Abstract:
A method and a device directed to the same, for stabilizing cobalt di-silicide/single crystal silicon, amorphous silicon, polycrystalline silicon, germanide/crystalline germanium, polycrystalline germanium structures or other semiconductor material structures so that high temperature processing steps (above 750° C.) do not degrade the structural quality of the cobalt di-silicide/silicon structure. The steps of the method include forming a di-silicide or germanide by either reacting cobalt with the substrate material and/or the codeposition of the di-silicide or germanide on a substrate, adding a selective element, either platinum or nitrogen, into the cobalt and forming the di-silicide or germanide by a standard annealing treatment. Alternatively, the cobalt di-silicide or cobalt germanide can be formed after the formation of the di-silicide or germanide respectively. As a result, the upper limit of the annealing temperature at which the di-silicide or germanide will structurally degrade is increased.

Description:
FIELD OF THE INVENTION 
     The present invention relates to semiconductor devices, and more particularly, to a method and an apparatus for improving the structural stability of cobalt di-silicides/germanides during high temperature annealing by the addition of one of platinum and nitrogen to cobalt di-silicide/germanide. 
     BACKGROUND OF THE INVENTION 
     It is well known by those skilled in the art that the electrical contact and structural stability of a cobalt di-silicide thin film in contact with a silicon substrate is critical for electrical conductivity and performance of semiconductor devices. It is also well known by those skilled in the art that the electrical contact and structural stability of a cobalt germanide thin film in contact with a germanium substrate is also critical. Therefore, a thin film of cobalt di-silicide or cobalt germanide which has enhanced resistance to structural degradation caused by thermal annealing is desirable. 
     Problems with cobalt di-silicide/silicon contact regions have been found when the cobalt di-silicide structure is exposed to high temperature annealing (over 750° C). Those skilled in the art have discovered that a low resistance cobalt di-silicide/silicon contact degrades upon heating over 750° C. resulting in the agglomeration and/or inversion of the cobalt di-silicide. This causes open or high resistance interconnections, high contact resistances and electrical leakage which lead to device failure. 
     For example, as known to those skilled in the art, one aspect of current silicon technology utilizes ion implantation for the formation of source/drain junctions followed by the deposition of a thin cobalt film and thermal annealing at 600° C. to 750° C. to convert cobalt into cobalt di-silicide. After di-silicide formation, a high temperature thermal anneal is performed (&gt;750° C.) to electronically activate the dopants in the source, gate and drain regions of the device. Low sheet resistance contacts and low leakage source and drain junctions are difficult to form by this technique because of the structural degradation of the cobalt di-silicide that occurs during the high temperature activation anneal. 
     One approach to solve these problems is to conduct the ion implantations and high temperature annealing steps (&gt;750° C.) required for the source, gate and drain electrical activation before the deposition and formation of the cobalt di-silicide/silicon contact. However, further processing of the silicon device after di-silicide formation must be limited to temperatures below the formation temperature of the cobalt di-silicide in order to avoid agglomeration of the cobalt di-silicide and device degradation. These temperature limitations are often difficult to maintain for processing of integrated silicon circuits. 
     Yet another shortcoming with the known technology occurs because the silicon used for the cobalt di-silicide/silicon contact is polycrystalline silicon. This would be the situation in local interconnects for wiring transistors together or gate structures for CMOS, bipolar or ASIC circuits. Thin film cobalt di-silicide/polycrystalline silicon contacts have less structural stability under thermal stressing than cobalt di-silicide/single crystal silicon structures. As a result, the thin film cobalt di-silicide/polycrystalline will agglomerate and invert with lower temperature anneals. As known to those skilled in the art, this problem is particularly acute in integrated schemes where the cobalt silicide/polycrystalline di-silicon gate contact is formed during the cobalt di-silicide/silicon source and drain formation and before high temperature dopant electronic activation anneals. 
     Some prior art designs provide alternate structures using di-silicide and silicon substrates. However, these designs fail to achieve a structure which can satisfy the above criteria for high temperature resistance of di-silicide to thermal agglomeration on a silicon substrate. 
     SUMMARY OF THE INVENTION 
     A method for forming a thin film on a substrate and a device directed to the same. The method includes forming a mixture of cobalt and platinum and/or nitrogen. Then reacting the mixture with the substrate to form the thin film which is thermally stable and resistant to agglomeration and layer inversion during elevated thermal cycles. The substrate is amorphous silicon, single crystal silicon, polycrystalline silicon, amorphous germanium, single crystalline germanium, polycrystalline germanium, or semiconductors or compound semiconductors of columns IIb, IIIa, IVa, Va, and VIa of the periodic table. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The aspects, features and advantages of the present invention will be more readily understood from the following detailed description when read in conjunction with the accompanying drawings, in which: 
     FIGS. 1(a)-1(e) show the steps in making a multilayered CMOS device having a source, drain and gate region of a CMOS multilayered device in accordance with the exemplary method of the present invention. 
     FIG. 2 is a cross-sectional schematic view of a CMOS device of the type shown in FIGS. 1(a)-1(e) after the deposition of a conducting cobalt thin film which contains platinum. 
     FIG. 3 is a cross-sectional schematic view of the exemplary embodiment of the present invention after the formation of the source, drain and gate contacts by thermal annealing and selective etching. 
     FIG. 4. is a cross-sectional schematic view of a a conventional CMOS device after the formation of the source, drain and gate contacts by thermal annealing and selective etching. 
     FIG. 5 is a cross-sectional schematic view of the effects of a high temperature anneal (&gt;750° C.) on the conventional CMOS device of FIG. 4. 
     FIG. 6 is an exploded view of the conventional CMOS device of FIG. 5 showing the distinct di-silicide and silicon phases resulting from a high temperature anneal. 
     FIG. 7(a) and (b) are a cross-sectional schematic of steps of the exemplary method of the present invention which uses nitrogen instead of platinum. 
     FIG. 8 is a graph showing the in situ resistance versus temperature behavior of 15 nm of pure cobalt on top of polycrystalline silicon and 15 nm of cobalt with two atomic percent platinum on top of polycrystalline silicon. 
     FIG. 9 is a graph showing the in situ resistance versus temperature behavior of 15 nm of cobalt with two atomic percent platinum on top of single crystal silicon. 
     FIG. 10 is a graph showing the final sheet resistance after annealing of cobalt-platinum thin films on top of polycrystalline silicon that are 5 to 35 nm thick and contain 0.1 to 6.5 atomic percent platinum. 
     FIG. 11 is a graph showing the in situ resistance versus temperature behavior of cobalt-selected element/polycrystalline silicon thin films where the selected element is either Pd, Ru, Rh or Pt. 
     FIG. 12 is a graph of the resistance versus temperature behavior of a CoSi 2  /polycrystalline silicon thin film with and without the addition of nitrogen by ion implantation into the CoSi 2  before any high temperature processing. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An exemplary method of the present invention will now be described more fully with reference to FIGS. 1(a)-(e)-3, in which a semiconductor device is shown during successive stages of manufacture. 
     As shown in FIG. 1(a), the starting material is a semiconductor substrate 11, in this example of n-type single crystal silicon, including a p-well 1, which was formed by doping the substrate 11. A silicon substrate material is preferred. Subsequently, as shown in FIG. 1(b), an oxide and/or nitride insulation layer 2 is formed on substrate 11 over p-well 1. The insulation layer 2, for example, is formed by thermal oxidation. 
     Then, on the insulation layer, a polysilicon layer 15 is formed as shown in FIG. 1(c), for example, by chemical vapor deposition. Subsequently, as shown in FIG. 1(d), by means of implantation, n+type source region 16 and drain region 17 are formed in p-well 1 and oxide layer 2 is selective removed above the source and drain regions to expose source region 16 and drain region 17. Then, nitride and/or oxide thin film spacers 13 are formed on either side of the polycrystalline silicon layer 15 as shown in FIG. 1(e). 
     Thus, the CMOS device shown in FIG. 1(e) is constructed. The CMOS includes a single crystal silicon substrate 11 with oxide and/or nitride insulation layers 12 which separate the active area of the CMOS device from adjacent devices. The gate region of the CMOS device includes gate oxide 14, polycrystalline silicon layer 15 and nitride and/or oxide thin film spacers 13. Thin film spacers 13 separate gate region from source region 16 and drain region 17. 
     Then, as shown in FIG. 2, a cobalt-alloy thin film layer 38 is then deposited over insulation layer 12, source region 16, drain region 17 and gate region. The cobalt-alloy includes platinum in concentrations less than 10 atomic percent. As will become more clear in the discussion hereinafter, the addition of platinum improves the structural stability of the formed cobalt di-silicide contacts under high temperature annealing. The thickness of the cobalt-alloy thin film layer 38 to be applied is in a range of 0-200 nm depending upon the relative dimensions, function, and intended use of the devices to constructed. However, a thickness range from 5 to 30 nm is preferred for the CMOS device. 
     Then, the CMOS device annealed in a typical annealing process at temperatures less than 750° C. to form cobalt silicide source contact 69, cobalt di-silicide drain contact 68, and cobalt di-silicide gate contact 70 as shown in FIG. 3. The cobalt-alloy layer reacts with the substrate 11 and the polycrystalline silicon gate 15, during the annealing process to form the cobalt-platinum di-silicide source 68, drain 69 and gate 70 contacts. Any remaining unreacted cobalt-alloy is etched away by methods known to those skilled in the art using, for example, either an acid or base solution in the presence of an oxidizing agent. 
     When the device shown in FIG. 3 is heated to temperatures in excess of 750° C., source 69, drain 68 and gate 70 di-silicide contacts do not structurally degrade. The structure of the di-silicide contacts are the same after the formation anneals below 750° C. and after any later anneals higher than 750° C. This is because platinum had been added to the cobalt di-silicide. 
     For comparison, FIG. 4 shows a conventional CMOS device in which platinum has not been added to the cobalt di-silicide contacts. The CMOS device shown in FIG. 4 has cobalt di-silicide source contact 19, cobalt silicide drain contact 18, and cobalt di-silicide gate contact 20 which were formed using a typical annealing process at temperatures less than 750° C. to form. 
     FIGS. 5 and 6 show the conventional CMOS device of FIG. 4 after an annealing at temperatures over 750° C. where the cobalt di-silicide/silicon source 19, drain 18 and gate 20 contacts shown in FIG. 4 degraded and which are shown in FIG. 5 as source 58, drain 59 and gate 50 contacts. In this case, the di-silicide layers shown in FIG. 4, including source 19, drain 18 and gate 20 contacts, agglomerated/inverted and formed a mixed di-silicide gate 50, source 58, and drain 59 layers having distinct phases of di-silicide 53 and silicon 54. FIG. 6 is an exploded view of FIG. 5 showing the mixed di-silicide 53 and silicon 54 phases. As known to those skilled in the art, anneals over 750° C. are required after di-silicide formation to electrically activate dopants in standard CMOS semiconductor devices. 
     In accordance with the exemplary embodiment shown in FIG. 3, the cobalt-alloy di-silicide gate 70, source 69 and drain 68 contacts will not break down into a mixed di-silicide and silicon layer when exposed to elevated temperatures as the device shown in FIG. 4 without platinum in the cobalt di-silicide layer. 
     FIG. 7(b) demonstrates an alternative exemplary method of the present invention. After the formation of cobalt or cobalt di-silicide contact layers, nitrogen is then added to the cobalt or cobalt di-silicide contact layers in order to improve the stability of the formed di-silicide contacts under high temperature stressing. In this method, pure cobalt 28 is deposited over the entire device first as shown in FIG. 7(a). Then, the pure cobalt 28 is reacted to form cobalt di-silicide on the exposed silicon areas and the unreacted cobalt is stripped away. Then, nitrogen is implanted into the formed cobalt di-silicide, but not into the silicon substrate underneath to form the CMOS device of FIG. 7(b). This forms cobalt di-silicide source 72, drain 71, and gate 70 contacts having nitrogen instead of platinum disposed therein. 
     When this alternative exemplary embodiment is exposed to an annealing source having a temperature over 750° C., the cobalt di-silicide contacts do not degrade. Both elements, platinum and nitrogen, prevent the physical degradation of the di-silicide layers when exposed to temperatures in excess of 750° C. 
     The exemplary embodiments of the inventive method as shown in FIGS. 1(a)-1(e) to FIG. 3 and FIG. 7(a)-7(b), involve adding platinum and/or nitrogen to cobalt before di-silicide formation by annealing below 750° C. and before subsequent high temperature processing and/or adding platinum and/or nitrogen to the formed cobalt di-silicide before it undergoes any high temperature processing (&gt;750° C). As will be demonstrated in further detail, the addition of platinum or nitrogen increases the temperature at which the structure of the cobalt di-silicide/silicon interface degrades from 750° C. to over 1000° C. 
     The inventive method also relates to germanium substrates where platinum and/or nitrogen is added to cobalt before germanide formation by annealing below 750° C. and before subsequent high temperature processing and/or where platinum and/or nitrogen is added to the formed cobalt germanide before it undergoes any high temperature processing (&gt;750° C). 
     In addition, other semiconductor or compound semiconductor materials may be used in place of silicon. These semiconductor or compound semiconductor materials may be selected from columns IIa, IIIa, IVa, Va, and VIa of the periodic table. For example, a compound semiconductor of Silicon Germanium, Si x  Ge x-y , where x and y are between 0 and 1, may also be used. 
     The method by which these elements are introduced into the cobalt and/or cobalt di-silicide is not limited to a particular method. To those skilled in the relevant art many different introduction techniques such as co-physical vapor deposition, ion implantation, electroplating and co-chemical vapor deposition may be used. For example, as shown in FIGS. 7(a)-(b), cobalt and one of platinum and nitrogen can be formed on the source, drain and gate electrodes using different deposition techniques as well as additional steps to deposit the cobalt and then platinum or nitrogen. The cobalt and the platinum or nitrogen could be deposited onto the device using co-physical vapor deposition or co-chemical vapor deposition or the cobalt layer could be first deposited upon the device and platinum or nitrogen could then be added to the cobalt layer either before or after the cobalt di-silicide/germanide layer is formed by annealing at temperatures below 750° C. 
     In addition, the cobalt layer having platinum or nitrogen could be used as an out-diffusion source by implanting dopant species into the cobalt layer having platinum or nitrogen and then subjecting the layer to a high thermal cycle to cause the dopant to out-diffuse into other layers. 
     FIG. 8 compares adding platinum at a level of 2 atomic percent to the cobalt before annealing to form a cobalt di-silicide thin film against the formation of cobalt di-silicide without adding platinum. Platinum and cobalt are deposited on the silicon substrate in a manner well known to those skilled in the art. The addition of platinum enhances the thermal stability of the formed cobalt di-silicide during high temperature processes. In FIG. 8, the in situ resistance versus temperature from 25° to 1200° C. is plotted for a cobalt film and a cobalt-2 atomic percent platinum film on polycrystalline silicon. For both films, changes from a linear resistance versus temperature slope below 700° C. indicate cobalt di-silicide formation. For the pure cobalt thin film at 900° C., there is a sharp increase in the resistance of the film, indicating, as known to those skilled in the art, that the discrete structure of the cobalt di-silicide on top of polycrystalline di-silicide has degraded and intermixed. 
     The addition of 2 atomic percent platinum into the cobalt layer before cobalt di-silicide formation as shown in the exemplary methods greatly enhances the stability of the cobalt di-silicide-polycrystalline silicon interface. The solid line in FIG. 8 shows that the temperature at which the cobalt di-silicide with platinum additions starts to degrade is near 1100° C. This is higher than the 900° C. degradation temperature observed for the pure cobalt sample. 
     Although the data presented in FIG. 8 has polycrystalline silicon substrate as the preferred type of silicon on which the cobalt di-silicide is formed, different types of substrates, for example, such as silicon, germanium or any other semiconducting layer and/or substrate in which cobalt and/or cobalt di-silicide/germanide is in contact with or formed thereon can be used. For example, in FIG. 9, similar results are observed when the substrate is single crystal silicon as opposed to polycrystalline silicon. FIG. 9 shows the stability of the sheet resistance of the di-silicide film on single crystal silicon at a temperature in excess of 900° C. 
     It should be noted that the exact composition of platinum or nitrogen in the cobalt or the cobalt di-silicide/germanide is not critical to obtain enhanced structural stability of the cobalt di-silicide/germanide. FIG. 10 is a plot of the resistance of a cobalt(platinum) di-silicide/polycrystalline silicon bilayer structure after being held at 930° C. for 5 minutes as a function of film thickness, where the platinum concentration in the alloy ranges from 0.15 to 6.5 percent. The silicon bilayer structure was annealed in He at a ramp temperature of 340 C./min up to 930° C. All these films have a low resistivity after the annealing treatment, indicating that they did not agglomerate. Cobalt di-silicide layer of comparable thickness formed using only cobalt was found to have structurally degraded (agglomerated) and to have an infinite resistance for this thermal cycle. 
     It should also be noted that the exemplary method of the present invention of adding small amounts of platinum to the cobalt and/or cobalt di-silicide/germanide to thermally stabilize the thin film does not depend on the initial distribution of platinum within the cobalt and/or cobalt di-silicide/germanide within the thin films. Various schemes have been tried for depositing platinum at various locations in the cobalt film. They all have significantly improved the thermal stability of the resultant films in a similar manner to a uniform distribution of platinum. The same results were found when nitrogen was added with various depositing schemes. 
     The use of platinum in the exemplary methods as an alloying element for cobalt and/or cobalt di-silicide/germanide is not arbitrary. Metal alloying elements with similar electron and chemical characteristics to platinum, will not enhance or could even possibly degrade the physical stability of cobalt di-silicide structures under high temperature annealing. FIG. 11 presents a plot of resistance versus time for several cobalt-alloy films on top of polycrystalline silicon as a function of time during a temperature ramp from room temperature to 930° C. The data demonstrates that platinum is the only alloy addition among similar elements (i.e. Pd, Ru, Rh) that improves the thermal stability of the formed cobalt di-silicide. The choice of nitrogen as an alloying element for cobalt and/or cobalt di-silicide/germanide is not necessarily limited. Other similar elements such as elements selected from column Va of the periodic table may be used to improve the stability of cobalt di-silicide/germanide films. More preferred are phosphorus and arsenic from column Va. Phosphorus and arsenic would be good substitutes for nitrogen because the elements have a similar chemical composition to nitrogen and are non-metals as nitrogen. However, nitrogen is the most preferred of the column Va elements. 
     Although cobalt di-silicide is the preferred embodiment for a stable contact layer, cobalt germanide or any intermetallic phase that could form between cobalt and a semiconducting material may be used to produce a conducting intermetallic phase that has enhanced structural stability under high temperature annealing conditions compared to a intermetallic phase that does not contain the addition of one of nitrogen and platinum. 
     EXAMPLE I 
     A 15 nm cobalt-2 atomic percent platinum film 38 is deposited onto a CMOS semiconductor device as shown in the exemplary method of FIG. 2. The cobalt-2 atomic percent platinum alloy is reacted with the exposed silicon interfaces which comprise the source 16, gate 15 and drain 17 regions of the device by annealing the sample at 500° C. for 10 minutes to form a CoSi intermetallic phase containing platinum impurities. A standard selective chemical etch, well known to those skilled in the art, is conducted on the semiconducting device, to remove the excess unreacted cobalt. The device is then heated at 900° C. for 5 minutes to convert the cobalt silicide into the desired low resistivity phase cobalt di-silicide CoSi 2 , and to electronically activate any dopants in the source 16, gate 15 and drain 17 regions. 
     END OF EXAMPLE I 
     The mechanism believed responsible for the effectiveness of the exemplary methods of the present invention which involves the addition of platinum as the to cobalt and/or cobalt di-silicide is the formation of a small amount of platinum silicide precipitates, upon heating, within the dominate cobalt di-silicide phase. These precipitates are located within the cobalt di-silicide grains and at its grain boundaries. The precipitates prevent the structural degradation of the majority cobalt di-silicide phase during high temperature annealing. These precipitates could include, for example, precipitates of at least one of CoY x , SiY x  and CoSi z  Y x , where Y is one of nitrogen and platinum and x and z are atomic fractions. 
     FIG. 12 demonstrates that a cobalt di-silicide sample on top of polycrystalline-silicon without platinum or nitrogen, will show a rapid increase in sheet resistance at 750° C., indicating the onset of structural degradation of the di-silicide. However, a cobalt di-silicide sample which contains nitrogen does not degrade for annealing temperatures up to 1000° C. 
     It should be noted that a method in which nitrogen could be introduced into the cobalt di-silicide, for example, would be by ion implantation into the di-silicide before any high temperature processing and at an acceleration voltage that has a corresponding ion range plus vertical straggling which is less than or equal to the di-silicide thickness. This is done in order to avoid any implantation damage of the underlying silicon semiconductor. It is preferred that nitrogen or platinum be implanted at an acceleration voltage so that the thickness of the di-silicide layer is no greater than the ion range plus vertical straggling but no less than the ion range minus vertical straggling. However, nitrogen or platinum may also be implanted at an acceleration voltage so that the thickness of the silicide layer is no greater than the ion range but no less than the ion range. 
     The dose rate for implanting implanting nitrogen is, for example, a dose rate from 5×10 14  to 5 ×10 17  /cm 2 . 
     It should also be noted that although the preferred embodiment for the introduction of nitrogen into the cobalt di-silicide is by ion implantation, one skilled the art could imagine a number of other introduction methods, such as physical vapor deposition or chemical vapor deposition of the cobalt and/or cobalt di-silicide in an environment that contains nitrogen, to introduce the nitrogen into the cobalt di-silicide layer. 
     Although nitrogen implantation into cobalt di-silicide is one preferred embodiment for the present invention, the present inventive method applies to the stability of any type of cobalt-intermetallic phase where the cobalt intermetallic phase is created by a reaction or a codeposition of cobalt and silicon, germanium or any other semiconductor materials. 
     EXAMPLE II 
     A cobalt film of 15 nm is deposited onto a CMOS semiconductor device. The cobalt is reacted with the exposed silicon interfaces which comprise the source, gate and drain regions of the device by annealing the sample at 500° C. for 10 minutes to form a CoSi intermetallic phase. A standard selective chemical etch, well know to those skilled in the art, is conducted on the semiconducting device, to remove the excess unreacted cobalt. Nitrogen ions are then implanted into the CoSi at a dose of 1×10 16  /cm 2  and at a energy of 80 keV. The device is then heated at 900° C. for 5 minutes to convert the cobalt silicide into the desired low resistivity phase cobalt di-sulicide, CoSi 2 , and also to electronically activate any dopants in the source, gate and drain regions. 
     END OF EXAMPLE II 
     The presence of nitrogen in the cobalt di-silicide does not cause stabilizing cobalt nitride precipitates to form, as would form in the case of TiSi 2  implanted with nitrogen. This is because the positive heat of formation of cobalt nitride (thermodynamically unfavorable formation) as opposed to the negative heat of formation (thermodynamically favorable formation) for titanium nitride. 
     It is emphasized that, according to common practice in semiconductor representation, the various layers of the drawing are not to scale. On the contrary, the width or length and thickness of the various layers are arbitrarily expanded or reduced for clarity. 
     While the overall invention has been described in terms of the preferred embodiments and examples, those skilled in the art will recognize that the specific configurations, steps and parameters may be varied in the practice of the invention by those skilled in the art without departing from the spirit of the invention, the scope of which is defined by the appended claims.