Sensitivity and contrast

The sensitivity, o, is related to the ability of a polymer to undergo structural modification on irradiation. 

Sensitivity is said to increase as the dose required to produce the lithographic image decreases. The sensitivity of a positive resist, Og, is the dose required to achieve complete solubility of the exposed region under conditions where the unexposed region remains completely insoluble. The sensitivity of a negative resist is conventionally defined as the dose at which 70% of the original film thickness has been retained after development, Oq 7. The required sensitivity varies with the type of irradiation and is expressed as energy/surface (e.g., J/m ). 

For narrow-UV radiation, values of the doses for the i- and g-line vary from 10-50 mj/cm and for deep-UV, from 10-20 mj/cm - For electron beam lithography, this dose is expressed as a current density around 5-10 mC/cm.  

Negative resists are in general more sensitive than positive resists but they exhibit a lower contrast (y l) (Table 6.2). A contrast higher than 3 is generally required for the high circuit density of new generation technologies such as the 256 Mb DRAM. 

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Both characteristic X-ray line and continuous spectra were used to evaluate the performances of the resists. To determine exposure parameters (i.e. sensitivity and contrast) irradiations were carried gut in this study using the aluminum Kot- 2 emission line at 8.3t A generated by means of a modified Vacuum Generators Limited model EG-2 electron beam evaporation gun. The resist samples were exposed through a mask (A) consisting of a range of aluminum foils of different thicknesses supported on an absorbing nickel frame in order to vary the X-ray flux. 

The lithographic performance, as measured by sensitivity and contrast for the TBS, TBMS and TBSS polymers containing two onium salt AG systems and the tosylate AG is given in Table V. The absorbance of these films at 248nm was -0.3 im-1 in each case. 

Sensitivity and Contrast Measurements. Imagewise exposure of films of the various copolymers containing from 5-10% triphenylsulfonium hexafluoroantimo-nate to UV light at 254 nm resulted in the crosslinking of the exposed areas as shown in Figure 4. 

Table 1 Lithographic sensitivity and contrast data for various copolymers...

Lithographic Characteristics. Based on the potential of crosslinker 3 to show high sensitivity and contrast and wide process latitude, it was of interest to evaluate its lithographic capability, using crosslinker 1 as the standard for comparison. Crosslinkers 1 and 3 (equal weight) were each incorporated into otherwise identical experimental AHR resist formulations. E-beam exposures were performed so that differences in DUV absorbance characteristics of the crosslinkers could be ignored. The e-beam sensitivities of the resists containing crosslinkers 1 and 3 were 6.2 and 4.2 (lC/cm2,... 

A higher sensitivity of SPP can be obtained using a more dilute TMAH solution, but at the expense of lower contrast. A solution more dilute than 0.6 wt% cannot completely dissolve the resist. The SPP exhibited a higher sensitivity and contrast than the novolac-based resist. 

Optimization of both resist sensitivity and contrast requires a fundamental appreciation of the radiation chemistry in addition to appreciation how polymer molecular parameters affect the lithographic behavior of the resist. The intent of this chapter is to further the readers understanding of the polymer and radiation chemistry that is associated with a large part of the microelectronics industry, and provide some of the necessary background to effect future developments. 

Effect of Proton Beam Energy on the Sensitivity and Contrast of Select Si-Containing Resists... 

TABLE II. Polymer and Processing Parameters that Affect Sensitivity and Contrast of a Resist... 

TABLE III. Hardware Parameters that Determine the Sensitivity and Contrast Requirements of a Resist. 

Sensitivity and contrast are conveniently measured experimentally by exposing areas of resist of known size to varying radiation doses and measuring the film thickness remaining after development for each area. In the case of negative resists, gel is not formed until a critical dose, denoted as the interface gel dose Dp, has been reached. At this dose no lithographi-... 

For a positive resist, the film thickness of the irradiated region after development decreases until eventually a critical dose Dp is reached which results in complete removal of the film 8,9). The sensitivity and contrast (7p) are evaluated in a manner similar to that for a negative resist. After they have been spin-coated and prebaked, a series of pads of known area are exposed to varying doses. The substrate is developed in a solvent that does not attack the unexposed film and the thickness of the film remaining in the exposed areas measured. The film thickness is normalized to the original thickness, and this value is plotted as function of log dose, as shown in Figure 5 where Dp represents the sensitivity of the positive resist. Contrast (7p) is determined from the extrapolated slope of the linear portion of the response curve as... 

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. 

Figure 14. Sensitivity and contrast plot for electron irradiation of Ge-Se inorganic resist. (Reproduced with premission from Ref. 109.)...

Electron Beam Microliihography Good sensitivity and contrast, acceptable plasma etching resistance less scattering of electrons aiul therefore better resolution negative and positive resists possible. 

These limitations of sensitivity and contrast have led researchers to develop hyperpolarization methods, where the nuclear polarization is greatly increased by the manipulation of spin states. Currently, the most general and popular hyperpolarization method is dynamic nuclear polarization (DNP), where the much higher polarization of an unpaired electron is transferred to the nuclei of interest. DNP was originally... 

In the case of a photoresist, the ultimate definable feature size together with the ability of the material to withstand either chemical etchants or plasma environments determines the domain of utility. The feature size is in turn determined by the wavelength required for exposure, the sensitivity and contrast of the resist, and the dimensional stability of the material during exposure, development, and subsequent processing. Adhesion of the resist to the substrate is critical both for patterning and use, and adhesion can be affected by surface preparations, and by residual stresses developed during deposition and cure. While photo-imagable polyimides have been introduced, their principal intended application is as a component of the finished part, either as passivant or interlevel dielectric (see below). 

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