Analog data-input device provided with a microelectromechanical pressure sensor

The present invention relates to an analog data-input device provided with a pressure sensor of a microelectromechanical type, in particular for use in a portable apparatus, such as a mobile phone, to which the ensuing description will make explicit reference, without this implying any loss of generality.

As is known, latest-generation mobile phones offer a plurality of advanced functions, such as e-mail and internet managing, displaying of electronic documents, acquiring and displaying of images, in addition to "standard" functions, such as managing of telephone books, phone calls and text messages. Graphic user interfaces (GUIs) enable simple and effective management of the various functions, via data-input devices (generally called trackpoints®) integrated in the mobile phones. The data-input devices enable scrolling of lists, making of selections, moving of a cursor displayed on the screen, or in general generation of actions within the graphic interface.

Data-input devices generally comprise an actuator element that can be actuated by a user, and a sensor mechanically coupled to the actuator element to detect its actuation and generate corresponding electrical signals. Such electrical signals, possibly amplified and filtered, are acquired by an electronic control circuit of the mobile phone, which thus generates the corresponding action within the graphic interface (for example, cursor displacement, or scrolling of a list).

In detail, the actuator element comprises one or more push-buttons, for example four arranged to form a cross. A direction of displacement within the graphic interface (for example, "Up", "Down", "Right", "Left") corresponds to each one of the push-buttons. Alternatively, instead of the push-buttons, the actuator element can comprise a single pin-shaped element (joystick), which is mobile in a number of directions.

The sensor comprises one or more sensitive elements, which can be of a digital type, or of an analog type. The sensitive elements of a digital type are switches, which are mechanically coupled to the actuator elements and close upon their actuation (for example, upon the pressure of a corresponding push-button, or else upon the displacement of the joystick in the corresponding direction). The sensitive elements of an analog type are piezoelectric or piezoresistive and comprise a mechanical element and an interface electronic circuit external to the mechanical element. The mechanical element undergoes a deformation following upon actuation of the actuator elements and generates an electrical quantity corresponding to the undergone deformation (a variation of electrical charge or of a resistivity). The interface electronic circuit generally comprises charge-amplifier circuits (in the case of piezoelectric sensitive elements), or bridge circuits (in the case of piezoresistive sensitive elements), and generates an electrical signal proportional to the deformation undergone by the mechanical element, which is acquired by the control circuit of the mobile phone.

If the data-input devices comprise sensitive elements of a digital type, the only information available to the control circuit of the mobile phone is the binary closing or opening state of the switches. Consequently, it is not possible to have a flexible control of the corresponding actions within the graphic interface; for example, it is possible to generate a displacement of a cursor in one or more directions, but it is not possible to regulate its speed of displacement. For this reason, operations such as the scrolling of a phone book, or else the zoom of an image are particularly laborious and far from immediate in so far as they require prolonged pressure on one and the same push-button, or else the displacement of the joystick in the same direction for a long time interval.

Instead, if the data-input devices comprise sensitive elements of an analog type, the control circuit of the mobile phone receives not only the information regarding the detection of an actuation of an actuator element, but also the information regarding the intensity of said actuation. Consequently, the control circuit provides a more flexible control of the actions generated within the graphic interface. For example, not only does it impart on the cursor a displacement in one or more directions, but also regulates its speed of displacement on the basis of the force with which the actuator elements have been actuated (and hence the amount of the corresponding deformation of the sensitive elements).

Known data-input devices comprising sensitive elements of an analog type have, however, the disadvantage of entailing a greater occupation of area and a greater complexity of implementation, both due to the presence of the mechanical element and the interface electronic circuit external to the mechanical element, and to the need for providing the corresponding electrical connections. Consequently, said devices are not particularly suited to integration in portable apparatuses, such as mobile phones.

US-A-5 844 287 discloses an electronic fingerprint sensor, which detects a fingerprint based on a greater pressure exerted by the ridge lines than by the valleys of the finger. The sensor includes a matrix of integrated pressure microsensors and a signal processing circuit, both made in monolithic form. A flexible protective layer is provided over the matrix of pressure microsensors, to interact with the finger.

Susumu Sugiyama et al: "A 32x32 (1k)-element silicon pressure-sensor array with CMOS processing circuits" Electronics & Communications in Japan, Part II - Electronics, Scripta Technica. New York, US, vol. 75, no. 1, January 1992 (1992-01), pages 64-75, XP000312022 ISSN: 8756-663X discloses a silicon pressure sensor array with integrated CMOS signal-processing circuits, that enables detection of a high-resolution pressure distribution. The pressure sensor array consists of an X-Y matrix-organized array of pressure-sensing cells and CMOS signal processing circuits formed around the array on one and the same chip.

WO 98/25115 discloses a micromechanical sensor for recording fingerprints, comprising an array of individual sensors, each provided with a micromechanical membrane. The membranes are provided with an actuation element to establish a contact with a supporting surface, and are made to vibrate by a driving circuit. The vibrations of the membranes are modified according to a pressure exerted by a finger exerting a pressure on the array, and this modification is indicative of the ridges or furrows of the finger structure.

JP 2001 174350 A discloses a pressure detecting device comprising a flexible membrane adapted to receive an external pressure to be detected and a semiconductor sensor, mechanically coupled to the membrane via a silicon oil. The semiconductor sensor is a microelectromechanical pressure sensor including a diaphragm and transducer elements integrated in the diaphragm.

US-A-5 661 245 discloses a force sensor assembly including a chamber filled with gel and housing a semiconductor pressure sensor. A flexible membrane coupled to a plunger key closes the chamber at the top and is configured to transmit an external force to the semiconductor sensor via the gel.

WO 00/68930 A1 discloses an electronic device including a user depressible surface with associated analog discrete pressure sensitive elements, mounted on a PCB together with a corresponding ASIC chip.

US 2004/112138 A1 discloses a load cell for measuring pressure, including MEMS sensor chips separately mounted on a common printed circuit board, on which an elastomer distributes an uniform pressure.

LAL R et al. "MEMS : technology, design, CAD and applications", Design Automation Conference, 2002. Proceedings of ASP-DAC 2002. 7th Asia and South Pacific and the 15th International Conference on VLSI Design., Proceedings. Bangalore, India 7-11 Jan. 2002, LOS Alamitos, CA, USA, IEEE Comput. Soc. US, 7 January 2002, pages 24-25 suggests the use of MEMS sensors in different fields of applications, among which that of portable devices.

The aim of the present invention is consequently to provide a data-input device that will enable the aforementioned problems and disadvantages to be overcome.

According to the present invention a data-input device is consequently provided, as defined in Claim 1.

For a better understanding of the present invention, there are now described preferred embodiments thereof, purely by way of non-limiting example and with reference to the attached drawings, wherein:

• Figure 1 shows a portable apparatus, in particular a mobile phone, comprising a data-input device;
• Figure 2 shows a schematic top view of a die of semiconductor material of a pressure sensor belonging to the data-input device of Figure 1, made according to a first embodiment of the present invention;
• Figure 3 shows a top view of a wafer of semiconductor material in an initial step of a process for manufacturing the die of Figure 2;
• Figure 4 shows a cross section at an enlarged scale of details of Figure 3;
• Figures 5-7 show cross sections through the wafer of Figure 3 in subsequent steps of the manufacturing process;
• Figure 8 shows a cross section of the pressure sensor of Figure 2, with the die included in a package;
• Figure 9 shows a cross section of the data-input device of Figure 1, according to one embodiment of the present invention;
• Figure 10 shows the circuit diagram of an interface electronic circuit belonging to the pressure sensor of Figure 2;
• Figure 11 shows a cross section of the data-input device of Figure 1 according to a different embodiment of the present invention; and
• Figures 12-14 show schematic top views of further embodiments of the pressure sensor die.

As shown in Figure 1, a portable apparatus, in particular a mobile phone, designated as a whole by 1, comprises a display 2, a plurality of function keys 3, and a data-input device 4. In a per se known manner, the function keys 3 enable execution of standard functions of the mobile phone 1, such as for example dialling of telephone numbers or composition of text messages, and on the display 2 a graphic interface 5 is displayed, made up of a plurality of icons, to each of which corresponds a given function (or set of functions) of the mobile phone 1. The data-input device 4 is, for example, located in a central portion of the body of the mobile phone 1, underneath the display 2, and enables a user to interact with the graphic interface 5. In particular, the data-input device 4 enables generation of displacement actions within the graphic interface 5 and selection and activation of particular functions of the mobile phone 1; moreover, the data-input device 4 controls movement of a cursor (not illustrated) in the display 2 in given operating conditions of the mobile phone 1.

The data-input device 4 comprises an actuator element 6, which is manually actuated by the user, and a pressure sensor 9 (see also Figures 2, 8 and 9), which is mechanically coupled to the actuator element 6 and detects its actuation, generating corresponding electrical signals. In particular, the pressure sensor 9 is located underneath the actuator element 6 so as to undergo mechanical deformations upon the actuation of the actuator element 6. In the example of Figure 1, the actuator element 6 comprises four push-buttons 8 arranged to form a cross, and corresponding to the four directions "Up", "Down", "Right", "Left" of generation of displacements within the graphic interface 5.

According to an embodiment of the present invention, illustrated schematically in Figure 2, the pressure sensor 9 comprises a die 10 of semiconductor material, in particular silicon, housing four sensitive elements 11, in particular of a microelectromechanical type, and an interface electronic circuit 12 connected to the sensitive elements 11. Connection pads 13 are provided on a surface of the die 10 for electrical connection of the sensitive elements 11 with an electronic control circuit (not shown) of the mobile phone 1. The sensitive elements 11 are arranged in a way corresponding to the push-buttons 8 of the actuator element 6, and thus, in the example, are arranged to form a cross. In addition, each sensitive element 11 detects, in a preferential way, the actuation of a corresponding push-button 8, in the sense that it supplies a maximum output when the corresponding push-button 8 is actuated. The interface electronic circuit 12 is formed in a region of the die 10 not occupied by sensitive elements 11; in the example, it is formed in a central position with respect to the sensitive elements 11 (but it is evident that other locations can be envisaged).

The process for manufacturing the die 10 is based upon the process described in the patent application EP-A-1 324 382, for manufacturing a SOI wafer, and the process described in the European patent application 04 425 197.3, filed in the name of the present applicant on March 19, 2004, for manufacturing a pressure sensor.

In detail, Figure 3, in an initial step of the manufacturing process, made on top of a wafer 15 of semiconductor material, for example silicon, comprising a substrate 16, for example of an N type, is a resist mask 17 (see also the cross section of Figure 4). The mask 17 has areas 18 having an approximately square shape, each comprising a plurality of hexagonal mask portions 17a that define a honeycomb lattice (as may be seen in the enlarged detail of Figure 3). For example, the distance t between opposite sides of the mask portions 17a is equal to 2 µm, whilst the distance d between facing sides of adjacent mask portions 17a is equal to 1 µm. In particular, the number of areas 18 of the mask 17 corresponds to the desired number of sensitive elements 11, and their arrangement on the surface of the wafer 15 corresponds to the desired arrangement of the sensitive elements 11 (in Figure 3 just two areas 18 are illustrated, for example corresponding to the sensitive elements 11 related to the "Right" and "Left" displacement directions within the graphic interface 5).

Then (Figure 4), using the mask 17, an anisotropic chemical etch of the substrate 16 is performed, following upon which trenches 19 are formed, having, for example, a depth of 10 µm, which are intercommunicating and delimit a plurality of silicon columns 20. In practice, the trenches 19 form an open region 21 of complex shape (corresponding to the honeycomb lattice of the mask 17), wherein the columns 20 (having a shape corresponding to the mask portions 17a) extend.

Next, the mask 17 is removed, and an epitaxial growth is performed in a deoxidizing environment (typically, in an atmosphere with a high concentration of hydrogen, preferably with trichlorosilane-SiHCl₃). Consequently, an epitaxial layer, for example of an N type and of thickness equal to 9 µm, grows above the columns 20 and closes the open region 21 at the top. Then a step of thermal annealing is performed, for example for 30 minutes at 1190°C, preferably in an atmosphere of hydrogen, or, alternatively, of nitrogen. As discussed in the aforementioned patent applications, the annealing step causes a migration of the silicon atoms, which tend to move into the position of lower energy. Consequently, and also thanks to the close distance between the columns 20, the silicon atoms migrate completely from the portions of the columns 20 within the open region 21, and a buried cavity 24 is formed (Figure 5), having a side for example equal to 500 µm. Above the buried cavity 24 a thin layer of silicon remains, which is constituted in part by epitaxially grown silicon atoms and in part by migrated silicon atoms, and forms a diaphragm 25. The diaphragm 25 is flexible and can deflect in the presence of external stresses. At the end of this step, as many diaphragms 25 are formed as are the areas 18 of the mask 17, which were previously defined (once again, just two are shown in Figure 5).

Next (Figure 6), piezoresistive elements 26 are formed in a surface portion of the diaphragm 25 opposite to the buried cavity 24, in particular at central peripheral portions of the diaphragm 25. In detail, the piezoresistive elements 26 are formed by P-type diffusion or implantation, for example of boron atoms, and are connected to one another in a Wheatstone-bridge configuration (in a way not illustrated in Figure 6). At the end of this step, the sensitive elements 11, each of which comprises a diaphragm 25 suspended above a buried cavity 24, and corresponding piezoresistive elements 26 are then formed.

As illustrated in Figure 7, the interface electronic circuit 12 is formed in the region of the wafer 15 comprised between two aligned sensitive elements 11. In particular, the interface electronic circuit 12 (by way of example, in Figure 7 just one bipolar transistor comprising a well region 29, a collector region 30, a base region 31, and an emitter region 32 is shown) is formed using manufacturing steps which are in common with those of the manufacturing process of the sensitive elements 11. Electrical insulation regions (not illustrated) can be provided for electrically insulating the interface electronic circuit 12 from the sensitive elements 11.

In a final step of the manufacturing process, the wafer 15 is then cut so as to obtain the die 10. In addition, the connection pads 13 are formed, for example via metallic deposition, and the corresponding connections with the interface electronic circuit 12 are formed.

As illustrated in Figure 8, the pressure sensor 9 further comprises a package 35, made for example of ceramic material, which includes the die 10. In particular, the die 10 is arranged in an open chamber 36 of the package 35; in particular, it is bonded to a bottom internal surface of the chamber 36 via a layer of adhesive material 38. The chamber 36 is filled with a coating gel 37, made up of an elastomer, for example a silicone elastomer, having a low Young's modulus, and is closed at the top by a membrane 39 made of flexible plastic material, which delimits a main top surface of the package 35. The coating gel 37 can, for example, be a silicone gel produced by the Company Dow Corning®.

The electrical connection between the connection pads 13 and the outside of the package 35 is made via metal leads 40, which are connected to the connection pads 13, inside the package 35, by means of wires 41.

The membrane 39, together with the coating gel 37, is an interface between the actuator element 6 (and in particular the push-buttons 8) and the sensitive elements 11, and enables transfer of the pressure deriving from the actuation of the actuator element 6 to the diaphragms 25 of the sensitive elements 11. In particular, the coating gel 37 protects the sensitive elements 11 from the external environment and from the direct pressure exerted by the user, which could cause damage thereto.

Figure 9 shows in more detail a possible embodiment of the data-input device 4. In particular, the actuator element 6 comprises a flexible structure 42, shaped to form keys so as to provide the push-buttons 8, arranged in contact with the membrane 39 of the pressure sensor 9 at a window 43 made in a casing 44 of the mobile phone 1. In particular, the flexible structure 42 is squeezed between an external portion 45 and an internal portion 46 of the casing 44, and the pressure sensor 9 is fixed to a printed circuit 47 of the mobile phone 1 via the metal leads 40.

Figure 10 shows in detail a possible embodiment of the interface electronic circuit 12.

The interface electronic circuit 12 comprises a number of circuit branches 50 equal to the number of sensitive elements 11 (represented here by the Wheatstone bridge formed by the corresponding piezoresistive elements 26). Each circuit branch 50 comprises an amplifier stage 51, comprising an instrumentation amplifier, which receives an unbalancing signal produced by the Wheatstone bridge of the respective sensitive element 11, and an analog-to-digital converter stage (ADC) 52 connected to the output of the instrumentation-amplifier stage 51, which receives the amplified signal and converts it into a digital signal. The interface electronic circuit 12 has a first output 54a of an analog type, constituted by the output of the amplifier stage 51, and a second output 54b of a digital type, constituted by the output of the analog-to-digital converter stage 52, in particular an eight-level digital signal (3 bits). Both of the outputs 54a, 54b are connected to the connection pads 13 and thereby to the control circuit of the mobile phone 1.

Operation of the data-input device 4 is the following.

Upon actuation of one of the push-buttons 8 of the actuator element 6 (i.e. upon exerting a pressure thereon), a pressure is applied to the membrane 39 of the pressure sensor 9, which is transferred, via the coating gel 37, in a preferential way, to the diaphragm 25 of the corresponding sensitive element 11 (i.e., the one arranged in a position corresponding to the push-button 8 that has been actuated). Consequently, the diaphragm 25 undergoes a deformation, causing a variation in the resistivity of the piezoresistive elements 26 and so an unbalancing of the Wheatstone bridge, which is detected by the interface electronic circuit 12, which generates corresponding output signals. Actually, also the diaphragms 25 of adjacent sensitive elements 11 undergo a certain deformation (in particular if the sensitive elements 11 are close to each another), which is in any case of a smaller amount with respect to the deformation of the diaphragm 25 of the sensitive element 11 corresponding to the actuated push-button 8. The electronic control circuit of the mobile phone 1 then receives the output signals from the interface electronic circuit 12 and determines the action to be generated within the graphic interface 5 according to the relation between the various signals received (in particular, it generates a displacement in the direction corresponding to the sensitive element 11 that has undergone the greatest deformation). Furthermore, the speed of said displacement is a function of the value of the actuation pressure of the actuator element 6. In fact, the greater the actuation pressure, the greater the deformations of the diaphragm 25 of the corresponding sensitive element 11, the unbalancing of the Wheatstone bridge, and consequently the output signals sent to the electronic control circuit of the mobile phone 1.

The advantages of the data-input device according to the present invention are clear from the foregoing description.

It is, in any case, emphasized that the integration in a single die 10 of the sensitive elements 11 and of the corresponding interface electronic circuit 12 enables a reduced area occupation and a simpler assembly, in so far as it is no longer necessary to envisage a purposely provided wiring between the sensitive elements and an external interface electronics.

In addition, the sensitive elements 11 detect the intensity of the pressure acting on the push-buttons 8 so that the control circuit of the mobile phone not only determines the corresponding action to be generated within the graphic interface, but, for example, also the speed thereof. This makes the use of the graphic interface of the mobile phone much more flexible and practical for the user.

Finally, it is clear that modifications and variations may be made to the data-input device described and illustrated herein, without thereby departing from the scope of the present invention, as defined in the attached claims.

In particular, as will be evident to the person skilled in the art, the shape and the structure of the actuator element 6 can be different. In particular, as illustrated in Figure 11, the flexible structure 42 may not be provided. In this case, the actuator element 6 is made by pressure areas defined directly on the surface of the membrane 39 of the pressure sensor 9 (for example, highlighted by purposely provided writings or symbols), which are directly accessible to the user through the window 43 in the casing 44 of the mobile phone 1. Alternatively, the actuator element 6 can comprise a single pin-shaped element (joystick), which is arranged in a central position with respect to the membrane 39 within the window 43 and is free to move with a number of degrees of freedom (in particular, as many as are the directions of displacement generation within the graphic interface 5).

Furthermore, a different number of sensitive elements 11 can be provided. For example, according to a further embodiment of the present invention, shown in Figure 12, the sensor 9 comprises an additional sensitive element 11 (for a total of five sensitive elements 11), integrated in the die 10, and arranged at the centre of the cross formed by the other sensitive elements 11. Pressure on the further sensitive element 11 can, for example, generate selection operations, or else zooming of an image displayed on the display 2. In this case, the interface electronic circuit 12 is arranged as a frame around said further sensitive element 11. Alternatively, as illustrated even more schematically in Figure 13, the die 10 can house a total of nine sensitive elements 11 of a microelectromechanical type, arranged in a regular way in an array of three rows and three columns. This configuration of the sensitive elements 11 enables generation of displacements also diagonally within the graphic interface 5. The sensitive element 11 in a central position once again can have the function of performing selections, or else zooming of an image. Even in this case, to a greater pressure corresponds a greater execution speed of the corresponding action in the graphic interface 5. As illustrated in Figure 14, the die 10 can even house just three sensitive elements 11, arranged in a triangle.

In addition, in another embodiment not claimed by the present invention, the sensitive elements 11 can each be integrated within a respective die of semiconductor material, possibly with a corresponding interface electronic circuit (corresponding to one of the circuit branches 50 of the interface electronic circuit 12). In this case, the various dies can be included within a single package having a structure similar to that of Figure 8, with electrical connections provided between the connection pads of the individual dies and the outside of the package.

In addition, the interface electronic circuit 12 can comprise further electronic components (not illustrated), for carrying out further processing operations on the output signals, for example for making a comparison thereof and determining the direction of displacement to be generated within the graphic interface 5.

Finally, the data-input device described herein can advantageously be used in any portable electronic apparatus provided with a display and a graphic interface with which it is necessary to interact, for example in a portable personal computer, a PDA, a game-computer console, etc., or else, in a remote control.