Jun.  18,2020  1  US 2020/0192221 Al  POSITIVE RESIST COMPOSITION AND  PATTERN FORMING PROCESS  CROSS-REFERENCE TO RELATED  APPLICATION  TECHNICAL FIELD  BACKGROUND ART  CITATION LIST  SUMMARY OF INVENTION  [0001]  This non-provisional application claims priority  under 35 U.S.C. § 119(a) on Patent Application No. 2018- 234513 filed in Japan on Dec. 14, 2018, the entire contents  of which are hereby incorporated by reference.  [0002]  This invention relates to a positive resist compo› sition and a patterning process using the composition.  [0003]  To meet the demand for higher integration density  and operating speed of LS Is, the effort to reduce the pattern  rule is in rapid progress. The wide-spreading flash memory  market and the demand for increased storage capacities  drive forward the miniaturization technology. As the  advanced miniaturization technology, manufacturing of  microelectronic devices at the 10-nm node by the immersion  ArF lithography has been implemented in a mass scale. The  candidates for the next generation 7-um node devices and  next-but-one generation 5-nm node devices include extreme  ultraviolet (EUV) lithography of wavelength 13.5 an and  double patterning version of the ArF lithography, on which  active research efforts have been made.  [0004]  The exposure system for mask manufacturing  made a transition from the laser beam exposure system to the  EB exposure system to increase the accuracy of line width.  Since a further size reduction became possible by increasing  the accelerating voltage of the electron gun in the EB  exposure system, the accelerating voltage increased from 10  kV to 30 kV and reached 50 kV in the current mainstream  system, with a voltage of 100 kV being under investigation.  [0005]  As the accelerating voltage increases, a lowering of  sensitivity of resist film becomes of concern. As the accel› erating voltage increases, the influence of forward scattering  in a resist film becomes so reduced that the contrast of  electron image writing energy is improved to ameliorate  resolution and dimensional control whereas electrons can  pass straightforward through the resist film so that the resist  film becomes less sensitive. Since the mask exposure tool is  designed for exposure by direct continuous writing, a low› ering of sensitivity of resist film leads to an undesirably  reduced throughput. Due to a need for higher sensitivity,  chemically amplified resist compositions are studied.  [0006]  As the feature size reduces, image blurs due to acid  diffusion become a problem. To insure resolution for fine  patterns with a size of 45 nm et seq., not only an improve› ment in dissolution contrast is important as previously  reported, but control of acid diffusion is also important as  reported in Non-Patent Document 1. Since chemically  amplified resist compositions are designed such that sensi› tivity and contrast are enhanced by acid diffusion, an attempt  to minimize acid diffusion by reducing the temperature  and/or time of post-exposure bake (PEB) fails, resulting in  drastic reductions of sensitivity and contrast.  [0007]  A triangular tradeoff relationship among sensitiv› ity, resolution, and edge roughness has been pointed out.  [0018]  Further, for improving the dissolution contrast,  recurring units having a carboxyl or phenolic hydroxyl  group in which the hydrogen is substituted by an acid labile  group are incorporated into the base polymer. There is  Specifically, a resolution improvement requires to suppress  acid diffusion whereas a short acid diffusion distance leads  to a loss of sensitivity.  [0008]  The addition of an acid generator capable of gen› erating a bulky acid is an effective means for suppressing  acid diffusion.  It  was then proposed to incorporate in a  polymer recurring units derived from an onium salt having  a polymerizable unsaturated bond. Since this polymer func› tions as an acid generator, it is referred to as polymer-bound  acid generator. Patent Document 1 discloses a sulfonium salt  having a polymerizable unsaturated bond, capable of gen› erating a specific sulfonic acid and a similar iodonium salt.  Patent Document 2 discloses a sulfonium salt having a  sulfonic acid anion directly attached to the backbone.  [0009]  Patent Documents 3 and 4 disclose resist materials  comprising a polymer comprising amino-containing recur› ring units. Polymeric amines are highly effective for sup› pressing acid diffusion. Patent Document 5 discloses a resist  material based on a polymer comprising recurring units of  acid generator and recurring units of amine.  It  is a single  component resist in which both an acid generator function  and a quencher function are assigned to a common polymer.  However, if the acid diffusion distance is too short, there  arises the problem that both dissolution contrast and sensi› tivity drop.  [0010]  Patent Document  1:  JP-A 2006-045311 (U.S. Pat.  No. 7,482,108)  [0011]  Patent Document 2: JP-A 2006-178317  [0012]  Patent Document 3: JP-A 2008-133312  [0013]  Patent Document 4: JP-A 2009-181062  [0014]  Patent Document 5: JP-A 2011-039266  [0015]  Non-Patent Document 1: SPIE Vol. 6520 65203L-1  (2007)  [0016]  An object of the present invention is to provide a  positive resist composition which exhibits a higher sensi› tivity and resolution than conventional positive resist com› positions, low edge roughness (LER, LWR) and small size  variation, and forms a pattern of good profile after exposure  and development, and a patterning process using the resist  composition.  [0017]  Making extensive investigations in search for a  positive resist material capable of meeting the current  requirements including high resolution, low edge roughness  and small size variation, the inventors have found the  following. To meet the requirements, the acid diffusion  distance should be minimized. This invites a lowering of  sensitivity and a drop of dissolution contrast, raising the  problem that the resolution of a two-dimensional pattern  such as hole pattern is reduced. Unexpectedly, when a  polymer comprising recturing units of nitrogen-containing  tertiary ester structure is used as a base polymer, the disso› lution contrast is increased and at the same time, the acid  diffusion distance is minimized. Better results are obtainable  using the polymer as a base polymer in a chemically  amplified positive resist composition. 