WO1998043465A1

WO1998043465A1 - Method for producing two communicating cavities in a substrate of monocrystalline material by chemical anisotropic etching

Info

Publication number: WO1998043465A1
Application number: PCT/FR1998/000562
Authority: WO
Inventors: Yves Fouillet; Patricia Touret
Original assignee: Commissariat A L'energie Atomique
Priority date: 1997-03-21
Filing date: 1998-03-20
Publication date: 1998-10-01
Also published as: FR2761199A1; FR2761199B1

Abstract

The invention concerns a method for producing at least two communicating cavities (3, 4) in a substrate (1) of monocrystalline material by chemical anisotropic etching, said two communicating cavities mutually communicating along an intersection. The invention is characterised in that it consists in: defining a connecting zone (5) for the two cavities (3, 4) corresponding to said intersection; etching the substrate (1) for producing the two cavities, except the connecting zone (5); etching the connecting zone until said intersection is produced. The invention is useful for producing hydrophones, and micro-apparatus for measuring or mixing.

Description

METHOD FOR PRODUCING TWO COMMUNICATING CAVITIES IN A SUBSTRATE OF SINGLE-CRYSTAL MATERIAL BY ENGRAVING

ANISOTROPIC CHEMICAL

The present invention relates to a method for producing two communicating cavities in a substrate made of monocrystalline material by anisotropic chemical etching.

Certain micro-machined structures in monocrystalline material are designed to present communicating cavities. This is particularly the case of hydrophones where a first cavity constituted by a microchannel allows a fluid, whose pressure is to be measured, to have access to a second cavity constituting an enclosure. In this type of sensor, a microchannel can ensure the flow of the fluid from a first enclosure to a second enclosure. An important parameter to take into consideration in the fluid flow is the pressure drop, especially when it comes to measuring a fluid pressure.

Consider a fluid which flows through a pipe from a first enclosure to a second enclosure, the enclosures being open or closed. Due to the presence of viscosity forces of the fluid, it is known that the pressure of the fluid tends to decrease along the pipeline. Thus, if the fluid flows from an enclosure A to an enclosure B, the pressure in the enclosure A is greater than the pressure in the enclosure B. It is then said that the pipe creates a pressure drop within flow or that the pipeline forms a restriction within the flow. Experience and theory of fluid mechanics show that in In many flow cases, it is possible to establish a simple relationship between the fluid pressures in the chambers and the volume flow of fluid in the pipeline. This relationship is solely a function of the properties of the fluid (density and viscosity), of the geometrical characteristics of the pipeline and possibly of the roughness of the walls of the pipeline.

In this case, it will be said that the pressure drop is calibrated if the tolerances of the geometric sizes of the pipe, and possibly the roughness of its walls, are controlled.

To properly control the flow in a pipeline, it is therefore necessary to pay particular attention to the dimensional tolerances of the section of the pipeline as well as its length. The tolerances on the section must in particular be particularly considered. Thus, for a cylindrical pipe and of circular section, it can be shown that, under certain conditions, the fluid flow rate is inversely proportional to the length of the pipe and proportional to the fourth power of its radius. This point is important to observe when choosing a process for making a microchannel for a hydrophone.

In the case of micromachined structures, most often one starts from thin plates, or substrates, of monocrystalline material (silicon or germanium). The microchannel is made either perpendicular to the substrate, or parallel to this substrate.

A cylindrical channel passing right through a substrate can be produced by deep reactive ion etching or by laser machining. These processes require efficient machines and are not always usable in an industrial context. On the other hand, the length of the channel is limited to the thickness of the substrate.

A channel parallel to the substrate can be produced by anisotropic chemical etching. This etching is commonly used to produce monocrystalline silicon microstructures. It results from a difference in attack speed between the crystallographic planes of this material. It is thus possible to etch a groove on the surface of a first substrate. A second substrate is then sealed on the etched surface of the first substrate so as to cover the groove and thus form a channel. The assembly formed by the first and the second substrate is then cut perpendicular to the groove so that the channel opens onto the cutting faces. Thanks to the perfect crystalline organization of monocrystalline materials, machining by anisotropic etching has the advantage of precisely controlling the dimensional tolerances of the section of the channel, which is well suited to obtaining a calibrated pressure drop. However, the cutting operation perpendicular to the groove has the following drawbacks. The precision over the length of the pipe to be obtained is given by the precision of the cutting machine. Then, during this cutting phase, there is a risk of pollution that can clog the pipe. Finally, the cutting operation can generate the formation of flakes on the intersection edges of the pipe and the cutting plane.

Different methods have been proposed in an attempt to improve the principle of opening the channel.

To limit the formation of flaking, certain improvements can be made to the cutting phase by intervening on the cutting parameters and on the type of tooling, as described in patent US-A-4 639 748. Another method of producing microchannels having no flake-like defect and limiting the risk of pollution is described in document FR-A-2 727 648. This document recommends the use of a protective layer deposited before the cutting phase. However, this process is not always applicable since it is not always possible to deposit a protective layer.

The methods described in documents US-A-4,639,748 and FR-A-2,727,648 do not completely solve the problem of the risk of pollution and the accuracy of the length of the channel. In view of the above, it seems preferable not to use a cutout for the opening of the channel. It seems more judicious to open the canal on "chimneys" crossing one of the two substrates. This solution is presented in FIG. 1 which shows a first substrate 10, one face 11 of which has a groove 12 obtained by etching, and a second substrate 20, the face 21 of which is sealed on the face 11 of the first substrate 10 to transform the groove into channel. Two chimneys 22 and 23 have been drilled in the second substrate 20 and communicate with the channel 12. Thus the useful length of the channel is defined by the position and the geometry of the chimneys 22 and 23. However, this solution requires machining the two substrates 10 and 20, which is not always possible. For example, many applications use a first monocrystalline substrate on which the micro-structures are etched while the second substrate is made of glass for which the machining is delicate and often imprecise. In this case, it is preferable to carry out the groove and chimneys on the same substrate as shown in Figure 2.

To make a chimney crossing a substrate right through, with a good positioning and by conventional techniques, it is preferable to make it by chemical etching as indicated above. Only such an engraving also allows collective manufacturing at low cost. FIG. 2 shows a first substrate 30, one face 31 of which has a groove 32 and which is crossed by a chimney 33 communicating with the groove 32. The face 41 of a second substrate 40 is sealed on the face 31 of the first substrate 30 for transform the groove into a channel.

The problem then arises of the appearance of rapid planes during the anisotropic etching phases, which can cause a significant modification of the geometry of the opening of the restriction. Depending on the type of etching bath used, the number of rapid planes and their orientations can be very different but we can schematize the result obtained by the representation given in Figure 3 which is a sectional view in the direction III-III indicated on Figure 2. The chimney 33 and the channel 32 are delimited by slow planes 34. On the other hand, the chimney-channel intersection consists of a succession of rapid planes 35 (dotted lines in the figure). The existence of these rapid plans poses two problems. First, we observe a funnel shape occupying a more or less important space, which can harm the miniaturization of the microsystem. For example, for a groove 60 μm deep, rapid planes can advance by more than 400 μm in the direction of the channel. Second, experience shows that the speed of fast planes is strongly dependent on the stability of the bath (concentration, temperature) and that a small crystalline disorientation can generate strong asymmetries. There will therefore be great uncertainty about the length of the channel. This problem is well known in micromachining, and in general it is possible to define compensation patterns to delay the appearance of fast shots on useful patterns as described in the article: "Corner Compensation Techniques in Anisotropic Etching of ( 100) -Silicon Using Aqueous KOH ", by H. SANDMAIER et al., IEEE, 1991, pages 456-459. These compensation reasons are well suited for most simple cases. However, in the particular case of a channel resulting from a V-shaped etching and leading to a very deep etching, the application of compensation patterns has been found to be inadequate. After various tests, it was not possible to demonstrate a compensation pattern allowing satisfactory and reproducible results to be obtained.

The present invention overcomes these drawbacks by proposing a method for producing at least two communicating cavities in a substrate made of monocrystalline material by anisotropic chemical etching, the two cavities communicating with each other at an intersection, characterized in what: - a connection zone is defined for the two cavities corresponding to said intersection,

- the substrate is etched in order to produce the two cavities, with the exception of the connection zone,

- the connection area is etched until the said intersection is obtained.

The etching of the substrate can be carried out so that one of the cavities constitutes a chimney crossing the substrate and that the other cavity constitutes a groove on one of the faces of the substrate. The invention also relates to a process for producing a hydrophone structure in a substrate made of monocrystalline material by anisotropic chemical etching to obtain three cavities in the substrate: a first cavity opening onto one of the main faces of the substrate and delimiting a membrane which separates it from the other main face of the substrate, a second cavity or chimney passing right through the substrate, a third cavity constituted by a groove formed on the face of the substrate where the first cavity opens and communicating with the first cavity by a first intersection and with the second cavity by a second intersection, characterized in that: - zones of connection are defined between the first cavity and the third cavity and between the second cavity and the third cavity, the connection zones corresponding to said intersections,

- the substrate is etched in order to produce the three cavities, with the exception of the connection zones,

- The connection areas are etched until the said intersections are obtained.

If the substrate is made of silicon, the cavities can be etched using masks as protective layers developed on the main faces of the substrate such as silicon oxide Siθ2 or silicon nitride Si3N4. In this case, the masks are advantageously produced by reactive ion etching of the protective layers.

The invention will be better understood and other advantages and particularities will appear in the description which follows, given by way of nonlimiting example, accompanied by the appended drawings among which: FIG. 1 is a view in longitudinal section of a micro-machined structure having a channel opening into two chimneys,

FIG. 2 is a view in longitudinal section of a micro-machined structure having a channel opening into a chimney, the channel and the chimney being etched in the same substrate,

FIG. 3 is a view along the plane III-III of the structure represented in FIG. 2, - FIGS. 4 to 6 are illustrative of the principle implemented in the method according to the present invention,

- Figure 7 illustrates the different steps of the method according to the present invention, in an industrial application.

FIG. 4 is a view in longitudinal section of a substrate 1 made of monocrystalline material, for example made of monocrystalline silicon, which has been partially etched from one of its main faces referenced 2. Anisotropic chemical etching has enabled, in a first step, to sketch a chimney 3, then, in a second step to sketch the groove 4 while deepening the chimney 3. We note that the engraving left intact an area 5, called connection area, between the sketches chimney and groove.

A third etching step is then carried out and the result obtained is illustrated by FIG. 5 which is a longitudinal view corresponding to FIG. 4. The third etching gives their final shape to the chimney 3 and to the groove 4 and places the chimney 3 and groove 4 by eliminating the connection area.

Figure 6 is a bottom view of the substrate 1 shown in Figure 5. The intersection chimney-groove consists of slow planes 6 shown in dotted lines. The etching times of the slow planes are minimum, which optimizes the positioning accuracy of these planes and in particular of those defining the section of the groove.

The width of the connection area 5 must be chosen according to the precision required for the dimensions of the groove. It is understood that the smaller this width, the better the groove will be calibrated.

A first substrate is thus obtained provided with a chimney, or possibly with two chimneys (reason for a chimney by end of groove), and with a groove. The sealing, on the face 2 of the substrate 1, of a second substrate makes it possible to transform the groove into a channel.

We will now describe a technically more detailed process for developing a micro-machined structure for hydrophone, with reference to FIG. 7. In this figure, the starting substrate is shown from the side.

We start from a substrate 50 made of monocrystalline silicon, of thickness 450 μm and orientation <100>. During a first step, protective layers are deposited on the substrate 50, made of silica SiO 2 or nitride

By way of example, the thickness of the layer 53 deposited on the upper face 51 of the substrate can be 1 μm and the thickness of the layer 54 deposited on the lower face 52 of the substrate can be 1.3 μm ( see sequence A in figure 7). The deposition technique can be a plasma assisted gas decomposition technique or PECVD.

The protective layers 53 and 54 are then masked and etched in several stages using the conventional techniques of photolithography. The etching of the protective layers can be carried out by a reactive ion etching technique.

The protective layer 54 is first etched, over its entire thickness, in zones 55 and 56 corresponding respectively to the membrane and to the chimney of the hydrophone. (see sequence B in Figure 7). Next, the part of the protective layer 54 remaining between the zones 55 and 56 is etched to a depth of approximately half the thickness of the layer 54, the etching preserving thicker end zones 57 and 58.

The end zones 57 and 58 undergo a new etching (see sequence C in FIG. 7) in order to reduce their thickness compared to the rest of the protective layer 54. This etching can be around 0.3 μm. The upper protective layer 53 is etched in turn, over its entire thickness, in a zone 59 corresponding to the chimney.

The substrate 50 can then be subjected to an anisotropic chemical etching using a KOH or TMAH (tetramethylammonium hydroxide) bath at about 80 ° C.

The sequence D of FIG. 7 represents the result obtained after etching of the two faces of the substrate 50. The parts of the unprotected faces of the substrate were etched to a depth of approximately 380 μm, producing a first cavity 61 in correspondence with the future membrane and a second cavity 62 corresponding to the future chimney. The lower protective layer 54 is then etched by reactive ion etching. This is done without masking. The etching is carried out until the protective layer disappears at the level of the future channel, that is to say between the end zones 57 and 58 (see sequence E of the figure 7). We then proceed to a second anisotropic etching of the substrate 50, 45 μm deep. In particular, a draft of the future channel 63 is obtained.

The areas 64 and 65 of the substrate, protected respectively by the end areas 57 and 58 coming from the lower protective layer - 54, constitute connection areas between the cavity 61 and the channel 63 on the one hand and between the channel or groove 63 and the chimney 62 on the other hand. The next step, illustrated by the sequence F of FIG. 7, consists in carrying out a reactive ion etching of the lower protective layer 54 until elimination of the end zones 57 and 58 (still visible in sequence E of the figure 7). This operation is done without masking. A third anisotropic chemical etching 5 μm deep is then carried out which allows the connection zones 64 and 65 to be removed (see sequence E). A channel 63 is thus obtained communicating, at one end, with the cavity 61 and, at its other end with the chimney 62. This third etching also made it possible to define the thickness of the membrane 65.

What remains of the protective layers 53 and 54 is eliminated, for example by reactive ion etching. The face 52 of the structure obtained can then be sealed on a support 70, for example made of glass, pierced with a through hole 71 (see sequence G in FIG. 7). The assembly can be obtained by an anodic sealing. The hole 71 of the support 70 opens into the cavity 61 and a communication is thus formed between the two faces of the assembly via the hole 71, the cavity 61, the groove 63 become channel and the chimney 62.

For example, the structure obtained can have a chimney with a cross section of 400 μm x 400 μm, a membrane with a surface area of 3 mm ² and a thickness of 20 μm and a channel of 400 μm in length for a triangular section of 70 μm in base, 50 μm in height.

Although FIG. 7 illustrates only the production of a single sensor, for the sake of simplification, it is obvious to those skilled in the art that the method makes it possible to produce a large number of sensors at the same time by a method. collective manufacturing. In this case, the individual cutting of the sensors takes place after the sealing step on the glass support.

The useful parts of the microstructure obtained are not produced by a cutting operation. There are therefore no pollution, defect and imprecision problems inherent in cutting operations. The method ensures good accuracy of the section of the channel and its length.

The invention also applies to the production of micro-structures other than hydrophones, for example micro-dosers and micro-mixers.

Claims

1. Method for producing at least two cavities (3, 4; 61, 63; 62, 63) communicating in a substrate (1; 50) in monocrystalline material by anisotropic chemical etching, the two cavities communicating with one other according to an intersection, characterized in that:

- a connection zone (5; 64; 65) is defined for the two cavities (3, 4; 61, 63; 62, 63) corresponding to said intersection,

- the substrate (l; 50) is etched in order to produce the two cavities, with the exception of the connection zone,

- the connection area is etched until the said intersection is obtained.

2. Method according to claim 1, characterized in that the etching of the substrate is carried out so that one of the cavities constitutes a chimney (3; 62) passing through the substrate (1; 50) and that the other cavity constitutes a groove (4; 63) on one of the faces (2; 52) of the substrate.

3. Method for producing a hydrophone structure in a substrate (50) of monocrystalline material by anisotropic chemical etching to obtain three cavities in the substrate: a first cavity (61) opening onto one (52) of the faces principal of the substrate (50) and delimiting a membrane

(65) which separates it from the other main face (51) of the substrate (50), a second cavity or chimney (62) passing right through the substrate, a third cavity constituted by a groove (63) formed on the face (52) of the substrate (50) where the first cavity (61) opens and communicating with the first cavity (61) by a first intersection and with the second cavity (62) by a second intersection, characterized in that:

- Connection zones (64,65) are defined between the first cavity (61) and the third cavity (63) and between the second cavity (62) and the third cavity (63), the connection zones (64,65 ) corresponding to said intersections,

- the substrate (50) is etched in order to produce the three cavities (61,62,63), with the exception of the connection zones (64,65),

- Etching the connection areas (64,65) until obtaining said intersections.

4. Method according to claim 3, characterized in that, the substrate being made of silicon, the cavities (61,62,63) are etched using masks as protective layers (53,54) developed on the main faces (51 , 52) of the substrate (50) such as silicon oxide Siθ2 or silicon nitride Si ₃ N ₄ .

5. Method according to claim 4, characterized in that the masks are produced by reactive ion etching of the protective layers (53,54)