Negative tone resists

Acid-C t lyzed Chemistry. Acid-catalyzed reactions form the basis for essentially all chemically amplified resist systems for microlithography appHcations (61). These reactions can be generally classified as either cross-linking (photopolymerization) or deprotection reactions. The latter are used to unmask acidic functionality such as phenohc or pendent carboxyhc acid groups, and thus lend themselves to positive tone resist apphcations. Acid-catalyzed polymer cross-linking and photopolymerization reactions, on the other hand, find appHcation in negative tone resist systems. Representative examples of each type of chemistry are Hsted below. 

The use of phenolic polymers in photocrosslinkable systems usually involves multicomponent systems which incorporate polyfunctional low molecular weight crosslinkers. For example, Feely et al. [9] have used hydroxymethyl melamine in combination with a photoactive diazonaphthoquinone which produces an indene carboxylic acid upon irradiation to crosslink a novolac resin. Similarly, Iwayanagi et al. [10] have used photoactive bisazides in combination with poly(p-hydroxy-sty-rene) to afford a negative-tone resist material which does not swell upon development in aqueous base. 

Note 3 In a positive-tone resist, also called a positive resist, the material in the irradiated area not covered by a mask is removed, which results in an image with a pattern identical with that on the mask. In a negative-tone resist, also called a negative resist, the non-irradiated area is subsequently removed, which results in an image with a pattern that is the complement of that on the mask. 

Positive-tone Resist Negative-tone Resist... 

The majority of organosilicon polymers crosslink efficiently upon e-beam exposure (5) and thus behave as negative-tone resists. Poly(alkenylsilane sulfone) s, on the other hand, have been found to degrade easily upon e-beam exposure, which places them in a small class of positive... 

Figure 4.28. Molecular structures and photoinduced reactions of common photoresists. Shown (top) is the positive tone resist containing the active diazonapthoquinone (DNQ) chromophore group. Chemical amplification (CAM) reactions are illustrated in (i)-(iii). Reaction (i) represents photoinduced acid generation step (ii) is an acid-catalyzed deprotection mechanism (positive tone resist) and step (iii) is an acid-catalyzed crosslinking mechanism (negative tone resist).

Chart 2.1. The negative-tone resists that were first used in semiconductor manufacturing were based on a matrix resin of synthetic rubber prepared by Ziegler-Natta polymerization of isoprene followed by acid-catalyzed cycliza-tion to improve the mechanical properties. This cyclized rubber was rendered photosensitive by addition of a bisarylazide that undergoes photolysis to produce a bisnitrene. The nitrene reacts with the cyclized rubber to create in-termolecular cross-links that render the exposed areas insoluble. 

The chemistry of the DQN materials has been modified such that they function as a high-resolution, high-contrast, negative-tone resist system that is as devoid of distortion due to swelling as the standard, positive DQN system 14, 15). This tone reversal of the DQN system is accomplished by addition of an appropriate base to the formulation. 

Sometimes, the last reaction is used to obtain a negative-tone resist mask by means of the DNQ-novo-lac positive photoresist.Imidazole, triethanol amine, and other bases can be used as tone-modifiers. 

Acid Catalyzed Rearrangements. An example of a photoresist based on an acid catalyzed rearrangement is the diaryliodonium salt photoinduced cyclization of cis-1,4-polyisoprene shown in Equation 18. This facile cyclization which has been reported previously (11) by non-photochemical processes results in a polycyclic polymer whose physical properties and solubility characteristics are considerably different than the initial polymer. Exploitation of these differences in the exposed and unexposed regions of the polymer film permit their use as either positive or negative tone resists. 

Figure 8. Negative tone resist patterns obtained with a matrix resist of poly(2-methyl-l-pentene sulfone) and Varcum resin.

Schedli, N. Muenzel, and H. Holzarth, 1,3 dioxolyl acetals as powerful crosslinkers of pheno lie resin, Proc. SPIE 1925, 109 (1993) W. S. Huang, K.Y. Lee, R.K. J. Chen, and D. Schepsis, Negative tone resist system using vinyl cyclic acetal crosslinker, Proc. SPIE 2724, 315 (1996). H. Ito, Chemical amplification resists for microlithography, Adv. Polym. Sci. 172, 155 (2005). 

Scheme 6.30 AII-supercritical-C02-processable (polymerization, casting, and development) negative-tone resist.

Other characteristics of DNQ/novolac resists that have contributed to their lasting success in the semiconductor industry include their high etch resistance and the fact that they can be developed in environmentally benign aqueous base developers. In addition, the cyclized rubber/bis-azide negative tone resists did not image well at the Hg g-line, the exposure wavelength of the earliest commercially available wafer steppers, the introduction of which effectively brought about the complete and wholesale conversion of the IC industry to novolac resists from the cyclized rubber/bis-azide. ... 

Figure 12.4 Contrast curves for (a) positive-tone and (b) negative-tone resists. The intercept of the curve and abscissa in positive-tone resists is called the dose to clear" and is designated as Do, while in negative resists, it marks the onset of cross-linking, and is designated as Dq. This should not be confused with the lithographic dose to print, which tends to be approximately 1.6-2.2 times higher. The absolute value of the slope of the tangent to the contrast curve at its intercept with the abscissa is defined as the resist contrast. It is usually defined in terms of an auxiliary dose value Di, which is obtained by continuing the above tangent line to the full resist film thickness (normalized to 1.0).

The conventional bilayer resist systems in which the top imaging layer (typically organosilicon polymer) also serves as an etch mask was first proposed by Hatzakis et al. in 1981, ostensibly for electron-beam lithography. Since then, a number of organosilicon resists for bilayer resist systems have been reported for use in near-UV, DUV, mid-UV, electron-beam, and x-ray applications, a good review of which has been provided by Ohnishi et al. In recent times, negative-tone resist systems and processes based on silicon-backbone polymers such as polysilanes,polysilynes, and plasma-deposited polymers have been developed for 193-nm lithography. 

The type of photoresist being used (e.g., positive-tone resist, negative-tone resist, diazonaphthoquinone-novolac resist, chemically amplified resist, single-layer resist, bilayer resist, etc.)... 

Semiconductor manufacturing applications, using 1.0 nm X-rays as the exposure source, require resist sensitivities on the order of 50-100 nJ/cm in order to meet the desired throughput. Conventional resists, where an interaction between the exposing radiation and resist directly defines sensitivity, have not been able to meet this need. Attention has turned to chemically-amplified resists where the exposure creates a species which catalyzes multiple chemical events during the post-exposure bake(PEB). There are a variety of resist systems that can demonstrate chemical amplification, but Shipley SAL 605 - a negative-tone resist showing sensitivities on the order of 100 nJ/cm2 - is the system chosen for this quantitative study. 

Analytical Techniques Used for the Quantitation. The specific details of the analytical techniques used in these studies and of the optimized processing conditions are described in a previous publications (4-5) and will only be reviewed here. The X-ray exposures were done on a negative-tone resist, SAL 605 (The Shipley Co.). 

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