Temperature Sensor and Method

The invention relates to sensors and, in particular, to temperature sensors and methods for measuring ambient temperature.

Known temperature sensors employ a variety of methods of measuring ambient temperatures. The thermocouple method takes advantage of the fact that, when two wires composed of dissimilar metals are joined at the ends and one of the ends is heated, there is a continuous current generated which flows in the thermo-electric circuit. If the circuit is broken at the center, an open circuit voltage is created which is a function of the junction temperature and the composition of the two dissimilar metal wires. However, the open circuit voltage to temperature relation is non-linear and the thermocouple typically exhibits poor sensitivity and stability characteristics making it difficult to employ in most situations.

The resistance temperature detector (RTD) takes advantage of the principal that the resistivity of metals is, to a small degree, dependent upon temperature. However, the RTD is typically very expensive (because the most commonly used metal for the RTD is platinum), has a low absolute resistance, and results in a small change in resistance relative to a change in temperature. One variation of the RTD is the metal film RTD. A thin metal film of platinum or metal-glass slurry is deposited or screened onto a small flat ceramic substrate, etched with a laser trim system and sealed. While the metal film RTD's are relatively easy to manufacture and offer increased resistance, they are less stable than the traditional RTD's.

The thermistor has also been employed to measure temperature. The thermistor is a temperature sensitive resistor and is generally composed of semi-conductor materials. While the thermistor is sensitive, it is also extremely non-linear and the response of the thermistor to temperature changes is highly dependent upon the process used to manufacture the thermistor. Additionally, the thermistor is extremely fragile and suffers from the additional problem of self-heating. That is, the thermistor generates sufficient energy to heat itself causing a concomitant increase in the resistivity of the thermistor.

The integrated circuit temperature sensor has also been used to measure temperatures. The integrated circuit sensor typically employs an integrated diode whose output characteristics are dependent upon temperature. Like the thermistor, the integrated circuit sensor suffers from self-heating and offers an extremely slow response to changes in temperature.

It is also known that integrated circuit resistors exhibit a resistivity which is dependent upon the temperature of the resistor and that this temperature dependence varies in accordance with the type and amount of impurity with which the resistor is doped. This temperature dependency has been considered a significant problem in integrated circuit technology because any integrated circuit employing these resistors exhibits some degree of temperature dependence requiring compensation.

In general terms, the invention provides a first embodiment of a temperature sensor for producing an electrical signal related to ambient temperature, and including temperature responsive means for generating a differential voltage output functionally related to ambient temperature, and amplifier means connected to the temperature responsive means for amplifying the differential output. Preferably, the invention provides a first embodiment of a temperature sensor including integrated thin film silicon or polysilicon resistors that are doped with selected concentrations of impurities such as boron, phosphorus, arsenic or antimony. The type and amount of dopant for the selected resistors is chosen so as to establish a predetermined temperature coefficient in at least some of the resistors so that the resistors exhibit a predetermined temperature dependence. This temperature dependence is utilized within the circuit of the temperature sensor to create a temperature sensor that is stable, accurate, and rugged and that has a generally linear output to temperature response.

The ability to select a predetermined temperature dependence allows varying degrees of linearity or non-linearity to be designed into the circuit. If a linear response is desired, the doped resistors are usually doped with boron at a concentration of between 5.times.10.sup.15 ions/cm.sup.2 to 5.times.10.sup.16 ions/cm.sup.2 or doped with phosphorus at a concentration of between 1.times.10.sup.15 ions/cm.sup.2 to 1.times.10.sup.16 ions/cm.sup.2.

The circuit of the temperature sensor includes a resistor bridge, in the form of a wheatstone bridge that includes a first resistor arm having serially connected resistors R1 and R2 with a first node between R1 and R2, and a second resistor arm having serially connected resistors R3 and R4 with a second node between R3 and R4. The resistors R1 and R2 have dissimilar temperature coefficients in order to provide at the first node a reference voltage that decreases with increasing temperatures. The resistors R3 and R4 also have dissimilar temperature coefficients in order to provide at the second node a voltage that increases with increasing temperatures. The first node is electrically connected to the inverting input of an operational amplifier and the second node is connected to the non-inverting input of the operational amplifier. The operational amplifier includes an output (V.sub.out) and a feedback resistor R.sub.f is connected between the V.sub.out and the first node or inverting input of the operational amplifier. R.sub.f has a temperature coefficient that results in a gain for the amplifier which increases with increasing temperature to provide an output ranging from zero to five volts in response to a temperature range of -40.degree. to +150.degree. centigrade with very little non-linearity. The gain of the amplifier is determined by the equation R.sub.f divided by the Thevenin equivalent resistance seen at the non-inverting input of the operational amplifier.

A principal advantage of the first embodiment of the temperature sensor is the provision of a method of measuring temperature and a temperature sensing circuit that uses thin film silicon or polysilicon resistors that are selectively doped with boron, phosphorus, or other impurities to create a resistor having a predetermined temperature coefficient such that the resistivity of the resistor responds to temperature in a predetermined way.

Another advantage of the first embodiment of the temperature sensor is the provision of an operational amplifier and biasing resistors for the operational amplifier that vary the gain of the amplifier in accordance with the ambient temperature.

Another advantage of the first embodiment of the temperature sensor is the provision of a method for measuring temperature and a temperature sensor circuit using integrated silicon and polysilicon resistors having a selected temperature coefficient.

Another advantage of the first embodiment of the temperature sensor is the provision of a method for measuring temperature and a circuit having a substantially linear output from zero to five volts in response to a temperature range of -40.degree. C. to +150.degree. C.

The invention also provides for a second embodiment of a temperature sensor wherein the sensor includes a feedback and biasing network comprised of three doped polysilicon resistors that are doped at the same level therefore making the gain not change with temperature. The temperature sensitive element of the sensor is a voltage divider made up of the two doped polysilicon resistors, one resistor having a positive temperature coefficient and the other resistor having a negative temperature coefficient. The temperature sensor operates with one half bridge responding to temperature and the other half bridge maintaining a constant gain and setting an offset voltage independent of temperature. Through the use of such a configuration, an operational amplifier can be utilized as a single ended amplifier due to the offset voltage created by the circuit.

Other features and advantages of the invention will become apparent to those of ordinary skill in the art upon review of the following detailed description, claims and drawings.

FIG. 1 is a schematic diagram of a first embodiment of a temperature sensor of the invention; and

FIG. 2 is a schematic diagram of a second embodiment of a temperature sensor of the invention.

Shown schematically in FIG. 1 is a first embodiment of a temperature sensor. The temperature sensor 10 includes temperature responsive means 14 for generating a differential voltage functionally related to ambient temperature, amplifier means 18 connected to the temperature responsive means 14 for amplifying the differential output, and means 22 for adjusting the gain of the amplifier means 18 in response to variations in the ambient temperature so as to generate an electrical signal at the output of the amplifier which signal is indicative of the ambient temperature.

In the preferred embodiment, the temperature sensor 10 is an integrated circuit which is preferably, though not necessarily, formed on a single substrate. The formation of the circuit on a single substrate assures that the individual elements of the circuit would be exposed to the same temperature variations and thereby provide a consistent reliable electrical output which is indicative of the ambient temperature to which the circuit is exposed.

In the illustrated embodiment of FIG. 1, the temperature responsive means 14 is a resistor bridge network that may be a half-bridge resistor network, a full-bridge resistor network or any other combination of silicon or polysilicon resistors that have an impedance that varies with temperature as a result of the selected doping of impurities in the resistors. The temperature responsive means 14 could even be a single resistor in a network that causes a varying voltage to develop across the resistor and causes the voltage to vary in response to the changes in ambient temperature.

In the embodiment shown in FIG. 1, the temperature responsive means 14 is a full-bridge resistor network including a first arm 26 having first and second serially connected resistors R1 and R2 respectively and a node 30 between the resistors R1 and R2. The resistors R1 and R2 are formed of a silicon or polysilicon material which is selectively doped with an impurity such as boron or phosphorus. Doping of the resistors with such impurities varies the temperature coefficients of the resistors so that the impedance of the resistors varies in accordance with the ambient temperature to which the resistors are exposed. The concentration of the dopant is typically expressed in ions/cm.sup.2. In the embodiment shown in FIG. 1, resistor R1 is doped with 1.8.times.10.sup.16 ions/cm.sup.2 phosphorous and the resistor R2 is doped with 5.times.10.sup.15 ions/cm.sup.2 boron. The use of different dopants or varying concentration of the dopants assures that the resistors R1 and R2 have dissimilar temperature coefficients so as to provide at the node 30 a reference voltage that decreases with increases in the ambient temperature. Many available impurities or concentrations of the impurities may be appropriate, and these will vary depending upon the desired output characteristics, for example, linear or non-linear, and the circuit topology which dictates the mathematical relationships between the individual components. In the embodiment shown in FIG. 1, the temperature coefficients are as follows:

TCR1=967 ppm/.degree.C.; where TCR1 is the temperature coefficient of R1;

TCR2=427 ppm/.degree.C.; where TCR2 is the temperature coefficient of R2.

These temperature coefficients result in temperature dependent impedances for R1 and R2 of:

The temperature responsive means 14 also includes a second arm 34 having a third resistor R3, a fourth resistor R4 connected in series with the third resistor R3, and a node 38 between the third and fourth resistors R3 and R4. Like the resistors R1 and R2 of the first arm 26, the resistors R3 and R4 are silicon or polysilicon resistors that are doped with impurities such as boron or phosphorus so that the resistors R3 and R4 each have a predetermined temperature coefficient. The predetermined temperature coefficients cause the impedances of the resistors R3 and R4 to vary according to temperature. In particular, the temperature coefficients of resistors R3 and R4 are precisely selected so as to generate a reference voltage at the node 38 that increases in accordance with increases in the ambient temperature. While many available impurities or concentrations of impurities are appropriate, the resistor R3 is doped with a concentration of 5.times.10.sup.15 ions/cm.sup.2 boron and the resistor R4 is doped with 1.8.times.10.sup.16 ions/cm.sup.2 phosphorous. In the embodiment shown in FIG. 1, the temperature coefficients of R3 and R4 are as follows:

TCR3=427 ppm/.degree. C.; where TCR3 is the temperature coefficient of R3;

TCR4=967 ppm/.degree. C.; where TCR4 is the temperature coefficient of R4.

These temperature coefficients result in temperature dependent impedances for R3 and R4 of:

The first arm 26 and second arm 34 are connected in parallel so that resistor R1 is connected to resistor R4 at node 42 and resistor R2 is connected to resistor R3 at node 46. Node 42 is connected to a voltage source V.sub.cc to provide power to the circuit. The voltage source is a regulated positive 5 volt direct current source. The node 46 is connected to a common or ground connection 48.

The output of the temperature responsive means 14 is the differential voltage across the nodes 30 and 38. Because the temperature responsive means 14 is a resistor network in which the output of the resistors R1, R2, R3 and R4 is dependant upon the interaction of the resistors, the discreet values of the resistors (at a selected reference temperature) are not as significant as the scaled or normalized values of the resistors R1, R2, R3 and R4 relative to one another. The normalized values of the resistors R1, R2, R3, and R4 of temperature responsive means 14 at 25.degree. C. are as follows:

R1=1.0 ohm

R2=1.0 ohm

R3=1.015 ohms

R4=1.0 ohm

The amplifier means 18 is preferably an operational amplifier 50 formed on the same integrated circuit substrate as the temperature responsive means 14. The operational amplifier 50 includes an inverting input 54 which is connected to the node 30, and a non-inverting input 58 which is connected to the node 38. Therefore, the operational amplifier 50 receives an input which is the differential of the reference voltage created by resistor R1 and R2 at the node 30 and the reference voltage created by resistors R3 and R4 at the node 38. As is known in the art, the operational amplifier 50 is also connected to V.sub.cc and to a ground or common connection 62. The operational amplifier 50 also includes an output V.sub.out and a feedback resistor R.sub.f connected between the output V.sub.out and the inverting input 54 of the operational amplifier 50. Though not necessary, the operational amplifier 50 also includes a trim or calibrating resistor R.sub.ftrim connected serially to feedback resistor R.sub.f to allow for calibration of the amplifier.

As is commonly known in the art, the amplifier means 18 has a gain which may be selectively established based on the components connected to the operational amplifier 50. The gain may be a voltage gain, a current gain, a transimpedance, or a transconductance and the gain may be a positive, a negative or an unity gain (ignoring temperature related adjustments). In the case of the temperature sensor 10 shown in FIG. 1, the amplifier is in a voltage gain configuration and the gain is determined by the equation

where R.sub.in equals the Thevenin equivalent resistance R.sub.thev seen at the inverting input 54 of the operational amplifier 50. At 25.degree. C., the Thevenin equivalent resistance R.sub.thev is determined by the following equation: ##EQU1##

Taking into account the temperature dependence of the resistors R1 and R2, the Thevenin equivalent resistance R.sub.thev at any temperature is: ##EQU2##

The Thevenin equivalent voltage source at any temperature is: ##EQU3##

As discussed above, the temperature sensor 10 also includes means 22 for adjusting the gain of the amplifier means 18 in response to variations in ambient temperature so as to generate an electrical signal at the output which is indicative of the ambient temperature. The means 22 for adjusting the gain of the amplifier is integral with the feedback resistor and the resistors R1 and R2 in the first arm 26 of the resistor network. The feedback resistor R.sub.f is a silicon or polysilicon thin film resistor that is doped with an impurity such as boron or phosphorus in select concentrations to establish a predetermined temperature coefficient. While many available impurities or concentrations of impurities are appropriate, the feedback resistor R.sub.f is doped with a concentration of 1.8.times.10.sup.16 ions/cm.sup.2 phosphorus which results in a temperature coefficient of: TCRF=967 ppm/.degree.C. and a temperature dependent resistance R.sub.tf which is defined by the equation:

Because of the predetermined temperature coefficient, the impedance of the feedback resistor R.sub.f varies in a predetermined way in accordance with changes in ambient temperature. Substituting R.sub.tf and R.sub.t thev into the gain equation results in the following temperature dependent mathematical relationship for the gain of operational amplifier 50 at any temperature: ##EQU4## and the temperature dependent output of the operational amplifier 50 is determined by: ##EQU5##

In operation, the temperature sensor 10 is designed to produce an output of zero volts d.c. at -40.degree. C. The output response of the temperature sensor 10 preferably, though not necessarily, increases linearly to a full-scale 5.0 volt d.c. output at a temperature of 150.degree. C. To calibrate the temperature sensor 10, the voltage at the node 38 is measured at 25.degree. C. and R3 is trimmed until that voltage measures 2.48 volts d.c. Next, the voltage at the node 30 is measured at 25.degree. C. and R1 is trimmed until that voltage is 2.5 d.c. Finally, the output voltage V.sub.out is measured at 25.degree. C. and the gain of operational amplifier 50 is adjusted by trimming R.sub.f trim until V.sub.out =1.731 volts d.c.

At -40.degree. C. the temperature sensor 10 generates an output of approximately zero volts d.c. As ambient temperature increases, the voltage at the node 30 decreases, the voltage at the node 38 increases and the gain of the operational amplifier 50 varies according to the above-identified temperature dependant gain equation so that at zero .degree.C., V.sub.out is approximately 1.049 volts d.c.; at 25.degree. C., V.sub.out is approximately 1.731 volts d.c.; at 80.degree. C., V.sub.out is approximately 3.182 volts d.c.; and at 150.degree. C., V.sub.out is approximately 5.0 volts d.c.

Referring now to FIG. 2, a second embodiment of the temperature sensor is illustrated. The second embodiment, sensor 100, is similar to the first embodiment of FIG. 1 with the following changes.

The temperature sensor 100 includes temperature sensing element such as a first arm 102 having first and second serially connected resistors R.sub.T1 and R.sub.T2 respectively and a node 104 therebetween. The resistors R.sub.T1 and R.sub.T2 are formed of a silicon or polysilicon material which is selectively doped with an impurity such as boron or phosphorus. Doping of the resistors with such impurities varies the temperature coefficients of the resistors so that the impedance of the resistors varies in accordance with the ambient temperature to which the resistors are exposed. While other dopants such as boron, arsenic and antimony may be appropriate, in the preferred embodiment, the resistor R.sub.T1 is implanted with low dose phosphorus at a level of 2.0.times.10.sup.15 cm.sup.-2 at 80 keV with an impedance value of approximately 240 ohms/square at 25 C. and the resistor R.sub.T2 is doped in situ with phosphorus during the polysilicon LPCVD with an impedance value of approximately 20 ohms/square at 25 C. The use of different dopants or varying concentrations of the dopants assures that the resistors R.sub.T1 and R.sub.T2 have dissimilar temperature coefficients so as to provide at the node 104 a voltage V.sub.T that changes with changes in ambient temperature.

The temperature sensor 100 also includes a resistor network such as a second arm 106 which includes a resistor R.sub.1 and a resistor R.sub.2 connected in series and a node 108 therebetween. The resistors R.sub.1 and R.sub.2 are silicon or polysilicon resistors that are doped with the same impurities at the same level so as to have the same temperature coefficient.

As shown in FIG. 2, both resistors R.sub.T1 and R.sub.1 are connected to a voltage source V.sub.cc to provide power. Preferably, V.sub.cc is a regulated positive 5 volt direct current source. Resistors R.sub.T2 and R.sub.2 are connected to a common or ground connection 110.

The temperature sensor of the second embodiment further includes an amplifier 112 and a feedback resistor R.sub.FB. The amplifier 112 is preferably a standard operational amplifier having an output V.sub.out. Due to the design of the temperature sensor 100, an instrumentation amplifier is not necessary. Node 104 is connected to the positive input of the amplifier 112 and node 106 is connected to the negative input of the amplifier 112. The feedback resistor R.sub.FB is connected between the output V.sub.out and the negative input of the amplifier 112.

The resistor R.sub.FB is doped at the same level with the same material as the resistors R.sub.1 and R.sub.2 so as to have the same temperature coefficient. For example, the resistors R.sub.1, R.sub.2 and R.sub.FB are implanted with phosphorus at level of 2.0.times.10.sup.15 cm.sup.-2 at 80 keV.

Thus, the feedback gain of the sensor 100 does not change with temperature. The resistors R.sub.1, R.sub.2 and R.sub.FB make up a feedback network to set the gain as follows:

The amplifier element, which includes the resistors R.sub.1, R.sub.2 and R.sub.FB and the operational amplifier, is a single input non-inverting input with the above gain A.

The temperature sensing element of sensor 100 is the voltage divider made up of the two resistors R.sub.T1 and R.sub.T2. The voltage V.sub.T varies with temperature and is amplified by the gain A such that:

wherein

T.sub.0 =some chosen arbitrary yet fixed temperature in the region of operation of the sensor, for example, room temperature;

T=temperature to be measured

R.sub.T20 =value of R.sub.T2 at temperature T.sub.0

R.sub.T10 =value of R.sub.T1 at temperature T.sub.0

wherein the first and second order temperature coefficients for the illustrated embodiment are as follows:

.alpha..sub.1 =-0.0011

.alpha..sub.2 =0.00101

.beta..sub.1 =3*10.sup.-8

.beta..sub.2 =6.4*10.sup.-7

wherein preferably

R.sub.T1 has a negative temperature coefficient, and

R.sub.T2 has a positive temperature coefficient.

Accordingly, as temperature increases, R.sub.T1 decreases, R.sub.T2 increases and V.sub.T increases. As the temperature decreases, R.sub.T1 increases, R.sub.T2 decreases and V.sub.T decreases.

The resistor network preferably includes a resistor divider made up of R.sub.1 and R.sub.2 to create an offset voltage which allows a single supply as compared to the two normally required for an operational amplifier.

The equation of operation of the sensor 100 is derived as below:

wherein V.sup.- =V.sub.T if the gain A of the operational amplifier is much larger than the closed loop gain: A>>R.sub.FB /(R.sub.1 //R.sub.2)+1. Therefore: ##EQU6## With the preferred embodiment, V.sub.out ranges from zero to five volts in response to a temperature range of -40.degree. to +150.degree. centigrade with very little non-linearity.

The resistors R.sub.T1 and R.sub.T2 are trimmed such that V.sub.T =V.sup.-. R.sub.1 and R.sub.2 are trimmed for proper offset. The resistor R.sub.FB is set to get the correct sensitivity to temperature. The temperature response is measured and R.sub.FB is trimmed.

As with the first embodiment, preferably the sensor 100 of the second embodiment is an integrated circuit which is preferably, though not necessarily, formed on a single substrate with no external resistors. To manufacture the sensor 100 as an integrated circuit, conventional integrated circuit manufacturing steps can be utilized.

The second embodiment of the temperature sensor uses all doped polysilicon resistors. Unlike the first embodiment, the sensor 100 does not use the full bridge as the temperature sensing element. The first arm or half-bridge 102 of the sensor 100 is temperature dependent while the second arm 106 is not temperature dependent. The two polysilicon resistors in the first arm 102 act as the temperature sensing element of the sensor 100 such that V.sub.T varies with changes in ambient temperature. The purpose of the second arm or half-bridge 106 is to set the gain. Therefore, the sensor 100 uses the half bridge 102 in combination with the resistor network of the second arm 106 and the feedback resistor R.sub.FB to modify the gain of the sensor 100. In this arrangement, only one operational amplifier can be used in the sensor 100 and turned into a single ended amplifier with the resistor network to modify the gain. The resistor network of R.sub.1 and R.sub.2 creates an offset voltage which allows for a single supply to the amplifier. It is important to note that the offset is trimmable independent of temperature.

Various other features and advantages of the invention are said forth in the following claims.