Method for inhibiting dopant out-diffusion

Abstract: A method for inhibiting out-diffusion of dopants from polycrystalline or single crystal silicon substrates (12) of high speed semiconductor devices into metal silicide conductive layers disposed on the substrate comprises interposing a refractory metal nitride layer (18) between the doped silicon substrate (12) and the refractory metal silicide conductive layer (20). Dopant out-diffusion is further retarded, and con­ tact resistance lowered, by adding a thin layer (16) of refrac­ tory metal between the refractory metal nitride layer (18) and the silicon substrate (12).

Description: Background of the Invention The present invention relates generally to processes for fabrication of semiconductor integrated circuit devices and more particularly to a process for inhibiting out-diffusion of dopants from polycrystalline or single crystal silicon substrates. The use of advanced semiconductor processing techniques, for example improved etching and photolithographic procedures, has enabled the reduction in the dimensions of semiconductor devices and a concomitant increase in their operating speed. However, this reduction in dimensions has caused a corresponding decrease in the cross-sectional area of interconnect regions which in turn has caused an increase in interconnection time delay resulting from material and circuit parameters. One solution to this increase in interconnection time delay is set forth in U.S. Patent No. 4,180,596, issued December 25, 1979 to Crowder et al. This solution comprises a method of placing a metal silicide layer on top of a doped polycrystalline silicon substrate in order to lower the sheet resistance of the polycrystalline silicon interconnections and gain increased circuit speed. Although the addition of the metal silicide layer on top of the polycrystalline layer lowers sheet resistance causing an increase in circuit speed, the dopants introduced into the polycrystalline silicon substrate tends to diffuse out from the substrate into the metal silicide layer during subsequent annealing and/or oxidation of the device. This causes the underlying polycrystalline silicon layer to become highly resistive; which, in turn makes the contact properties uncontrollable and creates undesirable effects on the operation of a device. A deleterious effect of this out-diffusion of the dopant from the polycrystalline silicon substrate into the metal silicide layer is shown in Figure 1. As diagrammatically depicted in Figure 1A, a conduction layer 100 of a refractory metal silicide is formed by, for example, by co-depositing an amorphous layer of tantalum and silicon on a polycrystalline silicon (poly Si) substrate 102 which has been doped with boron in this example. After annealing at 950°C for one hour in hydrogen, the conductive layer 100 is converted into tantalum disilicide (Ta Si₂). However, as diagrammatically represented by region 104 in Figure 1B, boron depletion occurs at the tantalum disilicide/poly Si interface. As the boron diffuses into the tantalum disilicide layer 100, the interfacial region goes from p+ (represented diagrammatically by region 106 in Figure 1B) to p- (represented by region 104) and the diode series resistance becomes excessively high. A similar effect occurs in the case of polycrystalline silicon base transistors where the base resistance increases due to boron out-diffusion to intolerably high levels rendering the device inoperable. This phenomena is shown in Figure 1C which depicts a plot of resistivity as a function of distance from the surface of the device (resistivity/depth profile). As can be seen by the dotted curve, which sets forth the resistivity/depth profile after the annealing process, the resistivity at the disilicide/poly Si interfacial region increases substantially over that plotted for the initial profile. Figure 2 shows the deterioration in performance of the diode characteristics of a semiconductor device which experiences this dopant out-diffusion. As can be seen in Figure 2A, the dopant out-diffusion (represented by arrow "d") manifests itself as a resistance R in series with the diode (Figure 2B). This causes a degradation in the characteristic current versus voltage curve of the device as shown in Figure 2C. Applied Physics Letters, vol. 36, #6, March 15 1980 , pages 456-458 discloses high-temperature contact structures for silicon semiconductor devices. They are obtained by a combination of a diffusion barrier consisting of a transition metal nitride and an intermetallic compound of aluminum, wherein preferably all layers are sputtered in one single operation on a platinum layer which forms the direct silicon contact. This system is said to have the disadvantage of increased sheet resistance compared with aluminum. It is the object of the present invention to provide a method for fabricating high-speed semiconductor devices based on a doped silicon substrate wherein the disadvantages of the prior art as referred to above are at least in part overcome. This object is achieved by the combination of features defined by claim 1 while the dependent claims define particular embodiments of this concept. Fig. 1 and 2 have been referred to above. Fig. 3 is a diagrammatic representation of the preferred method for constructing a device in accordance with the present invention, and Fig. 4 is a plot of resistivity versus distance from the surface in a prior art device as well as a device constructed in accordance with the present invention. Detailed Description of the Preferred Embodiment Although specific forms of the invention have been selected for illustration in the drawing, and the following description is drawn in specific terms for the purpose of describing these forms of the invention, this description is not intended to limit the scope of the invention which is defined in the appended claims. Referring to Figure 3A, there is shown, in diagrammatic form, a layer 12 of P-type polycrystalline type silicon (poly Si) which forms a substrate for a device made in accordance with the process of the present invention. The poly Si substrate 12 is fabricated by conventional growth techniques in the presence of a p-type dopant such as boron. Although the preferred embodiment includes a polycrystalline silicon substrate, a single crystal silicon substrate may be used and such a substrate is considered within the scope of the present invention. As shown in Figure 3B, islands 14, of silicon dioxide (SiO₂) are formed on the polycrystalline silicon substrate 12 in accordance with well known prior art techniques such as deposition or growth. Note that in the preferred embodiment set forth in this detailed description, the dopant is boron. However, the dopant could also be phosphorous arsenic or antimony. As shown diagrammatically in Figure 3C, a thin layer 16 of titanium (Ti) is disposed on the substrate 12 and islands 14 by conventional techniques such as physical or chemical vapor deposition. It is preferred that this layer 16 be on the order of 5 nm (50 Å) in thickness. Although the use of titanium is preferred, the layer could also be formed using other refractory metals such as tantalum, niobium, zirconium, chromium or hafnium. The layer 16 has been found to lower contact resistance by breaking down the thin native oxide layer formed on the polycrystalline silicon, after an elevated thermal heat treatment at 950°C for thirty minutes in hydrogen. This native oxide layer, which forms on the polycrystalline silicon after wet processing, inhibits good contact resistance if not removed or dissolved. Use of the layer 16 has been shown to reduce contact resistance to a value on the order of 10 to 20 ohms for contact windows having an area of 25 square microns. In addition, the layer 16 is so thin that any dopant out-diffusion into the layer is spatially minimal thereby precluding adverse effects of any consequence on the dopant profile of the device. Referring now to Figure 3D, a blanket layer 18 of tantalum nitride (Ta Nx) is preferably deposited over the titanium layer 16 by reactive evaporation or sputtering, preferably to a thickness of from about (20 nm - 100 nm) (200 Å to 1000 Å). It is preferred that the pressure of nitrogen in the ambient during this reaction process be typically in the range of from approximately (1 x 10⁻⁴ Pa to 1 Pa) (1 x 10⁻⁶ torr to 1 x 10⁻² torr). The pressure should be tuned to the deposition rate of the refractory metal, which is tantalum in this preferred embodiment, to ensure a fairly complete reaction of the tantalum with nitrogen. Other nitrogen bearing gases, such as ammonia, could be used in this process as a substitute for nitrogen. In addition, other refractory metals such as niobium, titanium, zirconium, chromium or hafnium could be used to form refractory barrier layers such as niobium nitride (NbN), titanium nitride (TiN), zirconium nitride (ZrN) or hafnium nitride (HfN). The blanket layer 18 may also be deposited using chemical vapor or plasma deposition techniques. Referring now to Figure 3E an amorphous co-deposited tantalum: silicon (Ta:Si) layer 20, preferably having a thickness on the order of 250 nm (2500 Å), is disposed on the tantalum nitride layer 18 without breaking vacuum. This process ensures a clean, oxide free interface between the nitride and amorphous silicide layers, 18 and 20 respectively, thereby minimizing Contact resistance between the silicide and nitride phases. It is preferred that the atomic ratio of silicon to tantalum in the co-deposit be in a range of from approximately 2:1 to approximately 4:1, the preferred ratio being approximately 2.5:1. Referring to Figure 3F, the Ta:Si/TaNx/Ti/poly Si structure is then annealed, preferably by heating at 950°C for 30 minutes in hydrogen, in order to transform the amorphous Ta:Si layer 20 to tantalum disilicide TaSi₂ and stabilize the interface between the disilicide and nitride layers 20 and 18 respectively. After transformation, the tantalum disilicide (TaSi₂) layer also contains pure Si grains which have nucleated from the amorphous layer 20 after the reaction. After plasma definition, this excess silicon can later be consumed during oxidation without generating voids in the newly formed TaSi₂ layer 20. It has been found that the excess silicon does not substantially change the resistivity of the disilicide layer. Although in the preferred embodiment a tantalum disilicide layer 20 is formed on the metal nitride layer 18, the layer 20 may alternatively be formed using a refractory metal such as molybdenum, tungsten or tantalum. Such a layer enables one to interconnect areas with lower resistivity pure metals instead of metal disilicides which require stricter control in their processing. Due to the inclusion of the metal nitride layer 18 in accordance with the present invention, the refractory metal layer 20 can be incorporated into the device without forming silicides, since the metal nitride layer blocks silicon transport and retains the chemical integrity of the metal without consumption of silicon from the underlying substrate. This leaves the silicon device characteristics intact. Referring now to Figure 3G, the structure is then photolithographically masked with photoresist and the TaSi₂/TaNx/Ti tri-layer structure is dry etched preferably in a flourine based plasma. With good end point detection, the etch can be made to stop on the polycrystalline silicon layer 12 as shown in Figure 3G. Depending on the device application, the poly Si layer, if desired, can be etched anisotropically with a chlorine based plasma. For example, the poly Si could be etched in an MOS gate application or left intact for bipolar contacts. Referring now to Figure 3H, a thin, thermal SiO₂ layer 24 is formed over the structure by high temperature oxidation in a wet H₂0 or dry 0₂ environment. Next, referring to Figure 3I, the structure is preferably covered with a layer 26 of PVX glass which is then reflowed at a temperature preferably in the range of approximately 825°C-1000°C. At this point, the device is ready for via generation and other standard back end wafer processes. Referring now to Figure 4A, there is shown a resistivity depth profile of a structure comprising a boron doped polycrystalline silicon (poly Si) layer disposed on a silicon dioxide (Si 0₂) substrate. The resistivity of the poly Si layer of this structure is substantially equal to 10⁻² ohm-cm. Figure 4B depicts a resistivity depth profile of a device structure comprising the structure depicted in Figure 4A having a tantalum disilicide (Ta Si₂) layer disposed on the doped poly Si layer thereof, after being subjected to a typical annealing heat cycle of 1000°C for thirty minutes. As shown in Figure 4B, the resistivity of the doped poly Si layer of this structure has increased to approximately 10⁻¹ ohm-cm from the pre-annealing value of 10⁻² ohm-cm for the structure depicted in Figure 4A. Figure 4C depicts a resistivity depth profile for a device structure of the present invention comprising tantalum nitride (TaNx) and titanium (Ti) layers disposed between the TaSi₂ and doped poly Si layers of the device structure depicted in Figure 4B, after being subjected to the same typical annealing heat cycle of 1000°C for thirty minutes. The resistivity of the doped poly Si layer of this structure remained substantially equal to the pre-annealing value of 10⁻² ohm-cm for the structure depicted in Figure 4A. This results in a resistivity which is approximately one order of magnitude lower than that of the doped poly Si layer of the prior art device. Furthermore, as the annealing time increases, the resistivity of the poly Si layer of the prior art device will continue to increase while the resistivity of the poly Si layer of the device of the present invention will remain substantially unchanged at approximately 10⁻² ohm-cm. It will be understood that various changes in the details, materials and arrangement of the parts which have been herein described and illustrated in order to explain the nature of this invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the following claims.

Claims: A method for fabricating a high speed semiconductor device, comprising: (a) providing a doped silicon substrate (12) having a first surface, (b) disposing a layer (16) of refractory metal over the first surface of said substrate, said layer having a predetermined thickness of substantially equal to 5 nm (50 angströms), (c) depositing refractory metal on said layer in an ambient environment having a predetermined concentration of nitrogen so as to form a barrier layer (18) of refractory metal nitride having a predetermined thickness, (d) placing an amorphous, co-deposited refractory metal/silicon layer having a predetermined atomic ratio of silicon to refractory metal on said barrier layer, and (e) transforming said co-deposited layer to a refractory metal disilicide layer (20) by heating at a predetermined temperature for a predetermined amount of time in a second predetermined ambient environment so as to form said refractory metal disilicide layer. The method of claim 1 wherein the refractory metal of said barrier layer (18) comprises tantalum, the silicon substrate dopant comprises boron and the refractory metal of said refractory metal layer (16) comprises titanium. The method of claim 1 or 2 wherein the refractory metal of said co-deposited layer comprises tantalum. The method of claim 3 wherein the thickness of said co-deposited layer is substantially equal to 250 nm (2500 Å). The method of claim 4 wherein said predetermined atomic ratio of silicon to tantalum is substantially equal to 2.5/1. The method of claim 5 wherein said amorphous tantalum/silicon layer is transformed to tantalum disilicide by heating at a temperature substantially equal to 950°C for a time substantially equal to 30 minutes in an ambient environment of hydrogen. The method of any one of claims 1 to 6 wherein the thickness of said barrier layer (18) is in a range of from approximately 20 to 100 nm 200 to 1000 Å. The method of any one of claims 1 to 7 wherein the pressure of nitrogen is in a range of from approximately 1 x 10 ⁻⁴ Pa to 1 Pa (1 x 10⁻ ⁶ to 1 x 10 ⁻² torr) The method of any one of claims 1 to 8 comprising the additional steps of: (a) etching through said metallic disilicide (20), barrier (18) and refractory metal (16) layers to said silicon substrate surface in predetermined locations on said first surface; (b) disposing a layer of silicon dioxide (24) over said etched and non-etched portions; (c) covering said layer of silicon dioxide with a layer of glass (26); and (d) reflowing said glass layer (26) at a predetermined temperature. The method of claim 9 characterized in that said glass (26) is reflowed at a temperature in the range of from approximately 825° C to 1000° C. The method of claim 1 characterized in that the silicon substrate dopant is selected from the group consisting of boron, phosphorus, arsenic and antimony. The method of claim 1 wherein said co-deposited refractory metal layer (20) comprises a refractory metal selected from the group consisting of molybdenum, tungsten and tantalum.

Inventors: KAPOOR ASHOK K, THOMAS MICHAEL E, VORA MADHUKAR B, KAPOOR, ASHOK K., THOMAS, MICHAEL E., VORA, MADHUKAR B.
Applicants: FAIRCHILD SEMICONDUCTOR, FAIRCHILD SEMICONDUCTOR CORPORATION
Application year: 1985
Main classification: H01L21
IPC codes: H01L29, H01L21, H01L23