# Patent

## Title
Semiconductor device with hollow structure

## Abstract

A device comprising a chip, which is held in casting compound and on which a hollow structure is arranged is disclosed.

## Background
RELATED APPLICATIONS
This present application is a divisional application of U.S. application Ser. No. 11/645,150, filed Dec. 22, 2006, which claims priority to German Patent Application No 10 2006 058 010.9, filed Dec. 8, 2006, the entirety of both of which are hereby incorporated by reference.


FIELD OF THE INVENTION
The invention relates to semiconductor devices with hollow structures.
BACKGROUND
Many chips, in particular those with electromechanical systems, require a hollow to ensure their function. Various techniques are known for creating such a hollow. It is possible to create the walls of the hollow by structuring a layer, subsequently dividing up the wafer into individual chips and then closing the hollow by applying a cover to the walls of the hollow. Another possibility is to produce the hollow by using a further wafer which has depressions in its surface, forming hollows over the active surface of the first wafer when the two wafers are joined together.
SUMMARY
Against this background, according to a first aspect, a method of producing devices in which hollow structures are produced on a wafer is provided. The wafer is divided into at least two chips. The two chips are covered with casting compound. By dividing the casting compound, at least two chips are separated.
According to a second aspect, an arrangement which has a number of chips which are together held in casting compound and on which hollow structures are arranged is provided.
According to a third aspect, a device which has a chip which is held in casting compound and on which a hollow structure is arranged is provided.



BRIEF DESCRIPTION OF THE DRAWINGS
The invention is explained in more detail below by way of example with reference to the drawings, in which:
FIG. 1 shows a schematic sectional representation of a wafer with hollow structures arranged over the active area of the wafer;
FIG. 2 shows a schematic sectional representation of the arrangement shown in FIG. 1 after the creation of contact elements;
FIG. 3 shows a schematic sectional representation of the arrangement shown in FIG. 2 after the wafer has been divided into individual chips with hollow structures arranged on them;
FIG. 4 shows a schematic sectional representation of two chips shown in FIG. 3, which have been placed on a carrier;
FIG. 5 shows a schematic sectional representation of the arrangement represented in FIG. 4 after casting with casting compound to produce a cast wafer;
FIG. 6 shows a schematic sectional representation of the subregion of the cast wafer that is shown in FIG. 5 after removal of the carrier;
FIG. 7 shows a schematic sectional representation of the subregion of the cast wafer that is shown in FIG. 6 after the creation of a wiring structure by the thin-film technique;
FIG. 8 shows a schematic sectional representation of the arrangement shown in FIG. 7 after the attachment of solder balls to external terminal areas of the wiring structure and after dividing the cast wafer into individual chip packages;
FIGS. 9-14 show schematic sectional representations of further exemplary embodiments of chip packages;
FIGS. 15A-15E show schematic sectional representations to explain a method of producing hollow structures;
FIG. 16 shows a flow diagram to explain a method of producing a chip package.



DETAILED DESCRIPTION OF THE DRAWINGS
Devices with a chip embedded in a casting compound are described below. The chips may be of extremely different types and contain for example electrical or electromechanical functional elements in the region of their active area. The chips may be configured as so-called MEMS (Micro-Electro Mechanical Systems), it being possible for micro mechanical structures such as for example bridges, membranes or tongue structures to be provided. The chips may be configured as sensors or actuators, for example BAW (Bulk Acoustic Wave) filters, pressure sensors, acceleration sensors, rotation sensors, microphones and so on and so forth. Chips in which such functional elements are embedded generally comprise electronic circuits which serve for driving the functional elements or further process signals generated by the functional elements. Like the chip, the functional elements accommodated in the hollow may consist of semiconductor material, but may also be produced from other materials, such as for example insulators, plastics or metals.
As explained in more detail below on the basis of examples, multichip devices with a number of functional elements, for example the combination of two BAW filters with the same resonant frequency or tuned to slightly different resonant frequencies may be provided. Other chips and/or passive electronic components may also be integrated in a device along with the chip with a hollow structure provided on top. For example, a chip with a hollow structure for realizing the MEMS functionality (for example a BAW filter) and a chip without a hollow structure for signal generation and/or signal evaluation (for example an LNA (low noise amplifier)) may be provided in a multichip device. Apart from realizing the hollow structure from a polymer in the way described in more detail below, the hollow structure may also consist of other materials, for example a semiconductor material or an insulator, for example glass.
FIG. 1 shows a semiconductor wafer 1, on the active surface 2 of which structures that are to be protected from environmental influences are formed, for example electromechanical functional elements 3. The functional elements 3 are respectively located in hollows 4, which are formed in a relatively thick (for example approximately 60 μm.) layer 6, which has previously been applied to the active surface 2 of the already processed semiconductor wafer 1.
The hollows 4 are produced in the wafer assembly. This may be performed for example by a polymer material being used for the layer 6 and structured by a photopatterning technique. The creation of the hollows 4 in a polymer layer 6 is explained later by way of example on the basis of FIGS. 15A to 15E. In FIG. 16, production of the hollow is denoted by the step S1.
Furthermore, it is also possible to use photolithographic means to produce free regions 8, in which wafer metallizations 7 are created, in electrically conductive connection (not represented) with one or more electromechanical functional elements 3, and consequently forming terminal regions for the operation of the electromechanical functional elements 3.
FIG. 2 shows the filling of the free regions 8 above the wafer metallizations 7 with a contact element 9. The contact element 9 extends from the wafer metallization 7 to the upper side 10 of the layer 6 and can, as represented in FIG. 2, protrude somewhat beyond the upper side 10 of the layer 6 and reach around the upper edge of the layer 6. The contact element 9 may be created for example by a galvanic process (“electroplating”) in a galvanic cell with a flow of electric current or by an electroless, selective deposition (“electroless metallization”) of a suitable metal, for example nickel, from a bath on the wafer metallization 7. Both processes are likewise carried out in the wafer assembly. Alternatively, it is also possible to dispense with the creation of the contact elements 9 at the point in time represented in FIG. 2 and to create corresponding contact structures only at a later time, after creating a wiring structure (see for example FIG. 7).
According to FIG. 3, the wafer 1 with the structured layer 6 arranged on it is divided up into individual chips 11. This produces hollow enclosures 5. The singulation into chips 11 may be performed on a film 12, which has previously been applied to the rear side of the wafer 1. In FIG. 16, this method step is denoted by S2.
It is pointed out that the structuring of the outer walls of the hollow enclosure 5 does not necessarily have to be performed by the singulation into chips 11, but in the same way as the creation of the free regions 8 can be carried out photolithographically in the wafer assembly. This allows any desired peripheral form of the hollow enclosures 5 to be achieved, and in particular the hollow enclosures 5 may be dimensioned in such a way that they are arranged only over a partial region of the chip area. For example, a structuring of the layer 6 may be performed along the dividing lines provided for the chip singulation. This has the effect that the layer 6 is not subjected to mechanical stress during the chip singulation. In this case, a free space that runs around in the manner of a frame may be present between the outer edge of the chip 11 and the contour line of the hollow enclosure 5.
The production of the hollow enclosures 5 and wafer metallizations 7 by suitable structuring of a layer 6 represents a low-cost method, since essentially only layer depositing and photolithographic techniques have to be used, and the creation of the hollows 4 is performed in the wafer assembly. The hollow enclosures 5 may, however, also be created in some other way, for example by a second wafer (for example made of glass or semiconductor material) being provided with suitable depressions and joined together with the semiconductor wafer 1 in such a way that the depressions form the hollows 4. In this case, wafer metallizations 7 may be realized for example by etching free appropriate regions in the second wafer, used as a covering.
According to FIG. 4, the chips 11 are then placed on a carrier 13. The carrier 13 may be made up of a carrier material 14 (for example metal, silicon, glass or a polymer) and an adhesive layer 15 running over it. The chips 11 are applied with the upper side 10 of the hollow enclosure 5 to the adhesive layer 15 of the carrier 13, spaced apart from one another. Subsequently, the chips 11 with hollow enclosures 5 arranged on the carrier 13 are cast with a casting compound (“mould compound”) (also referred to as “overmoulding”), see FIG. 5 and step S3 in FIG. 16. After the curing of the cast body formed in this way, which is referred to hereafter as cast wafer 16 (also referred to as “reconstituted wafer” or “artificial wafer”), the cast wafer 16 is detached from the carrier 13, see FIG. 6. This may be performed for example by supplying energy (for example heating), if an adhesive layer 15 that can be detached by supplying energy is used.
It is pointed out that the top region of the hollow enclosure 5 is not mechanically stressed by the casting step, since the upper side 10 of the hollow enclosure 5 is not encapsulated with casting compound. This makes it possible to prevent the hollow enclosure 5 being compressed by the casting step, which can very easily occur in the case of encapsulating the entire hollow enclosure 5, for example in the case of a hollow enclosure 5 consisting of polymer material. The free (i.e. unsupported) side wall regions of the hollow enclosure 5 are shorter than the free (i.e. unsupported) top region of the hollow enclosure 5 and may additionally be made stronger, so that no damage is caused by the casting operation in the side wall region surrounded by casting compound.
Before the cast wafer 16 is singulated into chip packages, further process steps may be carried out in the “cast wafer assembly”. Such processes can be carried out at particularly low cost, since the chip packages can be simultaneously processed while they are still together in the cast wafer 16. The cast wafer 16 forms as it were an artificial wafer, to which the production steps known from WLP (wafer level packaging) technology can be applied. In WLP technology, all the processes carried out before a wafer is divided into chips are carried out at wafer level. By analogy with WLP technology, it may be provided in the present case that, after the production of the cast wafer 16, all the processes carried out before the cast wafer 16 is singulated into chip packages are carried out at “cast wafer level”, i.e. for all the chip packages together on the intact cast wafer.
FIG. 7 shows how, by means of a thin-film technology applied to the cast wafer 16, the contact elements 9 of the chips 11 are wired into polymer layers 20, 21 lying over them. Firstly, the first polymer layer 20 is applied to the surface of the cast wafer 16, for example by “spin coating”. Subsequently, the first polymer layer 20 is structured, for example photolithographically, so that contacting openings to the contact elements 9 are created. Subsequently, a first metallization layer is deposited and photolithographically structured to form conducting tracks (wiring lines 23). Subsequently, a second polymer layer 21 is deposited on the wiring lines 23 and likewise structured, so that openings to the wiring lines 23 are created. There follows a further metallizing and structuring step for external terminals, with which external terminal areas 22 are created. Through the openings of the second polymer layer 21, the external terminal areas 22 are electrically connected to the wiring lines 22, and consequently to the contact elements 9.
The thin-film technique that is used allows the polymer layers 20, 21 and metallization layers to be very thin and the structuring to be very fine. Typically, the polymer layers are 5 to 20 micrometers thick and the metallization layers 2 to 15 micrometers thick. The polymer layers 20, 21 consist for example of polyimide and are deposited by a depositing process, for example spin coating, on that surface of the cast wafer 16 that is formed in certain regions by the upper sides 10 of the hollow enclosures 5. The contacting of the chips 11 by means of a thin-film technique is illustrated in FIG. 16 as step S4.
If, other than as represented in FIG. 2, the contact elements 9 have not already been introduced into the free regions 8, the contacting of the wafer metallizations 7 with the wiring lines 23 may also be performed at a later time. For this purpose, the polymer layers 20, 21 may be opened at the correct locations photolithographically and subsequently filled by a contact element metallization.
Regions of the chip package outside the chip contour are also available for the wiring, i.e. the external terminal areas 22 may lie with some or all their surface area outside the contour line (“footprint”) of the chip 11.
Subsequently, solder depots or solder balls 24 may be attached to the external terminal areas 22. This operation, referred to as ball attach, is likewise preferably performed on the intact cast wafer 16 and may be performed for example by means of templates which apply solder material to the external contact areas 22 at the correct locations.
After the performance of any further process steps there may be, the cast wafer 16 is divided up into individual chip packages 25, see FIG. 8 and step S5 in FIG. 16. The chip packages 25 may consist on the rear side and on their side wall regions 26 of casting compound. As already mentioned, the external terminal areas 22 may protrude beyond the chip contour or lie completely outside it, the increase in dimensions of the area available for the package contacting that can be achieved for the wiring being determined by the width of the side wall region 26. Wiring that is led out from the chip contour is also referred to as fan-out, as a result of which the chip package 25 may also be referred to as a “fan-out wafer level package”.
FIG. 9 shows a chip package 25.1, which differs from the chip package 25 shown in FIG. 8 essentially in that a greater number of external terminal areas 22 are provided, a first row of external terminal areas 22 being arranged underneath the chip 11 and a second row of external terminal areas 22 being arranged laterally outside the chip 11.
FIG. 10 shows a chip package 25.2, which differs from the chip package 25 shown in FIG. 8 essentially in that a number of hollow enclosures 5 or one hollow enclosure 5 with a number of hollows 4 are or is arranged on the chip 11.
A further variant is to provide a passage 30, which passes through the polymer layers 20, 21 and the top region of the hollow enclosure 5 and consequently connects the hollow 4 to the surroundings. The passage 30 may be created for example by photopatterning. This variant of a chip package 25.2, represented in FIG. 11, is suitable for example for pressure sensors.
FIG. 12 shows a chip package 25.3 without a wiring structure. In the case of this embodiment, no increase in the terminal geometry can be achieved. The solder balls 24 are attached directly to the end faces of the contact elements 9, it being possible for the attachment to be performed in the way already described on the still intact cast wafer 16. A further configuration, represented in FIG. 13, also dispenses with the attachment of solder balls to the end faces of the contact elements 9. The chip package 25.4, represented in FIG. 13, may be fastened by means of a reflow process directly to a printed circuit board or PCB, to which solder material has previously been applied at the correct locations, for example by suitable solder paste printing.
SiP (system in package) modules, which may contain a number of chips 11 or a single chip 11 and further passive or discrete devices in a chip package 25.5, can also be realized as a further configuration. FIG. 14 shows an example of such a SiP module. The two chips 11 are electrically coupled to each other by means of a common wiring structure, which may possibly also have common external terminal areas 22. It is also possible for the chips 11 to have no electrically coupled wiring structure and to interact with each other for example by means of other mechanisms (mechanically, magnetically etc). Furthermore, it may also be provided that only one of the chips 11 has a hollow enclosure 5, while one or more other chips (not represented) are configured in a conventional way without a hollow enclosure 5 and are enclosed by the casting compound.
FIGS. 15A to 15E illustrate by way of example a method of producing the hollow enclosures 5. Although FIGS. 15A to 15E only show the production of a hollow enclosure 5 on a chip 11, the technique explained below is used in the wafer assembly, i.e. for the simultaneous production of all the hollow enclosures 5 on the wafer 1.
As described in conjunction with FIG. 1, firstly a thick polymer layer 6, for example SU8, is deposited over the wafer 1. By means of a first photomask 40, those regions of the polymer layer 6 that are later intended to form the wall regions of the hollow enclosure 5 are exposed with light of a certain wavelength, see FIG. 15A. By means of a second photomask 41, a region of the polymer layer 6 that is intended to form the top region of the hollow enclosure 5 is exposed by means of light (possibly of a different wavelength). A hole portion 42 is thereby masked, see FIG. 15B.
The exposures have the effect of inducing in the polymer layer 6 a crosslinkage of the exposed regions with different depths that can be specifically set. The different depths may be accomplished for example by using different wavelengths of the light used for the exposure. The crosslinked regions 43 are represented in FIG. 15C.
In FIG. 15D, an ultrasound-assisted spray developing method is used to develop or etch free the hollow 4. The polymer layer 6 is thereby removed as far as the crosslinked regions 43. Within the crosslinked regions 43, the material removal is performed through the top hole 44, see FIG. 15D.
The top hole 44 may possibly be closed with a further polymer layer 45. The structure 5 represented in FIG. 15E represents an example of the hollow enclosure 5 described in the previous exemplary embodiments (in this case also with photopatterned outer walls).
It is true for all the exemplary embodiments that a high degree of flexibility is made possible with respect to arrangements and geometries of hollow enclosures 5 and external terminal areas 22. The production of the hollow enclosures 5 at wafer level makes production possible in a low-cost way. A further cost reduction is achieved by the use of low-cost “photoresist hollow enclosures” of a polymer, allowing a packaging with reliable process attributes to be achieved in spite of their low mechanical load-bearing capacity as a result of the top region 10 being left free during the casting. The multilayer thin-film technique allows a “fan-out design” to be achieved on the contact level, by which package terminals can be realized outside the chip area, thereby making it possible to achieve both an increase in the contacting reliability, by increasing the pitch spacing between external terminal areas, and an increase in the number of external terminal areas. This also opens up new possibilities in particular for the production of SiP modules of extremely varied types and designs.
It is therefore intended that the foregoing detailed description be regarded as illustrative rather than limiting, and that it be understood that it is the following claims, including all equivalents, that are intended to define the spirit and scope of this invention.

## Claims

1. A device comprising:
a chip comprising a first main surface, a second main surface and side surfaces connecting the first main surface and the second main surface;
a structure arranged over the first main surface of the chip wherein the structure comprises a first planar main surface, a second planar main surface, and side surfaces connecting the first planar main surface and the second planar main surface;
a hollow space arranged over the first main surface of the chip, wherein the entire hollow space is arranged between the first main surface of the chip and the first planar main surface of the structure, and wherein the entire hollow space is arranged between the side surfaces of the structure;
an encapsulant embedding the chip, wherein the encapsulant comprises a main surface and is disposed on the side surfaces of the chip, wherein the main surface of the encapsulant and the first planar main surface of the structure are arranged in a common plane; and
one or more dielectric layers with conductor tracks and external contact elements arranged over the main surface of the encapsulant, wherein at least one of the external contact elements is arranged completely outside of a contour of the first main surface of the chip.

2. The device of claim 1, further comprising a plurality of hollow spaces arranged over the first main surface.

3. The device of claim 1, further comprising solder depots attached to the external contact elements.

4. The device of claim 1, wherein a surface of the encapsulant is formed in regions by upper sides of the hollow space.

5. The device of claim 4, further comprising contact elements extending from the chip to the upper side of the hollow space for contacting an active area of the chip.

6. The device of claim 5, wherein the contact elements are provided with solder depots.

7. The device of claim 1, wherein the hollow space is formed in a polymer.

8. The device of claim 1, further comprising a plurality of hollow spaces.

9. The device of claim 1, wherein the hollow space defines a through-hole for connecting the hollow space to its surroundings.

10. The device of claim 1, wherein an entirety of the encapsulant is made of a mold compound.

11. The device of claim 1, wherein the main surface of the encapsulant, the first planar main surface of the structure and the first main surface of the chip are parallel to each other.

12. The device of claim 1, wherein at least one electrical contact is arranged over the first main surface of the chip.

13. The device of claim 1, wherein the second planar main surface of the structure and the first main surface of the chip are arranged in a common plane.

14. The device of claim 1, wherein a side surface of the chip and a side surface of the structure are arranged in a common plane.

15. The device of claim 1, further comprising:
at least one electrical connection extending from the first planar main surface of the structure to the second planar main surface of the structure.

16. The device of claim 1, wherein the at least one of the external contact elements is arranged completely outside of the contour of the first main surface of the chip when viewed in a direction perpendicular to the first main surface of the chip.