Patent Application
20090053401
1. Field of the Invention
The present invention relates to the field of BAW (bulk acoustic wave) resonators.
2. Prior Art
Piezoelectric resonators are frequently used for signal filtering and reference oscillators. These resonators are commonly referred to as BAW (bulk acoustic wave resonators). Other acronyms for the same or similar devices include FBAR (film bulk acoustic resonators) or SMR (solidly mounted resonators) or TFR (thin film resonators) or SCF (stacked crystal filters).
The resonators must be as efficient as possible in terms of limiting energy losses. These devices are not new and are well documented in the literature.
Standard IC fabrication methods are used for the basic manufacturing sequences, including depositions, photolithography, and etch processes. MEMS techniques may also be employed for packaging and resonator acoustic isolation from the substrate.
A Bragg mirror is used for acoustic isolation in SMR devices. In FBAR, the resonators are built upon a membrane. Both types of isolation are designed to prevent energy loss from the device.
The quality of a filter relies on an efficient piezoelectric transduction. This in turn depends on the quality of the piezoelectric material, usually AlN, deposited as a polycrystalline thin film on the wafer.
People trained in thin film processing know two ways of depositing a film with a controlled texture. One way is to provide an adequate substrate, itself with a well defined crystalline texture and a lattice match to the structure of the film to grow. This is called epitaxial or quasi-epitaxial growth. Another way is, on the opposite, to avoid for the substrate to have any influence on the film deposition: a crystalline phase can be obtained as the natural result of energy (entropy) optimization. This usually involves to prevent thermodynamic obstacles (provide enough energy and time to start with in the process for the film to self-organize as it grows).
The present invention relates to BAW resonators and filters fabricated using a process that allows an optimum growth of piezoelectric AlN film by means of a seed layer, itself made of AlN, and deposited with sputtering at lower temperature in an amorphous phase. Filters using these resonators can be designed to operate at a wide range of frequencies to address virtually all market filter applications (e.g., GSM, GPS, UMTS, PCS, WLAN, WIMAX, etc.).
Key aspects of a bulk acoustic wave resonator (BAW) are the quality factors (Q) and coupling coefficient keff2. The Q values are dominated by electrical and acoustic losses. The coupling coefficient is also dependent on both the intrinsic coupling kt2 of the piezoelectric layer active in the device and the choice and balance of materials used in the stack.
A good coefficient kt2 for AlN is obtained by controlling the film texture. The desirable AlN is a columnar polycrystalline film typically deposited by PVD. A columnar (0002)-oriented texture is desirable to maximize the film piezoelectric coefficient, or its coupling kt2. Any misoriented grain will not only decrease the piezoelectric efficiency of the resonator when functioning at its operating frequency, but potentially generates spurious modes that can be triggered by the existence of grains oriented in a direction distinct from the main texture of the film.
To foster an optimum (0002) orientation of the AlN, the film can either be deposited on a well oriented electrode, in the same way a mono-crystal can be grown over a mono-crystalline substrate with matching lattice structure, or in accordance with the present invention, be deposited over a amorphous substrate that would let the AlN self-organize into the desired columnar phase.
By using the multi-step AlN deposition recipe, a way has been defined to provide a thin amorphous and dielectric AlN interposing layer over the bottom electrode upon which piezoelectric AlN film can grow with the required quality.
Thus a BAW substrate is provided, consisting of a bottom electrode patterned over an acoustic isolation. In the case presented in
The substrate is then loaded into an AlN PVD deposition tool. Typically, the tool comes as a cluster with several chambers and allows movement of wafers from chamber to chamber without a vacuum break. A usual set-up combines a conditioning chamber (for degas and heating), a PVD deposition chamber for metal film (to process an electrode) and a second reactive PVD chamber to grow the piezoelectric film. Such a cluster is commercially available from companies like Aviza or Unaxis.
The process may be outlined as follows:
1. Deposit a thin (typically in the order of 50 A to 500 A). AlN film at low temperature (typically less than 200° C.). This film is amorphous, as not enough energy is provided to foster a crystalline orientation. Typically the process is a PVD one, with an Al target and a nitrogen rich plasma environment. The resulting stack is shown in
2. The wafer may be moved to the conditioning chamber in order to heat the wafer to a higher temperature, typically between 200° C. and 500° C.
3. The wafer is again moved either into same chamber as 1 above, or into another chamber from the cluster also suitable for AlN deposition. This time, the process aims at forming a crystalline film over the substrate. With the appropriate heat, enough energy is available for the AlN to self-organize as a polycrystalline textured film in a thermodynamically preferential phase: (0002). The result is illustrated in
Relevant points on the above include:
1. 1 and 3 above may or may not take place in the same chamber.
2. Amorphous AlN in 1 may or may not be stoichiometric.
3. Amorphous AlN deposited in 1 on a smooth surface provides in turn a smooth surface for crystalline AlN to grow in 3.
4. A vacuum break may or may not occur between 1 and 2.
5. AlN deposited in 1 is preferably as thin as possible to limit performance loss.
6. Well oriented AlN in step 3 can be grown at temperatures as low as 200° C.
7. The nature of the metal constituting the electrode has no influence on the AlN growth.
8. The growth of AlN in a crystalline texture is also the consequence of adequate choice of chamber pressure, power, and other typical parameters familiar to process engineers.
9. 1, 2 or 3 may or may not have to be followed in a row for each wafer. For instance a whole batch of wafers (typically a 25 wafer lot) can be processed through 1, then only individual wafers processed one at a time through 2 and 3.
There are several benefits of this invention:
1. The amorphous AlN film deposited, being dielectric, does not have to be patterned.
2. The amorphous AlN film encapsulates the underlying electrode surface and decouples the electric and acoustic function from the electrode, and the morphological function of the substrate (by opposition to the epi-like AlN growth for which electrode also needs to perform the function of a well oriented substrate). This alleviates difficulty for the whole process integration.
3. No extra chamber is required than the already required conditioning and AlN PVD deposition chamber.
4. Additional process time required for the AlN amorphous interposition layer deposition is short, and happens on potentially the same cluster tool as the piezoelectric deposition itself.
While preferred embodiments of the present invention have been disclosed and described herein for purposes of illustration and not for purposes of limitation, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention.
