Gilbert D. Feke

Realization of numerical aperture 2.0 using a gallium phosphide solid immersion lens

We report a study of a gallium phosphide, hemispherical, solid immersion lens through the imaging of 40-nm-diam fluorescent dye balls. A spatial resolution as small as 139 nm has been achieved at a wavelength of 560 nm, which is equivalent to a diffraction-limited system of numerical aperture 2.0. This resolution is a 33% improvement over conventional oil immersion objectives and previously reported solid immersion lenses, which typically have a numerical aperture around 1.5.

Imaging of photogenerated acid in a chemically amplified photoresist

Chemically amplified photoresists are widely used throughout the semiconductor industry due to the need for high throughput in the lithography process. Knowledge of the location of acid molecules in chemically amplified resists is of paramount importance for the process control of the image formation of almost all the lithographic techniques used in the semiconductor industry today. We have demonstrated a technique based on pH-dependent fluorescence which can measure directly the spatial distribution of the photoacid in photoresist films without the need of any other subsequent process after exposure. By adding a small amount of fluorescent material to the resist, a latent image can be formed when photogenerated acid molecules quench the fluorescence in exposed areas. We demonstrate images formed after exposure of SAL605 resist films to x-ray radiation, with no post-exposure bake, and show that the optical contrast is a function of dose. We also show that the same technique can be utilized for rapid evalu...

On-wafer photoacid determination and imaging technique for chemically amplified photoresists

A fundamental task of chemically amplified photoresists is to record the incident radiation by generating catalyst concentration gradients within the film. In many resists, the catalyst is a strong Bronsted acid which yields a latent image of pH within the exposed film. A number of mechanistic questions remain about acid generator efficiency and its mobility once generated and heated. We have developed a technique in which a pH-dependent fluorophore is incorporated into the resist (an undyed version of SAL 605 from the Shipley Company and similar formulations). The localized acid concentrations generated by exposure to x-rays are analyzed and imaged using fluorescence spectroscopy and microscopy. Initial experiments, the spectroscopic apparatus, and initial far-field imaging are reported elsewhere [S. J. Bukofsky, G. D. Feke, Q. Wu, R. D. Grober, P. M. Dentinger, and J. W. Taylor, Appl. Phys. Lett. 73, 3 (1998)]. In this article, several fluorophores are evaluated, and various criteria for successful imag...

On-wafer spectrofluorometric evaluation of the response of photoacid generator compounds in chemically amplified photoresists

The enhanced photospeed of chemically amplified photoresists is crucial for high throughput lithography required by the semiconductor industry. The photospeed depends directly on the efficiency of the generation of photoacid during exposure, which is a function of the properties of the photoacid generator compound used in the resist. We report a novel technique for photoacid generator evaluation which is convenient, fast, and robust. This technique involves “whole wafer” imaging of resist doped with pH-sensitive fluorophores and patterned with an array of fields of varying doses. A spatially encoded map of the response of the compound under study is thus obtained in a single camera image. We measure the amount of photoacid produced as a function of dose for three photoacid generator compounds with reference to the commercial resist Shipley SAL 605. The results demonstrate a one-to-one correspondence with lithographic performance as determined by the normalized remaining thickness technique.

Evaluation of Photoacid Generators in Chemically Amplified Resists for X-ray Lithography using an On-wafer Photoacid Determination Technique

This paper reports an on-wafer photoacid determination technique that can be used to quickly screen materials that function as photoacid generators (PAGs). The technique includes adding a small amount of a pH-sensitive fluorophore into the resist and exposing the resist to x-rays. The acid generated during exposure reacts with the fluorophore and quenches the fluorescence. The efficiency of photoacid generation is evaluated by comparing the degree of fluorescence quenching. This technique is nondestructive, fast, and does not significantly change the resist chemical properties given the low concentration of the added fluorophore. Six compounds that can generate hydrogen halides as potential PAGs were evaluated using this on-wafer technique and the lithographic performance was evaluated for comparison. The commercial resist, Shipley SAL 605, is used as a reference for comparison. The result showed that TBBPA gave higher photoacid generation efficiency that TCBPA and PBP, but lower than that in SAL 605. The results of fluorescence measurements agree with the results obtained using normalized remaining thickness measurements. The advantages, however, of this fluorescence technique are that it is simple, fast, and requires fewer processing steps.

A new laser Doppler system for examining optic nerve head circulation.

A new laser Doppler system for the noninvasive examination of the human optic nerve head microcirculation is described. The electro-optical component of the system consists of a retinal camera modified with laser input optics and a fiber optic light detection system for collection of the Doppler-shifted scattered light. Data acquisition is carried out in real time under computer control. Automated analysis of the data provides a quantitative measure of the speed of blood cells flowing through the capillaries of the optic nerve head. Unlike previous systems, this system provides results within a few minutes following data acquisition. It is thus appropriate for use in a clinical setting. Analysis of multiple measurements on a patient shows a coefficient of variation of 8.9%.

Calibration and validation of projection lithography in chemically amplified resist systems using fluorescence imaging

Coumarin 6 (C6), a pH sensitive fluorescent molecule were doped into commercial resist systems to demonstrate a cost-effective fluorescence microscopy technique for detecting latent photoacid images in exposed chemically amplified resist films. The fluorescenec image contrast is optimized by carefully selecting optical filters to match the spectroscopic properties of C6 in the resist matrices. We demonstrate the potential of this technique for two sepcific non-invasive applications. First, a fast, conventient, fluorescence technique is demonstrated for determination of quantum yeidsl of photo-acid generation. Since the Ka of C6 in the 193nm resist system lies wihtin the range of acid concentrations that can be photogenerated, we have used this technique to evaluate the acid generation efficiency of various photo-acid generators (PAGs). The technique is based on doping the resist formulations containing the candidate PAGs with C6, coating one wafer per PAG, patterning the wafer with a dose ramp and spectroscopically imaging the wafers. The fluorescence of each pattern in the dose ramp is measured as a single image and analyzed with the optical titration model. Second, a nondestructive in-line diagnostic technique is developed for the focus calibration and validation of a projection lithography system. Our experimental results show excellent correlation between the fluorescence images and scanning electron microscope analysis of developed features. This technique has successfully been applied in both deep UV resists e.g., Shipley UVIIHS resist and 193 nm resists e.g., Shipley Vema-type resist. This method of focus calibration has also been extended to samples with feature sizes below the diffraction limit where the pitch between adjacent features is on the order of 300 nm. Image capture, data analysis, and focus latitude verification are all computer controlled from a single hardware/software platform. Typical focus calibration curves can be obtained within several minutes.

Calibration and validation of projection lithography focusing by fluorescence detection of latent photoacid images in chemically amplified resist

We dope the commercial resist UVIII with the pH sensitive dye coumarin 6 (C6) to demonstrate a fluorescence microscopy technique for detecting latent photoacid images in exposed chemically amplified resist films. The spectroscopic properties of C6 in the resist matrix are investigated in order to select spectroscopic filters which optimize the fluorescence image contrast of exposed patterns. We apply this technique to focus calibration of a projection lithography system and show a significant correlation of the results to scanning electron microscope analysis of developed features. Hence, we demonstrate the utility of this technique as a fabrication line diagnostic for focus calibration and validation (i.e., proof of focus) prior to postexposure processing of the resist film.