Method for forming an epitaxial cobalt silicide layer on MOS devices

[0003] The present invention generally relates to a process for producing integrated circuit devices, and more particularly to a method for forming an epitaxial cobalt silicide layer on MOS devices.

[0004] Cobalt silicide (CoSi₂) has been widely accepted as an electrical contact and interconnect material for use in forming MOS devices. Significant reduction in parasitic electrical resistance is achieved with the formation of a CoSi₂layer, which therefore enhances drive-current and speed of the devices. One commonly known method of forming a CoSi₂layer is a self-aligned silicide (salicide) process, in which cobalt is deposited on a MOS structure, and reacts with the exposed silicon and polysilicon of the MOS structure in a heated environment to form a silicide layer. The unreacted metal is then removed from the MOS structure by using a selective etching process, leaving the silicide over the gate and the source/drain region.

[0005] As devices are scaled to sub-0.18 μm and beyond, the requirements for salicide process become more stringent. In order for devices to operate in the deep sub-micron regime, shallow source/drain junctions are incorporated to prevent junction “punch-through” and reduce the effect of drain-induced barrier lowering (DIBL). The presence of non-uniformity or interface roughness at the silicide-silicon interface is detrimental to the integrity of shallow junctions, and it could result in excessive junction leakage current. As a result, forming epitaxial CoSi₂has become of critical importance, especially for deep sub-micron devices.

[0006] One known method of forming epitaxial CoSi₂involves the use of a thin metal interlayer between the cobalt layer and silicon substrate. Another method includes depositing a thin titanium nitride or titanium tungsten film as a capping-layer on top of the cobalt layer. Both of these methods require subjecting their respective structures to heat treatment processes to form cobalt silicide. These known methods require additional processing steps and/or an additional sputtering chamber and target for depositing the interlayer or the capping-layer. Moreover, extra cleaning steps are also required to remove the additional interlayer and capping-layer upon silicide formation. Consequently, these methods further result in increased process cycle time due to the additional processing steps involved.

[0007] The present invention is directed to a method for forming an epitaxial cobalt silicide layer on a MOS device. The method includes the step of sputter depositing cobalt in an ambient, forming a first layer of cobalt silicide on gate and the source/drain regions of the MOS device. Subsequently, cobalt may be sputter deposited again in an ambient of argon to increase the thickness of the cobalt silicide layer to a second thickness.

[0008] FIGS. 1-3 are sectional views illustrating the steps for forming a cobalt silicide layer on a MOS device in accordance with one embodiment of the present invention.

[0009] Turning now to FIG. 1, a MOS device 10 includes a Si substrate 14, a gate electrode 16, and source/drain junctions 18 on both sides of the gate. Also provided on the substrate 14, adjacent to each of the source/drain junctions 18 are shallow trench isolations (STI) 20 that provide electrical isolations from neighboring devices. Spacers 22 are located adjacent both sides 24 of the gate electrode 16 to electrically isolate the gate from the source/drain junctions 18. The gate electrode 16 is formed by depositing a layer of polysilicon 25 over a thin layer of gate oxide 26 in a known manner. The source/drain regions 18 are also created in a known manner using ion-implantation of arsenic or boron ions, for example. The shallow trench isolations (STI) 20 are formed by etching the Si substrate 14, followed by oxide deposition and chemical-mechanical polishing (CMP) processes.

[0010] Turning now to FIG. 2, the first cobalt layer is sputter-deposited (indicated by arrows) in an ambient, preferably of an argon-nitrogen sputtering-gas mixture, onto the surface of the MOS device 10, including the gate electrode 16, the source/drain regions 18 and the STI 20, to form a layer 28 of nitrogen-doped cobalt silicide (CoN_xSi_y). The first thickness of the CoN_xSi_ylayer 28 can be from approximately 5 Å to about 100 Å, depending on the second thickness of CoSi₂required. Preferably, the first thickness of CoN_xSi_ylayer is at about 10 Å, after which about 100 Å of Co is deposited. The deposition of this first layer 28 can also be performed using a sputtering gas mixture of different combinations and concentration of inert gas (including argon, helium, neon, krypton, xenon) with, for example, nitrogen, oxygen, ammonia (NH₃) and carbon gas source (for example, CO₂, CH₄, C₂H₆). In one embodiment, an inert gas mixture of about 50% nitrogen and a remaining argon gas mixture may be implemented.

[0011] Turning now to FIG. 3, upon formation of the first CoN_xSi_ylayer 28 with the desired thickness, subsequent sputter deposition of cobalt can be performed in an ambient of argon within the same chamber without breaking vacuum. Optionally, there may be a break in time after the first layer is formed and before the start of the subsequent sputter deposition. Cobalt deposition is performed until the thickness of the cobalt silicide layer 28 increases to a second thickness of approximately 50 Å to approximately 1000 Å. Preferably, the second thickness of the CoSi₂is approximately 300 Å.

[0012] The growth of the cobalt silicide layer 28 during both the first and second formations is performed using known processes such as conventional solid phase epitaxy (SPE) or reactive deposition epitaxy (RDE). For RDE growth, the substrates are heated and maintained at a temperature ranging from 400 ° C. to 900° C. during Co deposition. For samples grown by SPE, an annealing step is carried out after Co deposition at temperatures ranging from 400° C. to 900° C. Pressure during deposition is about 20 mTorr. However, the pressure can vary from 5 mTorr to 100 mTorr. The deposition rate of Co is about 1 Å/s, but ranges from approximately 0.005 Å/s to 100 Å/s.

[0013] From the foregoing description, it should be understood that an improved method for forming an epitaxial CoSi₂layer on MOS devices has been shown and described which has many desirable attributes and advantages. The present method enables the formation of epitaxial CoSi₂with high quality silicide/silicon interface, which minimizes junction leakage. Moreover, the method in accordance with an embodiment of the present invention is fully compatible with the standard CMOS processing techniques without the requirement of adding additional processing steps.

[0014] While various embodiments of the present invention have been shown and described, it should be understood that other modifications, substitutions and alternatives are apparent to one of ordinary skill in the art. Such modifications, substitutions and alternatives can be made without departing from the spirit and scope of the invention, which should be determined from the appended claims.

[0015] Various features of the invention are set forth in the appended claims.