US 10,845,704 B2  9  10  different metal-containing precursor to incorporate different  metallic compound  207  ( different metal or metal oxide) into  the resist layer  206,  using different metal-containing precur› sor. For examples, the metallic compound  207  of zinc oxide  (ZnO) may use zinc acetate, zinc chloride, and zinc acety› lacetonate for metal-containing precursor. For the metallic  compound  207  of titanium oxide (TiO$_{2}$ ),  titanium iso› propoxide or titanium tetrachloride with water may be used  as metal-containing precursor. For the metallic compound  207  of tungsten (W), the metal-containing precursor may  include Bis(tert-butylimino )bis(tert-butylamino) tungsten,  Bis(tert-butylimino )bis( dimethylamino) tungsten, Bis( cy› clopentadienyl)tungsten, Tungsten hexacarbonyl, or Bis( cy› clopentadienyl)tungsten dihydride. For the metallic com› pound  207  of hafnium (Hf), the metal-containing precursor  may include HfC1$_{4 }$ or Hafnium isopropoxide isopropanol.  For the metallic compound  207  of tin (Sn), the metal› containing precursor may include Hexamethylditin, (CH$_{3}$ )  $_{6}$Sn$_{2}$ .  For the metallic compound  207  of zirconium (Zr), the  metal-containing precursor may include ZrC1$_{4}$ ,  or Zirconium  (IV) tert-butoxide.  In some embodiments, the infiltration process  106  includes an infiltration process using the metal-containing  precursor having two metallic compounds (such as two  metal oxides, two metals, or one metal plus one metal  oxide). Therefore, two metallic compounds are introduced  into the resist layer  206  by one infiltration process  106.  For  examples, the filtration process introduces a transparent  metallic compound and an absorbing metallic compound  with proper concentration ratio to enhance the EUV sensi› tivity of the resist layer  206.  A transparent metallic com› pound refers to a metallic compound transparent to the EUV  radiation of the exposing process in the EUV lithography;  and an absorbing metallic compound refers to a metallic  compound that absorbs the EUV radiation of the exposing  process in the EUV lithography. In various examples, the  transparent metallic compound includes hafnium (Hf), zir› conium (Zr), titanium (Ti$_{1}$ ), other transparent metal, corre› sponding metal oxide, or a combination thereof. In other  examples, the absorbing metallic compound includes tin  (Sn), aluminum (Al), other absorbing metal, corresponding  metal oxide or a combination thereof. The transparent metal- lic compound is transparent to and does not absorb EUV  radiation. However, it causes the EUV radiation to be  scattered and travelling longer, therefore increasing the  absorption of the EUV radiation by the absorbing metallic  compound. Accordingly, the EUV sensitivity of the resist  layer  206  is enhanced by the both metallic compounds and  is maximized when the ratio of the both is tuned to a proper  range. For example, if the absorbing metallic compound has  higher absorption, then the concentration of the transparent  metallic compound may need to be lower. Otherwise, the  concentration of the transparent metallic compound needs to  be higher to increase the absorption of the EUV radiation by  the absorbing metallic compound.  In some embodiments, the operation  106  includes two  infiltration processes implemented sequentially, such as  106  (1)  and  106(2),  each is designed with one metal-containing  precursor to introduce one respective metal different from  each other. For example, the first infiltration process  106(1)  introduces a transparent metallic compound and the second  infiltration process  106(2)  introduces an absorbing metallic  compound. Each infiltration process  106  may include steps  106A  through  106D  or a subset of the steps  106A  through  106D.  Referring to FIGS.  1 and SD,  the method  100  proceeds to  operation  108  by performing an exposing process to the  resist layer  206  utilizing an EUV radiation from a lithogra› phy system. In the present embodiment, the EUV radiation  has a wavelength of 13.5 nm. The operation  108  may be  performed in a vacuum. In some embodiments, the radiation  beam is directed to the resist layer  206  to form an image of  a circuit pattern defined on a photomask (such as a trans› missive mask or a reflective mask) in a proper exposing  mode such as step-and-scan. Various resolution enhance› ment techniques, such as phase-shifting, off-axis illumina- tion (OAI) and/or optical proximity correction (OPC), may  be used or implemented through the photomask or the  exposing process. For examples, the OPC features may be  incorporated into the circuit pattern on the photomask. In  another example, the photomask is a phase-shift mask, such  as an alternative phase-shift mask, an attenuated phase-shift  mask, or a chrome-less phase-shift mask. In yet another  example, the exposing process is implemented in an off-axis  illumination mode. In some other embodiments, the radia› tion beam is directly modulated with a predefined pattern  (such as an IC layout) without using a photomask (such as  using a digital pattern generator or direct-write mode). Since  the sensitivity of the resist layer  206  is enhanced by the  infiltration process and the exposing threshold of the resist  layer is lowered, such as lower than 20 mJ/cm 2 .  Accord- ingly, the exposing process is implemented with the dose  less than 20 mJ/cm 2 ,  according to the present example.  The operation  108,  after the exposing process, may fur› ther include other steps, such as thermal treatment. In the  present embodiment, the operation  108  includes a post- exposure baking (PEB) process to the semiconductor struc› ture  200,  especially to the resist layer  206  coated on the  substrate  202.  In a specific embodiment, the PEB process  may be performed in a thermal chamber at temperature  ranging between about 120°  C. to about 160°  C.  After the exposing process at the operation  108,  a latent  pattern is formed on the resist layer  206.  The latent pattern  of a resist layer refers to the exposed pattern on the resist  layer, which eventually becomes a physical resist pattern,  such as by a developing process. The latent pattern of the  resist layer  206  includes unexposed portions  206a  and  exposed portions  206b.  In the present case, the exposed  portions  206b  of the resist layer  206  are physically or  chemically changed. In some examples, the exposed por› tions  206b  are changed by polymerization, such as cross- linked as in negative-tone resist or depolymerized as in  positive-tone resist. In other examples, the exposed portions  206b  are de-protected, inducing polarity change for dual› tone imaging (to selectively remove exposed portions or  non-exposed portions by organic developing solution or  aqueous developing solution).  After the exposing process, the method  100  may also  include a post-exposure baking (PEB) process.  Referring to FIGS.  1  and SE, the method  100  then  proceeds to operation  110  by developing the exposed resist  layer  206  in a developer, constructed in accordance with  some embodiments. By the developing process, a patterned  resist layer  206’  is formed. In some embodiments, the resist  layer  206  is a negative-tone resist and the exposed portions  of the resist layer experience crosslinking and therefore  remain after the developing process. In some embodiments,  the resist layer  206  experiences a polarity change after the  operation  106,  and a dual-tone developing process may be  implemented. For examples, the exposed portions of resist  layer  206  are changed from a nonpolar state (hydrophobic  state) to a polar state (hydrophilic state), then the exposed  portions  206b  will be removed (positive tone imaging) by an  aqueous solvent, such as tetramethyl ammonium hydroxide 