EP0641934B1 - Manufacturing process for a micropump - Google Patents

Abstract

This process consists of machining a silicon piece (4) by means of selective oxidation operations and photolithography to form therein at least one cavity (7, 12) adapted to contain or convey a fluid, and of oxidizing the wall of the cavity to make this hydrophilic. The device is completed by fixing closing plates (1, 5) to its body thus formed. Prior to the machining operations the surfaces of the piece (4) adapted to be in contact with the closing plates (1, 5) are covered with a screening layer that resists these machining operations. Then, after these have been completed, the surfaces of the piece intended to be exposed to the fluid are oxidized to form therein an oxide layer favoring the wettability of these surfaces. The screening layer is then removed and the closing plates are fixed to the piece. The invention has applications, notably in micropumps.

Description

The present invention relates to a method for manufacturing devices produced by micromachining of silicon and called to contain or convey gaseous or liquid fluids. More particularly, the invention relates to the manufacture of silicon micropumps produced by photolithography machining techniques.

A particular construction of a silicon micropump excited by a piezoelectric element is known from patent application PCT-WO 91/07591. In this document, the problems which are linked to the fact that silicon is a hydrophobic material are evoked incidentally so that the surfaces of the silicon, in contact with the fluid to be pumped, have poor wettability. This problem is all the more difficult since, often, this kind of micropump is used to convey medicinal substances presented in the form of an aqueous solution. Under these conditions, and without taking any special precautions, correct filling of the pumping chamber and / or the chambers of the inlet and outlet valves is not possible.

The solution to this problem mentioned in the aforementioned international patent application, namely to make the surfaces in contact with the fluid to be conveyed hydrophilic, consists in oxidizing the pump body in silicon after its manufacture so as to form a very weak surface layer d silicon oxide which is hydrophilic and can thus considerably improve the wettability of the volumes of the pump in contact with the fluid to be conveyed. More specifically, in the aforementioned document, it is proposed to soak the completed pump body in boiling nitric acid for a period of time. sufficient to create a suitable thickness of the hydrophilic layer.

However, this procedure has the disadvantage that by oxidizing the pump body in this way, the entire exposed surface of the silicon undergoes the treatment, including the surfaces on which the cover glasses of the pump are subsequently welded. However, it is known that the welding of a glass on a silicon oxide surface is difficult or even impossible to achieve.

The presence of the oxide layer covering the silicon exposed to the fluid remains desirable, however, because it also has another advantage in that it makes it possible to protect the silicon against attack by the fluid provided that it naturally has an aggressive behavior. With respect to silicon, for example, one can imagine that the fluid consists of a corrosive gas, the harmful effects of which under these conditions are canceled under these conditions. Furthermore, the oxide layer can constitute an electrical insulator, when the fluid is electrically conductive.

The object of the invention is to remedy the above-mentioned drawback of the prior art and to propose a method for manufacturing micro-machined devices of the type indicated above which makes it possible to guarantee a good bond between the silicon body. of the device and the glass closure plates, while retaining an oxide layer on the expired surfaces with the fluid.

• usiner par des opérations d'oxydation sélective et de photolithographie une plaquette en silicium pour y former au moins une cavité destinée à contenir ou à véhiculer ledit fluide, et à oxyder la paroi de ladite cavité pour la rendre hydrophile, et
• achever ledit dispositif en assujetissant contre le corps de dispositif ainsi formé des plaques de fermeture,
• ce procédé étant caractérisé en ce qu'il consiste:
• préalablement auxdites opérations d'usinage, à recouvrir les surfaces de ladite plaquette destinées à être en contact desdites plaques de fermeture d'une couche-écran résistant auxdites opérations d'usinage;
• après achèvement desdites opérations d'usinage à oxyder les surfaces de ladite plaquette destinées à être exposées audit fluide pour y former une couche d'oxyde favorisant la mouillabilité de ces surfaces;
• à éliminer ladite couche d'écran; et
• à assujettir lesdites plaques de fermeture contre ladite plaquette.

The subject of the invention is therefore a method of manufacturing a micro-machined device intended to contain or convey liquid substances, this method consisting in:

• machine, by selective oxidation and photolithography operations, a silicon wafer to form at least one cavity therein intended to contain or conveying said fluid, and oxidizing the wall of said cavity to make it hydrophilic, and
• complete said device by subjecting closure plates against the device body thus formed,
• this process being characterized in that it consists:
• prior to said machining operations, covering the surfaces of said wafer intended to be in contact with said closure plates with a screen layer resistant to said machining operations;
• after completion of said machining operations to oxidize the surfaces of said wafer intended to be exposed to said fluid to form an oxide layer promoting the wettability of these surfaces;
• removing said screen layer; and
• securing said closure plates against said wafer.

According to another characteristic of the invention, said screen layer is made of silicon nitride and deposited on said wafer with the interposition of an intermediate oxide layer.

According to yet another characteristic of the invention, said intermediate oxide layer has a thickness less than that of said oxide layer promoting wettability, the method further consisting, after the elimination of said screen layer, in eliminating said intermediate oxide layer, while said oxide layer promoting wettability is exposed.

The invention also relates to a micro-machined device obtained by the method as defined above.

It follows from these characteristics that the assembly of the closing plates, an operation which completes the micro-machined device, remains easy to execute with great reliability of the result, while the silicon surfaces of the micro-machined device intended to be in contact with the fluid to be conveyed or to be sheltered, are hydrophilic and / or resistant to the possible aggression of this fluid.

• les figures 1a et 1b sont des vues schématiques en plan, respectivement de dessus et de dessous, d'un exemple de dispositif micro-usiné réalisé à l'aide du procédé selon l'invention, cet exemple concernant une micropompe à entraînement piézo-électrique à laquelle, toutefois l'invention n'est nullement limitée;
• la figure 2 est une vue en coupe transversale de la micropompe représentée aux figures 1a et 1b, vue qui est prise selon la ligne II-II de ces figures;
• la figure 3 montre, par une coupe partielle schématique selon la ligne III-III des figures 1a et 1b, les opérations successives nécessaires pour exécuter le procédé suivant l'invention.

Other characteristics and advantages of the present invention will become apparent during the description which follows, given solely by way of example and made with reference to the appended drawings in which:

• FIGS. 1a and 1b are schematic plan views, respectively from above and from below, of an example of a micro-machined device produced using the method according to the invention, this example concerning a micropump with piezoelectric drive to which, however, the invention is in no way limited;
• Figure 2 is a cross-sectional view of the micropump shown in Figures 1a and 1b, a view taken along line II-II of these figures;
• Figure 3 shows, in a schematic partial section along the line III-III of Figures 1a and 1b, the successive operations necessary to carry out the method according to the invention.

We will first refer to Figures 1a, 1b and 2 to describe, by way of example of the implementation of the method according to the invention, a piezoelectric drive micropump, an object which lends itself particularly well to be done using this process. Note that the terms "above" and "below" are used only for descriptive purposes, the pump can be used in any attitude in space.

The micropump comprises a base plate 1 or first closure plate, preferably made of glass and pierced in its thickness by two channels 2 and 3 which are respectively the inlet channel and the outlet channel of the micropump.

On this base plate 1 is fixed a plate 4 forming a pump body and made of silicon, this plate being micro-machined to form there, by the method of the invention, the various cavities and active members of the pump, as will be described below.

On the plate 4 forming the pump body is fixed in turn a third relatively thin plate 5 made of glass, preferably. This plate constitutes the second closing plate of the pump. It is surmounted by a piezoelectric transducer 6 extending over part of its outer surface, this transducer being intended, by virtue of its vibrational regime generated when it is excited by an electric voltage, to deform the second closure plate 5 and consequently to vary the volume of the pumping chamber of the pump during its operation.

To fix the ideas and by way of example only, it can be noted that a micropump thus constructed has a dimension in its general plane of 22 × 22 mm, the thicknesses of the plates 1, 4 and 5 being respectively 1.5 mm, 280 microns and 0.3 mm.

The intermediate plate 4 forming the pump body has an inlet chamber 7 (FIG. 2) communicating with the inlet channel 2 pierced in the base plate 1. This inlet chamber 7 surrounds an inlet valve 8, the l shutter 9 is formed by a thin and deformable veil machined in the silicon of the plate 4. The shutter 9 cooperates with a valve seat 10 which is not specially materialized, but is formed by the corresponding part of the surface of the base plate 1 on which the shutter rests. It will be noted that this shutter 9 has a ring-shaped lining 9a which is brought to it during the process of the invention, and which is intended to slightly arch the thin veil and thus guarantee proper application of the shutter 9 on its seat 10.

The shutter 9 is provided with a central communication hole 11 which opens, on the side of the veil opposite to the inlet chamber 7, in a pumping chamber 12 above which the piezoelectric transducer is placed 6. It is therefore the volume of this pumping chamber 12 which is caused to change periodically to obtain the action of pumping of the micropump.

The pumping chamber 12 is in communication with a transfer chamber 13 via a communication orifice 14, this transfer chamber surrounding a second valve of the pump which is the outlet valve 15 of the latter. This valve is constructed substantially in the same way as the inlet valve and therefore comprises a shutter 16, a shutter lining 16a, a seat 17 and a central communication orifice 18. The latter connects, if necessary it is ie when the outlet valve 15 is open, the transfer chamber 13 to an outlet chamber 19 located above the outlet valve 15. This outlet chamber 19 in turn communicates with the outlet channel 3 of the pump via a communication orifice 20.

The construction of the micropump which has just been described is known per se and we therefore refrain from describing its operation in detail as far as it can be easily reconstituted from the description which has just been given of this construction.

We will therefore now describe the process for manufacturing the pump body 4, emphasizing the essential characteristics of the present invention which, as already indicated at the beginning of this specification, aim to improve the hydrophilic properties and resistance to the aggressiveness of fluids. pump, surfaces of the pump body 4 in contact with this fluid during operation of the pump.

Figures 3a to 3j schematically show a partial sectional view of a pump body 4 taken along line III-III of Figures 1b, during the various stages of the method according to the invention. He is at note that in the description of the process which follows, the values of all the parameters such as layer thicknesses, temperatures, residence time in the ovens, etc., are only given by way of example and are not to be considered as limiting the present invention.

A silicon wafer 21, in which according to the usual technology, several pump bodies can be formed simultaneously, is first of all subjected to wet oxidation (step of FIG. 3a) which forms on its two faces a layer of oxide 22. The thickness of the layer can be 1 micron and the step can be carried out in an oven in which an atmosphere of water vapor prevails which is brought to a temperature of 1100 ° C. Water vapor can be generated in a bubbler in which oxygen is introduced at a flow rate of 0.5 l / min and nitrogen at a flow rate of 4 l / min.

The wafer thus provided with the oxide layers 22 is subjected to a conventional photolithography operation by which an attack of the oxide with hydrofluoric acid buffered with ammonium fluoride is carried out in a proportion of 1: 7 and at temperature ambient, through a photoresist mask, to keep only annular zones 23 intended to subsequently form the linings 9a and 16a of the valves. (It should be noted that Figures 3a to 3j show only the area corresponding to only one outlet valve 15).

The wafer resulting from the operation of the step of Figure 3b is then coated in its entirety with an oxide layer 24 of a predetermined thickness (in the example of 1000 Ångströms) by dry oxidation in a tubular oven at 1100 ° C in which a stream of oxygen circulates with a flow rate of 2 l / min. Then, the oxide layers thus obtained which have a role of bonding layer, are in turn coated with a layer 25 of silicon nitride (Si ₃ N ₄ ) by chemical vapor deposition (LPCVD) at 800 °. C and up to a thickness of 1500 Ångströms. Alternatively, the silicon nitride can be replaced by aluminum oxide (Al ₂ O ₃ ) of the same thickness.

The next step of the process, illustrated in FIG. 3d, consists in selectively removing the layers 24 and 25 in order to delimit expanses 26 and 27 on the wafer in which the various cavities of the pump are subsequently formed. With regard to FIGS. 3a to 3j, these are respectively the outlet chamber 19 and the transfer chamber 13. The annular zones corresponding to the linings 9a and 16a, respectively, are preserved. This step therefore comprises a conventional photolithography operation using a photoresist during which the silicon nitride is first removed selectively by plasma etching, then the oxide by etching with buffered hydrofluoric acid.

The wafer 21 is then again subjected to an oxidation operation on the two faces, outside the zones already covered by the silicon nitride to form the layers 28 (see FIG. 3e). This oxidation takes place in the same way as that which led to the formation of the layers 22 (see FIG. 3a), the thickness of the layers 28 being 3000 Ångströms, for example.

Then, an opening 29 of circular shape is made in the oxide layer 28 at the places where the central passages of the valve 8 and 15 must be located. This opening is produced by subjecting the wafer to photolithography operations using photoresist, the attack itself being carried out with buffered hydrofluoric acid. This results in the configuration shown in Figure 3f.

A cavity 30 is then formed in the silicon by subjecting the wafer to a KOH solution at a temperature between 40 and 60 ° C to attack it anisotropically until the depth of the cavity is approximately equal to 50 microns , after which we removes residual oxide which has not yet been removed by KOH attack, by again subjecting the wafer to a solution of hydrofluoric acid buffered with ammonium fluoride in a proportion of 1: 7 and at room temperature, until that all the oxide has disappeared on both sides of the wafer. This operation leads to the configuration shown in Figure 3g.

The wafer is then again subjected to an anisotropic attack with KOH by soaking in a solution of this compound for a sufficient time so that what has become the haze of each valve is only 50 microns thick. This operation also leads to the piercing of the plate in the center of the valve and to the formation of the various cavities provided for the pump, as shown in Figure 3h.

Then, the wafer is subjected to wet oxidation under the same conditions as those which led to the formation of layer 22 until an oxide layer 31 with a thickness of approximately 3000 Ångströms is obtained, this layer covering with oxide all the expanses of the pump intended to come into contact with the fluid. The zones which have remained covered with silicon nitride during all the stages of the process which have just been described are not affected by this oxidation operation, as shown in FIG. 3i.

The next step in the process consists in removing the silicon nitride from the layer still present on the wafer by subjecting it to an 85% solution of phosphoric acid at a temperature of around 180 ° C. and then to a solution. hydrofluoric acid buffered to remove the oxide from layer 24, previously underlying the silicon nitride. This last operation also leads to the partial removal of the oxide layer 31. However, since the oxide layer 24 had a thickness of approximately 1000 Ångströms, the operation of removal of oxide carried out last allows to remain a sufficient thickness on the surfaces exposed to the fluid (approximately 2000 Ångströms) so that these surfaces have sufficient wettability and are sufficiently protected against possible attacks by this fluid. This last operation leads to the configuration shown in FIG. 3j, where it can be seen that an oxide layer 32 has remained present.

It will be noted that this configuration corresponds to the completed pump body to which it then suffices to subject the closure plates 1 and 5 by anodic welding, as well as to set up the piezoelectric transducer to finalize the construction of the micropump.

Thus, as can be seen, the hydrophilic and protective layer 32 is provided during the process of making up the pump body without requiring subsequent dipping operations capable of oxidizing not only the surfaces which must actually be, but also the surfaces 33 against which the closure plates of the pump are to be fixed, as was the case in the prior art.

Finally, the method of the invention makes it possible to easily obtain a thicker oxide layer than was the case in the prior art, so that it can provide better electrical insulation.

Claims (4)

1. Process for the manufacture of a micromachined device adapted to contain or to convey fluid substances in particular liquids, this process consisting of :

   - machining a silicon plate (4) by means of selective oxidation and photolithographic operations to form therein at least one cavity (7, 12) adapted to contain or to convey said fluid, and to oxidize the wall of said cavity to render it hydrophilic, and

   - to complete said device by fixing closure plates (1, 5) to the body of the device thus formed,

   characterized in that it consists in :

   - preceding said machining operations by covering the surfaces of said plate (4) adapted to be in contact with said closure plates (1, 5) with a screening layer (25) resistant to said machining operations;

   - after completing said machining operations, oxidizing the surfaces of said plate adapted to be exposed to said fluid to form thereon an oxide layer (32) favouring the wettability of these surfaces;

   - removing said screening layer (25); and

   - fixing said closure plates (1, 5) to said plate (4).
2. Process according to claim 1, characterized in that said screening layer (25) is made of silicon nitride and deposited on said plate (4) with interposition of an intermediate oxide layer (24).
3. Process according to claim 2, characterized in that said intermediate oxide layer (24) has a thickness less than that said oxide layer (31) favouring the wettability, the process also consisting, after removal of said screening layer, of eliminating said intermediate oxide layer while said oxide layer favouring wettability is uncovered.
4. Device produced by micromachining silicon adapted to contain or convey a fluid, characterized in that it is obtained according to the process as defined in any one of claims 1 to 3.

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