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## IONIC THERMAL ACID GENERATORS FOR LOW TEMPERATURE APPLICATIONS 

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 

## FIELD 

## INTRODUCTION 

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. 

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. 

In one preferred aspect, the invention provides an ionic thermal acid generator represented by the formula: 

<img src='content_image/926337.jpg'>

in which 

A- is the anion of an organic or inorganic acid having a pKa of not more than 3; and 

(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. 

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. 

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. 

Substrates such as a semiconductor wafer also are pro› vided having coated thereon a composition of the invention. Other aspects are disclosed infra. 

## BRIEF DESCRIPTION OF THE DRAWINGS 

FIG. 1 is a scheme illustrating chemical trimming of a resist using a chemical trim overcoat (CTO) layer. 

This invention relates to thermal acid generators (TA Gs) and to compositions and methods using TAGs. 

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. 

In one preferred aspect, the invention provides an ionic thermal acid generator represented by the formula: 

<img src='content_image/926326.jpg'>

in which 

A- is the anion of an organic or inorganic acid having a pKa of not more than 3; and 

(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. 

## SUMMARY 

## DETAILED DESCRIPTION 

