EP1572578A2 - Method for making a planar suspended microstructure, using a sacrificial layer of polymer material and resulting component - Google Patents

Abstract

The invention concerns a method which consists in successively depositing a polymeric sacrificial layer (2), depositing, on at least part of the substrate (1) and the front surface of the sacrificial layer (2), an embedding layer (6), with a thickness greater than that of the sacrificial layer (2) and performing planarization such that the front surfaces of the sacrificial layer (2) and of the embedding layer (6) form a common planar surface. A forming layer (3) of a suspended structure (5) is deposited on the front face of the common planar surface. The planarization can include chemical mechanical polishing and etching the embedding layer (6). Etching the sacrificial layer (2) can be performed by means of a mask, formed on the front surface of a polymer material layer, removed during the planarization step.

Description

Method for producing a planar suspended micro-structure, using a sacrificial layer of polymer material and component obtained

Technical field of the invention

The invention relates to a method for producing an integrated microsystem-type component, comprising a planar suspended micro-structure, using a sacrificial layer of polymer material deposited on a substrate and having side walls delimiting the planar suspended structure, method comprising successively a planarization step, a step of depositing a formation layer of the suspended structure, a step of etching at least one opening of the formation layer up to the level of the front face of the sacrificial layer and a step dry etching of the sacrificial layer.

State of the art

Many integrated electromechanical micro-systems (“MEMS: micro electro-mechanical Systems”), include planar suspended micro-structures. This is for example the case of actuators, sensors, switches, variable capacitors, inductances (self) or acoustic wave resonators of suspended volume. In micro-technology or micro-electronics, suspended micro-structures are produced by the use of a sacrificial layer. The conventional steps for obtaining a suspended microstructure are shown, in simplified form, in FIGS. 1 to 5. In a first step shown in FIG. 1, a layer 2a is deposited on a substrate 1. The layer 2a is typically made of polymer material, silicon oxide or tungsten. The second step, shown in Figure 2, consists of lithographing and engraving the layer 2a so as to form a layer sacrificial 2 covering part of the substrate 1 on which the suspended structure must be formed. Then, in a third step represented in FIG. 3, a layer 3 for forming the suspended structure is deposited on the substrate 1 and on the sacrificial layer 2. The layer 3 for formation can be conductive or dielectric or constituted by a stack of several different layers. The fourth step, represented in FIG. 4, consists in lithographing and etching the formation layer 3 up to the level of the front face of the sacrificial layer, so as to delimit the suspended structure 5 by openings 4 in the layer 3 of training. In a fifth step, shown in FIG. 5, the sacrificial layer is removed by dry etching or wet etching, so as to constitute a free space between the substrate and the suspended structure 5, thus freeing the suspended structure.

The material constituting the sacrificial layer is chosen so that its etching is selective with respect to the material for making the microstructure. For example, the sacrificial layer can be made of silicon oxide (Si0 ₂ ) and the suspended structure can be made of polysilicon. A second combination comprises a sacrificial layer of polymer material and a suspended structure of Si0 ₂ . A third possibility is to use a sacrificial layer of polymer material and a suspended metal structure.

The use of a sacrificial layer which is removed by wet etching, for example Si0 ₂ in a chemical bath based on hydrofluoric acid (HF), poses problems of bonding of the structures during the release step. This problem is usually associated with capillary effects and surface forces. Consequently, a sacrificial layer of polymer material is increasingly used which is easily removed by plasma etching, for example of the oxygen plasma type. As this engraving is done dry, the bonding problems disappear. The geometric shape and sectional profile of the suspended structure has important consequences on the deformation or displacement of the suspended structure depending on an external excitation (electric, thermal, acceleration, pressure, etc.).

The outline of the suspended structure 5, in a plane perpendicular to FIG. 5, is perfectly defined during the lithography step. On the other hand, its profile in the plane of FIG. 5 depends on the lower layers, and in particular on the sacrificial layer, on which the suspended structure is built. In the case of the use of a sacrificial layer of polymer material, the profile is very often strongly accentuated by the creep of the material during annealing. However, the exact profile of the suspended structure affects system control. Indeed, undulations of the microstructure, brought about by the shape of the sacrificial layer, make it difficult to know the stiffness of the final device and its deformation as a function of the excitation conditions. The space between the suspended structure and the substrate is also influenced by the profile. Furthermore, the embedding of the micro-structure depends on the inclination of the suspended structure, which also depends on the profile. Lack of knowledge of the exact profile leads to a large gap between simulations and experimental measurements of the device and to the risk of stress concentrations at the embedding and on the mobile structure. Above all, this makes the devices extremely sensitive to process variations.

In order to better control the profile of the final suspended structure, it is desirable to go through a planarization step of the sacrificial layer. However, polymers are very difficult to planarise materials. Chemical mechanical polishing (CMP) tests show very poor results, for example the tearing of the resin during polishing, the irregularity of the planarization or the encrustation colloidal silica (contained in the CMP planarization product) in the polymer, found during the removal of the sacrificial layer.

Other tests in dry planarization (planarization on abrasive film) have also given poor results. Good rectification of the polymer was obtained, but at the cost of numerous scratches on the chip plane and tearing off on the polymer pads, as well as the encrustation of the abrasive material in the polymer.

The patents US6361402 and US6150274 propose processes for the planarization of polymers. However, these methods do not provide a simple solution. Furthermore, these methods are not suitable for all types of polymer (photosensitive resin, polyimide, etc.) and for all of the annealing conditions for these polymers. Indeed, in some cases, it may be necessary to anneal the polymer at a temperature above its temperature of use, for example by annealing at 300 ° C a photosensitive resin, the temperature of which is conventionally lower. at 200 ° C, to allow the use of a chemical vapor plasma deposition process ("PECVD: plasma enhanced chemical vapor deposition") at 300 ° C on the polymer. These heat treatments can lead to denaturing the polymer and making it almost impossible to planarize. In general, and in particular when they are annealed at high temperature, the polymers are very sensitive to tearing and tend to trap the abrasive compounds contained in the planarization products which are deposited under the mobile structure during the step. of release. Subject of the invention

The object of the invention is to remedy these drawbacks and, more particularly, to produce planar suspended structures using a planarized polymer sacrificial layer.

According to the invention, this object is achieved by the fact that, between the deposition of the sacrificial layer and the planarization step, a step of deposition, on at least part of the substrate and of the front face of the sacrificial layer, an embedding layer, having a thickness greater than the thickness of the sacrificial layer, so that, after the planarization step, the front faces of the sacrificial layer and the layer of embedding form a common flat surface, the formation layer of the suspended structure being deposited on the front face of the common flat surface.

According to a preferred embodiment, the planarization step successively comprises a sub-step of chemical mechanical polishing of the underlayment layer, and a sub-step of etching of the underlayment layer so that the faces before the sacrificial layer and the embedding layer form a common planar surface.

According to a development of the invention, the side walls of the sacrificial layer are delimited by etching by means of a mask formed on the front face of a layer of polymer material by deposition, lithography and etching of a temporary layer, the deposition of the embedding layer being carried out on the assembly constituted by the sacrificial layer and the mask, the mask being eliminated during the planarization step. According to another development of the invention, the component comprising elements projecting from the substrate, the method comprises, before the deposition of the sacrificial layer, successively a deposition on at least one area of the substrate, intended to be covered by the layer sacrificial and comprising projecting elements, of a base layer, having a thickness greater than the thickness of the projecting elements, and an additional planarization step, by chemical mechanical polishing, of the base layer, so as to that the front faces of the base layer and the projecting elements form a common flat surface.

According to one component, produced by a method according to the invention, the two faces of the formation layer of the suspended structure are completely flat.

Brief description of the drawings

Other advantages and characteristics will emerge more clearly from the description which follows of particular embodiments of the invention given by way of nonlimiting examples and represented in the appended drawings, in which:

Figures 1 to 5 show a method, according to the prior art, of a component comprising a suspended structure.

Figures 6 to 1 1 show different steps of a particular embodiment of a method according to the invention.

Figures 12 to 14 show steps of another particular embodiment of a method according to the invention.

Figures 15 to 19 show steps of a third particular embodiment of a method according to the invention. Figures 20 to 23 show steps of a fourth particular embodiment of a method according to the invention.

Description of particular embodiments

FIG. 6 represents a sacrificial layer 2 arranged on a substrate 1. The side walls 10 of the sacrificial layer 2 have been delimited by lithography and etching, as in FIG. 2. The planar suspended structure intended to be formed on the sacrificial layer 2 is delimited by the side walls 10 of the sacrificial layer 2. FIG. 7 represents a step of depositing, on at least a portion of the substrate and of the front face of the sacrificial layer 2, of an embedding layer 6, having a thickness greater than the thickness of the sacrificial layer. Typically the thickness of the embedding layer 6 is 1.7 times greater than the thickness of the sacrificial layer 2. The embedding layer 6 must be arranged so as to envelop the sacrificial layer 2 and to block lateral movement. of the sacrificial layer 2. The embedding layer 6 can completely cover and surround the sacrificial layer 2. It can also cover only a limited band of the sacrificial layer 2 and extend, at the ends of this band, over the zones adjacent to the substrate 1, on either side of the sacrificial layer 2. The material of the embedding layer 6 must be a material allowing the use of planarization process, in particular of CMP type, for example Si0 ₂ , silicon nitride or aluminum. As shown in the figure

8, a planarization step of the assembly of the embedding layer 6 and of the sacrificial layer 2 is carried out so that the front faces of the sacrificial layer 2 and of the embedding layer 6 form a flat surface common. The planarization stage must be stopped as soon as the front face of the sacrificial layer 2 is completely discovered. Thus, the thickness fluctuations of the sacrificial layer 2 are leveled and the sacrificial layer 2 and the embedding layer 6 form a common flat surface. Continuation of the planarization step beyond this limit increases the risk of deteriorating the quality of the surface of the sacrificial layer 2 and of deteriorating the flatness.

FIG. 9 represents a step of depositing a planar layer 3 for forming the structure suspended on the front face of the common planar surface of the sacrificial layer 2 and the embedding layer 6. Unlike the prior art (figure 3), the deposition of the formation layer 3 is done on a single plane. A fourth step, represented in FIG. 10, consists in etching at least one opening 4 in the formation layer 3 up to the level of the front face of the sacrificial layer 2. Then, in a fifth step, represented in FIG. 11 , the dry etching of the sacrificial layer 2 is carried out. The planar formation layer 3 then forms the planar suspended structure 5.

A component produced by the method according to the invention comprises a layer 3 for forming the suspended structure 5 having two flat faces, the front face and the rear face arranged on the embedding layer 6.

The planarization step may include a chemical mechanical polishing (CMP) and, in particular, consist only of a chemical mechanical polishing. A CMP type process consists, in known manner, of holding the object to be planarized against a rotary polishing plate wet in a polishing bath, containing abrasives and an acid or basic solution. Abrasives are typically particles based on aluminum or silicon. Thus, the layer intended to be planarized is chemically modified by the liquid and then removed by the particles of the abrasive. The application of a type CMP directly to the sacrificial layer 2 risks damaging the sacrificial layer 2, even in the presence of an embedding layer 6, in particular by encrustation of abrasive residues.

In another particular embodiment of the process of the invention, shown in FIGS. 12 to 14, contact of the sacrificial layer with the polishing bath is avoided. In fact, the initially deposited embedding layer 6 has a thickness approximately 1.7 times greater than the thickness of the sacrificial layer 2 (FIG. 12) and the planarization step includes a mechanical-chemical polishing sub-step. , making it possible to obtain a flat surface of the embedding layer 6 (FIG. 13), and a sub-step of etching the embedding layer 6 uncovering the sacrificial layer 2 so that the front faces of the layer sacrificial 2 and the embedding layer 6 form a common flat surface (Figure 14).

In another particular embodiment of a method according to the invention, represented in FIGS. 15 to 19, the initial etching of the sacrificial layer 2 is carried out by means of a mask 7 previously formed on the front face of the sacrificial layer 2 by deposition, lithography and etching of a temporary layer (Figure 15). The temporary layer can be of dielectric or metallic material (for example chrome, aluminum, etc.). The typical thickness of the temporary layer is between 10 and 50 nanometers. As shown in FIG. 16, the mask 7 makes it possible to delimit the side walls 10 of the sacrificial layer 2. The deposition of the embedding layer 6 is then carried out on the assembly constituted by the sacrificial layer 2 and the mask 7

(figure 17). The complete planarization stage is then carried out in two sub-stages. A first planarization sub-step can be carried out by a CMP type process, without risk of deterioration of the sacrificial layer 2, because the sacrificial layer 2 is protected by the mask 7 (FIG. 18). A second planarization sub-step consists in eliminating the mask 7, preferably by dry or wet etching, as shown in FIG. 19. Then the process for producing the suspended structure can be continued by the steps shown in Figures 9 to 11 , described above.

If the component comprises protruding elements 8 on the substrate 1, as shown in FIG. 20, the method for producing the suspended structure may include additional steps before the deposition of the sacrificial layer 2. In a particular embodiment, illustrated in FIG. 21, a base layer 9 is deposited on the substrate 1 and on the protruding elements 8 so as to completely fill the areas arranged between the protruding elements 8. The base layer 9 has a thickness greater than 1 thickness of the projecting elements (typically 1.7 times greater). The next step is planarization by chemical mechanical polishing of the base layer 9, so that the front faces of the base layer 9 and of the projecting elements 8 form a common flat surface (FIG. 22), serve as a substrate for the deposition of the sacrificial layer 2 (FIG. 23). If there is a risk that the protruding elements 8 are damaged during the planarization step, a CPM type planarization is followed by etching up to the level of the front face of the protruding elements 8.

The process is suitable for any type of polymer of the sacrificial layer (photosensitive resin, polyimide, PMMA, etc.) and independent of any treatment of the polymer of the sacrificial layer (polymer highly or slightly annealed or even not annealed, annealed under UV, having undergone ion implantation, etc.). The process makes it possible to produce any geometry of the sacrificial layer (narrow, wide, thick, thin, rectangular, round shape, etc.). There is no risk of scratches on the sacrificial layer and the substrate nor of risks tearing of the sacrificial layer during the planarization step, the sacrificial layer at no time exceeding the embedding layer.

The application of an etching sub-step during the planarization step (Figures 12 to 14) and / or the use of a temporary layer (mask 7) on the sacrificial layer 2 (Figures 15 to 19) allows in addition to eliminating any risk of deterioration of the sacrificial layer 2 by abrasives.

In the case where a heat treatment of the sacrificial layer is necessary (for example when the technological steps for producing the component include steps at high temperature, that is to say higher than the deposition temperature of the polymer), this will preferably be done before the etching step of the polymer in order to avoid its creep.

Claims

claims 1. Method for producing an integrated micro-system type component, comprising a planar suspended micro-structure (5), using a sacrificial layer (2) made of polymer material deposited on a substrate (1) and having side walls ( 10) delimiting the planar suspended structure (5), process comprising successively a planarization step, a step of depositing a formation layer (3) of the suspended structure (5), a step of etching at least one opening (4) from the formation layer (3) to the level of the front face of the sacrificial layer (2) and a step of dry etching of the sacrificial layer (2), process characterized in that it comprises, between depositing the sacrificial layer (2) and the planarization step, a step of depositing, on at least part of the substrate (1) and of the front face of the sacrificial layer (2), a layer of recess (6), having a thickness greater than the thickness of the layer e sacrificial (2), so that, after the planarization step, the front faces of the sacrificial layer (2) and the embedding layer (6) form a common planar surface, the formation layer ( 3) of the suspended structure (5) being deposited on the front face of the common flat surface. 2. Production method according to claim 1, characterized in that the planarization step comprises a chemical mechanical polishing. 3. Production method according to one of claims 1 and 2, characterized in that the planarization step successively comprises a sub-step of chemical mechanical polishing of the embedding layer (6), and a sub-step etching the embedding layer (6) so that the faces before the sacrificial layer (2) and the embedding layer (6) form a common planar surface. 4. Production method according to one of claims 1 and 2, characterized in that the side walls (10) of the sacrificial layer (2) are delimited by etching by means of a mask (7) formed on the front face a layer (2a) of polymer material by deposition, lithography and etching of a temporary layer, the deposition of the embedding layer (6) being carried out on the assembly constituted by the sacrificial layer (2) and the mask (7), the mask being removed during the planarization step. 5. Production method according to claim 4, characterized in that the planarization step comprises a step of etching the mask (7). 6. Production method according to any one of claims 1 to 5, characterized in that, the component comprising projecting elements (8) on the substrate (1), the process comprises, before the deposition of the sacrificial layer ( 2), successively a deposit on at least one area of the substrate (1), intended to be covered by the sacrificial layer (2) and comprising projecting elements (8), of a base layer (9), having a thickness greater than the thickness of the projecting elements (8), and an additional step of planarization, by chemical mechanical polishing, of the base layer (9), so that the front faces of the base layer ( 9) and projecting elements (8) form a common flat surface. 7. Component, produced by the method according to any one of claims 1 to 6, characterized in that the two faces of the formation layer (3) of the suspended structure (5) are completely flat.