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Scanning Tunneling Microscopy (STM)
STM-Specific Information and Operations
Rev. D Dimension 3100 Manual 181
11.3.1 STM Hardware
Some individual STM components are described below:
1. Tip holder—To keep the mass on the end of the tube low, a simple tip holder, designed to
take 0.010" diameter tips, is used. The holder is a stainless steel tube with a 0.012" inner
diameter mounted in ceramic. This design mounts the tip at the center of the piezo tube on a
ceramic support element. This design preserves the symmetry of the scan, although it makes
it difficult to see the tip-sample interface. This design is used on all large-range, “G” and “J”:
scanners.
2. Tips— The probes for the NanoScope STM must be less than 0.012" in diameter to fit into
the tip holder. The two most commonly used tips are made from either a platinum iridium
(PtIr alloy or tungsten. The PtIr tips are mechanically formed and can be purchased directly
from Veeco. At the end of this chapter, instructions are provided on how to etch tungsten tips
from tungsten wire with an electrochemical process. In general, PtIr tips provide better
atomic resolution than tungsten tips, but tungsten tips are more uniformly shaped. They may
perform better on samples with steeply sloped features such as compact or optical disks.
3. Preamp—Mounted on the tip holder is a circuit which contains the preamplifier for the
tunneling current and provides interconnections to the tube electrodes. A schematic for this
preamp is at the end of this section.
The preamplifier is an FET input amplifier with an input bias current of 25 picoamps, small,
compared to the nanoamps or fractions of nanoamps which are being measured. The preamp is
configured such that the tunneling tip is connected through a 1 megohm resistor to ground. The tip
is also connected to the input of the amplifier which is wired as a x100 non-inverting amplifier with
a cutoff frequency of 15KHz. The transimpedance gain of the input resistor/preamp combination is
100mV/nA with an input range from 0 to 100nA. The noise of the preamp is essentially the Johnson
noise in the 1 megohm resistor and the standard filtering is 2mV rms, equivalent to an input
tunneling current of 0.02nA rms.
A disadvantage of this amplifier configuration is that there is a voltage drop across the 1 megohm
resistor which raises the voltage of the tip above ground, reducing the effective bias voltage. The
actual bias voltage is equal to:
Vsample = Vbias - Itunneling x Rinput
This effect is accounted for in the NanoScope software so the actual bias voltage between the tip
and the sample agrees with the menu value.
Special low-noise preamps have been developed for spectroscopic functions. The preamps which
are included on scan heads designated as either AI or Veeco, have no voltage drop due to the
tunneling current.
Scanning heads with four maximum scan ranges are available for NanoScope. The “G” scanner-
based, Dimension heads is designed for a nominal 90µm scan size.