WO2005015196A1

WO2005015196A1 - Gas analysis sensor and method for the production thereof

Info

Publication number: WO2005015196A1
Application number: PCT/DE2004/001669
Authority: WO
Inventors: Arno Steckenborn
Original assignee: Siemens Aktiengesellschaft
Priority date: 2003-08-12
Filing date: 2004-07-23
Publication date: 2005-02-17
Also published as: DE10337692B3

Abstract

The invention relates to gas analysis sensor with sensor elements configured as a generator for surface waves (SAW). The invention also relates to a method for the production of sad gas analysis sensor using etching technology. According to the invention, etching of the substrate (11) is carried out in such a way that even wall surfaces (13) are produced in the monocrystalline substrate material (for instance, silicon) by means of an adequate combination of isotropic and anisotropic etching steps, said wall surfaces being suitable for the deposition of monocrystalline, piezoelectric layers (14) that are suitable as generator for surface waves in an appropriate connection with connecting electrodes (16). This production method advantageously enables the production of a gas analysis sensor, wherein the sensor element can be integrated in a space-saving manner in the channel for the passage of the gas that is to be analyzed. Said channel can be formed, for instance, by the separating column of a gas chromatograph.

Description

description

Gas analysis sensor and method for its production

The invention relates to a method for producing a gas analysis sensor with a channel for passing a gas to be analyzed and a sensor element, which is designed as a generator for surface waves (surface acoustic waves, SAW).

Such a method can be found, for example, in a publication by Electronic Sensor Technology, California, entitled "Electronic Nose Simulation of Olfactory Response Containing 500 Orthogonal Sensors in 10 Seconds" by Edward J. Staples. Thereafter, a gas analysis sensor can be formed by forming a surface wave generator on the surface of a quartz crystal. For gas analysis, a separation column of a gas chromatograph is used as a channel for the passage of the gas to be analyzed, the end of which is directed as an outlet opening for the gas to be analyzed onto the generator for surface waves. This generator then delivers a measurement signal which results from the change in its vibration behavior due to the accumulation of particles from the gas to be analyzed on the generator surface.

The object of the invention is to provide a method for producing a gas analysis sensor with a channel and a generator for surface waves, with which a gas analysis sensor can be generated which enables a measurement with comparatively small measurement errors. This object is achieved in that the

Channel is produced by etching in a substrate component with a single-crystal region in such a way that a flat wall surface of the channel with a crystal orientation suitable as a substrate for producing the sensor element is produced in the single-crystal region and the sensor element by coating the flat wall surface with a piezoelectric layer and through Manufacture of connection electrodes connected to the piezoelectric layer is produced. The monocrystalline region of the substrate component, which is used to produce the channel, is thus advantageously simultaneously available as a substrate for producing a sensor element with piezoelectric properties. The generation of piezoelectric properties in the coating requires their single-crystalline structure. Such a single-crystalline coating can be applied to the wall surface generated in the single-crystal region of the substrate component, e.g. B. form by deposition from the vapor phase. The crystal orientation of the surface of the single-crystalline area must be used for the generation of the piezoelectric

Layer be suitable in such a way that the crystal orientation of the wall surface enables a transfer of the crystal structure of the single-crystalline region to the piezoelectric layer to be produced.

The inventive method advantageously allows the generator for surface waves to be integrated into the channel for the passage of the gas to be analyzed. In this way, precisely defined flow conditions can be generated on the piezoelectric layer of the generator, which ensure high accuracy of the measured values generated by the sensor element. A miniaturization of the gas analysis sensor is also advantageous due to a high degree of integration. density of the individual components reached. The substrate component itself advantageously does not have to have any piezoelectric properties, since the function of the generator for surface waves, for which the piezoelectric properties are a prerequisite, is formed solely by the piezoelectric layer.

According to one embodiment of the method, it is provided that the substrate component has a cubic lattice structure and a surface with (100) orientation, the surface being anisotropically etched to form the channel with the formation of two flat wall surfaces with (111) orientation. In this case, a known method for anisotropic etching of so-called V-pits is advantageously used in order to produce flat wall surfaces with (111) orientation that meet the high requirements due to the different etching rates on the pit base and on the pit flanks in the V-pit to the flatness of the wall surface as a prerequisite for error-free growth of the piezoelectric layer. A piezoelectric layer, which is suitable as a generator for surface waves, can therefore be deposited on the wall surfaces formed in this way. In addition, by timely stopping the etching process for the V-pit, a channel with a trapezoidal cross section can be etched, as a result of which the formation of an acute angle between the channel walls can be avoided in order to prevent the development of unfavorable flow conditions in the channel.

According to another embodiment of the method, a multilayer substrate component can be used, in the surface of which the channel is first etched in a first layer down to the interface with a second layer and the second layer of which is exposed by a third layer. the flat layer forming the wall surface is etched.

In this case, a “silicon on insulator” substrate (hereinafter referred to as SOI substrate) can in particular be used as the substrate. The multi-layer structure advantageously ensures that the flat wall surface can be formed by the third layer, the second layer during the

Etching process for the actual channel cross section in the first layer protects the third layer from the etching medium.

The third layer can then be exposed using an etching medium which attacks the second but not the third layer. The exposed third layer then has the crystal orientation suitable for producing the sensor element and thus forms the single-crystalline, flat region of the substrate component.

It is particularly advantageous if the channel is etched into two substrate components such that when the substrate components are subsequently connected to one another, the recesses in the substrate components result in a single channel cross section. Half of the cross-section of the channel to be formed can be etched into one of the substrate components, the channel cross-section being created by sandwiching the substrate components. These can be fixed to one another and sealed, for example by gluing or wafer bonding. The already mentioned formation of V-pits which have not been fully etched allows a hexagonal channel cross section to be produced which is approximated to a circular cross section which is favorable in terms of flow technology.

However, it can also be advantageously provided that the substrate component is connected to a cover component, which is flat in the region of the channel, in order to close the channel is formed and carries an additional sensor element there. This allows a particularly simple and inexpensive manufacture of the channel to be achieved advantageously, since an etching treatment is only necessary for the substrate component, while the cover component already provides a further flat wall surface for the formation of a sensor element if the material is selected without pretreatment steps.

The invention further relates to a gas analysis sensor with a channel for passing a gas to be analyzed and a sensor element which is designed as a generator for surface waves (surface acoustic wafers, SAW). Such a gas analysis sensor can be found in the aforementioned publication by Edward J. Staples.

The object of the invention is to provide a gas analysis sensor with a channel and a generator for surface waves, which enables a measurement with comparatively small measurement errors.

This object is achieved according to the invention in that the channel is formed from two housing parts, a flat wall surface of the channel with a uniform crystal orientation being provided in at least one of the housing parts, which as sensor element bears a piezoelectric coating provided with connection electrodes, the crystal lattice of which is that of the flat wall surface equivalent. The advantages of a gas analysis sensor designed in this way have already been explained in connection with the method according to the invention. By forming a flat wall surface in the channel, it is in fact advantageously possible to provide this wall surface for integrally accommodating the sensor element in the channel to use. The channel itself does not necessarily have to be suitable for generating surface waves,

(This means that it does not have any piezoelectric properties), it only has to be ensured by the uniform crystal orientation of the wall surface that a piezoelectric coating can be formed in the interior of the channel.

An embodiment of the gas analysis sensor provides that the channel is designed as a separation column for a gas chromatograph in a micromechanical construction. As a result, an extremely compact design can advantageously be achieved, in which both the separation column of the gas chromatograph and the components for the gas analysis sensor can be formed in a single substrate.

A further embodiment of the invention provides that the separating column has a round cross-section which merges into a cross-section with the flat wall surface towards the area of the sensor element. In this way, it can be achieved in an advantageous manner that on the one hand there is a cross section without corners ideal for the separation of the gas components in the separation column and on the other hand the gas flow is not disturbed by a transition from the round to the cross section which is angular due to the flat wall surface. This advantageously further improves the accuracy of the measurement result. In addition to a circular cross section, other cross sections without corners, such as an oval cross section, are of course also possible as round cross sections.

It is particularly advantageous if a plurality of flat wall surfaces, each with a sensor element, are arranged on the circumference of the channel, the sensor elements via their connecting electrodes are particularly connected in series. As a result, the area made available by the connected sensor elements can be enlarged, which advantageously leads to an improvement in the sensitivity of the gas analysis sensor. At the same time, a comparatively short path length of the channel is required to accommodate the sensor surface required for the gas analysis sensor.

Furthermore, it is advantageous if two parallel channels are provided in the housing parts, which can be flowed through independently of one another. In particular in the case of production using etching technology, there is hardly any additional effort when producing an additional channel. The second channel can advantageously be used as a reference channel by introducing a reference gas without components to be analyzed. Under these conditions, the second channel is suitable, for example, for eliminating the influence of temperature on the measurement result.

Further details of the invention are described below with reference to the drawing. Show here

Figure 1 shows a substrate component with a subsequent channel component according to an embodiment of the invention in a perspective view

Figures 2 and 3 each embodiments of the gas analysis sensor according to the invention in section.

1 shows components of the invention

Remove the gas analysis sensor. The central component of the gas analysis sensor is formed from a substrate component 11, in which a channel 12 is produced by anisotropic etching. loading If the substrate component 11 is made, for example, of silicon with a (100) surface, the anisotropic etching step can be produced using wet chemistry, for example using KOH or TMAH.

The etching treatment creates flat wall surfaces 13 in the channel with a (111) surface, which is suitable as a substrate for the production of sensor elements 14. The sensor elements 14 consist of a piezoelectric layer 15, on the surface of which connection electrodes 16 and a sensitive one

Film 17 are applied. The piezoelectric layer 15 consists of aluminum nitride or zinc oxide, these materials being deposited from the vapor phase on the wall surfaces 13 to produce the piezoelectric layer 15, as a result of which the (111) orientation of the wall surfaces 13 is formed, forming a single-crystal structure of the piezoelectric layer 15 transmits this.

The connection electrodes 16 can be applied to the piezoelectric layer 15 by means of sputtering, for example. On the one hand, they serve to excite the piezoelectric layer for surface vibrations and, on the other hand, for detection by generating an electrical signal excited by the surface vibrations. The surface vibrations pass through the sensitive film 17 located between the connection electrodes 16, which leads to a measurable, characteristic change in the vibration behavior of the piezoelectric layer by taking up the gas components to be detected. The measuring principle and examples of various sensitive films can be found, for example, in the journal "Sensors and Actuators", 5 (1984), pages 307 to 325. The substrate component 11 can, for example, be part of a separation column 18 of a gas chromatograph indicated by dash-dotted lines. The separation column 18 is produced in a manner not shown in a micromechanical manner by etching in substrate components. The separating column 18 has a circular cross-section 19, a connecting component 20 being provided between the substrate component 11 and the indicated circular cross-section 19, which ensures a gradual transition from the circular cross-section 19 of the separating column 18 to the trapezoidal channel 12 of the substrate component 11. The cross-section of cover components, not shown, for the substrate component 11 and the connecting component 20, in which a semicircular cross-section is realized, which merges into the indicated circular cross-section 19, is also indicated by dash-dotted lines.

In the gas analysis sensor according to FIG. 2, an SOI substrate is used as the substrate component 11. This consists of a first layer 21 made of silicon with a (100) -oriented surface. A second layer 22 consists of silicon oxide and functions as an etching stop layer in the etching treatment of the first layer 21 by an anisotropic dry etching step (for example RIE process using CHF ₃ ). In a subsequent etching step, for example an isotropic etching step with diluted HF, the second

Layer etched away in the region of the channel 12, whereby the surface of a third layer 23 made of silicon with a (111) surface is exposed without being attacked by the etching medium itself. Therefore, this exposed layer is suitable as a wall surface 13 for a sensor element 14 corresponding to the construction according to FIG 12 can be charged with a gas. This channel 24 is used as a reference channel for eliminating temperature influences on the gas analysis sensor.

The substrate component 11 is closed with the aid of a cover component 25. The closed rectangular cross sections of the channels 12 and 24 are formed here. The cover component 25 also consists of silicon with a (111) surface, which is suitable in the area of the channels 12 and 24 as a substrate for an additional sensor element 26, which is of a structural design is identical to the sensor elements 14 according to FIG. 1.

The gas analysis sensor according to FIG. 3 consists of two identical substrate components 11, the etchings of which are combined to form a channel with a hexagonal cross section. The substrate components were subjected to an etching treatment, each combining the etching steps of the etching treatments according to FIGS. 1 and 2. In contrast to the method used in FIG. 2, the first layer 21 is anisotropically etched according to the method according to FIG. 1, so that wall surfaces 13 with (111) orientation are formed in the silicon according to the mechanism described in FIG. A further (111) silicon surface is exposed by exposing the respective third layer 23 (etching of the second layer 22 in accordance with the method in FIG. 2). In this way, the six emerging sides of the cross section of the channel 12 can be used as flat wall surfaces 13 for sensor elements 14, whereby an optimal use of the channel walls for accommodating sensor elements 14 over the entire circumference of the channel cross section is possible.

Claims

claims

1. A method for producing a gas analysis sensor with a channel (12) for passing a gas to be analyzed and a sensor element (14) which is designed as a generator for surface waves (SAW), characterized in that the channel (12) by etching in a substrate component (11) is produced with a single-crystalline region in such a way that a flat wall surface (13) of the channel with a crystal orientation suitable as a substrate for producing the sensor element (14) is produced in the single-crystal region and - the sensor element (14) by coating the flat wall surface ( 13) with a piezoelectric layer (15) and by producing connection electrodes (16) connected to the piezoelectric (15) layer.

2. The method according to claim 1, characterized in that the substrate component (11) has a cubic lattice structure and a surface with (100) orientation, the surface for forming the channel (12) with the formation of two flat wall surfaces (13) with (111 ) Orientation is anisotropically etched.

3. The method according to claim 1 or 2, characterized in that a multilayer substrate component (11) is used, in the surface of which the channel (12) is first etched down to the interface with a second layer (22) in a first layer (21) will and whose second layer (22) is etched exposing a third layer (23) forming the flat wall surface (13).

4. The method according to any one of the preceding claims, characterized in that the channel (12) is etched in two substrate components (11), such that when the substrate components (11) are subsequently connected to one another, the etchings in the substrate components result in a single channel cross section ,

5. The method according to any one of claims 1 to 3, characterized in that the substrate component (11) for closing the channel (12) is connected to a cover component (25) which is flat in the region of the channel (12) and there an additional Sensor element (26) carries.

6. Gas analysis sensor with a channel (12) for passing a gas to be analyzed and a sensor element (14) which is designed as a generator for surface waves (SAW), characterized in that the channel (12) consists of two housing parts (11, 25) is, a plane in at least one of the housing parts

Wall surface (13) of the channel (12) is provided with a uniform crystal orientation, which as sensor element (14) has a piezoelectric coating (15) provided with connection electrodes (16), the crystal lattice of which corresponds to that of the flat wall surface (13).

7. Gas analysis sensor according to claim 6, characterized in that the channel (12) is designed as a separation column (18) for a gas chromatograph in a micromechanical design.

8. Gas analysis sensor according to claim 7, so that the separating column (18) has a round cross section which merges with the area of the sensor element (14) into a cross section having the flat wall surface (13).

9. Gas analysis sensor according to one of claims 6 to 8, d a d u r c h g e k e n n z e i c h n e t that on the circumference of the channel (12) a plurality of flat wall surfaces (13) are arranged, each with a sensor element (14), the sensor elements being connected via their connecting electrodes (16).

10. Gas analysis sensor according to one of claims 6 to 9, d a d u r c h g e k e n n z e i c h n e t that in the housing parts (18) two parallel channels (12, 24) are provided, which can be flowed through independently of each other.