Nichrome resistors

A thin-film nichrome resistor in the form of a rectangle of dimensions 1 mmx5mm is deposited on to a glass substrate. Estimate the thickness of the film for the resistance measured between electrodes contacting the 1 mm edges to be 1.0 kfl The resistivity of nichrome may be taken to be 1.07 x 10 6Qm. 

Retention of moisture in polymer substrates or adhesives, especially in the presence of chloride or other ions, corrodes thin-film metallization such as nichrome resistors or aluminum conductors (further discussed in Chapter 6). Other limitations in using aqueous or semi-aqueous solvents include ... 

The original intent of this project was to identify and/or perfect vendor-supplied, state-of-the-art hydro-phobic polymer sealants, such that the end product could meet military criteria for hermeticity in hybrid microelectronic devices. Solubility and diffusivity constants and permeabilities for several types of hydrophobic polymers were determined. Data from the laboratory and theory from literature both indicate that moisture impermeable polymers are not attainable, at least to meet military standards. The project objective was then redirected towards finding a means of passivating the surface of a microelectronic device against moisture. It was found that interface modification, i.e., passivation of the chemically reactive surface species, provides significant protection in high temperature and humidity environments. Plasma polymerization of hexamethyIdi-silazane (HMDS) afforded phenomenal protection to moisture sensitive thin film nichrome resistors even under the most severe test conditions. 

Materials selected as interface modifiers for surface passivation were chosen by their potential to react with surface oxides and hydroxyls where moisture would likely interact. The three which were ultimately tested are polyphenylsilsesquioxane (PSS), hexamethyldisilazane (HMDS), and N- 3-(N-vinyIbenzylamino) ethyl-7-aminopropyl-trimethoxysilane-monohydrogen chloride ("silane ). The last material proved to be corrosive to the test nichrome resistors due to the chloride part of the molecule. Therefore, most of the study centered around the first two materials. 

The results for the temperature and humidity tests are given in Table 2. Failures were indicated by a high resistance value due to an open circuit. Usually this occurred as a nichrome resistor disintegrated from a reaction with the moisture, which was further enhanced by the power-on condition. Slight drifting of the circuit... 

Kuo, C.C.Y., Thick film nichrome resistors. Proceedings of the 7th European Passive Components Symposium, CARTS-EUROPE 93,1993, pp. 111-117. 

TABLE 4.1 Characteristics of Typical Tantalum Nitride and Nichrome Resistors... 

Resistors, which are passive devices limiting the flow of electrical current in proportion to the applied voltage and usually made of tantalum, nichrome, titanium, or tungsten. 

The bath temperature may be controlled by heating with an immersed loop of Nichrome resistance coil, such as the heating element from an electrical appliance. The coil is connected to a 110-volt line in series with a 27-ohm 5-ampere variable resistor and a 600-watt heater element, which may be cut out of the circuit to lower the fixed resistance. An ammeter in the circuit helps to determine the proper adjustment of the sliding contact of the resistor. To avoid an excessive load on the coil,... 

Corrosion of conductor metallizations, devices, wire, wire bonds and other connections, and platings occur by direct or indirect-chemical reactions. The metals most susceptible to chemical attack are the non-noble metals among which are thin-film aluminum used for conductors or bonding pads, thin-film nichrome used for precision resistors, tin-lead solders used for... 

Several such interface modifiers were found to have the potential of meeting the requirements of hermeticity and repairability. One, however, was outstanding in both categor ies. A plasma-polymer ized hexamethyldisilazane (HMDS) coating provided effective protection for nichrome thin film resistors in high temperature and humidity, power-on conditions for 1,000 hours. HMDS was a very thin coating (approximately 1000 A) which was easily removed for repair purposes. 

Fio. 2.7. Chlorination furnace TC, thermocouple positions 1, chlorine gas preheater 2, charcoal diffuser bed 3, resistor carbon 4, charge of briquettes 5, graphite electrode 6, split graphite-pipe top heater 7, feed hopper 8, nichrome heater for cross-over pipe 9, nickel-lined condenser 10, water-cooled iron aftercondenser 11, exhaust to scrubbers 12, condenser heating air blower 13, nichrome air heater (Stephens, W. W. and Gilbert H. L. Ref. 62). 

The most common types of sintering furnaces are electrical resistance furnaces in which a current carrying resistor, commonly called the furnace element or winding, provides the source of heat. In addition to size and cost, important considerations in the selection of a furnace are the maximum temperature capability and the atmosphere in which it can be operated for extended periods. Table 12.1 provides a selected list of common furnace elements. Several metal alloys (e.g., nichrome) can be used as furnace elements for temperatures up to 1200°C in both oxidizing and reducing atmospheres. For extended use in air, other furnace elements can deliver higher temperatures e.g., Pt (up to 1400°C) SiC (1450°C) ... 

The most common types of resistor material are nichrome (NiCr) and tantalum nitride (TaN). Although NiCr has excellent stability and TCR characteristics, it is susceptible to corrosion by moisture if not passivated by sputtered quartz or by evaporated silicon monoxide (SiO). TaN, on the other hand, may be passivated by simply baking in air for a few minutes. This feature has resulted in the increased use of TaN at the expense of NiCr, especially in military programs. The stabihty of passivated TaN is comparable to that of passivated NiCr, but the TCR is not as good unless annealed for several hours in a vacuum to minimize the effect of the grain boundaries. Both NiCr and TaN have a relatively low maximum sheet resistivity on alumina, about 400 S2/ for NiCr and 200 / for TaN. This requires lengthy and complex patterns to... 

Because the adhesion layer in thin film is usually a resistive material such as Nichrome, chrome, or titanium-tungsten, precision-integrated substrate resistors are available. 

If the subsequent assembly process calls for thermocompression or thermosonic wire bonding, plated copper under the wire-bond sites should be avoided. This can be accomplished by selectively plating the copper. When the adhesion layer is Nichrome or titanium-tungsten, then precision-deposited thin-film resistors can be fabricated. 

Resistor material -> characteristic T Tantalum nitride Nichrome... 

Thin-fikn circuits typically consist of three layers of material deposited on a substrate. The bottom layer serves two purposes It is the resistor material and it also provides the adhesion to the substrate. The adhesion mechanism of the film to the substrate is an oxide layer that forms at the interface between the two. The bottom layer must therefore be a material that oxidizes readily. The most common types of resistor material are nichrome (NiCr) and tantalum nitride (TaN). Gold and silver, for example, are noble metals and do not adhere well to ceramic surfaces. 

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