Another area of great interest for chemically selective imaging is that of chemically amplified polymer photoresists, used in the manufacturing of patterned semiconductor chips for electronics and computing. These photoresists are composed of two similar polymers in a pattern with few hundred nanometer spatial domains of each component. It would be advantageous to have a technique that could analyze these patterned photoresists quickly and easily for quality control purposes. Narrowband CARS microscopy has been used successfully to image single spectral peaks across model polymer photoresists (15), but to quantify multiple materials within the resist at the same time, a broader vibrational spectrum is desirable at each point in the microscope image.

CARS

CARS is a three-photon scattering process in which two intense laser fields, called the pump and Stokes fields, interact with sample molecules to induce coherent vibrations, and subsequent interaction with a probe pulse then results in the emission of a coherent fourth field at a higher frequency. This signal is strongly enhanced when the energy difference between the pump and Stokes pulses matches the energy of an intrinsic molecular vibrational level. Typically, the pump and probe pulses and the Stokes pulse are provided by two separate, synchronized picosecond lasers or two beams produced by a single laser. Single vibrational resonances can be probed one at a time with narrowband CARS microscopy if the difference between the pump and Stokes beams from the picosecond (narrowband) lasers is tuned to match these vibrational frequencies (18).

Figure 1: Energy level diagram of the multiplex CARS process using broadband pump and Stokes fields with a narrowband probe field.

By incorporating a broadband laser into this scheme, such that the broadband laser provides multiple Stokes frequencies simultaneously and a narrowband laser provides the pump and probe beams, a broad spectrum can be acquired simultaneously (multiplex CARS) without tuning over the individual resonances (Figure 1) (4,6,18). However, due to the high peak-power laser field applied to the sample, in addition to this vibrationally resonant CARS signal, purely electronic third-order frequency mixing processes in the sample also lead to a large nonresonant background across the entire spectrum (2,18,21). Because this nonresonant background results from the interaction of the laser field with the sample of interest, creative manipulations are necessary for suppression or elimination of this nonresonant background so that the much smaller intensity resonant spectrum can be obtained (1,3,4,13).

With phase and polarization pulse-shaping techniques, multiplex CARS spectroscopy can be accomplished with a single laser beam from an ultrafast laser, eliminating complications associated with tuning, timing, and alignment between two separate beams, and significantly reducing or eliminating the nonresonant background (1). The whole array of pump and Stokes frequencies is provided simultaneously by the broadband pulse, and the resulting coherent molecular vibrations are probed by a narrow spectral part of the total bandwidth within the same laser pulse. The probe part of the beam is differentiated by shifting the phase and polarization of that narrow spectral region of the laser pulse. Fingerprint-region CARS spectra with up to 700 cm^–1 of bandwidth have been obtained previously using single-pulse methods (1,3). With a variation of a pulse-shaping and signal-acquisition scheme developed recently in our laboratory (21), sensitivity improvements have been achieved that allow spectral images to be obtained in a shorter acquisition time with up to 1000 cm^–1 of bandwidth per spectrum (22).