Conducting polymers in microelectronics

Conjugated polymers in the nondoped and doped conducting state have an array of potential applications in the microelectronics industry. Conducting polymers are effective discharge layers as well as conducting resists in electron beam lithography, find applications in metallization (electrolytic and electroless) of plated through-holes for printed circuit board technology, provide excellent electrostatic discharge protection for packages and housings of electronic equipment, provide excellent corrosion protection for metals, and may have applications in electromagnetic interference shielding. This paper reviews some of these applications and briefly describes possible future applications of conducting polymers for use as interconnections or for electronic devices.

Introduction Microelectronics, the industry of information processing, has revolutionized our technological society. Electronic products in the form of home entertainment equipment, mobile electronic devices, desktop personal computers, and large supercomputers are pervasive in our everyday world. The electronics revolution began in the 1960s with the fabrication of the first integrated circuits (ICs) [1]. Since then, this industry has experienced remarkable growth resulting in significantly more complex ICs which are faster and smaller, and whose cost per function has decreased [2-9].

Conductors, semiconductors, and insulators, materials comprising the entire spectrum of conductivity, are integral to integrated circuit processing [6, 9, 10-12]. The active device components are composed of semiconductors. Conductors are extensively used for interconnection applications, for electrostatic discharge (ESD) protection of ICs, and for electromagnetic interference (EMI) shielding of electronic equipment. Insulators, most commonly polymers, are widely used as interlevel dielectrics, encapsulants, and materials for the packaging and housing of electronic equipment [6, 10-12].

Conducting polymers [13] offer a unique combination of properties that make them attractive alternatives for certain materials currently used in microelectronics. These polymers are made conducting, or "doped," by reacting the conjugated semiconducting polymer with an oxidizing agent, a reducing agent, or a protonic acid, resulting in highly delocalized polycations or polyanions [13]. The conductivity of these materials can be tuned by chemical manipulation of the polymer backbone, by the nature of the dopant, by the degree of doping, and by blending with other polymers. In addition, polymeric materials are lightweight, easily processed, and flexible.

Conducting polymers have potential applications at all levels of microelectronics (see Figure 1). This paper examines the use of conducting polymers in the area of lithography, with a subsequent discussion of their use for metallization, as corrosion-protecting coatings for metals, and as ESD-protective coatings for packages and housings of electronic equipment. Two areas of application for conducting polymers in the future are mentioned: their possible use in interconnection technology and as novel organic materials in electronic devices.

Lithography Background

Lithographic techniques delineate the intricate patterns necessary to form the doped regions of silicon on a chip, or their interconnections, or the interconnections on a package [4, 5, 12, 14]. Lithography relies on radiation-- sensitive polymers called resists which, when irradiated through a quartz/chrome mask containing the pattern to be transferred, undergo chain-scissioning, cross-linking, deprotection, or molecular rearrangement, thereby creating a difference in solubility between the irradiated or exposed areas and the non-irradiated or unexposed areas of the polymer [4, 5, 12, 14]. In a subsequent step, called develop, the more soluble regions of the resist are selectively removed, as shown in Figure 2. This pattern is subsequently transferred to the underlying substrate (e.g., silicon dioxide, silicon nitride, silicon, or metal) by various etching processes, followed by removal of the resist

[4, 5, 15].

Resists may be patterned with photons at different wavelengths (365 nm, 248 nm, 193 nm), electron beams, X-rays, and ion beams [16-18]. Photolithography has been the dominant technology in the industry to date. However, electron-beam (e-beam) technology is used to fabricate masks for photolithography [4] and for high-resolution, low-volume specialty chips, and it is currently being considered as a next-generation projection lithography option for semiconductor device fabrication [19].

The dimensions that must be delineated are rapidly decreasing. Current DRAM and logic devices require minimum feature size dimensions of less than 150 nm [16, 17]. As the industry continues to require improved resolution, new materials, processes, and tools must evolve to sustain this trend.

Charge dissipators for electron-beam lithography E-beam lithography is a direct-write method in which a focused beam of electrons is directly scanned over the resist [4, 5, 17, 18]; no mask is required because the pattern is computer-generated. It is a technology capable of extremely high resolution, since the beam of electrons can be focused to tens of nanometers [18], and capable also of excellent alignment of level-to-level pattern overlays. Recently, electron projection lithography has received considerable attention as a potential nextgeneration lithography option [16, 19].