Abstract:
A method and system to reduce the resistance of refractory metal layers by controlling the presence of fluorine contained therein. The present invention is based upon the discovery that when employing ALD techniques to form refractory metal layers on a substrate, the carrier gas employed impacts the presence of fluorine in the resulting layer. As a result, the method features chemisorbing, onto the substrate, alternating monolayers of a first compound and a second compound, with the second compound having fluorine atoms associated therewith, with each of the first and second compounds being introduced into the processing chamber along with a carrier gas to control a quantity of the fluorine atoms associated with the monolayer of the second compound.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of U.S. patent application Ser. No. 09/625,336, filed Jul. 25, 2000, now issued as U.S. Pat. No. 6,855,368, which is a divisional patent application of U.S. patent application Ser. No. 09/605,593, filed Jun. 28, 2000, now issued as U.S. Pat. No. 6,551,929. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to the processing of semiconductor substrates. More particularly, this invention relates to improvements in the process of depositing refractory metal layers on semiconductor substrates 
   2. Description of the Related Art 
   The semiconductor processing industry continues to strive for larger production yields while increasing the uniformity of layers deposited on substrates having increasing larger surface areas. These same factors in combination with new materials also provide higher integration of circuits per unit area of the substrate. As circuit integration increases, the need for greater uniformity and process control regarding layer thickness rises. As a result, various technologies have been developed to deposit layers on substrates in a cost-effective manner, while maintaining control over the characteristics of the layer. Chemical Vapor Deposition (CVD) is one of the most common deposition processes employed for depositing layers on a substrate. CVD is a flux-dependent deposition technique that requires precise control of the substrate temperature and precursors introduced into the processing chamber in order to produce a desired layer of uniform thickness. These requirements become more critical as substrate size increases, creating a need for more complexity in chamber design and gas flow technique to maintain adequate uniformity. 
   A variant of CVD that demonstrates superior step coverage, compared to CVD, is Atomic Layer Deposition (ALD). ALD is based upon Atomic Layer Epitaxy (ALE) that was originally employed to fabricate electroluminescent displays. ALD employs chemisorption to deposit a saturated monolayer of reactive precursor molecules on a substrate surface. This is achieved by alternatingly pulsing an appropriate reactive precursor into a deposition chamber. Each injection of a reactive precursor is separated by an inert gas purge to provide a new atomic layer additive to previous deposited layers to form a uniform layer on the substrate. The cycle is repeated to form the layer to a desired thickness. A drawback with ALD techniques is that the deposition rate is much lower than typical CVD techniques by at least one order of magnitude. 
   Employing the aforementioned deposition techniques it is seen that formation of a layer at a high deposition rate while providing adequate step coverage are conflicting characteristics often necessitating sacrificing one to obtain the other. This has been prevalent when depositing refractory metal layers to cover gaps or vias during formation of contacts that interconnect adjacent metallic layers separated by a dielectric layer. Historically, CVD techniques have been employed to deposit conductive material in order to inexpensively and quickly form contacts. Due to the increasing integration of semiconductor circuitry, tungsten has been used based upon the superior step coverage of tungsten. As a result, deposition of tungsten employing CVD techniques enjoys wide application in semiconductor processing due to the high throughput of the process. 
   Depositing tungsten in this manner, however, is attendant with several disadvantages. For example, blanket deposition of a tungsten layer on a semiconductor wafer is time-consuming at temperatures below 400° C. The deposition rate of tungsten may be improved by increasing the deposition temperature to, e.g., about 500° C. to about 550° C. Temperatures in this range may compromise the structural and operational integrity of the underlying portions of the integrated circuit being formed. Tungsten has also frustrated photolithography steps during the manufacturing process by providing a relatively rough surface having a reflectivity of 20% or less than that of a silicon substrate. Finally, tungsten has proven difficult to deposit uniformly. This has been shown by variance in tungsten layers&#39; thickness of greater than 1%, which frustrates control of the resistivity of the layer. Several prior attempts to overcome the aforementioned drawbacks have been attempted. 
   For example, in U.S. Pat. No. 5,028,565 to Chang et al., which is assigned to the assignee of the present invention, a method is disclosed to improve, inter alia, uniformity of tungsten layers by varying the deposition chemistry. The method includes, in pertinent part, formation of a nucleation layer over an intermediate barrier layer before depositing the tungsten layer via bulk deposition. The nucleation layer is formed from a gaseous mixture of tungsten hexafluoride, hydrogen, silane and argon. The nucleation layer is described as providing a layer of growth sites to promote uniform deposition of a tungsten layer. The benefits provided by the nucleation layer are described as being dependent upon the barrier layer present. For example, were the barrier layer formed from titanium nitride the tungsten layer&#39;s thickness uniformity is improved as much as 15%. Were the barrier layer formed from sputtered tungsten or sputtered titanium tungsten the benefits provided by the nucleation layer are not substantial. 
   A need exists, therefore, to provide techniques to improve the characteristics of refractory metal layers deposited on semiconductor substrates. 
   SUMMARY OF THE INVENTION 
   A method and system reduces the resistance of contacts of refractory metal layers by controlling the presence of fluorine contained therein. The present invention is based upon the discovery that when employing ALD techniques to form refractory metal layers on a substrate, the carrier gas employed impacts the presence of fluorine in the resulting layer. As a result, the method features chemisorbing onto the substrate alternating monolayers of a first compound and a second compound, with the second compound having fluorine atoms associated therewith, with each of the first and second compounds being introduced into the processing chamber along with a carrier gas; and controlling a quantity of the fluorine atoms associated with the monolayer of the second compound as a function of the carrier gas. Specifically, it was found that by introducing the first and second compounds employing H 2  as a carrier gas, the amount of fluorine present in the resulting refractory metal layer was substantially reduced, compared to employing either nitrogen, N 2 , or argon, Ar, as a carrier gas. 
   To that end, the system includes a processing chamber, having a holder, disposed therein to support the substrate. A gas delivery system and a pressure control system are in fluid communication with the processing chamber. A temperature control system is in thermal communication therewith. A controller is in electrical communication with gas delivery system, temperature control system, and the pressure control system. A memory is in data communication with the controller. The memory comprises a computer-readable medium having a computer-readable program embodied therein. The computer-readable program includes instructions for controlling the operation of the processing chamber. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of a semiconductor processing system in accordance with the present invention; 
       FIG. 2  is a detailed view of the processing chambers shown above in  FIG. 1 ; 
       FIG. 3  is a schematic view showing deposition of a first molecule onto a substrate during ALD; 
       FIG. 4  is a schematic view showing deposition of second molecule onto a substrate during ALD to form a refractory metal layer; 
       FIG. 5  is a graphical representation showing the concentration of gases introduced into the processing chamber shown above in  FIG. 2 , and the time in which the gases are present in the processing chamber, in accordance with the present invention; 
       FIG. 6  is a graphical representation showing the relationship between the number of ALD cycles and the thickness of a layer formed on a substrate employing ALD, in accordance with the present invention; 
       FIG. 7  is a graphical representation showing the relationship between the number of ALD cycles and the resistivity of a layer formed on a substrate employing ALD, in accordance with the present invention; 
       FIG. 8  is a graphical representation showing the relationship between the deposition rate of a layer formed on a substrate employing ALD and the temperature of the substrate; 
       FIG. 9  is a graphical representation showing the relationship between the resistivity of a layer formed on a substrate employing ALD and the temperature of the substrate, in accordance with the present invention; 
       FIG. 10  is a cross-sectional view of a patterned substrate having a nucleation layer formed thereon employing ALD, in accordance with the present invention; 
       FIG. 11  is a partial cross-sectional view of the substrate, shown above in  FIG. 10 , with a refractory metal layer formed atop of the nucleation layer employing CVD, in accordance with the present invention; 
       FIG. 12  is a graphical representation showing the concentration of gases shown above in  FIG. 5  in accordance with a first alternate embodiment of the present invention; 
       FIG. 13  is a graphical representation showing the concentration of gases shown above in  FIG. 5  in accordance with a second alternate embodiment of the present invention; 
       FIG. 14  is a graphical representation showing the fluorine content versus depth of a refractory metal layer formed on a substrate employing ALD either Ar or N 2  being a carrier gas; and 
       FIG. 15  is a graphical representation showing the fluorine content versus depth of a refractory metal layer formed on a substrate employing ALD with H 2  being a carrier gas. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIG. 1 , an exemplary wafer processing system includes two or more processing chambers  12  and  14  disposed in a common work area  16  surrounded by a wall  18 . The processing chambers  12  and  14  are in data communication with a controller  22  that is connected to one or more monitors, shown as  24  and  26 . The monitors typically display common information concerning the process associated with the processing chambers  12  and  14 . One of the monitors  26  is mounted to the wall  18 , with the remaining monitor  24  being disposed in the work area  16 . Operational control of the processing chambers  12  and  14  may be achieved use of a light pen, associated with one of the monitors  24  and  26 , to communicate with the controller  22 . For example, light pen  28  is associated with monitor  24  and facilitates communication with the controller  22  through monitor  24 . Light pen  30  facilitates communication with the controller  22  through monitor  26 . 
   Referring both to  FIGS. 1 and 2 , each of the processing chambers  12  and  14  includes a housing  30  having a base wall  32 , a cover  34 , disposed opposite to the base wall  32 , and a sidewall  36 , extending therebetween. The housing  30  defines a chamber  37 , and a pedestal  38  is disposed within the processing chamber  37  to support a substrate  42 , such as a semiconductor wafer. The pedestal  38  may be mounted to move between the cover  34  and the base wall  32 , using a displacement mechanism (not shown). Supplies of processing gases  39   a ,  39   b  and  39   c  are in fluid communication with the processing chamber  37  via a showerhead  40 . Regulation of the flow of gases from the supplies  39   a ,  39   b  and  39   c  is effectuated via flow valves  41 . 
   Depending on the specific process, the substrate  42  may be heated to a desired temperature prior to layer deposition via a heater embedded within the pedestal  38 . For example, the pedestal  38  may be resistively heated by applying an electric current from an AC power supply  43  to the heater element  44 . The wafer  40  is, in turn, heated by the pedestal  38 , and can be maintained within a desired process temperature range of, for example, about 20° C. to about 750° C. A temperature sensor  46 , such as a thermocouple, is also embedded in the wafer support pedestal  38  to monitor the temperature of the pedestal  38  in a conventional manner. For example, the measured temperature may used in a feedback loop to control the electrical current applied to the heater element  44  by the power supply  43 , such that the wafer temperature can be maintained or controlled at a desired temperature which is suitable for the particular process application. The pedestal  38  is optionally heated using radiant heat (not shown). A vacuum pump  48  is used to evacuate the processing chamber  37  and to help maintain the proper gas flows and pressure inside the processing chamber  37 . 
   Referring to  FIGS. 1 and 3 , one or both of the processing chambers  12  and  14 , discussed above may operate to deposit refractory metal layers on the substrate employing ALD techniques. Depending on the specific stage of processing, the refractory metal layer may be deposited on the material from which the substrate  42  is fabricated, e.g., SiO 2 . The refractory metal layer may also be deposited on a layer previously formed on the substrate  42 , e.g., titanium, titanium nitride and the like. 
   ALD proceeds by chemisorption. The initial surface of the substrate  42  presents an active ligand to the process region. A batch of a first processing gas, in this case Aa x , results in a layer of A being deposited on the substrate  42  having a surface of ligand x exposed to the processing chamber  37 . Thereafter, a purge gas enters the processing chamber  37  to purge the gas Aa x . After purging gas Aa x  from the processing chamber  37 , a second batch of processing gas, Bb y , is introduced into the processing chamber  37 . The a ligand present on the substrate surface reacts with the b ligand and B atom on the, releasing molecules ab and Ba, that move away from the substrate  42  and are subsequently pumped from the processing chamber  37 . In this manner, a surface comprising a monolayer of A atoms remains upon the substrate  42  and exposed to the processing chamber  37 , shown in  FIG. 4 . The process proceeds cycle after cycle, until the desired thickness is achieved. 
   Referring to both  FIGS. 2 and 5 , although any type of processing gas may be employed, in the present example, the processing gas Aa x  is WF 6  and the processing gas Bb y  is B 2 H 6 . Two purge gases were employed: Ar and N 2 . Each of the processing gases is flowed into the processing chamber  37  with a carrier gas, which in this example were one of the purge gases: WF 6  is introduced with Ar and B 2 H 6  is introduced with N 2 . It should be understood, however, that the purge gas may differ from the carrier gas, discussed more fully below. One cycle of the ALD technique in accordance with the present invention includes flowing the purge gas, N 2 , into the processing chamber  37  during time t 1 , which is approximately five seconds before B 2 H 6  is flowed into the processing chamber  37 . During time t 2 , the processing gas B 2 H 6  is flowed into the processing chamber  37  for approximately five seconds, along with a carrier gas, which in this example is N 2 . After five seconds have lapsed, the flow of B 2 H 6  terminates and the flow of N 2  continues during time t 3  for an additional five seconds, purging the processing chamber of B 2 H 6 . During time t 4 , the processing chamber  37  is pumped so as to remove all gases. The pumping process lasts approximately thirty seconds. After pumping of the process chamber  37 , the carrier gas Ar is introduced for approximately five seconds during time t 5 , after which time the process gas WF 6  is introduced into the processing chamber  37  for about five seconds, along with the carrier gas Ar during time t 6 . The flow of the processing gas WF 6  into the processing chamber  37  is terminated approximately five seconds after it commenced. After the flow of WF 6  into the processing chamber  37  terminates, the flow of Ar continues for five additional seconds, during time t 7 . Thereafter, the processing chamber  37  is pumped so as to remove all gases therein, during time t 8 . As before, the pumping process lasts approximately thirty seconds, thereby concluding one cycle of the ALD technique in accordance with the present invention. 
   The benefits of employing ALD are manifold, including flux-independence of layer formation that provides uniformity of deposition independent of the size of a substrate. For example, the measured difference of the layer uniformity and thickness measured between of 200 mm substrate and a 32 mm substrate deposited in the same chamber is negligible. This is due to the self-limiting characteristics of chemisorption. Further, the chemisorption characteristics contribute to near-perfect step coverage over complex topography. 
   In addition, the thickness of the layer A, shown in  FIG. 4 , may be easily controlled while minimizing the resistance of the same by employing ALD. With reference to  FIG. 6  it is seen by the slope of line  50  that the thickness of the tungsten layer A is proportional to the number of cycles employed to form the same. The resistivity of the tungsten layer, however, is relatively independent of the thickness of the layer, as shown by the slope of line  52  in  FIG. 7 . Thus, employing ALD, the thickness of a refractory metal layer may be easily controlled as a function of the cycling of the process gases introduced into the processing chamber with a negligible effect on the resistivity. 
   Referring to  FIG. 8 , control of the deposition rate was found to be dependent upon the temperature of the substrate  42 . As shown by the slope of line  54 , increasing the temperature of the substrate  42  increased the deposition rate of the tungsten layer A. For example, at  56 , the deposition rate is shown to be approximately 2 Å/cycle at 250° C. However at point  58  the deposition rate is approximately 5 Å/cycle at a temperature of 450° C. The resistivity of the tungsten layer, however, is virtually independent of the layer thickness, as shown by the slope of curve  59 , shown in  FIG. 9 . As a result, the deposition rate of the tungsten layer may be controlled as a function of temperature without comprising the resistivity of the same. However, it is preferred to perform many processing steps at temperatures well below 450° C. 
   To that end, a bifurcated deposition process may be practiced in which nucleation of the refractory metal layer occurs in a different chamber than the formation of the remaining portion of the refractory metal layer. Specifically, in the present example, nucleation of a tungsten layer occurs in chamber  12  employing the ALD techniques discussed above, with the substrate  42  being heated in the range of 200° C. to 400° C., and the processing chamber  37  being pressurized in the range of 1 to 10 Torr. A nucleation layer  60  of approximately 12 to 20 nm is formed on a patterned substrate  42 , shown in  FIG. 10 . As shown, the substrate  42  includes a barrier layer  61  and a patterned layer having a plurality of vias  63 . The nucleation layer is formed adjacent to the patterned layer covering the vias  63 . As shown, forming the nucleation layer  60  employing ALD techniques provides 100% step coverage. To decrease the time required to form a complete layer of tungsten, a bulk deposition of tungsten onto the nucleation layer  60  occurs using CVD techniques, while the substrate  42  is disposed in processing chamber  14 , shown in  FIG. 1 . The bulk deposition may be performed using recipes well known in the art. In this manner, a tungsten layer  65  providing a complete plug fill is achieved on the patterned layer with vias having aspect ratios of approximately 6:1, shown in  FIG. 11 . 
   As mentioned above, in an alternate embodiment of the present invention, the carrier gas may differ from the purge gas, as shown in  FIG. 12 . The purge gas, which is introduced at time intervals t 1 , t 3 , t 5  and t 7  comprises of Ar. The carrier gas, which is introduced at time intervals t 2  and t 6 , comprises of N 2 . Thus, at time interval t 2  the gases introduced into the processing chamber include a mixture of B 2 H 6  and N 2 , and a time interval t 6 , the gas mixture includes WF 6  and N 2 . The pump process during time intervals t 4  and t 8  is identical to the pump process discussed above with respect to  FIG. 5 . In yet another embodiment, shown in  FIG. 13 , the carrier gas during time intervals t 2  and t 6  comprises H 2 , with the purge gas introduced at time intervals t 1 , t 3 , t 5  and t 7  comprising of Ar. The pump processes at time intervals t 4  and t 8  are as discussed above. As a result, at time interval t 2  the gas mixture introduced into the processing chamber  37  consists of B 2 H 6  and H 2 , and WF 6  and H 2 , at time interval t 6 . 
   An advantage realized by employing the H 2  carrier gas is that the stability of the tungsten layer A may be improved. Specifically, by comparing curve  66  in  FIG. 14  with the curve  68  in  FIG. 15 , it is seen that the concentration of fluorine in the nucleation layer  60  is much less when H 2  is employed as the carrier gas, as compared with use of N 2  or Ar as a carrier gas. Specifically, the apex and nadir of curve  66  show that the fluorine concentration reaches levels in excess of 1×10 21  atoms per cubic centimeter and only as low as just below 1×10 19  atoms per cubic centimeter. Curve  68 , however, shows that the fluorine concentration is well below 1×10 21  atoms per cubic centimeter at the apex and well below 1×10 17  atoms per cubic centimeter at the nadir. Thus, employing H 2  as the carrier gas provides a much more stable film, i.e., the probability of fluorine diffusing into the substrate, or adjacent layer is reduced. This also reduces the resistance of the refractory metal layer by avoiding the formation of a metal fluoride that may result from the increased fluorine concentration. Thus, the stability of the nucleation layer, as well as the resistivity of the same, may be controlled as a function of the carrier gas employed. This is also true when a refractory metal layer is deposited entirely employing ALD techniques, i.e., without using other deposition techniques, such as CVD. 
   Referring again to  FIG. 2 , the process for depositing the tungsten layer may be controlled using a computer program product that is executed by the controller  22 . To that end, the controller  22  includes a central processing unit (CPU)  70 , a volatile memory, such as a random access memory (RAM)  72  and permanent storage media, such as a floppy disk drive for use with a floppy diskette, or hard disk drive  74 . The computer program code can be written in any conventional computer readable programming language; for example, 68000 assembly language, C, C++, Pascal, Fortran and the like. Suitable program code is entered into a single file, or multiple files, using a conventional text editor and stored or embodied in a computer-readable medium, such as the hard disk drive  74 . If the entered code text is in a high level language, the code is compiled and the resultant compiler code is then linked with an object code of precompiled Windows® library routines. To execute the linked and, compiled object code the system user invokes the object code, causing the CPU  70  to load the code in RAM  72 . The CPU  70  then reads and executes the code to perform the tasks identified in the program. 
   Although the invention has been described in terms of specific embodiments, one skilled in the art will recognize that various changes to the reaction conditions, i.e., temperature, pressure, film thickness and the like can be substituted and are meant to be included herein. In addition, other refractory metals may be deposited, in addition to tungsten, and other deposition techniques may be employed in lieu of CVD. For example, physical vapor deposition (PVD) techniques, or a combination of both CVD and PVD techniques may be employed. Therefore, the scope of the invention should not be based upon the foregoing description. Rather, the scope of the invention should be determined based upon the claims recited herein, including the full scope of equivalents thereof.