Thin-film and thick-film technologies in electronics
Thin-film and thick-film technologies are employed to provide high density interconnections in electronics applications ranging from engine control units and military hybrids to flat-panel displays and solar cells. They complement other interconnection technologies, such as rigid or flexible printed circuit boards, and direct bonded copper bonded substrates, for applications where high functional density, design flexibility and customisation are important factors. This knowledge summary provides a general review of thin-film and thick-film technologies as used in electronic packaging and interconnect.
Film in an electronics context can be described as a layer of a substance, or a coating, on a supporting material. The terms thin-film and thick-film have traditionally been applied when using ceramic, silicon or glass-based supporting material. The principal functional difference between the two film technologies is the relative thickness of deposition. Other differences include the processing methods and materials used for fabrication. These differences are summarised after a description of each of the technologies. A summary of these differences will follow a description of each of the technologies. References are provided at the conclusion where more detailed information can be found. Alternatively contact the Microtechnology Section at TWI.
|Thin-film on glass: |
(Courtesy of Intarsia/Dow Chemicals)
|Thick-film on ceramic|
Thin-film circuitry is often exploited when sharply defined features or pure metal layers are required. Sharply defined features are a function of the layer thickness and the control achieved during processing. The process used to apply the films is typically vacuum deposition. Films of various materials are sputtered or evaporated onto a substrate. Chemical processes such as sol-gel, plating and chemical vapour deposition (CVD) are also used. Photolithography and etching are then employed to define the circuit pattern features by cutting away - subtracting - unnecessary material. This subtractive process patterns all conductors, resistors, capacitors and the dielectric films.
Conductor materials in traditional thin-film include the pure metals such as gold, aluminium and copper. The first two metals provide compatibility with wire bonding used in electronics packaging. Conductive films for optical transparency applications are based on tin oxides (indium-tin-oxide and antimony-tin-oxide). Thin-film resistor materials are usually nickel chromium alloys or tantalum nitride. Dielectrics such as polyimide, SiO 2 and Si 3N 4 provide electrical isolation or allow multiple layers to be formed on one substrate. The substrates can include glass, silicon, sapphire, alumina and aluminium nitride.
Capital equipment costs for vacuum deposition, photolithography, etching, pattern transfer and mask design are relatively high. Thin-film is therefore most applicable when high volume, high density or high performance are required. Applications that take advantage of this high performance include sensors, flat panel displays, micro electro-mechanical systems (MEMS), biomedical devices and coatings, optical instruments, sensors, microwave and other integrated circuits and thin film integrated passive devices (IPDs). The high precision and very high resolution processes involved in the thin film technology can result in highly competitive products on a cost-per-function basis.
Thick-film technology is a highly flexible fabrication process for a wide range of circuit complexities from low and high volume applications to fast-turn and prototype products. Films for conductors, dielectrics, resistors and capacitors are fabricated as ceramic, glass and metal pastes. The pastes are applied as a pattern by screen printing on an insulating substrate then dried and fired. A screen or stencil is placed over a substrate with openings to define the circuit. A squeegee is then used to roll the paste into place at a height set by the screen or stencil. Once each layer is dried at around 100°C and then fired at around 850°C, the next layer is added.
A wide variety of conductor pastes exist to satisfy the bondability, solderablity and electrical performance requirements of the circuit: gold, platinum-gold, copper, silver, palladium-silver and platinum-silver are the most common. Thick-film dielectrics are used to provide electrical isolation, protection and to build multiple layers of circuitry. Common dielectric materials are alumina, glass and barium-titanium oxide. Resistors are formulated in a glass matrix using metal oxides, typically ruthenium dioxides. Substrate materials such as alumina, aluminium nitride and beryllia provide the mechanical support and electrical isolation necessary for the circuit.
Pastes in the thick-film process comprise three main components: the vehicle system, an inorganic binder phase and a functional phase. The vehicle system contains a solvent, organic binders and a wetting agent to hold the paste together and assist printing on the substrate. The inorganic binder phase is used to bind all the solids together and to promote adhesion to the substrate during firing. This last binder phase may consist of fritted glass used to lock into the substrate for strong adhesion. The functional phase is made up of metal or ceramic powders.
Based on simplicity in processing, thick-film circuits can be produced with low initial investment and low running costs. As such they are often utilised for low-volume, fast-turn applications. The relatively high printed volume of the films makes them useful in high power circuits where low resistivity is required. Examples of some applications include photovoltaic solar cells, chip resistors, gas analysis sensors, heaters, transducers and telecommunications switching.
Historically the difference between thin-film and thick-film technology could be explained by the thickness of the films. For example, greater than five micron thickness implied thick-film. The type and the resultant pitch and edge definition of processing has also been used as a differentiator. Subtractive, or etching, processes were used exclusively for thin film while thick-film employed a sequential or additive process. Today, however, these divisions have become blurred. Thin-film processing still achieves the thinnest films but the advent of photoimagable thick-film pastes has allowed thinner, more-closely spaced lines with better resolution than previously, while thin film technologies are employing plating technologies for increased conductor thickness while maintaining high precision.
A side-by-side comparison table is provided below for a number of film circuit characteristics.
|Characteristic||Thin-film (typical)||Thick-film (typical)|
|Film thickness (µm)||.01 - 1||5 - 40|
|Conductor line width/spaces (µm)||< 25||25|
|Processing technology||subtractive & additive||additive & subtractive|
|Conductor materials||Au, Al, Cu, ITO||Au, Pt-Au, Cu, Ag, Pd-Ag, Pt-Ag|
|Dielectric materials||polyimide, SiO 2, Si 3N 4||Al 2O 3, glass, ceramic, BaTiO 3|
|Resistor materials||NiCr, TaN||RuO 2, Bi 2 Ru 2O 7|
|Substrate||Al 2O 3, AlN, BeO, SiC, glass, Si, quartz, sapphire||Al 2O 3, AlN, BeO|
- Samuel J. Horowitz, et al., Advanced Ceramic Technology for HDI and Integrated Packaging, in Advanced Packaging, March 1999, p.40
- Aicha Elshabini, Fred D. Barlow, Thin Film Technology Handbook, 1997
- R. Tummala, et al., Thin-Film Packaging, in Microelectronics Packaging Handbook Part II, 1997, p. II-624
- Barbara Dziurdzia and Magorzata Jakubowska, Photoimageable Thick-Films in Microwaves, in Advancing Microelectronics, March/April 2002
- Leif Halbo, et al., Polymer Thick-Film Technology (PTF), Lobo Grafisk AS, 1990
- Harry K. Charles, et al., Thin-Film Hybrids, in Electronic Materials Handbook Vol 1 Packaging, 1989, p 313
- William Borland, Thick-Film Hybrids, in Electronic Materials Handbook Vol 1 Packaging, 1989, p 332
- G.V. Planar and L.S. Phillips, Thick Film Circuits, 1972