Abstract:
A method of fabricating a photomask having improved critical dimension (CD) uniformity that meets or exceeds 90 nanometer technology requirements. The method includes the steps of: providing a transparent substrate covered with a layer of opaque material and a layer of photoresist; patterning the layer of photoresist to expose an area of the layer of opaque material that has a shape that follows a contour of a main pattern area to be defined by the layer of opaque material; removing the exposed area to define the layer of opaque material into the main pattern area and an area that surrounds the main pattern area; removing the patterned layer of photoresist; and removing the surrounding area of the layer of opaque material.

Description:
FIELD OF THE INVENTION 
     The present invention relates to semiconductor device fabrication and, more particularly, to a method of fabricating a line pattern dominant photomask with improved critical dimension (CD) uniformity. 
     BACKGROUND OF THE INVENTION 
     Fabricating integrated circuits with faster and/or an increasing number of semiconductor devices is a continuing trend in integrated circuit technology. A very influential factor in this trend is technology scaling, i.e., the reduction in transistor feature sizes. Technology scaling has enabled transistors to become smaller, thus allowing for more dense integrated circuits in terms of the number of transistors packed on a chip. 
     Semiconductor devices are formed by performing a sequence of different processes on a semiconductor wafer. Some of these processes include doping, etching, oxidizing, or depositing various layers. Typically, only selected portions of the wafer are subjected to these processes at any given stage of fabrication. 
     The processing of selected portions of a semiconductor wafer can be accomplished using well known lithographic methods. In such methods, a photomask is used to transfer a desired pattern to the semiconductor wafer. The photomask may be a patterned chrome layer overlying a quartz substrate. 
     The pattern features of photomasks are becoming progressively smaller so as to keep up with the demand for more dense integrated circuits. These smaller pattern features are currently formed by a photomask fabrication method that employs electron-beam (e-beam) lithography, using positive or negative resists, dry etching and critical dimension scanning electron microscopy (CDSEM). 
     As is well known, the term critical dimension (CD) refers to the size of the smallest geometrical feature which can be formed during semiconductor device/circuit manufacturing using a given technology. The photomask fabrication method described above has many problems achieving CD uniformity requirements for 90 nanometer and smaller mask technology. Specifically, during e-beam lithography, local CD errors are induced by proximity effects, and global CD errors are caused by vacuum and heating effects. Global and local CD errors also occur during mask development and etching due to pattern loading differences and trench linearity problems. During CDSEM, where a negative resist has been used during e-beam lithography, the numerous non-conducting areas (the areas of the mask where the quartz substrate has been exposed by removal of the chrome layer) of the photomask become statically charged during handling, which causes CD measurement problems. The static charging also attracts dust and other particles to the photomask, which due to the high pattern density and the large size of the wafer dies, increases the probability of fatal defects in the individual wafers, thus resulting in lower process yields. 
     Accordingly, a photomask fabrication method is needed which is capable of achieving CD uniformity requirements of 90 nanometer or higher mask technology while eliminating or minimizing the above problems. 
     SUMMARY OF THE INVENTION 
     A method is disclosed herein for fabricating a photomask having improved critical dimension (CD) uniformity. The method comprises the steps of: providing a transparent substrate covered with a layer of opaque material and a layer of photoresist; patterning the layer of photoresist to expose an area of the layer of opaque material that has a shape that follows a contour of a main pattern area to be defined by the layer of opaque material; removing the exposed area to define the layer of opaque material into the main pattern area and an area that surrounds the main pattern area; removing the patterned layer of photoresist; and removing the surrounding area of the layer of opaque material. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A–1H  are plan views illustrating a first embodiment of the method of the present invention. 
         FIGS. 2A–2H  are plan views illustrating a second embodiment of the method of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention is a method of fabricating a line pattern dominant photomask with improved critical dimension (CD) uniformity that meets or exceeds 90 nanometer technology requirements. The method of the invention also resolves the problem of CD measurement and defect issues associated with convention mask fabrication methods. 
       FIGS. 1A–1H  illustrate a first embodiment of the method of the present invention, wherein a positive photoresist is used to fabricate a line pattern dominant photomask. As shown in  FIG. 1A , the method of the first embodiment commences with a transparent substrate  100  covered on a first side  102  with a first layer  104  of opaque material, and a second layer  106  of positive photoresist material. The substrate  100  is typically composed from quartz or glass, and may have a thickness of about 0.25 inches. The first layer  104  of opaque or light blocking material may be composed of chrome, chrome oxide, iron oxide, nickel or like masking material, and may have a thickness of about 700 angstroms. The second layer  106  of positive photoresist may be a chemically amplified resist, and may have a thickness of 3000 to approximately 4000 angstroms. If the mask is a phase shift mask, there may be a shifter layer (not shown), such as molybdenum silicide (MoSi), between the transparent substrate  100  and the layer  104  of opaque material. 
       FIG. 11B  shows a main geometrical pattern area  110  to be defined in the positive photoresist layer  106 . The main pattern area  110  may be created in the photoresist layer  106  using an electron-beam (e-beam) writer. The e-beam writer generates a geometrically constrained stream of electrons that irradiate selected areas of the photoresist. One of ordinary skill in the art will recognize that any other suitable writer may be used for irradiating selected areas of the photoresist layer  106 . In the case of positive photoresist, the irradiated areas are made soluble in developer and the non-irradiated areas remain insoluble in developer. 
     In  FIG. 1C , the e-beam writer is used to irradiate a frame-like area  112  of the photoresist layer  106 , that follows the profile or contour of the main pattern area  110 . This frame-like area or contour area  112  is written rigorously using a small shot size, i.e., generally a “spot” of electrons with a specified shape and size, and multiple passes, e.g., four passes, to reduce stitching and heating effects and maximize the CD performance of the main pattern  110 . Such rigorous writing methods are well known in the art. The irradiation makes the contour area  112  of the positive photoresist layer  106  soluble in developer, whereas the non-irradiated main pattern area  110  and surrounding area  114  of the positive photoresist layer  106  remain insoluble in developer. 
     After writing the main pattern area  110  to the photoresist layer  106 , the substrate  100  may be conventionally processed by sequentially performing the steps of post exposure baking, developing, dry etching, and photoresist stripping. The post exposure baking process quenches or activates the positive photoresist layer  106 . 
     As shown in  FIG. 1D , the developing process removes the soluble contour area  112  of the photoresist layer  106 , thereby exposing an area  120  of the underlying opaque layer  104 . The dry etching process removes the exposed area  120  of the opaque layer  104  from the substrate  100 , thereby exposing an area  130  of the first side  102  of the substrate  100 . The photoresist stripping process removes areas  110  and  114  of the photoresist layer  106  from the substrate  100 . 
       FIG. 1E  shows the substrate  100  after post exposure baking, developing, dry etching, and photoresist stripping. As can be seen, the opaque layer  104  now defines a main pattern area  140  and a surrounding area  150  which will be removed further on after measuring the CD of the main pattern area  140 . The exposed area  130  of the first side  102  of the substrate  100  is visible between the main pattern area  140  and surrounding area  150  of the opaque layer  104 . 
     Referring to  FIG. 1F , the CD of the main pattern area  140  is measured using conventional techniques, such as CDSEM. The CD measurement is advantageously performed at this stage of the method because the surrounding area  150  of the opaque layer  104  enhances the conductivity and reduces the charging effect during CDSEM. 
     In  FIG. 1G , a second layer  160  of photoresist has been formed over the substrate  100  and patterned conventionally to cover the main pattern area  140  of the opaque layer  104 . The second layer  160  of photoresist may be a positive or negative photoresist and preferably a positive or negative optical photoresist, as it is less expensive to use because it is written with a less expensive optical exposure process (as compared with the e-beam writer process). The patterned second layer  160  of photoresist should slightly overlap the main pattern area  140  to ensure that it is protected. 
     The surrounding area  150  of the opaque layer  104  is then removed from the substrate  100  using a dry etching process, or preferably, a less expensive wet etching process. After etching, the second layer of photoresist  160  is removed from the substrate  100  using any conventional photoresist stripping technique. 
       FIG. 1H  shows the completed mask after removing the surrounding area  150  of the opaque layer  104  and the patterned second layer  160  of photoresist from the substrate  100 . As can be seen, only the main pattern area  140  of the opaque layer  104  remains on the first side  102  of the substrate  100 . 
       FIGS. 2A–2H  illustrate a second embodiment of the method of the present invention, wherein a negative photoresist is used to fabricate a line pattern dominant photomask. As shown in  FIG. 2A , the method of the second embodiment commences with a transparent substrate  200  covered on a first side  202  with a first layer  204  of opaque material, and a second layer  206  of negative photoresist material. As in the first embodiment, the substrate  200  may be composed from quartz or glass, and may have a thickness of about 0.25 inches, and the first layer  204  of opaque or light blocking material may be composed of chrome, chrome oxide, iron oxide, nickel or like masking material, and may have a thickness of about 700 angstroms. The second layer  206  of negative photoresist may be a chemically amplified resist, and may have a thickness of 3000 to approximately 4000 angstroms. As in the first embodiment, if the mask is a phase shift mask, there may be a shifter layer (not shown), such as MoSi, between the transparent substrate  200  and the layer  204  of opaque material. 
       FIG. 2B , shows a main geometrical pattern area  210  to be defined in the negative photoresist layer  206 . As in the first embodiment, the main pattern area  210  in the second embodiment may be created in the photoresist layer  206  using electron-beam (e-beam) lithography. In the case of negative photoresist, the irradiated areas are made insoluble in developer and the non-irradiated areas remain soluble in developer. 
     In  FIG. 2C , the e-beam writer is used to irradiate the main pattern area  210  of the photoresist layer  206  and a surrounding area  214  of the photoresist layer  206 . The frame-like area or contour region  212  which follows the contour of the main pattern area  210  is not irradiated. The main pattern area  210  is written rigorously using a small shot size and multiple passes, e.g., four passes, to reduce stitching and heating effects and maximize the CD performance of the main pattern. The surrounding area  214  is written less rigorously with one or two passes using a large shot size and as fast as possible. The irradiation makes the main pattern area  210  and surrounding area  214  of the negative photoresist layer  206  insoluble in developer whereas the non-irradiated contour area  212  of the negative photoresist layer  206  remains soluble in developer. 
     The remaining steps of the method of the second embodiment are the substantially the same as the first embodiment. The substrate  200  may be conventionally processed by sequentially performing the steps of post exposure baking, developing, dry etching, and photoresist stripping. The post exposure baking process quenches or activates the negative photoresist layer  206 . The developing process removes the soluble contour area  212  of the photoresist layer  206 , thereby exposing an area  220  of underlying opaque layer  204  as shown in  FIG. 2D . The dry etching process removes the exposed area  220  of the opaque layer  204  from the substrate  200 , thereby exposing an area  230  of the first side  202  of the substrate  200 , and the photoresist stripping process removes areas  210  and  214  of the photoresist layer  206  from the substrate  200  as shown in  FIG. 2E . 
     Referring still to  FIG. 2E , as in the first embodiment, the opaque layer  204  now defines a main pattern area  240 , and a surrounding area  250  which will be removed further on after measuring the CD of the main pattern area  240 . The exposed area  230  of the first side  202  of the substrate  200  is visible between the main pattern area  240  and surrounding area  250  of the opaque layer  204 . 
     Referring to  FIG. 2F , the CD of the main pattern area  240  is measured using conventional techniques. 
     In  FIG. 2G , a second layer  260  of positive or negative photoresist and preferably, a less expensive positive or negative optical photoresist, has been formed over the substrate  200  and patterned conventionally to cover the main pattern area  240  of the opaque layer  204 . The patterned second layer  260  of photoresist should slightly overlap the main pattern area  240  to ensure that it is protected. 
     The surrounding area  250  of opaque layer  204  is then removed from the substrate  200  using a wet etching or similar process. After etching, the patterned second layer of photoresist  260  is removed from the substrate  200  using any conventional photoresist stripping technique. 
       FIG. 2H  shows the completed mask with only the main pattern area  250  of the opaque layer  204  remaining on the first side  202  of the substrate  200 . 
     The method of the present invention improves CD uniformity through improvements in e-beam processing, baking, developing, and etching. E-beam processing is improved because proximity effects are minimize, as every pattern has almost the same duty ratio (for positive photoresist embodiments only). For embodiments using positive photoresist, global CD uniformity is improved because the vacuum effect is substantially resolved. Developing and etching are improved through a reduction in the loading effect, as the loading of the whole mask is almost the same. 
     The method of the present invention also makes measuring the CD of the photomask pattern easier because the opaque, e.g., chrome, layer coverage is greatly increased. 
     In addition, the method of the present invention reduces the probability of photomask related defects. This is due to the fact that the open area percentage (the exposed areas of the substrate) is very small before and during dry etching. Accordingly, the number of particles which fall onto the photomask is reduced. Moreover, the second etching step used in the present invention removes any particles which do find their way to “surrounding” area of the photomask during the mask making process. 
     While the foregoing invention has been described with reference to the above embodiments, various modifications and changes can be made without departing from the spirit of the invention. Accordingly, all such modifications and changes are considered to be within the scope of the appended claims.