WO2005012893A1

WO2005012893A1 - Microstructured chemical sensor

Info

Publication number: WO2005012893A1
Application number: PCT/DE2004/001647
Authority: WO
Inventors: Heribert Weber; Doris Schielein; Christian Krummel; Christoph Schelling
Original assignee: Paragon Ag
Priority date: 2003-07-25
Filing date: 2004-07-23
Publication date: 2005-02-10
Also published as: KR20060055524A; EP1649271A1; JP2006528767A

Abstract

The invention relates to a sensor comprising a first metallization plane located on a substrate (1), a passivation layer (6) that is structured by contact holes (7) and is applied to said substrate and a sensitive ceramic layer (9) formed by thick-film technology on the passivation layer and in the contact holes (7). The aim of the invention is to improve the adhesion of the ceramic layer (9). To achieve this, the sensor is provided with an adhesion promoter layer (8) that is configured as a second metallization plane and is located between the passivation layer (6) and the ceramic layer (9).

Description

Microstructured chemical sensor

The invention relates to a chemical sensor with a first metallization level arranged on a substrate, in which an electrode structure is formed, a passivation layer applied on the first metallization level and structured by contact holes, and with one on the passivation layer and in the contact holes by means of dispensing. , Screen printing or inkjet process and subsequent sintering produced sensitive ceramic layer.

Chemical sensors in which the electrical resistance of a sensitive layer, usually comprising metal oxides, can be evaluated with the aid of an evaluation structure, the electrodes, are known in many designs, in particular as gas or moisture sensors. Porous ceramic layers, for example Sn0 ₂ or W0 ₃ , are generally used as sensitive layers for gas detection, the electrical surface conductivity of which changes when gases are adsorbed. The porous ceramic layers can be made selectively sensitive to certain gases by means of dopants.

The resistivities of such ceramics are very high. This means that the measuring resistances also become large. The evaluation structure therefore usually consists of an interdigital structure (IDT; "Interdigitated Transducers"), that is to say of two coplanar, interdigitated electrodes. This corresponds to a parallel connection of the resistances formed laterally between the individual fingers of different polarity and thus a reduced internal sensor resistance or an increased sensitivity of the sensor. In addition to the electrodes and the heating resistor, a temperature measuring resistor is often also provided on the sensor, it being possible for all elements of the metallization to be structured, for example from platinum, in one metallization level. In order to prevent "aging" in particular of the temperature measuring resistor, a passivation layer, typically silicon oxide, is often provided on the metallization level. The passivation layer is to be structured through contact holes in order to enable contact between the electrodes and the sensitive layer applied to the passivation layer.

While the structures of the metallization and the sensitive layer are conventionally applied to an aluminum oxide substrate, membrane sensors manufactured on the basis of a silicon substrate are also known micromechanically. The thermal decoupling of the sensor structures arranged on the membrane from the substrate results in a reduced power consumption of the sensor.

A microstructured silicon membrane sensor with a sensitive layer applied to an Si0 ₂ , Si ₃ N membrane is known, for example, from DE 197 10 358 AI. In the known sensor, however, unlike the generic sensor, only the heating and temperature measurement structures are passivated by means of a silicon oxide layer, to which an interdigital, three-dimensional electrode structure is then applied, into which a sensitive layer is filled using screen printing technology.

In general, when the sensitive ceramic layer is produced by screen printing, it has to be sintered after it has been applied as a thick-layer paste. Any mechanical tensions that occur or remain, in particular interfacial tensions, can lead to the detachment of layer material and particle generation. The effects of eg Sn0 ₂ or W0 ₃ on other micromechanical processes are unclear, so that there is a general risk of contamination. The porosity of the metal oxide ceramic produced during sintering is desirable on the one hand because the high sensitivity of the ceramic is due to the high surface-to-volume ratio. At the same time, however, the porosity has a negative effect on the mechanical stability of the layer. The most important requirement for stability is that the ceramic layers adhere to the substrate and the electrodes over the life of the sensor. In addition, the electrical contact between ceramic and electrodes must not degenerate.

The ceramic layers are currently produced directly on the passivation layers. It has been shown that the adhesion of the ceramics is often insufficient. The object of the invention is to improve the situation with regard to liability.

This object is achieved according to the invention by a chemical sensor according to claim 1. Further training and preferred measures result from the subclaims.

The invention is based on the generic chemical sensor in that an adhesion promoter layer is provided which is designed as a second metallization level and which is arranged between the passivation layer and the ceramic layer.

The upper metallization applied in two layers makes it possible to better bind the porous, sensitive ceramic layers to the substrate provided by the passivation layer. At the same time, the metallic adhesion promoter layer arranged between two contact hole openings in the upper level leads to a strong spatial limitation of the electrical field lines, and thus also to the current paths between two interdigital electrode fingers, which ultimately strongly limits the active zone in the sensitive ceramics, which is associated with the advantage of improved protection against sensor poisoning, for example through silicone. In addition, according to the invention, there is the possibility of active electrical use of the second metallization level, in particular, but not only, in connection with the sensor electrodes. The adhesion promoter layer can also be used in the bond area of the sensor - if it is suitable as a bonding material.

The second metallization level is preferably applied in such a way that it comes to lie in the contact holes on the first metallization level.

It can be useful if a further passivation layer is arranged between the adhesion promoter layer and the ceramic layer and is structured such that the adhesion promoter layer is partially passivated.

It can be provided that two coplanar electrodes are structured in the electrode structure of the first metallization level, and that the second metallization level is not at a defined electrical potential. In this case, the advantages mentioned result in the improvement of the adhesion and the narrowing of the functional ceramic zone by means of equipotential surfaces.

Alternatively, it can also be provided that the electrode structure of the first metallization level forms a first electrode, and that the second metallization level is designed as a second electrode and is at a defined electrical potential, so that the sensitive ceramic layer is provided with a vertical electrode arrangement. This results in a considerably shorter electrode spacing compared to the otherwise customary, lithographically specified, lateral electrode spacing. On the other hand, the lateral expansion of the ceramic layer can no longer be determined by technical requirements and can be reduced if necessary.

In this context, it is particularly useful to design the electrodes as interdigital electrodes, but this is also possible in all other embodiments.

In addition to the electrode structure, a heating structure and a temperature measurement structure are preferably formed in the first metallization level.

The structures of the metallization are preferably applied to the front of an Si substrate which has a membrane.

The invention is explained in more detail below on the basis of exemplary embodiments with reference to the figures. Show it:

FIG. 1 shows a cross section of a sensor according to the invention,

FIG. 2, in the same representation, a variant of the sensor,

FIG. 3, in the same representation, shows a schematically simplified section of the cross section according to FIGS. 1 and 2.

FIG. 1 shows a silicon substrate 1 in which, for example by etching a cavity 2 from the back of the substrate 1, a dielectric membrane 3 is produced, which can consist of a layer sequence of, for example, silicon dioxide and silicon nitride. A first metallization level, for example made of platinum, is located above the membrane 3. This metallization is structured in such a way that the heating structure 4 and the interdigital electrode fingers of different polarity IDT 1 and IDT 2, and optionally a temperature resistor 5, are formed for the chemical sensor. For better adhesion of the platinum metallization of the first metallization level, it is advantageous to converting the burst membrane layer, here silicon nitride, into a silicon oxide layer 3 'on the surface.

Above the first metallization level there is a passivation layer 6 (intermediate insulation layer) made of z. B. CVD

Oxide, nitride or oxynitride. There are holes 7 in the passivation layer 6, which serve for contacting the ceramic layer 9 and the bondlands. On the passivation layer 6 there is a second metallization level, the adhesion promoter layer 8, which serves to promote adhesion for the ceramic layer 9, and which, as described further below in connection with FIG. 3, is optionally set to a defined potential and then as a second interdigital electrode IDT 2 can serve, in which case the first metallization level with respect to the evaluation structure has only interdigital electrode fingers IDT 1 of the same polarity.

As FIG. 2 shows, the second metallization level can be applied in such a way that it comes to lie in the contact hole openings 7 on the first metallization level. A further passivation layer 10 can optionally lie on the second metallization 8. It in turn contains holes for connection purposes.

To produce the sensor according to the invention, the silicon raw wafer is first thermally oxidized. An LPCVD nitride is then deposited or an oxide is produced. The first metallization and its structuring into heater 4, temperature sensor 5 and interdigital electrode IDT are then deposited on the front of the wafer. A passivation layer 6 is then applied, for example a CVD oxide. The etching mask for the cavern etching for producing the membrane 3 is defined on the back of the wafer. The sensor can be controlled more quickly due to the reduced heat dissipation. Next, the adhesion promoter layer 8 or the second metallization level, for example made of the materials Au, is applied. Cr / Au, Pt, Pd, W or Sn. The adhesion promoter layer 8 is subsequently structured in such a way that it only remains in the area between the interdigital electrode fingers on which the ceramic is intended to adhere afterwards and, if appropriate, still on the bondlands.

The contact holes 7 are then etched into the passivation layer 6 on the front of the wafer, and the adhesion promoter layer 8 can partially serve as an etching mask. The application of the adhesive layer 8 and etching of the contact holes 7 can also be carried out in the reverse order. Finally, a protective lacquer is applied to the front and the caverns 2 are produced from the rear by anisotropic etching. Finally, the paste dot is applied and sintered in an oven to form the porous ceramic layer 9. Further details on the known process steps can be found in DE 197 10 358 AI mentioned above.

FIG. 3 shows to explain the function of the invention that goes beyond the improved mediation

Sensor a section of the cross section of the sensor. The z-scale is shown very high. The ceramic functional layer 9 is also not shown for better clarity:

A voltage is applied between the two interdigital electrodes IDT 1 and IDT 2. The measurement signal is tapped as current. If the upper, second metallization level, that is to say the structured adhesion promoter layer 8, is not at a defined potential, that is to say floating, both IDTs are implemented in the lower, first metallization level. Without the additional second metallization or adhesion promoter layer 8, the current paths between the IDTs would run exclusively over the ceramic layer, as indicated in FIG. 3 by the arrow shown in broken lines. With the adhesion promoter layer 8 according to the invention, however, they run, as shown, between the latter and the two IDTs. The current path then leads outside the contact holes 7 via the adhesion promoter layer 8. A change in the conductivity in the periphery of the ceramic dot or ceramic layer 9, for example due to poisoning, advantageously plays practically no role in this configuration, since the electrical field lines and thus the current paths are very limited in space.

If the upper, second metallization level (bonding agent layer 8) is set to a defined potential (e.g. 0V), it can be used as IDT 2. In this case, the electrode spacing is effectively approximately half as large as in the case of a floating adhesive layer 8.

Claims

claims

1. Chemical sensor, with a first metallization level arranged on a substrate (1), in which an electrode structure (IDT) is formed, a passivation layer (6) applied on the first metallization level and structured through contact holes (7), and with a sensitive ceramic layer (9) produced on the passivation layer (6) and in the contact holes (7), characterized in that an adhesion promoter layer (8) is provided which is designed as a second metallization level and which is between the passivation layer (6) and the ceramic layer ( 9) is arranged.

2. Chemical sensor according to claim 1, characterized in that the second metallization level is applied such that it comes to lie in the contact holes (7) on the first metallization level.

3. Chemical sensor according to claim 1 or 2, characterized in that a further passivation layer (10) is arranged between the adhesion promoter layer (8) and the ceramic layer (9) and is structured such that the adhesion promoter layer (8) is partially passivated.

4. Chemical sensor according to one of claims 1 to 3, characterized in that two coplanar electrodes (IDT 1, IDT 2) are structured in the electrode structure (IDT) of the first metallization level, and that the second metallization level is not at a defined electrical potential.

5. Chemical sensor according to one of claims 1 to 3, characterized in that the electrode structure (IDT) of the first metallization level forms a first electrode (IDT 1), and that the second metallization level is designed as a second electrode (IDT 2) and is at a defined electrical potential, so that the sensitive Ceramic layer (9) is provided with a vertical electrode arrangement.

6. Chemical sensor according to one of claims 1 to 5, characterized in that the electrodes (IDT 1, IDT 2) are designed as interdigital electrodes.

7. Chemical sensor according to one of claims 1 to 6, characterized in that a heating structure (4) and a temperature measurement structure (5) are formed in the first metallization level in addition to the electrode structure (IDT).

8. Chemical sensor according to one of claims 1 to 7, characterized in that the structures (4, 5, IDT) of the metallization are applied to the front of a Si substrate (1) which has a membrane (3).

9. Chemical sensor according to one of claims 1 to 8, characterized in that the material for the second metallization level comprises Au, Cr / Au, Pt, Pd, W or Sn.

10. Chemical sensor according to one of claims 1 to 9, characterized in that the application of the sensitive ceramic layer (9) can take place by means of screen printing, dispensing or inkjet methods.