Abstract:
A method of manufacturing an extreme ultra-violet lithography (EUVL) alternating phase-shift mask comprises preparing a substrate having a reflective layer, forming a light-shielding layer pattern on the reflective layer to cover part of the reflective layer while leaving a reflective region of the reflective layer exposed, forming a trench in a phase-shift region of the reflective layer by etching the reflective layer, and changing the physical structure of a non phase-shift region of the reflective region to lower its reflectivity with respect to extreme ultra-violet (EUV) light.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Embodiments of the present invention relate generally to methods of manufacturing an alternating phase-shift mask used for fabricating semiconductor devices. More specifically, embodiments of the invention relate to methods of manufacturing an extreme ultra-violet lithography (EUVL) alternating phase-shift mask. 
     A claim of priority is made to Korean Patent Application No. 2005-0032756, filed on Apr. 20, 2005, the disclosure of which is hereby incorporated by reference in its entirety. 
     2. Description of Related Art 
     Photolithography processes are commonly used to form minute patterns in electronic devices such as integrated circuits. In general, the minimum size of a pattern that can be formed by a photolithography process is limited by the resolution of photolithography equipment used to carry out the process. Where the desired critical dimension of a pattern approaches the resolution of the photolithography equipment, a proximity effect may occur. Briefly, the proximity effect includes undesirable structural interactions between adjacent features formed by the pattern. In the case of photolithography processes, the proximity effect generally results from electron scattering in an irradiated resist layer where the pattern is formed. 
     One proposed method for addressing the proximity effect in photolithography processes using a light source such as a krypton fluoride (KrF) or argon fluoride (ArF) laser is to shift the light source&#39;s phase using a transmitting phase-shift mask. Shifting the light source&#39;s phase introduces destructive interference which can prevent some of the electron scattering. One way to form the transmitting phase-shift mask is by etching a phase-shift region in a quartz substrate so that the quartz substrate will reflect the light source with respective phases of 0° and 180°. 
     One shortcoming of the above method is that light passing through the phase-shift region can be scattered, for example, by the sidewalls of the etched region. As a result, the intensity of light passing through the phase-shift region may be lower than the intensity of light passing through other portions of the quartz substrate. Due to this light intensity difference, a critical dimension (CD) difference (ΔCD) may arise between adjacent patterns transferred on a wafer. In addition, when the phase-shift deviates from 180°, a ΔCD reversal, which is also called an X-phenomenon, may occur. To address the X-phenomenon, an undercut is generally formed in the etched phase-shift region using an isotropic wet etching process to prevent light loss from occurring. 
     However, in next-generation EUVL exposure technology, because the absorbency of EUV light sources having a short wavelength is too high when transmitting masks are used, reflective masks are used instead of transmitting masks. To maximize the reflectivity of EUV with a wavelength of 13.5 nm, a reflective mask includes a reflective layer including two types of material alternately stacked a plurality of times. For example the reflective layer could comprise 40 pairs of alternately stacked molybdenum (Mo) and silicon (Si) layers with a chromium (Cr) shielding layer pattern formed thereon. Similar to the transmitting masks, the intensity of light reflected by a phase-shifting region of a reflecting phase-shift mask is generally lower than the intensity of light reflected by other portions of the reflecting phase-shift mask. Accordingly, the reflecting phase-shift mask also suffers from the ΔCD problem. Unfortunately, however, the ΔCD cannot be addressed by the same method used to reduce ΔCD in transmitting phase-shift masks. 
     Several methods have been proposed for reducing the ΔCD or X-phenomenon created by EUVL alternating phase-shift masks. Some of these methods are disclosed, for example, in the following two documents: “EUVL Alternating Phase-shift Mask Imaging Evaluation”, Pei-Yang Yan et al., Proc. Of SPIE Vol. 4889; and, “Phase-shift Mask in EUV Lithography”, Minoru Sugawara et al., SPIE Vol. 5037. 
     Unfortunately, all of the proposed methods are difficult and complicated to implement and therefore highly impractical. 
     SUMMARY OF THE INVENTION 
     Embodiments of the present invention recognize the general need to prevent a ΔCD or X-phenomenon from occurring in EUVL alternating phase-shift masks. Accordingly, embodiments of the invention provide various methods of forming EUVL alternating phase-shift masks so that the reflectivity of a non phase-shift region is lowered to be the same as the reflectivity of a phase-shift region. 
     According to one embodiment of the invention, a method of manufacturing an extreme ultra-violet lithography (EUVL) alternating phase-shift mask comprises preparing a substrate having a reflective layer. The reflective layer comprises a plurality of layers formed of a first material, alternately stacked with a plurality of layers formed of a second material. The method further comprises forming a light-shielding layer pattern on the reflective layer to cover part of the reflective layer while leaving a reflective region of the reflective layer exposed, forming a trench in a phase-shift region within the reflective layer by etching the reflective layer, and changing the physical structure of a non phase-shift region within the reflective region to lower a reflectivity of the non phase-shift region with respect to extreme ultra-violet (EUV) light. 
     According to another embodiment of the invention, a method of manufacturing an extreme ultra-violet lithography (EUVL) alternating phase-shift mask comprises preparing a- substrate having a reflective layer, forming a light-shielding layer pattern to cover part of the reflective layer while leaving a reflective region of the reflective layer exposed, forming a 180° phase-shift region by etching a first portion of the reflective region, and changing the physical structure of a second portion of the reflective region such that the second portion of the reflective region reflects extreme ultra-violet (EUV) light with the same intensity as the first portion. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is described below in relation to several embodiments illustrated in the accompanying drawings. Throughout the drawings like reference numbers indicate like exemplary elements, components, or steps. In the drawings: 
         FIGS. 1A through 1H  are sectional views illustrating exemplary method steps for manufacturing an EUVL alternating phase-shift mask according to several embodiments of the present invention. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       FIGS. 1A through 1H  are sectional views illustrating exemplary method steps used to manufacture an EUVL alternating phase-shift mask according to several embodiments of the present invention. 
     Referring to  FIG. 1A , reflective layers  112  and  116  are formed on a substrate  100 . Reflective layers  112  and  116  both have a multi-layered structure in which molybdenum and silicon (Mo/Si) or molybdenum and beryllium (Mo/Be) are alternately stacked a plurality of times. Preferably, each of the alternating layers is stacked  40  times in each of reflective layers  112  and  116 . 
     Typically, an etch stop layer  114  is formed between first and second reflective layer  112  and  116 . In some embodiments of the invention, however, etch stop layer  114  is omitted. First and second reflective layers  112  and  116  are preferably formed with the same materials and structure. 
     A light-shielding layer  120  is formed over reflective layers  112  and  116 . Light-shielding layer  120  is typically formed of tantalum nitride (TaN) or chromium (Cr). 
     Referring to  FIG. 1B , a first photoresist layer (not shown) is formed on light shielding layer  120 . The first photoresist layer is then patterned to form a first photoresist pattern  122  exposing a part of light-shielding layer  120 . 
     Referring to  FIG. 1C , light-shielding layer  120  is anisotropically dry etched using first photoresist pattern  122  as an etch-mask to form a light-shielding layer pattern  120   a  exposing a reflective region  100   a  on reflective layer  116 . Then, first photoresist pattern  122  is removed to expose a non-reflective region  100   b  on top of reflective layer  116 . 
     Referring to  FIG. 1D , a second photoresist layer (not shown) is formed on substrate  100  over light-shielding layer  120 . The second photoresist layer is patterned to form a second photoresist pattern  130  exposing a phase-shift region  132  in reflective region  100   a  of second reflective layer  116 . 
     Referring to  FIG. 1E , second reflective layer  116  is anisotropically-dry etched using photoresist pattern  130  as an etch-mask to form a trench (T) in second reflective layer  116 . Then, second photoresist pattern  130  is removed. Etch-stop layer  114  can be used as an etching endpoint when forming trench (T). 
     Trench (T) causes a 180° phase-shift to occur in light reflecting off of phase-shift region  132 . 
     Referring to  FIG. 1F , a third photoresist layer (not shown) is formed on substrate  100  over second photoresist pattern  130 . The third photoresist layer is patterned to expose a non phase-shift region  142  in reflective region  100   a  of second reflective layer  116 . 
     Referring to  FIG. 1G , a physical shock  150  is selectively applied to exposed non phase-shift region  142  using third photoresist pattern  140  as an etch mask. As a result of the physical shock, the structure of a portion of second reflective layer  116  defined by non phase-shift region  142  undergoes a physical transformation to form a low-reflectivity region  116   a.    
     One way to apply physical shock  150  to second reflective layer  116  is to use a heat treating method in which a localized focused electron beam (e-beam) is illuminated on non phase-shift region  142 . Preferably, the localized focused e-beam has a diameter between several μm and multiple hundreds of μm, an exposure time in the tens of milliseconds, a current about 5-50 nA, and an energy of about 10 KeV. 
     By illuminating the focused e-beam on non phase-shift region  142  of second reflective layer  116 , the illuminated region of second reflective layer  116  is thermally damaged, which causes its reflectivity to decrease. By selectively lowering the reflectivity of the non phase-shift region of second reflective layer  116 , the intensity of light reflected by non phase-shift region  142  can be lowered to the intensity level of light reflected by phase-shift region  132 . 
     Another way to apply physical shock  150  to reflective layer  116  is to illuminate a FIB on non phase-shift region  142  of second reflective layer  116 . By illuminating the FIB on non phase-shift region  142 , the surface of the illuminated region is slightly etched to form a small recess; The small recess causes the reflectivity of non phase-shift region  142  to decrease such that the intensity of light reflected by non phase-shift region  142  is the same as the intensity of light reflected by phase-shift region  132 . 
     Yet another way to apply physical shock  150  to reflective layer  116  is to use an ion-sputtering method. Preferably, the ion-sputtering method uses a gas for which the etching selection ratio between reflective layer  116  and third photoresist pattern  140  is about 5:1 or more. For example, the gas used for the ion-sputtering method may be argon gas. Preferably, the ion-sputtering method does not cause any damage to reflective region  100   a  exposed in non phase-shift region  142 . 
     Typically, the ion-sputtering method is performed by a sputtering apparatus having a source/bias power of tens or hundreds of watts. The bias power is preferably set higher than the source power to perform a bias sputtering. In one embodiment of the invention, the ion-sputtering method exposes non phase-shift region  142  of the second reflective layer  116  to Ar plasma for a period of several seconds to several tens of seconds under a pressure of several mTorrs to several tens of mTorrs. Throughout the ion-sputtering process, the sputtering temperature is preferably maintained constant to prevent diffusion of multi-layered reflective layers  112  and  116 . 
     When Ar ion-sputtering is applied to non phase-shift region  142  of second reflective layer  116 , a physical shock from the sputtering causes a physical change in non phase-shift region  142  of second reflective layer  116 , thereby lowering its reflectivity. By selectively lowering the reflectivity of second reflective layer  116  in non phase-shift region  142  using the ion-sputtering method, the intensity of EUV light illuminated on second reflective layer  116  can be selectively lowered such that the EUV light reflected from second reflective layer  116  in phase-shift region  132  and non phase-shift region  142  is substantially the same. 
     Referring to  FIG. 1H , third photoresist pattern  140  is removed to complete an EUVL alternating phase-shift mask. 
     According to selected embodiments of the present invention described above, methods of manufacturing an EUVL alternating phase-shift mask include physically changing the structure of a portion of a reflective layer in a non phase-shift region to lower the intensity of EUV light reflected thereon. The physical change can be brought about by several different techniques, such as those involving a focused e-beam, a FIB, or Ar sputtering. By physically changing the reflective layer in the non phase change region, the intensity of reflected EUV light in the non phase change region can be lowered to the intensity of reflected EUV light in a phase-shift region of the reflective layer. As a result, a ACD or X-phenomenon can be prevented from occurring between adjacent patterns printed on a wafer. In addition, unevenness of an aerial image can be relieved over the entire region of the phase-shift region and the non phase-shift region of an EUVL alternating phase-shift mask. 
     The foregoing preferred embodiments are teaching examples. Those of ordinary skill in the art will understand that various changes in form and details may be made to the exemplary embodiments without departing from the scope of the present invention as defined by the following claims.