Abstract:
A method of fabricating an integrated circuit using photolithography and an antireflective coating. An antireflective coating is formed on a substrate wherein the antireflective coating is electrically polarizable. A photoresist coating is formed on the antireflective coating on a side opposite from the substrate and the photoresist is exposed to activating radiation. The antireflective coating is subjected to an applied electric field at substantially the same time as the photoresist is exposed to activating radiation. The radiation absorption coefficient of said antireflective coating is increased and the refractive index of said antireflective coating is changed to be substantially equal to the refractive index of said photoresist coating.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates to semiconductor photolithography methods and, more particularly, to antireflection coatings for use with photolithography. 
     2. Description of the Related Art 
     One step in the fabrication of semiconductor devices such as integrated circuits is the formation of a substrate pattern over a semiconductor wafer surface through photolithographic masking and etching. A photoresist coating over a substrate is selectively exposed to activating radiation directed through a mask defining the desired conductor pattern. After photoresist development, the photoresist layer constitutes a relief image mask over the substrate. The relief image mask defines open areas over the substrate in a desired image pattern to be transferred to the substrate. The image is transferred to the surface of the substrate by surface modification of the substrate in a negative image of the pattern within the photoresist coating, such as by removal of a portion of the substrate by an etching process or by implantation of an atomic species into the substrate. The etching is often done in a plasma etch reactor in which a plasma of ions reacts with and etches away the exposed substrate. During these processes, the coating of the photoresist in the image pattern functions as a protective mask to prevent surface modification of the substrate underlying the photoresist mask. The resolution of the image transferred to the substrate is dependent upon the resolution within the imaged photoresist coating. 
     There are factors in addition to the resolution capability of the photoresist used that influence the quality or resolution of the image transferred to a photoresist masked substrate. For example, with reflective integrated circuit substrates, such as aluminum, exposure of a photoresist coating causes reflection of diffused activating radiation (light) from the integrated circuit substrate back into the photoresist coating. Standard photoresists are susceptible to surface reflections which degrade the fine-line images required for integrated circuit manufacture. This degradation occurs due to reflection of diffused light from the integrated circuit substrate back into the photoresist layer resulting in exposure of the photoresist layer in areas where imaging is not desired. Another common result of surface reflections is the formation of “notches” in conductive lines in certain regions because of unwanted exposure of photoresist by reflected light. These “notches” can cause the device to fail, or even worse, to be unreliable. 
     To prevent reflection of activating radiation into a photoresist coating, it is well known to provide antireflective layers (ARC&#39;s) between a substrate and a photoresist layer. These antireflective layers typically comprise an absorbing dye dispersed in a polymer binder though some polymers contain sufficient chromophores whereby a dye is not required. When used, the dye is selected to absorb and attenuate radiation at the wavelength used to expose the photoresist layer thus reducing the incidence of radiation reflected back into the photoresist layer. During the conventional processing of an integrated circuit substrate coated with the combination of an antireflective layer and a photoresist layer, the photoresist is exposed to activating radiation and developed to form a relief image, i.e., portions of the photoresist layer are removed by development with a liquid developer and portions remain as a mask defining a desired pattern. To alter the underlying substrate, the antireflective layer must be removed to bare the substrate in a desired image. Removal of the antireflective layer may be by dissolution with a liquid that simultaneously dissolves both the photoresist and the antireflective layer or by dry etching such as with an oxygen plasma. 
     Unfortunately, present antireflective coatings are less than 100 percent effective and are often difficult to remove. Furthermore, removal of the antireflective coating often results in degradation of important device properties and inconsistent performance of antireflection coatings due to thickness variation and other factors limits performance of photolithography. Therefore, it is desirable to provide an antireflective coating and a photolithographic process that results in up to 100 percent efficiency and that can be more easily removed from the underlying substrate. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method of fabricating an integrated circuit using photolithography and an antireflective coating. According to a preferred embodiment, an antireflective coating is formed on a substrate wherein the antireflective coating is electrically polarizable. A photoresist coating is formed on the antireflective coating on a side opposite from the substrate and the photoresist is exposed to activating radiation. The antireflective coating is subjected to an applied electric field at substantially the same time as the photoresist is exposed to activating radiation. The radiation absorption coefficient of said antireflective coating is increased and the refractive index of said antireflective coating is changed to be substantially equal to the refractive index of said photoresist coating. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself however, as well as a preferred mode of use, further objects and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
     FIG. 1 shows a flow chart depicting successive steps in the photolithographic masking and etching of a layer such as a metal layer overlying a semiconductor substrate. 
     FIGS. 2-8 show cross sections of a wafer at various stages in the fabrication process. 
     FIG. 9 shows a main chain polymer liquid crystal. 
     FIGS. 10A-10B show side chain polymer liquid crystals. 
     FIG. 11A depicts one type of main chain polymer liquid crystal. 
     FIG. 11B shows PHNA poly(hydroxynapthoic acid), a molecule suitable as a monomer. 
     FIG. 12A depicts a second type of main chain polymer liquid crystal. 
     FIG. 12B shows an example of a suitable mesogen, in this case PET poly(p-phenyleneterephthalate). 
     FIG. 13 depicts a side chain polymer liquid crystal. 
     FIG. 14 depicts an example of a side chain and mesogen. 
     FIG. 15 shows a twisted side chain polymer liquid crystal. 
     FIG. 16 shows an example of an aromatic ring suitable to form mesogens or monomers for polymer liquid crystals. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 1, there is shown a flow chart depicting successive steps in the photolithographic masking and etching of a layer such as a metal layer overlying a semiconductor substrate. Referring to FIG. 2, the method is intended to create a conductor pattern in a conductive film  202  which overlies a semiconductor substrate  201  and is typically insulated from substrate  201  by a dielectric layer  213  of a material such as silicon dioxide. Referring again to FIG. 1, the first step (step  110 ) of the method is to spin an antireflection coating (ARC)  203  on the upper surface of the conductive film  202 ; that is, to deposit a fluid antireflection coating on the surface of conductive film  202  and then distribute it evenly over the surface of conductive film  202  by spinning substrate  201  in a manner well known in the art. Referring to FIG. 2, the thickness of antireflective coating  203  is preferably between approximately 0.07 microns and approximately 0.15 microns with a most preferable thickness of approximately 0.1 microns. Antireflective coating  203  is comprised of a polymer liquid crystal material that is electrically polarizable and may also contain a solvent such as cyclohexadone to facilitate adhesion and spreading on the wafer. More detail about this material is provided below. 
     As shown in FIG. 1, the second step (step  120 ) is to permit the fluid antireflection coating  203  to settle and harden. If a polymer precursor is used, the hardening is the stop of polymerization, and if the fluid  203  is a polymer in a carrier, the hardening results from the evaporation of the carrier. Referring to FIG. 3, this step results in a planarized upper surface  233  of coating  203 . 
     Referring again to FIG. 1, the next step (step  130 ) is to coat the upper surface  233  with a layer of photoresist  204 . The photoresist is deposited and distributed, again, by spinning, as is well known in the art, which results in a photoresist layer  204  shown in FIG.  4 . Because of the planarization of the upper surface  233  of antireflection coating  203 , the photoresist layer  204  can be made to have a highly uniform thickness and a highly planar upper surface which are desired for some types of activating radiation. 
     The next step (step  140 ) of FIG. 1 is to selectively expose photoresist  204  to activating radiation  206  while subjecting antireflective coating  203  to an applied electric field  207  as shown in FIG. 5. A mask  205  with areas that are transmissive to the wavelength and type of activating radiation  206  chosen is placed between photoresist  204  and the source of activating radiation  206  so that photoresist  204  will only be exposed to activating radiation in desired areas. The directions of the applied electric field  207  should be approximately normal to the planes of the interfaces between adjacent layers as shown in FIG.  5 . It does not matter whether the electric field  207  is up (as shown in FIG. 5) or down as long as the applied electric field  207  is approximately normal to the planes of the interfaces between adjacent layers. The magnitude of applied electric field  207  is optimally between approximately 100 volts and 200 volts DC. The properties of antireflective coating  203  will be changed by this process such that the refractive index (n) of antireflective coating  203  will become approximately equal to the refractive index of photoresist  204  and the extinction coefficient (k ARC ) will be in the range of approximately 0.2 to approximately 0.5. In other words, n ARC ≅n resist  and k ARC =˜0.2 to ˜0.5. Thus the optical properties of antireflective coating  203  are tuned to photoresist layer  204  above it. 
     Next, photoresist  204  is developed (step  150 ). As is known in the art, development of photoresist coating  204  produces openings  254  in photoresist coating  204  describing the desired pattern to be formed in conducting layer  202  as shown in FIG.  6 . 
     Next, antireflection coating  203  coating and conductive layer  202  are plasma etched (step  160  of FIG.  1 ). The type of plasma utilized depends on the type of material used as conductive layer  202 . However, typical plasma etches may be oxygen or fluorine based etch chemistries of a type well known in the art. The etch stops on dielectric layer  213 . Finally, after plasma etch (step  160  of FIG.  1 ), photoresist layer  204  and ARC layer  203  are removed by etching, as is well known in the art, so as to leave the patterned conductive layer  202 . 
     Turning now to FIGS. 9-16, a more detailed description of the polymer liquid crystals suitable for use in ARC  203  is given. As mentioned above, ARC  203  comprises an electrically polarizable polymer liquid crystal. Polymer liquid crystals (“PLCs”) are a class of materials that combine the properties of polymers with those of liquid crystals. These “hybrids” show the same mesophases characteristic of ordinary liquid crystals, yet retain many of the useful and versatile properties of polymers. 
     In order for normally flexible polymers to display liquid crystal characteristics, rod-like or disk-like elements (called mesogens) must be incorporated into their chains. The placement of the mesogens plays a large role in determining the type of PLC that is formed. Main-chain polymer liquid crystals (“MC-PLCs”) are formed when the mesogens  910  are themselves part of the main chain of a polymer  920 . An example of the structure of a MC-PLC is shown in FIG.  9 . Conversely, side chain polymer liquid crystals (“SC-PLCs”) are formed when the mesogens  1020  are connected as side chains to the polymer  1010  by a flexible “bridge” (called the spacer)  1030 . Examples of SC-PLCs are shown in FIGS. 10A and 10B. 
     MC-PLCs are formed when rigid elements are incorporated into the backbone of normally flexible polymers. These stiff regions along the chain allow the polymer to orient in a manner similar to ordinary liquid crystals, and thus display liquid crystal characteristics. There are two distinct groups of MC-PLCs, differentiated by the manner in which the stiff regions are formed. 
     The first group of MC-PLCs is characterized by stiff, rod-like monomers. These monomers  1110  are typically made up of several aromatic rings  1120  that provide the necessary size. FIG. 11A shows an example of this kind of MC-PLC. FIG. 11B shows PHNA poly(hydroxynapthoic acid), a molecule suitable as monomer  1110 . 
     The second and more prevalent group of MC-PLCs is different because it incorporates a mesogen directly into the chain. The mesogen acts just like the stiff areas in the first group. Generally, the mesogenic units are made up of two or more aromatic rings that provide the necessary restriction on movement that allow the polymer to display liquid crystal properties. The stiffness necessary for liquid crystallinity results from restrictions on rotation caused by steric hindrance and resonance. Another characteristic of the mesogen is its axial ratio. The axial ratio is defined to be the length of the molecule divided by the diameter (x=L/d). Experimental results have concluded that these molecules must be at least three times as long as they are wide. Otherwise, the molecules are not rod-like enough to display the characteristics of liquid crystals. 
     This group is different from the first in that the mesogens are separated or “decoupled” by a flexible bridge called a spacer. Decoupling of the mesogens provides for independent movement of the molecules, which facilitates proper alignment. FIG. 12A shows a diagram of this type of MC-PLC. FIG. 12B shows an example of a suitable mesogen  1220 , in this case PET poly(p-phenyleneterephthalate). Notice the flexible spacer  1210  (methylene groups) and the stiff mesogen  1220  (aromatic ring and double bonds). 
     SC-PLCs have three major structural components: the backbone  1310 , the spacer  1330 , and the mesogen  1320 . FIG. 13 shows an example of a SC-PLC. The backbone  1310  of a SC-PLC is the element that the side chains  1320  and  1330  are attached to. The alignment of the mesogens  1320  causes the liquid crystal behavior. Usually, the mesogen  1320  is made up of a rigid core of two or more aromatic rings joined together by a functional group. FIG. 14 shows a diagram of a typical repeating unit (mesogen  1320 ) in a side chain polymer liquid crystal. Notice the spacer  1330  of methylene units and the mesogen  1320  of aromatic rings. 
     Like their main chain counterparts, mesogens  1320  attached as side groups on the backbone  1310  of SC-PLCs are able to orient because the spacer  1330  allows for independent movement. Notice, in FIG. 15, that even though the polymer backbone  1310  may be in a tangled conformation, orientation of the mesogens  1320  is still possible because of the decoupling action of the spacer  1330 . 
     Another example of an aromatic ring suitable to form mesogens  1320  and  1220  and monomers  1120  is the photochromic liquid crystal polyacrylate depicted in FIG.  16 . 
     In an alternate embodiment, ARC layer  203  is formed using chemical vapor deposition rather than a spin on method. In another alternate embodiment, photoresist layer  204  is formed using chemical vapor deposition. 
     In an alternate embodiment, the electrically polarizable liquid polymer is replaced with a magnetically polarizable material. The applied DC electric field in step  140  is replaced with a magnetic field or an alternating electric field with the field direction pointing in a direction substantially normal to the plane of the interface between ARC layer  203  and photoresist layer  204 . 
     Although the present invention has been described with reference to a conducting layer and has particular relevance to metals because of their highly reflective surfaces, the present teachings apply to patterning non-conductive and non-metallic layers as well. Furthermore, the present invention is not limited to the specific examples of polymer liquid crystals given. Any electrically polarizable polymer liquid crystal will suffice. However, the examples given are the presently preferred polymer liquid crystals. 
     The methods of forming individual layers in the wafer are given merely as examples. Other methods of forming layers other than chemical vapor deposition and spin-on techniques are applicable as well 
     The description of the preferred embodiment of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art. The embodiment was chosen and described in order to best explain the principles of the invention the practical application to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.