FIELD

The present invention relates to a bonding pad layer system, a gas sensor and a method for manufacturing a gas sensor.

BACKGROUND INFORMATION

Although applicable to arbitrary micromechanical components, the present invention and the problem underlying it are explained with reference to components having gas sensor chips.

During the manufacture of micromechanical gas sensors, a paste dot which, for example changes its resistance if certain gases are in its proximity, is generally deposited on the electrodes after completion of a micromechanical chip, which has a self-supporting dielectric membrane, in which a heating structure made of platinum and on which interdigital electrode structures are situated, as described for example in European Patent No. EP 085 92 31 A1.

The paste dot, once it is deposited, must be sintered in order to be able to remove organic paste components and to adjust the desired gas sensitivity. Sintering takes place generally at temperatures that may be significantly above 400° C., as described for example, in German Patent Application No. DE 10 2015 209 267 A1. Since the micromechanical substructure/sensor chip is already fully processed during sintering of the paste, the substructure must be able to withstand the sintering process with no functional impairment. This also means that the bonding pads and the bonding pad connections to conductor track levels must not degrade during the sintering process.

There are already micromechanically manufactured gas sensors on the market, in which a gold bonding wire is bonded onto the platinum layer of a tantalum/platinum bonding pad layer system. However, this system lacks sufficient process stability and has, for example, widely scattering bonding wire adhesion values.

It would be advantageous to design a system in which the bonding ability (bonding wire adhesion) has sufficient process stability in order to be able to ensure high-volume manufacturing.

When implementing such a layer system, the conductor track contacting method described in German Patent Application No. DE 198 24 400 A1 for a micromechanical air mass sensor chip is also to be used in order to become independent of potential conductor track breaks due to edge disintegration on contact hole edges. The air mass sensor chip, like the gas sensor chip, also has a self-supporting dielectric membrane, in which a heating structure made of platinum is integrated. In the conductor track contacting method described in German Patent Application No. DE 198 24 400 A1, an aluminum bonding pad is deposited largely in a contact hole and the electrical connection of the platinum conductor track leading to the heater is implemented with the aid of a ring contact inside the contact hole. The aluminum bonding pad is thus deposited inside this contact hole in part on a platinum layer as well as on a layer of the dielectric layer system situated below the platinum layer. This type of contacting ensures that edge disintegration on contact hole edges is unable to result in an interruption of the electrical contacting of the platinum heater structure due to negative etch edges.

SUMMARY

The present invention provides a bonding pad layer system, a gas sensor, and a method for manufacturing a gas sensor.

According to a first aspect, the present invention accordingly relates to a bonding pad layer system situated on a semiconductor chip. This semiconductor chip may, for example, be a micromechanical semiconductor chip, in which at least one self-supporting dielectric membrane with a heater made of platinum is integrated. During the manufacture of the bonding pad layer system, the deposition of a tantalum layer initially takes place, followed by the deposition of a first platinum layer, followed by the deposition of a tantalum nitride layer, followed by the deposition of a second platinum layer and followed by the deposition of a gold layer, which is used for implementing a bonding pad. The gold bonding pad may be connected to a bonding wire, the entire bonding pad layer system being able to be situated on a semiconductor chip, essentially in the area of the contact hole, in which the connection of a platinum conductor track leading to the heater is made with the aid of a ring contact. It is further possible that bonding pads, made up of the aforementioned bonding pad layer system are also situated outside a contact hole area on the semiconductor chip surface and are used for electrically contacting electrode structures.

According to a second aspect, the present invention relates to a gas sensor that includes at least a bonding pad layer system according to the first aspect, as well as a paste dot.

According to a third aspect, the present invention relates to a method for manufacturing a gas sensor that includes a bonding pad layer system and a paste dot, including the steps in the order: providing a semiconductor chip as a base, for example, a micromechanical semiconductor chip, in which at least one self-supporting dielectric membrane with a heater made of platinum is integrated, depositing a tantalum layer, depositing a first platinum layer, depositing a tantalum nitride layer, depositing a second platinum layer, depositing a gold layer, designing at least one bonding pad for connecting to a bonding wire on the gold layer, designing electrode structures, designing a paste dot and sintering the paste dot.

Advantageous refinements of the present invention are described herein.

The present invention provides a bonding pad layer system, a gas sensor and a corresponding manufacturing method, including a specific layer sequence, which is high-temperature stable and, after sintering of the paste dot in a back-end process, makes reliable, stable and high volume capacity wire bonding connections possible. “High-temperature-stable” is understood, in particular, to mean that the bonding pad layer system as well as the functionality of the gas sensor, remain stable with respect to their properties, even when they are exposed to temperatures at which paste dots used for manufacturing gas sensors are sintered. These are, in particular, temperatures in the range of 400° C. and above.

Thus, the present invention allows for the production of a high-temperature-stable bonding pad layer system, as it may be used, for example, for gas sensors in high volume manufacturing.

According to one preferred refinement of the present invention, the semiconductor chip is a micromechanical semiconductor chip, in which at least one self-supporting dielectric membrane that includes dielectric layers, a platinum conductor track and a heater made of platinum are integrated. This allows for a variety of potential applications of the layer system.

According to one preferred refinement of the present invention, at least a bonding pad is situated essentially in the area of a contact hole on the semiconductor chip, in which the connection to a platinum conductor track leading to the heater takes place with the aid of a ring contact.

According to one preferred refinement of the present invention, at least a bonding pad is situated essentially in the area outside the contact hole on the semiconductor chip. Via the bonding pad, a connection may take place to the heater and/or to additional electrode structures and conductor tracks.

As mentioned, the high-temperature stable bonding pad layer system has the layer sequence tantalum/platinum 1/tantalum nitride/platinum 2/gold. According to one preferred refinement of the present invention, the layer thickness of the tantalum layer is 2 to 200 nanometers, the layer thickness of the first platinum layer is 50 to 1000 nanometers, the layer thickness of the tantalum nitride layer is 2 to 200 nanometers, the layer thickness of the second platinum layer is 2 to 400 nanometers and the layer thickness of the gold layer is 50 to 1000 nanometers. The layer thicknesses may thus be optimally adapted to the sintering process in order, for example, to prevent undesirable diffusion processes or to ensure particular functions.

According to one preferred refinement of the present invention, the layer thickness of the tantalum layer is 5 to 50 nanometers, the layer thickness of the first platinum layer is 100 to 500 nanometers, the layer thickness of the tantalum nitride layer is 5 to 50 nanometers, the layer thickness of the second platinum layer is 10 to 150 nanometers and the layer thickness of the gold layer is 200 to 600 nanometers. With an optimum adaptation of the layer thicknesses to the desired function or to the applied sintering process, it is possible to achieve shorter process times during the deposition and lower material consumption.

According to one preferred refinement of the present invention, the layer composition of the tantalum nitride layer is made up of tantalum (Ta) and nitrogen (N2) in the ratio TaxNy, x being between 1 and 5 and y being between 0.04 and 6. With the selected layer composition, it is possible to influence or prevent the diffusion behavior of atoms through the tantalum nitride layer.

According to one preferred refinement of the present invention, the layer composition of the tantalum nitride layer is stoichiometric. As a result, it is possible to control the diffusion of atoms of other layers through the tantalum nitride layer during the sintering process.

According to one preferred refinement of the present invention, the tantalum layer and the first platinum layer of the bonding pad layer system are also used for manufacturing electrical conductor tracks and electrodes, for example, in the form of interdigital structures. Thus, it is possible to provide precise micro-electrical components having low power consumption in a minimum amount of space.

According to one preferred refinement of the present invention, at least one bonding pad is designed for connecting to a bonding wire outside the contact hole area on the first platinum layer of the bonding pad layer system. In this way, it is possible with the aid of a bonding wire to safely and with long-term stability, electrically contact the conductor tracks made up of the tantalum layer and the first platinum layer of the bonding pad layer system leading to the electrode structures. Whereas tantalum nitride layers are normally manufactured with the aid of reactive sputter technology, the tantalum nitride layers according to one preferred refinement of the method according to the present invention may also be formed by tempering applied tantalum layers at temperatures above 600° C. in an ammonia (NH3)/hydrogen (H2) atmosphere or in a nitrogen (N2)/hydrogen (H2) atmosphere. This allows for a particularly exact adjustment of the composition of the tantalum nitride layer.

According to one preferred refinement of the method according to the present invention, the tantalum and the first platinum layer are formed with the aid of a photoresist mask and of a subsequent etching step as conductor tracks and electrode structures, for example, as interdigital structures, making it possible for the etching to take place with the aid of IBE etching, plasma etching, wet chemical etching and combinations thereof.

