Diazoquinone-novolac resist

Diazoquinone-novolac resist, 506 Diazotization reaction, 941 DIBAH, see Diisobutylaluminum hydride... 

For conventional positive diazoquinone/novolac resists (Section 7.2.3), the exposure bleaches the photoactive compound (NDS) and therefore the problem of reflection becomes more severe. Three approaches to solving this problem all involve putting absorbing dyes in various layers of a multilayer configuration. Dyes may be placed directly in the photoresist layer, in the planarizing layer, or in a separate antireflection coating (ARC). The initial work with dyes in photoresists was in the ARC format, where a thin layer of dyed polymer is coated underneath the photoresist. The dyed layer may be directly on the substrate or on top of a planarizing layer. 

The polymer resist currently used in chip manufacturing is based on the two-component diazoquinone-novolac system. Novoiac resin is a soft, relatively low-molecular-weight poljnner made from methylphenol and formaldehyde, while the diazoquinone is a bicyclic (two-ring) molecule containing a diazo group (=N=N) adjacent to a ketone carbonyl (C=0). The diazoquinone-novolac mix is relatively insoluble when fresh, but on... 

The diazoquinone-novolac positive plates were only a moderate commercial success, but interest in the diazoqunone-novolac systems was revived in the 1970s when the resolution requirements of the semiconductor industry outpaced the capabilities of the negative bis-azide resists then in use. It was during this time that the diazoquinone resists established themselves as nonswelling, high-resolution imaging materials in the semiconductor industry. 

Figure 4 Schematic of the chemistry of diazoquinone/novolac-based positive resists...

Figure 2. Patterns in 50 nm Cr films employing 15 layers ( 30 nm) of LB novolac/diazoquinone resist, with (a) 10 wt% and (b) 20 wt% diazoquinone. The smallest arrays are nominally 1 pm lines-and-spaces.

Ultrathin LB PMMA and novolac/diazoquinone films have been demonstrated to act as high resolution electron beam and optical resists, respectively. Structural rearrangements in the LB PMMA films have been observed by using fluorescence spectroscopy. However, this rearrangement did not appear to influence the lithographic performance when seven or more layers of LB PMMA films were used as the resist. A more comprehensive study of the relationship between lithographic performance and LB film structure is currently underway. 

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. 

Wilkins and coworkers have redesigned both the sensitizer and the matrix resin (78-79). They have tested a variety of o-nitrobenzyl esters of cholic acid as sensitizers. These substances, like the diazoquinones, are insoluble in aqueous base but undergo a photo-reaction that yields base soluble products. The matrix resin chosen for the new sensitizer materials is a copolymer of methyl methacrylate and methacrylic acid that is far more transparent than novolac resins in the DUV. The new resist materials are reported to have useful sensitivity (ca. 00mJ/cm ) and extremely high contrast. The resist formulation is essentially aliphatic in nature and would be expected to be less stable to dry etching environments than the aromatic-based novolac resin materials (24). 

However, the photochemistry itself does not make a relief image. Rather it is used to modify the solubility of the polymeric binder. The diazoquinone compounds used in resists are referred to as dissolution inhibitors or photoactive components (PAC s). The addition of a diazoquinone molecule dramatically inhibits the dissolution rate of a thin film of a novolac resin. Upon exposure, the dissolution rate of the novolac based resist is considerably faster than the rate for the novolac alone. The accelerated dissolution rate may be caused by formation of acid eind its subsequent ionization during development or by enhauiced diffusion of the developer into the coating because of changes caused by the formation and fate of the nitrogen (2). 

Figure 5. Scanning electron micrograph depicting 1.0- xm images obtained in a silylated novolac-diazoquinone resist formulation after conventional exposure and development followed by pattern transfer by oxygen RIE.

Typical resists include cyclized polyisoprene with a photosensitive crosslinking agent (ex bisazide) used in many negative photoresists, novolac resins with diazoquinone sensitizers and imidazole catalysts for positive photoresists, poly(oxystyrenes) with photosensitizers for UV resists, polysilanes for UV and X-ray resists, and polymethacrylates and methacrylate-styrenes for electron-beam resists (Clegg and Collyer, 1991). Also note the more recent use of novolac/diazonaphthoquinone photoresists for mid-UV resists for DRAM memory chips and chemically amplified photoacid-catalysed hydroxystyrene and acrylic resists for deep-UV lithography (Choudhury, 1997). 

Although many types of compounds have been tested as sensitizers in phenolic host resins (Novolacs, Resols, etc.) (S), all commercial positive resists employ aromatic diazoquinones of some type which photochemically generate base soluble products via Wolff rearrangement initiated by the loss of nitrogen (6). A staggering variety of diazoketones have been synthesized and evaluated for lithographic purposes, but derivatives of J[ and 2 are most commonly employed (5). 

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