Semiconductor device and wiring method

Abstract: An iridium layer (16) is formed on an inter-layer insulation film (12) and in an opening (14). The iridium layer (16) is constituted with a part to be a lower electrode (16a) of a capacitor and a part to be a wiring (16b) for coming into contact with a drain zone (6). On part of the lower electrode (16a) of the iridium layer (16) is formed a ferroelectric layer (18) made of PZT on which is further formed an iridium layer (20) as an upper electrode. Since the melting point of iridium is higher than that of aluminum, there is no possibility of iridium melting even if heat treatment is carried out after forming the iridium layer (16). Since reactivity between iridium and silicon is low, unnecessary silicon compound is not produced on the interface to provide favorable contact.

Description: This invention relates to semiconductor devices, particularly the wiring of semiconductor devices. Heretofore, Al, W, Ta, Ti, their silicides, aluminum alloy (Al-Si-Cu) copper, copper alloy, polysilicon doped with impurities, have been used for wiring of semiconductors. However, when aluminum is used, there is a problem that heat treatment cannot be performed above 450 °C after forming with aluminum, because aluminum melts at temperatures above 450 °C. Polysilicon having a high melting point does not pose such a problem. However, since polysilicon has a high resistance, it is unsuitable for wiring of a long . distance or with fine wires. Alternatively when aluminum is formed on the silicon layer, aluminum enters (spikes) into silicon and may cause breakage of wires. In order to overcome such problems, a conventional method has been to form a thin layer of TiN as a barrier layer on the silicon layer over which aluminum layer is formed. The TiN barrier layer prevents diffusion (spiking) of silicon into aluminum. However, when the TiN barrier layer is formed and then heat-treated in an atmosphere containing O 2 , a possible problem is that Ti is oxidized to form nonconductive titanium oxide so that contacts cannot be made. Another problem is that since TiN is very hard, it may separate from metals or silicon because of difference in coefficients of thermal expansion. An object of this invention is to provide a semiconductor device with high performance wiring capable of solving the above-described problems. It is known to use either iridium or iridium oxide as wiring. This is discussed for example in European Patent Application EP-A-0629002. Therein is disclosed a semiconductor device of the type comprising: a semiconductor substrate having a semiconductor element zone; an insulation layer on said semiconductor substrate, said insulation layer having an opening so as to expose a part of said semiconductor element zone; and a wiring section, comprising both iridium and iridium oxide, on said insulation layer. Therein disclosed is a wiring section composed of a mixture of an elemental metal such as iridium and its conductive metal oxide. This wiring section does not come into direct electrical contact with the semiconductor element zone but is connected thereto by the intermediary of a silicon plug provided filling the opening in the covering insulator layer. An alternative arrangement is also disclosed in which connection is made by respective plugs of Cu and W in successive insulator layers between the wiring section and the semiconductor element zone. According to the present invention there is provided a semiconductor device as described in claim 1. By forming an iridium oxide layer in direct electrical contact with the silicon semiconductor element zone, good adhesion is assured. Also since iridium is used as the material of the main conducting layer, a low resistance connection is also assured. The reactivity between iridium oxide and silicon is very low, and so no problem of silicide formation arises. To facilitate further understanding of the invention a preferred embodiment of the invention will be described in detail. This discussion will follow the discussion of a background example given for ease of explanation. These descriptions are given with reference to the accompanying drawings wherein: FIG. 1 shows an example of a semiconductor device; FIGs. 2A through 2D show a manufacturing flow of the semiconductor device shown in FIG. 1; FIGs. 3A through 3C show a manufacturing flow of the semiconductor device shown in FIG. 1; FIG. 4 is a chart showing results of X-ray analysis to clarify non-reaction between iridium oxide and silicon; FIG. 5 shows an embodiment of a semiconductor device in accordance with the present invention; and FIG. 6 shows a conventional semiconductor device. FIG. 1 shows an example constitution of a semiconductor device by way of background to the present invention. In this example, a source zone (4) and a drain zone (6) are formed on a substrate (2). A gate electrode (10) is formed on a channel zone (8) between the source zone (4) and the drain zone (6) through an insulation film. An inter-layer insulation film (12) is formed over the above-mentioned components. An opening (14) is formed in the insulation layer (12) at a position above the drain zone (6). An iridium layer (16) is formed on the inter-layer insulation film (12) and inside the opening (14). The iridium layer (16) consists of a part to be a lower electrode (16a) and a part to be a wiring (16b) for contact with the drain zone (6). On part of the lower electrode (16a) of the iridium layer (16) is formed a ferroelectric layer (18) made of PZT over which is formed an iridium layer (20) as an upper electrode. A manufacturing process for the semiconductor device shown in FIG. 1 is shown in FIGs. 2 and 3. First, an element separation zone (30) is formed by LOCOS on the silicon substrate (2) and at the same time a gate oxidation film (32) is formed. On the gate oxidation film (32) is formed a gate electrode (10) using polysilicon or polycide. The gate electrode (10) is masked, ions are implanted and diffused, and the source zone (4) and the drain zone (6) are formed (See FIG. 2A). On the product prepared as described above is formed an inter-layer insulation film (12) of SiO 2 or BPSG to a thickness 600 nm (6000 angstroms) by the CVD process (See FIG. 2B). After that, in order to form an opening in the inter-layer insulation film (12) above the drain zone (6), a photoresist mask (34) is formed as shown in FIG. 2C. In compliance with the mask (34), the inter-layer insulation film (12) is etched to form the opening (14) (See FIG. 2D). In that state, an iridium layer (16) is formed over the entire surface by a sputtering, CVD, or vacuum deposition process, and then the product is subjected to a dry etching process such as RIE, ion milling or ECR to leave required parts only (See FIG. 3A) using gas such as Ar, and Cl 2 . By the way, the dry etching may be replaced with the wet etching. Next, on these iridium layer (16) and the inter-layer insulation film (12) a PZT film as a ferroelectric layer (18) is formed by a sol-gel process. As a starting material, mixed solution of Pb(CH 3 COO) 2 ·3H 2 O, Zr(T-OC 4 H 9 ) 4 , and Ti(i-OC 3 H 7 ) 4 is used. After applying the mixed solution to the product by spin coating, it is dried at 150 °C, and subjected to temporary baking at 400 °C for 30 seconds. This process is repeated five times and then the product is subjected to heat treatment in O 2 atmosphere at a temperature of 700 °C or higher. Thus the ferroelectric layer (18) of 250 nm thick is obtained. The above product is further coated with an iridium layer (20) as an upper electrode by sputtering, CVD or vacuum deposition. Then, unnecessary parts are removed by etching to leave the ferroelectric layer (18) and the iridium layer (20) only on the part above the lower electrode (16a) as shown in FIG. 3B. Next, a protection film (40) of Si 3 N 4 or SiO 2 is formed by plasma CVD (See FIG. 3C). In this example, the iridium layer (16) is used for the lower electrode (16a) and the wiring (16b). Since the melting point of iridium is higher than that of aluminum, there is no problem of iridium melting under heat treatment for PZT after forming the iridium layer (16). Therefore, the iridium layer (16) is used not only for the lower electrode (16a) but also for the wiring (16b) as shown in FIG. 1. Furthermore, even if iridium is oxidized during the heat treatment, since iridium oxide produced is electrically conducting, performance as the electrode and wiring is not affected. Another advantage is that the iridium layer (16) has a lower electrical resistance than polysilicon and others. In contrast to the above, if aluminum (24) is used for the wiring as shown in FIG. 6, the aluminum film (24) must be formed after the PZT film is formed. This makes an additional insulation layer (22) necessary and makes the constitution complicated. Furthermore, in the example shown in FIG. 1, the lower electrode (16a) and the wiring (16b) are simultaneously formed to facilitate manufacture. Another advantage is that the use of iridium for the lower electrode (16a) improves the dielectric property of the ferroelectric layer (18) formed over the lower electrode (16a) in comparison with electrodes made of platinum, etc. While the PZT film is formed as the ferroelectric layer (18) in the above example, PLZT, BaTiO 3 , BiSr 2 Ta 2 O 9 , may also be formed. Furthermore, in place of the ferroelectric layer (18), a dielectric layer having a high dielectric constant may be formed (such as a high dielectric constant thin film having a perovskite structure of SrTiO 3 and (Sr,Ba)TiO 3 ). While the iridium layer (16) is used for the wiring (16b) and the lower electrode (16a) in the above example, this may be replaced with an iridium oxide layer. Iridium oxide is preferable for wiring because of its good adhesion to silicon. Iridium oxide may be formed by a reactive sputtering process. Another advantage is that since the reactivity between iridium oxide and silicon is very low, there is no possible problem of silicide being produced on the boundary surface α (See FIG. 1) facing the drain zone (6). To prove this, various kinds of metals are formed on a silicon substrate, analyzed with X-rays, and the results are shown in the chart shown in FIG. 4. FIG. 4A shows the result for platinum, FIG. 4B for titanium covered with platinum, FIG. 4C for iridium, FIG. 4D for iridium oxide covered with iridium, and FIG. 4E for iridium oxide covered with platinum. As is clear from the comparison between FIGs. 4A and 4C, platinum produces a compound with silicon but iridium does not produce a compound with silicon. Alternatively, when iridium oxide is formed, it may also be formed under oxygen lacking condition as Ir0 2-x (o < χ < 2). In this way, oxide (such as silicon oxide), even if it is produced on the boundary surface, may be reduced (to silicon). In other words, undesirable oxides formed on the boundary surface may be eliminated. While simultaneous forming of the electrode and wiring is described in the above example, wiring only may be formed and the same advantages obtained. A semiconductor device according to the preferred embodiment of this invention is shown in FIG. 5. In this embodiment, a wiring (50) is formed with an iridium oxide layer (52) as a barrier layer and an iridium layer (54) formed as a main conducting layer formed on the iridium oxide layer (52). By forming the iridium oxide layer (52) as a barrier layer, adhesion to silicon is improved. In this embodiment, since the main conducting layer is formed with the iridium layer (54), electric resistance is lowered in comparison with the case in which the entire wiring is made with iridium oxide only. The iridium oxide layer (52) of this embodiment may be prevented from producing oxides by forming this layer as a non-stoichiometric oxide Ir0 2-x (o < χ < 2) under a condition of insufficient oxygen. In the semiconductor device aforesaid, wiring parts are made of iridium and iridium oxide. As a result, the wiring is less affected by high temperatures and no insulating substance is produced at the interface with silicon. Preferably the iridium oxide is made under a condition of insufficient oxygen. Therefore, oxides are prevented from being produced at the interface with silicon. Essentially, the wiring section is made up of a barrier layer and a main conducting layer, with the barrier layer being made of iridium oxide. The barrier layer then has a good barrier function and good adhesion to silicon.

Claims: A semiconductor device comprising: a semiconductor substrate (2) having a semiconductor element zone (6); an insulation layer (12) on said semiconductor substrate, said insulation layer having an opening (14) so as to expose a part of said semiconductor element zone; and a wiring section (16a,16b;50) on said insulation layer, said wiring section (16a,16b;50) comprising both a barrier layer (52) and a main conducting layer (54) on and in direct electrical contact with said barrier layer, said barrier layer covering and being in direct electrical contact with said part of said semiconductor element zone (6) of said semiconductor substrate (2); characterized in that : said semiconductor substrate (2) is of silicon; said barrier layer (52) is of irridium oxide and said main conducting layer (54) is of irridium. The semiconductor device of claim 1 wherein said iridium oxide is of non-stoichiometric composition IrO 2-x : o < χ < 2. The semiconductor device of either of claims 1 and 2, further comprising: a lower electrode (16a) on said insulation layer (12); and a dielectric layer (18) of ferroelectric or other high dielectric material on said lower electrode; wherein said lower electrode is an integral part of said wiring section (16a,16b). The semiconductor device of claim 3, further comprising: an upper electrode (20) on said dielectric layer (18). The semiconductor device of claim 4 wherein said upper electrode (20) comprises iridium or iridium oxide.

Inventors: NAKAMURA TAKASHI, NAKAMURA, TAKASHI, Nakamura, Takashi, c/o Rohm Co., Ltd.
Applicants: ROHM CO LTD, ROHM CO., LTD.
Application year: 1996
Main classification: H01L23
IPC codes: H01L23, H01L27, H01L21