Abstract:
An alternating source MOCVD process is provided for depositing tungsten nitride thin films for use as barrier layers for copper interconnects. Alternating the tungsten precursor produces fine crystal grain films, or possibly amorphous films. The nitrogen source may also be alternated to form WN/W alternating layer films, as tungsten is deposited during periods where the nitrogen source is removed.

Description:
BACKGROUND OF THE INVENTION 
   The invention relates to the MOCVD of barrier metal thin films, more specifically for use in connection with CVD copper interconnects. 
   Diffusion barriers are commonly used in integrated circuit (IC) fabrication to prevent interdiffusion of metals. For example, a TiN film is used to prevent diffusion of Al into Si at contact regions and along metal lines. As the dimensions of ICs, particularly contact regions and metal lines, continue to shrink, the requirements for the conducting barrier also become more stringent. Thinner barriers are required without substantially increasing resistivity. Barriers also need to be more resistant to diffusion of various new metals, which are being introduced into production processes. One of the metals that is being introduced is copper. Although few diffusion barriers materials effectively block the diffusion of copper, metal nitride, for example tungsten nitride, has been shown to act as good barriers against copper diffusion. 
   Tungsten Nitride (WN) films have been deposited in the past using reactive sputtering, chemical vapor deposition (CVD) from tungsten hexafluoride (WF 6 ) and ammonia (NH 3 ), and metal-organic CVD using tungsten hexacarbonyl W(CO) 6  and NH 3 . 
   SUMMARY OF THE INVENTION 
   Accordingly, a source alternating CVD (SACVD) process for forming WN barrier layers is provided. The SACVD process comprises heating a substrate within a CVD chamber and introducing an alternating source W(CO) 6  precursor into the CVD chamber, while providing an NH 3  source; whereby the W(CO) 6  and NH 3  react to deposit WN onto the heated substrate. The NH 3  source may be held steady to produce WN thin films, or alternated as well to produce alternating WN/W thing films. 
   The substrate is heated to a temperature between about 200° C. and 450° C. within a CVD chamber at a pressure of between about 200 mtorr and 1,000 mtorr. An alternating source W(CO) 6  precursor may be introduced into the CVD chamber by sublimating a solid W(CO) 6  precursor and delivering it into the CVD chamber by turning a hydrogen carrier gas, which has a flow rate of between about 10 and 200 sccm during the on-state, on and off according to a sequence with a on/off period that varies from about 25 seconds/5 seconds to 5 seconds/25 seconds. An NH 3  source is also introduced into the CVD chamber at a flow rate of between about 20 sccm and 50 sccm, whereby the W(CO) 6  and NH 3  react to deposit WN onto the heated substrate. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a schematic illustration of an MOCVD chamber using a solid precursor. 
       FIG. 2  shows a process cycle alternating over time. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   A source alternating CVD (SACVD) process is provided for depositing thin films for semiconductor applications. 
     FIG. 1  shows a schematic illustration of a CVD chamber  10  for performing an MOCVD process. A substrate  12  is placed within the CVD chamber  10 . The substrate  12  is introduced into the chamber  10  through a handler  14  and positioned on a chuck  15 . Precursor sources are shown schematically to illustrate the use of a solid precursor source  16 , and a gas precursor source  20 . 
   If a solid precursor is used, an ampule  22  and delivery lines  24  are heated to cause sublimation of the solid precursor into a precursor gas. The resulting precursor gas is then delivered into the chamber  10  by a carrier gas, such as hydrogen, nitrogen, or a noble gas, such as helium, neon, or argon. A first gas inlet valve  30  is used to control the flow of the carrier gas into the ampule  22 . A first chamber valve  32  is used to control the flow of the carrier gas and precursor into the chamber  10 . In addition, a first mass flow controller  34  is used to further regulate the flow rate into the chamber  10 . 
   Gas precursors may also be introduced through a second gas inlet valve  36 . The gas precursor is also controlled using a second chamber valve  38 , possibly in combination with a second mass flow controller  40 . 
   Both precursors, once in gas form, are distributed uniformly onto a heated substrate surface through a shower head  42 , also referred to as a gas distributor. 
   In the case of MOCVD WN, a solid precursor, such as tungsten hexacarbonyl W(CO) 6 , and a gaseous precursor NH 3  are used to deposit WN films through thermal activated reactions. The W(CO) 6  precursor is carried into a deposition chamber by a carrier gas, for example hydrogen. The NH 3  precursor is delivered into the chamber directly. 
   Pneumatic valves  30 ,  32 ,  36 , and  38 , as well as mass flow controllers (MFCs)  34 , and  40  are used to control the “on/off” and flow rate of H 2  and NH 3 .  FIG. 2  illustrates shows an example operating sequence. The H 2  can be turned on and off as indicated by sequence  60 . Alternating the H 2  causes the tungsten source that it carries to alternate. The NH 3  can also be turned on and off as indicated by sequence number  62 . Keeping the NH 3  on while H 2  alternates, as shown in region  70 , allows the incoming tungsten source time to fully react with the NH 3  for full nitridation of the resulting film. Therefore, the as deposited films have a high density. The alternating source will also produce a smaller grain size than that produced without alternating the source, due to the grain growth being on and off in accordance with the H 2  flow. 
   If the NH 3  is turned off, while allowing the H 2  to either remain on, or alternate, will produce a layer of W. By alternately, turning the NH 3  on and off, as shown in region  76 , it is possible to produce films of WN/W in alternating layers. These alternating layers will produce even finer grains, or possibly amorphous films, as well as providing films with low resistivity values. 
   As shown in  FIG. 2 , sequences of keeping NH 3  steady while alternating H 2 , region  70 , can be combined with sequences that alternate H 2  while keeping NH 3  steady, region  76 , to tailor the film, or film stack, and its corresponding properties to accommodate specific applications. 
   This source alternating CVD (SACVD) is flexible for tailoring the material composition and properties. SACVD has a higher deposition rate than ALCVD, since it does not require a self-limiting process of depositing single molecular layers at a time. The SACVD process is capable for producing layers that are more than a single molecular layer thick during each cycle. This also makes it easier to implement as the pressure and temperature ranges do not have to be as carefully controlled. 
   The SACVD process for W(CO) 6  and NH 3  can be done at temperatures in the range between about 200 and 450° C. The ability to use process temperatures in the range below about 400° C. is preferable for integrating WN barrier layers with porous low-k materials, which are being considered for interconnect applications below 0.1 micron generation. Through source alternation, the as-deposited films may have high density, low resistivity, and very fine crystalline grains, or an amorphous structure, desirable for copper barrier applications. Illustrative examples are provided below. 
   EXAMPLE 1 
   The chamber  10  is provided at a pressure of between about 200 mtorr and 1000 mtorr. The substrate  12  is placed on the chuck  15  and heated to a temperature of between about 351 and 450° C. 
   The Source Alternating CVD (SACVD) process is employed to deposit WN thin films. The solid precursor ampule  22  is provided with W(CO) 6 . The ampule and feed lines are heated to sublimate the W(CO) 6 . An H 2  flow rate is set between 10 to 200 sccm for the on-state to deliver the W(CO) 6  into the chamber  10 . The off-state is essentially 0 sccm, such that the flow is turned off. The H 2  flow rate has an “on/off” period that varies from 25 seconds on and 5 seconds off, to 5 seconds on and 25 seconds off during the deposition cycle. 
   An NH 3  source having a flow rate in the range of about 20 to 50 sccm is also provided to introduce a source of nitrogen into the chamber  10 . The NH 3  source remains on continuously to deposit WN. 
   The process continues until a WN film of between about 5 nm and 7 nm is deposited. The actual process time necessary to produce the desired thin film thickness will vary depending on specific values chosen form within the identified ranges, but can be readily ascertained without undue experimentation. 
   EXAMPLE 2 
   The chamber  10  is provided at a pressure of between about 200 mtorr and 1000 mtorr. The substrate  12  is placed on the chuck  15  and heated to a temperature of between about 351 and 450° C. 
   The Source Alternating CVD (SACVD) process is employed to deposit WN thin films. The solid precursor ampule  22  is provided with W(CO) 6 . The ampule and feed lines are heated to sublimate the W(CO) 6 . An H 2  flow rate is set between 10 to 200 sccm for the on-state to deliver the W(CO) 6  into the chamber  10 . The off-state is essentially 0 sccm, such that the flow is turned off. 
   An NH 3  source having a flow rate in the range of about 20 to 50 sccm in the on-state is also provided to introduce a source of nitrogen into the chamber  10 . The NH 3  source remains on during WN deposition and turned off to produce layers of W. The NH 3  source is turned off for between about 5 and 25 seconds. By alternating the NH 3  it is possible to produce alternating WN/W films. 
   The process continues until a WN/W film of between about 5 nm and 7 nm is deposited. The actual process time necessary to produce the desired thin film thickness will vary depending on specific values chosen form within the identified ranges, but can be readily ascertained without undue experimentation.