CH715946A2 - Coil spring and method of making a coil spring. - Google Patents

Abstract

Method for producing a spiral spring with a core (204) and a piezoelectric coating (207), wherein said piezoelectric coating is polarized after the coating by applying an electrical voltage.

Description

Technical area

The invention relates to a method for producing a coil spring with a substrate and a piezoelectric coating.

Such a spring can be used, for example, as a spiral spring in a mechanical watch, in which an electronic circuit regulates the rate of the clock by controlling the stiffness of the spiral spring.

State of the art

A balance with a spiral spring made of piezoelectric material and a small electronics regulating the course of the balance is used. Such a clockwork with a piezoelectric spiral spring is already known from JP2002228774A.

From the international application WO2011131784 and from CH20100001298, the content of which is hereby incorporated by reference, a method is described in which a piezoelectric spiral spring is produced by providing a spiral spring made of silicon with a piezoelectric coating and the corresponding electrodes. According to this application, the piezoelectric coating is produced by means of metal-organic chemical vapor deposition (MOCVD). After the spiral spring has been provided with the piezoelectric coating, the electrodes are structured by means of sputtering and subsequent etching. The disadvantage of this method is that the desired quality of the piezoelectric coating, in this case aluminum nitride AlN, can only be achieved at high temperatures in the range of 1100-1300 degrees Celsius. When the spiral spring cools down to room temperature, the different expansion coefficients of silicon and AlN cause great stresses.

These mechanical stresses can be partially reduced by growing intermediate layers of aluminum nitride alternately with layers of AlGaN or GaN. Due to the smaller lattice constant of the aluminum nitride, the AlGaN or GaN growing on it is slightly compressive, which counteracts the tensile stress that already arises during growth and, above all, during cooling. Thus, theoretically, one can obtain an almost stress-free material. In practice, however, it turned out that this is not that easy. In addition, GaN has a smaller piezo coefficient than AIN. Furthermore, it is hardly possible to dop the AlN with scandium, for example, which would significantly increase the piezo coefficient.

In the application CH20160000791 a method is described in which the piezoelectric coating by means of high-energy pulsed magnetron sputtering (English high power impulse magnetron sputtering, HiPIMS, or high power pulsed magnetron sputtering, HPPMS) are applied. In the same patent application it is also described that the aluminum nitride can be doped with scandium. This can be achieved by co-sputtering with a second target made of scandium, the first target is made of aluminum.

Sputtering of AIN with HiPIMPS works well, the spiral springs are much more stable after coating with the piezoelectric layer than the spiral springs coated by means of MOCVD. However, there are problems with the efficiency and polarization of the crystals. Furthermore, the induced tension of the spiral spring can become so high that a short circuit due to a breakdown can occur, since the breakdown strength is exceeded.

Presentation of the invention

The aim of the present invention is to propose a method for producing a piezoelectric spiral spring in which the piezoelectric coating has a better orientation of the crystals.

Another object is to increase the efficiency so that the piezo coil spring delivers the highest possible electrical power.

Another object of the invention is to increase the dielectric strength of the piezo coil spring in order to achieve a greater distance between the dielectric strength and the maximum induced voltage.

According to the invention, these problems are solved by coating the aluminum-scandium-nitride AI (1-x) Sc (X) N by means of high-energy pulse magnetron sputtering (HiPIMS, or high power pulsed magnetron sputtering, HPPMS), and that the Al (1-x) Sc (X) N coating is subsequently polarized.

Not only can aluminum nitride Al (1-x) Sc (X) be doped with scandium, it is also possible to dop gallium nitride or indium nitride with scandium. The corresponding materials are then Al (1-x) Sc (X), Ga (1-x) Sc (X) N and In (1-x) Sc (X) N. But it is also possible to dope aluminum gallium nitride with scandium. For the sake of simplicity, the invention is described below with Al (1-x) Sc (X).

[0013] Further advantageous embodiments are specified in the subclaims.

Brief description of the figures

The invention is explained in more detail with reference to the attached figure, wherein <tb> Fig.1a <SEP> shows a cross section through a single turn of the spiral spring. <tb> Fig.1b <SEP> A detail from the cross section of FIG. 1a. <tb> FIG. 2a <SEP> A longitudinal section through a section of the spiral spring after sputtering the electrodes and after breaking the first predetermined breaking points 3 according to a first variant of a manufacturing method. <tb> Fig.2a <SEP> A longitudinal section through a section of the spiral spring after the polarization process according to the first variant of a manufacturing process. <tb> Fig. 3a <SEP> A longitudinal section through a section of the spiral spring after sputtering the electrodes and after breaking the first predetermined breaking points 3 according to a second variant of a manufacturing method. <tb> Fig. 3b <SEP> a longitudinal section through a section of the spiral spring after the polarization process according to the second variant of a manufacturing process. <tb> Fig. 4 <SEP> A spiral spring from which no predetermined breaking point has been broken and which is still connected to the wafer. <tb> Fig. 5 <SEP> A spiral spring from which only certain predetermined breaking points have been broken so that the inner electrode can be contacted, whereby the spiral spring is still connected to the wafer. <tb> Fig. 6 <SEP> A spiral spring from which all predetermined breaking points have been broken and which has been removed from the wafer, so that the outer electrode is divided into two parts.

Ways of Carrying Out the Invention

Figures 1a and 1b show a cross section through a single turn of a spiral spring 20. The core of the spiral spring consists of a substrate 204 made of silicon. A layer 205 of silicon oxide with a thickness of 100 nm, for example, is applied thereon, for example by oxidizing the silicon wafer after the etching / structuring of the spiral spring 20. This has the advantage that the surface of the spiral spring is smoothed on the one hand, and at least partially achieves temperature compensation on the other so that the oscillation frequency of the balance / spiral spring combination remains essentially stable or changes only slightly even with temperature changes. In addition, through this layer of amorphous material, the crystal growth of Al (1-x) Sc (X) N is crystallographically decoupled from the silicon below.

An inner electrode 206, for example a layer of titanium or molybdenum, with a thickness of 10-50 nm, for example, is applied to this layer of amorphous silicon dioxide by means of cathode atomization (also called sputtering or sputtering). However, another conductive material can also be used. If the silicon oxide layer 205 is dispensed with, even the core of the spiral spring made of silicon can be used as the inner electrode, in which case only electrically conductive silicon has to be used. If the silicon dioxide layer 205 is thin, for example a few nanometers, and the core of the spiral spring is made of conductive silicon, no additional internal electrode is required; in this case, the core made of conductive silicon serves as the internal electrode.

A piezoelectrically active layer 207 is applied to the inner electrode 206 made of titanium or molybdenum, for example aluminum-scandium nitride Al (1-x) Sc (X) N with a layer thickness of 1000 nm. A proportion x of 0.27-0.43 scandium is advantageously used. Such an aluminum-scandium nitride has a piezo coefficient that is 2-5x higher than that of AlN. Finally, an external electrode 208, for example 50-200 nm made of chromium / nickel / gold, is applied to the layer of piezoelectrically active material. The outer electrodes 208 are also arranged on both side flanks of the spiral spring; there are preferably no electrodes on the top and bottom of the spiral spring 20.

The piezoelectric coating 207 made of Al (1-x) Sc (X) N is preferably applied by means of high-energy pulsed magnetron sputtering (English high power impulse magnetron sputtering, HiPIMS, or high power pulsed magnetron sputtering, HPPMS).

HiPIMS is a special magnetron sputtering process for the deposition of thin films. HiPIMS uses very high target power densities of a few kW · cm-2 in short pulses of a few tens of microseconds with a low duty cycle (on-off ratio) of less than 10%. A characteristic feature of the HiPIMS is the high degree of ionization of the sputtered donor material and the high rate of molecular gas dissociation. Since the pulses with HiPIMS only act on the target material for a very short time and this is followed by a relatively long "off time", the average cathode power is low (1-10 kW). In this way, the target material can cool down during the off-times and a better process stability is given.

Using HiPIMS, it is therefore possible to apply AI (1-x) Sc (X) N practically at room temperature. Therefore, in contrast to MOCVD, HiPIMS does not have the problem of thermal distortion or hardly any. Because of this, spiral springs that have been coated with HiPIMS are much more stable than spiral springs that have been coated with MOCVD.

Another advantage of HiPIMS is that the scandium content in the Al (1-x) Sc (X) N can be easily adjusted by using co-sputtering: One target is made of aluminum, the other target of scandium. Depending on how much power the individual targets are operated with, material is sputtered from this target accordingly.

But it is also possible to use a target with an alloy of aluminum and scandium, for example a target with 27% scandium and 73% aluminum. This then results in an Al0.73Sc0.27N.

Another advantage of HiPIMS is the possibility of being able to “stack” several different layers on top of one another. For example, the spiral spring can be oxidized first so that a layer of amorphous silicon dioxide is present on the entire surface of the spiral spring. This has the advantage that a spiral spring coated with silicon dioxide hardly changes the frequency when the temperature changes, since the variations in the modulus of elasticity of silicon and silicon dioxide more or less compensate one another. Another advantage of this layer of silicon dioxide is that the silicon dioxide is amorphous, and thus a crystallographic decoupling takes place between the silicon and the Al (1-x) Sc (X) N, both of which have a crystal structure.

With HiPIMS, Al (1-x) Sc (X) N 207 can be coated directly onto silicon dioxide 205. This is not possible with MOCVD, because at the high temperatures the aluminum reacts with the silicon dioxide and attacks the silicon dioxide layer or even dissolves it completely, resulting in a poor quality of the AIN or AI (1-x) Sc (X) N that has grown on it results.

When coating with HiPIMS, a spiral spring 20 made of silicon, the surface of which has been oxidized, can be coated with high quality Al (1-x) Sc (X) N without the silicon dioxide 205 being attacked during the coating process. Ideally, a thin layer 206 of, for example, 10-50 nm titanium or molybdenum is first applied to silicon dioxide 205, but a thin layer of 10-50 nm pure aluminum can also be sputtered on first. This layer 206 made of electrically conductive material serves as an internal electrode. A seed layer of preferably AlN of, for example, 50-100 nm is then first sputtered onto this thin conductive layer 206, and a 0.5-3 μm thick layer 207 made of Al (1-x) Sc (X) N is sputtered on top.

The Al (1-x) Sc (X) N sputtered by means of HiPIMS has a crystalline structure, preferably the growth is oriented c-axes, i.e. the orientation of the grown crystals is perpendicular to the surface on which the AIN grows. The orientation of the grown crystals is perpendicular to a surface on which the piezoelectric coating 207 grows.

Since the surface of the spiral spring 20 is curved, the Al (1-x) Sc (X) N cannot be grown as a monocrystal, but rather columnar polycrystalline Al (1-x) Sc (X) N is formed. Another columnar polycrystalline structure can be used. One problem, however, is that Al (1-x) Sc (X) N can grow to be nitrogen-polar or aluminum / scandium-polar. If 50% of the crystals have one polarity and 50% of the crystals have the other polarity, the induced voltages cancel each other out. The polarization can only be controlled with difficulty during the sputtering process, and it must always be expected that a substantial proportion of the crystals will not have the correct polarity after sputtering.

The piezoelectric coating 207 is preferably carried out on at least two sides of the core 204. The core is preferably coated everywhere and on all sides with the piezo material AI (1-x) Sc (X) N and with the outer electrode. The material for the outer electrode 208 is preferably subsequently removed from the upper and lower sides using a suitable method.

This situation is shown schematically on the simplified longitudinal section of FIGS. 2a and 3a, in which the core 204, the piezoelectric layer 207 and the outer electrodes 208 are shown. The relationships between the various layers 204, 207, 208 have been changed so that the figures can be better understood. For example, the layer 207, but also the layer 208, is shown greatly enlarged.

The above-described layers 205 and 206 can preferably also be used in all the embodiments of FIGS. 2a to 3c.

In the figures, the arrows in the layer 207 show the direction and the meaning of the polarization.

FIG. 2a and FIG. 3a show a longitudinal section of a section of the spiral spring, seen from above, immediately after sputtering. As the arrows show, some of the piezoelectric crystals have a first polarity, for example directed outwards, while the other crystals have the other polarity, for example directed inwards, so that the induced voltages partially cancel each other out.

This problem is solved in that the layer of Al (1-x) Sc (X) N is polarized after sputtering and after applying the electrodes so that all crystals have the same polarity, as shown in Figure 2b .

This polarization is achieved in that a high voltage is applied between the electrically conductive core 204 (or the inner electrode 206, if present) and the outer electrode 208 for a short time.

To polarize the layer, the silicon 204, onto which the Al (1-x) Sc (X) N has been sputtered, can be used as an internal electrode. The condition for this is that the silicon has good electrical conductivity. This can be achieved by using highly doped silicon for the spiral spring. However, a thin layer 206 of electrically conductive material, which is applied before the layer of Al (1-x) Sc (X) N is applied, can also be used as the inner electrode. For example, the thin layer of titanium or molybdenum that is described above could be used as the internal electrode. The disadvantage, however, is that the inner electrode is difficult to contact.

It was observed that with a proportion x of scandium in the Al (1-x) Sc (X) N of x = 27% to x = 43% Al (1-x) Sc (X) N polarize afterwards leaves. The higher the scandium content, the lower the voltage between electrodes 204 and 208 (or 206 and 208) that is required to polarize the Al (1-x) Sc (X) N. At x = 27%, polarization must be approx. 4.5MV / cm, at x = 43% with approx. 2MV / cm.

Another advantage of a high scandium content is that the coupling factor becomes higher; the higher the scandium content, the more electrical energy can be generated with the applied layer. With an increasing proportion of scandium, the dielectric constant of Al (1-x) Sc (X) N also increases. This is helpful in reducing the induced voltage, since the layer Al (1-x) Sc (X) N together with the electrodes forms a capacitor. The higher the dielectric constant of the insulator, the lower the voltage for the same amount of energy stored in the capacitor.

However, with increasing scandium content, the quality factor also decreases.

It is simpler if the electrically conductive silicon 204 is used as the internal electrode for the polarization. This can be implemented, for example, by integrating small predetermined breaking points 3 on the spiral roller into the design of the spiral spring. After the application of the piezoelectric coating 207, which is of course highly insulating, these predetermined breaking points 3 can be broken off so that the bare silicon 204 appears at the breaking point. The silicon 204 can then be contacted at this break point with an electrically conductive pin made of metal and used as an internal electrode for polarization and / or later for regulating the gait. If the silicon of the core 204 has to be used later as an inner electrode for regulating the gait, it can also best be contacted at the outer end of the spiral spring. The two outer electrodes 208 can be used as the second electrode, which is necessary for polarization. These can be contacted directly with a conductive material.

The piezoelectric layer 207 can be polarized when the spiral spring 20 has not yet been removed from the wafer 4, as shown in FIG. Only certain predetermined breaking points 3 are then broken out on the spiral roller so that contact can be made with the inner electrode 204 made of electrically conductive silicon. The resulting situation is shown in FIG. 5 (corresponding to FIGS. 2b and 3a); In this figure, the reference numeral 30 shows the areas from which predetermined breaking points 3 have been removed so that the outer layers 205 to 208 are no longer present and the core 204 is accessible as an inner electrode.

In this figure, the outer metallization 208 has only been removed from the areas 30; In the embodiment shown, these areas are all located in the inner hole in the virole 28 of the spiral spring 20. The outer electrode 208 on both side flanks of the turns of the spiral spring 20 is not yet interrupted.

At this stage, it is sufficient to apply a sufficiently high voltage between the contact points 30 inside the spiral roller 20 (conductive silicon, inner electrode 204) and at any point on the outer electrode 208 in order to polarize the piezoelectric coating, as shown schematically in Figure 2b shown.

According to the invention, however, another method can also be selected in order to make the inner electrode 204 accessible at the desired locations. For example, the outer layers 205-208 can be removed with a suitable laser so that contact can be made with the inner core as an inner electrode 204.

After this polarization, second predetermined breaking points 5 are removed so that the conductive outer electrode 208 on the inner side flank of the turns is galvanically separated from the conductive outer electrode 208 on the outer side flank of the turns. This can be done according to FIGS. 5 and 6 by removing a predetermined breaking point 5 on the outer side of the virole and breaking another, second predetermined breaking point 5 on the outer end of the spiral spring 20 by removing the spiral spring 20 from the wafer 4. As shown in FIG. 6, this results in a piezoelectrically controlled spiral spring with a first electrode 208A on the inner side of the turns and a second electrode 208B on the outer side of the turns; Between these two electrodes, the deformation of the piezoelectric material during operation of the watch creates a voltage with which an electronic circuit can be fed and which can be regulated in order to control the rigidity of the piezoelectric layer and thus the operation of the regulating element.

In this variant, the polarization of the piezoelectric layer 207 thus takes place with a voltage between the silicon core 204 (or the layer 206) as the inner electrode and the as yet uninterrupted outer electrode 208. During operation, the inner electrode 204 or 206 is not used, and the voltage between part 208A and part 208B of external electrode 208 is used. As a result, the inevitable contacting of the core 204 or the layer 206 is only required temporarily for the one-time polarization.

As a variant, however, it is also possible to polarize the piezoelectric layer 207 in two parts after the separation of the outer electrode 208, for example by applying a voltage between the electrode 208A on the inner side flank of the spiral spring and the electrode 208 on the outer one Side flank of the coil spring is attached, as shown in Figures 3A and 3C. In operation, the voltage between core 204 (or layer 206) and electrode 208A is then added to the voltage between core 204 (or layer 206) and electrode 208B.

With the polarization of the layer made of Al (1-x) Sc (X) N after the sputtering, the energy supplied by the piezospiral spring can be increased significantly.

However, one problem here is that the voltages can become so high that the dielectric strength is insufficient and an electrical breakdown occurs during polarization because the applied voltage is higher than the breakdown voltage. This can be prevented by carrying out the polarization in a medium which has a significantly higher breakdown voltage than air, for example transformer oil or distilled water. Distilled water has the advantage that it can be easily removed without leaving any residue. The disadvantage is that the spiral spring must be rinsed very cleanly after etching so that there are no more residues from the manufacture or processing of the electrodes. Otherwise the distilled water could become electrically conductive due to these residues.

Transformer oil has the advantage that it is non-corrosive, but is more difficult to remove than distilled water.

In the case of polycrystalline materials, cracks can occur at the boundaries of the crystallites under mechanical loads.

According to the invention, this can be achieved in that the sputtering parameters are selected in such a way that the Al (1-x) Sc (X) N grows under compression. Care must be taken that the compressive stress is only as great as is absolutely necessary; on the other hand, it could lead to cracks in the coating even without mechanical stress on the piezoelectric coating.

Another possibility to reduce the risk of cracks consists in adding a 10-100 nm thick layer of amorphous silicon nitride to the 1-3 μm thick layer of Al (1-x) Sc (X) N xN by means of atomic layer deposition ALD or a similar suitable material. On the one hand, this has the advantage that the Al (1-x) Sc (X) N is well protected against environmental influences, and that the spiral spring is much more break-proof, since the amorphous silicon nitride causes stress peaks in the Al (1-x) Sc (X) N can be reduced or even eliminated entirely. Another advantage is that when the coil spring is subsequently coated with electrodes by means of sputtering, no metal atoms can penetrate the AlN along the grain boundaries of the AlN and impair the electrical properties of the coil spring.

In order to counteract a possible deformation of the coil springs during the coating due to the effects of gravity, the wafer is brought into a different position one or more times during the manufacturing process. The effects of gravity during the coating process should therefore be reduced or eliminated entirely.

The coil spring can have a thickness of the windings that is not constant.

Claims (15)

1. Process for manufacturing a spiral spring with the following steps: Applying a piezoelectric coating (207) to a core (204), characterized in that the piezoelectric coating (207) is subsequently polarized by applying an electrical voltage. 2. The method according to claim 1, in which the piezoelectric coating (207) comprises at least one layer of aluminum-scandium-nitride Al (1-x) Sc (X) N or of gallium-scandium-nitride Ga (1-x) Sc ( x) N or of indium-scandium-nitride In (1-x) Sc (X) N. 3. The method according to claim 2, in which the piezoelectric coating made of Al (1-x) Sc (X) N with a scandium content of 0.27-0.43 is applied. 4. The method according to claim 2, in which the piezoelectric coating of Ga (1-x) Sc (X) N or of In (1-x) Sc (X) N with a scandium content of 0.2-0.5 is applied. 5. The method according to any one of claims 2 to 4, in which the thickness of the Al (1-x) Sc (X) N layer or the Ga (1-x) Sc (X) N layer (207) or the In ( 1-x) Sc (X) N layer has 0.5 to 3 µm. 6. The method according to any one of claims 1 to 5, in which the polarization of the piezoelectric coating (207) takes place in a medium which has a dielectric strength that is greater than 10 kV / mm. 7. The method according to any one of claims 1 to 6, in which the polarization of the piezoelectric coating (207) takes place in an environment of distilled water. 8. The method according to any one of claims 1 to 7, in which the core (204) is used as an electrode during the polarization process. 9. The method according to any one of claims 1 to 8, in which the core (204) is used as an electrode during the operation of the spiral spring for generating electrical energy. 10. The method according to any one of claims 1 to 9, in which at least one predetermined breaking point (3) is broken in order to make the core (204) accessible as an electrode and / or an inner electrode (206). 11. The method according to any one of claims 1 to 10, in which an outer electrode (208) is applied to a side flank of the spiral spring, and in which at least one predetermined breaking point (5) is broken around the outer electrode (208A) on the inner side flank of the spiral spring galvanically separated from the outer electrode (208A) on the outer side flank of the spiral spring. 12. The method according to any one of claims 1 to 11, in which a hole is made with a laser through the piezoelectric layer in order to make the core (204) accessible as an electrode and / or an inner electrode (206), and / or to to disconnect an electrode. 13. The method according to any one of claims 1 to 12, in which at least one electrode which is used to tap electrical energy generated by the piezoelectric coating (207) is also used for polarizing the same piezoelectric coating (207). 14. The method according to any one of claims 1 to 13, characterized in that the piezoelectric coating (207) grows under pressure. 15. Coil spring which is produced by the method according to any one of claims 1 to 14.