2. High-speed Scanning of Atomic Force Microscope

Nakahiro Yasuda¹ Mikio Yamamoto Mieko Kurano, Kuniaki Amemiya¹ Kazuyuki Uchikawa², Hiromi Kanamaru³ Yasuhisa Yamashita³ , Hiroyuki Takahashi¹ Masaharu Nakazawa¹ and Koichi Ogura⁴ (¹Univ. of Tokyo; ²Toho Univ.; ³Toyo Corporation ; ⁴Nihon Univ.)

Keywords: CR-39, atomic force microscope, high-speed scanning

The atomic force microscope (AFM) was introduced into the measurement of etch pits on CR-39 detector for the dosimetry under the high-density irradiation condition of the heavy ion beam. In this method, an individual etch pit can be measured even under the condition of 10⁸ ions/cm² by shortening etching time. Quick analysis and/or in situ observa tion in the irradiation can be expected, since pretreatment of the surface is not needed, even for the insulator quality like the CR-39 detector However, its scanning speed is too slow, it takes about TO minutes/image. A application to fields which need statistics such as dosimetry is prevented by this slow speed.

The AFM equipped with a 125m cantilever hav ing a typical probe tip length of 10m. The sample surface is scanned over using this cantilever. The cantilever is driven by ptezoceramuc actuators, which change their dimensions according to the applied voltage. The AFM is usually operated in air in the tapping mode that has been developed as a method to achieve high resolution without inducing destructive frictional force. However, in the tapping mode, the probe tip scans by oscillating, and the probe tip does not follow a rapid change III ruggedness of the sample surface for high-speed scanning. It has been considered that the measuring speed for obtaining a clear image is limited to about 10 minutes/image (scan rate = 1Hz) using the tapping mode.

Using the AFM (D3100: Digital Instruments) in the contact mode, we examined the effect on the image at high-speed scanning. In the contact mode, the probe tip is simply dragged across the surface. The dragging motion of the probe tip, combined with adhesive forces between the tip and the surface, can cause substantial damage to both sample and probe tip and create artifacts in image data. This scanntng method, however, may be able to withstand high-speed operation better than the tapping mode, since the sample always has contact with the probe tip The CR-39 sample was irradiated by 290 MeV/n C ions from HIMAC and etched for 40 minutes under the conditions of 7N Na0H solution and 7OC. The sample was measured over the area of 1O X 10m² which consisted of 512 X 512 pixels. Scanning speed was varied from 512 (scan rate = 1Hz) to 9 (58Hz) seconds/image. The piezo scanner was calibrated using the grating (1O.O0.3m pitch and 18O 5.4nm) at the every scanning speed. Figure 2 shows the image of the CR-39 surface with etch pits at the different scanning speeds (512, 17 and 9 s). The striped pattern (artifacts) that originally did not exist is observed, as the scanning rate is quickened. The diameter of etch pit is reduced in the scanning direction (the X direction in this case). The vertical for the scanning direction shows hardly any effect from the high-speed measurement. Since the probe tip does not follow the depth of an etch pit, the depth direction is obtained as an image which is shallower than the real shape. It is considered that the 17 s/ image (scan rate = 3OHz) is acceptable for the etch pit measurement. This scanning rate is almost equivalent to that of a scanning electron microscope. Recently, research on high-speed scanning is being widely carried out looking at the following two challenges: 1) a method with simultaneous scanning using multiple probes to increase the scanning area per time; 2) a method with feed back signal processing to the piezoactuator taking out information which is not easily affected by the scanning rate. By introducing the latter method, we are trying to construct the system that allows even higher measurement speed and the application of in vivo dosimetry for routine clinical usage of heavy ions and/or protons as radiation therapy modalities.

Fig.2. AFM images of CR-39 surface with etch pits. Scan rates are 1Hz(left), 30Hz(middle) and 50hz(right).

Publication :
K. Amemiya, H. Takahashi, M. Nakazawa, N Yasuda, M. Yamamoto, Y. Nakagawa, T. Kageji, M. Nakaichi, K. Ogura: Nucl. Inst. and Meth., B159 75-80 1999.