Method for fabricating a conducting electrode for semiconductor device

Abstract: Method for fabricating a semiconductor device, is disclosed, which is suitable for improving a resistivity, including the steps of forming a silicon layer on a substrate, forming a crystalline metal silicide layer on the silicon layer, forming an amorphous metal silicide layer by injecting ions into the crystalline metal silicide layer, and crystallizing the amorphous metal silicide by heat treating the amorphous metal silicide.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a method for fabricating a semiconductor device, which is suitable for improving a resistivity. Discussion of the Related Art In general, as the semiconductor devices are highly integrated, a width of wiring in the semiconductor devices is reduced, causing problems, such as drop in operation speed due to consequential increase of a resistance of the wiring. And, the formation of a thicker wiring as a wiring width is reduced used as a counter measure on the increased sheet resistivity of the wiring causes a problem that a fabrication process for the device becomes complicated with less yield because of a greater step coverage of the wiring. In order to solve these problems, a refractory metal silicide, such as tungsten silicide (WSi x ), titanium silicide(TiSi 2 ), or cobalt silicide(CoSi 2 ) is formed on a polysilicon layer in an intention for preventing an increase of resistivity (hereafter, a refractory metal silicide formed on a polysilicon layer is called "polycide"). However, though this can improve the resistivity and step coverage to a some extent, a method for forming a polycide which can improve further is in need. A conventional method for fabricating a semiconductor device will be explained with reference to the attached drawings. Figs. 1a ~ 1c illustrate sections showing process steps of a first conventional method for fabricating a semiconductor device, and Figs. 2a ~ 2c illustrate sections showing process steps of a second conventional method for fabricating a semiconductor device. A polycide fabrication process used for reducing a resistivity and a step coverage in a semiconductor device can be applied to a process for forming a gate electrode or a bitline. The process steps of the first conventional method for fabricating a semiconductor device in which the polycide fabrication process is applied to formation of a gate electrode will be explained. Referring to Fig. 1a, the process starts with deposition of a first oxide film 2 on a semiconductor substrate 1 and deposition of a polysilicon layer 3 on the first oxide film 2. The polysilicon layer 3 is doped with P type impurities and soluble in water. The doping of the polysilicon layer 3 may be done by ion injection after deposition of the polysilicon layer 3 is completed, or by deposition of POCl 3 , or by a continuous injection of a doping gas, such as PH 3 , while depositing the polysilicon layer. For removing a natural oxide film(or glass) which may remain on the polysilicon layer 3 during the process of forming the polysilicon 3 as shown in Fig. 1b, the process resultant is dipped into HF solution for cleaning. A chemical vapor deposition of SiH 4 or SiH 2 Cl 2 with tungsten hexafluoride(WF 6 ) gas is conducted to form a tungsten silicide layer 4, to form a polycide layer. As shown in Fig. 1c, the tungsten silicide layer 4, polysilicon layer 3 and the first oxide film 2 are subjected to photolithography to make an anisotropic etching of the layers 2, 3 and 4 with a mask for forming a gate electrode used, resulting to form a stack of a gate cap silicide layer 4a, a gate electrode 3a and a gate oxide film 2a. A lightly doped drain(LDD) region 5 is formed in the semiconductor substrate 1 on each side of the gate electrode 3a. A second oxide film is deposited on the entire surface and subjected to anisotropic etching to remove the second oxide film, forming sidewall insulating films 6 on both sides of the gate cap silicide layer 4a, gate electrode 3a and gate oxide film 2a. Portions of the semiconductor substrate 1 on outward sides of the sidewall insulating films 6, excluding a portion under the gate electrode 3a, are heavily doped, to form source/drain regions 7 therein. The process steps of the second conventional method for fabricating a semiconductor device in which the polycide fabricating process is applied to formation of a bitline will be explained. Referring to Fig. 2a, the process steps start with formation of an N type impurity doped layer 8 in a portion of a P type semiconductor substrate 1. A chemical vapor deposition is conducted to form an interlayer insulating layer 9 on the semiconductor substrate 1, which is selectively removed to form a contact hole 10 exposing the N type impurity doped layer 8. As shown in Fig 2b, a polysilicon layer 11 is formed on the entire surface. The polysilicon layer 11 is doped with P type impurities and soluble in water. The doping of the polysilicon layer 11 may be done by ion injection after deposition of the polysilicon layer 3 is completed, or by deposition of POCl 3 , or by a continuous injection of a doping gas, such as PH 3 , while depositing the polysilicon layer. For removing a natural oxide film(or glass) which may remain on the polysilicon layer 11 during the process of forming the polysilicon 11 as shown in Fig. 2c, the process resultant is dipped into HF solution for cleaning. A chemical vapor deposition of SiH 4 or SiH 2 Cl 2 with tungsten hexafluoride(WF 6 ) gas is conducted to form a tungsten silicide layer 12 on the polysilicon layer 11, which is subjected to selective patterning to form a bitline of the polycide layer. However, the conventional methods for fabricating a semiconductor device have the following problems. Because the gate electrode or the bitline in a semiconductor device formed by depositing tungsten silicide on a polysilicon layer according to the conventional methods shows the tungsten silicide which is not amorphous also with small granular sizes, the gate electrode or the bitline has a limitation in reducing the resisitivity of the polysilicon layer. Due to these reasons, the conventional methods for fabricating a semiconductor device cannot serve for reducing the resisitivity of the polysilicon in highly packed device to a size below 0.25 µm. JP 9-97771 shows a method comprising the steps of forming a semiconductor layer on a substrate, forming a high-melting silicide layer of WSi x that is a crystalline metal silicide layer, on the semiconductor layer, forming an amorphous metal silicide layer by implanting ions into said silicide layer to turn it amorphous; and crystallizing that amorphous silicide layer by a heat treatment. US 5,665,646 discloses a method for manufacturing a semiconductor device with low electric resistance silicide layer wherein a gate insulating film is formed on a substrate. A polysilicon layer is then formed on the insulating film. The polysilicon layer and the insulating film are patterned so as to form first and second gate electrode structures in active regions of NMOS and PMOS transistors. Thereafter, N-type impurities are ion implanted for forming source and drain regions on both sides of the gate first electrode structure, and P-type impurities are ion implanted for forming source and drain regions on both sides of the second gate electrode structure. It is known in the art (US 4,829,363) that dopent species of poly crystalline silicon attend to diffuse out from the layer into the over lying metal silicide layer during heat treatment of an electrode comprising these two adjacent layers. To reduce this diffusion problem usually an intermediate layer is deposited that acts as a diffusion inhibiting layer. Thereafter, a titanium film is deposited onto the entire surface of the resultant structure. This titanium film is converted into a titanium silicide film. Thus, a metal silicide layer or film is formed on the polysilicon gate electrodes and source and drain regions. After converting the titanium film partly into TiSi 2 those parts of the titanium film that are not converted into TiSi 2 are removed by etching. Consequently, the metal silicide layer remains only on the source and drain regions as well as on the gate electrodes. This metal silicide layer is turned amorphous by ion implanting. Thereafter heat treatment is carried out to convert the amorphous titanium silicide layers into recrystallized titanium silicide layers. US 5,338,701 discloses a method for fabrication of tungsten polycide-to-polysilicon capacitors with high linearity. Therefor, a first silicon layer is deposited on a gate oxide and field oxide regions. This polysilicon layer is doped by diffusion with phosphorous. Then, a tungsten silicide layer is deposited over doped polysilicon layer. Thereafter, the first polysilicon layer and the tungsten silicide layer are both patterned to form gate electrode structures in the active regions of an N-type and a P-type MOS transistor. SUMMARY OF THE INVENTION An object of the present invention is to provide a method for fabricating a semiconductor device which is suitable for improving a resistivity. This object is achieved by the method according to claim 1. BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention: In the drawings: Figs. 1a ~ 1c illustrate sections showing process steps of a first conventional method for fabricating a semiconductor device; Figs. 2a ~ 2c illustrate sections showing process steps of a second conventional method for fabricating a semiconductor device; Figs. 3a ~ 3c illustrate sections showing process steps of a method for fabricating a semiconductor device; Figs. 4a ~ 4f illustrate sections showing process steps of a method for fabricating a semiconductor device in accordance with a preferred embodiment of the present invention; Fig. 5 illustrates a graph showing an amorphous property of the doped tungsten silicide layer; Fig. 6 illustrates a graph showing ion dose vs. resistivity of the tungsten silicide layer; Fig. 7 illustrates a graph showing ion dose vs. resistivity of the tungsten silicide layer after subjecting to heat treatment at 900 deg. C for 30 minutes; Fig. 8 illustrates a graph showing line width vs. resistivity of the tungsten silicide layers of different thicknesses when P ion is used as dopant; and, Fig. 9 illustrates a graph showing resistivity of a gate electrode of a 0 25 µm line width formed under conditions identical to Fig. 8 DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. Figs. 3a ~ 3c illustrate sections showing process steps of a method for fabricating a semiconductor device in which a refractory metal silicide is formed and Figs. 4a - 4f illustrate sections showing process steps of a method for fabricating a semiconductor device in accordance with a preferred embodiment of the present invention. A polycide fabricating process used for reducing a resistivity of a semiconductor device can be applied to formation of a gate electrode or a bitline. An example in which the polycide fabricating process is applied to formation of a gate electrode will be explained. Referring to Fig. 3a, the process steps of the method for fabricating a semiconductor device starts with depositing a first oxide film 21 on a semiconductor substrate 20, on which a doped polysilicon layer 22 is deposited to a thickness of 1000 Å at 660 deg. C and 80 Torr using an ICT(Integrated Cluster Tool), thereafter. Then, an undoped polysilicon layer 23 is deposited on the doped polysilicon layer 22 to a thickness about 200 Å. In the deposition of the doped polysilicon layer 22, H 2 mixed with 50 % SiH 4 and 1% PH 3 is used as a source gas. A tungsten silicide layer 24 is formed on the undoped polysilicon layer 23 using WF 6 and SiH 2 Cl 2 or WF 6 and SiH 4 as source gases. The doped polysilicon layer 22 and the undoped polysilicon layer 23 are layered under the tungsten silicide layer 24, for preventing loss of ions injected into the tungsten silicide layer at converting the tungsten silicide layer 24 into an amorphous tungsten silicide layer by injecting the ions thereto at a later step. The doped polysilicon layer 22 may be deposited to a thickness of about 800 Å, the undoped polysilicon layer 23 may be deposited to a thickness of about 200 Å. and the tungsten silicide layer 24 may be deposited to a thickness of about 1000 Å. Or, alternatively, the doped polysilicon layer 22 may be deposited to a thickness of about 300 Å, the undoped polysilicon layer 23 may be deposited to a thickness of about 200 Å, and the tungsten silicide layer 24 may be deposited to a thickness of about 2000 Å. That is, the tungsten silicide layer 24 may be formed to have a thickness in a range of 1000 Å ~ 2000 Å. As shown in Fig. 3b, N type phosphorus ions are injected into the tungsten silicide layer 24 at an energy of 50 KeV to a dose of 1x10 15 ~ 8x10 15 cm -3 , to convert the tungsten silicide layer 24 into amorphous. In this instance, the energy injecting the phosphorus ions may be 80 KeV. Heat treatment is conducted at about 900 deg. C for approx. 30 minutes to recrystallize the amorphous tungsten silicide layer 24, to form a large grain sized tungsten silicide 24a. The heat treatment may be conducted at a temperature in a range of 400 ~ 1100 deg. C. In this instance, instead of the N type phosphorus ions, P type arsenic ions(As) or boron(B) ions may be injected at 70 KeV or 20 KeV respectively to a dose of 1x10 15 ~ 8x10 15 cm -3 , to form the large grain sized tungsten silicide 24a through the heat treatment. As shown in Fig. 3c, in order to form a gate electrode, a mask is used in an anisotropic etching the stack of the first oxide film, doped polysilicon layer 22, undoped polysilicon layer 23, large grain sized tungsten silicide layer 24a, to form a gate cap silicide layer 24b, the gate electrode 22a and 23a and gate oxide film 21 a. LDD(Lightly Doped Drain) regions 26 are formed in the semiconductor substrate 20 on both sides of the gate electrode 22a and 23a, on the entire surface of which a second oxide film is deposited and subjected to anisotropic etching to remove the second oxide film selectively, to form sidewall spacers 25 at sides of the gate cap silicide layer 25a, the gate electrode 21a and 22a and the gate oxide film 21a. Then, portions of the semiconductor substrate 21 on both sides of the sidewall spacers 25 excluding the gate electrode 22a and 23a are heavily doped to form source/drain regions 27. The tungsten silicide layer 24 may be deposited after deposition and HF cleaning of the doped polysilicon layer. Amorphous phases of the 1000 Å thick tungsten silicide layer 24 when it is ion injected at a dose of 5×10 15 cm -3 will be explained. Fig. 5 illustrates plots of XRD(X-ray diffraction) analysis data of the tungsten silicide layer 24 when different ions are injected thereinto, wherein (a) is in the case of no ion injection, (b) is in the case of phosphorus(P) ion injection (c) is in the case of boron(B) ion injection, (d) is in the case of arsenic ion injection, and (e) is argon ion injection, respectively. From the fact that X-rays appear more intensive when 2Θ(angle) of the tungsten silicide layer are 30 deg. and approx. 40 deg., it can be known that the X-rays are diffracted at those angles where the tungsten silicide has a crystal orientation of (111), which implies that the tungsten silicide layer is crystalline. Further, it can be known that the tungsten silicide layers 24 have been completely converted into amorphous phases in the cases of argon(Ar) and phosphorus(P), but the tungsten silicide layers 24 have not been completely converted into amorphous phases in the cases of boron(B) and arsenic(As). And, as shown in Fig. 6, when the dose is made differently, resistivity of the tungsten silicide layer 24 is sharply reduced in all the cases if the dose is over 1x10 15 cm -3 , and even in the cases when the dose is kept increased, the resistivity is reduced gradually, which implies that almost all portions of the tungsten silicide layer 24 is converted into amorphous phase at a dose of 1x10 15 cm -3 . As shown in Fig. 7, dose vs. resistivity of the tungsten silicide layer 24, doped and heat treated at 900 deg. C for 30 minutes, shows decrease of the resistivity as the dose increases in the cases of arsenic (As), boron(B), and phosphorus(P) ions, exhibiting the maximum extent of reduction at an injection energy of 80 KeV in the case of phosphorus(P) ions, but formation of a compound in the cases of arsenic (As) and boron(B) ion injections. On the other hand, in the case of argon ion injection, the resistivity is increased as the dose is increased because the argon(Ar), an inactive gas, is decomposed in the process of the heat treatment, to form voids therein. From examination of these experimental data, it can be known that phosphorus(P) ion injection at a dose over 5x10 15 cm -3 is preferable. Next, as shown in Figs. 8 and 9, if resistivities of the gate electrode 22a and 23a are compared in the cases of the tungsten silicide layer which is deposited to an 1000 Å thickness and injected of phosphorus(P) ions at 50 KeV, and deposited to an 1500 Å thickness and injected of phosphorus(P) ions at 80 KeV, and in the case when a minimum line width of the gate electrode 22a and 23a is set to be 0.25 µm while the other conditions are remained the same as the above, it can be known that the resistivity in the case of the tungsten silicide deposited to an 1500 Å thickness and injected of phosphorus(P) ions at 80 KeV is the lowest. A method for fabricating a semiconductor device in accordance with the present invention will be explained. An amorphous tungsten silicide layer formed in accordance with the method described above is used in formation of a CMOS transistor having a dual gate. Referring to Fig. 4a, the method for fabricating a semiconductor device in accordance with the second embodiment of the present invention starts with depositing a first oxide film 31 on a semiconductor substrate 30 to a thickness of 50 Å, on which undoped polysilicon layer 32 is deposited to a thickness of about 1000 Å. As shown in Fig. 4b, a first photoresist film 33 is coated on the undoped polysilicon layer 32 and subjected to selective patterning to expose a portion of the undoped polysilicon layer 32 in which an NMOS transistor is to be formed. The patterned photoresist film 33 is used as a mask in injecting N type phosphorus(P) ions into the portion of the undoped polysilicon layer 32, to form an N type polysilicon layer 32a. As shown in Fig. 4c, the first photoresist film 33 is removed, and a second photoresist film 34 is coated and subjected to exposure and development for selective patterning of the second photoresist film 34, to expose a portion of the undoped polysilicon layer 32 at a side of the N type polysilicon layer 32a in which a PMOS transistor is to be formed. P type arsenic(As) or boron(B) ions are injected into the exposed portion of the undoped polysilicon layer 32, to form a P type polysilicon layer 32b. Then, the second photoresist film 34 is removed. As shown in Fig. 4d, a titanium nitride(TiN) is deposited both on the N type polysilicon layer 32a and the P type polysilicon layer 32b to a thickness of about 100 Å, to form a diffusion preventing film 35. In the deposition of the 100 Å titanium nitride, a 50 Å of titanium nitride is deposited after deposition of a 50 Å of titanium nitride so that grain boundaries of the two films are not matched, to improve a diffusion preventing capability. Instead of the titanium nitride(TiN), tungsten nitride(WNx), tungsten silicon nitride(WSiN), tantalum silicon nitride(TaSiN), or the like may be deposited as the diffusion preventing film 35. Then, a tungsten silicide layer 36 is formed on the diffusion preventing film 35 to a thickness ranging 1000 Å ~ 2000 Å using WF 6 and SiH 2 Cl 2 or WF 6 and SiH 4 as source gases. The tungsten silicide layer 36 is formed after formation of the diffusion preventing film 35, for forming the dual gate which will not give influences to the dual gated CMOS device having a gate cap silicide layer 36b formed with the amorphous tungsten silicide layer 36 in a later step. As shown in Fig. 4e, N type phosphorus(P) ions are injected into the tungsten silicide layer 36 at an energy of 80 KeV to a dose in a range of 1x10 15 ~ 8x10 15 cm -3 , and, preferably, of more than 5x10 15 cm -3 , to convert the tungsten silicide into amorphous, which is subjected to heat treatment at about 900 deg. C for about 30 minutes, to recrystallize the tungsten silicide to form a large grain sized tungsten silicide layer 36a In this instance, the heat treatment may be conducted at a temperature in a range of 400 ~ 1100 deg. C The large grain sized tungsten silicide layer 36a may be formed through a heat treatment process after injecting P type arsenic(As) or boron(B) ions at 70 KeV or 20 KeV respectively to the same doses as the above. As shown in Fig. 4f, a mask for forming a gate electrode of an NMOS transistor is used in an anisotropic etching of a stack of the first oxide film 31, the N type polysilicon layer 32a, and the large grain sized tungsten silicide layer 36a, to form a gate cap silicide layer 36b, a first gate electrode 32c and a gate oxide film 31a. And, a mask for forming a gate electrode of a PMOS transistor is used in an anisotropic etching of a stack of the first oxide film 31, the P type polysilicon layer 32b, and the large grain sized tungsten silicide layer 36a, to form a gate cap silicide layer 36b, a second gate electrode 32d and a gate oxide film 31a. The tungsten silicide layer may be replaced with a titanium silicide layer or a tantalum silicide layer, which are refractory metal silicides. In the case of formation of the titanium silicide layer, TiCl 4 , TiI 2 , SiH 4 , or SiH 2 Cl 2 may be used as a source gas to form TiSi2, and, in the case of formation of the tantalum silicide layer, TaCl 5 , SiH 4 , or SiH 2 Cl 2 may be used as a source gas to form TaSi 2 . The method for fabricating a semiconductor device of the present invention as explained has the following advantages. First, the reduction in a resistivity of a gate electrode by forming a large grain sized silicide layer on the gate electrode by means of recrystallization allows an improvement of device performance Second, the diffusion preventing film formed on N type and P type doped polysilicon layers permits to improve a reliability of a dual gate device. It will be apparent to those skilled in the art that various modifications and variations can be made in the method for fabricating a semiconductor device of the present invention without departing from the scope of the invention as claimed.

Claims: A Method for fabricating a dual gate structure of a CMOS transistor, comprising the steps of: depositing an insulating film (31) on a substrate (30); forming an undoped polysilicon layer (32) on said insulating film (31); injecting ions into a first portion of said polysilicon layer (32) to form a first conductive type polysilicon layer (32a) in a first region thereof, in which an N-MOS transistor is to be formed injecting ions into a second portion of said polysilicon layer (32) to form a second conductive type polysilicon layer (32b) in a second region thereof, in which a P-MOS transistor is to be formed forming a diffusion preventing film (35) on said first and said second conductive type polysilicon layers (32a, 32b); forming a crystalline metal silicide layer (36) on said diffusion preventing film (35); forming an amorphous metal silicide layer (36a) by injecting ions into said crystalline metal silicide layer (36) to convert said crystalline metal silicide layer (36) into said amorphous metal silicide layer (36a); subjecting said amorphous metal silicide layer (36a) to heat treatment to form a re-crystallized metal silicide layer (36a); and patterning said re-crystallized metal silicide layer (36b), said diffusion preventing film (35), as well as said first and said second conductive type polysilicon layers (32a, 32b) to form a dual gate electrode structure of a CMOS transistor. A method as claimed in claim 1, wherein the diffusion preventing film is formed of titanium nitride (TiN), tungsten nitride (WNx), tungsten silicon nitride (WSiN), or tantalum silicon nitride (TaSiN). A method as claimed in claim 1, wherein the crystalline metal silicide layer is formed of tungsten (W), titanium (Ti), or tantalum (Ta). A method as claimed in claim 1, wherein the crystalline metal silicide layer is converted to be amorphous by injecting ions of phosphorus (P), boron (B), or arsenic (As). A method as claimed in claim 4, wherein a dose of the ions of phosphorus (P), boron (B), or arsenic (As) is in a range of 1E15 ~ 8E15 cm -3 . A method as claimed in claim 1, wherein the crystalline metal silicide layer is formed to a thickness ranging 100 nm - 200 nm. A method as claimed in claim 3, wherein the crystalline metal silicide layer is formed of tungsten using WF 6 and SiH 4 , or WF 6 and SiH 2 Cl 2 as source gases. A method as claimed in claim 1, wherein the heat treating of the amorphous metal silicide is conducted at a temperature ranging 400 ~ 1100°C. A method as claimed in claim 1, wherein the diffusion preventing film is formed to a thickness in the order of 10 nm. A method as claimed in claim 1, wherein the diffusion preventing film is formed by two times of depositions to a thickness of 5 nm in each of the depositions for improving a diffusion preventing property.

Inventors: BYUN JEONG SOO, LEE BYUNG HAK, BYUN, JEONG SOO, LEE, BYUNG HAK
Applicants: LG SEMICON CO LTD, LG SEMICON CO., LTD.
Application year: 1998
Main classification: H01L21
IPC codes: H01L21, H01L27