microlithography, for example, in photoresist formulations  and in antireflective coatings.  Further improvements in photolithography compositions  and processes are needed to achieve the formation of fine  patterns in electronic device fabrication and to avoid or  conspicuously ameliorate one or more of the foregoing  problems associated with the state of the art.  In one aspect, the present invention relates to new thermal  acid generators having a low onset temperature, and to  compositions comprising the thermal acid generators.  Photoresists are photosensitive films for transfer of  images to a substrate. They form negative or positive  images. After coating a photoresist on a substrate, the  coating is exposed through a patterned photomask to a  source of activating energy such as ultraviolet light to form  a latent image in the photoresist coating. The photomask has  areas opaque and transparent to activating radiation that  define an image desired to be transferred to the underlying  substrate. A relief image is provided by development of the  latent image pattern in the resist coating.  Positive-tone chemically amplified photoresists are con› ventionally used for high-resolution processing. Such resists  typically employ a resin having acid-labile leaving groups  and a photoacid generator. Exposure to actinic radiation  causes the acid generator to form an acid which, during  post-exposure baking, causes cleavage of the acid-labile  groups in the resin. This creates a difference in solubility  characteristics between exposed and unexposed regions of  the resist in an aqueous alkaline developer solution. Exposed  regions of the resist are soluble in the aqueous alkaline  developer and are removed from the substrate surface,  whereas unexposed regions, which are insoluble in the  developer, remain after development to form a positive  image.  One approach to achieving nm-scale feature sizes in  semiconductor devices is the use of short wavelengths of  light, for example, 193 nm or less, during exposure of  chemically amplified photoresists. To further improve litho› graphic performance, immersion lithography tools have  been developed to effectively increase the numerical aper› ture (NA) of the lens of the imaging device, for example, a  scanner having a KrF or ArF light source.  It  is possible to  achieve a 40 nm half-pitch resolution in a single exposure  process, thus allowing for improved design shrink. This  standard immersion lithography process, however, is gen› erally not suitable for manufacture of devices requiring  greater resolution, for example, for the 32 nm and 22 nm  half-pitch nodes.  To extend the practical resolution beyond that achieved  with standard photolithographic techniques, chemical trim› ming processes have been proposed for reducing the critical  dimension (CD) of a photoresist pattern. Chemical trimming  processes typically involve treating a photoresist pattern 55  with a composition including an acid or acid generator  capable of increasing solubility of a surface region of the  photoresist pattern in a developer solution. Such a process is  disclosed, for example, in U.S. Pat. No. 8,067,148, which  discloses a chemical trimming solution including a thermal 60  acid generator.  The inventors have found that the use of conventional  thermal acid generators having high onset temperatures in  chemical trimming processes can result in disadvantages  such as poor line width roughriess (LWR) of the trimmed  resist pattern. Use of such conventional thermal acid gen› erators can also be problematic in other materials used in  (BHt is the monoprotonated form of a nitrogen-contain› ing base B having a pKa between  O  and 5.0, and a boiling  point less than 170°  C.  A- is the anion of an organic or inorganic acid having a  pKa of not more than 3; and  in which  (A-)(BH)’  In one preferred aspect, the invention provides an ionic  thermal acid generator represented by the formula:  It  would be advantageous to have TAGs which are  capable of producing acid at lower temperatures ( e.g., below  100°  C.).  It  has now been found that improvements in line  width roughriess (LWR) in CTO processes are related to the  temperature of the bake (heating) step, with lower bake  temperatures generally providing lower (better) LWR val- ues.  This invention relates to thermal acid generators (TA Gs)  and to compositions and methods using TAGs.  FIG.  1  is a scheme illustrating chemical trimming of a  resist using a chemical trim overcoat (CTO) layer.  Substrates such as a semiconductor wafer also are pro› vided having coated thereon a composition of the invention.  Other aspects are disclosed infra.  In certain embodiments, A- is the anion of a fluoroalkyl› sulfonic acid, and in further embodiments, A- is the anion of  a perfluoroalkylsulfonic acid, including perfluorobutanesul› fonate. In yet another embodiment, A- is the anion of an  aromatic sulfonic acid, including p-toluenesulfonate.  In certain embodiments, B is substituted or unsubstituted  pyrimidine, pyrazine, oxazoline, or thiazoline, or difluorom› ethylammonia, or substituted pyridine, and in further  embodiments, B is substituted pyridine (such as 3-fluoro- pyridine ), or substituted or unsubstituted pyrimidine.  (BHt is the monoprotonated form of a nitrogen-contain› ing base B having a pKa between  O  and 5.0, and a boiling  point less than 170°  C.  A- is the anion of an organic or inorganic acid having a  pKa of not more than 3; and  in which  (A-)(BH)’  In one preferred aspect, the invention provides an ionic  thermal acid generator represented by the formula:  We have now discovered new ionic thermal acid genera› tors (TA Gs) having a low onset temperature. Such TA Gs are  useful in, e.g., antireflective coating compositions, photore› sist compositions, chemical trim overcoat (CTO) composi- tions, directed self assembly (DSA) patterning schemes, and  any other composition in which a lower-temperature TAG  would be beneficial.  IONIC THERMAL ACID GENERATORS FOR  LOW TEMPERATURE APPLICATIONS  FIELD  INTRODUCTION  SUMMARY  BRIEF DESCRIPTION OF THE DRAWINGS  DETAILED DESCRIPTION  US 9,541,834 B2  1  2 