KR20080009093A - Micromechanical pressure/force sensor and corresponding production method - Google Patents

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 sensors and corresponding manufacturing methods {MICROMECHANICAL PRESSURE / FORCE SENSOR AND CORRESPONDING PRODUCTION METHOD}

In consumer electronics, for example, touch-sensitive screens are used in PDAs and smartphones, and touch-sensitive surfaces are used in place of mice in notebook computers. The possibility that this type of use can be implemented lies in the use of a wire grid that is inserted into the surface and allows reading of the coordinates of the pin or finger by blocking the electrical contact upon contact. However, this type of device does not measure the applied force respectively.

Another possibility consists in the construction of a surface supported on the force-sensitive element at the edge as a rigid plate. The element can be implemented by a pressure sensor. It can be blocked at the location of the pin or finger by the ratio of forces (crowd law). The sum of the forces represents the pressing force, which can be considered, for example, the line thickness. Devices in this manner can likewise be used to improve handwriting recognition.

The problem here is the robustness of the force sensor. While the force to be determined with an accuracy of about 1% occurs in normal use of touch pads or other operating elements up to about 5N, the sensor must be overload protected up to about 50N against breakage and the like.

The possibility of transferring pressure onto the membrane or pressure from the touch pad or from an operating element manipulated directly with a finger is due to the use of small steel balls that remain loosely centered on the membrane by appropriate construction techniques. However, in this configuration, the relatively incorrect centering of loose balls adversely affects measurement accuracy.

The pressure / force transducer or method of making the same is based on a pressure sensor manufactured by a conventional method of surface micromechanical mechanics and comprises a membrane over a (vacuum) cavity, for example several micrometers high. Since the cavity is closed and the side towards the membrane side of the cavity is not open as in the case of pressure sensors made with conventional KOH-etching techniques, a mechanical mechanical stop is provided on the substrate for the membrane. This results in a surely improved overload protection when used as a force sensor. The curvature can also be limited to only a few micrometers by the structure of the cavity, so that the density of the cavity is not damaged even in the event of an overload, since no gaps can occur in the film at the strength of the correspondingly formed film.

Known pressure sensors operate on the surface micromechanically mechanical principle of measuring capacitance and require an evaluation circuit in the immediate vicinity of the measuring capacitance. This means a larger chip surface for each pressure sensor that can be used as a force sensor and, for use as a touch pad, typically means four, at least three members. In contrast, the present invention operates on the principle of piezo sensitivity by at least one piezo resistor provided into or on the membrane. In the case of the measuring principle of this manner, the measured raw signal can be transmitted without problem through a longer centimeter line and processed by each evaluation circuit separated from the micromechanical pressure / force transducer. Also in this case preferably the signals of the plurality of pressure / force transducers can be jointly processed by the respective evaluation circuit or the evaluation unit. It also simplifies the adjustments that need to be done only once in terms of modules.

The essence of the present invention is that pressure or force is not transmitted directly to the piezo resistors, but rather via a body provided on the membrane, such as a metal ball or solder ball. The use of the body in this manner, which is present on the membrane, has the advantage that the pressure is defined and guided on the membrane relative to the previously inserted piezo resistor.

In the configuration of the present invention, it is provided that the body-membrane bond is secured by fixing the body with an adhesive layer on the film. Thus, displacement of the body relative to piezo resistance can be ruled out during manufacture and / or in the use of pressure / force transducers. Therefore, measurement inaccuracies due to deformations appearing differently by the displacement of the main body on the film can also be prevented.

The body or metal balls / solder balls may be provided flat on the semiconductor substrate in the region of the support to prevent displacement of the body. The smoothing operation can take place before or during the provision from the used ball onto the membrane. It may also be provided in the refinement of the invention that the plate is mounted onto the body. It has also turned out to be desirable to flatten the upper part, ie the part of the ball facing the ball support on the semiconductor substrate, in order to obtain as good a mechanical support as possible for the plate.

For the creation of cavities, it has proved to be particularly preferred that the area in the semiconductor substrate, preferably consisting of silicon, is etched porous by a corresponding etching process. The semiconductor material is then surrounded by temperature treatment in the region to create a cavity. It is preferred that a film is formed over the cavity simultaneously with the enveloping. By the simultaneous formation of the cavity and the film, the cavity can be provided to be formed almost in a vacuum. If the parameters of the manufacturing process are selected correspondingly, a single crystal or a wide range of single crystal films can be formed by temperature treatment.

Also, assembly of expensive balls on a semiconductor substrate or chip can be replaced by the provision of solder bumps known from flip chip technology. Thus, the sensitivity of the pressure sensor that can be achieved can be increased by about 50 N compared to known force transducers in the low pressure region with respect to resolution and accuracy.

In another refinement of the invention, the body used on the membrane comprises a larger support or base surface than the lateral extensions of the membrane. This configuration allows the portion of the force acting on the body to be directed into the peripheral region of the membrane, so that the force measuring region is enlarged upwards before the membrane is considered.

In the use of micromechanical masking that is matched to each other in subsequent processing steps of the micromechanical manufacturing process, the position of the body or solder balls can be defined very accurately. Thus, the surface on the semiconductor substrate where the deformation occurs is reduced. In addition, the assembly into the plastic housing is facilitated by the fixing of the body or solder balls on the seed layer.

The use of different solder pastes, such as Pb- or Pb-free solder, can adapt the mechanical mechanical properties of the ball, in particular the desired use strength of the ball. Possible solder bonds are allowed, for example, alloys consisting of about 80% gold and about 20% tin, since the compounds are particularly hard.

The operation of the control element can be recorded using the invention. Preferably, however, in addition to simple digital switch on / off, the pressing force of the operating element can be measured and the display and / or control / adjustment can be carried out according to the pressing force.

In another refinement of the invention a plurality of pressure / force transducers are provided for use in a common device. The output signals of all pressure / force transducers can also be provided to the common evaluation unit, which controls the visual and / or audio display according to the detected signal. Various pressure / force transducers may include different structures for differentiating signals, such that the same force action on the body can be provided to be implemented with different values of the electrical signal generated by the piezo resistors. It may also be considered here that the film thickness is changed. However, as the evaluation unit includes different thresholds for all pressure / force transducers, the operation or control of the various force / pressure transducers may be distinguished from each other.

Typical areas of application for the present invention are in the field of daily living, such as, for example, mobile telephones (cellphones), (computer) keyboards, paddles, touch pads, etc., for example high pressure fluctuations or particularly harmful to environmental media / No special design of the force sensor is required. Thus, the proposed pressure / force transducers can be manufactured inexpensively by standard methods of micromechanics without expensive additional process steps.

Other advantages are given in the description of the examples or the dependent claims below.

1 shows a typical structure of a pressure / force transducer according to the present invention.

Figure 2 shows the force action leading to the maximum stop of the membrane.

3 illustrates a special embodiment.

4 shows the structure of the pressure / force transducer inside the housing.

By means of the invention, the operation of the operating element, as typically used in a keyboard or touch pad, can be measured using a pressure sensor made of a micromechanical structure based on piezo technology. In addition to the operation of the operating element as a digital on-off switch, direct measurement of the pressing force of the operating element is also possible. The configuration according to the invention thus allows the pressing force to be measured very accurately with an accuracy of about 1%. In this way an accurate measurement of the pressing force makes it possible to control the audio or visual display and other device units according to the measured parameters of the pressing force.

The starting point for the production of pressure / force transducers according to the invention is pressure sensors made by conventional micromechanical methods, for example known from DE 100 32 579 A1 or DE 101 14 036 A1. In this way a pressure sensor can be produced and the pressure sensor preferably comprises a film in the semiconductor substrate consisting of silicon and present on the cavity. It is also known, for example, from DE 101 35 216 A1 or DE 2004 007518 A1 that piezo resistors are produced in the membrane or in an additional layer on the membrane.

As shown in FIG. 1, in an advantageous embodiment the cavity 110 and the membrane 130 according to the method of manufacturing DE 100 32 579 A1 use subsequent temperature steps in the semiconductor substrate 100 under the formation of porous silicon. Is generated. Then onto the membrane 120 a piezo resistor 120 corresponding to DE 101 35 216 A1 or a wire strain gauge described in DE 10 2004 007518 A1 is provided by a masking step, an epitaxy step and a structuring step. . In order to achieve improved adhesion of the body 150 on the surface of the semiconductor substrate 100 and on the film 130, the adhesion region to the body 150 on the film 130 may be galvanized. Process steps in this manner produce a layer 140 that is represented as a seed layer. In order to enable particularly good adhesion of the body 150 on the substrate 100 or the film 130, the adhesion may be formed to be adapted to the material of the body 150 as well as the material of the film 130.

The piezo sensitive layer is first applied and then structured for the creation of the piezo resistive resistor 120. The application and structuring thus also takes place in a micromechanical standard method by means of corresponding masking, which allows for the precise positioning of the method steps successively following one another. Thereby, for example, the layer 140 produced after the piezo resistive resistor 125 of the same side on the substrate 100 can be positioned very accurately compared to the piezo resistive resistor 120. If a solder ball is used as the body 150, that is, a material that can simply be provided to the surface in a conventional micromechanical manner, the ball 150 is likewise associated with the resistor 120 by the use of masking techniques. Can be set fairly accurately. The orientation of the ball in this manner with respect to the piezo resistor 120 can significantly improve the accuracy of the force transducer.

However, in addition to the use of solder balls, metal balls may be located in layer 140. In this case, however, it should be noted that a positioning method should be chosen that allows for a similarly good orientation of the metal balls relative to the piezoresistive resistor 120. Care should also be taken to ensure that the metal balls are firmly connected with the semiconductor substrate 100 or layer 140 to prevent displacement of the balls during operation and thus alteration of the generated signals.

The meaning and purpose of the adhesive layer 140 in a semiconductor substrate is to prevent sliding of the body during manufacture and / or operation of the pressure / force transducer. In addition to the configuration of the main body as the ball 150, it is also proposed to use a main body similar to a pin. Forces or pressures of the sources spaced by the pin-like body may continue to be transmitted onto the pressure / force transducer. In general, by the body 150 and likewise by the plate 230, a relatively large area of the operating element on the body 150 can be connected with a relatively small effective area force receiving portion. Thus, for example, the operating element may comprise the area of the finger tip, whereas the pressure / force transducer belonging thereto forms only the fragmented portion of the area.

Corresponding regions may also be galvanized on the semiconductor substrate 100 to produce the layer 140 shown as the seed layer. Solder paste may also be applied onto the semiconductor substrate 100 by silk screen printing methods or other suitable manufacturing methods from flip chip technology. As a result, the solder paste is melted so that only the desired plane is applied and the solder balls are formed corresponding to the drawing 150 according to the amount of solder applied.

In other embodiments, the body 150 may be provided flat on the side that adheres to the seed layer 140 (see, eg, FIG. 1A). In order to prevent plastic deformation of the body 150 during operation when using the solder, the solder ball may be pre-compressed using the plate 230 at the time of assembly so that the solder ball is smooth on the top surface.

1 and 2 show a typical progression of the force transducer operation. 1A shows the force transducer in an inoperative state. In contrast, FIG. 1B shows a state in which the force 160 acting on the main body 150 is sufficient and the stop 130 is formed by the cavity base by the maximum bending of the membrane 130. Similarly, the piezo resistive resistor 120 is deformed by the curvature, thereby producing a measurable dislocation in the form of stress. The measured stress can then be assigned to the action 160 of the force in the corresponding evaluation unit.

In order to set the sensitivity of the pressure / force transducer, the semiconductor material or the thickness of the film 130 may be correspondingly selected or processed. Thus, it may be considered that the material may be deformed to some extent with the use of a pressure / force transducer. According to another configuration, the membrane 130 is formed into a single crystal, for example as a result of cavity formation with porous silicon. In addition, the depth and position of the piezoresistive resistor 125 in the semiconductor substrate may be selected according to the desired sensitivity.

When considering overload protection, ie protection of the force transducer from breakage by very large force action, the cavity depth can be adapted accordingly. The maximum curvature of the membrane is forcibly determined by the presetting of the cavity depth.

3 shows a possible configuration of the force transducer according to the present invention. In this case, a force transducer composed of the semiconductor substrate 100, the piezo resistive resistor 125, the seed layer and the main body 150 is accommodated in the housing 200. Thus, the electrical connection to the piezoresistive resistor 125 may be guided by the mating connection 210 to the contact point in the housing 200 or to the evaluation unit. In order to protect the electrical line from environmental influences or contamination, it may optionally be provided that the housing 200 is at least partially filled with a passivating gel or protective gel 220. As can be seen in FIG. 2, the body 150 is provided above the housing 200 so that the plate 230 can be positioned on the body. Since the plate 230 represents a manipulation element operable with a finger, for example, the body 150 may be pressed onto the semiconductor substrate 100 by applying a finger pressure to the manipulation element.

Possible areas of use for the force transducer are paddle areas for telecommunications applications, such as mobile phones (cellphones) and (computer) games or terminals. An advantage of the present invention is that a small size assembly structure of the micromechanical pressure / force transducer can be achieved. In addition, a measurable stress is generated in the piezo resistor 120 by the force applied to the body 150. Thus a separate voltage supply of the operating element is omitted.

In general a combination consisting of a force transducer and an evaluation unit can be provided for use as a switch in the detection of a voltage, for example by consideration of a threshold. It is also apparent that a plurality of thresholds and a plurality of circuit settings can be considered in the use of each force transducer.

In another embodiment, it may be provided that the signals of the plurality of micromechanical force transducers are supplied to the evaluation unit. In addition, the evaluation unit may generate an acoustic and / or visual signal in accordance with the measured signal of the force transducer. The evaluation unit may also use the measured signal for the control of another device.

As shown in FIG. 4, in another embodiment the base surface of the body 150 is provided with a greater than the lateral extension of the membrane 130. Also in an embodiment a layer 140 as seed layer covering at least the entire base surface of the body 150 is provided directly under the body 150, which provides a possible good adhesion of the body 150 to the semiconductor substrate 100. To make it possible. In the embodiment according to FIG. 1, the piezo resistor 120 is provided relatively broadly above the cavity 110 in the outer region of the membrane 130, while the piezo resistor 125 is provided directly below the body 150. Since this arrangement is based on the slight deformation of the membrane 130 in the embodiment of FIG. 3, the piezo resistor 125 must be provided near the position of maximum strain. Due to the slight deformation of the membrane 130, the pressure / force transducer according to FIG. 3 is less sensitive than the corresponding sensor according to FIG. 1. The sensitivity of the pressure / force transducer may be set by the ratio of the lateral extensions of the cavity 110, the thickness of the membrane 130, and the configuration of the base surface of the body 150.

Claims (14)

Creating a cavity 100 in the semiconductor substrate 100, creating a film 130 over the cavity 100, at least one piezo resistor 120, 125 in or on the film 130. A method of manufacturing a micromechanical pressure / force transducer, comprising the step of producing: A method of making a micromechanical pressure / force transducer, characterized in that a body (150) is provided on the membrane (130). The method of claim 1, wherein an adhesive layer 140 is formed in the piezo-sensitive resistors 120, 125 region on the upper surface of the semiconductor substrate prior to providing the body 150, and the body 150 is bonded to the semiconductor substrate 100. A method of making a micromechanical pressure / force transducer, characterized in that an adhesive layer (140) is provided to securely connect. 2. The metal ball or solder ball is provided on the semiconductor substrate, in particular the ball is in the region of the ball support on the semiconductor substrate 100 and / or on the side facing the ball support. A method of making a micromechanical pressure / force transducer, characterized in that it is provided to be smooth before or after provision. The solder paste of claim 2, wherein before the solder paste is formed into the solder ball in the melting process, the solder paste is preferably provided by a micromechanical deposition method for the production of the solder ball, in particular the solder paste is 80% gold and 20% Method of producing a micromechanical pressure / force transducer, characterized in that it comprises an alloy consisting of tin. The method of claim 1, wherein a step of generating a porous nicked cavity region and executing a temperature treatment of the semiconductor substrate are performed to create a cavity, wherein the temperature treatment is provided to form a cavity and preferably a single crystal film. Method for producing a micromechanical pressure / force transducer, characterized in that. In particular, at least one semiconductor substrate 100, one cavity 110 in the semiconductor 100 substrate, one film 130 on the cavity, and at least one piezo in or on the film 130 A micromechanical pressure / force transducer according to the method of any of claims 1 to 5 having resistors 120 and 125, Micromechanical pressure / force transducer, characterized in that the body (150) is provided on the membrane (130). 7. The micromechanical pressure / force transducer of claim 6, wherein the body (150) is firmly connected to the semiconductor substrate (100) by an adhesive layer (140). A metal ball or solder ball is provided on the semiconductor substrate, in particular the ball 150 being smooth on the semiconductor substrate 100 in the area of the ball support and / or on the side facing the ball support. Micromechanical pressure / force transducer, characterized in that provided. 7. The micromechanical pressure / force of claim 6, wherein the support surface of the body 150 includes a larger lateral extension on the semiconductor substrate 100 compared to the cavity 110 below the body 150. converter. 7. The micromechanical pressure / force of claim 6, wherein the semiconductor substrate comprises silicon and / or filmed single crystal semiconductor material and / or the body comprises an alloy of 80% gold and 20% tin. converter. At least one micromechanical pressure / force transducer according to any one of claims 6 to 10, wherein a micromechanical pressure / force transducer is disposed within the housing 200 and at least one portion of the body is external to the housing. In a device having a transducer, Wherein the electrical signal produced by the action of the force on the body is transmitted by means of a connecting conduit to an evaluation unit separate from the semiconductor substrate (100). 12. An apparatus with a plurality of micromechanical force transducers according to any one of claims 6 to 11, characterized in that each micromechanical force transducer delivers different values of electrical signals upon the same force action. An apparatus for use in a mobile telephone as claimed in claim 6. The device according to any one of claims 6 to 12, for using a force transducer in a touch pad.

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