Integrated heatable sensor

Abstract: A sensor for, e.g., the measurement of gas concentration, comprising a sensor element (8,10); a heating element (11) arranged in conjunction with the sensor, with which the sensor element can be brought to a temperature above the ambient gas atmosphere temperature; and an electronics circuitry section (18), with which the desired electronical properties of the sensor element (8,10) can be measured and the heating element (11) of the sensor (8,10) can be controlled. The sensor element (8,10) and the electronics circuitry (18) are on the same planar substrate which has openings around the sensor element area (8,10) to the immediate vicinity of the area so that the openings (5,13,17) extend through the substrate (1,7), whereby the sensor area (8,10) is connected to the surrounding part of the substrate and, thence, to the electronics circuitry section (18), only along thin isthmuses (14) remaining between the openings.

Description: This invention relates to a sensor, more particularly though not exclusively to an integrated heatable sensor for the measurement of gas concentration. Concentration measurements for different types of gases are performed to an increasing extent for both the living environment of man and industrial processes. Inflammable gases can be measured using sensors, which are operated with their surface temperature elevated above the ambient temperature. Inflammable gases in the ambient air are thereby oxidized on the sensor surface, and electrons released in the oxidization process alter the electrical conductivity of the sensor. Conventionally, this change in sensor resistance is converted into a voltage or current, whose magnitude is proportional to the gas concentration being measured. In prior art constructions, the sensors are usually fabricated as separate sensor element (or chips), which are bonded by electrical leads to the case encapsulating the sensor. In addition, the bonding leads act as springs, which support the sensor. Another method is to mount the sensor onto heat-insulating posts, while the electrical connections are bonded with thin conductors leads. The first mentioned construction has two principal drawbacks. Firstly, the spring leads must be designed with a sufficient thickness in order to provide protection for the sensor against mechanical stresses, e.g., those causes by impact shocks. By contrast, a thicker lead conducts so much heat away from the sensor element as to necessitate the application if increased heating power. Secondly, although mass production methods are being applied to the processing of the sensors, their fabrication into separated chips, individual mounting into the sensor case, and bonding to the attached electronics, makes the sensor manufacturing into a complicated and expensive process. A sensor is disclosed in GB-A-2158586 comprising a sensor element, a heating element for heating the sensor, electronics circuitry and a substrate on which the heating element, the sensor element and the electronics circuitry are placed. The sensor element comprises a resistive sensor element for providing a conductivity signal whose value is proportional to the gas concentration being measured and the heating element is arranged to heat the sensor element to a substantially constant temperature. The substrate is formed with openings to isolate the sensor element thermally from the rest of the substrate, the openings being situated around the sensor element and extending through the substrate. One object of the present invention is to provide an improved kind of integrated heatable sensor. More specifically, in accordance with the present invention we propose an integrated sensor for the measurement of gas concentration as defined in Claim 1. The invention makes it possible to integrate a heatable sensor onto, e.g., the substrate of the thick-film hybrid circuit so that the sensor and the substrate form such an integrated structure in which the heat flow from the sensor to the substrate is minimised. The sensor can be integrated onto a substrate containing the electronics circuitry during a single workphase in the mass production process. Further, an advantageous method, which is laser processing, can be applied to the fabrication of the openings, by virtue of combining this workphase with that of the thick-film resistor trimming operation performed during the calibration of the sensors. Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which:- Fig.1 is a plan view of an integrated sensor construction; Fig.2 is a plan view of another integrated sensor construction; and Figure 3 is an underside view of the sensor construction illustrated in Figure 2. With reference to the production methods of thick-film and thin-film hybrid circuits explicitly described in, e.g., such publications as Hammer D.W., Biggers J.V., Thick Film Hybrid Microcircuit Technology, Wiley-Interscience 1972, and Holmes P.J., Handbook of Thick Film Technology, Electrochemical Publications Ltd., 1976, the details of the aforementioned techniques are omitted herein. According to Figure 1, a tin-oxide based sensor 2 is fabricated onto an alumina substrate 1 in a conventional manner, while an additional workphase is also performed by simultaneously fabricating lead connections 3 of surface-mount electronical components 4 as well as resistors 6. The heating resistor of the sensor 2 may be placed on either the lower or the upper surface of the substrate. In the construction illustrated in the figure, the heating resistor is placed outside the illustrated area, under the sensor 2. In conjunction with the bonding of the surface-mount components 4, opening 5 are fabricated around the sensor area at appropriate places using, e.g., a laser. These openings 5 isolate the sensor 2 thermally from the rest of the substrate. A typical size of the sensor is a few square millimeters and the size of the substrate is, e.g., 2 * 3 cm². The width of the cut openings 5 can be, e.g, a few hundred micrometers. In accordance with Figure 2, metal electrodes 8 and conductors 12 are processed onto a substrate 7 using thick-film or thin-film techniques. A gas-sensitive layer 10 of appropriate sensor material, e.g., SnO₂ is sintered onto the electrodes 8. The entity called here a sensor 15 is defined to include a sensor material layer 10 and the electrodes 8. In the construction illustrated in the figure, the openings are arranged so that the sensor 15 is surrounded by an annular first set of inner openings, consisting of four L-shaped openings 17, which is further enclosed by a second set of outer openings, consisting of four rectangular openings 13. Conductors 12 of the electrodes 8 are routed via isthmuses 14 remaining between the openings 13 and 17. The ratio of the isthmuses to the circumference of the sensor 15 is about 10%, but this parameter may be varied from 5% to 15% as dictated by the material type and other needs. The electronic circuitry, together with its associated components, is not illustrated in the figure; however, its location is outlined in Figure 1. Illustrated in Figure 3 is the placement of a heating resistor 11 of the sensor to the bottom side of the substrate 7. Conductors 16 of the heating resistor are routed to the substrate along isthmuses remaining between openings 13 and 14. The shape of the openings may obviously vary within very large limits, since the most advantageous tool, the laser, offers an extremely wide latitude of control. Hence, the proposed rectangular shapes of openings can feasilby be replaced by freely selected shapes with curved contours. Furthermore, the ideally circular shape can be replaced by an elliptic contour.

Claims: An integrated sensor comprising a planar alumina substrate (1,7), a sensor element (8, 10) on one side of the alumina substrate, a heating element (11) for heating the sensor on the other side of the alumina substrate and electronics circuitry (18), the sensor element (8, 10) being a resistive sensor element for providing a conductivity signal whose value is proportional to the gas concentration being measured, the heating element (11) being arranged to heat the sensor element to a substantially constant temperature, the substrate being formed with openings (5, 13, 17) to isolate the sensor element thermally from the rest of the substrate (1, 7), the openings being situated around the sensor element (8, 10) and extending through the substrate (1, 7) such that thin isthmuses (14) between the openings (5, 13, 17) connect the sensor element (8, 10) to the surrounding part of the substrate and provide paths (12) for conductors to the electronics circuitry (18) and the electronics circuitry (18) being arranged to measure the desired electrical properties of the sensor element (8, 10) and to control the heating element (11), the electronics circuitry comprising components which are surface mounted on the alumina substrate. A sensor according to Claim 1, characterised in that the sensor element (8, 10) and the circuitry (18) are formed onto the planar alumina substrate (1, 7) using thick-film techniques. A sensor according to Claim 1 or 2 in which the heating element (11) of the sensor element (8, 10) is placed on the bottom surface of the alumina substrate (7) so as to be aligned with the sensor element (8, 10). A sensor according to Claim 1 or 2 in which the heating resistor (11) of the sensor element (8, 10) is placed on the top surface of the alumina substrate (7) so as to be aligned with the sensor element (8, 10). A sensor according to any preceding Claim, in which the openings (5, 3, 17) are formed using a laser. A sensor according to any preceding Claim, in which two sets of openings (13, 17) coaxially and annularly enclose the sensor element (8, 10). A sensor according to any preceding Claim, wherein the sensor element is attached to the rest of the alumina substrate (1, 7) along isthmuses by 5...15%, preferably 10%, of its circumference.

Inventors: LEHTO ARI, LEHTO, ARI
Applicants: ENGICOM NV, KEMIRA OY, NESTE OY, OUTOKUMPU OY, VAISALA OY, ENGICOM N.V.
Application year: 1989
Main classification: G01N27
IPC codes: G01N33, H05K1, G01N27