Patent Publication Number: US-2005130069-A1

Title: Resist pattern forming method

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a resist pattern used for micro-processing in a semiconductor manufacturing technique, etc., and more specifically, to a method for-forming the same.  
      2. Description of the Related Art  
       FIG. 1  is a schematic cross section showing a first conventional example of a resist pattern forming method, and the steps are carried out from  FIG. 1 [ 1 ] to  FIG. 1 [ 3 ]. Description will be provided-hereinafter by referring to the drawings.  
      The first conventional example is a single-layer resist process which is the most popularly used process. First, as shown in  FIG. 1 [ 1 ], an ordinal positive photoresist is applied onto a substrate  50  with an oxide film to form a resist film  51  with a film thickness of about 1 μm. Then, as shown in  FIG. 1 [ 2 ], prepared is a photomask  62  in which a light-shielding film  61  in a prescribed pattern is formed on a transparent substrate  60 . KrF laser light  63  is irradiated onto the resist film  51  through the photomask  62  for forming a photosensitive part  52 . At last, as shown in  FIG. 1 [ 3 ]; the photosensitive part  52  is removed through performing developing processing by an alkali developer solution for obtaining a resist pattern  53 .  
      However, in the positive photoresist used herein, a component such as Si for improving the resistance characteristic of the resist pattern  53  to dry etching cannot be contained. The reason is that Si is originally an impurity component which gives a bad influence on the resolution of the photoresist so that it is impossible to obtain a sufficient resolution performance with the film thickness (1 μm in this case) of the applied base oxide film, which is sufficient to be resistant to dry etching. In general, it is considered the practical film thickness of the Si-containing photoresist film which can obtain a sufficient resolution property is 200 nm or less. Thus, in the single-layer resist process, it has not been possible to obtain a resist pattern which comprises both the sufficient film thickness and the sufficient resistance to dry etching.  
      In the drawings, for conveniences&#39; sake, only a part of the substrate  50  is illustrated and the thicknesses of the substrate  50  and the transparent substrate  60  are thinned. It is also the same in other drawings.  
      Also, there is a bi-layer resist process (for example, Japanese Patent Unexamined Publication No. 3-283418) in which the resist film is not single-layered but bi-layered. The bi-layer resist process in which a Si-containing photoresist is used as an upper-layer resist film will be described next as a second conventional example.  
       FIG. 2  is a schematic cross section showing the second conventional example of a resist pattern forming method, and the steps are carried out from  FIG. 2 [ 1 ] to  FIG. 2 [ 4 ]. Description will be provided hereinafter by referring to the drawings.  
      First, as shown in  FIG. 2 [ 1 ], through applying a non-photosensitive resist and a Si-containing positive photoresist in order on a substrate  70  with an oxide film, a non-photosensitive lower-layer resist film  71  (film thickness of about 800 nm) and a Si-containing upper-layer resist film  72  (film thickness of about 200 nm) are formed. As the non-photosensitive resist, mainly used are novolak-based resin and acrylic-based resin, which do not contain a photosensitive component such as a photo-acid generator. Further, as the positive photoresist, mainly used are the Si-containing photoresist having polysiloxane-based resin or polyhydroxystyrene resin as a base.  
      Subsequently, as shown in  FIG. 2 [ 2 ], prepared is a photomask  82  in which a light-shielding film  81  in a prescribed pattern is formed on a transparent substrate  80 . KrF laser light  83  is irradiated onto the upper-layer resist film  72  through the photomask  82  for forming a photosensitive part  73  on the upper-layer resist film  72 .  
      Then, as shown in  FIG. 2 [ 3 ], through dissolving the photosensitive part  73  in an alkali developer solution, a resist pattern  74  comprising only the upper-layer resist  72  is obtained. The lower-layer resist  71  at this time is non-photosensitive so that it is not patterned by the alkali developer solution.  
      At last, as shown in  FIG. 2 [ 4 ], through performing a dry-developing processing using a mixed gas of SO 2 +O 2  with the resist pattern  74  being a mask, a complete resist pattern  75  comprising the lower-layer resist film  71  and the upper-layer resist film  72  is obtained.  
      With the bi-layer resist process, it is possible to obtain the resist pattern  75  having a sufficient resistant to etching without deteriorating the resolution.  
      As described, with the bi-layer resist process which uses the Si-containing photoresist as the upper-layer resist film  72 , it is possible to obtain the resist pattern  75  having a high resistance to dry etching, which cannot be achieved by the single-layer process used in an ordinal semiconductor manufacturing steps. Therefore, it is considered to be very effective to be applied to the steps for processing a thick base film, such as when etching a dielectric film.  
      However, there is such a shortcoming in the conventional bi-layer resist process that the numbers of the steps and the devices to be used are increased, since it requires a dry-developing processing performed on the lower-layer resist film  71 , totaling two developing steps together with the developing processing of the upper-layer resist film  72  using the alkali developer solution.  
      An object of the present invention therefore is to provide a resist pattern forming method with a bi-layer resist process, which only requires a simple developing step equivalent to that of a single-layer resist process.  
     SUMMARY OF THE INVENTION  
      The resist pattern forming method according to the present invention comprises steps of: forming a positive lower-layer resist film on a substrate which is a subject of etching; forming a positive upper-layer resist film on the lower-layer resist film; exposing the lower-layer resist film and the upper-layer resist film simultaneously; and developing the lower-layer resist film and the upper-layer resist film simultaneously. An etching resistance improving component for improving resistance to the etching is contained only in the upper-layer resist film.  
      First, on the substrate, the lower-layer resist film and the upper-layer resist film are formed in order. The lower-layer resist film and the upper-layer resist film are both the same positive type so that the both can be exposed simultaneously and also developed simultaneously. Thus, although it is a bi-layer resist process, it only requires a single developing step, which is the same as the case of a single-layer resist process. Further, the lower-layer resist film does not contain an etching resistance improving component so that it exhibits a good light-transmittance. On the other hand, the upper-layer resist film which is exposed to a substrate etchant contains the etching resistance improving component. Therefore, it enables to obtain a resist pattern having a sufficient etching resistance without deteriorating the resolution.  
      The resist pattern forming method according to the present invention further comprises a step of forming a diffusion preventive film between the lower-layer resist film and the upper-layer resist film. The diffusion preventive film prevents diffusion of the etching resistance improving component from the upper-layer resist film to the lower-layer resist film and transmits light at the time of the exposure, while having a characteristic of being eliminated by a developer solution at the time of the developing.  
      When the etching resistance improving component is diffused from the upper-layer resist film to the lower-layer resist film, the light-transmittance of the lower-layer resist film is deteriorated. Thus, the diffusion preventive film is provided as described above when the diffusion of the etching resistance improving component is an issue. The diffusion preventive film also transmits the light of the exposure and is eliminated by the developer solution at the time of developing. Therefore, there is no increase in the number of the exposing step and the developing step even if there is the diffusion preventive film being provided.  
      Further, in the resist pattern forming method according to the present invention, the lower-layer resist film is formed by photoresist which is more sensitive to the light of the exposure compared to the upper-layer resist film.  
      The light of the exposure transmits through the upper-layer resist film and reaches the lower-layer resist film so that it is slightly attenuated when reaching the lower-layer resist film. Especially, there is an etching resistance improving component contained in the upper-layer resist film so that the attenuation amount of the light is large. Therefore, the lower-layer resist film is set to have higher sensitivity than that of the upper-layer resist film so that the lower-layer resist film and the upper-layer resist film are uniformly exposed.  
      Further, in the resist pattern forming method according to the present invention, a polysiloxane-based resin or a polyhydroxystyrene based resin is used as a resin composing the lower-layer resist film and the upper-layer resist film, and silicon is used as the etching resistance improving component.  
      Described above are the specific examples of the lower-layer resist film and the upper-layer resist film, and that of the etching resistance improving component. Needless to say, the lower-layer resist film, the upper-layer resist film, and the etching resistance improving component are not limited to the specific examples.  
      Furthermore, in the resist pattern forming method according to the present invention, an alkali developer solution is used as the developer solution and the diffusion preventive film is made of a copolymer containing polyvinyl alcohol or polyvinyl pyrrolidone as a base, which is soluble to the alkali developer solution.  
      Described above are the specific examples of the developer solution and the diffusion preventive film. However, the developer solution and the diffusion preventive film are not limited to the specific examples.  
      A dissolution velocity of the diffusion preventive film with respect to the alkali developer solution is set by adjusting a copolymerization ratio of a soluble component to an insoluble component in a monomer used as a raw material of the diffusion preventive film.  
      The dissolution velocity of the diffusion preventive film with respect to the alkali developer solution is increased with a larger amount of the soluble component in the diffusion preventive film.  
      A negative lower-layer resist film and a negative upper-layer resist film may be used instead of the positive lower-layer resist film and the positive upper-layer resist film.  
      In general, the resolution of the negative type is lower than that of the positive type. The reason is that the negative becomes swollen by absorbing the developer solution at the time of the developing processing. Thus, the negative type may be used if the low resolution is not an issue.  
      Further, the resist pattern forming method according to the present invention comprises steps of: forming a lower-layer resist film having a high-sensitive optical property on a substrate; forming a diffusion preventive film having a non-photosensitivity on the lower-layer resist film; forming an upper-layer resist film having a low-sensitive optical property on the diffusion preventive film; exposing the formed films by irradiating exposure light; and developing the films to which the exposure is performed. In this case, a film thickness of the diffusion preventive film is set thinner than film thicknesses of the other resist films. Also, the lower-layer resist film and the upper-layer resist film are formed with a positive photoresist.  
      The present invention is a resist pattern forming method in which a positive lower-layer resist film (high-sensitive), a diffusion preventive film (non-photosensitive), a Si-containing positive upper-layer resist film (low-sensitive) are formed on a substrate in order, and the positive lower-layer resist film and the Si-containing positive upper-layer resist film are exposed and developed simultaneously.  
      As the lower-layer resist film, a positive photoresist having a photosensitivity is used. The Si-containing positive upper-layer resist film and the positive lower-layer resist film are simultaneously exposed and simultaneously developed by an alkali developer solution. Thus, a dry-developing processing of the lower-layer resist film, which has been conventionally performed, becomes unnecessary. However, in order to prevent the diffusion of Si, it is preferable to provide the diffusion preventive film in between the both resist films. Also, in order to make the sidewalls of the resist pattern perpendicular, it is preferable to set the sensitivity of the positive lower-layer resist film higher than that of the positive upper-layer resist film.  
      As described above, in a bi-layer resist process which uses the Si-containing photoresist as the upper-layer resist film, it is possible to omit the dry-developing processing for developing the lower-layer resist film. Therefore, the resist pattern having a high dry-etching resistance can be easily formed.  
      In the resist pattern forming method according to the present invention, provided are two layers comprising the positive lower-layer resist film which does not contain the etching resistance improving component and the positive upper-layer resist film which contains the etching resistance improving component, so that the films can be exposed simultaneously and also developed simultaneously. Thus, although it is the bi-layer resist process, it only requires a single developing step which is equivalent to that of the single-layer resist process. Moreover, the lower-layer resist film does not contain the etching resistance improving component so that it has a good light-transmittance, while the upper-layer resist film which is exposed to an etchant contains the etching resistance improving component. Therefore, it is possible to obtain a resist pattern with a sufficient etching resistance by a simple manufacturing process without deteriorating the resolution. Further, the present invention achieves the following effects according to each aspect.  
      In the resist pattern forming method according to an aspect of the present invention, the diffusion preventive film is formed between the lower-layer resist film and the upper-layer resist film. Thus, the light-transmittance of the lower-layer resist film is not deteriorated by diffusion of the etching resistance improving component from the upper-layer resist film. Furthermore, the diffusion preventive film transmits the light of the exposure and is eliminated by the developer solution at the time of developing. Therefore, the resolution can be improved without increasing the number of the exposure step and the developing step.  
      In the resist pattern forming method according to another aspect of the present invention, the lower-layer resist film provided under the upper-layer resist film containing the etching resistance improving component for blocking the transmittance of light is formed to have higher sensitivity than that of the upper-layer resist film. Thus, the lower-layer resist film is exposed together with the upper-layer resist film in the same manner so that the lower-layer resist film and the upper-layer resist film can be uniformly exposed. Therefore, the sidewalls of the lower-layer resist film after the developing can be formed perpendicular so that the etching precision of the substrate can be improved. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematic cross section showing the steps of a conventional resist pattern forming method in order;  
       FIG. 2  is a schematic cross section showing the steps of another conventional resist pattern forming method in order;  
       FIG. 3  is a flowchart showing a resist pattern forming method according to the present invention; and  
       FIG. 4  is a schematic cross section showing an embodiment of the resist pattern forming method according to the present invention shown in  FIG. 3  in the order of the steps. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       FIG. 3  is an illustration for describing the steps of an embodiment of the resist pattern forming method according to the present invention.  FIG. 4  is a schematic cross section of the embodiment, in which the steps are carried out in order from  FIG. 4 [ 1 ] to  FIG. 4 .[ 3 ].  
      In the followings, description will be provided by referring to the drawings.  
      The outline of the embodiment will be described by referring to  FIG. 3  and  FIG. 4 [ 1 ].  
      The resist pattern forming method according to the embodiment comprises the steps of: forming a positive lower-layer resist film  11  on a substrate  10  as a subject of etching ( 1 ); forming a diffusion preventive film  12  on the lower-layer resist film  11  ( 2 ); forming a positive upper-layer resist film  13  on the diffusion preventive film  12  ( 3 ); exposing the lower-layer resist film  11  and the upper-layer resist film  13  simultaneously ( 4 ); and developing the lower-layer resist film  11  and the upper-layer resist film  13  simultaneously ( 5 ).  
      Si as an etching resistance improving component for improving the resistance to etching is contained in the upper-layer resist film  13  but not in the lower-layer resist film  11 . The diffusion preventive film  12  prevents the diffusion of Si from the upper-layer resist film  13  to the lower-layer resist film  11  and transmits the light of the exposure, while having a characteristic of being eliminated by the developer solution at the time of developing.  
      Next, it will be described in more detail by referring to  FIG. 4 [ 1 ] to  FIG. 4 [ 3 ].  
      First, as shown in  FIG. 4  [ 1 ], the lower-layer resist film  11 , the diffusion preventive film  12  and the upper-layer resist film  13  are formed by applying a positive photoresist for the lower layer, a resin for the diffusion preventive film, and a positive photoresist for the upper layer in order on a substrate  10  with an oxide film. For the lower-layer resist film  11 , for example, used is a chemical-amplified photoresist of about 800 nm in film thickness for KrF excimer laser. The chemically-amplified photoresist is made of a mixed solution of a resin, acid generator, solvent and the like. The chemically-amplified photoresist used herein may be the most popular one which uses polyhydroxystyrene as a base. For the upper-layer resist film  13 , for example, used is a KrF-photosensitive Si-containing photoresist of about 200 nm in film thickness. This also may be the most popular Si-containing photoresist which uses a polysiloxane-based resin or polyhydroxystyrene resin as a base. Further, between the lower-layer resist film  11  and the upper-layer resist film  13 , the non-photosensitive diffusion preventive film  12  of about some nm to some ten nm in film thickness having a characteristic of transmitting the KrF light is formed by application. The diffusion preventive film  12  prevents the deterioration of the resolution performance due to a mutual diffusion of the both resist films  11  and  13  during pre-baking which is performed after the application of the resist and before the developing.  
      Then, as shown in  FIG. 4 [ 2 ], provided is a photomask  22  in which a light-shielding film  21  in a prescribed pattern is formed on a transparent substrate  20 . A photosensitive part  14  is simultaneously formed on the lower-layer resist film  11  and the upper-layer resist film  13  by performing the exposure using the photomask  22  and a KrF laser light  23 .  
      Subsequently, as shown in  FIG. 4 [ 3 ], a resist pattern  15  is obtained by dissolving the photosensitive part  14  formed by the lower-layer resist film  11  and the upper-layer resist film  13  using the alkali developer solution. The diffusion preventive film  12  is not photosensitive. At this time, although the diffusion preventive film  12  is not photosensitive, since its film thickness is thin, it is dissolved in the alkali developer solution along with the lower-layer resist film  11  and the upper-layer resist film  13  and patterned. Examples of a material used for the diffusion preventive film  12  which is soluble to the alkali developer solution may be a copolymer containing polyvinyl alcohol or polyvinyl pyrrolidone as a base. An appropriate dissolution velocity of the diffusion preventive film  12  with respect to the alkali developer solution may be about 8×10 −5  μm/s to 8×10 −4  μm/s. The dissolution velocity can be controlled by adjusting the polymerization ratio of the soluble component to insoluble component in a raw material (monomer). In order to make the sidewall of the resist pattern  15  perpendicular, it is necessary to set the photosensitivity of the lower-layer resist film  11  to the KrF light higher than that of the upper-layer resist film  13 .  
      When dry etching is performed on the substrate  10  with the oxide film using the resist pattern  15  obtained by the method described above, it enables to sufficiently obtain a selection ratio of an etching rate since there is the Si-containing layer with an excellent dry-etching resistance provided on the upper layer of the resist pattern  15 . Further, the lower-layer resist film  11  and the upper-layer resist film  13  are simultaneously patterned by the alkali developer solution so that it is possible to omit the dry-developing processing which is performed in a conventional bi-layer resist process for developing the lower-layer resist film.  
      Needless to say, the present invention is not limited to the above-described embodiment. For example, although the above-described embodiment illustrates a combination of the KrF-photosensitive positive photoresist along with the upper-layer resist film and the lower-layer resist film, it is possible to achieve the similar effect by a combination of positive photoresist which is photosensitive to a laser light other than the KrF (krypton fluoride) excimer laser light source (248 nm), such as ArF (argon fluoride) excimer laser light source (193 nm) or F 2  (fluorine dimer) excimer laser light source (157 nm). Further, the diffusion preventive film may be omitted if it tolerates the diffusion of Si.