Dissolution rate unexposed resist

Positive-Tone Photoresists based on Dissolution Inhibition by Diazonaphthoquinones. The intrinsic limitations of bis-azide—cycHzed mbber resist systems led the semiconductor industry to shift to a class of imaging materials based on diazonaphthoquinone (DNQ) photosensitizers. Both the chemistry and the imaging mechanism of these resists (Fig. 10) differ in fundamental ways from those described thus far (23). The DNQ acts as a dissolution inhibitor for the matrix resin, a low molecular weight condensation product of formaldehyde and cresol isomers known as novolac (24). The phenoHc stmcture renders the novolac polymer weakly acidic, and readily soluble in aqueous alkaline solutions. In admixture with an appropriate DNQ the polymer s dissolution rate is sharply decreased. Photolysis causes the DNQ to undergo a multistep reaction sequence, ultimately forming a base-soluble carboxyHc acid which does not inhibit film dissolution. Immersion of a pattemwise-exposed film of the resist in an aqueous solution of hydroxide ion leads to rapid dissolution of the exposed areas and only very slow dissolution of unexposed regions. In contrast with crosslinking resists, the film solubiHty is controUed by chemical and polarity differences rather than molecular size. 

Figure 8 shows the effect of the alkaline concentration in TMAH solutions on the contrast and sensitivity of the new resist. Sensitivity of the resist increases as the alkaline concentration increases, however, the contrast is maxima (4.72) at 0.83% TMAH solution. This means that the higher concentration over 0.83% cannot distinguish the difference of the dissolution rate between the unexposed and exposed resist film. For instance, the higher concentrated developer also attacks the exposed areas and the loss of resist thickness occurs. The alkaline concentration in TMAH solution, therefore, is optimized at 0.83%. This developer concentration was subjected to the following lithographic evaluation. 

An attempt to formulate a poly(4-hydroxy styrene)-based resist was less than completely successful because the difference in the rates of dissolution were too small to be used to give high contrast images. Other small molecules were added to the NDS/novolac resist and these were also found to have a profound effect upon the performance of the resist, particularly the development properties. When it was necessary to obtain higher dissolution rates, several triazoles and sulfonamides were found to improve the rate of development in the exposed areas without causing unacceptable thickness losses in the unexposed areas. Dyes incorporated to minimize problems of reflection and scattered light were also found to alter the dissolution behavior of the resist coating. 

The rate of development is always faster for the pHOSt but the discrimination between the exposed and the unexposed resists is smaller. Comparison of the dissolution rate curves for novolac and pHOSt in Figure 1 gives a good view of the difference in response of the two resist systems. It is difficult to formulate the pHOSt into a simple resist because there is too much thickness lost in the unexposed areas when a developer is used that is appropriate to develop the exposed areas in a reasonable time. 

The ultimate test of usefulness is, of course, the lithographic performance of a resist. The often-quoted sensitivity of PMMA of 50 pC/cm2 assumes conventional developing conditions and is measured by competitive dissolution rates of exposed and unexposed films. A secondary measure of sensitivity for positive-working resists is the G(s) value, the yield of chain scissions per 100 e.v. of absorbed energy. In this case, mea-... 

As stated earlier, photolysis of the DNQ-sulfonate ester PAC in novolac resins increases the solubility and the rate of development of the exposed areas of the latter in the aqueous base solution used for development relative to the unexposed areas (see Figs. 7.5 and 7.6). The difference between the dissolution rate of the unexposed area and the exposed area increases with DNQ loading. Good developer discrimination between the exposed and unexposed areas in this resist system produces high-contrast, high-resolution images (see Section 7.2.1.6). 

The model in Eq. (12.112) does not take into account the finite dissolution rate of unexposed resist r in that is independent of the exposed PAC. A common approach is simply to add this term to Eq. (113), giving... 

In a recent paper W.D. Hinsberg(4) demonstrated that the Nan-ion concentration plays a key role in the dissolution rate of both exposed and unexposed photoresist. While the exact machanism of this effect is not yet known, it was possible to obtain a function dissolution rate equation rate for unexposed resist(4) ... 

In order to confirm the effect predicted from the equation obtained by utilizing of data on sodium hydroxide/sodium chloride solutions, experiments were performed with the buffered Microposit series of developers (Na OH solution with NajBOj buffer) to obtain the Re (dissolution rate of exposed resist) and Ru (dissolution rate of unexposed resist). The results of these measurements are shown in Figures 6, 7, 8 as function of sodium chloride concentration. It can be seen that the effect of the molar sodium chloride solution is to enhance the dissolution rate of both the exposed and unexposed photoresist areas. When the ratio of Re/Ru is obtained (Table I), it is clear that the 5 1 developer dilution has significantly higher contrast and that the ratios are roughly constant for all sodium chloride concentration studied. 

Figure 8. Dissolution rate vs. NaCl concentration for an unexposed resist with a Microposit 351 developer concentration of 5 1 and 10 min in the developer.

The concept of acid labile crosslinked units in polymers for positive DUV resist was evaluated with poly-(4-hydroxystyrene/4-vinylcyclohexanol) and implemented on a poly-4-hydroxystyrene backbone. The principle to increase the molecular weight by a transacetalization reaction was used to improve acetal protected phenolic resins for advanced chemically amplified resists. The polymer chains were linked via diols with acetal bridges. This type of linkage can be applied to a wide range of phenolic polymer binders. Resolution and thermal stability of the photoresist patterns were improved. Gel permeation chromatography and dissolution rate measurements prove that the crosslinked units remain essentially intact in the unexposed areas, thus enhancing... 

The dissolution rates of the la-lc-S02 terpolymer films were also measured under the same conditions (bake 130 °C for 60 sec, developer 0.21 N TMAH) and plotted in Figure 5 as a function of the terpolymer composition. As the norbomene Ic bearing a butyl ester group is introduced into the polymer, the dissolution rate becomes exponentially smaller. The dissolution rate for la/lc=l/l is ca. 170 A/sec and the terpolymers with more than 55 mol% butyl ester do not dissolve at all in this developer. It thus appears that the concentration of the fluoroalcohol unit in the terpolymer must be below 45 % for an unexposed resist film to provide minimum thinning in the industry-standard 0.26 N TMAH solution. 

Stonewall Model. In the Stonewall Model, Hanabata etal (9) showed using gel permeation chromatography (GPC) that multifunctional DNQ can crosslink the novolac via a base catalyzed azo-coupling reaction (Figure 1). They explained that this reaction increases the polymer molecular weight, which results in a decrease in the dissolution rate in the unexposed areas of the resist film. 

In its turn, formation of surface relief in the process of selective dissolution of amorphous chalcodenide films is well known. The phenomenon is based on difference in dissolution rate of the exposed and unexposed areas of chalcogenide film surface in alkali developers (Teteris, 2002). This phenomenon of photo-induced changes in the dissolution rate of a large group of amorphous chalcogenide semiconductor films was the basis for an extensive development of a new class of inorganic resists. 

The main components of most commercially available positive photoresists are novolak as a binder and naphthoquinone-diazdde as a light-sensitive component. This light-sensitive compound is not base soluble and acts as a dissolution inhibitor for the novolak, which results in a very low dissolution rate of unexposed resist in aqueous base developer. Upon exposure a reaction is induced to yield indene carboxylic acid via a ketene ... 

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