Lithographic response

Figure 2 Typical lithographic response (contrast) curves for (a) positive and (b) negative resists.

Figure 3. Typical lithographic response or contrast plots for (a) positive resists and (b) negative resists in terms of the developed thickness normalized to initial resist thickness (p) as a function of log (dose).

We have seen that two important lithographic parameters of a resist are sensitivity and contrast. This leads to a consideration of the design features that must be incorporated into the resist in order to optimize these parameters and, in turn, requires a fundamental understanding of the interaction of radiation with matter and how the polymer molecular parameters affect lithographic response. These aspects have been extensively covered in the literature, (5,6) and only the conclusions relating to lithographic performance will be summarized. 

A variety of intermediate layers have been used. In addition to Si02, silicon, Si3N4, Ge and Ti have been used. In spite of the attractiveness of trilevel processing, viz., that it allows the imaging layer to be chosen purely on the value of its lithographic response rather than pattern transfer capabilities,... 

The critical properties of GMC-II produced from both laboratory and pilot systems are listed in Table 5. In comparing a 0.055-kg batch (62.8% monomer conversion) from laboratory synthesis and a 1.16-kg batch (54.1% monomer conversion) from the pilot synthesis, the material properties responsible for lithographic response are nearly identical. 

Figure 6. Lithographic response curve for sample 7Q (e-beam at 20 keV). Initial thickness 3300 A developed 30 s in 1 1 MEK/MIBK A 90 s in 1 3 MEK/MIBK.

Deep UV lithographic response curves for the graft copolymers as a function of graft MW at constant composition (25-32%). PMMA developed 45 s in MIBK. 

A variety of alternating copolymers based on H-allyl- and N-(3-ethynylphenyl)maleimides, with substituted styrenes and vinyl ethers, have been prepared and their response to x-ray irradiation studied. Broadband and monochromatic x-ray exposures were conducted at the Stanford Synchrotron Radiation Laboratory. Sensitivities were observed to correlate with mass absorption coefficients of the copolymers and were found to be as high as 5-10 mJ/cm2. Preliminary fine line lithographic studies indicate 0.5 ion resolution capabilities. 

There are two aspects of resist sensitometry 1) the measuiement of resist response to radiation which involves measurements designed to determine the intrinsic radiation sensitivity of the materials from which the resist is constituted and 2) lithographic sensitivity which is a measure of the efficiency with which these radiochemical conversions can be expressed in relief image formation. 

Sensitivity. Sensitivity is conventionally defined as the input incident energy (measured in terms of energy or the number of photons or particles (fluence) per unit area) required to attain a certain degree of chemical response in the resist that results, after development, in the desired relief image. This represents an operational, lithographic definition of sensitivity. 

Lithographic Characterization. Electron-beam exposures were conducted on an EBES system operating at 20 kV, with a beam address and spot size both equal to 0.25 ym. Electron response parameters were evaluated using linewidth control patterns. P(SI-CMS) was spray developed after exposure using an APT Model 915 resist processor in toluene-methanol (1 1) for 30 sec followed by a methanol rinse for 45 sec. Aqueous solutions of tetramethylammonium hydroxide (TMAH, 25% in water, Fluka Inc.) were used for the novolac resist development. Exposed films were dip-developed for 20 sec. in TMAH-water (1 2.5) solutions. 

Lithographic Characteristics. The exposure response curves for P(SI-CMS) and Sl-novolac containing PMPS (SI-NPR) are shown in Figure 4, and their lithographic characteristics are summarized in... 

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