Temperature Coefficient of Resistance The temperature coefficient of the input and output resistances will either be identical, or match very closely. Knowing the temperature variation of the input resistance is useful when designing the current source used to bias the transducer. For transducers biased with a constant current source, the bias voltage will be proportional to the transducer's resistance.
A bias circuit designed to drive a transducer with a particular resistance at room temperature may fail to do so at hot or cold extremes if variations in transducer resistance are not anticipated. For practical transducers the temperature coefficient of resistance can be quite high, often as much as 0. Noise In addition to providing a signal voltage, Hall-effect transducers also present electrical noise at their outputs.
For now we will limit our discussion to sources of noise actually generated by the transducer itself, and not those picked up from the outside world or developed in the amplifier electronics. The most fundamental and unavoidable of electrical noise sources is called Johnson noise, and it is the result of the thermally induced motion of electrons or other charge carriers in a conductive material.
It is solely a function of the resistance of the device and the operating temperature. Johnson noise is generated by any resistance including that found in a Hall transducer , and is described by: Equation where k is Boltzmann's constant 1. The wider the frequency range over which the signal is examined, the more noise will be seen. The two positive aspects are that it can be minimized by choice of transducer impedance, and that it is not usually of tremendous magnitude.
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Flicker noise, also known as llfnoise, is often a more significant problem than Johnson noise. This type of noise is found in many physical systems, and can be generated by many different and unrelated types of mechanisms. The common factor, however, is the resultant spectrum. The amount of noise per unit of bandwidth is, to a first approximation, inversely proportional to the frequency; this is why it's also referred to as llf noise.
Because many sensor applications detect DC or near-DC low-frequency signals, this type of noise can be especially troublesome. Unlike Johnson noise, which is intrinsic to any resistance regardless of how it was constructed, the flicker noise developed by a transducer is related to the specific materials and fabrication techniques used.
It is therefore possible to minimize it by improved materials and processes. The following sections describe the construction and characteristics of several types of Hall-effect transducers that are presently in common use. The transducer is cut and ground to the desired size and shape and the wires are attached by soldering or welding. One advantage of bulk-type devices is that one has a great deal of choice in selecting materials. Another advantage is that the large sizes of bulk transducers result in lower impedance levels and consequently lower noise levels than those offered by many other processes.
Some key characteristics of an instrument-grade bulk indium-arsenide transducer the F. Bell BH are shown in Table The thickness of the films used to fabricate these devices can be on the order of l pm or smaller. Figure Schematic top a and cross-section b views of thin-film Hall-effect transducer. The primary advantages of thin-film construction are: 0 Flexibility in material selection Small transducer sizes achievable Thin Hall-effect transducers provide more signal for less bias current Photolithographic processing allows for mass production 18 CHAPTER 2.
The details of the processing operations for each layer vary depending on the characteristics of the materials being used. Film deposition is commonly accomplished through a number of means, the two most common being evaporation and sputtering. In evaporation, the substrate to be coated and a sample of the coating material are both placed in a vacuum chamber, as shown in Figure The sample is then heated to the point where it begins to vaporize into the vacuum.
The vapor then condenses on any cooler objects in the chamber, such as the substrate to be coated. Because the hot vapor in many cases will chemically react with any stray gas molecules, a substantially good vacuum is required to implement this technique. The thickness of the deposited film is controlled by the exposure time.
Figure Schematic drawing of evaporative thin-film process. Sputtering is another method for coating substrates with thin films. In sputtering, the sample coating material is not directly heated. Instead, an inert gas, such as argon, is ionized into a plasma by an electrical source. The velocity of the ions of the plasma is sufficient to knock atoms out of the coating sample the target , at which point they can deposit themselves on the substrate to be coated.
As in the case of an evaporative coating system, film thickness is controlled by exposure time.
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Figure shows a schematic view of a sputtering system. Once a material has been laid down in a thin film, it is then patterned with a photoresist material, exposed to a photographic plate carrying the desired pattern, and the photoresist is then developed, leaving areas of the substrate selectively exposed. The substrate is then etched, often by immersion in a suitable liquid solvent or acid. Alternatively, the substrate can be plasma-etched by a process related to sputtering. In either case, after the etching step is finished, the remaining photoresist is stripped and the substrate is prepared to receive the next layer of film or readied for final processing.
The sequence of operations needed to process a layer of a thin film is summarized in Figure This device is made with two thin-film layers, a metal layer to provide contacts to the Hall-effect element, and an indium-arsenide thin film that forms the Hall-effect transducer itself. In addition, solder bumps are deposited on the copper to provide connection points to the outside world.
Wires may be soldered to these features, or the device may be placed face down on a printed circuit board or ceramic hybrid circuit, and reflow soldered into place. Key specifications for the HS transducer are listed in Table The major improvements over bulk devices are in the area of sensitivity and supply current; the thinfilm device is nearly as sensitive as the previously described bulk device BH , and obtains this level of sensitivity with an order of magnitude less supply current needed. The BH bulk device, however, is superior in the areas of offset error and drift over temperature.
The principal advantage of the thin-film device is potentially lower cost. Thin-film processing techniques allow a great number of devices to be fabricated simultaneously and separated into individual units at the end of processing. The trans- 2. The addition of electronics to the bare transducer allows sensor manufacturers to provide a very high degree of functionality and value to the end user, for a modest price. By simultaneously fabricating thousands of identical devices on a single wafer, it is possible to economically produce large numbers of high-quality sensors.
Figure shows an example of several hundred Halleffect sensors on a silicon wafer, before being separated and individually packaged. Figure Hall-effect sensor ICs on a silicon wafer. Courtesy of Melexis USA. While there are many layers and structures available in modem integrated circuit processes that can be exploited to fabricate Hall-effect transducers, we will illustrate the basics by considering one particular case.
Because of the complexity of integrated circuit fabrication processes, we will not even attempt to describe them here. For this example, we will consider what is known as an epitaxial Hall-effect transducer. We will begin by considering the structure of a related device, the epitaxial resistor. Figure shows top views and side views of an epitaxial resistor made with a typical bipolar process. The device is so named because it is built in the epitaxial N-type silicon layer. The raw wafer is usually of a P-type material, and the epitaxial layer is deposited on the surface of the wafer by a chemical vapor deposition CVD process, and can be doped independently of the raw wafer.
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P-type isolation walls are then implanted or diffused into the top surface of the epitaxial layer to form wells isolated islands of N-type material. Maintaining each of the wells at a positive voltage with respect to the P-type substrate causes the P-N junctions to be reverse-biased, thus electrically isolating the wells from each other.
By providing this junction isolation, one can build independent circuit components such as resistors, transistors, and Hall-effect transducers, in a single, monolithic piece of silicon, using the wells as starting points. The overall depth of epitaxial layers can vary from Fm for commonly available IC processes. In the case of an epitaxial resistor, the well defines the body of the component. The N-type material used typically has a resistance of about kS2 when measured across the opposite edges of a square section.
This allows one to readily construct resistors with values up to about kL2 by building long, narrow resistor structures.
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The whole IC is then covered with an insulating layer of SiO, silica glass , and holes called contact windows are then etched through this glass layer at specific points to allow for electrical contact to the underlying silicon. Their performance characteristics, however, are fairly awful, at least when compared to the discrete resistors most electronic designers commonly use. In addition, the effective thickness of the reverse-biased P-N junction that isolates an epi resistor from the substrate varies with applied voltage.
Despite the drawbacks of using the epitaxial layer when making resistors, it is quite useful for making Hall-effect transducers. Because the epitaxial layer is relatively thin 5 Fm is thin from a macroscopic perspective , and usually made from 2. This will give a general idea of the performance one can expect from such a device. Figure shows the details of this device. There are two reasons for doing this: the first is to maximize the sensitivity by ensuring that all the bias current flows between the sense terminals, and the second is to minimize ohmic offsets.
Metalization and contact windows, on the other hand, are fabricated in separate manufacturing steps, increasing the opportunities for contact misalignment.potbcirnasi.gq
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This is an amazingly high level of sensitivity. This sensor, however, will never operate at one ampere; 1 milliampere is a more realistic bias current. The next major question is that of input and output resistance. Because the bias current flows in a substantially uniform manner between the bias contacts, since the contacts extend across the width of the transducer, we can make a fairly good estimate of the input resistance by: x10dm R. Because of geometric factors, the output resistance cannot be as readily calculated as the input resistance.
If one were to apply a voltage between the output terminals, the lines of current flow would not be parallel and uniform and therefore amenable to back-of-envelope analysis. For purposes of designing a compatible front-end amplifier and noise calculations, one might assume that the resistance of the output is within a factor of two or three of that of the input.
Because the estimation of temperature sensitivities and ohmic offset is very difficult if not impossible even when one is working with a fully characterized process, we shall ignore them. Suffice it to say, however, that integrated Hall-effect transducers can be made with substantially good performance in these areas. Table Key characteristics of hypothetical silicon integrated Hall-effect transducer Continued 2. Integrated processes based on these other materials, however, do not provide the wealth of electronic device types that can be cofabricated on silicon processes.
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Silicon processes have another advantage: availability. High-quality Hall-effect transducers can be fabricated with many standard bipolar and CMOS integrated circuit processes with little or no modification. A number of semiconductor companies presently produce a vast array of Hall-effect integrated circuits. Figure shows an example of a silicon Hall-effect IC, containing a Hall-effect transducer and a number of other components such as transistors and resistors.
The Hall-effect transducer is the square-shaped object in the center.
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