DE102010031197A1 - Micro-electromechanical piezoresistive pressure sensor for use in motor car, has membrane arranged at insulating layer opposite side to piezoresistors and conductive paths that include metal contacts for detecting deformation of membrane - Google Patents

Abstract

The sensor has a membrane arranged in a silicon substrate (1) i.e. silicon-on-insulator substrate. An electrical insulating layer (3) is arranged on a silicon carbide layer opposite side of the substrate. The membrane is arranged at the insulating layer opposite side to piezoresistors (4) and conductive paths (5) provided with metal contacts for piezoresistive detection of pressure-effected deformation of the membrane. The carbide layer has thickness in a range between 1 and 20 micrometer, and is made of polycrystalline silicon carbide. The insulating layer is made of silicon dioxide. An independent claim is also included for a method for manufacturing a piezoresistive pressure sensor.

Description

The present invention relates to a piezoresistive pressure sensor and to a method for producing a piezoresistive pressure sensor.

State of the art

There are already known electromechanical systems (MEMS), which are designed as acceleration and pressure sensors.

Most MEMS pressure sensors operate on the same primary principle of operation, where a pressure difference across a thin membrane results in their deformation. A piezoresistive evaluation has proven itself for the metrological detection of the membrane deformation proportional to the pressure difference. In this case, the high piezoresistive effect of silicon is utilized in order to convert the mechanical stress state of the membrane or piezoresistors on the membrane into an electrically evaluable size.

Since the piezoresistive effect in p-doped silicon is higher than in n-doped silicon, the piezoresistors can generally be applied as p + -doped regions of a so-called n-well by doping. A frequently used operating temperature of such a sensor is in a range of about 150 ° C. For higher temperatures, up to about 500 ° C, piezoresistive pressure sensors made using SOI (Silicon to Insulator) technology can be used. The membranes of micromechanical pressure sensors are typically 10-100 microns thick.

Disclosure of the invention

The present invention is a piezoresistive pressure sensor, comprising a silicon substrate to which a silicon carbide layer forming a membrane is arranged, wherein further on the silicon carbide layer on the opposite side of the silicon substrate, an electrically insulating layer is disposed on the diaphragm opposite side piezoresistors and interconnects Metal contacts for piezoresistive detection of a pressure-induced deformation of the membrane are arranged.

According to the invention, a silicon substrate is thus provided which gives the pressure sensor as the base body sufficient stability even at high temperatures. The membrane is a silicon carbide (SiC) layer, which is arranged on the substrate. The substrate therefore has a recess in which the silicon carbide layer is elastically deformable as a membrane to react to a pressure.

The silicon carbide layer of the piezoresistive pressure sensor according to the invention behaves elastically in a usable thickness as a membrane to over 1000 ° C. It is therefore possible to produce silicon carbide sensors which allow operating temperatures of well over 500 ° C. Since, according to the invention, the silicon carbide has no electrical function but only a mechanical function, it is possible to dispense with expensive monocrystalline silicon carbide wafers. For example, the wafer costs of monocrystalline silicon carbide are about 200 times higher than silicon.

The pressure sensor according to the invention therefore has the advantage that it can be used for numerous fields of application of pressure sensors, in particular for applications in a motor vehicle, which require very high operating temperatures. This is the case, for example, with cylinder pressure measurements or pressure measurements along the exhaust gas line, which may be necessary for the fulfillment of future emission standards. The temperatures prevailing here on the pressure sensor element reach values of about 600 ° C for the cylinder pressure measurement and even 1100 ° C for the pressure measurement at the exhaust manifold.

Furthermore, leakage currents are almost completely avoided by the electrically insulating layer under the piezoresistors. The insulating layer does not affect the deformability of the membrane or hardly.

The provision of a membrane made of silicon carbide has the further advantage that silicon carbide shows a significantly reduced aging effect compared to pure silicon. The longevity of the membrane and the sensor can be greatly increased.

With the piezoresistive pressure sensor according to the invention, it is therefore possible to inexpensively produce micro-electro-mechanical pressure sensors for use at temperatures of far more than 500 ° C.

In a preferred embodiment, the silicon carbide layer has a thickness of 1 .mu.m to 20 .mu.m. In this thickness, the silicon carbide layer has sufficient stability, so as not to be unintentionally deformed even at high temperatures, or negatively affected in another way. In addition, an elastic deformation in the entire temperature range, in particular from room temperature up to 1000 ° C is possible, so that the membrane is sufficiently sensitive. Preferably, the pressure sensor according to the invention can thus be used in pressure ranges from 1 bar to 5000 bar.

In a further advantageous embodiment, the silicon carbide layer of polycrystalline silicon carbide (poly-SiC) is formed. Such a layer can be produced very inexpensively, for example, in a CVD process.

In the context of a further preferred embodiment, the piezoresistors and the conductor tracks are formed from p + -doped silicon. As a result, the piezoresistive effect of the resistors is particularly high. For example, the piezoresistive effect is higher in p-doped silicon than in n-doped silicon. P + doped in the context of the invention means that impurities or impurities, in particular in an amount of between 0.1 ppb and 100 ppm, are added to the silicon lattice. These foreign atoms specifically alter the properties of the starting material. For p + -doped silicon, trivalent elements, the so-called acceptors, are introduced into the silicon lattice and replace tetravalent silicon atoms. It thus creates a positive gap, also called hole or defect.

According to the invention, it is also advantageous if the electrically insulating layer is formed from silicon dioxide (SiO ₂ ). Silica is a suitable insulator even at high temperatures and can thus effectively reduce or prevent leakage currents in the entire desired temperature range.

• – Verwenden eines Siliziumsubstrats, das von einem Siliziumfilm durch eine elektrisch isolierende Schicht getrennt ist;
• – Strukturieren des Siliziumfilms zur Erzeugung von Piezowiderständen und Leiterbahnen;
• – Teilweises Entfernen des Siliziumsubstrats bis zur elektrisch isolierenden Schicht zur Erzeugung einer Membrangeometrie;
• – Abscheiden einer Siliziumcarbidschicht an der Membrangeometrie;
• – Anordnen von Metallkontakten an den Leiterbahnen.

The present invention further relates to a method for producing a piezoresistive pressure sensor comprising the steps:

• Using a silicon substrate separated from a silicon film by an electrically insulating layer;
• - structuring the silicon film to produce piezoresistors and tracks;
• - Partial removal of the silicon substrate to the electrically insulating layer to produce a membrane geometry;
• - depositing a silicon carbide layer on the membrane geometry;
• - Arranging metal contacts on the conductor tracks.

With the method according to the invention, piezoresistive pressure sensors can be produced in a particularly simple manner, so that the sensors according to the invention and described with reference to the sensor can be achieved with the sensors produced.

It is particularly advantageous if the structuring of the silicon film by reactive ion etching (RIE) takes place. In reactive ion etching, a plasma is ignited between two electrodes by high frequency in a vacuum over an etching gas. The silicon exposed to the plasma is etched by ion bombardment. Furthermore, a chemical reaction of the silicon with the etching gas take place. These two Ätzanteile can be specifically used to control the etching process.

As part of a further embodiment of the method according to the invention, the removal of the silicon substrate by KOH etching takes place. In such an anisotropic etching process, the silicon is dissolved out of the substrate as a hydroxy complex, whereby a very homogeneous process with high etching rates can be achieved.

In the context of a further embodiment of the method according to the invention, the deposition of the silicon carbide layer takes place via a PECVD silicon carbide deposition. Such a plasma-enhanced chemical vapor deposition (PECVD) is a method which can be used, in particular, for the deposition of silicon carbide, with which a silicon carbide layer can be deposited with high accuracy and homogeneous thickness.

It is also advantageous that the metal contacts are formed by sputtering, for example with a shadow mask. There is thus a deposition of a metal film on the conductor tracks, wherein the geometry of the metal contacts can be controlled by the shadow mask. By this method, it is possible to deposit the metal particularly homogeneously on the silicon.

Further advantages and advantageous embodiments of the subject invention are illustrated by the drawings and explained in the following description. It should be noted that the drawings have only descriptive character and are not intended to limit the invention in any way. Show it

1 a starting product for the production of the pressure sensor according to the invention;

2 an intermediate with a structured silicon layer;

3 another intermediate with defined membrane geometry;

4 another intermediate with deposited silicon carbide layer;

5 a finished piezoresistive pressure sensor according to the invention.

In the figures, a particularly preferred inventive production method for a piezoresistive according to the invention Pressure sensor shown. The pressure sensor according to the invention can be used for any application. In particular, the pressure sensor according to the invention is suitable for performing very accurate pressure measurements at high temperatures, such as in automotive applications, for example in the cylinder head or in the exhaust system, or in industrial metrology. With a sensor according to the invention, for example, both relative pressure measurements and absolute pressure measurements are possible.

In 1 the starting product for producing the piezoresistive pressure sensor according to the invention is shown. The starting material for the production of the pressure sensor according to the invention is a silicon carrier or a silicon substrate 1 intended. The silicon substrate 1 is designed as an SOI substrate (silicon to insulator). This results in shorter switching times and in particular lower power consumption. This is particularly advantageous in the present case, since the power consumption is low with respect to the leakage currents.

The silicon substrate 1 may consist of a conventional undoped silicon and have a thickness of about 700 microns. The silicon substrate 1 gives the sensor sufficient stability, both at low temperatures, such as at room temperature, but also at the possible high use temperatures according to the invention up to over 1000 ° C.

Next to the silicon substrate 1 as such is an SOI film, especially a silicon film 2 , intended. The silicon film 2 is preferably doped, in particular p + -doped. As a result, a particularly good piezoresistive effect can be achieved with the finished pressure sensor. The silicon film 2 may have a thickness of ≥ 0.5 to ≤ 20 μm. Particularly preferably, the film has a thickness of 1 to ≦ 10 μm. It is often found in doped semiconductors, such as doped silicon, that the piezoresistive effect is particularly pronounced especially at high temperatures, which is why an application at high temperatures, for example above 500 ° C, is particularly advantageous. The degree of doping of the silicon, which can be adjusted in the manufacturing process in a wide range, thereby determines the proportionality factor between strain of the membrane and resistance change of the semiconductor resistance.

The silicon film 2 is on an electrically insulating layer 3 arranged, which the silicon film 2 from the silicon substrate 1 electrically isolated. Preferably, this is electrically insulating layer 3 a silicon dioxide (SiO ₂ ) layer and is also referred to as buried oxide (BOX). However, other electrically insulating materials, such as sapphire, are also possible. The electrically insulating layer 3 may have a thickness of ≥ 50 nm to ≤ 2 μm. With particular preference, this layer has a thickness of ≥ 100 nm to ≦ 1 μm. By providing an electrically insulating layer 3 Leakage currents can be almost completely avoided.

In 2 the first method step for producing the piezoresistive pressure sensor according to the invention is shown. In this process step, the silicon film 2 structured. That means piezoresistors 4 and tracks 5 be generated. These are then arranged in particular in such a way as to piezoresistively detect a pressure-induced deformation of the membrane to be produced later on. The resistors 4 as well as the tracks 5 are therefore part of an electrical resistance bridge. For example, they may be arranged on the principle of a Wheatstone bridge.

The structuring of the silicon film 2 can be done for example by a reactive ion etching (reactive ion etching (RIE)) or reactive ion dry etching (deep reactive ion etching (DRIE)). By such methods, microstructures are in the silicon layer 2 With very good aspect ratios, ie the ratio of depth to width possible, with suitable structure depths can be achieved.

In 3 is shown a further intermediate step of the method according to the invention. In this process step, the silicon substrate 1 partially removed. This process step can be carried out particularly preferably by KOH etching. There is thus an aqueous leaching solution to the silicon substrate 1 which brings the silicon as hydroxy complex in solution. In this way, the silicon can be washed out of the basic structure and thus removed.

The partial removal of the silicon substrate in the context of the present invention is understood to mean that the silicon carbide only in the x-direction up to the electrically insulating layer 3 is completely removed. It creates a recess in this way 6 in the silicon substrate. As a result, a membrane geometry can be produced which is well suited for a membrane. For example, a reverse trough-shaped recess 6 arise as a membrane geometry, which is circular at its bottom of the tub. However, any desired geometries are possible which are advantageous for the membrane applied in a later method step.

In a further method step according to 4 is at the according 3 produced membrane geometry a silicon carbide layer 7 deposited. In particular, this is a layer of polycrystalline silicon carbide (poly-SiC). This can be realized by any possible method. Particularly preferred here is a PECVD deposition, ie a plasma-assisted chemical vapor deposition. As a result, a silicon carbide layer 7 be easily applied in a desired thickness. In particular, the silicon carbide layer 7 a thickness of 1 .mu.m to 20 .mu.m.

Because of that 3 a desired membrane geometry was generated, forms the silicon carbide layer 7 now, for example, on the tub bottom of the recess 6 , a membrane 8th out. This membrane 8th has a stability that is not significantly reduced to very high temperatures, such as up to 1000 ° C or even more. The maximum permissible measuring pressure can be influenced by the membrane thickness. For example, a maximum measuring pressure of 5000 bar can be achieved through a 20 μm thick membrane 8th realize.

In the last method step according to 5 be on according to 2 generated tracks 5 metal contacts 9 , in particular by sputtering with a shadow mask applied. The metal contacts 9 serve to electrically connect the sensor with a power source or evaluation unit so as to perform a piezoresistive pressure measurement can.

Consequently, in 5 a finished produced inventive piezoresistive pressure sensor shown.

The piezoresistive pressure sensor according to the invention thus comprises a silicon substrate 1 as a supporting body, on which a a membrane 8th forming silicon carbide layer 7 is arranged. The silicon carbide layer 7 thus forms the membrane 8th in a form which, according to the in 4 was selected process step shown. Above the substrate 1 or above the membrane 8th , that is on the silicon carbide layer 7 opposite side of the silicon substrate 1 , is an electrically insulating layer 3 arranged. This serves in particular to prevent leakage currents and thus to increase the accuracy of the measurement. Above the electrically insulating layer 3 So on the membrane 8th opposite side of the insulating layer 3 are piezoresistors 4 and tracks 5 with metal contacts 9 for piezoresistive detection of a pressure-induced deformation of the membrane 8th arranged. In particular, the piezoresistors 4 at the edge of the membrane 8th arranged.

Is on the sensor or on the membrane 8th exerted a pressure, the membrane deforms 8th by this pressure. At the edge of the membrane 8th the deformation is naturally most pronounced. By deformation of the membrane 8th gets on the piezoresistors 4 exerted a mechanical tension. The voltages in the piezoresistors 4 lead to resistance changes after the piezoresistive effect. By the preferred arrangement of the piezoresistors 4 at the edge of the membrane 8th be in the resistances 4 achieved so strong mechanical stresses, so that the triggered by the piezoresistive effect change in the resistance value is particularly strong. Thus, a particularly high sensitivity of the sensor is possible.

The resistance changes are pressure proportional, reversible and electronically evaluable. To the through the piezoresistors 4 and the tracks 5 trained measuring bridge is over the metal contacts 9 in particular a constant voltage of a few volts, about 5 V, can be applied in order to obtain good measurement results.

Claims (10)

Piezoresistive pressure sensor comprising a silicon substrate ( 1 ), on which a membrane ( 8th ) forming silicon carbide layer ( 7 ), wherein furthermore on the silicon carbide layer ( 7 ) opposite side of the silicon substrate ( 1 ) an electrically insulating layer ( 3 ) is arranged, at whose the membrane ( 8th ) opposite side piezoresistors ( 4 ) and printed conductors ( 5 ) with metal contacts ( 9 ) for piezoresistive detection of a pressure-induced deformation of the membrane ( 8th ) are arranged. Pressure sensor according to claim 1, characterized in that the silicon carbide layer ( 7 ) has a thickness of 1 micron to 20 microns. Pressure sensor according to claim 1 or 2, characterized in that the silicon carbide layer ( 7 ) is formed of polycrystalline silicon carbide. Pressure sensor according to one of claims 1 to 3, characterized in that the piezoresistors ( 4 ) and the tracks ( 5 ) are formed of p + -doped silicon. Pressure sensor according to one of claims 1 to 4, characterized in that the electrically insulating layer ( 3 ) is formed of silicon dioxide. Method for producing a piezoresistive pressure sensor comprising the steps: Using a silicon substrate ( 1 ) generated by a silicon film ( 2 ) by an electrically insulating layer ( 3 ) is separated; - structuring of the silicon film ( 2 ) for the production of piezoresistors ( 4 ) and printed conductors ( 5 ); Partial removal of the silicon substrate ( 1 ) to the electrically insulating layer ( 3 ) to produce a membrane geometry; Depositing a silicon carbide layer ( 7 ) on the membrane geometry; - Arranging metal contacts ( 9 ) on the tracks ( 5 ). Method according to claim 6, characterized in that the structuring of the silicon film ( 2 ) is carried out by reactive ion etching. Method according to claim 6 or 7, characterized in that the removal of the silicon substrate ( 1 ) by KOH etching. Method according to one of claims 6 to 8, characterized in that the deposition of the silicon carbide layer ( 7 ) via a PECVD silicon carbide deposition. Method according to one of claims 6 to 9, characterized in that the forming of the metal contacts ( 9 ) by sputtering, for example with a shadow mask occurs.

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