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HABI: Enabling Photo-Lithography in Electronics for 50 Years
March 19, 2013 |Estimated reading time: 7 minutes
Editor's Note: This article originally appeared in the December 2012 issue of The PCB Magazine.
Introduction to HABI
Hexaarylbiimidazole, better known as HABI, is an important photoactive material that rapidly initiates free radical polymerization. Discovered 50 years ago [1], it has been manufactured industrially for 40 years. The original applications were in lithographic printing plates, photoresists, and other light-sensitive systems. Since then, the digital revolution has changed the use and requirements of photopolymerization compositions in modern electronic systems, including PCBs [2]. To meet the need for increased photospeed and improved resolution in these systems, and as monochromatic lasers replaced traditional light sources, an optimization of the photoinitiator performance has been necessary. HABI has met the challenge in various ways. Modification of the basic HABI structure with substituents and the use of synergistic sensitizers and coinitiators have resulted in materials that are used in laser direct imaging (LDI) and holographic imaging in addition to their continued use in traditional applications.
The first HABI for large-scale use was o-Cl-HABI, developed for DuPont’s mono/multicolor proofing system known as Dylux®. This system was based on HABI’s oxidation of a leuco dye [3]. The use of HABI in photopolymerization was a later development and is now the primary use of HABI [4].
Manufacturing HABI
The triarylimidazole monomer with phenyl groups in the 2,4,5 positions known as lophine, or HABI monomer, is oxidatively dimerized. This dimer has six aryl groups, hence hexaarylbiimidazole(Figure 1).
Figure 1: Structure of hexaarylbiimidazole (HABI).
Numerous HABIs are possible by modifying the substituent groups “R” attached to the aryl groups of the molecule. For example, the structure is called a lophine dimer when “R” is hydrogen. However, when chlorine replaces hydrogen, the compound is called o-Cl-HABI. The addition of the Cl changes the physical properties and performance of the molecule. There are hundreds of HABIs that can be synthesized by the addition of functional groups to various and/or multiple positions on the aryl groups. Each may exhibit different physical properties and performance. The performance of each material, however, is system and application dependent.
The Properties of HABI
There are intrinsic properties of HABI that have led to 40 years of commercial applications including:
- The unique absorption spectrum;
- Its lack of sensitivity to oxygen;
- The formation of stable lophyl radicals; and
- The ability of HABI to interact with a large number of coinitiators and sensitizing dyes.
HABIs have been successfully utilized in commercial applications including PCBs, integrated circuits, holographic films, color proofing systems, lithographic plates, and semiconductor chips.
The Correlation Between the Chemical Structure of HABI and its Properties
The absorption spectrum of o-Cl HABI is shown in Figure 2. The addition of substituents has an effect on long wavelength absorption as shown by the example in Figure 3 (TCDM HABI). Electron-donating groups such as methoxy, or the substitution of a heterocyclic ring for a benzene ring, enhance the absorption in the 400nm region. Photophysical studies [5] of HABI show that irradiation at 275nm through 420nm efficiently yields imidazoyl radicals. Substituents at the ortho position of the 2-phenyl ring increase the rate constant of homolysis. Sensitization studies indicate [6] dissociation occurs through the singlet state; the absorption in the visible may be due to low-energy vibrational levels of the singlet [7]. Flash photolysis studies [8] indicate that while direct photolysis yields a lophyl radical, sensitized photolysis, presumably by an electron-transfer mechanism, yields an initial lophyl radical anion, which rapidly decays to a lophyl radical. Importantly, the lophyl radical is long-lived and stable and is also insensitive to oxygen.
Figure 2: The absorption spectrum of o-Cl HABI.
Figure 3: TCDM HABI.
An extension of the intrinsic yellow color of HABI is dye-sensitized photopolymerization, which extends it into the visible spectrum at the i, h, g lines. It was found that HABI initiated photopolymerization not only at 275nm, but also at 365 to 420nm. With a sensitizing dye, polymerization rate and depth of cure were improved.
Recent work [9] has focused on elucidating further the dynamics of HABI photodisassociation, as well as work on the exact nature of HABI/sensitizer interaction.
The Mechanism of HABI
The production of two imidazoyl radicals by photolysis is the mechanism of HABI’s photoactivity. The fate of these radicals in a designed chemical matrix is the backbone of a photosensitive composition for either color formation or photopolymerization [10].
The Value of HABI
Basic research established the unique properties of HABI as a source of free radicals; the next step was to make a technology out of it. As mentioned previously, this comprises color formation and photopolymerization. The following discussion will elucidate both the practical and the historic development of these technologies.
Color Formation
One of the most well-known examples of HABI being used in color formation is the Dylux® color proofing system developed by DuPont in the mid-1960s [11]. It was based on oxidation of leuco crystal violet and the tris 2-methyl analog by o-Cl HABI to a blue or violet color. Variations in the structure of the triphenylmethane led to a variety of colors and ultimately any leuco dye capable of being oxidized by a free radical could be used with HABI. Once the color was established, stability was the issue. In the same manner that a silver halide photograph needs to be fixed, a photo-activated redox system was incorporated into the Dylux® as a stabilizer [12]. Later work on Dylux® involved development of HABI analogs which increased photospeed and led to the development of Chromalin®, a four-color system [13], also developed by DuPont. In addition, it was found that HABI photospeed and efficiency could be improved by addition of additives, notably mercaptans such as 2-mercapto benzoxazole or ketones such as Michler’s keton [14.. Since DuPont’s development of Dylux® and its successors, other companies, including Kodak, Fuji and 3M, have also developed color formation compositions using HABI.
Color proofing systems have endured very well. With the advent of laser light sources, digital imaging can now be accomplished with HABI coupled with specialized substituents and sensitizers. In addition, recent work has made use of the photochromism of HABI in a variety of novel imaging applications such as smart glass and color-changing nail polish [15].
Photopolymerization
Photopolymerization is an extension of the use of radicals to generate images and dates back to the 1960s [16]. Early applications included printing plates and photoresists and are exemplified by the Kodak system using cinnamates and direct irradiation. In addition, DuPont’s Dycril system for printing plates evolved in 1971, when DuPont found HABI was able to initiate photopolymerization. As HABI is a free radical source, acrylates and methacrylates were used as monomers and the same additives that enhanced photospeed for color formations also increased photopolymerization rates. Eaton [17], at DuPont, elucidated that two mechanisms, hydrogen transfer and electron transfer, were operating with mercaptan and amine coinitiators. In addition, sensitizing dyes became more important with the evolution of laser technology by extending the operating light source into the visible. As previously mentioned, modifications of the basic HABI structure with substituents greatly affect photospeed and performance. Research on the mechanism of HABI photolysis continues to the present and has contributed to the development of more recent applications like authentication, high-resolution lithography and microelectronics [18].
HABI and Hampford Research, Inc (HRI)
HRI and HABI have a long relationship, based on 30 years of manufacturing, but also a technical R&D relationship that parallels the manufacturing evolution. Since its inception, HRI has worked jointly with industry leaders to develop more than 30 HABIs for a variety of applications, and today offers the largest selection of commercialized HABIs in the world. In addition, HRI has researched and scaled up many of the sensitizing dyes and coinitiators used with HABI in the applications referenced above.
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11. Cescon, Dessauer, U.S. 3,630,736 (1971)
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14. D. Eaton, J. Photochem. Photobiol., 373(58) (1991)
15. J. Abe, J. Phys. Chem. 116, 6792 (2012)
16. Kosar, Light Sensitive Systems, J. Wiley, (1965)
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18. Wang, J. Photopolymer Science & Technology 24, 611 (2011)
Stephen L. Finson is technical director of Hampford Research, Inc. and directs all R&D and technical activities for the firm. He joined founder Jack Hampford as the first employee 30 years ago. Previously, Finson worked at Ware Chemical and Synthetic Products. Finson has extensive experience synthesizing a variety of photoactive compounds and associated initiators and sensitizers.