Stigmator

A Stigmator is a variable strength Cylinder lens used to correct Astigmatism in Electron Microscopes, proposed by Otto Scherzer and first implemented in the AEG / Zeiss EM8 by O. Rang in 1948. This device greatly increased the resolving power of the Electron Microscope and reduced the necessity for frequent cleaning of the optics. The device corrected the inevitable asymmetric shape of the lens field caused by manufacturing the optics, and the charging based parasitic lenses caused by contamination buildup in the optics and beampipes. There are many ways a Stigmator can be created, these include Magnetic and Electric types, the first practical Stigmator, as designed by Rang was Electrostatic.

At the time of its first implementation, the EM8 was the highest resolution Electron Microscope world wide, the Stigmator came standard in the subsequent EM8 versions sold. Microscopes like the ÜM 100 from Siemens where retrofitted with Magnetic Stigmators of the Helical Moving Iron type, and where standard in later Iteration of the ÜM 100. Ever since, every Electron Microscope came standard with at least 1 Stigmator for the Objective Lens.

The Stigmator field has polar coordinates, and controls for such where historically in such, these being Amplitude and Angle. This was historically done by use of a Sin / Cos Potentiometer for the angle, and a linear potentiometer for the amplitude. Other machines like the Zeiss EM 10 featured 2 linear potentiometers controlling the strength of each Quadrupole separately. Most modern machines feature the XY implementation of the Stigmator control.

Types

Electrostatic Stigmator

Schematic principle Electrostatic Stigmator designed by O. Rang as seen in the AEG Zeiss EM8

This Stigmator is the type originally designed by O. Rang and has 3 sets of electrodes arranged radially around a central bore. 1 set has a span of 90° and 2 sets have a span of 45°. The 90° segments are usually connected to ground, and the 2 sets of 45° segments connected to bipolar power supplies, changing the polarity and the voltage of these segment sets allows the creation of a continually rotatable and amplitude adjustable electrostatic cylinder lens. This type of Stigmator was first implemented in the AEG / Zeiss EM8 which allowed this machine to reach the highest resolution achieved at the time.

Moving Iron Stigmator

2 Pol Piece

This Stigmator type, works by moving 2 iron Pieces mounted opposite to one another on a brass bevel gear, and is mounted in the pol Piece gap of a magnetic pol Piece lens, The iron pieces create an asymmetric magnetic circuit geometry, which allows the correction of asymmetric magnetic fields in the lens system. Changing the strength of the Stigmator is either not possible, or more typically is done by changing the distance of the iron pieces to the pol Piece gap, either in height or in radius. Typically this type of Stigmator has a single control that turns this setup through 360° while changing the strength as rotation Control, usually having an indicating Gauge as to its strength.

This setup dose not allow perfect correction of astigmatism, as the Angle and Amplitude are coupled. Example: Siemens ÜM 100 (Objective), Siemens Elmiskop I (Condenser)

4 Pol Piece

Structure and Operating Principle of the 4 Pol Piece Stigmator as seen in the Siemens Elmiskop I and IA

In the 4 Pol Piece setup, as featured in the Siemens Elmiskop I and Siemens Elmiskop IA, 2 sets of 2 iron pol pieces are mounted on 2 bevel gears, mechanically coupled in such a way, that both can turn together, allowing angle, and both running opposite to one another, thus allowing amplitude control. This setup is placed like the 2 Pol Piece setup, inside the magnetic circuit of the Objective lens (or other lens), where like the 2 pol piece, it is supplied by the flux of that lens. Maximum Stigmator power is present when both sets of pol pieces are in the same axis, and minimum Stigmator power is present when both sets are at 90° to one another.

This type of Stigmator has the disadvantage of requiring high precision in its manufacture to allow 0 Stigmator field.

Quadrupole Stigmator

The Quadrupole Stigmator of the BS242 showing its 4 pols (A,B,C,D) mounted on a azimuth bearing assembly controlled by a rod (E) going to an angle scales scribed on the outside of the column.

In a Quadrupole Stigmator, a single Quadrupole lens is used as the correcting element, this lens has 2 north and 2 south poles each opposing one another and oriented at 90° to each other, forming a cross shaped structure. This lens Focuses in 1 axis, and defocuses in the 90° opposing axis, thus allowing correction of astigmatism. Since this Aberration is rarely in the exact axis of the Quadrupole, means of rotating this lens are needed. Machines with such a Stigmator have a mechanical Angle control, usually allowing 90° of Rotation, and an electrical control for the field strength. A notable Example of this Stigmator type is the Tesla BS 242.

Dual Quadrupole Stigmator

In the Double Quadrupole Stigmator, 2 Quadropoles offset by 45° to one another are used to create an electrically rotatable and amplitude adjustable Cylinder lens. This setup is not to be confused with an Octopole lens which also features 8 pole Pieces. In this setup the Quadrupole field is formed by having 2 sets of poles at various ratios of power next to one another. Thus allowing full 360° rotation of the field. These 2 interleaved Quadrupoles are controlled either by a Sin / Cos Potentiometer for Angle, and a Linear Potentiometer for Amplitude, or by 2 Linear potentiometers for each Quadrupole. Example Zeiss EM 10