layer 5 by native oxidation, as shown in Fig. 1B. If desired, the alumina film 6 may be formed by plasma-oxidizing technique. Since the alumina film 6 has an oxidation-resisting function, the surface of the aluminum layer 5 is oxidized very thin. The thickness of the alumina film 6 is about 5 nm (50 Å). Subsequently, the silicon substrate 1 is set again into the sputtering equipment, and a tungsten silicide layer 7 is then deposited on the alumina film 6 with a thickness of about 50 nm (500 Å), as shown in Fig. 1C. The silicon substrate 1 is taken out of the sputtering equipment and is then covered with patterned photoresist layers 8-1 and 8-2 employed as an etching mask. The ion etching using CCℓ₄ gas is carried out to selectively remove the tungsten silicide layer 7, the alumina film 6 and the aluminum layer 5. As a result, interconnection lines 9-1 and 9-2 are formed as shown in Fig. 1D. The interconnection line 9-1 is in contact with the element region 2 through the contact hole 4 and is composed of aluminum layer 5-1, alumina film 6-1 and tungsten silicide layer 7-1, and the line 9-2 runs on the oxide layer 3 to contact with another element region (not shown) and is composed of aluminum layer 5-2, alumina film 6-2 and tungsten silicide layer 7-2. In a plan view (not shown), the interconnection line 9-2 extends in a direction perpendicular to a direction in which the line 9-1 is elongated. A heat treatment is thereafter carried out to enhance an ohmic contact between the aluminum layer 5-1 and the element region 2. The alumina layer 6-1 (6-2) does not disappear by this heat treatment. Accordingly, the tungsten silicide layer 7-1 (7-2) is separated from the aluminum layer 5-1 (5-2) by the alumina layer 6-1 (6-2). It has been confirmed by the inventor that this structure of tungsten silicide-alumina-aluminum (WSi₂-Aℓ₂O₃-Aℓ) prevents the hillock formation as well as the breaking down of an interconnection line caused by the electromigration. Devices having the inter-connection lines of the WSi₂-Aℓ₂O₃-Aℓ structure were tested at 200°C with electric currents supplied to the interconnection lines at a current density of 1x10⁶ A/cm². A time period until breaking down of each interconnection line was measured to get MTTF (mean time to failure). Then, the similar tests were conducted under the same condition for devices employing molybdenum silicide in place of tungsten silicide, devices having interconnection lines of a Ti-Aℓ two-layer structure, those having inter-connection lines of a Ta-Aℓ two-layer structure, those having interconnection lines of a Mo-Aℓ two layer structure, and those having interconnection lines an Aℓ single-layer structure, to measure respective values of MTTFs. The results are shown in Fig. 2. The MTTF of the Aℓ single-layer structure is as short as 20 hours and the MTTFs of the Ti-Aℓ, Ta-Aℓ, and Mo-Aℓ two-layer structures are 82 hours, 40 hours and 27 hours, respectively. In contrast, the MTTFs of the WSi₂-Aℓ₂O₃-Aℓ and MoSi₂-Aℓ₂O₃-Aℓ three-layer structures according to the invention are as large as 160 hours and 200 hours, respectively. Although the reason why the structure of the present invention provides such a long MTTF is not accurately known to the inventor, it seems that alumina layer would prevent the alloy reaction between aluminum thereunder and silicide thereover. It has been also confirmed by the inventor that the MTTF becomes short with a structure in which refractory metal is employed in place of its silicide, even though the alumina film is present. This would suggest that the refractory metal deoxidizes alumina of the intervening alumina film and changes the structure from refractory metal-alumina-aluminum three-layer structure to refractory metal-aluminum two-layer structure. Other silicides such as titanium silicide, tantalum silicide, chromium silicide, tungsten silicide and molybdenum silicide cause a similar result. The aluminum layer 5 shown in Fig. 1 may contain silicon or copper of about 1 wt.%. Referring to Fig. 3, another embodiment of the present invention has a multilevel interconnection structure. As shown in Fig. 3A, a first-level interconnection line 18 is formed, which is connected to an element region 12 formed in a silicon substrate 11 through a contact hole 14 provided in a silicon oxide layer 13. This line 18 is composed of an aluminum layer 15, an alumina film 16 and a tungsten silicide layer 17 and is produced in a manner described with reference to Fig. 1. A silicon nitride layer 19 having a thickness of 1 µm is deposited by plasma CVD (chemical vapor deposition) using ammonia and silane gases over the substrate with the first-level inter-connection line 18. The silicon nitride layer 19 is then selectively etched away by plasma etching technique using mixture gas of CF₄ and O₂, as shown in Fig. 3B. At this time, the tungsten silicide layer 17 is also etched. The alumina film 16 is then selectively etched by the reverse sputting employing argon ions. As a result, a contact hole 50 for an electrical connection between a second-level interconnection line and the first-level line 18 is formed. The contact hole 50 exposes a contact portion of the aluminum layer 15. Subsequently, an aluminum layer, an alumina film and a tungsten silicide layer are successively deposited in that order and patterning is then carried out. As a result, second-level interconnection lines 23-1 and 23-2 are formed as shown in Fig. 3C. The line 23-1 is in contact with the first-level interconnection line 18 through the contact hole 50 and is composed of aluminum 20-1, alumina 21-1 and tungsten silicide 22-1, while the line 23-2 crosses over the first-level interconnection line 18, isolated therefrom by the silicon nitride layer 19. The line 23-2 is also composed of aluminum 20-2, alumina 21-2 and tungsten silicide 22-2. The interconnection lines 23-1 and 23-2 are extended in directions perpendicular to each other. The tungsten silicide layer 17 of the first level line 18 prevents a short-circuit between the first-level and second-level interconnection lines 18 and 23-2 caused by the hillock formation, and alumina films 16, 21-1 and 21-2 suppress remarkably the electromigration of the interconnection lines 18, 23-1 and 23-2. Accordingly, an IC of high reliability is provided. Fig. 4 shows still another embodiment of the present invention. An interconnection line 38-1 is connected to an element region 32 formed in a silicon substrate 31 through a contact hole 34 provided in a silicon oxide layer 33. The line 38-1 is composed of an aluminum layer 35-1, an alumina film 36-1 and a silicon layer 37-1. Another interconnection line 38-2 is formed on the oxide layer 33 to be connected to another element region (not shown). The line 38-2 is composed of an aluminum layer 35-2, an alumina film 36-2 and a silicon layer 37-1. The alumina layer 36-1 (36-2) suppresses the alloy reaction between aluminum and silicon, or, the diffusion of silicon into the aluminum layer. In this embodiment, the silicon layer 37 is employed in place of the refractory metal silicide layer. The refractory metal and its silicide are expensive as compared with silicon, and therefore a cost of the IC of this embodiment is reduced. The silicon layer 37 has the substantially same effect as the refractory metal silicide with respect to the prevention of hillock formation. With respect to electromigration, however, the MTTF of the line 38 in this embodiment is shorter than the lines 9, 18 and 23 of Figs. 1 and 3. As described above, a device according to the prevent invention has extremely high reliability per cost.

Claims: A semiconductor device comprising a semiconductor substrate (1, 11, 31), at least one circuit element having a semiconductor region (2, 12, 32) formed in said semiconductor substrate, and an interconnection line (9-1, 18, 38-1) connected to said semiconductor region, said inter-connection line including a first layer (5-1, 15, 35-1) consisting essentially of aluminum and a second layer (7-1, 17, 37-1), characterized in that said interconnection line further includes an aluminum oxide layer (6-1, 16, 36-1) interposed between said first and second layers, and that said second layer is made of refractory metal silicide or silicon. The device as claimed in claim 1, characterized in that said refractory metal silicide is selected from tungsten silicide, molybdenum silicide, titanium silicide, tantalum silicide and chromium silicide. The device as claimed in claim 1 or 2, characterized in that said first layer (15) has a contact portion, that said aluminum oxide layer (16) is formed on said first layer except said contact portion, that said second layer (17) is formed on said aluminum oxide layer, and that said device further comprises an insulating film (19) covering said interconnection line and having a contact hole (50) exposing said contact portion of said first layer and an additional interconnection line (23-1) connected to said contact portion and elongated over said insulating film. The device as claimed in claim 3, characterized in that said additional interconnection line comprises a third layer (20-1) consisting essentially of aluminum, a fourth layer (21-1) made of aluminum oxide and a fifth layer (22-1) made of refractory metal silicide or silicon.

Inventors: KIKKAWA TAKAMARO, KIKKAWA, TAKAMARO
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1987
Main classification: H01L21
IPC codes: H01L23, H01L21