Abstract:
A method for forming an anti-reflective coating on a semiconductor substrate, including providing a first vessel containing an anti-reflective coating component and a second vessel containing a solvent. The anti-reflective coating component from the first vessel and the solvent from the second vessel are supplied to a mixing chamber. The anti-reflective coating component and the solvent are mixed in the mixing chamber to form a product. The product is transferred to the semiconductor substrate. The product is applied to the semiconductor substrate to form the anti-reflective coating. A system for forming an anti-reflective coating on a semiconductor substrate, including a first vessel for containing an anti-reflective coating component and a second vessel for containing a solvent. The system also includes a mixing chamber for mixing the anti-reflective coating component with the solvent to form a product, and a fluid transport system connecting the mixing chamber and the substrate for supplying the product from the mixing chamber to the semiconductor substrate to form the anti-reflective coating.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims priority to Provisional Application No. 60/175,729, filed on Jan. 12, 2000. This invention relates to organic chemicals for manufacturing, and more particularly to chemical delivery methods, and even more particularly to property control through delivery methods. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    Manufacturers in the semiconductor industry use organic materials for various fabrication steps. For example, anti-reflective coatings (ARC) are used in photolithography steps to suppress reflections caused by light reflected from underlying layers, thus facilitating accurate resolution of small features. For this type of application, the ARC needs to provide conformal coverage of underlying features. In certain other applications, ARC can also serve a dual role as a planarizing layer and an anti-reflective coating. An example of an ARC is DUV 30, available from Brewer Science, Inc., located in Rolla, Miss.  
           [0003]    Currently, one approach for providing an ARC is to synthesize a chromophore-linked novalac polymer, a cross-linker, and an appropriate solvent system. The manufacturer adds solvent to obtain an ARC viscosity necessary for producing the final thickness required by the customer&#39;s application. The manufacturer then hermetically seals the ARC within a bottle and ships it to the customer. The photolithography engineer obtains the final material form of the ARC by extracting material from the bottle, coating the substrate with the material, and baking the substrate. The organic ARC must be of appropriate thickness and optical density to actively suppress the reflective interference effects of the underlying films. In some applications, the ARC film is used to fill contact holes within a dielectric layer to an appropriate level to protect the substrate junctions during a dry etch for opening the ARC and a main etch.  
           [0004]    More specifically, the photolithography engineer receives a package containing an ARC material. The ARC comprises a polymer component and a cross-linker component in a solvent:  
                         
 
           [0005]    The polymer component is obtained, for example, by combining a polymer backbone, such as an epoxy novalac resin, and an actinic chromophore (see U.S. Pat. No. 5,919,598, the contents of which are incorporated herein by reference):  
           (B) novalac epoxy resin+actinic chromophore→polymer component  
           [0006]    Typically, the polymer component has a chromophore bonded to the backbone, or as part of the backbone design.  
           [0007]    An example of a cross-linker component is formed by dissolving POWDERLINK® 1174 (PL 1174, commercially available from Cytec Industries Inc., located in West Paterson, N.J.) in 1-methoxy-2-proponal (PGME), and adding toluenesulfonic acid (p-TSA.H 2 O). The package includes ethyl lactate, which reacts with the PL 1174, PGME, and p-TSA.H 2 O to form an active cross-linker (see U.S. Pat. No. 5,919,599, the contents of which are incorporated herein by reference):  
                         
 
           [0008]    [0008]FIG. 1 illustrates the conventional perception of the reaction path for forming the cross-linker. The chemical manufacturer starts with a solid powder, PL 1174, a methylated glycoluril-formaldehyde resin which acts as a cross-linker. The chemical structure of PL 1174 is illustrated in FIG. 1 as compound  1 . PL 1174 is dissolved in a solvent 1-methoxy-2-proponal (PGME). The chemical manufacturer adds p-TSA.H 2 O to the solution of PL 1174 and PGME. p-TSA.H 2 O is an acid that initiates the reaction of PL 1174 by first protonating the PL 1174 (compound  2 ). The chemical manufacturer also adds ethyl lactate (EL) to the solution. EL reacts with the protonated PL 1174 to form a final cross-linking reagent PL-EL (compound  3 ).  
           [0009]    This combination of the cross linker component and the polymer component with a solvent is the form of the ARC needed by photolithography engineers. The chemical manufacturer may add solvents to the mixture, such as ethyl lactate, 1-methoxy-2-proponal acetate, cyclohexanone, and n-methyl-pyrrolidone (NMP), to adjust the percentage of solids and to control the casting characteristics of the ARC, as well as to control the viscosity, and thereby control the thickness of the ARC layer when the ARC is used to coat a substrate.  
           [0010]    PL-EL is the expected final cross-linking reagent, needed to react with a polymer component to form the final, stable ARC material. It was believed that, during the ARC manufacturing process, the protonated PL 1174 compound immediately reacts with ethyl lactate to form PL-EL. Therefore, photolithography engineers expected that the material delivered by the chemical manufacturer would have particular properties. This properties would be characteristic of an ARC formed by combining PL-EL with a polymer component and solvents.  
           [0011]    Referring to FIG. 2, it has recently been determined that, in the presence of PGME, the protonated PL 1174 would be likely to form an adduct PL-PGME (compound  2   a ), before PL-EL is formed. The adduct PL-PGME is a high energy, unstable product. The introduction of this high energy product to another highly reactive alcohol or solvent, such as ethyl lactate, eventually results in a PL-EL product which has lower energy and is more stable than PL-PGME:  
           PL-PGME+EL→PL-EL+co-solvents.  
           [0012]    The rate of this reaction is somewhat slowed by steric hindrance of the PL-PGME molecule reaction site. Because of the initial formation of PL-PGME adduct and eventual conversion to PL-EL, the product shipped by the manufacturer to the end-user as an ARC comprising an ARC material and solvents is most likely a mixture of PL-PGME, PL-EL, ARC material, and solvents. Variation in the product composition creates problems for the end-user, in that the behavior of the product cannot be fully predicted.  
           [0013]    Conventionally, the chemical manufacturer delivers to the end-user a package containing ARC components mixed with solvent. It has been determined that this composition undergoes an aging-in process while the PL-EL is being formed. Because it was formerly believed that PL-EL forms instantly upon mixing ethyl lactate with the ARC components, the user unknowingly had the uncertainty of working with a material with varying composition, i.e. varying levels of PL-PGME adduct and PL-EL. In fact, because PL-PGME adduct reacts with ethyl lactate for a period of time, according to traditional methods, the material as delivered to the user may contain a great deal of the PL-PGME adduct, and nearly no PL-EL, which is the material the user expects. This leads to processing problems, because the ratios of PL-PGME adduct and PL-EL are critical for controlling the type of coverage provided by the ARC, e.g. conformal or planarizing.  
         SUMMARY OF THE INVENTION  
         [0014]    The invention is based on the principle of controlling the point at which cross-linker starting materials (e.g. PL 1174, PGME, p-TSA.H 2 O) are introduced to a solvent (e.g. ethyl lactate). The starting materials are delivered to a wafer fabrication facility separately, and then mixed with solvent, with or without artificial aging through mechanical means or heat. This procedure, thereby, provides the advantage of enabling the photolithography engineer to control output parameters. Also, the same starting materials can be used to produce both conformal and planarizing coatings. This discovery provides the advantage of eliminating the need for a fabrication facility to support multiple coating systems to obtain the required degrees of conformality and planarization. The invention provides a novel lithographic method for the formulation and delivery of a thin anti-reflective film with user-defined characteristics to a silicon wafer, on demand, with minimal set-up and conversion time through existing delivery system technology. The invention includes a procedure and a system for controlling the degree of conformality of an organic anti-reflective film coating. The conformality of an anti-reflective coating can be changed by changing the procedure in which the material formulation is completed and the sequencing of time and temperature. The anti-reflective material is mixed with an appropriate quantity of solvent, at an appropriate ratio to produce a desired viscosity. The product of either the as-mixed material or the aged material is then coated on to the surface of a substrate to produce a conformal coating or a planarized coating. If the product, without further treatment, is applied to a substrate, one obtains a conformal anti-reflective film coating which fills vias very poorly. If the product is aged before being applied to the substrate, the characteristics of the solution are permanently changed. The new characteristics produce planarizing coatings which are capable of filling vias completely. The degree of filling can be controlled through various conditions of aging.  
           [0015]    According to an aspect of the invention, a method for delivering a chemical to a substrate includes the following steps. A first vessel is provided for containing a first component of the chemical and providing a second vessel is provided for containing a second component of the chemical. The first component from the first vessel and the second component from the second vessel are supplied to a mixing chamber. The first and second components are mixed in the mixing chamber to form the chemical and the chemical is transferred from the mixing chamber to the substrate.  
           [0016]    In another aspect of the invention, a system for delivering a chemical to a substrate includes a first vessel for containing a first component of the chemical, a second vessel for containing a second component of the chemical, a mixing chamber for mixing the first and second components to form the chemical, and a fluid transport system connecting the mixing chamber and the substrate for transferring the chemical from the mixing chamber to the substrate.  
           [0017]    In another aspect of the invention, a method for forming an anti-reflective coating on a semiconductor substrate comprises providing a first vessel containing an ARC coating component, providing a second vessel containing a solvent, supplying the ARC component from the first vessel and the solvent from the second vessel to a mixing chamber, mixing the ARC component and the solvent in the mixing chamber to form a product, transferring the product to the semiconductor substrate, and applying the product to the semiconductor substrate to form the anti-reflective coating.  
           [0018]    Embodiments of the above aspects of the invention may include one or more of the following. The anti-reflective coating is a conformal coating. The anti-reflective coating has a degree of conformality and the degree of conformality is controlled by controlling a time between mixing the anti-reflective coating component and the solvent. The anti-reflective coating is a planarizing coating. The same anti-reflecting coating component and solvent is used to form a coating planarizing or conformal anti-reflective coating. The product is heated before it is transferred to the substrate. The anti-reflective coating has a degree of conformality and the degree of conformality is controlled by heating the product before transferring the product to the substrate. The product is transferred to the semiconductor substrate by a pump.  
           [0019]    Another aspect of the invention features a system for forming an anti-reflective coating on a semiconductor substrate. The system includes a first vessel for containing an anti-reflective coating component, and a second vessel for containing a solvent. It also includes a mixing chamber for mixing the anti-reflective coating component with the solvent to form a product and a fluid transport system connecting the mixing chamber and the substrate for supplying the product from the mixing chamber to the semiconductor substrate to form the anti-reflective coating.  
           [0020]    Embodiments of this aspect of the invention may include one or more of the following. The anti-reflective coating is a conformal coating. The anti-reflective coating is a planarizing coating. The same anti-reflective coating component and solvent are used to form the coating, with the coating being either planarizing or conformal anti-reflective coating. The system also includes a heat exchanger for heating the product before transferring the product to the substrate. The system also includes a pump for transferring the product to the semiconductor substrate.  
           [0021]    An advantage of an aspect of the invention is that it provides a method for forming conformal and planarizing films with a single product. The method provides an infinite range of film characteristics, with user-defined set-up properties. It enables significant process flexibility for manufacturing, not requiring tool-specific product. Conversion time lines for new product changeover can be accomplished in minutes.  
           [0022]    The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
       
    
    
     DESCRIPTION OF DRAWINGS  
       [0023]    [0023]FIG. 1 is a reaction scheme representing the formation of a cross-linking component, as is known in the prior art;  
         [0024]    [0024]FIG. 2 is a reaction scheme representing an alternate sequence in the formation of the cross-linking component of FIG. 1;  
         [0025]    FIGS.  3 A- 3 C are schematic cross-sectional views of a conformal anti-reflective coating formed on a substrate;  
         [0026]    FIGS.  4 A- 4 B are schematic cross-sectional views of a planarizing anti-reflective coating formed on a substrate;  
         [0027]    [0027]FIG. 5 is a schematic diagram of a chemical delivery system of the invention; and  
         [0028]    FIGS.  6 A- 6 D are micrographs of substrates with anti-reflective coatings, with varying degree of conformality. 
     
    
       [0029]    Like reference symbols in the various drawings indicate like elements.  
       DETAILED DESCRIPTION  
       [0030]    A system and approach for its use will be described in which the chemical manufacturer is not required to mix diluting solvents with ARC components in solution before the ARC material is shipped to the end-user. Instead, the end-user mixes the diluting solvents and ARC components himself in order to obtain desired material properties. For example, an ARC required by an end-user may include the product of a cross-linker component mixed with a polymer component and diluted in a solvent. The ARC cross-linker component is PL 1174 and p-TSA.H 2 O in a solution of PGME. Alternatively, the ARC cross-linker component is PL-EL, the ultimate reaction product of PL 1174 and p-TSA.H 2 O in a solution of PGME, in the presence of EL. The diluting solvents are kept separate from the ARC components in solution until the user introduces the diluting solvents and ARC components at the point of use. Alternatively, the user mixes the diluting solvents and ARC components in a bottle or reservoir at the point of use, and allows appropriate aging to form the cross-linker component PL-EL. In the latter alternative, the PL-EL is formed by either combining the appropriate components at room temperature and allowing enough time for the reaction to run to completion, or by artificial aging, such as by agitation. Finally, the ARC components and casting solvents are spun onto the substrate.  
         [0031]    The system described herein uses PGME as a mixing medium for the PL 1174 and polymer components. The diluting solvents can be ethyl lactate, cyclohexanone, and PGME, added at a specific ratio to produce an ARC composition with the desired final percentage of solids and with the required coating characteristics.  
       Conformal Arc  
       [0032]    Referring to FIG. 3A, a conformal ARC is formed over a substrate  10  having circuit features  12 ,  14  (e.g., polysilicon lines) defining a surface topography  16 . An ARC  18 , containing diluting solvents, e.g. ethyl lactate, and ARC components including cross-linker component comprising protonated PL, PGME, and PL-PGME adduct, as well as a polymer component, such as polyvinyl phenol, is delivered to substrate  10 , with the solvents and the ARC components present at an appropriate ratio. This ratio is a function of the desired viscosity and thickness. ARC  18  is spun on to substrate  10 , covering features  12 ,  14 .  
         [0033]    Referring to FIG. 3B, enthalpy is added (step  100 ) to cause ARC  18  to cross-link on substrate  10 . This post-coat bake  100  is performed at a temperature above the boiling point of the diluting solvents used to promote uniform spin coatings, i.e. casting solvents. The boiling point of the casting solvents EL, cyclohexanone, and PGME is approximately 154° C. The post-coat bake  100  is carried out at, e.g., 170° C. for 60 seconds. The initial stages of the bake cause the protonated PL and PL-PGME adduct to react with the available EL react to form PL-EL on substrate  10 .  
         [0034]    As the PL-PGME adduct transforms into PL-EL and solvent evaporates, increased shrinkage results in a conformal coating  20 . Bake step  100  effectively evaporates enough solvent to sufficiently reduce the mean free distance between polymer molecules and cross-linker molecules to enable cross-linking to occur. No cross linking occurs, however, until the PL-EL cross-linker component is formed. The solvent loss during the formation of the PL-EL cross-linker causes increased shrinkage due to the evaporation which takes place prior to the initiation of the cross linking. This solvent loss creates a conformal coating.  
         [0035]    Referring to FIG. 3C, heat is applied and a polymer resin stabilizes the ARC by cross-linking (step  200 ). The reaction on substrate  10  is as follows:  
                         
 
         [0036]    Converting the PL-PGME adduct to the PL-EL final product on substrate  10  reduces the effectiveness of the bake for the cross linking of the PL-EL and polymer. The reaction of PL 1174 with ethyl lactate, however, requires a lot of heat. The heat provided for this reaction also causes substantial amount of excess casting solvents, i.e. PGME, ethyl lactate, and cyclohexanone in EL, to evaporate. The reaction is somewhat slow because of steric hindrance (i.e., the size and shape of the molecules necessitate heat and time to get them into the appropriate location). As the solvents are driven off, the ethyl lactate and PL 1174 molecules get closer to each other, and the provided heat increases the kinetic energy of the molecules. A reaction between ethyl lactate and protonated PL 1174 then takes place. Heat is needed both to form the PL-EL cross-linker on the substrate  10  and also to cross-link the polymer.  
         [0037]    A conformal coating is normally desirable for better optical performance. ARC  18  is not completely opaque with respect to actinic light, i.e. light of an appropriate wavelength which causes desired photochemistry to occur, and substrate  10 . A conformal coating, therefore, provides a uniform optical thickness and reflectivity across substrate  10  because the ARC thickness is uniform. In accordance with the process described above, conformal coverage is obtained when the ARC cross-linker component is brought to the point-of-use containing primarily protonated PL 1174 and the PL-PGME adduct, the PL-EL cross-linker having not yet been formed. Conformal coverage is achieved by casting a very low viscosity material, having very low percentage of solids, across the whole wafer. As the solvents are evaporated, the material shrinks to form a conformal coating over the underlying topography. The solvents are driven off to form a dense film that mimics the topography of the underlying substrate. This process enables a shrinkage of the coating, controllable by varying the process parameters as well as the viscosity of the material.  
       Planarizing Arc  
       [0038]    Referring to FIG. 4A, in certain applications, a planarizing anti-reflective coating  22  is needed to planarize a surface  11  of a substrate  10  having topography  16  defined by circuit features  12 ,  14 . Planarization is facilitated by early cross linking of a cross linker, such as PL-EL and a polymer component. Thus, an ARC solution  24  containing PL-EL is spun on to substrate  10 . Substrate  10  is subjected to a bake step of, e.g., 170° C. for 60 seconds. The existence of the PL-EL cross linker allows the bake energy to directly accelerate solvent evaporation and cross linking. The solvent evaporates from surface  26  of the material. Thereby, the PL-EL cross linker and the polymer component cross link sooner than would be the case if PL-EL had not yet been formed. The cross linking sets the shape of the film surface. Referring to FIG. 4B, the result is a planarizing coating  22 . The reaction on substrate  10  is as follows:  
                         
 
         [0039]    More specifically, the initial process of adding heat to substrate  10  densities a surface  26  of ARC  24 . The PL-EL and polymer molecules at surface  26  come into contact quickly and set the surface nearly as it was coated, with very little shape changes. Once the surface skin is formed by cross-linking, the shape does not change, it is dense, and there is no further loss of thickness. Hence, one obtains a planarizing coating.  
         [0040]    In the planarizing ARC process, the reaction of cross-linking the polymers to one another through the powder link in PL-EL is a much quicker reaction than the conformal ARC process, in which PGME in the PL-PGME adduct has to be replaced with ethyl lactate Steric hindrances reduce the rate of the latter reaction. PL-EL, thereby, goes to a cross-linked final product much more easily than PL-PGME goes from PL-PGME to PL-EL to cross-linked product.  
         [0041]    A planarizing ARC coating is desirable, for example, in a case where one needs to selectively remove a portion or given thickness of an underlying layer, to expose some topographical features. Planarizing coatings can also be used to fill holes, thereby facilitating further lithographic or etching steps.  
         [0042]    Referring to FIG. 5, a system  300  for enabling the formation of both planarizing and conformal anti-reflective coatings from the same starting materials is shown. System  300  includes a vessel  310  for holding ARC components, e.g. a cross-linker component and a polymer component. System  300  also includes a vessel  320  containing solvents, e.g. ethyl lactate and cyclohexanone. A pump  330  transfers ARC components from vessel  310  to a mixing vessel  340  through tube  350 . Similarly, a pump  360  transfers solvents from vessel  320  to mixing vessel  340  through a tube  370 . The ARC components and solvents are mixed thoroughly, at a selected ratio, in mixing vessel  340 . According to a user specification, the resulting mixture is directed by a valve  380  to a path  390  to form a conformal coating, or to a path  400  to form a planarizing coating.  
         [0043]    The product, as it emerges from mixing vessel  340 , is in a form suitable for producing a conformal coating which has very little via fill. At that point, it has a relatively high amount of PL-PGME adduct and very little PL-EL. The product flows along path  390  towards pump  410 . Pump  410  pumps the product through a tube  420  and on to a substrate  430 , to form a conformal coating. A temperature-stabilizing jacket  435  is wrapped around tube  420  to ensure process reliability and repeatability.  
         [0044]    Alternatively, the product is transferred to path  400  and heated for an appropriate time and temperature to age in the material, causing the characteristics of the material to change. In this case, the change results in a planarizing coating which has nearly complete via filling capability. More specifically, the product, after entering path  400 , flows through a high temperature heat exchanger  440 . Heat exchanger  440  is electrically connected to a heater  450 . Heat exchanger  440  is set to an appropriate temperature, so that the product is altered to have the desired characteristics, e.g. a high ratio of PL-EL to PL-PGME. After the product is adequately aged, it is pumped by a pump  460  through tubes  470 ,  480 . Tubes  470 ,  480  are wrapped with temperature-stabilizing jackets  490 ,  500 . Jacket  490  cools the material to an appropriate working temperature, after the material has aged at a high temperature. The product passes through tube  480  to substrate  430  to form a planarizing coating. Temperatures, quantities, and flows at all process steps must be accurately controlled in order to effect the required changes in material characteristics.  
         [0045]    Referring to FIGS.  6 A- 6 D, aging of ARC affects via filling and planarity. In FIGS. 6A and 6B, at time 0, one has a conformal coating with poor via filling. As shown in FIG. 6C, a higher viscosity material forms a planarizing coating, after aging at room temperature for 21 days. Referring to FIG. 6D, aging a lower viscosity material, at room temperature for 21 days results in a coating transitioning from conformal to planarizing.  
         [0046]    The effect of aging in can be accelerated. The effect illustrated in FIGS.  6 A- 6 D can be seen at 1 hour with 60° C., or sooner at even higher temperatures. The option of accelerated aging allows one the opportunity to produce coatings which are either planarizing or conformal and can either fill vias to a high degree or to a lesser extent. As explained above, the reaction which controls the mechanism is initiated with the addition of the solvent system, a diluting medium, to the ARC components. Increasing the temperature increases the reaction rate and accelerates the aging.  
         [0047]    A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, the ARC components and solvents can be mixed through circulation or through other appropriate mixing methods. This procedure could be used to control many of the needed characteristics of chemistries used in the lithographic process by adapting the inherent properties or characteristics of a material through variations in quantities, times, temperatures, etc. Characteristics to be controlled could be via fill percentage, conformality, optical density, viscosity, and thickness. Further, the chemical product can be delivered to the substrate by various means other than a pump, such as by means of pressurized vessels, etc. The method of the invention can be used not only with ARC but also with various organic lithographic materials, such as spin-on glass or photoresist films. The method is appropriate to use with any material which has components which interact over time. Other materials, like spin-on-glass or photoresist films, may have other properties which have to be controlled, such as photospeed, ability to absorb other materials, like silation agents, or dielectric constants. The invention can be applicable to these materials, too. If one knows the mechanism by which the molecules are reacting, one can control the reaction and the characteristics at the final point of use.  
         [0048]    Accordingly, other embodiments are within the scope of the following claims: