Tip and cantilever

In the early years of AFM operation, the cantilevers were cut from a metal foil, and the tips were made from crushed diamond particles, picked up by a piece of eyebrow hair, and painstakingly glued manually on the cantilevers. This situation has changed completely since the methods for mass production of cantilevers with integrated tips were developed. A review of various methods for making cantilevers using standard microfabrication techniques was published by Albrecht et al. (1990), and an improved method is described by Akamine et al. (1990). Those AFM cantilevers with integrated tips are now available commercially. 

In this section, the method of making cantilevers with integrated tips of Si3N4 is briefly described (Albrecht et al., 1990). The process starts with a Si(IOO) wafer with a thermally grown Si02 layer, as shown in Fig. 15.2. The steps are as follows  

Using a photolithographic method, etch a square opening on the Si02 film. 

14 For example. Park Scientific Instruments, 1171 Borregas Ave., Sunnyvale, California 94089 Digital Instruments, Inc.. 6780 Cortona Drive, Santa Barbara, CA 93117. 

Use KOH solution to etch the part of the silicon wafer exposed through the square opening. The etch self-terminates at the Si(lll) planes, and a pyramidal pit is formed. 

---

In analogy to indentation experiments, measurements of the lateral contact stiffness were used for determining the contact radius [114]. For achieving this, the finite stiffness of tip and cantilever have to be taken into account, which imposes considerable calibration issues. The lateral stiffness of the tip was determined by means of a finite element simulation [143]. As noted by Dedkov [95], the agreement of the experimental friction-load curves of Carpick et al. [115] with the JKR model is rather unexpected when considering the low value of the transition parameter A(0.2Further work seems to be necessary in order to clarify the limits of validity of the particular contact mechanics models, especially with regard to nanoscale contacts. 

The drawback in the experimental set-up of all electrical measurements with the SFM is mainly due to the unshielded SFM tip and cantilever, regardless whether contact or non-contact approaches are used. Interesting solutions [439] for shielding the electrode are not yet on the market. 

The restriction to mica was overcome by a relatively recent technique the atomic force microscope (AFM). sometimes also called the scanning force microscope [69. AFMs are usually used to image solid surfaces. Therefore a sharp tip at the free end of a cantilever spring is scanned over a surface. Tip and cantilever are microfabricated. While scanning, surface features move the tip up and down and thus deflect the cantilever. By measuring the deflection of the cantilever, a topographic image of the surface can be obtained. 

The sensitivity of the optical deflection detection system is easily calibrated by recording an f-d curve on a stiff substrate, e.g., a glass slide or a piece of silicon. The slope of the hard wall contact region in this photodetector - piezo displacement plot must be unity, as for the movement of 1 nm in z direction the tip and cantilever move upwards 1 nm as well. This function is typically implemented in the AFM software (Fig. 2.29). 

Fig. 3.37 Schematic of AFM operation under liquid without rubber ring. A water drop, which is spanned between liquid cell and sample, encloses the imaged area incl. tip and cantilever completely...

Second, tip and cantilever can be used for preparative aspects to form defined nanoobjects such as molecular or atomic clusters, quantum dots, etc., as well as to stmcture or modify solid state siufaces in the nanometer range. Such studies belong to the preparative aspect of nanotechnology, which is still in the beginning. 

For AFM, commercial tips and cantilevers are used whereas for STM tips are prepared in various ways. W tips are easily obtained by electrochemical etching in NaOH. They have to be isolated with wax or varnish for electrochemical in situ studies to reduce electrochemical currents at the tip to less than 0.05 nA. Small electrochemical cells made of PCTFE (polycholorotri fluoroethylene) are pressed with a Viton O-ring onto the crystal surface [142]. They contain a Pt coimterelectrode and a small reference electrode. Often a Pt pseudoreference electrode is used, which yields a reproducible potential that can be calibrated using the characteristic features of the polarization curve for a given system. A simple Pt wire may be cleaned much better and does not introduce impurities to the small electrolyte volume. With oxygen present in the electrolyte, the Pt-wire potential is in the domain of the 02/OH reaction. 

---