Wide temperature range RTD

[0002] The present invention pertains to sensors and particularly to temperature sensors. More particularly, the invention pertains to resistance temperature devices.

[0003] Thermocouples produce small signals and require a cold junction for accurate temperature measurements. Thermocouples are unstable and require calibration. Thermistors have a limited range of temperature measurement because of a high temperature coefficient of resistance (TCR). Also, thermistors are not compatible with integrated circuitry. Related art resistance temperature devices (RTD's) typically are fabricated with platinum (Pt) metal wires or films. Several disadvantages of Pt RTD's are expensive Pt and low resistance. RTD's using metal conductor sensing elements have low base resistances, which are difficult to measure without heating the devices, and have different performance profiles at cold and hot temperatures.

[0004] The present invention is a RTD having a hafnium (Hf) nitride (N) sensing element. This RTD has a wide temperature operating range, has a simple temperature performance profile and is compatible with silicon processing and integrated circuitry. The present RTD may be passivated with merely a silicon nitride (SiN) film. The RTD is just a two-wire device in contrast to related-art RTD's that require three or four wires for useful measurements over a practical temperature range. The present RTD is quite manufacturable and commercially viable. This temperature sensor has application in thermal control and propulsion system monitoring of spacecraft, and for providing measurements of automotive and aircraft engines, brakes and emission control systems. The RTD also has cryogenic applications.

[0005] FIGS. 1a,1band 1cshow several embodiments of an HfN RTD.

[0006] FIG. 2 shows another embodiment of an HfN RTD.

[0007] FIG. 3 is a graph showing resistance versus temperature for an HfN RTD.

[0008] FIG. 4 shows a polynomial fit of resistance versus temperature for the HfN RTD.

[0009] FIG. 5 shows comparison data of resistance versus temperature for an HfN RTD, a Pt RTD and a thermistor.

[0010] FIG. 6 is a graph showing HfN resistor temperature cycling over time.

[0011] FIG. 7 is a schematic of an RTD and associated electronics.

[0012] FIG. 8 shows a diagram of interface electronics for an RTD.

[0013] FIG. 1ashows a layout of an RTD 10. A thin film serpentine resistor 11 fabricated from HfN is deposited, grown or formed a silicon die 12. Resistor 11 is connected to bonding pads 13 deposited, grown or formed on die 12. Typical bonding pads 15 are made from titanium-platinum (TiPt)/gold (Au) films. Die 12 is formed or placed on substrate 14. Substrate 14 may be alumina (Al₂O₃), ceramic, glass or other like material.

[0014] The substrate 12 may also be a mounted on a die header 31, in a configuration 40 shown in FIG. 1c,with the wire bond connections going from the chip pads to the header posts. Also, on substrate 14 are low resistance metal lines or contacts 15, which are composed of nickel-chromium (NiCr) metal but could also be composed of other metals such as Ni. Wires 16 bonded to pads 13 and metal lines or contacts 15 connect resistor 11 to strips 15. Wires 16 are typically gold but may also be aluminum or platinum. Leads or wires 17 are bonded to contacts, lines or strips 15 for providing connections of RTD 10 to the external devices. Leads or wires 17 may be composed of copper. In configuration 40 of FIG. 1c,chip 12 is mounted on a T08 header 31. The leads 32 are nickel wires spot-welded to the header leads 35. An HfN resistor 34 on the die is electrically attached to two header posts 36 with Au wires 33. The size and thickness of die 12 are about 0.5 millimeters (20 mils) thick and 3.2 millimeters (⅛ inch) square although smaller dies at 0.5 millimeter (mm) on a side are possible. The length and thickness of resistor 11 is about 50 nanometers (500 Angstroms) thick, 4 microns wide and 100 microns long. The bond pads 13 are about 0.1 mm (0.004 inch) in size. The sizes and thickness of strips 15 are about 25 to 50 microns wide and 0.25 to 2.5 mm long. The size and thickness of substrate 14 is about 3.2 mm thick and approximately 2.54 centimeters (cm) lateral dimension.

[0015] FIG. 1bshows a layout of RTD 20. Thin film serpentine resistor 11, having a composition of HfN, is deposited grown or otherwise formed on substrate 14. Substrate 14 may be made from alumina, ceramic, glass or other like material. Resistor 11 has its ends bonded or connected to low resistance metal pads, lines, strips or contacts 15. Leads or wires 17 are connected to contacts 15. Contacts 15 and wires 17 may be fabricated from similar materials as contacts 15 and wires 17 of RTD 10, respectively. Both substrates 14, or at least the portion of the substrate with resistor 11, may be covered and annealed with a passivation layer to seal resistor 11 from the ambient environment. An example of this passivation layer or thin film 18 is shown as a part of RTD 30 in FIG. 2. This layer 18 may be composed of silicon nitride (SiN). Passivation layer 18 makes RTD's 10, 20 and 30 more durable and robust for high temperature operation. A vacuum annealed HfN RTD results in excellent stability of accuracy over time.

[0016] RTD 30 of FIG. 2 has a serpentine thin film HfN resistor formed on substrate 14, which may be composed of silicon, ceramic, glass, alumina or other like material. Also, connected in series with resistor 11 is a ladder resistor 19 also composed of HfN. Resistor 19 may be trimmed with a laser light by cutting one of the straight sections of resistor 19, to calibrate the total resistance of resistors 11 and 19. This high resolution trimming of base resistance of resistors 11 and 12 is a precision-attaining procedure and is accomplished before passivation layer 18 is deposited or formed on resistors 11 and 19. The open resistor 11 and resistor 19 ends are bonded or connected to a pad 15. To pad 15 are bonded or connected leads or wires 17.

[0017] The composition of HfN used for resistors 11 and 19 may be HfN_x, where x may be between 1.0 and 2.0. Rutherford backscattering measurements on similar films have shown that the ratio of nitrogen to Hf is on the order of 1.25. The HfN is better characterized by its resistance in resistor 11 rather than the composition ratio of the material. Also, the configuration of resistor 11 need not be only of a serpentine fashion. Likewise, resistor 19 may have another configuration.

[0018] RTD's 10, 20 and 30 function with high accuracy in sensing temperatures from 20 degrees Kelvin (K) (minus 253 degrees Celsius (C.), minus 423 degrees Fahrenheit (F)) to 1400 degrees K (1130 degrees C., 2066 degrees F.). FIG. 3 is a graph showing the value of resistance of resistor 11 versus temperature in degrees Kelvin. Measurements on HfN resistor 11 have shown that the material can be formed with a resistance from 100 ohms per square to 1,000 kilohms (K-ohms) per square, and be patterned with 100 to 1,000 squares yielding ten K-ohm to one megohm (M-ohm) resistors.

[0019] FIG. 3 shows HfN resistor 11 to have a rather uniform resistance over temperature. Thin curve 21 reveals the heat measurements and thick curve 22 reveals the cool measurements. Resistor 11 for FIG. 3 has HfN resistance 500 ohms per square material.

[0020] FIG. 4 shows a close agreement between plots 23 of temperature at the measured resistance of resistor 11 with temperature calculated from one (fifth-order) polynomial fit 24 to the resistance. The fit is within 0.5 degrees K over the temperature range. Resistor 11 here has HfN resistance of 500 ohms per square material. The uniformity and high value of the resistance of resistor 11 over a large temperature range permits complete use of the present RTD's 10, 20, 30 with only two wires, which reduces complexity, weight and costs in applications.

[0021] Related-art RTD's, such as those having resistors made from Pt or like metals, have a low resistance in the range of 100 to 1000 ohms because of the low resistance of the metals. For useful measurements of practical accuracy, such RTD's require three or four point wire measurements or leads which increase complexity, weight and costs in applications. Further, the Pt RTD, for instance, has a different performance profiles at cold and hot temperatures having a TCR ranging from approximately 0.44 at minus 200 degrees C. to 0.29 at 850 degrees C. This performance requires the use of at least two polynomial fits to the Pt RTD output signal over temperature to obtain acceptable temperature measurements over a useful temperature range. FIG. 5 shows a comparison of resistance on a logarithmic scale versus temperature for the present HfN RTD 10, 20, 30, Pt RTD and a 3.9 percent thermistor. Curves 25, 26 and 27 show resistance versus temperature plots for the HfN RTD, Pt RTD and thermistor, respectively.

[0022] FIG. 6 shows RTD stability as a result of cold and hot temperature cycling of a passivated HfN resistor 11. Sensor 10 is (400-degree annealed) passivated with only an SiN film. Curve 28 is temperature versus time in hours. Curve 29 is the resistance of resistor 11 versus time. Cycling between 300 degrees C. and zero degrees C. shows a slight resistance drift of sensor 30. There is no resistance drift below zero degree C. The 300 degree C. drift is about 0.003 percent per hour for a 200 hour period. The high temperature drift is reflected in the zero degree C. drift. The hot-to-cold resistance ratios remain unchanged (0.00001 percent per hour) despite drift. Passivating RTD 30 more should result in more stability and a larger temperature range of operation.

[0023] FIG. 7 shows RTD 10, 20, 30 connected to processing electronics 38. A temperature indicator 39 is connected to processing electronics 31. FIG. 8 is a schematic revealing an analog buffering interface 41 for the RTD 10, 20, 30 output 37. Output 37 is connected to ground with a bias resistor 42 and to a non-inverting input of an operational amplifier 43. The operational amplifier is connected in a voltage follower configuration with output 44 fed back to the inverting input of operational amplifier 43. Output 44 has a voltage that is an indication of temperature measured by RTD 10, 20, 30. Output 44 may be connected to an analog-to-digital (A/D) converter. The digital output of the converter may go to a digital processor for signal processing or recording in a memory as desired. Interface 41, A/D converter, digital processor and memory are part of processing electronics 38. Also, a digital-to-analog converter, if desired, may be part of electronics 38. Indicator 39 may receive analog or digital signals to provide visual or electronic indications, for various applications, of the temperature measurements from the RTD or other processed information, such as from the memory.

[0024] Though the invention has been described with respect to a specific preferred embodiment, many variations and modifications will become apparent to those skilled in the art upon reading the present application. It is therefore the intention that the appended claims be interpreted as broadly as possible in view of the prior art to include all such variations and modifications.