HK1138252B - Process for manufacturing multilevel micromechanical parts made of silicon - Google Patents

Description

Method for producing a multi-layer silicon micromechanical component

Technical Field

The invention relates to a method for manufacturing a multilayer silicon micromechanical part and to a part obtained by this method, these parts being used in particular in the field of horology.

Background

The existing manufacturing techniques of silicon micromechanical parts allow the verticality of the sides of the parts to be controlled sufficiently well for using these parts as components of horological mechanisms. However, it is still very difficult to fabricate parts with multiple layers from a single silicon wafer using prior art techniques.

An alternative method of producing a multilayer part is proposed. The method describes how to obtain a multilayer silicon part from only a single layer silicon part produced using existing methods.

Disclosure of Invention

The aim of the invention is to provide a simple and economical solution for the manufacture of multilayer micromechanical parts from a single layer of commercially available silicon, said parts (whether fixed or mobile) being used in particular in the construction of a timepiece movement.

To this end, the invention relates to a method:

a) processing a first element in a first silicon wafer or processing a plurality of first elements on the same silicon wafer by adopting a chemical method, a physical method or a combination of the two methods;

b) repeating step a) with a second silicon wafer in order to process a second element having a different shape from the first element, or to process a plurality of said second elements on the same wafer;

c) stacking the first element and the second element or the first silicon slice and the second silicon slice face to face by means of a positioning device;

d) thermally oxidizing the assembly formed in step c), i.e. leaving it in the oxidizing agent mixture for about 2-4 hours; and

e) the parts are separated.

The method allows to precisely construct each element of the final part to be obtained, the technique adopted having been verified in silicon wafers with a thickness of less than 1 mm.

In step c), before the thermal oxidation to physically join the two elements together, different positioning means can be used to ensure the precise positioning of the two elements that should constitute the final part. In the detailed description section below, two examples of positioning means are given, which either use positioning means formed in each silicon wafer at the same time as the construction of the elements, which are thus connected to each other by means of material bridges, or use a mould that allows the positioning of the silicon wafer or of elements detached from the silicon wafer, one on the other.

Step d) for oxidizing the interface between the silicon wafers can be carried out by dry methods using dry oxygen or by wet methods using water vapor, for example in a furnace heated to a temperature between 900 ℃ and 1200 ℃ according to techniques well known to those skilled in the art, for example in the works "Semiconductor devices: physics and technology-edition John Wiley & Sons, ISBN 0-471-.

The oxidation may also be performed by any other means known to those skilled in the art, for example using laser amplified oxidation techniques.

It is also possible to obtain parts with more than two layers by processing a third silicon wafer, which may be included in step c) of the method, or repeating steps c), d), e) after step e).

The method of the invention allows the easy manufacture of multilayer silicon micromechanical parts, such as bearings for timepiece movements, cleats or slotted bridges, decelerating moving parts, etc.

Drawings

Other features and advantages of the present invention will become more apparent in the following description of specific embodiments thereof, given by way of illustration and not of limitation, with reference to the accompanying drawings, in which:

FIG. 1 is a schematic view of the steps of a method of obtaining a bearing with a porous stone and a trustee;

figure 2 shows another bearing that can be obtained by the present method; and

figure 3 shows a decelerated moving part that can be obtained by the present method.

Detailed Description

Fig. 1 is a schematic view of the steps of a method for obtaining a silicon bearing 10, the first element 3 of the silicon bearing 10 forming a "porous stone" and the second element 4 of the silicon bearing 10 forming a "travertine". For the size of these stones, the thickness is generally comprised in the range of 0.12-0.35mm and the diameter is generally comprised in the range of 0.70-1.80 mm.

In a first step a), a first element 3 is machined in a first silicon wafer 1, but preferably a plurality of said first elements 3 are machined. In practice, starting from commercially available silicon wafers, which may range from 75mm to about 300mm in diameter, it is possible to construct a plurality of elements having a diameter of less than 2 mm. The silicon wafer is typically less than 1mm thick, which is compatible with the dimensions of the watch piece, but can be machined to the exact dimensions desired by chemical etching. It is preferable to ensure that one side of the wafer is not polished, whether it is sold as such or lightly sanded.

The structuring of the elements 3 in the silicon wafer 1 is carried out by photolithography and etching of a shaped mask, using methods known to those skilled in the art. This etch may be performed according to other methods of processing silicon, which are also well known in the art. The method that allows to obtain the best aspect ratio, meaning that the cut plane is perpendicular to the surface of the part, is the RIE (reactive ion etching) technique.

This technique and the adaptations thereof that need to be made for each particular case are well known to those skilled in the art and will therefore not be described further.

In a first step, material bridges 5 are provided in order to keep the elements 3 connected to their supporting silicon wafer 1. This structuring method also forms elements in the silicon wafer 1 that position the silicon wafer 1 relative to the silicon wafer 2. These positioning means are constituted, for example, by two perforations 6, the reference 7 designating other positioning means, as will be explained below.

It is clear that depending on the positioning used, no material bridges 5 between the elements of each silicon wafer are necessary.

In step b), starting from the second wafer 2, the same procedure as in step a) is repeated in order to form the second element 4, i.e. the "stone", and the positioning means 6 coinciding with the positioning means 6 of the first wafer. In this example, the elements 3 and 4 have the same diameter.

When the silicon wafer 1 and the silicon wafer 2 used in the steps a) and b) have at least one unpolished surface, their assembly can be carried out without performing a preliminary surface treatment, only by stacking one unpolished surface on the other unpolished surface. If both sides of each wafer have been polished, it is preferable to carry out a preliminary surface treatment so that the two sides to be stacked together are either slightly rough or covered with a silicon oxide film slightly thicker than the naturally formed oxide film.

As shown in the left part of the figure, the purpose of step c) is to position the two silicon wafers 1, 2 one above the other and to fix this positioning, for example by means of pins 16, to form the assembly 11.

The right part of the figure shows a second example of positioning. First, the second element 4 is detached from the second silicon wafer 2 by breaking the material bridges 5, or the second element 4 is obtained without leaving the material bridges 5, and then the second element 4 is positioned in the corresponding cavity of the quartz mold 18. The first wafer 1 is then stacked and positioned by means of the slots 7 machined in its edges, said slots 7 engaging with the pins 17 of the die 18 to form the assembly 13. The first component 3 separated from the first silicon wafer 1 can also be obtained by means of a mold 18 comprising cavities having a suitable depth so as to be placed in the mold 18.

In step d), the series of assemblies 11 or 13 is placed on the carriage 15, the carriage 15 is then placed in the oven 8 at a temperature between 900 ℃ and 1200 ℃ and the oxidant mixture 9 is circulated in the oven 8 for about 2 to 4 hours. This oxidation step, which will join the two silicon wafers 1, 2 together by means of bonds of the silicon oxygen type, is carried out either by a dry process using a mixture comprising an inert carrier gas and dry oxygen, or by a wet process using water vapour instead of oxygen.

After this step is completed, the carriage 15 is taken out of the oven 18, the assembly 11 or 13 is allowed to return to ambient temperature, and the part 10 thus formed is then separated by breaking the remaining bridges 5 of material. The bearing 10 thus obtained is then used, mounted in the base 12 to receive the pivot 14 of the mobile part of the timepiece movement.

Fig. 2 shows a variant of the embodiment in which the stones 3, 4 of the bearing 20 do not have the same diameter. In this case, a variant of the method will preferably be adopted which uses a mould 18 which advantageously makes it possible to shape and position the multilayer cavities able to accommodate both elements 3 and 4 with respect to each other.

Fig. 3 shows another part 30 that can be obtained by the method according to the invention. This is a reduction moving part formed by assembly, in which element 3 is a pinion, element 4 is a gear, and elements 3 and 4 are passed through by the spindle hole 29.

The examples just described relate to the fabrication of a double-layer micromechanical part, but it is clear that more layers can be fabricated. For this purpose, it is only necessary to start with three or more silicon wafers or to repeat the process after step e) has been completed.

The invention is not limited to the examples just described, and a person skilled in the art can implement the method to obtain other micromechanical parts.

Nor is the invention limited to small-sized parts. The method can also be advantageously implemented for parts of larger dimensions, such as a timepiece movement bridge or a movement bridge, comprising a countersink. The thickness of such parts is in the order of 2mm and is not achievable from a single commercially available silicon wafer.

Claims (13)

1. Method for manufacturing a micromechanical part (10, 20, 30) for a horological mechanism, having at least two layers, from monocrystalline or polycrystalline silicon, comprising the following steps:

a) machining a first element (3) of said piece or a plurality of said first elements (3) in a first silicon wafer (1) by chemical means, physical means or both;

b) -repeating step a) with a second silicon wafer (2) in order to machine a second element (4) having a different shape from the first element, or to machine a plurality of said second elements (4);

c) -stacking the first and second elements (3, 4) or the first and second silicon wafers (1, 2) face to face by means of positioning means (6, 7);

d) thermally oxidizing the assembly formed in step c); and

e) separating the part (10) from the first and second wafers (1, 2);

wherein at least one of said first and second elements (3 or 4) is detached from its respective silicon wafer (1 or 2) after the end of step a) or b).

2. A method according to claim 1, characterized in that the oxidation of the assembly in step d) is carried out in a furnace (8) heated to a temperature between 900 ℃ and 1200 ℃ in the presence of an oxidant mixture (9) for 2-4 hours.

3. Method according to claim 1, characterized in that the positioning means (6, 7) of one (1) of the first and second silicon wafers with respect to the other (2) of the first and second silicon wafers are processed simultaneously with the processing of the first and second elements (3, 4).

4. A method according to claim 1, wherein each of the first and second elements is held on its respective silicon wafer by a material bridge (5).

5. Method according to claim 1, characterized in that the positioning means of the first and second silicon wafers (1, 2) consist of at least two holes (6) in which positioning pins (16) engage.

6. Method according to claim 1, characterized in that after the end of step a) or b) at least one element (3 or 4) of said first and second elements, detached from the respective wafer (1 or 2), is positioned in a quartz mould (18) in which the other wafer of the first and second wafers is superimposed on said at least one element in a recess reserved for this purpose, said other wafer of said first and second wafers comprising at its edge at least one slot (7) intended to engage with a corresponding rib (17) of the mould (18).

7. Method according to claim 1, characterized in that after the end of steps a) and b) both the first and the second element (3, 4) are released from their respective silicon wafers (1, 2) and positioned in a double quartz mould (18).

8. The method according to claim 1, characterized in that the surfaces of the silicon wafers (1, 2) which are stacked face to face in step c) are pre-oxidized.

9. The method according to claim 1, characterized in that the surfaces of the silicon wafers (1, 2) which are stacked face to face in step c) are pretreated in order to impart a certain roughness to said surfaces.

10. Method according to claim 1, characterized in that it further comprises at least one additional step of processing at least a third silicon wafer to form a third element, to be assembled with the two elements previously in steps c), d), e) so as to form at least a third layer.

11. A method according to claim 1, characterized in that the first element (3) is perforated to receive a pivot shaft, thereby forming a bearing pad.

12. A method according to claim 1, characterised in that both the first and the second element (3, 4) are perforated to receive the spindle and comprise teeth on their outer diameter to form the transmission moving part.

13. Method according to claim 12, characterized in that said first and second elements (3, 4) have outer diameters of different sizes so as to form a decelerating movable part.