Abstract:
A method is disclosed for repairing an attenuated phase shift mask. The mask initially has a mask substrate coated with a predetermined shift layer material, a mask pattern layer, and an energy beam resist layer sequentially. After forming a predetermined mask pattern in the mask pattern layer through an energy beam resist layer, the mask is inspected for detecting at least one missing pattern in the mask pattern layer. The predetermined mask pattern is repaired in a predetermined defect area for correcting the missing pattern. After the missing pattern is reformed. The predetermined mask pattern is transferred in the shift layer material.

Description:
BACKGROUND  
       [0001]     The present invention relates generally to photolithography masks, and more particularly to attenuated phase shift masks. Still more particularly, the present invention relates to methods for repairing defects in an attenuated phase shift mask.  
         [0002]     In a manufacturing process using a lithographic projection apparatus, a mask pattern is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as, priming, resist coating, and soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake, and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc.  
         [0003]     The basic lithography system consists of a light source, a stencil or photomask containing the pattern to be transferred to the wafer, a collection of lenses, and a means for aligning existing patterns on the wafer with patterns on the mask. Conventional photomasks consists of chromium (Cr) patterns on a quartz plate, allowing light to pass wherever the chromium has been removed from the mask. Light of a specific wavelength is projected through the mask onto the photoresist coated wafer, exposing the resist wherever hole patterns are placed on the mask. Exposing the resist to light of the appropriate wavelength causes modifications in the molecular structure of the resist polymers which, in common applications, allow a developer to dissolve and remove the resist in the exposed areas. Such resist materials are known as positive resists. (Negative resist systems allow only unexposed resist to be developed away.) The photomasks, when illuminated, can be pictured as an array of individual, infinitely small light sources which can be either turned on (points in clear areas) or turned off (points covered by chrome). If the amplitude of the electric vector which describes the light radiated by these individual light sources is mapped across a cross section of the mask, a step function will be plotted reflecting the two possible states in which each point on the mask can be found (light on, light off).  
         [0004]     The quality with which small images can be replicated in lithography depends largely on the available process window. That is, the amount of allowable dose and focus variation that still results in correct image size. Phase shifted mask (PSM) lithography improves the lithographic process window or allows operation at a lower k value by introducing an additional parameter on the mask, i.e., an electric vector. The electric vector, like any vector quantity, has a magnitude and direction, so, in addition to turning the electric field amplitude on and off, it can be turned on with a phase of about 0 degree or turned on with a phase of about 180 degree. This phase variation is achieved in PSMs by modifying the length that a light beam travels through the mask material. By recessing the mask to an appropriate depth, light traversing the thinner portion of the mask and light traversing the thicker portion of the masks will be 180° out of phase, that is, their electric field vector will be of equal magnitude but point in exactly the opposite direction so that any interaction between these light beams result in perfect cancellation.  
         [0005]     In recent years, the phase shift mask (PSM) has been gradually accepted by the industry as a viable alternative for sub-exposure-wavelength manufacturing. Two fundamental forms of PSM have been used the most, namely alternating PSM (altPSM) and attenuated PSM (attPSM). In the attenuated phase shift mask, the surface is mainly divided into two regions, which are the wholly transparent region in 0 degree phase and the attenuated transparent region in 180 degree phase. The wholly transparent region is mainly constructed of quartz, and the attenuated transparent region has an extra molybdenum silicide (MoSi), or similar material, layer. The transparency of the wholly transparent region is close to 100%. The transparency of the attenuated transparent region is much less than that of the wholly transparent region, and is typically less than 10%, possibly about 4 to 6%. The light that arrives at the photoresist on a wafer through the wholly transparent region of the photomask is 180 degrees out of phase with light that arrives at the photoresist on the wafer through the attenuated transparent region of the photomask. Where the two pattern regions appear adjacently, the phase difference produces destructive interference. The contrast between the wholly transparent region and the attenuated transparent region can be more pronounced, so that the resolution of the exposure process can be improved.  
         [0006]     Several types of defects are possible in the production process for attPSM. There may have been defects in the original Cr coating, defects in the original resist coating, particles on the resist coating, Cr peeling, or electron-beam (or laser beam) writing errors. All these possible problems may show up in the delineated Cr pattern and therefore, in the delineated mask pattern. Accordingly, there remains a need for a repair process that can restore the designed pattern.  
       SUMMARY  
       [0007]     In view of the foregoing, the present invention provides a method that can restore the designed pattern on a photomask such as an attenuated phase shift photomask.  
         [0008]     The mask initially has a mask substrate coated with a predetermined shift layer material, a mask pattern layer, and an energy beam resist layer sequentially. In one example, after forming a predetermined mask pattern in the mask pattern layer through an energy beam resist layer, the mask is inspected for detecting at least one missing pattern in the mask pattern layer. The predetermined mask pattern is repaired in a predetermined defect area for correcting the missing pattern. After the missing pattern is reformed. The predetermined mask pattern is transferred in the shift layer material.  
         [0009]     These and other aspects and advantages will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the disclosure. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIGS. 1A-1F  illustrate a normal attPSM production process up through inspection.  
         [0011]      FIGS. 2A-2C  illustrate a process for repair in accordance with one example of the present invention.  
         [0012]      FIGS. 3A-3C  illustrate a completion of the normal attPSM production process after inspection and repair in accordance with one example of the present invention.  
         [0013]      FIG. 4  illustrates a flow chart for attPSM production in accordance with one example of the present invention. 
     
    
     DESCRIPTION  
       [0014]     The present invention provides an improved method for repairing an attenuated phase shift mask (attPSM). Such a mask is utilized as a photomask in the production of semiconductor chip devices. In one example, production of an attPSM begins with a quartz plate coated first with a shift layer material such as molybdenum silicide (MoSi), second with a layer of chromium (Cr), and third with a layer of electron or laser beam resist. An electron beam or laser beam pattern-writing machine generates a primary pattern. The electron or laser beam, or any other similar beams, can be referred to as an energy beam for the purpose of this disclosure. The developed pattern defines the Cr etch, which defines the MoSi etch. There is a need for an inspection and pattern repair sequence to assure a correct pattern for MoSi etch.  
         [0015]     In  FIGS. 1A-1E , a process flow  100  illustrates normal attPSM production through inspection.  FIG. 1A  illustrates a typical process flow for the production of attPSM beginning with a blank quartz plate  102 , that is first coated with a shift layer material such as molybdenum silicide (MoSi)  104 , second coated with a mask pattern layer of chromium (Cr)  106 , and third coated with a layer of electron or laser beam resist  108 . This is the beginning of a process sequence that will produce a photomask that will be used to expose, with light, a design layer of an integrated circuit pattern into a photoresist coating on a semiconductor wafer. That photoresist pattern will contain many pattern elements that are sub-micron in size, in fact, smaller than the wavelength of the light used to expose the pattern in the photoresist. The generation of the primary pattern on the photomask is by a pattern-writing machine such as an electron-beam or laser-beam pattern-writing machine, which can generate, or write, pattern elements that are considerably smaller and more precisely defined than can be produced with a light exposure machine.  
         [0016]     The second step in the production of an attPSM, as illustrated in  FIG. 1B , is the exposure, or pattern writing, of the resist  108  by an energy beam such as an electron beam or laser beam. The resist  108  may be either positive-acting or negative-acting. The third step, as illustrated in  FIG. 1C , is the development of the written pattern  110  in the resist  108 . The fourth step, as illustrated in  FIG. 1D , is the etching of the Cr layer  106 , down to the shift material layer  104 , using the developed resist pattern  110  as an etch resistant layer. The fifth step, as illustrated in  FIG. 1E , is the stripping step for the resist. Now the resist pattern  110  is delineated in the Cr layer pattern  112 .  
         [0017]     At this point, several types of defects may become apparent. There may have been defects in the original Cr coating, defects in the original resist coating, particles on the resist coating, Cr peeling, or beam writing errors. All these possible problems may show up in the delineated Cr pattern.  
         [0018]     Now the invention makes recovery from such problems possible. Recovery, as illustrated in  FIG. 1F , begins with an inspection by an automatic machine, which compares the delineated Cr pattern  112  in the Cr layer  106 , containing possible defects  114 , with the original computer program from which it was generated or with a previously generated photomask pattern  110  shown in  FIG. 1C . This inspection requires a decision. If the decision is made that the attPSM is correctly patterned, then it is moved to MoSi etch, as illustrated in  FIG. 3A . If the decision is made that the attPSM is not patterned correctly, then it is moved to have the resist recoated, as illustrated in  FIG. 2A . The locations of any defects  114  are recorded. The defect location data is sent to the pattern-writing machine.  
         [0019]     In  FIGS. 2A-2C , a process flow  200  illustrates a repair process of the invention.  FIG. 2A  illustrates the photomask  102 , with the Cr  106  defects  114 , sent to be coated with a new layer of resist  202 , which is a repair resist formed in a predetermined area for correcting the defect. This time, the resist  202  is typically negative-acting, for example. In  FIG. 2B , the photomask  102 , recoated, with negative resist  202 , is sent to the pattern-writing machine, where only the specific areas containing defects are again exposed by the electron beam or laser beam. Pattern writing with an electron beam or laser beam is time consuming, but rewriting only the pattern areas that were previously defective only requires a small amount of time, since the area involved is a miniscule portion of the total area. This is the reason that negative-acting resist is preferable. In  FIG. 2B , the electron beam or laser beam exposes or rewrites only those areas so as to repeat the pattern  110 , as shown in  FIG. 1C , that was intended to be produced by the first process sequence. The pattern-writing parameters are slightly different from those of the pattern-writing in the normal process flow because there is not a complete continuous layer of Cr  106  on the MoSi  104 , so the conductive layers Cr  106  and MoSi  104  on the photomask  102  are slightly more resistive and they conduct the beam current away less efficiently. The parameters of the energy beam may be adjusted slightly.  FIG. 2C  illustrates a develop step, only in the locations that contained defects  114  the first time. Those locations now contain resist patterns  204  to redefine resist protection areas. At this point, the invention has reestablished a complete mask pattern, mostly defined in Cr pattern  112  in Cr layer  106 , and partly defined in the new resist pattern  204 . Now, the photomask  102  is returned to the normal process flow.  
         [0020]     The next step in the normal process flow is MoSi  104  etch as illustrated in  FIG. 3A . Most of the pattern is protected from this etch by the Cr pattern  112 . The patterns in the previously defective areas  114  are protected from this etch by the new negative-acting resist patterns  204 . In the normal process flow, there is no resist on the photomask  102  at this point. The miniscule total area of resist  204  in the repaired locations is tolerable. After the MoSi  104  etch, the resist pattern  204  along with the possibly defective Cr pattern  112 , fully repairs and reestablishes the intended pattern  110  of the original resist layer  108 . That originally intended pattern  110  is successfully delineated in the MoSi layer pattern  302 . Then, as illustrated in  FIG. 3B , the resist pattern  204  is stripped. The possibly defective Cr layer pattern  112  and the corrected MoSi layer pattern  302  remain. Then, as illustrated in  FIG. 3C , the Cr is stripped, leaving only the intended MoSi pattern  302 .  
         [0021]     The excursion from the normal process flow (represented by  FIGS. 2A-2C ) for the purpose of pattern inspection and repair may requires a minimum of extra process steps, and a minimum of extra time in the pattern-writing machine, after the necessary inspection step.  
         [0022]     A flow chart  400  illustrates an attPSM production process flow. The start of a quartz plate with coatings of MoSi, Cr, and resist is done in step  402 . In step  404 , a pattern-writing machine generates a primary pattern in resist. A development of the resist layer is carried out in step  406 . And in step  408 , an etching of the Cr layer is completed using an resist pattern as a mask. After a stripping of an resist layer (step  410 ), an inspection  412  is conducted. If the automated inspection shows a pattern defined in Cr to be defect-free, then an attPSM can be moved to an etching of MoSi step  420 . However, if the inspection shows that pattern defects are present in a pattern defined in Cr, then the attPSM enters a repair sequence. Step  414  illustrates a recoating of resist in a localized area to begin a repair sequence. Step  416  illustrates a rewriting of a repair pattern, which is a part of the originally desired primary pattern, in areas containing identified defects. Step  418  illustrates a development of the rewritten resist. Now the repair sequence ends with the return of a repaired attPSM to the normal production process flow to MoSi etching step  420 . This is the same process that is entered by an attPSM that inspection finds to be defect-free. After stripping of the resist after etch (step  422 ), step  424  performs an etching of Cr to leave a final desired pattern delineated in the MoSi layer. At this point, the production flow  400  is finished (step  426 ), and a repaired attPSM is produced that is composed of a quartz plate with an appropriate shift material layer that is free of a large class of production defects.  
         [0023]     As illustrated above, the inspection is conducted before the shift layer material (e.g. MoSi) is patterned so that any defects can be repaired before hand. If no inspection and repair is done at that point, the MoSi peeling defects can be severe and hard to repair later. Even it can be later repaired, the repair work impacts the quality of the finished mask. As using the methods provided above, the PSM production yield can be increased and the quality is enhanced.  
         [0024]     The above disclosure describes many different embodiments, or examples, for implementing different features of the disclosure. Specific examples of components, and processes are described to help clarify the disclosure. These are, of course, merely examples and are not intended to limit the invention from that described in the claims.  
         [0025]     Although illustrative embodiments of the disclosure have been shown and described, other modifications, changes, and substitutions are intended in the foregoing disclosure. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention, as set forth in the following claims.