Abstract:
The present invention relates to a method for removing a resist selective to a carbon hard mask including providing an etching plasma comprising of at least hydrogen at a predetermined temperature level and a predetermined pressure level in a reaction chamber, and etching the resist selectively to the mask with said plasma for a predetermined period of time.

Description:
TECHNICAL FIELD OF THE INVENTION 
   The present invention generally relates to semiconductor devices and more particularly to a method for removing a resist mask with high selectivity to a carbon hard mask used for semiconductor structuring. 
   BACKGROUND OF THE INVENTION 
   As is known in the art, mask layers, preferably hard mask layers, are deposited on semiconductor devices in order to structure the semiconductor devices in a predetermined manner. For photolithographically structuring the semiconductor device and/or the mask layer, a photo resist is commonly used. Such a photo resist is generally patterned by exposing the resist to electromagnetic waves of a predetermined wave-length range and patterned with a pattern device. 
   A cross-sectional view of a portion of a semiconductor device is shown in  FIG. 2  which illustrates a semiconductor device  10 , constituted preferably of Si, wherein as an example, a recess  11  has basically a rectangular shape in the cross-section of  FIG. 2 . A mask  12 , preferably a carbon hard mask with a thickness of 170 nm as an example, is deposited on the semiconductor substrate  10 . On top, a liner  13 , preferably SiON with a thickness of 60 nm as an example is deposited according to  FIG. 2 . 
   For the structure shown in  FIG. 2 , a known stripping process for removing a photo resist (not shown in  FIG. 2 ) mask was utilized. The resist was a carbon-type film which was removed by an oxygen-based etchant. The etchant plasma comprised for example 2000 sccm O 2  as well as 100 sccm N 2  at a pressure of e.g. 150 Pa and a temperature of 250° C. While stripping the photo resist on top of the liner layer  13 , e.g. SiON, cavities  14  were formed in the carbon hard mask  12  as a parasitic side effect. Such erosion cavities  14  in the carbon hard mask  12  are highly undesirable since it leads to the etching of undesired features during subsequent processing. The formation of the cavities  14  is based on the attack of the carbon hard mask  12  by the oxygen rich plasma process used during the resist rework step at areas of weakness, especially at edges, corners and strongly bent portions of liner layer  13   
     FIG. 3  shows a structure basically similar to the structure according to  FIG. 2  except that the cavities  14  and therefore the erosion of the carbon hard mask  12  on the semiconductor substrate  10  is substantially more pronounced. Leading to the structure of  FIG. 3 , a wet etch process was carried out three times to remove a photo resist (not shown in  FIG. 3 ) from the surface of liner  13 . Here, the vast cavities  14  formed in the carbon hard mask  12  on top of the semiconductor substrate  10  also result from the attack of the carbon hard mask by the wet etchant used during the resist stripping process at areas of weakness in the SiON liner layer  13 . 
   SUMMARY OF THE INVENTION 
   In an effort to reduce erosion of a mask layer during a resist strip process, it is desirable to develop a resist strip method with a high selectivity to the underlying mask, preferably a carbon hard mask. 
   A preferred embodiment of the present invention provides a method for removing a resist mask from a liner on a mask. The method includes, for example, providing a plasma comprising of hydrogen at a predetermined temperature level and a predetermined pressure level in a reaction chamber, and etching the resist selectively to the mask with the plasma for a predetermined period of time. 
   In accordance with a further preferred embodiment, the hydrogen plasma could be diluted with Nitrogen so as to obtain a cost-effective and safe Forming gas chemistry for the resist strip application. A 96:4 Nitrogen to Hydrogen gas mixture is a standard Forming gas chemistry used in the semiconductor industry as an example. 
   In accordance with a further preferred embodiment of the present invention, the plasma comprising of a predetermined amount of CF 4 , wherein the predetermined amount is for e.g. less than 5 per cent, preferably about 1 per cent. By the use of a small amount of CF 4 , the selectivity from the resist to the Carbon hard mask can be further enhanced. 
   In accordance with a further embodiment, the resist etching plasma is free from oxygen. This bears the advantage of a solely reductive etchant for the resist strip. 
   In accordance with a further preferred embodiment, the predetermined pressure level of the etching plasma is in the range of 50 to 300 Pa, preferably about 150 Pa. 
   In accordance with a further preferred embodiment, the predetermined temperature level is in the range of 150° C. to 350° C., preferably about 250° C. With these process parameters a high removal rate of the resist still supplying a desired selectivity from the mask to the resist is advantageously provided. 
   In accordance with a further preferred embodiment, the carbon hard mask deposited for example using a chemical vapor deposition technique is used as an etch mask for semiconductor structuring. 
   In accordance with a further preferred embodiment, the resist mask is a carbon-based photo resist material. 
   In accordance with a further preferred embodiment, the liner preferably SiON is deposited on the carbon hardmask prior to depositing and stripping the resist. 
   In accordance with a further preferred embodiment, the semiconductor substrate is a Si-substrate. Best results of the resist stripping process were identified with aforesaid advantageous materials. 
   In accordance with a further preferred embodiment, the selectivity of the mask to the resist is equal or higher than 10. 
   In accordance with a further preferred embodiment, the resist is stripped with an across wafer uniformity of &lt;3% one-sigma. Thereby, a highly selective removal of resist on a liner on top of a mask can be achieved with a high level of across wafer resist strip uniformity. 
   The foregoing section outlines rather broadly the features and technical advantages of embodiments of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the present invention will be henceforth described. It should be appreciated that the concepts and specific embodiments disclosed may be readily utilized by those skilled in the art for carrying out the same purposes outlined in the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following description taken in conjunction with the accompanying drawings, in which: 
       FIG. 1  illustrates a schematic cross-sectional view of a semiconductor substrate explaining a preferred embodiment method of the present invention. 
       FIG. 2  illustrates a schematic cross-sectional view of a semiconductor substrate explaining a prior art formation process. 
       FIG. 3  illustrates a schematic cross-sectional view of a semiconductor substrate explaining a further prior art formation process. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The making and using of the presently preferred embodiments are discussed in detail below. It should be appreciated, however, that the present invention provides a widely applicable inventive concept that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use the invention, and do not limit the scope of the invention. 
   The present invention will be described with respect to preferred embodiments in a specific context, namely carbon photo resist removal on a carbon hard mask deposited on a semiconductor substrate. The invention may also be applied, however, to other stripping processes, such as removal of a sacrificial layer on a semiconductor substrate. The concepts of the present invention can be used with a variety of semiconductor devices including memories, CPUs, digital signal processors and amplifying devices. 
   A first embodiment will now be described with respect to  FIG. 1 . In  FIG. 1 , an exemplified recess  11  is provided in a semiconductor substrate  10 . The semiconductor substrate  10  e.g. a semiconductor wafer is preferably a Si-semiconductor substrate. On the surface of the semiconductor substrate  10 , a mask  12  is deposited. The mask  12  is preferably a hard mask, such as a carbon hard mask preferably deposited by CVD (chemical vapor deposition) and extends for example about 200 nm above the surface of said semiconductor substrate  10 . The recess  11  in the semiconductor substrate as shown has a rectangular cross-section on which hard mask  12  is deposited. The convex shape of the hard mask  12  in the recess  11  is an unintentional result of the deposition of the hard mask  12  in the recess  11 . 
   The carbon hard mask  12  outside the recess  11  protrudes the vertical etch line of the semiconductor substrate  10  also unintentionally as a result of the formation process of the carbon hard mask  12 . An overlying liner  13 , preferably consisting of SiON, is deposited basically evenly on the surface of the carbon hard mask The liner  13  acts as a barrier liner separating the mask  12  from a overlying resist. A resist, preferably a carbon photo resist (not shown in  FIG. 1 ), which has been deposited on the shown structure is completely removed from the structure in accordance with  FIG. 1 . 
   For stripping the resist from the liner  13  overlying the carbon hard mask  12 , a reductive etchant comprising hydrogen is/was used. Preferably an etching plasma with a flow of 1000 sccm of forming gas, comprising 96 per cent nitrogen N 2  and 4 per cent hydrogen H 2 , was used for a predetermined time, for example 270 seconds, at a predetermined temperature level, for example 250° C., and a predetermined pressure level, for example 150 Pa. Using such an oxygen-free etching plasma, the selectivity between the liner  13  and the stripped resist of more than 10, can be reached. 
   As is apparent from  FIG. 1 , the carbon hard mask  12  shows no erosion symptoms, especially not in the areas of weakness existing in the liner layer  13  where said liner  13  is strongly bent around feature corners. Therefore, the stripping process to remove photo resist from a semiconductor wafer selectively to a mask  12  in accordance with the present invention using a reductive etchant comprising hydrogen is superior to the known stripping processes as described with reference to  FIG. 2  and  FIG. 3 . 
   In a further preferred embodiment, the etching plasma with 1000 SCCM of said nitrogen and hydrogen ratio, a predetermined amount of preferably below 4 per cent, especially 1 per cent of CF 4  equivalent to 10 sccm, is used to remove the photo resist from said liner  13 . With the etchant comprising about 1 per cent CF 4 , a selectivity from liner  12  to the resist of more than 16 is possible. While using said plasmas in accordance with the preferred embodiments of the present invention, an across wafer non-uniformity of less than 8 per cent, especially less than 4 per cent, could be obtained. 
   While not shown, it is understood that other elements could be included in the semiconductor substrate  10 . Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, manufacture, materials, methods or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, manufacture, materials, means, methods or steps. 
   REFERENCE SIGNS 
   
       
         10  semiconductor substrate/device, preferably Si-wafer 
         11  recess, preferably rectangular 
         12  mask, preferably carbon hard mask 
         13  liner, preferably SiON 
         14  cavity, especially mask erosion area