Abstract:
A method to redistribute solid copper deposited by PVD on a wafer topography. The deposited copper is solubilized in a fluid for redistribution. The copper redistribution prevents inherent nonuniformity of the deposited copper film thickness by improving the uniformity of thickness of the copper film on the covered surfaces, such as vertical and bottom surfaces. The method provides the advantages of good adhesion and good grain growth and orientation that are achieved with copper deposited by PVD, and also provides the good step coverage as achieved with copper deposited by CVD.

Description:
FIELD OF THE INVENTION 
     The invention relates generally to methods of physical vapor deposition of copper onto a substrate. 
     BACKGROUND 
     In the formation of integrated circuits (IC), thin films containing metal and metalloid elements are deposited upon the surface of a semiconductor substrate or wafer. The films provide conductive and ohmic contacts in the circuits and between the various devices of an IC. For example, a thin film of a desired metal might be applied to the exposed surface of a contact or via in a semiconductor substrate. The film, passing through the insulative layers of the substrate, provides plugs of conductive material for the purpose of making interconnections across the insulating layers. 
     One well known process for depositing a thin metal film is by chemical vapor deposition (CVD). In CVD, reactant or deposition gas precursors are pumped 
     One well known process for depositing a thin metal film is by chemical vapor deposition (CVD). In CVD, reactant or deposition gas precursors are pumped into proximity to a target and substrate inside a reaction chamber. The precursors are activated using either thermal energy or electrical energy, and subsequently undergo chemical reactions at the surface of the target and substrate. This results in one or more reaction by-products, which are deposited on the exposed substrate or wafer surface to form a film. 
     CVD of copper provides the desirable property of high step coverage of the substrate, that is, a high ratio of the coating thickness at the bottom of a feature such as a trench or via, relative to the thickness of the coating on the top surface of the substrate adjacent the feature. CVD of copper, however, suffers from several difficulties and limitations. Adhesion of films deposited by CVD on most substrates utilized for copper technologies is difficult because most copper precursors do not sufficiently nucleate. As a result, nucleation is uneven as well as spotty, leading to poor non-conformal film growth with poor grain size and orientation control. Deposition of copper with most precursors is also difficult to control because of the poor stability of the precursor sources as well as the frequent early decomposition of the precursor in the delivery device. 
     It has been shown that copper deposited by CVD using a hexafluoroacetylacetonate (hfac) type of copper precursor results in conformal film growth over topography with about 100% step coverage. On the other hand, it is also known that in most cases deposition is gas feed limited, which usually results in very poor conformality due to depletion of the precursor in the trench or via. These factors suggest that somehow the hfac type of precursor and other copper precursors have a unique ability to uniformly coat the topography of a device, regardless of what precursor concentrations or partial pressure exists. 
     Physical vapor deposition (PVD), whereby electrical energy creates a vapor of particles to deposit onto a substrate, provides several desirable properties when copper is deposited. Copper deposited by PVD onto commonly used substrates shows very good adhesion, and good grain size and orientation, which are key to reliable copper metallization schemes. However, PVD of copper has the disadvantage that, while PVD can be highly directional with generous amounts of copper on the top and bottom horizontal surfaces, there is relatively little copper deposited on the vertical wall surface of a feature, such as a trench or via. 
     A PVD method for depositing copper onto a substrate is desirable. Such a method will provide the good adhesion and good grain size and orientation advantages of PVD. 
     SUMMARY OF THE INVENTION 
     The invention is directed to a method to redistribute solid copper deposited by PVD on a wafer topography, to result in enhanced uniform coverage of copper. To this end, and in accordance with the principles of the present invention, the deposited copper is solubilized in a fluid under solubilizing-promoting conditions in the reaction chamber, transported and redeposited. These steps may be performed for as many times as necessary to achieve redistribution. The solubilizing fluid may be a gas or a liquid. The method may further include supplying an electric field to the wafer. Copper may be redeposited by changing the temperature and/or the pressure in the reaction chamber. 
     The invention is further directed to a method of redistributing copper deposited on a wafer surface having a topography. At least a part of the surface copper is volatilized. A concentration of the volatilized copper is maintained to diffuse into the topography for redeposition. Volatilizing may be obtained by flowing an etching agent above the wafer. The concentration of the removed copper may be maintained high by a copper plate positioned adjacent the wafer, and the temperature of the plate and chamber may be higher than the temperature of the wafer. 
     The invention improves the uniformity of thickness of the resulting copper film. These and other objects and advantages of the present invention shall be made apparent from the accompanying description and examples thereof. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic of deposited copper in a feature before redistribution and during redistribution; and 
     FIG. 2 is a schematic of deposited copper after redistribution. 
    
    
     DETAILED DESCRIPTION 
     A method is disclosed to redistribute copper that has been unevenly deposited onto a substrate by any PVD method or any low step coverage deposition method. In the method of the present invention, copper is redistributed from the substrate surfaces where it is in excess to the surfaces where it is lacking, particularly in recesses such as on the vertical walls of vias and trenches. 
     With reference to the wafer  14  of FIG. 1, solid copper  20  is deposited on substrate surfaces  22 ,  24 ,  26  by standard physical vapor deposition (PVD) methods, such as ionized physical vapor deposition (iPVD), or by any low step coverage deposition method. In the case of PVD, electrical energy creates a vapor of particles that are deposited onto substrate surfaces  22 ,  24 ,  26 . In iPVD, sputtered particles are passed through a dense plasma before deposition onto the substrate surfaces  22 ,  24 ,  26 . The dense plasma, containing positively charged ions coupled with an electric field or bias, attracts the particles so that they have a better chance of reaching the bottom surface  26  of a via  30  or other feature, such as a high aspect ratio feature, by a negative force or potential difference. 
     A method of depositing thin metal films on substrates, such as semiconductor wafers, by iPVD has been described. For example, U.S. Pat. Nos. 5,569,363 and 5,178,739 each describe an iPVD system which may be used in the present invention to achieve high metal ionizations. The result is deposition of copper  20  onto the surfaces  22 ,  24 ,  26  of a wafer  14  but in a non-uniform manner, as depicted in FIG.  1 . While adequate or even generous amounts of copper  20  are deposited on horizontal surfaces  22 ,  26  of the topography, relatively little copper  20  is deposited on vertical surfaces  24 , such as the wall of a via  30 . For ease of discussion, deposition will be said to occur by PVD methods, but it is to be understood that included therewith is any now known or hereafter developed low step coverage deposition method. 
     With further reference to FIG. 1, the wafer  14  containing solid copper  20  deposited by iPVD, for example, is submitted to a mixture of gases, such as Hhfac plus chlorine or oxygen and trimethylvinylsilane (tmvs), that will allow volatile copper compounds, such as Cu(hfac) 2  or Cu(hfac)tmvs, to form. It should be understood by those skilled in the art that other weak Lewis-base ligands may be substituted for tmvs without departing from the scope of the present invention. In FIG. 1, copper ligands A and B selectively adsorb onto copper surfaces  32  and  34  where the copper deposits are thicker. The exposure of wafer  14  to the gases occurs under conditions of operation such as low temperature, up to about 300° C., and high pressure, about 1-500 torr, that will allow migration of copper hfac or other molecules  36  away from the wafer  14  surface. Nevertheless, at the proper temperature and pressure in the reaction chamber, a thin film  38  of highly mobile copper hfac molecules  36  likely forms at the surface, allowing for redistribution of copper  40  by surface diffusion. 
     With further reference to FIG. 1, the thin film  38  of highly mobile copper hfac molecules  36  on the surfaces  32 ,  34  of the wafer  14  may be redistributed through fluids, such as liquids or gases. In one embodiment, the gas phase transportation of desorbed and readsorbed copper  40  is used, which takes advantage of the intrinsic trapping effect of molecules  36  in vias or trenches  30  by position to the rapidly flushed top horizontal surface  32 . Thus, in this embodiment, gaseous vectors transport copper  40  to other locations on the wafer  14  where copper was depleted or inadequately deposited, yielding a more uniform distribution of copper  20  on the substrate topography as depicted in FIG.  2 . 
     The thin film  38  of highly mobile copper hfac molecules  36  may also be redistributed by a liquid vector. The liquid dissolves the copper, and the resultant pseudo-liquid film flows into vias and trenches  30  driven by either gravity or surface tension mechanisms, and tends to fill the vias and trenches  30 . 
     Once the copper hfac molecules  36  are thought to have reached the desired location of the wafer  14  topography, operation conditions are changed to provoke the redeposition of solid copper  40  and evacuation of the hfac or other type of gaseous byproducts. For example, temperature, pressure, or other reaction conditions may be changed to provoke the following reaction: 
     
       
         Cu(hfac) 2 +H 2 →Cu↓+2Hhfac 
       
     
     Conditions of operation such as low temperature and high pressure are chosen to avoid migration of copper hfac molecules  36  or other precursor molecules away from the wafer  14  surface  32 . For instance, it could be recommended to operate the reaction chamber at 150° C. and 100 torr, which is the vapor pressure of Cu(hfac) 2  at 150° C. Nevertheless, at the proper temperature and pressure, a thin film  38  of highly mobile copper hfac molecules  36  will form at the surfaces  32 ,  34  of the copper, allowing for redistribution of copper  40  by surface diffusion. It is expected to see at least some of the top excess copper be transported along the side of a contact structure. 
     The net result is copper redistribution, as shown in FIG. 2, with a minimum loss of the copper  20  previously deposited by PVD. Redistribution of about 10%-20% of copper  20  deposited by PVD is generally sufficient. The copper  40  redistributed by the method of the present invention has the proper grain size and orientation provided by the PVD underlayer, i.e. a strong [111] orientation with micron-range sized grains, and also has improved step coverage associated with a copper film deposited by CVD. Moreover, the benefit of the excellent adhesion of the PVD copper film is advantageously exploited and nucleation difficulties of copper vapor precursors are less of a problem by the method of the invention. 
     EXAMPLE 1 
     Using liquid redistribution of copper, a solution of hfac and tmvs is dispensed onto a spinning wafer having a PVD-deposited Cu film thereon to evenly distribute the liquid across the surface. It should be understood by one skilled in the art that the optimum rate of spinning for even distribution will vary depending on the viscosity of the solution. The wafer is kept at a temperature at which dissolution of copper is very limited, around room temperature. The liquid fills the topography of the wafer and forms a thin film on the top surface of the wafer. At this point, the wafer is brought to a higher temperature, for example about 60° C., where the copper or the oxidized part of the copper reacts and dissolves in the solution, forming an in-situ copper-containing solution. Assuming that the copper molecules get redistributed uniformly in the solution, there will be a uniform supply of copper at any location of the topography. Then, the temperature to which the wafer is exposed is increased, for example to about 100-300° C., and the pressure is decreased, for example to lower than about 10 torr, to allow the copper molecules to decompose and result in even copper redeposition. The procedure may be repeated as many times as required to obtain the desired copper redistribution, preferentially in vias and trenches. 
     EXAMPLE 2 
     The same liquid solution containing copper molecules as described in Example 1 is used but is prepared ex-situ, such as spin-on metal solutions, which are well known to those of ordinary skill in the art. The solution is then dispensed onto a wafer, either with or without copper already deposited on the wafer, and the wafer is spun to obtain a homogenous liquid film on the surface. Subsequent treatments, such as temperature and pressure adjustment as described in Example 1, would allow for the copper to deposit from the solution and would allow the byproduct to be evacuated by evaporation of the solvent. Also, where copper is already deposited on the wafer, dissolution of copper into the ex-situ copper-containing solution may be invoked to increase the copper content in the liquid film. 
     EXAMPLE 3 
     The method of Example 1 or Example 2 is used with an electrical field added on the wafer. Copper deposition from the liquid solution is electro-assisted, as in electroplating, which is well known to those skilled in the art and, per se, forms no part of the present invention. 
     EXAMPLE 4 
     A two step process may be used, whereby copper is first deposited by PVD and then the copper is partially chemically etched, as described in reaction (1+): 
     
       
         Cu°+Cu 2+ (hfac) 2 +2(tmvs)→2Cu(hfac)(tmvs)  (1+) 
       
     
     By way of example, reaction (1+) is carried out at a temperature of about room temperature to about 300° C. and a pressure of about 0.5 torr to about 500 torr. The etched material is then used as a deposition precursor by reversing reaction (1+) by stopping the flow of Cu(hfac) 2  and tmvs, and copper is redeposited in a uniform fashion across the surface of the wafer according to reaction (1−): 
     
       
         2Cu(hfac)(tmvs)→Cu°+Cu 2+ (hfac) 2 +2(tmvs)  (1−) 
       
     
     By way of example, reaction (1−) is carried out at a temperature of about 100° C. to about 300° C. and a pressure of about 0.1 torr to about 10 torr. 
     The etching process (1+) is carried out in depletion so that relatively little of the Cu 2+  (hfac) 2  reaches the sidewalls and bottom of the features, and mainly most Cu 2−  (hfac) 2  reacts with the top surface material. Following the etching step, the system is placed into deposition mode by turning off the tmvs and/or the Cu(hfac) 2  thus stopping the (1+) etching reaction, but allowing for the produced Cu(hfac)(tmvs) to disproportionate according to the reaction (1−) that leads to the deposition of copper. In order to increase the residence time of Cu(hfac)(tmvs), which will in turn increase the deposition probability, the pumping of the gases will be slowed down or stopped. 
     EXAMPLE 5 
     An alternative chemistry for the etching reaction described in Example 4 is as follows: 
     
       
         Cu 2 O+2Hhfac→2Cu 2+ (hfac) 2 +H 2 O  (2) 
       
     
     The copper surface is first exposed to a low partial pressure of oxygen, for example of a few torr at about 100-200° C. for a few seconds, to oxidize only the top surface of the copper film and the copper overhang that typically forms at the top corners of the features. Hhfac is then introduced to etch the copper oxide, for example under conditions similar to the chemical reaction (1+) described in Example 4. Following the etching step, hydrogen is introduced into the reaction chamber to allow copper deposition to occur via hydrogen reduction of 2Cu 2+  (hfac) 2 , according to the reaction: 
     
       
         2Cu 2+ (hfac) 2 +H 2 →2Cu°+2Hhfac  (3) 
       
     
     for example, under conditions similar to the chemical reaction (1−) described in Example 4. All steps occur in a chamber with no pumping (static conditions) so that no copper is lost from the system. 
     EXAMPLE 6 
     Copper is redistributed from a wafer surface into recesses on the wafer by volatizing surface copper and maintaining a high concentration of gas phase copper precursor that diffuses into recesses and preferentially accretes therein. An etching agent, such as Hhfac or Cu(hfac) 2  and tmvs, is flowed above the wafer so that removal of copper at the surface and near the surface occurs. Copper removal and escape from deep recesses occurs to a lesser extent, since gas phase copper generated within the feature has a relatively large chance of striking and adhering back to the inner surfaces of the feature. Copper removal from recesses may also occur to a lesser extent because the etching conditions will be such that a minimum amount of etchants will be used, which will be just enough to saturate the top surfaces. 
     The gas phase above the wafer surface is maintained in a higher concentration (for example, at least doubled) of copper precursor through the use of a confining source  50 , such as an opposing copper plate or a wafer with a thick copper film adjacent, i.e., in close proximity (less than about 1 cm) to the wafer  14 , as shown in FIG.  1 . This limits escape of copper in the gas phase, as well as serving as an additional copper source. 
     Copper of the gas phase diffuses into recesses, and has a higher probability of adding to film thickness there than on the upper surface because of the relatively higher ratio of nucleation and growth sites in enclosed gaseous volume. The chamber (not shown) and opposing copper plate  50  may be kept at the higher temperature (for example, about 20-100° C. higher) than the product wafer  14  to enhance film growth. The higher temperature opposing copper plate  50  reacts with the etchants and contributes to the production of a higher amount of copper precursors. Copper redistribution is achieved at low cost in this embodiment since a conventional showerhead is not required. 
     It should be understood that the embodiments of the present invention shown and described in the specification are only preferred embodiments of the inventors who are skilled in the art and are not limiting in any way. Therefore, various changes, modifications or alterations to these embodiments may be made or resorted to without departing from the spirit of the invention and the scope of the following claims.