Development of a nanostructural microwave probe based on GaAs

Transcription

1 Microsyst Technol (2008) 14: DOI /s TECHNICAL PAPER Development of a nanostructural microwave probe based on GaAs Y. Ju Æ T. Kobayashi Æ H. Soyama Received: 18 June 2007 / Accepted: 25 November 2007 / Published online: 11 December 2007 Ó Springer-Verlag 2007 Abstract In order to develop a new structural microwave probe, we studied the fabrication of an AFM probe on a GaAs wafer. A waveguide was introduced by evaporating Au film on the top and bottom surfaces of the GaAs AFM probe where a tip 7 lm high with a 2.0 aspect ratio was formed and the dimensions of the cantilever were lm. The open structure of the waveguide at the tip of the probe was obtained by FIB fabrication. An AFM image and profile analysis for a standard sample, obtained by the fabricated GaAs microwave probe and a commercial Si AFM probe, indicate that the fabricated probe has a similar capability for measurement of material topography as compared to the commercial probe. developed. Recently, microwave microscopy has been of interest to many researchers (Steinhauer et al. 1999; Duewer et al. 1999; Tabib-Azar and Akinwande 2000; Ju et al. 2001), due to its potential for the evaluation of electrical properties of materials and devices. The advantage of microwave techniques is that the response of materials is directly relative to the electromagnetic properties of the materials. In this paper, the development of a nanostructural microwave probe was demonstrated. To restrict microwave attenuation in the probe, GaAs was used as the probe substrate. The new structural microwave probe is expected to be of use for measurement of electrical properties as well as the topography of materials and devices. 1 Introduction With the development of nanotechnology, measurement of electrical properties in localized areas of materials and devices has become of greater necessity. Although many different kinds of scanning probe microscopes have been developed for satisfying the requirements of nanotechonology, a microscopy technique that can determine electrical properties in a local area has still not been Y. Ju (&) Department of Mechanical Science and Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya , Japan ju@mech.nagoya-u.ac.jp T. Kobayashi H. Soyama Department of Nanomechanics, Graduate School of Engineering, Tohoku University, Aoba , Aramaki, Aoba-ku, Sendai , Japan 2 Probe fabrication To obtain the desired structure, wet etching was used to fabricate the probe. In contrast to dry etching, a sideetching occurs under the etching mask. Utilizing this property, a cone-shaped microtip can be obtained. Early studies suggested that a square resist pattern having 14 lm sides and one side at 45 to the\011[direction was found to be the most suitable mask for etching the tip of the probe (Ju et al. 2005). In the case of a single crystalline wafer such as Si and GaAs, the chemical activities are different for different crystalline planes, and thereby, the etch rates are also different. Therefore, the side plane obtained at the side of the mask pattern is the most inactive plane (that is, the plane having the lowest etching speed), which is parallel to the side of the mask pattern. Consequently, the result of etching is strongly affected by the direction of the mask pattern (Heisig et al. 1998; Iwata et al. 2004; MacFadyen 1983). By considering the chemical activities

2 1022 Microsyst Technol (2008) 14: at different crystalline planes, the length direction of the etching mask for forming the beam of the cantilever was patterned along the \011[ direction. Consequently, sideetching occurs under the resist mask, and mesa-type planes appear at both sides of the beam (45 inclined plane). On the other hand, an inverse-mesa type plane is formed at the end of the beam (60 75 inclined plane). Under the same conditions as the beam fabrication process, a holder was formed by backside etching. The etching mask was patterned on the bottom surface, and etching was carried out until the substrate was penetrated. The process for fabricating a GaAs AFM probe was described in detail in Ju et al. (2007). Au film of 50 nm thickness was deposited on the top and bottom surfaces of the probe to propagate a microwave signal in the probe. Both plane surfaces of the waveguide, which were made of evaporated Au film, were connected at the end of the beam. However, there was no Au film on the sides of the beam, since the formed inclined planes at the beam sides were not facing the direction of evaporation. By using focused ion beam (FIB) fabrication, a slit at the tip of the probe was formed to open the connection of the Au film on the two surfaces of the probe. 3 Probe evaluation Figure 1 shows scanning electron microscopy (SEM) photography of the tip of the fabricated microwave probe based on GaAs. The tip is observed as located near the front edge of the cantilever. It is 7 lm high and has an aspect ratio of 2.0, and the width of the microslit is about 200 nm. In order to confirm the resolution of the fabricated GaAs microwave probes, the AFM topography of two grating samples having 2,000 lines/mm and 17.9 nm step height were measured by the fabricated GaAs microwave probes and commercial Si AFM probes, respectively. A JSPM was used for measurement of the sample in noncontact mode and the properties of these AFM probes are given in Table 1. The resonance frequency was swept and the Q value was defined by the following relation, Q = f 0 /(f + - f - ), where f 0 is the peak frequency, f + and f - the shifted frequency from f 0 at 70.7% of peak intensity. The Q value indicates a resonance sharpness of the cantilever; the higher the Q value, the better stabilization of the oscillation. As shown in Table 1, fabricated GaAs microwave probes C and E have a higher Q value than the commercial Si probe. 3.1 Evaluation of topographies Figures 2, 3, 4 and 5 show the non-contact mode AFM topographies of the grating sample having 2,000 lines/mm Table 1 The properties of AFM probes in the atmosphere Probe The resonance frequency (khz) Q value Microwave probe A Microwave probe B Microwave probe C Microwave probe D Microwave probe E Commercial probe (Si) GaAs probe without FIB fabrication Fig. 1 SEM photograph of the tip and the microslit of the GaAs microwave probe Fig. 2 Surface topography of the grating sample obtained by the fabricated GaAs microwave probe A

3 Microsyst Technol (2008) 14: Fig. 3 Surface topography of the grating sample obtained by the fabricated GaAs microwave probe C Fig. 5 Surface topography of the grating sample obtained by the fabricated GaAs microwave probe E Fig. 4 Surface topography of the grating sample obtained by the fabricated GaAs microwave probe D as obtained by fabricated GaAs microwave probes A, C, D and E. The scan area was lm and the white spots in these figures are due to micro-dust on the sample surface. Higher resolution topographies were obtained by probes C and E, which have the highest Q values as compared to probes A and D. However, probe B did not yield scanning performance because of a lower Q value. Fig. 6 Surface topography of the grating sample obtained by the commercial Si probe 3.2 Comparison with Si AFM probe Figure 3 shows the topography of the sample obtained by the fabricated GaAs microwave probe C, which indicates that the grating depth is nm. Similar to probe C, the other GaAs microwave probes also can obtain good AFM topography. Figure 6 shows non-contact mode AFM topography obtained by using a commercial Si cantilever,

4 1024 Microsyst Technol (2008) 14: has a similar capability for sensing surface topography of materials as that of commercial AFM probes. 3.3 Evaluation of height accuracy In order to evaluate height accuracy, a grating sample having 17.9 ± 1 nm step height was measured by using probe C and the Si probe. As shown in Fig. 7, by analyzing the profile of the sample, microwave probe C reports the step height to be nm. In contrast, measurement results using the commercial Si probe indicate the step height to be nm, as shown in Fig. 8. These results suggest that the fabricated microwave probe has a similar height evaluation capability as that of the commercial AFM probe. 4 Conclusion Fig. 7 Profile analysis using by microwave AFM probe C GaAs microwave probes were fabricated on a GaAs wafer by using a wet etching process. A waveguide was introduced on the probe by evaporating Au film on both surfaces of the probe. The open structure of the waveguide at the tip of the probe was obtained by using FIB fabrication. AFM measurements were evaluated by comparison with those of the commercial Si AFM probe. Results indicate that the GaAs microwave probe has the capability to capture AFM topography of materials and has a high accuracy for height evaluation, similar to that of the commercial AFM probe. Acknowledgments This work was supported by the Japan Society for the Promotion of Science under Grant-in-Aid for Scientific Research (S) and (A) ; Ministry of Education, Culture, Sports, Science and Technology of Japan under Grant-in-Aid Exploratory Research References Fig. 8 Profile analysis using by commercial Si AFM probe in which the grating depth is shown to be nm. Even though the Q value is lower than that of probes C and E, the commercial Si probe still can obtain a higher resolution topography due to the higher aspect ratio of the tip. These results illustrate that the tip of the GaAs microwave probe Duewer F, Gao C, Takeuchi I, Xiang XD (1999) Tip-sample distance feedback control in a scanning evanescent microwave microscope. Appl Phys Lett 74(18): Heisig S et al (1998) Monolithic gallium arsenide cantilever for scanning near-field microscopy. Ultramicroscopy 71(1 4): Iwata N, Wakayama T, Yamada S (2004) Establishment of basic process to fabricate full GaAs cantilever for scanning probe microscope applications. Sens Actuat A 111(1):26 31 Ju Y, Saka M, Abé H (2001) NDI of delamination in IC packages using millimeter-waves. IEEE T Instrum Meas 50(4): Ju Y, Sato H, Soyama H (2005) Fabrication of the tip of GaAs microwave probe by wet etching. In: Proceeding of inter- PACK2005 (CD-ROM), IPACK Ju Y, Kobayashi T, Soyama H (2007) Fabrication of a GaAs microwave probe used for atomic force microscope. In: Proceeding of interpack2007 (CD-ROM), IPACK

5 Microsyst Technol (2008) 14: Steinhauer DE et al (1999) Imaging of microwave permittivity, tenability, and damage recovery in (Ba, Sr) TiO 3 thin films. Appl Phys Lett 75(20): Tabib-Azar M, Akinwande D (2000) Real-time imaging of semiconductor space-charge regions using high-spatial resolution evanescent microwave microscope. Rev Sci Instrum 71(3): MacFadyen DN (1983) On the preferential etching of GaAs by H 2 SO 4 H 2 O 2 H 2 O. J Electrochem Soc 130(9):