Characterization of Volatile Species Formed during Exposure of Photoresists to Ultraviolet Light

Abstract

A photochemical path for formation of volatile S and acidolytic paths for formation of volatile Si and/or C during exposure to ultraviolet light have been identified for two silicon-containing bilayer photoresists using atmospheric pressure ionization mass spectrometry (API MS) and secondary ion mass spectrometry (SIMS). Si and S are of particular concern because of their potential for irreversible adsorption onto nearby optical coatings inside a lithographic tool, causing permanent detuning. We describe the analytical techniques used and report estimates for absolute numbers of Si and S atoms lost from the resists. The data show that Si bound through oxygen is easily cleaved from the polymer by photogenerated acid while Si bound through carbon is not. Photodissociation of perfluorosulfonic acid from the photoacid generators results in significant loss of S. The data are compared to previous studies, and the advantages and limitations of these techniques for photoresist chemistry characterization are discussed.

LER Limitations of Resist Thin Films

“…[21][22][23][24][25][26] Since many PAGs are dissolution rate inhibitors, 7,27 the PAG segregation to the top of the film can cause surface inhibition. We reasoned that higher concentrations of PAG at the top of the resist films would result in flatter tops of the lines and thereby produce better LER.…”

Quantification of outgassing of C-, Si-, and S-containing products during exposure of photoresists

“…13 Briefly, both unexposed, postapply baked and exposed but not postexposure baked resist samples were placed in a Cameca SIMS instrument and depth profiled using Cs + ions. 13 Briefly, both unexposed, postapply baked and exposed but not postexposure baked resist samples were placed in a Cameca SIMS instrument and depth profiled using Cs + ions.…”

“…13 The thermogravimetric analysis was performed under flowing nitrogen using a temperature ramp of 20°C / min over a temperature range of 40-300°C. The instrument used, a Perkin-Elmer TGS-2 thermogravimetric analyzer coupled to an API3+ model Perkin-Elmer Biosystems Sciex atmospheric pressure ionization mass spectrometer, has been described previously.…”

“…9 However, with increasing wafer throughput and tightening requirements for imaging precision, the efficacy of the purge hood may no longer be sufficient. 12,13 In order for a useful and realistic maximum outgassing limit to be set to protect the optics, both resist chemists and lithographic tool designers would like to know which species outgas during exposure of a given photoresist, the characteristic time frame for desorption relative to the exposing laser pulse, which gives the rate of outgassing and the peak intensity of the desorbing flux, and which chemical species reach the optics and condense there. 3 Good resist polymer design can minimize the outgassing risk for Si; 10,11 however, emission of PAG fragments containing S during exposure is unavoidable since modern PAGs are sulfonic acid based.…”

“…13 It has the advantage of not requiring separate calibrations because the unexposed resist acts as a standard. 13 It has the advantage of not requiring separate calibrations because the unexposed resist acts as a standard.…”