EP1877745A1

EP1877745A1 - Micromechanical pressure/force sensor and corresponding production method

Info

Publication number: EP1877745A1
Application number: EP06708573A
Authority: EP
Inventors: Hubert Benzel; Matthias Illing; Simon Armbruster; Gerhard Lammel
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2005-04-28
Filing date: 2006-03-01
Publication date: 2008-01-16
Also published as: KR20080009093A; WO2006114346A1; DE102005020282A1

Abstract

Disclosed is a pressure/force transducer and a method for the production thereof, which are based on a pressure sensor produced by means of standard surface micromechanical processes. Said pressure/force transducer comprises a membrane over a (vacuum) cavity, e.g. having a height of several micrometers. The membrane is mechanically stopped on the substrate because the cavity is closed rather than being open on the side of the cavity facing away from the membrane, as is the case with pressure sensors produced according to the conventional KOH etching technique, resulting in significantly improved overload protection when used as a force sensor. In addition, the design of the cavity makes it possible to limit bending to a few micrometers such that the tightness of the cavity is not even affected in case of an overload as no fissure can be created in the membrane when the membrane is provided with adequate stiffness.

Description

Micromechanical pressure / force sensor and a corresponding manufacturing method

State of the art

In consumer electronics, touch-sensitive screens are used in PDAs and

Smartphones used, as well as touch-sensitive surfaces as a replacement for the mouse in laptops. One way to realize such applications is to use wire meshes which are introduced into the surface and which allow the contact of the pen or finger to be read by closing an electrical contact when touched. However, such an arrangement does not allow each spent

To detect contact force.

Another possibility is to carry out the surface as a rigid plate, which is mounted in the corners on force-sensitive elements. These can be realized by pressure sensors. By the ratio of the forces (lever law) can on the position of the

Pen or fingers are closed. The sum of the forces gives the contact force, which is e.g. can be considered as line width. Such an arrangement can also be used to improve handwriting recognition.

One problem here is the robustness of the force sensor. In the typical applications of

Touchpads or other control elements are subject to forces of up to approx. 5 N, which must be determined with approx. 1% accuracy, but the sensor must be overload-proof against breakage up to approx. One way of transmitting power from the touchpad or directly to the finger-operated control on the membrane of a pressure or force sensor is that a small steel ball is used, which is loosely held by a suitable construction technique in the middle of the membrane. The disadvantage of such a structure, however, the relatively inaccurate centering of the loose ball affects the accuracy of measurement.

Advantages of the invention

The present pressure / force transducer or its production method is based on a pressure sensor produced by means of conventional surface micromechanical methods and comprises a membrane over a (vacuum) cavity, for example with a height of a few micrometers. Since the cavern is closed and is not open to the membrane facing away from the cavity as in the conventional KOH etching pressure sensors, there is a mechanical stop on the substrate for the membrane.

This results in the application as a force sensor significantly improved overload protection. In addition, the design of the cavern allows to limit the deflection to a few micrometers, so that even in case of overload, the tightness of the cavern is not affected, since with a correspondingly shaped membrane stiffness no gap can occur in the membrane.

Known pressure sensors in surface micromechanics operate by means of a capacitive measuring principle and require an evaluation circuit in the immediate vicinity of the measuring capacity. This means a larger chip area for each of the pressure sensors which can be used as force sensors, as a rule 4, at least 3 pieces for use as a touchpad. in the

In contrast, the present invention operates on the piezoresitive principle by means of at least one piezoresistor, which is applied in or on the membrane. In such a measuring principle, the detected raw measurement signals over several centimeters long lines can be easily transferred and processed by a single, separate from the micromechanical pressure / force transducer evaluation. It is also advantageous here that the signals of a plurality of pressure / force transducers can be processed jointly by a single evaluation circuit or an evaluation unit. This also simplifies the adjustment, which then has to be performed only once at the module level. The essence of the invention lies in the fact that the pressure or the force is not passed directly to the piezoresistors, but via a body, such as a metal ball or a solder ball, which is applied to the membrane. The advantage of using such a body lying on the membrane lies in the defined guidance of the pressure on the membrane relative to the previously introduced piezoresistors.

In one embodiment of the invention, it is provided to fix the body on the membrane with an adhesive layer, so that the compound body membrane is rigid. A displacement of the body with respect to the piezoresistors in the manufacture and / or operation of the pressure / force transducer can thus be excluded. Thus, too

Measurement inaccuracies, which result from a displacement of the body on the membrane by differently occurring deformations are avoided.

In order to prevent a displacement of the body, it may also be provided to flatten the body or the metal ball / solder ball in the region of the support on the semiconductor substrate.

This flattening can be done with the ball used before or during the application to the membrane. In addition, in a development of the invention may also be provided to apply a plate on the body. It has been found to be advantageous to use the upper part, i. the part of the ball, which is the ball pad on the semiconductor substrate opposite also to flatten to the best possible mechanical support for the

To get plate.

To generate the cavern, a method has proved to be particularly advantageous in which first a region in the semiconductor substrate, preferably made of silicon, is etched porous by a corresponding etching process. The semiconductor material in this area is then rearranged by a temperature treatment, so that a cavern is formed. Advantageously, the membrane is simultaneously formed over the cavern with the rearrangement. Due to the simultaneous formation of the cavern and the membrane can be provided that the cavern is almost equipped with a vacuum. If the process parameters are selected appropriately, a monocrystalline or largely monocrystalline membrane can be formed by the temperature treatment.

Furthermore, the time-consuming mounting of the ball over the semiconductor substrate or the chip can be replaced by the application of solder bumps known from flip-chip technology. Consequently - A -

The achievable sensitivity of the pressure sensor can be increased in terms of resolutions and accuracy, especially in the low pressure range up to about 50 N compared to known force transducers.

In one embodiment of the invention, the body used on the membrane in the

Compared to the lateral extent of the membrane have larger support or foot surface. By such a configuration, a part of the force acting on the body in the surrounding region of the membrane can be derived, so that the force measuring range is increased upward before the membrane goes into attack.

When using coordinated micromechanical masking in subsequent process steps of the micromechanical manufacturing process, the position of the body or the solder ball can be defined very precisely. Thus, the area on the semiconductor substrate on which the deformation acts can be reduced. Due to the fixation of the body or the solder ball on the Seedlayer beyond the assembly in a

Plastic housing facilitates.

By using different solder pastes, e.g. Pb solder or Pb-free solder, it is possible to adapt the mechanical properties of the ball, in particular their hardness of the desired application. As a possible solder connection, for example, has a

Alloy of about 80% Au and about 20% Sn proved because this compound is particularly hard.

With the present invention, a way is shown to register the operation of a control element. Advantageously, however, in addition to a simple digital on / off the possibility also opens, to detect the contact pressure of the control element and make a display and / or control / regulation in response to this contact force. In a further development of the invention, it is provided to use a plurality of pressure / force transducers in a common device. In this case, the output signals of all pressure / force transducers can be led to a common evaluation unit, which controls a visual and / or acoustic display in dependence on the detected signals. For the differentiation of the signals it can be provided that the different pressure / force transducers have a different structure, so that an equal force on the body results in different values of the electrical signal generated by the piezoresistors. It is conceivable that the membrane thickness is varied. In addition, however, it is also possible for the evaluation unit to have different threshold values for each pressure / force transducer, so that thereby the actuation or control of the various

Pressure / force transducer can be distinguished from each other.

Since typical applications for the proposed invention are found in daily life, e.g. Mobile phone (mobile phone), (computer) keyboard, game consoles, touchpads, etc., is a special design of the pressure / force sensor, for example, compared to high

Temperature fluctuation or particularly aggressive environmental media not necessary. Thus, the proposed pressure / force transducer can be produced inexpensively in standard methods of micromechanics without additional complex process steps.

Further advantages will become apparent from the following description of

Embodiments or from the dependent claims.

drawings

1 shows a typical structure of a pressure / force transducer according to the invention is shown. FIG. 2 shows a force effect which leads to a maximum stop of the membrane. With the figure 3 is shown a particular embodiment. Figure 4 shows a structure of a pressure / force transducer within a housing. embodiment

With the present invention, a possibility is presented, the operation of a control element, as they are commonly used in keyboards or touchpads, with a pressure sensor manufactured in micromechanical construction on the basis of

To detect piezo technology. In this case, in addition to the operation of the control element as a digital on / off switch and a direct detection of the contact pressure of the control element is possible. Due to the configuration according to the invention, the contact pressure with approximately 1% accuracy can be detected very precisely. Such an accurate determination of the contact force makes it possible to provide an acoustic or visual display and a further device unit in

To control dependence on the detected size of the contact force.

The starting point for the production of a pressure / force transducer according to the invention is a pressure sensor produced by conventional micromechanical methods, as is known, for example, from DE 100 32 579 A1 or DE 101 14 036 A1. With these procedures is a

Pressure sensor can be produced, which has a cavern in a preferably made of silicon semiconductor substrate and a membrane located above the cavern. Furthermore, it is known, for example from the documents DE 101 35 216 A1 or DE 10 2004 007518 A1, to produce piezoresistors in a membrane or on an additional layer on the membrane.

In a preferred exemplary embodiment, as shown in FIG. 1, according to the production method of DE 100 32 579 A1, a cavern 110 and a membrane 130 are produced with the formation of porous silicon in a semiconductor substrate 100 with a subsequent annealing step. Piezo resistors 120, for example according to DE 101 35 216 A1 or strain gauges, as described in DE 10 2004 007518 A1, are then applied to membrane 120 by means of masking, epitaxy and structuring steps. One way to achieve better adhesion of the body 150 to the surface of the semiconductor substrate 100, and thus the membrane 130, is to electroplate the contact area to the body 150 on the membrane 130.

Such a process step creates a layer 140, referred to as a seed layer. In order to allow a particularly good adhesion of the body 150 on the substrate 100 or on the membrane 130, the layer 140 may be designed such that the adhesion is adapted to both the material of the membrane 130 and on the material of the body 150. To produce the piezoresistive resistors 120, it is provided that a piezosensitive layer is first applied and then patterned. The application and also the structuring takes place in micromechanical standard method by means of appropriate masks, which are accurate positioning of the successive

Allow procedural steps. As a result, for example, the layer 140, which is produced after the piezoresistive resistors 125 from the same side on the substrate 100, can be positioned very accurately relative to the piezoresistive resistors 120. If a solder ball is used as body 150, i. a material that can be easily brought to the surface in a common micromechanical process, so this ball

150 may also be positioned quite accurately with respect to the resistors 120 using the masking technique. By such an orientation of the ball with respect to the piezoresistors 120, the accuracy of the force transducer can be significantly increased.

In addition to the use of a solder ball, however, the positioning of a metal ball on the layer 140 is possible. It should be noted, however, that a method for positioning must be selected in which a similarly good orientation of the metal ball relative to the piezoresistive resistors 120 is possible. It should also be ensured that the metal ball is firmly connected to the semiconductor substrate 100 or to the layer 140 in order to prevent a displacement of the ball and thus a change in the signal generated when actuated.

The purpose of the adhesive layer 140 on the semiconductor substrate is to prevent slippage of the body during manufacture and / or operation of the pressure / force transducer. In this case, in addition to the design of the body as a ball 150 may also be provided to use a pen-like body. By such a pen-like body, the force or a pressure from a remote source to the pressure / force transducer can be forwarded. Generally, the body 150 and, optionally, a plate 230 on the body 150 should provide the ability to connect a relatively large area of control to a relatively small effective area of force. Thus, for example, the control element may have the area of a fingertip, whereas the associated pressure / force converter makes up only a fraction of this area. In order to produce a layer 140 called a seed layer, the corresponding area on the semiconductor substrate 100 may also be plated. In addition, a solder paste can be applied to the semiconductor substrate 100 by the screen printing method or another suitable manufacturing method of the flip-chip technology. The solder paste is then remelted, so that only the desired surface is wetted and depending on the amount of solder applied a solder ball corresponding to the figure 150 is formed.

In a further embodiment it can be provided that the body 150 is flattened on the side that is in contact with the seed layer 140 (see, for example, FIG. 1a). To cause plastic deformation of the body 150 while using solder during the process

To avoid operation, the solder ball can be pre-pressed during assembly with the plate 230 so that it is flattened on the top.

FIGS. 1 and 2 show a typical course of an actuation of the force transducer. Thus, Figure Ia shows the force transducer in the unactuated state. In contrast, FIG. 1b shows a state in which the force 160 acting on the body 150 is sufficient to maximally deflect the membrane 130, so that the stop is reached through the cavern floor. As a result of this bending, the piezoresistive resistors 120 are also deformed, so that they produce a measurable potential in the form of a voltage. The voltage thus detected can then be assigned to a force evaluation 160 in a corresponding evaluation unit.

To adjust the sensitivity of the pressure / force transducer, the semiconductor material of the membrane 130 or its thickness can be selected or treated accordingly. Thus, it is conceivable that depending on the application of the pressure / force transducer, the semiconductor material is more or less deformable. Another embodiment is that, for example, as a result of the cavern formation by means of porous silicon, the membrane 130 is formed monocrystalline. In addition, it is also conceivable to choose the depth and the position of the piezoresistive resistors 125 in the semiconductor substrate as a function of the desired sensitivity.

When considering the overload protection, ie the protection of the force transducer against destruction by excessive force, the cavern depth can be adjusted accordingly. By specifying the cavern depth so inevitably the maximum deflection of the membrane is set. A possible embodiment of a force transducer according to the invention is shown in FIG. In this case, the force transducer, consisting of the semiconductor substrate 100, the piezoresistive resistors 125, the seed layer and the body 150 is housed in a housing 200. The electrical connections to the piezoresistive resistors 125 can by means of

Bonded connections 210 are led to contact points in the housing 200 or to an evaluation unit. In order to protect the electrical lines against environmental influences or contamination, it may optionally be provided to at least partially fill the housing 200 with a passivating or protective gel 220. As can be seen from the figure 2, it is provided to leave the body 150 so beyond the housing 200 that a

Plate 230 can be placed on the body. This plate 230 represents, for example, a finger-operated operating element, so that the body 150 can be pressed onto the semiconductor substrate 100 by means of a finger pressure on the operating element.

Possible areas of application for the present force transducer are in the area of

Telecommunications, for example in mobile telephones (mobile phones) as well as in the console area for (computer) games or terminals. The advantage of using the present invention is that a small size of the micromechanical pressure / force transducer can be achieved. Furthermore, a detectable voltage is generated by the force exerted on the body 150 in the piezoresistors 120. This eliminates a separate power supply of the controls.

In general, it can be provided that a combination of force transducer and evaluation unit can be used as a switch, for example by taking into account a threshold value in the detection of the voltage. Of course, you can also do several

Thresholds and multiple circuit settings when using a single force transducer.

In a further exemplary embodiment it can be provided that the signals are fed to a plurality of micromechanical force transducers of an evaluation unit. It can in

Depending on the detected signals of the force transducer, the evaluation unit to generate an acoustic and / or optical signal. Moreover, it is also possible that the evaluation unit uses the detected signals to control other devices. As shown in Figure 4, is provided in a further embodiment, the foot surface of the body 150 to be significantly larger than the lateral extent of the membrane 130. Also in this embodiment, a layer 140 is provided directly below the body 150 as a seed layer, it covers at least the entire foot surface of the body 150 so as to allow the best possible adhesion of the body 150 to the semiconductor substrate 100. While the piezoresistors 120 in the exemplary embodiment according to FIG. 1 are provided relatively far in the outer region of the diaphragm 130 above the cavern 110, the piezoresistors 125 are provided directly underneath the body 150. This arrangement is due to the lower deformation of the diaphragm 130 in the embodiment of Figure 3, so that the piezoresistors 125 must be brought close to the place of maximum deformation. Due to the lower deformation of the diaphragm 130, a pressure / force transducer of Figure 3 is less sensitive than a corresponding sensor of Figure 1. On the design of the ratio of lateral extent of the cavity 110, thickness of the membrane 130 and foot surface of the body 150 can be Adjust the sensitivity of the pressure / force transducer.

Claims

claims

1. A method for producing a micromechanical pressure / force transducer, wherein for the preparation of the method steps

- Generating a cavern (110) in a semiconductor substrate (100), and

- Creating a membrane (130) above the cavern (110), and

- Producing at least one piezoresistor (120, 125) in or on the membrane (130), characterized in that on the membrane (130), a body (150) is applied.

2. The method according to claim 1, characterized in that prior to the application of the body (150) on the surface of the semiconductor substrate in the region of the piezosensitive resistor (120, 125) an adhesive layer (140) is produced, wherein it is provided that the adhesive layer ( 140) rigidly connects the body (150) to the semiconductor substrate (100).

3. The method according to claim 1, characterized in that on the semiconductor substrate, a metal ball or a solder ball is applied, wherein it is provided in particular that the ball (150)

is flattened in the region of the ball support on the semiconductor substrate (100) and / or on the opposite side of the ball support before or after application.

4. The method according to claim 2, characterized in that for the production of the solder ball preferably in a micromechanical deposition method, a solder paste is applied before the solder paste in a remelting process to a solder ball in particular, it is provided that the solder paste comprises an alloy of 80% Au and 20% Sn.

5. The method according to claim 1, characterized in that for generating the cavern, the method steps

- Creating a porous etched cavity region in the semiconductor substrate and

- Performing a temperature treatment of the semiconductor substrate are carried out, it being provided that the temperature treatment forms the cavern and the preferably monocrystalline membrane.

6. micromechanical pressure / force transducer, in particular according to a manufacturing method of claims 1 to 5, with at least

a semiconductor substrate (100), and a cavern (110) in the semiconductor substrate (100), and

- A membrane (130) above the cavern, and

- At least one piezoresistor (120, 125) in or on the membrane (130), characterized in that on the membrane (130), a body (150) is applied.

7. A micromechanical pressure / force transducer according to claim 6, characterized in that the body (150) by means of an adhesive layer (140) fixed to the semiconductor substrate (100) is connected.

8. micromechanical pressure / force transducer according to claim 6, characterized in that on the semiconductor substrate, a metal ball or a solder ball is applied, wherein it is provided in particular that the ball (150) in the region of the ball support on the semiconductor substrate (100) and / / or -flattened on the ball support opposite side.

9. micromechanical pressure / force transducer according to claim 6, characterized in that the bearing surface of the body (150) on the semiconductor substrate (100) in the

Compared to below the body (150) located cavern (110) has a greater lateral extent.

lO.Mikromechanischer pressure / force transducer according to claim 6, characterized in that

- The semiconductor substrate silicon and / or

- The membrane is a monocrystalline semiconductor material and / or

- The body has an alloy of 80% Au and 20% Sn.

11.Vorrichtung with at least one micromechanical pressure / force transducer according to one of claims 6 to 10, wherein it is provided that

- The micromechanical pressure / force transducer is arranged in a housing (200), and - at least a part of the body is outside the housing, characterized in that an electrical signal which is generated by the action of force on the body, by means of a connecting line to one of the semiconductor substrate (100) separated

Evaluation unit is forwarded.

12.Vorrichtung with a plurality of micromechanical force transducers according to one of claims 6 to 11, characterized in that each micromechanical

Force transducer with the same force a different value of the electric

Signal supplies.

13. Device for use in a mobile telephone with a force transducer according to one of claims 6 to 12.

H.Vorrichtung for use in a toucpad with a force transducer according to any one of claims 6 to 12.