Adjusting the frequency of film bulk acoustic resonators

A material may be removed from the top electrode of a film bulk acoustic resonator to alter the mass loading effect and to adjust the frequency of one film bulk acoustic resonator on a wafer relative to other resonators on the same wafer. Similarly, the piezoelectric layer or the bottom electrode may be selectively milled with a focused ion beam to trim the resonator.

BACKGROUND

This invention relates generally to front-end radio frequency filters including film bulk acoustic resonators (FBAR).

Film bulk acoustic resonators have many advantages compared to other techniques such as surface acoustic wave (SAW) devices and ceramic filters, particularly at high frequencies. For example, SAW filters begin to have excessive insertion losses above 2.4 gigahertz and ceramic filters are much larger in size and become increasingly difficult to fabricate at increased frequencies.

A conventional FBAR filter may include two sets of FBARs to achieve the desired filter response. The series FBARs have one frequency and the shunt FBARs have another frequency. The frequency of an FBAR is mainly determined by the thickness of its piezoelectric film which approximately equals the half wavelength of the acoustic wave. The frequencies of the FBARs need to be precisely set to achieve the desired filter response.

For example, for a 2 gigahertz FBAR, the thickness of the piezoelectric film may be approximately 1.8 micrometers. A one percent non-uniformity in piezoelectric film thickness may shift the frequency of the filter by approximately 20 megahertz which is not acceptable if a 60 megahertz pass bandwidth is required.

Generally, post-process trimming may be used to correct the frequency. One technique may involve etching the upper electrode or depositing more metal. Another technique involves adding a heating element. However, both of these approaches are problematic in high volume manufacturing, particularly since they are die-level processes that generally have low throughput. In addition, in-situ measurement may be required during the post-process trimming steps. Therefore, the costs are high and the throughput is relatively low.

Thus, there is a need for better ways to adjust the frequency of FBARs.

DETAILED DESCRIPTION

Referring toFIG. 1, a film bulk acoustic resonator (FBAR)10may include a top electrode12and a bottom electrode16sandwiching a piezoelectric layer14. The entire structure may be supported over a backside cavity24in a semiconductor substrate20. A dielectric film18may be interposed between the semiconductor substrate20and the remainder of the FBAR10. As shown inFIGS. 1 and 2, the top electrode12may be coupled to a contact18and the bottom electrode16may be coupled to a different contact18.

The frequency compensation may be done by altering the mass loading at the wafer level, achieving relatively high throughput without the need for in-situ measurement in some embodiments. Thus, each FBAR10on a wafer may have its frequency adjusted to achieve the originally designed frequency for each particular FBAR10. As necessary, across a wafer, each FBAR10may be individually compensated.

After FBAR10has been initially fabricated, frequency variations across the wafer may be adjusted. A focused ion beam “B” may be scanned across the wafer to trim the top electrode12, the bottom electrode16, or piezoelectric layer14or any combination of the above to the desired frequency by adjusting the thickness of a particular layer. The amount of trimming may be determined by the thickness profiles of layers across the wafer or by frequency measurements. Since the processes may be implemented at the wafer level, throughput may be relatively high. In some cases, the focused ion beam trimming may result in a small amount of ions, such as Ga+ ions being implanted.

In one embodiment, the FBARs10may be intentionally fabricated so that the highest frequency FBAR10is set equal to, but not exceeding, a target value. Then all of the FBARs10can be trimmed to meet the target frequency using focused ion beam trimming.

In embodiments in which the top electrode12is trimmed, variations of all three layers may be corrected in the most final step. Therefore, this approach may be cost effective and accurate in some embodiments. However, the thickness ratio of metal in the electrodes12and16to the piezoelectric layer14is slightly different after trimming. Thus, the tuning range may be limited in some embodiments.

In accordance with another embodiment of the present invention, trimming may be applied to the bottom electrode16at the final step, after turning the wafer over. This approach may be utilized in conjunction with trimming of the top electrode12, in some cases, to extend the tuning range.

In still another embodiment, the piezoelectric layer14may be trimmed. The thickness variation of the piezoelectric layer14is the main origin of frequency variation. So the metal to piezoelectric ratio may be relatively constant after trimming. However, pre-bias of the top electrode12variation may also need to be taken into account during the trimming process.

In still another embodiment, combinations of each of the above techniques may be utilized to achieve higher tuning range. However, trimming combinations of layers may raise throughput or cost issues.

The thickness profile of the piezoelectric layer14may be precisely measured after deposition. The amount of material that needs to be trimmed can be determined according to a thickness profile and electrode12,16variation. The focused ion beam conditions may be programmed according to the thickness profiles of the layers of the various FBARs10. The focused ion beam mills the layer or layers to tune the frequency of that FBAR10. As the focused ion beam advances to the next FBAR10across the wafer, it then anneals the appropriate amount of material from the next FBAR10to achieve uniform frequency. In one embodiment in situ radio frequency measurement may be used during trimming for better control.

Referring toFIG. 3, in accordance with one embodiment, the FBAR10may be formed by depositing a bottom electrode16as indicated in block30. A thickness profile of the bottom electrode may be measured as indicated in block32. Then the piezoelectric layer14may be deposited as indicated in block34. The thickness profile of the piezoelectric layer14may be measured as indicated in block36. Finally, a top electrode12may be deposited as indicated in block38.

The top electrode12may be trimmed using a focused ion beam in one embodiment of the present invention. In situ radio frequency testing may be used during the trimming process. The focused ion beam conditions may be programmed according to the previously measured thickness profiles. The focused ion beam may then be scanned across the wafer with a varying, programmable power to vary the amount of material that is removed, as indicated in block40. As a result, the focused ion beam may be scanned to achieve a relatively uniform frequency across the wafer as indicated in block42.