METHOD FOR MANUFACTURING A SUBSTRATE FOR A RADIOFREQUENCY FILTER

Abstract

Method for manufacturing a substrate for a radio frequency filter. A method for manufacturing a substrate for a radio frequency filter by assembling a piezoelectric layer onto a support substrate via an electrically insulating layer, characterized in that it comprises the steps of deposition of the electrically insulating layer by centrifugal coating of an oxide belonging to the SOG (Spin On Glass) family onto the surface of the piezoelectric layer to be assembled onto the support substrate, followed by annealing to densify said electrically insulating layer before assembly. Figure 1

Description

Title of the invention: Method for manufacturing a substrate for a radio frequency filter FIELD OF INVENTION

[0001] The present invention relates to a method for manufacturing a substrate for a radio frequency filter. STATE OF THE ART

[0002] It is known to manufacture a radio frequency (RF) device, such as a resonator or filter, on a substrate comprising successively, from its base to its surface, a support substrate, generally in a semiconductor material such as silicon, an electrically insulating layer and a piezoelectric layer.

[0003] Commonly used piezoelectric materials such as lithium niobate or lithium tantalate have relatively high coefficients of thermal expansion and are anisotropic, which makes assembly onto substrates such as silicon difficult. Problems of breakage or detachment during thermal annealing in processing, packaging, or cutting steps can render such assembled substrates unsuitable.

[0004] In addition, surface acoustic wave (SAW) filters typically comprise a thick piezoelectric layer (i.e., generally several tens of micrometers thick) and two electrodes in the form of two interlocking metal combs deposited on the surface of said piezoelectric layer. An electrical signal, typically a change in voltage, applied to one electrode is converted into an elastic wave that propagates across the surface of the piezoelectric layer. The propagation of this elastic wave is favored if the wave frequency corresponds to the frequency band of the filter. This wave is again converted into an electrical signal upon reaching the other electrode.

[0005] However, there are parasitic modes of wave propagation that extend through the thickness of the piezoelectric layer and are likely to be reflected at the interface with the underlying substrate. This phenomenon is called "rattle" in English.

[0006] To avoid these parasitic modes, it is known to ensure that the surface of the piezoelectric layer located at the interface with the electrically insulating layer is sufficiently rough to allow reflection of the parasitic waves in all directions (dispersion effect) and prevent their transmission into the substrate.

[0007] Given the wavelength considered, the roughness of the surface of the piezoelectric layer is very high, on the order of a few pm.

[0008] The fabrication of the substrate involves gluing the rough surface of the piezoelectric layer, optionally covered with the electrically insulating layer, onto the support substrate.

[0009] However, to ensure good adhesion between the piezoelectric layer and the supporting substrate despite such roughness, the current process requires a large number of successive steps, which make it long and expensive. Description of the invention

[0010] The present invention aims to overcome these limitations of the prior art by proposing a method for manufacturing a substrate for a radio frequency filter. This makes it possible to remedy the problems currently encountered.

[0011] The invention relates to a method of manufacturing a substrate for a radio frequency filter by assembling a piezoelectric layer on a support substrate via an electrically insulating layer characterized in that it comprises the steps of depositing the electrically insulating layer by centrifugal coating of an oxide belonging to the SOG (Spin On Glass) family on the surface of the piezoelectric layer to be assembled on the support substrate followed by annealing to densify said electrically insulating layer before assembly.

[0012] In advantageous embodiments the thickness of the piezoelectric layer is greater than 5 pm, preferably greater than 10 pm.

[0013] In advantageous embodiments the surface of the piezoelectric layer to be assembled on the support substrate has a rough surface suitable for reflecting a radio frequency wave.

[0014] In advantageous embodiments the rough surface of the piezoelectric layer has a roughness greater than 1 pm, preferably greater than 3 pm.

[0015] In advantageous embodiments the support substrate is made of silicon material.

[0016] In advantageous embodiments the silicon material support substrate includes a trapping layer towards the interface to be assembled with the piezoelectric layer.

[0017] In advantageous embodiments the piezoelectric layer (200, 200') is of lithium niobate or lithium tantalate. DESCRIPTION OF THE FIGURES

[0018] Other features and advantages of the invention will be better understood upon reading the detailed description that follows, with reference to the accompanying drawings in which: • Figure 1 illustrates a method for manufacturing a substrate for a radio frequency filter according to an embodiment of the invention; • Figure 2 illustrates a method for manufacturing a substrate for a radio frequency filter according to an embodiment of the invention;

[0019] To improve the readability of the figures, the different layers are not necessarily represented to scale.

[0020] DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0021] Figure 1 schematically illustrates the process for manufacturing a substrate for radio frequency filter according to an embodiment of the present invention. Figure 1 illustrates a support substrate 100 on which a piezoelectric layer 200 is assembled via an electrically insulating layer 300.

[0022] The electrically insulating layer is generally chosen from the family of SOGs (from the Anglo-Saxon term "Spin On Glass" meaning glass that can be deposited by centrifugation) which have the property of being in a liquid state at room temperature but can densify, and be made solid, through appropriate heat treatment.

[0023] This technique consists of rotating the substrate on which the electrically insulating layer 300 is to be deposited at a substantially constant and relatively high speed, in order to spread said layer in a liquid state uniformly over the entire surface of the substrate by centrifugal force. For this purpose, the substrate is typically placed and held by vacuum on a rotating platform.

[0024] A person skilled in the art is able to determine the operating conditions, such as the volume deposited on the surface of the substrate, the rotation speed of the substrate, and the minimum deposition time as a function of the desired thickness for the adhesive layer.

[0025] The thickness of the electrically insulating layer 300 is typically between 2 pm and 8 pm. In addition, the centrifugal coating technique used is advantageous in that the deposition of the layer 300 is carried out at room temperature and is followed by densification annealing at a temperature of approximately 250°C, and therefore does not cause deformation of the substrate on which the dielectric layer is formed.

[0026] An electrically insulating layer 300 of SOG type oxide makes it possible to maintain the acoustic performance of a radio frequency device obtained subsequently from the piezoelectric substrate at an optimal level.

[0027] According to a non-limiting example, the electrically insulating layer 300 can be chosen from families of SOGs of the "silicates" or "methylsilsexioquane" type, marketed for example under the references "20B" or "400F" by FILMTRONICS or "FOX 16" by DOW CORNING.

[0028] The piezoelectric layer 200, onto which the electrically insulating layer 300 has been deposited and densified, is bonded to the substrate 100 preferably by direct molecular adhesion. The bonding is preferably carried out at room temperature, approximately 20°C. However, it is possible to perform the bonding under heat at a temperature between 20°C and 50°C, and more preferably between 20°C and 30°C.

[0029] Furthermore, the bonding step is advantageously carried out at low pressure, i.e. at a pressure less than or equal to 5 mTorr, which allows the water to be desorbed from the surfaces forming the bonding interface, i.e. the surface of the electrically insulating layer 300 and the surface of the support substrate 100. Carrying out the bonding step under vacuum makes it possible to further improve the desorption of water at the bonding interface.

[0030] A heat treatment to strengthen the bonding interface can be carried out at low temperatures up to 300°C without the assembly undergoing excessive deformations causing breakage of the materials or detachment at the bonding interface.

[0031] The bonding energy obtained by direct bonding between the surface of the support substrate 100 and the electrically insulating layer 300 of the present invention is high and allows not only the thinning step of the piezoelectric layer by chemical-mechanical etching (CMP) but also the final cutting of the assembled plate in order to obtain radio frequency devices and this despite a significant difference in coefficient of thermal expansion of the piezoelectric materials of the piezoelectric layer 200 used as well as of the material of the support substrate 100 used.

[0032] According to a non-limiting example of the invention, the support substrate 100 can be made of silicon material.

[0033] According to another non-limiting embodiment, schematically illustrated in Figure 2, the support substrate 100 may be made of silicon material comprising, among other things, a trapping layer 400 at the interface to be assembled with the electrically insulating layer 300, enabling the trapping of electric carriers induced by the frequency operation of the radio frequency device. This layer thus makes it possible to reduce insertion losses and improve the performance of said devices.

[0034] According to a non-limiting example of the invention, the piezoelectric material can be chosen from lithium niobate and lithium tantalate.

[0035] The predetermined thickness of the piezoelectric layer 200 is preferably greater than 5 pm, or even more preferably greater than 10 pm.

[0036] Given the significant difference in thermal coefficient of expansion, such a high thickness generates stresses that are difficult to bear on the assembly of the piezoelectric layer 200 on the support substrate 100. The assembly according to the present invention makes it possible to obtain bonding energies which allow the substrate to remain mechanically intact and can thus undergo process steps such as thinning steps, electrode deposition steps involving temperatures up to 300°C, or cutting steps in order to isolate the radio frequency devices obtained on this assembled substrate.

[0037] According to a non-limiting example of the invention, the surface of the piezoelectric layer 200 to be assembled onto the substrate has a rough surface suitable for reflecting a radio frequency wave. This makes it possible to reduce the impact of parasitic waves reflected at any interfaces present within a hybrid substrate of a piezoelectric layer assembled to a support. In advantageous embodiments, the rough surface of the piezoelectric layer has a roughness greater than 1 pm, preferably greater than 3 pm, corresponding substantially to the wavelength of the parasitic waves. These parasitic waves also appear mainly for piezoelectric structures with a thickness greater than 5 pm, or even 10 pm, for which the assembly of the present invention offers advantages.It should be noted that the centrifugal coating technique has a smoothing effect and the envisaged thicknesses of the electrically insulating layer 300 between 2 pm and 8 pm not only allow for an assembly with good bonding energy but also avoid additional planarization and smoothing steps if this centrifugal coating deposit is applied to a rough surface as described previously.

Claims

Demands

1. A method for manufacturing a substrate for a surface acoustic wave radio frequency filter by assembling a piezoelectric layer (200, 200') onto a support substrate (100) via an electrically insulating layer (300) characterized in that it comprises the steps of depositing the electrically insulating layer (300) by centrifugal coating of an oxide belonging to the SOG (Spin On Glass) family onto the surface of the piezoelectric layer (200, 200') to be assembled onto the support substrate (100) followed by annealing to densify and solidify said electrically insulating layer before assembly.

2. A method according to the preceding claim wherein the thickness of the piezoelectric layer (200, 200') is greater than 5 pm, preferably greater than 10 pm.

3. A method according to any one of the preceding claims wherein the surface of the piezoelectric layer (200, 200') to be assembled on the support substrate (100) has a rough surface suitable for reflecting a radio frequency wave.

4. A method according to the preceding claim wherein the rough surface of the piezoelectric layer (200, 200') has a roughness greater than 1 pm, preferably greater than 3 pm.

5. A method according to any one of the preceding claims wherein the support substrate (100) is made of silicon material.

6. Method according to the preceding claim wherein the support substrate (100) of silicon material comprises a trapping layer (400) towards the interface to be assembled with the piezoelectric layer (200, 200').

7. A method according to any one of the preceding claims wherein the piezoelectric layer (200, 200') is of lithium niobate or lithium tantalate