Plane dispersion

Finally we remark that proper alignment of all diffractive-optic beam corrditioners normally reqrrires that they be adjusted with a common dispersion plane. This is defirred as the plane corrtaining the incident and diffracted beam (and therefore also the rrormals of the diffracting plane) for any given reflection. If the dispersion planes are rrot parallel then some intensity will be lost from the restriction this causes to the angular acceptance in the plane perpendicular to the dispersion plane. 

Tilt of the Bragg plane of the specimen, so that its dispersion plane is not parallel... 

Figure 2.22 A three-dimensional plot of the diffracted intensity as a function of specimen rotations both parallel ( ) and perpendicular ( j to the dispersion plane. The larger peak is that from the substrate GaAs and the smaller from the GaAIAs layer. CuK i, slit-limited from a single GaAs beam conditioner...

Figure 3.5 The projection of measured points onto the dispersion plane...

This measures the curvature about an axis perpendicular to the dispersion plane, i.e. the cylindrical curvature, and it may be necessary to rotate the wafer through 90° to get the orthogonal component. This may be related to absolute stress in the wafer with knowledge of the wafer thickness, diameter and elastic modulus. The most accurate method is to measure a number of points on a wafer and use a linear regression formula for the average curvature. 

The polarisation factor, C=D g. D j,. This is unity for polarisation, in which the electric field vector is perpendicular to the dispersion plane and cos(2 g) for polarisation in which the electric field vector lies in the dispersion plane. We may apply equation (4.20), which since h is real implies that Im(K o)=Im(K ), and, in order to express the solution simply, introduce the very important idea of deviation parameters, g and. These express the deviation of the incident and diffracted wavevectors from the kinematic assumption where k g = k ij =l/, and are defined as... 

Owing to the distance between somce and specimen, the divergence of the beam normal to the dispersion plane is small and with SR it is uimecessaty to perform tilt adjustments to bring the Bragg planes of specimen and beam conditioner exactly parallel. 

Fig. 15.1. Plan view (dispersive plane) schematic of a batch mode high current implanter, the Axcelis HC3...

In multiwafer configurations, which are common for high current tools, the wafers typically are mounted onto a rotating disk, which is then linearly translated across the ion beam. The rotation of the disk ensures that each wafer passes completely out of the beam in the direction of rotation. The linear translation must be at least as long as the wafer diameter plus the nominal beam diameter in that direction. Some high current configurations have the linear translation direction across the dispersive plane of the beam, while others have it across the nondispersive plane. Beam utilization efficiency (see Sect. 15.4) is directly coupled to the demands placed on this translation length. 

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