Patent Description:
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/or 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. <CIT> discloses a semiconductor device comprising a substrate and a transduction microstructure applied to the substrate. A cap is joined to the substrate and has an inner first face toward the substrate and an outer second face. The cap has a through opening extending from the second face to the first face and communicating with the transduction microstructure. <CIT> also discloses a method for manufacturing a substrate-level assembly comprising providing a device substrate of semiconductor material, forming a first integrated device within said device substrate, said first integrated device being provided with a membrane in the proximity of the substrate's top face, coupling a capping substrate to said device substrate above said top face so as to cover said first integrated device, said coupling comprising forming a first empty space in a position corresponding to said active area, and forming in said capping substrate a first access duct, fluidically connected to said first empty space and to the outside of said substrate-level assembly.

The aim of the present invention is 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.

According to the present invention a semiconductor device and a method for manufacturing a semiconductor device are provided, as defined in claims <NUM> and <NUM>, respectively.

For a better understanding of the invention, some embodiments thereof will now be described, purely by way of non-limiting example and with reference to the attached drawings, wherein:.

With reference to <FIG>, a semiconductor device according to an embodiment of the present invention, in particular a transducer designated by the number <NUM>, comprises a substrate <NUM>, a transduction microstructure <NUM> integrated in the substrate <NUM>, and a cap <NUM>. In the example illustrated, the transducer <NUM> is a pressure sensor, and the transduction microstructure <NUM> comprises an elastic transduction membrane <NUM> that closes a chamber <NUM> at 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 analysed and/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 membrane <NUM> opposite to the chamber <NUM> may be free or be coated with a passivation layer <NUM> and communicates with the external environment, as clarified in detail in what follows. The thickness and rigidity of the passivation layer <NUM> are in any case selected so as to enable deformation of the transduction membrane <NUM> as a result of pressure variations within a detection range.

In an embodiment, the transduction microstructure <NUM> is connected to the remaining part of the substrate <NUM> by elastic suspension elements <NUM>.

The cap <NUM> comprises a body <NUM> of semiconductor material, for example monocrystalline bulk silicon, and has a first face 5a adjacent to the substrate <NUM>, to which it is joined, and a second face 5b facing outwards. The cap <NUM> has a thickness such as to offer mechanical protection to the transduction microstructure <NUM> and has a thickness, for example, comprised between <NUM> and <NUM>.

Pedestals <NUM> extend from the first face 5a of the cap <NUM>, are coated with respective coupling coatings <NUM>, for example made of germanium, and are joined to respective bonding pads <NUM> of the substrate <NUM>. The pedestals <NUM> have a height with respect to the first face 5a of the cap <NUM> such as to create a gap <NUM> between the cap <NUM>, in particular the first face 5a, and the substrate <NUM>. For instance, the height of the pedestals <NUM> may be comprised between <NUM> and <NUM>. The transduction microstructure <NUM> communicates with the gap <NUM>.

In the embodiment of <FIG>, a channel <NUM> extends through the cap <NUM> from the second face 5b to the first face 5a. The channel <NUM> therefore has a first end open outwards and a second end open onto the gap <NUM> and communicating with the transduction microstructure <NUM>. The channel <NUM> has a diameter selected according to design preferences and for example comprised between <NUM> and <NUM>.

As illustrated in greater detail in <FIG>, the cap <NUM> comprises a protective membrane <NUM> made of porous polycrystalline silicon and permeable to the aeriform substances, arranged across the channel <NUM>. It is understood that the protective membrane <NUM> may be located on the second face 5b of the cap <NUM> at the first end of the channel <NUM>, or on the first face 5a of the cap <NUM> at the second end of the channel <NUM>, or in an intermediate position along the channel <NUM> between the first end and the second end, as in the example of <FIG>. In particular, the protective membrane <NUM> is part of a region <NUM> of porous polycrystalline silicon, which extends from the first face 5a of the cap <NUM> towards the inside of the channel <NUM> and coats the side surface of the channel <NUM> itself between the first face 5a and the protective membrane <NUM>. In one embodiment, the protective membrane <NUM> is released on both sides.

The porosity of the protective membrane <NUM> is selected so as to withhold granular solid materials, dust and particulate and enable passage of aeriform substances. In one embodiment, the pores <NUM> (<FIG>) of the polycrystalline silicon forming the protective membrane <NUM> may have an equivalent diameter comprised between <NUM> and <NUM>. By "equivalent diameter" here we mean the diameter of a duct having a circular section of passage and an area equal to the area of the mean section of passage of the pore. The pores <NUM>, which in <FIG> for 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 pores <NUM> of the protective membrane <NUM> are such that the empty/full ratio is comprised between <NUM>% and <NUM>%, for example <NUM>%. With these characteristics, the protective membrane <NUM> is 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 pores <NUM> in the absence of a pressure difference applied between the two sides of the protective membrane <NUM>.

With reference to <FIG>, in one embodiment the packaged semiconductor transducer here designated by <NUM>, comprises a plurality of channels <NUM>, which extend through the cap <NUM> from the second face 105b to the first face 105a. Each channel <NUM> is provided with a respective protective membrane <NUM> made of polycrystalline silicon. The protective membranes <NUM> are portions of regions <NUM> of porous polycrystalline silicon and permeable to the aeriform substances, which extend from the second face 105b of the cap <NUM> towards the inside of the respective channels <NUM> and coat the side surface of the channels <NUM> themselves between the second face 105b and the protective membranes <NUM>. The number, dimensions and position of the channels <NUM> are determined on the basis of the design preferences.

In the embodiment illustrated in <FIG>, in a packaged semiconductor transducer <NUM> the protective membrane <NUM> forms part of a region <NUM> of porous polycrystalline silicon, which extends from the second outer face 205b of the cap <NUM> towards the inside of the channel <NUM> and coats the side surface of the channel <NUM> itself between the second face 205b and the protective membrane <NUM>.

The packaged semiconductor transducer <NUM> of <FIG> may be manufactured following the method described in what follows with reference to <FIG>.

The transduction microstructure <NUM> is obtained using a microstructure semiconductor wafer <NUM>' (visible in <FIG>) according to a known method.

To manufacture the cap <NUM> of <FIG>, a cap semiconductor wafer <NUM>', which comprises a bulk layer <NUM>' of monocrystalline silicon, is etched through a first resist structure 20a, defined with a first mask 20b, as illustrated in <FIG>. The bulk layer <NUM>' is removed for a thickness corresponding to the height of the pedestals <NUM>, which are formed in regions protected by the first mask <NUM>.

The cap semiconductor wafer <NUM>' is then again dry-etched with a second resist structure 21a (defined with a second mask 21b) to open a trench <NUM>, as shown in <FIG>. 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>), a stop layer <NUM> and a protection layer <NUM> of porous polycrystalline silicon and permeable to aeriform substances, are formed in succession on the cap semiconductor wafer <NUM>', both on the first face 5a and inside the trench <NUM>. The stop layer <NUM> may 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 layer <NUM> is deposited on the stop layer <NUM> and may have a thickness comprised between <NUM> and <NUM>. In addition, the diameter of the pores and the empty/full ratio of the protective silicon layer <NUM> are selected on the basis of the design preferences for the protective membrane <NUM> described above. For instance, the protection layer <NUM> has pores of equivalent diameter comprised between <NUM> and <NUM>, the size and the density of the pores being such that the empty/full ratio is comprised between <NUM>% and <NUM>%.

The stop layer <NUM> and the protection layer <NUM> coat in a conformable way the first face 5a of the cap semiconductor wafer <NUM>', the side walls and the bottom of the trench <NUM>. The portion of the protection layer <NUM> on the bottom of the trench <NUM> is to form the protective membrane <NUM>.

After the stop layer <NUM> and the protection layer <NUM> have been formed, a resist structure 26a is deposited and defined via a third mask 26b so as to protect the inside of the trench <NUM>, as illustrated in <FIG>. The stop layer <NUM> and the protection layer <NUM> are selectively etched where they are not protected by the third resist structure 26a (<FIG>). The region <NUM> of porous polycrystalline silicon is thus obtained, which is separated from the layer of monocrystalline bulk silicon of the cap semiconductor wafer <NUM>' by a stop structure <NUM>', defined by a residual portion of the stop layer <NUM>.

After the third resist structure 26a has been removed, a coupling layer <NUM>, for example, of germanium, is deposited in a conformable way on the cap semiconductor wafer <NUM>' (<FIG>).

A fourth resist structure 29a is then deposited and defined via a fourth mask 29b (<FIG>). The fourth resist structure 29a coats the pedestals <NUM> and the coupling layer <NUM> around them. The coupling layer <NUM> is selectively etched and removed where it is not protected by the fourth resist structure 29a. The coupling coatings <NUM> are thus created, as illustrated in <FIG>.

With reference to <FIG>, the cap semiconductor wafer <NUM>' 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 trench <NUM>, until the stop structure <NUM>' is reached, which in this step protects the region <NUM> of porous polycrystalline silicon. The channel <NUM> is thus completed. The stop structure <NUM>' is then selectively etched. In particular, the portion of the stop structure <NUM>' exposed within the channel <NUM> is removed, thus releasing the protective membrane <NUM>. Consequently, the protective membrane <NUM> extends in a direction transverse to a longitudinal axis of the channel <NUM> (i.e., an axis directed from the first end to the second end of the channel <NUM>). The portions of the channel <NUM> on opposite sides of the protective membrane <NUM> communicate with one another through the protective membrane <NUM> of porous polycrystalline silicon.

The cap semiconductor wafer <NUM>' is finally joined to the bonding pads <NUM> of the microstructure semiconductor wafer <NUM>' to form a composite wafer (<FIG>), which is diced, thus obtaining a plurality of examples of the packaged semiconductor transducer <NUM> of <FIG>. Each example comprises a portion of the microstructure semiconductor wafer <NUM>', which defines the substrate <NUM> and contains the transduction microstructure <NUM>, and a portion of the cap semiconductor wafer <NUM>', which defines the cap <NUM>.

In order to manufacture the packaged semiconductor transducer <NUM> of <FIG>, the method described may be immediately adapted by modifying the masks <NUM> and <NUM> and 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 transducer <NUM> of <FIG>, instead, the cap semiconductor wafer is turned upside down after the pedestals <NUM> have been formed. The machining operation proceeds on the back of the cap semiconductor wafer (second face 205b), as already described until the stop layer <NUM> and the protective silicon layer <NUM> are etched to form the region <NUM> of porous polycrystalline silicon and the stop structure <NUM>'. 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 face 205a of the cap semiconductor wafer and with anisotropic etching of the bulk layer to form the channel <NUM>, followed by selective removal of the stop layer within the channel <NUM>. The cap semiconductor wafer is joined to the microstructure semiconductor wafer <NUM>' and, after singulation of the composite wafer thus obtained, examples of the packaged semiconductor transducer <NUM> of <FIG> are obtained.

With reference to <FIG>, a packaged semiconductor transducer <NUM> according to an embodiment of the present invention is designated by number <NUM> and comprises the substrate <NUM>, the transduction microstructure <NUM> integrated in the substrate <NUM> and a cap <NUM>.

The cap <NUM> comprises a bulk layer <NUM>, a stop layer <NUM>, a structural layer <NUM> and a protection layer <NUM> of porous polycrystalline silicon and permeable to aeriform substances. The stop layer <NUM>, of silicon oxide, is interposed between the bulk layer <NUM>, of monocrystalline silicon, and the structural layer <NUM>, of a material that can be selectively etchable with respect to the structural layer <NUM>, for example silicon nitride.

Channels <NUM> extend from a second face 305b to a first face 305a of the cap <NUM> through the bulk layer <NUM>, the stop layer <NUM>, and the structural layer <NUM>. The protection layer <NUM> forms protective membranes <NUM> in the channels <NUM>. More precisely, the protection layer <NUM>, which covers the first face 305a of the cap <NUM>, penetrates into the channels <NUM> coating the side surfaces substantially as far as an interface <NUM> between the structural layer <NUM> and the stop layer <NUM>. At the depth of the interface <NUM>, the protection layer <NUM> extends in a direction transverse to longitudinal axes of the channels <NUM> to form the protective membranes <NUM>.

The first face 305a of the cap <NUM> is joined to the substrate <NUM> by pedestals <NUM> that act as adhesion structures and, at the same time, have a height such as to create a gap <NUM> between the substrate <NUM> and the cap <NUM>.

The microstructure <NUM> communicates with the outside through the channels <NUM> and the protective membranes <NUM>, which enable passage of aeriform substances.

Whereas the transduction microstructure <NUM> is obtained using a microstructure semiconductor wafer <NUM>' as already described, according to a method for manufacturing the packaged semiconductor transducer <NUM> of <FIG> illustrated in <FIG>, a cap semiconductor wafer <NUM>' initially comprises the bulk layer <NUM>. The stop layer <NUM> and the structural layer <NUM> are formed in succession on the bulk layer <NUM> (<FIG>).

The structural layer <NUM> is then selectively etched using a mask <NUM> for opening cavities <NUM>, which extend as far as the stop layer <NUM>, as illustrated in <FIG>.

The protection layer <NUM> is then formed on the first face 305a of the cap semiconductor wafer <NUM>' (<FIG>), which is then turned upside down and etched on the back, i.e., on the side of the second face 305b, in positions corresponding (aligned) to respective cavities <NUM> (<FIG>). In particular, a markedly anisotropic trench etch is first carried out as far as the stop layer <NUM>, and then the stop layer <NUM> is etched where it is left exposed. In this way, the protective membranes <NUM> are freed and the channels <NUM> are completed.

The protection layer <NUM> is selectively etched to expose the structural layer <NUM> where the pedestals <NUM> are then formed.

The cap semiconductor wafer <NUM>' is finally joined to the bonding pads <NUM> of the microstructure semiconductor wafer <NUM>' to form a composite wafer, which is then diced, thus obtaining a plurality of examples of the packaged semiconductor transducer <NUM> of <FIG>. Each example comprises a portion of the microstructure semiconductor wafer <NUM>', which defines the substrate <NUM> and contains the transduction microstructure <NUM>, and a portion of the cap semiconductor wafer <NUM>', which defines the cap <NUM>.

The invention 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> shows an electronic system <NUM> that 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 device <NUM> may be a general-purpose or embedded processing system in a device, an apparatus or a further system.

The electronic system <NUM> comprises a processing unit <NUM>, memory devices <NUM>, a packaged semiconductor transducer, for example the packaged semiconductor transducer <NUM> of <FIG>, and may moreover be provided with input/output (I/O) devices <NUM> (for example a keyboard, a mouse or a touchscreen), a wireless interface <NUM>, peripherals <NUM>,. N and possibly further auxiliary devices (here not illustrated). The components of the electronic system <NUM> may be coupled in communication with one another directly and/or indirectly through a bus <NUM>. The electronic system <NUM> may moreover comprise a battery <NUM>. It should be noted that the scope of the present invention is not limited to embodiments necessarily having one or all of the devices listed.

The processing unit <NUM> may for example comprise one or more microprocessors, microcontrollers, and the like, according to the design preferences.

The memory devices <NUM> may 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.

Claim 1:
A semiconductor device comprising:
a substrate (<NUM>);
a transduction microstructure (<NUM>) integrated in the substrate (<NUM>);
a cap (<NUM>; <NUM>; <NUM>; <NUM>) joined to the substrate (<NUM>) and having a first face (5a; 105a; 205a; 305a) adjacent to the substrate (<NUM>) and an outer second face (5b; 105b; 205b; 305b);
a channel (<NUM>; <NUM>; <NUM>) extending through the cap (<NUM>; <NUM>; <NUM>; <NUM>) from the second face (5b; 105b; 205b; 305b) to the first face (5a; 105a; 205a; 305a) and communicating with the transduction microstructure (<NUM>); the semiconductor device characterised in further comprising
a protective membrane (<NUM>; <NUM>; <NUM>; <NUM>) made of porous polycrystalline silicon and permeable to aeriform substances, the protective membrane (<NUM>; <NUM>; <NUM>; <NUM>) being arranged across the channel (<NUM>; <NUM>; <NUM>).