Estimates of the total capacitive load

The capacitive load on the front-end is expected to change due to the presence of a ground plane close to the strips. The capacitance model, only including contributions from nearest-neighbours, is seen in the figure below.

The capacitive load is the sum of all capacitance to (AC) ground and is in the absense of a ground plane dominated by the capacitance to the nearest neighbours, C_ss in the figure. There is a small contribution from the capacitance to the backplane, C_sBP in the figure. For post-processed sensors there is a contribution from the capacitance to the ground plane, C_sG in the figure. Hence the following sum is a good estimate of the total capacitance:

C_tot = 2*C_ss + C_sG + C_sBP

These quantities depend on the thickness of the BCB layer between the sensor and the ground plane, and the configuration of the ground planes. These quantities are measured on the following page:

• The inter-strip capacitace C_is = 2* C_ss for the different configurations. Since the values are obtained by a four-wire measurement, all stray capacitance to ground is cancelled. Hence the value measured is the pure C_is.
• The capacitance to the ground plane C_sG is measured between the bias rail and the ground plane, and the value is divided by the number of strips.
• The capacitance to the back plane C_sBP is measured between the bias rail and the back plane (from C/V measurements) and divided by the number of strips.

The results for the different configurations can be summarised in the following table (C_sBP = 0.2 pF for all sensors). The thickness values in the table (6 and 12 um) refers to the thickness of the BCB layer below the ground plane.

The values marked with a * are not directly measured, they are interpolated estimates.

-- LarsEklund - 2010-06-28