K. Uehara

Fabrication of 5-GHz-band SAW filter with atomically-flat-surface AlN on sapphire

5-GHz-band surface acoustic wave (SAW) filters for mobile communications were fabricated on an atomically-flat-surface (0001)aluminum nitride/(0001)sapphire (AlN/Al/sub 2/O/sub 3/) combination. The SAW devices were fabricated using electron beam lithography and a lift-off method. Atomically-flat-surface AIN films were used to reduce SAW propagation loss. The center frequency of the fabricated SAW filter was 5.18 GHz. SAW velocity was 5688 m/s at normalized thickness by wave number (kH) of 9.9. The effective coupling coefficient was 0.1% and temperature-coefficient of delay was 9 ppm//spl deg/C at kH of 5.9. The SAW propagation loss was 0.0053 dB at 5.18 GHz. The atomically-flat-surface (0001)AlN/(0001)Al/sub 2/O/sub 3/ combination is promising for use in 5-GHz-band SAW filters for application to mobile communications.

Highly c-axis-oriented AlN film using MOCVD for 5GHz-band FBAR filter

We have successfully developed to deposit highly c-axis-oriented aluminum nitride (AlN) film using metal-organic-chemical-vapor deposition (MOCVD). Full-width at half-maximum (FWHM) of the deposited AlN(0002) film whose thickness was 1 /spl mu/m has found to be 2.98/spl deg/. The value of FWHM means the deposited film has the electromechanical coupling coefficient (K/sup 2/) of 6.4%. The conditions of deposition were substrate temperature of 1050/spl deg/C, pressure of 20Torr, and V-III ratio of 25000. Film-bulk-acoustic resonator (FBAR) band-pass filter for 5 GHz orthogonal-frequency-division multiplexing (OFDM) wireless local area network (WLAN) system has been designed using the c-axis-oriented AlN film. The designed band-pass filter has the sufficient bandwidth of more than 100 MHz, which is evaluated from butterworth-van dyke (BVD) equivalent circuit model of FBAR.

AlN epitaxial film with atomically flat surface for GHz-band SAW devices

We have successfully developed (00/spl middot/1) AlN film with atomically flat surface on (00/spl middot/1) sapphire substrate using metalorganic chemical vapor deposition (MO-CVD) method. The atomically flat surface of less than Ra=2/spl Aring/, Ra means mean roughness measured by atomic force microscope (AFM), within the thickness of 1.7 /spl mu/m has been achieved, whose conditions are high substrate temperature of 1200/spl deg/C, low pressure of 30Torr, low V-III ratio of 500 and the numerous flow rate of trimethylaluminum (TMA)-back-up H/sub 2/ gas of 5.0slm The temperature-coefficient-of-delay (TCD) of the fabricated surface-acoustic-wave (SAW) device on (00/spl middot/1)AlN/ (00/spl middot/1) Al/sub 2/O/sub 3/ combination with atomically flat surface are found to be 44.5 ppm//spl deg/C at kH=2.25 and 28.5 ppm//spl deg/C at kH=3.32, where kH is the normalized thickness by wave number, k is wave number and H is AlN film thickness. These measured TCD are agreed with simulated curve. AlN/Al/sub 2/O/sub 3/ combination with atomically flat surface has a potential for zero-TCD at kH=4.5.

Low propagation loss of atomically-flat surface aln with low dislocation density for 5-ghz band saw devices

Atomically-flat surface (0001) aluminum nitride (AlN) epitaxial films grown by metalorganic chemical vapor deposition (MOCVD) were investigated. To clarify the growth mechanism of the AlN film, dependences between growth rate and process parameters of MOCVD were evaluated. To investigate the relationship between surface roughness and dislocation, three types of dislocation were observed by transmission electron microscopy (TEM). It was found that suppression of the thermal convection was indispensable for growth of atomically-flat surface AlN. From TEM observation, periodical misfit-type dislocation was observed only within the 10- u A-thick interface between AlN and sapphire substrate. The density of edge-type dislocation in the AlN surface region was as low as 10 10 -10 11 cm −2 . Screw- type dislocation mostly stopped propagating along (0002)AlN within the thickness of 0.5 µm which agreed with the thickness of changing rough surface AlN to atomically-flat surface AlN. It is considered that suppression of screw-type dislocation was contributed to atomically-flat surface AlN.

Growth of AlN film on Mo/SiO/sub 2//Si (111) for 5 GHz-band FBAR using MOCVD

We fabricated a film-bulk-acoustic resonator with high c-axis oriented AlN film on Mo/SiO/sub 2//Si (100) using metalorganic chemical vapor deposition. The resonant frequency and anti-resonant frequency of the fabricated resonator were 3.189 GHz and 3.224 GHz, respectively. The quality factor and the effective electromechanical coupling coefficient were 24.7 and 2.65%, respectively. The conditions of AlN deposition were substrate temperature of 950/spl deg/C, pressure of 20 torr, and V-III ratio of 25000. We have successfully grown high c-axis oriented AlN film with 4/spl times/10/sup -5/ /spl Omega/cm resistivity of the Mo bottom electrode. The full width at half maximum (FWHM) of the AlN (0002) on Mo/SiO/sub 2//Si (100) and Mo/SiO/sub 2//Si (111) were 4/spl deg/ and 3.8/spl deg/, respectively. The FWHM values of the deposited AlN film satisfy the RF band pass filter specification for GHz-band wireless local area network.

AlN epitaxial film on 6H-SiC(0001) using MOCVD for GHz-band SAW devices

Aluminum nitride (AlN)(0001) epitaxial films on 6H-silicon carbide (SiC)(0001) were successfully grown using metal-organic-chemical-vapor deposition (MO-CVD). Crack-free on the surface of AlN films were obtained without preannealing for removal of polishing-induced damage from the 6H-SiC(0001) substrate. The crack-free AlN films were grown at 1100/spl deg/C under V-III ratio of 25000. A SAW device was fabricated on the AlN film using photolithography and reactive ion etching (RIE) techniques. The thickness of AlN film was 1.0 /spl mu/m. The direction of SAW propagation was a-axis of AlN. The line and space of IDT was 0.6 /spl mu/m. The center frequency was measured to be 2.747GHz. The phase velocity was calculated to be nearly equal 6600m/sec. The phase velocity in AlN(0001)/6H-SiC(0001) structure is larger than that of AlN(0001)/sapphire(0001) structure under same thickness and direction of SAW propagation.