Patent Publication Number: US-2011065273-A1

Title: Methods of Fabricating a Barrier Layer Over Interconnect Structures in Atomic Deposition Environments

Description:
CLAIM OF PRIORITY 
     This application is a divisional application, claiming priority under 35 USC 120, of U.S. application Ser. No. 11/591,310, filed on Oct. 31, 2001, and is herein incorporated by reference. 
    
    
     BACKGROUND 
     Integrated circuits use conductive interconnects to wire together the individual devices on a semiconductor substrate, or to communicate externally to the integrated circuit. Interconnect metallization for vias and trenches may include aluminum alloys and copper. As device geometry continued to scale down to 45-nm-node technology and sub-45-nm technology, the requirement of continuous barrier/seed layer with good step coverage along high aspect ratio geometry to provide void free copper filling becomes challenging. The motivation to go to ultra thin and conformal barrier in 45-nm-node or sub-45-nm-technology is to reduce the barrier&#39;s impact on via and line resistance. However, poor adhesion of copper to the barrier layer could cause delamination between the barrier layer and copper during processing or thermal stressing that poses a concern on electro-migration (EM) and stress-induced voiding. 
     Barrier overhang  104  near top of the interconnect structure  100 , as shown in  FIG. 1A , by conventional physical vapor deposition (PVD) process is known to cause copper voids in metal lines or vias during copper gap-fill due to poor step coverage. The limited deposition of barrier material in the lower corners  103 , as shown in  FIG. 1A , is also a known problem to cause copper diffusion, EM problem, and stress-induced voiding. To ensure sufficient barrier material in the lower corners, sufficient barrier materials need to be deposited in the interconnect structures, which would result in copper voids during copper gap-fill. Therefore, a more conformal barrier deposition is needed. 
     In addition to step coverage concern, barrier layer, such as tantalum nitride (TaN), adheres well to dielectric layer  150 ; however, the adhesion between TaN and copper is poor. TaN is a good copper diffusion barrier. In contrast, barrier layer, such as tantalum (Ta), adheres well to copper, but not as well to the dielectric layer. Although it&#39;s possible to deposit a TaN layer  111  to line the interconnect structure to allow the TaN to contact the dielectric material  150  and to deposit a Ta layer afterwards for copper  113  to be in contact with Ta  112 , as shown in  FIG. 1B . The Ta layer acts as a liner layer or a glue layer to copper. However, a two-step process is more complicated and the deposition of the first TaN makes the aspect ratio of the interconnect structure even higher, which worsen the step coverage issue of the following Ta layer. 
     In view of the foregoing, there is a need for systems and processes that deposit a thin and conformal barrier layer that can yield good adhesion with the dielectric layer surrounding the interconnect structure and also with the copper layer that covers the barrier layer to improve yield and electro-migration performance and to reduce the risk of stress-induce voiding of copper interconnect. 
     SUMMARY 
     Broadly speaking, the embodiments fill the need by providing improved processes and systems that produce a barrier layer with decreasing nitrogen concentration with the increase of film thickness. A barrier layer with decreasing nitrogen concentration with film thickness allows the end of barrier layer with high nitrogen concentration to have good adhesion with a dielectric layer and the end of barrier layer with low nitrogen concentration (or metal-rich) to have good adhesion with copper. It should be appreciated that the present invention can be implemented in numerous ways, including as a solution, a method, a process, an apparatus, or a system. Several inventive embodiments of the present invention are described below. 
     Methods of depositing a barrier layer on an interconnect structure in an atomic deposition environment are provided. One method includes depositing a barrier layer on the interconnect structure with a first nitrogen concentration during a first phase of deposition in the atomic deposition environment, The interconnect structure is formed in a dielectric layer. Then, continuing the deposition of the barrier layer on the interconnect structure with a second nitrogen concentration during a second phase deposition in the atomic deposition environment. The nitrogen concentration step-wisely decreases from the first nitrogen concentration in the first phase of the barrier layer to the second nitrogen concentration in the second phase of the barrier layer, and the first nitrogen concentration is highest where the barrier layer is in contact with the dielectric layer. A copper layer is then formed over the barrier layer, such that a nitrogen concentration in the barrier layer is lowest where the barrier layer is in contact with the copper layer. 
     In one embodiment, a method of depositing a barrier layer on an interconnect structure is provided. The method includes (a) providing an atomic layer deposition environment, (b) depositing a barrier layer on the interconnect structure with a first nitrogen concentration during a first phase of deposition in the atomic layer deposition environment. The method further includes (c) continuing the deposition of the barrier layer on the interconnect structure with a second nitrogen concentration during a second phase deposition in the atomic layer deposition environment. 
     In another embodiment, a method of depositing a metallic barrier layer on an interconnect structure in an atomic layer deposition system is provided. The method includes depositing the metallic barrier layer on the interconnect structure using atomic layer deposition, wherein the nitrogen concentration of the metallic barrier layer decreases with an increase of film thickness. 
     In yet another embodiment, a method of depositing a metallic barrier layer on an interconnect structure in an atomic layer deposition system is provided. The method includes depositing the metallic barrier layer on the interconnect structure using atomic layer deposition, wherein the nitrogen concentration of the metallic barrier layer decreases step-wide with an increase of film thickness. 
     Other aspects and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be readily understood by the following detailed description in conjunction with the accompanying drawings, and like reference numerals designate like structural elements. 
         FIG. 1A  (Prior art) shows an interconnect structure deposited with a barrier layer. 
         FIG. 1B  (Prior art) shows an interconnect structure deposited a two barrier layer and a copper layer. 
         FIG. 2  shows a dual damascene interconnect structure deposited with a barrier layer and a copper layer. 
         FIG. 3A  shows a cross section of an interconnect structure with a barrier layer sandwiched between a dielectric layer and a copper layer. 
         FIG. 3B  shows a molecular structure of TaN barrier layer deposited by an ALD process on a substrate surface 
         FIG. 3C  shows a barrier layer with continuously decreasing nitrogen concentration with increase of film thickness 
         FIG. 3D  shows a barrier layer with step-wise decreasing nitrogen concentration with increase of film thickness. 
         FIG. 4  shows an exemplary ALD deposition cycle. 
         FIG. 5A  shows an exemplary deposition pulse sequence of an ALD barrier layer with continuously decreasing nitrogen concentration with increase of film thickness. 
         FIG. 5B  shows another exemplary deposition pulse sequence of an ALD barrier layer with continuously decreasing nitrogen concentration with increase of film thickness. 
         FIG. 5C  shows another exemplary deposition pulse sequence of an ALD barrier layer with continuously decreasing nitrogen concentration with increase of film thickness. 
         FIG. 5D  shows another exemplary deposition pulse sequence of an ALD barrier layer with continuously decreasing nitrogen concentration with increase of film thickness. 
         FIG. 6A  shows an exemplary deposition pulse sequence of an ALD barrier layer with step-wise decreasing nitrogen concentration with increase of film thickness. 
         FIG. 6B  shows another exemplary deposition pulse sequence of an ALD barrier layer with step-wise decreasing nitrogen concentration with increase of film thickness. 
         FIG. 7A  shows an exemplary deposition pulse sequence of an ALD barrier layer with step-wise decreasing nitrogen concentration with increase of film thickness. 
         FIG. 7B  shows another exemplary deposition pulse sequence of an ALD barrier layer with step-wise decreasing nitrogen concentration with increase of film thickness. 
         FIG. 7C  shows another exemplary deposition pulse sequence of an ALD barrier layer with step-wise decreasing nitrogen concentration with increase of film thickness. 
         FIG. 8A  shows an exemplary ALD deposition system. 
         FIG. 8B  shows another exemplary ALD deposition system. 
     
    
    
     DETAILED DESCRIPTION 
     Several exemplary embodiments for depositing a metallic barrier layer with decreasing nitrogen concentration from initial deposition to final deposition to improve adhesion between the initial metal nitride barrier layer with dielectric and between the final tantalum barrier layer with copper are provided. It should be appreciated that the present invention can be implemented in numerous ways, including a process, a method, an apparatus, or a system. Several inventive embodiments of the present invention are described below. It will be apparent to those skilled in the art that the present invention may be practiced without some or all of the specific details set forth herein. 
       FIG. 2  shows an exemplary cross-section of an interconnect structure(s) after being patterned by using a dual damascene process sequence. The interconnect structure(s) is on a substrate  50  and has a dielectric layer  115 , which was previously fabricated to form a metallization line  101  therein. The metallization line is typically fabricated by etching a trench into the dielectric  106  and then filling the trench with a conductive material, such as copper. 
     In the trench, there is a metallic barrier layer  120 , used to prevent the copper material  122 , from diffusing into the dielectric  100 . The barrier layer  120  can be made of refractory metal compound, such as tantalum nitride (TaN), tantalum (Ta), or a combination of these films. Other barrier layer materials can also be used. Barrier layer materials may be other refractory metal compound including but not limited to titanium (Ti), tungsten (W), zirconium (Zr), hafnium (Hf), molybdenum (Mo), niobium (Nb), vanadium (V), ruthenium (Ru) and chromium (Cr), among others. Conventionally, a refractory metal is combined with reactive species, such as for example chlorine (Cl) or fluorine (F), and is provided with another gas to form a refractory metal compound. For example, titanium tetrachloride (TiCl 4 ), tungsten hexafluoride (WF 6 ), tantalum pentachloride (TaCl 5 ), zirconium tetrachloride (ZrCl 4 ), hafnium tetrachloride (HfCl 4 ), molybdenum pentachloride (MoCl 5 ), niobium pentachloride (NbCl 5 ), vanadium pentachloride (VCl 5 ), or chromium tetrachloride (CrCl 4 ) may be used as a refractory metal-containing compound gas. 
     A barrier layer  102  is deposited over the planarized copper material  122  to protect the copper material  122  from premature oxidation when via holes  114  are etched through overlying dielectric material  106  to the dielectric barrier layer  102 . The dielectric barrier layer  102  is also configured to function as a selective etch stop and a copper diffusion barrier. Exemplary dielectric barrier layer  102  materials include silicon nitride (SiN) or silicon carbide (SiC). 
     The dielectric layer  106  is deposited over the dielectric barrier layer  102 . The dielectric layer  106  can be made of an organo-silicate glass (OSG, carbon-doped silicon oxide) or other types of dielectric materials, preferably with low dielectric constants. Exemplary silicon dioxides can include, a PECVD un-doped TEOS silicon dioxide, a PECVD fluorinated silica glass (FSG), a HDP FSG, OSG, porous OSG, or a carbon-doped oxide (C-oxide). The dielectric constant of the low K dielectric material can be about 3.0 or lower. Commercially available dielectric materials including Black Diamond (I) and Black Diamond (II) by Applied Materials of Santa Clara, Calif., Coral by Novellus Systems of San Jose, Aurora by ASM America Inc. of Phoenix, Ariz., can also be used. Alternatively, the dielectric layer can be divided into a via dielectric layer and a trench dielectric layer. The via dielectric layer and the trench dielectric layer can be made of different materials. After the trench dielectric layer  106  is deposited, the substrate  50  that holds the structure(s) undergoes patterning and etching processes to form the vias holes  114  and trenches  116  by known art. 
     After the formation of via holes  114  and trenches  116 , a barrier layer  130  and a copper layer  132  are deposited to line and fill the via holes  114  and the trenches  116 . The barrier layer  130  can be made of refractory metal, such as tantalum nitride (TaN), tantalum (Ta), Ruthenium (Ru), or a hybrid combination of these films. Other barrier layer materials can also be used. Barrier layer materials may be a refractory metal compound including but not limited to titanium (Ti), tungsten (W), zirconium (Zr), hafnium (Hf), molybdenum (Mo), niobium (Nb), vanadium (V), and chromium (Cr), among others. Atomic layer deposition (ALD), pulsed CVD, or cyclic layer deposition processes can be used to achieve good step coverage of the barrier layer. While these are the commonly considered materials, other barrier layer materials can also be used. A copper film  132  is then deposited to fill the via holes  114  and the trenches  116 . 
     After copper film  132  fills the via holes  114  and trenches  116 , substrate  50  is planarized by chemical-mechanical polishing (CMP) to remove the copper material (or copper overburden) and barrier layer (or barrier overburden) over the surface of dielectric layer  106 . The metallic barrier layer can also be deposited to line a single-damascene contact, via, or a metal structure (not shown). 
       FIG. 3A  shows a schematic cross section  300  of layers of an interconnect structure. A metallic barrier layer  302  is sandwiched between a dielectric layer  301  and a copper layer  303 . As described above, the dielectric-barrier interfacial layer  304  needs to be nitrogen-rich to promote good adhesion between the metallic barrier layer  302  and the dielectric layer  301 . The barrier-copper interfacial layer  305  needs to be barrier-metal-rich to promote good adhesion between the metallic barrier layer  302  and the copper layer  303 . To ensure good step coverage of the barrier layer, the barrier layer is desirable to be deposited by an ALD process, since ALD process produces more conformal films with better step coverage. 
       FIG. 3B  depicts a cross-sectional view of an exemplary portion of substrate  350  in a stage of integrated circuit fabrication, and more particularly at a stage of barrier layer  355  formation by an ALD process. The exemplary barrier material depicted in  FIG. 3B  is tantalum nitride. Tantalum layer  352  is formed by chemisorbing a tantalum-containing compound on surface portion  351 T of substrate  351  by introducing a pulse of a tantalum-containing gas into an ALD process chamber. Afterwards, a layer  353  of nitrogen is illustratively shown as chemisorbed on tantalum layer  352  at least in part in response to introduction of a nitrogen-containing gas, such as ammonia. While not wishing to be bound by theory, it is believed that nitrogen layer  353  is formed in a similar self-limiting manner, as was tantalum layer  352 . Each tantalum layer  352  and nitrogen layer  353  in any combination and in direct contact with one another form a sublayer  354 , whether or not either or both or neither is a monolayer. The formation of sublayers  354  continues until a targeted thickness  355  is reached. 
     The nitrogen concentration in the barrier layer can continuously varying, as shown in  FIG. 3C , or step-wise decreasing with the thickness of the metallic barrier layer, as shown in  FIG. 3D .  FIG. 3D  shows that the nitrogen concentration decreases with film thickness in three steps. However, the number of steps could be two or greater than three. When the steps in  FIG. 3D  is numerous, the nitrogen variation pattern approaches the nitrogen concentration profile of  FIG. 3C . 
     ALD deposition is typically accomplished by using multiple pulses, such as two pulses, of reactants with purge in between, as shown in  FIG. 4 . For metallic barrier deposition, a pulse of barrier-metal-containing reactant (M)  401  is delivered to the substrate surface, followed by a pulse of purging gas (P)  402 . Examples of barrier-metal-containing reactant for barrier metal such as tantalum include but are not limited to pentaethylmethylamino-tantalum (PEMAT; Ta(NC 2 H 5 CH 3 ) 5 ), pentadiethylamino-tantalum (PDEAT, Ta[N(C 2 H 5 ) 2 ] 5 ), pentadimethylamino-tantalum (PDMAT, Ta[N(CH 3 ) 2 ] 5 ), and any and all of derivatives of PEMAT, PDEAT, or PDMAT. Other tantalum containing precursors include without limitation tertbutylimido-tris(diethylamido)-tantalum (TBTDET), tertbutylimido-tris(dimethylamido)-tantalum (TBTDMT), and tertbutylimido-tris(ethylmethylamido)-tantalum (TBTEMT), and all of derivatives of TBTDET, TBTDMT, and TBTEMT. Additionally, other tantalum containing precursors include without limitation tantalum halides for example TaX 5  where X is fluorine (F), bromine (Br) or chlorine (Cl), and derivatives thereof. Examples of purging gas include helium (He), neon (Ne), argon (Ar), hydrogen (H 2 ), nitrogen (N 2 ), and combinations thereof. The pulse of barrier-containing reactant  401  delivered to the substrate surface form a monolayer of barrier metal, such as Ta, on the substrate surface. In one embodiment, the pulse of purging gas is a plasma enhanced (or plasma assisted) gas. The barrier metal, such as Ta, bonds to the substrate surface, which is made of dielectric material. The purge gas  402  removes the excess barrier-metal-containing reactant  401  from the substrate surface. Following the pulse of the purging gas  402 , a pulse of reactant (B)  403  is delivered to the substrate surface. The reactant (B)  403  can be nitrogen-containing gas or a hydrogen-containing reducing gas. If the barrier material contains nitrogen, such as TaN, the reactant (B)  403  is likely to contain nitrogen. Examples of reactant (B)  403  include ammonia (NH 3 ), N 2 , and NO. Other N-containing precursors gases may be used including but not limited to N x H y  for x and y integers (e.g., N 2 H 4 ), N 2  plasma source, NH 2 N(CH 3 ) 2 , among others. If the barrier material contains little or no nitrogen, the reactant (B)  403  can be a hydrogen-containing reducing gas, such as H 2 . In one embodiment, the pulse of reactant (B)  403  is a plasma-enhanced (or plasma-assisted). Following pulse  403  is a pulse of purging gas  404 .  FIG. 4  shows one cycle of the barrier deposition pulses. After one cycle of the barrier deposition pulses, a thin layer (or phase) of layer (or phase) of barrier layer is deposited. Multiple cycles of pulses are applied until desired barrier layer thickness is achieved. The duration of the pulses is between about 100 mili-seconds to about 2 seconds. The total thickness of the barrier layer is between about 10 angstroms to about 50 angstroms, preferably between about 20 angstroms to about 30 angstroms. 
       FIG. 5A  shows an embodiment of pulses of reactants and purging gases as a function of time to achieve continuously decreasing nitrogen content in the barrier layer as shown in  FIG. 3C . 
     The deposition process starts by applying a pulse of barrier-metal-containing reactant (M)  501  on the substrate surface. Pulse  501  is followed by a pulse  502  of a purging gas (P) to remove excess barrier-metal-containing reactant  501  from the substrate surface. Afterwards, a pulse of reactant gas (B) is applied. Since the nitrogen content of the barrier metal layer decreases with film thickness, the reactant gas (B) used for pulse  503  should contain nitrogen. The reactant in pulse  503  react with the barrier-metal-containing reactant (M) on the substrate surface to form a metal nitride barrier layer, such as TaN. The examples of the nitrogen-containing gas include ammonia (NH 3 ), nitrogen (N 2 ), and nitrogen oxide (NO), and other nitrogen-containing reactant B described above. Pulses  501 ,  502 ,  503  and  504  constitute one cycle (cycle 1). The second cycle includes pulses  505 ,  506 ,  507 , and  508 . Pulse  505  is identical to pulse  501 . Pulse  506  is identical to pulse  502 . Pulse  508  is identical to pulse  504 . Pulse  507  uses the same reactant as pulse  503 , however, the duration of pulse  507  (t B2 ) is shorter than pulse  503  (t B1 ). The shorter pulse of the nitrogen-containing gas B makes the concentration of nitrogen in the barrier layer lower. The cycles continue to cycle N. In order for the concentration of nitrogen to decrease from cycle 1 to cycle 2 and continuing to cycle N, the pulse of reactant B from cycle 2 continuing to cycle N should not saturate (or under-saturate) the substrate surface. The duration of pulse with reactant B continues to decrease with each cycle to result in the nitrogen concentration in the deposited barrier layer. At the end of cycle N, the barrier layer reaches its targeted thickness. In one embodiment, the duration of the pulse with reactant B is zero second to make the top barrier surface metal-rich. 
     As described above, pulses with reactant B and purging gas P can be plasma enhanced. The plasma enhanced purging gas can densify the barrier layer and also can knock off the excess molecules attached to the barrier metal. For example, the M precursor PDMAT is a large molecules with chemical compound attached to the Ta molecule. The chemical compound can stay attached to Ta after Ta is chemisorbed on the substrate surface. The plasma of the purging gas or the reactant B can knock off the excessive molecules attached to the Ta to assist in the reaction of reactant B with Ta. 
     In one embodiment, there is a final plasma treatment to further reduce surface compound to be more metal-rich. The reducing plasma can include gas, such as hydrogen or ammonia. The reducing plasma can include an inert gas, such as Ar, or He. This final plasma treatment can also densify the barrier layer. In another embodiment, after the deposition cycles in the ALD process chamber, the substrate can be moved to a PVD process chamber for depositing a thin barrier metal layer, which is called a barrier metal flash. For example, if the barrier metal is Ta, the process is called Ta flash. This allows the top barrier surface to be Ta, which adhere well to copper. 
     Alternatively, the duration of the pulses of barrier-metal-containing reactant M increase with time, while the duration of the pulses of the nitrogen-containing gas B decrease with time, as shown in  FIG. 5B . The duration t M2  of pulse  505 ′ is longer than duration t M1  of pulse  501 ′, while the duration t B2  of pulse  507  is shorter than the duration t B1  of pulse  503 . The longer pulse of the barrier-metal-containing reactant M and shorter pulse of the nitrogen-containing gas B makes the concentration of nitrogen in the barrier layer deposited in cycle 2 lower than the nitrogen concentration in cycle 1. The cycles continue to cycle N. The duration of pulse with M continues to increase and the duration of pulse with reactant B continues to decrease with each cycle to result in the nitrogen concentration in the deposited barrier layer. At the end of cycle N, the barrier layer reaches its targeted thickness. In one embodiment, the duration of the pulse with reactant B is zero second to make the top barrier surface metal-rich. 
       FIG. 5C  shows another embodiment of a deposition process that would yield a barrier layer with decreasing nitrogen content in the barrier layer with film thickness, as shown in  FIG. 3C . In this embodiment, the M and P pulses stay the same through different deposition cycles. Reactant B is a nitrogen-containing compound. The pulse time to reactant B stays the same with different cycles. However, the concentration (or amount) of reactant B in the pulses of reactant B decreases with each deposition cycle. As shown in  FIG. 5C , the concentration of reactant B in pulse  553  is higher than pulse  557 . With decreasing concentration (or amount) of B with each deposition cycle, the nitrogen concentration decreases with film thickness. Similarly, the deposition cycles continue until cycle N when the target thickness has been reached. 
     Alternatively, the concentration of the barrier-metal-containing reactant M can be made to increase with cycles, while the concentration of the nitrogen-containing compound B is made to decrease with cycles, as shown in  FIG. 5D . Such combination can also result in decrease of nitrogen concentration in the barrier layer with film thickness. 
     As shown in  FIG. 3D , another way to create a barrier film with high concentration of nitrogen in the initial layer and low (or no) nitrogen concentration on the top surface is creating a barrier film with step-wise nitrogen concentration.  FIG. 3D  shows three concentration levels. However, other concentration levels are possible, as long as the nitrogen concentration decreases with film thickness.  FIG. 6A  shows an embodiment of pulses of reactants and purging gases as a function of time to achieve decreasing nitrogen content in the barrier layer as shown in  FIG. 3D . 
     The deposition process starts by applying a pulse of barrier-metal-containing reactant (M)  601  on the substrate surface. Pulse  601  is followed by a pulse  602  of a purging gas (P) to remove excess barrier-metal-containing reactant  601  from the substrate surface. Afterwards, a pulse of reactant gas (B) is applied. Since the nitrogen content of the barrier metal layer decreases with film thickness, the reactant gas (B) used for pulse  603  should contain nitrogen. The reactant in pulse  603  react with the barrier-metal-containing reactant (M) on the substrate surface to form a metal nitride barrier layer, such as TaN, with concentration C 1  shown in  FIG. 3D . The pulse duration of reactant B pulse  603  is t B1 . The examples of the nitrogen-containing gas include ammonia (NH 3 ), nitrogen (N 2 ), and nitrogen oxide (NO). Other types of nitrogen-containing gases described above are also possible. Pulses  601 ,  602 ,  603  and  604  constitute one cycle to deposit a barrier layer with nitrogen concentration at C 1  level. The cycles repeats until the film thickness reaches t 1  thickness (X cycles), which is shown in  FIG. 3D . 
     The second type of cycle includes pulses  611 ,  612 ,  613 , and  614  to deposit a barrier layer with C 2  nitrogen concentration. Pulse  611  is identical to pulse  601 . Pulse  612  is identical to pulse  602 . Pulse  614  is identical to pulse  604 . Pulse  613  uses the same reactant as pulse  603 , however, the duration t B2  of pulse  613  is shorter than the duration t B1  of pulse  603 . The shorter pulse of the nitrogen-containing gas B makes the concentration of nitrogen in the barrier layer lower. The cycles repeat until the barrier layer reaches the thickness of t 2  (Y cycles). Afterwards, the third type of cycle includes pulse  621 ,  622 ,  623 , and  624 . The duration T B3  of pulse  623  with reactant B is lower than duration t B2  of pulse  613 . The cycles repeat until the final barrier layer thickness is reached (Z cycles). At the end of Z cycles, the barrier layer reaches its targeted thickness. In one embodiment, the duration of the pulse with reactant B is zero second to make the top barrier surface metal-rich. As described above, pulses with reactant B and purging gas P can be plasma enhanced. The plasma enhanced purging gas can densify the barrier layer and also can knock off the excess molecules attached to the barrier metal. In one embodiment, there is a final plasma treatment to further reduce surface compound to be more metal-rich. The reducing plasma can include gas, such as hydrogen or ammonia. The reducing plasma can include an inert gas, such as Ar, or He. This final plasma treatment can also densify the barrier layer. In another embodiment, after the deposition cycles in the ALD process chamber, the substrate can be moved to a PVD process chamber for depositing a thin barrier layer, which is called a barrier flash. For example, if the barrier metal is Ta, the process is called Ta flash. This allows the top barrier surface to be Ta, which adhere well to copper. 
     The pulsing concentration and duration of the deposition reactants and purging gas of  FIG. 6A  is similar to  FIG. 5A , except the cycles of  FIG. 5A  are not repeated, while the cycles of  FIG. 6A  are repeated. Similarly, the concept described in  FIG. 5B , with both the durations of barrier-metal-containing reactant M and nitrogen-containing reactant B varying at the same time, can be applied to deposit barrier layer with step-wise-varying nitrogen concentration. Each cycle described in  FIG. 5B  is repeated in a manner shown in  FIG. 6A  to achieve step-wise variation in nitrogen concentration. 
       FIG. 6B  shows another embodiment of a deposition process that would yield a barrier layer with decreasing nitrogen content in the barrier layer with increase of film thickness, as shown in  FIG. 3D . In this embodiment, the pulse time to reactant B stays the same with different cycles. However, the concentration (or amount) of reactant B in the pulses of reactant B decreases with each cycle. As shown in  FIG. 6B , the concentration of reactant B in pulse  653  is higher than pulse  663 , which is higher than pulse  673 . Similarly, each deposition cycle continues until the target thickness has been reached. The decreasing concentration of reactant B, which contains nitrogen, in the three types of deposition cycles allows the deposited barrier layer to have decreasing nitrogen concentration with increase of film thickness, as shown in  FIG. 3D . 
     As described above for repeating cycles in  FIG. 5B  to deposit barrier layer with step-wise-varying nitrogen concentration, the concept described in  FIG. 5D , with both the concentrations of barrier-metal-containing reactant M and nitrogen-containing reactant B varying at the same time, can be applied to deposit barrier layer with step-wise-varying nitrogen concentration. Each cycle described in  FIG. 5D  is repeated in a manner shown in  FIG. 6A  to achieve step-wise variation in nitrogen concentration. 
       FIG. 7A  shows another embodiment of a deposition process that would yield a barrier layer with decreasing nitrogen content in the barrier layer with increase of film thickness, as shown in  FIG. 3D . The deposition process starts by applying a pulse of barrier-metal-containing reactant (M)  701  on the substrate surface. Pulse  701  is followed by a pulse  702  of a purging gas (P) to remove excess barrier-metal-containing reactant  701  from the substrate surface. Afterwards, a pulse of reactant gas (B) is applied. Since the nitrogen content of the barrier metal layer decreases with film thickness, the reactant gas (B) used for pulse  703  should contain nitrogen. The reactant (B) in pulse  703  react with the barrier-metal-containing reactant (M) on the substrate surface to form a metal nitride barrier layer, such as TaN, with nitrogen concentration at C 1  as shown in  FIG. 3D . Pulses  701 ,  702 ,  703  and  704  constitute one cycle. The cycles repeats until the film thickness reaches t 1  thickness (X cycles), which is shown in  FIG. 3D . 
     The second type of cycle includes pulses  711 ,  712 ,  713 , and  714  to deposit a barrier layer with C 2  nitrogen concentration. Pulse  711  is identical to pulse  701 . Pulse  712  is identical to pulse  702 . Pulse  714  is identical to pulse  704 . Pulse  713  uses a reactant C that would react with the barrier-metal-containing reactant M to produce a barrier layer with less nitrogen content C 2 , which is less than C 1  concentration resulting from reacting M with reactant B. The cycles of  711 ,  712 ,  713 , and  714  pulses repeat until the film thickness reaches t 2  thickness, which is shown in  FIG. 3D . The third type of cycle includes pulses  721 ,  722 ,  723 , and  724  to deposit a barrier layer with C 3  nitrogen concentration. Pulse  721  is identical to pulses  701  and  711 . Pulse  722  is identical to pulses  702  and  712 . Pulse  724  is identical to pulses  704  and  714 . Pulse  723  uses a reactant D that would react with the barrier-metal-containing reactant M to produce a barrier layer with less nitrogen content C 3 , which is less than C 2  concentration resulting from reacting M with reactant C. The cycles of  721 ,  722 ,  723 , and  724  pulses repeat until the film thickness reaches t 3  thickness, which is shown in  FIG. 3D . The pulse time and concentration for B, C, and D can be the same or varied. B, C, D processes can be thermal or plasma enhanced. Similarly, the process step of using purging gas P can be thermal or plasma enhanced. 
     As described above, pulses with reactant B and purging gas P can be plasma enhanced. The plasma enhanced purging gas can densify the barrier layer and also can knock off the excess molecules attached to the barrier metal. In one embodiment, there is a final plasma treatment to further reduce surface compound to be more metal-rich. The reducing plasma can include gas, such as hydrogen or ammonia. The reducing plasma can include an inert gas, such as Ar, or He. This final plasma treatment can also densify the barrier layer. In another embodiment, after the deposition cycles in the ALD process chamber, the substrate can be moved to a PVD process chamber for depositing a thin barrier layer, which is called a barrier flash. For example, if the barrier metal is Ta, the process is called Ta flash. This allows the top barrier surface to be Ta, which adhere well to copper. 
     An alternative embodiment of the embodiment shown in  FIG. 7  A is increasing the duration or concentration of the barrier-metal containing reactant M  701 ,  711 ,  721 , to react with B, C and D. The combination can also result in step-wise decrease in nitrogen concentration in the barrier layer. 
       FIG. 7B  shows another embodiment of a deposition process that would yield a barrier layer with decreasing nitrogen content in the barrier layer with increase of film thickness, as shown in  FIG. 3D . The deposition process starts by applying a pulse of barrier-metal-containing reactant (Ma)  751  on the substrate surface. Pulse  751  is followed by a pulse  752  of a purging gas (P) to remove excess barrier-metal-containing reactant  751  from the substrate surface. Afterwards, a pulse of reactant gas (B) is applied. Since the nitrogen content of the barrier metal layer decreases with film thickness, the reactant gas (B) used for pulse  753  should contain nitrogen. The reactant in pulse  753  reacts with the barrier-metal-containing reactant (Ma) on the substrate surface to form a metal nitride barrier layer, such as TaN. Pulses  751 ,  752 ,  753  and  754  constitute one cycle to deposit a barrier layer with nitrogen concentration at C 1  level. The cycles repeats until the film thickness reaches t 1  t 1  thickness (X cycles), which is shown in  FIG. 3D . 
     The second type of cycle includes pulses  761 ,  762 ,  763 , and  764  to deposit a barrier layer with C 2  nitrogen concentration. Pulse  761  uses a barrier-metal-containing reactant Mb that is different from Ma in pulse  751 . Pulse  762  is identical to pulse  752 . Pulse  764  is identical to pulse  754 . Pulse  763  uses a reactant C that would react with the barrier-metal-containing reactant Mb to produce a barrier layer with less nitrogen content C 2 , which is less than C 1  concentration resulting from reacting Ma with reactant B. The cycles of  761 ,  762 ,  763 , and  764  pulses repeat until the film thickness reaches t 2  thickness (Y cycles), which is shown in  FIG. 3D . The third type of cycle includes pulses  771 ,  772 ,  773 , and  774  to deposit a barrier layer with C 3  nitrogen concentration. Pulse  771  uses a barrier-metal-containing reactant Mc that is different from Ma in pulse  751  and Mb in pulse  761 . Pulse  772  is identical to pulses  752  and  762 . Pulse  774  is identical to pulses  754  and  764 . Pulse  773  uses a reactant D that would react with the barrier-metal-containing reactant Mc to produce a barrier layer with less nitrogen content C 3 , which is less than C 2  concentration resulting from reacting Mb with reactant C. The cycles of  771 ,  772 ,  773 , and  774  pulses repeat until the film thickness reaches t 3  thickness (Z cycles), which is shown in  FIG. 3D . The pulse time and concentration for B, C, and D can be the same or varied. B, C, D processes can be thermal or plasma enhanced. Similarly, the process step of using purging gas P can be thermal or plasma enhanced. 
     Examples of barrier-metal-containing reactant, Ma, Mb, and Mc, for barrier metal such as tantalum include but are not limited to pentaethylmethylamino-tantalum (PEMAT; Ta(NC 2 H 5 CH 3 ) 5 ), pentadiethylamino-tantalum (PDEAT, Ta[N(C 2 H 5 ) 2 ] 5 ), pentadimethylamino-tantalum (PDMAT, Ta[N(CH 3 ) 2 ] 5 ), and any and all of derivatives of PEMAT, PDEAT, or PDMAT. Other tantalum containing precursors include without limitation tertbutylimido-tris(diethylamido)-tantalum (TBTDET), tertbutylimido-tris(dimethylamido)-tantalum (TBTDMT), and tertbutylimido-tris(ethylmethylamido)-tantalum (TBTEMT), and all of derivatives of TBTDET, TBTDMT, and TBTEMT. Additionally, other tantalum containing precursors include without limitation tantalum halides for example TaX 5  where X is fluorine (F), bromine (Br) or chlorine (Cl), and derivatives thereof. Examples of reactants B, C, and D include ammonia (NH 3 ), N 2 , and NO. Other N-containing precursors gases may be used including but not limited to N x H y  for x and y integers (e.g., N 2 H 4 ), N 2  plasma source, NH 2 N(CH 3 ) 2 , among others. 
     As described above, pulses with reactant B and purging gas P can be plasma enhanced. The plasma enhanced purging gas can densify the barrier layer and also can knock off the excess molecules attached to the barrier metal. In one embodiment, there is a final plasma treatment to further reduce surface compound to be more metal-rich. The reducing plasma can include gas, such as hydrogen or ammonia. The reducing plasma can include an inert gas, such as Ar, or He. This final plasma treatment can also densify the barrier layer. In another embodiment, after the deposition cycles in the ALD process chamber, the substrate can be moved to a PVD process chamber for depositing a thin barrier layer, which is called a barrier flash. For example, if the barrier metal is Ta, the process is called Ta flash. This allows the top barrier surface to be Ta, which adhere well to copper. 
     An alternative embodiment, as shown in  FIG. 7C , of the embodiment shown in  FIG. 7B  is using one single nitrogen-containing reactant B in all deposition cycles of  FIG. 7B . Ma reacts with B to deposit a barrier layer with concentration at C 1  level after X cycles. Mb reacts with B to deposit a barrier layer with concentration at C 2 ′ (lower than C 1 ) level after Y cycles. Mc reacts with B to deposit a barrier layer with concentration C 3 ′ (lower than C 2 ′) level after Z cycles. 
       FIG. 8A  depicts a schematic illustration of an exemplary wafer processing system  800  that can be used to form one or more barrier layers in accordance with aspects of the present invention described herein. System  800  comprises process chamber  850 , gas panel  830 , along with other hardware components such as power supply  806  and vacuum pump  802 . For purposes of clarity, salient features of process chamber  850  are briefly described below. 
     Process chamber  850  generally houses a support pedestal  851 , which is used to support a substrate such as a semiconductor wafer  890  within process chamber  850 . Depending on process requirements, semiconductor wafer  890  can be heated to some desired temperature or within some desired temperature range prior to layer formation with heating power from power supply  806 . Wafer  890  may be maintained within a desired process temperature range of, for example, about 100° C. to about 400° C., preferably between about 150° C. to about 350° C. 
     Vacuum pump  802  is used to evacuate process gases from process chamber  850  and to help maintain a desired pressure or desired pressure within a pressure range inside chamber  850 . Orifice  820  through a wall of chamber  850  is used to introduce process gases into process chamber  850 . Sizing of orifice  820  conventionally depends on the size of process chamber  850 . 
     Gas pipe  831  is coupled to gas panel  830  to provide a process gas from three or more gas sources  835 ,  836 ,  838  to process chamber  850  through gap pipe  831 . Reactant sources  835 , and  836  may store precursors in a liquid phase at room temperature, which are later heated when in gas panel  830  to convert them to a vapor-gas phase for introduction into chamber  850 . Gas panel  830  is further configured to receive and then provide a purge gas from purge gas source  838  to process chamber  850 . In one embodiment, reactant source  835  stores M precursors, while reactant source  836  stores reactant B described above. For the embodiment with more than one M precursors (Ma, Mb, and Mc), multiple reactant sources  835  ( 835   a,    835   b,  and  835   c ) can be used. Similarly, for the embodiment, with more than one B reactant (B, C, and D), multiple reactant sources  836  ( 836 B,  836 C, and  836 D) can be used. 
     Alternatively, the reactive gases can be injected from the side of the process chamber.  FIG. 8B  depicts a schematic illustration of another exemplary wafer processing system  870  that can be used to form one or more barrier layers in accordance with aspects of the present invention described herein. In this embodiment, the gas pipe  831 ′ is coupled to gas panel  830  to provide a process gas through the side of the process chamber  850  to the surface of substrate  890 . In one embodiment, the reactive gas(es) is introduced to the surface of the substrate  890  in laminar flow. 
     As described above, during or after forming one or more of the barrier layer, substrate structure  890  may be subjected to plasma process gas for reaction or for plasma treatment. While not wishing to be bound by theory, the plasma treatment, such as an Ar sputtering, can help remove the organic compound attached to the barrier metal, such as Ta, after the barrier metal is chemisorbed on the substrate surface. The organic compound(s) attached to the barrier metal is part of the barrier metal precursor. The plasma treatment can help remove impurity from the barrier layer. The plasma treatment can also improve the quality and density of the barrier layer. Referring to  FIG. 8A , there is one or more RF power supplies  810  and  812 . RF power supply  810  is coupled to a showerhead  860 . 
     Showerhead  860  and wafer support pedestal  851  provide in part spaced apart electrodes. An electric field may be generated between these electrodes to ignite a process gas introduced into chamber  850  to provide a plasma. It will be appreciated that other non-chemically reactive gases with respect to the metallic barrier layer may be used for physically displacing nitrogen from metallic barrier layer, including but not limited to neon (Ne), xenon (Xe), helium (He), and hydrogen (H 2 ). Generally, for a plasma-gas that does not chemically react with a tantalum-nitride film, it is desirable to have a plasma-gas atom or molecule with an atomic-mass closer to N than to Ta in order to have preferential sputtering of the N. However, a chemically reactive process may be used where a gas is selected which preferentially reacts for removal of N while leaving Ta. 
     The concept described in the various embodiments above can also be used to deposit barrier layer with increasing or decreasing concentration of a compound in the barrier layer. For example, it might be desirable to have increasing nitrogen concentration in the barrier layer with increase of film thickness for other applications. Under the circumstance, the duration and/or concentration of B reactant are increased with deposition cycle, instead of decreasing as described above. The compound with increasing or decreasing concentration does not have to be nitrogen. Other applicable compounds can benefit from the concept. The concept applies to any ALD deposition that requires two reactants. 
     While this invention has been described in terms of several embodiments, it will be appreciated that those skilled in the art upon reading the preceding specifications and studying the drawings will realize various alterations, additions, permutations and equivalents thereof. Therefore, it is intended that the present invention includes all such alterations, additions, permutations, and equivalents as fall within the true spirit and scope of the invention. In the claims, elements and/or steps do not imply any particular order of operation, unless explicitly stated in the claims.