It is preferable that at least any one ofX 1 ,  X 2  and X 3  has  fluorine atom and/or X 3  is a trifluoromethyl group, from the  The introduction of the structural unit derived therefrom is  preferred because it is possible to enhance solubility in a  solvent, photosensitivity with a photoacid generator, adhe› sion to a substrate and compatibility with a photoacid gen- erator and other additives.  Examples thereof are, for instance, acrylic acid, meth› acrylic acid, a-fluoroacrylic acid, a-trifluoromethylacrylic  acid, acrylic acid esters, a-fluoroacrylic acid esters, meth› acrylic acid esters, a-trifluoromethylacrylic acid esters,  hydroxyethyl acrylate, hydroxyethyl methacrylate, glycidyl  acrylate, glycidyl methacrylate, acrylamides, methacryla› mides and the like.  and the like.  (mis an integer of from 1 to 10)  wherein X $^{1 }$and X 2  are the same or different and each is H or  F; X 3  is H, F, CH 3  orCF 3 ; X$^{4 }$andX$^{5 }$are the same or different  The structural unit derived from an ethylenic monomer  having functional group which is copolymerized with the  fluoroolefin and the monomer introducing the monocyclic  structure is a structural unit derived from an ethylenic mono› mer and represented by the formula (N-1):  Accordingly photosensitivity necessary in resist applica› tion and various useful functions necessary in other applica› tions can be imparted to the polymer.  The present inventors have found that a specific ethylenic  monomer having functional group can be copolymerized in  addition to the fluoroolefin and the above-mentioned mono› mer being capable of introducing a monocyclic structure,  thereby making it possible to introduce the functional group  to the fluorine-containing polymer having a monocyclic  structure in its trunk chain.  In the polymers of the formulae (Ma) and (Mb) of the  present invention, the structural unit Ml, the structural unit  M2a or M2b and the structural unit N are contained in  amounts of from 1 to 99% by mole, from 1 to 99% by mole  and from Oto 98% by mole, respectively. Provided that (Ml)+  (M2a) or (Ml)+(M2b) is 100% by mole, a percent by mole  ratio of (Ml)/(M2a) or (Ml)/(M2b) is preferably 80/20 to  20/80, more preferably 70/30 to 30/70, further preferably  60/40 to 40/60.  Among them, preferred are structural units derived from  tetrafluoroethylene  and chlorotrifluoroethylene  because  transparency and  dry  etching resistance can be improved in  resist application.  In the fluorine-containing polymer of the present inven› tion, the structural unit Ml derived from a fluoroolefin is at  least one structural unit selected from structural units derived  from fluorine-containing ethylenic monomers having 2 or 3  carbon atoms. Examples thereof are, for instance, tetrafluo› roethylene,  chlorotrifluoroethylene,  vinylidene fluoride,  vinyl fluoride, trifluoroethylene, hexafluoropropylene and the  like.  wherein X $^{1 }$and X 2  are the same or different and each is H or  F; X 3  is H, F, CH 3  or CF 3 ; R is selected from hydrogen atom,  a hydrocarbon group having 1 to 20 carbon atoms, a fluorine› containing alkyl group having 1 to 20 carbon atoms, a fluo› rine-containing alky  1  group having 2 to 100 carbon atoms and  ether bond or a fluorine-containing aryl group having 3 to 20  carbon atoms.  In the above-mentioned formula, examples of preferred  -Rare:  hydrogen atom,  Example of preferred derivative of carboxylic acid group is  one selected from carboxylic acid esters, functional groups  protected by a protective group which can convert the func- tional group to COOH group due to reaction with an acid, acid  halides and acid amides.  In the above-mentioned formulae (b-4) and (b-5), it is  preferable from the viewpoint of copolymerizability that Z 5  and Z 6  are the same or different and each is at least one  selected from COOH or a derivative of carboxylic acid group.  wherein Z 5  and Z$^{6 }$are as defined above, can be obtained.  (i) Structural unit derived from acrylic monomer and repre› sented by:  Examples thereof are, for instance, as follows.  and each is H, F or CF  3 ; Rf is a fluorine-containing alkylene  group having 1 to 40 carbon atoms or a fluorine-containing  alky lene group having 2 to 100 carbon atoms and ether bond;  a is 0 or an integer of from 1 to 3; b and c are the same or  different and each is 0 or 1; Z 2  is at least one functional group  selected from the group consisting of OH group, COOH  group, a derivative of carboxylic acid group and a functional  group protected by a protective group which can convert the  functional group to OH group due to reaction with an acid.  and/or the formula (b-5):  19  US 7,511,179 B2  20  (b- 5 )  (N-1) 60  ----CH(CF 3)2, -CH2CFHCF 3, -(CH2)m(CF  2)n  ----CF(CF 3)2,  (mis an integer of from 1 to 5, n is an integer of from 1 to  10)  -(CH 2 )m(CF $_{2}$ )n  -Cl, (mis an integer of from 1 to 5, n is an  integer of from 1 to 10)  ----C(CH3)3, -CH2CH2OH, -(CH2)m(CF  2)n  -F, -(CH2)m  (CF$_{2}$ )n-H,  -CH 2 CF(OCF2CF)m - F,  I  I  CF3  CF3 