JP2006528767A - Microstructured chemical sensors - Google Patents

Abstract

  A first metallization plane arranged on the substrate (1), a passivation layer (6) structured by contact holes (7) deposited on the first metallization plane, and this passivation layer ( 6) and a sensitive ceramic layer (9) formed by thick film technology in the contact hole (7), in order to improve the adhesion of the ceramic layer (9), the second meta It is proposed that a fixing agent layer (8) is provided which is formed as a planarization plane and is arranged between the passivation layer (6) and the ceramic layer (9).

Description

  The present invention is a chemical sensor comprising a first metallization plane disposed on a substrate, on which an electrode structure is formed, and a contact hole deposited on the first metallization plane. And a sensitive ceramic layer formed by a dispenser method, a screen printing method or an ink jet method and then by sintering in the contact hole and in the contact hole. About.

Chemical sensors, in which the electrical resistance of a sensitive layer, usually containing a metal oxide, can be evaluated by means of an electrode, which is an evaluation structure, are known in numerous configurations, in particular as gas sensors or humidity sensors. As a sensitive layer for gas detection, a porous ceramic layer, for example SnO ₂ or WO _3, is generally used. The electrical surface conductivity of the ceramic layer changes upon gas adsorption. The porous ceramic layer can be selectively made sensitive to a specific gas by a doping substance.

  Such ceramics have extremely high resistance. This also increases the measurement resistance. Therefore, the evaluation structure usually consists of an interdigitated structure (IDT; “Interdigitated Transducers”), ie two coplanar electrodes that engage in a finger shape. This corresponds to a parallel connection of resistors formed in the lateral direction between the individual fingers of different polarities and thus reduced sensor internal resistance or increased sensor sensitivity.

  Often, in addition to the electrodes and the heating resistor, a temperature measuring resistor is also provided on the sensor. In this case, all elements of the metallization layer, for example made of platinum, can be structured in one metallization plane. A passivation layer, typically silicon oxide, is often provided in the metallization plane, in particular to prevent “post-aging” of the temperature measuring resistor. This passivation layer can be structured by contact holes to allow contact between the electrode and a sensitive layer deposited on the passivation layer.

  The structure of the metallization layer and the sensitive layer are applied in a conventional manner to an aluminum oxide substrate, while in the meantime a diaphragm sensor manufactured by micromachining technology on the basis of a silicon substrate Is also known. By thermally isolating the sensor structure located on the diaphragm from the substrate, a reduced output consumption of the sensor is obtained.

Silicon diaphragm sensors microstructured with a SiO _2, Si layer of sensitive, which is applied to the diaphragm consisting of ₃ N ₄ are known, for example from German Patent Application No. 19710358. However, in this known sensor, unlike the sensor of the type described at the outset, only the heating structure and the temperature measuring structure are passivated by the silicon oxide layer. Thereafter, a three-dimensional comb-shaped electrode structure is deposited on the silicon oxide layer. The electrode structure is filled with a sensitive layer by screen printing techniques.

Generally, when forming a sensitive ceramic layer by screen printing technology, the ceramic layer must be further sintered after deposition as a thick film paste. The mechanical stresses generated or left behind in this case, in particular the interfacial tension, can cause delamination of the layer material and particle generation. The impact of, for example, SnO ₂ or WO _{3 on} other micromachining processes is unclear, which generally presents a contamination risk. On the other hand, the porosity of the metal oxide ceramic formed during sintering is desired. This is because the high sensitivity of ceramics is advantageously produced by a high ratio of surface to volume. At the same time, however, porosity has a negative effect on the mechanical stability of the layer. The most important requirement imposed on this stability is that the ceramic layer adheres to the foundation and electrodes over the lifetime of the sensor. Furthermore, the electrical contact between the ceramic and the electrode must not be altered.

  Currently, the ceramic layer is formed directly on the passivation layer. In this case, it has been found that the ceramics are often not sufficiently fixed. An object of the present invention is to improve the situation related to sticking.

  This object is achieved according to the invention by a chemical sensor according to claim 1. Improvements and advantageous measures are evident from the dependent claims.

  The present invention provides a chemical sensor of the type described at the outset by providing a fixer layer which is formed as a second metallization plane and which is arranged between the passivation layer and the ceramic layer. Is based.

  The upper metallization layer deposited on the two layers makes it possible to better bond the porous sensitive ceramic layer to the foundation provided by the passivation layer. At the same time, the metal fixer layer, which is arranged between each two contact hole openings in the upper plane, creates a significant spatial restriction of the electric field lines and thus the current path between the two comb electrode fingers. . This ultimately gives rise to a significant limitation of the area of activity in sensitive ceramics. This is accompanied by the advantage of improved protection against sensor poisoning, for example with silicone. Furthermore, according to the present invention, the possibility of an active electrical advantage of the second metallization plane, however, opens up, in particular, in conjunction with the sensor electrode. Incidentally, the fixing agent layer may be used in the bonding region of the sensor if it is suitable as a bonding material.

  Advantageously, the fixer layer formed as a second metallization plane is deposited so as to be located in the first metallization plane within the contact hole.

  It may be advantageous if another passivation layer is arranged between the fixing agent layer and the ceramic layer and is structured such that the fixing agent layer is partially passivated.

  It may be proposed that two coplanar electrodes are structured in the electrode structure of the first metallization plane and that the second metallization plane is not at a defined potential. This case results in the above-mentioned advantages with regard to the fixing and restriction of the functional ceramic region by the equipotential surface.

  However, alternatively, the electrode structure of the first metallization plane forms the first electrode and the second metallization plane is formed as the second electrode and is defined. It may be proposed that the sensitive ceramic layer is provided with a vertical electrode device. This makes it possible to obtain electrode intervals that are significantly shortened compared to the other normal electrode spacings set in the lateral direction by lithography. On the other hand, this also allows the lateral extension of the ceramic layer to no longer be set according to the measurement technical requirements and can be reduced in some cases.

  Combined with this, it is advantageous to form the electrodes as comb electrodes. However, this is possible with any other configuration.

  It is advantageous if a heating structure and a temperature measuring structure are additionally formed on the first metallization plane with respect to the electrode structure.

  Advantageously, the structure of the metallization layer is deposited on the surface of a Si substrate having a diaphragm.

  In the following, embodiments of the invention will be described in detail with reference to the drawings.

  In FIG. 1, a silicon substrate 1 is shown. A dielectric diaphragm 3 is formed on the silicon substrate 1 by, for example, etching the cavity 2 from the back surface of the substrate 1. This diaphragm 3 may consist, for example, of a layer sequence of silicon dioxide and silicon nitride. Above the diaphragm 3 is located a first metallization plane made of, for example, a platinum material. This metallization layer is structured such that a heating structure 4 for a chemical sensor, comb-shaped electrode fingers IDT1, IDT2 of different polarities and possibly a temperature resistor 5 are formed. . For better adhesion of the platinum metallization layer in the first metallization plane, it is advantageous to convert the uppermost diaphragm layer, here silicon nitride, into a silicon oxide layer 3 'on the surface.

  Above the first metallization plane is a passivation layer 6 (intermediate insulating layer) made of, for example, CVD oxide, CVD nitride or CVD oxynitride. Holes 7 are located in the passivation layer 6. The hole 7 serves for contact between the ceramic layer 9 and the bonding land. In the passivation layer 6, a fixing agent layer 8 which is a second metallization plane is located. This fixing agent layer 8 serves for fixing to the ceramic layer 9 and can be optimally placed at a defined potential, as will be described below in conjunction with FIG. It can serve as a comb electrode IDT2. In this case, in this case, the first metallization plane has only the comb electrode finger IDT1 of the same polarity with respect to the evaluation structure.

  As shown in FIG. 2, the fixer layer 8 formed as a second metallization plane may be deposited so as to be located in the first metallization plane within the contact hole opening 7. . Another passivation layer 10 may optionally be located on the second metallization layer 8. The passivation layer 10 also has holes for connection purposes.

  To fabricate a sensor according to the present invention, a silicon raw wafer is first thermally oxidized. Next, LPCVD nitride deposition or oxide formation is performed. Thereafter, the first metallization layer is deposited on the surface of the wafer and the first metallization layer is structured to the heater 4, the temperature feeler 5 and the comb electrode IDT. Thereafter, a passivation layer 6, for example CVD oxide, is deposited. An etching mask used for cavity etching for forming the diaphragm 3 is defined on the back surface of the wafer. Reduced heat derivation allows the sensor to be controlled more quickly. Subsequently, the fixing layer 8 made of, for example, the materials Au, Cr / Au, Pt, Pd, W or Sn or the second metallization plane is applied. Following this, the fixer layer 8 is structured so that it remains only in the area between the comb electrode fingers. Later, in this region and in some cases, ceramics are fixed to the bonding land.

  Thereafter, the contact hole 7 is etched into the passivation layer 6 on the surface of the wafer. In this case, the fixing agent layer 8 can partially serve as an etching mask. The deposition of the fixing agent layer 8 and the etching of the contact hole 7 may be performed in the reverse order. A protective lacquer is then applied to the surface, and the cavity 2 is fabricated from behind by anisotropic etching. Next, paste dots are applied and sintered in a furnace to form a porous ceramic layer 9. Further details on the method steps known per se can be taken from the above-mentioned German patent application DE 197 10 358.

  FIG. 3 shows a part of the cross section of the sensor in order to explain the function of the sensor according to the invention over the improved fixing. The z-axis scale is illustrated as being significantly excessively increased. In order to make the drawing easier to see, the ceramic functional layer 9 is not shown.

  A voltage is applied between the comb electrodes IDT1 and IDT2. The measurement signal is detected as a current. In the case where the upper second metallization plane, i.e. the structured fixer layer 8 is not at the defined potential, i.e. fluctuates, both IDTs are realized in the lower first metallization plane. ing. Without an additional second metallization layer or fixer layer 8, the current path between both IDTs may extend exclusively through the ceramic layer, as indicated by the dashed arrows in FIG. On the other hand, the current path extends between the fixing agent layer 8 and both IDTs as shown in the figure by the fixing agent layer 8 according to the present invention. In this case, the current path extends through the fixing agent layer 8 except for the contact hole 7. For example, the change in conductivity at the outer peripheral surface of the ceramic dots or the ceramic layer 9 due to poisoning is advantageously no longer a problem in this configuration. This is because the lines of electric force and thus the current paths are extremely spatially limited.

  In the case where the upper second metallization plane (fixer layer 8) is placed at a defined potential (eg 0V), the second metallization plane can be used as IDT2. In this case, the electrode spacing is effectively about half as large as in the case of the fusing agent layer 8 that fluctuates.

1 is a cross-sectional view of a sensor according to the present invention. It is a figure which shows the variation of a sensor in the same drawing. FIG. 3 shows a schematic part of a cross section according to FIG. 1 or FIG. 2 in the same drawing.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 Substrate, 2 Cavity, 3 Diaphragm, 3 ′ Silicon oxide layer, 4 Heater, 5 Temperature feeler, 6 Passivation layer, 7 Hole, 8 Fixing agent layer, 9 Ceramic layer, 10 Passivation layer, IDT1, IDT2 Comb electrode

Claims (10)

A chemical sensor, a first metallization plane disposed on a substrate (1), on which an electrode structure (IDT) is formed, and a contact hole deposited on the first metallization plane A type in which a passivation layer (6) structured by (7) and a sensitive ceramic layer (9) formed on the passivation layer (6) and in the contact hole (7) are provided. In
A chemical layer characterized in that it is formed as a second metallization plane and is provided with a fixer layer (8) arranged between the passivation layer (6) and the ceramic layer (9). Sensor.   The fixing agent layer (8) formed as a second metallization plane is deposited so as to be located in the first metallization plane within the contact hole (7). Sensor.   Another passivation layer (10) is arranged between the fixing agent layer (8) and the ceramic layer (9), and structured so that the fixing agent layer (8) is partially passivated. The sensor according to claim 1 or 2.   From the electrode structure (IDT) of the first metallization plane, two coplanar electrodes (IDT1, IDT2) are structured, and the second metallization plane is not at a defined potential. 4. The sensor according to any one of up to 3.   The electrode structure (IDT) of the first metallization plane forms the first electrode (IDT1) and the second metallization plane is formed as the second electrode (IDT2) 4. A sensor according to claim 1, wherein the sensitive ceramic layer (9) is provided with a vertical electrode device.   The sensor according to any one of claims 1 to 5, wherein the electrodes (IDT1, IDT2) are formed as comb-shaped electrodes.   The heating structure (4) and the temperature measurement structure (5) are additionally formed on the first metallization plane with respect to the electrode structure (IDT). The sensor according to item.   8. A sensor according to claim 1, wherein the metallization layer structure (4, 5, IDT) is applied to the surface of a Si substrate (1) with a diaphragm (3).   9. A sensor according to any one of the preceding claims, wherein the material for the second metallization plane comprises Au, Cr / Au, Pt, Pd, W or Sn.   10. The sensor according to claim 1, wherein the sensitive ceramic layer (9) is applied by a screen printing method, a dispenser method or an ink jet method.

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