EP1516167A1

EP1516167A1 - Sensor and method for producing a sensor

Info

Publication number: EP1516167A1
Application number: EP03759809A
Authority: EP
Inventors: Roland Guenschel; Richard Muehlheim; Bernhard Zappel; Stefan Pinter; Hubert Benzel; Joerg Muchow; Kurt Weiblen
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2002-06-12
Filing date: 2003-02-13
Publication date: 2005-03-23
Also published as: JP2005529347A; WO2003106955A1; DE10226034A1

Abstract

The invention relates to a sensor, which is suitable for measuring high pressure and a method for producing said sensor. According to said method, a cavity (500) with a high aspect ratio is configured in a semiconductor material (51).

Description

Sensor and method for producing a sensor

State of the art

The invention relates to a sensor made of a semiconductor material and a method for producing a sensor according to the preamble of the independent claims. Micromechanical silicon pressure sensors are already known, a membrane being produced by the introduction of a cavern in a silicon chip. Such a silicon sensor is disclosed, for example, in German published patent application DE 199 57 556. Here, the cavern is generated for example by anisotropic KOH etching.

Advantages of the invention

The sensor according to the invention and the method according to the invention with the features of the independent claims has the advantage that a simple and cost-effective design for the production of a sensor is proposed. In particular, the sensor according to the invention is used to measure high pressures, the sensor according to the invention nevertheless having a high overload safety. Furthermore, the sensor according to the invention has the advantage that the influence of temperature is low and the temperature hysteresis is small. The pressure sensor according to the invention is provided in particular as a piezoresistive pressure sensor. A high bursting safety of the pressure sensor according to the invention, ie the suitability of the pressure sensor for measuring high pressures, is achieved in particular by a large aspect ratio of the cavity taken out of the semiconductor material from its rear side. The inventive large aspect ratio is in particular by means of a Trench etching brought about. Furthermore, it is advantageous that when using a Trenchätzprozesses the transition radii from the cavern wall to the membrane are large compared to the anisotropic etching, whereby the mechanical stresses in the material can be reduced and thus the allowable pressure load can be increased. According to the invention, however, it is also envisaged to use an isotropic etching process, for example by means of etching with acids, in order to obtain large transition radii. In the case of isotropic etching, however, the transition radii are sometimes so great that the stress under pressure or the elastic deformation of the membrane on the upper side or front side of the semiconductor material is so small that the pressure sensitivity is thereby small.

The measures listed in the dependent claims advantageous refinements and improvements of the specified in the independent claims sensor and the method are possible.

It is particularly advantageous that means for measuring the deformation of the membrane region are provided on the front side of the semiconductor material. This is an accurate, relatively temperature-independent, stable and sensitive measurement of a force, which deforms the membrane, in particular a pressure, possible. As means for measuring the deformation of the membrane region, piezoelectric resistors are provided according to the invention in particular, but measuring resistors or measuring means based on another effect can also be provided.

According to the invention, it is particularly advantageous for an evaluation circuit to be monolithically integrated in the semiconductor material together with the membrane. As a result, the sensor according to the invention can be produced more cheaply and manufactured with greater accuracy.

drawing

Embodiments of the invention are illustrated in the drawings and explained in more detail in the following description.

1 shows a known micromechanical silicon pressure sensor according to the prior art, Figure 2 shows a semiconductor substrate with a sensor according to the invention, Figure 3 shows a first construction variant of the sensor according to the invention, Figure 4 shows a second construction variant of the sensor according to the invention, Figure 5 shows a third construction variant of the sensor according to the invention, Figure 6 shows a fourth construction variant of the sensor according to the invention and Figure 7 shows a fifth construction variant the sensor according to the invention.

Description of the embodiments

FIG. 1 shows the generally common structure of micromechanical silicon pressure sensors. A silicon substrate 150 is provided with a cavity 155, which leaves a membrane not provided with a reference numeral. The silicon substrate 150 is connected to a drilled glass 140, which is soldered to a base 120 with a solder 130. The base 120 is connected to a pressure connection pipe 110. Furthermore, measuring resistors not provided with a reference numeral and located on the upper side of the silicon substrate 150 are connected via one or more bonding wires 160 to a connection pin 170, which are electrically separated from the base 120 by means of an indentation 180.

Cavity 155 of the silicon sensor has a typical etched slope that is approximately truncated pyramidal. This results in a trapezoidal cross-section. This truncated pyramidal recess below the sensor membrane results from the use of a silicon substrate which has a (100) orientation because a KOH etch has different etch rates in different crystal directions. A disadvantage in the known silicon sensor proves that the pressure-engaging surface, the truncated pyramidal recess at its largest cross-section, ie at the back of the silicon substrate, is authoritative. Furthermore, it proves disadvantageous that an edge is formed in the transition region between the bevelled side surface of the truncated pyramidal recess and the membrane surface, which has a very small radius. As a result, very large stresses occur in the material, for example due to the formation of cracks. As a result, the silicon sensor in the prior art has only a low bursting pressure. In the structure of the silicon pressure sensor according to the prior art shown in Figure 1, the silicon chip 150 is anodically bonded to a glass intermediate layer 140 of sodium-containing glass, such as Pyrex and soldered by means of a solder 130 on a metal base 120. For an application at higher pressures, the following critical points result from the sensor structure according to the prior art:

The transition radii between the cavity edge and the membrane are very small due to the etching method used, for example KOH etching, which results in high mechanical stresses at the transition, which reduce the bursting strength. This is especially true for the time-etched membranes for higher pressure ranges, since there the radii are particularly small. The anisotropic etching process results in shallow cavern walls, i. Cavern walls with a low aspect ratio, in particular a slope of 54 degrees. This creates a large opening in the silicon. The larger this area, the more force is applied to the chip when pressure is applied. At the same time, the bonding area, i. the bonding area between the silicon sensor 150 and the glass 140 becomes smaller, thereby increasing the surface load in the pulling direction. This leads to a low pressure resistance. The construction of a pressure sensor by means of a glass intermediate layer therefore represents a limitation of the bursting strength.

For the generation of the cavern 155 or for the production of very thin membranes, a PN-etching stop is used. At high pressures, the membrane must be designed to have a very small edge length. This can only happen up to a certain limit, because the membrane can not be made smaller than the extent of the piezo-resistive resistors. In this case, the etching stop must be omitted and a "thick" membrane etched on time.

FIG. 2 shows the semiconductor sensor 50 according to the invention. A semiconductor substrate 51, which is provided in particular as a silicon substrate 51, has a front side 58 and a rear side 59. From its rear side 59, the semiconductor substrate 51 is treated such that it forms different thicknesses in different areas. The semiconductor substrate 51 is formed in a first region not denoted by a reference numeral in a first thickness provided with the reference numeral 52 and in a membrane region provided with the reference numeral 54 in a second thickness designated by the reference numeral 53 educated. Here, the first thickness 52 is greater than the second thickness 53. According to the invention, it is provided in particular that the first region extends around the membrane region 54. This is only indicated in FIG. 2, because FIG. 2 is a sectional view of the semiconductor substrate 51. Provided on the front side 58 of the semiconductor substrate 51 are means for measuring the deformation of the membrane region 54, which are provided with the reference numeral 57. These are, in particular, piezoresistive resistors 57. The depression provided on the rear side 59 of the semiconductor substrate 51 for producing the thinner membrane region 54 has in its "wall region" a transitional region provided with the reference numeral 55 in FIG Membrane region 54 has a radius provided with the reference numeral 550 in Figure 2. The wall of the transition region 55 is shown in an enlarged view in Figure 2, in which a specific surface structure 56 can be seen for the method used in the production of the recess or cavern, which is introduced into the rear side 59 of the silicon substrate 51 for producing the membrane region 54, is provided with the reference numeral 500 in FIG.

The transition area 55 between the first thickness 52 and the second thickness 53 has a high aspect ratio according to the invention, ie its depth is large compared to its lateral extent, so that the "walls" of the transition area 55 are comparatively steep When using the silicon substrate 51 as a pressure sensor, the pressing action comes from the back 59 forth, only a lower compressive force is applied to the semiconductor substrate 51 from the back 59 forth than in the truncated pyramid cavern 155 of the prior art, because the pressure application area around the 1. This is particularly the case when the height of the membrane region 54 is the same in each case Furthermore, the introduction of the cavern 500 into the semiconductor substrate 51 by means of a trench etching process has the advantage that the radii of curvature 550 large ge are nug, so that arise between the transition region 55 and the membrane region 54 in the load case no excessive voltage spikes in the substrate material. This makes it possible to increase the pressure load of the semiconductor sensor 50 and to make this technology available for higher pressures. Instead of the trench etching process, according to the invention, an isotropic etching process, for example by means of etching with acids, used to obtain large transition radii 550. However, these radii 550 are partially so great in isotropic etching that, because of the resulting lower deformation of the diaphragm region 54, a lower pressure sensitivity of the sensor 50 results.

In the use of a trench etching process, the edge region of the transition region 55 has a characteristic surface structure 56 which is caused by the fact that, in the trench etching process, the depth of the cavern 500 is alternately increased with accelerated charged particles and the surface of the cavity 500 is passivated. As a result of this alternating processing of the cavern material, the inventively large aspect ratio is caused because during the etching phases of the trench etching process, preferably the membrane region 54 is etched away from its rear side and the passivated sidewalls of the transition region 55 remain. Trenching with a high aspect ratio produces quasi-perpendicular cavern walls in the transitional region 55, as a result of which a high bursting compressive strength is achieved because essentially only the membrane region 54 is suitable as a pressure application surface.

According to the invention are therefore presented below on construction variants, which are particularly suitable for sensors that can withstand higher pressures. Such construction variants are characterized according to the invention in that comparatively large transition radii 550 are provided between the transition region 55 and the membrane region 54, that the cavern walls in the transition region 55 are steep or vertical, and that no glass is used in the construction technique or glass is avoided.

By avoiding glass in the assembly and connection technology, the pressure range for the sensor 50 according to the invention can be extended to the order of 1000 bar. As a result, the sensor according to the invention represents a cost-effective variant of previous sensors for this magnitude of pressures. For this purpose, it is advantageously possible to largely retain existing manufacturing processes of pressure sensors, such as the semiconductor process of the silicon chip 51 or existing sensor housing parts. Furthermore, it is inventively possible to build capacitive sensors also with this method and improve to avoid temperature-induced deformation of the sensor chip. In capacitive sensors leads a deformation of the sensor chip, ie in particular the Membrane region 54, to a deflection of the membrane, which ultimately means a change in the output signal.

In the construction variants of the present invention shown in FIGS. 3 to 7, it is provided that, despite the avoidance of glass, a temperature decoupling between the semiconductor substrate 51 and its surroundings is achieved. For this purpose, in particular intermediate plates are provided with an adapted temperature expansion coefficient. By cascading two or more intermediate plates this effect can be improved. The coefficient of thermal expansion of such intermediate plates on a pedestal should be chosen so that it lies between that of the base and the subsequent intermediate plate. The coefficient of thermal expansion of the intermediate plate connected to the silicon chip 50 should preferably match the coefficient of thermal expansion of the semiconductor material used, i. in particular of the silicon, in order to transmit as little as possible thermal expansion effects to the sensor 50. For connecting the intermediate plates to each other, to the base and to the chip, it can be soldered or glued. Another possibility is to weld together the base and the first intermediate plate and also to weld the other intermediate plates together.

FIG. 3 shows a first construction variant of the sensor according to the invention. The sensor chip 50 is fastened on a base 20 by means of a solder layer 30. In this case, the glass intermediate plate is dispensed with and the chip 50 is fastened directly, ie only by means of a thin intermediate layer 30, on the base 20. The chip 50 can be glued or metallized and soldered on the back. The material of the base 20 is particularly important because it must have a coefficient of thermal expansion close to that of the silicon chip 50. As preferred materials of the base 20, it is provided according to the invention to use iron-nickel steel in order to achieve a good strength with a low coefficient of thermal expansion. According to the invention, the materials "Kovar", "Invar", "Vakodil" are used singly or in combination with one another as the material for the base 20. According to the invention, the base 20 is in particular a TO8 socket or in another form, for example as a socket with a screw-in thread As is shown in FIG. 3, the base 20 is connected to a pressure connection tube 10, which no longer applies the pressure on the backside of FIG designated membrane region 54 passes. Furthermore, in FIG. 3 and in further FIGS. 4 to 6, one or more bonding wires 60 are provided, which comprise the measuring resistors provided on the front side of the semiconductor chip 50 and also not shown in FIG. 3 with a connection pin 70 or a printed circuit board 71 (FIG ), wherein the terminal pin 70 is provided by an insulation 80, which is provided in particular as a glaze, electrically isolated from the base material, if the terminal pin 70 is passed through the base 20. Any metal solder such as AuSn20 or SnCu3In0.5 or glass solder can be used as solder.

4 shows a second construction variant of the sensor according to the invention is shown, in which the base 20 made of a material with arbitrary

Thermal expansion coefficient, such as steel, may exist. Again, the pressure connection tube 10, connected to the base 20, is provided. There is provided between the base 20 and the sensor chip 50, a first intermediate plate 32, which consists of a material having an adapted temperature expansion coefficient. The intermediate plate 32 is either glued or soldered with respect to the sensor chip 50 or the base 20 or also, in particular by laser welding, welded. This results in the first intermediate layer 30 between the base 20 and the first intermediate plate 32 and the second intermediate layer 34 between the first intermediate plate 32 and the sensor chip 50th

The material of the first intermediate plate 32 may also consist of silicon or polysilicon according to the second embodiment of the invention. Furthermore, the connection between the silicon chip and the intermediate plate can also be effected by anodic bonding, if a pyrex layer applied to the chip 50 or the intermediate plate 32 is applied as the second intermediate layer 34. The connection between the first intermediate plate 32 and the base 20 can be filled with soft solder for thermal decoupling.

If the coefficient of thermal expansion of the base 20 is very far removed from the coefficient of thermal expansion of the semiconductor material used for the sensor 50, a structure according to a third construction variant, which is shown in FIG. 5, is suitable. In order to avoid high voltages between the base 20 and the semiconductor sensor 50, which by different Temperature expansions of different materials arise, is used in the third construction variant, a two-stage structure. The same reference numerals in FIG. 4 in FIG. 5 again denote the same components or parts of the sensor. FIG. 5 additionally shows a protective cap 90 which protects the structure of the sensor 50 on the base 20. Instead of the first intermediate layer 30, the first intermediate plate 32 and the second intermediate layer 34 as in FIG. 4, the first intermediate layer 30, the first intermediate plate 32, the second intermediate layer 34 are shown in FIG. 5 for better alignment of the temperature expansion coefficient of the sensor 50 with the coefficient of thermal expansion of the base 20 , the second intermediate plate 36, and the third intermediate layer 38. For the first intermediate plate 32, a material is preferably used which has a coefficient of thermal expansion which lies between the temperature expansion coefficient of the base 20 and the coefficient of thermal expansion of the second intermediate plate 36. This ensures that the different length expansions over the temperature of the materials used are distributed to the different levels of the intermediate plate stack, whereby the maximum mechanical stresses in the materials can be kept small. This leads to a low temperature influence of the sensor and a low temperature hysteresis or drift, at the same time high bursting strength. In Figure 5, the protective cap 90 is located, the open or pressure-tight closed, for example, vacuum-tight welded, may be designed to realize a reference, differential or absolute pressure sensor. For safety reasons, in the case of an absolute pressure sensor, the cap 90 can be designed so that its bursting strength is above the bursting strength of the silicon membrane, whereby leakage of the structure or bursting of the membrane can be avoided.

FIG. 6 shows a fourth construction variant of the pressure sensor. Here, the chip 50 is mounted on a discharge nozzle. The pressure connection can, as shown here by way of example, be carried out with a screw thread 6 and a cone seal 5, but also be realized by means of a nozzle with O-ring seal or the like. The same reference numerals in FIG. 5 again denote the same parts or elements of the sensor or the sensor structure. In contrast to FIGS. 3, 4 and 5, instead of the connecting pin 70, a printed circuit board 71 is provided in FIG. 6, which is connected by means of the or more bonding wires 60 to the pressure measuring means located on the front side of the sensor membrane. The chip 50 is thus electrically over the Circuit board 71 is connected, which in turn is connected to a plug 72 in the protective cap 90.

FIG. 7 shows a fifth construction variant with a brazed intermediate socket 33. The in turn existing base 20 and the intermediate socket 33 are erfmdungsgemäß in the fifth construction variant by means of the first intermediate layer 30 in particular brazed and connected together such that the first intermediate layer 30 is loaded with increasing pressure on the sensor 50 to pressure. By means of the second intermediate layer 34, the chip 50 is then connected to the intermediate socket 33. Here, the second intermediate layer 34 is provided in particular from a creep-resistant solder and also brazed. Since the solder of the first intermediate layer 30 is loaded in the fifth construction variant on pressure, it is according to the invention also possible to provide the first intermediate layer 30 with soft solder in the fifth construction variant, since this, in the event that it is subjected to pressure, can also withstand very high pressures.

Claims

claims

A sensor of a semiconductor material (51), wherein the semiconductor material (51) has a first thickness (52) and wherein the semiconductor material (51) in a membrane region (54) has a second thickness (53), wherein the second thickness (53 ) is provided smaller than the first thickness (52), wherein the sensor has a transition region (55), wherein the thickness of the semiconductor material (51) in the transition region (55) from the first thickness (52) to the second thickness (53) decreases , characterized in that in the transition region (55) a large aspect ratio is provided.

2. Sensor according to claim 1, characterized in that the transition region (55) has a surface structure (56), as is caused by trench etching.

3. Sensor according to claim 1 or 2, characterized in that the sensor is provided for pressure measurement.

4. Sensor according to one of the preceding claims, characterized in that as the semiconductor material (51) silicon is provided.

5. Sensor according to one of the preceding claims, characterized in that the semiconductor material (51) has a front side (58) and a rear side (59), wherein the transition region (55) is part of the back (59), wherein on the front ( 58) means (57) are provided for measuring the deformation of the membrane region (54).

6. Sensor according to one of the preceding claims, characterized in that in the semiconductor material (51) monolithically an evaluation circuit is provided integrated.

7. A method for producing a sensor from a semiconductor material (51), wherein a membrane region (54) is provided, wherein the thickness (52, 53) of the semiconductor material (51) is reduced, characterized in that the reduction of the thickness (52, 53) is performed by trench etching.