DE10118200A1 - Gas sensor element used, e.g., in biomedical analysis comprises a first and second metallic electrodes, nanotubes connecting the electrodes together, and a unit for determining the electrical resistance between the electrodes - Google Patents

Abstract

Providing an improved gas sensor element. Gas sensor element (200) comprises: a first metallic electrode (202); a second metallic electrode (204); nanotubes (203) connecting the electrodes together; and a unit for determining the electrical resistance between the electrodes. An Independent claim is also included for a process for the production of a gas sensor element, comprising forming nanotubes having a partially exposed surface, and forming electrodes so that they are connected by the nanotubes. Preferred Features: The nanotubes are carbon nanotubes or nanotubes doped with boron nitride. The nanotubes are arranged at a constant distance of 10-20 nm apart.

Description

The invention relates to a gas sensor element, a method for producing a gas sensor element and a method for Detection of gases.

Such a gas sensor element is e.g. B. in the form of a gas sensor based on semiconducting metal oxides and is used in particular for the detection of nitrogen dioxide (NO ₂ ) and ammonia (NH ₃ ) (cf. [1]). In addition to environmental measurement technology, possible areas of application include gas detection in chemical process controls or biomedical analyzes.

However, this gas sensor element has the disadvantage that for Achieve sufficient detection sensitivity for the Detecting gases relatively high operating temperatures in the A range of 200 to 600 ° C is required sufficient chemical reactivity between the gas molecules and to ensure the sensor materials.

It is also a gas sensor element in the form of Silicon components and a gas sensor element based known from conductive polymers. These gas sensor elements are also particularly suitable for the detection of Nitrogen dioxide or ammonia, however, have only one limited detection sensitivity. Furthermore is also a gas sensor element based on organic materials such as B. organic phthalocyanine semiconductors and  Carbon-polymer composites known, but one very high electrical resistance (from 10 GΩ and above) has.

All of the above gas sensor elements have therefore either too little detection sensitivity or need to achieve sufficient Detection sensitivity to temperatures of several hundred Degrees Celsius can be heated.

The invention is therefore based on the problem of a gas Sensor element, a method of manufacturing a gas Sensor element and a method for the detection of gases to indicate with or through which already at room temperature achieved a significant increase in detection sensitivity can be.

The problem is caused by the gas sensor element, the process to manufacture a gas sensor element and by the Method for the detection of gases with the features according to the independent claims solved.

A gas sensor element has a first metallic electrode and a second metallic electrode and at least one Nanotube on. The first and second metallic electrodes are electrical via the at least one nanotube interconnected, the at least one nanotube has an at least partially exposed surface.

Furthermore, a unit for determining the electrical Resistance between the first and the second metallic Electrode provided.  

The invention makes use of the knowledge that the electrical resistance of nanotubes changes by approximately three orders of magnitude within a few seconds if the nanotubes are exposed to a gas such as e.g. B. nitrogen dioxide (NO ₂ ) or ammonia (NH ₃ ).

It was demonstrated in [1] that an NH ₃ atmosphere leads to a reduced electrical conductivity of the nanotubes, which is explained by a shift in the valence band edge far below the Fermi level in the nanotubes and the resulting depletion of charge carriers in the form of holes has been.

Conversely, an increase in the electrical conductivity of the nanotubes by three orders of magnitude was found when they were exposed to a NO ₂ atmosphere at a concentration of about 200 ppm, which was explained by the fact that the Fermi energy in the region of the nanotubes shifted closer to the valence band and consequently the number of charge carriers in the form of holes in the nanotubes increases.

Thus, in the gas sensor element according to the invention Gas detection is done by the electrical Resistance between the first and the second metallic Electrode is measured because this is electrical Resistance due to the deposition of the gas molecules on the exposed surface of the nanotubes changes.

The gas detection is not limited to gases such as nitrogen dioxide (NO ₂ ) or ammonia (NH ₃ ), but other gases such as e.g. B. oxygen (02) can be detected, which lead to a change in the electrical resistance of the nanotubes.

The nanotubes can e.g. B. carbon nanotubes. It However, one or more boron nitride doped can Nanotubes are formed, which are characterized by that conductivity is reduced by applying an electrical Field can be influenced (so-called field effect).

Preferably at least one nanotube is between the first and second metallic electrodes are free-standing arranged.

The nanotubes are preferably in an ordered field between the first and second metallic electrodes arranged, being particularly advantageous in arranged essentially straight and parallel to each other are. The nanotubes are preferably in one constant, in the range between 10 and 20 nm Spaced from each other and z. B. a length of 1 to 10 µm. In this way, on the one hand high detection sensitivity for the gas sensor element passed gases reached, on the other hand sufficient gas flow at the same time is guaranteed.

The first and second metallic electrodes as well as the Nanotubes can, for example, on a semiconductor chip, z. B. a CMOS chip can be integrated. The unit for The electrical resistance can then be determined in particular one integrated on the semiconductor chip Include resistance sensor.  

Furthermore, a heating element for heating the at least a nanotube can be integrated on the semiconductor chip. about the heating element can turn the nanotube after a gas Detection at a temperature of, for example, 200 ° C be heated to cover the nanotubes with the Repeat molecules of the gas to be detected.

In a method of manufacturing a gas sensor element becomes at least one nanotube with an at least partially exposed surface and a first metallic electrode and a second metallic electrode are designed so that the first and the second metallic electrode over the nanotube (s) electrical are interconnected.

Here, an ordered array of nanotubes is preferred between the first and second metallic electrodes educated.

In particular, a gas sensor element can be manufactured as follows:
On a first insulating layer, a layer system consisting of a catalyst layer and a first metallic layer is applied. B. using MOCVD (metal-organic chemical vapor deposition = metal-organic vapor deposition) can be done in a particularly compliant manner.

The first insulating layer can e.g. B. from silicon dioxide or another insulating material, e.g. B. one other metal oxide are formed. In particular, a  deposited on a silicon substrate Silicon dioxide layer used as the first insulating layer become.

The catalyst layer is preferably made of cobalt, nickel or iron formed.

The first metallic layer can, for example, consist of Be formed of aluminum or another metallic have conductive material.

The layer system consisting of catalyst layer and first metallic layer is structured so that the Catalyst layer has a pore-structured surface having.

This can be done on the one hand in that after the Application of the first metallic layer on the Catalyst layer for forming the pore-structured Surface of the catalyst layer pores in the first I metallic layer are formed. According to the Evaporate the aluminum layer on the cobalt layer Paint mask with a corresponding pore structure on the Aluminum layer are applied, after which the Aluminum layer in a lithography step with one of the <Paint mask is provided corresponding pore-like structure. Alternatively, methods for nanostructuring of Substrates such as glass or silicon wafers are used, d. H. Processes using nano-particles made of metal on these Substrates are generated. In particular, colloid masks can be used as masks to create the pores.

Alternatively, however, the aluminum layer can also be applied first evaporated the first insulating layer and before the  subsequent vapor deposition of the cobalt layer on the Aluminum layer a pore-structured paint mask be applied so that the cobalt only in the pores of the Paint mask is deposited.

The pore structure in the paint mask depends on the desired structure of the growing nanotubes preferably in a constant, in the range between 10 to Pores located at a distance of 20 nm from one another.

A field of Nanotubes, e.g. B. carbon nanotubes or with boron nitride doped nanotubes in one of the pore structures of the Layer system consisting of catalyst layer and first metallic Layer corresponding arrangement grew up. This can e.g. B. by means of vapor deposition corresponding to that in [2] described procedures take place.

According to this method shown in [2] that is in the pore structure of the aluminum layer with a pore density of z. B. 1.1.10 ¹⁰ pores / cm ² deposited cobalt in the carbon monoxide (CO) flow at a temperature of 600 ° C reduced. Then acetylene is pyrolyzed to form a field of carbon nanotubes in the pore structure of the catalyst layer in a flow from an acetylene / nitrogen mixture at 700 ° C, whereby, for example, a regular hexagonal lattice of nanotubes with a uniform diameter of z. B. 50 nm can be obtained.

The length, diameter and arrangement of the Control nanotubes by making the pore structured Aluminum layer before applying the cobalt  Catalyst layer one in two explained in more detail in [2] Undergoes anodizing process.

After the formation of the nanotubes and before application a second insulating layer can advantageously on the first insulating layer, the layer system Catalyst layer and first metallic layer and the Nanotubes are deposited an adhesive layer, the Adhesive layer after the partial removal of the second metallic layer is also removed. The adhesive layer can be formed from silicon nitride, for example, but another material, such as B. titanium nitride or a multilayer system made of titanium and titanium nitride, exhibit.

A second insulating layer is placed on this adhesive layer covering the nanotubes, the first metallic ones Layer and the catalyst layer applied, the second insulating layer again silicon dioxide or another insulating material, especially a May have metal oxide.

In the second insulating layer, one is up to first metallic layer extending contact hole formed, which in particular by means of wet chemical etching or plasma etching can take place.

Then the second insulating layer and possibly the adhesive layer z. B. by etching with partial exposure of the nanotubes partially removed and in the second insulating layer becomes one to the first formed metallic layer extending contact hole.  

A second is applied to the second insulating layer metallic layer with complete filling of the Contact hole and covering the exposed portions of the Applied nanotubes. The second metallic layer can z. B. be made of tungsten, which makes a particularly compliant separation can be achieved. The second metallic However, layer can alternatively be any other Metallically conductive material such as B. copper or aluminum exhibit. The deposition of the second metallic layer can again be done in conformity with MOCVD.

The second metallic layer then turns again by etching, in particular wet chemical etching or Plasma etching, partially removed so that one with the first metallic layer electrically connected first Conductor section and one with the nanotubes electrical connected second conductor track section to form a first and second metallic electrodes remain.

Then the second insulating layer as well optionally the adhesive layer from the first insulating Layer, the layer structure of the catalyst layer and first metallic layer, as well as the carbon nanotubes at least partially removed by etching. The carbon Accordingly, nanotubes at least partially exposed surface on which there are gas molecules to deposit gas to be detected.

However, the second insulating layer is preferred and, if necessary, the adhesive layer is removed without residues, so that the nanotubes between those from the remaining Portions of the second metallic layer formed  free standing first and second metallic electrodes are arranged.

The remaining sections of the second metallic layer accordingly form a first conductor track section first metallic electrode and one the second conductor track section comprising a second metallic electrode, wherein the second metallic electrode with the carbon Nanotubes at their previously exposed end sections in Contact is made while the first metallic electrode is over the first metallic layer with the opposite End sections of the carbon nanotubes is in contact.

Via a resistance measurement between the first and the second electrode can now change the electrical Resistance of the nanotubes over on the nanotubes deposited gas molecules immediately determined and thus a too detecting, through the areas between the nanotubes gas passed through it can be detected.

In a method for the detection of gases in a Arrangement of a first metallic electrode, one second metallic electrode and at least one Nanotube, the first and the second metallic Electrode electrically via the at least one nanotube are interconnected and the at least one nanotube has an at least partially exposed surface, the at least one nanotube a gas to be detected exposed and the electrical resistance of the nanotube measured.

The at least one nanotube is then preferably connected heated to a temperature of 200 ° C or above to the  gas molecules attached to the nanotubes remove.

Embodiments of the invention are in the figures shown and are explained in more detail below.

Show it:

Figs. 1A to 1H are cross sections through a gas sensor element according to an embodiment of the invention at various states during its manufacture;

FIG. 2 shows a flow diagram for the method for producing a gas sensor element according to FIG. 1;

Figure 3 is a schematic layout for illustrating the individual steps lithography (L1-L6), which are used in the example shown in Figures 1 and 2 processes...; and

Fig. 4 is a schematic representation of a gas sensor element to explain the principle on which the gas sensor element according to the invention is based.

Referring to FIG. 1A, a method for manufacturing begins a gas sensor element 100 according to the embodiment with a silicon substrate 101, in which is etched in a first lithography step L1 in a known manner trenches 102 (corresponding to the step shown in in FIG. 2 S1 Flow chart). A first insulating layer 103 made of silicon dioxide is then deposited on the silicon substrate 101 and the trenches 102 by means of a CVD method (step S2 in FIG. 2).

Then, in a second lithography step L2, the surface of the first insulating layer 103 within the trenches 102 is covered to such an extent that only a region spaced apart from the side walls of the trenches 102 remains exposed. A catalyst layer 104 made of cobalt and a first metallic layer 105 made of aluminum are evaporated in succession on this exposed area (step S3 in FIG. 2).

In a further step (S4 in FIG. 2), a pore structure is produced in the first metallic layer 105 made of aluminum after application of a pore-structured lacquer mask to the first metallic layer 105 in a third lithography step L3, the pore spacing according to the exemplary embodiment being approximately 20 nm , The paint mask is then removed again. The structure obtained hereafter is shown in Fig. 1A.

In a next step, nanotubes, according to the exemplary embodiment made of carbon, are grown on the catalyst layer 104 structured according to FIG. 1A by means of gas phase deposition (CVD = "chemical vapor deposition") (step S5). According to the exemplary embodiment, the carbon nanotubes 106 have a length of approximately 10 μm and are arranged at a distance of approximately 20 nm from one another in accordance with the distance between the pores in the first metallic layer 105 .

Subsequently, on the first insulating layer 103, the layer structure of the catalyst layer 104 and porous structured first metal layer 105 and on the carbon nanotubes 106, a adhesion layer 107, according to the embodiment of silicon nitride (Si ₃ N ₄₎ in turn is deposited (by means of gas phase deposition step S6 in Fig. 2).

In a next step (S7), a second insulating layer 108 is deposited on the adhesive layer 107 by means of gas phase deposition, which according to the exemplary embodiment shown is formed from silicon dioxide like the first insulating layer 103 . The hereinafter structure obtained is shown in Fig. 1B and Fig. 1C, the illustration in Fig. 1B 1C a sectional view taken along the line BB in Fig. 3 corresponds to a section along the line AA in Fig. 3 and the depiction in FIG. ,

Subsequently, in a fourth lithography step L4, a contact hole 109 is etched into the second insulating layer 108 and the adhesive layer 107 , which extends from the surface of the second insulating layer 108 down to the first metallic layer 105 made of aluminum parallel to the carbon nanotubes 106 ( Step S8 in Fig. 2). The structure obtained after this is shown in FIG. 1D according to a section of the line BB in FIG. 3.

In a next step ( 59 ), the second insulating layer 108 and the adhesive layer 107 are etched back to such an extent that an end section 106 a of the carbon nanotubes 106 is exposed at its end facing away from the first metallic layer.

A second metallic layer 110 , according to the exemplary embodiment made of tungsten, is then deposited on the remaining second insulating layer 108 , the exposed sections 106 a of the carbon nanotubes 106 and the surface of the first metallic layer 105 exposed within the contact hole 109 by means of CVD (step S10 in Fig. 2). In this case, the second metallic layer 110 is deposited in such a thickness that the contact hole 109 is completely filled and the exposed sections 106 a of the carbon nanotubes 106 are completely covered by the second metallic layer 110 . The structure obtained after this is shown in FIG. 1E (corresponding to the section AA in FIG. 3) or FIG. 1F (corresponding to the section BB in FIG. 3).

In a next, fifth lithography step L5, the second metallic layer 110 made of tungsten is structured by etching in such a way that, according to the structure shown in FIG. 1G (corresponding to a section BB in FIG. 3), it is divided into two spatially separated sections 110a , 110 b is divided (step S11 in Fig. 2).

Here, the two sections 110 a, 110 b form two conductor tracks running on the surface of the second insulating layer 108 parallel to the section line AA in FIG. 3. The conductor track corresponding to section 110 b completely covers the exposed sections 106 a of the carbon nanotubes 106 , while the conductor track corresponding to section 110 a extends through the contact hole 109 down to the first metallic layer 105 .

The section 110 a corresponding conductor path is thus available over the first metallic layer 105 also connected to the carbon nanotube 106, but at the exposed portions 106 a opposite end portions in electrical contact.

In a next, sixth lithography step L6, the sections 110 a, 110 b of the second metallic layer 110 and the surface of the first insulating layer 103 are covered by means of a lacquer mask, whereupon the second insulating layer 108 is etched away using wet chemical etching and the lacquer mask is removed again (Step S12 in Fig. 2).

Subsequently, the adhesive layer 107 is also removed from the first insulating layer 103 , the layer structure comprising the catalyst layer 104 and the first metallic layer 105 , and the carbon nanotubes 106 by etching (step S13 in FIG. 2), so that the layer in FIG. 1H 3 according to a section BB in FIG. 3 of the finished structure of a gas sensor element 100 is obtained.

In the gas sensor element 100 according to FIG. 1H, the remaining sections of the second metallic layer 110 form a first metallic electrode 111 comprising the first interconnect section 110 a and a second metallic electrode 112 comprising the second interconnect section 110 b. Here, the first metallic electrode 111 is in electrical contact with the first metallic layer 105 and thus also with the carbon nanotubes 106 , while the second metallic electrode 112 is in electrical contact with the carbon nanotubes 106 at their end sections 106a .

The carbon nanotubes 106 themselves are now freestanding between the first and second metallic electrodes 111 , 112 after the removal of the second insulating layer 108 and the adhesive layer 107 and, via the first metallic layer 105, make the electrical contact between the first and the second metallic Electrode 111 or 112 .

The structure thus obtained provides an inventive gas sensor element 100 in a preferred embodiment wherein the through the first and second metallic electrode 111, 112, the electrical resistance of the assembly of free-standing carbon nanotubes 106 z. B. can be measured on a common CMOS chip integrated resistance sensors.

Based on the measured change in resistance, the detecting gases as explained above, whereupon the nanotubes to remove the deposited Gas molecules of the gas to be detected to a temperature of 200 ° C or above.

In FIG. 4, the essential elements of a gas sensor element 200 according to the invention are shown again for clarity, in a schematic principle sketch. Accordingly, a conductive bottom electrode 202 (corresponding to the first metallic electrode 111 from FIG. 1H) is arranged in a substrate 201 . Furthermore, a free-standing arrangement of nanotubes 203 is shown, which are in electrical contact with the conductive bottom electrode 202 and extend vertically therefrom in a straight line and parallel to one another to a likewise electrically conductive top electrode 204 , to which they are also electrically connected.

Further, 4 two side gates 205, 206 are shown in Fig., Which are arranged parallel to each other and extend from the conductive ground electrode 202 to the conductive top electrode 204 in a vertical arrangement to the electrodes 202, 204. The two side gates 205 , 206 are each electrically insulated from the top electrode 204 , so that the two electrodes 202 , 204 are in electrical contact only via the nanotubes 203 , as in the gas sensor element 100 .

A gas to be detected which is passed between the side gates 205 , 206 leads to a different rapid coverage of the nanotubes 203 in the gas sensor element 200 depending on the position of the side gates 205 , 206 , so that the working point is caused by the positioning of the side gates 205 , 206 of the gas sensor element 200 can be set.

The following publications are cited in this document:
[1] J. Kong et al., Nanotube Molecular Wires as Chemical Sensors, Science 287, pp. 622-625 ( 2000 ).
[2] JS Suh et al., Highly ordered two-dimensional carbon nanotube arrays, Appl. Phys. Lett. 75, pp. 2047-2049 ( 1999 ).

LIST OF REFERENCE NUMBERS

100

Gas sensor element

101

Silicon substrate

102

trenches

103

first insulating layer

104

Catalyst layer (Co)

105

first metallic layer (Al)

106

nanotubes

106

a End sections of the nanotubes

107

Adhesive layer (Si ₃

N ₄

)

108

second insulating layer

109

contact hole

110

second metallic layer (W)
110a, b conductor track sections

111

first metallic electrode

112

second metallic electrode

200

Gas sensor element

201

substratum

202

bottom electrode

203

nanotubes

204

top electrode

205

side gate

206

side gate

Claims (24)

1. Gas sensor element, comprising
a first metallic electrode;
a second metallic electrode;
at least one nanotube, the first and second metallic electrodes being electrically connected to one another via the at least one nanotube and the at least one nanotube having an at least partially exposed surface; and
a unit for determining the electrical resistance between the first and second metallic electrodes. 2. Gas sensor element according to claim 1, where the nanotube is a carbon nanotube or a is a nanotube doped with boron nitride. 3. Gas sensor element according to claim 1 or 2, in which the at least one nanotube between the first and the second metallic electrode is arranged free-standing is. 4. Gas sensor element according to one of the preceding claims, where an ordered array of nanotubes between the first and the second metallic electrode is arranged. 5. Gas sensor element according to claim 4, where the nanotubes are essentially straight and are arranged parallel to each other. 6. Gas sensor element according to claim 4 or 5,  where the nanotubes are in a constant, in the range spaced from 10 to 20 nm apart are. 7. Gas sensor element according to one of the preceding claims, in which the nanotubes have a length of 1 to 10 µm. 8. Gas sensor element according to one of the preceding claims, in which the first and second metallic electrodes and the at least one nanotube on a semiconductor chip together are integrated. 9. Gas sensor element according to claim 8, in which the unit for determining the electrical Resistance one integrated on the semiconductor chip Resistance sensor has. 10. Gas sensor element according to claim 8 or 9, in which a heating element for heating the at least one Nanotube is integrated on the semiconductor chip. 11. Method for producing a gas sensor element,
in which at least one nanotube is formed with an at least partially exposed surface and
in which a first metallic electrode and a second metallic electrode are formed in such a way that the first and the second metallic electrode are electrically connected to one another via the at least one nanotube. 12. The method according to claim 11, where an ordered array of nanotubes between the first and the second metallic electrode becomes.   13. The method according to claim 12,
in which a layer system comprising a first metallic layer and a catalyst layer is applied to a first insulating layer, the catalyst layer having a pore-structured surface;
in which a field of nanotubes corresponding to the pore structure is grown on the catalyst layer;
in which a second insulating layer is applied to cover the nanotubes and the layer system comprising the first metallic layer and the catalyst layer;
in which the second insulating layer is partially removed with partial exposure of the nanotubes and a contact hole extending to the first metallic layer is formed in the second insulating layer;
in which a second metallic layer is applied while filling the contact hole and covering the nanotubes; and
in which the second metallic layer is partially removed while leaving a first conductor section electrically connected to the first metallic layer and a second conductor section electrically connected to the nanotubes to form a first and second metallic electrode. 14. The method according to claim 13, in which the first and / or the second insulating layer Silicon dioxide are formed. 15. The method according to claim 13 or 14, in which the catalyst layer made of cobalt, nickel or iron is formed.   16. The method according to any one of claims 13 to 15, in which the first metallic layer is formed from aluminum becomes. 17. The method according to any one of claims 13 to 16, in which the second metallic layer is formed from tungsten becomes. 18. The method according to any one of claims 13 to 17, in that after the application of the first metallic layer on the catalyst layer to form the pore-structured surface of the catalyst layer pores are formed in the first metallic layer. 19. The method according to claim 18, in which the pores are in a constant range between 10 distance of up to 20 nm from each other can be formed. 20. The method according to any one of claims 13 to 19, in which the method of growing the nanotubes Vapor deposition is applied. 21. The method according to any one of claims 13 to 20, in which after the growth of the nanotubes and before Applying the second insulating layer an adhesive layer on the first insulating layer, the layer system Catalyst layer and first metallic layer and the Nanotubes is applied, the adhesive layer after the partial removal of the second metallic layer is also removed. 22. The method according to claim 21, in which the adhesive layer is formed from silicon nitride.   23. A method for the detection of gases in an arrangement of a first metallic electrode, a second metallic electrode and at least one nanotube, the first and the second metallic electrode being electrically connected to one another via the at least one nanotube and the at least one nanotube at least one partially exposed surface,
in which the at least one nanotube is exposed to a gas to be detected, and
in which the electrical resistance of the nanotube is measured. 24. The method according to claim 23, where the nanotube after measuring the electrical Resistance to a temperature of 200 ° C or above is heated.