Scanning Tunneling Microscopy (STM)
Low-Current STM
Rev. B
MultiMode SPM Instruction Manual
157
9.5.2 Hardware Description
The Low Current STM head, which allows STM measurements with I
tun
in the pA range is
constructed for operation with our MultiMode and AFM bases.
Low-current STM operation also requires the Picoamp Boost Box, which is installed between the
control unit or extender box and the MultiMode or AFM base.
Figure 9.5a
Low-Current Converter components: MultiMode head and Picoamp Boost Box.
The gain is switchable between 10
10
V/A or 10
11
V/A and the bandwidth is switchable among
400Hz, 1.5kHz and 4kHz. Lower bandwidths allow for lower noise levels which are essential for
operation at sub-picoampere
.
IMPORTANT
: The default calibration value of standard STM preampli
fi
er electronics is 10 nA/V.
This value
must
be changed to either 0.1 or 0.01nA/V for operation of the low-current head with
gain factors of either 10
10
and 10
11
respectively. To changes this value, click on
Realtime
>
Microscope
>
Calibrate
>
Z
. The
Current
sensitivity
parameter is listed on the
Z Calibration
panel and can be changed by simply entering the new value.
9.5.3 Precautions
Due to the low-current head’s highly sensitive electronic components, special precautions must be
taken.
1. The input stage operational amplifier is extremely vulnerable to electrostatic discharge
(ESD) and is easily destroyed. It is recommended that the user be grounded with a wristband
at all times that the low current head is handled, especially when tips are installed.
2. The feedback resistor and capacitor in the input stage are extremely vulnerable to
contamination. At no time should these delicate components ever be touched with the hands.
Preamp Setting
Current Sensitivity Setting
10
10
0.1nA/V
10
11
0.01nA/V
I
tun
