Patent Publication Number: US-2021185793-A1

Title: Adhesive material removal from photomask in ultraviolet lithography application

Description:
BACKGROUND 
     Field 
     Embodiments of the present disclosure generally relate to methods and apparatus for an adhesive layer removal process from a photomask. Particularly, embodiments of the present disclosure provide methods and apparatus for an adhesive layer removal process after a pellicle removal process on a photomask using a dielectric barrier discharge plasma process. 
     Description of the Related Art 
     In the manufacture of integrated circuits (IC), or chips, patterns representing different layers of the chip are created by a chip designer. A series of reusable masks, or photomasks, are created from these patterns in order to transfer the design of each chip layer onto a semiconductor substrate during the manufacturing process. Mask pattern generation systems use precision lasers or electron beams to image the design of each layer of the chip onto a respective mask. The masks are then used much like photographic negatives to transfer the circuit patterns for each layer onto a semiconductor substrate. These layers are built up using a sequence of processes and translate into the tiny transistors and electrical circuits that comprise each completed chip. Thus, any defects in the mask may be transferred to the chip, potentially adversely affecting performance. Defects that are severe enough may render the mask completely useless. Typically, a set of 15 to 30 masks is used to construct a chip and can be used repeatedly. 
     The increasing circuit densities have placed additional demands on processes used to fabricate semiconductor devices. For example, as circuit densities increase, the widths of vias, contacts and other features, as well as the dielectric materials between them, decrease to sub-micron dimensions, whereas the thickness of the dielectric layers remains substantially constant, with the result that the aspect ratios for the features, i.e., their height divided by width, increases. Reliable formation of high aspect ratio features is important to the success of sub-micron technology and to the continued effort to increase circuit density and quality of individual substrates. 
     Photolithography is a technique used to form precise patterns and structures on the substrate surface and then the patterned substrate surface is etched to form the desired device or features. The photolithographic technique utilizes a photolithographic substrate, such as a reticle, which has corresponding configures of features desired to be transferred to a target substrate, such as a semiconductor wafer. A light source emitting ultraviolet (UV) light or deep ultraviolet (DUV) light is transmitted through the photomask substrate to expose photoresist disposed on the substrate. Generally, the exposed resist material is removed by a chemical process to expose the underlying substrate material. The exposed underlying substrate material is then etched to form the features in the substrate surface while the retained resist material remains as a protective coating for the unexposed underlying substrate material. 
     Typically, one photomask, e.g., a reticle, may be repeatedly used to reproducibly print thousands of substrates. Typically, a photomask, e.g., a reticle, is typically a glass or a quartz substrate giving a film stack having multiple layers, including a light-absorbing layer and an opaque layer disposed thereon. While performing the photolithography process, a pellicle is used to protect the reticle from particle contamination. Pellicle is a thin transparent membrane which allows lights and radiation to pass therethrough to the reticle. Pellicles provide a functional and economic solution to particulate contamination by mechanically separating particles from the mask surface. After the photomask has been used for a number of cycles and the pellicle has become damaged or too dirty to use, the photomask is removed and the pellicle replaced. 
     Pellicles are typically supported and held on the reticle by an adhesive material, such as glue. However, when replacing the pellicle and the attachment feature from the photomask, residual adhesive material is often difficult to be removed from the reticle. Aggressive mechanical cleaning often results in reticle damage, surface roughness, or film stack and/or structure damage of the photomask. 
     Therefore, there is a need for apparatus and methods for removing or cleaning adhesive material from the attachment feature on the reticle after periodic use. 
     SUMMARY 
     Embodiments of the present disclosure generally provide apparatus and methods for removing an attachment feature, particularly for adhesive materials in the attachment feature, from a photomask. In one embodiment, an apparatus for processing a photomask includes an enclosure, a substrate support assembly disposed in the enclosure, and a dielectric barrier discharge (DBD) plasma generator disposed above the substrate support assembly, wherein the dielectric barrier discharge plasma generator further comprises a first electrode, a second electrode, wherein the first and the second electrodes are vertically aligned and in parallel, a dielectric barrier positioned between the first electrode and the second electrode, and a discharge space defined between the dielectric barrier and the second electrode. 
     In another embodiment, a method for processing a photomask includes removing an adhesive material from a photomask by a plasma generated from a dielectric barrier discharge plasma generator. 
     In yet another embodiment, a method for processing a photomask includes applying a power in a dielectric barrier discharge plasma generator disposed in an enclosure, directing a discharge gas in a discharge space defined in the dielectric barrier plasma generator to a surface of a photomask disposed in a substrate support assembly in the enclosure, generating a plasma in the discharge space toward the surface of the photomask, and removing an adhesive material on the photomask. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the above recited features of embodiments of the present disclosure can be understood in detail, a more particular description of the disclosure, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this disclosure and are therefore not to be considered limiting of its scope, for the disclosure may admit to other equally effective embodiments. 
         FIG. 1  schematically illustrates a lithography system in accordance with one embodiment of the present disclosure. 
         FIG. 2  schematically illustrates a cross sectional view of a photomask that may be used in the lithography system of  FIG. 1 . 
         FIG. 3A  schematically illustrates a cross sectional view of the photomask of  FIG. 2  after pellicle is removed from the photomask. 
         FIG. 3B  illustrates a top view of the photomask of  FIG. 2  after pellicle is removed from the photomask. 
         FIG. 4  depicts a flow diagram of an adhesive material removal process for removing an adhesive material from a reticle. 
         FIG. 5  illustrates cross sectional views of the photomask during different stages of the removal process of  FIG. 4 . 
         FIG. 6  depicts a cross sectional view of a photomask after an adhesive material is removed from the photomask. 
     
    
    
     To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements disclosed in one embodiment may be beneficially utilized on other embodiments without specific recitation. 
     DETAILED DESCRIPTION 
     Embodiments of the present disclosure generally provide apparatus and methods for removing an adhesive material in an attachment feature utilized to hold a pellicle from a photomask. The attachment feature is utilized to hold and/or support a pellicle to the photomask. The attachment feature includes an adhesive material attached between the pellicle and the photomask. An adhesive material removal apparatus is utilized to remove the adhesive material from the photomask. In one example, an adhesive material removal apparatus includes a dielectric barrier discharge (DBD) plasma generator that may generate plasma to react with the adhesive material, thus enabling removal of the adhesive material from the photomask. In one example, the dielectric barrier discharge (DBD) plasma may be performed in suitable pressure range, including under an atmospheric pressure (AP). 
       FIG. 1  depicts a photolithographic system  100 . The photolithographic system  100  includes a light source  112  providing an initial patterning radiation  114  through a back of a photomask (e.g., reticle)  202 . The initial patterning radiation  114  further passes through a projection lens  104 , providing a final patterning radiation  106  to a surface of a substrate  102 , such as a semiconductor substrate. The substrate  102  may have a photoresist layer (not shown) to assist exposing into a photoresist layer. The photomask  202  includes a pellicle  214  supported by an attachment fixture  216 . A pellicle  214  may be used to protect the surface of the photomask  202  from particle contamination or other sources of contamination while processing. The pellicle  214  may be supported by the attachment fixture  216  at a predetermined location  217 , such as a periphery region, of the photomask  202 . The pellicle  214  and the attachment fixture  216  may be removable and replaceable from the photomask  202 . The attachment fixture  216  may have an adhesive material to assist attach the attachment fixture  216  to the photomask  202 . Details of the attachment fixture  216  and the film stack formed on the photomask  202  will be further described in  FIG. 2 . The adhesives layer from the attachment fixture  216  along with the pellicle  214  may be typically fabricated from polymers or plastic materials with additives and/or solvents. As the pellicle  214  and adhesives material are exposed to radiation or light from the light source  112 , the material of the pellicle  214 , adhesive and solvents may outgas or evaporate, producing one or more types of residual organic compounds. The outgassed organic compounds may further reduce pellicle transparency, cause pellicle thinning and accelerate pellicle photo-degradation. 
     Furthermore, after a number of process has been performed and the pellicle  214  and the attachment fixture  216  is removed from the photomask  202 , some residual adhesive compounds may remain on the periphery region  217  of the photomask  202  where the attachment fixture  216  was supported, which often requires additional cleaning or adhesive removal process to remove the adhesive materials or compounds from the photomask  202 . 
       FIG. 2  depicts details of a film stack  204  disposed on the photomask  202 , such as a reticle. The photomask  202  includes a film stack  204  disposed on the photomask  202  having desired features formed therein. In the exemplary embodiment depicted in  FIG. 2 , the photomask  202  may be a quartz substrate (i.e., low thermal expansion silicon dioxide (SiO 2 )). The photomask  202  has a rectangular shape having sides between about 5 inches to about 9 inches in length. The photomask  202  may be between about 0.15 inches and about 0.25 inches thick. In one embodiment, the photomask  202  is about 0.25 inches thick. 
     The film stack  204  includes features  207  formed therein. The film stack  204  is formed in a center region  205  and a periphery region  217 . It is noted that the features  207  and the film stack  204  depicted in  FIGS. 2, 3A-3B and 5-6  are only for illustration purpose so that the features  207  and the film stack  204  may be in any form as needed. The film stack  204  includes an absorber layer  208  disposed on a phase shift layer  203 . The absorber layer  208  may be a metal containing layer, e.g., a chromium containing layer, such as a Cr metal, chromium oxide (CrO x ), chromium nitride (CrN) layer, chromium oxynitride (CrON), or multilayer with these materials, as needed. The phase shift mask layer  203  may be a molybdenum containing layer, such as Mo layer, MoSi layer, MoSiN, MoSiON, and the like. It is noted that the absorber layer  208  is predominately remained in the predetermined location, such as the periphery region  217 , of the photomask  202  so as to allow the attachment fixture  216  to be disposed thereon. The film stack  204  in the center region  205  of the photomask  202  predominately includes the phase shift mask layer  203 . 
     In the periphery region  217  of the photomask  202 , the attachment fixture  216  is formed thereon to support the pellicle  214 , as shown in  FIG. 2 . The attachment fixture  216  includes a pellicle frame  212  utilized to hold the pellicle  214  and an adhesive material  210 , such as a pellicle glue ring, utilized to assist attaching the pellicle frame  212  to the photomask  202 . The pellicle frame  212  may be made of any suitable material, such as metal containing materials, conductive materials, plastic materials, dielectric materials, or other materials suitable to hold the pellicle  214 . In one example, the pellicle frame  212  is a conductive material selected from a group consisting of titanium, aluminum, stainless steel, combinations thereof and alloys thereof. The adhesive material  210  may be any suitable glue layer, such as acrylic glue. The interface between the pellicle frame  212  and the pellicle  214  may include chemical adherence mechanical clamping mechanism to assist attaching the pellicle  214  securely on the pellicle frame  212  as needed. 
     Prior to the adhesive material removal process depicted in  FIG. 4 , the pellicle  214  and the pellicle frame  212  may be removed from the attachment fixture  216 , as shown in  FIG. 3A , by any suitable manner or mechanism.  FIG. 3B  depicts a top view of the photomask  202 .  FIG. 3A  depicts the cross sectional view along the cutting line A-A′ shown in  FIG. 3B . The attachment fixtures  216  are located at the periphery region  217  of the photomask  202 . As the pellicle frame  212  has been removed from the photomask  202 , the example depicted in  FIG. 3B  merely includes the adhesive material  210  remained on the absorber layer  208  disposed in the periphery region  217  of the photomask  202 . 
       FIG. 4  depicts an adhesive material removal process  400  that may be utilized to remove the adhesive material  210  from the photomask  202 .  FIG. 5  depicts cross sectional views of the photomask  202  in an adhesive material removal apparatus  550 . 
     The adhesive material removal process  400  starts at operation  402  by providing the photomask  202  in an adhesive material removal apparatus, such as the adhesive material removal apparatus  550  depicted in  FIG. 5 . The adhesive material removal apparatus  550  may provide an enclosure  551  that has a dielectric barrier discharge (DBD) plasma generator  503 . The adhesive material removal apparatus  550  is configured to remove the adhesive material  210  from the photomask  202 . The configuration (e.g., profile, shape, and/or contour) of the dielectric barrier discharge (DBD) plasma generator  503  may be in any profile or shape as needed to efficiently remove the adhesive material  210  from the photomask  202 . In the embodiment depicted herein, the dielectric barrier discharge (DBD) plasma generator  503  may have electrodes formed in rectangular shape/configuration to efficiently remove the adhesive material  210  (e.g., located at periphery region  217  of the photomask  202  in a rectangular arrangement as shown in  FIG. 3B ) disposed on the photomask  202 . 
     At operation  404 , a power is applied to the dielectric barrier discharge (DBD) plasma generator  503  disposed in the adhesive material removal apparatus  550  to generate a plasma. In one embodiment, the dielectric barrier discharge (DBD) plasma generator  503  includes a pair of electrodes  504 , such as a first electrode  504   a  and a second electrode  504   b  disposed in parallel and vertically aligned and a dielectric barrier  506  disposed against the first electrode  504   a . The first electrode  504   a  may be grounded. The electrodes  504  are attached to but insulated from the enclosure  551  (insulation not shown in the Figures). The dielectric barrier  506  is disposed between the first electrode  504   a  and the second electrode  504   b  defining an opening  508  (e.g., a discharge space) between the first and the second electrodes  504   a ,  504   b . The dielectric barrier  506  also maintains the first electrode  504   a  and the second electrode  504   b  in a spaced-apart relation. Though the example depicted in  FIG. 5  shows the dielectric barrier  506  is disposed against the first electrode  504   a , it is noted that the position of the dielectric barrier  506  may also be adjusted or changed to other positions, such as against the second electrode  504   b , as needed. 
     In one example, the first and the second electrodes  504   a ,  504   b  are an electrical conductive material that may generate electronic field when applying a power thereto. Suitable materials of the first and the second electrodes  504   a ,  504   b  include, but not limited to, aluminum, stainless steel, tungsten, copper, molybdenum, nickel, and other metal material. 
     Furthermore, in one embodiment, the first electrode  504   a  may be a conductive material as described above and coated with a dielectric layer to form the dielectric barrier  506 . Suitable materials of the dielectric layer include, but not limited to MgO, SiO 2 , Y 2 O 3 , La 2 O 3 , CeO 2 , SrO, CaO, MgF 2 , LiF 2 , and CaF 2 , among others. The conductive material could be indium tin oxide (ITO), SnO 2 , W, Mo, Cu, aluminum, alloys thereof, or another metal. 
     The dielectric barrier  506  acts as a current limiter during plasma generation process so as to assist generating plasma in a discharge gas supplied into the opening  508 . In one embodiment, the dielectric barrier  506  is a transparent dielectric material such as glass, quartz, ceramics, polymer materials or other suitable materials. 
     The opening  508  defined between the first and the second electrodes  504   a ,  504   b  is a discharge space that allows the discharge gas to be supplied thereto. A gas outlet  510  is coupled to a gas source  530  configured to supply the discharge gas into the opening  508 . The gas outlet  510  is disposed at a predetermined angle so as to inject the discharge gas predominately in the opening  508  defined between the first and the second electrodes  504   a ,  504   b . As a result, the center region  205  where the phase shift mask layer  203  is disposed on the photomask  202  would not be affected, reacted, or damaged by the discharge gas supplied into the adhesive material removal apparatus  550  during the adhesive material removal process. The gas outlet  510  is configured to continuously supply gas into the opening  508  so as to allow the plasma generated in the opening  508  to align with a location where the adhesive material  210  is formed on the photomask  202 . Similarly, the configuration of the electrodes  504  is also selected so as to confine the first and the second electrodes  504   a ,  504   b  in a manner that allows the plasma as generated to be flown in a direction toward the adhesive material  210  on the photomask, rather than the center region  205  of the film stack  204  on the photomask  202 , so as to dominantly react with the adhesive material  210  on the photomask without damaging other areas of the photomask  202 , including the absorber layer  208  disposed underneath the adhesive material  210 . 
     In one example, the opening  508  (e.g., the discharge space) has a selected discharging distance  560  (e.g., a width) creating a discharge volume to allow sufficient collisions among the electrons and the discharge gas executed in the opening  508 . The discharge volume is configured to sufficiently promote the collisions of the electrons and the discharge gas so that excited species, including excimers, may be created, therefore, generating the plasma as desired. In one embodiment, the discharging distance  560  of the opening  508  (e.g., the discharge space) is selected within an adequate range to promote the collisions in the opening  508 . In another embodiment, the discharging distance  560  of the opening  508  is selected between about 5 mm and about 50 mm, such as between about 10 mm and about 20 mm, for example, between about 2 mm and about 30 mm. 
     The collision of electrons with the discharge gas provides energy to the discharge gas creating reactive species including discharge plasma species and excimers. Such discharge plasma species and excimers reach to the adhesive material  210  disposed on the photomask  202 , activating the adhesive material  210  so as to soften and react with the adhesive material  210 , which may be removed from the photomask  202  in volatile state, or by further mechanical cleaning/scrubbing after the adhesive material removal process. 
     In one embodiment, the discharge gas may be oxygen gas (O 2 ), a hydrogen gas (H 2 ), or a nitrogen gas (N 2 ). In another embodiment, the discharge gas may be a gas mixture selected from a group including noble gases, such as xenon gas (Xe), krypton gas (Kr), argon gas (Ar), neon gas (Ne), helium gas (He) and the like. In yet another embodiment, the discharge gas may be a gas mixture including at least one of oxygen gas (O 2 ), a hydrogen gas (H 2 ), a nitrogen gas (N 2 ), a noble gas, a halogen containing gas, such as fluorine, bromine and chlorine gas, H 2 O, NH 3 , the combinations thereof, or the like. 
     A circuit arrangement  534  applies an operating voltage from a power supply  532  to the first electrode  504   a  and the second electrode  504   b . In operation, the voltage applied to the two electrodes  504   a ,  504   b  establishes an electric field that promotes the electrons being collided in the opening  508 . The electron collision generates energy to the discharge gas in the opening  508 , thus energizing the discharge gas into an excited state, forming a plasma, which often includes reactive species, discharge species, or excimers. The plasma promotes reaction between the reactive species from the plasma selective to the adhesive material  210  and relatively inert to the underlying absorber layer  208 , thus efficiently removing the adhesive material  210  from on the surface of the photomask  202  without damaging the underlying absorber layer  208 . In one example, the voltage applied by the circuit arrangement  534  from the power supply  532  is selected so that an electric field may be established that is sufficient to generate a plasma as described above. In one embodiment, the voltage may be applied between about 100 Volts or about 20,000 Volts. 
     An atmosphere control system  564  is coupled to the enclosure  551 . The atmosphere control system  564  includes throttle valves and pumps for controlling chamber pressure. The atmosphere control system  564  may additionally include gas sources for providing process or other gases to the interior volume of the adhesive material removal apparatus  550 . In one embodiment, the atmosphere control system  564  may assist controlling the pressure at a desired range during the adhesive material removal process. In one example, the pressure during the adhesive material removal process may be controlled at atmospheric pressure, such as at ambient pressure wherein the photolithographic system  100  is located. 
     During the operation  404 , a frequency of power supply between about 100 KHz and about 3 GHz may be applied to the dielectric barrier discharge (DBD) plasma generator  503  to generate a plasma in the opening  508  toward the adhesive material  210  for reaction. 
     At operation  406 , after the plasma is generated and flown toward the adhesive material  210 , the adhesive material  210  may be chemically reacted with the plasma, forming residuals in volatile state, pumping out of the adhesive material removal apparatus  550 . Furthermore, in some embodiments, a fluid wash process (e.g., suitable liquid precursors or gas precursors) to remove undesired precipitates, side product or residual adhesive materials, if any, from the photomask  202 . During the fluid wash process, an ultrasonic or megasonic energy may be applied during the process to assist dislodging the precipitates, side product or residual adhesive materials, if any, from the photomask  202 . 
     After adhesive material removal process, the adhesive material  210  is removed from the photomask  202 , as shown in  FIG. 6 . Although only a lithography process is described in accordance with the present disclosure, embodiments of the present disclosure may be applied to any suitable process and in any suitable processing tools that requires removal an adhesive material of an attachment feature from an object. 
     Thus, embodiments of the present disclosure generally provide apparatus and methods for removing an adhesive material of an attachment feature from a photomask. The methods and apparatus advantageously removing the adhesive material from the photomask by a dielectric barrier discharge (DBD) plasma under a desired pressure range control. Accordingly, the method and the apparatus provided herein advantageously facilitate fabrication of photomasks which is suitable for utilization in lithography applications. 
     While the foregoing is directed to embodiments of the present disclosure, other and further embodiments of the disclosure may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.