Abstract:
A coating is provided over a fresh layer of resist, such as a chemically amplified resist (CAR). The overcoat stabilizes process control and makes it possible to precoat the CAR on wafer or mask blanks some time prior to exposure.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a continuation-in-part of application Ser. No. 09/293,713, filed Apr. 16, 1999, which is hereby incorporated by reference in its entirety as if fully set forth herein. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    This invention relates the lithography and more specifically to lithography using chemically amplified resists.  
           [0004]    2. Description of the Related Art  
           [0005]    Lithography is a well known technique, especially in the semiconductor field, and involves coating a substrate which is, e.g., a semiconductor wafer or a reticle substrate with a layer of resist. The resist is sensitive to exposing energy which is typically either ultraviolet light, laser light, X rays or an electron beam. Portions of the resist are exposed and the remainder is not exposed. This is accomplished either by scanning a beam of the light or electrons across the resist to define patterns or, in the case of exposing certain types of wafers, applying the radiation through a partially transmissive mask, thereby to expose only non-masked portions of the resist.  
           [0006]    The resist is subsequently developed and the unexposed regions are either removed or remain, with the complementary exposed portions either remaining or being removed depending on whether the resist works in negative tone or positive tone, respectively. Thereby the exposure patterns the resist on the substrate.  
           [0007]    Subsequent steps typically involve ion implantation or etching or oxide growth so that the resist pattern is transferred into the underlying material. This is either the underlying substrate or, in the case of a mask, a thin layer of, for instance, chromium metal applied between the resist and the substrate which is thereby partially removed to form a mask.  
           [0008]    Lithography is thus used for making devices (for instance, either semiconductor devices or micro machined devices) and for making masks used in photolithography for exposure of other wafers. There are many well known formulations of resist for both electron beam exposure and light exposure at various wavelengths, as well as X-ray exposure. One category of enhanced sensitivity resists called chemically amplified resist (CAR) has been known for many years. CAR involves, e.g., an acid catalyzation process. Many variations of chemically amplified resists are commercially available primarily for 257 nm, 248 nm, and 193 nm deep ultraviolet (DUV) light lithography application. Many of these CARs have been used in electron beam light lithography.  
           [0009]    It is known that resists and especially CAR is sensitive to certain environmental contaminants, thus rendering their use for both wafer fabrication and for mask fabrication somewhat problematic and requiring special handling. This includes exposure and development very soon after application. It has been found that CAR deteriorates in terms of lithographic performance as soon as one hour (or less) after its application. Of course this undesirably increases cost. It also has limited use of the otherwise beneficial CAR.  
           [0010]    An additional problem associated with the use of resists such as chemically amplified resists is that of standing waves, which results from interference by a reflecting wave with the incoming wave. One method typically used to ameliorate the standing wave problem is to apply a post exposure bake (PEB). However, this does not always resolve the problem. Finally, certain resists may also be sensitive to variations in substrate stoichiometry.  
           [0011]    Examples of positive tone CAR are APEX, UVIIHS, rJV5, and UV6 manufactured by Shipley Co., Inc., AZ DX11000P, DX1200P and DX1300P manufactured by Clariant Corporation, ARCH 8010 and ARCH 8030 manufactured by Arch Chemicals, ODUR-1010 and ODtJR-1013 manufactured by Tokyo Ohka Kogyo Co., Ltd. and PEK11OA5 manufactured by Sumitomo Chemicals, Inc. Examples of negative tone CAR are SAL-60I, SAL-603 manufactured by Shipley Co., Inc., EN-009 PG manufactured by Tokyo Ohka Kogyo Co., Ltd., and NEB 22 manufactured by Sumitomo Chemicals, Inc.  
           [0012]    Therefore, it would be desirable to improve the usability and storability of chemically amplified resist applied on a substrate by finding ways to reduce the undesirable effects thereon of environmental contaminants.  
         SUMMARY OF THE INVENTION  
         [0013]    In accordance with this invention, the environmental sensitivity of resist is eliminated, or at least substantially reduced, by overcoating a chemically amplified (or other) resist with a thin coating of a protective but transmissive material. This allows long term storage (e.g., up to four months or longer) of unexposed resist applied to a substrate. The coating in some embodiments is an electric charge-dissipation (conductive) material. Although non-conductive material can also be used, it may be advantageous, particularly in electron beam exposure, to use a conductive overcoat.  
           [0014]    A conductive coating provides two desirable functions. These are, first, charge dissipation during electron beam exposure for accurate overlay of two successive layers in multilevel mask making, and, second, maintaining the shelf life and therefore stability of lithographic performance (in terms of critical dimension and integrity) of the resist, e.g., for a day, a week, a month, or months (at least four months as determined by experiment) after its application. Shelf life is not limited to mere storage, but includes, e.g., time spent in transit. This is a substantial improvement, since as stated above normally CAR formulations are subject to undesirable performance changes within minutes of application. Thus such an unexposed coated substrate (wafer or reticle) becomes an article of manufacture and of commerce rather than merely a transitory result of a process. This opens up a new business manufacturing opportunity of commerce (inter- or intracompany) in such articles of manufacture, not available heretofore.  
           [0015]    Thus desirably such overcoated resist can be prepared on the substrate (wafer or reticle) months before its actual exposure, in contrast to present use of CAR which requires application immediately prior to the exposure. Of course, this means that one company (or location) can manufacture the resist coated wafers or reticle blanks, and another company (or location) can then later perform the exposure, in contrast to present practice.  
           [0016]    An example of a charge dissipation coating material is any suitable conductive material which can be readily applied, for instance, a thin layer of an initially liquid organic conductive material (which dries) such as polyaniline, or a thin layer of a metal such as chromium or aluminum suitably applied. However, as noted above, any suitable material (charge dissipative or non-charge dissipative) which may be effective as a diffusion barrier (i.e., which may prevent diffusion of contaminants) may be employed as the overcoat. For example, in deep UV direct write laser mask fabrication, the material sold under the trade name AZ Aquatar III, sold by Clariant Corporation, may be employed to coat an entire mask prior to imaging.  
           [0017]    The exposing electron beam typically is operated 25 at or greater than 10,000 volts accelerating voltage and therefore can have a penetration range (through the coating material) on the order of about one micron to several microns below the resist surface. In the case of light exposure, the metal conductive coating layer will not be applicable. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    A better understanding of the invention is obtained when the following detailed description is considered in conjunction with the following drawings in which:  
         [0019]    [0019]FIG. 1 shows a mask blank with applied layers of CAR and charge dissipation material being exposed to a beam of exposing radiation in accordance with an embodiment of the present invention.  
         [0020]    [0020]FIG. 2 is a conceptual process flow diagram of a method in accordance with an embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]    The result of the above-described process of applying a layer of protective material over a CAR layer is illustrated in FIG. 1 which shows a conventional substrate  12  which, for instance, is the quartz or glass substrate (blank) used for making masks on which is conventionally a thin layer of metal such as chromium  14 , which is the mask layer to be patterned. Overlying these is the CAR layer  16  applied to a conventional thickness dependent on factors such as the CAR formulation and exposure technique.  
         [0022]    Structures  12 ,  14  and  16  may be wholly conventional. An additional layer  20  is applied over the CAR layer  16 . Layer  20  is in some embodiments of a charge dissipation material. This material is applied subsequent to application of the CAR layer  16  and while the CAR layerl 6  is fresh (before it is subject to environmental contamination). Then, later, structure  30  is exposed to actinic radiation, for instance a scanning electron beam or actinic (exposing) light in a conventional lithographic machine. In any case, the CAR is a formulation selected to be sensitive to the particular exposing radiation. Note that while a beam of exposing radiation is depicted here, this is not required. In lithography using a mask to expose a semiconductor wafer, the exposing radiation is not a focused beam.  
         [0023]    In the case of a semiconductor wafer, the chromium layer  14  is not present on the substrate  12 , which then would typically be crystalline silicon. However, in other respects use of the coating material layer  20  is the same in both the case of fabricating wafers and making a mask as depicted in FIG. 1. As described above, advantageously the protective layer  20  also provides electric charge dissipation during an electron beam exposure, since the electrons are dissipated through layer  20  rather than building up on the otherwise exposed upper surface of CAR layer  16 . (Resists are generally electrically insulative.) This charge dissipation has been found to be beneficial for accurate overlay when one is forming a mask with two successive layers in multilevel mask making, as in the fabrication of phase shift masks where some chromium is removed in the image areas, thereby rendering it nonconductive when the second level is exposed. (See Tan publication referenced below.) The protective layer is transmissive of the exposing radiation. A thin metal coating layer (typically 100-200A thick) is transmissive of an electron beam. If the actinic exposure is other than an electron beam, the overcoat material is chosen so that it is transmissive to the wavelength of exposure, such as 257 nm, 248 nm, or 193 nm deep ultraviolet light.  
         [0024]    Also, whether the exposing radiation beam is light or electrons, the presence of the protective layer improves the shelf life of the underlying CAR layer by shielding the CAR layer from environmental contaminants (including air and moisture). The coating layer  20  of course in any case is transmissive to the incident radiation.  
         [0025]    The following describes fabrication of the structure  30  of FIG. 1 and its use. The formation of chromium layer  14  on substrate  12  is conventional, as is the subsequent overlay of the CAR layer  16 . To the freshly prepared CAR layer  16  (which has typically been conventionally soft baked), a thin coating of a charge dissipation material  20  is applied. Examples of application of charge dissipation material are first spin coating a thin (800 to 2000A) layer of liquid organic conductive material (water-soluble conductive polymer), such as a polyaniline, commercially available as PanAquas (from IBM Corp) or Aquasave (from Nitto Chemicals). See “Conducting polyanilines: Discharge layers for electron-beam lithography”, Marie Angelopoulos et al.,  J.VAC.SCI.TECHNOL.B  7(6), (November/December 1989), pp.1519-1523, incorporated herein by reference in its entirety. Such water-soluble materials can be removed (after exposure of the resist) by rinsing in distilled water. Such a film has a conductivity of −0.1/ohm-cm.  
         [0026]    Alternatively, the charge dissipation coating is a thin metal layer  20  formed by evaporating or sputtering, for instance, to a thickness of 100 to 200A. Examples of suitable metals are chromium and aluminum. The coating material is selected to have no chemical effect on the resist. For further detail on an example of application of charge dissipation material on resist, see “Application of charge dissipation material on MEBES® phase shift mask fabrication”, Zoilo C. H. Tan et al.,  SPIE Vol.  2322  Photomask Technology and Management ( 1994), pp. 141-148, incorporated herein by reference in its entirety. Structure  30  is conventionally exposed (some time—minutes to months—later) using the electron beam or actinic light as in FIG. 1. Suitable systems for exposing the structure  30  include the MEBES and ALTA series systems, available from ETEC Systems, Inc., Hayward, Calif. A subsequent post exposure bake is also conventional.  
         [0027]    Then the upper layer  20  is stripped, e.g., by rinsing in deionized water which removes the organic conductive material. Another example, if the layer  20  is chromium, is stripping with a suitable acidic etching fluid. If layer  20  is aluminum, it similarly is removed by etching with alkaline etchant.  
         [0028]    Next is development of the exposed CAR layer  16 . This is conventional using whatever developer technique is suitable for the particular CAR formulation. If the development is performed using an alkaline developer formulation, this may by itself also remove the layer  20 , if layer  20  is aluminum. In other words, the application of the alkaline developer to structure  30  would initially dissolve the protective layer  20  and then perform the actual development of the underlying CAR layer  16 . This process therefore is exposure, bake, remove layer  20 , develop resist. Alternately, after exposure to actinic radiation, the upper layer  20  is stripped as described above, to be followed with post exposure bake and development of the underlying CAR layer  16  (expose, remove, bake, develop).  
         [0029]    As noted above, the coating may also be embodied as a non-charge dissipative layer and, in particular, any material suitable for use as a diffusion barrier (i.e., to prevent diffusion of airborne contaminants), for example, in direct write laser mask fabrication application. An example of such a material is AZ Aquatar III, available from Clariant Corporation. Use of such a material provides improvements in eliminating standing waves, protection of the resist from airborne contamination, and elimination of sensitivity to variations is substrate stoichiometry. In one embodiment, improved CD (critical feature) uniformity is achieved through selection of the material having an index of refraction matched to the index of refraction of the resist. For example, the index of refraction of the layer may be approximately equal to the square root of the index for the resist. In such an embodiment, light reflected off the substrate bottom and then internally back off the top of the protective layer and the top of the resist layer is generally equal in intensity.  
         [0030]    Fabrication of such a structure is explained with reference to FIG. 2. At  200 , a substrate  12  has applied to it a metal layer  14 . The substrate  12  may conventionally be fused silica. The metal layer  14  is the material in which the pattern is eventually formed. Typically, the metal layer is chromium and typically has a thickness of about 600 to 1000 angstroms. The chromium may be deposited by sputtering.  
         [0031]    In  202 , the resist  16 , such as a chemically amplified resist, is applied. The resist  16  may have a thickness of about 2500 to about 5000 angstroms and may be applied by spin coating. A suitable resist  16  is the DX1100 resist, available from Clariant Corporation. The mask is soft-baked at  204  (referred to as a “post apply bake (PEB)”) to remove solvents remaining in the resist film. Next, at  206 , the coating  22  is applied. For instance, a layer about 450 angstroms thick may be applied by spin coating at about  1550  RPM and spinning dry in air. As noted above, the coating  22  may be any material suitable for use as a diffusion barrier and, in particular, one such material is the material sold under the trade name AZ Aquatar III by Clariant Corporation. The coating  22  affords contaminant protection and critical dimension (CD) uniformity, as well as alleviating the standing wave problem.  
         [0032]    Next, at  208 , the mask is imaged using, for example, an ALTA laser writing system, available from ETEC Systems, Inc. As in the case of FIG. 1, the imaging may occur some time after the application of the resist and coating. In  210 , the mask is subject to a post-exposure bake (PEB). At  212 , the mask is then developed using a suitable developer. The developer may also be effective to remove the coating  22 . One advantage of a protective layer is that it may improve developer wetting and therefore achieve more optimal developing. Otherwise, the coating would be removed prior to developing. Finally, at  214 , the metal film is patterned, such as by planar plasma etching or reactive ion etching.  
         [0033]    This disclosure is illustrative and not limiting. The particular materials disclosed and the parameters of their use are also illustrative and not limiting; one of ordinary skill in the field will appreciate that various substitutions and modifications can be made. In any case, such modifications or substitutions are intended to fall within the scope of the appended claims.