Thick film devices screen printing

Applications of electrochemical transducers have relied on conventional and bulky disk (C, Au) or mercury drop electrodes, as well as on mass-producible, single-use, thick-film carbon screen-printed electrodes. The sensitivity of such devices, coupled to their compatibility with modern microfabrication technologies, portability, low cost (disposability), minimal power requirements, and independence of sample turbidity or optical pathway, make them excellent candidates for DNA diagnostics. In addition, electrochem-... 

Screen-printing allows the fast mass production of highly reproducible electrodes at low cost for disposable use. A variety of screen-printed thick-film devices can be produced and in Fig. 25.2 an example of carbon screen-printed electrode is shown. 

The LEC structure that involves the addition of ionic dopants and surfactants to the printable inks enables the ability to print a top electrode without restriction by the work function of the metal. Silver, nickel, or carbon particle-based pastes are generally the preferred printable electron injecting electrodes however, the shape and size of the particles combined with the softening properties of the solvent can create electrical shorts throughout the device when printed over a thin polymer layer that is only several hundred nanometers thick. For optimal performance, the commercially available pastes must be optimized for printing onto soluble thin films to make a fully screen-printed polymer EL display. 

The first miniaturized electrochemical device for measuring glucose in whole blood was a mediated system produced in thick-film technology by screen printing [73]. This disposable, single-shot system is produced and actually marketed widely by the company Medisense. Several other companies are now following with similar approaches [74,75]. 

MenU, F., Debeda, H., and Lukat, C., Screen-printed thick films From materials to functional devices, J. Eur. Ceram. Soc. 25 (2005) 2105-2113. 

Device structures adopted for resistor type sensors in practice, (a) Sintered block, (b) thin alumina tube-coated layer, (c) screen printed thick film, (d) small bead inserted with coil and needle electrodes, (e) small bead inserted with a single coil (heater and electrode), (f) practical sensor element assembling sensor device, metal cap and filter. 

Bare die and other chip devices are attached with electrically conductive or nonconductive adhesives to ceramic substrates having defined circuit patterns produced by thin-film vapor deposition and photoetching of metals or by screen-printing and firing of thick-film pastes. With recent advancements in fine-line printed-circuit boards, adhesives are also finding use in attaching bare die to PWBs, a technology known as chip-on-board (COB). 

Of the mass-transfer dispensing methods, screen printing and stencil printing are the oldest and most widely used. Screen printing has been used for over 40 years in the electronics industry to apply thick-film conductors, resistors, and dielectrics in fabricating circuits on ceramic and plastic-laminate substrates. Screen printing is also used as a batch process for depositing electrically conductive and insulative adhesives to interconnect devices on thin-film and thick-film hybrid microcircuits. 

Most commercially available anisotropically conductive adhesives are formulated on the bridging concept, as illustrated in Fig. 1. A concentration of conductive particles far below the percolation threshold is dispersed in an adhesive. The composite is applied to the surface either by screen printing a paste or laminating a film. When a device is attached to a PWB, the placement force displaces the adhesive composite such that a layer the thickness of a single particle remains. Individual particles span the gap between device and PWB and form an electrical interconnection. For successful implementation of anisotropically conductive adhesives, the concentration of metal particles must be carefully controlled such that a sufficient number of particles is present to assure reliable electrical conductivity between the PWB and the device (Z direction) while electrical isolation is maintained between adjacent pads (X,Y directions). 

The device consists of four layers substrate, transparent electrode, polymeric ink, and top electrode. The electroluminescent ink is screen-printed using multiple passes to result in a dry film thickness between about 100 nm and 1 jxm. 

Screen printing allows relatively thick films to be patterned, but lacks the lateral resolution of the other techniques presented. It seems likely that low-resolution OLED [24] and electrochromic matrix displays will be produced with this technique. It may also be used to apply protective (passivation) films to seal devices in order to prevent oxygen and water from reaching the sensitive electroactive films (see Section 4.6). 

Microfabrication has been the topic of a recent review in which thin-film (<1 pm, based on vacuum evaporation, sputtering or chemical vapor deposition) and thick-film (>10pm, based on screen printing or lamination) technologies are described for the mass production of potentiometric sensors and sensor arrays [80]. Current challenges include the cost of fabrication, especially for thin-film devices, the control of physical dimensions of the sensing elements, the incorporation of liquid reservoirs, and the stability of the integrated reference electrodes. 

Challenges facing the development of in vitro amperometric biosensors (interference rejection, rapid response, reproducibility, response range) have been met in many cases, and commercially available devices based on disposable test strips that incorporate miniature two-or three-electrode electrochemical cells are available for a variety of analytes (see Sect. 10.3.7). Thin-fihn and thick-film technology [80] have been used to mass-produce reproducible sensing elements, and amperometric detection in oxidase-based devices occurs by peroxide oxidation or the oxidation of freely diffusing mediators such as ferricyanide and ferrocene derivatives. The screen-printing process for disposable sensor preparation has also been reviewed [144]. 

Both AC and DC PDFs have been used to produce fuU-color, flat-panel large area dot-matrix devices. PDP systems operate in a memory mode. Memory operation is achieved through the use of a thick-fihn current-limiting series capacitor at each cell site. The internal cell memory capability eliminates the need for a refresh scan. The cell memory holds an image until it is erased. This eliminates flicker because each cell operates at a duty cycle of one. The series cell capacitor in an AC-PDP is fabricated using a thick-film screen-printed dielectric glass, as illustrated in Fig. 7.41. The gap separation between the two substrates is typically 4 mil. The surface of the thick-fihn dielectric is coated with a thin-film dielectric material such as magnesium oxide. 

---