Abstract:
A method of manufacturing a semiconductor device has the steps of: (a) preparing a semiconductor substrate formed with an insulating layer having a wiring recess; and (b) forming a conductive layer by chemical vapor deposition on a surface of the semiconductor substrate including an inner surface of the wiring recess, while lamp light is applied to the semiconductor substrate, the conductive layer being substantially made of copper. With this method, Cu wiring having a high adhesion force is formed by chemical vapor deposition.

Description:
[0001]    This application is based and claims priority on Japanese Patent Application 2000-242816, filed on Aug. 10, 2000, the entire contents of which are incorporated herein by reference.  
         BACKGROUND OF THE INVENTION  
         [0002]    a) Field of the Invention  
           [0003]    The present invention relates to a semiconductor device and its manufacture method, and more particularly to a semiconductor device with copper wiring and its manufacture method.  
           [0004]    In this specification, wiring made of copper or copper alloy consisting essentially of copper is called copper wiring. Copper wiring is intended to be inclusive of copper wiring with a barrier layer.  
           [0005]    b) Description of the Related Art  
           [0006]    Demands for high integration of semiconductor integrated circuits are high. Fine circuit elements such as active elements as well as fine wiring are therefore required. For fine wiring, in place of Al conventionally used as wiring material, attention has been paid to Cu which has a lower resistance and a higher tolerance of electromigration (EM).  
           [0007]    Cu is difficult to be worked for fine patterning. Namely, it is difficult to form a wiring line by forming a wiring layer and a mask such as resist on the wiring layer, and finely etching it, as opposed to Al wiring. In place of this method, a damascene process is performed for Cu wiring. With this process, a wiring trench is formed in an insulating layer, a Cu layer is embedded in the trench by plating or the like, and then an unnecessary Cu layer on the insulating layer is removed by chemical mechanical polishing (CMP) or the like.  
           [0008]    In a multi-layer wiring structure, a via conductor interconnecting wiring levels is required. A single damascene process and a dual damascene process are known. With the single damascene process, a via conductor and wiring are formed by different damascene processes, and with the dual damascene process, a via conductor and wiring are formed at the same time. From the viewpoint of process simplification, the dual damascene process is more effective.  
           [0009]    Cu has the nature that it diffuses into an insulating layer and degrades the insulation characteristics. In order to prevent Cu diffusion into an insulating layer, it is desired to form a barrier layer as an underlie layer. When a Cu layer is formed as a main wiring layer, electroplating is often used. It is desired to form a conductive seed layer for electroplating.  
           [0010]    A barrier layer has been formed by sputtering metal nitride such as TaN and TiN. Although this barrier layer is electrically conductive, if this layer is used as a seed layer, the efficiency of electroplating cannot be made high because this layer has a high resistance. From this reason, a Cu layer formed by sputtering has been used as a seed layer.  
           [0011]    As the degree of scaling-down proceeds, there is a tendency that the aspect ratio of a recess such as a via hole and a wiring trench becomes high. It is not easy to uniformly form a metal film through sputtering in a recess having a high aspect ratio. The higher the aspect ratio becomes, it becomes more difficult to uniformly form a metal film on the side wall of a recess.  
           [0012]    Chemical vapor deposition (CVD) is now becoming worth notice as a method of forming a uniform film in the recess having a high aspect ratio. A barrier layer capable of being formed by CVD may be a TiN layer, a WN layer, a ZrN layer, a TaN layer or the like. Reports on CVD growth of TiN layer, WN layer and ZrN layer have been given already. As a seed layer capable of being formed by CVD, a Cu layer has been reported which uses copper (I) trimethylvinyl-silylhexafluoroacethylacetonato (Cu (hfac) tmvs) or the like as source material.  
           [0013]    Although it is desired to form the barrier layer and seed layer for Cu wiring by CVD, this technology has not been established yet.  
         SUMMARY OF THE INVENTION  
         [0014]    It is an object of the present invention to provide a Cu wiring manufacturing method suitable for fine patterning.  
           [0015]    It is another object of the invention to provide a semiconductor device manufacturing method in which high purity Cu wiring is formed by CVD.  
           [0016]    It is another object of the invention to provide a semiconductor device manufacturing method in which Cu wiring having high adhesion force is formed by CVD.  
           [0017]    It is another object of the invention to provide a semiconductor device having Cu wiring formed by CVD and being less peeled off.  
           [0018]    According to one aspect of the present invention, there is provided a method of manufacturing a semiconductor device comprising the steps of: (a) preparing a semiconductor substrate formed with an insulating layer having a wiring recess; and (b) forming a conductive layer by chemical vapor deposition on a surface of the semiconductor substrate including an inner surface of the wiring recess, while lamp light is applied to the semiconductor substrate, the conductive layer consisting essentially of copper.  
           [0019]    With this method, a Cu layer having a low impurity concentration can be formed.  
           [0020]    A semiconductor device having high reliability Cu wiring can be formed.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]    [0021]FIGS. 1A and 1B are a cross sectional view illustrating a process of forming samples used by the experiments by the present inventors and a graph showing the impurity concentrations of samples.  
         [0022]    [0022]FIGS. 2A to  2 D are a schematic diagram showing the structure of a lamp heating CVD system, a diagram showing a molecular formula of organic copper compound used as Cu source material, and schematic cross sectional views showing modifications of a lamp heating CVD system.  
         [0023]    [0023]FIGS. 3A to  3 E are cross sectional views illustrating the precesses of forming wiring, and  
         [0024]    [0024]FIG. 3F is a cross sectional view showing the structure of a MOS transistor.  
         [0025]    [0025]FIGS. 4A and 4B are a cross sectional view illustrating a Cu film forming process by resistance heating CVD and a graph showing the characteristics of samples. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0026]    The present inventors first formed barrier layers and seed layers by CVD according to conventional techniques.  
         [0027]    [0027]FIGS. 4A and 4B are a cross sectional view illustrating a main process of forming samples and a graph showing the characteristics of samples.  
         [0028]    As shown in FIG. 4A, on a p-type Si substrate  10  with a silicon oxide film 1 about 10 nm in thickness formed on the surface thereof, a TiN barrier layer  2  about 50 nm in thickness and a Cu layer  3  having a thickness of 250 to 300 nm were formed by CVD.  
         [0029]    A resistance heating CVD system with a resistance heater RH shown in FIG. 4A was used for CVD and the silicon substrate  10  was placed on a susceptor SA made of aluminum nitride.  
         [0030]    Ti[N(CH 3 ) 2 ] 4  (TDMAT), Ti[N(C 2 H 5 )] 4  (TDEAT) and the like were used as the source material of TiN. In order to regulate the amount of nitrogen, ammonium was optionally added. Copper (I) trimethylvinylsilyl-hexafluoroacethylacetonato (Cu (hfac) tmvs) was used as the source material of Cu. Trimethylvinylsilane (tmvs) and hexafluoroacetylacetone dihydrate (Hhfac.2H 2 O, HDH) were added as reaction regulator.  
         [0031]    The conditions of forming the Cu layer were a film forming temperature of 220° C., a film forming pressure of 500 Pa, a carrier gas of H 2  (500 sccm), a Cu material of Cu (hfac) tmvs+tmvs (2.5 wt. %)+Hhfac.2H 2 O (0.4 wt. %), and a source material supply rate of 1.0 g/min.  
         [0032]    After the TiN layer  2  and Cu layer  3  were formed on the silicon nitride film  1  under the above conditions, a Cu layer was formed to a thickness of about 1 μm by electroplating. An adhesion force test was performed for the Cu layer by using tapes. With this adhesion force test, Cu layers were peeled off. If wiring is formed by using such a Cu layer having a low adhesion force, the finished wiring is very low in reliability.  
         [0033]    Samples were also formed at the CVD pressures of 300 Pa and 150 Pa when the Cu seed layer was deposited. Also in these samples, Cu layers were peeled off during adhesion force tests.  
         [0034]    A purity of each Cu layer formed by CVD was measured with a secondary ion mass spectrometer (SIMS). Most possible elements detectable as impurities are C, F, O and H, which are contained in the source material. Of these elements, the concentrations of C, F and O were measured.  
         [0035]    [0035]FIG. 4B is a graph showing the concentrations of O, C and F as a function of the film forming pressure. The abscissa represents a film forming pressure in the unit of Pa in linear scale, and the ordinate represents an impurity concentration in the unit of atoms/cm 3  in logarithmic scale. A curve c(O) indicates an impurity concentration of O, and curves c(C) and c(F) indicate impurity concentrations of C and F, respectively. The impurity concentrations of C and F were the same. Even if the film forming pressure was reduced, the impurity concentrations of O, C and F did not change greatly although they reduced a little, the impurity concentration of O being maintained higher than 1×10 20  cm −3  and those of C and F higher than 1×10 19  cm −3 .  
         [0036]    When the Cu film was formed by sputtering, the impurity concentrations of C, O and F were lower than the measurement limits (O: 5×10 18  atoms/cm 3 , C: 7×10 17  atoms/cm 3 , F: 1×10 16  atoms/cm 3 ). This result matches the fact that the source material does not contain these impurities.  
         [0037]    It can be presumed that a large amount of impurities left in the seed layer may cause an increased resistance of Cu wiring, a lowered adhesion force, and a degraded migration resistance. It is desired to reduce impurities in the Cu seed layer.  
         [0038]    [0038]FIGS. 1A and 1B are a cross sectional view illustrating a process of forming Cu wiring according to an embodiment of the invention and a graph showing the characteristics of Cu wiring formed by the embodiment method.  
         [0039]    As shown in FIG. 1A, a lamp heating CVD system used has a halogen lamp L disposed under a susceptor S made of quartz.  
         [0040]    [0040]FIG. 2A is a schematic diagram showing the overall structure of a lamp heating CVD system. Under a susceptor S made of quartz disposed in a chamber CH, a halogen lamp L is placed. An Si substrate  10  is placed on the quartz susceptor S. A shower head SH is disposed over the Si substrate to blow source gas over the whole surface of the substrate  10 . Liquid source LS is bubbled or vaporized by carrier gas CG (H 2  gas) in a carburetor or vaporizer V and supplied to the shower head SH as source gas. TDMAT and TDEAT are provided as the TiN source material, and a mixed liquid material of Cu (hfac) tmvs (97.1 wt. %)+tmvs (2.5 wt. %)+Hhfac.2H 2 O (0.4 wt. %) is provided as the Cu source material. A growth chamber is surrounded by the chamber CH, and the source gas after contribution to film growth is exhausted to the outside of the chamber CH via an exhaust port EX.  
         [0041]    Organic copper compound Cu (hfac) tmvs used as the Cu source material is expressed by the molecular formula and molecular structure as shown in FIG. 2B.  
         [0042]    As shown in FIG. 1A, on the surface of the p-type silicon substrate  10 , a silicon nitride film  1  was formed to a thickness of about 100 nm. This silicon substrate  10  was placed on the susceptor S, and CVD was performed by heating the substrate  10  with the halogen lamp L and flowing the source gas.  
         [0043]    First, a TiN layer  2  was grown to a thickness of about 50 nm by using source gas such as TDMAT and TDEAT. Thereafter, a mixed liquid of Cu (hfac) tmvs (97.1 wt. %), tmvs (2.5 wt. %), and Hhfac.2H 2 O (0.4 wt. %) was vaporized by H 2  gas in the vaporizer V to form source gas. A Cu layer having a thickness of about 250 to 300 nm was formed by CVD on the TiN layer  2  under the conditions of a substrate temperature of about 220° C., a source material supply amount of 1.0 g/min and a carrier H 2  flow rate of 500 sccm. At the film growth pressure of 500 Pa, the impurity concentrations of O, C and F in the deposited Cu film were 1×10 20  atoms/cm 3 , 1×10 19  atoms/cm 3 , and 2×10 18  atoms/cm 3 , respectively.  
         [0044]    At the reduced film growth pressure of 50 Pa and under the same other conditions, the impurity concentrations of O, C and F in the deposited Cu film were lower than 5×10 18  atoms/cm 3 , 7×10 17  atoms/cm 3 , and 1×10 16  atoms/cm 3 , respectively, which are the detection limits. These measurement results are shown in FIG. 1B. The impurity concentrations at the film forming pressure of 50 Pa are lower than the detection limits and have the values lower than those indicated by curves, as indicated by arrows. Impurity concentrations of O, C and F can be easily suppressed below 1×10 19  atoms/cm 3 . It will be desirable to conduct CVD of Cu with a growth pressure of 500 Pa or lower, preferably 100 Pa or lower, and more preferably 6 Pa or lower, for reducing the impurity concentration.  
         [0045]    For the purposes of comparison, the impurity concentrations of Cu films formed by resistance heating CVD are also shown in the graph of FIG. 1B. As compared with resistance heating, the impurity concentrations of Cu films formed by lamp heating are reduced at the same pressure. It can be presumed that as the film forming pressure is lowered, a difference of the impurity concentrations between resistance heating and lamp heating becomes large.  
         [0046]    After a Cu layer was formed by electroplating to a thickness of about 1 μm on the seed Cu layer formed by lamp heating, an adhesion force test by tapes was conducted. With the adhesion force test, the Cu layers were not peeled off.  
         [0047]    As described above, Cu films having a low impurity concentration were able to be formed by lamp heating CVD. It can be judged that the Cu film formed by lamp heating CVD provides a high adhesion force relative to peel-off and is more suitable for practical use.  
         [0048]    The reason for a lowered impurity concentration and an increased adhesion force obtained when a Cu film is formed by lamp heating CVD is not known to date. During resistance heating, the susceptor is first heated and then the Si substrate is gradually heated through thermal conduction, whereas during lamp heating, the Si substrate is heated directly and rapidly. The lowered impurity concentration and increased adhesion force may be ascribed first to this difference of heating methods. However, the reason for a different impurity concentration by the heating method is not clear to date. An Si substrate may be lamp-heated not only from the bottom surface of the substrate but also from the front surface.  
         [0049]    The lowered impurity concentration and increased adhesion force may be ascribed next to some chemical reaction to be caused by lamp light radiated to source gas, some chemical reaction to be caused by lamp light reflected and radiated to the substrate surface, or the like.  
         [0050]    In the latter case, heating the substrate by lamp light is not necessarily required, but the lamp light may be applied to supplied source gas or lamp light capable of chemical reaction may be applied to the substrate surface.  
         [0051]    In the above experiments, Cu (hfac) tmvs was used as the Cu source material. In addition to this organic copper compound, Cu (hfac).L having similar characteristics is expected to be usable, where L is (1) 1,5-cyclooctadiene, (2) bis(trimethylsilyl)acetylene, (3) 2-butyne, (4) triethoxyvinylsilane, and (5) trimethylallylsilane. Also for these materials, as additive, atom groups not containing Cu of these organic copper compounds or materials containing these atom groups are expected to be usable.  
         [0052]    Barrier layer is preferably formed of refractory metal such as Ti, Ta, W, Mo, Zr or their nitride. If the barrier layer is formed by CVD, a TiN layer, a WN layer, or a ZrN layer may be used as the barrier layer. A TaN layer is also expected to be usable.  
         [0053]    The lamp heating CVD system used for the experiments uses one halogen lamp as a light source.  
         [0054]    As shown in FIG. 2C, it is obvious that similar results will be obtained by using a plurality of lamps L 1 , L 2  and L 3 .  
         [0055]    As shown in FIG. 2D, it is also expected that similar results will be obtained by disposing a lamp above a substrate  10 .  
         [0056]    Similar results are expected not only by using a lamp only but also by radiating light to source gas while resistance heating is performed.  
         [0057]    The above-described preparatory experiments were conducted by forming a silicon oxide film, a barrier film and a seed layer on an Si substrate having a flat surface. A semiconductor device having a multi-layer structure has various geometrical structures in or on the surface layer of a semiconductor substrate.  
         [0058]    [0058]FIGS. 3A to  3 E are cross sectional views illustrating the manufacture processes for Cu damascene wiring of a semiconductor device.  
         [0059]    As shown in FIG. 3A, on the surface of an Si substrate  10  having a conductive region  12  and an isolation region  11 , an insulating layer  21  is formed and a damascene wiring trench is formed. After a barrier layer  22  and a Cu seed layer  23  are formed on the inner surface of the trench and the upper surface of the insulating layer  21 , a Cu layer  24  is formed on the surface of the seed layer  23  by electroplating. The Cu seed layer is formed by the method described earlier.  
         [0060]    Thereafter, the barrier layer, seed layer and Cu wiring layer deposited on the surface of the insulating layer  21  are removed by CMP. For example, if the conductive region  12  is the source/drain regions of a MOS transistor, damascene wiring for the source/drain regions is formed. This damascene wiring is, for example, a lead plug.  
         [0061]    [0061]FIG. 3F shows the structure of a MOS transistor. An insulated gate electrode structure is formed on a p-type region of a silicon substrate  10  having a shallow trench isolation region  11 . The insulated gate electrode structure includes a gate oxide film  50 , a polysilicon film  51  and a silicide film  52 . The side walls of the gate electrodes have side spacers  54  of silicon oxide and/or silicon nitride. Source/drain regions  12  are formed on both sides of the insulated gate electrode by ion implantation. Via holes are formed through an insulating layer  21  covering the gate electrode. Damascene plugs, each made of a barrier film  22  and copper region  24 , are embedded in the via holes.  
         [0062]    After the lower wiring layer is formed, on the surface of the first insulating layer  21 , a first etch stopper layer  26 , a second inter-level insulating film  27 , a second etch stopper layer  28  and a third inter-level insulating film  29  are formed. The etch stopper layers  26  and  28  may be SiN layers having a thickness of about 50 nm. The inter-level insulating layers  27  and  29  may be a CVD-SiO 2  film, a silicon oxide film added with P, B and P, F or the like, or other films.  
         [0063]    As shown in FIG. 3B, by using a resist mask, a via hole VH is formed through the third inter-level insulating film  29 , second etch stopper layer  28  and second inter-level insulating film  27 . The resist mask is thereafter removed.  
         [0064]    After a new resist mask is formed, a wiring trench or groove WG is formed through the third inter-level insulating layer  29 . The etch stopper layers  26  and  28  exposed on the bottoms of the via hole VH and wiring trench WG are etched and removed to expose the surface of the lower wiring plug  24 . The resist mask used is removed before or after the etch stopper layers are etched.  
         [0065]    As shown in FIG. 3C, a barrier layer  32  and seed layer  33  are formed on the inner surfaces of the via hole VH and wiring trench WG and on the upper surface of the third inter-level insulating film  29 . For example, the barrier layer  32  is a TiN layer made by metal organic CVD and the seed layer  33  is a Cu layer formed by CVD described earlier. The film deposition amount may be increased to bury the via hole VH as indicated by a broken line.  
         [0066]    As shown in FIG. 3D, a Cu layer  34  is formed on the seed layer  33  by electroplating, burying the via hole VH and wiring trench WG.  
         [0067]    As shown in FIG. 3E, the Cu wiring layer  34 , Cu seed layer  33  and TiN barrier layer  32  deposited on the upper surface of the third inter-level insulating film  29  are etched and removed to form a flat surface  35 .  
         [0068]    It is obvious to those skilled in the art that on this flat surface  35 , an etch stopper layer and an inter-level insulating film are repetitively formed to form similar damascene wiring.  
         [0069]    As an integration degree becomes high, the diameter of a via hole becomes small. In such a case, the via hole VH may be fully buried or embedded with the seed layer  33 . Dual damascene wiring may be formed only by CVD without using electroplating. In this case, the seed layer  33  and wiring layer  34  shown in FIG. 3E may be replaced by a single Cu layer.  
         [0070]    The present invention has been described in connection with the preferred embodiments. The invention is not limited only to the above embodiments. It will be apparent to those skilled in the art that various modifications, improvements, combinations, and the like can be made.