Patent Description:
As is known, for spraying inks and/or perfumes as well as in e-cigarettes or in inhalation medical devices, the use has been proposed of microfluidic devices of small dimensions, which may be obtained with microelectronic manufacturing techniques.

The delivery of known or unknown composition fluid is feasible with modified design, ink jet structures, described for example in <CIT>, <CIT>(corresponding to <CIT>), <CIT> or <CIT>.

In addition, <CIT> discloses a liquid ejecting apparatus and a manufacturing method thereof using a PZT bulk technology, wherein a thick metal plate is worked to form liquid passages and a chamber and a piezoelectric/electrostrictive element is fixed to the metal plate. Ejection is obtained by the piezoelectric/electrostrictive element that generates a pressure wave.

Another piezoelectric inkjet head is disclosed in <CIT>.

However, in some applications, such as in nebulizer applications, it is desired to spray drops of very small dimensions, as small as <NUM>. However, current semiconductor technologies allow manufacture of nozzles with diameters greater than <NUM>.

To solve this issue, for example <CIT> discloses a microfluidic device formed in a body accommodating a fluid containment chamber. An exemplary embodiment is shown in <FIG>. Here, a chamber <NUM> formed in a body <NUM> is coupled to a fluid access channel <NUM> and to a drop emission channel or nozzle <NUM> formed in a nozzle plate (not visible, overlying the chamber <NUM>). The drop emission channel <NUM> overlies the chamber <NUM> and is partially offset thereto, to define an intersection area <NUM> having smaller dimension than the hole area and thus defining an effective exit area. A heater <NUM> is formed in the body <NUM> under the chamber <NUM> and is configured to heat the fluid in the chamber <NUM> so as to generate a drop that is emitted through the drop emission channel <NUM>.

Thus, small drops may be obtained. In particular, the dimensions of the drops (diameter/volume) are directly linked to the nozzle diameter, as shown in <FIG> plotting the volume of the emitted drops as a function of the diameter of the nozzle (the effective exit area in <FIG>).

<CIT> discloses a similar microfluidic dispenser wherein small drops are obtained using offset nozzles openings having different shapes and arranged at different positions.

This solution has been successful in reducing the dimensions of the emitted drops but has caused further challenges regarding the operation of the device, in particular when it is desired to spray a high number of very small drops with high frequency.

In particular, for obtaining a sufficient volume of emitted fluid, test structures comprising a plurality of apertures arranged on the periphery of the chamber have been studied. However, it has been seen that this architecture may not be thermally efficient.

In fact, for example, microfluidic devices with peripheral offset nozzles with a diameter of <NUM>, configured to obtain drops of about <NUM> pL (picoliters) have been studied. This results in a drop volume that is less than <NUM>% of the chamber volume, and thus of fluid contained in the chamber (for example, <NUM> pL). Therefore, a much higher volume of fluid is heated than the volume of the actually ejected fluid. Consequently, it has been observed that heat energy builds up very quickly in the chamber and may cause the die, accommodating a plurality of adjacent chambers, to overheat.

In some cases, boiling of the fluid has been observed even before it enters the chambers, globally depriming the system. Therefore, in devices comprising many chambers each connected to a plurality of nozzles, with ignition at high frequency (even higher than <NUM>), the risk of a failure of the entire device due to global depriming exists. In addition, depriming may occur very quickly, destroying the device.

Thus, an aim of the invention is to provide an improved microfluidic device solving the problems of the prior art.

According to the present invention, there are provided a microfluidic device and a manufacturing process thereof, as defined in the attached claims.

For the understanding of the present invention, embodiments thereof are now described, purely as a non-limitative example, with reference to the enclosed drawings, wherein:.

Hereinbelow, embodiments of a microfluidic device will be described in detail. In the ensuing description, spatial indications such as "upper", "lower", "on", "over", "under", "top", "bottom", and so on are to be interpreted according to the discussed Figures and are not limitative.

<FIG> show a microfluidic device <NUM> (not belonging to the invention) manufactured using micro-manufacturing steps, as discussed more in detail hereinafter.

The microfluidic device <NUM> has a general structure shown in <FIG> and is formed in a body <NUM> including a substrate <NUM>, an insulating layer <NUM>, a chamber layer <NUM>, and a nozzle layer <NUM>.

The substrate <NUM>, the insulating layer <NUM>, the chamber layer <NUM> and the nozzle layer <NUM> extend over each other in a height direction, parallel to a vertical axis (first axis Z of a Cartesian reference system XYZ).

The substrate <NUM> is for example of semiconductor material, such as monocrystalline silicon. The insulating layer <NUM> is for example a multilayer including silicon oxide, silicon nitride and other insulating layers. The substrate <NUM> and the insulating layer form a base body portion <NUM>. The chamber layer <NUM> is for example a polymeric material such as dry film. The nozzle layer <NUM> may be formed by semiconductor material, such as monocrystalline silicon or a polymeric material such as dry film, as discussed hereinbelow.

The chamber layer <NUM> forms a plurality of chambers <NUM>, one chamber <NUM> being shown in <FIG>. The chambers <NUM> are laterally delimited by lateral walls <NUM> formed by the chamber layer <NUM>; in addition, the chambers <NUM> are delimited by a bottom base 17A formed by the insulating layer <NUM> and by an upper base 17B formed by the nozzle layer <NUM>. The bottom base 17A and upper base 17B extend along a first direction and a second direction, respectively, the second direction transverse to the first direction.

The insulating layer <NUM> accommodates a plurality of actuators, here heaters <NUM> (one shown). The heaters <NUM> are arranged below the chambers <NUM>, one heater <NUM> for each chamber <NUM>. However, in the alternative, more heater portions <NUM> may be arranged under each chamber <NUM>.

Each heater <NUM> is coupled to a firing circuit, not shown, through connection lines <NUM>.

Inlets <NUM> extend through the chamber layer <NUM> from opposite sides of the chamber <NUM>. The inlets <NUM> connect the chamber <NUM> with a fluid supply channel not shown here.

A plurality of nozzle openings <NUM> extend through the nozzle layer <NUM> along the periphery of each chamber <NUM>. Specifically, as clearly visible in <FIG>, the nozzle openings <NUM> partially overlay the chamber <NUM> and fluidically connect the chamber <NUM> with the outside of the microfluidic device <NUM>, for the ejection of liquid drops.

In practice, the microfluidic device <NUM> exploits the teaching of <CIT> discussed above, in order to reduce the exit area of the drops. Therefore, as shown in the enlarged detail in <FIG>, the nozzle openings <NUM> form each an intersection area <NUM> similar to intersection area <NUM> of <FIG>.

In <FIG>, the lateral walls <NUM> of the chamber <NUM> extend along a rectangle and the chamber <NUM> has a parallelepipedal shape with rectangular bottom base 17A, extending parallel to plane XY of the Cartesian reference system XYZ. In the top view of <FIG>, the bottom base 17A of the chamber <NUM> has long sides much longer than the short sides.

In particular, the length of the long sides of the bottom base 17C is greater than twice the length of the short sides; in <FIG>, the long sides of the rectangular bottom base 17C are four times longer than the short sides.

In <FIG>, the inlets <NUM> open in the chamber <NUM> at the short sides of the chamber <NUM>. The nozzle openings <NUM> extend adjacent and partially intersecting (that is, overlapping) the long sides.

The nozzle openings <NUM> are designed to have small intersection areas <NUM>, where the nozzle openings <NUM> and the chamber <NUM> overlap. Thereby, the drop volume is reduced, as visible from the plot of <FIG> showing the relationship between drop diameter and the effective exit area, that is the intersection area. Here, the interesting area is the one comprised between <NUM> and <NUM><NUM>.

In the shown example, the nozzle openings <NUM> have a triangular, almost isosceles shape, with an acute angle corner intersecting the chamber <NUM> and forming intersection area <NUM>. Thereby, for a triangle height Ht (<FIG>) of <NUM>, feasible with the present technology, an intersection area <NUM> of about <NUM><NUM> may be obtained, and consequently, a drop volume of about <NUM> pl.

In the microfluidic device <NUM>, the chamber <NUM> and the nozzle openings <NUM> are designed in order to have a volume ratio between drop volume and chamber volume that is higher than <NUM>%.

From study of the Applicant, it has been observed that, by designing the chamber <NUM> so as to maximize its perimeter (thereby, to have a higher number of small nozzle openings <NUM>) while reducing the volume of the chamber <NUM>, less overheating is obtained.

In particular, it has been demonstrated that, with a volume ratio higher than <NUM>%, a constant high flow of liquid from the inlets <NUM> to the nozzle openings <NUM> may be obtained, eliminating stationary liquid in the chamber <NUM> and thus chamber deprime.

For example, this may be obtained for a chamber <NUM> having a width W = <NUM>, a length L = <NUM> and thus a volume of <NUM><NUM>, eight nozzle openings <NUM> (with a total volume of emitted drops of <NUM> pL). The same ratio may be obtained with chambers <NUM> having an area that is an entire multiple n of the base chamber area and a number of nozzle openings <NUM> equal to 4n:.

<FIG> shows a device <NUM> (not forming part of the invention) including three groups of emitting portions <NUM>, each formed by a plurality of chambers <NUM> (here five), adjacent to each other.

Here each chamber <NUM> is formed by four basic cells (as indicated by the dashed lines) and thus has sixteen nozzle openings <NUM>.

The heaters <NUM> of the chambers <NUM> of a same group of emitting portions <NUM> are connected together, as indicated in <FIG> by lines <NUM>. In particular, as shown, the heaters <NUM> of a same group of emitting portions <NUM> are coupled between a firing circuit, supplying firing pulses Vo, e.g., 10V, and ground.

According to an embodiment of the present microfluidic device, a small intersection area may be obtained by forming small dimension features in the lateral wall <NUM> of the chamber <NUM>, instead of in the nozzle layer <NUM>. In fact, Applicant's tests have shown that alignment of the nozzle openings <NUM> with respect to the chambers <NUM> may be sometimes difficult. In addition, in some instances, drilling of very small nozzle openings <NUM> in the nozzle layer <NUM>, e. g, by laser, has been proved challenging and does not always bring to the formation of openings with constant dimension; in rare cases, partially closed nozzles were observed, thereby resulting in uneven intersection areas and not optimal behaviour.

Specifically, according to this embodiment, the lateral wall <NUM> is not smooth and straight, but has a plurality of protrusions of very small dimensions. Each nozzle opening has here an area (in top plan view) comparable with the chamber area and extend almost entirely offset with respect to an adjacent chamber <NUM> except for at the chamber protrusions, thereby defining a plurality of nozzle apertures of very small area.

For example, <FIG> show three different shapes of chambers and nozzle apertures that allow to obtain very small intersection areas in a simple way, using current macromachining techniques.

<FIG> shows a microfluidic device <NUM> comprising a chamber layer <NUM>.

Chamber layer <NUM> forms a plurality of chambers <NUM> (four visible) having here a generally rectangular area in top plan view (parallel to the plane XY). The chambers <NUM> are delimited by lateral walls <NUM> formed by the chamber layer <NUM>. The lateral walls <NUM> of each chamber <NUM> form a plurality of protrusions <NUM> (<FIG>) adjacent to each other, extending inside the chamber <NUM> and separated a corresponding plurality of indentations <NUM>.

Heaters <NUM> extend below the chambers <NUM> and are represented by dotted lines.

The protrusions <NUM> have here a generally square shape, with sides, e.g., of about <NUM>-<NUM>.

A nozzle layer <NUM> (represented by hatched lines) extends on the chamber layer <NUM> and upwardly closes the chambers <NUM>. The nozzle layer <NUM> has openings <NUM> offset to the chambers <NUM>, but intersecting (overlapping) the indentations <NUM>.

In particular, the openings <NUM> are vertically aligned to portions <NUM> of the chamber layer <NUM> extending between pairs of adjacent chambers <NUM>.

In more detail, each nozzle opening <NUM> extends between two adjacent chambers <NUM> and intersects the indentations <NUM> of the two adjacent chambers <NUM> at two different portions of its periphery.

Thereby, the openings <NUM> may have a large area, even larger than the chambers; therefore they may be obtained in a simple way and with high size accuracy.

Here, also the openings <NUM> are rectangular in top plan view.

Thereby the openings <NUM> and the indentations <NUM> form intersection areas <NUM> (<FIG>) of very small dimensions, and in particular, of a few µm<NUM>.

For example, if the openings <NUM> extend up to almost the entire length of the indentations <NUM> (along a second axis Y of the Cartesian reference system XYZ, parallel to the width dimension of the chambers <NUM>) an exit area of <NUM> x <NUM><NUM> may be obtained for each cavity <NUM>.

The intersection areas <NUM> are exit areas for a fluid contained in the chamber <NUM>, in an operating condition of the microfluidic device <NUM>. Thereby, the microfluidic device <NUM> is able to generate very small drops at each chamber <NUM> and, after application of a voltage pulse V to the heaters <NUM> (analogously to what shown in <FIG>), an aerosol of many, very small drops is obtained.

By virtue of the elongated shape of the chambers <NUM> (here having a length, along a third axis X of the Cartesian reference system XYZ, that is about four times the width, along the second axis Y), a volume ratio greater than <NUM>% may be obtained, thereby providing reliable operation of the microfluidic device <NUM>, without overheating or depriming of the microfluidic device <NUM>.

<FIG> also shows inlets <NUM> as well as pillars <NUM> formed in the inlets <NUM> to block any impurity possibly dragged by the entering liquid.

<FIG> shows a different shape of the chambers (here indicated by <NUM>) and of the openings (here indicated by <NUM>) in the nozzle layer <NUM>.

Here the chambers <NUM> have a generally oval or elliptic base area. Also here, the chambers <NUM> are delimited by lateral walls <NUM> forming a plurality of adjacent protrusions <NUM> and a corresponding plurality of indentations <NUM>. In addition, also here each chamber <NUM> has a greater dimension (length, measured along the third axis X) that is about twice the shorter dimension (width, measured along the second axis Y).

For example, the chambers <NUM> may have an elliptical shape with a first semiaxis length of <NUM> and a second semiaxis length of about <NUM>.

The heater, not shown, may have here again rectangular shape.

The protrusions <NUM> and the indentations <NUM> have here pointed tips.

A nozzle layer <NUM> (also represented by hatched lines) extends on the chamber layer <NUM> and has openings <NUM> that, in top plan view, are generally countershaped to the chambers <NUM>. In particular, the openings <NUM> are elongated in a direction parallel to the third axis X and have an arcuate, concave shape. Thus, in different cross-sections taken along the third axis X, the width of openings <NUM> is decreasing from the end (near one inlet <NUM> of the chambers <NUM>) toward a central portion of each opening <NUM>, and then increasing again toward the other end. The openings <NUM> also here at least partially extend over the protrusions <NUM> and the indentations <NUM>.

<FIG> shows another shape of the chambers (here indicated by <NUM>) and of the openings (here indicated by <NUM>) in the nozzle layer <NUM>.

Here, the chambers <NUM> have a general rectangular shape, in top plan view (parallel to plane XY) with point-tipped protrusions <NUM> separated by similarly shaped indentations <NUM>.

The openings <NUM> have a generally constant width (in a direction parallel to the second axis Y) with enlarged ends, with an aspect ratio of at least <NUM>:<NUM>.

In general, in further embodiments, the shape of the chambers, of the openings, of the projections and of the indentations therebetween may widely vary, as long as the openings have micrometric intersection areas with the indentations.

The microfluidic device <NUM> of <FIG> may be manufactured as discussed below with reference to <FIG>.

In these Figures, the manufacturing of a single microfluidic device <NUM> is described; in general however, many microfluidic devices are manufactured in a single wafer and separated at an intermediate or a final step, in a manner known in the art, even if not discussed in detail.

<FIG> shows a portion of a wafer <NUM> that has already been worked to form the heaters and the electrical connection structures.

In detail, <FIG>, the wafer <NUM> comprises a substrate <NUM>, for example of monocrystalline silicon, covered by an insulating layer <NUM> accommodating heaters <NUM>. The substrate <NUM> and the insulating layer <NUM> form a base body portion <NUM>.

Here, the insulating layer <NUM> is a multilayer including, e.g., an oxide layer <NUM>, for example of thermal oxide; a first intermediate dielectric layer <NUM>, for example BPSG (BoroPhosphoSilicate Glass); a second intermediate dielectric layer <NUM>, for example silicon nitride; and a protection layer <NUM>, for example USG (Undoped Silicon Glass).

A heater <NUM>, for example of TaSiN or TaAlN, extend between the first and the second intermediate dielectric layers <NUM> and <NUM>.

A metal layer <NUM>, for example Tantalium, extends here on the second intermediate dielectric layer <NUM> and forms a heat distribution layer. In some applications, however, the metal layer <NUM> may be missing.

The protection layer <NUM> covers the metal layer <NUM> and accommodates electric connection lines <NUM> (<FIG>), of conductive material, for example of Al, that are connected to the heaters <NUM> in openings (not visible) in the protection layer <NUM> and in the metal layer <NUM> and couple the heaters <NUM> to pads (not represented, for simplicity), arranged on the periphery of the microfluidic device.

The protection layer <NUM> is shaped to form chamber cavities <NUM> at locations where the chambers are to be formed. In particular, each chamber cavity <NUM> overlies a respective heater <NUM>. The shape of the chamber cavities <NUM> may be the same as the desired shape of the chambers or any, for example rectangular; in general, the area of the each first chamber cavity <NUM> is smaller than the chamber area.

In addition, the protection layer <NUM> forms tank connection cavities <NUM> (<FIG>), each extending near groups of first chamber cavities <NUM>, as explained better later on, and pad cavities <NUM> (<FIG>) overlying the pads (not shown).

Then, <FIG>, a lower chamber layer <NUM> is deposited and defined. The lower chamber layer <NUM> is for example of a photosensitive dry material that is spinned and defined to delimit lower chamber openings <NUM> vertically arranged over and in prosecution to the chamber cavities <NUM>, but slightly larger, as visible in <FIG>.

The lower chamber openings <NUM> are for example shaped as shown in <FIG> and visible in the enlarged detail of <FIG>. In particular, the lower chamber openings <NUM> are delimited by a wall forming lower indentations <NUM> separated by lower protrusions <NUM> (<FIG>).

In addition, the lower chamber layer <NUM> is shaped to form lower pillar portions <NUM> (<FIG>).

The lower chamber layer <NUM> is also removed to form lower tank connection openings <NUM> over the tank connection cavities <NUM> of <FIG> and to form lower pad opening <NUM> over the pad cavities <NUM>, as shown in <FIG>.

The lower chamber layer <NUM> is then baked and hardened.

In <FIG>, an upper chamber layer <NUM> is deposited and defined. The upper chamber layer <NUM> is for example of a photosensitive dry material, the same or different from the lower chamber layer <NUM>. The upper chamber layer <NUM> is, e.g., spinned and defined to delimit upper chamber openings <NUM> vertically arranged over and in prosecution (e.g., fluidically connected) to the lower chamber openings <NUM>. The lower and upper chamber layers <NUM>, <NUM> form chamber layer <NUM>.

Thereby, lateral walls <NUM> are formed (<FIG>).

As visible in the top plan view of <FIG>, the upper chamber openings <NUM> have a similar shape to the lower chamber openings <NUM>, but are slightly larger. In particular, the upper chamber openings <NUM> have upper indentations <NUM> that extend deeper in the lateral walls <NUM> than the lower indentations <NUM> and upper protrusions <NUM> that are about aligned with the lower protrusions <NUM>, as also visible by the dashed portions in <FIG>.

In addition, the upper chamber layer <NUM> is shaped to form upper pillar portions <NUM> (<FIG>), vertically aligned to the lower pillar portions <NUM>.

As indicated in <FIG>, the upper chamber openings <NUM> and the lower chamber openings <NUM> form chambers <NUM>; the upper protrusions <NUM> and the lower protrusions <NUM> form chamber protrusions <NUM>; the upper indentations <NUM> and the lower indentations <NUM> form chamber indentations <NUM>; the upper pillar portions <NUM> and the lower pillar portions <NUM> form pillars <NUM> (<FIG>).

As can be seen in particular in <FIG>, the lower and upper chamber layers <NUM>, <NUM> also form inlets <NUM>.

The upper chamber layer <NUM> also form upper tank connection openings <NUM> over the lower tank connection openings <NUM> of <FIG> as well as upper pad openings <NUM> over the lower pad openings <NUM> of <FIG>, as shown in <FIG>.

The upper chamber layer <NUM> is then baked and hardened.

Then, <FIG>, the substrate <NUM> is dry etched to remove the semiconductor material of the substrate <NUM> under the tank connection openings <NUM>, <NUM>. Thereby, fluid supply channels <NUM> are formed. The fluid supply channels <NUM> extend through the entire thickness of substrate <NUM>, laterally to the chambers <NUM>, and in fluid connection with the inlets <NUM>.

In <FIG>, a nozzle layer <NUM> is deposited and defined. The nozzle layer <NUM> is, e.g., of a photosensitive dry film, that may be the same or different from the lower and upper chamber layers <NUM>, <NUM>. The nozzle layer <NUM> is laminated and defined according to standard photolithographic techniques to form openings <NUM>, shaped as shown in <FIG> and <FIG>.

The openings <NUM> are offset with respect to the chambers <NUM>, as explained with reference to <FIG> and visible also in <FIG>, so that the nozzle layer <NUM> cover most of the area of the chambers <NUM> except for, at least, part of the chamber indentations <NUM>, forming intersection areas <NUM> (<FIG>).

The nozzle layer <NUM> also upwardly covers the inlets <NUM> and the fluid supply channels <NUM> and is removed over the lower and upper pad openings <NUM>, <NUM> (pad openings <NUM>, <FIG>), to allow electrical connection to the electric connection lines <NUM> (<FIG>).

Therefore, as visible in <FIG> and indicated by arrows L, fluid entering the fluid supply channels <NUM> from a lower face <NUM> of the substrate <NUM> may reach the inlets <NUM> and the chambers <NUM>, be heated by the heaters <NUM>, causing generation of bubbles, and be ejected through the intersection areas <NUM>, analogously to the operation described in above cited patent application <CIT>.

In particular, as shown by the arrows S in <FIG>, by virtue of the small dimensions of the intersection areas <NUM>, many small drops are ejected, ensuring a high total volume of the sprayed liquid with very small diameter drops.

Since the small features determining the dimension of the ejected drops are formed in the lower and upper chamber layers <NUM>, <NUM>, in particular in the upper chamber layer <NUM>, which may be defined in a simple way, using standard, reliable and well known photolithographic techniques, manufacturing of the microfluidic device <NUM> is simple and reliable.

The obtained geometry is thus well controlled and the microfluidic device <NUM> is able to operate in a desired manner.

By forming the chambers <NUM> so as to have smaller areas at the lower chamber openings <NUM> than at the upper chamber openings <NUM>, better ejection conditions may be obtained; in addition, the resulting chamber <NUM> is more easily complying the volume ratio of <NUM>% discussed above, all the other geometrical aspects being equal.

According to a different device, the nozzle layer <NUM> of <FIG> is formed by a separate wafer, that is bonded to the wafer accommodating the chambers <NUM>, as discussed below with reference to <FIG>.

With reference to <FIG>, a microfluidic device <NUM> not forming part of the invention comprises a lower wafer <NUM> and an upper wafer <NUM>.

The lower wafer <NUM> basically comprise the same structures as wafer <NUM> of <FIG>. Accordingly, the same elements are identified by reference numbers increased by <NUM> with respect to the correspondent elements in <FIG> and are not described in detail again.

In particular, the chamber layer, here identified by number <NUM>, may be formed by a single layer, e.g., of a polymeric material, as shown, or by a multiple layer, analogously to lower and upper chamber layers <NUM>, <NUM> of <FIG>. The chamber layer <NUM> forms chamber <NUM> delimited by a lateral wall <NUM>.

Upper wafer <NUM> is a semiconductor wafer shaped to form a plurality of nozzle openings <NUM>, extending for the entire thickness of the upper wafer <NUM>.

In particular, here, each nozzle opening <NUM> comprises a smaller section portion <NUM> and a larger section portion <NUM>.

Specifically, the upper wafer <NUM> has a lower main surface <NUM>, facing the lower wafer <NUM>, and an upper main surface <NUM>, opposite the lower main surface <NUM>. The smaller section portions <NUM> of the nozzle openings <NUM> extend from the upper main surface <NUM>; the larger section portions <NUM> extend from the lower main surface <NUM> and directly face the lower wafer <NUM>.

The smaller section portions <NUM> of the nozzle openings <NUM> may have a circular cross-section, with a diameter of about <NUM>; the larger section portions <NUM> may also have a circular cross-section, with a diameter of about <NUM>, and be concentric to the smaller section portions <NUM>.

The microfluidic device <NUM> of <FIG> is manufactured as described below, with reference to <FIG>.

Initially, <FIG>, a starting substrate <NUM> is used. Starting substrate <NUM> comprises a first semiconductor layer <NUM>, an intermediate layer <NUM> of insulating material, and a second semiconductor layer <NUM>. For example, first semiconductor layer <NUM> may be silicon with a thickness of about <NUM>, intermediate layer <NUM> may be silicon oxide with a thickness of about <NUM>, and second semiconductor layer <NUM> may be silicon with a thickness of about <NUM>-<NUM>.

In <FIG>, the second semiconductor layer <NUM> is etched using known photolithographic techniques to form the smaller section portions <NUM> of the nozzle openings <NUM>.

The smaller section portions <NUM> of the nozzle openings <NUM> may have the shapes shown in <FIG>. In the alternative, they may be arranged according the so-called showerhead arrangement, as shown in <FIG>, or in any other arrangement.

Then, <FIG>, thermal oxidation is performed; thus an etch stop layer <NUM> covers the surface of the second semiconductor layer <NUM>, including inside the smaller section portions <NUM> of the nozzle openings <NUM>. The etch stop layer <NUM> may be, e.g., <NUM> thick.

In <FIG>, a structural layer <NUM> of silicon is epitaxially grown on the etch stop layer <NUM> and then planarized, e.g. by CMP (Chemical Mechanical Polishing). The structural layer <NUM> grows on the thin covering layer <NUM> and may extend in the smaller section portions <NUM> of the nozzle openings <NUM>. The final thickness of the structural layer <NUM> may be <NUM>.

Then, the structural layer <NUM> is etched using a mask to form the larger portion sections <NUM> of the nozzle openings <NUM>.

Since the larger portion sections <NUM> are vertically centred with the smaller section portions <NUM> of the nozzle openings <NUM>, etching stops on the etch stop layer <NUM> and removes the silicon within the smaller section portions <NUM>.

<FIG> shows the resulting starting substrate <NUM> in a partially cut-away perspective view where intermediate layer <NUM> and etch stop layer <NUM> are not visible.

Simultaneously, before or after working the starting wafer <NUM>, the first wafer <NUM> is worked to obtain the structure of <FIG>. In a not visible manner, also fluid supply channels (<NUM> in <FIG>) have already been formed.

Then, <FIG>, the starting wafer <NUM> is turned upside down and bonded to the lower wafer <NUM>. Here, the chamber layer <NUM> acts as an adhesion layer that is directly bonded to the structural layer <NUM>, with the first semiconductor layer <NUM> arranged at the top.

Thereafter, the starting wafer <NUM> is thinned, e.g., by grinding the first semiconductor layer <NUM>, as shown by the dashed lines. For example, the first semiconductor layer <NUM> may be reduced to a thickness of about <NUM>.

In <FIG>, the first semiconductor layer <NUM> is completely removed, for example by dry etch; in addition, also the exposed portions of the intermediate layer <NUM> and of the etch stop layer <NUM> are removed by dry etch. The upper wafer <NUM> is thus obtained.

Thereby, the microfluidic device <NUM> of <FIG> and <FIG> is obtained.

<FIG> show prospective views of the microfluidic device <NUM>, showing the relative position of the chambers <NUM> and the nozzle apertures <NUM>, as well as of fluid supply channels <NUM>, inlets <NUM> and pillars <NUM>.

With the process of <FIG>, small features may be easily defined. In particular, in case of nozzle openings <NUM> forming a showerhead pattern, with a plurality of nozzle openings <NUM> for each chamber <NUM>, small dimensions may be obtained by dry etching the starting wafer <NUM>, in an easily definable way.

The same steps may however be used to form large dimension nozzle openings <NUM>, with small features formed in the chamber layer <NUM> as an alternative to the deposition of a photosensitive dry film, as discussed with reference to <FIG>.

Finally, it is clear that numerous variations and modifications may be made to the microfluidic device and the manufacturing steps described and illustrated herein, all falling within the scope of the invention as defined in the attached claims.

For example, the various embodiments described above can be combined to provide further embodiments.

In particular, the heaters <NUM>, <NUM>, <NUM> may be replaced by actuators operating according to a different principle; for example actuators of a piezoelectric material, for example PZT (Pb, Zr, TiO<NUM>) may be used, e.g., as disclosed in <CIT>.

Claim 1:
A microfluidic MEMS device (<NUM>; <NUM>; <NUM>) comprising:
a plurality of chambers (<NUM>; <NUM>; <NUM>; <NUM>), the chambers (<NUM>; <NUM>; <NUM>; <NUM>) having an elongated shape, with a length and a maximum width, wherein an aspect ratio between the length and the maximum width of the chambers is at least <NUM>:<NUM>;
a fluidic access channel (<NUM>; <NUM>, <NUM>) for each chamber, in fluidic connection with a respective chamber;
a plurality of nozzle apertures (<NUM>; <NUM>) for each chamber, in fluidic connection with the respective chamber;
an actuator (<NUM>; <NUM>) for each chamber, operatively coupled to the respective chamber and configured to cause ejection of drops of fluid through the nozzle apertures in an operating condition of the microfluidic MEMS device;
a chamber layer (<NUM>; <NUM>) and a nozzle layer (<NUM>; <NUM>; <NUM>; <NUM>), overlying each other, the chamber layer forming the plurality of chambers and the nozzle layer forming a plurality of nozzle openings (<NUM>; <NUM>; <NUM>; <NUM>),
each chamber being delimited by a lateral wall (<NUM>; <NUM>; <NUM>) having a plurality of indentations (<NUM>; <NUM>; <NUM>; <NUM>) and protrusions (<NUM>; <NUM>; <NUM>; <NUM>); and
the nozzle openings (<NUM>; <NUM>; <NUM>; <NUM>) being offset with respect to the chambers (<NUM>; <NUM>; <NUM>; <NUM>), with each nozzle opening extending between two adjacent chambers and intersecting the indentations of the two adjacent chambers at intersection areas forming the nozzle apertures (<NUM>; <NUM>; <NUM>).