Introduction

Due to the resolution, stability and light element perfor-mance of the SD EDS detector, there is increased pressure on EDS analysis to perform measurement on nanostruc-tures in SEM and TEM. This has brought on a discussion about using larger detector areas due to a number of issues related to nanoanalysis – low photon yields. However, what is far more important in this discussion is solid angle. Solid angle is the measurement which captures the actual number of photons that can be collected, or in a simpler way can be imagined as the angle of a slice of (round) cake.

In comparison, talking about detector area is like talking about the length of the “crust” of the cake. If the cake has a large diameter, the share of cake we get becomes smaller and inversely if the diameter gets smaller the share gets larger. However, using the slice angle (in case of the detector this is the solid angle) will always give the same percentage of cake, regardless of the diameter of the cake. The diameter of the cake is important, as it determines how much of the cake we actually get. With an EDS detec-tor, the diameter of the cake is equivalent of how close we get to the sample, which is determined by the detector finger size. Hence the total volume of cake is a combination of both diameter and solid angle.

Detector solid angle Ω

In an electron microscope and a given material, the count rate of an EDS detector can be achieved by roughly 2 fac-tors. These are probe current and solid angle. The simplest way to increase the count rate is by increasing the probe current of the microscope. This leads to larger spot size and hence deteriorates resolution. This method can also damage sensitive samples and is obviously not suitable for nano-analysis due to the poor resolution. The second option for optimization is increasing the solid angle.

How can this be achieved? Two ways are possible: 1. Increasing the active area of the detector, solid angle increases linearly with area (see figure below).

2. Moving the detector closer to the sample (X-ray source), this increases solid angle by the square law as distance decreases (see figure below).

Technical Information Note EDS #1Solid angle and EDS detector areaBackground information and solid angle optimization

Increasing detector active area

D is the distance between sample and detector, A the active area.

Increasing solid angle

Real world limitations

What are the limitations in reality? What are the factors that stop us from having an unimaginably large solid angle? With a few exceptions and special modifications to either spec-trometer or microscope, the limitations are based on the requirements or limits set by the microscope manufacturer.

1. Limitations of using larger area: � Only a number of chip sizes for SDDs with a maximum

around 150 mm² are available. � Larger chips mean a larger detector finger is required –

this limits how big a detector can be used at a particular sample to detector distance due to the pole piece.

� The finger diameter can be optimized for one large size, but using the same finger for smaller chips will limit the solid angle for the smaller chips.

� The performance of large single chips is poorer than that of small chips.

2. Limitations on moving closer: � For a given finger size, the detector can only get as close

as the take-off-angle, analytical working distance and pole piece allow – the larger the finger the further away the detector will be (see also figure „comparing finger sizes“).

� with some SEM manufacturers there are restrictions on moving below pole piece line – this means that even with a very small finger, the detector may not go any closer if it will be lower than the pole piece (see also figure „movement restrictions“).

The variable Z adapter

Bruker’s exclusive variable Z (VZ) feature assures that the detector can always be as close to the specimen as pos-sible to maximize count rate.

An unique mechanical design for tilting the detector in the vertical direction (Z) makes it possible to track samples over a range of working distances and to move the detector closer to the sample, past the pole piece, even when the chamber is under vacuum.

VZ gives an extra degree of freedom for positioning the detector. With VZ it is possible to truly optimize sample-to-detector geometry or adjust the EDS detector to accom-modate additional detectors, like backscatter or EBSD detectors.

Comparing finger sizes

Two detector finger sizes: the blue finger is large, the orange finger is small. TOA is the take-off angle, A.W.D the analytical working distance (also Z).

Movement restrictions

The red area shows the allowable area for detector movement, if positioning below the pole piece line is not permitted.

Function of the variable Z adapter

This figure shows a close-up of the Z range VZ can cover. The actual range is dependent on the microscope.

Conclusions

The discussion points hope to clarify that it is necessary to look at all factors when trying to get more counts from a lower beam current. Using larger areas is an easy but too simple a way to collect more X-rays, as its efficiency or increase in counts is limited by the diameter and length of the finger.

Sometimes an equal or higher solid angle (and hence count rate) can be achieved with a smaller area detector housed in a smaller finger. This is a question of geometry and require-ments set by the microscope manufacturer.

Moreover, improvement of count rates can be achieved by using the variable Z adapter, to tilt and move the detector into an optimum position.

Note: The “slice of cake” described in the introduction is more

of a cone-shaped volume with a spherically shaped bottom on it.

However, detectors are not spherically shaped, so the solid angle is

an approximation with a very small error. The formula used for solid

angle is Ω = active area / distance² sr.

The variable Z interface

The variable Z interface, the adjust knob is positioned on the lower section of the interface. The flange features a vacuum tight ball joint.

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