Semiconductor device and method for manufacturing a semiconductor device

A semiconductor device includes: a substrate; a transduction microstructure integrated in the substrate; a cap joined to the substrate and having a first face adjacent to the substrate and a second, outer, face; and a channel extending through the cap from the second face to the first face and communicating with the transduction microstructure. A protective membrane made of porous polycrystalline silicon permeable to aeriform substances is set across the channel.

BACKGROUND

Technical Field

The present disclosure relates to a semiconductor device and to a method for manufacturing a semiconductor device.

Description of the Related Art

As is known, electronic, microelectromechanical and microfluidic devices must be protected against external agents, such as dust, moisture and aggressive substances, which may cause damage and malfunctioning. In many cases, the devices may be sealed within packages that isolate them completely from the external environment from the mechanical and fluidic standpoint, while enabling electrical or electromagnetic coupling. However, devices exist, in particular some transducers, that by their very nature require a connection not only electrical with the outside world and consequently cannot benefit from sealed packages. For instance, electro-acoustic transducers (microphones and speakers), pressure sensors and sensors for detecting gases or volatile substances must be fluidically coupled to the environment for receiving and transmitting static pressures or pressure variations, according to the type of device and the operating principle. The packages for these devices are therefore provided with openings, which if, on the one hand, guarantee proper operation, on the other, reduce protection against penetration of potentially harmful external agents. Other examples of devices that cannot be sealed are some types of actuators and many microfluidic circuits and devices, such as micropumps and microfluidic valves.

Known solutions envisage either closing the openings by applying layers of transpirant polymeric fabric (for example, expanded polytetrafluoroethylene, ePTFE) or providing openings in the form of channels or vents having a very small cross-section so as to prevent entry of particulate and dust above a given diameter. However, both solutions have limitations. In the first case, applying layers of fabric entails considerable costs, because the operation can be carried out only by pick and place on the individual device and not during machining at the wafer level. The channels or vents may be provided during manufacture at the wafer level, but, given that to provide an adequate mechanical protection the packages normally have a considerable thickness, it may be difficult to reduce the diameter of the channels until the desired protection is guaranteed. Consequently, fine particulate could reach the device to be protected.

BRIEF SUMMARY

The present disclosure is directed to provide a semiconductor device and a method for manufacturing a semiconductor device that will enable the limitations described to be overcome or at least attenuated.

The present disclosure is directed to a device that includes a transduction microstructure in a substrate. There is a cap coupled to the substrate and having a first face facing the substrate and an outer second face. A channel extends through the cap from the second face to the first face and is in fluid communication with the transduction microstructure. The channel includes an internal sidewall. A protective membrane of porous polycrystalline silicon is across the channel and on the internal sidewall of the channel.

DETAILED DESCRIPTION

With reference toFIG.1, a semiconductor device according to an embodiment of the present disclosure, in particular a transducer designated by the number1, comprises a substrate2, a transduction microstructure3integrated in the substrate2, and a cap5. In the example illustrated, the transducer1is a pressure sensor, and the transduction microstructure3comprises an elastic transduction membrane6that closes a chamber7at reference pressure on one side. In other embodiments not illustrated, the semiconductor device may be a different type of transducer, such as, by way of non-limiting example, an electro-acoustic transducer (a microphone or a speaker) or a sensor for detecting gases or volatile substances; an actuator; or a microfluidic device comprising a microfluidic circuit and possibly microfluidic components, such as micropumps and microfluidic valves. For instance, the semiconductor device may be a so-called “Lab-On-Chip”, which requires at least one inlet for introducing a biological specimen to be analyzed or reagents into the microfluidic circuit from outside. Furthermore, it is understood that the transduction microstructure does not necessarily have to comprise mobile or deformable parts. For instance, in a sensor of volatile substances the transduction microstructure may comprise regions with fixed geometry obtained using materials with electrical properties that depend upon the concentration of the substances investigated.

The face of the transduction membrane6opposite to the chamber7may be free or be coated with a passivation layer8and communicates with the external environment, as clarified in detail in what follows. The thickness and rigidity of the passivation layer8are in any case selected so as to enable deformation of the transduction membrane6as a result of pressure variations within a detection range.

In an embodiment, the transduction microstructure3is connected to the remaining part of the substrate2by elastic suspension elements10.

The cap5comprises a body4of semiconductor material, for example monocrystalline bulk silicon, and has a first face5aadjacent to the substrate2, to which it is joined, and a second face5bfacing outwards. The cap5has a thickness such as to offer mechanical protection to the transduction microstructure3and has a thickness in a first direction, for example, comprised between 100 μm and 700 μm.

Pedestals or extensions11extend from the first face5aof the cap5, are coated with respective coupling coatings12, for example made of germanium, and are joined to respective bonding pads13of the substrate2. The pedestals11have a height with respect to the first face5aof the cap5such as to create a gap14between the cap5, in particular the first face5a, and the substrate2. For instance, the height of the pedestals11in the first direction may be comprised between 1 μm and 10 μm. The transduction microstructure3communicates with the gap14.

In the embodiment ofFIG.1, a channel15extends through the cap5from the second face5bto the first face5a. The channel15therefore has a first end open outwards and a second end open onto the gap14and communicating with the transduction microstructure3. The channel15has a diameter in a second direction that is transverse to the first direction, selected according to design preferences and for example comprised between 10 μm and 30 μm.

As illustrated in greater detail inFIG.2, the cap5comprises a protective membrane17made of porous polycrystalline silicon and permeable to the aeriform substances, arranged across the channel15. It is understood that the protective membrane17may be located on the second face5bof the cap5at the first end of the channel15, or on the first face5aof the cap5at the second end of the channel15, or in an intermediate position along the channel15between the first end and the second end, as in the example ofFIG.1. In particular, the protective membrane17is part of a region18of porous polycrystalline silicon, which extends from the first face5aof the cap5towards the inside of the channel15and coats the side surface of the channel15itself between the first face5aand the protective membrane17. In one embodiment, the protective membrane17is released on both sides.

The porosity of the protective membrane17is selected so as to withhold granular solid materials, dust and particulate and enable passage of aeriform substances. In one embodiment, the pores19(FIG.2) of the polycrystalline silicon forming the protective membrane17may have an equivalent diameter comprised between 5 nm and 50 nm. By “equivalent diameter” here we mean the diameter of a duct having a circular or round section of passage and an area equal to the area of the mean section of passage of the pore. The pores19, which inFIG.2for simplicity are represented as rectilinear passages with constant section, extend in fact along generally curvilinear paths with variable cross-section. Moreover, the dimensions and density of the pores19of the protective membrane17are such that the empty/full ratio is comprised between 5% and 30%, for example 10%. With these characteristics, the protective membrane17is able to withhold even drops of water. As a result of the surface tension, in fact, the drops of water are unable to penetrate into the pores19in the absence of a pressure difference applied between the two sides of the protective membrane17. The channel15is offset with respect to a center of the membrane6.

With reference toFIG.3, in one embodiment the packaged semiconductor transducer here designated by 100, comprises a plurality of channels115, which extend through the cap105from a second face105bto a first face105a. Each channel115is provided with a respective protective membrane117made of polycrystalline silicon. The protective membranes117are portions of regions118of porous polycrystalline silicon and permeable to the aeriform substances, which extend from the second face105bof the cap105towards the inside of the respective channels115and coat the side surface of the channels115themselves between the second face105band the protective membranes117. The number, dimensions and position of the channels115are determined on the basis of the design preferences.

The plurality of channels115are positioned between ones of the pedestals11. The channels115may be spaced by equal distances from adjacent channels. A first one of the channels, closer to a first one of the pedestals11is aligned to a first side of the membrane6. A second one of the channels is positioned overlapping the membrane6. A third one of the channels is on a second side of the membrane. The second channel is between the first and third channel. In the embodiment illustrated inFIG.4, in a packaged semiconductor transducer200the protective membrane217forms part of a region218of porous polycrystalline silicon, which extends from a second outer face205bof a cap205towards the inside of a channel215and coats a side surface of the channel215itself between the second face205band the protective membrane217. The membrane217includes a portion on the second face205band a portion on the side surface or internal sidewall of the channel215. The portion on the side surface extends partially into the channel such that more of the sidewall not covered by the portion of the membrane on the side surface217.

The membrane217is on the second face205band the pedestals are on the first face205a, opposite from each other.

The packaged semiconductor transducer1ofFIG.1may be manufactured following the method described in what follows with reference toFIGS.5-13.

The transduction microstructure3is obtained using a microstructure semiconductor wafer2′ (visible inFIG.14) according to a known method.

To manufacture the cap5ofFIG.1, a cap semiconductor wafer5′, which comprises a bulk layer4′ of monocrystalline silicon, is etched through a first resist structure20a, defined with a first mask20b, as illustrated inFIG.5. The bulk layer4′ is removed for a thickness corresponding to the height of the pedestals11, which are formed in regions protected by the first mask20. The height being from the surface5ato an outermost surface of the pedestal.

The cap semiconductor wafer5′ is then again dry-etched with a second resist structure21a(defined with a second mask21b) to open a trench23, as shown inFIG.6. In the example here illustrated, an anisotropic dry etch is used, but equally a wet etch could be used, according to the design preferences.

Then (FIG.7), a stop layer24and a protection layer25of porous polycrystalline silicon and permeable to aeriform substances, are formed in succession on the cap semiconductor wafer5′, both on the first face5aand inside the trench23. The stop layer24may be a layer of silicon oxide deposited or grown thermally. Alternatively, a different material may be used that may be etched in a selective way with respect to the polycrystalline silicon, such as a multilayer of silicon oxide and silicon nitride. The protection layer25is deposited on the stop layer24and may have a thickness comprised between 80 nm and 150 nm. In addition, the diameter of the pores and the empty/full ratio of the protective silicon layer25are selected on the basis of the design preferences for the protective membrane17described above. For instance, the protection layer25has pores of equivalent diameter comprised between 5 nm and 50 nm, the size and the density of the pores being such that the empty/full ratio is comprised between 5% and 30%.

The stop layer24and the protection layer25coat in a conformable way the first face5aof the cap semiconductor wafer5′, the side walls and the bottom of the trench23. The portion of the protection layer25on the bottom of the trench23is to form the protective membrane17.

After the stop layer24and the protection layer25have been formed, a resist structure26ais deposited and defined via a third mask26bso as to protect the inside of the trench23, as illustrated inFIG.8. The stop layer24and the protection layer25are selectively etched where they are not protected by the third resist structure26a(FIG.9). The region18of porous polycrystalline silicon is thus obtained, which is separated from the layer of monocrystalline bulk silicon of the cap semiconductor wafer5′ by a stop structure24′, defined by a residual portion of the stop layer24.

After the third resist structure26ahas been removed, a coupling layer28, for example, of germanium, is deposited in a conformable way on the cap semiconductor wafer5′ (FIG.10).

A fourth resist structure29ais then deposited and defined via a fourth mask29b(FIG.11). The fourth resist structure29acoats the pedestals11and the coupling layer28around them. The coupling layer28is selectively etched and removed where it is not protected by the fourth resist structure29a. The coupling coatings12are thus created, as illustrated inFIG.12.

With reference toFIG.13, the cap semiconductor wafer5′ is turned upside down and etched on the back with a markedly anisotropic etch, for example a trench etch, in a position corresponding (i.e., aligned) to the trench23, until the stop structure24′ is reached, which in this step protects the region18of porous polycrystalline silicon. The channel15is thus completed. The stop structure24′ is then selectively etched. In particular, the portion of the stop structure24′ exposed within the channel15is removed, thus releasing the protective membrane17. Consequently, the protective membrane17extends in a direction transverse to a longitudinal axis of the channel15(i.e., an axis directed from the first end to the second end of the channel15). The portions of the channel15on opposite sides of the protective membrane17communicate with one another through the protective membrane17of porous polycrystalline silicon.

The cap semiconductor wafer5′ is finally joined to the bonding pads13of the microstructure semiconductor wafer2′ to form a composite wafer (FIG.14), which is diced, thus obtaining a plurality of examples of the packaged semiconductor transducer1ofFIG.1. Each example comprises a portion of the microstructure semiconductor wafer2′, which defines the substrate2and contains the transduction microstructure3, and a portion of the cap semiconductor wafer5′, which defines the cap5.

In order to manufacture the packaged semiconductor transducer100ofFIG.3, the method described may be immediately adapted by modifying the masks21and26and the trench etch on the back of the cap semiconductor wafer so as to open the desired number of channels instead of just one.

In order to manufacture the packaged semiconductor transducer200ofFIG.4, instead, the cap semiconductor wafer is turned upside down after the pedestals11have been formed. The machining operation proceeds on the back of the cap semiconductor wafer (second face205b), as already described until the stop layer24and the protective silicon layer25are etched to form the region218of porous polycrystalline silicon and the stop structure24′. Before depositing the germanium coupling layer, the wafer is again turned over. Machining proceeds with formation and definition of the coupling layer on the first face205aof the cap semiconductor wafer and with anisotropic etching of the bulk layer to form the channel215, followed by selective removal of the stop layer within the channel215. The cap semiconductor wafer is joined to the microstructure semiconductor wafer2′ and, after singulation of the composite wafer thus obtained, examples of the packaged semiconductor transducer200ofFIG.4are obtained.

With reference toFIG.15, a packaged semiconductor transducer300according to an embodiment of the present disclosure is designated by number300and comprises the substrate2, the transduction microstructure3integrated in the substrate2and a cap305.

The cap305comprises a bulk layer304, a stop layer324, a structural layer341and a protection layer325of porous polycrystalline silicon and permeable to aeriform substances. The stop layer324, of silicon oxide, is interposed between the bulk layer304, of monocrystalline silicon, and the structural layer341, of a material that can be selectively etchable with respect to the structural layer341, for example silicon nitride.

Channels315extend from a second face305bto a first face305aof the cap305through the bulk layer304, the stop layer324, and the structural layer341. The protection layer325forms protective membranes317in the channels315. More precisely, the protection layer325, which covers the first face305aof the cap305, penetrates into the channels315coating the side surfaces substantially as far as an interface343between the structural layer341and the stop layer324. At the depth of the interface343, the protection layer325extends in a direction transverse to longitudinal axes of the channels315to form the protective membranes317.

The first face305aof the cap305is joined to the substrate2by pedestals311that act as adhesion structures and, at the same time, have a height such as to create a gap314between the substrate2and the cap305.

The microstructure3communicates with the outside through the channels315and the protective membranes317, which enable passage of aeriform substances.

Whereas the transduction microstructure3is obtained using a microstructure semiconductor wafer2′ as already described, according to a method for manufacturing the packaged semiconductor transducer300ofFIG.15illustrated inFIGS.16-19, a cap semiconductor wafer305′ initially comprises the bulk layer304. The stop layer324and the structural layer341are formed in succession on the bulk layer304(FIG.16).

The structural layer341is then selectively etched using a mask320for opening cavities345, which extend as far as the stop layer324, as illustrated inFIG.17.

The protection layer325is then formed on the first face305aof the cap semiconductor wafer305′ (FIG.18), which is then turned upside down and etched on the back, i.e., on the side of the second face305b, in positions corresponding (aligned) to respective cavities345(FIG.19). In particular, a markedly anisotropic trench etch is first carried out as far as the stop layer324, and then the stop layer324is etched where it is left exposed. In this way, the protective membranes317are freed and the channels315are completed.

The protection layer325is selectively etched to expose the structural layer341where the pedestals311are then formed.

The cap semiconductor wafer305′ is finally joined to the bonding pads13of the microstructure semiconductor wafer2′ to form a composite wafer, which is then diced, thus obtaining a plurality of examples of the packaged semiconductor transducer300ofFIG.15. Each example comprises a portion of the microstructure semiconductor wafer2′, which defines the substrate2and contains the transduction microstructure3, and a portion of the cap semiconductor wafer305′, which defines the cap305.

The disclosure described presents various advantages. From the structural standpoint, the presence of the protective membrane through the channel or channels prevents any contamination of the transduction microstructure with particulate and dust within the device, without jeopardizing the fluidic connection with the outside world, in particular for aeriform substances. Furthermore, the protective membrane prevents or at least hinders entry of small amounts of liquid, for example due to exposure to splashes or to the use during sports activity. In the case of water, for example, the diameter of the pores of the protective membrane is sufficient to prevent entry of drops as a result of the surface tension.

Proper operation of the transducer is ensured because the transduction microstructure, albeit remaining protected from contaminating agents, is in any case adequately exposed and coupled to the quantities to be detected (pressure, substances, etc.) thanks to the porosity of the protective membrane. Indeed, the constraints on the minimum diameter of the channel may even be at least in part relaxed because the barrier effect against contaminating agents is performed effectively by the protective membrane. Therefore, the presence of the protective membrane also affords a greater flexibility in design of the cap.

As regards the manufacturing method, the use of the stop layer between the layer of porous polycrystalline silicon and the bulk layer enables definition and release of the protective membrane within the channels in a simple way during machining at the wafer level. Operations for applying polymeric membranes at the device level following upon singulation can thus be avoided, with considerable reduction in costs. Moreover, the stop layer and the layer of porous polycrystalline silicon may be formed with standard techniques of machining of semiconductors and therefore in a reliable and inexpensive way. Also the characteristics of the protective membrane, such as porosity and thickness, may be easily controlled.

Etching of the bulk layer on both sides, before opening the trench on one side and then with anisotropic etching on the opposite side, makes it possible to obtain extreme shape ratios through the channel that traverses the cap. Furthermore, the channels can be formed without any need to thin the bulk layer of the cap wafer and therefore without degrading the mechanical resistance thereof. This adds flexibility to the design and to manufacturing of semiconductor transducers.

FIG.20shows an electronic system400that may be of any type, in particular, but not exclusively, a wearable device, such as a watch, a bracelet, or a smart band; a computer, such as a mainframe, a personal computer, a laptop or a tablet; a smartphone; a digital music player, a digital camera or any other device designed to process, store, transmit or receive information. The electronic device1may be a general-purpose or embedded processing system in a device, an apparatus or a further system.

The electronic system400comprises a processing unit402, memory devices403, a packaged semiconductor transducer, for example the packaged semiconductor transducer1ofFIG.1, and may moreover be provided with input/output (I/O) devices405(for example a keyboard, a mouse or a touchscreen), a wireless interface406, peripherals407.1, . . . ,407.N and possibly further auxiliary devices (here not illustrated). The components of the electronic system400may be coupled in communication with one another directly and/or indirectly through a bus408. The electronic system400may moreover comprise a battery409. It should be noted that the scope of the present disclosure is not limited to embodiments necessarily having one or all of the devices listed.

The processing unit402may for example comprise one or more microprocessors, microcontrollers, and the like, according to the design preferences.

The memory devices403may comprise volatile memory devices and non-volatile memory devices of various kinds, for example SRAMs and/or DRAMs for volatile and solid-state memories, magnetic disks and/or optical disks for non-volatile memories.

Finally, it is evident that modifications and variations may be made to the device and to the method described, without thereby departing from the scope of the present disclosure.

A semiconductor device may be summarized as including a substrate (2); a transduction microstructure (3) integrated in the substrate (2); a cap (5;105;205;305) joined to the substrate (2) and having a first face (5a;105a;205a;305a) adjacent to the substrate (2) and an outer second face (5b;105b;205b;305b); a channel (15;115;315) extending through the cap (5;105;205;305) from the second face (5b;105b;205b;305b) to the first face (5a;105a;205a;305a) and communicating with the transduction microstructure (3); a protective membrane (17;117;217;317) made of porous polycrystalline silicon and permeable to aeriform substances, the protective membrane (17;117;217;317) being arranged across the channel (15;115;315).

The protective membrane (17;117;217;317) may be located along the channel (15;115;315) at an intermediate position between the second face (5b;105b;205b;305b) and the first face (5a;105a;205a;305a).

The protective membrane (17;117;217;317) may have pores (19) with an equivalent diameter comprised between 5 nm and 50 nm.

A gap (14) may be defined between the first face (5a;105a;205a;305a) of the cap (5;105;205;305) and the substrate (2) and wherein the transduction microstructure (3) and the channel (15;115;315) communicate with the gap (14).

The device may include a region (18;118;218) of porous polycrystalline silicon that coats a side surface of the channel (15;115;215) between the protective membrane (17;117;218) and one between the first face (5a;105a) and the second face (205a), wherein the protective membrane (17) may be part of the region (18;118;218) of porous polycrystalline silicon.

The device may include a plurality of channels (115;315) and a plurality of protective membranes (117;317) of porous polycrystalline silicon and permeable to aeriform substances, each protective membrane (17;117;217;317) being arranged across a respective one of the channels (115;315).

An electronic system may be summarized as including a processing unit (402) and a semiconductor device (1;100;200;300).

A method for manufacturing a semiconductor device, may be summarized as including forming a transduction microstructure (3) in a microstructure semiconductor wafer (2′); forming a stop layer (24;324) on a bulk layer (4′;304′) of a cap semiconductor wafer (5′;305′); forming a protection layer (25;325) made of porous polycrystalline silicon and permeable to aeriform substances at least in part on the stop layer (24;324); forming a channel (15;115;315) through the cap semiconductor wafer (5′;305′) from a first face (5a;105a;205a;305a) to a second face (5b;105b;205b;305b) thereof, wherein forming the channel (15;115;315) comprises etching in an anisotropic way the bulk layer (4′;304′) as far as the stop layer (24;324) on one side of the cap semiconductor wafer (5′;305′) opposite to the protection layer (25;325); removing the stop layer (24;324) inside the channel (15;315); and joining the cap (5;105;205;305) to the microstructure semiconductor wafer (2′) so that the channel (15;115;315) will communicate with the transduction microstructure (3).

Forming the channel (15;315) may include forming a depression (23;345) in the cap semiconductor wafer (5′;305′) and wherein forming the protection layer (25;325) may include depositing the protection layer (25;325) within the depression (23;345).

Forming the depression (23) may include opening a trench in the bulk layer (4′).

Forming the depression (345) may include depositing a structural layer (341) on the stop layer (324) and selectively etching the structural layer (341).

Forming the protection layer (25;325) may include depositing the protection layer (25;325) directly in contact with the stop layer in the depression (23;345).

The protection layer (25;325) may have pores (19) with an equivalent diameter comprised between 5 nm and 50 nm and wherein the dimensions and density of the pores (19) may be such that an empty/full ratio of the protection layer (25;325) is comprised between 5% and 30%.

Forming the channel (15;115;315) may include etching in an anisotropic way the bulk layer (4′;304′) in a position corresponding to the depression (23;45).

The method may include forming a plurality of channels (115;315) through the cap semiconductor wafer (5′;305′) from the first face (5a;105a;205a;305a) to the second face (5b;105b;205b;305b) thereof.

The method may include forming pedestals (11) extending from the first face (5a;105a;205a;305a) of the cap semiconductor wafer (5′;305′); and forming coupling coatings (12) on the pedestals (11); wherein joining the cap semiconductor wafer (5′;305′) to the microstructure semiconductor wafer (2′) may include joining the pedestals (11) to the microstructure semiconductor wafer (2′); wherein the pedestals (11) may have a height with respect to the first face (5a;105a;205a;305a) of the cap semiconductor wafer (5′;305′) such as to create a gap (14) between the first face (5a;105a;205a;305a) and the microstructure semiconductor wafer (2′) of the substrate (2); and wherein the transduction microstructure (3) and the channel (15;115;315) communicate with the gap (14).