CROSS-REFERENCE TO RELATED APPLICATION
This application is a continuation of International Application PCT/EP99/04988, filed Jul. 14, 1999.
BACKGROUND OF THE INVENTION
The invention relates to a method for producing a temperature-dependent, platinum-containing resistor, configured as a temperature sensor, wherein a resistance layer is applied as a thick film on a substrate having a surface made of electrically insulating material. The outer surface of the resistance layer is covered by at least one layer made of an electrically insulating material which functions as a passivation layer and/or as a diffusion barrier. The invention also relates to an electric temperature sensor.
A rapid, platinum metal temperature sensor having a platinum resistance layer applied on a ceramic substrate and a passivation layer applied thereover is known from International Application publication WO 92/15101, wherein the passivation layer is constructed as a double layer made from a ceramic layer and a glass layer.
In order to manufacture such a temperature-dependent resistor as a temperature sensor, the resistance layer (Pt, meander structure) is applied as a thick film onto a substrate having a surface made of electrically insulating material, wherein the outer surface of the resistance layer is covered by a layer made of electrically insulating material, which functions as a passivation layer.
Furthermore, from European published patent application EP-A-0 543 413, a method is known for manufacturing a temperature-dependent, platinum-containing resistor configured as a temperature sensor, wherein an electrode is applied spaced from the resistance layer. An ion migration to the resistance layer, caused by current conduction, should thereby be avoided, and the electrode is connected electrically conducting with the resistance layer.
A method for manufacturing a temperature sensor is known from U.S. Pat. No. 5,202,665, wherein a platinum layer is applied on a substrate by thick-film technology. There, platinum powder is mixed with oxides and bonding agents and applied by screen printing; and then a tempering takes place in a temperature range between 1300 and 1350° C. A thus-produced temperature sensor having a platinum-containing layer on a substrate contains finely subdivided metallic platinum in an oxide ceramic and has a metallic platinum content in the range of 60 to 90 weight percent.
From European Patent EP 0 327 535 B1 a temperature sensor is known having a thin-film platinum resistance as a measuring element. A temperature measuring resistance made of platinum is formed on one surface of an electrically insulating substrate, wherein the resistance element is covered with a dielectric protective layer, which is preferably made of silicon dioxide and has a thickness in the range of 2000-4000 Angstroms. Furthermore, a diffusion barrier layer is provided as a cover layer, which is applied by deposition of titanium in an oxygen atmosphere to form titanium oxide. This barrier layer has a thickness in the range of 6000-12,000 Angstroms. Even if the diffusion barrier layer allows the admission of oxygen to the dielectric layer and thereby substantially prevents an attack of free metal ions released from the glass layer onto the platinum layer, under extreme environmental conditions it can nevertheless lead to an attack on the platinum layer, so that its physical behavior as a temperature measuring element is distorted.
Furthermore, an electric measuring resistor for resistance thermometers, as well as a method for producing such an electric measuring resistor, is known from U.S. Pat. No. 4,050,052 or the counterpart German Patent DE 25 27 739 C3.
SUMMARY OF THE INVENTION
An object of the invention is to protect the measuring resistor against external chemical or mechanical attacks and, in particular, to make certain that no sort of admission of contamination from the outside atmosphere into a platinum-containing resistance layer is possible.
The object of the invention is achieved in that, on the side of the resistance layer facing away from the substrate surface, an electrode is applied spaced from the resistance layer and is electrically insulated from the resistance layer by at least one layer of electrically insulating material.
It has proven to be especially advantageous that, on the one hand, a relatively reasonably priced manufacture is possible by applying a layer made of electrically insulating material as a diffusion barrier and/or as a passivation layer, while at the same time, because of the electrode preventing a contamination, a long service lifetime is obtained.
In a preferred embodiment, the resistance layer is applied onto a ceramic mass—preferably aluminum oxide—and then covered with a ceramic substance (likewise aluminum oxide) as a diffusion barrier or as a passivation layer. Here, the resistance layer can be applied on a fired ceramic substrate, whereby the advantage results that the geometry of the structure of the resistance layer remains unchanged. The diffusion barrier is preferably applied as an intermediate layer.
It is also possible, however, to apply the resistance layer onto a so-called “green” ceramic as a carrier, wherein after the application of the layer made of electrically insulating material as a passivation layer or as a diffusion barrier, this is sintered together with the carrier. Here, it is furthermore possible for a multiple layer system to also apply a laminated-on “green” ceramic as a diffusion barrier and/or as a passivation layer, which is then bonded to the carrier and resistance layer using a sintering process. For this, the use of an identical and/or similar material for carrier and covering of the resistance layer (passivation layer and/or diffusion barrier) proves to be especially advantageous, since a hermetically sealed embedding of the resistance layer and/or resistor structure is thereby possible.
In order to form the diffusion barrier and/or the passivation layer, ceramic powder can also be applied onto the resistance layer using a thick-film process and then sintered. An advantage that results therefrom is that this process is very cost-effective.
Furthermore, it is possible to apply ceramic powder onto the resistance layer of a fired substrate by a plasma spray process in order to form the diffusion barrier and/or the passivation layer. This has the advantage that the resulting layer, because of the high precipitation temperatures, also retains its stability at high temperatures that occur during later use.
In addition, it is possible to overglaze a plate made of ceramic as a diffusion barrier and/or as a passivation layer onto the resistance layer or to adhere it using a ceramic adhesive. Furthermore, the diffusion barrier and/or the passivation layer can be applied in a thin-film process using a PVD (Physical Vapor Deposition), IAD (Ion-Assisted Deposition), IBAD (Ion Beam-Assisted Deposition), PIAD (Plasma Ion-Assisted Deposition), or CVD (Chemical Vapor Deposition) magnetron sputtering process.
The electrode applied spaced from the resistance layer is preferably applied by a thick-film process, wherein it can be laid on by a screen-printing or stencil-printing process. This type of application proves to be advantageous by the structuring that results at the same time with the application. However, it is also possible to apply the electrode by a thin-film process.
The object of the invention is achieved with regard to the device for an electric temperature sensor with a platinum-containing resistance layer in thick-film technology, which is arranged as a measuring resistor provided with electrical contacts on an electrically insulating surface of a carrier constructed as ceramic substrate, wherein the resistance layer is covered with at least one layer made of an electrically insulating material for protection against contamination or damage, which material is constructed as a passivation layer and/or as a diffusion barrier, wherein on the side of the resistance layer facing away from the substrate surface, an electrode is applied spaced therefrom, wherein at least one part of a layer made of electrically insulating material is located between the electrode and the resistance layer.
The diffusion barrier is preferably constructed in the form of an intermediate layer. Proving to be advantageous are a cost-effective manufacture and a long servive lifetime of the temperature-dependent resistor. In one practical embodiment the thickness of the intermediate layer is in the range of about 0.2 μm to 50 μm.
The electrode is thus preferably arranged between the passivation layer and the intermediate layer. Furthermore, in one preferred embodiment, the electrode can be enclosed by the passivation layer. The electrode is preferably constructed as a platinum layer.
Furthermore, it is also possible to arrange the electrode on the side of the passivation layer facing away from the resistance layer. Here, the advantage results that the electrode, in the form of a platinum layer, protects the resistance layer from atmospheric poisoning in the sense of a “sacrificial electrode.”
In a further embodiment the electrode is provided with an electric connection. It is thereby possible to bias the electrode in an electrically negative manner against at least one connection of the resistance layer and/or the measuring resistor. It proves to be advantageous with an electrode in the form of a platinum layer that the platinum poisons (Si and metal ions), present as positive ions in extreme environmental conditions, are drawn to the negative platinum layer.
In one preferred embodiment according to the invention, the carrier is made of Al2O3. Furthermore, the diffusion barrier and/or the intermediate layer are also preferably made of Al2O3, MgO or a mixture of these two materials, wherein the weight percentage of Al2O3 lies in the range of about 20% to 70%. It is further possible to construct the diffusion barrier and/or the intermediate layer from a layer system with a layer sequence of at least two layers, which are formed respectively from at least one oxide selected from the group Al2O3, MgO, and Ta2O5. Here, at least one layer can be made from two of the oxides mentioned, wherein preferably a physical mixture of oxides is used. It is also possible, however, to use mixed oxides. In a further embodiment of the invention, the group of oxides consisting of Al2O3, MgO, Ta2O5 can be expanded to include hafnium oxide.
Preferably, the diffusion barrier and/or the passivation layer is made of a single-layer system according to Table 1 with the materials set forth in items 1 to 6 or of a multi-layer system according to Table 2, which has at least two layers 1 and 2, wherein however, on layer 2 one additional layer or several layers can be connected. The various layer materials are designated in the individual items or lines with the numbers 7 to 30.
|TABLE 1 |
|Single Layer System |
|1 ||Al2O3 only |
|2 ||MgO only |
|3 ||Ta2O5 only |
|4 ||Mixture Al2O3/MgO |
|5 ||Mixture Al2O3/Ta2O5 |
|6 ||Mixture MgO/Ta2O5 |
|TABLE 2 |
|Multi-Layer System |
| ||Layer 1 ||Layer 2 |
| || |
| 7 ||Al2O3 only ||Al2O3 only |
| 8 ||Al2O3 only ||MgO only |
| 9 ||MgO only ||MgO only |
|10 ||Ta2O5 only ||Ta2O5 only |
|11 ||Ta2O5 only ||Al2O3 only |
|12 ||Ta2O5 only |
|13 ||Mixture Al2O3/MgO ||Al2O3 only |
|14 ||Al2O3 only ||Mixture Al2O3/MgO |
|15 ||Mixture Al2O3/MgO ||Mixture Al2O3/MgO |
|16 ||Mixture Ta2O5/MgO ||Al2O3 only |
|17 ||Ta2O5 only ||Mixture Al2O3/MgO |
|18 ||Mixture Ta2O5/MgO ||Mixture Al2O3/MgO |
|19 ||Mixture Al2O3/Ta2O5 ||Al2O3 only |
|20 ||Al2O3 only ||Mixture Ta2O5/MgO |
|21 ||Mixture Al2O3/MgO ||Ta2O5 only |
|22 ||Ta2O5 only ||Mixture Al2O3/Ta2O5 |
|23 ||Al2O3 only ||Mixture Al2O3/Ta2O5 |
|24 ||Mixture Al2O3/MgO ||Mixture Ta2O5/MgO |
|25 ||Mixture Ta2O5/MgO ||Mixture Ta2O5/MgO |
|26 ||Mixture Al2O3/Ta2O5 ||Ta2O5 only |
|27 ||MgO only ||Mixture Al2O3/MgO |
|28 ||MgO only ||Mixture Al2O3/Ta2O5 |
|29 ||Mixture Al2O3/MgO ||MgO only |
|30 ||Mixture Al2O3/Ta2O5 ||MgO only |
The use of these materials proves to be especially advantageous, since these metal oxides are also stable even at high temperatures. The intermediate layer and/or the passivation layer is preferably produced using a PVD, IAD, IBAD, PIAD, or magnetron sputtering process.
Furthermore, the passivation layer has, according to both embodiments, a mixture made of SiO2, BaO and Al2O3, wherein the weight percentage of SiO2 lies in the range of about 20% to 50%. Here, it proves to be advantageous that this mixture has a high insulation resistance.