EP2718226A1

EP2718226A1 - Anodic bonding for a mems device

Info

Publication number: EP2718226A1
Application number: EP12740206.3A
Authority: EP
Inventors: François Bianchi
Original assignee: Debiotech SA
Current assignee: Debiotech SA
Priority date: 2011-06-08
Filing date: 2012-06-07
Publication date: 2014-04-16
Also published as: CN103608283A; EP2532619A1; US20140106095A1; JP2014524844A; WO2012168889A1

Abstract

The invention relates to a device comprising a wafer comprising a silicon area and a wafer comprising a glass area fastened to each other, the fastening zone thus formed between the wafers defining a multilayer structure comprising a first layer protecting the silicon from physical changes caused by attack of the surface, which layer covers the silicon area, and a second layer protecting the glass from physical changes caused by attack of the surface, which layer covers the glass area; said multilayer structure furthermore comprising at least one additional layer enabling anodic bonding between the two protective layers; said device containing at least one fluid channel protected by said protective layers and able to contain a solution temporarily.

Description

ANODIC WELDING FOR MEMS TYPE DEVICE

Field of the invention

The present invention is in the field of electromechanical microsystems commonly called MEMS devices (Micro Electro Mechanical Systems). It relates more precisely the anodic welding between a plate, one of whose faces is made of silicon and a plate, one of whose faces is made of glass. These two elements constitute the basic elements of most MEMS-type devices, a microsystem comprising one or more mechanical elements, which can use electricity if necessary as a source of energy, in order to perform a sensor function and / or actuator with at least one structure having micrometric dimensions, the function of the system being partly ensured by the shape of this structure.

Abbreviations used in this text

ALD Atomic Layer Deposition

CVD Chemical Vapor Deposition

LPCVD Low Pressure Chemical Vapor Deposition

MEMS Micro Electro Mechanical Systems

MI P Micro Implantable Pump

PECVD Plasma Enhanced Chemical Vapor Deposition

PVD Physical Vapor Deposition

a-Si amorphous silicon State of the art

A microfluidic system such as a pump or a flow regulator must be protected against chemical attack, especially if it is intended to be implanted for several years in a patient, for example as an active agent release system.

In general, the elements sensitive to such chemical attacks, such as silicon or glass plates, are covered with a protective layer. The assembly of these elements together is not always easy. Such a method of assembly has been the subject of a patent application relating to an implantable micro-fluidic system [R1 1].

For many years, several research groups have (successfully) [R1, R4] treated the possibility of welding on a glass surface (Pyrex 7740 in general) a silicon nitride-coated silicon surface.

Moreover, other research groups have demonstrated that it is possible to weld by anodic welding two plates of the same nature (silicon [R6, R7] or glass [R2, R5]) using intermediate layers. But as stated in the Knowles article [R3], the goal in these studies was to make possible the welding between two substrates that it was impossible or difficult to weld a priori, without recourse to one or intermediate layers.

Another approach used uses a direct soldering technique (no use of electrical voltage but use of pressure and surface preparation) to weld a silicon nitride-coated silicon plate against a glass plate [R8].

General description of the invention

The present invention consists of a MEMS device as defined in the claims. Preferably, said protective layers make it possible to protect said plates from surface attacks which may be, for example, chemical, electro-chemical, physical and / or mechanical. In particular it may be attack related to the pH of a solution in contact or a phenomenon of dissolution of the plate by a solution.

Contrary to the teaching of the state of the art [R1 1] which is limited to presenting the design of a fluidic resistance for an implantable pump having the shape of a capillary network having a protective layer pH better than glass or silicon, the problem addressed by our invention concerns the packaging of a MEMS by conventional anodic welding of a plate, one of whose surfaces is made of silicon with a plate, one of whose surfaces is made of glass - a material or a hard alloy more often consisting of silicon oxide (silica SiO 2, main constituent of the sand) and fluxes, while protecting them against surface attacks. Borosilicate, such as for example Pyrex 7740 or an equivalent material such as those described in Table 1 (see below), will be used more readily, a potential objective being to take advantage of its transparency.

To achieve this protection against surface attacks, silicon can be protected by a thin layer of Si ₃ N ₄ or TiN type (already described in the literature), or more complexly by a combination of two layers: TiO ₂ + If ₃ N ₄ , or Ti0 ₂ + a-Si. In this case the layer of a-Si or Si ₃ N ₄ which is deposited on the protective layer is not intended to protect the device against surface attacks but only to make the weld possible. This is also true for all the combinations of layers described in Table 2.

The special feature of Si3N4 is that it can act both as a protective layer and as an anodic solder layer. However, it is difficult to make deposits conform with Si3N4, especially on structured surfaces. These are nevertheless indispensable to the protective efficacy of such layers. Indeed, the slightest defect can become the weak point of the system where surface attacks will be concentrated. It is therefore preferred to choose TiO 2 as a protective material whose conformal deposition on a structured surface is easy since it is compatible with techniques such as ALD. Si3N4, which is not compatible with ALD-type deposition methods, can be deposited above the ΪΊ02, on the weld zone, in order to make anodic welding possible. It is known from the literature that anodic welding directly on titanium oxide deposited on silicon is not feasible. In this document, a compliant deposit is defined as a layer, deposited on a surface having a very high aspect ratio (such as hollows), conforming to said surface in a homogeneous manner.

If the glass is chemically inert with respect to a basic pH solution, the solubility of the silicon oxide, the essential component of the glass, sees its solubility increase significantly with the pH as illustrated in FIG. 1. Therefore, the deterioration of a glass structure exposed to basic pH solutions over the long term is a risk that is addressed in this invention. Thus, like silicon, the glass must be protected by a layer that can withstand a basic pH attack.

If the silicon nitride or titanium oxide can very well be deposited on the glass as a protective layer, it is however impossible to carry out a direct anode welding between one of these protected plates and a silicon surface having itself a protective layer. There is no protective layer to our knowledge applicable on a glass plate that is directly compatible with anodic solder. In the case of glass, For example, silicon nitride or titanium oxide will be used as a protective layer which will be combined with a thin layer of silicon oxide used as a solder layer.

This welding layer must allow on the one hand the realization of the anodic welding and on the other hand the maintenance of this weld over time despite its exposure to a basic pH solution. The weld layer must be thin enough to create capillary forces high enough to create a valve-type capillary stop, preventing infiltration of the basic solution into the weld area and thus avoiding the risk of delamination. Since anodic welding induces a chemical change of the material present in the weld zone by the creation of covalent bonds, the result of this chemical transformation can change the chemical / physical properties of the material and make it more resistant to basic solutions than its native form. before anodic welding.

Capillary valves or capillary stop valves make it possible to stop the flow of a solution within a microfluidic device by using a capillary pressure barrier when the geometry of the channel changes abruptly.

Fig. 2 shows an example of a complex structure that can be protected and soldered by the proposed technique. The following elements are present in this structure:

1. Borosilicate glass (Pyrex 7740 for example)

2. Functional layer on the surface

3. Protective layer (eg Ti0 ₂ )

4. Protective layer (eg Si0 ₂ )

5. Monocrystalline silicon

6. Protective layer (eg Ti0 ₂ )

7. Protective layer (eg Si ₃ N ₄ ) Despite the presence of intermediate layers, it remains possible to use conventional parameters of anodic welding.

To limit the risks of local failures (pinholes in English), one can use a compliant deposition technique called ALD (Atomic Layer Deposition) reputed without pinholes or when using a deposit by CVD (Chemical Vapor Deposition) proceed to a deposit in several stages.

Usually, the use of a method to obtain a compliant deposit does not make it possible to perform a weld between two plates. Except, the present invention makes it possible to weld two plates, at least one of which has a compliant deposit.

In addition, the device of the present invention is obtained by applying a protective layer against a surface attack on at least one silicon region and a protective layer against a surface attack on at least one glass region. The plates of the device may be structured before or after the application of said protective layers. Following the application of these protective layers, a thin layer of a material for performing anodic bonding is applied between the two layers of protection.

The assembly forming a device may comprise at least one fluidic path. This fluidic path allows a solution to circulate between the protective layers but also to cross all or part of said plates. It can be composed of channels, valve, sensor, means of pumping, ...

In addition to making it possible to weld said protective layers together, said solder layer, by its thickness, prevents the infiltration of said solution into the weld zone defining the lateral ends of said fluidic path through which said solution propagates. The conformal application of these layers can be done by various techniques: by deposition (ADL, LPCVD ...) or by growth (dry and wet oxidation)

The wafers thus structured and protected are assembled together to create a fluid path.

List of Figures

Fig. 1 Solubility curve of silica and quartz as a function of pH

Fig. 2 Sectional view of a complex structure protected from chemical attack by thin layers and sealed by conventional anodic welding.

Fig. 3 Test vehicle used to highlight the characteristics of protective layers. It consists of two fluidic inputs (the two circles) and a serpentine channel constituting a fluidic resistance.

Fig. 4 Channel diagram constituting the fluidic resistance with the different protective layers used. Layers (c) and (b) are deposited on the pyrex and layers (a) and (d) are deposited on the silicon. The layer (a) is preferably Si3N4 and may be directly welded to Pyrex, or layer (c) which is the solder layer deposited on the protective layer (b) which serves to protect the Pyrex. The layer (d) is a protective layer deposited on the silicon and which can be deposited by ALD but which is not directly weldable Pyrex or layer (c). Therefore, it is possible to add to layer (d) the layer (a) which this time makes it possible to weld silicon plate having a non-weldable protective layer to a Pyrex plate protected or not.

Fig 5, SEM image of the section of the fluidic path having been exposed to a solution of pH 1 2 for 8 days and having no protective layer or solder. As the fluid resistance of this channel has been halved by silicon etching, this value is used as a control over other channels that have undergone protection layer processing. The nominal depth of the channel is 16 μm.

Fig 6: Evolution of the fluidic resistance as a function of the exposure time for a channel with a 50 nm protection layer of Si3N4 deposited on the silicon wafer. The breaking point established with respect to the control corresponds to a reduction in the fluidic resistance by a factor of 2. Fig. 7 SEM image of the section of the fluidic path having been exposed to a solution of pH 12 for 28 days and having a 50 nm Si3N4 protective layer deposited on the silicon. Anisotropic attack is clearly observed at the intersection of the channels constituting the coil, while the bottom of the channels, protected by Si3N4, is not attacked. The observed failure suggests that a thickness of 50 nm does not provide good protection at the weld.

FIG. 8: Evolution of the fluid resistance as a function of the exposure time for a channel with a protective layer of 100 nm of Si 3 N 4 deposited on the silicon wafer. The breaking point established with respect to the control corresponds to a reduction in the fluidic resistance by a factor of 2.

Fig. 9 SEM image of the section of the fluidic path having been exposed to a solution of pH 12 for 48 days and having a Si3N4 100 nm protective layer deposited on the silicon. A pH attack is clearly observed at the interface of the plates defining the channels constituting the fluid resistance coil and thus creating a short circuit between two channels constituting the coil. On the other hand, no anisotropic Si attack is observed, thus showing that the thickness of 100 nm is sufficient to ensure good protection of the weld. An increase of 2 μm in the channel depth suggests that the Pyrex is eroded thus creating a decrease in fluid resistance.

Fig. 10: Evolution of the fluid resistance as a function of the exposure time for a channel with a protective layer of 200 nm Si3N4 deposited on the silicon wafer. The breaking point established with respect to the control corresponds to a reduction in the fluidic resistance by a factor of 2.

Fig 1 1: SEM image of the section of the fluidic path having been exposed to a solution of pH 12 for 140 days and having a protective layer of 100 nm Si3N4 deposited on the silicon. There is clearly a pH attack at the interface of the plates defining the channels that make up the coil the fluid resistance coil and an increase of more than 6 μm in the channel depth induced by a chemical attack of Pyrex. The protected part of the silicon does not seem to be attacked. In this case the distance between the channels constituting the coil is greater than 100 μm preventing the creation of short circuit between the channels and a too large fall of the fluidic resistance as is the case in FIG.

FIG. 12: Evolution of the fluidic resistance as a function of the exposure time for a channel with a 100 nm protection layer of Si 3 N 4 deposited on the silicon, a 200 nm protective layer of ΤΊ 2 O on the Pyrex and a layer 100 nm of SiO2 deposited on the ΤΊ02 layer for the anodic bonding. The breaking point established with respect to the control corresponds to a reduction in the fluidic resistance by a factor of 2.

FIG. 13: SEM image of the section of the fluidic path having been exposed to a solution of pH 12 for 140 days and having a SiN4 protection layer of 100 nm deposited on the silicon, a protective layer of 200 nm of TiO 2 on the Pyrex and a 100 nm layer of SiO2 deposited on the ΤΊ02 layer for anodic bonding. We can clearly see that the canal has kept its nominal depth. It is thus shown that the protective layers and the weld layer have fulfilled their role perfectly.

Detailed description of the invention

While using anodic welding technique with conventional parameters (350-400 ° C, 500-1000V) it is possible to weld a silicon wafer against a glass plate (Pyrex 7740) despite the presence of intermediate layers that serve to protection against surface attack.

Silicon can be protected by a thin TiN layer (<50nm) or by all kinds of silicon nitride (deposited by ALD, PECVD, LPCVD) with variable stoichiometries. By using two layers, it can also be protected by using an assembly Ti0 ₂ + Si ₃ N or Ti0 ₂ + a-Si. With thicknesses for TiO2 up to 250nm, up to 500nm for the additional layer of Si ₃ N ₄ (this thickness can be up to <1 μιη for silicon nitride alone) or <500nm for the additional layer of amorphous silicon. Pyrex can be protected by two layers: Ti0 ₂ followed by Si0 ₂ . These two layers can be deposited by ALD, Sputtering Reagent, PECVD (SiO2), LPCVD (SiO2). The usable thickness range is:

<500nm for Si0 ₂

<250nm for Ti0 ₂

The use of ALD (Atomic Layer Deposition) as a deposition technique is very interesting in our case since the number of point defects (pinholes in English) is reduced drastically compared to the other deposit techniques mentioned [R9].

The use of ALD also makes it possible to consider the protection of structures of very complex shapes since the technique is almost perfectly consistent (aspect ratio of 1: 1000 demonstrated [R1 0]).

In the case where the protective layer or layers are deposited by CVD (Chemical Vapor Deposition), it is useful to proceed in several distinct steps. This allows, by cutting the gap, to significantly reduce the risk of two defects (pinholes) superimposed.

It should also be noted that this anodic soldering technique does not require any pretreatment of the surfaces to prepare the weld. As long as the plates are free of particles greater than 0.5μηι it is easy to obtain a reliable and very sealed solder unlike many other welding techniques (Polymer-Bonding, Plasma-Activated-Bonding, etc.).

Pressure tests on the different weld configurations did not show any differences in weld strength. It therefore seems that regardless of the protective layers present on the Pyrex or on the silicon, the anodic welding is just as strong as in the case of a simple weld without a protective layer. Experimental results

I. compatible welding

Various experiments have demonstrated the difficulty of welding by anodic welding of silicon and glass by adding intermediate layers. The nature of the materials, the thickness of the layers, the order of deposition as well as the relative positions of the layers with respect to the plates are all key parameters that must be controlled to ensure reliable anodic welding. Among other things, the thickness of the solder layer is a decisive element in accomplishing this function. For example, a thickness of more than 500 nm of SiO 2 does not make it possible to perform a weld between the intermediate layers.

The following two examples show a tri-layer weld:

• Example 1: Welding tests were carried out on a Silicon - Pyrex assembly. A layer of silicon nitride of 100 nm was deposited on the silicon. On the Pyrex side, a thin layer of TiO 2 (50 nm) covers the substrate. On top of this set, 100 nm of SiO 2 were deposited. This set was soldered at 380 ° C at 750V. Scalpel tests demonstrated excellent adhesion.

• Example 2: A silicon plate was covered with a layer of 100 nm ΤΊ02 and a layer of 200 nm of Si. Similarly, a Pyrex plate was first covered with a 100 nm layer of ΤΊ02 and then another 100nm Si02. As before, a soldering at 380 ° C and 750V was performed. Welding results also demonstrated excellent adhesion between these two plates. Several phenomena are cited in the literature to explain anodic welding. Below we put our experimental results against these phenomena.

Firstly, oxidation at the interface is possible with oxygen coming from Pyrex (in particular dissociated NaOH by the electric field). In our case, we found that this theory is likely to explain some results:

- Silicon and / or Pyrex coated with ΤΊ02 is unlikely to be welded because ΤΊ02 prevents oxidation at the interface [R3]

- The silicon nitride is likely to prevent welding when it is deposited on the Pyrex by preventing the passage of oxygen while deposited on the silicon it is possible to oxidize and thus solder.

This explanation is, however, not entirely satisfactory in the case of Si \ TiO 2 / Si 3 N 4 versus SiO 2 / TiO 2 / Pyrex multi-layer welds which have been demonstrated. Indeed if the ΤΊ02 prevents the passage of oxygen Pyrex, how to explain that in this configuration the welding takes place? Does the SiO 2 layer deposited by PECVD above the TiO 2 sufficiently release oxygen to allow the weld to take place? But then why a thicker Si02 layer prevents soldering?

The second phenomenon highlighted by Veenstra R1 2 concerns the electrostatic force exerted on the interface. Linked to the oxidation of the layers at the interface, the electrostatic force is a key to understanding: in our case, the titanium deposited on the Pyrex very strongly reduces the electrostatic force at the interface.

A third phenomenon is the proximity between the two plates. This is the reason why an electric field is applied to obtain an electrostatic force sufficient to bring the two plates to be welded in close contact. In our case, roughness, which is one of the aspects of proximity, could have an impact: the difference in roughness between ALD and spray deposits is known, but does not seem to play an important role in our case. I I. Fluid resistance

In order to highlight the quality of protection and welding at high pH, a test vehicle constituting a fluidic resistance has been used (FIG. 3). The latter made it possible to highlight the failures and the performances of the different configurations used.

The test vehicle was subjected to a solution of pH 12 which represents an accelerated form of study compared with a less basic pH in the silicon etching. In addition, as regards glass, it is seen according to Fig 1, that from pH 9, the solubility of silicon dioxide increases exponentially. Therefore, the results obtained at pH 12 represent an acceleration factor of at least 1000 relative to a pH of 9 which may correspond to a more representative solution of a drug injection system.

The channel in question may have 4 different layers (a), (b), (c) and (d) as shown in Fig 4. As no protective layer (b) applied on the pyrex can be directly welded to the Silicon plate (and whatever the configuration (a) - (d)), the layer (b) must be automatically covered by a weld layer (c). In the case of silicon, the protective layer (d) can be directly welded to Pyrex without protection or to the solder layer (c). In the case where the silicon is covered by a layer (d) that can not be welded, the layer (a) can be deposited on the protective layer (d) and be used as a solder layer.

Depending on the type of material used for the solder layer, the latter may also undergo a surface attack of the solution passing through the fluid path. In addition, a thickness of 200 nm allows good anodic welding of the two plates but with such a thickness, the weld begins to quickly show weak points. Thus, a layer of 200 nm of SiO 2 partially prevents an infiltration of the basic solution in the weld zone, ultimately inducing a substantial risk of delamination. In order to allow a tight assembly of two wafers, the applied layers of SiO 2 can have a thickness of 50 nm to 100 nm. Fig. 5 shows the attack at pH 12 of a channel having no protection. Since the fluid resistance of this channel has decreased by a factor of 2, it is used as a control to determine the failure threshold for channel designs with protective layers.

Fig. 6 shows that a design comprising only a 50 nm Si3N4 (a) protection layer reaches the failure threshold after 22 days. As shown in Fig. 7, the failure is caused by anisotropic etching between the channels, thus showing that weakness is at the weld. On the other hand, the protective layer at the bottom of the channel does not seem to have been attacked with respect to the control of FIG.

Fig. 8 shows that a design comprising only a 100 nm Si3N4 (a) protection layer reaches the failure threshold after 48 days. As shown in Fig 9, the failure is caused by the creation of a short circuit between the channels. The channel, whose depth has increased by 2 microns, seems to have undergone this attack on the Pyrex plate side, while the side of the protected silicon plate seems intact. This result suggests that a thickness of 100 nm is sufficient to ensure good strength of the protective layer at the weld side of the silicon plate unlike the previously tested weld with 50 nm of Si3N4. The failure probably results from an unprotected Pyrex plate attack.

Fig. 13 shows that a design comprising a 100 nm SiN3N4 (a) protection layer on silicon, a 200 nm couche02 protection layer (b) and a 50 nm SiO2 solder layer (c) on the plate of Pyrex maintains a fluid resistance greater than or equal to its nominal value over time when exposed to a solution of pH 12. In fact over more than 140 days, the fluidic resistance of the coil does not decrease, contrary to previously experienced designs . The slight increase in the fluid resistance is instead attributed to set-up artifacts comprising a filter upstream of the chip which may, with time, partially lock to give the trend observed in FIG. As illustrated in FIG. 14, the channel forming the coil retains, after more than 140 days of incubation at a pH of 12, its nominal dimensions thus suggesting that all the protective layers, as well as that of the solder, have completely filled up. their function.

References

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Claims

claims

1. Device comprising a plate comprising a silicon surface and a plate comprising a glass surface fixed to each other, the fixing zone thus formed between the plates defining a multilayer structure comprising a first layer of protection against physical deterioration of the material caused by surface etching covering the silicon surface and a second layer of protection against physical alteration of the material caused by surface etching covering the glass surface; said multilayer structure further comprising at least one additional layer for effecting anodic bonding between the two protective layers; Said device having at least one fluidic path, protected by said protective layers, adapted to temporarily contain a solution; Said additional solder layer having a sufficiently thin thickness to form a capillary stop valve at the weld in case of attack of said additional solder layer.

2. Device according to claim 1, wherein said additional solder layer is of a thickness of less than 500 nm.

3. Device according to claim 2, wherein said additional layer of solder is of a thickness of less than 200 nm.

4. Device according to claim 3, wherein, preferably, said additional layer of solder is of a thickness between 50 and 100 nm.

5. Device according to claim 1, wherein at least one of said protective layers is a compliant deposit.

6. Device according to claim 1, wherein said at least one additional layer of solder is a compliant deposit.

7. Device according to claim 1, wherein said attacks can be chemical, electro-chemical, physical and / or mechanical.

8. Device according to claim 1, MEMS type, wherein the plates are machinable.

9. Device according to the preceding claims wherein the material constituting the protective layers which covers the surface of glass and silicon withstands acidic and / or basic pHs.

10. Device according to the preceding claims wherein said material constituting the protective layers may comprise titanium dioxide, titanium nitride or silicon nitride

1 1. Device according to the preceding claims wherein said solder layer is present only on the protective layer covering the glass plate

12. Device according to the preceding claims wherein the solder layer does not withstand a basic pH.

13. Device according to the preceding claims wherein the welding layer is composed of a material undergoing a chemical transformation at the weld during the anodic welding by making it resistant to basic solutions.

14. Device according to the preceding claims wherein the welding layer is silicon dioxide.

15. Device according to claims 1 to 10 and 14 wherein said solder layer is present only on the protective layer covering the silicon wafer

6. The device according to claims 1 to 10 and 14 to 15 wherein said solder layer is also a protective layer.

17. Device according to claims 1 to 10 and 15 to 16 whose solder layer is silicon nitride silicon

18. Device according to the preceding claims wherein the plate with a glass surface is borosilicate such as Pyrex or silicon.

19. Apparatus according to the preceding claims wherein the plate with silicon surface is silicon on insulator or glass.

20. Device according to one of the preceding claims wherein said multilayer structure has a thickness less than 1 μιη.

21. Device according to one of the preceding claims wherein the layers of protections and welds are biocompatible

22. Device according to the preceding claims adapted for use as a medical system.

23. Device according to the preceding claim adapted for use as an implantable medical system.

24. A method of manufacturing a MEMS device comprising the following steps:

a) applying a protective layer against a surface attack on at least one silicon region of a main face of a first plate,

b) applying a protective layer against a surface attack on at least one glass region of a main face of a second plate,

c) add a thin layer of a material that allows anodic welding between the two layers of protection while preventing infiltration a solution in the weld zone defining the lateral ends of a fluid path through which said solution propagates.

25. The method of claim 24 wherein at least one protective layer is deposited so that the deposit is compliant.

26. The method of claim 24 wherein said solder layer is deposited so that the deposit is compliant.

The method of claim 24 wherein said plates can be structured before and / or after steps a) and / or b).

28. The method of claim 24 wherein steps a) and b) are performed successively or simultaneously.

The method of claim 24 wherein the two protective layers, as well as the additional solder layer, are applied by one or the combination of the following techniques: Low Pressure Chemical Vapor Deposition (LPCVD), Plasma Enhanced Chemical Vapor Deposition (PECVD), Atomic Layer Deposition (ALD), oxidation, evaporation or sputtering.

30. A MEMS-type device obtained by a method comprising the following steps: a) applying a protective layer against a surface attack on at least one silicon region of a main face of a first plate,

c) adding at least one thin layer of a material which makes it possible to carry out anodic welding between the two protective layers and to prevent the infiltration of a solution into the weld zone defining the lateral ends of a fluid path from through which said solution propagates.

31. Apparatus according to claim 30 wherein at least one layer of

protection is deposited in such a way that the deposit is compliant.

32. Device according to claim 30 wherein said solder layer is deposited so that the deposit is compliant.

The device of claim 30 wherein said plates can be structured before and / or after steps a) and / or b).

34. Apparatus according to claim 30 wherein steps a) and b) are performed successively or simultaneously.

35. Device according to claim 30 wherein the two protective layers, as well as the additional solder layer, are applied by one or the combination of the following techniques: Low Pressure Chemical Vapor Deposition (LPCVD), Plasma Enhanced Chemical Vapor Deposition (PECVD), Atomic Layer Deposition (ALD), oxidation, evaporation or sputtering.