METAL STACK FOR INTEGRATED CIRCUIT HAVING TWO THIN LAYERS OF TITANIUM WITH DEDICATED CHAMBER DEPOSITIONS

Abstract: A metal stack (35) for use in an integrated circuit demonstrating improved electromigration properties. A base layer (31) of approximately 185 ANGSTROM of titanium is formed on an ILD followed by the formation of the bulk conductor layer (32) such as an aluminum-copper alloy layer. A capping layer (33) of approximately 185 ANGSTROM of titanium is formed on the bulk conductor layer (32). Finally, an antireflective coating (ARC) (34) of titanium nitride is formed on the capping layer (33).

Description: METAL STACK FOR INTEGRATED CIRCUIT HAVING TWO THIN LAYERS OF TITANIUM WITH DFDICATED CHAMBER DEPOSITIONS BACKGROUND OF THE INVENTION 1. Field of the Invention. The invention relates to metal stacks used for interconnecting structures in integrated circuits. 2. Related Application This application is related to co-pending application Serial No. 324,763, filed October 17, 1994, entitled "A Novel Via Hole Profile and Method of Fabrication", and assigned to the Assignee of the present application. DESCRIPTION OF THE PRIOR ART Modem integrated circuits often include millions of active and passive devices such as transistors, capacitors and resistors formed on a semiconductor substrate such as silicon. These devices, when initially fabricated, are isolated from one another on the substrate and are later interconnected to form functional circuits. The quality of these interconnecting structure drastically effects the performance and the reliability of the completed integrated circuit. Interconnections are increasingly determining the limits of performance and densities in modern ultra large scale integrated (ULSI) circuits. Often the interconnecting structure is fabricated from a metal stack which may include a base layer, bulk conductor layer and/or capping layer. The stack is formed on a dielectric layer generally by sputtering and then through use of photolithographic techniques is etched to define the interconnecting structure. In current production processes, multiple levels of interconnecting structures are used, for example, four layers of metal stacks may be used, each insulated from one another by a interlayer dielectric (ILD). More often than not, aluminum and aluminum alloys are used as the bulk conductor in metal stacks. Electromigration is a significant reliability problem for these thin- film conductors;. Aluminum, due to its low melting point, is more susceptible to electromigration than are other metals. As high currents pass through a conductor, atoms are transported and vacancies are generated at grain boundaries which coalesce into a network of voids. Void nucleation often occurs at the intersection of grain boundaries and conductor sidewalls. In aluminum-copper alloys selected boundaries remain intact, presumably where they are hardened by copper-rich planar precipitates, resulting in voids with several sharply defined edges. These variously shaped voids continue to enlarge until an open circuit terminates the process. Refractory metals are often used in conjunction with aluminum alloys to provide shunting layers, that is, an electrical path even in the presence of these voids. As will be discussed in connection with Figures 1 and 2, titanium and titanium nitride layers are sometimes used as shunting layers. Figure 1 shows one prior art metal stack used for an interconnecting structure. In Figure 1 the metal stack is formed on an interlayer dielectric (ILD) 10. The bulk conductor 1 1 comprises an aluminum-copper alloy layer 1 1. The thickness of this layer varies depending upon the current that the layer is required to carry; a typical layer may be 350θA thick. A layer of titanium nitride (TiN) is formed on the upper surface of the layer 1 1. This layer in the prior art stack described in Figure 1 is approximately 370A thick. Then a layer 13 of titanium approximately 100θA thick is sputtered on to the upper surface of layer 12. Following this an anti-reflective coating (ARC) 14 is formed on the upper surface of layer 13. This coating is 370A thick for the prior art example shown in Figure 1. Figure 2 shows another prior art metal stack used for an interconnecting structure which is formed on an ILD 20. First, a base layer 21 of titanium approximately 1000A thick is formed on the ILD 20. Following this the bulk conductor, again an aluminum-copper alloy layer 22 is formed on the upper surface of titanium layer 21. The thickness of this layer is, as before, determined by the amount of current that the layer is required to carry (e.g., 6000A to 12000A thick). An ARC 23 is formed on the upper surface of layer 22. Again layer 23 comprises a coating of TiN 370A thick As will be seen the present invention provides a different stack than those shown in Figures 1 and 2. The newly disclosed metal stack has been found to have superior qualities and, in particular, improved electromigration performance over the metal stacks shown in Figures 1 and 2 SUMMARY OF THE INVENTION An improved metal stack for use in an interconnecting structure of an integrated circuit is described The stack includes a thin base layer of titanium which, is approximately between 125A and 200A thick A bulk conductor layer is formed on the upper surface of the base layer In one embodiment this layer comprises an aluminum-copper alloy A capping layer of titanium approximately between 125A to 200A thick is formed on the upper surface of the bulk conductor layer An anti-reflective coating of titanium nitride is formed on the upper surface of the capping layer BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a cross sectional elevation view of a prior art metal stack used for an interconnecting structure in an integrated circuit Figure 2 is a cross sectional elevation view of another prior art metal stack used for an interconnecting structure in an integrated circuit Figure 3 is a cross sectional elevation view of a metal stack fabricated in accordance with the present invention Figure 4 is a cross sectional elevation view of two metal stacks formed in accordance with the present invention Figure 5 is a plan view of a sputter system used to fabricate the metal stacks of Figures 2 and 3 showing the sequence of wafer movement. Figure 6 is a process flow diagram illustrating the steps used to fabricate the metal stack of the present invention DETAILED DESCRIPTION OF THE PRESENT INVENTION The present invention describes a novel metal stack for use as an interconnecting structure in an integrated circuit In the following description numerous specific details are set forth, such as specific materials, processes and equipment in order to provide a thorough understanding of the present invention It will be apparent, however, to one skilled in the art that the present invention may be practiced without these specific details In other instances, well-known manufacturing materials, processes and equipment are not set forth in detail in order not to unnecessarily obscure the present invention In the cross sectional view of Figure 3 a portion of an interconnecting structure using the novel metal stack 35 of the present invention is illustrated Typically, the metal stack 35 of the present invention is formed on an ILD layer such as ILD 30 After the layers 31 - 34 of stack 35 have been formed on the dielectric, well-known photolithographic techniques are used to mask the stack and to etch away portions of the stack so as to form the interconnecting structures as will be discussed Vias are used to provide conductive paths between different levels of metal stacks and between the stacks and substrate regions One via for use with the metal stack of the present invention is described in a co- pending application entitled "A Novel Via Hole Profile and Method of Fabrication", serial no 327,763, filed October 17, 1994 assigned to the Assignee of the present invention The Present Invention as Shown bv the Stack of Figure 3 The stack 35 illustrated in Figure 3 includes a base layer 31 of titanium which is sputtered in a dedicated chamber onto the ILD layer 30 While titanium is preferred, other refractory metals may be used for this thin layer This shunting layer may be approximately between 125A and 20θΛ thick, although 185A is preferred A bulk conductor layer 32 which, in one embodiment ,uses an aluminum-copper alloy having approximately 0 5% copper is in contact with the upper surface of the base layer 31 Although an aluminum alloy layer is preferred because of its low resistivity and its well-known processes, it is to be appreciated that other low resistance materials may act as the bulk conductor. The thickness of the layer 32 is selected as a function of the amount of current that the layer 32 will carry. As will be described in conjunction with Figure 4, the thickness of this layer may be different in one level compared to another in a given integrated circuit. Typical values for the thickness of layer 32 range between approximately 5000A to 20,000A thick. A thin capping layer 33 of titanium is formed in contact with the upper surface of the layer 32. Layer 33 is sputtered titanium, in a dedicated chamber, preferably approximately 185A thick in the preferred embodiment. However, this layer may be approximately between 125A to 200A thick. Again, as in the case of layer 31 other refractory materials may be used for layer 33. Finally, an anti-reflective coating (ARC) 34 is formed on the upper surface of the layer 33. This layer of titanium nitride (TiN) is approximately 15θA thick. As is well-known this layer reduces reflections which would otherwise make masking of the metal stack more difficult. As may be noted from Figure 3 there are two interfaces in the stack 35 of aluminum alloy and titanium. Once such interface is between layers 31 and 32 and the other between layers 32 and 33. When titanium and aluminum are sufficiently heated, a reaction occurs to form titanium aluminide (TiAl3). In general, a complete reaction occurs between the titanium layers and the bulk conductor forming titanium aluminide layers at these interfaces. This is accomplished through the use of high temperature processing after formation of the metal stacks such as ordinarily occurs during ILD deposition, ILD annealing, high temperature ash cleaning steps and other steps. Although the original thickness of the titanium layers 31 and 33 are each approximately 185A in the currently preferred embodiment, 185A of Ti will react with 525A of AICu alloy and result in the formation of approximately 67θA of the TiAl3 layer. Performance measurements were compared for the metal stack shown in Figure 3 and the prior art stacks shown in Figures 1 and 2. The metal stacks of Figure 3 perform better in such areas as via resistance, metal undercutting, voiding, and sheet resistance. The stack of Figure 3 proved to be as manufacturable as the prior art stacks. The electromigration performance as measured by defect density for the stack of Figure 3 was found to be unexpectedly high when compared to the prior art stacks of Figures 1 and 2. Multiple Level of Figure 4 In a typical integrated circuit, the stack of Figure 3 is used at a plurality of different levels. This is partially shown in the cross section of Figure 4. A first metal stack 40 is formed on the ILD 43. The stack 40 is separated from a second metal stack 42 by an ILD 41 . Another ILD is formed on the stack 42 and may support an additional metal stack. In typical processing, after the stack 40 is formed it is patterned into an interconnecting structure using well-known photolithography and etch processes. Any well-known etching techniques such as reactive ion etching (RIE) with a chemistry comprising BCI3 and CI2 can be used to pattern the stack 40 to form the interconnecting structure. Following this an ILD 41 is formed over the pattern stack 40 using well-known chemical vapor deposition (CVD). For instance, a doped silicon dioxide layer is deposited to a thickness of approximately 10.000A. Also, as is well-known, the layer 41 is planarized using chemical etching or chemical-mechanical polishing to form a planar surface upon which the stack 42 may be formed. The stack 42 is then formed on the ILD 41 in the same manner as used to form the stack 35. Intermediate cleaning, via contact processing steps and other well-known steps have not been described. The stacks 40 and 42 each may be identical to the stack 35 of Figure 3 except that the thickness of the bulk conductor layer may vary. For instance, the stack 40 comprises a thin base layer of titanium 44 having a thickness in the preferred embodiment of approximately 185A. A bulk conductive layer 45 may have a thickness of, for example, 540θA. The thin capping titanium layer 46 which, in the preferred embodiment, has a thickness of approximately 185A is formed over the layer 45. The ARC coating 47 is then formed on the layer 46. After the patterning of stack 40 and planarization of the ILD 41 , the stack 42 is formed beginning with the base layer 48 of approximately 185A of titanium. Next the bulk conductor layer 49 of aluminum-copper alloy which may be thicker for the stack 42 (e.g., 740θA) is formed on layer 48. Then the second thin titanium capping layer 50 is formed on layer 49 (approximately 185A thick). Finally, another ARC coating 50 is formed on the layer 50 for the patterning of the stack 42. Processing of the Invented Metal Stack The metal stack of the present invention is formed using a commercially available cluster sputtering apparatus such as an AMAT 5500, Endura Sputter System. Such systems include, as shown in Figure 5, a central region 60 equipped with a robotic arm allowing wafers to be moved from one chamber to another chamber, such as between chambers 61 , 62, 63 and 64. Each of the chambers are separately controllable to allow different processing to occur in each of the chambers. For the formation of the stack of the present invention, a wafer is first transported as shown at 66 to a chamber 61. In chamber 61 the base layer of titanium is first sputtered onto an ILD. This is shown in Figure 6 as step 71 which follows the processing of the ILD 70. Following the formation of the approximately 185A thick base layer of titanium, the wafer is next moved to the chamber 63. in chamber 63 the bulk conductor such as aluminum-copper alloy is deposited on the base layer of titanium. This is shown in Figure 6 by the processing step 72. Now the wafer is moved to the chamber 64 where the capping layer of titanium is formed over the bulk conductor layer. Again as currently preferred, this capping layer is approximately 185A thick. The capping layer is shown in Figure 6 by the process step 73. Finally the wafer is transported to the chamber 62 where the ARC (TiN coating) is formed over the capping layer of titanium. This is shown in Figure 6 by step 74. Thus, a novel interconnecting structure formed from a metal stack comprising a thin layer of titanium, bulk conductor layer and thin capping layer of titanium has been described along with its method of fabrication.

Claims: CLAIMS 1. A metal stack for use as an interconnecting structure in an integrated circuit comprising: a base layer of titanium; a bulk conductor layer in contact with the base layer; and, a capping layer of titanium in contact with the bulk conductor layer. 2. The metal stack defined by claim 1 wherein the base layer is approximately between 125A and 20θA thick. 3. The metal stack defined by claim 1 wherein the capping layer is approximately between 125A and 20θA thick. 4. The metal stack defined by claim 1 wherein the base layer and capping layer are each approximately between 125A and 20θA thick. 5. The metal stack defined by claims 1 , 2, 3 or 4 wherein the bulk conductor layer comprises an aluminum-copper alloy. 6. The metal stack defined by claim 4 wherein an anti¬ reflective coating of titanium nitride is in contact with the upper surface of the capping layer. 7. A metal stack for use as an interconnecting structure in an integrated circuit comprising: a base layer of titanium having a thickness of approximately 185A; a bulk conductor layer in contact with the base layer; and, a capping layer of titanium in contact with the bulk conductive layer having a thickness of approximately 185A. 8. The metal stack defined by claim 7 wherein the bulk conductor layer comprises an aluminum-copper alloy. 9. The metal stack defined by claim 8 including an anti¬ reflective coating in contact with the upper surface of the capping layer. 10. The metal stack defined by claim 9 including an anti¬ reflective coating comprising titanium nitride formed on the capping layer. 1 1. A process for fabricating a metal stack on a dielectric layer comprising the steps of: sputtering a first layer of titanium on the dielectric layer; forming a bulk conductor layer on the first layer of titanium; and, sputtering a second layer of titanium on the bulk conductor layer. 12. The process defined by clam 1 1 wherein the first layer of titanium is approximately between 125A and 20θA thick. 13. The process defined by clam 12 wherein the second layer of titanium is approximately between 125A and 20θA thick. 14. The process defined by claim 13 wherein the forming step comprises the sputtering of an aluminum-copper alloy. 15. The process defined by claim 14 including the additional step of forming a layer of titanium nitride on the second titanium layer.

Inventors: AHMED SOHAIL, BAI PENG, MEYER WILLIAM K, RASTOGI RAJIV, AHMED, SOHAIL, BAI, PENG, MEYER, WILLIAM, K., RASTOGI, RAJIV
Applicants: AHMED SOHAIL, BAI PENG, INTEL CORP, MEYER WILLIAM K, RASTOGI RAJIV, AHMED, SOHAIL, BAI, PENG, INTEL CORPORATION, MEYER, WILLIAM, K., RASTOGI, RAJIV
Application year: 1996
Main classification: H01L23
IPC codes: H01L21, H01L23