To Next Page

To Previous Page

Page 20

iXon 3

Features & Functionality

During an acquisition using a conventional Frame Transfer CCD (FT CCD), the image area is exposed to light and an

image is captured. This image in the form of an electronic charge is then automatically shifted downwards behind the

masked region of the chip before being read out. To read out the sensor, charge is moved vertically into the readout

register, and then horizontally from the readout register into the output node of the amplier. As stated on the previous

page, the readout register is extended to include the multiplication (gain) register. The amplication occurs in this

tinybutniteprobabilitythatthechargesbeingtransferredcancreateadditionalchargebyaprocessknownas“impact

ionization”.Impactionizationoccurswhenachargehassufcientenergytocreateanotherelectron-holepairandhence

a free electron charge in the conduction band can create another charge. Hence, amplication occurs. To make this

process viable, EMCCD’s tailor the process in two ways, as follows:

1. Firstly, the probability of any one charge creating a secondary electron is increased by giving the initial electron

charge more energy. This is typically done by replacing one of the electrodes (phases) of this readout section with

two electrodes. The rst is held at a xed potential and the second is operated as normal, except that much higher

voltages are employed than are necessary for charge transfer alone. The large electric eld generated between the

xedvoltageelectrodeandtheclockedelectrodeissufcientlyhighfortheelectronstocause“impactionization”as

they transfer. The impact ionization causes the generation of new electrons, i.e. multiplication or gain.

2. Secondly, the EMCCD is designed with hundreds of cells or pixels in which impact ionization can occur and

although the probability of amplication or multiplication in any one pixel is small (only around x1.01 to x1.015 times)

over the entire length of the EM register the probability is very high and substantial gains of up to thousands can be

achieved.

Figure 12: Gain register operation

Andor Technology iXon3 Series User Manual

Questions and Answers: