WO2010112676A1

WO2010112676A1 - A casing of a sensor measuring oscillations, and a sensor for measuring oscillations

Info

Publication number: WO2010112676A1
Application number: PCT/FI2010/050247
Authority: WO
Inventors: Teuvo SILLANPÄÄ; Anu KÄRKKÄINEN; Panu Pekko
Original assignee: Valtion Teknillinen Tutkimuskeskus
Priority date: 2009-03-30
Filing date: 2010-03-29
Publication date: 2010-10-07
Also published as: FI20095340A0

Abstract

Oscillations are measured by an electromechanical sensor having a first surface with a groove (502) therein to separate a kinetic mass part (503) and a frame part (504) from each other. A casing encloses a sensor chip (112) and a circuit element (113, 322) for detecting a phenomenon occurring as the kinetic mass part moves. The sensor chip (112) is attached to an inner surface (506, 706) of the casing at the frame part side of the groove. The first surface (501) of the sensor chip, the inner surface (506, 706) of the casing, and the attachment (507, 707) of the sensor chip define a closed space into which the groove (502) opens.

Description

A casing of a sensor measuring oscillations, and a sensor for measuring oscillations

FIELD OF TECHNOLOGY

The present invention relates in general to casings of sensors. In particular, the invention relates to casings of micro- or nano-sized electromechanical sensors used for measuring oscillations such as acoustic emission.

BACKGROUND

Measurement of oscillations propagating inside structures gives useful information concerning not only the kinetic state of the structure but also its internal properties and structural changes. Measurement of acoustic emissions, for instance, is known to be a very useful method for monitoring the conditions of structures. Acoustic emission refers to an ultrasonic wave generated by a permanent and irreversible release of elastic energy inside a structure in which a crack begins to develop, for example. Structural damages which are detected may be leaks or bearing damages, for example. The frequencies involved are typically over 4O kHz₁ or between 100 kHz and 1 MHz according to some sources. Acoustic emission receivers are typically sensors in which the actual element which receives oscillation and converts it into an electric signal is based on microelectromechanical system (MEMS) technology. Sensor design depends on the frequency of the oscillation measured; at very high frequencies the sensor may be based on nanoelectromechanical system (NEMS) technology.

MEMS technology and acoustic emission measurements are discussed e.g. in W. Wu, D.W. Greve and I. Oppenheim: "Characterization and Noise Analysis of Capacitive MEMS Acoustic Emission Transducers", IEEE Sensors 2007 Conference, pp. 1152-1155, and in D. Ozevin, SP. Pessiki, D.W. Greve and I. Oppenheim: "A MEMS transducer for detection of acoustic emission events", IEEE Sensors 2005 Conference, pp. 776-779. Known MEMS sensors used for measuring acoustic emissions are typically packaged in steel casings. Casings are often hand-made, and the manufacturing costs of such sensors may be very high. Mass production of sensors and their large-scale use in industrial applications is not cost-effective unless the unit cost of a finished sensor be significantly brought down.

MEMS sensors tuned to frequencies lower than acoustic emission can be used for monitoring the kinetic state of an object, for example, or in other applications utilizing oscillations propagating in structures.

GENERAL DESCRIPTION OF THE INVENTION

An object of the present invention is to provide a packaging technique and a casing which enable low-cost manufacture of an electromechanical sensor used for measuring acoustic emission or other oscillation. Another object of the invention is that the industrial manufacture of electromechanical sensors used for measuring acoustic emission or other oscillation becomes cost-effective and suitable for an automated process. A further object of the invention is that an encased electromechanical sensor according to the invention is reliable, has a long life, and can be readily applied to different measurement tasks.

The objects of the invention are achieved by using the same type of technology which is known from casings of integrated circuits and which can be automated to a large extent also for the electromechanical sensor suitable for measuring acoustic emission or other oscillation. A sensor chip is preferably attached to the casing so as to constitute a sealed structure together with the casing material.

A sensor according to the invention is characterised by that which is specified in the characterizing part of the independent claim directed to a sensor.

The invention also concerns a method for manufacturing a sensor. A method according to the invention is characterized by that which is specified in the characterizing part of the independent claim directed to a method. BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 shows the structure of an LTCC casing,

Fig. 2 shows a section of a sensor in an LTCC casing,

Fig. 3 shows a section of a second sensor in an LTCC casing, Fig. 4 shows a section of a third sensor in an LTCC casing,

Fig. 5 shows the attachment of a sensor chip in a sensor in an LTCC casing,

Fig. 6 shows a section of a sensor in a plastic casing,

Fig. 7 shows the attachment of a sensor chip in a sensor in a plastic casing, and Fig. 8 illustrates a method according to an embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Throughout this description the following terminology is used. A "sensor" is a device designed to convert a phenomenon into an electric signal which can be transferred to a remote processing unit to be processed there. A MEMS or NEMS sensor typically includes a "sensor chip" which may also be called a transducer, and "circuit elements" connected directly therewith. The sensor chip, the circuit elements and the electrical couplings between them may also be called the functional parts of the sensor. Together they constitute a "sensor circuit". In addition to the sensor circuit a sensor typically has a "casing" designed to isolate the sensor circuit from the external environment, shield it from external interference, and enable the handling of the sensor and its attachment to a target of choice.

A sensor for measuring an acoustic signal must be attachable to the structure which is to be monitored in such a manner that the attachment is sufficiently integral or "fixed". The attachment must be such that it facilitates the propagation of the acoustic signal of interest from the examined structure into the sensor in such a manner that attenuation and distortion of the signal are as small as possible. In addition, the attachment should be such that it minimizes the effect of external interfering factors on the measurement. For example, a MEMS sensor for acoustic emissions measures extremely low capacitances so it is especially important to prevent the sensor from being coupled with external sources of stray capacitance.

A low temperature co-fired ceramic (LTCC) refers to a packaging technique for microelectronic circuits where the parts of the circuit entity to be encased are assembled in separate layers, aligned vertically, and sintered together at a relatively low temperature (usually below 1000⁰C). An advantage of the LTCC as compared with techniques requiring higher sintering temperatures, for example, is that the layers may contain circuit elements, metallic conductive areas, and other functional parts. A low sintering temperature guarantees that such parts will not become damaged by the heat of the sintering process. In the LTCC technique it is also possible to embed passive circuit elements, such as resistors, capacitors, and inductances, in the layer structure.

Fig. 1 shows some layers for constructing a packaged sensor according to an embodiment of the invention. Seen here is a five-layer LTCC structure. On the surface of the LTCC material layer which is the bottom layer in the Figure, there is a conductive area 102 which in a finished structure provides a grounded, electrically conductive shield layer beneath the sensor circuit. On the surface of the next LTCC material layer 111 there are circuit elements such as the sensor chip 112 and preamplifier 113. Some conductor paths, made of metal tape, can also be seen between circuit elements on the surface of layer 111. The next LTCC material layer 121 is a ring-like intermediate layer which leaves enough empty space for the circuit elements of the preceding layer. Along the perimeter of layer

121 there is a line of electrically conductive contacts 122 extending through layer 121 , ensuring that the electrically conductive shield layer extends substantially continuously around the functional parts of the sensor circuit.

The second uppermost LTCC material layer 131 in Fig. 1 shuts up the space reserved for the circuit elements so that circuit elements on the surface of layer

111 remain within the space defined by layers 111 , 121 , and 131. On the surface of layer 131 there is a conductive area 132 to which the contacts extending through layers 111 , 121 , and 131 are coupled at the upper ends. The whole structure is closed up by a cover layer 141 which has on its upper surface conductive areas 142 for providing electrically conductive connections to the functional parts inside the LTCC package.

Fig. 2 shows the same structure in a cross section. On the left in Fig. 2, the lower stack shows layers 101 , 111 , and 121. It shows how the electrically conductive shield layer, represented by a thick line, extends upwards from the conductive area 102 via through holes located at the perimeters of the structure. The upper stack shows the cover portion comprising layers 131 and 141. It shows the rest of the electrically conductive shield layer except for the hole through which the electrically conductive connections travel from the circuit elements to the conductive areas 142. The right-hand side of Fig. 2 shows an assembled structure where the circuit elements are surrounded at all sides by a hermetically sealed ceramic wall with an electrically conductive shield layer therewithin.

Fig. 3 shows a structure where the bottom comprises a stack made up of three LTCC material layers 301 , 311 , and 321. Layer 301 is to a large extent equivalent to layer 101 of Figs. 1 and 2, with a conductive area 302 on its surface, which ends up between layers 301 and 311 in a finished assembly. On top of layer 311 there is layer 321 on the surface of which there are circuit elements belonging to the functional parts of the sensor circuit. The purpose of layer 311 is to provide a route for wirings which utilize the through holes extending through the layers. We may assume that circuit element 112 is the sensor chip and circuit element 322 is some other circuit element belonging to the functional parts of the sensor circuit. In the example depicted in Fig. 3 there is a through hole 323 from circuit element 322 via layer 321 to the conductive area on the surface of layer 311 which leads via another through hole 324 to a contact pin 325 at the outer edge of the stack.

The cover portion, assembled on top of the stack comprising layers 301 , 311 , and

321 , comprises in Fig. 3 a conductive layer 331 and an insulating layer 332 on top of that. The conductive layer 331 may be made of a metal alloy, such as Kovar, which is a registered trademark of Carpenter Technology Corporation, or some other conductive material, and it may function as the supporting structure in the cover portion. The insulating layer 332 may be made of moulded epoxy or some other insulating material which can be easily processed in the manufacturing process. The primary purpose of the conductive layer 331 is to provide that portion of the electrically conductive shield layer which surrounds the functional parts of the sensor circuit on the side of the cover portion. The primary purpose of the insulating layer 332 is to prevent unnecessary contacts with the electrically conductive shield layer. The cover portion may comprise further layers or it may be made of a supporting insulating material with the necessary conductive areas metallized or otherwise created thereon.

Fig. 4 shows a cross section of a second LTCC embodiment of the invention. The main difference from the embodiment of Figs. 1 and 2 is that the sensor chip in this case is located in a different layer of the LTCC layer structure than some other circuit elements. The lowest two LTCC material layers 101 and 111 may be substantially the same as in Figs. 1 and 2. The third LTCC material layer 401 is ring-like but the aperture which it defines is much smaller than that defined by the ring-like intermediate layer 121 shown in Figs. 1 and 2. The thickness of layer 401 is of the same order of magnitude as that of the sensor chip 112 which means that the sensor chip can be easily electrically coupled with other circuit elements through the conductive areas on its upper surface using bonding or some other technique for providing electrically conductive connections. The bonding is represented by reference number 402 in Fig. 4. Reference number 403 in Fig. 4 represents an alternative method of electrical coupling where the conductive area on the lower surface of the sensor chip 112 is connected to the circuit elements on the upper surface of layer 401 via the conductive area on the surface of layer 111 and through hole in layer 401. Layer 401 in Fig. 4 is shown as a single LTCC material layer but it is obvious that a layer of LTCC material which has the height of a sensor chip may also comprise two or more separate layers.

The ring-like intermediate layer 121 may in the embodiment depicted in Fig. 4 be substantially identical to the layer 121 in Figs. 1 and 2. Its ring-like shape defines an area containing the functional parts of the sensor circuit. Also the layers 131 and 141 in the cover portion may be substantially similar to those of Figs. 1 and 2. Between them is a conductive area 132 which on its part encloses the functional parts of the sensor circuit within the electrically conductive shield layer. The coupling between the conductive areas 142 on the cover portion of the sensor casing and the functional parts of the sensor circuit may be like above or - if the thickness of a given circuit element and that of the ring-like intermediate layer 121 are mutually compatible - it can be realized direct from the lower surface of the lower layer 131 of the cover portion to the circuit element in question.

The enclosed space in which the sensor chip and the circuit elements of the sensor are at all sides surrounded by a hermetically sealed wall can be filled with a protective gel during manufacturing. It will make it more difficult for moisture and impurities to permeate the sensor even if the ceramic wall were to become slightly damaged.

Fig. 5 schematically shows an advantageous way of attaching the sensor chip to an inner surface of the LTCC casing. The sensor chip 112 has a first surface 501 which in the position depicted in the Figure can be called the lower surface. For the sensor to react to desired oscillation, there is a groove 502 on the first surface

501, separating the kinetic mass part 503 from the frame part 504. Their attachment to one another is elastic in a manner which depends on the properties of the material of the sensor chip and on the design of the isthmus that joins the kinetic mass part and frame part. When the sensor receives oscillations the kinetic mass part 503 moves elastically with respect to the frame part 504. The idea of the sensor is to detect and measure a phenomenon which occurs when the kinetic mass part 503 undergoes movement effected by the oscillation measured. In the embodiment depicted by Fig. 5 this phenomenon is the variation of capacitance in the gap which remains between the upper surface of the kinetic mass part 503 and the lower surface of the upper portion 505 of the sensor chip.

The sensor chip 112 is attached to an inner surface 506 of the LTCC casing at the frame part 504 side of the groove 502. The attachment is shown schematically in Fig. 5, using reference number 507. The attachment 507 circles around the whole length of the edge of the sensor chip so that the first surface 501 of the sensor chip, the inner surface 506 of the casing, and the attachment 507 define a closed space into which the groove 502 opens. The closed space significantly adds to the reliability of the sensor as the response to oscillatory inputs of the kinetic system comprised of the kinetic mass part 503 and frame part 504 could considerably change if moisture or impurities, for example, should make their way in the groove 502 or between the kinetic mass part 503 and the casing material.

Dimensional proportions in Fig. 5 are chosen such as to enhance graphical clarity. In a sensor following the structure of Fig. 5 the sensor chip 112 is rectangular, when viewed from above, and the lengths of its sides are 1.5mm. The thickness of the sensor chip, or the distance from its first surface 501 to the opposite second surface, is 0.78mm. The gap that remains between the first surface 501 of the sensor chip and the inner surface 506 of the casing must be large enough for the kinetic mass part 503 to move freely. In the above-mentioned sensor following the structure of Fig. 5 the movement of the kinetic mass part in a direction perpendicular to the first surface of the sensor chip is 500nm at the most.

In Fig. 5 it is assumed that the casing of the sensor comprises LTCC material layers joined together. The layer to which the sensor chip 112 is attached can be called a first LTCC material layer 111. To the same surface with it there is attached a second, ring-like LTCC material layer 401 which surrounds the sensor chip 112 from the sides and which is substantially of the same thickness as the sensor chip 112. Thus a second surface of the sensor chip 112, at the side opposite to said first surface 501 of the sensor chip, is level with the surface of said ring-like LTCC material layer 401. What is "substantially" level, depends mainly on the fact that the surfaces must allow the construction of an easy-to- make and reliable electrically conductive coupling across the gap separating the sensor chip and the ring-like LTCC material layer.

In Fig. 5 it is assumed that a circuit element (not shown) coupled to the sensor chip 112 is located on that side of the LTCC material layer which in Fig. 5 is the top surface, i.e. which is substantially level with the above-mentioned second surface of the sensor chip 112. The sensor has an electrically conductive coupling 509 from the second surface (coupling area 508) of the sensor chip to the circuit element mentioned above. Additionally the sensor has an electrically conductive coupling from the first surface 501 of the sensor chip to the above-mentioned circuit element via attachment 507 and metallizations 510. Naturally this calls for the assumption that the attachment 507 is electrically conductive. It may be done by soldering or using electrically conductive glue, for example. Couplings leaving from the bottom and top surfaces of the sensor chip may lead to the same circuit element(s).

Fig. 6 shows a section of an embodiment of the invention in which most of the material of the sensor casing is plastic. Such an embodiment can be called a moulded interconnect device (MID). The casing comprises two main parts: body 601 and cover portion 602. Of these, at least the body 601 , which constitutes the bulk of the casing, is plastic. It may be manufactured using the injection moulding technique, for instance. The side which in Fig. 6 is the top side, forms a hollow. The cover portion 602 fits on top of this hollow so that an enclosed space is formed in which the sensor chip 112 and its circuit elements (such as circuit element 322) are surrounded at all sides by the casing material.

Plastics suitable for injection moulding, say polyester, often contain a small amount of a copper complex compound which makes it possible to form a metal coating on desired spots of the plastic piece. The area to be metallized is treated with a laser which releases the copper and activates and roughens the surface processed. After the laser treatment, a metal layer, e.g. a layer of copper, can be chemically deposited on the area in question. The dashed line in Fig. 6 represents an electrically conductive shield layer 603 which is supported by the casing material and which surrounds in a substantially continuous manner the circuit elements of the sensor. In this case the phrase "in a substantially continuous manner" must be understood in such a way that, since at least some of the wirings needed in the sensor circuit are realized as conductive areas formed on a surface of the casing material, the electrically conductive shield layer 603 covers all parts of the inner surfaces of the above-mentioned closed space which are not needed for other purposes.

The conductive areas 604 for providing electrically conductive connections to the functional parts of the sensor circuit can be formed on desired surfaces of the casing. In the example depicted in Fig. 6 they are located at that part of the top surface of the body 601 which is not covered by the cover portion 602. The enclosed space in which the sensor chip and the circuit elements of the sensor are at all sides surrounded by the casing material can be filled with a protective gel during manufacturing. Plastic casing materials are generally not considered as reliable as ceramic ones, e.g. as regards keeping moisture off the functional parts of the sensor, so in the case of a sensor with a plastic casing the use of a protective gel may be even more justifiable than in the case of a sensor with a ceramic casing.

Fig. 7 schematically shows an advantageous way of attaching the sensor chip to an inner surface of a plastic casing. The sensor chip 112, its first surface 501 , the groove 502, the kinetic mass part 503, the frame part 504, and the upper portion

505 of the sensor chip may be the same as in Fig. 5. The sensor chip 112 is attached to an inner surface 706 of the plastic casing at the frame part 504 side of the groove 502. The attachment is shown schematically in Fig. 7, using reference number 707. The attachment 707 circles around the whole length of the edge of the sensor chip so that the first surface 501 of the sensor chip, the inner surface

706 of the casing, and the attachment 707 define a closed space into which the groove 502 opens. So, in this embodiment, too, closed space is used to ensure that no moisture or impurities enter the groove 502 or between the kinetic mass part 503 and the casing material.

The invention does not impose any limitations as to what kind of hollows, elevations and/or other surface formations there are on surfaces of the casing material. In Fig. 7 it is assumed that the sensor chip 112 is placed in a hollow in the casing material which is in the lateral direction only so much smaller than the sensor chip that installation and attachment of the sensor chip is easy to implement from a manufacturing point of view. Beside the sensor chip 112, the plastic casing material extends to the level of the upper surface of the sensor chip in the form of a side wall 708. Here, the upper surface or the second surface refers to that surface of the sensor chip which is on the opposite side to that surface (first or lower surface) of the sensor chip by which the sensor chip is attached to the casing material.

Placing the sensor chip, as shown in Fig. 7, in a hollow the depth of which substantially equals the thickness of the sensor chip brings some advantages from the electrical coupling standpoint. In the sensor of Fig. 7 there is an electrically conductive coupling 709 from the second surface of the sensor chip to a circuit element (not shown) of the sensor across the gap that separates the sensor chip and the plastic casing material. The coupling may comprise e.g. a bonding wire which at the sensor chip's side end is connected to a coupling area 508 on the upper surface of the sensor chip. From this we can deduce what the suitable depth for the hollow in the casing material is when it "substantially" equals the thickness of the sensor chip: the smaller the difference of height between two objects to be coupled, the easier it is to realize the bonding and many other electrical coupling techniques as well. The depth of the hollow "substantially" equals the thickness of the sensor chip when it is such that the above-mentioned electrical coupling becomes easy.

Fig. 7 also shows an electrically conductive coupling from the first (or lower) surface of the sensor chip to a circuit element (not shown) of the sensor along the surface of the plastic casing material. This coupling includes a conductive area 710, realized by metallizing a desired portion of the surface of the casing material, and the sensor chip attachment 707 which, in order to achieve this kind of coupling, must be at least partly electrically conductive. Suitable attachment methods include electrically conductive glue and soldering to the metallized area on the surface of the plastic casing material. Couplings leaving from the bottom and top surfaces of the sensor chip may lead to the same circuit element(s). Fig. 7 also schematically shows the protective gel 711 which fills up the free space in the closed space defined by the casing material where the sensor chip and circuit elements are located.

Fig. 8 illustrates a method according to an embodiment of the invention for manufacturing a sensor for measuring oscillations. Step 801 represents the manufacture of casing parts e.g. by producing the suitable LTCC layers, injection- moulded plastic parts, and/or metal cover parts. If conductor and coupling areas are formed within the casing parts and/or on surfaces thereof, this is also done in step 801. In step 802 the point of attachment(s) is/are prepared for the attachment of the sensor chip and possible other circuit elements. Depending on the method of attachment this may mean e.g. the spreading of reflow solder paste or electrically conductive glue on desired spots.

Step 811 represents the manufacture of the sensor chip, discussed in more detail in the same applicant's patent application FI-20095339 "A sensor for measuring acoustic emission". For the present invention it suffices to assume that step 811 results in a sensor chip which has a first surface and in that first surface a groove separating the kinetic mass part and the frame part. Fig. 812 separately shows the step 812 for the preparation of a point of attachment(s) in the sensor chip although this may be included in the manufacturing step 811 as well. Furthermore, if the sensor includes other circuit elements than the sensor chip, the method must include a step 821 for their manufacture.

In the insertion and attachment step 831 the sensor chip and possible other circuit elements are mounted in the casing part(s). The sensor chip is attached to an inner surface of the sensor casing at the frame part side of the groove so that after the attachment the first surface of the sensor chip, said inner surface of the casing, and the attachment to said inner surface of the casing define a closed space into which said groove opens. If a reflow method is used in the attachment and the casing is of the LTCC type, melting of the solder paste can be done in the same heat treatment in which the casing parts are sintered together. Sealing-up of the casing is shown as step 832 in Fig. 8. The above-described embodiments of the invention are examples only and do not limit the modifiability of the invention or the interpretation of the claims set forth below. For example, even though it was above assumed that the sensor chip and other circuit elements of the sensor are discrete components inside the sensor, the sensor chip and at least one other circuit element can be implemented monolithically so that the silicon or other crystalline material of which the kinetic mass part and frame part of the sensor chip are made also functions as a substrate for the rest of the circuit element. Another example of a variation falling into the scope of the invention is an embodiment of LTCC casing which utilizes a non-LTCC-type cover portion according to Fig. 3 but in which said cover portion is attached to a ring-like LTCC material layer in the same manner as the LTCC-type cover portion used in Figs. 1 and 2.

Claims

1. An electromechanical sensor for measuring oscillations, comprising: - a sensor chip (112) which has a first surface (501 ) and in said first surface a groove (502) which separates a kinetic mass part (503) and a frame part (504) from each other,

- at least one circuit element (113, 322) coupled to the sensor chip for detecting a phenomenon caused by the effect of oscillation measured as the kinetic mass part moves, and

- a casing which encloses the sensor chip (112) and the circuit element (113, 322);

characterized in that the sensor chip (112) is attached to an inner surface (506, 706) of the casing at the frame part side of the groove so that said first surface (501 ) of the sensor chip, said inner surface (506, 706) of the casing, and the attachment (507, 707) of the sensor chip to said inner surface of the casing define a closed space into which said groove (502) opens.

2. A sensor according to claim 1 , characterized in that the casing comprises LTCC material layers (101 , 111 , 121 , 131 , 141 , 301 , 311 , 321 , 401 ) joined to one another.

3. A sensor according to claim 2, characterized in that

- the sensor chip (112) is attached to a surface of a first LTCC material layer (111 , 321 ), to which surface there is also attached a second, ring-like LTCC material layer (121 , 401) which surrounds the sensor chip from the sides and which is substantially of the same thickness as the sensor chip (112).

- a second surface of the sensor chip, at the side opposite to said first surface (501 ) of the sensor chip, is level with the surface of said ring-like LTCC material layer (121 , 401).

4. A sensor according to claim 3, characterized in that - said circuit element (322) coupled to the sensor chip is located on that surface of said ring-like LTCC material layer (401 ) which is substantially level with said second surface of the sensor chip, and

- the sensor comprises an electrically conductive coupling (402, 509) from the second surface of the sensor chip to said circuit element across the gap that separates the sensor chip and the ring-like LTCC material layer (401).

5. A sensor according to claim 3, characterized in that

- said circuit element (322) coupled to the sensor chip is located on that surface of said ring-like LTCC material layer (401 ) which is substantially level with said second surface of the sensor chip, and

- the sensor comprises an electrically conductive coupling (403, 507, 510) from a first surface of the sensor chip to said circuit element through said ring-like LTCC material layer.

6. A sensor according to any one of claims 2 to 5, characterized in that - the sensor comprises a ring-like LTCC material layer (121 ) which surrounds the sensor chip and the circuit elements of the sensor, and

- the sensor comprises a cover portion attached to the ring-like LTCC material layer (121 ) to form a closed space in which the sensor chip and the circuit elements of the sensor are at all sides surrounded by a hermetically sealed wall.

7. A sensor according to claim 6, characterized in that it has at least partly inside the LTCC material an electrically conductive shield layer which surrounds in a substantially continuous manner the sensor chip and the circuit elements of the sensor.

8. A sensor according to claim 6 or 7, characterized in that said cover portion has LTCC material layers (131 , 141 ) in it.

9. A sensor according to any one of claims 2 to 5, characterized in that the sensor comprises a cover portion to form a closed space in which the sensor chip and the circuit elements of the sensor are at all sides surrounded by a hermetically sealed wall and that the cover portion has an electrically conductive layer (331) and outside that an insulating layer (332) which is made of something else than the LTCC material.

10. A sensor according to claim 1 , characterized in that a majority of the casing material is plastic.

11. A sensor according to claim 10, characterized in that

- beside the sensor chip (112), the plastic casing material extends to the level of the second surface of the sensor chip at the side opposite to said first surface (501 ) of the sensor chip, and - the sensor comprises an electrically conductive coupling (709) from the second surface of the sensor chip to said circuit element across a gap which separates the sensor chip and the plastic casing material.

12. A sensor according to claim 10 or 11, characterized in that the sensor comprises an electrically conductive coupling (707, 710) from the first surface of the sensor chip to said circuit element along the surface of the plastic casing material.

13. A sensor according to any one of claims 10 to 12, characterized in that the sensor comprises a cover portion (602) to form a closed space in which the sensor chip (112) and the circuit elements (322) of the sensor are at all sides surrounded by the casing material.

14. A sensor according to claim 13, characterized in that the sensor includes an electrically conductive shield layer (603) which is supported by the casing material and surrounds in a substantially continuous manner the sensor chip (112) and the circuit elements (322) of the sensor.

15. A method for manufacturing an electromechanical sensor for measuring oscillations, characterized in that in the method a sensor chip (112), which has a first surface (506) and in that first surface a groove (502) separating a kinetic mass part (503) and frame part (504), is attached (831 ) to an inner surface (506, 706) of the sensor casing at the frame part side of the groove so that after the attachment, said first surface (506) of the sensor chip, said inner surface (506, 706) of the casing, and the attachment (507, 707) of the sensor chip to said inner surface of the casing define a closed space into which said groove (502) opens.