Mpact I Onisation - Oxford Instruments ANDOR Newton CCD Hardware Manual

Newton
1 Exposure to light causes a pattern of charge (an electronic image) to build up on the frame (or Image Area) of the EMCCD-
chip.
2 Charge in the frame is shifted vertically by one row, so that the bottom row of charge moves into the shift register.
3 Charge in the shift register is moved horizontally by one pixel, so that charge on the endmost pixel of the shift register is
moved into the Gain register.
4 Charge is shifted into the output node of the amplifier.
5 The charge in the output node of the amplifier is passed to the analog-to-digital converter and is read out.
6 Steps 3 and 4 are repeated until the shift register is emptied of charge.
7 The frame is shifted vertically again, so that the next row of charge moves down into the shift register. The process is
repeated from Step 3 until the whole frame is read out.
6.1.2 i i
Mpact onisation
One of the electrodes (phases) in the Gain Register is replaced with two electrodes, the first is held at a fixed potential
and the second is clocked as normal, except that much higher voltages (between 40V and 60V amplitude) are used than
are necessary for charge transfer alone. The large electric field generated between the fixed voltage electrode and the
clocked electrode, is sufficiently high for the electrons to cause impact ionisation as they transfer.
The impact ionisation causes the generation of new electrons, i.e. multiplication or Gain. The multiplication per
transfer is actually quite small, only around x1.01 to x1.015 maximum, but when done over a large number of transfers
substantial gain is achieved. By inserting the electron amplification stage before the output amplifier the signal may be
increased above the readout noise, hence effectively reducing the readout noise.
The shift register runs below and parallel to the light collecting rows. It has the same number of pixels as a light-
collecting row, but is masked, to prevent light from falling on it. The gain register is also masked. When light falls on
an element, electrons (photoelectrons) are produced and, in normal operation, these electrons are confined to their
respective elements. Thus, if an image (or any light pattern) is projected on to the array, a corresponding charge pattern
will be produced. To capture the image pattern into computer memory, the charge pattern must be transferred off the
chip, and this is accomplished by making use of a series of horizontal (i.e. parallel to the rows/shift register) transparent
electrodes that cover the array.
By suitable 'clocking', these electrodes can be used to shift (transfer) the entire charge pattern, one row at a time, down
into the shift register. The shift register also has a series of electrodes (which are vertical, i.e. parallel to the columns)
that are used to transfer the charge packets, one element at a time, into the output node of the 'on-chip' amplifier. The
Version 1.4 rev 13 Oct 2017
Figure 5: Gain Register Operation
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