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 

