FIELD OF THE INVENTION

The invention relates in general to the fabrication of microfluidic probe heads and the resulting devices. In particular, it relates to the fabrication of vertical microfluidic probe heads.

BACKGROUND OF THE INVENTION

Microfluidics generally refers to microfabricated devices, which are used for pumping, sampling, mixing, analyzing and dosing liquids. Prominent features thereof originate from the peculiar behavior that liquids exhibit at the micrometer length scale. Flow of liquids in microfluidics may typically be laminar. Volumes well below one nanoliter can be reached by fabricating structures with lateral dimensions in the micrometer range. Reactions, which are limited at large scales (by diffusion of reactants) can thus be accelerated. Microfluidics are accordingly used for various applications. Many microfluidic devices have user chip interfaces and closed flowpaths. Closed flowpaths facilitate the integration of functional elements (e.g. heaters, mixers, pumps, UV detector, valves, etc.) into one device while minimizing problems related to leaks and evaporation.

A new and versatile concept of MFP was recently introduced: the vertical MFP (also referred to as vMFP in the literature), see G. Kaigala et ai, Langmuir, 27 (9), pp. 5686-5693, 2011 (http://pubs.acs.org/doi/abs/10.1021/la2003639). Vertical MFP heads have microfluidic features fabricated in-plane with a basis layer. Such heads are oriented vertically with the head apex (the processing surface) being parallel to the surface processed, in operation.

BRIEF SUMMARY OF THE INVENTION According to a first aspect, the present invention is embodied as a method of fabrication of microfluidic probe heads, the method comprising: providing a set of n microfluidic probe head layouts on a same bilayer substrate that comprises two layers, said layouts being annularly distributed on that bilayer substrate, and wherein each of said layouts comprises: a first layer, which corresponds to a portion of one of said two layers of the bilayer substrate; and a second layer, which corresponds to a portion of another one of said two layers of the bilayer substrate; and comprises at least one microchannel defined by a groove open on an upper surface of the second layer and closed by a portion of a lower surface of the first layer; machining a hole substantially at the center of the bilayer substrate, to create a cylinder wall that delimits said hole and intercepts each of the at least one microchannel of the layouts, such that said at least one microchannel of each of said layouts extends up to at least one respective aperture formed at an end of the groove at the level of said cylinder wall; and singulating each of the n layouts to obtain n microfluidic probe heads.

In embodiments, the method further comprises, prior to singulating, a step of polishing said cylinder wall.

Preferably, the method further comprises: prior to machining, a step of filling microchannels of said probe head layouts with a malleable material such as a material comprising wax, a polymer or a photoresist; and removing said malleable material after machining, the removal of malleable material being preferably carried out after a step of polishing said cylinder wall, and more preferably after singulating each of the n layouts.

In preferred embodiments, the method further comprises, prior to providing the layouts, fabricating said set of n microfluidic probe head layouts, which comprises grooving the at least one microchannel of each of the n layouts on the upper surface of said another one of said two layers. Preferably, grooving the microchannels is carried out by microfabrication, such as using photolithography or micromachining, and preferably comprises a step of wet or dry etching each microchannel.

In embodiments, the method further comprises a step of aligning and bonding said two layers after grooving each microchannel.

Preferably, fabricating further comprises machining, for each of the n layouts, at least one via connecting perpendicularly to said at least one microchannel, said least one via being preferably machined through said another one of said two layers.

In preferred embodiments: each of said two layers has substantially a disk shape; said one of said two layers of the bilayer substrate: has a smaller average diameter than said another one of said two layers of the bilayer substrate, and is aligned with respect to it such as to leave an outer portion of said another one of said two layers that is not covered by said one of said two layers, and the method further preferably comprises, prior to providing the set of microfluidic probe head layouts on the same bilayer substrate: at least partly metalizing said another one of said two layers such that at least said outer portion of said another one of said two layers is metalized.

Preferably, the method further comprises: providing, prior to machining, several sets of probe head layouts on respective bilayer substrates; and superimposing said respective bilayer substrates, wherein the step of machining comprises machining a hole substantially at the center of the superimposed bilayer substrates, through all the superimposed substrates, to create hole cylinder walls that intercept each of the at least one microchannel of the layouts in each of the superimposed bilayer substrates, and wherein, preferably, the method further comprises polishing the resulting hole cylinder walls.

In embodiments, providing comprises providing at least two concentric annular sets, comprising an inner set and an outer set, on a same bilayer substrate, each of the inner set and the outer set comprising probe head layouts annularly distributed in its respective set, the method comprising two steps of machining a hole, wherein: a first hole is machined to create a first cylinder wall intercepting microchannels of the inner set, and a second hole is machined by separating a portion of the bilayer substrate that comprises the inner set from a remaining portion of the bilayer substrate to create a second cylinder wall intercepting microchannels of the outer set.

According to another aspect, the invention is embodied as a microfluidic probe head obtained according to a method of according to any one of the above embodiments, wherein the head comprises: a first layer; and a second layer, which comprises: at least one microchannel defined by a groove open on an upper surface of the second layer and closed by a portion of a lower surface of the first layer; and at least one aperture at an end of said at least one microchannel, at the level of an edge of the second layer, said edge defining part of a processing surface of the head.

In preferred embodiments, at least part of said processing surface is concave, due to the machining of the hole in the fabrication method of the microfluidic probe head.

Preferably, the microfluidic probe head further exhibits two lateral edge portions at an angle of 2 π/η ± π/10, and preferably at an angle of 2 π/η ± π/20.

In embodiments, the microfluidic probe head further comprises at least two microchannels, and preferably further comprises at least two vias connecting perpendicularly to said at least two microchannels, respectively.

Preferably, an outer portion opposite to said edge defining the processing surface of the head is metalized.

Devices and methods embodying the present invention will now be described, by way of non- limiting examples, and in reference to the accompanying drawings. Technical features depicted in the drawings are not necessarily to scale.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

- FIG. 1 schematically depicts various steps involved in a method of fabrication of microfluidic probe heads, according to embodiments of the invention;

- FIG. 2 is a flowchart illustrating steps of a method of fabrication of microfluidic probe heads, according to embodiments of the invention; - FIG. 3 schematically illustrates microfluidic probe head layouts on a same bilayer substrate, as involved in embodiments of the invention;

- FIG. 4 illustrates a variant to FIG. 3;

- FIG. 5 shows geometrical specifications for layouts such as depicted in FIG. 3, as involved in embodiments;

- FIG. 6 schematically illustrates steps in a method of fabrication using two concentric annular sets of layouts, according to embodiments;

- FIG. 7 schematically illustrates steps in a method of fabrication where several bilayer substrates are superimposed before machining a central hole, according to embodiments; - FIG. 8 is a 3D view of a simplified representation of a microfluidic probe head, according to embodiments of the invention; and

- FIG. 9 illustrates surface processing by a microfluidic probe heads (2D view, simplified representation), according to embodiments.

DETAILED DESCRIPTION OF THE INVENTION

As evoked in introduction, vertical MFP heads have several advantages. Such heads are microfabricated. At least some of the fabrication steps (including the polishing) need be done either individually (i.e., repeated for each unit) or in groups of 3 - 4 heads. It can be realized that such fabrication steps are the limiting steps in the mass-manufacture of the MFP heads. Polishing of the heads (and more generally the preparation of the processing surface of the head) is particularly labor intensive and therefore contributes significantly to the fabrication cost. In addition, there are yield problems (e.g., not enough or too much polishing), polishing tools have a significant footprint and are expensive. Finally, misalignment of the heads during the polishing may result in different apex sizes. The present invention solves at least some of these problems (some embodiments address all of these problems) by introducing a new concept of fabrication, relying on annularly distributed MFP head layouts. This approach contrasts with the approaches known so far for vMFP layouts, where heads are diced from bi-dimensional arrays of head layouts. More in detail, and referring altogether to FIGS. 1, 2 and 6, 7 and 8, an aspect of the invention is first described, which concerns a method of fabrication of microfluidic probe heads (hereafter MFPs) 100.

Most generally, this new fabrication concept requires a set of n MFP layouts 14, which are arranged on a same bilayer substrate 10. This bilayer substrate comprises at least two layers 11, 12.

The layouts 14 are annularly distributed on the bilayer substrate 10. More in detail, each layout 14 comprises:

• a first layer 110, i.e., a portion of one 11 of the layers 11, 12 that form the bilayer substrate 10; and

· a second layer 120, which corresponds to a portion of another one of these layers 11, 12.

For clarity: a distinction is made between the layer portions 110, 120 that correspond to a single layout 14 (or eventually one MFP head 100), whereas layers 11, 12 are those larger layers that initially form the bilayer substrate 10, used for the fabrication of the MFP heads.

By bilayer substrate, it is meant any suitable substrate comprising at least two layers 11, 12. Each of the first and second layers 11, 12 may most practically be a disk, e.g., a wafer disk. However, the layers 11, 12 do not have to be circular: what is solely required is an annular distribution of the layouts 14. The two layers 11, 12 may of a same material (or not). Preferred materials are glass or silicon. Still, the materials of the layers 11, 12 may include (for any of the layers): plastics, ceramics, metal and/or any other hard material that is compatible with the present fabrication methods.

At the level of one layout 14, one layer (e.g., the second layer 120) shall typically comprise most of the microfluidic features (microchannels, vias, etc.) structured thereon. Strictly speaking, it comprises at least one microchannel 123, 124, although embodiments described below mostly assume two microchannels 123, 124, without prejudice. A microchannel is defined by a groove open on an upper surface 120w of the layer 120. The groove is closed by a portion of the lower surface 110/ of the other layer, which is here assumed to be the "first" layer 110 (see FIG. 8).

Owing to the annular distribution (or ring arrangement) of the layouts, one or more of the functional features of each of the n layouts 14 (e.g., microchannels, vias, etc.) shall typically be invariant under a 2 ln rotation with respect to a transverse symmetry axis passing through the center of the annular shape defined by the layouts 14. Thus, the substrate 10 may be invariant under a rotation by 2 π/η, at least for what concerns these functional features. Of course, this may no longer be the case if different layouts are provided on a same substrate 10a, as illustrated in FIG. 4.

Typically, more than 6, 12 or even 24 layouts can be provided on a same bilayer substrate 10. For yield reasons, one seeks to maximize the number n of layouts, e.g., n = 36, 48, or 72. Preferred numbers of layouts depends on the substrate size, the complexity of the layouts (which depends on the applications contemplated). Still, the fabrication methods discussed here can in principle be implemented with any number n≥ 2 of layouts (e.g., n≥ 3, 4, 5, 7, ...).

An important step of the present fabrication methods is the machining of a hole 16 (see step S20 in FIG. 1 or 2), substantially at the center of the bilayer substrate 10. Machining the hole creates a cylinder wall 18 (delimiting said hole). The layouts and the hole are designed such that the cylinder wall 18 intercepts microchannels of interest of the layouts 14. I.e., an aperture is created at the end of each of these microchannels. Thus, microchannels 123, 124 now extend up to respective apertures 121, 122 (which are formed when machining the hole 16), at the level of the cylinder wall 18, as best seen in FIGS. 1, 3 and 8.

Machining the hole 16 may include drilling, milling, cutting, etc. It may further involve a rotating cylinder or a laser, a water jet, etching, etc. Note that in embodiments where concentric layouts are used (see FIG. 6), the same machining technique may be used for machining the hole in the first (inner) annular ring (step S20z in FIG. 6) and for cutting out the inner ring, to obtain the second (outer) ring. In variants, different hole-machining techniques may be used for obtaining the two rings. The embodiment of FIG. 6 will be discussed later in detail.

Finally, and as illustrated in FIG. 1 , each of the n layouts is singulated (step S30 in FIGS. 1 , 2), to accordingly obtain n MFP heads 100. As the one skilled in the art may appreciate, the radial separation could involve dicing, cleaving, or singulating the layouts 14 by taking advantages or prefabricated precut lines, etc. FIG. 3 shows a pattern of dicing or cutting lines (dotted lines) that can typically be used in the present context. This fabrication methods discussed herein allow for wafer-level processing of the processing surfaces of the MFP heads, and thereby solve some of the issues of fabrication of vMFP heads noted earlier. With the present fabrication methods, the processing surfaces 310, 320 of several heads (typically many) can be obtained in a single step. In particular, the heads are arranged on the wafer in a way that polishing of all heads can be performed in a single step before singulating the heads (wafer-level polishing). In this respect, embodiments of the present fabrication methods may further comprises, prior to singulating, a step of polishing (S24 in FIGS. 1, 2) the cylinder wall 18. Still, polishing may be performed together with, or while machining the hole 16. Depending on the technique used for machining the hole 16, an additional, distinct step of polishing may actually be superfluous. Polishing may thus be concomitant with the machining step. Now, the polishing may not necessarily imply mechanical polishing means. Instead, means such as high-pressurized water jet cutting may be employed. In all cases, the machining and/or polishing steps may allow to obtain a clean processing surface 310, 320 for the resulting MFP heads 100, having low surface roughness, and well suited for typical vMFP applications.

Advantageously, present fabrication methods may further comprise, prior to machining, a step of filling SI 8 microchannels of the layouts with a malleable material, to protect the microchannels during the subsequent fabrication steps, in particular during the machining S20 of the hole 16. Similarly, other microfluidic features of the layouts may be filled with a malleable material. This material can be later removed S40 (after step S20 or later). The malleable material is preferably removed after polishing S24, if any. More preferably, it is only removed after singulation S30, in order to protect the microfluidic features also during the singulation.

By malleable material it is meant any material that can be used to fill and clog the microfluidic features, for protection. Typically, this malleable material can be removed by heating and melting it, followed by appropriate cleaning and rinsing of the microfluidic features. Such a material may comprise wax, a photoresist or, more generally, one or more polymers.

Photoresists are advantageous inasmuch as it makes it possible to clean the channels by light exposure to un-crosslink the resist and thereby make it very soluble, which enables simple subsequent removal. Preferably, the malleable material should not be soluble in liquids that are typically used in dicing operations (e.g., water might be used for cooling the substrate 10 and the heads). Photoresists are very clean and filtered; they are unlikely to leave any particulates. More generally, photosensitive materials may prove advantageous.

One may otherwise use a low temperature (ca. 60 - 80 °C) wax to fill and clog the channels. After machining the hole, wax can be heated and then removed using vacuum. It can also be dissolved using, e.g., heptane. Any clean low temperature wax that has low viscosity (e.g., at 80 °C) could potentially be suited for the present purpose.

In variants, other polymers can also be used, which can be dissolved and/or liquefied either by light, temperature or a solvent. So far, the most fundamental aspects of the fabrication methods have been described assuming that a prefabricated substrate was available (step S16 in FIGS. 1, 2). Still, embodiments of the present fabrication methods may include upstream fabrication steps (S8, S10, S12 in FIGS. 1, 2).

Notably, such steps may comprise grooving (step S12) the microchannels 123, 124, 224. The microchannels are grooved on the surface of one of the layers 11, 12, e.g., on the upper surface 120w of layer 12. It does in principle not matter on which of the two layers the channels are grooved, as long as they are closed by the other layer, e.g., after bonding.

Grooving S12 the microchannels is preferably carried out by microfabrication. This may involve photolithography or micromachining. The groove may for instance be engraved and/or milled by a tool directly on the upper surface of the base layer 120. It can have any appropriate section shape, e.g. rounded, square, U or V section. The required tool can be chosen according to the material of the base layer. In variants, laser ablation can be contemplated too. Advantageously yet, deep reactive ion etching (DRIE) is used for fabrication of microchannels. The microfabrication may otherwise typically involve steps of wet or dry etching each of the microchannels. Advantageously, the channels can be etched all at once, owing to the wafer-level approach proposed here. The fabrication may further comprise aligning and bonding the two layers 11, 12, cf. step S14 in FIG. 1 or 2, after the steps S12 of grooving the microchannels. Note that, in FIG. 1, the upper face 120w of layer 12 faces the reader's eyes, just like the lower face 110/ of layer 11 (in the bottom part of FIG. 1). However, the layer 11 is flipped before bonding S14, so that, at step S16, the lower face 110/ of layer 11 is facing the upper face 120w of layer 12 in FIG. 1.

As a first example, thermal bonding of glass layers 11, 12 can be done at 600°C for 4 hours (heating and cooling rate: 75C/hour). This results in fusion bonded (irreversible) of the glass substrates. When using glass substrates, it is preferable that the cooling rate does not exceed 100°C hour to avoid stress. Furthermore, the thermal expansion of the glass wafers needs to be equal.

As another example, assembly of two Si wafers 11, 12 can be achieved by spin coating ~3 μπι of a polyimide adhesive (HD Microsystems GmbH, Neu-Isenburg, Germany) onto the polished side of the lid wafer and by subsequently aligning and bonding both wafers. Bonding takes place at 320 °C with 2 bar pressure for 10 minutes (PRESSYS LE, Paul-Otto Weber GmbH, Remshalden, Germany). The MFP heads can then be diced and stored.

The upstream fabrication steps may further be directed to other microfluidic features, e.g., they may notably include machining S10 vias 111, 112. Namely, and for each of the n layouts 14, at least one via 111, 112 may be provided to connect perpendicularly to a respective microchannel 123, 124. For simplicity, vias are preferably machined as a through hole, through layer 12, such that they can easily be closed by the same layer 11 that already seal the channels 123, 124. Further equipment (tubing ports and tubes) can be provided to connect from the side opposite to the side 120w in layer 12, to simply enable vertical operation of the heads 100. Other fabrication steps may notably be directed to the creation of alignment holes 21, 22 in each layer 11, 12 (respectively steps S8, S6), for aligning the layers before bonding.

FIG. 5 gives an example of a set of geometry specifications that are particularly convenient for drilling glass layers 11, 12. Alignment holes 21, 22 of both layers can be drilled altogether. To that aim, the layers 11, 12 can be bonded together beforehand, e.g., with wax. The alignments holes 21, 22 can then be drilled through both layers 11, 22. Concerning the vias 111, 112: the drilling stops at the interface between both layers 11, 12, so that only one layer 12 comprises vias 111, When using glass wafers, a suitable holder may advantageously be used for precise glass drilling, e.g., in a computer numerical control (CNC) machine.

For example, one may use Schott Borofloat® 33 borosilicate glass wafer, having a wafer thickness of 500 μηι and a wafer size of 4 inch. Preferred smallest hole diameters are 0.25 mm. Preferred drilling parameters are in that case: small holes (0.4 mm) : 30 mm/min @ 30Ό00 rpm; and

alignment holes (1.5 mm): 25 mm/min @ 25 '000 rpm.

Drilling is preferably performed in aqueous cooling liquid. Diamond coated drills can be used.

The central hole can be drilled manually (20 mm diameter) using a standard milling machine and a diamond drill. Polishing of the central hole can be performed using a polishing pad and a 1 micrometer diamond paste. A standard dicing can be subsequently performed to singulate the heads.

The layouts 14 of a same set need not be all identical, as illustrated in FIG. 4, where four layouts of the substrate 10a exhibit additional microchannels, compared to the remaining layouts. Here, the microchannels are again arranged in a way that they intersect with a central hole, to be drilled and polished, at a later stage, to eventually form apertures. For this layout, the wafer size is 100 mm and the diameter of the center hole to be drilled is 20 mm.

In some MFP applications, electrodes need be designed at the head level. In that respect, the present fabrication methods can perfectly accommodate metallization. Metallization is preferably carried out on one of the layer 11, 12 only. It is for instance known to structure Pt/Ti patterns on glass. Electrodes can advantageously be implemented for heating, electrochemical sensing, etc.

In this regard, the present methods allow for simply fabricating outer electric pads. For instance, assuming that each of the two layers 11, 12 has substantially a disk shape, layer 11 can be provided with a smaller average diameter than layer 12. When aligning the two layers, an outer portion of layer 12 shall thus not be covered by layer 11. This allows for providing additional functional features at the level of this outer portion. In particular, one may metalize (at least partly, or selectively) the upper side 120w of layer 12, such as for the outer portion of layer 12 to be at least partly metalized. Metallization is typically carried out selectively on the entire larger layer. Microstructure such as heating structures or electrochemical electrodes can accordingly be obtained that can be connected through a metallized pad provided on the outer portion. The smaller wafer ensures free access to the electrode's contact pads. Again, the channels 123, 124 may be provided on any of the layers 11, 12, as their fabrication decouples from the metallization process. At present, refinements are discussed which allows the yield of the present fabrication methods to be multiplied, and this, in reference to FIGS. 6 and 7.

One way to increase the fabrication yield is to optimize the wafer surfaces, by exploiting concentric rings of layouts, FIG. 6. One may for example provide two (or more) concentric annular sets of layouts 14 (an inner set and an outer set) on the same bilayer substrate 10. Each of the inner and outer sets comprises probe head layouts 14/, 14o that are annularly distributed in its respective set. Then, a first hole can be machined (step S20/) to create a first cylinder wall intercepting microchannels of the inner set. Next, a second hole can be machined (step S20o) by separating the portion of the substrate 10 that comprises the inner set 14/ from the remaining portion of the substrate 10. This creates a second cylinder wall intercepting microchannels of the outer set 14o. As evoked earlier, same or identical machining techniques may be used to create the first and second cylinder walls.

Another way to increase the fabrication yield is to exploit the third dimension, i.e., perpendicularly to the wafer surfaces, e.g., by providing superimposed disks, prior to machining a hole therethrough, see FIG. 7. For instance, one may provide, at step SI 6, several sets of probe head layouts 14 on respective bilayer substrates 10. Then, at step S19, the substrates 10 can be superimposed, prior to machining S20 a hole substantially at the center of the superimposed substrates 10, through all the superimposed substrates. This results in creating a hole 16, whose walls 18 intercept microchannels in each of the superimposed substrates 10. If necessary, the cylinder walls of the resulting hole 16 are polished, as discussed earlier. The embodiments of FIGS. 6 and 7 can be combined. The fabrication yield multiplies as the number of concentric rings times the superimposed substrates. Referring now altogether to FIGS. 1 , 3 - 5, 8 and 9, another aspect of the invention is now described, which concerns MFPs obtained according to the present fabrication methods. Consistently with the fabrication methods discussed above, such an MFP shall comprise: a first layer 1 10 and a second layer 120. The latter exhibits one or more microchannels 123, 124, which are defined by respective grooves, open on the upper surface 120w of the second layer 120 and closed by a portion of the lower surface 1 10/ of the first layer 1 10, as best seen in FIG. 8. Also, apertures 121 are defined, thanks to the machining process S20, S24, at the ends of the microchannels, and this, at the level of the edge 320 of the layer 120 in which the channels are grooved. The edge surface 320 forms part of the processing surface defined by edge surfaces 310, 320 of each layer 1 10, 120 of the head. Of course, and as described earlier, such a MFP head may include other microfluidic features, such as vias 1 1 1 , 1 12 connecting to the microchannels.

Note that the obtained MFP heads are necessarily impacted by the fabrication methods discussed earlier:

First, and referring more particularly to FIG. 9, the processing surface 310, 320 the MFP heads 100 may be concave (if no additional, substantial surface processing is done to remove the concavity), due to the machining, step S20, of the hole 16;

Second, referring now to FIGS. 5, 8 and 9, the general shape of the heads 100 may furthermore reflect the initial annular distributions of the layout 14. For instance, the heads may have lateral edges (or at least portions thereof) that are at an angle close to 2 π/η, subject to a tolerance, e.g., ± π/10. The tolerance depends on the spacer stripes (see the dotted lines in FIG. 1 or 3) between the heads in the layout and the singulation technique used. However, the resulting edges should typically exhibit an angle of 2 π/η ± π/20, provided that sufficient care is taken during the singulation step;

The heads 100 may further exhibit other fingerprints of the present fabrication techniques, e.g., :

o The fine surface state of the lateral edges may differ from that of the processing surface, owing to the different techniques used (i.e., singulation for the lateral edges vs. machining/drilling/polishing for the processing surface); and o The sector-like shapes of the heads 100 (similar to sectors of a pie chart) and, more generally, residuals of the initial annular distribution of the layouts, the symmetry of a layout, the hole 16 machined to create the processing surface, etc., may result from the present fabrication techniques. Now, the heads 100 may, after singulation, undergo subsequent treatment, or processing, such that they may not necessarily retain all of the fabrication fingerprints mentioned above.

As said earlier, typical embodiments of the MFP involve at least two microchannels 123, 124, and consistently, at least two vias 111, 112 connecting perpendicularly to the microchannels, respectively. Also, an outer portion of one of the layers 11, 12 may be partly metalized, to provide electric pads. This outer portion is opposite to the processing surface 310, 320.

In addition to the microchannels 123, 124, lateral channels 224 could be provided too, as depicted in FIG. 9. Interestingly, advantage can be taken of the singulation steps to define apertures 222 at the end of the lateral channels.

FIG. 8 shows a view of the processing end of a bilayer MFP head 100 obtained according to embodiments of the present fabrication methods. The head 100 has a base layer 120, wherein processing liquid microchannels 123, 124 are provided together with immersion liquid microchannels 224 (only one of the lateral channels is depicted here). Each channel is in fluid communication with a respective aperture 121, 122, 222 and each aperture is located on a face of the base layer 120 in this example. The cover layer 110 closes the channels open on the upper face 120w of the base layer 120. Apertures are formed at the level of the edge surface 320 of the base layer 120. Owing to the fabrication process, the processing surface 310, 320 shall typically be acute, which allows for compact liquid deposition on a surface 200 of interest, and leaves rooms for easy optical monitoring. The concavity of the apex is not visible in FIG. 8.

The head may further be provided with tubing ports (not shown), to enable fluid connection with the vias 111 and 112 (not visible in FIGS. 8, 9). Vias and ports shall be configured to enable fluid communication from the ports to the apertures 121, 122, 222, through respective vias.

As an example of application, when moving the head in the vicinity of a surface 200, as schematically shown in FIG. 9, processing liquid PL can be dispensed through the aperture 121, which will combine with an immersion liquid IL (possibly provided via a lateral aperture of the head, not shown in FIG. 9). Note that the dimensions are not too scale, in particular the dimensions of the apertures, which are deliberately exaggerated, for clarity. The device 100 is preferably configured such as to be able to obtain laminar flows. Apertures' dimensions can in reality be, e.g., a few tens of micrometers. They are typically spaced a few tens to hundreds of micrometers. As pairs of processing channels/apertures are used here, the processing liquid PL can be aspirated at aperture 122 together with some of the immersion liquid IL. The flow path between apertures 121 and 122 can be inverted, i.e. processing liquid can be injected from aperture 122 while aperture 121 can aspirate liquid. The processing liquid is essentially located nearby the apertures 121 and 122, in operation, and is surrounded by immersion liquid that, typically, can be present only in the vicinity of the head 100.

Now, advantage can be taken of the concavity of the processing surface 310, 320 that naturally results from the fabrication process, to confine liquids in the convex space formed between the processing surface and the surface to be processed, as schematically depicted in FIG. 9. However, immersion liquid may still be required to hydrodynamically confine the processing liquid, and thereby avoid spreading of the processing liquid. Yet, the concave shape of the apex already ensures some confinement of the processing liquid flow within the concavity, when the device is in contact with the surface. No distance control is needed in this mode of operation. Note that the curvature of the apex can be altered to "design" a specific volume of liquid to be encapsulated. Finally, the curvature defines a unique flow resistance, effecting the geometry of the flow confinement.

MFP heads such as discussed above are particularly useful notably for surface processing applications. The latter, unlike biological applications, involve potentially smaller patterns and a broader range of liquids and chemicals. Employing a thin Si wafer (e.g. 100 μπι in thickness) to fabricate the basis layer 12, one may for instance fabricate well defined apertures with lateral dimensions of less than 10 μπι, using conventional DRIE or focused ion beam, which ensuring a mechanical strength of the head thanks to a Si lid 11 having a sufficient thickness. Multilayered heads such as discussed herein are also more amenable to using many processing liquids because apertures can be small and close to each other with horizontal microchannels sufficiently fanning out for leaving sufficient space for adding many ports on the layers 11, 12. More generally yet, the present MFP technology has a potential for patterning surfaces, processing materials, depositing and removing biomolecules and cells on surfaces, analyzing cells and biomolecules on surfaces, creating chemical gradients on surfaces, studying complex biological specimens such as tissue sections, and creating structures with unusual profiles such as tapered cavities. The methods described herein can be used in the fabrication of MFP heads and MFP chips. The resulting heads/chips can be distributed by the fabricator in raw form (that is, as a structured bilayer substrate or in a packaged form). In the latter case the chip can be mounted in a single chip package. In any case the head or chip can then be integrated with other elements, as part of either (a) an intermediate product or (b) an end product. The above embodiments have been succinctly described in reference to the accompanying drawings and may accommodate a number of variants. Several combinations of the above features may be contemplated. Examples of such combinations are given in the drawings. While the present invention has been described with reference to a limited number of embodiments, variants and the accompanying drawings, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the present invention. In particular, a feature (device-like or method-like) recited in a given embodiment, variant or shown in a drawing may be combined with or replace another feature in another embodiment, variant or drawing, without departing from the scope of the present invention. Various combinations of the features described in respect of any of the above embodiments or variants may accordingly be contemplated, that remain within the scope of the appended claims. In addition, many minor modifications may be made to adapt a particular situation or material to the teachings of the present invention without departing from its scope. Therefore, it is intended that the present invention not be limited to the particular embodiments disclosed, but that the present invention will include all embodiments falling within the scope of the appended claims. In addition, many other variants than explicitly touched above can be contemplated. For example, other materials than those explicitly mentioned herein could be used for each of the layers 11, 12. Similarly, the channels, vias, apertures could be provided with different dimensions. REFERENCE LIST

10, 10a bilayer substrate

100 Microfluidic probe (MFP) head(s)

I I, 12 substrate first and second layers

110 First (lid) layer of the head 100

1101 Lower surface of the first layer 11, 110

I I I, 112 Perpendicular vias

120 Second (basis) layer of the head 100

120u Upper surface of the second layer 12, 120 121, 122 MicroChannel apertures

123, 124 microchannels

14 MFP layouts

14i, 14o Concentric probe head layouts

16 Central hole

18 Cylinder wall delimiting the central hole

21, 22 Alignment holes

222 Lateral microchannel aperture

224 Lateral microchannel

310, 320 Head processing surface