GB2183344A

GB2183344A - Sensor

Info

Publication number: GB2183344A
Application number: GB8624094A
Authority: GB
Inventors: Masaya Hijikigawa; Shoei Kataoka
Original assignee: Sharp Corp
Current assignee: Sharp Corp
Priority date: 1985-10-08
Filing date: 1986-10-08
Publication date: 1987-06-03
Also published as: DE3634132C2; GB8624094D0; JPH0460549B2; JPS6283641A; GB2183344B; DE3634132A1

Abstract

A sensor comprises a substrate (11) and a sensing portion (12) formed thereon the sensing portion having a sensing surface comprising a plurality of upstanding and/or recessed portions which improve the interaction between the sensor and the parameter to be measured. The sensor may detect chemical or physical interaction with the object or quantity to be detected and may detect gases, humidity, ions, biological materials, electromagnetic waves or temperature, either directly or indirectly. Gases may be sensed by absorption effects or reactivity with the sensor causing resistance or EMF changes or by physical properties of the gas such as thermal conductivity or infrared absorption. Biological materials may be sensed with the sensing portion comprising an active material and a gas, light or heat change or luminescence being sensed. Temperature may be sensed by making the detector a thermistor, thermocouple or IR detector. <IMAGE>

Description

SPECIFICATION Sensor The present invention relates to a sensor which undergoes a chemical or physical interaction with an object oramountto be detected, and in particularto a sensor which is adapted for use in chemical sensor devices for detecting chemical amounts of gases, humidity, ions, etc., biosensor devices for detecting physiologically active substances such as uric acid and glucose, and physical sensor devices for detecting physical quantities of electromagnetic waves, temperature, etc. More particularly, the invention relates to a sensor which has a surface shaped to a suitably predetermined optimum form by an artificial reproducible method such as a finely processing orfine machining technique so as to exhibit improved performance.

The sensors embodying the present invention are not always limited to those acting to convert the object oramountto be detected directly to an electric signal but also includethose performing an indirectconvert ing action,forexample, by undergoing chemical or phsyical interaction with the objectoramountto convert the same to another chemical or physical quantity.

Gas sensor devices are adapted to detect a specified Component gas of a gaseous mixture in terms of an electricsignal,forexample, by (1 ) utilizing a phenomenon on the solid surface of a sensor resulting from the adsorption of the gas bythe surface, (2) utilizing the reactivity of a sensorwith the gas, (3) utilizing concentration polarization (electrnmotive force) due to a solid electrolyte, or (4) utilizing the physical properties (thermal conductivity, infrared absorption, etc.) of molecules ofthe gas.

Generally, semiconductor gas sensor devices are based on the principlethatwhen gas molecules (or radicals) are adsorbed by the surface of an n-type or p-type semiconductor (sensor) composed chiefly of a metallic oxide such as tin oxide (SnO2), zinc oxide (ZnO), nickel oxide (NiO) or cobalt oxide (CoO), transfer of electrons or uneven presence of charges occurs between the semiconductor and the adsorbed molecules (or radicals) to form a space-charge layer in thevicinity of the sem ico nductor su rface, consequently varying the electrical conductivity of the semiconductor, the variation in the conductivity thus indicating the presence of the gas.

For example, when a semiconductorofSnO2, ZnO or like metallic oxide exhibiting n-type conductivity adsorbs a combustible gas such as hydrogen, carbon monoxide ora hydrocarbon, transfer of charges between the adsorbed gas molecules and the semiconductor (donation of electrons by the adsorbed gas molec ules) gives increased electrical conductivity in the vicinity of the semiconductor surface. Thus, the variation in the electrical conductivity duetothe adsorption ofthe gas is detectable as a variation in thesurface electrical conductivity. This means that if the surface of the sensor is increased relative to its volume, for example, by reducing the thickness thereof, the variation ratio of the conductivity increases to renderthe sensor serviceable advantageously.

However, many actual semiconductor gas sensors are polycrystalline bodies obtained by sinerting a powderand have in the body ofthe sensor a contact portion or neck portion between crystal grains. Forexample, when there is a boundary 2 between crystal grains 1 in contact with each other as shown in Figure 7, a space-charge layer 3 is formed over the surface of the grains exposed to the atmosphere, owing to the influence of adsorbed oxygen (electron acceptor),so thatthetwo grains contact each otherthrough the space-charge layer3. Accordingly, an electron barrier indicated a curve 4 is formed between the grainsto impede the movement of electrons between the grains.It is thoughtthat when a combustible gas comes into contact with the grains, the adsorbed oxygen is consumed or removed by combustion, lowering the potential barriers indicated in a curve 5 in Figure7 and increasing the electrical conductivity.

With the actual semiconductor gas sensor, the contact portion atthe grain boudnarycontributes a great deal to the gas detecting mechanism, permitting the sensorto exhibit pronounced variations in the surface electrical conductivity. On the other hand, the output characteristics, i.e. the conductivity-gas concentration characteristics, of the semiconductor gas sensor are dependent on alterations in the minute structure ofthe contact portion or neck portion between the crystal grains. This is a greatfactorto cause the characteristicsto differ from sensorto sensor. Basically, therefore, it is necessary to control the size and shape of the grains and the state of fusion between the grains with good reproducibility.From the viewpoint of sensitivity to gas, it is also necessary to increase the area of adsorption of gas so that the space-charge layer formed in the vicinity of the sensor surface will greatly contribute to the conductivity as already described.

When sensors are to be prepared bya conventional method,forexample, bysintering a powder, the sintering temperature and time, addition of the sintering agent, gaseous sintering atmosphere, etc. are controlled empirically by controlling indirect conditions relative to one another. Nevertheless, when such indi rectcondition control is resorted to, it is not always possibleto control a singlefactoronly. Forexample, depending on the sintering time ortemperature, at least the size and shape of the crystal grains vary, and the state offusion between the grains (state of the grains bound together) also alters.For this reason, it is extremely difficultto produce with good reproducibility sensors which are identical in structure when viewed on a microscale.

Next, conventional biosensor devices will be described. Generally, biosensor devices comprise a sensor called a receptor which is prepared by fixing a living body associated substance, such as an enzyme, anti body ororganelle of the living body, to a suitable substrate (film), and a transducer for converting to an electric signal a gas orotherchemical substance or physical amount of light, heatorthe like resulting from,or eliminated by, the reaction of the receptor with the substance to be detected. For example, in the case of sensor devices for detecting glucose, the receptor is prepared by fixing an enzyme (glocose oxidase, GOD) to a high polymerfilm.When glucose contacts the enzyme GOD, hydrogen peroxide (H202) is formed according to the following reaction formula: Glucose +02

gluconic acid + H202 The H202 produced is detected in terms of an electric signal using electrochemical means (transducer) hav ing, for example, a platinum anode. Thus, the concentration ofthe substance to be detected, i.e. glucose, can be determined by detecting the amount ofthe resulting H202.

Whilethe chemical substance produced is detected by thetransducerinthis way, methods are also known of detecting emission of light or endothermic, exothermic or like thermal phenomenon resulting from the reaction between the enzyme and the substance to be detected. When the light emitting phenomenon is utilized, a photodetector is used as a transducer, while a thermistoror liketemperature sensor is usedfor detecting a thermal phenomenon. In any case, the lower limit of the detectable concentration of the object substanceis almost always dependent on the amount of reaction between the receptor and the substance.

Accordingly,the sensitivity of detection is increased by fixing the living body associated substance, such as enzyme, with the highest possible density, or by increasing the area of contact of the receptor with the substance to be detected. However, the increase in the area ofthe receptor is physically limited because the enzyme orthe like is fixed usuallyto a flat substrate and further because the size ofthe receptor needs to be considered relative to the size ofthe transducer.

Next, temperature sensors heretofore known will be described. Temperature sensors generally include those adapted to detect infrared rays, those utilizing the variation of electrical resistance with temperature and those utilizing thermal electromotive force. So-called thin film temperature sensors will be described below in which the temperature measuring element is a thin film of platinum, nickel or like material having a large temperature coefficient of resistance.

The temperature measuring elements forthin film temperature sensors mustfulfil the requirements of having a large temperature coefficient of resistance which is constant over a wide temperature range, being low in resistance value at a reference temperature (e.g. O C or 1000C) so so asto be interchangeable, having a resistancevalue in a readily usable rangefortemperature measurement (e.g. a resistancevalue Ro of 100 nor 1 kfl at 0' C), being small, etc.Accordingly, the conventional temperature measuring element is produced by forming a thin film of platinum, nickel or like metal having a high purity on a substrate of ceramics orthe like having a surface made planarto the greatest possible extent, by vacuum evaporation orsputtering, and thereafterforming the film into a pattern of lines having a specified width by photoetching or othertech nique.

To givethetemperature measuring elementa minimized area and a resistance value Rowhich is convenientto use, e.g. 100 or 1 kn, at a reference tem peratu re (e.g. O"C),the pattern is prepared in a zigzag (meandering) form with a minimized line width. However, because of the limitations actually involved in the processing technique especially when platinum is used, it is difficu Itto obtain a pattern up to several micrometres in linewidth and line spacing, consequently imposing limitations on minimizing the size ofthetem perature measuring element. With temperature measuring elements having a resistance valueR0 of 1 kQ,the lower limit of the size of elements is approximately 1 to 2 mm square.

The present invention provides a sensor device comprising a substrate and a sensing portion formed thereon, the sensing portion having a sensing surface which comprises a plurality of upstanding and/or recessed portions.

Byway of example only, specified embodiments of the present invention will now be described, with reference to the accompanying drawings, in which Figures 1(a) to (g) arefragmentary perspective views schematically showing embodiments of sensor in accordance with the present invention; Figure2 is a perspective view schematically showing a semiconductor sensor device incorporating one of the sensors shown in Figures 1 (a) to (g); Figures 3 and 4 are fragmentary diagrams illustrating other embodiments of the present invention; FigureS is a diagram in section showing a sensor in accordance with the present invention, as incorporated in a field-effecttransistor;; Figure 6is a fragmentary perspective view schematically showing an embodiment of sensor in accordance with the present invention, for use in a thin film temperature sensor device; and Figure 7 is a diagram illustrating the detection mechanism of a conventional semiconductor gas sensor.

Referring to the drawings, projections (indentations) are formed on the surface of a substrate by chemical or physical finely processing means or on the surface of the sensor by chemical or physical finely processing means. Alternatively, the projections are formed by selective film forming means or selective crystal growing means when the sensor is prepared. The projections are intended to provide a sensor having an optimum shape with respect to the sensing characteristics to exhibit improved performance. The present invention further assures good reproducibility in preparing sensors and therefore makes it possible to produce sensors in quantities with uniform operating characteristics.

Application to semiconductor gas sensor devices The embodiments of Figures 1(a) to 1(9) are semiconductor gas sensor devices which utilizethealteration of a phenomenon on the surface of a solid duetothe adsorption (desorption) of gas, especially variations in the electrical conductivity of a semiconductor sensor.

Figure 1 (a) is a perspective view showing a sensor for semiconductor gas sensor devices. A tin oxide film is selectively formed on the surface of an insulating substrate 11 of glass, ceramics orthe like by RFsputtering, vacuum evaporation or crystal growth, whereby projections (indentations) are formed on the surface of the tin oxide film. Alternatively, a uniform tin oxide film is formed over the surface of the substrate 11 by RF sputtering or vacuum evaporation and is thereafter finely processed into a wavelike form as illustrated by photolithography and a dry etching process such as chemical etching or plasma etching.Instead offinely processing the tin oxidefilm,the surface of the substrate 11 may be treated as for example bysandblastingto form fine trapezoidal or other shaped projections 13 thereon, followed by the deposition oftin oxide on the surface ofthe projections 13 to form a sensitive film 12 having projections or indentations. These three finely processing methods may be practiced at least twice repeatedly. The shape of the projections need not always be trapezoidal as illustrated in Figure 1 (a) but can, for example, be pyramidal, conical or semi-spherical.By way offurther example, Figure 1(b) shows rectangular elongate projections, Figure 1(c) showstrapezoidal elongate projections formed by anisotropic etching, Figure 1 (d) shows overhanging elongate projections formed by anisotropic etching, Figure 1 (e) shows projections in a mesh-like arrangement, and Figure 1 (f) shows indentations in a mesh-like arrangement. Further, as seen in Figure 1(g), projections or indentations are formed also at the outer peripheral portion ofthesubstrate 11 to preventor minimise peeling ofthe sensitive film.

The projections or indentations may be arranged periodically or otherwise. The shapes of projections or indentations shown in Figures 1 (a)to (f) are also usableforthe later-described embodiments.

With reference to Figure 1(a), the distanced between the substrate 11 and lowest point of the surface 14 of the indentation formed in the tin oxide sensitive film 12 by fine processing is preferably as large as the thickness (Debye length) ofthe space-charge layer. Because the Debye length varies with the energy position and density ofthe tin oxide at the surface level thereof, the distanced can not be determined specifically, but the optimum value thereof can usually be in the range offrom 0.1 to several micrometers. An improved detection sensitivity can be obtained when an element having a catalytic action, such as platinum or pal ladium, is added tothefilm forming material when the tin oxidesensitivefllm isto beformed.

Whenthesensoradsorbs a combustible gas, the potential barrier lowers as shown in the curve 5 in Figure7 to result in increased electrical conductivity.

The gas sensors having a finely processed surface as described above have the following advantages over the conventional gas sensors.

(1) The contact portion or neck portion between the crysal grains in the conventional gas sensor made ofa sintered body corresponds to the distance between the substrate 11 and the bottom surface 14 of indent ations ofthe present sensor, so that gas detecting characteristics can be obtained with good reproducibility by directly controlling the distanced by the fine processing.

(2) The pitch of the finely processed projections or indentations corresponds to the crystal grain of the conventional sintered body, and the projections or indentations can be suitably sized so as to havethe specified shape and dimensions. Consequently, the result achieved is equivalent to that obtained when the crystals have a uniform distribution in a plane. The gas detecting characteristics are controllable intentionally, while the desired characteristics are available with good reproducibility (with diminished variations).

(3) With an increased surface area given to the sensor per unit area of the substrate, the sensor has an increased area for adsorbing a gas, exhibiting an enhanced gas detection sensitivity, i.e. improved detecting characteristics.

(4) Sincethe dependence of the electrical conductivity not only on the concentration of gas but also on the kind of gas can be altered by varying the distanced, satisfactory gas selectivity is available.

Not only tin oxide but also zinc oxide, nickel oxide, cobalt oxide and like semiconductors of n-type or p-type are similarly usable forforming the gas sensor. From the viewpoint offinely processing techniques, however, itis desirableto use a single crystal, aggregate offine crystals or amorphous material.

Example 1 Figure2 is a diagram showing a semiconductor gas sensor device incorporating the above sensor.

Atin oxidefilm is formed on the uppersurface of a ceramic substrate 21 by RFsputtering, and the surface of the tin oxidefilm isfinely processed by photolithography and chemical etching. Platinum electrodes 23 are thereafter formed on the film at its opposite ends for measuring the electrical conductivity of the resulting sensor 22. A heater resistor 24 of platinum and electrodes 25 therefor are provided on the lower surface ofthe substrate for heating the sensor 22 during operation. Current is passed acrossthe electrodes 25to heat the resistor 24 and heat the sensor 22 to a suitable temperature. When the sensor 22 is placed in this state in a gas atmosphere, i.e. the object to be detected, the electrical resistance value of the sensor 22 varies with high sensitivity owing to the presence of the gas. The variation is detected via the opposed metal electrodes 23, whereby the presence of the gas is detectable.

Application to biosensordevices The present invention will be described next witch reference to semiconductor biosensor devices fordetect- ing a substance based on a reaction between the substance and a receptor (sensor) comprising an enzyme or other living body associated substance.

Figure 3 is a fragmentary diagram showing a sensorforthis purpose. Minute projections (and/or indent ations) arranged periodically or randomly as illustrated are formed at least on the surface of a substrate 31 of organic high polymer material such as polyvinyl alcohol (PVA) cellulose ester, for example, by chemical etching or plasma etching. Subsequently, a living body associated substance 32 such as, for example, glu cose oxidase, invertase, mutarotase, galactose oxidase, amino acid oxidase, urease, uricase or like enzyme or lactic acid bacterium, butyric acid bacterium, methane oxidizig bacterium or like microorganism isfixedto the finely processed substrate surface by the covalent bond method or adsorption method to obtain a recep tor (sensor).When thus prepared, the receptor has a larger amount of living body associated substance fixed to the substrate per unit area thereof and a larger area of contact with the substance to be detected than the conventional flat-surfaced receptor (sensor). Consequently, the sensor is given a higher detection sensitivity or can be madesmaller in its entirely than the conventional ones having the same sensitivity.

If it is impossible to fix the living body associated substance directly to the finely processed substrate surface or if it is difficu It to finely process a substrate suited for fixing, a sensor of double layer structure as shown in Figure 4 is usable as a receptor. With reference to the drawing, a substrate 41 is finely processed first, and a sensitive film 42 having fixed thereto the same living body associated substance 43 as mentioned above is formed on the substrate surface. The sensitive film 42 can be formed effectively on the substrate41, for example, by Langmuir-Blodgett'stechnique.

Further, when a single crystal is usable as the substrate 41, the substrate can be finely processed utilizing the dependence of chemical etching speed on the orientation of crystal, i.e. by the an isotropic etching tech nique. When the sensor is physically coupled to an electrochemical device or a transducer such as a photo detector or temperature sensor to provide a sensor device of conventional construction, the device is serviceable as a biosensor device having a higher detection sensitivity than those heretofore known.

The biosensor device can be adapted to give an electric output with high sensitivity when provided on the channel region of a field-effect transistor (FET) directly or with a gate insulating film formed therebetween.

Example 2 Two n+ regions spaced apart are formed in the surface of a p-type silicon substrate, and a gate insulating film 52 of silicon oxide (SiO2) is formed over the substrate surface between then + regions. A polyvinyl alcohol layer is formed over the gate insulating film 52. Minute projections (indentations) are randomly formed on the surface of the polyvinyl alcohol layer by plasma etching. Glucose oxidase is fixed to the etched surface by adsorption to provide a sensor 53. A reference electrode 54 is opposed to the sensor 53 as spaced therefrom by such a distance as to permitthe solution to be detected to pass therebetween. A predetermined potential is provided across the FET and the reference electrode 54 in the solution.The slight variation in the potential due to a reaction between the solution and the sensor is detected in terms of a variation in the gate current of the FET and is delivered as a greater variation in the drain current ID utilizing the amplification ofthe FET.

Thus, the sensor of the present embodiment is usable as electrically or physically connected to any kind of transducer heretofore known to provide a biosensor device of improved performance.

Example 3-Application to thin film temperature sensor device The invention will be described with reference to a thin film temperature sensor device wherein a thin film sensor of a material, such as platinum, having a large temperature coefficient of resistance is used as the temperature measuring element.

Figure 6 is a perspective view schematically showing the sensor device.

A substrate 61 having a sufficiently lower specific resistance than platinum film, for example, a highresistance silicon wafer, is finely processed by anisotropic etching at least over the surface thereof to the shape of flat-crested waves (and flat-bottomed troughs) having a definite or indefinite period as illustrated, whereby elongate projections or indentations oftrapezoidal cross-section are formed on or in the surface of the substrate 61. Subsequently, a platinum film 62 is formed over the etched surface ofthe substrate 61 by RF sputtering and is thereafter shaped into a pattern of zigzag lines of specified width and spacing by photoetching orsputtering-etching. The platinum film 62 thus prepared serves as a temperature measuring element, the resistance value of which varies with temperature.Since the resistance of the platinum film conductor 62 is greaterthan when the conventional flat substrate is used by an amount corresponding to the projections or indentations of the substrate surface, the temperature measuring element having a specified R0 value can be very much smallerthan the corresponding conventional ones. Furthermore, platinum conductors which are identical in film thickness and line width can be given different reference resistance values by altering the processed shape ofthe substrate or the pitch of projections. The substrate need not always have high resistance but may be madeto have a surface of increased resistance by being thermally oxidized afterthefine processing or by being covered with an additional insulating film.

As described in detail above, the embodiments of sensor device in accordance with the present invention comprise a crystalline, fine crystalline or amorphous sensor made of a semiconductor, metal, metallic oxide, dielectricororganic material and having projections or indentations formed on or in its surface with optional predetermined shape and dimensions by a finely processing technique. Thus, the sensors are characterized by their surface shaped to exhibit intentionally controlled sensing characteristics. As will be apparent from theforegoing embodiments, the reaction or interaction between the substance (amount) to be detected and the sensor can be enhanced by increasing the surface area and utilizing the shape ofthe surface. Accordingly, the present sensor is very useful in providing chemical sensor, biosensor or physical sensor devices of improved performance and function.

Claims

1. A sensor device comprising a substrate and a sensing portion formed thereon, the sensing portion having a sensing surface which comprises a plurality of upstanding and/or recessed portions.

2. A sensor device as claimed in claim 1,wherein the upstanding portions and/or recessed portions are in the form of projections and/or recesses respectively.

3. A sensor device as claimed in claim 1 or claim 2, wherein the upstanding and/or recessed portions are formed on the surface ofthe substrate by chemical or physical means, and the sensing surface generally follows the contours of at least a portion ofthe substrate.

4. A sensor device as claimed in any of claims 1 to 3, wherein the upstanding and/or recessed portions are formed in the sensing portions of the sensor device.

5. A sensor device as claimed in any of claims 1 to 4, wherein the upstanding and/or recessed portions are formed by chemical or physical finely processing means.

6. A sensor device as claimed in any of claims 1 to 5, wherein selective film forming means orselective crystal growing means is used when the sensing portion is formed.

7. A sensor device as claimed in any of claims 1 to 6, wherein the sensing portion is formed on the channel region of a field-effecttransistor.

8. A sensor device as claimed in any of the preceding claims, wherein the upstanding and/or recessed portions have predetermined shape and dimensions.

9. Asensordevice as claimed in anyofthe preceding claims, wherein the sensor portion is of one or more of metal, metal oxide, semiconductor, dielectric or organic material.

10. A sensor device substantially as herein described, with reference to, and as illustrated in, any of Figures 1 (a) to 1(g) or Figures 2 to 6 of the accompanying drawings.