Self-compensated ceramic strain gage for use at high temperatures

[0002] This application claims the benefit of Provisional application Ser. No. 60/131,584, filed Apr. 29, 1999.

[0003] 1. Field of the Invention

[0004] The invention relates to thin film strain gages.

[0005] 2. Description of the Relevant Art

[0006] The accurate measurement of both static and dynamic strain, at elevated temperatures is frequently required to determine the instabilities and life-times of various structural systems, and in particular, advanced aerospace propulsion systems. Conventional strain gages are typically applied to both stationary and rotating components for this purpose but are usually limited in scope due to their intrusive nature, severe temperature limitations and difficulties in bonding.

[0007] Thin film strain sensors are particularly attractive in the gas turbine engine environment since they do not adversely effect the gas flow over the surface of a component and do not require adhesive or cements for bonding purposes. Typically, thin film strain gages are deposited directly onto the surface of a component by rf sputtering on other known thin film deposition technology and as a result are in direct communication with the surface being deformed. In general, the piezo-resistive response or gage factor (g), of a strain gage is the finite resistance change of the sensing element when subjected to a strain and can result from (a) changes in dimension of the active strain element and/or (b) changes in the resistivity (p) of the active strain element. Further, the active strain elements used in a high temperature static strain gage, must exhibit a relatively low temperature co-efficient of resistance (TCR) and drift rate (DR) so that the thermally induced apparent strain is negligible compared to the actual mechanical applied strain.

[0008] One material of choice for high temperature thin film strain gages is a wide band semiconductor, e.g. indium-tin oxide (ITO), due to its excellent electrical and chemical stability and its relatively large gage factor at high temperature. When used alone it is usually limited by relatively high TCRs as is the case for many intrinsic semiconductors. However, as disclosed herein the TCR of a self-compensated ITO strain sensor can be reduced using a metal, e.g. Pt as a thin film resistor placed in series with the active ITO strain element.

[0009] However, the proper combination of materials, patterns and dimensions to fabricate a strain gage with a predetermined TCR is mostly a matter of empirical observation, i.e. trial and error.

[0010] With the present invention, knowing the temperature range that the sensor is to be operative at and the resistivities of the materials each at a working and a reference temperature one can automatically determine the TCR of a high temperature strain gage.

[0011] Broadly the invention comprises a self-compensated strain gage sensor having an automatically determined TCR including a TCR of essentially zero. The sensor comprises a wide band semiconductor deposited on a substrate. A metal is deposited on the substrate and in electrical communication with the semi-conductor functioning as serial resistors, the length, width and thicknesses of the semi-conductor and the metal are selected based on their resistivities at selected working and reference temperatures and the TCR automatically determined.

[0012] The semiconductors can be selected from the group consisting of silicon carbide, aluminum nitride, zinc oxide, gallium nitride, indium nitride, scandium nitride, titanium nitride, chromium nitride, zirconium nitirde, boron carbide, diamond, titanium carbide, tantalum carbide, zirconium carbide, gallium phosphide, aluminum gallium nitride, zinc oxide doped with alumina, cadmium telleride, cadmium selenide, cadmium sulfide, mercury cadium telleride, zinc selenide, zinc telleride, magnesium telleride, tin oxide, indium oxide, manganates-manganese oxides with iron oxides, iron oxide-zinc-chromium oxide, iron oxide-magnesium-chromium oxide, ruthenium oxide, lithium doped nickel oxide, tantalum nitride, indium-tin oxide-gallium oxide-tin oxide and combinations thereof.

[0013] The metal resistors can be selected from the group consisting of platinum, rhodium, palladium, gold, chromium, rhenium, irridium, tungsten, molybdenum, nickel, cobalt, aluminum, copper, tantalum, alloys of platinum and rhodium and combinations thereof.

[0014] A particularly preferred semi-conductor is indium tin oxide and a particularly preferred metal is platinum.

[0015] FIG. 1 is an illustration of a sensor design;

[0016] FIG. 2 is a simulated circuit of the design of FIG. 1; and

[0017] FIG. 3 is an illustration of an alternative sensor design; and

[0018] FIG. 4 is a graph of the resistance (signal) of the sensor of FIG. 1 changing with temperature.

[0019] Discussion

[0020] To establish adequate design rules for the self-compensated strain gage, the TCR of a self-compensated strain sensor is first modeled. The following approach is used to model the TCR of an ITO sensor with Pt self-compensation circuitry:

[0021] TCR_COMP=(R_COMP,f−R_COMP,0)/(R_COMP,0*ΔT)  (1)

[0022] Where R_COMP,fis compensated sensor resistance at a specific temperature, R_COMP,0is compensated sensor resistance at a reference temperature, ΔT is the temperature difference, and

[0023] R_COMP,f=R_Pt,f+R_ITO,f  (2)

[0024] R_COMP,0=R_Pt,0+R_ITO,0  (3)

[0025] Substituting equation (2) and (3) into equation (1), TCR_COMP,results in

[0026] TCR_COMP=((R_Pt,f+R_ITO,f)−(R_Pt,0+R_ITO,0))/((R_pt,0+R_ITO,0)*ΔT)  (4)

[0027] The resistance R is related to resistivity (ρ) which is a constant at a specific temperature,

[0028] R=ρ*L/(w*t)  (5)

[0029] Where L, w and t are length, width and thickness of sensor film. Substituting R into equation (4) TCR_COMP, results in a final format equation to model TCR_COMP.

[0030] TCR_COMP=(Δρ_Pt*A_pt+Δρ_ITO*A_ITO)/((ρ_Pt,0*A_Pt+ρ_ITO,0*A_ITO)*ΔT)  (6)

[0031] Where

[0032] Δρ_Pt=ρ_Pt,f−ρ_Pt,0  (7)

[0033] Δρ_ITO=ρ_ITO,f−ρ_ITO,0  (8)

[0034] A_Pt=L_Pt/(w_Pt*t_Pt)  (9)

[0035] A_ITO=L_ITO/(w_ITO*t_ITO)  (10)

[0036] ρ_Pt,f, ρ_Pt,0, ρ_ITO,f, ρ_ITO,0are resistivities of Pt and ITO at a working and reference temperatures. In equation (6), all resistivities and ΔT are constants, Δρ_Pt>0 and Δρ_ITO<0, a different length (L), width (w) and thickness (t) of ITO and Pt can be designed to let the TCR of self-compensated ITO-Pt sensor film be zero.

[0037] Also from equation (4), the TCR of self-compensated sensor can be related to TCR of Pt and ITO.

[0038] TCR_COMP=((R_Pt,f+R_ITO,f)−(R_Pt,0+R_ITO,0))/((R_Pt,0+R_ITO,0)*ΔT)  (4)

[0039] TCR_COMP={[(R_Pt,f−R_Pt,0)/(R_Pt,0*R_ITO,0*ΔT)]+[(R_ITO,f−R_ITO,0)/(R_Pt,0*R_ITO,0*ΔT)]}*B

[0040] Where

[0041] B=(R_Pt,0*R_ITO,0)/(R_Pt,0+R_ITO,0)

[0042] simplify this equation, the TCR of self-compensated sensor related to constant TCR of Pt and ITO.

[0043] TCR_COMP=(TCR_Pt*R_Pt,0+TCR_ITO*R_ITO,0)/(R_Pt,0+R_ITO,0)  (11)

[0044] The mathematical expressions set forth above were solved using commercially available software such as Mat Lab or Math Cad software loaded into a personal computer.

[0045] Sensor Preparation

[0046] A self-compensated ITO sensor was fabricated by sputtering Pt and ITO films that were subsequently patterned. A sensor design embodying the invention is shown in FIG. 1. A sensor is shown generally at 10 comprises a wide band semiconductor e.g. ITO, 12 and a metal, e.g. Pt, compensation circuit 14, deposited on a substrate S. For purposes of generating experimental data, there are four Pt bond pads 16a,16b,16cand 16c.

[0047] This self-compensated sensor 10 can be simulated as a circuit composed of resistors as shown in FIG. 2. Ch1 can measure the resistance of whole sensor, Ch2 is used to measure resistance of the Pt, Ch4 is for ITO part, Ch3 and Ch5 are for contact resistance between Pt and ITO.

[0048] Indium tin oxide (ITO) films were developed by rf reactive sputtering at low temperature using an MRC model 822 sputtering system. A high density target (12.7 cm in diameter) with a nominal composition of 90 wt % In₂O₃and 10 wt % SnO₂was used for all depositions. Oxygen partial pressure was 30% while an rf power density of 2.4 W/cm²and a total pressure of 9 mtorr was maintained during each sputtering run. Aluminum oxide constant strain beams were cut from rectangular plates (Coors Ceramics—99.9% pure) using a laser cutting technique. The constant strain beams were then sputter-coated with 4 μm of high purity alumina prior to the deposition of the ITO strain gages. After spin casing 4 μm of ITO, a 2 μm thick layer of positive photoresist was spin-coated onto the ITO film coating. After exposure and development, the ITO films were etched in concentrated hydrochloric acid to delineate the final device structure. Sputtered platinum films (1.1 μm thick) were used to form ohmic contacts to the active ITO strain elements.

[0049] The dimensions set forth above for the sensors were for experimental purposes to prove the concept and the mathematical model. One skilled in the art will recognize that the dimensions of a commercially viable sensor according to the teachings of the invention can be made at least an order of magnitude smaller using the current state-of-the-art microelectronic fabrication techniques.

[0050] Referring to FIG. 3, a sensor 20 is shown and comprises a wide band semiconductor 22 and a metal compensating circuit 24 on a substrate S. In this design, the G(−) of the semiconductor is maximized and the G(+) of the metal is minimized. Also shown are metal bond pads 26aand 26b.When the sensor is read, a monitor (not shown) connects to the bond pads 26.

[0051] High Temperature Strain Rig

[0052] Strain measurements were made using a cantilever bending fixture fabricated out of a machinable zirconium phosphate ceramic. A solid alumina rod was connected between an alumina constant strain beam and a linear variable differential transducer (LVDT) to measure deflection of the strain beam. Corresponding resistance changes were monitored using a four wire method with a 6 and ½ digit Hewlett Packard multimeter and a Keithley constant current source. The high accuracy LVDT, multimeter and constant current source were interfaced to an I/O board and an IBM PC employing an IEEE 488 interface. Lab Windows software was used for data acquisition.

[0053] High Temperature Strain Test Results

[0054] To evaluate the piezoresistive properties of the active ITO strain gage used over a wide temperature range, it is important to characterize the variation in electrical response with temperature. The electrical response of an ITO fim grown in a 30% O₂plasma as disclosed above and thermally cycled in air at temperatures up to 1200° C. was observed.

[0055] Broad band semiconductors over a defined temperature range may exhibit a single TCR or two or more TCRs. It will be understood that if two linear TCRs are exhibited in two distinct temperature ranges within the defmed temperature range that a sensor will be fabricated based on each distinct temperature range in order that the strain can be measured over the defmed temperature range.

[0056] It is known for ITO films that there can be two distinct TCRs depending upon temperature. T>800°, a linear response with a TCR of −210 ppm/° C. has been observed and that T>800° C., a TCR of −2170 ppm/° C. has been observed. More recently, an ITO with a single TCR of −300 to −1,500 ppm° C. has been measured.

[0057] In the example, a four-wire method was used connecting to the bond pads 16. This method is well known to one skilled in the art. The sensor was fabricated and tested as outlined in the sections above. Four cycles of heating and cooling were measured, the results are shown below and in FIG. 4. After the first heating, the resistance changing with temperature is almost identical in four cycles, thus it shows the reproducibility is good.

[0058] TCR_COMP=(R_COMP,f−R_COMP,0)/(R_COMP,0*ΔT)=(437-442/(437*1170)=−9.8(ppm/° C.)

[0059] TCR_Pt=(R_Pt,f−R_Pt,0)/(R_Pt,0*ΔT)=(379−160)/(160*1170)=+1169(ppm/° C.)

[0060] TCR_ITO=(R_ITO,f−R_ITO,0)/(R_ITO,0*ΔT)=(40−225)/(225*1170)=−702(ppm/° C.)

[0061] Resistivity of Pt

[0062] At 30° C.,

[0063] ρ_Pt,0=R_Pt,0*(w*t)/L=160*(0.6 mm*0.8×10⁻³)/500=1.535×10⁻⁴(Ω*m)

[0064] At 1200° C.,

[0065] ρ_Pt,f=R_Pt,f*(w*t)/L=379*(0.6*0.8×10⁻³)/500=3.639×10⁻⁴(Ω*m)

[0066] Resistivity of ITO

[0067] At 30° C.,

[0068] ρ_ITO,0=R_ITO,0*(w*t)/L=225*(5*4.4×10⁻³)/60=8.25×10⁻²(Ω*m)

[0069] At 1200° C.,

[0070] ρ_ITO,f=R_ITO,f*(w*t)/L=40*(5*4.4×10⁻³)/60=1.498×10⁻²(Ω*m)

[0071] from eq.(6)

[0072] TCR_COMP=(Δρ_Pt* A_Pt+Δρ_ITO*A_ITO)/((ρ_Pt,0*A_Pt+ρITO,0*A_ITO)*ΔT)  (6)

[0073] Where

[0074] Δρ_Pt=ρ_Pt,f−ρ_Pt,0=*(3.639−1.535)*10⁻⁴(Ω*mm)

[0075] Δρ_ITO=ρ_ITO,f−ρ_ITO,0=*(1.498=8.25)*10⁻²(Ω*mm)

[0076] A_Pt=L_Pt/(w_Pt*t_Pt)=500/(0.6*0.8×10⁻³)(mm⁻¹)

[0077] A_ITO=L_ITO/(w_ITO*t_ITO)=75/(5 mm*4.4×10⁻³)(mm⁻¹)

[0078] TCR_COMP=(Δρ_Pt*A_Pt+Δρ_ITO*A_ITO)/((ρ_Pt,0*A_Pt+ρ_ITO,0*A_ITO)*ΔT)=−21.32(ppm/° C.)

[0079] From equation (11)

[0080] TCR_COMP=(TCR_Pt*R_Pt,0+TCR_ITO*R_ITO,0)/(R_Pt,0+R_ITO,0)

[0081] TCR_COMP=−23.5(ppm/° C.)

[0082] The results of the self-compensated sensor are presented below. This sensor was thermally cycled to 1200° C. The experimental data shows that the TCR of self-compensated gage was almost zero (0 ppm/° C.±20 ppm/C) over the temperature range RT-1200° C.

[0083] The dimensions of the platinum resistor 14 were (0.6 mm×500 mm×0.8 μm thick) and the dimensions of the ITO sensor 12 placed in series with the platinum resistor were (5 mm×60 mm×4.4 μm thick). These dimensions correspond to the (width×length×thickness) of each resistor and the results in the table were obtained for these particular dimensions.

[0084] The room temperature resistances can be read from FIG. 4, approximately 240 ohms for the ITO resistor and 160 ohms for the platinum resistor.

[0085] The use of self-compensating resistors can be used in any electrical device which requires control of a TCR as a function of temperature, i.e. thermistors, temperature sensors, RTD's, etc.

[0086] The foregoing description has been limited to a specific embodiment of the invention. It will be apparent, however, that variations and modifications can be made to the invention, with the attainment of some or all of the advantages of the invention. Therefore, it is the object of the appended claims to cover all such variations and modifications as come within the true spirit and scope of the invention.