Resist photoactive compound

The thermal decomposition of the resist photoactive compound or radiation-sensitive components during prebake is modeled using first-order kinetics this often results in a change in the resist s optical properties, mostly the Dill A and B parameters. ... 

Figure 17 shows the sensitivity curves of the resist containing PM-5(run-6)(75 weight%) and 5-naphthoquinone diazide sulfonate of 2,3,4-trihydroxybenzophenone(25 weight%). The average esterification ratio of the photoactive compound was 2.5 units per three OHs in the benzophenone. The y value was 1.6 and the sensitivity was 90mJ/cm2 when developed with 0.34% TMAH aqueous solution for 120 sec at 20°C. The resist containing M-5(run 3)... 

The positive resist materials evolved from discoveries made by the Kalle Corporation in Germany who developed the first positive-acting photoresist based on the use of a novolac matrix resin and a diazoquinone photoactive compound or sensitizer. The original materials were designed to produce photoplates used in the printing industry. These same materials have been adopted by semi-conductor fabrication engineers and continue to function effectively in that more demanding application. 

At the present time, most of the positive photoresists used in the manufacture of microcircuits consist of a low molecular weight phenolic resin and a photoactive dissolution inhibitor. This composite system is not readily soluble in aqueous base but becomes so upon irradiation with ultraviolet light. When this resist is exposed, the dissolution inhibitor, a diazoketone, undergoes a Wolff rearrangement followed by reaction with ambient water to produce a substituted indene carboxylic acid. This photoinduced transformation of the photoactive compound from a hydrophobic molecule to a hydrophillic carboxylic acid allows the resin to be rapidly dissolved by the developer. (L2,3)... 

The most popular positive resists are referred to as DQN, corresponding to then-photoactive compound (DQ) and base material (N), respectively. DQ stands for diazo-quinone, and N stands for novolac resin. The repeat unit of a novolac resin is shown in Figure 7.47. Novolacs normally dissolve in aqueous solutions, but combinations of... 

Apart from multi-level layer resist systems, conventional positive-tone resists can be classified into two categories one-component and two-component systems. Classical examples of the former systems are polyfmethyl methacrylate), and poly (butene-1-sulfone) (2,3). Typical examples of the latter system are AZ-type photoresists, which are mixtures of cresol-formaldehyde-Novolak resins and a photoactive compound acting as a dissolution inhibitor... 

A mixture of three isomeric cresols is used in a commercially available cresol-formaldehyde Novolak resin. This mixed Novolak resin, Varcum resin (12), provides adequate properties as a host resin for near-UV- and mid-UV-photoresist applications. Gipstein and his co-workers prepared pure cresol-formaldehyde Novolak resin from each isomeric cresol and compared their spectroscopic and resist characteristics (13). Their data on the UV-absorption spectra of each cresol-formaldehyde Novolak resin together with the commercially available Varcum resin are as follows the absorbances of 0.2 jim thick Novolak films at 250 nm are 0.165(Varcum), 0.096(o-cresol), 0.092(m-cresol), and 0.055(p-cresol). The so-called "window" in the UV absorption at around 250 nm is a maximum with the p-cresol-formaldehyde Novolak resin, while the other isomeric cresol and formaldehyde Novolak resins yielded similar UV absorptions at this wavelength. The smallest UV absorption at 254 nm is an advantage for the p-cresol-formaldehyde Novolak when the resin is used for a deep UV photoresist with a suitable photoactive compound (14). 

The structural variations of Novolak resins also influence how well they mix or form solid solutions with a dissolution inhibitor when resist films are cast onto substrates. This is a crucial problem for resist formulation. Usually, cresol-formaldehyde Novolak resins mix well with photoactive compounds like a... 

Formulation of Resist Solutions. Forty grams of a Novolak resin was mixed with 10 g of the photoactive compound, and dissolved in 100 g of bis-2-methoxy-ethylether. After wafers were spin-coated, the samples were immediately placed on a hot plate at 82 C for 14 min. The formulation procedure of a composite resist of poly (2-methyl-1-pentene sulfone) in the Novolak resin is as follows the polysulfone was mixed with the resin (13 wt% solid), and then dissolved in 2-methoxyethyl acetate the films were spin-coated onto silicon wafers, and then baked at 100°C for 20 min prior to electron beam exposure. 

The preparation method is similar to that for the cresol-formaldehyde Novolak resin with a molar ratio of cresol/benzaldehyde = 1.1 in acidic conditions. We have prepared varieties of substituted m-cresol-benzaldehyde Novolak resins, and 1-, and 2-naphthol-4-hydroxybenzaldehyde Novolak resins in the same manner. Almost all of these benzaldehyde Novolak resins give excellent resist films when spin-coated onto silicon or silicon dioxide substrates after being dissolved, together with a photoactive compound, in a solvent like 2-... 

Typical resist images obtained after a mid UV-exposure with an UV3 Perkin Elmer Exposure System are shown in Figure 5 using the m-c resol-benzaldehyde Novolak and the diester photoactive compound. The experiments were not carried out under optimum conditions, and the exposure dose at 313 nm was 500 mJ/cm2. Although this resist system was not the fastest one at this wavelength region, it clearly provides usable patterns. 

Figure 5. Resist images made of m-cresol-benzaldehyde Novolak resin with a photoactive compound (15 wt%) 500 mJ/cm2 at 313 nm, and 70 sec in (1 4.5)...

These are naphthalene dlcizoquinones as photoactive compounds, benzo-triazoles as development enhancement agents, and dyes to reduce exposure from reflected light. The latter compounds have been added to resist formulations which already contain a photoactive compound in addition to the polymer binder. 

Most resists are designed for exposure at wavelengths longer than the 248.4 nm radiation provided by a KrF laser source. Wolf and coworkers O) have found that the choice of a positive resist for use at thTis wavelength is limited. They evaluated a number of positive resists. Only Microposit 2415, and its newer analog, Microposit 2400-17, were compatible with the anticipated exposure time of 0.5 to 1.0 seconds for resist sensitivity of 100 to 200 mj/cm needed with the new exposure tool developed by Pol and coworkers. (O The resist consists of three components a resin, a photoactive compound or PAG (which acts as a dissolution inhibitor), and a solvent. Upon exposure, the PAC is destroyed, and this allows the resist film to dissolve in the aqueous basic developer. 

The resist parameters A, B and C are normally determined from optical absorbance measurements of exposed and unexposed resist films. ( ) The B parameter is obtained from the absorbance of a film that is given a sufficient exposure dose to destroy all the photoactive compound. The quantity A+B is obtained from the absorbance of an unexposed resist film and C is determined from the initial slope of an absorbance versus exposure time plot. 

Material. The resist material photoactive compound (PAC), a diazonaphthoquinone derivative in a novolak resin matrix, was used as a control and also reformulated to obtain PAC/pyrene. Pyrene was obtained from Aldrich and was added to the PAC in this study in the quantity of 2.5% of the total solids content of the resist. The resists were mixed on a roller overnight and then filtered through a 0.8pm silver membrane filter. 

The photoresist is sensitive to the incident radiation and it undergoes (photo) chemical transformations. Photoresists are complex formulations consisting of organic solvent, polymer, photoactive compound (PAC), base, and other chemicals that confer it desired properties. In one-component resists, the polymer is the photoactive compound, whereas in two- or multicomponent resists the photoactive compound undergoes photochemical transformation resulting in new species that interact with the radiation inert polymer, triggering transformations that alter the solubility in the exposed areas. 

The resist system which supported the i-line technology for many years in an exclusive manner was so-called diazonaphthoquinone (DNQ)/novolac resists (Fig. 5). This type of resists originally invented for printing by Suss [2] is a two-component system consisting of a novolac resin and a photoactive compound (PAC), diazonaphthoquinone. The novolac resin is soluble in aqueous base in virtue of the acidic phenolic OH functionality. However, the lipophilic diazonaphthoquinone dispersed in the phenolic matrix inhibits the dissolution of the resin film in an aqueous base developer. UV irradiation of the photoactive compound results in formation of a highly reactive carbene, accompanied... 

The resist sensitivity is a very important parameter to be considered as it is directly related to wafer throughput and therefore device manufacturing cost. Thus, sensitivity enhancement was a primary research activity in the field of microlithography in the 1970s and early 1980s. However, the enhancement of sensitivity achieved at that time was too incremental and marginal. Quantum yields, expressed as the number of molecules transformed per photon absorbed, characterize the efficiency of photochemical events. Typical diazonaphthoquinone has a quantum yield of 0.2-0.3, which means that three to five photons are needed to convert a single molecule of the photoactive compound. An in-... 

Following exposure, the exposed resist film may be baked again, causing the catalytic photoacid generated from the photoactive compound contained in... 

It should be emphasized that one-component resists consisting of pure radiation-sensitive materials, which in modern times are comprised primarily of polymers that combine all of the necessary attributes of a resist, have now fallen out of favor. In contrast, modern advanced lithography relies almost exclusively on the multicomponent design concept in which resist functions are provided by separate components, comprising the resin/bmder on the one hand, and the photoactive compounds on the other. In these multicomponent systems, the resins/ binders are polymers and are typically inert to radiation, but can undergo radiation-induced reactions initiated by the photoactive components of the resists. ... 

Several negative resists based on polymeric resin binders and azides as the photoactive compound have also been reported. These include resists based on poly(vinyl phenol) as the polymeric resin and various mono-azides as the photosensitive component such as 4-azidochalcone and its homologs (XIV, XV, XVI). ... 

A review of synthetic methods for the most popular photoactive compounds used in diazoquinone resists—DNQ-5-sulfonate and DNQ-4-sulfonate—has been provided by Ershov et al. The synthesis typically begins with naphthalene derivatives, and proceeds via introduction of a sulfonic acid group, followed by diazotization and reaction with thionyl chloride to yield the sulfonic acid chloride (Scheme 7.2). In the next step, the chloride is reacted in a base-catalyzed esterification with a suitable ballast group or backbone, which usually is a multifunctional phenol, less frequently a monofunctional phenol or an aliphatic alcohol. ... 

Excimers (excited dimers) are formed by pairs of molecules or atoms that do not signihcantly interact in the ground state, but are weakly bonded in the excited state. The bonding in the excimer takes place between an excited molecule and a ground state molecule of the same species. Its origin is in the change of orbital symmetry that accompanies excitation and leads to cooperative (positive) orbital overlap and hence to bonding between the two systems. Examples in resist systems can be found in aromatic and heteroaromatic molecules used in photoactive compounds. Excimers were hrst observed by Eorster and Kasper in 1954 when they observed two kinds of fluorescence in fairly concentrated solutions (10 M) of pyrene. ... 

For a positive resist based on dissolution inhibition chemistry such as the DNQ/novolac resist system, the rate of the reaction tr between the developer with the resist (comprised of resin R, a photoactive compound that acts as a dissolution inhibitor M, but which is converted to product P on exposure to UV light, which in turn enhances the dissolution rate of the resin) is given by... 

C.A. Mack, Photoresist optimization, in KTI Microlithography Seminar Proc., pp. 153 167 (1987) P. Trefonas and C. A. Mack, Exposure dose optimization for a positive resist containing poly functional photoactive compound, Proc. SPIE 1466, 117 131 (1991). 

In order to understand the changes, it is necessary first to know the components of the resist and the chemistry of the exposure step. For positive photoresists, the mechanism was elucidated by Pacansky and Lyerla (14). Typical resists are a solution of a naphthoquinone diazide photoactive compound (PAC) and a cresol formaldehyde novolak resin in one or more high boiling point industrial solvents. The number average molecular weight of the resin is quite low, on the order of 1000, with a polydispersity of about 10 (13). During the normal exposure step, the PAC, in the presence of water, absorbs light and is transformed into a... 

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