Abstract:
Photolithographic processing is enhanced by employing a composite comprising two bottom anti-reflective coatings, wherein the extinction coefficient (k) of the upper anti-reflective coating is less than that of the underlying anti-reflective coating. The use of a composite bottom anti-reflective coating comprising partially transparent upper anti-reflective coating substantially reduces reflective notching in the photoresist layer, particularly when employing i-line or deep UV irradiation to obtain sub 0.35 μm features.

Description:
TECHNICAL FIELD 
     The present invention relates to a method of manufacturing a semiconductor device employing an anti-reflective coating during photolithography. The invention is particularly applicable to the manufacture of semiconductor devices having sub 0.35 μm circuitry. 
     BACKGROUND ART 
     The escalating requirements for high density and performance associated with ultra large scale integration require responsive changes in conductive patterns, which is considered one of the most demanding aspects of ultra large scale integration technology. High density demands for ultra large scale integration semiconductor wiring require increasingly denser arrays with minimal spacing between conductive lines. The increasing demands for high densification impose correspondingly high demands on photolithographic techniques. 
     During the manufacture of a semiconductor device, light from the stepper is passed through a mask and the pattern transferred to the underlying photoresist layer. However, when the substrate underlying the photoresist layer is highly reflective, e.g., metal and polysilicon layers, light reflections can destroy the pattern resolution by several mechanisms, including off-normal incident light reflected back from the photoresist that is intended to be masked, incident light reflected off device features exposing “notches” in the photoresist, and thin film interference effects leading to linewidth variations when photoresist thickness changes are caused by irregular wafer topography. 
     Photolithographic techniques conventionally employed during various phases in the manufacture of semiconductor devices comprise forming an anti-reflective coating (ARC), also characterized as an anti-reflective layer (ARL), typically a bottom ARC positioned between a substrate, e.g., a dielectric or conductive layer, and a photoresist layer. ARCs are conventionally made of various materials, including organic and inorganic materials. For example, inorganic materials conventionally employed for ARCs include silicon nitride, silicon oxynitride, α-carbon, titanium nitride, silicon carbide and amorphous silicon. Organic materials conventionally employed for ARCs include spin-on polyimides and polysulfones. Conventional ARCs are designed by appropriate adjustment of variables such as composition, deposition conditions and reaction conditions, to exhibit the requisite optical parameters, e.g., index of refraction (n) and extinction coefficient (k), to suppress multiple interference effects caused by the interference of light rays propagating in the same direction due to multiple reflections in the photoresist layer. The effective use of an ARC enables patterning and alignment without disturbance caused by such multiple interference effects, thereby improving linewidth accuracy and alignment, critical factors with respect to achieving fine line conductive patterns with minimal interwiring spacing. The use of an ARC is particularly significant when forming a via or contact hole over a stepped area, as when etching a dielectric layer deposited on a gate electrode and gate oxide formed on a semiconductor substrate in manufacturing a field effect transistor. The physics involved in ARCs is known and the use of ARCs is conventional and, hence, will not be set forth herein detail. See, for example, T. Tanaka et al., “A New Photolithography Technique with Antireflective Coating on Resist: ARCOR,” J. Electrochem. Soc., Vol. 137, No. 12, December 1990, pp. 3900-3905. 
     ARCs have improved the accuracy of ultra-violet and deep ultra-violet lithography, and expanded the use of ion beam, I-line, KrF and ArF excimer laser lithography. T. Ogawa et al., “SiO x N y :H, high performance anti-reflective layer for current and future optical lithography.” Efforts have been made to engineer the optical parameters of an ARC, as by adjusting process variables impacting the refractive index during plasma enhanced chemical vapor deposition (PECVD). T. Gocho et al., “Chemical Vapor Deposition of Anti-Reflective Layer Film for Excimer Laser Lithography,” Japanese Journal of Applied Physics, Vol. 33, January 1994, Pt. 1, No. 1B, pp. 486-490. 
     In copending application U.S. Pat. No. 5,710,067 filed on Jun. 7, 1996, an anti-reflective film comprising silicon oxime having the formula Si 1−(x+y+z) N x O y H z , wherein x, y and z represent the atomic percentage of nitrogen, oxygen and hydrogen, respectively, is disclosed for use as an ARC. The disclosed silicon oxime ARC typically comprises 15-20 at. % oxygen and about 10-20 at. % hydrogen, and is formed employing a stoichiometric excess of nitrogen sufficient to substantially prevent bonding between silicon atoms and oxygen atoms. 
     Conventional techniques for manufacturing a semiconductor employ various types of ARCs, including a bottom ARC formed beneath the photoresist layer to reduce substrate reflections, and a top ARC deposited over the photoresist layer to reduce second-auto reflections. Bottom ARCs have emerged as the most effective in reducing reflections while interfering the least with the photolithographic processes. A conventional application of a bottom dielectric ARC is schematically illustrated in FIG. 1, wherein dielectric bottom ARC  11  is formed on substrate  10 , which substrate  10  comprises either a dielectric layer or conductive layer. A photoresist layer  12  is formed on bottom ARC  11  and exposed through a patterned mask (not shown) to irradiation  13 . The reflected light, cancelled by a phase-shift cancellation at one-half wavelength, is shown by arrows  13 A and  13 B. 
     As design specifications are reduced below 0.35 μm, greater demands are placed upon the already strained requirements of photolithography. For example, as design features shrink below 0.35 μm, ARCs are required to suppress more than 99% of substrate-reflected light, meet stringent photoresist and device contamination requirements and operate at extended UV wavelengths. Such requirements cannot be met by conventional ARCs. For example, with features shrinking well below 0.35 μm and stepper productions systems shifting to shorter wavelengths, many conventional bottom ARCs result in reflective notching and no longer maintain acceptable linewidth variations. Conventional approaches resort to chemical vapor deposited dielectric ARCs and fine tuning optical parameters, such as the index of refraction (n) and extinction coefficient (k), as well as optimizing the thickness (d) of the ARC. See, for example, Benchor et al., “Dielectric Anti-Reflective Coatings for DUV Lithography,” Solid State Technology, March 1997, pp. 110-114. Notwithstanding such efforts, conventional photolithographic capabilities constitute a severe limiting factor in reducing the design rule or maximum dimension below 0.35 μm, particularly when forming a pattern on a substantially transparent dielectric substrate. 
     Conventional deep UV lithography utilizes bottom ARCs comprising both spin on films as well as various compounds of silicon, oxygen and/or nitrogen, e.g., silicon nitrides, silicon oxides and silicon oxynitrides. Such bottom anti-reflective coatings have been relatively successful in modifying or eliminating the amount of reflective energy from a substrate into a photoresist layer when the underlying substrate is optically infinite, i.e., when the substrate will only reflect incident energy, such as i-line or deep UV irradiation, from its top surface. However, a substrate or film that is not optically infinite will reflect and/or transmit energy at every interface within the substrate. Such reflections result in what is known as “reflective notching” which contributes to poor circuit design control. 
     Reflective notching is schematically illustrated in FIG. 2, wherein dielectric layer  20 , e.g., a layer of boron phosphorous tetraethoxy silicate (BPSG), is applied over structures  21  and  22 . Structure  21  comprises sequentially formed first polysilicon layer  21 A, dielectric layer  21 B, second polysilicon layer  21 C, metal silicide layer  21 D, such as tungsten silicide, and polysilicon layer  21 E. Structure  22  comprises sequentially formed first polysilicon layer  22 A, metal silicide layer  22 B, such as tungsten silicide, and polysilicon layer  22 C. Structures  21  and  22  extend to different heights above interface  23  on which dielectric layer  20  is formed. Photoresist layer  24  is deposited on dielectric layer  20  forming interface  25  therebetween. Upon exposure to irradiation  26 , it is apparent that reflected light from different surfaces, i.e.,  26 A,  26 B and  26 C, is scattered back to interface  25  at various angles along with random interference patterns causing undesirable reflective notching in photoresist layer  24 . 
     Accordingly, there exists a need for photolithographic techniques, particularly improved bottom ARCs for use in patterning substantially transparent substrates. There exists an even greater need for improved bottom ARCs to enable accurate control of submicron features, particularly below 0.35 microns, particularly for use in patterning a photoresist layer on a relatively transparent dielectric substrate. 
     DISCLOSURE OF THE INVENTION 
     An object of the present invention is a method of manufacturing a semiconductor device with submicron design features and high photolithographic reliability. 
     Another object of the present invention is a photolithographic technique employing a bottom ARC for improved control of fine line patterns and reduced reflective notching. 
     Additional objects, advantages and other features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from the practice of the invention. The objects and advantages of the invention may be realized and obtained as particularly pointed out in the appended claims. 
     According to the present invention, the foregoing and other objects are achieved in part by a method of manufacturing a semiconductor device, which method comprises: depositing a first anti-reflective coating on a substrate; depositing a second anti-reflective coating on the first anti-reflective coating, wherein the second anti-reflective coating exhibits greater transparency than the first anti-reflective coating; and depositing a photoresist layer on the second anti-reflective coating. 
     Another aspect of the present invention is a method of manufacturing a semiconductor device, which method comprises: depositing a first anti-reflective coating on a substrate; depositing a second anti-reflective coating on the first anti-reflective coating; wherein the second anti-reflective coating has an extinction coefficient (k), e.g., 1.0, which is less than the extinction coefficient (k), e.g., 0.3, of the first anti-reflective coating; and depositing a photoresist layer on the second anti-reflective coating. 
     A further aspect of the present invention is an intermediate article formed during manufacturing a semiconductor device, which intermediate article comprises: a dielectric substrate; a first anti-reflective coating on the substrate; a second anti-reflective coating on the first anti-reflective coating, wherein the second anti-reflective coating has an extinction coefficient (k) less than the extinction coefficient (k) of the first anti-reflective coating; and a photoresist layer on the second anti-reflective coating. 
     Additional objects and advantages of the present invention will become readily apparent to those skilled in this art from the following detailed description, wherein only the preferred embodiment of the invention is shown and described, simply by way of illustration of the best mode contemplated for carrying out the invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 schematically illustrates the use of a conventional bottom ARC. 
     FIG. 2 schematically illustrates the source of notching due to reflections from irregular topographical features in a substantially transparent dielectric layer. 
     FIG. 3 schematically illustrates the use of an optically infinite bottom ARC. 
     FIG. 4 illustrates an embodiment of the present invention employing a composite bottom ARC. 
     FIGS. 5 and 6 are graphs of the index of refraction (n) and extinction coefficient (k), respectively, as a function of wavelength for silicon oxime films. 
    
    
     DESCRIPTION OF THE INVENTION 
     The increasing demands for accurate fine line patterns, minimal interwiring spacings, and reliable interlayer contacts and vias, require expansion of the capabilities and improvements in the accuracy of photolithographic techniques. The limitations on design rule features imposed by photolithographic technology can be overcome by strategically structuring bottom ARCs. The present invention comprises the formation and use of a composite ARC having strategically designed optical qualities which enhance the accurate control of design features below 0.35 μm. The present invention is particularly effective in solving the reflective notching in a photoresist layer applied to a substantially transparent substrate containing various topographical features therein, such as topographical features at different depths within the substrate. 
     As the first approach to the reflective notching problem, an optically infinite bottom ARC was applied to a substantially transparent substrate containing different topographical features, such as the situation schematically illustrated in FIG.  2 . The approach employing an optically infinite bottom ARC is schematically illustrated in FIG. 3, wherein elements similar to elements in FIG. 2 bear similar reference numerals. Adverting to FIG. 3, an optically infinite bottom ARC  30  is formed on substantially transparent substrate  20  containing topographical structures  21  and  22  at different heights above substrate surface  23  upon which dielectric layer  20  is applied. Optically infinite bottom ARC  30  was formed of a silicon oxime film with an extinction coefficient (k) of about 0.3. Irradiating beams  31  impinge on optically infinite bottom ARC  30  and reflect from the surface thereof  30 A as reflected beams  31 A. The attempted solution illustrated in FIG. 3 did not prove successful, in that irregularities due to reflective etching were formed in the profile of the photoresist pattern. 
     After extensive research and experimentation, the reflective notching problem was successfully resolved by formulating a composite bottom ARC. In accordance with an embodiment of the present invention, a composite bottom ARC is formed comprising a first lower ARC on the surface of a relatively transparent substrate and a second upper ARC an the surface of the first lower ARC. The first and second ARCs are strategically formulated and optically designed to eliminate reflections from the interface with a photoresist layer into the photoresist layer by total destructive interference, thereby avoiding the occurrence of reflective notching. 
     In an embodiment of the present invention, a first ARC is formed on a substantially transparent dielectric substrate, and a second ARC, exhibiting greater transparency than the first ARC, is formed on the first ARC, as by conventional deposition techniques, e.g., chemical vapor deposition. A photoresist layer is formed on the composite bottom ARC. The photoresist layer is image-wise exposed, as by i-line or deep UV irradiation, through a photomask to define a pattern which is developed in a conventional manner. Etching is then conducted through the underlying composite bottom ARC to transfer the pattern to the underlying substantially transparent dielectric substrate. 
     The relative transparencies of the first and second ARCs of the composite bottom ARC of the present invention can be optimized in a particular situation depending upon, inter alia, the nature and degree of transparency of the underlying substrate, by suitable adjustment of the thickness and optical properties. For example, a second ARC can be advantageously formed with an extinction coefficient (k) less than the extinction coefficient (k) of first ARC. When employing a substantially transparent substrate and depositing a composite bottom ARC in accordance with the present invention, it has been found suitable to form the first ARC with an extinction coefficient (k) of about 1 to about 2, and to form the second ARC with an extinction coefficient (k) of about 0.3 to about 0.7. 
     The first and second ARCs of composite bottom ARC of the present invention can comprise any of various dielectric materials, such as silicon nitrides, silicon oxynitrides and silicon oximes, preferably silicon oxime such as the silicon oxime disclosed in copending application U.S. Pat. No. 5,710,067, the entire disclosure of which is incorporated herein by reference. 
     Accordingly, an embodiment of the present invention comprises the formation of a bottom composite ARC comprising first and second ARCs, either or both comprising silicon oxime having a formula: 
     
       
         Si 1−(x+y+z) N x O y :H z , 
       
     
     wherein x, y and z represent the atomic percentage of nitrogen, oxygen and hydrogen, respectively. Preferably the silicon oxime comprises about 10 to about 20 at. % hydrogen. 
     In forming a bottom composite silicon oxime ARC in accordance with the present invention, the first ARC can comprise substantially optically infinite silicon oxime, with an extinction coefficient (k) of about 1 to about 2, while the second ARC can comprise silicon oxime with an extinction coefficient (k) less than that of the first silicon oxime ARC, e.g., about 0.3 to about 0.7. In other embodiments of the present invention, the first ARC of the composite bottom ARC can comprise a substantially optically infinite material, such as a metal, polysilicon, amorphous silicon or silicon nitride. 
     The present invention also comprises an intermediate article formed during the manufacture of a semiconductor device. Such an intermediate article comprises a dielectric substrate, such as a substantially transparent dielectric substrate, with a composite bottom ARC formed thereon in accordance with the present invention, wherein the composite ARC comprises a first ARC on the dielectric substrate and a second upper ARC on the first ARC. The second upper ARC is formulated with an extinction coefficient (k) less than the extinction coefficient (k) of the first lower ARC. The intermediate article of the present invention further comprises a photoresist layer formed on the second upper ARC of the composite bottom ARC. 
     An embodiment of the present invention is depicted in FIG. 4, wherein elements similar to the elements illustrated in FIGS. 2 and 3 bear similar reference numerals. As schematically illustrated in FIG. 4, a composite bottom ARC  40 , is formed on transparent dielectric substrate  20  containing topographical structures  21  and  22  extending to different heights above interface  23  on which dielectric layer  20  is formed. Composite bottom ARC  40  comprises first ARC  41  and second ARC  42  formed on first ARC  41 . The second upper ARC  42  is formulated to exhibit greater transparency than the first lower ARC  41  which can be optically transparent. 
     The thickness of the first and second ARCs, particularly the thickness of the second ARC  42 , is optimized to achieve 180° out of phase reflections, thereby eliminating reflections into the photoresist layer  45 . As schematically illustrated in FIG. 4, beams  43 A are reflected at the interface  44  between the first and second ARCs of composite bottom ARC  40 , wherein total destructive interference occurs. By selecting an appropriate film thickness for second ARC  42  and optimizing the extinction coefficients for second ARC  42  vis-à-vis first ARC  41 , substantial interference is obtained between the phase of the energy into the photoresist layer  45  and initial reflections. As one having ordinary skill in the art would also understand, the thickness, index of refraction (n) and extinction coefficient (k) for a particular anti-reflective material can be optimized to achieve the desirable total destructive interference so that light energy does not leave the interface  44  between the photoresist layer  22  and the composite bottom ARC  40 . FIGS. 5 and 6 present indices of refraction (n) and extinction coefficients (k), respectively, for silicon oxime films ranging from substantially optically transparent to substantially optically infinite. 
     In carrying out the present invention, conventional materials and processing techniques can be employed and, hence, such conventional aspects are not set forth herein in detail. For example, etching of the underlying dielectric layer is conducted in a conventional manner. One having ordinary skill in the art could easily select suitable photoresist materials and etchants, and employ suitable deposition and etching techniques. 
     In accordance with the present invention, a composite bottom ARC is formed having a first lower ARC and a second ARC with optimized optical properties to substantially eliminate reflective notching, particularly reflective notching caused by different topographic features within an underlying substantially transparent dielectric substrate. The present invention is applicable to various phases of semiconductor manufacturing wherein a substrate, such as a dielectric layer, is etched, as in damascene techniques. 
     Only the preferred embodiment of the invention and but an example of its versatility is shown and described in the present disclosure. It is to be understood that the invention is capable of use in various other combinations and environments and is capable of changes or modifications within the scope of the inventive concept as expressed herein.