Coating dielectric/passivant

In electronics, polyimides are now extensively used in the form of self-standing films for flexible circuitry, deposited films for interlayer dielectrics, passivation and buffer coatings, moulding thermoplastic powders for PCBs, and adhesive pastes or tapes. The basic polyimide chemistry has been adapted to fulfill the specific requirements of these applications. A series of books provides complete information not only on the chemistry of polyimides but also on their utilization in electronics [4,23,24]. The following figures summarize the chemical formulae of the most important categories of polyimide precursors or precyclized polymers that are commonly used in electronics. 

Another example of a cold-wall reactor is shown in Fig. 5.9. It uses a hot plate and a conveyor belt for continuous operation at atmospheric pressure. Preheating and cooling zones reduce the possibility of thermal shock. The system is used extensively for high-volume production of silicon-dioxide coatings for semiconductor passivation and interlayer dielectrics. 

But metallic coatings have high absorption losses. But one can remedy them with dielectric coatings, which are necessary also to passivate the metal, avoiding oxidation. Aluminum is the best metal for the ultraviolet in addition it adheres on most substrates it needs passivation. Silver is easy to deposit it has the highest reflectance in the visible and the infrared since it tarnish rapidly, passivation is mandatory. Gold is the best material beyond 700 nm, and it is considered that it does not tarnish, which is not true actually because its surface is not that stable on the long term. 

Conditions have been defined for applying polyimide coatings onto the silicon wafer as passivation and/or dielectric. Processing variables studied included the critical areas of adhesion, cure cycle and thermostability. Aminosilane was shown to be effective adhesion promoter. The rate of imidization was followed by F.T.I.R. employing time lapse technique. 

Low-Pressure CVD Processes. Low-pressure CVD (LPCVD) (—101 Pa) is the main tool for the production of polycrystalline Si dielectric and passivation films used in Si IC (integrated-circuit) manufacture (1, 20, 21). The main advantage of LPCVD is the large number of wafers that can be coated simultaneously without detrimental effects to film uniformity. This capability is a result of the large diffusion coefficient at low pressures, which... 

Silicon Nitride. Silicon nitride produced by high-temperature (>700 °C) CVD is a dense, stable, adherent dielectric that is useful as a passivation or protective coating, interlevel metal dielectric layer, and antireflection coating in solar cells and photodetectors. However, these applications often demand low deposition temperatures (<400 °C) so that low-melting-point substrates or films (e.g., Al or polymers) can be coated. Therefore, considerable effort has been expended to form high-quality silicon nitride films by PECVD. 

The adsorption of hydrocarbon molecules on Si surfaces is an interesting topic of study under various viewpoints. For example, a thin hydrocarbon film coating Si may be applied as a low dielectric in microelectronics and may passivate the surface if covalent bonds are formed between Si atoms and the adsorbate species. Further, unsaturated hydrocarbons play an important role as precursor species for chemical vapor deposition (CVD) of diamond - like films on the Si surface, and of silicon carbide (SiC). 

The different PEC VD film/substrate systems are schematically presented in Figure 6a. The substrates correspond to 99.99% pure Al, mechanically polished with a 0.3 pm alumina powder, then finally electrolytically in a 70% methanol-30% nitric acid solution. When exposed to air, a native aluminum oxide of about 3 nm is produced. The substrates were coated with a dielectric film of a passivation material either SijN or Si02 4.5 wt.% P. These systems are, respectively, denoted as system A and system C. The SijN films were produced by plasma enhanced chemical vapor deposition at a temperature of 360°C, while the SiO 4.5 wt.% P films were chemically vapor deposited at a temperature of 420°C. For both passivation materials, the thickness of the films was 0.8 pm. 

Aromatic polyimides have found extensive use in electronic packaging due to their high thermal stability, low dielectric constant, and high electrical resistivity. Polyimides have been used as passivation coatings, (1) interlayer dielectrics, (2) die attach adhesives, (3) flexible circuitry substrates, (4) and more recently as the interlevel dielectric in high speed IC interconnections. (5) High speed applications require materials with a combination of low dielectric constant, flat dielectric response versus frequency and low water absorption. 

Passive oscillator mode Impedance analysis of the forced oscillation of the quartz plate provides valuable information about the coating even if the active mode is not applicable anymore. For impedance analysis, a frequency generator is used to excite the crystal to a constraint vibration near resonance while monitoring the complex electrical impedance and admittance, respectively, dependent on the applied frequency (Figure 2B). For low load situations near resonance, an equivalent circuit with lumped elements - the so-called Butterworth—van-Dyke (BVD) circuit — can be applied to model the impedance data. The BVD circuit combines a parallel and series (motional branch) resonance circuit. The motional branch consists of an inductance Lq, a capacitance Cq, and a resistance Rq. An additional parallel capacitance Co arises primarily from the presence of the dielectric quartz material between the two surface electrodes (parallel plate capacitor) also containing parasitic contributions of the wiring and the crystal holder (Figure 2B). 

Jayakrishnan R (2009) Dielectric coating agents for passivation and anti- reflection. Photo volt Intern 6 83-86... 

Passivation The process in which an insulating dielectric layer is formed over the surface of the die. Passivation is normally achieved by thermal oxidation of the siHcon and a thin layer of siHcon dioxide is obtained in this manner (a combination of PECVD oxide and PECVD nitride deposited at lower temperature (below 450° C)). Other passivation dielectric coatings may also be applied (used), such as siHcon glass (siHcon oxynitride). 

Organic polymers have attracted much interest in electrical and electronic applications because of their electrical insulating nature. Polyimides have gained much attention because of their excellent thermal stability and low dielectric constant. Polyimides have found applications in matrix resins for circuit boards, encapsulants, adhesives, passivation coatings, alpha particle barriers, ion implant masks and interlayer dielectrics."10... 

High performance polymer films and coating materials are increasing being required by the electronics industry for use as interlayer dielectrics and as passivation layers. Aromatic polyimides are general the polymers of choice for these applications because of their unique combination of chemical, physical, and mechanical properties (i). Another class of polymers which have been investigated for these applications are the po arylene ether)s that can be prepared by the nucleophilic displacement of activated aromatic dihalides alkali metal bisphenoxides. Heterocycles such as benzoxazoles (2), imidazoles (5), phenylquinoxalines (.4), and 1,2,4-triazoles (5) have been incorporated within poly(arylene ethers) utilizing this thetic procedure. 

Thin-film dielectric coatings for capacitors Thick-film substrates for hybrid circuits Glass passivation of semiconductor surfaces " ... 

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