Yury Oshmyansky

A film bulk acoustic resonator (FBAR) duplexer for USPCS handset applications

We will describe the design and measured performance of a duplexer based on film bulk acoustic resonators (FBARs) for the 1900 MHz PCS cellular phone market. Typical specifications for the duplexer require the Tx filter to attenuate frequencies in the Rx band by >40 dB while maintaining a worst-case insertion loss of 3.5 dB over the Tx band. VSWR must be better than 2.2 in the pass-band (8.5 dB return loss). The Rx filter must attenuate the Tx frequencies by >50 dB while maintaining a worst-case insertion loss of 4.2 dB in the Rx band. Again, VSWR must be better than 2.2.

A BAW antenna duplexer for the 1900 MHz PCS band

Miniature antenna duplexers are used in modem cellular telephone handsets. We present a new solution utilizing micro-machined, thin Film Bulk Acoustic Resonators (FBAR). A 1900 MHz duplexer with dimensions 5/spl times/8/spl times/2 mm is achieved. The duplexer, a three port device, achieves low loss from transmitter (Tx) to antenna (Ant) ports with a filter at 1880 MHz and from Ant to receiver (Rx) ports with a filter at 1960 MHz. High isolation from Tx to Rx port is achieved by coupling the two filters with a quarter wavelength line. Filter bandwidth is /spl sim/3%. The filters are realized as 21/2 or 31/2 section ladder connected FBARs with the shunt FBARs tuned 3% lower in frequency than the series FBARs to provide a flat pass band. Individual FEAR resonators are free membranes formed of Aluminum Nitride piezoelectric and metal electrode films micro-machined on a silicon substrate. Resonators with electro-acoustic coupling k/sub t//sup 2//spl sim/3 to 5% and Q/spl sim/300 are achieved. Filters fabricated in this manner achieve minimum insertion loss /spl sim/1 dB, out-of-band rejection of /spl sim/20 dB, 60 MHz bandwidth, and an input power capability of 2 Watts. Duplexers are assembled from one each Tx and Rx filter, and a lumped /spl lambda//4 line of two inductors and a capacitor. These duplexers have insertion loss <2 dB in the pass bands, and Tx to Rx isolation >45 dB.

A 5 mm /spl times/5 mm /spl times/1.37 mm hermetic FBAR duplexer for PCS handsets with wafer-scale packaging

We describe the design and measured performance of a 5 mm /spl times/5 mm /spl times/1.37 mm antenna duplexer for the U.S. PCS band (Tx: 1850-1910 MHz, Rx: 1930-1990 MHz) for cellular handsets based on FBAR (film acoustic resonator) technology. The FBARs are fabricated in a silicon-based IC process technology and are hermetically sealed in a wafer-level packaging process. Two dice, Tx and Rx, are attached to a 4-level printed circuit board, ball-bonded, and encapsulated in plastic to form the final product. Guaranteed worst-case Tx insertion loss is 3.5 dB, worst-case Rx insertion loss is 4.0 dB. Minimum rejection is 50/40 dB in the Tx/Rx bands, guaranteed isolation is >52/42 dB.

Manufacturability of highly doped aluminum nitride films

There have been several investigations [1], [2], [3], that demonstrated benefits of adding dopants such as (Sc) or combination of other materials, like Zr/Mg for example, to the aluminum nitride (AlN) films in order to increase coupling coefficient (kt^2) of the Bulk Acoustic Wave (BAW) devices. For concentrations below 10% atomic Sc, it is possible to use a single composite target with a standard magnetron design [4]. Most R&D systems that performed initial investigations on AlScN films with high concentration of Sc dopant, used two separate targets with two separate magnetrons: one with pure Al and one with pure Sc with different applying power to compensate for the large difference in sputtering rates of the two materials and get stoichiometric composition. Unfortunately, depositing from two different targets is only viable for low volume R&D experiments. The system described in this article uses standard dual conical magnetron with AC deposition source. Targets are cut into multiple segments as shown in Figure 1 [5]. Based on simple geometry of targets surface, deposited film composition is proportional to the surface of specific pieces of target material. Unfortunately, Al is eroded at much higher rate than Sc at the same potential and same magnetic field. Over the target life, concentration of Sc increases in the deposited films. In order to maintain same Sc composition over the entire target life, it is necessary to vary magnetic field locally over the surface of the Al and Sc pieces to provide same erosion rate of Al vs. Sc at the same target potential. Adjusting magnetic field for each segment of both Al and Sc allows for constant deposited film composition over the entire target life solves this problem.

Improving coupling coefficient distribution on BAW filters manufactured on 200mm wafers

In the past, most BAW and FBAR filters were produced on 150mm wafers. Typical yield on 150mm wafers was > 90% [1], [2]. In the last few years, most filter manufacturers started migrating to 200mm wafers in order to reduce the cost of manufacturing. Unfortunately, due to variation of coupling coefficient (kt^2) [3], [4], [5], yields on 200mm wafers were significantly lower than on 150mm wafers. Stress in the piezoelectric material is the largest variable that changes coupling coefficient [6]. Figure 1. Shows how coupling coefficient varies with stress for devices that use piezoelectric aluminum nitride (AlN) and molybdenum (Mo) electrodes. Depending on the exact processing involved in making a filter, the same AlN deposition process will produce a completely different stress/kt^2 variation across the wafer. Figure 2 illustrates a difference coupling coefficient variation across wafer, using tungsten (W) vs. Mo electrode material while leaving everything else the same. In order to be able to obtain the best coupling coefficient with different processing on 200mm wafer, it is critical to have independent control of film stress across wafer. In this paper, we will demonstrate a method of controlling coupling coefficient across wafer within less than +/-1% (total range) of the target.

Investigation of 20% scandium-doped aluminum nitride films for MEMS laterally vibrating resonators

This paper reports on the investigation of 1 μm thick films of 20% Scandium-doped Aluminum Nitride (ScAlN) for the making of piezoelectric MEMS laterally vibrating resonators (LVRs). The ScAlN films, which can be sputter-deposited such as undoped Aluminum Nitride (AlN) films, were used to demonstrate high performance resonators. These devices showed quality factor (Qs) in excess of 1000 in air centered around 250 and 500 MHz and enhanced electromechanical coupling (kt2) in the range of 3.2–4.5%. This kt2 is double the value of what has been achieved on similar resonators made out of AlN films. A 3-dB Qs of 1300 has been recorded both for 1-port and 2-port resonators at 250 and 500 MHz, while a maximum Qs of 1500 has been recorded for a 1-port resonator at 500 MHz. Along with experimental results from actual devices, this work also reports the etching characteristics of the piezoelectric material under Cl2/BCl3 chemistry to attain high selectivity and straight sidewall with a SiO2 hard mask. More broadly, enhancement of resonators design and fabrication process, suppression of spurious modes and increase in the concentration of Sc (theoretically up to 40%) will lead to significant performance improvements for many classes of piezoelectric MEMS, especially tunable filters.

AlN film stress and uniformity for BAW filters on 200mm wafers

Most standard processes only require controlling average stress on the sputter deposited films.Recently, MEMS devices have moved into high volume applications. Devices such as FBAR filters, MEMS microphones, and cantilever structures have much more sensitivity to stress variation across wafer and across the thickness of the deposited film. Sputter deposited aluminum nitride (AlN) films have polycrystalline and strongly oriented grain structure, starting with extremely compressive film at the initial few thousand angstroms thickness and becoming more tensile as grains get larger. Also most deposition systems have high variation of film stress across wafer due the magnetic field strength and electrical discharge at each racetrack not being selected for the best stress control across wafer. Because most systems have been designed with film thickness uniformity and average stress on a wafer as primary goals, stress across wafer and across film thickness is sometimes highly nonuniform. In this paper we will show how it is possible to control stress for all desired characteristics: average stress, stress uniformity across wafer and across entire film by using appropriate magnetic field design and in-situ ion beam trimming as part of the deposition process. Average stress control cross wafer is achieved by precise adjustment of magnetic field of inner and outer target ring magnetrons. Stress variation across wafer is achieved by varying the ratio of magnetic field strength between two magnetrons as shown in the paper. Stress variation across deposited films is achieved by using multistep deposition/ion mill trimming processes. As result, film stress variation across wafer and across film is maintained at less than +/-100MPa range with thickness uniformity on 200mm wafers maintained at below 0.2% one standard deviation.