Patent Publication Number: US-9897910-B2

Title: Treating a capping layer of a mask

Description:
PRIORITY INFORMATION 
     This application is a continuation of U.S. application Ser. No. 14/582,459 filed Dec. 24, 2014 and entitled “Treating a Capping Layer of a Mask,” the disclosure of which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     When fabricating integrated circuits, photolithography is often used to form various features such as metal lines into a semiconductor substrate. To form these features, photo-masks are used to form a pattern into a photo-resist layer. The regions where the photo-resist layer is removed expose the underlying substrate to an etching process used to form trenches where metal is subsequently placed. 
     One type of photolithography is Extreme Ultraviolet (EUV) lithography. In one example of an EUV mask, a patterned absorption layer is formed on a reflective multilayer. To expose a photoresist layer on a substrate, EUV light is projected onto the mask through a number of mirrors. The exposed portions of reflective layer then reflect light onto the substrate on which an integrated circuit is to be formed. The light thus exposes a photoresist layer deposited on that substrate. 
     An EUV mask typically includes a capping layer between the reflective layer and the absorption layer. The capping layer protects the reflective layer from various particles that accumulate on the mask during field operations. The capping layer, however, is subject to damage as well. For example, the EUV mask is generally cleaned after a certain number of uses. This cleaning process can cause damage to the capping layer over time. Additionally, the capping layer can be subject to oxidation, which also damages the capping layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Aspects of the present disclosure are best understood from the following detailed description when read with the accompanying figures. It is noted that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion. 
         FIGS. 1A-1D  are diagrams showing an illustrative process for treating a capping layer of a mask, according to one example of principles described herein. 
         FIGS. 2A-2B  are diagrams showing diffusion of a material into a capping layer, according to one example of principles described herein. 
         FIG. 3A  is a diagram showing an additional capping layer on a patterning layer, according to one example of principles described herein. 
         FIG. 3B  is a diagram showing a capping layer formed after the patterning layer, according to one example of principles described herein. 
         FIG. 4  is a flowchart showing an illustrative method for treating a capping layer of a mask, according to one example of principles described herein. 
     
    
    
     DETAILED DESCRIPTION 
     The following disclosure provides many different embodiments, or examples, for implementing different features of the provided subject matter. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed. 
     Further, spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper” and the like, may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The apparatus may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly. 
     As described above, the capping layer of a photolithography mask such as an EUV mask is subject to damage. According to principles described herein, the capping layer undergoes a treatment process to strengthen the capping layer so that it resists oxidation and is less prone to damage. Particularly, a secondary material is introduced into the capping layer to strengthen the capping layer. In one example, an implantation process is applied to the capping layer. The implantation process causes diffusion of a secondary material having a smaller atomic number than the material that forms the capping layer. In one example, the capping layer is made of ruthenium (Ru), which has an atomic number of 44. The ruthenium capping layer can be treated such that nitrogen is diffused into the capping layer. Nitrogen has an atomic number of 7. In some examples, any secondary material having an atomic number that is less than 15 is implanted into the capping layer. By doing so, the capping layer is stressed such that it is less prone to oxidation and other forms of damage. 
       FIGS. 1A-1B  are diagrams showing an illustrative process for fabricating a mask with a treated capping layer. According to the present example, the mask  100 , at a particular point during fabrication of the mask  100 , includes a substrate  101 , a reflective multilayer  102 , a capping layer  104 , a patterning layer  106 , and a photoresist layer  108 . 
     In one example, the mask  100  is an EUV mask. EUV lithography utilizes a reflective mask rather than a transmissive mask. EUV lithography utilizes scanners that emit light in the extreme ultraviolet (EUV) region, which is light having a wavelength of about 1-100 nm. Some EUV scanners provide 4× reduction projection printing, similar to some optical scanners, except that the EUV scanners use reflective rather than refractive optics, i.e., mirrors instead of lenses. EUV scanners provide the desired pattern on an absorption layer (“EUV” mask absorber) formed on a reflective mask. EUV lithography is similar to optical lithography in that it needs a mask to print wafers, except that it employs light in the EUV region, i.e., at 13.5 nm. At the wavelength of 13.5 nm or so, all materials are highly absorbing. Thus, reflective optics rather than refractive optics are used. 
     The substrate  101  is made of a suitable material, such as a Low Thermal Expansion Material (LTEM) or fused quartz. In various examples, the LTEM includes TiO 2  doped SiO 2 , or other suitable materials with low thermal expansion. In some examples, a conductive layer is additionally disposed under on the backside of the LTEM substrate  101  for the electrostatic chucking purpose. In one example, the conductive layer includes chromium nitride (CrN), though other suitable compositions are possible. 
     The mask  100  also includes a reflective multilayer  102  deposited on the substrate  101 . The reflective multilayer  102  includes a plurality of film pairs, such as molybdenum-silicon (Mo/Si) film pairs (e.g., a layer of molybdenum above or below a layer of silicon in each film pair). Alternatively, the reflective multilayer  102  may include molybdenum-beryllium (Mo/Be) film pairs, or other suitable materials that are configurable to highly reflect the EUV light. The characteristics of the reflective multilayer  102  are selected such that it provides a high reflectivity to specific electromagnetic radiation type/wavelength. Specifically, for the purpose of EUV lithography, the reflective multilayer  102  is designed to reflect light within the EUV range. The thickness of each layer of the reflective multilayer  102  depends on the EUV wavelength and the incident angle. Particularly, the thickness of the reflective multilayer  102  (and the thicknesses of the film pairs) is adjusted to achieve a maximum constructive interference of the EUV light diffracted at each interface and a minimum absorption of the EUV light. 
     The mask  100  further includes a capping layer  104  on the reflective multilayer  102 . The capping layer  104  is designed to be transparent to EUV light and to protect the reflective multilayer  102  from damage and/or oxidation. In one example, the capping layer is made of ruthenium. The ruthenium capping layer can be formed as a crystal structure. The thickness of the capping layer may be within a range of about 2 to 7 nanometers. 
     The mask  100  also includes a patterning layer  106 . In  FIG. 1A , the patterning layer  106  has not yet been patterned. The patterning layer  106  is used to form the desired exposure pattern  106  onto the mask. The patterning layer  106  may serve this purpose in a variety of ways. In one embodiment, the patterning layer  106  is an absorption material. In another embodiment, the patterning layer  106  is a phase shifting material, which is similar to the reflective multilayer  102  material. 
     In the case where the patterning layer  106  is an absorption material, the EUV mask  100  can be referred to as a Binary Intensity Mask (BIM). With a BIM, the remaining portions after the patterning layer  106  has been patterned are light absorbing, or opaque, regions. In the opaque regions, an absorber is present, and an incident light is almost fully absorbed by the absorber. In the reflective regions, the absorber is removed and the incident light is reflected by the underlying reflective multilayer  102 . In some examples, the absorption material is chromium or other suitable absorption material. 
     In the case where the patterning layer  106  is a second reflective multilayer, the EUV mask can be referred to as a Phase Shifting Mask (PSM). With a PSM, the patterning layer  106  is a second reflective layer patterned with the integrated circuit design. The second reflective layer is designed so as to cause a phase difference (such as 180° phase difference) between the light reflected from the reflective multilayer  106  and the light reflected from the reflective multilayer  102 . The phase shifting mask may be an alternating phase shifting mask or an attenuated phase shifting mask. In some examples, the second reflective multilayer may be similar to the first reflective multilayer, such as alternating Mo/Si films. 
     The photoresist layer  108  is used to pattern the patterning layer  108  with the integrated circuit design. Conventional methods for patterning the photoresist  108  may be used in accordance with principles described herein. Because, the mirrors used in the EUV lithography process may shrink the pattern, the patterning layer  106  may be patterned using such conventional techniques. In some examples, e-beam or laser writing may be used to pattern the patterning layer  106 . 
       FIG. 1B  is a diagram showing an illustrative etching process  110  used to pattern the patterning layer. The etching process  100  is an anisotropic etching process. Thus, the etching occurs primarily in one direction. The etching process  110  may be, for example, a dry etching process. During the etching process  110 , the portions of the patterning layer  106  that are not protected by the patterned photoresist  108  layer are removed. Dry etching removes material by exposing the material to a bombardment of ions. The dry etch process utilizes a plasma of reactive gases such as fluorocarbons, oxygen, chlorine, boron trichloride and other gases. The bombardment of ions dislodges portions of the exposed surface. 
     According to principles described herein, the dry etching process  110  can be modified so that during the dry etching process, the capping layer  104  is treated. According to one example of principles described herein, the etching process is modified to cause introduction of the secondary material into the capping layer in addition to removing the exposed portions of the patterning layer  106 . The secondary material that is introduced into the capping layer  104  has an atomic number that is less than the atomic number of the main material that forms the capping layer. In some examples, the secondary material is selected from the elements having an atomic number less than 15. For example, the secondary material may be carbon, nitrogen, or phosphorous. The secondary material may be introduced to the capping layer  104  through one of a variety of methods. In one example, the secondary material is introduced into the capping layer  104  during the same process used to etch away the patterning layer. 
     In the example where the secondary material is introduced into the capping layer in the same process of etching the patterning layer  106 , the gases used during the dry etching process  110  is selected accordingly. For example, the secondary material may be nitrogen, which has an atomic number of 7. In such a case, the dry etching process can utilize an etching gas that includes chlorine and a carrier gas that includes nitrogen. Thus, after the exposed portions of the patterning layer  106  are moved, the nitrogen within the carrier gas is introduced into the surface of the capping layer  104  to create strengthened portions  112  of the capping layer  104 . As will be described in further detail below, the strengthened portions  112  are less susceptible to oxidation. 
       FIG. 1C  illustrates another example in which the secondary material is introduced in a separate implanting process  114  after the patterning layer  106  has been patterned. Specifically, the dry etching process  110  may be a conventional dry etching process. Such processes may involve an etching gas that includes chlorine and a carrier gas that includes argon. Then, after the dry etching process  110  has completed and the capping layer  104  is exposed, nitrogen can be introduced by an ion implantation process using an implanting step  114 . The implanting step  114  involves the use of nitrogen plasma. This implantation process  114  can be performed in the same chamber that the dry etching process  110  is performed. 
     The introduction of the secondary material can be designed to introduce the secondary material in a manner such that the secondary material does not exceed a predetermined depth. In one example, the predetermined depth is based on a diffusion region between the capping layer  104  and the multilayer  102 . When the capping layer  104  is formed on the multilayer, there is a diffusion region between the capping layer  104  and the multilayer where materials from one layer diffuse into the other material. The secondary material of the capping layer  104  can be implanted such that it does not extend to the diffusion region between the capping layer  104  and the multilayer  102 . 
     The introduction process, whether done separate or with the dry etching process, can be performed at room temperature. By avoiding high temperatures, there is less change of inter-diffusion of the reflective multilayer  102 . Thus, the capping layer  103  can be effectively treated without damaging the reflective multilayer  102 . 
     In some examples, there may be other materials within the capping layer besides the main capping layer material and the secondary material. For example, in the case where the capping layer primarily includes ruthenium, and the secondary material is nitrogen, the capping layer may have various other elements such as carbon, oxygen, silicon, silicon oxide, and molybdenum. Such materials, however, may be in smaller concentrations compared to the ruthenium and the secondary material. 
     The secondary material in the capping layer  104  has a concentration higher enough to effectively strengthen the capping layer  104 . In some examples, the secondary material can be introduced such that it has a concentration within a range of 10-30 percent. For example, the concentration of the secondary material may be about 20 percent. In another example, the secondary material may have a concentration that is greater than 30 percent. Such concentrations can be selected such that capping layer is sufficiently strengthened to better prevent oxidation and other types of damage. 
     At some point in the fabrication process that is after the capping layer is exposed, the photoresist layer  108  is removed. The photoresist layer  108  is only in place for the patterning step and is thus no longer needed after the patterning layer  106  has been patterned. In some cases, the photoresist layer  108  is removed after the capping layer has been treated according to principles described herein. The photoresist layer may be removed by wet stripping or plasma ashing. 
       FIG. 1D  is a diagram showing an illustrative lithography system  120  that utilizes the mask  100  described in  FIGS. 1A-1C . The lithography system  120  includes a radiation source  122 , an illuminator  124 , the mask  100 , and a projection optics box  126 . The lithography system  120  is used to expose a pattern on a wafer  128 . 
     The lithography system  120  may also be generically referred to as a scanner that is operable to perform lithography exposing processes with respective radiation source and exposure mode. In the present example, the lithography system  120  is an extreme ultraviolet (EUV) lithography system designed to expose a resist layer on a wafer  128  by EUV light. The resist layer is a material sensitive to the EUV light. The EUV lithography system  120  employs a radiation source  122  to generate EUV light, such as EUV light having a wavelength ranging between about 1 nm and about 100 nm. In one particular example, the radiation source  122  generates an EUV light with a wavelength centered at about 13.5 nm. 
     The lithography system  120  also employs an illuminator  124 . In various examples, the illuminator  124  includes various refractive optic components, such as a single lens or a lens system having multiple lenses (zone plates). Additionally or alternatively, the illuminator includes reflective optics such as a single mirror or a mirror system having multiple mirrors in order to direct light from the radiation source  122  onto a mask stage  100 . The mask  100  may be secured on a mask stage  16  (not shown). In some examples, the illuminator  124  is operable to configure the mirrors to provide a proper illumination to the mask  100 . In one example, the mirrors of the illuminator  124  are switchable to reflect EUV light to different illumination positions. In some examples, a stage prior to the illuminator  124  may additionally include other switchable mirrors that are controllable to direct the EUV light to different illumination positions with the mirrors of the illuminator  124 . In some embodiments, the illuminator  124  is configured to provide an on-axis illumination to the mask  100 . 
     The lithography system  120  also includes a mask stage (not shown) configured to secure the mask  100 . In some embodiments, the mask stage includes an electrostatic chuck (e-chuck) to secure the mask  100 . This is because gas molecules absorb EUV light, and the lithography system for the EUV lithography patterning is maintained in a vacuum environment to avoid the EUV intensity loss. 
     The lithography system  120  also includes a projection optics module (or projection optics box (POB)  126  for imaging the pattern of the mask  100  on to a semiconductor substrate such as wafer  128 . The POB  126  also includes reflective optics. The light directed from the mask  100 , carrying the image of the pattern defined on the mask, is collected by the POB  126 . The illuminator  124  and the POB  126  are collectively referred to as an optical module of the lithography system  120 . The illuminator and the POB can be adjusted to shrink the pattern of the mask down by a factor of four before the pattern reaches the wafer  128 . 
       FIGS. 2A-2B  are diagrams showing diffusion of a secondary material into a capping layer. According to the present example,  FIG. 2A  illustrates an example of interstitial diffusion. The larger circles represent the primary capping layer material atoms  202 , for example, ruthenium atoms. The smaller circles represent the secondary material atoms  204 , which may be for example, nitrogen, carbon, or phosphorous atoms. The ruthenium capping layer forms a crystal structure and thus the atoms  204  are positioned in a periodic pattern. 
     Interstitial diffusion occurs when the impurity material (i.e., nitrogen) moves and becomes positioned between the locations of atoms of the host material (i.e., ruthenium). In this example, the nitrogen atoms  204  are placed at various locations between the ruthenium atoms  202 . The addition of the secondary material produces stress on the capping layer. This stress can provide a stronger capping layer that is less susceptible to oxidation. Particularly, it takes more energy to initiate the chemical reactions that cause oxidation. Thus, the capping layer will last longer during normal use of the mask. To best allow for interstitial diffusion, the atomic number of the secondary material should be smaller than the atomic number of the main material of the capping layer. As described above, in one example, the secondary material is selected from elements having an atomic number less than 15. For example, if the capping layer is made of ruthenium, which has an atomic number of 44, and the secondary material implanted into the ruthenium is nitrogen, which has an atomic number of 7, then the nitrogen atoms are sufficiently smaller than the ruthenium atoms to allow for interstitial diffusion. Furthermore, the nitrogen atoms substantially stay in the interstitial locations. 
       FIG. 2B  is a diagram showing substitution diffusion. Substitution diffusion occurs when impurity atoms replace atoms within the structure of the host material. For example, nitrogen atoms  204  replace the ruthenium atoms  202  within the ruthenium crystal structure. Again, other elements may be used instead of nitrogen, such as carbon, phosphorous, or other element having an atomic number less than 15. 
       FIG. 3A  is a diagram showing a mask  300  an additional capping layer  302  on the patterning layer  106 . In some examples, an additional capping layer  302  is formed on top of the patterning layer. This may be useful if the patterning layer is a phase shifting layer. Thus, the patterning layer itself is similar to the reflective multilayer  102 . Accordingly, it is desirable to protect such a material with a capping layer  302 . The treatment process used to treat the exposed portion of the capping layer  104  can also be used to treat the capping layer  302  formed on the patterning layer  106 . Thus, the capping layer  302  will be similar to the strengthened portions  112  of the underlying capping layer  104 . The capping layer  302  will thus also be more resistant to oxidation and other damage. 
       FIG. 3B  is a diagram showing a mask  310  with the capping layer  312  formed after the patterning layer. Thus, the capping layer is disposed on top of the features of the patterned patterning layer  106  and on the exposed portions of the reflective multilayer  102 . The capping layer  312  can then be treated as described above. Particularly, a secondary material is introduced into the capping layer to strengthen the capping layer. The strengthened capping layer  312  is thus less prone to oxidation and other forms of damage. 
       FIG. 4  is a flowchart showing an illustrative method for forming a mask. According to the present example, the method  400  includes a step  402  for providing a mask substrate. The mask substrate is formed of a LTEM material so that the mask will be less prone to warping during normal use of the mask. 
     The method  400  includes a step  404  for forming a reflective multilayer on the substrate. As described above, the reflective multilayer  102  includes a plurality of film pairs, such as molybdenum-silicon (Mo/Si) film pairs or molybdenum-beryllium (Mo/Be) film pairs. The characteristics of the reflective multilayer  102  are selected such that it provides a high reflectivity to specific electromagnetic radiation type/wavelength. Specifically, for the purpose of EUV lithography, the reflective multilayer  102  is designed to reflect light within the EUV range. The formation of the reflective multilayer may include chemical vapor deposition (CVD), physical vapor deposition (PVD), atomic layer deposition (ALD) or other suitable technique. 
     The method  400  further includes a step  406  for forming a capping layer on the reflective multilayer. The capping layer is designed to protect the reflective multilayer. The capping layer has a thickness within a range of 2-4 nanometers. The capping layer can be made of ruthenium. The formation of the capping layer may include CVD, PVD, ALD or other suitable technique. 
     The method  400  includes a step  408  for forming a patterning layer on the capping layer. The patterning layer may be an absorptive layer in the example where the mask is a binary intensity mask. The patterning layer may also be a phase shifting layer in the example where the mask is a phase shifting mask. 
     The method  400  includes two different options for treating the capping layer. In the first option, at step  410 , a dry etching process is performed to pattern the patterning layer. The dry etching process is designed to also introduce a secondary material into the capping layer. For example, if nitrogen is to be introduced into the capping layer, then the gases used for the dry etch process include nitrogen. For example, the gases may be a chorine and nitrogen mix. Thus, the dry etching process will proceed to remove the patterning layer. Upon reaching the capping layer, the dry etching process will begin to introduce the nitrogen into the exposed capping layer. 
     For the second option, at step  412 , a dry etching process is performed to pattern the patterning layer. Then, after the patterning process is complete, a separate step, step  414 , is used to perform an implanting process to implant the secondary material into the exposed portions of the capping layer. The implanting step, step  414 , can be performed in the same chamber in which the patterning step, step  412 , is performed. 
     The method  400  may further include a step  416  for using the mask to expose wafers. As described above, an EUV lithography process involves projecting EUV light onto the mask. The EUV light is then reflected off the EUV mask and on to a wafer to form a pattern on that wafer. The pattern is used to form part of an integrated circuit on the wafer. 
     The method  400  further includes a step  418  for cleaning the mask after a set number of uses. The EUV process often causes particles to accumulate on the mask. Such particles can reduce the effectiveness of the mask. Thus, the mask may be cleaned at certain intervals, such as after every 40 uses. The mask may be cleaned by being immersed in a cleaning fluid. In some examples, the cleaning process may also cause some damage to the capping layer. But, by using principles described herein, the capping layer of the mask is strengthened and is thus less prone to damage that can be caused by normal use of the mask, cleaning the mask, and oxidation of the mask. 
     According to one illustrative example, a method for forming a lithography mask includes forming a capping layer on a reflective multilayer layer, the capping layer comprising a first material, forming a patterned patterning layer on the capping layer, and introducing a secondary material into the capping layer, the secondary material having an atomic number that is smaller than 15. 
     According to one example, a method for fabricating a lithography mask includes providing a substrate, forming a reflective multilayer on the substrate, forming a capping layer on the reflective multilayer, the capping layer comprising a first material, forming a patterning layer on the capping layer, patterning the patterning layer to expose portions of the capping layer, introducing a second material into the capping layer, the second material having an atomic number small enough to allow for interstitial diffusion of the second material into the first material. 
     According to one example, a lithography mask includes a reflective multilayer, a capping layer formed on the reflective layer, the capping layer comprising ruthenium and a second material, the second material having an atomic number less than 15 and being dispersed in interstitial positions of the capping layer, and a patterning layer on the capping layer. 
     The foregoing outlines features of several embodiments so that those skilled in the art may better understand the aspects of the present disclosure. Those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure.