DE102004033597B4 - Highly sensitive hydrogen sensor for detecting fires/combustion has a thin-layer structure made from a semiconductor substrate, an insulator, fluoride ion conductors and electrodes - Google Patents

Abstract

An insulator layer (3) attaches to a semiconductor substrate (2). A fluoride ion conductor layer is applied to the insulator layer and a palladium layer/electrode (5) to the fluoride ion conductor layer (4). A second electrode (6) fits on the rear side of the semiconductor substrate. - Independent claims are included for: - (A) a hydrogen gas alarm with a hydrogen sensor; - (B) and for a hydrogen gas detector with a hydrogen sensor; - (C) and for a method for determining hydrogen gas concentration.

Description

The invention relates to a highly sensitive hydrogen sensor 1 , which is inexpensive to produce and has a very low energy consumption, since it works at room temperature. The hydrogen sensor according to the invention 1 is suitable as part of a hydrogen gas detector, inter alia, for fire detection, since it reliably measures the occurring low hydrogen concentrations and triggers alarm much earlier than the previously used and known optical smoke detection systems. It is of course also applicable for process control in industrial plants or in environmental protection.

The invention also provides a method for determining the hydrogen concentration, wherein the hydrogen sensor 1 is used as the gate region of a field effect transistor, as a capacitive semiconductor sensor or as a sensor based on the photoeffect in the semiconductor.

Hydrogen is used in many ways and at the same time is dangerous because of its tendency to explode. Hydrogen is also known as a gas suitable for fire detection ( US 4,088,986 and 5,856,780).

Hydrogen sensors are known in a variety of designs and with different principles of action. Lundström first described in Appl. Phys. Lett., 26 (1975) 55-57 discloses a metal / insulator / semiconductor (MIS) structure for hydrogen detection. The sensor operates at temperatures around 140 ° C, while its behavior is unstable at room temperature. In addition to Si, other semiconductors were used, such as SiC or GaN / AlGaN heterostructures at temperatures of 400-600 ° C. Microstructure technology was used to combine the sensors with heaters ( US 6,265,222 and 6,596,236).

adversely However, all of these sensors have an increased operating temperature from 140 ° C to over 600 ° C.

For many Potential applications is the associated energy consumption a decisive obstacle (battery-powered sensor systems, monitoring systems with power failure protection). Even with central monitoring systems is the power consumption of known fire gas sensors too high to the To enable operation with a power failure fuse, so that it only in Special cases used could become.

fire alarm are already used in the majority of households in the US. For Germany will be a similar one Development predicted and also a legal obligation to install discussed in private households.

The damage by fires are very big (100 annually Dead, 10,000 injured, € 2 billion material damage). By a early and reliable Warning could this situation will be significantly improved. The high power requirement, the price, cross-sensitivities and lack of long-term stability are the Main problems that the introduction prevent fire gas sensors.

A hydrogen sensor operating at room temperature is described by W. Moritz et al. in the abstract of the "54 Annual Meeting of the Infernational Society of Electrochemistry 31.08.-05.09.2003, Sao Pedro Brazil". It is a semiconductor device based on the field effect in the semiconductor, which consists of a layer system Si / SiO ₂ / Si ₃ N ₄ / LaF ₃ / Pt. If the response time slows down after a few days, the sensor is reactivated by using the platinum as a resistance heater. However, it has been found that this sensor is unsuitable for practical use because it is not stable enough, ie has a life of less than 3 months, and its hydrogen sensitivity changes with a change in relative humidity. Also, a lower detection limit of 10ppm for certain concerns in Schwellbrand- and fire detection may not be sufficient.

The US 2002/0187583 describes a method for producing a hydrogen sensor based on semiconductor or a sensor based on semiconductor, as second electrode has a palladium or platinum electrode, which has an enlarged surface, to the adsorption of the hydrogen gas and thus the sensitivity of the sensor to increase.

It The object of the present invention was to provide a highly sensitive and in the sensitivity of Degree of humidity independent, Stable at room temperature working and durable hydrogen gas sensor to disposal to provide, which is also suitable for Schwellbrand- and fire detection.

The object of the invention is achieved by a hydrogen sensor 1 which is a semiconductor device based on the field effect in the semiconductor and a semiconductor layer system 2 / Insulator 3 / Fluoride ion conductor 4 / Palladium electrode 5 and second electrode 6 consists and has a heating element. The hydrogen sensor 1 represents a thin-film structure, wherein the insulator layer 3 on the semiconductor substrate 2 , the fluoride ion conductor layer 4 on the insulator layer 3 and the palladium layer 5 on the fluoride ion conductor layer 4 are applied and the second electrode 6 on the back of the semiconductor substrate 2 is arranged, wherein the palladium electrode 5 and / or the second electrode 6 are formed as a resistance heating element.

The palladium electrode 5 and / or the second electrode 6 each have two temperature-stable, electrically conductive contacts 8th and 7 on, over which by applying a voltage of the sensor 1 can be heated. In a preferred embodiment, the sensor is via the palladium electrode 5 heated as a resistance heater. However, this heating is only operated via a pulse control, which is not used for the measurement, but only for reactivation necessary at longer intervals. Thus the sensor works at room temperature. It has surprisingly been found that for reactivating the sensor structure according to the invention already temperatures below 300 ° C are sufficient, preferably temperatures of 110-280 ° C. Reactivation is necessary at intervals of up to 7 days and for a period of 10 μs up to a maximum of 2 min.

The The invention therefore also relates to a method for determining the Hydrogen concentration according to claims 15 and 16. This mode of operation results in enormous energy savings across from constantly heated sensors and those heated periodically for measurement Need to become.

The hydrogen sensor 1 includes as a semiconductor substrate 2 a silicon single crystal, GaAs or amorphous silicon, preferably a silicon single crystal.

The insulator layer 3 may consist of SiO ₂ , of the layer combination SiO ₂ / Si ₃ N ₄ , of Al ₂ O ₃ or of Ta ₂ O ₅ , preferably of SiO ₂ / Si ₃ N ₄ . The layer thickness of the insulator layer should be from 30 nm to 90 nm. For the layer combination SiO ₂ / Si ₃ N ₄ , the ratio of about 40 nm / 40 nm is preferred.

The fluoride ion conductor layer 4 may consist of polycrystalline LaF ₃ , CaF ₂ or BaF ₂ , preferably LaF ₃ . The thickness of the fluoride ion conductor layer should be from 15 nm to 50 nm. In this area, a favorable impedance behavior was found.

As a second electrode 6 on the back of the semiconductor substrate 2 is preferably aluminum, platinum or gold in question. Particularly preferred is aluminum. The layer thickness is from 300 to 2,000 nm, preferably about 500 nm.

The palladium electrode 5 It consists of palladium or an alloy of Pd with Au, Ru, Ni, Fe, Co, Rh, Ir or Ag. The electrode 5 is preferably applied so that the layer thickness is up to 60 nm and by means of a metal mask a certain area and shape are given. In a preferred embodiment, the palladium electrode 5 two wider areas in the outer area, which are connected by a narrow strip. On the two wider areas in the outer area are the two contacts 8th applied.

In a very preferred embodiment, the hydrogen sensor 1 the layer structure Si / SiO ₂ / Si ₃ N ₄ / LaF ₃ / Pd on.

The production of the layer structure is carried out by methods well known to those skilled in the art. Thus, the fluoride ion conductor layer becomes 4 on the insulator 3 z. B. applied by thermal vapor deposition in a high vacuum or RF sputtering.

palladium electrode 5 and second electrode 6 can be applied by DC sputtering, thermal evaporation or electron beam evaporation.

The insulator layer 3 can on the semiconductor substrate 2 in a known manner, for. Example by means of chemical vapor deposition (chemical vapor deposition) are deposited, in the case of the preferred combination of layers SiO ₂ / Si ₃ N ₄ , both layers can be applied.

Surprisingly, the described sensor 1 with palladium electrode 5 highly sensitive and the sensitivity is in contrast to a sensor with identical Pt electrode not dependent on the change in humidity (see Example 5.). The lower detection limit of the sensor is about 2 ppm, making it ideal for measuring extremely low hydrogen concentrations.

The subject of the invention is therefore also a hydrogen gas detector comprising the described hydrogen sensor 1 , at least one voltage supply, means for measuring the capacitance of the photocurrent or the transistor drain / source current of the hydrogen sensor 1 , Hardware and software for calculating the hydrogen concentration and means for alarm triggering at hydrogen gas concentrations that are higher than the predetermined reference concentration. The invention further relates to a hydrogen gas detector, which is also provided for hydrogen measurement, but does not trigger an alarm, ie the means described for triggering alarm does not include, but z. B. is monitored online by computer.

The hydrogen sensor 1 can be used in various embodiments, namely as a capacitive semiconductor sensor, as a gate region of a field effect transistor or as a sensor based on the photoeffect in the semiconductor.

to Acquisition of capacity Therefore, for example, contain hydrogen gas detector and detector a frequency generator and an AC transducer, for photocurrent measurement in addition one Photodiode and for the transistor must have a constant voltage source.

When Signal control means are z. B. differential amplifier in Hydrogen gas detector and detector provided.

The in the hydrogen gas detector and detector existing power supply also includes a power supply control.

When Means for alarm triggering For example, a comparator circuit with a fixed setpoint used. Alternatively you can Also, intelligent systems can be used that reduce the variance use the long-term signal development for alarm definition.

According to the different applications of the hydrogen sensor 1 For signal detection, methods for determining the hydrogen concentration according to claims 12 to 14 are the subject of the invention.

Will the hydrogen sensor 1 used as a capacitive semiconductor sensor, the signal detection by means of the high-frequency capacitance / voltage measurement (RF-CF measurement), wherein the voltage between the second electrode 6 and the palladium electrode 5 is created.

In addition to DC voltage becomes an AC voltage of, for example, 10 kHz coupled. The capacitance is read out with a frequency-selective detector and from the steepness of the capacitance / voltage curve a potential change a concentration change recalculated.

Serves the hydrogen sensor 1 as a sensor based on the photoeffect in the semiconductor, modulated laser light is radiated into the semiconductor and the resulting photocurrent is evaluated analogously to the capacitance measurement.

Serves the hydrogen sensor 1 as a gate region of a field effect transistor, when applying a constant voltage to the drain / source electrodes (second electrode 6 and palladium electrode 5 ) evaluated the change of the drain / source current.

The hydrogen sensor is shown schematically in FIG 1 shown, in 1 mean:

1

Hydrogen sensor

2

Semiconductor substrate

3

insulator

4

Fluoride ion conductor

5

palladium electrode

6

Second electrode

7

electrical senior contacts

8th

electrical senior contacts

2 shows after reactivation, the response of the sensor of Example 2 to various hydrogen concentrations, baseline concentration 10 ppm, right scale: hydrogen concentration in ppm.

3 shows a comparison of the sensor signal of the sensor of Example 4a and the comparison sensor of Example 4b without Fluoridionenleiterschicht; 20 ppmH ₂ .

example 1

A sensor of a silicon single crystal with a 40 nm SiO ₂ layer and another 40 nm thick Si ₃ N ₄ layer was coated by thermal vapor deposition in a high vacuum at a deposition rate of 0.1 nm / s with LaF ₃ . The layer thickness of the ion conductor layer was 40 nm. By DC sputtering, a further layer consisting of Pd was deposited at a deposition rate of 1 nm / s up to a layer thickness of 50 nm. In this case, a Pd surface of 2 mm diameter was defined with a metal mask. A backside contact was realized by the deposition of Al (500 nm).

The sensor was characterized by high frequency capacitance / voltage measurement. Upon contact of the sensor with synthetic air of different hydrogen content, a sensitivity of 62 mV / μg P (H ₂ ) was found at room temperature.

Example 2

The sensor according to Example 1 was again measured after 60 days as above. There was only an extremely slow response. Subsequently, the sensor was heated for 10 seconds to 135 ° C and then measured again at room temperature in terms of hydrogen sensitivity. The result was the same behavior as shown in Example 1 for a new sensor. A ty The typical response is in 2 shown.

Example 3

Of the The hydrogen sensor described in Example 2 was used for detection of a test fire according to the European standard EN 54 (test fire 2) [published by the DIN German Institute for Standardization e. V., ref. No. DIN EN 54-7: 2001-03; DIN EN 54-5: 2001-03; DIN EN 54-1: 1996-10; DIN EN 54-7 / A1: 2002-09 and DIN EN 54-5 / A1: 2002-09] in comparison with a usual one optical smoke detection system tested. The fire was successful be detected, compared to the smoke detection system already 90 seconds earlier an alarm signal has been reached.

Example 4

A sensor a) according to Example 1 was prepared. Another sensor b) was prepared without Fluoridionenleiterschicht by on a silicon single crystal with a 40 nm SiO ₂ layer and another 40 nm thick Si ₃ N ₄ layer by DC sputtering with a deposition rate of 1 nm / s another layer consisting was deposited from Pd to a layer thickness of 50 nm. In this case, a Pd surface of 2 mm diameter was defined with a metal mask. A backside contact was realized by the deposition of Al (500 nm). The samples were characterized by high frequency capacitance / voltage measurement. Upon contact of the sensor with synthetic air of different hydrogen content at room temperature the in 3 found behavior, which demonstrates the advantage of using an additional ion conductor layer.

Example 5

One Sensor a) according to example 1 was produced.

A sensor b) with a Pt electrode was prepared by depositing a silicon single crystal with a 40 nm SiO ₂ layer and another 40 nm thick Si ₃ N ₄ layer by thermal vapor deposition under high vacuum at a deposition rate of 0.1 nm / s coated with LaF ₃ . The layer thickness of the ion conductor layer was 40 nm. By DC sputtering, a further layer consisting of Pt was deposited at a deposition rate of 1 nm / s up to a layer thickness of 50 nm. In this case, a Pt surface of 2 mm diameter was defined with a metal mask. A backside contact was realized by the deposition of Al (500 nm).

For both sensors, the influence of humidity on hydrogen sensitivity was investigated. While the sensor b) showed a change in sensitivity from 27 m / V / lg p (H ₂ ) to 20 mV / lg p (H ₂ ) with a relative humidity change from 33% to 80%, the sensitivity for Sensor a) constant.

Example 6

One Sensor according to example 1 was produced.

Of the Sensor was aged 2 months. From this chip 5 × 15 mm were 3 mm of the backside contact etched away. The remaining Al surfaces were contacted with a temperature-stable conductive adhesive and with a Voltage source connected. It was a tension for 1.5 minutes created by 17V. The subsequent Measurement of hydrogen sensitivity gave a result as in Example 2 shown.

Example 7

A sensor of a silicon single crystal with a 40 nm SiO ₂ layer and another 40 nm thick Si ₃ N ₄ layer was coated by thermal vapor deposition in a high vacuum at a deposition rate of 0.1 nm / s with LaF ₃ . The layer thickness of the ion conductor layer was 40 nm. By DC sputtering, a further layer consisting of Pd was deposited at a deposition rate of 1 nm / s up to a layer thickness of 50 nm. In this case, a Pd surface of 8 × 1 mm was defined with a metal mask. In back contact was realized by the deposition of Al (500 nm). The sample was aged for 2 months. The Pd was contacted with a temperature-stable conductive adhesive in two places at a distance of 4 mm and connected to a voltage source. A voltage of 70 V was applied for 10 ms. The subsequent measurement of the hydrogen sensitivity gave a result as shown in Example 2.

Claims (16)

Hydrogen sensor ( 1 ) comprising a thin-film structure of semiconductor substrate ( 2 ), Insulator ( 3 ), Fluoride ion conductors ( 4 ), Palladium electrode ( 5 ) and second electrode ( 6 ), wherein the insulator layer ( 3 ) on the semiconductor substrate ( 2 ), the fluoride ion conductor layer ( 4 ) on the insulator layer ( 3 ) and the palladium layer ( 5 ) on the fluoride ion conductor layer ( 4 ) and the second electrode ( 6 ) on the backside of the semiconductor substrate ( 2 ), wherein the palladium electrode ( 5 ) and / or the second electrode ( 6 ) are formed as a resistance heating element. Sensor according to claim 1, characterized in that palladium electrode ( 5 ) and second electrode ( 6 ) two temperature stable, elek trical contacts ( 8th ) and ( 7 ) exhibit. Sensor according to claim 1, characterized in that the palladium electrode ( 5 ) consists of palladium or its alloys with Au, Ru, Ni, Fe, Co, Rh, Ir or Ag. Sensor according to claim 1, characterized in that the semiconductor substrate ( 2 ) is a silicon single crystal, GaAs or amorphous silicon. Sensor according to claim 1, characterized in that the insulator layer ( 3 ) consists of SiO ₂ , of the layer combination SiO ₂ / Si ₃ N ₄ , of Al ₂ O ₃ or of Ta ₂ O ₅ . Sensor according to claim 1, characterized in that the fluoride ion conductor layer ( 4 ) consists of polycrystalline LaF ₃ , CaF ₂ or BaF ₂ . Sensor according to claim 1, characterized in that the layer thickness of the fluoride ion conductor layer ( 4 ) of 15 to 50 nm. Sensor according to claim 1, characterized in that the second electrode ( 6 ) consists of aluminum, platinum or gold. Hydrogen gas detector comprising a hydrogen sensor ( 1 ) according to claims 1 to 8, at least one voltage supply, means for measuring the capacitance, the photocurrent or the transistor drain / source current of the hydrogen sensor ( 1 ), Hardware and software for calculating the hydrogen concentration, and means for alarm triggering at hydrogen gas concentrations higher than the predetermined reference concentration. Hydrogen gas detector comprising a hydrogen sensor ( 1 ) according to claims 1 to 8, at least one voltage supply, means for measuring the capacitance, the photocurrent or the transistor drain / source current of the hydrogen sensor ( 1 ) and hardware and software for calculating the hydrogen concentration. Use of a hydrogen sensor ( 1 ) according to claims 1 to 8 for fire detection. Method for determining the hydrogen gas concentration, characterized in that a hydrogen sensor ( 1 ) according to claims 1 to 8 serves as a capacitive semiconductor sensor by between its palladium electrode ( 5 ) and the second electrode ( 6 ) applied a DC voltage and in addition a high-frequency AC voltage is coupled, measured by means of high-frequency capacitance / voltage measurement, the capacitance and the hydrogen concentration is determined. Method for determining the hydrogen gas concentration, characterized in that a hydrogen sensor ( 1 ) according to claims 1 to 8, modulated laser light into the semiconductor ( 2 ) of the sensor ( 1 ), the resulting photocurrent is measured and the hydrogen concentration is determined. Method for determining the hydrogen gas concentration, characterized in that a hydrogen sensor according to claims 1 to 8 serves as a gate region of a field-effect transistor, when a constant voltage is applied between its palladium electrode ( 5 ) and the second electrode ( 6 ) (Drain / source electrodes) the change in current is measured and the hydrogen concentration is determined. Process according to claims 12, 13 or 14, characterized in that the hydrogen sensor ( 1 ) is operated at room temperature during the measurement and the sensor ( 1 ) is reactivated outside the measurements by applying a voltage to the resistance heating element in a predetermined time regime ( 5 ) and or ( 6 ) is briefly heated to temperatures less than 300 ° C. Method according to claim 15, characterized in that that the reactivation at intervals of up to 7 days and for a Period from 10 μs to carried out for a maximum of 2 minutes becomes.