Metal shunt resistor

In one embodiment, a shunt resistor is provided, comprising two terminals, a semiconductor substrate embodying at least one temperature sensor comprising at least a temperature sensitive element comprising at least one pn-junction, and at least two metal layers above the semiconductor substrate, at least the upper of the metal layer comprising a path that electrically connects the two terminals, whereby the temperature sensor is below and within the periphery of the upper metal layer.

BACKGROUND

Shunt resistors may be used in current-sensing systems (CSS). In one example, a metal shunt resistor may be serially connected to a battery. Battery current flowing through the shunt resistor causes a voltage drop which is measured with the help of an analog-to-digital converter (ADC). The shunt resistor has a large temperature coefficient of about 0,35% /° C. and it heats by about 6° C. per 1 A current through the resistor. There might be shunt resistors with a different temperature coefficient. For example, depending on the composition of the shunt (copper/aluminum, via construction . . . ), this might vary. Furthermore, there are “precision” shunts available as external components with quite low temperature coefficients (nominally zero). However, this costs board space and money. The output signal of a temperature sensor is used in a correction circuit of the analog-to-digital converter to compensate for the resistor's self-heating. Even with that compensation scheme, an inaccuracy of the current sensing remains due to the temperature dependency of the shunt resistor. Basically there are two major influence factors: Inaccuracy of the temperature coefficient and inaccuracy in the measurement of the actual shunt temperature.

SUMMARY

In one embodiment, a shunt resistor is provided, comprising two terminals, a semiconductor substrate embodying at least one temperature sensor comprising at least a temperature-sensitive element comprising at least one pn-junction, and at least two metal layers above the semiconductor substrate, at least the upper of the metal layer comprising a path that electrically connects the two terminals, whereby the temperature sensor is below and within the periphery of the upper metal layer.

DETAILED DESCRIPTION

In the following detailed description, reference is made to the accompanying drawings, which are also part of the description. Unless otherwise noted, the description of successive drawings may reference features from one or more of the previous drawings to provide clearer context and a more substantive explanation of the current example embodiment. Still, the example embodiments described in the detailed description, drawings, and claims are not meant to be limiting. Other embodiments may be utilized, and other changes may be made, without departing from the spirit or scope of the subject matter presented herein. It will be readily understood that the aspects of the present disclosure, as generally described herein, and illustrated in the figures, may be arranged, substituted, combined, separated, and designed in a wide variety of different configurations, all of which are explicitly contemplated herein.

FIG. 1shows a schematic overview of the functional blocks of a current sensing system (CSS)1. The CSS1comprises a load2, a battery3and a shunt resistor4, an analog-to-digital converter (ADC)5, a correction circuit6and a bandgap reference circuit (BGR)21. The battery3provides a voltage, whereby one terminal of the battery3is connected to one terminal of the load, of which the second terminal is connected to ground33. The second terminal of the battery3is connected to a first terminal of the shunt resistor4, of which the second terminal is connected to ground33. Thus, the current through the battery also flows to the shunt resistor4. The voltage between the two terminals of the shunt resistor4is Vshunt. Since this is a series connection of elements, this series connection could also be in different order. For example, one could also think to connect the shunt to the positive terminal of the battery. However, the “battery” could be any source of energy. It could be also some electronic circuit, for example a transformer plus rectifier. Also, shunt resistor is used in a 4-wire sensing (Kelvin connection) configuration which illuminates the error in I-to-V conversion caused by shunt parasitic resistor.

The bandgap reference21comprises a bipolar first transistor9, a bipolar second transistor10, both being pnp transistors. The bases and the collectors of the transistors9and10are connected to ground33, whereas the emitter of the first transistor9is connected to a first terminal of a first current source8and the emitter of the second transistor10is connected to a first terminal of a second current source7. The voltage between collector and emitter of the second transistor10is the voltage VBE “diode” voltage with significant negative temperature coefficient and the voltage between the emitters of the first transistor9and the second transistor10is temperature-dependent voltage Delta VBE positive temperature coefficient. The current through the second transistor10is 16 times larger than the current through the first transistor. The dimensions of the current level, the area for the PNPs and the current density ratio should be very well-defined. For this purpose, dynamic element matching techniques may be used. The current density should not be too high where the transistor's parasitic resistor degrade the accuracy, and shouldn't be too low if the betas of the transistors are not the same.

The temperature sensor comprises a temperature sensitive element with at least one pn-junction. There are many possible variants: pnp could be npn, the transistors could be diodes, manipulation of positive and negative TC could be done in the analog domain or in the digital domain after the ADC.

In this case, these “bandgap voltages” serve to determine the temperature. But it is generally well known how to use bandgap elements for temperature measurement. It is also known how to use a single diode for temperature measurements (which in this case would replace the PNPs). In this embodiment, the bandgap voltage is considered to be the temperature sensor having the first and second transistors9and10to comprise a temperature sensitive element.

The ADC5receives the voltages Vshunt, Delta VBE and VBE. The ADC5converts, in a time-multiplexed manner, the voltages Vshunt and Delta VB into digital signals Vm and Tm, Vm representing the shunt voltage and Tm the temperature. In this case, Delta VBE and VBE are output signal being output from the temperature sensor. Thus the analog-to-digital converter5is coupled to measure a temperature signal being output by the at least one temperature sensor. The the analog-to-digital converter5further measures Vshunt.

One could also consider using two ADCs: one for shunt-measurements and the other for temperature measurements. This allows un-interrupted current measurements which might be important in dynamic situations.

The Signals Vm and Tm are input to the correction circuit that outputs a signal Vmc representing a temperature-compensated value for the current through the shunt resistor4.

Battery current Ibat is measured by digitizing the voltage drop Vshunt across a 10 mΩ metal shunt. Typically, the value of the shunt is chosen depending on the value of the maximum current to be measured resistor Rshunt. The shunt employs four metal layers (M2-to-M5) in parallel, and is quite large to facilitate low ohmic contacts to the outside world via eight bond pads. The four metal layers are stacked vertically when the substrate extends in the horizontal direction. The number of metal layer might differ for various embodiments. However, Rshunt suffers from process spread (up to ±15%) and a large temperature coefficient (about 0.35%/° C.). The former is corrected by room-temperature calibration, while the latter is mitigated by a temperature compensation scheme. This involves measuring the shunt's temperature (with the PNPs of the BGR) and then performing a polynomial correction on the digitized value of Vshunt.

FIG. 2shows a cross-section through the shunt resister4and the transistors9and10of the CSS1ofFIG. 1.FIG. 2shows a semiconductor substrate100that extends in the horizontal direction. The pnp transistors9and10are embedded in the substrate in a region14which is part of the upper surface of the substrate100. During production of the device, the p- and n-regions of in the region14were produced by doping the silicone in the upper surface101of the substrate100. Above the upper surface101of the substrate100, a first oxide layer191is provided which is interrupted by vias102. Above the first oxide layer191, a first metal layer M1is provided. The layers above the first metal layer M1are, in the vertical order from the bottom to the top, a second oxide layer192, a second metal layer M2, a third oxide layer193, a third metal layer M3, a fourth oxide layer194, a fourth metal layer M4, a fifth oxide layer195and a fifth metal layer M5. Neighbouring layers are electrically connected to each other by vias11.

The first metal layer M1comprises various conductors. The conductors15,16,17,18,19and20are connected via the vias102to terminals of the transistors9and10. The conductors103,104and105are connected to the second metal layer M2, However, it is a goal of an embodiment that the latter conductors do not conduct shunt current because they could have a different and not so well defined temperature coefficient. However, they should conduct the heat from the shunt to the temperature sensor in order to allow accurate temperature measurement of the shunt temperature. Conductors15-20serve to electrically connect the temperature sensing elements to the rest of the circuit (current sources, ADC).

Terminals for the shunt resistors4are the terminals T1and T2on the metal layer where the terminals may be connected by circuitry outside the shunt. The metal layers M2to MS are connected in parallel such that the shunt resistor has a low resistivity. A low resistivity means low energy losses in the shunt resistor. The first metal layer M1comprises conductors that are used to connect to terminals of the pnp transistors. Thus, a part of the metal layers that are vertically arranged is used to connect the temperature sensor and the other part is used to carry the current through the shunt resistor. The line103of first metal layer M1is not meant to carry any shunt current, but is used to electrically connect the pnp's and conduct heat from shunt to pnp's. This enables a short thermal coupling between the current conducting layers and the temperature sensor. Thus, the temperature at the sensor is close to the temperature of the conducting layers improving the accuracy of the temperature measurement. Further, providing the temperature sensor below the conducting metal layers saves space in comparison to circuits where the temperature sensor is provided outside of the periphery of the conducting metal layers. Even some of the conductors of the first metal layer M1are also used for current conducting. Thus, these conductors may also heat up if they conduct current.

There might be other embodiments in which lines of the first metal layer M1conduct shunt current.

The shunt employs the top metal layers (M2-to-M5), which allows the temperature-sensing PNPs to be located directly underneath it, thus improving a thermal coupling to the shunt. As shown inFIG. 2, this coupling is further improved by the use of thermal vias between the shunt and an M1sheet surrounding the PNPs. Measurements show that these modifications result in a three times higher improvement in the accuracy of the estimated shunt temperature compared to previous publications.

Joule heating in the parasitic resistance of the connections between the CSS and the outside world represents wasted power. Furthermore, it causes on-chip temperature gradients, which in turn introduce errors in the estimated shunt temperature. To minimize such parasitic resistances, the chip was mounted directly on a PCB, to which the on-chip shunt was connected by 32 short (<1 mm long) bond wires. The total parasitic series resistance is then less than 10 mOhms. In mass production, such low resistances can be achieved by the use of wafer-level packaging and ball-bonding.

To digitize both T and Ibat, the ADC is operated in incremental mode and time-multiplexed: with conversion times of 22.5 ms for Ibat and 2.5 ms for T, respectively. A temperature averaging scheme (TAS) in which the average of two adjacent T measurements is used to compensate each Ibat measurement, results in improved accuracy, especially during transients.

FIG. 3shows layout of the CSS. The CSS is integrated in an integrated circuit1000, which means that the parts of the CSS share a common silicon substrate, the substrate100shown inFIG. 2.

The components of the CSS that are arranged from the left to the right in the subsequent order are: shunt resister4, bandgap21, analog-digital-converter5and correction circuit6. The shunt resister4is a top-down view. The line A-A indicates the cross-section ofFIG. 2. The metal layer MS comprises an uninterrupted conductor23. On the top of the uninterrupted conductor23, terminals30are provided, which are connected by conductors26to bond pads27that are connected to the outside of the integrated circuit1000by wire bonds27. The terminals30define the periphery P. The current through the terminals of the shunt resistor flows within the periphery P. In other words, the path between the terminals is within the periphery P. The transistors9and10, these being the PNPs that are part of the “bandgap”, under the metal layer23are arranged in a middle position having about the same distances to the periphery lines. The transistors are within the periphery of the metal layers ensuring that they are—in the planar view ofFIG. 3—not outside of the metal conductors. Conductors18and20, which are part of the first metal layer18extend to the current sources7and8. The analog-to-digital converter is provided right of the bandgap21, receiving voltages from the bandgap21and from the shunt resistor4. Right of the analog-digital-converter is the digital correction circuit that compensates inter alia for the temperature-dependency of the shunt resistor4. The compensated output value the output the bond pad25, which is connected to a wire bond that connects external components to the integrated circuit1000.