MEMS device incorporating a fluidic path, and manufacturing process thereof

A MEMS device wherein a die of semiconductor material has a first face and a second face. A membrane is formed in or on the die and faces the first surface. A cap is fixed to the first face of the first die and is spaced apart from the membrane by a space. The die is fixed, on its second face, to an ASIC, which integrates a circuit for processing the signals generated by the die. The ASIC is in turn fixed on a support. A packaging region coats the die, the cap, and the ASIC and seals them from the outside environment. A fluidic path is formed through the support, the ASIC, and the first die, and connects the membrane and the first face of the die with the outside, without requiring holes in the cap.

BACKGROUND

1. Technical Field

The present disclosure relates to a MEMS device incorporating a fluidic path and to the manufacturing process thereof. In particular, the following description makes reference, without any loss of generality, to assembling of a MEMS pressure sensor of a packaged type.

2. Description of the Related Art

Sensors are known that include micromechanical structures made, at least in part, of semiconductor materials and using MEMS (micro-electro-mechanical systems) technology. Specifically, pressure sensors made using the MEMS technology typically find use in the medical field, in household apparatus, in consumer electronics (cellphones, personal digital assistants—PDAs), and in the automotive field. In particular, in the latter sector, pressure sensors are traditionally used for detecting the pressure of tires of vehicles, and are used by the control unit for signaling alarms. Pressure sensors are used also for monitoring the pressure of air-bags, for controlling the failure pressure of the ABS system, and for monitoring the engine-oil pressure, the fuel-injection pressure, etc.

A MEMS sensor generally comprises a micromechanical detection structure, which transduces a mechanical quantity to be detected (for example, a set of acoustic waves, a pressure, etc.) into an electrical quantity (for example, correlated to a capacitive variation); and an electronic reading circuit, usually made as an ASIC (Application-Specific Integrated Circuit), which performs processing operations (including amplification and filtering) of the electrical quantity and supplies an electrical output signal of an analogue type (for example, a voltage) or digital type (for example, a PDM (pulse-density modulation) signal. The electrical signal, possibly further processed by an electronic interface circuit, is then made available to an external electronic system, for example a microprocessor control circuit of the electronic apparatus incorporating the sensor.

To detect the mechanical quantity, the MEMS structure comprises a membrane formed in or on a semiconductor die and suspended over a cavity. The membrane moreover faces the external environment or is in communication with the latter through a fluidic path, as shown, for instance, in U.S. Pat. No. 8,049,287. filed in the name of the present applicant, disclosing a detection structure including a MEMS pressure sensor, of a differential capacitive type. In particular, in U.S. Pat. No. 8,049,287. the membrane faces a chamber formed in a protective cap fixed at the top to the die or faces a cavity etched from the back of the die and connected with the outside through a hole which extends through supporting elements.

The known MEMS structure, dedicated to detecting differential pressures, may be modified for detecting absolute pressures and may moreover undergo improvement as regards the simplicity of manufacture. In fact, the presence of a hole in the cap typically involves a complex molding of a packaging region, which has to be formed flush with the cap in order to prevent occlusion of the hole therein and is thus generally replaced by bonding of pre-shaped caps. Furthermore, the formation of the membrane facing the rear cavity is difficult to obtain since control of the thickness of the membrane, formed by deep etching from the back of the substrate, is complex.

BRIEF SUMMARY

One or more embodiments of the present disclosure a MEMS device is directed to incorporating a fluidic path and the corresponding manufacturing process. One embodiment is directed to a MEMS device comprising a first die of semiconductor material having a first face and a second face. A membrane is formed in or on the first die and facing the first face. A cap is fixedly coupled to the first die, facing the first face of the first die and spaced apart from the membrane by a space. A support is coupled to the first die and facing the second face of the first die. The MEMS device includes a fluidic path extending through the support and the first die and connecting the membrane to the outside of the MEMS device.

DETAILED DESCRIPTION

FIG. 1shows a MEMS device100, such as a packaged absolute pressure sensor, fixed on a supporting body101, formed, for example, by a printed-circuit board.

The MEMS device100is fixed to the supporting body101through adhesive regions, such as conductive adhesive regions102(for example, of Au, Cu or Sn), that keep the MEMS device100raised with respect to the supporting body101, thus creating a gap103between them. The adhesive regions102are spaced from each other so that the gap103is in fluid communication with an environment outside of the MEMS device100. The number and spacing of the adhesive regions102may vary. In general, the adhesive regions102provide suitable adhesion between the supporting body101and MEMS device100and adequately support the MEMS device100.

The MEMS device100comprises a die or chip1of semiconductor material, such as silicon, integrating a membrane2and electrical components (not shown). In detail, the die1has a first face1aand a second face1band the membrane2is flush with the first face1a(also defined hereinafter as “top face” of die1). The membrane2(see also the top plan view ofFIG. 7) is formed in a suspended region3separate from the rest of the die1(referred to hereinafter as “peripheral portion8” of the die1) through a trench4and supported by the peripheral portion8through elastic elements (also referred to as “springs5”).

In the example shown, the suspended region3and the trench4have a rectangular shape, in particular square, but other shapes, for example circular, may be envisaged. In the embodiment ofFIG. 1, the suspended region3has a smaller thickness than the peripheral portion8of the die1so that an air gap9extends underneath the suspended region3, laterally delimited by the peripheral portion8of the die1.

Furthermore, the membrane2is delimited at the bottom by a buried cavity6, which extends within the suspended region3.

A cap10covers at the top the suspended region3(including the membrane2), protects it from impact and external stresses and enables simplification of the manufacturing and assembly process, as described in greater detail hereinafter. The cap10is fixed to the first face1a of the die1via bonding regions11, for instance, of metal (Au, Sn, Cu, etc.) or glass frit, or polymeric materials, which are fixed to the peripheral portion8and extend over the top face1aof the die, outside the trench4. The cap10is then at a distance12from the cap10due to the thickness of the bonding regions11. In addition, in a way not shown, the side of the cap10facing the suspended region3can be etched so as to form a cavity of the cap facing the face1aof the die1.

The die1is fixed, on its second face lb, to a second die15, which may incorporate a processing circuit, for example an ASIC. To this end, a first adhesive layer16, of patternable material, such as a biadhesive film, for example a die-attach film (DAF), is arranged between the processing circuit15and the die1. In the example shown, the die1is connected to the processing circuit15by wire connection17, in a per se known manner.

In turn, the processing circuit15is fixed at the bottom to a support20, for example an organic multilayer substrate, such as a bismaleimide-triazine (BT) layer of, e.g., land-grid-array (LGA) type, via a second adhesive layer21, of patternable material, for example a biadhesive film, similar to that of the first adhesive layer16. The processing circuit15is electrically connected to the support20by wire connections22, in a per se known manner, and has an area (in top plan view) smaller than that of the support20.

A hole23extends through the support20, the second adhesive layer21, the processing circuit15, and the first adhesive layer16and sets the trench4in communication with the outside through the air gap9, as explained in greater detail hereinafter.

A packaging material25, for example a plastic material, such as resin, completely coats the die1, the cap10, and the processing circuit15and extends laterally flush with the support20so as to encapsulate and completely insulate the die from the external environment, except for the fluidic path including the hole23.

In particular, as indicated, the top surface of the membrane2is fluidically connected to the outside of the packaged sensor1through the trench4, the air gap9, the hole23, and the gap103and is sensitive to the pressure outside the MEMS device100.

The membrane2is provided, in a per se known manner and not shown, with transducer elements, for instance, piezoresistive elements, which, upon detection of a deformation of the membrane2as a result of the pressure acting on the membrane2itself (and equal to the external pressure, as indicated), generate an electrical signal supplied to the processing circuit15, which then generates an electrical signal indicating the detected pressure.

FIG. 2shows a MEMS device200wherein the suspended region3has a projecting portion or stem30, which extends into the air gap9towards the processing circuit15so that the total thickness of the suspended region3at the projecting portion30is substantially equal to that of the peripheral portion8of the die1. In this case, the hole23is offset with respect to the suspended region3, even though it is connected with the air gap9and the trench4, in order to provide the fluidic path. In this way, the bottom portion30forms an element limiting the movement in a vertical direction of the suspended region3in the event of impact or impulsive stresses from outside, which could cause failure of the springs5, by hitting upon the second die15.

FIG. 3shows a MEMS device300wherein the die1is thinned so that the peripheral portion8has the same thickness as the suspended region3. Here, the first adhesive layer16also forms a spacer so that between the die1and the processing circuit15a gap is created that enables fluids (air, gases, liquids) outside the MEMS sensor300to reach the membrane2. In this way, a separate element is not utilized for limiting the vertical displacement of the suspended region3, rather the underlying processing circuit15forms a stop element for possible excessive oscillations caused by impact or other impulsive stresses.

The MEMS devices100,200are manufactured as described hereinafter with reference toFIGS. 4-12.

As shown inFIG. 4, a first silicon wafer40is processed so as to form the buried cavities6that define the membranes2at the bottom. Formation of the buried cavities6may be obtained in different ways, for example, as taught in EP1577656.

As shown inFIG. 5, the first wafer40is etched from the back, using an appropriate mask (not shown), to form the air gaps9under and at a distance from the buried cavity6. For instance, etching may be carried out using dry processes such as deep silicon etching or in wet processes such as timed TMAH (tetramethyl-ammonium hydroxide).

As shown inFIG. 6, the first wafer40is etched from the front by deep silicon etching so as to define the trenches4and the springs5and release the suspended regions3, as may be seen also in the top plan view ofFIG. 7.

As an alternative to the above, front etching of the first wafer40to define the trenches4and the springs5may be carried out prior to forming the air gaps9, in practice reversing the flow described with reference toFIGS. 5 and 6. In this case, front etching may be timed to form the trenches (non-through trenches) as far as a given depth of the first wafer40, and release of the suspended regions3is obtained while forming the air gaps9, which extend as far as the trenches4.

As shown inFIG. 8, a second wafer50, having through cavities51of a rectangular shape, is fixed to the first wafer40using a known wafer-to-wafer bonding process, interposing the bonding regions11.

A composite wafer60is thus obtained, where the through cavities51have the purpose of enabling access to the pads (not shown) used for the electrical wire connection17.

As shown inFIG. 9, the second wafer50is thinned through a grinding step, and the composite wafer60is diced so that the caps10are no longer connected to the rest of the second wafer50. Consequently, a plurality of elements55is obtained, each formed by a die1and the corresponding cap10.

Separately (FIG. 10), using normal semiconductor manufacturing techniques, the processing circuit (ASIC)15is formed and then perforated in an appropriate area, for example via deep reactive ion etching (DRIE) of epitaxial silicon or by laser or via sand jet. A hole61is thus formed, having, for example, a diameter in the order of 10-100 μm. The hole61may be formed at wafer level, for example via DRIE, or after dicing of the second die15, for example by laser.

As shown inFIG. 11, the processing circuit15is fixed on the support20, which is also provided with an own hole62, so that the holes61,62of the processing circuit15and of the support20are arranged on top of one another and form the hole23ofFIG. 1. Etching is carried out by interposition of the second adhesive layer21, already perforated, to obtain a stack65, as represented inFIG. 11.

As shown inFIG. 12, the stack65is fixed to the element55to form a non-packaged pressure sensor66.

Customary steps follow for forming the wire connections17and22(FIG. 1), and the package25is then molded. In particular, due to the absence of holes in the cap10, molding may be carried out according to standard semiconductor techniques, avoiding complex techniques or bonding of pre-patterned caps for MEMS sensors of the type considered. The MEMS device100/200is thus obtained.

To manufacture the MEMS device300ofFIG. 3, it is possible to perform steps like the ones described above, except for forming the cavity9as described with reference toFIG. 5. In fact, for MEMS sensor300, this step is replaced by a thinning of the first wafer40, which is generally carried out after fixing the first wafer40on the second wafer50and thus after forming the trench4.

FIG. 13shows an embodiment wherein the membrane (here designated by70) extends over the first face1aof the die1. In this case, the manufacturing process differs from the one described only as regards the formation of the membrane70(in a per se known manner), and for the rest comprises steps similar to those described for the MEMS device100,200or300.

FIG. 14is a schematic illustration of an electronic apparatus150that uses the MEMS device100-400.

The electronic apparatus150comprises, in addition to the MEMS device100-300, a microprocessor154, a memory block155, connected to the microprocessor154, and an input/output interface156, which is also connected to the microprocessor154. Furthermore, a speaker158may be present, for generating a sound on one audio output (not shown) of the electronic apparatus150.

In particular, the electronic apparatus150is fixed to the supporting body101, here formed by a printed circuit, to which the MEMS device100-300and, moreover, the microprocessor154and the memory block155are mechanically and electrically coupled.

The electronic apparatus150is, for example, an apparatus for measuring blood pressure (sphygmomanometer), a household apparatus, a mobile communication device (cellphone, PDA, notebook) or an apparatus for measuring pressure that can be used in the automotive field.

The MEMS device100-300described herein has numerous advantages.

Due to the presence of the fluidic path formed by the hole23, the trench4, and possibly the air gap9, as well as the gap103, it is possible to expose the membrane2to the external environment even without the presence of a front hole in the cap10. This considerably facilitates the molding operations of the package25, since it is no longer necessary to protect the front hole during molding. It follows that molding can be carried out using a standard full-molding process, which is much less expensive and ensures a high yield.

Furthermore, as compared to the solutions where pre-patterned and glued caps are used, the final structure is more compact, which enables the use of the present MEMS sensor also in applications where space is critical.

Exposure to the external environment is obtained by keeping the membrane2on the front side (facing the cap10) of the die1, which enables use of known processes of surface machining of the silicon and implies a labyrinthine structure of the fluidic path. The labyrinthine structure reduces exposure of the membrane2to external contaminants, such as particles, dust, and moisture, which possibly get trapped along the fluidic path, without blocking it, and cannot reach the membrane2. It follows that the reliability and robustness of the MEMS sensor are enhanced, due also to the monolithic structure of the present MEMS sensor.

The MEMS sensor moreover is exposed to the external environment in the bottom area, as desired in certain solutions of assembly on the supporting body101, for example in the case of assembly on the board of a cellphone, with components mounted on an opposite side of the board and connected through holes in the board.

Separation between the peripheral portion8and the suspended region3, and thus the membrane2, of the die1due to the trench4prevents any assembly stresses from giving rise to deformation of the membrane and variation of the electrical parameters of the sensor, which would generate imprecision in reading.

Finally, it is clear that modifications and variations may be made to the MEMS sensor and to the manufacturing process described and illustrated herein, without thereby departing from the scope of the present disclosure.

For instance, the present MEMS sensor, instead of being a pressure sensor, could be a humidity sensor, a flow sensor, an environmental sensor (i.e., a combined pressure/humidity/temperature sensor), an air/gas sensor, a microfluidic device, or a miniaturized microphone.

The cap10, as has been mentioned, could have a cavity forming a chamber overlying the membrane2, as shown inFIG. 13, even though this entails an ad hoc processing operation. Furthermore, the cap10could be shaped so as to surround the die1and be fixed directly to the integrated circuit15or to the support20.

The connections between the die1and the integrated circuit15might not be of the wire type; for example, it is possible to use through vias and/or connections from the back.