Patent Publication Number: US-RE46628-E

Title: Pattern forming method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a reissue of U.S. Pat. No. 8,486,288, issued on Jul. 16, 2013 from U.S. patent application Ser. No. 13/049,419, which is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2010-65237, filed Mar. 19, 2010, the. The entire contents of which the above-identified applications are incorporated herein by reference. 
     FIELD 
     Embodiments described herein relate generally to a pattern forming method using nanoimprint lithography. 
     BACKGROUND 
     The advance of miniaturization of semiconductor devices has increased the difficulty in performing microfabrication by use of a conventional photolithography technique, and alternative techniques to replace the conventional technique have been demanded. Nanoimprint lithography (NIL) is gaining increasing attention as one of such alternative techniques. 
     According to the nanoimprint lithography technique, in a state that a mold (e.g., a template) having recesses and protrusions corresponding to a pattern to be transferred is brought into pressure contact with a light-curable substance applied to, for example, a substrate to be treated, the light-curable substance is cured by irradiation with light from the back surface of the mold; and thereby the pattern formed in the mold is transferred. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A to 1D  are sectional process drawings showing a pattern forming method according to a first embodiment. 
         FIGS. 2A to 2E  are sectional process drawings showing a pattern forming method according to the first embodiment. 
         FIG. 3  is a sectional process drawing showing a process included in a pattern forming method according to a second embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Before describing exemplary embodiments, descriptions are given of the context that has led the inventors to the exemplary embodiments. 
     In the nanoimprint lithography technique, when a template is brought into pressure contact with a light-curable organic material applied to, for example, an etching target layer (e.g., a silicon substrate) in an environment of air or a replacement gas, bubbles (of the air or the replacement gas) mixed in the organic material sometimes cause defects in the organic material. To discharge the bubbles for preventing the defects, the template needs to be kept in pressure contact with the organic material in, for example, a helium-gas atmosphere for a long time. 
     With this taken into consideration, the inventors have studied a method in which a porous layer (e.g., a porous oxide film) is formed immediately beneath the organic material, as a method for discharging the helium gas, which is mixed in the organic material, in a short time. The inventors formed the porous layer immediately beneath the organic material. As a result, the inventors experimentally confirmed that bubbles in the organic material can be reduced in a much shorter time by discharging the helium gas, which is mixed in the organic material, through the porous layer. The porous layer in this technique is formed between the organic material and the etching target layer. 
     When a desired pattern is formed in the etching target layer, however, the pattern needs to be transferred to the porous layer formed between the organic material and the etching target layer. The porous layer is less resistant to etching than the etching target layer. Hence, if dry etching (e.g., reactive ion etching (RIE)) is applied to the etching target layer by using the porous layer with the transferred pattern as a mask, it is likely that: the porous layer is etched before, for example, the etching target layer; and the desired pattern cannot be formed in the etching target layer. 
     In view of the circumstances described above, the below discussed exemplary embodiments provide a pattern forming method capable of forming a microscopic pattern even if a porous layer having lower etching resistance than an etching target layer is used immediately beneath the organic material. 
     Some exemplary embodiments are described below by referring to the drawings. In the descriptions that follow, the same portions are denoted by the same reference numerals throughout the drawings. In addition, the dimensional ratios in the drawings are not limited to illustrated ones. 
     Using sectional process drawings of  FIGS. 1 and 2 , a pattern forming method according to a first embodiment is described by giving an example of a method of forming a half-pitch pattern using a sidewall processing technique. 
     Firstly, as  FIG. 1A  shows, a light-curable organic material  13 a is applied to a transfer-receiving layer  11 a (hereafter referred to as a first transfer-receiving layer; for example a carbon layer) and a porous layer  12 a formed on and above an etching target layer  10 . In this respect, the porous layer  12 a is formed either by a chemical vapor deposition (CVD) process or by a coating process, for example, at a temperature of 300° C. to 350° C. To be more specific, as the porous layer  12 a, an oxide film (e.g., a laminate film of SiO 2  and SiO) having a lower density than an oxide film formed by a low-pressure chemical vapor deposition (LPCVD) process can be used. The density of the porous layer  12 a is, for example, approximately 1.5 g/cm 3  to 1.8 g/cm 3 . When the density of the porous layer  12 a is lower than 1.8 g/cm 3 , the time needed to discharge the helium gas can be reduced by half compared to the other cases. As the density is decreased, it becomes easier to discharge the helium gas, and the time needed to this end can be made shorter. 
     The organic material  13 a can be applied by spraying organic-material droplets by an inkjet technique, and  FIGS. 1A to 1D  show enlarged views of some of the droplets thus applied. 
     Then, as  FIG. 1B  shows, a template  14  with a desired pattern formed therein is brought into contact with the organic material  13 a. Then, as  FIG. 1C  shows, the template  14  is put in pressure contact with the organic material  13 a, and is held in that state until the organic material  13 b is spread into the entire pattern of the template  14  by capillary action. 
     Then, as  FIG. 1D  shows, the organic material  13 b is cured by, for example, irradiation of UV rays  15 . After that, as  FIG. 2A  shows, the template  14  is removed from the organic material  13 b. Thus, the organic material  13 b is formed in which a pattern inverted from the pattern of the template  14  is transferred (hereafter, the inverted pattern is referred to as the transferred pattern A). 
     Then, as  FIG. 2B  shows, an organic material  13 c is formed with either the dry-etching technique or the wet-etching technique. The resultant pattern of the organic material  13 c (hereafter the pattern is referred to as the transferred pattern B) is slimmed down with the sidewall portions etched laterally inward. The porous layer  12 a is processed with the dry etching (e.g., RIE) technique by using this organic material  13 c as a mask. Thus, a pattern (transferred pattern C) is formed as a porous layer  12 b. 
     Subsequently, as  FIG. 2C  shows, the organic material  13 c is removed, and then a transfer oxide film  16 a is formed on the first transfer-receiving layer  11 a and the porous layer  12 b. The transfer oxide film  16 a uses an oxide film with etching resistance higher than that of the porous layer  12 b, for example a SiO 2  film. The transfer oxide film  16 a is formed by, for example, a CVD process at room temperature. 
     Then, as  FIG. 2D  shows, the transfer oxide film  16 a is etched back until the top surface of the porous layer  12 b is exposed. To this end the transfer oxide film  16 a can be dry-etched with a CF-based gas (e.g. C 4 F 8  CHF 3 , CF 4 , or C 4 F 6 ). Note that a mixed gas containing any of the above-mentioned CF-based gas may be used. 
     As  FIG. 2E  shows, the porous layer  12 b alone is selectively dry-etched by using the difference in the etching resistance between the transfer oxide film  16 b and the porous layer  12 b. Hereafter, the resultant pattern formed in the transfer oxide film  16 c after the dry-etching is referred to as the processed pattern. 
     Then, the first transfer-receiving layer  11 a to which the processed pattern is transferred is formed with the transfer oxide film  16 c used as a mask. If the first transfer-receiving oxide film  11 a is, for example, a carbon film, the first transfer-receiving oxide film  11 a is dry-etched with an oxygen gas or the like. Then, the etching target layer  10  is dry-etched with this first transfer-receiving layer  11 a used as a mask. Thus formed is the etching target layer  10  with the processed pattern transferred thereto. 
     Note that, if the desired pattern can be formed in the etching target layer  10  with the transfer oxide film  16 c used as a mask, the first transfer-receiving layer  11 a need not be formed. 
     As has been described, in this embodiment, the etching target layer  10 , to which the processed pattern is transferred, is not formed using the porous layer  12 , but instead the transfer oxide film  16 . In the dry-etching of the first transfer-receiving oxide film  11 a and the etching target layer  10  by using the oxygen gas, the transfer oxide film  16  survives because the transfer oxide film  16  is not etched before the first transfer-receiving oxide film  11 a and the etching target layer  10  are etched. 
     Accordingly, this embodiment can provide a method capable of forming a microscopic pattern by using the nanoimprint lithography process even if the porous layer  12 , which is less resistant to etching than the etching target layer  10 , is formed between the etching target layer  10  and the organic material  13 . 
     In this embodiment, a half-pitch pattern is formed in the etching target layer  10  by the sidewall-processing technique. In a first possible modification, however, the pattern may be formed in the etching target layer  10 , to which the processed pattern that is the same as the transferred pattern is transferred, without using the sidewall-processing technique. 
     For example, after a transferred pattern E is formed in the porous layer  12  by using the organic material  13  with a transferred pattern D formed therein as a mask, the transfer oxide film  16  is formed on the porous layer  12 . Here, the transfer oxide film  16  is formed to bury the transferred pattern E of the porous layer  12 . Then, the transfer oxide film  16  is etched back until the top surface of the porous layer  12  is exposed. Then, the porous layer  12  is removed. Thus, the processed pattern is formed in the transfer oxide film  16 . 
     Then, the processed pattern is transferred to the etching target layer  10  with the transfer oxide film  16  used as a mask. 
     As has been described, as in the above-described first embodiment, the etching target layer  10  with the processed pattern transferred thereto is not formed using the porous layer  12 , but instead the transfer oxide film  16 . Accordingly, even if the porous layer  12 , which is less resistant to etching than the etching target layer  10 , is formed between the etching target layer  10  and the organic material  13 , a microscopic pattern can be easily formed by using nanoimprint lithography. 
     Next, a pattern forming method of a second embodiment is described by referring to a sectional process drawing of a process shown in  FIG. 3 . The pattern forming method of the second embodiment differs from that of the first embodiment in that, as  FIG. 3  shows, an oxide film  22  of the same kind as the transfer oxide film is formed beforehand between a first transfer-receiving layer  21  and a porous layer  24  (note that the oxide film  22  is also referred to as the transfer oxide film). The descriptions that follow focus on different points from the pattern forming method of the first embodiment. 
     As  FIG. 3  shows, on and over the etching target layer  10 , the first transfer-receiving layer  21  (e.g., a carbon layer), the transfer oxide film  22 , a transfer-receiving layer (hereafter referred to as the second transfer-receiving layer)  23 , and the porous layer  24  are formed in this order from the bottom. A light-curable organic material  25  is applied onto the porous layer  24 . A desired template (not illustrated) is brought into pressure contact with the organic material  25 , and then the organic material  25  is cured by, for example, irradiation of UV rays. Thus formed is the organic material  25  of a transferred pattern. 
     This transferred pattern of the organic material  25  is then transferred sequentially to the porous layer  24 , the second transfer-receiving layer  23 , the transfer oxide film  22 , and the first transfer-receiving layer  21 . 
     To be specific, using the organic material  25  as a mask, the porous layer  24  is dry-etched to transfer the transferred pattern to the porous layer  24 . Then, using the porous layer  24  as a mask, the second transfer-receiving layer  23  is dry-etched to transfer the transferred pattern to the second transfer-receiving layer  23 . When the porous layer  24  is dry-etched, a CF-based gas, such as C 4 F 8 , CHF 3 , CF 4 , and C 4 F 6  is used. On the other hand, when the second transfer-receiving layer  23  is dry-etched, either an oxygen gas or a mixed gas of oxygen and other gases is used if the second transfer-receiving layer  23  is made of, for example, a carbon film. 
     Then, the transfer oxide film  22  is dry-etched using the second transfer-receiving layer  23  as a mask, and thereby the processed pattern is formed. 
     Then, using the first transfer-receiving layer  21  as a mask, the processed pattern is transferred to the etching target layer  10 . 
     As has been described, in this embodiment, the etching target layer  10 , to which the processed pattern is transferred, is not formed using the porous layer  24 , but instead the transfer oxide film  22  as in the case of the first embodiment. In the dry-etching of the first transfer-receiving oxide film  21  and the etching target layer  10  with the oxygen gas, the transfer oxide film  22  survives because the oxide film  22  is not etched before the first transfer-receiving oxide film  21  and the etching target layer  10  are etched. 
     Accordingly, this embodiment can provide a method capable of forming a microscopic pattern by using the nanoimprint lithography process even if the porous layer  24 , which is less resistant to etching than the etching target layer  10 , is formed between the etching target layer  10  and the organic material  25 . 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.