CH710482A2 - A method of etching silicon dioxide. - Google Patents

Abstract

The present invention relates to a process for the chemical etching of a silicon dioxide material, such as quartz, which uses fluoride ions and a fluorinated polymer. This etching process can be used in particular in the manufacture of quartz oscillators intended to be integrated in all kinds of electronic devices such as timepieces, gyroscopes or pacemakers.

Description

The present invention relates to a method of chemical etching of a silicon dioxide-based material, such as quartz, which implements fluoride ions and a fluorinated polymer.

This etching process can be used in particular in the manufacture of quartz oscillators intended to be integrated in all kinds of electronic devices such as timepieces, gyroscopes or pacemakers.

PRIOR ART

[0003] Piezoelectric crystal resonators, in particular quartz oscillators, serve as a time base in electronic devices. In small portable devices such as watches, it is desirable to reduce the power consumption required for the vibration of the piezoelectric crystal, because the energy is provided by small size batteries. A low consumption resonator is generally in the form of a tuning fork made by a method comprising the steps of etching a quartz piece from a quartz substrate and then forming electrodes on the quartz piece. The tuning fork comprises two parallel branches, separated by a slot, each carrying a conductive deposit. The two electrodes thus formed, of opposite polarities, allow to create in the branches an alternating electric field causing the vibration of the tuning fork by piezoelectric deformation.

Since the oscillation frequency of a quartz oscillator depends essentially on the geometry of the quartz piece and the quality of the material, the step of etching the quartz from the substrate to form the tuning fork is a important step that conditions the performance of the quartz oscillator.

The etching process conventionally used to cut silicic materials such as quartz is a chemical etching process using hydrofluoric acid and an etching mask resistant to acid, usually gold.

However, the current etching masks are not sufficiently resistant: the hydrofluoric acid arrives in places to cross them and causes defects in the silicic substrate, which significantly reduces the production yield of the cut pieces.

The invention proposes to overcome this disadvantage by providing a chemical etching mask for silicic materials which is improved.

DESCRIPTION OF THE INVENTION

An object of the invention is a method for etching silicon dioxide comprising a step of manufacturing an etching mask using a fluorinated polymer, followed by a chemical etching step using fluoride ions. .

Another object of the present invention is a method of manufacturing a quartz oscillator which makes it possible to limit or avoid any defect that may be observed in a quartz substrate after treatment with hydrofluoric acid. the gaseous state or in the form of an aqueous solution.

The inventors have found that fluorinated polymers such as poly (vinylidene fluoride) can be used as an etching mask in a process for etching silicic materials with ions or radicals derived from fluorine, by fixing these polymers. fluorinated by covalent bonding to the substrate to be etched. This new mask decreases or even eliminates the defects that can be observed in the pieces of quartz that have been cut with acid in conventional etching processes that use gold as an etching mask.

In the etching process of the invention, the fluorinated polymer may advantageously completely or partially replace the gold generally used as an etching mask. The method of the invention thus makes it possible, advantageously, to overcome, if necessary, the use of a gold-based etching mask whose cost is high because of the cost of the raw material and the cost of material required for its vacuum deposition.

A first object of the invention therefore relates to a method of chemical etching a silicon dioxide-based substrate comprising the steps of: attaching a chemical etching mask in the form of a film to the surface of said silicon dioxide substrate; etching said substrate by exposing said substrate to fluoride ions for a time sufficient to cut said substrate into pieces of silicon dioxide.

According to this method, the chemical etching mask comprises a fluoropolymer and that the fluoropolymer can be immobilized by covalent bonds, in particular by a radiative treatment, preferably a VUV (Vacuum Ultra Violet) treatment.

The process of the invention applies to all materials containing silicon dioxide, preferably crystalline silicon dioxide. The fluoropolymer is preferably immobilized by creating covalent bonds with the silicon dioxide substrate using a radiative treatment.

Silicon dioxide is a solid material, preferably in crystalline form, which may contain some water molecules inserted into the crystal lattice formed by the silicon and oxygen atoms.

Among the substantially anhydrous silicon oxides in crystalline form used as substrate in the context of the etching process of the invention, mention may be made of quartz, tridymite, cristobalite, coesite, and fibrous silica.

According to one embodiment, the silicon dioxide substrate is a single-crystal synthetic quartz substrate whose crystalline structure allows it to be anisotropically etched, preferably in a plane perpendicular to the surface of the substrate. The quartz can be obtained hydrothermally at a temperature of 340 ° C and a pressure of 1000 atm. The growth is done on a small crystal initiator.

The fluoropolymer used in the process of the invention is preferably a carbon chain polymer comprising several carbon-fluorine bonds. In one embodiment, the fluoropolymer comprises a number of carbon-hydrogen bonds in a ratio of about 1: 1 to the number of carbon-fluorinated bonds. The polymer may be amorphous or semi-crystalline.

The fluoropolymer may be obtained by polymerization of at least one fluorinated monomer selected from the group consisting of vinyl fluoride, vinylidene fluoride, tetrafluoroethylene, hexafluoropropylene, perfluoropropylvinylether, perfluoromethylvinylether, and chlorotrifluoroethylene. . Other vinyl monomers such as ethylene and propylene may be copolymerized with the foregoing monomers.

According to one embodiment of the invention, the fluorinated polymer is a polyvinylidene fluoride (polyvinylidene fluoride, ISO standard PVDF, CAS Number: 24 937-79-9), for example a polymer sold under the reference Dyflor® (Evonik), Foraflon®, Kynar® (Arkema), Product Number: 182702 (Sigma Aldrich), or Solef5 (Solvay).

According to another embodiment, the fluorinated polymer is fluorinated ethylene-propylene obtained by copolymerization of tetrafluoroethylene and hexafluoropropylene, or PTFE (polytetrafluoroethylene).

The weight-average molecular weight of the fluoropolymer measured by GPC may be between 200,000 and 700,000 g / mol. According to a particular embodiment, the weight-average molecular weight of the fluoropolymer measured by GPC may be between 500,000 and 550,000 g / mol, and its density at 25 ° C. may be of the order of 1.7-1. , 8 g / mL. Its refractive index measured at 20 ° C is for example of the order of 1.4150-1.4250.

The fluoropolymer is advantageously in the form of a film deposited on the surface of the substrate and having a thickness of between 50 and 500 nm, preferably between 80 and 200 nm, for example between 80 and 100 nm.

The mask may be in the form of a film of thickness between 50 and 1000 nm, more preferably between 170 and 400 nm.

The chemical etching mask may comprise or consist of the stack of a fluoropolymer film and a film of gold, palladium or platinum having a thickness ranging from 50 to 500 nm, for example between 140 and 190 nm.

The adhesion of the layer of gold, palladium or platinum to the silicon dioxide substrate can be improved if necessary by interposing a chromium film with a thickness between 10 and 50 nm, for example from order of 30 nm, between the gold layer and the silicon dioxide substrate.

The chemical etching mask comprises or consists for example of a fluoropolymer film immobilized by covalent bonds with a gold film, which gold film is in contact with a chromium film, and which film of chromium is in contact with the substrate.

The gold, palladium or platinum film may be deposited by any method known to those skilled in the art, in particular by sputtering (English sputtering) or by physical vapor deposition (physical vapor deposition in English, PVD).

In another embodiment, the chemical etching mask consists of a fluoropolymer film immobilized by covalent bonds with the substrate.

The fluoropolymer may be deposited by any method known to those skilled in the art. When in the form of a film, the fluoropolymer film may be obtained by spin coating a solution of the polymer in an organic solvent, followed by evaporation of the solvent.

The process of the invention makes it possible to create covalent bonds between the silicon dioxide substrate and the fluoropolymer, or between gold (palladium or platinum) and the fluoropolymer.

The process for depositing the fluoropolymer is carried out on the substrate, covering the surface of the silicon dioxide or the surface of the gold (palladium or platinum), at a temperature which may advantageously be between 20 and 25.degree. 30 ° C. The method can be implemented on a conductive surface in the case of gold (palladium or platinum), or insulating in the case of silicon dioxide.

A process for depositing the fluoropolymer on silicon dioxide may comprise: <tb> 1) <SEP> contacting the surface of the silicon dioxide substrate with a solution of the fluorinated polymer in a solvent, and evaporation of the solvent, and <tb> 2) <SEP> attaching the fluoropolymer to the substrate by creating covalent bonds between the polymer and the substrate.

The step 1) of bringing a solution of fluorinated polymer into contact with the substrate is chosen from the group consisting of dip-coating, centrifugal coating (spin-coating). ), spray-coating, casting or jet printing. The evaporation of the solvent can be accelerated by raising the temperature so as to reduce the porosity of the film.

The step 2) of fixing the polymer may be carried out by a radiative treatment, for example a radiation treatment of ultraviolet rays, for example in the UV-c (Vacuum Ultra Violet or VUV). The treatment that is made of the fluoropolymer film deposited on the substrate to fix it by covalent bonding is advantageously devoid of the use of an activator of the surface of the silicon dioxide.

A method of immobilizing the fluoropolymer by a radiative treatment avoids a functionalization step of the surface with aryl salts for example.

A method of depositing the fluoropolymer on a gold film (palladium or platinum) may comprise an electrografting step in conditions known to those skilled in the art, for example by following the patent teaching. No. 8,784,635. For example, the fluorinated monomer, especially a vinylidene fluoride) will be dissolved in a DMF-type organic solvent in the presence of an aryl diazonium salt in order to form the electrografting solution.

In a particular embodiment, the chemical etching mask comprises the superposition of a chromium film, a gold (palladium or platinum) film and a fluoropolymer film, and the gold film (palladium or platinum) is deposited on the chromium film by spraying. In this embodiment, the fluoropolymer film is then attached to the surface of the gold (palladium or platinum) film by covalent bonding using a radiative treatment as previously described.

In the process of the invention, the silicon dioxide is etched chemically with fluoride ions.

We choose a fluoride ion concentration to achieve a compromise between the highest possible industrializable etching rate while ensuring the absence of defects in the quartz. Thus, the etching rate of the ions or radicals derived from fluorine is preferably between 10 and 1000 nm / mm, more preferably between 400 and 600 nm / min, at a temperature of between 20 and 90 ° C. preferably between 70 and 80 ° C in an open atmosphere.

The fluoride ions may be provided by HF in the gaseous state or in the form of an acidic aqueous solution.

The acidic aqueous solution containing fluoride ions comprises hydrofluoric acid, ammonium fluoride, or a mixture thereof. This solution may optionally contain other additive compounds such as surfactants.

In a particular embodiment, the etching solution comprises ammonium fluoride (NH4F), hydrofluoric acid (HF) and optionally additives. It is possible to use a volume ratio or a mass ratio between ammonium fluoride and hydrofluoric acid of between 10: 1 and 1: 1, so as to modulate the etching rate, depending on the thickness of the substrate. possible to use the commercial reference solution BOE®.

According to one embodiment, the etching is carried out by dipping the silicon dioxide substrate - in an aqueous solution of hydrofluoric acid at 40-60% buffered with ammonium fluoride - for 2 to 5 hours, at a temperature of between 70 and 90 ° C.

The shape of the etching mask comprising the fluoropolymer can be obtained by a method implementing a photolithography step, well known to those skilled in the art. The geometry of the mask is defined according to the shape and the size of the pieces of silicon dioxide that it is desired to obtain.

In a first step of the photolithography process, a photosensitive resin layer may be deposited on the entire surface of the film forming the mask comprising the fluoropolymer by using a spin coating method in order to deposit a resin film.

The term "photoresist", also called photoresist (photoresist in English), a photosensitive material used in the context of a photolithography process to form a protective coating openwork on the surface of the silicic material substrate. The photoresist can be "positive" or "negative". A positive photoresist becomes soluble in a developer solvent when exposed to light and is insoluble to the developer when it has not been exposed to light. A negative photoresist becomes insoluble in a developer solvent when exposed to light and is developer-soluble when not exposed to light. The photosensitive resins used in the context of the invention may be exposed to wavelengths of the ultraviolet spectrum.

The resin which has been deposited on the fluoropolymer film can then be subjected to a heat-curing treatment in an air circulation oven. The resin is, for example, a polymer such as the AZ-P1350 polymer sold by Clariant Co., or the Tokyo Ohka Co. resin (OFPR800).

According to the lithography method, an opaque impression piece is then disposed parallel to the surface of the quartz substrate integrally covered by the stack of several films, possibly including a Cr / Au film, a fluoropolymer film, and a photosensitive resin film, the etching mask being in contact with the substrate

The substrate is then exposed to light so that only the portions of the photosensitive material which have been exposed to light can be revealed by a solvent to allow the etching mask to be flush with a pattern complementary to that of the piece. fingerprint. The Cr / Au / fluoropolymer lines that are no longer coated with photoresist are then removed.

The fluoropolymer can be removed through a plasma 02, very often used to clean the patterns that have been developed through the photosensitive resin. To remove the gold film, I2 / KI / H2O can be used. The chromium film can be dissolved with a KMnO4 / NaOH / H2O mixture.

The photosensitive resin which has not been exposed is then removed using a solution of acetone or a solution of NMP for example, leaving the etching mask intact.

In a subsequent step, the assembly is treated with the fluoride ions to dissolve the slices of the quartz substrate which are no longer covered with the etching mask. The etching mask is finally removed in the same way as in the pattern definition step.

The present invention also relates to a method of manufacturing a quartz oscillator that uses the chemical etching process as described above. According to this method, the chemical etching mask is resistant to corrosion that can generate fluoride ions, especially in the form of a solution containing hydrofluoric acid. The invention also relates to a quartz oscillator obtained by this method, an electronic device selected from the group consisting of timepieces, gyroscopes and pacemakers, which apparatus comprises such a quartz oscillator.

The method of manufacturing a quartz oscillator according to the invention comprises the steps of forming an etching mask comprising a fluorinated polymer, and etching the quartz substrate with fluoride ions in order to cut the substrate of quartz in pieces.

The geometry of the etching mask can be defined by a photolithography method as described above, so as to obtain tuning forks of a size of the order of 450x900 microns.

The subject of the invention is also the use of a fluoropolymer, in particular a polyvinylidene fluoride, as a chemical etching mask, in a process of chemical etching of a silicon dioxide substrate, an acid solution of fluoride ions is used, said polymer making it possible to limit the occurrence of defects in said etched substrate. <tb> Fig. 1 <SEP> represents a top view of the quartz oscillators obtained according to the method of the invention, observed under a magnification microscope 40. <tb> Fig. 2 <SEP> represents a top view of the quartz oscillators obtained according to the method of the prior art, observed under a magnification microscope 40. The presence of small holes on the surface of the substrate is shown.

The invention is illustrated by the following examples. Unless otherwise stated, the experimental conditions are 25 ° C and 1 atm.

EXAMPLE 1 Gold / PVDF as a Chemical Etch Mask for Quartz Substrates

In this example, we used a mask composed of the stack of three films: Cr / Au / fluoropolymer.

Mask lines were created by photolithography with a photosensitive resin, and then the quartz was etched chemically with a solution of hydrofluoric acid pH buffered. The Cr / Au / fluoropolymer stacks were then removed from the obtained quartz pieces.

A. Material and equipment

The substrate that was used in this example is a quartz wafer 130 microns thick and 4 cm side, covered by the stack of a layer of chromium 30 nm, and a layer 175 nm gold deposited by sputtering.

A solution of poly (vinylidene fluoride) (Product Number 182702 manufactured by Sigma Aldrich) at a mass concentration of 2% was prepared by dissolving 2 g of polymer in 100 mL of DMF (dimethylformamide).

The chemical etching solution included the mixture of a 49% aqueous solution of HF and a 40% aqueous solution of NH4F, in a 1: 5 volume ratio.

B-method

i) Coating of the support with a PVDF polymer and immobilization by radiative treatment

The surface of a monocrystal quartz substrate covered with a chromium film and a gold film was cleaned by UV-ozone treatment for 5 minutes to remove any trace of organic contamination.

The PVDF solution was deposited homogeneously on the substrate using a micro-pipette to obtain a complete coating of the surface of the gold layer. The solvent was then evaporated hot in order to obtain a homogeneous film covering PVDF with a thickness typically of 80 to 100 nm.

The PVDF film was then irradiated by VUV (Vacuum Ultraviolet) radiation for 3 minutes at a distance of 7 cm in an air purged atmosphere with a dry nitrogen sweep, in order to graft it by insolation.

The characteristics of the VUV lamp were as follows: - Excimer lamp of OSRAM brand XERADEX model, - power 140 W, Wavelength ranging from 150 nm to 190 nm with a maximum at 172 nm.

ii) Formation of engraving vines

After deposition of the fluoropolymer layer, disjoint lines defining the pattern of the quartz etching mask were formed in the continuous layer Cr / Au / PVDF which covers the entire surface of the quartz. These lines were made by a conventional photolithography process using a photosensitive resin.

In a first step of the photolithography process, a layer of photoresist was deposited on the entire surface of the film forming the mask comprising the fluoropolymer using a spin coating method - at 2000 rpm for 30 seconds - so as to deposit a film with a resin thickness of about 5 microns.

The resin deposited on the fluoropolymer film was then subjected to a heat-curing treatment in an air circulation oven at 90.degree. According to the lithography method, an opaque impression piece was then placed parallel to the surface of the quartz substrate integrally covered by the Cr / Au / PVDF / photosensitive resin stack, the chromium being in contact with the substrate.

The substrate was then exposed to light so that only those portions of the photosensitive material which had been exposed to the light were revealed by a solvent to allow flush gold / PVDF lines in a complementary pattern. that of the impression room. An intensity of 50 to 60 mJ / cm <2> was set.

iii) Engraving quartz

The etching of the quartz is carried out by immersing the quartz substrate for 4 hours in the etching solution described above raised to 80 ° C.

The etching mask was then removed by repeating the steps used in the definition of the pattern of said mask.

C. Results achieved

The quality of quartz tunasons was verified by inspection under the electron microscope. The density of holes in quartz is greatly reduced. The polymer makes it possible to protect the quartz by decreasing the porosity of the gold with the fluoride ions during the chemical etching of the quartz, and thus to reduce the number of defects in the quartz after chemical etching. The appearance of the quartz pieces obtained, observed under a microscope, is represented in FIG. 1. In the process of the invention, the gold layer is not corroded by the etching solution.

We can achieve the same example using a quartz plate that has not been covered with gold and chrome. In this case, the fluoropolymer constitutes the etching mask. After formation of the fluorinated polymer lines by photolithography, and chemical etching of the quartz, the resulting hole density in the piece made of quartz should be greatly reduced as well. The polymer should help protect the quartz, and to overcome a deposit Cr / Au.

EXAMPLE 2 PVDF as an etching mask for quartz substrate chemical etching

A. Material and equipment

The substrate used in this example is a quartz wafer 130 microns thick and 4 cm side. In this example, the PVDF is used as a quartz etching mask. The quartz substrate is not covered with gold.

The PVDF solution and the etching solution are the same as those described in Example 1.

B-method

The PVDF polymer deposition conditions are identical to those described in Example 1.

After depositing the PVDF polymer on the quartz substrate, lines for describing the etching pattern of the quartz are made by photolithography.

After having etched the quartz under the same conditions as those of Example 1 and removed the polymer lines, the quartz pieces are observed by electron microscopy to detect any defects.

C-Results obtained

The polymer deposited on the quartz substrate has properties of resistance to fluorinated compounds present in the BOE solution. After formation of polymer lines and chemical etching of quartz, the resulting hole density in quartz is greatly reduced. The polymer helps to protect the quartz properly, to serve as a chemical etching mask.

COMPARATIVE EXAMPLE Gold constituting an etching mask for the chemical etching of quartz substrates

Example 1 was reproduced without using the PVDF polymer.

Due to the porosity of the chrome / gold stack, many holes were observed at the chrome / quartz interface after chemical etching. The high density of holes creates defects in the quartz which may drastically lower the output of the quartz tuning fork.

The appearance of the pieces of quartz obtained, observed under the microscope, is shown in FIG. 2.

Claims (15)

A method of etching a silicon dioxide substrate comprising the steps of: Fixing a chemical etching mask in the form of a film on the surface of said silicon dioxide substrate; Etching said substrate by exposing said substrate to fluoride ions for a time sufficient to cut said substrate into pieces of silicon dioxide, characterized in that the chemical etching mask comprises a fluoropolymer and the fluoropolymer is immobilized on the substrate by radiative treatment. 2. Method according to claim 1, characterized in that the fluoropolymer is immobilized by creating covalent bonds. 3. Method according to one of the preceding claims, characterized in that the fluoropolymer is a poly (vinylidene fluoride). 4. Method according to one of the preceding claims, characterized in that the radiative treatment is a treatment type Vacuum Ultra Violet (or VUV). 5. Method according to one of the preceding claims, characterized in that the fluoropolymer is in the form of a film with a thickness of between 80 and 200 nm. 6. Method according to one of the preceding claims, characterized in that the chemical etching mask comprises or consists of a fluoropolymer film immobilized by creating covalent bonds with a gold film, which gold film is in contact with a chromium film, and which chromium film is in contact with the substrate. 7. Method according to claim 6, characterized in that the gold film has a thickness between 50 to 500 nm. 8. Method according to one of claims 1 to 5, characterized in that the chemical etching mask consists of a fluoropolymer film immobilized by covalent bonds with the substrate. 9. The method of claim 5, characterized in that the fluoropolymer film is obtained by spin coating a solution of the polymer in an organic solvent. 10. Method according to one of the preceding claims, characterized in that the fluoride ions are provided in the form of an acidic aqueous solution comprising hydrofluoric acid, ammonium fluoride, or a mixture thereof. 11. Method according to one of the preceding claims, characterized in that the etching rate by fluoride ions is between 10 and 1000 nm / min at a temperature between 20 and 90 ° C. 12. A method of manufacturing a quartz oscillator which uses the chemical etching method according to one of claims 1 to 11. 13. Quartz oscillator obtained according to the method of one of claims 1 to 11. An electronic apparatus selected from the group consisting of timepieces, gyroscopes and pacemakers, which apparatus comprises a quartz oscillator obtained by the method of claim 13. 15. Use of polyvinylidene fluoride as a chemical etching mask in a process for the chemical etching of a silicon dioxide substrate using fluoride ions, said polymer making it possible to limit the occurrence of defects in said engraved substrate.