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Parasitic extraction techniques for touchscreens

28 Dec 2015  | Mohamed Elrefaee

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To characterise a full screen, a small sub-array is extracted and the results are extrapolated to represent the full screen. In the 5 by 5 array shown in figure 1, each crossover point between each x and y node represents a capacitance to be measured.


Figure 1: A 5 x 5 array of capacitors representing a portion of a touchscreen display.


The extraction tool computes all the capacitances in the matrix which, if plotted in a 3D graph, looks similar to the graph in figure 2. Ideally, the plane of this matrix is very flat, which is crucial to minimising errors when the finger touch or stylus is employed. The field solver solves for the electric potential in a window that has a mesh of points. While the window size cannot be controlled by the user, the density of the points inside that window can be specified. To ensure that the flatness of the plane is indeed characteristic of the panel design, we use the highest-accuracy mode in Calibre xACT 3D to force the solver to solve in more points inside the window, yielding more accurate results.


Figure 2: Extracted capacitance values across a touchscreen matrix.


A change in the measured capacitance at the touch point is seen when a touch tool is used. There is an electromagnetic field between conductors, due to the charges accumulated on them. These field lines go from one conductor to the other. If a third conductor (like a stylus) is present, some of those lines will terminate on that new conductor. This effect is called shielding, and results in a reduced coupling capacitance between the original two conductors. When the touch tool is placed at the X3Y3 node, the capacitance at that point goes down, due to the shielding effect of the touch tool, as shown in figure 3.


Figure 3: Change in measured capacitance at touch point.


The magnitude of the dip at the touch point depends on several factors, including the stylus size and model, among others. This capacitance change is what the circuit uses to detect the stylus position. The bigger the dip, the easier it is to detect. Optimally, the capacitance should be equal (or almost equal) over the entire array, except for the touch point and its neighbouring points. The biggest dip should be seen at the touch point, with smaller dips around the near points. A bad result would be when the designer sees no change in the capacitance at the touch point, or a very small change that cannot be detected by circuitry. One way to improve unacceptable results is by changing how the stylus is modelled.

Touchscreens are becoming a ubiquitous part of the electronics landscape. With the right tools and the right techniques, designers can ensure that their touchscreen designs will deliver the performance the market requires, while also ensuring they deliver their products to that market in a timely manner.


References
[1] "Touch Panel Industry and Cost Analysis Report-2014," IHS Technology.

[2] "Comparison of BEM with FEM," Integrated Engineering Software, 2014.

[3] Schrik, E., van der Meijs, N.P. "Combined BEM/FEM vs. 3DFEM Substrate Resistance Modeling,"

[4] Schrik, E., van Genderen, A.J., van der Meijs, N. P. "Combined BEM/FEM Resistance Modeling of Stratified Substrates with Layout-Dependent Doping Patterns in the Top Layer,"


About the author
Mohamed Elrefaee is a Technical Marketing Engineer in the Design to Silicon Division of Mentor Graphics in Cairo, Egypt. He is part of the Calibre parasitic extraction team, with a focus on developing extraction rule decks for foundries. Mohamed holds a B.S. in Electronics Engineering from The American University in Cairo.


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