GHz Range SAW Transducers Using Electron Beam Lithography

A cta Physicae Superßcierum • Vol П • 1990

G H z R A N G E SAW T R A N S D U C E R S U S IN G E L E C T R O N BEAM L IT H O G R A P H Y

K A ZU H IK O Y A M A NO UCH I, TOSHIYASU M EC U R O and YASUO CHO

Research Institute o f Electrical Communication, Toholcu University, Sendai, 980 Japan

Ab str a c t. Nanometer electrodes of the order of 0.085 ц т width have been fabricated using direct electron beam lithography, normal lift-off techniques and 0 ,-p la sm a cleaning system. These techniques have been applied to Inter-Digital Transducers (Ш Т ) as surface acoustic wave (SAW) transducers in the GHz-range. We performed SAW fdter experiments at GHz-range. Filters with minimum insertion loss of 16.0 dB at the center frequency ofS G Hz and 27 dB at 11 G H z were obtained.

1. IN TR O D U C T IO N

Surface A coustic W aves (SAW ’s) have been applied in various electronic devices, e.g. various kinds o f filters, stable high frequency oscillators, real time signal processing devices, convolvers and correlators used in spread spectrum com m unication systems, etc.

SAW devices can be fabricated by applying integrated circuit (IC) fabrication techniques. D evices operating at higher frequencies in the G H z range having better performance and greater density o f packaging are in heavy demand for electronics devices and new com m unication systems.

W e describe SAW filters in the G H z range which require nanom eter width of interdigital electrodes (ID T ’s). W e fabricated nanom eter electrodes utilizing direct electron beam lithography with the earth electrode on a resist for dielectric materials, ordinary lift-ofT techniques and 0 2-plasma cleaning system. These techniques have been applied to the ID T in the G H z range.

2. FABRICATION O F NA N O M ETER ELECTRODES

D irect wafer electron beam writing techniques give finest line width. There are various processes for fabricating nanom eter line width and in all o f them, lift-off techniques are more successful than chemical and dry etching techniques. Lift-off techniques have properties o f no mechanical dam age on the surface in contrast to the dry etching techniques, and alm ost n o surface contam ination as compared to other etching processes.

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ashing time of a few seconds, and (iv) with Al the resist pattern and to control the ashing

Figure 1 show s the flow-chart o f our lift-ofF technique used in the fabrication o f devices o f nanometer order o f line width. An earth electrode is required for a current pass of electron beams in the direct writing for the insurater. W e reported the m ethod [ 1 ] using thin metal film (Cr) on the dielectric materials as the earth electrode. N ow we propose new techniques with thin metal film (Au) on a resist as an earth electrode (shown in Fig. 1). In this case, the dam age of the resist is smaller than before. Also, in a normal lift-off technique, the developed portions of the pat­ tern are left with som e resist on them: this remaining resist is one o f the obstacles in the formation of fine pattern. The 0 2-plasma ashing technique is used to remove the left-over resists and clean the surface of the substrate. The conditions o f ashing are as follows, (i) R F power, 250 watts, (ii) in pure 0 2 gas, (iii) holder to avoid the degeneration o f depth of PM M A .

3. SAW DEVICES

We performed experiments o f the conventional types of ID T ’s at 5 G H z and 3-pairs groups and 4-pairs groups com bined type ID T [2 ] that can suppress the TTE (Triple Transit Echo in Band), shown in Fig. 2 and 10 G H z range conventional ID T. Figure 3 shows one o f the ID T patterns used in the experiments. The pattern o f this filter is fabricated by direct writing o f electron exposure (minimum beam spot size o f 0.02 ц т of beam) on PM M A resist using old lift-ofT techniques [1 ] with 0 2-pIasma ashing on 128 Y-ЛТ L iN b 0 3. The electrode widths and gaps are 0.187 ц т , as shown in Figure 3. Figure 4 shows the configuration o f 10 G H z conventional IDT. The ID T is fabricated by new lift-off techniques shown in Fig. 1 and minimum spot size of 0.008 ц т o f beams. Substrates used for the filter are also 128° rotated У-cut X -propagating L iN b O j. Figure 5 shows the frequency response o f the filter using 3 -4 pair ID T at the

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Fig. 6. SEM photo of 10 GHz ID T (0.085 цт).

center frequency o f 5.0 G H z with a line and space o f 0.187 |im and the electrode film thickness o f 500 Â with pair number of 40. The minimum insertion loss is about 16 dB with alm ost zero o f pass band ripples, because o f TTE suppression type of ID T . Figure 6 shows SEM pattern o f the 10 G H z ID T. The line widths and spaces are 0.085 ц т , respectively. Figure 7 shows the frequency response o f 10 G H z filter after removing signals o f direct through and TTE using a time dom ain

measurement system. The insertion loss is about 27 dB at 11.2 G H z. The lack o f signals o f the response below center frequency o f 11.2 G H z is caused by the reflection due to mass loading effects o f electrodes. T hese defects are removed by using 3 -4 com bined ID T [ 2 ] or unidirectional ID T or îarrow eap I.D .T [ 3 ] .

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Fig. 7. Frequency response o f 10 G Hz SAW filter.

C O NC LUSION

W e fabricated ID T electrodes of widths and spaces o f 0.085 ц т . These nanom eter electrodes were applied for SAW filters of real devices first proposed in this paper. N o w we are investigating the generation o f surface phonon using nanom eter ID T .

REFERENCES

[1 ] K. Yamanouchi, Z.H. Chen and T. Meguro, “GHz-range step-type unidirectional low-loss filters and 5 GHz-range conventional I.D.T. filters”, IEEE Ultrasonics Symposium Proceedings, Oct. 1986, pp. 81-84.

[2 ] H. Sato, K. Yamanouchi and K. Shibayama, Acoust. Soc. Jpn., Spring Meeting, 2-3-11 May 1975.

[3 ] K. Yamanouchi, T. Meguro and Z.H. Chen, “New surface acoustic wave interdigital transducers with narrow electrode gaps”, IEEE Ultrasonics Symposium(to be published), Oct. 1988.