EP0874379B1 - Magnetic microswitch and method of making - Google Patents

Description

The present invention relates to a microswitch with blades whose particular conformation ensures a reliable operation, both for closing a circuit electric by bringing two blades together the influence of a magnetic field, that for opening when the magnetic field is removed.

The invention also relates to a method of manufacture of such a microswitch by a method of galvanic growth from a substrate.

More generally, the invention belongs to well-known field of so-called "rod" contactors, and by "blade" extension, actuated by a magnetic field exterior may be either parallel to the rods or blades, or perpendicular to them. A contactor at parallel field stems is generally referred to as "reed" contactor. The standard model of such a contactor "reed" consists of a cylindrical glass bulb in which penetrates at each end a magnetizable rod and flexible, the free ends of each rod being able to, by their initial rapprochement, attracting themselves under the influence an external magnetic field to close a circuit electric, and be recalled to their original position by the elastic force of the rods, respectively of the blades, when the magnetic field is removed. The miniaturization of this standard model is necessarily limited by purely technical factors, causing the smallest "reed" contactors obtained still have a length of the order of 7.5 mm and a diameter of the order of 1.5 mm, while sometimes having mechanical stability questionable.

This standard model has therefore given rise to numerous improvements among which we will retain, in the framework of the present invention, on the one hand those which aim to reduce its size, for example to allow their integration into a micro-electronic assembly, such as a timepiece, on the other hand those who aim to make their magneto-mechanical behavior more reliable and more efficient.

With regard to the solutions provided for reducing the bulk, advantageously refer to US patent 5,430,421 which describes a method of manufacturing by galvanic growth from a substrate, making it possible to manufacture in batches, or "batch"", micro-contactors with very small blades, typically devices whose blades have a length L of approximately 500 μm, a width a of approximately 100 μm, for a thickness b and an air gap e of the order of about ten microns. In use, however, it appeared that certain micro-contactors from the same batch, that is to say micro-contactors manufactured in exactly the same conditions, did not meet the standards for ensuring operation reliable. Indeed, the construction of a metallic structure suspended by galvanic growth makes it possible to sufficiently control the geometry, and in particular the thickness of the deposits of a ferromagnetic material, but does not make it possible to predict with certainty in said deposits residual stresses which are, in known manner, greater at the start of galvanic growth. Taking into account the very small thickness of the blades, it follows that certain micro-contactors will, after elimination of the sacrificial layers, always in the closed position, or on the contrary will have an air gap too large for the blades to be brought into the closed position under the influence of the magnetic field should normally be applied.

To overcome the magneto-mechanical disadvantages of microswitches mentioned above, we sought, for blades obtained with a material having a modulus given elasticity and placed in a magnetic field given, on what building parameters it was possible to act to reduce or even eliminate residual stresses while promoting deflection and the contact pressure between the two blades.

By increasing the thickness b of the blade, we will reduce the influence of residual stresses and obtain better positioning of the two blades relative to each other, but at the same time we will increase their rigidity. To have the flexibility necessary for closing, the length L of the blade must then be increased, which does not correspond to the objective of miniaturization of the invention.

For devices placed in a magnetic field and having a very small air gap e , the deflection is approximately proportional to L ³ / b · r, L being the length of the blade, b its thickness and r the length of superposition of the two blades in the air gap e . All other parameters being equal, the contact pressure is approximately proportional to b ² / r ² .

Greater deflection can be achieved by increasing L and / or decreasing b . With an increase in L, the overall size of the micro-contactor increases, which does not correspond to the aims of the invention, and which also has the negative effect of increasing the dispersion of the magnetic field in the air gap. A decrease in b has the unfavorable effect, on the one hand of considerably reducing the contact pressure, on the other hand as indicated previously, of making the blade more sensitive to residual stresses.

Only the reduction in the superposition length r makes it possible to simultaneously increase the deflection and the contact pressure. However, the value of r must remain substantially equal to a few times the thickness b , failing which the effects of dispersion of the magnetic field cancel the advantage obtained.

It therefore appears from the preceding observations that the knowledge of the skilled person does not allow to provide a satisfactory solution to the drawbacks magneto-mechanical of a micro-contactor built by galvanic growth.

The object of the present invention is therefore to propose a solution in which, without modifying the size overall of the microswitch, an original geometry of at least less one blade increases the flexibility of said blade without modifying the maximum force obtained at its end.

To this end, the subject of the invention is a magnetic microswitch, produced by galvanic method from a substrate, comprising two conductive strips of length L and L 'and of width a , connected by their respective ends to means of electrical connection, and each comprising a distal part of respective section a · b and a · b ', the superposition of which over a length r determines an air gap of distance e , at least one of said blades being made of a magnetic material and consisting of an end integral with the substrate by means of a foot, of a median part and of a distal part of length L _O , flexible with respect to the distal part of the second blade between an open position in the absence of a magnetic field and a closed position in which the two blades are in contact with each other under the influence of the magnetic field, said micro-contactor being characterized in that said middle part of the flexible blade is shaped with a total cross section smaller than that of the distal part so as to have a lower resistance to flexion allowing the blade to have both a deflection of amplitude at least equal to e to establish contact under the influence of a magnetic field and sufficient return force to the open position in the absence of a magnetic field.

When the applied magnetic field is parallel blades, both blades are grown by growth galvanic of the same magnetic material.

By applying a magnetic field at saturation of the middle part it is then possible to increase the contact pressure between the blades by increasing the thickness b , respectively b ', of the distal part, so as to obtain reproducible contacts at low passage resistance while allowing the blade to have sufficient deflection.

According to a first embodiment, the flexible blade has a constant thickness b from its attachment to the foot to its distal part, and the middle part which forms the junction between these two ends is formed by one or more isthmus making the total cross section of said middle portion is smaller than the section of the distal portion, thereby allowing the blade to have greater flexibility without increasing bulk.

These isthmus can delimit one or more openings in the blade. In case there is only one single isthmus, it preferably occupies a position central by delimiting two notches on the edges of the blade. The isthmus can also have a section variable between the end fixed to the foot and the part distal, for example by forming contiguous openings substantially rectangular or square, having surfaces decreasing values from the attachment to the foot.

According to a second embodiment, the blade has neither opening nor notch, but its middle part has a thickness less than the thickness b of the distal part, by forming in a way a notch in the thickness of the blade, said notch which can be formed on either of the faces of the blade.

As already indicated, the middle part has only a slight influence on the magnetic behavior of the micro-contactor, especially when it is placed in a magnetic field parallel to the length of the blades. In other words, the active zone is the distal part of length L _o . In this case it is then advantageous, when the second strip is secured to the substrate, that its length L 'is equal to L _o and that its thickness b ' is equal to the thickness b of the flexible strip, so as to avoid maximum dispersion of the magnetic field.

When the microswitch is placed in a magnetic field perpendicular to the blades and the second blade is secured to the substrate, it is sufficient that the length L ′ of this second blade is equal to the covering length r , the material constituting it being able to be magnetic or not, and its thickness b ' may be greater than the thickness b of the flexible blade.

Instead of being integral with the substrate, the second blade can also be integral with said substrate by through another foot. This second blade will then also flexible and can be structured according to one of the modes described above, without having necessarily the same structure as the first blade.

The microswitch according to the invention also makes it possible, without modifying the overall size thereof, to act on the values b , b ′ of the thickness of the blades and on the value e of the air gap. Indeed, an increase in b , b ' causes a decrease in flexibility and correspondingly better relative positioning of the two blades to reduce the value e of the air gap.

• la figure 1 est une vue en perspective d'un premier exemple de micro-contacteur ayant une seule lame flexible, avec indication de toutes les longueurs caractéristiques;
• les figures 2 à 5 sont des vues en perspective de quatre autres exemples de réalisation dans lesquels une seule lame est flexible;
• la figure 6 est une vue en perspective d'un sixième exemple de réalisation dans lequel les deux lames sont flexibles;
• la figure 7 représente la coupe selon la ligne VII-VII de la figure 1, avant l'élimination des couches sacrificielles, et
• la figure 8 représente la coupe selon la ligne VIII-VIII de la figure 1, avant élimination des couches sacrificielles.

Other characteristics and advantages of the invention will appear on reading the detailed description of exemplary embodiments, given by way of non-limiting illustration, with reference to the appended figures in which:

• Figure 1 is a perspective view of a first example of a micro-switch having a single flexible blade, with indication of all the characteristic lengths;
• Figures 2 to 5 are perspective views of four other embodiments in which a single blade is flexible;
• Figure 6 is a perspective view of a sixth embodiment in which the two blades are flexible;
• FIG. 7 represents the section along line VII-VII of FIG. 1, before the elimination of the sacrificial layers, and
• Figure 8 shows the section along line VIII-VIII of Figure 1, before removal of the sacrificial layers.

Referring to Figure 1, there is shown a first example of a micro-contactor, once isolated from its manufacturing batch. We see that it has two blades 1, 2 supported by a substrate 10, from which it was built by galvanic growth like this will be explained later.

In this example, the micro-switch is intended to be subjected to a magnetic field parallel to the blades. The material forming the two blades should be ferromagnetic, for example an iron-nickel alloy exhibiting low magnetic hysteresis to allow a reproducible opening when the magnetic field is deleted.

Each of the two blades comprises means for connection to an electrical circuit, not shown, shown diagrammatically by the conductors 21 and 22, the skilled person being able to perfectly design other connection means, in particular when said micro-contactor is intended to be integrated into a more complex electronic assembly. The two blades have substantially the same width a , between 50 and 150 μm, for example 100 μm, and a thickness b, b ′ of the order of 10 μm. The strip 1, secured to the substrate 10 via a foot 9, has a total length L, typically between 300 and 900 μm, for example 500 μm. This blade 1 comprises three zones having substantially the same length and assuming different functions. One end 3 of the blade allows attachment to the base 9, the rest of the blade being suspended above the substrate 10. The other end 5, of length Lo, designated by "distal part", ensures magnetic operation. The middle part 4 ensures its mechanical functioning by making it possible to adjust the flexibility of the blade 1, that is to say in fact the maximum deflection of the distal end 5 in a given magnetic field. To this end, the middle part 4 has in its center a square opening 6 delimiting on the edges of the blade 1 two isthmus 8a and 8b connecting the end 3 integral with the foot to the distal part 5. In this middle part, the section total transverse is therefore less than the section a · b of the distal part 5, which gives the blade greater flexibility for a material having a given modulus of elasticity. The second blade 2, integral with the substrate, has a thickness b ' and a length L' and has no particular structure. However, its thickness b ′ will preferably be substantially equal to the thickness b of the flexible blade 1. The two blades are positioned relative to each other so that they overlap over a length r , defining between their facing surfaces an air gap e of between 10 and 50 μm, for example 5 μm. The length r of superposition of the two plates will preferably be equal to sometimes the thickness b , b ′ chosen for the plates, so as to reduce the effects of dispersion of the magnetic field.

Depending on its final destination, the micro-contactor can be encapsulated in air or controlled atmosphere, by example by means of a plastic cover not shown, glued or welded to the surface of the substrate, either by mounting in a suitable case.

We will now briefly describe, with reference to Figures 7 and 8 a method of producing the microswitch shown in Figure 1, by growth galvanic from a substrate 10. This process consists essentially to adapt at least one step in the process described in document US 5,430,421, which can be see for more details. In Figure 7, we have represented before elimination of the sacrificial layers a longitudinal section through an isthmus 8a of a single micro-contactor isolated from its manufacturing batch. The substrate 10 is in fact only a portion of a wafer, or "wafer" in an insulating, or semiconductor material even conductor covered with an insulating layer allowing to manufacture in a single batch a multitude of microswitches. We first perform by evaporation thermal deposition of a bonding layer 12a and 13a, for example titanium or chromium, then a layer of protection 12b and 13b for example in gold, so as to create two tracks 12 and 13 electrically isolated by etching of the surface according to known techniques. We deposit then, for example with a spinner, layers successive 14, 15 and 16 of thick photoresist, each photoresist layer being configured by means of a mask (not shown) to provide openings allowing growth to be carried out in stages galvanic. The first layer 14 is configured with two openings allowing the galvanic growth of a first stage 9a of foot 9 and of blade 2. The second layer 15 is configured with a single opening allowing to obtain by galvanic growth the second floor 9b of foot 9. Before depositing the third layer 16 of photoresist a new double metallization 17. This third layer 16 is configured to leave free for growth galvanic an opening corresponding to the end 3 integral with foot 9, distal part 5 and isthmus 8a and 8b, as appears more clearly on the figure 8. In this example, all the steps of galvanic growth can be conducted with the same ferromagnetic material, for example an iron-nickel alloy 20-80. It is also possible to improve the electrical contact of the blades when they are subjected to a magnetic field, covering their surfaces with gold opposite, i.e. after the first galvanic deposition and before the last galvanic deposit. The microstructure thus obtained is then subjected to an attack reagent to eliminate, at one or more times, the photoresist and the intermediate metallization layer 17 and release the microswitch. As already stated, all of these operations are carried out on a batch of micro-contactors that it is possible to encapsulate before isolating them by cutting, either individually or in groups according to a disposition determined according to their destination final.

Referring now to Figure 2, there is shown another example of micro-switch intended to be placed in a magnetic field parallel to the blades and in which there is always a single flexible blade. The middle part 4 of the flexible blade has two rectangular openings 6a and 6b, delimited by three isthmus 8a, 8b and 8c. As can be seen, by comparing FIGS. 1 and 2, the second strip 2 secured to the substrate has a length L '= Lo, the two strips having the same thickness b = b', of a value greater than that shown in the Figure 1, with a correspondingly smaller value for the air gap e .

The microswitch shown in Figure 3 is intended to be placed in a magnetic field perpendicular to the blades. In fact, as can be seen, the second strip 2 secured to the substrate can be reduced to a contact pad having a length L 'at least equal to the overlap length r of the two strips, and a thickness b' greater than the thickness b of the flexible blade. In this example, it is also possible to carry out the first growth step, to form the first stage of the base and the blade 2 with a non-magnetic material, for example gold. The middle part has three openings 6a, 6b and 6c substantially rectangular and contiguous, forming a single opening delimited on each edge of the blade by isthmus 8a and 8b composed of three zones s, m and 1 so the width increases from foot

In FIG. 4, the microswitch represented, intended to be placed in a parallel magnetic field with blades, has in the middle part of its blade flexible a single isthmus 8c delimiting notches 6d and 6th on the edges of the blade.

In the micro-contactor represented in FIG. 5, the increase in the flexibility of the movable blade relative to the blade 2 secured to the substrate 10 is obtained by configuring the middle part 4 with a thickness b " less than the thickness b of the distal part 5. In the example shown, this configuration corresponds to a notch 6f open towards the substrate. To achieve this micro-structure by galvanic growth, it will of course be necessary to perform an additional step to configure the notch 6f .

In Figure 6, there is shown a micro-contactor intended to be placed in a parallel magnetic field to the blades and in which the two blades are movable one compared to each other. A first blade 1 is integral of the substrate 10 via a foot 9 and comprises in its middle part an opening 6. A second blade 2 is secured to the substrate 10 by means of a foot 11. In the example shown, this second blade also has an opening in a middle part rectangular 7. This part can also have one any of the conformations previously described for the blade 1, or still have a constant total section of its end fixed to the foot 1 to its end distal. To achieve this micro-structure by growth galvanic, it will of course be necessary to carry out a additional step, to configure foot 11, and proceed with additional metallization before configure and grow by galvanic deposition the blade 2 and an additional stage of foot 9.

Without departing from the scope of the present invention, the skilled person is able to imagine other configurations of the middle part of at least one blade to have more flexibility and therefore obtain a microswitch with characteristics improved magneto-mechanical, as defined in the claims.

Claims (15)

1. Magnetic microswitch, made by galvanic growth from a substrate (10), including two conductive strips (1, 2) of length L and L' and of width a, connected by their respective ends (3, 3') to electric connection means (21, 22), and each including a distal portion (5, 5') of cross-section a · b and a · b' respectively, whose overlap over a length r determines an air gap of distance e, at least one of said strips (1) being made of magnetic material and consisting of one end (3) attached to the substrate via a foot (9), a median portion (4) and a distal portion (5) of length L_o, said strip being flexible with respect to the distal portion of the second strip (2) between an open position in the absence of a magnetic field and a closed position in which the two strips are in contact with each other under the influence of the magnetic field, characterized in that said median portion (4) of the flexible strip (1) is formed with a total cross-section less than that of the distal portion (5) so as to have a lesser bending resistance allowing the strip to have both deflection of an amplitude at least equal to e to make contact under the influence of a magnetic field and sufficient return force towards the open position in the absence of a magnetic field.
2. Microswitch according to claim 1, characterized in that the two strips (1, 2) are made of magnetic material when the magnetic field applied is parallel to said strips.
3. Microswitch according to claim 1, characterized in that the flexible strip (1) has a constant thickness b and in that the median portion (4) is formed by at least one isthmus (8a, 8b, 8c) connecting said distal portion (5) to the end (3) fixed to the foot (9).
4. Microswitch according to claim 3, characterized in that the median portion includes two isthmuses (8a, 8b) situated on the edges of the strip defining a single substantially rectangular or square opening (6).
5. Microswitch according to claim 3, characterized in that the median portion (4) includes more than two isthmuses (8a, 8b, 8c) extending parallel to the length of the strip forming several substantially rectangular or square openings (6a, 6b).
6. Microswitch according to claim 4, characterized in that the two isthmuses (8a, 8b) situated on the edge of the strip have cross-sections which decrease from between the zone of attachment to the foot towards the distal portion thereby forming several contiguous substantially rectangular or square openings (6a, 6b, 6c).
7. Microswitch according to claim 1, characterized in that the median portion (4) includes a single central isthmus (8c) delimiting scallopings (6d, 6e) on each of the edges of the strip.
8. Microswitch according to claim 1, characterized in that the thickness of the median portion (4) is less than the thickness b of the distal portion (5).
9. Microswitch according to claim 1, characterized in that the second strip (2) is attached to the substrate, has a constant cross-section and a length L' substantially equal to L_o when the magnetic field applied is parallel to the longitudinal axis of the strips (1, 2).
10. Microswitch according to claim 1, characterized in that the second strip (2) is attached to the substrate, has a constant cross-section and a length L' substantially equal to r when the magnetic field applied is perpendicular to the longitudinal axis of the strips (1, 2).
11. Microswitch according to claim 1, characterized in that each of the two strips (1, 2) is attached to the substrate via a foot (9, 11).
12. Microswitch according to claim 11, characterized in that the median portions of each strip are formed to have a lesser bending resistance.
13. Microswitch according to claim 1, characterized in that the two strips (1, 2) have distal portions having the same thickness b = b', when the magnetic field applied is parallel to the longitudinal axis of the strips (1, 2).
14. Microswitch according to claim 13, characterized in that an increase in the thickness b, b' for the strips (1, 2) allows a correlative decrease in the air gap e without modifying the global space requirement of said microswitch.
15. Method for manufacturing a microswitch according to claim 1, characterized in that it includes the following steps:

    forming two electrically insulated paths (12, 13) on a substrate (10);

    forming successive layers (14, 15, 16) of thick photoresist allowing the galvanic growth to be performed by steps;

    effecting an intermediate metallisation (17) of the entire surface of the structure already obtained, before each step of forming a strip (1, 2); and

    eliminating in one or more steps the photoresist and the intermediate metallisation layers by means of an etchant.

Images