Method for forming a plug metal layer

[0002] 1. Field of the Invention

[0003] The present invention relates generally to a method for forming a plug metal layer, and in particular to a method for forming a plug tungsten (W) layer.

[0004] 2. Description of the Prior Art

[0005] As semiconductor devices, such as Metal-Oxide-Semiconductor devices, become highly integrated, the area occupied by the device shrinks, as well as the design rule. With advances in semiconductor technology, the dimensions of the integrated circuit (IC) devices have shrunk to the deep sub-micron range. When the semiconductor device continuously shrinks to the deep sub-micron region, some problems described below are incurred due to the scaling down process.

[0006] Generally known in the art of integrated circuit fabrication is the use of titanium nitride layers. Titanium nitride layers within integrated circuit fabrication are most commonly employed as either barrier layers or adhesion promoter layers. When employed as barrier layers, titanium nitride layers are typically formed interposed between a conductor metallization layer and a doped silicon layer or a doped silicon semiconductor substrate. When formed in this location, a titanium nitride layer provides a barrier to inhomogeneous inter-diffusion and spiking from the conductor metallization layer into the doped silicon layer or the doped silicon semiconductor substrate. Titanium nitride layers, which are employed as barrier layers, are particularly well evolved within integrated circuit fabrication. Alternatively, when employed as adhesion promoter layers, titanium nitride layers within integrated circuits are typically formed as liner layers beneath blanket tungsten layers from which in turn are formed conductive contact and interconnection studs through patterned dielectric layers within those integrated circuits.

[0007] While the barrier layer characteristics of titanium nitride layers have made titanium nitride layers quite common within integrated circuit fabrication, methods through which such titanium nitride layers may be formed within integrated circuits are not entirely without problems. In particular, within the titanium nitride layers the integrated circuit device dimensions have decreased, and the aspect ratios of apertures has increased within those integrated circuits. It has become increasingly difficult to form, through conventional physical vapor deposition (PVD) sputtering methods, titanium nitride layers with adequate step coverage. The difficulty derives from the inherent line-of-sight deposition characteristics of conventional physical vapor deposition (PVD) sputtering methods employed in forming integrated circuit layers of titanium nitride, as well as other materials. The line-of-sight deposition characteristics typically provide only limited sidewall and bottom coverage of titanium nitride within a narrow high aspect ratio aperture (i.e., an aperture with a width less than about 0.5 microns and an aspect ratio greater than about 3). A titanium nitride layer is desired in comparison with titanium nitride coverage upon the surface of the integrated circuit layer (typically a dielectric layer) within which is formed the aperture.

[0008] In response to the step coverage limitations inherent in forming titanium nitride layers through physical vapor deposition (PVD) sputtering methods, there has alternatively been proposed and disclosed the use of chemical vapor deposition (CVD) methods for forming titanium nitride layers within integrated circuits. Titanium nitride layers formed through chemical vapor deposition (CVD) methods have inherently superior step coverage within narrow high aspect ratio apertures within integrated circuits since chemical vapor deposition (CVD) methods, in general, proceed through a surface diffusion deposition phenomenon rather than a line-of-sight deposition phenomenon.

[0009] Chemical vapor deposition (CVD) methods may be employed within integrated circuits to provide titanium nitride layers with superior step coverage for narrow high aspect ratio apertures. Chemical vapor deposition (CVD) methods are also not entirely without problems when forming titanium nitride layers within integrated circuits with optimally desirable properties. In that regard, it is difficult to deposit titanium nitride layers at comparatively low temperatures (i.e., less than a temperature of about 550 degrees centigrade at which aluminum containing conductor metallization layers deteriorate) through low pressure chemical vapor deposition (LPCVD) methods. Simultaneously, it is also difficult to deposit titanium nitride layers with a low resistivity and impurity concentration through metal organic chemical vapor deposition (MOCVD) methods. Particularly undesirable impurities formed within titanium nitride layers deposited through metal organic chemical vapor deposition (MOCVD) methods are carbon, oxygen, and hydrogen. These impurities increase the difficulty of the subsequent process, for example, a gap fill process.

[0010] In general, a titanium layer and a titanium nitride layer are first formed to form a barrier layer in the process for forming a plug tungsten layer. Next, the titanium nitride layer is treated by way of using a plasma process, and then the tungsten process is performed. Nevertheless, there is the directional issue when the conventional plasma process for the treatment of the barrier layer is used to treat the surface of the barrier layer in the via hole. That is, the treatment of the conventional plasma process cannot overall suffuse the barrier layer and, hence, the resulting treatment of the titanium nitride layer on the sidewall of the via hole is incomplete. Furthermore, if the titanium nitride layer is formed by way of metal organic chemical vapor deposition (MOCVD), the titanium nitride layer on the sidewall of the via hole without plasma will have residual volatility solvent therein, therefore causing “out-gassing.”

[0011] The issues above result from the most commonly used precursors for metal organic chemical vapor deposition (MOCVD) which are TDMAT and TDEAT, and which might end up with a TiN layer having carbon and/or hydrogen byproducts after a thermal dissociation process. A conventional method used in solving the above problem is to use nitrogen (N₂) and/or hydrogen (H₂) plasma treatment to reduce the byproduct content within the TiN layer, as well as reducing its resistivity and water absorption.

[0012] Nevertheless, anisotropic plasma treatment against deeper trenches, that is, trenches having high aspect ratios, cannot effectively remove the residing byproducts in the sidewalls of the via/contact holes. Thus, the byproduct contents at the sidewall and at the top of the via/contact holes are very different, which result in different metal deposition rates over the treated/untreated TiN barrier layer at later stages, that is, selectivity of the metal chemical vapor deposition. As shown in FIG. 1, the selectivity of the metal chemical vapor deposition is a ratio of the thickness of the metal nucleation, wherein the ratio (b/a) is the thickness of the metal nucleation on the top of the via hole (with the treatment of plasma) and the sidewall of the via hole (without the treatment of plasma). In the conventional process for the metal chemical vapor deposition, the ratio of b to a is about 40 to 60% or 50 to 70% that is due to the fact that the sidewall of the via hole cannot be treated with the plasma process and, hence, discontinuous and insufficient nucleation sites.

[0013] In such a situation, the side-wall deposition rate is less than the top deposition rate, as the plug metal is laid down, it tends to cover the via/contact holes before the metal layer covering the opposite sides of the holes meets. This forms an open area in the holes called a “seam” phenomena. When the plug metal layer etch back is performed, the void is opened, which forms an irregular upper surface on the metal plug in the well. It is very difficult to form a good contact between the irregular upper surface of the open via/contact plug and the interconnect line. Metal applied to such irregular surfaces tends to crack or break over time, such as an over-high resistance constant (RC), which can cause a defective integrated circuit and can create major reliability problems. Especially for device generation smaller than 0.18 micrometer, the process integration is very difficult owing to the resistance constant (RC) being too high and, hence, unstable.

[0014] In accordance with the above description, a new and improved method for forming a plug metal layer is therefore necessary so as to raise the yield and quality of the follow-up process.

[0015] In accordance with the present invention, a method is provided for forming a plug metal layer that substantially overcomes the drawbacks of the above mentioned problems that arise from conventional methods.

[0016] Accordingly, it is a main object of the present invention to provide a method for forming a plug metal layer. This invention can perform the cycle step of an atomic layer deposition (ALD) to form a continuous metal seed layer (CMSL), so as to raise the selectivity of the metal. Furthermore, the steps of the atomic layer deposition (ALD) of the present invention is to individually transport the reactive gas in order to prevent the issue of competitive adsorption whereby a thin continuous metal seed layer (CMSL ) with continuous and dense nucleation sites is formed. The thickness of the continuous metal seed layer (CMSL) is only about 20 to 40 Å, wherein the thin continuous metal seed layer (CMSL) can substitute for the thick nucleation layer that is about more than 500 Å. It is necessary to be formed in the conventional process, because the nucleation sites are insufficient. Moreover, the continuous metal seed layer (CMSL) also can be reformed before the nucleation layer is formed to reduce the thickness of the nucleation layer, and this can make for a better gap-filling process. Therefore, the present invention makes cost reductions that correspond to a positive economic effect and are appropriate for deep sub-micron technology.

[0017] In accordance with the present invention, a new process for forming a plug metal layer is disclosed. First of all, a semiconductor substrate is provided. Then a dielectric layer is deposited on the semiconductor substrate. Next, forming and defining a photoresist layer on the dielectric layer. An etching process is then performed by the photoresist layer as an etched mask to form a via hole. After removing the photoresist layer, a barrier layer on the dielectric layer and the sidewall and bottom of the via hole is formed. Afterward, the barrier layer is treated by way of a plasma process. Subsequently, the formation of an atomic layer deposition (ALD) at least once is performed to form a continuous metal seed layer (CMSL) on the barrier layer, wherein the atomic layer deposition comprises: a mixing gas of hydrogen and silane, such as hydroxy silane or tetrahydroxy silane, is transported on the barrier layer. Next, a purge/vacuum process is performed. Then a reactive gas, such as WF₆, is transported to form the continuous metal seed layer (CMSL). The cycle step of the atomic layer deposition (ALD) can be repeated to form the thickness of the continuous metal seed layer (CMSL) about 20 to 40 Å. Then a nucleation layer is formed on the continuous metal seed layer (CMSL). Afterward, a blanket metal layer is formed on the nucleation layer. Finally, the blanket metal layer is etched back by way of a chemical mechanical polishing process (CMP) and a plug metal layer is formed on the via hole.

[0018] The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:

[0019] FIG. 1 shows cross-sectional views illustrative of depositing the nucleation layer on the top (with the plasma process) and the sidewall (without the plasma process) of the via hole in accordance with the conventional process;

[0020] FIG. 2A to FIG. 2C show cross-sectional views illustrative of various stages for forming the continuous metal seed layer (CMSL) by way of an atomic layer deposition (ALD) in accordance with the first embodiment of the present invention;

[0021] FIG. 3A to FIG. 3C show cross-sectional views illustrative of various stages for forming the continuous metal seed layer (CMSL) by way of an atomic layer deposition (ALD) in accordance with the second embodiment of the present invention;

[0022] FIG. 4A to FIG. 4F show cross-sectional views illustrative of various stages for forming the plug metal layer by way of a continuous metal seed layer (CMSL) in accordance with the third embodiment of the present invention, wherein the FIG. 4C shows a flowchart of the atomic layer deposition (ALD); and

[0023] FIG. 5A to FIG. 5F show cross-sectional views illustrative of various stages for forming the plug tungsten layer by way of a continuous tungsten seed layer (CWSL) in accordance with the fourth embodiment of the present invention, wherein the FIG. 5C shows a flowchart of the atomic layer deposition (ALD).

[0024] Preferred embodiments of the present invention will now be described in greater detail. Nevertheless, it should be recognized that the present invention can be practiced in a wide range of embodiments besides those explicitly described, and the scope of the present invention is expressly not limited except as specified in the accompanying claims.

[0025] As illustrated in FIG. 2A to FIG. 2C, in the first embodiment of the present invention, a semiconductor substrate 200 having a dielectric layer 210, is provided. There is a via hole 220 in the dielectric layer 210, and a barrier layer 230 is formed on the dielectric layer 210 and sidewall and bottom of the via hole 220, wherein the method for forming the barrier layer 230 comprises a metal organic chemical vapor deposition (MOCVD). Then the barrier layer 230 is treated by way of using a plasma process 240. Afterward, performance of an atomic layer deposition (ALD) 250 at least one time to form a continuous metal seed layer (CMSL) 260 on the barrier layer 230, wherein the cycle step of the atomic layer deposition (ALD) 250 comprises a mixing gas of hydrogen and silane, such as hydroxy silane or tetrahydroxy silane, being transported on the barrier layer. Next, a purge/vacuum process is performed. Then a reactive gas having a metal is transported to form the continuous metal seed layer (CMSL) 260. The cycle step of the atomic layer deposition (ALD) 250 can be repeated to form the thickness of the continuous metal seed layer (CMSL) 260 about 20 to 40 Å. Finally, a blanket metal layer 270 is formed on the continuous metal seed layer (CMSL) 260.

[0026] As illustrated in FIG. 3A to FIG. 3C, in the second embodiment of the present invention, a semiconductor substrate 300 having a dielectric layer 310 is provided. There is a via hole 320 in the dielectric layer 310. A barrier layer 330 is formed on the dielectric layer 310, sidewall and the bottom of the via hole 320. The method for forming the barrier layer 330 comprises a metal organic chemical vapor deposition (MOCVD) and a physical vapor deposition (PVD). Then the barrier layer 330 is treated by way of a plasma process 340. Afterward, an atomic layer deposition (ALD) 350 is performed at least one time to form a continuous metal seed layer (CMSL) 360 on the barrier layer 330, wherein the cycle step of the atomic layer deposition (ALD) 350 comprises a mixing gas of hydrogen and silane, such as hydroxy silane or tetrahydroxy silane, being transported on the barrier layer. Next, a purge/vacuum process is performed to remove unnecessary matter after the mixing gas is absorbed on the barrier layer 330. Then a reactive gas having a metal is transported to form the continuous metal seed layer (CMSL) 360. The cycle step of the atomic layer deposition (ALD) 350 can be repeated to form the thickness of the continuous metal seed layer (CMSL) 360 about 20 to 40 Å. Afterward, a metal nucleation layer 370 is formed on the continuous metal seed layer (CMSL) 360. Finally, a blanket metal layer 380 is formed on the metal nucleation layer 370 by way of a chemical vapor deposition (CVD).

[0027] As illustrated in FIG. 4A and FIG. 4B, in the third embodiment of the present invention, first of all, a semiconductor substrate 400 is provided. Then a dielectric layer 410 is formed on the semiconductor substrate 400. Next, a photoresist layer 415 is formed and defined on the dielectric layer 410. An etching process is then performed by the photoresist layer 415 as an etched mask to form a via hole 420 in the dielectric layer 410. After removing the photoresist layer 415, a barrier layer 425 is formed on the dielectric layer 410, the sidewall and bottom of the via hole 420, wherein the method for forming the barrier layer 425 comprises a titanium layer 430 being conformed on the dielectric layer 410 and the sidewall and bottom of the via hole 420 by way of a depositing process. A metal organic chemical vapor deposition (MOCVD) is performed to conform a nitride titanium layer (MOCVD-TiN) 435 on the titanium layer 430, whereby Ti/MOCVD-TiN (430/435) stack the barrier layer 425. Afterward, the barrier layer 425 is treated by way of a plasma process, wherein the plasma process comprises a nitrogen plasma process.

[0028] As illustrated in FIG. 4C to FIG. 4F, in this embodiment, an atomic layer deposition (ALD) 445 is performed at least once to form a continuous tungsten seed layer (CWSL) 450 on the barrier layer 425, wherein the cycle step of the atomic layer deposition (ALD) 445 comprises a mixing gas 455 of hydrogen and silane, such as hydroxy silane or tetrahydroxy silane, being first transported on the barrier layer 425, the mixing gas 455 being absorbed on the barrier layer 425. Next, a purge/vacuum process 460 is performed to remove unnecessary matter after the mixing gas 455 is absorbed on the barrier layer 425. Then a reactive gas 465 having a tungsten, such as WF₆, is transported to form a mono-seed layer with tungsten by a mono-layer substitution reaction that reacts with the reactive gas 465 and the mixing gas 455 on the barrier layer 425. The cycle step of the atomic layer deposition (ALD) 445 can be repeated to form the thickness of the continuous metal seed layer (CMSL) 450 about 20 to 40 Å, wherein the continuous metal seed layer (CMSL) 450 gets much better step coverage that would get lower and stable resistance constant (RC) with the barrier layer 425. Further, the selectivity for forming the tungsten can be raised to about 100% . Afterward, a tungsten layer 470A is formed on the continuous tungsten seed layer (CWSL) 450. Finally, the tungsten layer 470A is etched back to form a plug tungsten layer 470B in the via hole 420.

[0029] As illustrated in FIG. 5A and FIG. 5B, in the third embodiment of the present invention, first of all, a semiconductor substrate 500 is provided. Then a dielectric layer 510 is deposited on the semiconductor substrate 500. Next, a photoresist layer 515 is formed and defined on the dielectric layer 510. An etching process is then performed by the photoresist layer 515 as an etched mask to form a via hole 520 in the dielectric layer 510. After removing the photoresist layer 515, a barrier layer 525 is formed on the dielectric layer 510 and the sidewall and bottom of the via hole 520, wherein the method for forming the barrier layer 525 comprises a titanium layer 530 being conformed on the dielectric layer 510 and the sidewall and bottom of the via hole 520 by way of using a depositing process. A metal organic chemical vapor deposition (MOCVD) is performed to conform a first nitride titanium layer (MOCVD-TiN) 535 on the titanium layer 530. A second nitride titanium layer (PVD-TiN) 540 is formed on the first nitride titanium layer (MOCVD-TiN) 535 by way of using a physical vapor deposition (PVD), whereby the Ti/MOCVD-TiN/PVD-TiN (530/535/540) stack the barrier layer 525. Afterward, the barrier layer 525 is treated by way of a plasma process, wherein the plasma process comprises a nitrogen/hydrogen plasma process.

[0030] As illustrated in FIG. 5C to FIG. 5F, in this embodiment, an atomic layer deposition (ALD) 550 is performed at least once to form a continuous tungsten seed layer (CWSL) 555 on the barrier layer 525, wherein the cycle step of the atomic layer deposition (ALD) 550 comprises a mixing gas 560 of hydrogen and silane, such as hydroxy silane or tetrahydroxy silane, being first transported on the barrier layer 525, so as to absorb the mixing gas 560 on the barrier layer 525. Next, a purge/vacuum process 565 is performed to remove unnecessary matter or the remainder after the mixing gas 565 is absorbed on the barrier layer 525. Then a reactive gas 570 with a WF₆is transported on the barrier layer 525 having the mixing gas to form a mono-seed layer with tungsten by a mono-layer substitution reaction that reacts with the reactive gas 570 and the mixing gas 560 on the barrier layer 525. The cycle step of the atomic layer deposition (ALD) 550 can be repeated to form the thickness of the continuous tungsten seed layer (CWSL) 555 about 20 to 40 Å, wherein the continuous tungsten seed layer (CWSL) 555 gets much better step coverage that would get lower and stable resistance constant (RC) with the barrier layer 525, further, the selectivity for depositing the tungsten can be raised to about 100%. Afterward, a nucleation layer 575 is formed on the continuous tungsten seed layer (CWSL) 555. Subsequently, a tungsten layer 580A is formed on the nucleation layer 575 by way of a chemical vapor deposition (CVD). Finally, the tungsten layer 580A is etched back by way of a chemical mechanical polishing process to form a plug tungsten layer 580B in the via hole 520.

[0031] In these embodiments of the present invention, as discussed above, this invention can perform an atomic layer deposition (ALD) to form a continuous metal seed layer (CMSL) on the barrier, so as to raise the selectivity for depositing the metal. Furthermore, the absorption rate of the reactive gas with the metal is greater than the same one of the mixing gas with the hydrogen/silane. If both gases are transported at the same time, the reactive gas with the metal will be absorbed first on the barrier layer. Hence, the substitution reaction is difficult to react with the reactive gas and the mixing gas and will result in discontinuous and insufficient nucleation sites. Accordingly, in this invention, firstly, the cycle step of the atomic layer deposition (ALD) transports the mixing gas, so as to absorb the mixing gas on the barrier layer, and then transports the reactive gas to proceed to the substitution reaction with the mixing gas on the barrier layer, whereby the issue of the competitive adsorption can be prevented to perform the substitution reaction completely. Therefore, this invention can provide continuous and high-density nucleation sites.

[0032] Moreover, this invention can raise the depositing selectivity in the follow-up process for depositing the metal to about 100%. A thin continuous metal seed layer (CMSL) of about 20 to 30 Å is only necessary for this invention to avoid a thick nucleation layer from forming at about 500 Å, this is due to the discontinuous and insufficient nucleation sites in the conventional process. On the other hand, the continuous metal seed layer (CMSL) produced by this invention is also able to substitute for the thick nucleation layer produced by a conventional process, and that can make a direct deposit of the metal layer thereon thereby making for a sound cap-filling process. Therefore, the present invention makes cost reductions that correspond to positive economic effect and appropriate for deep sub-micron technology.

[0033] Of course, it is possible to apply the present invention to form the tungsten layer with the chemical vapor depositing process, and to any process for forming the plug metal layer in the process of the semiconductor device. Also, this invention can be applied to form the continuous metal seed layer (CMSL) with a thin thickness by the atomic layer deposition (ALD) concerning an increase of the nucleation sites used for forming the plug tungsten layer has not been developed at present. The method of the present invention is the best process for forming the plug metal layer compatible process for a deep sub-micron process.

[0034] Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, that the present invention may be practiced other than as specifically described herein.

[0035] Although the specific embodiments have been illustrated and described, it will be obvious to those skilled in the art that various modifications may be made without departing from what is intended to be limited solely by the appended claims.