This letter describes the procedure to manufacture

high-performance surface acoustic wave (SAW) resonators on

AlN/diamond heterostructures working at frequencies beyond

10 GHz. In the design of SAW devices on AlN/diamond systems,

the thickness of the piezoelectric layer is a key parameter. The inﬂuence of the ﬁlm thickness on the SAW device response has been studied. Optimized thin ﬁlms combined with advanced e-beam lithographic techniques have allowed the fabrication of one-port SAW resonators with ﬁnger width and pitch of 200 nm operating in the 10–14 GHz range with up to 36 dB out-of-band rejection

Brink, Dietmar

Calle Gómez, Fernando

Fuentes Iriarte, Gonzalo

Pedrós Ayala, Jorge

Rodriguez Madrid, Juan

Williams, Oliver A.

This letter describes the procedure to manufacture high-performance surface acoustic wave (SAW) resonators on AlN/diamond heterostructures working at frequencies beyond 10 GHz. In the design of SAW devices on AlN/diamond systems, the thickness of the piezoelectric layer is a key parameter. The influence of the film thickness on the SAW device response has been studied. Optimized thin films combined with advanced e-beam lithographic techniques have allowed the fabrication of one-port SAW resonators with finger width and pitch of 200 nm operating in the 10-14 GHz range with up to 36 dB out-of-band rejection

Rodriguez-Madrid, J. G.

Iriarte, G. F.

Pedros, J.

Williams, Oliver Aneurin

Brink, D.

Calle, F.

Online Research @ Cardiff

Super-High-Frequency SAW Resonators on AlN/Diamond

J. G. Rodriguez-Madrid

G. F. Iriarte

J. Pedros

O. A. Williams

D. Brink

F. Calle

Crossref

IEEE Electron Device Letters

Servicio de Coordinación de Bibliotecas de la Universidad Politécnica de Madrid

Super-High-Frequency SAW Resonators 
on AIN/Diamond 
J. G. Rodríguez-Madrid, G. F. Marte, J. Pedros, O. A. Williams, D. Brink, and F. Calle 
Abstract—This letter describes the procedure to manufacture 
high-performance surface acoustic wave (SAW) resonators on 
AlN/diamond heterostructures working at frequencies beyond 
10 GHz. In the design of SAW devices on AlN/diamond systems, 
the thickness of the piezoelectric layer is a key parameter. The in-
fluence of the film thickness on the SAW device response has been 
studied. Optimized thin films combined with advanced e-beam 
lithographic techniques have allowed the fabrication of one-port 
SAW resonators with finger width and pitch of 200 nm operating 
in the 10-14 GHz range with up to 36 dB out-of-band rejection. 
Index Terms—AlN/diamond, surface acoustic wave (SAW) 
resonator, super-high-frequency band, thickness influence. 
I. INTRODUCTION 
T HE INCREASING volume of information in data trans-mission systems results in a growing demand of applica-
tions working in the super-high-frequency band (3-30 GHz). 
Also, in gas sensor applications, the sensitivity is proportional 
with the square of the resonance frequency. In order to increase 
the frequency, two approaches may be followed: 1) to improve 
lithography resolution and 2) to exploit confined modes in 
proper layer-substrate combinations (in particular, slow-on-fast 
structures). In this letter, both approaches have been done using 
the high sound velocity of some modes in AlN/diamond [1], 
[2] and submicrometer interdigital transducer (IDT) periods 
[3], [4]. The AlN/diamond system has attracted considerable 
interest recently not only for the combination of the strong 
piezoelectricity of A1N with the SAW velocity of diamond (of 
about 12 000 m/s [5], the largest of all materials) but also for 
its suitability for harsh environments [6]. In such a layered 
system, the electromechanical coupling coefficient (K2) and 
the propagation velocity v of the acoustic waves depend on the 
film-thickness-to-wavelength ratio (H/X). At a small H/X, the 
SAW extends far into the underlying solid, and v approaches 
the large velocity value of the bare substrate, although K2 is 
reduced due to the nonpiezoelectricity of diamond. Conversely, 
at a large H/X, the SAW is mostly concentrated in the film 
region. Thus, K2 is large, but v is limited by the smaller 
velocity value of the A1N overlayer. In addition to the intrinsic 
limitations imposed by the dispersion of the material system, 
the crystal quality and orientation of the sputtered A1N thin 
films, and thus their piezoelectricity, are limited by the film 
thickness [7], [8]. Moreover, the insulating character of A1N 
makes the e-beam patterning of the submicrometer IDTs more 
difficult. Therefore, the design of high-frequency and low-loss 
AIN/diamond-based SAW devices requires careful selection 
of the film thickness, which should be a tradeoff among the 
different aspects mentioned earlier. 
In this letter, we report on the influence of the film thick-
ness on the response of super-high-frequency SAW devices 
fabricated on AlN/diamond systems using nanometer-scale 
IDTs. One-port SAW resonators working in the 10-14-GHz 
regime with good out-of-band rejection have been developed 
on tailored AlN/diamond structures. These frequencies are the 
highest obtained for this material system and almost double the 
maximum value of 7.4 GHz reported so far [5], [9], [10]. 
II. EXPERIMENTAL 
A1N Alms with thickness ranging from 150 nm to 1.2 ¡j,ra 
were deposited in a home-built balanced magnetron sputter 
deposition system on polished microcrystalline diamond sub-
strates. The specific parameters used for the deposition of 
very thin (0002)-textured A1N Alms are described elsewhere 
[7], [H].Fifty-nanometer-thickAluminiumIDTs and reflecting 
gratings were fabricated by e-beam lithography. The nominal 
period and metallization ratio were 800 nm and 0.5, respec-
tively, i.e., finger width and pitch of 200 nm. The number of 
Anger pairs was 100 for both the IDT and the reAector, whose 
Anger length was 6.2 ¡j,ra. The dimensions of the nanometer-
scale patterns were conArmed by scanning electron microscopy 
(SEM) and atomic force microscopy (AFM) analysis, as de-
picted in Fig. 1. 
III. RESULTS AND DISCUSSION 
In order to study the inAuence of the piezoelectric Aim 
thickness on the SAW response, one-port SAW resonators with 
a period of 800 nm were manufactured using 150-, 300-, 600-, 
and 1200-nm-thick A1N Alms on diamond (kH = 2-KH/X = 
1.18, 2.36, 4.71, and 9.42, respectively). 
Fig. 1. (a) SEM and (b) AFM images of a A = 800 nm IDT on AIN/diamond. 
The quality of the AIN film, and hence its piezoelectricity, 
is thickness-dependent. The compromise between selecting the 
right thickness while maintaining the crystallographic quality 
of the piezoelectric film is evident with the experimental data 
presented in this letter. Thus, for an optimal design structure 
(IDT/piezoelectric fllm/diamond substrate), the kH providing 
a mode with the best combination of v and K2 is selected. This 
choice implies a compromise between the frequency and the 
intensity of the targeted resonances, i.e., a larger frequency can 
be achieved at the cost of larger losses or vice versa due to the 
K2 behavior discussed later. Once kH is chosen and since / = 
v/X, the smallest A that can be reliably patterned is selected. 
This also requires a film thickness large enough to ensure a 
good crystallographic quality. Since, as shown elsewhere [11], 
this is a limiting factor, a compromise should be made again 
between the achievable frequency and intensity (loss) of the 
targeted resonances. 
Rayleigh and Sezawa (confined) modes are presented in 
similar structures as the one presented here due to the higher 
v of the substrate in comparison with the v of the piezoelectric 
film [12], [13]. Fig. 2 shows the reflection spectra of the fabri-
cated devices. For the SAW resonators on the 150-, 300-, and 
600-nm-thick AIN Alms, the main resonance corresponds to 
the first-order Sezawa mode (labeled as S\) at 14.0, 12.6, and 
10.9 GHz, respectively. However, for the SAW device on the 
1200-nm-thick film, the main resonance (at 11.5 GHz) corre-
sponds to the second Sezawa mode (S2). These frequencies 
are much higher than the largest value of 7.4 GHz obtained 
for this material system so far [10]. In addition, the quality 
factor Q and the out-of-band rejection for the 5i mode in the 
300-nm-thick film are 12 560 and 36.5 dB, respectively. These 
values are among the largest reported for a SAW resonator [9]. 
The identification of the acoustic modes is based on the 
numerical calculation of the dispersion of their velocity us-
ing a method based on the Green's function formalism [14], 
which was developed to calculate surface Brillouin spectra. In 
addition, the theoretical values of K2 were calculated using 
the transfer matrix approach [15]. The material parameters 
used are listed elsewhere [16]. Fig. 3 shows the calculated 
dispersion of the SAWs and pseudoSAWs propagating in 
the AlN(0001)/diamond(lll) heterostructure. The spectrum of 
SAWs comprises the Rayleigh (R) and the Sezawa (5¿) modes 
that propagate with velocities below that of the transverse bulk 
wave in the substrate [12]. The gray scale represents the magni-
tude of the shear vertical component of the waves. In this scale, 
brighter areas indicate larger magnitude. The experimental data, 
0 
-10 
-20 
-30 
-*0 
0-
-10 
-20 
-30 
to 
~ -40 
s, v^ 
(a) 1 2 0 0 n m 
w 0' 
-10 
-20 
-30 
-40 
(b) 
X ^ S3 
600 nm 
(c) 
Y~~ 
s, 
300 n m 
¿L 0 
-10 
-20 (d) 
-30 
|s. 
• 1 5 0 n m 
10 11 12 13 
Frequency (GHz) 
14 15 
Fig. 2. Reflection coefficient (¿>ii) spectra for identical (A = 800 nm) one-
port SAW resonators on (a) 1200-, (b) 600-, (c) 300-, and (d) 150-nm-thick 
AIN films on diamond. R and Si denote the Rayleigh and Sezawa modes, 
respectively. 
18000-
16000-
- J 14000-
i 
j f r 12000-
O 
O 
¡S 10000-
8000-
6000-
\ S 
1 
t 
PB \ 
f 1 • 
\ ; 
* • * ; -
PB, \ N vN 
• . -••• •••• : • 
- - + - - -Ife -^* * * + * + , ,-*.*.. -«V-
 v 
*•. " . 
* > ' * * . • * . 
V R - - . " «• _ 
r n * i N 
'ST AIN 
kH 
Fig. 3. Dispersion of the velocity of the Rayleigh (_R), Sezawa (Si), and 
pseudobulk (PBi) modes on AIN/diamond. The gray-scale traces correspond 
to the calculated modes, whereas the experimental data are represented by white 
solid stars. 
obtained from the resonance frequencies in Fig. 2, are indicated 
by white solid stars. 
The velocity of the modes decreases from kH = 0 (dia-
mond) to kH = ex) (AIN) indicating that the thickness of the 
piezoelectric film needs to be reduced in order to obtain higher 
frequencies. However, the dispersion of K2 is not a monotonic 
function of kH, as shown elsewhere [9], [17]. Fig. 4 shows the 
1,2-
0,9 
0,6 
0,3 
0,0 
i 
t 
t 
t 
1 
• 
i 
f 
1 
t ! 
I 
1 
' ' 
^ 
^ 
^ 
' 
— R 
• • * - - S 2 
s 3 
_ 
-
4 
10 
kH 
Fig. 4. Calculated electromechanical coupling coefficient (K2) for the 
Rayleigh (R) and Sezawa (Si) modes as a function of the normalized piezo-
electric film thickness on the AIN/diamond structure (kH). The wavelength is 
800 nm in all the cases. 
calculated K2 for the different kH studied here. The best K2 
value for the 5i mode (1.21%) is obtained for the 300-nm-thick 
AIN film (kH = 2.36), in agreement with previous calculations 
[9], [17]. On the other hand, from the point of view of the 
film quality, the degree of the c-axis orientation of the AIN 
film, and thus its piezoelectricity, decreases as its thickness is 
reduced [11]. According to our results and the results of [18] 
on sputtered AIN, the full-width at half-maximum (FWHM) of 
the (0002)-rocking curve must typically be smaller than 4° in 
order to yield a strong piezoelectric response. Therefore, the 
300-nm-thick AIN film (kH = 2.36) corresponds to the opti-
mum case, where a very high frequency is achieved while 
keeping a maximum K2 for the S\ mode and an FWHM of the 
AIN layer just below 4° [11], both conditions ensuring low loss. 
IV. CONCLUSION 
AIN films of different thickness have been deposited by 
reactive sputtering on diamond substrates in order to fabricate 
tailored SAW resonators for high-frequency applications. We 
have demonstrated that for very-high-frequency (12-14 GHz) 
and low-loss SAW devices, the film thickness should be kept 
well below the SAW wavelength (kH = 1.2-2.4) but has to 
be large enough to ensure a rocking-curve FWHM value of 
the A1N(0002) reflection of at least 4° as measured by X-ray 
diffraction. In particular, for a A of 800 nm, the optimum AIN 
layer thickness is 300 nm. This structure provides an Si mode 
operating at 12.6 GHz with a Q factor of 12 560 and an out-of-
band rejection of 36.5 dB. The frequency of this mode has been 
shifted to 14.0 GHz using a 150-nm-fhick AIN film, although 
the reduced crystal quality and K2 yield lower Q factor and 
out-of-band rejection values. 
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Super-high-frequency SAW resonators on AlN/Diamond

This letter describes the procedure to manufacture high-performance surface acoustic wave (SAW) resonators on AlN/diamond  heterostructures working at frequencies beyond 10 GHz. In the design of SAW devices on AlN/diamond systems, the thickness of the piezoelectric layer is a key parameter. The inﬂuence of the ﬁlm thickness on the SAW device response has been studied. Optimized thin ﬁlms combined with advanced e-beam lithographic techniques have allowed the fabrication of one-port SAW resonators with ﬁnger width and pitch of 200 nm operating in the 10–14 GHz range with up to 36 dB out-of-band rejection

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Super-high-frequency SAW resonators on AlN/Diamond

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