Method to form MOSFET with an elevated source/drain for PMOSFET

[0002] This is a continuation-in-part application of an application filed under the title of “ELEVATED SOURCE/DRAIN MOSFET WITH SOLID PHASE DIFFUSED SOURCE/DRAIN EXTENSION” having a Ser. No. 09/033,526 filed at Mar. 2, 1998, now U.S. Pat. No. 5,979,521, which is assigned to the same assignee and having the same inventor as he present application.

[0003] The present invention relates to the manufacturing of transistors in the semiconductor fabrication, and more specifically, to a method of forming a metal oxide semiconductor field effect transistor (MOSFET) having an elevated source/drain for PMOSFETs (p-type MOSFETs).

[0004] From the first invention of integrated circuits in 1960, the number of devices on a chip has grown at an explosively increasing rate. The technologies of the semiconductor industry has been researched continuously for almost four decades. The progress of the semiconductor integrated circuits has stepped into the ULSI (ultra large scale integration) level or even a higher level. The capacity of a single semiconductor chip increased from several thousand devices to hundreds of million devices, or even to billions of devices. The integrated circuit devices like the transistors, the capacitors, and the connections must be greatly narrowed simultaneously. The increasing packing density of the integrated circuits has generated numerous challenges to the semiconductor manufacturing process. Every device needs to be formed within a smaller size without damaging the characteristics and operations. The demands of high packing density, low heat generation, and low power consumption devices with good reliability and long operation life must be maintained without any degradation in their functions. These achievements are expected to be reached with the five key aspects of the semiconductor manufacturing, including photography, etching, deposition, ion implantation, and thermal processing technologies. The continuous increase in the packing density of the integration circuits must be accompanied with a smaller feature size. With the present semiconductor manufacturing technology, the processes with generally a quarter micrometer in size is widely utilized. For making the next generation devices, the technologies focusing mainly on one-tenth micrometer and even nanometer feature sizes are highly required.

[0005] Transistors, or more particularly the metal oxide semiconductor field effect transistors (MOSFET), are the most important and frequently employed devices. The MOSFET is widely employed in the integrated circuits with its high performance. However, with the continuous narrowing of device size, sub-micron scale MOS transistors have to face many risky challenges. As MOS transistors become narrower and thinner accompanied by shorter channels, problems like junction punchthrough, leakage, and contact resistance cause the reduction in the yield and the reliability of the semiconductor manufacturing processes.

[0006] For developing future MOS devices with a sub-micrometer or even smaller feature size, the ultra shallow junctions are required to suppress the short channel effects encountered with the down scaling sizes. On the other hand, new challenges arise with a narrowed size. The preparation of an extremely shallow source/drain junction is much harder. The conventional ion implantation process is unable to form a shallow junction with high dopant concentration. In the work by K. Takeuchi et al. (“High Performance Sub-tenth Micron CMOS using Advanced Boron Doping and WSi₂Dual Gate Process”, 1995 Symposium on VLSI technology Digest of Technical Papers), the problem is addressed. The ion implantation is hard to form shallow and high concentration source/drain. The defect-induced anomalous diffusion of boron in the channel region becomes a problem. Local boron depletion near the source/drain junctions will directly enhance short channel effects. A CMOS fabrication method is also disclosed in their work.

[0007] In addition, a device degradation problem is found to come from the boron penetration into the thin gate oxide with the formation of a doped polysilicon gate. S. L. Wu (the inventor of the present invention), C. L. Lee, and T. F. Lai consider the problem in their work “Suppression of Boron Penetration into an Ultra-Thin Gate Oxide (≦7 nm) by Using a Stacked-Amorphous-Silicon (SAS) Film” (IEDM 93329 1993 IEEE). The p+ polysilicon has been widely used as the gate material of pMOSFET to avoid short-channel effects. The BF₂-implant is typically used in forming both the gate and the junction. However, the F-incorporation will enhance the boron penetration through the thin gate oxide into the silicon substrate. The penetration also results in a large threshold voltage shift. An SAS gate structure is proposed to suppress the F-incorporation-induced boron penetration effect in their work.

[0008] A method of forming a metal oxide semiconductor field effect transistor (MOSFET) with an elevated source/drain is provided in the present invention. The short channel effects can be suppressed with the elevated junction. The boron penetration problem is minimized by the protection of a dielectric layer. An extended ultra-shallow source/drain junction is formed by using a spacer structure as a diffusion source. The effects accompanied by the small feature size devices are eliminated by the extended ultra-shallow junction.

[0009] The method of the present invention in forming a transistor, more specifically a MOSFET, on a semiconductor substrate includes the following steps. A gate insulator layer is formed over the semiconductor substrate and a first silicon layer is then formed over the gate insulator layer. An anti-reflection layer is formed over the first silicon layer. A gate region is defined by removing a portion of the gate insulator layer, of the first silicon layer, and of the anti-reflection layer. A portion of the gate insulator layer is removed to have undercut spaces under the first silicon layer. A dielectric layer is then formed on the semiconductor substrate, on the sidewalls of the gate region, and within the undercut spaces. A spacer structure containing first type dopants is then formed on the gate region. Following the removal of the anti-reflection layer, a second silicon layer containing second type dopants is formed over the semiconductor substrate and the first silicon layer. Finally, a thermal process is performed to the semiconductor substrate for diffusing the first type dopants and the second type dopants into the semiconductor substrate.

[0010] The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated and better understood by referencing the following detailed description, when taken in conjunction with the accompanying drawings, wherein:

[0011] FIG. 1 illustrates a cross sectional view of the formation of a gate insulator layer, a first silicon layer, and an anti-reflection layer over the semiconductor substrate in the present invention.

[0012] FIG. 2 illustrates a cross-sectional view of defining a gate region over the semiconductor substrate in the present invention.

[0013] FIG. 3 illustrates a cross-sectional view of etching undercut spaces, and forming a dielectric layer on the semiconductor substrate, on the sidewalls of the gate region, and within the undercut spaces in the present invention.

[0014] FIG. 4 illustrates a cross-sectional view of the formation of a spacer structure on the gate region in the present invention.

[0015] FIG. 5 illustrates a cross-sectional view of forming a second silicon layer over the semiconductor substrate and the first silicon layer in the present invention.

[0016] FIG. 6 illustrates a cross-sectional view of performing a thermal process to diffuse the dopants into the semiconductor substrate in the present invention.

[0017] A method of forming a metal oxide semiconductor field effect transistor (MOSFET) having an elevated source/drain is provided in the present invention. The elevated source/drain junction is formed through the deposition of a second silicon layer as a diffusion source. The short channel effects is suppressed by using the elevated junction and an additional dielectric layer under the gate region. A stacked silicon layer structure is utilized as the gate structure. The boron penetration problem is minimized with the protection of the dielectric layer. An extended ultra-shallow source/drain junction is formed by using a spacer structure as a diffusion source. The hot electron effect is eliminated by the extended ultra-shallow junction.

[0018] The method and the steps in the present invention applied on a semiconductor wafer can create different types of transistors and numerous devices at a time. For a clear illustration, the steps for forming mainly a single PMOS transistor are illustrated. Since the variations in the processes for incorporating the formation of the other types of transistors are well known in the art, the details are not described. Referring to FIG. 1, the same semiconductor substrate 10 with a preferable single crystalline silicon in a <100> direction is provided. An isolation region using field oxide isolation 12 or other isolation technology like trench isolation (not shown) is formed on the semiconductor substrate 10. A gate insulator layer 14, like an oxide layer, is formed. The oxide layer 14 can be grown thermally in an oxygen containing ambient. The oxide layer 14 is grown from the semiconductor substrate 10 with a thickness ranging from about 15 angstroms to 300 angstroms.

[0019] A first silicon layer 16 is then formed over the oxide layer 14. Preferably, an undoped polysilicon layer can be deposited as the first silicon layer 16. The undoped polysilicon layer 16 can be formed by chemical vapor deposition with a thickness of about 300 angstroms to 2000 angstroms. The forming of the undoped polysilicon layer 16 subsequently over the oxide layer 14 can eliminate the problem of the oxide degradation from the penetration of ions. An anti-reflection layer 18 is formed over the undoped polysilicon layer. The anti-reflection layer 18 effectively reduces the unwanted reflections in the lithography process and increases the accuracy of the pattern transfer. One choice of the anti-reflection layer 18 is a nitride layer formed by a chemical vapor deposition process. The anti-reflection characteristic of the nitride layer 18 is nominated in the work of T. P. Ong et al. (“CVD SiNx Anti-reflective Coating for Sub-0.5 μm Lithography”, 1995 Symposium on VLSI Technology Digest of Technical Papers”) The integration of a LPCVD (low pressure chemical vapor deposition) silicon-rich nitride film as a bottom anti-reflective coating (BARC) is recommended in their work. Their integration has been successfully demonstrated for deep-UV lithography with the details set forth in the article.

[0020] A gate region is formed by a series of steps to remove a portion of the oxide layer 14, of the undoped polysilicon layer 16, and of the nitride layer 18, as shown in FIG. 2. A lithography and an etching process can be applied in doing the work. A photoresist layer 20 is formed and patterned for defining a gate pattern. The oxide layer 14, the undoped polysilicon layer 16, and the nitride layer 18 can be sequentially removed with a reactive ion etching (RIE) under the generation of plasma, using the gate pattern as a mask. The photoresist layer 20 is then removed after the gate region is defined.

[0021] Next, another portion of the oxide layer 14 is removed, preferably with a wet etching using buffered oxide etchant (BOE) or diluted hydrofluoric acid, to have undercut spaces under the undoped polysilicon layer 16, as shown in FIG. 3. A dielectric layer 22 is then formed on the semiconductor substrate 10, on sidewalls of such gate region, and within the undercut spaces. In the case, an oxynitride film thermally grown from the semiconductor substrate 10 and the undoped polysilicon layer 16 in a nitrogen and oxygen-containing gas ambient, like a N₂O or NO ambient, is used as the dielectric layer 22. The oxynitride film 22 is also grown on the sidewalls of the oxide layer 14 by the diffusion of silicon through the oxide layer 14 from the semiconductor substrate 10. The thin oxynitride film can be grown with a ultra-thin thickness between about 5 angstroms to 100 angstroms.

[0022] The formation of the oxynitride film 22 by consuming surface silicon material recovers the etching damage on the surface of the semiconductor substrate 10. The dielectric layer 22 covering the sidewalls of the oxide layer 14 can further reduce the penetration of undesired contamination into the oxide layer 14 from a later-formed doped spacers. The degradation of the oxide layer 14 which damages the device performance can be minimized. Furthermore, by replacing a portion of the gate oxide layer with an oxynitride film having better dielectric characteristics, the operation of the MOSFET is further raised by reduced short channel effect and increased reliability of the oxynitride film.

[0023] Referring to FIG. 4, a series of steps can be employed in forming a spacer structure 24 on the gate region. The spacer structure 24 is formed with first type dopants contained within. In the present embodiment, a boron containing oxide or a boron-doped glass (BSG) can be used as the spacer structure 24. A BSG layer is deposited and etched off to form the BSG spacers. The doping concentration in the BSG spacers 24 is about 5E19 atoms/cm³to about 5E22 atoms/cm³. A portion of the oxynitride film 22 uncovered by the BSG spacers 24 is also removed during the etching process.

[0024] Following the formation of the BSG spacers 24, the nitride layer 18 within the BSG spacers 24 is removed, or etched off. Referring to FIG. 5, a second silicon layer 26 is formed over the semiconductor substrate 10 and the undoped polysilicon layer 16, with second type dopants contained within. Preferably, a boron-doped silicon layer is employed. The boron-doped silicon layer 26 can be formed in a chemical vapor deposition (CVD) with the introducing of boron containing gases. Preferably, a UHV (ultra high vacuum) CVD process is employed. The doping concentration in the doped silicon layer 26 is about 5E19 atoms/cm³to about 5E22 atoms/cm³.

[0025] A portion of the boron-doped silicon layer 26 within the spacer structure 24 is combined with the undoped polysilicon layer 16 to form a stacked gate structure. The conductivity of the gate structure can be maintained with the boron doped silicon layer 26. The degradation of the oxide layer 14 can be prevented by eliminating the possible boron penetration. Finally, a thermal process is performed to the semiconductor substrate. The boron dopants in the boron-doped silicon layer 26 and the BSG spacers 24 are diffused to form respectively an ultra-shallow source/drain junction 28 and an extended source/drain junction 30, as shown in FIG. 6.

[0026] The details of the silicon-boron layer as a solid diffusion source can be acquired by referencing the investigation of T. F. Lei et al. (“Low-Temperature Growth of Silicon-Boron Layer as Solid Diffusion Source for Polysilicon Contacted p⁺-n Shallow Junction”, IEEE Transactions on Electron Devices, Vol. 42, No. 12, 1995) In this work, it is found that the silicon-boron source diode has a much shallower junction and is less sensitive to thermal budget than the BF2⁺ source diodes. A uniform ultra-shallow p⁺-n junction can be obtained by using a thin silicon-boron layer as a diffusion source in their investigation.

[0027] A MOSFET having an elevated source/drain and solid phase diffused source/drain extension is formed by the method provided in the present invention. The elevated source/drain junction is formed with ultra-shallow dimension and uniform size by employing the boron doped silicon layer as a diffusion source. The short channel effect is greatly suppressed by using the elevated shallow junction and the addition of the oxynitride film as a portion of the gate insulation. A stacked silicon layer structure is utilized as the gate structure to solve the problem of the degradation of the gate oxide. Thus the operational characteristics of the devices can be increased. The boron penetration problem is minimized with the protection of the dielectric layer. An extended ultra-shallow source/drain junction is formed by using the BSG spacers as a diffusion source. The unwanted effects accompanied by the narrowed channel region of the small size devices are eliminated by the extended ultra-shallow junction.

[0028] As is understood by a person skilled in the art, the foregoing descriptions of the preferred embodiment of the present invention is an illustration of the present invention rather than a limitation thereon. It is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims. The scope of the claims should be accorded to the broadest interpretation so as to encompass all such modifications and similar structures. While the preferred embodiment of the invention has been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention.