Abstract:
DUV lithography process that eliminates post exposure baking of a photoresist. Thick photoresist may be processed to obtain enhanced sidewall profiles for microelectronic devices.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to U.S. Provisional Patent Application No. 61/808,160 entitled “Post Exposure Bake Removal for DUV Photoresist Process” filed on Apr. 3, 2013 for Xianzhong Zeng, et al. which is incorporated herein by reference. 
    
    
     BACKGROUND 
       FIG. 1  illustrates a conventional process  100  for preparing a photoresist. In process  100  a photoresist is provided on a substrate to form a coating in block  12 , and then the photoresist coating is soft baked via block  14 . Exposure of the coating through a mask is then performed via block  16 . Process  100  proceeds to bake the resist coating after exposure via block  18 , a procedure known as post exposure bake. A post exposure bake procedure is considered necessary to forman image in the resist. The conventional process concludes with development block  19 , in which the pattern on the mask is transferred to the resist coating. 
     Features patterned with photoresist masks prepared in accordance with process  100  have demonstrated deformed sidewalls in certain photolithography processes.  FIG. 2A  depicts a photoresist  30  on substrate  32 . Photoresist  30  has been baked after exposure in accordance with the method of  FIG. 1 . As a result of performing a post exposure bake, the top and bottom resist critical dimension (CD) shrink to form the curved side walls  35  of  FIG. 2A . After development, sidewall  35  of photoresist  30  has a deformed profile that deviates from the substantially vertical sidewalls that are desirable for fabricating microelectronic structures. Thus, process  100  generates photoresists that are unsuitable for forming microelectronic structures with predetermined critical dimensions. 
     A photoresist processed in accordance with the method of  FIG. 1  was used to define the side shields  20  of  FIG. 2B . On substrate  22 , side shields  20  are shown flanking cavity  25  in an intermediate structure. Intermediate structure may be used to fabricate, for example, a magnetic recording head. Cavity  25  has a narrower opening f near top  23 , and a much wider opening g near base  28 . Specifically,  FIG. 2B  illustrates an intermediate structure having a width f near the top  23  of cavity  25  and a width g near base  28  of cavity  25 . Since width f is significantly dissimilar to width g, the sidewall profile of features patterned with photoresists prepared by process  100  adversely impacts the critical dimensions of patterned devices. 
     It is believed that post exposure baking can deform photoresist sidewalls prepared in accordance with process  100 . Therefore, photoresists prepared in accordance with process  100  impartan undesirable curvature to side shields  20  in  FIG. 2B . 
     In light of the aforementioned problems, there is a need for improving the methods for processing photoresists for high resolution lithography. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a flowchart of a conventional method for producing a photoresist. 
         FIG. 2A  depicts a photoresist produced in accordance with the prior method of  FIG. 1 . 
         FIG. 2B  depicts a side shield produced in accordance with the prior method of  FIG. 1 . 
         FIG. 3  is a flowchart of a method for processing a photoresist in accordance with one embodiment of the disclosure. 
         FIG. 4A  depicts a photoresist produced in accordance with the process of  FIG. 3 . 
         FIG. 4B  depicts a side shield produced with a photoresist processed in accordance with an embodiment of the method of  FIG. 3 . 
         FIG. 5  illustrates a trench formed using a photoresist mask prepared in accordance with an embodiment of the method of  FIG. 3 . 
         FIGS. 6A-6B  illustrate a sequence of figures representing a process for forming a side shield in accordance with another embodiment of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Several embodiments of the invention will be described in reference to  FIGS. 3-6 . The figures are not drawn to the scale of an actual device or system, and are merely illustrative of the embodiments described herein. 
     One embodiment of the invention is directed to forming a pattern in a deep ultraviolet (DUV) lithography process. The flowchart of  FIG. 3  summarizes a process  300  of an embodiment of the disclosure. Process  300  begins at block  315  in which a thick photoresist is placed on a substrate to form a coating. Then the coating is soft baked or heated in block  325  to remove residual solvent. After soft baking, the coating can have a thickness of about 0.25 to about 5 micrometers in some embodiments. In other embodiments, the coating will have a thickness of about 1 to about 3 micrometers after block  325 . 
     Following the process of  FIG. 3 , the photoresist is then exposed via block  335 . After exposure, the photoresist is developed in accordance with block  345 , without any intervening baking performed between the exposure and development blocks. In certain embodiments, by adopting the process of  FIG. 3 , approximately 10-20% of processing time can be saved. 
     The process of  FIG. 3  may be implemented in several different ways. In one embodiment, a resist is coated onto a substrate in block  315 . Then the resist is baked as shown in block  325 . Thereafter, the resist is exposed at a high dosage in block  335 . During exposure, a ranging dosage ranged from 160-200 mJ/cm 2  is provided by a laser to expose portions of the positive photoresist. Since the resist can be irradiated with wavelengths of less than 300 nm, suitable light sources encompass DUV with wavelengths of 248 nm. Specifically, the light source may be a laser such as a krypton fluoride laser. After exposure, the resist is developed in block  345 . In this embodiment, post exposure baking is not performed. Following development, the exposed regions of the resist are removed in block  345 . Acid diffusion of the photoresist may occur at room temperature. Despite the absence of post exposure baking from the process of  FIG. 3 , the contrast and sensitivity of the resulting photoresists was acceptable. It is believed that a combination of a high dosage exposure and omission of post exposure baking forms smooth patterned features in certain embodiments. 
     Yet another embodiment of  FIG. 3  is directed to forming a patterned feature in a DUV lithography process. This embodiment comprises providing a photoresist on a substrate; heating the photoresist prior to exposure; exposing a portion of the photoresist to an illumination dose greater than or equal to 160 mJ/cm 2 ; and then developing the photoresist to form a pattern. In this embodiment, a post exposure bake (PEB) prior to block  245  is not performed. 
       FIG. 4A  illustrates a photoresist  40 , on substrate  42 , produced in accordance with one embodiment of the process of  FIG. 3 . When processed in accordance with this embodiment, a substantially straight sidewall profile  35  for photoresist  40  is obtained. By removing PEB in the process flow and/or controlling exposure, substantially straight photoresist side walls  35  were obtained, as shown in  FIG. 4A . 
     In certain embodiments, the photoresist is a positive photoresist. A suitable photoresist is Shinetsu I051, available from Shin-etsu, MicroSi, Inc. Although, in certain embodiments a positive photoresist is used, other embodiments of the process could employ a negative photoresist instead. 
     The positive photoresist is a combination of a suitable polymer with a photoactive compound (PAC). The PAC absorbs light energy during block  335 , resulting in the generation of an acid. The acid reacts with the polymer to break some of the polymeric bonds. In certain embodiments, the absence of PEB does not preclude PAC from absorbing sufficient amount of light energy to cleave chemical groups from the polymer. Thus chemical amplification occurs in the deep UV process in some embodiments of the present disclosure. The exposed photoresist film is developed with a basic chemical developer in block  345  to transfer the pattern from a mask to the photoresist. 
       FIG. 4B  depicts a structure such as a side shield  45  on a substrate  43  that is patterned with a photoresist produced in accordance with an embodiment of  FIG. 3 .  FIG. 4B  illustrates side shields  45  in an intermediate structure that may be used to fabricate a magnetic recording head. Side shields  45  flank cavity  44 . Cavity  44  has an upper portion  41  with a width j and a lower portion  47  with a width k. Width j of cavity  44  substantially equals width k of cavity  44 . The consistent widths for cavity  44  are an improvement over the side shield profile obtained in  FIG. 2B . Thus, by using a photoresist processed in accordance with embodiments of the disclosure, substantially straight side shields  45  may be obtained. 
     Features patterned with a photoresist  50  produced in accordance with  FIG. 3  will now be discussed in association with  FIG. 5 . After forming a photoresist  50  with straight side walls  58  in accordance with an embodiment of the disclosure, further processing can be performed. For example,  FIG. 5  illustrates a trench  55  patterned in intermediate layer  57  using photoresist  50  as a mask. The resulting trench  55  is produced with a smooth sidewall profile  53 . 
     Yet another example of patterning with a photoresist produced in accordance with an embodiment of  FIG. 3  is shown in  FIGS. 6A and 6B , where common elements have been omitted from for clarity.  FIG. 6A  illustrates an intermediate structure that includes a write pole  70  that has been ion milled and planarized to a predetermined width. Layer  66  is provided adjacent to sidewalls  61  of write pole  70  to separate pole  70  from intermediate layer (interm layer)  64 . On an upper surface of intermediate layer  64  and write pole  70  lies a remnant of mask  60  which was used to define write pole  70 . Seed layer  68  can be deposited across the structure of  FIG. 6A  in preparation for depositing shield material in a manner that is known to the skilled artisan. The deposition of seed layer  68  creates channels  65 . 
     After the structure in  FIG. 6A  is formed, a photoresist mask  67 , prepared in accordance with an embodiment of  FIG. 3 , is disposed on seed layer  68 .  FIG. 6B  illustrates the substantially straight side walls  77  of photoresist mask  67 . Photoresist mask  67  is used to define side shield  75  when side shield material is deposited onto seed layer  68  to extend within channels  65 . Due to the substantially straight profile of photoresist mask  67 , side shields  75  are formed with substantially straight side walls  72 . 
     Example 1 
     A solution of Shinetsu I051 was spun onto a wafer, and then heated at 110° C. After baking, the photoresist had a thickness of between about 1-4 microns. Using an ArF laser, the resist coating was exposed at a wavelength of 248 nm and an energy of 170 mJ/cm 2 . Then, the coating was developed to completely transfer the pattern from the mask to the substrate. A PEB was not performed prior to development. 
     The above detailed description is provided to enable any person skilled in the art to practice the various embodiments described herein. While several embodiments have been described, it should be understood that these are for illustration purposes only and should not be taken as limiting the scope of the invention. 
     Various modifications to these embodiments will be readily apparent to those skilled in the art, and generic principles defined herein may be applied to other embodiments. Thus, many changes and modifications may be made to the embodiments, by one having ordinary skill in the art, without departing from the spirit and scope of the invention.