US 2016/0334706 Al  32  Nov. 17, 2016  diffusion regulator. This is because the counter anion of the  onium salt is a conjugated base of weak acid. As used herein,  the "weak acid" indicates an acidity insufficient to deprotect  an acid labile group from an acid labile group-containing  unit in the base resin. The onium salt having formula (9a) or  (9b) functions as a quencher when used in combination with  an onium salt type PAG having a conjugated base of a strong  acid, typically a sulfonic acid which is fluorinated at a-po› sition as the counter anion. In a system using a mixture of  an onium salt capable of generating a strong acid (e.g.,  a-position fluorinated sulfonic acid) and an onium salt  capable of generating a weak acid (e.g., a-position non› fluorinated sulfonic acid or carboxylic acid), if the strong  acid generated from the PAG upon exposure to high-energy  radiation collides with the unreacted onium salt having a  weak acid anion, then a salt exchange occurs whereby the  weak acid is released and an onium salt having a strong acid  anion is formed. In this course, the strong acid is exchanged  into the weak acid having a low catalysis, incurring apparent  deactivation of the acid for enabling to control acid diffu› sion. In this way, the onium salt having formula (9a) or (9b)  functions as the quencher.  [0110]  If a PAG capable of generating a strong acid is an  onium salt, an exchange from the strong acid generated upon  exposure to high-energy radiation to a weak acid as above  can take place, but it rarely happens that the weak acid  generated upon exposure to high-energy radiation collides  with the unreacted onium salt capable of generating a strong  acid to induce a salt exchange. This is because of a likeli› hood of an onium cation forming an ion pair with a stronger  acid anion.  [0111]  An  appropriate amount of the onium salt having  formula (9a) or (9b) added is 0 to 40 parts, and if used,  preferably 0.1 to 40 parts, and more preferably 0.1 to 20  parts by weight per 100 parts by weight of the base resin.  An  excess of the onium salt may cause a degradation of reso› lution or leave foreign particles after resist development or  stripping.  [0112]  In the resist composition, a photo-decomposable  onium salt having a nitrogen-containing substituent group  may also be used together, if desired. This compound  functions as a quencher in the unexposed region, but as a  so-called photo-degradable base in the exposed region  because it loses the quencher function in the exposed region  due to neutralization thereof with the acid generated by  itself. Using a photo-degradable base, the contrast between  exposed and unexposed regions can be further enhanced.  With respect to the photo-degradable base, reference may be  made to JP-A 2009-109595, 2012-046501 and JP-A 2013- 209360, for example.  [0113]  An  appropriate amount of the photo-degradable  base added is Oto 40 parts, and if used, preferably 0.1 to 40  parts, and more preferably 0.1 to 20 parts by weight per 100  parts by weight of the base resin.  An  excess of the base may  cause a degradation of resolution or leave foreign particles  after resist development or stripping.  [0114]  Component (E) may be any organic solvent as long  as the polymer, PAG, quencher and other additives are  soluble therein. Examples of the organic solvent include  ketones such as cyclohexanone and methyl-2-n-pentyl  ketone; alcohols such as 3-methoxybutanol, 3-methyl-3- [0108]  The onium salt having formula (9a) or (9b) may be  any of arbitrary combinations of cations with anions, both as  exemplified above. The onium salt may be readily synthe› sized by ion exchange reaction according to the well-known  organic chemistry procedure. With respect to the ion  exchange reaction, reference may be made to JP-A 2007- 145797, for example.  [0109]  In the resist application, the onium salt having  formula (9a) or (9b) functions as the quencher or acid  (E) Organic Solvent  r- w  C  >- s  Q  -W- I  °’  -W- I  -continued 