EP1615021A1 - Hydrogen sensor and use thereof - 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 an extremely low energy consumption, since it operates at room temperature. The hydrogen sensor 1 according to the invention is suitable as part of a hydrogen gas detector, inter alia, for fire detection, since it reliably measures the low hydrogen concentrations occurring 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, in which 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 also used, such as SiC or GaN / AlGaN heterostructures Temperatures of 400-600 ° C. The microstructure technique was used to combine the sensors with heaters (US 6,265,222 and 6,596,236).

A disadvantage of all these sensors, however, is that they require an increased operating temperature of 140 ° C to over 600 ° C.

For many potential applications, the associated energy consumption is a major impediment (battery-powered sensor systems, power fail-safe monitoring systems). Even with central monitoring systems, the power consumption of known fire gas sensors is too high to allow operation with a power failure fuse, so that they could only be used in special cases.

Fire detectors are already used in the majority of households in the USA. For Germany, a similar development is predicted and also a legal obligation to install in private households discussed.

The damage caused by fires is very high (100 dead every year, 10,000 injured, € 2 billion material damage). An early and reliable warning could greatly improve this situation. The high power requirements, the price, cross sensitivity and lack of long-term stability are the main problems that prevent the introduction of 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 lifetime of less than 3 months, and its hydrogen sensitivity with a change in the relative Humidity changes. Also, a lower detection limit of 10ppm for certain concerns in Schwellbrand- and fire detection may not be sufficient.

It was therefore the object of the present invention to provide a highly sensitive and stable in the sensitivity of the degree of humidity, stable at room temperature and long-lived hydrogen gas sensor available, 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 component based on the field effect in the semiconductor and consists of a layer system semiconductor 2 / insulator 3 / fluoride ion conductor 4 / palladium electrode 5 and second electrode 6 and has a heating element. The hydrogen sensor 1 is a thin film structure in which the insulator layer 3 is deposited 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 is disposed on the back surface of the semiconductor substrate 2 Palladiumelektrode 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 8 and 7, via which the sensor 1 can be heated by applying a voltage. In a preferred embodiment, the sensor is heated via the palladium electrode 5 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.

Therefore, the invention also relates to a method for determining the hydrogen concentration according to claims 15 and 16. From this mode of operation results in enormous energy savings compared to constantly heated sensors and those that need to be heated periodically for measurement.

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

The insulator layer 3 can 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 is made of palladium or an alloy of Pd with Au, Ru, Ni, Fe, Co, Rh, Ir and / or Ag. The electrode 5 is preferably applied in such a way that the layer thickness is up to 60 nm and a specific area and shape are predetermined by means of a metal mask. In a preferred embodiment, the palladium electrode 5 has two wider areas in the outer area, which are connected by a narrow strip are. On the two wider surfaces in the outer region, the two contacts 8 are applied.

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

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

Palladium electrode 5 and second electrode 6 may be deposited 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 according to the invention with palladium electrode 5 is highly sensitive and the sensitivity is not dependent on the change in the air humidity, in contrast to a structurally identical sensor with Pt electrode (compare Example 5). The lower detection limit of the sensor is about 2 ppm, making it ideal for measuring extremely low hydrogen concentrations.

The invention therefore also relates to a hydrogen gas detector comprising the described hydrogen sensor 1, 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. The invention further relates to a Hydrogen gas detector, which is also intended for hydrogen measurement, but does not trigger an alarm, that does not include the means described for triggering the alarm, but z. B. is monitored online by computer. That is, this hydrogen gas detector comprises the described hydrogen sensor 1, at least one power 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.

The hydrogen sensor 1 according to the invention 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.

For detecting the capacitance, therefore, the hydrogen gas detector and detector include, for example, a frequency generator and an AC transducer, a photodiode for photocurrent measurement, and a constant voltage source for the transistor.

As a means of signal control z. B. Differential amplifier in the hydrogen gas detector and detector provided.

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

As a means for alarm triggering a comparator circuit is used with a fixed setpoint, for example. Alternatively, intelligent systems can be used, which use the deviation from the long-term signal development for alarm definition.

In accordance with the various possible 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.

If the hydrogen sensor 1 is used as a capacitive semiconductor sensor, signal detection takes place by means of the high-frequency capacitance / voltage measurement (HF-CF measurement technique), the voltage being applied between the second electrode 6 and the palladium electrode 5. In addition to the DC voltage, an AC voltage of 10 kHz, for example, is injected. The capacitance is read out with a frequency-selective detector and a change in potential with a change in concentration is recalculated from the slope of the capacitance / voltage curve.

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

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

• 1 Wasserstoffsensor
• 2 Halbleitersubstrat
• 3 Isolator
• 4 Fluoridionenleiter
• 5 Palladiumelektrode
• 6 Zweite Elektrode
• 7 elektrisch leitende Kontakte
• 8 elektrisch leitende Kontakte

The hydrogen sensor according to the invention is shown schematically in FIG. 1, wherein in FIG. 1:

• 1 hydrogen sensor
• 2 semiconductor substrate
• 3 insulator
• 4 fluoride ion conductors
• 5 palladium electrode
• 6 Second electrode
• 7 electrically conductive contacts
• 8 electrically conductive contacts

Fig. 2 shows after reactivation, the response of the sensor according to the invention from Example 2 to different Hydrogen concentrations, baseline concentration 10 ppm, right-hand scale: hydrogen concentration in ppm.

FIG. 3 shows a comparison of the sensor signal of the sensor according to the invention from example 4a and the comparison sensor from example 4b without fluoride ion conductor layer; 20 ppmH ₂ .

Fig. 4a shows the scheme of the hydrogen gas detector of the invention for measuring the drain / source current of the transistor (see Example 8 of the invention).

U_G

Gate-Spannungsquelle

U_DS

Drain-/Source-Spannungsquelle

A

Amperemeter

In Fig. 4a means:

U _G

Gate-voltage source

U _DS

Drain / source voltage source

A

ammeter

Figure 4b shows the scheme of the hydrogen gas detector of the invention for measuring capacitance (see Example 9 of the invention).

U_G

Gate-Spannungsquelle

U_AC

Wechselspannungsquelle

C

Kapazitätsmesser

In Fig. 4b mean:

U _G

Gate-voltage source

U _AC

AC voltage source

C

capacitance meter

Figure 4c shows the scheme of the hydrogen gas detector of the invention for measuring the photocurrent (see Example 10 of the invention).

U_G

Gate-Spannungsquelle

A

Amperemeter

In Fig. 4c mean:

U _G

Gate-voltage source

A

ammeter

embodiments 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 / Ig 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 typical response is shown in Fig. 2.

Example 3

The hydrogen sensor described in Example 2 was used to detect a test fire according to European Standard EN 54 (Testfire 2) [published by the DIN Deutsches Institut für Normung 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] compared with a standard optical smoke detection system. The fire was successfully detected, whereas in comparison to the smoke alarm system an alarm signal was reached 90 sec earlier.

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, the behavior shown in FIG. 3 was found at room temperature, which demonstrates the advantage of using an additional ion conductor layer.

Example 5

A sensor a) according to Example 1 was prepared.

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 Ai (500 nm).

For both sensors, the influence of humidity on hydrogen sensitivity was investigated. While sensor b) showed a change in sensitivity from 27 m / V / Ig 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

A sensor according to Example 1 was produced.
The sensor was aged for 2 months. From this 5 x 15 mm chip, 3 mm of the backside contact was etched away. The remaining Al surfaces were contacted with a temperature-stable conductive adhesive and connected to a voltage source. A voltage of 17V was applied for 1.5 minutes. The subsequent measurement of the hydrogen sensitivity gave a result as shown in Example 2.

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.

Example 8 Method for determining the hydrogen concentration using a hydrogen sensor consisting of n-Si / SiO ₂ / Si ₃ N ₄ / LaF ₃ / Pd according to Example 1 as a gate region of a field effect transistor (see Fig. 4a)

The hydrogen sensor described above is brought into contact with a gas 1 of known hydrogen concentration. A constant drain / source voltage is applied and the resulting drain / source current 1 is measured.

In a second step, the gas is replaced by a gas 2 having a different known hydrogen concentration, and the resulting drain / source current 2 is measured using the same voltage conditions.

In the following, the gas of unknown hydrogen concentration is brought into contact with the sensor and under the constant voltage conditions, the drain / source current 3 is measured.

For small concentration ranges, a linear relationship between the gas concentrations and the drain / source current describes the sensor behavior. Therefore, using the gas concentrations 1 and 2 and the drain / source currents 1 and 2, a linear equation can be established. Using this equation and the drain / source current 3, the unknown hydrogen concentration can then be determined.

Example 9 Method for determining the hydrogen concentration using a hydrogen sensor consisting of n-Si / SiO ₂ / Si ₃ N ₄ / LaF ₃ / Pd according to Example 1 as a capacitive semiconductor sensor (see Fig. 4b)

The hydrogen sensor described above is brought into contact with a gas 1 of known hydrogen concentration. A constant gate voltage and a small signal AC voltage are applied and the capacitance 1 of the structure is measured.

In a second step, the gas is replaced by a gas 2 having a different known hydrogen concentration, and the capacitance 2 is measured using the same voltage conditions.

In the following, the gas of unknown hydrogen concentration is contacted with the sensor and the capacitance 3 is measured under the constant voltage conditions.

For small concentration ranges, a linear relationship between gas concentrations and capacitance describes the sensor behavior. Therefore, using the gas concentrations 1 and 2 and the capacities 1 and 2, a linear equation can be established. Using this equation and the capacity 3, the unknown hydrogen concentration can then be determined.

Example 10 Method for determining the hydrogen concentration using a hydrogen sensor consisting of n-Si / SiO ₂ / Si ₃ N ₄ / LaF ₃ / Pd according to Example 1 as a sensor based on the photoelectric effect in the semiconductor (see Fig. 4c).

The hydrogen sensor described above is brought into contact with a gas 1 of known hydrogen concentration. A constant gate voltage is applied, modulated laser light is irradiated. The resulting photocurrent 1 of the structure is measured.

In a second step, the gas is replaced by a gas 2 having a different known hydrogen concentration and the photocurrent 2 is measured using the same voltage conditions.

In the following, the gas of unknown hydrogen concentration is brought into contact with the sensor and under the constant voltage conditions the photocurrent 3 is measured.

For small concentration ranges, a linear relationship between the gas concentrations and the photocurrent describes the sensor behavior. Therefore, using the gas concentrations 1 and 2 and the photocurrents 1 and 2, a linear equation can be established. Using this equation and the photocurrent 3, the unknown hydrogen concentration can then be determined.

Claims (16)

A hydrogen sensor (1) comprising a thin-film structure of semiconductor substrate (2), insulator (3), fluoride ion conductor (4), palladium electrode (5) and second electrode (6), wherein the insulator layer (3) on the semiconductor substrate (2), the fluoride ion conductor layer (3). 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 side of the semiconductor substrate (2) is arranged, wherein the palladium electrode (5) and / or 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) each have two temperature-stable, electrically conductive contacts (8) and (7). 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 and / 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) is from 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 Alarm triggering means for hydrogen gas concentrations higher than the predetermined reference concentration. A 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 a DC voltage is applied between its palladium electrode (5) and the second electrode (6) 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 is used, modulated laser light is radiated into the semiconductor (2) of the sensor (1), the resulting photocurrent is measured and the hydrogen concentration is determined. A 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, upon application of a constant voltage between its palladium electrode (5) and the second electrode (6) (drain / source electrodes) the change the current is measured and the hydrogen concentration is determined. Method 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 of the measurements by briefly applying a voltage to the resistance heating element (5) and / or (6) in a predetermined time regime less than 300 ° C is heated. Method according to claim 15,
characterized in that
the reactivation is carried out at intervals of up to 7 days and for a period of 10 μs up to a maximum of 2 min.

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