Furthermore, by using a chain transfer agent such as  HS----CH$_{2}$----CH$_{2}$----CH$_{2}$----C(CF$_{3}$ ) $_{2}$---0H during the above 50  polymerization, a ----C(CF$_{3}$ ) $_{2}$---0H group can be introduced  at the terminals of the component (Al). Such a copolymer  having an introduced hydroxyalkyl group in which some of  the hydrogen atoms of the alkyl group have been substituted  with fluorine atoms is effective in reducing developing 55  defects and line edge roughness (LER: unevenness in the side  walls of a line pattern).  US 9,164,380 B2  103  104  There are no particular limitations on these other structural  units, provided they cannot be classified as one of the above  structural units (al) to (a4), and any of the multitude of  conventional structural units used within resist resins  designed for use withArF excimer lasers, KrF excimer lasers,  EB or EUV or the like can be used.  The component (Al) is preferably a polymer that includes  the structural unit (al), and is more preferably a copolymer  containing the structural unit (al), and at least one structural  unit selected from among the structural units (a2) and (a3).  Among such copolymers, a copolymer that includes the struc› tural units (al) and (a2) is preferable, and a copolymer con› taining the structural unit (al), the structural unit (a2) and the  structural unit (a3) is particularly desirable.  Examples of these polymers and copolymers include  copolymers consisting of the structural units (al) and (a2),  copolymers consisting of the structural units (al) and ( a3 ),  copolymers consisting of the structural units (al), (a2) and  (a3), and copolymers consisting of the structural units (al),  (a2), (a3) and (a4).  The structural units (al), (a2), (a3) and (a4) within these  polymers and copolymers may each include a single type of  structural unit or a combination of two or more types.  The weight-average molecular weight (Mw) (the polysty› rene equivalent value determined by gel permeation chroma- tography (GPC)) of the component (Al) is not particularly  limited, but is preferably within a range from 1,000 to 50,000,  more preferably from 1,500 to 30,000, and most preferably  from 2,000 to 20,000. When the weight-average molecular  weight is not more than the upper limit of the above range, the  component (Al) exhibits satisfactory solubility in a resist  solvent when used as a resist. On the other hand, when the  weight-average molecular weight is at least as large as the  lower limit of the above range, the dry etching resistance and  the cross-sectional shape of the resist pattern are improved.  Further, although there are no particular limitations on the  dispersity (Mw/Mn) of the component (Al), the dispersity is  preferably from 1.0 to 5.0, more preferably from 1.0 to 3.0,  and most preferably from 1.0 to 2.5. Here, Mn represents the  number-average molecular weight.  A single component (Al) may be used alone, or a combi› nation of two or more types of the component (Al) may be  used.  The component (Al) can be obtained, for example, by a  conventional radical polymerization or the like of the mono- mers corresponding with each of the structural units, using a  radical polymerization initiator such as azobisisobutyroni› trile (AIBN).  In terms of the monomers used for forming each of the  structural units, either commercially available monomers  may be used, or the monomers may be synthesized using  conventional methods.  { Component (A2)}  The component (A2) is preferably a low molecular weight  compound having a molecular weight of at least 500 but less  than 4,000, containing a hydrophilic group and an acid-dis- sociable group such as those mentioned above in the descrip› tion of the component (Al). Specific examples of the com- The resist composition of the present invention also  includes an acid (G) (hereafter referred to as "component  (G)") having a pKa of 4 or less.  <Component (G)>  There amount of the component (A) within the resist com› position of the present aspect may be adjusted in accordance  with factors such as the thickness of the resist film that is to be  formed.  The amount of the component (Al) within the component  (A), based on the total weight of the component (A), is pref› erably at least 25% by weight, more preferably 50% by  weight or more, still more preferably 7 5% by weight or more,  and may even be 100% by weight. When this amount is at  least 25% by weight, lithography properties such as the mask  error factor (MEF) and the circularity are improved, and  roughness can be further reduced.  Among the various possibilities described above, the com› ponent (A) preferably includes the component (Al).  In the resist composition of the present invention, a single  component (A) may be used alone, or a combination of two or  more types of the component (A) may be used.  A single component (A2) may be used alone, or a combi› nation of two or more types of the component (A2) may be  used.  There are no particular limitations on the acid-dissociable  group, and examples include the same groups as those  described above within the description relating to the com› ponent (Al).  In particular, a phenolic compound having 2 to 6 triphenyl› methane structures is preferable as the low molecular weight  phenolic compound, as such compounds yield superior levels  ofresolution and line width roughness (LWR).  Examples of these low molecular weight phenolic com› pounds include bis( 4-hydroxyphenyl)methane, bis(2,3,4-tri› hydroxyphenyl)methane,  2-( 4-hydroxyphenyl)-2-( 4’-hy› droxyphenyl)propane,  2-(2,3,4-trihydroxyphenyl)-2-(2’,3’,  4 ’-trihydroxypheny  !)propane,  tris( 4-hydroxyphenyl)  methane,  bis( 4-hydroxy-3,5-dimethylphenyl)-2- hydroxyphenylmethane,  bis( 4-hydroxy-2,5- dimethylphenyl)-2-hydroxyphenylmethane, bis(4-hydroxy- 3,5-dimethylphenyl)-3,4-dihydroxyphenylmethane,  bis(4- hydroxy-2,5-dimethylphenyl)-3,4- dihydroxyphenylmethane, bis( 4-hydroxy-3-methylphenyl)- 3,4-dihydroxyphenylmethane, bis(3-cyclohexyl-4-hydroxy- 6-methylphenyl)-4-hydroxyphenylmethane,  bis(3- cyclohexyl-4-hydroxy-6-methylphenyl)-3,4- dihydroxyphenylmethane,  1-[1-( 4-hydroxyphenyl)  isopropyl]-4-[1, 1-bis( 4-hydroxyphenyl)ethyl]benzene,  and  dimers, trimers, tetramers, pentamers and hexamers of for› malin condensation products of phenols such as phenol,  m-cresol, p-cresol and xylenol. Needless to say, the low  molecular weight phenol compound is not limited to these  examples.  Examples of the component (A2) include low molecular  weight phenolic compounds in which a portion of the  hydroxyl group hydrogen atoms have each been substituted  with an aforementioned acid-dissociable group. These types  of compounds are known, for example, as sensitizers or heat  resistance improvers for use in non-chemically amplified  g-line or i-line resists, and any of these compounds may be  used.  ponent (A2) include compounds containing a plurality of  phenol structures in which a portion of the hydrogen atoms of  the hydroxyl groups have each been substituted with an afore- mentioned acid-dissociable, group. 