Method for manufacturing BAW resonators on a semiconductor wafer

Abstract

A method for manufacturing a wafer on which are formed resonators, each resonator including, above a semiconductor substrate, a stack of layers including, in the following order from the substrate surface: a Bragg mirror; a compensation layer made of a material having a temperature coefficient of the acoustic velocity of a sign opposite to that of all the other stack layers; and a piezoelectric resonator, the method including the successive steps of: a) depositing the compensation layer; and b) decreasing thickness inequalities of the compensation layer due to the deposition method, so that this layer has a same thickness to within better than 2%, and preferably to within better than 1%, at the level of each resonator.

Claims

1. A method, comprising: forming a Bragg mirror on a substrate, the forming of the Bragg mirror including: forming a first conductive layer having a temperature coefficient of acoustic velocity of a first sign; forming a compensation layer on the first conductive layer, the compensation layer having a temperature coefficient of acoustic velocity of a second sign that is opposite to that of the first sign; and decreasing thickness inequalities of the compensation layer at least until the compensation layer has a thickness variation less than 2%; and forming a piezoelectric resonator on the compensation layer, the forming of the piezoelectric resonator including: forming a first electrode on the compensation layer; forming a piezoelectric layer on the first electrode; and forming a second electrode on the piezoelectric layer.

2. The method of claim 1 wherein the forming of the Bragg mirror includes: forming a first dielectric layer on the substrate; forming the first conductive layer on the first dielectric layer; forming a second dielectric layer on the first conductive layer; and forming a second conductive layer on the second dielectric layer.

3. A method, comprising: forming a stack of layers on a semiconductor substrate, the forming of the stack of layers including: forming a Bragg mirror on the substrate, the Bragg mirror including: a first conductive layer on the substrate; a first dielectric layer on the first conductive layer; a second conductive layer on the first dielectric layer; and a second dielectric layer on the second conductive layer, the first and second conductive layers having a temperature coefficient of acoustic velocity (TCV) of a first sign; depositing a compensation layer on the second dielectric layer of the Bragg mirror, the compensation layer having a TCV of a second sign that is opposite to that of the first sign; and decreasing thickness inequalities of the compensation layer at least until the compensation layer has a thickness variation less than 2%; and forming a piezoelectric resonator on the compensation layer, the forming of the piezoelectric resonator including: forming a first electrode on the compensation layer; forming a layer of a piezoelectric material on the lower electrode; and forming a second electrode on the layer of the piezoelectric material.

4. The method of claim 3 wherein decreasing thickness inequalities includes decreasing the thickness inequalities until at least the compensation layer has less than a 1% variation in thickness.

5. The method of claim 3 wherein decreasing thickness inequalities includes decreasing by ion etching of overthicknesses of the compensation layer caused by the depositing.

6. The method of claim 3 wherein an upper layer of the Bragg mirror and the compensation layer are a single layer of a same material.

7. The method of claim 3 wherein the compensation layer includes silicon oxide.

8. The method of claim 3 wherein the first and second electrodes includes molybdenum.

9. The method of claim 3 wherein the layer of piezoelectric material includes aluminum nitride.

10. The method of claim 3 wherein the first and second conductive layers each have a first acoustic impedance and the first and second dielectric layers each have a second acoustic impedance smaller than the first acoustic impedance.

11. The method of claim 10 wherein the first and second conductive layers are tungsten and the first and second dielectric layers are silicon oxide.

12. The method of claim 3, further comprising forming a frequency adjustment layer on the resonator, the frequency adjustment layer having a thickness capable of compensating for a frequency shift due to manufacturing dispersions.

13. A method, comprising: forming a Bragg mirror, including: forming a first dielectric layer on a substrate; forming a first conductive layer on the first dielectric layer; forming a second dielectric layer on the first conductive layer; and forming a second conductive layer on the second dielectric layer; forming a temperature compensation layer on the second conductive layer of the Bragg mirror; and decreasing thickness inequalities of the temperature compensation aver at east until the compensation layer has a thickness variation less than 2%; and forming a piezoelectric resonator on the temperature compensation layer, the forming the piezoelectric resonator including: forming a first electrode on the temperature compensation layer; forming a piezoelectric material layer on the first electrode; and forming a second electrode on the piezoelectric material layer.

14. The method of claim 13 further comprising forming a frequency adjustment layer on the second electrode of the piezoelectric resonator.

15. The method of claim 13 wherein the temperature compensation layer has a temperature coefficient of acoustic velocity (TCV) of a first sign and the first and second conductive layers of the Bragg mirror have a TCV of a second sign that is opposite to the first sign.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

(1) The foregoing objects, features, and advantages of the present disclosure will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings.

(2) FIG. 1, is a cross-section view schematically showing a known BAW resonator;

(3) FIG. 2 is a cross-section view schematically showing an example of a temperature-compensated BAW resonator according to an embodiment of the present disclosure;

(4) FIG. 3 illustrates a step of an example of a method for forming temperature-compensated BAW resonators of the type described in relation with FIG. 2; and

(5) FIGS. 4A and 4B illustrate the variation of the resonance frequency of BAW resonators according to temperature.

DETAILED DESCRIPTION

(6) For clarity, the same elements have been designated with the same reference numerals in the different drawings and, further, as usual in the representation of microcomponents, the various drawings are not to scale.

(7) FIG. 2 is a cross-section view a BAW resonator 11 having a temperature-compensated frequency response according to an embodiment of the present disclosure.

(8) BAW resonator 11 of FIG. 2 is identical to resonator 1 of FIG. 1, but for the fact that a temperature compensation layer 13, for example, made of silicon oxide, is provided between upper layer 7b of reflector 7 and lower electrode 5a.

(9) By analyzing current temperature compensation modes, the present inventors have determined that, among the different layers forming a BAW resonator, the silicon oxide temperature compensation layer 13 is one of the layers with the most inaccurate deposition. As an example, thickness variations on the order of 9% (maximum-minimum) can be observed on this layer, at the scale of a semiconductor wafer, which amounts to a standard deviation on the order of 2%. As a comparison, the thickness variations of piezoelectric layer 5b for example are on the order of 2% (standard deviation on the order of 0.4%) for aluminum nitride.

(10) The inventors have further observed that, among the different layers of BAW resonator 11, silicon oxide layer 13 is that for which thickness variations have the greatest influence upon the resonator TCF. As an example, a thickness variation of 1% of this layer can shift the TCF of the resonator by 0.8 ppm/ C. As a comparison, a thickness variation of 1% of piezoelectric layer 5b only causes a small TCF variation, on the order of 0.2 ppm/ C. Similarly, a thickness variation of 1% of electrodes 5a and 5c causes a TCF variation on the order of 0.1 ppm/ C.

(11) An aspect of an embodiment of the present disclosure is to provide, on manufacturing, a step of leveling of the thickness of temperature compensation layer 13, so that this layer has a constant thickness to within better than 2% (standard deviation on the order of 0.5%) at the level of each resonator. According to a preferred embodiment of the present disclosure, layer 13 has a same thickness to within better than 1% (standard deviation on the order of 0.2%) at the level of each BAW resonator.

(12) FIG. 3 illustrates a step of an example of a method for forming temperature-compensated BAW resonators of the type described in relation with FIG. 2. FIG. 3 is a cross-section view schematically showing a portion of a semiconductor wafer on which are formed elements 7a, 7b of the Bragg mirror and the temperature compensation layer 13.

(13) After the deposition of the compensation layer 13, a step where the thickness of this layer is made even by etching of the overthicknesses due to the deposition method is provided. This thickness leveling step may advantageously be performed by ion etching, like the final frequency adjustment step described in relation with FIG. 1. As an example, the semiconductor wafer on which the resonators are formed is scanned by an ion beam 21. The scan speed is locally controlled on this wafer to etch the compensation layer more or less strongly. At the end of the leveling step, temperature compensation layer 13 has a same thickness to within better than 2% (standard deviation on the order of 0.5%), and preferably to within better than 1% (standard deviation on the order of 0.2%), at the level of each BAW resonator.

(14) FIGS. 4A and 4B show the variation of frequency drift (f/f) of the BAW resonators, expressed in parts per million (ppm), according to the temperature in the operating temperature range [Tmin, Tmax].

(15) FIG. 4A shows the variation of the frequency drift according to temperature for BAW resonators of the type described in relation with FIG. 2, but for which the step of leveling of the temperature compensation layer has not been carried out. In such resonators, the thickness of temperature compensation layer 13 is tainted with an uncertainty on the order of 9% (standard deviation on the order of 2%).

(16) Curve 31, in dotted lines, shows the ideal temperature behavior, that is, the temperature behavior of a resonator in which the different layers, and especially temperature compensation layer 13, would exhibit no thickness uncertainty.

(17) Curves 33 and 35 illustrate the temperature behavior of two resonators formed from a same semiconductor wafer. Curve 33 for example corresponds to the case of a resonator formed in an area of the semiconductor wafer where the thickness of the temperature compensation layer 13 is maximum. Curve 35 for example corresponds to the case of a resonator formed in an area of the semiconductor wafer where the thickness of the temperature compensation layer 13 is minimum.

(18) Significant differences of the temperature behavior of the resonance frequency can be observed. It can further be observed on curve 35 that for certain resonators, the resonance frequency variation according to temperature is not linear.

(19) FIG. 4B shows the variation of the resonance frequency according to temperature for BAW resonators of the type described in relation with FIG. 2. In such resonators, the thickness of temperature compensation layer 13 exhibits an uncertainty smaller than 2% (standard deviation of 0.5%).

(20) Curve 41, in dotted lines, shows the ideal temperature behavior, that is, the temperature behavior of a resonator in which the different layers, and especially the temperature compensation layer, would exhibit no thickness uncertainty.

(21) Curves 43 and 45 illustrate the temperature behavior of resonators formed from a same semiconductor wafer. Curve 43 for example corresponds to the case of a resonator formed in an area of the semiconductor wafer where the thickness of the temperature compensation layer 13 is maximum. Curve 45 for example corresponds to the case of a resonator formed in an area of the semiconductor wafer where the thickness of the temperature compensation layer 13 is minimum.

(22) It can be observed that the temperature behavior of the operating frequency is substantially the same for all resonators and close to the ideal behavior. It can further be observed that the temperature behavior of the operating frequency is substantially linear across the entire wafer.

(23) Of course, the slopes of the dotted lines and of the tangents to the curves of FIGS. 4A and 4B may be inverted (thus corresponding to a TCF of opposite sign).

(24) An advantage of the provided embodiment is that it enables to obtain a particularly accurate and linear temperature compensation. In particular, it can be observed that resonators manufactured identically on one or several semiconductor wafers have a TCF which is substantially identical and constant in the range of use temperatures of the resonator. Thus, an accurate linear drift of the frequency according to temperature is guaranteed at the scale of a substrate wafer. This especially enables to simplify the steps of calibration of the circuits comprising BAW resonators. This is particularly advantageous in the case of time reference oscillators based on BAW resonators.

(25) Further, if upper layer 7b of the Bragg mirror is made of silicon oxide, it may in practice, advantageously, form one and the same layer with temperature compensation layer 13.

(26) Specific embodiments of the present disclosure have been described. Various alterations and modifications will occur to those skilled in the art. In particular, the present disclosure is not limited to the materials mentioned in the above description. In particular, it is within the abilities of those skilled in the art to implement the desired operation by using other piezoelectric materials, for example, potassium niobate or zinc oxide, and other conductive materials, for example, copper, tungsten, or aluminum to form the resonant core. It will further be within the abilities of those skilled in the art to use other materials having high and low acoustic impedances, for example, silicon nitride or aluminum nitride, capable of forming an isolation reflector between the resonant core and the substrate. Finally, other materials than silicon oxide may be used to form the frequency adjustment and temperature compensation layers, for example, SiON.

(27) Further, the present disclosure is not limited to the use of a local ion beam to make the thickness of the temperature compensation layer even. It will be within the abilities of those skilled in the art to implement the desired operation by using other adapted leveling methods.

(28) Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present disclosure. Accordingly, the foregoing description is by way of example only and is not intended to be limiting.

(29) These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.